Two-Dimensional Transition Metal Disulfides for ... - MDPI

chemosensors Review

Two-Dimensional Transition Metal Disulfides for Chemoresistive Gas Sensing: Perspective and Challenges Tae Hoon Kim 1 , Yeon Hoo Kim 1 , Seo Yun Park 1 , Soo Young Kim 2 and Ho Won Jang 1, * 1

2

*

Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Korea; [email protected] (T.H.K.); [email protected] (Y.H.K.); [email protected] (S.Y.P.) School of Chemical Engineering and Materials Science, Chung-Ang University, Seoul 06974, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-2880-1720

Academic Editors: Giovanni Neri and Salvatore Gianluca Leonardi Received: 5 April 2017; Accepted: 3 May 2017; Published: 5 May 2017

Abstract: Transition metal disulfides have been attracting significant attentions in recent years. There are extensive applications of transition metal disulfides, especially on gas sensing applications, due to their large specific surface-to-volume ratios, high sensitivity to adsorption of gas molecules and tunable surface functionality depending on the decoration species or functional groups. However, there are several drawbacks such as poor gas selectivity, sluggish recovery characteristics and difficulty in the fabrication of large-scale devices. Here, we provide a review of recent progress on the chemoresistive gas sensing properties of two-dimensional transition metal disulfides. This review also provides various methods to enhance the gas sensing performance of two-dimensional disulfides, such as surface functionalization, decoration receptor functions and developing nanostructures. Keywords: transition metal disulfides; two-dimensional; chemoresistive gas sensors; sensitivity; selectivity

1. Introduction The Internet of Things (IoT), which is frequently mentioned along with “connected devices” and “smart devices”, is the inter-networking of vehicles, buildings, physical devices and other items, which include the electronics, actuators, sensors software and network connectivity that enable these objects to collect and exchange data [1]. One of the key components enabling the IoT is sensors, as they provide enormous information by sensing things and monitoring the environment. Gas sensors are devices or instruments that produce a measurable signal output in order to determine the detectable presence, concentration or quantity of a particular target chemical substance (analyte) present in gas. Such gas sensors are necessary for various applications of IoT, including building automation, healthcare and life sciences, consumer and home automation, transportation, industrial, environment, security and public safety and retail and logistics. Gas sensors for use in the IoT should possess basic performance, like high sensitivity, good selectivity and stability, and meet special requirements, such as low cost, low power consumption, small size and easy integration into existing technologies [2]. Among various types of gas sensors, chemoresistive gas sensors based on semiconductors have been considered as suitable candidates for the IoT due to low cost and small size [3]. Gas sensors based on nanostructured semiconducting metal oxides (typically spherical nanoparticles) lead to high responses to various gases [4–7]. They should operate at elevated temperatures with external heaters in order to keep the materials sensitive to the target analytes. However, the use of an external heater not only increases the power consumption of the sensor, but also causes thermal stability problems,

Chemosensors 2017, 5, 15; doi:10.3390/chemosensors5020015

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obstructing practical applications for the IoT [2]. To overcome the intrinsic problems of semiconducting metal oxides for chemoresistive gas sensors, two-dimensional (2D) materials, including graphene, reduced graphene oxide and MoS2 , have attracted remarkable interest for their potential as active materials for gas sensors [8–11]. Due to the high surface-to-volume ratios and easiness in device fabrication, the application of many 2D materials was made on gas sensing in their early development stage. While graphene is metallic with zero band gap energy, many transition metal disulfides, such as MoS2 , WS2 and SnS2 , are semiconducting with tunable bandgap energies depending on the thickness [12]. In this respect, the transition metal disulfides are inherently more desirable than graphene for chemoresistive gas sensing applications. In this review, gas sensors based on 2D transition metal disulfides are introduced thoroughly with strong emphasis on the sensing material and underlying gas sensing mechanism. While the gas sensors based on individual 2D transition metal disulfides are successfully demonstrated for fundamental research, obstacles for practical applicability are discussed to mention challenges to be addressed for commercializing 2D transition metal disulfide gas sensors. Some crucial issues including long-term stability, selectivity of the sensors and array fabrication are discussed. 2. Gas Sensors Based on Transition Metal Disulfides 2.1. General Bulk transition metal dichalcogenides, MX2 , have layered structures of X-M-X, where M represents a layer of transition metal atoms, which is hexagonally packed and sandwiched between two X, which represent chalcogen atoms. In general, the thickness of each layer is about 6–7 Å [13]. About 40 different layered transition metal dichalcogenide compounds exist, as shown in Figure 1a [12]. The highlighted elements in the periodic table present the transition metals and the three chalcogen elements (S, Se and Te), which have a predominant tendency to crystallize in X-M-X layered structures. Co, Rh, Ir and Ni are partially highlighted, which present that only a portion of the dichalcogenides has layered structures. For instance, NiTe2 is reported to have a layered compound, while NiS2 shows a pyrite structure. The layers are connected by weak van der Waals forces as the M-X bonds within layers are typically covalent. This results in the easy splitting of the crystal along the layer boundaries. Four electrons donated from the metal fill the bonding state of a transition metal dichalcogenide. Therefore, the metal and sulfur atoms have +4 and −2 of the oxidation states, respectively. The lone pair electrons of the sulfur atoms are placed at the end of the layer surfaces, and the layers have been stable against the interaction between environmental species with the absence of dangling bonds. The metal of layered transition metal dichalcogenides can have the coordination either in the trigonal prismatic structure or the octahedral structure, determined by the arrangement of the metal and chalcogen elements (Figure 1b,c). Several polymorphs can be adopted to the transition metal dichalcogenides by the type of layer stacking orders. Trigonal, hexagonal and rhombohedral are the most common polymorphs, which can be represented by 1T, 2H and 3R, while the numbers represent the number of X-T-X units in the unit cell, in other words, the number of layers in the stacking sequence. Some transition metal dichalcogenides can be formed in several polymorphs and stacking sequences. For instance, the 2H phase is the most common polymorph of MoS2 in nature, as it is the most stable form, while the 3R phase is often adopted to the synthetic MoS2 to form a stacking sequence of AbA BaB and AbA CaC BcB, respectively, if we symbolize the chalcogen atom layers by capital letters and the metal atom layers by lower-case letters. TiS2 is largely formed in the 1T phase and the stacking sequence of AcB.

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Figure 1. monolayered transition metal dichalcogenides. (a) About 40 different layered Figure 1. 1. Structure Structure of of metal dichalcogenides. dichalcogenides. (a) (a) About About 40 40 different different layered layered Figure Structure of monolayered monolayered transition transition metal transition metal dichalcogenide compounds exist. (b,c) c-axis and [11–20] section view of single-layer transitionmetal metaldichalcogenide dichalcogenidecompounds compoundsexist. exist.(b,c) (b,c)c-axis c-axisand and[11–20] [11–20] section section view view of single-layer transition transition metal dichalcogenides with trigonal prismatic (b) and octahedral (c) coordinations. transition metal metal dichalcogenides dichalcogenideswith withtrigonal trigonalprismatic prismatic(b) (b)and andoctahedral octahedral(c) (c)coordinations. coordinations. Atom Atom transition color code: purple, metal; yellow, chalcogen. The labels AbA and AbC represent the stacking sequence metal; yellow, chalcogen. The labels AbA and AbC represent the stacking color code: code:purple, purple, metal; yellow, chalcogen. The labels AbA and AbC represent the sequence stacking where upperand letters represent chalcogen and metal where the the upperand lower-case lower-case lettersletters represent chalcogen and metal elements, respectively sequence where the upperand lower-case represent chalcogen and metalelements, elements, respectively respectively (Reproduced with permission of [12]. Copyright Nature Publishing Group, 2013.). (Reproduced with with permission permission of of [12]. [12]. Copyright Copyright Nature Nature Publishing Publishing Group, Group, 2013.). 2013.). (Reproduced

(a) (a) NumberofofPublications Publications Number

80 80

60 60

(b) (b)

TaS TaS22 TiS TiS22 1% 1% 1% 1%

WS WS22 14% 14%

SnS SnS22 9% 9%

40 40 20 20

MoS MoS22 75% 75%

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Figure 2. forms of 2D transition metal disulfides used for (a) Number of publications and (b) element forms of of 2D2D transition metal disulfides used for Figure 2. (a) (a)Number Numberof ofpublications publicationsand and(b) (b)element element forms transition metal disulfides used gas sensor applications in the publications since 2011. The publication search was performed on gasgas sensor applications in in thethe publications since for sensor applications publications since2011. 2011.The Thepublication publicationsearch search was was performed performed on 23 March 2017 using the Science Citation Index Expanded database of the Web of Science provided 23 March ofof thethe Web of Science provided by March 2017 2017 using usingthe theScience ScienceCitation CitationIndex IndexExpanded Expandeddatabase database Web of Science provided by Thomson Reuters. All possible keywords from combinations of gas sensor and 2D transition metal Thomson Reuters. All possible keywords from combinations of gas sensor and 2D transition metal by Thomson Reuters. All possible keywords from combinations of gas sensor and 2D transition metal disulfides (MoS (MoS disulfides SnS222,, ,TaS TaS222,, ,TiS TiS22)2) )were wereused usedfor forthe thesearch. search. (MoS222,, WS WS222,,, SnS SnS TaS TiS were used for the search.

