Surface Functionalization of Layered Molybdenum Disulfide for the

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Functional Inorganic Materials and Devices

Surface Functionalization of Layered Molybdenum Disulfide for the Selective Detection of Volatile Organic Compounds at Room Temperature Winston Yenyu Chen, Chao-Chun Yen, Sichuang Xue, Haiyan Wang, and Lia Stanciu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b13827 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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ACS Applied Materials & Interfaces

Surface Functionalization of Layered Molybdenum Disulfide for the Selective Detection of Volatile Organic Compounds at Room Temperature Winston Yenyu Chen,†,‡ Chao-Chun Yen,§ Sichuang Xue,† Haiyan Wang,† and Lia A. Stanciu†,‡,* †School

‡Birck

§Department

of Materials Engineering, Purdue University, West Lafayette, IN 47907, USA. Nanotechnology Center, Purdue University, West Lafayette, IN 47907 USA

of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

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ABSTRACT Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) are considered promising sensing materials due to the high surface-to-volume ratio and active sensing sites. However, the reported strategies for 2D TMDCs towards sensing of volatile organic compounds (VOCs) present with some drawbacks. These include high operation temperatures, low gas response, and complex fabrication, limiting the development of room-temperature gas sensors. In this study, 2D MoS2 nanoflakes were prepared by liquid phase exfoliation (LPE), and their surface was functionalized with Au nanoparticles (NPs) through a facile solution mixing method. MoS2 decorated with Au NPs with average size of 10 nm were used as material platform for an electrochemical sensor to detect a wide variety of VOCs at room temperature. Through dynamic sensing tests, the enhancement of gas-sensing performance in terms of response and selectivity, especially in detecting oxygen-based VOCs (acetone, ethanol, and 2-propanol) was demonstrated. After Au functionalization, the response of the gas sensor to acetone improved by 131% (changing from 13.7% for pristine MoS2 to 31.6% for MoS2-Au(0.5)). Sensing tests under various relative humidity values (10-80%), bending or long-term conditions indicated the sound robustness and flexibility of the sensor. Density function theory (DFT) simulations suggested that the adsorption energy of VOC molecules on MoS2-Au is significantly higher than that on pristine MoS2, contributing to the gas-sensing enhancement; a VOC sensing mechanism for Au-decorated MoS2 nanoflakes was proposed for the first time for the highly sensitive and selective detection of oxygen-based VOCs. KEYWORDS: molybdenum disulfide (MoS2), liquid phase exfoliation, surface functionalization, gold nanoparticle, volatile organic compound (VOC), gas sensor 2 ACS Paragon Plus Environment

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1.

INTRODUCTION Volatile organic compounds (VOCs) are prevalent as air pollutants and carcinogens and are

thus harmful to human health.1 Furthermore, some VOCs from human breath serve as biomarkers for various diseases (e.g., acetone as breath biomarker for diabetes).2 Therefore, it is important to develop a facile and effective method to monitor VOCs, especially at room temperature. Furthermore, the development of gas sensors on flexible substrates for wearable electronics is becoming an important area of research, which has been proven to have a high potential for Internet of Things (IoTs).3 Recently, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention in various applications because they possess excellent structural, optical, and semiconducting properties.4-6 Particularly, 2D molybdenum disulfide (MoS2) is considered to be a promising sensing material because its high specific surface area not only offers elevated adsorption sites for gas molecules, but also contributes additional active sites for surface modification.7-9 MoS2 gas sensors have been used to monitor inorganic gases (NH3 and NO2) based on a charge transfer mechanism, and these characteristics offer clues for understanding the sensing behavior of MoS2 devices.10 However, to date, its sensing behavior toward VOCs has not been 3 ACS Paragon Plus Environment


