High-performance Humidity Sensors Research Articles

Stable and Fast-Response Capacitive Humidity Sensors Based on a ZnO Nanopowder/PVP-RGO Multilayer.

In this paper, capacitive-type humidity sensors were prepared by sequentially drop-coating the aqueous suspensions of zinc oxide (ZnO) nanopowders and polyvinyl pyrrolidone–reduced graphene oxide (PVP-RGO) nanocomposites onto interdigitated electrodes. Significant improvements in both sensitivity and linearity were achieved for the ZnO/PVP-RGO sensors compared with the PVP-RGO/ZnO, PVP-RGO, and ZnO counterparts. Moreover, the produced ZnO/PVP-RGO sensors exhibited rather small hysteresis, fast response-recovery time, and long-term stability. Based on morphological and structural analyses, it can be inferred that the excellent humidity sensing properties of the ZnO/PVP-RGO sensors may be attributed to the high surface-to-volume ratio of the multilayer structure and the supporting roles of the PVP-RGO nanocomposites. The results in this work hence provide adequate guidelines for designing high-performance humidity sensors that make use of the multilayer structure of semiconductor oxide materials and PVP-RGO nanocomposites.

A high performance humidity sensor based on surface acoustic wave and graphene oxide on AlN/Si layered structure

A surface acoustic wave (SAW) humidity sensor based on AlN/Si (doped) layered structure and graphene oxide (GO) sensing layer is proposed for high sensitivity and low temperature coefficient of frequency. With the GO thin film, the sensitivity of the humidity sensor is increased to 42.08 kHz/RH% when the relative humidity is greater than 80%RH. The humidity sensor performs well even at both very low (≤10%RH) and very high (≥90%RH) detection range of humidity. The present sensor has low hysteresis, excellent short term repeatability and long term stability (variation less than ± 2%). The sensor also shows a fast response and short recovery time. Moreover, using the AlN/Si (doped) layered structure, the thermal stability of the sensor is significantly improved. After being covered with GO thin film, the temperature coefficient of frequency (TCF) of the sensor is further reduced to −22.1 ppm/°C, much smaller than the previously reported SAW humidity sensors.

Correction to "Organic Field-Effect Transistors with Macroporous Semiconductor Films as High-Performance Humidity Sensors".

Correction to "Organic Field-Effect Transistors with Macroporous Semiconductor Films as High-Performance Humidity Sensors".

Facile fabrication of high-performance QCM humidity sensor based on layer-by-layer self-assembled polyaniline/graphene oxide nanocomposite film

This paper demonstrates a layer-by-layer self-assembled polyaniline (PANI)/graphene oxide (GO) film on quartz crystal microbalance (QCM) for humidity sensing. The morphological and compositional properties of PANI/GO composite films were examined by means of SEM, XRD and FT-IR. The humidity sensing properties of the PANI/GO nanocomposite-based QCM sensor were investigated by exposing to a broad range of 0–97%RH. The experimental results showed that the presented PANI/GO nanocomposite sensor has a high sensitivity, a good repeatability and fast response/recovery characteristics, which surpasses the previous reported counterparts. Moreover, the underlying humidity sensing mechanism of the PANI/GO-based QCM sensor was discussed by using Langmuir adsorption model. This work highlights the unique advantage of layer-by-layer self-assembled route for QCM sensor fabrication.

Facile Fabrication of High-Performance Humidity Sensors by Flash Reduction of GO

Graphene is a promising sensing material that has been widely used for high-performance sensors. However, the development of graphene-based sensing device is limited by complex manufacturing procedures. Here, we report a facile fabrication of humidity sensors by flash reduction of graphene oxides (GO) with the help of a photographic camera flash. The flash treatment triggers a drastic deoxygenation of oxygen functional groups (OCGs) from the GO sheets, which not only restores its conductivity partly but also induces the formation of a highly porous structure. The reduced GO sensor shows high sensitivity at the relative humidity atmosphere of 11%–95%, small hysteresis, rapid response, and enhanced durability. The sensitive mechanism was discussed through the investigation of complex impedance plots. This facile flash reduction strategy holds great promise for chemical-free production of graphene-based sensing devices.

