ArticleConstruction of LSPR-enhanced 0D/2D CdS/MoO3−x S-scheme heterojunctions for visible-light-driven photocatalytic H2 evolution
Graphical Abstract
The MoO3-x/CdS heterojunctions exhibit improved photocatalytic H2 generation activity, which is accredited to an LSPR-enhanced S-scheme charge transfer mechanism.
Introduction
The consumption of fossil fuels and the environmental degradation produced by the combustion of fuels have become two significant challenges in the 21st century. Therefore, more and more attention has been paid to the research and development of clean and renewable energy [1, 2, 3, 4, 5, 6, 7]. Semiconductor-based photocatalytic technology is considered a potential approach to converting solar energy into clean and pollution-free energy in the form of H2 to meet the growing global energy demand [8, 9, 10, 11, 12]. For a typical photocatalytic H2 evolution reaction, semiconductor photocatalysts such as TiO2, g-C3N4, and CdS adsorb photons to produce photo-induced electrons and holes to take part in redox reactions [13, 14]. However, due to restricted light response and rapid recombination of photo-induced electron-hole pairs, the aforementioned pristine semiconductors usually demonstrate unsatisfactory water splitting activities [15, 16, 17, 18].
Recently, plasmonic semiconductor-based photocatalysts have attracted significant interest owing to their broad light-harvesting capacities, which range from visible to near-infrared light. Furthermore, they can harvest low-energy photons to generate extra high-energy “hot electrons,” allowing for full-spectrum-driven water reduction reactions [19]. Among the reported plasmonic semiconductors, nonstoichiometric molybdenum oxide with abundant oxygen vacancies (MoO3−x) demonstrates a broad and intense localized surface plasmon resonance (LSPR) absorption centered at 700 nm, with adsorption tailing to 2000 nm. Accordingly, MoO3−x has become one of the most attractive candidates for harvesting visible to near-infrared light for photocatalytic H2 generation [20, 21, 22, 23]. Nevertheless, the rapid recombination of charge carriers in pristine MoO3−x still needs to be improved. The construction of MoO3−x-based composite photocatalysts with type-II, Z-scheme, or S-scheme heterostructures has been evidenced as an effective tactic to simultaneously realize the expansion of light adsorption and the enhancement of spatial charge separation to improve photocatalytic H2 evolution performance [24, 25, 26, 27, 28]. The advantage of Z-scheme and S-scheme heterostructures over traditional type-II heterostructures is that the photoexcited electrons preserved at higher reduction potentials would take part in water-splitting reactions, which is beneficial for achieving higher photocatalytic reaction efficiency [29, 30, 31, 32]. It is worth mentioning that the migration distance between the electrons and holes can be further shortened in the S-scheme due to band bending caused by the internal electric field of the semiconductors, leading to faster separation of the photo-induced carriers in comparison to the Z-scheme [33]. The first S-scheme heterojunction, 2D/2D WO3/g-C3N4, was published by Yu et al. [34], and it has been suggested that the construction of S-scheme heterojunctions relies on the band offset of the two semiconductors. Among the various semiconductors reported, CdS is a potential candidate as a photocatalyst due to its suitable bandgap (Eg = 2.4 eV) and high conduction band position, which make it able to reduce water to H2 [35, 36, 37, 38, 39, 40, 41]. Moreover, CdS possesses an appropriate energy level that matches with MoO3−x, satisfying the requirements for the construction of an S-scheme for photocatalytic H2 evolution.
Interface design is another critical factor in the construction of highly efficient S-scheme hybrid materials. 0D/2D heterojunctions have demonstrated some unique features, with 0D nanoparticles having short charge-migration distances while 2D nanosheets act as supports that improve contact area and prevent the self-aggregation of 0D nanoparticles [42, 43, 44, 45, 46]. Inspired by this, there is motivation to rationally design and investigate 0D/2D CdS/MoO3−x S-scheme heterojunctions to enhance the visible-light photocatalytic activity to levels suitable for possible practical utilization. In the present work, a 0D/2D CdS/MoO3−x S-scheme heterojunction composing of 0D CdS nanoparticles and 2D MoO3−x elliptical nanosheets were prepared by a simple coprecipitation route. The photocatalytic H2 generation performances of the photocatalysts were evaluated under visible light irradiation (including monochromic 420, 450, 550, and 650 nm light). The CdS/MoO3−x composite containing 15 wt% MoO3−x was recognized to be the most active photocatalyst for H2 evolution (7.44 mmol·g−1·h−1) under visible-light irradiation, which is about 10.3 times as much as that of pristine CdS nanoparticles. The improvement in photocatalytic efficiency can be accredited to a LSPR-enhanced S-scheme charge transfer mechanism.
Section snippets
Materials
Molybdenum metal powder (99.5%) was bought from Aladdin Industrial Corporation. Hydrogen peroxide (H2O2, 30 wt%), 1-butanol (≥ 99.5%), Cd(NO3)2·4H2O (≥ 99.0%), Na2S·9H2O (≥ 98.0%), and lactic acid (≥ 85.0%) were bought from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without purification.
Sample preparation
MoO3−x elliptical nanosheets were synthesized via a solvothermal process [23]. Two mmol of molybdenum metal powder was intermixed with 24 mL of 1-butanol in a beaker of capacity 50 mL, and then
Results and discussion
Fig. 1(a) shows the XRD patterns of MoO3−x, CdS, and the CdS/MoO3−x composites. The XRD peaks of pure MoO3−x can be indexed as orthorhombic MoO2.69 (JCPDS# 70-0615), and the peaks at 23.4°, 25.5°, 27.2°, 33.8°, 38.7°, and 39.6° can be assigned to the (110), (040), (021), (111), (060), and (002) planes, respectively [23]. Only three diffraction peaks could be found for pristine CdS at 26.7°, 43.9°, and 51.9°. These are attributed to the (111), (220), and (311) crystal reflection planes of cubic
Conclusions
In summary, an LSPR-enhanced 0D/2D CdS/MoO3−x heterojunction composed of 0D CdS nanoparticles grown on 2D plasmonic MoO3−x elliptical nanosheets has been successfully fabricated. The 0D/2D CdS/MoO3−x heterojunction exhibits improved photocatalytic H2 generation activity under a wide range of visible light irradiation, including using 420, 450, 550, and 650 nm monochromic light, and the optimal CdS/MoO3−x demonstrates 10.3 times higher photocatalytic H2 evolution (7.44 mmol·g−1·h−1) than that of
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This work is supported by the National Natural Science Foundation of China (51672113, 21975110, 21972058) and Taishan Youth Scholar Program of Shandong Province.
Published 5 January 2021
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These two authors contributed equally.