Robert W. J. Scott

Plasmonic Enhancement of Dye Sensitized Solar Cells in the Red-to-near-Infrared Region using Triangular Core–Shell Ag@SiO2 Nanoparticles

Recently, plasmonic metal nanoparticles have been shown to be very effective in increasing the light harvesting efficiency (LHE) of dye-sensitized solar cells (DSSCs). Most commonly, spherical nanoparticles composed of silver or gold are used for this application; however, the localized surface plasmon resonances of these isotropic particles have maxima in the 400-550 nm range, limiting any plasmonic enhancements to wavelengths below 600 nm. Herein, we demonstrate that the incorporation of anisotropic, triangular silver nanoprisms in the photoanode of DSSCs can dramatically increase the LHE in the red and near-infrared regions. Core-shell Ag@SiO2 nanoprisms were synthesized and incorporated in various quantities into the titania pastes used to prepare the photoanodes. This optimization led to an overall 32 ± 17% increase in the power conversion efficiency (PCE) of cells made using 0.05% (w/w) of the Ag@SiO2 composite. Measurements of the incident photon-to-current efficiency provided further evidence that this increase is a result of improved light harvesting in the red and near-infrared regions. The effect of shell thickness on nanoparticle stability was also investigated, and it was found that thick (30 nm) silica shells provide the best protection against corrosion by the triiodide-containing electrolyte, while still enabling large improvements in PCE to be realized.

Non-aqueous synthesis of mesostructured tin dioxide

Novel sodium glycostannate precursors have been synthesized from sodium stannate and glycol reactants. These anionic precursors were investigated as potential starting materials towards the assembly of mesostructured tin oxide. The glycostannate precursors could be charge balanced with cationic surfactants, such as cetyltrimethylammonium bromide, in non-aqueous solutions to give composite structures in which the glycostannate anions remained structurally intact. Subsequent hydrolysis and condensation of these structures with acidic aqueous solutions gave mesostructured tin oxide materials. The initial formation and hydrolysis/condensation of these surfactant–glycostannate composites was followed by solid-state 119Sn NMR, thermal gravimetric analysis, powder X-ray diffraction, and transmission electron microscopy. Calcination of the final materials at 400 °C led to collapse of the mesostructure. Future attempts to stabilize such mesostructures to calcination utilizing silane, sulfate, or phosphate protecting groups should allow the formation of electroactive mesoporous tin oxide materials.

Synthesis of metal sulfide materials with controlled architecture

Recent advances in the synthesis of metal sulfides with controlled architecture, exemplified by microporous and mesostructured materials, intercalates, nanowires, and nanotubes are described. These illustrate a paradigm shift from traditional solid-state methods to rational, directed assembly of materials with hierarchical design.

Probing the Relative Stability of Thiolate- and Dithiolate-Protected Au Monolayer-Protected Clusters

Dithiolate ligands based on (+/-)-alpha-lipoic acid derivatives have been investigated as ligands for both Au monolayer-protected clusters (MPCs) and mixed alkanethiol/dithiolate Au MPCs. The oxidative and thermal stability of the MPCs were investigated by a combination of UV-vis spectroscopy, TEM, and (1)H NMR experiments. Results show that the dithiolate-protected MPCs are much more prone to oxidation by oxygen under ambient conditions than their alkanethiolate-protected MPC analogues; in addition, the Au core of the dithiolate-protected Au MPCs could be etched by KCN at much faster rates than both alkanethiolate-protected and mixed monolayer MPCs. These results suggest that strategies to increase ligand-metal interactions by incorporating more thiolate linkers into the ligand must also take into account the packing efficiency and/or stability of such ligands on the metal surface, which can make them much more prone to oxidation under ambient conditions.

