Study Of TiO₂-based Semiconductors:Photo-generated Carriers’ Separation And Visible Light Response

As a potential process to deal with energy crisis and environmental issues,heterogeneous photocatalysis has drawn great attention in the past decades.In heterogeneous photocatalysis,holes in the valence band and electrons in the conducting band are created when the semiconductors are illuminated by photons with energy higher than their band gap.TiO₂,owing to its chemical stability,non-toxicity and commercial availability etc.,attracts great attension,and it is still regarded as a benchmark photocatalyst under UV light.However,the high recombination rate of photo-generated hole-electron pairs and the wide band gap greatly reduce the photocatalytic efficiency and limit its application under solar light.Herein,some strategies are developed to solve the above-mentioned problems,as described in the following:1.The sole effect of surface/bulk defects of TiO₂ samples on the photogenerated carrier recombination was first investigated.Nano-sized anatase and rutile TiO₂ were prepared and their surface/bulk defects were adjusted simply by calcination at different temperatures,i.e.400-700 oC.High temperature calcinations induced the growth of crystalline sizes and the decrease in the surface areas,while the crystalline phase and the exposed facets were kept unchanged during calcination,as indicated by the characterization results from XRD,Raman,nitrogen adsorption/desorption,TEM and UV-Vis spectra.The existence of surface/bulk defects in calcined TiO₂ samples was confirmed by photoluminescence and XPS,and the surface/bulk defect ratio was quantitatively analyzed according to positron annihilation results.The photocatalytic activity of calcined TiO₂ samples was evaluated in the photocatalytic hydrogen production from water splitting and the photocatalytic oxidation of α-phenethyl alcohol.On the basis of the characterization and catalytic results,a direct correlation between the surface specific photocatalytic activity and the surface/bulk defect density ratio could be drawn for both anatase and rutile TiO₂.The surface defects of TiO₂ could promote the separation of photo-generated electron-hole pairs under irradiation.2.To prevent the recombination of the photogenerated carrier,heterojunction system Nb₂O₅-TiO₂ was fabricated.In this section,in situ hydrolysis-loading of ultra-fine niobium oxide nanoparticles on the surface of rutile TiO₂ was developed as a new strategy to synthesize Nb₂O₅/TiO₂ heterojunctions.The physico-chemical properties of Nb₂O₅/TiO₂ heterojunctions were characterized by XRD,Raman,UV-Vis,XPS and TEM.The separation efficiency of photo-generated electron-hole pairs on NbOx/TiO₂ heterojunctions under irradiation was investigated by PL and ESR spectroscopy.The activity of Nb Ox/TiO₂ heterojunctions was examined in the selective photocatalytic oxidation of α-phenylethanol and the photocatalytic hydrogen production from water splitting.In both reactions,NbOx/TiO₂ heterojunctions exhibited distinct higher photocatalytic activity than pure rutile TiO₂ or Nb₂O₅.The photocatalytic activity of NbOx/TiO₂ heterojunctions was relevant with Nb/Ti ratio and the optimal activity was obtained at Nb/Ti=0.12 with the highest separation efficiency of photo-generated electron-hole pairs.3.The surface plasmon resonance（SPR）of Au was employed to realize visible light absorbance on semiconductors.Au nanoparticles were loaded on the surface of TiO₂ via photo-deposition and TiO₂ surface defect concentration was adjusted to control the Au particle size.Bigger Au particles with wide distribution could be obtained using anatase TiO₂ with relative low surface defect concentration as the support,while smaller Au particles with narrow distribution obtained using TiO₂ with higher surface defect concentration.The obtained Au/TiO₂ showed an obvious visible response.On the other hand,the synergetic promotion of photocatalytic activity of TiO₂ by gold deposition under UV-visible light irradiation was observed for the first time.4.Owing to the controllable particle size and morphology,colloidal gold are always chosen for SPR application.However,the surface-capping agents on Au nanoparticles will limit the SPR-induced energy transition from Au nanoparticles to semiconductor.A simple and green photo-deposition route was developed for the selective deposition of Au nanoparticles on anatase TiO₂ {001} facets.The photo-deposited Au/TiO₂ photocatalysts were characterized by Quasi-situ UV-Vis spectrum,XPS and the photo-deposition efficiency was close to 100% and the surface organic molecules on Au could be completely removed during the photodeposition process.The close contact between Au and TiO₂ support in the as-prepared Au/TiO₂ system was clearly distinguished by our lattice-resolved HRTEM image,UV-Vis spectrum,Au 4f and C 1s XPS.Enhanced photocatalytic hydrogen generation and methylene blue decomposition over as-prepared Au/TiO₂ under visible light were clearly observed.5.The localized SPR over substoichiometric tungsten oxide was studied for solar light harvesting.WO₃ single crystal nanosheets were successfully prepared via the exfoliation of layered tungstic acid to tungsten oxide nanosheets and subsequent introduction of oxygen vacancies.The as-prepared sub-stoichiometric tungsten oxide nanosheets showed dramatically enhanced performance in both photocurrent responses and photocatalytic water oxidation.The introduction of oxygen vacancies to pristine tungsten oxide nanosheets could induce the surface plasmon resonance,which not only created the light harvesting via LSPR in the near-infrared region but also promote the light harvesting in the ultraviolet and visible region.6.TiO₂ is a promising photocatalyst for water splitting,but it suffers from low visible light activity due to its wide band gap.Doping can narrow the band gap of titanium dioxide;however,new charge carrier recombination centers will be introduced.Here a solution to this problem was developed by designing sub-10 nm rutile titanium dioxide nanoparticles,with increased amount of surface/sub-surface defects to overcome the negative effects from bulk defects.Abundant defects could not only upward shift the valence band top of rutile titanium dioxide for band gap narrowing but also promote the charge carrier separation.The role of Ti3+ was to enhance,rather than initiate,the visible-light-driven water splitting.The sub-10 nm rutile nanoparticles exhibited the state-of-the-art activity among TiO₂-based semiconductors for visible-light-driven water splitting and the concept of ultra-small nanoparticles with abundant defects can be extended to the design of other robust semiconductor photocatalysts.