Publications by authors named "Iradwikanari Waluyo"

32 Publications

Interface Sensitivity in Electron/Ion Yield X-ray Absorption Spectroscopy: The TiO-HO Interface.

J Phys Chem Lett 2021 Oct 14;12(41):10212-10217. Epub 2021 Oct 14.

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

To understand corrosion, energy storage, (electro)catalysis, etc., obtaining chemical information on the solid-liquid interface is crucial but remains extremely challenging. Here, X-ray absorption spectroscopy (XAS) is used to study the solid-liquid interface between TiO and HO. A thin film (6.7 nm) of TiO is deposited on an X-ray-transparent SiN window, acting as the working electrode in a three-electrode flow cell. The spectra are collected based on the electron emission resulting from the decay of the X-ray-induced core-hole-excited atoms, which we show is sensitive to the solid-liquid interface within a few nm. The drain currents measured at the working and counter electrodes are identical but of opposite sign. With this method, we found that the water layer next to anatase is spectroscopically similar to ice. This result highlights the potential of electron-yield XAS to obtain chemical and structural information with a high sensitivity for the species at the electrode-electrolyte interface.
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http://dx.doi.org/10.1021/acs.jpclett.1c02115DOI Listing
October 2021

Xenon Trapping in Metal-Supported Silica Nanocages.

Small 2021 10 31;17(39):e2103661. Epub 2021 Aug 31.

Materials Science and Chemical Engineering Department, State University of New York at Stony Brook, 100 Nicolls Rd, Stony Brook, NY, 11794, USA.

Xenon (Xe) is a valuable and scarce noble gas used in various applications, including lighting, electronics, and anesthetics, among many others. It is also a volatile byproduct of the nuclear fission of uranium. A novel material architecture consisting of silicate nanocages in contact with a metal surface and an approach for trapping single Xe atoms in these cages is presented. The trapping is done at low Xe pressures and temperatures between 400 and 600 K, and the process is monitored in situ using synchrotron-based ambient pressure X-ray photoelectron spectroscopy. Release of the Xe from the cages occurs only when heating to temperatures above 750 K. A model that explains the experimental trapping kinetics is proposed and tested using Monte Carlo methods. Density functional theory calculations show activation energies for Xe exiting the cages consistent with experiments. This work can have significant implications in various fields, including Xe production, nuclear power, nuclear waste remediation, and nonproliferation of nuclear weapons. The results are also expected to apply to argon, krypton, and radon, opening an even more comprehensive range of applications.
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http://dx.doi.org/10.1002/smll.202103661DOI Listing
October 2021

Thermally Aged Li-Mn-O Cathode with Stabilized Hybrid Cation and Anion Redox.

Nano Lett 2021 May 14;21(10):4176-4184. Epub 2021 May 14.

Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.

Though low-cost and environmentally friendly, Li-Mn-O cathodes suffer from low energy density. Although synthesized LiMnO-like overlithiated spinel cathode with reversible hybrid anion- and cation-redox (HACR) activities has a high initial capacity, it degrades rapidly due to oxygen loss and side-reaction-induced electrolyte decomposition. Herein, we develop a two-step heat treatment to promote local decomposition as LiMnO → 2LiMnO + LiMnO + 1/2 O↑, which releases near-surface reactive oxygen that is harmful to cycling stability. The produced nanocomposite delivers a high discharge capacity of 225 mAh/g and energy density of over 700 Wh/kg at active-material level at a current density of 100 mA/g between 1.8 to 4.7 V. Benefiting from suppressed oxygen loss and side reactions, 80% capacity retention is achieved after 214 cycles in half cells. With industrially acceptable electrolyte amount (6 g/Ah), full cells paired with LiTiO anode have a good retention over 100 cycles.
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http://dx.doi.org/10.1021/acs.nanolett.0c04920DOI Listing
May 2021

Enhanced Catalysis under 2D Silica: A CO Oxidation Study.

Angew Chem Int Ed Engl 2021 May 30;60(19):10888-10894. Epub 2021 Mar 30.

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, 11973, USA.

