Humidity-sensing Properties Research Articles

Humidity-Sensing Properties of a BiOCl-Coated Quartz Crystal Microbalance.

The performance of a bismuth oxychloride (BiOCl)-based quartz crystal microbalance (QCM) humidity sensor was studied using an oscillating circuit method. The BiOCl powder was prepared by a hydrolysis method. Scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy were used to characterize the BiOCl sample. Its humidity-sensing property was analyzed by combining it with a QCM at room temperature (25 °C). Experimental results indicated that the BiOCl-based QCM sensor showed good humidity characteristics from 11.3 to 97.3%, such as good logarithmic frequency response to humidity levels (R2 = 0.994), fast response time (5.2 s)/recovery time (4.5 s), good reversibility, stability, repeatability, and low humidity hysteresis. In addition, the response to human nose breaths showed excellent practicability. Finally, the humidity sensing mechanism of the BiOCl-based QCM humidity sensor was discussed in detail. This work demonstrates that BiOCl is a promising candidate material for humidity detection.

Surface Engineering of 1T/2H-MoS2 Nanoparticles by O2 Plasma Irradiation as a Potential Humidity Sensor for Breathing and Skin Monitoring Applications

For improving the sensor performance of molybdenum disulfide (MoS2), the modification of surface properties is essential. In this study, the surfaces of MoS2 nanoparticles with a mixture of 1T (metallic) and 2H (semiconductor) (1T/2H-MoS2) prepared solvothermally were modified by O2 plasma irradiation to enhance their humidity-sensing properties. The O2 plasma irradiation of MoS2 induced MoO3 formation on the surfaces. The amount of MoO3 increased with increasing O2 plasma irradiation time. The specific surface area, as well as total pore volume, increased after O2 plasma irradiation from 28 m2/g and 0.089 cm3/g to 185 m2/g and 0.279 cm3/g, respectively. The pore size distribution shifted to a lower value after O2 plasma irradiation. These results suggest that the O2 plasma irradiation can modify the surface nature of 1T/2H-MoS2 nanoparticles. The humidity-sensing properties of the obtained samples were evaluated with various relative humidity (RH) values. When the O2 plasma-irradiated MoS2 sensor was exposed to a higher RH, a higher response was obtained. The sensor response of the O2 plasma-irradiated MoS2 nanoparticles was significantly enhanced to around 100% at 95% RH compared with the sample before the O2 plasma irradiation. As a demonstration of a practical application, the materials were used for the sensing of human breath. The breath testing indicated a good response, and the sensor also distinguished wet and dry hands clearly. These results strongly suggested that the O2 plasma-irradiated MoS2 can be applied for breath testing as well as in skin monitoring devices.

Adsorption mechanism of water molecules on the surface of ZnO-SAW sensors

In the present study, the ZnO thin film was deposited on the surface acoustic wave (SAW) chip for sensitive humidity sensors by using radio frequency magnetron sputtering method. The microstructure of the ZnO thin film was characterized by using scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). The ZnO film exhibited rough morphology with the average thickness of about 0.3 μm. The humidity-sensing properties of the sensors were examined by changing the relative humidity (RH) within the range of 20%–60% RH at ambient temperature. The resonant frequency shifts increased gradually with the rising of RH levels. It was found that the ZnO-SAW sensors showed high sensitivity, mainly owing to the formation of the liquid water layer and superposition effect of water molecules (WMs). Moreover, the density functional theory (DFT) calculation revealed that the adsorption and desorption of WMs on the ZnO film were physical processes.

Facile and low-cost fabrication of a humidity sensor using naturally available sepiolite nanofibers

Much effort has focussed on enhancing the humidity-sensing performances of humidity sensors, but their fabrication using facile and low-cost methods is also desirable. In this work, a humidity sensor based on a naturally available nanomaterial, sepiolite nanofibers (SNFs), was facilely fabricated without any expensive raw materials or complex processes. Characterization results show that SNFs have a natural slender nanofiber structure (diameter 20–50 nm) and abundant hydrophilic functional groups (–OH). The results of humidity-sensing tests show that the SNF humidity sensor has outstanding humidity-sensing properties (i.e. large response, good linearity and repeatability) within the relative humidity range from 10.9% to 91.5% at room temperature (25 °C). This work presents a moderate and cost-effective strategy for the fabrication of high-performance humidity sensors using the natural SNF nanomaterial.

