I. INTRODUCTION

Importantly, the depletion of fossil fuels and emissions of hazardous gases such as NO$_{\mathrm{x}}$ and CO$_{\mathrm{x}}$ have highlighted alternative energy fuels. Various fuels are being developed, but the development of high-performance fuels is insufficient. Recently, H$_{2}$ fuel energy is rapidly gaining attention as a clean energy source. H$_{2}$ has colorless and odorless properties when the concentration in the atmosphere is 4% or higher, making it difficult for human senses to detect explosiveness ^[1-3]. H$_{2}$ gas has wide flammability and metal corrosiveness. Therefore, real-time sensing and monitoring of H$_{2}$ leaks in various H$_{2}$ infrastructures and fields are important. In addition, the sensitivity and response/recovery time in detecting H$_{2}$ under various environment & temperature conditions are critical issues that need to be addressed.

H$_{2}$ sensing technology has been extensively studied based on catalytic combustion, electrochemical, and semiconductor metal oxide (SMO) sensors. Table 1. describes the characteristics and mechanisms of each sensor. To improve the sensing parameters of H$_{2}$ sensors, various strategies have been developed for nanomaterial-based H$_{2}$ gas sensors, which have gained attention for their high sensing performance and simple structure ^[12,13]. Firstly, Pd-NP ^[14], palladium nanowires (Pd-NW), and palladium nanotubes (Pd-NT) have been developed as a Pd network not only reduce the activation energy of H$_{2}$ adsorption but also exhibit high catalytic properties for H$_{2}$ sensing ^[15]. Secondly, SMO (TiO$_{2}$ ^[10], Nb$_{2}$O$_{5}$ ^[16], ZnO)-based H$_{2}$ gas sensors are known to have high reactivity and catalytic activation to detect H$_{2}$. Lastly, 1D single-walled carbon nanotubes (SWCNTs) and 2D transition metal dichalcogenides (TMDs) nanosheet-based H$_{2}$ gas sensors also show excellent H$_{2}$ sensing performance at low operating temperatures and promising detection platforms in practical H$_{2}$ applications ^[17-19].

In this study, Fig. 1 presents nanocomposites H$_{2}$ gas sensor decorated with Pd on MoS$_{2}$ layer as a channel. We analyze the H$_{2}$ sensing mechanism according to MoS$_{2}$ synthesis and Pd coating techniques.

Fig. 1. Proposed nanocomposites H$_{2}$ gas sensor structure.

Table 1. Comparison of various H$_{2}$ gas sensors

II. NANO MATERIALS

Nanomaterial technology is being actively researched in various industries and applications. Among the various infrastructures, MoS$_{2}$ of 2D materials is also attracting attention in the field of gas sensors. MoS$_{2}$ nanosheets allow gas molecules to penetrate and diffuse between layers through van der Waals forces. MoS$_{2}$ is also highly reactive to toxic gases due to its high surface area-to-volume. However, MoS$_{2}$ is insensitive to nonpolar molecular sensing such as H$_{2}$ gas ^[20]. To effectively detect H$_{2}$, Pd with high catalytic properties for H$_{2}$ was decorated. Fig. 2 illustrates the operation of Pd-H$_{2}$ detection. When H$_{2}$ is injected into a gas sensor, the absorbed H$_{2}$ reacts with Pd to produce a Pd hybrid (PdH$_{\mathrm{x}}$), which combines with O$_{2}$ in the air to form H$_{2}$O. The technology based on Pd nanoparticles operates on the principle that H$_{2}$ is dissociated within the lattice of Pd to form PdH$_{\mathrm{x}}$ and induce a change in resistance. Also, the Pd-H$_{2}$ mechanism enables a fast response time for H$_{2}$ detection through electrochemical modulation. As the high sensing specific surface area of MoS$_{2}$ increases, it exhibits high sensing sensitivity and excellent selectivity for H$_{2}$ detection. Moreover, the high electron mobility of MoS$_{2}$ activates the reaction of H$_{2}$ as it induces rapid oxygen ionization within the MoS$_{2}$ channel. Fig. 1(a) proposed an H$_{2}$ sensor by applying a nanocomposite of Pd-MoS$_{2}$. The optimization of Pd decoration and MoS$_{2}$ synthesis is critical. If the Pd deposit thickness is as 1 nm, Pd molecules on MoS$_{2}$ interfaces may be unclear and Pd nanoparticles may have low coverage density ^[22]. When Pd nanoparticles are deposited more than 8 nm, the distance between particles disappears and the H$_{2}$ reaction volume immersion decreases. The formation of Pd nanosheets not only reduces the H$_{2}$ detection area of MoS$_{2}$ but also weakens the H$_{2}$ sensing characteristics. The bandgap changes according to the number of MoS$_{2}$ deposited layers. Monolayer MoS$_{2}$ is a semiconductor with a direct bandgap of 1.8 eV, whereas multilayer MoS$_{2}$ has an indirect bandgap that decreases to 1.2 eV as the number of layers increases. Additionally, the number of layers in MoS$_{2}$ controls the interaction properties of electrons and the electrical conductivity characteristics. In the case of multilayer MoS$_{2}$, there is increased electron mobility, leading to a decrease in the bandgap and improved electrical properties. Through the optimization of the fabrication process, various Pd-MoS$_{2}$ gas sensor samples were fabricated, and I-V characteristics and sensing I-T curve were analyzed.