Interfacially confined microenvironments have recently gained attention in catalysis, as they can be used to modulate reaction chemistry. The emergence of a 2D nanospace at the interface between a 2D material and its support can promote varying kinetic and energetic schemes based on molecular level confinement effects imposed in this reduced volume. We report on the use of a 2D oxide cover, bilayer silica, on catalytically active Pd(111) undergoing the CO oxidation reaction. We "uncover" mechanistic insights about the structure-activity relationship with and without a 2D silica overlayer using in situ IR and X-ray spectroscopy and mass spectrometry methods. We find that the CO oxidation reaction on Pd(111) benefits from confinement effects imposed on surface adsorbates under 2D silica. This interaction results in a lower and more dispersed coverage of CO adsorbates with restricted CO adsorption geometries, which promote oxygen adsorption and lay the foundation for the formation of a reactive surface oxide that produces higher CO formation rates than Pd alone.
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http://dx.doi.org/10.1002/anie.202013801DOI Listing
May 2021

A Surface Se-Substituted LiCo[O Se ] Cathode with Ultrastable High-Voltage Cycling in Pouch Full-Cells.

Adv Mater 2020 Dec 10;32(50):e2005182. Epub 2020 Nov 10.

Department of Nuclear Science and Engineering and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA.

Cycling LiCoO to above 4.5 V for higher capacity is enticing; however, hybrid O anion- and Co cation-redox (HACR) at high voltages facilitates intrinsic O (α < 2) migration, causing oxygen loss, phase collapse, and electrolyte decomposition that severely degrade the battery cyclability. Hereby, commercial LiCoO particles are operando treated with selenium, a well-known anti-aging element to capture oxygen-radicals in the human body, showing an "anti-aging" effect in high-voltage battery cycling and successfully stopping the escape of oxygen from LiCoO even when the cathode is cycled to 4.62 V. Ab initio calculation and soft X-ray absorption spectroscopy analysis suggest that during deep charging, the precoated Se will initially substitute some mobile O at the charged LiCoO surface, transplanting the pumped charges from O and reducing it back to O to stabilize the oxygen lattice in prolonged cycling. As a result, the material retains 80% and 77% of its capacity after 450 and 550 cycles under 100 mA g in 4.57 V pouch full-cells matched with a graphite anode and an ultralean electrolyte (2 g Ah ).
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http://dx.doi.org/10.1002/adma.202005182DOI Listing
December 2020

Nucleation and Initial Stages of Growth during the Atomic Layer Deposition of Titanium Oxide on Mesoporous Silica.

Nano Lett 2020 Sep 25;20(9):6884-6890. Epub 2020 Aug 25.

Department of Chemistry, University of California, Riverside, California 92521, United States.

A chemical approach to the deposition of thin films on solid surfaces is highly desirable but prone to affect the final properties of the film. To better understand the origin of these complications, the initial stages of the atomic layer deposition of titania films on silica mesoporous materials were characterized. Adsorption-desorption measurements indicated that the films grow in a layer-by-layer fashion, as desired, but initially exhibit surprisingly low densities, about one-quarter of that of bulk titanium oxide. Electron microscopy, X-ray diffraction, UV/visible, and X-ray absorption spectroscopy data pointed to the amorphous nature of the first monolayers, and EXAFS and Si CP/MAS NMR results to an initial growth via the formation of individual tetrahedral Ti-oxide units on isolated Si-OH surface groups with unusually long Ti-O bonds. Density functional theory calculations were used to propose a mechanism where the film growth starts at the nucleation centers to form an open 2D structure.
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http://dx.doi.org/10.1021/acs.nanolett.0c02990DOI Listing
September 2020

Catalytic Oxidation of CO on a Curved Pt(111) Surface: Simultaneous Ignition at All Facets through a Transient CO-O Complex*.

Angew Chem Int Ed Engl 2020 Nov 31;59(45):20037-20043. Epub 2020 Aug 31.

Centro de Física de Materiales CSIC/UPV-EHU-, Materials Physics Center, Manuel Lardizabal 5, 20018, San Sebastian, Spain.

The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.
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http://dx.doi.org/10.1002/anie.202007195DOI Listing
November 2020

Protonic solid-state electrochemical synapse for physical neural networks.

Nat Commun 2020 06 19;11(1):3134. Epub 2020 Jun 19.

Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA.

