Abstract
We introduced atomic sulfur passivation to tune the surface sites of heavy metal-free ZnSe nanorods, with a Zn2+-rich termination surface, which are initially capped with organic ligands and under-coordinated with Se. The S2− ions from a sodium sulfide solution were used to partially substitute a 3-mercaptopropionic acid ligand, and to combine with under-coordinated Zn termination atoms to form a ZnS monolayer on the ZnSe surface. This treatment removed the surface traps from the ZnSe nanorods, and passivated defects formed during the previous ligand exchange process, without sacrificing the efficient hole transfer. As a result, without using any co-catalysts, the atomic sulfur passivation increased the photocurrent density of TiO2/ZnSe photoanodes from 273 to 325 μA/cm2. Notably, without using any sacrificial agents, the photocurrent density for sulfur-passivated TiO2/ZnSe nanorod-based photoanodes remained at almost 100% of its initial value after 300 s of continuous operation, while for the post-deposited ZnS passivation layer, or those based on ZnSe/ZnS core–shell nanorods, it declined by 28% and 25%, respectively. This work highlights the advantages of the proper passivation of II-VI semiconductor nanocrystals as an efficient approach to tackle the efficient charge transfer and stability of photoelectrochemical cells based thereon.
Highlights
It is ever important to establish efficient photocatalytic water splitting systems, in order to obtain clean and storable hydrogen fuels for solving energy and environment issues [1,2]
The large surface area of the colloidal nanoparticles provides abundant surface reaction sites and promotes their contact with electron donors and/or acceptors in PEC cells. This leads to a high ratio of under-coordinated surface termination atoms, which are usually passivated with organic capping ligands, mostly long-chain alkyl amines, or alkyl acids in the case of NCs synthesized in organic solvents [18,19,20,21,22]
For the colloidal semiconductor NCs to be applied in PEC, in order to promote charge transfer and facilitate their efficient loading on metal oxide substrates, their original long-chain alkyl-based ligands are commonly exchanged by short chain bifunctional ligands, such as thioglycolic acid (TGA) or 3-mercaptopropionic acid (3-MPA) [27]
Summary
It is ever important to establish efficient photocatalytic water splitting systems, in order to obtain clean and storable hydrogen fuels for solving energy and environment issues [1,2]. For the colloidal semiconductor NCs to be applied in PEC, in order to promote charge transfer and facilitate their efficient loading on metal oxide substrates, their original long-chain alkyl-based ligands are commonly exchanged by short chain bifunctional ligands, such as thioglycolic acid (TGA) or 3-mercaptopropionic acid (3-MPA) [27] This ligand exchange process can introduce additional surface traps, causing inefficient charge carrier transfer, which in turn results in the degradation of the anode materials and the overall PEC performance [28,29,30]. An improper shell thickness would hinder the charge carrier transport in core–shell NCs, while the lattice mismatch between the core and the shell materials may introduce defects at the core–shell interface, which would compromise the optoelectronic properties [36] Another frequently applied method is the deposition of the metal chalcogenide (ZnSe or ZnSe) on an NC-based photoelectrode via a successive ionic layer adsorption and reaction (SILAR) or chemical bath deposition (CBD) [37,38]. Photoelectrochemical characterization has shown that an improved charge transfer and a suppressed charge recombination was achieved with the aid of S2− passivation
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