- Email: [email protected]
Single crystal growth and photoelectrochemical study of copper tungstate
ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) e651–e656 www.elsevier.com/locate/jcrysgro
Single crystal growth and photoelectrochemical study of copper tungstate S.K. Arora, Thomas Mathew, Bhupendra Chudasama, Anjana Kothari Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120 Gujarat, India Available online 15 December 2004
Abstract Single crystals of copper tungstate have been grown by employing double-decomposition chemical reaction between cupric chloride and sodium tungstate taken in the ratio 3:1 at 1000 1C, followed by slow and controlled/programmed cooling up to 600 1C. The kinetics of crystallization have been investigated at 800 and 850 1C by arresting the growth at different intervals from 1 to 16 h. The diffusion controlled character of crystallization predominates up to an extent of 61–62%. As an application, the grown crystals have been annealed and then employed as a photoanode and so copper tungstate crystal has been used to decompose water electrolytically. A higher EMF of 30 mV has been obtained. A three-electrode system containing the semiconducting photoanode and a calomel electrode placed in H2 SO4 =KCl electrolyte of pH 1.5 has been fabricated. r 2004 Elsevier B.V. All rights reserved. PACS: 82.50.Nd; 81.10.Dn; 72.10.Bg Keywords: A1. Characterization; A2. Single-crystal growth; B1. Oxides; B1. Tungstates; B2. Nonlinear optic materials
1. Introduction Copper tungstate belongs to a family of structurally related divalent transition metal tungstates and has received particular prominence because of its potential technological significance in applications such as scintillation detectors, laser host, photoanodes, optical fibers, etc [1–4]. With these applications in mind, a wide range of studies Corresponding author. Fax: 91 2692 236475.
E-mail address: [email protected] (S.K. Arora).
such as crystal growth [5,6], optical, electrical and electrochemical characteristics [7–10], hydrogen reduction [11], photoelectrochemical investigations [12], magnetic excitations and short-range order [13–17], ferroelastic phase transitions [18,19] and solid-phase synthesis [20] on this material have been made. One can find in the literature some studies, in addition to crystal growth of CuWO4 [5,6], made with regard to the behavior of semiconducting electrodes for photoelectrolysis of water in terms of physical properties of the semiconductor [21] where photocatalysts are em-
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.046
ARTICLE IN PRESS e652
S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
ployed to harvest the light energy and induce the hydrogen and oxygen redox reactions [22]. We thought it worthwhile to report the growth of larger and well-shaped single crystals of copper tungstate and study their kinetics of crystallization. Further, a lot of work is reported [23–28] to have been carried out on photoelectrolysis, an effort by Benko et al. [29] concerns photoelectrolysis using polycrystalline indium-doped copper tungstate. However, there is no report with single crystalline CuWO4 samples and hence this also finds place in the paper.
2. Experimental and results 2.1. Crystal growth The growth of large single crystals of CuWO4 ; modifying the procedure of double decomposition reaction [6] has been carried out. This is, in principle, a flux reaction technique. The weighed quantities of CuCl2 (8.79 g) and Na2 WO4 (26.39 g) taken in the ratio of 3:1, rather than their stoichiometric combination were mixed thoroughly in pestle and mortar and loaded into a 100 ml platinum crucible with loosely fitting lid. The charge was heated in a horizontal tube furnace kept at 1000 1C. After 2 h, when the reaction was complete, it was fast cooled at a rate of 100 1C/h down to 820 1C. Later, slow cooling at a uniform rate of 5 1C/h was carried out down to 600 1C, after which the furnace power was shut off. The involved chemical reaction CuCl2 þ Na2 WO4 ! CuWO4 # þ2NaCl is essentially not complicated by the formation of binary or ternary compounds. The quantity of Na2 WO4 ; taken initially in excess of the stoichiometric composition, along with the by-product NaCl serves as a high-temperature flux system for single crystal growth of CuWO4 : The resulting CuWO4 : NaCl: Na2 WO4 system, during the slow cooling process, gives rise to the supersaturation needed to allow slow precipitation of CuWO4 ; which is the only stable solid insoluble phase under the described growth conditions. The grown crystals are displayed in Fig. 1. An increase in
Fig. 1. Some typical as-grown single crystals of CuWO4 (scale cm).
