Protonic Titanate Nanotube–Reduced Graphene\nOxide Composites for Hydrogen Sensing

Abstract

Hydrogen sensors are of tremendous\ntechnological demand, which\nrequires more selective and responsive sensors over a wide concentration\nrange (ppm to percentage). Here, we report a giant enhancement in\nsensor performance in diffusion-limited hydrogen response of protonic\ntitanate nanotubes (TNTs, H₂Ti₃O₇) by the addition of reduced graphene oxide (RGO). Unlike TiO₂, the electrical conductivity of TNTs decreases upon heating\nup to ∼120 °C due to the loss of chemically adsorbed water,\nwhich imparts protonic conduction below 100 °C. Thus, TNTs are\nvery sensitive and selective to hydrogen gas due to protonic conduction.\nHowever, their response kinetics is dominated by slow diffusion of\nhydrogen, leading to large response times (∼1000 s for 1000\nppm). We show that an ex situ fabricated sensor using a TNT–RGO\nphysical hybrid exhibits a gigantic 950% change in current upon exposure\nto 1000 ppm hydrogen gas at 30 °C with half the response time\nof nearly 200 s, whereas the phase-separated TNT–RGO composite\nmade in situ shows 1.5 times enhancement and a further lower response\ntime of ∼40 s without losing the selectivity offered by pristine\nnanotubes. The dynamic range as well as the response time of the titanate\nnanotubes is improved due to type I heterostructure formation at the\ninterface of TNTs and RGO as seen from X-ray photoelectron spectroscopy.\nThe sensor response shows two distinct time constants in both response\nand recovery, depicting the two processes involved, which is also\nconfirmed by impedance spectroscopy. The bulk diffusion-dominated\nTNTs and surface-dominated RGO along with their heterostructures are\nidentified as key factors for enhanced sensor properties, particularly\nfaster saturation and recovery. In our paper, we not only have made\ncomposites and physical hybrids but show that effective mixing is\nnecessary to achieve better sensing properties.