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Films of metal oxides such as WO₃, TiO₂, ZnO, and SnO₂ have been widely used as sensing materials for hydrogen detection [20- 22]. However, these metal oxide-based sensors require high operating temperatures for sufficient sensing performance. To overcome this limitation, various works have studied the combination of graphene materials with metal oxide nanoparticles [23,24]. Although pure graphene materials do not sensitively detect hydrogen gas, combining them with metal oxide nanoparticles enables highly sensitive and fast hydrogen sensing at room temperature. Zhang et al. investigated a room-temperature hydrogen sensor based on SnO₂-decorated graphene transistors [25]. They used the mechanical exfoliation method to prepare graphene and transferred the exfoliated graphene to a SiO₂/Si substrate. Tin oxide nanoparticles were formed by annealing the sample after metal film deposition. The device fabrication process was completed by the formation of electrodes using electron beam lithography and Cr/Au deposition. The prepared SnO₂ nanoparticledecorated graphene gas sensors exhibited a fast response to hydrogen and short recovery times of less than 1 s at room temperature. The high sensitivity and fast response are explained by the small energy barrier between SnO₂ and graphene and the high carrier mobility of graphene. Kathiravan et al. reported the gas sensing properties of a self-assembled hierarchical ZnO nanotube/graphene-based sensor. The sensor exhibited high response and stability, and a fast reaction to hydrogen at room temperature [26]. The graphene was synthesized by the chemical vapor deposition (CVD) method and transferred to a SiO₂/Si substrate. Then, vertically aligned ZnO nanotubes were grown on the prepared graphene using the hydrothermal method. Finally, Pt interdigitated electrodes were fabricated on the hydrogen sensors. The sensors were prepared using various ZnO nanotube growth times. Fig. 1 shows the response curves of the sensors under 100 ppm hydrogen gas at room temperature and their responses to other gases. The sensing characteristics were altered depending on the growth condition and structural characteristics. In particular, the sensor based on defect-free ZnO nanotubes and graphene exhibited the largest response to hydrogen gas. The superb hydrogen sensing properties are attributed to the multiple depletion layers induced by the formation of metalized regions in the ZnO nanotube/graphene interfaces.

Fig. 1. 
(a) Response and recovery curves of the sensors with growth periods of 8 (red), 12 (blue), and 18 h (black line). (b) Response of each sensor to various gases. Reprinted with permission from [26]. Copyright © 2017, American Chemical Society.

Decoration with nanoparticles of noble metals such as Au, Ag, Pd, and Pt is a promising strategy for enhancing the sensitivity and selectivity of graphene-based gas sensors through functionalizing the surface chemistry [27,28]. The noble metal nanoparticles introduced on the graphene-based materials change the band gap and work function of graphene and induce carrier transfer. In addition, the noble metal nanoparticles modify the surface morphology and function as catalytic centers, resulting in variations of the sensing properties [29-31]. In particular, surface functionalization using Pt and Pd nanoparticles is a well-known method for enhancing the detection of hydrogen gas. The catalytic effect of the nanoparticles accelerates the dissociation of hydrogen molecules into more reactive atomic forms [32,33].

Kaniyoor et al. investigated room-temperature hydrogen gas sensors based on Pt-decorated graphene sheets and Pt-decorated multi-walled carbon nanotubes (MWCNTs) [34]. The graphene sheets were prepared by a thermal exfoliation method, and the MWCNTs were synthesized by a catalytic CVD method. Pt nanoparticle decoration was then achieved using chemical reduction from a Pt salt. The prepared samples were ultrasonically dispersed and drop-casted onto the electrodes to complete the sensor fabrication. The gas sensor based on the Pt-decorated graphene sheets showed a two-fold increase in the response compared to the sensor based on Pt-decorated MWCNTs. The sensing mechanism is the decrease in the work function due to hydrogen exposure. The lowered work function transfers electrons from the Pt nanoparticles to the graphene-based materials, which causes a decrease in the hole carriers and an increase in the sensor resistance. Because the graphene sheets contain more hole carriers than the carbon nanotubes, the recombination occurs at a increased rate in the Pt-decorated graphene-based sensor, resulting in a higher hydrogen sensitivity.

