Zinc oxide (ZnO) nanoparticle UV-vis and scattering

In UV–visible spectroscopy, ZnO, a wide-gap semiconductor (band gap about 3.3 eV): its nanoparticles absorb strongly below about 375 nm, with an excitonic absorption edge near 360–375 nm, and are transparent in the visible apart from scattering by larger particles. Quantum confinement shifts the edge to shorter wavelengths for particles under about 7 nm.

Particles of 20–50 nm scatter almost no visible light (less than 2 % of the optimum per gram, table above), so the cream looks clear, while their band-gap absorption still blocks UVA and UVB: at 350 nm nearly all of a small particle's extinction is absorption. Larger pigment-grade ZnO (a few hundred nanometres) looks white.

About 2.0 at 550 nm for crystalline ZnO (ordinary ray; the extraordinary is slightly higher), falling to about 1.93 in the near-infrared. Porous films and powders measure lower (1.6–1.8). The calculators offer Bond (1965) crystal data and Aguilar et al. (2019) thin-film data with UV absorption.

Its refractive index (about 2.0) is lower than TiO₂'s (2.5–2.7), so the contrast with the binder is smaller and each gram scatters less; the best size is also larger, about 540 nm versus 270 nm. See the TiO₂ page.

Yes: zinc oxide can be used for spectra and optical properties of nanospheres and core–shell nanoparticles (add a shell), near-field enhancement maps, nanoparticle arrays and lattice resonances, particles on a substrate and laser heating.