C Buddie Mullins

Publications of C Buddie Mullins

• Incorporation of Mo and W into nanostructured BiVO(4) films for efficient photoelectrochemical water oxidation.

  Authors: Sean P Berglund, Alexander J E Rettie, Son Hoang, C Buddie Mullins

  Physical chemistry chemical physics : PCCP. 04/2012;

  Porous, nanostructured BiVO(4) films are incorporated with Mo and W by simultaneous evaporation of Bi, V, Mo, and W in vacuum followed by oxidation in air. Synthesis parameters such as thePorous, nanostructured BiVO(4) films are incorporated with Mo and W by simultaneous evaporation of Bi, V, Mo, and W in vacuum followed by oxidation in air. Synthesis parameters such as the Bi : V : Mo : W atomic ratio and deposition angle are adjusted to optimize the films for photoelectrochemical (PEC) water oxidation. Films synthesized with a Bi : V : Mo : W atomic ratio of 46 : 46 : 6 : 2 (6% Mo, 2% W) demonstrate the best PEC performance with photocurrent densities 10 times higher than for pure BiVO(4) and greater than previously reported for Mo and W containing BiVO(4). The films consist of a directional, nanocolumnar layer beneath an irregular surface structure. Backside illumination utilizes light scattering off the irregular surface structure resulting in 30-45% higher photocurrent densities than for frontside illumination. To improve the kinetics for water oxidation Pt is photo-deposited onto the surface of the 6% Mo, 2% W BiVO(4) films as an electrocatalyst. These films achieve quantum efficiencies of 37% at 1.1 V vs. RHE and 50% at 1.6 V vs. RHE for 450 nm light.

• Improving the Stability of Nanostructured Silicon Thin Film Lithium-Ion Battery Anodes through Their Controlled Oxidation.

  Authors: Paul R Abel, Yong-Mao Lin, Hugo Celio, Adam Heller, C Buddie Mullins

  ACS nano. 03/2012; 6(3):2506-16.

  Silicon and partially oxidized silicon thin films with nanocolumnar morphology were synthesized by evaporative deposition at a glancing angle, and their performance as lithium-ion battery anodes wasSilicon and partially oxidized silicon thin films with nanocolumnar morphology were synthesized by evaporative deposition at a glancing angle, and their performance as lithium-ion battery anodes was evaluated. The incorporated oxygen concentration was controlled by varying the partial pressure of water during the deposition and monitored by quartz crystal microbalance, X-ray photoelectron spectroscopy. In addition to bulk oxygen content, surface oxidation and annealing at low temperature affected the cycling stability and lithium-storage capacity of the films. By simultaneously optimizing all three, films of ∼2200 mAh/g capacity were synthesized. Coin cells made with the optimized films were reversibly cycled for ∼120 cycles with virtually no capacity fade. After 300 cycles, 80% of the initial reversible capacity was retained.

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

  Authors: Son Hoang, Sean P Berglund, Nathan T Hahn, Allen J Bard, C Buddie Mullins

  Journal of the American Chemical Society. 02/2012; 134(8):3659-62.

  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.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.

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

  Authors: David W Flaherty, Nathan T Hahn, R Alan May, Sean P Berglund, Yong-Mao Lin, Keith J Stevenson, Zdenek Dohnalek, Bruce D Kay, C Buddie Mullins

  Accounts of chemical research. 10/2011;

  Porous, high surface area materials have critical roles in applications including catalysis, photochemistry, and energy storage. In these fields, researchers have demonstrated that thePorous, 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 charac-terization 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.

• Reactive ballistic deposition of alpha-Fe2O3 thin films for photoelectrochemical water oxidation.

  Authors: Nathan T Hahn, Heechang Ye, David W Flaherty, Allen J Bard, C Buddie Mullins

  ACS nano. 04/2010; 4(4):1977-86.

  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 forWe 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.

• CO dissociation induced by adsorbed oxygen and water on Ir(111).

  Authors: Ming Pan, Son Hoang, Jinlong Gong, C Buddie Mullins

  Chemical communications (Cambridge, England). 12/2009;

  Although CO does not dissociate on the clean Ir(111) surface, the addition of atomic oxygen induces CO dissociation at low temperatures (lower than 400 K); similarly, CO dissociation has also beenAlthough CO does not dissociate on the clean Ir(111) surface, the addition of atomic oxygen induces CO dissociation at low temperatures (lower than 400 K); similarly, CO dissociation has also been observed on water co-adsorbed Ir(111) or water and oxygen co-adsorbed Ir(111).

