Design of Hetero-Nanostructures on MoS2 Nanosheets To Boost NO2

Design of Hetero-Nanostructures on MoS2 Nanosheets To Boost NO2 Room-Temperature Sensing ... Publication Date (Web): June 13, 2018 ... To the best of ...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

Design of Hetero-Nanostructures on MoS2 Nanosheets To Boost NO2 Room-Temperature Sensing Yutong Han, Da Huang, Yujie Ma, Guili He, Jun Hu, Jing Zhang, Nantao Hu, Yanjie Su, Zhihua Zhou, Yafei Zhang, and Zhi Yang* Key Laboratory of Thin Film and Microfabrication (Ministry of Education), Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China

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ABSTRACT: Molybdenum disulfide (MoS2), as a promising gas-sensing material, has gained intense interest because of its large surface-to-volume ratio, air stability, and various active sites for functionalization. However, MoS2-based gas sensors still suffer from low sensitivity, slow response, and weak recovery at room temperature, especially for NO2. Fabrication of heterostructures may be an effective way to modulate the intrinsic electronic properties of MoS2 nanosheets (NSs), thereby achieving high sensitivity and excellent recovery properties. In this work, we design a novel p−n hetero-nanostructure on MoS2 NSs using interface engineering via a simple wet chemical method. After surface modification with zinc oxide nanoparticles (ZnO NPs), the MoS2/ZnO hetero-nanostructure is endowed with an excellent response (5 ppm nitrogen dioxide, 3050%), which is 11 times greater than that of pure MoS2 NSs. To the best of our knowledge, such a response value is much higher than the response values reported for MoS2 gas sensors. Moreover, the fabricated hetero-nanostructure also improves recoverability to more than 90%, which is rare for roomtemperature gas sensors. Our optimal sensor also possesses the characteristics of an ultrafast response time of 40 s, a reliable long-term stability within 10 weeks, an excellent selectivity, and a low detection concentration of 50 ppb. The enhanced sensing performances of the MoS2/ZnO hetero-nanostructure can be ascribed to unique 2D/0D hetero-nanostructures, synergistic effects, and p−n heterojunctions between ZnO NPs and MoS2 NSs. Such achievements of MoS2/ZnO hetero-nanostructure sensors imply that it is possible to use this novel nanostructure in ultrasensitive sensor applications. KEYWORDS: molybdenum disulfide nanosheets, zinc oxide nanoparticles, hetero-nanostructure, p−n heterojunctions, nitrogen dioxide gas sensors

1. INTRODUCTION Chemiresistive sensing devices have been widely studied because of their characteristics of miniaturization, ease of fabrication, simple operation, and low production cost. Among various sensitive materials, two-dimensional (2D) materials with excellent properties show exciting prospects for use in chemical sensor devices.1−3 First, the large surface-to-volume ratio and abundant active sites can provide active adsorption sites for gas molecules.4 Second, the electronic properties of 2D materials can be tuned by changing the number of their layers.3 Most importantly, 2D material-based gas sensors can work at room temperature, and this is very appealing because of their low power consumption, simplified fabrication processes, and reduced operational costs.5 Nitrogen dioxide (NO2) has been recognized as an air pollutant that may bring a variety of environmental effects and threaten the human health.6 Detecting low concentrations of NO2 in air is essential for ensuring air quality and our health. Many 2D materials such as graphene,7 reduced graphene oxide,8 transition-metal dichalcogenides (TMDs),9−13 SnS2,14 and black phosphorus15,16 have been explored for use as sensitive materials in chemiresistors to detect NO2. Molybde© 2018 American Chemical Society

num disulfide (MoS2) is a typical TMD material and has attracted intense attention because of its air stability, high-yield preparation process, and various active sites (sulfur defects, vacancies, and edge sites) for functionalization.17,18 Previously reported MoS2 gas sensors have mostly been focused on field effect transistors (FETs), which are expensive and the fabrication of which is elaborate. Low sensitivity, slow response, and weak recovery at room temperature are also obstacles of MoS2 gas sensors.9,11,19,20 Many methods have been exploited to improve the sensing performances, such as exposing the edge sites of MoS2,21 ligand conjugation,22 chemical doping,23 post annealing,24 and cooperating with noble metals such as Pt,25 Au,26 and other nanomaterials (graphene,27 carbon dots,28 SnO229,30 and Co3O431). However, auxiliary methods such as heating, lighting, or adding bias voltage are still required for better sensing properties. Consequently, there is a great deal of room for improving the high sensing performance of MoS2-based gas sensors, Received: April 11, 2018 Accepted: June 13, 2018 Published: June 13, 2018 22640

DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

Research Article

ACS Applied Materials & Interfaces especially for response time and recoverability at room temperature. Fabrication of p−n heterostructures is an effective way to modulate the intrinsic electronic properties of MoS2 nanosheets (NSs), thereby achieving high sensitivity and excellent recovery properties. As for many p−n hetero-nanostructures of metal oxide semiconductors, the synergistic effects, spontaneous electron transfer, and formed heterojunction barriers at the interfaces play important roles in the improved sensing performances.32−34 Among different types of p−n heteronanostructures, the 2D/0D hetero-nanostructures can ensure the penetration and diffusion of gas molecules, which may boost the sensitivity, response time, and recovery time of sensors.35 To the best of our knowledge, such 2D/0D p−n hetero-nanostructures on MoS2 NSs have not been synthesized for high-performance NO2 gas sensors. Thus, 2D/0D p−n hetero-nanostructures are proposed with the goal of achieving excellent-sensing performance MoS2 NS-based gas sensors. Herein, we provide a report on fabricating unique p−n MoS2/ZnO hetero-nanostructures for detecting low concentrations of NO2. MoS2 NSs were prepared using a liquid-phase exfoliation method with a mixture of ethanol and water.18,36−38 Surface modification of zinc oxide nanoparticles (ZnO NPs) was obtained via a facile wet chemical method. In this process, MoS2 NSs serve as one part of the hetero-nanostructures and also as substrates for ion adsorption, crystal nucleation, and subsequent growth of ZnO NPs. It is expected that the fabricated MoS2/ZnO p−n hetero-nanostructures can detect NO2 molecules accurately, reliably, and quickly at room temperature. Experimental results have proved that the optimal hetero-nanostructure exhibits an enhanced sensitivity of 3050% for 5 ppm NO2, an ultrafast response time of 40 s, an excellent recovery of more than 90%, and a reliable long-term stability within 10 weeks toward NO2 at room temperature. The unique structure of MoS2/ZnO hetero-nanostructures also provides guidance for other 2D materials that might be used in gassensing applications.

2.4. Gas-Sensing Measurements. The NO2 sensing measurements were carried out by a homemade gas-sensing detection system previously reported by our group,8,28 which is equipped with gas distribution equipment and a data information acquisition system. Different concentrations of NO2 are obtained by mixing initial NO2 with certain amounts of dry compressed air. Mass flow controllers (MFC) are employed here to control the flow of NO2 and dry compressed air. The flow rate of the test gases was controlled at 1 SLM (standard liter per minute). A precise semiconductor parameter analyzer (Agilent 4156C) is utilized to receive the real-time currents of gas sensors under a working voltage of 500 mV. We define the response time (τres) and recovery time (τrec) as the time required to reach 90% of the full response and recovery values, respectively. The response value (R) is calculated by the following formula:

2. EXPERIMENTAL SECTION

Scheme 1. Illustration of the Formation of MoS2 NSs and MoS2/ZnO Hetero-Nanostructure

R = (Igas − I0)/I0 = ΔI /I0

(1)

where I0 is the current value of gas sensor in dry air and Igas is the current value of gas sensor exposed to NO2 molecules. 2.5. Characterizations. Field emission scanning electron microscopy (Ultra Plus, Carl Zeiss, Germany) and transmission electron microscopy (TEM, JEM-2100, JEOL, Japan) were performed to observe the morphologies of different samples. High-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) and elemental mapping were carried on an FEI Talos F200X microscope (USA). Chemical compositions and the distributions of different elements were analyzed by an energydispersive X-ray spectroscope (Oxford Instruments INCA PentaFET×3, model: 7426). The thickness distribution of the MoS2 NSs was tested by an atomic force microscope (Veeco, USA). An X-ray diffractometer (XRD, D8 ADVANCE, Bruker, Germany), a Raman spectrometer (inVia, Renishaw, UK), and an X-ray photoelectron spectrometer (XPS, Kratos Axis Ultra DLD, Japan) were used to characterize the phase, vibrational properties, and surface compositions properties of samples, respectively. The surface areas were determined by Brunauer−Emmett−Teller (BET) instruments (Micromeritics ASAP 2020 M, USA).

