Continuous Electrical Conductivity Variation in M3 ...
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Continuous Electrical Conductivity Variation in M 3 (Hexaiminotriphenylene) 2 (M = Co, Ni, Cu) MOF Alloys Tianyang Chen, Jin-Hu Dou, Luming Yang, Chenyue Sun, Nicole J. Libretto, Grigorii Skorupskii, Jeffrey T. Miller, and Mircea Dincă * Cite This: J. Am. Chem. Soc. 2020, 142, 12367-12373 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: We report on the continuous fine-scale tuning of band gaps over 0.4 eV and of the electrical conductivity of over 4 orders of magnitude in a series of highly crystalline binary alloys of two-dimensional electrically conducting metal−organic frameworks M 3 (HITP) 2 (M = Co, Ni, Cu; HITP = 2,3,6,7,10,11-hexaimino- triphenylene). The isostructurality in the M 3 (HITP) 2 series permits the direct synthesis of binary alloys (M x M′ 3−x )(HITP) 2 (MM′ = CuNi, CoNi, and CoCu) with metal compositions precisely controlled by precursor ratios. We attribute the continuous tuning of both band gaps and electrical conductivity to changes in free- carrier concentrations and to subtle differences in the interlayer displacement or spacing, both of which are defined by metal substitution. The activation energy of (Co x Ni 3−x )(HITP) 2 alloys scales inversely with an increasing Ni percentage, confirming thermally activated bulk transport. ■ INTRODUCTION The ability to change the electronic energy levels of the valence and conduction bands in semiconductors is one of the foundational principles of modern electronics. In principle, the same applies to electrically conducting metal−organic frameworks (MOFs), a class of emerging crystalline porous conductors. 1,2 Finely tuning their electronic properties, be it their absolute conductivity or the activation energy for charge transport, is crucial for optimizing their performance as active materials in supercapacitors, 3−5 batteries, 6 electrocatalysis, 7−11 chemiresistive sensors, 12−17 and thermoelectrics. 18,19 However, this has proven difficult in conducting MOFs, where changes in the conductivity are most often enacted by varying the structure rather than the composition. Whereas different structures indeed lead to different conductivities, structural changes rarely allow for continuous fine-tuning of electronic energy levels. Notable exceptions involve redox doping, 20 such as in tetracyanoquinodimethane-doped Cu 3 (BTC) 2 , a system that allows smooth variation of the conductivity of over 6 orders of magnitude, 21 or more commonly via I 2 doping of relatively insulating host frameworks. 17 In inorganic semiconductors, a much more successful strategy toward band engineering is isostructural alloying, such as in Si 1−x Ge x . We show here that one of the prototypical electrically conducting MOFs, Ni 3 (HITP) 2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), is amenable to similar compositional tuning. Binary alloys (M x M′ 3−x )(HITP) 2 (MM′ = CuNi, CoNi, and CoCu) allow precise control over the M/ M′ ratios and are isostructural with the parent compounds M 3 (HITP) 2 (M = Co, Ni, Cu), here reported with significantly improved crystallinity compared to published procedures (see Figure 1). Alloying enables continuous variation of electrical conductivity of over 4 orders of magnitude from 5.8 × 10 −3 to 55.4 S/cm. ■ RESULTS AND DISCUSSION Although pure M 3 (HITP) 2 (M = Ni, Cu, Co) have been reported with varying degrees of crystallinity and poros- ity, 9,12,22 systematic investigations of potential mixed-metal phases required the development of consistent synthetic procedures for high-quality crystalline and porous powders of all three MOFs. Generally, the synthesis of M 3 (HITP) 2 involves the coordination of amino groups to metal ions, subsequent deprotonation of amino groups (typically by aqueous ammonia), and followed by oxidation in air. The absence of air prevents the formation of any precipitate (Figure S1) and air is thus considered critical. However, replacing ammonia with a weaker base, such as CH 3 CO 2 Na led to significant improvements in the crystallinity for Ni 3 (HITP) 2 Received: April 29, 2020 Published: June 12, 2020 Article pubs.acs.org/JACS © 2020 American Chemical Society 12367 https://dx.doi.org/10.1021/jacs.0c04458 J. Am. Chem. Soc. 2020, 142, 12367−12373 Downloaded via MASSACHUSETTS INST OF TECHNOLOGY on August 11, 2020 at 15:19:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Transcript of Continuous Electrical Conductivity Variation in M3 ...
Continuous Electrical Conductivity Variation inM3(Hexaiminotriphenylene)2 (M = Co Ni Cu) MOF AlloysTianyang Chen Jin-Hu Dou Luming Yang Chenyue Sun Nicole J Libretto Grigorii SkorupskiiJeffrey T Miller and Mircea Dinca
Cite This J Am Chem Soc 2020 142 12367minus12373 Read Online
ACCESS Metrics amp More Article Recommendations sı Supporting Information
ABSTRACT We report on the continuous fine-scale tuning ofband gaps over 04 eV and of the electrical conductivity of over 4orders of magnitude in a series of highly crystalline binary alloys oftwo-dimensional electrically conducting metalminusorganic frameworksM3(HITP)2 (M = Co Ni Cu HITP = 23671011-hexaimino-triphenylene) The isostructurality in the M3(HITP)2 series permitsthe direct synthesis of binary alloys (MxMprime3minusx)(HITP)2 (MMprime =CuNi CoNi and CoCu) with metal compositions preciselycontrolled by precursor ratios We attribute the continuous tuningof both band gaps and electrical conductivity to changes in free-carrier concentrations and to subtle differences in the interlayerdisplacement or spacing both of which are defined by metalsubstitution The activation energy of (CoxNi3minusx)(HITP)2 alloysscales inversely with an increasing Ni percentage confirming thermally activated bulk transport
INTRODUCTION
The ability to change the electronic energy levels of the valenceand conduction bands in semiconductors is one of thefoundational principles of modern electronics In principlethe same applies to electrically conducting metalminusorganicframeworks (MOFs) a class of emerging crystalline porousconductors12 Finely tuning their electronic properties be ittheir absolute conductivity or the activation energy for chargetransport is crucial for optimizing their performance as activematerials in supercapacitors3minus5 batteries6 electrocatalysis7minus11
chemiresistive sensors12minus17 and thermoelectrics1819 Howeverthis has proven difficult in conducting MOFs where changes inthe conductivity are most often enacted by varying thestructure rather than the composition Whereas differentstructures indeed lead to different conductivities structuralchanges rarely allow for continuous fine-tuning of electronicenergy levels Notable exceptions involve redox doping20 suchas in tetracyanoquinodimethane-doped Cu3(BTC)2 a systemthat allows smooth variation of the conductivity of over 6orders of magnitude21 or more commonly via I2 doping ofrelatively insulating host frameworks17
In inorganic semiconductors a much more successfulstrategy toward band engineering is isostructural alloyingsuch as in Si1minusxGex We show here that one of the prototypicalelectrically conducting MOFs Ni3(HITP)2 (HITP =23671011-hexaiminotriphenylene) is amenable to similarcompositional tuning Binary alloys (MxMprime3minusx)(HITP)2 (MMprime= CuNi CoNi and CoCu) allow precise control over the M
Mprime ratios and are isostructural with the parent compoundsM3(HITP)2 (M = Co Ni Cu) here reported with significantlyimproved crystallinity compared to published procedures (seeFigure 1) Alloying enables continuous variation of electricalconductivity of over 4 orders of magnitude from 58 times 10minus3 to554 Scm
RESULTS AND DISCUSSIONAlthough pure M3(HITP)2 (M = Ni Cu Co) have beenreported with varying degrees of crystallinity and poros-ity91222 systematic investigations of potential mixed-metalphases required the development of consistent syntheticprocedures for high-quality crystalline and porous powders ofall three MOFs Generally the synthesis of M3(HITP)2involves the coordination of amino groups to metal ionssubsequent deprotonation of amino groups (typically byaqueous ammonia) and followed by oxidation in air Theabsence of air prevents the formation of any precipitate (FigureS1) and air is thus considered critical However replacingammonia with a weaker base such as CH3CO2Na led tosignificant improvements in the crystallinity for Ni3(HITP)2
Received April 29 2020Published June 12 2020
ArticlepubsacsorgJACS
copy 2020 American Chemical Society12367
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
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and Cu3(HITP)2 (see Figures S3 and S4 and relateddiscussions in section S3 in the Supporting Information)Most strikingly Co3(HITP)2 crystallizes only in the presenceof CH3CO2Na with ammonia yielding an essentiallyamorphous phase (Figures S2 and S5minusS7) The crystallinityof all three compounds can be further improved by employingcoordinating solventsH2O mixtures (eg NN-dimethylfor-mamide (DMF) for Co3(HITP)2 dimethyl sulfoxide (DMSO)for Ni3(HITP)2 and NN-dimethylacetamide (DMA) forCu3(HITP)2) The organic cosolvents modulate the rate ofdeprotonation thereby retarding nucleation23 and have higherdonicitydonor numbers than water24 which affects thereversibility of the metalminusligand bond formation and furthercontrols crystal growth23
Crystallinity improvements also correlate with enhancedpermanent porosity N2 adsorption isotherms at 77 K forM3(HITP)2 samples activated by heating at 90 degC underdynamic vacuum gave BrunauerminusEmmettminusTeller (BET)surface areas of 8055 plusmn 05 8847 plusmn 09 and 4954 plusmn 13m2g for the Co Ni and Cu materials respectively (FigureS10) These are considerably higher than previously reportedvalues and confirm the improved synthetic procedures andmaterial quality developed here918 Uniform pore sizedistributions were found in M3(HITP)2 by a pore sizedistribution analysis (Figure S11) A thermogravimetricanalysis (TGA) revealed that M3(HITP)2 samples undergopronounced weight losses above 200 degC likely due todecomposition (Figure S12)Scanning electron microscopy (SEM) revealed that black
Ni3(HITP)2 and Cu3(HITP)2 powders consist of hexagonal1minus2 μm-long rod-like crystals whereas Co3(HITP)2 consistsof polycrystalline flakes several hundreds of nanometers wide(Figures S6 and S7) Their morphology was further confirmedby low-magnification transmission electron microscopy(Figures S16minusS18) All materials exhibit sufficiently highcrystallinity for the Pawley refinement of their respectivepowder X-ray diffraction (PXRD) data (Figure 2a) Surpris-ingly PXRD data for all materials gave best fits for theorthorhombic space group Cmcm (Figure 2a and Table S1)confirming a lower symmetry than the previously reportedhexagonal systems1222 The lower symmetry is also aconsequence of an improved crystallinity and indicates lessaveraging of the stacking arrangement in the c directionIndeed the synchrotron PXRD data allows the observation ofsubtle differences among the three materials for both the
interlayer displacement (D) along the b direction and theinterlayer spacing (S) along the c direction Thus Ni3(HITP)2exhibits the largest D value 156(2) Aring whereas Cu3(HITP)2has the smallest S value of only 316(8) Aring The latter issignificantly smaller than the previously reported value 330 Aringwhich was derived from a more poorly crystalline sample11
The S spacing is evidently connected with the strength of theinteraction between the 2D sheets which has recently beenshown to contribute significantly to the electrical conductivityin layered lanthanide MOFs25 precise measurements of thisvalue are important for future computations involving thesematerialsStructural details from the Pawley refinements were
confirmed by high-resolution transmission electron microscopy(HRTEM) with all three materials exhibiting hexagonallattices (Figure 3) Importantly the Fast-Fourier transform(FFT) analysis of the HRTEM micrographs provided lattice
Figure 1 Synthesis of M3(HITP)2 (M = Co Ni Cu) and(MxMprime3minusx)(HITP)2 alloys
Figure 2 (a) Synchrotron PXRD patterns and corresponding Pawleyrefinements for Co3(HITP)2 Ni3(HITP)2 and Cu3(HITP)2 Insetsshow the simulated structures (both the ab direction and c direction)The red dashed lines indicate interlayer displacement (b) Continuouschanges of the crystal lattice parameters as evidenced by shifts in the(110) peak position for (CoxCu3minusx)(HITP)2 (CoxNi3minusx)(HITP)2and (CuxNi3minusx)(HITP)2 alloys Solid lines are a guide to the eyedashed lines show the linear fit to the Vegardrsquos law with R2 values of098 092 and 098 respectively
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parameters which are in excellent agreement with thoseobtained from PXRD (Tables S2minusS4) HRTEM also allowsprecise measurements of πminusπ stacking distances of 325 Aring forNi3(HITP)2 and 312 Aring for Cu3(HITP)2 again in excellentagreement with those obtained from the PXRD analysis(Figures S16minusS18)Owing to their nearly identical structures M3(HITP)2 (M =
Co Ni Cu) can be produced from binary mixtures of metalprecursors with continuously variable compositions Toillustrate this we targeted three binary series (MxMprime3minusx)-(HITP)2 5 compositions in the CuNi space 5 for CoNiand 6 for CoCu The final MMprime ratios were determined byinductively coupled plasma-mass spectrometry (ICP-MS) X-ray photoelectron spectroscopy (XPS) andor energy-dispersive X-ray spectroscopy (EDS) all of which confirmedthat the final metal compositions were similar to those of theprecursors (Figure S19)A series of spectroscopic and analytical techniques
confirmed the uniform composition of the MOF alloys with(CoxNi3minusx)(HITP)2 as a representative example STEMelemental mapping of (CoxNi3minusx)(HITP)2 samples revealed auniform distribution of Co and Ni atoms within singlecrystallitesgrains confirming the formation of true mixedphases rather than individual particles or phase-segregatedislets of Co3(HITP)2 and Ni3(HITP)2 (Figures S20minusS25)Tellingly the morphology of (CoxNi3minusx)(HITP)2 particles alsoevolves gradually from nanoflakes for the cobalt-rich(Co238Ni062)(HITP)2 to nanorods for the nickel-rich(Co060Ni240)(HITP)2 (Figures S8 S9) Finally monotonousshifts of the (110) peak position in the corresponding PXRDpatterns confirm a systematic increase in the b unit cellparameter of (CoxNi3minusx)(HITP)2 with increasing Ni content(Figure 2b) Similar changes are found in the (CuxNi3minusx)-(HITP)2 and (CoxCu3minusx)(HITP)2 series (Figures 2b and S26and Table S5) all of which follow Vegardrsquos law an empiricalformula describing solid solutions26
XPS and X-ray absorption spectroscopy (XAS) confirmedthat all metals are in the +2 formal oxidation state and exhibitsquareminusplanar coordination geometry The XPS Co 2p32 peakof Co3(HITP)2 has a binding energy of 7809 eV with aprominent satellite at 7867 eV (Figure S29) These areconsistent with Co2+ compounds as satellite features rarelyappear for Co3+ compounds427 Furthermore Co3(HITP)2resides in the Co(II) region in the Wagner plot28 of Co based
on the Co 2p32 binding energy and Co LMM kinetic energy(7697 eV Figure S30) The XPS spectra of Ni3(HITP)2 andCu3(HITP)2 exhibit single Ni 2p32 and Cu 2p32 peaks at8552 and 9335 eV respectively as expected for Ni2+ and Cu2+
(Figure S29)2930 The weak satellite accompanying the Ni2p32 peak suggests Ni2+ is diamagnetic in Ni3(HITP)2
