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. 2025 Dec 29;11(1):1092–1100. doi: 10.1021/acsomega.5c08316

Enhanced Humidity Sensing Characteristics of MoS2 through Benzyl Viologen Dichloride Doping

Jamiela Sakuddin †,‡, Seyoung Park †, Hyung-il Jang †, Jongsun Maeng §, Ho-Joong Kim ‡, Ja-Yeon Kim ∥, Min-Ki Kwon †,*
PMCID: PMC12809765  PMID: 41552589

Abstract

Accurate humidity monitoring is essential for industries such as healthcare, agriculture, and manufacturing, where precise environmental control directly impacts quality and safety. Molybdenum disulfide (MoS2) is a promising candidate for humidity sensors due to its high surface area and favorable electronic properties, but its response characteristics require further enhancement for real-time applications. In this study, we demonstrate that benzyl viologen dichloride (BVD) doping significantly improves the performance of MoS2-based humidity sensors. BVD doping increases carrier concentration, enabling superoxide-mediated water dissociation and hydroxyl adsorption, which collectively enhance conductivity, proton transport, and surface hydrophilicity. The BVD-doped MoS2 humidity sensor exhibited a 16-fold enhancement in relative response compared to undoped MoS2, with response times reduced from 2.18 to 0.699 s and recovery times shortened from 15.2 to 6.56 s. Structural and electronic confirmation of successful n-type doping was obtained via Raman spectroscopy, UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrical measurements. Contact angle analysis further indicated enhanced hydrophilicity of BVD-doped MoS2, supporting improved water adsorption. These results highlight BVD doping as an effective strategy to advance MoS2-based humidity sensors toward next-generation applications in industrial, environmental, and medical fields.

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Introduction

Humidity, defined as the amount of water vapor in the atmosphere, fluctuates significantly due to variations in temperature, seasonal changes, and other environmental factors. With the growing impact of climate change and the increasing demand for enhanced living standards, monitoring humidity has become critical across a wide range of industries, including semiconductor fabrication, automotive manufacturing, food production, pharmaceuticals, and agriculture. This growing demand has led to the development of portable, highly sensitive, low-cost, and fast-response humidity sensors for applications requiring precise control, such as microfabrication and greenhouse management. In industrial processes, maintaining optimal humidity is essential for ensuring product quality. For instance, in textile manufacturing, low humidity can lead to electrostatic charges that cause materials to stick together, whereas dry condition is necessary for certain steps in silicon wafer fabrication and electronics assembly. Additionally, humidity sensors play an important role in automotive systems, such as rear window defogging, and are vital in agricultural operations to maintain suitable conditions for crop growth and food preservation. The medical field also relies on these sensors for applications such as respiratory equipment, sterilizers, and incubators.

Various transduction techniques have been employed for humidity sensing, including resistive, capacitive, , optical fiber-based, and field-effect transistor methods. Among these resistive sensors are simpler in structure and can be more easily integrated with complementary metal-oxide-semiconductor (CMOS) technology, positioning them as a promising alternative for future developments in humidity sensing. ,

Numerous materials have been explored for use in humidity sensors, including polymers, metal oxides, carbon nanotubes, and composites. These materials each possess distinct advantages and application conditions. The emergence of two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), and MXenes, has drawn significant attention in research, particularly for their application in next-generation electronics. Among the 2D materials, molybdenum disulfide (MoS2), has attracted substantial interest in recent years for its potential in next-generation electronic and sensing applications. MoS2 is especially promising for humidity sensors due to its unique properties: it has mechanical flexibility, tunable bandgap, high carrier mobility, and optical transparency. These characteristics make MoS2 suitable for diverse applications, from displays to optical and environmental sensors. In humidity sensing specifically, MoS2’s large surface area enhances sensitivity, as it provides ample sites for water molecule adsorption, a critical aspect of sensor performance.

Despite its promise, MoS2-based humidity sensors face several challenges that limit their performance, particularly in terms of response and recovery times, stability, and sensitivity under varying environmental conditions. Variations in MoS2’s physical properties based on the number of layers, combined with high contact resistance due to the Schottky barrier at MoS2-metal interfaces, reduce device efficiency and reliability. Additionally, existing MoS2 sensors often exhibit slower response and recovery times, impacting their utility in real-time applications. To address these issues, researchers have explored various doping methods and appropriate dopants to modulate MoS2’s electronic properties and improve its sensor performance.