Colossal permittivity in (Li + Nb) co-doped Fe2O3 ceramics

(Li + Nb) co-doped (Li + Nb)xFe2-xO3 (with x = 0.0005, 0.005, 0.05, and 0.1) ceramics were prepared by sol-gel method. Their structural, dielectric, humidity, and magnetic properties were investigated. Colossal permittivity (~104) was approached or achieved in all doped samples even with a very small doping level of x = 0.0005. The colossal permittivity behavior is composed of two dielectric relaxations with the low-temperature one being a polaron relaxation due to electrons hopping between Fe3+ and Fe4+ ions and the high-temperature one being a Maxwell-Wagner relaxation caused by humidity-sensing properties.

Poly(vinyl alcohol)-based thin films for optical humidity sensing

The optical and humidity-sensing properties are studied of hydrophobically modified PVA, namely poly(vinyl alcohol-co-vinyl acetal)s of varied copolymer composition. Copolymers are synthesized by reacting PVA with acetaldehyde in aqueous solution, thus introducing cyclic acetal functionalities in the polymer chain. Thin polymer films are deposited by spin-coating and their optical properties are studied as a function of the copolymer composition, i.e., degree of acetalization. Reflectance measurements at different relative humidities in the range 5 – 95 % RH are conducted in order to probe the sensing behavior. The comparison with neat PVA thin films confirms the influence of the copolymer’s acetal fraction on the optical and sensing properties and degree of hysteresis. The feasibility of applying poly(vinyl alcohol-co-vinyl acetal) thin films for optical sensing of humidity is demonstrated and discussed.

Diamine decorated graphene oxide film on quartz crystal microbalance for humidity-sensing analysis

Graphene oxide (GO) materials using various synthetic processes and amination were deposited on quartz crystal microbalance (QCM) device for humidity sensor application. Characteristics of various GO materials including surface morphology and functional groups were investigated by SEM images equipped with EDX, Fourier transform infrared spectra (FTIR) and Raman spectra. Hydrophilic property was identified by contact angle measurement. Mechanism of humidity-sensing performance are were adopted to analyze the moisture sensitivity of GO loaded QCM sensors ranging from 23% to 92% relative humidity. In this study, GO produced by strong reductant (NaNO3) and temperature control combined with amination process could generate much intercalate functional groups and high hydrophilic property. High adsorption of water molecules in graphene oxide produce large interlayer expansion of GO layers, resulting in crystal motional resistance increase, energy loss and broaden peak pattern. In humidity-sensing property, amine GO exhibits quick humidity response/recovery (10 s/11 s) sensitivity and high Q-factor, thus amination of GO film can be a candidate for future humidity sensor material.

Comparative Study of Structural, Morphological and Humidity Sensing Properties of Pure WO3 and Cu2O-WO3 Nanocomposite

This work reports the enhancement of humidity sensing properties of tungsten trioxide (WO3) by the formation of nanocomposite with cuprous oxide (Cu2O). A 0.1 g of Cu2O was mixed with 0.9 g of WO3. Pellets of pure WO3 and the obtained mixture are prepared at pressure of 250 MPa applied for 0.5 h. The pellets were then annealed at 300 ºC, 400 ºC, 500 ºC and 600 ºC. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyzed the crystallinity and morphology of as-prepared pellets surface. Humidity sensing application of pellets is studied in a specially designed chamber. It is observed that as relative humidity increases, there is decrease in the resistance of the pellets. The humidity-sensing investigation (10-99%RH) showed that the nanocomposite of Cu2O with WO3 annealed at 600 ºC, is having the best sensitivity, low hysteresis, less ageing and high reproducibility than pure WO3. It is also observed that as annealing temperature increases from 300 to 600 ºC, sensitivity increases. The present investigation could be useful for fabrication of resistive type humidity sensors for commercial applications.

