Enhanced performances of PbS quantum dots modified MoS2

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Enhanced performances of PbS quantum dots modified MoS2 composite for NO2 detection at room temperature Xin Xin, Yong Zhang, Xiaoxiao Guan, Juexian Cao, Wenli Li, Xia Long, and Xin Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Applied Materials & Interfaces

Enhanced performances of PbS quantum dots modified MoS2 composite for NO2 detection at room temperature Xin Xina, Yong Zhanga,b, * , Xiaoxiao Guana, Juexian Cao a, b, * , Wenli Lia, Xia Long a, Xin Tan a a

School of Physics and Optoelectronics, Xiangtan University, Xiangtan 411105, PR China

b

Hunan Institute of Advanced Sensing and Information Technology, Xiangtan University, Xiangtan 411105, PR China

KEYWORDS: Heterojunctions; MoS2/PbS; NO2 gas-sensing properties; Density functional theory; Competitive adsorption

Abstract

The modification of the material surface by the second phase particles enables the electron interaction on the Fermi level or the energy band between different materials, which can achieve the improvement of gas sensing properties. Herein, a novel composite of PbS quantum dotsmodified MoS2 (MoS2/PbS) is synthesized by hydrothermal method combining with chemical precipitation, and fabricated into the gas sensor to investigate its enhanced gas sensing properties caused by the modification of PbS quantum dots at room temperature. It is found that the

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responsivity of MoS2/PbS is obviously higher than that of pure MoS2 gas sensor throughout the whole test range, and MoS2/PbS gas sensor is of better selectivity compared with pure MoS2 gas sensor at room temperature. The response of MoS2/PbS gas sensor is about 50 times higher than that of MoS2 gas sensor at 100 ppm NO2 concentration. The recovery behavior is greatly improved, and the resistance of MoS2/PbS gas sensor can return completely with almost no drift (The recovery ratio is more than 99 %). The enhanced gas sensing properties of MoS2/PbS, which are superior to that of pure MoS2, are ascribe to the large surface area of MoS2 combine with the high responsivity of PbS QDs for NO2, and the formation of heterojunctions leads to the competitive adsorption of the target gases, which can prevent MoS2 from being oxidized, further improving the stability of gas sensor. Furthermore, in order to profoundly discuss the enhanced performances and the sensing mechanism, the molecular models of adsorption systems are constructed to calculate the adsorption energies and the diffusion characters of NO2 via density functional theory (DFT). We expect that our work can offer a useful guideline for enhancing the gas sensing properties at room temperature.

1. Introduction In recent years, MoS2 has attracted much attention in gas sensors due to the distinct layered structure and high surface-to-volume ratio. 1-3 However, MoS2 is strongly influenced by oxygen adsorption in air, which might lead to instability of the MoS2 sensor device, 4 and the carrier mobility dramatically degrade with the accumulative exposure to ambient oxygen, further reducing gas sensing properties of materials. 5 It has been reported that the second phase particles modification on the surface of MoS2 can enhance the stability, 5 and the modification of the material surface can be also used to enhance gas sensing selectivity and sensitivity by reason of synergistic effects, 6,7 such as the interfacial electron transfer, the large specific surface area, etc.8


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For example, Roya Maboudian et al. reported that the MoS2/graphene heterostructure shows ultra-sensitivity, good selectivity, and fast response and recovery behaviors for NO2 detection, due to the large specific surface area, high electrical conductivity of graphene and good NO2 sensing properties of MoS2. 9 Li Li et al. found that In2O3-composited SnO2 nanorods sensor shows the high sensing response of 8.98 for 100 ppm NOx with fast response time 4.67 s, which is over 11 times higher than that of pristine SnO2 nanorods at room temperature, and the enhanced performances of In2O3-composited SnO2 nanorods sensor are ascribed to the electrons transfer between In2O3 and SnO2 and the formation of more electronic transfer channels.

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Moreover, the number of heterojunction, namely the number of electronic transfer channels, is another important parameter related to gas sensing properties, which is related to the number of active sites. Sang Sub Kim et al. found that the enhanced H2S-sensing properties of ZnO-CuO composite nanofibers are ascribed to the increasing of the number of heterojunctions due to the small nanograin size.

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PbS quantum dots (QDs) are reported to be of high response for

detecting NO2 at room temperature, 12 but it agglomerates easily and seriously, resulting in the decrease of effective surface.

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Hence, PbS QDs can be used as the suitable second phase

particles for modifying MoS2, and the modification process can improve the gas sensing properties of MoS2 and avoid agglomeration caused by the direct contact between PbS QDs. Compare with the modified materials with large size, the modification of MoS2 by PbS QDs can increase the number of p-n junctions and the interfacial surface area, which may be beneficial to improve the adsorption of gas molecules on the surface of material and surface activity.

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Moreover, the resistance-type gas sensors worked at room temperature can be easier fabricated and show the less cost, lower power consumption, higher environmental safety and longer life times than the traditional gas sensors with high operated temperature (150-400 °C). 15-17 As a


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result, based on the effects of modification of the material surface, seeking an effective way to improve the gas sensing performance of MoS2 and even to realize the room temperature gas detection of MoS2, are of great significance for realizing the practical application of MoS2. In our work, the pure MoS2 and PbS QDs-modified MoS2 (MoS2/PbS) composite are synthesized by hydrothermal method combining with chemical precipitation. The properties of gas sensors based on MoS2 and MoS2/PbS are compared to investigate the influence of the material surface modification on the gas sensing properties at room temperature. Based on the characterizations such as crystal structures, morphologies, the surface chemical states and compositions, as we expected, the modification of PbS QDs has greatly improved the gas sensing properties of MoS2. 2. Experiment details The pure MoS2 is synthesized by hydrothermal method, and the detailed preparation process is shown in Figure 1 (a). Firstly, 0.194 g Na2MoO4·2H2O and 0.15 g CH3CSNH2 were dissolved in 60 ml DI water, and stirred for 20 min to obtain a clear and transparent homogeneous solution. Secondly, the mixture was ultrasonicated for 5 min and then transferred into a Teflon-lined autoclave which was continuously heated at 200 °C for 36 hours. 18 After natural cooling to the room temperature, a solid (MoS2)-liquid mixture with a fuzzy layered interface is obtained. Lastly, the MoS2 was collected by centrifuge with the speed of 2000 rad/min, and washed with DI water and ethanol for removing the unreacted solution. The residual DI water and ethanol were removed after drying at 60 °C for 12 h.


