Transformation of amorphous TiO2 to a hydronium oxofluorotitanate and applications as an HF sensor

•A hydronium oxofluorotitanate phase is formed by reaction of gaseous HF with amorphous titania.•The reaction product is characterized by ellipsometry, XPS, Auger spectroscopy, and XRD.•The extent of reaction is found to depend on both HF concentration and relative humidity.•Gaseous HF at concentrations as low as 1ppm was detectable at moderate relative humidity.