Mechanism Investigation On Oxygen-Vacancy-Based Photo-thermal Coupling Water Splitting

Since ancient times,the sun has been considered as the source of all things,and has been used by human directly or indirectly.The Industrial Revolution made the use of fossil fuels the dominant source of energies,but the Energy Revolution has made the development of clean,renewable energy a major goal at the moment.Hydrogen energy shows important value because of its high calorific value,environmentally friendly and its ability to meet the needs of industrial.Oxygen Vacancy(V_O)is a normal defect form in photochemistry and electrochemistry,which should be produced by extreme high temperature(approximately>1000?)or ultraviolet light.Recent work has showed the powerful reducibility of V_Os for dissociating the water molecules.The photo-induced V_Os are generated during the photo-reaction,and further react with H₂O for H₂yield during the thermo-reaction,achieving the stepwise response of light and heat energy at the lattice oxygen(O_L).V_O-based photo-thermal coupling water splitting is an innovative,clean and promising method to use solar energy with advantages of full spectrum utilization,high energy efficiency and high exergy efficiency.The TiO₂-based enhanced mechanism has been built and confirmed in this work as follow:hv+2 Ti⁴⁺+O_L^2-?2 Ti³⁺+V_O+1/2 O₂(Photochemistry)2 Ti³⁺+V_O+H₂O?2 Ti⁴⁺+O_L^2-+H₂(Thermochemistry)Based on TiO₂ nanomaterials synthesized by various preparation methods,the performance of pure water splitting in photo-thermal coupling was tested on a variety of topography TiO₂ samples and its promoted form with Fe ions doping.Compared with hydrothermal nanosheets and nanowires,the nanoparticles prepared by sol-gel own abundant defects,resulting in a decline in photo-induced V_Os yield.Fe doping method successfully enhances the generation of photo-induced V_Os.Meanwhile,defects in nanomaterials promote effective doping of Fe atoms,realizing the classic V_Os yield on Fe-doped nanoparticle TiO₂(Fe-TiO₂).The hydrogen yield shows a positive correlation with the ability of photo-induced V_Os formation,indicating that the number of photo-induced V_Os is the key factor to determine the water decomposition efficiency.Moreover,the 0.5%mass ratio of Fe doping own the best performance by comparing the various doping ratio.In order to compare the thermal reactivity of photo-induced V_Os,a batch of TiO₂nanoparticles doped with 0.5%transition metals(Fe,Cu,Co,Ni,Zn)were prepared.The metal doping was introduced into the nanomaterials for boosting the photo-induced V_Os production,with the advantages of extended photo-response,enhanced electron-hole pairs separation,prolonged carriers lifetime and decreased impedance.The Fe-TiO₂ own the highest H₂ yield of 9.73?mol/g among all M-TiO₂ samples.hv+Ti⁴⁺+M^x++O_L^2-?Ti³⁺+M^(x-1)++V_O+1/2 O₂(Photochemistry)Ti³⁺+M^(x-1)++V_O+H₂O?Ti⁴⁺+M^x++O_L^2-+H₂(Thermochemistry)According to the first principle method,the simulation calculation was carried out on the basis of the experiment,and the H₂O decomposition pathway has been deeply explored.The calculation showed that the V_Os suffer different energy barriers in various M-TiO₂ models,pointing out the V_Os reactivity in the thermal reaction should be another key factor in H₂ yield besides the V_Os generation in the photo reaction.These conclusions demonstrate the obvious impact of quantity and quality of photo-induced V_Os in V_O-based reactions.To ameliorate the loss of reactivity in the decrease of V_O formation energy,the Ni-loaded Cu-doped TiO₂(NCT)material has been proposed.The heterostructure of Ni oxides and Cu-TiO₂ promotes charge separation,prolongs the carrier life,and further improves the photo-induced V_O generation.More importantly,the interlaced energy band structure made the photo-induced V_Os appeared in Ni oxides,which own a lower H₂ formation energy compared to Ti oxides.The highest H₂ yield has been achieved on NCT sample of 30.6?mol/g,which is almost 40 times to the bare TiO₂.The enhanced photoreaction mechanism has been built as follow:hv?h⁺+e^-Ti⁴⁺+Cu²⁺+2e^-?Ti³⁺+Cu⁺Ni²⁺+5h⁺+2 O_L^2-?Ni³⁺+V_O+O₂Meanwhile,the anchoring effect of surface defects was found,and the phenomenon of ion agglomeration and the transformation of loading form(NiO to Ni₂O₃)with the increase of Ni solubility was observed.In the photo-thermal coupling water splitting,high-valence Ni oxide forms strong band with OH*,further prevents the breakage of OH*,resulting in H₂ production presents“volcanic”type with Ni loading.In the methanol-water mixture reaction,high-valence Ni oxide accelerates the reaction with advantage of abundant surface positive charges,resulting in H₂ production increases linearly with Ni loading.The above material design work provides a new research hotspot in the field of material engineering,and gives great expectations for the V_O-based photo-thermal coupling hydrogen production reaction in the field of solar energy utilization.