Photoelectrochemical CO2 Reduction with a Rhenium Organometallic Redox Mediator at Semiconductor/Aqueous Liquid Junction Interfaces

Atomic/molecular layer deposition strategies for enhanced CO2 capture, utilisation and storage materials

Elevated levels of carbon dioxide (CO2) in the atmosphere and the diminishing reserves of fossil fuels have raised profound concerns regarding the resulting consequences of global climate change and the future supply of energy. Hence, the reduction and transformation of CO2 not only mitigates environmental pollution but also generates value-added chemicals, providing a dual remedy to address both energy and environmental challenges. Despite notable advancements, the low conversion efficiency of CO2 remains a major obstacle, largely attributed to its inert chemical nature. It is imperative to engineer catalysts/materials that exhibit high conversion efficiency, selectivity, and stability for CO2 transformation. With unparalleled precision at the atomic level, atomic layer deposition (ALD) and molecular layer deposition (MLD) methods utilize various strategies, including ultrathin modification, overcoating, interlayer coating, area-selective deposition, template-assisted deposition, and sacrificial-layer-assisted deposition, to synthesize numerous novel metal-based materials with diverse structures. These materials, functioning as active materials, passive materials or modifiers, have contributed to the enhancement of catalytic activity, selectivity, and stability, effectively addressing the challenges linked to CO2 transformation. Herein, this review focuses on ALD and MLD's role in fabricating materials for electro-, photo-, photoelectro-, and thermal catalytic CO2 reduction, CO2 capture and separation, and electrochemical CO2 sensing. Significant emphasis is dedicated to the ALD and MLD designed materials, their crucial role in enhancing performance, and exploring the relationship between their structures and catalytic activities for CO2 transformation. Finally, this comprehensive review presents the summary, challenges and prospects for ALD and MLD-designed materials for CO2 transformation.

Voltammetric Measurement of Rates and Energetics for Surface Methoxylation of Si(100) in Methanol with Dissolved Electron Acceptors Using Si Ultramicroelectrodes

The steady-state voltammetric responses of n-type Si(100) semiconductor ultramicroelectrodes (SUMEs) immersed in air- and water-free methanolic electrolytes have been measured. The response characteristics of these SUMEs in the absence of illumination were modeled and understood through a framework that describes the distribution of the applied potential across the semiconductor/electrolyte contact using four discrete regions: the semiconductor space charge, surface, Helmholtz, and diffuse layers. The latter region was described by the full Gouy-Chapman model. This framework afforded insight on how relevant parameters such as the semiconductor band edge potentials, the reorganization energies for charge transfer, the standard potential of redox species in solution, the density and energy of surface state populations, and the presence of an insulating (tunneling) layer individually and collectively dictate the observable current-potential responses. With this information, the methoxylation of Si surfaces was evaluated by analysis of the change in voltammetric responses during the course of prolonged immersion in methanol. The electrochemical data were consistent with a surface methoxylation mechanism that depended on the standard potential of redox species dissolved in solution. Estimates of the enthalpies of adsorption as well as the potential-dependent rate constant for surface methoxylation were obtained. Collectively, these measurements supported the contention that the rates of Si surface reactions can be systematically tuned by exposure to dissolved outer-sphere electron acceptors. Moreover, the data represent the quantitative utility of voltammetry with SUMEs for the measurement of semiconductor/liquid contacts.

Plasmonic Heterostructure Functionalized with a Carbene-Linked Molecular Catalyst for Sustainable and Selective Carbon Dioxide Reduction

Hybridization of homogeneous catalytic sites with a photoelectrode is an attractive approach to highly selective and tunable photocatalysis using heterogeneous platforms. However, weak and unclear surface chemistry often leads to the dissociation and irregular orientation of catalytic centers, restricting long-term usability with high selectivity. Well-defined and robust ligands that can persist under harsh photocatalytic conditions are essential for the success of hybrid-type photocatalysis. Here, we introduce N-heterocyclic carbene as a durable linker for the immobilization of a Rubpy complex-based CO₂ reduction site (cis-dichloro-(4,4'-diphosphonato-Rubpy)(p-cymene) (RuCY)) on a p-type gallium nitride/gold nanoparticle (p-GaN/AuNP) heterostructure. The p-GaN/AuNPs/RuCY photocathode was coupled with a hematite photoanode to drive photoelectrochemical CO₂ reduction along with water oxidation. Highly selective CO₂ reduction into formates, up to 98.2%, was achieved utilizing plasmonic hot electrons accumulated on AuNPs. The turnover frequency was 1.46 min^-1 with a faradic efficiency of 96.8% under visible light illumination (243 mW·cm^-2). This work demonstrates that the N-heterocyclic carbene-mediated surface functionalization with homogeneous catalytic sites is a promising approach to increase the sustainability and usability of hybrid catalysts.

Recent Advances in TiO2-Based Photocatalysts for Reduction of CO2 to Fuels

Titanium dioxide (TiO₂) has attracted increasing attention as a candidate for the photocatalytic reduction of carbon dioxide (CO₂) to convert anthropogenic CO₂ gas into fuels combined with storage of intermittent and renewable solar energy in forms of chemical bonds for closing the carbon cycle. However, pristine TiO₂ possesses a large band gap (3.2 eV), fast recombination of electrons and holes, and low selectivity for the photoreduction of CO₂. Recently, considerable progress has been made in the improvement of the performance of TiO₂ photocatalysts for CO₂ reduction. In this review, we first discuss the fundamentals of and challenges in CO₂ photoreduction on TiO₂-based catalysts. Next, the recently emerging progress and advances in TiO₂ nanostructured and hybrid materials for overcoming the mentioned obstacles to achieve high light-harvesting capability, improved adsorption and activation of CO₂, excellent photocatalytic activity, the ability to impede the recombination of electrons-holes pairs, and efficient suppression of hydrogen evolution are discussed. In addition, approaches and strategies for improvements in TiO₂-based photocatalysts and their working mechanisms are thoroughly summarized and analyzed. Lastly, the current challenges and prospects of CO₂ photocatalytic reactions on TiO₂-based catalysts are also presented.