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Silicon microbridge structures have attracted the attention of researchers in humidity sensing area owing to their small size and ease of integration with the integrated circuit (IC). However, the preparation of silicon microbridge humidity sensors with high response are still a challenge. This paper proposed a silicon microbridge humidity sensor based on an edge-constrained sandwich sensing structure, in which a graphene oxide core layer with small area was covered on the surface of the silicon bridge by a larger area of Nafion film. Due to the constraint of the peripheral edge, the swelling deformations of this sandwich structure in the vertical direction were effectively magnified during humidity detection. Hence, the proposed humidity sensor exhibited a higher humidity sensitivity (109.97 µV/%RH) compared to the silicon microbridge sensors with general sandwich sensing structure or monolayer sensing film. Meanwhile, the novel sensor also demonstrated a small humidity hysteresis and good linearity in humidity range of 11.3–97.3%. [Display omitted] • A new edge-constrained sandwich structure on the silicon microbridge was proposed. • The sensitivity of the sensor is improved by inserting the core layer. • GO core layer was constrained by Nafion film to make the expansion of sensitive film mainly occur in vertical direction. [ABSTRACT FROM AUTHOR]

In this article, a Nafion/MoS2 hybrid thin film was deposited on quartz crystal microbalance (QCM) for detecting humidity, in which moderate MoS2 nanosheets with small size were dispersed into a Nafion membrane to improve its humidity sensitivity. The experimental results indicate that a hybrid film QCM humidity sensor with the best volume ratio of Nafion/MoS2 exhibits a considerably higher shift frequency (−11 879 Hz) than a pure Nafion film QCM humidity sensor, even more than most of the reported QCM humidity sensors. It could be that the molybdenum disulfide nanosheets dispersed in the Nafion film would form some nano-assembly near the hydrophilic group of Nafion, which greatly enhances the ability of the composite film to establish water channels, thus promoting hydrophilicity of the composite film. The highly sensitive humidity sensor could clearly distinguish tiny humidity difference near the back and palm of a hand. Therefore, it has potential application in human–robot collaboration. [ABSTRACT FROM AUTHOR]

In this work, we, for the first time, propose a quartz crystal microbalance (QCM) humidity sensor that is prepared by a one-step method based on pencil drawing. The scanning electron microscopy (SEM), atomic force microscope (AFM), and Raman spectra confirm some graphite flakes, and a layer of sensing film is formed, demonstrating the successful preparation of the QCM sensor. A series of experiments were performed by means of oscillating circuit and impedance analysis at room temperature. The impedance analysis method demonstrates that the QCM humidity sensor exhibits an excellent stability with a quality factor (${Q}$) that only drops from 47 923 to 43 849 in the whole humidity range, which is better than ever reported QCM humidity sensors. In addition, the sensor shows a good long-term stability with a maximal frequency deviation of less than 5% in a long-term test. Furthermore, the sensor displays a rapid response time ~8s and a recovery time less than 5 s. The underlying reasons for these excellent properties of the proposed sensor are analyzed and discussed. This research demonstrates that the pencil-drawn method is a simple, green, and cost-effective way for constructing a QCM humidity sensor with high performance. [ABSTRACT FROM AUTHOR]

This paper reports a humidity sensor based on TiO2 nanowires. The humidity sensor prepared with a simple and easy technology exhibits obviously high sensitivity and fast response/recovery speed. A series of analyses including scanning electron microscopy, specific surface area analysis, water contact angle, and complex impedance spectrum were used to analyze superior humidity sensing performance of TiO2 nanowires. The results demonstrate that higher sensitivity of the sensor probably stems from larger specific area of nanowires, and its faster response might be attributed to the continuity of nanowires and their intricate connection network, which offer highways for water molecules diffusion and charges transport. Besides, the current-voltage characteristic of the sensor presents a negative differential resistance effect (NDR) with a very low bias voltage under certain humidity level at room temperature, which facilitates compatibility with some low voltage integrated circuit. The tip enhancement of electric field caused by TiO2 nanowire end is suggested to explain the mechanism of NDR at very low bias voltage. The high-performance humidity sensor provides a way to study the gender difference in moisture loss through skin evaporation with a non-contact manner, which can avoid the interference of contact state between the sensor and the skin. [ABSTRACT FROM AUTHOR]

