INTRODUCTION:

As natural resources are being exhausted at ever-increasing pace and when all natural resources of energy will be completely exhausted then man will have no other alternative then to utilize solar energy. It is not only new, harmless and low cost source of energy but also this alternative source of energy will let us all to overcome energy crisis.

During last decade photochemical production of compounds with high energy had been a fascinating field of research. Davis et al.¹ investigated the photoredox reaction of metal ions for photochemical solar energy conversion. An expanded conjugation photosensitizers with two different adsorbing group of cell was investigated by Yao et al.² Matsumura et al.³ studied sensitization of zinc oxide and TiO₂ electrodes by xanthene dyes and tetraphenyl porphyrins. Zinc and magnesium prophyrins and their polymers as sensitizers had been reported by Minami et al.⁴Pichat et al.⁵ investigated photovoltage determining mechanism in dye sensitized solar cells. Ameta et al.⁶ studied use of micelles in photochemical conversion of solar energy using Azur A - Glucose system. Hara et al.⁷ design new coumarin dyes having thiophene moities for highly efficient organic dye sensitized solar cells. Dye sensitized photoelectrochemical and solid solar cells, charge seperation transport and recombination was observed by Tennakone et al.⁸

Alkaitis et al.⁹have explained tunneling of photoelectrons from micelles to aqueous phase. Use of toluidine blue-mannitol system in a photogalvanic cell for solar energy conversion was observed by Ameta et al.¹⁰. Efficient dye-sensitized photoelectrochemical cells for the direct conversion of sunlight to electricity has been reported by Kalyansundram et al.¹¹ Enhancement in power output of solar cell consisting of mixed dyes was observed by Jana et al.¹² Fruit extracts and ruthenium polypyridine dyes for sensitization of TiO₂ in photoelectrochemical solar cell was reported by Gracia et al.¹³ Quaternary self organisation of porphyrine and fullerene units by clusterization with gold nanoparticles on SnO₂ electrode for organic solar cell was studied by Hasobe et al.¹⁴. In the present work we have extended our studies to methylene blue dye.

EXPERIMENTAL:

Methylene blue (Reidel), EDTA (Merck), sodium hydroxide (Qualigens) and sodium lauryl sulphate (Hi media) were used. All the solutions were prepared in double distilled water and kept in amber coloured containers to protect them from light. A mixture of solutions of dye (1.33 x 10^-5 M), EDTA (5.33 x 10^-3M), NaOH water was filled in the H-shaped glass cell. Platinum electrode (1.0 x 1.0 cm²) was dipped in one limb of the cell and saturated calomel electrode (SCE) in the other. The platinum electrode was exposed to a 200 W tungsten lamp (Philips) and the limb containing SCE was kept in the dark. A water filter was used to avoid thermal effects. The intensity of the light was measured with the help of a solarimeter (Solarimeter Model 501 CEL) in the units of mWcm^-2. In another experiment under same conditions NaLS was also added.

The photopotential and photocurrent generated by the system methylene Blue/EDTA/NaLS/OH/hn were measured by a digital conductance multimeter (SYSTRONICS 435) and micro ammeter (Kew) respectively. The current voltage (i-V) characteristics of the cell were studied by using an external load (linear 470) in the circuit. Although a lot of work has been done on use of dye-reductant system in photogalvanic cell but the performance (output) of the cell is low. It was observed that use of surfactant will drastically increase the output of cell. This was the motive behind present work. Anionic, cationic and neutral micelles were used but it was observed that when methylene dye was incorporated into anionic surfactant it will drastically increase the probability of photoejection of electrons. The anionic micelle was found to give better output than the cell without micelles.

EFFECT OF DYE AND REDUCTANT CONCENTRATION:

Dependence of photopotential and photocurrent on the concentration of the dye and reductant was studied and the results are summarized in the Table 1 and 2 respectively.

As evident from Table 2 that when concentration of dye is lower the photopotential and photocurrent have lower values because fewer dye molecules are available for excitation and consecutive donation of the electrons to the platinum electrode. When the concentration of dye is increased, photopotential and photocurrent increases due to an increase in the number of dye molecules undergoing excitation and electron donation to electrode.

