Susan E. Habas

Nanocrystal bilayer for tandem catalysis

Supported catalysts are widely used in industry and can be optimized by tuning the composition and interface of the metal nanoparticles and oxide supports. Rational design of metal-metal oxide interfaces in nanostructured catalysts is critical to achieve better reaction activities and selectivities. We introduce here a new class of nanocrystal tandem catalysts that have multiple metal-metal oxide interfaces for the catalysis of sequential reactions. We utilized a nanocrystal bilayer structure formed by assembling platinum and cerium oxide nanocube monolayers of less than 10 nm on a silica substrate. The two distinct metal-metal oxide interfaces, CeO(2)-Pt and Pt-SiO(2), can be used to catalyse two distinct sequential reactions. The CeO(2)-Pt interface catalysed methanol decomposition to produce CO and H(2), which were subsequently used for ethylene hydroformylation catalysed by the nearby Pt-SiO(2) interface. Consequently, propanal was produced selectively from methanol and ethylene on the nanocrystal bilayer tandem catalyst. This new concept of nanocrystal tandem catalysis represents a powerful approach towards designing high-performance, multifunctional nanostructured catalysts.

Selective Growth of Metal and Binary Metal Tips on CdS Nanorods

Here, we demonstrate an approach for the selective growth of Pt, PtNi, and PtCo on CdS nanorods. The hybrid nanostructures prepared via an organometallic synthesis have promise for photocatalytic and magnetic applications.

Dendrimer Templated Synthesis of One Nanometer Rh and Pt Particles Supported on Mesoporous Silica: Catalytic Activity for Ethylene and Pyrrole Hydrogenation.

Monodisperse rhodium (Rh) and platinum (Pt) nanoparticles as small as approximately 1 nm were synthesized within a fourth generation polyaminoamide (PAMAM) dendrimer, a hyperbranched polymer, in aqueous solution and immobilized by depositing onto a high-surface-area SBA-15 mesoporous support. X-ray photoelectron spectroscopy indicated that the as-synthesized Rh and Pt nanoparticles were mostly oxidized. Catalytic activity of the SBA-15 supported Rh and Pt nanoparticles was studied with ethylene hydrogenation at 273 and 293 K in 10 torr of ethylene and 100 torr of H 2 after reduction (76 torr of H 2 mixed with 690 torr of He) at different temperatures. Catalysts were active without removing the dendrimer capping but reached their highest activity after hydrogen reduction at a moderate temperature (423 K). When treated at a higher temperature (473, 573, and 673 K) in hydrogen, catalytic activity decreased. By using the same treatment that led to maximum ethylene hydrogenation activity, catalytic activity was also evaluated for pyrrole hydrogenation.

Highly Selective Synthesis of Catalytically Active Monodisperse Rhodium Nanocubes

Monodisperse sub-10 nm Rh nanocubes were synthesized with high selectivity (>85%) by a seedless polyol method. The {100} faces of the Rh NCs were effectively stabilized by chemically adsorbed Br- ions from trimethyl(tetradecyl)ammonium bromide (TTAB). This simple one-step polyol route can be readily applied to the preparation of Pt and Pd nanocubes. Moreover, the organic molecules of PVP and TTAB that encapsulated the Rh nanocubes did not prevent catalytic activity for pyrrole hydrogenation and CO oxidation.

