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Synthesis of nanocrystalline TiO2 in alcohols
Powder Technology 125 (2002) 39 – 44 www.elsevier.com/locate/powtec
Synthesis of nanocrystalline TiO2 in alcohols Cheng Wang a, Zhao-Xiang Deng a, Guohui Zhang a, Shoushan Fan b,c, Yadong Li a,c,* a
Department of Chemistry, Tsinghua University, Beijing, 100084, PR China b Department of Physics, Tsinghua University, Beijing, 100084, PR China c Center of Atomic and Molecular Science, Tsinghua University, Beijing, 100084, PR China Received 15 June 2001; received in revised form 14 November 2001; accepted 27 November 2001
Abstract A solvothermal synthetic method to nanocrystalline titania has been carefully investigated in alcohol solutions. The selection of crystal structures, grain sizes and morphologies could be achieved through simply varying the alcohols and other reaction conditions. It is believed that HCl plays a significant role in determining the possible formation mechanism and the crystal structures. Although GC – MS and FT-IR results can give some information about the synthetic process, the formation of titania in these systems still remains unclear and both the hydrolytic and non-hydrolytic mechanisms might be involved. The products were also characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman as well as UV – VIS spectroscopy. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Solvothermal process; Titania; Nanomaterials
1. Introduction The chemistry of titania has been the focus of many research interests during the past decade due to its scientific and technological importance. It has been demonstrated to be useful in various areas, such as pigments, gas and humidity sensors, dielectric ceramics, catalyst supporter, solar cells [1 –9] and so on. Its performance in applications depends to a large extent on its physical and chemical properties, which are related to the synthetic conditions. These conditions dictate the properties such as crystal structure, morphology, grain size, thermal stability, surface structure, etc. There are many methods reported in the literatures for the synthesis of titania. One is the oxidation method. This could be fulfilled in gas, liquid and solid phase and the substance to be oxidized could be metallic titanium or its precursors [10 – 12]. Since titanium precursors such as halides and alkoxides Ti(OR)4 (OR is an alkoxy group) are very reactive towards nucleophilic reagents such as water, hydrolytic process has been accepted as the main means. As one example of hydrolytic process, sol – gel synthesis is widely * Corresponding author. Department of Chemistry, Tsinghua University, Beijing, 100084, PR China. Tel.: +86-10-62772350; fax: +86-1062788765. E-mail address: [email protected] (Y. Li).
used for fabricating transition metal oxides (including titania) with nanoscale microstructures and provides for excellent chemical homogeneity and the possibility deriving unique metastable structures at low reaction temperature. However, sol –gel-derived precipitates of titania precursors are amorphous in nature. In order to obtain crystalline products, further heat treatment at high temperature is compulsory. This calcination process will inevitably cause the grain growth, reduction in specific surface area of particles and even induce phase transformation. Hydrothermal synthesis in which chemical reactions can occur in aqueous or organic media under the self-produced pressure at low temperature (usually lower than 250 jC), can solve those problems encountered during sol – gel process. Usually, hydrothermal processing of amorphous titania yields non-aggregated anatase nanocrystals in water in the presence of acids, bases and chelating agents as dispersants. However, rutile and brookite can also be obtained by changing hydrothermal processing variables such as sol composition and pH, reaction temperature and pressure, aging time and nature of solvent and additives [13 – 23]. Very recently, selective synthesis of anatase and rutile through ultrasound irradiation as energy input, also based on hydrolytic process was reported [24]. Particles obtained from hydrolytic processes contain hydroxyl groups on their surfaces and these hydroxyls could influence material properties. The surface hydroxyl groups
0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 1 ) 0 0 5 2 3 - X
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C. Wang et al. / Powder Technology 125 (2002) 39–44
can either be replaced by other functional group [25] or eliminated by sintering at extremely high temperature. A direct solution to this is the use of ether or alkoxides instead of water as nucleophilic agents to react with titanium precursors, which is called non-hydrolytic sol – gel methods and has recently received considerable attention even though it is relatively old [26 –35]. In this paper, we carefully investigated the synthesis of titania in alcohols through solvothermal process. Titania with different crystal structures, grain sizes and morphologies is achievable by simply varying the alcohols and other reaction conditions. The formation of titania in these systems is suggested based on our observation and compared with previous reported results. It is believed that HCl plays a significant role in determining the possible formation mechanism and the crystal structures.
