Synthesis of Numerous Edge Sites in MoS2 via SiO2 Nanorods

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Functional Nanostructured Materials (including low-D carbon)

Synthesis of Numerous Edge Sites in MoS2 via SiO2 Nanorods Platform for Highly Sensitive Gas Sensor Young-Seok Shim, Ki Chang Kwon, Jun Min Suh, Kyoung Soon Choi, Young Geun Song, Woonbae Sohn, Seokhoon Choi, Koo Tak Hong, Jong-Myeong Jeon, SeungPyo Hong, Sangtae Kim, Soo Young Kim, Chong-Yun Kang, and Ho Won Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08114 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Applied Materials & Interfaces

Synthesis of Numerous Edge Sites in MoS2 via SiO2 Nanorods Platform for Highly Sensitive Gas Sensor Young-Seok Shim1,†, Ki Chang Kwon2,†, Jun Min Suh2, Kyoung Soon Choi3, Young Geun Song1, Woonbae Sohn2, Seokhoon Choi2, Kootak Hong2, Jong-Myeong Jeon2, Seung-Pyo Hong2, Sangtae Kim1, Soo Young Kim6, Chong-Yun Kang1,4,*, Ho Won Jang2,* 1

Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul 02791, Republic of Korea 2

Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea 3

Advanced Nano-Surface Research Group, Korea Basic Science Institute (KBSI), Daejeon 34133, Republic of Korea 4

KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea

5

School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974, Republic of Korea

*E-mail: C. Y. K. [email protected], H. W. J. [email protected] †

These authors equally contributed to this work.

KEYWORDS: MoS2, Nanostructure platform, Edge site, Gas sensor, Chemical vapor deposition.

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ABSTRACT The utilization of edge sites in two-dimensional (2D) materials including transition metal dichalcogenides (TMDs) is an effective strategy to realize high-performance gas sensors due to their high catalytic activity. Herein, we demonstrate a facile strategy to synthesize the numerous edge sites of vertically aligned MoS2 and larger surface area via SiO2 nanorod (NRs) platforms for highly sensitive NO2 gas sensor. The SiO2 NRs encapsulated by MoS2 film with numerous edge sites and partially vertical-aligned regions synthesized using simple thermolysis process of [(NH4)2MoS4]. Especially, the vertically aligned MoS2 prepared on 500 nm-thick SiO2 NRs (500MoS2) shows approximately 90 times higher gas sensing response to 50 ppm NO2 at room temperature than the MoS2 film prepared on flat SiO2, and the theoretical detection limit is as low as ~2.3 ppb. Additionally, it shows reliable operation with reversible response to NO2 gas without degradation at an operating temperature of 100 °C. The use of the proposed facile approach to synthesize vertically aligned TMDs using nanostructured platform can be extended for various TMD-based devices including sensors, water splitting catalysts, and batteries.


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INTRODUCTION In the past decades, innovative concepts have emerged in information and communication technologies with the development of smart objects. Ubiquitous communication and connectivity, pervasive computing, and ambient intelligence have brought about the concept of Internet of Things (IoT).1-4 A plethora of technologies have been intensively developed based on popular computing platforms, because over two billion people have access to portable devices.5-7 Recently, various sensors such as temperature, humidity, pressure, accelerometer, digital compass, gyroscope, and motion sensors have been embedded in portable devices.8,9 Tremendous efforts have been made to integrate an increasing number of functional sensors in portable devices, because they can play a critical role in IoT by continuously monitoring, providing, and sharing dynamic surrounding conditions and information.10,11 Gas sensors that provide chemical information on the presence and concentration of particular gas species in the ambient atmosphere are used in a wide range of applications, such as food processing, medical diagnostics, automotive fuel control, and aerospace vehicles.12 Furthermore, in the modern society, the quality of human life is directly related to that of indoor air; therefore, it is essential to monitor and manage the quality of indoor air.13-15 Besides excellent gas sensing performance, miniaturized size with capability of integration into existing circuits is mainly required to adopt gas sensors in portable devices. In addition, power consumption should be extremely low (< 10 mW) considering limited battery capacity, since the sensors operate continuously for real-time detection of harmful gases in a standby state.16 To date, various types of gas sensors, including optical sensors, electrochemical sensors, surface acoustic wave sensors, and metal oxide-based gas sensors have been



