Elsevier

Renewable Energy

Volume 156, August 2020, Pages 469-477
Renewable Energy

The enhanced photocatalytic hydrogen production of nickel-cobalt bimetals sulfide synergistic modified CdS nanorods with active facets

https://doi.org/10.1016/j.renene.2020.04.053Get rights and content

Highlights

  • High index facets can promote photo-electron aggregating to increase HER activity.

  • Pt-like Ni–Co–S can promote photo-electron diffusing into water quickly.

  • CdS nanorods and Ni–Co–S nanoparticles can provide sufficient HER active sites.

Abstract

Nickel-cobalt bimetals sulfide synergistic modified CdS nanorods with active facets are fabricated by a simple continuously hydrothermal method, there, CdS nanorods are preliminarily fabricated by the hydrothermal method, and then Ni–Co–S nanoparticles are deposited on the surface of CdS nanorods. Evaluated by the photocatalytic hydrogen production, the photocatalytic performance of CdS/Ni–Co–S (∼6.56 mmol/g∙h) exhibits an obvious enhancement of about ∼240 folds than typical CdS. The main reasons for the HER enhancement are ascribed to that, the active facets can promote the photo-generated electron aggregating at surface to increase the HER activities, Pt-like behavior Ni–Co–S can promote the photo-generated electrons transferring/diffusing into the water, the small size of CdS nanorods and Ni–Co–S nanoparticles can provide sufficient HER active sites and shorten the transferring route, which can be supported by the electrochemical measurements.

Introduction

With the dilemmas of the environment-energy, the green and sustainable energy has been an urgent demand. Nowadays, lots of ways have been reported [1,2], including the solar cells, windows or other new energy [[3], [4], [5]]. There, H2, with no pollution and high energy density, has been a hot topic in current researches [[6], [7], [8]], e.g. electrocatalytic hydrogen production or petroleum cracking, etc. Among these, the photocatalytic hydrogen production, with the clean, low cost and free solar energy, is considered as a promising issue [9,10].

Up to now, lots of achievements have been reported [11,12], including TiO2, SnO2 or ZnS, etc. There, CdS, with remarkable visible light response, sufficient active sites and appropriate reduction potential of H+/H2, are reported as an ideal photocatalyst [13], such as Qin et al. have prepared TiO2/CdS nanorods with enhanced photocatalytic hydrogen evolution performance [14], and so on [15]. However, restricted by the photo-generated charge carrier efficiency, the photocatalytic HER activity of CdS is expected to be advanced further [16,17]. As known, the HER performance mainly depends on the recombination and transport of the photo-generated charge carriers, which can be ascribed to the photo-generated carriers diffusing at the solid/solution interface and transferring in photocatalyst interior [18,19], including the high active facets and active sites.

Hence, the co-catalyst modification, with the low over potential and remarkable solid/solution interface, is reported as an efficient method [20,21], because that, which can promote the photo-generated electron diffusing into water quickly and act as an acceptor to capture the photo-generated electron to accelerate its separation, such as Hassanzadeh-Tabrizi groups have prepared Fe2O3/Pt/Au nanocomposite modified g-C3N4 with enhanced HER performance [22], etc. [23,24]. However, vast of reported achievements are about noble metal co-catalyst, though the excellent electrochemical performance are attractive, but the exorbitant price makes it being prohibitive in practical application, so the exploiting of low cost and stable co-catalyst is a significant issue. Herein, transition metal sulfides [25], with low over potential, superior solid/solution interface and diversified microcosmic morphology, have been the current hot topics, such as Parida groups have fabricated MoS2/NiFe nanocomposites with the remarkable photocatalytic activities of RhB degradation and H2 evolution [26], Xi groups have prepared N–NiMoO4/NiS2 to boost the overall water splitting hydrogen production [27], and so on [28,29]. Particularly bimetals sulfides [30], own the unique bimetals synergistic effect and tunable potential, exhibit more excellent HER performance during the continuous catalysis, e.g. NiMoSx [31], NiFeSx [32], etc. There, nickel-cobalt bimetals sulfide (Ni–Co–S), with the easy preparation and potential tunability induced by Ni–Co synergy, is deemed as an ideal candidate [33]. Especially the similar physical-chemical properties and ionic radius of Ni/Co, which make the displacement doping of Co obtaining less lattice mismatch and can form Ni–Co sulfide with any percentage [34,35], that is beneficial for the charge carriers transport and crystal regulation, and has been reported by lots of electrocatalysis, such as Dong and Chai groups have fabricated NiCoS/nickel foam for enhancing the electrocatalytic overallwater splitting activities [36], and so on [37]. Though nickel-cobalt bimetals sulfide is rarely reported by the photocatalysis, but it can still provide a new sight for the design of photocatalyst.

