High-performance Humidity Sensors Research Articles

All-inorganic lead-free halide perovskite Cs2TeBr6 enables real-time touchless human breath and finger related humidity monitoring

Halide perovskite (HP) materials have attracted much attention in recent years in the fields of smart home and medical detection due to their low fabrication cost, simple preparation process, controllable properties, and adjustable morphology. However, the inherent instability and toxicity of lead has been barriers limiting the practicality in electronical devices. Here, we demonstrate a high-performance impedimetric humidity sensor based on an all-inorganic lead-free HP Cs2TeBr6 for the first time. The Cs2TeBr6 sensor achieves ultra-high response value of 5 orders of magnitude (8.89 × 105) at 11–97 % relative humidity (RH), short response time (4 s), favorable repeatability, and good long-term stability. Moreover, high humidity sensing performance enables the Cs2TeBr6 sensor to enjoy great potentials in real-time touchless human breath and finger detections. In addition, the mechanism of moisture sensitivity is explained by complex impedance spectra at different humidity levels. This work contributes an attracting candidate material for humidity sensors, and furthermore provides new insights into next-generation sensory devices using lead-free HPs for human-related detections.

High sensitivity and excellent durability of wearable microenvironmental humidity sensors inspired by the spider-web

In this study, we report a high-performance microenvironmental humidity sensor inspired by a spider-web structure, where a carbon nanotube network with cellulose nodes is constructed on the surface of cotton fabric by micro-dissolution technology. The obtained MWCNTs/cotton fabrics, as wearable humidity sensors, show excellent sensitivity when compared to other cotton fabric-based humidity sensors (maximum response of 99.6 %). In addition, the cellulose interlocking structure between the spider-web humidity-sensitive layer and the cotton substrate, provides the sensor with a strong interfacial adhesion. The sensor exhibits excellent durability (50 times cycling, 500 times folding, 20 times friction and stability over 3 months). As a theoretical validation, we simulated baby nappy moisture detection. The sensor shows significantly higher sensitivity and accuracy than commercial hygrometers and nappy discoloration strips, indicating that our sensor has great potential for humidity detection and early warning in body-worn microenvironments.

Synthesis and characterization of PVDF/PMMA/ZnO hybrid nanocomposite thin films for humidity sensor application

This work aimed PVDF/PMMA thin films in a range of ZnO NPs weight ratios that were successfully manufactured via spin coating as high-performance humidity sensors. The films have been characterized to calculate the structure, functional groups, transmittance and optical energy band gap. X-ray diffraction (XRD) structural studies reveal that the prepared films are polycrystalline with hexagonal crystal structures. It was found that the grain size of these films decreases with the increasing weight ratios of zinc oxide nanoparticles. Field emission scanning electron microscopy reveals the homogeneous distribution of zinc oxide nanoparticles in the PMMA/PVDF matrix. EDX was used to investigate the chemical composition of the PVDF/PMMA/ZnO nanocomposite. FTIR test was performed to identify the functional groups in the prepared samples as well as the detection of doping which is added to the samples. The optical energy band gap values were calculated and types of optical transmission and absorption of the films were determined from the optical transmittance spectra. The optical absorption studies in the wavelength range (200−700) nm show that energy band gap values of the prepared films decrease from (4.98–4.28) eV with increasing the ZnO NPs weight ratio. A sensor for relative humidity within the range (31−100) % RH was manufactured by studying the change of electrical properties (capacitive, resistance) and simulating them theoretically. It was found that the PVDF/PMMA/ZnO NPs nanocomposite is highly sensitive to humidity due to the high porosity of the material. The parameters of the sensors, such as response time and retrieval time, were also studied, and it was found that the manufactured sensors have acceptable values for these parameters.

