Oxygen vacancy-mediated sandwich-structural TiO_2−x /ultrathin g-C₃N₄/TiO_2−x direct Z-scheme heterojunction visible-light-driven photocatalyst for efficient removal of high toxic tetracycline antibiotics

Highlights

• A defect TiO_2−x/ultrathin g-C₃N₄/TiO_2−x direct Z-scheme photocatalyst is fabricated.

• Z-scheme, oxygen vacancy and 3D structure improve the photocatalytic activity.

• It exhibits remarkable removal ability for TCH in actual wastewater or ion disturbed.

• The removal path of TCH and Z-scheme photocatalytic mechanism are clarified.

Abstract

A surface defect sandwich-structural TiO_2−x/ultrathin g-C₃N₄/TiO_2−x direct Z-scheme heterojunction photocatalyst is successfully constructed. The results manifest the existence of oxygen vacancies, sandwich structure and direct Z-scheme heterojunction. Noticeably, TiO_2−x/ultrathin g-C₃N₄/TiO_2−x efficiently eliminates high toxic tetracycline hydrochloride by means of·O₂⁻, h⁺ and·OH, whose removal rate is 87.7% during 90 min and the pseudo-first-order rate constant reaches up to 31.7 min⁻¹ × 10⁻³. The extraordinary performance can be attributed to the special 3D structure, Z-scheme heterojunction expediting charge transfer and promoting the generation of active species, meanwhile the oxygen vacancies enhancing the spatial separation of photo-induced carriers. Moreover, various environmental factors are systematically explored by statistics. SO₄²⁻, NH₃-N and pH exhibit an obvious impact on removal rate. Meanwhile, TiO_2−x/ultrathin g-C₃N₄/TiO_2−x could also effectually remove tetracycline hydrochloride from complex actual-wastewater and exhibit high stability. Besides, the photocatalytic mechanism and degradation path of tetracycline hydrochloride are also elucidated.

Introduction

Tetracycline antibiotics have played an important role in safeguarding human health for many decades (Subbiah et al., 2020, Zhu et al., 2020). However, the overuse of them also causes a range of harm, such as breaking the ecological balance, threatening the human body and seriously interfering with the ecosystem (Berti and Hirsch, 2020, Henrickson, 2019). Noticeably, tetracycline hydrochloride (TCH), which produced by hydrochloric acid and tetracycline, exhibits higher water solubility. It could be stable in water environment for a long time as a refractory antibiotic substance. Hence a great deal of research has been done to solve the environmental problem (Shan et al., 2019, Ai et al., 2019). Among all techniques, visible-light photocatalysis is considered to be a promising one (Liu et al., 2020, Wang et al., 2019b). Excited by visible light, electrons in the valence band (VB) of visible-light photocatalyst can transfer to the corresponding conduction band (CB), forming photogenerated electron-hole pairs. Due to their separation, a series of strong oxidizing species (SOSs) would be generated and effectively remove TCH (Subudhi et al., 2019, Behera et al., 2019a).

As a traditional photocatalyst, TiO₂ has demonstrated its excellent value in engineering. However, it also has some disadvantages that cannot be ignored, such as too wide band gap leading extremely poor visible-light activity (Pang et al., 2014). Therefore, scholars keep trying modification methods to inject new vitality into TiO₂ (Chen et al., 2018, Jin et al., 2020). The proposal of oxygen vacancies (O_V) breaks this deadlock (Chen et al., 2015). It essentially changes the lattice structure of the catalyst. Meanwhile the resulting lattice defects can act as shallow electron traps for inhibiting photogenerated electron hole pairs recombination. Besides, O_V can enhance the ability of catalysts to absorb dissolved oxygen in water, thereby increasing the production of SOSs (Kim et al., 2019, Liu et al., 2019b). In addition, as for TiO₂, the generation of O_V would significantly change the position of band gaps and the color of catalysts, which could distinctly extend the wavelength absorption range (Zhao et al., 2019, Yang et al., 2018). Hence, the value of O_V cannot be ignored.

Since 2009, when Wang introduced g-C₃N₄ to the field of photocatalysis (Wang et al., 2009), research on g-C₃N₄ has gained increasing attention (Liu et al., 2019a, Zhang et al., 2020). As an organic semiconductor, the narrow band gap of g-C₃N₄ results in a stronger visible-light response. Therefore, g-C₃N₄ has great potential in the field of water remediation. Nevertheless, the same study by Wang and colleagues, has also pointed out the key constraints on its development, which is the photocatalytic efficiency of g-C₃N₄ greatly decrease due to the high recombination rate of photoelectron pairs (M. Chen et al., 2020). In view of this, researchers have continuously tried to overcome the problem by changing the microscale of g-C₃N₄, resulting in zero-dimensional (0D) g-C₃N₄ quantum dots (Yin et al., 2020), one-dimensional (1D) g-C₃N₄ nano-fibers (Zeng et al., 2019), two-dimensional (2D) g-C₃N₄ nano-sheets and three-dimensional (3D) mesoporous g-C₃N₄ (Tian et al., 2018, Dou et al., 2020). 2D ultrathin materials have attracted widespread attention at present. Huang et al. (2018). With regard to 2D materials, the thinner thickness, the more catalytic active site, leading to better photocatalytic activity (S. Wang et al., 2019). Therefore, 2D ultrathin g-C₃N₄ (UCN) is a good choice to solve the problem of antibiotic pollution.

Based on the above, if TiO₂ with O_V (TiO_2−x) could combine with UCN to form heterojunction which is used to solve the pollution of tetracycline, excellent results would be achieved. To emphasized, distributing TiO_2−x nanoparticles on both sides of UCN could form a sandwich-like 3D structure, a meaningful morphology which is usually formed during 2D laminar catalyst combining with other materials (Wu et al., 2020, Zhang et al., 2010). The formation of sandwich structure may effectively shorten the carrier transport pathway, provide more reaction sites and improve the photocatalytic performance. In addition, high VB and low CB are easier to generate direct Z-scheme, which has been testified to be a prospective modification for enhancing the photocatalytic performance (Xue et al., 2019, Tang et al., 2020). The idea could not only keep the respective advantages of TiO_2−x and UCN, but also obtain unexpected microscopic characteristics, significantly promoting the removal of TCH from water (P. Ding et al., 2020).

Herein, in this study, the overarching aim is to design an O_V-mediated sandwich-structural TiO_2−x/ultrathin g-C₃N₄/TiO_2−x direct Z-scheme heterojunction visible-light-driven photocatalyst (S-TUCNov) to efficiently remove TCH from water. Among them, the synthesis process mainly includes the method of sol-gel combined with secondary-calcination, meanwhile planetary grinding and in situ reduction are used for compositing and introducing O_V. After characterization, many environmental factors are explored, such as dose, miscellaneous ions, pH, etc. Against this backdrop, the removal rate of TCH in actual wastewater is investigated. Finally, mechanisms of photocatalytic and TCH decomposition are deeply elucidated. It is believed that the introduction of direct Z-scheme heterojunction, O_V and sandwich structure could attract more attention in terms of photocatalysis, environmental restoration and other applications.