Humidity-sensing Properties Research Articles

Dielectric response, non-Ohmic behaviors and humidity-sensing characteristics: Tin doping in sodium yttrium copper titanate ceramics

The impact of tin (Sn4+) doping on sodium yttrium copper titanate (Na0.5Y0.5Cu3Ti4O12) ceramics, prepared using a conventional mixed-oxide route, was thoroughly investigated in terms of microstructure, dielectric properties, electrical response, non-linear behaviors, and humidity-sensing characteristics. All sintered ceramics showed a dense micro-structure and a pure Na0.5Y0.5Cu3Ti4O12 phase without any secondary phases. The doped Na0.5Y0.5Cu3Ti4−xSnxO12 ceramics, with x = 0.05, demonstrated excellent dielectric properties, including a low dielectric loss tangent (∼0.032) and a giant dielectric permittivity (∼1.8 × 104) across a wide temperature range. Sn4+ doping improved the non-linear behaviors at 25 °C in Na0.5Y0.5Cu3Ti4O12, with a reduced dielectric loss tangent corresponding to an enhanced grain boundary (GB) response. The colossal dielectric response is attributed to the internal barrier layer capacitor model, which relates to the Schottky barrier height (ΦB) at the GBs. Notably, the ΦB values for the Na0.5Y0.5Cu3Ti4O12 ceramic increased with Sn4+ ion doping. Additionally, an in-depth study of the humidity-sensing properties of the Sn4+-doped Na0.5Y0.5Cu3Ti4O12 material revealed that the capacitance at 1 kHz increased with increasing relative humidity levels, from 30 % to 90 %, suggesting potential applications in humidity-sensing technologies.

Designing High Performance Carbon/ZnSn(OH)6-Based Humidity Sensors.

In this work, pure phase and carbon/ZnSn(OH)6 samples were synthesized by a hydrothermal method. The composite sample's structure, morphology, and functional groups were investigated by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Subsequently, ZnSn(OH)6 samples were modified with different carbon contents, and their humidity-sensing properties were investigated. The introduction of carbon increased the specific surface area of pure ZnSn(OH)6 samples, thus significantly improving the sensors' humidity sensing response. The C10-ZnSn(OH)6 sensor exhibited a high response, up to three orders of magnitude, a humidity hysteresisof 13.5%, a fast response time of 3.2 s, and a recovery time of 24.4 s. The humidity sensor's possible humidity sensing mechanism was also analyzed using the AC complex impedance puissance method with a simulated equivalent circuit. These results revealed that ZnSn(OH)6 can effectively detect ambient humidity and that the introduction of carbon significantly improves its humidity-sensing performance. The study provides an effective strategy for understanding and designing ZnSn(OH)6-based humidity sensors.

Synthesis of Li doped MgFe2O4 nanoparticles for humidity sensor applications

Recently, there has been a greater emphasis on researching the potential use of ferrite nanoparticles as humidity-sensing materials. We report on the humidity-sensing properties of Mg1-yLiyFe2O4 (y = 0, 0.01, 0.03 and 0.05), which were synthesized using the solution combustion synthesis route. According to the XRD analysis, it was observed that both the unit cell volume and the crystallite size increased as the concentration of lithium-ion increased. The crystallite size was measured to be 17–22 nm, indicating the presence of nanomaterials. Moreover, the material exhibited a single phase with the Fd-3m space group, suggesting its structural integrity and uniformity. The FE-SEM images revealed that the porous nature of the material became more pronounced with higher concentrations of Li, indicating potential benefits for sensing applications. The synthesized powder demonstrated promising characteristics for use in humidity sensors. Specifically, it was noted that increasing the Li composition led to a notable increase in resistance, particularly significant for the Li = 0.05 concentration, which exhibited the highest average sensitivity. This suggests that Li doping at this concentration effectively enhances resistance, a crucial aspect for sensor functionality. The response and recovery times of thin film humidity sensors fabricated from these materials were determined to be 9 and 12 s, respectively. These times indicate rapid and efficient sensing capabilities, essential for real-time monitoring applications. Moreover, the newly discovered sensing material exhibited exceptional stability and reproducibility, further highlighting its potential for practical sensor applications. In summary, the synthesized materials show great promise for humidity sensor applications due to their enhanced sensitivity, rapid response and recovery times, as well as their stability and reproducibility. These findings open up avenues for the development of advanced sensing technologies with improved performance and reliability.

