Preparation and Research of a High-Performance ZnO/SnO2 Humidity Sensor

A high-performance flexible humidity sensor based on a TiO2-MWCNT nanocomposite for human healthcare applications

The present work shows the improved humidity sensing characteristics of TiO2 nanoparticles in the form of a nanocomposite with multiwalled carbon nanotubes (MWCNTs) prepared by a hydration-dehydration method. The structural and morphological characterizations of TiO2-MWCNTs confirm the nanocomposite formation without any other impurities and with an improved surface area. The pure TiO2 and nanocomposite films are deposited on IDE coated flexible poly-ethylene terephthalate (PET) substrates by a drop casting method. The nanocomposite shows improved sensitivity (1246.2 MΩ/%RH) and an ultrafast response/recovery time (2 s/1 s) with a minimal hysteresis of 0.27%RH. Further, the flexible nanocomposite sensor is tested for human healthcare applications including respiratory monitoring, apnea like situations, and skin moisture detection. The sensor can distinguish different breath patterns like normal, fast, deep and apnea like situations significantly. Skin moisture detection can also be performed using the nanocomposite sensor in a non-invasive manner. Overall, this study represents an environmentally friendly, easy to fabricate, flexible TiO2-MWCNT nanocomposite based improved humidity sensor for application in human healthcare and wearable devices.

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In this work, we develop a novel capacitive humidity sensor based on Al-Si acceptor-donor co-doped SnO2 for real-time monitoring of ambient humidity and human respiration. XRD measurements reveal that all samples exhibit a tetragonal rutile phase and the crystallite size of SnO2 decreases with increasing Al-Si content. The high intensity of the Raman peak at 762 cm-1 confirms the presence of bridging mode oxygen vacancies in (Al + Si)0.02Sn0.98O2. The EPR results show that the amount of singly ionized oxygen vacancies increases after the introduction of Al-Si. Both types and amounts of oxygen vacancy defects are particularly sensitive to the adsorption of water molecules. Moreover, according to DFT calculations, the contribution of the Si 3s orbital and Al 3s orbital to the band edge verifies the formation of acceptor-donor complexes in Al-Si co-doped SnO2. The humidity sensing results reveal that the (Al + Si)0.02Sn0.98O2 humidity sensor shows high sensitivity (S = 839), low hysteresis (1.94%) and fast response/recovery times (25 s/5 s). The respiratory intervals during shallow, medium and deep breathing states of (Al + Si)0.02Sn0.98O2 were measured at 2.8 s, 3.8 s and 4.5 s, respectively. The chemical mechanism for the enhancement of humidity sensing performance corresponding to the oxygen vacancy defects induced by Al-Si interplay is proposed.

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.

Synthesis of Guanine/Vermiculite Two-Dimensional Nanocomposites for Wireless Humidity Sensing in Nut Storage Environment

Two-dimensional (2D) nanostructures have the advantages of high specific surface area, easy surface functionalization, abundant active sites, and good compatibility with device integration and can be assembled into three-dimensional structures, which are key to the development of high-performance gas sensors. In this study, 2D vermiculite (VMT) nanosheets and guanine (G), two renewable resources with unique chemical structures, were organically combined to fully use the specificity of their molecular structures and functional activities. Driven by the regulation of 2D VMT nanosheets, guanine/vermiculite (G/VMT)-based 2D nanocomposites with controllable pore structure, multiple binding sites, and unobstructed mass transfer were designed and synthesized. The G/VMT nanocomposite material was used as a quartz crystal microbalance (QCM) electrode-sensitive film material to build a QCM-based humidity sensor. G/VMT-based QCM humidity sensor had good logarithmic linear relation (0.9971), high sensitivity (24.49 Hz/% relative humidity), low hysteresis (1.75% RH), fast response/recovery time (39/6 s), and good stability. Furthermore, with a QCM sensor and a specially designed wireless circuit, a wireless humidity detection system transmitting via Wi-Fi allows real-time monitoring of nut storage. This study presents an environmentally friendly, high-performance, miniature 2D nanocomposite sensor strategy for real-time monitoring.

