SnS2 Nanograins on Porous SiO2 Nanorods Template for Highly

Feb 25, 2019 - ... chemoresistive gas sensors for Internet of Everything applications, low ... reported for SnS2-based gas sensors to the best of our ...
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SnS Nanograins on Porous SiO Nanorods Template for Highly Sensitive NO sensor at Room Temperature with Excellent Recovery 2

Ki Chang Kwon, Jun Min Suh, Tae Hyung Lee, Kyoung Soon Choi, Koo Tak Hong, Young Geun Song, Young-Seok Shim, Mohammadreza Shokouhimehr, Chong-Yun Kang, Soo Young Kim, and Ho Won Jang ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b01526 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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SnS2 Nanograins on Porous SiO2 Nanorods Template for Highly Sensitive NO2 sensor at Room Temperature with Excellent Recovery Ki Chang Kwon,†abc Jun Min Suh,†a Tae Hyung Lee,a Kyoung Soon Choi,d Kootak Hong,a Young Geun Song,ef Young-Seok Shim,g Mohammadreza Shokouhimehr,a Chong-Yun Kang,eh Soo Young Kim*b and Ho Won Jang*a a. Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea. b. School of Chemical Engineering and Materials Science, Integrative Research Center for Two-Dimensional Functional Materials, Institute of Interdisciplinary Convergence Research, Chung-Ang University, Seoul 06974, Republic of Korea. c. SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, College of Optoelectronic Engineer-ing, Shenzhen University, Shenzhen 518060, China. d. Advanced Nano-Surface Research Group, Korea Basic Science Institute (KBSI), Daejeon 34133, Republic of Korea. e. Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea. f. Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul 02841, Republic of Korea. g. Department of Materials Science and Engineering, KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. h. KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea. KEYWORDS: tin disulfide, room temperature, gas sensor, nitrogen dioxide, glancing angle deposition

ABSTRACT: In order to develop high performance chemoresistive gas sensors for Internet of Everything applications, low power consumption should be achieved due to the limited battery capacity of portable devices. One of the most efficient ways to reduce the power consumption is to lower the operating temperature to room temperature. Herein, we report superior gas sensing properties of SnS2 nanograins on SiO2 nanorods toward NO2 at room temperature. The gas response is as high as 701 % for 10 ppm NO2 with excellent recovery characteristics and the theoretical detection limit is evaluated to be 408.9 ppb at room temperature, which have not been reported for SnS2-based gas sensors to the best of our knowledge. The SnS2 nanograins on the template used in this study have excessive sulfur component (Sn:S = 1:2.33) and exhibit p-type conduction behavior. These results will provide a new perspective of nanostructured 2-dimensional materials for gas sensor applications on demand.

With the technology development in data acquisition and processing, there have been increasing demands on high performance sensors for the data collection. Especially, the sensors for detecting gaseous substances have been one of the most intensively studied fields since the data they can provide directly benefit human health and life. For examples, indoor air quality data prevent people from exposure to toxic gases like volatile organic compounds, which are usually emitted in very low concentration but continuous exposure can lead to severe damage in human body.1 In addition, the examination of the exhaled breath from people can provide helpful information for diagnosis of their potential diseases.2 In order to col-

lect the gaseous data from various sources in real-time and process them into useful information through network under Internet of Everything (IoE) platform, the gas sensors should be small-sized for portability and consume very low power due to a limited battery capacity.3 To date, various types of gas sensors have been reported including electrochemical sensors, optical sensors, surface acoustic wave sensors, mass sensitive sensors, and chemoresistive sensors.4 Compared to other types of gas sensors, chemoresistive gas sensors are developed in very simple structure and relatively cheap in economic point of view, having strength in future commercialization. The chemoresisitve gas sensors detect the sensor signals from