2. Experimental
Fig. 1 is the X-ray diffraction (XRD) pattern of the particles produced in some alcohols, the titania obtained in ethanol, n-propanol and isopropanol could be indexed as anatase phase, while in n-butyl and n-octyl alcohols the products were rutile phase. The particle sizes, using the well-known Scherrer equation based on the half widths of (101) diffraction peak for anatase and (110) diffraction peak for rutile are, 8, 31 and 46 nm, for the titania prepared in ethanol, n-propanol and n-octyl alcohol systems, respectively. Transmission electron microscopy images (TEM, Fig. 2) show that the products prepared in these systems consist of spherical particles with particle sizes consistent with the calculated results. TEM (Fig. 2c and e) also shows agglomerated tenuous fibers of rutile and anatase products prepared in n-octyl alcohol and isopropanol, respectively. Fig. 1 also shows that particles obtained in methanol are anatase phase with very poor crystallinity and those in ethylene glycol are mixture of anatase and rutile with poor crystallinity when heated at 100 jC (Fig. 1a) and 150 jC (Fig. 1g), respectively. However, there is still no precipitate
A general procedure for preparing titania is: about 2 mL TiCl4 was transferred into stainless steel autoclaves with Teflon liner (50 mL capacity) containing 40 mL distilled primary alcohols in a glove box. The pipets and autoclaves were dried in vacuum before use. All manipulations were carefully done in a glove box in the presence of flowing dry nitrogen. After being sealed, the autoclaves were heated at 100 jC (160 jC for n-octyl alcohol and isobutyl alcohol) for 24 h. The products were collected by filtration, washed by absolute ethanol and acetone and dried under vacuum, consequently. The filtered solutions were also collected and characterized by GC – MS. The collected products were characterized by XRD, TEM, IR, UV – vis, as well as Raman spectroscopy. X-ray diffractions (XRD) were carried out with a Regaku D/max ˚ ) incigA X-ray diffractometer with Cu Ka (k = 1.5418 A dent radiation at room temperature over the 2h range 10 – 70j. The morphologies and particle sizes were investigated with transmission electron microscopy (TEM) (Hitachi800). GC – MS were recorded on Perkin Elmer Autosystem XL (GC) and Perkin Elemer Turbo Mass (MS). UV – VIS absorptions were investigated using Shimadzu UV-2100S spectrometer. Raman absorptions were obtained on Perkin Elemer Spectrometer (GX/FT-Raman spectrometer).
3. Results and discussion The reactions in all alcohols are believed to be complete because no filtered solutions could give white precipitate through the addition of diluted NaOH aqueous solution (0.1 mol/l). The yields of titania in methanol, ethanol, n-propanol and isopropanol systems, based on the amounts of TiCl4 added, were nearly quantitative, while the yields in the other alcohol systems were low, since some of the products remained dispersed in the liquid media.
Fig. 1. X-ray diffraction patterns of: (a) anatase phase obtained in methanol heated at 100 jC for 24 h, (b) anatase phase obtained in ethanol heated at 100 jC for 24 h, (c) anatase phase obtained in propanol heated at 100 jC for 24 h, (d) rutile phase obtained in n-butyl alcohol heated at 100 jC for 24 h, (e) rutile phase obtained in n-octyl alcohol heated at 160 jC for 24 h, (f) anatase phase obtained in isopropanol heated at 100 jC for 24 h, (g) mixed phase of anatase and rutile phase of titania in ethylene glycol at 160 jC for 24 h.
