Michael H. Huang

Surface Plasmonic Effects of Metallic Nanoparticles on the Performance of Polymer Bulk Heterojunction Solar Cells

We have systematically explored how plasmonic effects influence the characteristics of polymer photovoltaic devices (OPVs) incorporating a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). We blended gold nanoparticles (Au NPs) into the anodic buffer layer to trigger localized surface plasmon resonance (LSPR), which enhanced the performance of the OPVs without dramatically sacrificing their electrical properties. Steady state photoluminescence (PL) measurements revealed a significant increase in fluorescence intensity, which we attribute to the increased light absorption in P3HT induced by the LSPR. As a result, the rate of generation of excitons was enhanced significantly. Furthermore, dynamic PL measurements revealed that the LSPR notably reduced the lifetime of photogenerated excitons in the active blend, suggesting that interplay between the surface plasmons and excitons facilitated the charge transfer process. This phenomenon reduced the recombination level of geminate excitons and, thereby, increased the probability of exciton dissociation. Accordingly, both the photocurrents and fill factors of the OPV devices were enhanced significantly. The primary origin of this improved performance was local enhancement of the electromagnetic field surrounding the Au NPs. The power conversion efficiency of the OPV device incorporating the Au NPs improved to 4.24% from a value of 3.57% for the device fabricated without Au NPs.

Synthesis of Cu2O Nanocrystals from Cubic to Rhombic Dodecahedral Structures and Their Comparative Photocatalytic Activity

In this study, a new series of Cu(2)O nanocrystals with systematic shape evolution from cubic to face-raised cubic, edge- and corner-truncated octahedral, all-corner-truncated rhombic dodecahedral, {100}-truncated rhombic dodecahedral, and rhombic dodecahedral structures have been synthesized. The average sizes for the cubes, edge- and corner-truncated octahedra, {100}-truncated rhombic dodecahedra, and rhombic dodecahedra are approximately 200, 140, 270, and 290 nm, respectively. An aqueous mixture of CuCl(2), sodium dodecyl sulfate, NaOH, and NH(2)OH·HCl was prepared to produce these nanocrystals at room temperature. Simple adjustment of the amounts of NH(2)OH·HCl introduced enables this particle shape evolution. These novel particle morphologies have been carefully analyzed by transmission electron microscopy (TEM). The solution color changes quickly from blue to green, yellow, and then orange within 1 min of reaction in the formation of nanocubes, while such color change takes 10-20 min in the growth of rhombic dodecahedra. TEM examination confirmed the rapid production of nanocubes and a substantially slower growth rate for the rhombic dodecahedra. The rhombic dodecahedra exposing only the {110} facets exhibit an exceptionally good photocatalytic activity toward the fast and complete photodegradation of methyl orange due to a high number density of surface copper atoms, demonstrating the importance of their successful preparation. They may serve as effective and cheap catalysts for other photocatalytic reactions and organic coupling reactions.

Au Nanocube-Directed Fabrication of Au−Pd Core−Shell Nanocrystals with Tetrahexahedral, Concave Octahedral, and Octahedral Structures and Their Electrocatalytic Activity

In this study, we have successfully developed a facile method for the high-yield fabrication of Au-Pd core-shell heterostructures with an unusual tetrahexahedral (THH) morphology using Au nanocubes as the structure-directing cores. The lattice orientations of the Au nanocubes match those of the Pd shells. Structural analysis establishes that the THH nanocrystals are bounded by high-index {730} facets. A substantial lattice mismatch between Au and Pd, oxidative etching in the presence of chloride and oxygen, the use of cetyltrimethylammonium chloride (CTAC) surfactant, and the reaction temperature (30-60 °C) were identified to be key factors facilitating the formation of the THH core-shell nanocrystals. Intermediate products have also been examined to follow the growth process. By selecting cubic gold cores with sizes of 30-70 nm and varying the volume of the gold core solution used, THH Au-Pd core-shell nanocrystals with continuously adjustable sizes from 56 to 124 nm can be readily obtained. Their UV-vis spectra display progressive red-shifted bands. Interestingly, novel concave octahedral and octahedral Au-Pd core-shell nanocrystals can be prepared by lowering the reaction temperature and prolonging the reaction time. The concave octahedra show depressions on all the {111} faces. Electrocatalytic activity of the three Au-Pd core-shell structures for the oxidation of ethanol has been investigated. The THH nanocrystals with entirely high-index {730} facets were found to exhibit the best electrocatalytic activity. These size-tunable THH Au-Pd core-shell nanocrystals may be valuable for catalyzing other organic reactions.

