Metal nanoparticles of multi-principal element alloys (MPEA) with a single crystalline phase have been synthesized by flash heating/cooling of nanosized metals encapsulated in micelle vesicles dispersed in an oil phase (e.g., cyclohexane). Flash heating is realized by selective absorption of a microwave pulse in metals to rapidly heat metals into uniform melts. The oil phase barely absorbs microwave and maintains the low temperature, which can rapidly quench the high-temperature metal melts to enable the flash cooling process. The precursor ions of four metals, including Au, Pt, Pd, and Cu, can be simultaneously reduced by hydrazine in the aqueous solution encapsulated in the micelle vesicles. The resulting metals efficiently absorb microwave energy to locally reach a temperature high enough to melt themselves into a uniform mixture. The duration of microwave pulse is crucial to ensure the reduced metals mix uniformly, while the temperature of oil phase is still low to rapidly quench the metals and freeze the single-phase crystalline lattices in alloy nanoparticles. The microwave-enabled flash heating/cooling provides a new method to synthesize single-phase MPEA nanoparticles of many metal combinations when the appropriate water-in-oil micelle systems and the appropriate reduction reactions of metal precursors are available.

Nanoparticles of refractory compounds represent a class of stable materials showing a great promise to support localized surface plasmon resonances (LSPRs) in both visible and near infrared (NIR) spectral regions. It is still challenging to rationally tune the LSPR band because of the difficulty to control the density of charge carriers in individual refractory nanoparticles and maintain the dispersity of nanoparticles in the processes of synthesis and applications. In this work, controlled chemical transformation of titanium dioxide (TiO₂) nanoparticles encapsulated with mesoporous silica (SiO₂) shells to titanium nitride (TiN) via nitridation reaction at elevated temperatures is developed to tune the density of free electrons in the resulting titanium-oxide-nitride (TiO_xN_y) nanoparticles. Such tunability enables a flexibility to support LSPR-based optical absorption in the synthesized TiO_xN_y@SiO₂ core-shell nanoparticles across both the visible and NIR regions. The silica shells play a crucial role in preventing the sintering of TiO_xN_y nanoparticles in the nitridation reaction and maintaining the stability of TiO_xN_y nanoparticles in applications. The LSPR-based broadband absorption of light in the TiO_xN_y@SiO₂ nanoparticles exhibits strong photothermal effect with photo-to-thermal conversion efficiency as high as ~ 76%.

A strategy has been developed for analyzing growth kinetics of colloidal metal nanoparticle quantitatively by focusing both the very early and the very late growth stages, at which the size of growing nanoparticles and the reaction time follow linear functions. Applying this extreme-condition model to a microwave-assistant synthesis of colloidal silver nanoparticles, for the first time, results in the determination of intrinsic kinetics parameters involving in the growth of the silver nanoparticles. The diffusion coefficient (D) of the precursor species containing Ag⁺ is 4.9 × 10^–14 m²/s and the surface reaction rate constant (k) of the precursor species on the surface of the growing silver nanoparticles is 8.7 × 10^–8 m/s in an ethylene glycol solution containing 0.15 M polyvinylpyrrolidone at 140 ℃. The extreme-condition model is ready to deconvolute the intrinsic kinetics parameters of growing colloidal nanoparticles once the enlargement rate of the nanoparticles can be experimentally measured in real time and with high temporal resolution. Availability of the high-fidelity values of k and D will provide the crucial information to guide the design and synthesis of colloidal metal nanoparticles with the desirable properties.

The optical absorption of semiconducting AgBr nanocubes is significantly increased by up to 5 times in the measured spectral range when they are bonded to the surface of dielectric SiO₂ nanospheres through electrostatic interaction. The absorption enhancement factor depends on the wavelength and the size of the SiO₂ nanoparticles (NPs). Finite-difference time-domain calculations provide the nearfield intensity mapping of a heterostructure that is composed of a AgBr nanocube in close contact with a SiO₂ nanosphere. The electric-field distributions indicate the field enhancement near the SiO₂/AgBr interface due to light scattering and absorption enhancement in the AgBr nanocube, implying that the enhanced scattering nearfield increases the absorption cross section of the AgBr nanocube. The absorption cross-section spectra calculated using Mie theory agree with the experimental observations. This discovery sheds light on the utilization of dielectric spherical particles to increase the absorption in semiconductor NPs, thus improving the light-harvesting efficiency for solar-energy conversion.

The design of efficient artificial photosynthetic systems that harvest solar energy to drive the hydrogen evolution reaction via water reduction is of great importance from both the theoretical and practical viewpoints. Integrating appropriate co-catalyst promoters with strong light absorbing materials represents an ideal strategy to enhance the conversion efficiency of solar energy in hydrogen production. Herein, we report, for the first time, the synthesis of a class of unique hybrid structures consisting of ultrathin Co(Ni)-doped MoS₂ nanosheets (co-catalyst promoter) intimately grown on semiconductor CdS nanorods (light absorber). The as-synthesized one-dimensional CdS@doped-MoS₂ heterostructures exhibited very high photocatalytic activity (with a quantum yield of 17.3%) and stability towards H₂ evolution from the photoreduction of water. Theoretical calculations revealed that Ni doping can increase the number of uncoordinated atoms at the edge sites of MoS₂ nanosheets to promote electron transfer across the CdS/MoS₂ interfaces as well as hydrogen reduction, leading to an efficient H₂ evolution reaction.