well explored due to complicated and synergistic mechanisms, involving dipole-induced scattering, charge transfer, physisorption, and chemisorption.11,12 Moreover, in terms of desired electronic structure, a multilayered MoS2 is preferred for gas sensing to the single-layered counterpart due to the advantages of scalable fabrication process and stable sensing response.13,14 Various exfoliation methods are used to obtain 2D MoS2 nanoflakes among which liquid phase exfoliation (LPE) is a facile, cost-effective method for mass production of MoS2 nanoflakes.15-17 Furthermore, MoS2 nanoflakes fabricated by LPE provide more active sites (i.e., sulfur defects, vacancies, and edge sites) which are beneficial to gas adsorption,18 thus achieving a better recognition of specific analytes for gas sensing devices. Semiconducting-metal oxides (SMOs) and SMO/carbon-based composites are the two key materials for room-temperature sensing of VOCs which, as demonstrated by the selected literature reports in Table S1 (Supporting Information), typically detect one or a few VOCs. As reviewed recently, various approaches have been proposed to improve the SMO’s sensitivity and selectivity to target analytes, including hybridization with metal oxides (e.g., SnO2, ZnO, or WO3), conducting polymers or carbon-based materials, surface functionalization with noble-metal (e.g., Ag, Au, Nb, Pt) nanoparticles (NPs), and designing heterostructures.19-21 Similar strategies to 4 ACS Paragon Plus Environment

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enhance the sensing performance of 2D TMDCs have recently been reported.22,23 However, the reported VOC sensing strategies present with some drawbacks. These include high operation temperatures, low gas response, and complex fabrication, limiting the development of roomtemperature gas sensors. Notably, incorporation of noble-metal NPs is considered as an alternative way of modification of 2D TMDCs.24,25 Noble-metal NPs not only provide resistance to environmental oxidation and corrosion of sensing materials, but also offer a catalytic effect,26-28 leading to enhancements of sensing reactions and sensing performance.29-31 Additionally, noblemetal NPs act as electron traps that impede the rapid recombination of electrons and holes, which also benefits the electronic properties of sensing materials.32 In this work, we use a facile solution mixing strategy to functionalize MoS2 with Au NPs, integrating them into high-performance flexible VOC sensors. Both pristine MoS2 and Au functionalized MoS2 gas sensors show good VOC sensing performance at room temperature. Two types of VOCs, oxygen-based (acetone, ethanol, and 2-propanol) and hydrocarbon-based (hexane, toluene, and benzene) vapors were introduced to investigate sensing mechanisms, and a new sensing mechanism in conjunction with density function theory (DFT) simulations for the adsorption of Au NPs and VOC molecules was proposed. We observed an improvement of gas 5 ACS Paragon Plus Environment


sensing performance in terms of gas response and selectivity when using optimally Au-decorated MoS2 in comparison to devices constructed with pristine MoS2. We proposed a sensing mechanism that is likely involved in the detection of different VOCs and that explains the superior sensing performance induced by Au decoration in the detection of oxygen-based VOCs. 2. Materials and Methods 2.1. Fabrication of MoS2-nanoflake based sensors Figure 1 shows a schematic flow chart for the preparation of MoS2-based nanoflakes and gas sensors: (a) liquid-phase exfoliation of bulk MoS2 forming 2D-MoS2 nanoflakes, (b) functionalizing MoS2 nanoflakes with Au NPs by a solution mixing method, and (c) fabricating sensors by drop-casting the Au decorated (denoted MoS2-Au) nanoflakes onto interdigitated electrodes (IDEs) incorporated polyethylene terephthalate (PET) substrate.

Figure 1. Schematic process flows for the fabrication of MoS2-based nanoflakes and gas sensors: (a) liquid-phase exfoliation of bulk MoS2, (b) funcationalization of 2D MoS2 nanoflakes with Au NPs, and (c) fabrication of Au-deccorated MoS2 sensors for VOC detections. 6 ACS Paragon Plus Environment

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30 mg of 99% purity MoS2 powder with sizes < 2 μm from Sigma-Aldrich was dispersed in 15 mL of 45 vol% ethanol/water solution, and sonicated in a Branson® CPX (2800H) ultrasonic bath using high power mode at 0 °C for 4 h, followed by centrifugation at 6000 rpm for 1 h at room temperature. Then, the supernatant containing exfoliated MoS2 nanoflakes was collected. 99.9% gold(III) chloride hydrate (HAuCl4∙3H2O) with trace metal basis from Sigma-Aldrich was dissolved in deionized water. The water-HAuCl4 aqueous solution was then added to the MoS2 dispersion to modify MoS2 nanoflakes with Au NPs. Molar ratios of MoS2 to HAuCl4 were adjusted over a wide range of values from 1:0.25, 1:0.5, 1:1 and 1:2 for the preparation of MoS2Au nanoflakes and fabrication of gas sensors, denoted hereafter as MoS2-Au(0.25), MoS2-Au(0.5), MoS2-Au(1), and MoS2-Au(2), respectively. The broad range of Au decoration allows us to evaluate the effect of size and distribution of the Au particles on sensing performance. Metalon JS-B25HV conductive silver ink was purchased from NovaCentrix for printing of eight pairs of silver IDEs on a PET substrate using a Dimatix DMP-2850 inkjet printer. The total active area of the IDEs was 10 mm × 10 mm. The gas-sensing devices were fabricated by dropcasting each of the MoS2-Au and MoS2 (reference) aqueous dispersions onto the electrode surface,

