Sustainable design and tuning of nanostructured WO₃ via green anodization in pure lactic acid for photoelectrochemical water splitting

Abstract

In this work, we demonstrate the use of pure, environmentally friendly lactic acid as an electrolyte for the controlled, green synthesis of WO₃ layers by anodic oxidation. The influence of anodization voltage (80–110 V), electrolyte concentration (150 and 300 mM), and process temperature (5–25 °C) was systematically investigated to identify optimal conditions for producing nanostructured, porous WO₃ layers with a thickness below 700 nm. Detailed characterization of the materials' morphology and composition was carried out using FE-SEM, EDS, XRD, XPS, and Raman spectroscopy confirming their high purity, and crystallinity. Optical (UV–Vis DRS), semiconducting (Mott-Schottky, EIS) and photoelectrochemical (under simulated sunlight and monochromatic illumination) analyses revealed an optical band gap of about 2.9 eV and an electrochemical band gap of about 2.7 eV, along with high donor density and efficient incident photon-to-current efficiency (IPCE). The WO₃ layers synthesized at 15 °C exhibited the best photoelectrochemical performance and long-term stability in solar-driven water oxidation. The optimal photoanodes exhibit a photocurrent of up to approximately 0.80 mA cm⁻² (at 1.23 V vs. RHE) under simulated sunlight and retain about 80 % of their initial photocurrent after 4 h of continuous illumination. This study provides a novel, simple, one-step green synthesis strategy for fabricating photoanodes, eliminating the need for hazardous, fluoride-based electrolytes and offering a sustainable route for photoelectrochemical water splitting applications.