J O U R N A L O F C H E M I S T R Y Materials The thio-sol–gel synthesis of titanium disulfide and niobium disulfide Part 1.—Materials chemistry Mandyam A. Sriram and Prashant N. Kumta* Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh PA 15213, USA Received 3rd April 1998, Accepted 3rd August 1998 Investigations pertaining to the synthesis of titanium disulfide (TiS2) have so far been focused on solid state reactions, low temperature chemical techniques and vapor phase reactions using titanium tetrachloride (TiCl4) as the starting material.In this paper, we have investigated the potential of titanium tetraalkoxides [Ti(OR)4], which have been widely used for the synthesis of oxides by the sol–gel approach, for the synthesis of TiS2 via the thiosol –gel process.The mechanism of the reaction of titanium isopropoxide {Ti(OPri)4} with H2S in benzene has been studied using infrared spectroscopy (FTIR), gas chromatography (GC) and chemical analysis. Based on these studies, it has been determined that the precipitate obtained from the reaction forms following a thiolysis–condensation mechanism similar to the hydrolysis–condensation mechanism that operates in the oxide sol–gel process.The precipitate, which is an alkoxysulfide, can be converted to TiS2 by heat treatments in flowing H2S. The influence of modifying agents, the role of solvents and the alkoxy group [in Ti(OR)4] on the formation of the alkoxysulfide precipitate have also been presented and discussed. Finally, the applicability of this process for the synthesis of NbS2 has also been demonstrated.by these processes with respect to control of the morphology 1 Introduction or stoichiometry of the sulfide. Considering the strong impact Titanium disulfide (TiS2) has been studied as a useful cathode of all these factors on the performance of the sulfide as a material for Li intercalation or insertion batteries.It exhibits cathode in rechargeable lithium batteries, it is useful to identify a strong potential for inserting lithium into the crystal to form a process that provides this flexibility to control both the LixTiS2, where 0<x<1, without causing any phase changes. microstructure (morphology, particle and crystallite size) and It also has a good electronic conductivity (ca. 28 S cm-1) and defect concentration of the final sulfide powder. displays a high reversibility of the intercalation reaction.1 Metal alkoxides are unique in these sense that they oVer the These properties of the material have made it a viable cathode possibility to react with nucleophilic sulfidizing agents to yield material for both bulk and thin film rechargeable lithium precursors which can be converted to form the sulfide with batteries.The performance of the sulfide as a cathode material control of defect concentration and microstructure. Metal depends on the eYcacy of the intercalation reaction, which in alkoxides have been extensively investigated for the synthesis turn depends on a number of factors such as morphology, of oxide ceramics, glasses and thin films via the sol–gel process. crystallite and particle sizes, and defect concentration.All However, there has been little work reported on the potential these factors are strongly influenced by the nature of the of these compounds for the synthesis of non-oxides.12–22 In processes and the processing conditions used to synthesize the first part of this two-part series, we report mainly the the material.solution and precursor chemistry pertaining to the synthesis There are several methods reported in the literature for of TiS2 via the thio-sol–gel process. In the second part (see synthesizing titanium disulfide in both powder and film form. following paper), the morphology, defect concentration and These methods include: (a) high temperature solid state reac- electrochemical characteristics of the thio-sol–gel synthesized TiS2 powders are presented and discussed.tions,2,3 (b) vapor phase reactions4–9 and (c) low temperature chemical precipitation techniques.10,11 The simplest method is the synthesis of the sulfide by melting the individual elements 2 Experimental procedure in an evacuated sealed quartz tube at high temperatures.Several vapor phase reactions have also been investigated to The reactions of titanium alkoxides were studied using diVerent synthesize TiS2 powders and thin films, the most common sulfidizing agents. At the same time, the influence of modifying being the reaction of TiCl4 with H2S (incidently, this was also the alkoxide, the alkyl group and the solvent system on the the first vapor phase reaction to be investigated for the sulfidization reaction were also studied.Unless otherwise mensynthesis of TiS2 powders4). Room temperature chemical tioned, the source and purity of the chemicals and the instruprecipitation routes mostly using chlorides and various inor- ments used for the analyses are: Ti(OPri)4 (as received from ganic10 and organic11 sulfidizing agents have also been success- Johnson Matthey Alfa, 97%), benzene (as received anhydrous fully implemented to synthesize fine particles of TiS2. benzene from Aldrich, 99.8%), KBr (FT-IR grade from All the methods described above result directly in the Aldrich, 99+%), CsI (Aldrich, 99.9%), H2S (Mattson gases, formation of TiS2.The low temperature precipitation routes CP grade), benzenesulfonic acid (Aldrich, tech.grade), acetoresult in a poorly crystalline sulfide which can be crystallized nitrile (anhydrous, as received from Aldrich, 99.8%), ethyl by heating the material at a higher temperature. These precipi- alcohol (absolute, McCormick Distillation Co., Pekin, IL), tates are often contaminated by chloride (Cl-) ions and by titanium tetraethoxide (Aldrich, tech.grade), titanium ninorganic by-products of the reaction. More importantly, the butoxide (Aldrich, 99%), titanium 2-ethyl hexoxide (Aldrich, composition of the material (and therefore its defect structure), 95%), dimethyl disulfide (Aldrich, 99%), hexamethyldisilthiane its morphology and particle size are all determined at the (Fluka); Gas chromatography (Hewlett Packard model 5830A), mass spectrometry (VG Analytical, model 7070), precipitate stage itself. Therefore there is little flexibility oVered J.Mater. Chem., 1998, 8(11), 2441–2451 2441does not chelate the metal atom (unlike carboxylic acids which chelate the metal atom due to the presence of the highly polar CNO group). In order to confirm the reaction of Ti(OPri)4 with BSA, the reactions were conducted in a benzene solvent (in an acid5alkoxide molar ratio, n=1510, 151 and 251). The isopropanol liberated from the reaction was azeotropically distilled under ultrahigh purity (UHP) nitrogen and quantitatively analyzed using GC. 2.2.1 Modification using the ratio n=0.1 (reaction B). The procedure adopted to study the molecular processes that occur in the reaction of modified alkoxide with H2S was very similar to that developed for the first reaction.The alkoxide was modified with BSA (n=acid5alkoxide molar ratio=0.1) in benzene and reacted with H2S. The precipitate, filtrate and distillate were analyzed as described in section 2.1. 2.2.2 Modification using the ratio n=1 and n=2. Fig. 1 Schematic flow chart showing the procedure for synthesizing Benzenesulfonic acid was added to Ti(OPri)4 in benzene in a TiS2 using the reaction of titanium alkoxide with H2S.molar ratio of 151. When H2S was passed through the solution for about 10 minutes, there was only a change in the color of scanning electron microscopy (CamScan series 4). All manipu- the solution from colorless to dark brown, without the appearlations and handling of reactants and products were executed ance of any precipitate.The vessel was left sealed for about under an argon or a nitrogen atmosphere. All reactions were 18 h after which the liquid turned black without the formation conducted in glassware dried in vacuum at 120 °C after rinsing of any precipitate. Ti(OPri)4 was also modified using 2 moles with hexane. of BSA per mole of alkoxide.When H2S was bubbled through this solution in benzene, the solution changed from colorless 2.1 Reaction of titanium isopropoxide with H2S (reaction A) to yellow without the formation of any precipitate. These reactions were not studied any further. The flow chart in Fig. 1 describes the experimental procedure. Ti(OPri)4 was dissolved in anhydrous benzene, and H2S was 2.3 Influence of solvent on the reaction of Ti(OPri)4 with H2S bubbled through it at room temperature.A black precipitate was observed within a minute, but the bubbling was continued Acetonitrile (ACN). Both reaction A (plain alkoxide+H2S) for 10 min to ensure completion of the reaction. The reaction and reaction B (modified alkoxide+H2S, using a modification vessel was sealed and isolated for 12 h.ratio n=0.1) were conducted in anhydrous ACN. In both The precipitate was then collected and washed thoroughly cases, there was immediate precipitation without any changes with anhydrous benzene in a Soxhlet extractor, and dried in in the color of the solution. The precipitates were collected vacuum at 40 °C for 3 h after which it was perceived to be using a Soxhlet extractor, washed with ACN and dried at pyrophoric and extremely air-sensitive.This precipitate formed 40 °C for 3 h. The precipitates were weighed to estimate the the precursor for synthesizing titanium disulfide (TiS2). yield of the reaction and their Ti and S contents were analyzed Chemical analysis of the precipitate was conducted by to estimate the extent of the sulfidization reaction.Galbraith Laboratories Inc. (Knoxville, TN), and an IR spectrum was collected in the 4000 to 650 cm-1 wavenumber Ethyl alcohol. Neither the plain alkoxide nor the modified range in a KBr pellet using a Fourier transform infrared alkoxide reacted with H2S in a solution of absolute ethyl spectrometer (FTIR spectrometer, Bruecker Instruments) alcohol. A slight color change to light yellow was observed equipped with an MCT detector and in the 500 to 200 cm-1 but no precipitate appeared even after allowing the reaction range in a CsI pellet using an FTIR spectrometer (Biorad) to proceed for several days.equipped with a DTGS detector and CsI optics. The filtrate obtained was dark brown indicating the presence 2.4 Influence of alkoxy group on the reaction of alkoxide with of molecular species containing TiMS bonds.The filtrate was H2S azeotropically distilled and gas chromatography (GC) per- Three other alkoxides were reacted with H2S in ben- formed on the distillate to quantify the products of the zene and in acetonitrile: titanium ethoxide {Ti(OC2H5)4}, reaction. After ensuring that all the excess benzene was distilled n-butoxide {Ti(OCH2CH2CH2CH3)4} and 2-ethylhexoxide oV, chemical analysis was performed on the dark brown liquid.{Ti[OCH2CH(C2H5)(CH2)3CH3]4}. The observations are The liquid was also analyzed employing electron impact mass shown in Table 1. The solids collected from the reactions spectroscopy. shown in Table 1 were analyzed for their Ti and S contents. The powders were also weighed to estimate the yield of the 2.2 Influence of modification on the reaction of Ti(OPri)4 with reactions.H2S Modification of titanium isopropoxide cannot be accomplished 2.5 Reaction of titanium isopropoxide with other sulfidizing by aqueous acids because of the extreme sensitivity of the agents alkoxide to water. However, it has been shown that alkoxides react with organic acids to form the metal acylates with the 2.5.1 Ti(OPri)4+dimethyldisulfide (DMDS; reaction C).release of alcohol. For example with a carboxylic acid:23 Ti(OPri)4 was dissolved in anhydrous benzene and excess dimethyl disulfide (DMDS) was added to it. The mixture was M(OR)n+m RCOOHAM(OR)n-m(OOCR)m+m ROH refluxed for 24 h at 70 °C, after which a small amount of H2S (1) gas was passed over the surface of the liquid. This resulted in the formation of a black precipitate.The precipitate was In this study, benzenesulfonic acid (BSA, C6H5SO3H) was used to modify the alkoxide for the following reasons: (a) washed with benzene and dried under vacuum at 40 °C for 2 h. The mechanism of the reaction, however, has not been BSA is a strong organic acid and (b) it is monofunctional and 2442 J.Mater. Chem., 1998, 8(11), 2441–2451Table 1 Reaction of titanium alkoxides in benzene and acetonitrile on the powders to verify the phases present and the powders were examined in an SEM. Observations Reaction of H2S with niobium(V) ethoxide modified with Alkoxy group In benzene In acetonitrile benzenesulfonic acid. Niobium ethoxide was reacted with BSA Ethoxy Orange soln., Insoluble.Introduction of in the molar ratio of 1051, alkoxide to acid and dissolved in no ppt. H2S for 10 min led to a thick acetonitrile. H2S gas was bubbled into the solution. The viscous liquid. Some solid reaction proceeded in the same way as described above. A collected by heating the black precipitate was formed, which was also dried at 40 °C liquid under vacuum at ca.for 2 h under vacuum. IR analysis and further heat treatments 60 °C. of the precipitate were conducted in the same manner as n-Butoxy Dark brown liquid, Insoluble. Introduction of no ppt. H2S for ca. 5 min led to a described above. black precipitate. The precipitate was washed with 3 Results and Discussion acetonitrile and dried at 40 °C for 3 h. In this section, the mechanisms of the reaction of titanium 2-Ethylhexoxy No reaction No reaction isopropoxide with various sulfidizing agents are presented and discussed along with the influence of solvents and the alkoxy group. This is followed by a study of the conversion of the studied.A similar scheme has been attempted by Guiton precipitates obtained from these reactions to crystalline TiS2.et al.,24 where they observed that the reaction of diethylzinc In order to facilitate the discussion, we have used the following with dibenzyl trisulfide in the presence of H2S led to an abbreviations for the various reactions. Reaction A: the reacenhancement in the kinetics of precipitation of ZnS. tion of titanium isopropoxide with H2S conducted in benzene. The same reaction conducted in acetonitrile will be termed 2.5.2 Ti(OPri)4+hexamethyldisilthiane (HMDST, reaction ‘reaction A in acetonitrile’.Reaction B: the reaction of titanium D). Owing to the extreme moisture sensitivity, coupled with isopropoxide modified with BSA using a modification ratio the toxicity and stench of hexamethyldisilthiane (HMDST), a n=0.1, conducted in benzene. Reaction C: the reaction of cannula technique was used inside a glove bag for all transfer titanium isopropoxide with dimethyldisulfide (DMDS) cataprocedures.HMDST was added to Ti(OPri)4 in anhydrous lyzed by H2S using benzene as a solvent. Reaction D: the benzene and the vessel was sealed and isolated for 12 h. A reaction of titanium isopropoxide with hexamethyldisilthiane change in the color of the mixture from colorless to bright (HMDST) conducted in benzene.yellow was observed within about 3 min and after a series of color changes, from yellow to dark brown, a black precipitate 3.1 Reaction of titanium isopropoxide with H2S (Reaction A) was finally observed after ca. 3 h. The precipitate was collected, The IR absorption spectrum collected on the precipitate washed and dried as described in previous sections.An FTIR obtained from the reaction is displayed in Fig. 2, showing the spectrum was collected on the precipitate and chemical analysis 1800 to 900 cm-1 and 500 to 200 cm-1 (inset) spectral ranges. was conducted to estimate the amounts of Ti, S, C, H and Si The doublet at 1377 and 1360 cm-1 is characteristic of the in the precipitate.gem-dimethyl structure of the isopropoxy group.25,26 Bands at 1160, 1127 and 1013 cm-1 are also characteristic of the isopro- 2.6 Conversion of the precursors to TiS2 poxy groups bonded to Ti.26 The inset shows the typical TiMS The dried powders obtained from the four reactions (reactions stretching absorption10 at 300 cm-1 (hence indicating the A–D) were then heat treated in flowing H2S at 600, 700 and formation of TiMS bonds due to the reaction of H2S with the 800 °C for 6 h (heated at 15 °Cmin-1 and furnace cooled).At alkoxide in solution), and the TiMO stretching absorption at each stage, an X-ray diVractogram was collected using Cu-Ka 450 cm-1 (owing to the unreplaced isopropoxy groups25,26). radiation from 2h= 10° to 90° (Rigaku h/h diVractometer equipped with a diVracted beam graphite monochromator), and the powders were observed under an SEM.Samples for SEM examination were prepared by ultrasonicating the powders in hexane for 5 min and placing a few drops of the suspension on a graphite surface. 2.7 Synthesis of niobium disulfide Reaction of H2S with niobium(V) ethoxide. The applicability of the thio-sol–gel process was also extended to the synthesis of NbS2. Niobium pentaethoxide [Nb(OC2H5)5; 10 g] was dissolved in 200 cm3 acetonitrile. H2S was passed through the solution for 10 min.A black precipitate was observed to form in 20 s and the vessel was left sealed for 24 h. A Soxhlet extractor was used to filter the suspension and wash the powders using acetonitrile (HPLC grade, used as received from Fisher Chemicals).In this case too, the filtrate was dark brown to black. The black precipitate obtained after filtration was dried under vacuum at 40 °C for 2 h. Absorption IR spectra were collected on the precipitate using the CsI pellet Fig. 2 The IR spectrum collected on the as-precipitated alkoxy-sulfide method (using a Mattson Galaxy Series spectrometer equipped powder (prepared using reaction A) in the wavenumber range with CsI optics) in the wavelength range from 4000 to 4000–650 cm-1 showing the presence of unreacted isopropoxy groups 200 cm-1.The precipitated powder was then heat treated at (1377, 1360, 1160, 1122, 1013 cm-1) bonded to titanium. The inset 700 and 800 °C for 6 h in flowing H2S (heated at 15 °Cmin-1 shows Ti–O and Ti–S absorptions in the far IR range.Note: the absorbance axis has arbitrary units. and furnace cooled). X-Ray diVraction patterns were collected J. Mater. Chem., 1998, 8(11), 2441–2451 2443Table 2 Chemical analyses and quantitative GC (reaction A) with SH, (b) GC in conjunction with chemical analyses confirm condensation of thiols to be the dominant mechanism. (a) Chemical analyses of liquid and solid products Chemical analysis of the liquid product (Table 2) showed a S/Ti molar ratio of about 0.06.In addition, the mass fragmen- Ti S C H O (diVerence) Weight/g tation pattern of the liquid was identical to that of titanium Solid (wt%)a 33.6 32.3 17.3 3.5 13.3 0.754 isopropoxide, implying that the dark liquid was in fact Ti Solid (mol%) 9.4 13.5 19.3 46.8 11 isopropoxide, which had not undergone any significant reac- Liquid (wt%)b 7.3 0.3 76.8 8.1 7.5 30.088 tion with H2S.Some of the alkoxide molecules remained Liquid (mol%) 1 0.06 42.3 53.5 3.1 unreacted, possibly due to (a) association in inert solvents like (b) Correlation of isopropanol replacement from chemical analyses benzene (titanium isopropoxide is known to have an average and GC molecular complexity of 1.3 in benzene28), or (b) due to the formation of partially reacted soluble oligomers which do not Moles of titanium isopropoxide used in the reaction 0.0504 undergo any further reaction in benzene because of steric Moles of isopropanol detected in the distillate 0.025 hindrance to nucleophilic attack by H2S.Experiments conduc- Moles of isopropanol that should be liberated, calculated 0.021 from chemical analyses and assuming reaction (4) for all ted in acetonitrile (a coordinating solvent which is known to thiol groups be a better medium for the dissociation of H2S), have shown more than a five-fold increment in product yield, providing aThe solid precipitate was analyzed after drying at 40 °C for 2 h in vacuum.bThe liquid was analyzed after removing benzene by support for this hypothesis.Increase in the product yield when distillation. the reaction was conducted in ACN led us to explore the eVects of solvent and alkoxy groups on the yield of the reaction. These results are presented in the following sections. 3.2 Influence of modification Based on the chemical analysis of the precipitate (shown in Table 2) and the IR results, it can be concluded that the solid Titanium isopropoxide dissolved in benzene was modified is an alkoxysulfide, containing (a) unreacted isopropoxy groups using BSA in the modification ratios (n=molar ratio of acid and (b) sulfur bonded to Ti.to alkoxide) of 0.1, 1 and 2. The isopropanol liberated from Isopropanol was also detected in the distillate implying the the reaction was azeotropically distilled and quantitatively replacement of isopropoxy groups (OPri) by thiol groups (SH) analyzed using gas chromatography.Based on the number of from H2S. This attack of the alkoxy groups would form the moles of isopropanol liberated, we could confirm that the basis of a thiolysis reaction very similar to the hydrolysis following modification reaction went to completion: reactions in the sol–gel process, as illustrated by Livage:27 Ti(OC3H7)4+n C6H5SO3HATi(OC3H7)4-n(C6H5SO3)n Pri +nC3H7OH (n=0.1, 1, 2) (5) | H2S+Ti(OPri)4 A H2SATi(OPri)4 A (HS) (OPri)3TiBOH The alkoxides, modified to diVerent extents, were reacted with H2S using anhydrous benzene as a solvent.A Ti(OPri)3SH+PriOH (2) 3.2.1 Modification using the ratio n=0.1 (reaction B) The The overall reaction can therefore be written as infrared spectrum of the precipitate obtained from the reaction Ti(OPri)4+nH2S A Ti(OPri)4-n(SH)n+nPriOH (n<4) of the alkoxide (partially modified by benzenesulfonic acid in (3) the molar ratio of 10 to 1, alkoxide to acid) with H2S is shown in Fig. 3. The most important feature