Capacitive-type Humidity Sensor Research Articles

Dicarbocyanine-based organic humidity sensors

The utilization of organic dicarbocyanine dye-conjugated molecules, specifically 1,1′-diethyl-4,4′-dicarbocyanine iodide (DDCI-4), as the sensing film in capacitive-type humidity sensors has been demonstrated. The structural, morphological, and wettability characteristics of the sensing films as well as the performance of the electrode gaps of the devices were analyzed. Aluminum (Al)/DDCI-4/Al capacitive humidity sensors were fabricated using solution-processed spin coating techniques in a planar geometry. In this study, a surface profiler was used to measure the size of electrode gaps, which were 94.87 ± 1.20 µm, 391.00 ± 1.53 µm, 902.00 ± 2.08 µm, and 1906.67 ± 3.33 µm. Compared to those with larger gaps (391 µm, 902 µm, and 1907 µm), the device with the smallest gap measured 45 ± 9.1 fF/%RH with the highest sensitivity. In addition, the hysteresis with response and recovery performances has been studied, revealing the lowest values of 0.31%, 6 s, and 9 s, respectively. It is anticipated that the 1,1′-diethyl-4,4′-dicarbocyanine iodide's electronic, ionic, and dipole polarizations are responsible for the excellent performance of dye-based organic humidity sensors.

Effect of different deposition techniques of PCDTBT:PC71BM composite on the performance of capacitive-type humidity sensors

The performance and hence pragmatic use of organic humidity sensors requires various factors to be addressed outside of progress in high-performance organic-based sensing materials. While important, evaluation of fabrication techniques also requires attention to optimise performance. Herein we report the effect of different application techniques on the humidity sensing performance of poly[N-90-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) organic composites. Spin-coating, immersion and drop-casting techniques have been applied to prepare sensors. Field-emission scanning electronic microscopy (FESEM) analysis of the prepared films revealed different types of self-assembled nanorods and nanotubes to be formed. The morphological and self-assembled differences could be attributed to various crystallisation processes as a result of different deposition techniques. Capacitive responses of all three sensors have been studied as a function of relative humidity at room temperature. PCDTBT:PC71BM composites prepared through the immersion technique showed superior sensing performance including higher sensitivity (62.34 pF/%RH) than sensors prepared through the spin coating (0.2 pF/%RH) and drop-casting (7.5 pF/%RH) techniques. The superior sensing performance of the sensor prepared through the immersion technique can be ascribed to both the nanotube and porous morphology exhibited.

Study of humidity sensing properties of TiO2/Ethyl cellulose (EC) composite

Monitoring environment parameters such as humidity have received wide attention from the past up until now. One of the main factors to ensure the accuracy and response speed of the humidity sensor is the selection of appropriate materials and its fabrication process. The selection of suitable materials should be based on materials that exhibit good sensitivity over the entire humidity range, low hysteresis, and are resistant to various chemicals vapor. In this study, we report a capacitive type humidity sensor has been fabricated by combining titanium dioxide (TiO2) with Ethyl Cellulose (EC) polymer. The analysis of the structure of the material was characterized by XRD. Humidity sensing performance studies on TiO2/EC sensor sample were measured at various humidity levels from 11% to 96% relative humidity (RH) at a frequency of 1 kHz. The measurement results and analysis show that the dynamic response of the TiO2/EC sensor capacitance with time tends to increase at higher RH, the linearity of the increase in capacitance response is relatively good, the sensor hysteresis is less than 4% from the adsorption–desorption process of water molecules, and the sensitivity of the sensor to RH is good compared to some previously reported studies, reaching up to 600% at 96% RH. Based on the results obtained, it can be concluded that TiO2 mixed with EC is very suitable to be applied to humidity sensors.

