Highly sensitive two-dimensional MoS2 gas sensor decorated with Pt nanoparticles

A two-dimensional molybdenum disulfide (MoS2)-based gas sensor was decorated with Pt nanoparticles (NPs) for high sensitivity and low limit of detection (LOD) for specific gases (NH3 and H2S). The two-dimensional MoS2 film was grown at 400°C using metal organic gas vapour deposition. To fabricate the MoS2 gas sensor, an interdigitated Au/Ti electrode was deposited using the electron beam (e-beam) evaporation method with a stencil mask. The MoS2 gas sensor without metal decoration sensitively detects NH3 and H2S gas down to 2.5 and 30 ppm, respectively, at room temperature (RT). However, for improved detection of NH3 and H2S gas, we investigated the functionalization strategy using metal decoration. Pt NP decoration modulated the electronic properties of MoS2, significantly improving the sensitivity of NH3 and H2S gas by 5.58× and 4.25×, respectively, compared with the undecorated MoS2 gas sensor under concentrations of 70 ppm. Furthermore, the Pt NP-decorated MoS2 sensor had lower LODs for NH3 and H2S gas of 130 ppb and 5 ppm, respectively, at RT.


Introduction
Metal oxide-based gas sensors possess many merits, such as relatively high sensitivity and low cost, which has garnered much attention to the material [1]. However, to obtain such gas-sensing properties for a specific gas, these gas sensors generally suffer from several issues, such as thermal instability and high power consumption, due to poor performance at low operating temperatures [1]. of material, new materials that possess excellent characteristics for adsorption of gas molecules at low operating temperatures are in great demand as a way to overcome these deficiencies.
In this work, we investigated the performance of a two-dimensional MoS 2 -based gas sensor and the effects of functionalization using Pt nanoparticle (NP) decoration. Metal organic chemical vapour deposition (MOCVD), which can control the number of two-dimensional material layers and synthesize scalable two-dimensional-layered materials with high quality, was employed to grow the twodimensional MoS 2 film. Moreover, MOCVD is capable of synthesizing high-quality two-dimensional MoS 2 directly on the sensor substrate to easily fabricate the MoS 2 gas sensor. The MoS 2 sensor consisted of the MOCVD-grown MoS 2 film as the active channel and an interdigitated Au/Ti electrode fabricated using the electron beam (e-beam) evaporation method with a stencil mask. At room temperature (RT), the MoS 2 sensor could sensitively detect down to 2.5 and 30 ppm for NH 3 and H 2 S gas, respectively. However, the detection of NH 3 and H 2 S gas still requires higher sensitivity and a lower limit of detection (LOD) owing to the inhalation toxicology of both gases.
Because both gases exhibit inhalation toxicology that leads to lung and organ injury or even death as well as skin irritation, several international standards have been developed as guidelines to assist in the control of these health hazards. For example, the permissible exposure limits, i.e. regulatory limits, in the work place stipulated by the Occupational Safety and Health Administration (OSHA) are 50 ppm for NH 3 and 10 ppm for H 2 S. However, in many cases, the recommended limits of NH 3 and H 2 S gas, which are characterized by an extremely irritating odour, lie below the regulatory limit stipulated by OSHA, so studies for high sensitivity and low LOD are still required. Thus, we investigated a functionalization strategy, i.e. metal decoration, to obtain excellent gas-sensing characteristics (e.g. higher sensitivity and lower LOD) for NH 3 and H 2 S gas. To improve the sensitivity and LOD for NH 3 and H 2 S gas, which act as electron donors on the MoS 2 , Pt metal NPs, which resist corrosion and oxidation, yielding stable doping as a noble metal [27], are used as the decorating material. Furthermore, Pt NPs double the p-type doping effect compared with Au NPs [27], which commonly act as a p-dopant [12], so Pt NPs could improve the sensing characteristics for NH 3 and H 2 S gas. Even though various studies have already been reported [15][16][17][18][19]34] on the detection of various toxic gases (e.g. NO 2 , NH 3 , H 2 and CO 2 ) using two-dimensional TMDCbased sensors, most use flake or bulk two-dimensional TMDC instead of two-dimensional MoS 2 film at RT. In this work, H 2 S detection using a two-dimensional MoS 2 sensor in addition to the detection of well-known gases (e.g. NH 3 , NO 2 , H 2 and volatile organic compounds) is reported. The H 2 S response is lower than the NH 3 response on the MoS 2 sensor, confirming that the MoS 2 sensor has lower charge transfer for the detection of H 2 S than NH 3 gas. Compared to an undecorated MoS 2 sensor, the Pt NPdecorated MoS 2 sensor detected NH 3 and H 2 S gas with high sensitivity down to 600 ppb and 5 ppm, respectively. In addition, NH 3 detection experiments were conducted at concentrations of 600 ppb or less in order to determine the LOD of the Pt-decorated MoS 2 sensor for NH 3 gas.