High-performance fibre-optic humidity sensor based on a side-polished fibre wavelength selectively coupled with graphene oxide film

A high-performance humidity sensor based on a side-polished single-mode fibre (SPF) coated with graphene oxide film was fabricated with a wheel side-polishing technique and spontaneous evaporation. The wavelength-selective coupling gave rise to a loss dip in the transmitted spectrum when a phase match condition was satisfied at a given resonant wavelength. The swelling effect of GO under high RH, which was confirmed by XRD and a spectroscopic ellipsometer, could increase the GO film thickness and led to a redshift at resonant wavelength. Experimental results show that the GO-film-coated SPF (GFC-SPF) is capable of working in two modes: tracing-wavelength mode and intensity variation mode. For the two working modes, the GFC-SPF has high sensitivity, good reversibility, good repeatability and excellent linearity. For the tracing-wavelength mode, a very high sensitivity of 0.145nm/RH% and a high linear correlation of 99.6% were achieved in a low RH range of 32%–85%, while an ultrahigh sensitivity of 0.915nm/RH% and a linear correlation of 98.7% were achieved in a high RH range of 85%–97.6%. For the intensity variation mode, a high sensitivity of 0.427dB/RH% and an excellent linearity correlation of 99.8% were achieved in a wide RH range of 58.2%–92.5%. Such a high performance allows GFC-SPF to have widespread potential applications, such as in the fields of biology, chemical processing and food processing.

Linear bi-layer humidity sensor with tunable response using combinations of molybdenum carbide with polymers

A high performance humidity sensor for environmental and health monitoring has been fabricated using various combinations of molybdenum carbide (Mo2C) with polyacrylamide (PAM) and polyvinyl alcohol (PVA). The humidity sensing properties of pure Mo2C were compared with those of its composites with PAM and PVA. The response curves and working principle were investigated and novel bi-layered sensors were then fabricated to achieve highly linear and stable response for a wide range of humidity sensing (0% RH to 90% RH). The sensors having a bi-layer combination of Mo2C with PAM give a linear response with a sensitivity of 775Ω/%RH and excellent repeatability. The working mechanism of the sensors allow them to be used with frequency response conversion read-out circuits and the results ascertain their linear and stable behavior. The response-time and recovery-time of the sensors were measured that gave average values of ∼1s and ∼2.5s respectively for the composites and ∼1.8s and 2.3s for the bi-layer configuration. The sensors are aimed to replace low performance complex and expensive sensors in the market for environmental and health monitoring applications.

Annealing temperature and bias voltage dependency of humidity nanosensors based on electrospun KNbO3 nanofibers

Humidity sensors based on nanofibers have been actively explored due to their unique structures comprising grains and grain boundaries. However, most of the resistance-type sensors lack linearity in sensing response and are operated at high voltages. In this report, resistance-type humidity nanosensors were fabricated from electrospun perovskite KNbO3 nanofibers and the effects of annealing temperature of nanofibers was investigated. A modified TGA setup was employed to test the absorption/desorption behaviour of the as-annealed nanofibers. KNbO3 nanofibers annealed at 550 °C displayed the highest sensitivity up to 104 for the humidity change from 15% to 90% RH. The I-V curves with respect to relative humidity (RH) measured at different biasing conditions revealed that disassociation of water molecules was dependent on the applied bias voltage thereby affecting the sensitivity. At higher RH environments, biasing the humidity nanosensor at higher voltage improved the linearity in the sensing response. Furthermore, the conductivity values of the as-fabricated humidity nanosensor were reproducible regardless of humidification and dehumidification process. The test outcomes from this study could offer better understanding on the design of high performance humidity sensors based on nanofibers.

