Humidity sensor based on poly(lactic acid)/PANI-ZnO composite electrospun fibers

The electrospinning technique has been successfully used to prepared micro-fibers of the poly(lactic acid)/polyaniline–zinc oxide (PLA/PANI–ZnO) composite. The polyaniline–zinc oxide (PANI–ZnO) nanocomposites are synthesized by hydrothermal and in situ polymerization methods. X-ray diffraction techniques are used to study the structural properties of the PLA/PANI–ZnO composite fibers and the PANI–ZnO nanocomposite. The average crystallite size of the PANI–ZnO nanocomposite is found to be 36 nm. The morphology and diameter of the composite fibers are analyzed by scanning electron microscopy (SEM). The average fiber diameter of the pure poly(lactic acid) (PLA) fiber is around 2.5 μm and that of the PLA/PANI–ZnO composite fiber is around 1.4 μm. Differential scanning calorimetry (DSC) provides the thermal properties of the PLA/PANI–ZnO composite fibers. The melting temperature (T_m) for the pure PLA is observed at 149.3 °C, and it is shifted to 153.0 °C for the PLA/PANI–ZnO composite fibers. The enhanced thermal properties of the composite fibers are due to the interaction between the polymer and the nanoparticles. The water contact angle measurements probe the surface hydrophilicity of the PLA/PANI–ZnO composite fibers. The role of the PANI–ZnO nanocomposite on the sensing behavior of PLA fibers has also been investigated. The humidity sensing properties of the composite fiber based sensor are studied in the relative humidity (RH) range of 20–90% RH. The experimental results show that the composite fiber exhibited good response (85 s) and recovery (120 s) times. These results indicate that the one-dimensional (1D) fiber structure enhances the humidity sensing properties.

The electrospinning technique has been successfully used to prepared micro-fibers of the poly(lactic acid)/polyaniline–zinc oxide (PLA/PANI–ZnO) composite for humidity sensor application.

TiO2/(K,Na)NbO3 Nanocomposite for Boosting Humidity-Sensing Performances

Nanomaterials of TiO₂, (K_0.5Na_0.5)NbO₃, and the TiO₂/(K_0.5Na_0.5)NbO₃ nanocomposite were successfully synthesized by a hydrothermal method. Impedance-type humidity sensors were fabricated based on these materials. Our results reveal that the impedance of the TiO₂/(K_0.5Na_0.5)NbO₃ sensor changes by 5 orders of magnitude with an ultrahigh sensing response of S_f = 166 470 recorded at 100 Hz in the tested relative humidity (RH) range of 12-94%. This value is almost 2 and 4 orders of magnitude larger than that of the (K_0.5Na_0.5)NbO₃ and TiO₂ sensors, respectively. Interestingly, satisfactory response/recovery time (25/38 s, within 5 min), very small hysteresis (<5%), excellent stability, and good repeatability were also achieved in the TiO₂/(K_0.5Na_0.5)NbO₃ sensor. The improved sensing properties are ascribed to the synergistic effect of TiO₂/(K_0.5Na_0.5)NbO₃ heterojunction, which contributes the impedance that is susceptible to environmental humidity. This work underscores that it is a facile way to boost humidity-sensing performance by constructing proper nanocomposites.

Highly Sensitive and Selective H2S Chemical Sensor Based on ZnO Nanomaterial

ZnO is worth evaluating for chemical sensing due to its outstanding physical and chemical properties. We report the fabrication and study of the gas sensing properties of ZnO nanomaterial for the detection of hydrogen sulfide (H2S). This prepared material exhibited a 7400 gas sensing response when exposed to 30 ppm of H2S in air. In addition, the structure showed a high selectivity towards H2S against other reducing gases. The high sensing performance of the structure was attributed to its nanoscale size, morphology and the disparity in the sensing mechanism between the H2S and other reducing gases. We suggest that the work reported here including the simplicity of device fabrication is a significant step toward the application of ZnO nanomaterials in chemical gas sensing systems for the real-time detection of H2S.

Hierarchical Self-Assembled SnS2 Nanoflower/Zn2SnO4 Hollow Sphere Nanohybrid for Humidity-Sensing Applications

At present, humidity sensors have promising prospects in disease monitoring, family life, environmental protection, and so on. Flexible humidity sensor is more and more popular because of its flexibility and portability. In our work, a flexible humidity sensor based on a tin disulfide (SnS₂) nanoflower and a zinc stannate (Zn₂SnO₄) hollow sphere film was fabricated though layer-by-layer self-assembly technique. The humidity performance showed that the SnS₂/Zn₂SnO₄ hybrid film sensor was ultrasensitive to humidity at room temperature. The test results demonstrated superior response, fast response/recovery behavior, and excellent repeatability. Moreover, compared to the single SnS₂ and single Zn₂SnO₄ nanomaterials, the SnS₂/Zn₂SnO₄ hybrid film sensor exhibited great improvement in humidity sensing. In addition, complex impedance spectroscopy was adopted to further explore the sensing mechanisms of the SnS₂/Zn₂SnO₄ hybrid to various humidities. Human respiration, palm sweat, urine, and water droplets were delicately detected by the SnS₂/Zn₂SnO₄ humidity sensor, indicating its great potential in multifarious application fields.