Panchromatic Enhancement of Light-Harvesting Efficiency in Dye-Sensitized Solar Cells Using Thermally Annealed Au@SiO2 Triangular Nanoprisms

Plasmonic enhancement is an attractive method for improving the efficiency of dye-sensitized solar cells (DSSCs). Plasmonic materials with sharp features, such as triangular metal nanoparticles, show stronger plasmonic effects than their spherical analogues; however, these nanoparticles are also often thermally unstable. In this work, we investigated the thermal stability of Au@SiO2 triangular nanoprisms by annealing at different temperatures. Morphological changes were observed at temperatures greater than 250 °C, which resulted in a blue shift of the localized surface plasmon resonance (LSPR). Annealing at 450 °C led to a further blue shift; however, this resulted in better overlap of the LSPR with the absorption spectrum of black dye. By introducing 0.05% (w/w) Au@SiO2 nanoprisms into DSSCs, we were able to achieve a panchromatic enhancement of the light-harvesting efficiency. This led to a 15% increase in the power conversion efficiency from 3.9 ± 0.6% to 4.4 ± 0.4%.

Highly Stable Noble‐Metal Nanoparticles in Tetraalkylphosphonium Ionic Liquids for in situ Catalysis

Gold and palladium nanoparticles were prepared by lithium borohydride reduction of the metal salt precursors in tetraalkylphosphonium halide ionic liquids in the absence of any organic solvents or external nanoparticle stabilizers. These colloidal suspensions remained stable and showed no nanoparticle agglomeration over many months. A combination of electrostatic interactions between the coordinatively unsaturated metal nanoparticle surface and the ionic-liquid anions, bolstered by steric protection offered by the bulky alkylated phosphonium cations, is likely to be the reason behind such stabilization. The halide anion strongly absorbs to the nanoparticle surface, leading to exceptional nanoparticle stability in halide ionic liquids; other tetraalkylphosphonium ionic liquids with non-coordinating anions, such as tosylate and hexafluorophosphate, show considerably lower affinities towards the stabilization of nanoparticles. Palladium nanoparticles stabilized in the tetraalkylphosphonium halide ionic liquid were stable, efficient, and recyclable catalysts for a variety of hydrogenation reactions at ambient pressures with sustained activity. Aerial oxidation of the metal nanoparticles occurred over time and was readily reversed by re-reduction of oxidized metal salts.

Electronically addressable SnO2 inverted opal gas sensors fabricated on interdigitated gold microelectrodes

Electronically addressable thin films of tin oxide gas sensors with well-defined opaline microstructures and reproducible sensor-to-sensor responses have been fabricated on interdigitated gold microelectrodes through self-assembly growth of a monodisperse polystyrene latex film onto the electrodes followed by infiltration of tin tert-butoxide and calcination of the film.

Selective Hydrogenations with AgPd Catalysts Prepared by Galvanic Exchange Reactions

The structural characterization and catalytic activity of bimetallic nanoparticles is a topic of high current research attention. Core-shell nanoparticle catalysts are promising because of the possibility of achieving high-atom economies with respect to precious metals if the core is composed of a less precious material. Several groups have already investigated the viability of such an approach by using Ni Pt and Ni Pd core-shell systems. In this communication we report the synthesis of putative Ag core-Pd shell nanoparticles with a core diameter less than 3 nm through galvanic exchange reactions between Ag “seed” nanoparticles and Pd salts. The nanoparticles were catalytically active for the selective hydrogenation of substrates such as allyl alcohol, 3-buten-1-ol, and 3-hexyn-1-ol, and their actual structures were investigated by characterization by using TEM, UV/Vis spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The motivation for creating core-shell Ag Pd nanoparticle catalysts is based not only on increasing the atom-economy of Pd, but also on the possibility of enhanced selectivity, owing to the presence of the sub-surface Ag; others have shown that PdAg alloys have excellent selectivity for the partial hydrogenation of acetylene to ethylene. 13] Indeed, Freund et al. describe the ideal bimetallic PdAg system as having a Pd-rich surface to dissociate H2 and catalyze hydrogenation reactions and an Ag-rich core to prevent the occurrence of sub-surface H. The Ag Pd core-shell nanoparticles were synthesized via galvanic reactions between poly(vinylpyrrolidone) (PVP) stabilized Ag nanoparticle seeds and K2PdCl4. Briefly, the Ag seeds were synthesized by the reduction of AgNO3 by NaBH4 in the presence of the PVP polymer stabilizer under nitrogen. Ag Pd core-shell materials were synthesized through the addition of stoichiometric amounts of K2PdCl4 to the Ag 0 seeds at 85 8C (all Ag/Pd ratios reported are synthetic ratios of the metals). Shown in Figure S1 are the UV/Vis spectra of the Ag particles before and after the addition of K2PdCl4 (Ag/Pd = 2:1). The Ag seeds show a strong plasmon band at approximately l= 400 nm, which is characteristic of Ag nanoparticles in this size range. After the addition of K2PdCl4, the plasmon band blueshifts from l= 400 to 383 nm, broadens, and dampens in intensity. The partial galvanic exchange reaction, (2 Ag+Pd + !2 Ag+Pd), yields particles that are similar in size to the original Ag seeds (3–4 nm). A series of AgPd nanoparticles with varying Ag to Pd ratios were synthesized (Table 1); AgCl

