Accepted Manuscript

Synthesis of diselenide nanosheets and its ethanol-sensing mechanism

Shaolin Zhang, Weibin Zhang, Thuy Hang Nguyen, Jiawen Jian, Woochul Yang

PII: S0254-0584(18)30727-2

DOI: 10.1016/j.matchemphys.2018.08.062

Reference: MAC 20905

To appear in: Materials Chemistry and Physics

Received Date: 25 October 2017

Accepted Date: 21 August 2018

Please cite this article as: Shaolin Zhang, Weibin Zhang, Thuy Hang Nguyen, Jiawen Jian, Woochul Yang, Synthesis of molybdenum diselenide nanosheets and its ethanol-sensing mechanism, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.08.062

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Synthesis of molybdenum diselenide nanosheets and its

ethanol-sensing mechanism

Shaolin Zhanga,b,*, Weibin Zhangc, Thuy Hang Nguyena, Jiawen Jianb, Woochul Yanga,*

aDepartment of Physics, Dongguk University, Seoul 04620, Korea bFaculty of Information Science and Engineering, Ningbo University, Ningbo 315211, P. R.

China cSchool of Physic and Optoelectronic Engineering, Yangtze University, Jingzhou 434023, P.

R. China

*To whom correspondence should be addressed:

Email: [email protected]; [email protected]; Tel: +82-2-2290-1397; Fax: +82-2-2260-8713

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Abstract

Molybdenum diselenide (MoSe2) nanosheets thin film gas sensor was firstly fabricated and its sensing potential to ppm-level ethanol vapor at low operating temperature was investigated. Ultrathin MoSe2 nanosheets were prepared in large scale through a facile liquid-phase exfoliation method using low boiling temperature solvent. The exfoliated MoSe2 nanosheets exhibited high purity and crystallinity with few atomic layer thickness. Systematical gas sensing tests demonstrated that MoSe2 nanosheets based thin film could be utilized as ethanol gas sensor with linear response, quick recovery, and good

o repeatability at 90 C. The sensing mechanism of MoSe2 toward ethanol was investigated based on first principle calculation. The adsorption behavior of ethanol molecule on MoSe2 surface was revealed in light of adsorption orientation, adsorption energy, charge transfer, projected electronic density of state, and molecular orbital. The calculation well matched with experimental results. It is found the quick and complete recovery of MoSe2 nanosheets sensor was benefited by the appropriate physical interaction between ethanol and MoSe2 surface. This finding offers a competitive option instead of conventional sensor for ethanol gas detection at low temperature.

Keywords: Liquid-phase exfoliation; Molybdenum diselenide; Low temperature; Ethanol sensor; First principle calculation

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1. Introduction

Metal oxide semiconductors have been widely utilized as gas sensor materials for last few decades because of their high sensitivity and relatively low cost. However, these conventional sensors generally require high-temperature operation (e.g. 200-600oC) which is not applicable for the next-generation gas sensor requring wearable and ubiquitous properties [1-3]. Inspired with this obstacle, numerous alternative materials have been investigated for developing gas sensor operated at room or low temperature, such as polymer, carbon nanotubes, graphene, and two dimensional (2D) transition metal dichalcogenides (TMDs) [4-15]. Specifically, as analogues of graphene, TMDs have recently attracted increasing attention for use in next-generation sensor devices owing to their high surface-to-volume ratio and excellent semiconducting properties [16, 17].

Among TMDs, molybdenum diselenide (MoSe2), as an emerging semiconducting material with a direct band gap (1.5 eV) has rarely been investigated for sensor application. MoSe2 is composed of Se-

Mo-Se layers in which a plane of Mo atoms is sandwiched between two planes of Se atoms by covalent interaction, and different Se-Mo-Se layers are stacked together by weak van der Waals interaction [18-

20]. Late et al. firstly investigated mechanically exfoliated single-layer MoSe2 and demonstrated its high sensing performance to ppm-level NH3 gas [21]. Very recently Baek et al. also developed a MoSe2 multilayer based field-effect (FET) for detecting NO2 gas [22]. Their multilayer MoSe2 thin film was prepared through a chemical vapor deposition (CVD) method followed by a mechanical exfoliation process. Both pioneer studies have successfully demonstrated the excellent potential of 2D

MoSe2 as a sensor material. However, considering the nonrepeatability of mechanical exfoliation process as well as complicate and time-consuming CVD growth, it is necessary to develop an economical method to mass produce MoSe2, and then fabricate reproducible thin-film device with reliable property.