C. Wang et al. / Powder Technology 125 (2002) 39–44
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Fig. 2. Transmission electron microscopy (TEM) images of rutile and anatase phase of titania prepared in some alcohols: (a) spherical anatase phase particles prepared in ethanol at 100 jC, (b) spherical anatase phase particles prepared in n-propanol at 100 jC, (c) tenuous fibers of rutile phase prepared in n-butyl alcohol at 100 jC, (d) spherical rutile phase particles prepared in n-octyl alcohol at 150 jC, (e) tenuous fibers of anatase phase prepared in isopropanol at 100 jC, (f) spherical anatase phase particles with larger size prepared in ethanol at 150 jC.
in glycerol even when heated at 150 jC. An interesting result is fiber-like anatase with several centimeters in length and averagely 50 Am in diameter (Fig. 3), could be obtained through evaporating the mixture of the unfiltered reacted npropanol solution at 70 jC added with 50 mL deionized water. It is well-known that TiCl2(OR)2 could be readily formed when TiCl4 is introduced into alcohols with the liberation of HCl. After stirring these solutions for 2 h to allow the completion of HCl evolution, Vioux et al. obtained titanium precursors through alcoholysis. These precursors were allowed to be gelated at 110 jC within a period of 0.5 –18 h. After calcination in air at 500 and 950 jC, they studied the influence of source alcohols on the
crystal structure of the final products [30]. In our process, we did not stir the solutions sealed in the autoclave, and there are a large amount of HCl dissolved in the alcohols through forming oxonium salts between HCl and alcohols. The pH test paper detection shows that the solution is acidic in nature. This dissolved HCl can act as catalyst for the etherification of alcohols in the systems. All original alcohols and their etherification products can be detected by GC –MS (Table 1). The formation of ethers could be the result of etherification of alcohols or decomposition of titanium precursors (TiClx(OR)4 x, where x = 0 or 1). We also noticed, although very little, alkyl chlorides also appeared in heavier alcohol solutions such as isobutyl, n-
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Fig. 3. Scan electron microscopy (SEM) of anatase titania obtained through evaporating a mixed solution of unfiltered reacted n-propanol and 50 ml deionized water at 70 jC. Scale bar: 290 Am.
butyl and n-octyl alcohol. Alkyl chlorides might also be formed in reactions with smaller alcohols, but the corresponding alkyl chlorides evolved from solutions before GC – MS measurement. Alkyl chlorides could also be decomposed from titanium precursors mentioned earlier or chlorine replacement of hydroxyl groups in source alcohols. The above GC – MS results could not give us a clear image about the formation of titania in these alcohol solutions since we cannot exclude either hydrolytic or non-hydrolytic process from each other. If etherification reactions occurred in the system, water as a by-product may be produced and can play a role as oxygen donor in the synthesis of titania. Thus, the surface of titania would be covered by adsorbed hydroxyl. FT-IR results seem to support this hydrolytic mechanism for the formation of titania (Fig. 4). However, non-hydrolytic products can also adsorb the so formed water. However, the FT-IR results do suggest the involvement of etherification reactions of alcohols in our
process. The products were also characterized by XPS, UV and Raman spectra. Some of these results are comparable with those reported in literatures [36 – 39]. As a byproduct during the hydrolysis of TiCl4 in hydrolytic process, HCl is favorable for the control of crystallite size. It plays a role as catalyst in promoting the hydrolysis process and as electrolyte in preventing particle growth or agglomeration through electrostatic repulsion and delaying the gelation process [40,41]. In this solvothermal process to nanocrystalline titania, besides as catalyst for etherification and the roles similar to hydrolytic process, it is speculated that it might also control the crystal structure of titania based on our series of experimental investigations. Among the three natural crystalline polymorphs, rutile is the thermodynamically stable polymorph while anatase and brookite are kinetically stable phases. The transformation from anatase to rutile has been the subject of considerable interest and the focus of research activities over the years [42]. Many factors, for example, annealing temperature, peptization temperature [43 – 45] and doping ions [46,47], might influence the phase change. When TiCl4 was added to alcohols