Comparative study of microstructural and humidity-sensing properties of graphene-based nanocomposite thin film

This article reports enhancement of the humidity-sensing properties of graphene (Gr) by the formation of nanocomposites with silver nanoparticles (AgNPs) and poly-methyl methacrylate (PMMA). Ultraviolet/visible spectroscopic study of the prepared materials confirms the formation of Gr and its nanocomposites. The relative capacitance/resistance, sensitivity and response/recovery time of the surface type Cu/Gr/Cu, Cu/Gr + AgNPs/Cu and Cu/Gr + AgNPs + PMMA/Cu humidity sensors are investigated under controlled relative humidity (RH) levels at 300 K. Comb-type interdigitated copper electrodes with a gap of 50 µm are thermally evaporated on a clean plastic substrate and a thin film of Gr and its nanocomposites is deposited on the surface of the pre-deposited interdigitated metallic electrodes using the drop-casting method. Analysis by x-ray diffraction and scanning electron microscopy shows the crystallinity and morphology of the as-deposited thin film surface. Raman spectroscopy is used to analyse the chemical structure, polymorphism and number of layers of the deposited thin films. The humidity-sensing investigation (30%–95% RH) shows an increasing capacitive and resistive response of the fabricated sensors due to the hydrophobic nature of the material at various input frequencies. The response and recovery times for capacitive and resistive Gr-based sensors are much shorter than the earlier reported values in the literature. The present investigation could be useful for fabrication of sensors for commercial applications.

Effect of Zn-Doping on the Structural, Optical, and Humidity Sensing Properties of Sol-Gel Synthesized NiO Thin Film

This paper presents the fabrication process and characterization process of zinc (Zn) doped nickel oxide (NiO) thin film. The main objective of this research is to investigate the electrical and structural properties of Zn-doped NiO thin film using sol-gel spin-coating method. Zn-doped NiO thin film was prepared by using spin-coating method. The samples were prepared at different doping parameters, which are undoped NiO, 0.5 at.% Zn-doped NiO, 1.0 at.% Zn-doped NiO, 1.5 at.% Zn-doped NiO and 2.0 at.% Zn-doped NiO. The studies of structural properties of the samples were conducted by using an X-ray diffraction. The optical properties of the samples were characterized by using ultraviolet-visible spectroscopy. The spectra of transmittance and absorbance of the samples were studied. The humidity sensing properties of the samples were conducted using humidity chamber and sensor measurement system.

Multifunctional and High-Sensitive Sensor Capable of Detecting Humidity, Temperature, and Flow Stimuli Using an Integrated Microheater.

A multifunctional sensor comprising humidity, temperature, and flow detection capabilities is fabricated with a facile, single-layered device structure. A microheater based on serpentine Pt microlines plays key roles in both humidity and flow sensing at the hot state by introducing an efficient Joule heating effect, and meanwhile functions as a reliable thermistor at the cold state for accurate temperature measurement. For the first time, the strong temperature-dependent humidity-sensing properties of graphene oxide (GO) are revealed using the microheater platform. The GO-based humidity sensor displays ultrahigh sensitivity [124/% relative humidity (RH)], fast response time (3 s), wide detection range (8-95% RH) at room temperature, while the sensitivity drops at elevated temperatures, indicating the non-negligible temperature effect. Interestingly, a linear relationship between sensitivity and voltage is observed for the flow sensor, indicating the capability to manipulate sensitivity by conveniently modifying the voltage applied on the microheater. Because the three sensors work independently with distinguishable output signals, multiparametric sensing is enabled to monitor various human activities, such as respiration, noncontact sensation, and so forth. This work develops a simple, cost-effective, and useful multiparametric-sensing platform using a microheater for potential applications in the growing fields of internet of things, healthcare monitoring, and human-machine interfaces.

