Enhanced Photocatalytic Reduction of CO2to CO through TiO2Passivation of InP in Ionic Liquids

Impact of quantum size effects to the band gap of catalytic materials: a computational perspective

The evolution of nanotechnology has facilitated the development of catalytic materials with controllable composition and size, reaching the sub-nanometer limit. Nowadays, a viable strategy for tailoring and optimizing the catalytic activity involves controlling the size of the catalyst. This strategy is underpinned by the fact that the properties and reactivity of objects with dimensions on the order of nanometers can differ from those of the corresponding bulk material, due to the emergence of quantum size effects. Quantum size effects have a deep influence on the band gap of semiconducting catalytic materials. Computational studies are valuable for predicting and estimating the impact of quantum size effects. This perspective emphasizes the crucial role of modeling quantum size effects when simulating nanostructured catalytic materials. It provides a comprehensive overview of the fundamental principles governing the physics of quantum confinement in various experimentally observable nanostructures. Furthermore, this work may serve as a tutorial for modeling the electronic gap of simple nanostructures, highlighting that when working at the nanoscale, the finite dimensions of the material lead to an increase of the band gap because of the emergence of quantum confinement. This aspect is sometimes overlooked in computational chemistry studies focused on surfaces and nanostructures.

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.

Integrated CO2 capture and electrochemical upgradation: the underpinning mechanism and techno-chemical analysis

Coupling post-combustion CO2 capture with electrochemical utilization (CCU) is a quantum leap in renewable energy science since it eliminates the cost and energy involved in the transport and storage of CO2. However, the major challenges involved in industrial scale implementation are selecting an appropriate solvent/electrolyte for CO2 capture, modeling an appropriate infrastructure by coupling an electrolyser with a CO2 point source and a separator to isolate CO2 reduction reaction (CO2RR) products, and finally selection of an appropriate electrocatalyst. In this review, we highlight the major difficulties with detailed mechanistic interpretation in each step, to find out the underpinning mechanism involved in the integration of electrochemical CCU to achieve higher-value products. In the past decades, most of the studies dealt with individual parts of the integration process, i.e., either selecting a solvent for CO2 capture, designing an electrocatalyst, or choosing an ideal electrolyte. In this context, it is important to note that solvents such as monoethanolamine, bicarbonate, and ionic liquids are often used as electrolytes in CO2 capture media. Therefore, it is essential to fabricate a cost-effective electrolyser that should function as a reversible binder with CO2 and an electron pool capable of recovering the solvent to electrolyte reversibly. For example, reversible ionic liquids, which are non-ionic in their normal forms, but produce ionic forms after CO2 capture, can be further reverted back to their original non-ionic forms after CO2 release with almost 100% efficiency through the chemical or thermal modulations. This review also sheds light on a focused techno-economic evolution for converting the electrochemically integrated CCU process from a pilot-scale project to industrial-scale implementation. In brief, this review article will summarize a state-of-the-art argumentation of challenges and outcomes over the different segments involved in electrochemically integrated CCU to stimulate urgent progress in the field.

Plasmon-Enhanced Photocatalytic CO2 Reduction for Higher-Order Hydrocarbon Generation Using Plasmonic Nano-Finger Arrays

The carbon dioxide reduction reaction (CO2RR) is a promising method to both reduce greenhouse gas carbon dioxide (CO₂) concentrations and provide an alternative to fossil fuel by converting water and CO₂ into high-energy-density chemicals. Nevertheless, the CO2RR suffers from high chemical reaction barriers and low selectivity. Here we demonstrate that 4 nm gap plasmonic nano-finger arrays provide a reliable and repeatable plasmon-resonant photocatalyst for multiple-electrons reactions: the CO2RR to generate higher-order hydrocarbons. Electromagnetics simulation shows that hot spots with 10,000 light intensity enhancement can be achieved using nano-gap fingers under a resonant wavelength of 638 nm. From cryogenic ¹H-NMR spectra, formic acid and acetic acid productions are observed with a nano-fingers array sample. After 1 h laser irradiation, we only observe the generation of formic acid in the liquid solution. While increasing the laser irradiation period, we observe both formic and acetic acid in the liquid solution. We also observe that laser irradiation at different wavelengths significantly affected the generation of formic acid and acetic acid. The ratio, 2.29, of the product concentration generated at the resonant wavelength 638 nm and the non-resonant wavelength 405 nm is close to the ratio, 4.93, of the generated hot electrons inside the TiO₂ layer at different wavelengths from the electromagnetics simulation. This shows that product generation is related to the strength of localized electric fields.