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Figure 1. Detailed preparation processes of (a) pure MoS2 and (b) MoS2/PbS composites. (c) Fabricated technological process diagram of gas sensor. The MoS2/PbS was synthesized by hydrothermal method combining with chemical precipitation as shown as Figure 1 (b). 0.0138 g Pb(NO3)2 and 0.01 g Na2S∙9H2O were respectively dissolved in 20 mL ethanol solution (50%vol DI water with 50%vol anhydrous ethanol) to form the solution A and solution B. 0.1 g of as-prepared MoS2 was dispersed in 10 mL ethanol by ultrasonication for 30 min. The solution A and the MoS2 dispersion liquid were mixed and constantly stirred via the magnetic stirrer to make Pb2+ adsorb uniformly onto the


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surfaces of MoS2, and the solution B was dropwise added into mixed solution until reacting completely. Then, a suspension solution is obtained, and the MoS2/PbS was collected by using the same process as mentioned in the last step of MoS2 preparation. The crystal structures were characterized by X-ray diffraction (XRD, Rigaku, D/Max 2500PC) with Cu Kα radiation (λ=0.15418nm). The morphologies were determined by field emission scanning electron microscopy (FE-SEM, Hitachi SU5000) with the energy dispersive spectroscope (EDS, Bruker QUANTAX-400) and the transmission electron microscopy (TEM, JEOL, JEM-2100). The surface chemical states and compositions were performed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi) using an Al Kα excitation source. In order to measure the gas sensing properties at room temperature, the MoS2 and MoS2/PbS gas sensors are fabricated and the technological process diagram is shown as Figure 1(c). The MoS2 and MoS2/PbS were respectively dispersed in ethanol by ultrasonication, then the dispersion liquids were dropped on the Al2O3 substrate with interdigital electrodes to form a film which was subsequently dried at 60 °C for 30 min. The dropped and dried processes were repeated three times. After drying the substrates with sensing film at 60°C for 2 h, the MoS2 and MoS2/PbS gas sensors without heating unit were obtained, and the as-prepared sensors were aged with DC voltage of 3.5 V at room temperature of 25°C for 24 h to improve their stability. The thickness of the sensing film is about 3 μm, which is measured by film thickness meter (KLA tencor D120). The performance of gas sensors were measured on a CGS-8 intelligent gas sensing analysis system (Beijing Elite Tech Co., Ltd, China) which could acquire the sensor resistance automatically. The applied voltage in our work is the DC of 4.5 V. The NO2 gas sources of different concentrations (5 ppm, 10 ppm, 20 ppm, 50 ppm, 100 ppm, 200 ppm, and 400 ppm, 21%vol O2 with 79%vol N2 as balanced gas) were bought from Dalian Special Gases co. LTD


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(Dalian, China), which had been calibrated by Fourier transform infrared spectrometer (USA, spectrum 100). The response can be defined as S =

Ra-Rg Ra

100 ( ), where Rg and Ra are the

resistances of the sensors in target gas and air, respectively. 19-21 The response and recovery times are defined as the time taken by the gas sensor to achieve 90% of the total resistance change in the case of adsorption and desorption. 22 Compared with traditional side-heating gas sensors that require a certain operating temperature (150-400°C), 6 all gas sensors in this work are operated at room temperature (25°C) without heating device. 3. Results and discussion

Figure 2. (a) XRD patterns of MoS2 and MoS2/PbS. (b) FE-SEM of pure MoS2 and (c) MoS2/PbS composites. SEM mapping of (d) Mo, (e) S and (f) Pb in MoS2/PbS composites. The inset of (d) shows the area of SEM mapping.


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The XRD patterns of MoS2 and MoS2/PbS are shown in Figure 2 (a). It can be seen that the diffraction peaks of MoS2 are well indexed to the standard JCPDS card (06-0097), revealed the MoS2 belongs a typical hexagonal structure. The XRD pattern of the MoS2/PbS is superimposed by the diffraction peaks of MoS2 and PbS respectively (the standard JCPDS card of MoS2 (060097) and PbS (05-0592)), and there is no characteristic peak for impurity in the XRD pattern, indicating that the composite is consisted by the MoS2 and PbS only. The FE-SEM images are used to confirm the morphologies of MoS2 and MoS2/PbS as shown in Figure 2 (b-c). Figure 2(b) shows that the morphology of the pure MoS2 is self-assembled by the MoS2 sheet to form a fluffy ball-like structure. Compared with the pure MoS2, the modification of PbS does not affect its morphology, and the MoS2/PbS composite still maintains the original fluffy ball-like structure. Unlike pure MoS2, there are many small nanoparticles attached on the fluffy ball-like structure surface, indicating that PbS is successfully deposited onto the surface of MoS2 to form the MoS2/PbS composite. To further demonstrate the distribution of PbS on the MoS2 surface, the SEM mappings of MoS2/PbS are performed as shown in Figure 2 (d-f), and the inset of Figure 2 (d) is the area of the SEM mapping. The distributions of Mo, S and Pb are homogeneous, but the distribution density of Pb is lower than that of Mo and S obviously. This phenomenon is caused by the difference in component proportion between main materials (MoS2) and second phase materials (PbS). Moreover, the homogeneous distribution of Pb means that many PbS nanoparticles are dispersed uniformly on the surface of MoS2 (Figure 2 (f)), namely, many MoS2/PbS heterojunctions may be well formed on the MoS2 surface, which is in favor of the increase of active sites number on MoS2 surface. 23


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Figure 3. (a) TEM image and (b) high magnification image of MoS2. (c) TEM and (d) HRTEM images of MoS2/PbS composites. Figure 3 shows the TEM and HRTEM images of MoS2 and MoS2/PbS. From Figure 3(a), the pure MoS2 presents the fluffy ball-like microstructure which can provide the abundant area to deposit the PbS, and the average diameter of microstructure is about 600 nm. The high magnification image of pure MoS2 edge is shown in Figure 3(b). Compared with the central part of MoS2 stacked by many MoS2 nanosheets, the edge of pure MoS2 exhibits gauzy nanosheet structure, and the wrinkles appear clearly at the edge of pure MoS2, which is caused by the stack of few MoS2 nanosheets. From Figure 3(c), many second phase materials (PbS) are dispersed on the MoS2 surface, and the sizes of PbS are only in the range of 5 to 10 nm, which can be deemed that the PbS is attached to the MoS2 in the form of QDs. 24 In addition, PbS QDs


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are densely attached on the MoS2 surface, and it means that there are abundant heterojunctions formed in MoS2/PbS composites due to the size effect of PbS QDs. 11 The HRTEM image of MoS2/PbS composites is further performed, and it can be found two different lattice fringes spacing (0.27 nm and 0.30 nm) in Figure 3(d). According to the XRD analysis results (The lattice parameters of MoS2: a=3.160 Å, b=3.160 Å and c=12.295 Å. The lattice parameters of PbS: a =5.936 Å, b=5.936 Å and c=5.936 Å), it is calculated that the lattice spacing of the MoS2 (100) and PbS (200) are about 0.273 nm and 0.297 nm, respectively. Therefore, the two different lattice fringes spacing (0.27 nm and 0.30 nm) belong to the crystal planes (100) of the MoS2 and (200) of PbS, respectively. The exposed crystal planes is the main factor to affect the gas sensing properties,