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changes in electric current that continuously flows along the sensing materials, and the most widely adopted sensing materials have been the metal oxide semiconductors like SnO2, or WO3 due to their abundant surface defects.57 However, they require very high operating temperature over 300 °C for activation of the surface defects and meaningful sensor signals followed by inevitable high power consumption. As an alternative, two-dimensional (2D) materials including graphene, transition metal dichalcogenides (TMDs), or metal nanosheets, have been attracting a great amount of attention for their high surface-to-volume ratio with high surface energy, which are beneficial for room-temperature operation.8-10 Among various 2D materials, tin disulfide (SnS2) has been intensively studied for gas sensor application due to its earth-abundant, nontoxic, and heavy-metal-free characteristics with layered 2D structure just like TMDs like MoS2, WS2, or VS2 as well as superior mechanical properties and remarkable electronic properties.11 J. Ou et al. reported NO2 gas sensors based on SnS2 with high selectivity and reversibility.12 According to them, SnS2 has larger electronegativity compared to MoS2, leading to potentially increased active sites for gas adsorption. Their SnS2 nanoflakes prepared by solution methods exhibited the response of 27.2 to 10 ppm NO2 but still required external heater for operating temperature of 80 °C. K. Xu et al. reported SnO2-SnS2 hybrids for NH3 gas sensors operating at room temperature.13 Partially oxidized SnS2 with utilization of heterojunctions between SnS2 and SnO2 was effective in room temperature operation but exhibited relatively low response of 1.16 to 10 ppm NH3. In order to achieve gas sensors based on SnS2 operating at room temperature with high performance, further efforts are evidently required. Herein, we report highly sensitive NO2 sensors operating at room temperature with excellent recovery using SnS2 nanograins synthesized on porous SiO2 nanorods (NRs). This strategy was adopted from our recent study applied to MoS2.14 We deposited 500 nm of SiO2 NRs by glancing angle deposition (GAD) method using e-beam evaporator. The precursor solution for SnS2 was spincoated on the SiO2 NRs and sulfurization process using chemical vapor deposition (CVD) system was conducted to synthesize SnS2. Uniformly distributed SnS2 on SiO2 NRs exhibited the gas response of 701% to 10 ppm NO2 at room temperature with a theoretical detection limit of 408.9 ppb. This promising gas sensing properties with excellent recovery at room temperature can provide a new perspective towards real application of SnS2-based gas sensors to IoE.

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IDEs for a selective deposition on the active area. SiO2 grains (Kojundo Chemistry) were used for the deposition of SiO2 NRs. In order to fabricate SiO2 NRs, evaporation was conducted at a glancing angle of 80° with substrate rotation speed of 80 rpm. The substrate was located 50 cm above the crucible with SiO2 grains. The base pressure was maintained at 1 × 10-6 Torr and growth rate was 2.0 Å/s. Synthesis of SnS2 nanograins on SiO2 NRs. The precursor solution was prepared by dissolving tin (IV) chloride pentahydrate (SnCl4∙5H2O, Sigma-Aldrich, 98%) in ethylene glycol at a concentration of 1 M. The precursor solution was spin-coated on prepared IDEs and SiO2/Si substrates at 3500 rpm for 60 s. Hydrogen (H2) and nitrogen (N2) gases with high purity were used for the thermolysis process in a thermal CVD system with dual furnaces. Firstly, the temperature of CVD furnace (Heater #2 in Figure 1) for precursor-coated sample was increased to 500 °C under the flow of H2 and N2 at 1 Torr during 20 min. The flow rate of H2 and N2 was set at 100 and 500 sccm, respectively, by using mass flow controllers. After 20 min, the CVD furnace (Heater #1 in Figure 1) with temperature of 300 °C for S powder was moved through sliding rail to the set position to evaporate the S powder for the sulfurization process. The sulfurization process was started slowly under constant flow of H2 and N2 gas and maintained for 30 min. After 30 min, both furnaces were cooled down to room temperature with rate of 20 °C/min. Characterization. The morphologies of the fabricated the bare SiO2 NRs and the SnS2 nanograins on SiO2 NRs were characterized by a field-emission scanning electron microscopy (FESEM, SU-70) using an acceleration voltage of 5 kV and a working distance of 8 mm. Bright field and high-resolution images were obtained with a fieldemission transmission electron microscopy (FETEM, JEM2100F) in order to investigate the morphology and structure of the bare SiO2 NRs and the SnS2 nanograins on SiO2 NRs. The chemical mapping of the SnS2 nanograins on SiO2 NRs was also obtained by energy dispersive X-ray spectroscopy (EDS). The X-ray photoemission spectroscopy (XPS) was carried out using AXIS Ultra DLD instrument (Kratos, U.K.) in an advanced in-situ surface analysis system (AISAS; KBSI, Korea) operating at a base pres-

