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Optical inhomogeneities in nickel ion containing silica gels
J O U R N A L OF
ELSEVIER
Journal of Non-Crystalline Solids 194 (1996) 210-212
Letter to the Editor
Optical inhomogeneities in nickel ion containing silica gels S. Roy, D. Kundu, D. Ganguli * Sol-Gel Laboratory, Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullick Road, Calcutta 700 032, India Received 22 May 1995; revised 31 August 1995
Abstract
Lateral variation in the concentration of Ni 2÷ in a nickel-containing, flat silica gel plate, caused by migration of the dopant during drying (60°C), was measured spectrophotometrically as a function of absorption of Ni 2÷ at 393 nm. Compositional variations along individual lines of measurement could be expressed as second order polynomials, and the overall distribution took the form of a shallow basin with a minimum at the central region of the gel plate.
Sol-derived silica gel monoliths have proven to be a convenient host for the doping of cations with a view to examining the optical properties of the latter or, more importantly, fabricating gel or gel-derived glass optics [1-3]. It has, however, been observed that, as a rule, such products are rendered inhomogeneous by the migration of cations toward the surface, even during drying of the monolithic xerogels [4,5]. Various steps have been suggested for stopping such migration [4,5]. However, there has been no systematic study on this migration and the consequent generation of compositional inhomogeneities which are expected. Considering that such a facile generation of a gradient may have practical implications, an attempt has been made here to examine this behaviour of these doped gels. Divalent nickel, having an absorption band at about 393 nm due to the transition 3A2(F)--~ 3T1(P), has been selected as a probe. Monolithic plates (75 mm × 50 mm with 4.17-
* Corresponding author. Tel: +91-33 473 3469. Telefax: +9133 473 0957.
4.33 mm thickness) of silica gel were prepared from alcohol-free (initially) sols with a molar ratio of tetraethyl orthosilicate (TEOS):H 20:HC1 -= 1:14:0.01 with 8.0 equivalent mol% NiO from nickel nitrate. The required amount of nickel nitrate was dissolved in acidified water, which was then added to TEOS under stirring. Initially, a two-phase mixture was obtained which changed into a single-phase sol after evolution of heat and generation of ethyl alcohol [6,7]. After cooling to about 25°C, the sol was filtered and the final pH was adjusted to 3.5 by addition of ammonium hydroxide solution (NH4OH:H20 = 1:50 v / v ) . The sol-gel transition took place in polythene rectangular moulds placed on flat perspex benches made horizontal with a spirit level. After aging in syneresis liquid for 10 days, the gels were dried at 60 + I°C for 7 days. The carefully collected syneresis liquid was analyzed by a wet chemical method [8]. The dried gel plate was cut into fifteen rectangular pieces, according to a sampling grid (Fig. 1). Spectral absorption was recorded for all the pieces on a spectrophotometer (Hitachi 3210 UV-VIS). To obtain a better correlation between absorbance and
0022-3093/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 5 3 2 - 3
S. Roy et a l . / Journal of Non-Crystalline Solids 194 (1996) 210-212
•
4
32 0
I"
I0
II 0
1.5
sured several times within the 4 mm marked outline with slide callipers, and the mean value (standard deviation --- 0.0023-0.0162) was recorded. From the above data, the linear absorption coefficients were calculated using the Lambert-Beer law:
° 2
3
4
9
8
7
12 I 3.0
13 4,5
211
Optical density (OD) --
14 6,0
15 7.5
Length (cm) Fig. 1. Measurement grid showing the observation points corresponding to the gel containing 8.0 equivalent NiO mol%.
