Accurate Measurement of the Molecular Thickness of Thin Organic Shells on Small Inorganic Cores Using Dynamic Light Scattering

Dynamic light scattering (DLS) has become a primary nanoparticle characterization technique with applications from material characterization to biological and environmental detection. With the expansion in DLS use from homogeneous spheres to more complicated nanostructures comes a decrease in accuracy. Much research has been performed to develop different diffusion models that account for the vastly different structures, but little attention has been given to the effect on the light scattering properties in relation to DLS. In this work, small (core size < 5 nm) core–shell nanoparticles were used as a case study to measure the capping thickness of a layer of dodecanethiol (DDT) on Au and ZnO nanoparticles by DLS. We find that the DDT shell has very little effect on the scattering properties of the inorganic core and, hence, can be ignored to a first approximation. However, this results in conventional DLS analysis overestimating the hydrodynamic size in the volume- and number-weighted distributions. With the introduction of a simple correction formula that more accurately yields hydrodynamic size distributions, a more precise determination of the molecular shell thickness is obtained. With this correction, the measured thickness of the DDT shell was found to be 7.3 ± 0.3 Å, much less than the extended chain length of 16 Å. This organic layer thickness suggests that, on small nanoparticles, the DDT monolayer adopts a compact disordered structure rather than an open ordered structure on both ZnO and Au nanoparticle surfaces. These observations are in agreement with published molecular dynamics results

Photocatalysis with Pt–Au–ZnO and Au–ZnO Hybrids: Effect of Charge Accumulation and Discharge Properties of Metal Nanoparticles

Metal–semiconductor hybrid nanomaterials are becoming increasingly popular for photocatalytic degradation of organic pollutants. Herein, a seed-assisted photodeposition approach is put forward for the site-specific growth of Pt on Au–ZnO particles (Pt–Au–ZnO). A similar approach was also utilized to enlarge the Au nanoparticles at epitaxial Au–ZnO particles (Au@Au–ZnO). An epitaxial connection at the Au–ZnO interface was found to be critical for the site-specific deposition of Pt or Au. Light on–off photocatalysis tests, utilizing a thiazine dye (toluidine blue) as a model organic compound, were conducted and confirmed the superior photodegradation properties of Pt–Au–ZnO hybrids compared to Au–ZnO. In contrast, Au–ZnO type hybrids were more effective toward photoreduction of toluidine blue to leuco-toluidine blue. It was deemed that photoexcited electrons of Au–ZnO (Au, ∼5 nm) possessed high reducing power owing to electron accumulation and negative shift in Fermi level/redox potential; however, exciton recombination due to possible Fermi-level equilibration slowed down the complete degradation of toluidine blue. In the case of Au@Au–ZnO (Au, ∼15 nm), the photodegradation efficiency was enhanced and the photoreduction rate reduced compared to Au–ZnO. Pt–Au–ZnO hybrids showed better photodegradation and mineralization properties compared to both Au–ZnO and Au@Au–ZnO owing to a fast electron discharge (i.e. better electron-hole seperation). However, photoexcited electrons lacked the reducing power for the photoreduction of toluidine blue. The ultimate photodegradation efficiencies of Pt–Au–ZnO, Au@Au–ZnO, and Au–ZnO were 84, 66, and 39%, respectively. In the interest of effective metal–semiconductor type photocatalysts, the present study points out the importance of choosing the right metal, depending on whether a photoreduction and/or photodegradation process is desired