2D Materials for Gas Sensing Applications: A Review on Graphene Oxide, MoS₂, WS₂ and Phosphorene

Abstract

After the synthesis of graphene, in the first year of this century, a wide research field on two-dimensional materials opens. 2D materials are characterized by an intrinsic high surface to volume ratio, due to their heights of few atoms, and, differently from graphene, which is a semimetal with zero or near zero bandgap, they usually have a semiconductive nature. These two characteristics make them promising candidate for a new generation of gas sensing devices. Graphene oxide, being an intermediate product of graphene fabrication, has been the first graphene-like material studied and used to detect target gases, followed by MoS₂, in the first years of 2010s. Along with MoS₂, which is now experiencing a new birth, after its use as a lubricant, other sulfides and selenides (like WS₂, WSe₂, MoSe₂, etc.) have been used for the fabrication of nanoelectronic devices and for gas sensing applications. All these materials show a bandgap, tunable with the number of layers. On the other hand, 2D materials constituted by one atomic species have been synthetized, like phosphorene (one layer of black phosphorous), germanene (one atom thick layer of germanium) and silicone (one atom thick layer of silicon). In this paper, a comprehensive review of 2D materials-based gas sensor is reported, mainly focused on the recent developments of graphene oxide, exfoliated MoS₂ and WS₂ and phosphorene, for gas detection applications. We will report on their use as sensitive materials for conductometric, capacitive and optical gas sensors, the state of the art and future perspectives.

Keywords: MoS2; WS2; gas sensors; graphene oxide; phosphorene.

Figure 1

Number of published papers vs. year of publication for “graphene oxide”, “MoS₂”, “WS₂” and “phosphorene” or “exfoliated black phosphorus”. (Source: Scopus, 28 September 2018).

Figure 2

Schematic illustration of a chemiresistor (adapted from ref. [42], Copyright 2013, with permission from Elsevier, Amsterdam, The Netherlands).

Figure 3

Panel (a): schematic illustration of a FET sensor based on reduced graphene oxide; Panel (b): SEM image of the device, the brightest regions are the metal electrodes; Panel (c): I_ds vs. V_g curves before (black curve) and after (blue curve) exposure to NH₃; Panel (d): I_ds vs. V_g curve after exposure to NO₂ (adapted with permission from [43]. Copyright 2011, American Chemical Society, Washington, DC, USA).

Figure 4

Panel (a): schematic of the impedance sensor with a 1T-WS₂ sensing layer; Panel (b): selectivity studies of 1T-WS₂ sensor, impedance phase spectra (adapted with permission from [45]. Copyright 2015, John Wiley and Sons, Hoboken, NJ, USA).

Figure 5

Absorbance change of rGO/Au NPs sample exposed to 10,000 ppm H₂, 10,000 ppm CO and 1 ppm NO₂. The incident wavelength is 528 nm (adapted from [46], Copyright 2013, with permission from Elsevier, Amsterdam, The Netherlands).

Figure 6

(Left panel) photograph of the sprayed GO on Ag electrodes. Only the Ag electrodes are visible, due to the transparency of the deposited ultrathin GO film; (Central panel) Nyquist plots of the GO flakes recorded at different RH values; (Right panel) response of three GO sensors with different heights to wet air, compared with an ultrafast commercial sensor (adapted with permission from ref. [69]. Copyright 2013, American Chemical Society, Washington, DC, USA).

Figure 7

Panel (a): chemical structure of GO flakes; Panel (b): schematic image of the GO flakes deposited on the Si membrane and the embedded Wheatstone bridge; Panel (c): piezoresistive Wheatstone-bridge circuit; Panel (d): response curve to humidity of the 65 nm thick GO layer deposited on the Si microbridge (black curve) and of the bare Si microbridge (red curve) (adapted from [76], Copyright 2012, with permission from Elsevier, Amsterdam, The Netherlands).