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developed; but, none of them have satisfied the above criteria for application in portable devices.17-20 Two-dimensional (2D) materials such as graphene-based materials and transition metal dichalcogenides (TMDs) have been receiving significant attention for gas sensing applications because their atomically thin layered structure induced by van der Waals forces is considered ideal for adsorption and desorption of gas molecules at room temperature.21,22 Furthermore, it has inherently large surface-volume ratios and possess high densities of catalytically active edge sites for chemical reactions.23 Transition metal dichalcogenides, especially group VI metals (Mo and W), have sufficient bandgap with semiconducting behavior which is effective for modulating charge transport characteristics to enhance gas sensing performance.23 Despite these advantages, the low response and the incomplete recovery have been reported as major drawbacks that limit practical applications.24,25 Recently, Lee et al. have reported the effective approach to easily improve the gas response using reduced graphene oxide formed on high aspect-ratio micro-pillars.26 However, they could not show the remarkable enhancement of gas response because they only focused on the increase of surface area of graphene. In addition, it has been reported that the vacancies of chalcogen atoms and the edge sites of TMDs can act as catalytic active sites in water splitting and gas adsorption and desorption.27 According to theoretical and experimental studies, the edge sites of TMDs with high d-orbital electron density, can significantly enhance the gas sensing properties by chemical catalytic activity.27 Although the excellent catalytic activity of the Mo terminated edge sites, they have not been successfully demonstrated the significant enhancement of gas sensing properties which is gas responsivity and recovery. Since the vertically-aligned MoS2 films are normally too dense to efficiently utilized the edge sites, the novel approach to enhance the gas sensing properties of edge site of MoS2 for the practical applications of TMD-based gas senssors.25 4 ACS Paragon Plus Environment

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Herein, we report a facile strategy to utilize the numerous edge sites of vertically aligned MoS2 with porous structures for highly sensitive gas sensing applications. We employ the porous SiO2 nanorods (NRs) platform to increase the number of exposed edge sites for improving gas adsorption on the synthesized vertically aligned MoS2. The fabricated structure (MoS2/SiO2 NR) has much larger surface area and the partially vertical-aligned MoS2 to react with the target gas molecules compared to the plane MoS2 on flat SiO2 substrate. The morphologies of the SiO2 NRs encapsulated by MoS2 and their crystallographic orientations observed by transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. X-ray photoelectron spectroscopy (XPS) measurements were carried out to examine the chemical state of all the samples. Upon exposure to 50 ppm NO2, the gas response of SiO2 NRs encapsulated by MoS2 was approximately 90 times higher than that of MoS2 on flat SiO2 at room temperature (20 °C). In addition, the recovery of SiO2 NRs encapsulated by MoS2 improved as the operating temperature was increased. When the operating temperature reached 100 °C, the fabricated sensors showed approximately 100% recovery, and the theoretical detection limit was below 8.84 ppb. From these results, we strongly believe that the proposed facile fabrication technique for numerous edge sites of MoS2 would be useful for extending the potential of TMD-based devices, for examples, hydrogen evolution catalysts in water splitting, cathodic materials in battery, and charge transport layers in optoelectronic devices.



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RESULT AND DISCUSSION Vertically ordered SiO2 NRs, which have extremely large surface-to-volume ratio, excellent accessibility of target gases, and aggregation-free geometrical morphologies, were fabricated on the patterned platinum-interdigitated electrode (Pt-IDE) on SiO2/Si substrates using GAD method. The MoS2 layers used in this study were synthesized by a simple thermolysis of ammonium tetrathiomolybdate [(NH4)2MoS4] solution precursor with a concentration of 200 mM.28 To make a uniform precursor layer, the ultraviolet-ozone (UV-O3) treatment was performed on the SiO2 NRs/Pt-IDE substrate. Then, the precursor thin layer was spin-coated onto the cleaned SiO2/Si substrate and the SiO2 NRs prepared at different thicknesses. The synthesis of MoS2 using chemical vapor deposition (CVD) is divided into two steps. The precursor layer was first annealed at 500 °C with N2/H2 flow to prevent carbon contamination from the residual ethylene glycol solvent. In addition, annealing at 800 °C was conducted to improve the crystallinity of MoS2 and transform MoS3 complex to MoS2.29 The detailed procedures are summarized in the experimental section. The schematic illustrations of the facile fabrication process and the synthesized MoS2 on SiO2 NRs are shown in Figure 1(a). To identify the lengths and porosities of fabricated samples, scanning electron microscopy (SEM) studies were conducted (Supporting information, Figure S1). By increasing the deposition time, the length and the diameter of SiO2 NRs can be controlled. As the thickness of the SiO2 NRs increases, the porosity of SiO2 NRs is increased. It is observed in the topview SEM images that the as-deposited SiO2 NRs are separated from each other. The SiO2 NRs are densely encapsulated by the MoS2 after thermolysis process using CVD method, and its diameter becomes larger than as-deposited SiO2 NRs. The thicknesses of the SiO2 NRs were found to be approximately 120, 250, and 500 nm, respectively. These values do not change and the SiO2 NRs are not delaminated from substrate after thermolysis process 6 ACS Paragon Plus Environment