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subsequently followed by drying treatment at 60 °C using a hot plate. The average thickness of asprepared films was 1.34  0.21 m, as measured by surface profilometry (P-7 Profiler, KLA). 2.2. Characterization and gas-sensing measurements The surface morphology, crystallinity and microstructure of MoS2 nanoflakes were first examined by scanning electron microscopy (SEM; S-4800, Hitachi), high-resolution transmission electron microscopy (HRTEM; Talos 200X, FEI), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Raman spectroscopy using 532 nm laser (LabRAM HR, Horiba) and X-ray photoelectron spectroscopy (XPS; AXIS Ultra DLD, Kratos) surface analysis were used to investigate the effect of Au NPs on electronic structure and composition of the MoS2 nanoflakes. The performance of the MoS2 and various MoS2-Au gas sensors in the detection of various VOCs was measured with a homemade gas-sensing system. (See Supporting Information, Figure S1 and Table S2) Briefly, the sensors were placed in a sensing chamber equipped with gas inlet and outlet. Mass flow controllers were used to control the concentrations of VOC analytes by adjusting the flow rates of the VOC analytes and dilution gas (dry air), with a total flow rate fixed at 500 ml/min. The bubbler containing the analyte was controlled at a given temperature to 8 ACS Paragon Plus Environment

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maintain a stable vapor pressure. The target gas and purging gas (dry air) were exposed for 5 and 10 min for each cycle of gas sensing testing to achieve a steady-state measurement. Humidity interference tests were performed by introducing a VOC analyte gas into deionized water and the relative humidity (RH) was monitored with a commercial humidity sensor (HDC 2010, Texas Instruments). The gas concentrations were calibrated with a commercial VOC sensor (Honeywell, ToxiRAE Pro PID). The input voltage of + 0.1V was applied to the electrodes of the sensors and the real time electrical signals (currents) were measured with a Keithley 2400 sourcemeter. The response of a MoS2-based sensor upon the adsorption of a VOC analyte is defined by the following equation: Response (%) = I/I0  100 = (Ig  I0)/I0  100, where Ig and I0 represent the current values of the sensor in the presence of VOC analyte and dry air, respectively. The response time is defined as the time taken to reach 90% of the maximum conductance value after the introduction of a VOC analyte. The recovery time is defined as time taken to return to 90% of an initial conductance value after the removal of target analyte.



2.3. Adsorption energy calculations via density function theory (DFT) simulations Here, acetone is used as a model gas, and the adsorption energy (Ea) of an acetone molecule on pristine MoS2 and MoS2-Au was calculated by performing the Vienna ab initio software package (VASP).33-35 The electron-ion interactions of each atom were expressed by a projector augmented wave method.36 Perdew-Burke-Ernzerhof function with generalized gradient approximation (CGA) was employed to deal with the electron exchange and correlation.37 A planewave basis set with an energy cutoff at 600 eV was conducted with a force threshold of 0.0001 eV/Å for the ionic relaxation. The 6 × 6 × 1 Monkhorst-Pack k-point grids were performed for geometric optimization of static energy and electronic structure calculations.38 3. RESULTS AND DISCUSSION 3.1. Morphology and microstructure analysis of pristine MoS2 and Au-decorated MoS2 Top-view SEM imaging was used to observe all of the nanoflake samples, including pristine MoS2, MoS2-Au(0.25), MoS2-Au(0.5), MoS2-Au(1), and MoS2-Au(2), and representative results are displayed in Figure 2a-c. These micrographs reveal that MoS2 nanoflakes have an average size of around 250 nm, and the size and coverage of Au NPs increase with increasing of HAuCl4 molar concentration. Extremely small Au NPs with an average size of 10 nm are observable from MoS210 ACS Paragon Plus Environment

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Au(0.5) in Figure 2b. In addition, excess coverage of large Au NPs of approximately 55 nm in size can be observed from MoS2-Au(2) nanoflakes (Figure 2c). Notably, the deposition of Au NPs onto MoS2 layers results from a redox reaction between the MoS2 and noble metal ions, as shown in Figure 2d. The work function of exfoliated MoS2 is 5.2 eV, and its Fermi energy level is well above the reduction potential of AuCl4¯ (+1.002 versus standard hydrogen electrode; SHE).39 Therefore, the Au nanoparticles can spontaneously attach onto the MoS2 surface via a reduction reaction between MoS2 and AuCl4¯.

