Designed synthesis of a novel BiVO₄–Cu₂O–TiO₂ as an efficient visible-light-responding photocatalyst

Abstract

A novel visible-light-responding BiVO₄–Cu₂O–TiO₂ ternary heterostructure composite was successfully fabricated via the preparation of BiVO₄–TiO₂ followed by coupling with Cu₂O through facile wet chemistry methods based on the strategy of energy gap engineering. The as-fabricated composite was characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, UV–vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. Benefited from the rational design and construction, BiVO₄–Cu₂O–TiO₂ exhibits a significantly enhanced photocatalytic activity for the degradation of rhodamine B (RhB) under the visible-light irradiation as compared with Cu₂O and Cu₂O–TiO₂. Specifically, under the irradiation with an ordinary 9 W energy-saving fluorescent lamp for 8 h, the photocatalytic degradation ratio of RhB for 5 wt%BiVO₄–40 wt%Cu₂O–TiO₂ reaches 97.8%. The enhanced photocatalytic activity of BiVO₄–Cu₂O–TiO₂ can be ascribed to the matched band edge positions of BiVO₄, Cu₂O and TiO₂, the heterojunction formations among them as well as the lower charge transfer resistance, favoring the separation of the photo-generated electron–hole pairs. A possible mechanism of the visible-light photocatalytic degradation of RhB is also proposed.

Introduction

Semiconductor photocatalysis has been extensively investigated in the organic pollutant degradation and water splitting due to its outstanding catalytic performance, mild reaction conditions, wide availability and no secondary pollution [1], [2], [3], [4], [5], [6]. Among various semiconductors, titanium dioxide (TiO₂), the most intensively studied photocatalyst, has all of these cited advantages together with non-toxicity, relative inexpensiveness and strong oxidizability [7]. However, its wide bandgap (3–3.2 eV) and fast recombination of the photo-generated electron–hole pairs lead to the limited utilization of solar energy (less than 5%) and low quantum efficiency, enormously restricting its wide and practical applications in photocatalysis [8]. Therefore, enhancing its visible-light photocatalytic efficiency and suppressing the recombination of the photo-generated electron–hole pairs are two of the most urgency issues in the photocatalysis area. During the last decade, intensive researches have been focused on achieving these two goals, and some strategies such as doping TiO₂ with N, C and S [9], [10], [11], [12], and sensitizing TiO₂ with quantum dots or dyes [13], [14], [15] have been proposed. Among these strategies, coupling TiO₂ with other narrow bandgap semiconductors such as CdS, CdSe, PbS and WO₃ and constructing heterojunctions have been demonstrated to be efficacious to improve its visible-light photocatalytic properties [16], [17], [18], [19], [20], [21], which serve as visible-light sensitizers to transfer the excited-state electrons to TiO₂.

The p-type semiconductor cuprous oxide (Cu₂O) with a direct bandgap of 2.0 eV is undoubtedly an attractive visible-light-responding photocatalyst thanking to its high visible-light absorption capability, environmental benignity, low cost and easy preparation [22], [23], [24], [25]. However, the photocatalytic activity of pure Cu₂O is usually comparatively low because of the easy recombination of the photo-generated electron–hole pairs [26], [27]. To overcome this deficiency, coupling Cu₂O with n-type semiconductors such as TiO₂ or zinc oxide (ZnO) is deemed as a good strategy since their band structures are favorably positioned, in which both the conduction and valance band levels of Cu₂O are more cathodic than those of TiO₂ or ZnO, and numerous studies have been spurted during the last decade [28], [29], [30], [31], [32]. Albeit those binary hybrids exhibited enhanced photocatalytic performances to some extent, we think that there still exists some scope to improve the photocatalytic performance of Cu₂O–TiO₂ through further optimizing its energy band construction.

As an inexpensive and environmentally friendly multi-component bismuth-based n-type semiconductor, bismuth vanadate (BiVO₄) has attracted considerable attention recently owing to its excellent performance in water splitting [33], [34], [35] and pollutant degradation [36], [37] under visible-light irradiation. Generally, there exist three different phases of BiVO₄: tetragonal zircon, monoclinic scheelite and tetragonal scheelite. In most instances, the monoclinic scheelite structure BiVO₄ (m-BiVO₄) with a bandgap of 2.5 eV displays the best photocatalytic performances under visible-light irradiation [38]. But the rapid recombination rate of the photo-generated electron–hole pairs and poor adsorptive capability still give rise to a low photocatalytic activity for m-BiVO₄ [39]. A considerable number of researchers have also realized that coupling BiVO₄ with other desirable semiconductors can effectively enhance its photocatalytic activity [40], [41], [42], among which TiO₂–BiVO₄ [43] and Cu₂O–BiVO₄ [44] heterogeneous nanostructures have been reported exhibiting much higher visible-light photocatalytic activities in comparison with the individual components because of their matching band structures.

Recently, quite a few researches have demonstrated that the formations of heterojunctions especially p–n heterojunctions by coupling one semiconductor with another semiconductor can pronouncedly promote the photo-generated electron–hole pairs separation and therefore relieve their recombination due to the tunable electronic structures and existence of inner electric fields of heterojunctions [45], [46]. Herein, aiming at further improving the photocatalytic performances of TiO₂-based photocatalysts and based on the strategy of energy gap engineering, we constructed a novel ternary heterostructure composite via coupling p-type Cu₂O and BiVO₄ semiconductors with n-type TiO₂ semiconductor. Additionally, considering that almost all visible-light resources employed for the investigations of photocatalytic performances of photocatalysts are large-power lamps such as Xe lamps and tungsten lamps with waltages exceeding 300 W, it is highly essential to develop photocatalysts with high photocatalytic activities suitable for the ordinary visible-light resources such as fluorescent lamps for the sake of feasible uses of photocatalysts and energy conservation. Therefore, in this work, an ordinary 9 W energy-saving fluorescent lamp was used as the visible-light resource for the photocatalytic degradation of rhodamine B (RhB). Furthermore, the possible mechanism for the separation of photo-generated electron–hole pairs among BiVO₄, Cu₂O and TiO₂ was also discussed for the purpose of understanding the enhanced photocatalytic activity of BiVO₄–Cu₂O–TiO₂. To the best of our knowledge, no works have been reported on the BiVO₄–Cu₂O–TiO₂ ternary composite for the visible-light photodegradation of organic pollutants.