Material and methods
2.1. MOCVD growth of two-dimensional MoS 2 monolayer film with bilayer islands As previously reported [35], the two-dimensional MoS 2 film was grown using MOCVD. The MoS 2 film was grown by a showerhead-type reactor using Mo(CO) 6 (better than 99.9% purity, Sigma-Aldrich), rsos.royalsocietypublishing.org R. Soc. open sci. 5: 181462 high-purity H 2 (99.999%), Ar (99.999%) and H 2 S (99.9%) with an operating temperature of 4008C. A p-type silicon wafer (1-10 V cm) with 300 nm silicon dioxide was used as the substrate for growth. The piranha solution (H 2 SO 4 : H 2 O 2 ¼ 3 : 1) was used for pretreatment to passivate dangling bonds [35] and make a hydrophilic substrate, resulting in the control of more nucleation sites on the substrate; the piranha treatment was performed by immersing the substrate in the piranha solution for 10 min. After immersion in the piranha solution, the substrate was thoroughly rinsed in deionized water through immersion and sonication, and it was blow-dried with high-purity N 2 (99.999%). After cleaning this, the substrate was placed on a silicon carbide-coated graphite susceptor and immediately loaded into a load-lock chamber, which was connected with the MOCVD process chamber to prevent any contamination. Subsequently, the substrate on the graphite susceptor was loaded into the process chamber to grow the two-dimensional MoS 2 film. MoS 2 film was grown using sublimed Mo(CO) 6 precursor vapour with high-purity H 2 S, Ar and H 2 flow for high-quality MoS 2 at 3008C by the reaction between sulfur and gas with the Mo-precursor under a pressure of 5 Torr. Consequently, the fully covered monolayer MoS 2 film was synthesized with bilayer islands without empty spaces of MoS 2 .

Fabrication of MoS 2 -based devices and decoration with metal NPs
To prevent any contamination by chemical substances during the fabrication of the MoS 2 gas sensor, we chose a solution-free process, as shown in figure 1a. The MoS 2 gas sensor, which consisted of the MOCVD two-dimensional MoS 2 active channel and the interdigitated electrode, was fabricated using the e-beam evaporation method with a stencil mask. To use the MoS 2 as the active material of the gas sensor, half of the MoS 2 film was removed by the scotch-tape method. The interdigitated electrode was patterned on the remaining MoS 2 film using the e-beam evaporation method and a stencil mask with the interdigitated pattern. Additionally, the electrode pad for resistance measurement was patterned on Si with SiO 2 where the MoS 2 was removed. The interdigitated electrode region was made from 5 nm Ti and 50 nm Au using an e-beam evaporation method under a pressure of approximately 10 27 Torr. Figure 1a also shows an optical image of the active channel (MoS 2 film, 70 mm in width) and the interdigitated Au/Ti electrode of the MoS 2 gas sensor. The active channel material detects gas molecules, and the interdigitated electrode collects charge. In this work, two MoS 2 -based devices (one decorated with Pt NPs and one without Pt NPs) were made to investigate the effect of Pt NP decoration. The e-beam evaporation method was used to randomly distribute Pt NPs on the MoS 2 . The Pt NPs (approx. 1 nm in thickness) were monitored with a crystal thickness sensor of e-beam evaporation under a pressure of approximately 10 27 Torr.

