Plasmonic Ag-Decorated Few-Layer MoS2 Nanosheets Vertically Grown on Graphene for Efficient Photoelectrochemical Water Splitting

Abstract

A controllable approach that combines surface plasmon resonance and two-dimensional (2D) graphene/MoS₂ heterojunction has not been implemented despite its potential for efficient photoelectrochemical (PEC) water splitting. In this study, plasmonic Ag-decorated 2D MoS₂ nanosheets were vertically grown on graphene substrates in a practical large-scale manner through metalorganic chemical vapor deposition of MoS₂ and thermal evaporation of Ag. The plasmonic Ag-decorated MoS₂ nanosheets on graphene yielded up to 10 times higher photo-to-dark current ratio than MoS₂ nanosheets on indium tin oxide. The significantly enhanced PEC activity could be attributed to the synergetic effects of SPR and favorable graphene/2D MoS₂ heterojunction. Plasmonic Ag nanoparticles not only increased visible-light and near-infrared absorption of 2D MoS₂, but also induced highly amplified local electric field intensity in 2D MoS₂. In addition, the vertically aligned 2D MoS₂ on graphene acted as a desirable heterostructure for efficient separation and transportation of photo-generated carriers. This study provides a promising path for exploiting the full potential of 2D MoS₂ for practical large-scale and efficient PEC water-splitting applications.

Keywords: Graphene; Molybdenum disulfide; Photoelectrocatalysis; Surface plasmon resonance.

Fig. 1

Schematic of preparation of plasmonic Ag-decorated vertically aligned few-layer MoS₂ nanosheets on graphene

Fig. 2

a Tilted-view SEM, b planar-view SEM, and c planar-view TEM images of vertically aligned MoS₂ nanosheets on graphene (G/MoS₂). The inset in a is the Raman spectrum of G/MoS₂. TEM images of Ag-decorated vertically aligned few-layer MoS₂ nanosheets on graphene: d G/MoS₂/Ag-2, e G/MoS₂/Ag-4, and f G/MoS₂/Ag-8. g High-resolution lattice TEM image of an Ag NP in G/MoS₂/Ag-4

Fig. 3

XPS spectra of a Ag 3d, b Mo 3d, and c S 2p core levels in G/MoS₂ and G/MoS₂/Ag-4

Fig. 4

a TRPL results of ITO/MoS₂ and G/MoS₂. The inset shows the corresponding PL spectra. b Nyquist plots of ITO/MoS₂ and G/MoS₂ in the dark and under illumination. The inset shows the equivalent Randles circuit. c Photo- and dark current densities versus the potential curves of PEC cells with working electrodes of ITO/MoS₂ and G/MoS₂

Fig. 5

a Raman spectra, b UV–Vis absorption, c TRPL results, and d Nyquist plots of G/MoS₂, G/MoS₂/Ag-2, G/MoS₂/Ag-4, and G/MoS₂/Ag-8. The inset in c shows the carrier lifetimes based on the corresponding TRPL measurements. The inset in d shows the R_ct (dark)/R_ct (photo) values of G/MoS₂, G/MoS₂/Ag-2, G/MoS₂/Ag-4, and G/MoS₂/Ag-8

Fig. 6

Simulated UV–Vis absorption spectra and electric field distribution contour plots at λ_SPR for a AgNP-3, b AgNP-15, and c AgNP-60

Fig. 7

a UPS secondary electron cutoff and b valence spectra of G/MoS₂. c PEC water-splitting working principle of plasmonic Ag-decorated vertically aligned few-layer MoS₂ nanosheets on graphene. The photographs in c show gas bubbling on the dark cathodes (Pt) for ITO/MoS₂ and G/MoS₂/Ag-4 during PEC measurement

Fig. 8

a Photocurrent density–potential curves of PEC cells with various working electrodes (G/MoS₂, G/MoS₂/Ag-2, G/MoS₂/Ag-4, and G/MoS₂/Ag-8) in 0.3 M KH₂PO₄ + 0.3 M KOH solution. b Photoconversion efficiencies and c photocurrent–time plots for G/MoS₂, G/MoS₂/Ag-2, G/MoS₂/Ag-4, and G/MoS₂/Ag-8 in 0.3 M KH₂PO₄ + 0.3 M KOH solution and G/MoS₂/Ag-4 in 0.5 M Na₂SO₃ + 0.5 M Na₂SO₄ solution

Fig. 9

a IPCE plots and b hydrogen evolution amounts for 15 min of PEC cells with various working electrodes (ITO/MoS₂, G/MoS₂, and G/MoS₂/Ag-4) in 0.5 M Na₂SO₃ + 0.5 M Na₂SO₄ solution