Sheng Peng

Facile Synthesis of Sunlight-Driven AgCl:Ag Plasmonic Nanophotocatalyst

Highly efficient plasmonic photocatalysts of AgCl:Ag hybrid nanoparticles are successfully synthesized via a one-pot synthetic approach involving a precipitation reaction followed by polyol reduction. The as-synthesized nanoparticles exhibit high catalytic performance under visible light and sunlight for decomposing organics, such as methylene blue.

Reversing the size-dependence of surface plasmon resonances.

The size-dependence of surface plasmon resonances (SPRs) is poorly understood in the small particle limit due to complex physical/chemical effects and uncertainties in experimental samples. In this article, we report an approach for synthesizing an ideal class of colloidal Ag nanoparticles with highly uniform morphologies and narrow size distributions. Optical measurements and theoretical analyses for particle diameters in the d ≈ 2–20 nm range are presented. The SPR absorption band exhibits an exceptional behavior: As size decreases from d ≈ 20 nm it blue-shifts but then turns over near d ≈ 12 nm and strongly red-shifts. A multilayer Mie theory model agrees well with the observations, indicating that lowered electron conductivity in the outermost atomic layer, due to chemical interactions, is the cause of the red-shift. We corroborate this picture by experimentally demonstrating precise chemical control of the SPR peak positions via ligand exchange.

Plasmonic/Magnetic Bifunctional Nanoparticles†

An amorphous seed-mediated strategy has been developed for the synthesis of hybrid nanoparticles that are composed of silver (yellow) and iron oxide (blue) nanodomains and exhibit unique optical properties. These properties originate from both the strong surface plasmon resonance of the silver and the strong superparamagnetic responses of the iron oxide nanodomains.

Synthesis and Characterization of Wurtzite ZnTe Nanorods with Controllable Aspect Ratios

ZnTe nanorods with controllable aspect ratios were synthesized using polytellurides a tellurium precursor. The use of polytellurides which allow nucleation and growth at relatively low temperature is the key to formation of wurtzite phase and controlled anisotropic growth along c-axis. The aspect ratio of the resulting ZnTe nanorods was controlled by tuning the temperature that in turn controls the kinetics of the nanocrystal growth. A diameter dependent quantum confinement effect in ZnTe nanorods was observed by UV-vis absorption spectroscopy. Transient absorption measurements show ultrafast charge injection dynamics from ZnTe nanorods, suggesting their strong potential for applications in photocatalysis.

Quantitative 3D evolution of colloidal nanoparticle oxidation in solution

Watching nanomaterials transform in time Real-time analysis of chemical transformations of nanoparticles is usually done with electron microscopy of a few particles. One limitation is interference by the electron beam. Sun et al. monitored the oxidation of iron nanoparticles in solution by using small- and wide-angle x-ray scattering and molecular dynamics simulations (see the Perspective by Cadavid and Cabot). These methods revealed the formation of voids within the nanoparticles, diffusion of material into and out of the nanoparticles, and ultimately the coalescence of the voids. Science, this issue p. 303; see also p. 245 Transformation of iron nanoparticles into hollow iron oxide structures through the Kirkendall effect is observed using x-rays. Real-time tracking of the three-dimensional (3D) evolution of colloidal nanoparticles in solution is essential for understanding complex mechanisms involved in nanoparticle growth and transformation. We used time-resolved small-angle and wide-angle x-ray scattering simultaneously to monitor oxidation of highly uniform colloidal iron nanoparticles, enabling the reconstruction of intermediate 3D morphologies of the nanoparticles with a spatial resolution of ~5 angstroms. The in situ observations, combined with large-scale reactive molecular dynamics simulations, reveal the details of the transformation from solid metal nanoparticles to hollow metal oxide nanoshells via a nanoscale Kirkendall process—for example, coalescence of voids as they grow and reversal of mass diffusion direction depending on crystallinity. Our results highlight the complex interplay between defect chemistry and defect dynamics in determining nanoparticle transformation and formation.

Surface chemistry: a non-negligible parameter in determining optical properties of small colloidal metal nanoparticles

Surface chemistry can become pronounced in determining the optical properties of colloidal metal nanoparticles as the nanoparticles become so small (diameters <20 nm) that the surface atoms, which can undergo chemical interactions with the environment, represent a significant fraction of the total number of atoms although this effect is often ignored. For instance, formation of chemical bonds between surface atoms of small metal nanoparticles and capping molecules that help stabilize the nanoparticles can reduce the density of conduction band electrons in the surface layer of metal atoms. This reduced electron density consequently influences the frequency-dependent dielectric constant of the metal atoms in the surface layer and, for sufficiently high surface to volume ratios, the overall surface plasmon resonance (SPR) absorption spectrum. The important role of surface chemistry is highlighted here by carefully analyzing the classical Mie theory and a multi-layer model is presented to produce more accurate predictions by considering the chemically reduced density of conduction band electrons in the outer shell of metal atoms in nanoparticles. Calculated absorption spectra of small Ag nanoparticles quantitatively agree with the experimental results for our monodispersed Ag nanoparticles synthesized via a well-defined chemical reduction process, revealing an exceptional size-dependence of absorption peak positions: the peaks first blue-shift followed by a turnover and a dramatic red-shift as the particle size decreases. A comprehensive understanding of the relationship between surface chemistry and optical properties is beneficial to exploit new applications of small colloidal metal nanoparticles, such as colorimetric sensing, electrochromic devices, and surface enhanced spectroscopies.

Ripening of bimodally distributed AgCl nanoparticles

Ripening of AgCl nanoparticles with a bimodal size distribution has been carefully studied in ethylene glycol containing poly(vinyl pyrrolidone) (PVP) as capping molecules and at elevated temperatures (e.g., 160 °C). The resulting AgCl particles exhibit high uniformity in size and cube-tetrapod morphology that are significantly different from the original AgCl nanoparticles. In addition, enhanced reducing ability of ethylene glycol at high temperature partially reduces AgCl to form Ag nanocrystalline domains in the AgCl particles, leading the AgCl particles to be efficiently absorbing visible light and to serve as a class of visible-light-driven photocatalysts due to the strong surface plasmon resonance (SPR) associated with the Ag nanocrystallites.

Reversible Modulation of Surface Plasmons in Gold Nanoparticles Enabled by Surface Redox Chemistry

Switchable surface redox chemistry is demonstrated in gold@iron/iron oxide core-shell nanoparticles with ambient oxidation and plasmon-mediated reduction to modulate the oxidation state of shell layers. The iron shell can be oxidized to iron oxide through ambient oxidation, leading to an enhancement and red-shift of the gold surface plasmon resonance (SPR). This enhanced gold SPR can drive reduction of the iron oxide shell under broadband illumination to reversibly blue-shift and significantly dampen gold SPR absorption. The observed phenomena provide a unique mechanism for controlling the plasmonic properties and surface chemistry of small metal nanoparticles.