Plasmonic-enhanced polymer photovoltaic devices incorporating solution-processable metal nanoparticles

We have explored the effect of gold nanoparticle (Au NP)-induced surface plasmons on the performance of organic photovoltaic devices (OPVs). The power conversion efficiency of these OPVs was improved after blending the Au NPs into the anodic buffer layer. The addition of Au NPs increased the rate of exciton generation and the probability of exciton dissociation, thereby enhancing the short-circuit current density and the fill factor. We attribute the improvement in device performance to the local enhancement in the electromagnetic field originating from the excitation of the localized surface plasmon resonance.

Fabrication of Truncated Rhombic Dodecahedral Cu2O Nanocages and Nanoframes by Particle Aggregation and Acidic Etching

We report a simple approach for the fabrication of cuprous oxide (Cu2O) nanocages and nanoframes. An aqueous solution of CuCl2, sodium dodecyl sulfate (SDS) surfactant, NH2OH·HCl reductant, HCl, and NaOH was prepared with reagents introduced in the order listed. Rapid seed particle aggregation and surface reconstruction of the intermediate structures resulted in the growth of type I nanoframes with just the {110} skeleton faces and empty {100} faces 45 minutes after mixing the reagents. Continued crystal growth for additional 75 min produced the nanocages with filled {100} faces. The nanocages have diameters of 350–400 nm, and their walls are thicker than those of the nanoframes. Selective acidic etching over the {110} faces of the nanocages by HCl via the addition of ethanol and then sonication of the solution led to the formation of type II nanoframes with elliptical pores on the {110} faces. The morphologies of these nanoframes have been carefully examined by electron microscopy. Without adding ethanol, random etching of the nanocages can occur at a slow rate. These composite materials should display interesting properties and functions.

Seed-Mediated Synthesis of Palladium Nanorods and Branched Nanocrystals and Their Use as Recyclable Suzuki Coupling Reaction Catalysts

In this study, we have used a simple seed-mediated synthesis process to prepare uniform Pd nanorods with average lengths of approximately 200 and 300 nm through the addition of 50-100 microL of 0.004 M copper acetate solution into the growth solution for the first time. Because of their long lengths, they nicely settled to the bottom of the reaction vial and can easily be separated from the suspended faceted particles. The nanorods have an average diameter of approximately 20 nm, so they have high aspect ratios of 10-15 or more. They can readily self-assemble into high-density packing structures on substrates. By increasing the volume of the copper acetate solution added to 250 microL, extensively branched Pd nanocrystals were obtained. The crystal structures of both particle morphologies have been examined. The nanorods possess a penta-twinned structure. Different growth directions have been found for the branched nanocrystals. The growth mechanism of these nanostructures was studied in great detail. A mixture of short Pd rods and faceted particles was formed first and elongated into long rods or branched nanocrystals with the assistance of copper atom deposition. A reduction potential of copper lower than that of palladium leads to periodic deposition and reoxidation of copper atoms on the growing rods and faceted particles. Use of other metal ions did not form long nanorods and branched nanocrystals. UV-vis spectra of the 200 nm Pd nanorods showed a significant absorption band in the near-infrared region with band maximum at approximately 1800 nm, while the 300 nm rods have band maxima of 2000-2100 nm. Both the nanorods and branched nanocrystals were found to serve as highly efficient and recyclable catalysts for catalyzing a Suzuki coupling reaction between phenylboronic acid and iodobenzene. They are also thermally stable under the reflux condition and can be easily removed from the product solution. These novel Pd nanostructures should find use in other palladium-catalyzed reactions.

Au Nanocrystal-Directed Growth of Au-Cu2O Core-Shell Heterostructures with Precise Morphological Control

Formation of metal-semiconductor core-shell heterostructures with precise morphological control of both components remains challenging. Heterojunctions, rather than core-shell structures, were typically produced for metal-semiconductor composites. Furthermore, growth of semiconductor shells with systematic shape evolution using the same metal particle cores can also present a significant challenge. Here, we have synthesized Au-Cu(2)O core-shell heterostructures using gold nanoplates, nanorods, octahedra, and highly faceted nanoparticles as the structure-directing cores for the overgrowth of Cu(2)O shells by a facile aqueous solution approach. The gold nanoparticle cores guide the growth of Cu(2)O shells with morphological and orientation control. Systematic shape evolution of the shells can be easily achieved by simply adjusting the volume of reductant added. For example, truncated cubic to octahedral Cu(2)O shells were produced from octahedral gold nanocrystal cores. Unusual truncated stellated icosahedral and star column structures have also been synthesized. The heterostructures were found to be formed via an unusual hollow-shell-refilled growth mechanism not reported before. The approach has potential toward the preparation of other complex Cu(2)O structures with well-defined facets.

