Heterogeneous reduction of carbon dioxide by hydride-terminated silicon nanocrystals

Photoelectrochemical CO2 Reduction to CO Enabled by a Molecular Catalyst Attached to High-Surface-Area Porous Silicon

A high-surface-area p-type porous Si photocathode containing a covalently immobilized molecular Re catalyst is highly selective for the photoelectrochemical conversion of CO2 to CO. It gives Faradaic efficiencies of up to 90% for CO at potentials of -1.7 V (versus ferrocenium/ferrocene) under 1 sun illumination in an acetonitrile solution containing phenol. The photovoltage is approximately 300 mV based on comparisons with similar n-type porous Si cathodes in the dark. Using an estimate of the equilibrium potential for CO2 reduction to CO under optimized reaction conditions, photoelectrolysis was performed at a small overpotential, and the onset of electrocatalysis in cyclic voltammograms occurred at a modest underpotential. The porous Si photoelectrode is more stable and selective for CO production than the photoelectrode generated by attaching the same Re catalyst to a planar Si wafer. Further, facile characterization of the porous Si-based photoelectrodes using transmission mode FTIR spectroscopy leads to highly reproducible catalytic performance.

Supra-Photothermal CO2 Methanation over Greenhouse-Like Plasmonic Superstructures of Ultrasmall Cobalt Nanoparticles

Improving the solar-to-thermal energy conversion efficiency of photothermal nanomaterials at no expense of other physicochemical properties, e.g., the catalytic reactivity of metal nanoparticles, is highly desired for diverse applications but remains a big challenge. Herein, a synergistic strategy is developed for enhanced photothermal conversion by a greenhouse-like plasmonic superstructure of 4 nm cobalt nanoparticles while maintaining their intrinsic catalytic reactivity. The silica shell plays a key role in retaining the plasmonic superstructures for efficient use of the full solar spectrum, and reducing the heat loss of cobalt nanoparticles via the nano-greenhouse effect. The optimized plasmonic superstructure catalyst exhibits supra-photothermal CO2 methanation performance with a record-high rate of 2.3 mol gCo -1 h-1 , close to 100% CH4 selectivity, and desirable catalytic stability. This work reveals the great potential of nanoscale greenhouse effect in enhancing photothermal conversions through the combination with conventional promoting strategies, shedding light on the design of efficient photothermal nanomaterials for demanding applications.

Enhancing CO2 reduction through the catalytic effect of a novel silicon haeckelite-inspired 2D material

We propose a novel 2D material based on silicon haeckelite (Hck), whose structure contains a silicon atom arranged in a periodic pattern of pentagons and heptagons. Stacking the two layers gives rise to a planar geometry of the layers that compose it. This new structure presents a semiconductor character with a band gap of 0.17 eV. Furthermore, we studied CO2 reduction using molecular hydrogen to form formic acid, carbon monoxide, formaldehyde, methanol, and methane. All these have been studied theoretically at the Grimme D3BJ corrected TPSS/def2-SVP level. A massive biflake containing 132 Si atoms was used to model the Hck surface. According to the results, CO2 capture with Hck is a spontaneous step; in contrast, the same process for silicene mono- and bi-flakes studied previously was endergonic. After the capture of CO2, the addition of H2 to the substrate passes through an intermediate containing a Si-H bond. The formation of Si-H intermediates is the origin of the catalytic effect, facilitating H2 dissociation and acting as the hydrogen atom donor for the substrate. These intermediates are transformed by adding hydrogen atoms and losing water molecules, producing formic acid and formaldehyde as the most probable products, with rate-controlling steps of 29.2 and 27 kcal mol-1, whose values were less than those exhibited by the silicene biflake. This means that the silicon haeckelite biflake presents better catalytic activity than the silicene biflake. The results show that the novel 2D silicon hackelite material has remarkable potential for CO2 capture and reduction. The theoretical analysis of this innovative 2D structure provides valuable insights into the potential applications of silicene-based materials.

