Nathan T. Hahn

Enhancing visible light photo-oxidation of water with TiO2 nanowire arrays via cotreatment with H2 and NH3: synergistic effects between Ti3+ and N.

We report a synergistic effect involving hydrogenation and nitridation cotreatment of TiO(2) nanowire (NW) arrays that improves the water photo-oxidation performance under visible light illumination. The visible light (>420 nm) photocurrent of the cotreated TiO(2) is 0.16 mA/cm(2) and accounts for 41% of the total photocurrent under simulated AM 1.5 G illumination. Electron paramagnetic resonance (EPR) spectroscopy reveals that the concentration of Ti(3+) species in the bulk of the TiO(2) following hydrogenation and nitridation cotreatment is significantly higher than that of the sample treated solely with ammonia. It is believed that the interaction between the N-dopant and Ti(3+) is the key to the extension of the active spectrum and the superior visible light water photo-oxidation activity of the hydrogenation and nitridation cotreated TiO(2) NW arrays.

Visible Light Driven Photoelectrochemical Water Oxidation on Nitrogen-Modified TiO2 Nanowires

We report hydrothermal synthesis of single crystalline TiO(2) nanowire arrays with unprecedented small feature sizes of ~5 nm and lengths up to 4.4 μm on fluorine-doped tin oxide substrates. A substantial amount of nitrogen (up to 1.08 atomic %) can be incorporated into the TiO(2) lattice via nitridation in NH(3) flow at a relatively low temperature (500 °C) because of the small cross-section of the nanowires. The low-energy threshold of the incident photon to current efficiency (IPCE) spectra of N-modified TiO(2) samples is at ~520 nm, corresponding to 2.4 eV. We also report a simple cobalt treatment for improving the photoelectrochemical (PEC) performance of our N-modified TiO(2) nanowire arrays. With the cobalt treatment, the IPCE of N-modified TiO(2) samples in the ultraviolet region is restored to equal or higher values than those of the unmodified TiO(2) samples, and it remains as high as ~18% at 450 nm. We propose that the cobalt treatment enhances PEC performance via two mechanisms: passivating surface states on the N-modified TiO(2) surface and acting as a water oxidation cocatalyst.

Reactive Ballistic Deposition of α-Fe2O3 Thin Films for Photoelectrochemical Water Oxidation

We report the preparation of alpha-Fe2O3 electrodes using a technique known as reactive ballistic deposition in which iron metal is evaporatively deposited in an oxygen ambient for photoelectrochemical (PEC) water oxidation. By manipulating synthesis parameters such as deposition angle, film thickness, and annealing temperature, we find that it is possible to optimize the structural and morphological properties of such films in order to improve their PEC efficiency. Incident photon to current conversion efficiencies (IPCE) are used to calculate an AM1.5 photocurrent of 0.55 mA/cm(2) for optimized films with an IPCE reaching 10% at 420 nm in 1 M KOH at +0.5 V versus Ag/AgCl. We also note that the commonly observed low photoactivity of extremely thin hematite films on fluorine-doped tin oxide substrates may be improved by modification of annealing conditions in some cases.

Effects of Composition and Annealing Conditions on Catalytic Activities of Dealloyed Pt–Cu Nanoparticle Electrocatalysts for PEMFC

Dealloyed Pt 25 Cu 75 bimetallic nanoparticle electrocatalysts exhibit up to six times higher oxygen reduction reaction activities than pure nanoparticle Pt catalysts at 0.9 V/ reversible hydrogen electrode (RHE). The active form of the catalyst is formed in situ from Pt-Cu precursor material using voltammetric dealloying. The effects of composition of precursors as well as effects of the annealing temperature and duration on the catalyst activity are studied. We vary the composition between Pt 25 Cu 75 and Pt 75 Cu 25 and change the annealing conditions from 600 to 950°C and for 7 and 14 h. X-ray diffraction and electrochemical analyses are used to obtain insight on the structural details of the catalyst samples. Information regarding the extent of alloying, atomic ordering, the Pt and Cu compositions, and distributions on the nanoparticles and particle (crystallite) sizes is correlated with the trends observed from mass and specific activities of the catalysts. It was found that an annealing duration of 14 h offers little or no benefit to catalytic activities compared to 7 h. Dealloyed Pt 25 Cu 75 annealed for 7 h, at 800°C yielded an optimal active material with respect to the extent of alloying and particle size growth and exhibited the highest Pt mass-based and favorable specific catalytic oxygen reduction reaction (ORR) activity. The occurrence and role of a noncubic Pt 50 Cu 50 Hongshiite phase is discussed.

