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Preparation and properties of uniform colloidal indium compounds of different morphologies
Colloids and Surfaces,
50 (1990)
Elsevier Science Publishers
281
281-293 B.V., Amsterdam
Preparation and Properties of Uniform Colloidal Indium Compounds of Different Morphologies
KEITA YURA*, KARL C. FREDRIKSON** Department
of Chemistry,
(Received 7 February
Clarkson
University,
1990; accepted
and EGON MATIJEVIC Potsdam,
NY 13699 (U.S.A.)
19 March 1990)
ABSTRACT Colloidal indium compounds, consisting of uniform particles of narrow size distribution of different morphologies were prepared by precipitation from homogeneous solutions at elevated temperatures. The forced hydrolysis of indium nitrate and chloride solutions yielded prismatic In (OH )3 crystals. In the presence of sulfate ions, aging of solutions resulted in dispersions of different particle shapes, which depended on the reaction time, temperature and the pH. These solids contained sulfate ion which could not be removed by washing. A mechanism for the formation of such dispersions is suggested. All powders, so prepared, were transformed by calcination to In,O,, while the particle shape was preserved.
INTRODUCTION
The interest in compounds of indium and indium oxide has been based on their semiconductor and optical properties. Despite such uses in high technology, relatively little work has been reported on the preparation of these materials in a finely dispersed state. Ultrafine powders were obtained either by vapor deposition [ 11 or by precipitation in alcoholic solutions [ 2 1, but the particles so obtained were neither uniform in size nor in shape. In this work precipitation procedures are described, which yield spherical and rod-like particles of narrow size distributions by precipitation from homogeneous solutions. Depending on the experimental conditions, these solids were either indium hydroxide or indium basic sulfate. On calcination, both kinds of powders changed to In,O, while preserving their morphology. *On leave from Kobe Steel, Ltd, Kobe, Japan. **Participant in the FASTEC program.
0166.6622/90/$03.50
0 1990 -
Elsevier Science Publishers
B.V.
282 EXPERIMENTAL
Materials Indium nitrate pentahydrate (Alfa, Aldrich) and anhydrous indium chloride
( Alfa) were used without further purification. The stock solution of indium nitrate in doubly distilled water (0.1 mol dnp3 ), acidified to pH 2.5 by adding 1.0 mol dnp3 nitric acid, was stable for two months. Indium chloride stock solutions (5.0.10 -2 mol dmp3 ) were sufficiently acidic as prepared. All solutions were filtered through 0.2 pm pore size membranes in order to remove any possible particulate contaminants. Preparation and characterization of the dispersions In general, the preparation of colloidal dispersions consisted of aging, at elevated temperatures, indium salt solutions containing different additives. Before treatment, the systems were checked for the absence of any particles by a Tyndall beam. The solutions were contained in tightly sealed 10 cm3 Pyrex test tubes and heated in an oven at 75 or 85 oC for different periods of time. When urea was used, the aging took place either at 85 or 120’ C. On completion of the reaction, the tubes were quenched to room temperature in cold water. The pH measurements were carried out before and after heating. To separate the particles, dispersions were centrifuged for 15 min at 1600 rpm and the supernatant solution was discarded. The solids were redispersed in doubly distilled water in an ultrasonic bath and separated again. This purification procedure was repeated twice. To calcine the powders, the solids were first dried in a desiccator for approximately 24 h. Some samples were then kept at 75’ C in a vacuum oven for N 8 h. Calcinations at higher temperature (up to 800” C) in air were carried out in a tubular furnace for varying periods of time. The rate of temperature increase was 10°C min-l. The size and shape of the particles were determined by transmission (TEM) and scanning (SEM ) electron microscopy. The structure and composition were evaluated by X-ray diffraction (XRD) and infrared (IR) spectroscopy. The changes on heating of the solids were followed by thermogravimetric analysis (TGA). The content in indium in the resulting powders was assayed by atomic absorption, while the sulfur was analyzed by the radiotracer technique. In the latter case, the isotope 35S was introduced into the reacting solution and the amount of incorporated sulfur in the particles was then determined by dissolving the solids in HCl and measuring the activity in a scintillation counter.
