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Growth of anatase (TiO2) crystals by chemical transport reactions with HBr and HCl
Journal of Crystal Growth 47 (1979) 139—144 © North-Holland Publishing Company
GROWTH OF ANATASE (Ti02) CRYSTALS BY CHEMICAL TRANSPORT REACTIONS WITH HBr AND HCI F. IZUMI, H. KODAMA and A. ONO National Institute for Researches in Inorganic Materials, Namiki 1-1, Sakura-mura, Niihari-gun, Ibaraki 300-31, Japan Received 2 April 1979
The influence of Group HIB metals on the polymorphic crystallization of Ti02 from vapor has been investigated. When highly pure rutile was used as a starting material, Ti02 was transported to colder zones of closed tubes by means of HBr and HCI to give incorporated into growing anatase crystals far more readilyor than Ga:~ Twomaterial possible produced mechanisms have only rutile. 3+ Bywas contrast, the addition of a small amount of Al203, Ga203, 1n203 toand the1n3+. starting anatase crystals. been proposed A1 for the metastable formation of anatase in the presence ofthe foreign metals.
I. Introduction
tubes by means of HBr and HC1. Such reaction systems must favor rutile formation in the absence of any additives stabilizing anatase since reducing atmospheres predominate with hydrogen halides as
Vapor-phase oxidation and hydrolytic precipitation of Ti4~species yield two tetragonal modifications of Ti0 2: stable rutile and metastable anatase. Factors influencing the polymorphic crystallization of Ti02 have been widely investigated. Previous work [1,2] has shown that fluoride, sulfate, and phosphate ions dissolved in parent solutions favor anatase formation under hydrothermal conditions. Suyama and co-workers [3,4] have established that, in vaporphase oxidation of TiCl4, reducing environments tend to facilitate rutile formation. The literature contains some references to chemical vapor deposition of the Ti02 modifications in the presence of foreign materials. Davtyan [5] reported that, in the chemical transport of Ti02 with TeCI4 as transporting agent, addition of Al203 to a solid starting material makes it possible to grow single crystals of anatase. His finding however, conflict with the descriptions of someis,patents [3],inwhich indicate that the rutile content in Ti0 2 powders produced by the so-called chloride process is increased by adding impurities, including Aid3 and ZrCl4. The present investigation was undertaken to shed light on the effect of Group IIIB metals, i.e., Al, Ga, and In, upon the polymorphic crystallization of Ti02. In our experiments, Ti02 was transported in closed
transporting agents [6]. Therefore, the two transporting agents are expected to be suitable for determining whether or not anatase formation prevails under the influence of the Group IIIB metals.
2. Experimental Rutile powder obtained from Rare Metallic Co., Ltd. was 99.99% pure. NH4Br and NH4C1 were of a GR grade. The starting rutile (150—350 mg), and NH4Br or NH4C1 were charged into an ampule of quartz glass ca. 10 mm in inner diameter and 9.5—12 cm long, with or without an additive. Thevolume amounts the 3 of tube wereof6.8— ammonium 9.8 mg cm3salts forper NH1 cm 3 for 4Br and 4.1—4.4 mg cm NH4CI. The ampule was evacuated, sealed, and then heated in a horizontal tubular furnace with two independent heaters. HBr and HC1 were generated in the ampule by thermal decomposition of NH4Br and NH4CI, respectively. Ti02 was transported from a region of higher temperature T2 to a region of lower temperature T1 (L~T “2 100 K). —
139
F. izurni Ct al.
140
/ Growth of anatase crystals
Before the transport process was started, the walls of the crystallization zone were purified from fine particles of Ti02 which had adhered to it during the introduction of the starting material. For this purpose, these particles were allowed to migrate in the reverse direction, that is, from the crystallization zone (T2) to the source zone (T1) for at least 15 h. The products that formed spontaneously at the crystallization zone were identified by X-ray powder diffraction using nickel-filtered Cu Ka radiation. The lattice parameters of anatase were determined from a least-squares refinement of d spacings for indexed reflections; Si of 99.99% purity was used as an internal standard. Some crystals were examined with a Hitachi HU-1 lD electron microscope (accelerating voltage: 100 kV) and a JEOL JXA-5A electron probe X-ray microanalyzer. For the determination of Al in anatase, the sample (ca. 20 mg) was fused with 1.5 g of K2S207. The 3 sulfuric resulting cake was dissolved in 9 mol dm acid. After dilution to 50 cm3, 10 cm3 aliquots were drawn from this solution. Ti4~was masked by adding 3 cm3 of 30% H 202, and then the Al concentration was determined by extraction-spectrophotometry with 8-quinolinol [7].