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Among the three chalcogen elements, sulfur is the most favorable atom since it is the most Among the three them chalcogen elements, sulfur the most favorable atom since it is the are mostless Earth-abundant among and most of the MS2 is(transition metal disulfide) compounds Earth-abundant among them and most of the MS2 (transition metal disulfide) compounds are less toxic and more stable in air than MSe2 and MTe2 . Figure 2a shows the number of publications on toxic and more stable in air than MSe2 and MTe2. Figure 2a shows the number of publications on gas gas sensors based on transition metal disulfides since 2011. The number of publications is rapidly sensors based on transition metal disulfides since 2011. The number of publications is rapidly increasing, and it is expected that the number will be over 100 this year. MoS2 is one of the most increasing, and it is expected that the number will be over 100 this year. MoS2 is one of the most noted noted transition metal disulfide materials studied in gas sensing applications, as shown in Figure 2b. transition metal disulfide materials studied in gas sensing applications, as shown in Figure 2b. Seventy-five percent focused on onMoS MoS2 2along alongwith withWS WS 2 (14%), SnS2 Seventy-five percentofofpublications publicationsup upto tonow now were were focused 2 (14%), SnS 2 (9%), etc.etc. The predominance transitionmetal metaldisulfides disulfidesisisdue due fact that (9%), The predominanceofofMoS MoS22 over over other other transition toto thethe fact that thethe material is the easiest material to synthesize and the most stable. material is the easiest material to synthesize and the most stable. 2.2.2.2. Molybdenum MolybdenumDisulfide Disulfide MoS one of the most notable transition metal disulfides, which is a layered compound, similar 2 is MoS 2 is one of the most notable transition metal disulfides, which is a layered compound, similar to to graphite. Each of Mo Moatoms atomssandwiched sandwichedbetween between planes S atoms. graphite. Eachlayer layerisiscomposed composed of of aa plane plane of planes of Sofatoms. Compared with graphite and graphene, the MoS semiconductor has an indirect bandgap (~1.2 Compared with graphite and graphene, the MoS22 semiconductor has an indirect bandgap (~1.2 eV).eV). It It also whichare areeffective effective hydrodesulfurization works alsohas hascatalytic catalytic properties, properties, which forfor hydrodesulfurization [14]. [14]. SeveralSeveral works that that have been reported recently have shown indirect band gap, a property bulk MoS have been reported recently have shown thatthat an an indirect band gap, a property of of bulk MoS 2 2 semiconductor,can canbebemodified modifiedby bymaking making aa single single monolayer from anan indirect semiconductor, monolayerof ofMoS MoS2:2it : itconverts converts from indirect band gap directband bandgap gapsemiconductor semiconductor (~1.9 efficiency [15,16]. band gap totoa adirect (~1.9eV) eV)with withsuperior superiorluminescence luminescence efficiency [15,16]. It also can appliedfor forthe thetransport transportchannel channel of a field on/off It also can bebeapplied field effect effecttransistor transistor(FET) (FET)with withananexcellent excellent on/off ratio [17] and noticeable photoresponse [18,19]. ratio [17] and noticeable photoresponse [18,19].

Figure Opticalmicroscope microscope image image of of mechanically-exfoliated mechanically-exfoliated bilayer 2 films ononSi/SiO 2. Figure 3. 3.(a)(a)Optical bilayerMoS MoS Si/SiO 2 films 2. AFM imageofofthe thebilayer bilayer(2L) (2L)(thickness: (thickness: 1.5 nm) current response after (b)(b) AFM image nm) MoS MoS22film. film.(c) (c)Real-time Real-time current response after exposureofofthe the2L2LMoS MoS 2 FET to NO with increased concentration. Inset: A typical adsorption and exposure 2 FET to NO with increased concentration. Inset: A typical adsorption and desorption process of NO the2L2LMoS MoS 2 FET. (d) Plot of the percent change in current as a function desorption process of NO ononthe 2 FET. (d) Plot of the percent change in current as a function of of NO concentration (Reproduced with permission [20]. CopyrightJohn JohnWiley Wileyand andSons, Sons,2011.). 2011.). NO concentration (Reproduced with permission of of [20]. Copyright


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Li et al. demonstrated the gas sensing ability of FET based on a single- and multi-layer MoS2 films deposited onto 300 nm-thick SiO2 layer/Si substrates [20]. Single- and multi-layer MoS2 nanosheets are fabricated by the mechanical exfoliation technique (Figure 3a,b) [17,21,22]. The prepared MoS2 FET devices have an n-type semiconductor behavior. The current on/off ratio of bilayer (2L), trilayer (3L) and quadrilayer (4L) MoS2 devices is higher than 103 (Vg from −10–10 V), while the single-layer (1L) MoS2 FET has smaller values (≈ 102 ). The prepared single- and multi-layer MoS2 FETs are measured with NO gas at room temperature. The results show that the FET sensors fabricated with 2L, 3L and 4L MoS2 films have a pronounced sensitivity to NO and that the detection limit is 0.8 ppm (parts-per-million). By comparison, the monolayer MoS2 device exhibits a rapid, but unstable response. Figure 3c shows a typical current response of a 2L MoS2 FET device during the exposure to NO with a concentration range from 0.3 to 2 ppm. The p-doping effect contributes to the decrease of the current, which means the increase of the resistance during the exposure to NO [8,10,11,23]. It is similar to the charge transfer mechanism that occurs in the gas sensors based on 2D graphene [24] and 1D carbon nanotube [25]. The desorption and adsorption processes of NO can be split into two steps; one is a rapid and the other is a slow process. This phenomenon also can be found in the gas sensors based on graphene and explained by the different binding energies originating from each interaction between gas molecules and the different defect sites on the surface of the sensing materials [10]. Perkins et al. reported that single monolayer MoS2 operates with high performance as a chemical sensor at room temperature, showing high selectivity to various analytes [26]. They produced sensors with planar structures composed with a monolayer MoS2 channel on SiO2 /Si substrates, as shown in Figure 4a,b. The sensor with the MoS2 monolayer shows a distinct response to triethylamine (TEA-N(CH2 CH3 )3 ) exposure, which is a laboratory-safe decomposition product from the V series nerve gas agents. The degree of the conductivity variation rises as the TEA concentration increases. The response of the device to a sequence of pulses is shown in Figure 4d. Over the sequence of pulses, the TEA concentration increases from 0.002% P0 (1 ppm) to 0.2% P0 (100 ppm) in 450 s. The response of the device to a sequence of acetone (CH3 )2 CO pulses is shown in Figure 4e. The acetone concentration increases from 0.02% P0 (50 ppm) to 2% P0 (5000 ppm). The incremental trend of conductivity is highly correlated with the sequence of exposure pulses, while the amplitude of ∆G/G0 is on a par with a given concentration. Compare to the relatively high responses to the exposure of the TEA and the acetone, the MoS2 sensor shows almost no variation in conductivity to the exposure of analytes that are considered to be electron acceptors or weak donors. For instance, Figure 4f shows the response to a sequence of nitrotoluene (NT) pulses, a laboratory-safe simulant for the explosive trinitrotoluene (TNT). The MoS2 monolayer exhibits no variation in conductivity as the NT concentration increases from 0.01% P0 –1% P0 . For the sake of contrast, the response of the CNT network sensor is presented. The conductivity of the CNT increases significantly to the exposure. They hypothesized that the interaction mechanism between the analyte and the device is regulated by the localized lone pair orbitals of the sulfur end units. The Mo 3dyz orbitals and S 2p orbitals are placed on the surface plane as the charge density distributions make the Mo 3dyz slightly reduced (negatively charged) and the S 2p slightly oxidized (positively charged) [12]. Therefore, they can easily interact with the target analytes. They suspected that the SiO2 substrate under the MoS2 monolayer has its own positive charge; therefore, the negative charge of the Mo 3dyz orbitals can be compensated. As a result, the positively-charged S is only available for the interaction between the gas molecules and the surface. Hence, the MoS2 sheet has a significant interaction with donor-like analytes, corresponding to the experimental results.


