Charge Transfer Characteristics Of Electrochemically Induced Surface Polaron States

Energy and environment are the foundation of human society development.Since the industrial revolution,the extensive use of fossil energy has brought tremendous pressure to the environment,and the development of renewable clean energy has become an inevitable requirement for human sustainable development.Hydrogen energy is a high-energy-density clean energy with a wide range of uses,which is expected to replace traditional fossil energy.Efficient production of hydrogen energy is one of the effective ways to solve the environmental and energy crisis.Among them,the photoelectrochemical（PEC）water splitting to hydrogen through the semiconductor photoelectrode can convert solar energy into hydrogen,which is an efficient energy conversion technology.When a semiconductor electrode absorbs photons that exceed its band gap energy,electron-hole pairs will be generated.Under the separation of the space charge layer,the photo-generated electrons and holes will undergo reduction and oxidation reactions respectively to complete the process of water splitting.The space charge layer is mainly formed by the bending of the energy band,which is generated by the semiconductor electrode/electrolyte contact,so the separation and reaction of the charge are mainly carried out at the electrode/electrolyte interface.Therefore,exploring the semiconductor photoelectrode/electrolyte interface charge transfer mechanism and enhancing the separation and transfer of photogenerated carries are the focus of research on photoelectrode material selection and modification.Due to the intrinsic defects and the influence of some external factors of the semiconductor material,there exsit surface states which can capture electrons on the surface of the semiconductor electrode.In this case,the separation and transfer of interface charges are not only affected by the space charge layer,but also by the surface states.Therefore,it is necessary to conduct in-depth research on the formation of surface states and the mechanism that affects the interface charge transfer ability,which has important guidance for understanding the charge transfer mechanism of new semiconductors and the modification of traditional semiconductors.Electrochemical doping is one of the methods of introducing surface states on the electrode surface.Through the combined action of electrons and protons,surface polaron states can be generated on the electrode surface,which improves the PEC performance of the electrode.However,the current research on the surface polaron states of semiconductor photoelectrodes is not mature enough,and there are unclear parts of the effect of electrochemical doping on the electrode.Therefore,this article takes the single crystal rutile TiO₂ nanorod array electrode as the research electrode to explore the specific mechanism of the surface polaron states induced by electrochemical doping,which affects the interface charge separation and transfer.With a certain understanding of electrochemical doping,this method is applied to the protonation of metal oxide semiconductor powder to improve their catalytic performance.The main research contents are as follows:1.The surface polaron states on single-crystal TiO₂ nanorod arrays enhance charge separation and transfer.In previous studies of electrochemical doping,there is a view that the improvement of the PEC performance of the electrode is due to the passivation of the surface grain boundary,which reduces the recombination of photogenerated carriers.Therefore,in order to avoid the interference of the grain boundary passivation effect,the single-crystal rutile TiO₂ nanorod array electrode was applied as the research electrode to study the influence of electrochemical doping on the electrode.After the electrode is electrochemically doped with a negative bias,the saturated photocurrent density of the electrode is significantly increased.The characterization confirmed that the structure of the surface polaron states induced by electrochemical doping is Ti³⁺-OH,and the PEC measurement and in-situ spectroscopy analysis confirmed that the surface polaron states can capture and store the photogenerated charges,and the applied bias can adjust the density of polaron states to separate and transfer the carriers.The research results confirm that the surface polaron states induced by electrochemical doping is the main reason to enhance the PEC performance of the electrode.And the comparison results of flat single crystal electrodes show that the charge separation and transfer through polaron states are closely related to the area of the electrode/electrolyte interface.2.The electrochemical protonation of metal oxide semiconductor powder enhances surface electron transfer.Generally,metal oxide semiconductor powders are chemically stable and require extreme conditions to complete the hydrogenation process.In previous research,we found that the TiO₂ electrode can complete the surface protonation process through electrochemical methods.So,a similar electrochemical method was applied to the surface hydrogenation process of TiO₂ powder.After the characterization of the processed powder,it is found that this electrochemical method can obtain the powder with high catalytic activity,and the degree of protonation is closely related to the applied bias.At the same time,this electrochemical doping method was applied to other semiconductor powders,and it was found that these powders had completed surface hydrogenation to a certain extent,which proved the universality of electrochemical protonation.In the PEC test,it was found that the charge transfer resistance of the processed sample was reduced,and the PEC performance was improved.This electrochemical doping method combines the action of electrons and protons to form polaron states on the surface of the semiconductor powder,which enhances the catalytic performance of the powder.This method does not require extreme reaction conditions and can be precisely controlled by adjusting the doping bias,which provides a simple and cost-effective green route for the protonation of semiconductor powders.