High Entropy Alloy Nanoparticles Decorated, p-type 2D-Molybdenum Disulphide (MoS2) and Gold Schottky Junction Enhanced Hydrogen Sensing

Molybdenum Disulphide (MoS2) is an interesting material which exists in atomically thin multilayers and can be exfoliated into monolayer or a few layers for multiple applications. It has emerged as a promising material for development of such efficient sensors. Here, we have exfoliated and decorated MoS2 flakes with novel, single phase High Entropy Alloy (HEA) nanoparticles using facile and scalable cryomilling technique, followed by sonochemical method. It is found that the decoration of HEA nanoparticles impart surface enhanced Raman scattering effect and reduction in the work function of the material from 4.9 to 4.75 eV as measured by UV photoelectron spectroscopy. This change in the work function results in a schottky barrier between the gold contact and HEA decorated MoS2 flakes as a result of drastic changes in the surface chemical non-stoichiometry. The response to hydrogen gas is studied at temperatures 30 to 100 oC and it shows unusual p-type nature due to surface adsorbed oxygen species. The nanoscale junction formed between HEA and MoS2 shows 10 times increase in the response towards hydrogen gas at 80 oC. The experimental observations are further explained with DFT simulation showing more favourable hydrogen adsorption on HEA decorated MoS2 resulting for enhanced response.


Introduction
The high surface area of the atomically thin materials (2D materials) are utilized for different kinds of applications starting from electronics, energy generation, biomedical and sensors 1-6 .
Further, the varied and tunable electronic properties of these materials make them a hot cake for various optoelectronic applications. The transition metal dichalcogenides (TMDs) are one of the exciting family of 2D materials, which are reported to be interesting for its varied physical and chemical properties. Particularly in the area of chemical sensors, with the discovery of graphene and TMDs, the onus has been shifted from metal oxide materials to them due to their excellent sensing properties such as sensitivity at relatively low operating temperatures [7][8] . A suitable low operating temperature sensor device, which does not need a power-hungry heater assembly and can make them energy efficient, is required. The high chemical sensitivity of the molybdenum disulphides is utilized for the development of different class of gas sensors [8][9][10] . However, the major issue of the MoS2 based sensors is their lower sensitivities and large response time i.e., time taken by sensor to reach 90% of the saturation for a given concentration of gas as well as recovery time i.e., time taken to come back to 90% of the initial value upon withdrawing the test gas. This large recovery times arise due to high binding energies of the gas molecules on to the highly reactive MoS2 surface 8,11 . This could be addressed by making heterostructures of MoS2 with different materials having better electronic properties such as nanoparticles of metals, alloys, oxides, other chalcogenides etc [Ref].
However, the methods of synthesis of such heterostructures are quite laborious, involves multistep. Thus, a simple, facile and scalable method is always desirable.
Recently developed, multicomponent High Entropy Alloys (HEA) have attracted lot of attention due to its unique properties, which are distinctly different than that of its individual components [12][13] . The structural stability and high chemical activity of these HEA has been utilized in structural and energy generation applications. In current work, we have synthesized 3 a hybrid consisting nanoparticles of HEA and MoS2. The HEA nanoparticles are uniformly decorated on the 2D sheet of MoS2. The hybrid is further used for gas sensing properties. In order to gain insights into efficient hydrogen sensing we have performed DFT calculations. It was found that there is a significant structural change at the interface of HEA and MoS2, due to charge transfer between the two components, which leads to its stability. The hydrogen adsorption was more favourable at HEA sites rather than that at MoS2, which could explain the enhanced hydrogen gas sensing by HEA-MoS2 composite.

Materials
The Molybdenum disulphide (99%) was obtained from Thermo Fisher Scientific (USA) and used for cryomilling without any processing. Similarly, the metals purchased from Thermo Fisher Scientific (USA), Copper, Silver, Gold, Palladium, and Platinum were used to synthesize HEA NPs using arc melting, followed by cryomilling. The DMF(N,N-Dimethylformamide) 99% was also purchased from Thermo Fisher.