EXPERIMENTAL SECTION Fabrication of SiO2 NRs. Interdigitated electrodes (IDEs) with 5 μm spacing were fabricated by depositing Pt/Ti (100 nm/ 30 nm thick) on SiO2/Si substrate using photolithography and e-beam evaporation. The fabricated electrodes were sonicated in acetone, isopropanol and distilled water in sequence. Then the electrodes were concealed with scotch tape except the active area with

ACS Paragon Plus Environment Figure 1. Schematic images of fabrication procedure for SnS2 nanograins on SiO2 NRs.

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sure of 1.6 × 10-10 mbar at 300 K with a monochromatic Al Kα line at 1486.69 eV to discover the chemical composition of synthesized SnS2 nanograins on SiO2 NRs. Gas sensing measurement. Gas sensing measurement was performed in a quartz tube with external heater for consistent room temperature of 25 °C. The dry air and target gases (balanced with dry air, Sinjin Gases) were injected in repeating sequences through automated program. A constant flow of 1000 sccm was used for both dry air and target gases. The concentration of all gases used in this study was fixed at 500 ppm except NO2 which was 10 ppm and H2 which was 10000 ppm. The change in electric current by gas sensors was precisely measured by a sourcemeter (Keithley 2400) under DC bias voltage of 1 V.

RESULTS AND DISCUSSION The GAD method using e-beam evaporator was utilized to deposit SiO2 NRs on Pt-interdigitated electrodes (IDEs) with different thicknesses (125, 250, and 500 nm) for gas sensor application. By changing deposition angle and time, the diameters, lengths, and degree of porosity of SiO2 NRs can be controlled. The deposition angle of 80° and thicknesses of 125, 250, and 500 nm were chosen based on our previous studies and the detailed procedures for GAD method can also be found in our previous studies.15,16 After deposition of SiO2 NRs, 1 M tin (IV) chloride

pentahydrate (SnCl4∙5H2O) precursor solution which is dissolved in ethylene glycol was spin-coate d on the SiO2 NRs and sulfurization process using CVD system was conducted to synthesize the SnS2 nanograins on porous SiO2 NRs (Figure 1). The sulfur (S) powder and precursor coated-SiO2 NRs were placed inside the quartz tube with individual heater for each of them. The evaporated S powder reacted with spin-coated SnCl4∙5H2O precursor solution on SiO2 NRs with the following chemical equation: SnCl4 + S (excess) + 2H2 → SnS2 + HCl + H2S

(1)

The H2 gas was used as a dilute gas to get rid of chlorine radicals and the N2 gas was used as a carrier gas.17 Figure 2a-c show cross-sectional scanning transmission electron microscopy (STEM) images of SnS2 nanograins on SiO2 NRs in 125, 250, and 500 nm thicknesses, respectively. The synthesized SnS2 nanograins can be clearly seen through the rough surface of the SiO2 NRs for all thicknesses. From SEM images in Figure S1, the SnS2 nanograins were distributed not on the individual SiO2 NR but on a group of SiO2 NRs. The decrease in the concentration of SnCl4∙5H2O precursor solution from 1 M can help SnS2 distribution on individual SiO2 NR but the electrical re-

