Catalytic Overlayer Research Articles

Exclusive detection of ethylene using metal oxide chemiresistors with a Pd–V2O5–TiO2 yolk–shell catalytic overlayer via heterogeneous Wacker oxidation

A bilayer design based on yolk–shell structured Pd loaded V2O5–TiO2 and hollow structured In2O3 showed unprecedentedly high selectivity and response toward ethylene through concurrent Wacker oxidation of ethylene and filtering of interfering gases.

Ru/Al2O3 on Polymer-Derived SiC Foams as Structured Catalysts for CO2 Methanation

The catalytic methanation of CO2 via the strongly exothermic equilibrium Sabatier reaction requires the development of structured catalysts with enhanced mass- and heat-transfer features to limit hot-spot formation, avoid catalyst deactivation, and control process selectivity. In this work, we investigated the use of polymer-derived SiC open-cell foams as structured carriers onto which γ-Al2O3 was applied by either dip-coating or pore-filling methods; eventually, Ru was dispersed by impregnation. The formation of an undesired insulating SiO2 layer on the surface of the SiC struts was prevented by a pyrolysis treatment under an inert atmosphere at temperatures varying from 800 up to 1800 °C. SiC foam substrates and their corresponding structured catalysts were characterized by SEM, XRD, N2 physisorption, and compressive strength measurements, and their CO2 methanation activity was tested at atmospheric pressure in a fixed bed flow reactor operated in the temperature range from 200 to 450 °C. SiC foams obtained at intermediate pyrolysis temperatures (1000–1200 °C) showed good mechanical strength and high compatibility with the Ru/Al2O3 active catalytic overlayer.

Reduced Electrocatalyst Loading on Vertical Graphene for Lithium Sulfur Batteries

Lithium–sulfur batteries (LSBs) represent a promising energy conversion and storage technique for a multitude of applications, ranging from stationary grid storage to mobile electric vehicles and electronic devices. However, the dissolution of intermediate lithium polysulfides (LPSs) into the electrolyte causes the shuttle phenomenon of the soluble LiPSs and the subsequent adverse effects that result in low columbic efficiency and poor cycling stability, among other issues. Numerous methods have been investigated to attack the polysulfide shuttle effect, from physically blocking the diffusion of LPS to chemically immobilize the LPS to catalytic surface[1]. The later strategy is more effective in curbing the LPS diffusion and can also facilitate the LPS redox reaction by reducing their active energies. In this research, one of the promising catalytic materials, nickel cobalt sulfide (NiCo2S4, NCS) was investigated as catalyst to enhance the LSB performance by grafting it to vertical graphene (VG) grown on carbon nanofiber film (CNF) substrate. Different from other reported metal sulfides materials[2], the studied NCS nanosheets possess ultrathin and edge oriented structure, which is realized by the structural regulation function of VG. As a result, this NCS/VG material not only significantly reduces the utilization of NCS catalyst but also enormously improves the conductivity of the hybrid material. When using this free-standing film as catalytic overlayer on top of sulfur cathode, the derived LSB exhibited attractive electrochemical performances of 1000 mAh g-1 specific capacity at 0.3C and 800 mAh g-1 at 1C within 400 cycles for 2 mg cm-2 of sulfur loading and 850 mAh g-1 specific capacity at 0.25C for 6 mg cm-2 of sulfur loading. An areal capacity of 4.5 mAh cm-2 was demonstrated at the 100th cycle at 0.3 C.[1] S. Li, Z. Fan, Encapsulation methods of sulfur particles for lithium-sulfur batteries: A review. Energy Storage Materials, 2020, 34, 107-127.[2] M. Zhang, W. Chen, L. Xue, Y. Jiao, et al., Adsorption-Catalysis Design in the Lithium-Sulfur Battery. Adv. Energy Mater., 2020, 10, 1903008.

