Irreversible n to p Transition and Corresponding Performance Improvement of RGO/TiO2 Nanotubes Hybrid Vapor Sensor Devices by Varying Electrophoretic Deposition Time

Abstract

In this paper, an irreversible n-type to p-type conductivity transition is correlated with the corresponding vapor sensing performance improvement of reduced graphene oxide (RGO) and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotubes-based hybrid vapor sensor devices. The RGO was deposited on top of electrochemically grown TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanotube (NT) array by electrophoretic deposition technique. The deposition time was varied from 0.5 to 3 h and it was established through XPS and electrochemical impedance spectroscopy (EIS) measurement that the conductivity of the RGO changed from n- to p-type, in an irreversible manner, when the deposition time exceeded 2.6 h. Characterizations such as an FESEM and an RAMAN spectroscopy study revealed that with increase in electrodeposition time, the percentage coverage of the RGO on TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> NT matrix (n-type in nature) was increased monotonically reaching almost 90% coverage in 2.6 h, causing the overall device conductivity to change from n- to p-type (i.e., the overall conductivity is governed by the conductivity of p-type RGO, as it covered 90% of the nanotube matrix). The sensing performance of the hybrid structure was also investigated and the device with higher electrodeposition time (p-type) was found to offer better vapor sensing performance towards acetone (as the test species) compared to its n-type counterpart (lower electro-deposition time). Such improvement in sensing performance was possibly attributed to the additional adsorption sites and higher electron mobility of p-RGO over n-RGO.