Photoelectrochemical Properties of Eumelanin Nanoparticle-TiO₂ Bilayer Electrodes

Melanin, the most abundant natural pigment, possesses unique and not yet fully understood physicochemical properties that make it attractive for applications in photocatalysis, optoelectronics, and bio-optoelectronics. We employed cyclic voltammetry (CV) to investigate the redox properties of lab-synthesized eumelanin nanoparticles and eumelanin-based donor-acceptor constructs. Interestingly, the CV of eumelanin, eumelanin-TiO₂, and TiO₂ films revealed distinct redox peaks that indicated reversible electron transfer processes over several cycles. The eumelanin electrode shows a broad reduction wave with no sharp peak, which reflects its broad optical absorptions across UV-NIR and multiple overlapping Quinone-based redox states, Quinone, Semiquinone, Hydroquinone, which produces a broad, distributed reduction wave from ∼0.3 V to -0.8 V. The characteristic low charge mobility of melanin results in a lower photocurrent magnitude than that of the hybrid and a broad, capacitive-like behavior. Decisive signatures are observed for the eumelanin-TiO₂-FTO electrode, including significantly larger photocurrent, particularly at negative potentials, indicating increased electron population in the electrode, improved extraction, and reduced recombination. CV under spectrally selective illumination (Xe lamp, λ > 420 nm or λ > 580 nm) shows the photocurrent at ∼-0.5 V to -0.9 V is influenced by irradiation spectral region, emphasizing the wavelength-sensitive photoresponse of the constructs. For all electrodes, under solar light irradiation, more intense, sharper redox peaks with higher current were observed than under ambient light. Electrochemical impedance spectroscopy (EIS) suggests that the series resistance (R _S) and charge transfer resistance (R _CT) were smaller in eumelanin-TiO₂ bilayer electrodes compared to those for eumelanin and TiO₂ electrodes.