Gas Sensing Differences Depending on Various Noble Metals Decorated on Graphene: A Study on NH3 Response Mechanism

Noble metal nanoparticles (NPs) are often decorated on graphene (Gr) to achieve enhanced gas sensing performances. However, the underlying mechanisms and the roles that various noble metal NPs may play in the improvement of gas sensing have not been elucidated to date. In the present work, we seek to study the mechanism of Gr-loaded noble metal NPs composites for sensing NH3 for example by using Au, Ag, Pd, and Pt NPs as the model metals. The analysis results of morphological and Langmuir adsorption isotherm indicate that the morphological differences, which are defined as chemical sensitization, might not be responsible for the gas sensing differences among various noble metals so decorated. Alternatively, the work function and electronic structure of Gr, which is known as the electronic sensitization, could be effectively tuned upon the decoration of different noble metals might serve as the key factors governing the different performances of composited for gas sensing. In addition to the electron depletion around Ag NPs on Gr surfaces, the strong hybridization between Pt or Pd NPs and Gr carriers might change the electronic of states, thus promoting the adsorption of gas molecules on the Gr surfaces. More importantly, the reduced local work functions at the interfaces between Gr carriers and Pd, Pt, and Ag NPs might be more beneficial to the donated electrons transferring from adsorbed NH3 to Gr substrates, in contrast to the reverse one for Gr-loaded Au NPs. The underlying mechanism of gas sensing differences is thereby proposed to be attributed to the increased density of state that might enhance the gas adsorption and the reduced barrier height at the metal NPs/Gr interfaces. This study may provide a guide for the design and morphology optimization of noble metal NPs/Gr composites.