size (more than 2 cm in the present case) compared to the previous report [6] is noteworthy. 2.2. Characterization and crystallization kinetics The EDAX spectra of grown crystals gave prominent Cu and W peaks. Their XRD analysis allowed crystalline habit to be axinite-type triclinic, and the band gap as determined was 3.53 eV. The data on band location, band bending, electron affinity, PEC solar cell, etc. are already available [9,12]. The kinetics of CuWO4 crystallization have been investigated employing isothermal evaporation of the charge CuWO4 : NaCl: Na2 WO4 at 800 and 850 1C, arresting the growth at different intervals varying from 1 to 16 h. The average crystal length (along macroaxis, b) and the width (along microaxis, a) were measured with the help of a traveling microscope (least count 0.001 cm) fitted with a filar eyepiece. The graphical plots of ‘t (cm) and wt (cm) versus t (h) at the two constant temperatures are shown in Fig. 2. The results shown are an average of the number of crystals obtained in three growth runs. One observes that (i) the crystal size increases regularly with increasing crystallization time, (ii) the higher the growth temperature the larger is the growth rate, and (iii) beyond 16 h of isothermal evaporation, the crystal
ARTICLE IN PRESS e653
2
0.8
1.5
0.6
1
0.4
0.5
0.2
0
wt (cm)
lt (cm)
S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
0 0
5
10 t (h)
15
20
Fig. 2. Average crystal length ‘t and width wt with time (; m — 800 1C; ; D — 850 1C).
growth is found far too slowed down to produce a measurable increase in the crystal size above the values, ‘fin and wfin : Since ‘fin (or wt ) is a function of growth time, it is worthwhile to study the degree of crystallization a (i.e. at‘ or atw ), which is measured by the ratio of substance crystallized to the amount able to crystallize, and is given in terms of macrodimensions [30] as at‘ ¼
‘3t ; ‘3fin
atw ¼
w3t ; w3fin
(1)
where ‘t ; wt and ‘fin ; wfin are the instantaneous and the final crystal lengths and widths, respectively. This relationship at 800 and 850 1C reveals (graphs not shown) that at‘ ; for example, as of course atw also, increases monotonically with time, reaching much greater than 0.8 after 15 h of crystallization. The rate of growth of the longer side of the crystal face at any time can be expressed by the relation [30] d‘ 2D‘ DC 0 ð1 aÞ ¼ cm s1 ; dt ‘t
(2)
where D‘ is the rate constant for diffusioncontrolled growth of the longer side of crystal, and DC t ¼ DC 0 ð1 aÞ: A typical solution of Eq. (2) for all at values, as derived by Nielson [31] is Z a a1=3 ð1 aÞ1 da (3) ID ¼ 0
"
# 12D‘ DC 0 ¼ t ‘2t
(4)
Here, I D ðaÞ is to be regarded as the ‘dimensionless time’, called ‘chronomal’ in the literature [31]. This is a characteristic parameter for the diffusioncontrolled growth processes. The significance of I D ðaÞ is that if the size of a particle is known at a certain instant of time, then one can calculate back at constant concentration the time at which it started with zero size. The diffusion chronomel, I D ðat‘ Þ; corresponding to the macroaxis b would be ! 12D‘ DC 0 I D ðat‘ Þ ¼ t: (5) ‘2fin The values of I D ðat‘ Þ obtained with the help of Nielson’s table [31] and plotted against time (graphs not shown), show linear increase of I D ðat‘ ) upto 0.58 for 800 1C and 0.64 for 850 1C, respectively, and beyond that I D ðat‘ ) increases too rapidly with time. Likewise, the rate of diffusioncontrolled growth of the shorter side (along the microaxis, a) i.e., the width wt of the crystal can be expressed as 12Dw DC 0 I D ðatw Þ ¼ t: (6) w2fin The values of I D ðatw ) versus tðhÞ as obtained, exhibit linear increase upto 0.58 for 800 1C and 0.67 for 850 1C, respectively, beyond which I D ðatw ) is observed to increase too fast. Further, the experimental curves of a2=3 and a1=3 versus t (h) have been plotted as shown in Fig. 3(a,b), 2=3 2=3 indicating noticeable increase of both at‘ and atw with time, which is typical of a diffusion-controlled
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
e654
2
0.8
1.6
0.6
1.2
0.4
0.8
0.2
0.4
0
(αtw)2/3
(αtl)2/3
1
0 0
4
8
12
16
(αtl)1/3
1
1.6
0.8
1.2
0.6
0.8
0.4
0.4
0.2
0 0
2
4
6
8 t (h)
(b) 2=3
(αtw)1/3
t (h)
(a)
2=3
10
12
14
16 1=3
1=3
Fig. 3. (a) Variation of at‘ and atw versus time. (; m — 800 1C; ; D — 850 1C), (b) Variation of at‘ and atw versus time. (; m — 800 1C; ; D — 850 1C).
