Regular Article
Room-temperature synthesis of carnation-like ZnO@AgI hierarchical nanostructures assembled by AgI nanoparticles-decorated ZnO nanosheets with enhanced visible light photocatalytic activity

https://doi.org/10.1016/j.jcis.2017.04.080Get rights and content

Abstract

The preparation of highly efficient visible-light-driven photocatalyst for the photodegradation of organic pollutants has received much attention due to the increasing global energy crises and environmental pollution. In this study, carnation-like ZnO@AgI hierarchical nanostructures assembled by AgI nanoparticles-decorated ZnO nanosheets were successfully prepared via a room-temperature route. The as-prepared ZnO@AgI nanostructures exhibited highly efficient photocatalytic activity under visible light irradiation (λ > 400 nm). Under optimized AgI content, the ZnO@AgI-5% sample showed high photocatalytic activity, which was 25.7 and 1.5 times the activity of pure ZnO and pure AgI, respectively. Mechanism studies indicated that superoxide anion radicals (O2-) was the main reactive species in the photocatalytic process. The high photocatalytic activity of the ZnO@AgI nanostructures is attributed to the highly active AgI nanoparticles and the heterojunction between AgI nanoparticles and ZnO nanosheets. The heterojunction structure reduced the recombination of the photogenerated electron-hole pairs in the conduction band (CB) and valence band (VB) of AgI nanoparticles by transferring the electrons from the CB of AgI nanoparticles to the CB of ZnO nanosheets. The composite of ZnO and AgI not only improves photocatalytic efficiency but also reduces photocatalyst cost, which is beneficial for practical application.

Graphical abstract

Carnation flower-like ZnO@AgI hierarchical nanostructures assembled by AgI nanoparticles-decorated ZnO nanosheets with enhanced visible light photocatalytic activity.

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Introduction

Environmental protection and remediation, especially waste water treatment has been attracting worldwide attention [1], [2], [3], [4]. A number of ways have been developed for the purification of wastewater and semiconductor-based heterogeneous photocatalysis has been proved to be a promising technique for the treatment of wastewater which is contaminated by organic pollutants [5], [6], [7], [8], [9], [10], [11], [12]. Among various semiconductors, TiO2 and ZnO have been widely studied and recognized to be preferable photocatalysts [2], [3], [4], [9], [13], [14], [15]. However, both TiO2 and ZnO have wide band gaps and can only absorb a small portion of solar light in the UV region [16]. Furthermore, a high recombination rate of the photogenerated electron-hole pairs also decreases the photocatalytic efficiency, for only a small amount of the charge carries can be transferred to the surface to conduct the photocatalytic reaction. One of the approaches to address these issues is to combine TiO2 and ZnO with narrow gap semiconductors, which not only can expand the light absorption to visible light region, but also can decrease the electron-hole recombination [17], [18].

Recently, a new type of visible-light-driven plasmonic photocatalyst, AgX@Ag (X = Cl, Br and I) has been developed and found to possess excellent photocatalytic activity under visible light irradiation due to the strong surface plasmon resonance effect of metallic Ag nanoparticles [19], [20], [21], [22], [23], [24], [25]. However, silver belongs to noble metals, it is of being rare and expensive, and thus the compounds of silver halides (AgX) are relatively expensive compared with ZnO and TiO2. The high cost of AgX photocatalyst hinders their practical application in large-scale photocatalytic processes. Consequently, preparation of AgX with low-cost semiconductors such as TiO2 and ZnO not only can extend the light response into visible light region but also cut down the cost in practical application. Over the past few years, much attention has focused on the coupling of TiO2 and ZnO with AgCl [26], [27], [28], [29], [30] and AgBr [31], [32], [33], [34], and rare attention has been paid to the fabrication and photocatalytic investigation of ZnO@AgI composites [35]. Vignesh and co-workers prepared ZnO@AgI composites by a multi-step method [36]. First, ZnO precursors were obtained by the reaction between ZnSO4·7H2O and NaHCO3. Then ZnO nanoparticles were prepared by calcining the ZnO precursors at 350 °C for 3 h. Finally, ZnO@AgI composite was fabricated by the deposition of AgI on the surface of ZnO by a precipitation method at room temperature for 12 h. Lu and co-workers prepared ZnO@AgI by immersing ZnO nanorod arrays into a mixed solution of AgNO3 and KI for 8 h. [37]. Recently, Shaker-Agjekandy and Habibi-Yangjeh fabricated ZnO@AgI composites by using an one-pot refluxing method at temperature of 96 °C [38] or a one-pot microwave-assisted methodology [35]. However, some of the reported synthesis procedures are multi-steps and time-consuming, and some need high temperature or special instrument. Therefore, to find a cost-effective and time-saving methodology for the preparation of ZnO@AgI composite is still of great importance.

In this study, carnation flower-like ZnO@AgI hierarchical nanostructures assembled by AgI nanoparticles-decorated ZnO nanosheets were successfully fabricated at room temperature without using any special instruments. First, carnation-like ZnO hierarchical nanostructures consist of ZnO nanosheets with thickness of about 25 nm were prepared using citrate as complex reagent. Then AgI nanoparticles with diameter of about 20 nm were decorated onto the surface of the ZnO nanosheet with the assistance of polyvinylpyrrolidone (PVP, K-30) and ethylene glycol. The AgI-decorated ZnO nanosheets were cross-linked and assembled into sphere-like nanostructures with lots of meso-pores and macro-pores. This unique hierarchical structure is beneficial for enlarging the accessible surface area, creating more photocatalytic active sites, facilitating the transfer of electrons, reducing the recombination of the photogenerated electron-hole pairs, and consequently leading to the enhanced photocatalytic performance.

Section snippets

Materials preparation

Carnation-like ZnO hierarchical nanostructures were prepared by the reaction of zinc acetate (Zn(CH3COO)2·2H2O) with sodium hydroxide (NaOH) in the presence of trisodium citrate (C6H5Na3O7·2H2O) [39]. All of the reagents were used as received without further purification. Deionized (DI) water was used in all experiments. In a typical synthesis, 3 mmol of zinc acetate and 5 mmol of trisodium citrate were dissolved into 50 mL DI water under magnetic stirring at room temperature, then 15 mmol of NaOH

Characterization

Fig. 1 shows the XRD patterns of ZnO, AgI and ZnO@AgI samples. The ZnO sample shows diffraction peaks at 2θ of 31.74°, 34.34°, 36.16°, 47.56°, 56.54°, 62.82°, 66.48°, 67.96°, 69.02° correspond well to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal planes of of ZnO (JCPDS No. 36-1451) with hexagonal wurtzite crystal structure (a = b = 3.250 Å, and c = 5.207 Å) [15]. The AgI sample shows diffraction peaks at 2θ of 22.36°, 23.68°, 25.36°, 32.94°, 39.22°, 42.66° and 46.28°,

Conclusions

In summary, ZnO@AgI composites were successfully prepared by a facile deposition-precipitation method at room temperature and characterized by XRD, SEM, DRS and XPS techniques. The photocatalytic activity of the composites was evaluated by the degradation of methyl orange under visible-light irradiation. The activity of the composites was first increased and then gradually decreased with increasing the mole fraction of AgI. The superior activity was observed for the ZnO@Ag-5% sample in which

Acknowledgement

This work was supported by the Innovation Training Project for University Students in the College of Chemistry and Chemical Engineering at China West Normal University (2015, H. Huang), the Open Project of Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province (CSPC2016-3-2), and the Innovation Team Project of the Education Department of Sichuan Province (15TD0018).

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