31 Thecomplementary Cu LMM peak with a kinetic energy of 9182eV also locates Cu3(HITP)2 in the Cu(II) region of theWagner plot of Cu (Figure S31) We note that the improvedsynthesis of Cu3(HITP)2 leads to pure Cu(II) samples incontrast with previous reports where mixed valence contribu-tions to the conductivity could not be ruled out12 XPS N 1sspectra of M3(HITP)2 can be deconvoluted into two peaks at3996 and 3979 eV which are attributed to the anilinic amine(NH) and the quinoid imine (N)3233 and correlatewell with the proposed partial oxidation of HATP to formHITP (Figure S29) The resulting charge neutrality of theframeworks is demonstrated by the absence of possibleadditional charge-balancing ions (eg Na+ and OAcminus FiguresS32 and S33)The X-ray absorption near-edge structure (XANES) analysis
of Co3(HITP)2 revealed a pre-edge feature at 77092 keV andan edge energy of 77204 keV well-matched with the valueexpected for Co2+ (Figure S34) The weak pre-edge feature at89712 keV in the XANES spectrum of Cu3(HITP)2 (FigureS37) is characteristic of Cu2+ which has been unambiguouslyassigned to a 1s rarr 3d transition This transition is notobserved in Cu+ with fully occupied 3d orbitals34minus36 TheXANES spectra of M3(HITP)2 and corresponding modelcomplexes MPc (Pc = phthalocyanine) exhibit similar featuresindicating that they have a similar squareminusplanar coordinationenvironment (Figures S34minusS37) The fit for the extended X-ray absorption fine structure (EXAFS) spectrum ofCo3(HITP)2 demonstrates the squareminusplanar coordinationmode of Co with four CoN bonds (with a fittedcoordination number of 39 Table S6) of 184 Aring (FigureS35) matching well with the structure modeled from PXRDdata The simulated CoN bond length is also comparable tothe 1824(5) Aring averaged CoN bond length of the Co(II)(s-bqdi)2 complex (s-bqdi = semi-o-benzoquinonediimine)37
Importantly the +2 oxidation states and squareminusplanarcoordination environment of the metal ions are conserved inall (MxMprime3minusx)(HITP)2 alloys as verified by XPS (section S16)X-band electron paramagnetic resonance (EPR) spectrosco-
py confirmed the presence of HITP-centered organic radicalswith g = 200 Peaks at g = 564 and sim19 in the EPR spectrumof Co3(HITP)2 are typical for high-spin Co2+ ions38 Similarlyg peaks at 217 and 202 observed for Cu3(HITP)2 are typicalfor squareminusplanar Cu2+ ions39 Expectedly for non-KramersNi2+ ions the X-band EPR spectrum of Ni3(HITP)2 exhibitsonly the signal for the ligand-based radical (Figure S42)Optical spectroscopy provided additional insight into the
electronic structures of (MxMprime3minusx)(HITP)2 Diffuse reflectanceUVminusvisible (DR-UVminusvis) spectroscopy revealed a clearabsorption edge for Co3(HITP)2 with broader absorptionedges in the near-infrared (IR) region for Ni3(HITP)2 andCu3(HITP)2 (Figure 4) The increase of the background IRabsorption generally associated with free-carrier absorption indegenerate or narrow-gap semiconductors40 suggests anenhancement of the free-carrier concentration fromCo3(HITP)2 to Ni3(HITP)2 (Figure 4a) Indeed diffusereflectance infrared Fourier transform spectra (DRIFTS) of(MxMprime3minusx)(HITP)2 also reveal a continuous increase of the
Figure 3 HRTEM images of Co3(HITP)2 (a and d) Ni3(HITP)2 (band e) and Cu3(HITP)2 (c and f) Insets show the FFT analysis ofthe micrographs in (d) (e) and (f)
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background absorption with an increasing Cu or Ni contentsuggesting the continuous elevation of the free-carrierconcentration (Figure S43) Plotting the DR-UVminusvis spectraof M3(HITP)2 in Tauc coordinates41minus43 revealed optical bandgaps Eo of 0719 0429 and 0291 eV for Co3(HITP)2Cu3(HITP)2 and Ni3(HITP)2 respectively The progressivelynarrower band gap when transitioning from Co to Cu and Ni isalso in agreement with a decreased free carrier concentrationfor the pure cobalt MOF Most importantly DR-UVminusvisspectra of (MxMprime3minusx)(HITP)2 (Figure S43) confirm that theoptical band gaps of the alloys can also be continuously tunedover 04 eV (cf MossminusBurstein shift)4144 They narrowprogressively from 0662 eV for (Co238Ni062)(HITP)2 to0395 eV for (Co060Ni240)(HITP)2 in the CoNi series from0762 eV for (Co247Cu053)(HITP)2 to 0513 eV for(Co052Cu248)(HITP)2 in the CoCu series and from 0392eV for (Cu232Ni068)(HITP)2 to 0326 eV for (Cu050Ni250)-(HITP)2 in the CuNi series (Figure 4c) Finally a DRIFTSanalysis of M3(HITP)2 and (MxMprime3minusx)(HITP)2 showed theexpected vibrational bands corresponding to HITP but nofeatures related to electronic transitions below the absorptionedges The influence of defects on optical properties is thusnegligibleThe smooth variation in electronic structures derived from
optical spectroscopy is also reflected in electrical conductivitymeasurements for the pure-phase M3(HITP)2 and the(MxMprime3minusx)(HITP)2 alloys Four-contact probe measurementsof polycrystalline pellets of M3(HITP)2 revealed bulkconductivity values σ of 0024 075 and 554 Scm for thepure Co Cu and Ni MOFs respectively at 296 K These arehigher than the bulk conductivities reported previously for thesame compounds1222 which were already among the mostconductive MOFs The metal ions exert an obvious influenceon electrical conductivity at least partially by contributing indifferent extents to the valence and conduction bands density
of states The electronic nature of the metal ion notwithstand-ing we attribute the differences in σ to the slight structuralvariations among the three materials In particular theinterlayer displacement D and stacking S exert an importanteffect on the transport normal to the sheets D appears to beoptimal in Ni3(HITP)2 (D asymp 17 Aring in the optimized calculatedstructure22) which also shows the highest bulk conductivityWith D deviating to 139(0) Aring for Co3(HITP)2 and 086(9) Aringfor Cu3(HITP)2 these materials exhibit lower conductivitiesSimilarly the higher σ of Cu3(HITP)2 relative to Co3(HITP)2can be traced to its smaller interlayer spacing 316(8) Aringcompared to 329(8) Aring for Co3(HITP)2In line with the spectroscopic and electrical data for the pure
phases the conductivity of (MxMprime3minusx)(HITP)2 alloys can betuned precisely for over 4 orders of magnitude from 58 times 10minus3
Scm in (Co247Cu053)(HITP)2 to 554 Scm in Ni3(HITP)2(Figure 5a) Variations in the electrical conductivity among the
16 different compositional alloys are in line with all of thediscussions above and can be traced to combinations ofsystematic changes in optical band gaps free carrierconcentrations and structural parameters S and D Forinstance increasing the Ni content in the (CoxNi3minusx)(HITP)2series shifts the D and S values in such a way as to enhanceinterlayer πminusπ interactions which in turn leads to higher σvalues Apparent deviations from these trends such as with(Co247Cu053)(HITP)2 are also in line with the slightly lowercrystallinity for that particular composition itself likely a resultof the relatively more pronounced structural mismatchbetween Co3(HITP)2 and Cu3(HITP)2 (Table S1 and FigureS26)Notably control measurements of mechanically blended
mixtures of pure-phase M3(HITP)2 samples behave differentlythan the alloyed phases with the more abundant sampledominating the transport in a nonlinear fashion For instancemixtures of Co3(HITP)2 and Ni3(HITP)2 are nearly as
Figure 4 DR-UVminusvis and DRIFT spectra of M3(HITP)2 and(MxMprime3minusx)(HITP)2 (a) Normalized KubelkaminusMunk-transformedspectra of M3(HITP)2 (b) Normalized Tauc plots from the spectrain (a) Dashed lines indicate best linear fits to the absorption edges(c) Optical band gaps Eo of (MxMprime3minusx)(HITP)2 Solid lines areguides to the eye dashed lines are linear fits of Eo
Figure 5 (a) Electrical conductivity data for (MxMprime3minusx)(HITP)2Solid lines are guide to the eye dashed lines are linear fits (b)Continuous changes of Ea in (CoxNi3minusx)(HITP)2 The dashed line isa linear fit with R2 = 098
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conductive as the former when the Ni is low and jump to aconductivity closer to that of the latter when the Ni reaches40 Surprisingly all blended Co3(HITP)2Cu3(HITP)2mixtures exhibit lower σ values than Co3(HITP)2 likely aconsequence of the work functionenergy level mismatchbetween individual particles of the two pure phases whichincreases the grain boundary resistance (Figure S43)Variable-temperature (VT) conductivity measurements
performed for the pure phases and for (CoxNi3minusx)(HITP)2as a representative example of the alloyed samples revealed atemperature-activated bulk transport from 200 to 350 K in allcases (Figure S44) The corresponding activation energies fortransport Ea correlate well with the σ values 157 meV forCo3(HITP)2 653 meV for Cu3(HITP)2 and 157 meV forNi3(HITP)2 Ea values for (CoxNi3minusx)(HITP)2 alloys varylinearly with the Ni content (Figure 5b) decreasing from 136meV for (Co238Ni062)(HITP)2 to 254 meV for (Co060Ni240)-(HITP)2 providing a final confirmation of continuouscompositional band engineering through metal substitution
CONCLUSIONSA considerable need exists for the improved synthesis ofconductive MOFs Here synthetic improvements in theproduction of pure-phase M3(HITP)2 (M = Co Ni Cu) ledto more crystalline and porous materials with higherconductivity values than previously reported These advancesalso enable a systematic investigation into the influence of themetal ion on physical properties in (MxMprime3minusx)(HITP)2 alloyswhich allow continuous tuning of optical and electronic bandgaps and precise modulation of electrical conductivity of over 4orders of magnitude These results highlight the importance ofsynthetic advances for a deeper understanding of structureminusfunction relationships that are crucial for further materialdevelopments in this field
ASSOCIATED CONTENTsı Supporting InformationThe Supporting Information is available free of charge athttpspubsacsorgdoi101021jacs0c04458
Additional experimental details and characterization data(PDF)
AUTHOR INFORMATIONCorresponding AuthorMircea Dinca minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0002-1262-1264Email mdincamitedu
AuthorsTianyang Chen minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0003-3142-8176
Jin-Hu Dou minus Department of Chemistry Massachusetts Instituteof Technology Cambridge Massachusetts 02139 UnitedStates orcidorg0000-0002-6920-9051
Luming Yang minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Chenyue Sun minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Nicole J Libretto minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates
Grigorii Skorupskii minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Jeffrey T Miller minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates orcidorg0000-0002-6269-0620
Complete contact information is available athttpspubsacsorg101021jacs0c04458
NotesThe authors declare no competing financial interest
ACKNOWLEDGMENTSThis work was supported by the Army Research Office (AwardNo W911NF-17-1-0174) Part of this work (SEM part ofXPS and HAADF-STEM) made use of the MRSEC SharedExperimental Facilities at MIT supported by the NationalScience Foundation under Award No DMR-1419807 Wethank Dr Yong Zhang for their assistance with HAADF-STEM This research used resources (aberration-correctedTEM) of the Center for Functional Nanomaterials which is aUS Department of Energy Office of Science Facility atBrookhaven National Laboratory under Contract No DE-SC0012704 Use of the Advanced Photon Source at ArgonneNational Laboratory was supported by the US Department ofEnergy Office of Science Office of Basic Energy Sciencesunder Contract No DE-AC02-06CH11357 Raman spectros-copy and part of XPS were performed at the Harvard Centerfor Nanoscale Systems (CNS) a member of the NationalNanotechnology Infrastructure Network (NNIN) which issupported by the National Science Foundation under AwardNo ECS-0335765
REFERENCES(1) Sun L Campbell M G Dinca M Electrically ConductivePorous MetalminusOrganic Frameworks Angew Chem Int Ed 2016 553566minus3579(2) (a) Xie L S Skorupskii G Dinca M Electrically ConductiveMetalminusOrganic Frameworks Chem Rev 2020 120 (b) Guo L SunJ Wei J Liu Y Hou L Yuan C Conductive MetalminusOrganicFrameworks Recent Advances in Electrochemical Energy-relatedApplications and Perspectives Carbon Energy 2020 2 203(3) Sheberla D Bachman J C Elias J S Sun C-J Shao-HornY Dinca M Conductive MOF Electrodes for Stable Supercapacitorswith High Areal Capacitance Nat Mater 2017 16 220minus224(4) Park J Lee M Feng D Huang Z Hinckley A CYakovenko A Zou X Cui Y Bao Z Stabilization ofHexaaminobenzene in a 2D Conductive MetalminusOrganic Frameworkfor High Power Sodium Storage J Am Chem Soc 2018 140 10315minus10323(5) (a) Feng D Lei T Lukatskaya M R Park J Huang Z LeeM Shaw L Chen S Yakovenko A A Kulkarni A Xiao JFredrickson K Tok J B Zou X Cui Y Bao Z Robust andConductive TwominusDimensional MetalminusOrganic Frameworks withExceptionally High Volumetric and Areal Capacitance Nat Energy2018 3 30minus36 (b) Sun J Guo L Sun X Zhang J Liu Y HouL Yuan C Conductive Co-based MetalminusOrganic FrameworkNanowires A Competitive High-rate Anode Towards Advanced Li-ion Capacitors J Mater Chem A 2019 7 24788minus24791(6) Shinde S S Lee C H Jung J-Y Wagh N K Kim S-HKim D-H Lin C Lee S U Lee J-H Unveiling Dual-Linkage 3DHexaiminobenzene MetalminusOrganic Frameworks towards Longminus
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Lasting Advanced Reversible Zn-Air Batteries Energy Environ Sci2019 12 727minus738(7) Clough A J Yoo J W Mecklenburg M H Marinescu S CTwo-Dimensional MetalminusOrganic Surfaces for Efficient HydrogenEvolution from Water J Am Chem Soc 2015 137 118minus121(8) Miner E M Fukushima T Sheberla D Sun LSurendranath Y Dinca M Electrochemical Oxygen ReductionCatalysed by Ni3(Hexaiminotriphenylene)2 Nat Commun 2016 710942(9) Dong R Zheng Z Tranca D C Zhang J ChandrasekharN Liu S Zhuang X Seifert G Feng X Immobilizing MolecularMetal DithioleneminusDiamine Complexes on 2D MetalminusOrganicFrameworks for Electrocatalytic H2 Production Chem minusEur J2017 23 2255minus2260(10) Downes C A Clough A J Chen K Yoo J W MarinescuS C Evaluation of the H2 Evolving Activity of BenzenehexathiolateCoordination Frameworks and the Effect of Film Thickness on H2Production ACS Appl Mater Interfaces 2018 10 1719minus1727(11) Miner E M Wang L Dinca M Modular O2 Electro-reduction Activity in TriphenyleneminusBased MetalminusOrganic Frame-works Chem Sci 2018 9 6286minus6291(12) Campbell M G Sheberla D Liu S F Swager T M DincaM Cu3(Hexaiminotriphenylene)2 An Electrically Conductive 2DMetalminusOrganic Framework for Chemiresistive Sensing Angew ChemInt Ed 2015 54 4349minus4352(13) Campbell M G Liu S F Swager T M Dinca MChemiresistive Sensor Arrays from Conductive 2D MetalminusOrganicFrameworks J Am Chem Soc 2015 137 13780minus13783(14) Smith M K Mirica K A SelfminusOrganized Frameworks onTextiles (SOFT) Conductive Fabrics for Simultaneous SensingCapture and Filtration of Gases J Am Chem Soc 2017 139 16759minus16767(15) Rubio-Gimenez V Almora-Barrios N Escorcia-Ariza GGalbiati M Sessolo M Tatay S Marti-Gastaldo C Origin of theChemiresistive Response of Ultrathin Films of Conductive MetalminusOrganic Frameworks Angew Chem Int Ed 2018 57 15086minus15090(16) Meng Z Aykanat A Mirica K A Welding Metal-lophthalocyanines into Bimetallic Molecular Meshes for Ultra-sensitive LowminusPower Chemiresistive Detection of Gases J AmChem Soc 2019 141 2046minus2053(17) Aubrey M L Kapelewski M T Melville J F Oktawiec JPresti D Gagliardi L Long J R Chemiresistive Detection ofGaseous Hydrocarbons and Interrogation of Charge Transport inCu[Ni(23-Pyrazinedithiolate)2] by Gas Adsorption J Am Chem Soc2019 141 5005minus5013(18) Sun L Liao B Sheberla D Kraemer D Zhou J Stach EA Zakharov D Stavila V Talin A A Ge Y Allendorf M DChen G Leonard F Dinca M A Microporous and NaturallyNanostructured Thermoelectric MetalminusOrganic Framework withUltralow Thermal Conductivity Joule 2017 1 168minus177(19) Erickson K J Leonard F Stavila V Foster M E SpataruC D Jones R E Foley B M Hopkins P E Allendorf M DTalin A A Thin Film Thermoelectric MetalminusOrganic Frameworkwith High Seebeck Coefficient and Low Thermal Conductivity AdvMater 2015 27 3453minus3459(20) Xie L S Sun L Wan R Park S S DeGayner J AHendon C H Dinca M Tunable Mixed-Valence Doping TowardRecord Electrical Conductivity in a Three-Dimensional MetalminusOrganic Framework J Am Chem Soc 2018 140 7411minus7414(21) Talin A A Centrone A Ford A C Foster M E StavilaV Haney P Kinney R A Szalai V El Gabaly F Yoon H PLeonard F Allendorf M D Tunable Electrical Conductivity inMetalminusOrganic Framework ThinminusFilm Devices Science 2014 34366minus69(22) Sheberla D Sun L Blood-Forsythe M A Er S Wade CR Brozek C K Aspuru-Guzik A Dinca M High ElectricalCondutivity in Ni3(23671011-Hexaiminotriphenylene)2 a Semi-conducting MetalminusOrganic Graphene Analogue J Am Chem Soc2014 136 8859minus8862