mechanism of crystal growth [31]. The fact that 1=3 at‘ values increase rather relatively slowly with time up to 4 h of crystallization, and later increasing almost linearly throughout, implies the presence of polynuclear layer mechanism of crystal growth. We are inclined to infer that diffusion-controlled character of crystallization predominates up to an extent of 61–62%, while the remaining 38–39% of growth may, however, be surface reaction rate controlled, approximated by the relation ‘3t ¼ ‘30 þ Rk ðt t0 Þ;
w3t ¼ w30 þ Rk ðt t0 Þ;
(7)
where Rk is the surface reaction rate constant, obviously different from K D : The remaining 38–39% of growth that would occur by surface reaction could not be done, because it was not possible to measure the surface energy of the reacting solid interfaces. 2.3. Photoelectrolysis of water The ability of a semiconductor photoelectrode to drive a water-splitting reaction is determined by its band gap and the position of valence and
conduction band edges relative to water redox reactions. Solar energy cannot be utilized for production of hydrogen by the direct decomposition of water because this involves a threshold of about 6.5 eV. Fujishima and Honda [32] have shown that the threshold can be greatly reduced if the decomposition is accomplished by photoelectrolysis using a catalyst semiconductor. The grown single crystals of CuWO4 have been found suitable for the purpose. A meticulously chosen flat (devoid of microtopographical features) crystal was first annealed in argon atmosphere at 800 1C for 80 h. Its surface was connected to a copper wire of gauge 33 SWG using silver paste and it was mounted on a clean glass plate by applying clear epoxy resin along the bounding edges, so as to insulate the back crystal surface as well as the connecting wire from the electrolyte. Immersed in an aqueous electrolyte composing 0.5 M KCl and 0.1 N H2 SO4 and having pH 1.5, the crystal was illuminated by an incandescent 6 V 15 W filament lamp. Thus a cell has been fabricated for photoelectrolysis of water (Fig. 4). In an unbiased cell which refers to an ideal situation, where the energy level EðHþ =H2 Þ coincides with Fermi level E F ;
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
Fig. 4. A biased photoelectrolysis cell: (1) CuWO4 crystal anode, (2) platinum counter electrode, (3) cell compartment, (4) aqueous electrolyte (0.1 N H2 SO4 and 0.5 M KCl), (5) glass encapsulation, (6) jackets for collecting oxygen and hydrogen, (7) saturated calomel electrode.
there was no gas evolution. Therefore, one needs to shift higher the Fermi level in metal, for effective electron transfer to occur. This is accomplished in practice by adding an external bias that brings about an efficient electron–hole separation, entailing greater cell efficiency. The semiconductor CuWO4 remains chemically inert during the reaction, serving only to interact with light to produce holes and electrons. Thus one writes E g ¼ ðE c E v Þ4EH2 O=H2 EH2 O=O2 ¼ 1:23 eV: (8) Since the Gibb’s free energy is only 1.23 eV, the total cell should be able to supply additional energy to surmount the ohmic losses, overpotentials of electrodes, etc. It was noticed that at 2.13 V SCE threshold, the oxygen evolution started, and the rate of evolution was observed to increase with increasing external bias. The discharge current, J (mA cm2 ) was found to decrease with time and to saturate at around 60 s, irrespective of the magnitude of external bias. As an alternative method to shift higher the Fermi level, a concentration cell (Fig. 5) has been developed. Here the electromotive force
e655
Fig. 5. A photoelectrolysis cell using concentration gradient for self-biasing: (1) CuWO4 crystal anode, (2) platinum counter electrode, (3) cell compartment, (4) cathodelyte (KCl), (5) anodelyte (H2 SO4 ), (6) saturated calomel electrode, (7) glass stopper, (8) jackets for collecting hydrogen and oxygen.
of the cell can be enhanced by making more positive the potential for hydrogen evolution at the cathode and by making more negative the potential for oxygen evolution at the anode. The former can be attained by increasing the proton concentration and the latter by increasing the pH value of the electrolyte. Thus, the use of KCl solution in the CuWO4 electrode compartment and H2 SO4 in the platinum cathode compartment brings about higher e.m.f. of the photocell. Consequently, we employ a two-compartment unit with strong acidic anodelyte (0.1 N H2 SO4 ; pH ¼ 1:6) and a strong basic cathodelyte (0.5 M KCl, pH ¼ 5:7), separated by a glass stopper, thereby giving effective appearance of a U-tube apparatus. The concentration gradient serves as the self bias. The results of this cell obtained by adjusting the external resistance are presented in Fig. 6. One can notice that the maximum efficiency is obtained by a combination of 0.1 N H2 SO4 and 0.5 M KCl (curve a), the open circuit voltage being 30 mV with reference to saturated calomel electrode. In other cases (curves b, c, d and e), with greater concentration of both H2 SO4 and KCl, the increase in efficiency is lesser. Besides, the concentration cell is inappropriate for long-term use, since the pH difference between the two compartments decreases as the neutralization of the solutions proceeds with photoelectrolysis.
