High-performance Humidity Sensors Research Articles

Interfacially Fabricated Covalent Organic Framework Membranes for Film-Based Fluorescence Humidity Sensors and Moisture Driven Actuators.

Rapid, on-site measurement of ppm-level humidity in real time remains a challenge. In this work, we fabricated a few micrometer thick, β-ketoenamine-linked covalent organic framework (COF) membrane via interfacially confined condensation of 1,3,5-tris-(4-aminophenyl)triazine (TTA) with 1,3,5-tri-formylphloroglucinol (TP). Based on the super-sensitive and reversible response of the COF membrane to water vapor, we developed a high-performance film-based fluorescence humidity sensor, depicting unprecedented detection limit of 0.005 ppm, fast response/recovery (2.2 s/2.0 s), and a detection range from 0.005 to 100 ppm. Remarkably, more than 7,000-time continuous tests showed no observable change in the performance of the sensor. The applicability of the sensor was verified by on-site and real-time monitoring of humidity in a glovebox. The superior performance of the sensor was ascribed to the highly porous structure and unique affinity of the COF membrane to water molecules as they enable fast mass transfer and efficient utilization of the water binding sites. Moreover, based on the remarkable moisture driven deformation of the COF membrane and its composition with the known polyimide films, some conceptual actuators were created. This study brings new ideas to the design of ultra-sensitive film-based fluorescent sensors (FFSs) and high-performance actuators.

Ultrasensitive and durable borophene-based humidity sensors for advanced human-centric applications

Borophene, a novel two-dimensional (2D) nanomaterials with high surface-to-volume ratio and abundant active sites, show great promise for humidity sensing applications, which are crucial for improving human comfort, public health, and device safety. However, traditional fabrication methods, such as drop-coating, face limitations including uncontrollable sensing layer thickness, poor water-damage resistance, and inherent variability between devices. In this work, we present a high-performance humidity sensor fabricated using borophene glass. The sensor demonstrates an impressive detection range (11–97 % RH), exceptional sensitivity (up to 1.83×105% at 97 % RH), low hysteresis (1.245 %), excellent repeatability, and relatively fast response (28.8 s)/recovery (2.6 s) times. Notably, the sensor exhibits thickness-dependent sensing performance, long-term stability, high selectivity, and water-damage resistance. First-principles calculations show that borophene exhibits an adsorption energy of 0.208 eV for H2O molecules, with the hole doping effect following adsorption driving the sensor’s response to varying relative humidity levels. The sensor’s sustained heightened sensitivity under high humidity conditions is attributed to the Grotthuss mechanism. Additionally, the sensor has been successfully integrated into various human-centric applications, including speech recognition, wireless monitoring of respiratory patterns, and non-contact switches. These advancements pave the way for the integration of borophene humidity sensors into smart devices and non-contact control systems, driving innovation in human–computer interaction and healthcare technologies.

Environmentally stable, Adhesive, Self-healing, Stretchable and Transparent Gelatin based Eutectogels for Self-Powered Humidity Sensors

Intrinsic environmental and stability limitations of hydrogels have inhibited their practical applications as a flexible wearable device due to water evaporation or freezing in complex environments such as low temperatures and arid environments. In this work, a multifunctional gelatin based ionic conductive eutectogel with double network structure is designed via ternary deep eutectic solvent (DES) (acrylic acid (AA), choline chloride (ChCl) and ethylene glycol (EG)). In this system, the introduction of ethylene glycol (EG) can be used to dissolve gelatin. The resulting DESG eutectogel exhibited excellent adhesion, mechanical robustness, anti-freeze, anti-drying, and self-healing. Interestingly, the DESG gels showed high humidity sensitivity in a wide humidity detection range (11 %–83 %), which can be assembled as a self-powdered humidity sensor to monitor human mouth and nose breathing. This work is expected to bring new prospect to construct high performance humidity sensors using gelatin based humidity-responsive materials for a wide range of potential applications in respiratory diagnostics, sleep monitoring, electronic skin and wearable electronics.