In this paper, a novel and flexible humidity sensor based on nanodiamond (ND) sensing film was fabricated on a polyimide substrate. A series of flexibility-related humidity testing experiments is also performed. The experimental results demonstrated that the prepared sensor exhibited a subsecond response/recovery time and showed good repeatability under different bending states. It was found that different bending radii had an influence on the humidity sensor’s response/recovery time. The response/recovery time is reduced with the increase in bending radius, which is suspected to be related to the interspace change of nanodiamond sensing membrane under different bending radii. Furthermore, due to good flexibility and rapid response, this humidity sensor can be directly attached on finger or wrist for human breath detection, which can provide a more convenient monitoring means for human health activities such as sports and fitness. [ABSTRACT FROM AUTHOR]

Highlights • The rGO/ND nanocomposite film was fabricated by hydrothermal reduction method. • The synthesized sensor exhibited an ultrahigh sensitivity, low hysteresis, fast response/recovery time, excellent repeatability and good long-term stability. • Compared with the pure rGO and pure ND film sensors, the sensitivity and hysteresis properties of rGO/ND film sensor were prominently improved. • The humidity sensing mechanism of the rGO/ND film sensor was analyzed and discussed. Abstract This paper demonstrated the preparation of reduced graphene oxide/nanodiamond (rGO/ND) nanocomposite by a facile hydrothermal reduction method and its application as an ultrahigh performance humidity sensor. The synthesized rGO/ND suspension was deposited on Au interdigitated microelectrodes for sensor fabrication. The nanostructures, morphologies, and composition properties of the prepared rGO/ND nanocomposite were characterized by scanning electron microscopy (SEM), atomic force microscope (AFM), Raman spectrum, and Fourier Transform infrared spectroscopy (FTIR). The experimental results demonstrated that the rGO/ND based humidity sensor exhibited an ultrahigh sensitivity (13,086 pF/%RH), which surpasses the existed conventional humidity sensors and is more than 30 times higher than that of the pure rGO and pure ND based humidity sensors. The hysteresis characteristic of rGO/ND based humidity sensor is also dramatically improved when compared with pure rGO and pure ND based humidity sensors. Meanwhile, the sensor shows a fast response/recovery time, excellent repeatability and good long-term stability. The humidity sensing mechanism was analyzed via complex impedance spectrum (CIS). Lastly, a sensing mechanism model was established to detailedly explain the improved hysteresis properties. This research revealed that rGO/ND composite film is a promising candidate material for fabricating a high-performance humidity sensor. [ABSTRACT FROM AUTHOR]

This paper described a quartz crystal microbalance (QCM) humidity sensor based on fullerene (C 60 )/graphene oxide (GO) nanocomposites. Humidity sensing properties such as the quality factor, frequency response property, response/recovery time, and humidity hysteresis were measured and quantitatively compared with a GO based QCM humidity sensor. The experiments provided evidences that the GO film was viscous even at low relative humidity, which had a large impact on the measurable result of a GO coated QCM humidity sensor. As a result, the proposed sensor showed better performances at quality factor and dynamic response/recovery behavior than the GO based QCM humidity sensor. Based on the admittance analysis and frequency response analysis of the sensors, it was speculated that the introduction of C 60 molecules into GO reduced the aggregation of the GO sheets and formed some hydrophobic isolation layers between the GO sheets, which could inhibit the water molecules permeation and maintain the mechanical stiffness of the C 60 /GO film. [ABSTRACT FROM AUTHOR]

In this paper, a high-stability QCM humidity sensor coated with nanodiamond (ND)/multiwalled carbon nanotubes (MWCNTs) nanocomposite film was purposed based on good dispersion of MWCNTs in ND solution. The experimental results demonstrated that the sensor not only had a relative high frequency shift with humidity change, but also maintained a high quality (Q) factor compared with that of pure ND film coated sensor. In high humidity, the Q factor of ND/MWCNTs coated sensor is about four times larger than that of pure ND coated sensor, and can retain negligible change as long as several months. The possible reason was analyzed and discussed. Moreover, the sensor exhibited low hysteresis, fast response/recovery time, and good repeatability. [ABSTRACT FROM AUTHOR]

