Abstract
In this work, a three-layered heterostructure Cu2O/CuO/CuS was obtained through a low-cost and large-area fabrication route comprising electrodeposition, thermal oxidation, and reactive annealing in a sulfur atmosphere. Morphological, microstructural, and compositional analysis (AFM, SEM, XRD, EDS, XPS) were carried out to highlight the surface modification of cuprous oxide film after oxidation and subsequent sulfurization. Impedance, voltammetric, and amperometric photoelectrochemical tests were performed on Cu2O, Cu2O/CuO, and Cu2O/CuO/CuS photocathodes in a sodium sulfate solution (pH 5), under 100 mW cm−2 AM 1.5 G illumination. A progressive improvement in terms of photocurrent and stability was observed after oxidation and sulfurization treatments, reaching a maximum of − 1.38 mA cm−2 at 0 V versus RHE for the CuS-modified Cu2O/CuO electrode, corresponding to a ~ 30% improvement. The feasibility of the proposed method was demonstrated through the fabrication of a large area photoelectrode of 10 cm2, showing no significant differences in characteristics if compared to a small area photoelectrode of 1 cm2.
Highlights
Cuprous oxide (Cu2O) is one of the most investigated p-type semiconductor material in the framework of photoelectrochemical hydrogen production[1,2,3], mainly because of its optical properties and potentially lowcost synthesis[4]
Photoelectrochemical characterization and testing were performed in a threeelectrode cell comprising Ag/AgCl as a reference electrode and a Ru-based mixed metal oxide (MMO) net as a counter for both linear sweep voltammetry (LSV) and amperometric test (Amel 2559 potentiostat/galvanostat)
While the majority of the studies report the growth of cuprous oxide on Au-sputtered substrate at relatively high pH values[29], on FTO the plating conditions vary more among different literature works[30]
Summary
Cuprous oxide photoelectrode showed the typical curve reported in the literature, characterized by a significant reductive photocurrent (p-type conductivity) at high cathodic bias but with a relatively small immersion potential under illumination and, more importantly, an onset potential close to the hydrogen evolution reaction one. The photoelectrochemical characterization was carried for Cu2O films having a relatively wide range of charge deposition density (0.5–1.25 C cm−2) (Fig. S6) where no significant variations were reported in the immersion potential under illumination, indicating the same chemical/phase composition. Carrying out a shorter oxidation treatment (1 h) showed no variation in the immersion potential, indicating that the cupric oxide layer grew homogeneously all over the surface only after 1 h of air annealing; the photocurrent recorded was lower (− 0.75 mA cm−2 at 0 V vs RHE) (Fig. S7), in agreement with previous reports[18,21]. The amperometric tests (Fig. 10) showed no differences between the two samples, indicating that the modification of the cupric oxide layer through reactive annealing was reliable as demonstrated to homogeneously modify the overall surface oxide surface
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