Full Length ArticleNiS and graphene as dual cocatalysts for the enhanced photocatalytic H2 production activity of g-C3N4
Graphical abstract
Introduction
Nowadays, photocatalytic water splitting to produce hydrogen by sunlight has attracted immense attention because it provides a promising strategy to solve environmental issues and energy crisis [1]. Currently, TiO2 has been widely studied as photocatalyst since the exploration of water photolysis by Fujishima and Honda in 1972 [2]. However, the low utilization efficiency of visible light and poor charge carrier transfer vastly limit its application [3]. Up to now, the rational design and development of sustainable and high-efficient visible light-responsive photocatalyst are still great challenges.
Among the numerous photocatalysts, graphitic carbon nitride (g-C3N4) stands out because of its low cost, good chemical stability, suitable band edge potential and unique porous structure [4], [5], [6], [7]. Nevertheless, pristine g-C3N4 suffers from shortcomings such as high recombination rate of photoinduced charge carriers, small specific surface area, insufficient active sites and low visible light absorption, which greatly restrict its development and practical application in photocatalytic H2 generation [8], [9], [10], [11], [12]. In order to solve the above-mentioned problems and obtain excellent photocatalytic activity, various strategies have been proposed, including surface modification [13], [14], doping [15], [16], [17], [18] and design of heterojunction composite [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Although great success has been achieved in the last few decades, feasible strategies to improve the separation rate of photogenerated electron–hole pairs and photocatalytic activity of g-C3N4 are still urgently needed.
Noble metals, including Pt, Au and Ag, have been widely used as cocatalyst to enhance the photocatalytic H2 evolution rate of g-C3N4 [30]. However, on account of the high cost and extreme rarity, the large-scale application of noble metal-modified photocatalytic systems is limited [31]. By contrast, it is more practical to develop low-cost noble-metal-free cocatalyst. In recent years, several earth-abundant metal compounds, such as MoS2 [32], [33], NiSx (x = 1 and 2) [34], [35], [36], CoSx [37], [38] and Ni(OH)2 [39], have emerged as alternative cocatalysts for g-C3N4. The employment of these compounds indeed reduces the cost and enhances the photocatalytic activity, while the increase in photocatalytic activity is still unsatisfactory.
Graphene, a two-dimensional (2D) monolayer consisting of sp2-hybridized carbon atoms, has attracted great attention owing to its large specific surface area and high electron mobility rate. Particularly, it can be used as a transfer mediator for electrons, which is beneficial for the separation of photogenerated charge carriers. Therefore, great efforts have been made to endow semiconductors with excellent photocatalytic performance by combining them with graphene. For example, Quan and coworkers reported that the graphene oxide/g-C3N4 hybrid synthesized by sonochemical approach displayed enhanced photocatalytic activity under visible light irradiation towards the degradation of rhodamine B and 2,4-dichlorophenol [40].
In this work, we demonstrate the modification of g-C3N4 using NiS and graphene as dual cocatalysts via a facile one-step hydrothermal method. The photocatalytic H2-production performance of the g-C3N4/0.25%RGO/3%NiS sample is remarkably improved under visible light irradiation due to the enhanced separation of photogenerated electron–hole pairs. The 2D graphene with π-conjugation structure acts as an excellent electron acceptor and transporter. This study enriches the development of high-performance g-C3N4-based photocatalysts.
Section snippets
Sample preparation
All chemical reagents were purchased and used without any further purification. Graphene oxide (GO) was prepared by a modified Hummers’ method using natural graphite powder as precursor [41]. Bulk g-C3N4 was prepared by one-step thermal polymerization of urea in a muffle furnace under 550 °C for 2 h with a ramp rate of 5 °C min−1. In a typical synthesis process, 0.1 g of g-C3N4, 0.03 mmol of Ni(NO3)2·6H2O and 0.03 mmol of thioacetamide were dissolved in 50 mL ethanol to form a homogeneous
Results and discussion
XRD patterns (Fig. 1) were used to investigate the phase structures of the samples. It is found that all the prepared powders exhibit two characteristic diffraction peaks centered at around 13.2° and 27.4°, which are consistent with the typical (1 0 0) and (0 0 2) facets of g-C3N4, respectively [42], [43]. These diffraction peaks are ascribed to the structural packing of tri-s-triazine heterocycles and the regular graphite-like interlayer stacking, respectively [44]. Besides, a broad peak in
Conclusions
In summary, g-C3N4/0.25%RGO/3%NiS photocatalyst is conveniently synthesized by a facile one-step hydrothermal method. Compared with the g-C3N4/3%NiS binary composite and pure g-C3N4, the obtained ternary composite photocatalyst exhibits better photocatalytic H2 generation performance with an appropriate RGO content under visible light irradiation. Herein, graphene plays a significant role due to its high light utilization, superior charge separation and electron transfer efficiency, which
Acknowledgements
This work was supported by the NSFC (21801091), National Postdoctoral Program for Innovative Talents (BX20180231), Opening Fund of Hubei Key Laboratory of Processing and Application of Catalytic Materials (201828903), Science Technology Development Planning of Jilin Province (20170520069JH) and Education Department Project of Jilin Province (JJKH20180556KJ).
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