• Oxygen exchange in the selective oxidation of 2-butanol on oxygen precovered Au(111).

  Authors: Ting Yan, Jinlong Gong, C Buddie Mullins

  Journal of the American Chemical Society. 11/2009; 131(44):16189-94.

  Direct evidence for C-O bond cleavage in the partial oxidation of 2-butanol on oxygen precovered Au(111) is provided using temperature programmed desorption (TPD) and molecular beam reactiveDirect evidence for C-O bond cleavage in the partial oxidation of 2-butanol on oxygen precovered Au(111) is provided using temperature programmed desorption (TPD) and molecular beam reactive scattering (MBRS) under ultrahigh vacuum (UHV) conditions. The oxygen precovered Au(111) surface can promote the partial oxidation of 2-butanol into 2-butanone with near 100% selectivity at low oxygen coverages, while 2-butanol adsorbs and desorbs molecularly on the clean Au(111) surface. Both C(2)H(5)C(16)OCH(3) and C(2)H(5)C(18)OCH(3) are observed in TPD after 2-butanol (C(2)H(5)CH(16)OHCH(3)) was dosed onto Au(111) precovered with (18)O(a). This oxygen exchange phenomenon serves as strong evidence for the C-O bond cleavage in 2-butanol partial oxidation to 2-butanone. Two surface intermediates are proposed for the selective oxidation of 2-butanol: 2-butoxide and eta(2)-aldehyde. As oxygen coverage increases, full oxidation is activated in addition to selective partial oxidation.

• Surface Science Investigations of Oxidative Chemistry on Gold.

  Authors: Jinlong Gong, C Buddie Mullins

  Accounts of chemical research. 08/2009;

  Because of gold's resistance to oxidation and corrosion, historically chemists have considered this metal inert. However, decades ago, researchers discovered that highly dispersed gold particles onBecause of gold's resistance to oxidation and corrosion, historically chemists have considered this metal inert. However, decades ago, researchers discovered that highly dispersed gold particles on metal oxides are highly chemically active, particularly in low-temperature CO oxidations. These seminal findings spurred considerable interest in investigations and applications of gold-based materials. Since the discovery of gold's chemical activity at the nanoscale, researchers found that bulk gold also has interesting catalytic properties. Thus, it is important to understand and contrast the intrinsic chemical properties of bulk gold with those of nanoparticle Au. Despite numerous studies, the structure and active site of supported Au nanoclusters and the active oxygen species remain elusive, and model studies under well-controlled conditions could help identify these species. The {111} facet has the lowest surface energy and is the most stable and prevalent configuration of most supported gold nanoparticles. Therefore, a molecular-level understanding of the physical properties and surface chemistry of Au(111) could provide mechanistic details regarding the nature of Au-based catalysts and lead to improved catalytic processes. This Account focuses on our current understanding of oxidative chemistry on well-defined gold single crystals, predominantly from recent investigations on Au(111) that we have performed using modern surface science techniques. Our model system strategy allows us to control reaction conditions, which assists in the identification of reaction intermediates, the determination of the elementary reaction steps, and the evaluation of reaction energetics for rate-limiting steps. We have employed temperature-programmed desorption (TPD), molecular beam reactive scattering (MBRS), and Auger electron spectroscopy (AES) to evaluate surface oxidative chemistry. In some cases, we have combined these results with density functional theory (DFT) calculations. By controlling the reaction parameters that determine product selectivity, we have examined the chemical properties of bulk gold. Based on our investigations, the surface-bound oxygen atoms are metastable at low temperature. We also demonstrate that the oxygen atoms and formed hydroxyls are responsible for some of the distinct chemical behavior of gold and participate in surface reactions either as a Brønsted base or a nucleophilic base. We observe similar reaction patterns on gold surfaces to those on copper and silver surfaces, suggesting that the acid-base reactions that have been observed on copper and silver may also occur on gold. Our model chemical studies on gold surfaces have provided intrinsic fundamental insights into high surface area gold-based catalysts and the origin of the reactive oxygen species.

