Preparation and characterisation of CoS2 nanomaterial in aqueous cationic

surfactant medium of cetyltrimethylammonium bromide (CTAB)

Indranil Chakraborty, Pradip K. Malik and Satya P. Moulik*Department of Chemistry, Centre for Surface Science, Jadavpur University, Kolkata, 700 032, India; *Authorfor correspondence (Tel.: +91-33-2414-6411; Fax: +91-33-2414-6266; E-mail: spmcss@yahoo.com)

Received 31 May 2005; accepted in revised form 18 August 2005

Key words: CoS2 nanoparticle, CTAB micelles, surface capping, thiophenol, characterization, colloids,dispersion

Abstract

Cobalt disulfide is an important nanomaterial in the field of material chemistry due to its interestingcatalytic and electromagnetic properties. We herein report the simple solution phase preparation ofnanoparticles of cobalt disulfide in micellar medium of cationic surfactant cetyltrimethylammonium bro-mide (CTAB) and also by way of capping with thiophenol. The product has been characterized usingspectroscopic and electron microscopic and magnetic moment measurements. The presence of disulfidebond is clearly evident from the 480 cm)1 peak in the infrared spectrum. The thiol passivated particlesprepared in aqueous and ethanol medium has been found to have different compositions as observed fromthe ESIMS study.

Introduction

Preparation and characterization of nanocrystal-lites are of priority interest in recent material sci-ence research for possible size quantized unusualoptelectronic properties (Alivisatos, 1996). Thefield has strong interdependence between basic andapplied research. The crystal growth diminishesthe surface area to volume ratio. The growth isaugmented by decreased surface free energy andrelease of lattice energy. The defects in surfacestructure acts as efficient traps for photogeneratedelectron/hole/electron-hole pair (Wang & Herron,1991) to manifest interesting photophysical prop-erties. Surface capping by organic thiols and otherligands can stabilize the particles against coagu-lation and can prevent photoinduced reactionswith solvent and environment (Murray, 1993;

Stigerwald et al., 1998; Balasubramanian et al.,2002).Synthesis of nanomaterials in compartments of

micelles, microemulsions, gels and coacervateshas been found to be convenient and simplecompared to vapor-phase reaction method underhigh temperature. Among the microheteroge-neous compartments used, water-in-oil (w/o)microemulsions have been fairly employed indifferent laboratories (Pileni, 1998; Moulik et al.,1999; Panda et al., 2000, 2001). Addition of thi-phenol, dodecanethiol etc. as capping agents candestabilize the microemulsion and facilitate iso-lation of the capped nanoparticle (Pileni, 1993;Chakraborty & Moulik, 2005b). Preparation bysol-gel method using SiO2 matrix (Battisha,2002), polymer-surfactant gel (Chakraborty &Moulik, 2005a) and membrane (Pattabi, 2000,

Journal of Nanoparticle Research (2006) 8:889–897 � Springer 2006DOI 10.1007/s11051-005-9032-y

2003; Rollins, 2000) as templates have been alsoreported. In this direction preparation andcharacterization of semiconductor nanomaterialsviz., CdS, CdSe, ZnS, PbS have been amplyreported.Due to its catalytic, electrical and magnetic

properties, cobalt disulfide (CoS2) nanoparticleshave received recent attention. This 3d transitionmetal disulfide crystallizes in pyrite structure. Itspreparation by solvent thermal method has beenreported (Qian et al., 1998). Its synthesis in poly-vinylpyrrolidone has also been reported (Bi et al.,2003).The reported methods of preparation of CoS2

nanoparticles are not straightforowardly simple;they require high temperature for annealing. Veryrecently, we have reported synthesis of nanopar-ticles of PbS (Chakraborty & Moulik, 2004), ZnS(Mitra et al., 2005) and that of CdS and HgS(both core-shells and composites) (Chakrabortyet al., 2005) in aqueous micellar media of Na-bis(2-ethylhexyl sulphosuccinate (AOT), sodiumdodecyl sulfate (SDS) and cetyltrimetylammoni-um bromide (CTAB), respectively. Micellar solu-tion is the simplest template (environment) intowhich the metal ions react with sulfide ions toproduce sulfides in nanometer dimensions. Inaqueous CTAB micellar medium, the CoS2 prep-aration occurs at room temperature and is thussimpler compared to the procedures reportedearlier. The procedure for the said preparationand characterization of the product by differentmethods (spectroscopy, electron microscopy etc.)have been described and discussed. Capping of theCoS2 nanoparticles by thiophenol in both aqueousand aquo-ethalnolic media has been also pre-sented.

