Preparation And Visible Light-activated Photocatalytic Properties Of Ag-based Semiconductor Photocatalysts

In recent years, with the fast development of economy and society, people have paid close attention to the issues of energy and environment. Semiconductor photocatalytic technology, which can fully utilize the solar energy for water decomposition, air cleaning, organic matter degradation and carbon dioxide photoreduction, has become a significant tactic for solving the energy shortage and global environmental pollution. Tremendous efforts have been made to develop efficient semiconductor photocatalyst, such as TiO2 because of its stability, non-toxicity and low cost. Nevertheless, it’s not suitable for large-scale industrial production and practical applications in the social life due to its low quantum efficiency and wide bandgap, which could only adsorb 4% of solar energy. Consequently, developing novel efficient photocatalysts and broadening the photocatalyst responses will be a hot topic in the field of semiconductor photocatalysis.During the past decades, Ag-based photocatalysts, such as silver orthophosphate, silver vanadate, silver oxide, silver halides (AgCl, AgBr, AgI), have attracted public attention due to their visible response and excellent photocatalytic activity. However, single-component semiconductors have poor photocatalytic efficiency in visible light range due to insufficient charge-separation ability and photocorrosion. To address these problems, a wide variety of multi-component nanocomposites have been synthesized, such as TiO2/Ag3PO4, Ag3PO4/MoS2,AgCl/BiOCl, AgVO3/g-C3N4, etc. According to the results of extensive researches, it is well-known that multi-component photocatalysts display better photocatalytic activity than single-component photocatalysts in degrading organic contaminants. The main reason is that the heterojunction between different semiconductors with matching energy band structure can promote efficient separation of photogenerated charge carriers and enhance solar light absorption in the visible region. In this paper, valuable explorations have been implemented as follows:(1) preparation of AgVO3/AgI composited photocatalyst (2) preparation of TiO2-x-NS/Ag/Ag3PO4 composited photocatalyst (3) preparation of GO/Ag3PO4/g-C3N4 composited photocatalyst. The main content could be summarized as follows:1. AgVO3 was synthesized using AgN03 and NH4VO3 by hydrothermal synthesis method. A series of AgVO3/AgI composite photocatalysts were prepared with the in situ anion-exchange method by adjusting the concentration of KI solution. With increasing KI concentration, more AgVO3 turned into AgI particles, and the absorption of AgVO3/Agl showed slight red shift. According to the results of photocatalytic activity experiments under visible light irradiation, when the mole ratio of AgVO3/AgI is 3:1, the AgVO3/AgI composite showed the best photocatalytic activity, which is 4.5 and 9 times more than AgVO3 and AgI, respectively. At last, a possible mechanism was proposed.2. TiO2 nanosheet (NS) was prepared by a facile solvothermal method using tetrabutyl titanate as a source of titanium and hydrofluoric acid (HF) as the structure director. TiO2-x-NS/Ag was synthesis through a redox reaction between the reductive TiO2 with oxygen vacancies (TiO2-x) and AgNO3 solution. TiO2-x-NS/Ag/Ag3PO4 composite was prepared by in situ precipitation method. The photocatalytic performance of different photocatalysts was measured with the degradation of methylene blue (MB) at room temperature under visible light irradiation. The photocatalytic activity experiments indicated that TiO2-x-NS/Ag/Ag3PO4 exhibited much higher photocatalytic activity than Ag3PO4 and TiO2-x-NS/Ag3PO4. This could be attributed to the surface plasmon resonance (SPR) of Ag particles and matching energy band structure between TiO2-x-NS and Ag3PO4, which could promote efficient separation of photogenerated charge carriers and enhance solar light absorption in the visible region.3. According to previous research, GO owns unique properties, such as its special surface with hydroxyl and carboxyl groups, which can adsorb positive ion Ag+, and g-C3N4 can be phosphorylated by Na2HPO4. Thus, graphene oxide (GO)/Ag3PO4/g-C3N4 composite was prepared by a simple precipitation method. The photocatalytic degradation of RhB showed that GO/Ag3PO4/g-C3N4 exhibits the best photo-degradation activity, which is 162 times of pure g-C3N4 and 8 times of pure Ag3PO4. The enhanced photocatalytic activity was attributed that photogenerated electrons and holes could be transferred to GO and g-C3N4 sheets, respectively. GO not only expand the light absorption range, but also enhance the adsorption of pollutants. In addition, GO/Ag3PO4/g-C3N4 was more stable than pure Ag3PO4. Finally, the controlled experiment proved that the degradation of RhB was mainly attributed to h+ and ·O2- in the whole photodegradation process. On the basis of band-structure analysis and photocatalytic performance, a possible photocatalytic mechanism was proposed.