Designing catalysts for water splitting based on electronic structure considerations

Abstract

The disproportionation of H2O into solar fuels H2 and O2, or water splitting, is a promising strategy for clean energy harvesting and storage but requires the concerted action of absorption of photons, separation of excitons, charge diffusion to catalytic sites and catalysis of redox processes. It is increasingly evident that the rational design of photocatalysts for efficient water splitting must employ hybrid systems, where the different components perform light harvesting, charge separation and catalysis in tandem. In this topical review, we report on the recent development of a new class of hybrid photocatalysts that employs M x V2O5 (M = p-block cation) nanowires in order to engineer efficient charge transfer from the photoactive chalcogenide quantum dots (QDs) to the water-splitting and hydrogen evolving catalysts. Herein, we summarize the oxygen-mediated lone pair mechanism used to modulate the energy level and orbital character of mid-gap states in the M x V2O5 nanowires. The electronic structure of M x V2O5 is discussed in terms of density functional theory and hard x-ray photoelectron spectroscopy (HAXPES) measurements. The principles of HAXPES are explained within the context of its unique sensitivity to metal 5(6)s orbitals and ability to non-destructively study buried interface alignments of quantum dot decorated nanowires i.e., M x V2O5/CdX (X = S, Se, Te). We illustrate with examples how the M x V2O5/CdX band alignments can be rationally engineered for ultra-fast charge-transfer of photogenerated holes from the quantum dot to the nanowires; thereby suppressing anodic photo-corrosion in the CdX QDs and enabling efficacious hydrogen evolution.

Electronic properties and high-pressure behavior of wolframite-type CoWO4

“…, being CoWO 4 the wolframites with the lowest band gap so far. 11,18 Interestingly, our result is in line with the predictions of Lacomba-Perales et al, which highlight a correlation between the band gap and the cationic radii in AWO 4 wolframites.11 The value we determined for the band gap of CoWO 4 (2.25(1) eV), makes it an excellent candidate as semiconductor electrode for efficient visible light photolysis of water 63. Tauc plot of the absorbance spectrum of wolframite CoWO 4 at ambient pressure.…”

Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020

“…A recent review paper by Razek et al also emphasised the benefits of laboratory HAXPES to accelerate material screening, in particular for energy applications. [106] Finally, laboratory HAXPES systems have been employed to study buried layers and interfaces in device-relevant multilayer samples. Results on Si-based and post-transition metal oxide device multilayers and transistor structures have demonstrated the ability to probe the core level spectra of buried layers and extract information on their chemical state in-situ without the need of sputter depth profiling as is necessary in SXPS studies.…”

“…[89] Several of the studies mentioned here also exemplify the complementarity of laboratory-and synchrotron-based HAXPES systems to enable a comprehensive and efficient exploration of samples of interest. [85,87,89,106] In contrast to synchrotron-or FEL-based HAXPES, the excitation energy in laboratory-based HAXPES systems is fixed to the characteristic X-ray emission of the anode material. This has important implications for the information depth (see general discussion on this topic in Section 3.1.1.…”

Fabrication of Highly Textured 2D SnSe Layers with Tunable Electronic Properties for Hydrogen Evolution

“…Water splitting is considered to be the most promising pathway for hydrogen production, as it is recyclable, clean, and abundant. According to the thermodynamics of water splitting, the energy required to break one mole of water is 237 kJ [ 3 , 4 ], and the potential needed is at least 1.23 V [ 5 , 6 ]. To dissociate water molecules into hydrogen and oxygen by means of photoelectrochemical catalysis, various semiconductor materials have been used as catalysts, such as TiO 2 [ 7 , 8 ], SrTiO 3 [ 9 , 10 ], BiVO 4 [ 10 , 11 , 12 ], and chalcogenide compounds [ 13 , 14 ].…”