(23) Van Vleet M J Weng T Li X Schmidt J R In Situ TimeminusResolved and Mechanistic Studies of MetalminusOrganic FrameworkNucleation and Growth Chem Rev 2018 118 3681minus3721(24) Marcus Y The Effectivity of Solvents as Electron Pair DonorsJ Solution Chem 1984 13 599minus624(25) Skorupskii G Trump B A Kasel T W Brown C MHendon C H Dinca M Efficient and Tunable OneminusDimensionalCharge Transport in Layered Lanthanide MetalminusOrganic Frame-works Nat Chem 2020 12 131minus136(26) Denton A R Ashcroft N W Vegardrsquos Law Phys Rev A AtMol Opt Phys 1991 43 3161minus3164(27) McIntyre N S Cook M G X-ray Photoelectron Studies onSome Oxides and Hydroxides of Cobalt Nickel and Copper AnalChem 1975 47 2208minus2213(28) Moretti G Auger Parameter and Wagner Plot in theCharacterization of Chemical States by Xminusray PhotoelectronSpectroscopy A Review J Electron Spectrosc Relat Phenom 199895 95minus144(29) Matienzo J Yin L I Grim S O Swartz W E Jr XminusrayPhotoelectron Spectroscopy of Nickel Compounds Inorg Chem1973 12 2762minus2769(30) Frost D C Ishitani A McDowell C A Xminusray PhotoelectronSpectroscopy of Copper Compounds Mol Phys 1972 24 861minus877(31) Yin L Adler I Tsang T Matienzo L J Grim S OParamagnetism and Shakeminusup Satellites in X-ray PhotoelectronSpectra Chem Phys Lett 1974 24 81minus84(32) Tan K L Tan B T G Kang E T Neoh K G XminusrayPhotoelectron Spectroscopy Studies of the Chemical Structure ofPolyaniline Phys Rev B Condens Matter Mater Phys 1989 398070minus8073(33) Mohtasebi A Chowdhury T Hsu L H Biesinger M CKruse P Interfacial Charge Transfer Between Phenylminuscapped AnilineTetramer Films and Iron Oxide Surfaces J Phys Chem C 2016 12029248minus29263(34) Bair R A Goddard W A III Ab initio Studies of the XminusrayAbsorption Edge in Copper Complexes I Atomic Cu2+ andCu(II)Cl2 Phys Rev B Condens Matter Mater Phys 1980 222767minus2776(35) Sano M Komorita S Yamatera H XANES Spectra ofCopper (II) Complexes Correlation of the Intensity of the 1s rarr 3dTransition and the Shape of the Complex Inorg Chem 1992 31459minus463(36) Chaboy J Munoz-Paez A Carrera F Merkling P MarcosE S Ab initio X-ray Absorption Study of Copper Kminusedge XANESSpectra in Cu(II) Compounds Phys Rev B Condens Matter MaterPhys 2005 71 134208(37) Peng S-M Chen C-T Liaw D-S Chen C-I Wang YEstablishment of the Bond Patterns of o-Benzoquinonediimine andSemi-o-Benzoquinonediimine Crystal Structures of Metal Complexes[FeII(bqdi)3](PF6)2[Co
II(s-bqdi)2] and [CoIIICl(s-bqdi)2)] InorgChim Acta 1985 101 L31minusL33(38) Jimenez H R Salgado J Moratal J M Morgenstern-Badarau I EPR and Magnetic Susceptibility Studies of Cobalt (II)-and Nickel (II)-substituted Azurins from Pseudomonas AeruginosaElectronic Structure of the Active Sites Inorg Chem 1996 35 2737minus2741(39) Garribba E Micera G The Determination of the Geometry ofCu (II) Complexes An EPR Spectroscopy Experiment J Chem Educ2006 83 1229minus1232(40) Amalric Popescu D Herrmann J-M Ensuque A Bozon-Verduraz F Nanosized Tin Dioxide Spectroscopic (UV-Vis NIREPR) and Electrical Conductivity Studies Phys Chem Chem Phys2001 3 2522minus2530(41) Chu J Sher A Physics and Properties of Narrow GapSemiconductors Springer New York NY 2008(42) Nowak M Kauch B Szperlich P Determination of EnergyBand Gap of Nanocrystalline SbSI Using Diffuse ReflectanceSpectroscopy Rev Sci Instrum 2009 80 No 046107
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(43) Gibbs Z M LaLonde A Snyder G J Optical Band Gap andthe Burstein-Moss Effect in Iodine Doped PbTe Using DiffuseReflectance Infrared Fourier Transform Spectroscopy New J Phys2013 15 No 075020(44) Kudman I Seidel T Absorption Edge in Degenerate pminusTypeGaAs J Appl Phys 1962 33 771minus773
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and Cu3(HITP)2 (see Figures S3 and S4 and relateddiscussions in section S3 in the Supporting Information)Most strikingly Co3(HITP)2 crystallizes only in the presenceof CH3CO2Na with ammonia yielding an essentiallyamorphous phase (Figures S2 and S5minusS7) The crystallinityof all three compounds can be further improved by employingcoordinating solventsH2O mixtures (eg NN-dimethylfor-mamide (DMF) for Co3(HITP)2 dimethyl sulfoxide (DMSO)for Ni3(HITP)2 and NN-dimethylacetamide (DMA) forCu3(HITP)2) The organic cosolvents modulate the rate ofdeprotonation thereby retarding nucleation23 and have higherdonicitydonor numbers than water24 which affects thereversibility of the metalminusligand bond formation and furthercontrols crystal growth23
Crystallinity improvements also correlate with enhancedpermanent porosity N2 adsorption isotherms at 77 K forM3(HITP)2 samples activated by heating at 90 degC underdynamic vacuum gave BrunauerminusEmmettminusTeller (BET)surface areas of 8055 plusmn 05 8847 plusmn 09 and 4954 plusmn 13m2g for the Co Ni and Cu materials respectively (FigureS10) These are considerably higher than previously reportedvalues and confirm the improved synthetic procedures andmaterial quality developed here918 Uniform pore sizedistributions were found in M3(HITP)2 by a pore sizedistribution analysis (Figure S11) A thermogravimetricanalysis (TGA) revealed that M3(HITP)2 samples undergopronounced weight losses above 200 degC likely due todecomposition (Figure S12)Scanning electron microscopy (SEM) revealed that black
Ni3(HITP)2 and Cu3(HITP)2 powders consist of hexagonal1minus2 μm-long rod-like crystals whereas Co3(HITP)2 consistsof polycrystalline flakes several hundreds of nanometers wide(Figures S6 and S7) Their morphology was further confirmedby low-magnification transmission electron microscopy(Figures S16minusS18) All materials exhibit sufficiently highcrystallinity for the Pawley refinement of their respectivepowder X-ray diffraction (PXRD) data (Figure 2a) Surpris-ingly PXRD data for all materials gave best fits for theorthorhombic space group Cmcm (Figure 2a and Table S1)confirming a lower symmetry than the previously reportedhexagonal systems1222 The lower symmetry is also aconsequence of an improved crystallinity and indicates lessaveraging of the stacking arrangement in the c directionIndeed the synchrotron PXRD data allows the observation ofsubtle differences among the three materials for both the
interlayer displacement (D) along the b direction and theinterlayer spacing (S) along the c direction Thus Ni3(HITP)2exhibits the largest D value 156(2) Aring whereas Cu3(HITP)2has the smallest S value of only 316(8) Aring The latter issignificantly smaller than the previously reported value 330 Aringwhich was derived from a more poorly crystalline sample11
The S spacing is evidently connected with the strength of theinteraction between the 2D sheets which has recently beenshown to contribute significantly to the electrical conductivityin layered lanthanide MOFs25 precise measurements of thisvalue are important for future computations involving thesematerialsStructural details from the Pawley refinements were
confirmed by high-resolution transmission electron microscopy(HRTEM) with all three materials exhibiting hexagonallattices (Figure 3) Importantly the Fast-Fourier transform(FFT) analysis of the HRTEM micrographs provided lattice
Figure 1 Synthesis of M3(HITP)2 (M = Co Ni Cu) and(MxMprime3minusx)(HITP)2 alloys
Figure 2 (a) Synchrotron PXRD patterns and corresponding Pawleyrefinements for Co3(HITP)2 Ni3(HITP)2 and Cu3(HITP)2 Insetsshow the simulated structures (both the ab direction and c direction)The red dashed lines indicate interlayer displacement (b) Continuouschanges of the crystal lattice parameters as evidenced by shifts in the(110) peak position for (CoxCu3minusx)(HITP)2 (CoxNi3minusx)(HITP)2and (CuxNi3minusx)(HITP)2 alloys Solid lines are a guide to the eyedashed lines show the linear fit to the Vegardrsquos law with R2 values of098 092 and 098 respectively
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parameters which are in excellent agreement with thoseobtained from PXRD (Tables S2minusS4) HRTEM also allowsprecise measurements of πminusπ stacking distances of 325 Aring forNi3(HITP)2 and 312 Aring for Cu3(HITP)2 again in excellentagreement with those obtained from the PXRD analysis(Figures S16minusS18)Owing to their nearly identical structures M3(HITP)2 (M =
Co Ni Cu) can be produced from binary mixtures of metalprecursors with continuously variable compositions Toillustrate this we targeted three binary series (MxMprime3minusx)-(HITP)2 5 compositions in the CuNi space 5 for CoNiand 6 for CoCu The final MMprime ratios were determined byinductively coupled plasma-mass spectrometry (ICP-MS) X-ray photoelectron spectroscopy (XPS) andor energy-dispersive X-ray spectroscopy (EDS) all of which confirmedthat the final metal compositions were similar to those of theprecursors (Figure S19)A series of spectroscopic and analytical techniques
confirmed the uniform composition of the MOF alloys with(CoxNi3minusx)(HITP)2 as a representative example STEMelemental mapping of (CoxNi3minusx)(HITP)2 samples revealed auniform distribution of Co and Ni atoms within singlecrystallitesgrains confirming the formation of true mixedphases rather than individual particles or phase-segregatedislets of Co3(HITP)2 and Ni3(HITP)2 (Figures S20minusS25)Tellingly the morphology of (CoxNi3minusx)(HITP)2 particles alsoevolves gradually from nanoflakes for the cobalt-rich(Co238Ni062)(HITP)2 to nanorods for the nickel-rich(Co060Ni240)(HITP)2 (Figures S8 S9) Finally monotonousshifts of the (110) peak position in the corresponding PXRDpatterns confirm a systematic increase in the b unit cellparameter of (CoxNi3minusx)(HITP)2 with increasing Ni content(Figure 2b) Similar changes are found in the (CuxNi3minusx)-(HITP)2 and (CoxCu3minusx)(HITP)2 series (Figures 2b and S26and Table S5) all of which follow Vegardrsquos law an empiricalformula describing solid solutions26
XPS and X-ray absorption spectroscopy (XAS) confirmedthat all metals are in the +2 formal oxidation state and exhibitsquareminusplanar coordination geometry The XPS Co 2p32 peakof Co3(HITP)2 has a binding energy of 7809 eV with aprominent satellite at 7867 eV (Figure S29) These areconsistent with Co2+ compounds as satellite features rarelyappear for Co3+ compounds427 Furthermore Co3(HITP)2resides in the Co(II) region in the Wagner plot28 of Co based
on the Co 2p32 binding energy and Co LMM kinetic energy(7697 eV Figure S30) The XPS spectra of Ni3(HITP)2 andCu3(HITP)2 exhibit single Ni 2p32 and Cu 2p32 peaks at8552 and 9335 eV respectively as expected for Ni2+ and Cu2+
(Figure S29)2930 The weak satellite accompanying the Ni2p32 peak suggests Ni2+ is diamagnetic in Ni3(HITP)2
31 Thecomplementary Cu LMM peak with a kinetic energy of 9182eV also locates Cu3(HITP)2 in the Cu(II) region of theWagner plot of Cu (Figure S31) We note that the improvedsynthesis of Cu3(HITP)2 leads to pure Cu(II) samples incontrast with previous reports where mixed valence contribu-tions to the conductivity could not be ruled out12 XPS N 1sspectra of M3(HITP)2 can be deconvoluted into two peaks at3996 and 3979 eV which are attributed to the anilinic amine(NH) and the quinoid imine (N)3233 and correlatewell with the proposed partial oxidation of HATP to formHITP (Figure S29) The resulting charge neutrality of theframeworks is demonstrated by the absence of possibleadditional charge-balancing ions (eg Na+ and OAcminus FiguresS32 and S33)The X-ray absorption near-edge structure (XANES) analysis
of Co3(HITP)2 revealed a pre-edge feature at 77092 keV andan edge energy of 77204 keV well-matched with the valueexpected for Co2+ (Figure S34) The weak pre-edge feature at89712 keV in the XANES spectrum of Cu3(HITP)2 (FigureS37) is characteristic of Cu2+ which has been unambiguouslyassigned to a 1s rarr 3d transition This transition is notobserved in Cu+ with fully occupied 3d orbitals34minus36 TheXANES spectra of M3(HITP)2 and corresponding modelcomplexes MPc (Pc = phthalocyanine) exhibit similar featuresindicating that they have a similar squareminusplanar coordinationenvironment (Figures S34minusS37) The fit for the extended X-ray absorption fine structure (EXAFS) spectrum ofCo3(HITP)2 demonstrates the squareminusplanar coordinationmode of Co with four CoN bonds (with a fittedcoordination number of 39 Table S6) of 184 Aring (FigureS35) matching well with the structure modeled from PXRDdata The simulated CoN bond length is also comparable tothe 1824(5) Aring averaged CoN bond length of the Co(II)(s-bqdi)2 complex (s-bqdi = semi-o-benzoquinonediimine)37
Importantly the +2 oxidation states and squareminusplanarcoordination environment of the metal ions are conserved inall (MxMprime3minusx)(HITP)2 alloys as verified by XPS (section S16)X-band electron paramagnetic resonance (EPR) spectrosco-
py confirmed the presence of HITP-centered organic radicalswith g = 200 Peaks at g = 564 and sim19 in the EPR spectrumof Co3(HITP)2 are typical for high-spin Co2+ ions38 Similarlyg peaks at 217 and 202 observed for Cu3(HITP)2 are typicalfor squareminusplanar Cu2+ ions39 Expectedly for non-KramersNi2+ ions the X-band EPR spectrum of Ni3(HITP)2 exhibitsonly the signal for the ligand-based radical (Figure S42)Optical spectroscopy provided additional insight into the
electronic structures of (MxMprime3minusx)(HITP)2 Diffuse reflectanceUVminusvisible (DR-UVminusvis) spectroscopy revealed a clearabsorption edge for Co3(HITP)2 with broader absorptionedges in the near-infrared (IR) region for Ni3(HITP)2 andCu3(HITP)2 (Figure 4) The increase of the background IRabsorption generally associated with free-carrier absorption indegenerate or narrow-gap semiconductors40 suggests anenhancement of the free-carrier concentration fromCo3(HITP)2 to Ni3(HITP)2 (Figure 4a) Indeed diffusereflectance infrared Fourier transform spectra (DRIFTS) of(MxMprime3minusx)(HITP)2 also reveal a continuous increase of the
Figure 3 HRTEM images of Co3(HITP)2 (a and d) Ni3(HITP)2 (band e) and Cu3(HITP)2 (c and f) Insets show the FFT analysis ofthe micrographs in (d) (e) and (f)
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background absorption with an increasing Cu or Ni contentsuggesting the continuous elevation of the free-carrierconcentration (Figure S43) Plotting the DR-UVminusvis spectraof M3(HITP)2 in Tauc coordinates41minus43 revealed optical bandgaps Eo of 0719 0429 and 0291 eV for Co3(HITP)2Cu3(HITP)2 and Ni3(HITP)2 respectively The progressivelynarrower band gap when transitioning from Co to Cu and Ni isalso in agreement with a decreased free carrier concentrationfor the pure cobalt MOF Most importantly DR-UVminusvisspectra of (MxMprime3minusx)(HITP)2 (Figure S43) confirm that theoptical band gaps of the alloys can also be continuously tunedover 04 eV (cf MossminusBurstein shift)4144 They narrowprogressively from 0662 eV for (Co238Ni062)(HITP)2 to0395 eV for (Co060Ni240)(HITP)2 in the CoNi series from0762 eV for (Co247Cu053)(HITP)2 to 0513 eV for(Co052Cu248)(HITP)2 in the CoCu series and from 0392eV for (Cu232Ni068)(HITP)2 to 0326 eV for (Cu050Ni250)-(HITP)2 in the CuNi series (Figure 4c) Finally a DRIFTSanalysis of M3(HITP)2 and (MxMprime3minusx)(HITP)2 showed theexpected vibrational bands corresponding to HITP but nofeatures related to electronic transitions below the absorptionedges The influence of defects on optical properties is thusnegligibleThe smooth variation in electronic structures derived from