Quantum confinement in chalcogenides 2D nanostructures from first principles

We investigated the impact of quantum confinement on the band gap of chalcogenides 2D nanostructures by means of density functional theory. We studied six different systems: MoS₂, WS₂, SnS₂, GaS, InSe, and HfS₂and we simulated nanosheets of increasing thickness, ranging from ultrathin films to ∼10-13 nm thick slabs, a size where the properties converge to the bulk. In some cases, the convergence of the band gap with slab thickness is rather slow, and sizeable deviations from the bulk value are still present with few nm-thick sheets. The results of the simulations were compared with the available experimental data, finding a quantitative agreement. The impact of quantum confinement can be rationalized in terms of effective masses of electrons and holes and system's size. These results show the possibility of reliably describing quantum confinement effects on systems for which experimental data are not available.

Construction of Heterostructured Sn/TiO2 /Si Photocathode for Efficient Photoelectrochemical CO2 Reduction

Using renewable energy to convert CO₂ into liquid products, as a sustainable way to produce fuels and chemicals, has attracted intense attention. Herein, a novel heterostructured photocathode composed of Si wafer, TiO₂ layer, and Sn metal particles has been successfully fabricated by combining of a facile hydrothermal and electrodeposition method. The obtained Sn/TiO₂ /Si photocathode shows enhanced light absorption performance by the surface plasmon resonance effect of Sn metal. Especially, the Sn/TiO₂ /Si photocathode together with rich oxygen vacancy defects jointly promote photoelectrochemical CO₂ reduction, harvesting a high faradaic efficiency of HCOOH and a desirable average current density (-4.72 mA cm^-2 ) at -1.0 V vs. reversible hydrogen electrode. Significantly, the photocathode Sn/TiO₂ /Si also shows good stability due to the design of protecting layer TiO₂ . This study provides a facile strategy of constructing an efficient photocathode to improve the light absorption performance and the electron transfer efficiency, exhibiting great potential in the CO₂ reduction.

Infinite possibilities of ultrathin III-V semiconductors: Starting from synthesis

Ultrathin III-V semiconductors have been receiving tremendous research interest over the past few years. Owing to their exotic structures, excellent physical and chemical properties, ultrathin III-V semiconductors are widely applied in the field of electronics, optoelectronics, and solar energy. However, the strong chemical bonds in layers are the bottleneck of the two-dimensionalization preparation process, which hinders the further development of ultrathin III-V semiconductors. Some effective methods to synthesize ultrathin III-V semiconductors have been reported recently. In this perspective, we briefly introduce the structures and properties of ultrathin III-V semiconductors firstly. Then, we comprehensively summarize the synthetic strategies of ultrathin III-V semiconductors, mainly focusing on space confinement, atomic substitution, adhesion energy regulation, and epitaxial growth. Finally, we summarize the current challenges and propose the development directions of ultrathin III-V semiconductors in the future.

Photo-Driven Reduction of Carbon Dioxide: A Sustainable Approach Towards Achieving Carbon Neutrality Goal

The photo-driven reduction of carbon dioxide (CO2) into green and valuable solar fuels could be a promising solution to simultaneously address energy- and environmental-related problems. This approach could play an integral role in achieving a sustainable energy economy by closing the carbon cycle and allowing the storage and transportation of intermittent solar energy within the chemical bonds of hydrocarbon molecules. This Perspective discusses the latest technological advancements in photo-driven CO2 conversion via various pathways, namely photocatalysis, photoelectrocatalysis and photovoltaic-integrated systems. In addition to providing an outlook on unresolved issues concerning the said technologies, this Perspective also spotlights new trends and strategies in the structural engineering of materials to meet the demands for prominent CO2 photoreduction activity as well as spearhead the ground-breaking advances in the field that lead to the translation of CO2 photo-driven technologies from the laboratory to industrial-scale applications.

