Resistive Humidity Sensor Research Articles

The Development of a Flexible Humidity Sensor Using MWCNT/PVA Thin Films.

The exponential demand for real-time monitoring applications has altered the course of sensor development, from sensor electronics miniaturization, e.g., resorting to printing techniques, to low-cost, flexible and functional wearable materials. Humidity sensing has been used in the prevention and diagnosis of medical conditions, as well as in the assessment of physical comfort. This paper presents a resistive flexible humidity sensor composed of silver interdigitated electrodes (IDTs) screen printed onto polyimide film and an active layer of multiwall carbon nanotubes (MWCNT) dispersed in a water-soluble polymer, polyvinyl alcohol (PVA). Different MWCNT/PVA sensor sizes and MWCNT percentages are tested to study their effect on the initial electrical resistance (Ri) values and sensor response at different humidity percentages. The results show that the Ri values decrease with the increase in % MWCNT. The sensor size did not influence the sensor response, while the % MWCNT affected the sensor behavior upon relative humidity (RH) increments. The 1% MWCNT/PVA sensor showed the best response, reaching a relative electrical resistance, ΔR/R0, of 509% at 99% RH. Comparable with other reported sensors, the produced MWCNT/PVA flexible sensor is simpler, greener and shows a good sensitivity to humidity, being easily incorporated in wearable monitoring applications, from sports to medical fields.

Synthesis-in-place of V2O5 nanobelts for wide range humidity detection

Resistive type humidity sensors offer advantages in simplicity, low cost, compact size, and remote operation. Due to the benefits, they have great potential for use in a variety of environments and future applications, including the Internet of Everything (IoE). However, resistive humidity sensors have limitations in detecting extreme humidity levels below 10 % or above 90 % relative humidity (RH) and are also vulnerable to high temperatures, which hinders their use in harsh environments applications. For broader applications, humidity sensors with reliable performance across the full range of humidity levels and high stability at elevated temperatures need to be developed. Here, we report high performance resistive humidity sensors based on V2O5 nanobelts operating at high temperatures. The V2O5 nanobelts are directly deposited between Pt electrodes by O2-assisted physical vapor transport (PVT) method. The V2O5 nanobelt humidity sensor exhibits reliable humidity sensing properties over a wide range from 1 % to 100 % RH. Furthermore, long-term stability and reproducibility of V2O5 nanobelts are demonstrated through 25 consecutive humid air pulses and measurements obtained after 4 months of storage. The high performance of the V2O5 nanobelt humidity sensor is advantageous for practical use in applications ranging from IoE environments to harsh industrial conditions.

Synthesis of copper-containing metal–polymer nanocomposites and their use as a humidity sensor

Recently, the synthesis of new gas-sensitive materials for use in resistive humidity sensors has attracted considerable interest. In the study, copper-containing metal–polymer nanocomposites were obtained by thermolysis of copper fumarate (I) and its complexes with 2,2′-dipyridyl (II) and 1,10-phenanthroline (III). The nanocomposites were characterized by Fourier transform infrared (FTIR) spectroscopy, elemental analysis, energy-dispersive X-ray (EDX) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The most common particle sizes of the thermolysis products of compounds I, II, and III were 18.7, 8.3, and 20.7 nm, respectively. The manufactured sensor samples exhibited good sensitivity to the relative humidity (RH) of air: 2.48%/%RH, 3.77%/%RH, and 3.11%/%RH for the thermolysis products of compounds I, II, and III, respectively. Because of the high porosity and moisture absorption of the film, the maximum sensitivity was approximately 0.005 MΩ/%RH, which indicates fairly effective behavior of the film with respect to humidity. The response and recovery times were 23.7, and 37.3 s; 24.7, and 35.8 s; 32.4, and 58.4 s, respectively. The experiment had 88%–97% reproducibility. The fabricated sensors have great potential as humidity-sensing elements for humidity monitoring.

