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Water poisoning has been a long-standing problem in oxide semiconductor gas sensors. Herein, for the first time, we report that pure and Pr-doped Ce4W9O33 provide humidity-independent gas sensing characteristics. Ce4W9O33 and Pr-doped Ce4W9O33 powders with a porous structure have been successfully prepared by ultrasonic spray pyrolysis and subsequent annealing at low temperature (600 °C). Interestingly, these p-type oxide semiconductors exhibited nearly the same gas sensing characteristics at 300 °C regardless of humidity variation, whereas pure WO3 showed a significant decrease of sensor resistance and gas response when the atmosphere is changed from dry to relative humidity 80%. Furthermore, Ce4W9O33-based sensors showed highly selective and sensitive detection of ppm-level trimethylamine (TMA). Moisture-endurant gas sensing characteristics were discussed in relation to surface regeneration through the hydroxyl scavenging reaction assisted by abundant Ln3+ (Ln = Ce, Pr) in (Ce1−xPrx)4W9O33 (x = 0–0.3) and TMA selectivity was explained by the acid–base interaction between the analyte gas and sensing material. Phase-pure ternary or quaternary oxides with a decreased oxidation state of lanthanide components provide a new and general strategy to design humidity-independent gas sensors with new functionality.

Pure and 3-12 at. % Pr-doped In2O3 macroporous spheres were fabricated by ultrasonic spray pyrolysis and their acetone-sensing characteristics under dry and humid conditions were investigated to design humidity-independent gas sensors. The 12 at. % Pr-doped In2O3 sensor exhibited approximately the same acetone responses and sensor resistances at 450 °C regardless of the humidity variation, whereas the pure In2O3 exhibited significant deterioration in gas-sensing characteristics upon the change in the atmosphere, from dry to humid (relative humidity: 80%). Moreover, the 12 at. % Pr-doped In2O3 sensor exhibited a high response to acetone with negligible cross responses to interfering gases (NH3, CO, benzene, toluene, NO2, and H2) under the highly humid atmosphere. The mechanism for the humidity-immune gas-sensing characteristics was investigated by X-ray photoelectron and diffuse reflectance infrared Fourier transform spectroscopies together with the phenomenological gas-sensing results and discussed in relation with Pr3+/Pr4+ redox pairs, regenerative oxygen adsorption, and scavenging of hydroxyl groups.

Nearly monodisperse hollow hierarchical Co3O4 nanocages of four different sizes (∼0.3, 1.0, 2.0, and 4.0 μm) consisting of nanosheets were prepared by controlled precipitation of zeolitic imidazolate framework-67 (ZIF-67) rhombic dodecahedra, followed by solvothermal synthesis of Co3O4 nanocages using ZIF-67 self-sacrificial templates, and subsequent heat treatment for the development of high-performance methylbenzene sensors. The sensor based on hollow hierarchical Co3O4 nanocages with the size of ∼1.0 μm exhibited not only ultrahigh responses (resistance ratios) to 5 ppm p-xylene (78.6) and toluene (43.8) but also a remarkably high selectivity to methylbenzene over the interference of ubiquitous ethanol at 225 °C. The unprecedented and high response and selectivity to methylbenzenes are attributed to the highly gas-accessible hollow hierarchical morphology with thin shells, abundant mesopores, and high surface area per unit volume as well as the high catalytic activity of Co3O4. Moreover, the size, shel...

Reduced graphene oxide (rGO)-In 2 O 3 hybrid materials were prepared by a solvothermal reaction of an In precursor containing rGO sheets, which were coated onto flexible substrates for gas sensors. The rGO-In 2 O 3 flexible sensors showed a high and reversible response (resistance ratio = 22.3) to 500 ppb NO 2 at 150 °C and negligible cross-responses to C 2 H 5 OH, CO, NH 3 , toluene, H 2 , and HCHO. The ultrahigh response and selectivity of rGO-In 2 O 3 hybrid materials to NO 2 were attributed to the chemical affinity of rGO to NO 2 and the extension of the electron depletion layer in n -type In 2 O 3 forming a p - n junction with the p -type rGO.

Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders and phase-pure ZnCr 2 O 4 powders were prepared by the galvanic replacement of Zn in ZnO hollow spheres by Cr and subsequent heat treatment. The Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders prepared by galvanic replacement consisted of nanocrystalline Cr 2 O 3 and ZnCr 2 O 4 particles, which showed a high response (resistance ratio) of 69.2 to 5 ppm xylene at 275 °C and excellent xylene selectivity. In contrast, both Cr 2 O 3 and ZnO powders showed selectivity to ethanol, and no significant selectivity to a specific gas was found for the ZnCr 2 O 4 powders. The Cr 2 O 3 /ZnCr 2 O 4 composite powders consisting of coarse particles prepared by solid-state reaction showed relatively low response and selectivity to xylene. The high selectivity and response to xylene of the Cr 2 O 3 /ZnCr 2 O 4 nanocomposite powders were attributed to the synergistic promotion of the methylbenzene-sensing reaction by two sensing materials with different catalytic activities and the high chemiresistive variation of the small particles, respectively, both of which result from the intimate and uniform mixing of nanocrystalline Cr 2 O 3 and ZnCr 2 O 4 particles.

The pure and 0.3 wt% Pt-doped SnO2 hollow nanospheres were prepared by the oxidation of pure and Pt-doped Sn nanoscrystals embedded in carbon matrix and their gas sensing characteristics were investigated. The formation of hollow morphology was attributed to the nanoscale Kirkendall effect due to rapid outward diffusion of Sn ions and relatively slow inward diffusion of oxygen. Pure SnO2 hollow nanospheres showed a high response (resistance ratio) of 93.3 when exposed to 5 ppm ethanol. The response to 5 ppm ethanol was significantly increased to 1399.9 with doping 0.3 wt% Pt. In addition, selectivity to ethanol was also enhanced by Pt doping. Ultrasensitive and selective detection of ethanol in pure and Pt-doped SnO2 nanospheres is explained by the effective electron depletion in hollow structures and catalytic promotion of gas sensing reaction.

Highly porous NiO–gadolinium-doped ceria (GDC) nano-composite powders are synthesized by a one-pot glycine nitrate process and applied to the fabrication of Ni–YSZ (yttria-stabilized zirconia)-supported tubular solid oxide fuel cells (SOFCs) with a cell configuration of Ni–YSZ/Ni/Ni–GDC/GDC/LSCF (La0.6Sr0.4Co0.2Fe0.8O3−δ)–GDC/LSCF. The power density of the cell is as high as 413mWcm−2 at 600°C, which is 1.37 times higher than that of an identically configured cell fabricated using ball milling-derived NiO–GDC powders (301mWcm−2). The high porosity of the powders and the good mixing between the NiO and GDC primary nanoparticles due to the abrupt combustion of the precursors effectively suppress the densification, coarsening, and agglomeration of NiO and GDC particles during sintering, resulting in a highly porous Ni–GDC anode layer with good dispersion of Ni and GDC particles and a cell with significantly enhanced power density.

Highly sinterable gadolinia doped ceria (GDC) powders are prepared by carbonate coprecipitation and applied to the GDC interlayer in Ni–YSZ (yttria stabilized zirconia)-supported tubular solid oxide fuel cell in order to prevent the reaction between YSZ electrolyte and LSCF (La0.6Sr0.4Co0.2Fe0.8O3−δ) cathode materials. The formation of highly resistive phase at the YSZ/LSCF interface was effectively blocked by the low-temperature densification of GDC interlayer using carbonate-derived active GDC powders and the suppression of Sr diffusion toward YSZ electrolyte via GDC interlayer by tuning the heat-treatment temperature for cathode fabrication. The power density of the cell with the configuration of Ni–YSZ/YSZ/GDC/LSCF–GDC/LSCF was as high as 906 mW cm−2, which was 2.0 times higher than that (455 mW cm−2) of the cell with the configuration of Ni–YSZ/YSZ/LSM(La0.8Sr0.2MnO3−δ)–YSZ/LSM at 750 °C.

Punched ZnO nanobelt (NB) network sensors with strong adhesion to Au electrodes were fabricated by the vapor-phase growth of ZnS NB networks and subsequent thermal annealing. The punched ZnO NB networks showed sensitive and selective detection of trimethylamine (TMA), while no significant sensitivity or selectivity to a specific gas was observed in dense ZnO NB networks. This high TMA response can be explained by abundant chemoresistive contacts, effective electron depletion, and high surface area to volume ratio of punched ZnO NB networks.

