Titanium dioxide (TiO2) is proposed to be effective photocatalyst for wastewater treatment, air purification, and self-cleaning ability, because of its strong oxidation and superhydrophilicity. In order to conquer the limits of TiO2, a variety of methods have been used. This paper presents a critical review of novel research and achievements in the modification of TiO2 nanoparticles with silver halide (AgX,
Water is one of the most essential substances for all life. However, an increasing number of organic pollutants are being discharged into aqueous environment along with wastewater. Various treatment techniques and processes have been used to remove the organic pollutants from wastewater. Among all the approaches proposed, the photocatalysis technology for the treatment of contaminated water has attracted significant attention in the past few decades, because of its high stability, low cost, and nontoxicity toward both humans and the environment [
In response to these deficiencies, several approaches for TiO2 modification have been developed: doping by metals (noble metals, transition metals, lanthanide metals, alkaline and alkaline earth metals, cadmium sulphide, etc.) and nonmetals (nitrogen, fluorine, sulfur, carbon, etc.) and modification with dye-sensitization and composite semiconductors.
After the phenomenon of surface plasmon resonance (SPR) was discovered, a novel photocatalyst named plasmonic photocatalyst was developed, which was a composite photocatalyst composed of noble-metal nanoparticles and a polar semiconductor, such as silver halide (AgX). In the photographic process, an electron is liberated by photon adsorption and electron-hole pair creation. The liberated electron will combine with an Ag+ cation to form an Ag0 atom. Continuous irradiation will bring limitless photons, which will be absorbed by the AgX particles to form a cluster of Ag0 atoms, and this phenomenon is the very basis for chemical photography. Considering its instability under sunlight, AgX is scarcely ever used as a photocatalyst. In the study of the photocatalyzed H2 production from a CH3OH aqueous solution in the presence of AgBr dispersed on a silica support, Kakuta et al. observed that Ag0 atoms take place on the surface of AgBr particles and AgBr particles are not destroyed under UV illumination [
With a direct band gap of 5.6 eV and an indirect band gap of 3.25 eV, silver chloride (AgCl) is widely acknowledged as photosensitive material and is one of the source materials of photographic films [
The composite also can be made into thin films. Zhou et al. fabricated the Ag/AgCl/TiO2 nanocomposite thin films by a sol-gel and photochemical reduction method. The samples show high visible light photocatalytic activity for degradation of 4-chlorophenol aqueous solution and recycling stability [
Chemical doping of TiO2 with nonmetallic elements (C, N, B, S, or F) to extend their visible absorbance is a commonly used method [
Compared with the commercial P25 photocatalyst, Sangchaya et al. [
Updating the synthesis methods or changing conditions of the degradation progress can not only increase the photocatalytic activity, but also reveal the mechanism, although there are still some details in the reaction that remain unclear.
Previous research showed that silver bromide (AgBr) with a band gap of 2.6 eV is an inorganic photosensitive semiconductor material, which will have high photographic sensitivity under visible light irradiation and can be used to modify TiO2 to have visible light activity [
Hu et al. prepared Ag/AgBr/TiO2 nanocomposites by the deposition-precipitation method. The catalyst showed high visible light photoexcited efficiency when investigated for the decomposition of azodyes and killing of
Ag-AgBr/TiO2 composites prepared by a sol-gel method followed by photoreduction showed intrinsic antibacterial activities against
Graphene is a single layer which has many outstanding properties, such as high theoretical specific surface area, superior mechanical, chemical stabilities, and so forth. Most recently, Wang et al. synthesized a new photocatalytic nanocomposite, Ag-AgBr/TiO2 supported on reduced graphene oxide (Ag-AgBr/TiO2/RGO). The four-component nanocomposite (Ag-AgBr/TiO2/RGO) exhibited a much higher photocatalytic activity for the degradation of penicillin G (PG) under white light-emitting diode (LED-W) irradiation, compared with the single-component (TiO2), two-component (Ag-AgBr, Ag/TiO2, and TiO2/RGO), and three-component (Ag-AgBr/RGO, Ag-AgBr/TiO2 and Ag/TiO2/RGO) nanocomposites [
To well understand the properties of photogenerated charges and their effects on the photocatalytic performance, a detailed mechanism schematic of transfer and separation of photogenerated charges is presented, as shown in the Figure
The mechanism of AgBr/TiO2.
In order to investigate the mechanism of AgBr/TiO2, Velmurugan and Swaminathan [
In order to promote the photocatalytic activity or stability, an appropriate synthesis approach and the ration of the different component are all the key aspects. Consequently, optimization of these two matters in the final results. And if there is a selected substrate to support the material, it will be more efficient under irradiation.
As a member of the silver halides, silver iodide (AgI) has three different phases: at room temperature, it is usually a mixture of a hexagonal
It has been reported that the inner electric field of heterojunction catalyst can decrease the recombination of the electron-hole pairs and prompt the photocatalytic activity of semiconductor photocatalysts [
Cao et al. prepared a composite catalyst AgI/AgCl/TiO2 by ion exchange method. And the result indicates that when the molar percentage of AgI to initial AgCl is 20%, the composite catalyst has the maximal degradation efficiency of MO [
Specialized morphology of the particles may lead to specific properties, so the 3D nanoparticles with high specific surface area have an enhanced ability of photocatalysis. And multisemiconductors’ common effect also may enhance the initial properties of the single material.
AgX is instable when exposed to sunlight alone. And TiO2 shows relative activity under UV light. However, AgX/TiO2 system showed high photocatalytic activity and stability under visible light. Great efforts have been made to develop AgX/TiO2 preparation methods in recent years. In this review, AgX/TiO2 composite semiconductors with different morphologies, such as Ag/AgX composite, nanotube or other nanostructures, thin film fabrication, and reduced graphene oxide supported AgX/TiO2, were prepared to enhance their visible light activity and circulation stability through hydrothermal, ion-exchange methods or precipitation processes.
It was assumed that properties, such as stability, electron-hole recombination rate, and visible light photocatalytic activity, of AgX/TiO2 strongly depended on the preparation method used. However, the properties of AgX/TiO2 cannot be improved by a single preparation method. Therefore, a major area of future research would be the development of AgX/TiO2 preparation method, such as preparation of TiO2 coupled with various types of AgX, ternary composite of AgX/TiO2 and other semiconductors, and AgX/TiO2 loaded on supported materials. Such materials preparation together with the development of technically application is crucial for broader scale utilization of photocatalytic systems in commercial application.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Xiaojia Wan and Ting Wang contributed equally to this work and share first authorship.