A comparative study on humidity response of graphene oxide with reduced graphene oxide and its multifunctional applications

In this paper, we present a comparative study of Graphene Oxide (GO) and chemically reduced Graphene Oxide (rGO) based resistive type humidity sensors fabricated on Ti/Au Inter-Digital Electrodes (IDEs) coated SiO2 substrates. The sensing performance, including sensitivity, response/recovery time, and hysteresis has been evaluated for both sensors. The GO film exhibited quick response (0.3 s) and high sensitivity (104.7 kΩ/%RH) with remarkable repeatability and negligible hysteresis as compared to rGO film sensor. The excellent humidity sensing properties of GO are attributed to the presence of abundant oxygen-containing polar functional groups on GO relative to rGO. To demonstrate the multifunctional applications of developed GO based sensor, we have also demonstrated its use as respiratory monitoring, non-contact proximity detector, and speech event detector through the detection of humidity perturbations. The work suggests GO sensitive material has high potential for the development of wearable and integrated health monitoring platforms and non-contact sensing devices.

Synthesis of nickel-containing metal-polymer nanocomposites and their use as a humidity sensor

In recent years, the synthesis of new gas-sensitive materials for resistive humidity sensors has attracted significant interest. In the present paper, nickel-containing metal-polymer nanocomposites were obtained by thermolysis of nickel itaconate (I) and its complexes with 2,2′-dipyridyl (II), 1,10-phenanthroline (III) and 4′-phenyl-2,2′:6′,2''-terpyridine (IV). The formed nanocomposites were characterized by FTIR, elemental analysis, EDX, SEM, TEM and XRD studies. The most common particle sizes are 28, 12.8, 10.7 and 12.3 nm for the thermolysis products of compounds I–IV respectively. The manufactured sensor samples have good sensitivity to air relative humidity (RH) of 3.24 %/%RH, 4.72 %/%RH, 2.18 %/%RH and 4.37 %/%RH for thermolysis products of compounds I–IV with the polymer binder polyacrylamide, respectively. The average initial resistance of the sensors is 4.54, 15.33, 20.8 and 35.75 MΩ, respectively. Due to the high porosity and moisture absorption of the film, the maximum sensitivity was about 0.003 MΩ/%RH, which indicates a fairly effective behavior of the film with respect to humidity. The response times were 27, 25, 33, 30 s, and the recovery time 47, 44, 51, 49 s, respectively, and repeating the experiment resulted in 87–92 % reproducible results. The fabricated sensors have great potential for use as a humidity sensing element in sensors that can be used for humidity monitoring.

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.

Ultrafast response humidity sensor based on titanium dioxide quantum dots/silica and its multifunctional applications

Humidity sensors with ultrafast response and recovery speed are very popular for humidity monitoring in dynamic environments such as complex operating conditions and human body monitoring. In this work, we prepared titanium dioxide quantum dots by hydrothermal synthesis and silica microspheres by Stober method. In addition, the electrostatic self-assembly method was used to prepare the two films with ultrafast response for moisture sensitivity. Due to the superhydrophilic properties of TiO2 QDs, water molecules can be adsorbed and resolved quickly on their surface. While the addition of SiO2 microspheres further increases the specific surface area of moisture-sensitive films and improves the moisture-sensitive properties of TiO2 QDs. The TiO2 QDs/SiO2 resistive humidity sensors not only exhibit excellent stability and repeatability, but also have fast water adsorption/desorption capacity. Between 11 % and 97 % relative humidity, the resistance change of the sensor reaches 6 orders of magnitude. In order to simulate the actual application environment, an experimental platform was built to verify the ultrafast response of the humidity sensor, and the response/recovery time of the sensor was as short as 22.4 ms/40.8 ms, which can quickly detect the change of humidity. The TiO2 QDs/SiO2 ultrafast humidity sensors proposed in this paper have great application potential in rapidly changing humidity environments such as complex working conditions, human body detection, humidity alarm and voice recognition.

Patterned, flexible, self-supporting humidity sensor with core-sheath structure for real-time sensing of human-related humidity

Self-supported nanofiber membranes were fabricated by the rhombus-patterned collector in the electrospinning process in this work. The rhombus nanofiber networks were used to retain the mechanical flexibility of the device and stable responses such as bending and winding deformation. A resistive humidity sensor was developed by applying hydrophilic PANI with varying contents onto nanofiber surfaces through in situ polymerization, serving as electrodes. In this context, the self-supported membranes serve dual roles as both the sensing material and the substrate for the sensor. A comprehensive assessment was conducted to analyze how variations in relative humidity affect the sensing characteristics of the acquired sensors. By facilely regulating the ratio of dopant and oxidant concentration to monomer concentration in the polymerization process, the response of the sensors was increased from 49% to 83% with increasing relative humidity from 13% to 93%, good linearity (R2 = 0.97) and a short response/recovery times (33/88 s). The prepared sensors exhibited appreciable mechanical properties and showed outstanding stability after 1000 cycles of bending and the resistances only slightly increased by 2.5%. Moreover, the sensor can monitor both human respiration and non-contact skin humidity. Its exceptional performance makes it easily integrable into textiles for the creation of wearable electronics.