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Figure Figure4.4.(a) (a)Schematic Schematicof ofaasingle single monolayer monolayer of of MoS22 supported supported on on a SiO22/Si /Sisubstrate substrateand andcontacted contacted with Au Au contact contact pads. pads. (b) (b) An An optical optical image image of of the the processed processed devices devices showing showing the the monolayer monolayer MoS MoS22 with flakes electrically contacted by multiple Au leads. (c) Histogram of MoS and CNT-network sensor flakes electrically contacted by multiple Au leads. (c) Histogram of MoS22and CNT-network sensor responsestotovarious variousanalytes. analytes. Change in conductivity ofmonolayer the monolayer channel responses (d)(d) Change in conductivity of the MoS2MoS sensor channel upon 2 sensor upon exposure to a series of pulses in which TEA concentration increasesfrom from0.002% 0.002%PP0–0.2% exposure to a series of pulses in which the the TEA concentration increases 0 –0.2%PP0.0 . (e) Change Change in in conductivity conductivity of of the the monolayer monolayer MoS MoS22 sensor sensorchannel channelupon uponexposure exposureto toaa sequence sequenceof of (e) pulses in which the acetone concentration increases from 0.02% P –2% P (black line). (f) Response pulses in which the acetone concentration increases from 0.02% P0–2% P0 (black line). (f) Response of 0 0 of sensors to nitrotoluene. The monolayer (red line) no shows no response to analytes, sensors to nitrotoluene. The monolayer MoS2 MoS sensor (red line) shows response to analytes, such as 2 sensor such as nitrotoluene, which tend act as acceptors, the CNT-network sensor (black line) nitrotoluene, which tend to act as to acceptors, while the while CNT-network sensor (black line) shows a shows a pronounced The nitrotoluene concentration fromP00.01% –1%isP0shown and is pronounced response.response. The nitrotoluene concentration increasesincreases from 0.01% –1% PP0 0and shown inverted for convenience to facilitate comparison sensor response(Reproduced (Reproducedwith with inverted for convenience to facilitate comparison withwith the the sensor response permission of [26]. Copyright American Chemical Society, 2013.). permission of [26]. Copyright American Chemical Society, 2013.).

Late Late et et al. al. reported reported the the sensing sensing performance performanceof of aa MoS MoS22 thin-layered thin-layeredstructure structuretransistor transistorto to the the exposure of NO 2 and NH 3 in several conditions such as gate bias at room temperature [27]. As shown exposure of NO2 and NH3 in several conditions such as gate bias at room temperature [27]. As shown in inFigure Figure5a, 5a, the theelectron-beam electron-beam lithography lithography is is used used for for patterning patterning Ti/Au Ti/Auelectrodes electrodeson ontop topof ofisolated isolated MoS 2 sheets, and the back gate geometry is also used for the fabrication of the gas-sensing devices. MoS2 sheets, and the back gate geometry is also used for the fabrication of the gas-sensing devices. The The single-layer single-layer MoS MoS22device deviceshows showsunstable unstablecurrent currentvariation variationover overtime timeas aspresented presentedearlier earlier[20], [20], while multilayer MoS 2 devices show better stable responses. They measured sensing behavior with while multilayer MoS2 devices show better stable responses. They measured sensing behavior with and andwithout withoutapplying applying back back gate gate voltage voltage (+15 (+15V) V)for fortwo-layer two-layerand andfive-layer five-layerMoS MoS22FETs FETswhen whenexposed exposed to 2. Figure 5c,d shows the variation of the sensitivity as the concentration increases. The toNH NH3 3and andNO NO . Figure 5c,d shows the variation of the sensitivity as the concentration increases. 2 five-layer MoS 2 sensor shows the highest sensitivity to the exposure of NO 2 when operated with The five-layer MoS2 sensor shows the highest sensitivity to the exposure of NO2 when operated with positive positive gate gate voltage. voltage. The The highest highest sensitivity sensitivity to to the the exposure exposure of of NH NH33 is is shown shown on on the the five-layer five-layer MoS MoS22 sample sample when when operated operated without without gate gate voltage. voltage. Their Their sensitivities sensitivities to to 1000 1000 ppm ppm NO NO22 and and NH NH33 were were determined to be 1372 and 86%, respectively. NO 2 acts as an electron acceptor (p-type doping), while determined to be 1372 and 86%, respectively. NO2 acts as an electron acceptor (p-type doping), while NH NH33acts actsas asan anelectron electrondonor donor (n-type (n-type doping). doping). Thus, Thus, for for n-type n-type semiconductors, semiconductors, such such as as MoS MoS22,, NO NO22 increases the resistance while NH 3 decreases the resistance based on a charge transfer mechanism. increases the resistance while NH3 decreases the resistance based on a charge transfer mechanism. As As the the NO NO22isisthe theelectron electronacceptor, acceptor, itit has has more more electrons electrons to to accept accept when when the the electron electron accumulation accumulation occurred occurred at at the the interface interface of of MoS MoS22 and andSiO SiO22by byapplying applyingthe thepositive positivegate gatevoltage. voltage. However, However, as as NH NH33 isis the electron donor, the electric field built up at the interface repels NH 3 to donate an electron, the electron donor, the electric field built up at the interface repels NH3 to donate an electron,which which results resultsin in the the decreases decreases of of the the gas gas sensitivity. sensitivity.


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Figure Schematicofofthe theMoS MoS2transistor-based transistor-based NO Low-magnification Figure 5. 5. (a)(a) Schematic NO22gas-sensing gas-sensingdevice. device.(b)(b) Low-magnification 2 TEM image; the inset shows the high-resolution TEM image. (c) Sensitivity as a a function of of TEM image; the inset shows the high-resolution TEM image. (c) Sensitivity as function concentration (in ppm) for two-layer and five-layer MoS 2 sheets for NH3. (d) Sensitivity as a function concentration (in ppm) for two-layer and five-layer MoS2 sheets for NH3 . (d) Sensitivity as a function of of concentration (in ppm) for two-layer and five-layer MoS2for sheets NO2 (Reproduced with concentration (in ppm) for two-layer and five-layer MoS2 sheets NO2 for (Reproduced with permission permission of [27]. Copyright American Chemical Society, 2013.). of [27]. Copyright American Chemical Society, 2013.).