Figure 2. Selective top-view SEM images of (a) pristine MoS2, (b) MoS2-Au(0.5), and (c) MoS2Au(2) nanoflakes, showing evolution of surface morphologies by the decoration of Au NPs. (d) The energy diagram showing the relation between Fermi energy of MoS2 and the reduction 11 ACS Paragon Plus Environment


potential of Au3+ (+1.002 V versus SHE), resulting in a spontaneous redox reaction between MoS2 and AuCl4¯. (e) HRTEM image and (f) high angle annual dark-field STEM mapping, with Mo, S, and Au elements, from the same region of MoS2-Au(0.5) sample showing uniform distribution and even size of typically 10 nm for Au decorated MoS2. (g) Enlarged HRTEM image of Au NPs on MoS2 revealing typical particle size of around 9 nm. (h) HRTEM images of MoS2 (top) and Au (bottom) from region squared in (e) indicating the lattice distances of 2.7 Å and 2.3 Å, corresponding to MoS2 (100) and Au (111) planes, respectively.

The size distribution of noble metal NPs is a factor affecting gas-sensing performance.40,41 For example, Au NPs with smaller particle size and uniform distribution on ZnO or WO3 sensing materials lead to an enhancement in gas sensing.42,43 To understand the effect of size and distribution of Au NPs on MoS2, Au NPs decorated on MoS2-Au(0.25), MoS2-Au(0.5), MoS2Au(1), and MoS2-Au(2) were characterized by SEM imaging (Figure 2a-c), revealing average sizes of approximately 7, 10, 25 and 55 nm, respectively (see Supporting Information, Figure S2). Clearly, the size of Au NPs increases with increasing HAuCl4 concentration. The gas-sensing response signal tested for all of the Au-decorated MoS2 sensors indeed showed that MoS2-Au(0.5) sensor with adequate size distribution of Au NPs yields the best gas-sensing performance. 12 ACS Paragon Plus Environment

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The high-resolution TEM and high-angle annual dark-field STEM micrographs of representative MoS2-Au(0.5) nanoflakes, presented in Figure 2e and 2f, respectively, reveal that equiaxed Au NPs of sizes typically 10 nm across are uniformly decorated on MoS2 nanoflakes. Moreover, the presence of the Au NPs with average size of 10 nm can be clearly identified on MoS2 through elemental mapping (in yellow) of the annual dark-field STEM image. Figure 2g, a lattice fringe image enlarged from the square region in Figure 2e, shows 9-nm-sized Au NPs. As shown in Figure 2h, spacing measurements of the lattice fringes in HRTEM images of MoS2 (top) and Au (bottom) reveal the lattice distances of 2.7 Å and 2.3 Å, corresponding to MoS2 (100) and Au (111) planes, respectively. The magnified HRTEM images indicate an excellent crystallinity of MoS2-Au hybrid materials examined here. To identify the effect of decorating Au NPs on altering surface structures of MoS2, the Mo 3d and S 2p XPS spectra of MoS2 and MoS2-Au(0.5) nanoflakes are shown in Figure 3a and 3b, respectively. Additionally, as shown in Figure 3c, Au 4f5/2 and Au 4f7/2 peaks were respectively observed at 87.68 and 84.02 eV, confirming the decoration of MoS2 with Au NPs. After Au functionalization, the binding energies of Mo 3d3/2 and Mo 3d5/2 shift from 232.03 to 232.39 eV, and 228.91 to 229.25 eV, respectively. Similarly, the binding energies of S 2p1/2 and S 2p3/2 shift 13 ACS Paragon Plus Environment


from 162.93 to 163.23 eV and 161.75 to 162.01 eV, respectively. The upshifts of these peaks are directly attributed to the result of electron-donating effect of Au since its Fermi level located at the zero energy shifts toward the conduction band edge.44,45

