27 research outputs found
Microstructure Effects on the Water Oxidation Activity of Co<sub>3</sub>O<sub>4</sub>/Porous Silica Nanocomposites
We
investigate the effect of microstructuring on the water oxidation
(oxygen evolution) activity of two types of Co<sub>3</sub>O<sub>4</sub>/porous silica composites: Co<sub>3</sub>O<sub>4</sub>/porous SiO<sub>2</sub> core/shell nanoparticles with varying shell thicknesses and
surface areas, and Co<sub>3</sub>O<sub>4</sub>/mesoporous silica nanocomposites
with various surface functionalities. Catalytic tests in the presence
of RuĀ(bpy)<sub>3</sub><sup>2+</sup> as a photosensitizer and S<sub>2</sub>O<sub>8</sub><sup>2ā</sup> as a sacrificial electron
acceptor show that porous silica shells of up to ~20 nm in thickness
lead to increased water oxidation activity. We attribute this effect
to either (1) a combination of an effective increase in catalyst active
area or consequent higher local concentration of RuĀ(bpy)<sub>3</sub><sup>2+</sup>; (2) a decrease in the permittivity of the medium surrounding
the catalyst surface and a consequent increase in the rate of charge
transfer; or both. Functionalized Co<sub>3</sub>O<sub>4</sub>/mesoporous
silica nanocomposites show lower water oxidation activity compared
with the parent nonfunctionalized catalyst, likely because of partial
pore blocking of the silica support upon surface grafting. A more
thorough understanding of the effects of microstructure and permittivity
on water oxidation ability will enable the construction of next generation
catalysts possessing optimal configuration and better efficiency for
water splitting
Solution-Grown Sodium Bismuth Dichalcogenides: Toward Earth-Abundant, Biocompatible Semiconductors
Many
technologically relevant semiconductors contain toxic, heavily regulated
(Cd, Pb, As), or relatively scarce (Li, In) elements and often require
high manufacturing costs. We report a facile, general, low-temperature,
and size tunable (4ā28 nm) solution phase synthesis of ternary
APnE<sub>2</sub> semiconductors based on Earth-abundant and biocompatible
elements (A = Na, Pn = Bi, E = S or Se). The observed experimental
band gaps (1.20ā1.45 eV) fall within the ideal range for solar
cells. Computational investigation of the lowest energy superstructures
that result from ācoloringā, caused by mixed cation
sites present in their rock salt lattice, agrees with other better-known
members of this family of materials. Our synthesis unlocks a new class
of low cost and environmentally friendly ternary semiconductors that
show properties of interest for applications in energy conversion
How Robust are Semiconductor Nanorods? Investigating the Stability and Chemical Decomposition Pathways of Photoactive Nanocrystals
Anisotropic
IIāVI semiconductor nanostructures are important
photoactive materials for various energy conversion and optical applications.
However, aside from the many available surface chemistry studies and
from their ubiquitous photodegradation under continuous illumination,
the general chemical reactivity and thermal stability (phase and shape
transformations) of these materials are poorly understood. Using CdSe and CdS nanorods as model systems, we have investigated the
behavior of IIāVI semiconductor nanorods against various conditions
of extreme chemical and physical stress (acids, bases, oxidants, reductants,
and heat). CdSe nanorods react rapidly with acids, becoming oxidized
to Se or SeO<sub>2</sub>. In contrast, CdSe nanorods remain mostly
unreactive when treated with bases or strong oxidants, although bases
do partially etch the tips of the nanorods (along their axis). Roasting
(heating in air) of CdSe nanorods results in rock-salt CdO, but neither
CdSe nor CdO is easily reduced by hydrogen (H<sub>2</sub>). Another
reductant, <i>n</i>-BuLi, reduces CdSe nanorods to metallic
Cd. Variable temperature X-ray diffraction experiments show that axial
annealing and selective axial melting of the nanorods precede particle
coalescence. Furthermore, thermal analysis shows that the axial melting
of IIāVI nanorods is a ligand-dependent process. In agreement
with chemical reactivity and thermal stability observations, silica-coating
experiments show that the sharpest (most curved) IIāVI surfaces
are most active against heterogeneous nucleation of a silica shell.
