Recent Advances in Graphene-Based Humidity Sensors

Abstract

Humidity sensors are a common, but important type of sensors in our daily life and industrial processing. Graphene and graphene-based materials have shown great potential for detecting humidity due to their ultrahigh specific surface areas, extremely high electron mobility at room temperature, and low electrical noise due to the quality of its crystal lattice and its very high electrical conductivity. However, there are still no specific reviews on the progresses of graphene-based humidity sensors. This review focuses on the recent advances in graphene-based humidity sensors, starting from an introduction on the preparation and properties of graphene materials and the sensing mechanisms of seven types of commonly studied graphene-based humidity sensors, and mainly summarizes the recent advances in the preparation and performance of humidity sensors based on pristine graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, and a wide variety of graphene based composite materials, including chemical modification, polymer, metal, metal oxide, and other 2D materials. The remaining challenges along with future trends in high-performance graphene-based humidity sensors are also discussed.

Keywords: chemical modified graphene; graphene; graphene oxide; graphene quantum dots; graphene/2D materials; graphene/metal oxide; graphene/polymer; humidity sensors; reduced graphene oxide.

Figure 1

Schematics of the synthesis mechanism of chemical vapor deposition (CVD) graphene on Cu foil (a) and the roll-to-roll transfer process of graphene (b). Reproduced with permission from [42]. Copyright Wiley-VCH, 2016. Schematic of the preparation of graphene oxide and reduced graphene oxide by the reduction of graphene oxide (c). Reproduced with permission from [63]. Copyright Wiley-VCH, 2011.

Figure 2

Schematic of a graphene-based field-effect transistor (FET) humidity sensor with source-drain voltage, V_ds, and gate voltage, V_gs, control.

Figure 3

Schematic of a graphene-based humidity sensor in a resistive, impedance, or capacitive working mode depending on the measuring parameter.

Figure 4

Schematic view of a surface acoustic wave sensor with a clean surface and covered with a graphene oxide (GO) film. Reproduced with permission from [79], Copyright Elsevier B.V., 2017.

Figure 5

Photograph and structure of the quartz crystal microbalance (QCM) sensor. Reproduced with permission from [86], Copyright Elsevier B.V., 2017.

Figure 6

Classification of optical fiber humidity sensors [98].

Figure 7

Changes in resistivity caused by graphene’s exposure to various gases diluted in concentration to 1 ppm (a). Reproduced with permission from [57], Copyright Nature Publishing Group, 2007. The values of maximum resistivity observed, and the time taken to achieve 90% of the saturation value for different values of absolute humidity (b). Reproduced with permission from [107], Copyright Wiley-VCH, 2010. Resistance change in the graphene device versus the relative humidity (%RH) for a device placed in a vacuum chamber (1) and the same device placed in a humidity chamber (2) (c) and the interaction of water molecules with the graphene surface (d). Reproduced with permission from [108], Copyright Royal Society of Chemistry, 2015. Sensitivity of the humidity sensors under current and capacitance modes (e). Reproduced with permission from [111], Copyright Wiley-VCH, 2017. Resistance response of the double-layer graphene device in comparison with the %RH response from a commercial humidity sensor as well as the response from a commercial pressure sensor during three consecutive cycles of pumping air from the environment into and out of the vacuum chamber (f). Reproduced with permission from [112], Copyright Elsevier B.V., 2017.

Figure 8

Schematic illustration of the water adsorption and desorption on surfaces of CVD-growth flat graphene (a) and wrinkled graphene (b), and breathing signals recorded by wrinkled graphene sensors with alternating breathing speed and depth during physical activity (c). Reproduced with permission from [113], Copyright Wiley-VCH, 2018.

Figure 9

SEM image of the device (a), schematic diagram of the humidity testing system graphene oxide film as a humidity sensing material was placed on the two sets of interdigitated electrodes (b), and the output capacitances of sensors as a function of RH (c). Adapted from [76]. Photograph of a sprayed GO sensing element (d), and normalized response of the different sensors to a modulated humid air flow at 1 Hz (e). Reproduced with permission from [74], Copyright American Chemical Society, 2013. GO-based humidity sensor performance (capacitance vs RH) (f) and real-time RH sensing of the GO-based humidity sensor at specific RH (g). Reproduced with permission from [118], Copyright Wiley-VCH, 2016. The humidity sensing layer through drop-casting of GO followed by the measurement of relative humidity via conductance (h) and normalized conductance of three different humidity sensors as a function of RH (i). Reproduced with permission from [119], Copyright Elsevier B.V., 2017.

Figure 10

Schematic of CO₂ laser-patterning of free-standing hydrated GO films to fabricate rGO–GO–rGO devices with in-plane geometry (a) and the dependence of ionic conductivity on the exposure time to vacuum and air (b). Reproduced with permission from [75], Copyright Macmillan Publishers Limited, 2011. Schematic image of the laser direct writing system for the single-step fabrication of all-graphene noncontact sensors (c), optical images of the sensor where the rGO electrodes appear in black and the brown thin film corresponds to the GO sensing material (d), plots of impedance as a function of RH at different operation frequencies (e), and (f) upper panel: real-time moisture sensing with different RH ranges (all starting from 11% RH) and repeated RH detection between 11% RH and 90% RH for four cycles, and lower panel: response−recovery curve of the sensor with RH switching between 11% and 90%. Reproduced with permission from [135], Copyright American Chemical Society, 2017.

