SrTiO3–TiO2 Heterostructured Nanotube Arrays for Ultrafast Ethanol Sensing

Abstract

Diverse metal oxide semiconductors have bloomed in chemical sensing applications providing opportunities and challenges. In this work, SrTiO3–TiO2 heterostructured nanotube arrays were well designed via the electrochemical synthesis of TiO2 nanotubes, followed by a facile one-step hydrothermal route by varying the amount of Sr(OH)2·8H2O precursor (0.25–25 mM) to grow different quantities of SrTiO3 on TiO2 nanotubes. The crystalline nature, morphology, and synthesis mechanism of the nanotube array were fully elucidated. Vertically aligned SrTiO3–TiO2 heterostructured nanotube arrays were then sandwiched between the Ti bottom electrode (substrate as well) and Au top electrode to fabricate the metal–insulator–metal type sensors. SrTiO3–TiO2 nanotube sensors exhibited ethanol-selective behavior where the highly defective SrTiO3 layer acted as the main sensitive material that interacted with target species like ethanol. The SrTiO3–TiO2 heterostructured nanotube array sensor synthesized with 0.25 mM Sr(OH)2 precursor exhibited an excellent response magnitude (Ra/Rg) of ∼556 with an ultrafast response time of 0.4 s toward 50 ppm ethanol at an operating temperature of 150 °C. Moreover, the sensor exhibits excellent stability, a low detection limit of 2.94 ppb, and good selectivity. All the SrTiO3–TiO2 heterostructured nanotube array sensors showed promising humidity-tolerant behavior, and negligible change in response was obtained in the presence of 80% humid ambient air. The mechanism behind the modulated VOC sensing characteristics was explained with the comprehensive effects of the larger specific surface area, high surface defects, and modulated oxygen vacancies coming from the SrTiO3 modification in the nanotube structures.