Insights into the Impurities of BiWO Synthesized Using the Hydrothermal Method

Bismuth tungstate (BiWO) nanomaterials are widely used as visible-light driven photocatalysts. However, limited attention has been paid to the purity of prepared BiWO nanoparticles, which may affect the photocatalytic performance and hinder in-depth study of BiWO. In this work, the impurities of BiWO formed during the hydrothermal process under a wide range of acid-base conditions (from 1.5 M HNO to 0.5 M NaOH) were qualitatively analyzed and accurately quantified for the first time. After confirmation of BiWO 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 BiWO prepared in different acid-base environments, and to facilitate qualitative analysis of impurities. The hydrolytic properties of raw materials used for the synthesis of BiWO were also analyzed with UV-Vis transmittance observation. Following these analyses, the types and contents of impurities in BiWO prepared by the hydrothermal method under different acid-base conditions were determined. Results show that the primary impurity is WO·0.33HO (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 BiWO has relatively high purity, and the impure products were identified as BiONO. BiO began to appear when pH reached 9.01 and it reached 18.88% when pH was 12.98. The final product was BiO 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.