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Experimental Research On Nano-composite System Based On TiO2 Heterostructure For Photo-electro Properties

Posted on:2016-01-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z F ShaoFull Text:PDF
GTID:1108330479478624Subject:Optics
Abstract/Summary:PDF Full Text Request
Owing to its low cost, non-toxicity, highly surface-activity, and high stability, a wide range of functional applications of titanium dioxide(TiO2) has been extensively explored, ranging from photocatalysis, solar energy conversion, electrochromic effects, humidity sensors, antibacterial application, and spintronic devices, to its use as a biomaterial. However, for practical applications, pure TiO2 is intrinsically not a good candidate due to the large band gap energy of 3.2 e V, which makes it can not absorb visible light effectively(it is only photo-active under irradiation with wavelength less 388 nm), thus limiting its practical application for photocatalytic and photo-electricity conversion processes. TiO2-based semiconductor nano-heterojunctions, which integrate the advantages of nano-materials and semiconductor heterojunctions, have attracted wide application in many areas. For environmental pollution controlling and other applications, TiO2 heterojunction nano-composites possess the potential application, because they can overcome the low quantum efficiency of bulk counterparts(as a result of the rather fast recombination of photo-generated carriers). Meanwhile, nano-complex heterostructure of TiO2 semiconductor composite with different band gap semiconductor can expand the solar absorption spectrum to visible light region. As we see, this work opens up a promising potential of TiO2 for green energy source production, as well as elimination of environmental pollutants from water and air, including the photo-induced killing of micro-organisms.In this thesis, Au/TiO2 nano-heterostructure, which is composited by TiO2 nanotube array(TNA) decorated with Au nanoparticles(NPs), and porous silicon(PS)/TiO2 nano-heterostructure were chosen as two typical and widely used heterostructure materials. By means of steady-state and nanosecond time-resolved transient photoluminescence spectra techniques, the transient competition between separation and photogenerated carriers recombination in TiO2-based heterostructure nano-composite under ultraviolet(UV) and visible-light irradiation were studied, and the effects of Au NPs and PS on photocatalytic activity of TiO2 semiconductor were analyzed. These researches are helpful for studing surface(interfacial) charge transfer kinetics of TiO2 heterostructure nano-composite in photo-electro-chemicl processes. Moreover, it further improves the applications of TiO2 heterostructure nano-composite system in the fields of opto-electronics, photovoltaics and photochemistry.Firstly, in order to match the surface plasmon resonance(SPR) effect of Au NPs with visible-light absorption, vacuum magnetron sputtering and high temperature annealing method were utilized to prepare homogeneous and uniform Au NPs; deposition time of the vacuum magnetron sputtering method of the Au films were adjusted to controll the particle size of Au NPs and adjust the SPR peaks position.Secondly, highly ordered TiO2 nanotube array(TNA) films were fabricated by an anodic oxidation method. Au NPs films are decorated onto the top of TNA with the aid of magnetron sputtering and high temperature annealing method to constitute Au/TiO2 heterostructure nano-composite. An enhanced photocatalytic activity under 266 nm light irradiation is obtained in comparison with the pristine TNA, which is convinced by the steady-state photoluminescence spectra. Furthermore, a distinct blue-shift in the nanosecond time-resolved transient photoluminescence spectra is observed. Such a phenomenon could be well explained by considering the competition between the surface photocatalytic process and recombination of photogenerated carriers. The enhanced UV photocatalytic activities of Au/TiO2 heterostructure nano-composite were evaluated through photodegradation of methyl orange(MO) in aqueous solution by UV-vis absorption spectrometry.Thirdly, the steady-state and the nanosecond time-resolved transient photoluminescence spectra, as well as X-ray photoelectron spectroscopy(XPS) and Raman analysis, confirm the presence of Ti3+ valence states in the prepared TNA films. The UV-vis absorption spectra show that the photo-response of as-prepared samples is extended from UV to visible light region. From the 400 nm excited nanosecond time-resolved transient photoluminescence spectra, Ti3+-related photoluminescence intensity is observed to vary distinctly with the deposition time of Au NPs. Such a phenomenon could be explained by considering the modulation of oxygen vacancies density and charge states in TNA films by surface loading of Au NPs. The enhanced visible-light photocatalytic activities of Au/TiO2 heterostructure nano-composite were evaluated through photodegradation of MO in aqueous solution by UV-vis absorption spectrometry. The decoration of Au NPs plays an essential role in enhancing visible-light photocatalytic activity, because energetic photoelectrons are able to inject to the conduction band of TiO2 owing to the SPR effect. It is hoped that our current work would provide a simple strategy to synthesize defect-related composite for photocatalytic application.Finally, highly ordered porous silicon(PS) films were fabricated by an electrochemical etching method. TiO2 NPs are decorated onto the top of PS with the aid of magnetron sputtering and anodic oxidation method to constitute PS/TiO2 heterostructure nano-composite. The distinct blue-shift in the nanosecond time-resolved transient photoluminescence spectra is observed under both 266 nm and 400 nm light irradiation. Meanwhile, X-ray photoelectron spectroscopy characterization confirms the presence of Si3+ and Ti3+ valence states in the PS/TiO2 heterostructure nano-composite. Such a phenomenon could be well explained by considering the competition between the surface separation process and recombination of photogenerated carriers. The above results will be helpful for guiding the synthesis of new type and highly efficient photocatalysts.
Keywords/Search Tags:Au nanoparticles, surface plasmon resonance, TiO2 heterostructure nano-composite, charge transfer, oxygen vacancy defect state, photocatalysis
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