optical spectroscopy is also reflected in electrical conductivitymeasurements for the pure-phase M3(HITP)2 and the(MxMprime3minusx)(HITP)2 alloys Four-contact probe measurementsof polycrystalline pellets of M3(HITP)2 revealed bulkconductivity values σ of 0024 075 and 554 Scm for thepure Co Cu and Ni MOFs respectively at 296 K These arehigher than the bulk conductivities reported previously for thesame compounds1222 which were already among the mostconductive MOFs The metal ions exert an obvious influenceon electrical conductivity at least partially by contributing indifferent extents to the valence and conduction bands density
of states The electronic nature of the metal ion notwithstand-ing we attribute the differences in σ to the slight structuralvariations among the three materials In particular theinterlayer displacement D and stacking S exert an importanteffect on the transport normal to the sheets D appears to beoptimal in Ni3(HITP)2 (D asymp 17 Aring in the optimized calculatedstructure22) which also shows the highest bulk conductivityWith D deviating to 139(0) Aring for Co3(HITP)2 and 086(9) Aringfor Cu3(HITP)2 these materials exhibit lower conductivitiesSimilarly the higher σ of Cu3(HITP)2 relative to Co3(HITP)2can be traced to its smaller interlayer spacing 316(8) Aringcompared to 329(8) Aring for Co3(HITP)2In line with the spectroscopic and electrical data for the pure
phases the conductivity of (MxMprime3minusx)(HITP)2 alloys can betuned precisely for over 4 orders of magnitude from 58 times 10minus3
Scm in (Co247Cu053)(HITP)2 to 554 Scm in Ni3(HITP)2(Figure 5a) Variations in the electrical conductivity among the
16 different compositional alloys are in line with all of thediscussions above and can be traced to combinations ofsystematic changes in optical band gaps free carrierconcentrations and structural parameters S and D Forinstance increasing the Ni content in the (CoxNi3minusx)(HITP)2series shifts the D and S values in such a way as to enhanceinterlayer πminusπ interactions which in turn leads to higher σvalues Apparent deviations from these trends such as with(Co247Cu053)(HITP)2 are also in line with the slightly lowercrystallinity for that particular composition itself likely a resultof the relatively more pronounced structural mismatchbetween Co3(HITP)2 and Cu3(HITP)2 (Table S1 and FigureS26)Notably control measurements of mechanically blended
mixtures of pure-phase M3(HITP)2 samples behave differentlythan the alloyed phases with the more abundant sampledominating the transport in a nonlinear fashion For instancemixtures of Co3(HITP)2 and Ni3(HITP)2 are nearly as
Figure 4 DR-UVminusvis and DRIFT spectra of M3(HITP)2 and(MxMprime3minusx)(HITP)2 (a) Normalized KubelkaminusMunk-transformedspectra of M3(HITP)2 (b) Normalized Tauc plots from the spectrain (a) Dashed lines indicate best linear fits to the absorption edges(c) Optical band gaps Eo of (MxMprime3minusx)(HITP)2 Solid lines areguides to the eye dashed lines are linear fits of Eo
Figure 5 (a) Electrical conductivity data for (MxMprime3minusx)(HITP)2Solid lines are guide to the eye dashed lines are linear fits (b)Continuous changes of Ea in (CoxNi3minusx)(HITP)2 The dashed line isa linear fit with R2 = 098
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conductive as the former when the Ni is low and jump to aconductivity closer to that of the latter when the Ni reaches40 Surprisingly all blended Co3(HITP)2Cu3(HITP)2mixtures exhibit lower σ values than Co3(HITP)2 likely aconsequence of the work functionenergy level mismatchbetween individual particles of the two pure phases whichincreases the grain boundary resistance (Figure S43)Variable-temperature (VT) conductivity measurements
performed for the pure phases and for (CoxNi3minusx)(HITP)2as a representative example of the alloyed samples revealed atemperature-activated bulk transport from 200 to 350 K in allcases (Figure S44) The corresponding activation energies fortransport Ea correlate well with the σ values 157 meV forCo3(HITP)2 653 meV for Cu3(HITP)2 and 157 meV forNi3(HITP)2 Ea values for (CoxNi3minusx)(HITP)2 alloys varylinearly with the Ni content (Figure 5b) decreasing from 136meV for (Co238Ni062)(HITP)2 to 254 meV for (Co060Ni240)-(HITP)2 providing a final confirmation of continuouscompositional band engineering through metal substitution
CONCLUSIONSA considerable need exists for the improved synthesis ofconductive MOFs Here synthetic improvements in theproduction of pure-phase M3(HITP)2 (M = Co Ni Cu) ledto more crystalline and porous materials with higherconductivity values than previously reported These advancesalso enable a systematic investigation into the influence of themetal ion on physical properties in (MxMprime3minusx)(HITP)2 alloyswhich allow continuous tuning of optical and electronic bandgaps and precise modulation of electrical conductivity of over 4orders of magnitude These results highlight the importance ofsynthetic advances for a deeper understanding of structureminusfunction relationships that are crucial for further materialdevelopments in this field
ASSOCIATED CONTENTsı Supporting InformationThe Supporting Information is available free of charge athttpspubsacsorgdoi101021jacs0c04458
Additional experimental details and characterization data(PDF)
AUTHOR INFORMATIONCorresponding AuthorMircea Dinca minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0002-1262-1264Email mdincamitedu
AuthorsTianyang Chen minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0003-3142-8176
Jin-Hu Dou minus Department of Chemistry Massachusetts Instituteof Technology Cambridge Massachusetts 02139 UnitedStates orcidorg0000-0002-6920-9051
Luming Yang minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Chenyue Sun minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Nicole J Libretto minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates
Grigorii Skorupskii minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Jeffrey T Miller minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates orcidorg0000-0002-6269-0620
Complete contact information is available athttpspubsacsorg101021jacs0c04458
NotesThe authors declare no competing financial interest
ACKNOWLEDGMENTSThis work was supported by the Army Research Office (AwardNo W911NF-17-1-0174) Part of this work (SEM part ofXPS and HAADF-STEM) made use of the MRSEC SharedExperimental Facilities at MIT supported by the NationalScience Foundation under Award No DMR-1419807 Wethank Dr Yong Zhang for their assistance with HAADF-STEM This research used resources (aberration-correctedTEM) of the Center for Functional Nanomaterials which is aUS Department of Energy Office of Science Facility atBrookhaven National Laboratory under Contract No DE-SC0012704 Use of the Advanced Photon Source at ArgonneNational Laboratory was supported by the US Department ofEnergy Office of Science Office of Basic Energy Sciencesunder Contract No DE-AC02-06CH11357 Raman spectros-copy and part of XPS were performed at the Harvard Centerfor Nanoscale Systems (CNS) a member of the NationalNanotechnology Infrastructure Network (NNIN) which issupported by the National Science Foundation under AwardNo ECS-0335765
REFERENCES(1) Sun L Campbell M G Dinca M Electrically ConductivePorous MetalminusOrganic Frameworks Angew Chem Int Ed 2016 553566minus3579(2) (a) Xie L S Skorupskii G Dinca M Electrically ConductiveMetalminusOrganic Frameworks Chem Rev 2020 120 (b) Guo L SunJ Wei J Liu Y Hou L Yuan C Conductive MetalminusOrganicFrameworks Recent Advances in Electrochemical Energy-relatedApplications and Perspectives Carbon Energy 2020 2 203(3) Sheberla D Bachman J C Elias J S Sun C-J Shao-HornY Dinca M Conductive MOF Electrodes for Stable Supercapacitorswith High Areal Capacitance Nat Mater 2017 16 220minus224(4) Park J Lee M Feng D Huang Z Hinckley A CYakovenko A Zou X Cui Y Bao Z Stabilization ofHexaaminobenzene in a 2D Conductive MetalminusOrganic Frameworkfor High Power Sodium Storage J Am Chem Soc 2018 140 10315minus10323(5) (a) Feng D Lei T Lukatskaya M R Park J Huang Z LeeM Shaw L Chen S Yakovenko A A Kulkarni A Xiao JFredrickson K Tok J B Zou X Cui Y Bao Z Robust andConductive TwominusDimensional MetalminusOrganic Frameworks withExceptionally High Volumetric and Areal Capacitance Nat Energy2018 3 30minus36 (b) Sun J Guo L Sun X Zhang J Liu Y HouL Yuan C Conductive Co-based MetalminusOrganic FrameworkNanowires A Competitive High-rate Anode Towards Advanced Li-ion Capacitors J Mater Chem A 2019 7 24788minus24791(6) Shinde S S Lee C H Jung J-Y Wagh N K Kim S-HKim D-H Lin C Lee S U Lee J-H Unveiling Dual-Linkage 3DHexaiminobenzene MetalminusOrganic Frameworks towards Longminus
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Lasting Advanced Reversible Zn-Air Batteries Energy Environ Sci2019 12 727minus738(7) Clough A J Yoo J W Mecklenburg M H Marinescu S CTwo-Dimensional MetalminusOrganic Surfaces for Efficient HydrogenEvolution from Water J Am Chem Soc 2015 137 118minus121(8) Miner E M Fukushima T Sheberla D Sun LSurendranath Y Dinca M Electrochemical Oxygen ReductionCatalysed by Ni3(Hexaiminotriphenylene)2 Nat Commun 2016 710942(9) Dong R Zheng Z Tranca D C Zhang J ChandrasekharN Liu S Zhuang X Seifert G Feng X Immobilizing MolecularMetal DithioleneminusDiamine Complexes on 2D MetalminusOrganicFrameworks for Electrocatalytic H2 Production Chem minusEur J2017 23 2255minus2260(10) Downes C A Clough A J Chen K Yoo J W MarinescuS C Evaluation of the H2 Evolving Activity of BenzenehexathiolateCoordination Frameworks and the Effect of Film Thickness on H2Production ACS Appl Mater Interfaces 2018 10 1719minus1727(11) Miner E M Wang L Dinca M Modular O2 Electro-reduction Activity in TriphenyleneminusBased MetalminusOrganic Frame-works Chem Sci 2018 9 6286minus6291(12) Campbell M G Sheberla D Liu S F Swager T M DincaM Cu3(Hexaiminotriphenylene)2 An Electrically Conductive 2DMetalminusOrganic Framework for Chemiresistive Sensing Angew ChemInt Ed 2015 54 4349minus4352(13) Campbell M G Liu S F Swager T M Dinca MChemiresistive Sensor Arrays from Conductive 2D MetalminusOrganicFrameworks J Am Chem Soc 2015 137 13780minus13783(14) Smith M K Mirica K A SelfminusOrganized Frameworks onTextiles (SOFT) Conductive Fabrics for Simultaneous SensingCapture and Filtration of Gases J Am Chem Soc 2017 139 16759minus16767(15) Rubio-Gimenez V Almora-Barrios N Escorcia-Ariza GGalbiati M Sessolo M Tatay S Marti-Gastaldo C Origin of theChemiresistive Response of Ultrathin Films of Conductive MetalminusOrganic Frameworks Angew Chem Int Ed 2018 57 15086minus15090(16) Meng Z Aykanat A Mirica K A Welding Metal-lophthalocyanines into Bimetallic Molecular Meshes for Ultra-sensitive LowminusPower Chemiresistive Detection of Gases J AmChem Soc 2019 141 2046minus2053(17) Aubrey M L Kapelewski M T Melville J F Oktawiec JPresti D Gagliardi L Long J R Chemiresistive Detection ofGaseous Hydrocarbons and Interrogation of Charge Transport inCu[Ni(23-Pyrazinedithiolate)2] by Gas Adsorption J Am Chem Soc2019 141 5005minus5013(18) Sun L Liao B Sheberla D Kraemer D Zhou J Stach EA Zakharov D Stavila V Talin A A Ge Y Allendorf M DChen G Leonard F Dinca M A Microporous and NaturallyNanostructured Thermoelectric MetalminusOrganic Framework withUltralow Thermal Conductivity Joule 2017 1 168minus177(19) Erickson K J Leonard F Stavila V Foster M E SpataruC D Jones R E Foley B M Hopkins P E Allendorf M DTalin A A Thin Film Thermoelectric MetalminusOrganic Frameworkwith High Seebeck Coefficient and Low Thermal Conductivity AdvMater 2015 27 3453minus3459(20) Xie L S Sun L Wan R Park S S DeGayner J AHendon C H Dinca M Tunable Mixed-Valence Doping TowardRecord Electrical Conductivity in a Three-Dimensional MetalminusOrganic Framework J Am Chem Soc 2018 140 7411minus7414(21) Talin A A Centrone A Ford A C Foster M E StavilaV Haney P Kinney R A Szalai V El Gabaly F Yoon H PLeonard F Allendorf M D Tunable Electrical Conductivity inMetalminusOrganic Framework ThinminusFilm Devices Science 2014 34366minus69(22) Sheberla D Sun L Blood-Forsythe M A Er S Wade CR Brozek C K Aspuru-Guzik A Dinca M High ElectricalCondutivity in Ni3(23671011-Hexaiminotriphenylene)2 a Semi-conducting MetalminusOrganic Graphene Analogue J Am Chem Soc2014 136 8859minus8862
(23) Van Vleet M J Weng T Li X Schmidt J R In Situ TimeminusResolved and Mechanistic Studies of MetalminusOrganic FrameworkNucleation and Growth Chem Rev 2018 118 3681minus3721(24) Marcus Y The Effectivity of Solvents as Electron Pair DonorsJ Solution Chem 1984 13 599minus624(25) Skorupskii G Trump B A Kasel T W Brown C MHendon C H Dinca M Efficient and Tunable OneminusDimensionalCharge Transport in Layered Lanthanide MetalminusOrganic Frame-works Nat Chem 2020 12 131minus136(26) Denton A R Ashcroft N W Vegardrsquos Law Phys Rev A AtMol Opt Phys 1991 43 3161minus3164(27) McIntyre N S Cook M G X-ray Photoelectron Studies onSome Oxides and Hydroxides of Cobalt Nickel and Copper AnalChem 1975 47 2208minus2213(28) Moretti G Auger Parameter and Wagner Plot in theCharacterization of Chemical States by Xminusray PhotoelectronSpectroscopy A Review J Electron Spectrosc Relat Phenom 199895 95minus144(29) Matienzo J Yin L I Grim S O Swartz W E Jr XminusrayPhotoelectron Spectroscopy of Nickel Compounds Inorg Chem1973 12 2762minus2769(30) Frost D C Ishitani A McDowell C A Xminusray PhotoelectronSpectroscopy of Copper Compounds Mol Phys 1972 24 861minus877(31) Yin L Adler I Tsang T Matienzo L J Grim S OParamagnetism and Shakeminusup Satellites in X-ray PhotoelectronSpectra Chem Phys Lett 1974 24 81minus84(32) Tan K L Tan B T G Kang E T Neoh K G XminusrayPhotoelectron Spectroscopy Studies of the Chemical Structure ofPolyaniline Phys Rev B Condens Matter Mater Phys 1989 398070minus8073(33) Mohtasebi A Chowdhury T Hsu L H Biesinger M CKruse P Interfacial Charge Transfer Between Phenylminuscapped AnilineTetramer Films and Iron Oxide Surfaces J Phys Chem C 2016 12029248minus29263(34) Bair R A Goddard W A III Ab initio Studies of the XminusrayAbsorption Edge in Copper Complexes I Atomic Cu2+ andCu(II)Cl2 Phys Rev B Condens Matter Mater Phys 1980 222767minus2776(35) Sano M Komorita S Yamatera H XANES Spectra ofCopper (II) Complexes Correlation of the Intensity of the 1s rarr 3dTransition and the Shape of the Complex Inorg Chem 1992 31459minus463(36) Chaboy J Munoz-Paez A Carrera F Merkling P MarcosE S Ab initio X-ray Absorption Study of Copper Kminusedge XANESSpectra in Cu(II) Compounds Phys Rev B Condens Matter MaterPhys 2005 71 134208(37) Peng S-M Chen C-T Liaw D-S Chen C-I Wang YEstablishment of the Bond Patterns of o-Benzoquinonediimine andSemi-o-Benzoquinonediimine Crystal Structures of Metal Complexes[FeII(bqdi)3](PF6)2[Co
