High-performance Humidity Sensors Research Articles

The microstructure and humidity sensing properties of the Mn3.15Co0.3Ni0.8O4 thin film with a three-dimensional nano-network structure

The Mn3.15Co0.3Ni0.8O4 (MCN) thin film with a three-dimensional nano-network structure was synthesized by a sol–gel technique. X-ray diffraction analysis was used to determine the film׳s composition and spinel structure. Scanning electron microscopy measurements revealed that the film exhibited a three-dimensional nano-network structure. The usefulness of the MCN film as a humidity sensor was investigated at room temperature at relative humidity (RH) values of 12–95%. The impedance of the MCN film humidity sensor was varied by approximately three orders of magnitude in the RH range from 12% to 95% with a high level of linearity. The response and recovery times of the sensor were approximately 30s and 50s, respectively. Furthermore, the MCN film humidity sensor was highly stable and reliable, suggesting potential applications of MCN films with nano-network structures in high performance humidity sensors.

Facile development of high performance QCM humidity sensor based on protonated polyethylenimine-graphene oxide nanocomposite thin film

In this paper, a protonated polyethylenimine (P-PEI)-graphene oxide (GO) nanocomposite thin film, referred as P-PEI-GO, was deposited on the quartz crystal microbalance (QCM) by a facile dipping process for the development of high performance humidity sensors. The morphological and structural characteristics of P-PEI-GO thin film were analyzed by scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscope. The humidity-sensing properties, such as response and recovery, sensitivity, repeatability, selectivity, humidity hysteresis and long-term stability, were investigated in a wide range of working humidity (11–97% RH). The resultant P-PEI-GO nanocomposite thin film possesses much larger specific surface area and richer hydrophilic groups than pure P-PEI film, leading to higher sensitivities and shorter response/recovery times of the sensor. Especially, the sensor preserves excellent durability, small hysteresis, good selectivity and stability compared with individual P-PEI or GO film sensor, which could be ascribed to the high mechanical modulus of GO and weak interaction between P-PEI and GO. Finally, the humidity sensing mechanism of proposed sensor was discussed in details. This research provides a feasible and effective approach to construct high performance humidity sensor operated at room temperature based on GO nanocomposites.

High-performance humidity sensors based on quartz crystal micro-balance coated with mesoporous silica MCM-41 thin film

Improved humidity sensors based on quartz crystal micro-balance coated with mesoporous silica MCM-41 thin film were fabricated. XRD, BET, TEM and SEM were employed to characterize sensing films so as to study the effects of its pore structure on the humidity sensing properties. The results reveal that sensitivity over a wide humidity range and especially for low humidity conditions. The high sensitivity, well reproducibility, short response and recovery times obtained were ascribed to the ordered pore arrays and high surface area of the MCM-41 films. This study has significance in tailoring the moisture sensitivity in the design of diverse sensors for practical applications.

Layered K(K1.5Eu0.5)Ta3O10 for humidity sensor

Dion–Jacobson structure layered perovskite K(K1.5Eu0.5)Ta3O10 was synthesized using a traditional solid-state reaction. A humidity sensor made from the obtained K(K1.5Eu0.5)Ta3O10 reveals that the sensitivity exhibits good linear response, and the response and recovery time is only about 6s and 2s at 85% RH respectively, showing fast response and recovery characteristics. Furthermore, the humidity hysteresis is only 7% RH. All these results indicate that K(K1.5Eu0.5)Ta3O10 can be utilized in high-performance humidity sensors.

Highly chemoresistive humidity sensing using poly(ionic liquid)s.

A novel resistance type humidity sensor was fabricated using poly(ionic liquid)s PEVIm-Br, which not only exhibited high sensitivity at a relative humidity (RH) in the range of 11-98%, but also featured short response and recovery time, as well as small hysteresis and good repeatability, proving poly(ionic liquid)s to be promising sensing materials for high-performance humidity sensors.

