Journal ofMaterials Chemistry C

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View Article OnlineView Journal

Organotitanias: a

aLaboratorio de Nanotecnologıa Molecular

Universidad de Alicante, Carretera San

E-mail: j.garcia@ua.es; Web: http://www.nabDepartamento de Quımica-Centro de Inv

Universidad de La Rioja, Madre de Dios, 51

berenguer@unirioja.es; Web: https://cisq.un

† Electronic supplementary information (Efurther discussion about the formatcharacterization (nitrogen adsorption/deand absorption spectra of the photodeg10.1039/c4tc01704h

Cite this: DOI: 10.1039/c4tc01704h

Received 1st August 2014Accepted 7th September 2014

DOI: 10.1039/c4tc01704h

www.rsc.org/MaterialsC

This journal is © The Royal Society of

versatile approach for band gapreduction in titania based materials†

Marisa Rico-Santacruz,a Angel E. Sepulveda,b Elena Serrano,a Elena Lalinde,b

Jesus R. Berenguer*b and Javier Garcıa-Martınez*a

The reduction of the band gap of titania is critically important to fully utilize its photocatalytic properties.

Two main strategies, i.e. doping and partial reduction of Ti(IV), are the main alternatives available to date.

Herein, we report a new synthesis strategy based on one-pot co-condensation of in situ prepared

polymetallic titanium-alkoxide complexes with titanium tetrabutoxide. Using this direct reaction, it is

possible to introduce organic compounds in the anatase phase, causing site distortions in the crystalline

structure of the network. By using this strategy, a yellow and a black titania have been produced, with

the latter showing a remarkable photocatalytic activity under visible-light.

Introduction

Titania (TiO2) is one of the most widely used materials in lightharvesting systems, such as dye-sensitized solar cells (DSCs), insolar water splitting and purication of air and water,1 due to itschemical stability, biocompatibility, non-toxicity and goodoxidative characteristics.2 Notwithstanding, due to its largeband gap between the conduction and valence bands (3.0 eV forrutile and 3.2 eV for anatase phases), this material only harvestsabout ve per cent of solar radiation (UV). In this area, thesearch for titania based materials with a smaller band gap,enabling absorption in the 400–900 (visible and near-IR) spec-tral range, is of prime importance for improving their efficiency.

Visible light-activated titanias have been prepared bypartially reducing TiO2 and metal-ion implantation or non-metal doping. Along this line, a remarkable interest has beendevoted to the so-called “black titanias”, dark colored materialscapable of absorbing a wide range of wavelengths.2a,3However, achallenge encountered in the design of these titania basedmaterials is the predictability of their structural and electronicphotocatalytic properties, because they have been found to belargely dependent on the synthesis conditions and procedures,which normally include high temperature treatments.2b–d,4

, Departamento de Quımica Inorganica,

Vicente s/n, E-03690, Alicante, Spain.

nomol.es

estigacion en Sıntesis Quımica (CISQ),

, E-26006, Logrono, Spain. E-mail: jesus.

irioja.es/gmmo.php

SI) available: Synthesis procedures andion of organotitanias, full set ofsorption isotherms, XRD, and TEM)radation reactions of R6G. See DOI:

Chemistry 2014

The band gap of these titania-based materials can be alsomodied by sensitizing TiO2 with the appropriate organic orcoordination dyes, which are able to inject electrons into theconduction band upon absorption of visible (or near IR) light.This sensitization is normally a post-synthesis procedure(impregnation or supercial covalent graing),2c,4a,5 thussuffering from inherent problems (poor control over the loca-tion of the functionality, decrease in the surface area, partialpore blocking, leaching, ., etc.). In these sensitized systems, ithas been stated that the electronic transference is more efficientwhen the charge-transfer dye is covalently attached to thesurface of a semiconductor substrate.5b This could be due to thefact that the properties of these materials depend not only onthe inorganic semiconductor or the charge-transfer dye, butalso on the interface between both phases.

For that reason, it would be highly desirable to get additionalknowledge on the design of new low-cost and reproduciblesynthesis pathways for the preparation of efficient hybrid pho-tocatalyst titania materials, in which the charge-transfer dyesshould be more closely integrated in the semiconductor matrix.

