Single-Particle Insights into Plasmonic Hot Carrier Separation Augmenting Photoelectrochemical Ethanol Oxidation with Photocatalytically Synthesized Pd-Au Bimetallic Nanorods.

Understanding the nature of hot carrier pathways following surface plasmon excitation of heterometallic nanostructures and their mechanistic prevalence during photoelectrochemical oxidation of complex hydrocarbons, such as ethanol, remains challenging. This work studies the fate of carriers from Au nanorods before and after the presence of reductively photodeposited Pd at the single-particle level using scattering and emission spectroscopy, along with ensemble photoelectrochemical methods. A sub-2 nm epitaxial Pd0 shell was reductively grown onto colloidal Au nanorods via hot carriers generated from surface plasmon resonance excitation in the presence of [PdCl4]2-. These bimetallic Pd-Au nanorod architectures exhibited 14% quenched emission quantum yields and 9% augmented plasmon damping determined from their scattering spectra compared to the bare Au nanorods, consistent with injection/separation of intraband hot carriers into the Pd. Absorbed photon-to-current efficiency in photoelectrochemical ethanol oxidation was enhanced 50× from 0.00034% to 0.017% due to the photodeposited Pd. Photocurrent during ethanol oxidation improved 13× under solar-simulated AM1.5G and 40× for surface plasmon resonance-targeted irradiation conditions after photodepositing Pd, consistent with enhanced participation of intraband-excited sp-band holes and desorption of ethanol oxidation reaction intermediates owing to photothermal effects.

Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation.

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot electrons upon SPR irradiation drive reductive deposition of Pd on colloidal AuNR in the presence of [PdCl4]2-. Plasmon-driven reduction of secondary metals potentiates covalent, sub-wavelength deposition at targeted locations coinciding with electric field "hot-spots" of the plasmonic substrate using an external field (e.g., laser). The process described herein details a solution-phase deposition of a catalytically-active noble metal (Pd) from a transition metal halide salt (H2PdCl4) onto aqueously-suspended, anisotropic plasmonic structures (AuNR). The solution-phase process is amenable to making other bimetallic architectures. Transmission UV-vis monitoring of the photochemical reaction, coupled with ex situ XPS and statistical TEM analysis, provide immediate experimental feedback to evaluate properties of the bimetallic structures as they evolve during the photocatalytic reaction. Resonant plasmon irradiation of AuNR in the presence of [PdCl4]2- creates a thin, covalently-bound Pd0 shell without any significant dampening effect on its plasmonic behavior in this representative experiment/batch. Overall, plasmonic photodeposition offers an alternative route for high-volume, economical synthesis of optoelectronic materials with sub-5 nm features (e.g., heterometallic photocatalysts or optoelectronic interconnects).

Engineering Photosystem I Complexes with Metal Oxide Binding Peptides for Bioelectronic Applications.

Conventional dye-sensitized solar cells comprise semiconducting anodes sensitized with complex synthetic organometallic dyes, a platinum counter electrode, and a liquid electrolyte. This work focuses on replacing synthetic dyes with a naturally occurring biological pigment-protein complex known as Photosystem I (PSI). Specifically, ZnO binding peptides (ZOBiP)-fused PSI subunits (ZOBiP-PsaD and ZOBiP-PsaE) and TiO2 binding peptides (TOBiP)-fused ferredoxin (TOBiP-Fd) have been produced recombinantly from Escherichia coli. The MOBiP-fused peptides have been characterized via western blotting, circular dichroism, MALDI-TOF, and cyclic voltammetry. ZOBiP-PSI subunits have been used to replace wild-type PsaD and PsaE, and TOBiP-Fd has been chemically cross-linked to the stromal hump of PSI. These MOBiP peptides and MOBiP-PSI complexes have been produced and incubated with various metal oxide nanoparticles, showing increased binding when compared to that of wild-type PSI complexes.

Photoelectrochemistry of photosystem I bound in nafion.