Figure 3. XPS surface analysis of MoS2 and MoS2-Au(0.5) with binding energy peaks of (a) Mo 3d3/2 and Mo 3d5/2, (b) S 2p1/2 and S 2p3/2, and (c) Au 4f5/2 and Au 4f7/2. The upshift of the peaks after Au functionalization is indicated by a black dash line. (d) Raman spectra of pristine MoS2, 14 ACS Paragon Plus Environment

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MoS2-Au(0.5), and MoS2-Au(2) nanoflakes showing E12g (in plane vibration) and A1g (out-offplane vibration) for all samples. The blue shift is attributed to the electron-donating effect of Au NPs. (e) The energy band diagram for MoS2 and Au, indicating electron transfer from Au NPs.

Figure 3d shows the Raman spectra of moderately Au-decorated MoS2 and heavily Audecorated MoS2, i.e., MoS2-Au(0.5) and MoS2-Au(2), using pristine MoS2 as reference. The E12g peak results from the opposite in-plane vibration of two S atoms with respect to the Mo atom, while the A1g peak is induced by the out-off-plane vibration of only S atoms in opposite directions.46 Using the peaks of MoS2 as reference, the frequencies of both E12g and A1g peaks were blue-shifted by 2.2 and 4.98 cm-1 for MoS2-Au(0.5) and MoS2-Au(2) nanoflakes, respectively, indicating the electron-donating of MoS2 via incorporating of Au NPs.47 Indeed, the degree of energy shifting is increased with increasing of size and density of Au NPs due to their electron transfer behavior. The energy band diagram for MoS2 and Au is shown in Figure 3e. The direction of electron flow induced by incorporation of a noble metal on MoS2 strongly depends on the work functions of the metal and MoS2.47 The work functions (and band gaps) of MoS2 nanoflakes range from 5.2 to 5.4 eV (and 1.3 to 1.9 eV), depending on the number of layers.48



Given that the work function of Au is 5.1 eV, electrons transfer from Au to MoS2, rendering the electron-donating effect. 3.2. Gas sensing performance of 2D MoS2-based nanoflakes The impact of altering size and distribution of incorporated Au NPs (by varying MoS2-toHAuCl4 molar ratios) on sensing performance was first examined by comparing the response curves of pristine MoS2 and the whole set of Au-decorated MoS2 sensors towards the detection of 120-ppm acetone, as shown in Figure 4a. The pristine MoS2 sensor yielded a reasonable response (16.3%) to acetone, indicating that the 2D nanostructure of MoS2 nanoflakes indeed provides large specific surface area for effective gas diffusion. The response increased to 31.6% for MoS2Au(0.25) sensor with relatively low coverage of 7-nm-sized Au NPs. The highest response of 42.2% towards 120-ppm acetone was obtained for the MoS2-Au(0.5) sensor. This enhancement is attributed to a uniform coverage of adequately 10-nm-sized Au NPs on the MoS2 sensing materials that induced electronic sensitization, ultimately leading to a distribution of large numbers of electrons on the MoS2 surfaces, making them available for gas response enhancement.32,49 However, the MoS2-Au(1) and MoS2-Au(2) sensors both exhibited no response to acetone due to the large size ( 25 nm) and excessive number of Au NPs. This resulted in a metallic behavior of 16 ACS Paragon Plus Environment

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MoS2 channels, with high current values, and thus the adsorption of VOC molecules on channel surfaces had little effect on the conductance of the sensors. The sensing behaviors of the MoS2-Au(0.5) sensor were further investigated by exposure to acetone (oxygen-based) and hexane (hydrocarbon-based) gases from 10 to 120 ppm, and the results are presented in Figure 4b. It can be observed that the response increases rapidly while the gas analytes are injected, indicating that the Au decorated MoS2 nanoflakes are sensitive to target gases. As expected, the responses decrease with decreasing of VOC concentrations. However, the sensor still reveals good response to acetone (15.22%) and hexane (2.97%) vapors, even at low concentration of 10 ppm. The various levels of gas response to acetone and hexane are determined by complicated molecular interactions between sensing materials and VOC molecules, which are dominated by different sensing mechanisms as proposed later. Figure 4c shows the response versus concentration plots of acetone and hexane analytes. The saturation point is approached at higher VOC concentrations, and thus resulting in decrease in the slopes of both curves. At low VOC concentrations, when most of active sites are available, the charge transfer is directly proportional to the concentration. The response and recovery times of the MoS2-Au(0.5) sensor against different concentrations of acetone is shown in Figure 4d. It can 17 ACS Paragon Plus Environment


be observed that the response time decreases and the recovery time increases as the acetone concentration increases from 10 to 120 ppm. The decrease in response time may be caused by the large availability sites on the film for the gas adsorption; the increase in recovery time may result from the chemisorption of acetone molecules50 and their reaction products, which do not immediately desorb from the interface.25,51,52