While ion implantation is widely used for doping silicon-based electronics due to its precision, it is unsuitable for MoS2 due to the significant damage it causes to the atomic lattice of the thin layers. In contrast, chemical treatment offers a more practical solution by modulating carrier concentrations through the exchange of charges between the dopant and the MoS2 material. Dopants in this context can include surface adatoms, ions, molecules, particles, or substrates. BVD, in particular, offers several advantages such as high stability, reproducibility, and a significant reduction potential, making it a highly suitable dopant for low-dimensional materials. Compared to conventional metal adatom or polymer-assisted doping methods, BVD doping provides more stable charge transfer and better environmental stability, which highlights its potential as a robust doping strategy. Pradhan et al. reported a marked increase in current for BVD-doped MoS2 compared to undoped samples, suggesting that the material’s n-doping effect reduces the barrier height at the MoS2 heterojunction, thereby improving its performance. Additionally, Jo et al. found that BV-treated MoS2 nanosheets achieve higher carrier concentrations and enhanced electrical conductivity (2.28 × 10–1 S m–1) due to effective n-doping from BV molecules.

Generally, the humidity sensing mechanism of MoS2 primarily relies on the adsorption and desorption of water molecules on its surface. At low humidity levels, superoxide ions (O2 –), formed by the interaction of adsorbed oxygen with free electrons from the MoS2 surface, play a critical role in water dissociation. These superoxide ions facilitate the dissociation of water molecules, generating hydroxyl ions (OH–) on the MoS2 surface. This process enhances the conductivity of the MoS2 as a measurable response to humidity changes. However, the formation of superoxide ions is limited by the available carrier concentration in undoped MoS2, which restricts the overall sensitivity and response and recovery speed of the sensor.

In this study, we propose benzyl viologen dichloride (BVD) doping as a method to enhance MoS2’s humidity sensing performance by increasing the carrier concentration, which in turn boosts the formation of superoxide ions on the surface. The added free electrons from BVD doping facilitate more effective oxygen ionization, producing a higher concentration of reactive superoxide ions. This increased superoxide availability promotes more efficient water dissociation, resulting in a hydroxyl enriched surface that serves as the foundation for more water adsorption at higher humidity, enabling a quicker initiation of proton hopping, which is more effective and rapid in its conductivity mechanism. The successful integration of BVD into MoS2 was confirmed through electrical measurements and multiple characterization techniques, including Raman spectroscopy, UV–vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). Contact angle measurement was employed to investigate the effect of doping on the sample’s surface wettability. These findings demonstrate the potential of BVD-doped MoS2 for advanced humidity sensor applications, offering improved response times, recovery times, and overall sensitivity.

Experimental Section

Synthesis of MoS2, Doping with BVD, and Fabrication of Sensor

Figure illustrates the schematic of the hydrothermal synthesis process used to grow MoS2 and the procedure of device fabrication. In this procedure, 0.45 g of sodium molybdate dihydrate and 0.9 g of thioacetamide were dissolved in 50 mL of deionized water, with continuous stirring using a magnetic stirrer. The solution was heated at 220 °C for 9 h under high pressure in an autoclave with a limited amount of air gas. Following the synthesis, the MoS2 solution was centrifuged to separate the precipitate from the supernatant, which was discarded. The precipitate was then redispersed in deionized water. To ensure uniform distribution of the MoS2 particles, the solution was sonicated before drop-casting onto the substrate, followed by drying at 100 °C for 10 min.

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Fabrication process steps for the MoS2-based humidity sensor. The inset shows the SEM and substrate images.

Sensor Measurements

In this study, the current response of the sensors to changes in humidity was measured. In a controlled environment using tunable humidity chamber (a TH3-ME Bench Top Humidity Chamber), the MoS2 thin film was exposed to varying humidity levels. Humidity changes were monitored using a commercially available capacitive/resistive type humidity sensor (CENTER 314 Datalogger Dual Input Humidity Temperature Meter), while the sensor’s current response was measured using a Keithley 2636b Parameter Analyzer.