Recently, Prof. Schubert rst pointed out the possibility ofusing titanium coordination compounds [Ti(OR)a(L)b]n, con-taining adequate functionalized organic ligands (L), as appro-priate precursors in the preparation of hybrid materials by sol–gel processing.6a–d Inspired by this idea, and taking into accountour previous experience in the design of hybrid periodic mes-oporous metal complex-silica,7 herein we report the successfuladaptation of our “Sol–Gel Coordination Chemistry” approach7c

to the synthesis of mesoporous organotitania (anatase). Thesematerials display a remarkable reduction in their band gap andexcellent photocatalytic activity. This methodology constitutes anew and environmentally friendly alternative to doping andTi(IV) partial reduction. Also, it should be noted that, comparedwith the extensive research carried out on mesoporous

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Journal of Materials Chemistry C Paper

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organosilicas, the introduction of functionalities into titania-based sol gels materials by using functional-modied organic orcoordination compounds remains essentially unexplored.6,8,9

Results and discussion

As depicted in Scheme 1, mixing of ethanolic solutions oforganic compounds (4,6-dihydroxypyrimidine, DHP-H2 and p-phenylenediamine, PPD) with tetrabutyl orthotitanate, TBOT(organic compound/TBOT 0.05–0.2 molar ratio) leads to theformation of polymetallic titanium-alkoxide intermediatespecies homogeneously dispersed in the nal mixture (i, A or B,Scheme 1). Room temperature hydrolysis of these mixtures for24 hours (ii, Scheme 1) affords a gel (white for DHP, pale-greyfor PPD), which crystallizes adopting an anatase phase byheating the suspension at 80 �C for 24 hours (iii, Scheme 1).10

These organotitanias are obtained as a mesoporous materialwithout the concurrence of surfactants. The obtained samplesare denoted TiO2-DHP-x and TiO2-PPD-x, where x indicates thewt% of the nominal organic compound incorporated (x ¼ a, band c for 6, 12 or 22 wt%, respectively, see Table 1 and S1, ESI†).

In spite of using the same reaction conditions, and despiteTiO2-DHP-x organotitanias being pale-yellow solids, thismethod yields black organotitanias in the case of PPD (see thephotographs in Scheme 1). In the context of these results, it isworth noting that Boyle et al. have reported that the stoichio-metric reaction of DHP-H2 with titanium-alkoxides Ti(OR)4evolves yielding alkoxy-titanium(m-dihydroxypyrimidinate)complexes [{LxL

04�xTi}2(m-DHP)],6e similar to the dimetallic

organotitania precursor A proposed in Scheme 1. Undersubsequent hydrolysis conditions, these species are able to co-condensate with the rest of the TBOT precursors affording a gel(ii, Scheme 1).

The case of p-phenylenediamine is more complex. Theplausible intermediate species generated (i, B, Scheme 1) areless straightforward. We tentatively propose a dinuclear speciesB stabilized by Ti/N and O/H–N bonding interactions,similar to related reported dimetallic or polymeric amineadducts of stoichiometry [Ti2(OR)8(NH2R0)2].6a

At this point, it is worth pointing out the different behaviorsof the PPD in an aqueous suspension or in the TBOT mixture. Itis well known that free PPD is prone to autoxidation in aqueousmedia to give dark materials, which have been used as typicalingredients in hair dyes or in a large variety of coloured prod-ucts, like textiles, cosmetics, black rubber or even temporarytattoos. This oxidation gives the highly reactive p-phenyl-enequinone diimine (NH]C6H4]NH, PPQD), which furtherundergoes self-conjugation reactions to form a mixture ofdimers and trimers (like the Brandrowski's base), see ESI,Fig. S1.†11 In contrast with this behaviour, the gel, precursor ofthe black titania, which is generated aer 24 hours of hydrolysisat room temperature is a pale-grey solid (ii, Scheme 1 bottom).This observation suggests that the p-phenylenediamine (PPD)obtains an effective protection against the oxidation of PPD inthe synthesis gel, which further supports the formation of theproposed intermediate species. In fact, the darkening of thematerial, and thus the oxidation of a part of the PPD molecules

J. Mater. Chem. C

included in the gel, mainly takes place during the crystallizationstep (iii, Scheme 1 bottom). We cannot state the extent of theoxidation, which probably leads to a mixture of organic deriv-atives (OPPD, oxidized p-phenylenediamine), but it is reason-able to assume that if the PPD molecules are protected duringthe formation of the gel and homogeneously dispersed in themixture, it should be difficult for them to oligomerize in theoxidation process. For this, we propose tentatively that theorganic species present in the black organotitanias are mainlymolecules of PPD and PPQD (see the ESI† for moreinformation).

