Oxalic acid

Firstly, 0.001 mol niobium chloride (NbCl₅, 99.9%, Macklin) and 0.001 mol vanadium (III) 2,4-pentanedionate (C₁₅H₂₁O₆V, 99.9%, Macklin) were added to the mixed solution of 35 mL ethanol and 5 mL deionized water to form a suspension. Then, added 0.6 g urea (CH₄N₂O, 99.9%, Macklin), 0.63 g oxalic acid (C₂H₂O₄, 99.5%, Macklin) and 0.063 g F127 (99.9%, Sigma-Aldrich) under vigorous stirring, kept stirring at room temperature for 30 min until the solution became clear. The obtained green transparent solution was transferred to a 100 mL Teflon autoclave and reacted at 180 °C for 18 h. After cooling to room temperature, the mixture was filtered and washed with ethanol (C₂H₅OH, 99.9%, Damao chemical reagent factory) for 5 times, and put into a vacuum drying oven for overnight drying. After that, the obtained precursor was transferred to corundum crucible, kept at 600 °C for 3 h in Ar protected tubular furnace, and then cooled naturally to obtain black VNbO₄ powder. For comparison, T-Nb₂O₅ was also prepared via same synthesis process without V source.

A reagent grade sodium tungstate dihydrate (Na₂WO₄·2H₂O, 99.5%), citric acid (C₆H₈O₇·H₂O, 99.5%), oxalic acid (C₂H₂O₄·2H₂O, 99.5%), tartaric acid (C₄H₆O₆, 95%, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China), hydrochloric acid (HCl, 37%, Da Mao Chemical Reagent Factory, Tianjin, China) and ethanol (C₂H₅OH, 99.5%) were used directly without further purification. All reagents except those indicated are from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China).