283 RESULTS
Preparation of particles To establish the best conditions for the preparation of uniform dispersions and to evaluate the effect of experimental conditions on the composition, size, and shape of the resulting particles, a number of parameters was systematically varied, including the concentration of different indium salts, the addition of sodium sulfate and urea, the pH, the temperature, and the aging time. Forced hydrolysis of indium salts Aging either In (NO, )3 or InCl, aqueous solutions at elevated temperatures in the absence of additives promoted the hydrolysis process sufficiently to cause precipitation of crystalline particles. Scanning electron micrographs in Fig. 1 illustrate powders obtained after keeping the respective electrolyte solutions for 12 h at 85 “C. While In (NO,), yielded, as a rule, cubic particles, prismatic crystals appeared when InCl, was used. Analogous results were obtained with different salt concentrations as indicated in Fig. 2 (bottom line). Experiments carried out at 75 oC gave similar results.
Fig. 1. Scanning electron micrographs (SEM) of In(OH)3, obtained by: (a) aging for 12 h at 85”C,a7~10~3moldm-3solutionofIn(NO,),attheinitialpH2.8; (b)for2hat85”C,a1*10-2 mol drne3 solution of InCl, at the initial pH 3.2. The longer bar equals 1 pm.
284
0
0.1 --A-
A(P) -
A(P)-
A(P)
A(P)
P(P)-----
I P(P)-----
P(P)
I P
I 3 0.05 --
P-
0--p--
I PiPi-
1
1
7
1
3
In (NO,), Fig. 2. Precipitation 85’ C, as a function
or
I 5
InCI,
P(,P) 1
I 7
P(,Pl I
I
I
10
/ lo3 mol dme3
domains for In(NO,), or InCl, (in parentheses) solutions, aged for 2 h at of the urea concentration. Symbols: P, prisms; A, agglomerated particles.
In(NOJ), or Id&-urea systems To promote hydrolysis of the indium ion, urea was added in different concentrations and then the aqueous solutions aged at 85 and 120” C, respectively. The results at the lower temperature are summarized in Fig. 2 for In (NC,), and InC& solutions at the initial pH of - 3.5. At higher urea concentrations mostly a mixture of spindle-type and agglomerated particles was found, while at lower concentrations prismatic crystals were obtained by heating for 2 h. A series of experiments was carried out with indium chloride solutions containing urea at lower pH values (0.8-1.6)) but at a higher temperature ( 120’ C ) . In these systems rod-like particles crystallized over the pH range 1.3-1.6 (acidified with HCl) at urea concentrations ranging from 0.1 to 0.3 mol dm-3 at [InCl,] =5*10B3 mol dmP3. Figure 3 illustrates two such dispersions. When precipitation occurred at a still lower pH value ( - 0.8) at higher urea concentrations, only irregular particles were formed. The effect of the sulfate ion It was shown before that the sulfate ion had a major effect on the composition and morphology of inorganic precipitates [ 3-61. For this reason, a series of experiments was carried out with solutions of In (NO, ) 3 to which different amounts of Na,SO, were added. Figure 4 displays transmission electron micrographs of particles obtained at pH 3.5 20.2, aged at 85°C for 2 h. Unlike the systems described in previous sections, the particles so prepared are of spheroidal shape, clearly showing an internal substructure; i.e., they consist of
285
Fig. 3. Transmission electron micrographs (TEM) of rod-like particles obtained by aging for 2 h at 120°C a solution 5*10-3 mol dmm3 in InCl, and 0.1 mol dmw3 in urea at the initial pH 1.6 (a) and 1.3 (b).
much smaller subunits. Only at the very lowest Na,SO, concentration used (0.1 mol dmb3), prismatic crystals were produced. In general, the particle shape and size depended on the sulfate concentration, pH, and time of aging. Transmission electron micrographs in Fig. 5 show the effect of different reaction times on systems, at the original pH of 2.9, at two different sulfate concentrations. It is apparent that at [ Na,SO, ] = 1. lop4 mol dmP3, the crystalline character of the particles becomes more pronounced with the length of heating at 85°C (Figs 5c and d). At the higher content in Na,SO, (5*10m4 mol dmm3), the particles grow somewhat larger with an indication of a change in the structure (Figs 5a and b). The variation of the pH, at otherwise the same conditions, affects strongly the particle properties. Figure 6 shows that the average size decreases substantially with increasing initial pH.