In this paper, a quartz crystal resonator (QCR) with high quality factor and a humidity-sensitive capacitor were combined to fabricate a high-sensitivity and good-stability humidity sensor. Through the humidity sensor, the perturbation of the humidity-sensitive capacitor which was caused by different humidity levels could be transformed into the deviation of the resonant frequency of the QCR. The expression between the resonant frequency of the sensor and relative humidity was derived. The test curve of the resonant frequency responses of the sensor versus relative humidity changes was measured and showed a similar trend to the theoretical result. In addition, the influences of different sensitive materials on the sensor performance have also been researched. And the humidity sensors exhibited a high sensitivity (116.2 Hz/%RH) and a good stability with 0.038-Hz standard deviation of the output frequency in the range of 11.13%RH–97.3%RH. The properties of humidity hysteresis as well as response and recovery have also been investigated. [ABSTRACT FROM PUBLISHER]

A novel quartz crystal microbalance (QCM) humidity sensor using nanodiamond (ND) as sensing films has been presented in this paper. For evaluating the performance of ND-coated QCM humidity sensor, a series of humidity-sensing experiments were carried out by using both oscillating circuit and impedance analysis methods. Based on the large specific surface area and high mechanical modulus, the ND-coated sensor exhibited large humidity-sensing response and good stability. In addition, the resultant sensors displayed low hysteresis, good repeatability, and short response time. The influence of the depositing amount of ND on the sensor's performance has also been discussed. The result indicated that the stability of QCM humidity sensor showed an extreme decrease when the deposition amount of ND on QCM is large enough. The possible reason for this anomalous phenomenon is analyzed and discussed. This study suggests that ND could be a promising candidate material for developing high-performance QCM humidity sensor. [ABSTRACT FROM PUBLISHER]

Abstract: In this work, we present an approach to use graphene oxide-silicon bi-layer flexible structure as stress-based humidity sensors. By the spin-coating method, graphene oxide thin films were deposited onto silicon microbridge as a humidity sensing layer. Upon expose to humid environment, graphene oxide thin films swells and leads to the bending of silicon membrane. Then, the full piezoresistive Wheatstone-bridge embedded in silicon microbridge was used to transform the deformation into a measurable output voltage. The humidity sensing properties of the bi-layer flexible structure, such as sensitivity, repeatability, humidity hysteresis, response and recovery, were investigated in the wide relative humidity range of 10–98%. The test results show that graphene oxide-silicon bi-layer flexible structure exhibits high humidity sensitivity, good repeatability, small humidity hysteresis and clear and fast response–recovery. Moreover, the dependence of the thin films thickness of graphene oxide on the response properties was also examined. At last, the humidity sensing mechanism of the proposed bi-layer structure was discussed in detail. [Copyright &y& Elsevier]

Abstract: In this paper, we demonstrated that chemically derived graphene oxide (GO) thin film as a humidity sensitive coating deposited on quartz crystal microbalances (QCMs) for humidity detection. By exposing GO thin film coated QCMs to various relative humidity (RH) environments at room temperature, the humidity sensing characteristics of the QCMs such as sensitivity and linearity, response and recovery, humidity hysteresis were investigated. The experiment results show that GO thin film coated QCMs exhibit an excellent humidity sensing performance. Moreover, the possible humidity sensing mechanism of GO thin film coated QCMs was also investigated by monitoring the crystal''s motional resistance change. It is suggested that the frequency response of the QCMs is dependent on water molecules adsorbed/desorbed masses on GO thin film in the low RH range, and on both water molecules adsorbed/desorbed masses on GO thin film and variations in interlayer expansion stress of GO thin film derived from swelling effect in the high RH range. [Copyright &y& Elsevier]