in general as we observed, the reactions between TiCl4 and alcohols became less vigorous with the increase of carbon number in the alcohols and so does the evolution of HCl. This phenomenon could be explained as follows: the steric hindrance of alcohols increase with the increase of carbon number in the alcohols, which caused the replacement of chlorine in the TiCl4 by OR more difficult. Since the reaction system involves etherification of alcohols, water as one by-product would be produced. The water would make the hydrolysis of titanium precursors feasible if the formation of titania is a hydrolytic process and dissolve more HCl produced from the reaction between TiCl4 and alcohols. Because the reactions were realized in sealed autoclaves, the retention of HCl in the reaction system
Table 1 GC – MS results of the filtrates of some alcohols after reactions Alcohols
Retention time (min)
Detected molecular
Isopropanol
2.734 3.505 3.622 4.098 2.66 3.85 7.343 7.691 15.130
(CH3)2CHOCH(CH3)2 CH3CH2CH2CH2Cl CH3CH2CH2CH2OH CH3(CH2)3O(CH2)3CH3 (CH3)3CCl (CH3)2CHCH2OCH2CH(CH3)2 CH3(CH2)7Cl CH3(CH2)7OH CH3(CH2)7O(CH2)7CH3
n-butyl alcohol
Isobutyl alcohol n-octyl alcohol
Fig. 4. FTIR spectra of titania. R: rutile phase obtained in n-butyl alcohol, A: anatase phase obtained in isopropanol.
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increases with the increase of carbon number in the alcohols, and after the reactions the acidity of these systems became higher. This explanation is partly supported by the pH check after reactions. Accompanying with this, the crystal structure of the so prepared nanocrystalline titania changes from anatase to rutile, demarcated from carbon number between three and four. In order to correlate these two experimental results, we surmise it is HCl or in another word, the acidity of the media determines the crystal structure of titania: lower concentration of HCl favors the formation of anatase while higher concentration favors the rutile phase. As a further proof, 10 mL TiCl4 in 30 mL absolute ethanol, 30 mL ethanol aqueous solution (V/ V > 95%) and 30 mL distilled water correspondingly gave anatase, rutile and rutile phase when heated at 100 jC. However, the output in distilled water is less than 60%. Reaction temperature should also be considered in our method. Its effect on morphology and crystal structure has not been observed for any given alcohol system in the range from 100 to 190 jC, but it does have an effect on the initiation of reactions and particle sizes. For n-CmH2m + 1 OH, with the increase of CH2 in the formula of alcohol, the steric hindrance increases and the reaction rate decreases. If m V 4, the synthesis of titania could be easily fulfilled at 100 jC, when m = 8, the reaction temperature should be higher (160 jC), otherwise no product could be obtained. When we used isobutyl alcohol, a simplest primary alcohol with branch and higher steric hindrance, only rutile (confirmed by XRD after centrifugation) dispersed in the liquid medium was attainable when heated at 160 jC. This means that the steric hindrance is a key factor to be considered for the initiation of the reactions in our method. As to the effect of temperature on particle sizes, for example, spherical anatase particles with 8 and 17 nm mean diameter were obtained in ethanol when heated at 100 and 150 jC, respectively.
4. Conclusion In summary, nanocrystalline titania could be successfully prepared in alcohols under solvothermal conditions. The amount and configuration of CH2 in alcohols and the reaction temperature play two key roles in controlling the crystal structures, grain sizes and morphologies of the final products. HCl plays a significant role in determining the possible formation mechanism and the crystal structures. This method might be found applicable in the syntheses of other metal oxides and mixed metal-oxide ceramics.
products. We also thank Prof. Ruji Wang for recording XRD pattern and valuable discussion, and Dr. Chengdui Yang for assistance with GC – MS analysis.
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Acknowledgements [38]
This project was supported by National Nature Science Foundation of China (No. 29871028) and State Key Project of Fundamental Research. We are indebted to Mrs. Meijuan Zhao for her endeavor in recording TEM photos for our
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[41] C.-C. Wang, J.Y. Ying, Chem. Mater. 11 (1999) 3113. [42] J. Ovenstone, K. Yanagisawa, Chem. Mater. 11 (1999) 2770, and the references therein. [43] R.R. Basca, M. Gra¨tzel, J. Am. Ceram. Soc. 79 (1996) 2185. [44] B.L. Bischoff, M. Anderson, Chem. Mater. 7 (1995) 1772.
