Preparation Of Metal Sulfides For Photocatalytically Reducing CO₂ With Methanol

The increase of atmospheric carbon dioxide（CO₂） and the depletion of fossil fuels raise serious concerns about global warming and energy crisis. Utilizing the abundant solar energy to convert CO₂ into organic chemicals would be an expedient accomplishment to resolve both problems, simultaneously. In this paper, a series of metal sulfides photocatalysts such as ZnS, ZnIn₂S₄ and Bi2S3 were synthesized and used for photocatalytically reducing CO₂ to produce methyl formate in methanol, which worked as both solution of absorbing CO₂ and sacrificial reagent. This work aims to enhance the photocatalytic performance of metal sulfides from the point of metal doping, crystalline structure, photocatalyst surface morphology, and composite semiconductor photocatalysts. The main contents are as follows:Firstly, a series of ZnS photocatalysts were prepared via simple precipitation, ion-exchange and hydrothermal methods, respectively; meanwhile, various Ni-doped ZnS photocatalysts were synthesized by hydrothermal method. The experimental results showed that the ZnS photocatalysts prepared by hydrothermal method had higher photocatalytic activity, which increased rapidly with the increasing hydrothermal time and reached a maximum at 24 h. Besides, the doping of Ni2+ could further enhance this activity, the optimal content of Ni dopant was about 0.3 wt% and the corresponding methyl formate production rate was 121.4 μmol g-1 h-1. The high photocatalytic activity could be attributed to that the Ni2+ doping facilitated the separation of photogenerated electron-hole pairs and reduced their recombination probability. The spectra caught by on-line ATR-FTIR spectroscopy confirmed CO₂ to be consumed in the reaction.Secondly, ZnIn₂S₄ nanosheets with hexagonal and cubic structures, have been prepared through a liquid ultrasonic exfoliation method and another strategy involving a lamellar hybrid, respectively. It was revealed that ZnIn₂S₄ could be used as an effective photocatalyst for CO₂ reduction, and both hexagonal and cubic ZnIn₂S₄ nanosheets exhibited high photocatalytic activity due to their large exposed surface area and thin thickness. Besides, hexagonal ZnIn₂S₄ showed higher activity and better stability for methyl formate production. The electronic structures of the two phases of ZnIn₂S₄ were investigated in the light of density functional theory. In addition to the narrower band gap, the conduction bands of hexagonal ZnIn₂S₄ were slightly more dispersive than those of cubic ZnIn₂S₄, indicating that the photogenerated electrons in hexagonal ZnIn₂S₄ possessed a smaller effective mass and therefore higher migration ability, leading to promoting the photoreduction activity.Thirdly, different shape of bismuth sulfide（Bi2S3）, including nanoparticles, and urchin-like, microspheres hierarchical nanostructures, have been successfully fabricated using a facile and template-free solvothermal method. The electron microscopy observations showed that both the sulfur sources and solvents greatly affected the morphologies of as-prepared Bi2S3. Compared with Bi2S3 nanoparticles, the hierarchical architectures exhibited higher photocatalytic activity, and Bi2S3 microspheres showed the highest activity. This was attributed to their special hierarchical structure, good permeability and high light-harvesting capacity.Fourthly, Bi2S3-ZnIn₂S₄ composite photocatalysts were synthesized by two step hydrothermal method, and used for the photocatalytic reduction of CO₂ with methanol. The results showed that the obtained Bi2S3-ZnIn₂S₄ composite photocatalysts had better photocatalytic activities than pure ZnIn₂S₄ nanosheet, and the optimal Bi2S3 loading amount was 2 wt%. The improved photocatalytic activity induced by the Bi2S3 loading was attributed to the interfacial transfer of photo-induced carriers（i.e., electrons and holes） between Bi2S3 and ZnIn₂S₄ nanosheets, which leads to effective charge separation. Furthermore, semiconductor component with narrow band gap could also broaden the light response of the composite photocatalyst.