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Preparation and physical properties of V2O5 aerogel
]OURNA
Journal of Non-Crystalline Solids 147&148 (1992) 386-388 North-Holland
L OF
NON-CRYSTALLSOLIDS INE
Preparation and physical properties of V205 aerogel Kazumi Sudoh and Hiroshi Hirashima Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223, Japan
Monolithic aerogels of hydrated V205 containing 0, 5 and 10 mol% GeO 2 were prepared from metal alkoxides by hydrolysis and supcrcritical drying. The porosity of the aerogels was > 90%. The aerogels consisted of microfibrils. Although temperature was increased to 250°C during drying, higher than the crystallization temperature of V205 xerogels, X-ray diffraction peaks of crystalline vanadium oxides were not observed for the aerogels. The microhardness of the aerogels increased with addition of GeO 2. DC conductivity of aerogels was lower than that of bulk xerogels by one order of magnitude and was comparable with that of the xerogel coatings in the direction perpendicular to the surface.
1. Introduction Hydrated VzO 5 gels are known to be semiconductors with high anisotropic conductivity [1]. They can be applied to conductive thin coatings, switching devices, electrochromic films, electrodes for Li-batteries, catalysts, etc. For some applications, such as catalysts, electrode materials and gas sensors, micro-porous gels may be appropriate because of their extremely high surface area. The structure of micro-porous gels is also interesting. I n this study, V20 5 aerogels were p r e p a r e d from vanadyl ethoxide. Many results for silica and silicate aerogels made by supercritical drying have been reported. However, few papers on aerogels of non-silicate oxides have b e e n published. Silica aerogels usually consist of granular particles. On the other hand, V20 5 gels are known to have a fibrous structure. The authors previously reported preparation of V20 5 gels from vanadyl alkoxides and their microstructure [2,3]. Effects of additive oxides on physical properties of V20 5 gels have been also reported [3,4]. Electrical conductivity decreased with addition of TiO a or G e O 2, but mechanical strength was improved by these additives. In this study, the effects of G e O z additions on the physical properties of V20 5 aerogels are also discussed.
2. Experimental procedure V20 s wet gels containing 0, 5 and 10 mol% G e O 2 were prepared from VO(C2H5O) 3 and Ge(C2HsO)4, 99.9%, supplied by Soekawa Rikagaku Co., Tokyo, by hydrolysis in ethanol solution without catalyst [2]. After aging for 1-3 weeks, the wet gels were supercritically dried using an autoclave, at 255°C and 210 atm for 1 h. Bulk density, volume change and X-ray diffraction of the dried gels were measured. Pore size distribution and specific surface area of the aerogels were measured by Hg-porosimetry and B E T method using N 2, respectively. D C conductivity of the aerogels was measured by the two-terminal method using vacuum-evaporated Au films as electrodes. The microstructure was observed by scanning electron microscopy (SEM). Dynamic microhardness of the aerogels was measured using a trigonal d i a m o n d pyramid indenter (Shimadzu, DUH-200). Bulk xerogels, dried under an atmospheric pressure, were also prepared for comparison.
3. Results During aging, shrinkage was observed for the wet gels. The shrinkage during aging for 1 week
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
K. Sudoh, H. Hirashima / V205 aerogel
387
Fig. 1. SEM photographs of fracture surface of the aerogels. (a) V205 aerogel, (b) 90V205 • 10GeO 2 aerogel.
was 25-30% in volume. Shrinkage during supercritical drying was < 10% in volume. The bulk density of aerogels was about 0.07 g / c m 3. This value is about 4% of the density of V205 xerogels (2.0 g/cm3). The porosity of aerogels was larger than 90%. The specific surface area of the aerogels was 140-200 m2/g. The aerogels were black in color. This suggests that the aerogels contain reduced V ions. The fraction of reduced V ions, [Vt4otal] determined by wet chemical analysis or estimated from the weight gain by heating in air, was 0.2-0.25. These values are nearly the same as those of xerogels prepared from vanadyl alkoxides. The structure of the aerogels (fig. 1), consists of micro-fibrils of less than 10 nm in diameter. '(a )
=r•O= c
i
T
r
% °°- ~ ,- - ~ - ~
The diameter and the length of the micro-fibrils seem to decrease with G e O 2 addition. Although the temperature in an autoclave went up to 255°C, no X-ray diffraction peak of crystalline vanadium oxides was observed for the aerogels (fig. 2). The aerogels exhibited a bimodal pore size distribution; one is < 20 nm in diameter while the other is > 100 nm in diameter (fig. 3). SEM photographs also suggest that there are two types of pores in the aerogels, small pores of the same size as the diameter of micro-fibrils and larger pores (fig. 1). The dynamic microhardness of the aerogels was very low in comparison with ordinary glasses (table 1). The microhardness increased remarkably with G e O 2 addition. The mechanical strength of the aerogels without additives was too low to prepare samples for conductivity measurements. DC conductivity of bulk aerogels was lower than that of bulk xerogels by about one order of
o
~60 F- - I
el
,
,
,
o
0
>~0
I
. 10
.
