Surface engineering enabled highly sensitive humidity sensors in two-dimensional hexagonal boron nitride nanosheets

Highly sensitivity and wide-range flexible humidity sensor based on LiCl/cellulose nanofiber membrane by one-step electrospinning

Development of highly sensitive relative humidity sensor based on nanoporous PCPDTBT thin film

Two-dimensional (2D) materials and their composites have gained significant importance as the functional layer of various environmental sensors and nanoelectronics owing to their unique properties. This work reports for the first time a highly sensitive, fast, and stable humidity sensor based on the bi-layered active sensing area composed of graphene flower (GF) and poly (vinyl alcohol) PVA thin films for multifunctional applications. The GF/PVA humidity sensor exhibited stable impedance response over 15 days, for a relative humidity (RH) range of (40–90% RH) under ambient operating conditions. The proposed bi-layered humidity sensor also exhibited an ultra-high capacitive sensitivity response of the 29 nF/%RH at 10 kHz and fast transient response of 2 s and 3.5 s, respectively. Furthermore, the reported sensor also showed a good response towards multi-functional applications such as non-contact skin humidity and mouth breathing detection.

Highly sensitive CMUT-based humidity sensors built with nitride-to-oxide wafer bonding technology

Superhydrophilic ZnO nanoneedle array: Controllable in situ growth on QCM transducer and enhanced humidity sensing properties and mechanism

One-pot synthesis of AC/ZnO nanocomposites for highly sensitive, repeatable and fast response humidity sensor

Highly sensitive humidity sensors based on Li-C3N4 composites on porous graphene flexible electrodes

We developed a resistive humidity sensor based on a heterojunction of silver sulfide (Ag2S) quantum dots (QDs) and TiO2 because of its specificity to water vapor adsorption and its insensitivity to environmental gases. The QDs were grown on a mesoporous TiO2 layer using the successive ionic layer adsorption and reaction (SILAR) method. The boundary condition between TiO2 and Ag2S provides a tunable energy gap by adjusting the number of SILAR cycles. Besides, the large surface-to-volume ratio of QDs provides a strong water vapor adsorption ability and electron transfer. Nano-silver precipitated during the SILAR process provides free electrons and lowers the Fermi level to between n-type TiO2 and p-type Ag2S. The resistance response increased significantly to 4600 and the reaction equilibrium time decreased greatly to 7 seconds due to the presence of nano-silver. Finally, the Ag2S QDs possess a best sensing range of 13–90%. To sum up, Ag2S QDs are high sensitivity and selectivity humidity sensors.

Self-powered humidity sensors based on friction electric and piezoelectric principles have been proposed in recent years. However, the complicated structure and preparation processes usually involved in these power generation humidity sensors limit their wider application. Herein, we developed a self-powered flexible humidity sensor with a simple structure and facile preparation process based on the primary battery principle. The self-powered humidity sensor consists of copper conductive tape as the positive electrode, nickel conductive tape as the negative electrode, and a carboxymethyl cellulose film dissolved with lithium chloride and sodium chloride as the sensing layer. The sensor exhibits good sensing linearity (R2 = 0.99791) in a wide relative humidity range (11-95%) with a satisfying response voltage of 41 mV (RH 95%) and excellent flexibility. Furthermore, the conduction mechanism of the sensing film was investigated by the complex impedance and phase angle-frequency spectral measurement and analysis. Multifunctional applications of human respiration monitoring to determine the respiratory rate and type, noncontact sensing, diaper and soil moisture detection, and power generation were demonstrated. The low-cost and facile preparation method in this work could provide a useful strategy for developing a self-powered, flexible, and multifunctional humidity sensor.

The utilization of printing techniques for the development of high-performance humidity sensors holds immense significance for various applications in the fields of the Internet of Things, agriculture, human healthcare, and storage environments. However, the long response time and low sensitivity of current printed humidity sensors limit their practical applications. Herein, a series of high-sensing-performance flexible resistive-type humidity sensors is fabricated by the screen-printing method, and hexagonal tungsten oxide (h-WO3 ) is employed as the humidity-sensing material due to its low cost, strong chemical adsorption ability, and excellent humidity-sensing ability. The as-prepared printed sensors exhibit high sensitivity, good repeatability, outstanding flexibility, low hysteresis, and fast response (1.5s) in a wide relative humidity (RH) range (11-95%RH). Furthermore, the sensitivity of humidity sensors can be easily adjusted by altering the manufacturing parameters of the sensing layer and interdigital electrode to meet the diverse requirements of specific applications. The printed flexible humidity sensors possess immense potential in various applications, including wearable devices, non-contact measurements, and packaging opening state monitoring.

All range highly linear and sensitive humidity sensor based on 2D material TiSi2 for real-time monitoring

Natural cellulose nanocrystals (CNCs) possess outstanding properties, such as biodegradability, hydrophilicity, large specific surface areas, and are low in cost. A highly stable and sensitive humidity sensor based on a quartz crystal microbalance (QCM) was prepared using CNCs as the sensing film. To investigate the impact of CNCs loading on the sensor performance, a series of humidity-sensitive performance tests, including linearity, response and recovery times, repeatability, hysteresis characteristics, selectivity, and long-term stability tests, was conducted. The experimental results indicated that the humidity sensor with CNCs loading of 2 μg (QCM-2) exhibited excellent logarithmic linearity, high sensitivity (32.35 Hz/% relative humidity (RH)), excellent reversible behavior, and long-term stability at a RH of 11–84%. The contact angle of QCM-2 was 12.5°, revealing its suitable hydrophilicity. Thus, we demonstrate that CNCs are great prospects for potential application in humidity sensors with high performance and low cost.

Carbon nanotube (CNT) humidity sensors are potential for the application in e-skins. However, the sensitivity and response/recovery speed are not satisfied. Herein, we design a composite of CNTs and hydrophilic maltodextrin. The CNT/maltodextrin humidity sensor demonstrates a sensitivity of 69% and a short response and recovery time of 56 s/3 s. The CNT/maltodextrin humidity sensor showed strong robustness during peeling test and cyclic bending process. Finally, the flexible humidity sensor was demonstrated to detect finger movement.

Sensitive humidity sensor using Quartz Crystal Microbalance (QCM) is presented. A humidity change is detected by an oscillation frequency change depending on the mass of the absorbed H/sub 2/O molecule. Obtained coefficient of humidity-to-frequency shift is about 0.8%/Hz. We developed a crystal oscillator type humidity sensor using a thin Cr/sub 2/O/sub 3/ film. As a basic characteristic, sensitivity was obtained in relative humidity, and it was concluded that the presented sensor can serve as a practical humidity sensor.

A facile nanocasting synthesis of mesoporous Ag-doped SnO2 nanostructures with enhanced humidity sensing performance