The Application Of Metal Compound Semiconductors For Hydrogen Generation By Photoelectrochemical Water-splitting

With world energy demands rapidly increasing,hydrogen emerges as a sustainable,pollution-free,high energy density alternative to fossil fuels.Present hydrogen usage as a power source has been limited to a few highly developed locations,but new developments in the technology and techniques in the coming years may improve the technical and economic viability of hydrogen fuel for energy production around the globe.Photoelectrochemical?PEC?water-splitting hydrogen evolution,since the first report by Japanese scientist Fujishima in 1971,is recognized as a promising method to produce economical,clean hydrogen.Typically,a PEC water-splitting hydrogeneration device is composed of a photoanode and photocathode,which is separated by a membrane.Among these parts,the photoelectrode is the most important component for the reaction.In the process of PEC water-splitting,photons with energy higher than the bandgap of semiconductor are absorbed,and electrons in the valence band are excited to the conduction band.This is followed by the separation of electron-hole pairs due to built-in electron filled or applied voltage.The electrons move to photocathode to react with water to produce hydrogen,while the holes simultaneously transfer to photoanode to generate oxygen.Considering the energy level and cost,here,the photoelectrodes are mostly composed by metal compound semiconductors.For the past decades,significant effort has been spent to modify and discover old and new semiconductor materials to improve the performance of the photoelectrode.The performance,however,is still limited to a small fraction of the theoretical value.In practice,the performance of metal composed semiconductor is limited by the contradiction of efficient carriers'generation and collection,as well as the loss of carriers in both inside and outside transfer processes.To investigate the contradiction of generation and collection of carriers,in this thesis,we grow SnO₂ and ITO single crystal nanowire arrays on the FTO substrate using the chemical vapor deposition method as a high-speed conductive network for carrier's transfer,which can inhibit the scatter effect of carriers caused by grain boundaries.After,semiconductors with a small bandgap to work as light absorbers were deposited on the surface of a single crystal conductive network to form a core-shell heterostructure photoanode nano-array.In this design,the new dimension?vertical?can convert the two-dimensional light absorption to three-dimensional,while keeping the carrier diffusion length around 20 nm.As for the carrier's transfer,in this thesis we separate it into two parts:the inside transfer and outside transfer.The inside transfer refers to the carriers transferring inside the semiconductor where the single crystal core of the core-shell heterostructure photoanode can work as high-speed electron transfer pathway.The outside transfer refers to the carrier transferring at the interface of semiconductor/electrolyte,which can be improved using a charge transfer layer and co-catalyst layer to modify the surface of photoanode.Through the use of carbon-based charge transfer materials,the previous point connection formed at semiconductor/electrolyte interface can convert to a facial connection,which can significantly accelerate charge transfer at the interface of the semiconductor/charge transfer layer/co-catalyst layer/electrolyte.In this thesis,we focus on the main limitations of photoelectrochemical water-splitting and present our approach to solve the problems.The results have been confirmed and analyzed with experimental and theoretical techniques.The idea of single crystal,core-shell nanostructure photoelectrode in this thesis can broadly extend the structure design of semiconductor photoelectrode.Additionally,the use of carbon-based charge transfer materials significantly improves the performance of metal compound photoelectrodes,and is a productive step for further developing semiconductor PEC water-splitting devices.