Nanostructured silicon photocatalysts for solar-driven fuel production

Solar-driven production of fuels such as hydrogen, hydrocarbons, and ammonia using semiconducting photocatalysts has the potential to be a sustainable alternative to current chemical processes. In recent years, silicon (Si) nanostructures have been recognized as a promising photocatalyst for hydrogen generation and organic oxidation reactions owing to its abundance, biocompatibility, and cost. While bulk Si has been studied extensively, on the nanoscale, plenty of opportunities exist to understand and engineer optimally performing Si photocatalysts. This perspective will highlight key results on the use of Si nanostructures for photocatalytic H₂ production, CO₂ reduction via light and heat-driven chemical looping, and current challenges in utilizing it for fuel-forming reactions. A brief guide on how these challenges can be addressed in the future and other unexplored questions that remain in the field are also discussed.

Water-borne, durable and multicolor silicon nanoparticles/sodium alginate inks for anticounterfeiting applications

Recently, water-borne fluorescent inks have attracted extensive attention in anti-counterfeiting applications due to their convenient implementation and eco-friendliness. However, due to poor service durability, the latent authorization information from the inks is easily damaged, and even disappears when encountering water. Moreover, most of the existing fluorescent inks are monochromic, toxic, and allergic to skin, thus are unsuitable for their sustainability during real-life applications. Herein, this work presents environment-friendly, durable, and multicolor fluorescent anti-counterfeiting silicon nanoparticles (SiNPs)/sodium alginate (SA) inks. The multicolor SiNPs are synthesized by a one-pot method with defined morphologies and optical properties. Subsequently, SA is employed as the binder to prepare the fluorescent inks with optimized rheological properties. Practicability results show that the SiNPs/SA inks not only exhibit excellent printability, but also impart authentic information with superior covert performance. More notably, spraying solution of calcium dichloride can further improve fluorescent fastnesses of the SiNPs/SA inks by ionic crosslinking.

Postproduction Approach to Enhance the External Quantum Efficiency for Red Light-Emitting Diodes Based on Silicon Nanocrystals

Despite bulk crystals of silicon (Si) being indirect bandgap semiconductors, their quantum dots (QDs) exhibit the superior photoluminescence (PL) properties including high quantum yield (PLQY > 50%) and spectral tunability in a broad wavelength range. Nevertheless, their low optical absorbance character inhibits the bright emission from the SiQDs for phosphor-type light emitting diodes (LEDs). In contrast, a strong electroluminescence is potentially given by serving SiQDs as an emissive layer of current-driven LEDs with (Si-QLEDs) because the charged carriers are supplied from electrodes unlike absorption of light. Herein, we report that the external quantum efficiency (EQE) of Si-QLED was enhanced up to 12.2% by postproduction effect which induced by continuously applied voltage at 5 V for 9 h. The active layer consisted of SiQDs with a diameter of 2.0 nm. Observation of the cross-section of the multilayer QLEDs device revealed that the interparticle distance between adjacent SiQDs in the emissive layer is reduced to 0.95 nm from 1.54 nm by “post-electric-annealing”. The shortened distance was effective in promoting charge injection into the emission layer, leading improvement of the EQE.

CO2 conversion to formamide using a fluoride catalyst and metallic silicon as a reducing agent

Metallic silicon could be an inexpensive, alternative reducing agent for CO₂ functionalization compared to conventionally used hydrogen or hydrosilanes. Here, metallic silicon recovered from solar panel production is used as a reducing agent for formamide synthesis. Various amines are converted to their corresponding amides with CO₂ and H₂O via an Si-H intermediate species in the presence of a catalytic amount of tetrabutylammonium fluoride. The reaction system exhibits a wide substrate scope for formamide synthesis. Spectroscopic analysis, including in situ Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), N₂ adsorption/desorption analyses, and isotopic experiments reveal that the fluoride catalyst effectively oxidizes Si atoms on both surface and interior of the powdered silicon particles. The solid recovered after catalysis contained mesopores with a high surface area. This unique behavior of the fluoride catalyst in the presence of metallic silicon may be extendable to other reductive reactions, including those with complex substrates. Therefore, this study presents a potential strategy for the efficient utilization of abundant resources.