Wearable humidity sensors based on bar-printed poly(ionic liquid) for real-time humidity monitoring systems

Wearable chemical sensors can provide crucial human-physiology information by monitoring humidity in various environments. Capacitive-type humidity sensors based on poly(ionic liquid) (PIL) as the sensing material, which was synthesized via the polymerization of an ionic liquid (IL) were developed. The PIL films fabricated through a facile bar-printing process in ambient air, exhibited outstanding capacitive characteristics over a broad relative humidity (RH) range of 10%−80% at frequencies between 100 Hz and 1 MHz. Notably, high sensitivity (1.8 nF/%RH), good linearity (R2 = 0.980), low hysteresis, and fast response (~20 ms) of the developed humidity sensors were achieved. The corresponding sensing mechanism of the PIL for capacitive-type humidity sensors was studied by investigating the chemisorption of water molecules on the PIL surface and proton hopping through multiple layers of water molecules physisorbed on chemisorbed water molecules through single hydrogen bonding. In addition, the operational stability of the wearable humidity sensors developed in this study was investigated by analyzing the capacitive characteristics of the PIL sensing material at various temperatures (20–60 °C), even under a bending state and after 20,000 bending-stress cycles with a strain of 30%. Finally, a real-time humidity monitoring system integrated with signal processing and wireless communication modules was demonstrated by analyzing real-time capacitive signals acquired from a wearable humidity sensor attached to a human wrist and by visualizing processed humidity information on a user’s smartphone.

Target properties optimization on capacitive-type humidity sensor: Ingredients hybrid and integrated passive devices fabrication

Optimized hygroscopic film preparation and high-precision manufacturing guarantee the humidity sensor expected properties on sensitivity, hysteresis, response/recovery speed, reliability, etc. This paper demonstrated a comprehensive prioritization scheme on the premise of industrial production. Raw materials mixtures such as pyromellitic dianhydride (PMDA)/p-phenylenediamine (p-PDA), and PMDA/oxydianiline (ODA) were investigated to solve the issue of film surface collapse. Polyacrylonitrile (PAN) is adopted as an additive to enhance the film reliability (maximum of 0.15 % measurement error rate in 120 h continuous testing) as well as assistance in the film coating process. Rutile TiO2 with the average particle size of 0.33 μm was employed as doping elements to reduce the hysteresis to 0.95 % relative humidity (RH), increase the sensitivity to 0.94 pF/% RH, and limit the response/recovery time within 40 s. The film-growth quality and the consistency of production performance were assured by the integrated-passive-device (IPD) fabrication process.

Enhanced sensitivity of zinc phthalocyanine-based microporous humidity sensors by varying size of electrode gaps

The use of organic materials has become an increasingly important issue in sensing devices in recent times. Phthalocyanine is among the most promising materials in this undertaking. Zinc phthalocyanine (ZnPc) based microporous device was fabricated and its capacitance was utilized as the sensing mechanism for a humidity sensor. The effect of the electrode gap of the device on the electrical properties was investigated along with the correlation between the device’s performances and the morphology of the sensing film. Using the solution-processed spin coating method, the capacitive type humidity sensor devices have been fabricated in a planar geometry of Al/ZnPc/Al with the presence of a microporous template. The size of electrode gaps measured with a surface profiler was 53.00 ± 0.06 μm, 119.00 ± 0.03 μm and 286.00 ± 0.01 μm. The surface morphology was characterized by using transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS). Analysis of the experimental results showed that the device with the shortest electrode gap (53.00 μm) produced the best sensitivity of 1.03 ± 0.04 pF/%RH than that of the longer gaps. Additionally, hysteresis as well as response and recovery performances have also been investigated.

Capacitive humidity sensing performance of naphthalene diimide derivatives at ambient temperature

We report for the first-time the development of capacitive type humidity sensors employing naphthalene diimide derivatives (NDI) as sensing layer. Three different naphthalene diimide derivatives bearing imide side chains of different hydrophilicity were designed, synthesised and characterised. X-ray diffraction and thermogravimetric analyses gave useful information about structural and thermal behaviour of the newly developed materials, indicating their crystallinity and stability. Atomic force microscopy analysis revealed a variety of morphologies in thin films as a result of the structural properties of the NDIs. Devices bearing NDI layers were fabricated on ceramic substrates with gold interdigitated electrodes spaced 200 µm apart. Humidity sensing performance, as a change in capacitance, was studied upon exposure to a wide range of relative humidity levels (0–95%) at ambient temperature. Importantly, an increase in the capacitance of the sensors was recorded with an increase in relative humidity. The developed sensors exhibited high sensitivity, good long-term stability, excellent reproducibility, and low hysteresis. The sensor performance was also tested against different operating frequencies (250 Hz–2 kHz) to improve linearity, illustrating directions for optimised performance. These results confirm that sensors based on NDIs possess better sensing performance to other types of reported capacitive humidity sensors.

