Facile synthesis of Co(Ⅱ)-BiOCl@biochar nanosheets for photocatalytic degradation of p-nitrophenol under vacuum ultraviolet (VUV) irradiation

Highlights

• Co(Ⅱ)-BiOCl@biochar hybrid catalysts were hydrothermally synthesized by the assist of ethylene glycol.

• The as-synthesized Co(Ⅱ)-BiOCl@biochar photocatalyst presented excellent VUV-light-driven photocatalytic activities.

• The Co²⁺ doping and biochar loading on BiOCl exhibited synergistic effect for PNP degradation.

• The synergistic effect of ·OH, h⁺, ·O₂⁻, and e_aq^- caused the degradation of PNP.

Abstract

In this study, cobalt-doped and biochar-cocatalyst bismuth oxychloride (Co(Ⅱ)-BiOCl@biochar) photocatalysts were hydrothermally synthesized for the photodegradation of p-nitrophenol (PNP) and characterized via a series of analytical techniques. Results indicated that the addition of Co²⁺ and biochar to BiOCl resulted in 99.4% degradation of PNP in 90 min under vacuum ultraviolet (VUV) illumination. It was found that 5% doping of Co²⁺ and 30% loading of biochar significantly enhanced the photocatalytic performance of the as-prepared catalysts. After five cycles, the obtained hybrid catalysts still exhibited a good stability. Although the structure of BiOCl was not changed by the modification, the growth face changed from (0 0 1) to (1 1 0), which led to the narrow band gap, wide wavelength of the light source, effective separation of e^-/h⁺ pairs, and high photocatalytic activity. The vast improvement was related to the formation of ·OH, h⁺, e⁻, ·O₂⁻, and e_aq⁻, which was confirmed by the radical-trapping experiments. Furthermore, a plausible mechanism for the photocatalysis of PNP over CO(Ⅱ)-BiOCl@biochar hybrid catalysts under VUV irradiation was also proposed.

Introduction

With the continuous advancement of the industrial revolution, the environment on which human beings depend for survival has suffered serious damage. It has triggered a series of environmental problems, such as water, soil, and air pollution, and several notorious fatal-polluting environmental accidents [1], [2], [3], [4]. Among them, the disposal of refractory pollutants in the aquatic ecosystem has left researchers perplexed. Their existence will not only increase the difficulty of the conventional treatment process but also seriously harm human health [5], [6]. Thus, it is urgent to seek alternative technologies to solve this problem.

As is known, vacuum ultraviolet (VUV) method is a promising technology to degrade organic pollutants because water can be rapidly cracked into hydroxyl radical (·OH) by absorbing light of 185 nm wavelength [7], [8]. However, the removal efficiency for refractory pollutants with electron-withdrawing groups is still limited, as described in our previously report [9]. Recently, photocatalysis technology based on TiO₂, ZnO, BiOCl, WO₃, etc., has attracted increasing attention in the environmental protection field due to the mass production of reactive oxygen species (ROS) and environment-friendly operation systems [10], [11]. In particular, bismuth oxychloride (BiOCl) is identified as one of the most promising photocatalysts exhibiting outstanding photocatalysis. It has been reported that BiOCl possesses a unique layered structure with [Bi₂O₂]²⁺ layer sandwiched between two [Cl₂]²⁻ layers in the skeleton. When BiOCl is stimulated, numerous photo-induced electron-hole (e⁻/h⁺) pairs are produced to render good photocatalytic activity [12], [13], [14]. Importantly, BiOCl is a typical UV-light-driven photocatalyst which shows no response towards visible light. Therefore, the powerful combination of BiOCl and VUV irradiation may realize a synergistic effect. However, some modifications need to be performed to enhance the photocatalytic activity of BiOCl [15], [16]. Mokhtari and Tahmasebi [17] reported that W-doped BiOCl eliminated more than 90% of Rhodamine B after 180 min under visible light. Not only was the wavelength of the light source broadened, but the band gap energy was also lowered. Cao et al. [18] investigated the photocatalytic performance of Mn-doped BiOCl and achieved 91.6% removal efficiency of metronidazole after 30 min photodegradation under visible light. The band gap was 2.89 eV lower than that of pure BiOCl (3.12 eV). Wang et al. [19] synthesized a modified BiOCl doped with cobalt through a simple hydrothermal route, which exhibited good photocatalytic activity. They found that the new Co-doped BiOCl could not only increase light absorption in the visible region, but could also enhance the charge separation between e⁻/h⁺ pairs. Mei et al [20] synthesized a heterojunction of BiPO₄/BiOCl (P-BOC) to degrade hazardous organic pollutants under visible-light irradiation. They found that suitable conduction band match between BiPO₄ and BiOCl in P-BOC heterojunctions readily facilitated the subsequent electron transfer from excited RhB to P-BOC surface, and formed various ROS. Li et al. [21] employed a low-cost and easily accessible carbon material biochar to modify BiOX (X = Cl, Br) and developed biochar/BiOX (X = Cl, Br) composite photocatalysts via a facile in-situ deposition method. Their results demonstrated that biochar/BiOX (X = Cl, Br) composites exhibited remarkably enhanced visible-light-driven photocatalytic activity toward degradation of target molecules. According to the above report, the introduction of metallic and non-metallic elements, oxygen vacancies, and even coupling with multiple semiconductors all promote the separation of e⁻/h⁺ pairs, broaden the wavelength of the light source, and improve the photocatalytic property.

Moreover, the application of biochar, which has extensive source, lower cost, and excellent catalytic ability, also plays an important role in enhancing the photocatalytic performance [22], [23]. Thus, the loading of biochar and doping of metal ion can both be employed to tune the photocatalytic activity of BiOCl. In this work, a facile hydrothermal synthesis method was applied to obtain modified BiOCl with doping of Co²⁺ and loading of biochar. The samples were characterized via X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), ultraviolet-visible diffuse reflectance spectrometry (UV-Vis DRS), and X-ray photoelectron spectroscopy (XPS). The photocatalytic performance of the samples was determined in the degradation of p-nitrophenol (PNP) as the target pollutant, including photocatalytic efficiency, effect-factor of synthetic conditions, radical-quenching, and recycling experiment. In addition, the photocatalytic mechanism of the as-prepared catalyst under VUV irradiation was also investigated.