Grave-to-cradle upcycling of Ni from electroplating wastewater to photothermal CO2 catalysis

Treating hazardous waste Ni from the electroplating industry is mandated world-wide, is exceptionally expensive, and carries a very high CO₂ footprint. Rather than regarding Ni as a disposable waste, the chemicals and petrochemicals industries could instead consider it a huge resource. In the work described herein, we present a strategy for upcycling waste Ni from electroplating wastewater into a photothermal catalyst for converting CO₂ to CO. Specifically, magnetic nanoparticles encapsulated in amine functionalized porous SiO₂, is demonstrated to efficiently scavenge Ni from electroplating wastewater for utilization in photothermal CO₂ catalysis. The core-shell catalyst architecture produces CO at a rate of 1.9 mol·g_Ni^-1·h^-1 (44.1 mmol·g_cat^-1·h^-1), a selectivity close to 100%, and notable long-term stability. This strategy of upcycling metal waste into functional, catalytic materials offers a multi-pronged approach for clean and renewable energy technologies.

CO2 Footprint of Thermal Versus Photothermal CO2 Catalysis

Transformation of CO₂ into value-added products via photothermal catalysis has become an increasingly popular route to help ameliorate the energy and environmental crisis derived from the continuing use of fossil fuels, as it can integrate light into well-established thermocatalysis processes. The question however remains whether negative CO₂ emission could be achieved through photothermal catalytic reactions performed in facilities driven by electricity mainly derived from fossil energy. Herein, we propose universal equations that describe net CO₂ emissions generated from operating thermocatalysis and photothermal reverse water-gas shift (RWGS) and Sabatier processes for batch and flow reactors. With these reactions as archetype model systems, the factors that will determine the final amount of effluent CO₂ can be determined. The results of this study could provide useful guidelines for the future development of photothermal catalytic systems for CO₂ reduction.

Thermal Disproportionation for the Synthesis of Silicon Nanocrystals and Their Photoluminescent Properties

In the past decades, silicon nanocrystals have received vast attention and have been widely studied owing to not only their advantages including nontoxicity, high availability, and abundance but also their unique luminescent properties distinct from bulk silicon. Among the various synthetic methods of silicon nanocrystals, thermal disproportionation of silicon suboxides (often with H as another major composing element) bears the superiorities of unsophisticated equipment requirements, feasible processing conditions, and precise control of nanocrystals size and structure, which guarantee a bright industrial application prospect. In this paper, we summarize the recent progress of thermal disproportionation chemistry for the synthesis of silicon nanocrystals, with the focus on the effects of temperature, Si/O ratio, and the surface groups on the resulting silicon nanocrystals’ structure and their corresponding photoluminescent properties. Moreover, the paradigmatic application scenarios of the photoluminescent silicon nanocrystals synthesized via this method are showcased or envisioned.

Silica-copper catalyst interfaces enable carbon-carbon coupling towards ethylene electrosynthesis

Membrane electrode assembly (MEA) electrolyzers offer a means to scale up CO2-to-ethylene electroconversion using renewable electricity and close the anthropogenic carbon cycle. To date, excessive CO2 coverage at the catalyst surface with limited active sites in MEA systems interferes with the carbon-carbon coupling reaction, diminishing ethylene production. With the aid of density functional theory calculations and spectroscopic analysis, here we report an oxide modulation strategy in which we introduce silica on Cu to create active Cu-SiOx interface sites, decreasing the formation energies of OCOH* and OCCOH*-key intermediates along the pathway to ethylene formation. We then synthesize the Cu-SiOx catalysts using one-pot coprecipitation and integrate the catalyst in a MEA electrolyzer. By tuning the CO2 concentration, the Cu-SiOx catalyst based MEA electrolyzer shows high ethylene Faradaic efficiencies of up to 65% at high ethylene current densities of up to 215 mA cm-2; and features sustained operation over 50 h.

Boosting thermo-photocatalytic CO2 conversion activity by using photosynthesis-inspired electron-proton-transfer mediators

Natural photosynthesis proceeded by sequential water splitting and CO2 reduction reactions is an efficient strategy for CO2 conversion. Here, mimicking photosynthesis to boost CO2-to-CO conversion is achieved by using plasmonic Bi as an electron-proton-transfer mediator. Electroreduction of H2O with a Bi electrode simultaneously produces O2 and hydrogen-stored Bi (Bi-Hx). The obtained Bi-Hx is subsequently used to generate electron-proton pairs under light irradiation to reduce CO2 to CO; meanwhile, Bi-Hx recovers to Bi, completing the catalytic cycle. This two-step strategy avoids O2 separation and enables a CO production efficiency of 283.8 μmol g-1 h-1 without sacrificial reagents and cocatalysts, which is 9 times that on pristine Bi in H2 gas. Theoretical/experimental studies confirm that such excellent activity is attributed to the formed Bi-Hx intermediate that improves charge separation and reduces reaction barriers in CO2 reduction.

