Characterisation and activity of sol–gel-prepared TiO<SUB>2</SUB> photocatalysts modified with Ca, Sr or Ba ion additives

TiO₂ (anatase)-based photocatalyst powders containing up to 20 mol% calcium, strontium or barium ions were prepared from α-titanic acid by calcining gels prepared from triethanolamine-based sols at 600 °C. The powders were characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance ultraviolet spectrophotometry, nitrogen sorption porosimetry and in situ infrared spectroscopy to examine surface adsorbed species. Compositions containing greater than 15 mol% alkaline earth ions resulted in largely amorphous materials. The residual anatase showed decreased crystallite sizes and increased crystallographic cell volumes with increasing concentration of alkaline earth ions, while the BET surface areas of the materials increased from around 80 m² g⁻¹ (no additive) to 160 m² g⁻¹ at higher levels of additive. Electronic spectroscopy showed that the band gaps of the materials increased with increasing Ca²⁺ content, due to the decreasing particle sizes. Under the synthesis conditions used, no other crystalline phase was observed, except rutile (&lt; 1% w.r.t. anatase). The photocatalytic decomposition of oxalic acid was used as a model system to determine the relative influences of the additives on the photocatalytic activities. Titania containing 20 mol% alkaline earth ions showed approximately double the photocatalytic activity of similarly prepared anatase without additive. Half of the enhancement occurred for titania samples containing only 2 mol% alkaline earth ions. Comparisons of the physico-chemical properties of the photocatalysts with oxalic acid decomposition rates were carried out to determine the factors influencing the photocatalytic activity.