Multi-functional ultrathin coating layers on nanoparticles via atomic layer deposition

Abstract

There is strong demands to functionalize nanoparticles for many different industrial and scientific application. Especially ultrathin coating for nanoparticles has attracted considerable interest in field of functionalization for nanoparitlces. However, it is not easy to form ultrathin film coating on individual nanoparticles due to the limit of existing coating techniques. The methods to coat nanoparticles can be categorized into two: 1) liquid phase synthesis (LPS) and 2) gas-phase synthesis (GPS). In LPS, organic and inorganic precursors or both are used in the liquid phase for synthesizing and coating nanoparticles through a variety of techniques, such as hydrolysis, sol–gel, reduction, hydrothermal, etc. The main drawback of LPS is the difficulty in the accurate control of the coating thickness at the atomic range. Another demerit of LPS is also that the coating process varies with respect to the material to be coated, which is relatively more complex than the GPS. Nevertheless, most of the reports on the coating layer until now are based on the chemical synthetic approach owing to its popularity and simplicity. On the other hand, the GPS method of coating particles provides a superior controllability of the coating thickness at the atomic range over the LPS because it utilizes a gas phase chemical reaction, which was first developed by using the chemical vapor deposition (CVD) technique, which was later continued by using the atomic layer deposition (ALD) technique. The CVD method is found to be inadequate for particle deposition because the coating thickness cannot be easily controlled and the uniformity of the coating was poor compared to the ALD technique. In addition, the CVD method causes particle agglomeration in fluidized bed reactors (FBRs) and the excessive growth generated additional particles of coating material, which contaminated the reactor. On the other hand, the ALD technique facilitates the growth of conformal thin films with an accurate thickness in the atomic range on large areas through a self-limiting surface reaction mechanism. However, the ALD coating method on nanoparticles with a very high surface area by using FBRs still has a serious problem due to the short life time of the reactants in the particle bed after being pumped out, resulting in the fact that a large quantity of reactants has to be supplied for achieving a high coating efficiency. McCormic et al. suggested that the introduction of rotary reactors can increase the surface reaction efficiency by using static reactant exposure, as closing the main valve between the particle ALD chamber and the pump. In order to improve the agitation of the nanoparticles and the coating efficiency, we herein introduce a modified rotary reactor for the ALD coating on nanoparticles. Herein, I studied ultrathin surface coating for the main material in cosmetics and photocatalysis fields. In cosmetics work, for the elimination of the phototoxicity of TiO2 nanoparticles, ultrathin ALD Al2O3 ultra-thinfilm was coated using modified rotary reactor layer. The performance of Al2O3 layer for elimination of TiO2 phototoxicity was demonstrated for UV-protection application. The coated Al2O3 is suggested to play an important role in degrading the photocatalytic activity as preventing charge carriers generated by UV from moving to the surface of the TiO2 nanoparticles. We revealed that the phototoxicity of the particles was crucially affected by the thickness and growth temperature of the Al2O3 coating layer, and the related modeling was suggested. In photocatalysis study, in order to solve the problem of high recombination rate of photo-generated charge carriers of g-C3N4 photocatalyst, TiO2, ZnO, ZnS coating was performed on the surface of g-C3N4. To the best of our knowledge, coating on g-C3N4 via ALD has not been reported except our group yet. Successful uniform TiO2, ZnO and ZnS coating on g-C3N4 via ALD was demonstrated by using SEM-EDS, TEM. From the photocatalytic dye degradation test, we found that TiO2, ZnO, ZnS coating for g-C3N4 forms uniform heterojunction, resulting in enhancing the photocatalytic activity of g-C3N4. Thus, nanoparticle coating using powder ALD system can be highly potential technique for nanoparticle functionalization.