Construction of direct-Z-scheme heterojunction photocatalyst of g-C3N4/Ti3C2/TiO2 composite and its degradation behavior for dyes of Rhodamine B

Abstract

Direct-Z-scheme g-C3N4/Ti3C2/TiO2 photocatalyst with giant internal electric field was prepared by one-step aqueous sonication self-assembly method using g-C3N4 and MXene of Ti3C2 as the source materials. The chemical composition and structure of the catalysts was characterized by FT-IR, XRD, SEM, TEM, and XPS. The XPS characterization indicated that Ti3C2 was partially oxidized to TiO2 during the composite process. As a result, an efficient direct-Z-scheme heterojunction structure consisting of the g-C3N4 and TiO2 with Ti3C2 as an electron bridge was constructed. The photocatalytic performance of the prepared catalysts was evaluated by degrading the Rhodamine B (RhB) wastewater. Compared with the single g-C3N4, the g-C3N4/Ti3C2/TiO2 composite photocatalyst exhibited efficient and stable photocatalytic degradation ability, with a degradation efficiency as high as 99.2% for RhB under optimal conditions (2% Ti3C2, pH = 3). The high degradation performance of g-C3N4/Ti3C2/TiO2 for RhB was attributed to the combination of Ti3C2, TiO2, and g-C3N4 components, forming a direct-Z-scheme heterojunction with a high-speed electron transport channel structure. The role of Z-scheme heterojunctions in electron transport is verified by photoelectrochemical characterization, along with photoluminescence (PL). Our research provides a simple method to design photocatalysts by constructing direct-Z-scheme electron transport channels for highly efficient treatment of dye wastewater.

1. Introduction

Water pollution has become a significant topic in modern environmental science [1]. Over the past few decades, a considerable amount of persistent organic pollutants (POPs) have been discharged into water bodies. Due to their high structural and chemical stabilities and their elevated pathogenicity and carcinogenicity, POPs have severe environmental impacts on water environments. These pollutants, such as antibiotics, dyes, and hormones, not only endanger the stability of the ecological environment but also threaten human health and safety [2]. To address the increasingly serious environmental pollution crisis, a photocatalytic technology utilizing solar energy has received widespread attention [3].

g-C3N4 is a 2D layered non-metallic polymer with a structure similar to graphene, in which carbon (C) and nitrogen (N) atoms are hybridized by sp2 to form a C/N six-membered ring. The remaining lone pairs of electrons of C and N atoms on the six-membered ring bond with each other to form a conjugated p bond, thus creating a highly delocalized p electron conjugated system on a 2D network structure [4,5]. As an n-type semiconductor, g-C3N4 has a band gap of 2.7 eV, conductivity potential of -1.3 eV, and valence potential of 1.4 eV. These characteristics enable g-C3N4 to have the ability to photodegrade water into hydrogen and promote the oxidative degradation of organic pollutants [6]. So, the metal-free polymer graphite carbon nitride (g-C3N4) is considered an attractive photocatalytic catalyst for water treatment. g-C3N4 possesses special semiconductor characteristics, such as low energy density, and good electron affinity. Meanwhile, it also has the ability to absorb visible light and is highly resistant to photocorrosion phenomena, and has an easily adjustable structure, thus has the potential to serve as an excellent photocatalyst.

However, due to the electron coupling effect, the electron-hole pairs generated by g-C3N4 under light excitation are easy to recombine, which greatly limits the catalytic activity and efficiency of the materials [7]. In order to overcome these limitations, various approaches have been proposed to enhance the photocatalytic performance of bulk g-C3N4, including forming hybrid structures [8], stripping into nanosheets [9], preparing nanocomposites with other semiconductors [10], dye sensitization [11], photocatalytic membrane materials [12], and doping to alter its band structure [13]. Following these modification methods, g-C3N4 was combined with other semiconductors (such as TiO2, WO3, ZnO, a...