Kim et al. investigated the oxygen sensing behavior of MoS2 chemoresistive gas sensors [28]. The Kim et al. prepared investigated the oxygen sensing behavior gas sensors [28]. sensors were by liquid exfoliation methods from of theMoS MoS22 chemoresistive single crystal and dropcasting The sensorson were prepared by liquid exfoliation the MoS and dropcasting 2 single methods Pt interdigitated electrodes (IDEs). methods Figure 6afrom shows the response ofcrystal the liquid-exfoliated methods on Pt interdigitated electrodes (IDEs). Figure 6a shows the response of the liquid-exfoliated MoS2 sensor to four successive pulses of 100% O2 at 300 °C. The device exhibits pronounced stability MoS to four successive of 100% O22 without at 300 ◦ C. The device exhibits pronounced stability and complete recovery to the pulses four pulses of O shifting the base resistance or responses. 2 sensor and complete recovery to the four pulsesofofthe O2sensor without shifting the of base resistance Figure 6b shows the measured responses to the exposure 2%–100% of O2oratresponses. 300 °C. Figure 6b shows the measured responses of the sensor to the10.83%, exposure of 2%–100% of O2 23.98%, at 300 ◦ C. The liquid-exfoliated MoS2 sensor shows responses of 8.69%, 12.25%, 13.73%, 17.4%, 29.96%, 50.28% and 63.73% to 2%, 5%, 7%,responses 10%, 15%,of 25%, 50%,10.83%, 75% and12.25%, 100% O13.73%, 2, respectively. The liquid-exfoliated MoS2 sensor shows 8.69%, 17.4%,From 23.98%, Figure50.28% 6c, the slope the plotted line shows value of 5453.6 , which indicates the 29.96%, and of 63.73% to 2%, 5%, 7%,the 10%, 15%, 25%, ppm 50%,−175% and 100% O respectively. 2 , sensitivity −1 , role of the device. calculations were used to investigate theppm critical of the surfacethe From Figure 6c,The thefirst-principles slope of the plotted line shows the value of 5453.6 which indicates configurations of MoS 2 particles prepared by using liquid-exfoliation techniques, which means the of sensitivity of the device. The first-principles calculations were used to investigate the critical role edge sites attract more O 2 molecules than clean surfaces. They examined the electronic structure of the surface configurations of MoS2 particles prepared by using liquid-exfoliation techniques, which the clean S monomers with and without 2 adsorption. Figure 6e shows the band means the Mo-edges edge siteswith attract more O2 both molecules than cleanOsurfaces. They examined the electronic structure (left) and the density of states projected on the edge atoms (right) each model. The 6e structure of the clean Mo-edges with S monomers both with and without O2 for adsorption. Figure metallic character of the clean Mo–S bridges is indicated by the intersection of the energy band and shows the band structure (left) and the density of states projected on the edge atoms (right) for each the Fermi level in the band structure. The localized metallic states along the Mo-edges are indicated model. The metallic character of the clean Mo–S bridges is indicated by the intersection of the energy by the color intensity at the intersection of the energy band and the Fermi energy. The charge density band and the Fermi level in the band structure. The localized metallic states along the Mo-edges are distribution below the figure indicates that the metallic states result principally from the d orbitals of indicated by the color intensity at the intersection of the energy band and the Fermi energy. The charge Mo atoms on the edges [29]. In other words, there are significant changes in the electronic structure density distribution below the figure indicates that the metallic states result principally from the d with the adsorption of O2 molecules on the Mo–S bridge sites [30–36]. The metallic band is orbitals of Mo flattened, atoms on which the edges [29]. Inthe other words,electron there are significant changes in the significantly means that effective mass is much higher than thatelectronic for the structure with the adsorption of O molecules on the Mo–S bridge sites [30–36]. The metallic band is 2 clean edges. According to the semiclassical Drude model, the higher effective electron mass causes significantly flattened, which means that the effective electron mass is much higher than that for the


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Chemosensors 2017, 5, 15 of 19 clean edges. According to the semiclassical Drude model, the higher effective electron mass8causes the decrease in the electrical conductivity. The localized electronic states along the Mo-edges are also the decrease in the electrical conductivity. The localized electronic states along the Mo-edges are also demonstrated by the charge distribution at the bottom of Figure 6e. demonstrated by the charge distribution at the bottom of Figure 6e.

Figure 6. (a) Response curves of the liquid-exfoliated MoS2 gas sensor to four pulses of 100% of O2 at Figure 6. (a) Response curves of the liquid-exfoliated MoS2 gas sensor to four pulses of 100% of O2 300 ◦°C. (b) Response curves to different O2 concentrations at 300 °C. ◦(c) Linear fit of the responses as at 300 C. (b) Response curves to different O2 concentrations at 300 C. (c) Linear fit of the responses a function of O2 at 300 °C. (d) Stable sites of O2 adsorption. Locally stable configurations of O2 as a function of O2 at 300 ◦ C. (d) Stable sites of O2 adsorption. Locally stable configurations of O2 adsorbed on clean Mo-edges and Mo-edges with S monomers. The adsorption free energy at 300 °C adsorbed on clean Mo-edges and Mo-edges with S monomers. The adsorption free energy at 300 ◦ C and 1 atm (Gad (300 °C, 1 atm)) is displayed. (e) Band structure, density of states (DOS) of edge atoms and 1 atm (Gad (300 ◦ C, 1 atm)) is displayed. (e) Band structure, density of states (DOS) of edge and charge density distribution near the Fermi level of clean Mo-edges with S monomers and O2 atoms and charge density distribution near the Fermi level of clean Mo-edges with S monomers and adsorbed Mo-edges with S monomers on the Mo–S bridge site. The color intensity in the band O2 adsorbed Mo-edges with S monomers on the Mo–S bridge site. The color intensity in the band structure is proportional to the weight of the corresponding state on the edge atoms (Reproduced structure is proportional to the weight of the corresponding state on the edge atoms (Reproduced with with permission of [28]. Copyright Royal Society of Chemistry, 2016.). permission of [28]. Copyright Royal Society of Chemistry, 2016.).

2.3. Tungsten Disulfide 2.3. Tungsten Disulfide Tungsten disulfide is a layered structure similar to MoS2, which has a trigonal prismatic Tungsten disulfide is W a layered structure similarantoappreciable MoS2 , which prismatic coordination sphere with atoms. As WS2 possesses band has gap,ait trigonal is appropriate to use as active materials electronic, photonic and gas sensing devices. The large surface to volume coordination sphere withinW atoms. As WS2 possesses an appreciable band gap, it is appropriate ratioasofactive WS2 in a few layered structures also reveals its suitability for sensing to use materials in electronic, photonic and gas sensing devices. Theapplications. large surface to volume O’Brien et al. reported the aptness of plasma-assisted synthesis of WS 2 thin films for sensing ratio of WS2 in a few layered structures also reveals its suitability for sensing applications. applications The pre-structured WOof3 films were deposited on the center a SiO2 substrate and O’Brien et[37]. al. reported the aptness plasma-assisted synthesis of WSof 2 thin films for sensing then sulfurized by the H2S plasma.WO ICP-activated atmosphere, a mixture of 10% 2S/90% Ar, was applications [37]. The pre-structured films were deposited on the center of aH SiO 3 2 substrate and introduced to the sputtered films. The plasma was generated in a one-inch quartz tube, which uses a then sulfurized by the H2 S plasma. ICP-activated atmosphere, a mixture of 10% H2 S/90% Ar, was copper coil that is attached to a 13.56-MHz power supply and associated match network. To convert introduced to the sputtered films. The plasma was generated in a one-inch quartz tube, which uses the WO3 to WS2, an exposure time of one hour at 500 °C was used for the synthesizing condition. a copper coil that is attached to a 13.56-MHz power supply and associated match network. To convert Then, the IDE electrodes were aligned on top of the WS2 film as shown in Figure 7a. The electrical the WO3 to WS2 , an exposure time of one hour at 500 ◦ C was used for the synthesizing condition. response of the WS2 films while being exposed to NH3 vapor was measured by continuous Then, the IDE electrodes were aligned on top of the WS2 film as shown in Figure 7a. The electrical observation of the variation in the resistivity of the WS2 films during gas exposure at room response of the WS2 films while being exposed to NH3 vapor was measured by continuous observation temperature. Figure 7b shows the relative resistance change to the initial resistance of the films, which of the variation in the resistivity of the WS2 films during gas exposure at room temperature. Figure 7b is measured before the NH3 injection. The thicker samples show almost no response to the exposure shows the3, relative resistance change initial resistance the films, the which is measured of NH which can be attributed to to thethe existence of oxides. of Meanwhile, 2-, 5and 10-nmbefore devicesthe


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NH3 injection. The thicker samples show almost no response to the exposure of NH3 , which can be Chemosensors 2017, 5, 15 9 of 19 attributed to the existence of oxides. Meanwhile, the 2-, 5- and 10-nm devices exhibit a small increase in conductivity to the exposure of NH3 . This thatofNH a charge the WS2 achannel exhibit a small increase in conductivity to themeans exposure NH33.gives This means thattoNH 3 gives charge and indicates WS2 thin films tested showinn-type semiconductor behaviors, to thethat WSthe 2 channel and indicates that in thethis WSexperiment 2 thin films tested this experiment show n-type semiconductor similar to the bulk [20,26,38,39]. Thissensors indicatesbased that the sensors similar to the bulk behaviors, crystals [20,26,38,39]. Thiscrystals indicates that the gas ongas WS 2 have the based on WS 2 have the potential to have selectivity for specific gas molecules, as they exhibit potential to have selectivity for specific gas molecules, as they exhibit distinguishing properties in distinguishing properties in charge transfer to WS2 due to their difference in tendency of donating or charge transfer to WS 2 due to their difference in tendency of donating or accepting charge from the accepting charge from the WS2 surface. The 2-nm device exhibits a superior response to NH3 WS2 surface. The 2-nm device exhibits a superior response to NH3 exposure, but electrical noise is exposure, but electrical noise is observed, which is not found in 5- and 10-nm devices. Figure 7c shows observed, which is not found in 5- and 10-nm devices. Figure 7c shows the response curves of the the response curves of the 2-nm gas sensor at various concentrations from 1–5 ppm 2-nm(parts-per-million) gas sensor at various from 1–5 ppm (parts-per-million) of NH3 exposure. of NHconcentrations 3 exposure.

