Nanocrystal composite

A nanocrystal composite comprising a connected aggregate including a plurality of nanocrystal fragments connected to one another, each nanocrystal fragment having a main surface and an end surface, and nanoparticles supported on the connected aggregate, wherein

• the plurality of nanocrystal fragments each have a flaky shape;
• the plurality of nanocrystal fragments have gaps between the main surfaces;
• the gaps are arranged so as to open to the outside of the connected aggregate, wherein the nanoparticles are supported in the gaps;
• the nanoparticles have a metallic element different from that of the plurality of nanocrystal fragments;
• and
• a proportion of a visual field area of the nanoparticles with respect to a visual field area of the plurality of nanocrystal fragments is 2% or more and 50% or less,
• wherein the visual field area of the plurality of nanocrystal fragments is an area in the visual field when the plurality of nanocrystal fragments are observed by a microscope,
• wherein the visual field area of the nanoparticles is an area in the visual field when the nanoparticles are observed by the microscope,
• wherein the proportion is calculated based on an elemental mapping of the plurality fragments and the nanoparticles of nanocrystal using SEM-EDS,
• wherein the minimum dimension of the main surface of the nanocrystal fragment is 10 nm or more and less than 1 μm, and the thickness t of the nanocrystal fragment is 1/10 or less of the minimum dimension of the main surface,
• wherein the thickness t of the nanocrystal fragment is 1 nm or more,
• wherein the plurality of nanocrystal fragments are a first kind metal oxide and the nanoparticles are a second kind metal oxide different from the first kind metal oxide,
• wherein the nanoparticles are dispersedly present in a proportion of 1 to 2 particles with respect to 20 nanocrystal fragments, and
• wherein the second kind metal oxide comprises a nanoparticle of a mixture of cerium oxide and zirconium oxide, wherein the nanoparticles are present in said proportion between the nanocrystal fragments.