Organic Field-Effect Transistors with Macroporous Semiconductor Films as High-Performance Humidity Sensors.

In this study, we designed and fabricated a high-performance humidity sensor based on a donor-acceptor polymer transistor. To improve its sensing performance, a polymeric semiconductor film with macroporous structure was prepared using a facilitated phase-separation method. The relationship between the sensing performance and the pore size was systematically investigated by testing the humidity-sensing performance. The results suggested that the sensitivity of the sensor was improved with increasing pore size within a certain range. The sensor based on the macroporous film with an average pore size of 154 nm exhibited a sensitivity of 415 and a response time of 0.68 s, as the low relative humidity (RH) changed from 32% RH (9146 ppm) to 69% RH (20 036 ppm). These sensitivity values are better than those obtained by other reported humidity sensors based on organic field-effect transistors.

Highly sensitive BEHP-co-MEH:PPV + Poly(acrylic acid) partial sodium salt based relative humidity sensor

A high performance humidity sensor for environmental and health monitoring has been fabricated using a composite of BEHP-co-MEH:PPV co-polymer and super hydrophilic Poly(acrylic acid) partial sodium salt (PAASS) as the sensing layer. The device was fabricated using all printing methods. Reverse offset printing was used to fabricate silver electrodes on LiNbO3 piezoelectric substrate while spin coating was used to deposit the composite active layer. Four samples with different concentrations of PAASS were fabricated to find out the effect of super hydrophilic material concentration on the performance of the sensors. Physical, chemical, and electrical characterizations were performed for the sensors and their response was recorded for varying humidity levels. The results showed that the response of sensors towards humidity change is very sensitive, quite stable, and can effectively measure low humidity levels with reasonable response and recovery times. The capacitance of the sensors was measured between 0% RH and 80% RH at 1kHz and 10kHz test frequencies. The maximum absolute change in capacitance was ∼2750pF (34pF per%RH sensitivity) for the sample with maximum concentration of PAAPSS recorded at 1kHz test frequency. Different composition ratios resulted in different performance parameters, with some of them improved, and the others compromised. The concentrations can be optimized with application specific requirements.

Capacitive humidity sensing properties of CdS/ZnO sesame-seed-candy structure grown on silicon nanoporous pillar array

Utilizing silicon nanoporous pillar array (Si-NPA) as substrate, a kind of CdS/ZnO sesame-seed-candy structure was prepared by successively growing ZnO nanorods and CdS nanograins via chemical vapor deposition and successive ion layer adsorption reaction method, respectively. The capacitive humidity sensing properties of CdS/ZnO/Si-NPA were studied by constructing coplanar interdigital electrodes. With the relative humidity (RH) increasing from 11% to 95%, a capacitance response over 201,530% was achieved, which indicates that CdS/ZnO/Si-NPA is highly sensitive to humidity. The response and the recovery times were determined to be ∼110 s and 32 s, respectively, with a maximum hysteresis being ∼2.67% at 75% RH. The high humidity sensitivity of CdS/ZnO/Si-NPA is attributed to the enlarged specific surface area induced by its unique nanostructure, while the short response/recovery times and the small hysteresis were due to the channel network brought by Si-NPA. These results indicate that CdS/ZnO/Si-NPA might be an ideal material system for fabricating high-performance humidity sensors.

High-performance humidity sensor based on a polyvinyl alcohol-coated photonic crystal cavity.

We demonstrate a high-performance relative humidity (RH) sensor by coating a photonic crystal (PC) cavity with polyvinyl alcohol (PVA). Because a PC cavity's evanescent field strongly interacts with the coated moisture-sensitive PVA film, the resonant wavelength is modified remarkably under varying RH levels ranging from 30% to 90%. In a PC cavity coated with a 720 nm thick PVA, the sensor exhibits a linear spectrum sensitivity exceeding 129 pm/%RH over 40-90%RH, and the power interrogation presents a high sensitivity as 0.77 dB/%RH. The resolvable humidity variation could be much less than 0.1%RH. Relying on the sub-micron thick PVA, the sensor promises a response time less than 300 ms and good repeatability. The dependence of the sensor performances on the PVA thickness is studied as well, indicating a tradeoff between the sensing dynamic range and the response time.

