Chapter 1. Introduction
1.5 Research Motivation
The motivation of this research is to explore the efficient conversion of sunlight into chemical fuels through photoelectrochemical water splitting, which has the potential to generate sustainable hydrogen fuel. Our research focuses on the various strategies to improve the performance of the photoelectrochemical water splitting for ZnO nanorod-array-based photoanodes.
ZnO is chosen as a semiconductor material in the phototelectrochemical cell based on several reasons. First, our research proposes that a photoelectrode in the photoelectrochemical cell can achieve the overall water-splitting reaction. The CB and valence band position of the semiconductor should straddle the hydrogen (0 V vs. NHE) and oxygen (1.23 V vs. NHE) redox potential. The commonly used semiconductors for overall water splitting in photoelectrochemical cells are TiO2
and ZnO. The CB of ZnO is at a more negative position than that of TiO2, such that the photogenerated electrons are better able to reduce protons because they can obtain a larger driving force, resulting in greater solar energy conversion
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efficiency. Second, ZnO may become an alternative to TiO2 for fabricating photoelectrodes because of its higher carrier mobility and high chemical stability in the neutral condition. Third, the morphology of ZnO can be easily controlled through adjusting synthesis conditions. Tuning the nanostructure of the ZnO may increase light absorbance and charge carrier separation.
Three main approaches are employed to improve the performance of photoelectrochemical water splitting. First, CdTe and InP quantum dots are used to sensitize the ZnO photoanode. Second, doping strategies are used to narrow the bandgap of the ZnO. Third, plasmic nanoparticles are placed on the ZnO to investigate the plasmonic-enhanced photoelectrochemical water-splitting reaction.
The 1D nanostructure of the ZnO can be a potential solution for the following reasons: (1) light absorption is raised because of the increasing light path in the photoelectrode; (2) the surface area of the photoelectrode is dramatically increased, resulting in an increased loading amount of quantum dots or nanoparticles; and (3) an electronic transport pathway is provided to improve electron transfer efficiency.
Chapter 2 describes all the analysis techniques used in this study.
Chapter 3 discusses a QD-sensitized nanowire photodevice based on the photosensitization of ZnO nanowires with CdTe and InP QDs through different strategies. CdTe with a more favorable CB energy (ECB= 1.0 V vs. NHE) can inject more energetic electrons into ZnO faster than CdSe (ECB= -0.6 V vs. NHE).
Additionally, InP QDs are typically more stable than chalcogenides because an oxide layer forms in air on the surface of the InP nanocrystal. The InP is also a non-toxic nanomaterial. We expect to harvest more visible-light regions using quantum dots in the solar spectrum to enhance the photoelectrochemical water-splitting reaction.
Chapter 4 examines the homogeneous doped ZnOxS1-x photoelectrochemical
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water splitting. Traditional doping methods and alternative sensitization methods for solar hydrogen generation are provided to considerably enhance photocurrents.
Chapter 5 discusses the ZnO nanorod-array photoelectrode decorated with gold nanoparticles, and investigates the plasmon-induced effect in the photoelectrochemical cell. Measurement methods (described in Chapter 2) will be attempted to analyze the contribution of each plasmon-induced effect in the photoelectrochemical water-splitting reaction.
Chapter 6 using gold nanorods to decorate on the surface of the ZnO nanorod-array as photocathode, combining with the CdTe quantum dots sensitized TiO2
nanorods array as photoanode to investigate the plasmon-induced effect in the photocathode reaction.
In Chapter 7, the ZnO nanorod array was grown on the silver nanoparticle pattern. The usage of silver metallic nanostructures can support the desired surface plasmon effects to increase absorbance and achieve efficient charge-carrier collection inside the photoelectrode; moreover, the surface plasmon resonance of spherical silver nanostructures commonly occurs in a near-ultraviolet region to couple with the photoexcitaion of ZnO nanorods.[69] Both localized surface plasmon resonance in metal nanoparticles and plasmon polaritons propagating at the metal/semiconductor interface can improve the capture of sunlight and the collection of charge carriers.
Chapter 8 presents the NIR-driven photoelectrochemical water splitting using ZnO nanorod array decorated with CdTe quantum dots and plasmon-enhanced UCNs, which were employed to harvest and convert NIR light into high-energy photons. The high-energy photons excited the CdTe QDs to generate high-energy electron–hole pairs. These excited electrons are transferred to the conduction band
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(CB) of ZnO nanorods, conducted to the Pt foil through the external circuit, and made to react with water to generate hydrogen, thereby producing the photocurrent.
The photogenerated holes are oxidized with water to form oxygen.
The associated fabrication and the factors that affect the photoelectrochemical performance of these strategies will also be discussed in each chapter. The controlling parameters connected with the strategies of each chapter are summarized in Figure 1-11.
Figure 1-11. Strategies of controlling parameters for this study.
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