thickness data, a circular aperture of 4 mm diameter was placed immediately behind the sample in the sample compartment of the spectrophotometer, so that the beam from the light source entered the sample only through this circular aperture. After recording the optical data, an outline of the aperture was marked in situ on each gel sample. At the end of the measurement, the gel samples were re-assembled and the coordinates of the points of observation (i.e., the centre of the marked circle) on each sample were measured with respect to the same of the central piece (number 8 of Fig. 1) taken as (0,0). For each sample, the thickness was mea-
ad = ecd,
(1)
where a is the linear absorption coefficient (cm-l), d is the thickness (cm), c is the concentration (mol/1) and e is the extinction coefficient (cm-1 mol-1 1). The nickel released via syneresis liquid was subtracted from the total nickel content, and the volume of the dried gel and the concentration of equivalent NiO in m o l / l was evaluated. As the concentration of the Ni ion in the gel was very high, determination of • in this concentration range was essential for obtaining the Ni ion concentration in the gel using the Lambert-Beer law. The value of • was calculated from the spectra of the solutions with the same Ni concentration as that of the gel. The value of a was evaluated for each sample from the recorded value of OD. Substituting the value of e in the obtained value of a , the values of the concentrations of Ni at different points of the gel were evaluated. Table 1 assembles the coordinates ( x , y ) of the central point of measurement for each gel piece, the thickness and the corresponding calculated concentration of Ni 2+. If the concentrations were expressed
Table 1 The value of nickel concentration (tool/l) as a function of the distance from the centre of the gel corresponding to 8.0 equivalent NiO mol% Grid position
Position of the observation points (mm) x y
Concentration of NiO ( m o l / l )
Corresponding thickness (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
- 33.0 - 17.0 0.0 15.0 29.0 27.0 14.0 0.0 - 18.0 - 35.0 - 32.0 - 15.0 0.0 14.0 25.0
1.7468 1.6539 1.5972 1.6143 1.6398 1.6387 1.6044 1.5817 1.6135 1.7275 1.7701 1.6318 1.6078 1.6306 1.6819
4.22 4.26 4.25 4.33 4.27 4.26 4.30 4.18 4.27 4.18 4.17 4.23 4.25 4.30 4.24
-
18.0 20.0 18.0 18.0 14.0 0.0 0.0 0.0 0.0 0.0 15.0 13.0 15.0 15.0 17.0
212
S. Roy et al./ Journal of Non-Crystalline Solids 194 (1996) 210-212
0.5
o
0
d t
Fig. 2. Concentration gradient of equivalent NiO along the x - y plane of the gel body.
as z, the following relationship could be used to define the distribution of Ni 2+ along the grid lines of Fig. 1: Z = a p 2 + b p + c,
(2)
where p = (x,y) and a, b and c are constants. In fact, these equations, giving rise to a series of similar additional equations, define the concentration surface of the original (uncut) dry gel plate. The basin-like surface is shown in Fig. 2. It has been shown theoretically by Scherer [9] that, during drying of a flat plate, evaporation of the pore liquid takes place essentially along the z-axis (i.e., vertically), and not along x-or y-axis. In the present case, a gradient in Ni concentration is, therefore, expected along the z-axis. It was difficult to measure the concentration gradient along the thickness of the plate by the spectrophotometric method. However, Table I and Fig. 2 show that the lateral concentration of Ni 2÷ did decrease progressively from the edges of the plate toward the inside, coming to a minimum in the central region (sample no. 8 in Fig. 1). This slope indicates that the evaporation of liquid taking place from the pores opening laterally also caused a supply of further liquid carrying the hydrated/solvated nickel from the inside, thus de-
pleting the central part of the Ni concentration; the fugitive liquid, carrying the nickel ions towards the lateral surface, deposited them on the pore surface before turning into vapour and escaping the gel body [5]. Note that, in spite of the relatively high concentration, no deposition of nickel salts on the gel surface was noted. The nature of the concentration gradient, with the minima precisely at the central part of the gel (the latter, expectedly, a result of casting on horizontal flat benches), allows us to calculate the concentration of the dopant at any point with some precision if the basic data are known. The shapes of the curves and the resulting concentration basin can be manipulated by changing the size of the gel, drying temperature and time, dopant concentration and, obviously, the dopant type. Such gradient formation can be utilized in obtaining functionally gradient materials by simple sol-gel processing for optical and other applications.
Acknowledgements The authors are thankful to Dr B.K. Sarka, Director, for his kind permission to publish this communication. They also thank the Computer Section of the Institute for generating Fig. 2.
References [1] D.R. Ulrich, J. Non-Cryst. Solids 100 (1988) 174. [2] M. Yamane and M. Inami, J. Non-Cryst. Solids 147&148 (1992) 606. [3] L.C. Klein, ed., Sol-Gel Optics: Processing and Applications (Kluwer, Boston, 1994). [4] I.M. Thomas, S.A. Payne and G.D. Wilke, J. Non-Cryst. Solids 151 (1992) 183. [5] M, Yamane, H. Kawazoe, A. Yasumori and T. Takahashi, J. Non-Cryst. Solids 99 (1988) 160. [6] D. Avnir and V.R. Kaufman, J. Non-Cryst. Solids 92 (1987) 180. [7] S. Roy and D. Ganguli, J. Non-Cryst. Solids 151 (1992) 203. [8] A.I. Vogel, A Textbook of Quantitative Inorganic Analysis, 3rd Ed. (English Language Book Society and Longman, 1960) p. 435. [9] G.W. Scherer, J. Am. Ceram. SOC.73 (1990) 3.