Applications of Atomic Layer Deposition in Design of Systems for Energy Conversion

There is a huge demand for clean energy conversion in all industries. The clean energy production processes include electrocatalytic and photocatalytic conversion of water to hydrogen, carbon dioxide reduction, nitrogen conversion to ammonia, and oxygen reduction reaction and require novel cheap and efficient photo- and electrocatalysts and their scalable methods of fabrication. Atomic layer deposition is a thin film deposition method that allows to deposit thin layers of catalysts on virtually any surface of any shape, size, and porosity in an even and easy to control manner. Here the state of the art in applications of atomic layer deposition in the clean energy production and the opportunities it represents for the whole field of the photo- and electrocatalysis for a sustainable future are reviewed.

Rational Design of Semiconductor Heterojunctions for Photocatalysis

Electronic structure calculations provide a useful complement to experimental characterization tools in the atomic-scale design of semiconductor heterojunctions for photocatalysis. The band alignment of the heterojunction is of fundamental importance to achieve an efficient charge carrier separation, so as to reduce electron/hole recombination and improve photoactivity. The accurate prediction of the offsets of valence and conduction bands in the constituent units is thus of key importance but poses several methodological and practical problems. In this Minireview we address some of these problems by considering selected examples of binary and ternary semiconductor heterojunctions and how these are determined at the level of density functional theory (DFT). The atomically precise description of the interface, the consequent charge polarization, the role of quantum confinement, the possibility to use facet engineering to determine a specific band alignment, are among the effects discussed, with particular attention to pros and cons of each one of these aspects. This analysis shows the increasingly important role of accurate electronic structure calculations to drive the design and the preparation of new interfaces with desired properties.

Band offset in semiconductor heterojunctions

Semiconductor heterojunctions are widely applied in solid-state device applications, including semiconductor lasers, solar cells, and transistors. In photocatalysis they are of interest due to their capability to hinder charge carriers' recombination. A key role in the performance of heterojunctions is that of the alignment of the band edges of the two units composing the junction. In this work, we compare the performances of three widely applied approaches for the simulation of semiconductors heterostructures, based on density functional theory calculations with hybrid functionals. We benchmark the band offsets of ten semiconductors heterostructures for which experimental values are available: AlP/GaP, AlP/Si, AlAs/GaAs, AlAs/Ge, GaAs/Ge, GaP/Si, ZnSe/Ge, ZnSe/AlAs, ZnSe/GaAs, and TiO₂/SrTiO₃. The methods considered are (i) the alternating slabs junction (ASJ), (ii) the surface terminated junction (STJ), and (iii) the independent units (IU) approach. Moreover, two different ways to determine a common reference have been considered, (i) the plane averaged electrostatic potential, and (ii) the energy of the core levels. Advantages, drawbacks and overall performances of each method are discussed. The results suggest that the accuracy in the estimation of the band offsets is ∼0.2 eV when the ASJ method is applied. The STJ approach provides a similar accuracy, while the neglection of any interface effect, as in the IU method, provides only a qualitative estimate of the band offset and can result in significant deviations from the experiment.

Role of TiO2coating layer on the performance of Cu2O photocathode in photoelectrochemical CO2reduction

TiO₂is usually employed as a protective layer for Cu₂O in photoelectrocatalytic CO₂reduction. However, the role of TiO₂layer on CO₂reduction activity and selectivity is still elusive. In this work, a systematic investigation is carried out to probe the impact of the deposition parameters of TiO₂overlayer, including the temperature and thickness, on CO₂reduction performance. Compositional and (photo-)electrochemical analysis is performed to explore the property of TiO₂overlayers. Carrier behavior, including donor density and electron energy, and stability of TiO₂are demonstrated to be influenced by atomic layer deposition conditions and thus play a role in controlling CO₂reduction reaction. Specifically, as the thickness of the TiO₂layer increases from 2 to 50 nm, the electron energy tends to be lowered accompanying the electron transfer mode from tunneling for TiO₂thin layers to type II for thick TiO₂, leading to a decrease in CO₂reduction selectivity. With an increase of the TiO₂deposition temperature, the stability increases with a loss of conductivity. Cu₂O coated with 2 nm TiO₂at 150 °C is proven to be the optimized candidate in this work for photoelectrochemical reduction of CO₂to CO, HCOOH and CH₃COOH under an applied bias of -0.4 versus RHE.