Physical neural networks made of analog resistive switching processors are promising platforms for analog computing. State-of-the-art resistive switches rely on either conductive filament formation or phase change. These processes suffer from poor reproducibility or high energy consumption, respectively. Herein, we demonstrate the behavior of an alternative synapse design that relies on a deterministic charge-controlled mechanism, modulated electrochemically in solid-state. The device operates by shuffling the smallest cation, the proton, in a three-terminal configuration. It has a channel of active material, WO. A solid proton reservoir layer, PdH, also serves as the gate terminal. A proton conducting solid electrolyte separates the channel and the reservoir. By protonation/deprotonation, we modulate the electronic conductivity of the channel over seven orders of magnitude, obtaining a continuum of resistance states. Proton intercalation increases the electronic conductivity of WO by increasing both the carrier density and mobility. This switching mechanism offers low energy dissipation, good reversibility, and high symmetry in programming.
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http://dx.doi.org/10.1038/s41467-020-16866-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7371700PMC
June 2020

CO Oxidation Mechanisms on CoO-Pt Thin Films.

J Am Chem Soc 2020 May 23;142(18):8312-8322. Epub 2020 Apr 23.

Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

The reaction of CO and O with submonolayer and multilayer CoO films on Pt(111), to produce CO, was investigated at room temperature in the mTorr pressure regime. Using operando ambient pressure X-ray photoelectron spectroscopy and high pressure scanning tunneling microscopy, as well as density functional theory calculations, we found that the presence of oxygen vacancies in partially oxidized CoO films significantly enhances the CO oxidation activity to form CO upon exposure to mTorr pressures of CO at room temperature. In contrast, CoO films without O-vacancies are much less active for CO formation at RT, and CO only adsorbed in the form of carbonate species that are stable up to 260 °C. On submonolayer CoO islands, the carbonates form preferentially at island edges, deactivating the edge sites for CO formation, even while the reaction proceeds inside the islands. These results provide a detailed understanding of CO oxidation pathways on systems where noble metals such as Pt interact with reducible oxides.
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http://dx.doi.org/10.1021/jacs.0c01139DOI Listing
May 2020

Bi-directional tuning of thermal transport in SrCoO with electrochemically induced phase transitions.

Nat Mater 2020 Jun 24;19(6):655-662. Epub 2020 Feb 24.

Laboratory for Electrochemical Interfaces, Massachusetts Institute of Technology, Cambridge, MA, USA.

Unlike the wide-ranging dynamic control of electrical conductivity, there does not exist an analogous ability to tune thermal conductivity by means of electric potential. The traditional picture assumes that atoms inserted into a material's lattice act purely as a source of scattering for thermal carriers, which can only reduce thermal conductivity. In contrast, here we show that the electrochemical control of oxygen and proton concentration in an oxide provides a new ability to bi-directionally control thermal conductivity. On electrochemically oxygenating the brownmillerite SrCoO to the perovskite SrCoO, the thermal conductivity increases by a factor of 2.5, whereas protonating it to form hydrogenated SrCoO effectively reduces the thermal conductivity by a factor of four. This bi-directional tuning of thermal conductivity across a nearly 10 ± 4-fold range at room temperature is achieved by using ionic liquid gating to trigger the 'tri-state' phase transitions in a single device. We elucidated the effects of these anionic and cationic species, and the resultant changes in lattice constants and lattice symmetry on thermal conductivity by combining chemical and structural information from X-ray absorption spectroscopy with thermoreflectance thermal conductivity measurements and ab initio calculations. This ability to control multiple ion types, multiple phase transitions and electronic conductivity that spans metallic through to insulating behaviour in oxides by electrical means provides a new framework for tuning thermal transport over a wide range.
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http://dx.doi.org/10.1038/s41563-020-0612-0DOI Listing
June 2020

Morphology and chemical behavior of model CsO/CuO/Cu(111) nanocatalysts for methanol synthesis: Reaction with CO and H.

J Chem Phys 2020 Jan;152(4):044701

Chemistry Department, Stony Brook University, Stony Brook, New York 11794, USA.

Cs is a promoter of Cu-based catalysts for the synthesis of alcohols from CO hydrogenation. Scanning tunneling microscopy and ambient-pressure x-ray photoelectron spectroscopy were used to study the morphology and chemical properties of surfaces generated by the deposition of cesium on CuO/Cu(111) and Cu(111) substrates. CsO nanostructures were formed after Cs metal was deposited on CuO/Cu(111) at 300 K. The formed CsO protrude over the surface of copper oxide by 2-4 Å, with the dimension at the base of the nanostructures being in the range of 1-3 nm. Heating to elevated temperature induced significant changes in the size and dispersion of the CsO nanostructures, and there was a clear reconstruction of the copper oxide substrate, which then exhibited long range order with a hexagonally packed structure. The as-deposited and annealed surfaces of CsO/CuO/Cu(111) were more reactive toward CO than plain CuO/Cu(111) or clean Cu(111). However, none of them were stable in the presence of H, which fully reduced the copper oxide at 400-450 K. In CsO/Cu(111), the CsO nanoclusters were dispersed all over the metallic copper in no particular order. The CsO species had an average width of 2 nm and ∼1 Å height. The CsO/Cu(111) systems exhibited the highest activity for the binding and dissociation of CO, suggesting that the CsO-copper interface plays a key role in alcohol synthesis.
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http://dx.doi.org/10.1063/1.5129152DOI Listing
January 2020

Ultrafine CoO nanoparticles as an efficient cocatalyst for enhanced photocatalytic hydrogen evolution.