The effectiveness in the homogeneous incorporation of theorganic entities into the structure of mesoporous titania hasbeen conrmed using elemental analysis, FTIR and XPSanalyses.

The nal organic contents of the solids are ca. 5 wt% (organiccompound/TiO2 ca. 0.05 molar ratio) independent of the initialreaction ratio employed (Tables 1 and S1, ESI†). FTIR spectra ofboth organotitania materials show bands characteristic of eachof the organic moieties (see Fig. 1). In the spectra of TiO2-DHP,one absorption at 1410 cm�1 is clearly observed, correspondingto the d(CH) associated with the ring of the pyrimidine. Thespectra of the black TiO2-PPD show several absorptions due ton(C]C)arom (1623, 1511, and 1402 cm�1), the correspondingone to n(C–N) at 1268 cm�1 and a distinctive medium absorp-tion at 1573 cm�1 ascribable to n(C]N)imine,12 supporting thepresence of oxidized organic species. In fact, this band appearsonly as a very weak shoulder in the spectrum of the pale-grey gel(see Fig. S2†), supporting again that the iminic species aremainly generated in the crystallization step (iii, Scheme 1).

XPS spectra of both types of organotitania display symmetricTi(2p) features, 2p3/2 and 2p1/2, corresponding to Ti(IV) coordi-nation sites (Fig. 2, top le). These bands do not show addi-tional lower binding energy components associated with thepresence of reduced Ti3+ centres, which is a rather commonsituation in non-metal atom doped titanias.4a Both sampleshave similar Ti 2p and O 1s binding energies than those oforganic-free titania (Fig. 2, top), conrming the octahedralcoordination of Ti(IV) in our samples.8,13

XPS spectra of the N 1s region (aer 20 seconds of etching toassure the elimination of adventitious N2,13 see the ESI, Fig. S3†)show a symmetric band with a maximum at 400.23 eV for TiO2-DHP and a broad symmetric band with a maximum at ca. 400eV, which can be deconvoluted into two peaks at ca. 400.2 and399.25 eV (Fig. 2, bottom). The assignment of XPS peaks innitrogen-containing titanias is still under debate.4a,13 The N 1ssignal at ca. 400 eV is usually ascribed to nitrogen species in ahigher oxidation state (NC, C–N]C or NCO bonds, NO sites orinterstitial NH species), as well as to adsorbed molecularnitrogen.13 Therefore, we tentatively relate the aforementionedbands to the incorporation of one type of organic molecule inthe yellow TiO2-DHP and to the simultaneous presence of twotypes of species (PPD and oxidized PPQD) for the black titaniaTiO2-PPD.

These novel organotitanias (TiO2-DHP and TiO2-PPD)present Type IV isotherms with a shape similar to the one of thecontrol TiO2 (Fig. 3 le, Table 1 and ESI, Fig. S4†). The XRD

This journal is © The Royal Society of Chemistry 2014

Scheme 1 Synthesis and photographs of TiO2-DHP-x and TiO2-PPD-x organotitanias (organic compound/TBOT 0.05–0.2 molar ratio),including the photographs of the organic free control titania.

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patterns of these samples (Fig. 3 right and ESI, Fig. S5†) showthe characteristic peaks of the anatase structure (2q ¼ 25.3�,37.8� and 48.05�),14a as expected by the generation of a numberof distorted sites in the crystalline structure. The texturalproperties of all samples are also very similar, supporting the

This journal is © The Royal Society of Chemistry 2014

incorporation of the organic moieties in the framework of themesoporous titania, and not between the titania nanoparticles.Related modications of the mesoporous TiO2 anatase phase byincorporation of benzene siloxane have been recently reportedby Fan et al.14b

J. Mater. Chem. C

Table 1 Textural and structural parameters of the mesoporous organotitania materials, as compared with the organic-free titania, TiO2

