The edge sites of molybdenum disulfide (MoS 2 ) have been shown to be efficient electrocatalysts for the hydrogen evolution reaction (HER). To utilize these structures, two main strategies have been proposed. The first strategy is to use amorphous structures, which should be beneficial in maximizing the area of the edge-site moieties of MoS 2 . However, these structures experience structural instability during HER. The other strategy is nanostructuring, in which, to enhance the resulting HER performance, the exposed surfaces of MoS 2 cannot be inert basal planes. Therefore, MoS 2 may need critical nanocrystallinity to produce the desired facets. Here, we first describe that when atomic layer deposition (ALD) is applied to layered materials such as MoS 2 , MoS 2 exhibits the nonideal mode of ALD growth on planar surfaces. As a model system, the ALD of MoCl 5 and H 2 S was studied. This nonideality does not allow for the conventional linear relationship between the growth thickness and the number of cycles. Instead, it provides the ability to control the relative ratios of the edge sites and basal planes of MoS 2 to the exposed surfaces. The number of edge sites produced was carefully characterized in terms of the geometric surface area and effective work function and was correlated to the HER performance, including Tafel slopes and exchange current densities. We also discussed how, as a result of the low growth temperature, the incorporation of chlorine impurities affected the electron doping and formation of mixed 2H and 1T phases. Remarkably, the resulting 1T phase was stable even upon thermal annealing at 400 °C. With the simple, planar MoS 2 films, we monitored the resulting catalytic performance, finding current densities of up to 20 mA cm −2 at −0.3 V versus the reversible hydrogen electrode (RHE), a Tafel slope of 50−60 mV/decade, and an onset potential of 143 mV versus RHE.
We describe the direct preparation of crystalline NiS thin films via atomic layer deposition (ALD) techniques at temperatures as low as 250 °C without postthermal treatments. A new ALD chemistry is proposed using bis(1-dimethylamino-2-methyl-2-butoxy) nickel(II) [Ni(dmamb)] and HS as precursors. Homogeneous and conformal depositions of NiS films were achieved on 4 in. wafers (both metal and oxide substrates, including Au and SiO). The resulting crystalline NiS layers exhibited highly efficient and stable performance as electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline solutions, with a low overpotential of 300 mV and a high turnover frequency for HER and an overpotential of 400 mV for OER (at a current density of 10 mA/cm). Using our NiS films as both the cathode and the anode, two-electrode full-cell electrolyzers were constructed, which showed stable operation for 100 h at a current density of 10 mA/cm. The proposed ALD electrocatalysts on planar surfaces exhibited the best performance among NiS materials for overall water splitting recorded to date.
A composite chalcogenide with bulk layered heterojunctions exhibits an excellent catalytic activity for hydrogen production.
Cadmium sulfide (CdS) is a semiconducting absorber for photoelectrochemical (PEC) hydrogen production with suitable electronic band structures. However, it suffers from severe photocorrosion and rapid charge recombination during the desired PEC reactions. Herein, we describe the identification of the optimal junction thickness of CdS/MoS 2 core/sheath heterojunction nanostructures by employing atomic layer deposition (ALD) techniques. ALD-grown MoS 2 sheath layers with different thicknesses were realized on single-crystalline CdS nanorod (NR) arrays on transparent conducting oxide substrates. We further monitored the resulting solar H 2 evolution performance with our heterojunction photoanodes. The results showed that the junction thickness of MoS 2 plays a key role in the reduction of photocorrosion and the enhanced photocurrent density by optimizing the charge separation. A better saturation photocurrent (∼46%) was obtained with the 7 nm-thick MoS 2 @CdS NRs than that with the bare CdS NRs. Moreover, the external quantum efficiency was increased twofold over that of the pristine CdS NRs. The ALD-grown MoS 2 @CdS heterojunction structures provides an efficient and versatile platform for hydrogen production when combining ALD-grown MoS 2 with ideal semiconducting absorbers.
We describe the direct formation of mixed-phase (1T and 2H) MoS2 layers on Si as a photocathode via atomic layer deposition (ALD) for application in the photoelectrochemical (PEC) reduction of water to hydrogen. Without typical series-metal interfaces between Si and MoS2, our p-Si/SiOx/MoS2 photocathode showed efficient and stable operation in hydrogen evolution reactions (HERs). The resulting performance could be explained by spatially genuine device architectures in three dimensions (i.e., laterally homo and vertically heterojunction structures). The ALD-grown MoS2 overlayer with the mixed-phase 1T and 2H homojunction passivates light absorber and surface states and functions as a monolithic structure for effective charge transport within MoS2. It is also beneficial in the operation of p-i-n heterojunctions with inhomogeneous barrier heights due to the presence of mixed-phase cocatalysts. The effective barrier heights reached up to 0.8 eV with optimized MoS2 thicknesses, leading to a 670 mV photovoltage enhancement without employing buried Si p-n junctions. The fast-transient behaviors via light illumination show that the mixed-phase layered chalcogenides can serve as efficient cocatalysts by depinning the Fermi levels at the interfaces. A long-term operation of ~70 h was also demonstrated in a 0.5 M H2SO4 solution.
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