Facile one-step preparation of Co2CrO4 spinel from heterometallic compounds – Structural, magnetic, electrical and photocatalytic studies

Two isostructural heterometallic complex salts with mononuclear units, [NH(CH3)2(C2H5)][Cr(terpy)2][CoCl4]2 (1) and [NH(CH3)(C2H5)2][Cr(terpy)2][CoCl4]2 (2), were synthesized using a building block approach and characterized by single-crystal and powder X-ray diffraction (XRD), impedance spectroscopy and magnetization measurements. They exhibit particular humidity-sensing properties and very high proton conductivity at room temperature. The magnetization of complexes shows the high spin state of 3/2 for tetrahedrally coordinated CoII ions, which on cooling remain in the paramagnetic state within the CrIII network, which is antiferromagnetically correlated. The prepared compounds were thoroughly explored as single-source precursors for the formation of the spinel oxide Co2CrO4 by their thermal decomposition. By optimizing the annealing temperature, heating/cooling rates and holding times, the phase composition of thermal processing of 1 or 2 was evaluated: phase pure spinel Co2CrO4 was obtained by thermal treatment already at 450 °C. The structure, morphology, and optical properties of spinel oxides prepared at different temperatures were characterized using powder XRD, field emission scanning electron microscopy (SEM) and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), while electrical and magnetic by impedance spectroscopy and magnetization measurements. The determined band gap energy and nanosized crystallites prompted us to investigate the photocatalytic activity of spinel oxide produced at 450 °C in the degradation of dyes Rhodamine B (RhB) and Methylene blue (MB) under Vis irradiation without and with assistance of hydrogen peroxide (H2O2). The results show that the Co2CrO4/H2O2 system exhibits a significantly higher photoactivity response; the degradation efficiency of MB and RhB was found to be 86.1% and 80.3%, respectively. The electrical conductivity spectra and the activation energy for the DC conductivity demonstrate the electronically semiconducting behaviour of the Co2CrO4 oxide. The Co2CrO4 spinel has a ferrimagnetic ground state, with mixed CoII and CoIII ions on the tetrahedral and octahedral sites: CoII having a high spin state on both sites and CoIII having a high spin state on the tetrahedral site and a low spin state on the octahedral site. Different thermal treatments affect the magnetization according to the different sizes of the obtained crystallites.

Interconnected SnO2 nanoflakes decorated WO3 composites as wearable and ultrafast sensors for real-time wireless sleep quality tracking and breath disorder detection

Monitoring human breath is critical for determining human well-being. Most reported humidity sensors are unsuitable for breath monitoring because of their long response and recovery time, poor stability, and high operating temperature. To address these challenges, robust tungsten oxide (WO3) and tin oxide (SnO2) composite sensors were fabricated, and their humidity-sensing properties were investigated. The SnO2/WO3 composites were optimized by altering the SnO2 fraction to achieve high humidity sensitivity. The SnO2/WO3 composite exhibited superior sensing performance (88) than pristine WO3 and SnO2 under 97 % relative humidity. The sensor demonstrated good linearity while altering humidity from 15 % to 97 %, with an R2 = 0.9729. The sensor's rapid response (0.6 s) and recovery (0.6 s) time allow for breath rate monitoring and breath disorder detection. The wearable sensor demonstrated multifunctional human breath detections such as varied breath rates (5–60 breaths per minute), nose blocking for separate nostrils, wireless sleep quality tracking for more than 7 h continuously, and sleep apnea-hypopnea detection via a smartphone. The capabilities of the humidity sensor allow for robust application in sleep quality, sleep apnea, and hypopnea detection.