Experimental

Materials

Cobalt sulphate was a product of E-Merck, Ger-many. CTAB and sodium sulfide were purchasedfrom Loba Chemicals, India. Thiophenol was aproduct of SRL, India. Ethanol was purchasedfrom Tedia, USA. All the chemicals were of ana-lytical grade and were used as received. Water usedin this study was doubly distilled and filteredthrough 0.22 l Millipore membrane filter paper

and kept in a thoroughly cleaned and steamedglass container.

Methods

The absorption spectra were recorded in a UV1601 (Shimadzu, Japan) instrument operating indual beam mode. Matched pair of quartz cells ofoptical path length 1 cm were used. The FTIRspectra were recorded in KBr pellets in a Perkin-Elmer instrument. All the electrospray ionizationmass spectroscopic (ESI-MS) study was carriedout under positive mode in a Thermo FinniganLCQ Advantage instrument (USA) with capillarytemperature=250�C, capillary voltage=10 V, ionspray voltage=5 kV and using N2 as the sheathgas.The transmission electron microscopic (TEM)

experiments were carried out using 50 kV accel-erating voltage in a Hitachi, Japan, model H600instrument equipped with SAED technique (cam-era length 0.8 m) using carbon-coated copper grid.A drop of the solution was placed on the grid anddried in two ways: by spontaneous evaporationand by rapid vacuum evaporation. The scanningelectron microscopic (SEM) experiments weredone with a Hitachi, Japan, model S-415Ainstrument. The magnetic moment of the uncap-ped material was measured in a Guoy balance at25�C. XRD of the uncapped CoS2 material wasdone in a Philips PW 1830 instrument operating ata voltage of 40 kV and a current of 30 mA withCu-Ka radiation by drying a drop of the disper-sion on a glass slide.

Preparation of nanoparticle

About 100 ll of 10 mM cobalt sulfate solutionwas added to 5 ml of 10 mM aqueous solution ofthe surfactant, CTAB (critical micellar concentra-tion (cmc)=1 mM at 298 K). Solid Na2S was thenadded to maintain the [Co2+]:[S2)]=1:10. Themixture was then stirred thoroughly and thesolution turned black. The product was thenallowed to age for 3 days, and then centrifuged at40,000 rpm for 1 h to collect the dispersed solidmass.For the preparation of the capped particles in

aqueous medium, thiophenol (acting as oil inaqueous medium) was added to the previouslyprepared solution in 1:1 volume ratio (as done in

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our previous work on PbS nanoparticle prepara-tion in reverse micellar medium) and shaken well.There was an immediate phase separation. Onlong standing for �1 month, a layer of blackparticles appeared in the interfacial layer. It wasseparated and dried. FTIR study evidenced thematerial to be thiophenol-capped nanomaterial ofCoS2.Surface passivation of the product was also

done with 1:1 (v/v) water–ethanol medium. In thisprocedure, the emulsion formed after addition ofthiophenol was stable for a day. Ethanol, there-fore, acted as the emulsion stabilizer. Within 24 h,a violet precipitate (easily redispersible in themedium) of the nanomaterial was obtained.Enhanced rate of capping in aqueous-ethanolicmedium compared to that in aqueous medium wasprobably due to better stabilization of the emul-sion by ethanol preventing it from breaking. Upondrying the precipitate by evaporation, its violetcolor was retained. On vacuum drying, the sampleturned black. Thus, the presence of ethanol on thesurface (as also evidenced from the FTIR experi-ment) was responsible for the purple color of thematerial.

Results and discussion

It was observed that bellow cmc (1 mM), theaqueous CTAB medium could not stabilizethe nanodispersion. At [CTAB]=�4 mM, theparticles precipitated within a day at CTAB:Co2+

mole ratio of 25. Even it was not stable at[CTAB]=9.9 mM, at the same mole ratio of 25.At the ratio=50, the dispersion was stable for atleast 2 weeks at [CTAB]=9.9 mM.A probable mechanism of formation of CoS2 in

CTAB solution (pH�4), formation of its nanoag-gregate and their stabilization by the amphiphile isshown bellow.