Nanoscale 2019 Sep 13;11(33):15633-15640. Epub 2019 Aug 13.

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China.

In order to further enhance the performance of photocatalysts, cocatalysts are used to accelerate the photocatalytic reactions. Herein, ultrafine cobalt oxide (CoO) nanoparticles are synthesized through a novel bottom-up strategy and explored as an efficient non-noble cocatalyst to dramatically promote the photocatalytic hydrogen evolution rate of CdS nanorods. CdS/CoO heterostructures, consisting of highly dispersed 3-5 nm CoO nanoparticles anchored on the CdS nanorods, can provide a high photocatalytic hydrogen evolution rate of 6.45 mmol g h (∼36 times higher than that of bare CdS nanorods) in the visible-light region (>420 nm). Combined X-ray photoelectron spectroscopy and X-ray absorption near edge spectroscopy analyses suggest Co-S bond formation between CoO and CdS, which guarantees efficient migration and separation of photogenerated charge carriers. This work provides a new avenue for adopting CoO as an effective cocatalyst for enhanced photocatalytic hydrogen production in the visible-light region.
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http://dx.doi.org/10.1039/c9nr05086hDOI Listing
September 2019

Improving the Electrochemical Performance and Structural Stability of the LiNiCoAlO Cathode Material at High-Voltage Charging through Ti Substitution.

ACS Appl Mater Interfaces 2019 Jul 21;11(26):23213-23221. Epub 2019 Jun 21.

Department of Materials Science , Fudan University , Shanghai 200433 , China.

LiNiCoAlO (NCA) has been proven to be a good cathode material for lithium-ion batteries (LIBs), especially in electric vehicle applications. However, further elevating energy density of NCA is very challenging. Increasing the charging voltage of NCA is an effective method, but its structural instability remains a problem. In this work, we revealed that titanium substitution could improve cycle stability of NCA under high cutoff voltage significantly. Titanium ions with a relatively larger ion radius could modify the oxygen lattice and change the local coordination environment of NCA, leading to decreased cation migration, better kinetic and thermodynamic properties, and improved structural stability. As a result, the Ti-substituted NCA cathode exhibits impressive reversible capacity (198 mA h g at 0.1C) with considerable cycle stability under a cutoff voltage up to 4.7 V. It is also revealed that Ti could suppress oxygen release in the high-voltage region, benefitting cycle and thermal stabilities. This work provides valuable insight into the design of high-voltage layered cathode materials for high-energy-density LIBs.
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http://dx.doi.org/10.1021/acsami.9b05100DOI Listing
July 2019

Ultrathin Amorphous Titania on Nanowires: Optimization of Conformal Growth and Elucidation of Atomic-Scale Motifs.

Nano Lett 2019 06 7;19(6):3457-3463. Epub 2019 May 7.

Department of Materials Science and Chemical Engineering , Stony Brook University , Stony Brook , New York 11794 , United States.

Due to its chemical stability, titania (TiO) thin films increasingly have significant impact when applied as passivation layers. However, optimization of growth conditions, key to achieving essential film quality and effectiveness, is challenging in the few-nanometers thickness regime. Furthermore, the atomic-scale structure of the nominally amorphous titania coating layers, particularly when applied to nanostructured supports, is difficult to probe. In this Letter, the quality of titania layers grown on ZnO nanowires is optimized using specific strategies for processing of the nanowire cores prior to titania coating. The best approach, low-pressure O plasma treatment, results in significantly more-uniform titania films and a conformal coating. Characterization using X-ray absorption near edge structure (XANES) reveals the titania layer to be highly amorphous, with features in the Ti spectra significantly different from those observed for bulk TiO polymorphs. Analysis based on first-principles calculations suggests that the titania shell contains a substantial fraction of under-coordinated Ti ions. The best match to the experimental XANES spectrum is achieved with a "glassy" TiO model that contains ∼50% of under-coordinated Ti ions, in contrast to bulk crystalline TiO that only contains 6-coordinated Ti ions in octahedral sites.
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http://dx.doi.org/10.1021/acs.nanolett.8b04888DOI Listing
June 2019

Deconvolution of octahedral PtNi nanoparticle growth pathway from in situ characterizations.