These results provide valuable insights into the fate and possible
ways to enhance the stability and improve the use of IIāVI
semiconductor nanostructures in the fields of optics, magnetism, and
energy conversion
Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/IIāVI Semiconductor Epitaxy
Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character <i>via</i> strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent IIāVI, IIIāV and IVāVI semiconductor quantum dots. Here, we use relatively unexplored IV/IIāVI epitaxy as a way to enhance the photoluminescence and improve the optical stability of colloidal Ge nanocrystals. Selected on the basis of their relatively small lattice mismatch compared with crystalline Ge, we explore the growth of epitaxial CdS and ZnS shells using the successive ion layer adsorption and reaction method. Powder X-ray diffraction and electron microscopy techniques, including energy dispersive X-ray spectroscopy and selected area electron diffraction, clearly show the controllable growth of as many as 20 epitaxial monolayers of CdS atop Ge cores. In contrast, Ge etching and/or replacement by ZnS result in relatively small Ge/ZnS nanocrystals. The presence of an epitaxial IIāVI shell greatly enhances the near-infrared photoluminescence and improves the photoluminescence stability of Ge. Ge/IIāVI nanocrystals are reproducibly 1ā3 orders of magnitude brighter than the brightest Ge cores. Ge/4.9CdS core/shells show the highest photoluminescence quantum yield and longest radiative recombination lifetime. Thiol ligand exchange easily results in near-infrared active, water-soluble Ge/IIāVI nanocrystals. We expect this synthetic IV/IIāVI epitaxial approach will lead to further studies into the optoelectronic behavior and practical applications of Si and Ge-based nanomaterials
Shape-Programmed Nanofabrication: Understanding the Reactivity of Dichalcogenide Precursors
Dialkyl and diaryl dichalcogenides are highly versatile and modular precursors for the synthesis of colloidal chalcogenide nanocrystals. We have used a series of commercially available dichalcogenide precursors to unveil the molecular basis for the outcome of nanocrystal preparations, more specifically, how precursor molecular structure and reactivity affect the final shape and size of IIāVI semiconductor nanocrystals. Dichalcogenide precursors used were diallyl, dibenzyl, di-<i>tert</i>-butyl, diisopropyl, diethyl, dimethyl, and diphenyl disulfides and diethyl, dimethyl, and diphenyl diselenides. We find that the presence of two distinctively reactive CāE and EāE bonds makes the chemistry of these precursors much richer and interesting than that of other conventional precursors such as the more common phosphine chalcogenides. Computational studies (DFT) reveal that the dissociation energy of carbonāchalcogen (CāE) bonds in dichalcogenide precursors (RāEāEāR, E = S or Se) increases in the order (R): diallyl < dibenzyl < di-<i>tert</i>-butyl < diisopropyl < diethyl < dimethyl < diphenyl. The dissociation energy of chalcogenāchalcogen (EāE) bonds remains relatively constant across the series. The only exceptions are diphenyl dichalcogenides, which have a much lower EāE bond dissociation energy. An increase in CāE bond dissociation energy results in a decrease in RāEāEāR precursor reactivity, leading to progressively slower nucleation and higher selectivity for anisotropic growth, all the way from dots to pods to tetrapods. Under identical experimental conditions, we obtain CdS and CdSe nanocrystals with spherical, elongated, or tetrapodal morphology by simply varying the identity and reactivity of the dichalcogenide precursor. Interestingly, we find that precursors with strong CāE and weak EāE bond dissociation energies such as PhāSāSāPh serve as a ready source of thiol radicals that appear to stabilize small CdE nuclei, facilitating anisotropic growth. These CdS and CdSe nanocrystals have been characterized using structural and spectroscopic methods. An intimate understanding of how molecular structure affects the chemical reactivity of molecular precursors enables highly predictable and reproducible synthesis of colloidal nanocrystals with specific sizes, shapes, and optoelectronic properties for customized applications
High-Precision Tracking with Non-blinking Quantum Dots Resolves Nanoscale Vertical Displacement
Novel non-blinking quantum dots (NBQDs) were utilized
in three-dimensional
super-localization, high-precision tracking applications under an
automated scanning-angle total internal reflection fluorescence microscope
(SA-TIRFM). NBQDs were randomly attached to stationary microtubules
along the radial axis under gliding assay conditions. By automatically
scanning through a wide range of incident angles with different evanescent-field
layer thicknesses, the fluorescence intensity decay curves were obtained.
Fit with theoretical decay functions, the absolute vertical positions
were determined with sub-10-nm localization precision. The emission
intensity profile of the NBQDs attached to kinesin-propelled microtubules
was used to resolve the self-rotation of gliding microtubules within
a small vertical distance of ā¼50 nm. We demonstrate the applicability
of NBQDs in high-precision fluorescence imaging experiments
High-Precision Tracking with Non-blinking Quantum Dots Resolves Nanoscale Vertical Displacement
Novel non-blinking quantum dots (NBQDs) were utilized
in three-dimensional
super-localization, high-precision tracking applications under an
automated scanning-angle total internal reflection fluorescence microscope
(SA-TIRFM). NBQDs were randomly attached to stationary microtubules
along the radial axis under gliding assay conditions. By automatically
scanning through a wide range of incident angles with different evanescent-field
layer thicknesses, the fluorescence intensity decay curves were obtained.
