Prof. Dr. Tong Li

Institute for Materials
Ruhr-Universität Bochum

Author IDs

  • A review of the metastable omega phase in beta titanium alloys: the phase transformation mechanisms and its effect on mechanical properties
    Ballor, JoAnn and Li, Tong and Prima, Frédéric and Boehlert, Carl J. and Devaraj, Arun
    International Materials Reviews 68 (2023)
    Since its discovery in 1954, the omega (ω) phase in titanium and its alloys has attracted substantial attention from researchers. The β-to-ω and ω-to-α phase transformations are central to β-titanium alloy design, but the transformation mechanisms have been a subject of debate. With new generations of aberration-corrected transmission electron microscopy and atom probe tomography, both the spatial resolution and compositional sensitivity of phase transformation analysis have been rapidly improving. This review provides a detailed assessment of the new understanding gained and related debates in this field enabled by advanced characterization methods. Specifically, new insights into the possibility of a coupled diffusional-displacive component in the β-to-ω transformation and key nucleation driving forces for the ω-assisted α phase formation are discussed. Additionally, the influence of ω phase on the mechanical properties of β-titanium alloys is also reviewed. Finally, a perspective on open questions and future direction for research is discussed. © This material is authored by Battelle Memorial Institute with the US Department of Energy under Contract No. DE-AC05-76RL01830. The US Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, and irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
    view abstract10.1080/09506608.2022.2036401
  • Atomic-Scale Insights into Morphological, Structural, and Compositional Evolution of CoOOH during Oxygen Evolution Reaction
    Luan, C. and Corva, M. and Hagemann, U. and Wang, H. and Heidelmann, M. and Tschulik, K. and Li, T.
    ACS Catalysis 13 (2023)
    view abstract10.1021/acscatal.2c03903
  • Interface formation evolution of the hot-forged copper-(Cr)diamond composite and its thermal conductivity
    Jia, S.Q. and Bolzoni, L. and Li, T. and Yang, F.
    Journal of Alloys and Compounds 943 (2023)
    Due to the distinction of (111) and (100) faces on the diamond particle, the preferential formation of interface layers on the diamond particle is discovered when adding metal additives like titanium, chromium, aluminum in copper/diamond composites. The formed interface would affect the thermal conductivity of the copper/diamond composites. In this study, copper-55 vol%(Cr-coated) diamond composites were successfully produced by hot forging under 800 ℃, 900 ℃ and 1050 ℃. Heat treatment of Cr-coated diamond powders was conducted under a series of temperatures for studying the evolution of the carbide interface formation on surfaces of diamond particles. It was found that the average grain size of the Cr7C3 on diamond (111) faces was larger than that on (100) faces when the temperature is below 850 ℃, and the opposite trend was observed when the heat treatment temperature is above 850 ℃. The Cr layer was fully transformed into Cr7C3 when the temperature was above 950 ℃. Unlike the results obtained after heat treatment, the interface layers was directly transformed into Cr3C2 due to the non-equilibrium phase transformation after hot forging. The Cr3C2 interface layers were preferentially kept on (100) diamond faces, and exfoliated on (111) faces after hot forging. The grain size of the formed Cr3C2 on diamond (100) faces was increased with raising the hot forging temperature. The 1050 ℃-hot-forged sample showed the highest thermal conductivity, attributed to large grain sizes of the Cr3C2 interface layer. This study elaborates the mechanisms of the evolution of the interface layer formed under different temperatures on the diamond surfaces in copper-(Cr-coated) diamond composites, and the phenomenon of preferential formation of the interface layers is verified. © 2023 Elsevier B.V.
    view abstract10.1016/j.jallcom.2023.169133
  • Linking Composition, Structure and Thickness of CoOOH layers to Oxygen Evolution Reaction Activity by Correlative Microscopy
    Luan, Chenglong and Angona, Johanna and Bala Krishnan, Arjun and Corva, Manuel and Hosseini, Pouya and Heidelmann, Markus and Hagemann, Ulrich and Batsa Tetteh, Emmanuel and Schuhmann, Wolfgang and Tschulik, Kristina and Li, Tong
    Angewandte Chemie - International Edition 62 (2023)
