High photothermal conversion efficiency of RF sputtered Ti4O7 Magneli phase thin films and its linear correlation with light absorption capacity

RF-sputtering is used to deposit Ti₄O₇-Magneli-phase films onto various substrates at deposition temperatures (T_s) ranging from 25 to 650 °C. Not only the structural, but also electrical conductivity, optical absorbance and photothermal properties of the Ti₄O₇ films are shown to change significantly with T_s. A T_s of 500 °C is pointed out as the optimal temperature that yields highly-crystalized pure-Ti₄O₇-Magneli phase with a densely-packed morphology and a conductivity as high as 740 S/cm. The Ti₄O₇ films deposited at T_s = 450-500 °C also exhibited the highest optical absorption over all the broad (200-1500) nm range. The absorbed sunlight (AM1.5) was efficiently converted into heat by raising the temperature of the Ti₄O₇ films up to ~ 54 °C. Thus, the external photothermal efficiency (η_ext) of the Ti₄O₇ films, was found to be as high as ~ 74%. This is the highest η_ext reported so far for sputtered-Ti₄O₇ coatings (just ~ 450 nm-thick), highlighting their significant potential for photothermal applications such as desalination, deicing and/or smart windows. Finally, the η_ext of the Ti₄O₇ coatings is demonstrated, for the first time, to be linearly correlated to their integrated light absorption coefficient. This fundamental relationship paves the way towards the design and optimization of highly efficient solar-thermal conversion devices.