In general, a high specific surface area is important for catalysts to provide as many catalytic sites as possible. The final core-shell catalyst particles exhibit a BET surface area of 9.4 ± 0.1 m 2 g -1 which is three times larger than the specific surface area of the bare support particles. This increase in surface area is attributed to the additional iridium nanoparticles that form during the isopropanol reduction step. Although the BET surface of the TiO2 support was very low with only 3 m² g -1 , according to our calculation, the iridium-specific surface area of the coreshell catalyst is ~24 m 2 gIr -1 , only slightly lower than that of the commercial catalysts (~ 33-35 m 2 gIr -1 ; BET surface area values are shown in Fig. ). We emphasize that this finding justifies the use of low surface area supports for OER catalysts since the iridium structures contribute the necessary surface area. After having clarified the origins of the different catalyst features, we now turn to investigate the morphology and oxidation state of the novel core-shell catalyst. For this purpose, bright field (HR)TEM and ED analyses were performed on a thin lamella of the catalyst layer which was prepared by FIB-SEM lift out from a CCM and subsequent FIB milling to ~ 100 nm thickness. As shown in Fig. , the additional iridium nanoparticles are evenly distributed around the TiO2 support. Notably, in contrast to HAADF-STEM depicted above, here bright field (HR)TEM is employed. Consequently, the iridium phase appears now much darker than the TiO2 support. While the inter-particle electrical percolation in the catalyst layer (CL) in Fig. seems to be imperfect, it should be kept in mind that the FIB lamella only shows a thin slice of the CL and percolation might exist in the third dimension. A small gap in the shell is observed in Fig. , which could be attributed to an incomplete coverage during synthesis or to dewetting during FIB lamella preparation or during imaging as we witnessed beam-induced dewetting of the iridium shell at high electron doses. A closer inspection of the iridium layer at higher magnification reveals that lattice fringes of metallic iridium are present, corresponding to their larger scale equivalent measured by ED (Fig. ). Moreover, HRTEM (Fig. ) shows that that the Ir phase consists of many randomly oriented small crystalline grains in the range of 3 -8 nm. This is confirmed by a Rietveld refinement analysis of the XRD measurement (Fig. ) that yields an average crystallite size of 6.2 ± 0.2 nm for the crystalline iridium metal phase still present after annealing. The composition of the support based on XRD shows mainly the rutile structure of TiO2 with small amounts of anatase. Interestingly, the iridium phase appears to be mainly metallic iridium, as both ED patterns (Fig. and Fig. ) and XRD measurements (Fig. ) reveal. Although no full oxidation can be expected from an annealing temperature of 350 °C, it is interesting to note that crystalline IrO2 could not be detected at all. Nonetheless, some degree of oxidation is expected, and it is likely that an amorphous IrOx phase developed on the surface of the iridium nanoparticles that cannot be detected in our diffraction measurements. To further investigate the oxidation state of the iridium, XPS measurements were performed. In Fig. , the XPS Ir 4f spectrum of the synthesized TiO2@IrOx catalyst is shown. The deconvolution of the peaks reveals a surface with a mixed iridium oxidation state of Ir(0), Ir(III), and Ir(IV) present in the order of Ir(IV) > Ir(0) > Ir(III) (values shown in Table ). Combining this finding about the surface composition with the detected metallic crystalline iridium in XRD measurements suggests that the dispersed nanoparticles consist of an IrOx/Ir structure with a metallic core and an amorphous and substoichiometric IrOx surface. To explicitly investigate the oxidation states of the photodeposited seeds and the thin shell, additional XPS analyses were performed on samples after only photodeposition (cf. HAADF-STEM image in Fig ) and photodeposition followed by annealing (cf. HAADF-STEM image in Fig S3 ), respectively. In Fig. , the XPS spectrum of the photodeposited seeds is shown and reveals the presence of mostly Ir(III) and Ir(IV) species. This is in contrast with the detected acetone in the reaction solution which suggests the reduction of the iridium precursor to Ir(0), as discussed above. A possible explanation for the presence of Ir(III) and Ir(IV) species in the seed particles could lie in anodic oxidization of Ir(0) during the concurrent water photooxidation. This mechanism would be analogous to the formation of a hydrous iridium oxide surface during electrochemical water splitting which is reported for metallic iridium catalysts in literature. The XPS spectrum of only the thin IrOx shell on the support is shown in Fig. and exhibits a high fraction of Ir(IV) species pointing towards a high oxidation state of the shell. As the Ir(IV) species were not detected as crystalline IrO2, and also Ir(0) and Ir(III) are present, we regard the phase of the shell as amorphous IrOx with x being close to 2. All our findings about the synthesis steps, morphologies and oxidation states are formulated in the schematic illustration in Fig. .