Hydrogenation of CO2 Promoted by Silicon-Activated H2S: Origin and Implications

Unlike the usual method of CO_x (x = 1, 2) hydrogenation using H₂ directly, H₂S and HSiSH (silicon-activated H₂S) were selected as alternative hydrogen sources in this study for the CO_x hydrogenation reactions. Our results suggest that it is kinetically infeasible for hydrogen in the form of H₂S to transfer to CO_x at low temperatures. However, when HSiSH is employed instead, the title reaction can be achieved. For this approach, the activation of CO₂ is initiated by its interaction with the HSiSH molecule, a reactive species with both a hydridic H^δ− and protonic H^δ+. These active hydrogens are responsible for the successive C-end and O-end activations of CO₂ and hence the final product (HCOOH). This finding represents a good example of an indirect hydrogen source used in CO₂ hydrogenation through reactivity tuned by silicon incorporation, and thus the underlying mechanism will be valuable for the design of similar reactions.

Flash Solid-Solid Synthesis of Silicon Oxide Nanorods

1D silicon-based nanomaterials, renowned for their unique chemical and physical properties, have enabled the development of numerous advanced materials and biomedical technologies. Their production often necessitates complex and expensive equipment, requires hazardous precursors and demanding experimental conditions, and involves lengthy processes. Herein, a flash solid-solid (FSS) process is presented for the synthesis of silicon oxide nanorods completed within seconds. The innovative features of this FSS process include its simplicity, speed, and exclusive use of solid precursors, comprising hydrogen-terminated silicon nanosheets and a metal nitrate catalyst. Advanced electron microscopy and X-ray spectroscopy analyses favor a solid-liquid-solid reaction pathway for the growth of the silicon oxide nanorods with vapor-liquid-solid characteristics.

First-Principles Study on the Molecular Mechanism of Solar-Driven CO2 Reduction on H-Terminated Si

Solar-driven conversion of CO₂ with H-terminated silicon has recently attracted increasing interest. However, the molecular mechanism of the reaction is still not well understood. A systematic study of the mechanism has been carried out with first-principles calculations. The formation energies of the intermediates are found to be insensitive to the structure of the surface. On the fully H-terminated Si(111) surface, several pathways for the conversion of CO₂ into CO at a coordinatively saturated Si site are studied, including the conventional COOH* pathway and the direct insertion of CO₂ into Si-H and Si-Si bonds. Although the barrier of the COOH* pathway is lowest among the three pathways, it is higher than that for OH* elimination, which suggests that CO₂ should be converted by other types of active site. The reaction at the isolated and dual coordinatively unsaturated (CUS) Si sites, which can be generated by light illumination, heat, and Pd loading, are then studied. The results suggest that the most efficient pathway to convert CO₂ is to convert it into CO and O* at an isolated CUS Si site before O* reacts with a terminating H* to form adsorbed OH* and generate new isolated CUS Si sites. Therefore, the CUS Si site catalyzes the reaction until all H* is converted into OH*. The results provide new insight into the mechanism of the reaction and should be helpful for the design of more efficient Si-based catalysts for CO₂ conversion.

A metal-free radical technique for cross-linking of polymethylhydrosiloxane or polymethylvinylsiloxane using AIBN

A new method was developed for the metal-free cross-linking of silicone rubbers. This process uses azobisisobutyronitrile (AIBN) to selectively react with Si-H and vinyl groups as a free-radical initiator for the thermal curing of polymethylhydrosiloxane (PMHS) and polymethylvinylsiloxane (PMVS). The AIBN-initiated curing reaction between the Si-H groups of PMHS generated Si-O-Si and Si-Si cross-links. In contrast, PMVS was cured via the formation of C-C bonds through "methyl-vinyl" and "vinyl-vinyl" mechanisms. Curing reactions were performed at 80-120 °C in air and confirmed by 13C and 29Si solid state NMR analyses and swelling trials.