Characterization of an Impedance-Type Humidity Sensor Based on Porous SnO2/TiO2 Composite Ceramics Modified with Molybdenum and Zinc

In this study, we report on the room-temperature characteristics of an impedance-type humidity sensor based on porous tin oxide/titanium oxide (SnO2/TiO2) composite ceramics modified with Mo and Zn. The SnO2/TiO2-based composites synthesized in the solid-state processing technique have been structurally characterized using X-ray diffraction, scanning electron microscopy, energy dispersive, and Raman spectroscopy. Structural analysis indicated the desired porous nature of the synthesized ceramics for sensing applications, with an average crystallite size in the nano range and a density of about 80%. The humidity-sensing properties were evaluated within a wide relative humidity range from 15% to 85% at room temperature, and the results showed that a better humidity response had a sample with Mo. This humidity-sensing material exhibits a linear impedance change of about two orders of magnitude at the optimal operating frequency of 10 kHz. Furthermore, fast response (18 s) and recovery (27 s), relatively small hysteresis (2.8%), repeatability, and good long-term stability were also obtained. Finally, the possible humidity-sensing mechanism was discussed in detail using the results of complex impedance analysis.

A Flexible Humidity Sensor with Wide Range, High Linearity, and Fast Response Based on Ultralong Na2Ti3O7 Nanowires

A flexible humidity sensor with wide sensing range, superior sensitivity, high linearity, and advanced response/recovery capabilities is extremely desirable for practical applications in human body-related (HBR) monitoring and human-machine interaction (HMI). However, the practical sensor lacks a versatile nanomaterial integrated with sensing capabilities and mechanical flexibility to meet the criteria. Herein, a comprehensive flexible humidity sensor with ultralong Na₂Ti₃O₇ nanowires (>120 μm) is subtly constructed for the first time. Owing to the distinguish nanowires network structure, the sensor exhibits wide sensing range (11-95% RH), high sensitivity (>10³), high linearity (R² > 0.98), and fast response/recovery capability (8.9/2.1 s), as well as excellent respiratory stability (>5000 s). In addition, the Na₂Ti₃O₇-based humidity sensor demonstrates superior flexibility and antibacteria capabilities, and exhibits diverse applications in respiration monitoring, noncontact detection, as well as dynamic interactive display. This work provides a multifunctional humidity sensor with excellent practicability, suggesting the great potential in next-generation human-related flexible/wearable devices.

KIT-5-Assisted Synthesis of Mesoporous SnO2 for High-Performance Humidity Sensors with a Swift Response/Recovery Speed

Developing highly efficient semiconductor metal oxide (SMOX) sensors capable of accurate and fast responses to environmental humidity is still a challenging task. In addition to a not so pronounced sensitivity to relative humidity change, most of the SMOXs cannot meet the criteria of real-time humidity sensing due to their long response/recovery time. The way to tackle this problem is to control adsorption/desorption processes, i.e., water-vapor molecular dynamics, over the sensor's active layer through the powder and pore morphology design. With this in mind, a KIT-5-mediated synthesis was used to achieve mesoporous tin (IV) oxide replica (SnO₂-R) with controlled pore size and ordering through template inversion and compared with a sol-gel synthesized powder (SnO₂-SG). Unlike SnO₂-SG, SnO₂-R possessed a high specific surface area and quite an open pore structure, similar to the KIT-5, as observed by TEM, BET and SWAXS analyses. According to TEM, SnO₂-R consisted of fine-grained globular particles and some percent of exaggerated, grown twinned crystals. The distinctive morphology of the SnO₂-R-based sensor, with its specific pore structure and an increased number of oxygen-related defects associated with the powder preparation process and detected at the sensor surface by XPS analysis, contributed to excellent humidity sensing performances at room temperature, comprised of a low hysteresis error (3.7%), sensitivity of 406.8 kΩ/RH% and swift response/recovery speed (4 s/6 s).