Table 1 Chemical transport of Ti02 by means of HBr and HCI 1 a) Run No. Additive (mol%) Carrier wV cm3) (mg 1 2 3 4 5 6 7 8 9 10 11 12
3. Results Table 1 shows the experimental conditions and products of chemical-transport reactions. In order to avoid the anatase—rutile transformation, these runs were carried out at relatively low temperatures: T1 = 1011—1073 K, T2 = 1121—1175K. When only pure rutile was used as a starting material, dark blue or dark green crystals of rutile formed exclusively, regardless of carriers. By contrast, brownish white, opaque crystals of anatase separated out when 1—9.8 mol% of Al203 was added to the starting material (fig. 1). They did not have facets and their maximum dimensions were 6 mm. The presence of A1203 appreciably slowed down the rate of Ti02 transport. On being heated at 1270 K, the anatase, obtained by adding 1 mol% of A12O3, transformed into rutile extremely slowly (percentage of conversion: 38%was afterreadily 35 h;6l% after 61onto h). the surface of Al3~ adsorbed anatase and subsequently incorporated into its crystal lattice during crystal growth. A scanning X-ray image of Al Ka revealed the fairly ununiform distribution of Al in an anatase crystal (fig. 2). The products from runs No. 3 and No. 5, respectively, contained 0.19%
T (K) 1
1 b) wt (pg h1)
Products c)
T2 (K)
1029 1023 1023 1021 1017 1073 1028 1025 1011 1017 1023 1020
1128 1127 1132 1125 1126 1175 1133 1133 1121 1129 1131 1125
810 200 310 190 220 180 40 2.8 11 4.4 15 14
Ru Ru An ~‘ Mu An a~Mu An ~‘ Mu An a- Mu Ani’ Mu An An Ru An An
NH
Al~O3(1.0) Al~O3(3.7) M203 (9.8) A1203 (1.0) Al203 (1.0) Ga203 (1.0) Ga203(1.9) Ga203 (2.1) ln~O3(1.0) ln203 (1.0)
4Br NFI4C1 NH4Br NH4Br NH4 Br NH4Br NH4CI NH4Br NH4Br NH4CI NH4Br NH4C1
8.2 4.1 7.7 8.6 9.8 6.8 4.4 7.9 8.8 4.1 8.7 4.1 3 of tube volume.
a) Rate b) Amounts of Ti0 of the ammonium salts per 1 cm 2 transport. c) Ru, rutile; An, anatase; Mu, titanium-containing mullite.
F. Izumi et al. / Growth of anatase crystals
141
Fig. 1. An anatase crystal grown by adding 1 molil of Al
203.
One division = 1 mm.
and 0.30% Al203. The lattice constants of the anatase obtained in run No.4 were: a = 378.5 pm; c = 950.8 pm. These values are essentially the same as determined for the anatase prepared in the presence of 1 mol% Al203. The c axis dimension was a little smaller than that reported for pure anatase (951.4 pm) [8], whereas the value for the a axis remained rouohlv the same Pulverized rutile prepared without any additive was tinged with blue owing to the deviations reduction 4~togreenish Ti3~accompanying slight of some Ti
________________________________________________
scanning picture of the Al distribution in the crosssection of an anatase crystal grown by adding 1 mol% of Fig. 2. A
£4 A
_____
_,.
3 mm Fig. 3. A scanning electron micrograph of mullite crystals deposited on an anatase crystal.
from ideal stoichiometry on the side of metal excess [9,10]. On the other hand, aluminum-containing anatase ground to powders was always pure-white. These observations can be understood on the basis that A13~ions are admitted into the anatase crystal lattice by replacing Ti4~ions in six-coordination and creating charge-compensating anion vacancies [10, 11]; if these same sites were occupied by Ti3~ions, the crystal would appear blue or black. Anatase was always accompanied by a slight amount of radial aggregates of needle crystals, which deposited not only on the walls of the ampule but also on anatase crystals (fig. 3). This by-product was isomorphous with mullite, Al 6Si2O13. Electron microscopic observations showed that the axes of the needles overlapped with the orthorhombic c axis (fig. 4). Its typical composition as determined by X-ray microanalysis was : A1203, 75.2 wt%; Si02, 4~is captured by mullite 21.6 wt%; Ti02, 2.3 wt%. Ti evidently because of its higher charge. Ti4~ions presumably substitute for A13~ions which have as
A1 203.