Developing a solid state Photosystem I (PSI) modified electrode is attractive for photoelectrochemical applications because of the quantum yield of PSI, which approaches unity in the visible spectrum. Electrodes are constructed using a Nafion film to encapsulate PSI as well as the hole-scavenging redox mediator Os(bpy)2Cl2. The photoactive electrodes generate photocurrents of 4 µA/cm(2) when illuminated with 1.4 mW/cm(2) of 676 nm band-pass filtered light. Methyl viologen (MV(2+)) is present in the electrolyte to scavenge photoelectrons from PSI in the Nafion film and transport charges to the counter electrode. Because MV(2+) is positively charged in both reduced and oxidized states, it is able to diffuse through the cation permeable channels of Nafion. Photocurrent is produced when the working electrode is set to voltages negative of the Os(3+)/Os(2+) redox potential. Charge transfer through the Nafion film and photohole scavenging at the PSI luminal surface by Os(bpy)2Cl2 depends on the reduction of Os redox centers to Os(2+) via hole scavenging from PSI. The optimal film densities of Nafion (10 µg/cm(2) Nafion) and PSI (100 µg/cm(2) PSI) are determined to provide the highest photocurrents. These optimal film densities force films to be thin to allow the majority of PSI to have productive electrical contact with the backing electrode.

Comparative photoactivity and stability of isolated cyanobacterial monomeric and trimeric Photosystem I.

Photoactivity of native trimeric, and non-native monomeric Photosystem I (PSI) extracted from Thermosynechococcus elongatus is compared in a photoelectrochemical system. The PSI monomer is isolated by disassembling a purified PSI trimer using a freeze-thaw technique in presence of the short-chain surfactant, octylthioglucoside. Photoactive electrodes are constructed with PSI, functioning as both light absorber and charge-separator, embedded within a conductive polymer film. Despite structural differences between PSI trimers and monomers, electrodes cast with equal chlorophyll-a concentration generate similar photoactivities. Photoaction spectra show that all photocurrent derived from electrodes of PSI and conductive polymer originates solely from PSI with no photocurrent caused by redox mediators in the conductive polymer film. Longevity studies show that the two forms of PSI photodegrade at the same rate while exposed to equal intensities of 676 nm light. Direct photo-oxidation measurements indicate that PSI's monomeric form has fewer light harvesting antennae than trimer, and also shows energy sharing between monomeric subunits in the trimer. The structure of PSI is also found to impact cell performance when applying light at incident powers above 1.5 mW/cm(2) due to the reduced optical cross-section in the monomer, causing saturation at lower light intensities than the trimer.

In situ atomic force microscopy of lithiation and delithiation of silicon nanostructures for lithium ion batteries.

Using electron beam lithography, amorphous Si (a-Si) nanopillars were fabricated with a height of 100 nm and diameters of 100, 200, 300, 500, and 1000 nm. The nanopillars were electrochemically cycled in a 1 M lithium trifluoromethanesulfonate in propylene carbonate electrolyte. In situ atomic force microscopy (AFM) was used to qualitatively and quantitatively examine the morphology evolution of the nanopillars including volume and height changes versus voltage in real-time. In the first cycle, an obvious hysteresis of volume change versus voltage during lithiation and delithiation was measured. The pillars did not crack in the first cycle, but a permanent volume expansion was observed. During subsequent cycles the a-Si roughened and deformed from the initial geometry, and eventually pillars with diameters >200 nm fractured. Furthermore, a degradation of mechanical properties is suggested as the 100 and 200 nm pillars were mechanically eroded by the small contact forces under the AFM probe. Ex situ scanning electron microscopy (SEM) images, combined with analysis of the damage caused by in situ AFM imaging, demonstrate that during cycling, the silicon became porous and structurally unstable compared to as-fabricated pillars. This research highlights that even nanoscale a-Si suffers irreversible mechanical damage during cycling in organic electrolytes.

Tin-coated viral nanoforests as sodium-ion battery anodes.