Additionally, the sensor’s response to human respiration was tested. During the measurement, the fabricated sensor was exposed to human breath, and the resulting change in humidity was measured using a commercially available capacitance-type moisture sensor (TR-72WB). The sensor’s corresponding current response was then measured using a Keithley 4200-SCS Parameter Analyzer.

Material Characterization

To examine the optical properties, micro-Raman scattering spectroscopy (RSS) was conducted on the dimple-etched sample surfaces using a 532 nm laser with a power density of 0.75 W/m2 (a 100 μm2 beam size and a 0.75 μW laser intensity) with a spectrometer (XPER RAM C, Nanobase). An optical microscope (BX-43, Olympus) and scanning electron microscope (SNE-4500M, SEC) were used to analyze the surface morphology of the samples. Contact angle measurements were performed using a Phoenix MT (M.A.T) Contact Angle Analyzer (SEO, Korea) to examine the surface wettability of the samples. A microsyringe was used to dispense deionized water onto the sample surface under ambient conditions. The droplet images were captured using the SEO Phoenix software, and contact angles were determined by analyzing the images with ImageJ software. UV–vis Spectrophotometer (Agilent Technologies) was used to examine the optical absorption properties of the undoped and BVD-doped MoS2 samples in the wavelength range of 200–1200 nm. X-ray photoelectron spectroscopy (K-Alpha+ system, Thermo Fisher Scientific) was used to analyze the chemical bonding states and phase evolution of the MoS2 samples. XPS spectra were collected using a 400 μm diameter X-ray beam operated at 6 mA and 12 kV under constant analyzer energy mode. Survey scans were recorded with a pass energy of 200 eV, while high-resolution spectra were obtained in snapshot acquisition mode using a pass energy of 150 eV with a collection time of 5 s per region. Surface charging was neutralized with a dual-function flood gun emitting low-energy electrons and 20 eV argon ions. Spectral acquisition and analysis were performed using Thermo Scientific Avantage software, with energy scale calibration carried out through an automated routine using internal reference standards of gold (Au), silver (Ag), and copper (Cu).

Results and Discussion

Benzyl viologen dichloride (BVD) is known for having one of the lowest reduction potentials among electron-donor organic molecules, making it an ideal choice for electron donation. It has been reported that the reduction potentials for the BV0/BV+ and BV+/BV2+ pairs are −0.79 V and −0.33 V, respectively and the conduction band edge of MoS2 is reported to be approximately 0 V versus the standard hydrogen electrode (SHE). Due to this energy level offset, BVD can readily transfer electrons to the acceptor material, MoS2, as seen in Figure S1b.

To investigate the charge transfer interaction between BVD and MoS2, electrical measurements, Raman spectroscopy, UV–vis spectroscopy, and X-ray photoelectron spectroscopy were employed. As shown in Figure a, the Raman spectra for undoped MoS2 display characteristic peaks corresponding to the E1 2g (in-plane vibration) and A1g (out-of-plane vibration) modes at around 379.5 cm–1 and 405.6 cm–1, respectively. With BVD doping introduced, both peaks shift toward lower wavenumbers, indicating that electron donation from the dopant molecules to MoS2 leads to the softening of the A1g vibrational mode. This is in agreement with a study conducted by Pradhan et al., where they reported a red-shift in the A1g peak position in their BVD-doped MoS2 samples. In addition to the shift in peak position, the intensity of both Raman peaks gradually decreases with increasing BVD content, suggesting a disruption in the MoS2 lattice or reduced crystallinity. As the BVD amount increased from 0.01 g to 0.03 g, a progressive red-shift was observed, as shown in Figure a. Interestingly, when the BVD amount reached 0.04 g (Figure S2a), the characteristic MoS2 peaks were no longer observed. Strong electron doping in MoS2 with 0.04 g BVD fundamentally suppresses Raman-active vibrational modes by maximizing electron–phonon coupling. As doping concentration increases, abundant free electrons drastically reduce the phonon lifetime, causing significant broadening and intensity reduction of Raman peaks such as E1 2g and A1g. If the doping level surpasses a certain threshold, these interactions quench the Raman signal entirely. This phenomenon is closely associated with phonon mode softening, asymmetric broadening, and Fermi level shifts, all stemming from the altered electronic environment and disrupted lattice dynamics in heavily doped MoS2. Additionally, the UV–vis absorption spectra of doped and undoped MoS2 samples are shown in Figure b. The band gap energy can be calculated using the Tauc equation.

