245 Sonoelectrochemical synthesis of polypyrrole-cadmium sulfide-graphene oxide nanocomposite as an efficient photocatalyst for photoelectrochemical hydrogen evolution

Considering the consuming of fossile fuel and growing of environmental pollution, hydrogen energy has attracted great interest because of its environmentally clean nature and high value of combustion. Today electrochemical energy conversion is growing dramatically due to the growth in global demand for energy sources. However, due to the existance of a large overpotential for hydrogen evolution reaction, suitable catalysts are necessary to increase the hydrogen production efficiency. Platinium metal is commonly used as a well-stablished electrocatalyst owing to its low overpotential. However, Because of high price and limited world-wide supply of noble metals, their amounts used in energy technologies should be decreased. In recent years, research on semiconductor-based photocatalysts has received more and more attention because of the problems of solar energy and environmental pollution. Among semiconductor photocatalysts, CdS is one of the most efficient visible-light-driven photocatalysts because its bandgap is narrow (2.4 eV) and its condction band edge is more negative than the H2O/H2 electrode potential. However, the bare CdS still suffers from the quick recombination of photogenerated charge carriers and the instablity owing to photocorrosion. It is commonly accepted that co-catalysts play an important role in transfer photoinduced electron and improve dispersion of photocatalyst. Conducting polymers such as polypyrrole have aroused great interest to the researchers because of their excellent electronic, magnetic and optical properties and using conducting polymers as photosensitizers to modify semiconductors photocatalysts has been studied. Graphen oxide (GO) due to its high specific surface area, high carrier mobility and also high content of the oxygen containing functional groups in the basal planes and the edges can provide anchoring groups wich caused to bind inorganic materials onto GO [1-6]. The present article reports the sonoelestrosynthesis of PPy-CdS-GO nanocomposite as a photocatalyst for H2 evolution reaction. For this purpose, CdS nanospheres synthesized by sonochemical method with 13th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 22- 23 Nov, 2017 246 average size distribution about 25.86 nm that was determind by XRD and SEM techniques. Then GO was synthesized by modified Hummer method and atlast an in situ sonoelectrochemical method was employed to synthesis of PPy-CdS-GO nanocomposite on anodized copper substrate. After design, preparation and chracterizaton of nanocomposite, its photoelectrochemical performance for photoelectrochemical hydrogen evolution was investigated in neutral solution under visible light irradiation by linear sweep voltametry, electrochemical impedance spectroscopy and chronoamperometry. Results of electrochemical investigations proved that nanocomposites containing CdS nanoparticles are sensitive to visible light. Also investigations showed the decrease of overpotential and enhancement of current density for nanocomposite compared with polypyrrole coating. The overpotential for PPy-CdS-GO nanocomposite, 0.9 V/SHE was determined. Tafel slope evaluation exhibit that hydrogen desorption from the substrate obeys from Volmer-Tafel mechanism. Also mechanism of photocatalytic hydrogen evolution discussed that graphene oxide nanosheets have a predomonant effect on charge transfer and enhancment of H2 evolution on electrode surface. In fact in this study an efficient photocatalyst with composed of conductive polymer polypyrrole, cadmium sulfide semicoductor and graphene oxide nanosheets was prepared and their synergistic effect considerd.