Effects of Al incorporation on structural and humidity-sensing properties of SnO2 sensor

Effects of Al incorporation on structural and humidity-sensing properties of SnO2 sensor

Next-generation self-adhesive dressings: Highly stretchable, antibacterial, and UV-shielding properties enabled by tannic acid-coated cellulose nanocrystals

Hydrogels with excellent high-water uptake and flexibility have great potential for wound dressing. However, pure hydrogels without fiber skeleton faced poor water retention, weak fatigue resistance, and mechanical strength to hinder the development of the dressing as next-generation functional dressings. We prepared an ultrafast gelation (6 s) Fe3+/TA-CNC hydrogel (CTFG hydrogel) based on a self-catalytic system and bilayer self-assembled composites. The CTFG hydrogel has excellent flexibility (800% of strain), fatigue resistance (support 60% compression cycles), antibacterial, and self-adhesive properties (no residue or allergy after peeling off the skin). CTFG@S bilayer composites were formed after electrospun silk fibroin (SF) membranes were prepared and adhesive with CTFG hydrogels. The CTFG@S bilayer composites had significant UV-shielding (99.95%), tensile strain (210.9 KPa), and sensitive humidity-sensing properties. Moreover, the integrated structure improved the mechanical properties of electrospun SF membranes. This study would provide a promising strategy for rapidly preparing multifunctional hydrogels for wound dressing.

Effect of Sb Doping on Structure and Humidity Sensing Properties of SnO2 Synthesized by Hydrothermal Method

AbstractThe tetragonal rutile Sb doped Sn O2 (0, 0.5, 1 at.%) is successfully synthesized by hydrothermal method. The XRD analysis shows that the average crystallite size of SnO2 decreases from 29.59 to 20.53 nm with the increase of the doping concentration. SEM images depict the spherical grain morphology of the SnO2 sample. With the increase of the doping ratio, the optical bandgap value of the SnO2 samples increases from 3.6 to 3.86 eV and then decreases to 3.47 eV, showing a trend of first increasing and then decreasing. XPS results indicate that oxygen vacancy defects and substitutional defects(SbSn)are formed into the SnO2 lattice, which changes the electronic effect of Sn element to tune the oxygen vacancy concentration. The humidity‐sensitive testing data shows that 1 at.% Sb‐doped SnO2 humidity sensor presented excellent sensitivity and linearity at a working frequency of 100 Hz as compared to the pure SnO2 sample.

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.

Fabrication of Micro-Cantilever Sensor Based on Clay Minerals for Humidity Detection.

In this paper, novel humidity sensors based on montmorillonite, kaolinite, and composite films coated on micro-cantilevers were prepared to measure the relative humidity (RH) values by the deflection of a micro-cantilever (MC) at room temperature. The humidity-sensing properties, such as response and recovery, sensitivity, repeatability, humidity hysteresis, and long-term stability, were investigated in the range of working humidity (10-80% RH). The humidity response in the close humidity range of 10% RH to 80% RH revealed a linear increase in water absorption of montmorillonite, kaolinite, and montmorillonite/kaolinite mixed dispersant (1:1) as a function of RH with linear correlation factors between the humidity change and deflection estimated to be 0.994, 0.991, and 0.946, respectively. Montmorillonite's sensitivity was better than kaolinite's, with the mixed-clay mineral film's response falling somewhere in between. This research provides a feasible and effective approach to constructing high-performance MC humidity sensors that can be operated at room temperature based on clay minerals.

ZIF-90-Derived Porous ZnO Coated Optical Microfiber Interferometer Sensor for Enhanced Humidity Sensing and Breath Monitoring.

Humidity plays an important role in many fields, and the realization of high sensitivity and fast response simultaneously for humidity detection is a great challenge in practical application. In this work, we demonstrated a high-performance relative humidity (RH) sensor made by supporting zeolitic imidazolate framework-90 (ZIF-90)-derived porous zinc oxide (ZnO) onto an optical microfiber Sagnac interferometer (OMSI). The ZIF-90-modified OMSI (ZIF-90-OMSI) sensor was in situ heated at different temperatures to obtain porous ZnO, and their humidity-sensing properties were investigated ranging from 25 to 80% RH. The experimental results showed that the porous ZnO fiber sensor prepared at 500 °C (Z500-OMSI) exhibited best humidity-sensing performance with a high sensitivity of 96.2 pm/% RH (25-45% RH) and 521 pm/% RH (50-80% RH) and ultrafast response/recovery time (62.37/206.67 ms) at 22.3% RH. These performances were attributed to the complete transformation of ZIF-90 to ZnO at 500 °C. The obtained Z500 not only retained the high porosity and specific surface area of ZIF-90 but also exhibited the exceptional hydrophilicity of ZnO. In addition, the signals of the proposed Z500-OMSI sensor changed with different breathing patterns, indicating the possibility for human respiration monitoring. This work provided a reliable candidate for an effective RH monitoring system with potential application in medical diagnoses, industrial production, environmental detection, and human health monitoring.

