Rational Design Of Ag₃PO₄-based Heterostrctures With Superior Photocatalytic Performance

Ag₃PO₄ as a novel visible light responsive semiconductor photocatalytic materials are widely used in water treatment, the photocatalysis of water splitting, waste gas treatment and sterilization and other fields. It has a high photocatalytic activity, morphology adjustable, etc., photocatalytic water splitting quantum efficiency up to 90%, the rate of oxygen evolution three times and eight times than WO3 were Bi VO4, respectively, so that it has an important significance and use prospects. Recently, people through some modifications of Ag₃PO₄, such as facet engineering, morphology and size controlling to further enhanced the Ag₃PO₄ photocatalytic activity. However, the wide application of Ag₃PO₄ is still restricted by the inherent drawbacks, including the high recombination rate of photo-generated electron-hole pairs, the high cost of Ag-containing chemicals as well as the serious photocorrosion in photocatalysis process. Therefore, the researchers focused on the areas of the Ag₃PO₄-based composites. Mainly based on the following method to construct composites: protection strategy, Type II heterojunction strategy and Z-scheme strategy.This article focus on the above three major shortcomings of Ag₃PO₄, selecting graphene, Si C nanowires（Si C NWs） and Ti O₂ which were abundant, cheap and readily available materials as the composite object, through rational design prepared Ag₃PO₄/Ti O₂/Ag-r GO, Ag₃PO₄/Si C and 3D Ag₃PO₄/GAs. The structure and catalytic mechanism of light microscopic composites were carried out a detailed characterization and in-depth discussions, the results obtained are as follows:（1） Silver nanoparticles（Ag NPs） modified reduced graphene oxide wrapped Ag₃PO₄/Ti O₂（Ag₃PO₄/Ti O₂/Ag-r GO, Ag-ATG） photocatalysts have been developed through a rational design that combines the optimization of charge generation, separation and transfer in the composites, which helps to increase the photocatalytic performance. The Ag-ATG composites possess novel microstructure, in which Ti O₂ mesoporous spheres of hundreds of nanometers in size are decorated with dense nano-sized Ag₃PO₄ to form pinecone-liked Ag₃PO₄/Ti O₂ particles, which were further wrapped by r GO sheets that are selectively decorated with Ag NPs. The Ag-ATG composites exhibit improved photocatalytic performance toward degradation of methylene blue（MO） and methyl orange（MB） under visible light compared to bare Ag₃PO₄ and Ag₃PO₄/Ti O₂/r GO（ATG）. The underlying mechanism has been studied based on the results of reactive oxygen species capture experiment, photoluminescence（PL） spectra, and photocurrent measurements under visible light and monochromatic lights. The improved photocatalytic performance is mainly ascribed to the efficient spatial separation of photo-induced electrons and holes in Ag-ATG, i.e., the electrons in Ag₃PO₄ transfer to Ag-r GO, meanwhile the holes in Ag₃PO₄ transfer to Ti O₂. Ag NPs play an important role in the hybrid structure owing to the synergistic effect of Ag NPs and r GO, which not only enhance the light harvest but also increase the capacity of electron accepting from Ag₃PO₄. Meanwhile, active photo-induced electrons at the plasmonic Ag NPs can facilitate the formation of O₂?- radicals for photocatalysis. As a result, both the stability and photocatalytic active of Ag-ATG are significantly improved.（2） Ag₃PO₄ nanoparticles loaded on the ultralong Si C NWs via in situ precipitation method by tuning the concentration of the reactants, resulted the loading capacity and particle size controlled Ag₃PO₄/Si C nanocomposite. In addition, the composite remains a structural characteristic of ultralong nanowires, and can processing into fiber membrane by a simple film-forming operation. The microstructure of Ag₃PO₄/Si C studied by XRD, SEM, TEM and FTIR, Raman. The photocatalytic properties of composite materials studied by MO and p-nitrophenol（PNP） photocatalysis under visible light. And the mechanism of the Ag₃PO₄/Si C were also studied by the reactive oxygen species capture experiment, PL spectra, and photocurrent measurements under visible light and monochromatic lights, as well as electrochemical impedance spectroscopy（EIS）。（3） Three dimensional（3D） Ag₃PO₄/graphene hydrogels（Ag₃PO₄/GHs） has been first synthesized via a facile two-step strategy, where GHs prepared by hydrothermal method, then Ag+ and PO43- successively passed through the GHs and precipitation of nano-sized Ag₃PO₄ throughout the surface of GHs porous sturcture. Assisited by feeze drying, 3D Ag₃PO₄/graphene aerogels（GAs） could be prepared. Owing to the excellent electronic properties, GAs significantly increase the electrical conductivity of 3D Ag₃PO₄/GAs nanocomposite that largely facilitates the electrons draining from Ag₃PO₄ resulting longer photo-induced carrier lifetime compared to bare Ag₃PO₄. As photocatlystic material for water spliting, 3D Ag₃PO₄/GAs exhibts enhanced oxygen evolution rate compared to bare Ag₃PO₄. This efficient continuous ion adsorption method is expected to expand the use of other nanomaterials in complex systems, such as Ag2CO3/GAs, Ag X（X = Br, I） / GAs etc. The underlying mechanism has been studied based on the results of time-resolved fluorescence spectra, EIS and photocurrent measurements under visible light.