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
e656
Cell voltage mV (VSCE)
35 30 25 a
20
b
15 d
10 5
c
e
0 0
1
2
3
4
5
6
7
Discharge current J (µAcm-2) Fig. 6. Variation of cell voltage with discharge current with CuWO4 anode compartment and Platinum cathode compartment: (a) 0.5 M KCl, 0.1 N H2 SO4 ; (b) 0.5 M KCl, 0.5 N H2 SO4 ; (c) 0.5 M KCl, 1.0 N H2 SO4 ; (d) 1.0 M KCl, 0.5 N H2 SO4 ; (e) 1.5 M KCl, 0.5 N H2 SO4 :
3. Conclusion Single crystal growth of CuWO4 via flux reaction technique is a diffusion-controlled process. The grown crystal works as a suitable photoanode to decompose water electrolytically. The cell voltage and the discharge current (under illumination) are functions of the composition of the cathodic and the anodic electrolyte solutions. References [1] R.H. Gillette, Rev. Sci. Instrum. 21 (1950) 294. [2] M. Peter, Phys. Rev. 113 (1959) 801. [3] L.G. VanUitert, S.T. Preziosi, J. Appl. Phys. 33 (1962) 2908. [4] R.P. Rastogi, B.L. Dubey, I. Das, J. Sci. Ind. Res. 37 (1978) 172. [5] L.G. VanUitert, R.B. Soden, J. Appl. Phys. 31 (1964) 328. [6] S.K. Arora, Thomas Mathew, N.M. Batra, J. Crystal Growth 88 (1988) 379. [7] S.K. Arora, Thomas Mathew, N.M. Batra, J. Phys. Chem. Solids 50 (1989) 665. [8] S.K. Arora, T. Mathew, Phys. Stat. Solidi (a) 116 (1989) 405. [9] S.K. Arora, Thomas Mathew, N.M. Batra, J. Phys. D 23 (1990) 460. [10] T. Mathew, N.M. Batra, S.K. Arora, J. Mater. Sci. 27 (1992) 4003. [11] A.K. Basu, F.R. Sale, J. Mater. Sci. 13 (1979) 91. [12] J.P. Doumerc, J. Hejtmanek, J.P. Chaminade, M. Pouchard, M. Krussanova, Phys. Stat. Solidi (a) 82 (1984) 285.
[13] A.G. Anders, A.I. Zvyagin, Sov. Phys. JETP 36 (1973) 115. [14] A.I. Zvyagin, A.G. Anders, Sov. Phys. JETP 67 (1974) 309. [15] B. Lake, D.A. Tennant, R.A. Cowley, J.D. Axe, J. Phys.: Condens. Matter 8 (1996) 8613. [16] B. Lake, R.A. Cowley, D.A. Tennant, J. Phys.: Condens. Matter 9 (1997) 10951. [17] B. Lake, D.A. Tennant, Physica B 234-6 (1997) 557. [18] P.F. Schofield, S.A.T. Redfern, J. Phys.: Condens. Matter 4 (1992) 375. [19] Simon A.T. Redfern, Phys. Rev. B 48 (1993) 5761. [20] L.P. Dorfman, D.L.Houck, M.J. Scheithauer, J.N. Dann, H.O. Fassett, J. Mater. Res.—Report 2001, Materials Research Society, USA. [21] M.A. Butler, J. Appl. Phys. 48 (1997) 1914. [22] Yu. V. Pleskov, Photoelectrochemical Solar Energy Conversion, Moscow, Khimiya, 1990, p. 176. [23] A. Fujishima, K. Honda, S. Kikuelu, J. Chem. Soc. Japan 72 (1969) 108. [24] J.G. Mavroides, D.I. Techernev, J.A. Kafalas, D.F. Kolesar, Mater. Res. Bull. 10 (1975) 1023. [25] D.S. Ginley, M.A. Butler, J. Appl. Phys. 48 (1977) 2019. [26] A. Shamsi, W.E. Wallace, Mater. Res. Bull. 18 (1983) 389. [27] R.F. Alimova, L.T. Gottsova, V.A. Garkusheva, L.V. Kravchenko, L.B. Maksimaldo, M.P. Elkamin, V.A. Goltsov, V.K. Popov, Int. J. Hydrog. Energ. 13 (1989) 111. [28] P. Maruthamuthu, M. Ashokkumar, Int. J. Hydrog. Energ. 14 (1989) 275. [29] F.A. Benko, C.L. Maclanrin, F.P. Koffybeng, Mater. Res. Bull (USA) 17 (1982) 133. [30] B.N. Roy, A. Packter, J. Crystal Growth 49 (1980) 368. [31] A.E. Nielson, Kinetics of Precipitation, Pergamon Press, Oxford, 1994 (Chapter 3 and 4). [32] A. Fujishima, K. Honda, Nature 238 (1972) 37.
