Carbon spheres supported visible-light-driven CuO-BiVO₄ heterojunction: Preparation, characterization, and photocatalytic properties

Abstract

To utilize visible light more effectively in photocatalytic reactions, carbon-supported CuO-BiVO₄ (CuO-BVO@C) composite photocatalyst was prepared by hydrothermal process and impregnation technique. The photocatalytic activities of as-prepared catalysts were evaluated by degradation of methylene blue (MB) in aqueous solution under visible light irradiation, it was found that CuO-BVO@C exhibits the highest photocatalytic degradation activity with the pseudo-first-order rate constant K_a five times higher than pure BiVO₄, which could be assigned to the synergistic effect of CuO-BiVO₄ heterojunction and carbon spheres. The characterization of photocatalysts by a series of joint techniques, including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV–vis diffuse reflectance spectra, PL spectra and electrochemistry technology, discloses that carbon spheres play two crucial roles in enhancing of photocatalytic activity. One is to act as a dispersing support to suppress the grain growth, the other is to act as a photosensitizer to transfer the electrons to CuO-BiVO₄ heterojunction, which narrows the band gap of BiVO₄, hinders the electron–hole pair's recombination, extends the absorption range of visible light, and improves the photocatalytic performance of catalyst. The photocatalytic degradation pathways mainly involve the formation and reaction of OH radicals. Based on the experimental results of electron spin-resonance spectroscopy, a reasonable mechanism was also proposed to elucidate the role of carbon spheres in the CuO-BVO@C composite as a photocatalyst for degradation of organic pollutants.

Introduction

Recently, significant interest has been devoted to designing semiconductor–carbon composite materials, aiming at cooperative or synergistic effects between the metal oxides and carbon phases to meet the requirements imposed by specific application, such as solar energy utilization and heterogeneous photocatalysis. Besides the modification of catalysts by incorporating alloying components, the application of appropriate support is another way to improve the performance of catalysts. Various research techniques have been developed and applied towards coating carbon supports for this purpose because a composite product of carbon and semiconductor photocatalysts could potentially create many active sites for photocatalytic degradation [1], [2]. Indeed, besides the action as adsorbent or support, some studies have proven that carbon can act as sensitizer and transfer electrons to the semiconductors, triggering the formation of very reactive radicals to improve photocatalytic activity of semiconductors for target reactions. This may be responsible for extending photocatalytic activity of semiconductors into the visible light range [3], [4], [5], [6], [7], [8]. Among carbon supports of similar variety, carbon spheres (CSs) synthesized with an easy hydrothermal approach have aroused considerable interest in a variety of scientific fields because of their wide applications as adsorbents [9], catalysts [10], and electrode materials [11]. Wang et al. [12] have prepared perfect spherical carbon with uniform nanopores by a hydrothermal route, in which sugar was used as carbon sources. In view of environmentally benign and inexpensive properties of carbon spheres [13], [14], it would be more meaningful to hydrothermally synthesize effective photocatalysts combined with carbon spheres to further optimize the photocatalytic activity.

The development of visible-light-driven photocatalysts focused on photocatalytic splitting of water or degradation of organic pollutants has inspired a great deal of research interest to utilize solar energy effectively [15], [16], [17]. It has been reported that photocatalytic composite oxides containing bismuth such as BiVO₄ (BVO) normally have strong response to visible light due to the change of electronic structure in the composites. BiVO₄ can exist in three crystalline phases, i.e., tetragonal zircon, monoclinic scheelite, and tetragonal scheelite. These three crystal types can undergo phase transition under different thermal conditions [18]. It has been found that BiVO₄ with a monoclinic scheelite structure shows excellent photocatalytic performance under visible light irradiation [17], [18], [19]. Compared with TiO₂ photocatalyst, which band gap is 3.2 eV, monoclinic BiVO₄ has a band gap energy of 2.4 eV and can adsorb the solar spectrum up to blue light fraction of ca. 520 nm [20]. Experimental results on the photocatalytic evolution of oxygen [21], [22], [23] and the photocatalytic degradation of organic pollutants [24], [25], [26] using monoclinic BiVO₄ proved that BiVO₄ was an effective photocatalyst under visible light. Nevertheless, the photocatalytic activity of pure BiVO₄ is limited due to its poor adsorptive abilities and difficult migration of electron–hole pairs [25]. Therefore, it is necessary to find a suitable way like heterostructure fabrication to facilitate the BiVO₄ driven photooxidation of organic pollutants through rapid transfer or separation of photoinduced electron–hole pairs [27].

Copper oxide (CuO), a p-type semiconductor with band gap of 1.70 eV, has been widely used for diverse applications such as heterogeneous catalysts [24], [28], [29], gas sensors [30], [31], lithium ion electrode materials [32], [33], and field-emission emitter [34], [35]. When copper oxide is in contact with an n-type semiconductor such as BiVO₄, a p–n-type heterojunction is formed and the recombination of electron–hole pairs is suppressed. Jiang et al. [24] fabricated CuO-BiVO₄ photocatalyst through solution combustion method and impregnation technique, they ascribed the mechanism of enhanced photocatalytic activities to the p-type CuO dispersed on the surface of n-type BiVO₄ to constitute a heterojunction composite, which can separate the electron–hole pairs efficiently.

In this paper, the over goal was to prepare and characterize a class of carbon spheres supported BiVO₄ (BVO@C) by a hydrothermal method towards the photocatalytic oxidation of methylene blue (MB) under visible light irradiation, and finally improve the photocatalytic ability by synthesizing CuO-BiVO₄ (CuO-BVO) heterojunction. Herein, we synthesized carbon-deposited CuO-BiVO₄ (CuO-BVO@C) composite photocatalyst and demonstrated the physical properties and the enhanced photocatalytic activities. The role of carbon spheres and the mechanism underlying the enhanced photocatalytic activity of CuO-BVO@C were also discussed.