Ammonia Sensing Using Gold-Decorated rGO: Effects of Synthesis Routes on the Sensing Mechanism

Understanding how metal functionalization modifies charge transport in graphene can facilitate the rational design of high-performance graphene-based sensors. In this work, we investigate the role of gold functionalization in reduced graphene oxide (rGO)-based ammonia (NH3) sensors by comparing two distinct gold-functionalized rGO channel architectures: one consisting of rGO decorated with gold nanoparticles (rGO-AuNP) and the other based on an rGO-Au composite formed through simultaneous gold functionalization and particle synthesis (rGO-AuμF). Electrical measurements revealed opposite responses toward NH3 exposure, with the rGO-AuNP device exhibiting an increase in current, whereas the rGO-AuμF device showed a decrease. In the rGO-AuNP device, catalytic oxidation of NH3 removes adsorbed oxygen species from the surface, accompanied by energy-band realignment that lowers the electron transport barrier. In contrast, the rGO-AuμF device favors direct NH3 adsorption, where electron donation partially compensates the hole carriers, resulting in a decrease in current. Analysis of the transfer characteristics revealed that these contrasting responses are associated with differences in the Dirac voltage, consistent with modulation of the effective flat-band voltage. Gold functionalization primarily induces shifts in the effective flat-band voltage, while the extent of direct carrier doping may be limited by metal-induced gap states (MIGS) and associated Fermi-level pinning at the contacts. Density functional theory (DFT) simulations further support experimental observations. These findings provide mechanistic insights into the role of gold in tuning charge transport in rGO-based gas sensors.