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Abstract
The development of advanced relative humidity (r.h.) sensors holds immense potential for wearable applications where precise monitoring of environmental or physiological conditions is crucial. In this study, we explore the tunability of graphene oxide/reduced graphene oxide/titania nanocrystal (GO/rGO/TNC) hybrid films via photocatalytic reduction for resistive humidity sensing. First, we introduce a facile layer-by-layer spin-coating (LbL-SC) technique for the efficient fabrication of thin GO/TNC films (15-150 nm) on various substrates, using titania nanorods (TNRs) or nanoplates (TNPs). Second, we study the tunability of the films' electrical, optical, and structural properties set by the photocatalytic activity of the TNCs upon UV exposure (254 nm), by varying the illumination time, TNC type, and film thickness. We show that the electrical conductance of the films dramatically increases in presence of TNCs, and that the GO/rGO and (GO/rGO)/TNC ratio can be adjusted, with prolonged UV treatment resulting in the degradation of GO/rGO with rates depending on the films' composition and thickness. Third, we employ this method to tune the resistive sensing behavior of (GO/rGO)/TNC films on flexible substrates towards r.h. changes (ranges 35 – 85% and 1 – 80%). TNP-based films show superior humidity sensing performance to (GO/rGO)/TNR- and pristine GO/rGO films, with a sensitivity of up to 2.2 and response times down to 1 s. Depending on the film composition and the r.h. level, positive and negative responses are obtained. Gravimetric measurements reveal a similar mass uptake of water in all tested hybrid films, indicating that distinct resistive responses arise from differences in their chemical and electrical properties. Further, we demonstrate the use of these sensors for the detection of fluctuations in body-related humidity. Our findings highlight the potential of (GO/rGO)/TNC films and combinations thereof with tailored sensing characteristics as versatile platform for the design of advanced humidity sensors.
Supplementary materials
Title
Supporting Information
Description
TNC characterization (XRD, TGA), GO characterization (AFM, UV-vis-, FTIR-spectrscopy), estimation of GO and TNC content in hybrid films, characterization of DUV radiation source, spectroscopic characterization of photocatalytically reduced (GO/rGO)/TNC films (UV-vis-, FTIR-spectroscopy), TNC concentration-dependent reduction, reference experiments of DUV-induced GO reduction of pure GO films (IV curves, FTIR-spectroscopy, AFM, humidity sensing), additional XPS data of (GO/rGO)/TNR films, reference humidity measurements with a commercial sensor, comparison of sensor performance with GO/rGO resistive sensors reported in the literature, resistive sensor timetraces, QCM sensor timetraces.
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