• Selective oxidation of propylamine to propionitrile and propionaldehyde on oxygen-covered gold.

  Authors: Jinlong Gong, Ting Yan, C Buddie Mullins

  Chemical communications (Cambridge, England). 03/2009;

  Oxidative dehydrogenation of amines using heterogeneous gold catalysts has unanticipated potential; chemisorbed atomic oxygen is used to activate propylamine, producing propionitrile and/orOxidative dehydrogenation of amines using heterogeneous gold catalysts has unanticipated potential; chemisorbed atomic oxygen is used to activate propylamine, producing propionitrile and/or propionaldehyde on a single-crystal Au(111) surface.

• Selective oxidation of ethanol to acetaldehyde on gold.

  Authors: Jinlong Gong, C Buddie Mullins

  Journal of the American Chemical Society. 01/2009; 130(49):16458-9.

• Selective Oxidation of Propanol on Au(111): Mechanistic Insights into Aerobic Oxidation of Alcohols.

  Authors: Jinlong Gong, David W Flaherty, Ting Yan, C Buddie Mullins

  Chemphyschem : a European journal of chemical physics and physical chemistry. 12/2008;

• Carbonate formation and decomposition on atomic oxygen precovered Au(111).

  Authors: Rotimi A Ojifinni, Jinlong Gong, Nathan S Froemming, David W Flaherty, Ming Pan, Graeme Henkelman, C Buddie Mullins

  Journal of the American Chemical Society. 09/2008; 130(34):11250-1.

  Experimental results supported by density functional theory calculations show carbonate formation and reaction on atomic oxygen precovered Au(111). Oxygen mixing is observed in temperature-programmedExperimental results supported by density functional theory calculations show carbonate formation and reaction on atomic oxygen precovered Au(111). Oxygen mixing is observed in temperature-programmed desorption measurements when a Au(111) precovered with 16O is exposed to isotopically labeled CO2 (C18O2). The presence of 16O18O is attributed to surface carbonate formation and decomposition at surface temperatures ranging from 77-400 K and initial oxygen coverages ranging from 0.18-2.1 ML. A reaction probability on the order of 10(-4) and an activation energy of -0.15+/- 0.08 eV are estimated for this reaction.

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• Water-enhanced low-temperature CO oxidation and isotope effects on atomic oxygen-covered Au(111).

  Authors: Rotimi A Ojifinni, Nathan S Froemming, Jinlong Gong, Ming Pan, Tae S Kim, J M White, Graeme Henkelman, C Buddie Mullins

  Journal of the American Chemical Society. 06/2008; 130(21):6801-12.

  Water-oxygen interactions and CO oxidation by water on the oxygen-precovered Au(111) surface were studied by using molecular beam scattering techniques, temperature-programmed desorption (TPD), andWater-oxygen interactions and CO oxidation by water on the oxygen-precovered Au(111) surface were studied by using molecular beam scattering techniques, temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. Water thermally desorbs from the clean Au(111) surface with a peak temperature of approximately 155 K; however, on a surface with preadsorbed atomic oxygen, a second water desorption peak appears at approximately 175 K. DFT calculations suggest that hydroxyl formation and recombination are responsible for this higher temperature desorption feature. TPD spectra support this interpretation by showing oxygen scrambling between water and adsorbed oxygen adatoms upon heating the surface. In further support of these experimental findings, DFT calculations indicate rapid diffusion of surface hydroxyl groups at temperatures as low as 75 K. Regarding the oxidation of carbon monoxide, if a C (16)O beam impinges on a Au(111) surface covered with both atomic oxygen ( (16)O) and isotopically labeled water (H 2 (18)O), both C (16)O (16)O and C (16)O (18)O are produced, even at surface temperatures as low as 77 K. Similar experiments performed by impinging a C (16)O beam on a Au(111) surface covered with isotopic oxygen ( (18)O) and deuterated water (D 2 (16)O) also produce both C (16)O (16)O and C (16)O (18)O but less than that produced by using (16)O and H 2 (18)O. These results unambiguously show the direct involvement and promoting role of water in CO oxidation on oxygen-covered Au(111) at low temperatures. On the basis of our experimental results and DFT calculations, we propose that water dissociates to form hydroxyls (OH and OD), and these hydroxyls react with CO to produce CO 2. Differences in water-oxygen interactions and oxygen scrambling were observed between (18)O/H 2 (16)O and (18)O/D 2 (16)O, the latter producing less scrambling. Similar differences were also observed in water reactivity toward CO oxidation, in which less CO 2 was produced with (16)O/D 2 (16)O than with (16)O/H 2 (16)O. These differences are likely due to primary kinetic isotope effects due to the differences in O-H and O-D bond energies.