13, 25

and the materials with the high-energy exposed crystal planes usually exhibit

greater reactivity than those with low-energy exposed crystal planes. 26, 27 So, MoS2 with the exposed crystal plane (100) is more active than other crystal planes, 28 which are beneficial to gas molecular adsorption, and the PbS QDs can provide more active sites further resulting the improvement of MoS2/PbS gas sensing properties. To investigate the surface composition and chemical states of MoS2 and MoS2/PbS, the XPS spectra of them are shown in Figure 4. As can be seen from Figure 4(a), the XPS spectrum of pure MoS2 only contains the peaks of Mo, S, C and O, while the peaks of Pb can be found in that spectrum of MoS2/PbS besides the elements contained in MoS2. In the XPS spectra, C 1s peak (284.8 eV) is used to calibrate all the binding energy values. The high-resolution XPS spectra of Mo 3d, S 2p and O 1s are shown in Figure 4(b-d), respectively. As observed from Figure 4(b) and (c), the Mo 3d and S 2p peaks in MoS2/PbS shift to the higher binding energy compared with that of pure MoS2 due to the existence of the electron transfer between MoS2 and PbS QDs, 29


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indicating that there is an interaction between MoS2 and PbS QDs through the formation of MoS-Pb bond. 30

Figure 4. (a) XPS spectra of MoS2 and MoS2/PbS. High-resolution XPS spectra of (b) Mo 3d, (c) S 2p and (d) O 1s. In Figure 4(d), the peaks of O 1s can be found in the MoS2 and MoS2/PbS XPS spectra, which are caused by influence of adsorbed oxygen. Due to the asymmetry of the O 1s peak of MoS2, it can be fitted into two peaks at 532.2 eV and 528.2 eV which belong to the adsorbed oxygen and lattice oxygen respectively, and the proportion of lattice oxygen is about 8% in O 1s peak of MoS2. Because MoS2 is easily influenced by oxygen adsorption, 31, 32 this phenomenon can be


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explained that oxygen atoms occupy the vacancy on the MoS2 surface and a small part of MoS2 is oxidized by oxygen in air, 33 which may lead to instability of the MoS2 sensor. The O 1s peak of MoS2/PbS only can be fitted into one peak at 532.4 eV, which is assigned as the adsorbed oxygen, and the absence of lattice oxygen in MoS2/PbS means MoS2 is not oxidized in air after modification by PbS QDs. Moreover, the atomic ratios of Mo and S in MoS2 and MoS2/PbS are 1: 2.19 and 1: 2.06, respectively. The difference in atomic ratio of Mo and S between MoS2 and MoS2/PbS is ascribe to the fact that some Pb atoms occupy the Mo vacancies on the MoS2 surface and decrease the number of surface defects. Therefore, the modification of PbS QDs on MoS2 can prevent MoS2 from being oxidized and enhance the stability of MoS2.

Figure 5. (a) Dynamic response-recovery curves of MoS2 and MoS2/PbS gas sensors without heating device at different NO2 concentrations. The inset of (a) shows typical I-V curves of MoS2 and MoS2/PbS gas sensors in air and 100 ppm NO2. (b) Response versus NO2 concentrations curves of MoS2 and MoS2/PbS gas sensors. The inset of (b) shows linear fitting curve of MoS2/PbS gas sensor at 5-100 ppm NO2.


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In order to investigate the enhanced gas sensing properties of PbS QDs-modified MoS2 at room temperature, the dynamic response-recovery curves of MoS2 and MoS2/PbS gas sensors without heating device at different NO2 concentrations are shown in Figure 5(a). The pure MoS2 gas sensor exhibits the low responses for NO2 at all concentrations (5-400 ppm), and it shows different response behaviors with increase of NO2 concentrations. At low NO2 concentrations (550 ppm), the MoS2 gas sensor shows a negative response, while it exhibits positive response at high NO2 concentrations (100-400 ppm) as shown in Figure 5(a). After modifying the PbS QDs on MoS2, it can be seen that the MoS2/PbS gas sensor shows a positive response and the responses against NO2 are greatly improved. The gas sensing properties of pure PbS gas sensor are shown in Figure S1, and it shows a positive response to NO2. Although the PbS gas sensor exhibits a high response at low NO2 concentration, it shows a low sensitivity due to the similar response at each NO2 concentration. The response and recovery times of the PbS gas sensor are about 200 s and 1300 s, which are too long for NO2 detection. The responses of MoS2/PbS gas sensor are much higher than that of pure MoS2 at all NO2 concentrations, for instances, it is about 50 times higher than that of pure MoS2 gas sensor at 100 ppm NO2. The enhanced in responsivity of the MoS2/PbS gas sensor can be due to the synergistic effects which combine the high responsivity of PbS QDs with the large specific surface area of MoS2.

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Furthermore,

compared with the modification materials with large size, the modification with small size can form more heterojunctions and electronic transfer channels. 10 So, the modification via PbS QDs with small size can greatly provide abundant MoS2/PbS heterojunctions, namely, it can increase the numbers of electronic transfer channels between MoS2 and PbS, which is beneficial to further enhance the MoS2/PbS gas sensing properties. The inset of Figure 5(a) shows the typical I-V curves of MoS2 and MoS2/PbS gas sensors in air and 100 ppm NO2, respectively. It can be


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observed that the currents of all gas sensors exposed to air and NO2 are increased linearly with the increasing applied voltages, meaning that the good ohmic contacts are formed between the sensing layer and electrodes, and the upcoming gas sensing behaviors are caused by the sensing material. 35 The response versus NO2 concentrations curves of MoS2 and MoS2/PbS gas sensors are shown in Figure 5(b). It can be clearly seen that the responsivity and sensitivity of MoS2 gas sensor against NO2 are all small, which are obviously less than that of MoS2/PbS gas sensor. To illustrate the reliability of the data, the data are repeatedly measured many times under the same conditions, and the error bars of responses for MoS2 and MoS2/PbS gas sensors are calculated by the standard deviation formula (

).