Calcination
Calcination of powders at temperatures up to 800’ C for 4 h resulted in some shrinkage of the particles without a change in shape. Furthermore, no sintering was detected.
286
InCN03&
/IO3 mol
dme3
Fig. 4, TEM of particles obtained by aging solutions for 2 h at 85 ’ C at the initial pH 3.5 2: 0.2 at different concentrations of In(NO,), and Na,SO,.
287
Fig. 5. TEM of particles obtained by aging at 85°C solutions 7*10m3 mol dme3 in In(N03)3 and 5-10-4 mol drnm3 in Na,SO, for 15 min (a) and for 2 h (b). Solutions 7-10m3 mol drnm3 in In(N03)3
and l-10W4 mol drnm3 in Na,SO,
for 15 min (c) and for 2 h (d).
Fig. 6. TEM of particles obtained by aging at 85’ C for 2 h solutions 7 - 10e3 mol drnm3in In (NO3 )3 under the conditions given in Table 1.
Characterizations
The X-ray diffraction pattern of the powder obtained by forced hydrolysis of In (NC&) 3 solutions (sample shown in the electron micrograph la) is given
TABLE 1 Conditions under which the particles in Fig. 6 were obtained Micrograph No 6a 6b 6c 6d
Na,SO, (mol dmm3)
PH
5*10W4 5*10-4 l.lOP l*lOP
Initial
Final
2.6 3.0 2.7 3.0
2.2 2.3 2.3 2.3
In(OH)3
In2’3
Fig. 7. X-ray diffraction patterns of particles shown in Fig. la (a), 6a (b), and (c) of particles obtainedbyagingat85”Cfor2hasolution7~10~3moldm~3inIn(NO~)~and1~10~4moldm-3 in Na,SO, and then calcined up to 1000°C in the TGA instrument.
r
It
L .““(r
1,“”
l,l”U
,“I
cm-’ Fig. 8. IR absorption spectra of the particles shown in Fig. 6c (a) and of the same sample calcined at 800°C for 4 h (b).
in Fig. 7 (top) which is identified as In (OH), [ 71. The weight loss on calcination to In,O,, as determined by the TGA, was found to be 16.5% which corroborates the composition of the solids before and after the heat treatment. Samples synthesized in the presence of the sulfate ion had a different composition. The XRD pattern (Fig. 7, middle) of particles illustrated in Fig. 6b shows characteristics of a crystalline material, but the spectrum could not be related to any reported indium compounds. On treating the same sample up to 1000’ C, the powder was converted to pure In,O, as revealed in Fig. 7 (bottom ) . The IR spectrum of solids precipitated in the sulfate containing solutions shows clearly presence of this anion (Fig. 8 top, 1080-1130 cm-‘), which peak is absent when the same powder is heated at 800’ C (Fig. 8, bottom). The radiotracer analysis of the above samples indeed confirmed that the sulfate ion was incorporated in the particles in amounts which increased with the concentration of Na,SO,. For example, precipitates obtained in 7~10~” In(N03)3 solutions with 5*10W4and 1*10P4 mol dmm3 Na,SO, contained 1.7 and 1.2% (by wt) of sulfur, respectively. An independent analysis by atomic absorption spectroscopy of powders shown in Figs 6a and b gave 62% (by wt )
291
In, while the amount of In estimated by the weight loss (TGA) was N 66%. This discrepancy may be due to incomplete dehydration of the powder in the course of heating. The analysis of a sample calcined for an extended period of time (4 h) at 800°C gave 82% In, which compares well with the expected composition for In20,. DISCUSSION
This study has shown that colloidal indium compounds of different morphologies of narrow size distribution can be obtained by precipitation from homogeneous solutions. Several observations regarding dispersions containing indium are worth mentioning, especially in comparison with other elements of the same group (Al and Ga ) . Depending on conditions, both the composition and the morphology of finely dispersed indium compounds can be varied. In this respect, the nature of anions plays an essential role. Aging of aqueous indium nitrate solutions at elevated temperatures yielded crystalline indium hydroxide particles, either cubic or prismatic. In comparison, by an analogous procedure, Ga precipitates consisted of a rod-like hydrous oxide [ 81, while aluminum nitrate did not give uniform