Gadolinia-doped ceria (GDC, Gd0.1Ce0.9O1.95) powders with high sinterability are prepared by carbonate coprecipitation and applied in the fabrication of Ni–YSZ (yttria-stabilized zirconia)-supported tubular solid oxide fuel cells (SOFCs) with a cell configuration of Ni–YSZ/Ni/Ni–GDC/GDC/LSCF(La0.6Sr0.4Co0.2Fe0.8O3−δ)–GDC/LSCF. A high power density (594 mW cm−2) at 600 °C is achieved using the cell (diameter: 6 mm) prepared by sintering of the electrolyte at 1400 °C. This result is attributed to the dense electrolyte and good adherence between GDC and the electrode. The carbonate-derived GDC powders can be sintered to 97% of the theoretical density even at 1050 °C, which demonstrates that the fabrication temperature of tubular SOFCs can potentially be lowered by using cost-effective Ni–YSZ supports.

Highly sensitive detection of volatile organic compounds, such as C2H5OH and HCHO, has been achieved by decorating p-type NiO nanoparticles on n-type ZnO nanowire networks, whereas the incorporation of Ni into the lattice of the ZnO nanowires deteriorated the gas sensing characteristics.

Manganese gallium oxide (MnGa2O4) one-dimensional nanostructures were synthesized by the thermal evaporation of MnCl2/Ga/Zn. The single-crystalline spinel-structured MnGa2O4 nanowires are preferentially grown in the [111] direction. In contrast, the Zn (10%)-doped MnGa2O4 nanostructures adopt zigzag structures. Some of them are nanosprings that consist of four equivalent [011] growth directions with the axial [001] direction. The nanobelts often have two zigzagged [011] directions along the long axis parallel to the [112] direction. We discuss the growth model of these unique zigzagged nanostructures. The MnGa2O4 nanostructures exhibit the emission at 2.5 eV (7 K), which possibly originated from the d−d transition of the tetrahedrally coordinated Mn2+ ions, and the decreased intensity with the Zn doping. The magnetization measurements indicate that these MnGa2O4 nanostructures exhibit virtually paramagnetic behaviors.

High-purity N-doped double-walled carbon nanotubes (DWNTs) were synthesized by chemical vapor deposition, and investigated by synchrotron X-ray photoelectron spectroscopy (XPS). As the photon energy increases, the N content increases up to 3 at.%, indicating that the inner wall contains the higher N content than the outer wall. The graphite-like and pyridine-like N structures exist as about 1:1 ratio. The self-consistent charge-density-functional-based tight-binding calculation shows that the N-doping of the inner wall yields the more stable DWNT than that of the outer wall, which supports the XPS result.

ZnO nanowires doped with a high concentration Ga, In, and Sn were synthesized via thermal evaporation. The doping content defined as X/(Zn + X) atomic ratio, where X is the doped element, is about 15% for all nanowires. The nanowires consist of single-crystalline wurtzite ZnO crystal, and the average diameter is 80 nm. The growth direction of vertically aligned Ga-doped nanowires is [001], while that of randomly tilted In- and Sn-doped nanowires is [010]. A correlation between the growth direction and the vertical alignment has been suggested. The broaden X-ray diffraction peaks indicate the lattice distortion caused by the doping, and the broadening is most significant in the case of Sn doping. The absorption and photoluminescence of Sn-doped ZnO nanowires shift to the lower energy region than those of In- and Ga-doped nanowires, probably due to the larger charge density of Sn.

We report synthesis of wurtzite structured gallium nitride nanowires via a chemical vapor deposition of gallium, gallium nitride, and boron oxide mixture under ammonia atmosphere. All nanowires exhibit the same [0 0 1] growth direction. The majority of nanowires are extremely thin and long; the diameter is 5–10 nm and the length is 40–50 μm. The larger diameter nanowires exhibit a zigzag configuration with amorphous boron outerlayers. X-ray diffraction and Raman spectroscopy reveal no significant strains inside the nanowires.

Strategies for the enhancement of gas sensing properties, and specifically the improvement of gas selectivity of metal oxide semiconductor nanowire (NW) networks grown by chemical vapor deposition and thermal evaporation, are reviewed. Highly crystalline NWs grown by vapor-phase routes have various advantages, and thus have been applied in the field of gas sensors over the years. In particular, n-type NWs such as SnO2, ZnO, and In2O3 are widely studied because of their simple synthetic preparation and high gas response. However, due to their usually high responses to C2H5OH and NO2, the selective detection of other harmful and toxic gases using oxide NWs remains a challenging issue. Various strategies—such as doping/loading of noble metals, decorating/doping of catalytic metal oxides, and the formation of core–shell structures—have been explored to enhance gas selectivity and sensitivity, and are discussed herein. Additional methods such as the transformation of n-type into p-type NWs and the formation of catalyst-doped hierarchical structures by branch growth have also proven to be promising for the enhancement of gas selectivity. Accordingly, the physicochemical modification of oxide NWs via various methods provides new strategies to achieve the selective detection of a specific gas, and after further investigations, this approach could pave a new way in the field of NW-based semiconductor-type gas sensors.