A: formation of CoS2:

4Hþ þ 2Br� þ SO2�4 ! SO2 þ 2H2Oþ Br2

ð1Þ(Dutta, 1998)

S2� þ Br2 ! Sþ 2Br� ð2Þ(E0=2.579)

S2� þ S! S2�2 ð3Þ

(Cotton & Wilkinson, 1969)

Co2þ þ S2�2 ! CoS2 ð4Þ

B: Formation of Aggregate:

nðCoS2Þ ! ðCoS2ÞnC: Stabilization of nanoaggregate by the amphi-

phile:

ðCoS2Þn þmCTAþ ! ðCoS2ÞnðCTAþÞm

A schematic representation of the stabilized nano-aggregate is shown in Figure 1.

The CTAB has a weak absorption in the 200–300 nm UV region. Co2+ in CTAB solution hasalmost no absorption measured against CTABcontrol. Figure 2a illustrates the absorption spec-tra of CoS2 in CTAB measured against Co2+–CTAB control. The spectra evidenced a broadabsorption over the entire region with two humpsnear 305 and 420 nm (4.068 and 2.954 eV,respectively). The capped particles prepared inethanol medium resulted a sharp peak at 560 nm(2.22 eV) and a shoulder at 440 nm (2.82 eV) asshown in Figure 2b. The red shift of the opticalspectrum of the capped particle compared to theuncapped particle was due to the charge transfer

Figure 1. Pictorial representation of CTA+ stabilized CoS2nanoparticle.

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from S (PhS)) to Co-core (Cristou et al., 1982;Wachhold et al., 2000) and/or the coupling of thecore electronic states of Co(II) with the p-electronsystem of the thiophenolate aromatic ring (Collieret al., 1998) on the surface. The spectra wasasymptotic bellow this region. The band gap of the

bare material has been evaluated from the Taucrelation (Tauc & Menth, 1972)

ðehmÞ ¼ Cðhm� egÞm

For m=1/2, eg is the direct allowed band gap. Form=higher multiple of 1/2, eg gives some forbiddenand indirect band gaps. CoS2 is a direct band gapmaterial and we assume m=1/2. The data withinthe absorption range of 250–500 nm was consid-ered. From the linear part of the (ehm)2 vs. hm plot(Inset, Figure 2a), a band gap of 4 eV wasobtained. The curvature near the absorption edgemight be due to defected sites or indirect transi-tions. The sample did not fluoresce when excited atits absorption maximum.The FTIR spectrum of the solid capped samples

prepared in aqueous and ethanolic media are givenin Figure 3a and b, respectively. Both the spectrawere very similar excepting the difference in the

Figure 2. (a) UV–Vis spectra of 19.6 lM CoS2 prepared in10 mM CTAB medium using Co2+/CTAB solution wasused as the control. Inset: Tauc plot for determination ofthe band gap of the formed CoS2. (b) UV–Vis spectra ofthiophenol capped CoS2 prepared in 10 mM CTAB inwater/ethanol (1:1 v/v) medium using aquo-ethanol med-ium as the control.

Figure 3. FTIR spectra of (a) thiophenol capped CoS2nanoparticle prepared in 10 mM aqueous CTAB medium,and (b) thiophenol capped CoS2 nanoparticle prepared inaquo ethanol CTAB medium.

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transmitance at the 3410 cm)1 peak. This peak is acharacteristic of hydrogen bonding (Hobbs &Hafner, 1999). The stronger peak in case of thematerial prepared in ethanol clearly evidenced thepresence of ethanol on the surface of the material.This was also confirmed from the ESI-MS studydiscussed later. The 480 cm)1 peak correspondedto the S–S stretching (Silverstein et al., 1991) of thedisulfide group. This along with the 1020 cm)1

peak clearly indicated the formation of cobaltdisulfide20. The absence of S–H stretching peak(2430–2590 cm)1) confirmed coordination fromthiophenol moiety and consequent surface passiv-ation as evidenced from the absorption spectrumand ESI-MS study. The 733 cm)1 peak was char-acteristic of out of plane deformation of the ring-hydrogen in the thiophenol ring. The 1430 and1630 cm)1 peaks corresponded to C=C skeletalvibration, and the �3020 cm)1 peak correspondedto the C–H stretching. The C–S stretching wasevidenced by 684 and 730 cm)1 peaks. The FTIRspectra also confirmed the stability of the preparedCoS2 against aerial oxidation by the absence ofany CoO peak at 670 and 580 cm)1 in the FTIRspectrum (Pejova et al., 2001). On vacuum drying,the intense violet sample turned into black mass.The 3410 cm)1 peak in the FTIR spectra becameless intense, showing desorption of ethanol fromthe surface. The presence of ethanol was, there-fore, solely responsible for the intense violet col-oration of the capped sample. A possibleexplanation of the color is the increased electrondensity on S of thiophenolate capping groupinduced by way of radical formation (Ganapathiet al., 2000). This increased electron density on theS led to electron transfer from the S to the Co-core, resulting in the purple color. Purple thi-ophenol stabilized cobalt complex was alsoreported in the case of tetrahedral [Co8S6(SPh)8]

4)

prepared in acetonitrile medium by Christou et al.(1982).The ESI-MS experiments on the prepared CoS2

prepared in the aqueous and ethanolic mediaresulted in strong peaks at m/z=602 and 786,respectively (Figure 4a, b) confirming the forma-tion of a compound in the mole ratioCoS2:

)SPh=4:1 and CoS2:)SPh:EtOH=4:1:4,

respectively in the two media. The presence of fourEtOH molecules per complex made a difference inits spectral properties. The peaks at m/z=740 and678 in Figure 4b were probably due to desorption

of ethanol and thiophenolate ion (PhS)), respec-tively from the complex under the condition ofhigh temperature and voltage in the experiment.The SEM micrograph of the uncapped CoS2

particles is presented in Figure 5; accumulatedspherical particles in close size range wereobserved. The TEM pictures of the uncapped andcapped samples of CoS2 are illustrated in Figure 6.

Figure 4. ESI-MS for (a) thiophenol capped particle pre-pared in aqueous medium and (b) thiophenol capped CoS2particle prepared in aquo-ethanol medium.

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The uncapped sample processed by rapid evapo-ration of a drop of the sample on the carbon-coated copper grid in vacuum is presented in

Figure 6a. The corresponding sample prepared bysimple evaporation on the grid is depicted in Fig-ure 6b. The rapidly dried sample evidenced nearspherical (d=20 nm) and well separated particles,whereas in slow drying condition particle growthand association leading to polydisperse parallele-piped type agglomeration was observed. In Fig-ure 6c, rapidly evaporated vacuum dried sample ofcapped CoS2 is presented. The slowly evaporatedcapped sample is presented in Figure 6d. Thecapped particles (Figure 6c) have evidenced fractallike association of two-dimensional arrays ofspherical particles with fairly narrow size distri-bution (d=20 nm). A few large agglomerates werealso observed in the figure. Formation of necklacetype chain of associated spherical particles is evi-denced in Figure 6d. The electron diffraction(SAED) patterns of the uncapped and cappedparticles (Figure 7a and b, respectively) having five

Figure 5. SEM micrograph of uncapped CoS2 particlesprepared in 10 mM aqueous CTAB medium.

Figure 6. TEM micrograph of the CoS2 particle prepared in 10 mM aqueous CTAB medium. Conditions: Uncapped particle,(a) Vacuum dried, (b) Dried by evaporation. Thiophenol capped particles (c) dried in vacuum, (d) dried by evaporation. Thesize distribution of the particles in Figure 5c has been shown in inset using Gaussian fitting.

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circular rings suggested cubic arrangement of Coatoms forming a face centered cubic structure sit-ting at the center of the sulphur octahedron in thepyrite structure of CoS2. The pattern of the cappedparticle was more pronounced compared to theuncapped particles suggesting better crystallinityof the capped material. CoS2 is a ferromagneticmaterial with magnetic moment 0.84 lB per Coatom at 0 K (Benoit, 1995; Qian et al., 1998;Hobbs & Hafner, 1999) and Curie temperature

near 120 K (Qian et al., 1998; Bi et al., 2003). Themagnetic moment suggested one unpaired electroncorresponding to the low spin configuration understrong octahedral field of the sulfur ligand envi-ronment. Relatively little work has been done onthe magnetic moment of nanoparticles of CoS2.The measured magnetic moment of CoS2 found inthis study was 3.77 lB per Co atom signifying highspin d7 configuration (Passaretti et al., 1981;Huheey et al., 1993). This was probably due to

Figure 7. SAED patterns for uncapped (a) and thiophenol capped CoS2 particles (b).

Figure 8. XRD pattern of the uncapped CoS2 nanomaterial showing pyrite structure.

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transition from low to high spin state at roomtemperature as the Curie temperature of thematerial was sufficiently low. The peaks at2h=37.7, 45.6 and 54.8� in Figure 8 conclusivelysupported the pyrite structure of uncapped CoS2with (210), (220), (311) planes in the XRD pattern(Zhaoling, 2003).

Conclusion

Nano CoS2 particles can be conveniently preparedin self-organized micellar solution of CTAB. Thethiol-capped CoS2 particles prepared in aqueousand aquo-ethanol media have evidenced charac-teristic difference. The composition of the cappednano CoS2 particles has been proposed from theFTIR and ESI-MS results.

Acknowledgements

IC thanks CSIR, Government of India, for Jou-nior Research Fellowship and SPM thanks INSA,Govt. of India for a Senior Scientist Position. Helpreceived from Mr. A. Sanyal of National ChemicalLaboratory, Pune, India for XRD measurementsis acknowledged with thanks.

References

Alivisatos A.P., 1996. Semiconductor clusters, nanocrystals,

and quantum dots. Science 271, 933–937.

Balasubramanian R., B. Kim, S.L. Tripp, X. Wang, M.

Lieberman & A. Wei, 2002. Dispersion and stability studies

of resorcinarene-encapsulated gold nanoparticles. Langmuir

18, 3676–3681.

Battisha I.K., 2002. Physical properties of nanoparticle silica

gel doped with CdS prepared by sol-gel technique. Fizika A

11, 61–70.

Benoit, R., 1995. J. Chim. Phys., 52, 119.

Bi H., X. Jiang, C. Yang & J. Hong, 2003. Synthesis of cobalt

disulfide nanoparticles in polymer matrix. Mater. Lett. 57,

2606–2611.

Chakraborty I. & S.P. Moulik, 2005. Preparation and

characterization of nanoscale semiconductor particles of

ZnS, CdS and PbCrO4 in polymer surfactant gel matrix. J.

Dispersion Science and Technol 25, 849–859.

Chakraborty I. & S.P. Moulik, 2004. Preparation and

characterization of PbS nanoparticles in AOT micellar

medium. J. Nanoparticle Res. 6, 233–240.

Chakraborty I., D. Mitra & S.P. Moulik, 2005. Spectroscopic

studies on nanodispersions of CdS, HgS, their core-shells and

composites prepared in micellar medium. J. Nanoparticle

Res. 7, 227–236.

Chakraborty I. & S.P. Moulik, 2005. On PbS nanoparticles

prepared in the compartments of water/AOT/n-heptane

microemulsion. J. Nanoparticle Res. 7, 237–247.

Cotton, F.A., G. Wilkinson, Advanced Inorganic Chemistry,

Wiley Eastern Pvt. Ltd., 2nd Edn., p 528, 1969, New Delhi.

Cristou G., K.S. Hagen & R.H. Holm, 1982. Synthesis,

structure, and properties of [Co8S6(SC6H5)]4) containing an

octanuclear Co8S6 rhombic dodecahedron related to that of

cobalt pentlandite. J. Am. Chem. Soc. 104, 1744–45.

Collier C.P., T. Vossmeyer & J.R. Heath, 1998. Nanocrystal

superlattice. Annu. Rev. Phys. Chem. 49, 371–404.

Dutta, P.K., General and Inorganic Chemistry. Sarat Book

House, 9th edn., Vol II, p. 38 1998, Calcutta.

Ganapathi M.R., R. Hermann, S. Naumov & O. Brede, 2000.

Free electron transfer from several phenols to radical cations

of non-polar solvents. Phys. Chem. Chem. Phys. 2, 4947–

4955.

Hobbs D. & J. Hafner, 1999. Magnetism and magneto-

structural effects in transition-metal sulfides. J. Phys.: Con-

dens. Mater. 11, 8179–8222.

Huheey J.E., E.A. Keiter & R.L. Keiter, 1993. Inorganic

Chemistry (4th edn.). New York: Harper Collins College

Publishers 465 Ch. 11.

Mitra D., I. Chakraborty & S.P. Moulik, 2005. Studies on ZnS

nanoparticles prepared in aqueous sodium dodecylsulfate

(SDS) micellar medium. Colloid J. 67, 494–499.

Murray C.B., D.J. Norris & M.G. Bawendi, 1993. Synthesis

and characterization of nearly monodisperse CdE (E = S,

Se, Te) semiconductor nanocrystallites. J. Am. Chem. Soc.

