Semiconductor Photocatalyst: Possibilities and Challenges

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2013, Article ID 371356, 8 pages http://dx.doi.org/10.1155/2013/371356 Review Article Ag3PO4 Semiconductor Photocatalyst: Possibilities and Challenges Gui-Fang Huang, Zhi-Li Ma, Wei-Qing Huang, Yong Tian, Chao Jiao, Zheng-Mei Yang, Zhuo Wan, and Anlian Pan Department of Applied Physics, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082, China Correspondence should be addressed to Wei-Qing Huang; Received 30 January 2013; Accepted 8 March 2013 Academic Editor: Yongzhu Fu Copyright © 2013 Gui-Fang Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Ag3 PO4 as a photocatalyst has attracted enormous attention in recent years due to its great potential in harvesting solar energy for environmental purification and fuel production. The photocatalytic performance of Ag3 PO4 strongly depends on its morphology, exposed facets, and particle size. The effects of morphology and orientation of Ag3 PO4 on the catalytic performance and the efforts on the stability improvement of Ag3 PO4 are reviewed here. This paper also discusses the current theoretical understanding of photocatalytic mechanism of Ag3 PO4 , together with the recent progress towards developing Ag3 PO4 composite photocatalysts. The crucial issues that should be addressed in future research activities are finally highlighted. 1. Introduction The development of efficient photocatalysts is very important and desirable in environmental pollution mediation and solar energy conversion [1–6]. Over the past decades, fundamental progress has been made in developing novel photocatalysts, particularly visible light response catalysts for the efficient utilization of solar energy. A great deal of photocatalysts, including inorganic, molecular, and hybrid organic/inorganic materials, have been explored to meet specific requirements such as a light-absorbing wavelength modification, photoinduced charge separation, and a faster photocatalytic reaction. Among the various photocatalysts developed, TiO2 is undoubtedly the most popular and widely used photocatalyst since it is of low cost, high photocatalytic activity, chemical and photochemical stability [7, 8]. However, TiO2 is not ideal for all purposes and performs rather poorly in processes associated with solar photocatalysis due to its wide band gap (3–3.2 eV), thus making impractical overall technological process based on TiO2 . To design visible-light-driven photocatalysts, two strategies have been proposed. One is to modify the wide band gap photocatalysts (such as TiO2 , ZnS) by doping or by producing hetero-junctions between them and other materials [4, 7– 12], and the other involves exploration of novel semiconductor materials capable of absorbing visible light. Various compounds, such as BiVO4 [13], Bi2 WO6 [1, 14], CaBi2 O4 [15], PbBi2 Nb2 O9 [16], Bi4 Ti3 O12 [17], and Ag@AgCl [18], and others have been reported to be promising photocatalysts under visible light irradiation [19–22]. Despite many of these photocatalysts being effective for the degradation of organic pollutants and water splitting, up to date, the present achievements are still far from the ideal goal. Yi and his coworkers [23] have recently presented the pioneering work on exploring the photocatalytic properties of Ag3 PO4 that exhibit extremely high photooxidative capabilities for the O2 evolution from water and the decomposition of organic dyes under visible-light irradiation. Actually, the photodegradation rate of organic dyes over Ag3 PO4 is dozens of times faster than the rate over BiVO4 and commercial TiO2−𝑥 N𝑥 [23, 24]. Moreover, the most interesting is that this novel photocatalyst can achieve a quantum efficiency of up to 90% at wavelengths greater than 420 nm, which is significantly higher than the previous reported values. This finding potentially opens an avenue for solving current energy crisis and environment problems with abundant solar light, and the 2 Journal of Nanomaterials O P Ag (a) (b) Figure 1: Unit-cell structure of cubic Ag3 PO4 , showing (a) ball and stick and (b) polyhedron configurations. Red, purple, and blue spheres represent O, P, and Ag atoms, respectively [24]. research of Ag3 PO4 is thus attracting considerable interest. Since then, many efforts have been devoted to further improving and optimizing their photoelectric and photocatalytic properties. Despite the fact that Ag3 PO4 is a promising candidate for environmental remediation and renewable energy, the consumption of a large amount of noble metal and the low structural stability of pure Ag3 PO4 strongly limits its practical environmental applications. Therefore, it is a highly crucial task to improve the photocatalytic stability of Ag3 PO4 while maintaining its high photocatalytic activity. In this paper, the effects of morphology and orientation of Ag3 PO4 on the catalytic performance and the current theoretical understanding of key aspects of Ag3 PO4 photocatalysts are presented. Also reviewed is the effort on the photocatalytic stability of Ag3 PO4 and Ag3 PO4 composite photocatalyst. 2. The Structure of Ag3 PO4 Ag3 PO4 is of body-centred cubic structure type with space group P4-3n and a lattice parameter of ∼6:004 Å. The structure consists of isolated, regular PO4 tetrahedra (P–O distance of ∼1.539 Å) forming a body-centred-cubic lattice. The six Ag+ ions are distributed among twelve sites of twofold symmetry [25]. This indicates that each Ag atom at (0.25, 0, 0.50) actually occupies one of the two sites at (𝑥, 0, 0.50) and (0.5 − 𝑥, 0, 0.50) on the 2-fold axis. The unit-cell structure of cubic Ag3 PO4 is shown in Figure 1, in which the Ag atom experiences 4-fold coordination by four O atoms [24]. The P atoms have 4-fold coordination surrounded by four O atoms, while the O atoms have 4-fold coordination surrounded by three Ag atoms and one P atom. 3. Morphology Control and Catalytic Properties of Ag3 PO4 Since Ag3 PO4 was first reported as visible light response photocatalyst by Yi and his coworkers [23], much research has been devoted to investigate this active photocatalyst [26– 29], and different methods have been developed to synthesize various Ag3 PO4 and its composites [28, 30–32]. The investigation of photocatalytic activity shows that all of the prepared Ag3 PO4 exhibit excellent photocatalytic activity under UV light irradiation or visible-light illumination, which is much more excellent than that of commercial P25 TiO2 [33, 34] or N-doped TiO2 [30, 35, 36] as shown in Figure 2. It is well known that the morphology of materials is closely related to the exposed facets of the crystals, which directly affect the properties of the catalysts. Various Ag3 PO4 nanostructures including spherical morpholo (...truncated)