particles. It was demonstrated on a number of examples that the addition of sulfate ion has a major effect on the properties of the dispersions obtained by precipitation. This anion can be included in the solid phase in a stoichiometrically well defined manner, such as in the case of colloidal alunites [ 9,101, or it may give basic compounds of varying metal to sulfate ratios [ 51. Finally, in some instances, the sulfate ion causes particles to appear in spherical shape, in which this anion is present as a leachable contaminant [ 3,4]. In the case of indium compounds, the sulfate ion does affect the nature of the particles, which differs with its concentration. If the latter is low, distinct crystals are obtained, but at somewhat higher concentrations nearly spherical, internally composite, particles are found. Importantly, the X-ray analysis of all these powders shows peaks indicating crystalline nature of an, as yet not identified, indium compound. On calcination these solids converted to In,O,. It should be noted that aging of aluminum salt solutions resulted in amorphous spherical particles which remained stable as long as the dispersed solids were kept in the mother liquor [ 31. Gallium solutions also gave spherical particles, but these were originally amorphous and showed some crystalline characterization on prolonged aging ( N 18 h) [ 81. In all cited cases, the sulfate ion was present in the precipitates, but could be removed either by dialysis or by calcination. There are many speculations, but only a few analyses of the systems that quantitatively explain the role of the sulfate ion in the precipitation of uniform
292
colloidal dispersions. Mostly, it is suggested that this anion promotes polymerization of solute precursors to the solid phase formation. Obviously, such complexes cannot be the same with different metal ions, since the properties of the resulting dispersions differ. The finding that the original spherical particles of indium compounds, containing the sulfate ion, consist of a larger number of small subunits is not unique. A similar observation was made with other metals such as Ce (IV) [ 51 and Sn (IV) [ 111, although in these cases, metal oxides were precipitated rather than the basic sulfates. The chemical analysis of In3+ and SO:- in solutions at the completion of the precipitation process showed that the loss of these species agreed, within the experimental error, with the amount found in the solids after extensive washings. This finding would indicate that the sulfate group is strongly bound in the crystalline solids and cannot be removed by leaching. Such behavior is expected, if particle formation is caused by interaction of relatively simple complexes rather than a continuous polymerization of hydrolyzed species. From the literature data [ 121 solutes of the composition, In3+, In (OH)‘+, and InSO,+ predominate in the presence of sulfate ions at room temperature. Although no speciation diagrams are available for higher temperatures of interest in this work, one may assume that with heating, the hydrolysis is promoted. Since in the solid phase [ In3+ ] / [SO:- ] - 10, it would appear that the nucleation involves InSO,+ and indium hydrolyzed complexes, while further growth is dominated by the latter species. Livage et al. have used a “partial charge model” to interpret the formation of metal (hydrous) oxides in aqueous media [ 131. The model is more difficult to apply when anions other than OH- are involved in the solid phase formation. Following the procedure by Livage, the partial charge was estimated for two reactions as follows: In2(OH),(S04)
(OH,), %Inz(OH),(OH,)~+
+SOi-
(1)
and InZ(OH),(HZS04)
(OHz)2=Inz(OH)6(OHZ)2
(2)
+HZSOd
yieldingvaluesfor6(SO,)=-0.32 (> -2) and6(H,SO,)=-0.18 (
(H,O),+ +In,(OH),(SO,)
(H,O),
+3H30+
(3)
293
Further growth of the particles could proceed through the condensation hydrolyzed indium ions with the above formulated nuclei.