Nat Commun 2018 10 26;9(1):4485. Epub 2018 Oct 26.

Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH, 44325, USA.

Understanding the growth pathway of faceted alloy nanoparticles at the atomic level is crucial to morphology control and property tuning. Yet, it remains a challenge due to complexity of the growth process and technical limits of modern characterization tools. We report a combinational use of multiple cutting-edge in situ techniques to study the growth process of octahedral PtNi nanoparticles, which reveal the particle growth and facet formation mechanisms. Our studies confirm the formation of octahedral PtNi initiates from Pt nuclei generation, which is followed by continuous Pt reduction that simultaneously catalyzes Ni reduction, resulting in mixed alloy formation with moderate elemental segregation. Carbon monoxide molecules serve as a facet formation modulator and induce Ni segregation to the surface, which inhibits the (111) facet growth and causes the particle shape to evolve from a spherical cluster to an octahedron as the (001) facet continues to grow.
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http://dx.doi.org/10.1038/s41467-018-06900-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6203767PMC
October 2018

Strongly correlated perovskite lithium ion shuttles.

Proc Natl Acad Sci U S A 2018 09 13;115(39):9672-9677. Epub 2018 Aug 13.

School of Materials Engineering, Purdue University, West Lafayette, IN 47907.

Solid-state ion shuttles are of broad interest in electrochemical devices, nonvolatile memory, neuromorphic computing, and biomimicry utilizing synthetic membranes. Traditional design approaches are primarily based on substitutional doping of dissimilar valent cations in a solid lattice, which has inherent limits on dopant concentration and thereby ionic conductivity. Here, we demonstrate perovskite nickelates as Li-ion shuttles with simultaneous suppression of electronic transport via Mott transition. Electrochemically lithiated SmNiO (Li-SNO) contains a large amount of mobile Li located in interstitial sites of the perovskite approaching one dopant ion per unit cell. A significant lattice expansion associated with interstitial doping allows for fast Li conduction with reduced activation energy. We further present a generalization of this approach with results on other rare-earth perovskite nickelates as well as dopants such as Na The results highlight the potential of quantum materials and emergent physics in design of ion conductors.
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http://dx.doi.org/10.1073/pnas.1805029115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6166818PMC
September 2018

Imaging the ordering of a weakly adsorbed two-dimensional condensate: ambient-pressure microscopy and spectroscopy of CO molecules on rutile TiO(110).

Phys Chem Chem Phys 2018 May;20(19):13122-13126

Chemistry Department, Brookhaven National Laboratory, Bldg. 555A, P.O. Box 5000, Upton, New York 11973, USA. and Department of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA.

Disorder-Order transitions in a weakly adsorbed two-dimensional film have been identified for the first time using ambient-pressure scanning tunneling microscopy (AP-STM) and X-ray photoelectron spectroscopy (AP-XPS). As of late, great effort has been devoted to the capture, activation and conversion of carbon dioxide (CO2), a ubiquitous greenhouse gas and by-product of many chemical processes. The high stability and non-polar nature of CO2 leads to weak bonding with well-defined surfaces of metals and oxides. CO2 adsorbs molecularly on the rutile TiO2(110) surface with a low adsorption energy of ∼10 kcal mol-1. In spite of this weak binding, images of AP-STM show that a substantial amount of CO2 can reside on a TiO2(110) surface at room temperature forming two-dimensionally ordered films. We have employed microscopic imaging under in situ conditions, soft X-ray spectroscopy and theory to decipher the unique ordering behavior seen for CO2 on TiO2(110).
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http://dx.doi.org/10.1039/c8cp01614cDOI Listing
May 2018

Enhanced, robust light-driven H generation by gallium-doped titania nanoparticles.

Phys Chem Chem Phys 2018 Jan;20(3):2104-2112

Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA.