Journal of Crystal Growth 275 (2005) e651–e656 www.elsevier.com/locate/jcrysgro
Single crystal growth and photoelectrochemical study of copper tungstate S.K. Arora, Thomas Mathew, Bhupendra Chudasama, Anjana Kothari Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388120 Gujarat, India Available online 15 December 2004
Abstract Single crystals of copper tungstate have been grown by employing double-decomposition chemical reaction between cupric chloride and sodium tungstate taken in the ratio 3:1 at 1000 1C, followed by slow and controlled/programmed cooling up to 600 1C. The kinetics of crystallization have been investigated at 800 and 850 1C by arresting the growth at different intervals from 1 to 16 h. The diffusion controlled character of crystallization predominates up to an extent of 61–62%. As an application, the grown crystals have been annealed and then employed as a photoanode and so copper tungstate crystal has been used to decompose water electrolytically. A higher EMF of 30 mV has been obtained. A three-electrode system containing the semiconducting photoanode and a calomel electrode placed in H2 SO4 =KCl electrolyte of pH 1.5 has been fabricated. r 2004 Elsevier B.V. All rights reserved. PACS: 82.50.Nd; 81.10.Dn; 72.10.Bg Keywords: A1. Characterization; A2. Single-crystal growth; B1. Oxides; B1. Tungstates; B2. Nonlinear optic materials
1. Introduction Copper tungstate belongs to a family of structurally related divalent transition metal tungstates and has received particular prominence because of its potential technological significance in applications such as scintillation detectors, laser host, photoanodes, optical fibers, etc [1–4]. With these applications in mind, a wide range of studies Corresponding author. Fax: 91 2692 236475.
E-mail address: [email protected] (S.K. Arora).
such as crystal growth [5,6], optical, electrical and electrochemical characteristics [7–10], hydrogen reduction [11], photoelectrochemical investigations [12], magnetic excitations and short-range order [13–17], ferroelastic phase transitions [18,19] and solid-phase synthesis [20] on this material have been made. One can find in the literature some studies, in addition to crystal growth of CuWO4 [5,6], made with regard to the behavior of semiconducting electrodes for photoelectrolysis of water in terms of physical properties of the semiconductor [21] where photocatalysts are em-
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.046
ARTICLE IN PRESS e652
S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
ployed to harvest the light energy and induce the hydrogen and oxygen redox reactions [22]. We thought it worthwhile to report the growth of larger and well-shaped single crystals of copper tungstate and study their kinetics of crystallization. Further, a lot of work is reported [23–28] to have been carried out on photoelectrolysis, an effort by Benko et al. [29] concerns photoelectrolysis using polycrystalline indium-doped copper tungstate. However, there is no report with single crystalline CuWO4 samples and hence this also finds place in the paper.
2. Experimental and results 2.1. Crystal growth The growth of large single crystals of CuWO4 ; modifying the procedure of double decomposition reaction [6] has been carried out. This is, in principle, a flux reaction technique. The weighed quantities of CuCl2 (8.79 g) and Na2 WO4 (26.39 g) taken in the ratio of 3:1, rather than their stoichiometric combination were mixed thoroughly in pestle and mortar and loaded into a 100 ml platinum crucible with loosely fitting lid. The charge was heated in a horizontal tube furnace kept at 1000 1C. After 2 h, when the reaction was complete, it was fast cooled at a rate of 100 1C/h down to 820 1C. Later, slow cooling at a uniform rate of 5 1C/h was carried out down to 600 1C, after which the furnace power was shut off. The involved chemical reaction CuCl2 þ Na2 WO4 ! CuWO4 # þ2NaCl is essentially not complicated by the formation of binary or ternary compounds. The quantity of Na2 WO4 ; taken initially in excess of the stoichiometric composition, along with the by-product NaCl serves as a high-temperature flux system for single crystal growth of CuWO4 : The resulting CuWO4 : NaCl: Na2 WO4 system, during the slow cooling process, gives rise to the supersaturation needed to allow slow precipitation of CuWO4 ; which is the only stable solid insoluble phase under the described growth conditions. The grown crystals are displayed in Fig. 1. An increase in
Fig. 1. Some typical as-grown single crystals of CuWO4 (scale cm).
