Lone but Not Alone: Precise Positioning of Lone Pairs for the Design of Photocatalytic Architectures

Abstract

With current economic growth and consumption trends projected to bring about a precipitous and rapid rise of the global temperature, the world stands at a crossroads with regards to climate change. The rate at which greenhouse gas emissions from fossil fuels, industry, and land-use is curtailed over the next decade will determine the trajectory of global warming for the rest of the century. It is increasingly apparent that far-reaching decarbonization of the transportation infrastructure will need to be supplemented by extensive carbon capture, storage, and utilization. Taking a leaf from Nature's playbook, photocatalytic architectures that can utilize water or CO 2 in conjunction with energy harvested from sunlight and store it in the form of energy-dense chemical bonds represent an attractive proposition. Harnessing solar irradiance, through solar energy conversion involving photovoltaics, as well as the photocatalytic generation of solar fuels, and the photocatalytic reduction of CO 2 have emerged as urgent imperatives for the energy transition. Functional photocatalysts must be capable of efficiently absorbing sunlight, effectively separating electronhole pairs, and ensuring they are delivered at appropriate potentials to catalytic sites to mediate redox reactions. Such photocatalytic architectures must further direct redox events down specific pathways to yield desired products, and ensure the transport of reactants between catalytic sites; all with high efficiency and minimal degradation. In this Perspective, we describe a palette of heterostructures designed to promote robust and efficient direct solar-driven water splitting and CO 2 reduction. The heterostructures comprise M x V 2 O 5 or M x M y ′V 2 O 5 , where M is a p-block cation, M′ is an s-, p-, or d-block cation, and V 2 O 5 represents one of multiple polymorphs of this composition interfaced with semiconductor quantum dots (QDs, binary or ternary II−VI or III−V QDs). The stereochemically active 5/6s 2 electron lone pairs of p-block cations in M x V 2 O 5 give rise to filled midgap electronic states that reside above the O 2p-derived valence band. Within heterostructures, the photoexcitation of QDs results in the transfer of holes to the midgap states of M x V 2 O 5 or M x M y ′V 2 O 5 on subpicosecond time scales. Ultrafast charge separation minimizes the photoanodic corrosion of QDs, which has historically been a major impediment to their use in photocatalysis, and enables charge transport and the subsequent redox reactions underpinning photocatalysis to compete with electron−hole recombination. The energy positioning and dispersion of lone pair states is tunable through multiple chemical and compositional levers accessible across the palette of M x V 2 O 5 or M x M y ′V 2 O 5 compounds: choice of lone-pair cation M and its stoichiometry x, atomic connectivity of V 2 O 5 polymorphs, cointercalation of M′ cations in "quaternary" vanadium oxide bronzes, anionic substitution, and alternative lone pair vanadate frameworks with altogether different c...

Photocatalytic and Photocurrent Responses to Visible Light of the Lone-Pair-Based Oxysulfide Sr6Cd2Sb6S10O7

“…A theoretical work has recently demonstrated the potential of heteroleptic and polar coordination environments in oxysulfides (in which the photoactive cation M is coordinated by both oxide and sulfide anions) to enhance the photocatalytic performance, but this has not yet been explored experimentally . A second feature that may be found in photocatalytic and photovoltaic materials is the presence of a (heavy) “lone pair” cation such as Bi 3+ , Sb 3+ , Sn 2+ , and so forth − The presence of a 5s 2 or 6s 2 lone pair of electrons, and its energy relative to anion p orbitals, can determine the energy (and degree of delocalization) of states at the top of the valence band which are key to explaining the optical response of these materials. , …”

Research progress of vanadium pentoxide photocatalytic materials

“…In addition, there were alternative semiconductor materials to recombine with V 2 O 5 , such as CdS quantum dots, 30,31 ZnO, 32 and others (Fig. 12).…”

Linker-Assisted Assembly of Ligand-Bridged CdS/MoS2 Heterostructures: Tunable Light-Harvesting Properties and Ligand-Dependent Control of Charge-Transfer Dynamics and Photocatalytic Hydrogen Evolution

“…15 SILAR yields high loadings of QDs on MoS 2 with direct, intimate contact for charge transfer. 3,14,35 However, the ability to control the size distribution of QDs and the properties of the QD/MoS 2 interface is limited, which in turn limits control over optical properties and interfacial electronic structure, real-space charge separation, and electronic coupling. 3,8,35 Linker-assisted assembly (LAA), in which multifunctional ligands tether material components to each other, 3,8,36 is a particularly attractive approach to synthesize QD-containing heterostructures.…”

“…3,14,35 However, the ability to control the size distribution of QDs and the properties of the QD/MoS 2 interface is limited, which in turn limits control over optical properties and interfacial electronic structure, real-space charge separation, and electronic coupling. 3,8,35 Linker-assisted assembly (LAA), in which multifunctional ligands tether material components to each other, 3,8,36 is a particularly attractive approach to synthesize QD-containing heterostructures. When presynthesized colloidal QDs are incorporated into heterostructures, their sizes are generally unperturbed, enabling enhanced control over electronic and optical properties.…”

“…When presynthesized colloidal QDs are incorporated into heterostructures, their sizes are generally unperturbed, enabling enhanced control over electronic and optical properties. 3,36,37 Moreover, interfacial distance can be tuned with the properties of linking ligands, and throughbridge electronic coupling can accelerate electron transfer. 3,36,38,39 LAA has been used to tether colloidal QDs to a range of semiconducting and metallic substrates, including TiO 2 , gold, aluminum, silicon, and, more recently, V 2 O 5 and various cation-intercalated V 2 O 5 polymorphs.…”