Enhanced visible-light photocatalytic degradation and disinfection performance of oxidized nanoporous g-C₃N₄ via decoration with graphene oxide quantum dots

Abstract

Oxidized nanoporous g-C₃N₄ (PCNO) decorated with graphene oxide quantum dots (ox-GQDs) was successfully prepared by a facile self-assembly method. As co-catalysts, the ultrasmall zero-dimensional (0D) ox-GQDs can achieve uniform dispersion on the surface/inner channels of PCNO, as well as intimate contact with PCNO through hydrogen bonding, π-π, and chemical bonding interactions. In contrast with PCNO, the ox-GQDs/PCNO composite photocatalysts possessed improved light-harvesting ability, higher charge-transfer efficiency, enhanced photooxidation capacity, and increased amounts of reactive species due to the upconversion properties, strong electron capturing ability, and peroxidase-like activity of the ox-GQDs. Therefore, the visible-light photocatalytic degradation and disinfection performances of the ox-GQDs/PCNO composite were significantly enhanced. Remarkably, the composite with a 0.2 wt.% deposited amount of ox-GQDs (ox-GQDs-0.2%/PCNO) exhibited optimum amaranth photodegradation activity, with a corresponding rate about 3.1 times as high as that of PCNO. In addition, ox-GQDs-0.2%/PCNO could inactivate about 99.6% of Escherichia coli (E. coli) cells after 4 h of visible light irradiation, whereas only ˜31.9% of E. coli cells were killed by PCNO. Furthermore, h⁺, •O₂⁻, and •OH were determined to be the reactive species generated in the photocatalytic process of the ox-GQDs/PCNO system; these species can thoroughly mineralize azo dyes and effectively inactivate pathogenic bacteria.

Introduction

Urbanization and rising living standards have led to increasingly serious environmental pollution and energy crisis issues [1, 2]. The wastewater produced by a variety of industries contains various organic and inorganic pollutants as well as pathogenic bacteria, which seriously damage the environment and threaten human health [3, 4, 5, 6]. In particular, azo dyes, as the most common dyestuffs in wastewater produced by textile and food industries, have caused serious ecological damage because of their toxicity, non-biodegradability, potential carcinogenicity, and mutagenic nature [⁷]. Moreover, the presence of azo dyes in water significantly increases the chemical and biochemical oxygen demands (COD and BOD, respectively), which further leads to potential damage to the aquatic life [¹]. On the other hand, many kinds of pathogenic bacteria exist in domestic, food processing, and hospital wastewater including Escherichia coli (E. coli), Shigella, and Campylobacter, which could affect aquatic ecosystems and pose a threat to human health [⁸]. Moreover, drinking water containing excessive amounts of pathogenic bacteria can cause severe inflammation and even death [⁹]. Therefore, the development of a green, efficient, and safe technology to remove both azo dyes and pathogenic bacteria from wastewater represents an urgent task. Compared to traditional treatments, photocatalytic technologies have advanced rapidly in the environmental remediation field, due to superior features such as low cost, strong oxidation ability, excellent recycling ability, and lack of secondary pollution [¹⁰]. Significant efforts have been made in the study of novel semiconductor photocatalysts, in order to enhance the photocatalytic performance in two aspects: (i) improving the solar energy utilization, extending the light-harvesting region of the photocatalyst from the ultraviolet (UV) to the near-infrared (NIR) window, thus improving the efficiency of the photocatalytic process [11, 12, 13]; (ii) increasing the photogenerated charge separation efficiency, which can result in higher numbers of reactive species participating in the photocatalytic process [14, 15, 16].