• The humidity sensitivity properties of metal nanoparticles loaded fullerene were significantly improved and the sensitivity was several orders of magnitude higher than that of pure fullerenes. • The mechanism of humidity-sensitive enhancement of metal nanoparticles is explained, and different MNPs can be used to improve the performance of C 60 humidity sensor according to different actual needs In this paper, the response characteristics of capacitive humidity sensors based on fullerene (C 60) loaded with metal nanoparticles (MNPs) were investigated. Experimental results demonstrated that the humidity sensitive properties of all the sensors are significantly improved after loading MNPs. This may be related to the hydrophilicity change of fullerene composite membranes induced by MNPs, which is confirmed by the test results of water contact angle and complex impedance spectrum. Moreover, the enhancement effect of humidity sensitivity is more obvious with the increase of MNPs content, but when the MNPs content exceeds a certain value, the enhancement effect of humidity sensitivity will decrease, which may be attributed to the agglomeration of MNPs. Meanwhile, for samples loaded with different MNPs (AuNPs, AgNPs, CuNPs), the improvement effects are quite different. Among the three samples studied, the C 60 /Ag sample exhibit the best response characteristics with a sensitivity of 5,812.8 pF/%RH, which is more than 200 times higher than that of the unloaded sample. The C 60 /Cu sample has no obvious response in low relative humidity (RH), but its sensitivity at high humidity is even higher than that of the C 60 /Ag sample, which shows the characteristics similar to a humidity sensitive switch. The C 60 /Au sample is one order of magnitude more sensitive than pure C 60 sample and exhibits the best stability among the three samples. The above results suggest that different MNPs can be selected to develop humidity sensors with different characteristics according to the application situations. [ABSTRACT FROM AUTHOR]

This paper suggests a novel humidity sensor based on graphene oxide sensing film supported by TiO 2 nanorods which are grown between Au electrodes via hydrothermal method. The supported structure allows water molecules to pass through TiO 2 nanorods and spread to the lower surface of the sensing film, which increases the utilization of the lower surface of graphene oxide (GO) film for humidity detection. Hence, water molecules can diffuse rapidly on both sides of GO film, which will assist in reducing the response time of the sensor. The SEM morphology characterization shows that the supported structure is feasible, and the response curve proves that the response speed of the humidity sensor is improved. The complex impedance spectra (CIS) results are utilized to analyse the response characteristics of the sensor at low humidity level. Unlabelled Image • In this work, we fabricated a novel humidity sensor based on GO film supported by TiO 2 nanorods between electrodes. • The supported TiO 2 nanorods increased the utilization of the lower surface of GO film for humidity detection. • The supported TiO 2 nanorods structure reduced the response time and improved the performance of the fabricated sensor. [ABSTRACT FROM AUTHOR]

• A flower-like MoS 2 /ND nanocomposite film was fabricated via a facile hydrothermal method. • Compared with pure MoS 2 film sensor, the synthesized sensor exhibited a high sensitivity, low hysteresis, ultrafast response/recovery time, excellent repeatability. • The MoS 2 /ND film sensor was used for non-contact sensing control investigation via monitoring water evaporation on the skin. A flower-like molybdenum disulfide (MoS 2)/nanodiamond (ND) nanocomposite was successfully prepared via an easy hydrothermal synthesis method and fabricated into humidity sensors to investigate its humidity sensing characteristics. The morphological, nanostructural, and compositional characteristics of the prepared MoS 2 /ND nanocomposite were characterized by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), confirming that the MoS 2 /ND nanocomposite was successfully prepared. The experimental results demonstrate the MoS 2 /ND composite film sensor has high sensitivity as compared to pure MoS 2 based humidity sensor, which could be ascribed to the formed 3D hierarchical structure of MoS 2 /ND nanocomposite. Meanwhile, the sensor exhibits good repeatability, rapid response, and low hysteresis. Moreover, the detailed humidity sensing mechanism was analyzed and discussed using complex impedance spectra and bode plots. Based on good sensitivity and rapid response, the sensor was employed to control the different degree of brightness of LED with noncontact sensing method via monitoring water evaporation on the skin, which could provide a potential and more advantageous method to control the brightness of display screen, such as cellphone and computer screens. [ABSTRACT FROM AUTHOR]

• In this paper, proper amount of Ag nanoparticles was added into TiO 2 nanoparticles as sensing film, which could increase hydrophilicity and absorption ability of the sensing film. • The enhancement effect of Ag on humidity performance of the sensors was better than lots of most reports. • This high-sensitivity humidity sensor proposed in this work could enable the nondestructive detection of wet paper and has potential applications in the preservation of ancient manuscripts, historical calligraphies and paintings. In this paper, a capacitive-type humidity sensor based on TiO 2 /Ag nanoparticle composites film was fabricated on interdigital electrodes. Experimental results showed that a small amount of Ag nanoparticles mixed with TiO 2 nanoparticles could greatly improve the sensor sensitivity. Compared with pure TiO 2 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. [ABSTRACT FROM AUTHOR]