II(s-bqdi)2] and [CoIIICl(s-bqdi)2)] InorgChim Acta 1985 101 L31minusL33(38) Jimenez H R Salgado J Moratal J M Morgenstern-Badarau I EPR and Magnetic Susceptibility Studies of Cobalt (II)-and Nickel (II)-substituted Azurins from Pseudomonas AeruginosaElectronic Structure of the Active Sites Inorg Chem 1996 35 2737minus2741(39) Garribba E Micera G The Determination of the Geometry ofCu (II) Complexes An EPR Spectroscopy Experiment J Chem Educ2006 83 1229minus1232(40) Amalric Popescu D Herrmann J-M Ensuque A Bozon-Verduraz F Nanosized Tin Dioxide Spectroscopic (UV-Vis NIREPR) and Electrical Conductivity Studies Phys Chem Chem Phys2001 3 2522minus2530(41) Chu J Sher A Physics and Properties of Narrow GapSemiconductors Springer New York NY 2008(42) Nowak M Kauch B Szperlich P Determination of EnergyBand Gap of Nanocrystalline SbSI Using Diffuse ReflectanceSpectroscopy Rev Sci Instrum 2009 80 No 046107
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(43) Gibbs Z M LaLonde A Snyder G J Optical Band Gap andthe Burstein-Moss Effect in Iodine Doped PbTe Using DiffuseReflectance Infrared Fourier Transform Spectroscopy New J Phys2013 15 No 075020(44) Kudman I Seidel T Absorption Edge in Degenerate pminusTypeGaAs J Appl Phys 1962 33 771minus773
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parameters which are in excellent agreement with thoseobtained from PXRD (Tables S2minusS4) HRTEM also allowsprecise measurements of πminusπ stacking distances of 325 Aring forNi3(HITP)2 and 312 Aring for Cu3(HITP)2 again in excellentagreement with those obtained from the PXRD analysis(Figures S16minusS18)Owing to their nearly identical structures M3(HITP)2 (M =
Co Ni Cu) can be produced from binary mixtures of metalprecursors with continuously variable compositions Toillustrate this we targeted three binary series (MxMprime3minusx)-(HITP)2 5 compositions in the CuNi space 5 for CoNiand 6 for CoCu The final MMprime ratios were determined byinductively coupled plasma-mass spectrometry (ICP-MS) X-ray photoelectron spectroscopy (XPS) andor energy-dispersive X-ray spectroscopy (EDS) all of which confirmedthat the final metal compositions were similar to those of theprecursors (Figure S19)A series of spectroscopic and analytical techniques
confirmed the uniform composition of the MOF alloys with(CoxNi3minusx)(HITP)2 as a representative example STEMelemental mapping of (CoxNi3minusx)(HITP)2 samples revealed auniform distribution of Co and Ni atoms within singlecrystallitesgrains confirming the formation of true mixedphases rather than individual particles or phase-segregatedislets of Co3(HITP)2 and Ni3(HITP)2 (Figures S20minusS25)Tellingly the morphology of (CoxNi3minusx)(HITP)2 particles alsoevolves gradually from nanoflakes for the cobalt-rich(Co238Ni062)(HITP)2 to nanorods for the nickel-rich(Co060Ni240)(HITP)2 (Figures S8 S9) Finally monotonousshifts of the (110) peak position in the corresponding PXRDpatterns confirm a systematic increase in the b unit cellparameter of (CoxNi3minusx)(HITP)2 with increasing Ni content(Figure 2b) Similar changes are found in the (CuxNi3minusx)-(HITP)2 and (CoxCu3minusx)(HITP)2 series (Figures 2b and S26and Table S5) all of which follow Vegardrsquos law an empiricalformula describing solid solutions26
XPS and X-ray absorption spectroscopy (XAS) confirmedthat all metals are in the +2 formal oxidation state and exhibitsquareminusplanar coordination geometry The XPS Co 2p32 peakof Co3(HITP)2 has a binding energy of 7809 eV with aprominent satellite at 7867 eV (Figure S29) These areconsistent with Co2+ compounds as satellite features rarelyappear for Co3+ compounds427 Furthermore Co3(HITP)2resides in the Co(II) region in the Wagner plot28 of Co based
on the Co 2p32 binding energy and Co LMM kinetic energy(7697 eV Figure S30) The XPS spectra of Ni3(HITP)2 andCu3(HITP)2 exhibit single Ni 2p32 and Cu 2p32 peaks at8552 and 9335 eV respectively as expected for Ni2+ and Cu2+
(Figure S29)2930 The weak satellite accompanying the Ni2p32 peak suggests Ni2+ is diamagnetic in Ni3(HITP)2
31 Thecomplementary Cu LMM peak with a kinetic energy of 9182eV also locates Cu3(HITP)2 in the Cu(II) region of theWagner plot of Cu (Figure S31) We note that the improvedsynthesis of Cu3(HITP)2 leads to pure Cu(II) samples incontrast with previous reports where mixed valence contribu-tions to the conductivity could not be ruled out12 XPS N 1sspectra of M3(HITP)2 can be deconvoluted into two peaks at3996 and 3979 eV which are attributed to the anilinic amine(NH) and the quinoid imine (N)3233 and correlatewell with the proposed partial oxidation of HATP to formHITP (Figure S29) The resulting charge neutrality of theframeworks is demonstrated by the absence of possibleadditional charge-balancing ions (eg Na+ and OAcminus FiguresS32 and S33)The X-ray absorption near-edge structure (XANES) analysis
of Co3(HITP)2 revealed a pre-edge feature at 77092 keV andan edge energy of 77204 keV well-matched with the valueexpected for Co2+ (Figure S34) The weak pre-edge feature at89712 keV in the XANES spectrum of Cu3(HITP)2 (FigureS37) is characteristic of Cu2+ which has been unambiguouslyassigned to a 1s rarr 3d transition This transition is notobserved in Cu+ with fully occupied 3d orbitals34minus36 TheXANES spectra of M3(HITP)2 and corresponding modelcomplexes MPc (Pc = phthalocyanine) exhibit similar featuresindicating that they have a similar squareminusplanar coordinationenvironment (Figures S34minusS37) The fit for the extended X-ray absorption fine structure (EXAFS) spectrum ofCo3(HITP)2 demonstrates the squareminusplanar coordinationmode of Co with four CoN bonds (with a fittedcoordination number of 39 Table S6) of 184 Aring (FigureS35) matching well with the structure modeled from PXRDdata The simulated CoN bond length is also comparable tothe 1824(5) Aring averaged CoN bond length of the Co(II)(s-bqdi)2 complex (s-bqdi = semi-o-benzoquinonediimine)37
Importantly the +2 oxidation states and squareminusplanarcoordination environment of the metal ions are conserved inall (MxMprime3minusx)(HITP)2 alloys as verified by XPS (section S16)X-band electron paramagnetic resonance (EPR) spectrosco-
py confirmed the presence of HITP-centered organic radicalswith g = 200 Peaks at g = 564 and sim19 in the EPR spectrumof Co3(HITP)2 are typical for high-spin Co2+ ions38 Similarlyg peaks at 217 and 202 observed for Cu3(HITP)2 are typicalfor squareminusplanar Cu2+ ions39 Expectedly for non-KramersNi2+ ions the X-band EPR spectrum of Ni3(HITP)2 exhibitsonly the signal for the ligand-based radical (Figure S42)Optical spectroscopy provided additional insight into the
electronic structures of (MxMprime3minusx)(HITP)2 Diffuse reflectanceUVminusvisible (DR-UVminusvis) spectroscopy revealed a clearabsorption edge for Co3(HITP)2 with broader absorptionedges in the near-infrared (IR) region for Ni3(HITP)2 andCu3(HITP)2 (Figure 4) The increase of the background IRabsorption generally associated with free-carrier absorption indegenerate or narrow-gap semiconductors40 suggests anenhancement of the free-carrier concentration fromCo3(HITP)2 to Ni3(HITP)2 (Figure 4a) Indeed diffusereflectance infrared Fourier transform spectra (DRIFTS) of(MxMprime3minusx)(HITP)2 also reveal a continuous increase of the
Figure 3 HRTEM images of Co3(HITP)2 (a and d) Ni3(HITP)2 (band e) and Cu3(HITP)2 (c and f) Insets show the FFT analysis ofthe micrographs in (d) (e) and (f)
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background absorption with an increasing Cu or Ni contentsuggesting the continuous elevation of the free-carrierconcentration (Figure S43) Plotting the DR-UVminusvis spectraof M3(HITP)2 in Tauc coordinates41minus43 revealed optical bandgaps Eo of 0719 0429 and 0291 eV for Co3(HITP)2Cu3(HITP)2 and Ni3(HITP)2 respectively The progressivelynarrower band gap when transitioning from Co to Cu and Ni isalso in agreement with a decreased free carrier concentrationfor the pure cobalt MOF Most importantly DR-UVminusvisspectra of (MxMprime3minusx)(HITP)2 (Figure S43) confirm that theoptical band gaps of the alloys can also be continuously tunedover 04 eV (cf MossminusBurstein shift)4144 They narrowprogressively from 0662 eV for (Co238Ni062)(HITP)2 to0395 eV for (Co060Ni240)(HITP)2 in the CoNi series from0762 eV for (Co247Cu053)(HITP)2 to 0513 eV for(Co052Cu248)(HITP)2 in the CoCu series and from 0392eV for (Cu232Ni068)(HITP)2 to 0326 eV for (Cu050Ni250)-(HITP)2 in the CuNi series (Figure 4c) Finally a DRIFTSanalysis of M3(HITP)2 and (MxMprime3minusx)(HITP)2 showed theexpected vibrational bands corresponding to HITP but nofeatures related to electronic transitions below the absorptionedges The influence of defects on optical properties is thusnegligibleThe smooth variation in electronic structures derived from
optical spectroscopy is also reflected in electrical conductivitymeasurements for the pure-phase M3(HITP)2 and the(MxMprime3minusx)(HITP)2 alloys Four-contact probe measurementsof polycrystalline pellets of M3(HITP)2 revealed bulkconductivity values σ of 0024 075 and 554 Scm for thepure Co Cu and Ni MOFs respectively at 296 K These arehigher than the bulk conductivities reported previously for thesame compounds1222 which were already among the mostconductive MOFs The metal ions exert an obvious influenceon electrical conductivity at least partially by contributing indifferent extents to the valence and conduction bands density
of states The electronic nature of the metal ion notwithstand-ing we attribute the differences in σ to the slight structuralvariations among the three materials In particular theinterlayer displacement D and stacking S exert an importanteffect on the transport normal to the sheets D appears to beoptimal in Ni3(HITP)2 (D asymp 17 Aring in the optimized calculatedstructure22) which also shows the highest bulk conductivityWith D deviating to 139(0) Aring for Co3(HITP)2 and 086(9) Aringfor Cu3(HITP)2 these materials exhibit lower conductivitiesSimilarly the higher σ of Cu3(HITP)2 relative to Co3(HITP)2can be traced to its smaller interlayer spacing 316(8) Aringcompared to 329(8) Aring for Co3(HITP)2In line with the spectroscopic and electrical data for the pure
phases the conductivity of (MxMprime3minusx)(HITP)2 alloys can betuned precisely for over 4 orders of magnitude from 58 times 10minus3
Scm in (Co247Cu053)(HITP)2 to 554 Scm in Ni3(HITP)2(Figure 5a) Variations in the electrical conductivity among the
16 different compositional alloys are in line with all of thediscussions above and can be traced to combinations ofsystematic changes in optical band gaps free carrierconcentrations and structural parameters S and D Forinstance increasing the Ni content in the (CoxNi3minusx)(HITP)2series shifts the D and S values in such a way as to enhanceinterlayer πminusπ interactions which in turn leads to higher σvalues Apparent deviations from these trends such as with(Co247Cu053)(HITP)2 are also in line with the slightly lowercrystallinity for that particular composition itself likely a resultof the relatively more pronounced structural mismatchbetween Co3(HITP)2 and Cu3(HITP)2 (Table S1 and FigureS26)Notably control measurements of mechanically blended
mixtures of pure-phase M3(HITP)2 samples behave differentlythan the alloyed phases with the more abundant sampledominating the transport in a nonlinear fashion For instancemixtures of Co3(HITP)2 and Ni3(HITP)2 are nearly as
Figure 4 DR-UVminusvis and DRIFT spectra of M3(HITP)2 and(MxMprime3minusx)(HITP)2 (a) Normalized KubelkaminusMunk-transformedspectra of M3(HITP)2 (b) Normalized Tauc plots from the spectrain (a) Dashed lines indicate best linear fits to the absorption edges(c) Optical band gaps Eo of (MxMprime3minusx)(HITP)2 Solid lines areguides to the eye dashed lines are linear fits of Eo
Figure 5 (a) Electrical conductivity data for (MxMprime3minusx)(HITP)2Solid lines are guide to the eye dashed lines are linear fits (b)Continuous changes of Ea in (CoxNi3minusx)(HITP)2 The dashed line isa linear fit with R2 = 098
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conductive as the former when the Ni is low and jump to aconductivity closer to that of the latter when the Ni reaches40 Surprisingly all blended Co3(HITP)2Cu3(HITP)2mixtures exhibit lower σ values than Co3(HITP)2 likely aconsequence of the work functionenergy level mismatchbetween individual particles of the two pure phases whichincreases the grain boundary resistance (Figure S43)Variable-temperature (VT) conductivity measurements
performed for the pure phases and for (CoxNi3minusx)(HITP)2as a representative example of the alloyed samples revealed atemperature-activated bulk transport from 200 to 350 K in allcases (Figure S44) The corresponding activation energies fortransport Ea correlate well with the σ values 157 meV forCo3(HITP)2 653 meV for Cu3(HITP)2 and 157 meV forNi3(HITP)2 Ea values for (CoxNi3minusx)(HITP)2 alloys varylinearly with the Ni content (Figure 5b) decreasing from 136meV for (Co238Ni062)(HITP)2 to 254 meV for (Co060Ni240)-(HITP)2 providing a final confirmation of continuouscompositional band engineering through metal substitution
CONCLUSIONSA considerable need exists for the improved synthesis ofconductive MOFs Here synthetic improvements in theproduction of pure-phase M3(HITP)2 (M = Co Ni Cu) ledto more crystalline and porous materials with higherconductivity values than previously reported These advancesalso enable a systematic investigation into the influence of themetal ion on physical properties in (MxMprime3minusx)(HITP)2 alloyswhich allow continuous tuning of optical and electronic bandgaps and precise modulation of electrical conductivity of over 4orders of magnitude These results highlight the importance ofsynthetic advances for a deeper understanding of structureminusfunction relationships that are crucial for further materialdevelopments in this field
ASSOCIATED CONTENTsı Supporting InformationThe Supporting Information is available free of charge athttpspubsacsorgdoi101021jacs0c04458
Additional experimental details and characterization data(PDF)
AUTHOR INFORMATIONCorresponding AuthorMircea Dinca minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0002-1262-1264Email mdincamitedu
AuthorsTianyang Chen minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0003-3142-8176
Jin-Hu Dou minus Department of Chemistry Massachusetts Instituteof Technology Cambridge Massachusetts 02139 UnitedStates orcidorg0000-0002-6920-9051
Luming Yang minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Chenyue Sun minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Nicole J Libretto minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates
Grigorii Skorupskii minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Jeffrey T Miller minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates orcidorg0000-0002-6269-0620
Complete contact information is available athttpspubsacsorg101021jacs0c04458
NotesThe authors declare no competing financial interest
ACKNOWLEDGMENTSThis work was supported by the Army Research Office (AwardNo W911NF-17-1-0174) Part of this work (SEM part ofXPS and HAADF-STEM) made use of the MRSEC SharedExperimental Facilities at MIT supported by the NationalScience Foundation under Award No DMR-1419807 Wethank Dr Yong Zhang for their assistance with HAADF-STEM This research used resources (aberration-correctedTEM) of the Center for Functional Nanomaterials which is aUS Department of Energy Office of Science Facility atBrookhaven National Laboratory under Contract No DE-SC0012704 Use of the Advanced Photon Source at ArgonneNational Laboratory was supported by the US Department ofEnergy Office of Science Office of Basic Energy Sciencesunder Contract No DE-AC02-06CH11357 Raman spectros-copy and part of XPS were performed at the Harvard Centerfor Nanoscale Systems (CNS) a member of the NationalNanotechnology Infrastructure Network (NNIN) which issupported by the National Science Foundation under AwardNo ECS-0335765