ZnO-carbon nanofibers for stable, high response, and selective H2S sensors

Hydrogen sulfide (H₂S), as a typical atmospheric pollutant, is neurotoxic and flammable even at a very low concentration. In this study, we design stable H₂S sensors based on ZnO-carbon nanofibers. Nanofibers with 30.34 wt% carbon are prepared by a facial electrospinning route followed by an annealing treatment. The resulting H₂S sensors show excellent selectivity and response compared to the pure ZnO nanofiber H₂S sensors, particularly the response in the range of 102-50 ppm of H₂S. Besides, they exhibited a nearly constant response of approximately 40-20 ppm of H₂S over 60 days. The superior performance of these H₂S sensors can be attributed to the protection of carbon, which ensures the high stability of ZnO, and oxygen vacancies that improve the response and selectivity of H₂S. The good performance of ZnO-carbon H₂S sensors suggests that composites with oxygen vacancies prepared by a facial electrospinning route may provide a new research strategy in the field of gas sensors, photocatalysts, and semiconductor devices.

Enhanced moisture sensing properties of a nanostructured ZnO coated capacitive sensor

This work reports the enhancement in sensitivity of a simple and low-cost capacitive moisture sensor using a thin film of zinc oxide (ZnO) nanoparticles on electrodes.

Co3O4–SnO2 nanobox sensor with a PN junction and semiconductor–conductor transformation for high selectivity and sensitivity detection of H2S

Uniform Co₃O₄–SnO₂ nanoboxes have been synthesized successfully by a facile annealing treatment.

Hierarchical ZnO Nanowires-loaded Sb-doped SnO2-ZnO Micrograting Pattern via Direct Imprinting-assisted Hydrothermal Growth and Its Selective Detection of Acetone Molecules

We propose a novel synthetic route by combining imprinting transfer of a Sb-doped SnO₂ (ATO)-ZnO composite micrograting pattern (MP), i.e., microstrip lines, on a sensor substrate and subsequent hydrothermal growth of ZnO nanowires (NWs) for producing a hierarchical ZnO NW-loaded ATO-ZnO MP as an improved chemo-resistive sensing layer. Here, ATO-ZnO MP structure with 3-μm line width, 9-μm pitch, and 6-μm height was fabricated by direct transfer of mixed ATO and ZnO nanoparticle (NP)-dispersed resists, which are pre-patterned on a polydimethylsiloxane (PDMS) mold. ZnO NWs with an average diameter of less than 50 nm and a height of 250 nm were quasi-vertically grown on the ATO-ZnO MP, leading to markedly enhanced surface area and heterojunction composites between each ATO NP, ZnO NP, and ZnO NW. A ZnO NW-loaded MP sensor with a relative ratio of 1:9 between ATO and ZnO (1:9 ATO-ZnO), exhibited highly sensitive and selective acetone sensing performance with 2.84-fold higher response (R_air/R_gas = 12.8) compared to that (R_air/R_gas = 4.5) of pristine 1:9 ATO-ZnO MP sensor at 5 ppm. Our results demonstrate the processing advantages of direct imprinting-assisted hydrothermal growth for large-scale homogeneous coating of hierarchical oxide layers, particularly for applications in highly sensitive and selective chemical sensors.

Rational tailoring of ZnSnO₃/TiO₂ heterojunctions with bioinspired surface wettability for high-performance humidity nanosensors

We developed a novel kind of branched heterostructure by hydrothermal growth of ZnSnO3 nanostructures on TiO2 electrospun nanofibers, and demonstrated its enhanced ability to sense humidity through a sequential cactus-inspired tailoring of the ZnSnO3 nanostructures. Combining these results with first-principles calculations, it is deduced that the concentration of water molecules adsorbed on the ZnSnO3/TiO2 heterojunction surface can be increased by reducing the surface potential barrier. Meanwhile, the bioinspired ZnSnO3 nanoneedles, which form branches on the heterostructures, can further boost their adsorption abilities for water molecules via a water collection process. The adsorbed water molecules on the tips of the ZnSnO3 nanoneedles desorb easily in a low-humidity environment due to the small area of the tips (1.5-2.5 nm). Thus, the optimal ZnSnO3/TiO2 heterostructure exhibits response and recovery times of ∼2.5 s and ∼3 s, respectively. Its good sensitivity may enable it to detect tiny fluctuations in moisture and relative humidity that may surround any high-precision instrument.

Hydrothermal synthesis of Co–ZnO nanowire array and its application as piezo-driven self-powered humidity sensor with high sensitivity and repeatability

Self-powered humidity sensor with high sensitivity and repeatability has been fabricated from Co-doped ZnO NW arrays. Such a high performance can be attributed to the piezo-surface coupling effect and more active sites introduced by the Co dopants.

High-performance humidity sensors from Ni(SO4)0.3(OH)1.4 nanobelts

Ni(SO4)0.3(OH)1.4 nanobelts were synthesized by a facile hydrothermal method. Humidity sensors based on Ni(SO4)0.3(OH)1.4 nanobelts were fabricated and exhibited high sensitivity and a fast response. They also showed good long-term stability. The high performance could be related to the high surface-to-volume ratio of nanobelts and the chemical composition of Ni(SO4)0.3(OH)1.4.