Study on nanoporous CuBi2O4 photocathode coated with TiO2 overlayer for photoelectrochemical CO2 reduction

Investigation on the electrode/electrolyte interface of nanotextured electrodes is very challenging but critical to understand the CO₂ reduction reactions. Here we present that the nanoporous CuBi₂O₄ photocathodes coated by a TiO₂ overlayer with gradient thickness show improved CO₂ reduction performance compared to pristine CuBi₂O₄. Different activity and selectivity for photoelectrochemical CO₂ reduction are observed when the thickness of TiO₂ coating layer is set at two extreme values, i.e., less and longer than its Debye length. The CuBi₂O₄ with a thin TiO₂ layer shows higher CO/H₂ yield ratio, and the one with a thick layer exhibits lower CO/H₂ ratio but higher yield for both CO and H₂. All these are elucidated based on the results of Mott-Schottky plots, photocurrent response and SEM/TEM images. Results indicate that the TiO₂ overlayer on CuBi₂O₄ is in favor of the generation and separation of electron/hole pairs, and thus facilitate CO and H₂ production, while the nanoporous structure and the nonuniform TiO₂ overlayer on CuBi₂O₄ have a great impact on mass transport, local pH, and exposed active sites and, thereby, on the CO production selectivity.

Tartrate-stabilized titanium–oxo clusters containing sulfonate chromophore ligands: synthesis, crystal structures and photochemical properties

In tartrate-stabilized TOCs, aniline-sulfonate ligands can extend the absorption edge to the visible light region.

Quantum confinement in group III-V semiconductor 2D nanostructures

In this work we investigate the role of quantum confinement in group III-V semiconductor thin films (2D nanostructures). To this end we have studied the electronic structure of nine materials (AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb) by means of Density Functional Theory (DFT) calculations using a screened hybrid functional (HSE06). We focus on the structural and electronic properties of bulk and the (110) surfaces, for which we evaluate and rationalize the impact of system size to the band gap and band edge positions. Our results indicate that when the quantum confinement is strong, it mainly affects the position of the Conduction Band Minimum (CBM) of the semiconductor, while the Valence Band Maximum (VBM) is almost insensitive to the system size. The results can be rationalized in terms of electron and hole effective masses. Our conclusions, based on slabs, can be generalized to other cases of quantum confinement such as quantum dots, overcoming the need for an explicit consideration and calculation of the properties of semiconductor nanoparticles.

Plasmonic photosynthesis of C1-C3 hydrocarbons from carbon dioxide assisted by an ionic liquid

Photochemical conversion of CO₂ into fuels has promise as a strategy for storage of intermittent solar energy in the form of chemical bonds. However, higher-energy-value hydrocarbons are rarely produced by this strategy, because of kinetic challenges. Here we demonstrate a strategy for green-light-driven synthesis of C₁–C₃ hydrocarbons from CO₂ and H₂O. In this approach, plasmonic excitation of Au nanoparticles produces a charge-rich environment at the nanoparticle/solution interface conducive for CO₂ activation, while an ionic liquid stabilizes charged intermediates formed at this interface, facilitating multi-step reduction and C–C coupling. Methane, ethylene, acetylene, propane, and propene are photosynthesized with a C₂₊ selectivity of ~50% under the most optimal conditions. Hydrocarbon turnover exhibits a volcano relationship as a function of the ionic liquid concentration, the kinetic analysis of which coupled with density functional theory simulations provides mechanistic insights into the synergy between plasmonic excitation and the ionic liquid.

While light-driven conversion of CO₂ and H₂O directly into fuels affords an attractive means to store sunlight in chemical bonds, few systems produce high-value hydrocarbons. Here, authors show gold nanoparticles to reduce CO₂ to multi-carbon products using visible light, ionic liquids, and H₂O.

Resonant and Selective Excitation of Photocatalytically Active Defect Sites in TiO2