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• Chemistry. Taking a selective bite out of methane.

  Authors: C Buddie Mullins, Greg O Sitz

  Science (New York, N.Y.). 03/2008; 319(5864):736-7.

• Reactivity of molecularly chemisorbed oxygen on a Au/TiO2 model catalyst.

  Authors: James D Stiehl, Jinlong Gong, Rotimi A Ojifinni, Tae S Kim, Sean M McClure, C Buddie Mullins

  The journal of physical chemistry. B. 11/2006; 110(41):20337-43.

  We present results of an investigation into the reactivity of molecularly chemisorbed oxygen with CO on a Au/TiO2 model catalyst at 77 K. We previously discovered that exposing the model catalystWe present results of an investigation into the reactivity of molecularly chemisorbed oxygen with CO on a Au/TiO2 model catalyst at 77 K. We previously discovered that exposing the model catalyst sample to a radio-frequency-generated plasma jet of oxygen results in co-population of both atomically and molecularly chemisorbed oxygen species on the sample. We tested the reactivity of the molecularly chemisorbed oxygen by comparing the CO2 produced from a sample populated with both species to the CO2 produced from a sample that has been cleared of molecularly chemisorbed oxygen employing collision-induced desorption. Samples that are populated with both species consistently result in greater CO2 produced than samples with only atomic oxygen. We interpret this result to indicate that molecularly chemisorbed oxygen on the sample can directly participate in the CO oxidation reaction. The reactivity of molecularly chemisorbed oxygen has been investigated for five different gold coverages (0.5, 0.75, 1, 1.25, and 2 ML), and we observe that there is a greater fractional difference in the CO2 produced (difference between sample populated with both molecularly and atomically adsorbed oxygen and sample populated solely with atomically adsorbed oxygen) for the 1 ML Au coverage than for the other coverages for equivalent oxygen plasma-jet exposures. However, it is not possible to unambiguously conclude that this observation is directly related to a particle size effect on the chemistry since the absolute O(2,a) and O(a) content on the various surfaces is different for all the coverages studied because of the plasma-jet technique that we employed for populating the surfaces with oxygen. Unfortunately, this precludes a direct comparison of the reactivity of molecular oxygen in the carbon monoxide oxidation reaction as a function of gold coverage and hence particle size.

• Transport in amorphous solid water films: implications for self-diffusivity.

  Authors: Sean M McClure, Evan T Barlow, Minta C Akin, Douglas J Safarik, Thomas M Truskett, C Buddie Mullins

  The journal of physical chemistry. B. 10/2006; 110(36):17987-97.

  Thermal desorption spectroscopy is employed to examine transport mechanisms in structured, nanoscale films consisting of labeled amorphous solid water (ASW, H(2)(18)O, H(2)(16)O) and organic spacerThermal desorption spectroscopy is employed to examine transport mechanisms in structured, nanoscale films consisting of labeled amorphous solid water (ASW, H(2)(18)O, H(2)(16)O) and organic spacer layers (CCl(4), CHCl(3)) prior to ASW crystallization (T approximately 150-160 K). Self-transport is studied as a function of both the ASW layer and the organic spacer layer film thickness, and the effectiveness of these spacer layers as a bulk diffusion "barrier" is also investigated. Isothermal desorption measurements of structured films are combined with gas uptake measurements (CClF(2)H) to investigate water self-transport and changes in ASW film morphology during crystallization and annealing. CCl(4) desorption is employed as a means to investigate the effects of ASW film thickness and heating schedule on vapor-phase transport. Combined, these results demonstrate that the interlayer mixing observed near T approximately 150-160 K is inconsistent with a mechanism involving diffusion through a dense phase; rather, we propose that intermixing occurs via vapor-phase transport through an interconnected network of cracks/fractures created within the ASW film during crystallization. Consequently, the self-diffusivity of ASW prior to crystallization (T approximately 150-160 K) is significantly smaller than that expected for a "fragile" liquid, indicating that water undergoes either a glass transition or a fragile-to-strong transition at a temperature above 160 K.