TABLE – 1: EFFECT OF EDTA CONCENTRATION

[Methylene Blue] = 1.33 x 10^-5M

Temperature = 303 K

pH = 11.10

Intensity = 35.0 mWcm^-2

[EDTA] x 10³M	Photopotential (mV)	Photocurrent (µA)
4.73	551.0	54.0
4.86	630.0	60.0
5.06	666.0	65.0
5.33	692.0	71.0
5.52	558.0	67.0
5.78	484.0	63.0

TABLE2: EFFECT OF METHYLENE BLUE CONCENTRATION

[EDTA] = 5.33 x 10^-3M

Temperature = 303 K

pH = 11.10

Intensity = 35.0 mWcm^-2

[Methylene Blue]x10⁵M	Photopotential (mV)	Photocurrent (µA)
0.66	575.0	60.0
1.33	692.0	71.0
2.66	640.0	68.0
4.10	594.0	66.0
5.33	542.0	63.0
6.00	489.0	57.0

On the other hand, if concentration of dye is further increased, a decrease in photopotential and photocurrent is observed. It is due to fact that only a small fraction of light reaches the dye molecules present near the electrode. Dye molecules present in the bulk of the solution absorb a major portion of the light. Therefore, electron transfer from dye molecules to electrode is retarded, when results in decrease in power output. A maximum photocurrent (71.0 µA) and photopotential (692.0 mV) is generated at an optimum value of dye concentration (1.33 x 10^-5 M).

Similarly an optimum value for reductant concentration (5.33 x 10^-3 M) was observed at which maximum power was generated. If concentration of reductant is lower than optimum value, fall in power output has been observed; as very few reductant molecules are available for electron donation to the dye molecules. On the other hand, higher concentration of reductant causes hindrance to dye molecules to approach the electrode in the desired time limit. Thus higher concentration of reductant also results in decrease in power output of cell.

EFFECT OF LIGHT INTENSITY:

It was observed that the photocurrent shows a linear relationship with an increase in the intensity of light whereas photopotential increases with increasing light intensity in a logarithmic manner. The results are given in Table 3.

TABLE – 3: EFFECT OF LIGHT INTENSITY

[Methylene Blue] = 1.33 x 10^-5M

Temperature = 303 K

[EDTA] = 5.33 x 10^-3M

pH = 11.1

Light intensity (mWcm^-2)	Photopotential (mV)	log V	Photocurrent (µA)
15.0	573.0	2.7581	55.0
20.0	604.0	2.7810	59.0
25.0	630.0	2.7993	63.0
30.0	661.0	2.8202	66.0
35.0	692.0	2.8401	71.0

The number of photons per unit area (incident power), striking the dye molecules around the platinum electrode, increase with an increase in the light intensity and there is a rise in photopotential and photocurrent. However, an increase in light intensity will also raise the temperature of the cell. Therefore, intensity of medium order (35.0 mW cm^-2) was used for all investigation.

TABLE – 4: EFFECT OF DIFFUSION LENGTH

[Methylene Blue] = 1.33 x 10^-5M

Temperature = 303 K

[EDTA] = 5.33 x 10^-3M

Intensity = 35.0 mWcm^-2

pH = 11.1

Diffusion length D_L (cm)	Maximum photocurrent i_max (µA)	Equilibrium photocurrent i_eq (µA)
1.0	66.0	65.0
1.5	67.0	64.0
2.0	68.0	63.0
2.5	69.0	63.0
3.0	71.0	62.0

EFFECT OF DIFFUSION LENGTH:

The effect of variation of diffusion length on current parameters (i_max, i_eq) was also studied. i_max was found to increase as diffusion length was increased but i_eq showed negligible small decreasing behaviour with an increase in diffusion length. The results are summarized in Table 4.

Current voltage characteristics of the cell:

The open circuit voltage (V_oc) and short circuit current (i_sc) of this cell was measured from digital pH meter (keeping the circuit open) and from multimeter (keeping the circuit closed), respectively. The electrical parameters in between two extremes were determined with the help of a carbon pot (Linear 470 K) connected in the circuit of the multimeter, through which an external load was applied. The corresponding values of potential with respect to different current values are given in Table 5 and the i-V characteristics of the all containing methylene blue-EDTA system is shown graphically in Figure 1.

The value of potential and current at power point is presented as V_pp and i_pp respectively. With the help of the I-V curve, the fill factor and conversion efficiency of the cell were determined using the formula,

Fill factor (η) = i pp x V pp

The fill factor and conversion efficiency of the cell was 0.28 and 0.5974 % respectively. The same experiment was performed in the presence of surfactant (NaLS) and the results are graphically represented in Figure 2. Further, the fill factor and conversion efficiency of the cell in this case was calculated as 0.40 and 0.9874%, respectively.

The performance of the cell was studied by applying the external load necessary to have current and potential at power point after removing the course of light. It was observed that the cell can be used without surfactant in the dark at its power point for 25 minutes, and with surfactant it can be used for 32 minutes. The photovoltaic cell cannot be used in the dark even for a second whereas the photogalvanic system has an additional advantage of being used in the dark of course with lower conversion efficiency.