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The largest standard deviation of the

response is 0.55 for MoS2/PbS at 20 ppm, indicating the data of responses are reliability in all concentrations. The curve of MoS2/PbS gas sensors rises dramatically at 5-100 ppm while rises slowly beyond 100 ppm. It indicates that the MoS2/PbS gas sensors have the best sensitivity against NO2 at the range of 5-100 ppm for the whole test range. Based on the least square method, the inset of Figure 5(b) shows the linear fitting curve of MoS2/PbS gas sensor at 5-100 ppm NO2, which can be represented as Y = 0.15431X + 7.47079, R2 = 0.93151, where Y is response, X is NO2 concentration, R2 is the correlation coefficient. The value of R2 is 0.93151 (> 0.9), indicating the MoS2/PbS gas sensor is of good linearity at 5-100 ppm NO2. Moreover, the response curves of MoS2/PbS gas sensor to 100 ppm NO2 under different humidity condition are given as shown in Figure S2. Compared with the slight variation at low relative humidity (RH), the response of MoS2/PbS gas sensor shows obvious change at high RH. The MoS2/PbS gas sensor is more or less affected by humidity in NO2 gas detection, especially at high RH. This is because the interaction between water vapors and pre-adsorbed oxygen will decrease the resistance variation, 37 and this issue is being studied by many researchers.


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Figure 6. Schematic diagrams of the possible gas sensing mechanism for (a) MoS2 and (b) MoS2/PbS gas sensors. The schematic diagrams of the possible gas sensing mechanism for MoS2 and MoS2/PbS gas sensors are shown in Figure 6. From Figure 6(a), when the MoS2 gas sensor is exposed to NO2, NO2 as main electron acceptor will continuously capture the electrons from MoS 2 surface because of its even higher electron affinity (220 kJ mol-1) compared to oxygen (42 kJ mol-1) .38 According to the reaction：NO2 + e- → NO2−, at the low concentration of NO2 (5-50 ppm), there are enough electrons to be captured by NO2, resulting in the increase of resistance, so the MoS2 gas sensor shows the negative response at low NO2 concentration. When the NO2 concentration increases to the high concentration range (100-400 ppm), NO2 will trap all the electrons from


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MoS2 and a depletion layer is formed at the MoS2 surface. Moreover, because the adsorption oxygen species is in the form of O2− at the temperatures less than 150°C,

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when the MoS2 gas

sensor is exposed to the air at room temperature, the adsorbed oxygen on the surface of MoS 2 captures the electrons from MoS2 in the form of O2−. Another possible cause for the n-p transition is the electron capturing interaction between NO2 and negatively charged oxygen, as illustrated by the following reactions: 40, 41 NO2 (g) + O2(ads) − + e− → 2 NO3(ads)−. Then, the everincreasing NO2 continuously traps the electrons from MoS2 due to its high electron affinity, and the electron capturing interaction between NO2 and negatively charged oxygen can simultaneously reduce the number of electrons in the MoS2, resulting the hole concentration is higher than electron concentration in MoS2. Therefore, the conduction of MoS2 is dominated by holes at this moment. When the NO2 continually capture the electron from MoS2 with the NO2 concentration increasing, the hole concentration will increase and the corresponding resistance will decrease, so that the MoS2 gas sensor exhibits the change from the negative response to the positive response at high concentration of NO2 (100-400 ppm). From Figure 6(b), the enhanced performance caused by PbS QDs modification could be attributed to the following two factors. Firstly, the response against target gas is generally a surface controlled process, 42 and the gas sensing mechanism can be explained by the target gas chemisorption and desorption on the materials surface. 43, 44 According to the XPS results, the electrons can transfer from MoS2 to PbS, which indicates more electrons can be captured by NO2 molecules reacted on PbS surface, and the electronic transmission behavior between MoS 2 and PbS can improve the target gas chemisorption on the MoS2/PbS to a certain extent, resulting a great change in the resistance of the gas sensors from air to NO2. Therefore, the formation of heterojunction between MoS2 and PbS can enhance the MoS2/PbS gas sensor response. Secondly,


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due to the small size of PbS QDs combining with the large specific surface area of fluffy balllike MoS2, there are many heterojunctions formed in the MoS2/PbS composites, namely, many electronic transfer channels are formed on the MoS2/PbS composites, further enhancing the adsorption active site against target gases. Hence, more electrons can transfer from MoS2 to PbS to further improve the MoS2/PbS gas sensing properties. Moreover, charge transfer between other material and MoS2 (functionalizing the MoS2 surface) can cause P-type behavior of MoS2. 45, 46

In our work, many PbS QDs modified on the MoS2 lead to more electronic transmission

from MoS2 to PbS, which may cause the electron depletion in MoS2. When the MoS2/PbS composites are exposed to NO2, NO2 as the electron acceptor can further consume the electrons from MoS2/PbS, resulting in the increase of hole concentration. Therefore, the response behavior of the MoS2/PbS composites is the same as that of PbS QDs (Figure S1) which exhibits positive response behavior in detecting NO2 gas, and the gas sensor based on MoS2/PbS composites shows the resistance decrease upon exposure to NO2. Figure 7(a) shows transient response characteristic of MoS2/PbS gas sensor at 100 ppm NO2. The resistance value of MoS2/PbS gas sensor is stable at 75 MΩ in air, and then the resistance decreased with the exposure of the MoS2/PbS gas sensor to 100 ppm NO2. The resistance subsequently arrived to the initial value when the MoS2/PbS gas sensor switch from NO2 to air, and the response and recovery times of MoS2/PbS are about 30 s and 235 s. As can be observed from the inset of Figure 7(a), with the MoS2 gas sensor switching between air and 100 ppm NO2, the resistance decreases from the stable original value to a low resistance value and then turn back to about half of the original value subsequently, which the corresponding response and recovery times are about 50 s and 47 s. Here, the recovery time of pure MoS2 gas sensor is shorter than that of MoS2/PbS gas sensor, but it should be noted that the resistance of pure MoS2


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gas sensor does not fully recover to the original value, only recovers to 50% of the original value. From Figure S3, it can be seen that the recovery time of pure MoS2 gas sensor is shorter than that of MoS2/PbS gas sensor at each NO2 concentration, which is caused by incomplete recovery behavior of pure MoS2. In order to compare the recovery behavior between MoS2 and MoS2/PbS, the recovery ratio is calculated as follows. 47 Reco er ratio

=

Rg Rr Rg Ra

100

Here, Ra and Rg are the resistances of the gas sensor before and after exposing to the target gas, and Rr is the resistance of the gas sensor exposed to air for a given recovery time. The recovery ratios of MoS2 and MoS2/PbS gas sensors at different NO2 concentration are given in Table S1. It shows that the recovery ratios of MoS2/PbS gas sensors are much better than that of the pure MoS2 gas sensor, especially at high NO2 concentrations, and this is mainly caused by the modification of PbS QDs. Generally, the NO2 and O2 are physically absorbed onto the MoS2 surface in theoretical aspect, which means these gas molecules can easily fast desorb from the MoS2 surface. 48, 49 However, according to the XPS results mentioned above, many defects may exist on the MoS2 surface, which means it can lead to a strong chemisorption between MoS2 and gas molecules, so that the NO2 and O2 are difficult to desorb from the MoS2 gas sensor.48 Moreover, the oxidation of MoS2 confirmed by the XPS results might lead to instability of the MoS2, so that the MoS2 gas sensor does not fully recover to the original value.