(Plenary) Rational Design of Oxide Chemiresistors for Next-Generation Gas Sensors and Artificial Olfaction

Oxide semiconductor chemiresistors have been widely used to detect harmful gases because of their advantages such as high sensitivity, facile integration, and cost effectiveness, and their applications are being expanded to pollutant monitoring, medical diagnosis from exhaled breath, food quality assessment, and smart farming. The collection of big gas sensing data becomes easier than before thanks to the progresses in wireless communication, Internet of Things, and sensor integration. Furthermore, machine learning algorithm and enhanced computing capability are supporting the artificial olfaction. Despite the growth of market demand and relevant technology, oxide semiconductor gas sensors and electronic noses using their array need further improvement since oxide-based chemiresistors often show low gas selectivity and the sensors in the array are frequently insufficient to cover all the complex chemicals. To date, various nanostructures with high surface area to volume ratio have been explored to enhance the gas response and sensing materials have been loaded with noble metal or oxide catalysts for tailoring gas selectivity. Nevertheless, for demand-based design of high performance gas sensors and artificial olfaction, many issues still remain unsolved, which include the detection of ultralow concentration of analyte gas, highly selective detection of a specific gas, moisture-independent gas sensing, and the establishment of distinctive gas sensing library toward numerous analyte gases as well as complex odors [1]. How can we overcome above challenges? The diversity of gas sensing materials provides a rational solution for the complexity of chemicals, which can be accomplished by introducing new sensing materials, designing new nano-architectures, making new methods to tune the reforming/oxidation of analyte/interference gases, establishing new strategies to maximize/control catalytic activity, and using different ways to control electronic/chemical sensitization. In this presentation, various new, promising and general strategies to design sensitive, selective, and reliable chemiresistors for next-generation gas sensors and artificial olfaction will be suggested, which are as follows: Hollow and hierarchical nanostructures with high gas accessibility for rapid gas detection [2]P-type oxide semiconductors with high catalytic activity for detecting new gas species [3]Multi-valence additives for humidity-independent oxide semiconductor gas sensors [4,5]Catalyst-loaded or catalytic micro-reactors for highly selective detection of aromatic compounds [6]Bilayer sensors consisting of sensing layer and nanoscale catalytic overlayer for separating catalytic and sensing reactions and modulating gas sensing behaviors [7,8]Hetero-nanostructures composed of two different materials with dissimilar catalytic activity [9]Monolayer oxide nanofiber sensor with tailored exposure of catalytic electrode for controlling gas selectivity [10] References [1] S.-Y. Jeong, J.-S. Kim, J.-H. Lee, Rational Design of Semiconductor‐Based Chemiresistors and their Libraries for Next‐Generation Artificial Olfaction, Adv. Mater. (online published) doi.org/10.1002/adma.202002075[2] J.-H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: Overview, Sens. Actuators B 140 (2009) 319-336. doi:10.1016/j.snb.2009.04.026.[3] H.-J. Kim, J.-H. Lee, Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview, Sens. Actuators B 192 (2014) 607-627. doi:10.1016/j.snb.2013.11.005.[4] H.-R. Kim, A. Haensch, I.-D. Kim, U. Weimar, J.-H. Lee, Role of NiO doping in reducing the humidity impact on the performance of SnO2-based gas sensors: synthesis strategies, phenomenological and spectroscopic studies, Adv. Funct. Mater. 21 (2011) 4229-4240. doi:10.1002/adfm.201101154.[5] J.‐W. Yoon, J.‐S. Kim, T.‐H. Kim, Y. J. Hong, Y. C. Kang, J.‐H. Lee, A new strategy for humidity independent oxide chemiresistors: Dynamic self‐refreshing of In2O3 sensing surface assisted by layer‐by‐layer coated CeO2 nanoclusters, Small 12 (2016) 4456-4463. doi:10.1002/smll.201601507.[6] J.-W. Yoon, Y.J. Hong, G.D. Park, S.-J. Hwang, F. Abdel-Hady, A.A.Wazzan, Y.C. Kang, J.-H.Lee, Kilogram-scale synthesis of Pd-loaded quintuple-shelled Co3O4 microreactors and their application to ultrasensitive and ultraselective detection of methylbenzenes, ACS Appl. Mater. Interfaces 7 (2015) 7717-7723. doi: 10.1021/acsami.5b00706.[7] S.-Y.Jeong, Y.K. Moon, T.-H. Kim, .S-W. Park, K. B. Kim, Y.C. Kang, J.-H. Lee, A New Strategy for Detecting Plant Hormone Ethylene Using Oxide Semiconductor Chemiresistors: Exceptional Gas Selectivity and Response Tailored by Nanoscale Cr2O3 Catalytic Overlayer ethylene sensor Adv. Sci. 7 (2020) 1903093. 10.1002/advs.201903093[8] S.‐Y. Jeong, Y.K. Moon, J.K. Kim, S.‐W. Park, Y.K. Jo, Y.C. Kang, J.‐H. Lee, A General Solution to Mitigate Water Poisoning of Oxide Chemiresistors: Bilayer Sensors with Tb4O7 Overlayer, Adv. Funct. Mater. (online published) 10.1002/adfm.202007895[9] T.-H. Kim, C.-H. Kwak, J.-H. Lee, NiO/NiWO4 composite yolk−shell spheres with nanoscale NiO outer layer for ultrasensitive and selective detection of sub ppm-level p-xylene, ACS Appl. Mater. Interfaces 9 (2017) 32034-32043. doi:10.1021/acsami.7b10294.[10] K. Lim, Y.-M. Jo, J.-W. Yoon, J.-H. Lee, Metal oxide patterns of one-dimensional nanofibers: on-demand, direct-write fabrication, and application as a novel platform for gas detection, J. Mater. Chem. A 7 (2019) 24919-24928. doi:10.1039/C9TA09708B.

