Noble-Metal-Free Reduced Graphene Oxide Platforms for Room-Temperature H2 Sensing in High-Humidity Conditions

Abstract

Reduced graphene oxide (rGO) sensors─free from noble-metal nanoparticles─were prepared and tested at room temperature (21 °C) against hydrogen (H2) in high-humidity conditions (above 90% r.h.). The rGO material was obtained through a simple chemical/thermal protocol from graphene oxide (GO) as the starting material, using ascorbic acid as the reducing agent. The degree of reduction of GO was tuned by modifying the chemo-thermal conditions, which allowed shaping the sensing response. The latter was demonstrated to scale with the sp2 carbon (Csp2) content of the rGO structure, which varied from ∼40%-Csp2 for the least-reduced sample to ∼70%-Csp2 for the highest-reduced material. Current-voltage measurements revealed the influence of the reduction on the electronic conduction mechanisms, evidencing a shift from a strong nonlinear behavior (with a component characteristic of a space charge limited conduction mechanism, SCLC) to a pure ohmic-like response as the reduction of GO proceeded. To evaluate the selectivity of the sensors to the target molecule, different oxidizing and reducing gases other than H2 (CO, CO2, and NO2) were also tested. The highest-reduced sample showed a superior response when exposed to a range of H2 concentrations in an atmosphere with humidity levels above 90% r.h. In these conditions, a negligible cross-sensitivity to CO, CO2, and NO2 was achieved. For the top-performance sensor, the response (t66) and recovery (trec) times were calculated to be 72 and 374 s (at 0.05% H2), respectively, with a sensitivity of 0.7% and an estimated detection limit of 2.5 ppm of H2.