Self-Assembled Nanostructured Tin Oxide Thin Films at the Air–Water Interface for Selective H2S Detection

Simple, inexpensive, and scalable strategies for metal oxide thin film growth are critical for potential applications in the field of gas sensing. Here, we report a general method for the synthesis of free-standing oxide thin films via a one-step, surfactant-free hydrothermal reaction wherein the oxide film forms at the air–water interface. Using SnO2 and PdO as model systems, we show that the thin films, thus formed, have lateral dimensions of the order of centimeters and thickness of the order of tens of nanometers. Transmission electron microscopy (TEM) has been used to understand the growth mechanism of the films. On the basis of these studies, we propose that the metal oxide particles formed in the bulk of the solution move to the interface and get trapped to form a continuous, polycrystalline film. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) measurements have been performed to understand the structure, morphology, and thickness of the films. Thickness tunability by varying the precursor concentration has been explored, which in turn affects optical and gas sensing properties. Thin SnO2 films (30 nm) revealed an ultrasensitive response (R) of 25000% to 6 ppm of H2S at 150 °C while demonstrating 25 ppb (R = 19.3%) as the experimental lowest limit of detection. The selectivity of these nanostructured films toward H2S stands tall among the other interfering gases by exhibiting an ∼2 orders higher response magnitude. Furthermore, these thin films are highly stable at elevated temperatures.