Tailoring of Gas Sensing Characteristics Using Bilayer Sensor with Oxide Semiconductor Sensing Layer and Nanoscale Au Catalytic Overlayer

Introduction Gas sensing characteristics of oxide semiconductor chemiresistors can be enhanced or controlled by loading noble metal catalysts to the entire sensing materials. Although uniform catalyst loading generally promotes the sensing reaction between analyte gas and ionized oxygen on the surface, it may induce other adverse effects such as 1) the excessive increase of sensor resistance depending on work function of metal catalyst, 2) the gas oxidation at the upper part of sensing film, and 3) the gas reforming during the gas transport within sensing film. Excessively high sensor resistance hampers the cost-effective signal measurement using conventional electric circuits, while it is difficult to separate the gas sensing, reforming, and oxidation reactions, leading to low controllability of gas sensing characteristics. Herein, a novel bilayer design with an oxide chemiresistors sensing layer (SnO2, ZnO, and Co3O4) and nanoscale catalytic Au overlayer was suggested to solve above problems. Bilayer sensor design effectively separated the sensing and catalytic reaction, leading to the excellent control of gas sensing characteristics without affecting the sensor resistance in air. Method Sensing materials SnO2 hollow spheres were prepared using ultrasonic spray pyrolysis of mixed solution consisting of 0.1 M of Tin (II) chloride dihydrate (SnCl2•2H2O, ≥ 99.99%) and 0.025 M of citric acid monohydrate (C6H8O7•H2O, ≥ 99.0%). Droplets of this source solution were then generated by six ultrasonic transducers (resonance frequency: 1.7 MHz) and transferred into a quartz tube (inner diameter: 55 mm) in an electric furnace heated to 700 ºC in air (flow rate : 10 L/min) for pyrolysis. As-prepared precursor powders were converted into SnO2 hollow spheres via heat treatment at 600 ºC for 2 h in air. SnO2 sensing films were fabricated by screen printing onto Pt interdigitated electrode (IDE) (electrode gap: 100 mm) patterns on SiO2/Si substrates (size = 8 mm x 8 mm). Catalytic Au nanoparticles were deposited on the SnO2 thick films through e-beam evaporation of Au grains (99.998%) in a vacuum chamber (below 2×10-6 Torr) at a rate of 0.1 Å sec−1 (nominal thicknesses: 0.5, 1, and 3 nm), as calibrated by a quartz crystal microbalance, which were converted into discrete configurations of Au nanoparticles through heat treatment at 550 ºC for 2 h. Additionally, SnO2 thick films with 1-nm-thick Au overlayer were also heat treated at 550 ºC for 0.5 and 4 h to investigate the effects of Au configuration on gas sensing characteristics. Results and Conclusions The configuration of Au nanoparticle overlayer was changed by controlling the Au-coating thickness and annealing time and investigated the gas sensing characteristics of SnO2 sensors with various configurations of Au nanoparticles overlayer. The Au nanoparticle overlayer significantly enhanced the methylbenzene selectivity and response of SnO2 thick film sensor by reforming of less reactive methylbenzenes into more reactive species and suppressing the cross-responses to relatively reactive interference gases (e.g. ethanol and HCHO) through oxidative filtering. Furthermore, the methylbenzene selectivity and response of ZnO and Co3O4 were also significantly enhanced by introducing the Au nanoparticle overlayer, confirming the general validity on the role of Au nanoparticle overlayer. In all the sensing films, the gas transport as well as sensor resistance is hardly influenced by nanoscale catalytic Au overlayer, demonstrating that the bilayer sensor is promising platform to control the gas sensing characteristics by separating catalytic reaction and sensing reaction and by tuning the catalyst-induced gas reforming and oxidation reaction. These sensors can be used for various sensor applications with new functionality.