.
. 30
.
.
(c)l
/o
/
°/°/
a0 .
50
2.,~ldegree __
Fig. 2. X-ray diffraction patterns of V205 gels ( C u K a ) . (a) %1205 xerogel, heat treated at 260°C for 2 h. 'o' - V205 (orthorhombic); (b) V205 aerogel, as dried; (c) 90V205. 10GeO 2 aerogel, as dried.
I
2
3 4 log (pore radius/~, )
5
Fig. 3. Pore size distribution of V205 aerogel. Pore volume: [cumulative volume of pores]/[total pore volume].
K. Sudoh, H. Hirashima / V205 aerogel
388 Table 1 Dynamic microhardness of aerogels Sample
Dynamic microhardness (gf/m 2)
V20 s aerogel 90V2Os. 10GeO 2 aerogel Microscope slide glass Silica glass
3.73 10.99 160 250
i
-I
o
Eu_3
•
i
i
I~
o o
ol
o
0..5_ -5
` o
i
0
i
low in comparison with that of silica aerogels, which has been often reported larger than 500 m 2 / g [5]. This result could be attributed to the difference in microstructure of the aerogels. V205 aerogels consist of micro-fibrils while silica aerogels consist of granular particles. Further study is needed to clarify the reason for the low specific surface area of the V205 aerogels. The dynamic microhardness of the aerogels increased with GeO 2. The addition of GeO 2, which can make a 3-dimensional network structure, may introduce a bridge structure in the layer-like structure of V205 gels, and enable the hardness of the gels to increase. The electrical conductivity of the aerogels was isotropic and low. The low conductivity of the aerogels may be attributed to high porosity.
~ [
5 10 [GeO~l / tool*l,
Fig. 4. DC conductivity of V 2 0 s gels at 25°C. 1, xerogel coating, parallel to the surface; 2, xerogel coating, perpendicular to the surface; 3, bulk xerogel; 4, bulk aerogel.
magnitude (fig. 4), and was near to the value of the xerogel coatings measured in the direction perpendicular to the surface (the low-conductivity direction).
4. Discussion
Several broad and weak diffraction peaks were observed for V205 aerogels (fig. 2(b)). The diffraction pattern was different from that of vanadium oxide such as V205, V30 7 and VO 2. The molar ratio of [H20]/[V205] of the aerogels, kept in a desiccator with silica gel, was about 0.5, nearly the same as that of the xerogels dried at 120°C. The fraction of reduced V ion was nearly the same as that of the xerogels. V205 xerogels crystallized when heated at temperatures higher than 200°C (fig. 2(a)). These results suggest that hydrated V205 gel is stabilized by high pressure during supercritical drying. The porosity of the V205 aerogels was as high as 96%. However, the specific surface area was
5. Conclusions
(1) V205 aerogels were obtained by supercritical drying of wet gels prepared from vanadyl ethoxide. The porosity was > 90%. (2) Microhardness of the aerogels was very low, but increased with the addition of GeO 2. (3) DC conductivity of the bulk aerogels was isotropic and lower than that of the bulk xer0gels by one order of magnitude. The authors are very grateful to Professor M. Yamane, Dr A. Yasumori and Mr M. Iwasaki of Tokyo Institute of Technology for their useful advice and help to prepare aerogels.
References [1] J. Livage, in: Better Ceramics Through Chemistry, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1984) p. 125. [2] H. Hirashima, K. Tsukimi and R. Muratake, SeramikkusuRonbun-Shi 97 (1989) 235. [3] H. Hirashima, S. Kamimura, R. Muratake and T. Yoshida, J. Non-Cryst. Solids 100 (1988) 394. [4] H. Hirashima and K. Sudoh, J. Non-Cryst. Solids 121 (1990) 68. [5] J. Zarzycki and T. Woignier, in: Aerogels, ed. J. Fricke (Springer, Berlin, 1986) p. 42.
