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Energy & Fuels 2009, 23, 1024–1031
Comparative Study of Photosensitizing Dyes in Photogalvanic Cells for Solar Energy Conversion and Storage: Brij-35-Diethylenetriamine Pentaacetic Acid (DTPA) System K. R. Genwa*,† and Nemi Chand Khatri‡ Department of Chemistry, Jai Narain Vyas (JNV) UniVersity, Jodhpur 342033, India, and Jodhpur Engineering College and Research Centre, Jodhpur 342001, India ReceiVed September 6, 2008. ReVised Manuscript ReceiVed NoVember 20, 2008
The photogalvanic effect has been studied in three systems using photogalvanic cells: Brij-35-diethylenetriamine pentaacetic acid (DTPA)-Safranine, Brij-35-DTPA-Bismark Brown, and Brij-35-DTPA-Methyl Orange systems. The photopotential and photocurrent generated by these systems were 842, 786, and 625 mV and 155, 115, and 95 µA, respectively. The effects of different parameters on the electrical outputs of the cell have been observed, and current-voltage characteristics of the cell have been studied. A mechanism has been proposed for the generation of photocurrent in photogalvanic cells. The conversion efficiencies for Safranine, Bismark Brown, and Methyl Orange were 0.6438, 0.5192, and 0.2707%, and storage capacities were 122, 117, and 94 min, respectively.
Introduction Non-renewable sources, such as coal, oil, natural gas, etc., are limited, and demand for these sources is increasing day by day; therefore, there is an urgent need to increase the use of renewable sources to fulfill the increasing energy demand. Solar cell technology has been proven as the best alternative source to replace the use of fossil fuels. Direct and indirect sources of solar energy, such as photovoltaic technology, wind power technology, and others, will become more popular for the generation of electricity in the coming decades as the cost of fossil fuel electricity generation becomes higher. Presently, the large-scale use of photovoltaic devices for electricity generation is prohibitively expensive. In these solar cells, the light absorber is made of silicon, which acts as a semiconductor. The problem with silicon cells is that they are expensive to make because the silicon needs to be very pure. A photogalvanic cell is a type of device in which light absorbed within a highly absorbing electrolyte to provide energy for a reaction. Electrical power is generated by subsequent charge transfer to the electrolyte by photo-oxidized or photoreduced molecules diffusing from the bulk of the electrolyte. The photogalvanic cells are based on chemical reactions, which give rise to high-energy products upon excitation by a photon. The photogalvanic cell is working on the photogalvanic effect. The photogalvanic effect was first observed by Rideal and Williams1 and systematically investigated by Rabinowitch2,3 in the iron-thionine system, and later on, it was followed by various workers4-8 time to time. Hoffman and Lichtin9 have * To whom correspondence should be addressed. E-mail: krg2004@ rediffmail.com. † JNV University. ‡ Jodhpur Engineering College and Research Centre. (1) Rideal, E. K.; Williams, D. C. J. Chem. Soc. 1925, 258. (2) Rabinowitch, E. J. Chem. Phys. 1940, 8, 551. (3) Rabinowitch, E. J. Chem. Phys. 1940, 8, 560. (4) Potter, A. C.; Thaller, L. H. Sol. Energy 1959, 3. (5) Anisworth, S. J. Phys. Chem. 1960, 64, 715.
