Enhancing Hydrogen Evolution Reaction via Synergistic Interaction between the [Mo3S13]2– Cluster Co-Catalyst and WSe2 Photocathode

A thiomolybdate [Mo3S13]2– nanocluster is a promising catalyst for hydrogen evolution reaction (HER) due to the high number of active edge sites. In this work, thiomolybdate cluster films are prepared by spin-coating of a (NH4)2Mo3S13 solution both on FTO glass substrates as hydrogen evolving electrodes and on highly 00.1-textured WSe2 for photoelectrochemical water splitting. As an electrocatalyst, [Mo3S13]2– clusters demonstrate a low overpotential of 220 mV at 10 mA cm–2 in 0.5 M H2SO4 electrolyte (pH 0.3) and remain structurally stable during the electrochemical cycling as revealed by in situ Raman spectroscopy. Moreover, as a co-catalyst on WSe2, [Mo3S13]2– clusters enhance the photocurrent substantially by more than two orders of magnitude (from 0.02 to 2.8 mA cm–2 at 0 V vs RHE). The synergistic interactions between the photoelectrode and catalyst, i.e., surface passivation and band bending modification by the [Mo3S13]2– cluster film, promoted HER catalytic activity of [Mo3S13]2– clusters influenced by the WSe2 support, are revealed by intensity-modulated photocurrent spectroscopy and density functional theory calculations, respectively. The band alignment of the WSe2/[Mo3S13]2– heterojunction, which facilitates the electron injection, is determined by correlating UV–vis with photoelectron yield spectroscopy results.


XRD experimental:
A Bruker AXS D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) was used to obtain X-ray diffractograms (XRD). The thin film analysis was measured under grazing incidence configuration with an incidence angle of 0.5°. The detection angle (2θ) was varied from 5° to 90°, while powder samples were measured under θ-2θ coupling from 5° to 60°. casting and spin-coating. In principle, hydrogen evolution is a two-step process.
The first setup featuring a proton reduction (also known as Volmer step): associated with a Tafel slope b = 2.3RT/αF ≈ 120 mV.
The second is the hydrogen desorption step with two alternatives proposed: either by the Heyrovsky step: or the Tafel step formulated as: where the asterisks * in these equations stand for active sites on the catalyst surface, R is the ideal gas constant (8.314 J K -1 mol -1 ), T is the temperature in Kelvin, and α is the barrier symmetry factor (~0.5 for metals). [1] The sample prepared by drop-casting using methanol (MeOH) as solvent has a slightly higher Therefore after 25 min, the electrochemical test was stopped manually to prevent the electrode from damage. The green curve of the Raman spectra was taken afterwards. Finally in the last step, we kept the electrode at a moderate potential (-0.2 V vs. RHE) for 30 min so that the gas bubbles won't block the whole chamber and then measured the Raman spectra (purple). As shown by all the spectra in Figure S4b

XPS experimental:
X-ray photoelectron spectroscopy (XPS) was carried out with a monochromatic Al Kα X-ray source (1486.74 eV, Specs Focus 500 monochromator). A hemispherical analyser (Specs Phoibos 100) in an ultrahigh vacuum system (base pressure of about 10 -8 mbar) was used to study the composition and the valence states of different elements.
S-8   In IMPS measurements, the illumination intensity is modulated by an amplitude of 10%. The photocurrent at a fixed potential was recorded as a function of frequency which varies from 100 kHz to 1 Hz. The overall modulated photocurrent consists of both the minority charge and the majority charge current, where the latter normally precedes the minority current. Therefore, under fluctuated illumination, the resulted photocurrent could be expressed as a real and an imaginary part. In a typical IMPS spectrum, the imaginary photocurrent is plotted as a function of the real part of the photocurrent. The IMPS spectra of WSe2 and WSe2/[Mo3S13] 2electrodes, measured at different applied potentials, are shown in Figure S12A and S12B. In these two figures, the intercepts of high frequency semi-circles are normalized to 1 for easier comparison between spectra measured at different applied potentials by dividing both the real and imaginary photocurrent with the electron current. The electron current can be demonstrated by the absolute intercepts of the high frequency semicircles with the x-axis, which corresponds to the flux of electrons that arrive at the semiconductor interface before they recombine or be transferred to the electrolyte (see Figure S13). [3] The low frequency upper semi-circle (note that the sign of the y-axis is inverted) is dominant by recombination processes. The intercept of this semi-circle stands for ktr / (ktr + krec) which equals to ηCT according to Eq. 1. Besides, the imaginary photocurrent reaches its maximum when the frequency matches the characteristic relaxation constant of the system as depicted by Eq. S4 (see also Figure S13). ωmax = ktr + krec (S4)

S-12
In Figure S12A, it is shown that the recombination semi-circles varying the potentials applied for the bare WSe2 electrode almost have the same shape and size and the intercepts of the recombination semi-circles are close to zero. This behavior suggests that the performance of the bare WSe2 photocurrent is largely suppressed by surface recombination. The high recombination rate could be caused by palladium in the layer which was used as promoter for WSe2 crystallization. Pd forms a PdSex phase due to the reaction with H2Se, which is a component of the reacting gas in the sputtering system. The PdSex phase mainly accumulated on the WSe2 surface which could act as recombination centers. [4] From Figure S12B, it can clearly be seen that the the upper semicircles indicate a lower recombination proportion compared to bare WSe2 in Figure S12A,    The work function of the [Mo 3 S 13 ] 2cluster film was determined by Eq. S5 expressed as below: The CPD value could be determined by the mean value in Figure S15, which is around -213    Further details of simulation on the single Mo3S13 cluster are described in our previous publication [5] : "t" stands for terminal sulfur, and "b" for bridging sulfur of S2 2units from the Mo3S13 cluster (shown in Figure S17(B, C)). Table S2. The adsorption energy of the possible adsorption sites in Figure S18 Table S3. The Bader charge of the Mo 3 S 13 cluster before and after the adsorption on WSe 2 in Figure 6(A).

Before adsorption
After adsorption