WO3/BiVO4 Photoanodes: Facets Matching at the Heterojunction and BiVO4 Layer Thickness Effects

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO3/BiVO4 heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO3/BiVO4 interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO4 layer over a WO3 scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO4 growth over the WO3 layer along the BiVO4 {010} face, driven by the formation of a stable interface with new covalent bonds, with a favorable band alignment and band bending between the two oxides. This crystal facet phase matching allows a smooth transition between the electronic states of the two oxides and may be a key factor ensuring the high efficiency attained with this heterojunction. The photoelectrochemical activity of the WO3/BiVO4 photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-absorbing BiVO4 layer, a 75 nm thick BiVO4 layer on WO3 being best performing.


Cross section FESEM images and extinction coefficient of BiVO4
Figure S1. Cross section FESEM images (a) of clean FTO and of BiVO4 photoanodes prepared by successive deposition of (b) 2, (c) 4 and (d) 6 BiVO4 layers. The scale bar is 500 nm.
We performed the FESEM cross section analysis of the films prepared by 2, 4 and 6 successive BiVO4 depositions (Figure S1); the cross section of the clean conductive glass substrate was used to determine the thickness of the pristine FTO layer. From the absorption spectra of the three electrodes, reported in Figure 2, and the evaluated thickness of the BiVO4 overall layers in BiVO4 photoanodes, we calculated the absorption coefficient of BiVO4 at 420 nm (α 420 ) using the following equation: 420 = α 420 × d, where d is the average thickness in cm and 420 the absorbance at 420 nm ( Figure 2A). The absorption coefficient 420 = 6.7 10 4 cm -1 was obtained as the slope of the absorbance at 420 nm vs. film thickness plot shown below.
The calculated α 420 allowed us to estimate the thickness of the individual BiVO4 films (either deposited on FTO or on FTO/WO3) from the electrode absorbance at 420 nm. The thickness of the BiVO4 layer was found to increase almost linearly with the number of coated layers with an average increment of ca. 20 nm per coated layer, as shown by the data reported in Table S1. Table S1. Absorbance (Abs) at 420 nm and thickness of the BiVO4 layers in single BiVO4 and composite WO3/BiVO4 photoanodes, obtained upon deposition of a different number of BiVO4 coated layers.

Cutoff limits, convergence criteria and thresholds, and reciprocal space sampling
The cutoff limits in the evaluation of Coulomb and exchange series were set to 10 -7 for both Coulomb overlap and penetration, 10 -7 for exchange overlap and pseudo-overlap in the direct space, and 10 -14 for exchange pseudo-overlap in the reciprocal space. An SCF was considered converged when the energy difference between two consecutive steps was below 10 -6 au. The reciprocal space of heterostructure models was sampled by adopting a working shrinking factor equal to 4. For all atoms, the thresholds for the maximum and the root mean square forces were set to 0.00045 and 0.00030 au and those for the maximum and the root mean square atomic displacements to 0.00180 and 0.00120 au, respectively.

a. Bulk WO3 and BiVO4
The calculated bulk structural and electronic properties of WO3 and BiVO4 are reported in Table S2, in comparison with available experimental data.  Table S3.

c. WO3/BiVO4 interfaces models
The WO3 (001) slabs have been interfaced with the three BiVO4 (010) models described above. All composite models have been fully optimized. The calculated lattice parameters are reported in Table S4, together with the calculated adhesion energy Ead, defined as: The valence band (VBMWO3, VBMBiVO4) and conduction band (CBMWO3, CBMBiVO4) edges of independent components can be aligned by using, as a common reference, the calculated plane averaged electrostatic potential (V) along the non-periodic dimension z. [1][2][3][4] In  The formation of the interface leads to a ~0.3 eV enhancement of the band offsets, which is expected to improve the driving force of the electrons migration toward WO3 and holes migration towards BiVO4. This 0.3 eV band offsets increase is similar to what obtained in oxide interfaces. 5