Introduction of crystalline hexagonal-C₃N₄ into g-C₃N₄ with enhanced charge separation efficiency

Highlights

• A novel crystalline hexagonal-C₃N₄/g-C₃N₄ heterophase junction photocatalyst is firstly designed.

• The obtained heterophase junction exhibits perfect interfacial contact and high hydrophilicity.

• The proposed mechanism of photocatalytic hydrogen evolution was studied in-depth.

Abstract

Polymeric carbon nitride is a promising candidate for photocatalytic hydrogen evolution but mostly just shows moderate activity because of its inefficient charge separation and sluggish surface reaction kinetics. Phase-based heterostructures, especially crystal-phase heterostructures that are composed of identical compositions with different crystal phases, can endow nanomaterials with promising properties and efficient photocatalysis applications. In this study, a novel crystalline hexagonal-C₃N₄/g-C₃N₄ hetero-phase junction was synthesized using a direct in situ alkali salt template coupled with organic solvents strategy. High hydrophilicity was obtained because of the synergistic effect between alkali salt and organic solvents in the polycondensation process. The greatly increased separation and transfer efficiency of charge carriers and the improved proton absorption in the surface reaction of the photocatalyst were achieved by the constructed phase junctions between the crystalline hexagonal-C₃N₄ and amorphous g-C₃N₄ decorated with numerous hydrophilic groups. Thus, the carbon nitride hetero-phase junction exhibited dramatically enhanced photocatalytic performance in hydrogen evolution and excellent cycling stability under visible light irradiation. This work presents a novel insight into phase engineering based on carbon nitride materials with surface functional modification for an efficient photocatalytic activity.

Introduction

Semiconductor photocatalysts, with their potential applications, have gained increasing attention in energy storage and environmental issues [1], [2], [3]. Photocatalytic water splitting is demonstrated to be an appealing pathway to produce hydrogen for the future energy sources [4], [5]. Over the past forty years, inspired by the pioneer work from Fujishima, who reported that photoelectrochemical water splitting could be realized using TiO₂ as an electrode, many efforts have been made to explore the desired photocatalysts with high photocatalytic performance [6]. In particular, a metal-free photocatalyst, polymeric carbon nitride (PCN), has attracted increasing attention because of its easily synthesized processes (i.e. pyrolysis, refluxing, solvothermal, microwave-assisted methods), extraordinary stability, tunable electronic structure and has been widely used in various fields, such as pollutant degradation, water splitting, organic photosynthesis as well as photoelectrochemical sensor [7], [8], [9], [10], [11]. However, the practical application of pristine carbon nitride in photocatalysis still faces great challenges due to its low quantum efficiency caused by the rapid recombination of charge carriers, insufficient light-harvesting ability, and limited surface reaction kinetics, which was largely affected by the inherent electronic and structural features of the material [12], [13], [14]. Therefore, many strategies have been developed to overcome these drawbacks of pristine carbon nitride, such as nanostructure fabrication [15], [16], [17], defect creation [18], [19], elemental doping [20], [21], [22], surface functionalization [23], [24] and construction of heterostructure junction [25], [26], [27], aiming to improve the separation and transfer efficiency of charge carriers and regulate the electronic structure of photocatalysts. To date, great progress has been made to develop highly efficient carbon nitride photocatalysts, and many studies have focused on solving the following issues: 1) lack of efficient interfacial channels for fast charge transfer and 2) insufficient contact with reactants due to poor hydrophilic ability.

