Electrodeposited CuO thin-film for enhanced photo-electrochemical glucose oxidation

Abstract

Cuprous oxide has been used as a non-enzymatic glucose sensor for decades. As such, there are a multitude of reports which focus on its electro-catalytic properties towards glucose oxidation though few make use of its intrinsic photocatalytic attribute. In this work, a visible light activated CuO porous film was developed using a three-step synthesis process. Optimization of each synthesis stage was achieved by comparing the sensors anodic peak currents in the dark to those captured under illumination from an LED light source. The physical characterization performed on selected samples included XRD, XPS, SEM, Hall Effect, Raman and UV-Vis to elucidate the morphology, bonding, optical and charge transfer properties. The electro-catalytic and photocatalytic oxidation of glucose was investigated by means of Cyclic Voltammetry, Electrochemical Impedance Spectroscopy and Chronoamperometric measurements under both the light and dark scenarios. Optimization of the synthesis parameters showed the greatest difference between light and dark current density using a combination of 4 V for 2 min electrodeposition accompanied by 450 ℃ calcination for 1 h and an etching time of 2 min in concentrated NH3 solution. XPS indicated two Cu 2p core levels peaks at 933.4 eV and 953.2 eV which confirms the presence of the Cu2+ species and hence, CuO. SEM images revealed trapezoidal crystal shapes on the FTO surface with increased porosity following etching. The optimized sensor presented a monoclinic structure following calcination and etching which suggests only the exposed facets were affected by NH3. Analysis of the UV-Vis data found that the band gap (𝐸𝑔) decreased from 2.07 eV to 1.88 eV following etching. Hall measurements indicate high charge carrier densities following n-type semiconductor behavior. The CV results demonstrated that with the addition of light, the current density in 1 mM glucose solution increases by as much as 0.382 mA/cm2 with all else equal. The electrochemical oxidation of glucose was found to be an irreversible electron transfer process controlled by diffusion in both the light and dark with regression coefficients 𝑅2 of 0.993 and 0.999 respectively. EIS confirmed that the charge transfer resistance in the light decreases by several order of magnitude. Periodic illumination revealed persistent photoconductivity that’s presence is as a result of deep-trap levels between the valence and conduction bands. Good amperometric performance was obtained for the CuO film with a 4 second response time with negligible interference from other species present in human blood. The sensor produced three linear ranges spanning over a 0.0-2.77 mM, 2.77-9.95 mM and 9.95-29.12 mM respectively. The linear range was unaffected by the incorporation of light however the sensitivity increased by 33.5% at a lower LOD.