Figure 11

Schematic illustration for the preparation of rGO patterns by laser direct writing (a), interdigitated pattern of rGO/GO/rGO prepared on a flexible poly(ethylene terephthalate) (PET) film (b), SEM image of the structures obtained by laser direct writing (c), Raman spectra at different positions of a laser-irradiated line (d), electronic circuit of the rGO/GO/rGO humidity sensor (e), output waves of the humidity sensor responding to a rectangular alternating current (ac) wave with a (peak) pk−pk voltage of 1 V at 0.04 Hz (f), response and recovery of the humidity sensor at 40 Hz (g), and the change of the sensing peak voltages toward RH at different frequencies (h). Reproduced with permission from [131], Copyright American Chemical Society, 2018.

Figure 12

Schematic of the reproducible exfoliation process for the fabrication of r-GO ultrathin films (a), a typical photograph (b) and SEM image of an r-GO ultrathin film on a PET substrate (c), transmittance spectra of 13 r-GO ultrathin films on PET substrates (d), real-time response of one sensor in the flexible matrix device to RH from 4.3% to 75.7% (e), real-time-repeated response of the sensor at 4.3% RH for eight cycles (f), and the 3D mapping of matrix device when the fingertip approaches the relative center area of the device (g). Reproduced with permission from [149], Copyright Wiley-VCH, 2014.

Figure 13

Schematic illustration of the fabricated flexible sensor (inset is an optical image of a flexible array of sensors) (a), SEM image of the drop-casted sensing layer (b), the sensor response to different levels of humidity (inset shows sensor resistance as a function of time upon exposure to 60% RH during subsequent cycles) (c), and response of the fabricated sensor to human breath (d). Reproduced with permission from [151], Copyright The Royal Society of Chemistry, 2017.

Figure 14

Schematic representation of the graphene/methyl-red composite based humidity sensor (a), and capacitance versus relative humidity (% RH) characteristics curves of the graphene/methyl-red composite, methyl-red only, and graphene only based humidity sensors measured in the humidity chamber at a 1 kHz frequency (b). Reproduced with permission from [161], Copyright, 2016 Elsevier B.V. Optical images of the comb electrode before and after coating a layer of hydrophobin (HFBI) wrapped rGO flakes (c), real-time responses of HFBI wrapped rGO sensor (left) and bare rGO sensor (right) to RH ranging from 2% to 50%, respectively (d), and relative resistance change upon the exposure to water molecules at the maximum equilibrium vs. RH values (e). Reproduced with permission from [162], Copyright Elsevier B.V., 2017. Schematic of the supramolecular assembly of Pyr-rGO sheets with the corresponding physical image in dispersion (f), the impedance curves of Pyr-rGO based humidity sensors measured at different frequencies under different RH levels (g), and the five-cycle response-recovery curve of Pyr-rGO (h). Adapted from [163].

Figure 15

Photographs of a flexible rGO/LS-1 thin film (a), cross-sectional SEM images of rGO (b) and rGO/LS-1 (c) thin-film, real-time resistance measurement of the rGO/LS thin-films under switching RH (d), time-dependent humidification-dehumidification curves to a relative humidity pulse between 0% and 33%, 52%, 75%, and 97%, respectively (e), long-term stability of rGO/LS thin-film at 33%, 52%, 75%, and 97% RH (f), and schematic image of the humidity sensing mechanism of an rGO/LS thin-film at the initial stage and the saturation stage under a humidity environment, respectively (g). Reproduced with permission from [170], Copyright Elsevier B.V., 2017.

Figure 16

Schematic fabrication layer-by-layer stacking process of the graphene nanochannels confined poly(dopamine) (GNCP) high order superlattice sensing junction structure (a), photograph of a drop-casted GNCP sensing element and atomic force microscope (AFM) image of nano-size layered structure poly(dopamine) (PDA)/graphene film on it (b), illustration of superlattices composed of alternating atomic scale PDA/graphene layers (c), AFM image of graphene before (up) and after PDA modification (down) (d), the RH dependent resistance response range of PDA/graphene sensors as a function of PDA content (e), histogram plots of absorbed water of PDA/graphene at different humidity atmospheres (f), the derived RH dependent resistance changes of PDA/graphene and its magnified curve of the low RH region from 0% to 35% (g), and the changes in the measured current from the film at 1 V as RH was switched between dry air (RH ≈ 10%) and humidity air (RH ≈ 80%) (right) (h). Estimated results showed the ultrafast response (20 ms) and recovery (17 ms) times (left). Reproduced with permission from [176], Copyright American Chemical Society, 2018.

Figure 17

(a) Schematic illustration of a humidity sensor for human exhaled air detection during speaking. (b) Repeated responses of a PDA/graphene sensor to three different words. (c) Responses of a PDA/graphene sensor to the song “Twinkle Twinkle Little Star” sung by two different volunteers. Reproduced with permission from [176], Copyright American Chemical Society, 2018.

Figure 18

Schematic illustration of as-fabricated sensor prototype (a), sensitivity comparison between SnO₂/rGO composite and rGO towards humidity (b), and response and recovery curves of the SnO₂/rGO composite sensor towards an RH pulse from dry air to other RH levels (c). Reproduced with permission from [77], Copyright Elsevier B.V., 2015. AFM images of GO-Ag sheets (d), time-dependent response and recovery curve of an rGO scroll meshes-based device (e) and an rGO-Ag scroll meshes-based device (f) at different humidity. Reproduced with permission from [195], Copyright The Royal Society of Chemistry, 2017.

Figure 19

Fabrication procedure of the rGO@MoS₂ humidity sensor (a), response curves of rGO, RGMS 1, RGMS 5, RGMS 10, and MoS₂ to 50% RH at 25 °C (b), response curves to different RHs of 5%, 25%, 45%, 65%, and 85% at 1 V (c), and Schematic of the mechanism with enhanced humidity sensing properties of the enhanced depletion region on (d) bare rGO and (e) RGMS. Reproduced from [199] with permission from The Royal Society of Chemistry.