of the spectrum is the After thiolysis, the precipitates could have formed due to existence of absorptions at 1000, 1020 and 1040 cm-1 all of condensation–polymerization of the alkoxythiols by the liberwhich correspond to the sulfonyl vibrations of the acid.29,30 ation of H2S as explained in the following paragraphs: In addition to this, other vibrations due to the alkoxy groups 2p {Ti(OPri)4-n(SH)n} A {(PriO)4-n(SH)n-1Ti-S-Ti(SH)n-1(OPri)4-n}p+pH2S (n<4) (4) The condensation of alkoxy groups is less likely due to the highly polar character of the TiMO bonds as well as steric hindrance caused by the bulky isopropoxy groups.As shown in Table 2, 0.025 moles of alcohol per 0.0504 moles of starting Ti isopropoxide, was detected by quantitative GC. It should be noted that the alcohol released is a result of the alkoxythiol (OPri<SH) replacement reaction responsible for the formation of both the liquid and the solid products.In order to confirm the formation of the precipitate by the above mentioned condensation mechanism [reaction (4)], the isopropanol liberated was calculated from the chemical analysis of the liquid and solid products. Assuming that the precipitates are formed as a result of condensation of all the thiol groups leaving sulfur formally bonded to two titanium atoms, calculations indicate an expected release of 0.021 moles of isopropanol, which is in good agreement with the isopropanol content estimated by quantitative GC.This, therefore, vali- Fig. 3 The IR spectra of the powders obtained from the reaction of dates the assumed condensation mechanism.Two facts, there- modified alkoxide with H2S (reaction B). In comparison to Fig. 5 the fore, become clear from the above analysis: (a) infrared additional vibrations at 1000, 1020 and 1040 cm-1 are all due to the spectroscopy and GC have indicated that the thiolysis reaction sulfonyl groups of the acid. Note: the absorbance axis has arbitrary units.as shown in reaction (3) causes partial replacement of OPri 2444 J. Mater. Chem., 1998, 8(11), 2441–2451Table 3 Chemical analyses and quantitative GC (reaction B) sation of which could be sterically hindered due to the presence of the bulky benzenesulfonyl groups. (a) Chemical analysis of liquid and solid products 3.2.2 Modification using the ratio n=1 and n=2. When H2S Ti S C H O (diVerence) Wt./g gas was bubbled into a solution of Ti(OPri)4 in benzene Solid (wt%)a 32.3 30.3 20.8 3.5 13.1 1.234 modified using a modification ratio of n=1, a series of color Solid (mol%) 8.8 12.4 22.7 45.3 10.8 changes was observed, from colorless to yellow to dark brown, Liquid (wt%) 16.4 1.8 48.5 8.4 24.9 12.3b without the formation of any precipitate.The change in color Liquid (mol%) 2.4 0.4 28.0 58.4 10.8 indicated the possibility of some thiolysis reaction occurring (b) Correlation of replacement of isopropanol from chemical analysis in solution leading to the formation of some TiMSH linkages and GC without any condensation.Similarly, the alkoxide solution modified using the ratio n=2 changed from colorless to yellow Moles of titanium isopropoxide used in the reaction 0.0504 when reacted with H2S, again without the formation of any Moles of isopropanol detected in the distillate ignoring the 0.016 precipitate. This occurs mainly due to steric hindrance oVered isopropanol liberated from the modification reaction Moles of isopropanol that should be liberated, calculated 0.027 by the bulky acid groups.In the case of H2S, even though from chemical analysis assuming complete thiol based thiolysis may occur, condensation does not occur because of condensation mechanism steric hindrance.These observations are similar to the oxide aThe solid precipitate was analyzed after drying at 40 °C for 2 h in sol–gel process, wherein the hydrolysis reaction of water with vacuum. bCalculated by conserving titanium.a modified alkoxide is considerably slower than that with an unmodified alkoxide. 3.3 Influence of solvent bonded to Ti can be observed (CMO vibrations at 1120 and To evaluate the eVect of solvent, reactions A and B were 1160 cm-1) indicating also the existence of unreacted alkoxy conducted in a non-polar solvent (anhydrous benzene, groups in the solid precipitate from the sulfidization reaction.described above), a polar protic solvent (absolute ethyl The replacement of some alkoxy groups by SH is of course alcohol ) and a polar aprotic solvent (anhydrous acetonitrile). again confirmed by the presence of the TiMS vibration at The yield of each of the reactions was measured along with 300 cm-1 (see inset in Fig. 3).10 This replacement has been the S/Ti molar ratios in the precipitates obtained from the verified in a manner similar to reaction A, namely by the reactions (see Table 4).identification of isopropanol as a by-product of the reaction It is well known that the HS- ion is a stronger nucleophile using a combination of quantitative GC and chemical analysis than H2S. A simple calculation of partial charges on the (see Table 3).electronegative species shows that S in H2S has a partial The observations discussed above imply that: (a) the acid charge dS=-0.13, while in HS-, dS=-0.6. Therefore, a reacts to completion with the alkoxide to modify one tenth of larger concentration of HS- ions in solution should strongly the titanium alkoxide molecules as shown in reaction (5) for aVect the thiolysis reaction.The dissociation of H2S to H+ n=0.1 and (b) the sulfonyl group is not replaced by H2S and HS- is higher in solvents with a higher relative permit- during the sulfidization reaction, but the molecules of alkoxide tivity. In benzene, molecular H2S acts as the nucleophile in its that are modified do participate in the thiolysis reactions (no reaction with the alkoxide.sulfonyl group vibrations could be detected in the IR spectrum It is likely that during thiolysis in benzene, intermediate of the liquid obtained from the reaction) as do those that are oligomers form, which are resistant to nucleophilic attack not modified: because of steric hindrance hence lowering the overall yield. H2S+Ti(OPri)3(C6H5SO3) A H2SATi(OPri)3(C6H5SO3) A Electron impact mass spectrometry of the liquid product from the first reaction indeed showed that the mass fragmentation (HS) (OPri)2(C6H5SO3)TiBOH A Ti(C6H5SO3)(OPri)2SH pattern of the liquid fraction from the reaction was similar to that of the plain alkoxide.In acetonitrile, the dissociation of | +PriOH H2S causes the formation of the stronger nucleophile, HS-, Pri (6) which can react even with the intermediate species leading to and condensation.An interesting point to note is that although ethyl alcohol has a relatively large relative permittivity, com- Pri parable to that of acetonitrile, (implying the formation of | more HS- species), the reaction of titanium isopropoxide does H2S+Ti(OPri)4 A H2SATi(OPri)4 A (HS) (OPri)3TiBOH not proceed to any significant extent in this solvent.This is A Ti(OPri)3SH+PriOH (7) because in protic solvents, the anions are solvated by hydrogen bonding from the solvent.31 Therefore in alcohols, the HS- After thiolysis, the precipitates form due to the condensationions are solvated by alcohol molecules, which prevent them polymerization of the alkoxythiols by the