discussed various problems encountered in the development of this field. A detailed literature survey reveals that different photosensitizers have been used in photogalvanic cells along with reductants and surfactants.10-12 Gangotri et al.,13-17 Ghosh and Bhattacharya,18,19 and Sharma et al.20,21 observed the photogalvanic effect in cells containing dyes, reductants, and surfactants, but negligible attention has been paid to the use of Safaranine, Bismark Brown, and Methyl Orange as photosensitizers in photogalvanic cells with Brij-35 as the surfactant and diethylenetriamine pentaacetic acid (DTPA) as the reductant for solar energy conversion and storage; therefore, the present work was undertaken. Experimental Section DTPA (Loba), Brij-35 (s.d. fine), Safranine (Loba), Bismark Brown (Loba), Methyl Orange (Loba), and sodium hydroxide (s.d. (6) Sakata, T.; Suda, Y.; Tanka, J.; Tsubomura, H. J. Phys. Chem. 1977, 81, 537. (7) Kaneko, M.; Yamada, A. J. Phys. Chem. 1977, 81, 1213. (8) Fox, M. A.; Kabir-Ud-Din, J. Phys. Chem. 1980, 83, 1800. (9) Hoffman, M. Z.; Litchin, N. N. Sol. Energy 1979, 153. (10) Ameta, S. C.; Jain, P. K.; Janoo, A. K.; Ameta, R. Energy J. 1985, 58, 8. (11) Ameta, S. C.; Ameta, R.; Sharma, D.; Dubey, T. D. Hung. J. Ind. Chem. 1987, 15, 377. (12) Ameta, S. C.; Kamesra, S.; Gangotri, K. M.; Seth, S. J. Phys. Chem. 1990, 271, 427. (13) Gangotri, K. M.; Regar, O. P.; Chhagan Lal; Kalla, P.; Genewa, K. R.; Meena, R. Int. J. Energy Res. 1996, 20, 581. (14) Gangotri, K. M.; Regar, O. P.; Chhagan Lal; Kalla, P.; Genewa, K. R.; Meena, R. Arabian J. Sci. Eng. 1997, 22, 115. (15) Gangotri, K. M.; Meena, R. C.; Meena, R. J. Photochem. Photobiol., A 1999, 123, 93. (16) Gangotri, K. M.; Regar, O. P. J. Indian Chem. Soc. 2000, 77, 347. (17) Gangotri, K. M.; Chhagan Lal, Energy Sources 2001, 23, 267. (18) Ghosh, J. K.; Bhattacharya, S. C. J. Mol. Liq. 2002, 95, 87–98. (19) Ghosh, J. K.; Ghosh, S. K.; Bhattacharya, S. C. J. Oleo Sci. 2004, 53, 73–77. (20) Madhwani, S.; Vardia, J.; Punjabi, P. B.; Sharma, V. K. J. Power Energy 2007, 221, 33. (21) Madhwani, S.; Ameta, R.; Vardia, R.; Punjabi, P. B.; Sharma, V. K. Energy Sources, Part A 2007, 29, 721.
10.1021/ef800747w CCC: $40.75 2009 American Chemical Society Published on Web 02/02/2009
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Energy & Fuels, Vol. 23, 2009 1025 Absorption spectra of dye solutions have also been taken with the help of Chemito’s UV-vis 2600 spectrophotometer. Photochemical bleaching of the dyes was studied potentiometrically. A digital pH meter (Systronics 335) and a microammeter (INCO-65) were used to measure the potential and current generated by the system, respectively. The current-voltage characteristics were studied by applying an external load with the help of a carbon pot (log 470 K) connected in the circuit. Structure of the Compounds Used.
Figure 1. Experimental setup. Table 1. Effect of the Surfactanta [Brij-35] (×105 M) Brij-35-DTPA system Safranine
photopotential (mV) photocurrent (µA) power (µW) Methyl Orange photopotential (mV) photocurrent (µA) power (µW) Bismark Brown photopotential (mV) photocurrent (µA) power (µW)
5.2
5.8
6.4
6.9
7.2
478 80 38.24 589 50 29.45 560 68 38.08
762 120 91.44 610 73 44.53 681 95 64.69
842 155 130.51 625 95 59.37 786 115 90.39
730 108 78.84 617 82 50.59 655 100 65.50
385 70 26.95 602 64 38.52 405 65 26.32
a [Safranine], 4.0 × 10-6 M; [Bismark Brown], 3.2 × 10-6 M; [Methyl Orange], 9.6 × 10-6 M; [DTPA], 2.0 × 10-2 M; light intensity, 10.4 mW cm-2.
Figure 2. Absorption spectra of dyes.
fine) were used in the present work. All solutions were prepared in double-distilled water and were kept in amber-colored containers for protection from sunlight. Concentrations of Safranine, Bismark Brown, and Methyl Orange dyes used were 4.0 × 10-6, 3.2 × 10-6, and 9.6 × 10-6 M, respectively. A mixture of solutions of dye, reductant, surfactant, and sodium hydroxide was taken in a H-type glass tube. A platinum electrode (1.0 × 1.0 cm2) was immersed in one limb of the H-tube, and a saturated calomel electrode (SCE) was immersed in the other limb. The experimental setup of the photogalvanic cell is shown in Figure 1. The whole system was first placed in the dark until a stable potential was attained, and then the limb containing the platinum electrode was exposed to a 200 W tungsten lamp (Philips), while the other with the SCE was kept in the dark. A water filter was used to cut off thermal radiations.
