ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2013, том 47, № 6, с. 456-462
ФОТОХИМИЯ
SURFACE MODIFICATION OF SEMICONDUCTOR PHOTOELECTRODES FOR BETTER PHOTOELECTROCHEMICAL PERFORMANCE
© 2013 Ruby Upadhyay*, Mridula Tripathi**, Ashutosh Pandey***
* **Department of Chemistry, C. M. P. College, Allahabad, India * ***Department of Chemistry, MotilalNehru National Institute of Technology, Allahabad, India E-mail: cmpau2010@gmail.com Поступила в редакцию 07.05.2013 г. В окончательном виде 15.06.2013 г.
The present paper describes the modification and solar hydrogen production studies employing a new semiconductor-septum (SC—SEP) photoelectrode ns-TiO2/In2O3 based photoelectrochemical solar cell. The current-voltage characteristics of the above SC—SEP cell revealed that an enhancement in short-circuit current (ISC) up
to three times (5 ~ 14.6 mA cm-2). The optimum hydrogen production rate was found to be 11.8 lh-1 m-2 for 5 M H2SO4 and with a further increase in H2SO4 concentration, the hydrogen production rate was found to be invariant.In yet another part of our study instead of using new SC—SEP solar cell design, we used another new oxide material form such as ns-TiO2/WO3. The ns-TiO2/WO3 exhibited a high photo-current and photovoltage of 15.6 mA cm-2, 960 mV, respectively. The ns-TiO2/WO3 electrode exhibited a higher hydrogen gas evolution rate of13.8 lh-1 m-2. Evidences and arguments are put forward to show that, whereas for the bare ns-TiO2 electrode, the improvement in the performance of this photo-electrode compared with its original form was due to the higher quantum yield. In the case of ns-TiO2/In2O3 and ns-TiO2/WO3 photo-electrodes, the improvement is due to the improved spectral response resulting from decrease of energy band gap.
DOI: 10.7868/S0023119713060112
Presently, renewable energy contributes only about 5% of the commercial hydrogen production primarily via water electrolysis, while other 95% hydrogen is mainly derived from fossil fuels [1]. Renewable Hydrogen production is not popular yet because the cost is still high. Photovoltaic water electrolysis may become more competitive as the cost continues to decrease with the technology advancement; however the considerable use of small band gap semiconducting materials may cause serious life cycle environmental impacts. Photocatalysis is expected to make a great contribution to both environmental treatment (emission cleaning and water purification) and renewable energy.
The exploitation of solar energy is now a day at the focus of many energy discussions, with most attention given to applications where solar radiation is used to heat water or generate steam. Hydrogen (H2) is widely considered to be the future clean energy carrier in many applications, such as environmentally friendly vehicles, domestic heating and stationary power generation. Photo catalytic Hydrogen (H2) production from water is one of the most promising ways to realize a hydrogen economy [2]. Hydrogen is known to be an advantageous fuel source due to its clean burning qualities, abundance, storage potential and high energy capacity [3, 4].
In nature Hydrogen (H2) is always present in bound form, in organic compound and water. Hydrogen (H2) can be produced from different sources eg. Coal, natural gas, liquefied petroleum gas, propane, methane, gasoline, light diesel, dry biomass as well as from water. The photoelectrochemical process produces Hydrogen (H2) in one step, splitting of water by illuminating a water immersed semiconductor with sunlight. However this technology is also in the early stage of development.
The nanoparticle-based semiconductor thin film photoelectrode can start a new era in the development of efficient PEC solar cells. Nano-crystalline thin films of metal oxide such as TiO2, ZnO, CuO and SnO2 with highly porous structures have drawn considerable interest in recent years for their use in PEC cells as photosensitive semiconductor electrodes [5, 6].
One special feature about PEC cells is that the active semiconductor component in the form of photo-electrode can be easily surface modified; often with electrochemical process. This results in improved conversion efficiency and greater resistant to corrosion. It seems that that after an extensive search for suitable semiconductor during 1972—1985, the interest has returned back to TiO2 [7]. Several attempts have been
made to bring spectral response of TiO2 into visible or near visible region. It is known that the spectral response of the TiO2 films can be improved through admixing with appropriate oxides [8, 9].
The motivation of the present work was to prepare an electrode having high effective surface area and hence better quantum yield and improved PEC activity. Yet another modification introduced for getting improved TiO2 photo-electrode was admixing In2O3 and WO3 with the nano-structured TiO2 photo-electrode. The surface morphology, structural and photo-electrochemical characterization of the bare TiO2 as well as the TiO2 overlaid with In2O3 and WO3 thin film admixtures have been investigated in relation to hydrogen production through SC—SEP photo-electrochemical solar cell.
