Efficient Photorecovery of Noble Metals from Solution Using a γ‑SiW₁₀O₃₆/Surfactant Hybrid Photocatalyst

In recent years, the recovery of noble metals from waste has become very important because of their scarcity and increasing consumption. In this study, we attempt the photochemical recovery of noble metals from solutions using inorganic–organic hybrid photocatalysts. These catalysts are based on polyoxometalates such as PMo₁₂O₄₀^3‑, SiW₁₂O₄₀^4‑, and γ-SiW₁₀O₃₆^8‑ coupled with a cationic surfactant, dimethyldioctadecylammonium (DODA). The three different photocatalysts dissolved in chloroform were successful in photoreducing gold ions dissolved in water in a two-phase (chloroform/water) system under UV irradiation (λ < 475 nm). The γ-SiW₁₀O₃₆/DODA photocatalyst exhibited the best activity and recovered gold from solution efficiently. It was suggested that one-electron reduced γ-SiW₁₀O₃₆^9‑ formed by the UV irradiation reduced gold ions. As a result, large two-dimensional particles (gold nanosheets) were produced using the γ-SiW₁₀O₃₆/DODA photocatalyst, indicating that the reduction of gold ions occurred at the interface between chloroform and water. The γ-SiW₁₀O₃₆/DODA photocatalyst was able to recover metals such as platinum, silver, palladium, and copper from deaerated solutions. The selective recovery of gold is possible by controlling pH and oxygen concentration in the reaction system

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 (<i>S</i>) exceeded <i>S</i> = 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

WO₃ Nanolamella Gas Sensor: Porosity Control Using SnO₂ Nanoparticles for Enhanced NO₂ Sensing

Tungsten trioxide (WO₃) is one of the important multifunctional materials used for photocatalytic, photoelectrochemical, battery, and gas sensor applications. Nanostructured WO₃ holds great potential for enhancing the performance of these applications. Here, we report highly sensitive NO₂ sensors using WO₃ nanolamellae and their sensitivity improvement by morphology control using SnO₂ nanoparticles. WO₃ nanolamellae were synthesized by an acidification method starting from Na₂WO₄ and H₂SO₄ and subsequent calcination at 300 °C. The lamellae were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), which clearly showed the formation of single-crystalline nanolamellae with a <i>c</i>-axis orientation. The stacking of each nanolamella to form larger lamellae that were 50–250 nm in lateral size and 15–25 nm in thickness was also revealed. From pore size distribution measurements, we found that introducing monodisperse SnO₂ nanoparticles (ca. 4 nm) into WO₃ lamella-based films improved their porosity, most likely because of effective insertion of nanoparticles into lamella stacks or in between assemblies of lamella stacks. In contrast, the crystallite size was not significantly changed, even by introducing SnO₂. Because of the improvement in porosity, the composites of WO₃ nanolamellae and SnO₂ nanoparticles displayed enhanced sensitivity (sensor response) to NO₂ at dilute concentrations of 20–1000 ppb in air, demonstrating the effectiveness of microstructure control of WO₃ lamella-based films for highly sensitive NO₂ detection. Electrical sensitization by SnO₂ nanoparticles was also considered

Graphene Oxide Membrane Reactor for Electrochemical Deuteration Reactions

The deuteration of organic molecules is considerably important in organic and medicinal chemistry. An electrochemical membrane reactor using proton-conducting graphene oxide (GO) nanosheets was developed to synthesize valuable deuterium-labeled products via an efficient hydrogen-to-deuterium (H/D) exchange under mild conditions at ambient temperature and atmospheric pressure. Deuterons (D+) formed by the anodic oxidation of heavy water (D2O) at the Pt/C anode permeate through the GO membrane to the Pt/C cathode, where organic molecules with functional groups (CC and CO) are deuterated with adsorbed atomic D species. Deuteration occurs in outstanding yields with high levels of D incorporation. We also achieved the electrodeuteration of a drug molecule, ibuprofen, demonstrating the promising feasibility of the GO membrane reactor in the pharmaceutical industry

Solid Electrolyte Gas Sensor Based on a Proton-Conducting Graphene Oxide Membrane