Journal of Non-Crystalline Solids 147&148 (1992) 386-388 North-Holland
L OF
NON-CRYSTALLSOLIDS INE
Preparation and physical properties of V205 aerogel Kazumi Sudoh and Hiroshi Hirashima Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223, Japan
Monolithic aerogels of hydrated V205 containing 0, 5 and 10 mol% GeO 2 were prepared from metal alkoxides by hydrolysis and supcrcritical drying. The porosity of the aerogels was > 90%. The aerogels consisted of microfibrils. Although temperature was increased to 250°C during drying, higher than the crystallization temperature of V205 xerogels, X-ray diffraction peaks of crystalline vanadium oxides were not observed for the aerogels. The microhardness of the aerogels increased with addition of GeO 2. DC conductivity of aerogels was lower than that of bulk xerogels by one order of magnitude and was comparable with that of the xerogel coatings in the direction perpendicular to the surface.
1. Introduction Hydrated VzO 5 gels are known to be semiconductors with high anisotropic conductivity [1]. They can be applied to conductive thin coatings, switching devices, electrochromic films, electrodes for Li-batteries, catalysts, etc. For some applications, such as catalysts, electrode materials and gas sensors, micro-porous gels may be appropriate because of their extremely high surface area. The structure of micro-porous gels is also interesting. I n this study, V20 5 aerogels were p r e p a r e d from vanadyl ethoxide. Many results for silica and silicate aerogels made by supercritical drying have been reported. However, few papers on aerogels of non-silicate oxides have b e e n published. Silica aerogels usually consist of granular particles. On the other hand, V20 5 gels are known to have a fibrous structure. The authors previously reported preparation of V20 5 gels from vanadyl alkoxides and their microstructure [2,3]. Effects of additive oxides on physical properties of V20 5 gels have been also reported [3,4]. Electrical conductivity decreased with addition of TiO a or G e O 2, but mechanical strength was improved by these additives. In this study, the effects of G e O z additions on the physical properties of V20 5 aerogels are also discussed.
2. Experimental procedure V20 s wet gels containing 0, 5 and 10 mol% G e O 2 were prepared from VO(C2H5O) 3 and Ge(C2HsO)4, 99.9%, supplied by Soekawa Rikagaku Co., Tokyo, by hydrolysis in ethanol solution without catalyst [2]. After aging for 1-3 weeks, the wet gels were supercritically dried using an autoclave, at 255°C and 210 atm for 1 h. Bulk density, volume change and X-ray diffraction of the dried gels were measured. Pore size distribution and specific surface area of the aerogels were measured by Hg-porosimetry and B E T method using N 2, respectively. D C conductivity of the aerogels was measured by the two-terminal method using vacuum-evaporated Au films as electrodes. The microstructure was observed by scanning electron microscopy (SEM). Dynamic microhardness of the aerogels was measured using a trigonal d i a m o n d pyramid indenter (Shimadzu, DUH-200). Bulk xerogels, dried under an atmospheric pressure, were also prepared for comparison.
3. Results During aging, shrinkage was observed for the wet gels. The shrinkage during aging for 1 week
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
K. Sudoh, H. Hirashima / V205 aerogel
387
Fig. 1. SEM photographs of fracture surface of the aerogels. (a) V205 aerogel, (b) 90V205 • 10GeO 2 aerogel.
was 25-30% in volume. Shrinkage during supercritical drying was < 10% in volume. The bulk density of aerogels was about 0.07 g / c m 3. This value is about 4% of the density of V205 xerogels (2.0 g/cm3). The porosity of aerogels was larger than 90%. The specific surface area of the aerogels was 140-200 m2/g. The aerogels were black in color. This suggests that the aerogels contain reduced V ions. The fraction of reduced V ions, [Vt4otal] determined by wet chemical analysis or estimated from the weight gain by heating in air, was 0.2-0.25. These values are nearly the same as those of xerogels prepared from vanadyl alkoxides. The structure of the aerogels (fig. 1), consists of micro-fibrils of less than 10 nm in diameter. '(a )
=r•O= c
i
T
r
% °°- ~ ,- - ~ - ~
The diameter and the length of the micro-fibrils seem to decrease with G e O 2 addition. Although the temperature in an autoclave went up to 255°C, no X-ray diffraction peak of crystalline vanadium oxides was observed for the aerogels (fig. 2). The aerogels exhibited a bimodal pore size distribution; one is < 20 nm in diameter while the other is > 100 nm in diameter (fig. 3). SEM photographs also suggest that there are two types of pores in the aerogels, small pores of the same size as the diameter of micro-fibrils and larger pores (fig. 1). The dynamic microhardness of the aerogels was very low in comparison with ordinary glasses (table 1). The microhardness increased remarkably with G e O 2 addition. The mechanical strength of the aerogels without additives was too low to prepare samples for conductivity measurements. DC conductivity of bulk aerogels was lower than that of bulk xerogels by about one order of
o
~60 F- - I
el
,
,
,
o
0
>~0
I
. 10
.
