SnO2–ZnO composite nanofibers have been fabricated on Si/SiO2/Ti/Pt substrates by a novel method named as “stepwise-heating electrospinning” in this paper. The Si/SiO2/Ti/Pt substrates were fabricated by typical MEMS technology including some technological processes of thermal oxidation, photolithography, sputtering, and lift-off. Comparing with normal ceramic tube sensor fabrication process, spin coating or grinding was not needed during the sensors fabrication using silicon planar technology, which avoided destroying the original morphologies of nanomaterials. The ZnO-modified SnO2 shows good sensing properties to methanol due to the presence of N–N heterojunction at the interface of ZnO and SnO2 grains. The efficient charge separation of SnO2–ZnO heterojunction in the gas sensing performance was discussed from the perspective of energy band and formation of electronic accumulation layer as well as depletion layer. A detailed description of the change of band bending and potential barrier height of SnO2–ZnO composite nanofibers was also given, as well as a specific sensing mechanism in the process of methanol adsorption and desorption.

La0.7Sr0.3FeO3 nanoparticles assembled nanowires were synthesized by a hydrothermal method assisted with cetyltrimethylammonium bromide (CTAB). The hydrothermal temperature was 180 °C and the annealed temperature was 700 °C. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the morphology, composition and structural properties of the materials. The results showed that the La0.7Sr0.3FeO3 nanoparticles assembled nanowires had a high aspect ratio (the largest aspect ratio >100); the size of the nanoparticles was about 20 nm and the diameter of the nanowires was about 100–150 nm. The growth mechanism of La0.7Sr0.3FeO3 nanowires was discussed. Gas sensors were fabricated by using La0.7Sr0.3FeO3 nanowires. Formaldehyde gas sensing properties were carried out in the concentration range of 0.1–100 ppm at the optimum operating temperature of 280 °C. The response and recovery times to 20 ppm formaldehyde of the sensor were 110 s and 50 s, respectively. The gas sensing mechanism of La0.7Sr0.3FeO3 nanowires was investigated.Highlights► High aspect ratio La0.7Sr0.3FeO3 nanoparticles assembled nanowires were synthesized by a CTAB assisted hydrothermal method. ► Formaldehyde with low concentration (0.1–100 ppm) was used for gas sensing study. ► The growth mechanism of La0.7Sr0.3FeO3 nanowires was reported.

Y3+-doped and Mg2+-doped zirconia thick film humidity sensors were investigated. The humidity sensors exhibited good sensing characteristics. The sensor got a high sensitivity with the impendence changed five orders of magnitude from 108 to 103 Ω in the relative humidity (RH) range of 11–98% at 20 °C. The response time is about 30 s for Y3+ doped sensor, and 5 s for Mg2+ doped one, and recovery time is 5 s for both sensors. Small humidity hysteresis is about 3% RH and 4% RH for Y3+ and Mg2+ doped ZrO2 sensors, respectively. Moreover, good repeatability, linearity and temperature properties of the both sensors were also exhibited. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and defect structure were used to analyze the influence of dopant on humidity sensitive properties. And the mechanism of humidity sensing properties was also discussed.Research highlights▶ Y3+ doped and Mg2+ doped zirconia thick film humidity sensors were developed. ▶ The sensors exhibited high sensitivity, good linearity and small hysteresis. ▶ Rapid response-recovery behavior and less effect of temperature were also exhibited. ▶ The influence of dopant on the sensors was discussed by XRD, XPS and defect structure. ▶ The humidity sensing mechanism is investigated.

Formaldehyde is a kind of hazardous gases dangerous to human health. Hence, gas sensor is an essential device to monitor formaldehyde in air, especially in indoor ambient. Semiconductor metal oxides are studied as gas-sensing material to detect most of key gases for decade years. For the purpose of actual application and meeting a variety of conditions, diverse additives added into host material are expected to improve the performance of gas sensors. The formaldehyde gas-sensing characteristics of undoped and NiO-doped SnO2 (NSO) nanofibers synthesized via a simple electrospinning method were investigated in this study. It is noticed that the addition of NiO causes the distortion at the surface of SnO2 nanofibers, which is responsible to adjust activation energy, grain sizes and chemical states of host material. The sensors fabricated from NSO nanofibers exhibited good formaldehyde sensing properties at operating temperature 200 °C, and the minimum-detection-limit was down to 0.08 ppm. The response time and recovery time of the sensors were about 50 s and 80 s to 10 ppm formaldehyde, respectively. The sensor shows a good long-term stability in 90 days. The simple preparation and excellent properties significantly advance the viability of electrospun nanofibers as gas sensing materials. The sensing mechanisms of NSO nanofibers to formaldehyde were discussed. The results indicated that NSO nanofibers could be used as a candidate to fabricate formaldehyde sensors in practice.

Nanometer zirconia (ZrO2) with grain size 20 nm was used to make thick film humidity sensor on silicon substrate. The impedance of the sensor changed from 106 Ω to 102 Ω when the relative humidity (RH) varied from 11% to 98%. The curve of impedance vs frequency showed good linearity when the measured frequency was in between 100 Hz and 1 kHz. The temperature influenced the impedance of the sensor. Complex impedance (Nyquist) diagrams of the sensor at different relative humidities and temperatures were drawn. An equivalent circuit with resistors and capacitors was built to explain the conduction process of the sensor. In low RH range, the conduction process was dominated mainly by conduction and polarization of the grains of nanometer zirconia, while in high RH range, by decomposition and polarization of the absorbed water.

