Research available as a pre-proof in the journal Materials Chemistry and Physics demonstrates the feasibility of using molybdenum disulfide/graphene oxide quantum dot (MoS2/GOQD) nanocomposite films as sensing materials in humidity sensors.
Study: High-sensitive humidity sensor based on MoS2/graphene oxide quantum dot nanocomposite. Image Credit: PRO Stock Professional/Shutterstock.com
Importance of Effective Humidity Sensors
Humidity control and measurement have attracted considerable attention in several applications, such as electronic device fabrication. The effectiveness of humidity sensors is heavily dependent on the sensitivity of the sensing film. Two-dimensional (2D) nanomaterials and metal oxide semiconductors are used extensively as sensing films for humidity detection.
Among them, 2D transition metal sulfides have gained more prominence compared to others for sensor fabrication owing to their high surface-to-volume ratio and exceptional carrier mobility.
MoS2 is more suitable for humidity detection compared to the other transition metal sulfides. However, slow recovery/response time and low sensitivity are the major drawbacks of the MoS2 sensing films, which must be addressed to improve the humidity detection performance of the humidity sensors based on MoS2 films.
Recently, several studies have demonstrated that the humidity sensing performance of MoS2 humidity sensors can be improved by modifying MoS2 films with metal oxide and metal nanocomposites. However, modified MoS2-based humidity sensors typically require higher operation temperature, which increases the overall fabrication cost of the sensor.
The incorporation of graphene and its derivatives, such as GO in MoS2 films is increasingly becoming an effective way to improve the sensing performance of MoS2 humidity sensors. For instance, MoS2/GO nanocomposites with high proton conductivity were used to successfully fabricate highly sensitive resistive humidity sensors.
GOQD, comprised of some quantum scale GO material fragments, is more suitable compared to GO for humidity sensing applications owing to its larger specific surface area and excellent hydrophilicity. Additionally, sensing films fabricated by the GOQD stack possess many interwafer voids, which promote the water molecule permeation process inside the moisture-sensitive film and accelerate the moisture-sensitive response of the sensor.
Thus, a MoS2/GOQD-based humidity sensor fabricated by combining GOQD and MoS2 can potentially deliver a better performance in terms of sensitivity and response/recovery time.
Fabrication of the Proposed MoS2/GOQD Composite Film-based Humidity Sensor
In this study, researchers synthesized a MoS2/GOQD composite film-based capacitive humidity sensor for the first time and investigated its humidity sensing performance.
Initially, the interdigitated electrodes (IDEs) were prepared through the semiconductor fabrication process. Initially, 400 nanometers thick silicon dioxide layer was prepared on an N-type silicon wafer through thermal oxidation, and then 300 nanometers/100 nanometers thick gold/titanium layers were deposited on the top of the silicon dioxide layer through magnetron sputtering. The gold electrodes with 20 micrometers wide gap were fabricated through photoetching and wet etching. The IDEs were cleaned using ethanol and deionized water for 30 minutes at an ultrasonic washing unit and dried before the fabrication of humidity sensing films on them.
The MoS2/GOQD nanocomposite film was synthesized by a simple solution composite method. Aqueous suspensions of GOQD and MoS2 were mixed at fixed ratios and then sonicated for one hour to obtain the MoS2/GOQD composite suspension. Subsequently, a few drops of MoS2/GOQD suspension were drop coated on the gold IDEs through a micro-pipette and dried for six hours at 45 degree Celsius to obtain a MoS2/GOQD composite film-based humidity sensor.
Characterization and Evaluation of the Synthesized Samples
Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and high-resolution TEM (HR-TEM) were used to characterize the MoS2/GOQD nanocomposite film and pure MoS2 film. Researchers also evaluated the long-term stability, hysteresis error, recovery/response time, and repeatability of the synthesized nanocomposite sensor. Pure GOQD and MoS2-based sensors were also fabricated to utilize them as the control in all experiments.
Research Findings
A humidity sensor based on MoS2/GOQD nanocomposite film was fabricated successfully. The elemental forms of sulfur, molybdenum, oxygen, and carbon were observed in the synthesized MoS2/GOQD composite film, indicating the successful fabrication of the composite.
The GOQD and MoS2 were in compact contact in the MoS2/GOQD composite film, which facilitated the transfer of carriers and significantly promoted the desorption and adsorption of water molecules.
The humidity sensor performance was investigated at 100 Hertz frequency as the sensor capacitance displayed a greater variation towards RH exposure at this frequency. The MoS2/GOQD film-based sensor displayed a significantly higher response to different RH levels compared to pure GOQD and MoS2 film-based sensors. The sensitivity of the MoS2/GOQD composite film sensor was also substantially higher at 369 picofarads per relative humidity compared to pure GOQD and MoS2 sensors.
Moreover, the sensor displayed good long-term stability, rapid response/recovery speed, low hysteresis, and good repeatability. Furthermore, the nanocomposite sensor fabricated in this study displayed better sensing properties compared to previously reported humidity sensors, which indicated the potential of these sensors in practical applications.
To summarize, the findings of this study demonstrated that MoS2/GOQD composite film could be an effective sensing material for the fabrication of humidity sensors.
Reference
Li, W., Li, X., Gou, C. et al. (2022) High-sensitive humidity sensor based on MoS2/graphene oxide quantum dot nanocomposite. Materials Chemistry and Physics https://www.sciencedirect.com/science/article/pii/S0254058422004527?via%3Dihub
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