Abdinoor A. Jelle

Photomethanation of Gaseous CO2 over Ru/Silicon Nanowire Catalysts with Visible and Near‐Infrared Photons

Gaseous CO2 is transformed photochemically and thermochemically in the presence of H2 to CH4 at millimole per hour per gram of catalyst conversion rates, using visible and near‐infrared photons. The catalyst used to drive this reaction comprises black silicon nanowire supported ruthenium. These results represent a step towards engineering broadband solar fuels tandem photothermal reactors that enable a three‐step process involving i) CO2 capture, ii) gaseous water splitting into H2, and iii) reduction of gaseous CO2 by H2.

Electrochromic Bragg Mirror: ECBM

IO N We describe herein the fi rst example of an electrochromic Bragg mirror (ECBM), combining nanoporous multilayers made of NiO and WO 3 nanoparticles. Because NiO and WO 3 are complementary in their coloration effects (e.g. cathodic coloration for WO 3 and anodic coloration for NiO) [ 1 ] and their corresponding change in refractive index, tunability can be achieved by combining these electrochromic components in a 1D Bragg mirror tandem arrangement. The high nanoporosity of this ECBM allows protons and electrons to be quickly shuttled into and out of the multilayers, altering the mix of intervalence charge transfer optical effects within the layers and Bragg diffraction effects between the layers. A proper choice of electrolyte guarantees cycling of the optical properties with negligible degradation. Electrochromic [ 2 , 3 ] devices change their electronic structure and color via electrically-induced storage of ions and electrons in the material, which can be reversed by applying an opposing electrical bias. In comparison, photonic crystals change their color by alterations in the dimension and/or refractive index of the photonic lattice, which can also be changed electrically and reversibly. A prominent example for reversible color changes emanating from alterations of the geometrical structure are voltage-driven, swellable and shrinkable 3D inverse opals built from cross-linked polyferrocenylsilane. [ 3–5 ] This effect is known as the electrophotonic effect. [ 4 , 5 ] Electrochromic materials present themselves as a good alternative, as the coloration usually comes with a change in the refractive index. In the fi eld of electrochromic devices, tungsten trioxide W(VI)O 3 and nickel oxide Ni(II)O are the inorganic material archetypes, and function according to Equations 1–2 . [ 6–8 ]

Heterogeneous reduction of carbon dioxide by hydride-terminated silicon nanocrystals

Silicon constitutes 28% of the earths mass. Its high abundance, lack of toxicity and low cost coupled with its electrical and optical properties, make silicon unique among the semiconductors for converting sunlight into electricity. In the quest for semiconductors that can make chemicals and fuels from sunlight and carbon dioxide, unfortunately the best performers are invariably made from rare and expensive elements. Here we report the observation that hydride-terminated silicon nanocrystals with average diameter 3.5 nm, denoted ncSi:H, can function as a single component heterogeneous reducing agent for converting gaseous carbon dioxide selectively to carbon monoxide, at a rate of hundreds of μmol h−1 g−1. The large surface area, broadband visible to near infrared light harvesting and reducing power of SiH surface sites of ncSi:H, together play key roles in this conversion. Making use of the reducing power of nanostructured hydrides towards gaseous carbon dioxide is a conceptually distinct and commercially interesting strategy for making fuels directly from sunlight.

Nanostructured Indium Oxide Coated Silicon Nanowire Arrays: A Hybrid Photothermal/Photochemical Approach to Solar Fuels

The field of solar fuels seeks to harness abundant solar energy by driving useful molecular transformations. Of particular interest is the photodriven conversion of greenhouse gas CO2 into carbon-based fuels and chemical feedstocks, with the ultimate goal of providing a sustainable alternative to traditional fossil fuels. Nonstoichiometric, hydroxylated indium oxide nanoparticles, denoted In2O3-x(OH)y, have been shown to function as active photocatalysts for CO2 reduction to CO via the reverse water gas shift reaction under simulated solar irradiation. However, the relatively wide band gap (2.9 eV) of indium oxide restricts the portion of the solar irradiance that can be utilized to ∼9%, and the elevated reaction temperatures required (150-190 °C) reduce the overall energy efficiency of the process. Herein we report a hybrid catalyst consisting of a vertically aligned silicon nanowire (SiNW) support evenly coated by In2O3-x(OH)y nanoparticles that utilizes the vast majority of the solar irradiance to simultaneously produce both the photogenerated charge carriers and heat required to reduce CO2 to CO at a rate of 22.0 μmol·gcat(-1)·h(-1). Further, improved light harvesting efficiency of the In2O3-x(OH)y/SiNW films due to minimized reflection losses and enhanced light trapping within the SiNW support results in a ∼6-fold increase in photocatalytic conversion rates over identical In2O3-x(OH)y films prepared on roughened glass substrates. The ability of this In2O3-x(OH)y/SiNW hybrid catalyst to perform the dual function of utilizing both light and heat energy provided by the broad-band solar irradiance to drive CO2 reduction reactions represents a general advance that is applicable to a wide range of catalysts in the field of solar fuels.

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

Abstract 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.

