Semiconducting WO3 is a photocatalyst in the UV and visible spectral range, for water splitting and H2 production. We introduce a porous nanostructured bilayered WO3 photoelectrode grown by reactive radio frequency (RF) plasma sputtering in diode configuration. Enhanced charge transport and photocatalytic response is due to the formation of a n-n buried junction, issued by changing the total gas pressure during growth.
Sputtering was performed in 40%O2/Ar atmosphere from metallic W target. By subsequently setting the total gas pressure at 3 Pa and 1.7 Pa, respectively, two WO3 coatings (about 1 μm in total) were deposited on top of each other on a W foil substrate (purity 99%) at RT and then calcined at 600 °C for 2 h . The crystalline structure was characterized by XRD. SEM analysis revealed a nanostructured porous double layer surmounting a columnar basement. The band-gap was probed by Mott-Schottky analysis, UV-vis-NIR diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. XPS also assessed oxide stoichiometry and the nature of defects, also probed by photoluminescence (PL) spectroscopy.
Samples grown at 1.7 Pa show a 0.1 V wider bandgap (2.94 eV) than those prepared at higher pressure (2.82 eV). Both Mott-Schottky analysis and XPS valence band spectra showed that this difference is essentially due to a shift of the conduction flat band (CB) potential. Thus, the total gas pressure during WO3 deposition affects the position of the CB energy, by inducing different density of oxygen vacancies and a related different extent of crystal structure distortion . The equivalent n-n junction at the interface of the bilayer creates a built-in electric field that helps electron transfer, while the columnar innermost layer introduces percolation paths for efficient electron transport toward the conductive W foil. Both phenomena add to decrease the interfacial charge transfer resistance, as measured by impedance spectroscopy under irradiation, and lead to up a 30% increase in the photoelectrocatalytic (PEC) performance - evaluated by Incident Photon-to-Current Efficiency (IPCE) measurements and by water splitting - compared to monolayered and inverted bilayered coatings. Moreover, a 93% faradaic efficiency was achieved over the bilayer sample, that 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 system, much larger than the usually observed current doubling effect.
This work received financial support from the Regione Lombardia and Cariplo Foundation co-funded “SmartMatLab Centre” project (Grant No. 2013-1766)
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