Previously, our group had developed a liquid-phase exfoliation method to fabricate ultrathin graphene and MoS2 nanosheets using low-boiling-temperature solvents [23, 24]. This facile method allows obtaining large yield of nanoscale products, and also has potential to be adapted for exfoliating other TMDs materials. In the present work, we exfoliate bulk MoSe2 into nanosheets using this modified

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process. The gas sensing properties of MoSe2 sensors before and after exfoliation toward different concentrations of ethanol vapor with numerous operating temperatures are systematically investigated.

The gas sensing mechanism of MoSe2 was carefully interpreted through study on the adsorption behavior of ethanol on bilayer MoSe2 surface using first principle calculation for the first time. It is expected our findings can offer a promising candidate material for gas sensor application instead of graphene, and enlighten more studies on MoSe2 based sensing materials.

2. Experimental and methods

2.1 Experimental details

The experimental route was modified from previous developed liquid phase exfolaition processes

[23, 24]. Typically, 4 g of MoSe2 powder was dispersed in 200 ml of acetonitrile, and ultrasonically treated for 1 hour using horn probe sonic tip (Sonic VCX 750) with temperature below 5oC throughout.

Subsequently, the dark brown dispersion was centrifuged at 3000 rpm for 30 minutes. After centrifugation, the red-brown supernatant dispersion was carefully decanted. For the characterization, a drop of solution containing the produced graphene was placed on a Si/SiO2 substrate and then dried in a furnace at 100oC for 6 hours before it was observed by scanning electron microscope (SEM, JSM

7100F), X-ray Diffraction (XRD Rigaku RINT2200), atomic force microscopy (AFM, Bruker Nano

N8), and Raman spectroscopy (XperRam 200).

Concentrated MoSe2 nanosheets solution was drop-casted on an alumina substrates (4 x 4 mm) with interdigital Pt electrodes. The sensor was then heated at 100oC in oven for 1 hour and sintered in an oven at 200oC in Ar gas for 1 hour. Subsequently, the sensor was placed into a sealed chamber with a built-in hot plate. Nitrogen was used as a carrier gas, and manufactured standard ethanol gas as target gases. The accurate concentration control of the target gases was done by using a mixing system equipped with mass flow controllers. The electrical conductance signal of the sensor was collected and recorded by a data acquisition under ambient conditions. The sensing response was defined as:

푅퐸 ‒ 푅푁 S(%) = × 100 (1), 푅푁

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where RN and RE represented the resistance of the sensor upon exposure to nitrogen and ethanol gases, respectively. The response and recovery time were defined as the time required for the signal to reach

90% of the final value of sensitivity during the adsorption and desorption process of ethanol, respectively.

2.2 Calculation methods

First principles calculations based on the density functional theory (DFT) and the generalized gradient approximation (GGA) with a plane-wave pseudo-potential basis were performed using the

CASTEP (Cambridge Serial Total Energy Package) code [25]. The Perdew Wang (1991) (PW91) type gradient-corrected density functional was employed for the exchange correlation potential Geometry optimization was performed by the Broyden Fletcher Goldfarb Shanno (BFGS) routine and an ultrasoft pseudopotential [26]. A 4x4x1 supercell model consists of a bilayer MoSe2 with an ethanol molecule adsorbed on it has been built, as shown in Fig. 1. A large vacuum of 20 Å along the z-direction has been taken to prevent the interaction between the adjacent replicas. Ethanol molecule was optimized in a large periodic cubic box with a cell parameter of 10×10×10 Å3. A cutoff energy of 270 eV was adopted, and the Brillouin zone was done using a 3x3x1 k-point mesh. The convergence tolerance of the energy was set to10-5 eV/atom, and the maximum allowed force, maximum stress and displacement were 0.05 eV/Å, 0.05 GPa and 10-3 Å, respectively.

Optimization calculation was conducted for the gas molecule structure and the supercell to adjust their structure parameters in the model to be as close to the experimental parameters as possible. The system of gas molecules adsorbing on MoSe2 was calculated and optimized to figure out the most stable adsorption geometry, with which a series of physical and chemical characteristic parameters, including adsorption energy (ΔEa), adsorption distance (d), and net charge transfer (ΔQ) were evaluated. The adsorption energy is defined as the change of total energy in the adsorption process of gas molecules on MoSe2 surface to reach the most stable adsorption system, and calculated using the following equation [27, 28]:

ΔE a = E(total) - E(MoSe2) – E(ethanol) (2),

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where E(total), E(MoSe2), and E(ethanol) represent the total energy of the system after ethanol adsorbed on

MoSe2, the energy of MoSe2, and the energy of ethanol molecule, respectively.

