Ag₃PO₄/g-C₃N₄ Z-scheme composites with enhanced visible-light-driven disinfection and organic pollutants degradation: Uncovering the mechanism

Highlights

• A mesoporous Ag₃PO₄/g-C₃N₄ Z-scheme photocatalyst was prepared successfully.

• Ag₃PO₄/g-C₃N₄ exhibited excellent activity for disinfection and degradation.

• The practical application of Ag₃PO₄(8)/g-C₃N₄ in natural water was discussed.

• The photocatalytic mechanism of Ag₃PO₄/g-C₃N₄ was studied systemically.

Abstract

A novel Z-scheme Ag₃PO₄/g-C₃N₄ heterostructures was constructed successfully for Escherichia coli inactivation and organic pollutants degradation under visible light irradiation. The Ag₃PO₄(8)/g-C₃N₄ possessed the highest photocatalytic effect with more than 7 log of live Escherichia coli (E. coli) totally inactivated within 75 min and complete BPA degradation within 180 min. By mimicking the components of natural water individually, humic acid (HA) and inorganic anion were found to deeply affected the photocatalytic disinfection activity. Moreover, the Ag₃PO₄(8)/g-C₃N₄ revealed significantly boosted visible-light-driven photocatalytic performance for ciprofloxacin and sulfadiazine degradation. The satisfying degradation effects of bisphenol A in nature waters were also obtained. The improved photocatalytic efficiency of Ag₃PO₄/g-C₃N₄ may be attributed to the formation of Z-scheme heterostructure structure and matching valence band and conduction band, resulting in rapid separation of photo-induced carrier, enhanced electronic transport capacity and prolonged carrier lifetime. The possible mechanism was studied using radical quenching tests and electron spin resonance, suggesting that the hole, electron and hydroxyl radical were the paramount active species during Escherichia coli disinfection. This work pioneers a novel strategy for upgrading the photocatalytic disinfection and degradation capacities of g-C₃N₄ based catalysts.

Introduction

The modern society suffers from a series of environmental problems such as water, air and soil pollution, which have threatened the environmental and public health. Although obtaining safe drinking water is the most basic requirement for living in the world, more than one billion people have no access to safe drinking water [1]. Waterborne illnesses caused by bacteria, enteric virus infections and organic pollutant have led to serious illness and death in the world especially these developing countries or water-scarce countries [2], [3]. Traditionally, safe drinking water is achieved by many strategies such as chlorination, adsorption, biological technology and chemical oxidation [4], [5]. Among them, photocatalytic technology by employing various semiconductor photocatalysts such as g-C₃N₄ [6], Co₂TiO₄ [7], Ag₃PO₄ [8] and carbon quantum dots (CQDs) [9], lanthanide metal oxides and their ion-doped magnetic ferrite nanoparticles [10], [11], [12], [13] has spurred much attention because of its excellent organic degradation and disinfection performance.

Graphitic carbon nitride (g-C₃N₄) has received much attention because of its unique 2D nanosheet morphology and intrinstic π-conjugated polymer functions. It has displayed excellent performances in environmental remediation, CO₂ reduction and photocatalytic hydrogen evolution [14], [15], [16]. Since it can be easily synthesized via thermal polymerization from urea or melamine, g-C₃N₄ is considered to be a promising photocatalyst because of its cheapness, bulk availability, easy modification, and outstanding stability [17]. Nevertheless, the small specific surface area, narrow absorption range and serious charge recombination limit the photocatalytic activity of pristine g-C₃N₄ [18]. Since the morphology and structure of a nanomaterial is closely correlated to its activity, the modification in morphology can be utilized to enhance the photocatalytic effect of g-C₃N₄ [19]. Mesoporous g-C₃N₄-based catalysts are researched extensively due to their large specific surface area, accessible porous framework, copious paths for mass transfer and reactive sites for photogenerated electron-hole pairs separation, and higher photocatalytic efficiency [20], [21]. Although the porous g-C₃N₄ has been proved to be a good way to improve its photocatalytic capability, it is required to be modified further to meet the practical application. How to suppress the fast recombination of photoexcited electron-hole pairs is the major obstacle in improving its photocatalytic efficiency. According to the previous literature, some strategies had been proposed to solve the above problem, including depositing of precious metals [22], modifying with carbon materials [23], and coupling with other semiconductors [24].

Silver orthophosphate (Ag₃PO₄), as a promising visible-light-driven photocatalyst showed excellent performance in O₂ evolution from water and environmental decontamination due to its narrow bandgap (2.45 eV) [25], [26]. However, pristine Ag₃PO₄ was easy to aggregate, not stable under visible light illumination, and presented fast recombination of photoinduced carriers. All these drawbacks weakened its photocatalytic activity and thus restricted its broad application. Recently, massive efforts have been made to solve the above problems. Constructing heterostructure has been deemed to an effective method for promoting the photocatalytic performance of Ag₃PO₄ [27]. As far as we know, two semiconductor materials with matching energy band can be constructed to a heterojunction photocatalyst with enhanced photocatalytic activity. Reports have proven that Z-scheme heterostructure has aroused extensive attention due to its strong redox capability. Z-scheme photocatalytic catalyst can accelerate the photo-induced carrier separation and possess high redox capacity [28], [29]. Therefore, constructing Z-scheme heterostructure for Ag₃PO₄ can mitigate the easy photocorrosion and aggregation of bare Ag₃PO₄ [30]. For example, Bi₂WO₆ [31], MoS₂ [3], BiVO₄ [32], WO₃ [33], AgBr [34] and C₃N₄ [35] have been employed to build heterostructure with Ag₃PO₄ and considered as effective strategies for enhancing the organics degradation performance. Among these semiconductors, g-C₃N₄ has received much attention because of its cheapness. It has been reported that Ag₃PO₄/g-C₃N₄ photocatalysts manifested enhanced visible light photocatalytic performance for the photocatalytic degradation [19], [36] and hydrogen production [37], [38]. However, few literatures about the photoactivity for Escherichia coli inactivation under visible light irradiation of Ag₃PO₄/g-C₃N₄ composites were reported.

In this study, to solve the disadvantages of bare g-C₃N₄ and Ag₃PO₄, a novel Z-scheme mesoporous Ag₃PO₄/g-C₃N₄ photocatalyst was synthesized. The disinfection effects of prepared samples were evaluated through the inactivation of E. coli under visible light irradiation. Inorganic salts and HA exist in natural water body, which can have influence on bactericidal action. Therefore, it is necessary to discuss the influence of HA or inorganic anions on the disinfection activity of Ag₃PO₄/g-C₃N₄. In addition, bisphenol A (BPA), ciprofloxacin (CIP) and sulfadiazine (SD) were used to affirm the photocatalytic performance. The photocatalytic degradation effect of Ag₃PO₄/g-C₃N₄ in natural waters was explored. Finally, the underlying mechanism for the E. coli disinfection over the Ag₃PO₄/g-C₃N₄ was investigated.