nearest neighbors six
02
ions arranged octahedrally,
142
1~Izumi et al.
a
/ Grosvth of anatase crystals
2 ilm ___
Fig. 4. (a) An electron micrograph ol a mullite crystal and (b) a selected-area diffraction pattern corresponding to (A).
with a result that Al3~ions occupy tetrahedral spaces in excess, at the expense of Si4~ions to preserve electrical neutrality [12]. The Si4~lattice ions must have been supplied through the attack of corrosive halide vapors on quartz vessels [131. Ga2 03 and 1n203 were also effective additives for crystallizing anatase. The formation of rutile in run No. 10 (table 1) was an exception. Products other than Ti02 never occurred in these runs. The morpho3~ logy of anatase crystals obtained with of Ga and In3~ was widely different from thatthe of aid aluminumcontaining anatase. They were frequently twinned and characterized by transparent, almost colorless plates flattened on {00l} (fig. 5). Unlike Al3~ions, neither Ga3~nor In3~ions were incorporated into growing anatase crystals to such an extent that they were detectable by X-ray microanalysis. This marked contrast reflects the fact that A13~fits in the lattice of anatase more readily, and with greater decrease of free energy, than Ga3~and In3~.It
is, nevertheless, almost certain that traces of Ga3~and In3~do invade the anatase lattice and replace Ti4~ since anatase crystals prepared by adding Ga 203 and 1n203 were neither black nor blue owing to the _____
______ ______
____________________ ____________
_______
______
Fig. 5. Anatase crystals obtained in run No. 11. One clivisian
=
1 mm.
F. Izumi et al.
/ Growth of anatase crystals
143
2C
absence of Ti3~responsible for the color of Ti02.
(A)
As table 1 shows, the rate of Ti02 transport was decreased one or two orders of magnitude on funcaddi3~and In3~may tion of Ga2O3 and 1n203. Ga tion as surface poisons by being chemisorbed on active growth sites such as kinks and thereby inhibit the spread of monomolecular steps on the crystal surface; they do not enter the anatase crystal lattice to any great extent in the course of growth.
4. Discussion Ti0 2 is believed to be transported along temperature gradients (T2 -* T5) according to the endothermic, reversible reaction [9,14]: Ti02(c) + 4 HX(g) TiX4(g) + 2 H2 0(g) (X: Br or Cl) (1) The free enregy change accompanying the above reaction is the difference in the standard free energies of the products and the reactants: -
=
z~G~.(rutile) 4 L~G~(HX). —
The condition for the equilibrium is given by rp(T1x 21 4 ~ 4) P(H2o) = —R Tin Kp = —R TIn P(Hx)
[
I
I
1400
For plotted further Fig. 6. Partial pressures of (A) TiBr4 and (B) 3. TiCL4 against see temperature; n°(HX)/V = 0.1 mmol/cm details, text.
tion, from which P(TiX4) for a given set of tempera(2) (3)
(the activity of rutile being assumed as unity), where Kp denotes the equilibrium constant of reaction (1) expressed by the partial pressures of the three gaseous species. Eq. (1) states that the stoichiometric ratio 2TiX4 to H20 is specified for these gases: 2.P(TiX4) =P(H20)
I
1200 i/K
Eqs. (2)—(5) can be combined into a single equaz~G~’(TiX 4) + 2 z~G.(H20) —
800
(4)
-
The material balance for the species bearing Br or Cl in the system is given by the relation n°(HX)= n(HX) + 4n(TiX4)
.