3. RESULTS AND DISCUSSION 3.1. Structural Properties of MoS2/ZnO HeteroNanostructures. MoS2/ZnO hetero-nanostructures were obtained via a wet chemical route, as illustrated in Scheme 1.

2.1. Preparation of MoS2 NSs. First, 100 mg of MoS2 powders with a certain amount of acetonitrile was ground for 2 h using an agate mortar.38 Next, the black powders were dried in a vacuum oven at 60 °C overnight. Subsequently, the powders were dispersed into the ethanol/water mixture (45 vol %, 50 mL) and then sonicated for 3 h at 200 W.37,39 Finally, MoS2 NSs were collected by extracting the supernatant after centrifuging at 1500 rpm for 20 min. 2.2. Synthesis of MoS2/ZnO Hetero-Nanostructures. First, 20 mg of the as-prepared MoS2 NSs was dispersed into 20 mL of N,Ndimethylformamide (DMF) solvent. Then, 3 mg of zinc acetate dihydrate [Zn(Ac)2·2H2O] was added. The mixing solvent was heated at 200 °C for 2 h with magnetic stirring and refluxed. In particular, the procedures were protected by the flow of nitrogen to prevent the oxidation of MoS2 NSs. The products were named 3-ZM, 5-ZM, 7-ZM, 10-ZM, and 20-ZM corresponding to 3, 5, 7, 10, and 20 mg of added Zn(Ac)2·2H2O, respectively. The sediments were collected by centrifuging at 8000 rpm and washed with deionized water and ethanol several times to ensure the removal of reactants. 2.3. Gas Sensor Fabrication. The standard interdigital electrodes with a spacing of 20 μm were prepared by the lift-off microfabrication process.8 Two microliters of MoS2 NSs and MoS2/ZnO heteronanostructure ethanol solutions with an appropriate concentration were dropped to the interdigital electrodes fixed on customized copper bases. Finally, the fabricated sensors were naturally air-dried at first and then further dried in a vacuum drying oven at 60 °C for 2 h.

At the beginning, the MoS2 NSs were prepared by a liquidphase exfoliation method. Then, the exfoliated MoS2 NSs were dispersed into DMF, which is a Lewis base. When Zn[Ac]2· 2H2O is added, the Zn2+ precursors for the reaction may be in the form of Zn(NH3)42+. Gedanken et al. have reported that Zn(NH3)42+ is likely generated in a medium of DMF even without the use of ammonia.40 After heating at 200 °C for 2 h, the ZnO NPs are formed on the surface of MoS2 NSs. Especially, the in-plane defects and edge sites with higher potential energy are active for the nucleation of ZnO NPs.26 Figure 1a,b shows the scanning electron microscopy (SEM) images of exfoliated MoS2 NSs. Compared with commercial MoS2 (Figure S1), both the lateral sizes and the thicknesses of 22641

DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

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photograph of Figure 1a, Tyndall test indicates that MoS2 NSs have good dispersion in ethanol/water solution. When Zn(Ac)2·2H2O is added in a small amount, the separated ZnO NPs are uniformly distributed on the surfaces of MoS2 NSs for 3-ZM, 5-ZM, and 7-ZM, as shown in Figures S3a,b and 1d, respectively. With the further increase of Zn(Ac)2· 2H2O, the ZnO NPs are gradually linked together, as shown in the SEM images of 10-ZM and 20-ZM in Figure S3c,d. Energydispersive X-ray spectroscopy (EDXS) spectra were recorded to obtain the elemental composition of MoS2/ZnO heteronanostructures. As shown in Figure S4, the presence of Mo, S, Zn, and O elements in 3-ZM, 5-ZM, 7-ZM, 10-ZM, and 20ZM is confirmed and the atomic contents of Zn element significantly increase from 1.88% (3-ZM) to 8.64% (20-ZM). TEM was utilized to observe more detailed microstructure information of the MoS2 NSs and MoS2/ZnO heteronanostructures. Figure 2a displays the TEM image of MoS2 NSs with a lateral size about 300 nm. The curled edge seen in the high-resolution TEM (HRTEM) image indicates that the obtained MoS2 NS has five layers, as shown in Figure 2b. The edge is not continuous because some defects were generated in the exfoliation processes. In Figure 2c, the selected area electron diffraction (SAED) pattern demonstrates the single crystalline characteristic of MoS2 NSs. The diffraction spots correspond to the (100) and (110) planes of the MoS2 NSs. From the TEM image of 7-ZM in Figure 2d, ZnO NPs with sizes about 10 nm are uniformly separated on the surface of MoS2 NSs. A greater density of ZnO NPs form on the edge sites because the edge area defects are more active for nucleation of ZnO NPs. With a further increase in Zn2+ concentration, ZnO NPs begin linking together, as shown in the TEM image of 20-ZM (Figure S5). The lattice spacing is measured to be 0.28 nm, which is consistent with the (100)

Figure 1. SEM images of (a,b) MoS2 NSs and (c,d) 7-ZM at different magnifications. The inset photograph in (a) is Tyndall phenomenon of exfoliated MoS2 with the centrifuging speeds of 5000, 3000 and 1500 rpm from left to right.

MoS2 NSs are reduced after grinding and sonication processes. Edges of the MoS2 NSs have some curled areas because of their thinness. The thickness distribution of MoS2 NSs was investigated by atomic force microscopy (AFM) imaging and thickness measurements. Figure S2 shows that the majority of MoS2 NSs are less than 10 nm in thickness. In the inset

Figure 2. (a) TEM image of MoS2 NSs. (b) HRTEM image of MoS2 NSs. (c) SAED pattern of MoS2 NSs. (d) TEM image of 7-ZM. (e) HRTEM image of 7-ZM. (f) SAED pattern of 7-ZM. The inset in (e) is the fast Fourier transform pattern of 7-ZM. (g) HAADF-STEM image and elemental mapping of (h) Mo, (i) S, and (j) Zn of 7-ZM. 22642

DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

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Figure 3. (a) XRD patterns of MoS2, 3-ZM, 7-ZM, 10-ZM, and 20-ZM. (b) Raman spectra of bulk MoS2, exfoliated MoS2 NSs, and 7-ZM. XPS spectra of (c) Mo, (d) S, (e) Zn, and (f) O elements.

NSs, two main peaks of E2g 1 and A1g are located at 378.9 and 403.5 cm−1, respectively. The distance between two characteristic peaks of 24.6 cm−1 further confirms the successfully exfoliated MoS2 NSs with a few-layered nanostructure,42 which is also proved by AFM and HRTEM analysis. The E2g 1 and A1g modes of 7-ZM are slightly red-shifted compared with the corresponding modes of MoS2 NSs, which means that MoS2 NSs appear to be compressed lightly in the lateral direction and that there is tensile deformation on the vertical site due to the interaction with ZnO NPs.43 The surface compositions and chemical states of 7-ZM were measured using XPS. From the spectrum of Mo element in Figure 3c, the main peaks at 232.65 and 229.55 eV are consistent with the binding energies of Mo4+ 3d5/2 and Mo4+ 3d3/2, respectively. Except for two main peaks, the peaks at 235.78 and 233.05 eV can be contributed to Mo6+ 3d5/2 and Mo6+ 3d3/2. The formation of Mo6+ is mainly attributed to the defects and vacancies with highly active sites for oxygen molecules.44 Figure 3d displays the spectra of S element. The main peaks at 162.28 and 163.38 eV are ascribed to S2− 2p3/2 and S2− 2p1/2, respectively. In Figure 3e, the peaks at 1045.25 and 1022.45 eV of the Zn 2p spectrum are assigned to Zn2+ 2p1/2 and Zn2+ 2p3/2, respectively, showing the Zn element in the oxide state. Three components are included in the O 1s spectrum shown in Figure 3f. The three components are Zn− O (oxygen ions in the crystal lattice of ZnO, 531.35 eV), O2− (adsorption of oxygen, 532.65 eV), and −OH (hydroxyl groups on the surface, 533.95 eV),45 and the proportions of

planes of hexagonal ZnO in Figure 2e. Both the fast Fourier transform pattern (the inset in Figure 2e) and the SAED pattern (Figure 2f) of 7-ZM exhibit two sets of diffraction spots, which can be attributed to MoS2 and ZnO, respectively. The well-defined hexagonal symmetry is assigned to the (100) planes of MoS2, whereas the other set of spots with a lattice spacing of 0.28 nm belongs to the (100) planes of ZnO. A HAADF-STEM image and the elemental mapping data are shown in Figure 2g−j, which also confirms the even distribution of Zn element and the formation of the MoS2/ ZnO hetero-nanostructure. To further verify the phase of MoS2 and MoS2/ZnO heteronanostructures, XRD patterns are recorded. In Figure 3a, the pattern of 20-ZM contains the main characteristic peaks of 2HMoS2 (JCPDS card no. 37−1492) and wurtzite ZnO (JCPDS card no. 36−1451), which indicates the successful growth of ZnO NPs on the surface of MoS2 NSs. For other samples, the characteristic peaks of ZnO are not observed because the small grain sizes and low contents cannot be identified using XRD.34 Combined with the TEM results, both the same hexagonal crystalline structure and the similar lattice spacing between wurtzite ZnO and 2H-MoS2 provide the guarantee of the successful synthesis of MoS2/ZnO hetero-nanostructures.41 Raman spectroscopy is an effective way to understand the vibrational properties of 2D materials. Figure 3b displays the Raman spectra of bulk MoS2, exfoliated MoS2 NSs, and 7-ZM. For bulk MoS2, the strong signals at 376.5 and 403.5 cm−1 can be ascribed to the in-plane E2g 1 and out-of-plane A1g vibration modes, respectively. When the bulk MoS2 is exfoliated to MoS2 22643

DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

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Figure 4. Nitrogen adsorption−desorption isotherms of (a) MoS2 NSs and (b) 7-ZM.

Figure 5. (a) Schematic illustration of the gas-sensing test system. (b) Schematic illustration of the fabricated sensing device. (c) Sensing response of MoS2/ZnO hetero-nanostructures with different amounts of ZnO NPs at 5 ppm NO2. (d) Response and recovery curves of MoS2 NSs and 7ZM at 5 ppm NO2.

Table 1. Comparison of Gas-Sensing Performances of the MoS2/ZnO Hetero-Nanostructure toward NO2 with Previous Works sensing materials

sensor type

NO2 (ppm)

operation temperature

ΔI/I0 (%)

recovery ability

refs

monolayer MoS2 bilayer MoS2 five-layer MoS2 vertically aligned MoS2 Pt/MoS2 graphene/MoS2 annealed MoS2 SnO2 NC/MoS2 NS 3D MoS2 SnO2/graphene MoS2/graphene SnS2 Ag/WS2 MoS2/ZnO hetero-nanostructure

FET FET FET FET FET FET chemiresistor FET chemiresistor chemiresistor FET chemiresistor FET chemiresistor

0.1 2 100 100 5 100 1 10 50 60 3 5 500 5

room temperature room temperature room temperature room temperature room temperature 150 °C 200 °C room temperature 150 °C 70 °C 200 °C 120 °C room temperature room temperature

50 80 100 11 20 80 580 28 78 108 17 2210 667 3050

incomplete incomplete incomplete incomplete incomplete incomplete complete complete complete complete complete complete complete complete

49 9 11 21 10 25 24 29 50 51 27 14 52 this work

is higher than that of MoS2 NSs of 12.6 m2/g. The larger surface area of the MoS2/ZnO hetero-nanostructure is attributed to the small sizes of ZnO NPs and the reduced agglomeration of MoS2 NSs via surface modification. 3.2. Gas-Sensing Properties. The NO2 sensing performances were measured by a homemade gas delivery system in Figure 5a. The sensitive thin films of different samples were formed on Si substrates printed with interdigital microelectrodes by a drop-casting method (Figure 5b). Figure S6

these are estimated to be 20.83, 56.29, and 22.88%, respectively. All of the above characterizations reveal the presence of interfacial effects in MoS2/ZnO hetero-nanostructures. Growth of ZnO NPs on the surface of MoS2 NSs creates tight contact between ZnO and MoS2 and also limits the growth of ZnO. Consequently, the specific surface area of the MoS2/ZnO hetero-nanostructures is enlarged. Figure 4 shows that the BET specific surface area of 7-ZM is 25.7 m2/g, which 22644

DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

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Figure 6. (a) Dynamic curves of 7-ZM to different NO2 concentrations. (b) Sensitivity variations of 7-ZM with the increase of NO2 concentration. (c) Five successive sensing cycles of 7-ZM to 5 ppm NO2. (d) Long-term stability of 7-ZM to 5 ppm NO2 within 10 weeks.

state within 1000 s at room temperature without other auxiliary means (Figure 5d), and this is extremely difficult to achieve with room-temperature NO2 gas sensors.9−11,21 Thus, 7-ZM displays superior performance not only with an excellent response of 3050% to 5 ppm NO2 with a fast response time of 40 s, but also with an outstanding recovery ability. All of the above properties of 7-ZM are competitive with the reported MoS2 gas sensors in Table 1. To further investigate the gas-sensing properties of 7-ZM, the dynamic sensing curves are recorded with the NO2 concentration ranging from 50 ppb to 5 ppm in Figure 6a. This sensor has a clear response to 50 ppb NO2 and an increasing sensitivity at higher NO2 concentrations. Figure 6b shows that the response values gradually increase from 8.2 to 32.3 with the increase of NO2 concentration. The fitting equation between the response value Y and lg(NO 2 concentration) of X can be illustrated as a linear relationship with a coefficient of determination (R2) value of 0.99, as shown in Figure S10a. The linear relationship between response values and response concentrations is convenient for practical applications of gas sensors. Figure 6c presents the five successive cycles of 7-ZM to 5 ppm NO2, demonstrating a good cycle stability. The response values increase slightly during cycling, which may be attributed to the residual NO2 molecules on the surface of 7-ZM after the recovery process of prior cycle, although which recoverability up to more than 90%. In practical applications, long-term stability is also essential for the gas-sensing performance. 7-ZM shows no apparent decline of sensitivity and exhibits excellent long-term stability within 10 weeks, as illustrated in Figure 6d. Furthermore, we have studied the effects of humidity and some other gases on the sensor. It is noteworthy that, as shown in Figure S10b, the conductivity of 7-ZM decreases by 20% when exposed to an environment with 60% relative humidity. This 20% decreased conductivity is negligible compared with the high response value of 3050% toward NO2. As shown in Figure S11, the response value of 3050% to 5 ppm NO2 is much higher than the values of 43.4, 46.4, and 81.2% for 1000 ppm NH3, H2S, and SO2, respectively. Moreover, it also

shows that the sensitive materials are uniformly distributed on the surface of electrodes, and thus, conductive paths are formed between sensitive materials and electrodes. In Figure S7, the response value of 7-ZM keep continually increasing even though 5 ppm NO2 is introduced to the testing chamber after 1000 s, which shows the adsorption process for roomtemperature gas sensors to saturated state is relatively slow. Because the majority of response process is complete after 300 s, a suitable response time of 300 s is chosen for the sensors to facilitate comparison. Then, the effect of ZnO NPs on the sensing performances was investigated. When exposed to 5 ppm NO2, the response values of sensors based on pure MoS2, 3-ZM, 5-ZM, 7-ZM, 10-ZM, and 20-ZM are shown in Figure 5c. The response values increase at the beginning and then decrease with an increase in the amount of ZnO NPs. 7-ZM is endowed with the highest response value of 3050%, which is 11 times higher than that of pristine MoS2 NSs. As far as we know, such a response value is much higher than the values reported for MoS2 sensors in Table 1. Interestingly, both pristine MoS2 NSs and MoS2/ZnO hetero-nanostructures exhibit positive conductivity variation on exposure to electrophilic NO2 (Figure S8), and this indicates that MoS2 NSs and MoS2/ZnO hetero-nanostructures have p-type semiconducting behaviors. As reported previously, the p-type behavior of MoS2 NSs is attributed to several reasons: substrate-induced charge transfer, charge transfer between the metal and MoS2, and ptype dopants on MoS2 flakes. In our study, oxygen adsorbed on the MoS2 NSs or the interaction between in-plane MoS2 and the SiO2/Si substrate may contribute to p-type doping in MoS2 NSs.16,24,26,46−48 In Figure S9, the presence of O element and the additional +6 oxidation state peaks of Mo 3d are ascribed to some oxygen adsorbed on the surface of MoS2 NSs, which may induce the p-type MoS2 NSs. The p-type characters of MoS2/ZnO hetero-nanostructures with any ratio indicate that MoS2 NSs act as major charge carriers for these systems. Figure 5d displays the representative response and recovery curves of MoS2 NSs and 7-ZM at 5 ppm NO2, which illustrates that τres of 7-ZM (40 s) is much shorter than that of pure MoS2 (211 s). Moreover, 7-ZM returns nearly to the fully recovered 22645

DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

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After being combined with n-type ZnO NPs, the majority of high energy binding sites on the surface of MoS2 are occupied by the nucleation of ZnO. The defect-dominated adsorption is restricted, and the electronic effect of formed p−n junction becomes prominent. To gain a better understanding of high sensing performances of MoS2/ZnO hetero-nanostructures, a schematic diagram of the band structures of ZnO and MoS2 before equilibrium is displayed in Figure S12. The electrons of n-type ZnO near the p−n interface tend to diffuse into p-type MoS2, and this results in the formation of both a depletion layer on the surface of ZnO and a positively charged area. Meanwhile, the holes of MoS2 near the p−n interface have a tendency to diffuse into the surface of ZnO, which leaves a negatively charged region. The electron−hole diffusion continues until the Fermi level of this system reaches a balanced state (see Scheme 2b,c). As a result, the region closest to the p−n interface creates an internal electric field, which acts as a potential barrier for further carrier diffusion. The potential barriers at the interfaces of MoS2/ZnO and depletion layers on the surface of ZnO contribute to the low conductivity of the MoS2/ZnO hetero-nanostructures in air. MoS2 NSs still serve as the conductive path in the sensors; thus, the changes in the surface depletion of ZnO do not have a remarkable influence on the electron density and mobility of the sensors, which are significantly affected by the potential barrier between ZnO and MoS2. The potential barrier acts as a lever for sensing performance, and electron transport is facilitated or restrained with respect to different kinds of gases. When the sensor is exposed to NO2, as shown in Scheme 2b, NO2 molecules gain electrons from ZnO NPs and decrease their carrier concentration. The balance of the built-in electric field is broken, and extra holes are no longer restricted by the electrons of the ZnO NPs and return to MoS2 NSs until a new equilibrium is formed. During this adsorption process, more holes accumulate at the surface of MoS2 NSs (Scheme 2c) and the width of the heterojunction barriers is decreased. Thus, the conductivity of MoS2/ZnO hetero-nanostructures greatly increases, which contributes to the enhanced response values.51,53−58 Besides, ZnO NPs on the surface of MoS2 NSs not only act as n-type decorations but also enlarge the specific surface area of the MoS2/ZnO hetero-nanostructures. The MoS2/ZnO hetero-nanostructures with a larger surface area can provide more absorbed sites for NO2 gas molecules and contribute to a further increase in the sensitivity. Accordingly, the enhanced charge transfer and increased active sites boost the sensing performances of the MoS2/ZnO hetero-nanostructure.

exhibits excellent selectivity for some volatile organic compound vapors, such as acetone and methanol. Therefore, the 7-ZM-based gas sensor exhibits excellent selectivity to NO2. Undoubtedly, the gas sensor can be applied to the actual environment for several months without obvious degradation of performances and the interference of other gases and humidity. The MoS2/ZnO hetero-nanostructure proves to be an excellent candidate for an ultrasensitive NO2 roomtemperature sensor, with an unparalleled response value, a fast response time, an excellent recovery, a reliable long-term stability, and an outstanding selectivity. 3.3. Gas-Sensing Mechanisms. The interaction between ZnO NPs and MoS2 NSs is proved by the above characterizations of SEM, TEM, XRD, Raman spectroscopy, and XPS. For sensing performances, the MoS2/ZnO hetero-nanostructure-based gas sensors exhibit p-type characters, which indicates that MoS2 NSs act as major charge carriers and ZnO NPs serve as active decorations. On the basis of these data, the sensing mechanisms will be explained as follows. For pristine p-type MoS2 NSs, the defects on the surface of MoS2 act as active sites for NO2 molecules, and this defectdominated process contributes to the slow rates of response and recovery due to high adsorption energy.27 During this process, electrophilic NO2 molecules capture electrons from the conduction band of MoS2, leading to an increased conductivity of the sensor (Scheme 2a). Scheme 2. Schematic of Sensing Mechanisms of (a) Pure MoS2 NSs and (b) MoS2/ZnO Hetero-Nanostructures to NO2 Molecules and (c) Energy Band Structure of MoS2/ ZnO Hetero-Nanostructures in Air and a NO2 Atmosphere

