Ultrapure water

Sodium tungstate dihydrate (Na₂WO₄·2H₂O) was purchased from Alfa Aesar India, and tin (II) chloride hydrate (SnCl₂·2H₂O) was purchased from Alfa Aesar United States. Hydrochloric acid (HCl, ~35% v/v) and ammonium sulfate ((NH₄)₂SO₄) were purchased from Duksan. Carbon cloth (WOS1009) was obtained from CeTech Co., Ltd., and sodium sulfate (Na₂SO₄, anhydrous, ≥99%) was from Honeywell/Fluka. Potassium ferricyanide (K₃Fe(CN)₆, 99%) was purchased from Acros Organics. All of these chemicals were used without further purification. Aqueous solutions were freshly prepared with ultrapure water (resistivity of 18.2 MΩcm; Yamato, Japan) throughout the experiments. Characterization was performed over the range 2θ = 10° to 60° at room temperature with an X-ray diffractometer (XRD, Bruker, D2 phaser Karlsruhe, German) using a copper radiation source (λ = 0.154 nm), operating at 40 kV and 30 mA, with a step size of 0.05 and step time of 5 sec.; X-ray photoelectron spectroscopy (XPS, VG scientific ESCALAB 250, Birmingham, UK); and field emission scanning electron microscopy (FE-SEM, at 15 kV acceleration voltage) equipped with an energy-dispersive X-ray spectroscopy (EDS) analyzer (JSM-6500F, JEOL, Tokyo, Japan).

Titanium(iv) isopropoxide (TTIP, ACROS ORGANICS, CHINA), isopropanol (≥99.8% (GC), Honeywell|Riedel-de Haën™, France), sodium tungstate oxide dihydrate (Na₂WO₄·2H₂O, 99.0–101.0%, Thermo Fisher Scientific.), ethanol (≥99.9%, Honeywell|Fluka™, Germany), nitric acid (65% w/w), hydrochloric acid (37%, reagent grade), ethylene glycol (99.5%, ACROS ORGANICS), methylene blue (ACROS ORGANICS, INDIA), and methyl orange (ACROS ORGANICS, INDIA) were used without further purification. Para-benzoquinone (p-BQ) and ethylenediaminetetraacetic acid disodium salt (Na₂-EDTA) were purchased from Sigma-Aldrich and Fisher Chemical, respectively. Ultrapure water (resistivity of 18.2 mΩ cm, Yamato, Japan) was used throughout the experiments.