Two-dimensional gersiloxenes with tunable bandgap for photocatalytic H2 evolution and CO2 photoreduction to CO

The discovery of graphene and graphene-like two-dimensional materials has brought fresh vitality to the field of photocatalysis. Bandgap engineering has always been an effective way to make semiconductors more suitable for specific applications such as photocatalysis and optoelectronics. Achieving control over the bandgap helps to improve the light absorption capacity of the semiconductor materials, thereby improving the photocatalytic performance. This work reports two-dimensional -H/-OH terminal-substituted siligenes (gersiloxenes) with tunable bandgap. All gersiloxenes are direct-gap semiconductors and have wide range of light absorption and suitable band positions for light driven water reduction into H2, and CO2 reduction to CO under mild conditions. The gersiloxene with the best performance can provide a maximum CO production of 6.91 mmol g-1 h-1, and a high apparent quantum efficiency (AQE) of 5.95% at 420 nm. This work may open up new insights into the discovery, research and application of new two-dimensional materials in photocatalysis.

Emerging Atomic Energy Levels in Zero-Dimensional Silicon Quantum Dots

Driven by the emergence of colloidal semiconductor quantum dots (QDs) of tunable emission wavelengths, characteristic of exciton absorption peaks, outstanding photostability and solution processability in device fabrication have become a key tool in the development of nanomedicine and optoelectronics. Diamond cubic crystalline silicon (Si) QDs, with a diameter larger than 2 nm, terminated with hydrogen atoms are known to exhibit bulk-inherited spin and valley properties. Herein, we demonstrate a newly discovered size region of Si QDs, in which a fast radiative recombination on the order of hundreds of picoseconds is responsible for photoluminescence (PL). Despite retaining a crystallographic structure like the bulk, controlling their diameters in the 1.1-1.7 nm range realizes the strong PL with continuous spectral tunability in the 530-580 nm window, the narrow spectral line widths without emission tails, and the fast relaxation of photogenerated carriers. In contrast, QDs with diameters greater than 1.8 nm display the decay times on the microsecond order as well as the previous Si QDs. In addition to the five-orders-of-magnitude variation in the PL decay time, a systematic study on the temperature dependence of PL properties suggests that the energy structure of the smaller QDs does not retain an indirect band gap character. It is discussed that a 1.7 nm diameter is critical to undergo changes in energy structure from bulky to molecular configurations.

Defects in nanosilica catalytically convert CO2 to methane without any metal and ligand

Active and stable metal-free heterogeneous catalysts for CO₂ fixation are required to reduce the current high level of carbon dioxide in the atmosphere, which is driving climate change. In this work, we show that defects in nanosilica (E' centers, oxygen vacancies, and nonbridging oxygen hole centers) convert CO₂ to methane with excellent productivity and selectivity. Neither metal nor complex organic ligands were required, and the defect alone acted as catalytic sites for carbon dioxide activation and hydrogen dissociation and their cooperative action converted CO₂ to methane. Unlike metal catalysts, which become deactivated with time, the defect-containing nanosilica showed significantly better stability. Notably, the catalyst can be regenerated by simple heating in the air without the need for hydrogen gas. Surprisingly, the catalytic activity for methane production increased significantly after every regeneration cycle, reaching more than double the methane production rate after eight regeneration cycles. This activated catalyst remained stable for more than 200 h. Detailed understanding of the role of the various defect sites in terms of their concentrations and proximities as well as their cooperativity in activating CO₂ and dissociating hydrogen to produce methane was achieved.

Organic-inorganic hybrids for CO2 sensing, separation and conversion

Motivated by the air pollution that skyrocketed in numerous regions around the world, great effort was placed on discovering new classes of materials that separate, sense or convert CO₂ in order to minimise impact on human health. However, separation, sensing and conversion are not only closely intertwined due to the ultimate goal of improving human well-being, but also because of similarities in material prerequisites -e.g. affinity to CO₂. Partly inspired by the unrivalled performance of complex natural materials, manifold inorganic-organic hybrids were developed. One of the most important characteristics of hybrids is their design flexibility, which results from the combination of individual constituents with specific functionality. In this review, we discuss commonly used organic, inorganic, and inherently hybrid building blocks for applications in separation, sensing and catalytic conversion and highlight benefits like durability, activity, low-cost and large scale fabrication. Moreover, we address obstacles and potential future developments of hybrid materials. This review should inspire young researchers in chemistry, physics and engineering to identify and overcome interdisciplinary research challenges by performing academic research but also - based on the ever-stricter emission regulations like carbon taxes - through exchanges between industry and science.