Results and Discussion Our effort is aimed at bringing down the comparative cost of solar cells. For this purpose, easily available, stable, lowcost, and indigenous materials should be used, and hence, suitable dyes, reductant, and surfactant have been selected for the present investigation. The chief observations about different systems are given in Table 5, which reflect the overall outcome of the present studies and justify the significance of these cells from the solar energy conversion and storage point of view. Absorption spectra of used dye solutions Safranine, Bismark Brown, and Methyl Orange are given in Figure 2.
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Table 2. Effect of the Reductant Concentrationa [DTPA]
(×102
1.0
1.5
2.0
2.5
3.0
610 102 62.22 528 63 33.26 490 89 43.61
770 130 100.10 585 82 47.97 700 108 75.60
842 155 130.51 625 95 59.37 786 115 90.39
725 115 83.37 576 79 45.50 681 105 71.50
550 90 49.50 536 66 35.37 450 82 36.90
Brij-35-DTPA system Safranine
photopotential (mV) photocurrent (µA) power (µW) Methyl Orange photopotential (mV) photocurrent (µA) power (µW) Bismark Brown photopotential (mv) photocurrent (µA) power (µW)
M)
a [Safranine], 4.0 × 10-6 M; [Bismark Brown], 3.2 × 10-6 M; [Methyl Orange], 9.6 × 10-6 M; [Brij-35], 6.4 × 10-5 M; light intensity, 10.4 mW cm-2.
Table 3. Effect of the Temperaturea temperature (K) Brij-35-DTPA system
298.0 303.0 308.0 313.0 318.0
Safranine
photopotential (mV) photocurrent (µA) Bismark Brown photopotential (mV) photocurrent (µA) Methyl Orange photopotential (mV) photocurrent (µA)
880 152 835 109 647 87
842 155 786 115 625 95
805 160 755 122 611 97
763 165 710 128 588 102
718 170 674 133 570 108
a [Safranine], 4.0 × 10-6 M; [Bismark Brown], 3.2 × 10-6 M; [Methyl Orange], 9.6 × 10-6 M; [Brij-35], 6.4 × 10-5 M; [DTPA], 2.0 × 10-2 M; light intensity, 10.4 mW cm-2.
Table 4. Effect of the Light Intensitya light intensity (mW cm-2) Brij-35-DTPA system
3.1
5.2
10.4
15.6
26.0
Saframine photopotential (mV) photocurrent (µA) log V Methyl photopotential (mV) Orange photocurrent (µA) log V Bismark photopotential (mV) Brown photocurrent (µA) log V
795 147 2.9003 609 83 2.7846 771 98 2.8870
815 150 2.9111 612 87 2.7867 780 104 2.8920
842 155 2.9253 625 95 2.7958 786 115 2.8954
853 160 2.9304 631 97 2.8000 808 125 2.9074
880 170 2.9444 647 102 2.8109 832 147 2.9201
a [Safranine], 4.0 × 10-6 M; [Bismark Brown], 3.2 × 10-6 M; [Methyl Orange], 9.6 × 10-6 M; [Brij-35], 6.4 × 10-5 M; [DTPA], 2.0 × 10-2 M; temperature, 303 K.