Reactive Ballistic Deposition of Nanostructured Model Materials for Electrochemical Energy Conversion and Storage

Porous, high surface area materials have critical roles in applications including catalysis, photochemistry, and energy storage. In these fields, researchers have demonstrated that the nanometer-scale structure modifies mechanical, optical, and electrical properties of the material, greatly influencing its behavior and performance. Such complex chemical systems can involve several distinct processes occurring in series or parallel. Understanding the influence of size and structure on the properties of these materials requires techniques for producing clean, simple model systems. In the fields of photoelectrochemistry and lithium storage, for example, researchers need to evaluate the effects of changing the electrode structure of a single material or producing electrodes of many different candidate materials while maintaining a distinctly favorable morphology. In this Account, we introduce our studies of the formation and characterization of high surface area, porous thin films synthesized by a process called reactive ballistic deposition (RBD). RBD is a simple method that provides control of the morphology, porosity, and surface area of thin films by manipulating the angle at which a metal-vapor flux impinges on the substrate during deposition. This approach is largely independent of the identity of the deposited material and relies upon limited surface diffusion during synthesis, which enables the formation of kinetically trapped structures. Here, we review our results for the deposition of films from a number of semiconductive materials that are important for applications such as photoelectrochemical water oxidation and lithium ion storage. The use of RBD has enabled us to systematically control individual aspects of both the structure and composition of thin film electrodes in order to probe the effects of each on the performance of the material. We have evaluated the performance of several materials for potential use in these applications and have identified processes that limit their performance. Use of model systems, such as these, for fundamental studies or materials screening processes likely will prove useful in developing new high-performance electrodes.

Lithium Self-Discharge and Its Prevention: Direct Visualization through In Situ Electrochemical Scanning Transmission Electron Microscopy

To understand the mechanism that controls low-aspect-ratio lithium deposition morphologies for Li-metal anodes in batteries, we conducted direct visualization of Li-metal deposition and stripping behavior through nanoscale in situ electrochemical scanning transmission electron microscopy (EC-STEM) and macroscale-cell electrochemistry experiments in a recently developed and promising solvate electrolyte, 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane. In contrast to published coin cell studies in the same electrolyte, our experiments revealed low Coulombic efficiencies and inhomogeneous Li morphology during in situ observation. We conclude that this discrepancy in Coulombic efficiency and morphology of the Li deposits was dependent on the presence of a compressed lithium separator interface, as we have confirmed through macroscale (not in the transmission electron microscope) electrochemical experiments. Our data suggests that cell compression changed how the solid-electrolyte interphase formed, which is likely responsible for improved morphology and Coulombic efficiency with compression. Furthermore, during the in situ EC-STEM experiments, we observed direct evidence of nanoscale self-discharge in the solvate electrolyte (in the state of electrical isolation). This self-discharge was duplicated in the macroscale, but it was less severe with electrode compression, likely due to a more passivating and corrosion-resistant solid-electrolyte interphase formed in the presence of compression. By combining the solvate electrolyte with a protective LiAl0.3S coating, we show that the Li nucleation density increased during deposition, leading to improved morphological uniformity. Furthermore, self-discharge was suppressed during rest periods in the cycling profile with coatings present, as evidenced through EC-STEM and confirmed with coin cell data.

Controlled Electrochemical Li Cycling in a TEM to Observe Li Morphology Evolution

To meet the increased demand for high power energy storage for grid and transportation applications, a stable highly efficient Li metal battery electrode is being investigated. The system is dependent on the formation of a solid electrolyte interphase that is capable of suppressing Li dendrite formation, which is the limiting characteristic that prevents application of high capacity Li metal anodes in current lithium ion batteries (LIBs). The SEI layer that is formed between the electrolyte and the Li metal electrode is dependent on the breakdown of the Li containing electrolyte. We investigated lithium bis(fluorosulfonyl)imide (LiFSI) in dimethoxyethane (DME) for suppressed Li dendrite formation [1]. This electrolyte is targeted for stable stripping of lithium at current densities up to 10 mA/cm 2 and Coulombic efficiencies above 99.1%. The morphological evolution of the Li deposition and stripping on copper electrodes was monitored using quantitative in situ scanning transmission electron microscopy (STEM) in a custom fabricated electrochemical cell.