Trans. Nonferrous Met. Soc. China 25(2015) 112-121

Research progress of Ag₃PO₄-based photocatalyst: Fundamentals and performance enhancement

Yun-zhu MA, Fan CHENG, Wen-sheng LIU, Juan WANG, Yi-kai WANG

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Received 10 February 2014; accepted 12 May 2014

Abstract: Ag₃PO₄ is found to be a highly efficient photocatalyst and receives great attention. The high activity of the photocatalyst is credited to the intrinsic electronic structure. The morphology control and nano-composite fabrication are used to improve the performance and practicability. This paper reviews the structure, properties and some theoretical aspects of Ag₃PO₄ single crystal. Also, the major strategies, namely the morphology control and hetero-nanostructure construction, as ways to improve the performance of Ag₃PO₄-based photocatalysts, are summarized with the aid of some typical instances.

Key words: Ag₃PO₄-based photocatalyst; electronic structure; morphology control; hetero-structure construction; preparation; photocatalytic activity

1 Introduction

Pollutants released by both civilian and military sectors have been imposing great threats on the water quality worldwide [1-3]. Thus, on behalf of the sustainable development of human society, large amount of scientific and technological efforts are paid toward water purification. Among those strategies, heterogeneous semiconductor photocatalysis has been recognized generally as a promising green route because it paves a possible way of utilizing the solar irradiation or artificial indoor illumination to decompose contaminants [4].

Since Fujishima [5] discovered the photocatalytic phenomenon of TiO₂ electrode in 1972, TiO₂-based photocatalysts have been most comprehensively studied. TiO₂ has the merits of low cost, strong redox capacity, relatively high activity and stability, etc [5-8]. However, due to such intrinsic limitations as wide band gap (3-3.2 eV), it is not adequate for all conditions especially under visible light irradiation [9-11]. Though doping can decrease the band gap, it will simultaneously introduce states that act as combining centers [12-16], which lowers the quantum efficiency. Thus, the development of novel visible light responsive photocatalysts has become an urgent issue.

Simple metal oxides (Fe₂O₃, WO₃, SnO₂, Bi₂O₃, etc) [17-20], multi-metal oxides (Bi₂WO₆, BiVO₄, CaBi₂O₄, InVO₄, etc) [21-24], solid solutions (ZnO_x-GaN_1-x, NaNbO₃-AgNbO₃, BiTa_1-xNb_xO₄, etc) [25-27] have been tried in terms of obtaining the desired band gap to achieve visible absorption. Among these, Ag⁺-based multi-metal oxides, especially those derived by combining Ag₂O and oxides of p-block elements (Ag₃PO₄, AgAlO₂, AgGaO₂, etc), were found to be promising. The improved photocatalytic performance can be credited to their similar electronic structures [28-30].

As a unique one, Ag₃PO₄ is so far the only compound that incorporates nonmetallic p-block specie into Ag₂O. More importantly, it possesses ultra-high photocatalytic activity in O₂ evolution and organic contaminant degradation [31]. Since YI et al [31] first used Ag₃PO₄ for photocatalysis and demonstrated the high activity, lots of researches have been devoted to Ag₃PO₄-based photocatalysts. Thus, a clear and complete summery of the present progress is essential. In this review, the structure, properties and some theoretical aspects of Ag₃PO₄ single crystal are introduced, while the major strategies, namely the morphology control and hetero-nanostructure construction, are concluded with the aid of some reliable examples.

2 Structure and properties of Ag₃PO₄

Ag₃PO₄ possesses a cubic crystal structure. The space group is . As Fig. 1 [32] illustrated, the PO₄ tetrahedron and AgO₄ tetrahedron constitute the lattice [33,34]. By theoretical optimization, the cell parameters obtained are a=b=c=6.010 and α=β=γ=90° [35]. This nearly fits with the experimental data of a=b=c=6.026 and α=β=γ=90°.