ture and concentration of HX can be calculated. Fig. 6 shows P(TiX4) versus Tdata for n°(HX)/Vof 3. Thermochemical reported in 0.1 ref. mmol [15] cm were employed to calculate P(TiX The chemical transport of Ti0 4). 2 with HBr has never been studied, as far as is known to us. Fig. 6A suggests that HBr may be of somewhat advantage than HC1 for a transporting agent. The experimental results given in table 1 are in accord with this expectation. Two hypotheses can be advanced to explain the deposition of Ti02 as anatase, rather than more stable rutile, by the action of the Group IIIB metals: (1) The initial stage of vapor-phase oxidation of titanium (IV) halides is always the formation of nuclei (clusters) with structuresrelated to anatase [3,4]. Without any additives stabilizing the anatase structure, they
Here n°(HX)is the initial number of moles of HX, n(HX) the number of moles of unreacted HX, and n(T1X4) the number of moles of TiX4. Using the perfect-gas equation, we write
are unstable towards their transition to rutile nuclei under reducing atmospheres. The Group IIIB metals are, more or less, incorporated into (or attached to) the anatase nuclei and interfere with the transition. (2) The vapor deposition of Ti02 does not necessarily
n°(HX) P(HX) + 4P(TiX4) V RT
involve anatase nucleation in the absence of any additives. Addition of the Group IIIB oxides enables
(5)
144
F. Izumi Ct al.
/ Growth
anatase to be nucleated selectively because anatase nuclei are themodynamically stabilized relative to rutile ones as a result of the uptake of the foreign metals. In other words, the relative stabilities for rutile and anatase of large particle size are reversed in the nuclear state in this case. One of the authors [2] has put this type of interpretation on the effectiveness of dissolved fluoride, sulfate, and phosphate ions in promoting anatase crystallization under hydrothermal conditions. Stabilization of the anatase structure by the uptake of the Group IIIB metals is a prerequisite for both hypotheses. Rao et a!. [16] found that the Al3~ ion has a strong retarding effect on the anatase— rutile transformation. In support of their finding, the aluminum-containing anatase prepared in the present work was converted into rutile most sluggishly at a temperature as high as 1270 K. Although no work has been published on the influence of Ga3~and In3~on the phase transition, it is reasonable to presume that the presence of these two cations has a stabilizing effect on anatase as well. The very small content of Ga and In in anatase implies that its nuclei contain Ga3~or In3~to a much smaller extent than they do Al3~.In3~is too large to be readily incorporated in anatase nuclei (ionic radius: Ti4~,69 pm; In3~,88 pm); substantially all En3~ ions are believed to be attached to their surfaces. The exact function of the Group IIIB metals in the crystallization and stabilization of anatase are difficult to determine at present. Extensive investigations of the transformation behavior and lattice defects of anatase doped with these foreign metals will provide additional insight into the mechanism of anatase formation from vapor.
of anatase crystals
Acknowledgments The authors express their gratitude to Y. Yajima for Al analyses, M. Tsutsumi for obtaining the scanning electron micrograph, and Y. Sekikawa for electron microscopic observations. Their thanks are also due to F. Okamura for a valuable comment on the mullite structure. References [1] F. Izumi (1976) 709.and Y. Fujiki, Bull. Chem. Soc. Japan 49 12] F. Izumi, Bull. Chem. Soc. Japan 51(1978)1771. [3]Y. Suyama, K. Ito and A. Kato, J. Inorg. Nuci. Chem. 37 (1975) 1883. [4]Y. Suyama, K. Ohmura and A. Kato, Nippon Kagaku Kaishi (1976) 584. [5] GD. Davtyan, Kristallografiya 21(1976)869. [61P. Peshev, l.Z. Babievskaya and V.A. Krenev, Mater. Res. Bull. 12(1977)1035.
[71H.
Hashitani and K. Motojinia, Bunseki Kagaku 7 (1958) 478. [8]Powder Diffraction File, File No. 21-1272. [9] E. Wäsch, Kristall Tech. 7 (1972) 187. 1101 M.D. Beats, in: High Temperature Oxides, Vol. 5-11, Ed. AM. Alper (Academic Press, New York, 1970) p. 99. 1111 K.J .D. MacKenzie, Trans. J. Brit. Ceram. Soc. 74 (1975) 29. [12] R.W.G. Wyckoff, Crystal Structures, Vol. 4, 2nd ed. (lnterscience, New York, 1968) p. 187. [13] H. Schafer, Z. Anorg. Allgem. Chem. 445 (1978) 129. [14] T. Niemyski and W. Piekarczyk, J. Crystal Growth 1 (1967) 177. [15] 1. Barin and 0. Knacke, Thermochemical Properties of Inorganic Substances (Springer, Berlin, and Stahleisen, Düsseldorf, 1973). [16] C.N.R. Rao, A. Turner and J.M. Honig, J. Phys. Chem. Solids 11(1959)173.
