We report the production of efficient, ultra-low Pt loading catalyst layers on fuel cell electrodes via pulse potential deposition (PPD) from a H2Pt(OH)6 plating solution. We show that by optimizing the off time in the PPD pulse sequence, it is possible to deposit very small, crystalline Pt nanoparticles with narrow size distribution (1.36 ± 0.360 nm) on amorphous carbon supports. Electrodes fabricated from these high surface area electrocatalysts display increased fuel cell performance compared to commercial electrodes with up to an order of magnitude less Pt. The improved performance of our electrodes is attributed to a combination of the optimized size of the deposited nanoparticles as well as the decreased thickness of the catalyst layer. In our deposited electrodes the catalyst layers are very thin (<10 μm) and as a result achieve up to twelve times the gravimetric power of commercial electrodes.

Evidence suggests that oligomers of the 42-residue form of the amyloid β-protein (Aβ), Aβ42, play a critical role in the etiology of Alzheimer’s disease (AD). Here we use high resolution atomic force microscopy to directly image populations of small oligomers of Aβ42 that occur at the earliest stages of aggregation. We observe features that can be attributed to a monomer and to relatively small oligomers, including dimers, hexamers, and dodecamers. We discovered that Aβ42 hexamers and dodecamers quickly become the dominant oligomers after peptide solubilization, even at low (1 μM) concentrations and short (5 min) incubation times. Soon after (≥10 min), dodecamers are observed to seed the formation of extended, linear preprotofibrillar β-sheet structures. The preprotofibrils are a single Aβ42 layer in height and can extend several hundred nanometers in length. To our knowledge this is the first report of structures of this type. In each instance the preprotofibril is associated off center with a single layer of a dodecamer. Protofibril formation continues at longer times, but is accompanied by the formation of large, globular aggregates. Aβ40, by contrast, does not significantly form the hexamer or dodecamer but instead produces a mixture of smaller oligomers. These species lead to the formation of a branched chain-like network rather than discrete structures.

Amorphous carbon is widely used as a support for Pt nanoparticle catalysts. We show here that catalytic performance can be greatly improved by functionalizing the carbon support with a nitrogen-containing molecule, in conjunction with a new method for the in situ synthesis of nanocrystalline Pt. Vulcan® carbon black is covalently functionalized with 4-aminomethylpyridine (4AMP) via formation of an acid chloride on the surface followed by amidation. The resulting 4AMP-functionalized Vulcan® (4AMP-VC) was thoroughly characterized and shown to contain N at the surface of the Vulcan® carbon support. Pt nanoparticles grown on the 4AMP-VC have a smaller average size and much narrower size distribution than Pt nanoparticles grown on bare Vulcan® (VC). In addition, the Pt/4AMP-VC catalysts show higher catalytic activity and are more durable than their Pt/VC counterparts. We infer through careful analysis of X-ray photoelectron spectra that the Pt nanoparticles bind preferentially to the pyridinic nitrogen of the 4AMP.

We report the results of a systematic study of the catalytic activity of mass-selected vanadium oxide clusters deposited on rutile TiO2 surfaces under ultrahigh vacuum (UHV) conditions. Our results show that supported V, VO, and VO2 clusters are not catalytically active for the oxidative dehydrogenation of methanol to formaldehyde but can be made catalytically active by postoxidation. In addition, we found that the postoxidized VO/TiO2 produces the most formaldehyde. Scanning tunneling microscopy (STM) imaging of the postoxidized VO/TiO2 reveals isolated clusters with height and width indicative of VO3 bound to the TiO2 surface. Our results are consistent with previous density functional theory (DFT) calculations that predict that VO3 will be produced by postoxidation of VO and that VO3/TiO2 is an active catalyst.