ELSEVIER
Journal of Non-Crystalline Solids 194 (1996) 210-212
Letter to the Editor
Optical inhomogeneities in nickel ion containing silica gels S. Roy, D. Kundu, D. Ganguli * Sol-Gel Laboratory, Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullick Road, Calcutta 700 032, India Received 22 May 1995; revised 31 August 1995
Abstract
Lateral variation in the concentration of Ni 2÷ in a nickel-containing, flat silica gel plate, caused by migration of the dopant during drying (60°C), was measured spectrophotometrically as a function of absorption of Ni 2÷ at 393 nm. Compositional variations along individual lines of measurement could be expressed as second order polynomials, and the overall distribution took the form of a shallow basin with a minimum at the central region of the gel plate.
Sol-derived silica gel monoliths have proven to be a convenient host for the doping of cations with a view to examining the optical properties of the latter or, more importantly, fabricating gel or gel-derived glass optics [1-3]. It has, however, been observed that, as a rule, such products are rendered inhomogeneous by the migration of cations toward the surface, even during drying of the monolithic xerogels [4,5]. Various steps have been suggested for stopping such migration [4,5]. However, there has been no systematic study on this migration and the consequent generation of compositional inhomogeneities which are expected. Considering that such a facile generation of a gradient may have practical implications, an attempt has been made here to examine this behaviour of these doped gels. Divalent nickel, having an absorption band at about 393 nm due to the transition 3A2(F)--~ 3T1(P), has been selected as a probe. Monolithic plates (75 mm × 50 mm with 4.17-
* Corresponding author. Tel: +91-33 473 3469. Telefax: +9133 473 0957.
4.33 mm thickness) of silica gel were prepared from alcohol-free (initially) sols with a molar ratio of tetraethyl orthosilicate (TEOS):H 20:HC1 -= 1:14:0.01 with 8.0 equivalent mol% NiO from nickel nitrate. The required amount of nickel nitrate was dissolved in acidified water, which was then added to TEOS under stirring. Initially, a two-phase mixture was obtained which changed into a single-phase sol after evolution of heat and generation of ethyl alcohol [6,7]. After cooling to about 25°C, the sol was filtered and the final pH was adjusted to 3.5 by addition of ammonium hydroxide solution (NH4OH:H20 = 1:50 v / v ) . The sol-gel transition took place in polythene rectangular moulds placed on flat perspex benches made horizontal with a spirit level. After aging in syneresis liquid for 10 days, the gels were dried at 60 + I°C for 7 days. The carefully collected syneresis liquid was analyzed by a wet chemical method [8]. The dried gel plate was cut into fifteen rectangular pieces, according to a sampling grid (Fig. 1). Spectral absorption was recorded for all the pieces on a spectrophotometer (Hitachi 3210 UV-VIS). To obtain a better correlation between absorbance and
0022-3093/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 5 3 2 - 3
S. Roy et a l . / Journal of Non-Crystalline Solids 194 (1996) 210-212
•
4
32 0
I"
I0
II 0
1.5
sured several times within the 4 mm marked outline with slide callipers, and the mean value (standard deviation --- 0.0023-0.0162) was recorded. From the above data, the linear absorption coefficients were calculated using the Lambert-Beer law:
° 2
3
4
9
8
7
12 I 3.0
13 4,5
211
Optical density (OD) --
14 6,0
15 7.5
Length (cm) Fig. 1. Measurement grid showing the observation points corresponding to the gel containing 8.0 equivalent NiO mol%.