(αhv)1/2=A(hv−Eg) 1

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(a) Raman spectra, (b) absorption spectra, the gate voltage characteristics of (c) undoped and (d) BVD-doped MoS2 samples, and the XPS spectra of the (e) Mo 3d region and (f) S 2p region for undoped and BVD-doped MoS2.

where n, α, A, hν, and E g correspond to the transition type (n = 2: indirect, n = 0.5: direct) absorption coefficient, constant, photon energy (eV), and optical band gap, respectively. When applying the n = 2 (indirect) fitting, the estimated bandgap values for both the undoped and BVD-doped samples appeared below 1.0 eV, which is significantly lower than the typical bandgap of MoS2. In contrast, using the n = 1/2 (direct) model yielded bandgap values of approximately 1.58 ± 0.2 eV, closely matching the known optical bandgap of MoS2. Therefore, it is concluded that the prepared samples exhibit direct bandgap characteristics. In addition, it can be seen that with BVD-doping, there is a noticeable decrease in absorbance, as well as a shift to longer wavelengths. Moreover, as BVD is introduced at increasing amounts, as seen in Figure S2b, the absorbance decreased and shifted to longer wavelengths. This red shift, characterized by the movement of absorption peaks toward longer wavelengths, signifies a decrease in the band gap energy of MoS2, which can be seen by the inset graph showing a clear decrease in band gap energy with increasing BVD amount.

Additionally, the n-doping effect of BVD in MoS2 thin films can also be seen in Figure c,d, which shows an increase in drain current with rising gate voltage and fixed drain voltage (V D = 5 V) in BVD-doped MoS2 sample, in contrast to undoped MoS2 which showed a decrease in current with rising gate voltage. In general, hydrothermally grown MoS2 tends to contain a large number of sulfur vacancies (V s) due to sulfur deficiency during the growth process. These sulfur vacancies act as p-type dopants, introducing acceptor-like states within the bandgap. As shown in Figure c, the drain current decreases with increasing positive gate voltage, indicating p-type conduction behavior arising from these sulfur vacancies in the MoS2 thin film. Upon BVD doping, the number of free electrons increases, and consequently, as shown in Figure d, the drain current increases with positive gate bias, signifying enhanced electron accumulation in the channel. This clearly confirms that effective n-type doping has been successfully achieved through BVD molecular doping.

Moreover, the high-resolution XPS spectra for the Mo­(3d) and S 2p peaks of undoped and BVD-doped MoS2 is shown in Figure e,f, showing an upshift in the binding energy of core levels in BVD-doped MoS2 samples, an indication of electron transfer from benzyl viologen dichloride to MoS2. The observed shift in XPS binding energies can be attributed to a change in the Fermi level, which is known to influence these spectral features. In the case of n-type doping, such as with BVD, an upward shift of the Fermi level can lead to an increase in binding energy. This interpretation aligns with previously reported trends in other studies involving BVD-doped MoS2, where similar upshifts were observed.

The sensing performance of both undoped and BVD-doped MoS2 sensors was evaluated under varying relative humidity (RH) conditions, ranging from 30 to 90% RH, at a constant bias voltage of 5 V and temperature (30 °C). The electrical conductivity of all sensors increased with rising RH. This increase in conductivity suggests that the adsorbed water molecules act as electron donors on both the doped and undoped MoS2 sensors. Notably, compared to its undoped counterpart, the current in the BVD-doped sensors increased significantly with higher humidity levels. To understand this enhanced current response, we calculated the sensitivities of the sensors. Figure presents the sensitivities of both undoped and BVD-doped sensor as a function of RH. In this study, all humidity-sensing measurements were carried out at a constant ambient temperature of 30 °C, while the relative humidity (RH) was systematically varied from 30% to 90% using a controlled humidity chamber.

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Sensing relative response as a function of RH of the undoped and BVD-doped MoS2 samples.