115, 8706–8715.

Moulik S.P., G.C. De, A.K. Panda, B.B. Bhowmik & A.R. Das,

1999. Dispersed molecular aggregates. 1. Synthesis and

characterization of nanoparticles of Cu2[Fe(CN)6] in H2O/

AOT/n-Heptane water-in-oil microemulsion medium. Lang-

muir 15, 8361–8367.

Panda A.K., B.B. Bhowmik, A.R. Das & S.P. Moulik, 2000.

Dispersed Molecular Aggregates. 3. Synthesis and character-

ization of colloidal lead chromate in watersodium Bis

(2-ethylhexyl)sulphosuccinate/n-Heptane water-in-oil micro-

emulsion medium. Langmuir 17, 7982–7987.

Panda A.K., S.P. Moulik, B.B. Bhowmik & A.R. Das, 2001.

Dispersed Molecular Aggregates. 2. Synthesis and character-

ization of nanoparticles of tungstic acid in H2O/(TX-

100+Alkanol)/n-Heptane W/O Microemulsion Medium. J.

Colloid Interface Sci. 235, 218–226.

Passaretti J.D., K. Dwight, A. Wold, W.J. Croft & R.R.

Chianelli, 1981. Preparation and properties of poorly crys-

tallized cobalt disulfide and ruthenium disulfide. Inorg.

Chem. 20, 2631–2634.

Pattabi M. & J. Uchil, 2000. Synthesis of cadmium sulfide

nanoparticles. Solar Energy Materials and Solar Cells 63,

309–314.

Pattabi M. & J. Uchil, 2003. The effect of precursor concen-

tration on the size of the CdS nanoparticles synthesized in a

896

chicken egg membrane. Solar Energy Materials and Solar

Cells 76, 323–330.

Pejova B., A. Isahi, M. Najdoski & I. Grozdanov, 2001.

Fabrication and characterization of nanocrystalline cobalt

oxide thin films. Mater. Res. Bull. 36, 161–170.

Pileni M.P., 1998. Fabrication and Properties of nanosized

material made by using colloidal assemblies as templates.

Cryst. Re. Technol. 33, 1155–1186 and references therein.

Pileni M.P., 1993. Reverse micelles as microreactors. J. Phys.

Chem. 97, 6961–6973.

Qian X.F., X.M. Zhang, C. Wang, K.B. Tang, Y. Xie & Y.T.

Qian, 1998. Solvent-thermal preparation of nanocrystalline

pyrite cobalt disulfide. J. Alloys. Comps. 278, 110–112.

Rollins H.W., T. Whiteside, G.J. Shafer, J-J. Ma, M-H. Tu,

J-T. Liu, D.D. DesMarteau & Y-P. Sun, 2000. Nanoscale

metal sulfides in perfluorinated ionomer membranes. J.

Mater. Chem. 10, 2081–2084.

Silverstein R.M., G.C. Bassler & T.C. Morril, 1991. Spectro-

metric Identification of Organic Compounds (5th edn.). New

York Inc.: John Willy and Sons.

Steigerwald M.L., A.P. Alivisatos, J.M. Gibson, T.D. Harris,

R. Kortan, A.J. Muller, A.M. Thayer, T.M. Duncan, D.C.

Douglass & L.E. Brus, 1998. Surface derivatization and

isolation of semiconductor cluster molecules. J. Am. Chem.

Soc. 110, 3046–3050.

Tauc J. & A. Menth, 1972. Semiconductor band-gap. Non-

cryst. Solids 569, 8–10.

Wachhold M., K.K. Rangan, S.J.L. Billinge, V. Petkov, J.

Heising & M.G. Ksntzidis, 2000. Mesostructured Non-oxidic

solids with adjustable wormhole shaped pores: M-Ge-Q

(Q=S, Se) frameworks based on tetrahedral [Ge4Q10]4)

Clusters. Adv. Mater. 12, 85–91.

Wang Y. & N. Herron, 1991. Nanometer sized semiconductor

clusters: materials synthesis, quantum size effects and

photophysical properties. J. Phys. Chem. 95, 525–532.

Zhaoling Z., M. Jun & Y. Xiyao, 2003. Separate/simultaneous

catalytic reduction of sulfur dioxide and/or nitric oxide by

carbon monoxide over TiO2-promoted cobalt sulfides. J.

Molecular Catalysis, A: Chem. 195, 189–200.

897