Well-designed three-dimensional hierarchical hollow tubular g-C₃N₄/ZnIn₂S₄ nanosheets heterostructure for achieving efficient visible-light photocatalytic hydrogen evolution

Abstract

Photocatalytic water splitting for hydrogen production is an important strategy to achieve clean energy development. In this report, a novel three-dimensional (3D) hierarchical hollow tubular g-C₃N₄/ZnIn₂S₄ nanosheets (HTCN/ZIS) type-Ⅱ heterojunction photocatalyst was successfully prepared and applied for photocatalytic hydrogen production under visible light irradiation. The experimental results reveal that the optimal proportion of HTCN/ZIS with the remarkable photocatalytic H₂ evolution rate of 20738 μmol h⁻¹ g⁻¹ was obtained. The main reasons for the improvement of hydrogen production activity are as follows: (i) this unique tubular hollow structure can effectively enhances the light capturing ability by the multiple light scattering/reflection of incident light in the inner cavity; (ii) the shorten the phase plane transmission distance could reduce the path of charge transfer; (iii) the surface coated a large number of scaly ZnIn₂S₄ nanosheets can provide abundant reactive sites. Combining the various characterization tests, the enhanced spatial segregation of charge carriers could owning to the intimately interfacial contact and well-matched band gaps structure between g-C₃N₄ and ZnIn₂S₄ through the type-II heterojunction. This work provides a new prospect for the construction of a novel 3D hierarchical type-II heterojunction photocatalyst for highly efficient photocatalytic hydrogen production.

Introduction

Utilization of semiconductor-based photocatalysis to achieve solar water splitting can be an effective mean to convert renewable energy into chemical fuels [1], [2], [3], [4]. In particular, the high-energy density, stored and pollution-free hydrogen produced from water splitting under sunlight is significant for global energy shortages and sustainable development [5], [6], [7], [8]. Although various photocatalysts for water splitting to produce hydrogen have been developed, their instability, toxicity, low solar utilization and rapid electron-hole recombination are far below the requirements of practical applications [9], [10]. Consequently, constructing a highly efficient and stable photocatalyst with a visible light response is highly expected, yet challenging.

In recent years, the typical ternary metal sulfide of two-dimensional (2D) ZnIn₂S₄ nanosheets have been widely used in photocatalytic field owning to its excellent chemical stability, strong visible light absorption and suitable band gap [11], [12]. Since Lei et al. firstly used ZnIn₂S₄ as a visible-light-driven photocatalyst for photocatalytic hydrogen evolution, the modification of ZnIn₂S₄ to build high-efficiency photocatalytic performance has attracted great attention [13]. Unfortunately, the single-phase ZnIn₂S₄ nanosheets commonly exhibit the serious agglomeration during the reaction process and tend present a micro-flower structure composed of nanosheets, which seriously affects its photocatalytic activity due to the cover of active sites and limited the effective use of photocarriers [14]. In addition, the photocatalytic activity of pure ZnIn₂S₄ is also affected by the low separation efficiency of photo generated electron hole pairs and narrow range of visible light utilization, leading to the low photocatalytic performance in practical application [15], [16]. For the purpose to improve its photocatalytic activity, it is a common method to construct heterojunction by coupling with another semiconductor photocatalyst with matching energy band structure, which provides a possibility that the photogenerated carriers in each individual component could be effectively and spatially separated [17], [18].

As a non-metal conjugated semiconductor polymer, graphitic carbon nitride (g-C₃N₄) has become an ideal photocatalytic semiconductor for scientific researchers due to its remarkable chemical stability, unique band gap structure, easy synthesis and rich Earth resources [19], [20], [21], [22], [23], [24]. However, the disadvantage inherent to the bulk g-C₃N₄ (BCN) is poor visible light utilization, lower than surface area, and severe recombination of photogenerated electron-hole pairs, which greatly constrain their photocatalytic activity [25], [26], [27], [28]. In general, the photocatalytic process occurs on the surface of the photocatalyst, so to obtain satisfactory photocatalytic properties of g-C₃N₄, morphological control has become an effective way to modify g-C₃N₄ to have better optical and chemical properties than bulk g-C₃N₄ [29], [30], [31], [32]. It can be said that while obtaining the specific surface area and structure of g-C₃N₄, adjusting the band structure, which greatly improves the electron hole transfer efficiency of g-C₃N₄ [33], [34], [35]. To date, copious efforts have been devoted to construct with special morphology, such as nanosphere [36], porous structure [37], nanosheets [38] and nanotubes [39], etc. It is worth noting that the one-dimensional (1D) hollow tubular g-C₃N₄ has received extensive attention in recent years due to its unique electron transmission method and large specific surface area. In particular, the unique hollow tube structure can allow the multiple refract/reflect of incident light on the inner tube wall, which improves the light capturing ability [40], [41]. Moreover, 1D g-C₃N₄ nanorods/nanotubes can transport photo generated carriers in 1D radial direction, which makes the electrons transfer directly and quickly along the specific direction, so as to achieve effective space charge separation [42], [43]. Meanwhile, a larger specific surface area can effectively provide abundant reaction sites for photocatalytic reaction, and further improve the utilization rate of visible light [44], [45], [46]. Therefore, combining the above analysis and the various controllability of the g-C₃N₄material, the carefully morphological designed one-dimensional hollow tube g-C₃N₄ as the base material can be well matched with the ZnIn₂S₄.

In this work, a low-temperature solvothermal method was used to grow 2D ZnIn₂S₄ nanosheets in situ on a 1D hollow tubular g-C₃N₄ substrate support. This well-designed novel hollow tubular g-C₃N₄/ZnIn₂S₄ heterojunction photocatalyst was constructed for the first time and applied to photocatalytic water splitting for hydrogen evolution under visible light irradiation. The matched band structure of g-C₃N₄ and ZnIn₂S₄ can form a Type-Ⅱ heterojunction, which can effectively separate the photogenerated carriers in space and promote the redox reaction of the photocatalyst. Thus, the optimized g-C₃N₄/ZnIn₂S₄ exhibits an ultra-high hydrogen production rate (20738 μmol h^-1g⁻¹) under visible light irradiation, which is 2.7 times higher than pure ZnIn₂S₄ (7613 μmol h^-1g⁻¹). Moreover, the as-prepared samples were analyzed by various characterization methods and further proposed the possible photocatalytic reaction mechanism.