A high-sensitive microwave humidity sensor based on one-step laser direct writing of dielectric silver nanoplates

Microwave technology has emerged as an appealing alternative for building humidity sensors with multiple detection parameters. This paper demonstrates the one-step laser direct writing fabrication of microwave humidity sensors. Silver nanoplates were used as both the humidity-sensitive material and a precursor to highly conductive Ag electrodes. Particularly, the unirradiated silver nanoplates film shows an unusual lossy dielectric nature, with the effective complex dielectric permittivity being dramatically tuned by humidity stimuli. Through a laser-induced sintering mechanism, laser irradiation bursts the conductivity by approximately 10 orders of magnitude with a maximum conductivity value of 9.7 × 106 S/m. A prototype microwave humidity sensor is designed and fabricated and its humidity sensing performances are comparatively studied and discussed. The measured results show that the sensor displays a high humidity sensitivity of up to 12.61 MHz/RH or 0.288 dB/RH. This work suggests a good candidate approach for the realization of high-performance microwave humidity sensors with a fast and simplified advantage.

Ferroelectric Polarization and Oxygen Vacancy Synergistically Induced an Ultrasensitive and Fast Humidity Sensor for Multifunctional Applications.

With the arrival of the Internet of Things and artificial intelligence, humidity sensors monitoring water emissions from human metabolism have attracted great attention in the fields of smart wearable devices and noncontact human-machine interaction. However, their application is seriously limited by the trade-off between the sensitivity and response speed for traditional humidity sensors. Herein, to overcome it, a self-powered high performance humidity sensor is developed on the basis of the electric-poled and oxygen vacancy-rich BiFeO3 (BFO) ferroelectric material. The synergistic effect of ferroelectric polarization and oxygen vacancy provides a strong driving force and active adsorption sites for an abundance of OH/H2O adsorption, resulting in an ultrahigh response (∼104) and ultrafast response/recovery speed (∼84/376 ms). Benefiting from its promising advantages, the wearable humidity sensor can accurately record the respiration rate/depth and recognize different human respiratory behaviors in real-time. Importantly, by utilizing the moisture from mouth-blowing and skin, the sensors are successfully applied to noncontact control of a robotic car, noncontact switch, and noncontact interface for visualization applications. This work provides an effective strategy for developing excellent humidity sensors that meet the requirement of noncontact interaction for next-generation intelligent electronics.

Smart wearable band-aid integrated with high-performance micro-supercapacitor, humidity and pressure sensor for multifunctional monitoring

Band-aids, which possess breathable woven fabric and a compressible cotton pad, can be a potential substrate for smart wearable electronics. However, integrating energy storage modules and multiple functions into commercial band-aids through a simple and practical method is still a huge challenge. In this work, we design a multifunctional intelligent wearable band-aid, which integrates the high-performance micro-supercapacitor (MSC) with humidity and pressure sensors, through a facile laser-scribing and single-wall carbon nanotubes (CNT) drop-casting strategy. As a proof of concept, the fabricated laser-induced sulfur-doped porous graphene-based MSC on the band-aid surface offers an excellent areal capacitance of 68.6 mF cm−2 and good cyclic stability. Also, the designed humidity sensor has a fast and reversible real-time response and a wide relative humidity (RH) sensing range (11–97% RH). Remarkably, the obtained capacitive pressure sensor has a high sensitivity of 25.15 kPa−1, an ultra-broad linear range (0 ∼ 300 kPa) and a low limit of detection of 2.92 Pa. This proposed band-aid based multifunctional integrated system paves a new avenue for the development of smart wearable and flexible electronics.

Flexible humidity sensors composed with electrodes of laser induced graphene and sputtered sensitive films derived from poly(ether-ether-ketone)

Currently, there are still many challenges to fabricate flexible humidity sensors with excellent properties in large-scale production. In this paper, a new and convenient method was proposed to prepare flexible humidity sensors, which were composed with interdigital electrode (LIG-IDE) of laser-induced graphene on flexible substrates of poly(ether-ether-ketone) (PEEK) and impedance-type hygroscopic film by PEEK sputtering and polymerization. The humidity sensors displayed good humidity sensing properties with high sensitivity, fast response/recovery time (4.2/6.8 s), and a wide operating range of relative humidity (11% RH∼97% RH). In the mechanism study of PEEK sputtered and polymerized humidity sensors, we found that the impedance-humidity response can be attributed to hydrophilic functional groups formed on the polymer chains by moist aging with oxidation. The above humidity sensitive advantages extended the flexible sensor's versatility, including contactless humidity sensing and breath detection for human applications. Therefore, a high-performance flexible humidity sensor entirely based on the PEEK polymer material is reported here, which provides a novel approach for the development of flexible gas and humidity sensors.

