Halloysite nanotubes (HNTs) can be used as stable support materials for gold and silver nanoparticles (Au/Ag-NPs) without distinction given to the morphology of the tubular HNTs, i.e. polygonal prismatic or cylindrical. The present study aimed to use direct visualization techniques to observe and compare the adsorption sites and behaviors of the two tubular morphologies with both Au- and Ag-NPs. Au/Ag-NPs were surface engineered with methyl-, carboxylate-, or phosphonate-terminated linker groups and reacted with halloysite nanotubes with two different morphologies, forming composite materials HNTs@Au-NPs and HNTs@Ag-NPs. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy were used to investigate the sorption reactions. Au-NPs disperse well into individual particles while the Ag-NPs show a tendency to agglomerate, making them less suitable probes for adsorption onto HNTs. The results also indicate that polygonal prismatic HNTs show greater sorption than cylindrical HNTs, where NPs were detected on the external walls of the nanotubes. This sorption is suggested to occur at the steps and edges of the particles, at the termini of the tubes, and in the hollow inner lumina. For the varying linker groups, carboxylate- and phosphonate-derivatized Au-NPs show similar sorption behavior on HNTs, while no surface sorption was observed for the methyl-terminated linkers. This justifies the hypothesis that the carboxylate and phosphonate moieties chemisorb to the hydroxyls of the aluminol groups at the edge sites, steps, and lumina of polygonal prismatic HNTs, identifying these as key anion bonding sites on HNTs, while non-ionic, methyl-terminated Au-NPs do not adsorb. Identification of these specific anionic bonding sites on two different HNT morphologies highlights the importance of HNT morphology in their use as stable support materials in applications such as catalysis, anion sorption, and drug delivery, amongst others.

Using nanoparticulate TiO2 films, the photocatalytic growth of Ag nanoparticles (NPs) in the AgNO3 aqueous solution has been studied in terms of reduction, nucleation, and coalescence. It was proved that Ag primary particles were formed in a growth time of <1 s after the photocatalysis started. The growth dynamics was found to be critical for isotropic and anisotropic growth of Ag NPs, depending on the AgNO3 concentration and surface properties of TiO2 films. In the AgNO3 solutions of ≤300 mg/L, the isotropic growth dominates the growth dynamic behavior, producing irregularly spherical Ag NPs. In the AgNO3 solutions of ≥400 mg/L, the increased reduction rate promotes the formation of Ag nanoplates in the product. Ostwald ripening and oriented attachment were suggested to be the mechanisms dominating the isotropic and anisotropic growth, respectively. A photocatalytic growth model of Ag NPs was proposed by taking Ag atom and Ag+ ion diffusion into consideration. The plasmonic properties of the Ag–TiO2 films were studied in terms of extinction, surface enhanced Raman scattering, and fluorescence enhancement.

Ag–TiO2 hybrids are useful in various applications, such as photocatalysis, solar energy conversion, and biosensoring. In this study, oil-decorated TiO2 films were used to induce the formation of Ag nanoplates in AgNO3 solution via a photocatalytic method. Ag nanoplates in the products can be controlled by changing the oil-decoration time of films or changing the AgNO3 concentration of the solution. Oil decoration was found to be necessary in the formation of Ag nanoplates, and a critical concentration of AgNO3 solution was needed. The oil layer on the TiO2 films was demonstrated to play a role in the prevention of the reoxidation of the Ag atoms, and a growth model was proposed to interpret the formation of Ag nanoplates on the oil-decorated TiO2 films.