Humidity sensor applications based on mesopores LaCoO3

Humidity sensor plays a pivotal role in determining the quality of products and the precision of instruments. In order to measure and control humidity, LaCoO3 mesopores sensor was prepared by a modified citrate technique. The sample was characterized by X-ray diffraction, Field emission scanning electron microscopy, High resolution transmission electron micrographs, Energy dispersive X-ray spectroscopy and Brunauer–Emmet–Teller. LaCoO3 exhibits rhombohedral distorted perovskite crystal structure with R3c space group. The humidity-sensing properties were investigated in a wide range of working humidity (11–97% RH) and frequency (100 Hz–100 kHz). The obtained results confirm that the optimum measuring frequency is 1 kHz. A great advantage of LaCoO3 is its porosity, which is essential for a humidity sensor. These pores serve as humidity adsorption sites. Another advantage is its high resistivity, which is reduced by approximately eight times its normal magnitude with increasing the surrounding humidity. The average crystallite size and surface area of the sample are 12.88 nm and 154.125 m2/g respectively. According to VSM results, the hysteresis loop of the sample indicates its paramagnetic nature. Furthermore, the ferroelectric hysteresis loop, that was observed at room temperature, implies the antiferroelectric nature of LaCoO3 nanoparticles. The obtained data confirms that the LaCoO3 sample is very sensitive to humidity and can be commercially used as a humidity sensing element.

Preparation and properties of humidity sensor based on K-doped ZnO nanostructure

KxZn1−xO (X = 0%, 3%, 5%, 10%) nanowires have been synthesized through hydrothermal method and characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. Dielectrophoresis nano-manipulation technique was employed to arrange the materials on pre-designed Ti/Au electrodes to fabricate the humidity sensors, and the humidity sensing properties of sensors were investigated. The experimental results show that K-doped ZnO humidity sensors exhibit more excellent humidity sensing than the undoped ZnO humidity sensor. Especially, 5% K-doped ZnO humidity sensor show the highest sensitivity, the response time reduced from 32 to 12 s, and have lower hysteresis and better reproducibility. The improvement of humidity sensing performance is explained by the increase of oxygen vacancy defects due to the K doping process. In addition, the sensing mechanism was analyzed by complex impedance spectroscopy and multilayer adsorption theory. These results demonstrate the potential application of K-doped ZnO nanowires for fabricating high performance humidity sensors.

Effect of Tin Element on the Structural, Optical and Humidity Sensing Properties of Cerium Oxide Nanoparticles

In this work, pristine and tin substituted cerium oxide nanoparticles (NPs) namely Ce1−xSnxO2, x = 0.0, 0.2, 0.4, 0.6, were prepared by a simple microwave synthesis method for humidity sensing studies. Different characterization methods such as scanning electron microscopy, transmission electron microscopy and energy-dispersive x-ray spectroscopy analysis demonstrated the formation of spherical NPs with an approximate crystalline size of 5–10 nm, desirable chemical composition and presence of oxygen defects. Also, optical measurements results demonstrated the increase of the band gap energy in tin substituted NPs due to formation of intermediate energy levels as a result of tin substitution. Furthermore, humidity sensing studies showed a high response of the fabricated sensors to humidity as a result of very small particle sizes along with the presence of tin in cerium oxide NPs. Finally, the fabricated humidity sensors showed a very good reproducibility of 95%. The results of this study confirm the possibility of realization of highly sensitive humidity sensors based on tin substituted cerium oxide NPs for application in real environments.

Effects of Cr Doping on Structural, Morphological, Optical, Humidity Sensing and Catalytic Properties of Magnetic Co3O4

Effects of Cr Doping on Structural, Morphological, Optical, Humidity Sensing and Catalytic Properties of Magnetic Co₃O₄

Improvement of Gas and Humidity Sensing Properties of Organ-like MXene by Alkaline Treatment.

Ti3C2T x MXene with an organ-like structure was synthesized from Ti3AlC2 (MAX phase) through the typical hydrofluoric (HF) acid etching method. Ti3C2T x MXene was further alkaline-treated with a sodium hydroxide solution to obtain alkalized Ti3C2T x. Room-temperature planar-type gas- and humidity-sensing devices were also fabricated by utilizing Ti3C2T x MXene and alkalized Ti3C2T x sensing material based on the dip coating method, respectively. The intercalation of the alkali metal ion (Na+) and the increase of the surface terminal oxygen-fluorine ratio ([O]/[F]) in Ti3C2T x can effectively improve humidity- and gas-sensing properties at room temperature. The developed alkalized Ti3C2T x sensor exhibited excellent humidity-sensing characteristics (approximately 60 times response signal change) in the relative humidity (RH) with a range of 11-95% and considerable NH3 sensing performance (28.87% response value to 100 ppm of NH3) at room temperature. The improvement of NH3 and humidity-sensing properties indicated that alkalized Ti3C2T x has great potential in chemical sensors, especially in NH3 and humidity sensors.