Improvement of capacitive humidity sensors using tris(8-hydroxyquinoline) gallium (Gaq3) nanofibers as a dielectric layer

In this research work, we improved the sensing response of the organic-based co-planar humidity sensors using tris(8-hydroxyquinoline) gallium (Gaq3) nanofibers as a dielectric material. The humidity sensor was fabricated by a drop-casting solution in the gap between pre-deposited silver (Ag) electrodes to form Ag/Gaq3/Ag-based capacitive-type humidity sensor. The morphology of the Gaq3 films has been investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). At room temperature, the capacitive response of the sensor is evaluated in the range of 0 to 95%RH at 120 Hz & 1 kHz. Notably, at 1 kHz frequency, the sensor shows fast response, good sensitivity, and low hysteresis. The response & recovery times were found to be 6 s each, which are way smaller than those reported in the literature. Such features demonstrate that Gaq3 nanofibers are an up-and-coming candidate for making very sensitive and fast humidity sensors.

Aerosol deposited BaTiO3 film based interdigital capacitor and squared spiral capacitor for humidity sensing application

In this article, ultra-high sensitive aerosol deposited BaTiO3 film based inter-digital capacitors (IDCs), and a squared spiral capacitor (SSC) have been investigated for humidity sensing application. BaTiO3 based sensing film is prepared at room temperature, then post-annealed at 200 °C and 400 °C to enhance material's sensing properties. Thermal exposure of BaTiO3 sensing film increases electron density, improves material defects, enhances grain to grain connectivity, and escalates hydrophilicity. The sensitivity of all fabricated devices is evaluated by comparing capacitive variation at different relative humidity (RH) levels. The electric field distribution and open area ratio (OAR) are scrutinized to understand the effect of these parameters on the BaTiO3 integrated capacitive type humidity sensor. The obtained results indicate that the IDC-1 with large OAR reveals high sensitivity due to the availability of the wider interaction area between fringing electric field intensity and water vapors adsorbed on sensing material surface. Similarly, the SSC structure reveals high sensitivity over IDC-2 and IDC-3 due to the existence of concentrated electric field intensity and large OAR. The intensified electric field polarizes the water vapor molecules and sensing material ions that upsurge the device capacitance at various RH levels, which enhances sensitivity. This study reveals that the design structure, electric field intensity, and sensing material property are equally responsible for high sensitivity in capacitive humidity sensors.

Capacitive type humidity sensor based on PANI decorated Cu–ZnS porous microspheres

Porous microstructure materials are considered good candidates for the development of highly sensitive and fast humidity sensors. In this regard, we prepared polyaniline (PANI) decorated Cu–ZnS porous microsphere structures (PMSs) for the fabrication of humidity sensors. PANI coated Cu–ZnS PMSs were synthesized by a hydrothermal method and in situ polymerization process. The synthesized PMSs were characterized by different techniques to study the structural, morphological and surface absorption properties. Several compositions for the PANI/Cu–ZnS PMS were investigated, which were then compared with pure PANI. The experimental observations demonstrate that a PANI/1%Cu–ZnS PMS has better sensitivity, fast response and good stability compared to pure PANI and other PANI/CuZnS compositions. Finally, a PANI/1% Cu–ZnS PMS was found to be optimized for humidity sensors due to its well distributed roughness, porosity and hydrophilicity. The average response and recovery times for PANI/1% Cu–ZnS were found to be 42 s and 24 s, respectively, which outperform recent results.