Fabrication and characterization of an ultrasensitive humidity sensor based on metal oxide/graphene hybrid nanocomposite

This paper demonstrated a flexible humidity sensor based on tin dioxide/reduced graphene oxide (RGO) nanocomposite film. The humidity sensor was fabricated on a polyimide substrate with microelectrodes by using a facile one-step hydrothermal route. The hydrothermal synthesized SnO2 nanoparticles and SnO2/RGO hybrid nanostructures were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The humidity sensing properties of the presented SnO2/RGO nano-hybrid sensor were investigated by exposing it to a broad humidity range of 11–97%RH at room temperature. Compared with traditional humidity sensors, the SnO2 modified graphene sensor demonstrated an ultrahigh sensitivity and a rapid response/recovery characteristic over a full humidity range measurement, highlighting the unique advantages of hydrothermal synthesis for sensors fabrication. Finally, the possible humidity sensing mechanism of the proposed sensor was discussed by using complex impedance spectra and bode diagrams. These observed results demonstrate that RGO modified with metal oxide is promising nanomaterials for constructing high performance humidity sensors in widespread applications.

Fast response with high performance humidity sensing of Ag–SnO2/SBA-15 nanohybrid sensors

In this work, a simple and straight forward technology of loading Ag nanoparticles in SnO2/SBA-15 nanocomposite for measuring room temperature relative humidity (RH) is demonstrated. The nanocomposite SnO2/SBA-15 comprising Ag nanoparticles exhibit several fundamental characteristics, such as high specific surface area, open porous structure and good interconnectivity. Therefore, the RH sensors based on Ag–SnO2/SBA-15 hybrid nanocomposite shows very high sensitivity and ultrafast response/recovery time to humidity, which outperform current porous structure based sensors. The impedance of synthesized nanocomposite sensor varies from 108 Ω to 102 Ω between 11 RH% and 98 RH%, the humidity range required for a sensor to operate at ambient humidity. Moreover, the resultant sensors also display relatively small hysteresis and long-term stability. This study demonstrates that Ag–SnO2/SBA-15 complex hybrid material is a step towards fabricating futuristic high performance humidity sensors.

One pot synthesis of mesoporous ZnO–SiO 2 nanocomposite as high performance humidity sensor

Abstract In this work, we report a facile synthesis of ZnO loaded mesoporous silica nanocomposite for detecting relative humidity (RH) at room temperature. ZnO was loaded into mesoporous silica using ex situ and in situ loading processes. Response of the nanocomposite sensors synthesized using different loading process were investigated by exposing them to humid environments in wide range of 11–98%RH at 25 °C. The sensor utilizing in situ loading process exhibits a ∼5 orders impedance change along with excellent linearity, quick response time, rapid recovery time, negligible hysteresis, good repeatability and stability. AC nyquist plots of the sensor at different RHs were analyzed to understand the humidity-sensing mechanism. Our study can open a new way for realizing ZnO–SiO 2 hybrid nanocomposite for fabrication of high performance RH sensors.

Humidity sensor based on electrospun (Na0.5Bi0.5)0.94TiO3–Ba0.06TiO3 nanofibers

(Na0.5Bi0.5)0.94TiO3–Ba0.06TiO3 (NBT–BT6) nanofibers were synthesized by electrospinning and characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The humidity sensor was fabricated by coating NBT–BT6 nanofiber on a ceramic substrate with Ag–Pd interdigitated electrodes. At 100Hz, the impedance of the humidity sensor changed by more than five orders of magnitude with the relative humidity (RH) changing from 11% to 95%. The response time and recovery time were about 2s and 50s, respectively, and the maximum hysteresis was around 5% RH. The results indicate that NBT–BT6 nanofibers are of great potential for high-performance humidity sensor applications.

Nano titania loaded mesoporous silica: Preparation and application as high performance humidity sensor

Relative humidity (RH) sensing response was evaluated using TiO2 supported mesoporous SBA-15 at room temperature in the 11–98 RH% range. The TiO2/SBA-15 hybrid nanocomposites were prepared by wet impregnation and direct loading of titanium species in mesoporous silica, SBA-15 under extreme acidic conditions. Humidity response of the nanocomposite sensor, including linearity, hysteresis, response/recovery time and stability, were investigated by exposing sensors to humid environments in wide range of 11–98 RH% at 25°C. It was found that 10wt% TiO2 loaded in SBA-15 nanocomposite produced by direct synthesis method exhibits improved sensing response toward RH change, along with excellent linearity, fast response/recovery time, excellent repeatability, negligible hysteresis and outstanding stability. The complex impedance spectra of the sensor at different RHs were analyzed to explore the humidity sensing mechanism. The investigation of humidity sensing characteristics of TiO2/SBA-15 nanocomposite indicates that the direct synthesis of mesoporous hybrid material can be used for the fabrication of high performance RH sensors.