Graphitic carbon nitride (g-C₃N₄), a metal-free π-conjugated semiconductor, has attracted considerable attention in the field of photocatalysis due to its low cost, easy availability, high stability, non-toxicity, and simple tunability [17, 18, 19]. Furthermore, g-C₃N₄ also possesses suitable band gap and valence/conduction band positions, which are appropriate for harvesting UV-visible (UV-vis) light for water splitting, pollutant degradation, and disinfection [20, 21, 22]. However, poor vis-NIR light utilization, low specific surface area, reduced number of active sites, and fast charge recombination have limited further applications of g-C₃N₄ [23, 24]. Several methods have been used to address the above problems, including elemental doping to tune the energy band structure [25, 26], morphology control to increase the surface area [27, 28], and heterojunction formation to accelerate charge separation [29, 30]. Among these approaches, obtaining g-C₃N₄ with nanoporous morphology is an effective method to enhance its photocatalytic performance, due to the advantages associated with the increased surface area and the creation of additional inner nanoporous channels [³¹]. Although porous materials provide more active sites, they also introduce higher numbers of surface defect states, resulting in rapid charge recombination [³²]. Moreover, the light-harvesting ability of these materials cannot be improved to a sufficient level [³³]. Compared to bulk or two-dimensional (2D) nanosheet photocatalysts, it is harder to achieve full contact of the nanoporous photocatalysts with the modification material, which can only contact the outer surface rather than being deposited into the nanoporous channels [³²]. Therefore, to meet the above requirements we choose ultrasmall zero-dimensional (0D) quantum dots as the co-catalyst. The decoration with quantum dots can accelerate the charge transfer, regulate the band structure, and improve the light-harvesting ability, thereby greatly enhancing the photocatalytic activity of the nanoporous semiconductors [³⁴].

As an emerging 0D graphene-based material with sub-5 nm size, graphene oxide quantum dots (ox-GQDs) consist of highly crystalline, few-atom-thick graphene planes and abundant oxygen-containing functional groups including hydroxyl (–OH), epoxy (–O–), methoxy (–OCH₃), carbonyl (–C=O), and carboxyl (-COOH) [³⁵]. These oxygen-containing groups result in improved specific surface area and solubility of ox-GQDs, as well as changes in their optical and electronic properties compared to those of graphene quantum dots (GQDs) [³⁶]. Due to pronounced quantum confinement and edge effects, ox-GQDs possess unique physical and chemical properties, such as high conductivity, low cytotoxicity, good surface grafting, strong electron capturing ability, characteristic upconverted photoluminescence (PL), and unique peroxidase-like properties [37, 38]. Among these features, the high conductivity and strong electron capturing ability of ox-GQDs can make the access to electrons and the electron diffusion process much easier, effectively improving the charge transfer efficiency of the photocatalysts [³⁹]. Furthermore, the upconversion properties of ox-GQDs can convert absorbed long-wavelength light into short-wavelength light, improving the light-harvesting ability of the photocatalysts [³⁵]. In addition, ox-GQDs also exhibit peroxidase-like activity, and can catalyze the decomposition of H₂O₂ (low oxidation activity) to •OH (high oxidation activity) [³⁸]. This special property provides a new route to increase the amount of produced reactive species and enhance the photooxidation capability of the photocatalysts.

In this study, in order to achieve increased solar energy utilization, higher charge-transfer efficiency, and enhanced photooxidation ability, oxidized nanoporous g-C₃N₄ (PCNO) was decorated with ox-GQDs by a facile self-assembly method. We employed a mild hydrothermal approach to introduce specific amounts of oxygen-containing groups into the g-C₃N₄ framework and fabricate the nanoporous structure of PCNO. A top-down strategy was also adopted to directly cut graphene oxide into ox-GQDs via an acidic exfoliation method. The morphology, structure, and properties of the ox-GQDs/PCNO composite were systematically investigated by various techniques. The amaranth and E. coli degradation and disinfection activities of the composite photocatalysts under visible light irradiation were carefully evaluated and found to be markedly enhanced in comparison to those of PCNO. Moreover, the important roles played by the ox-GQDs and the generated reactive species in the photocatalytic process of the ox-GQDs/PCNO system were also elucidated in detail.