Functionalized multi-walled carbon nanotubes and carbon nanofibers as potential sensing materials for gas sensor application

Multi-walled carbon nanotubes (MWCNT) and carbon nanofibers (CNF) have been demonstrated as excellent sensing materials for sensitive chemical gas sensor because of the high surface area to volume ratio and their remarkable electrical properties. However, MWCNT and CNF tend to agglomerate and difficult to disperse uniformly in most solvent matrix due to their strong sp2 bonding and high van der Waals attractive forces. These characteristics limited their potentials and applications in detecting various types of gases. Hence, functionalization of MWCNT and CNF with carboxyl, ester and amide group can enhance the dispersibility and chemical reactivity towards material surroundings. MWCNT and CNF were functionalized with carboxyl group via sulphonitric treatment, and further modified with ester and amide functional groups via Fischer esterification. Characterization analysis using FESEM, EDX, FT-IR, Raman spectroscopy and CHNOS elemental analyser techniques shows that MWCNT and CNF were successfully functionalized and able to maintain their suspension in DMF solvent for few days. The potential of pristine and modified MWCNT and CNF as sensing material in detecting acetone, ammonia, benzene, hydrogen and methane gas were investigated in air tight gas chamber. The concentration of gases varied and their sensitivity, response time, reproducibility, reusability, recovery time and selectivity at room temperature were observed. The increment of MWCNT and CNF resistance upon detection of tested gases suggested that MWCNT and CNF are p-type semiconducting sensing material, and gases acts as electron donating group, which transferred their electron towards sensing layer. The increased number of mobile carrier charge, reduced the hole carriers and therefore, increased their resistance. Based on highest sensitivity at the lowest concentration, MWCNT-amide is the best sensing material (32.12 %) in detection of acetone gas, CNF-ester is the best sensing material (54,488.00 %) in detection of ammonia gas and MWCNT-carboxyl is best sensing material (521.31 %) in detection of benzene gas. While, CNF-amide is the best sensing material in detection of hydrogen (47.12 %) and methane (11.09 %) gas. Functionalized MWCNT and CNF showed high selectivity towards ammonia gas as compared to other gases. It was also observed that approximately all functionalized MWCNT and functionalized CNF showed response time and recovery time less than 60 s. The morphology and structure of CNF which has highest number of defects contributed to excellent performance as sensing material in most gases. Functionalized MWCNT and CNF exhibited outstanding performance by present fast response, high sensitivity and short recovery time as compared to pristine MWCNT and pristine CNF.