Effective Charge Carrier Utilization In Semiconductor Photocatalysts

Tremendous efforts have been devoted to searching for sustainable energy resources to alleviate the upcoming energy and environment crises.Among various types of new energy resources,solar energy has been considered as one of the most promising choices,since it is abundant,sustainable,and clean.Conversion of solar energy into chemical energy is of great importance which could potentially provide continuous and flexible energy supplies.However,the conversion efficiency is still relatively low which prevents it from practical applications.Therefore,designing of stable photocatalysts with high activities is urgently needed.Generation,transport,recombination,and reaction of photogenerated charge carriers are involved in semiconductor based photocatalytic solar energy conversions.Thus,it could be inferred that the efficiency of charge carrier utilization is the core issue to determine the activity of semiconductor photocatalysts.Strategies to impove the efficiency of charge carrier utilization includes extending the light absorption ranges of photocatalysts,acceleration of the charge transport,suppression of recombination centers,and enhancement of surface reaction kinetics.Each strategy to enhance the charge carrier utilization efficiency is discussed in the thesis to improve the efficiencies of photocatalytic solar energy conversions.Firstly,tantalum nitride（Ta₃N₅）and hematite（Fe₂O₃）are investigated which are all visible light responsive photocatalysts having high theoretical solar energy conversion efficiencies.Secondly,Ta₃N₅ nanotube array anode with strong connections between the photoactive materials and the substrate are obtained by improving the anodization conditions as well as adjusting the anodization duration and bias during the synthesis.Therefore the conduction of charge carriers from the Ta₃N₅ nanotube arrays to the counter electrode is accelerated,resulting in the improved activity.Thirdly,atomic layer deposition（ALD）is used to grow ultrathin titanium oxide（TiO₂）onto the surface of Ta₃N₅ thin film photoanode to passivate the surface states.Therefore,surface recombination centers can be partly removed to reduce the overpotential.It was found that ten cycles of ALD could result in the formation of thin film of TiO₂ which showed the best passivation effect.Finally,carbon nanodots（Cdots）and cobalt oxide（Co₃O₄）cocatalysts were integrated onto Fe₂O₃ thin film photoanode,which showed enhanced activity owning to the fast surface reactions.By detecting the hydrogen peroxide（H₂O₂）after the reaction with Fe₂O₃ photoanodes with different types of coatalysts,the synthetic effect between the two cocatalysts were investigated.It was considered that the two-step-two-hole photooxidation process of H₂O was accelerated when two cocatalysts were loaded simultaneously.