High performance humidity sensors based on CeO2 nanoparticles

CeO2 is a good humidity sensitive material, which has been widely investigated by using direct current (d.c.) analysis method. In this paper, we used the complex impedance analysis method to analyze CeO2 nanoparticles (NPs). It was found that the impedance as a function of humidity exhibited the best linearity and smallest hysteresis at 1KHz than the capacitance or resistance did. The complex impedance modulus (|Z|) varied by four orders of magnitude from 3.8×107Ω to 4.3×103Ω in the humidity range of 11–98% RH. The value of the linearity fit correlation (R2) was 0.997. The sensor exhibits a rapid and reversible response characterized by a very small hysteresis (∼2% RH). The response time and recovery time were 10s and 3s, respectively. These results further demonstrated promising application of CeO2 as humidity sensor.

Effect of calcination temperature on sensing properties of Bi3NbTiO9

With Bi2O3, TiO2 and Nb2O5 as the starting materials, the conventional solid-state reaction method was used to fabricate Bi3NbTiO9 powders. The mixture of Bi2O3, TiO2 and Nb2O5 was calcinated at different temperatures. After being characterised using X-ray diffraction, differential thermal analysis and scanning electron microscopy techniques, the as-prepared powders were used to fabricate humidity sensors for investigating the possible effects of crystal structure on the electric properties. The results indicated that the calcination temperatures had significant effects on the humidity properties of Bi3NbTiO9. In comparison with the 773 and 1073 K samples, the humidity hysteresis of the 573, 873 and 1173 K samples were smaller. Besides, among all the samples, the response and recovery time of the 773 K sample was found to be the best. Further studies revealed the discrepancy in humidity properties was attributed to the structural evolution with temperature increasing and Bi3NbTiO9 prepared by the conventional method can be used for high-performance humidity sensor by controlling the calcination process.

Humidity sensing properties of CeO2–NiO nanocomposite materials

CeO2–NiO composites were prepared by a simple grind method followed by a heat-treatment process and characterized by X-ray diffraction, transmission electron microscope and nitrogen adsorption–desorption test. Humidity sensing properties of CeO2–NiO composites with different mass ratios were studied. The optimal result was obtained for the sample with CeO2–NiO mass ratio of 0.5. The impedance of the sensor changes more than five orders of magnitude within the humidity range from 11 to 95 % relative humidity (RH) at 100 Hz. The response and recovery time are about 3 and 19 s respectively, and the maximum hysteresis is <2 % RH. The results indicate the potential applications of CeO2–NiO composite (CeO2–NiO mass ratio of 0.5) for fabricating high performance humidity sensors.

Wide range high speed relative humidity sensor based on PEDOT:PSS–PVA composite on an IDT printed on piezoelectric substrate

A high performance humidity sensor for environmental and health monitoring was fabricated with a polymer–polymer composite sensing layer. PEDOT:PSS/PVA composite was synthesized by physically mixing PEDOT:PSS aqueous solution with PVA aqueous solution and was then sonicated and stirred at elevated temperatures. Sensors were fabricated by depositing the sensing layer on an IDT printed on a piezoelectric substrate. The sensors’ electrical characterizations were performed to find out the response toward varying humidity levels. The results showed that the composite's response toward humidity change is very stable and can effectively measure low humidity levels with fast response time. The AC capacitance and impedance of the sensor were measured between 0%RH and 80%RH. The maximum change in impedance was from 137kΩ to 110kΩ and for capacitance from 96pF to 124pF. The sensors were deployed in a real time humidity measurement and the readings were converted into corresponding %RH. The results indicated that the sensors show promising response with ±1% reading error. The response and recovery times of the sensor were measured to be 0.625s and 0.53s respectively showing excellent performance of the sensor toward increasing and decreasing humidity levels.

Humidity sensor based on AlPO4-5 zeolite with high responsivity and its sensing mechanism

AlPO4-5 zeolite was successfully employed in a resistance type humidity sensor. AlPO4-5 zeolite humidity sensor was fabricated by coating nanosized materials over interdigitated electrodes. The sensor shows good linearity over a relative humidity (RH) range from 11% to 95%. At the frequency of 100Hz, response and recovery time are both about 2s, and the maximum hysteresis is <3%. In nanoscale confinement range, the sorbed water quantity in AlPO4-5 channels is exceptionally large and the water sorption phenomenon looks like “capillary condensation” phase transition. The complex impedance at different RH was investigated to understand the sensing mechanism. The results indicate the potential application of AlPO4-5 zeolite for fabricating high-performance humidity sensors.