liberation of H2S in the same way as shown in reaction (4). The reaction mechanism can be elucidated further by (a) Table 4 Molar yield of titanium from reactions A and B in benzene, ethyl alcohol and acetonitrile performing quantitative GC analysis of the benzene–isopropanol azeotrope obtained upon distillation of the filtrate from Modified the reaction and (b) a correlation of this analysis to the results Plain isopropoxide isopropoxidea of chemical analysis in a manner similar to that described in (reaction A) (reaction B) the previous section.These results are displayed in Table 3. Relative Solvent permittivity Yield (%) S/Tib Yield (%) S/Tib The isopropanol observed in the distillate is lower than that calculated from chemical analysis (assuming complete conden- Benzene 2.3 ca. 15 1.4 ca. 15 1.3 sation, i.e. the condensation of all the TiMSH groups to form Ethyl alcohol 32.7 — — — — TiMSMTi bonds). This implies that part of the sulfur (approxi- Acetonitrile 37.5 ca. 90 1.3 ca. 90 1.5 mately one third of the number of moles of S) in the solid aModified in a ratio of 1051, isopropoxide to benzenesulfonic acid. and the liquid are still not condensed completely leading to bMolar ratio of sulfur to titanium in the precipitated powder.the presence of some TiMSH linkages in the solid, the conden- J. Mater. Chem., 1998, 8(11), 2441–2451 2445Table 6 Chemical analysis of precursors obtained from reactions C from attacking the Ti center of the alkoxide (again by steric and D hindrance). On the other hand, in aprotic solvents like acetonitrile, the cation (H+) is strongly solvated by solvent mol- Reaction Ti S C H Si O (diVerence) ecules because the exposed N atom in acetonitrile carries a significant negative partial charge.31 The positive end of the Reaction C 34.9 28 15.5 3.3 — 18.3 (DMDS), wt.% dipole is buried in the alkyl group which makes it ineVective Reaction D 32 22.5 20 3.3 4.3 17.9 in solvating the negative ions (HS-).This leaves the anions (HMDST), wt.% free to participate in the reaction. Therefore, the best solvent system for the thio-sol–gel process involving H2S as a sulfidizing agent is an aprotic polar solvent. reaction of H2S with alkoxides. Alkoxides with the smallest alkoxy group on the metal center and having the least molecu- 3.4 Influence of alkoxy group lar complexity exhibit the lowest steric hindrance to nucleophilic attack. Such an alkoxide would be most likely to react A total of four diVerent alkoxides of titanium were reacted with H2S and undergo thiolysis and condensation reactions to with H2S in benzene and in acetonitrile: ethoxide, isopropoxform solid precursors useful for the synthesis of sulfides.ide, n-butoxide and 2-ethylhexoxide. The yields of the reactions Similarly, the most appropriate solvent system is a polar in benzene and in acetonitrile are shown in Table 5, along with aprotic solvent having a large relative permittivity (for their molecular complexities.The molecular complexity of example, acetonitrile) which is an eVective medium for the titanium ethoxide is high because of the small size of the dissociation of H2S to form HS- species, which are stronger ethoxy group. The larger alkoxy groups show a lower molecunucleophiles in comparison to H2S.lar complexity, because complexity itself is reduced by steric hindrance. If we consider the reactions conducted in benzene, 3.5 Reactions of titanium isopropoxide with DMDS and the isopropoxide is the only alkoxide of titanium which shows HMDST any reaction with H2S.The ethoxide shows no reaction because of steric hindrance caused by a higher molecular complexity The precipitates that resulted from reactions C and D were and the larger alkoxides (n-butoxide and ethylhexoxide) show very air sensitive and extra precautions were taken while no reaction because of hindrance from the bulky alkyl groups, handling these powders.Table 6 shows the results of chemical although the larger alkoxides tend to exist as monomers in analysis performed on these precursors. It can be observed benzene. In the case of titanium, the isopropoxy group oVers that the sulfur contents in these precipitates are considerably the best combination of size, alkyl group and molecular lower, indicating a lower extent of replacement of the alkoxy complexity.Hence, the sulfidization reaction is seen to occur groups. In addition, the reaction of the alkoxide with HMDST to considerable extents using Ti isopropoxide in both polar results in a powder contaminated with Si. aprotic and in non-polar solvents. The as-precipitated powders were amorphous, while the IR The ethoxide, n-butoxide and ethylhexoxide do not dissolve spectra of the powders showed the same characteristic in acetonitrile. However, when H2S is bubbled through the vibrations corresponding to the presence of isopropoxy groups suspension, the ethoxide and the n-butoxide do react. The (in the region of 1800 to 900 cm-1) as seen in the powders ethoxide forms a thick viscous liquid precursor, which forms prepared using the first two reactions.TiMO and TiMS a glassy solid when dried under vacuum at 60 °C. The solid vibrations were also seen at 450 and 300 cm-1 respectively. In shows a S to Ti molar ratio of 1.5, which implies a significant addition to these absorptions, the powder prepared using extent of the reaction with H2S (similar to reaction A in HMDST as the sulfidizing agent also showed nSiMO and benzene or ACN using titanium isopropoxide).It was also dSi(CH3)3 vibrations at 933 and 757 cm-1 respectively.33 observed that the liquid precursor could be easily coated on In reaction C (using DMDS as a sulfidizing agent) there is glass (by dip-coating or spin-coating) to form films which a possibility of heterolytic cleavage of the SMS bond due to appeared to be quite stable in air.These preliminary obser- the presence of H2S, causing the formation of RS- species vations indicate a potential application of this process for the which attack the alkoxide.34 In reaction D the following formation of TiS2 thin films. The reaction of H2S with titanium sequence could operate: n-butoxide also proceeds to a significant extent, the precipitates OTiOPri+R3Si-S-SiR3A OTi-S-SiR3+R3Si-OPri (8) showing a molar ratio of S to Ti of 1.5 (comparable to reaction A using titanium isopropoxide).The yield from this reaction OTi-S-SiR3+OTi-OPriAOTi-S-TiO+R3Si-OPri (9) is also high, although not as high as what is observed in the (R=methyl group) case of the reaction of H2S with titanium isopropoxide in the same solvent. Titanium 2-ethylhexoxide does not react At present, the proposed reactions paths are hypothetical and with H2S either in benzene or in acetonitrile because of steric have been proposed based on the properties of the sulfidizing hindrance from the bulky ethylhexoxy group.agents and the IR results of the solid precipitates obtained In summary, steric hindrance plays a significant role in the from the reactions.More detailed studies to elucidate the exact reaction mechanisms were not conducted owing to the benefits exhibited by the use of H2S in reactions A and B. Table 5 Reactions of various titanium alkoxides with H2S in benzene and in acetonitrile 3.6 Conversion of precursors to TiS2 Reaction in Reaction in