Effect of the Variation of pH. It is seen that all three systems work effectively in the strong alkaline range. The working range for the present work was pH 10.7-13.2. The potential of the system is found to increase as the pH increases, reaching a maximum value for a particular pH, and then decreases upon further increases in pH. It is quite interesting to observe that pH at the optimum condition for reductant has a relation with its pKa value; i.e., the desired pH values should be slightly higher than their pKa values (pH ) pKa + 1-3). Effect of Dyes. In the present work, three different dyes (Safranine, Bismark Brown, and Methyl Orange) were used in photogalvanic cells containing the Brij-35-DTPA system. The effects of variation of the concentration of the three dyes have been studied. It is observed that there is an increase in photopotential (∆V) and photocurrent (isc) values upon increasing the concentration of the dyes. On the lower side of the concentration range of dyes, there are a limited number of dye molecules to absorb the major portion of the light in the path and, therefore, there is low electrical output, whereas a higher concentration of dyes does not permit the desired light intensity to reach the molecules near the electrodes and, hence, there is a corresponding fall in the power of the cell. Effect of the Surfactant. The most important properties of micellar systems are the ability to solubilize a variety of molecules and substantial catalytic effect on many chemical
reactions.22,23 Photoinduced electron-transfer processes in micellar systems are potentially important for efficient energy conversion and storage because surfactant micelles help to achieve the separation of photoproducts by hydrophilichydrophobic interaction of the products with the micellar interface.24,25 In the present work, a non-ionic surfactant (Brij-35) was used and the effect of variation of its concentration in all three systems was studied and reported in Table 1. It was observed that electrical output of the cell is found to be increasing upon increasing the concentration of surfactant, reaching a maximum value, and then, there was a decrease in the electrical output of the cell upon further increasing the concentration of surfactant (Brij-35). Effect of the Reductant Concentration. The results showing the effects of variation of the concentration of reductant (DTPA) in all three systems have been given in Table 2. It was observed that, upon increasing the concentration of the reductant, the electrical output of these cells was found to increase, which reaches a maximum value, and then, there was a decrease in electric output of the cell upon further increasing the concentration of reductant. The fall in power output with a decrease in the concentration of reductant because of the decreased number of molecules available for electron donation to the cationic form of dye, on the other hand, the movement of dye molecules, may be hindered by the higher concentration of reductant to reach the electrode in the desired time limit, and it will also result in a decrease in electrical output. Effect of the Diffusion Length. The effect of variation of the diffusion length on the electrical output (imax) and initial rate of generation of the current of the photogalvanic cell were observed by using a H-cell with different dimensions. The diffusion length (distance between the electrodes) greatly affects all three systems. imax was found to increase with diffusion length. The dye- (D-) and dye (D) are electroactive species in the illuminated and dark chambers, respectively. In the illuminated chamber, D- strikes at Pt electrode and donates an electron to the electrode, which moves through the external circuit to reach CE (dark chamber). In the dark chamber, eletroactive species (D) strike at CE and accept an electron from it and, then, through diffusion, D- reaches Pt and D diffuses to the dark chamber. Thus, the electrical output depends upon the diffusion and conductivity of D. The conductivity of electroactive species depends upon its population between electrodes. As the diffusion length is increased, the volume of dye solution and intern population of dye molecules (D) increased, leading to higher imax. The electroactive nature of D/D- is proven by the fact that imax increases with the diffusion length. Therefore, it may be concluded that the main electroactive species are the leuco or semi-leuco form of dye (D-) and the dye (D) in the illuminated and dark chambers, respectively. The reductant and its oxidation product act only as electron carriers in the path. Effect of the Temperature. Photopotential and photocurrent of photogalvanic cells have been measured at different temperatures, and the effect of the temperature on electrical output in (22) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (23) Attwood, D.; Florence, A. T. Surfactant Systems; Chapman and Hall: New York, 1983. (24) Moroi, Y.; Infelte, P. P.; Gratzel, M. J. Am. Chem. Soc. 1979, 101, 573–579.
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Figure 3. Variation of the photocurrent and photopotential with temperature for the Brij-35-DTPA-Safranine system.
Figure 4. Variation of the photocurrent and log V with light intensity for the Brij-35-DTPA-Safranine system.
all three systems has been studied. The results are given in Table 3 and Figures 3, 5, and 7. On the basis of the obtained results, it is clear that there is a linear relation between electrical output of the cell and temperature. It is also observed that, upon increasing the temperature, the photocurrent increases but the photopotential decreases. This is due to the fact that the internal resistant of the cell decreases at a higher temperature, resulting in a rise in photocurrent and, correspondingly, there is a fall in the photopotential in all of the systems.
Effect of the Light Intensity. Light source of different intensities (different wattages) were used to observe the effect of the light intensity on the output of the cell. It is observed that the photocurrent shows linearly increasing behavior with an increase in the intensity of light, whereas photopotential increases with an increasing light intensity in a logarithmic manner (i.e., the plot of log V versus i is linear). (25) Rohatgi-Mukherjee, K. K.; Chaudhuri, R.; Bhowmik, B. B. J. Colloid Interface Sci. 1985, 106, 45–51.
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Figure 5. Variation of the photocurrent and photopotential with temperature for the Brij-35-DTPA-Bismark Brown system.
Figure 6. Variation of the photocurrent and log V with light intensity for the Brij-35-DTPA-Bismark Brown system.
