Surface Termination and CO₂ Adsorption onto Bismuth Pyrochlore Oxides

The catalytic activity and gas-sensing properties of a solid are dominated by the chemistry of the surface atomic layer. This study is concerned with the characterization of the outer atomic surfaces of a series of cubic ternary oxides containing Bi­(III): Bi₂M₂O₇ (M = Ti, Zr, Hf), using low-energy ion scattering spectroscopy. A preferential termination in Bi and O is observed in pyrochlore Bi₂Ti₂O₇ and related cubic compounds Bi₂Zr₂O₇ and Bi₂Hf₂O₇, whereas all three components of the ternary oxide are present on the surface of a Bi-free pyrochlore oxide, Y₂Ti₂O₇. This observation can be explained based on the revised lone-pair model for post-transition-metal oxides. We propose that the stereochemically active lone pair resulting from O 2p-assisted Bi 6s–6p hybridization is more energetically favored at the surface than within a distorted bulk site. This leads to reduction of the surface energy of the Bi₂M₂O₇ compounds and, therefore, offers a thermodynamic driving force for the preferential termination in BiO_<i>x</i>-like structures. CO₂ adsorption experiments <i>in situ</i> monitored by diffuse reflectance IR spectroscopy show a high CO₂ chemisorption capacity for this series of cubic bismuth ternary oxides, indicating a high surface basicity. This can be associated with O 2p–Bi 6s–6p hybridized electronic states, which are more able to donate electronic density to adsorbed species than surface lattice oxygen ions, normally considered as the basic sites in metal oxides. The enhanced CO₂ adsorption of these types of oxides is particularly relevant to the current growing interest in the development of technologies for CO₂ reduction

Pd₂Ga-Based Colloids as Highly Active Catalysts for the Hydrogenation of CO₂ to Methanol

Colloidal Pd₂Ga-based catalysts are shown to catalyze efficiently the hydrogenation of CO₂ to methanol. The catalysts are produced by the simple thermal decomposition of Pd­(II) acetate in the presence of Ga­(III) stearate, which leads to Pd⁰ nanoparticles (ca. 3 nm), and the subsequent Pd-mediated reduction of Ga­(III) species at temperatures ranging from 210 to 290 °C. The resulting colloidal Pd₂Ga-based catalysts are applied in the liquid-phase hydrogenation of carbon dioxide to methanol at high pressure (50 bar). The intrinsic activity is around 2-fold higher than that obtained for the commercial Cu-ZnO-Al₂O₃ (60.3 and 37.2 × 10^–9 mol_MeOH m^–2 s^–1), respectively, and 4-fold higher on a Cu or Pd molar basis (3330 and 910 μmol mmol_Pd or Cu^–1 h^–1). Detailed characterization data (HR-TEM, STEM/EDX, XPS, and XRD) indicate that the catalyst contains Pd₂Ga nanoparticles, of average diameters 5–6 nm, associated with a network of amorphous Ga₂O₃ species. The proportion of this Ga₂O₃ phase can be easily tuned by adjusting the molar ratio of the Pd:Ga precursors. A good correlation was found between the intrinsic activity and the content of Ga₂O₃ surrounding the Pd₂Ga nanoparticles (XPS), suggesting that methanol is formed by a bifunctional mechanism involving both phases. The increase in the reaction temperature (190–240 °C) leads to a gradual decrease in methanol selectivity from 60 to 40%, while an optimum methanol production rate was found at 210 °C. Interestingly, unlike the conventional Cu-ZnO-Al₂O₃, which experienced approximately 50% activity loss over 25 h time on stream, the Pd₂Ga-based catalysts maintain activity over this time frame. Indeed, characterization of the Pd/Ga mixture postcatalysis revealed no ripening of the nanoparticles or changes in the phases initially present

Reversible Redox Cycling of Well-Defined, Ultrasmall Cu/Cu₂O Nanoparticles

Exceptionally small and well-defined copper (Cu) and cuprite (Cu₂O) nanoparticles (NPs) are synthesized by the reaction of mesitylcopper­(I) with either H₂ or air, respectively. In the presence of substoichiometric quantities of ligands, namely, stearic or di­(octyl)­phosphinic acid (0.1–0.2 equiv vs Cu), ultrasmall nanoparticles are prepared with diameters as low as ∼2 nm, soluble in a range of solvents. The solutions of Cu NPs undergo quantitative oxidation, on exposure to air, to form Cu₂O NPs. The Cu₂O NPs can be reduced back to Cu(0) NPs using accessible temperatures and low pressures of hydrogen (135 °C, 3 bar H₂). This striking reversible redox cycling of the discrete, solubilized Cu/Cu­(I) colloids was successfully repeated over 10 cycles, representing 19 separate reactions. The ligands influence the evolution of both composition and size of the nanoparticles, during synthesis and redox cycling, as explored in detail using vacuum-transfer aberration-corrected transmission electron microscopy, X-ray photoelectron spectroscopy, and visible spectroscopy

Nanoscale structure-property relationships in low temperature solution-processed electron transport layers for organic photovoltaics

Here we elucidate the nanostructure–property relationships in low-temperature, solution-processed ZnO based thin films employed as novel electron transport layers (ETLs) in organic photovoltaic (OPV) devices. Using a low-cost zinc precursor (zinc acetate) in a simple amine–alcohol solvent mix, high-quality ETL thin films are prepared. We show that at a processing temperature of 110 °C the films are composed of nanoparticles embedded in a continuous organic matrix consisting of ZnO precursor species and stabilizers. Using a combination of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), we study the thermally induced morphological and compositional changes in the ETLs. Transient optoelectronic probes reveal that the mixed nanocrystalline/amorphous nature of the films does not contribute to recombination losses in devices. We propose that charge transport in our low-temperature processed ETLs is facilitated by the network of ZnO nanoparticles, with the organic matrix serving to tune the work function of the ETL and to provide excellent resistance to current leakage. To demonstrate the performance of our ETLs we prepare inverted architecture OPVs utilizing Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7): [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) as active layer materials. The low-temperature ETL devices showed typical power conversion efficiencies (PCEs) of >7% with the champion devices achieving a PCE > 8%