The sensing relative response (S R) is defined as

SR=IH−I1II×100% 2

where I H is the current in the presence of humidity, and I I is the baseline current at 30% RH. ,

As shown in Figure S4, BVD doping significantly enhances sensor relative response. Across RH values of 30–60%, although all samples exhibited increased relative response with rising humidity, BVD-doped samples showed higher relative response compared to undoped MoS2. Specifically, beyond 70% RH, the relative response of the BVD-doped samples significantly increased in comparison to undoped MoS2, where the highest relative response is reported to be around 500% at 90% RH. Notably, the sensor doped with BVD (0.03 g) exhibits the highest relative response around 8000% at 90% RH. Sixteen-times more than the relative response of undoped MoS2 at this relative humidity. This pronounced enhancement in relative response indicates that 0.03 g of BVD is the optimal doping concentration for humidity sensing applications. However, when the BVD doping amount reached 0.04 g, a significant decrease in relative response was observed. This may be linked to the Raman spectra of the samples (Figure S2a), where the characteristic MoS2 peaks disappear at 0.04 g BVD doping amount, suggesting that excessive dopant incorporation induces substantial structural disorder. Such disorder likely arises from dopant aggregation or uneven distribution, which could not only disrupt the crystallinity of MoS2 but also impairs the effective interaction between the sensor surface and water molecules, ultimately compromising humidity sensing performance.

To further explore the cause of this enhancement, we conducted contact angle measurements to assess changes in surface wettability as shown in Figure . For the contact angle measurement, each sample was measured three times at different positions, and the average value with standard deviation was calculated. The pristine surface exhibited a contact angle of 65.45 ± 1.2°, while the BVD doped surface showed 57.12 ± 1.0°. In general, materials with contact angles below 90° are considered hydrophilic, and smaller contact angles indicate stronger surface hydrophilicity. , Although this reduction suggests improved wettability upon BVD doping, the change is relatively modest and cannot fully explain the dramatic increase in relative response. Therefore, it is more plausible that synergistic effects, particularly electronic modulation induced by BVD n-doping, dominate in enhancing sensing performance.

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Contact angles of (a) undoped and (b) BVD-doped MoS2 samples.

The enhancement of the humidity sensing capability of BVD-doped sensors can be attributed to a combination of interrelated sensing mechanisms. The sensing mechanism begins when the sensor material is first exposed to air. The oxygen molecules from the surrounding environment are adsorbed onto the MoS2 surface, and become ionized by capturing free electrons from the conduction band, forming superoxide ions (O2 –) at temperatures below 100 °C. This is expressed in this reaction:

O2⁡(adsorbed)+e−→O2− 3

At room temperature, superoxides play a key role in dissociating water molecules through the following reaction:

H2O+O2−+e−↔4OH−⁡(adsorbed) 4

The sensing mechanism of MoS2 is illustrated in Figure . At low relative humidity (RH), hydroxyl ions (OH–) generated from dissociated water molecules chemisorb onto defect sites on the MoS2 surface, acting as electron donors and increasing the sensor’s conductivity. BVD doping enhances this process by increasing the availability of free charge carriers, which facilitates more efficient oxygen ionization and promotes greater superoxide (O2 –) formation. As a result, BVD-doped MoS2 surfaces exhibit a higher concentration of adsorbed hydroxyl ions than undoped samples, driven by the increased formation of superoxide ions that catalyze water molecule dissociation.

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Schematic of the humidity sensing mechanism on the surface of the fabricated sensor.

As humidity increases, physisorption of water molecules begins to form a thin, continuous water layer atop the chemisorbed layer, commonly referred to as the first physisorbed water layer. According to the Grotthuss mechanism, proton (H+) hopping can occur on this thin water layer, as shown in the following formula:

H++H2O→H2O+H+ 5

However, at low RH, protons cannot hop freely on the discontinuous first physisorbed water layer. At this stage, the primary mechanism for increasing conductivity is the charge transfer from dissociated water molecules to the sensing layer. As humidity levels rise, more water molecules dissociate and adsorb onto the MoS2 surface. Additional water molecules condense on top of the first physisorbed layer, forming a second, less ordered physisorbed layer, enabling more efficient proton hopping and a marked increase in conductivity. , In the BVD-doped MoS2 samples, the increase in the hydroxyl ions adsorbed on its surface accelerates the formation of both physisorbed layers, allowing the proton hopping mechanism to initiate at lower humidity levels. This behavior is reflected in Figure , where the BVD-doped MoS2 sample exhibited a significant sensing relative response increase at RH values exceeding 70%, in contrast to the undoped MoS2.