thickness data, a circular aperture of 4 mm diameter was placed immediately behind the sample in the sample compartment of the spectrophotometer, so that the beam from the light source entered the sample only through this circular aperture. After recording the optical data, an outline of the aperture was marked in situ on each gel sample. At the end of the measurement, the gel samples were re-assembled and the coordinates of the points of observation (i.e., the centre of the marked circle) on each sample were measured with respect to the same of the central piece (number 8 of Fig. 1) taken as (0,0). For each sample, the thickness was mea-
ad = ecd,
(1)
where a is the linear absorption coefficient (cm-l), d is the thickness (cm), c is the concentration (mol/1) and e is the extinction coefficient (cm-1 mol-1 1). The nickel released via syneresis liquid was subtracted from the total nickel content, and the volume of the dried gel and the concentration of equivalent NiO in m o l / l was evaluated. As the concentration of the Ni ion in the gel was very high, determination of • in this concentration range was essential for obtaining the Ni ion concentration in the gel using the Lambert-Beer law. The value of • was calculated from the spectra of the solutions with the same Ni concentration as that of the gel. The value of a was evaluated for each sample from the recorded value of OD. Substituting the value of e in the obtained value of a , the values of the concentrations of Ni at different points of the gel were evaluated. Table 1 assembles the coordinates ( x , y ) of the central point of measurement for each gel piece, the thickness and the corresponding calculated concentration of Ni 2+. If the concentrations were expressed
Table 1 The value of nickel concentration (tool/l) as a function of the distance from the centre of the gel corresponding to 8.0 equivalent NiO mol% Grid position
Position of the observation points (mm) x y
Concentration of NiO ( m o l / l )
Corresponding thickness (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
- 33.0 - 17.0 0.0 15.0 29.0 27.0 14.0 0.0 - 18.0 - 35.0 - 32.0 - 15.0 0.0 14.0 25.0
1.7468 1.6539 1.5972 1.6143 1.6398 1.6387 1.6044 1.5817 1.6135 1.7275 1.7701 1.6318 1.6078 1.6306 1.6819
4.22 4.26 4.25 4.33 4.27 4.26 4.30 4.18 4.27 4.18 4.17 4.23 4.25 4.30 4.24
-
18.0 20.0 18.0 18.0 14.0 0.0 0.0 0.0 0.0 0.0 15.0 13.0 15.0 15.0 17.0
212
S. Roy et al./ Journal of Non-Crystalline Solids 194 (1996) 210-212
0.5
o
0
d t
Fig. 2. Concentration gradient of equivalent NiO along the x - y plane of the gel body.
as z, the following relationship could be used to define the distribution of Ni 2+ along the grid lines of Fig. 1: Z = a p 2 + b p + c,
(2)
where p = (x,y) and a, b and c are constants. In fact, these equations, giving rise to a series of similar additional equations, define the concentration surface of the original (uncut) dry gel plate. The basin-like surface is shown in Fig. 2. It has been shown theoretically by Scherer [9] that, during drying of a flat plate, evaporation of the pore liquid takes place essentially along the z-axis (i.e., vertically), and not along x-or y-axis. In the present case, a gradient in Ni concentration is, therefore, expected along the z-axis. It was difficult to measure the concentration gradient along the thickness of the plate by the spectrophotometric method. However, Table I and Fig. 2 show that the lateral concentration of Ni 2÷ did decrease progressively from the edges of the plate toward the inside, coming to a minimum in the central region (sample no. 8 in Fig. 1). This slope indicates that the evaporation of liquid taking place from the pores opening laterally also caused a supply of further liquid carrying the hydrated/solvated nickel from the inside, thus de-
pleting the central part of the Ni concentration; the fugitive liquid, carrying the nickel ions towards the lateral surface, deposited them on the pore surface before turning into vapour and escaping the gel body [5]. Note that, in spite of the relatively high concentration, no deposition of nickel salts on the gel surface was noted. The nature of the concentration gradient, with the minima precisely at the central part of the gel (the latter, expectedly, a result of casting on horizontal flat benches), allows us to calculate the concentration of the dopant at any point with some precision if the basic data are known. The shapes of the curves and the resulting concentration basin can be manipulated by changing the size of the gel, drying temperature and time, dopant concentration and, obviously, the dopant type. Such gradient formation can be utilized in obtaining functionally gradient materials by simple sol-gel processing for optical and other applications.
Acknowledgements The authors are thankful to Dr B.K. Sarka, Director, for his kind permission to publish this communication. They also thank the Computer Section of the Institute for generating Fig. 2.
References [1] D.R. Ulrich, J. Non-Cryst. Solids 100 (1988) 174. [2] M. Yamane and M. Inami, J. Non-Cryst. Solids 147&148 (1992) 606. [3] L.C. Klein, ed., Sol-Gel Optics: Processing and Applications (Kluwer, Boston, 1994). [4] I.M. Thomas, S.A. Payne and G.D. Wilke, J. Non-Cryst. Solids 151 (1992) 183. [5] M, Yamane, H. Kawazoe, A. Yasumori and T. Takahashi, J. Non-Cryst. Solids 99 (1988) 160. [6] D. Avnir and V.R. Kaufman, J. Non-Cryst. Solids 92 (1987) 180. [7] S. Roy and D. Ganguli, J. Non-Cryst. Solids 151 (1992) 203. [8] A.I. Vogel, A Textbook of Quantitative Inorganic Analysis, 3rd Ed. (English Language Book Society and Longman, 1960) p. 435. [9] G.W. Scherer, J. Am. Ceram. SOC.73 (1990) 3.