Sunlight photocatalytic activity enhancement and mechanism of novel europium-doped ZnO hierarchical micro/nanospheres for degradation of phenol

Highlights

• Novel Eu-doped hierarchical ZnO micro/nanospheres were fabricated.

• The hierarchical structure was accumulated by lots of interleaving nanosheets.

• The formation of hierarchical micro/nanospheres was discussed.

• Eu doping can effectively trap the e_cb⁻ and inhibit the recombination with h_vb⁺.

• Effective degradation and mineralization of phenol under natural sunlight.

Abstract

Europium-doped ZnO hierarchical micro/nanospheres (Eu/ZnO) were synthesized for the first time via a facile and surfactant-free chemical solution route. The as-synthesized products were characterized by X-ray diffraction, field-emission scanning electron microscopy, energy dispersion X-ray analysis, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV–visible diffuse reflectance spectroscopy, nitrogen adsorption–desorption and photoluminescence spectroscopy. The results showed that the as-synthesized products were well-crystalline and accumulated by large amount of interleaving nanosheets. It was also observed that the Eu doping increased the light absorption ability of Eu/ZnO and a red shift for Eu/ZnO appeared when compared to pure ZnO. Under natural sunlight irradiation, the Eu/ZnO exhibited much higher photocatalytic activity than those of pure ZnO, Eu-doped ZnO nanorods (Eu/ZNRs) and commercial TiO₂ for the degradation of phenol. The photocatalytic enhancement of Eu/ZnO products was attributed to their high charge separation efficiency and hydroxyl radical generation ability as evidenced by the photoluminescence spectra. By using several radical scavengers, hydroxyl radical was determined to play a pivotal role for the phenol degradation. Furthermore, the Eu/ZnO could be easily separated and reused, showing great potential for practical applications in environmental cleanup and solar energy conversion.

Introduction

During the last two decades, different applications for advanced science and technology relating to industrial processes and agricultural productions have led to considerably increase in a number of phenols, pesticides, dyes, solvents and other organic pollutants existing in various natural resources. Most of them were resistant to biodegradation and may undergo natural reductive anaerobic degradation to yield potentially carcinogenic intermediates [1]. The photocatalytic reaction has become a desirable method to transform the organic pollutants into nontoxic molecules to eliminate the environmental pollution. Many oxide semiconductors, such as ZnO [2], [3], [4], TiO₂ [1], [5], Ag₃PO₄ [6], WO₃ [7] and Bi₂O₃ [8] have been reported as significant photocatalytic materials in various organic pollutants degradation and antimicrobial application. ZnO is a wide band gap (∼3.3 eV) semiconductor which has been extensively used because of its catalytic and photochemical properties along with its low cost [3]. Moreover, there are many reports of ZnO having higher photocatalytic activities than other semiconductors in both air and aqueous media [9], [10].

The surface area of the photocatalyst played a significant role because the photocatalytic reactions mainly occurred at the interface between the catalyst surfaces and organic pollutants. The nanoscale ZnO has shown the larger specific surface area-to-volume ratio and higher photocatalyic performance than that of bulk materials [11]. Much effort has been devoted to the synthesis of nanostructured ZnO with tunable size and shape parameter to effectively degrade numerous pollutants in the wastewaters [11], [12], [13]. However, it should be mentioned that the low dimensional nanoscaled building blocks (such as nanoparticles, nanorods and nanosheets) tend to aggregate during the preparation and photocatalysis processes, resulting in reduction of their surface area and photocatalytic efficiency. Therefore, three-dimensional (3D) semiconductor materials with hierarchical structure have aroused great concern due to their fine structure and larger size that can avoid the aggregation of particles during application as photocatalysts [4], [14]. For example, flower-like ZnO hierarchical microarchitectures exhibited an enhanced photocatalytic performance compared with the other nanostructured ZnO powders of nanoparticles, nanorods and nanosheets [4]. Hitherto, most of 3D hierarchical structures were developed via the surfactants or structure-directing reagents assisted route, and assemblage of nanoscaled building blocks into the 3D structured morphologies without any surfactants still remains an intricate challenge.

The solar energy utilization efficiency could be improved by modifying the ZnO through doping of metal or non-metal ions, deposition of noble metals as well as use of coupled semiconductors [15], [16], [17]. Recently, some studies have reported ZnO doping with rare earth (RE) ions showed enhancement in light absorption due to the creation of impurity energy levels within the band gap [18], [19]. Moreover, RE ions doping produced traps for photogenerated charge carriers, thus lowering the electron-hole pairs recombination rate leading to an increase in the photocatalytic efficiency in ZnO [16], [18]. Khatamian et al. [18] reported that doping with La³⁺, Nd³⁺ and Sm³⁺ improved the photoactivity of ZnO in degradation of 4-nitrophenol under UV irradiation. A study by Karunakaran et al. [19] also showed that the doping of Ce ions in ZnO enhanced the photooxidation of cyanide under solar irradiation. In fact, doping of ZnO is a subject of broad interest for the past few years. To the best of our knowledge, however, the doping of RE ions particularly Eu³⁺ in ZnO and their photocatalytic activities under natural sunlight irradiation have never been reported so far.

On the basis of the above consideration, this work reports for the first time on the synthesis of Eu-doped ZnO hierarchical micro/nanospheres (Eu/ZnO) by a simple chemical solution route without any organic solvent or surfactant. The morphology evolution and mechanism on the formation of the 3D ZnO hierarchical structure were investigated. The as-synthesized ZnO products were characterized by different techniques and used for the photocatalytic degradation of phenol under natural sunlight irradiation. Moreover, the stability of as-synthesized Eu/ZnO was studied through successive four cycles of experiments. The mechanism for the enhanced photoactivity of Eu/ZnO was finally discussed based on the separation process of electron-hole pairs and the active species detection.