Regular ArticleInternal electric field induced S–scheme heterojunction MoS2/CoAl LDH for enhanced photocatalytic hydrogen evolution
Graphical abstract
IEF-induced S-scheme charge transfer mechanism combined with the advantages of the unique architecture endows the MoS2/CoAl LDH heterojunction with an improved photocatalytic H2 generation activity under visible-light irradiation.
Introduction
Photocatalysis, especially photocatalytic water splitting for hydrogen (H2) generation when solar irradiations are incident, is seen as one of most auspicious maneuvers to treat the global energy and environmental concerns [1], [2], [3], [4], [5], [6], [7]. Since the first discovery of photo-electrochemical water splitting on a TiO2 electrode [8], multifarious photocatalysts, such as g-C3N4 [9], [10], CdS [11], [12], Bi2WO6 and ect., have been developed for their feasibility and efficiency for H2 production. Among them, two-dimensional (2D) nanosheets of photocatalysts demonstrates unique advantages of large specific surface area and abundant active sites [13], [14], [15]. Especially, layered double hydroxides (LDHs, [M2+1−xM3+x(OH)2] [A n−x/n·mH2O], where M2+ and M3+ represent divalent and trivalent cations, and An− is an anion) [16], [17], [18], as a typical 2D layered clay photocatalysts, have aroused widespread attention for their large specific surface and tunable band gap for the photocatalysis application. For example, Parida et al. have fabricated the ZnCr LDH hybrids for the dual-function of tetracycline degradation and H2 evolution [19]; Kim et al have designed a novel NiFe LDH composite with improved photocatalytic H2 evolution performanc [20]; Ogale et al have synthesized the NiAl-LDH composite with remarkable photocatalytic CO2 reduction performance under visible-light irradiation [21]. It is noted that numerous LDHs having divalent metal cations, such as Mg, Co, Ni, and Zn, and trivalent cations such as Al, In, Ga, and Cr within their interlayers have been researched to refine the photocatalytic performance. CoAl LDH with high conduction band (ECB = − 0.75 eV) [18] and abundant Co active sites is proven effective in various catalysis reactions (photodegradation [19], H2 production [22] and CO2 reduction [23]). However, pristine CoAl LDH generally manifests feeble quantum efficiency in the presence of solar irradiations as a result of sluggish charge carrier mobility and speedy recombination of (e−-h+) combinations [24].
To better overcome the shortcomings of pristine CoAl LDH, the construction of efficient heterojunction by employing CoAl LDH with other appropriate semiconductors is an efficient manner to extend the light absorption, inhibit the charge recombination and boost the photocatalytic performance [25]. Liu et al. has fabricated a 0D/2D Type-II heterostructure consisting of ultrathin CoAl LDH nanosheets and CdS nanoparticles, which can induce more efficient charge transfer and improved photodegradation activity [26]. However, such Type-II heterostructure weakens the redox capabilities of e− and h+, which is inauspicious for efficient H2 evolution and stipulates high redox potential of H2/H+ [27]. In a contrary manner, S–scheme composite consisting of two semiconductors can maintain light-induced charges with firm redox capability distinctly on two semiconductors, fulfilling the criteria of H2 evolution. Especially, S–scheme photocatalytic system driven by an interfacial internal electric field (IEF), whereas the band potential energies of the two-component semiconductors were bended to offer an easier way for electrons transfer at the interface, accordingly, efficiently improved light-induced (e−-h+) detachment and strong redox ability can be accomplished contemporaneously [28], [29].
Choosing a suitable semiconductor to couple with CoAl LDH is an important factor to construct IEF-induced S–scheme heterojunction for enhanced photocatalytic activity. Among those reported semiconductors, 2D MoS2 with relatively confined band gap with layered structure, is considered to be a promising material for coupling with other semiconductors. The band energy structures of the CoAl LDH (ECB = − 0.75 eV, EVB = 1.35 eV) and MoS2 (ECB = − 0.12 eV, EVB = 1.78 eV)[30], [31] are in accordance with the establishment of an S–scheme heterojunction. Besides that, coupling 2D MoS2 with 2D CoAl LDH would form a 2D/2D heterojunction, in which the interfacial contact area can be increased and the migration distance for charge transport from bulk to surface can be reduced. Accordingly, in this paper, 2D/2D MoS2/CoAl LDH heterojunctions are fabricated employing 2D CoAl LDH nanosheets serving as a supporting platform for the in-situ growth of 2D MoS2 to form tight interfacial contact. The electrons migration from CoAl LDH to MoS2 took place, engendering the creation of an internal electric field (IEF). Consequently, IEF-induced S–scheme charge transfer eventuated upon exposure to light irradiation, where the electrons generated in MoS2 would combine with the holes in CoAl LDH and thus photocatalytic H2 evolution performance of the 2D/2D MoS2/CoAl LDH heterojunction could be enhanced.
Section snippets
Materials and reagents
Polyvinylpyrrolidone (PVP, Mw ≈ 55000), Ammonium molybdate ((NH4)6Mo7O24·4H2O, 99%), thiourea (CH4N2S, 99%), cobalt nitrate (Co(NO3)2·6H2O, 99%), aluminium nitrate (Al(NO3)2·9H2O, 99%), urea (CH4N2O, 99%), ammonium fluoride (NH4F, 99%) were purchased from Aladdin Industrial Co., China. All the above-mentioned reagents were used without further purification.
Synthesis of MoS2/CoAl LDH
Usually (NH4)6Mo7O24·4H2O (Aladdin, 99%) (0.2 g), thiourea (Aladdin, 99%) (0.18 g) and polyvinylpyrrolidone (Mw ≈ 55000, Aladdin, 99%)
Results and discussions
The SEM and TEM micrographs of the MoS2 and CoAl LDH are presented in Fig. 1. Pristine MoS2 demonstrates a spherical morphology with an average size of 200 nm (Fig. 1a), and these spheres are assembled with 2D MoS2 nanosheets (NSs). The observed lattice fringes on the edges of MoS2 NSs with calculated fringe spacing of 0.19 nm belonging to (2 1 4) plane of MoS2[30], as depicted by the HRTEM image of MoS2 (Fig. 1b). The CAM-5 composite (Fig. 1c and d) exhibits a carnations morphology comprising of
Conclusion
In summary, 2D/2D MoS2/CoAl LDH heterojunction with IEF-induced S–scheme charge transfer was prepared by a simple hydrothermal method. SEM and TEM results convinced that MoS2 nanospheres were entangled between the layers of the CoAl LDH nanosheets, forming an intimate interface between 2D MoS2 nanosheets and 2D CoAl LDH nanosheets. Accordingly, an internal electric field pointing from CoAl LDH to MoS2 was developed, leading to the IEF-induced S–scheme charge transfer mechanism. Benefiting from
CRediT authorship contribution statement
Junnan Tao: Conceptualization, Writing - original draft, Formal analysis. Xiaohui Yu: Formal analysis, Data curation. Qinqin Liu: Validation, Visualization, Writing - review & editing. Guiwu Liu: Supervision, Visualization. Hua Tang: Writing - review & editing, Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (51672113, 21975110 and 21972058) and Prof. Hua Tang also appreciates the support from Taishan Youth Scholar Program of Shandong Province.
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These two authors contribute equally to this paper.