Figure (a) Optical image 2 devicescontacted contacted with electrodes. (b) Percentile Figure 7. (a)7.Optical image of of WSWS with interdigitated interdigitated electrodes. (b) Percentile 2 devices resistance change versus time at given bias voltages for the 2-, 5and 10-nm WS 2 films withwith NH3NH gas gas resistance change versus time at given bias voltages for the 2-, 5- and 10-nm WS2 films 3 injections over 180 min. (c) Percentile resistance change versus time for the 2-nm film at a bias voltage injections over 180 min. (c) Percentile resistance change versus time for the 2-nm film at a bias voltage of 0.9 V, upon interval NH3 exposures at various concentrations from 1 ppm–5 ppm (Reproduced of 0.9 V, upon interval NH3 exposures at various concentrations from 1 ppm–5 ppm (Reproduced with with permission of [37]. Copyright Chemical Physics Letters, 2014.). permission of [37]. Copyright Chemical Physics Letters, 2014.).

The improved gas-sensing properties of 2D WS2 nanosheets through Ag nanowires (NW) functionalization was demonstrated by Ko etofal.2D [40].WS WS22 nanosheets nanosheets with a large Ag areananowires and Ag NWs(NW) The improved gas-sensing properties through were synthesized the sulfurization of [40]. atomicWS layer deposition (ALD) with WO 3 and a Ag functionalization wasbydemonstrated by process Ko et al. nanosheets with a large area and 2 modified CuCl 2-mediated polyol process, respectively [41,42]. Ag NW-functionalized (ANF) WS2 NWs were synthesized by the sulfurization process of atomic layer deposition (ALD) with WO3 and a was formed through spin-coating usingrespectively the Ag NWs[41,42]. above. The gas sensors were fabricated modified CuCl2 -mediated polyol process, Ag NW-functionalized (ANF) with WS2 was thermally evaporated metal contacts (Cr 5 nm, Au 50 nm). The gas sensing properties were measured formed through spin-coating using the Ag NWs above. The gas sensors were fabricated with thermally at 100 °C. Figure 8a,b shows the gas-sensing property of the ANF WS2 gas sensor with a 4L WS2 evaporated metal contacts (Cr 5 nm, Au 50 nm). The gas sensing properties were measured at 100 ◦ C. nanosheet. After Ag NW functionalization, a current level of 4L WS2 gas sensor exhibits a result Figure 8a,b shows two the gas-sensing ANF WS 4L2 WS nanosheet. 2 2gas approximately orders lower property than that of of the the pristine WS gassensor sensor.with ANFaWS gas 2sensors Afterconsisting Ag NW functionalization, a current of compared 4L WS2 gas sensor a result approximately of 1L and 2L WS2 show similarlevel results to those of exhibits the pristine WS2 sensor. This two orders lower than the pristine WS gas sensor. ANF WS sensors consisting of 1Lby and 2L lowered current of that ANFof WS 2 gas sensors is 2probably attributed to the n-type doping effect caused 2 gas the electrons from Ag NWs to WS [43–45]. As shown in Figure 8c, the pristine and ANF WS2 show similartransferred results compared to those of2the pristine WS2 sensor. This lowered current of ANF 4L WS 2 gas sensors show attributed a high percent recovery greater than 90%,caused which implies good recovery to WS2 gas sensors is probably to the n-type doping effect by the electrons transferred acetone sensing. On the other hand, as shown in Figure 8d, the pristine WS 2 gas sensor exhibits from Ag NWs to WS2 [43–45]. As shown in Figure 8c, the pristine and ANF 4L WS2 gas sensors show relatively low percent recovery ranging from 15%–82% for NO2 exposure (25–500 ppm). However, a high percent recovery greater than 90%, which implies good recovery to acetone sensing. On the the ANF 4L WS2 gas sensor shows significantly improved percent recovery for NO2 sensing (over other hand, as shown in Figure 8d, the pristine WS2 gas sensor exhibits relatively low percent recovery 90%) because the low surface energy of Ag enhances the chemisorption of nitrogen and oxygen ranging from 15%–82% for NO2 exposure (25–500 ppm). However, the ANF 4L WS2 gas sensor shows species on the Ag surface during the recovery process [46]. significantly improved percent recovery for NO2 sensing (over 90%) because the low surface energy of


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Ag enhances the chemisorption of nitrogen and oxygen species on the Ag surface during the recovery process [46]. 2017, 5, 15 Chemosensors 10 of 19 An ammonia sensor based on WS2 nanoflakes was reported by Li et al., which showed a remarkable An ammonia based on WS2Compared nanoflakes was reported by Li et al., whichMoS showed performance at room sensor temperature [47]. to other materials including 2 , thea WS2 remarkable performance roomand temperature [47]. Compared to other materials including MoS120 2, thes and nanoflake sensor exhibited at good full recovery. The response and recovery time was WS 2 nanoflake sensor exhibited good and full recovery. The response and recovery time was 120 s by 4+ 150 s, respectively. It seems that a lower adsorption energy of W in the WS2 for ammonia caused and 150 s, respectively. It seems that a lower adsorption energy of W4+ in the WS2 for ammonia caused a larger radius results in the faster recovery. The sensor exhibits a high selectivity to other gases such by a larger radius results in the faster recovery. The sensor exhibits a high selectivity to other gases as ethanol, methanol, acetone, benzene and formaldehyde with almost no responses in the air with a such as ethanol, methanol, acetone, benzene and formaldehyde with almost no responses in the air humidity 40%. of 40%. with aofhumidity

Figure 8. Gas-sensing results of the Ag nanowires-functionalized (ANF) WS2 gas sensors consisting

Figure 8. Gas-sensing results of the Ag nanowires-functionalized (ANF) WS2 gas sensors consisting of of 4L WS2 nanosheet upon (a) acetone exposure (0.5, 1, 2, 4 and 10 ppm) and (b) NO2 exposure (25, 4L WS nanosheet upon (a) acetone exposure (0.5, 1, 2, 4 and 10 ppm) and (b) NO exposure (25, 50, 50,2 100, 200 and 500 ppm). Percent recovery of the pristine and ANF WS2 gas sensor 2for (c) acetone 100, 200 5002 ppm). Percent recoveryofofgas theconcentration pristine and (Reproduced ANF WS2 gas sensor for (c) acetone and and (d) NO exposure as a function with permission of [40]. and (d) NO as a function ofSociety, gas concentration (Reproduced with permission of [40]. Copyright 2 exposure Copyright American Chemical 2016.). American Chemical Society, 2016.). 2.4. Tin Disulfide

