Enhanced ethanol sensing properties of TiO2/ZnO core–shell nanorod sensors

TiO₂-core/ZnO-shell nanorods were synthesized using a two-step process: the synthesis of TiO₂ nanorods using a hydrothermal method followed by atomic layer deposition of ZnO. The mean diameter and length of the nanorods were ～300 nm and ～2.3 μm, respectively. The cores and shells of the nanorods were monoclinic-structured single-crystal TiO₂ and wurtzite-structured single-crystal ZnO, respectively. The multiple networked TiO₂-core/ZnO-shell nanorod sensors showed responses of 132–1054 % at ethanol (C₂H₅OH) concentrations ranging from 5 to 25 ppm at 150 ^°C. These responses were 1–5 times higher than those of the pristine TiO₂ nanorod sensors at the same C₂H₅OH concentration range. The substantial improvement in the response of the pristine TiO₂ nanorods to C₂H₅OH gas by their encapsulation with ZnO may be attributed to the enhanced absorption and dehydrogenation of ethanol. In addition, the enhanced sensor response of the core–shell nanorods can be attributed partly to changes in resistance due to both the surface depletion layer of each core–shell nanorod and the potential barriers built in the junctions caused by a combination of homointerfaces and heterointerfaces.     

UV-activated gas sensing properties of ZnS nanorods functionalized with Pd

Pd-functionalized ZnS nanorods were prepared for use as gas sensors. Scanning electron microscopy revealed the diameters and lengths of the nanorods ranging from 30 to 80 nm and from 2 to 5 μm, respectively. The diameter of Pd nanoparticles ranged from 2 to 5 nm. Transmission electron microscopy revealed that ZnS nanorods and Pd nanoparticles were monocrystalline and amorphous, respectively. The responses of multiple networked ZnS nanorods sensors to 1–5 ppm NO₂ were increased substantially by a combination of Pd functionalization and UV irradiation. Pristine ZnS nanorod sensors at room temperature in the dark showed a response (∼100%) almost independent of NO₂ concentration in a NO₂ concentration range of 1–5 ppm. Pristine ZnS nanorod sensors showed enhanced responses of 214–603% to 1–5 ppm NO₂ at room temperature under UV illumination. Pd-functionalized ZnS nanorods sensors showed further enhanced responses of 355–1511% to 1–5 ppm NO₂ at room temperature under UV illumination. The NO₂ gas sensing mechanism of the Pd-functionalized ZnS nanorods sensors under UV illumination is discussed in depth.     

Room temperature hydrogen sensing of multiple networked ZnO/WO3 core–shell nanowire sensors under UV illumination

ZnO/WO₃ core–shell nanowires were synthesized by thermal evaporation of a mixture of ZnO and graphite powders (ZnO:C = 1:1) followed by sputter-deposition of WO₃. The sensing properties of multiple networked ZnO-core/WO₃-shell nanorod sensors toward H₂ gas was examined. The responses of pristine ZnO and ZnO-core/WO₃-shell nanorods to 1000 ppm H₂ at room temperature under UV illumination were ∼236% and ∼645%, respectively. The responses of the core–shell nanowires increased from ∼118 to ∼645% with increasing the UV illumination intensity from 0 mW/cm² to 1.2 mW/cm². The enhanced sensing performance of the ZnO-core/WO₃-shell nanowires induced by encapsulation with WO₃ was explained based on a combination of surface depletion and potential barrier-controlled carrier transport models. The origin of the enhanced sensing properties of ZnO-core/WO₃-shell nanorods toward H₂ under UV illumination was also discussed.     

The sensitivity and dynamic response of field ionization gas sensor based on ZnO nanorods

Field ionization gas sensors based on ZnO nanorods (50–300 nm in diameter, and 3–8 μm in length) with and without a buffer layer were fabricated, and the influence of the orientation of nano-ZnO on the ionization response of devices was discussed, including the sensitivity and dynamic response of the ZnO nanorods with preferential orientation. The results indicated that ZnO nanorods as sensor anode could dramatically decrease the breakdown voltage. The XRD and SEM images illustrated that nano-ZnO with a ZnO buffer layer displayed high c-axis orientation, which helps to significantly reduce the breakdown voltage. Device A based on ZnO nanorods with a ZnO buffer layer could distinguish toluene and acetone. The dynamic responses of device A to the NO _x compounds presented the sensitivity of 0.045 ± 0.007 ppm/pA and the response speed within 17–40 s, and indicated a linear relationship between NO _x concentration and current response at low NO _x concentrations. In addition, the dynamic responses to benzene, isopropyl alcohol, ethanol, and methanol reveals that the device has higher sensitivity to gas with larger static polarizability and lower ionization energy.     

Structure and NO2 gas sensing properties of SnO2-core/In2O3-shell nanobelts

SnO₂-core/In₂O₃-shell nanobelts were fabricated by a two-step process comprising thermal evaporation of Sn powders and sputter-deposition of In₂O₃. Transmission electron microscopy and X-ray diffraction analyses revealed that the core of a typical core–shell nanobelt comprised a simple tetragonal-structured single crystal SnO₂ and that the shell comprised an amorphous In₂O₃. Multiple networked SnO₂-core/In₂O₃-shell nanobelt sensors showed the response of 5.35% at a NO₂ concentration of 10 ppm at 300 °C. This response value is more than three times larger than that of bare-SnO₂ nanobelt sensors at the same NO₂ concentration. The enhancement in the sensitivity of SnO₂ nanobelts to NO₂ gas by sheathing the nanobelts with In₂O₃ can be accounted for by the modulation of electron transport by the In₂O₃–In₂O₃ homojunction.     

Dependence of the selectivity of SnO2 nanorod gas sensors on functionalization materials

Effects of functionalization materials on the selectivity of SnO₂ nanorod gas sensors were examined by comparing the responses of SnO₂ one-dimensional nanostructures functionalized with CuO and Pd to ethanol and H₂S gases. The response of pristine SnO₂ nanorods to 500 ppm ethanol was similar to 100 ppm H₂S. CuO-functionalized SnO₂ nanorods showed a slightly stronger response to 100 ppm H₂S than to 500 ppm ethanol. In contrast, Pd-functionalized SnO₂ nanorods showed a considerably stronger response to 500 ppm ethanol than to 100 ppm H₂S. In other words, the H₂S selectivity of SnO₂ nanorods over ethanol is enhanced by functionalization with CuO, whereas the ethanol selectivity of SnO₂ nanorods over H₂S is enhanced by functionalization with Pd. This result shows that the selectivity of SnO₂ nanorods depends strongly on the functionalization material. The ethanol and H₂S gas sensing mechanisms of CuO- and Pd-functionalized SnO₂ nanorods are also discussed.     

Sensors for the nitrogen oxides, NO2, NO and N2O, based on In2O3 and WO3 nanowires

Sensing characteristics of ZnO, In₂O₃ and WO₃ nanowires have been investigated for the three nitrogen oxides, NO₂, NO and N₂O. In₂O₃ nanowires of ∼20 nm diameter prepared by using porous alumina membranes are found to have a sensitivity (defined as the ratio of the sensor resistance in the gas concerned to that in air) of about 60 for 10 ppm of all the three gases at a relatively low temperature of 150 °C. The response and recovery times are around 20 s. The sensitivity of these In₂O₃ nanowires is around 40 for 0.1 ppm of NO₂ and N₂O at 150 °C. WO₃ nanowires of 5–15 nm diameter, prepared by the solvothermal process show a sensitivity of 20–25 for 10 ppm of the three nitrogen oxides at 250 °C. The response and recovery times are 10 s and 60 s, respectively. The sensitivity is around 10 for 0.1 ppm of NO₂ at 250 °C. The sensitivity of In₂O₃ and WO₃ nanowires is not affected by humidity even up to 90% relative humidity. The study also reveals that the sensing mechanism for the three nitrogen oxides have a commonality in that the desorption of oxygen is a crucial step in all the cases. PACS 07.07.Df; 85.35.-p; 82.35.Np     

Enhancement of ethanol sensing of TeO2 nanorods by Ag functionalization

Gas sensors based on Ag–TeO₂ composite nanorods were fabricated using thermal evaporation and sputtering techniques. The morphology, structure and phase composition of the as-prepared nanofibers were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD), respectively. TEM and XRD showed that the nanorods and nanoparticles on them were tetragonal-structured single crystal TeO₂ and a mainly amorphous phase, respectively. The multiple-networked bare TeO₂ nanorod sensors exhibited a response of ～219% at 25 ppm C₂H₅OH at 300 °C, whereas the Ag-functionalized TeO₂ nanorod sensors showed a response of ～808% under the same conditions. The mechanism by which the sensing properties of the TeO₂ nanorods were enhanced by functionalization with Ag is also discussed.     

Controlled synthesis of oriented ZnO nanorod arrays by seed-layer-free electrochemical deposition

Oriented ZnO nanorod arrays were successfully prepared on transparent conductive substrates by seed-layer-free electrochemical deposition in solution of Zn(NO₃)₂ at a low temperature of 70 °C without using any catalysts, additives, and additional seed crystals. The effects of the Zn(NO₃)₂ concentration, deposition time and applied current on the localized nanorod arrays are investigated. X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) were used to characterize the structures and the morphologies of ZnO nanorod arrays. The heights and diameters of ZnO nanorods can be tuned by controlling the electrodeposition parameters.     

Low-temperature growth and ethanol-sensing characteristics of quasi-one-dimensional ZnO nanostructures

ZnO nanorods, nanobelts, nanowires, and tetrapod nanowires were synthesized via thermal evaporation of Zn powder at temperatures in the range 550-600 °C under flow of Ar or Ar/O₂ as carrier gas. Uniform ZnO nanowires with diameter 15-25 nm and tetrapod nanowires with diameter 30-50 nm were obtained by strictly controlling the evaporation process. Our experimental results revealed that the concentration of O₂ in the carrier gas was a key factor to control the morphology of ZnO nanostructures. The gas sensors fabricated from quasi-one-dimensional (Q1D) ZnO nanostructures exhibited a good performance. The sensor response to 500 ppm ethanol was up to about 5.3 at the operating temperature 300 °C. Both response and recovery times were less than 20 s. The gas-sensing mechanism of the ZnO nanostructures is also discussed and their potential application is indicated accordingly.     

Enhanced ethanol sensing properties of TeO2/In2O3 core–shell nanorod sensors

TeO₂/In₂O₃ core–shell nanorods were fabricated using thermal evaporation and sputtering methods. The multiple networked TeO₂/In₂O₃ core–shell nanorod sensor showed responses of 227–632%, response times of 50–160 s, and recovery times of 190–220 s at ethanol (C₂H₅OH) concentrations of 50–250 ppm at 300 °C. The response values are 1.6–2.9 times higher and the response and recovery times are also considerably shorter than those of the pristine TeO₂ nanorod sensor over the same C₂H₅OH concentration range. The origin of the enhanced ethanol sensing properties of the core–shell nanorod sensor is discussed.     

Porous WO3 from anodized sputtered tungsten thin films for NO2 detection

In this paper, porous WO₃ films were prepared by anodic oxidation of metallic tungsten (W) films deposited on alumina substrates. The structural and morphological properties of the porous WO₃ films were investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). A large number of cracks appeared on the surface of films after anodization, which makes the films porous. The porous WO₃ sensors achieved their maximum response values to NO₂ at a low operating temperature of 150 °C. The porous WO₃ sensors showed high response values, great stability and fast response-recovery characteristics to different concentration of NO₂ gas due to the high specific surface area and special structural and morphological properties.     

Promotion effect of Pt on a SnO2–WO3 material for NOx sensing

Metal-oxide nanocomposites were prepared over screen-printed gold electrodes to be used as room-temperature NO_x (nitric-oxide (NO) and nitrogen dioxide (NO₂)) sensors. Various weight ratios of SnO₂–WO₃ and Pt loadings were used for NO sensing. The sensing materials were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET surface analysis. The NO-sensing results indicated that SnO₂–WO₃ (1:2) was more effective than other materials were. The sensor response (S=resistance of N₂/resistance of NO=R_N2/R_NO) for detecting 1000 ppm of NO at room temperature was 2.6. The response time (T₉₀) and recovery time (TR₉₀) was 40 s and 86 s, respectively. By further loading with 0.5% Pt, the sensor response increased to 3.3. The response and recovery times of 0.5% Pt/SnO₂–WO₃ (1:2) were 40 s and 206 s, respectively. The linearity of the sensor response for a NO concentration range of 10–1000 ppm was 0.9729. A mechanism involving Pt promotion of the SnO₂–WO₃ heterojunction was proposed for NO adsorption, surface reaction, and adsorbed NO₂ desorption.     

Deposition and microstructure characterization of atmospheric plasma-sprayed ZnO coatings for NO2 detection

This work studied the possibility of using a sensor based on plasma-sprayed zinc oxide (ZnO) sensitive layer for NO₂ detection. The atmospheric plasma spray process was employed to deposit ZnO gas sensing layer and the obtained coating structure was characterized by scanning electron microscopy and X-ray diffraction analysis. The influences of gas concentration, working temperature, water vapor in testing air on NO₂ sensing performance of the ZnO sensors were studied. ZnO sensors showed a good sensor response and selectivity to NO₂ at an optimal working temperature.     