Designed as a high-capacity alloy host for Na-ion chemistry, a forest of Sn nanorods with a unique core-shell structure was synthesized on viral scaffolds, which were genetically engineered to ensure a nearly vertical alignment upon self-assembly onto a metal substrate. The interdigital spaces thus formed between individual rods effectively accommodated the volume expansion and contraction of the alloy upon sodiation/desodiation, while additional carbon-coating engineered over these nanorods further suppressed Sn aggregation during extended electrochemical cycling. Due to the unique nanohierarchy of multiple functional layers, the resultant 3D nanoforest of C/Sn/Ni/TMV1cys, binder-free composite electrode already and evenly assembled on a stainless steel current collector, exhibited supreme capacity utilization and cycling stability toward Na-ion storage and release. An initial capacity of 722 mA·h (g Sn)(-1) along with 405 mA·h (g Sn)(-1) retained after 150 deep cycles demonstrates the longest-cycling nano-Sn anode material for Na-ion batteries reported in the literature to date and marks a significant performance improvement for neat Sn material as alloy host for Na-ion chemistry.

Photocurrent generation from surface assembled photosystem I on alkanethiol modified electrodes.

Photosystem I (PSI) is a key component of oxygenic photosynthetic electron transport because of its light-induced electron transfer to the soluble electron acceptor ferredoxin. This work demonstrates the incorporation of surface assembled cyanobacterial trimeric PSI complexes into a biohybrid system for light-driven current generation. Specifically, this work demonstrates the improved assembly of PSI via electrophoretic deposition, with controllable surface assembled PSI density, on different self-assembled alkanethiol monolayers. Using artificial electron donors and acceptors (Os(bpy)(2)Cl(2) and methyl viologen) we demonstrate photocurrent generation from a single PSI layer, which remains photoactive for at least three hours of intermittent illumination. Photoelectrochemical comparison of the biohybrid systems assembled from different alkanethiols (hexanethiol, aminohexanethiol, mercaptohexanol, and mercaptohexanoic acid) reveals that the PSI generated photocurrent is enhanced by almost 5 times on negatively charged SAM surfaces as compared to positively charged surfaces. These results are discussed in light of how PSI is oriented upon electrodeposition on a SAM.

Architecturing hierarchical function layers on self-assembled viral templates as 3D nano-array electrodes for integrated Li-ion microbatteries.

This work enables an elegant bottom-up solution to engineer 3D microbattery arrays as integral power sources for microelectronics. Thus, multilayers of functional materials were hierarchically architectured over tobacco mosaic virus (TMV) templates that were genetically modified to self-assemble in a vertical manner on current-collectors, so that optimum power and energy densities accompanied with excellent cycle-life could be achieved on a minimum footprint. The resultant microbattery based on self-aligned LiFePO(4) nanoforests of shell-core-shell structure, with precise arrangement of various auxiliary material layers including a central nanometric metal core as direct electronic pathway to current collector, delivers excellent energy density and stable cycling stability only rivaled by the best Li-ion batteries of conventional configurations, while providing rate performance per foot-print and on-site manufacturability unavailable from the latter. This approach could open a new avenue for microelectromechanical systems (MEMS) applications, which would significantly benefit from the concept that electrochemically active components be directly engineered and fabricated as an integral part of the integrated circuit (IC).

The growth of CuSi composite nanorod arrays by oblique angle co-deposition, and their structural, electrical and optical properties.

Different CuSi composite nanorods with 0-100 at.% Cu were fabricated by an oblique angle co-deposition technique. The effects of increasing Cu during deposition on the morphologies, structures and properties were investigated. During co-evaporation, the addition of Cu decreases the nanorod width and height but increases the nanorod tilting angle. The polarized optical transmission spectra reveal that all the nanorod samples show a remarkable anisotropic response to visible light with an eccentricity e ≈ 1, whereas their optical response to NIR light depends strongly on the Cu composition, and the related eccentricity increases monotonically with the increase of Cu. The obtained amorphous Si film has a resistivity of approximately 4.9 × 10(4) Ω cm. The incorporation of 5-75 at.% Cu increases the electrical conductance from two to eight orders of magnitude. The improved conductance and the unique optical properties of the Si-based nanocomposites could have potential applications for Li-ion battery anode and optical design.

Porous three-dimensional nanorod arrays through selective chemical etching of nanocomposites.

Three-dimensional Cu-Si and Cu-SiO(2) nanorod arrays containing ~68 at% Cu have been fabricated by a glancing angle co-deposition technique. By selectively etching Cu in 0.05 M KCN methanol solution, porous nanorods with different shapes form, which are promising for applications in sensors, catalysts, and as medical capsules that are able to be loaded with functional materials.