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Enhanced photopromoted electron transfer over a bilayer WO₃ n–n heterojunction prepared by RF diode sputtering

Chiarello, Gian Luca, Bernareggi, Massimo, Pedroni, Matteo, Magni, Mirko, Pietralunga, Silvia M., Tagliaferri, Alberto, Vassallo, Espedito, Selli, Elena
Journal of materials chemistry A 2017 v.5 no.25 pp. 12977-12989
coatings, diodes, electric current, electric field, electron transfer, foil, hydrogen, methanol, oxygen, photocatalysis, radio waves, scanning electron microscopy, tungsten oxide, Scotland
A bilayer WO₃ photoelectrode was obtained by radio frequency (RF) plasma sputtering in a reactive 40%O₂/Ar atmosphere by depositing two successive WO₃ coatings on a tungsten foil at two different total gas pressures (3 Pa and 1.7 Pa, respectively), followed by calcination at 600 °C. Two monolayer samples deposited at each of the two pressures and a bilayer sample deposited at inverted pressures were also prepared. Their photoelectrocatalytic (PEC) activity was evaluated by both Incident Photon-to-Current Efficiency (IPCE) measurements and separate evolution of H₂ and O₂ by water splitting in a two-compartment PEC cell. SEM analysis revealed that the photoanodes have a nanostructured porous double layer surmounting a columnar basement (Staffa-like morphology, after the name of the Scottish island). Mott–Schottky analysis showed that the single layer deposited at 3 Pa has a conduction flat band potential 0.1 V more positive than that deposited at 1.7 Pa. The equivalent n–n heterojunction at the interface of the double-layer creates a built-in electric field that facilitates the photopromoted electron transfer toward the lower lying conduction band material, while the columnar innermost layer introduces percolation paths for efficient electron transport toward the conductive tungsten foil. Both phenomena contribute to decrease the interfacial charge transfer resistance (Rcₜ) and lead up to a ca. 30% increase in the PEC performance compared to the monolayer and the inverted bilayer coatings and to a 93% faradaic efficiency, which is among the highest reported so far for WO₃ photoanodes. Upon methanol addition an outstanding 4-fold photocurrent density increase up to 6.3 mA cm⁻² was attained over the bilayer WO₃ photoanode, much larger than the usually observed current doubling effect.