Facile synthesis and high activity of novel BiVO₄/FeVO₄ heterojunction photocatalyst for degradation of metronidazole

Highlights

• BiVO₄/FeVO₄ was synthesized by a facile one-step hydrothermal method.

• BiVO₄/FeVO₄ displays superior photocatalytic activity.

• The photocatalytic mechanism were discussed in detail.

Abstract

The novel BiVO₄/FeVO₄ heterojunction photocatalyst was successfully synthesized by a facile hydrothermal method firstly. The physical and chemical properties of as-prepared samples were characterized based upon XRD, XPS, BET, SEM, EDS, TEM, UV–vis DRS and fluorescence spectrum techniques. The TEM images showed a clear interface between BiVO₄ and FeVO₄, indicating that a heterojunction between BiVO₄ and FeVO₄ was formed during the hydrothermal reaction. In addition, the photodegradation activity of metronidazole (MNZ) was used as a measurement for photocatalytic performance of BiVO₄, FeVO₄ and BiVO₄/FeVO₄ heterojunction photocatalyst. It indicated that under visible light irradiation the photocatalytic activity of BiVO₄/FeVO₄ heterojunction photocatalyst was very effective, and moreover, much higher than the single BiVO₄ or single FeVO₄. The possible photocatalytic mechanism has been discussed on the basis of the theoretical calculation of the electronic structure, and the experimental results.

Introduction

In recent years, semiconductor photocatalysis has been a hot research topic due to its potential to solve the energy crisis and environment problems [1], [2], [3], [4], [5]. TiO₂ has been the most widely used photocatalyst [6], [7], [8] because of its optical and electronic properties, low cost, low toxicity and relatively high chemical stability. However, the relatively wide band gap (3.2 eV) of TiO₂ limits its photocatalytic applications. Since UV light irridated from the Sun to reach the earth surface only is ∼4% of total the Sun light, visible light are more than 35%. Therefore, great efforts have been devoted to design the highly efficient photocatalysts within the range of visible light irradiation [9], [10], [11], [12], [13].

Constructing heterojunction photocatalysts composed of multi-semiconductors is an efficient method to facilitate the separation of photoexcited electronehole pairs and improve photocatalytic properties, a number of heterojunction photocatalysts have been constructed through various routes such as ball milling [14], hydrothermal [15], and coprecipitation [16], which have greatly improved the photocatalytic activities compared to their single constituents. For example, Zhao et al. have developed a novel heterojunction photocatalyst CuFe₂O₄/Bi₄Ti₃O₁₂ by ball milling; the photocatalytic activity of CuFe₂O₄/Bi₄Ti₃O₁₂ was higher than that of Bi₄Ti₃O₁₂ alone. The enhanced photocatalytic activity could be attributed to the formation of a heterojunction between CuFe₂O₄ and Bi₄Ti₃O₁₂, which suppressed the recombination of photogenerated electronehole pairs [17]. Chen et al. also reported other heterojunction photocatalysts, such as NiO/ZnO, ZnO/TiO₂, CaFe₂O₄/Ag₃VO₄, Bi₂O₃/NaNbO₃, CuO/BiVO₄ and InVO₄/BiVO₄ [18], [19], [20], [21], [22], [23], [24]. These experimental results suggest that the catalytic performance of heterojunction photocatalysts often can be more efficient than that of single semiconductors.

Compared with the classic photocatalysts, BiVO₄ belongs to a new type of visible-light-driven photocatalysts and has long been recognized as an important semiconductor photocatalyst due to its unique properties such as small bad gap, ionic conductivity, photocatalytic activities for water splitting, and pollutant decomposing. BiVO₄ exists in three main crystallographic forms: monoclinic scheelite, tetragonal scheelite and tetragonal zircon. Monoclinic BiVO₄ has attracted extensive interest as a promising visible-light-driven photocatalyst material with a narrow band gap of 2.4 eV [25], [26], [27], [28]. Great efforts such as heterojunction construction and element doping, have been made to overcome the poor quantum yield due to rapid recombination of photogenerated electrons and holes and to enhance its performance for the practical application. For example, Li et al. have successfully constructed an efficient g-C₃N₄/BiVO₄ heterojunction photocatalyst, which exhibits superior visible light photocatalytic activities in degradation of MB [29]. Gao et al. have reported that the as-prepared BiVO₄/Bi₂S₃ hollow discoids exhibit significantly enhanced photoelectrochemical current response and photocatalytic activity for reduction of Cr^VI under visible-light illumination [30].

Comparing with BiVO₄, FeVO₄ constituting of earth-abundant elements has a low band gap semiconductor, and indicates inherent visible light absorption onset at about 600 nm, corresponding to an optical band gap of around 2.05 eV [31], [32]. Meanwhile, FeVO₄ is a highly stable and highly selective catalyst that finds many applications including photocatalytic degradation of the organic pollutants [33] and catalytic dehydrogenation [34]. Moreover, FeVO₄ has the suitable conduction and valence band levels to match BiVO₄ for forming the composite photocatalyst interfaces.

According to the above surveys, it is recognized that the combination of FeVO₄ and BiVO₄ could increase separation efficiency of electron–hole pairs and enhance the photocatalytic activity of BiVO₄/FeVO₄ heterojunction photocatalyst. To the best of our knowledge, although the photocatalytic properties of BiVO₄-based composite materials have been extensively studied, the heterojunction photocatalyst materials of BiVO₄/FeVO₄ and its photocatalytic degradation of metronidazole in water environmment have not been reported.

Metronidazole (MNZ, 2-methyl-5-nitroimidazole-1-ethanol) is a kind of nitroimidazole antibiotic, which is not only commonly used to treat infections caused by anaerobic bacteria and various protozoans, but is also added to fish and poultry feed to promote weight production [35], [36], [37], [38], [39]. Continuously increasing consumption and emission (incomplete absorption and metabolism) of metronidazole drugs will lead to its accumulation in organisms and environment. Even more importantly, it may pose health risks to humans [39], [40]. Due to its widely used, highly soluble, nonbiodegradable and suspectedly carcinogenic, more attention has been paid to the environmental impact and health risks caused by metronidazole [41]. Many technologies were reported to degrade metronidazole present in environmental waters, including ultraviolet (UV) irradiation [42] and oxidation processes using UV, UV/H₂O₂, H₂O₂/Fe²⁺, UV/H₂O₂/Fe²⁺, UV/Zn₂GeO₄ [43]. In general, these processes are expensive and complicated [44], [45]. In comparison, visible light irradiation is more easy to obtain and more cost-effective [46].

In this work, the BiVO₄/FeVO₄ heterojunction photocatalyst was successfully synthesized by one-step hydrothermal methods. The as-prepared samples were characterized for their physical and chemical properties based upon XRD, SEM, EDS, TEM, UV–vis DRS and fluorescence spectrum techniques. The photodegradation activity of metronidazole (MNZ) was used as a measurement for photocatalytic performance of BiVO₄, FeVO₄ and BiVO₄/FeVO₄ heterojunction photocatalyst samples. Based on the experimental results and the theoretical calculation of the electronic structure, the enhanced photocatalytic activity of BiVO₄/FeVO₄ heterojunction photocatalyst was discussed in detail.