Sample Org. cont.a (wt%) dpb (nm) Vp

c (cm3 g�1) ABETd (m2 g�1) DXRD,e (nm) FTEM,f (nm) dXRD101

,g (nm) dTEM101,h (nm)

TiO2 — 5.6 (2–9) 0.29 245 6.4 8.7 0.350 0.37TiO2-DHP-a 3.9 4.6 (2–9) 0.26 260 5.7 8.8 0.354 0.38TiO2-DHP-b 5.1 3.8 (1–9) 0.21 258 5.5 8.5 0.354 0.38TiO2-DHP-c 5.0 3.4 (1–9) 0.20 218 5.7 9.1 0.352 0.38TiO2-PPD-a 5.2 6.1 (2–10) 0.27 210 6 8.9 0.353 0.37TiO2-PPD-b 4.6 6.0 (2–11) 0.27 210 5.6 8.7 0.353 0.38TiO2-PPD-c 9.2 6.1 (2–11) 0.31 225 5.6 9.9 0.352 0.38

a Organic content from elemental analysis. b Average mesopore diameters. c Mesopore volume. d BET surface area from N2 isotherms at 77 K.e Crystalline domain size (calculated from XRD). f Particle size (calculated from TEM). g Anatase spacing from XRD. h Anatase spacing fromTEM. See the ESI for details.

Fig. 1 FTIR spectra of organotitanias prepared with 12 wt% of nominalorganic compounds, as compared with the control titania, TiO2, andthe organic compounds.

Fig. 2 (Top) Ti 2p XPS spectra (left) and O 1s XPS spectra (right) of theorganotitanias prepared with 12 wt% nominal organic compounds, ascompared to the organic-free mesoporous titania TiO2. (Bottom) Thecorresponding N 2p XPS spectra after 20 s of etching.

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The most signicant feature observed in the XRD diffracto-grams is the broadening of the peaks in the organic-containingsamples, which corresponds to a reduction in the crystallinedomain (measured using the Scherrer equation) of the orga-notitanias synthesized, when compared with the mesoporousorganic-free TiO2 (this reduction is on average around 0.7 nm,Table 1). However, a constant value for the particle size of ca. 8.7nm has been determined by TEM for all the synthesizedsamples. This suggests that the particle size is determined bythe synthesis conditions and not modied by the presence oforganic moieties, which however causes a decrease in thecrystalline domain size.

The TEM analysis conrms the mesoporous nature and thecrystalline structure of these materials (Fig. 4a and ESI,Fig. S6†), having a lattice spacing (obtained by using Gatansoware) of about 0.37–0.38 nm, close to the distance between(101) crystal planes of the anatase phase (ca. 0.35 nm). Contraryto that observed by TEM by other authors for titanias sensitizedby carbonaceous species,15 no evidence of a core–shell structurehas been detected aer a careful examination of TEM images.So, these results suggests that the organic moieties, homoge-neously distributed in the gels, give rise to the formation of

J. Mater. Chem. C

defect sites in the crystalline network, thus reducing the crys-talline domain without modifying its particle size.8

Tauc plots and DRUV spectra and of representativematerials are shown in Fig. 4b. As shown in the inset ofFig. 4c, the only difference of the yellow titania TiO2-DHP inrelation to the control TiO2 is a slight red-shi in theabsorption band edge related to the direct band gap transi-tion (3.17 vs. 3.24 eV from the corresponding Tauc plots. Bycontrast, the DRUV spectra of the black titania (TiO2-PPD)

This journal is © The Royal Society of Chemistry 2014

Fig. 3 Nitrogen adsorption/desorption isotherms at 77 K (left) andnormalized XRD patterns (right) of the organotitanias prepared with 12wt% nominal organic compounds, as compared to the organic-freemesoporous titania TiO2.

Fig. 4 (a) Representative TEM image of the TiO2-PPD-b sample, (b)valence band XPS spectra, (c) Tauc plot of the transformed Kubelka–Munk function versus the energy of light adsorbed (the inset repre-sents the DRUV spectra of samples), and (d) schematic illustration ofthe DOS of the organotitanias TiO2-DHP-b and TiO2-PDD-b ascompared with the control TiO2.