High-performance humidity sensor based on plasma modified graphene

Humidity monitoring has flourished in many advanced applications, such as environmental monitoring, industries, and medical health. Humidity devices with fast response and high sensitivity are in high demand for complicated application conditions. In this work, we bridge this gap by employing oxygen plasma treatment in a graphene-based humidity sensor. The sensitivity can be regulated from −99.2% to 153.1%, meanwhile retaining a high-speed response with rise and fall times ∼58 ms and ∼661 ms. This work provides a promising strategy for humidity detection with fast response and high sensitivity.

A free-standing humidity sensor with high sensing reliability for environmental and wearable detection

Humidity measurement has been of extreme importance in both conventional environment monitoring and emerging digital health management. State-of-the-art flexible humidity sensors with combined structures, however, lack sensing reliability when they subject to high humidity with condensation and/or liquid water invasion. Here, we report a free-standing humidity sensor by creating a stable open porous graphite structure and controlling the number of oxygen-related groups at the molecular level. The sensor indicates high sensing reliability against water liquid wetting and machine washing as well as many mechanical deformations. We demonstrate the sensor has wide potential applications in challenge environmental monitoring and wearable body area sensing networks. Particularly, the concept of the humidity sensing strategy is applicative to not only cellulose-based materials such as cotton and linen, but also protein-based materials such as silk, paving a new route for producing high-performance and cost-effective humidity sensors.

Impedance Analysis of Chitin Nanofibers Integrated Bulk Acoustic Wave Humidity Sensor with Asymmetric Electrode Configuration.

This paper fabricated a high-performance chitin nanofibers (ChNFs)-integrated bulk acoustic wave (BAW) humidity sensor with an asymmetric electrode configuration. The ChNFs were successfully prepared from crab shells and used as moisture-sensitive materials to compare the performance of quartz crystal microbalance (QCM) humidity sensors with symmetric and asymmetric electrode structures. The QCM humidity sensor with a smaller electrode area exhibited high sensitivity of 58.84 Hz/%RH, competitive response/recovery time of 30/3.5 s, and low humidity hysteresis of 2.5% RH. However, it is necessary to choose a suitable electrode diameter to balance the stability and sensitivity because the impedance analysis result showed that the reduction of the electrode diameter leads to a sharp decrease in the Q value (stability). Next, the possible humidity-sensitive mechanism of the ChNFs-integrated asymmetric n-m electrode QCM humidity sensor was discussed in detail. Finally, the reasons for the highest sensitivity of the asymmetric n-m electrode QCM humidity sensors having a smaller electrode diameter were analyzed in detail in terms of both mass sensitivity and fringing field effect. This work not only demonstrates that the chitin nanofiber is an excellent potential material for moisture detection, but also provides a new perspective for designing high-performance QCM humidity sensors.

Integrated Capacitive- and Resistive-Type Bimodal Relative Humidity Sensor Based on 5,10,15,20-Tetraphenylporphyrinatonickel(II) (TPPNi) and Zinc Oxide (ZnO) Nanocomposite.