Synthesis and characterization of MgAl2O4 micro-rods by a molten salt method

Large amounts of MgAl2O4 micro-rods were successfully synthesized using the molten-salt technology. The effect of KCl contents on the formation of MgAl2O4 micro-rods was investigated. The structure and morphology of MgAl2O4 were investigated by means of powder X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy, respectively. The experimental results showed that the contents of KCl significantly influenced the formation of MgAl2O4 micro-rods. MgAl2O4 micro-rods could be prepared at 1150°C with a weight ratio of 100:1 between the salt and the starting materials. The formation of MgAl2O4 micro-rods could be suggested to be due to the inhomogeneous nucleation and orientated growth perpendicularly to the surfaces of Al2O3 grains. An impedance-type humidity sensor was finally fabricated based on the as-prepared MgAl2O4 micro-rods. According to tests of the humidity performance, MgAl2O4 micro-rods might be suitable for high-performance humidity sensors.

Room-temperature humidity-sensing performance of SiC nanopaper.

Centimeters-long SiC nanowire could be a strong "bridge" between microworld and macroworld due to its unique morphology, excellent chemical stability, and intriguing physical properties. Here we present a novel "paperlike" material-free-standing SiC nanopaper fabricated by acetone-assisted compression of centimeters-long SiC nanowires. The resistance of this new paperlike material linearly increases with increasing environmental relative humidity in a very short time. We suggest that adsorption of water molecules on SiC nanopaper surface led to fast electron transfer between SiC nanopaper and water layer, which indicates that SiC nanopaper could be applied to high-performance humidity sensor in harsh environment.

Humidity sensing properties of novel graphene/TiO2 composites by sol–gel process

This study prepared a novel sensing graphene/TiO2 (G/TiO2) material by using a sol–gel method with 1wt%, 5wt%, 10wt%, and 20wt% of graphene. The structure and morphology of the materials were characterized using X-ray diffraction, Fourier transform infrared spectrometry, Transmission electron microscopy, and N2 adsorption–desorption technology. The humidity-sensing properties were measured using an inductance–capacitance–resistance analyser. At a relative humidity of 12–90%, the sample with 10% of graphene exhibited more favourable humidity-sensing properties compared with the other samples. The prepared sensor exhibited a higher sensing response (S=151) and larger Brunauer–Emmett–Teller surface area (169.5m2/g) than sensors reported in previous studies. Moreover, the humidity hysteresis value (<0.39%) was very small at all tested humidity conditions. Our results demonstrate the potential for applying G/TiO2 composite materials to the fabrication of high-performance humidity sensors.

Humidity sensing properties of MoO3-NiO nanocomposite materials

MoO3-NiO composites were prepared by a simple one-step electrospinning method and were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and nitrogen adsorption-desorption test. Humidity sensing properties of MoO3-NiO composites with different mass ratios were studied, the best result was obtained for the sample with MoO3-NiO mass ratio of 0.5 (S1) under the same preparation condition. The impedance of the sensor changed more than three orders of magnitude when relative humidity ranged from 11% to 95%. The response and recovery time were about 2s and 10s respectively with maximum hysteresis of <2%RH (relative humidity). The above findings suggest potential applications of the MoO3-NiO composite (S1) in high performance humidity sensors.

Humidity sensing properties of K0.5Na0.5NbO3 powder synthesized by metal organic decomposition

K0.5Na0.5NbO3 (KNN) powder is synthesized via a metal-organic decomposition (MOD) method and characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and energy dispersive spectrometer (EDS). A humidity sensor, which is consisted of five pairs of AgPd interdigitated electrodes and an Al2O3 ceramic substrate, is fabricated by spin-coating the KNN powder on the substrate. The humidity sensing properties of the KNN humidity sensor are investigated at room temperature within the relative humidity (RH) range of 11–95%. The variations of the KNN humidity sensor impedance are about four orders of magnitude within the whole humidity range from 11% to 95% RH at the frequencies of 60Hz and 100Hz. The response time and recovery time of the KNN humidity sensor are all about 8s and 18s, and their maximum hystereses are all around 2% RH at 60Hz and 100Hz. Furthermore, at low RH and frequency conditions, the dielectric dissipation factor (DF) values of the KNN humidity sensor are small and hardly change, indicating that the KNN humidity sensor is of good insulating properties and reliability. The KNN powder is of broader potential applications for fabricating high performance humidity sensors.