Figure 8

Panel (a): SEM image of the device. The lighter stripes are the Pt interdigitated electrodes on Si₃N₄ substrate; Panel (b): XPS C 1s core level spectrum of the deposited GO flakes. The grey lines are the fitting curves, labelled with their own relative C chemical bond (adapted with permission from [80]. Copyright 2013 American Chemical Society, Washington, DC, USA); Panel (c): SEM image of the device at higher magnification than (a); Panel (d): normalized resistance of a GO-based conductometric gas sensor exposed to various NO₂ concentrations (ranging from 20 to 400 ppb) at different RH (adapted with permission from [81], © IOP Publishing, Bristol, United Kingdom. Reproduced with all permission. All rights reserved.).

Figure 9

Panel (a): AFM image of the pristine GO flakes. The height profile of a flake is reported in the inset; Panel (b): AFM image of the tailored GO flakes. The height profile of a flake is reported in the inset; Panel (c): SEM image of the interdigitated electrodes on the FET device; Panel (d): SEM image of the GO flakes bridging the electrodes; Panel (e): response to different concentrations of SO₂ of pristine GO (red curve) and tailored GO flakes (black curve); Panel (f): current vs. SO₂ concentration graph of the pristine GO (red curve) and tailored GO flakes (black curve); Panel (g): ten cycles of the tailored GO flakes for response to 500 ppm of SO₂; Panel (h): real time response of the tailored GO flakes to other gases. (Adapted and reproduced with permission of RSC Pub., Cambridge, United Kingdom, from [87]; permission conveyed through Copyright Clearence Center, Inc.).

Figure 10

Panel (a): sketch of the device showing the electrodes and the dielectric porous GO (pGO) between them; Panel (b): SEM image of the pGO network. The graphs report the responses of the not-functionalized (pGO) and phenyl-, dodecyl-, ethanol-functionalized GO sensors to different gas vapours, indicated according to colour code (concentration 180 ppm) and 75% RH. (Adapted and reproduced with permission of RSC Pub., Cambridge, United Kingdom, from [95]; permission conveyed through Copyright Clearence Center, Inc.).

Figure 11

Panel (a): response of the rGO-CDs sensor to NO₂ concentrations ranging from 50 ppb to 25 ppm; Panel (b): calibration curve of the rGO-CDs sensor vs. NO₂ concentrations; Panel (c): reproducibility tests of the rGO-CDs sensor; Panel (d): stability of the sensor response over 90 days; Panel (e): response curve upon exposure to 10 ppb of NO₂; Panel (f): selectivity of the rGO-CDs sensor: all the bars but the first and the last, are the response of the sensor to 2% of the saturated vapour pressure (SVP) of the labelled gas; the first to 1% SVP chloroform and the last to 25 ppm of NO₂. (Adapted and reproduced with permission of RSC Pub., Cambridge, United Kingdom, from [121]; permission conveyed through Copyright Clearence Center, Inc.).

Figure 12

Schematic representation of the one-headed POF sensor covered with GO-rGO (a); Panel (b): fabrication process of the GO-rGO POF sensor by converting GO into rGO with sunlight; Panel (c): plot of the selectivity of one headed GO-rGO POF to THF, dichloromethane and ethanol. (Adapted with permission from Nature, Scientific Reports, London, United Kingdom, [86] copyright 2013).

Figure 13

Panel (a): schematic image of the three-layers CVD grown MoS₂ device in dark conditions; Panel (b): response of the MoS₂ sensor to NO₂ concentrations ranging from 120 ppb to 1 ppm; Panel (c): calibration curve of the MoS₂ sensor; Panel (d): OT dependence of the response of the MoS₂ sensor to 1200 ppb of NO₂; Panel (e): reproducibility tests; Panel (f): results of the selectivity tests. (Reprintedwith permission from [161]. Copyright 2015 American Chemical Society, Washington, DC, USA).

Figure 14

Top panel: dynamic response in dry air of the MoS₂ device annealed at 250 °C to NO₂ concentrations ranging from 20 ppb to 1 ppm, at OT = 200 °C; Bottom panel: calibration curve of the device. (Reprinted from [164], with permission from Elsevier, Amsterdam, The Netherlands).