Increasing light intensities increase the number of photons per unit area (incident power), striking the dye molecules around the platinum electrode, and therefore, an increase in the electrical output occurs. An increase in light intensity also increases the temperature of the cell, and hence, in our experiment, a water filter was used to cut off the thermal radiations. The results are given in Table 4 and are also presented in Figures 4, 6, and 8. i-V Characteristics of the Cell. A digital pH meter was used to measure the open-circuit voltage Voc (keeping the other circuit open), whereas the short-circuit current (isc) was measured
with a microammeter (keeping the other circuit closed). The electrical parameters between these two extreme values (Voc and isc) were determined with the help of a carbon pot (log 470 K) in the circuit of the microammeter, through which an external load was applied. The corresponding values of the potential with respect to different current values for all three systems were studied. It was observed that, in the entire three systems, the i-V curves deviated from their expected regular rectangular shapes. The power point (a point on the curve where the product of
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Figure 7. Variation of the photocurrent and photopotential with temperature for the Brij-35-DTPA-Methyl Orange system. Table 5. DTPA-Brij-35 Systema observations open-circuit voltage (Voc) (mV) photopotential (∆V) (mV) equilibrium photocurrent (ieq) (µA) maximum photocurrent (imax) (µA) short-circuit current (isc) (µA) current at power point (ipp) (µA) potential at power point (Vpp) (mV) power at power point (PP) (µW) rate of generation of current (µA min-1) fill factor (η) charging time (min) t1/2 (min) conversion efficiency (%)
Safranine
Bismark Brown
1048.0 842.0 155.0 200.0 155.0 900.0 744.0 66.96 25.00
970.0 786.0 115.0 140.0 115.0 80.0 675.0 54.00 15.55
735.0 625.0 95.0 120.0 95.0 55.0 512.0 28.16 10.00
0.480 135.0 117.0 0.5192
0.400 150.0 94.0 0.2707
0.410 125.0 122.0 0.6438
Methyl Orange
a [Safranine], 4.0 × 10-6 M; [Bismark Brown], 3.2 × 10-6 M; [Methyl Orange], 9.6 × 10-6 M; [Brij-35], 6.4 × 10-5 M; [DTPA], 2.0 × 10-2 M; light intensity, 10.4 mW cm-2; temperature, 303 K.
potential and current is at the maximum) on these i-V curves was determined, and their fill factors were also calculated. i-V curves are given in Figure 9. On the basis of fill factor calculated, the most efficient system is Bismark Brown-Brij35--DTPA, followed by Safranine and Methyl Orange. Performance and Storage Capacity of the Cell. Performance of photogalvanic cells was studied by applying the desired external load to obtain the potential and current corresponding to the power point after removing the source of illumination. Time t1/2 was determined after removing the source of light. It is the time taken to reach half the value of power. Performances of various cells were studied, and comparative values are summarized in Table 5. On the basis of the observed data, Safranine-Brij-35-DTPA is the most efficient system from the power generation point of view (solar energy conversion) and also the most efficient from
the performance point of view (solar energy storage), followed by the Brij-35-DTPA-Bismark Brown and Brij-35DTPA-Methyl orange systems. In our understanding, the photogalvanic cells are devices that undergoes cyclical charging and discharging processes. The charging of the cell occurs only in the presence of the illuminating source. The discharging of the cell takes place only when we apply the external circuit for electron transfer. As long as there is no external circuit, the cell will keep light energy stored. For how long the cell will store light energy depends upon the stability of the excited state of dye and the population of excited dye molecules. The storage capacity of the cell will be higher if excited dye molecules are more stable because of its bulkiness or amount of delocalization of excited electrons on it or if, for any reason, because of the recombination of Dand R+ as a result of diffusion, the concentration of dye is reduced, the storage capacity of the cell is also reduced. Therefore, we are planning to carry out further research with a view to stop this recombination of D- and R+. In this direction, Ghosh and Bhattacharya26 used a H-shaped cell, in which illuminated and dark chambers were separated by using a Pyrex sintered glass membrane to prevent recombination of D- and R+. Conversion Efficiency of the Cell. Conversion efficiencies of all of these systems were calculated using the outputs at the power point and the intensity of the incident radiation. The systems (at their optimum conditions) were also exposed to sunlight. The conversion efficiency and sunlight conversion data for these three systems are reported in Table 5. On the basis of these observations, the highest conversion efficiency was found in the Brij-35-DTPA-Safranine, followed (26) Ghosh, J. K.; Bhattacharya, S. C. J. Indian Chem. Soc. 2002, 79, 225–230.