size (more than 2 cm in the present case) compared to the previous report [6] is noteworthy. 2.2. Characterization and crystallization kinetics The EDAX spectra of grown crystals gave prominent Cu and W peaks. Their XRD analysis allowed crystalline habit to be axinite-type triclinic, and the band gap as determined was 3.53 eV. The data on band location, band bending, electron affinity, PEC solar cell, etc. are already available [9,12]. The kinetics of CuWO4 crystallization have been investigated employing isothermal evaporation of the charge CuWO4 : NaCl: Na2 WO4 at 800 and 850 1C, arresting the growth at different intervals varying from 1 to 16 h. The average crystal length (along macroaxis, b) and the width (along microaxis, a) were measured with the help of a traveling microscope (least count 0.001 cm) fitted with a filar eyepiece. The graphical plots of ‘t (cm) and wt (cm) versus t (h) at the two constant temperatures are shown in Fig. 2. The results shown are an average of the number of crystals obtained in three growth runs. One observes that (i) the crystal size increases regularly with increasing crystallization time, (ii) the higher the growth temperature the larger is the growth rate, and (iii) beyond 16 h of isothermal evaporation, the crystal
ARTICLE IN PRESS e653
2
0.8
1.5
0.6
1
0.4
0.5
0.2
0
wt (cm)
lt (cm)
S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
0 0
5
10 t (h)
15
20
Fig. 2. Average crystal length ‘t and width wt with time (; m — 800 1C; ; D — 850 1C).
growth is found far too slowed down to produce a measurable increase in the crystal size above the values, ‘fin and wfin : Since ‘fin (or wt ) is a function of growth time, it is worthwhile to study the degree of crystallization a (i.e. at‘ or atw ), which is measured by the ratio of substance crystallized to the amount able to crystallize, and is given in terms of macrodimensions [30] as at‘ ¼
‘3t ; ‘3fin
atw ¼
w3t ; w3fin
(1)
where ‘t ; wt and ‘fin ; wfin are the instantaneous and the final crystal lengths and widths, respectively. This relationship at 800 and 850 1C reveals (graphs not shown) that at‘ ; for example, as of course atw also, increases monotonically with time, reaching much greater than 0.8 after 15 h of crystallization. The rate of growth of the longer side of the crystal face at any time can be expressed by the relation [30] d‘ 2D‘ DC 0 ð1 aÞ ¼ cm s1 ; dt ‘t
(2)
where D‘ is the rate constant for diffusioncontrolled growth of the longer side of crystal, and DC t ¼ DC 0 ð1 aÞ: A typical solution of Eq. (2) for all at values, as derived by Nielson [31] is Z a a1=3 ð1 aÞ1 da (3) ID ¼ 0
"
# 12D‘ DC 0 ¼ t ‘2t
(4)
Here, I D ðaÞ is to be regarded as the ‘dimensionless time’, called ‘chronomal’ in the literature [31]. This is a characteristic parameter for the diffusioncontrolled growth processes. The significance of I D ðaÞ is that if the size of a particle is known at a certain instant of time, then one can calculate back at constant concentration the time at which it started with zero size. The diffusion chronomel, I D ðat‘ Þ; corresponding to the macroaxis b would be ! 12D‘ DC 0 I D ðat‘ Þ ¼ t: (5) ‘2fin The values of I D ðat‘ Þ obtained with the help of Nielson’s table [31] and plotted against time (graphs not shown), show linear increase of I D ðat‘ ) upto 0.58 for 800 1C and 0.64 for 850 1C, respectively, and beyond that I D ðat‘ ) increases too rapidly with time. Likewise, the rate of diffusioncontrolled growth of the shorter side (along the microaxis, a) i.e., the width wt of the crystal can be expressed as 12Dw DC 0 I D ðatw Þ ¼ t: (6) w2fin The values of I D ðatw ) versus tðhÞ as obtained, exhibit linear increase upto 0.58 for 800 1C and 0.67 for 850 1C, respectively, beyond which I D ðatw ) is observed to increase too fast. Further, the experimental curves of a2=3 and a1=3 versus t (h) have been plotted as shown in Fig. 3(a,b), 2=3 2=3 indicating noticeable increase of both at‘ and atw with time, which is typical of a diffusion-controlled
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
e654
2
0.8
1.6
0.6
1.2
0.4
0.8
0.2
0.4
0
(αtw)2/3
(αtl)2/3
1
0 0
4
8
12
16
(αtl)1/3
1
1.6
0.8
1.2
0.6
0.8
0.4
0.4
0.2
0 0
2
4
6
8 t (h)
(b) 2=3
(αtw)1/3
t (h)
(a)
2=3
10
12
14
16 1=3
1=3
Fig. 3. (a) Variation of at‘ and atw versus time. (; m — 800 1C; ; D — 850 1C), (b) Variation of at‘ and atw versus time. (; m — 800 1C; ; D — 850 1C).
