Combinatorial Atmospheric Pressure Chemical Vapor Deposition of F:TiO2; the Relationship between Photocatalysis and Transparent Conducting Oxide Properties



Combinatorial atmospheric pressure chemical vapor deposition (APCVD) is used to deposit anatase TiO2 with a graded level of F-doping between 1.10 ≤ F:Ti (at%) ≤ 2.57 from the reaction of titanium tetrachloride, ethyl acetate and trifluoroacetic acid at 500 °C on glass. The photocatalytic activity and electrical resistivity of 200 allotted positions across a grid are screened using high-throughput techniques. A blue region of film is singled out for containing the lowest electrical resistivities of any previously reported doped or undoped TiO2-based system formed by APCVD (ρ ≈ 0.22–0.45 Ω cm, n = 0.8–1.2 × 1018 cm−3, μ = 18–33 cm2 V−1 s−1). The blue region contains a lower fluorine doping level (F:Ti ≈ 1.1–1.6%, Ebg ≈ 3.06 eV) than its neighboring colorless region (F:Ti ≈ 2.3–2.6%, Ebg ≈ 3.15–3.21 eV, ρ ≈ 0.61–1.3 Ω cm). State-of-the-art hybrid density functional theory calculations were employed to elucidate the nature of the different doping behaviors. Two distinct fluorine doping environments were present. At low concentrations, F substituting for O (FO) dominates, forming blue F:TiO2. At high concentrations, negatively charged fluorine interstitials (Fi−1) begin to dominate, forming transparent F:TiO2.