Fig. 1 Illustration of crystal structure of Ag₃PO₄ with ball-stick(a) and polyhedron(b)

To understand the origin of the high photocatalytic property of Ag₃PO₄, a comprehension of the electronic structure of the compound is needed. Experimentally, Ag₃PO₄ possesses an indirect band-gap of 2.36 eV as well as a direct transition of 2.43 eV, which was deduced from the ultraviolet-visible diffuse reflectance spectrum (Fig. 2) [31]. Thus, Ag₃PO₄ is able to absorb irradiation with a wavelength shorter than 530 nm, well extending into the visible region.

Fig. 2 Ultraviolet-visible diffusive reflectance spectrum of Ag₃PO₄ samples [31]

Also, theoretical studies were carried out. UMEZAWA et al [28] employed the DFT-based methods to reveal that due to the strong P—O bonds of PO₄ tetrahedral units, the covalent nature of Ag—O bonds is weakened. This impairs the Ag d and O p states to blend. Thus, the d character is excluded from the conduction-band minimum (CBM), remaining highly dispersive Ag s—Ag s hybrid bands. This leads to a small effective mass of the electron, which is advantageous for the carrier transfer to surface. MA et al [33] used first-principle DFT combined with the LDA+U formalism to show that Ag₃PO₄ has a large dispersion of conduction band, which facilitates the separation of charge carriers. Moreover, high concentration of Ag vacancies in Ag₃PO₄ lattice has a significant effect on the separation of electron-hole pairs and optical absorbance in the visible-light region. In addition to the calculations performed by the above two studies, LIU et al [35] used hybrid density functional method to more precisely get the electronic structure of Ag₃PO₄ photocatalyst (Figs. 3 and 4). The results reveal a band gap of 2.43 eV, which agrees well with the experimental result. The conduction bands are credited to Ag 5s and 5p states, while the valence bands mainly consist of O 2p and Ag 4d states. The VBM potential was 2.67 eV (vs normal hydrogen electrode), which indicates an adequate driving force for water oxidation or pollutants degradation.

On the other hand, the photocatalytic redox mechanism is illustrated with reference to CBM, VBM and the potentials of the relevant redox pairs (Fig. 5) [31]. It is evident that Ag₃PO₄ is able to decompose organic pollutants or split water to produce O₂ when AgNO₃ is present as the sacrificial agent. Moreover, Ag₃PO₄ shows a much higher photocatalytic activity in comparison with WO₃, BiVO₄, etc [28,31,32], which implies that Ag₃PO₄ is potentially a highly efficient photocatalyst. Consequently, much attention has been paid to the improvement of the property of Ag₃PO₄ (morphology control and hetero-structure fabrication).

Fig. 3 Band structure of Ag₃PO₄ calculated using PBE0 approach (a) and magnified view of band structure near Fermi level (b) [35]

Fig. 4 TDOS and PDOS of Ag₃PO₄ using PBE0 approach [35]

Fig. 5 Photon induced oxidation over Ag₃PO₄ under visible light as well as illustration of redox potentials of Ag₃PO₄ [31]

3 Morphology control strategy

As a common sense in the field of photocatalysis, the morphology (size, shape and kind of exposed facets, etc) of the photocatalysts has an influence on the efficiency and activity [4]. This also applies, within expectation, to the Ag₃PO₄-based materials. Since Ag₃PO₄ is a promising photocatalyst, lots of researches have been conducted to the morphology tuning. Such novel shapes as branch, tetrapod, nanorod, triangular prism [36], pine tree [37] and porous microcubes [38] are successfully obtained. Generally, those preparation methods involve precipitation (with the aid of Ag⁺-ligant complex, organic additives or templates) [37,39], chemical or electrochemical oxidation of metallic Ag into Ag⁺ (subsequently captured by PO^4-) [40], ligant- assisted anion exchange process [41] and hydrothermal synthesis [42] etc. Except for modulating the inner conditions of the reactions, those outside factors like ultrasound were also demonstrated to be influential to the morphology [36].