It has been known for several decades that defects are largely responsible for the catalytically active sites on metal and semiconductor surfaces. However, it is difficult to directly probe these active sites because the defects associated with them are often relatively rare with respect to the stoichiometric crystalline surface. In the work presented here, we demonstrate a method to selectively probe defect-mediated photocatalysis through differential alternating current (ac) photocurrent (PC) measurements. In this approach, electrons are photoexcited from the valence band to a relatively narrow distribution of subband gap states in TiO₂ and then subsequently to the ions in solution. Because of their limited number, these defect states fill up quickly, resulting in Pauli blocking, and are thereby undetectable under direct current or continuous wave excitation. In the method demonstrated here, the incident light is modulated with an optical chopper, whereas the PC is measured with a lock-in amplifier. Thin (5 nm) films of TiO₂ deposited by atomic layer deposition on various metal films, including Au, Cu, and Al, exhibit the same wavelength-dependent PC spectra, with a broad peak centered around 2.0 eV corresponding to the band-to-defect transition associated with the hydrogen evolution reaction (HER). While the UV-vis absorption spectra of these films show no features at 2.0 eV, photoluminescence (PL) spectra of these photoelectrodes show a similar wavelength dependence with a peak of around 2.0 eV, corresponding to the subband gap emission associated with these defect sites. As a control, alumina (Al₂O₃) films exhibit no PL or PC over the visible wavelength range. The ac PC plotted as a function of electrode potential shows a peak of around -0.4 to -0.1 V versus normal hydrogen electrode, as the monoenergetic defect states are tuned through a resonance with the HER potential. This approach enables the direct photoexcitation of catalytically active defect sites to be studied selectively without the interference of the continuum interband transitions or the effects of Pauli blocking, which is limited by the slow turnover time of the catalytically active sites, typically on the order of 1 μs. We believe that this general approach provides an important new way to study the role of defects in catalysis in an area where selective spectroscopic studies of these are few.

Semiconductor-Based Photoelectrochemical Conversion of Carbon Dioxide: Stepping Towards Artificial Photosynthesis

The photoelectrochemical (PEC) carbon dioxide reduction process stands out as a promising avenue for the conversion of solar energy into chemical feedstocks, among various methods available for carbon dioxide mitigation. Semiconductors derived from cheap and abundant elements are interesting candidates for catalysis. Whether employed as intrinsic semiconductors or hybridized with metallic cocatalysts, biocatalysts, and metal molecular complexes, semiconductor photocathodes exhibit good performance and low overpotential during carbon dioxide reduction. Apart from focusing on carbon dioxide reduction materials and chemistry, PEC cells towards standalone devices that use photohybrid electrodes or solar cells have also been a hot topic in recent research. An overview of the state-of-the-art progress in PEC carbon dioxide reduction is presented and a deep understanding of the catalysts of carbon dioxide reduction is also given.

Zinc Oxide Coating Effect for the Dye Removal and Photocatalytic Mechanisms of Flower-Like MoS2 Nanoparticles

Flower-like MoS₂ nanoparticles (NPs) consist of ultra-thin MoS₂ nanosheets are synthesized via a facile one-pot hydrothermal method. The MoS₂/ZnO p-n heterostructure is formed by coating n-type ZnO on the surface of flower-like MoS₂ NPs through the seed-mediate route and post-annealing treatment. The effects for the dye removal and photocatalytic performances after ZnO coating are systematically investigated. The results demonstrated that the coating of ZnO nanoparticles has a positive promotion to the photodegrading properties while negative effect on the adsorption capacity of the MoS₂/ZnO heterostructures. The related mechanisms on the relationship of adsorption capacity and photocatalysis are discussed in detail.

Understanding the Influence of [EMIM]Cl on the Suppression of the Hydrogen Evolution Reaction on Transition Metal Electrodes

We have studied the influence of low concentrations (0.1 M) of the ionic liquid 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) on suppressing the hydrogen evolution reaction (HER) using polycrystalline Ag, Cu, and Fe electrodes in aqueous acidic and basic media. HER suppression is generally desired when aiming to catalyze other reactions of interests, e.g., CO₂ electro-reduction. Cyclic voltammetry and chronoamperometry measurements were performed at potentials between -0.2 and -0.8 V versus the reversible hydrogen electrode (RHE) to investigate HER activity in a simulated CO₂ electrolysis environment without the CO₂. In an acidic electrolyte, a decrease in HER activity was observed for all three electrodes with the largest effect being that of Fe, where the HER activity was suppressed by 75% at -0.5 V versus RHE. In contrast to the effect of [EMIM]Cl in an acidic electrolyte, no HER suppression was observed in basic media. Using ¹H nuclear magnetic resonance spectroscopy on the electrolyte before and after electrolysis, it was determined that [EMIM]Cl breaks down at both the working and counter electrodes under reaction conditions under both acidic and basic conditions. These results underscore the challenges in employing ionic liquids for electrochemical reactions such as CO₂ reduction.