2.4. Tin Disulfide The crystal phase of tin disulfide (SnS2) exists in a layer structure [48]. This phase consists of Sn atoms and sulfur atoms; the Sn atoms are sandwiched between two layers of closely-packed sulfur The crystal phase of tin disulfide (SnS2 ) exists in a layer structure [48]. This phase consists of Sn atoms, which are disposed hexagonally. The neighbor sulfur layers are linked by the weak atoms and sulfur atoms; the Sn atoms are sandwiched between two layers of closely-packed sulfur van der Waals forces (Figure 9a). SnS2 has a larger electronegativity, so it has the potential to increase atoms, which are disposed hexagonally. The neighbor sulfur layers are linked by the weak van der gas adsorption sites compared to 2D MoS2 [49]. Due to the relatively high temperature dependency Waalsonforces (Figureband 9a). structure SnS2 hasofaSnS larger electronegativity, so it has the potential to increase gas the electronic 2, it is able to be used as an optimizing sensing response and adsorption sites compared to 2D MoS [49]. Due to the high temperature dependency on the 2 be used for up stepping recovery kinetics at a range of relatively fair elevated temperatures [50,51]. electronicOu band SnS to befor used as an and optimizing response be used et al.structure presentedofan important progress selective reversiblesensing NO2 sensing basedand on the 2 , it is able charge transfer between physisorbed NO2ofgas molecules two-dimensional (2D) tin disulfide for up stepping recovery kinetics at a range fair elevatedand temperatures [50,51]. (SnSet 2) flakes at low operating temperatures [52]. The of 2D SnS2 flakesNO was done by a wet Ou al. presented an important progress for preparation selective and reversible 2 sensing based on chemical synthesis technique, and an HRTEM image is shown in Figure 9b. Dropcasting the charge transfer between physisorbed NO2 gas molecules and two-dimensional (2D)was tinused disulfide for the fabrication of the 2D SnS2 gas sensors. The solution containing 2D SnS2 flakes was dropcast on (SnS2 ) flakes at low operating temperatures [52]. The preparation of 2D SnS2 flakes was done by a wet the substrates made of alumina with interdigitated electrode (IDE) patterns on the surface. As shown chemical synthesis technique, and an HRTEM image is shown in Figure 9b. Dropcasting was used for in Figure 9c, the sensor exposed to the NO2 gas with concentrations ranging from 0.6–10 ppm has the fabrication of the 2D SnS sensors. The solution containing was dropcast the 2 gas 2 flakes dynamic performance at the optimum operation temperature of 1202D °C.SnS As the concentration of NOon 2 substrates made of alumina with interdigitated electrode (IDE) patterns on the surface. As shown in increases, more surface dipoles are formed before the NO2 coverage reaches its maximum, which Figure 9c, the exposed to the NO concentrations ranging from 0.6–10 ppm has dynamic results in sensor more electron transfer from2 gas SnS2with to NO 2. Excellent sensor reversibility is observed with a performance the of optimum 120 ◦ C. As the concentration recovery at time less thanoperation 180 s at temperature the optimum of operating temperature regardlessofofNO the2 increases, NO2 From this work, the 2D SnS flakes showed strong selectivity to NO2, aswhich it has results only moreconcentration. surface dipoles are formed before the2 NO reaches its maximum, in 2 coverage


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more electron transfer from SnS2 to NO2 . Excellent sensor reversibility is observed with a recovery time of less than 180 s at the optimum operating temperature regardless of the NO2 concentration. 15 SnS2 flakes showed strong selectivity to NO2 , as it has only minimal11 of 19 From Chemosensors this work,2017, the5,2D responses ◦ toward other gases. At 120 C, response factors at industrially-meaningful concentrations for H2 minimal responses toward other gases. At 120 °C, response factors at industrially-meaningful (1%), CH4 (10%), CO2 (10%) and H2 S (56 ppm) are found to be ~1.0, ~1.1, ~1.1 and ~1.3, respectively, concentrations for H2 (1%), CH4 (10%), CO2 (10%) and H2S (56 ppm) are found to be ~1.0, ~1.1, ~1.1 in comparison to ~36 for NO2 (10 ppm)to(Figure 9d). The unique selectivity is formed by the strong and ~1.3, respectively, in comparison ~36 for NO2 (10 ppm) (Figure 9d). The unique selectivity is physical affinity of paramagnetic NO gas molecules toward SnSmolecules Furthermore, there is a 2 2 surfaces.toward formed by the strong physical affinity of paramagnetic NO2 gas SnS2 surfaces. relatively preferredthere position between preferred the Fermiposition level ofbetween SnS2 and occupied orbitals Furthermore, is a relatively thepartially Fermi level of SnS2molecular and partially (POMO) of NO occupied molecular orbitals (POMO) of NO2. 2.

Figure 9. (a) Top and cross-sectional schematics of SnS2. (b) Zoomed-in HRTEM figure indicating that Figure 9. (a) Top and cross-sectional schematics of SnS2 . (b) Zoomed-in HRTEM figure indicating that the interlayer spacing for hexagonal SnS2 is 0.587 nm. (c) Dynamic sensing performance of 2D SnS2 the interlayer spacing for hexagonal SnS2 is 0.587 nm. (c) Dynamic sensing performance of 2D SnS2 flakes toward NO2 gas at concentrations ranging from 0.6–10 ppm under the operation temperature flakes toward NO2 gas at concentrations ranging from 0.6–10 ppm under the operation temperature of of 120 °C. (d) Measured cross-talk of 2D SnS2 flakes toward H2 (1%), CH4 (10%), CO2 (10%), H2S 120 ◦ C. (d) Measured cross-talk of 2D SnS2 flakes toward H2 (1%), CH4 (10%), CO H2 S (56 ppm) 2 (10%), Chemical (56 ppm) and NO2 (10 ppm) (Reproduced with permission of [52]. Copyright American and NO ppm) (Reproduced with permission of [52]. Copyright American Chemical Society, 2015.). 2 (10 2015.). Society,

2.5. Tantalum Disulfide 2.5. Tantalum Disulfide Tantalum disulfide displays a similarstructure structure to to molybdenum It has a number of of Tantalum disulfide displays a similar molybdenumdisulfide. disulfide. It has a number polytypes with a tantalum atom either in trigonal prismatic or octahedral coordination and a number polytypes with a tantalum atom either in trigonal prismatic or octahedral coordination and a number of different stacking orders. All of these types are layered structures with three-coordinate sulfide of different stacking orders. All of these types are layered structures with three-coordinate sulfide centers and trigonal prismatic metal centers. centers and trigonal prismatic metal centers. He et al. reported a dosage meter for toxic gas based on TaS2 nanosheets [53]. The TaS2 thin film He et al. reported a dosagefiltration meter for toxic gas based on TaS nanosheets [53]. The TaS2 thin film is prepared via the vacuum of TaS 2 aqueous suspension2obtained from the electrochemical is prepared via the vacuum filtration of TaS aqueous suspension obtained from the electrochemical 2 fabrication lithium-intercalation method [54–56]. The of a planar chemoresistive sensor was lithium-intercalation method [54–56]. fabrication of a planar chemoresistive sensor was processed processed by placing the TaS 2 strip The (rectangular shaped) onto Si/SiO 2 (300 nm) substrate. Then, the deposition of silver paint on both ends of the onto rectangularly-shaped strip was doneThen, for thethe usedeposition of drain of by placing the TaS (rectangular shaped) Si/SiO2 (300 nm) substrate. 2 strip source electrodes. The TaS 2 gas sensing performance was evaluated bythe theuse conditions of and bias source of silverand paint on both ends of the rectangularly-shaped strip was done for of drain 0.4 V at room temperature, exposing to NO gas with controlling the concentration. The device electrodes. The TaS2 gas sensing performance was evaluated by the conditions of bias of 0.4 V at room performed a current response of the NO exposure irreversibly. Furthermore, there was no current temperature, exposing to NO gas with controlling the concentration. The device performed a current recovery at all, even after a long time purge with N2 or heating the sample. At very low response of the NO exposure irreversibly. Furthermore, there was no current recovery at all, even after concentrations, the sensor based on TaS2 film showed a linear current response and responded a longimmediately time purgetowards with N2the or change heatingofthe sample. At very low concentrations, the sensor based on TaS2 concentration. Due to these properties, detecting the total dosage film showed a linear current response and towards the change of concentration. of NO under fluctuating concentration responded is possible. immediately Figure 10a shows the current responses of TaS2 Due to theseagainst properties, detecting the totalconcentrations dosage of NOforunder fluctuating is possible. devices NO exposure at different 10 min. The currentconcentration response and the NO concentration have a quasi-linear relation at ppm level, as shown in Figure 10b. Due to this linear


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Figure 10a shows the current responses of TaS2 devices against NO exposure at different concentrations for 10 min. The current response and the NO concentration have a quasi-linear relation at ppm level, as shown in Figure 10b. Due to this linear current profile, the TaS2 sensor can realize the integration of Chemosensors 2017, 5, 15 of 19 the total dosage of NO at fluctuating concentration. The current responses of a typical12TaS 2 device upon continuous exposure of NO at five different concentrations were tested (Figure 10c). The current profile, the TaS2 sensor can realize the integration of the total dosage of NO at fluctuating rate of currentconcentration. decrease (orThe thecurrent sensitivity of the increased with the escalation ofofNO responses of adevice) typical TaS 2 device upon continuous exposure NOconcentration at five different concentrations were tested (Figure between 10c). The rate of current decrease and (or the of load for from 0.19 ppm to 4.9 ppm. The linear relation the current response thesensitivity total toxic the device) increased with the escalation of NO concentration from 0.19 ppm–4.9 ppm. The linear the TaS2 device was demonstrated. (Figure 10d). Thus, the TaS2 -based device meets the requirements relation between the current response and the total toxic load for the TaS2 device was demonstrated. for a dosage meter. From the current profile, total toxic load can be directly read out. Since the (Figure 10d). Thus, the TaS2-based device meets the requirements for a dosage meter. From the currentcurrent response upon NO exposure is irreversible, it is available to record the toxic load of NO gas profile, total toxic load can be directly read out. Since the current response upon NO exposure continuously in the itsub-ppm level. is irreversible, is available to record the toxic load of NO gas continuously in the sub-ppm level.