Formation of a Nanorod-Assembled TiO2 Actinomorphic-Flower-like Microsphere Film via Ta Doping Using a Facile Solution Immersion Method for Humidity Sensing

This study fabricated tantalum (Ta)-doped titanium dioxide with a unique nanorod-assembled actinomorphic-flower-like microsphere structured film. The Ta-doped TiO₂ actinomorphic-flower-like microsphere (TAFM) was fabricated via the solution immersion method in a Schott bottle with a home-made improvised clamp. The samples were characterised using FESEM, HRTEM, XRD, Raman, XPS, and Hall effect measurements for their structural and electrical properties. Compared to the undoped sample, the rutile-phased TAFM sample had finer nanorods with an average 42 nm diameter assembled to form microsphere-like structures. It also had higher oxygen vacancy sites, electron concentration, and mobility. In addition, a reversed double-beam photoacoustic spectroscopy measurement was performed for TAFM, revealing that the sample had a high electron trap density of up to 2.5 μmolg^-1. The TAFM showed promising results when employed as the resistive-type sensing film for a humidity sensor, with the highest sensor response of 53,909% obtained at 3 at.% Ta doping. Adding rGO to 3 at.% TAFM further improved the sensor response to 232,152%.

SnO2-Based Ultra-Flexible Humidity/Respiratory Sensor for Analysis of Human Breath

Developing ultraflexible sensors using metal oxides is challenging due to the high-temperature annealing step in the fabrication process. Here, we demonstrate the ultraflexible relative humidity (RH) sensor on food plastic wrap by using 808 nm near-infrared (NIR) laser annealing for 1 min at a low temperature (26.2-40.8 °C). The wettability of plastic wraps coated with sol-gel solution is modulated to obtain uniform films. The surface morphology, local temperature, and electrical properties of the SnO₂ resistor under NIR laser irradiation with a power of 16, 33, and 84 W/cm² are investigated. The optimal device can detect wide-range RH from 15% to 70% with small incremental changes (0.1-2.2%). X-ray photoelectron spectroscopy reveals the relation between the surface binding condition and sensing response. Finally, the proposed sensor is attached onto the face mask to analyze the real-time human breath pattern in slow, normal, and fast modes, showing potential in wearable electronics or respiration monitoring.

Polyantimonic acid-based materials evaluated as moisture sensors at ambient temperature

Humidity sensors are in high demand for many applications, such as environmental monitoring and air and food quality control. Despite many inorganic and organic materials exhibit moisture sensing properties, the electrical response of many existing sensors is not stable along the time. Polyantimonic acid (PAA) is characterized by elevated proton conductivity and by high thermal stability: consequently, it is seen as promising proton conductor for usage in humidity sensing devices. In this work, for the first time, PAA-based bulk solid membranes were produced and tested as potential materials for relative humidity (RH) detection and their moisture sensitivity was evaluated. Two different amounts of binder were used for moulding the solid sensors: the ones with 10% of binder were designated as 90PAA, while the ones with 20% were named 80PAA. The structures of the solid samples were investigated by X-ray diffraction (XRD) technique, adsorption–desorption curves via Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) techniques. The electrical behaviour was examined at ambient temperature by electrical impedance spectroscopy in the entire relative humidity (RH) interval (0–100%) and in the frequency range of 40 Hz up to 60 MHz. Electrical response of the materials was correlated with the structural features of the membranes. Both 90PAA and 80PAA sensors showed total resistance 3 × 105 and 3.5 × 105 Ω at 10% RH, respectively. A linear decrease of the resistance on RH was observed in the range 30–90% RH for both sensors. The electrical response of the evaluated PAA-based sensors displays good repeatability and reproducibility: the ones with lower binder content showed higher moisture sensitivity as well as very good time stability over 1 year.