One-Step Direct Fixation of Atmospheric CO2 by Si-H Surface in Solution

The efficient conversion of carbon dioxide (CO2) into useful chemicals has important practical significance for environmental protection. Until now, direct fixation of atmospheric CO2 needs first extraction from the atmosphere, an energy-intensive process. Silicon (or Si-H surface), Earth-abundant, low-cost and non-toxic, is a promising material for heterogeneous CO2 chemical fixation. Here we report one-step fixing of CO2 directly from the atmosphere to a paraformaldehyde-like polymer by Si-H surface at room temperature. With the assistance of HF, commercial silicon powder was used as a heterogeneous reducing agent, for converting gaseous CO2 to a polymer of fluorine substituted polyoxymethylene and hydroxyl substituted polyoxymethylene alternating copolymer (F-POM). Making use of the Si-H surface toward the fixation of atmospheric gaseous CO2 is a conceptually distinct and commercially interesting strategy for making useful chemicals and environmental protection.

The next big thing for silicon nanostructures – CO2 photocatalysis

Silicon nanostructures for the catalytic conversion of CO₂ to value-added products.

Industrial carbon dioxide capture and utilization: state of the art and future challenges

This review covers the sustainable development of advanced improvements in CO₂ capture and utilization.

Silicene catalysts for CO2 hydrogenation: the number of layers controls selectivity

Hydrogenation of carbon dioxide (CO2) is among the most promising approaches for reclaiming the major greenhouse gases to produce fuels and chemicals. Developing catalysts composed of natural abundant, economical and eco-friendly elements is critical for the industrialization of this technology. Silicon satisfies all these requirements but lacks activity. Using first-principles calculations, we show for the first time that the two-dimensional phase of silicon, i.e., mono- and few-layer silicene supported by a Ag(111) substrate, exhibits superior catalytic activity for CO2 hydrogenation, with selectivity being intrinsically controlled by the number of layers. The supported silicene monolayer as a catalyst leads to the formation of carbon monoxide, formic acid and formaldehyde, while the formation of methanol and methane is favored on bilayer silicene on the Ag substrate. The key parameters governing activity and selectivity are the densities and energy levels of surface dangling bond states, which in turn are mediated by the substrate coupling and covalent interaction between silicene layers. These theoretical results elucidate the fundamental principles for tailoring the catalytic properties of non-metal materials by controlling the number of layers and manipulating the surface states and will advance the development of silicon-based catalysts for renewable energy technologies.

Towards Solar Methanol: Past, Present, and Future

This work aims to provide an overview of producing value-added products affordably and sustainably from greenhouse gases (GHGs). Methanol (MeOH) is one such product, and is one of the most widely used chemicals, employed as a feedstock for ≈30% of industrial chemicals. The starting materials are analogous to those feeding natural processes: water, CO2, and light. Innovative technologies from this effort have global significance, as they allow GHG recycling, while providing society with a renewable carbon feedstock. Light, in the form of solar energy, assists the production process in some capacity. Various solar strategies of continually increasing technology readiness levels are compared to the commercial MeOH process, which uses a syngas feed derived from natural gas. These strategies include several key technologies, including solar-thermochemical, photochemical, and photovoltaic-electrochemical. Other solar-assisted technologies that are not yet commercial-ready are also discussed. The commercial-ready technologies are compared using a technoeconomic analysis, and the scalability of solar reactors is also discussed in the context of light-incorporating catalyst architectures and designs. Finally, how MeOH compares against other prospective products is briefly discussed, as well as the viability of the most promising solar MeOH strategy in an international context.

Silicon-doped graphene edges: an efficient metal-free catalyst for the reduction of CO2 into methanol and ethanol

Si doped graphene as a metal-free catalyst to convert CO₂ to methanol and ethanol with high selectivity and activity.