5

Through

modifying the PbS QDs on the MoS2, Pb atoms can fill the vacancy on the MoS2 surface, which will weaken the gas molecules chemisorption caused by the existence of defects on MoS2. Furthermore, NO2 and O2 may be more easily adsorbed on the PbS surface rather than MoS2, which is the so-called competitive adsorption of gas by materials. So the abundant PbS QDs modified on the MoS2 surface will effectively hinder the adsorption of gas molecular (such as O2)


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on the MoS2 to make MoS2 difficult to oxidize. Additionally, the response and recovery curves of MoS2 and MoS2/PbS gas sensors between air and 100 ppm NO2 for three cycles are measured to evaluate the reproducibility as shown in Figure S4. Among the three cycles, MoS2/PbS gas sensor has better reproducibility than MoS2 gas sensor which is quite poor reproducibility. To verify the stability difference between MoS2 and MoS2/PbS gas sensors, the long-term stabilities of MoS2 and MoS2/PbS gas sensors are measured at room temperature as shown in Figure S5. It can be seen that the MoS2 gas sensor shows a large resistance variation in air. On the contrary, the MoS2/PbS gas sensor exhibits a relatively stable resistance in the same conditions, which has better stability than MoS2 gas sensor. The inset of Figure S5 shows the response of MoS2 and MoS2/PbS gas sensors to 100 ppm NO2 in the long-term measurement, which exhibits the longterm stability of MoS2/PbS gas sensor is better than that of MoS2 gas sensor. In summary, we can conclude that the modification of PbS QDs on MoS2 can not only improve the surface defects of MoS2, but also effectively prevent the oxidation of MoS2, further greatly enhancing the stability and recovery behavior of gas sensor. The selectivity of the MoS2 and MoS2/PbS gas sensors to detect 100 ppm different gases are shown in Figure 7(b). Compared with the pure MoS2 gas sensors, the MoS2/PbS gas sensors show better selectivity for NO2 at room temperature, and it illustrates that the modification of PbS QDs is main reason for improving the high response and selectivity of MoS2 against NO2. In addition, the pure MoS2 gas sensors exhibit better response against NH3 than MoS2/PbS, and this phenomenon is due to the fact that the PbS QDs covered on the surface of MoS2 will hinder the direct adsorption of NH3 on the MoS2 surface. Therefore, the modification of PbS QDs improves the gas sensing properties of MoS2 against NO2.


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Figure 7. (a) Transient response characteristic of MoS2/PbS gas sensor at 100 ppm NO2. (b) Selectivity of the MoS2 and MoS2/PbS gas sensors for 100 ppm different gases To further investigate the mechanical of the enhanced performance for PbS QDs covered MoS2, the adsorption energies and the diffusion characters of NO2 are calculated. All calculations are performed based on the density-functional theory as implemented in the Vienna ab-initio simulation package (VASP). 50, 51 The exchange-correlation potential is treated with the PerdewBurke-Eznerh of generalized-gradient approximation (PBE-GGA).

52

We used the projector

augmented wave (PAW) method for the description of the electron–ion interaction. 53, 54 A slab model with 3 × 3 supercell for MoS2 and 4 × 4 supercell for PbS are used in our calculations. A 6 6 Γ-centered k-point mesh produced by Monkhost-Pack method was used to sample the Brillouin zone of the supercell and the cutoff energy for the plane wave is set to be 500 Ev. 55 For geometry optimization, all the internal coordinates are fully relaxed until the Hellmann-Feynman forces are less than 0.01eV/Å. Table 1. Adsorption energies of NO2 on MoS2 and PbS


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Substrate

MoS2

Configuration

Ead (in meV)

Mo-top

-26

hollow

-31

S-top

-38

Pb-top(downward)

-846

hollow(downward) -797 PbS

hollow(upward)

-699

Pb-top(upward)

-596

S-top

-341

The adsorption energies for NO2 on MoS2 and PbS are given in Table 1. The adsorption energy (Ead) is defined as Ead = E(NO2/sub)-E(sub)-ENO2, where sub indicate the substrate MoS2 or PbS. The E(NO2/sub), E(sub), ENO2 refer to the total energies for NO2 adsorbed on MoS2 and PbS, the prefect MoS2 or PbS and the isolated NO2 molecule, respectively. From the Table 1, one can find that adsorption energies is -26, -31 and -38 meV for the NO2 adsorbed on the Mo-top, hollow and Stop site of MoS2. The low adsorption energy indicates that the NO2 molecule is physical adsorbed on MoS2 surface. Differently, the calculated adsorption energies for NO2 adsorbed at the Pb-top, hollow and S-top site of PbS are -846 (-596), -797 (-699) and -341 meV, indicating that the NO2 adsorption on PbS is chemisorption. Obviously, the adsorption energy for NO2 on the PbS is quite larger than that on the MoS2. 56 To further understand the responsibility of PbS QDs modified on MoS2 as gas sensors, the diffusion paths and their barriers are also calculated. Our calculations show that the diffusion barrier is only dozens of milli-electron volt for NO2 on MoS2. It is revealed that NO2 gas molecules adsorbed on MoS2 may easily diffuse at a high rate of speed on MoS2 surface in addition to desorption. There are two patterns for NO2 adsorbed on PbS. One is “upward-Vshape” with N atom linked to the PbS surface and the other is “downward-V-shape” with the O


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atom linked to the PbS surface as the atomic arrangement shown in Figures 8. The transition barrier for “upward-V-shape” to the “downward-V-shape” is about 220 meV, and it means that NO2 is difficult to transform between two patterns at room temperature. The most stable adsorption site is NO2 at the top site of Pb with “downward-V-shape” as shown Figure 8(a), and the adsorption energy is -846 meV. For NO2 with the “downward-V-shape” adsorbed on the PbS, the diffusion barriers is about 30 meV. For the upward-V-shape adsorption pattern, the NO2 prefers to occupy the hollow site of the PbS with adsorption energy -699 meV as shown in Figure 8(c). One can find that the diffusion barrier is about 100 meV from Figure 8(f). Although NO2 gas molecules adsorbed on the PbS surface are difficult to diffuse at room temperature in a “upward-V-shape to downward-V-shape” pattern, they can easily diffuse in single “upward-Vshape” pattern or “downward-V-shape” pattern. Our calculations clearly demonstrated that the NO2 can freely diffusion on the MoS2 and PbS. It means that the adsorbed NO2 on MoS2 surface can be easily captured by PbS due to the freely diffusion of NO2 on MoS2 surface and the large adsorption energy of NO2 on the PbS. Therefore, as the MoS2/PbS composites are exposed to NO2, NO2 is more prone to capture the electrons in the form of chemisorption from PbS surface instead of MoS2 surface. That is to say, after modifying the PbS QDs on MoS2, there is a competitive adsorption of NO2 between MoS2 and PbS which will lead to more NO2 molecules adsorbed on the PbS surface of the MoS2/PbS composites instead of MoS2, further enhancing the responses and stabilities of MoS2/PbS gas sensors.


