Research Journal of Recent Sciences ______ ______________________________ ______ ____ ___ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 76 - 81 ( 201 3 ) Res. J. Recent . Sci. International Science Congress Association 76 Study of Photogalvanic effect in Photogalvanic cell containing mixed Surfactant (NaLS+Tween - 80), Methylene blue as a Photosensitizer and Xylose as Reductant Lal Mohan and Gangotri K.M. Department of Chemistry, Jai Narain Vyas University, Jodhpur, Rajasth an, INDIA Available online at: www.isca.in Received 1 st Dcember 2012, revised 21 st January 201 3 , accepted 23 rd January 2012 Abstract Photogalvanic effect was studied in a photogalvanic cell containing methylene blue - xylose - NaLS+Tween - 80 system. The photo potential and photocurrent were observed 645.0 mV and 210.0  A respectively. The conversion efficiency of the system was observed 0.5313 % and fill factor was determined as 0.3024. The cell performance was observed100 .0 minutes in dark. The effects of different parameters on the electrical output of the cell were observed and current - voltage (i - V) characteristics of the cell were also studied. Keywords: Photopotenial, photocurrent , methylene blue, xylose , Tween - 80, f ill factor, conversion efficiency . Introduction Energy is an integral part of Nation. The world needs all form of energy to perform different tasks. The quality of human life depends to large de gree of availability of energy. First of all the photogalv anic effect was observed by Rideal and Williams 1 and then was systematically investigated by Rabinowitch 2 - 3 and later by various other workers time to time. 4 – 8 Studies in photogalvanic cells for solar energy conversion and storage reported by Ameta et al 9 – 11 . Effect of dyes along with surfactant was studied by Gangotri et al 12 – 13 . Genwa and Gangotri have reported a comparative studies in anionic cationic and nonionic surfactant and Azur B – NTA – CPC system in photogalvanic cell 14 . Studies of surfactants in Ph otogalvanic cell – NaLS – EDTA – Azur – B system studied by Meena et al 15 . Gangotri and Gangotri studied micellar effect of photogalvanics for solar energy conversion and storage using Safranine O - EDTA - CTAB - 80 system. 16 Gangotri and Bhimwal carried out a comparati ve study on the performance of photogalvanic cell with different photosensitizer for solar energy conversion and storage 17 . A detailed Literature survey reveals that different photo sensitizers and reductants have been used in photo galvanic cell for devel opment of photo galvanic system 18 - 26 . Material and Methods Methylene blue as photosensitizer, xylose as reductant and mixed surfactant (NaLS+Tween - 80) were used in this system. All solutions were prepared in doubly distilled water and were kept in amber c oloured containers to protect them from sun light. A mixture of solutions of dye, reluctant, mixed surfactant and sodium hydroxide were taken in an H - type glass tube which is blackened by black carbon paper and white. A shiny platinum foil electrode was im mersed in one limb of the H - tube and a saturated calomel electrode (SCE) was immersed in the other limb. The whole system was first placed in the dark till a stable potential was attained, then the limb containing the platinum electrode was exposed to a 20 0 W tungsten lamp (Philips). A water filter was used to cut off thermal radiation. A digital pH meter and micro ammeter were used to measure the photo potential and photocurrent generated by the system respectively. The current voltage characteristics wer e studied by applying an external load with the help of a carbon pot (log 470 K) connected in the circuit. Over all experimental set up is given in figure 1. Results and Discussion Effect of variation of photosensitizer (methylene bl ue) concentration on t he system: Dependence of photo potential and photocurrent on the concentration of photosensitizer (Methylene blue) was studied. It was observed that lower concentration of photosensitizer resulted into a fall in photopotential and photocurrent because fewe r photosensitizer (methylene blue) molecules are available for the excitation and consecutive donation of the electrons to the platinum electrode. A greater concentration of dye again resulted into a decrease into electrical output as the intensity of ligh t reaching the dye molecules near the electrode decrease due to absorption of the major portion of the light by dye molecules present in the path. The observed results are summarized in table - 1. Effect of variation of reductant (xylose) concentration on t he system : The electrical output of the photogalvanic cell was affected by the variation of reductant (xylose) concentration on the system. Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 76 - 81 (201 3 ) Res. J. Recent. Sci . International Science Congress Association 77 Figure - 1 Experimental set up Lower concentration of reducing agent resulted into a fall in electrical output be cause fewer reducing agent molecules were available for electron donation to photosensitizer (methylene blue) molecule. Large concentration of reducing agent again resulted into a decrease in electrical output, because the large number of reducing agent molecules hinders the dye molecules from reaching the electrode in the desired time limit. The observed results are summarized in table - 1. Effect of variation of mixed surfactant (NaLS+Tween - 80 ) concentration on the system: For photo galvanic cell having methylene blue – xylose - NaLS+Tween - 80 system, the photo potential and photocurrent varied with variation in concentration of mixed surfactants i.e. NaLS and Tween - 80. In one case concentration of NaLS was kept constant and concentration of Tween - 80 was varie d. In other case the concentration of Tween - 80 was kept constant and concentration of NaLS was varied. In both cases a maxima was found for a particular value of NaLS and Tween - 80 concentration above which decrease in electrical output of photogalvanic cel l was obtained. The observed results are summarized in table 1. Effect of variatio n of pH on the system: The electrical output of the photogalvanic cell was affected by the variation of pH on the system. It can be observed from the table 1 that there is a n increase in electrical output of the cell with the increase in pH values. At pH 12.84 a maxima was obtained. On further increase in pH, there was a decrease in photo potential and photocurrent. Thus, photogalvanic cells containing the methylene blue - xylo se - NaLS+Tween - 80 system were found to be quite sensit ive to the pH of the solutions. Table - 1 Effect of Variation of Methylene blue, Xylose, NaLS, Tween - 80 and pH Parameters Photopotential (mV) Photocurrent (  A) [Methylene blue] 10 - 5 M 3.60 548.0 142.0 3.80 597.0 181.0 4.00 645.0 210.0 4.20 604.0 177.0 4.40 562.0 136.0 [Xylose] 10 - 3 M 1.96 543.0 138.0 1.98 598.0 185.0 2.00 645.0 210.0 2.02 602.0 177.0 2.04 554.0 143.0 [NaLS] 10 - 3 M 6.36 536.0 144.0 6.38 599.0 185.0 6.40 645.0 210.0 6. 42 592.0 182.0 6.44 512.0 138.0 [Tween - 80] 10 - 4 M 5.60 562.0 157.0 5.90 603.0 185.0 6.20 645.0 210.0 6.50 597.0 177.0 6.80 555.0 148.0 pH 12.78 538.0 145.0 12.81 587.0 178.0 12.84 645.0 210.0 12.87 592.0 181.0 12.90 542.0 152.0 Light Intens ity = 10.4 mWcm - 2 Temp. = 303 K Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 76 - 81 (201 3 ) Res. J. Recent. Sci . International Science Congress Association 78 It was observed that the pH for the optimum condition has a relation with pKa of the reductant and the desired pH is higher than in pKa value (p�HpKa). The reason may be the availability of the reductant in its anionic form , which is a better donor form. The observed results are summarized in table 1. Effect of diffusion length : Effect of variation of diffusion length (distance between the two electrodes) on the current parameter of the cell (i max ) has been studied using H - shaped cells of different dimensions. It is observed that in the first few minutes of illuminations there is sharp increase in the photocurrent. As a consequence, the maximum photocurrent (i max ) of photocurrent increase with increase in diffusion length, b ut this is not observed experimentally. Therefore, it may be concluded that the main electroactive species are the leuco or semi - leuco form of dye (photosensitizer) and the dye in illuminated and dark chamber respectively. The reductant and its oxidation p roduct act only as electron carriers in the path. The results are summarized in table 2. Effect of electrode a rea: The effect of electrode area on the current parameters of the cell has also been studied. It was observed that with the increase in the elec trode area the value of maximum potential (i max ) is found to increase. The results are summarized in table 3. Current – voltage (i - V) characteristics of the photogalvanic cell: The photogalvanic cell containing methylene blue - xylose - NaLS+Tween - 80 system, th e short circuit current (i sc ) and open circuit voltage (V oc ) of the photogalvanic cells were measured with the help of a microammeter (keeping the