| Energy and environmental issues at a global level are important topics. To solve the issues, converting sunlight to usable electricity or fuel is the most viable way for producing environmentally, friendly, renewable, and clean energy. Over the past sev-eral decades, as a next-generation energy carrier, hydrogen production obtained by photocatalytic overall water splitting using solar energy has attracted much research attention. On the other hand, with the rapid development of computer hardware and sci-entific software, first-principles calculations based on density functional theory (DFT) have been widely and successfully used in various fields (i.e. condensed matter, quan-tum chemistry, and materials science). Of course, DFT also is a powerful theoretical tool to study the solar photocatalysts and their photocatalytic mechanism of water split-ting. In this dissertation for Ph. D degree, we focus on semiconductor photocatalysts and layered nanocomposite by performing extensive first-principles calculations. This dissertation contains the following chapters.In Chapter1, theoretical knowledge related to this thesis is briefly introduced, including Born-Oppenheimer approximation, one-electron approximation, Hohenberg-Kohn theorem, Kohn-Sham equation, as well as various functionals for exchange and correlation. Finally, we also introduce several DFT-based computational packages.In Chapter2, we briefly review the research progress in photocatalytic H2pro-duction. Firstly, we summarize fundamental mechanism and main processes of photo-catalytic H2generation, and introduce some UV-active photocatalysts for water split-ting. Then, some approaches to tune the band structures for visible light harvesting are introduced in details. Finally, we review several effective approaches for efficient photogenerated carrier separation.In Chapter3, we examine the charge-compensated n-p codoped anatase TiO2sys-tems. Compensated n-p codoping means that the number of electrons from the n-type dopants equal to the number of holes contributed by the p-type dopants. Previous investigations of n-p codoping focus on the transition metal-based codoping, and lit- tle attention has been paid to the noble metal-based codoping. This chapter includes two parts. In the first part, we explore the (Rh+F) codoping effect on electronic structures and photocatalytic activities of anatase TiO2. We find that the stable charge-compensated donor-acceptor pair (Rh+F) codoping in TiO2can effectively reduce the band gap by forming delocalized and filled intermediate bands within the band gap. Interestingly, the band edge alignment in the (Rh+F) codoped TiO2is desirable for water splitting. Moreover, the calculated optical absorption curve of (Rh+F) codoped TiO2verifies that it has significantly improved visible light absorption. In the second part, we explore the (Rh+F) codoping effect on electronic structures and photocat-alytic activities of anatase TiO2(101) surface. Our calculated results clearly reveal that single noble metal (Rh) dopant can be stably doped at the upmost layer in this surface with the aid of the codoped F atom. In addition, we find that the (Rh+F) codoping effect on the electronic structures and optical properties of anatase TiO2(101) surface is similar to the results of this codoped bulk case, which indicates that it is easy to realize in experiments according to these theoretical findings.In Chapter4, we explore the double hole doped anatase TiO2systems. In the previous works, the dopants mainly include the non-metal elements, such as C, N, P, and S. Only few studies have addressed the double-hole-mediated coupling in metal and non-metal codoped anatase TiO2. Here, we extend the concept of of double-hole-mediated coupling of dopants, and examine the (Sc+P) and (In+P) codoping effects on electronic structures and photocatalytic activities of anatase TiO2. It is found that the coupling of P dopant with the second-nearest neighboring O atom assisted by acceptor metals (Sc/In) leads to the fully occupied and delocalized intermediate bands within the band gap of anatase TiO2, which is driven by the P-O antibonding states. This metal-assisted P-O coupling can prevent the recombination of photogenerated electron-hole pairs and effectively reduce the band gap of TiO2. Moreover, the band edge alignments in (Sc+P) and (In+P) codoped anatase TiO2are desirable for water splitting. The calculated optical absorption curves indicate that (Sc+P) and (In+P) codoping in anatase TiO2can also effectively enhance the visible light absorption. These findings indicate that band structure engineering of anatase TiO2by the metal-assisted P-O coupling, namely, the double-hole-mediated coupling of acceptor metal and acceptor non-metal, is a promising method for enhanced photoelectrochemical water splitting.In Chapter5, we explore the photocatalytic mechanism of the hybrid g-C3N4/MoS2nanocomposite. The coupling betweewn semiconductors has been shown to be an ef-fective method for enhancing the efficiency of photogenerated carrier separation, im-proving the stability of photocatalyst and extend the extent of visible-light absorp-tion. Currently, semiconducting nanocomposite photocatalysts have attracted a lot of attention, especially, g-C3N4-based nanocomposites. Here, we explore the enhanced photocatalytic mechanism of the hybrid g-C3N4/MoS2nanocomposite. The calculated results show that it is a type-II band alignment between g-C3N4monolayer and MoS2sheet. Interestingly, the charge transfer between MoS2and g-C3N4results in a polar-ized field within the interface region, which will benefit the separation of photogenerat-ed carriers. In addition, this proposed layered nanocomposite is a good light-harvesting semiconductor. In addition, we find that a g-C3N4bilayer covering a MoS2sheet also displays desirable properties. These results and findings provides useful information for designing new semiconductor nanocomposite photocatalyst.In Chapter6, we explore the single-layer II-VI metal chalcogenide photocatalysts. It is essential to develop stable, low-cost and high efficient photocatalysts under vis-ible light for practical and mass hydrogen production. Here, using a first-principles design approach, we explore the single-layer II-VI metal chalcogenide photocatalyst-s(MX, M=Cd and Zn, X=S, Se and Te). Firstly, we find that the single-layer II-VI metal chalcogenides exhibit low formation energies and large range of band gaps from2.20to4.08eV. Next, calculations using a PBE and HSE06functional determine the conduction and valence band edge positions. Comparing the band edge positions with the redox potentials of H2O shows that single-layer II-VI metal chalcogenides are po-tential photocatalysts for water splitting. Moreover, the bandgaps, band edge positions, and optical absorption of the single-layer II-VI metal chalcogenides can be tuned by biaxial strain to increase the efficiency of solar energy conversion. |
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