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Figure 8. (a-b) “upward-V-shape” with N atom and (c-e) “downward-V-shape” with the O atom linked to the PbS surface. (f) Diffusion barriers of NO2 on PbS surface between different adsorption pattern 4. Conclusions In summary, we have successfully synthesized the MoS2 and MoS2/PbS composite by hydrothermal method combing with chemical precipitation, and the gas sensing properties of them at the whole test range are compared to investigate the enhanced performance caused by the modification of PbS QDs. The results show that the responsivity of MoS2/PbS gas sensor is


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obviously enhanced compared with pure MoS2 gas sensor throughout the whole test range, and the resistance of MoS2/PbS gas sensor can fully recover to the original value at room temperature, indicating the modification of PbS QDs can effectively enhance the responsivity and recovery behavior to NO2 gas of pure MoS2 gas sensor. The enhanced performances of MoS2/PbS gas sensor could be attributed to the following two factors. On the one hand, the formation of many heterojunctions can provide many electron transfer channels to make more electrons transfer between MoS2 and PbS, further enhancing the response of MoS2 gas sensor. On the other hand, the simulation analysis of gas adsorption based on the DFT is used to confirm that the competitive adsorption of MoS2/PbS against target gas is the main reason to prevent MoS2 from being oxidized and enhance the stability of MoS2. ASSOCIATED CONTENT Supporting Information Dynamic response-recovery curve of PbS gas sensor at different NO2 concentration, MoS2/PbS gas senor under different RH conditions. Recovery times, recovery ratios and long-term stabilities of MoS2 and MoS2/PbS gas sensors. These files are available free of charge on the Web at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] (Y. Zhang). * E-mail address: [email protected] (J. Cao) Funding Sources


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This work is supported by National Natural Science Foundation of China (51502255, 11474245 and 11772285) and Hunan Provincial Natural Science Foundation of China (2018JJ2404). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (51502255, 11474245 and 11772285) and Hunan Provincial Natural Science Foundation of China (2018JJ2404). REFERENCES (1) Cho, S. Y.; Kim, S. J.; Lee, Y.; Kim, J. S.; Jung, W. B.; Yoo, H. W.; Kim, J.; Jung, H. T. Highly Enhanced Gas Adsorption Properties in Vertically Aligned MoS2 Layers. ACS nano 2015, 9, 9314-9321. (2) Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, P. M.; Jernigan, G. G.; Jonker, B.T. Chemical Vapor Sensing with Monolayer MoS2. Nano Letters 2013, 13, 668-673. (3) Xu, M. S.; Liang, T.; Shi, M. M.; Chen, H. Z. Graphene-like Two-dimensional Materials. Chem. Rev. 2013, 113, 3766-3798. (4) Liu, X. H.; Ma, T. T.; Pinna, N.; Zhang, J. Two-dimensional Nanostructured Materials for Gas Sensing. Adv. Funct. Mater. 2017, 27, 1702168.


25


Page 26 of 42

(5) Cui, S. M.; Wen, Z. H.; Huang, X. K.; Chang, J. B.; Chen, J. H. Stabilizing MoS2 Nanosheets through SnO2 Nanocrystal Decoration for High-performance Gas Sensing in Air. Small 2015, 11, 2305-2313 (6) Zhang, J.; Liu, X.; Neri, G.; Pinna, N. Nanostructured Materials for Room-Temperature Gas Sensors. Adv. Mater. 2016, 28, 795-831. (7) Joshi, N.; Hayasaka, T.; Liu, Y.; Liu, H.; Oliveira, O. N.; Lin, L. A Review on Chemiresistive Room Temperature Gas Sensors Based on Metal Oxide Nanostructures, Graphene and 2D Transition Metal Dichalcogenides. Microchimica Acta. 2018, 185, 213. (8) Fu, D.; Zhu, C.; Zhang, X.; Li, C.; Chen, Y. Two-dimensional Net-like SnO2/ZnO Heteronanostructures for High-performance H2S Gas Sensor. J. Mater. Chem. A 2016. 4, 1390-1398. (9) Long, H.; Harley‐Trochimczyk, A.; Pham, T.; Tang, Z.R.; Shi, T. L.; Zettl, A.; Carraro, C.; Worsley, M.A.; Maboudian, R. High Surface Area MoS2/Graphene Hybrid Aerogel for Ultrasensitive NO2 Detection. Adv. Funct. Mater. 2016, 26, 5158-5165. (10) Xu, S.; Gao, J.; Wang, L.; Kan, K.; Xie, Y.; Shen, P.; Li, L.; Shi, K. Role of the Heterojunctions in In2O3-composite SnO2 Nanorod Sensors and their Remarkable GasSensing Performance for NOx at Room Temperature. Nanoscale 2015, 7, 14643-14651. (11) Katoch, A.; Choi, S. W.; Kim, J. H.; Lee, J. H.; Lee, J. S.; Kim, S. S. Importance of the Nanograin Size on the H2S-sensing Properties of ZnO-CuO Composite Nanofibers. Sens. Actuators B 2015, 214, 111-116.


