To discriminate between the chemical states of elements originating from CoPOM and present either as a dopant or a co-catalyst, XPS analysis has been carried out and the Co 2p and W 4f XP spectra are shown in Figure and Figure , Supporting Information. As a reference, XPS analysis of the pristine CoPOM powder shows, as expected, the presence of Co 2+ species at the binding energies of 797.0 eV and 781.1 eV for the Co 2p1/2 and Co 2p3/2 spin-orbit coupling peaks , respectively (Figure , Supporting Information). In contrast, the low-intensity Co 2p XPS data of the doped Composite 1 sample (Figure ) show the Co 2p1/2 and Co 2p3/2 peaks located at 795.2 eV and 779.1 eV, which indicates the presence of Co 3+ . This suggests that Co 2+ from the CoPOM dopant is being oxidized to Co 3+ during the photoanode annealing process. After additional surface modification with CoPOM, the resulting Composite 2 photoanode exhibits signals characteristic of both Co 2+ and Co 3+ (Figure ). Taken together, these data indicate that while the Co dopant within the BiVO4 bulk is present as Co 3+ , the surface-deposited CoPOM retains its original Co 2+ oxidation state, which is also in line with our previous study on CoPOM as co-catalyst. In addition, the oxidation state of tungsten in all samples determined from the high-resolution XP spectra of W 4f was W 6+ (Figure , Supporting Information), as expected for CoPOM derivatives. The morphology and film thickness of as-prepared BiVO4, Composite 1 and Composite 2 photoanodes were investigated by field emission scanning electron microscopy (FESEM). As shown in Figure -2c, all of the films are composed of wormlike nanoparticles forming a three-dimensional mesoporous structure. After the CoPOM doping and surface modification with CoPOM, the worm-like particle shape does not change significantly, as shown in Figure and, retaining the typical porous structure. However, after the CoPOM doping of BiVO4, the average length of worm-like particles increases from 160 nm to 277 nm (see Figure and S5e, Supporting Information) and the particles become more interconnected. The cross-sectional views (Figure , Supporting Information) show that the BiVO4 layers have a comparable thickness of 520 ± 40 nm in all photoanodes. High-resolution transmission electron microscopy (HR-TEM) of Composite 1 and Composite 2 shows the presence of crystalline regions with a lattice d-spacing of 0.31 nm, which can be assigned to the (-121) plane of BiVO4 (JCPDS 14-0688) (Figure and). As compared to Composite 1 photoanodes (Figure ), additional surface modification with CoPOM leads to the presence of homogenously distributed nanospheres with a size of ~5 nm, as indicated by the orange dotted lines in Figure . This suggests the formation of a CoPOM-derived surface layer on the Composite 1 photoanodes, in line with our previous results. To investigate the distribution of CoPOM, energy dispersive X-ray spectroscopy (EDX) elemental mapping was carried out. This analysis (Figure , Supporting Information) indicates the presence of Co and W elements, which is in line with the homogeneous distribution of CoPOM within the BiVO4 bulk and on the BiVO4 surface since the structure of the Composite 1 material is porous and the CoPOM was deposited via impregnation.