Studies on Catalyst Decorated Reduced Graphene Oxide for Selective Gas Sensing

Abstract

The selectivity of catalytic nanoparticles (NPs) decorated reduced graphene oxide (rGO) based gas sensor is tuned by varying the reduction method, operating temperature, catalyst and catalyst concentration to detect reducing gases (H2, CO and CH4). Graphene oxide (GO) is initially synthesized by oxidative treatment of natural graphite flakes following the modified Hummer’s method followed by reduction via chemical and thermal approach. The variation of the reducing method induces variation in the oxygen functionalities of rGO. Catalyst (Pd, Au and CeO2) nanoparticles are prepared using the hydrothermal method. The structural, chemical and morphological study of pure catalysts, base materials and catalyst-decorated composites are done by using Electron microscopy (EM), Fourier transforms infrared (FTIR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XRD and HRTEM confirm the formation of catalyst nanoparticles. The higher degree of defects of CRGO and its composites is indicated by Raman spectroscopy. FTIR reveals the difference in the functional groups of chemically reduced graphene oxide (CRGO) and thermally reduced graphene oxide (TRGO), which is also supported by XPS spectra. The XPS spectra of composite materials reveal the presence of various oxidation states of metal catalysts. The FESEM result reveals the morphology of materials and the role of the high temperature thermal reduction on the particle size of catalysts. The role of different oxygen functionalities and decoration of different catalyst nanoparticles on CRGO and TRGO surfaces is studied for selective reducing gases (H2, CO & CH4) sensing at temperatures (RT-150ºC). The selectivity study reveals the selectivity of CRGO-Pd (Pd = 1%) and CRGO-Au (Au = 1%) to CH4 gas at temperatures>50ºC. In contrast, CRGO-CeO2 (CeO2 =1%) shows selectivity to CO gas. TRGO-Pd (Pd =1%) composites are selective to H2 gas at temperature >50ºC whereas TRGO-Au (Au =1%) and TRGO-CeO2 (CeO2 =1%) shows no selectivity at all temperatures. This is ascribed to the difference in the functionalities of CRGO and TRGO. The increase in the catalyst concentration further tunes the selectivity and is ascribed to the concentration of various chemical states in the catalysts.