Surface Properties of Water-Soluble Glycine-Cysteamine-Protected Gold Clusters

We report the synthesis of water-soluble, nearly monodisperse glycine-cysteamine (Gly-CSA) gold monolayer protected clusters (MPCs) via base deprotection of Fmoc-Gly-CSA MPCs. The resulting Gly-CSA MPCs, which have terminal primary amine groups, are fully characterized by (1)H and (13)C NMR, UV-vis spectroscopy, and TEM, and their surface properties were probed by dynamic light scattering and acid-base titrations. The characterization methods indicate that the as-synthesized particles are nearly monodisperse with an average particle size of 1.8 +/- 0.3 nm, but are only stable to aggregation in water at pHs of 4 and below. Acid-base titrations of the Gly-CSA MPCs show that the primary ammonium groups have a pK(a) of approximately 5.5, which is several orders of magnitude lower than the pK(a2) for the ammonium group of glycine (9.6). Thus, the particles are only partially protonated at intermediate pHs, which then drives the aggregation of the nanoparticles via hydrogen-bond formation. Dynamic light scattering results confirm the pH-driven aggregation of the nanoparticles, and studies with ninhydrin confirm that the primary amine groups are reactive and have potential for further functionalization. These results show that amine-terminated MPCs can be synthesized; however, their aggregation at intermediate pHs can limit their utility as building blocks for multifunctional nanoparticle syntheses.

Optimization of transition metal nanoparticle- phosphonium ionic liquid composite catalytic systems for deep hydrogenation and hydrodeoxygenation reactions†

A variety of metal nanoparticle (NP)/tetraalkylphosphonium ionic liquid (IL) composite systems were evaluated as potential catalysts for the deep hydrogenation of aromatic molecules. Particles were synthesized by reducing appropriate metal salts by LiBH4 in a variety of ILs. Gold NPs were used as probes to investigate the effect of both chain lengths of the alkyl substituents on the phosphonium cation and the nature of anions, on the stability of NPs dispersed in the ILs. The presence of three medium-to-long alkyl chains (such as hexyl) along with one long alkyl chain (such as tetradecyl) in the IL, coupled with highly coordinating anions (such as halides, or to a smaller extent, bis-triflimides) produced the most stable dispersions. These ILs also showed maximum resistance to heat-induced sintering; for example, TEM studies of Pt NPs heated under hydrogen to 120 °C showed only moderate sintering in trihexyl(tetradecyl)phosphonium chloride and bis(triflimide) ILs. Finally, olefinic hydrogenations, aromatic hydrogenations, and hydrodeoxygenation of phenol were carried out with Ru, Pt, Rh and PtRh NPs using hydrogen at elevated pressures. From preliminary studies, Ru NPs dispersed in trihexyl(tetradecyl)phosphonium chloride emerged as the catalyst system of choice. The presence of borate Lewis-acid by-products in the reaction medium (from the borohydride reduction step) allowed for partial phenol hydrodeoxygenation.