Fig. 1 Schematic model of ethanol molecule adsorbed on bilayer MoSe2 surface, (a) side view, (b) top view. The yellow and green balls represent and molybdenum atom, respectively.

3. Results and discussion

3.1 Characterization of the exfoliated MoSe2

During liquid-phase exfoliation process, black MoSe2 powder was processed to dark red-brown dispersion, as shown in Fig. 2(a) and (b). The color change was attributed to the size dependent light absorption property of MoSe2. Observation of well-defined Tyndall effect also evidenced it. As shown in Fig. 2(c), a light column appeared when a light was applied, suggesting the existence of nanoscale material in the dispersion. The exfoliated MoSe2 product was further characterized by SEM observation.

As shown in the inset in Fig. 2(d), the MoSe2 powder before exfoliation exhibited bulk morphology with lateral size over 10 um and thickness over 2 um. Impressively, after exfoliation, the bulk MoSe2 was efficiently exfoliated. In Fig. 2(d), large number of sheet-like nanostructured MoSe2 products with lateral size about 600-800 nm and ultrathin thickness were observed. Clearly, the liquid-phase exfoliation process was highly efficient for obtaining ultrathin MoSe2 nanosheets from bulk powder. It is worthwhile to note that the solvent applied here is with low boiling temperature giving an advantage

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over previous work, where high-boiling-temperature solvent was commonly used [29-31]. The use of low-boiling-temperature solvent would greatly facilitate the removal of solvent in the following fabrication of thin-film device as well as efficiently avoid the aggregation during annealing treatment.

Furthermore, the EDS analysis demonstrated that except Si and O elements, which were contributed by the substrate, only Mo and Se elements existed after drying process, as shown in Fig. 2(e), suggesting the completed elimination of solvent at 100oC.

Fig. 2 Photographs of (a) black MoSe2 powder before exfoliation and (b) dark red-brown MoSe2 nanosheets dispersion after exfoliation. (c) Tyndall effect. (d) SEM image of exfoliated MoSe2 nanosheets. Inset is the bulk counterpart. (e) EDS spectrum taken from the square area in (d). Inset table is the elemental composition.

The structure of the MoSe2 product was further characterized by XRD and Raman measurement.

As shown in Fig. 3(a), all the diffraction peaks of bulk and exfoliated MoSe2 could be assigned to the hexagonal MoSe2 (JCPDS card no. 29-0914). The suppression of (002) peak after exfoliation could be attributed to the preferential exfoliation along c direction. Similar phenomena was observed in previous study on the exfoliation of MoS2 [6]. Fig. 3(b) presents the Raman spectra of the MoSe2 before and after

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1 exfoliation. Both spectra displayed characteristic A1g (out-of-plane), E 2g (in-plane) and E1g (in-plane) peaks, which are in good agreement with earlier reports [18, 19]. It was observed that the A1g exhibited a red shift after exfoliation. This is attributed to the interlayer coupling weakened as the thickness of

MoSe2 became ultrathin and consequently the restoring force reduced [19, 32-34].

Fig. 3 (a) XRD patters of bulk and exfoliated MoSe2. (b) Raman spectra of MoSe2 before and after exfoliation.

Fig. 4(a) and (b) give the AFM details of a typical MoSe2 nanosheet, which indicates the thickness was around 2 nm. Given that the thickness of a MoSe2 monolayer is about 0.65-0.7 nm, this suggests that the obtained MoSe2 nanosheets consisted of 3 layers. Statistical analysis of thickness distribution indicates that most of MoSe2 nanosheets (over 70%) after exfoliation were of layer number less than

10, as shown in Fig. 4(c).

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Fig. 4 (a) AFM image of a typical MoSe2 nanosheet. (b) Height profile along the line shown in (a). (c)

Histogram of the counts of MoSe2 nanosheets captured by AFM as a function of the thickness per nanosheet.

3.2 Sensing performance of MoSe2 nanosheets thin film

Fig. 5(a) presents the typical responses of the MoSe2 nanosheets based thin film sensor toward 20 ppm ethanol with different operating temperatures. The conductance of the MoSe2 sensor decreased upon exposure of ethanol vapor suggesting a p-type response. Considering the electron-donating property of reducing ethanol molecules, exposure to ethanol would increase/decrease the conductance of n-type/p-type semiconducting materials. Similar phenomena was reported in previous study when