Synthesis of Lead Chalcogenide Alloy and Core–Shell Nanowires†

Control over the dimensions and shape of nanostructures represents one of the main challenges in modern materials science. Morphology control of a variety of materials can be achieved using vapor–liquid–solid or solution–liquid–solid techniques to obtain one-dimensional (1D) systems. The unique optical and electrical properties of 1D nanostructures make them one of most important building blocks for nanoscience and nanotechnology applications, and provide the opportunity for their integration in electronic, photonic, thermoelectric, and sensor-based devices. Size control has been traditionally important and necessary to tune the optical and electrical properties of nanomaterials by changing the band gap. This is particularly important in the strong confinement region, where one of the dimensions is smaller than the corresponding excitonic Bohr diameter. Semiconductor alloy and core–shell nanowire systems represent another interesting direction towards functional nanostructures with enhanced structural and property tunability. Herein, we focus on preparing novel 1D heterostructures of IV–VI semiconductor nanomaterials. Lead chalcogenides are known to be good materials for thermoelectrics due to their low thermoconductivity. Pseudobinary (e.g. PbSeTe) and pseudoternary alloys (e.g. PbSnSeTe) have even lower lattice thermal conductivities than the binary compounds due to disorder-induced phonon scattering processes. Lead chalcogenide materials are also good candidates for multiexciton-generation (MEG) solar cells. For example, previous reports showed quantum efficiencies as high as 300% and 700% for PbSe nanoparticles. Heterostructured alloy and core–shell nanomaterials have previously been shown for various materials, mainly II–VI semiconductor nanocrystals. For example, a quasi 1D system of CdSe–ZnS has been reported, other systems include PbSe–PbS core–shell and alloy spherical nanoparticles developed by Lifshitz and co-workers. In addition, Talapin et al. have demonstrated the growth of PbS and Au onto PbSe nanowires. The physical properties of these heterostructured nanosystems are of interest for various applications as shown by the electronic structure calculations carried out by different groups. Here we demonstrate the formation of lead chalcogenide heterostructure nanowires by a solution-phase synthesis at moderate temperatures (see the Experimental Section). Two types of heterostructures (alloy and core–shell) were prepared by changing the concentration and temperature of the reaction. We were able to control the composition of the alloy and the thickness of the shell by changing the growth parameters. Three different systems, PbSexS1 x alloys, and PbSe–PbS and PbSe–PbTe core–shell nanowires were prepared. Achieving these three targeted structures is nontrivial due to various competitive processes such as ripening and formation of pure PbS (PbTe) nanoparticles. The synthesis of PbSe nanowires is based on a previous report by Murray and co-workers. The same procedure was used to prepare the PbSe nanowires used here as templates for further growth to give the alloy and core–shell nanostructures. The diameter of the core nanowires could be controlled and varied from 4 nm up to 100 nm, with a length of a few tens of micrometers. The PbSe nanowires (Figure 1A) were used as templates to form PbSexS1 x alloy wires. Figure 1B shows PbSe0.4S0.6 alloy nanowires that were prepared by the slow addition of Pb and S precursors to a hot solution containing PbSe nanowires. (a detailed description of the synthesis can be found in the Experimental Section). The diameter of the alloy nanowires increased from 6 nm (pure PbSe nanowires) to ca. 10 nm, indicating the incorporation of additional material into the nanowires. Structural characterization of the alloy system was carried out using various methods as shown in Figure 1. Figure 1D shows a high-resolution transmission electron microscopy (HRTEM) image of the PbSe0.4S0.6 nanowires. The latticeresolved image indicates that the nanowires are growing along the h100i direction. X-ray diffraction (XRD) measurements of the alloy nanowires are shown in Figure 1C. The pattern can be indexed to a structure intermediate between the cubic PbSe and cubic PbS bulk phases, which strongly supports the formation of an alloyed structure. An energydispersive X-ray (EDX) spectrum (Figure 1E) taken on a small area of the alloy nanowire, shown in Figure 1D, indicates the presence of Se from the original PbSe nanowires, Pb from the original and added materials, and Cu from the TEM grid. However, due to overlap between the Pb and S peaks, electron energy loss spectroscopy (EELS) was necessary to detect the incorporation of S. The energy loss peak for S was observed at 165 eV (Figure 1F), providing clear evidence for the existence of S in the alloy nanowires. The EDX and EELS spectra were taken from the same area of the nanowire shown in Figure 1D. Tuning the alloy composition can be achieved by simply controlling the reaction conditions. For example, altering the S concentration will act to tune the alloy composition. The actual composition was determined by [*] Dr. T. Mokari, S. E. Habas, Prof. P. Yang Department of Chemistry, University of California Berkeley, CA 94720 (USA) Fax: (+1)510-642-7301 E-mail: p_yang@berkeley.edu

Surface Chemistry Exchange of Alloyed Germanium Nanocrystals: A Pathway Toward Conductive Group IV Nanocrystal Films.

We present an expansion of the mixed-valence iodide reduction method for the synthesis of Ge nanocrystals (NCs) to incorporate low levels (∼1 mol %) of groups III, IV, and V elements to yield main-group element-alloyed Ge NCs (Ge1-xEx NCs). Nearly every main-group element (E) that surrounds Ge on the periodic table (Al, P, Ga, As, In, Sn, and Sb) may be incorporated into Ge1-xEx NCs with remarkably high E incorporation into the product (>45% of E added to the reaction). Importantly, surface chemistry modification via ligand exchange allowed conductive films of Ge1-xEx NCs to be prepared, which exhibit conductivities over large distances (25 μm) relevant to optoelectronic device development of group IV NC thin films.

