Synthesis and photocatalytic application of Au/Ag nanoparticle-sensitized ZnO films
Highlights
► A new Au–Ag-ZnO film photocatalyst was fabricated by a facile three-step synthesis approach. ► The photocatalyst was applied in MO degradation with higher photocatalytic ability. ► Photocatalytic process with excellent stability was established.
Introduction
In the past decades, environmental problems have been increasingly serious with the development of the industry of human society. Photocatalytic degradation of organic pollutants using nanoparticulate semiconductors has been extensively investigated to solve environmental problems [1], [2]. Various photocatalysts have been synthesized and studied, such as TiO2 [3], ZnO [4], BiVO4 [5], AgNBO3 [6], and ZnWO4 [7]. ZnO is a wide-band gap oxide semiconductor with a direct energy gap (EG) of about 3.3 eV, and as a consequence, it absorbs UV radiation due to the band-to-band transitions [8]. ZnO, as an alternative nanomaterial, has a competitive photocatalytic activity (PA) greater, in some cases, than TiO2; such as on the discoloration of reactive blue 19, a textile anthraquinone dye, in aqueous suspension [9] and in the oxidation of protocatechuic acid [10]. By comparison with ZnO nano-powders, the film-form catalyst has the advantage for its thorough separation after the photocatalytic reaction and can be applied to the industrial applications [11], while it has smaller surface area and subsequently a lower photocatalytic activity. So it is important to find the way to enlarge the surface area with respect to the efficiency of photocatalytic processes. Therefore, the film surface treatment is currently a focus of attention, linked to the improvement of photocatalytic properties of ZnO film. Notably, many groups have documented that the control over nano or micro-scale structures on ZnO film surfaces is an effective method to enhance the surface area and different techniques have been developed such as metal organic chemical vapor deposition [12], sol–gel [13], [14], thermal evaporation, oxidation and anodizing [15], [16], [17]. In comparison to the other methods, electrodeposition has the advantage that it represents a low-cost and low-temperature method [18].
The modification of semiconductors with noble metals has attracted significant attention especially in heterogeneous photocatalysis [12], [19], [20], [21], [22], [23]. Incorporating silver in ZnO is now an exciting area in research for developing electronic applications [24], [25]. The modification with silver has influenced the photocatalytic activity of photocatalysts for Ag has special optical and electronic properties [26], [27], [28]. Silver can trap the photogenerated electrons from the semiconductor and allow the holes to react with the surface-bound H2O or OH− to produce hydroxyl radicals that result in the degradation reaction of organic species present. The main drawback of the silver interface is its chemical instability. Silver rapidly oxidizes when exposed to air, and the process would be accelerated in aqueous solutions, making it difficult to get reliable optical signals and to perform long time measurements [29]. Silver-based interfaces can only be employed when covered with a protecting layer, stabilizing the interface while keeping the favorable optical properties of silver. Merely, many approaches were used up to date to protect silver surfaces. One approach takes advantage of the spontaneous formation of self-assembled monolayers (SAMs) on silver [30], [31], [32], [33], [34], [35]. Other strategies are based on the spin-coating of thin films of polyionene (20 nm) [36] or aluminum tris-(8-hydroxyquinoline) (30 nm) [37]. A quite different approach to protect silver is based on the formation of bimetallic silver/gold layers taking advantage of the stable property of gold. The gold overlayer, being in contact with the solution, protects the silver due to its high chemical stability [38], [39], [40], [41].
In this work, Ag and Au were modified on ZnO sheetlike film and applied in the photocatalytic degradation of MO. Scheme 1 shows the simulative process. Because of the differences that the not-annealed and the annealed ZnO had brought, the effect of annealing step was discussed in the article. The relationship between the photocatalytic activity and the structure of prepared catalysts was discussed through a systematic characterization analysis in detail.
Section snippets
Materials
HAuCl4 was obtained from Sigma–Aldrich Chemie, Inc. All other reagents of analytical grade were obtained from commercial sources and used as received. Deionized water was used for preparation of all aqueous solutions.
Fabrication of Au–Ag-ZnO composites
ZnO was electrodeposited on indium tin oxide (ITO, In:SnO2) glass substrates in an aqueous electrolyte containing 0.05 M Zn(NO3)2 and 0.1 M KCl with a three-electrode electrochemical configuration with ITO (10 Ω cm−2, Nippon sheet glass) as the work electrode, a platinum wire counter
Characterization
Fig. 1 shows the SEM images of ZnO with/without deposition of Au and Ag. As shown in Fig. 1a, the morphology of ZnO is sheetlike. According to the work of Wang group [42], the nanodeposits should be the mixture of ZnO and Zn5(OH)8Cl2·H2O. After being annealed in 400 °C for 10 min, the surface of ZnO nanosheet is turned to a tough surface as shown in Fig. 1b, probably due to the pyrolysis of Zn5(OH)8Cl2·H2O (Eq. (1)):Zn5(OH)8Cl2·H2O → 5ZnO + 4H2O↑ + 2HCl↑
The BET surface area of the hierarchically porous
Conclusions
Au–Ag-ZnO architecture was fabricated through a facile three-step modification of Au–Ag on a removable substrate. The properties including photocatalytic activity are investigated in the photodegradation of MO. The modifications of Au–Ag can significantly increase both the photocatalytic activity and the stability. This work suggests a new direction to improve photocatalysis of nanostructure combined with noble metals.
Acknowledgements
We gratefully acknowledge the National Basic Research Program of China (Grant No. 2009CB421601), and the National Science Foundation of China (Grant No. 21175038) for financial support. We thank the editor and reviewers for helpful comments and suggestions.
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