Figure 4. (a) Dynamic response curve for the room-temperature sensing of 120-ppm acetone using MoS2, MoS2-Au(0.25), MoS2-Au(0.5) and MoS2-Au(1) sensors. (b) Real-time monitoring of


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varying concentrations (10120 ppm) of acetone (top) and hexane (bottom) using MoS2-Au(0.5) sensor. (c) Response versus concentration plots of acetone and hexane for the MoS2-Au(0.5) sensor. (d) Response and recovery times versus concentration curves of acetone for the MoS2Au(0.5) sensor derived from (b).

To realize further the benefit of decorating MoS2 with Au NPs in the detection of various VOCs, pristine MoS2 and MoS2-Au(0.5) sensors were exposure to various oxygen-containing (acetone, ethanol, and 2-propanol) and hydrocarbon-based (toluene, hexane, and benzene) VOCs at 40 ppm, and their gas responses are presented in Figure 5a and b, respectively. The corresponding bar chart (Figure 5c) reveals that the MoS2-Au(0.5) sensor exhibits high selectivity and sensitivity towards sensing of oxygen-containing VOCs, which is attributed to the optimal surface decoration with Au NPs. For example, the response of acetone analyte for pristine MoS2 and MoS2-Au(0.5) sensors increases from 13.7% to 31.6%, corresponding to an increase of 131% by the incorporation of Au NPs in MoS2. The reasons for the improvement of sensing the oxygencontaining VOCs will be fully discussed later in terms of different dominant sensing mechanisms. Gas sensors for real-life application require good reproducibility and stability besides high sensitivity and selectivity. Figure 5d (top) shows the room-temperature responses of the MoS219 ACS Paragon Plus Environment


Au(0.5) sensor tested under 40 ppm of acetone for five consecutive cycles. Repeatable sensing responses of 31.6% were observed. As shown in Figure 5d (bottom), the long-term stability of the MoS2-Au(0.5) sensor was also conducted to evaluate the operational shelf life. The sensor was tested under 40 ppm of acetone at an interval of five days. After a month, the response of the flexible MoS2-Au(0.5) sensor exhibited 30.5% of response to 40 ppm of actone, revealing only a 3.5% decrease compared to the initial response value. Therefore, the experimental results indicate that the Au-decorated MoS2 sensors show a good consistency and stability for practical sensing applications.


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Figure 5. Dynamic response curves for the room-temperature sensing of various VOCs at 40 ppm using (a) pristine MoS2 and (b) MoS2-Au(0.5) sensors. (c) Corresponding response bar chart showing selectivity of MoS2 sensors is significantly enhanced by adequate decoration of Au NPs. (d) Five consecutive sensing cycles (top) and long-term stability of responses over a month (bottom) under 40 ppm of acetone for the MoS2-Au(0.5) sensor.



Humidity is an important parameter that might impact greatly the performance of 2D chemiresistive sensors.53 Thus, dynamic response curves to10-ppm acetone using the MoS2Au(0.5) sensor under various RH levels (1080%) were recorded. As shown in Figure S3, the responses of the MoS2-Au(0.5) sensor to acetone slightly decrease at a rate of 0.15 per percentage of RH over the range from 10 to 60% RH. However, the responses dramatically decrease at the rate of 0.37 per percentage of RH from 60 to 80% RH, which is attributed to the excessive occupancy of water molecules on the sensing surface, causing degradation of sensing performance.54 Furthermore, the MoS2-Au(0.5) sensor was subjected to cyclic bending tests to demonstrate its robustness and flexibility. As shown in Figure S4, after 1000 bending cycles with a bending radius of 5 mm, the MoS2-Au(0.5) sensor exhibited only a small reduction of gas response, from 15.2% to 14.3%; this was presumably due to the robust mechanical properties of 2D MoS2 that assisted to minimize cracks and damage during the bending test. Compared to the previous TMD-based VOC sensing materials listed in Table S3, the MoS2Au(0.5) sensors shows excellent sensing performance such as high responsivity and roomtemperature detection, along with flexibility and facile process. Moreover, in contrast to other methods, such as atomic layer deposition and chemical vapor deposition, the solution methods 22 ACS Paragon Plus Environment

