Hydrous iridium oxide (IrOx) nanoparticles (NPs) with an average diameter of 1.7 ± 0.3 nm were prepared via photochemical hydrolysis of iridium chloride in alkaline medium at room temperature. The photoinduced hydrolysis was monitored by time-dependent ultraviolet-visible (UV-vis) spectroscopy, and the effects of the incident wavelength and irradiation time on the production of IrOx NPs were systematically investigated. It was found that UV-vis irradiation is crucial for the generation of IrOx NPs during the hydrolysis of IrCl3, and once the irradiation was turned off, the hydrolysis reaction stopped immediately. The production rate of IrOx NPs greatly depended on the incident wavelength. There is a critical wavelength of 500 nm for the hydrolysis reaction, and IrOx NPs can only be produced under the illumination with an incident wavelength shorter than 500 nm. Moreover, the shorter the incident wavelength, the faster the growth rate of IrOx NPs. The obtained IrOx NPs were highly stable during two months of storage at 4 °C. The Ir/IrOx nanocomposites were prepared by surface reduction of IrOx NPs with NaBH4. The microstructure of the Ir/IrOx composite was characterized by transmission electron microscopy (TEM), and the presence of zero-valence Ir was confirmed by the X-ray diffraction (XRD) result. The Ir/IrOx nanocomposite exhibited good catalytic activity and high recycling stability toward the reduction of 4-nitrophenol. The catalytic activity per unit surface area of the Ir/IrOx composite catalyst was increased by a factor of 15 compared to that of pure Ir catalyst. The presence of the Ir/IrOx interfaces in the composite catalyst is believed to be responsible for the high activity.

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