Humidity Diode Sensors Based on 1D Nanosized Silicon Structures

Introduction. Humidity measurement is essential in microelectronics, aerospace, biomedical, and food industries, as well as in households for climate control. Currently, various types of devices have been used as humidity sensors: capacitive, resistive, diode, gravimetric, optical structures, field-effect transistors and devices based on surface acoustic waves.Problem Statement. Today, there is a need to develop IC-compatible humidity sensors that have high sensitivityand low cost. To this end, silicon nanowires have been successfully used in resistive and capacitive humidity sensors. However, there is a lack of research on the nanowire effect on device parameters of diode-type humidity sensors.Purpose. To develop diode sensors based on silicon nanowires and to determine the effect of process parameters of synthesis and structural features of nanowires on the performance of humidity sensors.Materials and Methods. The process of sensor fabrication includes several steps: chemical cleaning of silicon wafer, synthesis of silicon nanowires using standard or modified metal-assisted chemical etching, phosphorus diffusion to create a p-n junction, front and back metallization. The surface morphology of the nanostructures has been studied by scanning electron microscopy. The humidity-sensitive characteristics have been studied with theuse of salt hygrostats.Results. It has been shown that the addition of one-dimensional silicon nanostructures to the diode-type sensor signifi cantly improves its characteristics. The rectification ratio increases from 161 to 1807, the response ups from 4.5 to 25, the sensitivity grows from 1.6 to 4.02 (%RH)–1, while the response time and recovery time are reduced from 85/90 to 25/30 s, the hysteresis value goes down from 75 to 16%, the signal deviation after cycling drops from 15to 3%, and the signal fluctuation during continuous device operation decreases from 17 to 15%.Conclusions. The results have shown that the use of a simple and cheap nanowire synthesis technology is effective to produce humidity sensors.

Low temperature approach towards synthesis and fabrication of resistive humidity sensor elicited from SnO2-modified mesoporous FDU-12

Low temperature approach towards synthesis and fabrication of resistive humidity sensor elicited from SnO2-modified mesoporous FDU-12

Graphene films grown by chemical vapor deposition and their applications

In this article we provide the results of the synthesis of graphene films and discuss their potential applications in electronic structures. Graphene films were synthesized on copper foil using the CVD method at 1050 °C. During the initial stage of synthesis, graphene domains with hexagonal shapes and an average size of 10 μm were formed. The orientation and size of the graphene domains are based on the synthesis parameters and the copper foil. As the synthesis time increases, domain cross-linking occurs, resulting in polycrystalline continuous graphene film formation. Graphene films have areas up to 100 cm2 and thicknesses ~ 1 nm to 5 nm. To measure the Raman spectra, graphene films were transferred to SiO2/Si substrates. Graphene films exhibit an intense 2D peak that significantly exceeds the G peak of crystalline graphite. Flexible transparent conductive touch panels were created on the basis of the grown graphene films. A lamination method has been used to create graphene films that can be transferred from copper foil to polymer substrates. A laboratory touch screen with a graphene film acting as a capacitive touch sensor was constructed on the basis of the transferred film, and transparent electrodes for molybdenum disulfide-based photosensitive elements were also created. Resistive humidity sensors based on graphene films were developed and transferred to SiO2/Si and epoxy/Si substrates. The graphene humidity sensor has a low response, high temperature stability, and is highly reliable.