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(liquid phase exfoliation for 2D materials and solution mixing for functionalization) used here have the advantages of scalable production with high yield even at room temperature. The solutionintegrated method thus has a high potential of fabricating 2D-TMDC based materials for mass production of flexible sensors. Therefore, we can state that the Au nanoparticle functionalized MoS2 using this cost-effective method shows outstanding sensing performance at room temperature in terms of both response and detection limit. This finding indicates that 2D TMDCs functionalized with noble-metal NPs is a promising method for room-temperature detecting of VOCs molecules. 3.3. Gas sensing mechanisms of 2D MoS2-based nanoflakes To understand the gas-sensing behavior, we used density function theory (DFT) calculations. First, we obtained an optimized lattice parameter of 3.15 Å for a MoS2 model and a 4 × 4 × 1 supercell with a vacuum layer of 20 Å thickness, which prevents the interaction between the supercell and adjacent slabs. The binding energies (Eb) and distances of an Au atom on tops of Mo (TMo) and S (TS) atoms of MoS2 are summarized in Table S4. The binding energy and distance of Au on Mo atom were 0.582 eV and 3.04 Å, while those of Au on S were stronger (0.730 eV) and shorter (2.47 Å). These results imply that transition metal Au atoms tend to locate on the top 23 ACS Paragon Plus Environment


sites of S atoms achieving the most stable adsorption configuration, which is in good agreement with other studies.55,56 The geometric optimization and the lowest energy structure were selected for the following calculations of gas adsorption energies. The interaction of a given gas molecule with MoS2-Au sensing materials depends on its surface adsorption energy (Ea), which was calculated by the following equation: Ea = EMoS2 - Au + gas ― (EMoS2 - Au + Egas) where EMoS2 - Au + gas is the total energy of the gas molecule adsorbed on MoS2-Au, EMoS2 - Au is the energy of Au atom decorated on MoS2 layer, and Egas is the energy of an isolated gas molecule. To understand the favorable gas adsorption configurations, acetone was selected as model gas and its initial adsorption orientation was set horizontal and vertical with respect to the basal plane of the MoS2-Au layer. The calculated adsorption energies and heights of the acetone molecule for MoS2 and MoS2-Au are summarized in Table S5, revealing that horizontal configurations are favored for both samples. Moreover, the adsorption energy of acetone on MoS2Au (0.573 eV) is significantly stronger than that of MoS2 (0.293 eV), consistent with the extremely short equilibrium distance of 2.17 Å between acetone and MoS2-Au. Figure 6 shows (a) side view and (b) top view of the most stable molecular configuration for an acetone molecule 24 ACS Paragon Plus Environment

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laying parallel to the surface on MoS2-Au layer through DFT simulations. Larger adsorption energy leads to a greater amount of adsorbed gases and stronger interaction between gases and sensing materials, which is consistent with our experimental results showing a superior acetone sensing with the MoS2-Au sensor. The corresponding sensing mechanisms will be discussed below.

Figure 6. (a) Side view and (b) top view of minimum energy configurations for an acetone molecule adsorbed on MoS2-Au surface based on density function theory (DFT) calculations.

Figure 7a depicts the mechanism of sensing the hydrocarbon-based gases (hexane toluene, and benzene) using MoS2 sensors: these VOC molecules are interacting with MoS2 surfaces through physisorption, and the electron clouds of their methyl groups induce merely dipole scattering with a smaller change in electronic signals,57,58 subsequently yielding the lower response as observed here. 25 ACS Paragon Plus Environment


