Interaction of alkaline metal cations with oxidic surfaces: effect on the morphology of SnO2 nanoparticles

Electroreduction of CO to 2.8 A cm⁻2 C2+ Products: Maximizing Efficiency with Minimalist Electrode Design Featuring a Mesopore-Rich Hydrophobic Copper Catalyst Layer

This work shows how hydrophobicity and porosity can be incorporated into copper catalyst layers (CLs) for the efficient electroreduction of CO (CORR) in a flow cell. Oxide-derived (OD) Cu catalysts are synthesized using K+ and Cs+ as templates, termed respectively as OD-Cu-K and OD-Cu-Cs. CLs, assembled from OD-Cu-K and OD-Cu-Cs, exhibit enhanced CORR performance compared to "unmodified" OD-Cu CL. OD-Cu-Cs can notably reduce CO to C2+ products with Faradaic efficiencies (FE) as high as 96% (or 4% FE H2). During CO electrolysis at -3000 mA cm-2 (-0.73 V vs reversible hydrogen electrode), C2+ products and the alcohols are formed with respective current densities of -2804 and -1205 mA cm- 2. The mesopores in the OD-Cu-Cs CL act as barriers against electrolyte flooding. Contact angle measurements confirm the CL's hydrophobicity ranking: OD-Cu-Cs > OD-Cu-K > OD-Cu. The enhanced hydrophobicity of a catalyst is proposed to allow more triple-phase (CO-electrolyte-catalyst) interfaces to be available for CORR. This study shows how the pore size-hydrophobicity relationship can be harvested to guide the design of a less-is-more Cu electrode, which can attain high CORR current density and selectivity, without the additional use of hydrophobic polytetrafluoroethylene particles or dopants, such as Ag.

Gram-scale selective synthesis of WO3-x nanorods and (NH4)xWO3 ammonium tungsten bronzes with tunable plasmonic properties

Localized surface plasmon resonance properties in unconventional materials like metal oxides or chalcogenide semiconductors have been studied for use in signal detection and analysis in biomedicine and photocatalysis. We devised a selective synthesis of the tungsten oxides WO_3-x and (NH₄)_xWO₃ with tunable plasmonic properties. We selectively synthesized WO_3-x nanorods with different aspect ratios and hexagonal tungsten bronzes (NH₄)_xWO₃ as truncated nanocubes starting from ammonium metatungstate (NH₄)₆H₂W₁₂O₄₀·xH₂O. Both particles form from the same nuclei at temperatures >200 °C; monomer concentration and surfactant ratio are essential variables for phase selection. (NH₄)_xWO₃ was the preferred reaction product only for fast heating rates (25 K min^-1), slow stirring speeds (∼150 rpm) and high precursor concentrations. A proton nuclear magnetic resonance (¹H-NMR) spectroscopic study of the reaction mechanism revealed that oleyl oleamide, formed from oleic acid and oleylamine upon heating, is a key factor for the selective formation of WO_3-x nanorods. Since oleic acid and oleylamine are standard surfactants for the wet chemical synthesis of many metal and oxide nanoparticles, the finding that oleyl oleamide acts as a chemically active reagent above 250 °C may have implications for many nanoparticle syntheses. Oriented attachment of polyoxotungstate anions is proposed as a model to rationalize phase selectivity. Magic angle spinning (MAS) ¹H-NMR and powder X-ray diffraction (PXRD) studies of the bronze after annealing under (non)inert conditions revealed an oxidative phase transition. WO_3-x and (NH₄)_xWO₃ show a strong plasmon absorption for near infra-red light between 800 and 3300 nm. The maxima of the plasmon bands shift systematically with the nanocrystal aspect ratio.