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(Supporting Information, Figure S1). To specifically investigate the changes in the morphologies, we converted the top-view SEM images using MATLAB (Supporting information, Figure S2a). The porosity of all the samples was calculated using the ratio of black (blank) and red (flat SiO2/Si substrate or SiO2 NRs encapsulated by MoS2) colors (Supporting information, Figure S2b). The SiO2 NRs encapsulated by MoS2 showed lower porosity values than the bare SiO2 NRs, indicating that the MoS2 layers are well-grown onto the SiO2 NRs after thermolysis process. Figures 1(b)–(d) shows the TEM images of the fabricated SiO2 NRs encapsulated by MoS2 with different SiO2 lengths. The SiO2 NRs with 120, 250, and 500 nm thickness are well-defined, and the MoS2 layers are vertically formed along the surface of the SiO2 NRs from the top to the bottom, regardless of the thickness of the SiO2 NRs. Energy dispersive spectroscopy (EDS) was performed to identify the elements present in the SiO2 NRs encapsulated by MoS2. Figure 1(e) shows the EDS mapping images of Si, O, Mo, and S elements. A uniform distribution of Si and O elements over the SiO2 NRs encapsulated by MoS2 is observed at the 120, 250, and 500-nm-thick SiO2 NR platforms. In contrast, the mapping images of Mo and S elements show non-uniform distribution, indicating that the MoS2 layers are located densely near the surface of the SiO2 NRs. To observe the surface morphologies of synthesized MoS2 on flat SiO2 and SiO2 NRs, the TEM was conducted. The synthesized MoS2 layers are observed with the low magnification images in Figures 2(a)–(d). The cross-sectional TEM images clearly show that the MoS2 layers are located at the surface of SiO2 NRs. The lengths of deposited SiO2 NRs are also well-defined in these images. In the case of MoS2 synthesized on the flat SiO2 substrate, the hexagonal atomic structure of MoS2 is well-defined in top-view of TEM images, as shown in Figure 2(e). The selected-area-electron-diffraction (SAED) patterns was clearly displayed in plane MoS2 on flat SiO2 substrate. Because the synthesis of MoS2 in this experiment is thermolysis of solution-based precursor, there are some grain boundaries and poly-crystalline 7 ACS Paragon Plus Environment


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regions in synthesized MoS2, which is well-consistent with previous report.29 Interestingly, the partially vertical-aligned MoS2 were observed in SiO2 NRs encapsulated MoS2 samples as shown in cross-sectional view TEM images, Figure 2(f). The inset figures in Figure 2(e) and 2(f) display the transmission direction of electron beam onto the samples. The high-resolution TEM (HR-TEM) images show that the d-spacing of vertically aligned MoS2, 0.618 nm, which is consistent with that reported previously.30 The vertically aligned MoS2 on the SiO2 NRs with regardless of its lengths were also observed (Supporting Information, Figure S3). No interfacial compounds were found near the MoS2/SiO2 interfaces. While most of the MoS2 are in-plane orientated on the surface of the flat SiO2/Si substrate, the vertically stacked (001) plane of the MoS2 is randomly oriented on the SiO2 NRs after the thermolysis process. From these results, it can be inferred that the SiO2 NRs are well-covered with basal planeorientated and edge-exposed (vertically-aligned) MoS2. Additionally, the fabricated 1D SiO2 NRs can act as a platform to expose the edge sites of the MoS2. As observed in the TEM images, the edge-terminated vertically aligned MoS2 could play a crucial role in catalytic active sites (gas adsorption and desorption in gas sensors), indicating that the fabricated sample would show excellent gas sensing performance. To characterize the MoS2 synthesized on the SiO2 NRs with different thicknesses, the X-ray diffraction (XRD) studies were performed. There are no significant peaks in the XRD pattern of flat SiO2/Si substrate, however, a broad peak corresponding to amorphous SiO2 is observed near 21° (Supporting Information, Figure S4). All the SiO2 NRs encapsulated by MoS2 samples exhibit a strong (002) peak near 13° (JCPDS #88-0550), which is consistent with the TEM results. Raman spectroscopy has been widely used to identify molecular vibration in 2D materials such as graphene and TMDs. There are two distinctive peaks in the Raman spectrum of MoS2; in-plane vibration mode (E12g) and out-of-plane vibration mode (A1g). The E12g and the A1g peaks are observed at approximately 383 and 408 cm-1, respectively in the Raman spectrum 8 ACS Paragon Plus Environment