Figure 2. STEM images of SnS2 nanograins on SiO2 NRs with various thicknesses of SiO2 NRs: (a) 125 nm, (b) 250 nm, and (c) 500 nm. (d-g) EDS mapping of (d) Si, (e) O, (f) Sn, and (g) S, (h) HRTEM, and (i) SAED pattern images of SnS 2 nanograins on SiO2 NRs with 500 nm thickness. ACS Paragon Plus Environment

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sistance increases at the same time. The resistance increases with decreasing concentration of precursor solution due to a limited amount of SnS2 at the conduction path of inter-spacing between SiO2 NRs. The balance between the grouping of SnS2 nanograins on SiO2 NRs and electrical conductivity were met at the concentration of 1 M solution. EDS analysis was conducted on SnS2 nanograins on SiO2 NRs with 500 nm thickness and EDS mappings of Si, O, Sn, and S are shown in Figure 2d-g. The uniformly distributed Sn and S components on SiO 2 NRs were clearly identified. EDS mappings for SnS2 nanograins on SiO2 NRs with 125 and 250 nm thickness are also presented in Figure S2. High resolution transmission electron microscopy (HRTEM) image and corresponding selected area diffraction (SAED) image are shown in Figure S3. Nano-sized grains of SnS2 can be found on the surface of SiO 2 NRs through the HRTEM image. The SnS2 nanograins on the surface of the SiO2 NRs had a fringe interval of 0.32 nm for d-spacing of (100) and 0.20 nm for d-spacing of (003). The magnified HRTEM and SAED pattern images in Figure 2h and i clearly show d-spacing of 0.32 nm for SnS2. AFM and XPS analyses were done to further character-

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ize synthesized SnS2 (Figure S4). In Figure 3a and b, the atomic force microscopy (AFM) images of SnS2 on flat SiO2 substrates were obtained to examine thickness and rms roughness of the synthesized SnS2, and they were 17 nm and 0.688 nm, respectively. The XPS analysis was conducted to investigate the chemical composition of the synthesized SnS2. Sn 3d and S 2p core level spectra of SnS2 synthesized on flat SiO2 and SiO2 NRs with different thicknesses are shown in Figure 3c and d, respectively. The peaks for Sn 3d3/2 and Sn 3d5/2 were observed at approximately 495.8 and 487.5 eV, respectively. Those for S 2p3/2 and S 2p1/2 were located at approximately 165.3 and 164.0 eV, respectively. A little peak shift of Sn 3d peaks, as shown in Figure 3c, originated from the structural factor in the SnS2 synthesized onto SiO2 NRs compared with that on flat SiO2 substrates. From peak intensity and atomic sensitivity factors, atomic composition ratio was calculated for Sn and S. As shown in Figure S5, the ratio between Sn and S was consistent for all substrates with ratio of approximately 30 to 70, which can be reduced to 1:2.33 (Sn:S). Although HRTEM and SAED pattern data supported well-synthesized SnS2, actual atomic ratio between Sn and S was a little bit higher than 1:2.

Figure 3. (a) Thickness and (b) rms roughness measurement through AFM analysis of synthesized SnS2 thin film on the flat SiO2 substrate. The XPS core level spectra of (c) Sn 3d and (d) S 2p.

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Table 1. Comparison of NO2 sensing performance of the previously reported gas sensors based on SnS2.

Material

Structure

Synthesis method

Temp (oC)

Target gas

Balance gas

Concen.

Response (%)

Ref.