The development of high-performance humidity sensors to cater for a plethora of applications, ranging from agriculture to intelligent medical monitoring systems, calls for the selection of a reliable and ultrasensitive sensing material. A simplistic device architecture, robust quantification of ambient relative humidity (% RH), and compatibility with the contemporary integrated circuit technology make a bimodal (capacitive and resistive) surface-type sensor to be a prominent choice for device fabrication. Herein, we have proposed and demonstrated a facile realization of a 5,10,15,20-tetraphenylporphyrinatonickel (II)–zinc oxide (TPPNi-ZnO) nanocomposite-based bimodal surface-type % RH sensor. The TPPNi macromolecule and ZnO nanoparticles have been synthesized by an eco-benign microwave-assisted technique and a thermal-budget chemical precipitation method, respectively. It is speculated from the morpohological study that specific surface area improvement, via the provision of ZnO nanoparticles on micro-pyramidal structures of TPPNi, may reinforce the sensing properties of the fabricated humidity sensor. The relative humidity sensing capacitive and resistive characteristics of the sensor have been monitored in 40–85% relative humidity (% RH) bandwidth. The fabricated sensor under the biasing conditions of 1 V of applied bias (Vrms) and 500 Hz AC test frequency exhibits a significantly higher sensitivity of 387.03 pF/% RH and 95.79 kΩ/% RH in bimodal operation. The average values of both the response and recovery times of the capacitive sensor have been estimated to be ∼30 s. It has also been debated why this high degree of sensitivity and considerable reduction in response/recovery time has been obtained. In addition, the intense and wide bandwidth spectral response of the TPPNi-ZnO nanocomposite indicates that it may also be utilized as a potential light-harvesting heterostructured nanohybrid in future studies.

High performance humidity sensor based on crystalline copper tungstate nanoparticles at room temperature

This work demonstrates the synthesis and fabrication of Copper tungstate (CuWO4) nanoparticles-based humidity sensor. The as synthesized sample was characterized by X-ray diffraction, Raman spectroscopy, photoluminescence and high resolution transmission electron microscopy to study structural and morphological nature. The humidity sensing performance of CuWO4 nanoparticles based sensor device was tested at room temperature which was exposed to 11%-97% relative humidity levels. The sensitivity of the device was found to be ∼767 at 97% relative humidity level. The CuWO4 nanoparticles sensing device showed ultra-low response and recovery time of ∼39 sec and ∼18 sec respectively. The high sensitivity and response/recovery was likely due to the high adsorption energy, fast charge transfer rate and low bonding energy of water molecules with CuWO4 nanoparticles. Our results indicate that CuWO4 nanoparticles-based humidity sensor device has great promising application as a portable device for control humidity detection.

High Sensitive Humidity Sensors Based on Biomass Ionogels

This work proposes humidity sensors based on ionic liquid loaded in situ crosslinked ionogels. Stable ionogels were constructed by the synergistic effect of k-carrageenan and gelatin to ensure the uniformly dispersion of ionic liquid, which is conducive to the stability of humidity sensors. The humidity sensitive electrolyte with different contents were introduced into the ionogels. By controlling the ratio of ionic liquid in the crosslinked ionogels, the optimal sensor exhibits a high sensitivity in the low humidity range (5% - 35% RH). The resultant sensors exhibit excellent repeatability, and possess good stability in long-term tests up to 60 days. The introduction of ionic liquid to the sensing film is beneficial for enhancing the water adsorption capacity and ionic conductivity, which is of great significance for high-performance humidity sensors.

Carbon nanodot-based humidity sensor for self-powered respiratory monitoring

Developing low-cost, portable, and high-performance humidity sensors for respiratory monitoring has received considerable attention in recent years. Herein, humidity sensors have been fabricated based on carbon nanodots (CDs), which exhibit high sensitivity (5318%) at 94% relative humidity (RH) and excellent long-time stability under the RH range of 11% to 94%. The high sensitivity can be attributed to the adsorption of the water molecules by the massive hydrophilic functional groups on the surface of CDs. By introducing a breath-driven triboelectric nanogenerator (TENG), the self-powered humidity sensor is developed for the first time, exhibiting a wide sensing range (11–94% RH) and excellent stability. The maximum output voltage of the TENG is up to 200 V and the maximum short-circuit current is about 9.2 μA. Furthermore, the self-powered humidity sensor further demonstrates the potential ability for real-time respiration monitoring, which can detect different breathing statuses. This work provides a convenient and low-cost strategy for constructing sensitive CDs-based humidity sensors, and prospects their applications in respiratory monitoring systems.

