Elucidating the Mechanistic Origins of Photocatalytic Hydrogen Evolution Mediated by MoS₂/CdS Quantum-Dot Heterostructures

Abstract

Solar fuel generation mediated by semiconductor heterostructures represents a promising strategy for sustainable energy conversion and storage. The design of semiconductor heterostructures for photocatalytic energy conversion requires the separation of photogenerated charge carriers in real space and their delivery to active catalytic sites at the appropriate overpotentials to initiate redox reactions. Operation of the desired sequence of light harvesting, charge separation, and charge transport events within heterostructures is governed by the thermodynamic energy offsets of the two components and their photoexcited charge-transfer reactivity, which determine the extent to which desirable processes can outcompete unproductive recombination channels. Here, we map energetic offsets and track the dynamics of electron transfer in MoS 2 /CdS architectures, prepared by interfacing two-dimensional MoS 2 nanosheets with CdS quantum dots (QDs), and correlate the observed charge separation to photocatalytic activity in the hydrogen evolution reaction. The energetic offsets between MoS 2 and CdS have been determined using hard and soft X-ray photoemission spectroscopy (XPS) in conjunction with density functional theory. A staggered type-II interface is observed, which facilitates electron and hole separation across the interface. Transient absorption spectroscopy measurements demonstrate ultrafast electron injection occurring within sub-5 ps from CdS QDs to MoS 2 , allowing for creation of a long-lived charge-separated state. The increase of electron concentration in MoS 2 is evidenced with the aid of spectroelectrochemical measurements and by identifying the distinctive signatures of electronphonon scattering in picosecond-resolution transient absorption spectra. Ultrafast charge separation across the type-II interface of MoS 2 /CdS heterostructures enables a high Faradaic efficiency of ∼99.4 ± 1.2% to be achieved in the hydrogen evolution reaction (HER) and provides a 40-fold increase in the photocatalytic activity of dispersed photocatalysts for H 2 generation. The accurate mapping of thermodynamic driving forces and dynamics of charge transfer in these heterostructures suggests a means of engineering ultrafast electron transfer and effective charge separation to design viable photocatalytic architectures.

Spontaneous Enhanced Visible-Light-Driven Photocatalytic Water Splitting on Novel Type-II GaSe/CN and Ga2SSe/CN vdW Heterostructures

“…According to the different energy band arrangements, 2D single-layer semiconductor heterostructures can be divided into type I, type II, and type III. Compared with type I with a staggered band and type III with a broken band, the band of type-II heterostructure is symmetrically staggered. − Light-excited electron transitions cause electrons and holes to accumulate on different semiconductor monolayers, thereby achieving the effective separation of electrons and holes. − In particular, some recent studies have reported that the experimentally synthesized type-II heterostructure exhibits excellent photocatalytic activity. Tang et al have confirmed that the type-II heterostructure formed by the vertical growth of ZnO 2D nanosheets sensitized by nonmetallic carbon (C) dots helps to improve the charge transfer between C dots and ZnO and enhance the photoanodic performance .…”

Recent Progress of Perovskite Oxide in Emerging Photocatalysis Landscape: Water Splitting, CO₂ Reduction, and N₂ Fixation

“…In this respect, hydrogen is considered as a promising candidate for next-generation energy carriers due to its large energy content of ~122 kJ•g −1 and clean emission [221][222][223] . Compared with the environmentally unfriendly steam reforming of natural gas and the expensive electrolysis of water, photocatalytic water splitting is considered to be a promising approach for the production of H2 due to its effective utilization of solar energy 29, [224][225][226][227][228][229][230][231][232][233][234][235] . Since Fujishima and Honda 94 proposed the first photoelectrochemical water splitting system in 1972, photocatalytic water splitting has drawn immense attention and leading-edge achievements emerged in an endless stream to date, and the typical works are concluded in Table 3 114,143,147, 236-240. The basic steps in the process of photocatalytic water splitting on a semiconductor photocatalyst are shown in Fig.…”

Review on Colloidal Quantum Dots Luminescent Solar Concentrators

“…Much attention has been given to QDs as ideal luminophores for their wide absorption spectra, controllable emission spectra, and high stability [24–30] . In addition to applications in LSC technology, the controllable emissions spectra of QDs have enabled the development of QDs as a photocatalyst in hydrogen production [31–36] . The color conversion properties of QDs lead to their application in high‐efficiency LEDs [37–43] .…”