Enhanced activity of ZnS (111) by N/Cu co-doping: Accelerated degradation of organic pollutants under visible light
Graphical Abstract
Introduction
With the rapid development of industrialization, many new organic pollutants are attached to wastewater, including persistent organic pollutants (POPs), pharmaceutical and personal care products (PPCPs), endocrine disrupting chemicals (EDCs) (Han and Currell, 2017, Vasseghian et al., 2021; Wang et al., 2021). 2,4-Dichlorophenol (2,4-DCP) is a typical non-biodegradable POPs in wastewater, even very low concentrations can cause serious harm to the environment (Yang et al., 2020). Tetracycline (TC) is a typical PPCPs and the second most widely used antibiotic in the world. Due to the high stability and low biodegradability, it is often found in different water sources and drinking water (Wang et al., 2020). Therefore, it is urgent and meaningful to remove these non-biodegradable pollutants from wastewater. With its non-selective oxidation performance and the ability to degrade organic pollutants gradually and completely in wastewater, photocatalysis technology has attracted the attention and is considered as a green technology to solve global environmental problems in the future (Huang et al., 2016, 2017; Wang et al., 2019). In the process of photocatalytic degradation of organic pollutants, new by-products may be produced, and their toxicity may be greater than the initial pollutants. Therefore, it is essential to explain the photocatalytic degradation pathway (Zhou et al., 2020). Since the advent of photocatalytic technology, many kinds of photocatalysts have been developed to deal with the challenges of water pollution, such as sulfides (Ren et al., 2020; Zhu et al., 2021), oxides (Zu et al., 2019), carbon materials (Li et al., 2019) and their composite products (Feng et al., 2020). However, the application of oxide-based photocatalysts is often limited due to low quantum efficiency and easy photo corrosion. Therefore, sulfide-based photocatalysts have attracted the attention of researchers (Tie et al., 2019).
ZnS is an important II-VI photocatalyst semiconductor, which attracts people's interest because of its good thermal stability, non-toxic, insoluble in water and relatively low cost (Xu et al., 2018; Lee and Wu, 2017). Because the band gap of ZnS is about 3.5 eV, it can only absorb ultraviolet light with a wavelength below 380 nm (Mehrabian and Esteki, 2017). However, the radiation energy of visible light, infrared light and ultraviolet light is 40.3%, 51.4% and less than 10%, respectively. How to adjust the band gap of ZnS to make it absorb visible light and even infrared light, is the key issues of the study. Element doping is an effective method to control the band gap of semiconductor (Erwin et al., 2005). The doping of transition elements can introduce impurity energy levels in the forbidden band of intrinsic semiconductors, reduce the energy required for electronic excitation transition, thereby reducing the band gap (Eg) of the photocatalyst and finally absorbing the visible light, such as Cu (Liao and Carter, 2013) and Mo (Zhang et al., 2017). Generally, the donor and acceptor energy levels introduced in the band gap are full or empty energy levels, which is not easy to become the recombination center of electrons and holes (Dong et al., 2015). In addition, if the impurity energy level introduced by transition elements can be hybridized with the energy level at the top of valence band or the bottom of conduction band of the intrinsic photocatalyst simultaneously, the doping will become a better way of doping. The non-metallic element N is also an excellent doping element, which can reduce the Eg of ZnS, improve the responsiveness of ZnS to visible light (Muruganandham and Kusumoto, 2009), and N-doped ZnS also has excellent stability (Popov et al., 2018). According to literature surveys, so far, there are still few studies on N-doped ZnS. The main reason is that the preparation conditions are relatively high, and ZnS is often calcined at 800°C in an ammonia atmosphere, which makes the preparation process very dangerous. Therefore, how to prepare N-doped ZnS under friendly conditions is significant research.
Morphology is also a key factor affecting the photocatalytic performance of ZnS. It is well known that when Eg is close, the uniform morphology and highly controllable structure can make the semiconductor have stronger photocatalytic performance and be more stable. Due to the increased research on nanomaterials, it is possible to prepare nanometer photocatalytic materials with uniform and stable morphology, such as spherical (Zhu et al., 2011; Castillo et al., 2010), layered (Li et al., 2020), and flower-shaped (Chowdari et al., 2020; Jiang et al., 2020). The separation of photogenerated electrons and holes can also be accelerated by adjusting the particle size of photocatalyst. And smaller size can reduce the distance between photogenerated electrons and holes to the surface, thereby increasing the ratio of photogenerated electrons and holes participating in the reaction (Hong et al., 2009). According to the literature survey, there is no report about the research on the simultaneous doping of ZnS and morphology control by non-metal elements and transition elements. Although there are many ways to improve the photocatalytic performance of ZnS, most of the studies still use ultraviolet light in the photocatalytic degradation of organic pollutants (Ahadi et al., 2016). Therefore, it is still a great challenge to make ZnS efficiently degrade organic pollutants under visible light.
In this study, for the first time, the N/Cu co-doped ZnS nanosphere photocatalyst (N/Cu-ZnS) was synthesized by a simple hydrothermal method. The characterization analysis and DFT calculation results of N/Cu-ZnS showed that the catalytic activity of ZnS (111) crystal plane was enhanced and more (111) crystal planes were exposed. The (111) crystal plane with high catalytic activity accelerated the adsorption of O2, resulting in more •O2− production during the reaction, which made N/Cu-ZnS show excellent degradation performance for 2,4-DCP and TC under visible light (>420 nm). Finally, the possible photocatalytic degradation pathways of 2,4-DCP and TC by N/Cu-ZnS were proposed.
Section snippets
Preparation of N/Cu-ZnS nanospheres
Scheme 1 shows the preparation process of N/Cu-ZnS. Firstly, N-doped ZnS was prepared. Mixed solution (50 mL) of ethanol and glycol (mixed ratio of 3:1) was token out, and 0.545 g of ZnCl2 was added to it. After ultrasonic treatment for 2 hr, 0.601 g of thioacetamide and 0.214 g of NH4Cl were added into this solution, and the ultrasonic treatment was continued for 2 hr. The resulting solution was taken to a 100 mL Teflon-lined autoclave, and reacted at 160°C for 16 hr. Then, the product was
Catalyst characterization
To study the influence of N and Cu doping on the structure and morphology of ZnS, ZnS, N-ZnS and N/Cu-ZnS were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). Fig. 1a-b shows that the ZnS samples are solid nanosphere with a smooth surface and a diameter of about 1 µm. After N and Cu doping (Fig. 1c-f), the spherical morphology of the sample does not change, but the diameter of the sphere is reduced to about 800 and 700 nm, and the compactness of the
Conclusions
In this study, for the first time, N/Cu co-doped ZnS nanospheres are prepared by a simple hydrothermal method. The DFT calculation results show that the Cu 3p, Cu 3d and N 2p orbitals are hybridized with S 3p orbitals at the valence band, and the N 2s and N 2p orbitals are hybridized with Zn 4p and S 3p orbitals at the conduction band. These hybridizations reduce the band gap of N/Cu-ZnS, thus achieving the purpose of responding to visible light. Under visible light (> 420 nm), compared with
Acknowledgments
This work was supported by CNPC safety and environmental protection key technology research and promotion project (No. 2017D-4613), Sub project of national science and technology major project (No. 2016ZX05040-003) and China University of Petroleum (East China) Graduate Innovative Engineering Project (No. YCX2020039).
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