mechanism of crystal growth [31]. The fact that 1=3 at‘ values increase rather relatively slowly with time up to 4 h of crystallization, and later increasing almost linearly throughout, implies the presence of polynuclear layer mechanism of crystal growth. We are inclined to infer that diffusion-controlled character of crystallization predominates up to an extent of 61–62%, while the remaining 38–39% of growth may, however, be surface reaction rate controlled, approximated by the relation ‘3t ¼ ‘30 þ Rk ðt t0 Þ;
w3t ¼ w30 þ Rk ðt t0 Þ;
(7)
where Rk is the surface reaction rate constant, obviously different from K D : The remaining 38–39% of growth that would occur by surface reaction could not be done, because it was not possible to measure the surface energy of the reacting solid interfaces. 2.3. Photoelectrolysis of water The ability of a semiconductor photoelectrode to drive a water-splitting reaction is determined by its band gap and the position of valence and
conduction band edges relative to water redox reactions. Solar energy cannot be utilized for production of hydrogen by the direct decomposition of water because this involves a threshold of about 6.5 eV. Fujishima and Honda [32] have shown that the threshold can be greatly reduced if the decomposition is accomplished by photoelectrolysis using a catalyst semiconductor. The grown single crystals of CuWO4 have been found suitable for the purpose. A meticulously chosen flat (devoid of microtopographical features) crystal was first annealed in argon atmosphere at 800 1C for 80 h. Its surface was connected to a copper wire of gauge 33 SWG using silver paste and it was mounted on a clean glass plate by applying clear epoxy resin along the bounding edges, so as to insulate the back crystal surface as well as the connecting wire from the electrolyte. Immersed in an aqueous electrolyte composing 0.5 M KCl and 0.1 N H2 SO4 and having pH 1.5, the crystal was illuminated by an incandescent 6 V 15 W filament lamp. Thus a cell has been fabricated for photoelectrolysis of water (Fig. 4). In an unbiased cell which refers to an ideal situation, where the energy level EðHþ =H2 Þ coincides with Fermi level E F ;
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
Fig. 4. A biased photoelectrolysis cell: (1) CuWO4 crystal anode, (2) platinum counter electrode, (3) cell compartment, (4) aqueous electrolyte (0.1 N H2 SO4 and 0.5 M KCl), (5) glass encapsulation, (6) jackets for collecting oxygen and hydrogen, (7) saturated calomel electrode.
there was no gas evolution. Therefore, one needs to shift higher the Fermi level in metal, for effective electron transfer to occur. This is accomplished in practice by adding an external bias that brings about an efficient electron–hole separation, entailing greater cell efficiency. The semiconductor CuWO4 remains chemically inert during the reaction, serving only to interact with light to produce holes and electrons. Thus one writes E g ¼ ðE c E v Þ4EH2 O=H2 EH2 O=O2 ¼ 1:23 eV: (8) Since the Gibb’s free energy is only 1.23 eV, the total cell should be able to supply additional energy to surmount the ohmic losses, overpotentials of electrodes, etc. It was noticed that at 2.13 V SCE threshold, the oxygen evolution started, and the rate of evolution was observed to increase with increasing external bias. The discharge current, J (mA cm2 ) was found to decrease with time and to saturate at around 60 s, irrespective of the magnitude of external bias. As an alternative method to shift higher the Fermi level, a concentration cell (Fig. 5) has been developed. Here the electromotive force
e655
Fig. 5. A photoelectrolysis cell using concentration gradient for self-biasing: (1) CuWO4 crystal anode, (2) platinum counter electrode, (3) cell compartment, (4) cathodelyte (KCl), (5) anodelyte (H2 SO4 ), (6) saturated calomel electrode, (7) glass stopper, (8) jackets for collecting hydrogen and oxygen.
of the cell can be enhanced by making more positive the potential for hydrogen evolution at the cathode and by making more negative the potential for oxygen evolution at the anode. The former can be attained by increasing the proton concentration and the latter by increasing the pH value of the electrolyte. Thus, the use of KCl solution in the CuWO4 electrode compartment and H2 SO4 in the platinum cathode compartment brings about higher e.m.f. of the photocell. Consequently, we employ a two-compartment unit with strong acidic anodelyte (0.1 N H2 SO4 ; pH ¼ 1:6) and a strong basic cathodelyte (0.5 M KCl, pH ¼ 5:7), separated by a glass stopper, thereby giving effective appearance of a U-tube apparatus. The concentration gradient serves as the self bias. The results of this cell obtained by adjusting the external resistance are presented in Fig. 6. One can notice that the maximum efficiency is obtained by a combination of 0.1 N H2 SO4 and 0.5 M KCl (curve a), the open circuit voltage being 30 mV with reference to saturated calomel electrode. In other cases (curves b, c, d and e), with greater concentration of both H2 SO4 and KCl, the increase in efficiency is lesser. Besides, the concentration cell is inappropriate for long-term use, since the pH difference between the two compartments decreases as the neutralization of the solutions proceeds with photoelectrolysis.