26

Page 27 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Liu, H.; Li, M.; Voznyy, O.; Hu, L.; Fu, Q.; Zhou, D.; Xia, Z.; Sargent. E. H.; Tang, J. Physically flexible, Rapid-response Gas Sensor based on Colloidal Quantum Dot Solids. Adv. Mater. 2014, 26, 2718-2724. (13) Gao, X.; Zhang, T. An overview: Facet-dependent Metal Oxide Semiconductor Gas Sensors. Sens. Actuators B 2018, 277, 604-633. (14) Zhang, B.; Li, M.; Song, Z.; Kan, H.; Yu, H.; Liu, Q.; Zhang, G.; Liu, H. Sensitive H2S Gas Sensors Employing Colloidal Zinc Oxide Quantum Dots. Sens. Actuators B 2017, 249, 558-563. (15) Sun, L.; Luan, W. L.; Wang, T. C.; Su, W. X.; Zhang, L. X. Room-Temperature CO Thermoelectric Gas Sensor based on Au/Co3O4 Catalyst Tablet. Nanotechnology 2017, 28, 075501. (16) Rigoni, F.; Maiti, R.; Baratto, C.; Donarelli, M.; MacLeod, J.; Gupta, B.; Lyu, M.; Ponzoni, A.; Sberveglieri, G.; Motta, N.; Faglia, G. Transfer of CVD-grown Graphene for Room Temperature Gas Sensors. Nanotechnology 2017, 28, 414001. (17) Bai, S.; Tian, Y.; Cui, M.; Sun, J.; Tian, Y.; Luo, R.; Chen, A.; Li, D. Polyaniline@SnO 2 Heterojunction loading on Flexible PET Thin Film for Detection of NH3 at Room Temperature. Sens. Actuators B 2016, 226, 540-547 (18) Huang, Z.; Qi, Y.; Yu, D.; Zhan, J. Radar-like MoS2 Nanoparticles as A Highly Efficient 808 nm Laser-induced Photothermal Agent for Cancer Therapy. RSC Adv. 2016, 6, 31031-31036.


27


Page 28 of 42

(19) Kaniyoor, A.; Jafri, R. I.; Arockiadoss, T.; Ramaprabhu, S. Nanostructured Pt Decorated Graphene and Multi Walled Carbon Nanotube based Room Temperature Hydrogen Gas Sensor. Nanoscale 2009, 1, 382-386. (20) Zhang, D.; Wu, J.; Li, P.; Cao, Y. Room-temperature SO2 Gas-sensing Properties based on a Metal-doped MoS2 Nanoflower: an Experimental and Density Functional Theory Investigation. J. Mater. Chem. A 2017, 5, 20666-20677. (21) Singh, E.; Meyyappan, M.; Nalwa, H. S. Flexible Graphene-based Wearable Gas and Chemical Sensors. ACS Appl. Mater. Interfaces 2017, 9, 34544-34586. (22) Chen, R.; Wang, J.; Xia, Y.; Xiang, L. Near Infrared Light Enhanced Room-temperature NO2 Gas Sensing by Hierarchical ZnO Nanorods Functionalized with PbS Quantum Dots. Sens. Actuators B 2018, 255, 2538-2545. (23) Tang, W.; Wang, J.; Yao, P.; Li, X. Hollow Hierarchical SnO2-ZnO Composite Nanofibers with Heterostructure based on Electrospinning Method for Detecting Methanol. Sens. Actuators B 2014, 192, 543-549. (24) Zhuang, L.; Guo, L.; Chou, S. Y. Silicon Single-electron Quantum-dot Transistor Switch Operating at Room Temperature. Appl. Phys. Lett. 1998, 72, 1205-1207. (25) Shang, Y.; Guo, L. Facet-Controlled Synthetic Strategy of Cu2O-Based Crystals for Catalysis and Sensing. Adv. Sci. 2015, 2, 1500140. (26) Kuang, Q.; Wang, X.; Jiang, Z.; Xie, Z.; Zheng, L. High-energy-surface Engineered Metal Oxide Micro- and Nanocrystallites and their Applications. Accounts chem. Res. 2013, 47, 308-318.


28

Page 29 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Gurlo, A. Nanosensors: Towards Morphological Control of Gas Sensing Activity. SnO2, In2O3, ZnO and WO3 Case Studies. Nanoscale 2011, 3, 154-165. (28) Liu, Y.; Zhu, Y.; Fan, X.; Wang, S.; Li, Y.; Zhang, F.; Zhang, G.; Peng, W. (0D/3D) MoS2 on Porous Graphene as Catalysts for Enhanced Electrochemical Hydrogen Evolution. Carbon 2017, 121, 163-169. (29) Wang, D. H.; Pan, J. N.; Li, H. H.; Liu, J. J.; Wang, Y. B.; Kang, L. T.; Yao, J. N. A Pure Organic Heterostructure of μ-oxo Dimeric Iron (III) Porphyrin and Graphitic-C3N4 for Solar H2 Roduction from Water. J. Mater. Chem. A 2016, 4, 290-296. (30) Wang, D. H.; Jia, L.; Wu, X. L.; Lu, L. Q.; Xu, A. W. One-step Hydrothermal Synthesis of N-doped TiO2/C Nanocomposites with High Visible Light Photocatalytic Activity. Nanoscale 2012, 4, 576-584. (31) Park, W.; Park, J.; Jang, J.; Lee, H.; Jeong, H.; Cho, K.; Hong, S.; Lee, T. Oxygen Environmental and Passivation Effects on Molybdenum Disulfide Field Effect Transistors. Nanotechnology 2013, 24, 095202. (32) Wang, Z.; von dem Bussche, A.; Qiu, Y.; Valentin, T. M.; Gion, K.; Kane, A. B.; Hurt, R. H. Chemical Dissolution Pathways of MoS2 Nanosheets in Biological and Environmental Media. Environ. Sci. Technol. 2016, 50, 7208-7217. (33) Fan, S.; Shen, W.; An, C.; Sun, Z.; Wu, S.; Xu, L.; Sun, D.; Hu, X.; Zhang, D.; Liu, J. Implementing Lateral MoSe2 P-N Homojunction by Efficient Carrier-Type Modulation. ACS Appl. Mater. Interfaces 2018, 10, 26533-26538.


29


Page 30 of 42

(34) Ng, S.; Kuberský, P.; Krbal, M.; Prikryl, J.; Gärtnerová, V.; Moravcová, D.; Sopha, H.; Zazpe, R.; Yam, F. K.; Jäger, A.; Hromádko, L.; Beneš, L.; Hamáček. A.; Macak, J. M. ZnO Coated Anodic 1D TiO2 Nanotube Layers: Efficient Photo-Electrochemical and Gas Sensing Heterojunction. Adv. Eng. Mater. 2018, 20, 1700589. (35) Hu, Y.; Zhou, J.; Yeh, P. H.; Li, Z.; Wei, T. Y.; Wang, Z. L. Supersensitive, FastResponse Nanowire Sensors by Using Schottky Contacts. Adv. Mater. 2010, 22, 33273332. (36) Zhang, H. G.; Han, X. J.; Yao, B. F.; Li, G. X. Study on the Effect of Engine Operation Parameters on Cyclic Combustion Variations and Correlation Coefficient between the Pressure-related Parameters of a CNG Engine. Appl. Energy 2013, 104, 992-1002. (37) Gao, H.; Zhao, L.; Wang, L.; Sun, P.; Lu, H.; Liu, F.; Chuai, X.; Lu, G. Ultrasensitive and Low Detection Limit of Toluene Gas Sensor based on SnO2-Decorated NiO Nanostructure. Sens. Actuators B 2018, 255, 3505-3515. (38) Gu, F.; Nie, R.; Han, D.; Wang, Z. In2O3-graphene Nanocomposite based Gas Sensor for Selective Detection of NO2 at Room Temperature. Sens. Actuators B 2015, 219, 94-99. (39) Yamazoe, N.; Fuchigami, J.; Kishikawa, M.; Seiyama, T. Interactions of Tin Oxide Surface with O2, H2O and H2. Surf. Sci. 1979, 86, 335-344. (40) Wetchakun, K.; Samerjai, T.; Tamaekong, N.; Liewhiran, C.; Siriwong, C.; Kruefu, V.; Wisitsoraat, A.; Tuantranont, A.; Phanichphant, S. Semiconducting Metal Oxides as Sensors for Environmentally Hazardous Gases. Sens. Actuators B 2011, 160, 580-591.