Electrospun PVA/TiC Nanofibers for High Performance Capacitive Humidity Sensing

In this work, high performance capacitive type humidity sensors are fabricated based on polymeric PVA/TiC nanofibers by reinforcing TiC NPs into the polyvinyl alcohol matrix following the most cost-effective electrospinning technique. The fabricated nanofibers are then characterized by SEM, XRD and FTIR which reveal the strong bonding interaction of both the precursors. The humidity sensing capability of PVA/TiC nanofibers mat is determined at room temperature in the frequency range of 100 Hz - 100 kHz. The fabricated humidity sensor displays an excellent capacitive humidity response in terms of linear capacitive behavior, short response/recovery time of 2.8/1.7 s, high sensitivity of 1,277,355.0%, good repeatability and excellent stability, at the frequency of 100 Hz. Moreover, in the humidification and dehumidification cycle, the sensor exhibits a small hysteresis of 2.8% at 100 Hz. The fabricated nanofibers-based sensor also indicates high sensitivity to every 1% RH variation from 1% to 90% RH. These favorable characteristics of PVA/TiC nanofibers enable the fabricated humidity sensor to determine the human respiration and can differentiate between slow and fast breath from the nose and mouth or before and after the exercise, respectively.

The enhanced performance of capacitive-type humidity sensors based on ZnO nanorods/WS2 nanosheets heterostructure

Zinc oxide (ZnO) is a promising candidate for humidity-sensing materials due to its low-cost preparation, superior chemical and thermal stability, controllable surface morphology, and low water solubility. However, pristine ZnO-based humidity sensors suffer from poor response and large hysteresis that limit their application. In this study, n-type semiconducting tungsten disulfide (WS2) was utilized to form ZnO nanorods/WS2 nanosheets heterostructure grown on indium tin oxide coplanar electrode-coated glass substrate. The capacitive-type humidity sensing characteristics were investigated at room temperature. The results show that for humidity ranges of 18–85 % RH, three deposition cycles of WS2 nanosheets onto ZnO nanorods produced significant improvements in the response, sensitivity, and hysteresis with an unchanged response and recovery times compared to pristine ZnO sensors. This improved sensor performance might be due to the ability of WS2 to provide more water molecule adsorption sites. The formation of an n-n junction between ZnO and WS2 created interfaces with high local charge density and built an internal electric field that increase the water dissociation rate. The improved hysteresis might be due to water molecule adsorption on WS2 nanosheets is physical adsorption that facilitates the desorption process.

Resistive- and capacitive-type humidity and temperature sensors based on a novel caged nickel sulfide for environmental monitoring

In this work for the first time, a new bis(4-benzylpiperazine-1-carbodithioato-k2S,Sʹ)nickel(II) complex (hereafter caged nickel sulfide) has been used to fabricate the capacitive-type and resistive-type sensor. The surface consisted of 2D plates, pores and pore-channels of various shapes and size. These 2D plates and pores played a pivotal role in the sensing mechanism of the sensor. The conduction mechanism is based on Von Grotthuss mechanism. In the relative humidity (RH) range 30–90%, the resistance of the sensor was decreased by two orders of magnitude (from 2.94 × 108 Ω at 30%RH to 2.34 × 106 Ω at 90%RH at operational frequency of 120 Hz). While at applied frequency of 120 Hz, capacitance of the sensor was increased from 15.95 pF to 38.1 pF in the range of 30–90%RH. At higher frequency (10 kHz) the capacitance of the sensor is reduced to 6.285 pF. The maximum hysteresis of 1.54% is noted which is less than the reported in the literature. The response and recovery time of the sensor were 25 and 30 s, respectively, which are either far smaller or greater than the response and recovery time of the various sensors reported in the literature.

Silk fibroin based transparent and wearable humidity sensor for ultra-sensitive respiration monitoring

Recently, the development of biocompatible and transparent devices for high sensitivity respiration monitoring has become a hot topic. Herein, a capacitive-type humidity sensor based on the silk fibroin dielectric sensing film was proposed. Both the breathing rates and intensities were recognized through the capacitance changes induced by vapor penetration. A low-cost and facile patterning method was proposed to prepare the Ag nanowires interdigitated electrodes (IDEs). The sensors showed rapid responses toward environmental humidity variations and different breathing conditions, covered the breathing rate up to 4 Hz. The sensor had a transmittance of above 85%. Mechanical and electrical stabilities of the sensor were confirmed through a bending test. A portable kit integrated with conventional electronics was attempted, demonstrating the practical use is on the way.