Fig. 2. Pd-H$_{2}$ of reaction mechanism.

III. FABRICATION METHOD

Fig. 2(a) shows an overview of the fabrication process of the Pd-MoS$_{2}$ based H$_{2}$ sensor. Fig. 3(b) shows the channel and metal electrode deposition size of the fabricated nanocomposites H$_{2}$ gas sensor. After P$^{+}$ (Boron) implantation a Si wafer substrate, SiO$_{2}$ layer thickness of 300 nm was accumulated on the Si body by wet oxidation. To deposit the channel, MoS$_{2}$ is synthesized through chemical vapor deposition (CVD). Monolayer MoS$_{2}$ was deposited approximately 4 layers (0.7 nm) and multilayer MoS$_{2}$ was deposited about 8 layers (1.4 nm). Fig. 4(b) measured a sample of H$_{2}$ gas sensor by Raman spectroscopy of MoS$_{2}$. After the pattern is formed through a lithography process, Pd is decorated on MoS$_{2}$ using DC sputter as shown in Fig. 4(c). The sputtering process is conducted under two conditions: 4 seconds and 10 seconds at 100 W and Ar flow rate of 30 CC. If the Pd thickness exceeds 10 nm, the Pd nanoparticles are formed into Pd nanosheets. As shown in Fig. 4(d), Pd nanoparticles were decorated with 3 nm and 8 nm. As the last step, a two-terminal Ti/Au electrode is deposited with thicknesses of 10 nm/40 nm.

Fig. 3. (a) The fabrication process flow of Pd-MoS$_{2}$ network gas sensor; (b) The schematic drawing of proposed H$_{2}$ gas sensor structure.

Fig. 4. (a) The schematic diagram of MoS$_{2}$ synthetic method; (b) Raman spectroscopy of the MoS$_{2}$ on SiO2/Si, showing a distribution from 350-450 cm$^{-1}$; (c) The schematic diagram of DC sputter fabrication; (d) Pd thickness conditions as deposition time for DC sputtering.

IV. GAS SENSOR MECHANISM

A detailed H$_{2}$ sensing mechanism is presented in Fig. 5. Prior to the injection of H$_{2}$ gas, oxygen molecules are adsorbed onto the surface of MoS$_{2}$ and converted into oxygen ions. When the gas sensor is exposed to H$_{2}$ gas, the Pd coating on the surface adsorbs H$_{2}$. As the concentration of H$_{2}$ gas increases, H$_{2}$ diffuses from Pd to the MoS$_{2}$ surface through the spillover effect. Subsequently, the oxygen ions and H$_{2}$ within the MoS$_{2}$ channel react, resulting in the production of H$_{2}$O and electrons. These chemical reactions induce changes in resistance in the MoS$_{2}$ channel. When observing the MoS$_{2}$-Pd network gas sensor, the Schottky barrier is lowered, allowing current to flow in response by H$_{2}$ injection.

Fig. 5. H$_{2}$ sensing mechasim of Pd decorated MoS$_{2}$ gas sensor: (a) When H$_{2}$ gas is not exposed to the sensor; (b) When H$_{2}$ gas is injected to the sensor.