Responsible Humidity Sensor by Direct Laser Writing on Cork Substrate

AbstractA novel laser‐induced graphene (LIG) resistive relative humidity (RH) sensor is successfully fabricated by direct laser writing on a common natural cork sheet. In this work, a straightforward fabrication is presented where LIG, porous multilayer graphene, is generated by a laser photothermal process on a carbon‐based substrate, in this case, an agglomerate cork sheet. The formation of LIG material is revealed by structural and morphological characterization using various analyses, including Raman spectroscopy, X‐ray Photoelectron spectroscopy (XPS), and Scanning Electron Microscopy (SEM). The electrical analyses are conducted to measure the resulting sheet resistance, getting results as low as 31 Ω sq−1. With the laser parameter optimized, a LIG resistive humidity transducer is manufactured and characterized in a climate chamber. This biodegradable sensor shows good linearity with a sensitivity of 0.015%/%RH from 40 to 80% RH. Additionally, the influence of temperature is studied and demonstrated a low impact on the sensor's response toward RH. Furthermore, a proof of concept is successfully implemented by integrating the transducer onto a cork stopper. Considering all, the prospect of creating an easy, fast, scalable, and cost‐effective transducer combined with the use of natural and abundant materials, leads the way for future large‐scale production of sustainable sensors.

Single-layer graphene based resistive humidity sensor enhanced by graphene quantum dots

Graphene is broadly applied as sensitive sensing material results from its superb features. Concurrently, as a derivative of graphene with 0D structure, graphene quantum dots (GQDs) offer more possibilities as a supportive sensing material due to its adjustable size and functional group modification. In this work, GQDs are introduced to single-layer graphene (SLG) based humidity sensor to enhance the sensing performance. Specifically, consistent resistance response to relative humidity (RH) is extended from the range of 10%–60% to 10%–90% by contrary to original SLG based sensor. Parallelly, effect of the amount of GQDs is investigated by means of multiple GQDs deposition. As the resultant higher binding efficiency between water molecules and the functional groups of GQDs, improved response rate is observed. For the case of 4-time deposition of GQDs, the response rate ( ΔR/R ) reaches ∼130% in RH range of 10%–90%. Besides, the response time and recovery time are ∼0.7 s and ∼1.1 s, respectively. The fluctuation of the resistance change of the sensor under constant humidity is less than 5% over a month which demonstrates long-term reliability.

Efficient Electron Transfer through Interfacial Water Molecules across Two-Dimensional MoO3 for Humidity Sensing.

Resistive humidity sensors are required in flexible and integrated devices. Two-dimensional MoO3 offers a large interface area, enabling the modulation of its electrical properties over a wide range. In this study, 2D MoO3 was synthesized via liquid-phase exfoliation for humidity-sensing tests. In terms of high sensitivity, negligible hysteresis, linearity, and stability, the humidity-sensing performance of MoO3 is superior to those of other materials. The sensitivity reaches 9794 Ω/RH at 25 °C. The sensing mechanism of MoO3 was investigated by using impedance spectra and voltage-current scans under different humidity levels. The results indicate that the resistance change of MoO3 due to humidity originates from the interfacial conductance. Interfacial H2O adsorption induces efficient conducting paths via hydrogen bonding, decreases the potential barrier for electron transfer, and supplies additional electron states to the valence bands. In this study, electronic humidity sensing was investigated in depth, and a new perspective was proposed for electronic humidity sensing.

Graphene oxide boosted high surface area CeO2 for humidity sensing

Precise measurement of humidity is of high significance in diverse fields like instrumentation, automated systems, agriculture and climatology. Herein as a first ever attempt, we report ceria-graphene oxide (CeO2-GO) composites as resistive type humidity sensor, that shows attractive response to moisture. High surface area ceria is synthesised via ammonia mediated precipitation from salt precursor, and CeO2-GO composites are achieved via hydrothermal route. The composites are well characterized using various analytical tools. Experiments using different mass ratios of GO in the composite show that there is an optimum amount of GO to realise maximum efficiency in the test parameters. It is further noted that the performance of the sensor is controlled by the active adsorption sites, which is proportional to surface area of the composite. An optimized CeO2-GO matrix yielded a response and recovery time of 19 s and 10 s respectively. The narrow hysteresis in moisture adsorption-desorption study and excellent stability for about 60 days are also observed for the system, warranting the successful utility of the system for practical applications.