Figure 15

Sensing performances of 5-layers (red curve) and bilayers (green curve) MoS₂ sensing device to NH₃ (panel (a)) and NO₂ (panel (b)). Gas concentrations are 100, 200, 500 and 1000 ppm. Inset: SEM image of the 2-layer MoS₂ transistor device (scale bar 20 µm). (Adapted with permission from [166]. Copyright 2013 American Chemical Society, Washington, DC, USA).

Figure 16

Panel (a): vertically aligned MoS₂ flakes have higher resistance due to cross-plane hopping of the carriers; Panel (b): resistance change to 1000 ppm of ethanol for horizontally (black curve), vertically (blue curve) and mixed aligned MoS₂ flakes; Panel (c): resistance change to 100 ppm of NO₂ for horizontally (black curve), vertically (blue curve) and mixed aligned MoS₂ flakes; Panel (d): relative resistance change of the horizontally, vertically and mixed aligned MoS₂ flakes to 0.1–100 ppm NO₂; Panel (e): schematic representation of the adsorption of NO₂ molecules on edge sites and basal plane of the MoS₂ flakes. (Reproduced with permission from [173]. Copyright 2015 American Chemical Society, Washington, DC, USA).

Figure 17

Panel (a): the MoS₂-PdCl₂ solution; Panel (b): MoS₂-Pd FET, with gold electrodes; Panel (c): SEM image of the MoS₂-Pd composite; Panel (d): comparison between the electrical responses of pristine MoS₂ and Pd-MoS₂ nanosheets to 50,000 ppm of H₂; Panel (e): electrical responses of the Pd-MoS₂ sensor exposed to H₂ concentrations ranging from 50,000 to 500 ppm; Panel (f): selectivity of the Pd-MoS₂ device to different target gases. (Adapted with permission from [182]. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany).

Figure 18

(Left panel) images of the MoS₂-graphene heterostructure device; (Right panel) response of the device before and after bending to NO₂ and NH₃. (Reproduced with permission from [183]. Copyright 2015 American Chemical Society, Washington, DC, USA).

Figure 19

(Left panel) NO₂ sensing responses of the WS₂ sensors annealed at different temperatures; (Right panel) sensing responses to NH₃ and H₂ of the 150 °C annealed device. The inset reports the calibration curves for NH₃ (blue curve) and H₂ (red curve). (Adapted and reprinted from [193], with permission from Elsevier, Amsterdam, The Netherlands).

Figure 20

Panel (a): resistance change of the WS₂/GA sensor to cyclic exposure to NO₂, at different RH values; Panel (b): NO₂ response of the device at different RH values; Panel (c): electrical response of the WS₂/GA device to different concentrations of NO₂ at RH = 60%; Panel (d): calibration curve of the device exposed to NO₂ at RH = 60%. Inset: sketch of the resistive WS2/GA device. (Adapted and reprinted from [196], with permission from Elsevier, Amsterdam, The Netherlands).

Figure 21

(Left panel) optical image of the phosphorene-based FET. The phosphorene flake is bounded by a dotted black line; (Right panel) conductance change of the phosphorene flakes exposed to different NO₂ concentrations. (Reproduced with permission from [204]. Copyright 2015 American Chemical Society, Washington, DC, USA).

Figure 22

Panel (a): SEM image of the suspended phosphorene flake. Inset: optical image of the suspended flake; Panel (b): schematic illustration of the target gas molecules adsorbing on the suspended and supported phosphorene flake; Panel (c): responses of the suspended and the supported phosphorene flakes to increasing NO₂ concentrations (from 25 to 200 ppm). (Adapted and reprinted from [211], with permission from Elsevier, Amsterdam, The Netherlands).

Figure 23

Panel (a): RT sensing responses of exfoliated BP flakes to H₂ (black curve) and NH₃ (blue curve); Panel (b): RT sensing responses of exfoliated BP flakes to NO₂. (Reproduced with permission from [212], © IOP Publishing, Bristol, United Kingdom. Reproduced with all permission. All rights reserved).

Figure 24

Panel (a): resistance variation of BP, MoS₂ and graphene sensors exposed to 0.1, 1, 5, 10 and 50 ppm of NO₂; Panel (b): the maximal resistance change onto various analytes of BP, MoS₂ and graphene sensors. (Adapted and reprinted from [213], with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany).