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Porous silicon nanowires (SiNWs) is considered as one of promising candidates for high-performance NO2 sensor application in human health and quality life owing to its fast response and stable operation at room temperature. However, the development of SiNWs-based gas sensors for stable detection of ppb-level NO2 in practical high-humidity environments remains a challenge. In this study, octadecyltrichlorosilane (OTS) was utilized to modify partially insensitive SiNWs surface with the Si-O bond, yielding an immunity to high humidity in gas detection process. The modified OTS provides an umbrella-like barrier blocking water molecules attraction on the nanowire surface which disinhibits the main adsorption sites and sensing channels of the sensor. The OTS-modified porous SiNWs (OTS/SiNWs) sensor exhibited a practical capability to detect 5 ppb NO2 at humidity of 25%–55% RH and a response of 1.8% to 50 ppb NO2 at 75% RH at room temperature. Moreover, the fast response/recovery characteristics and good reversibility of the OTS/SiNWs sensor was observed even under high humidity conditions. In addition, the OTS/SiNWs sensor displayed excellent stability to humidity variations. The enhanced gas sensing performance is attributed to a marriage of porous structure of SiNWs and OTS. The simple modification for SiNWs-based gas sensor will broaden new avenues, not only in chemical trace detection, but also in high humidity health monitoring.

Surface modification by noble metal nanoparticles has been proved highly effective on response enhancement of a semiconductor gas sensor. In this work, the adsorption of NO2 on Ag-modified silicon nanowire (Ag@SiNW) was investigated by means of density functional theory (DFT) calculations with aim to explore the nature and theoretical mechanism of Ag modification for sensing response enhancement. Ag surface modification tunes the electronic structure of SiNW considerably, which is further moderated by the spontaneous adsorptions of NO2 on surface Si site and on the modified Ag atom. The modified Ag as a donor creates an n-doping state, activating the surface Si atom and promoting the adsorption of Si surface to NO2 gas. Nevertheless, Mulliken charge transfer calculations clarify that the enhanced response observed experimentally for an Ag@SiNW sensor is mainly attributed from the gas adsorption right on Ag atom. The adsorbed NO2 on modified Ag atom captures 1.27 e from the nanowire surface, which is more than twice as much as that with Si site adsorption. The calculations on geometry and charge density difference reveal the catalyst nature of the modified Ag for NO2 adsorption and further clarify theoretically the possible reaction of NO2, i.e., dissociation and spillover, occurring on the Ag atom. Besides, a defect-induced preferential etching mechanism of metal (Ag)-assisted chemical etching (MACE) for SiNW was proposed from a new perspective based on the lattice distortion analysis induced by Ag modification.

Layered double hydroxides (LDHs) are attracting increased attention in the field of gas sensor owing to their high specific surface and interlayer modifiability. However, the general tendency to restacking and poor conductivity significantly affect the performance of the gas sensor based on LDHs towards rarefied gas. To resolve these problems, we successfully constructed the MXene (Ti3C2Tx)-supported ZnTi-LDHs (LDHs/Ti3C2Tx) nanocomposite with loose heterostructure by one-step hydrothermal method based on electrostatic ordered self-assembly. The LDHs uniformly wrapped on Ti3C2Tx nanosheets form a highly synergistic similar metal-semiconductor contact with Ti3C2Tx, and the formed anti-barrier layer (ABL) has a trapping effect on the electrons generated by NH3-sensing. Meanwhile, the formation of tiny LDHs might be ascribed to the effect of Ti atoms in Ti3C2Tx, which is confirmed by first-principles calculations. Benefiting from similar metal-semiconductor heterostructure and the effect of Ti atoms in Ti3C2Tx, the electrical conductivity and the adsorption active sites are prominently improved, which are essential for the enhancement of sensitivity to rarefied gas. The gas sensor based on ZnTi-LDHs/Ti3C2Tx nanocomposite exhibits about 5 and 9-fold enhancement in response value to 50 ppm NH3 at room temperature (RT) compared with the pure LDHs and Ti3C2Tx, respectively. Notably, the as-fabricated gas sensor could detect as low as 100 ppb NH3 with a response of 1.26 at RT. Furthermore, the LDHs/Ti3C2Tx sensor exhibits fast response/recovery time, high selectivity, acceptable humidity tolerance and outstanding long-term stability towards NH3. Above all, this work provides a feasible route to develop excellent gas-sensing materials of sensor via interfacial design.

A facile and cost-effective preparation process for Ag nanoparticles modified Si nanowires (Ag NPs@RNWs) was first developed based on the metal-assisted chemical etching (MACE)-produced Ag dendrites. Partial removal of the wrapped Ag dendrites and a crucial tetramethylammonium hydroxide (TMAH) post-etching treatment are sequentially involved in the process. The crucial step of TMAH-etching induces a loose array of needle-like SiNWs with highly rough surface (RNWs), and causes redistribution of the attached Ag NPs. HRTEM observations reveal that numerous tiny Ag NPs with size of 2–6 nm are firmly inlaid on the concave-convex surface of RNWs. Comparative investigations of NO 2 -sensing properties among the bare-SiNWs, RNWs and Ag NPs@RNWs were carried out at room temperature. The Ag NPs@RNWs sensor exhibits considerably enhanced response (about five times higher) compared with bare-SiNWs and can rapidly response ultrararefied NO 2 with deep ppb level. Above results demonstrate the considerable effect of the Ag NPs modification process developed here; the resulted rough and loose array promotes the response speed of the SiNWs sensors and the formed tiny Ag NPs contributes dramatic enhancement in response sensitivity. The enhanced response of Ag NPs@RNWs is analyzed based on Ag modification-induced electronic sensitization and TMAH etching-induced structural modulation.

Conducting polymer of polypyrrole (PPy) was employed to functionalize the ordered silicon nanowire (SiNWs) with the aim to enhance gas-sensing response at room temperature (RT). A dual-MACE process was first developed to form loose SiNWs (LNWs) array facilitating uniform attachment of PPy on whole nanowire surface. The LNWs array functionalized with continuous PPy ultrathin film and discrete PPy nanoparticles, i.e., PPy-shell@LNWs and PPy-NPs@LNWs, were prepared respectively via vapor chemical polymerization (VCP) and liquid chemical polymerization (LCP) of pyrrole monomer (Py). Comparative investigations on the NH3-sensing properties of the pristine SiNWs before and after dual-MACE treatment, and PPy-functionalized SiNWs of PPy-shell@LNWs and PPy-NPs@LNWs were carried out at RT. It was found that PPy functionalization enhances the NH3-sensing response of SiNWs-based sensors effectively. Formation of continuous PPy shell film is further found to result in more pronounced response enhancement when compared to the modification by discrete PPy NPs. The PPy-shell@LNWs and PPy-NPs@LNWs respectively exhibit about 27 and 6 times response enhancement to 10 ppm NH3 gas compared to the pristine one. The underlying NH3-sensing mechanism for the PPy-functionalized SiNWs, especially for the remarkable response enhancement of PPy-shell@SiNWs was demonstrated.

The group-IV monochalcogenide of SnS is explored promising for gas sensor applications due to its unique two-dimensional puckered monolayer and superior oxidation resistance. This work focuses on the gas selectivity regulation of SnS monolayer with adjustable doping. To this end, detailed studies for the effects of nonmetallic dopants (X) of Si, P, or Cl in SnS lattice on the selective adsorption of NO2, NH3, SO2 are first performed by first-principles calculations. It reveals that the SnS monolayers with Si and P doping exhibit stronger adsorption capability and rapid recovery characteristic towards NH3 and SO2 at room temperature, respectively, while Cl-doped SnS could realize the reversible detection of NO2 at elevated temperature. These theoretical findings were further verified in experiment. Gas-sensing evaluations show the obvious gas selectivity of SnS with different doping of Si, P and Cl towards NH3, SO2, and NO2, respectively. The response magnitude exhibited by the doped SnS is revealed to be positively correlated with the calculated electron transfer. In terms of recovery performance of the doped SnS, intrinsic recovery characteristic could be predicted by the calculation on recovery time, while the measured recovery time reflects the combined effect of material characteristic and environmental factors.

The gas sensing characteristics of porous tungsten disulfide (WS 2 ) aerogel are investigated. The sensor is fabricated by integrating WS 2 aerogel onto a low power polycrystalline silicon microheater platform to provide control over the operating temperature. The WS 2 aerogel is characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermal gravimetric analysis. The sensing performances of the WS 2 aerogel-based sensor to NO 2 , O 2 , NH 3 , H 2 , and humidity are investigated, indicating a p-type behavior. The optimum sensing temperature is found to be about 250 °C, when considering sensitivity, power consumption and response time. The role of O 2 in the sensor performance is probed and is found to be helpful for enhancing the sensitivity and recovery of the sensor in H 2 , humidity and NH 3 .

Silicon nanowires (SiNWs) is considered as one of the most promising candidates for gas sensing application due to its capability of operating at room temperature (20 °C) and compatibility with modern semiconductor process. However, there still remains a challenge that SiNWs is poorly sensitive to volatile organic compounds (VOCs). In this study, we reveal that the co-decoration of zeolite imidazole frameworks-8 (ZIF-8) and Ag nanoparticles can improve the gas-sensing response to ethanol and enhance the humidity resistance at 20 °C. Due to the inversion modulation introduced by nano-Ag modificaiton and the enhanced gas adsorption provided by ZIF-8 modification, the resultant co-decorated sensor of ZIF-8&Ag@SiNWs shows obviously enhanced response upon exposure to ethanol with wide concentration range. It meanwhile exhibits a practical capability to sense rarefied ethanol vapor low to 125 ppb at 20 °C and relative humidity of 25 %. Especially, the sensor exhibits a low limit of detection of 5.45 ppb of ethanol vapor. When working at high humidity of 85 %, the developed ZIF-8&Ag@SiNWs sensor still remain a significant response. The superior moisture resistance performance is clarified by first-principles caculations to be highly related to the hydrophobicity characteristic of the modified ZIF-8 and the hygroscopic center formed by Ag nanoparticles. A resistance model was proposed to demonstrate the enhanced sensing response of the ZIF-8&Ag@SiNWs. Further, a two-level moisture resistance mechanism is demonstrated based on the first-principles caculations and the unique microstructure of ZIF-8&Ag@SiNWs. The beneficial effect of ZIF-8&nano-Ag co-modification demonstrated in present work suggest an effective strategy for developing superior gas sensors capable of sensitive detection of VOCs with strong stability in humditiy ambient.

For gas sensor applications, a unique heterostructure array of W18O49/TiO2 core-shell nanowires with high alignment and uniform shell layer were fabricated by thermal oxidation of W film followed by sputter deposition and annealing of TiO2. The ordering of the rough aligned W18O49–core nanowires formed from thermal oxidation of metallic W film is found to be modulated considerably by the sputter deposition of shell layer. Thicker shell results in better alignment. The gas-sensing characteristics of the as-prepared W18O49/TiO2 nanowire sensor are evaluated at room temperature to 150 °C by measuring the dynamic response over NO2 concentration ranging from 0.5 to 5 ppm. The W18O49/TiO2 core-shell nanowires show temperature-dependent p-n response characteristic reversal. At room temperature, it behaves as an abnormal p-type semiconductor and exhibits good NO2-sensing performances including high sensitivity, good selectivity and excellent dynamic response-recovery characteristics. It is found that, with improved alignment, the heteronanowires array responses to sub-ppm level of NO2 with ultrafast response and recovery rate at room temperature. The measured response times are shorter than 5 s and the sensor can be recovered completely within 50s. The underlying gas-sensing mechanism correlated to the p-type response inversion at room temperature is analyzed in detail.

ZnO nanosheets exhibit notably interesting gas sensing performances due to their great carrier transport and high special surface area. However, not only the 2D nanosheets tend to stack suppressing the surface area, but also the pristine ZnO cannot show desired ethanol sensing properties. Herein, segregated ZnO-In2O3 porous nanosheets are synthesized facilely via one-pot solvothermal method. The Indium doping stems the porous nanosheets stacking, as well as induces lots of heterojunctions. Due to the specific porous structure for high gas accessibility, thickness less than double Debye length, abundant reaction sites and heterojunctions, the segregated ZnO-In2O3 nanosheets exhibit remarkable ethanol sensing performance (LOD of 100 ppb, response/recovery time of 3/8 s, and great repeatability and selectivity under natural humidity for a long period). This study provides a prospect for practical applications of ZnO-In2O3 ethanol sensors.

SnS is a two-dimensional black phosphorus-like material with potential in gas sensor applications. The vacancy defects generally existed in SnS structure could show a marked impact on its surface, physicochemical and electronic properties. This work focuses on the effect of vacancy defects in SnS, including S-vacancy and Sn-vacancy, on its gas adsorption and selectively sensing characteristics. To this end, we carry out first-principles calculations on vacancy-defected SnS and corresponding adsorption configurations with different gas molecules. Adsorption energy, density of states and Mulliken population for pristine SnS and defective SnS respectively with S-vacancy and Sn-vacancy were calculated and analyzed to clarify the correlation of adsorption characteristic with surface structure. The results revealed that the presence of S-vacancy in SnS creates a preferring surface for NH3 adsorption, while the defective SnS with Sn-vacancy is capable of enhanced adsorption and selective detection for NO2. For CH2O, CO, H2S and CH4, weak adsorption and low sensitivity are demonstrated to occur on SnS and defective SnS according to the adsorption energy calculations. The adsorption difference of the defective SnS with S- and Sn-vacancy towards NH3 and NO2 gases is deeply clarified by analyzing the charge density difference of corresponding defective structure.

The adsorption of ethanol on V 2 O 5 (010) surface was investigated by means of density functional theory (DFT) with a combined generalized gradient approximation (GGA) plus Hubbard U approach to exploit the potential sensing applications. The adsorption configurations were first constructed by considering different orientations of ethanol molecule to V and O sites on the “Hill”- and “Valley”-like regions of corrugated (010) surface. It is found that ethanol molecule can adsorb on whole surface in multiple stable configurations. Nevertheless the molecular adsorption on the “Hill”-like surface is calculated to occur preferentially, and the single coordinated oxygen on “Hill”-like surface (O 1(H) ) acting as the most energetically favorable adsorption site shows the strongest adsorption ability to ethanol molecule. Surface adsorption of ethanol tunes the electronic structure of V 2 O 5 and cause an n-doping effect. As a consequence, the Fermi levels shift toward the conductive bond increasing the charge carrier concentration of electrons in adsorbed V 2 O 5 . The sensitive electronic structure and the multiple stable configurations to ethanol adsorption highlight the high adsorption activity and then the potential of V 2 O 5 (010) surface applied to high sensitive sensor for ethanol vapor detection. Further Mulliken population and Natural bond orbital (NBO) calculations quantify the electron transfer from the adsorbed ethanol to the surface, and correlates the adsorption ability of surface sites with the charge donation and dispersion.

Gas sensors based on aligned arrays of W 18 O 49 nanowires were formed directly via a novel route of in situ thermal oxidation of sputtered W film on the substrate attached patterned Pt electrodes. The well-developed nanowires have diameter of 10–20 nm and show roughly aligned morphology. It is found that the duration of oxygen exposure during thermal annealing plays a crucial role to harvest a pure phase of W 18 O 49 nanowire with desired length. The roughly aligned W 18 O 49 nanowires show favourable microstructure for gas adsorption and rapid gas diffusion. The NO 2 -sensing properties of aligned W 18 O 49 nanowires sensor were evaluated at 50 °C up to 200 °C over NO 2 concentration ranging from 250 ppb to 2.5 ppm. The results indicate that the W 18 O 49 nanowire arrays sensor exhibits good NO 2 -sensing performances at its optimal operating temperature of 150 °C, especially perfect stability and fast response–recovery characteristics. The reliable interface performance and fast gas adsorption–desorption properties of the directly assembled vertically aligned nanowire array attribute to the superior stability and quick response/recovery. The growth mechanism of aligned W 18 O 49 nanowires is proposed based on the direct SEM observations on the intermediate products, and meanwhile the sensing mechanism of the corresponding sensor is analyzed.

Tungsten oxide (WO 3 ) nanocrystals with various nanomorphologies were synthesized by solvothermal method using tungsten hexachloride (WCl 6 ) as a raw material and pure ethylene glycol (EG) or water–EG as reaction solvent, and the NO 2 -sensing properties of the WO 3 nanocrystals were studied. The morphology and crystal structure were investigated by field emission scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The NO 2 gas sensing properties of WO 3 nanocrystals were investigated at different temperatures ranging from room temperature (∼25 °C) to 250 °C over NO 2 concentration ranging from 0.1 to 3 ppm. The results indicate that the morphology and crystal phase of the WO 3 nanocrystals depended on the water content in water–EG mixed solvent. With the increase of water content, the crystalline phase transformed from hexagonal to monoclinic. At the operating temperature below 55 °C, the sensor synthesized in EG solvent showed an abnormal p-type conductive behavior. It is found that all the sensors exhibit high sensor responses and rapid response characteristics to different concentrations of NO 2 , and their highest sensor responses are achieved at 100 or 50 °C.

The gas response of oxide-based sensors strongly depends on its surface properties. To explore the potential sensing ability of one-dimensional (1D) WO3 nanowire under existence of oxygen vacancy, the adsorption of NO2 molecule on the oxygen vacancy defected surface of WO3 nanowire was studied using density functional theory (DFT) calculations. Three kinds of stable oxygen vacancies were considered, and the three corresponding most energetically favorable adsorption configurations were constructed based on the calculation adsorption energy. The electronic properties including density of states, band structure and atomic Mulliken population were further studied. It is found that the existence of oxygen vacancies benefits NO2 adsorption on nanowire surface, and makes the interaction between molecule and vacancy-defected surface strengthened markedly. Consequently, the electronic structures and electronic properties of the defective WO3 nanowires were tuned obviously, causing a distinct change of the density of state at Fermi level and much more electrons extracted from the defective surface. The transferred electrons from the vacancy-defected WO3 nanowire was four to six times more, depending on different oxygen vacancies, than that from the stoichiometric nanowire, indicating a positive effect of oxygen vacancy on NO2-sensing response. The results highlight the possibility to develop high sensitive NO2 gas sensors through introducing oxygen vacancy defects in WO3 nanowire.

In this paper, well-ordered one-dimensional single crystalline hexagonal WO 3 nanorods were synthesized directly on the porous silicon substrates by a seed-induced hydrothermal method without using any template, catalyst. And the effect of the annealing temperature of WO 3 seed layers on the microstructure and NO 2 -sensing properties was studied. The morphology and crystal structure of the porous silicon/WO 3 nanorods composites were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The gas-sensing properties of the porous silicon/WO 3 nanorods composite gas sensors to NO 2 with the concentration ranging from 100 ppb to 3 ppm were examined. The results indicate that well-ordered WO 3 nanorods can greatly improve the gas-sensing properties of the sensor. The sensor exhibited a high response (∼3.38) and excellent selectivity toward 1 ppm NO 2 at room temperature. And the NO 2 -sensing mechanism of this sensor was further explained.

A novel composite structure of p-porous silicon/p-TeO2 nanowires has been successfully synthesized using Te powder as source materials and porous silicon as growth substrate by thermal evaporation method. The morphological feature and crystal structure of the TeO2 nanowires grown on porous silicon were characterized by scanning electron microscopy, X-ray diffraction and transmission electron microscopy. As prepared TeO2 nanowires with diameters ranging from 100 to 200 nm and lengths up to 15 μm were covered by a distinctly snow-like over layer along whole length. Gas sensing properties of both composite structure sensor and pristine porous silicon sensor were examined at the working temperature of 26–150 °C with NO2 concentration ranging from 0.05 to 3 ppm. Next, we made a comparison of gas sensing performances between porous silicon/TeO2 nanowires and other reported TeO2 nanowires. Besides, the repeatability and selectivity of the new sensor were also tested respectively. The results revealed that the composite structure sensor exhibited high response, excellent repeatability and good selectivity to NO2 at room temperature.

A novel tungsten oxide nanowires/porous silicon composite has been successfully synthesized via a convenient thermal evaporation method with no catalyst. The morphology and crystal structure of products obtained were investigated by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The diameters and lengths of nanowires were 40–60 nm and 20–30 μm, respectively, and the aspect ratio (length/diameter) of nanowires can be in range of 500–750. The factor influenced the morphology was substrate temperature. The diameter of nanowires decreased as the substrate temperature increased. The sensor made of tungsten oxide nanowires/porous silicon composite exhibited a high response (∼3.32), fast response/recovery (∼175/44 s) and excellent selectivity toward 2 ppm NO2 at a low operating temperature of 100 °C. Modulations of the depletion width along the nanowires are likely to be the reasons for the low-temperature gas sensing properties. Furthermore, modulations of the potential barriers at both networked nanowires homojunctions and heterojunctions between porous silicon and tungsten oxide are also responsible for the good gas sensing properties at a low operating temperature.

