The Applications Of Nano-TiO₂ And Their Nanocomposites In Photoelectrochemistry

Recently,in response to the increasing demands for solving the energy crisis and environment pollution,which severely threaten the human's survival, considerable efforts in this aspect have to be directly connected with a challenging search for new clean and regenerative energy sources with high efficiencies and no pollution to replace the pollutive and consumptive energy sources such as petroleum and coal.It is generally accepted that breakthroughs in photoelectrochemical technologies hold the key to new generations of the direct solar energy-based photovoltaic cells,not only for applications in providing clean and regenerative energy but especially for eliminating environmental pollution.To date,due to their unique photoelectrochemical properties,excellent chemical stability,bioinertness, innocuity and easy preparation,semiconductor nanomaterials especially TiO₂ nanomaterials have been widely used in the photoelectrochemical fields such as solar photo-electric conversion and photocatalytic treatment of the pollutants.However,TiO₂ nanomaterials still have several drawbacks,such as wide bandgap,short-lifetime and easy recombination of the photogenerated electron-hole pairs and the limited photoresponding range,which to a great extent hinder their photoelectrochemical activities as well as their applications. In this dissertation,pursuing these efforts towards improving the photoresponding range and the photoelectrochemical activities of TiO₂ nanomaterials,we have explored the feasibility to design and optimize the nano-TiO₂ architectures for potential photoelectrochemical applications by means of dye-sensitization,semiconductor hybridization,fabrication of ordered surface,utilization of novel electrolytes,and so on,which were characterized by TEM,SEM,XRD and UV-vis measurements.Their photoelectrochemical performances have been evaluated by means of various techniques including electrochemical impedance spectroscopy(EIS),Mott-Schottky(MS) spectroscopy,photocurrent and photocatalytic measurements,and the corresponding photoelectrochemical mechanisms were to some extent investigated.It is expected that the research work in this dissertation can provide useful and important theoretical and practical guidances for the effective enhancement and control of photoelectrochemical performances of nano-TiO₂ architectures.The main contents in this work are listed in the following:1.The stepwise assembly of negatively charged dye molecules and positively charged TiO₂ colloids on pretreated fluorin-dopped tin oxide(FTO) conductive glass substrate surfaces utilizing the layer-by-layer(LbL)approach was successfully fabricated,which was characterized by UV-vis spectrophotometry and electrochemistry.Photocurrent measurements demonstrated that the photocurrent response within the UV and visible wavelength range were observed in these hybrid thin films,and the generated photocurrent increased linearly as the deposited bilayers increased,which could be attributed to the dye-sensitized effect of the layered dye molecules and the linearly ordered assembly of hybrid thin films.It was demonstrated that electrostatic LbL films were attractive systems for the photoelectrochemical investigation,and the effective control of the photoelectrochemical performances of nano-TiO₂ architectures could be achieved by the structure of the multilayered films.2.Growing attention has been paid to quasi-solid dye-sensitized solar cells(DSSCs)using polymer gel electrolytes due to their unique hybrid network structure and favorable properties such as thermal stability, non-flammability,negligible vapor pressure and easy solidification,when compared with the liquid electrolytes.To pursue high conversion efficiency of DSSCs,it is necessary and pivotal to enhance the ionic conductivities of these polymer gel electrolytes.However,conventional polymer electrolytes exhibit very low ambient ionic conductivity because of the severe crystallinity of polymers.In this respect,a novel composite polymeric gel comprising room-temperature ionic liquids(1-butyl-3-methyl-imidazoliumhexafluorophosphate, BMImPF₆)and heteropolyacids(phosphotungstic acid, PWA)in polymer matrix was successfully prepared and employed as a quasi-solid state electrolyte in DSSCs.These composite polymer electrolytes offered specific benefits over the ionic liquids,and heteropolyacids,which effectively enhanced the ionic conductivity of the composite polymer electrolyte.The performances of the unsealed DSSCs employing these composite polymer electrolytes were also enhanced to a great extent.It is expected that these composite polymer electrolytes are new attractive alternative electrolytes for the fabrication of the long-term stable quasi-solid state or solid state DSSCs.3.In efforts to make use of unprecedented physical and chemical characteristics of TiO₂ nanomaterials and effectively control their unique photoelectrochemical properties,it is essential to affix them on surfaces or arrange them in an organized network.In this work,patterned TiO₂ microarrays with different features were successfully fabricated by using a simple,fast and cheap patterning technique,and their photoelectrochemical performances could be adjusted by the control of the feature size of TiO₂ patterns.As an alternative to typical pattern transfer techniques for microfabrication,this work employed a standard microcontact printing(μCP) process for the fabrication of patterned TiO₂ microarrays onto FTO substrates. During theμCP process,the TiO₂ precursor was used as the "ink" and transferred from a pattern-featured poly(dimethylsiloxane)(PDMS)"stamp" onto the pretreated FTO substrate.Following the subsequent thermal oxidation,patterned TiO₂ microarrays with different features were successfully achieved.It was demonstrated that their photoelectrochemical properties were dependent on the feature size of the TiO₂ patterns.With the decrease of the feature size,the photocurrent activity and photocatalytic ability of the patterned TiO₂ thin film increased,which was due to the increased TiO₂ surface area as well as the increased optical path length within the patterned TiO₂ thin film, resulting from multiple reflection of incident light.This work indicates that patterned TiO₂ thin films are attractive systems for surface tailoring and also provide a novel method to effectively control the photoelectrochemical properties of nanostructured TiO₂ thin films with promising applications in microsystem devices for solar energy conversion,photocatalysis,sensing,and SO on,4.In order to enhance the photogenerated charge separation and the photoresponding range of nano-TiO₂,considerable interest has focused on TiO₂-based nanocomposite.Generally,there has been considerable effort made toward the investigation of nanocomposites comprising zero-dimensional(1D)nanoparticles or one-dimensional(1D)nanostructures; however,to the best of our knowledge,little attention has been paid to the coupling of 1D nanostructures with 0D nanoparticles.In this work,novel coupled bicomponent nanocomposites consisting of anatase TiO₂ nanoparticles and wurtzite ZnO nanorods were successfully prepared using a one-step hydrothermal method for the first time,and their structures and morphologies could be controlled by adjusting the Ti/Zn molar ratio of the precursors.Compared with the mere TiO₂ nanoparticles or ZnO nanorods,the coupling of TiO₂ nanoparticles and ZnO nanorods produced a significant effect on its properties,such as surface morphologies,surface areas,electronic properties,and photoelectrochemical properties.The generated photocurrent of the coupled ZnO-TiO₂ nanocomposite was largely enhanced with several orders of magnitude higher intensities than that of the mere TiO₂ nanoparticles or ZnO nanorods.The photocatalytic activities of the coupled TiO₂-ZnO nanocomposites were also significantly enhanced.The enhancement of the photocurrent actions and photocatalytic activities might arise from the increased surface area,which can enhance the light harvest and the ability of generating photoinduced electron-hole pairs of active sites,and the favorable electron-transfer properties of the heterojunctions TiO₂/ZnO in the coupled ZnO-TiO₂ nanocomposites.It is expected that the present work is of notable significance for understanding the unique properties that result from the coupled nanocomposites and designing new nanocomposites with advanced functions in photoelectrochemical applications.