30

Page 31 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Xia, Y.; Wang, J.; Xu, J. L.; Li, X.; Xie, D.; Xiang, L.; Komarneni, S. Confined Formation of Ultrathin ZnO Nanorods/Reduced Graphene Oxide Mesoporous Nanocomposites for High-Performance Room-Temperature NO2 Sensors. ACS Appl. Mater. Interfaces 2016, 8, 35454-35463. (42) Xu, J.; Pan, Q.; Tian, Z. Grain Size Control and Gas Sensing Properties of ZnO Gas Sensor. Sens. Actuators B 2000, 66, 277-279. (43) Huang, X. J.; Choi, Y. K. Chemical Sensors based on Nanostructured Materials. Sens. Actuators B 2007, 122, 659-671. (44) Brynzari, V.; Korotchenkov, G.; Dmitriev, S. Simulation of Thin Film Gas Sensors Kinetics. Sens. Actuators B 1999, 61, 143-153. (45) Min, S. W.; Yoon, M.; Yang, S. J.; Ko, K. R.; Im, S. Charge-Transfer-Induced p-Type Channel in MoS2 Flake Field Effect Transistors. ACS Appl. Mater. Interfaces 2018, 10, 4206-4212. (46) Agrawal, A. V.; Kumar, R.; Venkatesan, S.; Zakhidov, A.; Yang, G.; Bao, J.; Kumar, M.; Kumar, M. Photoactivated Mixed In-Plane and Edge-Enriched p-Type MoS2 Flake-Based NO2 Sensor Working at Room Temperature. ACS sensors 2018, 3, 998-1004. (47) Ko, K. Y.; Song, J. G.; Kim, Y.; Choi, T.; Shin, S.; Lee, C. W.; Lee, K.; Koo, J.; Lee, H.; Kim, J.; Lee, T.; Park, J.; Kim, H. Improvement of Gas-Sensing Performance of LargeArea Tungsten Disulfide Nanosheets by Surface Functionalization. ACS nano 2016, 10, 9287-9296.


31


Page 32 of 42

(48) Nan, H.; Wang, Z.; Wang, W.; Liang, Z.; Lu, Y.; Chen, Q.; He, D.; Tan, P.; Miao, F.; Wang, X.; Wang, J.; Ni, Z. Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding. ACS nano 2014, 8, 5738-5745. (49) Lee, E.; Yoon, Y. S.; Kim, D. J. Two-Dimensional Transition Metal Dichalcogenides and Metal Oxide Hybrids for Gas Sensing. ACS sensors 2018, 3, 2045-2060. (50) Kresse, G.; Furthmüller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors using a Plane-wave Basis Set. Comp. Mater. Sci. 1996, 6, 15-50. (51) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab initio Total-energy Calculations using a Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169. (52) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (53) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. B 1994, 50, 17953. (54) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B 1999, 59, 1758. (55) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-zone Integrations. Phys. Rev. B 1976, 13, 5188. (56) Zhang, X.; Yu, L.; Wu, X.; Hu, W. Experimental Sensing and Density Functional Theory Study of H2S and SOF2 Adsorption on Au-Modified Graphene. Adv. Sci. 2015, 2, 1500101.


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Abstract Graphic Enhanced performances of PbS quantum dots modified MoS2 composite for NO2 detection at room temperature


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Figure 1. Detailed preparation processes of (a) pure MoS2 and (b) MoS2/PbS composites. (c) Fabricated technological process diagram of gas sensor. 208x208mm (300 x 300 DPI)


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Figure 2. (a) XRD patterns of MoS2 and MoS2/PbS. (b) FE-SEM of pure MoS2 and (c) MoS2/PbS composites. SEM mapping of (d) Mo, (e) S and (f) Pb in MoS2/PbS composites. The inset of (d) shows the area of SEM mapping. 189x115mm (300 x 300 DPI)



Figure 3. (a) TEM image and (b) high magnification image of MoS2. (c) TEM and (d) HRTEM images of MoS2/PbS composites. 173x173mm (300 x 300 DPI)


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Figure 4. (a) XPS spectra of MoS2 and MoS2/PbS. High-resolution XPS spectra of (b) Mo 3d, (c) S 2p and (d) O 1s. 223x212mm (300 x 300 DPI)



Figure 5. (a) Dynamic response-recovery curves of MoS2 and MoS2/PbS gas sensors without heating device at different NO2 concentrations. The inset of (a) shows typical I-V curves of MoS2 and MoS2/PbS gas sensors in air and 100 ppm NO2. (b) Response versus NO2 concentrations curves of MoS2 and MoS2/PbS gas sensors. The inset of (b) shows linear fitting curve of MoS2/PbS gas sensor at 5-100 ppm NO2. 223x106mm (300 x 300 DPI)


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Figure 6. Schematic diagrams of the possible gas sensing mechanism for (a) MoS2 and (b) MoS2/PbS gas sensors. 188x140mm (300 x 300 DPI)



Figure 7. (a) Transient response characteristic of MoS2/PbS gas sensor at 100 ppm NO2. (b) Selectivity of the MoS2 and MoS2/PbS gas sensors for 100 ppm different gases 192x85mm (300 x 300 DPI)


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Figure 8. (a-b) “upward-V-shape” with N atom and (c-e) “downward-V-shape” with the O atom linked to the PbS surface. (f) Diffusion barriers of NO2 on PbS surface between different adsorption pattern 212x175mm (300 x 300 DPI)



Enhanced performances of PbS quantum dots modified MoS2 composite for NO2 detection at room temperature 148x89mm (300 x 300 DPI)


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Enhanced performances of PbS quantum dots modified MoS2

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