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of the MoS2 synthesized on the flat SiO2/Si substrate, as shown in Figure 3(a). A peak shift is observed in the Raman spectra of SiO2 NRs encapsulated by MoS2, indicating that the synthesized MoS2 on the SiO2 NRs lost their initial photon energies owing to scattering effects in the nanostructures. Furthermore, the intensity ratio (E12g/A1g) significantly decreases when the MoS2 are synthesized on the SiO2 NRs compared to flat SiO2/Si substrate, as shown in Figure 3(b). This suggests that out-of-plane vibrations (A1g) in vertical aligned MoS2 have more degrees of freedom for their lattice vibration; these results are wellconsistent with previous report.31 The Raman spectrum of TMDs with a small domain size can provide sufficient information about the phonon dispersion near the Brillouin-zone center. This usually gives an asymmetric broadening of the Raman peak.32 The full-width-halfmaximum (FWHM) of SiO2 NRs encapsulated MoS2 in Raman spectra, as shown in Figure 3(b), is significantly increased compared to the MoS2 thin film on flat SiO2 substrate, indicating that synthesized MoS2 have many small domains in their structure which are vertically aligned regions. X-ray photoemission spectroscopy (XPS) was conducted to identify the chemical composition of the synthesized MoS2. The Mo 3d and S 2p core level spectra of MoS2 synthesized on the SiO2 NRs with different thicknesses are shown in Figures 3(c) and 3(d), respectively. The peaks observed at approximately 229 and 232.5 eV correspond to Mo 3d5/2 and Mo 3d3/2, respectively, and a small Mo 2s peak is located at approximately 226.5 eV. In the case of the S 2p spectrum, S 2p3/2 and S 2p1/2 peaks are observed at approximately 162.3 and 163.5 eV, respectively. There is no peak shift in the XPS spectra of the different samples, and no oxidation peaks are observed in the Mo 3d spectra, indicating that the MoS2 are well-synthesized on the SiO2 NRs. For the gas sensing measurements, four different types of samples, plane MoS2 (PMoS2), 120 nm-, 250 nm-, and 500 nm-thick SiO2 NRs encapsulated by MoS2 (named as 120MoS2, 250MoS2, and 500MoS2, respectively) were prepared on Pt-IDE patterned SiO2/Si substrates. 9 ACS Paragon Plus Environment


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Figure 4(a) shows the baseline resistance of all the samples in air. The measurement system is displayed in Figure S5 (Supporting Information). Although the same amount of solution precursor was used to prepare the MoS2 by spin-coating onto SiO2 NRs, the baseline resistance of all the samples was different. Compared with PMoS2 (1.57 × 104 Ω), the baseline resistance of 250MoS2 is slightly higher (2.43 × 104 Ω), and 500MoS2 shows the highest baseline resistance (3.71 × 106 Ω). On the other hand, 120MoS2 (1.29 × 104 Ω) shows a lower baseline resistance than PMoS2. For 500 nm-thick-SiO2 NRs, the [(NH4)2MoS4] solution precursor was entirely coated on SiO2 NRs without residual solution precursor. However, there were significant residual [(NH4)2MoS4] solution precursors after spin coating, in case of 120- and 250-nm-thick SiO2 NRs samples, because of its shorter NR lengths (Supporting Information, Figure S6). In this reason, the residual [(NH4)2MoS4] solution precursors were sunk to bottom, resulting in formation of MoS2 film same as plane MoS2 on its bottom, which is consistent with TEM image in Figures 1(b)–(d). The responses of all the samples (defined as (Rgas – Rair)/Rair × 100, where Rair and Rgas denote the resistance of the sensor in dry air and on exposure to target gas) to 50 ppm NO2 were measured at room temperature, and the results are shown in Figure 4(b). Upon exposure to 50 ppm NO2 at room temperature, the resistances of all the samples decrease abruptly, which is consistent with previously reported gas sensing properties of MoS2, indicating that the fabricated samples are p-type semiconducting gas sensors.33 Figure 4(c) shows the responses of all the samples to 50 ppm NO2. The responses of PMoS2, 120MoS2, 250MoS2, and 500MoS2 are 4.5, 137, 210, and 390%, respectively. Thus, as the thickness of the SiO2 NRs increases, the gas response also increases (Supporting information, Figure S7). Interestingly, the 500MoS2 sample exhibits approximately 90 times higher response than PMoS2. To the best of our knowledge, there are no studies demonstrating such a high


