Deciphering the\nOptical Absorption Spectra of Ultrafine\nMetal Nanoparticles Dispersed on Submicron Silica Spheres

Abstract

Optical absorption of metal nanoparticles on dielectric\nsupports\nthat do not absorb light becomes complicated due to the convolution\nof light scattering of the dielectric supports. Identifying the origin\nof optical absorption characteristics in metal/dielectric composite\nmaterials is vital to designing materials with desirable optical absorption\nproperties for interesting applications such as photocatalysis. In\nthis work, we systematically investigated the optical absorption of\nultrafine metal nanoparticles (ufMNPs) supported on silica spheres\n(SiO_<i>x</i>-SPs) using absorption-sensitive\ndiffuse reflectance spectroscopy (DRS) and theoretical modeling/calculations.\nThe comparisons between experimental measurements and theoretical\npredictions reveal that the SiO_<i>x</i>-SPs in\nufMNPs/SiO_<i>x</i>-SP composite powders exhibit\nthree types of scattering to determine the optical absorption of the\nsupported ufMNPs. First, localized light-scattering resonances near\nthe surface of individual SiO_<i>x</i>-SPs with\nlarge sizes generate enhanced electric fields to promote strong optical\nabsorption in the ufMNPs with well-defined peaks. Second, nonunidirectional\ndiffuse light scattered from individual SiO_<i>x</i>-SPs with relatively small sizes penetrates the composite powders\nto be absorbed by the ufMNPs, exhibiting uphill and peakless absorption\nprofiles. Third, the diffuse light can be modulated by the ordered\npacking of the composite particles that exhibit photonic reflection\nat the stop band, resulting in a valley in the DRS spectrum of a composite\npowder. The deconvolution of optical absorption in the ufMNPs/SiO_<i>x</i>-SP composite particles opens an avenue to\nrationally design composite materials with favorable light absorption\nproperties, for example, nonlinearity of localized absorption versus\nlinearity of diffuse absorption.