M-H. Choi, J. Park, H. Noh, G. Min and S.B. Kaemmer
Park Systems inc.,
United States
Keywords: TiO2 nanoparticle, Heterodyne KPFM, contact potential difference (CPD), photo-induced surface potential
Summary:
Titanium dioxide (TiO₂) nanoparticles are widely used in photocatalysis, solar energy conversion, and environmental remediation, where performance is governed by the generation, separation, and persistence of photo-induced charges at the surface under illumination.1,2 Understanding these processes at the single-particle level remains essential for optimizing nanoscale functionality. Kelvin probe force microscopy (KPFM) was originally implemented in amplitude-modulation mode,3 which provides quantitative surface potential measurements but is limited by long-range electrostatic interactions and reduced spatial resolution. Frequency-modulation KPFM4 improved accuracy and lateral resolution by detecting electrostatic force gradients via resonance frequency shifts. Heterodyne KPFM further advances the technique by separating the mechanical drive and electrical modulation frequencies, significantly enhancing signal-to-noise ratio, spatial resolution, and measurement speed under ambient conditions.5,6 These advantages make heterodyne KPFM particularly well suited for probing UV-induced surface potential changes in TiO₂ nanoparticles. In this work, the photo-induced surface potential of TiO₂ nanoparticles is investigated as a model system using heterodyne KPFM, enabling time- and size-resolved mapping of contact potential difference (CPD) with nanometer-scale spatial resolution. Upon UV irradiation, a distinct CPD signal from an individual TiO₂ nanoparticle appears immediately, resulting in surface photovoltage formation and efficient charge separation. Systematic measurements performed on nanoparticles (78 - 220 nm diam.) reveal clear size-dependent behavior in both CPD magnitude and relaxation dynamics. These results demonstrate the capability of heterodyne KPFM as a powerful tool for nanoscale photo-physics and provide design insights for optimizing TiO₂ nanoparticle size in photocatalytic and photoelectrochemical applications, such as correlations between spatial CPD hotspots and local electrochemical reactivity.