Nanocomposites of Titania/Reduced Graphene Oxide: Flexible Humidity Sensors Tuned via Photocatalytic Reduction

Abstract

In this study, we demonstrate the tunability of hybrid graphene oxide/reduced graphene oxide/titania nanocrystal (GO/rGO/TNC) films for resistive humidity sensing through photocatalytic reduction. Using a layer-by-layer spin-coating (LbL-SC) technique, we fabricate GO/TNC nanofilms with titania nanorods (TNRs) or nanoplates (TNPs) on various substrates, achieving high uniformity and precise control over the film thickness (15–150 nm). We investigate the evolution of the electrical, optical, and structural properties of these films, modulated by the photocatalytic activity of TNCs under UV exposure (254 nm) while varying the illumination time, TNC type, and film thickness. The inclusion of TNCs enhances the films’ conductivity by several orders of magnitude compared to pure GO films under UV illumination and enables precise adjustment of the GO/rGO and (GO/rGO)/TNC ratios. This approach is used for tuning the sensitivity, response time, and response polarity of (GO/rGO)/TNC resistors on flexible substrates to changes in relative humidity (RH). TNP-based films demonstrate superior performance, achieving sensitivities of up to 2.2 and response times as short as 1 s over a broad range of RH levels (∼35 to 85% and ∼1 to 80%). Depending on the composition and RH level, the sensors exhibit both positive and negative resistive responses to increasing humidity. Gravimetric analyses show that films with varying GO/rGO ratios exhibit the same change in water mass uptake, indicating that the differences in resistive behavior are driven by UV-induced alterations in their chemical and electrical properties. Finally, we propose the use of these sensors to detect body-related humidity fluctuations, demonstrating their suitability for wearable electronics. Our results highlight the potential applicability of (GO/rGO)/TNC nanocomposites as highly customizable humidity sensors.