Montmorillonite-hybridized g-C3N4 composite modified by NiCoP cocatalyst for efficient visible-light-driven photocatalytic hydrogen evolution by dye-sensitization
Graphical abstract
Introduction
The use of sunlight for the generation of hydrogen from water splitting is considered as a low-cost but promising technology to convert solar energy into chemical energy and support sustainable development [1], [2], [3], [4]. At present, many semiconductor-based photocatalysts, such as TiO2, CdS, bimetallic sulfides, ZnO, BiOI, Bi2WO6, and graphitic carbon nitride (g-C3N4), have been explored for such process [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Among these semiconductors, g-C3N4 with flexible two-dimensional (2D) layered structure is one of the most promising photocatalysts for H2-evolution from water splitting because of its suitable band-edge positions and low cost [23]. However, the shortcomings of the easy agglomeration of g-C3N4 nanosheets, poor visible-light adsorption intensity, and limited electron-hole separation efficiency impair its H2-evolution activity.
To overcome the easy agglomeration of g-C3N4 nanosheets, a commonly employed methods is the coupling of g-C3N4 with a support, such as montmorillonite (MMT) [24], bentonite [25], [26], or kaolinite [27]. In particular, the coupling of g-C3N4 with 2D materials to fabricate 2D/2D heterostructures with tight interfaces has been remarkably encouraged, since it can provide a direct path for efficient interfacial charge transfer [28], [29]. Particularly, MMT is a hydrated magnesium-aluminum-silicate clay mineral with a 2D sheet-like morphology. Sun et al. fabricated the g-C3N4/MMT composites for the photo-degradation of rhodamine B and tetracycline [29]. Li et al. fabricated the g-C3N4/MMT composites via the calcination method resulting in an enhanced photocatalytic reduction of methyl blue [26]. In our previous work, we fabricated the MMT/MoS2/NiCo heterostructure for photocatalytic water reduction to produce H2 [30]. However, the visible-light response of MMT is low, which restricts its application in photocatalysis.
To solve the issue due to the fast recombination of electron-hole, one of the typical methods is to decorate g-C3N4 with suitable cocatalysts, which can not only assemble the photoinduced electrons from g-C3N4 but also lower the activation potential of H2-evolution reaction [31]. Transition metal phosphides, such as CoP, FeP, Cu3P, and Ni2P, were employed to enhance the photocatalytic H2 generation of g-C3N4 [32], [33], [34], [35], [36], [37], [38], [39]. Among these cocatalysts, CoP and Ni2P are especially attractive for H2-evolution reaction because of their low cost and high light-adsorption intensity. Many studies reported the use of CoP or Ni2P as a cocatalyst for the photocatalytic H2 generation [32], [33], [36], [37], [38], [39], [40]. Bimetallic phosphides offer enhanced charge transfer between different ions, resulting in a dramatically increased HER activity [41]. NiCoP is such an example as it has been used for the photocatalytic H2 evolution [42], [43], [44].
Dye sensitization could increase the photocatalyst absorption in the visible-light region; however, high stability of dye during H2 generation combined with excellent cocatalysts accelerating electrons transfer from excited dye molecules to active sites are required. A common method to stabilize the dye is to immobilize it on a porous support. Typical cocatalysts that facilitate electron transfer are noble metals (especially Pt) [45]. Cocatalysts consisting of transition metal phosphides are also very promising and popular because of their abundancy and excellent ability to capture photo-induced charges in non-dye sensitized photocatalysts [31], [46]. However, their capability and mechanisms to transfer electrons from excited dye to the active sites have not been investigated in detail.
Taking all this into account, we have used NiCoP as a cocatalyst to decorate g-C3N4 simultaneously hybridized with MMT for H2 production from water splitting in this work. Compared to MMT/g-C3N4 and g-C3N4/NiCoP, the so-fabricated MMT/g-C3N4/NiCoP composites exhibit a higher H2-evolution rate of 12.50 mmol h−1 g−1 at pH of 11 under 1.0 mmol L−1 of EY-sensitization. We also propose a mechanism of the improved H2-evolution performance of the MMT/g-C3N4/NiCoP catalyst.
Section snippets
Preparation of MMT/g-C3N4/NiCoP composite
For the preparation of the MMT/g-C3N4 composite, first, 1 g of MMT was dispersed in 100 mL of distilled water and ultrasonically treated for 30 min. Then, 20 mL 75 mg mL−1 of melamine hydrochloride solution was added. Details concerning the synthesis procedure are provided in the Supplementary Materials. The suspension was stirred at room temperature for 2 h and collected by centrifugation. The obtained powder was placed into a crucible covered and heated at 550 °C for 4 h in a muffle furnace
Catalyst characterization
XRD patterns of the prepared g-C3N4, MMT, MMT/g-C3N4, MMT/g-C3N4/15%NiCoP composites are shown in Fig. 1. The g-C3N4 bulk displays two diffraction peaks at 2θ = 13.34° and 27.21°, corresponding to the (100) and (002) planes, respectively (Fig. 1d) [47]. XRD peaks of MMT appear at the 2θ angles 6.05°, 19.74°, 28.51°, 34.89°, and 61.91°, which correspond to the (001), (100), (004), (110), and (300) planes of MMT (JCPDS No. 43-0688) [24]. Quartz has been found in MMT as testified by the XRD peak
Conclusions
In summary, a low-cost photocatalyst was developed using NiCoP as a cocatalyst to enhance the H2-evolution activities of g-C3N4 hybridized with MMT. Compared to g-C3N4/15%NiCoP and MMT/g-C3N4, MMT/g-C3N4/15%NiCoP exhibited the highest photocatalytic activity with an H2-evolution rate equal to 12.50 mmol g−1 h−1 under EY-sensitization and an apparent quantum yield of 40.3% at 420 nm and pH value equal to 11. The enhanced photocatalytic activity can be attributed to the loosely stacked g-C3N4
Acknowledgments
The authors would like to thank the National Natural Science Foundation of China for their financial support under grants No. 51404143, 51372125, 21571112 as well as Taishan Scholars Program, the Foundation of Key Laboratory of Clay Mineral Applied Research of Gansu Province, Lanzhou Institute of Chemical Physics and Chinese Academy (CMAR-05).
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