Phase-Control-Enabled Enhancement in Hydrophilicity and Mechanical Toughness in Nanocrystalline Tungsten Oxide Films for Energy-Related Applications

We report on the phase-control-enabled enhancement in the hydrophilicity and mechanical properties of nanostructured WO3 films for energy-related applications. Nanostructured WO3 films were fabricated by utilizing reactive magnetron sputter deposition onto silicon (100) substrates with varying deposition temperature (Ts = 25–500 °C) at a fixed oxygen partial pressure of ∼4 mTorr. Extensive characterization performed indicates that the fundamental surface/interface structure–phase–hydrophilicity–mechanical property correlation enables tailoring nanocrystalline WO3 films to meet the requirements of various technological applications. Crystal structure, surface/interface morphology, and microstructure characterization indicate the deposition processing conditions induce phase transformations and surface/interface quality variation, which in turn controls the hydrophilicity and mechanical behavior, in WO3 films. Carefully tuned processing conditions induce an amorphous to crystalline structural transformation, which progresses through amorphous to monoclinic to tetragonal phases, coupled with variation in surface roughness and crystallite size. Mechanical characterization using nanoindentation reveals that the mechanical response, in terms of hardness (H), elastic modulus (E), and scratch parameters, of WO3 films is highly sensitive to their phase and microstructure evolution. Nanostructured WO3 films crystallized in monoclinic phase exhibit superior mechanical response compared to either amorphous or tetragonal phase WO3 films. The maximum hardness (∼38 GPa) and elastic modulus (∼320 GPa) values were obtained for WO3 films deposited at Ts = 400 °C. The phase transformation sequence coupled with surface/interface structure affects the contact angle values significantly. The contact angle decreases significantly from 70° to nearly 5° with variation in phase, microstructure, and surface/interface quality of WO3 films. A direct structure–phase–microstructure–hydrophilicity–mechanical property relationship found suggest that tuning properties of WO3 films for photoelectrochemical, photocatalytic, and energy-related applications can be achieved by tuning the deposition conditions and controlling the phase at the nanoscale dimensions.