Study Of Preparation And Photoelectric Catalysis Properties Of TiO₂Composite Nanostructures

Titanium dioxide （TiO₂） is an important semiconductor material and has attractedconsiderable attention in the fields of photoelectric conversion and photocatalysisbecause of its good chemical stability, anti-photocorrosion, nontoxic and low-cost.However, the wide band gap of TiO₂limits them to UV radiation, which is only4%ofthe solar spectrum energy. Moreover, the high recombination rate of photo-electron andhole limits its photoelectrochemical catalytic application. So, the development of TiO₂nano-photocatalysts with visible-light response as well as its new applications is one ofthe most important works in the photocatalytic fields．Because traditional nanometer-scale powders are likely of aggregate and theconversion efficiencies of nanoparticle polycrystalline films is limited, TiO₂nanorodarray and nanotube array films are studied in this work. The vertically aligned TiO₂nanotube arrays （NTs） have been prepared by anodizing method on Ti foil and Pt/TiO₂electrocatalyst composite electrodes have been prepared and used for the electrooxidationof ethanol in acidic and alkaline media. Vertically aligned TiO₂nanorod arrays （NRs）were synthesized by a facile hydrothermal method, and followed by depositing CdS,Co₃O₄and In₂O₃nanoparticles on TiO₂NRs by different methods. The surfacemorphology, structure, optical and photoelectrochemical behaviors of these CdS/TiO₂、Co₃O₄/TiO₂、In₂O₃/TiO₂composite films are studied in detail. Modified rutile TiO₂NRswere synthesized by being treated using the noble metal （Pt） deposition andsemiconductor （CdS） compounding. The photocatalytic-activities for hydrogenevolution were firstly studied and compared with TiO₂NRs, Pt/TiO₂NRs andCdS-Pt/TiO₂NRs electrodes in the electrochemical system under the visible light （orUV-light） irradiation. The details are described as follows:1) The highly ordered TiO₂nanotube arrays （NTs） have been prepared byanodizing method on Ti foil. The morphology and composition of the as-preparedsamples were characterized by X-ray diffraction, field-emission electron scanningmicroscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy,ultraviolet-visible reflection spectrophotometry, and X-ray photoelectron spectroscopy.The formation mechanism of TiO₂NTs was discussed in detail.（Chapter2）2) TiO₂nanorod arrays （NRs） were synthesized by a facile hydrothermal methodand were characterized by XRD, FESEM, TEM, UV-vis spectrum, and XPS respectively. The influences of hydrothermic factor, such as solution concentration，hydrothermal temperature，and hydrothermal time on the structure and morphology ofthe samples were systematically studied. Then, the growth mechanisms of the sampleswere proposed. The TiO₂NRs was applied to construct photoelectric devices, by whichhighly sensitive and steady photocurrent responses were obtained. The electrodes showan instantaneous change in current upon light illumination and keep stable after20s.The current retracts to its original value almost instantaneously once the illumination isswitched off. The highly sensitive responses and excellent stability to the UV lightreveal that the TiO₂NRs can be used to fabricate the UV sensors/switches.（Chapter3）3) CdS/TiO₂composite films have been fabricated via a two-step method.Vertically aligned TiO₂NRs were synthesized by a facile hydrothermal method, andfollowed by depositing CdS nanoparticles on TiO₂NRs by successive ionic layeradsorption and reaction （SILAR） method. The surface morphology, structure, opticaland photoelectrochemical behaviors of the CdS/TiO₂nanocomposite were considered.The CdS nanoparticles with a diameter of3-5nm are uniformly dispersed on the TiO₂NRs. The TiO₂NRs coated by CdS nanoparticles show higher photocurrent value thanthat of pure TiO₂NRs. The enhanced photoelectrochemical behaviors can be attributedto the coated CdS nanoparticles, which increase the probability of electron–holeseparation and extend the range of the TiO₂photoresponse from ultraviolet to visibleregion due to the low band gap of2.56eV.A novel heterostructure of Co₃O₄/TiO₂was fabricated by Co₃O₄nanoparticlescoated on the TiO₂NRs using a photochemical coating method. The composition andmorphologies of samples were characterized. HRTEM showed that Co₃O₄nanoparticleswith mean size of2–3nm are coated on TiO₂nanorod, could not be ‘‘peeled’’ off fromTiO₂nanorod even under sonication for10min. The gap energy3.10and2.15eV isestimated corresponding to TiO₂and Co₃O₄in the Co₃O₄/TiO₂heterostructure,respectively, which is smaller than that of the reported pure TiO₂due to the interfacialincorporation and matched band edges between the semiconductor TiO₂and Co₃O₄. Thephotoelectrochemical properties have been investigated under visible-light irradiationand the results showed remarkably enhanced visible light PEC response of theCo₃O₄/TiO₂heterostructure due to the coupling effects of Co₃O₄and TiO₂incomparison with the pure TiO₂NRs, offering attractive perspectives for applications ofthe Co₃O₄/TiO₂heterostructure in solar cells or photocatalysts.The In₂O₃/TiO₂nanocomposite films have been prepared by dip-coating process. The composition and morphologies of samples were characterized. In₂O₃nanoparticleswith mean size of30–40nm adhered to TiO₂NRs and the density of the nanoparticles isdramatically increased with the number of cycles of dip-coating process. Thephotoelectrochemical properties of the TiO₂NRs photoelectrodes before and after beingsensitized with In₂O₃nanoparticles were investigated under visible light illumination.The results show that the photocurrent of In₂O₃/TiO₂is dramatically enhancedcompared with that of the TiO₂NRs or In₂O₃nanoparticles under visible lightillumination. The enhanced photocurrent is attributed to the coupling effects of In₂O₃nanoparticles and TiO₂NRs. However, it also shows that when excess In₂O₃nanoparticles are deposited on the TiO₂NRs, there is a decrease of photocurrent undervisible light illumination.（Chapter4）4) CdS-Pt/TiO₂composite nanostructure has been fabricated via a two-step method,which was used for the hydrogen production under visible-light irradiation. Pt/TiO₂wassynthesized by Pt nanoparticles deposited on TiO₂NRs, and followed by depositing CdSnanoparticles on Pt/TiO₂by SILAR method. The composition and morphologies ofobtained CdS-Pt/TiO₂nanocomposites were characterized by XRD, FESEM, TEM,UV-vis spectrum, and XPS respectively. The results show that the Pt nanoparticles with adiameter of2-5nm are uniformly dispersed on the TiO₂NRs and the CdS nanoparticles arealso uniformly dispersed on the Pt/TiO₂. It is found that the CdS nanoparticlesembedded in the interstices of the Pt/TiO₂formed intimate contacts between the NRs,which benefit to significantly enhance the charge separation and then the photocatalyticactivity. The photoactivity for the hydrogen production of CdS-Pt/TiO₂, Pt/TiO₂andTiO₂electrodes were assessed comparatively in terms of the photocurrent collected. TheCdS-Pt/TiO₂electrode shows the biggest photocurrent (0.30mA cm^-2) at1.0V undervisible-light illumination, which is about3.1times larger than that of the Pt/TiO₂electrode and5.45times larger than that of the TiO₂electrode, respectively. Thephotoelectricity efficiency CdS-Pt/TiO₂electrode is1.90％, which is about3.0timeslarger than that of the Pt/TiO₂electrode and10.0times larger than that of the TiO₂electrode, respectively. The electrochemical impedance spectroscopies were studied,and the results indicate that the electrical conductivity of CdS-Pt/TiO₂electrode is betterthan that of Pt/TiO₂and TiO₂electrodes. And the possible mechanism of the obtainedCdS-Pt/TiO₂nanocomposite as a photocatalyst for hydrogen production was proposed.（Chapter5）5) The TiO₂NTs were prepared by anodizing method and Pt/TiO₂electrocatalyst composite electrodes were prepared by photodeposition method. FESEM and TEMobservations reveal that the Pt nanoparticles about3-10nm in diameter are uniformlydispersed on the TiO₂NTs, both inside and outside of the tubes. Electrocatalytic activityand stability for the electrooxidation of ethanol were studied by cyclic voltammetry,chronoamperometry and electrochemical impedance spectroscopy. Because of thethree-dimensional open nanostructure of the TiO₂support for well-dispersed Ptnanoparticles, the Pt/TiO₂electrocatalyst has a larger active surface area for faciletransport of ethanol in comparison with the Pt/C and Pt/Ti electrocatalysts. Meanwhile,Pt/TiO₂catalysts show a relatively higher activity and anti-poisoning capability forethanol electrooxidation in both acidic and alkaline media than the Pt/C and Pt/Tielectrocatalysts due to the stronger synergistic interaction between TiO₂support and Ptnanoparticles with smaller particle size.（Chapter6）...