Figure 10. (a) The time-dependent current response of TaS2 sensors at different concentrations of NO.

Figure 10. (a) The time-dependent current response of TaS2 sensors at different concentrations of NO. (b) Plot of the detection sensitivity versus the concentration of NO. Standard error was obtained by (b) Plottesting of thefive detection sensitivity versus the concentration of NO. Standard error was obtained by devices from the same batch of TaS 2 film (c) Current profile of device under continuous testing exposure five devices from the same batch filmconcentration (c) Currentprofile profile of device under continuous of NO. The dotted red lines in of (c) TaS show of exposed NO. (d) Plot of 2 the theof current versus thelines integrated load. withprofile permission of [53]. Copyright exposure NO. response The dotted red in (c) toxic show the(Reproduced concentration of exposed NO. (d) Plot of He etresponse al. TaS2 nanosheet-based room-temperature meter for with nitric permission oxide, AIP Materials, 2, the current versus the integrated toxic load.dosage (Reproduced of [53]. Copyright 092506, 2014.). He et al. TaS2 nanosheet-based room-temperature dosage meter for nitric oxide, AIP Materials, 2, 092506, 2014.). 2.6. Heterojunction Previous theoretical studies have reported that heterojunctions with some transition metal disulfide monolayers can align their band structures to be suitable for potential application in versatile theoretical applications. The investigations have demonstrated that the monolayer transition metal metal Previous studies have reported that heterojunctions withofsome transition disulfide sheets is a sensitive detector for various gas molecules due to the change in the resistivity disulfide monolayers can align their band structures to be suitable for potential application in versatile by the gas adsorption. It is expected that the heterojunctions have electrical resistivity changes by the applications. The investigations have demonstrated that the monolayer of transition metal disulfide adsorption of molecules especially on the in-plane interfaces. sheets is a sensitive detector forthe various due to the in change the resistivity Xu et al. have revealed roles ofgas the molecules heterogeneous interface the gasinsensing propertiesby of the gas adsorption. It is2 expected the heterojunctions have [57]. electrical resistivity changes by the adsorption SnO2-SnS hybrids tothat ammonia at room temperature SnS2-SnO 2 hybrids are synthesized by oxidizing pristine SnS in air at 300 °C with different times from 0.5–4 h (Figure 11a,b). To form a of molecules especially on 2the in-plane interfaces.

2.6. Heterojunction

Xu et al. have revealed the roles of the heterogeneous interface in the gas sensing properties of SnO2 -SnS2 hybrids to ammonia at room temperature [57]. SnS2 -SnO2 hybrids are synthesized by


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oxidizing pristine SnS2 in air at 300 ◦ C with different times from 0.5–4 h (Figure 11a,b). To form Chemosensors 2017, 5, 15 13 of 19 a suspension of SnS2 nanosheets, they were dispersed uniformly in ethanol by ultrasonication for 15 min. The suspension was dropcast onto the Au interdigitated electrodes. The sensors were named suspension of SnS2 nanosheets, they were dispersed uniformly in ethanol by ultrasonication for as SnO -SnS where X indicates the different oxidation time inelectrodes. hours. The inset of Figure 11c shows 2 2 -X,suspension 15 min. The was dropcast onto the Au interdigitated The sensors were named the responses of the sensors, which are measured by the I /I (represent the current in NH g ain hours. The inset of Figure 11c shows 3 and dry as SnO2-SnS2-X, where X indicates the different oxidation time air) the values. The response of thewhich sensors increases then decreases frominSnO responses of the sensors, areinitially measured by the Igand /Ia (represent the current NH32 -SnS and dry 2 -0.5 to SnOair) -SnS -4. The maximum is for SnO -SnS -1. The evolution of chemical bonds has the same change values. The response of the sensors initially increases and then decreases from SnO 2 -SnS 2 -0.5 to 2 2 2 2 SnO 2 -SnS 2 -4. The maximum is for SnO 2 -SnS 2 -1. The evolution of chemical bonds has the same change trend, which indicates that the performance of the gas-sensing is highly dependent on the number trend, which indicates the performance of 2the gas-sensing highly number of chemical bonds at thethat interface. SnO2 -SnS -0.5, SnO2 -SnSis2 -1 and dependent SnO2 -SnSon also showofgood 2 -2the chemical bonds the response. interface. SnO -SnS2-0.5, SnO2-SnS 2-1 and SnO2-SnS2-2 also show good reproducibility and at quick The2typical response recovery characteristics of SnO2 -SnS2 -0.5, reproducibility and quick response. The typical response recovery characteristics of SnO2-SnS 2-0.5, SnO2 -SnS2 -1 and SnO2 -SnS2 -2 to 100 ppm of NH3 , which appear moderately reproducible, are shown SnO2-SnS2-1 and SnO2-SnS2-2 to 100 ppm of NH3, which appear moderately reproducible, are shown in Figure 11c. Figure 11d indicates the response and recovery characteristics of SnO2 -SnS2 -1 to NH3 in Figure 11c. Figure 11d indicates the response and recovery characteristics of SnO2-SnS2-1 to NH3 with concentrations of 10, 25, 50, 100, 200 and 500 ppm at room temperature. The sensitivity and with concentrations of 10, 25, 50, 100, 200 and 500 ppm at room temperature. The sensitivity and reproducibility characteristics of the sensor both show high performance when exposed to various reproducibility characteristics of the sensor both show high performance when exposed to various NH3NH concentrations. The response is about 1.16 to the exposure of 10 ppm NH3 , indicates which indicates 3 concentrations. The response is about 1.16 to the exposure of 10 ppm NH3, which that thataalow lowdetecting detecting limit achieved by using as the gas sensing 2 hybrids limit cancan be be achieved by using SnO2SnO -SnS22 -SnS hybrids as the gas sensing material.material. The The comparison comparisonofofthe theseries seriesofofSnO SnO oxidized range of 0.5–4 h verifies 2-SnS 2 hybrids oxidized for for the the timetime range of 0.5–4 h verifies the the 2 -SnS 2 hybrids dependence between the response of SnO -SnS hybrids to NH at room temperature and the interface dependence between the response of SnO 2 2-SnS22 hybrids to NH3 at 3 room temperature and the interface positioned chemical bonds. positioned chemical bonds.

Figure 11. (a) SEM image of SnO2-SnS2-0.5. (b) Schematic illustration of SnO2-SnS2 contacts. (c) The

Figure 11. (a) SEM image of SnO2 -SnS Schematic illustration of SnO -SnS2 contacts. (c) The 2 -0.5. (b) inset shows the response of the obtained samples to a NH3 with 100 ppm.2Repetitive dynamic insetresponse-recovery shows the response samples to a NH3 with 100 ppm. Repetitive dynamic curvesofofthe SnOobtained 2-SnS2-0.5, SnO2-SnS2-1 and SnO 2-SnS2-2. (d) The response-recovery response-recovery curves of SnO -SnS -0.5, SnO -SnS -1 and SnO -SnS -2. (d)The Theinset response-recovery 2 2 2 2 2 curves of SnO2-SnS2-1 to NH3 in the concentration range from 10–5002ppm. shows the curves of SnO -SnS -1 to NH in the concentration range from 10–500 ppm. The inset shows the 2 3 3 concentration (Reproduced with permission of [57]. response as2 a function of NH Copyright response as a function of NH concentration (Reproduced with permission of [57]. Copyright American American Chemical Society, 3 2015.). Chemical Society, 2015.). Gu et al. demonstrated a gas sensor based on the SnO2/SnS2 heterojunction, which has a significant to the exposure of NO2 based at low temperature 80 °C [58]. The heterojunctions Gu et al.response demonstrated a gas sensor on the SnOof 2 /SnS2 heterojunction, which has were prepared by the in situ high temperature oxidation method, and the SnO ◦ 2 nanoparticle-decorated

a significant response to the exposure of NO2 at low temperature of 80 C [58]. The heterojunctions


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were prepared by the in situ high temperature oxidation method, and the SnO2 nanoparticle-decorated 2017, 5, better 15 of 19 SnS2 Chemosensors sensor showed performance to the exposure of NO2 than the pristine SnS2 , such14as higher response, lower operating temperature and better selectivity toward other gas molecules. SnS2 sensor showed better performance to the exposure of NO2 than the pristine SnS2, such as higher response, lower operating temperature and better selectivity toward other gas molecules.