MoSe2 was exposed to NH3 [21]. The p-type property could be attributed to the existence of large number of Mo vacancies in MoSe2. This elemental deficiency was also confirmed by the composition analysis obtained from EDS measurement, as shown in the inset table in Fig. 2(e). Fig. 5(b) summarizes the response and recovery time of MoSe2 nanosheets sensor toward 20 ppm ethanol with various operating temperatures. The response increased as the operating temperature increased, and presented a saturation when the temperature over 90oC. The increased response may be owing to the lowered adsorption barrier of ethanol molecules at higher temperature. Moreover, it was observed that the MoSe2 nanosheets took long time to restore its original state at room temperature. The difficult desorption of gas molecules is a universal fault in 2D materials based sensor device, and generally overcome through high-temperature annealing or vacuum treatment [35, 36]. Impressively, MoSe2 nanosheets sensor showed completed recovery within 5 min when the working temperature was over 90oC, which well surpassed the graphene based sensor (typical recovery temperature 200oC) [36].

Fig. 6(a) shows the comparison test of MoSe2 nanosheets sensor and its bulk counterpart toward

o ethanol at 90 C. Remarkably, exfoliated MoSe2 sensor achieved double response compared with its bulk counterpart. This improved performance is certainly attributed to the increased surface area brought by the efficient exfoliation. Moreover, MoSe2 nanosheets sensor responded well as the input concentration increased, as shown in Fig. 6(b). The sensing performance of nanosheets sensor was also

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investigated as a function of ethanol concentrations. Fig. 6(c) displays the log-log plot of concentration

o dependent response of MoSe2 nanosheets sensor at 90 C. Impressively, the calibration curve (response vs. concentration) obey to the power law [37]:

S = αCβ (3), where α and β are constants depending on many factors and varies among different gas species. The more importance is that there is a good linearity between concentration and response to ethanol in large

o concentration range from 10 ppm to 200 ppm at 90 C. Fig. 6(d) shows the response curve of MoSe2 nanosheets sensor toward 20 ppm ethanol for continuous four cycles. No obvious deterioration was observed. These results indicate that liquid-phase exfoliated MoSe2 nanosheets may serve not only as a sensitive material for ethanol detection but also a powerful tool to qualitatively examine ethanol at ppm level.

Fig. 5 (a) Typical sensing response curves of MoSe2 nanosheets sensor toward 20 ppm ethanol with different operating temperatures. (b) Response and recovery time of MoSe2 nanosheets sensor as function of operating temperatures.

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o Fig. 6 (a) Comparison test of bulk and exfoliated MoSe2 based sensors toward 10 ppm ethanol at 90 C.

(b) Responses of MoSe2 nanosheets sensor toward different concentrations of ethanol gases. (c) Linear response of nanosheets sensor as function of ethanol concentrations. (d) Repeated response toward 20 ppm ethanol at 90oC.

3.3 Sensing mechanism of MoSe2

Generally, there are two dominant control factors in 2D nanomaterials based gas sensor, i.e. contacts (or interjunctions) and surface. The contacts bridging the electrical conduction between nanoflakes play an important role in gas sensing performance. It is believed that the effective exfoliation of MoSe2 resulted in large number of nanosheets contacts which partially contributed to the excellent sensing performance. On the other hand, as a typical 2D material, the main sensing mechanism of

MoSe2 is based on the charge transfer process which involves its surface interaction with molecules. In general, the sensing process of MoSe2 could be explained as follows: upon exposure, the ethanol molecule with low electron affinity serves as electron donor, and transfer its electrons to the conduction band of p-type MoSe2, thus leading to a decreased electrical conductivity [21, 38]. However, the details

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in this process are still lack. Numerous efforts have been donated to the first principle calculation of graphene in order to reveal the mechanism of graphene for gas sensor application [27, 39-41], while rare of them involved with MoSe2. Herein, for the first time, we apply first principle calculation to gain insight into the surface of MoSe2 to study the adsorption behavior of ethanol on MoSe2 and their effect on sensing performance.

Table 1. Ethanol on MoSe2: the adsorption energy (ΔEa), the distance between adsorbed ethanol molecule and the MoSe2 surface (d), and the net charge transfer from the molecule to MoSe2 (ΔQ) for different adsorption orientations.

Table 1 presents numerous physical and chemical characteristic parameters of ethanol molecule on bilayer MoSe2 surface. Four kinds of orientations of the ethanol molecule with respect to the MoSe2 surface were examined, i.e. vd, vu, pd, pu, where v presents the O-C bond sits vertically to MoSe 2 surface, p presents the O-C bond sits parallelly to MoSe 2 surface, d presents the O-C bond pointing down, and u presents the O-C bond pointing up, respectively. The adsorption energies of all geometries presented negative values. A negative ΔEa indicates that the molecule adsorption is a spontaneous and exothermic process and thus the adsorption system is energetically stable. It was found the ‘vu’ orientation presented strongest ΔEa suggesting its best adsorption geometry of the molecule. With this

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geometry, the ΔEa was closed to the critical value of chemical adsorption which is 800 meV indicating the interaction between MoSe2 and ethanol was strong physical adsorption [42]. It is known that the adhesivity of a molecule on sensing material is essentially important. The slow and incomplete recovery of conventional graphene based gas sensors is due to their strong chemical interaction with molecules.