REFERENCES(1) Sun L Campbell M G Dinca M Electrically ConductivePorous MetalminusOrganic Frameworks Angew Chem Int Ed 2016 553566minus3579(2) (a) Xie L S Skorupskii G Dinca M Electrically ConductiveMetalminusOrganic Frameworks Chem Rev 2020 120 (b) Guo L SunJ Wei J Liu Y Hou L Yuan C Conductive MetalminusOrganicFrameworks Recent Advances in Electrochemical Energy-relatedApplications and Perspectives Carbon Energy 2020 2 203(3) Sheberla D Bachman J C Elias J S Sun C-J Shao-HornY Dinca M Conductive MOF Electrodes for Stable Supercapacitorswith High Areal Capacitance Nat Mater 2017 16 220minus224(4) Park J Lee M Feng D Huang Z Hinckley A CYakovenko A Zou X Cui Y Bao Z Stabilization ofHexaaminobenzene in a 2D Conductive MetalminusOrganic Frameworkfor High Power Sodium Storage J Am Chem Soc 2018 140 10315minus10323(5) (a) Feng D Lei T Lukatskaya M R Park J Huang Z LeeM Shaw L Chen S Yakovenko A A Kulkarni A Xiao JFredrickson K Tok J B Zou X Cui Y Bao Z Robust andConductive TwominusDimensional MetalminusOrganic Frameworks withExceptionally High Volumetric and Areal Capacitance Nat Energy2018 3 30minus36 (b) Sun J Guo L Sun X Zhang J Liu Y HouL Yuan C Conductive Co-based MetalminusOrganic FrameworkNanowires A Competitive High-rate Anode Towards Advanced Li-ion Capacitors J Mater Chem A 2019 7 24788minus24791(6) Shinde S S Lee C H Jung J-Y Wagh N K Kim S-HKim D-H Lin C Lee S U Lee J-H Unveiling Dual-Linkage 3DHexaiminobenzene MetalminusOrganic Frameworks towards Longminus
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Lasting Advanced Reversible Zn-Air Batteries Energy Environ Sci2019 12 727minus738(7) Clough A J Yoo J W Mecklenburg M H Marinescu S CTwo-Dimensional MetalminusOrganic Surfaces for Efficient HydrogenEvolution from Water J Am Chem Soc 2015 137 118minus121(8) Miner E M Fukushima T Sheberla D Sun LSurendranath Y Dinca M Electrochemical Oxygen ReductionCatalysed by Ni3(Hexaiminotriphenylene)2 Nat Commun 2016 710942(9) Dong R Zheng Z Tranca D C Zhang J ChandrasekharN Liu S Zhuang X Seifert G Feng X Immobilizing MolecularMetal DithioleneminusDiamine Complexes on 2D MetalminusOrganicFrameworks for Electrocatalytic H2 Production Chem minusEur J2017 23 2255minus2260(10) Downes C A Clough A J Chen K Yoo J W MarinescuS C Evaluation of the H2 Evolving Activity of BenzenehexathiolateCoordination Frameworks and the Effect of Film Thickness on H2Production ACS Appl Mater Interfaces 2018 10 1719minus1727(11) Miner E M Wang L Dinca M Modular O2 Electro-reduction Activity in TriphenyleneminusBased MetalminusOrganic Frame-works Chem Sci 2018 9 6286minus6291(12) Campbell M G Sheberla D Liu S F Swager T M DincaM Cu3(Hexaiminotriphenylene)2 An Electrically Conductive 2DMetalminusOrganic Framework for Chemiresistive Sensing Angew ChemInt Ed 2015 54 4349minus4352(13) Campbell M G Liu S F Swager T M Dinca MChemiresistive Sensor Arrays from Conductive 2D MetalminusOrganicFrameworks J Am Chem Soc 2015 137 13780minus13783(14) Smith M K Mirica K A SelfminusOrganized Frameworks onTextiles (SOFT) Conductive Fabrics for Simultaneous SensingCapture and Filtration of Gases J Am Chem Soc 2017 139 16759minus16767(15) Rubio-Gimenez V Almora-Barrios N Escorcia-Ariza GGalbiati M Sessolo M Tatay S Marti-Gastaldo C Origin of theChemiresistive Response of Ultrathin Films of Conductive MetalminusOrganic Frameworks Angew Chem Int Ed 2018 57 15086minus15090(16) Meng Z Aykanat A Mirica K A Welding Metal-lophthalocyanines into Bimetallic Molecular Meshes for Ultra-sensitive LowminusPower Chemiresistive Detection of Gases J AmChem Soc 2019 141 2046minus2053(17) Aubrey M L Kapelewski M T Melville J F Oktawiec JPresti D Gagliardi L Long J R Chemiresistive Detection ofGaseous Hydrocarbons and Interrogation of Charge Transport inCu[Ni(23-Pyrazinedithiolate)2] by Gas Adsorption J Am Chem Soc2019 141 5005minus5013(18) Sun L Liao B Sheberla D Kraemer D Zhou J Stach EA Zakharov D Stavila V Talin A A Ge Y Allendorf M DChen G Leonard F Dinca M A Microporous and NaturallyNanostructured Thermoelectric MetalminusOrganic Framework withUltralow Thermal Conductivity Joule 2017 1 168minus177(19) Erickson K J Leonard F Stavila V Foster M E SpataruC D Jones R E Foley B M Hopkins P E Allendorf M DTalin A A Thin Film Thermoelectric MetalminusOrganic Frameworkwith High Seebeck Coefficient and Low Thermal Conductivity AdvMater 2015 27 3453minus3459(20) Xie L S Sun L Wan R Park S S DeGayner J AHendon C H Dinca M Tunable Mixed-Valence Doping TowardRecord Electrical Conductivity in a Three-Dimensional MetalminusOrganic Framework J Am Chem Soc 2018 140 7411minus7414(21) Talin A A Centrone A Ford A C Foster M E StavilaV Haney P Kinney R A Szalai V El Gabaly F Yoon H PLeonard F Allendorf M D Tunable Electrical Conductivity inMetalminusOrganic Framework ThinminusFilm Devices Science 2014 34366minus69(22) Sheberla D Sun L Blood-Forsythe M A Er S Wade CR Brozek C K Aspuru-Guzik A Dinca M High ElectricalCondutivity in Ni3(23671011-Hexaiminotriphenylene)2 a Semi-conducting MetalminusOrganic Graphene Analogue J Am Chem Soc2014 136 8859minus8862
(23) Van Vleet M J Weng T Li X Schmidt J R In Situ TimeminusResolved and Mechanistic Studies of MetalminusOrganic FrameworkNucleation and Growth Chem Rev 2018 118 3681minus3721(24) Marcus Y The Effectivity of Solvents as Electron Pair DonorsJ Solution Chem 1984 13 599minus624(25) Skorupskii G Trump B A Kasel T W Brown C MHendon C H Dinca M Efficient and Tunable OneminusDimensionalCharge Transport in Layered Lanthanide MetalminusOrganic Frame-works Nat Chem 2020 12 131minus136(26) Denton A R Ashcroft N W Vegardrsquos Law Phys Rev A AtMol Opt Phys 1991 43 3161minus3164(27) McIntyre N S Cook M G X-ray Photoelectron Studies onSome Oxides and Hydroxides of Cobalt Nickel and Copper AnalChem 1975 47 2208minus2213(28) Moretti G Auger Parameter and Wagner Plot in theCharacterization of Chemical States by Xminusray PhotoelectronSpectroscopy A Review J Electron Spectrosc Relat Phenom 199895 95minus144(29) Matienzo J Yin L I Grim S O Swartz W E Jr XminusrayPhotoelectron Spectroscopy of Nickel Compounds Inorg Chem1973 12 2762minus2769(30) Frost D C Ishitani A McDowell C A Xminusray PhotoelectronSpectroscopy of Copper Compounds Mol Phys 1972 24 861minus877(31) Yin L Adler I Tsang T Matienzo L J Grim S OParamagnetism and Shakeminusup Satellites in X-ray PhotoelectronSpectra Chem Phys Lett 1974 24 81minus84(32) Tan K L Tan B T G Kang E T Neoh K G XminusrayPhotoelectron Spectroscopy Studies of the Chemical Structure ofPolyaniline Phys Rev B Condens Matter Mater Phys 1989 398070minus8073(33) Mohtasebi A Chowdhury T Hsu L H Biesinger M CKruse P Interfacial Charge Transfer Between Phenylminuscapped AnilineTetramer Films and Iron Oxide Surfaces J Phys Chem C 2016 12029248minus29263(34) Bair R A Goddard W A III Ab initio Studies of the XminusrayAbsorption Edge in Copper Complexes I Atomic Cu2+ andCu(II)Cl2 Phys Rev B Condens Matter Mater Phys 1980 222767minus2776(35) Sano M Komorita S Yamatera H XANES Spectra ofCopper (II) Complexes Correlation of the Intensity of the 1s rarr 3dTransition and the Shape of the Complex Inorg Chem 1992 31459minus463(36) Chaboy J Munoz-Paez A Carrera F Merkling P MarcosE S Ab initio X-ray Absorption Study of Copper Kminusedge XANESSpectra in Cu(II) Compounds Phys Rev B Condens Matter MaterPhys 2005 71 134208(37) Peng S-M Chen C-T Liaw D-S Chen C-I Wang YEstablishment of the Bond Patterns of o-Benzoquinonediimine andSemi-o-Benzoquinonediimine Crystal Structures of Metal Complexes[FeII(bqdi)3](PF6)2[Co
II(s-bqdi)2] and [CoIIICl(s-bqdi)2)] InorgChim Acta 1985 101 L31minusL33(38) Jimenez H R Salgado J Moratal J M Morgenstern-Badarau I EPR and Magnetic Susceptibility Studies of Cobalt (II)-and Nickel (II)-substituted Azurins from Pseudomonas AeruginosaElectronic Structure of the Active Sites Inorg Chem 1996 35 2737minus2741(39) Garribba E Micera G The Determination of the Geometry ofCu (II) Complexes An EPR Spectroscopy Experiment J Chem Educ2006 83 1229minus1232(40) Amalric Popescu D Herrmann J-M Ensuque A Bozon-Verduraz F Nanosized Tin Dioxide Spectroscopic (UV-Vis NIREPR) and Electrical Conductivity Studies Phys Chem Chem Phys2001 3 2522minus2530(41) Chu J Sher A Physics and Properties of Narrow GapSemiconductors Springer New York NY 2008(42) Nowak M Kauch B Szperlich P Determination of EnergyBand Gap of Nanocrystalline SbSI Using Diffuse ReflectanceSpectroscopy Rev Sci Instrum 2009 80 No 046107
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(43) Gibbs Z M LaLonde A Snyder G J Optical Band Gap andthe Burstein-Moss Effect in Iodine Doped PbTe Using DiffuseReflectance Infrared Fourier Transform Spectroscopy New J Phys2013 15 No 075020(44) Kudman I Seidel T Absorption Edge in Degenerate pminusTypeGaAs J Appl Phys 1962 33 771minus773
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12373
background absorption with an increasing Cu or Ni contentsuggesting the continuous elevation of the free-carrierconcentration (Figure S43) Plotting the DR-UVminusvis spectraof M3(HITP)2 in Tauc coordinates41minus43 revealed optical bandgaps Eo of 0719 0429 and 0291 eV for Co3(HITP)2Cu3(HITP)2 and Ni3(HITP)2 respectively The progressivelynarrower band gap when transitioning from Co to Cu and Ni isalso in agreement with a decreased free carrier concentrationfor the pure cobalt MOF Most importantly DR-UVminusvisspectra of (MxMprime3minusx)(HITP)2 (Figure S43) confirm that theoptical band gaps of the alloys can also be continuously tunedover 04 eV (cf MossminusBurstein shift)4144 They narrowprogressively from 0662 eV for (Co238Ni062)(HITP)2 to0395 eV for (Co060Ni240)(HITP)2 in the CoNi series from0762 eV for (Co247Cu053)(HITP)2 to 0513 eV for(Co052Cu248)(HITP)2 in the CoCu series and from 0392eV for (Cu232Ni068)(HITP)2 to 0326 eV for (Cu050Ni250)-(HITP)2 in the CuNi series (Figure 4c) Finally a DRIFTSanalysis of M3(HITP)2 and (MxMprime3minusx)(HITP)2 showed theexpected vibrational bands corresponding to HITP but nofeatures related to electronic transitions below the absorptionedges The influence of defects on optical properties is thusnegligibleThe smooth variation in electronic structures derived from
optical spectroscopy is also reflected in electrical conductivitymeasurements for the pure-phase M3(HITP)2 and the(MxMprime3minusx)(HITP)2 alloys Four-contact probe measurementsof polycrystalline pellets of M3(HITP)2 revealed bulkconductivity values σ of 0024 075 and 554 Scm for thepure Co Cu and Ni MOFs respectively at 296 K These arehigher than the bulk conductivities reported previously for thesame compounds1222 which were already among the mostconductive MOFs The metal ions exert an obvious influenceon electrical conductivity at least partially by contributing indifferent extents to the valence and conduction bands density
of states The electronic nature of the metal ion notwithstand-ing we attribute the differences in σ to the slight structuralvariations among the three materials In particular theinterlayer displacement D and stacking S exert an importanteffect on the transport normal to the sheets D appears to beoptimal in Ni3(HITP)2 (D asymp 17 Aring in the optimized calculatedstructure22) which also shows the highest bulk conductivityWith D deviating to 139(0) Aring for Co3(HITP)2 and 086(9) Aringfor Cu3(HITP)2 these materials exhibit lower conductivitiesSimilarly the higher σ of Cu3(HITP)2 relative to Co3(HITP)2can be traced to its smaller interlayer spacing 316(8) Aringcompared to 329(8) Aring for Co3(HITP)2In line with the spectroscopic and electrical data for the pure
phases the conductivity of (MxMprime3minusx)(HITP)2 alloys can betuned precisely for over 4 orders of magnitude from 58 times 10minus3
Scm in (Co247Cu053)(HITP)2 to 554 Scm in Ni3(HITP)2(Figure 5a) Variations in the electrical conductivity among the
16 different compositional alloys are in line with all of thediscussions above and can be traced to combinations ofsystematic changes in optical band gaps free carrierconcentrations and structural parameters S and D Forinstance increasing the Ni content in the (CoxNi3minusx)(HITP)2series shifts the D and S values in such a way as to enhanceinterlayer πminusπ interactions which in turn leads to higher σvalues Apparent deviations from these trends such as with(Co247Cu053)(HITP)2 are also in line with the slightly lowercrystallinity for that particular composition itself likely a resultof the relatively more pronounced structural mismatchbetween Co3(HITP)2 and Cu3(HITP)2 (Table S1 and FigureS26)Notably control measurements of mechanically blended
mixtures of pure-phase M3(HITP)2 samples behave differentlythan the alloyed phases with the more abundant sampledominating the transport in a nonlinear fashion For instancemixtures of Co3(HITP)2 and Ni3(HITP)2 are nearly as
Figure 4 DR-UVminusvis and DRIFT spectra of M3(HITP)2 and(MxMprime3minusx)(HITP)2 (a) Normalized KubelkaminusMunk-transformedspectra of M3(HITP)2 (b) Normalized Tauc plots from the spectrain (a) Dashed lines indicate best linear fits to the absorption edges(c) Optical band gaps Eo of (MxMprime3minusx)(HITP)2 Solid lines areguides to the eye dashed lines are linear fits of Eo
Figure 5 (a) Electrical conductivity data for (MxMprime3minusx)(HITP)2Solid lines are guide to the eye dashed lines are linear fits (b)Continuous changes of Ea in (CoxNi3minusx)(HITP)2 The dashed line isa linear fit with R2 = 098
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conductive as the former when the Ni is low and jump to aconductivity closer to that of the latter when the Ni reaches40 Surprisingly all blended Co3(HITP)2Cu3(HITP)2mixtures exhibit lower σ values than Co3(HITP)2 likely aconsequence of the work functionenergy level mismatchbetween individual particles of the two pure phases whichincreases the grain boundary resistance (Figure S43)Variable-temperature (VT) conductivity measurements
performed for the pure phases and for (CoxNi3minusx)(HITP)2as a representative example of the alloyed samples revealed atemperature-activated bulk transport from 200 to 350 K in allcases (Figure S44) The corresponding activation energies fortransport Ea correlate well with the σ values 157 meV forCo3(HITP)2 653 meV for Cu3(HITP)2 and 157 meV forNi3(HITP)2 Ea values for (CoxNi3minusx)(HITP)2 alloys varylinearly with the Ni content (Figure 5b) decreasing from 136meV for (Co238Ni062)(HITP)2 to 254 meV for (Co060Ni240)-(HITP)2 providing a final confirmation of continuouscompositional band engineering through metal substitution
CONCLUSIONSA considerable need exists for the improved synthesis ofconductive MOFs Here synthetic improvements in theproduction of pure-phase M3(HITP)2 (M = Co Ni Cu) ledto more crystalline and porous materials with higherconductivity values than previously reported These advancesalso enable a systematic investigation into the influence of themetal ion on physical properties in (MxMprime3minusx)(HITP)2 alloyswhich allow continuous tuning of optical and electronic bandgaps and precise modulation of electrical conductivity of over 4orders of magnitude These results highlight the importance ofsynthetic advances for a deeper understanding of structureminusfunction relationships that are crucial for further materialdevelopments in this field
ASSOCIATED CONTENTsı Supporting InformationThe Supporting Information is available free of charge athttpspubsacsorgdoi101021jacs0c04458
Additional experimental details and characterization data(PDF)
AUTHOR INFORMATIONCorresponding AuthorMircea Dinca minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0002-1262-1264Email mdincamitedu
AuthorsTianyang Chen minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0003-3142-8176
Jin-Hu Dou minus Department of Chemistry Massachusetts Instituteof Technology Cambridge Massachusetts 02139 UnitedStates orcidorg0000-0002-6920-9051