Reaction A. The precipitates obtained from reactions A–D benzene acetonitrile Molecular were amorphous to X-rays.The X-ray traces obtained on the Alkoxide complexitya Yield (%) S/Tib Yield (%) S/Tib precipitate obtained from reaction A after conducting diVerent heat treatments are shown in Fig. 4 indicating the formation Ethoxide 2.5 — — —c 1.5c of crystalline TiS2 with increasing temperature. At 600 °C, Isopropoxide 1.3 15 1.4 90 1.3 crystalline TiS2 was observed along with the presence of rutile n-Butoxide 1.4 — — 80 1.5 and anatase phases of TiO2.The formation of TiO2 could be 2-Ethylhexoxide 1 — — — — explained by the favored high temperature condensation reac- aReferences 28,32. bMolar ratio of sulfur to titanium in the solid. cA tion of the unreacted isopropoxy groups: thick viscous liquid was obtained from the reaction.A solid was obtained by heating the liquid under vacuum at 60 °C. Ti-OPri+PriO-Ti A Ti-O-Ti+hydrocarbons (10) 2446 J. Mater. Chem., 1998, 8(11), 2441–2451Fig. 4 The X-ray diVractograms of the powders prepared from reaction A, heat treated at 600, 700 and 800 °C for 6 h respectively. The TiS2 peaks appear at 600 °C along with the rutile and anatase peaks of TiO2 marked by ‘O’.The oxide peak (rutile) is of low intensity at 700 °C and is eliminated at 800 °C. This process could have occurred at the drying stage as well as during the initial stages of heat treatment. At 700 °C after 6 h in H2S, only a small amount of oxide is present as Fig. 5 SEM micrographs of the powders; (a) as-prepared powders evidenced by the relatively low intensity peak visible at 2h= from reaction A, and heat treated to (b) 600 °C, (c) 700 °C and 27.5, implying the reduction of the TiMO bonds by H2S in (d) 800 °C for 6 h each in flowing H2S.The micrographs (b)–(d) the vicinity of this temperature. We have observed that con- clearly indicate changes in morphology occurring with the crystalliztinued heat treatment at 700 °C for about 10 h can eliminate ation of TiS2.the oxide phase. At 800 °C, the oxide is completely eliminated, yielding single phase TiS2. Based on these results, a mechanism 15 m2 g-1 makes it more sensitive to handling in air, hence for the formation of the sulfide could be postulated as follows. several precautions were taken to minimize its exposure to air. (1) The sulfur containing thiol groups attached to titanium in At 600 °C, as shown in Fig. 5(b), small platelets of TiS2 are the amorphous precursor transform in the presence of H2S to seen to form, which are separated from the spherical particles. form crystalline TiS2 at 600 °C, while the unreacted alkoxy At 700 and 800 °C however, there are very few spherical groups attached to titanium undergo condensation reactions particles and some sintering has occurred between the platelets to form the oxide.(2) The oxide then reacts with H2S at as indicated in the micrographs in Fig. 5(c), (d). The size of 700 °C to form the sulfide. (3) It is also possible that some of the platelets is not uniform and they range in width from 0.5 the sulfur remains bonded to Ti as an oxysulfide. The presence to 1 mm.Surface area measurements of the powder heat treated of such oxysulfides of titanium has been reported.35 Thus, at 700 °C have indicated a more than threefold increase to a although a portion of the unreacted alkoxide transforms to value of about 52 m2 g-1. the sulfide via the gas–solid reaction, the formation of single phase TiS2 at 800 °C in 6 h is an indication of the enhanced Reaction B.Conversion of the precipitates obtained from kinetics of the sulfidization reaction due to the incorporation reaction B to TiS2 was again accomplished by heat treatment of sulfur in the initial stages leading to the formation of the in flowing H2S. The X-ray diVraction patterns of the powders alkoxysulfide precursor. The formation of titania led us to heat treated to various temperatures are shown in Fig. 6. At conduct some control experiments employing fine particles of 600 °C the peaks are broad (in comparison to the correspond- TiO2 obtained by hydrolyzing titanium isopropoxide in a ing pattern shown in Fig. 4 for the products of reaction A) solution of acetonitrile via a sol–gel reaction. The powders indicating the presence of fine crystallites.Peaks characteristic were heat treated in H2S employing the same conditions of rutile and anatase also appear at this temperature, however (800 °C for 6 h) and the results showed TiO2 to be the major heat treatments at higher temperatures of 700 and 800 °C yield phase. Similar observations have been reported on the formasingle phase hexagonal TiS2. tion of cubic La2S312–15 from the sulfidization of alkoxides.The SEM micrographs of the precipitates and heat treated The partially sulfidized lanthanum oxysulfide precursors transform to the crystalline sulfide (La2S3) at reduced temperatures and in much shorter reaction times than those reported for the conventional routes involving high temperature sulfidization of sol–gel derived oxide gels in H2S. The alkoxide approach therefore oVers a novel route to the synthesis of sulfide ceramics of reactive metals.The evolution of morphology of the powders appears to follow an interesting path as displayed in Fig. 5(a)–(d). The as-precipitated powder is spherical and monodispersed with a particle size of about 0.5 mm. The monodispersed particle size distribution points to the fact that the rate of thiolysis is much slower than the rate of condensation.36 This helps maintain the concentration of the condensing species [shown in reaction (3)] below the critical concentration required for homogenous nucleation.Therefore any new species formed merely condense on the particles already formed during the initial burst of Fig. 6 Powders derived from reaction B heat treated at 600, 700 and nucleation, hence causing growth.In addition to its molecular 800 °C for 6 h each in flowing H2S. Peaks marked ‘O’ are due to TiO2 (anatase and rutile). structure, the high surface area of the precursor of about J. Mater. Chem., 1998, 8(11), 2441–2451 2447Fig. 7 Morphology of the powders obtained from reaction B (modified alkoxide+H2S), (a) precipitated powder and the powder heat treated at (b) 600 °C, (c) 700 °C and (d) 800 °C for 6 h each in flowing H2S.powders are shown in Fig. 7(a)–(d). The as-precipitated powder consists of agglomerated spherical particles about 2 mm in diameter which are about four times as large as the Fig. 8 (a) X-Ray diVractograms of the powders obtained from reaction particles obtained from reaction A. On heat treatment at C (alkoxide+H2S+DMDS) heat treated at 600, 700 and 800 °C for 600 °C, it can be seen that the TiS2 crystallites begin to form 6 h each in flowing H2S.TiS2 is formed at 600 °C with relatively small from individual spherical particles with their prismatic planes amounts of oxide (peak marked ‘O’), (b) X-ray diVractograms of the directed radially outwards. In this transformation, the spherical powders obtained from reaction D (alkoxide+HMDST) heat treated shape of the original particles, however, is retained.At 700 °C, in flowing H2S under the same conditions. the platelets grow further along both basal and prismatic planes while maintaining the overall shape of the