The impressive full recovery of MoSe2 nanosheets sensor achieved within 5 min at low operating temperature is owing to the appropriate adsorption energy of ethanol molecule on MoSe2.

The net charge transfer from ethanol molecule to the MoSe2 was also much dependent on the adsorption orientation. The ‘vu’ orientation doubtlessly presented largest charge transfer owing to its strongest interaction. The ethanol molecule acted as an electron donor during electron transfer.

However, reverse phenomena was observed upon ‘pu’ orientation in spite of the reducing nature of ethanol.

Abovementioned discussion revealed that the electrons transfer direction is decided by the adsorption orientation of molecule. Further insights into the ethanol sensing mechanism of MoSe2 surface was conducted through projected electronic density of state (PDOS) analysis and molecular orbital theory. Fig. 7 (a) displays the PDOS of ethanol molecule adsorbed on MoSe2 surface with ‘vu’ orientation. It was found the interaction of molecule and MoSe2 surface was mainly achieved by the overlapping of O 2p and Se 4p orbitals at -0.5 to -3.5 eV range, while the H and C had slight interaction with MoSe2. This result partially explained the strong adsorption energy of ethanol molecule on MoSe2 with ‘vu’ orientation.

Fig. 7 (b) presents the molecular orbitals of ethanol molecule. It was observed that the highest occupied molecular orbital (HOMO) of the ethanol molecule is mostly located on the O atom and its adjacent H atoms. The lowest unoccupied molecular orbital (LUMO) mostly distributes at the end side of the ethanol molecule. Thus, upon ethanol adsorption with ‘vu’ orientation, the HOMO played a dominant role in the interaction with MoSe2 surface. It is known that there are two charge transfer mechanism between molecule and semiconductor [27, 43]. One is the energy level dependent charge transfer. Electrons transfer from molecule to semiconductor if the HOMO of the molecule is above the

Fermi level of the semiconductor, and reversed transfer happens if the LUMO is below the Fermi level.

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Another transfer mechanism between molecule and semiconductor is determined by the mixing

(overlapping) of the HOMO and LUMO with electron orbitals of semiconductor. In the adsorption geometry with ‘vu’ orientation, the HOMO near the O atom full with electrons would donate electrons to the MoSe2, and result in considerable charge transfer. While in other adsorption geometries, the contributions from HOMO and LUMO would cancel each other out resulting in small charge transfer, or LUMO dominated the interaction resulting in reversed charge transfer. Despite the orientation dependent different charge transfer direction, however, the total charge transfer is mainly decided by the ‘vu’ adsorption orientation, i.e. ethanol donates electron to MoSe2, owing to its relatively strong adsorption energy.

Fig. 7 (a) Projected electronic density of state of ethanol molecule on MoSe2 surface. (b) The HOMO

and LUMO of ethanol (the carbon atoms are gray, the hydrogen atoms are white, and oxygen is red).

4. Conclusion

In summary, large scale few-layer MoSe2 nanosheets were fabricated through liquid-phase exfoliation method using a low boiling-temperature solvent. MoSe2 nanosheets prepared by this method exhibited high crystallinity and ultrathin thickness. The MoSe2 nanosheets based thin film sensor exhibited promising potential in detecting ethanol at low temperature. Taking advantage of economical and reproducible fabrication process, as well as low working temperature, liquid-phase exfoliated

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MoSe2 nanosheets provides competitive advantages compared with conventional graphene based sensor.

Moreover, the first principle calculation was successfully conducted to interpret the origin of the sensing mechanism of MoSe2 toward ethanol molecule. It was revealed that the reaction between ethanol molecule and MoSe2 surface was strong physical adsorption, which benefits the complete and quick recovery of MoSe2 nanosheets sensor at low working temperature. The charge transfer mostly originated from the orbital overlapping between O and Se atoms. The calculation results well matched with the experimental results.

Acknowledgments

This research was supported by the Basic Science Research Program through the National

Research Foundation of Korea (NRF) funded by the Ministry of Education

(No.2015R1D1A1A01058991, No. 2016R1A6A1A03012877, and No. 2014R1A1A2059913).

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