The relation between proton exchange membrane (PEM) hydration and fuel cell performance has been well documented at the macroscopic scale. Understanding how changes in membrane water content affect the organization of proton conducting domains at the micrometer and submicrometer scales is a sought-after goal in the rational design of higher performing PEMs. Using atomic force microscopy phase and current imaging, we have resolved proton conducting domains at the surface of Nafion membranes at dehydrated, ambient, and hydrated conditions, observing a unique morphology at each membrane water content. At ambient conditions, Nafion’s surface morphology resembles that proposed in the parallel-pore and bicontinuous network models, with the exception that hydrophilic domains are larger at the surface of Nafion compared to the bulk. At hydrated conditions, a network of wormlike, insulating domains extends several micrometers over Nafion’s surface with more conductive, water-rich regions found between these fibrillar features. Neither the surface morphology observed at ambient conditions nor at hydrated conditions persists in dehydrated membranes, which instead exhibit a low coverage of isolated hydrophilic surface domains that, remarkably, are similar in size to such domains at ambient conditions. These observations affirm properties distinct to Nafion’s surface and provide morphological evidence for the low conductivity observed in Nafion at dehydrated conditions and the high conductivity observed at hydrated conditions.

We report the electrochemical deposition of Pt nanoparticles from platinic acid H2[Pt(OH)6] using pulse potential deposition (PPD). We are able to control the size, morphology, and loading of platinum nanoparticles from H2[Pt(OH)6] by controlling the deposition parameters such as the pH of the plating solution, the pulse potential, the pulse width, and the duty cycle of the pulse sequence. We show that a high density of Pt nanoparticles electrodeposited can be produced on both planar and nonplanar electrode supports with high surface area and high catalytic activity. Finally, we show that fuel cell electrodes can be produced using H2[Pt(OH)6] as the source of Pt via our optimized PPD technique. The fuel cells produced from these electrodes are highly efficient with less than half the Pt content of commercially available fuel cells, which results in a gravimetric power more than twice that of fuel cells produced by using commercially available electrodes.

We use ultra-high vacuum scanning tunneling microscopy (UHV–STM) to probe, at the atomic level, the structure of mass-selected isolated V1, V2, VO and VO2 clusters deposited on rutile TiO2(110) by ion soft landing. All four species interact differently with the TiO2 surface and the ultimate binding site and configuration strikes a balance between the gas-phase structure and the ligation of this cluster by the TiO2 surface. Our results show that vanadium atoms prefer to bind in the upper threefold hollow sites on the surface and have a slight tendency to pair along the [1–10] direction, while vanadium dimers bind to the surface oriented along the [001] direction exclusively. VO clusters bind with the vanadium atom in the upper threefold hollow site and with the oxygen atom bound to an adjacent fivefold coordinated Ti atom (5c-Ti). The VO2 cluster also binds with the vanadium atom in the upper threefold hollow site and with both oxygen atoms bound to adjacent 5c-Ti atoms or with only one oxygen bound to the surface and the other directed out of the plane of the surface.Research Highlights► Vanadium monomers bind preferentially to upper threefold hollow sites on TiO2(110). ► Vanadium dimers are oriented along the bridging oxygen rows on TiO2(110). ► Evaporative deposition of vanadium results in a mixture of monomers and dimers. ► The vanadium atoms in VO and VO2 clusters bind to upper threefold hollow sites. ► VO2 clusters on TiO2 are longer along the [001] surface direction than VO clusters.

The nature of tip−sample interaction forces in atomic force microscopy (AFM) phase imaging strongly affects the resolution of proton conducting domains mapped at the surface of Nafion membranes. Images acquired in repulsive mode overestimated the area of individual proton conducting domains by a factor of 4 (360 vs 90 nm2) and underestimated the number of these domains by a factor of 3 (0.9 domains per 1000 nm2 vs 2.7 domains per 1000 nm2) compared to attractive mode. When the cantilever was driven above resonance or when the combination of scan parameters resulted in an AFM feedback loop that was not fully optimized, phase contrast arose not from proton conducting domains but instead from changes in topography. In attractive mode, phase contrast did not correlate with either topography or changes in topography, and the resulting images most accurately represent the fluorocarbon and aqueous domains at the surface of Nafion membranes.