Recently, the hetero-phase junction design for PCN, which is usually formed during the traditional thermal polycondensation process, has been reported to be the most effective way for improved charge separation and transfer by building nano-junctions at the interfaces [25], [28], [29], [30]. Compared with heterojunctions constructed using distinct semiconductors, hetero-phase junctions composed of materials with identical chemical compositions are more effective because they provide more chances to build ideal interfaces and promote charge migration across the interfaces more efficiently [31], [32], [33], [34]. Moreover, with the increasing understanding of the intrinsic properties of PCN, many researches have found that crystallinity is an important factor affecting the photoreactivity of PCN, as high crystallinity reduces the recombination centers for charge carriers. Recent progress from some research groups has revealed that the use of suitable alkali salts to mediate the polymerization process significantly decreases the structural defects and improves the crystalline of carbon nitride, which exhibits excellent solar conversation [35], [36], [37], [38], [39]. What’s more, salt-assisted synthesis provides more opportunities to synthesize carbon nitride into new allotropes and iso-forms, with many coming with changeable energy levels [37], [39]. Therefore, it is worthwhile to rationally design PCN-based heterostructure systems containing the allotropes of carbon nitride with high crystallinity. For example, Zeng and co-authors successfully synthesized tris-s-triazine/triazine crystalline C₃N₄ hetero-phase junctions using the secondary growth method [32]. Benefiting from their suitable band structures and perfect interfacial contacts based on the simultaneous growth of tris-s-triazine C₃N₄ into the matrix of triazine C₃N₄, the high charge separation and transfer efficiency can be realized, thus causing an outstanding photocatalytic hydrogen evolution. Moreover, Xie’s group conducted an in-depth analysis by taking the semicrystalline C₃N₄ isotype junction with abundant order–disorder interfaces as an example to explain the excellent performance for the enhanced dissociation and migration of charge carriers [34]. Nevertheless, improved crystallinity is usually accompanied with a dramatic decrease in surface area compared with g-C₃N₄, which greatly reduces the number of exposed active sites and affects photocatalytic performance because of less proton absorption on the surface of catalyst materials and sluggish interfacial reaction rate [32], [40], [41]. Therefore, it is highly necessary to develop facile strategies for achieving efficient proton (H⁺) absorption and subsequent H₂O/H⁺ activation over reactive sites.

Surface hydrophilic modification is one of the effective strategies for improving contact between the catalyst surface and the reactants. The introduction of hydrophilic groups into catalysts not only enhances the dispersibility and hydrophilicity of materials, but also works as interfacial active sites to improve proton absorption and activation before the subsequent hydrogen evolution. Consequently, several attempts have been made to develop well-soluble g-C₃N₄ materials [42], [43], [44], [45]. Yu et al. developed a controllable functional modification strategy on the surface of PCN using a simple hydrothermal treatment followed by a vacuum freezing-drying process [40]. They found that the introduction of hydrophilic groups into the PCN structure can greatly improve the hydrophilicity of g-C₃N₄ nanosheets. Nonetheless, these previously reported strategies usually need a post-treatment synthetic procedure based on the prepared bulk carbon nitride, which is prejudicial to further structural engineering. In addition, the synchronous development of in situ construction of hetero-phase junction and surface functional modification of PCN to achieve efficient charge separation and meanwhile enhanced reactants adsorption has been rarely reported before. Thus, exploring a simple method to in situ construct a novel carbon nitride based hetero-phase junction photocatalyst with high hydrophilicity is a meaningful and highly challenging subject.

In this study, we designed and constructed a one-step synthesis of crystalline hexagonal-C₃N₄/amorphous g-C₃N₄ hetero-phase junction through a simple thermal condensation of melamine assisted by solid salt coupled with organic solvent. Due to the synergistic effect between alkali salt and organic solvents in polycondensation process, high hydrophilicity of catalyst was obtained. The organic solvent used here not only acts as a solvent, but also serves as a co-reagent during the thermal polymerization process for supplying more oxygen-containing groups, which is beneficial for the formation of more hydrophilic groups in hexagonal-C₃N₄/g-C₃N₄ hetero-phase junction during the process of pyrolysis. Thus, the obtained photocatalytic system exhibited a dramatically enhanced activity for hydrogen evolution compared with pristine g-C₃N₄. The interactive mechanism of the enhanced activity for crystalline hexagonal-C₃N₄/amorphous g-C₃N₄ hetero-phase junction was discussed in detail.