Graphene oxide (GO) is an ultrathin carbon nanosheet with various oxygen-containing functional groups. The utilization of GO has attracted tremendous attention in a number of areas, such as electronics, optics, optoelectronics, catalysis, and bioengineering. Here, we report the development of GO-based solid electrolyte gas sensors that can continuously detect combustible gases at low concentrations. GO membranes were fabricated by filtration using a colloidal solution containing GO nanosheets synthesized by a modified Hummers’ method. The GO membrane exposed to humid air showed good proton-conducting properties at room temperature, as confirmed by hydrogen concentration cell measurements and complex impedance analyses. Gas sensor devices were fabricated using the GO membrane fitted with a Pt/C sensing electrode. The gas-sensing properties were examined by potentiometric and amperometric techniques. The GO sensor showed high, stable, and reproducible responses to hydrogen at parts per million concentrations in humid air at room temperature. The sensing mechanism is explained in terms of the mixed-potential theory. Our results suggest the promising capability of GO for the electrochemical detection of combustible gases

Pulse-Driven Micro Gas Sensor Fitted with Clustered Pd/SnO₂ Nanoparticles

Real-time monitoring of specific gas concentrations with a compact and portable gas sensing device is required to sense potential health risk and danger from toxic gases. For such purposes, we developed an ultrasmall gas sensor device, where a micro sensing film was deposited on a micro heater integrated with electrodes fabricated by the microelectromechanical system (MEMS) technology. The developed device was operated in a pulse-heating mode to significantly reduce the heater power consumption and make the device battery-driven and portable. Using clustered Pd/SnO₂ nanoparticles, we succeeded in introducing mesopores ranging from 10 to 30 nm in the micro gas sensing film (area: ϕ 150 μm) to detect large volatile organic compounds (VOCs). The micro sensor showed quick, stable, and high sensor responses to toluene at ppm (parts per million) concentrations at 300 °C even by operating the micro heater in a pulse-heating mode where switch-on and -off cycles were repeated at one-second intervals. The high performance of the micro sensor should result from the creation of efficient diffusion paths decorated with Pd sensitizers by using the clustered Pd/SnO₂ nanoparticles. Hence we demonstrate that our pulse-driven micro sensor using nanostructured oxide materials holds promise as a battery-operable, portable gas sensing device

Solution-Processed Cu₂ZnSnS₄ Nanocrystal Solar Cells: Efficient Stripping of Surface Insulating Layers Using Alkylating Agents

Solution-processed photovoltaic (PV) devices based on semiconductor nanocrystals (NCs) such as Cu₂ZnSnS₄ (CZTS) and CuInS₂ (CIS) are attracting much attention for use in next-generation solar cells. However, the performance of NC-based devices is hindered by insulating surface-capping ligands that limit transfer/transport of charged carriers. Here, to remove surface-capping ligands (long-chain fatty amines) from NCs, we use the strong alkylating agent methyl iodide, which converts primary amines to quaternary amines that have low coordinating affinity to the NC surface. X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy analyses confirm the successful removal of capping ligands from the CZTS surface after treatment with methyl iodide without changing the crystal structure of CZTS. CZTS and CIS NC-based devices treated with methyl iodide exhibit a reproducible PV response under simulated sunlight. The developed route can potentially enhance the performance of NC-based devices used in a broad range of applications

Tunable Graphene Oxide Proton/Electron Mixed Conductor that Functions at Room Temperature

Graphene oxide (GO) and reduced graphene oxide exhibit proton and electron (or hole) conduction, respectively. Owing to this, the conductivity of GO can be controlled via reduction because its electron conductivity increases and its proton conductivity depends on the concentration of epoxide groups. Herein, we report the successful control of the proton and electron conductivities of GO using the photoirradiation and thermal reduction processes. The proton conductivity decreases when the epoxide content and layer distance decreases, whereas the electron conductivity drastically increases with decreasing oxygen content. Both the electron and proton conduction mechanisms for GO are discussed based on the concentrations of various functional groups and defects, changes in the interlayer distance, and the activation energy associated with proton conduction. Finally, we determined the most suitable degree of reduction for obtaining a good mixed conductor useful as an electrode material and a hydrogen separation membrane that functions at room temperature

Water Vapor Electrolysis with Proton-Conducting Graphene Oxide Nanosheets

Hydrogen production by membrane water electrolysis has attracted tremendous attention because of its benefits, which include easy separation of hydrogen and oxygen, no carbon emissions, and the possibility to store hydrogen fuel as an electricity source. Here, we study water vapor electrolysis using a proton-conducting membrane comprising graphene oxide (GO) nanosheets. The GO membrane shows good through-plane proton conductivity, as confirmed by concentration-cell measurements, complex impedance spectroscopy, and hydrogen pumping experiments. The results also confirm that most carriers in the GO membrane are protons. The GO membrane fitted with Pt/C and IrO₂–Al₂O₃ as the cathode and the anode, respectively, efficiently electrolyzes humidified air to produce hydrogen and oxygen at room temperature, which indicates bright prospects for this carbon-based electrochemical device