GROWTH OF ANATASE (Ti02) CRYSTALS BY CHEMICAL TRANSPORT REACTIONS WITH HBr AND HCI F. IZUMI, H. KODAMA and A. ONO National Institute for Researches in Inorganic Materials, Namiki 1-1, Sakura-mura, Niihari-gun, Ibaraki 300-31, Japan Received 2 April 1979
The influence of Group HIB metals on the polymorphic crystallization of Ti02 from vapor has been investigated. When highly pure rutile was used as a starting material, Ti02 was transported to colder zones of closed tubes by means of HBr and HCI to give incorporated into growing anatase crystals far more readilyor than Ga:~ Twomaterial possible produced mechanisms have only rutile. 3+ Bywas contrast, the addition of a small amount of Al203, Ga203, 1n203 toand the1n3+. starting anatase crystals. been proposed A1 for the metastable formation of anatase in the presence ofthe foreign metals.
I. Introduction
tubes by means of HBr and HC1. Such reaction systems must favor rutile formation in the absence of any additives stabilizing anatase since reducing atmospheres predominate with hydrogen halides as
Vapor-phase oxidation and hydrolytic precipitation of Ti4~species yield two tetragonal modifications of Ti0 2: stable rutile and metastable anatase. Factors influencing the polymorphic crystallization of Ti02 have been widely investigated. Previous work [1,2] has shown that fluoride, sulfate, and phosphate ions dissolved in parent solutions favor anatase formation under hydrothermal conditions. Suyama and co-workers [3,4] have established that, in vaporphase oxidation of TiCl4, reducing environments tend to facilitate rutile formation. The literature contains some references to chemical vapor deposition of the Ti02 modifications in the presence of foreign materials. Davtyan [5] reported that, in the chemical transport of Ti02 with TeCI4 as transporting agent, addition of Al203 to a solid starting material makes it possible to grow single crystals of anatase. His finding however, conflict with the descriptions of someis,patents [3],inwhich indicate that the rutile content in Ti0 2 powders produced by the so-called chloride process is increased by adding impurities, including Aid3 and ZrCl4. The present investigation was undertaken to shed light on the effect of Group IIIB metals, i.e., Al, Ga, and In, upon the polymorphic crystallization of Ti02. In our experiments, Ti02 was transported in closed
transporting agents [6]. Therefore, the two transporting agents are expected to be suitable for determining whether or not anatase formation prevails under the influence of the Group IIIB metals.
2. Experimental Rutile powder obtained from Rare Metallic Co., Ltd. was 99.99% pure. NH4Br and NH4C1 were of a GR grade. The starting rutile (150—350 mg), and NH4Br or NH4C1 were charged into an ampule of quartz glass ca. 10 mm in inner diameter and 9.5—12 cm long, with or without an additive. Thevolume amounts the 3 of tube wereof6.8— ammonium 9.8 mg cm3salts forper NH1 cm 3 for 4Br and 4.1—4.4 mg cm NH4CI. The ampule was evacuated, sealed, and then heated in a horizontal tubular furnace with two independent heaters. HBr and HC1 were generated in the ampule by thermal decomposition of NH4Br and NH4CI, respectively. Ti02 was transported from a region of higher temperature T2 to a region of lower temperature T1 (L~T “2 100 K). —
139
F. izurni Ct al.
140
/ Growth of anatase crystals
Before the transport process was started, the walls of the crystallization zone were purified from fine particles of Ti02 which had adhered to it during the introduction of the starting material. For this purpose, these particles were allowed to migrate in the reverse direction, that is, from the crystallization zone (T2) to the source zone (T1) for at least 15 h. The products that formed spontaneously at the crystallization zone were identified by X-ray powder diffraction using nickel-filtered Cu Ka radiation. The lattice parameters of anatase were determined from a least-squares refinement of d spacings for indexed reflections; Si of 99.99% purity was used as an internal standard. Some crystals were examined with a Hitachi HU-1 lD electron microscope (accelerating voltage: 100 kV) and a JEOL JXA-5A electron probe X-ray microanalyzer. For the determination of Al in anatase, the sample (ca. 20 mg) was fused with 1.5 g of K2S207. The 3 sulfuric resulting cake was dissolved in 9 mol dm acid. After dilution to 50 cm3, 10 cm3 aliquots were drawn from this solution. Ti4~was masked by adding 3 cm3 of 30% H 202, and then the Al concentration was determined by extraction-spectrophotometry with 8-quinolinol [7].