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response using pristine MoS2 without p-n heterojunctions, metal additives, or surface functionalization (Supporting information, Table S1). At room temperature, the critical drawbacks of MoS2-based sensors are their extremely slow response, incomplete recovery to initial base line after a sensing event, and incapability to produce reliable sensing output upon exposure to the same concentration of target gas molecules; similar problems observed in other 2D materials based gas sensors. To investigate the temperature dependency of the gas sensing properties, 500MoS2 was exposed to 50 ppm NO2 at various temperatures, ranging from room temperature to 200 °C, as shown in Figure 5(a). As the operating temperature is increased, the response of 500MoS2 gradually decreases. On the other hand, the recovery rate increases and eventually falls below the baseline at temperatures above 150 °C. Interestingly, when the operating temperature reaches 250 °C, the resistance of 500MoS2 abruptly increases to approximately ~107 Ω, and the sample no longer reacts to NO2 (Supporting information, Figure S8). These results can be explained using the following proposed mechanism. In chemoresistive gas sensors, the amount of ionized oxygen species (O2-, O-, O2-) play a critical role in the gas sensing performance of MoS2 owing to charge transfer.9 The adsorption kinematics are explained by the following reaction paths: O2 (gas) ↔ O2 (absorbed)

(1)

O2 (adsorbed) + e- ↔ O2-

(2)

O2- + e- ↔ 2O-

(3)

O- + e- ↔ O2-

(4)

The oxygen ions, O2-, O-, and O2-, are stable below 100 °C, between 100 and 300 °C and above 300 °C, respectively. Since most adsorption sites on MoS2 might have been occupied with pre-adsorbed O2 molecules as the operating temperature increased, less active sites on MoS2 were left for NO2 at the elevated temperatures (Supporting information, Figure S9). At room temperature, numerous edge sites exist on MoS2; hence, the adsorption of NO2 is dominant and continues during gas exposure. In contrast, desorption is extremely sluggish because of the strong bonging between NO2 and MoS2.21 The binding energies between each different MoS2 configurations and NO2 molecules have been revealed using DFT calculations in previous report.25 In the basal plane, S edge, and Mo edge, the computed binding energy values are – 0.248, – 0.927, and – 4.530 eV, respectively. It means that the NO2 gas 11 ACS Paragon Plus Environment


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molecules could be absorbed well to vertical-aligned and Mo-edge terminated MoS2. As the temperature is increased to 200 °C, the number of edge sites is decreased by adsorption of more oxygen species, leading to low response and resistance. Simultaneously, NO2 desorption is enhanced for better recovery. At an operating temperature of 250 °C, the surface of MoS2 is partially oxidized to MoO3. The response and the recovery of 500MoS2 to 50 ppm NO2 were measured as functions of the operating temperature and are plotted in Figure 5(b). We compared the stability and the gas sensing properties at room temperature and 100 °C. Upon multiple exposures to 50 ppm NO2, the base resistance at room temperature increases. On the other hand, 500MoS2 attains the initial base resistance at 100 °C completely, as shown in Figure 5(c). Therefore, in addition to reliable gas sensing properties, the operating temperature of 100 °C is considered effective in terms of sensor power consumption.32 In order to estimate the selectivity of the 500MoS2 to NO2, we measured response characteristics of the 500MoS2 to 50 ppm CH3COCH3 and NH3 at room temperature and 100 °C (Supporting information, Figure S10). The response of CH3COCH3 is negligible at both room temperature and 100 °C. Moreover, the response to NH3 at both room temperature and 100 °C is much lower and that recovery to the based resistance is very poor. This result demonstrates the high selectivity of the 500MoS2 to NO2. To evaluate the theoretical detection limit of 500MoS2, the responses of the sensor to 1‒5 ppm of NO2 at room temperature and 100 °C were measured, and the results are shown in Figure 5(d). The ((R/R0) ‒ 1) values proportionally increase with increase in the NO2 concentration, where the R is measured resistance and R0 is baseline resistance. The response values are plotted as a function of gas concentration in the inset of Figure 5(d). A simple linear relationship between the response and the gas concentration was obtained: y = 15.569x + 0.7689 and y = 8.509x + 1.777 at room temperature and 100 °C, respectively, where y is the response and x is the concentration of NO2. The measure of goodness-of-fit of the linear regression, R-Square, was calculated to be 12 ACS Paragon Plus Environment