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Synthesis of nanocrystalline TiO2 in alcohols Cheng Wang a, Zhao-Xiang Deng a, Guohui Zhang a, Shoushan Fan b,c, Yadong Li a,c,* a
Department of Chemistry, Tsinghua University, Beijing, 100084, PR China b Department of Physics, Tsinghua University, Beijing, 100084, PR China c Center of Atomic and Molecular Science, Tsinghua University, Beijing, 100084, PR China Received 15 June 2001; received in revised form 14 November 2001; accepted 27 November 2001
Abstract A solvothermal synthetic method to nanocrystalline titania has been carefully investigated in alcohol solutions. The selection of crystal structures, grain sizes and morphologies could be achieved through simply varying the alcohols and other reaction conditions. It is believed that HCl plays a significant role in determining the possible formation mechanism and the crystal structures. Although GC – MS and FT-IR results can give some information about the synthetic process, the formation of titania in these systems still remains unclear and both the hydrolytic and non-hydrolytic mechanisms might be involved. The products were also characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman as well as UV – VIS spectroscopy. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Solvothermal process; Titania; Nanomaterials
1. Introduction The chemistry of titania has been the focus of many research interests during the past decade due to its scientific and technological importance. It has been demonstrated to be useful in various areas, such as pigments, gas and humidity sensors, dielectric ceramics, catalyst supporter, solar cells [1 –9] and so on. Its performance in applications depends to a large extent on its physical and chemical properties, which are related to the synthetic conditions. These conditions dictate the properties such as crystal structure, morphology, grain size, thermal stability, surface structure, etc. There are many methods reported in the literatures for the synthesis of titania. One is the oxidation method. This could be fulfilled in gas, liquid and solid phase and the substance to be oxidized could be metallic titanium or its precursors [10 – 12]. Since titanium precursors such as halides and alkoxides Ti(OR)4 (OR is an alkoxy group) are very reactive towards nucleophilic reagents such as water, hydrolytic process has been accepted as the main means. As one example of hydrolytic process, sol – gel synthesis is widely * Corresponding author. Department of Chemistry, Tsinghua University, Beijing, 100084, PR China. Tel.: +86-10-62772350; fax: +86-1062788765. E-mail address: [email protected] (Y. Li).
used for fabricating transition metal oxides (including titania) with nanoscale microstructures and provides for excellent chemical homogeneity and the possibility deriving unique metastable structures at low reaction temperature. However, sol –gel-derived precipitates of titania precursors are amorphous in nature. In order to obtain crystalline products, further heat treatment at high temperature is compulsory. This calcination process will inevitably cause the grain growth, reduction in specific surface area of particles and even induce phase transformation. Hydrothermal synthesis in which chemical reactions can occur in aqueous or organic media under the self-produced pressure at low temperature (usually lower than 250 jC), can solve those problems encountered during sol – gel process. Usually, hydrothermal processing of amorphous titania yields non-aggregated anatase nanocrystals in water in the presence of acids, bases and chelating agents as dispersants. However, rutile and brookite can also be obtained by changing hydrothermal processing variables such as sol composition and pH, reaction temperature and pressure, aging time and nature of solvent and additives [13 – 23]. Very recently, selective synthesis of anatase and rutile through ultrasound irradiation as energy input, also based on hydrolytic process was reported [24]. Particles obtained from hydrolytic processes contain hydroxyl groups on their surfaces and these hydroxyls could influence material properties. The surface hydroxyl groups
0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 1 ) 0 0 5 2 3 - X
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C. Wang et al. / Powder Technology 125 (2002) 39–44
can either be replaced by other functional group [25] or eliminated by sintering at extremely high temperature. A direct solution to this is the use of ether or alkoxides instead of water as nucleophilic agents to react with titanium precursors, which is called non-hydrolytic sol – gel methods and has recently received considerable attention even though it is relatively old [26 –35]. In this paper, we carefully investigated the synthesis of titania in alcohols through solvothermal process. Titania with different crystal structures, grain sizes and morphologies is achievable by simply varying the alcohols and other reaction conditions. The formation of titania in these systems is suggested based on our observation and compared with previous reported results. It is believed that HCl plays a significant role in determining the possible formation mechanism and the crystal structures.