SnS2

Nanograin (SiO2 NRs platform)

Chemical vapor deposition

RT

NO2

Air

10 ppm

701

This Work

SnS2

Nanoflake

Wet chemical

120

NO2

Air

10 ppm

3533

12

SnS2

Nanoparticle

Precipitation

300

NO2

Air

1 ppm

~78

21

SnS2

Nanoflake

Wet chemical

150

NO2

Air

1 ppm

60

22

SnS2

Nanosheet

Purchased

80

NO2

Air

8 ppm

~65.3

18

SnS2

Nanopowder

High-energy ball milling

250

NO2

N2

10 ppm

1190.1

23

SnS2

Nanosheet

Hydrothermal method

RT

NO2

Air

10 ppm

~30

24

SnS2

Nanoflower

Hydrothermal method

120

NO2

Air

0.5 ppm

2440

25

This sulfur-rich (S-rich) nature of SnS2 can be attributed to nano-sized grains of SnS2 with numerous exposed Stermination, which will be discussed later with their effects on gas sensing properties. The SnS2 nanograins on SiO2 NRs with different thicknesses were exposed to 10 ppm NO2 at room temperature to investigate their gas sensing properties. The constant voltage of 1 V was applied to the SnS2 nanograins on SiO2 NRs under repeated pulses of synthetic air and NO2 gas. The gas response was calculated by the ratio between the resistance change upon exposure to target gas molecules and the resistance under target gas, or (Rair - Rgas) / (Rgas) × 100, where Rair and Rgas stand for the resistance under ambient air and target gas, respectively. As shown in Figure 4a, the gas response to 10 ppm NO2 increased for the SnS2 distributed on thicker SiO2 NRs and SnS2 on SiO2 NRs with 500 nm thickness exhibited the highest gas response of 701 %. To the best of author’s knowledge, there have been no report on such a high gas response at room temperature using SnS2 without any additives or heterojunctions according to previous reports (Table 1).12,13,18-25 In addition, SnS2 nanograins on SiO2 NRs exhibited excellent recovery after exposure to target gas molecules. The response time and recovery time were calculated to be 272.8 s and 3800.4 s, respectively, which are relatively slow in practical aspects, but still promising considering operation at room temperature. The inset in Figure 4b shows the baseline resistance increment upon increase in the thickness of SiO2 NRs. Since the thicker SiO2 NRs provide the larger surface area for larger amount of SnS2 nanograins to be synthesized on, more gas adsorption sites are present as well as the higher porosity for the better accessibility of the gas molecules. Moreover, our pervious study revealed that SiO2 NRs template induced more edge sites of 2D materials to be exposed through smaller-sized grains, which is also realized through nano-sized SnS2 grains in this study.14 Therefore, more exposed edge sites of SnS2 nanograins on SiO2 NRs are expected to have contributed to the enhanced gas sensing properties. At the same time, increased conduction path along the longer SiO2 NRs increased electrical resistance. Interestingly, the resistance

decreased after exposure to 10 ppm NO2 gas and increased after exposure back to ambient air, meaning p-type semiconducting characteristics. According to previous studies, SnS2 is widely known as n-type semiconductor.26 For ntype TMDs, S vacancies are present as defects on the basal plane of TMDs and function as electron donors, indicating that the Fermi level is shifted toward the conduction band minimum (CBM).27 On the other hand, when S atoms are rich and metal atoms is relatively deficient, the Fermi level is shifted away from the CBM, exhibiting ptype semiconducting properties. Therefore, the p-type semiconducting characteristics of SnS2 nanograins on SiO2 NRs synthesized in this study can be attributed to Srich nature, which was already revealed through atomic ratio (Sn:S = 1:2.33) obtained from XPS analysis. In addition to it, Cho and Hahm et al. reported n-type MoS2 changing to p-type MoS2 after transferring from sapphire glass to SiO2/Si substrate.28 The dangling oxygen bonds on SiO2/Si substrate can modulate the effective Fermi levels within the band gap of MoS2, leading to change in the conduction type. Since SnS2 used in this study was also synthesized on SiO2 NRs, the same phenomenon can possibly have contributed to the p-type semiconducting nature of the SnS2 nanograins. For the further investigation on gas sensing properties of SnS2 nanograins on SiO2 NRs, the detection limit and stability upon repeated exposure to the target gas were tested. As shown in Figure 4c, SnS2 nanograins on SiO2 NRs were exposed to five consecutive pulses of NO2 gas with 1, 2, 3, 4, and 5 ppm concentration at room temperature. The gas responses exhibited a linear relationship with the gas concentration, indicating their reliable operation over the tested concentration range (Figure 4d). The slope of the concentration-toresponse curve was calculated to be 0.462 ppm-1 using a linear least-squares fit.29 Although 1 ppm was the lowest concentration of the tested NO2 gas, the theoretical detection limit (signal-to-noise ratio higher than 3) was calculated to be approximately 408.9 ppb.