Gas sensors based on V2O5 hierarchical structure networks were formed through a direct seed-induced hydrothermal growth of nanostructured V2O5 on the substrate attached a pair of patterned Pt electrodes. Porous flower- or honeycomb-like V2O5 hierarchical structures could be controlledly synthesized under different precursor (NH4VO3) concentrations. The as-prepared hierarchical structures of V2O5, especially the flower-like network with radially oriented ultrathin nanoneedles and nanoribbons and the overlaped nanowires as constituents, show favorable microstructure features for gas-sensing application. The ethanol gas sensing properties of V2O5 network-structured sensors were investigated at room temperature (20 °C) up to 300 °C over ethanol concentration ranging from 5 to 1000 ppm. The sensor based on V2O5 hierarchical structure network showed temperature-dependent p- to n-type response characteristic reversal, resulting in dual working temperature characteristic with the dual response extremes reached at room temperature (20 °C) and 250 °C respectively. The flower-like V2O5 network sensor exhibits perfect reversibility, high response value and fast response–recovery characteristic to ethanol gas at the dual working temperatures, due to the good interface performance and gas adsorption–desorption properties of the directly assembled porous network. At 250 °C, the flower-like V2O5 network sensor is much sensitive to both ethanol gas and NH3, while at room temperature, the sensor presents very good selectivity to ethanol gas.

Solvothermally synthesized W18O49 nanowires were modified by addition of titanium. The effect of the concentration and the introduction procedure of Ti additive on the microstructure and NO2 sensing properties of W18O49 nanowires were investigated. In order to study the influence of the additive introduction procedure, Ti addition was carried out by physical impregnation process and chemical synthesis process, in which the additive was introduced after and during nanowire synthesis, respectively. Introduction of Ti additive by chemical synthesis process suppressed the growth of one-dimensional nanowires along their axis direction and resulted in the secondary assembly of nanowires bundles forming unfavorable structure for gas sensing. By contrast, Ti addition preformed to as-synthesized nanowires with a proper additive concentration (2 at% in the present study) could create a favorable spotty configuration of tiny additive particles on the nanowire surface. The Ti-added W18O49 nanowires showed temperature-dependent response characteristic reversal, which behave as a p-type semiconductor at operating temperature below 100 °C. Ti addition not only lowered the optimal operating temperature of W18O49 nanowires sensors from 150 °C to room temperature but also increased the maximum value of sensor response. At room temperature, the Ti-added nanowires exhibited rapid response characteristic to NO2 gas.

In this paper, a novel porous WO3 sensor was prepared by anodic oxidation of DC magnetron sputtered metallic tungsten (W) film deposited on alumina substrate. The structural and morphological properties of these films are investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Coral-like porous crystalline WO3 film with a grain size of about 9.3 nm was obtained after annealing of the anodized W film. The porous WO3 sensor achieved its maximum response value to NO2 at a low operating temperature of 150 °C. In comparison to sputtered WO3 sensor, the porous WO3 sensor showed markedly higher responses, much better response–recovery characteristics and lower optimal operating temperature to different concentration of NO2 gas due to its high specific surface area and small grain size.

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.

Quasi-orienting W 18 O 49 nanowire bundles were synthesized by solvothermal method. The microstructures and the NO 2 -sensing properties of the as-synthesized nanowires annealed at different temperatures were studied. To characterize the morphology and crystalline structure of the annealed tungsten oxide, field emission scanning electron microscope, X-ray diffraction and transmission electron microscope were employed. It was found that with annealing temperature increasing, the nanowire bundles became straighter and slightly thicker, and eventually a nonobelt-like structure was formed via the nanowires coalescence at 450 °C. Meanwhile, the monoclinic W 18 O 49 was transformed to monoclinic WO 3 when annealing temperature rising from 350 to 450 °C. In comparison to the W 18 O 49 nanowire bundles-based sensor, the sensor based on the 450 °C annealing-induced WO 3 nanobelt-like structure exhibited markedly higher response value and gas selectivity, as well as much better response-recovery characteristics to NO 2 gas due to its favorable microstructure feature to gas-sensing.

