Synthesis, Microstructure and Photodegradation Activity of Bismuth Tungsten Oxides

On the influence of hydrothermal treatment pH on the performance of Bi2WO6 as photocatalyst in the glycerol photoreforming

Solar driven semiconductor-based photoreforming of biomass derivatives, such as glycerol is a sustainable alternative towards green hydrogen evolution concerted with production of chemical feedstocks. In this work, we have investigated the influence of the pH of the hydrothermal treatment on the efficiency of Bi₂WO₆ as photocatalyst in the glycerol photoreforming. Bi₂WO₆ is pointed as a promising material for this application due its adequate band gap and the ability to promote hole transfer directly to glycerol without formation of non-selective ^⋅OH radicals. Samples prepared at neutral to moderate alkaline conditions (pH = 7-9) are highly crystalline, while those prepared in acidic media (pH = 0-2) exhibit higher concentrations of oxygen vacancies. At pH = 13, the non-stoichiometric Bi(III)-rich phase Bi_3.84W_0.16O_6.24 is formed. All samples were fully characterized towards their optical and morphological properties. UV-Vis irradiation of the photocatalysts modified with 1% m/m Pt and in the presence of 5% v/v aqueous glycerol solution leads to H₂ evolution and glycerol oxidation. The sample prepared at pH = 0 exhibited the highest photonic efficiency (ξ) for H₂ evolution (1.4 ± 0.1%) among the investigated samples with 99% selectivity for simultaneous formic acid formation. Similar performance was observed for the non-stoichiometric Bi_3.84W_0.16O_6.24 sample (ξ = 1.2 ± 0.1% and 88% selectivity for formic acid), whereas the more crystalline sample prepared at pH = 9 was less active (ξ = 0.9 ± 0.1%) and leads to multiple oxidation products. The different behaviors were rationalized based on the role of oxygen vacancies as active adsorption and redox sites at the semiconductor surface, stablishing clear relationships between the semiconductor structure and its photocatalytic performance. The present work contributes for the rational development of specific photocatalysts for glycerol photoreforming.

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.