.
. 30
.
.
(c)l
/o
/
°/°/
a0 .
50
2.,~ldegree __
Fig. 2. X-ray diffraction patterns of V205 gels ( C u K a ) . (a) %1205 xerogel, heat treated at 260°C for 2 h. 'o' - V205 (orthorhombic); (b) V205 aerogel, as dried; (c) 90V205. 10GeO 2 aerogel, as dried.
I
2
3 4 log (pore radius/~, )
5
Fig. 3. Pore size distribution of V205 aerogel. Pore volume: [cumulative volume of pores]/[total pore volume].
K. Sudoh, H. Hirashima / V205 aerogel
388 Table 1 Dynamic microhardness of aerogels Sample
Dynamic microhardness (gf/m 2)
V20 s aerogel 90V2Os. 10GeO 2 aerogel Microscope slide glass Silica glass
3.73 10.99 160 250
i
-I
o
Eu_3
•
i
i
I~
o o
ol
o
0..5_ -5
` o
i
0
i
low in comparison with that of silica aerogels, which has been often reported larger than 500 m 2 / g [5]. This result could be attributed to the difference in microstructure of the aerogels. V205 aerogels consist of micro-fibrils while silica aerogels consist of granular particles. Further study is needed to clarify the reason for the low specific surface area of the V205 aerogels. The dynamic microhardness of the aerogels increased with GeO 2. The addition of GeO 2, which can make a 3-dimensional network structure, may introduce a bridge structure in the layer-like structure of V205 gels, and enable the hardness of the gels to increase. The electrical conductivity of the aerogels was isotropic and low. The low conductivity of the aerogels may be attributed to high porosity.
~ [
5 10 [GeO~l / tool*l,
Fig. 4. DC conductivity of V 2 0 s gels at 25°C. 1, xerogel coating, parallel to the surface; 2, xerogel coating, perpendicular to the surface; 3, bulk xerogel; 4, bulk aerogel.
magnitude (fig. 4), and was near to the value of the xerogel coatings measured in the direction perpendicular to the surface (the low-conductivity direction).
4. Discussion
Several broad and weak diffraction peaks were observed for V205 aerogels (fig. 2(b)). The diffraction pattern was different from that of vanadium oxide such as V205, V30 7 and VO 2. The molar ratio of [H20]/[V205] of the aerogels, kept in a desiccator with silica gel, was about 0.5, nearly the same as that of the xerogels dried at 120°C. The fraction of reduced V ion was nearly the same as that of the xerogels. V205 xerogels crystallized when heated at temperatures higher than 200°C (fig. 2(a)). These results suggest that hydrated V205 gel is stabilized by high pressure during supercritical drying. The porosity of the V205 aerogels was as high as 96%. However, the specific surface area was
5. Conclusions
(1) V205 aerogels were obtained by supercritical drying of wet gels prepared from vanadyl ethoxide. The porosity was > 90%. (2) Microhardness of the aerogels was very low, but increased with the addition of GeO 2. (3) DC conductivity of the bulk aerogels was isotropic and lower than that of the bulk xer0gels by one order of magnitude. The authors are very grateful to Professor M. Yamane, Dr A. Yasumori and Mr M. Iwasaki of Tokyo Institute of Technology for their useful advice and help to prepare aerogels.
References [1] J. Livage, in: Better Ceramics Through Chemistry, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1984) p. 125. [2] H. Hirashima, K. Tsukimi and R. Muratake, SeramikkusuRonbun-Shi 97 (1989) 235. [3] H. Hirashima, S. Kamimura, R. Muratake and T. Yoshida, J. Non-Cryst. Solids 100 (1988) 394. [4] H. Hirashima and K. Sudoh, J. Non-Cryst. Solids 121 (1990) 68. [5] J. Zarzycki and T. Woignier, in: Aerogels, ed. J. Fricke (Springer, Berlin, 1986) p. 42.