skip to main content

Title: Designing catalysts for water splitting based on electronic structure considerations

The disproportionation of H2O into solar fuels H2and 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 MxV2O5(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 MxV2O5nanowires. The electronic structure of MxV2O5is 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., MxV2O5/CdX (X = S, Se, Te). We illustrate with examples how the MxV2O5/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.

more » « less
Award ID(s):
1919704 1627583
Author(s) / Creator(s):
; ; ; ; ; ; ; ;
Publisher / Repository:
IOP Publishing
Date Published:
Journal Name:
Electronic Structure
Page Range / eLocation ID:
Article No. 023001
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    2D early transition metal carbide and nitride MXenes have intriguing properties for electrochemical energy storage and electrocatalysis. These properties can be manipulated by modifying the basal plane chemistry. Here, mixed transition metal nitride MXenes, M‐Ti4N3Tx(M = V, Cr, Mo, or Mn; Tx= O and/or OH), are developed by modifying pristine exfoliated Ti4N3TxMXene with V, Cr, Mo, and Mn salts using a simple solution‐based method. The resulting mixed transition metal nitride MXenes contain 6–51% metal loading (cf. Ti) that exhibit rich electrochemistry including highly tunable hydrogen evolution reaction (HER) electrocatalytic activity in a 0.5mH2SO4electrolyte as follows: V‐Ti4N3Tx> Cr‐Ti4N3Tx> Mo‐Ti4N3Tx> Mn‐Ti4N3Tx> pristine Ti4N3Txwith overpotentials as low as 330 mV at −10 mA cm−2with a charge‐transfer resistance of 70 Ω. Scanning electrochemical microscopy (SECM) reveals the electrochemical activity of individual MXene flakes. The SECM data corroborate the bulk HER activity trend for M‐Ti4N3Txas well as provide the first experimental evidence that HER results from catalysis on the MXene basal plane. These electrocatalytic results demonstrate a new pathway to tune the electrochemical properties of MXenes for water splitting and related electrochemical applications.

    more » « less
  2. Efficient charge separation and transportation are key factors that determine the photoelectrochemical (PEC) water‐splitting efficiency. Here, a simultaneous enhancement of charge separation and hole transportation on the basis of ferroelectric polarization in TiO2–SrTiO3core–shell nanowires (NWs) is reported. The SrTiO3shell with controllable thicknesses generates a considerable spontaneous polarization, which effectively tunes the electrical band bending of TiO2. Combined with its intrinsically high charge mobility, the ferroelectric SrTiO3thin shell significantly improves the charge‐separation efficiency (ηseparation) with minimized influence on the hole‐migration property of TiO2photoelectrodes, leading to a drastically increased photocurrent density (Jph). Specifically, the 10 nm‐thick SrTiO3shell yields the highestJphand ηseparationof 1.43 mA cm−2and 87.7% at 1.23 V versus reversible hydrogen electrode, respectively, corresponding to 83% and 79% improvements compared with those of pristine TiO2NWs. The PEC performance can be further manipulated by thermal treatment, and the control of SrTiO3film thicknesses and electric poling directions. This work suggests a material with combined ferroelectric and semiconducting features could be a promising solution for advancing PEC systems by concurrently promoting the charge‐separation and hole‐transportation properties.

    more » « less
  3. Abstract

    The large‐scale hydrogen production and application through electrocatalytic water splitting depends crucially on the development of highly efficient, cost‐effective electrocatalysts for oxygen evolution reaction (OER), which, however, remains challenging. Here, a new electrocatalyst of trimetallic Fe–Co–Ni hydroxide (denoted as FeCoNiOxHy) with a nanotubular structure is developed through an enhanced Kirkendall process under applied potential. The FeCoNiOxHyfeatures synergistic electronic interaction between Fe, Co, and Ni, which not only notably increases the intrinsic OER activity of FeCoNiOxHyby facilitating the formation of *OOH intermediate, but also substantially improves the intrinsic conductivity of FeCoNiOxHyto facilitate charge transfer and activate catalytic sites through electrocatalyst by promoting the formation of abundant Co3+. Therefore, FeCoNiOxHydelivers remarkably accelerated OER kinetics and superior apparent activity, indicated by an ultra‐low overpotential potential of 257 mV at a high current density of 200 mA cm−2. This work is of fundamental and practical significance for synergistic catalysis related to advanced energy conversion materials and technologies.

    more » « less
    more » « less
  5. Abstract

    Data‐intensive discovery of water‐splitting catalysts can accelerate the development of sustainable energy technologies, such as the photocatalytic and/or electrocatalytic production of renewable hydrogen fuel. Through computational screening, 13 materials were recently predicted as potential water‐splitting photocatalysts: Cu3NbS4, CuYS2, SrCu2O2, CuGaO2, Na3BiO4,Sr2PbO4, LaCuOS, LaCuOSe, Na2TeO4, La4O4Se3, Cu2WS4, BaCu2O2, and CuAlO2. Herein, these materials are synthesized, their bandgaps and band alignments are experimentally determined, and their photoelectrocatalytic hydrogen evolution properties are assessed. Using cyclic voltammetry and chopped illumination experiments, 9 of the 13 materials are experimentally found to have bandgaps and band alignments that straddle the potentials required for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), as computationally predicted. During photocatalytic testing, 12 of the materials yield a measurable photocurrent. However, only three are found to be active for the HER, with Cu3NbS4, CuYS2, and Cu2WS4producing H2in amounts comparable to bare TiO2; a benchmark photocatalyst. This study provides experimental validation of computational bandgap and band alignment predictions while also successfully identifying active photocatalysts.

    more » « less