High photocatalytic property and crystal growth of spindle-like ZnO microparticles synthesized by one-step hydrothermal method

Highlights

• The morphology of ZnO microparticles was spindle-like.

• A simple one-step hydrothermal method was used to synthesize the specimen.

• The product presented a high photocatalytic performance and excellent recycle performance.

• The growth mechanism of the spindle-like ZnO microparticles was discussed.

Abstract

The spindle-like ZnO microparticles were successfully synthesized by a simple hydrothermal method using triethanolamine (TEA) as the surfactant. The phase components, particle sizes and morphologies of ZnO samples derived from different contents of TEA (3, 4, 5, 6 and 7 mL) were characterized by XRD, SEM, TEM, EDS and BET. The photocatalytic activity of ZnO microparticles with different morphologies was evaluated by degradation of methyl orange (MO). The photocatalytic degradation process was monitored in terms of decolorization and total organic carbon (TOC) removals. The results indicated that the spindle-like ZnO exhibited higher photodegradation efficiencies under UV light irradiation than other ZnO structures due to the special morphology. The photodegradation could be described as the pseudo-first-order kinetics with apparent rate constants ranging from 3.59 × 10⁻³ to 1.04 × 10⁻² min⁻¹, which were based on the morphology of the structures. The sample presented a good recycle performance.

Introduction

Recently, water pollution is one of the major threats to public health. Incorrect and excessive use of pesticides, fertilizers, and pigments has caused serious water pollution. The traditional methods for wastewater treatment, e.g., adsorption, membrane separation, and extraction, are inefficient and cost high energy. Therefore, the finding of a “green” technology for removal of pollutants from wastewater is a main challenging [1], [2], [3]. In the last decades, heterogeneous photocatalysts for application in wastewater treatment had been developed. Titanium dioxide (TiO₂) is one of the most commonly used photocatalytic materials, which can be applied efficiently to degrade the organic pollutants in the wastewater [4]. However, the photoelectrons and photoholes of the TiO₂ are inclined to combine during the photocatalytic process, thus the production rate of light quantum is less than 10%. So the photocatalytic efficiency of TiO₂ is unsatisfactory, which restrict the practical applications of this photocatalyst. Zinc oxide (ZnO) possesses the similar band gap of TiO₂, furthermore this semiconductor oxide is non-toxic and has high photocatalytic efficiency. So it can also be used as an effective candidate photocatalytic materials [5].

Zinc oxide is a direct band-gap (E_g = 3.37eV) semiconductor with good optics, electrical and structural characteristics [6]. ZnO nano-materials with different structures and morphologies, such as nanorods, nanowires, nanoflowers, nanowhiskers [7], [8], [9], [10], show many special properties, which has attracted much attention in recent years [11]. ZnO with different morphologies has been synthesized by various methods such as solvothermal method, template method, precipitation method and green synthesis method [12], [13], [14], [15], [16], [17] and has been used in many fields, for instance, solar cell, stealth material, luminescent material, sensor, photocatalyst and wastewater treatment [18], [19], [20], [21], [22], [23], [24]. The morphology of ZnO is affected by many factors, for example, reaction temperature, surfactant, zinc salt, solution concentration and the doping of semiconductors is demonstrated to be a promising method because of its high efficiency, convenience a briefness [25], [26], [27], [28]. Among of them, surfactant is one of the most important factors affecting ZnO morphology. In previous studies, K.thingSsuriwong et al. [29] used the amino alcohols as the surfactant to synthesize ZnO microrods, which exhibited excellent band gap. The photocatalytic activity of ZnO is strongly related to the types of crystal, in the research of photocatalysis for methylene blue. Mclaren et al. [30] found that the catalytic performances of the polar surfaces (001) and (00$\overline{1}$) were higher than that of the non-polarized planes (010) and (210). However, the morphology of the microrods is irregular and uneven, so that those studies may be more reasonable if they had considered this situation. As we all know, photocatalytic activity could be enhanced by altering the shape and size distribution of ZnO, thus it is of great importance to find a straightforward method to prepare the samples with regular shape and uniform size.

Compared with the above synthesis method in the economic evaluation, we think the synthesis of the spindle-like ZnO samples with hydrothermal method presented obvious advantages of uniform, stable, high yield and low cost. In this paper, a simple hydrothermal reaction with triethanolamine (TEA) as surfactant was used to synthesize spindle-like ZnO microstructures possessing superexcellent photocatalysis. The effect of the dosage of the TEA on the morphology and the photocatalytic efficiency of the samples was investigated. Furthermore, the photocatalytic activities of the samples prepared with various amount of TEA were characterized with methyl orange (MO) as reacting substance. In addition, the formation mechanism of spindle-like ZnO was also discussed according to the evolution of morphologies from SEM.