of the
REFERENCES
8 9
10 11 12 13
M. Takeuchi, K. Inoue, S. Ozawa and H. Nagasaka, Appl. Surf. Sci., 33/34 (1988) 905. A. Fojtik, A. Henglein, L. Katsikas and H. Weller, Chem. Phys. Lett., 138 (1987) 535. R. Brace and E. MatijeviC, J. Inorg. Nucl. Chem., 35 (1973) 3691. A. Bell and E. MatijeviC, J. Inorg. Nucl. Chem., 37 (1975) 907. W.P. Hsu, L. Rijnnquist and E. Matijevic, Langmuir, 4 (1988) 31. A. Garg and E. Matijevic, J. Colloid Interface Sci., 126 (1988) 243. Powder Diffraction File, JCPDS International Centre for Diffraction Data, Swarthmore, PA, 1981. S. Hamada, K. Bando and Y. Kudo, Bull. Chem. Sot. Jpn., 59 (1986) 2063. E. Matijevic, R.S. Sapieszko and J.B. Melville, J. Colloid Interface Sci., 50 (1975) 567. R.S. Sapieszko, R.C. Pate1and E. MatijeviC, J. Phys. Chem., 81 (1977) 1061. M. Ocafia and E. Matijevic, J. Mater. Res., 5 (1990) 1083. L.G. Sill&nand A.E. Mar-tell,Chem. Sot. Spec. Publ., 17 (1964). J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 18 (1988) 259.
50 (1990)
Elsevier Science Publishers
281
281-293 B.V., Amsterdam
Preparation and Properties of Uniform Colloidal Indium Compounds of Different Morphologies
KEITA YURA*, KARL C. FREDRIKSON** Department
of Chemistry,
(Received 7 February
Clarkson
University,
1990; accepted
and EGON MATIJEVIC Potsdam,
NY 13699 (U.S.A.)
19 March 1990)
ABSTRACT Colloidal indium compounds, consisting of uniform particles of narrow size distribution of different morphologies were prepared by precipitation from homogeneous solutions at elevated temperatures. The forced hydrolysis of indium nitrate and chloride solutions yielded prismatic In (OH )3 crystals. In the presence of sulfate ions, aging of solutions resulted in dispersions of different particle shapes, which depended on the reaction time, temperature and the pH. These solids contained sulfate ion which could not be removed by washing. A mechanism for the formation of such dispersions is suggested. All powders, so prepared, were transformed by calcination to In,O,, while the particle shape was preserved.
INTRODUCTION
The interest in compounds of indium and indium oxide has been based on their semiconductor and optical properties. Despite such uses in high technology, relatively little work has been reported on the preparation of these materials in a finely dispersed state. Ultrafine powders were obtained either by vapor deposition [ 11 or by precipitation in alcoholic solutions [ 2 1, but the particles so obtained were neither uniform in size nor in shape. In this work precipitation procedures are described, which yield spherical and rod-like particles of narrow size distributions by precipitation from homogeneous solutions. Depending on the experimental conditions, these solids were either indium hydroxide or indium basic sulfate. On calcination, both kinds of powders changed to In,O, while preserving their morphology. *On leave from Kobe Steel, Ltd, Kobe, Japan. **Participant in the FASTEC program.
0166.6622/90/$03.50
0 1990 -
Elsevier Science Publishers
B.V.
282 EXPERIMENTAL
Materials Indium nitrate pentahydrate (Alfa, Aldrich) and anhydrous indium chloride
( Alfa) were used without further purification. The stock solution of indium nitrate in doubly distilled water (0.1 mol dnp3 ), acidified to pH 2.5 by adding 1.0 mol dnp3 nitric acid, was stable for two months. Indium chloride stock solutions (5.0.10 -2 mol dmp3 ) were sufficiently acidic as prepared. All solutions were filtered through 0.2 pm pore size membranes in order to remove any possible particulate contaminants. Preparation and characterization of the dispersions In general, the preparation of colloidal dispersions consisted of aging, at elevated temperatures, indium salt solutions containing different additives. Before treatment, the systems were checked for the absence of any particles by a Tyndall beam. The solutions were contained in tightly sealed 10 cm3 Pyrex test tubes and heated in an oven at 75 or 85 oC for different periods of time. When urea was used, the aging took place either at 85 or 120’ C. On completion of the reaction, the tubes were quenched to room temperature in cold water. The pH measurements were carried out before and after heating. To separate the particles, dispersions were centrifuged for 15 min at 1600 rpm and the supernatant solution was discarded. The solids were redispersed in doubly distilled water in an ultrasonic bath and separated again. This purification procedure was repeated twice. To calcine the powders, the solids were first dried in a desiccator for approximately 24 h. Some samples were then kept at 75’ C in a vacuum oven for N 8 h. Calcinations at higher temperature (up to 800” C) in air were carried out in a tubular furnace for varying periods of time. The rate of temperature increase was 10°C min-l. The size and shape of the particles were determined by transmission (TEM) and scanning (SEM ) electron microscopy. The structure and composition were evaluated by X-ray diffraction (XRD) and infrared (IR) spectroscopy. The changes on heating of the solids were followed by thermogravimetric analysis (TGA). The content in indium in the resulting powders was assayed by atomic absorption, while the sulfur was analyzed by the radiotracer technique. In the latter case, the isotope 35S was introduced into the reacting solution and the amount of incorporated sulfur in the particles was then determined by dissolving the solids in HCl and measuring the activity in a scintillation counter.