Meanwhile, analyzing human breath to monitor breathing rate, depth, and exhaled air components is crucial for assessing human health. Given its high sensing relative response, a respiration sensor using 0.03 g BVD-doped MoS2 was fabricated for this study. An undoped MoS2 sensor was also prepared for comparison. During the respiration analysis, a constant voltage of 5 V was applied, and the current response over time was measured. As shown in Figure , the undoped MoS2 sensor exhibited a response time of 2.18 s and a recovery time of 15.2 s during normal breathing. In contrast, the BVD-doped MoS2 sensor displayed faster response and recovery times of 0.699 and 6.56 s, respectively. Additionally, we also examined the reactivity of the sensor with respect to the distance between the nose and the sensor with measurements performed at distances of 3, 5, and 10 cm. It can be seen on Figure c that as the distance is increased, the current value decreased; however, the respiratory response was maintained.

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Sensor response and recovery times for (a) undoped and (b) BVD-doped MoS2 sensors and (c) current response depending on the distance between the nose and sensor.

Table presents a comprehensive review of studies on MoS2-based humidity sensors, employing various transduction techniques such as resistive, capacitive, and impedance methods, each demonstrating unique performance under different humidity conditions. Although FET and impedance-based MoS2 sensors exhibit high sensing relative response, they face limitations regarding response and recovery times. ,− Capacitive sensors, on the other hand, offer fast response and recovery times but tend to lack the sensing relative response seen in other methods. , In contrast, resistive based humidity sensors show superior sensing relative response and faster response and recovery times. ,,,,,−

1. Comparative Study of Various MoS2-Based Humidity Sensors.

Sensing material Sensor type Detection range (%RH) Sensing relative response (%) Response time (s) Recovery time (s) Reference
MoS2 FET 0–35 104 (35%RH) 10 60
MoS2 QDs synthesized in NMP Impedance 10–95 2.27 × 106 (2.21 MΩ/%RH) 14 172
Dendritic MoS2 Impedance 11–95   11 17
2D MoS2–PEDOT:PSS Impedance 0–80 4000 (50 kΩ/%RH) 0.5 0.8
MoS2/nanodiamond Capacitive 11–97 ∼3500 (100%RH) <1 0.9
MoS2/SnO2 Capacitive 0–97 3.3 × 106 (156.97 Ω/%RH) 5 13
rGO/MoS2 Chemoresistive 5–85 2494 (85%RH) 6.3 30.8
Pt decorated MoS2 Resistive 35–85 4000 (85%RH) 91.2 153.6
MoS2 flakes Resistive 10–95 5.3 × 106 (95%RH) 8 22
AMHS-based Resistive 20–85 668 (85%RH) 0.47 0.81
MoS2 Resistive 0–60 3 (60%RH) 9 17
MoS2/GO Resistive 25–85 1600 (85%RH) 43 37
MoS2 Resistive 30–60   2.14 3.96
MoS2 Resistive 45–55   0.12 0.21
BVD-doped MoS2 Resistive 30–90 8000 (90%RH) 0.70 6.56 This work
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As seen on the tabulated literature, BVD-doped resistive device in this study demonstrates distinct advantages in sensor response and simplicity of fabrication. For instance, a study employed rGO/MoS2 van der Waals composites to achieve enhanced sensor response and selectivity. While the composite approach benefits from improved surface area and porosity afforded by the graphene oxide matrix, it requires more complex synthesis. By contrast, the present BVD doping strategy achieves a maximum sensor response of 8000% at 90% RH through charge transfer doping, which electronically modulates carrier concentration without altering the intrinsic structure, thus representing a simpler and more direct route.

Similarly, Pereira et al. fabricated aerosol-printed MoS2 films with high surface area, which offered considerable sensor response improvements. Their performance, however, was strongly influenced by film morphology and porosity arising from the printing process. In contrast, the BVD-doped sensor benefits primarily from doping-induced electronic effects, with only modest contributions from wettability changes, suggesting that the observed enhancement is not morphology-dependent.

