Enhanced photocatalytic activity and photoinduced stability of Ag-based photocatalysts: The synergistic action of amorphous-Ti(IV) and Fe(III) cocatalysts

https://doi.org/10.1016/j.apcatb.2016.01.011Get rights and content

Highlights

  • The amorphous Ti(IV) and Fe(III) are loaded on the Ag-based photocatalyst surface.

  • Photocatalytic activity of Ag-based materials can be improved by loading Ti(IV).

  • The poor stability of Ti(IV)/AgBr can be improved by loading Fe(III) cocatalyst.

  • A synergistic action of amorphous-Ti(IV) and Fe(III) cocatalysts was proposed.

Abstract

In recent years, Ag-based materials have attracted a great deal of attentions due to their excellent photocatalytic performance. However, the rapid recombination of photogenerated charges and the poor photostability cause an obvious decrease of their photocatalytic performance. In this study, amorphous Ti(IV) as a hole cocatalyst was first successfully loaded on the surface of AgBr photocatalyst by a facile impregnation method. It was found that the photocatalytic activity of AgBr could be greatly improved by a factor of 1.5 when the loading amount of Ti(IV) cocatalyst was 0.05 wt%. Moreover, in addition to the AgBr, the amorphous Ti(IV) could also be used as an effective hole cocatalyst to greatly improve the photocatalytic performance of other Ag-based materials (such as AgCl, AgI, Ag2O, Ag2CO3, and Ag3PO4). However, owing to the rapid transfer of photogenerated holes by Ti(IV) cocatalyst, more photogenerated electrons were accumulated on the conduction band of AgBr, causing an obvious deactivation due to the reduction of surface lattice Ag+ ions to metallic Ag. In this case, after the further surface modification by Fe(III) as an electron cocatalyst, the photoinduced stability and photocatalytic activity of Ti(IV)/AgBr could be significantly enhanced. The possible reason is due to the synergistic action of amorphous Ti(IV) and Fe(III) cocatalysts, namely, Ti(IV) cocatalyst acts as a hole-capture center to efficiently transfer holes to oxidize organic contaminants, while Fe(III) cocatalyst functions as a reduction active site to reduce oxygen efficiently. Compared with the expensive noble metal cocatalyst (such as Au, Pt, and RuO2), the surface modification by low-cost transition metal cocatalysts (such as Ti and Fe) is a significant method to develop highly efficient photocatalytic materials.

Introduction

Photocatalysis technology plays a crucial role in the treatments of wastewater and air pollution owing to its strong oxidizing ability and environmentally friendly property [1], [2], [3], [4], [5], [6]. In recent years, Ag-based compounds as the typical visible-light-driven photocatalysts have received growing attentions, and various Ag-based materials such as AgCl [7], [8], [9], [10], AgBr [11], [12], [13], AgI [14], [15], Ag3PO4 [16], [17], [18], [19] and Ag2O [20], [21] have been widely reported. Compared with the typical visible-light N-TiO2 photocatalyst, the Ag-based photocatalysts usually show a higher photocatalytic performance for the degradation of various organic contaminants [22], [23]. However, the rapid recombination rate of photogenerated electron-hole pairs in the photocatalysts results in a low photocatalytic efficiency. Therefore, to further enhance the photocatalytic performance of Ag-based photocatalysts, abundant strategies have been performed, such as morphology modulation [24], [25], composite materials [26], [27], and cocatalyst modification [12], [28]. Among these reported methods, cocatalyst modification has been demonstrated to be one of the most important strategies to enhance the photocatalytic performance of various materials by effectively separating photogenerated charges. More importantly, only a small amount of cocatalyst can drastically increase the photocatalytic activity by some simple synthetic strategies [12], [17], [28].

For the efficient photocatalytic degradation of organic contaminants, the rapid transfer of photogenerated holes onto the photocatalyst surface and their following effective oxidation for organic substances are highly required. As a consequence, the surface modification by loading hole cocatalyst is an efficient route to improve the photocatalytic performance via the rapid capture of interfacial holes and promoting their rapid oxidization reaction. At present, the well-known hole cocatalysts such as RuO2 [29], IrO2 [30] and PdS [31] have been widely investigated and applied in photocatalytic reactions. However, the above reported noble metals are usually expensive and scarce, thus largely limiting their practical applications. Hence, it is necessary and important to develop some nontoxic and economical hole-cocatalyst materials. In fact, some low-cost hole cocatalysts such as CoOx [32], Fe2O3 [33] and B2O3−xNx [34] have been developed and applied in various photocatalytic systems. Recently, amorphous TiO2 (Ti(IV)) was also found to show excellent ability to capture photogenerated holes. For example, Liu et al. have demonstrated that amorphous Ti(IV) can work as a hole-trapping center on the surface of rutile TiO2 to effectively decompose organic pollutants [35]. In addition, the amorphous TiO2 film has also been reported to be a very good carrier of photogenerated holes on semiconductor photoelectrodes such as BiVO4 and GaAs [36], [37]. However, the relevant reports about amorphous Ti(IV) as the hole cocatalyst are still very limited. In view of its nontoxic, low-cost and abundant in natural resources, it is very meaningful and interesting to investigate whether amorphous Ti(IV) can act as a general cocatalyst to improve the photocatalytic performance of other photocatalysts.