Shin et al. reported a method for fabricating flower-like Pd nanocluster-decorated graphene electrodes and the sensitive and reversible hydrogen sensing properties of these electrodes at room temperature [35]. First, CVD graphene was transferred to a poly(ethylene naphthalate) substrate and chemically functionalized by reacting with 1,5-diaminonaphthalene. Then, Pd nanoparticles were deposited directly on the graphene surface by electrodeposition. Flower-like palladium nanoclusters were formed as the deposition time increased. Fig. 2(a) shows the response curves of the electrodes with various functionalization solutions containing 0.01, 0.1, and 1M 1,5-diaminonaphthalene under exposure to increasing hydrogen gas concentrations. The response increased with the number of flower-like palladium nanoclusters. This indicates that the palladium nanoclusters act as active materials for reacting with hydrogen molecules (Fig. 2(b)). When the decorated graphene is exposed to hydrogen molecules, the phase of the palladium nanoclusters changes from Pd to Pd hydride (PdH_x). The phase transfer decreases the work function and facilitates the flow of more electrons, resulting in a decrease of the resistance (Fig. 2(c)).

Fig. 2. 
Response curves at room temperature upon (a) exposure to various concentrations of hydrogen gas (0.1 to 100 ppm) and (b) periodic exposure to 10 ppm hydrogen gas of pristine graphene (green) and flower-like Pd nanocluster-decorated graphene with 0.01 (black), 0.1 (blue) and 1 M (red) 1,5-diaminonaphthalene. (c) Scheme of hydrogen gas sensing mechanism in flower-like Pd nanocluster- decorated graphene. Reprinted with permission from [35]. Copyright © 2015, Dong Hoon Shin et al.

Although gold in the form of atoms and molecules is one of the most inactive noble metals, Au nanoparticles with sizes of less than 10 nm have ideal catalytic activity when supported on metal oxide or graphene-based materials [36,37]. Diverse studies on utilizing the catalytic activity of Au nanoparticles have investigated various aspects, such as surface modification and heterostructure formation. Unlike platinum and palladium nanoparticles, which have shown a remarkable enhancement of the hydrogen detection, no research on improving the hydrogen sensing properties of Au nanoparticles has been reported so far. In 2019, Kim et al. reported the enhancement of the roomtemperature hydrogen sensing properties of a graphene-based sensor by using a Au-decorated graphene microchannel. Three-layer graphene was prepared by the CVD method and micropatterned to form a narrow micro-sized graphene channel [38]. Three-layer graphene was then transferred to a flexible and transparent polyimide substrate for the fabrication of the self-activated graphene-based gas sensor. Au (1 nm) was deposited on the graphene microchannel using an electron beam evaporator and agglomerated by Joule heating to form Au nanoparticles. Figs. 3(a) and (b) show a comparison of the hydrogen sensing behaviors of the Au-decorated graphene sensor and the pristine graphene sensor upon exposure to 50 ppm H₂ in the self-activation state. The Au-decorated graphene sensor exhibited a sensing response that is 48.8 times larger than that of the pristine graphene sensor, indicating that the Au nanoparticle decoration selectively enhanced the hydrogen sensing performance. First-principles density functional theory (DFT) calculations were performed for various geometries to understand the unusual enhancement of hydrogen sensing by Au decoration. The DFT analysis showed that the adsorption energies of the hydrogen atoms are notably high at the vertex and interface vertex sites of the Au nanoparticles, resulting in enhanced hydrogen detection despite the binding of hydrogen gas to the Au or graphene surface being energetically unfavorable (Figs. 3(c) and (d)). The sensing parameters of the several graphene-based hydrogen gas sensors and the commercial hydrogen gas sensor are summarized and listed in Table 1. More studies are needed for the commercialization of the graphene-based hydrogen sensors, especially the graphene-based sensors without incorporation of the metal-oxide semiconductors.

Fig. 3. 
(a) Response curves of Au-decorated and pristine graphene sensors under different voltages. (b) Thermographic images and measured temperatures of Au-decorated and pristine graphene sensors. (c) Binding energies of hydrogen molecules and hydrogen atoms on (111), (110), and (100) Au surfaces and edge, interface edge (edge-i), vertex, and interface vertex (vertex-i) sites adsorption sites in Au nanoclusters on graphene. (d) Schematic of the charge density after adsorption of hydrogen atoms. Reprinted with permission from [38]. Copyright © 2019, Royal Society of Chemistry