Time-Resolved Photoluminescence for Surface Characterization of Modified Metal Oxides

Inorganic coatings (ICs) on metal oxides and semiconducting nanoparticles are essential for a broad range of applications. They allow tailoring of the interface properties, increase stability, and passivate the surface. Quantitative characterization of these ICs is often complicated and relies on involved surface characterization methods such as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). We present time-resolved photoluminescence (TR-PL) spectroscopy of surface-bound fluorescent probe molecules as a versatile method for the semiquantitative characterization of ICs that can determine shell thicknesses and coverage. The method relies on coating-dependent fluorescence quenching of the fluorophores through interfacial electron transfer. It eliminates the need for selective fluorophore binding to specific surfaces and enables rapid characterization of coating properties in situ on large scales. The approach is demonstrated on Al2O3 coatings on TiO2. However, the method can be extended to other ICs on semiconducting and potentially metal nanoparticles, as well as core–shell structures. The semiquantitative method requires calibration with reference samples synthesized through an improved process for the controlled deposition of Al2O3-closed shells and patches on TiO2 nanoparticle surfaces using wet chemical deposition.