agglomerates. At 800 °C, the platelets seem to have grown in both the prismatic and basal plane directions. The morphologies that have formed in this case are distinctly diVerent from those observed in reaction A due to the pseudomorphous transformation of the spherical precipitates to form TiS2.There is therefore a strong influence of modification of the alkoxide on the evolved morphology of the TiS2 platelets. Reactions C and D. X-Ray diVractograms of the precipitates subjected to heat treatments in flowing H2S are shown in Fig. 8(a), (b). In the case of reaction C (Ti isopropoxide+DMDS+H2S), the conversion of the alkoxysulfide precipitate to TiS2 is close to completion upon heat treatment at 600 °C for 6 h (note the low relative intensity of the peak characteristic of titania). The broad TiS2 peaks grow and sharpen at higher temperatures while the oxide peaks are completely eliminated.On the other hand, the powder obtained from reaction D (Ti isopropoxide and HMDST) shows sharp TiS2 peaks at 600 °C superimposed on an amorphous back- Fig. 9 SEM micrographs of the powders from reaction C; (a) as ground in addition to the oxide (rutile and anatase) peaks of precipitated, showing monodispersed spherical particles, (b) heat TiO2 [see Fig. 8(b)]. At 700 and 800 °C, however, single phase treated at 600 °C, showing the formation of TiS2 platelets, (c) heat TiS2 is formed. treated at 700 °C and (d) at 800 °C. All heat treatments done for 6 h. The SEM micrographs of the precipitated precursor and the heat treated powders (obtained from reaction C) are displayed in Fig. 9(a)–(d). The precursor powder shows spherical mono- particles are spherical, but are also polydispersed.They range in size from 0.5 to about 2 mm. At 600 °C, TiS2 platelets dispersed particles of about 0.5 mm in diameter, similar to the powders obtained from reaction A [compare with Fig. 5(a)]. measuring 1 to 2 mm along the basal plane can be seen along with spherical particles which exhibit rough surface features.At 600 °C, however, there are no spherical particles remaining, since most of them have transformed (probably along a path At 700 °C, however, the morphology is similar to Fig. 7(c), where platelets (about 0.5 to 1 mm) have grown from the similar to that observed in the case of reaction B) to form agglomerates of fine platelets about 0.3 to 0.5 mm along the spherical precursor particles.On further heat treatment at 800 °C the morphology of the powders show a controlled basal plane. At higher temperatures (700 and 800 °C) there is random growth of the platelets which have basal plane dimen- growth of the platelets in both the basal and prismatic plane directions similar to the characteristic morphology exhibited sions ranging from a fraction of a micrometer to several micrometers.The as-precipitated precursor powders from reac- by the powders obtained from reaction B using BSA [see Fig. 7(d)]. tion D [shown in Fig. 10(a)], however, are quite diVerent; the 2448 J. Mater. Chem., 1998, 8(11), 2441–2451Fig. 11 IR spectra of precipitates obtained by the reaction of (a) Nb(OEt)5 with H2S in acetonitrile and (b) Nb(OEt)5 modified with BSA reacted with H2S in acetonitrile.ref. 29): MMO stretching vibration at 585 cm-1; terminal Fig. 10 SEM micrographs of the powders obtained from reaction D; (a) as-precipitated and heat treated for 6 h at (b) 600 °C, (c) 700 °C CMO vibrations at 925, 1264 and 1100 cm-1; vibrations and (d) 800 °C. characteristic of the CH3 group at 1376, 1440, 2872 and 2962 cm-1; and vibrations characteristic of the CH2 group at 1464, 2853 and 2926 cm-1.In addition, the precipitate The SEM analysis of the precursors and the transformed obtained from the modified alkoxide [Fig. 11(b)] shows crystalline powders obtained from reactions A–D reflect the additional absorptions at 1000 (sharp), 1020 and 1040 consequences of the diVerent reaction mechanisms in solution (shoulders) cm-1 which are characteristic of the sulfonyl group instrumental for the formation of the precursor itself.Thus present in BSA.29,30 From these observations, it is clear that the four diVerent reactions exhibit diVerences in chemical the precipitates are formed by a partial thiolysis reaction of composition and structure, in addition to the various particle Nb(OEt)5 with H2S and condensation of the thiol groups, a sizes and their distributions (due to the competing rates of the mechanism similar to what has been determined in the case of formation of condensable species and their subsequent condentitanium isopropoxide.In this case the extent of the conden- sation reactions). These structural and compositional varisation reaction has not been determined, but complete conden- ations could provide diVerent reaction pathways for the sation is not expected due the five-fold coordination of Nb formation of the crystalline sulfide during the H2S treatments.which could oVer steric hindrance to condensation. The contrast is clearly seen in the intermediate morphology of When the precipitates from both the reactions were heat the powders for example, obtained from reactions A and B at treated in flowing H2S at 700 and 800 °C for 6 h, they showed 600 °C and at higher temperatures.The composition and the formation of single phase NbS2 (Fig. 12). At 700 °C, both structure allow diVerent responses to the sulfur potentially powders show the presence of small crystallites of NbS2. leading to variations in the morphology of the product, which SEM micrographs of the precipitates and the heat treated is clearly unique to the thio-sol–gel process.The heat treatment powders are shown in Fig. 13 and 14. In both cases, the conditions can be used to control the defect structure and precipitates consist of spherical particles ranging in size from morphology of the TiS2 powders, which in turn strongly 0.75 to 1 mm in diameter. At 700 °C, the powders derived from influence the electrochemical properties of the material.These the modified alkoxide [Fig. 14(b)] clearly show a pseudo- aspects are further discussed in the second part of this morphous transformation, causing the formation of NbS2 two-part series. platelets growing from the spherical particles. In the plain alkoxide derived powder, the platelets are not clearly dis- 3.7 Synthesis of niobium disulfide tinguishable [Fig. 13(b)]. The particles have retained their Nb(OEt)5, plain (or unmodified) as well as modified with spherical shape with increased roughness on the surface of the BSA (in a molar ratio of 1510, acid to alkoxide) were particles. At 800 °C [Fig. 13(c) and 14(c)], however, both separately dissolved in acetonitrile and reacted with H2S at powders show the presence of platelets ranging in size from a room temperature, causing the formation of a black precipi- fraction of a micrometer to several micrometers.This is also tate. Chemical analysis of the precipitate (for Nb and S) is approximately reflected in the extensive broadening at the base shown in Table 7.Infrared spectra collected on these precipi- of the diVraction peaks in the XRD patterns obtained for the tates in the range of 4000 to 200 cm-1 are shown in Fig. 11. powders heat treated at 800 °C. Both spectra show the formation of NbKS bonds as evidenced by the characteristic absorption at 357 cm-1.10 These bonds 4 Summary and conclusions could have formed through a thiolysis–condensation mechanism similar to what was observed in the case of titanium.The reaction of Ti(OPri)4 with H2S results in an alkoxysulfide There are also unreplaced alkoxy groups in the precipitates as precipitate through a thiolysis–condensation mechanism simievidenced by the following absorptions (ref. 23, pp. 114–122; lar to the hydrolysis–condensation mechanism seen in the oxide-sol–gel process.The unique features of this reaction {Ti(OPri)4+H2S} are that the condensation reaction is rapid Table 7 Chemical analysis of precursors obtained by the reaction of niobium ethoxide with H2S in acetonitrile and all thiol groups that are bonded to titanium during the thiolysis of the isopropoxide condense to form TiMSMTi Reaction Nb(wt.