Micro-formaldehyde gas sensor of palladium doped tin dioxide was fabricated on silicon substrate. Finite element software ANSYS was used to analyze the thermal field distribution. In order to obtain a uniform thermal field distribution in the micro-gas sensor, the width of the marginal four strips of the heating electrode was designed to be 50 μm and the width of the central seven strips 25 μm. Undoped and 1 mol% Pd-doped SnO2 thin films were synthesized by sol–gel method on silicon substrates. XRD and XPS spectra of the films were analyzed. The response of the 1 mol% Pd-doped SnO2 micro-gas sensor to formaldehyde was much higher than that of the undoped SnO2 micro-gas sensor. This experimental observation was also supported by the XPS O (1s) patterns of the films which showed that doping 1 mol% Pd into SnO2 increased the value of Oa/Ol, i.e., the ratio of adsorption and lattice oxygens. Formaldehyde of 0.03 ppm concentration was detected by the 1 mol% Pd-doped SnO2 micro-gas sensor.

The Mn-doped ZnO piezoelectric films were prepared by sol–gel method. The ZnO films with perfect c-axis orientation were obtained in the annealing temperature range of 470–700 °C when 1% Mn ion (molar percent) was doped into precursor sol. The resistivity of the ZnO films annealed at 600 °C increased from 800 Ω cm (undoped) to 2 × 107 Ω cm (1% Mn-doped). The XPS spectra of Mn-doped ZnO films were analysed.

Thin films of Mn-doped ZnO with different doping concentration (0.8, 1, 3, 5 at%) were prepared on Pt/Ti/SiO2/Si substrates by using sol–gel method. The effects of the doping concentration on the structural properties, electrical characteristics and element binding energy in films were investigated. X-ray diffraction (XRD) results showed that the c-axis orientation of ZnO films was affected by Mn2+ content. Current–voltage (I–V) measurements indicated that resistivities of ZnO films were observably enhanced by dopant of Mn2+ and the resistivities value increased with a doping level up to 5 at% Mn. X-ray photoelectron spectroscopy (XPS) patterns suggested that the binding energies of O1s and ZnL3M45M45 were affected by the content of Mn2+.

The humidity sensing mechanism of the composite material of nanometer barium titanate and polymer quaternary acrylic resin (QAR) is discussed in terms of d.c. property, dielectric loss and thermally stimulated depolarization current (TSDC). Various conducting carriers in different humidity ranges are identified by the direct current curves. The dielectric loss may be caused by the polarizations of dipole orientation and interface charge. The TSDC patterns show that the active energy and relaxation time of the dipoles in the composite materials take continuous, rather than discrete, values.

In order to fabricate qualified ZnO piezoelectric thin films by sol–gel method, the relationships between the heat treatment temperatures (preheating temperature and annealing temperature) and the quality characteristics of ZnO piezoelectric thin films (c-axis orientation, residual stress, grain size, roughness and resistivity) were investigated. The chemical composition of the precursor sol and the intermediate produced in the films heating process were analyzed by TGA–SDTA and FT-IR. The c-axis orientation and the residual stress of the ZnO thin film were identified by XRD. The morphologies, roughness and grain size were observed and estimated by AFM. The I–V characteristics were measured by using semiconductor characterization system. Experimental results show that the c-axis orientation is determined by both preheating and annealing temperatures, and that the residual stress, grain size, roughness and resistance of the ZnO thin films are mainly influenced by the annealing temperature. A qualified ZnO piezoelectric thin film has been prepared by using sol–gel with preheating temperature 400 °C for 10 min and annealing temperature 700 °C for 30 min.

Resistive- (surface-) and capacitive- (sandwich-) types of humidity sensors of nanometer barium titanate were fabricated. The impedance, capacitance and dielectric loss properties of the sensors were tested under different temperatures. In the frequency range of 10–105 Hz, both types of sensors got the dielectric loss nearly in all the relative humidity range, and the peaks of the dielectric loss moved to the high frequency direction as the relative humidity (RH) or temperature increased, respectively. The results indicated that not only the conduction carriers but also the polarizations of the sensing material and/or the adsorbed water existed for both types of the humidity sensors. It is mainly the sensing material rather than the structure or the electrode form that determines the properties of the humidity sensors.

The sensing properties of the humidity sensor made of composite material of nanocrystalline lanthanum ferrite (LaFeO3) and polymer quaternary acrylic resin are investigated and compared with those of nanocrystalline lanthanum ferrite, including the sensitivity, the hysteresis, and the response and recover times. The measurement frequency influences both the linearity of the curves of resistance via relative humidity (RH) and the relation between capacitance and RH. By coating the ethyl cellulose on the humidity sensor as protecting films, the water resistance property of the sensor is improved. Humidity sensing mechanisms of the sensors are discussed.

•The SnO2/SnS2 heterojunctions based gas sensor exhibits excellent sensitivity to NO2 from 1 ppm to 8 ppm at the lower temperatures.•The enhanced NO2 sensing was attributed to the increased active sites for NO2 adsorption and formed heterojunctions inducing extra electrons.•The sensor shows an excellent selectivity compared to several gas vapors such as formaldehyde, alcohols, acetone, benzene, and methylbenzene.SnS2 nanosheets decorated with SnO2 nanocomposites have been successfully prepared by in-situ high-temperature oxidizing pristine SnS2 in air at 300 °C. The formed SnO2/SnS2 heterojunction based chemiresistive gas sensor exhibits an excellent sensitivity and selectivity to different concentrations of NO2 from 1 ppm to 8 ppm at 80 °C. The higher response of the as-fabricated SnO2/SnS2 heterojunction based sensor to NO2 compared with that of the pure SnS2 based one could be attributed to the extra charge transfer between SnO2/SnS2 interfaces.