V. MEASUREMENT RESULT

Fig. 6 shows the I-V characteristics of the fabricated Pd-decorated MoS$_{2}$ gas sensor. The I-V curves of the typical sensor in air exhibit obvious nonlinear operation at room temperature by vacuum probe station. A voltage DC sweeps of -5 V to +5 V was applied to the two-terminal electrodes. A significant difference of 1,000 times [3 orders] was observed when comparing the two devices coated with Pd 3 nm, with multilayer MoS$_{2}$ showing a larger difference compared to monolayer MoS$_{2}$. The number of MoS$_{2}$ layers not only affects the bandgap but also leads to different resistance values. Moreover, Pd 8nm decorated multilayer MoS$_{2}$ demonstrates characteristics similar to a selector device in resistive random-access memory.

Fig. 7(a) and 5(b) are the results of extracting H$_{2}$ detection responses of monolayer MoS$_{2}$ and multilayer MoS$_{2}$ deposited with Pd 3 nm. To perform I-T curve measurements, a nanocomposite H$_{2}$ gas sensor was placed inside the gas probe station chamber. The N$_{2}$ gas was injected for 1 minute at a flow rate of 100 standard cubic centimeters per minute (SCCM) to minimize the reaction with other gases. When injecting H$_{2}$ gas at a 20 SCCM, all gases were controlled by a mass flow controller (MFC). During the measurement, a voltage of 3~V was applied to the Au/Ti electrode at room temperature. The gas concentrations were supplied at 10000 ppm, 20000 ppm, 30000 ppm, and 40000 ppm to detect the LOD. The sensitivity was calculated as R$_{0}$/R$_{\mathrm{g}}$, where R$_{\mathrm{g}}$ denotes the channel resistance value of the sensor after H$_{2}$ gas injection, and R$_{0}$ presents the resistance of the gas sensor in the presence of an N$_{2}$ gas without H$_{2}$. To extract the response time, N$_{2}$ is injected to stabilize the resistance. To extract the response time, N$_{2}$ was injected to stabilize the resistance. The response time was measured as the time exceeding 90% of the initial resistance change from the injection of H$_{2}$ gas.

With the supply of H$_{2}$ gas, Fig. 7(a) shows that the sensitivity of the Pd 3 nm - monolayer MoS$_{2}$ gas sensor is about 22.19 and the response time is about 77.4 sec. In Fig. 7(b), the sensitivity of the Pd 3 nm - multilayer MoS$_{2}$ gas sensor was 6.513, and the response time was extracted as 220.8 sec. Therefore, it may be seen that MoS$_{2}$ as multilayer structure has excellent reaction characteristics of H$_{2}$ gas. On the other hand, Fig. 7(c) shows that there is no reactivity to H$_{2}$ gas when the Pd thickness is 8 nm. If deposited as Pd nanosheets rather than Pd nanoparticles, the reaction area of the sensing gas is reduced, and the catalytic properties of H$_{2}$ are degraded ^[21].

Fig. 6. I-V characteristics of the Pd decorated MoS$_{2}$ network gas sensors measured by a typical DC double sweep.

Fig. 7. Dynamic I-T curves of (a) Pd 3 nm – monolayer MoS$_{2}$ gas sensor; (b) Pd 3 nm – multilayer MoS$_{2}$ gas sensor; (c) Pd 8 nm – multilayer MoS$_{2}$ gas sensor as 1% to 4% H$_{2}$ concentration sensing.

V. CONCLUSIONS

In this study, we proposed a resistive-type H$_{2}$ gas sensor with Pd-NP decorated on MoS$_{2}$ as channel network. Then, the chemiresitive H$_{2}$ detection mechanism in the sensing layer according to MoS$_{2}$ CVD synthesis and Pd nanoparticles coating technique is introduced. The sensitivity and response time of the fabricated H$_{2}$ gas sensors are measured through the input voltage signal. We demonstrated high H$_{2}$ reaction volume density properties through deposition of Pd nanoparticles. In addition, we compared the sensing parameters of H$_{2}$ gas following the interaction of electrons according to different energy band gaps with MoS$_{2}$. Finally, the H$_{2}$ sensor was optimized through Pd nanoparticle application technology, and the electrical H$_{2}$ gas sensing behavior according to the number of MoS$_{2}$ layers was confirmed through the input voltage.