Mixed Solvothermal Synthesis Of Hierarchical ZnIn₂S₄and The Photocatalytic Active Sites

Energy crisis is the major issues faced in twenty-first century sustainabledevelopment. Recent years, semiconductor photocatalytic hydrogen productionprovides an attractive way for the development and implementation of an idealfuture use of solar energy. ZnIn₂S₄with a narrow band gap of about2.4eV （CB：-0.29eV vs NHE； VB：2.11eV vs NHE） has a strong visible-light harvesting,appropriate band structure and high photochemical stability, proving to be a broadapplication prospects. But its current catalytic performance is much below thepotential, this may be due to the high recombination rate of photo-generated carriers.Recently, the reactivity of exposed crystal facets in semiconductor catalyst receivedincreasing attention. Experimental and theoretical exploration consistently showedthat exposing appropriate plane of the samples can greatly increase its catalyticactivity. This thesis mainly focus on the synthesis of hierarchical ZnIn₂S₄spheresphotocatalyst, exposed crystal facets, carrier diffusion and photocatalyticperformance, and covering the following aspects.1) Mixed solvothermal synthesis of hierarchical ZnIn₂S₄photocatalyst.Hierarchical ZnIn₂S₄spheres composed of tiny nanosheets are synthesized bysolvothermal treatment with Zn（CH₃COO）₂·2H₂O, InCl₃·4H₂O and CH₃CSNH₂inethanol/water mixture. The influences of solvent composition and reactionconditions on the morphology, composition, structure and optical absorptionproperties of ZnIn₂S₄are investigated by means of X-ray diffraction, scanningelectron microscopy, transmission electron microscopy, nitrogenadsorption-desorption isotherms, EDS spectra and UV-visible diffuse reflectancespectroscopy. The photocatalytic activity of the sample is tested in Na₂S-Na₂SO₃aqueous solution under visible light （λ>400nm）. It is found that the formation of themicrosphere comprises two phases: the precursor complexes (In_3.8Zn_0.2S_1.6Cl)generation and in situ recrystallization. Lattice mis match at the interfaces betweenIn_3.8Zn_0.2S_1.6Cl and ZnIn₂S₄mainly contributes to the formation of such ahierarchical architecture. This allows the fine synthesis of hierarchical structurewithout surfactant.2) Exposed crystal facets of hierarchical ZnIn₂S₄microspheres. Crystal surfaceis considered a key factor affecting the performance of photocatalysts. Recent years,the study on active sites of photocatalyst was always applied on model catalystwithout effective and practical means to characterize complex structurephotocatalysts. Herein, TEM, XRD and dye adsorption are applied to partially observate, overall analysis and verification the exposed crystal facets of hierarchicalZnIn₂S₄, respectively. To a randomly orientated sample, the XRD peak intensity of alattice plane is proportional to the number of the lattice plane. Peak intensity ratioreflects the proportion of the lattice plane number in different direction whichdetermines the average geometrical shape of a sample. Therefore, XRD can be usedto study the overall exposed crystal facets of the sample. Anionic dye molecules canbe adsorbed on the crystal surface with positive ions which is not sensitive to theshape of a catalyst. So it can be used to verify the change of surface facets in ahierarchical catalyst. The analys is showed that the nanosheets are enclosed by {100}and {001} facets, and the photocatalytic activity tests reveal that hydrogen evolutionrate of the product is positive correlative with {100} crystal faces. The result of thisstudy provides a viable method to exposed crystal surface characterization incomplex ZnIn₂S₄samples, which is potentially applicable to other systems.3) In order to clarify the facet-dependent photocatalytic activity in ZnIn₂S₄andin-depth understanding of their carrier migration process, electrostatic potential of aunit cell of ZnIn₂S₄is calculated by DMol3module of Materials Studio. Uniquelayered structure of ZnIn₂S₄makes the electrostatic potential between layersfluctuate strongly to form periodic dipoles aligned along [001], and the electrostaticpotential inner-layer is much smaller. Photogenerated electrons and holes tend totransport in In-S and Zn-S sub-layer with lower electrical barrier, and to {100}crystal plane of the sample finally. The synergy between electron transfer processand {100} crystal faces makes the {100} planes become active sites of hydrogenevolution. Meanwhile, electrostatic potential calculation provides a general methodfor visualizing the internal electric fie ld and dipoles. The research bring about a newinsight for understanding of photoexcited charge carriers diffus ion andphotocatalytic active sites, and contribute to the design of high-performancephotocatalyst.