Probing compositional variation within hybrid nanostructures.

We present a detailed analysis of the structural and magnetic properties of solution-grown PtCo-CdS hybrid structures in comparison to similar free-standing PtCo alloy nanoparticles. X-ray absorption spectroscopy is utilized as a sensitive probe for identifying subtle differences in the structure of the hybrid materials. We found that the growth of bimetallic tips on a CdS nanorod substrate leads to a more complex nanoparticle structure composed of a PtCo alloy core and thin CoO shell. The core-shell architecture is an unexpected consequence of the different nanoparticle growth mechanism on the nanorod tip, as compared to free growth in solution. Magnetic measurements indicate that the PtCo-CdS hybrid structures are superparamagnetic despite the presence of a CoO shell. The use of X-ray spectroscopic techniques to detect minute differences in atomic structure and bonding in complex nanosystems makes it possible to better understand and predict catalytic or magnetic properties for nanoscale bimetallic hybrid materials.

Direct write metallization for photovoltaic cells and scaling thereof

Atmospheric solution processing can help toward a significant cost reduction of photovoltaics. We investigate the use of direct write deposition approaches for deposition of metallization for a variety of solar cell materials. We are studying inkjet printing and aerosol spraying of metal contacts for Si, CIS/CIGS and organic photovoltaics. We have developed metal organic decomposition inks for metals such as: silver, nickel, copper and aluminum. All of these can be deposited in lines with 30–40 µm width and conductivities close to that of bulk metals. For silicon photovoltaics materials have been developed to facilitate Ohmic contact formation through an anti reflection coating. Initial research has been focusing on small cells, but in order to transfer the technology to production it has to be demonstrated on large area cells as well. For this the Atmospheric Processing Platform (APP) was developed at NREL. This platform allows us to scale the deposition of the developed inks and processing to large area (Up to 157 mm × 157 mm) and prototype contact patterns. The APP consists of several deposition, processing and characterization units, most located in a controlled environment. The atmospheric deposition tools in the APP are: inkjet printing, aerosol spraying and ultrasonic spraying. A rapid thermal processing unit is integrated for thermal processing. XRF and XRD can be accessed without leaving the controlled environment to determine the composition and structure of the deposited material. Sputter deposition and evaporation are also part of the APP, even though these techniques are not atmospheric. Details of the individual platforms in the APP will be given together with results of direct write contacts on large area cells.

Functionalising carbon nanotubes

The realisation of efficient functionalisation strategies is critical to the further development of CNT science and technology. The area has attracted a great deal of attention and a host of innovative approaches developed. However, as the examples highlighted here illustrate, many challenges remain.

An investigation into support cooperativity for the deoxygenation of guaiacol over nanoparticle Ni and Rh2P

The production of hydrocarbon fuels from biomass pyrolysis requires the development of effective deoxygenation catalysts, and insight into how the properties of the support influence performance is critical for catalyst design. In this report, nanoparticles of Ni and Rh2P were synthesized using solution-phase techniques and dispersed on high surface area supports. The supports included a relatively inert material (C), an acidic reducible metal-oxide (TiO2), an acidic irreducible metal-oxide (Al2O3), and a basic irreducible metal-oxide (MgO). The eight active phase/support combinations were investigated for the deoxygenation of guaiacol, a pyrolysis vapor model compound, under ex situ catalytic fast pyrolysis conditions (350 °C, 0.44 MPa H2). Compared to the baseline performance of the C-supported catalysts, Ni/TiO2 and Rh2P/TiO2 exhibited higher guaiacol conversion and lower O : C ratios for C5+ products, highlighting the enhanced activity and greater selectivity to deoxygenated products derived from the use of an acidic reducible metal-oxide support. The Al2O3-supported catalysts also exhibited higher conversion than the C-supported catalysts and promoted alkylation reactions, which improve carbon efficiency and increase the carbon number of the C5+ products. However, Ni/Al2O3 and Rh2P/Al2O3 were less selective towards deoxygenated products than the C-supported catalysts. The MgO-supported catalyst exhibited lower conversion and decreased yield of deoxygenated products compared to the C-supported catalysts. The results reported here suggest that basic metal-oxide supports may inhibit deoxygenation of phenolics under CFP conditions. Contrastingly, support acidity and reducibility were demonstrated to promote conversion and selectivity to deoxygenated products, respectively.