Humidity sensing studies on ZnO/bismuth borate glass heterostructure thin films

Abstract

Semiconducting bismuth borate glasses in xBi₂O₃–(1 − x)B₂O₃ system where x = 0.25, 0.50, and 0.75 (in mol.), and ZnO-glass heterostructures are studied for humidity sensing. Glass samples are prepared using melt-quenching method, whereas pristine ZnO is synthesized by sol–gel process. Heterostructure samples are obtained by adding different weight fractions of pulverized bismuth borate glass to the ZnO sol. Conducting silver paint and graphite pencil are used to make electrodes on silicon wafer and flexible paper substrates, respectively. ZnO, glass, and ZnO-glass heterostructure thin films are deposited on both the substrates through drop-casting method. Structural and microstructural changes of heterostructure films are studied using X-ray powder diffraction (XRD), and scanning electron microscope (SEM). Pristine ZnO, glass and ZnO-glass heterostructure thin film samples are tested for humidity sensing at room temperature by monitoring changes in the resistance of the samples. ZnO-glass heterostructures have shown enhanced humidity sensitivity with the lowest response and recovery times (12 and 16 s, respectively), which indicates their promising nature for humidity sensing applications. Enhanced sensing properties are attributed to the unique microstructural features of ZnO nanoparticles which are grown on glass particles through a heterogeneous nucleation process and semiconducting ZnO/glass heterostructure mechanism.