Fig. 5 (Left) Representation of the pseudophotocatalytic constant (k0)of the organotitania samples: TiO2-DHP-b and TiO2-PPD-b ascompared with neat titania, TiO2, in the degradation reaction of anaqueous solution of Rhodamine 6G (5 � 10�5 M) under UV (125 W,upper part) or visible (400W, bottom part) irradiation. (Right) Recyclingtests under the same conditions.

Fig. 6 Schematic representation of the photo-induced chargetransfers and the mechanism of the photocatalytic process.

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show two absorption band edges. The low energy featurecovering the visible region with a tail to ca. 900 nm is ascribedto transitions from the nal species generated by the oxida-tion of a part of the PPD, whereas the higher energy one isrelated to the direct band gap transition. Interestingly in thisblack titania, the energy of the direct band gap is foundstrongly red-shied (2.74 eV), lying in the visible (453 nm),and the HOMO–LUMO gap associated with the oxidizedorganic moieties decreases to 1.24 eV. For comparison, wehave also examined the DRUV of a solid sample of the pale-grey gel precursor of the black titania TiO2-PPD. In agree-ment with its unstructured nature, its Tauc plot (ESI,Fig. S7†) shows a clear blue-shi in the absorption band edgerelated to the direct band gap transition (3.44 eV vs. 3.24 eVfor anatase). In addition, the low energy feature, associatedwith the presence of oxidized phenylenediamine, is clearly

This journal is © The Royal Society of Chemistry 2014

less intense than that observed for the black titania, againconrming that the oxidation mainly takes place during thecrystallization step.

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Table 2 Constant values of the photocatalytic activity under UV and visible irradiation, regression coefficients and conversions at different timesof the organotitania samples as compared with the organic-free titania, TiO2

Samples Ega (eV) k0 � 103b (min�1) Rc

% Conversiond

k0/k0TiO2

e1 h 2 h 3 h

UVTiO2 3.24 7.2 � 1.9 (6.3) 0.9897 31.1 52.8 67.7 —TiO2-DHP-b 3.17 12.3 � 2.4 (9.8) 0.9857 43.6 66.5 84.2 1.7TiO2-PPD-b 2.74 20.0 � 3.4 (23.4) 0.9868 75.7 94.0 98.5 2.8

VisibleTiO2 3.24 2.5 � 0.7 (2.6) 0.9640 20.2 31.7 39.0 —TiO2-DHP-b 3.17 2.4 � 0.7 (2.6) 0.9563 15.0 24.3 39.5 1.0TiO2-PPD-b 2.74 5.6 � 1.2 (6.1) 0.9842 22.1 50.6 63.4 2.3

a Band gap energies calculated from the intercept of the tangent to the (F(R0)hn)1/2 versus the (hn) plot. b 1st order reaction rate pseudophotocatalyticconstant of the degradation of an aqueous solution of rhodamine 6G (5 � 10�5 M) obtained as the average of a minimum of three runs. The valuesin brackets indicate the k value used for the determination of the degree of conversion. c Regression coefficient for the same degradation reaction asparagraph (b). d Degree of conversion (%) achieved by samples for 1 h, 2 h and 3 h. e Ratio between the average k0 values of the as-synthesizedorganotitanias as compared with the average k0 value determined from the control TiO2 sample.

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The organic-free titania VB XPS spectrum shows the edge ofthe maximum energy at about 2.28 eV, similar to that found forother titanias,3 occurring the conduction band minimum (CB)at ca. �0.96 eV (Fig. 4b and d). Although not shown, voltam-perometry measurements gave a similar conduction bandminimum. In both cases, the incorporation of the organicmoieties in the structure of anatase induces a narrowing in theband gap and a slight shi to higher binding energies (ca. 0.17eV) in the VB maximum energy. Nevertheless, while for TiO2-DHP-b the band gap only decreases around 0.07 eV, in the caseof black organotitania (TiO2-PPD-b), the band gap remarkablydecreases up to 2.74 eV.