Variable-temperature scanning tunneling microscopy (STM) is used to show that Au1+ deposited onto a TiO2(110)-(1 × 1) surface under soft-landing conditions at 600 K results in a surface decorated with isolated gold atoms bound to oxygen vacancies. This result is in sharp contrast to the large, sintered islands which form from Au1+ deposited onto a hydroxylated TiO2(110)-(1 × 1) surface under soft-landing conditions at 300 K. The position of the isolated Au atoms prepared by deposition at 600 K changes from directly above the bridging oxygen rows to directly above 5c-Ti atoms when the substrate is allowed to cool from 600 to 300 K. The binding site of the Au atoms returns to directly over the bridging oxygen rows when the temperature is returned to 600 K, indicating that this process is reversible. We attribute the change in binding site to a competition between the Au atom and an adsorbed water molecule for an oxygen vacancy on the reduced TiO2 surface. Using density functional theory (DFT), we show that dissociative adsorption of water occurs at an oxygen vacancy occupied by an Au atom, displaces the Au atom, and forms a stable OH−Au−TiO 2 complex on the surface.

Polarization anisotropy is investigated in single porous silicon nanoparticles containing multiple chromophores. Two forms of nanoparticle samples are studied; low current density (LCD) and high current density (HCD). Photoluminescence measurements reveal that LCD samples exhibit red-shifted spectra and HCD particles display a blue-shifted spectrum. We utilize single molecule spectroscopy to detect the polarization effects of spatially isolated individual nanoparticles, and show that LCD nanoparticles demonstrate strong polarization anisotropy, whereas a dynamic polarization response is collected from HCD nanoparticles.

Polarization anisotropy is investigated in single porous silicon nanoparticles containing multiple chromophores. Two classes of nanoparticles, low current density and high current density, are studied. Low current density samples exhibit red-shifted spectra and contain only one or two chromophores. High current density particles, on average, contain less than four chromophores and display a blue-shifted spectrum. We utilize single-molecule spectroscopy to probe the polarization effects of the particles, and we show that both classes of particles are influenced by a polarized excitation source. These results are exciting at the fundamental level for understanding coupled quantum dot emitters as well as for applications involving single-photon sources or silicon-based polarization-sensitive detectors.Keywords: polarization anisotropy; porous Si; silicon nanocrystals; single-molecule fluorescence

We probe the fluorescence from single molecules of a new class of tetrahedral oligo(phenylenevinylene) (OPV) molecules. Our results show that the tetrahedral molecules contain multiple chromophores with limited inter-arm coupling, but significant molecular motion about the central carbon results in fluctuations in the polarizability axis of the molecule. Loss in luminescence intensity is also observed during the fluctuations which is attributed to inter-arm coupling occurring when adjacent arms come close together. These fluctuations occur on the timescale of 100 ms to 10 s and are shown to be absent in the ‘arm’ molecules alone.

We applied shear force microscopy, an analog to attractive-mode atomic force microscopy, and near-field scanning optical microscopy (NSOM) to the study of surface roughness and nanoscale morphology in polyelectrolyte self-assembled layers. Our data show that the surface roughness of multilayer films on glass grows linearly with the number of polyelectrolyte bilayers for the first 10 bilayers, and then asymptotes at a surface roughness of 4 nm. The surface of these films is characterized by bumps of 100–500 nm in diameter and 25–50 nm tall. In addition, the size and density of the bumps for each bilayer are uncorrelated to the previous surface. This result is in sharp contrast to what is observed for other self-assembled layer structures, such as metal-phosphonate and Langmuir–Blodgett self-assembly, where the surface roughness linearly increases with the number of layers. Subsurface morphology in thin films was observed via fluorescence NSOM of a dye-doped polyelectrolyte film. The NSOM images show domains of higher and lower fluorescence intensity, which could be assigned to local changes in the film thickness, suggesting that the dye molecules are uniformly distributed within the film. We also show that both the initial and asymptotic surface roughness can be dramatically decreased if the bare glass surface is treated with a high charge-density polyelectrolyte, such as poly(ethyleneimine) (PEI), prior to the bilayer formation. Finally, we observed that if the initial substrate is rough, growth of the polyelectrolyte layers acts to smooth the surface until the equilibrium value is reached.