Fit with theoretical decay functions, the absolute vertical positions
were determined with sub-10-nm localization precision. The emission
intensity profile of the NBQDs attached to kinesin-propelled microtubules
was used to resolve the self-rotation of gliding microtubules within
a small vertical distance of ā¼50 nm. We demonstrate the applicability
of NBQDs in high-precision fluorescence imaging experiments
Surface Doping Quantum Dots with Chemically Active Native Ligands: Controlling Valence without Ligand Exchange
One remaining challenge in the field of colloidal semiconductor
nanocrystal quantum dots is learning to control the degree of functionalization
or āvalenceā per nanocrystal. Current quantum dot surface
modification strategies rely heavily on ligand exchange, which consists
of replacing the nanocrystalās native ligands with carboxylate-
or amine-terminated thiols, usually added in excess. Removing the
nanocrystalās native ligands can cause etching and introduce
surface defects, thus affecting the nanocrystalās optical properties.
More importantly, ligand exchange methods fail to control the extent
of surface modification or number of functional groups introduced
per nanocrystal. Here, we report a fundamentally new surface ligand
modification or ādopingā approach aimed at controlling
the degree of functionalization or valence per nanocrystal while retaining
the nanocrystalās original colloidal and photostability. We
show that surface-doped quantum dots capped with chemically active
native ligands can be prepared directly from a mixture of ligands
with similar chain lengths. Specifically, vinyl and azide-terminated
carboxylic acid ligands survive the high temperatures needed for nanocrystal
synthesis. The ratio between chemically active and inactive-terminated
ligands is maintained on the nanocrystal surface, allowing to control
the extent of surface modification by straightforward organic reactions.
Using a combination of optical and structural characterization tools,
including IR and 2D NMR, we show that carboxylates bind in a bidentate
chelate fashion, forming a single monolayer of ligands that are perpendicular
to the nanocrystal surface. Moreover, we show that mixtures of ligands
with similar chain lengths homogeneously distribute themselves on
the nanocrystal surface. We expect this new surface doping approach
will be widely applicable to other nanocrystal compositions and morphologies,
as well as to many specific applications in biology and materials
science
Selective Alcohol Dehydrogenation and Hydrogenolysis with Semiconductor-Metal Photocatalysts: Toward Solar-to-Chemical Energy Conversion of Biomass-Relevant Substrates
Photocatalytic conversion of biomass is a potentially
transformative
concept in renewable energy. Dehydrogenation and hydrogenolysis of
biomass-derived alcohols can produce renewable fuels such as H<sub>2</sub> and hydrocarbons, respectively. We have successfully used
semiconductor-metal heterostructures for sunlight-driven dehydrogenation
and hydrogenolysis of benzyl alcohol. The heterostructure composition
dictates activity, product distribution, and turnovers. A few metal
(M = Pt, Pd) islands on the semiconductor (SC) surface significantly
enhance activity and selectivity and also greatly stabilize the SC
against photoinduced etching and degradation. Under selected conditions,
CdS-Pt favors dehydrogenation (H<sub>2</sub>) over hydrogenolysis
(toluene) 8:1, whereas CdS<sub>0.4</sub>Se<sub>0.6</sub>-Pd favors
hydrogenolysis over dehydrogenation 3:1. Photochemically generated,
surface-adsorbed hydrogen is useful in tandem catalysis, for example,
via transfer hydrogenation. We expect this work will lead to new paradigms
for sunlight-driven conversions of biomass-relevant substrates
High-Precision Tracking with Non-blinking Quantum Dots Resolves Nanoscale Vertical Displacement
Novel non-blinking quantum dots (NBQDs) were utilized
in three-dimensional
super-localization, high-precision tracking applications under an
automated scanning-angle total internal reflection fluorescence microscope
(SA-TIRFM). NBQDs were randomly attached to stationary microtubules
along the radial axis under gliding assay conditions. By automatically
scanning through a wide range of incident angles with different evanescent-field
layer thicknesses, the fluorescence intensity decay curves were obtained.
Fit with theoretical decay functions, the absolute vertical positions
were determined with sub-10-nm localization precision. The emission
intensity profile of the NBQDs attached to kinesin-propelled microtubules
was used to resolve the self-rotation of gliding microtubules within
a small vertical distance of ā¼50 nm. We demonstrate the applicability
of NBQDs in high-precision fluorescence imaging experiments