3-D Edge-Oriented Electrocatalytic NiCo 2 S 4 Nanoflakes on Vertical Graphene for Li-S Batteries

Polysulfide shuttle effect, causing extremely low Coulombic efficiency and cycling stability, is one of the toughest challenges hindering the development of practical lithium sulfur batteries (LSBs). Introducing catalytic nanostructures to stabilize the otherwise soluble polysulfides and promote their conversion to solids has been proved to be an effective strategy in attacking this problem, but the heavy mass of catalysts often results in a low specific energy of the whole electrode. Herein, by designing and synthesizing a free-standing edge-oriented NiCo 2 S 4 /vertical graphene functionalized carbon nanofiber (NCS/EOG/CNF) thin film as a catalytic overlayer incorporated in the sulfur cathode, the polysulfide shuttle effect is largely alleviated, revealed by the enhanced electrochemical performance measurements and the catalytic function demonstration. Different from other reports, the NiCo 2 S 4 nanosheets synthesized here have a 3-D edge-oriented structure with fully exposed edges and easily accessible in-plane surfaces, thus providing a high density of active sites even with a small mass. The EOG/CNF scaffold further renders the high conductivity in the catalytic structure. Combined, this novel structure, with high sulfur loading and high sulfur fraction, leads to high-performance sulfur cathodes toward a practical LSB technology.

3-D Edge-Oriented Electrocatalytic NiCo2S4 nanoflakes on Vertical Graphene for Lithium Sulfur Batteries