Gas-sensing experiments
A gas-sensing system that allows the users to customize experimental conditions according to their need was set up, as shown in figure 1b. The system consisted of analyte gas (NH 3 and H 2 S), dilution (or purging) gas (N 2 ), a pneumatic valve for opening/closing gas flow, a mass flow controller for regulating analyte gas and dilution gas, a sensing chamber for the gas detection process and a rotary pump for purging the atmosphere after gas-sensing measurements. The gas-sensing experiments were conducted under analyte gas (NH 3 or H 2 S) diluted with N 2 in a sensing chamber, and the concentration of analyte gas was controlled by modulating the flow rate of analyte gas and N 2 dilution gas. The fabricated MoS 2 gas sensor was placed in the sensing chamber for gas detection.
After loading one of the MoS 2 gas sensors (i.e. bare MoS 2 or Pt-decorated MoS 2 ), the gas-sensing system was sufficiently stabilized under N 2 gas in the chamber and then the gas-sensing experiments were repeatedly performed by automatically controlling the purging step and analysis step for 10 and 5 min, respectively. The experiments were done with analyte concentrations of 600 ppb and 1, 2.

Results and discussion
To examine the morphology and grain size of the as-grown MoS 2 , the MoS 2 film was characterized using scanning electron microscopy (SEM) (Hitachi S-4800), as shown in figure 2a-d: figure 2a shows smaller monolayer MoS 2 clusters (growth time: 1 h), figure 2b shows larger monolayer MoS 2 clusters with a little bit of space (growth time: 2 h) and figure 2c shows monolayer MoS 2 with bilayer islands (growth time: 3 h), which were obtained by controlling coverage using the growth time without a change of other process conditions. As the growth time increased, the coalescence of these monolayer islands led to a fully covered MoS 2 film. Under a highly strong sulfiding condition, the layer atoms are more strongly attracted to the substrate than to themselves, thereby accelerating two-dimensional growth.  Figure 2d shows that a monolayer two-dimensional MoS 2 film with bilayer islands was grown using the MOCVD method. Thus, MOCVD two-dimensional MoS 2 was successfully grown at 3008C by the reaction between the sulfur and gas reaction with the Moprecursor under the sulfiding conditions. The grain size of the bilayer islands on the fully formed monolayer film was about 100 nm. Because the adsorption of gas molecules can be attributed to the influence of the many activation sites caused by the presence of the many edges site of the bilayer islands, the fully formed monolayer MoS 2 film with bilayer islands was used as the active material of the gas sensor in this work.