Nanoparticles preparation
Equiatomic (Au0.20Ag0.20Cu0.20Pd0.20Pt0.20) HEA nanoparticles have been prepared using arc melting and casting followed by cryomilling method. The all five metals in equal molar ratio have been arc melted and cast as ingot in an inert atmosphere and homogenized at 1000 o C for 24 h. Subsequently, The cast ingot has been parted into smaller pieces and cryomilled until the formation of finely dispersed nanoparticles and the detailed process can be found elsewhere 49 .

Exfoliation of MoS2
The MoS2 powder has also been cryomilled for 7 hours, and the sample has been collected 2, 4, and 7 hours for characterization. After 7 hours of cryomilling, the MoS2 powder has been dispersed in 50:50 ratio water/DMF (Dimethylformamide) using ultrasonication. In addition, 4 1 wt% AgAuCuPdPt HEA nanoparticles also added and sonicated for 30 hours using ultrasonicator operated at 40 Hz, 150 watt for exfoliation of multilayers MoS2.

Characterizations
The MoS2 powders has been collected after 2, 4, and 7 hours of cryomilling are characterized using X-ray diffraction (Panalytical, Cu target, λ = 0.154026 nm). The exfoliated MoS2 in 50:50% DMF/water has been washed with methanol using ultracentrifugation and dispersed in methanol. The one drop of washed and dispersed MoS2-NPs was placed over carbon supported TEM grid (600 mesh Cu) and dried overnight before Transmission Electron Microscopic

Results and Discussion
The MoS2 nano-flakes were prepared using inhouse designed low temperature (<123 K) grinding. The layered MoS2 structure was massively been fractured/exfoliated at extremely low temperature and forms 2D-MoS2. Subsequently, the nanosheet of MoS2 are decorated using HEA (AgAuCuPdPt) NPs allow on its surface. It is confirmed using Raman spectra of MoS2 nanosheet with and without NPs as shown in Figure 1b. In case of HEA NPs decoration, Raman band has been found to be red shifted by ~2.5 cm -1 , which could possibly be due to functionalization of AgAuCuPdPt NPs over MoS2. The intensity E2g (381 cm -1 ; in-plane vibration) and A1g (406.7 cm -1 ; out of plane vibration) of the bulk MoS2 with MoS2 nanoflakes after 30 hours sonication shows quite higher intensity. The process of cryomilling in successive hours (2, 4, 7 hours) has progressively led to reduction of the MoS2 Raman intensity as shown in Figure S1(a), [Supplementary information] which is considered the evidence of nanostructure formation by cryomilling. In addition, the process of the 7 hours cryomilled followed by 30 hours sonication has led to drastic decrease of the intensity of E2g and A1g band as shown in Figure S1(a), which reveals the nanostructured formation with few layers of MoS2.
However, the addition of high entropy alloy nanoparticles (AgAuCuPdPt) in 7 hours cryomilled MoS2 cause of enhancement of the intensity, indicating that the HEA alloy NPs impart surface enhance Raman scattering effects possibly due to electromagnetic field redistribution 14-15 . 6 The X-ray diffraction (XRD) pattern of MoS2 nanoflakes with NPs reveals presence of constituent phases as shown in Figure 1c. Successive hour of milling as shown in Figure S1(c) [Supplementary information] reveals that the intensity peaks in the XRD pattern continuously decreasing as time for cryomilling increases due to massive fracture multilayer MoS2 16 , which could result in reduced intensity in diffraction pattern as shown in inset of Figure S1(c) (002) peak. Besides, it was observed that the FWHM of the (002) increases with increasing cryomilling time in Figure S1(c).
The photo-absorption (UV-visible) of nanostructured MoS2 is shown in Figure 1d, and it is found to be weak as compared to that of bulk counterparts 17-18 . It is further weakening in intensity as milling time increases as shown in Figure S1 In order to study the effect of addition of HEA NPs on chemical stoichiometry and electronic structure, the X-ray and UV photoelectron spectroscopy of both the samples were conducted and the results obtained are analysed and presented in Figure 3. It is seen from Figure 3(a), that the Mo 3d core level XPS spectra is quite complex unlike CVD grown single MoS2 sheets 20 .
Several peaks were observed in this region which have been attributed to core levels of S 2s character as found in literature 21-23 . Similarly, S 2p core level spectra shown in Figure 3(b) reveals more than one oxidation states for S (S 2-, S2 2-) as well [24][25] . Along with this, metal sulphide peaks could also be seen at lower binding energy i.e.,159.80 eV. In addition to anionic sulphur, several peaks have been found on higher binding energy which reflect more positively charged nature of sulphur ions and hence have been attributed to oxidized sulphur i.e., SOx.
Hence, this supports the existence of surface oxidation with observed higher Mo 6+ ratio in the same. It is known from the literature that when MoS2 is made by physical methods such as sputtering, laser ablation etc. and exposed to air, it results in surface oxidation 15 Where, I is the current obtained at applied bias V, T is absolute temperature, q is the charge of electron, k is Boltzmann constant, A* is Richardson constant and S is the area of the device.
The same equation (2) may be written in the form of equation (3) as Thus, from Eq (3) when ln( / 4 ) is plotted as a function of 1/T, the slope gives the value of barrier height = |Φ KLMNO − Φ PLKQRSTUVRMSW |. Ideally, the slope value obtained is further plotted against the value of potential applied to get the y-intercept i.e. in the limit of zero bias [30][31] . However, we have observed that the slope is practically independent of bias (See figure S2 in supplementary information) i.e. the barrier is sufficiently high and the change in barrier with application of electric field is minimal. This is also evident from the three orders of magnitude lower current in comparison to that of the ohmic contact formed without HEA nanoparticles.
Thus the barrier height, value obtained from Figure 4