Hydrothermal growth mechanism of SnO2 nanorods in aqueous HCl

Rutile-type nanorods of SnO2 were obtained in a one-pot hydrothermal synthesis starting from SnCl4·5H2O and HCl in a temperature range between 200 and 240°C. Although the nanorods are polydisperse, the average length of the nanorods could be adjusted from 13 to 65 nm by varying of the reaction temperature. The resulting anisotropic nanocrystals were characterized using powder X-ray diffraction (PXRD), (high resolution-) transmission electron microscopy (HR-TEM), and selected area electron diffraction (SAED). The particle growth proceeds via a dissolution-recrystallization process with soluble [SnCl5(H2O)]− intermediates, as confirmed by PXRD, Raman spectroscopy, and magic angle spinning nuclear magnetic resonance (MAS-NMR).

Study of structural properties and defects of Ni-doped SnO2 nanorods as ethanol gas sensors

An ethanol gas sensor with enhanced sensor response was fabricated using Ni-doped SnO₂ nanorods, synthesized via a simple hydrothermal method. It was found that the response (R = R ₀/R _g) of a 5.0 mol% Ni-doped SnO₂ (5.0Ni:SnO₂) nanorod sensor was 1.4 × 10⁴ for 1000 ppm C₂H₅OH gas, which is about 13 times higher than that of pure SnO₂ nanorods, (1.1 × 10³) at the operating temperature of 450 °C. Moreover, for 50 ppm C₂H₅OH gas, the 5.0Ni:SnO₂ nanorod sensor still recorded a significant response reading, namely 2.0 × 10³ with a response time of 30 s and recovery time of 10 min. To investigate the effect of Ni dopant (0.5-5.0 mol%) on SnO₂ nanorods, structural characterizations were demonstrated using field emission scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, x-ray diffraction (XRD) analysis, x-ray photoelectron spectroscopy and an ultraviolet-visible spectrometer. XRD results confirmed that all the samples consisted of tetragonal-shaped rutile SnO₂ nanorods. It was found that the average diameter and length of the nanorods formed in 5.0Ni:SnO₂ were four times smaller (∼6 and ∼35 nm, respectively) than those of the nanorods formed in pure SnO₂ (∼25 and 150 nm). Interestingly, both samples had the same aspect ratio, ∼6. It is proposed that the high response of the 5.0Ni:SnO₂ nanorod sensor can be attributed to the particle size, which causes an increase in the thickness of the charge depletion layer, and the presence of oxygen vacancies within the matrix of SnO₂ nanorods.

Ultrasensitive Detection of Volatile Organic Compounds by a Pore Tuning Approach Using Anisotropically Shaped SnO2 Nanocrystals

Gas sensing with oxide nanostructures is increasingly important to detect gaseous compounds for safety monitoring, process controls, and medical diagnostics. For such applications, sensor sensitivity is one major criterion. In this study, to sensitively detect volatile organic compounds (VOCs) at very low concentrations, we fabricated porous films using SnO₂ nanocubes (13 nm) and nanorods with different rod lengths (50-500 nm) that were synthesized by a hydrothermal method. The sensor response to H₂ increased with decreasing crystal size; the film made of the smallest nanocubes showed the best sensitivity, which suggested that the H₂ sensing is controlled by crystal size. In contrast, the responses to ethanol and acetone increased with increasing crystal size and resultant pore size; the highest sensitivity was observed for a porous film using the longest nanorods. Using the Knudsen diffusion-surface reaction equation, the gas sensor responses to ethanol and acetone were simulated and compared with experimental data. The simulation results proved that the detection of ethanol and acetone was controlled by pore size. Finally, we achieved ultrahigh sensitivity to ethanol; the sensor response (S) exceeded S = 100 000, which corresponds to an electrical resistance change of 5 orders of magnitude in response to 100 ppm of ethanol at 250 °C. The present approach based on pore size control provides a basis for designing highly sensitive films to meet the criterion for practical sensors that can detect a wide variety of VOCs at ppb concentrations.

Flexible electronics based on inorganic nanowires

This review summarizes the latest research for exploiting the flexible electronic applications of inorganic nanowires.