Seed-Mediated Synthesis of Gold Nanocrystals with Systematic Shape Evolution from Cubic to Trisoctahedral and Rhombic Dodecahedral Structures

We report a seed-mediated synthesis method for the preparation of gold nanocrystals with systematic shape evolution from truncated cubic to cubic, trisoctahedral, and rhombic dodecahedral structures in aqueous solution for the first time. Nanocrystals with transitional morphologies were also synthesized. The combination of using cetyltrimethylammonium chloride (CTAC) surfactant and a very small amount of NaBr to control the bromide concentration in the growth solution was found to be critical to the formation of nanocubes. Variation in the volume of ascorbic acid added to the growth solution enabled the fine control of nanocrystal morphology. Nanocubes and rhombic dodecahedra with controlled sizes of 30-75 nm were prepared by adjusting the volume of the seed solution added to the growth solution. They can self-assemble into ordered packing structures on substrates because of their uniform sizes. XRD, TEM, and UV-vis absorption characterization of the different products synthesized have been performed. By increasing the bromide concentration 5-fold that used to make the nanocubes, unusual right bipyramids of gold bounded by six {100} faces were produced. The high product purity and excellent size control of this facile synthetic approach should make these novel gold nanostructures be readily available for a wide range of studies.

Facet-Dependent and Au Nanocrystal-Enhanced Electrical and Photocatalytic Properties of Au-Cu2O Core-Shell Heterostructures

We report highly facet-dependent electrical properties of Cu(2)O nanocubes and octahedra and significant enhancement of gold nanocrystal cores to the electrical conductivity of Au-Cu(2)O core-shell octahedra. Cu(2)O nanocubes and octahedra and Au-Cu(2)O core-shell cubes and octahedra were synthesized by following our reported facile procedures at room temperature. Two oxide-free tungsten probes attached to a nanomanipulator installed inside a scanning electron microscope made contacts to a single Cu(2)O nanocrystal for the I-V measurements. Pristine Cu(2)O octahedra bounded by {111} facets are 1100 times more conductive than pristine Cu(2)O cubes enclosed by {100} faces, which are barely conductive. Core-shell cubes are only slightly more conductive than pristine cubes. A 10,000-fold increase in conductivity over a cube has been recorded for an octahedron. Remarkably, core-shell octahedra are far more conductive than pristine octahedra. The same facet-dependent electrical behavior can still be observed on a single nanocrystal exposing both {111} and {100} facets. This new fundamental property may be observable in other semiconductor nanocrystals. We also have shown that both core-shell cubes and octahedra outperform pristine cubes and octahedra in the photodegradation of methyl orange. Efficient photoinduced charge separation is attributed to this enhanced photocatalytic activity. Interestingly, facet-selective etching occurred over the {100} corners of some octahedra and core-shell octahedra during photocatalysis. The successful preparation of Au-Cu(2)O core-shell heterostructures with precise shape control has offered opportunities to discover new and exciting physical and chemical properties of nanocrystals.

Fabrication of Au–Pd Core–Shell Heterostructures with Systematic Shape Evolution Using Octahedral Nanocrystal Cores and Their Catalytic Activity

By using octahedral gold nanocrystals with sizes of approximately 50 nm as the structure-directing cores for the overgrowth of Pd shells, Au-Pd core-shell heterostructures with systematic shape evolution can be directly synthesized. Core-shell octahedra, truncated octahedra, cuboctahedra, truncated cubes, and concave cubes were produced by progressively decreasing the amount of the gold nanocrystal solution introduced into the reaction mixture containing cetyltrimethylammonium bromide (CTAB), H(2)PdCl(4), and ascorbic acid. The core-shell structure and composition of these nanocrystals has been confirmed. Only the concave cubes are bounded by a variety of high-index facets. This may be a manifestation of the release of lattice strain with their thick shells at the corners. Formation of the [CTA](2)[PdBr(4)] complex species has been identified spectroscopically. Time-dependent UV-vis absorption spectra showed faster Pd source consumption rates in the growth of truncated cubes and concave cubes, while a much slower reduction rate was observed in the generation of octahedra. The concave cubes and octahedra were used as catalysts for a Suzuki coupling reaction. They can all serve as effective and recyclable catalysts, but the concave cubes gave higher product yields with a shorter reaction time attributed to their high-index surface facets. The concave cubes can also catalyze a wide range of Suzuki coupling reactions using aryl iodides and arylboronic acids with electron-donating and -withdrawing substituents.