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Figure 4e shows a stable gas response upon seven consecutive exposures to 10 ppm NO2 gas at room temperature, demonstrating a stability of the fabricated gas sensors. The selectivity toward various gases were also tested for the SnS2 nanograins on SiO2 NRs (Figure 4f). The fabricated gas sensors were exposed to H2 (10000 ppm), C2H5OH (500 ppm), CO (500 ppm), and NH3 (500 ppm) at room temperature. Since NO2 is widely known as one of the most reactive oxidizing gas and other gases are expected to exhibit very low response when the concentration was fixed at the same concentration to NO2 gas (10 ppm), above gases with the concentration higher than 500 ppm were used for the examination. However, the gas responses to H2, C2H5OH, CO, and NH3 were significantly lower than the gas response to 10 ppm NO2 even with the higher concentration indicating strongly selective behavior toward NO2 by the SnS2 nanograins on SiO2 NRs. Additionally, the effects of humidity and external heat supply on gas sensing properties of the SnS2 nanograins on SiO2 NRs were investigated as shown in Figure S6 and S7. With exposure to relative humidity of 50% (RH50), the base resistance increased and the recovery characteristic was improved. Since ambient atmosphere is always composed of certain amount of humidity, this can be promis-

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ing results considering future practical application of SnS2 nanograins on SiO2 NRs. With external heat supply, the response time and recovery time were both improved, but the gas response decreased with increasing temperature. The baseline resistance decreased with increasing operating temperature, which is probably due to thermally activated charge carriers. The promising gas sensing properties of the SnS2 nanograins on SiO2 NRs can be attributed to three different factors: i) nanograins of SnS2 with numerous Stermination ii) O2-repulsive surface characteristics of SnS2, and iii) 1D vertically aligned nanost ructures of SnS2 nanograins on SiO2 NRs. As previously mentioned, S-rich condition was the main reason for the p-type semiconducting characteristics of the synthesized SnS2 used in this study. When SnS2 experiences S-rich or Sn-deficient condition, the structural defects are formed according to McDonnell et al.27 They studied non-uniform stoichiometry over a single MoS2 flake, and revealed that S-rich region exhibits p-type semiconducting properties. In addition, a high concentration of dark defects at the S-rich region observed using scanning tunnelling microscopy (STM) measurements are expected to be missing layers of

Figure 4. (a) Response transient of SnS2 on each different substrate to 10 ppm NO2 at room temperature. (b) Responses depending on the thickness of SiO2 NRs, and inset indicates resistance depending on the thickness of SiO2 NRs. (c) Response curves and (d) calibration of responses for SnS2 nanograins on SiO2 NRs toward 1-5 ppm NO2 at room temperature. Responses of SnS2 nanograins on SiO2 NRs to (e) 7 pulses of 10 ppm NO2 and (f) various gases at room temperature.