High-performance humidity sensor for multipurpose applications by recycling of potato peel bio-waste

This study suggests the development of a biocompatible and eco-friendly humidity sensor fabricated by using a naturally available potato peel bio-waste as the substrate and the humidity sensitive material. Entire fabrication process of this potato peel-based humidity sensor (PPHS) took less than two minutes owing to the facile and handmade patterning of silver interdigitated electrodes on the surface of dry potato peel. The fabricated humidity sensor yielded a high sensitivity (70 kΩ/ % RH) due to the presence of abundant polar hydrophilic functional groups. The achieved results were stable for more than seven days and repeatable data was obtained for three different samples. Furthermore, a quick response/recovery time of 0.96/1.65 s was recorded for nose breathing with a small hysteresis of only 0.21 % for adsorption and desorption process. The sensor exhibited a good linearity (R2 = 0.997) in the practical applications-based humidity sensing range of 40–85 % RH. Efficient use of PPHS was successfully demonstrated in multiple applications including medical diagnosis, skin care, real-time environment monitoring and touchless sensing owing to its biocompatibility and nonhazardous nature. In addition to the above listed multifunctional applications of PPHS, economic use of potato peel in commercial devices can help in recycling the worldwide wastage of this bio-waste.

Ultrafast, highly sensitive, flexible textile-based humidity sensors made of nanocomposite filaments

Wearable electronics are promising next-generation smart textiles for human-computer interaction, health monitoring, and other applications. However, integrating electronic functions into strong, highly deformable electronic textiles and maintaining their functionalities during wear is a great challenge. Herein, we report ultrafast, highly sensitive, textile-based humidity sensors made of single-walled carbon nanotube (SWNT)/polyvinyl alcohol (PVA)/lithium chloride (LiCl) nanocomposite filaments fabricated via a wet-spinning and a solvent exchange process. The SWNT/PVA/LiCl textile-based humidity sensors exhibited excellent stabilities during deformation and high sensitivities with 6-fold resistance variations over a wide range of relative humidity (RH) conditions. The ultrafast responses of the sensors were attributed to the quick deliquescence of LiCl during contact with water molecules and the simultaneous conductivity change caused by the generated ions. The as-prepared sensors responded well to human breathing and real-time humidity changing, which could distinguish different breaths and microclimate changes. Such outstanding sensing performances of our textile-based sensors, in combination with their mechanical flexibilities, enable the facile integration of high-performance humidity sensors onto textile substrates for real-time microclimate monitoring.

Morphology-controlled BaTiO3 nanostructures with high sensitivity and fast response for respiratory monitoring

As a typical perovskite oxide, BaTiO3 has been fully proved to be an excellent impedance humidity sensing material, meanwhile its application in human respiratory monitoring needs to be further researched. In this contribution, BaTiO3 nanorods (BT-NR), nanowires (BT-NW) and micron particle (BT-MP) were successfully synthesized by the molten salt synthesis (MSS) method. Various characterization techniques were used to analyze the structure, morphology and composition of these materials. As-synthesized BaTiO3 samples had been assembled as impedimetric humidity sensors and then tested systematically under relatively humidity (RH) range of 11%–97%. All the three sensors exhibited high response values greater than three orders of magnitude, which are much higher than the commercial BaTiO3 (BT-C) sensor (S97% = 4.90 × 102). Thereinto, the BT-NR humidity sensor holds the fastest response-recovery times (1 s and 22 s) and the smallest hysteresis (ΔH = 0.67%). Excellent sensing performance of BT-NR can be attributed to synergistic effects of larger specific surface area and smaller pore size. In addition, the BT-NR humidity sensor shows considerable potential in the field of human respiratory monitoring. This work provides a new perspective to high-performance humidity sensors for human body-related detection through morphology-controlled perovskite nanostructures.

Aerosol-Printed MoS2 Ink as a High Sensitivity Humidity Sensor.