2. Experimental
Fig. 1 is the X-ray diffraction (XRD) pattern of the particles produced in some alcohols, the titania obtained in ethanol, n-propanol and isopropanol could be indexed as anatase phase, while in n-butyl and n-octyl alcohols the products were rutile phase. The particle sizes, using the well-known Scherrer equation based on the half widths of (101) diffraction peak for anatase and (110) diffraction peak for rutile are, 8, 31 and 46 nm, for the titania prepared in ethanol, n-propanol and n-octyl alcohol systems, respectively. Transmission electron microscopy images (TEM, Fig. 2) show that the products prepared in these systems consist of spherical particles with particle sizes consistent with the calculated results. TEM (Fig. 2c and e) also shows agglomerated tenuous fibers of rutile and anatase products prepared in n-octyl alcohol and isopropanol, respectively. Fig. 1 also shows that particles obtained in methanol are anatase phase with very poor crystallinity and those in ethylene glycol are mixture of anatase and rutile with poor crystallinity when heated at 100 jC (Fig. 1a) and 150 jC (Fig. 1g), respectively. However, there is still no precipitate
A general procedure for preparing titania is: about 2 mL TiCl4 was transferred into stainless steel autoclaves with Teflon liner (50 mL capacity) containing 40 mL distilled primary alcohols in a glove box. The pipets and autoclaves were dried in vacuum before use. All manipulations were carefully done in a glove box in the presence of flowing dry nitrogen. After being sealed, the autoclaves were heated at 100 jC (160 jC for n-octyl alcohol and isobutyl alcohol) for 24 h. The products were collected by filtration, washed by absolute ethanol and acetone and dried under vacuum, consequently. The filtered solutions were also collected and characterized by GC – MS. The collected products were characterized by XRD, TEM, IR, UV – vis, as well as Raman spectroscopy. X-ray diffractions (XRD) were carried out with a Regaku D/max ˚ ) incigA X-ray diffractometer with Cu Ka (k = 1.5418 A dent radiation at room temperature over the 2h range 10 – 70j. The morphologies and particle sizes were investigated with transmission electron microscopy (TEM) (Hitachi800). GC – MS were recorded on Perkin Elmer Autosystem XL (GC) and Perkin Elemer Turbo Mass (MS). UV – VIS absorptions were investigated using Shimadzu UV-2100S spectrometer. Raman absorptions were obtained on Perkin Elemer Spectrometer (GX/FT-Raman spectrometer).
3. Results and discussion The reactions in all alcohols are believed to be complete because no filtered solutions could give white precipitate through the addition of diluted NaOH aqueous solution (0.1 mol/l). The yields of titania in methanol, ethanol, n-propanol and isopropanol systems, based on the amounts of TiCl4 added, were nearly quantitative, while the yields in the other alcohol systems were low, since some of the products remained dispersed in the liquid media.
Fig. 1. X-ray diffraction patterns of: (a) anatase phase obtained in methanol heated at 100 jC for 24 h, (b) anatase phase obtained in ethanol heated at 100 jC for 24 h, (c) anatase phase obtained in propanol heated at 100 jC for 24 h, (d) rutile phase obtained in n-butyl alcohol heated at 100 jC for 24 h, (e) rutile phase obtained in n-octyl alcohol heated at 160 jC for 24 h, (f) anatase phase obtained in isopropanol heated at 100 jC for 24 h, (g) mixed phase of anatase and rutile phase of titania in ethylene glycol at 160 jC for 24 h.