283 RESULTS
Preparation of particles To establish the best conditions for the preparation of uniform dispersions and to evaluate the effect of experimental conditions on the composition, size, and shape of the resulting particles, a number of parameters was systematically varied, including the concentration of different indium salts, the addition of sodium sulfate and urea, the pH, the temperature, and the aging time. Forced hydrolysis of indium salts Aging either In (NO, )3 or InCl, aqueous solutions at elevated temperatures in the absence of additives promoted the hydrolysis process sufficiently to cause precipitation of crystalline particles. Scanning electron micrographs in Fig. 1 illustrate powders obtained after keeping the respective electrolyte solutions for 12 h at 85 “C. While In (NO,), yielded, as a rule, cubic particles, prismatic crystals appeared when InCl, was used. Analogous results were obtained with different salt concentrations as indicated in Fig. 2 (bottom line). Experiments carried out at 75 oC gave similar results.
Fig. 1. Scanning electron micrographs (SEM) of In(OH)3, obtained by: (a) aging for 12 h at 85”C,a7~10~3moldm-3solutionofIn(NO,),attheinitialpH2.8; (b)for2hat85”C,a1*10-2 mol drne3 solution of InCl, at the initial pH 3.2. The longer bar equals 1 pm.
284
0
0.1 --A-
A(P) -
A(P)-
A(P)
A(P)
P(P)-----
I P(P)-----
P(P)
I P
I 3 0.05 --
P-
0--p--
I PiPi-
1
1
7
1
3
In (NO,), Fig. 2. Precipitation 85’ C, as a function
or
I 5
InCI,
P(,P) 1
I 7
P(,Pl I
I
I
10
/ lo3 mol dme3
domains for In(NO,), or InCl, (in parentheses) solutions, aged for 2 h at of the urea concentration. Symbols: P, prisms; A, agglomerated particles.
In(NOJ), or Id&-urea systems To promote hydrolysis of the indium ion, urea was added in different concentrations and then the aqueous solutions aged at 85 and 120” C, respectively. The results at the lower temperature are summarized in Fig. 2 for In (NC,), and InC& solutions at the initial pH of - 3.5. At higher urea concentrations mostly a mixture of spindle-type and agglomerated particles was found, while at lower concentrations prismatic crystals were obtained by heating for 2 h. A series of experiments was carried out with indium chloride solutions containing urea at lower pH values (0.8-1.6)) but at a higher temperature ( 120’ C ) . In these systems rod-like particles crystallized over the pH range 1.3-1.6 (acidified with HCl) at urea concentrations ranging from 0.1 to 0.3 mol dm-3 at [InCl,] =5*10B3 mol dmP3. Figure 3 illustrates two such dispersions. When precipitation occurred at a still lower pH value ( - 0.8) at higher urea concentrations, only irregular particles were formed. The effect of the sulfate ion It was shown before that the sulfate ion had a major effect on the composition and morphology of inorganic precipitates [ 3-61. For this reason, a series of experiments was carried out with solutions of In (NO, ) 3 to which different amounts of Na,SO, were added. Figure 4 displays transmission electron micrographs of particles obtained at pH 3.5 20.2, aged at 85°C for 2 h. Unlike the systems described in previous sections, the particles so prepared are of spheroidal shape, clearly showing an internal substructure; i.e., they consist of
285
Fig. 3. Transmission electron micrographs (TEM) of rod-like particles obtained by aging for 2 h at 120°C a solution 5*10-3 mol dmm3 in InCl, and 0.1 mol dmw3 in urea at the initial pH 1.6 (a) and 1.3 (b).
