Structure Regulation And Visible Photocatalytic H₂ Evolution Of CuInS₂ And Cu₂ZnSnS₄

Photocatalytic H2 evolution by solar energy is one of the ideal approaches to solving the current energy crisis and environmental issues. The conventional photocatalytic H2 evolution systems usually employed wide bandgap semiconductors, which could only absorb UV and partial visible lightirradiation. To efficiently utilize solar energy, the exploration of visible-light-driven photocatalysts with high efficiency and good stability has been received great attractions. As important direct band-gap semiconductors, both CuInS2 and Cu2ZnSnS4, with low band-gap energy of ~1.5 eV(well matching the solar spectrum) and high absorption coefficient of ~105 cm-1, are promising materials for photocatalytic H2 evolution. Recently, the theoretical and experimental studies on the applications of CuInS2 and Cu2ZnSnS4 in the fields of inorganic thin film solar cells and dye-sensitized solar cells have achieved great advance. However, as excellent photocatalysts, the researches on CuInS2 and Cu2ZnSnS4 in photocatalytic H2 evolution are just in the beginning. Especially, few works have been reported on the relationship between the structures and the photocatalytic H2 evolution activities of CuInS2 and Cu2ZnSnS4. In this dissertation, we focused on the photocatalytic H2 evolution activities of CuInS2 with different morphological structuresand Cu2ZnSnS4 with different crystalline structures, aiming at exploring high efficient CuInS2 and Cu2ZnSnS4 photocatalysts. This dissertation consists of four parts as follow:(1) CuInS2 particles with different morphological structures were synthesized by a polyvinylpyrrolidone(PVP)-assisted solvothermal route in ethylene glycol with metal chlorides and thiourea as precursors. PVP increased the viscosity of reaction system, thus inhibited the growth of CuS nucleus and the adsorption of Cl- on <1012 > facet of CuS nucleus. On the other hand, PVP is advantageous to the diffusion of In3+ into CuS nucleus to form CuInS2 nucleus. CuS nucleus mainly grow along <1012 > direction, while the growth of Cu InS2 nucleus dominated <001> direction. The different growth direction of CuS and CuInS2 nucleus caused the mircostructure of CuInS2 tuned from flower-like to sphere-like by increasing the dosage of PVP. The flower-like CuInS2 was mainly exposed with {001} facets, and the sphere-like one was mainly exposed with {100}facets. The sphere-like CuInS2 exhibited higher photocatalytic H2 evolution activity, suggesting {100} facet possessed more active sites for H2 evolution than {001} facet.(2) Wurtzite Cu2ZnSnS4 nanospindles were firstly synthesized via a facile one-pot method using copper(II) acetylacetonate, zinc acetate, tin(IV) acetate and 1-dodecanethiol as precursors. By using a simple annealing process, mixed wurtzite-kesterite and pure kesterite Cu2ZnSnS4 were obtained. Cu2ZnSnS4 has been found to exhibit phase-dependent photocatalytic H2 evolution under visible light irradiation, which is probably attributed to the different electric structure(band structure) and surface atomic configuration of wurtzite and kesterite Cu2ZnSnS4. Comparing with wurtzite Cu2ZnSnS4, kesterite Cu2ZnSnS4 have more dispersive band structure, thus the photogenerated holes and electrons have smaller effective masses and higher migration abilities. Meanwhile, kesterite Cu2ZnSnS4 possesses much more polar and charge imbalanced {112} facets, which would be stabilized by absorbing the cations and/or the anions and be favorable for the surface chemical reaction. Moreover, during the facet change from wurtzite {100} gradually into kesterite {112}, some instable atom formed on the kesterite surface, which could also enhance the adsorption of H+ and S2-, thus enhanced the photocatalytic H2 evolution.(3) Kesterite Cu2ZnSnS4 with different crystallinities were obtained by high temperature recrystallization of as-prepared kesterite Cu2ZnSnS4 nanocrystals under elevated temperature. The crystallinity of Cu2ZnSnS4 was improved as the increase of annealing temperature. The improved crystallinity is helpful for reducing the native defects of Cu2ZnSnS4, thus decreasing the recombination of photogenerated carriers and increasing the carriers’ concentration, and then significantly enhancing the surface specific photocatalytic H2 evolution rate. The crystallinity and the surface specific area are two key factors that determine the overall photocatalytic H2 evolution activity of kesterite Cu2ZnSnS4 photocatalyst. Too high temperature would promote the fast growth of Cu2ZnSnS4 nanocrystals, resulting in dramaticdecrease ofthe surface specific area, thus reduce the overall photocatalytic H2 evolution activity. A rapid heating and rapid cooling process, which would induce the improvement of crystallinity without significant loss of specific surface area, could further enhance the overall photocatalytic H2 evolution activity ofkesteriteCu2ZnSnS4 photocatalyst.(4) A seriers of Cu2ZnSnS4/SiO2 were fabricated by loadingkesteriteCu2ZnSnS4 nanocrystals on SiO2 spheres followed by high temperature annealing. The fast growth ofkesteriteCu2ZnSnS4 nanocrystals was efficiently suppressed by the loaded-on SiO2. All Cu2ZnSnS4/SiO2 exhibited superior photocatalytic H2 evolution. Especially, the photocatalytic H2 evolutin rate per unit mass Cu2ZnSnS4 was significantly enhanced, which was attributied the less loss of specific surface area during annealing process.