Enhanced photocatalytic performance of CuBi2O4 particles decorated with Ag nanowires

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Abstract

CuBi2O4 particles (350–700 nm) were decorated with Ag nanowires (30 nm in diameter and 0.8–1.5 µm in length) by a hydrothermal route. The as-prepared Ag-CuBi2O4 composite was systematically characterized by XRD, TEM, XPS, UV–vis DRS, EIS, PL spectroscopy and photocurrent response. It is revealed that Ag-CuBi2O4 composite displays an enhanced separation efficiency of photogenerated electron-hole pairs, which is due to the migration of electrons from CuBi2O4 particles to Ag nanowires. RhB and phenol were chosen as the target organic pollutant to evaluate their degradation behavior over the samples under simulated-sunlight irradiation. It is observed that Ag-CuBi2O4 composite shows a photocatalytic activity much higher than that of bare CuBi2O4. The enhanced photocatalytic activity of the composite can be explained by the efficient separation of photoinduced charges and the increased availability of the charges for the photocatalytic reactions. The reactive species were determined by investigating the effect of ethanol, BQ and AO on the degradation of RhB and the formation of •OH radicals. It is concluded from the experimental results that •OH is the dominant reactive species causing the dye degradation. The photocatalytic mechanism involved was discussed.

Introduction

Spinel-type compounds with general formula of AB2O4 (where A represents a divalent metal cation and B represents a trivalent metal cation) have attracted extensive interest due to their diverse properties and wide applications in electronic devices, catalysis and energy storage. Copper bismuth oxide (CuBi2O4), packed with square planar CuO4 groups linked to distorted trigonal BiO6 polyhedra [1], is one of the outstanding representatives of spinel-type compounds. CuBi2O4 is a p-type semiconductor and holds many promising physicochemical properties including magneticity, dielectricity, high-temperature heat capacity, electrochemical capacitance, photoelectrochemical property and catalysis [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. In particular, CuBi2O4 has a relatively small bandgap energy of 1.5–1.9 eV, and thus it can largely absorb the visible part of the solar spectrum. As the photocatalytic applications, Bi-contained semiconductors are particularly interesting [25], [26], [27], and furthermore, the conduction band potential of CuBi2O4 is more negative than the redox potential of H+/H2. Due to these attractive features, CuBi2O4 has been extensively studied as a photocathode for solar water splitting into H2 [11], [12], [13], [14], [15]. It has been shown that CuBi2O4 can be also used as a promising visible-light-responsive photocatalyst for the degradation of organic dye pollutants [19], [20], [21], [22], [23], [24].

The photocatalytic behavior of a semiconductor is closely associated with photogenerated electrons (e) and holes (h+). For a bare semiconductor, however, most of the photogenerated electrons and holes will undergo rapid geminate recombination and only a small portion of the carriers migrate to the semiconductor surface for participating in the photocatalytic reactions. Efficient separation of electron-hole pairs is crucial to achieve a good photocatalytic activity of the semiconductor. Construction of heterostructured composites from two semiconductors is one of the most important strategies to promote the electron-hole pair separation and hence improve the photocatalytic performance of semiconductors. Recently, much work has been devoted to the heterostructure composite photocatalysts constructed from CuBi2O4 and other semiconductors, such as CuBi2O4/TiO2, CuBi2O4/BiVO4, CuBi2O4/SrO, CuBi2O4/CeO2, CuBi2O4/NaTaO3 and CuBi2O4/Bi2WO6 [28], [29], [30], [31], [32], [33], [34]. In these composites, photogenerated electrons or holes tend to migrate from one semiconductor to another. Due to this carrier transfer process, the recombination rate of electron-hole pairs can be effectively inhibited, and thus more carriers are available for the photocatalytic reactions. It has been shown that these composite photocatalysts exhibit significantly enhanced photocatalytic activity compared to individual semiconductors. Decoration of the semiconductor with noble metals (e.g. Ag, Au, Pt, etc.) is another promising strategy to improve its photocatalytic activity [35], [36], [37]. Noble metals that act as efficient electron sinks can capture photogenerated electrons, leaving behind holes on the semiconductor surface. As a result, photogenerated holes are increasingly available to participate in the photocatalytic reactions. Up to now, there has been little work concerned with the photocatalytic performance of noble metal-decorated CuBi2O4 photocatalyst. In this work, we adopted a hydrothermal route to grow Ag nanowires onto the surface of CuBi2O4 particles. The photocatalytic performance and mechanism of the as-prepared Ag-CuBi2O4 composites was investigated by degrading rhodamine B (RhB) and phenol under simulated-sunlight irradiation.

Section snippets

Preparation of CuBi2O4 particles

CuBi2O4 particles were synthesized via a polyacrylamide gel route as described in the literature [38]. In a typical synthesis process, 4.8507 g (0.01 mol) of Bi(NO3)3·5H2O and 1.208 g (0.005 mol) of Cu(NO3)2·3H2O were dissolved in 30 mL of dilute nitric acid solution. Then 3.377 g (0.0225 mol) of tartaric acid, 20 g of glucose, and 9.5958 g (0.135 mol) of acrylamide were successively added to the above solution. The molar amount of tartaric acid and acrylamide was 1.5 and 9 times the amount of total Cu2+

Results and discussion

Fig. 2 shows the XRD patterns of CuBi2O4 and Ag-CuBi2O4 samples, along with the standard XRD line patterns for CuBi2O4 tetragonal structure with P4/ncc space group (PDF #42-0334) and Ag cubic structure with Fm-3m space group (PDF #04-0783). It is seen that all the diffraction peaks for CuBi2O4 can be indexed according to the standard diffraction pattern, implying that the particles crystallize into single CuBi2O4 tetragonal phase. For Ag-CuBi2O4, the XRD pattern shows that CuBi2O4 undergoes no

Conclusions

Ag nanowires of 30 nm in diameter and 0.8–1.5 µm in length were successfully assembled onto the surface of CuBi2O4 particles (350–700 nm) via a hydrothermal reaction route. In the as-prepared Ag-CuBi2O4 composite, photogenerated electrons tend to migrate from CuBi2O4 particles to Ag nanowires. This electron transfer process leads to a decrease in the recombination of electron-hole pairs, and thus more photogenerated holes and electrons are able to participate in the photocatalytic reactions. As a

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 51262018 and 51662027).

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