High Proton Conductivity of Magnetically Ordered 2D Oxalate-Bridged [MnIICrIII] Coordination Polymers with Irregular Topology.

Two heterometallic coordination polymers {[NH(CH3)2(C2H5)]8[Mn4Cl4Cr4(C2O4)12]}n (1) and {[NH(CH3)-(C2H5)2]8[Mn4Cl4Cr4(C2O4)12]}n (2) were obtained by slow evaporation of an aqueous solution containing the building block [A]3[Cr(C2O4)3] [A = (CH3)2(C2H5)NH+ or (CH3)(C2H5)2NH+] and MnCl2·2H2O. The isostructural compounds comprise irregular two-dimensional (2D) oxalate-bridged anionic layers [Mn4Cl4Cr4(C2O4)12]n8n- with a Shubnikov plane net fes topology designated as (4·82), interleaved by the hydrogen-bonded templating cations (CH3)2(C2H5)NH+ (1) or (CH3)(C2H5)2NH+ (2). They exhibit remarkable humidity-sensing properties and very high proton conductivity at room temperature [1.60 × 10-3 (Ω·cm)-1 at 90% relative humidity (RH) of 1 and 9.6 × 10-4 (Ω·cm)-1 at 94% RH of 2]. The layered structure facilitates the uptake of water molecules, which contributes to the enhancement of proton conductivity at high RH. The better proton transport observed in 1 compared to that in 2 can be tentatively attributed to the higher hydrophilicity of the cations (CH3)2(C2H5)NH+, which is closely related to their affinity for water molecules. The original topology of the anionic networks in both compounds leads to the development of interesting magnetic phases upon cooling. The magnetically ordered ground state can be described as the coupling of ferromagnetic spin chains in which Mn2+ and Cr3+ ions are bridged by bis(bidentate) oxalate groups into antiferromagnetic planes through monodentate-bidentate oxalate bridges in the layers, which are triggered to long-range order below temperature 4.45 K via weaker interlayer interactions.

Fast response humidity sensor based on chitosan/graphene oxide/tin dioxide composite

In this paper, Chitosan(CS)/graphene oxide(GO)/SnO2 composite is proposed as the sensing material of an impedance-type humidity sensor. The composite was synthesized via a facile solution mixing method. And it was formed by adding CS and SnO2 to GO aqueous solution. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FT-IR) were executed to characterize the structure and morphology of the composite. At room temperature, the CS/GO/SnO2 composite shows more favorable humidity-sensing properties compared with the CS, CS/GO, and CS/SnO2 material. The resulted sensor exhibits high response (72683%), high sensitivity (402.5 kΩ/%RH), rapid response and recovery time (8 s/8 s). Moreover, small humidity hysteresis value (<7%) under maximum impedance difference, good repeatability and long-term stability are demonstrated. Therefore, the CS/GO/SnO2 composite is a promising candidate for humidity sensing.

Ultra-Sensitive and Fast Humidity Sensors Based on Direct Laser-Scribed Graphene Oxide/Carbon Nanotubes Composites.