High-performance flexible humidity sensor based on B-TiO2/ SnS2 nanoflowers for non-contact sensing and respiration detection

The development of flexible humidity sensors is essential for the advancement of wearable devices and electronic skin. However, the process of preparing these sensors is typically complex, and they often suffer from poor stability and a limited sensing range, which fails to meet the requirements for practical applications. Here, we prepared SnS2 with nanoflower morphology by hydrothermal method, and successfully prepared a high-performance flexible humidity sensor with a high response range (20-95 %) by compositing it with B-TiO2 nanoparticles with oxygen defects after sodium borohydride reduction and coating it on a PET flexible substrate. This sensor demonstrates an exceptional response (753.2 at 90 % RH) in high humidity environments and exhibits rapid response (13 s) and recovery times (19 s). Moreover, the sensor accurately detects human breathing patterns and frequencies, making it suitable for non-contact sensing applications. This research offers valuable insights into the simplified preparation and advancement of flexible humidity sensors.

All-printed MXene/WS2-based flexible humidity sensor for multi-scenario applications

As non-contact sensing, non-invasive medical diagnostics and environmental humidity monitoring technologies advance, the demand for high-performance humidity sensors is becoming increasingly apparent. Ti3C2Tx MXene is an attractive humidity-sensitive candidate due to its excellent chemical activity and hydrophilic surface. However, achieving fast responsiveness and high sensitivity of dynamic changes in humidity is still challenging. Herein, this work introduced a WS2-modified Ti3C2Tx MXene composite material as humidity sensing material with superior sensing performance. Then sodium carboxymethyl cellulose solution was added to the Ti3C2Tx MXene/WS2 composite material to adjust viscosity of the composite material, enabling the formulation of printable humidity-sensitive ink. Utilizing screen-printing technology, a mass-producible humidity sensor (CMXW2) was successfully fabricated. Experimental results show that the sensor has short response and recovery times of 4.65 s/1.33 s in the relative humidity of 11–97 % with a high humidity response of 1707 % and excellent mechanical properties. The efficacy of the humidity sensor for skin moisture monitoring, real-time respiration monitoring, and non-contact switch was also evaluated. The successful demonstration of multi-scenario applications using the CMXW2 humidity sensor opens up new possibilities and applications in the fields of smart healthcare and non-contact sensing.

Relative humidity sensor based on a nano-film deposited LPFG with hydrophilic polymer composite coating

A novel high-performance relative humidity (RH) sensor is proposed by using a hydrophilic polymer composite film-coated nano-film deposition deposited long-period fiber grating (LPFG). A high refractive index (RI) TiO2 nano-film is deposited on the LPFG as an inner layer. The film thickness is selected to maximize the surrounding refractive index (SRI) sensitivity of the grating when the SRI is approaching the cladding index. A composite film made of polyvinyl alcohol (PVA) and polyethylene glycol (PEG) is used as the an external humidity-to-RI transducer. The concentration and proportion of the PVA/PEG solutions are appropriately chosen to tune the refractive index (RI) RI of the polymer coating to the transition range during the RH varying from 60 % to 95 %. For the LPFG coated with two layers film, the a RH sensitivity up to 3.3 nm/% is achieved. The variation in RH is found to have a linear response. The proposed device offers a reliable alternative for higher RH environment sensing.

Nanofluidic sensing inspired by the anomalous water dynamics in electrical angstrom-scale channels

Manipulation of confined water dynamics by voltage keeps great importance for diverse applications. However, limitations on the membrane functions, voltage-control range, and unclear dynamics need to be addressed. Herein, we report an anomalous electrically controlled gating phenomenon on cation-intercalated multi-layer Ti3C2 membranes and reveal the confined water dynamics. The water permeation rate was improved rapidly following the application and rise of voltage and finally reached a maximum rate at 0.9 V. The permeation rate starts to decrease from 0.9 V. Below 0.9 V, the electric field affects the charge and polarity of water molecules and then leads to ordered and denser rearrangement in the two-dimensional (2D) channel to accelerate the permeation rate. Above 0.9 V, with the assistance of metal cations, the surge in current induced aggregation of water molecules into clusters, thereby limiting the water mobility. Based on these findings, a high-performance humidity sensor was developed by simultaneously optimizing the response and recovery speeds through electric manipulation. This work provides flexible strategies in intelligent membrane design and nanofluidic sensing.

