Thermoelectric and optical properties of nanoporous CaxCoO2 and CaxCoO2·yH2O thin films

• Metastable nanoporous Ca x CoO 2 and Ca x CoO 2 ·yH 2 O thin films prepared through oxidative insertion. • Intercalation of water in CoO 2 layers in Ca x CoO 2 thin films result in the formation of Ca x CoO 2 ·yH 2 O. • Altered thermoelectric properties compared with single-phase nanoporous Ca x CoO 2 thin films. • Design strategy for the thermoelectric and optical properties of the layered cobaltates. Metastable nanoporous Ca x CoO 2 and Ca x CoO 2 ·yH 2 O thin films have been prepared through oxidative insertion by solvent reaction of single-phase nanoporous Ca x CoO 2 thin film. The intercalation of water in CoO 2 layers in Ca x CoO 2 thin films result in the formation of Ca x CoO 2 ·yH 2 O in a reversible process. Nanoporous hydrated Ca x CoO 2 /Ca x CoO 2 ·yH 2 O thin films show enhanced thermoelectric properties compared with single-phase nanoporous Ca x CoO 2 thin films. The room-temperature power factor of nanoporous Ca x CoO 2 /Ca x CoO 2 ·yH 2 O (0.8 μWcm −1 K −2 ) is a factor of 60 higher than reference nanoporous Ca x CoO 2 thin films (0.013 μWcm −1 K −2 ). The optical band gaps of the nanoporous Ca x CoO 2 /Ca x CoO 2 ·yH 2 O thin film and single-phase Ca x CoO 2 thin film are 2.38 and 2.5 eV, respectively. This work thus offers a design strategy for the thermoelectric and optical properties of the layered cobaltates.