High performance humidity sensor based on metal organic framework MIL-101(Cr) nanoparticles

Metal organic framework MIL-101(Cr) nanoparticles were successfully HF- free synthesized via hydrothermal method and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Nitrogen adsorption-desorption technology. Then resistive humidity sensor was fabricated to investigate humidity sensing properties. The results indicate that the sensor shows high sensitivity and rapid response-recovery time. The impedance changes more than three orders of magnitude in the range of 33–95% RH at 100 Hz. The response time and recovery time are 17 s and 90 s respectively. Finally, the humidity sensing mechanism was discussed through complex impedance analysis. The results illustrate that porous MIL-101(Cr) nanoparticles have great potential to be used as humidity sensing materials.

High performance humidity sensor and photodetector based on SnSe nanorods

Tin selenide (SnSe) nanorods were synthesized using a one-step solvothermal route and their humidity sensing and photodetection performance at room temperature were investigated. The results depict that SnSe nanorod-based humidity and photosensors have good long-term stability, are highly sensitive and have fast response and recovery times. In the case of the humidity sensor it was observed that the resistance of the films decreased with increasing relative humidity (RH). The humidity sensing behaviors were investigated in the range 11–97% RH at room temperature. A response time of ∼68 s and recovery time of ∼149 s were observed for the humidity sensor. The photosensing behavior showed typical response /recovery times of ∼3 s with highly reproducible behavior.

Fabrication of resistance type humidity sensor based on CaCu3Ti4O12 thick film

Abstract A resistance type humidity sensor has been fabricated from an assembly of CaCu 3 Ti 4 O 12 thick film, Ag interdigitated electrodes, and an Al 2 O 3 ceramic substrate. The humidity sensing properties were measured using the direct current (DC) analysis method. The results show that the electrical properties of the CaCu 3 Ti 4 O 12 thick film are dependent on humidity and applied voltage. At low humidity, the film exhibited low conductivity and behaved as an insulator. However, at high humidity, the conductivity of the film increased due to the enhancement of ion conduction. These outcomes indicate that the measured resistance is highly dependent on the applied bias voltage within the whole humidity range i.e. 20–90% relative humidity (RH) at ambient temperature. The response and recovery times as well as sensitivity were determined to be around 2.8 min, 25 min, and 98.2%, respectively. Therefore, it is concluded that CaCu 3 Ti 4 O 12 thick film has good humidity sensing properties and has high potential in the application for fabrication of high-performance humidity sensors.

High Performance Humidity Sensor Based on ZnO Nanoparticles Synthesized by Co-Precipitation Method

ZnO nanoparticles were successfully synthesized by a low cost co-precipitation method using zinc nitrate and sodium hydroxide as the raw materials. It was observed that the synthesized temperatures greatly effect on the size of ZnO nanoparticles. The lower synthesized temperatures resulted in the smaller nanoparticles. By adjusting the mole ratio of sodium hydroxide, the size of ZnO nanoparticles was also changed. The smallest ZnO particles was 47 nm obtained with 0.7 mole of sodium hydroxide. The smallest ZnO nanoparticles from each synthesized temperatures were fabricated as humidity sensor, showing an impressive performance under different relative humidity (17-94% RH). It should be noticed that the ZnO nanoparticles humidity sensor synthesized at 75 °C exhibited high response for 2 times higher than that of synthesized at 95 °C. This is attributed to the higher surface area of ZnO nanoparticles for absorbed water molecule.

Design and Development for Capacitive Humidity Sensor Applications of Lead-Free Ca,Mg,Fe,Ti-Oxides-Based Electro-Ceramics with Improved Sensing Properties via Physisorption.