In this paper, we successfully modified Ag-based materials (such as AgCl, AgBr, AgI, Ag2O, Ag2CO3, and Ag3PO4) with amorphous Ti(IV) as the hole cocatalyst by a simple impregnation method. After the Ti(IV) modification, all the resultant Ti(IV)/Ag-based photocatalysts exhibited an obviously enhanced photocatalytic performance for the photodegradation of phenol, suggesting that amorphous Ti(IV) can be used as a general hole cocatalyst to greatly improve the photocatalytic performance of various Ag-based materials. However, owing to the rapid transfer and capture of photogenerated holes by Ti(IV) cocatalyst, more photogenerated electrons were accumulated on the conduction band (CB) of AgBr, causing an obvious deactivation due to the reduction of surface lattice Ag+ ions to metal Ag. To prevent the deactivation (the rapid reduction of lattice Ag+ ions) in Ti(IV)/AgBr and to improve its photoinduced stability, the well-known Fe(III) electron-cocatalyst was further loaded on its surface to prepare the Fe(III)-Ti(IV)/AgBr photocatalyst. In this case, it is expected that the Fe(III) electron-cocatalyst can inhibit the deactivation progress of Ti(IV)/Ag-based photocatalysts and improve their photoinduced stability by rapidly trapping excess photogenerated electrons. In fact, it was found that both the photocatalytic activity and photostability of Ti(IV)/Ag-based photocatalysts could be significantly enhanced by loading Fe(III) cocatalyst. To the best of our knowledge, this is the first report about the enhanced photocatalytic activity and photostability of Ag-based photocatalysts by the simultaneous modification of amorphous Ti(IV) and Fe(III) cocatalysts. This work may open a new sight for the development of low-cost and highly efficient photocatalytic materials.

Section snippets

Preparation of AgBr photocatalyst

In a typical synthesis of AgBr photocatalyst, 50 mL of AgNO3 solution (0.01 mol L−1) was dropped into 50 mL of NaBr solution (0.01 mol L−1) under magnetic stirring at room temperature. Then the mixture was continuously stirring for another 30 min to obtain a yellow suspension. The resulting suspension was maintained at 60 °C for 2 h. Finally, the precipitate was harvested by filtration, rinsed with distilled water, and dried at room temperature.

To obtain the Ag/AgBr composite, the as-obtained AgBr

Morphology and microstructures of Ti(IV)/AgBr photocatalyst

The FESEM and TME images of the as-prepared samples are shown in Fig. 2. It can be seen that the size of AgBr (Fig. 2A and B) is in the range of 0.2–1 μm due to a simple precipitation method, and the particle surface is smooth. The corresponding XRD pattern (Fig. 3a) clearly suggests the formation of AgBr phase (JCPDS no. 06-0438). After loading Ti(IV) cocatalyst (0.05 wt%) on its surface (Fig. 2C), the particle morphology and size show no obvious difference compared with the pure AgBr (Fig. 2A).

Conclusions

In summary, Ti(IV) was first demonstrated be an effective and general hole cocatalyst to significantly improve the photocatalytic performance of various Ag-based photocatalysts such as AgCl, AgBr, AgI, Ag2O, Ag2CO3, and Ag3PO4. However, owing to the rapid transfer of photogenerated holes by Ti(IV) cocatalyst, more photogenerated electrons were accumulated on the CB of AgBr, causing an obvious deactivation due to the reduction of surface lattice Ag+ ions to metallic Ag. After the further surface

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51472192, 21277107, 21477094, and 61274129) and 973 Program (2013CB632402). This work was also financially supported by program for new century excellent talents in university (NCET-13-0944), Wuhan Youth Chenguang Program of Science and Technology (2014070404010207), and the Fundamental Research Funds for the Central Universities (WUT 2015IB002).

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