Polysulfide shuttling between the sulfur cathode and the lithium anode leads to low Columbic efficiency and cycling stability, which hinders the development of practical lithium sulfur batteries (LSBs). Introducing catalytic nanostructures to stabilize the otherwise soluble polysulfides and promote their conversion to solids has been proved to be an effective strategy in addressing this problem, but the heavy mass of catalysts often results in a low specific energy of the whole electrode.In this work, nickel cobalt sulfide (NiCo2S4, NCS), one of the promising catalytic materials, was investigated to enhance the LSB performance by grafting it to vertical graphene (VG) grown on carbon nanofiber film(CNF) substrate. Different from previous studies, here NCS nanosheets possess ultrathin and edge-oriented structure, inherited from the underlying VG. As a result, this NCS/VG material not only significantly reduces the needed mass of NCS catalyst but also enormously improves the conductivity of the hybrid structure. When using this free-standing film as catalytic overlayer on top of the sulfur cathode, the polysulfide shuttle effect is largely alleviated, as revealed by the enhanced electrochemical performance and the catalytic function demonstration. The assembled LSB exhibited attractive electrochemical performances of 1000 mAh g-1 specific capacity at 0.3C and 800 mAh g-1 at 1C within 400 cycles for 2 mg cm-2of sulfur loading and 850 mAh g-1 specific capacity at 0.25C for 6 mg cm-2 of sulfur loading. An areal capacity of 4.5 mAh cm-2 was demonstrated at the 100th cycle at 0.3 C.

Tailoring of Gas Sensing Characteristics Using Bilayer Sensor with Oxide Semiconductor Sensing Layer and Nanoscale Au Catalytic Overlayer

Introduction Gas sensing characteristics of oxide semiconductor chemiresistors can be enhanced or controlled by loading noble metal catalysts to the entire sensing materials. Although uniform catalyst loading generally promotes the sensing reaction between analyte gas and ionized oxygen on the surface, it may induce other adverse effects such as 1) the excessive increase of sensor resistance depending on work function of metal catalyst, 2) the gas oxidation at the upper part of sensing film, and 3) the gas reforming during the gas transport within sensing film. Excessively high sensor resistance hampers the cost-effective signal measurement using conventional electric circuits, while it is difficult to separate the gas sensing, reforming, and oxidation reactions, leading to low controllability of gas sensing characteristics. Herein, a novel bilayer design with an oxide chemiresistors sensing layer (SnO2, ZnO, and Co3O4) and nanoscale catalytic Au overlayer was suggested to solve above problems. Bilayer sensor design effectively separated the sensing and catalytic reaction, leading to the excellent control of gas sensing characteristics without affecting the sensor resistance in air. Method Sensing materials SnO2 hollow spheres were prepared using ultrasonic spray pyrolysis of mixed solution consisting of 0.1 M of Tin (II) chloride dihydrate (SnCl2•2H2O, ≥ 99.99%) and 0.025 M of citric acid monohydrate (C6H8O7•H2O, ≥ 99.0%). Droplets of this source solution were generated by ultrasonic transducers (resonance frequency: 1.7 MHz) and transferred into a quartz tube (inner diameter: 55 mm) in an electric furnace heated to 700 ºC in air (flow rate : 10 L/min) for pyrolysis. As-prepared precursor powders were converted into SnO2 hollow spheres via heat treatment at 600 ºC for 2 h in air. SnO2 sensing films were fabricated by screen printing onto Pt interdigitated electrode (IDE) (electrode gap: 100 µm) patterns on SiO2/Si substrates (size = 8 mm x 8 mm). Catalytic Au nanoparticles were deposited on the SnO2 thick films through e-beam evaporation of Au grains (99.998%) in a vacuum chamber (below 2×10-6 Torr) at a rate of 0.1 Å sec−1 (nominal thicknesses: 0.5, 1, and 3 nm), which were converted into discrete configurations of Au nanoparticles through heat treatment at 550 ºC for 2 h. Additionally, SnO2 thick films with 1-nm-thick Au overlayer were also heat treated at 550 ºC for 0.5 and 4 h to investigate the effects of Au configuration on gas sensing characteristics. Results and Conclusions The configuration of Au nanoparticle overlayer was changed by controlling the Au-coating thickness and annealing time and investigated the gas sensing characteristics of SnO2 sensors with various configurations of Au nanoparticles overlayer. The Au nanoparticle overlayer significantly enhanced the methylbenzene selectivity and response of SnO2 thick film sensor by reforming of less reactive methylbenzenes into more reactive species and suppressing the cross-responses to relatively reactive interference gases (e.g. ethanol and HCHO) through oxidative filtering. Furthermore, the methylbenzene selectivity and response of ZnO and Co3O4 were also significantly enhanced by introducing the Au nanoparticle overlayer, confirming the general validity on the role of Au nanoparticle overlayer. In all the sensing films, the gas transport as well as sensor resistance is hardly influenced by nanoscale catalytic Au overlayer, demonstrating that the bilayer sensor is promising platform to control the gas sensing characteristics by separating catalytic reaction and sensing reaction and by tuning the catalyst-induced gas reforming and oxidation reaction. These sensors can be used for various sensor applications with new functionality.