To test the photocatalytic activity of the mesoporous orga-notitanias, Rhodamine 6G (R6G) was degraded by the catalystsunder UV and visible light irradiation (see methods). Interest-ingly, and despite showing similar textural and structuralproperties than the organic-free TiO2, the observed photo-catalytic activity of both organotitanias under UV irradiation isenhanced up two- or three-fold (Fig. 5, top). Thus, with blackTiO2-PPD-b, the characteristic absorption peak at 525 nm ofR6G practically disappears with concomitant decolouration ofthe solution aer three hours of irradiation time (see ESI,Fig. S8 and S9†), contrasting with the required 12 h for thebleaching of the dye in the presence of control TiO2. It is well-known that UV irradiation produces the formation of a hole–electron pair (the hole way). While the positive holes are scav-enged by the hydroxyl groups (or H2O molecules) present in thesurface, the electrons are transferred to the conduction band ofthe semiconductor and subsequently trapped by the absorbedO2, leading, in both cases, to the formation of cOH radicals,which promote the degradation of the organics (Fig. 6).16 In thiscontext, the good photocatalytic response of our organotitaniascould be attributed to the smaller domain size of particles (from6.4 nm in TiO2 to ca. 5.5 nm in both organotitanias), which mayreduce the recombination rate of the charge carriers.

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TOC measurements of dye adsorption carried out in theabsence of light point out that the reaction time for thecomplete degradation of organic species is longer than forcomplete disappearance of Rhodamine 6G in UV-vis spectra.Notwithstanding, during the catalytic reactions, the maximumat 525 nm does not shi while decreasing, suggesting that thenally formed species in solution presumably do not containaromatic conjugated rings, that usually absorb in this region(see ESI, Fig. S8†).17

The photocatalytic activity under visible light was alsoexamined (Fig. 5, bottom and Table 2, and Fig. S10 in the ESI†).While, the yellow TiO2-DHP-b showed similar activities than thecontrol titania, the black TiO2-PPD-b exhibits a more than two-fold enhanced activity, giving decomposition rates of about 60%aer three hours of irradiation. It should be noted that, in thecase of the photodegradation of R6G under visible light usingTiO2 and TiO2-DHP-b, the ltration of radiation with wave-length shorter than 400 nm precludes the hole way.

Nevertheless, due to their band gap of 2.23 eV, the R6Gmolecules are excited by visible light, generating cationic cR6G+

radicals and electrons. As has been previously noted, theseelectrons can be transferred to the conduction band of titaniabecause the R6G LUMO energy is higher than the titania CB byabout 1.4 eV (the dye way), with TiO2 acting only as an electronmediator in the decomposition of R6G, in accordance with theirsimilar observed photocatalytic activity (Fig. 6, top).2b

By contrast, the improved photoactivity under visible light ofthe black titania is likely due to the synergistic combination ofits reduced band gap (the hole way) and the redox properties ofthe aromatic oxidized dyes (OPPD) or even of the R6G (the dyeway) (Fig. 6, bottom). To gain a better understanding of thedegree of contribution of each of these two mechanisms to theactivity of the photocatalyst, we decided to study the inuenceof some scavengers in the process.2b We observed that while theaddition of isopropanol, as a hydroxyl radical scavenger, onlycaused a slight decrease in the photodegradation rate,

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practically no catalytic degradation was found by the addition oftriethanolamine as a hole scavenger (see ESI, Fig. S11†). Theseresults seem to suggest that the dominant mechanism is thehole way, at least under the experimental conditions used.

Notably, as observed in Fig. 5 (top), both organotitanias stillshow an efficiency of almost 90% aer seven photodegradationcycles under UV irradiation, and the good degradation effi-ciency of black organotitania under visible light did not declineaer being reused ve times (Fig. 5, bottom). These resultsindicate that both the organotitanias are relatively stable,exhibiting a negligible degradation during the photocatalyticoxidation of R6G.

Conclusions

Herein we report a new synthesis approach to reduce the bandgap of titania via crystal disruption. This environmentallyfriendly one-pot method allows the synthesis of highly activeand stable photoactive materials through the one-pot co-condensation of in situ prepared polymetallic titanium-alkoxidespecies with titanium tetrabutoxide, allowing the incorporationof organic moieties (crystal disruptors) in the crystal structure oftitania, which causes distortion of the anatase network inlocalized sites, thus reducing the crystalline domain withoutmodifying its particle size. By using p-phenylenediamine as adisruptor, a black titania has been obtained with a remarkablereduction of its band gap (from 3.24 eV to 2.74 eV), exhibitingoutstanding photocatalytic activity under visible light.

The use of crystal disruptors to reduce the band gap oftitania is a new tool to tune the optical properties of photoactivematerials with a wide range of applications. Further work isbeing performed to clarify the relationship between the struc-ture of the organic moiety and the band gap of the nalorganotitania.