Table 1 Chemical transport of Ti02 by means of HBr and HCI 1 a) Run No. Additive (mol%) Carrier wV cm3) (mg 1 2 3 4 5 6 7 8 9 10 11 12
3. Results Table 1 shows the experimental conditions and products of chemical-transport reactions. In order to avoid the anatase—rutile transformation, these runs were carried out at relatively low temperatures: T1 = 1011—1073 K, T2 = 1121—1175K. When only pure rutile was used as a starting material, dark blue or dark green crystals of rutile formed exclusively, regardless of carriers. By contrast, brownish white, opaque crystals of anatase separated out when 1—9.8 mol% of Al203 was added to the starting material (fig. 1). They did not have facets and their maximum dimensions were 6 mm. The presence of A1203 appreciably slowed down the rate of Ti02 transport. On being heated at 1270 K, the anatase, obtained by adding 1 mol% of A12O3, transformed into rutile extremely slowly (percentage of conversion: 38%was afterreadily 35 h;6l% after 61onto h). the surface of Al3~ adsorbed anatase and subsequently incorporated into its crystal lattice during crystal growth. A scanning X-ray image of Al Ka revealed the fairly ununiform distribution of Al in an anatase crystal (fig. 2). The products from runs No. 3 and No. 5, respectively, contained 0.19%
T (K) 1
1 b) wt (pg h1)
Products c)
T2 (K)
1029 1023 1023 1021 1017 1073 1028 1025 1011 1017 1023 1020
1128 1127 1132 1125 1126 1175 1133 1133 1121 1129 1131 1125
810 200 310 190 220 180 40 2.8 11 4.4 15 14
Ru Ru An ~‘ Mu An a~Mu An ~‘ Mu An a- Mu Ani’ Mu An An Ru An An
NH
Al~O3(1.0) Al~O3(3.7) M203 (9.8) A1203 (1.0) Al203 (1.0) Ga203 (1.0) Ga203(1.9) Ga203 (2.1) ln~O3(1.0) ln203 (1.0)
4Br NFI4C1 NH4Br NH4Br NH4 Br NH4Br NH4CI NH4Br NH4Br NH4CI NH4Br NH4C1
8.2 4.1 7.7 8.6 9.8 6.8 4.4 7.9 8.8 4.1 8.7 4.1 3 of tube volume.
a) Rate b) Amounts of Ti0 of the ammonium salts per 1 cm 2 transport. c) Ru, rutile; An, anatase; Mu, titanium-containing mullite.
F. Izumi et al. / Growth of anatase crystals
141
Fig. 1. An anatase crystal grown by adding 1 molil of Al
203.
One division = 1 mm.
and 0.30% Al203. The lattice constants of the anatase obtained in run No.4 were: a = 378.5 pm; c = 950.8 pm. These values are essentially the same as determined for the anatase prepared in the presence of 1 mol% Al203. The c axis dimension was a little smaller than that reported for pure anatase (951.4 pm) [8], whereas the value for the a axis remained rouohlv the same Pulverized rutile prepared without any additive was tinged with blue owing to the deviations reduction 4~togreenish Ti3~accompanying slight of some Ti
________________________________________________
scanning picture of the Al distribution in the crosssection of an anatase crystal grown by adding 1 mol% of Fig. 2. A
£4 A
_____
_,.
3 mm Fig. 3. A scanning electron micrograph of mullite crystals deposited on an anatase crystal.
from ideal stoichiometry on the side of metal excess [9,10]. On the other hand, aluminum-containing anatase ground to powders was always pure-white. These observations can be understood on the basis that A13~ions are admitted into the anatase crystal lattice by replacing Ti4~ions in six-coordination and creating charge-compensating anion vacancies [10, 11]; if these same sites were occupied by Ti3~ions, the crystal would appear blue or black. Anatase was always accompanied by a slight amount of radial aggregates of needle crystals, which deposited not only on the walls of the ampule but also on anatase crystals (fig. 3). This by-product was isomorphous with mullite, Al 6Si2O13. Electron microscopic observations showed that the axes of the needles overlapped with the orthorhombic c axis (fig. 4). Its typical composition as determined by X-ray microanalysis was : A1203, 75.2 wt%; Si02, 4~is captured by mullite 21.6 wt%; Ti02, 2.3 wt%. Ti evidently because of its higher charge. Ti4~ions presumably substitute for A13~ions which have as
A1 203.