Ethylene Sensors: A New Strategy for Detecting Plant Hormone Ethylene Using Oxide Semiconductor Chemiresistors: Exceptional Gas Selectivity and Response Tailored by Nanoscale Cr2O3 Catalytic Overlayer (Adv. Sci. 7/2020)

Ethylene Sensors: A New Strategy for Detecting Plant Hormone Ethylene Using Oxide Semiconductor Chemiresistors: Exceptional Gas Selectivity and Response Tailored by Nanoscale Cr₂O₃ Catalytic Overlayer (Adv. Sci. 7/2020)

A New Strategy for Detecting Plant Hormone Ethylene Using Oxide Semiconductor Chemiresistors: Exceptional Gas Selectivity and Response Tailored by Nanoscale Cr2O3 Catalytic Overlayer.

A highly selective and sensitive detection of the plant hormone ethylene, particularly at low concentrations, is essential for controlling the growth, development, and senescence of plants, as well as for ripening of fruits. However, this remains challenging because of the non‐polarity and low reactivity of ethylene. Herein, a strategy for detecting ethylene at a sub‐ppm‐level is proposed by using oxide semiconductor chemiresistors with a nanoscale oxide catalytic overlayer. The SnO2 sensor coated with the nanoscale catalytic Cr2O3 overlayer exhibits rapid sensing kinetics, good stability, and an unprecedentedly high ethylene selectivity with exceptional gas response (R a/R g − 1, where R a represents the resistance in air and R g represents the resistance in gas) of 16.8 at an ethylene concentration of 2.5 ppm at 350 °C. The sensing mechanism underlying the ultraselective and highly sensitive ethylene detection in the unique bilayer sensor is systematically investigated with regard to the location, configuration, and thickness of the catalytic Cr2O3 overlayer. The mechanism involves the effective catalytic oxidation of interfering gases into less‐ or non‐reactive species, without limiting the analyte gas transport. The sensor exhibits a promising potential for achieving a precise quantitative assessment of the ripening of five different fruits.

Water Oxidation Catalysts from Waste Metal Resources: A Facile Metal-Organic Electrochemical Approach

Abstract A fabrication route is presented by a novel metal-organic electrochemical approach, allowing facile preparation of electrocatalytic metal and metal oxide thin films from solid metals at room temperature: divalent transition metals such as iron, nickel and cobalt can be deposited as amorphous oxyhydroxide films. Among them, nickel- and cobalt-prepared samples are showing high activity as water oxidation catalysts in alkaline electrolytes. The applicability to waste metal material is exemplified using one of the most abundant waste metal alloys, i.e. steel: with a suitable multi-layer architecture, comprising, a large surface-area iron oxyhydroxide core-geometry and a steel-derived catalytic overlayer, the overpotential for oxygen evolution (at 10 mA cm−2) could be improved to only 370 mV. Chemical analysis is provided to elucidate the reaction pathway, encompassing metal halogenation, formation of meta-stable metal-organic intermediates or direct electrochemical reduction, respectively. Structural peculiarities of the derived films are finally demonstrated upon development of a photoactive nickel oxyhydroxide/silicon junction realizing a photocurrent of 1 mA cm−2 at the thermodynamic potential for oxygen evolution and short-term stabilization in the range of several hours.

