Introduction of oxygen vacancies and fluorine into TiO2 nanoparticles by co-milling with PTFE

Solid-state processes of introducing oxygen vacancies and transference of fluorine to n-TiO2 nanoparticles by co-milling with poly(tetrafluoroethylene) (PTFE) powder were examined by diffuse reflectance spectroscopy (DRS) of UV, visual, near- and mid-IR regions, thermal analyses (TG-DTA), energy-dispersive X-ray spectroscopy (EDXS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The broad absorption peak at around 8800 cm−1 (1140 nm) was attributed to the change in the electronic states, viz. electrons trapped at the oxygen vacancies (Vo) and d–d transitions of titanium ions. Incorporation of fluorine into n-TiO2 was concentrated at the near surface region and amounted to ca. 40 at% of the total fluorine in PTFE, after co-milling for 3 h, as confirmed by the F1s XPS spectrum. The overall atomic ratio, F/Ti, determined by EDXS was 0.294. By combining these analytical results, a mechanism of the present solid state processes at the boundary between PTFE and n-TiO2 was proposed. The entire process is triggered by the partial oxidative decomposition of PTFE. This is accompanied by the abstraction of oxygen atoms from the n-TiO2 lattices. Loss of the oxygen atoms results in the formation of the diverse states of locally distorted coordination units of titania, i.e. TiO6−nVon, located at the near surface region. This leads subsequent partial ligand exchange between F and O, to incorporate fluorine preferentially to the near surface region of n-TiO2 particles, where local non-crystalline states predominate.

Graphical abstractScheme of the reaction processes: (a) pristine mixture, (b) oxygen abstraction from TiO2 and (c) fluorine migration from PTFE to TiO2.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlightsTransfer of fluorine from PTFE to n-TiO2 in a dry solid state process was confirmed. ► 40% of F in PTFE was incorporated to the near surface region of n-TiO2 nanoparticles. ► The transfer process is triggered by the oxidative decomposition of PTFE. ► Fluorine incorporation is mediated by the formation of oxygen vacancies. ► The sequential mechanisms are verified by XPS, EDXS, HRTEM, TG and DRS.