Size-Tunable Photothermal Germanium Nanocrystals

Germanium nanocrystals (ncGe) have not received as much attention as silicon nanocrystals (ncSi). However, Ge has demonstrated superiority over Si nanomaterials in some applications. Examples include, high charge-discharge rate lithium-ion batteries, small band-gap opto-electronic devices, and photo-therapeutics. When stabilized in an oxide matrix (ncGe/GeOx ), its high charge-retention has enabled non-volatile memories. It has also found utility as a high-capacity anode material for Li-ion batteries with impressive stability. Herein, we report an organic-free synthesis of size-controlled ncGe in a GeOx matrix as well as freestanding ncGe, via the thermal disproportionation of GeO prepared from thermally induced dehydration of Ge(OH)2 . The photothermal effect of ncGe, quantified by Raman spectroscopy, is found to be size dependent and superior to ncSi. This advance suggests applications of ncGe in photothermal therapy, desalination, and catalysis.

Tailoring Surface Frustrated Lewis Pairs of In2O3−x(OH)y for Gas‐Phase Heterogeneous Photocatalytic Reduction of CO2 by Isomorphous Substitution of In3+ with Bi3+

Abstract Frustrated Lewis pairs (FLPs) created by sterically hindered Lewis acids and Lewis bases have shown their capacity for capturing and reacting with a variety of small molecules, including H2 and CO2, and thereby creating a new strategy for CO2 reduction. Here, the photocatalytic CO2 reduction behavior of defect‐laden indium oxide (In2O3− x(OH)y) is greatly enhanced through isomorphous substitution of In3+ with Bi3+, providing fundamental insights into the catalytically active surface FLPs (i.e., In—OH···In) and the experimentally observed “volcano” relationship between the CO production rate and Bi3+ substitution level. According to density functional theory calculations at the optimal Bi3+ substitution level, the 6s2 electron pair of Bi3+ hybridizes with the oxygen in the neighboring In—OH Lewis base site, leading to mildly increased Lewis basicity without influencing the Lewis acidity of the nearby In Lewis acid site. Meanwhile, Bi3+ can act as an extra acid site, serving to maximize the heterolytic splitting of reactant H2, and results in a more hydridic hydride for more efficient CO2 reduction. This study demonstrates that isomorphous substitution can effectively optimize the reactivity of surface catalytic active sites in addition to influencing optoelectronic properties, affording a better understanding of the photocatalytic CO2 reduction mechanism.

Enhanced Photothermal Reduction of Gaseous CO2 over Silicon Photonic Crystal Supported Ruthenium at Ambient Temperature

Solar-driven CO2 hydrogenation can provide a renewable source of fuels and reduce greenhouse gas emissions if operated at industrial scales. Herein we investigate the photomethanation (light-driven Sabatier reaction) rates over Ru films sputtered onto silica opal (Ru/SiO2) and inverted silicon opal photonic crystal (Ru/i-Si-o) supports at ambient temperature under solar-simulated radiation as a function of incident light intensity. Photomethanation rates over both the Ru/SiO2 and Ru/i-Si-o catalysts increase significantly with increasing light intensity, and rates as large as 2.8 mmol g−1 h−1 are achieved over the Ru/i-Si-o catalyst. Furthermore, the quantum efficiency of the photomethanation reaction is almost three times larger when measured over the Ru/i-Si-o catalyst as compared to the Ru/SiO2 catalyst. The large photomethanation rates over the Ru/i-Si-o catalyst are attributed to its exceptional light-harvesting properties. Moreover, we perform DFT analysis to investigate the potential role of photo-induced charges on the Ru surface. The results from the simulation indicate that charged Ru surfaces can destabilize adsorbed CO2 molecules and adsorb and dissociate H2 such that it can readily react with CO2, thereby accelerating the Sabatier reaction.

Pd@HyWO3–x Nanowires Efficiently Catalyze the CO2 Heterogeneous Reduction Reaction with a Pronounced Light Effect

The design of photocatalysts able to reduce CO2 to value-added chemicals and fuels could enable a closed carbon circular economy. A common theme running through the design of photocatalysts for CO2 reduction is the utilization of semiconductor materials with high-energy conduction bands able to generate highly reducing electrons. Far less explored in this respect are low-energy conduction band materials such as WO3. Specifically, we focus attention on the use of Pd nanocrystal decorated WO3 nanowires as a heretofore-unexplored photocatalyst for the hydrogenation of CO2. Powder X-ray diffraction, thermogravimetric analysis, ultraviolet-visible-near infrared, and in situ X-ray photoelectron spectroscopy analytical techniques elucidate the hydrogen tungsten bronze, H yWO3- x, as the catalytically active species formed via the H2 spillover effect by Pd. The existence in H yWO3- x of Brønsted acid hydroxyls OH, W(V) sites, and oxygen vacancies (VO) facilitate CO2 capture and reduction reactions. Under solar irradiation, CO2 reduction attains CO production rates as high as 3.0 mmol gcat-1 hr-1 with a selectivity exceeding 99%. A combination of reaction kinetic studies and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements provide a valuable insight into thermochemical compared to photochemical surface reaction pathways, considered responsible for the hydrogenation of CO2 by Pd@H yWO3- x.