nearest neighbors six
02
ions arranged octahedrally,
142
1~Izumi et al.
a
/ Grosvth of anatase crystals
2 ilm ___
Fig. 4. (a) An electron micrograph ol a mullite crystal and (b) a selected-area diffraction pattern corresponding to (A).
with a result that Al3~ions occupy tetrahedral spaces in excess, at the expense of Si4~ions to preserve electrical neutrality [12]. The Si4~lattice ions must have been supplied through the attack of corrosive halide vapors on quartz vessels [131. Ga2 03 and 1n203 were also effective additives for crystallizing anatase. The formation of rutile in run No. 10 (table 1) was an exception. Products other than Ti02 never occurred in these runs. The morpho3~ logy of anatase crystals obtained with of Ga and In3~ was widely different from thatthe of aid aluminumcontaining anatase. They were frequently twinned and characterized by transparent, almost colorless plates flattened on {00l} (fig. 5). Unlike Al3~ions, neither Ga3~nor In3~ions were incorporated into growing anatase crystals to such an extent that they were detectable by X-ray microanalysis. This marked contrast reflects the fact that A13~fits in the lattice of anatase more readily, and with greater decrease of free energy, than Ga3~and In3~.It
is, nevertheless, almost certain that traces of Ga3~and In3~do invade the anatase lattice and replace Ti4~ since anatase crystals prepared by adding Ga 203 and 1n203 were neither black nor blue owing to the _____
______ ______
____________________ ____________
_______
______
Fig. 5. Anatase crystals obtained in run No. 11. One clivisian
=
1 mm.
F. Izumi et al.
/ Growth of anatase crystals
143
2C
absence of Ti3~responsible for the color of Ti02.
(A)
As table 1 shows, the rate of Ti02 transport was decreased one or two orders of magnitude on funcaddi3~and In3~may tion of Ga2O3 and 1n203. Ga tion as surface poisons by being chemisorbed on active growth sites such as kinks and thereby inhibit the spread of monomolecular steps on the crystal surface; they do not enter the anatase crystal lattice to any great extent in the course of growth.
4. Discussion Ti0 2 is believed to be transported along temperature gradients (T2 -* T5) according to the endothermic, reversible reaction [9,14]: Ti02(c) + 4 HX(g) TiX4(g) + 2 H2 0(g) (X: Br or Cl) (1) The free enregy change accompanying the above reaction is the difference in the standard free energies of the products and the reactants: -
=
z~G~.(rutile) 4 L~G~(HX). —
The condition for the equilibrium is given by rp(T1x 21 4 ~ 4) P(H2o) = —R Tin Kp = —R TIn P(Hx)
[
I
I
1400
For plotted further Fig. 6. Partial pressures of (A) TiBr4 and (B) 3. TiCL4 against see temperature; n°(HX)/V = 0.1 mmol/cm details, text.
tion, from which P(TiX4) for a given set of tempera(2) (3)
(the activity of rutile being assumed as unity), where Kp denotes the equilibrium constant of reaction (1) expressed by the partial pressures of the three gaseous species. Eq. (1) states that the stoichiometric ratio 2TiX4 to H20 is specified for these gases: 2.P(TiX4) =P(H20)
I
1200 i/K
Eqs. (2)—(5) can be combined into a single equaz~G~’(TiX 4) + 2 z~G.(H20) —
800
(4)
-
The material balance for the species bearing Br or Cl in the system is given by the relation n°(HX)= n(HX) + 4n(TiX4)
.
