Environmental-friendly synthesis of heterojunction photocatalysts g-C₃N₄/BiPO₄ with enhanced photocatalytic performance

Highlights

• Heterojunction photocatalysts g-C₃N₄/BiPO₄ with varying loadings of g-C₃N₄ were fabricated by the ball milling method.

• The superior performance was achieved from heterostructure g-C₃N₄/BiPO₄ with 5 wt% g-C₃N₄ and ball milling time of 18 h.

• h⁺ and OH were proven to be the main active species in the degradation process.

• The possible photocatalytic mechanism was clarified by semiconductor energy band theory.

Abstract

Heterojunction photocatalysts g-C₃N₄/BiPO₄ with varying loadings of g-C₃N₄ were synthesized by ball milling process in different time, the synthetic method of which was a simple, rapid, economical, and easy-controlled way. The evaluation for the performance of as-prepared g-C₃N₄/BiPO₄ was conducted by the degradation of tetracycline (TC) in stimulated sunlight condition. The superior performance was achieved from g-C₃N₄/BiPO₄ heterostructure photocatalyst with 5 wt% g-C₃N₄ and ball milling time of 18 h, which could decompose 97% TC after 120 min illumination. The enhanced photocatalytic activity could be ascribed to the synergistic effects, including changes in morphology and structure after ball milling, increased specific surface area, outstanding visible-light absorption, and efficient separation of electron-hole pairs. The possible degradation mechanism was clarified by semiconductor energy band theory. This work may present a novel strategy for preparing heterojunction photocatalysts with high photoactivity and provide the analysis method of mechanism by combining theoretical calculations and experimental observations.

Introduction

Excessive use of veterinary antibiotics (VA) has caused a global environmental problem in the past decades. Tetracycline (TC), most basic compound in tetracycline family of antibiotics, has been widely detected in water and soil environment [1], [2], [3], which poses potential risks to human and ecosystem health due to biotoxicity and induction of antibiotic resistance genes [4], [5]. Thus, a series of technologies have been exploited to decompose antibiotics [6], [7], [8], [9], [10], [11]. Among the approaches, photocatalysis has attracted substantial attention owing to its low cost, nontoxicity and high efficiency. Up to date, TiO₂ has been recognized as one of most extensively used photocatalysts in the fields owing to its excellent stability and nontoxicity. However, recombination of charge carriers and the low efficiency of light utilization restrains its applications [12]. To overcome the two drawbacks, enormous efforts have been undertaken on developing visible-light-driven materials with highly efficient separation of charge carriers [13], [14], [15].

Recently, bismuth-based photocatalysts have been proven to perform a splendid photocatalytic activity in both UV and visible-light regions due to their layered structure and plate-like appearance [16], [17], [18], [19], [20]. Bismuth phosphate (BiPO₄), one of bismuth-based photocatalysts, was first reported by Pan’s group [21] and then approved as an excellent photocatalyst with high photocatalytic activity, strong stability, unique electronic properties and low cost. The inherently induced effect of phosphate radical is beneficial to the separation of electron-hole pairs, which is considered to be the reason for enhanced photocatalytic activity compared to TiO₂ [22], [23], [24]. Nevertheless, the band bap of BiPO₄ is appropriately 3.85 eV, which could only be excited by UV light. Extensive efforts have been exploited to improve the visible light utilization, such as doping metal or nonmetal, coupling with noble metal, forming heterojunctions, etc. Constructing heterostructure between BiPO₄ and another semiconductor is an effective and most used method to separate photogenerated electrons-hole pairs. Heterojunction semiconductors like BiPO₄/BiOCl [25], BiPO₄/BiVO₄ [26], BiPO₄/Ag₃PO₄ [27], Ag/BiPO₄ [28] and Bi₂O₃/BiPO₄ [29], have been reported for enhancing the photocatalytic efficiency under visible light condition.

Graphite-like carbon nitride (g-C₃N₄) is of increasing interest because of its special electronic structure, good thermo-chemical stability and facile preparation. Moreover, the appropriate band gap (appropriately 2.7 eV) makes g-C₃N₄ respond to visible light up to 460 nm [30]. However, the poor visible light response and low separation efficiency of photogenerated electrons and holes limited the photocatalytic activity of g-C₃N₄ [31]. Constructing heterojunction is an effective approach to resolve the problem mentioned above. For instance, the g-C₃N₄/BiPO₄ nanocomposite showed better photocatalytic performance under visible light region than that of each component. Pan et al. [32] fabricated g-C₃N₄/BiPO₄ photocatalyst with core-shell structure, the photocatalytic activity of which was significantly enhanced for its effective charge separation at the heterojunction constructed between g-C₃N₄ and BiPO₄. Zou et al. [28] combined BiPO₄ with g-C₃N₄ through a wet impregnation method followed by calcination. The composite g-C₃N₄/BiPO₄ presented the better photocatalytic performance for the removal of gaseous toluene in comparison to single BiPO₄ under visible light. Xia et al. [33] synthesized the composite g-C₃N₄/BiPO₄ via a facile solvothermal process. The composite showed higher photocatalytic performance than single BiPO₄ for the removal of methylene blue and ciprofloxacin. As mentioned, different methods have been used to prepare g-C₃N₄/BiPO₄ heterojunctions, including hydrothermal treatment, ultrasonic dispersion, solvothermal treatment and calcination.

In this work, we aimed at synthesizing the BiPO₄/g-C₃N₄ heterojunction photocatalyst by a simple, rapid, economical and easy-controlled method. BiPO₄ nanostructures was first synthesized via a facile and rapid microwave irradiation method within 15 min. Then the BiPO₄/g-C₃N₄ heterojunction photocatalyst was successfully fabricated through ball milling technique, which is a practical and green (with no or minimal solvent) approach to synthesize heterojunction photocatalysts [34]. The BiPO₄/g-C₃N₄ composites exhibited distinctively enhanced photocatalytic activity beyond that of BiPO₄, as demonstrated by the degradation of tetracycline under visible light irradiation. In addition, the potential mechanisms for the formation of heterojunction and separation of charge carriers at the heterojunction interface were studied by the method of combining theoretical calculations and experimental observations.