Ultrathin sulfur-doped holey carbon nitride nanosheets with superior photocatalytic hydrogen production from water

Highlights

• Ultrathin sulfur-doped holey carbon nitride nanosheets were successfully prepared via self-templating approach.

• Optimized S-CN(0.1) performed superior hydrogen evolution rate of 6225.4 μmol g⁻¹ h⁻¹ (λ> 420 nm), almost 45 times higher than the pristine bulk one.

• An apparent quantum yield of 10 % at 420 nm was achieved for hydrogen production.

• A reliable and universal method was developed to realize morphological evolution of graphitic carbon nitride with increasing reaction sites.

Abstract

Surface engineering is an efficient way to enhance photoabsorption, promote charge separation and boost photocatalysis. Herein, sulfur-doped holey g-C₃N₄ nanosheets have been prepared through a universal self-templating approach with thiocyanuric acid as the single-precursor. By subtly controlling the feeding amount of precursor, the synthesized sulfur-doped holey g-C₃N₄ nanosheets exhibit excellent visible-light driven photocatalytic hydrogen production activity. The optimized catalyst presents a hydrogen evolution rate of 6225.4 μmol g⁻¹h⁻¹, with an apparent quantum yield of 10 % at 420 nm. Comprehensive characterizations and theoretical calculations suggest that the enhanced photocatalysis is attributed to the synergy of the enlarged surface area, the negatively-shifted conduction band, and the narrowed bandgap due to sulfur-doping and ultra-thin two-dimensional topology. This work highlights the importance of controlling the precursor dosage and inducing sulfur doping into the polymer, providing a promising and reliable strategy to simultaneously regulate the nanostructural and electronic structure of g-C₃N₄ for highly efficient photocatalysis.

Introduction

Highly efficient photocatalysts have attracted tremendous attention due to their potential applications in renewable energy supply and environmental remediation [[1], [2], [3]]. The key is the development of efficient photocatalysts. Among various photocatalysts, graphitic carbon nitride (g-C₃N₄) has emerged as a promising metal-free visible-light responsive photocatalyst due to its moderate bandgap and high stability [[4], [5], [6]]. Thermolysis of various nitrogen-rich precursors could prepare g-C₃N₄ in large-scale, but the insufficient photoabsorption, high charge recombination rate and low quantum yield still suppress its photocatalytic performance [[7], [8], [9]]. To improve the efficiency, great efforts including nanostructural and surface modification have been devoted [[10], [11], [12], [13], [14], [15]].

Two-dimensional nanosheets are a novel category of nanostructural materials and prevailing because of their unique layered features of intriguing surface, optical and electronic properties [[16], [17], [18]]. Exfoliation of the bulk was a post-synthetic method to break down the interlayer van der Waals' forces into two-dimensional nanosheets, which had been demonstrated as an efficient route for shortening the charge transfer pathway, increasing the surface area and providing more active sites [19,20]. Through ultrasonic-, thermal- and chemical-assisted exfoliation, pristine g-C₃N₄ nanosheets were prepared [19,21,22]. However, the enlarged surface area would result in an increased bandgap energy due to the quantum confinement effect, decreasing the photoabsorption ability. Moreover, the severe charge recombination could hardly be suppressed through simple morphological modification [23]. Therefore, developing a green and reliable strategy for low-cost assembly of two-dimensional g-C₃N₄ nanosheets as well as optimizing its electronic structure is still highly desired.

Anion doping is generally an alternative and important approach owing to the effectiveness in regulating the electronic structure and broadening the region of light absorption. With anion doping like B, O, C, P, I, the atomic and electronic properties could be positively optimized through the injection of localized states from dopants [[24], [25], [26], [27], [28], [29], [30], [31]]. As the conduction and valence band of g-C₃N₄ were theoretically confirmed to be primarily derived from the p_z orbitals of carbon and nitrogen [32], the substitution of these atoms would primarily lead to the delocalization of big π-conjugated system, boosting the conductivity of g-C₃N₄. The more negative conduction band would exhibit stronger photoreduction ability and provide stronger driving force for hydrogen evolution. Nevertheless, heteroatom dopants might also cause doping asymmetry and serve as new charge recombination sites [33,34]. Thus, to enhance the charge separation and maintain suitable photoabsorption, subtle regulation of the type and location of dopants is of great significance.

In this work, sulfur-doped holey g-C₃N₄ nanosheets were fabricated through a controllable self-templating approach without any additives. Based on the nucleation-growth mechanism, tuning the amount of precursor would result in the optimized concentration in a semi-closed synthesis system for thermally-driven polymerization, thereafter generating g-C₃N₄ nanosheets. With thiocyanuric acid as the precursor, ultrathin sulfur-doped holey nanosheets were prepared. The dramatically enhanced photoactivity for hydrogen production is attributed to the enlarged surface area, the enhanced photoabsorption, and the suppressed charge recombination derived from the synergy of sulfur-doping and ultrathin holey nanosheet topology.