Enhanced Visible-Light-Driven H₂ Production From Water On Cadmium Sulfide Nanorods With Novel Cocatalysts Made Of Earth Abundant Elements ?Cobalt And Nickel?

Sun is the source of free, non-pollutant, abundant and renewable source of energy in the universe. Therefore, solar energy got much attention as a promising solution to the future energy and environmental issues. Among various strategies to utilize solar energy, photocatalytic hydrogen production has gathered huge attention due to its storage and transportation advantages. In addition, chemical burning energy of H2 is higher than that of other chemical fuels and its burning is environmentally benign. Cadmium sulfide ?CdS? has been extensively explored in various photocatalytic systems due to its low cost and suitable band gap to the visible light. However, CdS undergoes photocorrosion and charge recombination processes which hamper its activity. Usually, highly efficient photocatalytic systems for H2 production consist of noble metals or their composites. High price and scarcity of noble metals rendered the use of the highly active photocatalytic systems for large scale applications. In this thesis we developed highly efficient, stable and economically favored photocatalytic systems consist of earth abundant elements?cobalt and nickel? to overcome the aforementioned problems.In the first chapter, the importance of photocatalytic H2 production was briefly discussed. In addition, the mechanism and challenges to develop highly enhanced non-noble metal based photocatalytic systems were also enclosed. In the second chapter, we described the use of cobalt based bidentate complexes as molecular cocatalysts on CdS nanorods ?NRs? to enhance the photocatalytic H2 production. An cocatalyst can capture the photogenerated electrons from the photoexcited CdS and suppress the rapid recombination of charge carriers. Photoluminescence spectra were obtained to confirm the electron transfer process between the Co-complex and CdS NRs photosensitizer. The system showed highly enhanced H2 production and good stability for > 12 hours. UV-visible spectroscopy was used to study the mechanistic role of Co-complex in enhanced photocatalytic H2 evolution. In the third chapter, we integrated Co-salen complex with ZnO/CdS heterojunction to study the synergistic effect of molecular cocatalyst and heterojunction. SEM, TEM, HRTEM and XRD were used to characterize the ZnO/CdS NRs heterojunction. On the basis of spectroscopic results, we proposed two electron transfer pathways mechanism for highly enhanced photocatalytic H2 production. Formation of ZnS due to Na₂S/Na₂SO₃ sacrificial electron donors was proved during irradiation. ZnS can also enhance H2 production by providing charge transfer site to the CdS.In the fourth chapter, we discussed the successful employment of NiSe as non-noble metal cocatalysts on CdS NRs. NiSe/CdS NRs composites were prepared using a facile aqueous solution method. Photoluminescent spectra and photocurrent responses proved the efficient transfer of electrons at the interface of NiSe and CdS NRs which leads to enhanced H2 production. Furthermore, we loaded NiSe as cocatalyst on ZnSe/CdS NRs heterojunction to study the synergistic effect ?in the fifth chapter?. The synergistic effect of NiSe and ZnSe effectively prevents the charge recombination in CdS and remarkably enhances photocatalytic H2 evolution.In the sixth chapter, we developed noble-metal-free photocatalytic system based on Ni3C/CdS composites. Photoluminescent spectra and photocurrent responses were studied to elucidate the role of Ni3C in enhanced photocatalytic performance.Loading of Ni3C on CdS NRs accelerates the electron transfer process and decreases the rapid recombination of photogenerated electron-hole pairs. Finally, we successfully developed noble-metal-free photocatalytic systems using various strategies to produce highly enhanced H2 production with stability.