Fabrication of an anodized porous alumina relative humidity sensor with improved sensitivity

Sensitivity is an important parameter of any humidity sensor. In recent times, nanostructure materials have been used to fabricate the capacitive-type humidity sensors to improve the sensitivity. However, for the nanostructure sensors, in most cases, the sensitivity improvement is not guaranteed although extra efforts are made in terms of time and cost. Our aim is to improve the sensitivity of an anodized porous alumina capacitive humidity sensor by a simple method. A nanostructure porous film has been formed on each side of a pure aluminum substrate (99% purity) by an electrochemical anodization method. Two parallel plate capacitors are formed by depositing metal plate on each side of the metal oxide film with bulk aluminum as the common electrode. The sensitivity has been increased by connecting the two parallel plate capacitive sensors in parallel. Thus, two capacitive sensors on single substrate are fabricated and connected in parallel to form the third capacitor, which is used as the humidity sensor. The sensitivity, response time, recovery time, and hysteresis of the sensors are determined with variation in the range from 10% to 99% relative humidity (RH). The sensitivity of the sensor obtained by connecting two capacitors in parallel has been increased by almost two times compared to the sensitivity of an individual sensor fabricated on each side.

High Sensitivity Humidity Sensor and Its Application in Nondestructive Testing for Wet Paper

In this paper, a capacitive-type humidity sensor based on TiO2/Ag nanoparticle composites film was fabricated on interdigital electrodes. Experimental results showed that a small amount of Ag nanoparticles mixed with TiO2 nanoparticles could greatly improve the sensor sensitivity. Compared with pure TiO2 sample, the improved capacitance variation of sensor raised more than two orders of magnitude. The reduction in water contact angle measurements proved that the hydrophilicity of the film had increased, and the appearance of Warburg impedance at 11% RH in the complex impedance spectra plot indicated that adding Ag improved the absorption ability of composite film. This high-sensitive humidity sensor proposed in this work could enable the nondestructive detection of wet paper and had potential applications in the preservation of ancient manuscripts, historical calligraphies and paintings.

Planar capacitive type humidity sensor fabricated using PTB7-Th by facile solution processing approach

A capacitive humidity sensor using coplanar electrodes coated with poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl](PTB7-Th) thin film (Al/PTB7-Th/Al) has been fabricated and investigated. In the present work, we show that increased sensitivity is obtained by rough and non-uniform surface morphology of the polymer thin film which comprises distinct peaks and valleys. The sensor shows good sensing behavior with enhanced sensitivity, better linearity and larger bandwidth. The device showed sensitivity over an appreciable range of relative humidity levels (between 20 and 95% RH). Typical adsorption and desorption response times were measured to be 20 s each.

A Capacitive Type Humidity Sensor with Potassium Ion Doped TiO2 Thin Film as Humidity Sensing Material

A Capacitive Type Humidity Sensor with Potassium Ion Doped TiO₂ Thin Film as Humidity Sensing Material

Stable and Fast-Response Capacitive Humidity Sensors Based on a ZnO Nanopowder/PVP-RGO Multilayer.

In this paper, capacitive-type humidity sensors were prepared by sequentially drop-coating the aqueous suspensions of zinc oxide (ZnO) nanopowders and polyvinyl pyrrolidone–reduced graphene oxide (PVP-RGO) nanocomposites onto interdigitated electrodes. Significant improvements in both sensitivity and linearity were achieved for the ZnO/PVP-RGO sensors compared with the PVP-RGO/ZnO, PVP-RGO, and ZnO counterparts. Moreover, the produced ZnO/PVP-RGO sensors exhibited rather small hysteresis, fast response-recovery time, and long-term stability. Based on morphological and structural analyses, it can be inferred that the excellent humidity sensing properties of the ZnO/PVP-RGO sensors may be attributed to the high surface-to-volume ratio of the multilayer structure and the supporting roles of the PVP-RGO nanocomposites. The results in this work hence provide adequate guidelines for designing high-performance humidity sensors that make use of the multilayer structure of semiconductor oxide materials and PVP-RGO nanocomposites.

Ultrahigh humidity sensitivity of graphene oxide combined with Ag nanoparticles

Novel capacitive-type humidity sensors with ultrahigh sensitivity based on graphene oxide (GO) combined with Ag nanoparticles (AgNPs) in different concentrations are reported in this study.