%) S(wt.%) S/Nba linkages in the solid.Modification of the alkoxide by benzenesulfonic acid, the Plain alkoxide+H2S in acetonitrile 43.6 29.6 2.0 solvent system and the alkoxy group bonded to titanium all Modified alkoxide+H2S in acetonitrile 44.2 24.7 1.6 significantly influence the reaction of the alkoxide with H2S. aMolar ratio of S to Nb. In the case of Ti(OPri)4 modified with benzenesulfonic acid, J.Mater. Chem., 1998, 8(11), 2441–2451 2449Fig. 14 SEM micrographs of (a) precipitates obtained by the reaction Fig. 12 X-Ray diVraction patterns of precipitates obtained by (a) the of modified niobium ethoxide with H2S in acetonitrile, (b) the reaction of niobium ethoxide with H2S in acetonitrile and (b) the precipitates heat treated at 700 °C for 6 h in flowing H2S and (c) the reaction of niobium ethoxide modified with benzenesulfonic acid (in precipitates heat treated at 800 °C for 6 h in flowing H2S.a molar ratio of n=1510, acid to alkoxide), heat treated in flowing H2S at 700 and 800 °C for 6 h respectively. chemical and spectroscopic analysis of the solid precipitate and of the liquid obtained from the reaction has revealed that the condensation reaction does not go to completion, possibly due to steric hindrance by the bulky benzenesulfonyl groups.The solvent used for the reaction has a significant influence on the yield of the reaction. A comparison of the reaction of titanium isopropoxide and H2S in diVerent solvents has shown that acetonitrile is the best solvent system for the reaction because it facilitates the decomposition of H2S to form highly nucleophilic SH- species in solution.A comparison of the reaction of various alkoxides with H2S has shown that steric hindrance plays a significant role in the reaction. Consequently, alkoxides with smaller alkoxy groups having lower molecular complexities in solution are more likely to undergo any significant thiolysis and condensation reactions with H2S.Preliminary investigations have shown that the viscous liquid that forms from the reaction of titanium ethoxide with H2S can be spin coated to form thin films. Titanium isopropoxide has also been reacted with dimethyldisulfide and hexamethyldisilthiane. Both reactions also lead to the formation of an alkoxysulfide precipitate although with a lower extent of sulfidization in comparison to H2S.All the reactions yield spherical alkoxysulfide precipitates that exhibit significant variations in particle size indicating diVerences in the rates of thiolysis and condensation reactions. Heat-treatment in flowing H2S results in the formation of crystalline TiS2 powder at diVerent temperatures exhibiting striking variations in crystallite morphology.In particular, distinct diVerences can be observed between TiS2 formed using plain Ti(OPri)4 and that formed using Ti(OPri)4 modified with benzenesulfonic acid. In the former, a wide platelet size distribution with random orientations have been observed. In the latter, a pseudomorphous transformation from the spheri- Fig. 13 SEM micrographs of (a) precipitates obtained by the reaction cal precursor particles has been observed, leading to a platelet of niobium ethoxide with H2S in acetonitrile, (b) the precipitate heat morphology which retains the overall spherical shape of the treated at 700 °C for 6 h in flowing H2S and (c) the precipitate heat treated at 800 °C for 6 h in flowing H2S.precursor particles. 2450 J. Mater. Chem., 1998, 8(11), 2441–245115 P.N.Kumta, V. P. Dravid and S.H. Risbud, Philos. Mag. B, 1993, Finally, the thio-sol–gel process has been used to synthesize 68, 67. NbS2 in order to demonstrate the applicability of this process 16 Y. Han and M. Acink, J. Am. Ceram. Soc., 1991, 74, 2815. for the synthesis of other transition metal sulfides. In this 17 M. A. Sriram and P. N. Kumta, Mater. Res. Soc. Symp. Proc., system too, alkoxysulfide precipitates form, which can be 1993, 327, 15–22.converted to NbS2 upon heat treatment in H2S. NbS2 synthe- 18 M. A. Sriram and P. N. Kumta, J. Am. Ceram. Soc., 1994, 77, 1381. sized by this method also exhibits variations in morphology 19 M. A. Sriram, K. S. Weil and P. N. Kumta, Appl. Organomet. similar to that observed in TiS2. Chem., 1997, 11, 163. 20 M. A. Sriram and P. N. Kumta, Ceram. Trans., 1996, 65, 163. 21 J. Y. Kim, M. A. Sriram, P. H. McMichael, P. N. Kumta, This work has been supported by the US National Science B. L. Phillips and S. H. Risbud, J. Phys. Chem. B, 1997, 101, 4689. Foundation Grant DMR 9301014 and a Research Initiation 22 V. Stanic, A. C. Pierre, T. H. Etsell and R. J. Mikula, J. Mater. Award (RIA) from the US National Science Foundation Res., 1996, 11, 363. Grant CTS 9309073, and CTS 9700343. The authors would 23 D. C. Bradley, R. C. Mehrotra and D. P. Gaur, Metal Alkoxides, Academic Press, London, 1978. also like to acknowledge the technical support of Dr. George 24 T. A. Guiton, C. L. Checkaj and C. G. Pantano, J. Non-Cryst. E. Blomgren of Eveready Battery Co. Solids, 1990, 121, 7. 25 J. V. Bell, J. Heisler, H. Tannenbaum and J. Goldenson, Anal. Chem., 1953, 25, 1720. References 26 C. T. Lynch, K. S. Mazdiyasni, J. S. Smith and W. J. Crawford, Anal. Chem., 1964, 36, 2332. 1 S. D. Jones, J. R. Akridge and F. K. Shokoohi, Solid State Ionics, 27 J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1994, 69, 357. 1988, 18, 259. 2 M. S. Whittingham and J. A. Panella, Mater. Res. Bull., 1981, 28 D. C. Bradley, R. C. Mehrotra and W. Wardlaw, J. Chem. Soc., 16, 37. 1952, 5020. 3 R. P. Clement, W. B. Davies, K. A. Ford, M. L. H. Green and 29 L. J. Bellamy, The Infra-red Spectra of Complex Molecules, John A. J. Jacobson, Inorg. Chem., 1978, 17, 2754. Wiley and Sons Inc., New York, 1958, pp. 364–367. 4 V. W. Biltz and P. Ehrlich, Z. 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Bormoy, in Microionics, Solid State Integrable 10 R. R. Chianelli and M. B. Dines, Inorg. Chem., 1978, 17, 2758. Batteries, ed. N. Balkanski, North Holland, New York, 1991, 11 A. Bensalem and D. M. Schleich, Mater. Res. Bull., 1988, 23, 857. pp. 73–95. 12 P. N. Kumta and S. H. Risbud, Mater. Sci. Eng., B, 1989, 2, 281. 36 E. A. Barringer and H. K. Bowen, Langmuir, 1985, 1, 414. 13 P. N. Kumta and S. H. Risbud, Mater. Sci. Eng., B, 1993, 18, 260. 14 P. N. Kumta and S. H. Risbud, Prog. Crystal Growth Charact., 1991, 22, 321. Paper 8/02564I J. Mater. Chem., 1998, 8(11), 2441–2451 2451