C. Wang et al. / Powder Technology 125 (2002) 39–44
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Fig. 2. Transmission electron microscopy (TEM) images of rutile and anatase phase of titania prepared in some alcohols: (a) spherical anatase phase particles prepared in ethanol at 100 jC, (b) spherical anatase phase particles prepared in n-propanol at 100 jC, (c) tenuous fibers of rutile phase prepared in n-butyl alcohol at 100 jC, (d) spherical rutile phase particles prepared in n-octyl alcohol at 150 jC, (e) tenuous fibers of anatase phase prepared in isopropanol at 100 jC, (f) spherical anatase phase particles with larger size prepared in ethanol at 150 jC.
in glycerol even when heated at 150 jC. An interesting result is fiber-like anatase with several centimeters in length and averagely 50 Am in diameter (Fig. 3), could be obtained through evaporating the mixture of the unfiltered reacted npropanol solution at 70 jC added with 50 mL deionized water. It is well-known that TiCl2(OR)2 could be readily formed when TiCl4 is introduced into alcohols with the liberation of HCl. After stirring these solutions for 2 h to allow the completion of HCl evolution, Vioux et al. obtained titanium precursors through alcoholysis. These precursors were allowed to be gelated at 110 jC within a period of 0.5 –18 h. After calcination in air at 500 and 950 jC, they studied the influence of source alcohols on the
crystal structure of the final products [30]. In our process, we did not stir the solutions sealed in the autoclave, and there are a large amount of HCl dissolved in the alcohols through forming oxonium salts between HCl and alcohols. The pH test paper detection shows that the solution is acidic in nature. This dissolved HCl can act as catalyst for the etherification of alcohols in the systems. All original alcohols and their etherification products can be detected by GC –MS (Table 1). The formation of ethers could be the result of etherification of alcohols or decomposition of titanium precursors (TiClx(OR)4 x, where x = 0 or 1). We also noticed, although very little, alkyl chlorides also appeared in heavier alcohol solutions such as isobutyl, n-
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Fig. 3. Scan electron microscopy (SEM) of anatase titania obtained through evaporating a mixed solution of unfiltered reacted n-propanol and 50 ml deionized water at 70 jC. Scale bar: 290 Am.
butyl and n-octyl alcohol. Alkyl chlorides might also be formed in reactions with smaller alcohols, but the corresponding alkyl chlorides evolved from solutions before GC – MS measurement. Alkyl chlorides could also be decomposed from titanium precursors mentioned earlier or chlorine replacement of hydroxyl groups in source alcohols. The above GC – MS results could not give us a clear image about the formation of titania in these alcohol solutions since we cannot exclude either hydrolytic or non-hydrolytic process from each other. If etherification reactions occurred in the system, water as a by-product may be produced and can play a role as oxygen donor in the synthesis of titania. Thus, the surface of titania would be covered by adsorbed hydroxyl. FT-IR results seem to support this hydrolytic mechanism for the formation of titania (Fig. 4). However, non-hydrolytic products can also adsorb the so formed water. However, the FT-IR results do suggest the involvement of etherification reactions of alcohols in our
process. The products were also characterized by XPS, UV and Raman spectra. Some of these results are comparable with those reported in literatures [36 – 39]. As a byproduct during the hydrolysis of TiCl4 in hydrolytic process, HCl is favorable for the control of crystallite size. It plays a role as catalyst in promoting the hydrolysis process and as electrolyte in preventing particle growth or agglomeration through electrostatic repulsion and delaying the gelation process [40,41]. In this solvothermal process to nanocrystalline titania, besides as catalyst for etherification and the roles similar to hydrolytic process, it is speculated that it might also control the crystal structure of titania based on our series of experimental investigations. Among the three natural crystalline polymorphs, rutile is the thermodynamically stable polymorph while anatase and brookite are kinetically stable phases. The transformation from anatase to rutile has been the subject of considerable interest and the focus of research activities over the years [42]. Many factors, for