High-Performance Humidity Sensor Capable for Monitoring Low-Moisture Levels in Energy Industries

Humidity sensors are extensively used in industrial processing and environmental control. There are two categories of humidity sensors: High-humidity (relative humidity) measurement and Low-humidity (moisture) measurement. The proper units for low-humidity measurement are dew point and parts per million in volume (ppmv) or parts per billion in volume (ppbv). It is very important to monitor moisture levels in natural gas during processing and transportation in pipelines. It is of critical importance to monitor moisture levels in lithium-ion battery manufacturing. In these cases, the moisture levels are limited to be less than -50°C dew point or 40 ppmv. The conventional relative humidity sensors cannot be used because of their low sensitivity. The dew point sensors on the market are polymer-based and gamma-phase-alumina based. The polymer-based sensors have low sensitivity, which can only accurately detect moisture levels of -60°C dew point and higher. The gamma-phase alumina sensors suffer from long-term drift due to phase change.We present novel high-performance humidity sensors based on porous alpha-phase alumina and silicon dioxide nanostructures, which are capable of detecting moisture levels as low as -100°C dew point or 13 ppbv with excellent long-term stability. The porous alpha-phase alumina films were produced by anodic-spark-deposition on aluminum substrates in which electric sparks were produced due to high-voltage anodization (>120V). Local high-temperature (about 2200°C) at the points of sparks converted the amorphous alumina into alpha-phase alumina (Sapphire). It provides an extremely stable structure for humidity sensors. Hydrophilic silicon dioxide films deposited by atomic layer deposition was used as sensing materials. The sensors’ sensitivity, temperature coefficient, response speed, and long-term stability were studied in details. The sensors have great promise for monitoring moisture levels in natural gas industry and lithium-ion battery manufacturing.

High performance humidity sensor based on 3-D mesoporous SnO2 derived via nanocasting technique

High performance humidity sensor based on 3-D mesoporous SnO2 derived via nanocasting technique

High-performance humidity sensors based on SnO2/Ti3C2Tx nanocomposites coated on porous graphene electrodes

This study proposes the synthesis of SnO2/Ti3C2Tx composites on porous graphene electrodes (PGEs) for high-performance humidity sensors. In the heterojunction structure of SnO2/Ti3C2Tx composites, SnO2 nanoparticles prevented the layered structure of Ti3C2Tx to collapse and restack. Moreover, the high specific surface area of the SnO2/Ti3C2Tx composites provided more adsorption sites that improved their response. Porous graphene was fabricated by a fiber laser-induced process on the polyimide film as an electrode layer for flexible humidity sensors. The characteristics of the SnO2/Ti3C2Tx composites and PGEs were examined and investigated. SnO2/Ti3C2Tx composites were drop-cast on PGEs to fabricate the humidity sensor. Furthermore, the response of humidity sensors was tested in various RHs. The performance of the proposed humidity sensor demonstrated a wide detection range of 11–97 % RH, low hysteresis of 2.8 % RH, low response/recovery times of 14/49 s, high sensitivity of 862.19 kΩ/%RH, excellent long-time stability and repeatability, and good flexibility. In the future, the proposed sensor can be widely applied in human respiration monitoring, non-contact switch, and detection of volatile organic compounds.

High-Performance Self-Cleaning Triboelectric Mini Humidity Sensor Enabled by Facet Engineered Titania Nanocrystals Adorned Nylon-6,6 Electrospun Nanofibers

High-Performance Self-Cleaning Triboelectric Mini Humidity Sensor Enabled by Facet Engineered Titania Nanocrystals Adorned Nylon-6,6 Electrospun Nanofibers

A high-sensitive capacitive humidity sensor based on chitosan-sodium chloride composite material

Chitosan, as a hydrophilic, renewable, biodegradable, and cost-effective natural polymer, holds great promise in humidity sensor applications. This study proposes a green high-performance capacitive humidity sensor based on chitosan (CS)-sodium chloride (NaCl) composites. The CS-NaCl films were fabricated through a simple drip coating method, and their surface morphology and structure were characterized using SEM, XRD, FTIR and XPS techniques. The pure chitosan humidity sensor demonstrates an impressive response of 1180.17 % with excellent linearity. The introduction of NaCl markedly boosted the sensor response, with CS-50 wt% NaCl composites exhibiting a 72-fold increase (86, 492.84 %) in capacitance response compared to pure CS films. The contact angle test demonstrated that the chitosan film exhibited good hydrophilicity, which was further enhanced by the addition of NaCl. XRD and FTIR characterization confirmed that NaCl altered the structural properties of chitosan, disrupted hydrogen bonds, and consequently increased the mobility of polarizing groups and the dielectric constant. A water adsorption model was developed to elucidate the humidity sensing mechanism of the CS-NaCl sensor. The CS-NaCl composite material presents advantages such as easy preparation, high sensitivity, eco-friendliness, and promising practical applications in humidity sensing.