Detection of hydrogen at room temperature with catalyst-coated multiple ZnO nanorods

A variety of different metal catalyst coatings (Pt, Pd, Au, Ag, Ti and Ni) deposited on multiple ZnO nanorods have been compared for their effectiveness in enhancing sensitivity for detecting hydrogen at room temperature. Pt-coated nanorods show a relative response of up to 8% in room-temperature resistance upon exposure to a hydrogen concentration in N₂ of 500 ppm. This is a factor of two larger than that obtained with Pd and more than an order of magnitude larger than that achieved with the remaining metals. The power levels for these sensors were low, ∼0.4 mW for the responses noted above. Pt-coated ZnO nanorods easily detected hydrogen down to 100 ppm, with a relative response of 4% at this concentration after 10-min exposure. The nanorods show a return to their initial conductance upon switching back to a pure-air ambient with time constants of the order of a few minutes at room temperature. This slow response at room temperature is a drawback in some applications, but the sensors do offer low-power operation and ppm detection sensitivity. PACS 78.66.Hf; 73.61.Ga; 73.40.Qv     

Detection of hydrogen with SnO2-coated ZnO nanorods

SnO₂-coated ZnO nanorods on c-plane sapphire substrates were synthesized by pulsed laser deposition. The thickness of the polycrystalline SnO₂ was ∼10 nm, as determined by high-resolution transmission electron microscopy, while the diameter of the ZnO nanorods was ∼30 nm. The sensitivity of the SnO₂/ZnO structures to hydrogen was tested by depositing Ti/Au Ohmic contacts on a random array of the nanorods and measuring the current at fixed voltage. There was no response to 500 ppm H₂ in N₂ at room temperature, but we obtained a sensitivity of ∼70% at 400 °C. The SnO₂/ZnO structures exhibit drift in their recovery characteristics and for sequential detection of hydrogen, as generally reported for SnO₂ thin film sensors.     

Highly sensitive and selective trimethylamine sensors based on WO3 nanorods decorated with Au nanoparticles

One-dimensional tungsten oxide (WO₃) gas sensing materials have been widely used for the detection of trimethylamine (TMA) gas. Furthermore, it is believed that an effective method to improve the gas sensing performance is to introduce noble metals into sensing materials. In this work, a novel gas sensing material was prepared by decorating Au nanoparticles on WO₃ nanorods. Based on field emission scanning electron microscopy (FESEM/EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM), the morphology and microstructure of as-prepared samples were characterized. Results show that Au nanoparticles with diameter of 13–15 nm are loaded on the surface of WO₃ nanorods with length of about 1–2 µm and width of 50–80 nm. Gas sensing tests reveal that the Au@WO₃ sensor has remarkably enhanced response to TMA gas compared with pure WO₃ nanorods. In addition, and the gas sensing mechanism has been investigated based on the experimental results. The superior sensing features indicate the present Au@WO₃ nanocomposites are promising for gas sensors, which can be used in the detection of the trimethylamine gas and this work provides insights and strategies for the fabrication of sensing materials.     

水热法合成ZnO纳米棒图案及其场增强效应

利用水热合成方法在图案化的Au岛上合成了ZnO纳米棒图案,采用的溶液体系为六次甲基四胺和硝酸锌溶液,ZnO纳米棒的基底是ITO导电玻璃上的有序Au岛. 由于ZnO的异相成核速度在Au和ITO基底上具有不同的成核速度,因此ZnO优先生长在成核速度快的Au岛上,同时由于受到了溶液中前驱物种扩散的限制,纳米棒继续生长也被受到了约束. 通过调控六次甲基四胺和硝酸锌的浓度,可以调整不同的图案. 此外,利用X射线衍射、光致发光谱和场发射特性性能对水热合成的ZnO纳米棒图案进行了研究. ZnO纳米棒表现出良好的场增强性     

Effect of zinc nitrate concentration on the structural and the optical properties of ZnO nanostructures

ZnO nanorods and nanodisks were formed on indium-tin-oxide-coated glass substrates by using an electrochemical deposition method. Scanning electron microscopy images showed that the ZnO nanorods were transformed into nanodisks with increasing Zn(NO₃)₂ concentration. X-ray diffraction patterns showed that the ZnO nanostructures had wurzite structures. The full widths at half maxima of the near band-edge emission peak of photoluminescence spectra at 300 K for ZnO nanorods were small, indicative of the high quality of the nanorods. These results indicate that the structural and the optical properties of ZnO nanostructures vary by changing Zn(NO₃)₂ concentration.     

Amplification of raman scattering by localized plasmons in silver nanoparticles on the surface of zinc oxide nanorods

The magnetron sputtering of Ag nanoparticles onto ZnO nanorod arrays is studied. The lateral faces of the nanorods are coated with nanoparticles at a much lower density as compared to the flat faces at comparable sputtering times. The silver density is high on the edges of the lateral faces of the nanorods. The plasmon absorption in the synthesized arrays of nanorods coated with individual Ag nanoparticles is maximal at 450?C500 nm. The appearance of local plasmon excitations increases the intensity of the multiphonon processes with the participation of ZnO polar modes in Raman spectra. The cross section of resonance Raman scattering for A ₁(LO) phonon overtones increases with the equivalent Ag film thickness.