Overall, while composite-based sensors (e.g., MoS2/GO, MoS2/SnO2) enhance adsorption capacity through increased surface area and porosity, and decorated structures (e.g., Pt–MoS2) benefit from catalytic activity, the BVD doping strategy achieves comparable or even superior response primarily through electronic tuning. This not only simplifies the synthesis process but also underscores doping as a practical alternative to composites, particularly in applications where reproducibility, scalability, and cost are critical. Nevertheless, composites can provide additional advantages such as improved mechanical stability and long-term durability, which could be synergistically combined with doping in future designs.

Although the BVD-doped MoS2 sensors exhibit outstanding sensing relative response and response characteristics, further improvements in performance could be achieved through structural optimization. The incorporation of an anodic aluminum oxide (AAO)-assisted MoS2 honeycomb structure (AMHS) may significantly increase the available surface area for hydroxyl adsorption, thereby promoting faster water interaction and further improving response and recovery times. This hybrid approach offers a promising direction for the development of next generation humidity sensors with enhanced responsiveness, sensing relative response, and application versatility. The relative response of ∼8000% at 90% RH for the BVD-doped MoS2 chemiresistive sensor significantly exceeds not only that of previously reported academic prototypes (typically below 4000%) but also outperforms state-of-the-art commercial humidity sensors, such as polymer-based capacitive or metal-oxide resistive types, which generally exhibit sensitivities in the range of 100–1000% under similar humidity ranges. This remarkable enhancement can be attributed to the synergistic contribution of (i) improved surface wettability and (ii) BVD-induced electronic modulation, which collectively facilitate faster charge transfer and more efficient water-molecule adsorption–desorption dynamics.

Conclusions

This study demonstrates that benzyl viologen dichloride (BVD) doping significantly enhances the humidity sensing performance of MoS2 by increasing its carrier concentration and promoting the formation of superoxide ions on its surface. These superoxides catalyze the dissociation of water molecules, resulting in hydroxyl-rich surfaces that facilitate the formation of a continuous physisorbed water layeressential for activating the Grotthuss proton hopping mechanism. As a result, the BVD-doped sensor exhibited a sensor response of 8000% at 90% RH, representing a ∼16-fold enhancement compared to undoped MoS2. Moreover, response time was reduced from 2.18 to 0.699 s (approximately 3-fold faster), and recovery time improved correspondingly. Contact angle measurements further confirmed enhanced surface hydrophilicity in doped samples, correlating with improved adsorption and faster charge transport. These synergistic effects collectively explain the marked increase in sensor response beyond 70% RH. Spectroscopic and electrical characterizations validated successful n-type doping, confirming the role of BVD in modulating electronic structure. Overall, BVD doping establishes MoS2 as a high-performance material for next-generation humidity sensors, with demonstrated potential for biomedical, environmental, and industrial applications, including real-time respiratory monitoring

Supplementary Material

ao5c08316_si_001.pdf (359.7KB, pdf)

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (RS-2025-02233037), the Regional Innovation System & Education (RISE) program through the Gwangju RISE Center, funded by the Ministry of Education (MOE) and the Gwangju Metropolitan City, Republic of Korea.(2025-RISE-05-013), and the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (RS00411221), HRD Program for Industrial Innovation.

Glossary

Abbreviations

BVD

benzyl viologen dichloride

MoS2

molybdenum disulfide

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c08316.

  • Additional schematic illustrations of the BVD doping and electron transfer mechanism, Raman and UV–vis absorption spectra of undoped and BVD-doped MoS2 samples with varying BVD dopant amounts, comparative humidity sensing response curves, and relative response behavior as a function of relative humidity (PDF)

All authors contributed to the development of this manuscript. M.-K.K. conceptualized and supervised the study. J.S. carried out the synthesis and BVD doping of MoS2, as well as the sensor fabrication and measurements. H.-i.J. and S.P. performed additional material characterizations. Data analysis was conducted by J.M., H.-J.K., and J.-Y.K. J.S. prepared the initial draft of the manuscript. All authors reviewed and approved the final version for publication.

The authors declare no competing financial interest.

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