circuit closed) and with a digital pH meter (keeping the other circuit open), respectively. The current and p otential values in between these two extreme values were recorded with the help of a carbon pot (log 470 K) connected in the circuit of microammeter, through which an external load was applied. The Current – Voltage (i - V) characteristics of the photogalvanic cell containing Methylene blue - Xylose - NaLS+Tween - 80 system is reported in figure 2. It was observed that current – voltage (i - V) curve deviated from their regular rectangular shapes. A point in i - V curve, called power point (pp) was determined where the pr oduct of current and potential was maximum and the fill - factor was calculated using the following formula: (1) Where V pp and i pp represent the value of potential and current at power point, respectively and V oc , i sc rep resent open circuit voltage and short circuit current, respectively. The value of fill factor (  ) = 0.3024 was obtained and the power point of cell (pp) = 55.25  W was determined on the system. Cell performance and conversion efficiency: The performance o f the photogalvanic cell was observed by applying an external load (necessary to have current at power point) after termination the illumination as soon as the potential reaches a constant value. The performance was determined in terms of t 1/2 , (figure - 3) i.e., the time required in fall of the output (power) to its half at power point in dark. It was observed that the cell can be used in dark for 100 minutes. The conversion efficiency of the cell was determined as 0.5313 % using the following formula: (2) Where V pp, i pp and A are photopotential at power point, Photocurrent at power point and electrode area respectively. Table – 2 Effect of diffusion length Diffusion length (mm) Maximum photocurrent i max (  A) Equilibri um photocurrent i eq (  A) Rate of initial generation of photocurrent (  A min - 1 ) 35.0 232.0 218.0 7.25 40.0 236.0 214.0 7.38 45.0 240.0 210.0 7.50 50.0 244.0 206.0 7.63 55.0 249.0 202.0 7.78 [Methylene blue] = 4.0 10 - 5 M Light Intensity = 10.4 mW c m - 2 [Xylose] = 2.0 10 - 3 M Temperature = 303 K [NaLS ] = 6.40 10 - 3 M pH = 13.20 [Tween - 80] = 6.2 10 - 4 M Table - 3 Effect of electrode area Methylene blue - Xylose - NaLS+Tween - 80 System Electrode Area (cm 2 ) 0.70 0.85 1.00 1.15 1.30 Maximum photocur rent i max (  A) 232.0 236.0 240.0 244.0 248.0 Equilibrium photocurrent i eq (  A) 222.0 218.0 210.0 203.0 196.0 [Methylene blue] = 4.0 10 - 5 M Light Intensity = 10.4 mW cm - 2 [Xylose] = 2.0 10 - 3 M Temperature = 303 K [NaLS ] = 6.40 10 - 3 M pH = 13.20 [Tween - 80] = 6.2 10 - 4 M Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 76 - 81 (201 3 ) Res. J. Recent. Sci . International Science Congress Association 79 Fig ure - 2 Current - Voltage (i - V) curve of the cell Fig ure - 3 Performance of the Cell Research Journal of Recent Sciences ______ _ _ _______________________________ ______________ _ ________ ISSN 2277 - 2502 Vol. 2 ( ISC - 2012 ), 76 - 81 (201 3 ) Res. J. Recent. Sci . International Science Congress Association 80 Mechasnism : On the basis of above investigations the mechanism of the photocurrent generation in the photogalvanic cell may be proposed as follows: Illuminate Chamber h v Dye Dye * (i) Dye * + R Dye - (semi or leuco) + R + (ii) At platinum electrode: Dye - Dye + e - (iii) Dark Chember At counter el ectrode: Dye + e - Dye (semi or leuco) (iv) Dye - + R + Dye + R (v) Here Dye , Dye - , R and R + are the dye (methylene blue), its leuco form, reductant (xylose) and its oxidized form, respectively. Conclu sion On the basis of observed results, conclusively mixed surfactant affected photogalvanic to a large extent. The mixed surfactant have not only enhance the conversion efficiency but storage capacity of photo galvanic cells also and exhaustive efforts sti ll have the scope to enhance electrical output as well as storage capacity of photo galvanic cells along with reduction in there cost to get commercial viability. The conversion efficiency, t 1/2 and fill factor are recorded as 0.5313%, 100.0 min. and 0.30 24 respectively in methylene blue - xylose - NaLS+Tween - 80 system. Acknowledgment Authors are thankful to Prof. Mrs. (Dr.) Sunita Kumbhat, Head, Department of Chemistry, J.N.V. University, Jodhpur for providing necessary facilities and one of the author (Moha n Lal) is specially thanks to Dr. K.R. Genwa, Dr. P. Koli and Dr. P. P. Solanki for critical and scientific analysis. References 1. 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