13
The gas sensing response of the devices was studied for various concentrations of hydrogen gas. As can be seen from Figure 5 The Faster response may be due to catalytic activity of the HEA NPs which helps in improving the number as well as rate of the surface adsorption of the target gas molecules. For practical applications it is ideal that the sensor responds and recovers faster as shown in Figure 5(d) for various concentrations of hydrogen. These obtained time constants with high response are much smaller than those reported to date to the best of our knowledge 9, 30 . Hydrogen is a reducing gas, i.e. it donates an electron when it surface adsorbs. This electron when received in an n-type semiconductor, leads to increase in its carrier concentration and hence resistance drops. While in p-type semiconductors, it recombines with some of the holes and thus decreases the carrier concentration and hence resistance increases. The rise in resistance observed in this study denotes that the observed behaviour is that of the p-type semiconductor and this p-type behaviour was attributed to the surface oxygen adsorption on MoS2 flakes 35 . Further, the type of carriers is also confirmed by hall measurements. The lower work function implies that the fermi level (zero binding energy line) moving closer to conduction band (away from valance band), which will make the barrier height more for n-type 15 material. On the other hand, the fermi level moving closer to valance band will make the barrier height higher for p-type semiconductor.
High entropy alloys are solid solutions of five metal atoms which are otherwise highly catalytically active materials. Whenever such catalytically active metal atoms are attached to semiconductor gas sensor material, it is known to enhance the sensor response by two mechanisms, i.e. controlling the fermi level and spill over mechanism. The fermi level control results due to changes in electronic transport as a result of formation of a depletion region, while the spill over refers to the catalytic effect of these particles [36][37][38] . The spill over model is  Where \S]^_Z`a , \S]^, and Z`a are the total energies of MoS2-HEA composite, MoS2, and HEA, and is the area of the unit cell (Table S1). The value for [ was found to be -74.02 meV/Å 2 , indicating its high stability, in agreement with the experiments. This is further corroborated by the charge density difference ( UQcc ) plot shown in Figure 6(a), which was calculated using: where MSM , dWQPMQTL , and Z^a re the total energies of H adsorbed system, pristine (bare) system, and isolated H2 gas molecule, respectively (Table S1). The values for Z * corresponding to MoS2 and MoS2-HEA composite are shown in Figure 1, where it can be seen that Z * on MoS2-HEA composite is exothermic, while that on MoS2 is endothermic.
Therefore, H will tend to adsorb faster on MoS2-HEA composite compared to that on bare MoS2, hence the response time will be lower for the MoS2-HEA composite, as is also observed experimentally.

Conclusion
To summarise, we have proposed a facile method to decorate/functionalise 2-D materials.