The splitting of water into molecular hydrogen and oxygen with the use of renewable solar energy is considered one of the most promising routes to yield sustainable fuel. Herein, we report the H evolution performance of gallium doped TiO photocatalysts with varying degrees of Ga dopant. The gallium(iii) ions induced significant changes in the structural, textural and electronic properties of TiO nanoparticles, resulting in remarkably enhanced photocatalytic activity and good stability for H production. Ga ions can act as hole traps that enable a large number of excited electrons to migrate towards the TiO surface, thereby facilitating electron transfer and charge separation. Additionally, the cationic dopant and its induced defects might introduce a mid-gap state, promoting electron migration and prolonging the lifetime of charge carrier pairs. We have discovered that the optimal Ga dopant concentration was 3.125 at% and that the incorporation of platinum (0.5 wt%) as a co-catalyst further improved the H evolution rate up to 5722 μmol g h. Pt not only acts as an electron sink, drastically increasing the electron/hole pair lifetime, but it also creates an intimate contact at the heterojunction between Pt and Ga-TiO, thus improving the interfacial electron transfer process. These catalyst design strategies provide new ways of designing transition metal photocatalysts that improve green fuel production from renewable solar energy and water.
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http://dx.doi.org/10.1039/c7cp04155aDOI Listing
January 2018

Investigation of Water Dissociation and Surface Hydroxyl Stability on Pure and Ni-Modified CoOOH by Ambient Pressure Photoelectron Spectroscopy.

J Phys Chem B 2018 01 19;122(2):810-817. Epub 2017 Sep 19.

Department of Chemical and Biological Engineering, Princeton University , Princeton, New Jersey 08544, United States.

Water adsorption and reaction on pure and Ni-modified CoOOH nanowires were investigated using ambient pressure photoemission spectroscopy (APPES). The unique capabilities of APPES enable us to observe water dissociation and monitor formation of surface species on pure and Ni-modified CoOOH under elevated pressures and temperatures for the first time. Over a large range of pressures (UHV to 1 Torr), water dissociates readily on the pure and Ni-modified CoOOH surfaces at 27 °C. With an increase in HO pressure, a greater degree of surface hydroxylation was observed for all samples. At 1 Torr HO, ratios of different oxygen species indicate a transformation of CoOOH to CoOH in pure and Ni-modified CoOOH. In temperature dependent studies, desorption of weakly bound water and surface dehydroxylation were observed with increasing temperature. Larger percentages of surface hydroxyl groups at higher temperatures were observed on Ni-modified CoOOH compared to pure CoOOH, which indicates an increased stability of surface hydroxyl groups on these Ni-modified surfaces.
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http://dx.doi.org/10.1021/acs.jpcb.7b06960DOI Listing
January 2018

Hydrogenation of CO on ZnO/Cu(100) and ZnO/Cu(111) Catalysts: Role of Copper Structure and Metal-Oxide Interface in Methanol Synthesis.

J Phys Chem B 2018 01 30;122(2):794-800. Epub 2017 Aug 30.

Chemistry Division, Brookhaven National Laboratory , Upton, New York 11973, United States.

The results of kinetic tests and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) show the important role played by a ZnO-copper interface in the generation of CO and the synthesis of methanol from CO hydrogenation. The deposition of nanoparticles of ZnO on Cu(100) and Cu(111), θ < 0.3 monolayer, produces highly active catalysts. The catalytic activity of these systems increases in the sequence: Cu(111) < Cu(100) < ZnO/Cu(111) < ZnO/Cu(100). The structure of the copper substrate influences the catalytic performance of a ZnO-copper interface. Furthermore, size and metal-oxide interactions affect the chemical and catalytic properties of the oxide making the supported nanoparticles different from bulk ZnO. The formation of a ZnO-copper interface favors the binding and conversion of CO into a formate intermediate that is stable on the catalyst surface up to temperatures above 500 K. Alloys of Zn with Cu(111) and Cu(100) were not stable at the elevated temperatures (500-600 K) used for the CO hydrogenation reaction. Reaction with CO oxidized the zinc, enhancing its stability over the copper substrates.
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http://dx.doi.org/10.1021/acs.jpcb.7b06901DOI Listing
January 2018

Cu supported on mesoporous ceria: water gas shift activity at low Cu loadings through metal-support interactions.

Phys Chem Chem Phys 2017 Jul 27;19(27):17708-17717. Epub 2017 Jun 27.

Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA.