much smaller subunits. Only at the very lowest Na,SO, concentration used (0.1 mol dmb3), prismatic crystals were produced. In general, the particle shape and size depended on the sulfate concentration, pH, and time of aging. Transmission electron micrographs in Fig. 5 show the effect of different reaction times on systems, at the original pH of 2.9, at two different sulfate concentrations. It is apparent that at [ Na,SO, ] = 1. lop4 mol dmP3, the crystalline character of the particles becomes more pronounced with the length of heating at 85°C (Figs 5c and d). At the higher content in Na,SO, (5*10m4 mol dmm3), the particles grow somewhat larger with an indication of a change in the structure (Figs 5a and b). The variation of the pH, at otherwise the same conditions, affects strongly the particle properties. Figure 6 shows that the average size decreases substantially with increasing initial pH.
Calcination
Calcination of powders at temperatures up to 800’ C for 4 h resulted in some shrinkage of the particles without a change in shape. Furthermore, no sintering was detected.
286
InCN03&
/IO3 mol
dme3
Fig. 4, TEM of particles obtained by aging solutions for 2 h at 85 ’ C at the initial pH 3.5 2: 0.2 at different concentrations of In(NO,), and Na,SO,.
287
Fig. 5. TEM of particles obtained by aging at 85°C solutions 7*10m3 mol dme3 in In(N03)3 and 5-10-4 mol drnm3 in Na,SO, for 15 min (a) and for 2 h (b). Solutions 7-10m3 mol drnm3 in In(N03)3
and l-10W4 mol drnm3 in Na,SO,
for 15 min (c) and for 2 h (d).
Fig. 6. TEM of particles obtained by aging at 85’ C for 2 h solutions 7 - 10e3 mol drnm3in In (NO3 )3 under the conditions given in Table 1.
Characterizations
The X-ray diffraction pattern of the powder obtained by forced hydrolysis of In (NC&) 3 solutions (sample shown in the electron micrograph la) is given
TABLE 1 Conditions under which the particles in Fig. 6 were obtained Micrograph No 6a 6b 6c 6d
Na,SO, (mol dmm3)
PH
5*10W4 5*10-4 l.lOP l*lOP
Initial
Final
2.6 3.0 2.7 3.0
2.2 2.3 2.3 2.3
In(OH)3
In2’3
Fig. 7. X-ray diffraction patterns of particles shown in Fig. la (a), 6a (b), and (c) of particles obtainedbyagingat85”Cfor2hasolution7~10~3moldm~3inIn(NO~)~and1~10~4moldm-3 in Na,SO, and then calcined up to 1000°C in the TGA instrument.
r
It
L .““(r
1,“”
l,l”U
,“I
cm-’ Fig. 8. IR absorption spectra of the particles shown in Fig. 6c (a) and of the same sample calcined at 800°C for 4 h (b).
in Fig. 7 (top) which is identified as In (OH), [ 71. The weight loss on calcination to In,O,, as determined by the TGA, was found to be 16.5% which corroborates the composition of the solids before and after the heat treatment. Samples synthesized in the presence of the sulfate ion had a different composition. The XRD pattern (Fig. 7, middle) of particles illustrated in Fig. 6b shows characteristics of a crystalline material, but the spectrum could not be related to any reported indium compounds. On treating the same sample up to 1000’ C, the powder was converted to pure In,O, as revealed in Fig. 7 (bottom ) . The IR spectrum of solids precipitated in the sulfate containing solutions shows clearly presence of this anion (Fig. 8 top, 1080-1130 cm-‘), which peak is absent when the same powder is heated at 800’ C (Fig. 8, bottom). The radiotracer analysis of the above samples indeed confirmed that the sulfate ion was incorporated in the particles in amounts which increased with the concentration of Na,SO,. For example, precipitates obtained in 7~10~” In(N03)3 solutions with 5*10W4and 1*10P4 mol dmm3 Na,SO, contained 1.7 and 1.2% (by wt) of sulfur, respectively. An independent analysis by atomic absorption spectroscopy of powders shown in Figs 6a and b gave 62% (by wt )
291
In, while the amount of In estimated by the weight loss (TGA) was N 66%. This discrepancy may be due to incomplete dehydration of the powder in the course of heating. The analysis of a sample calcined for an extended period of time (4 h) at 800°C gave 82% In, which compares well with the expected composition for In20,. DISCUSSION