ARTICLE IN PRESS S.K. Arora et al. / Journal of Crystal Growth 275 (2005) e651–e656
e656
Cell voltage mV (VSCE)
35 30 25 a
20
b
15 d
10 5
c
e
0 0
1
2
3
4
5
6
7
Discharge current J (µAcm-2) Fig. 6. Variation of cell voltage with discharge current with CuWO4 anode compartment and Platinum cathode compartment: (a) 0.5 M KCl, 0.1 N H2 SO4 ; (b) 0.5 M KCl, 0.5 N H2 SO4 ; (c) 0.5 M KCl, 1.0 N H2 SO4 ; (d) 1.0 M KCl, 0.5 N H2 SO4 ; (e) 1.5 M KCl, 0.5 N H2 SO4 :
3. Conclusion Single crystal growth of CuWO4 via flux reaction technique is a diffusion-controlled process. The grown crystal works as a suitable photoanode to decompose water electrolytically. The cell voltage and the discharge current (under illumination) are functions of the composition of the cathodic and the anodic electrolyte solutions. References [1] R.H. Gillette, Rev. Sci. Instrum. 21 (1950) 294. [2] M. Peter, Phys. Rev. 113 (1959) 801. [3] L.G. VanUitert, S.T. Preziosi, J. Appl. Phys. 33 (1962) 2908. [4] R.P. Rastogi, B.L. Dubey, I. Das, J. Sci. Ind. Res. 37 (1978) 172. [5] L.G. VanUitert, R.B. Soden, J. Appl. Phys. 31 (1964) 328. [6] S.K. Arora, Thomas Mathew, N.M. Batra, J. Crystal Growth 88 (1988) 379. [7] S.K. Arora, Thomas Mathew, N.M. Batra, J. Phys. Chem. Solids 50 (1989) 665. [8] S.K. Arora, T. Mathew, Phys. Stat. Solidi (a) 116 (1989) 405. [9] S.K. Arora, Thomas Mathew, N.M. Batra, J. Phys. D 23 (1990) 460. [10] T. Mathew, N.M. Batra, S.K. Arora, J. Mater. Sci. 27 (1992) 4003. [11] A.K. Basu, F.R. Sale, J. Mater. Sci. 13 (1979) 91. [12] J.P. Doumerc, J. Hejtmanek, J.P. Chaminade, M. Pouchard, M. Krussanova, Phys. Stat. Solidi (a) 82 (1984) 285.
[13] A.G. Anders, A.I. Zvyagin, Sov. Phys. JETP 36 (1973) 115. [14] A.I. Zvyagin, A.G. Anders, Sov. Phys. JETP 67 (1974) 309. [15] B. Lake, D.A. Tennant, R.A. Cowley, J.D. Axe, J. Phys.: Condens. Matter 8 (1996) 8613. [16] B. Lake, R.A. Cowley, D.A. Tennant, J. Phys.: Condens. Matter 9 (1997) 10951. [17] B. Lake, D.A. Tennant, Physica B 234-6 (1997) 557. [18] P.F. Schofield, S.A.T. Redfern, J. Phys.: Condens. Matter 4 (1992) 375. [19] Simon A.T. Redfern, Phys. Rev. B 48 (1993) 5761. [20] L.P. Dorfman, D.L.Houck, M.J. Scheithauer, J.N. Dann, H.O. Fassett, J. Mater. Res.—Report 2001, Materials Research Society, USA. [21] M.A. Butler, J. Appl. Phys. 48 (1997) 1914. [22] Yu. V. Pleskov, Photoelectrochemical Solar Energy Conversion, Moscow, Khimiya, 1990, p. 176. [23] A. Fujishima, K. Honda, S. Kikuelu, J. Chem. Soc. Japan 72 (1969) 108. [24] J.G. Mavroides, D.I. Techernev, J.A. Kafalas, D.F. Kolesar, Mater. Res. Bull. 10 (1975) 1023. [25] D.S. Ginley, M.A. Butler, J. Appl. Phys. 48 (1977) 2019. [26] A. Shamsi, W.E. Wallace, Mater. Res. Bull. 18 (1983) 389. [27] R.F. Alimova, L.T. Gottsova, V.A. Garkusheva, L.V. Kravchenko, L.B. Maksimaldo, M.P. Elkamin, V.A. Goltsov, V.K. Popov, Int. J. Hydrog. Energ. 13 (1989) 111. [28] P. Maruthamuthu, M. Ashokkumar, Int. J. Hydrog. Energ. 14 (1989) 275. [29] F.A. Benko, C.L. Maclanrin, F.P. Koffybeng, Mater. Res. Bull (USA) 17 (1982) 133. [30] B.N. Roy, A. Packter, J. Crystal Growth 49 (1980) 368. [31] A.E. Nielson, Kinetics of Precipitation, Pergamon Press, Oxford, 1994 (Chapter 3 and 4). [32] A. Fujishima, K. Honda, Nature 238 (1972) 37.