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Figure 8. Variation of the photocurrent and log V with light intensity for the Brij-35-DTPA-Methyl Orange system.
Figure 9. Current-voltage (i-V) curve of the Brij-35-DTPA system.
by the Brij-35-DTPA-Bismark Brown and Brij-35DTPA-Methyl Orange systems. Mechanism. In the dark, no reaction between dyes and reductants takes place. It may be concluded that the redox
potential of reductant (DTPA) is much higher than the dye(s) used in the present work, because rapid fall in potential is observed when the platinum electrode is illuminated, and after some time, a constant value was obtained. Upon removing the
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source of light, the change in potential is reversed, but it never reaches the initial value. It suggests that the main reversible photochemical reaction is also accompanied by some irreversible side reactions. Gomer27 and Clark and Eckert28 have discussed the electroactive species in a well-established system, such as the thionine iron(II) system, where ferric ions were considered as the reactive species at the dark electrode. In all three systems, the electroactive species are the leuco dyes and dye themselves at the illuminated and dark electrodes, respectively. According to observed results, the most likely rate-determining process for ieq should be the recycling reaction of the oxidation product of the reducing agent and the semi-leuco or leuco dyes (photosensitizers). Some experimental evidence has been obtained by Wildes and Lichtin29 and Wyart et al.,30 supporting the participation of the leuco form of the dyes as electroactive species. On the basis of these observations, a mechanism is suggested for the generation of photocurrent in the photogalvanic cell as below. Illuminated Chamber. Upon irradiation, dye molecules attain the excited form D f D* Excited dye molecules accept an electron each from the reductant and are converted into the semi-leuco or leuco form of the dye, and the reductant is converted into its oxidized form as D* + R f D- (semi-leuco or leuco) + R+ At the platinum electrode, the semi-leuco or leuco form of the dye loses an electron and is converted into the original dye molecule D- f dye + eDark Chamber. At the counter electrode, dye molecules accept an electron each from the electrode and are converted to semi-leuco or leuco form D + e- f D- (semi-leuco or leuco) Finally, the leuco or semi-leuco form of the dye and the oxidized form of the reductant combine to give the original dye and reductant molecule, and the cycle goes on. D- + R+ f D + R
D, D-, R, and R+ are the dye, excited from of the dye (semileuco or leuco form), the reductant, and the oxidized form of the reductant, respectively. A schematic of the photocurrent mechanism is shown as
Conclusions Scientists have studied the harvesting of solar energy in various forms of solar cells, such as photoelectrochemical, photovoltaic, and photogalvanic cells. Photovoltaic cells are widely used in most countries for conversion and storage of solar energy, but owing to their low storage capacity, photogalvanic cells are preferred because they have the added advantage of inherent storage capacity. On the basis of results obtained in the present work by using three dyes (Safranine, Bismark Brown, and Methyl Orange), we have demonstrated the higher storage capacity of these photogalvanic cells, as well as their greater electrical output, in comparison to those previously prepared with azur B, Tween-80, and NaLS, used by Genwa and Gangotri.31 In these systems, the value of conversion efficiency is 0.2177% and storage capacity is 74.0 min. Other dyes, such as methylene blue, with oxalic acid and NaLS, were used by Gangotri and Meena32 and Gangotri and Lal;33 their values of electrical output (conversion efficiency ) 0.1211 and 0.42% and cell performance ) 35.0 and 30.0 min, respectively) and other parameters are lower, whereas the present values of electrical output are reasonably higher. Therefore, these systems are more efficient than existing photogalvanic cell systems. Efforts will be made in the future to enhance both of these factors along with exploring their commercial viability. EF800747W
(27) (28) (29) (30) 3, 303.
Gomor, R. Electrochim. Acta 1975, 20, 13. Clark, W. D. K.; Eckert, J. A. Sol. Energy 1975, 17, 147. Wildes, P. D.; Litchtin, N. N. J. Phys. Chem. 1978, 52, 981. Wyart Romy; Memaeker, A. K. D.; Naslelski, NouV. J. Chem. 1979,
(31) Genwa, K. R.; Gangotri, K. M. Afinidad 2001, 58, 492. (32) Gangotri, K. M.; Meena, R. C. J. Photochem. Photobiol. 2001, A141, 175. (33) Gangotri, K. M.; Lal, C. J. Power Energy 2005, 219, 315–320.