Despite the many attractive potential uses of ceramic materials as humidity sensors, some unavoidable drawbacks, including toxicity, poor biocompatibility, long response and recovery times, low sensitivity and high hysteresis have stymied the use of these materials in advanced applications. Therefore, in present investigation, we developed a capacitive humidity sensor using lead-free Ca,Mg,Fe,Ti-Oxide (CMFTO)-based electro-ceramics with perovskite structures synthesized by solid-state step-sintering. This technique helps maintain the submicron size porous morphology of the developed lead-free CMFTO electro-ceramics while providing enhanced water physisorption behaviour. In comparison with conventional capacitive humidity sensors, the presented CMFTO-based humidity sensor shows a high sensitivity of up to 3000% compared to other materials, even at lower signal frequency. The best also shows a rapid response (14.5 s) and recovery (34.27 s), and very low hysteresis (3.2%) in a 33%–95% relative humidity range which are much lower values than those of existing conventional sensors. Therefore, CMFTO nano-electro-ceramics appear to be very promising materials for fabricating high-performance capacitive humidity sensors.

First-principle and experiment investigation of MoS2@SnO2 nano-heterogeneous structures with enhanced humidity sensing performance

In this work, we report the First-principle investigation and synthesis of MoS2@SnO2 heterostructure as high-performance humidity sensor by a two-step hydrothermal method. The first-principles calculations were performed to explain water molecule adsorption mechanism by applying density of state model to simulate the interaction between water molecule and sensing base material. The higher specific surface and the lower adsorption energy theoretically predicted the improvement on humidity sensing performance, which was confirmed by experiments testing. The MoS2@SnO2 heterostructure exhibited promoted humidity sensing characteristics on response time of 53 s and recovery time of 21 s, while switching the humidity between 11% relative humidity (RH) and 95% RH. The corresponding humidity sensing mechanisms of MoS2@SnO2 were elaborately interpreted. This work could bring forward a new design method on practical humidity sensing devices with an excellent stability and fast response by using MoS2@SnO2 heterostructure.

Synthesis and humidity sensing properties of the VO2(B)@ZnO heterostructured nanorods

Hierarchical VO2(B)@ZnO heterostructure with ZnO nanorods radially growing on the surface of the VO2(B) cores has been successfully fabricated for the first time. The novel structure could be used as humidity materials. The sensor based on the as-synthesized product shows high sensitivity, rapid response and recovery, small hysteresis, and good stability. These improved properties are greatly ascribed to the unique structure of the 3D architectures with a large specific surface area and a synergistic effect. It is found that the resistance of the sensor obviously decreases with increasing the relative humidity. The results demonstrate that VO2(B)@ZnO nanostructures are potential to be used as effective and high performance humidity sensors.

Organic field-effect transistors as high performance humidity sensors with rapid response, recovery time and remarkable ambient stability

The stability of organic field-effect transistors (OFETs) under very high humidity condition has been the bottleneck in many applications. In this work, remarkable enhancement in ambient stability and performance of CuPc based OFETs are observed by exploiting the polarization of hydroxyl groups in Poly (vinyl alcohol) (PVA) dielectric layer, which is sandwiched between Al2O3 and Poly (methyl methacrylate) (PMMA) layer. The devices are used to fabricate OFETs based humidity sensors, which show exceptional ambient stability and rapid response to the water molecules in moisture. In a controlled experiment, the sensors demonstrate 0.73 s as response time and 0.52 s as recovery time. Such results are the fastest responses observed on humidity sensors fabricated based on OFETs. The enhanced responses of the sensors are due to the systematic polarization of the hydroxyl groups present in PVA layer by the additional dipole field arising from the adsorbed water molecules, which are also polarized under gate-field. The devices show no variation in threshold voltage as well as field-effect carrier mobility, measured throughout a year under ambient exposure. The specific design of the sensors with tri-layer dielectric system plays crucial role in enhancing the stability and moisture sensitivity, which can make the devices technologically very relevant.