Photocatalytic CO2 reduction by H2O: insights from modeling electronically relaxed mechanisms

Understanding of the ground-state surface reaction mechanism for photocatalytic CO₂ reduction and new connections between catalyst surface reactivity and experimentally observed activity and selectivity are presented to facilitate the development of catalysts that exhibit improved activity, controlled product distributions, and enhanced quantum yield.

Communication: Photoinduced carbon dioxide binding with surface-functionalized silicon quantum dots

Nowadays, the search for efficient methods able to reduce the high atmospheric carbon dioxide concentration has turned into a very dynamic research area. Several environmental problems have been closely associated with the high atmospheric level of this greenhouse gas. Here, a novel system based on the use of surface-functionalized silicon quantum dots (sf-SiQDs) is theoretically proposed as a versatile device to bind carbon dioxide. Within this approach, carbon dioxide trapping is modulated by a photoinduced charge redistribution between the capping molecule and the silicon quantum dots (SiQDs). The chemical and electronic properties of the proposed SiQDs have been studied with a Density Functional Theory and Density Functional Tight-Binding (DFTB) approach along with a time-dependent model based on the DFTB framework. To the best of our knowledge, this is the first report that proposes and explores the potential application of a versatile and friendly device based on the use of sf-SiQDs for photochemically activated carbon dioxide fixation.

Si nanocrystal solution with stability for one year

Colloidal silicon nanocrystals (SiNCs) are a promising material for next-generation nanostructured devices. High-stability SiNC solutions are required for practical use as well as studies on the properties of SiNC. Here, we show a solution of SiNCs that was stable for one year without aggregation. The stable solution was synthesized by a facile process, i.e., pulsed laser ablation of a Si wafer in isopropyl alcohol (IPA). The long-term stability was due to a large ζ-potential of −50 mV from a SiNC passivation layer composed of oxygen, hydrogen, and alkane groups, according to the results of eight experiments and theoretical calculations. This passivation layer also resulted in good performance as an additive for a conductive polymer film. Namely, a 5-fold enhancement in carrier density was established by the addition of SiNCs into an organic conductive polymer, poly(3-dodecylthiophene), which is useful for solar cells. Furthermore, it was found that fresh (<1 day) and aged (4 months) SiNCs give the same enhancement. The long-term stability was attributed to a great repulsive energy in IPA, whose value was quantified as a function the distance between SiNCs.

A stable nanocrystal for one year without aggregation in a liquid is synthesized by one-step, one-pot, and one-hour process.

Photothermal Catalyst Engineering: Hydrogenation of Gaseous CO2 with High Activity and Tailored Selectivity

This study has designed and implemented a library of hetero-nanostructured catalysts, denoted as Pd@Nb2O5, comprised of size-controlled Pd nanocrystals interfaced with Nb2O5 nanorods. This study also demonstrates that the catalytic activity and selectivity of CO2 reduction to CO and CH4 products can be systematically tailored by varying the size of the Pd nanocrystals supported on the Nb2O5 nanorods. Using large Pd nanocrystals, this study achieves CO and CH4 production rates as high as 0.75 and 0.11 mol h-1 gPd-1, respectively. By contrast, using small Pd nanocrystals, a CO production rate surpassing 18.8 mol h-1 gPd-1 is observed with 99.5% CO selectivity. These performance metrics establish a new milestone in the champion league of catalytic nanomaterials that can enable solar-powered gas-phase heterogeneous CO2 reduction. The remarkable control over the catalytic performance of Pd@Nb2O5 is demonstrated to stem from a combination of photothermal, electronic and size effects, which is rationally tunable through nanochemistry.

Controlling selectivities in CO2 reduction through mechanistic understanding

Catalytic CO₂ conversion to energy carriers and intermediates is of utmost importance to energy and environmental goals. However, the lack of fundamental understanding of the reaction mechanism renders designing a selective catalyst inefficient. Here we show the correlation between the kinetics of product formation and those of surface species conversion during CO₂ reduction over Pd/Al₂O₃ catalysts. The operando transmission FTIR/SSITKA (Fourier transform infrared spectroscopy/steady-state isotopic transient kinetic analysis) experiments demonstrates that the rate-determining step for CO formation is the conversion of adsorbed formate, whereas that for CH₄ formation is the hydrogenation of adsorbed carbonyl. The balance of the hydrogenation kinetics between adsorbed formates and carbonyls governs the selectivities to CH₄ and CO. We apply this knowledge to the catalyst design and achieve high selectivities to desired products.