Metal Oxide Gas Sensors with Au Nanocluster Catalytic Overlayer: Toward Tuning Gas Selectivity and Response Using a Novel Bilayer Sensor Design.

Noble metals or oxide catalysts have traditionally been loaded or doped to enhance the gas sensing properties of oxide semiconductor chemiresistors. However, the selective detection of various chemicals for a wide range of new applications remains a challenging problem. In this paper, we propose a novel bilayer design with an oxide chemiresistor sensing layer and nanoscale catalytic Au overlayer to provide high controllability for gas sensing characteristics. The Au nanocluster overlayer significantly enhances the methylbenzene response of a SnO2 thick film sensor by reforming gases into more reactive species and suppresses the responses to reactive interference gases through oxidative filtering, leading to excellent selectivity to methylbenzene. Gas sensing characteristics can be tuned by controlling the morphology, amount, and number density of Au nanoclusters through the variation in the Au coating thickness (0.5-3 nm) and thermal annealing conditions (0.5-4 h at 550 °C). Furthermore, the general validity of the proposed Au-coated bilayer sensor design was confirmed through the enhancement of response and selectivity toward methylbenzenes by coating Au nanoclusters onto ZnO and Co3O4 sensors. The sensing mechanism, advantages, and potential applications of bilayer sensors are discussed from the perspective of the separation of sensing and catalytic reactions, as well as the reforming and oxidation of analyte gases in association with the configuration of the sensing layer and Au catalytic overlayer.

Metal Oxide/(oxy)hydroxide Overlayers as Hole Collectors and Oxygen-Evolution Catalysts on Water-Splitting Photoanodes.

Solarwatersplitting provides a mechanism to convert and store solar energy in the form of stable chemical bonds. Water-splitting systems often include semiconductor photoanodes, such as n-Fe2O3 and n-BiVO4, which use photogenerated holes to oxidize water. These photoanodes often exhibit improved performance when coated with metal-oxide/(oxy)hydroxide overlayers that are catalytic for the water-oxidation reaction. The mechanism for this improvement, however, remains a controversial topic. This is, in part, due to a lack of experimental techniques that are able to directly track the flow of photogenerated holes in such multicomponent systems. In this Perspective, we illustrate how this issue can be addressed by using a second working electrode to make direct current/voltage measurements on the catalytic overlayer during operation in a photoelectrochemical cell. We discuss examples where the second working electrode is a thin metallic film deposited on the catalyst layer, as well as where it is the tip of a conducting atomic-force-microscopy probe. In applying these techniques to multiple semiconductors (Fe2O3, BiVO4, Si) paired with various metal-(oxy)hydroxide overlayers (e.g., Ni(Fe)O xH y and CoO xH y), we found in all cases investigated that the overlayers collect photogenerated holes from the semiconductor, charging to potentials sufficient to drive water oxidation. The overlayers studied thus form charge-separating heterojunctions with the semiconductor as well as serve as water-oxidation catalysts.

Substantial effect of palladium overlayer deposition on the H2 sensing performance of TiO2/ITO nanocomposite

The fabrication of hybrid Pd overlayerd TiO2/ITO nanocomposite films by a novel two step deposition process was demonstrated. The influence of Pd overlayer deposition on the structural, morphological and optical properties of the composite were investigated and optimized for use as H2 sensor. The sample prepared with 4 min Pd deposition time revealed nanospherical grains with coconut grated morphology favorable for H2 adsorption. The reduced band gap of optimized samples was attributed to the large crystallite size exhibited by the Pd: TiO2 particles. The response of the TiO2/ITO sensor with Pd catalytic overlayer on the sensitivity was observed. The fast recovery time of 15 s recorded for our sensor was attributed to the desorption activity of the nano porous Pd/TiO2 sensing layer achieved by multilayer deposition.

Gas Selectivity Control in Co3O4 Sensor via Concurrent Tuning of Gas Reforming and Gas Filtering using Nanoscale Hetero-Overlayer of Catalytic Oxides.