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0.846 and 0.990 at room temperature and 100 °C, respectively. The responses of 500MoS2 to 1, 2, 3, 4, and 5 ppm NO2 were 16.2, 36.6, 51.2, 63.4, and 80.6, respectively at room temperature and 7.7, 13.5, 18.5, 24.13, and 44.9, respectively at 100 °C. Although 1 ppm of NO2 was the lowest detection limit obtained experimentally, the theoretical detection limits of 500MoS2 were calculated to be approximately 2.31 and 8.84 ppb at room temperature and 100 °C, respectively.9 The sub-ppb level detection limits to NO2 suggest the potential of 500MoS2 for a variety of applications, such as environmental monitoring and respiratory analysis, especially asthma (> 100 ppb NO2) diagnosis.12 Since the MoS2 are gradually transformed to MoO3 at high temperatures, the changes in the gas sensing properties upon exposure to NO2 gas were studied, as shown in Figure 3(c). To identify the correlation between the changes in the gas sensing behavior and the oxidation of MoS2 under high operating temperatures, the XPS studies were carried out, as shown in Figure 6(a). The Mo 3d core level spectra consist of four peaks: Mo 3d5/2 (MoS2), Mo 3d3/2 (MoS2), Mo 3d5/2 (MoO3), and Mo 3d3/2 (MoO3). The shape of the peaks changed, and two additional peaks near 234 and 236 eV corresponding to Mo 3d5/2 (MoO3) and Mo 3d3/2 (MoO3), are observed in the spectra, compared with the Mo 3d core level spectra of the assynthesized samples in Figure 2(c). The oxygen atoms from ambient air could be adsorbed to the surface of the MoS2 at high temperatures, transforming MoS2 to MoO3 at the surface. The peak intensities of Mo 3d5/2 (MoO3) and Mo 3d3/2 (MoO3) gradually increases with respect to increase of operating temperatures. Furthermore, the atomic ratio of each sample that underwent measurement at different temperatures significantly changes, as shown in Figure 6(b). These changes strongly affect the sensing behavior of MoS2 at different temperatures. To understand the unprecedented enhancement in the gas sensing properties of SiO2 NRs encapsulated by vertically aligned MoS2, we considered three basic factors including i) transducer function, ii) utility factor, and iii) receptor function, that strongly affect the sensing 13 ACS Paragon Plus Environment


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performance of chemoresistive gas sensors.34 The transducer function refers to the ability to convert the signal caused by chemical adsorption of the target surface into an electrical signal and the utility factor refers to the ability of inner grains of target materials to access the target gas. Lastly, the receptor function refers to the ability of the oxide surface to interact with the target gas. The schematic illustration (Supporting Information, Figure S9) shows the underlying mechanism for the enhancement in the gas response of SiO2 NRs encapsulated by MoS2. While the plane MoS2 on flat SiO2 substrate (Figure 7(a)) exposes restricted number of edge sites at the grain boundaries due to large domain size corresponding to Figure 2(e), the vertically aligned MoS2 on SiO2 NRs (Figure 7(b)) exposes much larger number of edge sites due to more grain boundaries and Mo-terminated edges resulting from smaller domain sizes of synthesized MoS2 as shown in TEM images (Figure S3). Since the edge sites of MoS2 have high d-orbital electron density, they can act as the highly catalytic sites for gas adsorption and desorption (receptor function). The larger number of edge sites for SiO2 NRs encapsulated by MoS2 was also experimentally supported by the intensity ratio (E12g/A1g) and broadening of FWHM in Raman spectroscopy as already explained in Figure 3. In addition, vertically ordered 1D NRs with extremely large surface-to-volume ratio provide excellent accessibility of target gases for high sensing performance as already demonstrated in our previous studies (utility factor).9,14 More the gas molecules get accessed, higher the gas response and the number of edge sites participating in the reaction. Moreover, the current pathways between the SiO2 NRs encapsulated by MoS2 are localized to more effectively modulate the potential barrier upon exposure to target gas molecules (transducer function). Therefore, SiO2 NRs encapsulated by MoS2 with high porosity (48%) contributes to improvement in all three basic factors and is a very effective structure for the enhancement of the gas sensing properties. 14 ACS Paragon Plus Environment

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CONCLUSION In summary, we demonstrated significantly enhanced NO2 sensing properties in SiO2 NRs encapsulated by MoS2 prepared by a facile process. The sensors showed extremely high sensing behaviors with a detection limit of ~2.3 ppb at room temperature. Upon exposure to 50 ppm NO2, the gas response of 500MoS2 (500-nm-thick SiO2 NR) was 90 times higher than that of PMoS2 (flat SiO2 substrate) at room temperature. Additionally, an operating temperature at 100 °C, the fabricated sensor exhibited reliable full recovery after gas sensing with a detection limit of ~8.84 ppb. Our results clearly show that well-fabricated 1D SiO2 NRs with porous structure act as a platform to expose the edge sites of MoS2. Furthermore, the vertically aligned (edge site exposed) MoS2 along the SiO2 NRs play a crucial role as receptors in gas adsorption and desorption. The proposed facile fabrication technique for numerous edge sites of MoS2 would be useful for extending the potential of TMD-based devices, for examples, hydrogen evolution catalysts in water splitting, cathodic materials in battery, and charge transport layers in optoelectronic devices.