We have synthesized and tested a highly active Cu doped mesoporous CeO catalyst system for the low temperature water-gas shift (WGS) reaction. While typical oxide-supported copper WGS catalysts are characterized by high copper loadings (30-40%), the morphological properties of the mesoporous CeO material enable high catalytic activity at copper loadings as low as 1%. Operando X-ray diffraction, in situ X-ray absorption near-edge structure spectroscopy (XANES), and operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) methods were used to probe the interactions between the metal and mesoporous oxide components under reaction conditions. Copper was observed to undergo reduction from oxide to metal under WGS conditions at 150 °C, while the CeO lattice was observed to expand upon heating, indicating Ce formation correlated with CO production. The active state of the catalysts was confirmed by in situ XANES to contain Cu and partially reduced CeO. DRIFTS analysis revealed carboxyl species bound to copper during reduction, as well as formate and carbonate surface species on ceria. Lower concentrations of copper were observed to foster enhanced metal-support interactions.
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http://dx.doi.org/10.1039/c7cp02378bDOI Listing
July 2017

Dry Reforming of Methane on a Highly-Active Ni-CeO2 Catalyst: Effects of Metal-Support Interactions on C-H Bond Breaking.

Angew Chem Int Ed Engl 2016 06 4;55(26):7455-9. Epub 2016 May 4.

Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY, 11794, USA.

Ni-CeO2 is a highly efficient, stable and non-expensive catalyst for methane dry reforming at relative low temperatures (700 K). The active phase of the catalyst consists of small nanoparticles of nickel dispersed on partially reduced ceria. Experiments of ambient pressure XPS indicate that methane dissociates on Ni/CeO2 at temperatures as low as 300 K, generating CHx and COx species on the surface of the catalyst. Strong metal-support interactions activate Ni for the dissociation of methane. The results of density-functional calculations show a drop in the effective barrier for methane activation from 0.9 eV on Ni(111) to only 0.15 eV on Ni/CeO2-x (111). At 700 K, under methane dry reforming conditions, no signals for adsorbed CHx or C species are detected in the C 1s XPS region. The reforming of methane proceeds in a clean and efficient way.
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http://dx.doi.org/10.1002/anie.201602489DOI Listing
June 2016

Ambient pressure XPS and IRRAS investigation of ethanol steam reforming on Ni-CeO2(111) catalysts: an in situ study of C-C and O-H bond scission.

Phys Chem Chem Phys 2016 Jun;18(25):16621-8

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, USA.

Ambient-Pressure X-ray Photoelectron Spectroscopy (AP-XPS) and Infrared Reflection Absorption Spectroscopy (AP-IRRAS) have been used to elucidate the active sites and mechanistic steps associated with the ethanol steam reforming reaction (ESR) over Ni-CeO2(111) model catalysts. Our results reveal that surface layers of the ceria substrate are both highly reduced and hydroxylated under reaction conditions while the small supported Ni nanoparticles are present as Ni(0)/NixC. A multifunctional, synergistic role is highlighted in which Ni, CeOx and the interface provide an ensemble effect in the active chemistry that leads to H2. Ni(0) is the active phase leading to both C-C and C-H bond cleavage in ethanol and it is also responsible for carbon accumulation. On the other hand, CeOx is important for the deprotonation of ethanol/water to ethoxy and OH intermediates. The active state of CeOx is a Ce(3+)(OH)x compound that results from extensive reduction by ethanol and the efficient dissociation of water. Additionally, we gain an important insight into the stability and selectivity of the catalyst by its effective water dissociation, where the accumulation of surface carbon can be mitigated by the increased presence of surface OH groups. The co-existence and cooperative interplay of Ni(0) and Ce(3+)(OH)x through a metal-support interaction facilitate oxygen transfer, activation of ethanol/water as well as the removal of coke.
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http://dx.doi.org/10.1039/c6cp01212dDOI Listing
June 2016

Long-range ion-water and ion-ion interactions in aqueous solutions.

Phys Chem Chem Phys 2015 Apr 28;17(13):8427-30. Epub 2015 Jan 28.

Department of Chemistry, Stanford University, Stanford, California 94305, USA.

Using small-angle X-ray scattering (SAXS), we obtained direct experimental evidence on the structure of hydrated polyatomic anions, with hydration effects starkly different from those of cations (J. Chem. Phys., 2011, 134, 064513). We propose that the size and charge density of the naked ions do not sufficiently account for the differences in the SAXS curves. For cations, the ion-ion contribution gives a prominent first-order diffraction peak, whereas for anions, the low-Q enhancement in the SAXS curves indicates density inhomogeneities as a result of ion-water interactions.
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http://dx.doi.org/10.1039/c4cp04759aDOI Listing
April 2015

A different view of structure-making and structure-breaking in alkali halide aqueous solutions through x-ray absorption spectroscopy.