SnO2@ZnO nanocomposites doped polyaniline polymer for high performance of HTM-free perovskite solar cells and carbon-based

Herein, at first, green SnO₂@ZnO nanocomposites were synthesized using Calotropis plant extract as an electron transfer material (ETM) to fabricate low-temperature-processed perovskite solar cells (PSCs). Then, the polyaniline (PANI) polymer was applied as an efficient additive to improve perovskite film quality. Under the effects of the small content of PANI additive, the quality of perovskite films is enhanced, which showed higher crystallinity in (110) crystal plane; also, the perovskite grains were found to be enlarged from 342 to 588 nm. The power conversion efficiency (PCE) of the prepared PSCs with SnO₂@ZnO.PANI nanocomposites electron transfer layer (ETL) increased by 3.12%, compared with the PCE of SnO₂@ZnO nanocomposites. The perovskite devices using SnO₂@ZnO.PANI nanocomposites ETL have shown good stability during 480 h of tests. Furthermore, the optimal PSCs were fabricated by the mp-TiO₂/SnO₂@ZnO.PANI nanocomposites as ETL, which has a power conversion efficiency of 15.45%. We expect that these results will boost the development of low-temperature ETL, which is essential for the commercializing of high-performance, stable, and flexible perovskite solar cells.

Annealing Temperature Effects on Humidity Sensor Properties for Mg0.5W0.5Fe2O4 Spinel Ferrite

The effects of annealing temperature on the structural, physical and humidity sensing properties of stoichiometric Mg_0.5W_0.5Fe₂O₄ spinel ferrite are investigated. In order to highlight the influence of sintering temperature on the structural, magnetic and electrical properties, ferrite samples were sintered for 2 h at 850 °C, 900 °C, 950 °C, 1000 °C and 1050 °C and the physical properties and humidity influence on magnesium-tungsten ferrite materials were analyzed. X-ray diffraction investigations confirmed the formation of magnesium-tungsten ferrite in the analyzed samples. SEM micrographs revealed the influence of annealing temperature on the microstructures of the samples and provided information related to their porosity and crystallite shape and size. This material, treated at different temperatures, is used as an active element in the construction of capacitive and resistive humidity sensors, whose characteristics were also investigated in order to determine the most suitable sintering temperature.

An Improved Humidity Sensor with GO-Mn-Doped ZnO Nanocomposite and Dimensional Orchestration of Comb Electrode for Effective Bulk Manufacturing

The measurement and control of humidity is a major challenge that affects the sensing properties of sensors used in high-precision equipment manufacturing industries. Graphene Oxide(GO)-based materials have been extensively explored in humidity sensing applications because of their high surface area and functional groups. However, there is a lack of effective bulk-manufacturing processes for the synthesis of 2D-based nanocomposites with comb electrodes. Moreover, water intercalation within the layers of 2D materials increases recovery time. This work demonstrates the enhanced sensing characteristics of a capacitive/resistive GO-MnZnO nanocomposite humidity sensor produced using a cost-effective single-pot synthesis process. The in-plane sensing layer consistently improves sensitivity and reduces response time for a wide range of relative humidity measurements (10% to 90%). Interdigitated gold electrodes with varying numbers of fingers and spacing were fabricated using photolithography on a Si/SiO₂ for a consistent sensor device platform. The choice of nanomaterials, dimension of the sensor, and fabrication method influence the performance of the humidity sensor in a controlled environment. GO nanocomposites show significant improvement in response time (82.67 times greater at 40% RH) and sensitivity (95.7 times more at 60% RH). The response time of 4.5 s and recovery time of 21 s was significantly better for a wider range of relative humidity compared to the reduced GO-sensing layer and ZnMnO. An optimized 6 mm × 3 mm dimension sensor with a 28-fingers comb was fabricated with a metal-etching process. This is one of the most effective methods for bulk manufacturing. The performance of the sensing layer is comparable to established sensing nanomaterials that are currently used in humidity sensors, and hence can be extended for optimal bulk manufacturing with minimum electrochemical treatments.