3. Challenges

3. Challenges The sensing mechanism of 2D transition metal disulfides gas sensors is based on a charge transfer process [59–61]. Whenmechanism the sensorsofare to metal reactive gas, thegas gassensors molecules are adsorbed (mainly The sensing 2Dexposed transition disulfides is based on a charge physically at process temperatures 100 ◦ C) the surfaces of 2D transition metal disulfide materials transfer [59–61].lower When than the sensors areon exposed to reactive gas, the gas molecules are adsorbed (mainly physically at temperatures lower than 100 °C)adsorption on the surfaces 2Dconsequent transition metal disulfide resulting in the variation of sensor resistance. The gas andof the resistance change materials resulting in the variation of sensor resistance. The gas adsorption and the consequent are affected by three basic factors, namely receptor function, transducer function and utility factor resistance change are by three basicisfactors, namely receptor transducer function (Figure 12a) [62–67]. Theaffected receptor function concerned with howfunction, each particle (grain) responds and utility factor (Figure 12a) [62–67]. The receptor function is concerned with how each particle to the adsorbed gas, so it is closely related to the sensitivity and selectivity of the gas sensor [68]. (grain) responds to the adsorbed gas, so it is closely related to the sensitivity and selectivity of the gas When catalysts are incorporated on the surface of each particle, the gas adsorption and the sequential sensor [68]. When catalysts are incorporated on the surface of each particle, the gas adsorption and charge transfer can be enhanced. For the selective adsorption of specific gas molecules, the adoption of the sequential charge transfer can be enhanced. For the selective adsorption of specific gas molecules, appropriate catalysts is very important. transducer is related to how the variation of each the adoption of appropriate catalysts isThe very important.function The transducer function is related to how the particle is transduced into device resistance. The transducer function is played by the contacts between variation of each particle is transduced into device resistance. The transducer function is played by each particle. A typical gaseach sensing strategy that is suggested based on thisis recognition and the the contacts between particle. A typical gas sensing strategy that suggested based on concepts this and thesemiconductor concepts from other traditional semiconductor devices is composed electron and from recognition other traditional devices is composed of electron depletion of theofparticles depletion ofbarrier the particles and double Schottky barrier formation across12a. theThe contact, as factor shownexplains in double Schottky formation across the contact, as shown in Figure utility Figure 12a. The utility factor explains how the resistance change is attenuated in an actual porous how the resistance change is attenuated in an actual porous sensing body with the dissipation of the sensing body with the dissipation of the target gas while it diffuses in to the sensing sites. To target gas while it diffuses in to the sensing sites. To maximize the utility factor, the sensing film should maximize the utility factor, the sensing film should be very porous, and then, the target gas molecules be very porous, and then, the target gas molecules diffuse into the bottom layer of the sensing film diffuse into the bottom layer of the sensing film where the sensor electrode exists, which leads to the whereeffective the sensor electrode exists, leadsfilm. to the effective resistance variation of the sensing film. resistance variation of which the sensing

Figure 12. (a) Three basic factors controlling semiconductor gas sensors. (b) STEM- annular dark-field

Figure 12. (a) Three controlling semiconductor gas sensors. (b) STEM- annular dark-field (ADF) image of abasic MoSfactors 2 grain boundary with its common dislocations highlighted and close-ups of (ADF)the image of a highlighted MoS2 grain (Reproduced boundary with its common dislocations highlighted and close-ups regions with permission of [69]. Copyright Nature Publishingof the regions highlighted Group, 2013.). (Reproduced with permission of [69]. Copyright Nature Publishing Group, 2013.).


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Figure 12b is a scanning transmission electron microscope-annular dark-field (STEM-ADF) image of a MoS2 monolayer, a well-studied material representing transition metal disulfides [69]. The grain boundary and its dislocation cores are highlighted. The boundary has a significant relation to the transducer function. When the electron moves across the boundary, it should overcome the double Schottky barrier between two grains. This means that the pre-existing grain boundaries are transducers where the resistance change by the gas adsorption is amplified. For the monolayer MoS2 film, tailoring the utility factor is meaningless. Since the material is atomically thin, the adsorption of gas molecules is not related to the diffusion through the material inside. In other words, the utility factor is already maximized for the 2D material. Therefore, an appropriate approach to develop high performance gas sensors based on 2D transition metal disulfides is to achieve proper receptors that enhanced the interaction between target gas molecules and the sensing material. Thus, the sensitivity and selectivity of chemoresistive gas sensors based on 2D transition metal disulfides largely rely on the receptors on the sensing materials. The receptors can be incorporated on the material by various methods to decorate the surface such as physical vapor deposition, chemical vapor deposition and wet and dry chemical processes. In addition, the creation of point defects, including dopants and vacancies, in the sensing material can be effective since the defects existing on the surface can act as effective receptors. When multiple receptors that selectively respond to each specific gas molecules are obtained, a sensor array based on 2D transition metal disulfides can be realized, in which various target gases can be detected and monitored for environmental and indoor air quality control [70,71]. Since atomically 2D transition metal disulfides are transparent and flexible, the application of 2D-transition-metal-disulfide-based sensors will find new areas that were not reached by the conventional chemoresistive gas sensors. Developing 3D nanostructures based on 2D transition metal disulfides is promising to increase gas response [72,73]. Most of the previously-performed gas sensor devices based on the 2D transition metal disulfide materials had to be measured under a controlled environment, as the existence of other gases in realistic environmental conditions has an influence on the sensitivity of the sensing devices. Hence, following studies need to focus on achieving gas sensing devices with high sensitivity toward the target gas in practical conditions. Under practical conditions, the humidity poisoning effect by which water vapor adsorption reduces the gas sensing properties for most chemoresistive gas sensors should be considered. The fabrication of reliable gas sensors is one of the most challenging issues. Normally, most of the gas sensors are fabricated by a vapor deposition or microfabrication process. These processes are complicated and time consuming, and a reproducible process dealing with transition metal disulfide materials should be established. Furthermore, displaying the intrinsic properties of the sensing materials can fail. Thus, developing a facile, effective and reliable strategy for gas sensors still remains a huge challenge. The capability of large-scale manufacturing of 2D materials with a large area, constant and better quality and reproducibility is yet another big challenge [12,74]. 4. Conclusions In this article, we review typical 2D transition metal disulfides for gas sensing applications. Transition metal disulfides (MoS2 , WS2 , SnS2 and TaS2 ) possess tunable band gaps dependent on their layer thickness, and their excellent advantages in nanostructuring and heterostructuring make them attractive candidates for the fabrication of gas sensors. The results described that 2D transition metal disulfides are suitable materials for the fabrication of high performance gas sensors operated in diverse environments due to their particular characteristics. There are some reports on the theoretical simulations of gas adsorption on transition metal disulfides; still, the works are very limited and lacking. Following research in this field needs to understand the details of the specific properties of nanostructured two-dimensional materials and the change of the electronic properties with the adsorption of analytes to enhance the performance of the gas sensors based on these materials. First-principles calculations can be used to identify the appropriate modification of these materials for the higher performance of the gas sensors. The gas sensing mechanisms of devices based on 2D transition metal disulfides have not been completely explained till now. Once the mechanism is


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clarified, 2D transition metal disulfides gas sensors could perform significant sensitivity, ultrahigh selectivity, fast response, good reversibility, room temperature operation, outstanding stability, etc., in the near future. Acknowledgments: This work was financially supported by the Future Material Discovery (NRF-2016M3D1A1027666), the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) (2016M3A7B4910) and the International Energy Joint R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (20168510011350). Author Contributions: T.H.K. and H.W.J. conceived of and wrote the paper. Y.H.K., S.Y.P and S.Y.K. contributed to finding the references. Conflicts of Interest: The authors declare no conflict of interest.

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