ture and concentration of HX can be calculated. Fig. 6 shows P(TiX4) versus Tdata for n°(HX)/Vof 3. Thermochemical reported in 0.1 ref. mmol [15] cm were employed to calculate P(TiX The chemical transport of Ti0 4). 2 with HBr has never been studied, as far as is known to us. Fig. 6A suggests that HBr may be of somewhat advantage than HC1 for a transporting agent. The experimental results given in table 1 are in accord with this expectation. Two hypotheses can be advanced to explain the deposition of Ti02 as anatase, rather than more stable rutile, by the action of the Group IIIB metals: (1) The initial stage of vapor-phase oxidation of titanium (IV) halides is always the formation of nuclei (clusters) with structuresrelated to anatase [3,4]. Without any additives stabilizing the anatase structure, they
Here n°(HX)is the initial number of moles of HX, n(HX) the number of moles of unreacted HX, and n(T1X4) the number of moles of TiX4. Using the perfect-gas equation, we write
are unstable towards their transition to rutile nuclei under reducing atmospheres. The Group IIIB metals are, more or less, incorporated into (or attached to) the anatase nuclei and interfere with the transition. (2) The vapor deposition of Ti02 does not necessarily
n°(HX) P(HX) + 4P(TiX4) V RT
involve anatase nucleation in the absence of any additives. Addition of the Group IIIB oxides enables
(5)
144
F. Izumi Ct al.
/ Growth
anatase to be nucleated selectively because anatase nuclei are themodynamically stabilized relative to rutile ones as a result of the uptake of the foreign metals. In other words, the relative stabilities for rutile and anatase of large particle size are reversed in the nuclear state in this case. One of the authors [2] has put this type of interpretation on the effectiveness of dissolved fluoride, sulfate, and phosphate ions in promoting anatase crystallization under hydrothermal conditions. Stabilization of the anatase structure by the uptake of the Group IIIB metals is a prerequisite for both hypotheses. Rao et a!. [16] found that the Al3~ ion has a strong retarding effect on the anatase— rutile transformation. In support of their finding, the aluminum-containing anatase prepared in the present work was converted into rutile most sluggishly at a temperature as high as 1270 K. Although no work has been published on the influence of Ga3~and In3~on the phase transition, it is reasonable to presume that the presence of these two cations has a stabilizing effect on anatase as well. The very small content of Ga and In in anatase implies that its nuclei contain Ga3~or In3~to a much smaller extent than they do Al3~.In3~is too large to be readily incorporated in anatase nuclei (ionic radius: Ti4~,69 pm; In3~,88 pm); substantially all En3~ ions are believed to be attached to their surfaces. The exact function of the Group IIIB metals in the crystallization and stabilization of anatase are difficult to determine at present. Extensive investigations of the transformation behavior and lattice defects of anatase doped with these foreign metals will provide additional insight into the mechanism of anatase formation from vapor.
of anatase crystals
Acknowledgments The authors express their gratitude to Y. Yajima for Al analyses, M. Tsutsumi for obtaining the scanning electron micrograph, and Y. Sekikawa for electron microscopic observations. Their thanks are also due to F. Okamura for a valuable comment on the mullite structure. References [1] F. Izumi (1976) 709.and Y. Fujiki, Bull. Chem. Soc. Japan 49 12] F. Izumi, Bull. Chem. Soc. Japan 51(1978)1771. [3]Y. Suyama, K. Ito and A. Kato, J. Inorg. Nuci. Chem. 37 (1975) 1883. [4]Y. Suyama, K. Ohmura and A. Kato, Nippon Kagaku Kaishi (1976) 584. [5] GD. Davtyan, Kristallografiya 21(1976)869. [61P. Peshev, l.Z. Babievskaya and V.A. Krenev, Mater. Res. Bull. 12(1977)1035.
[71H.
Hashitani and K. Motojinia, Bunseki Kagaku 7 (1958) 478. [8]Powder Diffraction File, File No. 21-1272. [9] E. Wäsch, Kristall Tech. 7 (1972) 187. 1101 M.D. Beats, in: High Temperature Oxides, Vol. 5-11, Ed. AM. Alper (Academic Press, New York, 1970) p. 99. 1111 K.J .D. MacKenzie, Trans. J. Brit. Ceram. Soc. 74 (1975) 29. [12] R.W.G. Wyckoff, Crystal Structures, Vol. 4, 2nd ed. (lnterscience, New York, 1968) p. 187. [13] H. Schafer, Z. Anorg. Allgem. Chem. 445 (1978) 129. [14] T. Niemyski and W. Piekarczyk, J. Crystal Growth 1 (1967) 177. [15] 1. Barin and 0. Knacke, Thermochemical Properties of Inorganic Substances (Springer, Berlin, and Stahleisen, Düsseldorf, 1973). [16] C.N.R. Rao, A. Turner and J.M. Honig, J. Phys. Chem. Solids 11(1959)173.