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MoS2 to relieve surface strain induced by excessive S. Since the 2D nanostructures of S-sandwiched metal atoms is equivalent for all TMDs including MoS2 and SnS2, those defects with missing layers will also be expected to exist on the surface of S-rich SnS2 and these phenomena can lead to the formation of nano-sized grains of SnS2 grown on SiO2 NRs. Moreover, as depicted in Figure 5a, nanograins of SnS2 can expose either Sn-termination or Stermination. However, clean Sn-termination is energetically not favourable and therefore, Sn atoms prefer to adsorb additional S atoms in their coordination.30 J. V. Lauritsen et al. studied edge terminations of MoS2 and concluded that S dimers or S monomers at Mo-edge are two possible energetically favourable configurations based on STM images and density functional theory (DFT) calculations.31 These two configurations of fully coordinated six S atoms to each Mo atom can also be applied to SnS2 since it has just the same 2D structures as MoS2, as previously mentioned (Figure 5b and c). Therefore, energetically stable SnS2 contains either Sterminations or additional S-adsorbed Sn-terminations, leading to a little higher amount of S at the edge sites.

These resulted in overall high Sn:S (1:2.33) ratio exhibited by nano-sized grains of SnS2 synthesized and utilized in this study. At the same time, S-termination sites can act as energetically more active sites for the chemical reactions and maximized S-terminations of SnS2 nanograins resulted in the high gas sensitivity toward NO2 gas.32 In addition, SnS2 has been reported to have positive binding energy toward O2 and repels O2 molecules, indicating a different charge transfer mechanism compared to typical metal oxide semiconductors.12 The metal oxide semiconductors adsorb O2 components (O2-, O-, or O2- depending on operating temperature) on the surface and charge transfer occurs between adsorbed O2 components and the gas molecules.33 On the other hand, SnS2 does not utilize O2 components for charge transfer, but SnS2 itself interacts with the target gas molecules. This can also be supported by the data of gas sensing curves shown in Figure 4a. The initial stabilization process for typical metal oxide semiconductors under constant flow of dry air exhibits saturation of electrical resistance after gradual increase or decrease of the resistance. However, SnS2 exhibited no such stabilization process but consistent resistance inde-

Figure 5. (a) Ball model of a hypothetical bulk truncated SnS2 hexagon exposing both Sn- and S-termination. (b-c) Top-view and side-view ball models of Sn-termination with (b) S dimers and (c) S monomers per Sn-termination. (d) Schematic illustration of bare SiO2 NRs and SnS2 nanograins on SiO2 NRs.

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pendent of exposure to dry air, indicating no chemical interaction with O2 molecules in dry air. Under these unique surface characteristics of SnS2, NO2 has the strongest adsorption energy to SnS2 and exhibits distinct charge transfer from SnS2 compared to other gases according to the DFT calculation and electronic density of states calculation done by J. Z. Ou et al.12 Therefore, highly sensitive and selective detection of NO2 by SnS2 nanograins on SiO2 NRs could be achieved. The excellent recovery characteristics of SnS2 nanograins on SiO2 NRs can be attributed to p-type semiconducting nature of SnS2 synthesized in this study. When NO2 molecules are adsorbed on SnS2, electrons are transferred from SnS2 to NO2 to lower the resistance through hole generation. Since S-rich SnS2 has less electrons due to lacking electron donors of S-vacancies than typical SnS2, desorption process of NO2 is much more accelerated to supplement lacking electrons from NO2, contributing to the excellent recovery characteristics at room temperature. Moreover, as mentioned before, the utilization of SiO 2 NRs template resulted in more exposed edge sites of SnS2 on SiO2 NRs. This unique configuration of SnS2 on SiO2 NRs is also expected to have influenced better recovery characteristics along with electron deficient nature. Lastly, 1D vertically aligned nanostructure of nanograins on SiO2 NRs provided an effective platform for gas molecules to get accessed to SnS2 as illustrated in Figure 5d. The 1D nanostructures with high porosity provided maximized surface area for SnS2 and chance of more gas molecules to get adsorbed to SnS2 for extremely high gas responses toward NO2 gas.