Rapid-response humidity sensors based on ultra-thin films stacked with single-layer graphene oxide

Monitoring and control of humidity is important for industry, agriculture and human activities. Currently, it is achallengeto develop highly sensitive humidity sensors with both the response time and recovery time less than 1 s.Therefore, it isdifficult to dynamically monitor rapid changes in humidity. Here, we report a resistive humidity sensor based on an ultrathin graphene oxide (GO) film, which is formed by stacked single-layered GO. Both the response and recovery time of the sensor is less than 1 s in the range of 10 %–95 % relative humidity (RH). The shortest response and recovery time of the reported GO-based resistive humidity sensor can be 0.33 s at 10 % RH, indicating the fastest response and recovery property to date. Moreover, the sensor is sensitive, and it can accurately detect the distance between the sensor and the human finger as well as the strength of the human breath. The simple preparation, microscale structure, and extremely fast response make the GO-based sensor a new alternative for integrated humidity sensors with real-time monitoring functions.

A fast response and highly sensitive flexible humidity sensor based on a nanocomposite film of MoS2 and graphene oxide.

Graphene oxide (GO)-based humidity sensors are attracting widespread attention due to their high responsivity and low cost. However, GO-based humidity sensors generally suffer from slow response and recovery as well as poor stability. Here, we report a flexible resistive humidity sensor based on a MoS2/GO composite film that was fabricated by mixing different volumes of MoS2 and GO dispersions with adjustable volume ratios. The MoS2/GO composite film has been used as a sensing layer on screen-printed interdigital electrodes. The results show that the best device performance was achieved at a dispersion volume of 0.05 mL with a MoS2/GO volume ratio of 5 : 1, featuring a high responsivity (∼98%), a fast response/recovery time (1.3/12.1 s), excellent stability and low cost. Furthermore, the humidity sensor exhibits good linearity over a wide humidity range (33%RH-98%RH) at room temperature (25 °C) and can be fabricated easily and feasibly. The application of the humidity sensors we prepared in human respiration detection and human fingertip proximity detection has been demonstrated. These findings indicate the great potential of the MoS2/GO composite in developing next generation high-performance humidity sensors.

A thin film resistive humidity sensor based on polymer and carbon black nanoparticle composites

This paper proposes a resistive humidity sensor that uses a carbon-black and polyvinyl alcohol composites thin film, fabricated with a unique film coating method for thinner thickness and higher sensitivity. Improving the sensitivity of sensing films is still of great importance in the research field of gas sensors. The humidity sensor devices with thin composite film and microelectrode structure are fabricated on the glass substrate for a low cost and a simple fabrication process. The sensor gives a rapid response for humidity levels from 10.9% relative humidity (RH) to 73.7% RH, and the response time is about 5.77 s. Experimental results reveal that the sensor has good sensitivity, reproducibility, fast reaction time, and wide range. In addition to humidity, the sensor also responds well to gases such as ethanol. The proposed gas sensor in this paper can be applied to the other combinations of polymers and nanoparticles to form new gas sensors, which have the potential to be used as a gas sensor array for detecting the composition of complex gases such as volatile organic components.

Fabrication of rare earth(R[dbnd]Ce,Gd, Ho and Sm) doped CoCr2-x RxO4 pigments for capacitive and resistive humidity sensor applications

Rare earth (Ce, Gd, Ho & Sm) doped CoCr2O4 green pigments with an extremely porous nature powder were produced using a quick and affordable chemical synthesis procedure called solution combustion. X-ray diffraction analyses revealed the phase composition and average crystallite size. SEM images of the samples that had been annealed at 600°C revealed the existence of cuboids and soft agglomerations. Using an LCR metre to analyse the dielectric properties of as-synthesised samples, it is discovered that the small crystallite size exhibits a higher dielectric constant. The results were impressive when permittivity and electrical resistivity fluctuated with frequency under humid circumstances. By varying the relative humidity from 0 to 100%, electrical measurements were performed to investigate the rare earth pigments' capabilities for sensing humidity. Sample electrical resistance decreases over the entire humidity range. At room temperature, the resistive responses were measured at various relative humidity (RH) levels ranging from 0% to 98% and at various frequencies ranging from 20 Hz to 20 MHz. The rare earth-doped green pigments used in the humidity sensor have a linear response, high sensitivity, and low hysteresis. In addition, the sensor has 35 and 240 s reaction and recovery times, respectively. Our research indicates that this material could be employed as a capacitive and resistive humidity sensor. The results of this work open the door for the use of rare earth doped cobalt chromate pigments in applications for humidity sensing.

Laser-Induced Graphene on Chitosan: An Enabling Technology for Sustainable Resistive Humidity Sensors

Laser-Induced Graphene on Chitosan: An Enabling Technology for Sustainable Resistive Humidity Sensors