A high-density array of vertically aligned ZnGa(2)O(4) nanowires has been synthesized on Si substrates via CVD of ZnO-Ga at 1000 degrees C consisting of a single-crystalline cubic spinel structure grown in a [111] direction and exhibiting strong photoluminescence and cathodoluminescence in the blue wavelength region.

Highly selective and sensitive detection of trimethylamine (TMA) was achieved by the decoration of discrete p-type Cr(2)O(3) nanoparticles on n-type ZnO nanowire (NW) networks. Semielliptical Cr(2)O(3) nanoparticles with lateral widths of 3-8 nm were deposited on ZnO NWs by the thermal evaporation of CrCl(2) at 630 °C, while a continuous Cr(2)O(3) shell layer with a thickness of 30-40 nm was uniformly coated on ZnO NWs at 670 °C. The response (R(a)/R(g): R(a), resistance in air; R(g), resistance in gas) to 5 ppm TMA of Cr(2)O(3)-decorated ZnO NWs was 17.8 at 400 °C, which was 2.4 times higher than that to 5 ppm C(2)H(5)OH and 4.3-8.4 times higher than those to 5 ppm p-xylene, NH(3), benzene, C(3)H(8), toluene, CO, and H(2). In contrast, both pristine ZnO and ZnO (core)-Cr(2)O(3) (shell) nanocables (NCs) showed comparable responses to the different gases. The highly selective and sensitive detection of TMA that was achieved by the deposition of semielliptical Cr(2)O(3) nanoparticles on ZnO NW networks was explained by the catalytic effect of Cr(2)O(3) and the extension of the electron depletion layer via the formation of p-n junctions.

Highly crystalline CoO and Co3O4 nanowires were prepared by vapor-phase conversion of ZnO nanobelts. The ZnO nanobelts were successfully transformed into highly crystalline CoO nanowires by thermal evaporation of CoCl2 at 700 °C in Ar, which were subsequently converted to Co3O4 nanowires by oxidative annealing at 600 °C. The reactions at 500 and 550 °C in 0.5% O2 led to the formation of well-defined oxide p–n junction nanostructures such as Co3O4-decorated ZnO nanobelts and ZnO–ZnCo2O4 core–shell nanocables, respectively, as the reaction intermediates. The evolutions of phase and residual Zn component during the conversion were investigated in relation to the reaction temperature and oxygen partial pressure.

Magnetite (Fe3O4) hollow spheres were prepared by solvothermal reaction of ethanol solution containing Fe-acetate and L-lysine, and were subsequently transformed into hematite (Fe2O3) hollow spheres with nanoscale (20–30 nm) thin shells by heat treatment at 500 °C for 2 h. Both the as-prepared and heat-treated hollow spheres contained another small sphere within each shell, which was attributed to the following solvothermal self-assembly reactions: (1) the nucleation of Fe3O4 spheres, (2) lysine capping on the nuclei, (3) the growth of lysine-capped particles by cross-linking between lysine molecules, and (4) the formation of Fe shell layers by the interaction between Fe ions and outer lysine molecules. In the assembly reaction, L-lysine with amino and carboxyl radicals played the key role. The heat-treated Fe2O3 hollow spheres showed significantly enhanced C2H5OH sensing characteristics and promising Li-ion intercalation behaviors.

Rh-loaded In2O3 hollow spheres with diameters of ∼2 μm were prepared by a one-pot hydrothermal reaction of aqueous solution containing indium nitrate, rhodium chloride, and glucose and subsequent heat treatment at 500 °C for 2 h. The response to 100 ppm C2H5OH (Ra/Rg, Ra: resistance in air, Rg: resistance in gas) of 1.67 at% Rh-loaded In2O3 hollow spheres was 4748, which was ∼180 times higher than that of pure In2O3 hollow spheres. Rh loading decreased the temperature for maximum gas response from 475 °C to 371 °C, which also enhanced the selectivity to C2H5OH 15.1–24.7 times and recovery speed. The ultrahigh sensitivity and selectivity to C2H5OH, the lower sensing temperature, and the reduced recovery time were attributed to electronic interactions between Rh and In2O3 and the promotion of catalytic dissociation of C2H5OH into reactive gases.