High-performance humidity sensors based on reduced graphene oxide sheets decorated with cobalt and iron doped ZnO nanorods

Humidity sensors based on doped zinc oxide (ZnO) / reduced graphene oxide (rGO) composites have shown potential since the doped ZnO/rGO material is sensitive to changes in humidity. Here, the present work report of one-step hydrothermal synthesis of ZnO/rGO and doped ZnO/rGO nanocomposites where ZnO has been doped with cobalt and iron at concentrations of 2 %. X-ray diffraction validates the substitution of transition metal cations in the ZnO lattice. Scanning electron microscopy depicts a graphene-layered structure with intact ZnO rods. Humidity sensors based on Fe doped ZnO/rGO composite showed response and recovery periods of 27 s and 24 s, respectively, in the range of 11–97 % relative humidity. The composite has the sensitivity of 2.2 kΩ at 100 Hz which is the highest as compared to ZnO/rGO and Co doped ZnO/rGO. Because of the greatest sensitivity and quick reaction and recovery timer, Fe doped ZnO/rGO composites appear to be ideal for use as humidity sensors.

Biosynthesis, structural characterization and humidity sensing properties of cellulose/ZnO nanocomposite

The present work demonstrates the applications of a nanocomposite of the cellulose polymer and ZnO nanoparticles with 1:1 weight ratio, prepared by a green assisted precipitation method for a high-performance resistive type humidity sensor. The morphology and nanostructure of prepared cellulose/ZnO composite were characterization by X-ray diffraction, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and transmission electron microscopy are confirmed the decoration of ZnO nanoparticles on cellulose polymer matrix surface. The humidity sensing martials was coated onto the interdigital electrodes. The sensitivity, response/recover and stability studies performance of fabricated humidity sensors have been monitored in 5 to 98% relative humidity (%RH) at room temperature (37 °C). The response and recovery times of the fabricated sensors are observed as ≈ 26 s and ≈ 53 s respectively, and the sensitivity factor (Sf) is 2656 ± 103 Ω. Possible mechanisms of the humidity sensor based on water-induced conductivity increase are discussed. Also, the energy band of cellulose/ZnO nanocomposite was simulated by density functional theory (DFT) studies. The cellulose/ZnO nanocomposite humidity sensor has great potential for practical field applications.

Preparation and Mechanism Analysis of High-Performance Humidity Sensor Based on Eu-Doped TiO2.

TiO2 is a typical semiconductor material, and it has attracted much attention in the field of humidity sensors. Doping is an efficient way to enhance the humidity response of TiO2. Eu-doped TiO2 material was investigated in both theoretical simulations and experiments. In a simulation based on density functional theory, a doped Eu atom can increase the performance of humidity sensors by producing more oxygen vacancies than undoped TiO2. In these experiments, Eu-doped TiO2 nanorods were prepared by hydrothermal synthesis, and the results also confirm the theoretical prediction. When the doping mole ratio is 5 mol%, the response of the humidity sensor reaches 23,997.0, the wet hysteresis is 2.3% and the response/recovery time is 3/13.1 s. This study not only improves the basis for preparation of high-performance TiO2 humidity sensors, but also fills the research gap on rare earth Eu-doped TiO2 as a humidity-sensitive material.

High-Performance Flexible Humidity Sensors Based on MCl (M = Li, Na, K) Doped PVP/PVDF Self-Supporting Films for Boosted Real-Time Noncontact Moisture Monitoring

High-Performance Flexible Humidity Sensors Based on MCl (M = Li, Na, K) Doped PVP/PVDF Self-Supporting Films for Boosted Real-Time Noncontact Moisture Monitoring

High-Performance and Sensitive Humidity Sensors Based on Reduced Graphene Oxide/Nickel Ferrite Nanocomposites

This work reports the fabrication of highly sensitive humidity sensors using reduced graphene oxide (rGO), nickel ferrite nanoparticles (NiFe2O4 NPs), and their nanocomposites. rGO and NiFe2O4 NPs, obtained by the modified Hummers method and the wet chemical coprecipitation method, respectively, were used to synthesize rGO/NiFe2O4 nanocomposites at two different compositions. XRD analysis confirmed both the spinel structure of NiFe2O4 and the reinstatement of π-conjugated structure of graphene. Average crystallite sizes, calculated by XRD, were 17.80 nm, 13.60 nm, and 8.22 nm for NiFe2O4 NPs, rGO/NiFe2O4 (70/30), and rGO/NiFe2O4 (90/10), respectively. FTIR affirmed the successful reduction of graphene oxide to rGO. Optical bandgaps of 1.83 eV and 2.18 eV were obtained for rGO and NiFe2O4 respectively using Tauc plot. SEM confirmed the films’ porosity, providing more active sites for the adsorption of water molecules. Our results demonstrate that NiFe2O4 NPs show the highest response and sensitivity within the 35–90 RH% range. Additionally, the rGO/NiFe2O4 (70/30) nanocomposite achieved the fastest response/recovery time of 8/4 s at 100 Hz, with good repeatability and stability. These findings suggest that rGO/NiFe2O4 NCs are promising candidates for advanced humidity sensing applications due to their rapid response, sensitivity, and long-term reliability.