Understanding the mechanism of CO₂ reduction on a catalyst surface is essential for achieving the desired product selectivity. Here, the authors show an operando kinetic analysis of CO₂ hydrogenation over a palladium catalyst in order to address the factors governing the selectivity of the process.

Prominent Electron Penetration through Ultrathin Graphene Layer from FeNi Alloy for Efficient Reduction of CO2 to CO

The chemical transformation of CO₂ is an efficient approach in low-carbon energy system. The development of nonprecious metal catalysts with sufficient activity, selectivity, and stability for the generation of CO by CO₂ reduction under mild conditions remains a major challenge. A hierarchical architecture catalyst composed of ultrathin graphene shells (2-4 layers) encapsulating homogeneous FeNi alloy nanoparticles shows enhance catalytic performance. Electron transfer from the encapsulated alloy can extend from the inner to the outer shell, resulting in an increased charge density on graphene. Nitrogen atom dopants can synergistically increase the electron density on the catalyst surface and modulate the adsorption capability for acidic CO₂ molecules. The optimized FeNi₃ @NG (NG=N-doped graphene) catalyst, with significant electron penetration through the graphene layer, effects exceptional CO₂ conversion of 20.2 % with a CO selectivity of nearly 100 %, as well as excellent thermal stability at 523 K.

Perovskite nanocomposites as effective CO2-splitting agents in a cyclic redox scheme

We report iron-containing mixed-oxide nanocomposites as highly effective redox materials for thermochemical CO₂ splitting and methane partial oxidation in a cyclic redox scheme, where methane was introduced as an oxygen "sink" to promote the reduction of the redox materials followed by reoxidation through CO₂ splitting. Up to 96% syngas selectivity in the methane partial oxidation step and close to complete conversion of CO₂ to CO in the CO₂-splitting step were achieved at 900° to 980°C with good redox stability. The productivity and production rate of CO in the CO₂-splitting step were about seven times higher than those in state-of-the-art solar-thermal CO₂-splitting processes, which are carried out at significantly higher temperatures. The proposed approach can potentially be applied for acetic acid synthesis with up to 84% reduction in CO₂ emission when compared to state-of-the-art processes.

Powering a CO2 Reduction Catalyst with Visible Light through Multiple Sub-picosecond Electron Transfers from a Quantum Dot

Photosensitization of molecular catalysts to reduce CO₂ to CO is a sustainable route to storable solar fuels. Crucial to the sensitization process is highly efficient transfer of redox equivalents from sensitizer to catalyst; in systems with molecular sensitizers, this transfer is often slow because it is gated by diffusion-limited collisions between sensitizer and catalyst. This article describes the photosensitization of a meso-tetraphenylporphyrin iron(III) chloride (FeTPP) catalyst by colloidal, heavy metal-free CuInS₂/ZnS quantum dots (QDs) to reduce CO₂ to CO using 450 nm light. The sensitization efficiency (turnover number per absorbed unit of photon energy) of the QD system is a factor of 18 greater than that of an analogous system with a fac-tris(2-phenylpyridine)iridium sensitizer. This high efficiency originates in ultrafast electron transfer between the QD and FeTPP, enabled by formation of QD/FeTPP complexes. Optical spectroscopy reveals that the electron-transfer processes primarily responsible for the first two sensitization steps (Fe^IIITPP → Fe^IITPP, and Fe^IITPP → Fe^ITPP) both occur in <200 fs.

Integration of Photothermal Effect and Heat Insulation to Efficiently Reduce Reaction Temperature of CO2 Hydrogenation

The photothermal effect is applied in CO₂ hydrogenation to reduce the reaction temperature under illumination by encapsulating Pt nanocubes and Au nanocages into a zeolitic imidazolate framework (ZIF-8). Under illumination, the heat generated by the photothermal effect of Au nanocages is mainly insulated in the ZIF-8 to form a localized high-temperature region, thereby improving the catalytic activity of Pt nanocubes.