rsos.royalsocietypublishing.org R. Soc. open sci. 5: 181462
To examine the number of the MoS 2 layers, photoluminescence and Raman spectroscopy (Renishaw inVia Raman microscope) were used. A laser with an excitation wavelength of 488 nm was used. The spectra were also normalized at the Si peak (520.7 cm -1 ). As a result, the photoluminescence spectrum of the MoS 2 has an adsorption peak at 659 nm (1.88 eV). The Raman spectrum represents the in-plane vibration (E 1 2g ) and the out-of-plane vibration (A 1g ) from the MoS 2 . The peak position difference between E 1 2g and A 1g was 21.81 cm -1 , which indicates that the MoS 2 has two layers based on the laser wavelength.
To investigate the effects of Pt NP decoration, we made two sensors: an undecorated MoS 2 sensor and a Pt NP-decorated MoS 2 sensor. Henceforth, the undecorated MoS 2 and Pt NP-decorated MoS 2 sensors will be denoted as bare MoS 2 and Pt : MoS 2 , respectively. Before the gas-sensing experiments, a transmission electron microscope (TEM) (FEI Tecnai G2 F30 S-Twin) was used to characterize the morphology and structure of the as-obtained Pt : MoS 2 . The decorated Pt NPs on MoS 2 were confirmed by TEM images. As shown in figure 3, a number of Pt NPs indicated by yellow circles and the TEM image clearly show small NPs with a diameter around 5 nm. The TEM image of the Pt : NP-decorated MoS 2 reveals the monolayer MoS 2 film (brightest), the triangular bilayer islands (brighter) and the Pt NPs (darker). Moreover, since Pt NPs were physically decorated by the e-beam evaporation method, the TEM image showed that the Pt NPs were randomly distributed on the MoS 2 .  figure 4. With incrementally increasing concentrations (600 ppb and 1, 2.5, 5, 15, 30 and 70 ppm) of analyte gas, the gas responses of two sensors were investigated under NH 3 and H 2 S gas. The sensitivity to the analyte gas was calculated using DR/R p ¼ (R a 2 R p )/R p , where R p and R a represent the resistances of the device under the N 2 gas ( purging gas) and the analyte gas, respectively. When exposed to the NH 3 or H 2 S gas, the MoS 2 sensors showed decreasing resistance (negative sensitivity). This means that both NH 3 and H 2 S gas act as electron donors, transferring electrons to the conduction band of MoS 2 [15]. Thus, this leads to increased electron concentration and conductivity, and the resulting n-doping brings the Fermi level closer to the conduction band edge. Figure 4a shows that the LOD of NH 3 gas was as low as 2.5 ppm and the absolute sensitivity of NH 3 gas ranged from 2.5 to 70 ppm on the bare MoS 2 gas sensor at RT. Moreover, the LOD of H 2 S gas was as low as 30 ppm, and the sensitivity of H 2 S gas ranged from 30 to 70 ppm at RT, as shown in figure 4b. The twodimensional MoS 2 gas sensor is the first film sensor to exhibit H 2 S gas-sensing characteristics    We concluded that detection of H 2 S gas can be attributed to the influence of many activation sites caused by the presence of bilayer islands on the MoS 2 gas sensor. As the concentration of analyte gas (NH 3 or H 2 S) increased, the absolute sensitivity of the MoS 2 sensor increased at RT (figure 4c,d). Because the bare MoS 2 gas sensor has higher sensitivity and lower LOD for NH 3 than H 2 S gas sensor, it experiences a smaller charge transfer from H 2 S than from NH 3 . To confirm the LOD of NH 3 gas for the Pt : MoS 2 gas sensor, we performed an additional gas-sensing experiment under analyte concentrations of 600 ppb or less. This experiment demonstrates that the Pt : MoS 2 gas sensor has an LOD of NH 3 gas down to 130 ppb, as shown in figure 5. Table 1 summarizes the gas-sensing characteristics of many TMDC-based gas sensors for NH 3 or H 2 S gas at RT. It shows that the Pt : MoS 2 gas sensor has high sensitivity and low LOD for both NH 3 and H 2 S gas. Figure 6 shows a schematic of the energy band diagram of MoS 2 and Pt, where the work function (F m ) of Pt is approximately 5.65 eV [36]

Conclusion
In summary, Pt NPs were decorated on MOCVD two-dimensional MoS 2 to obtain high sensitivity and low LOD for NH 3 and H 2 S gases. The electronic properties of MoS 2 were tuned through decoration with Pt NPs, improving the sensitivity and lowering the LOD of the gas sensor for the target gases. The Pt NPs act as a p-dopant, depleting the electron carriers of the MoS 2 film, thereby improving the sensitivity to NH 3 and H 2 S gas by 5.58Â and 4.25Â, respectively, compared to the bare MoS 2 gas sensors under concentrations of 70 ppm. The Pt : MoS 2 gas sensor exhibited LODs of 130 ppb and 5 ppm for NH 3 and H 2 S gas, respectively, which are far lower than those for the bare MoS 2 gas sensor (2.5 and 30 ppm, respectively). Even though all gas-sensing experiments were carried out at RT, the MoS 2based gas sensors showed excellent gas-sensing characteristics. Therefore, functionalization using metal decoration on a two-dimensional gas sensor can improve the sensitivity and LOD for a specific gas by modulating the electronic properties of the two-dimensional material. This strategy opens an avenue for effective toxic gas detection with two-dimensional gas sensors.
Data accessibility. This article does not contain any additional data and all data are provided in the main text. Authors' contributions. J.P. and J.M. conceived the study. J.P. designed and performed the calculations, and wrote the Adv.