Experimental sectionSynthesis of the control titania

Mesoporous titania was prepared according to the procedurereported by Y. Wang et al.,14a but without the aid of a surfactant(Triton X-100). TiO2 samples prepared with and without asurfactant show type IV isotherms, typical of mesoporousmaterials with similar textural properties and XRD diffracto-grams, thus the use of a surfactant was avoided during thiswork.10 In a typical synthesis, 5 g (14.7 mmol) of titaniumprecursor (tetrabutyl orthotitanate, TBOT) were dissolved in35.37 ml of absolute ethanol under magnetic stirring. Then,123.5 g (6.86 mol) of water was added drop-wise causing theprecipitation of the solid. The molar ratio of the synthesis gelwas 1 TBOT : 41.3 EtOH : 467 H2O. The mixture was thenreacted at room temperature for 24 h under vigorous magneticstirring, followed by treatment in an oven at 80 �C for 24 hours.The obtained solid product was washed with water and acetone,ltered and dried in an oven at 100 �C for 8 hours.

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Synthesis of organotitanias

The synthesis of hybrid mesoporous organotitanias was carriedout by co-condensation of the titania precursor, tetrabutylorthotitanate (TBOT), with different amounts of 4,6-dihydrox-ypyrimidine (DHP-H2) and p-phenylenediamine (PPD) (6–24wt%). The synthesis was carried out using only water and smallamounts of ethanol as solvents and without the concurrence ofsurfactants. In a typical synthesis, the organic compounds wererst dissolved in absolute ethanol for 2 hours at 40 �C undermagnetic stirring. Then, 5 g (14.7 mmol) of TBOT were added tothe solution and the mixture was magnetically stirred for 40min at room temperature. Finally, 123.5 g (6.86 mol) of waterwere added drop-wise causing the precipitation of the solid. Themixture was then kept at room temperature for 24 h undervigorous magnetic stirring, followed by treatment at 80 �C for 24hours in a drying oven. The obtained solid product was washedwith water and acetone, successively, ltered and dried in anoven at 100 �C for 8 hours. The molar ratio of the synthesis gelwas 1 TBOT : z Org. : 41.3 EtOH : 467 H2O, where Org. refers toan organic compound, with z being varied between 0.05 and 0.2.

Control titania, i.e. organic-free titania (TiO2 sample), wassynthesized following the same procedure described above forthe synthesis of the organotitanias, avoiding the organiccompound addition step (see the ESI†).

General methods

The incorporation of both organic compounds in the titaniasamples was evaluated by FTIR and XPS spectroscopy tech-niques, and the content of both organic compounds in the naltitania materials was analyzed by elemental analysis. XRDanalysis was carried out in order to study the crystalline struc-ture of the titania materials and TEM was used to study theirmorphology, porosity and crystallinity. The particle size wascalculated by TEM, using the soware Gatan, being estimatedas an average of the size of 100 particles. Porous textureparameters (mesoporosity volume, surface area and porediameter) were characterized by N2 adsorption at 77 K and theresults were analyzed using the soware package AUTOSORB.DRUV spectra were recorded in order to determinate the bandgap of the titania materials. XPS spectra of these materials wereobtained in the �10 to 2 eV region to calculate the position ofthe maximum of their valence bands. Both techniques werecombined to determine the maximum of the conduction bandposition of the titania materials and to obtain the density ofstates (DOS) scheme.

Photocatalytic activity of the synthesized materials wasevaluated by photocatalytic degradation of Rhodamine 6Gmolecules in aqueous solution under UV or visible radiation.The samples irradiated with UV light were studied with a totalorganic carbon analyzer to determine the TOC content of thesynthesized samples before and aer bleaching of the samples.A calibration curve using R6G with different concentrations wasused for determining the TOC contents of samples.

All equipment and experimental conditions used aredescribed in more detail in the ESI.†

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Acknowledgements

We thank the Spanish MINECO (Projects CTQ2011-28954-C02-01, CTQ2008-06669-C02-02/BQU and CTQ2013-45518-P) fornancial support. E.S. acknowledges nancial support from theUniversity of Alicante, Spain (GRE12-39).

Notes and references

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This journal is © The Royal Society of Chemistry 2014