Size-Dependent Phononic Properties of PdO Nanocrystals Probed by Nanoscale Optical Thermometry

With the advent of novel nanoscale devices, fast and reliable thermal mapping with high spatiotemporal resolution is imperative for probing the characteristics of phonons and evaluating the local temperature at the nanoscale. In this work, Raman spectroscopy is employed as a rapid and noncontact optical thermometry technique to investigate phononic properties of macroscopic assemblies of monodisperse palladium oxide (PdO) nanocrystals. PdO has been extensively employed in high temperature catalytic devices; however, the phonon behavior which determines the thermal stability of PdO remains unexplored thus far. Our study focuses on homogeneous, large-scale assemblies of monodisperse 4 and 10 nm nanocrystals synthesized using colloidal chemistry to understand size-dependent effects on the measured thermal properties. By monitoring the Raman peak shifts, peak broadening, and alterations in peak intensities as a function of laser power and particle concentration, a size-dependent trend is observed attributable to confinement of optical phonons within nanocrystal grain boundaries and laser-induced heating, both influenced by nanocrystal size. This study correlates size-dependent single-particle heating effects with size-dependent interparticle heat transfer under laser irradiation and is enabled by controlled nanocrystal synthesis

Engineered Porous Silicon Counter Electrodes for High Efficiency Dye-Sensitized Solar Cells

In this work, we demonstrate for the first time, the use of porous silicon (P-Si) as counter electrodes in dye-sensitized solar cells (DSSCs) with efficiencies (5.38%) comparable to that achieved with platinum counter electrodes (5.80%). To activate the P-Si for triiodide reduction, few layer carbon passivation is utilized to enable electrochemical stability of the silicon surface. Our results suggest porous silicon as a promising sustainable and manufacturable alternative to rare metals for electrochemical solar cells, following appropriate surface modification

Morphology-Directed Catalysis with Branched Gold Nanoantennas

We synthesized multibranched gold nanoantennas (MGNs) of two morphologies by varying the core-to-branch ratio. We compared their efficacy in catalytic reduction of <i>p</i>-nitrophenol (PNP) to <i>p</i>-aminiphenol (PAP). We observed that MGNs with shorter protrusions had a faster induction time and higher apparent rate constant, <i>k</i>_app, for PNP catalysis relative to the MGNs with longer protrusions. By examining the reaction as a function of temperature, we observed significantly lower activation energy for the MGNs with shorter protrusions (80 J/g) compared to MGNs with longer protrusions (200 J/g). The Langmuir–Hinshelwood model was used to fit the change in <i>k</i>_app as a function of increasing [PNP], which demonstrated more efficient PNP adsorption on the surfaces of MGNs with shorter protrusions. For the MGNs with longer protrusions, PNP adsorption is affected by the heterogeneity of the surface sites resulting in a lower adsorption coefficient. We attributed the improved efficiency of the MGNs with shorter protrusions to the presence of {100} and {110} crystal planes, which have a high density of atomic steps and kinks that promote higher catalytic activity for PNP degradation. MGNs with long protrusions are bound by low index {111} facets; the highly coordinated atoms of {111} reduce the adsorption efficiency of PNP

Solution Assembled Single-Walled Carbon Nanotube Foams: Superior Performance in Supercapacitors, Lithium-Ion, and Lithium–Air Batteries

We demonstrate a surfactant-free, solution processing route to form three-dimensional freestanding foams of pristine single-walled carbon nanotubes (SWCNTs) and explore the diverse electrochemical energy storage applications of these materials. This route utilizes SWCNT dispersions in organic <i>n</i>-methylpyrrolidone solvents and subsequent electrophoretic assembly onto a metal foam sacrificial template which can be dissolved to yield surfactant-free, binder-free freestanding SWCNT foams. We further provide a comparison between surfactant-free foams and conventional surfactant-based solvent processing routes and assess performance of these foams in supercapacitors, lithium-ion batteries, and lithium–air batteries. For pristine SWCNT foams, we measure up to 83 F/g specific capacitance in supercapacitors, specific capacity up to 2210 mAh/g for lithium-ion batteries with up to 50% energy efficiency, and specific discharge capacity up to 8275 mAh/g in lithium–air batteries. For lithium–air batteries, this corresponds to a total energy density of 21.2 and 3.3 kWh/kg for the active mass and total battery device, respectively, approaching the 12.7 kWh/kg target energy density of gasoline. In comparison, SWCNT foams prepared with surfactant exhibit poorer gravimetric behavior in all devices and compromised Faradaic storage that leads to depreciated amounts of usable, stored energy. This work demonstrates the broad promise of SWCNTs as lightweight and highly efficient energy storage materials but also emphasizes the importance of clean nanomanufacturing routes which are critical to achieve optimized performance with nanostructures

Geometry-Dependent Plasmonic Tunability and Photothermal Characteristics of Multibranched Gold Nanoantennas

Plasmon resonances of anisotropic multibranched nanostructures are governed by their geometry, allowing morphology-directed selective manipulation of the optical properties. In this work, we have synthesized multibranched gold nanoantennas (MGNs) of variable geometry by a one-step seedless approach using 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) as a capping and reducing agent. This approach enables us to modulate the MGNs’ geometry by controlling three different parameters: concentration of HEPES, concentration of Au³⁺, and pH of HEPES buffer. By altering the MGNs morphology with minimal increase in the overall dimensions, the plasmon resonances were tuned from the visible to the near-infrared. The MGNs plasmon resonances demonstrated a nonintuitive blue-shift when pH > p<i>K</i>_a of HEPES which we attributed to emergence of charge transfer oscillations formed when MGNs cluster to dimers and trimers. Further, due to the presence of multiple sharp protrusions, the MGNs demonstrated a refractive index sensitivity of 373 nm/RIU, which is relatively high for this class of branched nanostructures of similar size. Finally, the sharp protrusions of MGNs also give rise to intense photothermal efficiencies; ∼53 °C was achieved within 5 min of laser illumination, demonstrating the efficacy of MGNs in therapeutic applications. By modulating the mass density of MGNs, the laser flux, and time of illumination, we provide a detailed analysis of the photothermal characteristics of MGNs

Ultrafast Spectral Dynamics of CsPb(Br_<i>x</i>Cl_1–<i>x</i>)₃ Mixed-Halide Nanocrystals

In this work we investigated the spectral dynamics of cesium lead mixed-halide, CsPb­(Br_<i>x</i>Cl_1–<i>x</i>)₃ perovskite nanocrystals probed with complementary spectral techniques: time-resolved photoluminescence and transient absorption spectroscopy. Mixed-halide perovskite nanocrystals were synthesized via a hot-injection method followed by anion exchange reactions. Our results show that increased Cl content in perovskite nanocrystals (<i>a</i>) diminished the photoluminescence quantum yield and gave rise to rapid radiative recombination of carriers; (<i>b</i>) resulted in rapid thermalization of hot carriers and low carrier temperatures, which suggests weaker hot-phonon bottleneck and Burstein–Moss effects; (<i>c</i>) decreased the bandgap renormalization energy, which suggests high exciton binding energy and poor charge extraction in Cl substituted perovskite nanocrystals; and (<i>d</i>) increased the number of carriers undergoing Auger losses, where Auger processes dominate over trap-assisted recombination. These findings provide a generalized framework to guide researchers as to when mixed-halide perovskite nanocrystals would be useful for optoelectronic technologies and when they would be detrimental to device performance