Molybdenum disulfide (MoS2) is attractive for use in next-generation nanoelectronic devices and exhibits great potential for humidity sensing applications. Herein, MoS2 ink was successfully prepared via a simple exfoliation method by sonication. The structural and surface morphology of a deposited ink film was analyzed by scanning electron microscopy (SEM), Raman spectroscopy, and atomic force microscopy (AFM). The aerosol-printed MoS2 ink sensor has high sensitivity, with a conductivity increase by 6 orders of magnitude upon relative humidity increase from 10 to 95% at room temperature. The sensor also has fast response/recovery times and excellent repeatability. Possible mechanisms for the water-induced conductivity increase are discussed. An analytical model that encompasses two ionic conduction regimes, with a percolation transition to an insulating state below a low humidity threshold, describes the sensor response successfully. In conclusion, our work provides a low-cost and straightforward strategy for fabricating a high-performance humidity sensor and fundamental insights into the sensing mechanism.

A Nb2CTx/sodium alginate-based composite film with neuron-like network for self-powered humidity sensing

Existing wearable sensing electronics generally require to connect a battery for the power supply. Despite many years of effort, it still remains a great challenge to develop self-powered sensing systems without external power supplies. Herein, we develop a neuron-like MXene Nb2CTx/sodium alginate (Nb2CTx/SA) composite films prepared by an electrospinning approach. The Nb2CTx/SA composite film not only can be used to fabricate a high-performance electronic humidity sensor, but also to construct a moisture-triggered nanogenerator with a sustainable voltage output. Importantly, the humidity sensor is demonstrated to be portable and compatible for applications in respiration assessment, noncontact monitoring and intelligent alarm. More importantly, by simply integrating the Nb2CTx/SA film-based sensor/nanogenerator with a capacitor and light-emitting diode (LED), a prototype of self-powered sensing system capable of indicating different relative humidity (i.e., low, medium or high) through the LED’s brightness without external power supplies is demonstrated. This work not only demonstrates the design and fabrication of a smart self-powered humidity sensing system, but also opens a new avenue for development of next-generation intelligent wearable electronics.

Highly Conductive and Mechanically Robust Cellulose Nanocomposite Hydrogels with Antifreezing and Antidehydration Performances for Flexible Humidity Sensors.

Conductive hydrogels are emerging as an appealing material platform for flexible electronic devices owing to their attractive mechanical flexibility and conductive properties. However, the conventional water-based conductive hydrogels tend to inevitably freeze at subzero temperature and suffer from continuous water evaporation under ambient conditions, leading to a decrease in their electrical conductivities and mechanical properties. Thus, it is extremely necessary, but generally challenging, to create an antifreezing and antidehydration conductive gel for maintaining high and stable performances in terms of electrical conductivity and mechanical properties. Herein, we fabricated a cellulose nanofibril (CNF)-reinforced and highly ion-conductive organogel featuring excellent antifreezing and antidehydration performances by immersing it in the CaCl2/sorbitol solution for solvent displacement. The incorporation of a rigid CNF serving as a dynamic connected bridge provided a hierarchical honeycomb-like cellular structure for the obtained CS-nanocomposite (NC) organogel networks, facilitating significant mechanical reinforcement. The synergy effects of sorbitol and CaCl2 allowed high-performance integration with excellent antifreezing tolerance, antidehydration ability, and ionic conductivity. Strong hydrogen bonds were formed between water molecules and sorbitol molecules to impede the formation of ice crystals and water evaporation, thereby imparting the CS-NC organogels with extreme-temperature tolerance as low as -50 °C and pre-eminent antidehydration performance with over 90% weight retention. Furthermore, this CS-NC organogel exhibited high humidity sensitivity in a wide humidity detection range (23∼97% relative humidity) because of the ready formation of hydrogen bonds between water molecules and numerous hydrophilic groups in the binary solvent and elaborated polymer chains, which can be assembled as a stretchable humidity sensor to monitor human respiration with a fast response. This work provides a new prospect for fabricating intrinsically stretchable and high-performance humidity sensors using cellulose-based humidity-responsive materials for the emerging wearable applications.