This study has shown that colloidal indium compounds of different morphologies of narrow size distribution can be obtained by precipitation from homogeneous solutions. Several observations regarding dispersions containing indium are worth mentioning, especially in comparison with other elements of the same group (Al and Ga ) . Depending on conditions, both the composition and the morphology of finely dispersed indium compounds can be varied. In this respect, the nature of anions plays an essential role. Aging of aqueous indium nitrate solutions at elevated temperatures yielded crystalline indium hydroxide particles, either cubic or prismatic. In comparison, by an analogous procedure, Ga precipitates consisted of a rod-like hydrous oxide [ 81, while aluminum nitrate did not give uniform particles. It was demonstrated on a number of examples that the addition of sulfate ion has a major effect on the properties of the dispersions obtained by precipitation. This anion can be included in the solid phase in a stoichiometrically well defined manner, such as in the case of colloidal alunites [ 9,101, or it may give basic compounds of varying metal to sulfate ratios [ 51. Finally, in some instances, the sulfate ion causes particles to appear in spherical shape, in which this anion is present as a leachable contaminant [ 3,4]. In the case of indium compounds, the sulfate ion does affect the nature of the particles, which differs with its concentration. If the latter is low, distinct crystals are obtained, but at somewhat higher concentrations nearly spherical, internally composite, particles are found. Importantly, the X-ray analysis of all these powders shows peaks indicating crystalline nature of an, as yet not identified, indium compound. On calcination these solids converted to In,O,. It should be noted that aging of aluminum salt solutions resulted in amorphous spherical particles which remained stable as long as the dispersed solids were kept in the mother liquor [ 31. Gallium solutions also gave spherical particles, but these were originally amorphous and showed some crystalline characterization on prolonged aging ( N 18 h) [ 81. In all cited cases, the sulfate ion was present in the precipitates, but could be removed either by dialysis or by calcination. There are many speculations, but only a few analyses of the systems that quantitatively explain the role of the sulfate ion in the precipitation of uniform
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colloidal dispersions. Mostly, it is suggested that this anion promotes polymerization of solute precursors to the solid phase formation. Obviously, such complexes cannot be the same with different metal ions, since the properties of the resulting dispersions differ. The finding that the original spherical particles of indium compounds, containing the sulfate ion, consist of a larger number of small subunits is not unique. A similar observation was made with other metals such as Ce (IV) [ 51 and Sn (IV) [ 111, although in these cases, metal oxides were precipitated rather than the basic sulfates. The chemical analysis of In3+ and SO:- in solutions at the completion of the precipitation process showed that the loss of these species agreed, within the experimental error, with the amount found in the solids after extensive washings. This finding would indicate that the sulfate group is strongly bound in the crystalline solids and cannot be removed by leaching. Such behavior is expected, if particle formation is caused by interaction of relatively simple complexes rather than a continuous polymerization of hydrolyzed species. From the literature data [ 121 solutes of the composition, In3+, In (OH)‘+, and InSO,+ predominate in the presence of sulfate ions at room temperature. Although no speciation diagrams are available for higher temperatures of interest in this work, one may assume that with heating, the hydrolysis is promoted. Since in the solid phase [ In3+ ] / [SO:- ] - 10, it would appear that the nucleation involves InSO,+ and indium hydrolyzed complexes, while further growth is dominated by the latter species. Livage et al. have used a “partial charge model” to interpret the formation of metal (hydrous) oxides in aqueous media [ 131. The model is more difficult to apply when anions other than OH- are involved in the solid phase formation. Following the procedure by Livage, the partial charge was estimated for two reactions as follows: In2(OH),(S04)
(OH,), %Inz(OH),(OH,)~+
+SOi-
(1)
and InZ(OH),(HZS04)
(OHz)2=Inz(OH)6(OHZ)2
(2)
+HZSOd
yieldingvaluesfor6(SO,)=-0.32 (> -2) and6(H,SO,)=-0.18 (
(H,O),+ +In,(OH),(SO,)
(H,O),
+3H30+
(3)
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Further growth of the particles could proceed through the condensation hydrolyzed indium ions with the above formulated nuclei.
of the
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