• Selective catalytic oxidation of ammonia to nitrogen on atomic oxygen precovered Au(111).

  Authors: Jinlong Gong, Rotimi A Ojifinni, Tae S Kim, John M White, C Buddie Mullins

  Journal of the American Chemical Society. 08/2006; 128(28):9012-3.

  We demonstrate ammonia oxidation promoted by an atomic oxygen precovered Au(111) surface. The selectivity of the catalytic oxidation of ammonia to NO or N2 on Au(111) is tunable by the atomic oxygenWe demonstrate ammonia oxidation promoted by an atomic oxygen precovered Au(111) surface. The selectivity of the catalytic oxidation of ammonia to NO or N2 on Au(111) is tunable by the atomic oxygen coverage. We propose that N2 and NO are produced via the recombination reactions of Nad + Nad and Nad + Oad.

• Evidence that amorphous water below 160 K is not a fragile liquid.

  Authors: Sean M McClure, Douglas J Safarik, Thomas M Truskett, C Buddie Mullins

  The journal of physical chemistry. B. 07/2006; 110(23):11033-6.

  We have examined transport mechanisms in amorphous solid water (ASW) by studying thermal desorption of layered nanoscale films of CCl4 and labeled ASW. The interlayer mixing observed near TWe have examined transport mechanisms in amorphous solid water (ASW) by studying thermal desorption of layered nanoscale films of CCl4 and labeled ASW. The interlayer mixing observed near T approximately 150-160 K is inconsistent with a mechanism involving diffusion through a dense phase. Instead, intermixing occurs via vapor-phase transport through an interconnected porous network created within the film during crystallization. As a consequence, the self-diffusivity of ASW is significantly smaller than previously thought, indicating that water undergoes either a glass transition or a fragile-to-strong transition at a temperature above 160 K.

• Water activated by atomic oxygen on Au(111) to oxidize CO at low temperatures.

  Authors: Tae S Kim, Jinlong Gong, Rotimi A Ojifinni, J M White, C Buddie Mullins

  Journal of the American Chemical Society. 06/2006; 128(19):6282-3.

  The Au(111) surface was populated with atomic oxygen [16O] followed by oxygen-labeled water [H218O] at surface temperatures as low as 77 K. When a CO beam was impinged on this surface, both [C16O16O]The Au(111) surface was populated with atomic oxygen [16O] followed by oxygen-labeled water [H218O] at surface temperatures as low as 77 K. When a CO beam was impinged on this surface, both [C16O16O] and [C16O18O] were produced. The results strongly suggest the direct involvement and promoting role of water in CO oxidation on oxygen covered Au(111) at low temperatures.

• Formation of molecularly chemisorbed oxygen on TiO2-supported gold nanoclusters and Au(111) from exposure to an oxygen plasma jet.

  Authors: James D Stiehl, Tae S Kim, Sean M McClure, C Buddie Mullins

  The journal of physical chemistry. B. 05/2005; 109(13):6316-22.

  We present results of an investigation into the low-temperature formation of molecularly chemisorbed oxygen on a Au/TiO(2) model catalyst and on a Au(111) single crystal during exposure to a plasmaWe present results of an investigation into the low-temperature formation of molecularly chemisorbed oxygen on a Au/TiO(2) model catalyst and on a Au(111) single crystal during exposure to a plasma jet of oxygen. Through the use of collision-induced desorption measurements and isotopic mixing experiments we show evidence suggesting that at least some of the molecular oxygen is formed as a result of recombination of oxygen atoms on the samples during the plasma exposure. Of course, adsorption of excited molecular oxygen directly from the gas phase may also take place. We also present evidence showing that the adsorption of oxygen atoms on the surface assists in the molecular chemisorption of oxygen on the Au/TiO(2) model catalyst samples. Thus, oxygen molecules impinging on the samples during plasma-jet exposures (plasma jet has approximately 40% dissociation fraction) could have an enhanced probability of adsorption due to simultaneous oxygen atom adsorption.