Protonic Titanate Nanotube-Reduced Graphene Oxide Composites for Hydrogen Sensing

Hydrogen sensors are of tremendous technological demand, which requires more selective and responsive sensors over a wide concentration range (ppm to percentage). Here, we report a giant enhancement in sensor performance in diffusion-limited hydrogen response of protonic titanate nanotubes (TNTs, H2Ti3O7) by the addition of reduced graphene oxide (RGO). Unlike TiO2, the electrical conductivity of TNTs decreases upon heating up to similar to 120 degrees C due to the loss of chemically adsorbed water, which imparts protonic conduction below 100 degrees C. Thus, TNTs are very sensitive and selective to hydrogen gas due to protonic conduction. However, their response kinetics is dominated by slow diffusion of hydrogen, leading to large response times (similar to 1000 s for 1000 ppm). We show that an ex situ fabricated sensor using a TNT-RGO physical hybrid exhibits a gigantic 950% change in current upon exposure to 1000 ppm hydrogen gas at 30 degrees C with half the response time of nearly 200 s, whereas the phase-separated TNT-RGO composite made in situ shows 1.5 times enhancement and a further lower response time of similar to 40 s without losing the selectivity offered by pristine nanotubes. The dynamic range as well as the response time of the titanate nanotubes is improved due to type I heterostructure formation at the interface of TNTs and RGO as seen from X-ray photoelectron spectroscopy. The sensor response shows two distinct time constants in both response and recovery, depicting the two processes involved, which is also confirmed by impedance spectroscopy. The bulk diffusion-dominated TNTs and surface-dominated RGO along with their heterostructures are identified as key factors for enhanced sensor properties, particularly faster saturation and recovery. In our paper, we not only have made composites and physical hybrids but show that effective mixing is necessary to achieve better sensing properties.