CONCLUSIONS We demonstrated an effective and efficient strategy to dramatically enhance gas sensing properties of SnS2 through nano-sized grains of SnS2 and highly porous platform of SiO2 NRs. The SnS2 nanograins on SiO2 NRs exhibited a superior gas sensing performance toward NO2 gas with remarkable gas response and excellent recovery characteristics with extremely high selectivity at room temperature. We attribute them to S-rich nature of SnS2 induced by numerous S-termination of SnS2 nanograins, unique surface characteristics of SnS2, and 1D nanostructures of SnS2 nanograins on SiO2 NRs. We strongly believe that this study can provide a new perspective toward utilization of nanostructured 2D materials for gas sensor application.

ASSOCIATED CONTENT Supporting Information. Top view and cross-sectional SEM images for bare SiO2 NRs and SnS2 nanograins on SiO2 NRs, EDS mapping for SnS2 nanograins on SiO2 NRs with various thicknesses, HRTEM and SAED pattern images of SnS2 nanograins on SiO2 NRs, wide scan XPS spectra for SnS2 nanograins on SiO2 NRs, the atomic ratio of synthesized SnS2 on each different substrate, time-transient gas response curves under humidity and external heat supply. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *Prof. Ho Won Jang, Associate Professor of Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea; Office: (+82)2-880-1720; E-mail: [email protected] *Prof. Soo Young Kim, Professor of School of Chemical Engineering and Materials Science, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Republic of Korea; Office: (+82)2-820-5875; E-mail: [email protected]

Author Contributions H. W. Jang and S. Y. Kim conceived and supervised the project. K. C. Kwon and J. M. Suh fabricated the samples used in this research and analyzed the results. T. H. Lee conducted TEM analysis and K. S. Choi conducted XPS analysis. K. Hong conducted AFM analysis. Y. G. Song, Y.-S. Shim, M. Shokouhimehr participated in gas sensing measurements and discussion on the mechanisms. C.-Y. Kang provided facilities for the gas sensing measurements. The manuscript was mainly written by K. C. Kwon, J. M. Suh, S. Y. Kim and H. W. Jang. All authors discussed the results and commented on the manuscript. † These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT K. C. Kwon and J. M. Suh contributed equally to this work. This work was financially supported by the Basic Science Research Program (2017R1A2B3009135), Future Material Discovery Program (2016M3D1A1027666), and the Nano·Material Technology Development Program (2016M3A7B4910) through the National Research Foundation of Korea, and the International Energy Joint R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (20168510011350). J. M. Suh acknowledges the Global Ph.D. Fellowship Program through the National Research Foundation of Korea funded by the Ministry of Education (2015H1A2A1033701).

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Figure 1. Schematic images of fabrication procedure for SnS2 nanograins on SiO2 NRs. 190x161mm (300 x 300 DPI)

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Figure 2. STEM images of SnS2 nanograins on SiO2 NRs with various thicknesses of SiO2 NRs: (a) 125 nm, (b) 250 nm, and (c) 500 nm. (d-g) EDS mapping of (d) Si, (e) O, (f) Sn, and (g) S, (h) HRTEM, and (i) SAED pattern images of SnS2 nanograins on SiO2 NRs with 500 nm thickness. 190x203mm (300 x 300 DPI)

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Figure 4. (a) Response transient of SnS2 on each different substrate to 10 ppm NO2 at room temperature. (b) Responses depending on the thickness of SiO2 NRs, and inset indicates resistance depending on the thickness of SiO2 NRs. (c) Response curves and (d) calibration of responses for SnS2 nanograins on SiO2 NRs toward 1-5 ppm NO2 at room temperature. Responses of SnS2 nanograins on SiO2 NRs to (e) 7 pulses of 10 ppm NO2 and (f) various gases at room temperature. 156x199mm (300 x 300 DPI)

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Figure 5. (a) Ball model of a hypothetical bulk truncated SnS2 hexagon exposing both Sn- and Stermination. (b-c) Top-view and side-view ball models of Sn-termination with (b) S dimers and (c) S monomers per Sn-termination. (d) Schematic illustration of bare SiO2 NRs and SnS2 nanograins on SiO2 NRs. 190x152mm (300 x 300 DPI)

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