Flexible Humidity Sensor Based on a Graphene Oxide-Carbon Nanotube-Modified Co3O4 Nanoparticle-Embedded Laser-Induced Graphene Electrode.

To meet evolving humidity monitoring needs, the development of flexible, high-performance humidity sensors is crucial. This study introduces an innovative flexible humidity sensor using a single-step laser scribing technique to fabricate a flexible in situ Co3O4 nanoparticle-embedded laser-induced graphene (Co3O4-LIG) composite electrode. Compared to conventional LIG electrodes, the Co3O4-LIG electrode exhibits improved conductivity and hydrophilicity, enhancing charge transfer and water molecule affinity. The unique two-dimensional structure and exceptional water permeability of graphene oxide (GO) combine with the rapid water response and high specific surface area of carboxylated multiwalled carbon nanotubes (MWCNTs), thereby assuming a crucial function in the modification and optimization of the performance of humidity sensors. Through the application of a homogenously blended aqueous solution comprising GO and MWCNTs in precise proportions onto the Co3O4-LIG composite electrode, an excellent humidity-responsive layer is established, culminating in the realization of a cutting-edge GO-MWCNTs@Co3O4-LIG flexible humidity sensor. Noteworthy attributes of this sensor include a heightened sensitivity [959.1% (ΔR/R0)], rapid response and recovery times (within 5 and 26 s, respectively), and a noteworthy linearity (R2 = 0.994) across a relative humidity range of 14 to 95%. The findings presented herein offer valuable insights and a practical blueprint for the design and production of flexible humidity sensors.

A capacitive humidity sensor based on nanodiamond/silver nanoparticles (ND/Ag) nanocomposite with high stability and rapid response for respiratory monitoring

In this work, a high-performance capacitive humidity sensor based on nanodiamond/silver nanoparticles (ND/Ag) nanocomposite is proposed. The influence of silver nanoparticles with different content in nanodiamond is investigated, and the optimum ratio of 5 wt% Ag in the composite shows the maximum humidity sensing performance, which is three times than that of the pure nanodiamond based humidity sensor. Moreover, the proposed sensor exhibits a superior repeatability, ultrafast response time (<2 s) and recovery time (<1 s). More importantly, the sensor can maintain a good repeatability and rapid response speed when exposure to high humidity for a long time, demonstrating the excellent stability of the proposed sensor. Furthermore, complex impedance spectroscopy was applied to account for the underlying sensing mechanisms of the ND/Ag composite film humidity sensor. Finally, we design a circuit to realize non-contact breathing control by using the fabricated humidity sensor, which demonstrate that the fabricated humidity sensor could be applied to some auto-control systems by human-breath. This work demonstrated that ND/Ag composite film is an ideal candidate material for constructing high-performance humidity sensors for respiratory applications.

Preparation and properties study of high performance Eu2Sn2O7–SnO2 composites humidity sensor

In the work, a high-performance Eu2Sn2O7–SnO2 (ES-2) humidity sensors were prepared through a one-step hydrothermal method. Eu2Sn2O7 is a bimetallic material can increase the adsorption capacity to water molecules and promote electron transfer, thus improving the sensitivity of the sensor. The introduction of Eu2Sn2O7 promotes the increase of the content of oxygen vacancies in the ES-2 composite material, which promotes the accelerated decomposition of water molecules, and facilitates the formation of the chemisorbed layer to improve the sensitivity of the sensor and the response time. The increase in specific surface area of ES-2 provided more adsorption sites for H2O and also provided a good transport channel for water molecules to diffuse into the interior of the material. ES-2 humidity sensor exhibits high sensitivity (22,405 ± 363), small humidity hysteresis (1 ± 0.3 %), short response/recovery time (7/6 ± 0.5 s), good repeatability and stability (11%–95 % RH). This study not only promotes the development of SnO2-based humidity sensors, but provides new ideas for the preparation of high-performance humidity sensors.