The deposition of an atomically precise nanocluster, for example, Ag44(SR)30, onto a large‐band‐gap semiconductor such as TiO2 allows a clear interface to be obtained to study charge transfer at the interface. Changing the light source from visible light to simulated sunlight led to a three orders of magnitude enhancement in the photocatalytic H2 generation, with the H2 production rate reaching 7.4 mmol h−1 gcatalyst−1. This is five times higher than that of TiO2 modified with Ag nanoparticles and even comparable to that of TiO2 modified with Pt nanoparticles under similar conditions. Energy band alignment and transient absorption spectroscopy reveal that the role of the metal clusters is different from that of both organometallic complexes and plasmonic nanoparticles: A type II heterojunction charge‐transfer route is achieved under UV/Vis irradiation, with the cluster serving as a small‐band‐gap semiconductor. This results in the clusters acting as co‐catalysts rather than merely photosensitizers.

The atomically precise bimetallic nanocluster (NC), Au24Ag20(PhCC)20(SPy)4Cl2 (1) (Py=pyridine), was employed for the first time as a stable photosensitizer for photoelectrochemical applications. The sensitization of TiO2 nanotube arrays (TNA) with 1 greatly enhances the light‐harvesting ability of the composite because 1 shows a high molar extinction coefficient (ϵ) in the UV/Vis region. Compared to a more standard Au25(SG)18‐TNA (2‐TNA; SG=glutathione) composite, 1‐TNA shows a much better stability under illumination in both neutral and basic conditions. The precise composition of the photosensitizers enables a direct comparison of the sensitization ability between 1 and 2. With the same cluster loading, the photocurrent produced by 1‐TNA is 15 times larger than that of 2‐TNA. The superior performance of 1‐TNA over 2‐TNA is attributed not only to the higher light absorption ability of 1 but also to the higher charge‐separation efficiency. Besides, a ligand effect on the stability of the photoelectrode and charge‐transfer between the NCs and the semiconductor is revealed. This work paves the way to study the role of metal nanoclusters as photosensitizers at the atomic level, which is essential for the design of better material for light energy conversion.