Nanowires and nanosheets of tungsten oxide were synthesized by solvothermal method with different tungsten hexachloride (WCl 6 ) concentrations in 1-propanol solvent. The morphology and crystal structure of the tungsten oxide nanostructures were investigated by means of field emission scanning electron microscope, X-ray diffraction and transmission electron microscope. The specific surface area and pore size distribution were characterized by Brunauer–Emmett–Teller gas-sorption measurements. One-dimensional W 18 O 49 nanowire bundles were synthesized at the WCl 6 concentration of 0.01 M. With the concentration increasing to 0.02 M, the structure of the pure two-dimensional WO 3 nanosheets was formed. The NO 2 gas sensing properties of W 18 O 49 nanowires and WO 3 nanosheets were investigated at 100 °C up to 250 °C over NO 2 concentration ranging from 1 to 20 ppm. Both nanowires and nanosheets exhibit reversible response to NO 2 gas at different concentrations. In comparison to WO 3 nanosheets, W 18 O 49 nanowire bundles showed a much higher response value and faster response–recovery characteristics to NO 2 gas, especially a much quicker response characteristic with response time of 19 s at the concentration of 5 ppm.

The field emission properties of electrophoretic deposition(EPD) carbon nanotubes (CNTs) film have been improved by depositing CNTs onto the titanium (Ti)-coated Si substrate, followed by vacuum annealing at 900 °C for 2 h, and the enhanced emission mechanism has been studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. Field emission measurements showed that the threshold electric field was decreased and the emission current stability was improved compared to that of EPD CNTs film on bare Si substrate. XRD and Raman spectroscopy investigations revealed that vacuum annealing treatment not only decreased the structural defects of CNTs but made a titanium carbide interfacial layer formed between CNTs and substrate. The field emission enhancement could be attributed to the improved graphitization of CNTs and the improved contact properties between CNTs and substrate including electrical conductivity and adhesive strength due to the formed conductive titanium carbide.

Carbon nanotubes (CNTs) were modified by depositing a thin layer of titanium film on the surface using magnetron sputtering method, followed by vacuum annealing at 900 °C for 2 h. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirmed that the as-deposited thin titanium film reacted with carbon atoms to form titanium carbide after annealing. The experiment results show that the thickness of sputter-deposited titanium film has significant effect on the field emission J–E characteristic of modified CNTs film. The titanium carbide-modified CNTs film obtained by controlling the titanium sputtering time to 2 min showed an improved field emission characteristics with a significant reduction in the turn-on electric field and an obvious increase in the emission current density as well as an improvement in emission stability. The improvement of field emission characteristics achieved is attributed to the low work function and good resistance to ion bombardment of titanium carbide.

A new preparation process for carbon nanotubes (CNTs) cold cathode was studied through the replacement of traditional organic or inorganic binder with Ag nano-particles. This method has the advantages of low preparation temperature and fine electrical contact between CNTs paste and substrate. A mixture paste of CNTs, Ag nano-particles and other organic solvents was spreaded on Si substrate. By melting and connecting of Ag nano-particles after sintered 30 min at 250 °C, a flat CNTs films with good field emission properties was obtained. The measurements reveal that the turn on electric field and the threshold electric field of as-prepared CNTs cathode are 2.1 and 3.9 V/μm respectively and the field emission current density is up to 41 mA/cm 2 at an applied electric field of 4.7 V/μm.

The effects of reoxidation annealing on the electrical properties and the positive temperature coefficient of resistance (PTCR) effects of multilayer Ba1.005(Ti1−x Nb x )O3 (BTN) ceramics sintered at different temperatures from 1100 to 1160 °C for 2 h in a reducing atmosphere and reoxidized at various temperatures in the range 500–900 °C were investigated. The results indicated that the room-temperature (RT) resistance of the BTN ceramics reoxidized at different temperatures first increased slowly and then quickly with the increase in the reoxidation temperature. Meanwhile, the resistance jump of the samples increased first and then decreased with the increase in the reoxidation temperature. In addition, the PTCR effect could be improved by prolonging the reoxidation time. Finally, the samples reoxidized at a low temperature of 600 °C after sintering at 1160 °C in a reducing atmosphere exhibited a significant PTCR effect, with a resistance jump of 3.5 orders of magnitude, along with a low RT resistance of 0.38 Ω. Additionally, the influences of the reoxidation temperature and time on the grain resistance and grain-boundary resistance were investigated using complex-impedance spectroscopy.