Humidity Sensing Properties of Metal Organic Framework-Derived Hollow Ball-Like TiO2 Coated QCM Sensor

This paper demonstrates three kinds of TiO2 with hollow ball, nanosphere, and nanoflower nanostructures coated quartz crystal microbalance (QCM) humidity sensors, which were synthesized by metal organic framework (MOF)-derived, hydrothermal and solvothermal route. The compositional and morphological characteristics of three kinds of TiO2 nanostructures were characterized by means of X-ray diffraction, specific surface area analysis, scanning electron microscopy, and water contact angle measurement. We found that the surface structure plays a crucial role in the humidity-sensing response properties of QCM sensors. The humidity sensing experiment was performed at room temperature. The MOF-derived hollow ball-like TiO2 coated QCM sensor exhibited excellent humidity sensing characteristics over a wide RH range (0%–97%RH), such as high sensitivity (33.8 Hz/%RH), good repeatability, and short dynamic response/recovery times (5 s/2 s). In addition, Langmuir adsorption model was developed to explain the humidity-sensing properties of the TiO2-based QCM sensor. This paper proves that MOF-derived hollow-ball TiO2 is a potential candidate nanomaterial for humidity sensor fabrication.

Optical, Magnetic and Humidity Sensing Properties of MnWO4–WO3 Composites

Optical, Magnetic and Humidity Sensing Properties of MnWO₄–WO₃ Composites

Effective enhancement on humidity sensing characteristics of sulfonated poly(ether ether ketone) via incorporating a novel bifunctional metal–organic–framework

Recently, sulfonated poly(ether ether ketone) (SPEEK) has been used as the humidity sensing material due to its simple processing, environmental stability, and large humidity responding range. However, intrinsic shortcomings such as large hysteresis and lags in response restrict its practical application. Thus, a novel bifunctional metal–organic–framework, MNS (abbreviation for MIL-101-NH2-SO3H) was prepared as an inorganic nano-filler to improve sensing properties of humidity sensor based on SPEEK, especially small hysteresis. Humidity sensitive properties of sensors were investigated by measuring the complex impedance spectra of sensors at different relative humidity (RH). The organic-inorganic humidity sensor based on the composite material (SPEEK/MNS-30%) showed the best sensing properties, such as quick response (absorption: 9s) and small hysteresis (<3% RH, almost eighth of SPEEK sensor). Besides, the mechanism of the enhancement caused by MNS was proposed and proved by investigating the surface structure and microstructure of humidity sensing film via scanning electron microscopy and transmission electron microscopy, respectively. In addition, the equivalent circuit was built up by complex impedance spectra to study the sensing behavior of the composite humidity sensor. The good reproducibility and stability of the humidity sensor fabricated by SPEEK/MNS-30% indicated that it had the potential to be applied for practical humidity detection.

An urchin-like SnO2/NaNbO3 nanocomposite with stable humidity-sensing properties at room temperature

An urchin-like nano-composite of SnO2/NaNbO3 was fabricated by the hydrothermal method and the influence of the Sn/Nb ratio in the samples on their humidity-sensing properties was investigated. The phases and morphologies of these composites and their elemental distributions were analyzed by X-ray diffraction scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The sensor based on the optimum Sn/Nb ratio of 1:0.4 exhibited remarkable humidity-sensing properties, including a good response (S = 4823.8) that was 8.72 times higher than that of the pure SnO2 sensor (S = 553.01). The composites showed rapid response and recovery times (3/9 s), as well as good stability, linearity, and excellent selectivity. The fabrication and humidity-sensing mechanisms were systematically analyzed using analog computations and Nyquist diagrams, respectively. Compared with pure NaNbO3 and SnO2 sensors, our easily prepared SnO2/NaNbO3 sensor demonstrates good sensing properties and holds great promise for use in humidity-sensing applications.