example, annealing temperature, peptization temperature [43 – 45] and doping ions [46,47], might influence the phase change. When TiCl4 was added to alcohols in general as we observed, the reactions between TiCl4 and alcohols became less vigorous with the increase of carbon number in the alcohols and so does the evolution of HCl. This phenomenon could be explained as follows: the steric hindrance of alcohols increase with the increase of carbon number in the alcohols, which caused the replacement of chlorine in the TiCl4 by OR more difficult. Since the reaction system involves etherification of alcohols, water as one by-product would be produced. The water would make the hydrolysis of titanium precursors feasible if the formation of titania is a hydrolytic process and dissolve more HCl produced from the reaction between TiCl4 and alcohols. Because the reactions were realized in sealed autoclaves, the retention of HCl in the reaction system
Table 1 GC – MS results of the filtrates of some alcohols after reactions Alcohols
Retention time (min)
Detected molecular
Isopropanol
2.734 3.505 3.622 4.098 2.66 3.85 7.343 7.691 15.130
(CH3)2CHOCH(CH3)2 CH3CH2CH2CH2Cl CH3CH2CH2CH2OH CH3(CH2)3O(CH2)3CH3 (CH3)3CCl (CH3)2CHCH2OCH2CH(CH3)2 CH3(CH2)7Cl CH3(CH2)7OH CH3(CH2)7O(CH2)7CH3
n-butyl alcohol
Isobutyl alcohol n-octyl alcohol
Fig. 4. FTIR spectra of titania. R: rutile phase obtained in n-butyl alcohol, A: anatase phase obtained in isopropanol.
C. Wang et al. / Powder Technology 125 (2002) 39–44
increases with the increase of carbon number in the alcohols, and after the reactions the acidity of these systems became higher. This explanation is partly supported by the pH check after reactions. Accompanying with this, the crystal structure of the so prepared nanocrystalline titania changes from anatase to rutile, demarcated from carbon number between three and four. In order to correlate these two experimental results, we surmise it is HCl or in another word, the acidity of the media determines the crystal structure of titania: lower concentration of HCl favors the formation of anatase while higher concentration favors the rutile phase. As a further proof, 10 mL TiCl4 in 30 mL absolute ethanol, 30 mL ethanol aqueous solution (V/ V > 95%) and 30 mL distilled water correspondingly gave anatase, rutile and rutile phase when heated at 100 jC. However, the output in distilled water is less than 60%. Reaction temperature should also be considered in our method. Its effect on morphology and crystal structure has not been observed for any given alcohol system in the range from 100 to 190 jC, but it does have an effect on the initiation of reactions and particle sizes. For n-CmH2m + 1 OH, with the increase of CH2 in the formula of alcohol, the steric hindrance increases and the reaction rate decreases. If m V 4, the synthesis of titania could be easily fulfilled at 100 jC, when m = 8, the reaction temperature should be higher (160 jC), otherwise no product could be obtained. When we used isobutyl alcohol, a simplest primary alcohol with branch and higher steric hindrance, only rutile (confirmed by XRD after centrifugation) dispersed in the liquid medium was attainable when heated at 160 jC. This means that the steric hindrance is a key factor to be considered for the initiation of the reactions in our method. As to the effect of temperature on particle sizes, for example, spherical anatase particles with 8 and 17 nm mean diameter were obtained in ethanol when heated at 100 and 150 jC, respectively.
4. Conclusion In summary, nanocrystalline titania could be successfully prepared in alcohols under solvothermal conditions. The amount and configuration of CH2 in alcohols and the reaction temperature play two key roles in controlling the crystal structures, grain sizes and morphologies of the final products. HCl plays a significant role in determining the possible formation mechanism and the crystal structures. This method might be found applicable in the syntheses of other metal oxides and mixed metal-oxide ceramics.
products. We also thank Prof. Ruji Wang for recording XRD pattern and valuable discussion, and Dr. Chengdui Yang for assistance with GC – MS analysis.
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This project was supported by National Nature Science Foundation of China (No. 29871028) and State Key Project of Fundamental Research. We are indebted to Mrs. Meijuan Zhao for her endeavor in recording TEM photos for our
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