Figure 7. (a) Schematic illustration of equivalent resistance models for different distribution situations of ZnO NPs on the surface of MoS2 NSs. (b) I−V curves of MoS2 NSs, 3-ZM, 5-ZM, 7-ZM, 10-ZM, and 20-ZM. 22646

DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

Research Article

ACS Applied Materials & Interfaces ORCID

Next, we discuss the distinct effects that different amounts of ZnO NPs have on sensing properties. For the formed heterojunction barriers, when the ZnO NPs are dispersed on the surfaces of MoS2 NSs (3-ZM, 5-ZM and 7-ZM), as shown in Figure 7a, the equivalent resistance is increased from RM (the resistance of MoS2 NSs) to RZM (the resistance of MoS2/ ZnO hetero-nanostructures). With a further increase in the ZnO NP content, a conducting path formed between the particles, and this is confirmed from SEM and TEM images of 10-ZM and 20-ZM (Figures S3c,d and S5b). The equivalent resistance of the ZnO NP conducting path is defined as RZ, which is paralleled to and larger than RZM because of the smaller sizes of ZnO NPs. In this case, the effects of p−n junctions are counteracted until being ignored. The decreased conductivities of 3-ZM to 7-ZM (as seen from the I−V curves in Figure 7b) can be attributed to the increasing heterojunction barriers between MoS2 NSs and ZnO NPs. For 10ZM and 20-ZM, the formed conductive path of the ZnO NPs leads to increased conductivities of the samples. Thus, the hetero-nanostructure of 7-ZM makes full use of the effects of p−n heterojunctions and has the optimal sensing performance. Additionally, the dispersed ZnO NPs on the surface of 7-ZM ensure the penetration and diffusion of gas molecules, and this may boost the sensitivity and shorten the response time of the sensors.

Yutong Han: 0000-0003-3726-6315 Yanjie Su: 0000-0003-2193-5473 Zhi Yang: 0000-0002-0871-5882 Notes

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b05811. SEM images of commercial MoS2 powders, 3-ZM, 5ZM, 10-ZM, 20-ZM, and 7-ZM on the interdigital electrodes; AFM image of MoS2 NSs; TEM images of 7ZM and 20-ZM; XPS of MoS2 NSs; EDXS characterizations of MoS2 and MoS2/ZnO hetero-nanostructures; response/recovery curves of MoS2 and MoS2/ZnO hetero-nanostructures; selectivity of 7-ZM; and band diagrams of MoS2 and ZnO (PDF)

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ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support of the National Natural Science Foundation of China (61671299), National Key Research and Development Program of China (2016YFC0102700), Shanghai Science and Technology Grant (16JC1402000), Natural Science Foundation of Shanghai (17ZR1414100), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (GZ2016005), and Shanghai Jiao Tong University New Youth Teacher Initiative Project (17X100040074). They also acknowledge analysis support from the Instrumental Analysis Center of Shanghai Jiao Tong University and the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University.

4. CONCLUSIONS In summary, we have demonstrated a facile fabrication of MoS2/ZnO hetero-nanostructures via a wet chemical route. After surface functionalization of ZnO NPs, MoS2/ZnO hetero-nanostructures exhibit significantly enhanced NO2 sensing performances. The optimal p−n hetero-nanostructure has a response value (5 ppm NO2, 3050%) that is 11 times greater than that of pristine MoS2. The recoverability is also obviously improved to more than 90%, which ensures the high reliability and sustainability of the sensors. Moreover, the sensor has an ultrafast response time of 40 s, an excellent selectivity, a reliable long-term stability for 10 weeks, and a low detection concentration of 50 ppb. The comprehensive sensing mechanisms are also investigated in detail. The special 2D/0D hetero-nanostructure, synergistic effects, and p−n heterojunctions between ZnO and MoS2 contribute to the enhanced NO2 sensitivity. Thus, constructing p−n hetero-nanostructures for 2D materials has been proved to be a versatile solution for achieving excellent sensing performances.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 22647

DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649

Research Article

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DOI: 10.1021/acsami.8b05811 ACS Appl. Mater. Interfaces 2018, 10, 22640−22649