Insights into the impurities of Bi₂WO₆ synthesized using the hydrothermal method

Bismuth tungstate (Bi₂WO₆) nanomaterials are widely used as visible-light driven photocatalysts. However, limited attention has been paid to the purity of prepared Bi₂WO₆ nanoparticles, which may affect the photocatalytic performance and hinder in-depth study of Bi₂WO₆. In this work, the impurities of Bi₂WO₆ formed during the hydrothermal process under a wide range of acid-base conditions (from 1.5 M HNO₃ to 0.5 M NaOH) were qualitatively analyzed and accurately quantified for the first time. After confirmation of Bi₂WO₆ stability, the impurities were dissolved using acid or base treatment, followed by measurements of the ion concentrations using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Furthermore, various characterization techniques including XRD, FE-SEM, TEM, UV-Vis DRS, XPS and FTIR were implemented to explore the change in morphology and optical properties of Bi₂WO₆ prepared in different acid-base environments, and to facilitate qualitative analysis of impurities. The hydrolytic properties of raw materials used for the synthesis of Bi₂WO₆ were also analyzed with UV-Vis transmittance observation. Following these analyses, the types and contents of impurities in Bi₂WO₆ prepared by the hydrothermal method under different acid-base conditions were determined. Results show that the primary impurity is WO₃·0.33H₂O (41.09%) for the precursor prepared in 1.5 M nitric acid solution. When the pH of the precursor was in the range of 0.97-7.01, the synthesized Bi₂WO₆ has relatively high purity, and the impure products were identified as BiONO₃. Bi₂O₃ began to appear when pH reached 9.01 and it reached 18.88% when pH was 12.98. The final product was Bi₂O₃ exclusively for the precursor conditioned in 0.5 M NaOH solution. In addition, the accuracy of the proposed quantitative method using ICP-MS was validated for several scenarios by weight difference experiments.