In this paper, the relative humidity sensor properties of graphene oxide (GO) and graphene oxide/multiwalled nanotubes (GO/MWNTs) composites have been investigated. Composite sensors were fabricated by direct laser scribing and characterized using UV-vis-NIR, Raman, Fourier transform infrared, and X-ray photoemission spectroscopies, electron scanning microscopy coupled with energy-dispersive X-ray analysis, and impedance spectroscopy (IS). These methods confirm the composite homogeneity and laser reduction of GO/MWNT with dominant GO characteristics, while ISresults analysis reveals the circuit model for rGO-GO-rGO structure and the effect of MWNT on the sensor properties. Although direct laser scribing of GO-based humidity sensor shows an outstanding response (|ΔZ|/|Z| up to 638,800%), a lack of stability and repeatability has been observed. GO/MWNT-based humidity sensors are more conductive than GO sensors and relatively less sensitive (|ΔZ|/|Z| = 163,000%). However, they are more stable in harsh humid conditions, repeatable, and reproducible even after several years of shelf-life. In addition, they have fast response/recovery times of 10.7 s and 9.3 s and an ultra-fast response time of 61 ms when abrupt humidification/dehumidification is applied by respiration. All carbon-based sensors' overall properties confirm the advantage of introducing the GO/MWNT hybrid and laser direct writing to produce stable structures and sensors.

Humidity Sensing Properties of (In+Nb) Doped HfO2 Ceramics

(In+Nb) co-doped HfO2 ceramics, Hf1-x(In0.5Nb0.5)xO2 (x = 0, 0.005, 0.05, and 0.1), were prepared via a solid-state reaction method. Dielectric measurements reveal that the environmental moisture has an obvious influence on the dielectric properties of the samples. The best humidity response was found in a sample with the doping level of x = 0.005. This sample was therefore selected as a model sample to further investigate its humidity properties. In doing so, nanosized particles of Hf0.995(In0.5Nb0.5)0.005O2 were fabricated via a hydrothermal method and the humidity sensing properties of this material were studied in the relative humidity range of 11-94% based on impedance sensor. Our results show that the material exhibits a large impedance change of nearly four orders of magnitude over the tested humidity range. It was argued that the humidity-sensing properties were related to the defects created by doping, which improves the adsorption capacity for water molecules.

Humidity and Temperature Sensing of Mixed Nickel–Magnesium Spinel Ferrites

Temperature- and humidity-sensing properties were evaluated of NixMg1-x spinel ferrites (0 ≤ x ≤ 1) synthesized by a sol-gel combustion method using citric acid as fuel and nitrate ions as oxidizing agents. After the exothermic reaction, amorphous powders were calcined at 700 °C followed by characterization with XRD, FTIR, XPS, EDS and Raman spectroscopy and FESEM microscopy. Synthesized powders were tested as humidity- and temperature-sensing materials in the form of thick films on interdigitated electrodes on alumina substrate in a climatic chamber. The physicochemical investigation of synthesized materials revealed a cubic spinel Fd3¯m phase, nanosized but agglomerated particles with a partially to completely inverse spinel structure with increasing Ni content. Ni0.1Mg0.9Fe2O4 showed the highest material constant (B30,90) value of 3747 K and temperature sensitivity (α) of −4.08%/K compared to pure magnesium ferrite (B30,90 value of 3426 K and α of −3.73%/K) and the highest average sensitivity towards humidity of 922 kΩ/%RH in the relative humidity (RH) range of 40–90% at the working temperature of 25 °C.

Giant dielectric response, nonlinear characteristics, and humidity sensing properties of a novel perovskite: Na1/3Sr1/3Tb1/3Cu3Ti4O12.

In this study, we unveil a novel perovskite compound, Na1/3Sr1/3Tb1/3Cu3Ti4O12, synthesized through a solid-state reaction method, exhibiting remarkable giant dielectric response, nonlinear characteristics, and humidity sensing capabilities. This research highlights the emergence of a Cu-rich phase, the properties of which undergo significant alterations depending on the sintering conditions. The optimization of sintering parameters, encompassing a temperature range of 1040-1450 °C for 1-8 h, resulted in substantial dielectric permittivity (ε') values (∼2800-6000). The temperature dependence of ε' demonstrated relationship to the particular sintering conditions utilized. The acquired loss tangent values were situated within encouraging values, ranging from 0.06 to 0.16 at 1 kHz. Furthermore, the material revealed distinct nonlinear electrical characteristics at 25 °C, with the nonlinear coefficient values of 5-127, depending on ceramic microstructures. Additionally, we delved deeply into the humidity-sensing properties of the Na1/3Sr1/3Tb1/3Cu3Ti4O12 material, showing a considerable variation in ε' in response to fluctuations in relative humidity, thereby indicating its prospective application in humidity sensing technologies. The hysteresis error and response/recovery times were calculated, highlighting the versatility of this compound and its promising potential across multiple applications. The Na1/3Sr1/3Tb1/3Cu3Ti4O12 not only shows remarkable giant dielectric responses but also portrays significant promise for nonlinear and humidity sensing applications, marking it as a significant participant in the advancement of perovskite-based functional materials.