Figure 7. Schmatic diagrams elucidating differences in sensing mechanisms of (a) hydrocarbonbased VOCs and (b) oxygen-based VOCs using MoS2, as well as using Au-decorated MoS2 to detect oxygen-based VOCs, (c) and (d). (a) Hydrocarbon-based VOC molecules adsorbed onto MoS2 creating only dipole scattering through electron clouds of their methyl groups and thus contributing to a small change in electronic signals. (b) Upon exposure to oxygen-based VOCs, (right) the injection of electrons occurring by the electron-donating properties of ethanol; (left) the interaction between oxygen species (O2¯ and O¯) adsorbed on MoS2 and oxygen-based VOCs creating volatile species (CO2 and H2O), subsequently releasing electrons back to MoS2, both mechanisms contributing to the increases of gas response for pristine MoS2 sensor. (c) decoration of Au NPs resulting in an increasing of electron concentration on MoS2, causing an increase of 26 ACS Paragon Plus Environment

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adsorbed oxygen species and thus (d) for the Au-decorated MoS2 to have higher responsivity and selectivity toward oxygen-based VOCs because of the returning of more electrons captured by adsorbed oxygen species on MoS2 channel. On the other hand, the response of MoS2 sensor to oxygen-based VOCs (acetone, ethanol, and 2-propanol) is rather complicated and has not been well explored. Herein, we propose the sensing mechanisms of oxygen-based VOCs for understanding the sensing behavior of pristine MoS2 and Au-decorated MoS2 sensors. As shown on the right-hand side of Figure 7b, upon adsorption to MoS2 surfaces, the injection of electrons occurs through direct charge transfer mechanism induced by electron-donating property of ethanol, thus resulting in an increase in current.50 Another mechanism crucial to the sensing improvement involving the interaction between MoS2, adsorbed oxygen species, and VOC molecules is also proposed on the left-hand side of Figure 7b. First, the oxygen species, such as O2¯ and O¯, are adsorbed onto MoS2 surfaces during the step of purging air (containing 20% O2 and 80% N2). Following the exposure of oxygenbased VOCs (e.g., acetone), the adsorbed active oxygen species (O2¯ and O¯) react with acetone molecules to form volatile gases (H2O and CO2), subsequently releasing electrons to MoS2 channel and resulting in an increase in sensing signals. 27 ACS Paragon Plus Environment


As depicted in Figure 7c, the electron-donating effect of Au NPs (as confirmed from XPS and Raman analysis) significantly increases electron concentrations in Au-decorated MoS2 channel, resulting in a further increase in adsorbed oxygen species (O2¯ and O¯) on MoS2 surface; higher concentrations of oxygen species in turn trap more electrons from MoS2. As shown in Figure 7d, when exposed to oxygen-based VOCs, MoS2 channel receives more electrons from an electron-donating process via the chemical interaction between ethanol and the adsorbed oxygen species, thus yielding a much higher response. This is the dominant sensing mechanism for enhancing the response and selectivity in detection of oxygen-containing VOCs using Audecorated MoS2 as the superior sensing materials. 4. CONCLUSIONS We have successfully developed flexible gas sensors based on Au-functionalized MoS2 nanoflakes using a facile solution mixing method, which have great potential for scalable production. Size distribution of Au NPs is crucial for the sensors to have high performance towards detecting of various hydrocarbon- and oxygen-based VOCS at room temperature. With 10-nmsized Au NPs, the MoS2 sensors exhibit high selectivity and excellent response to oxygen-based VOCs because of the electron-donating effect of Au NPs and synergistic effects between MoS2, 28 ACS Paragon Plus Environment

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Au NPs, ambient oxygen species, and VOC analytes. An enhancement of response by 131% towards detection of acetone is achieved by incorporation of Au NPs onto MoS2 nanoflakes. Bending and long-term dynamic sensing tests suggest that functionalization of 2D TMDCs with Au NPs is a promising strategy for developing high-performance sensors towards roomtemperature sensing of VOCs. Density function theory simulations reveal that the adsorption of VOC molecules on MoS2-Au are stronger than that on pristine MoS2, contributing to the gassensing enhancement. The sensing mechanism of oxygen-based VOCs for Au-decorated MoS2 is proposed for the first time as being dominated by the interaction between adsorbed oxygen species on MoS2 and oxygen-containing VOCs. ASSOCIATED CONTENT Supporting Information Available: A schematic of a homemade gas-sensing system. A table summarized recent studies of sensing volatile organic compounds based on chemiresistive gas sensors. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Lia Stanciu); Phone: +1-765-49-63552. 29 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors wish to thank the Birck Nanotechnology Center, Purdue University, and the help of Dr. D. Zemlyanov for providing equipment and technical support. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.


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Surface Functionalization of Layered Molybdenum Disulfide for the

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