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

Abstract

A bilayer WO3 photoelectrode was obtained by radio frequency (RF) plasma sputtering in reactive 40%O2/Ar atmosphere by depositing on a tungsten foil two successive WO3 coatings 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 laying 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 to 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.

Multilayer WO3/BiVO4 Photoanodes for Solar-Driven Water Splitting Prepared by RF-Plasma Sputtering

“…The WO 3 and BiVO 4 coatings prepared as above described display the characteristic XRD peaks of the orthorhombic and monoclinic crystal structures, respectively, as already reported in our previous works [24,25]. The SEM micrograph of the cross section of the multilayer WO 3 /BiVO 4 /FTO photoanode ( Figure 1a) confirms an overall film thickness of 1.28 µm due to a ca.…”

“…50 cm from the light source in order to have an incident power of 100 mWcm −2 on the photoactive surface. The incident photon to current efficiency (IPCE) curves were measured using the set-up described elsewhere [25]. The incident power was measured with a calibrated Thorlabs S130VC photodiode connected to a Thorlabs PM200 power meter.…”

“…In the present work, a BiVO 4 thin film was deposited by radio-frequency (RF) plasma co-sputtering [24] over a bilayer WO 3 n-n heterojunction already optimized in our previous work [25]. The latter was obtained by plasma reactive sputtering, by depositing two consecutive layers of WO 3 at different pressures.…”

Imaging photoinduced surface potentials on hybrid perovskites by real-time Scanning Electron Microscopy

“…The advantage of SEM, if compared to other measurement techniques, is the combination of surface selectivity and 2D mapping over a wide range of view fields with its lateral resolutions at the nanoscale. Such features are specifically useful when studying optical beam sensitive photoelectrical and photovoltaic materials, where a major role in defining the figures of merit for energy conversion is played by photo-physics and photo-chemistry at surfaces and interfaces (Chiarello et al, 2017;Lanzani, 2012;Wuerfel, 2009). The idea of detecting electrons emitted from the illuminated surface of a semiconductor as a non-contact means to probe local photovoltage values actually traces back to the 1990s.…”

Tailored Assembly of Molecular Water Oxidation Catalysts on Photoelectrodes for Artificial Photosynthesis

“…The stability issues can be classified as the following: (1) degradation of the LbL‐assembled film by electrochemical reactions on photoelectrodes and (2) detachment or delamination of the film. Nevertheless, we believe there will be many opportunities for non‐covalent, LbL approaches, because it has more flexibility in design and fabrication and also can be readily combined with other conventional approaches, such as doping,, defect engineering,, and heterojunction formation, , of an underlying photoelectrode, for the practical application of artificial photosynthesis.…”