Co3O4 sensors with a nanoscale TiO2 or SnO2 catalytic overlayer were prepared by screen-printing of Co3O4 yolk-shell spheres and subsequent e-beam evaporation of TiO2 and SnO2. The Co3O4 sensors with 5 nm thick TiO2 and SnO2 overlayers showed high responses (resistance ratios) to 5 ppm xylene (14.5 and 28.8) and toluene (11.7 and 16.2) at 250 °C with negligible responses to interference gases such as ethanol, HCHO, CO, and benzene. In contrast, the pure Co3O4 sensor did not show remarkable selectivity toward any specific gas. The response and selectivity to methylbenzenes and ethanol could be systematically controlled by selecting the catalytic overlayer material, varying the overlayer thickness, and tuning the sensing temperature. The significant enhancement of the selectivity for xylene and toluene was attributed to the reforming of less reactive methylbenzenes into more reactive and smaller species and oxidative filtering of other interference gases, including ubiquitous ethanol. The concurrent control of the gas reforming and oxidative filtering processes using a nanoscale overlayer of catalytic oxides provides a new, general, and powerful tool for designing highly selective and sensitive oxide semiconductor gas sensors.

Deposition of Nickel on Electrodeposited Cu2O Films at Potentials More Positive than the Nernst Potential of Ni2+/Ni0

Deposition of nickel on electrodeposited Cu2O films was investigated at potentials more positive than the estimated Nernst equilibrium potential of Ni2+/Ni0 for the purpose of developing a monolayer deposition method that will act as a protective and catalytic overlayer on the oxide films. Cu2O films were subjected to 0.08 mM nickel acetate solutions for 60 s at potentials more positive than the estimated Nernst equilibrium potential of Ni2+/Ni0. The quantities of Ni recovered from the Cu2O films were determined by atomic absorption spectrometry. The oxidation state of Ni deposited on the Cu2O was determined to be +3 via X-ray photoelectron spectroscopy. The absence of evidence of reduction of Ni2+ to Ni0 suggests that the nature of the nickel deposition is adsorption, and it is not characterized as underpotential deposition. Photoelectrochemical measurements showed that deposited Ni protected Cu2O films from the photodecomposition to Cu0 by interfering with H+ binding to the O2− surface sites, and indicated the possibility of catalytic activity of deposited nickel for H+ reduction. The catalytic activity of deposited nickel was explained by the shift of the Cu2O Fermi level by 50 mV in negative direction upon nickel deposition.

Ultrahigh PEMFC Performance of a Thin‐Film, Dual‐electrode Assembly with Tailored Electrode Morphology

A dual-electrode membrane electrode assembly (MEA) for proton exchange membrane fuel cells with enhanced polarization under zero relative humidity (RH) is fabricated by introducing a phase-separated morphology in an agglomerated catalyst layer of Pt/C (platinum on carbon black) and Nafion. In the catalyst layer, a sufficient level of phase separation is achieved by dispersing the Pt catalyst and the Nafion dispersion in a mixed-solvent system (propane-1,2,3-triol/1-methyl-2-pyrrolidinone).The high polymer chain mobility results in improved water uptake and regular pore-size distribution with small pore diameters. The electrochemical performance of the dual-film electrode assembly with different levels of phase separation is compared to conventional electrode assemblies. As a result, good performance at 0 % RH is obtained because self-humidification is dramatically improved by attaching this dense and phase-separated catalytic overlayer onto the conventional catalyst layer. A MEA prepared using the thin-film, dual-layered electrode exhibits 39-fold increased RH stability and 28-fold improved start-up recovery time during the on-off operation relative to the conventional device. We demonstrate the successful operation of the dual-layered electrode comprised of discriminatively phase-separated agglomerates with an ultrahigh zero RH fuel-cell performance reaching over 95 % performance of a fully humidified MEA.