Elsevier

Journal of Alloys and Compounds

Volume 710, 5 July 2017, Pages 102-113
Journal of Alloys and Compounds

Morphological evolution of Cu doped ZnO for enhancement of photocatalytic activity

https://doi.org/10.1016/j.jallcom.2017.03.150Get rights and content

Highlights

  • ZnO with cube, corn seed and rod like morphology were prepared by a facile reflux method.

  • Controlled Cu doping could be achieved at low temperature.

  • The substitution of Cu2+ in ZnO corn seed was confirmed by various characterization techniques.

  • 0.5 mol% Cu doped ZnO exhibits 3.5 times higher photocatalytic activity than pure ZnO (Z5).

  • Based on scavengers test, both superoxide and hydroxyl radicals are mainly actives species.

Abstract

The structure, morphology and chemical doping in metal oxide are important factors that can influence on its photocatalytic activity. In this work, a facile and template free reflux method has been used to synthesize ZnO nanostructures for the morphology control of cubes, maize corn seeds, and rods at low temperature. The photocatalytic activity of ZnO with different morphology was evaluated towards degradation of methyl orange under UV light illumination. Among all the morphology controlled ZnO, the maize corn seed shaped ZnO shows the superior photocatalytic activity. Moreover, the influence of Cu contents on the structural, optical, and photocatalytic activity of ZnO maize corn seed was also investigated systematically. The photocatalytic experiment towards the degradation of methyl orange revealed that the 0.5 mol% Cu doped ZnO exhibits 3.5 fold higher photocatalytic activity than pure corn seed shaped ZnO. The enhancement of photocatalytic activity by Cu ions substituting in ZnO lattices is attributed to synergetic effect of Cu and ZnO, increase the separation of photogenerated electron-hole pairs, oxygen vacancy, decrease in crystallite size and the higher surface to volume ratio. Based on scavengers test, it was found that both superoxide and hydroxyl radicals are mainly actives species involved for the degradation of methyl orange.

Introduction

Environmental and energy issues are recessing towards serious concerns for human community, which have attracted over worldwide attention [1], [2], [3], [4], [5]. Among the various emergent techniques dealing with the energy and environmental problems, the photocatalysis offers several advantages such as facile, reusable and complete elimination of dyes from waste waters without causing any damage to the environment [6]. Hence, photocatalysis is considered as a promising, efficient and green chemical technique [7]. In order to enhance the photocatalytic activity of metal oxide semiconductors numerous efforts have been paved in material sciences like adjusting their size, morphology, orientation and other means [8], [9], [10]. The control of shapes and structures of transition metal oxides has been attracted significant interest because their catalytic properties are function of morphologies and crystallographic form [11].

Among the transition metal oxide library, ZnO, n-type II–VI semiconductor metal oxide has been studied extensively in the recent years. ZnO has its great potential for a variety of practical applications, such as photocatalyst, gas sensors, antimicrobial agents, photoanode materials for dye-sensitized solar cells, light emitting diodes, and so on due to its unique properties and the availability of a variety of growth methods resulting in a number of different morphologies and a wide range of material properties of synthesized nanostructures [12], [13], [14]. ZnO material has been considered as a suitable and promising alternate for TiO2photocatalyst because of its many fascinating properties such as wide band gap, unique chemical and physical properties, environmental stability, non-toxicity, abundant availability, ease of morphology control and low cost [15], [16]. However, the relatively rapid recombination rate due to the lower quantum efficiency of ZnO, is a major obstacle to limit the use of pure ZnO for commercial application in the photocatalytic degradation of contaminants [17]. There are numerous flyovers to overcome this limitation, among those doping of impurity in particular, Cu metal ion in ZnO is a powerful tool to enhance the separation of charge carries, increase in surface area, tuning the band gap, and generate lattice defects in semiconductor photocatalyst [18], [19]. Milenova et al. [20] used many dopants such as Mn, Co, Ni, Cu, and Ag to enhance the photocatalytic efficiency of ZnO, among these Cu was found to be one of the most effective doping elements for improving the photocatalysis response. The literature reveals that the prepared Cu-doped ZnO offers great potential in application of photocatalysis field.

To date, Cu-doped ZnO nanoparticles (NPs) have been synthesized through the various approaches such as sol-gel, vapor transport, hydrothermal, precipitation, and in-situ polymerization [21], [22], [23], [24], [25], [26], [27], [28]. Among these, solution-based techniques is the simplest approach because of the mild synthesis conditions, potential for scale-up, economic factors, simple, and ease of operation [29]. Conversely, the reported preparation methods were required complicated equipments, high calcinations, longer processing time and most of them involved environmentally malignant chemicals, which limits for material commercialization [23]. The higher degree calcinations lead to Oswald repining, which cause the depletion of specific surface area of material. The hydrothermal approaches demand high pressure apparatus, which bring along additional safety concerns.

Therefore, searching new methodology with low-cost, green and mass-production method is quite challenging and yet demanding to develop low temperature, an easier-to-operate, template-free, solution-based, and morphology-controllable approach [30]. The reflux method is more advantageous compare to others, in the context of easy operation, safety, lower processing temperature and high yield [31]. Although there are many reports for the ZnO, still rare work has been focused on the preparation of Cu-doped ZnO nanostructures with a reflux method for photocatalysis.

In this work, simply ZnO synthesized with different morphology demonstrate corn seed showed superior photocatalytic activity. We have doped Cu at optimal condition and investigate the effects of Cu doped ZnO with different Cu contents on the physical properties like structural, morphological and optical properties. Moreover, the consequences of various operational parameters including morphology, Cu doping contents and reflux reaction time on photocatalytic activities have been performed to achieve maximum photodegradation efficiency for Methyl Orange (MO). The effect of scavenger on photocatalytic activity is examined to investigate active species mainly involved in the degradation of MO.

Section snippets

Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), copper chloride dihydrate (CuCl2·2H2O), and ethanol were purchased from Sigma-Aldrich. Ammonia solution, 28–30% extra pure was obtained from Samchun. Methyl Orange was obtained from Daejung Chemicals & Metals Co Ltd (120240:KOSDAQ). All the reagents and chemicals were of analytical grade and used without further purification. All solutions were prepared in deionized (DI) water obtained from Millipore water system.

Synthesis of ZnO and Cu doped ZnO

0.1 M Zn(NO3)2·6H2O solution was

XRD analysis

In order to determine the crystal structure, crystallite size and phase purity of as synthesized Z2, Z5 and Z8 samples XRD patterns were examined as shown in Fig. 1. From Fig. 1, it clearly seen that all the diffraction peaks correspond to (100), (002), (101), (102), (110), (103), (200), (112), (201), and (202) planes can be assigned to hexagonal wurtzite structured ZnO accordance with (JCPDS card No. 89-7102) [32]. No additional peaks related to metallic zinc or any other forms of zinc

Conclusions

In this work, we have successfully synthesized template free ZnO with various architectures such as cubes, corn seeds and rods by reflux method at lower temperature. The concentrations of ammonia solution plays a vital role to control the morphology of ZnO. Moreover, to boost photocatalytic performance of ZnO, we doped Cu with various dopant content at low temperature. The successful substitution of Cu2+ in ZnO was confirmed by various characterization techniques such as XRD, FESEM, UV–Vis.

Acknowledgements

This research work supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (NRF-2015R1A1A1A05027848).

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