EXPERIMENTAL TECHNIQUES
The TiO2 was prepared in the form of a sol gel by the hydrolysis process. For preparing the sol gel, Ti[OCH(CH3)2]4 solution was added slowly to propanol drop by drop. De-ionized water was slowly added under vigorous stirring during 10 min. During the addition, a white precipitate was formed, then 1 mL of 70% HNO3 was added to the mixture. The mixture was then stirred for 15 min at 80°C. The propanol together with some water was allowed to evaporate during this time. In this way, stable TiO2 colloidal solution was formed. This TiO2 solution was then concentrated by evaporation of water in vacuum at 25°C, until a viscous liquid was obtained. Carbowax M-20.000 (40% by weight of TiO2) was added and a viscous dispersion was obtained.
Titanium sheet which was used as the conducting substrate was ~2 mm thick and ~1 cm2 area. This was cleaned by rubbing with emery paper and ultrasonical-ly cleaned in acetone for 15 min. at a frequency of 40 KHz, so that the TiO2 particles did not agglomerate. Spin-on technique employing photo-resist spinner was used for film deposition. Ultrasonically cleaned substrate was placed at the center of the rotating substrate holder of the spinner. One drop of the sol gel was put on the substrate and spin-coated at 3000 rpm for 30 s. The film on the substrate was fired in air (oven) at 150°C for 5—10 min. The process of deposition and subsequent firing was repeated 5 times. The formed film was annealed in Argon (Ar) at a predetermined temperature of 700°C for achieving structural homogeneity. Finally, in order to increase the conductivity of the TiO2 films, they were exposed to reduction atmosphere by annealing in hydrogen at 300°C for duration of ~3 h. In addition to TiO2, the photo electrodes corresponding to ns-TiO2/In2O3 and ns-TiO2/WO3 were also prepared for obtaining improved spectral response. To achieve this, In2O3 was electro-deposited by cathodically polarizing the TiO2
electrode in 5 M In2(SO4)3 solution at a field of 1 V/cm for 2 min. WO3 was deposited on the ns-TiO2 coated photo-electrode by thermal evaporation of WO3 powder from an electrical coated molybdenum boat kept at 1823 K in vacuum.
Later on these electrodes were annealed in oxygen at 500 ± 10°C. The photo-electrochemical behavior of both the ns-TiO2/WO3 and ns-TiO2/In2O3 PEC cells were investigated.
The structural characterization of ns-TiO2, ns-TiO2/ In2O3 and ns-TiO2/WO3 electrode materials was carried out through XRD by employing a Philips PW 1710 Diffractometer equipped with a graphite mono-chromator.
The micro-structural characteristics of the as synthesized mixed oxide ns-TiO2/In2O3 and ns-TiO2/WO3 electrodes were also performed by employing scanning electron microscope (SEM) (GEOL JXA-8100 EPMA).
In order to explore the spectral response the absorption spectra of ns-TiO2, ns-TiO2/In2O3 and ns-TiO2/WO3 on ITO glass substrate were carried out through double beam spectrophotometer (Systronics 2201).
Rectangular SC—SEP, PEC cell made of lucite had a quartz window of (1 cm2) for illumination. The cell was divided into two compartments by a lucite separator, which had a hole in its center on which the ns-TiO2/In2O3 admixed/Ti and ns-TiO2/WO3 ad-mixed/Ti septum electrodes were glued with araldite (Figure 4). It should be pointed out that we have tried several possible placements of Pt counter electrode (PtCE). It was found that the best results in regard to hydrogen production were achieved when the PtCE was put in the dark compartment. This is in contrast to earlier studies where both PtWE and PtCE were not placed in the dark compartment [10, 11]. The configuration of the present SC—SEP, PEC cell consisting of two chambers connected through ns-TiO2/In2O3 admixed/Ti or ns-TiO2/WO3 admixed/Ti septum electrode is:
SCE/1 M NaOH/ns-TiO2/Ti/H2SO4 + K2SO4/PtCE,
PtWE
SCE/1 M NaOH/ns-TiO2/In2O3 admixed/Ti/H2SO4 +
+ K2SO4/PtCE, PtWE SCE/1 M NaOH/ns-TiO2/WO3 admixed/Ti/H2SO4 + + K2SO4/PtCE, PtWE
where CE = counter electrode; SCE = saturated calomel electrode; and WE = working electrode.
The electrolytes in the illuminated compartment facing ns-TiO2/In2O3 admixed/Ti was 1 M NaOH and in the dark compartment this 6 M H2SO4 + 1 M K2SO4 5 M H2SO4 + 1 M K2SO4 4 M H2SO4 + 1 M K2SO4, 3 M H2SO4 + 1 M H2SO4, 2 M H2SO4 + 1 M K2SO4 and 1 M H2SO4 + 1 M K2SO4. The area of SC-SEP
(а)
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* TiO2
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10
20
30
40 50 29 degree
60
70
10
20
30
(c)
40 50 29 degree
60
70
* TiO2
c TiO2-In2O3
10
20
30
40 50 29 degree
60
70
Fig. 1. XRD diagrams of (a) TiO2, (b) TiO2-WO3 and (c) TiO2-In2O3.
photo-electrode (ns-TiO2/In2O3 admixed) was ~1 cm2. All electrochemical measurements were carried out by using a Princeton Applied Research (PAR) model 173 Potentiostat/Galvanostat PAR 175 universal programm
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