On the other hand, the high active facet is another significant factor for the HER performance [38,39], there, the different facets obtain different potentials and chemical activities, which can regulate the distribution of the photo-generated electron and improve the HER active sites, both can promote H+ reduction and are beneficial for HER [40], such as Zhao and Zhang groups have prepared TiO2 nanocrystals with coexposed {001} and {101} facets for enhancing the photocatalytic CO2 reduction activity [41], Zhang and Ji groups have enhanced the photocatalytic performance of MoS2/anatase TiO2 by the co-exposed {101} and {001} facets [42], etc [43]. For CdS, compared with the {001} facets, the (101) and other high active facets obtain higher chemical activities and lower relative hydrogen potentials [44,45], which can accelerate the photo-generated electron aggregation and increase the HER active sites, and is regarded as a significant factor for the HER. Thus, the regulation and preferential growth of the (101) and other high active facets would be an important issue for enhancing the HER performance.

Additionally, the microcosmic morphology is also another important factor for the photocatalytic performance [46,47]. There, with the decreasing of size, the rapidly increased specific surface area can provide sufficient HER active sites and increase the solid/solution interface efficiently [[48], [49], [50], [51], [52]], and have reported by series of works, including nanosheets, nanosphere or nanotubes, etc. Especially for nanorods [53], the clubbed crystal can provide a decent route for the photo-generated electron transferring, such as Fan groups have prepared K+ doped ZnO nanorods with remarkable visible light photocatalytic performance [54], and so on [55]. Moreover, Ni–Co–S nanoparticles with 0D structure can increase the specific surface areas further and shorten the photo-generated electron transferring route for enhancing the photocatalytic HER performance further, such properties can also inhibit the photocorrosion for enhancing the photocatalytic stability [[56], [57], [58]]. What’s more, during the process of the hydrothermal method, CdS nanorods and Ni–Co–S nanoparticles would expose a mass of edge S atoms [[59], [60], [61]], which is deemed as the efficiently HER sites and supported by previous literatures [[62], [63], [64]].

In this work, nickel-cobalt bimetals sulfide synergistic modified CdS nanorods with active facets are fabricated via the simple continuous hydrothermal methods. Evaluated by the HER activities, the photocatalytic performance of Ni–Co–S modified CdS nanorods with active facets exhibit an obvious enhancement of about ∼240 folds than typical CdS. Further, the mechanism of Ni–Co–S modified CdS nanorods with active facets is studied.

Section snippets

Experimental

Nickel-cobalt bimetals sulfide synergistic modified CdS nanorods with active facets are fabricated via the simple continuous hydrothermal method. There, CdS nanorods are preliminarily fabricated by the hydrothermal method, and then the Ni–Co–S nanoparticles are deposited on the surface of CdS nanorods by the secondary hydrothermal method. CdS nanorods modified with different ratios of Ni–Co–S nanoparticles are labelled as CdS/Ni–Co–S-1 (5%), CdS/Ni–Co–S-2 (10%), CdS/Ni–Co–S-3 (15%) and

Results and discussions

Fig. 1 reveals the XRD pattern of Ni–Co–S modified CdS nanorods, including typical CdS and hollow cubic CdS. It’s obvious that, hollow cubic CdS and CdS nanorods obtain better crystallinity than typical CdS. As demonstrated, the peaks at 24.85°, 26.59°, 28.23°, 36.72°, 43.76°, 47.92°, 51.01°, 51.88°, 52.85°, 54.68°, 58.35°, 60.96°, 66.85°, 69.36°, 70.99°, 72.54°, 75.63° and 77.66° are ascribed to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202), (104), (203),

Conclusions

We have prepared nickel-cobalt bimetals sulfide synergistic modified CdS nanorods with active facets by the simple continuously hydrothermal methods. Evaluated by the HER activities, the photocatalytic performance of CdS/Ni–Co–S (∼6.56 mmol/g∙h) exhibits an obvious enhancement of about ∼240 folds than typical CdS. There, the high active facets of CdS and bimetals Ni–Co–S co-catalyst modification are considered as the significant reasons, in which, the high active facets can promote the

CRediT authorship contribution statement

Jiaqi Pan: Investigation, Formal analysis, Conceptualization, Data curation, Writing - original draft. Hongli Li: Investigation, Formal analysis, Conceptualization, Writing - original draft. Shi Li: Investigation. Wei Ou: Investigation. Yanyan Liu: Formal analysis. Jingjing Wang: Formal analysis. Changsheng Song: Data curation. Yingying Zheng: Conceptualization. Chaorong Li: Conceptualization, Data curation, Writing - original draft.

Declaration of competing interest

There are no conflicts of interest to declare.

Acknowledgments

This work was supported by the Natural Science Foundation of China (Nos. 51672249, 51802282 and 11804301), the Fundamental Research Funds of Zhejiang Sci-Tech University (No. 2019Q062).

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