Luming Yang minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Chenyue Sun minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Nicole J Libretto minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates
Grigorii Skorupskii minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Jeffrey T Miller minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates orcidorg0000-0002-6269-0620
Complete contact information is available athttpspubsacsorg101021jacs0c04458
NotesThe authors declare no competing financial interest
ACKNOWLEDGMENTSThis work was supported by the Army Research Office (AwardNo W911NF-17-1-0174) Part of this work (SEM part ofXPS and HAADF-STEM) made use of the MRSEC SharedExperimental Facilities at MIT supported by the NationalScience Foundation under Award No DMR-1419807 Wethank Dr Yong Zhang for their assistance with HAADF-STEM This research used resources (aberration-correctedTEM) of the Center for Functional Nanomaterials which is aUS Department of Energy Office of Science Facility atBrookhaven National Laboratory under Contract No DE-SC0012704 Use of the Advanced Photon Source at ArgonneNational Laboratory was supported by the US Department ofEnergy Office of Science Office of Basic Energy Sciencesunder Contract No DE-AC02-06CH11357 Raman spectros-copy and part of XPS were performed at the Harvard Centerfor Nanoscale Systems (CNS) a member of the NationalNanotechnology Infrastructure Network (NNIN) which issupported by the National Science Foundation under AwardNo ECS-0335765
REFERENCES(1) Sun L Campbell M G Dinca M Electrically ConductivePorous MetalminusOrganic Frameworks Angew Chem Int Ed 2016 553566minus3579(2) (a) Xie L S Skorupskii G Dinca M Electrically ConductiveMetalminusOrganic Frameworks Chem Rev 2020 120 (b) Guo L SunJ Wei J Liu Y Hou L Yuan C Conductive MetalminusOrganicFrameworks Recent Advances in Electrochemical Energy-relatedApplications and Perspectives Carbon Energy 2020 2 203(3) Sheberla D Bachman J C Elias J S Sun C-J Shao-HornY Dinca M Conductive MOF Electrodes for Stable Supercapacitorswith High Areal Capacitance Nat Mater 2017 16 220minus224(4) Park J Lee M Feng D Huang Z Hinckley A CYakovenko A Zou X Cui Y Bao Z Stabilization ofHexaaminobenzene in a 2D Conductive MetalminusOrganic Frameworkfor High Power Sodium Storage J Am Chem Soc 2018 140 10315minus10323(5) (a) Feng D Lei T Lukatskaya M R Park J Huang Z LeeM Shaw L Chen S Yakovenko A A Kulkarni A Xiao JFredrickson K Tok J B Zou X Cui Y Bao Z Robust andConductive TwominusDimensional MetalminusOrganic Frameworks withExceptionally High Volumetric and Areal Capacitance Nat Energy2018 3 30minus36 (b) Sun J Guo L Sun X Zhang J Liu Y HouL Yuan C Conductive Co-based MetalminusOrganic FrameworkNanowires A Competitive High-rate Anode Towards Advanced Li-ion Capacitors J Mater Chem A 2019 7 24788minus24791(6) Shinde S S Lee C H Jung J-Y Wagh N K Kim S-HKim D-H Lin C Lee S U Lee J-H Unveiling Dual-Linkage 3DHexaiminobenzene MetalminusOrganic Frameworks towards Longminus
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Lasting Advanced Reversible Zn-Air Batteries Energy Environ Sci2019 12 727minus738(7) Clough A J Yoo J W Mecklenburg M H Marinescu S CTwo-Dimensional MetalminusOrganic Surfaces for Efficient HydrogenEvolution from Water J Am Chem Soc 2015 137 118minus121(8) Miner E M Fukushima T Sheberla D Sun LSurendranath Y Dinca M Electrochemical Oxygen ReductionCatalysed by Ni3(Hexaiminotriphenylene)2 Nat Commun 2016 710942(9) Dong R Zheng Z Tranca D C Zhang J ChandrasekharN Liu S Zhuang X Seifert G Feng X Immobilizing MolecularMetal DithioleneminusDiamine Complexes on 2D MetalminusOrganicFrameworks for Electrocatalytic H2 Production Chem minusEur J2017 23 2255minus2260(10) Downes C A Clough A J Chen K Yoo J W MarinescuS C Evaluation of the H2 Evolving Activity of BenzenehexathiolateCoordination Frameworks and the Effect of Film Thickness on H2Production ACS Appl Mater Interfaces 2018 10 1719minus1727(11) Miner E M Wang L Dinca M Modular O2 Electro-reduction Activity in TriphenyleneminusBased MetalminusOrganic Frame-works Chem Sci 2018 9 6286minus6291(12) Campbell M G Sheberla D Liu S F Swager T M DincaM Cu3(Hexaiminotriphenylene)2 An Electrically Conductive 2DMetalminusOrganic Framework for Chemiresistive Sensing Angew ChemInt Ed 2015 54 4349minus4352(13) Campbell M G Liu S F Swager T M Dinca MChemiresistive Sensor Arrays from Conductive 2D MetalminusOrganicFrameworks J Am Chem Soc 2015 137 13780minus13783(14) Smith M K Mirica K A SelfminusOrganized Frameworks onTextiles (SOFT) Conductive Fabrics for Simultaneous SensingCapture and Filtration of Gases J Am Chem Soc 2017 139 16759minus16767(15) Rubio-Gimenez V Almora-Barrios N Escorcia-Ariza GGalbiati M Sessolo M Tatay S Marti-Gastaldo C Origin of theChemiresistive Response of Ultrathin Films of Conductive MetalminusOrganic Frameworks Angew Chem Int Ed 2018 57 15086minus15090(16) Meng Z Aykanat A Mirica K A Welding Metal-lophthalocyanines into Bimetallic Molecular Meshes for Ultra-sensitive LowminusPower Chemiresistive Detection of Gases J AmChem Soc 2019 141 2046minus2053(17) Aubrey M L Kapelewski M T Melville J F Oktawiec JPresti D Gagliardi L Long J R Chemiresistive Detection ofGaseous Hydrocarbons and Interrogation of Charge Transport inCu[Ni(23-Pyrazinedithiolate)2] by Gas Adsorption J Am Chem Soc2019 141 5005minus5013(18) Sun L Liao B Sheberla D Kraemer D Zhou J Stach EA Zakharov D Stavila V Talin A A Ge Y Allendorf M DChen G Leonard F Dinca M A Microporous and NaturallyNanostructured Thermoelectric MetalminusOrganic Framework withUltralow Thermal Conductivity Joule 2017 1 168minus177(19) Erickson K J Leonard F Stavila V Foster M E SpataruC D Jones R E Foley B M Hopkins P E Allendorf M DTalin A A Thin Film Thermoelectric MetalminusOrganic Frameworkwith High Seebeck Coefficient and Low Thermal Conductivity AdvMater 2015 27 3453minus3459(20) Xie L S Sun L Wan R Park S S DeGayner J AHendon C H Dinca M Tunable Mixed-Valence Doping TowardRecord Electrical Conductivity in a Three-Dimensional MetalminusOrganic Framework J Am Chem Soc 2018 140 7411minus7414(21) Talin A A Centrone A Ford A C Foster M E StavilaV Haney P Kinney R A Szalai V El Gabaly F Yoon H PLeonard F Allendorf M D Tunable Electrical Conductivity inMetalminusOrganic Framework ThinminusFilm Devices Science 2014 34366minus69(22) Sheberla D Sun L Blood-Forsythe M A Er S Wade CR Brozek C K Aspuru-Guzik A Dinca M High ElectricalCondutivity in Ni3(23671011-Hexaiminotriphenylene)2 a Semi-conducting MetalminusOrganic Graphene Analogue J Am Chem Soc2014 136 8859minus8862
(23) Van Vleet M J Weng T Li X Schmidt J R In Situ TimeminusResolved and Mechanistic Studies of MetalminusOrganic FrameworkNucleation and Growth Chem Rev 2018 118 3681minus3721(24) Marcus Y The Effectivity of Solvents as Electron Pair DonorsJ Solution Chem 1984 13 599minus624(25) Skorupskii G Trump B A Kasel T W Brown C MHendon C H Dinca M Efficient and Tunable OneminusDimensionalCharge Transport in Layered Lanthanide MetalminusOrganic Frame-works Nat Chem 2020 12 131minus136(26) Denton A R Ashcroft N W Vegardrsquos Law Phys Rev A AtMol Opt Phys 1991 43 3161minus3164(27) McIntyre N S Cook M G X-ray Photoelectron Studies onSome Oxides and Hydroxides of Cobalt Nickel and Copper AnalChem 1975 47 2208minus2213(28) Moretti G Auger Parameter and Wagner Plot in theCharacterization of Chemical States by Xminusray PhotoelectronSpectroscopy A Review J Electron Spectrosc Relat Phenom 199895 95minus144(29) Matienzo J Yin L I Grim S O Swartz W E Jr XminusrayPhotoelectron Spectroscopy of Nickel Compounds Inorg Chem1973 12 2762minus2769(30) Frost D C Ishitani A McDowell C A Xminusray PhotoelectronSpectroscopy of Copper Compounds Mol Phys 1972 24 861minus877(31) Yin L Adler I Tsang T Matienzo L J Grim S OParamagnetism and Shakeminusup Satellites in X-ray PhotoelectronSpectra Chem Phys Lett 1974 24 81minus84(32) Tan K L Tan B T G Kang E T Neoh K G XminusrayPhotoelectron Spectroscopy Studies of the Chemical Structure ofPolyaniline Phys Rev B Condens Matter Mater Phys 1989 398070minus8073(33) Mohtasebi A Chowdhury T Hsu L H Biesinger M CKruse P Interfacial Charge Transfer Between Phenylminuscapped AnilineTetramer Films and Iron Oxide Surfaces J Phys Chem C 2016 12029248minus29263(34) Bair R A Goddard W A III Ab initio Studies of the XminusrayAbsorption Edge in Copper Complexes I Atomic Cu2+ andCu(II)Cl2 Phys Rev B Condens Matter Mater Phys 1980 222767minus2776(35) Sano M Komorita S Yamatera H XANES Spectra ofCopper (II) Complexes Correlation of the Intensity of the 1s rarr 3dTransition and the Shape of the Complex Inorg Chem 1992 31459minus463(36) Chaboy J Munoz-Paez A Carrera F Merkling P MarcosE S Ab initio X-ray Absorption Study of Copper Kminusedge XANESSpectra in Cu(II) Compounds Phys Rev B Condens Matter MaterPhys 2005 71 134208(37) Peng S-M Chen C-T Liaw D-S Chen C-I Wang YEstablishment of the Bond Patterns of o-Benzoquinonediimine andSemi-o-Benzoquinonediimine Crystal Structures of Metal Complexes[FeII(bqdi)3](PF6)2[Co
II(s-bqdi)2] and [CoIIICl(s-bqdi)2)] InorgChim Acta 1985 101 L31minusL33(38) Jimenez H R Salgado J Moratal J M Morgenstern-Badarau I EPR and Magnetic Susceptibility Studies of Cobalt (II)-and Nickel (II)-substituted Azurins from Pseudomonas AeruginosaElectronic Structure of the Active Sites Inorg Chem 1996 35 2737minus2741(39) Garribba E Micera G The Determination of the Geometry ofCu (II) Complexes An EPR Spectroscopy Experiment J Chem Educ2006 83 1229minus1232(40) Amalric Popescu D Herrmann J-M Ensuque A Bozon-Verduraz F Nanosized Tin Dioxide Spectroscopic (UV-Vis NIREPR) and Electrical Conductivity Studies Phys Chem Chem Phys2001 3 2522minus2530(41) Chu J Sher A Physics and Properties of Narrow GapSemiconductors Springer New York NY 2008(42) Nowak M Kauch B Szperlich P Determination of EnergyBand Gap of Nanocrystalline SbSI Using Diffuse ReflectanceSpectroscopy Rev Sci Instrum 2009 80 No 046107
Journal of the American Chemical Society pubsacsorgJACS Article
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
12372
(43) Gibbs Z M LaLonde A Snyder G J Optical Band Gap andthe Burstein-Moss Effect in Iodine Doped PbTe Using DiffuseReflectance Infrared Fourier Transform Spectroscopy New J Phys2013 15 No 075020(44) Kudman I Seidel T Absorption Edge in Degenerate pminusTypeGaAs J Appl Phys 1962 33 771minus773
Journal of the American Chemical Society pubsacsorgJACS Article
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
12373
conductive as the former when the Ni is low and jump to aconductivity closer to that of the latter when the Ni reaches40 Surprisingly all blended Co3(HITP)2Cu3(HITP)2mixtures exhibit lower σ values than Co3(HITP)2 likely aconsequence of the work functionenergy level mismatchbetween individual particles of the two pure phases whichincreases the grain boundary resistance (Figure S43)Variable-temperature (VT) conductivity measurements
performed for the pure phases and for (CoxNi3minusx)(HITP)2as a representative example of the alloyed samples revealed atemperature-activated bulk transport from 200 to 350 K in allcases (Figure S44) The corresponding activation energies fortransport Ea correlate well with the σ values 157 meV forCo3(HITP)2 653 meV for Cu3(HITP)2 and 157 meV forNi3(HITP)2 Ea values for (CoxNi3minusx)(HITP)2 alloys varylinearly with the Ni content (Figure 5b) decreasing from 136meV for (Co238Ni062)(HITP)2 to 254 meV for (Co060Ni240)-(HITP)2 providing a final confirmation of continuouscompositional band engineering through metal substitution
CONCLUSIONSA considerable need exists for the improved synthesis ofconductive MOFs Here synthetic improvements in theproduction of pure-phase M3(HITP)2 (M = Co Ni Cu) ledto more crystalline and porous materials with higherconductivity values than previously reported These advancesalso enable a systematic investigation into the influence of themetal ion on physical properties in (MxMprime3minusx)(HITP)2 alloyswhich allow continuous tuning of optical and electronic bandgaps and precise modulation of electrical conductivity of over 4orders of magnitude These results highlight the importance ofsynthetic advances for a deeper understanding of structureminusfunction relationships that are crucial for further materialdevelopments in this field
ASSOCIATED CONTENTsı Supporting InformationThe Supporting Information is available free of charge athttpspubsacsorgdoi101021jacs0c04458
Additional experimental details and characterization data(PDF)
AUTHOR INFORMATIONCorresponding AuthorMircea Dinca minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0002-1262-1264Email mdincamitedu
AuthorsTianyang Chen minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States orcidorg0000-0003-3142-8176
Jin-Hu Dou minus Department of Chemistry Massachusetts Instituteof Technology Cambridge Massachusetts 02139 UnitedStates orcidorg0000-0002-6920-9051
Luming Yang minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Chenyue Sun minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Nicole J Libretto minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates
Grigorii Skorupskii minus Department of Chemistry MassachusettsInstitute of Technology Cambridge Massachusetts 02139United States
Jeffrey T Miller minus Davidson School of Chemical EngineeringPurdue University West Lafayette Indiana 47907 UnitedStates orcidorg0000-0002-6269-0620
Complete contact information is available athttpspubsacsorg101021jacs0c04458
NotesThe authors declare no competing financial interest
ACKNOWLEDGMENTSThis work was supported by the Army Research Office (AwardNo W911NF-17-1-0174) Part of this work (SEM part ofXPS and HAADF-STEM) made use of the MRSEC SharedExperimental Facilities at MIT supported by the NationalScience Foundation under Award No DMR-1419807 Wethank Dr Yong Zhang for their assistance with HAADF-STEM This research used resources (aberration-correctedTEM) of the Center for Functional Nanomaterials which is aUS Department of Energy Office of Science Facility atBrookhaven National Laboratory under Contract No DE-SC0012704 Use of the Advanced Photon Source at ArgonneNational Laboratory was supported by the US Department ofEnergy Office of Science Office of Basic Energy Sciencesunder Contract No DE-AC02-06CH11357 Raman spectros-copy and part of XPS were performed at the Harvard Centerfor Nanoscale Systems (CNS) a member of the NationalNanotechnology Infrastructure Network (NNIN) which issupported by the National Science Foundation under AwardNo ECS-0335765
REFERENCES(1) Sun L Campbell M G Dinca M Electrically ConductivePorous MetalminusOrganic Frameworks Angew Chem Int Ed 2016 553566minus3579(2) (a) Xie L S Skorupskii G Dinca M Electrically ConductiveMetalminusOrganic Frameworks Chem Rev 2020 120 (b) Guo L SunJ Wei J Liu Y Hou L Yuan C Conductive MetalminusOrganicFrameworks Recent Advances in Electrochemical Energy-relatedApplications and Perspectives Carbon Energy 2020 2 203(3) Sheberla D Bachman J C Elias J S Sun C-J Shao-HornY Dinca M Conductive MOF Electrodes for Stable Supercapacitorswith High Areal Capacitance Nat Mater 2017 16 220minus224(4) Park J Lee M Feng D Huang Z Hinckley A CYakovenko A Zou X Cui Y Bao Z Stabilization ofHexaaminobenzene in a 2D Conductive MetalminusOrganic Frameworkfor High Power Sodium Storage J Am Chem Soc 2018 140 10315minus10323(5) (a) Feng D Lei T Lukatskaya M R Park J Huang Z LeeM Shaw L Chen S Yakovenko A A Kulkarni A Xiao JFredrickson K Tok J B Zou X Cui Y Bao Z Robust andConductive TwominusDimensional MetalminusOrganic Frameworks withExceptionally High Volumetric and Areal Capacitance Nat Energy2018 3 30minus36 (b) Sun J Guo L Sun X Zhang J Liu Y HouL Yuan C Conductive Co-based MetalminusOrganic FrameworkNanowires A Competitive High-rate Anode Towards Advanced Li-ion Capacitors J Mater Chem A 2019 7 24788minus24791(6) Shinde S S Lee C H Jung J-Y Wagh N K Kim S-HKim D-H Lin C Lee S U Lee J-H Unveiling Dual-Linkage 3DHexaiminobenzene MetalminusOrganic Frameworks towards Longminus