WANG et al [43] first managed to synthesize uniform tetrapod-like Ag₃PO₄ microcrystals (T-Ag₃PO₄) with a simple hydrothermal method without adding any template or surfactant. The precursor was phosphoric acid while the pH value of the reaction system was tuned by urea. By varying the amount of the urea, reaction time and temperature, the morphology of the product was successfully tuned. As the SEM images (Figs. 6(a) and (b)) shown, four arms of the tetrapod are cylindrical microrods with an average diameter of 5 μm and a length of 15-30 μm. As the XRD patterns shown (Fig. 6(c)), the intensity ratios of (110)/(200) and (222)/(321) for T-Ag₃PO₄ are 2.9 and 1.6, which are remarkably higher than those (0.56 and 0.9) of the irregular counterpart, respectively. From the XRD pattern, the high exposing rate of the (110) facet is demonstrated. The origin for higher intensity ratio of (110)/(200) is credited to the high surface energy of (110). The activity of the product was demonstrated in RhB degradation (Fig. 6(d)). In comparison with N-doped TiO₂, the Ag₃PO₄ samples exhibited higher catalytic activities, while the T-Ag₃PO₄ possessed the highest activity. The highest activity of T-Ag₃PO₄ was resulted from the higher surface energy of (110) facets than those of (200) facets.

A special example, with regard to the preparation method, is worth mentioning. JIAO et al [44] fabricated various shaped Ag₃PO₄ microcrystals based on the heteroepitaxial growth procedure, in which different seeds were added into the reaction system before precipitation happened. This is a procedure well recognized in the field of nano fabrication, in which the nucleation and crystal growth are separated in terms of space and time [45]. Figure 7 [44] illustrates the preparation procedure as well as the SEM images of the products. Actually, considering the fact that the structure diversity of the crystal nucleus will lead to drastically different shaped products, the usage of pure single sorts of seeds are beneficial to forming pure, uniform shaped crystals as well as to obtaining the desired morphology.

Though an absence of comparison in terms of activity between individual literatures, nearly all of them showed an enhanced reaction rate with regard to the spherical counterpart and N-doped TiO₂. Considering the large amount of the researches, some of the reliable and representative instances are listed in Table 1 with regard to the morphology, preparation method and photocatalytic activity.

Fig. 6 SEM images of T-Ag₃PO₄ at different magnifications (a, b), XRD patterns of tetrapod-like and irregular Ag₃PO₄ (c) and degradation of RhB with tetrapod-like Ag₃PO₄, irregular Ag₃PO₄ and N-doped TiO₂ under visible-light (λ>420 nm) (d) [43]

Fig. 7 Schematic illustration of growth process of Ag₃PO₄ with different morphologies fabricated by seed-mediated method using different seeds (a) and SEM images of four kinds of products (b-e) [44]

4 Hetero-nanostructure construction

Fabricating hetero-structure is a common strategy to enhance the activity of photocatalysts as well as to overcome some application barriers [4]. Such attempts were also made in the case of Ag₃PO₄. Ag₃PO₄-based composite photocatalysts including TiO₂/Ag₃PO₄ (decreasing Ag content to reduce cost) [48], Fe₃O₄/ Ag₃PO₄ (magnetic separable) [49], In(OH)₃/Ag₃PO₄ (enhancing absorption by tuning surface electric property) [50], Ag₃PO₄/carbon nanotube-stabilized pickering emulsion (enhancing activity by surface-chemical design of novel micro-reaction system) [51], Ag₃PO₄-graphene [52], Ag@(Ag₂S/Ag₃PO₄) (facilitating migration of charge carriers) (enhancing activity via synergistic effect of Ag and Ag₂S) [53], AgX/ Ag₃PO₄ (improving stability via core-shell structure) [54] and so forth have been successfully synthesized and studied. Each of the synthesized photocatalysts shows exclusive characters (described in the brackets behind) as well as common features shared with others like the promotion of charge carrier separation, increase of surface area and so forth. To see the literatures as a whole, one would be puzzled by the amount as well as the void of a complete comparison between the activities of the photocatalysts (variation of testing condition). To simplify, only selective instances are briefly introduced, while others are listed in Table 2.

