Unraveling of cocatalysts photodeposited selectively on facets of BiVO4 to boost solar water splitting

Bismuth vanadate (BiVO4) has been widely investigated as a photocatalyst or photoanode for solar water splitting, but its activity is hindered by inefficient cocatalysts and limited understanding of the underlying mechanism. Here we demonstrate significantly enhanced water oxidation on the particulate BiVO4 photocatalyst via in situ facet-selective photodeposition of dual-cocatalysts that exist separately as metallic Ir nanoparticles and nanocomposite of FeOOH and CoOOH (denoted as FeCoOx), as revealed by advanced techniques. The mechanism of water oxidation promoted by the dual-cocatalysts is experimentally and theoretically unraveled, and mainly ascribed to the synergistic effect of the spatially separated dual-cocatalysts (Ir, FeCoOx) on both interface charge separation and surface catalysis. Combined with the H2-evolving photocatalysts, we finally construct a Z-scheme overall water splitting system using [Fe(CN)6]3−/4− as the redox mediator, whose apparent quantum efficiency at 420 nm and solar-to-hydrogen conversion efficiency are optimized to be 12.3% and 0.6%, respectively.

(hole scavenger, j sulfite ) are given in Supplementary Fig. 11a. According to the equations: η inj = j water /j sulfite and η sep = j sulfite / jabs , where j abs is the maximum photocurrent density that a photoanode can achieve (jabs = 5.7 mA/cm 2 determined by the light absorption of BiVO 4 , Supplementary Fig. 11b).

Supplementary Fig. 12 Schematic of the whole OER mechanism on the FeCoO x /BiVO 4 and CoO x /BiVO
respectively.  When using the Ir-CoO x (Imp.)/BiVO 4 with Ir and Co randomly dispersion or BiVO 4 as the OEP, the OWS will not be achieved.

Photocatalytic reactions
The photocatalytic reactions were carried out in a Pyrex top irradiation-type reaction vessel connected to a closed gas circulation system. Before reactions, the mixed solution containing catalysts was evacuated and then irradiated using a 300 W Xenon lamp with a cut-off filter (Hoya, L-42; λ ≥ 420 nm). A flow of cooling water was used to maintain the reaction system at 288 K. The gases evolved were analyzed by gas chromatography (Agilent; GC-7890A, MS-5A column, TCD, Ar carrier).

X-ray absorption fine structure (XAFS) measurements
The XAFS spectra of Fe K-edge were collected at BL14W1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) while Co K-edge were collected at 1W1B beamline of Beijing Synchrotron Radiation Facility (BSRF). The data were collected in fluorescence mode using a Lytle detector while the corresponding reference samples were collected in transmission mode. The samples were grinded and uniformly daubed on the special adhesive tape.

XAFS analysis and results
The acquired EXAFS data were processed according to the standard procedures using the ATHENA module of Demeter software packages.
The extended X-ray absorption fine structure (EXAFS) spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing it with respect to the edge-jump step. Subsequently, the χ(k) data were Fourier transformed to real (R) space using a hanning windows (dk=1.0 Å -1 ) to separate the EXAFS contribution from different coordination shells. To obtain the quantitative structural parameters around central atoms, least-squares curve parameter fitting was performed using the ARTEMIS module of Demeter software packages The following EXAFS equation was used: the theoretical scattering amplitudes, phase shifts and the photoelectron mean free path for all paths calculated. S 0 2 is the amplitude reduction factor, F j (k) is the effective curved-wave backscattering amplitude, N j is the number of neighbors in the j th atomic shell, R j is the distance between the X-ray absorbing central atom and the atoms in the j th atomic shell (backscatterer), λ is the mean free path in Å, ϕ j (k) is the phase shift (including the phase shift for each shell and the total central atom phase shift), σ j is the Debye-Waller parameter of the j th atomic shell (variation of distances around the average R j ). The functions F j (k), λ and ϕ j (k) were calculated with the ab initio code FEFF9. The additional details for EXAFS simulations are given below.
All fits were performed in the R space with k-weight of 2 while phase correction was also 22 applied in the first coordination shell to make R value close to the physical interatomic distance between the absorber and shell scatterer. The coordination numbers of model samples were fixed as the nominal values. The obtained S 0 2 was fixed in the subsequent fitting. While the internal atomic distances R, Debye-Waller factor σ 2 , and the edge-energy shift Δ were allowed to run freely.

Computational details
The density-functional theory (DFT) 1 DFT-D3 method 8,9 was adopted to consider van der Waals correction.

Theoretical Evaluation of Activity
The theoretical overpotentials for FeCoO x /BiVO 4 and CoO x /BiVO 4 interfaces were determined, assuming the conventional oxygen evolution reaction (OER) mechanism 10 . The computational hydrogen electrode model 11 was used for the expression of the chemical potentials of protons and electrons at any given pH and applied potential as in previous OER works 12,13