Journal of the American Chemical Society pubsacsorgJACS Article
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
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Lasting Advanced Reversible Zn-Air Batteries Energy Environ Sci2019 12 727minus738(7) Clough A J Yoo J W Mecklenburg M H Marinescu S CTwo-Dimensional MetalminusOrganic Surfaces for Efficient HydrogenEvolution from Water J Am Chem Soc 2015 137 118minus121(8) Miner E M Fukushima T Sheberla D Sun LSurendranath Y Dinca M Electrochemical Oxygen ReductionCatalysed by Ni3(Hexaiminotriphenylene)2 Nat Commun 2016 710942(9) Dong R Zheng Z Tranca D C Zhang J ChandrasekharN Liu S Zhuang X Seifert G Feng X Immobilizing MolecularMetal DithioleneminusDiamine Complexes on 2D MetalminusOrganicFrameworks for Electrocatalytic H2 Production Chem minusEur J2017 23 2255minus2260(10) Downes C A Clough A J Chen K Yoo J W MarinescuS C Evaluation of the H2 Evolving Activity of BenzenehexathiolateCoordination Frameworks and the Effect of Film Thickness on H2Production ACS Appl Mater Interfaces 2018 10 1719minus1727(11) Miner E M Wang L Dinca M Modular O2 Electro-reduction Activity in TriphenyleneminusBased MetalminusOrganic Frame-works Chem Sci 2018 9 6286minus6291(12) Campbell M G Sheberla D Liu S F Swager T M DincaM Cu3(Hexaiminotriphenylene)2 An Electrically Conductive 2DMetalminusOrganic Framework for Chemiresistive Sensing Angew ChemInt Ed 2015 54 4349minus4352(13) Campbell M G Liu S F Swager T M Dinca MChemiresistive Sensor Arrays from Conductive 2D MetalminusOrganicFrameworks J Am Chem Soc 2015 137 13780minus13783(14) Smith M K Mirica K A SelfminusOrganized Frameworks onTextiles (SOFT) Conductive Fabrics for Simultaneous SensingCapture and Filtration of Gases J Am Chem Soc 2017 139 16759minus16767(15) Rubio-Gimenez V Almora-Barrios N Escorcia-Ariza GGalbiati M Sessolo M Tatay S Marti-Gastaldo C Origin of theChemiresistive Response of Ultrathin Films of Conductive MetalminusOrganic Frameworks Angew Chem Int Ed 2018 57 15086minus15090(16) Meng Z Aykanat A Mirica K A Welding Metal-lophthalocyanines into Bimetallic Molecular Meshes for Ultra-sensitive LowminusPower Chemiresistive Detection of Gases J AmChem Soc 2019 141 2046minus2053(17) Aubrey M L Kapelewski M T Melville J F Oktawiec JPresti D Gagliardi L Long J R Chemiresistive Detection ofGaseous Hydrocarbons and Interrogation of Charge Transport inCu[Ni(23-Pyrazinedithiolate)2] by Gas Adsorption J Am Chem Soc2019 141 5005minus5013(18) Sun L Liao B Sheberla D Kraemer D Zhou J Stach EA Zakharov D Stavila V Talin A A Ge Y Allendorf M DChen G Leonard F Dinca M A Microporous and NaturallyNanostructured Thermoelectric MetalminusOrganic Framework withUltralow Thermal Conductivity Joule 2017 1 168minus177(19) Erickson K J Leonard F Stavila V Foster M E SpataruC D Jones R E Foley B M Hopkins P E Allendorf M DTalin A A Thin Film Thermoelectric MetalminusOrganic Frameworkwith High Seebeck Coefficient and Low Thermal Conductivity AdvMater 2015 27 3453minus3459(20) Xie L S Sun L Wan R Park S S DeGayner J AHendon C H Dinca M Tunable Mixed-Valence Doping TowardRecord Electrical Conductivity in a Three-Dimensional MetalminusOrganic Framework J Am Chem Soc 2018 140 7411minus7414(21) Talin A A Centrone A Ford A C Foster M E StavilaV Haney P Kinney R A Szalai V El Gabaly F Yoon H PLeonard F Allendorf M D Tunable Electrical Conductivity inMetalminusOrganic Framework ThinminusFilm Devices Science 2014 34366minus69(22) Sheberla D Sun L Blood-Forsythe M A Er S Wade CR Brozek C K Aspuru-Guzik A Dinca M High ElectricalCondutivity in Ni3(23671011-Hexaiminotriphenylene)2 a Semi-conducting MetalminusOrganic Graphene Analogue J Am Chem Soc2014 136 8859minus8862
(23) Van Vleet M J Weng T Li X Schmidt J R In Situ TimeminusResolved and Mechanistic Studies of MetalminusOrganic FrameworkNucleation and Growth Chem Rev 2018 118 3681minus3721(24) Marcus Y The Effectivity of Solvents as Electron Pair DonorsJ Solution Chem 1984 13 599minus624(25) Skorupskii G Trump B A Kasel T W Brown C MHendon C H Dinca M Efficient and Tunable OneminusDimensionalCharge Transport in Layered Lanthanide MetalminusOrganic Frame-works Nat Chem 2020 12 131minus136(26) Denton A R Ashcroft N W Vegardrsquos Law Phys Rev A AtMol Opt Phys 1991 43 3161minus3164(27) McIntyre N S Cook M G X-ray Photoelectron Studies onSome Oxides and Hydroxides of Cobalt Nickel and Copper AnalChem 1975 47 2208minus2213(28) Moretti G Auger Parameter and Wagner Plot in theCharacterization of Chemical States by Xminusray PhotoelectronSpectroscopy A Review J Electron Spectrosc Relat Phenom 199895 95minus144(29) Matienzo J Yin L I Grim S O Swartz W E Jr XminusrayPhotoelectron Spectroscopy of Nickel Compounds Inorg Chem1973 12 2762minus2769(30) Frost D C Ishitani A McDowell C A Xminusray PhotoelectronSpectroscopy of Copper Compounds Mol Phys 1972 24 861minus877(31) Yin L Adler I Tsang T Matienzo L J Grim S OParamagnetism and Shakeminusup Satellites in X-ray PhotoelectronSpectra Chem Phys Lett 1974 24 81minus84(32) Tan K L Tan B T G Kang E T Neoh K G XminusrayPhotoelectron Spectroscopy Studies of the Chemical Structure ofPolyaniline Phys Rev B Condens Matter Mater Phys 1989 398070minus8073(33) Mohtasebi A Chowdhury T Hsu L H Biesinger M CKruse P Interfacial Charge Transfer Between Phenylminuscapped AnilineTetramer Films and Iron Oxide Surfaces J Phys Chem C 2016 12029248minus29263(34) Bair R A Goddard W A III Ab initio Studies of the XminusrayAbsorption Edge in Copper Complexes I Atomic Cu2+ andCu(II)Cl2 Phys Rev B Condens Matter Mater Phys 1980 222767minus2776(35) Sano M Komorita S Yamatera H XANES Spectra ofCopper (II) Complexes Correlation of the Intensity of the 1s rarr 3dTransition and the Shape of the Complex Inorg Chem 1992 31459minus463(36) Chaboy J Munoz-Paez A Carrera F Merkling P MarcosE S Ab initio X-ray Absorption Study of Copper Kminusedge XANESSpectra in Cu(II) Compounds Phys Rev B Condens Matter MaterPhys 2005 71 134208(37) Peng S-M Chen C-T Liaw D-S Chen C-I Wang YEstablishment of the Bond Patterns of o-Benzoquinonediimine andSemi-o-Benzoquinonediimine Crystal Structures of Metal Complexes[FeII(bqdi)3](PF6)2[Co
II(s-bqdi)2] and [CoIIICl(s-bqdi)2)] InorgChim Acta 1985 101 L31minusL33(38) Jimenez H R Salgado J Moratal J M Morgenstern-Badarau I EPR and Magnetic Susceptibility Studies of Cobalt (II)-and Nickel (II)-substituted Azurins from Pseudomonas AeruginosaElectronic Structure of the Active Sites Inorg Chem 1996 35 2737minus2741(39) Garribba E Micera G The Determination of the Geometry ofCu (II) Complexes An EPR Spectroscopy Experiment J Chem Educ2006 83 1229minus1232(40) Amalric Popescu D Herrmann J-M Ensuque A Bozon-Verduraz F Nanosized Tin Dioxide Spectroscopic (UV-Vis NIREPR) and Electrical Conductivity Studies Phys Chem Chem Phys2001 3 2522minus2530(41) Chu J Sher A Physics and Properties of Narrow GapSemiconductors Springer New York NY 2008(42) Nowak M Kauch B Szperlich P Determination of EnergyBand Gap of Nanocrystalline SbSI Using Diffuse ReflectanceSpectroscopy Rev Sci Instrum 2009 80 No 046107
Journal of the American Chemical Society pubsacsorgJACS Article
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
12372
(43) Gibbs Z M LaLonde A Snyder G J Optical Band Gap andthe Burstein-Moss Effect in Iodine Doped PbTe Using DiffuseReflectance Infrared Fourier Transform Spectroscopy New J Phys2013 15 No 075020(44) Kudman I Seidel T Absorption Edge in Degenerate pminusTypeGaAs J Appl Phys 1962 33 771minus773
Journal of the American Chemical Society pubsacsorgJACS Article
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
12373
Lasting Advanced Reversible Zn-Air Batteries Energy Environ Sci2019 12 727minus738(7) Clough A J Yoo J W Mecklenburg M H Marinescu S CTwo-Dimensional MetalminusOrganic Surfaces for Efficient HydrogenEvolution from Water J Am Chem Soc 2015 137 118minus121(8) Miner E M Fukushima T Sheberla D Sun LSurendranath Y Dinca M Electrochemical Oxygen ReductionCatalysed by Ni3(Hexaiminotriphenylene)2 Nat Commun 2016 710942(9) Dong R Zheng Z Tranca D C Zhang J ChandrasekharN Liu S Zhuang X Seifert G Feng X Immobilizing MolecularMetal DithioleneminusDiamine Complexes on 2D MetalminusOrganicFrameworks for Electrocatalytic H2 Production Chem minusEur J2017 23 2255minus2260(10) Downes C A Clough A J Chen K Yoo J W MarinescuS C Evaluation of the H2 Evolving Activity of BenzenehexathiolateCoordination Frameworks and the Effect of Film Thickness on H2Production ACS Appl Mater Interfaces 2018 10 1719minus1727(11) Miner E M Wang L Dinca M Modular O2 Electro-reduction Activity in TriphenyleneminusBased MetalminusOrganic Frame-works Chem Sci 2018 9 6286minus6291(12) Campbell M G Sheberla D Liu S F Swager T M DincaM Cu3(Hexaiminotriphenylene)2 An Electrically Conductive 2DMetalminusOrganic Framework for Chemiresistive Sensing Angew ChemInt Ed 2015 54 4349minus4352(13) Campbell M G Liu S F Swager T M Dinca MChemiresistive Sensor Arrays from Conductive 2D MetalminusOrganicFrameworks J Am Chem Soc 2015 137 13780minus13783(14) Smith M K Mirica K A SelfminusOrganized Frameworks onTextiles (SOFT) Conductive Fabrics for Simultaneous SensingCapture and Filtration of Gases J Am Chem Soc 2017 139 16759minus16767(15) Rubio-Gimenez V Almora-Barrios N Escorcia-Ariza GGalbiati M Sessolo M Tatay S Marti-Gastaldo C Origin of theChemiresistive Response of Ultrathin Films of Conductive MetalminusOrganic Frameworks Angew Chem Int Ed 2018 57 15086minus15090(16) Meng Z Aykanat A Mirica K A Welding Metal-lophthalocyanines into Bimetallic Molecular Meshes for Ultra-sensitive LowminusPower Chemiresistive Detection of Gases J AmChem Soc 2019 141 2046minus2053(17) Aubrey M L Kapelewski M T Melville J F Oktawiec JPresti D Gagliardi L Long J R Chemiresistive Detection ofGaseous Hydrocarbons and Interrogation of Charge Transport inCu[Ni(23-Pyrazinedithiolate)2] by Gas Adsorption J Am Chem Soc2019 141 5005minus5013(18) Sun L Liao B Sheberla D Kraemer D Zhou J Stach EA Zakharov D Stavila V Talin A A Ge Y Allendorf M DChen G Leonard F Dinca M A Microporous and NaturallyNanostructured Thermoelectric MetalminusOrganic Framework withUltralow Thermal Conductivity Joule 2017 1 168minus177(19) Erickson K J Leonard F Stavila V Foster M E SpataruC D Jones R E Foley B M Hopkins P E Allendorf M DTalin A A Thin Film Thermoelectric MetalminusOrganic Frameworkwith High Seebeck Coefficient and Low Thermal Conductivity AdvMater 2015 27 3453minus3459(20) Xie L S Sun L Wan R Park S S DeGayner J AHendon C H Dinca M Tunable Mixed-Valence Doping TowardRecord Electrical Conductivity in a Three-Dimensional MetalminusOrganic Framework J Am Chem Soc 2018 140 7411minus7414(21) Talin A A Centrone A Ford A C Foster M E StavilaV Haney P Kinney R A Szalai V El Gabaly F Yoon H PLeonard F Allendorf M D Tunable Electrical Conductivity inMetalminusOrganic Framework ThinminusFilm Devices Science 2014 34366minus69(22) Sheberla D Sun L Blood-Forsythe M A Er S Wade CR Brozek C K Aspuru-Guzik A Dinca M High ElectricalCondutivity in Ni3(23671011-Hexaiminotriphenylene)2 a Semi-conducting MetalminusOrganic Graphene Analogue J Am Chem Soc2014 136 8859minus8862
(23) Van Vleet M J Weng T Li X Schmidt J R In Situ TimeminusResolved and Mechanistic Studies of MetalminusOrganic FrameworkNucleation and Growth Chem Rev 2018 118 3681minus3721(24) Marcus Y The Effectivity of Solvents as Electron Pair DonorsJ Solution Chem 1984 13 599minus624(25) Skorupskii G Trump B A Kasel T W Brown C MHendon C H Dinca M Efficient and Tunable OneminusDimensionalCharge Transport in Layered Lanthanide MetalminusOrganic Frame-works Nat Chem 2020 12 131minus136(26) Denton A R Ashcroft N W Vegardrsquos Law Phys Rev A AtMol Opt Phys 1991 43 3161minus3164(27) McIntyre N S Cook M G X-ray Photoelectron Studies onSome Oxides and Hydroxides of Cobalt Nickel and Copper AnalChem 1975 47 2208minus2213(28) Moretti G Auger Parameter and Wagner Plot in theCharacterization of Chemical States by Xminusray PhotoelectronSpectroscopy A Review J Electron Spectrosc Relat Phenom 199895 95minus144(29) Matienzo J Yin L I Grim S O Swartz W E Jr XminusrayPhotoelectron Spectroscopy of Nickel Compounds Inorg Chem1973 12 2762minus2769(30) Frost D C Ishitani A McDowell C A Xminusray PhotoelectronSpectroscopy of Copper Compounds Mol Phys 1972 24 861minus877(31) Yin L Adler I Tsang T Matienzo L J Grim S OParamagnetism and Shakeminusup Satellites in X-ray PhotoelectronSpectra Chem Phys Lett 1974 24 81minus84(32) Tan K L Tan B T G Kang E T Neoh K G XminusrayPhotoelectron Spectroscopy Studies of the Chemical Structure ofPolyaniline Phys Rev B Condens Matter Mater Phys 1989 398070minus8073(33) Mohtasebi A Chowdhury T Hsu L H Biesinger M CKruse P Interfacial Charge Transfer Between Phenylminuscapped AnilineTetramer Films and Iron Oxide Surfaces J Phys Chem C 2016 12029248minus29263(34) Bair R A Goddard W A III Ab initio Studies of the XminusrayAbsorption Edge in Copper Complexes I Atomic Cu2+ andCu(II)Cl2 Phys Rev B Condens Matter Mater Phys 1980 222767minus2776(35) Sano M Komorita S Yamatera H XANES Spectra ofCopper (II) Complexes Correlation of the Intensity of the 1s rarr 3dTransition and the Shape of the Complex Inorg Chem 1992 31459minus463(36) Chaboy J Munoz-Paez A Carrera F Merkling P MarcosE S Ab initio X-ray Absorption Study of Copper Kminusedge XANESSpectra in Cu(II) Compounds Phys Rev B Condens Matter MaterPhys 2005 71 134208(37) Peng S-M Chen C-T Liaw D-S Chen C-I Wang YEstablishment of the Bond Patterns of o-Benzoquinonediimine andSemi-o-Benzoquinonediimine Crystal Structures of Metal Complexes[FeII(bqdi)3](PF6)2[Co
II(s-bqdi)2] and [CoIIICl(s-bqdi)2)] InorgChim Acta 1985 101 L31minusL33(38) Jimenez H R Salgado J Moratal J M Morgenstern-Badarau I EPR and Magnetic Susceptibility Studies of Cobalt (II)-and Nickel (II)-substituted Azurins from Pseudomonas AeruginosaElectronic Structure of the Active Sites Inorg Chem 1996 35 2737minus2741(39) Garribba E Micera G The Determination of the Geometry ofCu (II) Complexes An EPR Spectroscopy Experiment J Chem Educ2006 83 1229minus1232(40) Amalric Popescu D Herrmann J-M Ensuque A Bozon-Verduraz F Nanosized Tin Dioxide Spectroscopic (UV-Vis NIREPR) and Electrical Conductivity Studies Phys Chem Chem Phys2001 3 2522minus2530(41) Chu J Sher A Physics and Properties of Narrow GapSemiconductors Springer New York NY 2008(42) Nowak M Kauch B Szperlich P Determination of EnergyBand Gap of Nanocrystalline SbSI Using Diffuse ReflectanceSpectroscopy Rev Sci Instrum 2009 80 No 046107
Journal of the American Chemical Society pubsacsorgJACS Article
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
12372
(43) Gibbs Z M LaLonde A Snyder G J Optical Band Gap andthe Burstein-Moss Effect in Iodine Doped PbTe Using DiffuseReflectance Infrared Fourier Transform Spectroscopy New J Phys2013 15 No 075020(44) Kudman I Seidel T Absorption Edge in Degenerate pminusTypeGaAs J Appl Phys 1962 33 771minus773
Journal of the American Chemical Society pubsacsorgJACS Article
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
12373
(43) Gibbs Z M LaLonde A Snyder G J Optical Band Gap andthe Burstein-Moss Effect in Iodine Doped PbTe Using DiffuseReflectance Infrared Fourier Transform Spectroscopy New J Phys2013 15 No 075020(44) Kudman I Seidel T Absorption Edge in Degenerate pminusTypeGaAs J Appl Phys 1962 33 771minus773
Journal of the American Chemical Society pubsacsorgJACS Article
httpsdxdoiorg101021jacs0c04458J Am Chem Soc 2020 142 12367minus12373
12373