Towards Improved Humidity Sensing Nanomaterials via Combined Electron and NH3 Treatment of Carbon-Rich FEBID Deposits.

Focused Electron Beam Induced Deposition (FEBID) is a unique tool to produce nanoscale materials. The resulting deposits can be used, for instance, as humidity or strain sensors. The humidity sensing concept relies on the fact that FEBID using organometallic precursors often yields deposits which consist of metal nanoparticles embedded in a carbonaceous matrix. The electrical conductivity of such materials is altered in the presence of polar molecules such as water. Herein, we provide evidence that the interaction with water can be enhanced by incorporating nitrogen in the deposit through post-deposition electron irradiation in presence of ammonia (NH3). This opens the perspective to improve and tune the properties of humidity sensors fabricated by FEBID. As a proof-of-concept experiment, we have prepared carbonaceous deposits by electron irradiation of adsorbed layers of three different precursors, namely, the aliphatic hydrocarbon n-pentane, a simple alkene (2-methyl-2-butene), and the potential Ru FEBID precursor bis(ethylcyclopentadienyl)ruthenium(II). In a subsequent processing step, we incorporated C-N bonds in the deposit by electron irradiation of adsorbed NH3. To test the resulting material with respect to its potential humidity sensing capabilities, we condensed sub-monolayer quantities of water (H2O) on the deposit and evaluated their thermal desorption behavior. The results confirm that the desorption temperature of H2O decisively depends on the degree of N incorporation into the carbonaceous residue which, in turn, depends on the chemical nature of the precursor used for deposition of the carbonaceous layer. We thus anticipate that the sensitivity of a FEBID-based humidity sensor can be tuned by a precisely timed post-deposition electron and NH3 processing step.

Synthesis and Study of Highly Porous Nature Gadolinium Doped CoCr2O4: Focus on the Structural, Microstructural, Electric, and Humidity Sensing Properties

Synthesis and Study of Highly Porous Nature Gadolinium Doped CoCr2O4: Focus on the Structural, Microstructural, Electric, and Humidity Sensing Properties

Optimization of Aluminum Dopant Amalgamation Immersion Time on Structural, Electrical, and Humidity-Sensing Attributes of Pristine ZnO for Flexible Humidity Sensor Application

This study synthesized pristine and aluminum (Al)-doped zinc oxide (Al:ZnO) nanostructures through a simplistic low-temperature ultrasonicated solution immersion method. Al:ZnO nanostructures were synthesized as a sensing material using different immersion times varying from two to five hours. The Al:ZnO nanostructured-based flexible humidity sensor was fabricated by employing cellulose filter paper as a substrate and transparent paper glue as a binder through a simplistic brush printing technique. XRD, FESEM, HRTEM, EDS, XPS, a two-probe I–V measurement system, and a humidity measurement system were employed to investigate the structural, morphological, chemical, electrical, and humidity-sensing properties of the pristine ZnO and Al:ZnO nanostructures. The structural and morphological analysis confirmed that Al cations successfully occupied the Zn lattice or integrated into interstitial sites of the ZnO lattice matrix. Humidity-sensing performance analysis indicated that the resistance of the Al:ZnO nanostructure samples decreased almost linearly as the humidity level increased, leading to better sensitivity and sensing response. The Al:ZnO-4 h nanostructured-based flexible humidity sensor had a maximum sensing response and demonstrated the highest sensitivity towards humidity changes, which was noticeably superior to the other tested samples. Finally, this study explained the Al:ZnO nanostructures-based flexible humidity sensor sensing mechanism in terms of chemical adsorption, physical adsorption, and capillary condensation mechanisms.