Herein, the synthesis route of the TiO2@IrOx core-shell particles comprises three individual steps, which are schematically depicted in Fig. . The first step is a photodeposition process exploiting the semiconducting properties of the TiO2 support particles: By using photons with an energy exceeding that of the semiconductor's bandgap (Eg (TiO2) = 3.0 -3.2 eV), electrons inside the semiconductor can be excited from the valence to the conduction band. Usually, the electron-hole pairs would recombine and dissipate heat. However, if a hole scavenger is introduced, it is readily oxidized by the hole in the semiconductor, so that an electron remains in the conduction band of the semiconductor. This electron is available for reactions on the surface of the TiO2 particles. In this work, isopropanol was chosen as hole scavenger. Oxidation of isopropanol yields acetone, while Ir 3+ is reduced to metallic Ir 0 as schematically shown in Fig. . This process is known as reductive photodeposition. Alternatively, oxidative photodeposition for iridium has also been reported with a direct deposition of IrO2. The photodeposition mechanism in this work is described in detail in the following. To utilize all the iridium precursor, the second synthesis step employs the remaining isopropanol as a reduction agent at 100 °C. This approach was chosen since it requires no additional chemicals, rendering the synthesis method easily scalable. As a third step, the catalyst is annealed at 350 °C under oxidative conditions. This temperature was chosen as a compromise between activity and stability which is a well-known trade-off for iridium-based catalysts. The TiO2@IrOx core-shell particles synthesized via this three-step method are shown in HAADF-STEM images in Fig. . The core-shell catalyst is based on a TiO2 support with a comparably large particle size of 750 ± 360 nm. Two different iridium structures make up the active catalyst: A 2.1 ± 0.4 nm thin and homogeneous shell of iridium phase (iridium oxide, as evidenced below) covers the whole TiO2 support particle and on top of this shell, nanoparticles with a sponge-like morphology and a size of 34 ± 9 nm are formed. The thin shell is particularly important because it forms an electrically percolating pathway that connects the dispersed iridium nanoparticles with each other and with the porous transport layer (PTL). In other words, this thin IrOx shell enables conductive TiO2@IrOx catalysts with low iridium contents. The additional iridium nanoparticles, on the other hand, can benefit the catalyst by introducing a high surface area for the catalytic water splitting. The rationale behind choosing TiO2 particles with a large diameter is their low specific surface area, which allows to get fully iridium-covered core-shell particles at a low iridium content. For the TiO2 particles used here, a BET surface of 3.0 m 2 g -1 was measured. Assuming a 2.1 nm thin shell of iridium oxide, only 6 wt% iridium in the catalyst powder is needed to complete the core-shell structure (for this calculation, ρ (IrO2) = 11.66 g cm -3 was used). The measured iridium content after the photodeposition is only 2.7 ± 0.6 wt% (XRF measurements) which equates to an average shell thickness of ~ 1 nm. This discrepancy in iridium content and the observed shell thickness should be further investigated. In this work, an iridium content of 40 wt% was chosen to guarantee an excess of iridium for the shell formation and additional active sites of the nanoparticles. Experimentally, this iridium content was confirmed to be 39.8 ± 2.5 wt%. This amount is still much lower than the iridium content in commercial catalysts which is usually between 75 and 100 wt% (e.g., Umicore Elyst Ir75 or iridium black). To gain insights into the formation mechanisms of the different catalyst structures, the particles were analyzed after each individual synthesis step by HAADF-STEM and EDX spectrum imaging. As shown in Fig. , there is no formation of larger (>10 nm) iridium structures after the photodeposition step. Looking at the surface of the TiO2 support particles, however, reveals the existence of small iridium seeds with an average size of 0.9 ± 0.2 nm (Fig. ). We conclude that these seeds are the result of the photodeposition. The participation of isopropanol as hole scavenger in the photodeposition was evidenced by detecting its oxidized form (acetone) in the 1 H NMR spectrum of the reaction filtrate (Fig. and). We hypothesize that the photodeposition process is self-limiting because Ir/TiO2 compounds are known as effective catalysts for photoelectrochemical water splitting which might compete with the photodeposition process. Moreover, the formed iridium particle layer could hinder the UV-light entering the TiO2 particles. The second synthesis step uses the isopropanol for the chemical reduction of the iridium ions, which has been reported by only a few other studies. Again, the reaction pathway was confirmed by detecting an increased amount of acetone in the 1 H NMR spectrum of the filtrate after the second reaction step (Fig. ). During this second step, iridium nanoparticles with an average size of 34 ± 9 nm have formed on top of the seeds from the first reaction step (Fig. ). It should be noted that also some unsupported iridium particle clusters were observed. However, as the measured iridium content is very close to the calculated value (as described above), we conclude that the majority of the particles formed on the support particles. On the surface of the TiO2 support, the iridium seeds from the first synthesis step increase in size to 1.4 ± 0.4 nm, but generally maintain their morphology (Fig. ). After the third synthesis step, the iridium nanoparticles retained their size and morphology (Fig. ). Importantly, a continuous shell of iridium oxide on the TiO2 core has formed (Fig. ; more HAADF-STEM and EDX spectrum images are shown in Fig. ). Therefore, we conclude that the closed shell is formed during the annealing step. To verify that the shell forms from the iridium seeds previously grown by photodeposition, we performed the synthesis route as described but without the second synthesis step (isopropanol reduction). As shown in Fig. , a continuous IrOx shell is also observed for these particles that only underwent photodeposition and annealing. Although the formation of the shell can thus be clearly attributed to these two synthesis steps, the exact mechanism behind the shell formation has yet to be elucidated. A possible explanation could be the fulfillment of a wetting condition during the annealing step. As an additional validation step of the synthesis, another batch of particles was synthesized with an omitted photodeposition step. The iridium content of this catalyst is measured to be 38 ± 3 wt%, which is similar to the batch that underwent all synthesis steps. These particles, which only underwent isopropanol reduction and annealing, do not exhibit a shell as shown in Fig. . This observation emphasizes the importance of the photodeposition step for the formation of the core-shell structure.