| Meeting the growing global energy demand is one of the important challenges of the 21st century. Currently over 80% of the world’s energy requirements are supplied by the combustion of fossil fuels, which promotes global warming and has deleterious effects on our environment. Moreover, fossil fuels are non-renewable energy and will eventually be exhausted due to the high consumption rate. A new type of alternative energy that is clean, renewable and inexpensive is urgently needed. Among sveral candidates, solar energy is particularly attractive because it is essentially clean and inexhaustible. Photocatalysis and photovoltaics are two of the most important routes for the utilization of solar energy. Meanwhile, environmental protection is also critical to realize a sustainable future, and water pollution is a serious problem of current society. Photocatalysis is also an essential route for the degradation of organic dyes in waste water. As a new type of solar cell possessing great potential and broad prospects, quantum dot-sensitized solar cell (QDSC) is at a stage of rapid development. Rencently, the semiconductor nanomaterials have been wildly used in many fields such as photocatalysis and photovoltaics due to their unique properties of optics, electronics and photoelectronics. Among various semiconductor materials, zinc oxide (ZnO) has been regarded as one of the most promising photocatalysts owing to its wide band gap (Eg= 3.37 eV), large exciton binding energy, high carrier mobility, non-toxicity, abundant availability, simple tailoring of the nanostructures, and easy modification of the surface structure. Nevertheless, very poor response to visible light and high recombination ratio of photoinduced electron-hole pairs and photocorrosion has hindered the application of ZnO in photocatalysis. In this study, we designed and prepared three ZnO-based composite photocatalysts with high efficient visible-light harvesting capability and charge separation efficiency as well as excellent chemical stability, and the main content and results are presented in Chapter 3, 4 and 5. As the most important part of QDSCs, the photoanodes composing of semiconductor oxide directly affects the harvesting of sunlight and transfer of photo-excited electrons, and subsequently influences the final photoelectric conversion efficiency. The conventional TiO2 nanoparticles with a large number of surface defect state and interface barriers will limit the transport of photo-excited carriers. TiO2/Au hybrid photoelectrode with highly efficient light harvesting capability and transmission property of photo-induced electrons was designed and developed to finally improve the photoelectric conversion efficiency of QDSCs. And the corrospoding work is discussed in Chapter 6.1. A visible light-responsive TiO2-coated ZnO:I nanorod (ZnO:I/TiO2 NRs) array film vertically aligned on indium tin oxide (ITO) via a hydrothermal method and a subsequent wet-chemical process is reported. The fabricated I-doped photocatalytic ID nanorod structure densely coated with a layer of TiO2 was found to exhibit enhanced light absorption intensity, effective separation of photogenerated e-h, and excellent chemical stability. These photochemical and photoelectrochemical characteristics distinctly enhance the performance of the ZnO-based photocatalyst, which is further evidenced by the degradation of rhodamine B (RhB), a widely used model pollutant. More importantly, the TiO2 coating promotes high stability and photoactivity for the ZnO:I NRs films in both acidic and alkaline solutions. Furthermore, the recycled degradation results showed that TiO2-coated ZnO:I NRs could serve as an efficient photocatalytic material to degrade organic pollutants in aqueous eco-environments.2. A multifunctional visible-light-driven photocatalyst composed of iodine-doped ZnO aggregates (ZnO:I) film post decorated by TiO2 (ZnO:I@TiO2) on fluorine-doped tin oxide-coated (FTO) glass via hydrothermal method and subsequent wet-chemical process is demonstrated. A series of ZnO:I with various iodine concentrations were firstly prepared to study iodine dopant amount dependent-photocatalytic activity. The photocatalytic measurement results showed that the ZnO:I film with an optimum I-doping ratio of 5.0 mol% achieved degragation efficiency of 93.7% toward RhB, which is much higher than that of undoped ZnO (only ～54.3%). Furthermore, the ZnO:I@TiO2 exhibited enhanced light absorption and charge separation efficiency of photogenerated e-h, as characterized by UV-vis absorption spectra and the photoelectrochemical characterization by EIS, which is originating from iodine doping and TiO2 post modification. Owing to these synergic advantages, the ZnO:I@TiO2 composite photocatalyst exhibited significantly enhanced decomposition activity for RhB (～97%after 4 h of irradiation, a 79% increase over pure ZnO) under visible irradiation. Additionally, the ZnO:I@TiO2 films exhibited enhanced chemical stability in both acidic and alkaline solutions in comparison to ZnO:I, which was further verified by repeated photodegradation experiments under visible light irradiation. These results indicate that the prepared ZnO:I@TiO2 could serve as an efficient photocatalytic material to degrade organic pollutants in aqueous eco-environments.3. A high-efficiency visible-light-driven photocatalyst composed of homogeneously distributed Au nanoparticles (AuNPs) well-defined on hierarchical ZnO microspheres (ZMS) via a controllable layer-by-layer self-assembly technique is demonstrated. The gradual growth of the characteristic absorption bands of Au loaded on ZnO in visible region with increasing number of assemblies indicates the enhancement of the light harvesting ability of the ZMS/Au composites as well as the reproducibility and controllability of the entire assembly process. Results on the photoelectrochemical performance characterized by EIS and transient photocurrent response spectra indicate that the ZMS/Au composites possess increased photoinduced charge separation and transfer efficiency compared to the pure ZMS film. As a result, the hybrid composites exhibited enhanced decomposition activity for methylene blue and salicylic acid as well as the antibacterial activity in killing S. aureus and E. coli under visible irradiation. Notably, well-distributed Au components even at a rather low Au/ZnO weight ratio of～1.2% also exhibited extraordinary photocatalysis. Such a facile and controllable self-assembly approach may be viable for preparing high-performance visible-light-driven ZMS/Au photocatalyst in a simple and controllable way, and consequently, extendable to other plasmon-enhanced heterostructures made of nanostructured semiconductors and noble metals for great potential application in environmental protection.4. In order to solve the drawbacks of the TiO2 mesoporous photoelectrode on light harvesting and electron transport, monodisperse Au nanoparticles (NPs) was incorporated in TiO2 film via an in situ assembly process for plasmon-enhanced CdS/CdSe-sensitized solar cells. An overall efficiency of 6.0% was obtained for TiO2/Au QDSC, which was 50% higher than that of the pure TiO2 cell. Furthermore, the TiO2/Au exhibited enhanced light absorption and electron transport efficiency of photogenerated e-h, as characterized by UV-vis absorption spectra and the photoelectrochemical characterization by EIS, which is originating from the introduction of AuNPs. Notably, Quantum dots (QDS) surrounding AuNPs form QDs@Au core-shell structure. The AuNPs in photoelectrode work as electron relay, facilitating the photogenerated electron transfer to the conduction band of TiO2 and inhibiting the recombination of electrons in the conduction band of TiO2 photoanode transfer back to react with Sn2- in electrolyte. |
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