Engineering unique architectures at the nanoparticle and colloidal scales represents a promising strategy for harnessing physicochemical interparticle interactions, particularly to enhance near-field light focusing. Although electric fields tend to concentrate at regions of high curvature, such as sharp tips, the presence of the latter features alone does not substantially strengthen the near-field enhancement. Instead, directly assembling two sharp tips in a tip-to-tip configuration represents an effective way to maximize near-field focusing by generating highly localized electromagnetic "hot spots". To achieve this goal, we introduce an innovative approach for obtaining a tip-to-tip assembly of octahedral nanoparticles. This strategy involves encapsulating solid octahedral nanoparticles within cubic shells, serving as structural building blocks, to form point contacts between the flat surfaces of the cubic shell and the sharp tips of the octahedron. By arranging these distinctive structures in a serial configuration, we achieve a controlled tip-to-tip alignment. Within this architecture, the inner tips induce charge concentration on the flat planes, while the serial arrangement further enhances near-field focusing across adjacent building blocks. This configuration exhibits distinct near-field characteristics compared to assemblies composed of simple solid cubes or isolated octahedral nanoparticles, thus providing a novel strategy for optimizing near-field interactions in nanoscale systems.
Titanium (Ti) alloys have low thermal conductivity, suffer from tool wear and deformation of workpieces and are difficult-to-machine metals. This contributes to surface roughness, Sa > 240 nm of Ti alloys after mechanical polishing with a low material removal rate (MRR). With the addition of assisting energy fields, the MRR is usually lower than 7 μm h-1. Nevertheless, there is a high demand to achieve Sa < 50 nm on a free surface blade to save energy and reduce the resistance of fluids. To address this challenge, novel photocatalytic shear-thickening chemical mechanical polishing (PSTCMP) was developed using a custom-made polisher. The new PSTCMP slurry contained ceria, corn starch, sodium bicarbonate and deionized water. After PSTCMP, the Sa and thickness of the damaged layer of a free surface blade of a Ti alloy decreased from 501.71 to 38.46 nm and from 634.79 to 7.83 nm, respectively, representing reductions of 92% and 99%. The MRR is 12.52 μm h-1. To the best of our knowledge, both the Sa and MRR are the best published to date for a Ti alloy blade with a free surface. PSTCMP mechanisms were interpreted using first-principles molecular dynamics, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Hydroxyl radicals were generated under ultraviolet irradiation on ceria with a size of 4.2 nm, oxidizing the surface of the Ti alloy and forming Ti-OH and Ti-O groups. A Ce-O-Ti interface bridge was produced between Ti-OH and Ce-OH, induced by the hydrolysis of ceria. Our findings provide a new way to fabricate nanometer-scale surface roughness on a free surface blade of a Ti alloy with a high MRR.
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