J Chem Phys 2014 Jun;140(24):244506

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, P.O. Box 20450, Stanford, California 94309, USA.

X-ray absorption spectroscopy measured in transmission mode was used to study the effect of alkali and halide ions on the hydrogen-bonding (H-bonding) network of water. Cl(-) and Br(-) are shown to have insignificant effect on the structure of water while I(-) locally weakens the H-bonding, as indicated by a sharp increase of the main-edge feature in the x-ray absorption spectra. All alkali cations act as structure-breakers in water, weakening the H-bonding network. The spectral changes are similar to spectra of high density ices where the 2nd shell has collapsed due to a break-down of the tetrahedral structures, although here, around the ions, the breakdown of the local tetrahedrality is rather due to non-directional H-bonding to the larger anions. In addition, results from temperature-dependent x-ray Raman scattering measurements of NaCl solution confirm the H-bond breaking effect of Na(+) and the effect on the liquid as similar to an increase in temperature.
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http://dx.doi.org/10.1063/1.4881600DOI Listing
June 2014

Observation of Tunneling in the Hydrogenation of Atomic Nitrogen on the Ru(001) Surface to Form NH.

J Phys Chem Lett 2013 Nov 23;4(21):3779-86. Epub 2013 Oct 23.

Department of Chemistry, University of Illinois at Chicago , 845 West Taylor Street, Chicago, Illinois 60607, United States.

The kinetics of NH and ND formation and dissociation reactions on Ru(001) were studied using time-dependent reflection absorption infrared spectroscopy (RAIRS). Our results indicate that NH and ND formation and dissociation on Ru(001) follow first-order kinetics. In our reaction temperature range (320-390 K for NH and 340-390 K for ND), the apparent activation energies for NH and ND formation were found to be 72.2 ± 1.9 and 87.1 ± 1.8 kJ/mol, respectively, while NH and ND dissociation reactions between 370 and 400 K have apparent activation barriers of 106.9 ± 4.1 and 101.8 ± 4.8 kJ/mol, respectively. The lower apparent activation energy for NH formation than that for ND as well as the comparison between experimentally measured isotope effects with theoretical results strongly indicates that tunneling already starts to play a role in this reaction at a temperature as high as 340 K.
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http://dx.doi.org/10.1021/jz4020585DOI Listing
November 2013

Solvation structures of protons and hydroxide ions in water.

J Chem Phys 2013 Apr;138(15):154506

Department of Chemistry, Stanford University, Stanford, California 94305, USA.

X-ray Raman spectroscopy (XRS) combined with small-angle x-ray scattering (SAXS) were used to study aqueous solutions of HCl and NaOH. Hydrated structures of H(+) and OH(-) are not simple mirror images of each other. While both ions have been shown to strengthen local hydrogen bonds in the hydration shell as indicated by XRS, SAXS suggests that H(+) and OH(-) have qualitatively different long-range effects. The SAXS structure factor of HCl (aq) closely resembles that of pure water, while NaOH (aq) behaves similar to NaF (aq). We propose that protons only locally enhance hydrogen bonds while hydroxide ions induce tetrahedrality in the overall hydrogen bond network of water.
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http://dx.doi.org/10.1063/1.4801512DOI Listing
April 2013

Increased fraction of low-density structures in aqueous solutions of fluoride.

J Chem Phys 2011 Jun;134(22):224507

Department of Chemistry, Stanford University, Stanford, California 94305, USA.

X-ray absorption spectroscopy (XAS) and small angle x-ray scattering (SAXS) were utilized to study the effect of fluoride (F(-)) anion in aqueous solutions. XAS spectra show that F(-) increases the number of strong H-bonds, likely between F(-) and water in the first hydration shell. SAXS data show a low-Q scattering intensity increase similar to the effect of a temperature decrease, suggesting an enhanced anomalous scattering behavior in F(-) solutions. Quantitative analysis revealed that fluoride solutions have larger correlation lengths than chloride solutions with the same cations but shorter compared to pure water. This is interpreted as an increased fraction of tetrahedral low-density structures in the solutions due to the presence of the F(-) ions, which act as nucleation centers replacing water in the H-bonding network and forming stronger H-bonds, but the presence of the cations restricts the extension of strong H-bonds.
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http://dx.doi.org/10.1063/1.3597606DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3188619PMC
June 2011
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