Table 1 Preparation and photocatalytic properties of Ag₃PO₄ with different morphologies

HOU et al [58] prepared graphene-supported Ag₃PO₄/Ag/AgBr via photoassisted deposition- precipitation method. This was followed by subsequent hydrothermal treatment. Under the irradiation of visible light, the O₂-evolution rate of the nano-composite was two times of that of the bare Ag₃PO₄ powder. Compared with unsupported Ag₃PO₄/Ag/AgBr, graphene supported bare Ag₃PO₄ and Ag/AgBr, it also performed improved activity (Fig. 8). The depletion of the conduction band electrons of Ag₃PO₄, downshift of the Ag₃PO₄ valence band influenced by silver and charge transferring onto the graphene support were responsible for the enhanced activity (Fig. 9).

Another special case, in terms of the preparation method, is the work of YU et al [59]. Necklace-like Ag₃PO₄– polyacrylonitrile (PAN) hetero-nanofibers were successfully fabricated through electrospinning technique (Fig. 10). The product exhibited excellent photocatalytic activities for the degradation of organic contaminants under visible light irradiation. By tuning the mass ratio (between Ag₃PO₄ and PAN) and applied voltage, the morphology of the product can be changed correspondently. Since electrospun polyacrylonitrile is used in clothing production, this research implies a possibility to make the clothing material photocatalytic self-cleanable.

5 Conclusion and perspective

In summery, Ag₃PO₄, with characteristic electronic structure, is an efficient photocatalyst, which can harness the visible light to oxidize water as well as decompose organic pollutants in aqueous solution. From the point view of quantum chemistry, Ag₃PO₄ is the result of inserting a nonmetallic p block element-phosphorus-into the simple narrow band gap Ag₂O semiconductor. The introduction of p block elements (Al, Ga, Ge, As, and Sb, etc) into Ag₂O implicates a new strategy of designing high efficiency photocatalyst. The two typical strategies, namely morphology control and hetero-structure construction, are employed to improve the activity and practicability of Ag₃PO₄ based photocatalysts, which are proved to be highly effective.

Table 2 Preparation and photocatalytic properties of Ag₃PO₄-based hetero-structures

Fig. 8 Photocatalytic O₂ evolution under visible light irradiation (wavelength > 420 nm) over bare Ag/AgBr, Ag₃PO₄, Ag₃PO₄/RGO, Ag₃PO₄/Ag/AgBr, and Ag₃PO₄/Ag/AgBr/RGO (from bottom to top) (a) and transient photocurrent responses of electrodes functionalized with Ag₃PO₄-based materials in the same order (bottom to top) as in panel (b) (Measurements proceeded in a 0.01 mol/L Na₂SO₄ aqueous solution under visible light irradiation (wavelength > 420 nm, I₀=64 mW/cm²) at 0.5 V (vs SCE) bias [58]

Fig. 9 Model explaining increase of activity of Ag₃PO₄ upon functionalization with Ag/AgBr and RGO [58]

Fig. 10 Schematic illustration of formation process of Ag₃PO₄–PAN necklace-like nanofibers prepared by electrospinning (a) and SEM images of products (b, c) [59]

Future effort should be paid to: 1) the comparison between individual works in terms of activity of the catalysts; 2) development of novel preparation method which is environmental friendly and cost saving; 3) fabricating novel composites which possess high activity.

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Ag₃PO₄体系光催化材料的研究进展: 基础与改性

马运柱，成 帆，刘文胜，王 娟，王依锴

中南大学 粉末冶金国家重点实验室，长沙 410083

摘  要：Ag₃PO₄是一种高效的光催化剂，因而受到了极大的关注。其高光催化活性归功于其内禀电子结构。采用形貌控制和纳米复合体构筑的方法提高其性能和实用性。综述Ag₃PO₄单晶的晶体结构、性能及其基础理论，并用一些典型例子来说明提高其光催化活性的主要策略，即形貌控制和纳米复合体的构筑。

关键词：Ag₃PO₄体系光催化剂；电子结构；形貌控制；异质结构构筑；制备；光催化活性

(Edited by Wei-ping CHEN)

Corresponding author: Yun-zhu MA; Tel: +86-731-88877285; E-mail: zhuzipm@mail.csu.edu.cn

DOI: 10.1016/S1003-6326(15)63585-3