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EXPERIMENTAL METHODS Synthesis of MoS2 on SiO2 NRs. SiO2 (300 nm)/Si wafers were cleaned using conventional cleaning procedures followed by ultra-sonication in acetone, isopropyl alcohol, and deionized (DI) water. The SiO2 NRs, with thickness of 120, 250, and 500 nm, were deposited using the glancing angle deposition (GAD) method with the e-beam evaporator.12 A rotation speed of 80 rpm and a tilt angle of 83° were used to deposit the SiO2 NRs on the SiO2/Si substrate with the Pt-IDEs. To obtain clean and hydrophilic surfaces of SiO2/Si wafers, UV-O3 surface treatments were performed for 20 min. The solution precursor was prepared by dissolving ammonium tetrathiomolybdate [(NH4)2MoS4] powder (Sigma-Aldrich, 99.97% purity) in ethylene glycol (Sigma-Aldrich, 99.8% purity, anhydrous) at a concentration of 200 mM. The solution precursor was spin-coated onto the flat Pt-IDEs and the SiO2 NRs (120, 250, and 500 nm) deposited on the Pt-IDEs at 3500 rpm for 60 s. The precursor-coated substrates were annealed at 50 °C to evaporate the residual solvent. High-purity (99.999%) H2 and N2 gases were used for the thermolysis process in a thermal CVD system. To avoid carbon contamination from the solvent, initially, the temperature of the CVD chamber was increased to 500 °C, and this temperature was maintained for 40 min under the flow of H2 and N2 at 1 Torr. Then, the temperature of the CVD chamber was increased to 900 °C, and this temperature was maintained for 40 min. The sublimation of sulfur powder was initiated in the other heating zone, where the temperature was set to 300 °C. The thermolysis process was carried out for 1 h. After the thermolysis process, the CVD chamber was cooled down to room temperature under the flow of N2/H2 gas mixture. Transfer process of MoS2 on the flat SiO2 substrate. The as-synthesized MoS2 thin film was transferred onto arbitrary substrates such as SiO2/Si wafer, glass substrate, and Si wafer. For this, poly methylmethacrylate (PMMA) supporting polymer solution (2 g/50 mL in 17 ACS Paragon Plus Environment


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chlorobenzene) was spin-coated onto the surface of the as-synthesized MoS2 thin films. The PMMA/MoS2/SiO2/Si substrates were then immersed into a mixed solution of hydrofluoric acid and buffered oxide etchant in a 1:3 volume ratio to separate the MoS2 thin films from the SiO2/Si wafer. The separated PMMA/MoS2 thin films were rinsed with DI water 7 to 9 times to remove the residual etchants and were transferred onto an arbitrary substrate. After the PMMA/MoS2 thin film was perfectly adhered to the substrate, the PMMA layer was removed using an acetone and toluene bath at 120 °C for 1 h. Material characterizations. The Raman spectra of the synthesized MoS2 were recorded with a Lab RAM HR (Horiba JobinYvon, Japan) at an excitation wavelength of 532 nm. The morphologies of the MoS2 fabricated on the SiO2 NRs were characterized by field-emission SEM (MERLIN-Compact) at an acceleration voltage of 1 kV and a working distance of 3 mm. For cross-sectional TEM analysis, conventional mechanical polishing was followed by Ar ion milling. Bright- and dark-field high resolution images were obtained by using a fieldemission TEM (JEM-2100F) to investigate the morphology and the structure of the MoS2 fabricated on the SiO2 NRs with different thicknesses. Furthermore, the crystalline orientations were measured by XRD (D8 Advance), and elemental mapping of the fabricated samples was performed by EDS. Measurements of gas sensing properties. The gas sensing measurements were performed in a quartz tube with an external heat source. Through an automated system, a repeating sequence of dry air and target gas (balanced with dry air, Sinjin Gases) injection occurred. A constant flow of 1000 sccm was used for both dry air and target gas. The concentration of the NO2 gas used in this study was fixed at 50 ppm. The response to NO2 gas was precisely measured at a DC bias voltage of 1 V using a sourcemeter (Keithley 2400). The gas flow was


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controlled using a mass flow controller, and all the measurements were recorded by LabVIEW using a general purpose interface bus.