EFFECT OF SURFACTANT CONCENTRATION:

The effect of sodium lauryl sulphate concentration on photopotential and photocurrent in the photogalvanic cell was also studied. The results obtained are shown in Table 5. It was observed that as the concentration of sodium lauryl sulphate was increased that was corresponding increase in photopotential and photocurrent. Further increase in concentration of this surfactant resulted into decrease in the output. The charge of micelle will also play an important role to decide the efficiency of photo ionization process. The negative potential inside the anionic micelle aggregate will favor the process of photo ionization and therefore, it will increase the efficiency of the cell.

TABLE – 5: EFFECT OF NaLS CONCENTRATION

[Methylene Blue] = 2.66 x 10^-5M

Temperature = 303 K

[EDTA] = 5.33 x 10^-3M

Intensity = 35.0 mWcm^-2

pH = 10.0

[NaLS] x 10⁴ M	Photopotential (mV)	Photocurrent (µA)
-	692.0	71.0
3.33	415.0	101.0
4.00	590.0	102.0
4.66	744.0	104.0
5.33	677.0	103.0
6.00	614.0	101.0

CONDITIONS:

PREPARATION OF SOLUTIONS:

The stock solution of all the chemicals were prepared in doubly distilled water and kept in amber coloured containers to protect them from light.

MEASUREMENT OF pH:

The present investigations were carried out in strongly alkaline medium. The pH of the system was adjusted by adding desired volume of sodium hydroxide solution, which was standardized against oxalic acid solution. The pH of the reaction mixture was measured by a digital pH meter (Systronics Model 335) after stabilizing the dark potential.

MEASUREMENT OF LIGHT INTENSITY:

Solar intensity meter (Solarimeter Model 501 CEL) was used for measurement of light intensity in terms of mWcm-2.

MEASUREMENT OF PHOTOPOTENTIAL:

To measure potential of the system digital multimeter (Systronics 435) was used. When the system attains a constant potential value in dark then the platinum electrode was illuminated till another stable potential was obtained. The photopotential was calculated from the difference between the two potentials. Photopotential = Stable potential after illumination - Dark potential

MEASUREMENT OF CURRENT:

Microammeter (KEW) was used to measure photocurrent generated by the system. The change in current with respect to time, the maximum photopotential and the current at equilibrium were noted in some sets only. In all other cases, the current was measured after obtaining a stable potential on illumination.

FIG. 1 EXPERIMENTAL SET UP OF PHOTOGALVANIC CELL

S = Source of light, SCE = Saturated calomel electrode

Pt = Platinum electrode, R = Resistance

K = Key, A = microammeter, mV = millivoltmeter, W = Water

SET-UP OF THE PHOTOGALVANIC CELL: (Fig.-1)

An ‘H’ type tube was used in the present work, which was filled with a mixture of known amount of solutions of dye, sodium hydroxide and reducing agent. The total volume of the mixture was always kept 75.0 mL. A platinum electrode (1.0 x 1.0 cm2) was immersed in one arm of ‘H’ tube and saturated calomel electrode was kept in the other. The whole cell was kept in the dark. The potential (in mV) was measured in dark as soon as it reaches a stable value. Then platinum electrode was exposed to light. A 200 W (Philips) tungsten lamp was used for illumination. The intensity of light was varied by changing distance between the source of light and the platinum electrode. A water filter was used for cutting off thermal radiations. The temperature of the system was kept 303 K

i-V CHARACTERISTICS

In the circuit an external load (linear 470 k) was used to study currentvoltage (i-V) characteristics of the cell.

VARIATION OF DIFFUSION LENGTH

Cells of different dimensions (diffusion length = 1.0, 1.5, 2.0, 2.5, 3.0 cm) were used to observe the effect of diffusion length on the electrical parameters of the cell.

PERFORMANCE OF THE CELL

The performance of the cell was observed at power point using desired load and after removing the source of illumination. Time required for a fall in the product of potential and photocurrent to its half is called performance. The fill factor is the one of the measure of performance of the cell and can be calculated by formula –

Fill factor (η)= i pp x V pp

――――

isc x V oc

Where, ipp = Current at power point

Vpp = Voltage at power point

isc = Short circuit current

Voc = Open circuit voltage

CONVERSION EFFICIENCY OF THE CELL

The conversion efficiency of the cell was determined with the help of current and potential values at the power point and the incident power of radiation by using formula.

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Received on 20.07.2010 Modified on 30.07.2010

Asian J. Research Chem. 4(1): January 2011; Page 71-74