简单湿化学法制备ZnO纳米结构

Abstract 

Flower-like ZnO nanostructures were synthesized by a facile wet chemical method. Structural, optical and photocatalytic properties of these nanostructures have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) and UV–vis absorption spectroscopy. SEM and TEM studies revealed flower-like structures consisting of nanosheets, formed due to oriented attachment of ZnO nanoparticles. Flower-like ZnO structures showed enhanced photocatalytic activity towards sun-light driven photodegradation of methylene blue dye (MB) as compared to ZnO nanoparticles. XRD, UV–vis absorption, PL, FTIR and TEM studies revealed the formation of Zn(OH)2  surface layer on ZnO nanostructures upon ageing. We demonstrate that the formation of a passivating Zn(OH)2  surface layer on the ZnO nanostructures upon ageing deteriorates their efficiency to photocatalytically degrade of MB.

Introduction

Nanostructured semiconductor photocatalysts such as ZnO and TiO2 have attracted significant attention in recent years because of their wide-spread application in environmental remediation [1,2]. These photocatalysts have a high efficiency for the degradation of toxic organic pollutants that originate from the effluents of textile and dyeing industries. Since the sun is an abundantly available natural energy source, its light can be conveniently utilized for the photodegradation of organic dyes [3-8]. ZnO with a band gap of 3.37 eV has received much attention for the complete mineralization and degradation of environmental pollutants. ZnO nanostructures with different morphologies have been synthesized by wet chemical methods [9-13] and used for various applications such as photocatalytic degradation of organic dyes [14-24], dye sensitized solar cells [25-28], gas sensors [29,30], clean energy applications [31] and UV detection [32]. Xia et al. [12] synthesized nanostructured ZnO flowers made up of bundled nanochains that could detect dopamine in the presence of L-ascorbic acid with high sensitivity and selectivity. Flower-shaped ZnO nanostructures were synthesized by Umar et al. [31] for an efficient photocatalysis and the fabrication of efficient dye sensitized solar cells. Shi et al. [33] fabricated flower-like ZnO on ZnO nanorods without use of any surfactant. Self-supported ZnO photocatalysts in the form of plates were prepared by Yassitepe et al. [24] by the tape casting method. These ZnO plates showed a good photocatalytic activity for azo dyes that depended on their surface area. Shen et al. [34] have shown that depositing ZnO on silica nanoparticles is a simple and effective method to prepare photocatalysts that could degrade 90% methylene blue (MB) in 60 min. ZnO nanoparticles (NP) that were synthesized by wet chemical methods can be passivated by a Zn(OH)2 layer during ageing. Hong et al. [35] have shown that the photocatalytic activity of ZnO NP is drastically reduced when the is surface modified with polysterene. In this paper, we have studied the effects of ageing on structural, optical and photocatalytic properties of flower-like ZnO nanostructures, which were prepared by a simple wet chemical method. We have demonstrated that the formation of a passivating layer of Zn(OH)2  on the ZnO nanostructures due to ageing leads to a drastic decrease in the efficiency of the sunlight driven photocatalytic degradation of MB.

Results and Discussion 

Figure 1 shows the XRD patterns of the as-synthesized samples S1, S2 and S3 (see Experimental section for the naming scheme). The observed well-defined peaks in the spectra can be indexed to the hexagonal wurtzite structure of bulk crystalline ZnO [JCPDS no. 36-1451]. No extra peaks related to any impurity were observed. This confirms that the as-synthesized products are pure wurtzite-type ZnO.

Fig1

Conclusion 

In summary, we have synthesized flower-like ZnO nanostructures by using a facile wet chemical method. The structural, optical and photocatalytic properties of ZnO nanostructures have been investigated. As compared to spherical ZnO nanoparticles, flower-like ZnO nanostructures exhibited an enhanced photocatalytic efficiency towards the degradation of methylene blue dye under irradiation with sunlight. The effect of ageing of flower-like ZnO structures on their photocatalytic efficiency has been investigated. XRD, FTIR, PL and UV–vis absorption studies confirmed the formation of a Zn(OH)2 layer on ZnO nanostructures upon ageing. Photocatalytic studies showed that Zn(OH)2 surface passivating layer leads to a drastic reduction in the efficiency of flower-like ZnO structures for sunlight induced photocatalytic degradation of MB dye.