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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10/1021/acsnano. Figure S1 shows the top-view and the cross-sectional SEM images of the fabricated samples which are 120, 250, 500 nm-thick SiO2 NRs encapsulated by MoS2. Figure S2 shows the porosity of the SiO2 NRs encapsulated by MoS2 obtained using MATLAB. Figure S3 displays the HR-TEM images of vertically aligned MoS2 with 120-, 250-, and 500 nm-thick SiO2 NRs. Figure S4 displays the XRD analysis of MoS2 on flat SiO2 and SiO2 nanorods with different lengths. Figure S5 shows the measurement systems, and Figure S6 displays the effect of thickness of SiO2 NR on solution precursor spin-coating. Figure S7 shows the response curves of the fabricated samples to 50 ppm NO2 at room temperature. Figure S8 displays the gas sensing properties of the fabricated samples at different temperatures. Figure S9 shows schematic illustration of the proposed gas sensing mechanism about the fabricated samples. Figure S10 displays the selectivity measurement (CH3COCH3 and NH3) of fabricated samples This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *C.-Y. K. E-mail: [email protected] *H. W. J. E-mail: [email protected]

Author Contributions †Y.-S. Shim and K. C. Kwon contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Multi-Ministry Collaborative R&D Program (Development of Techniques for Identification and Analysis of Gas Molecules to Protect Against Toxic Substances) through the National Research Foundation (NRF) of Korea funded by the KNPA, MSIT, MOTIE, ME, and NFA (2017M3D9A1073501), and the Nano-material Technology Development Program through the NRF of Korea funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4910). Jun Min Suh acknowledges the Global Ph.D. Fellowship Program through the National Research Foundation of Korea funded by the Ministry of Education (2015H1A2A1033701). 20 ACS Paragon Plus Environment

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[Figure 1]

Figure 1. (a) Schematic illustration of the fabrication process of SiO2 NRs encapsulated by MoS2. Cross-sectional TEM images of the (b) 120-, (c) 250-, and (d) 500 nm-thick SiO2 NRs encapsulated by MoS2. (e) EDS mapping images of the SiO2 NRs encapsulated by MoS2.


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[Figure 2]

Figure 2. Transmission electron microscopy (TEM) images with low magnification for (a) plane MoS2, (b) MoS2 on 120 nm SiO2 NRs, (c) MoS2 on 250 nm SiO2 NRs, and (d) MoS2 on 500 nm SiO2 NRs. High-resolution transmission electron microscopy (HR-TEM) images of (e) plane MoS2 synthesized on flat SiO2/Si substrate and (f) vertically aligned MoS2 synthesized on SiO2 nanorod with 500 nm length.



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[Figure 3]

Figure 3. (a) Raman spectra of MoS2 on flat SiO2 and SiO2 nanorods with different lengths. (b) Intensity ratio of E12g to A1g (left, red) and the full-width-half-maximum values (right, blue and pink) plotted against with different SiO2 nanorod lengths. XPS core level spectra of (c) Mo 3d and (d) S 2p for SiO2 nanorod encapsulated by MoS2 with various lengths of SiO2 nanorods.


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[Figure 4]

Figure 4. (a) Baseline resistance of each samples in ambient air. (b) Response transient of MoS2 on flat SiO2 substrate (PMoS2) and SiO2 nanorod encapsulated by MoS2 with different lengths (120MoS2, 250MoS2, and 500MoS2) to 50 ppm NO2 at room temperature. (c) The gas response of each samples in (b).



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[Figure 5]

Figure 5. (a) Response transients, (b) response (red, left) and recovery (blue, right) of the SiO2 nanorods encapsulated by MoS2 (500MoS2) to 50 ppm NO2 vs. operating temperature. (c) Response transients of the SiO2 nanorods encapsulated by MoS2 (500MoS2) to 3 pulses of 50 ppm NO2 at room temperature and 100 °C. (d) Response transients of the SiO2 nanorods encapsulated by MoS2 toward 1‒5 ppm NO2. Inset in (d) show the calibration for the response as function of NO2 concentration.


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[Figure 6]

Figure 6. (a) XPS core level spectra of Mo 3d with various operating temperatures. (b) Atomic ratio of SiO2 nanorods encapsulated by MoS2 (500MoS2) after annealing at different temperatures (50, 100, 150, and 200 °C) in ambient air.



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[Figure 7]

Figure 7. Schematic illustration of (a) the plane MoS2 on flat Pt-IDE/SiO2/Si substrate and (b) the SiO2 NRs encapsulated by partially vertical aligned (edge site exposed) MoS2. The gas adsorption and desorption in vertically aligned MoS2 is more reactive than plane MoS2.


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[TOC Figure]


Synthesis of Numerous Edge Sites in MoS2 via SiO2 Nanorods

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