    The role of β-CoOOH crystallographic orientations in catalytic activity for the oxygen evolution reaction (OER) remains elusive. We combine correlative electron backscatter diffraction/scanning electrochemical cell microscopy with X-ray photoelectron spectroscopy, transmission electron microscopy, and atom probe tomography to establish the structure–activity relationships of various faceted β-CoOOH formed on a Co microelectrode under OER conditions. We reveal that ≈6 nm β-CoOOH(01 (Figure presented.) 0), grown on [(Figure presented.) 0]-oriented Co, exhibits higher OER activity than ≈3 nm β-CoOOH(10 (Figure presented.) 3) or ≈6 nm β-CoOOH(0006) formed on [02 (Figure presented.) - and [0001]-oriented Co, respectively. This arises from higher amounts of incorporated hydroxyl ions and more easily reducible CoIII−O sites present in β-CoOOH(01 (Figure presented.) 0) than those in the latter two oxyhydroxide facets. Our correlative multimodal approach shows great promise in linking local activity with atomic-scale details of structure, thickness and composition of active species, which opens opportunities to design pre-catalysts with preferred defects that promote the formation of the most active OER species. © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.
    view abstract10.1002/anie.202305982
  • 3D atomic-scale imaging of mixed Co-Fe spinel oxide nanoparticles during oxygen evolution reaction
    Xiang, W. and Yang, N. and Li, X. and Linnemann, J. and Hagemann, U. and Ruediger, O. and Heidelmann, M. and Falk, T. and Aramini, M. and DeBeer, S. and Muhler, M. and Tschulik, K. and Li, T.
    Nature Communications 13 (2022)
    The three-dimensional (3D) distribution of individual atoms on the surface of catalyst nanoparticles plays a vital role in their activity and stability. Optimising the performance of electrocatalysts requires atomic-scale information, but it is difficult to obtain. Here, we use atom probe tomography to elucidate the 3D structure of 10 nm sized Co2FeO4 and CoFe2O4 nanoparticles during oxygen evolution reaction (OER). We reveal nanoscale spinodal decomposition in pristine Co2FeO4. The interfaces of Co-rich and Fe-rich nanodomains of Co2FeO4 become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of CoFe2O4. However, the activity of Co2FeO4 drops considerably due to concurrent irreversible transformation towards CoIVO2 and pronounced Fe dissolution. In contrast, there is negligible elemental redistribution for CoFe2O4 after OER, except for surface structural transformation towards (FeIII, CoIII)2O3. Overall, our study provides a unique 3D compositional distribution of mixed Co-Fe spinel oxides, which gives atomic-scale insights into active sites and the deactivation of electrocatalysts during OER. © 2022, The Author(s).
    view abstract10.1038/s41467-021-27788-2
  • A review of the metastable omega phase in beta titanium alloys: the phase transformation mechanisms and its effect on mechanical properties
    Ballor, J. and Li, T. and Prima, F. and Boehlert, C.J. and Devaraj, A.
    International Materials Reviews (2022)
    Since its discovery in 1954, the omega (ω) phase in titanium and its alloys has attracted substantial attention from researchers. The β-to-ω and ω-to-α phase transformations are central to β-titanium alloy design, but the transformation mechanisms have been a subject of debate. With new generations of aberration-corrected transmission electron microscopy and atom probe tomography, both the spatial resolution and compositional sensitivity of phase transformation analysis have been rapidly improving. This review provides a detailed assessment of the new understanding gained and related debates in this field enabled by advanced characterization methods. Specifically, new insights into the possibility of a coupled diffusional-displacive component in the β-to-ω transformation and key nucleation driving forces for the ω-assisted α phase formation are discussed. Additionally, the influence of ω phase on the mechanical properties of β-titanium alloys is also reviewed. Finally, a perspective on open questions and future direction for research is discussed. © This material is authored by Battelle Memorial Institute with the US Department of Energy under Contract No. DE-AC05-76RL01830. The US Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, and irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
    view abstract10.1080/09506608.2022.2036401
  • Atomic-scale characterization of (electro-)catalysts and battery materials by atom probe tomography
    Li, T. and Devaraj, A. and Kruse, N.
    Cell Reports Physical Science 3 (2022)
    view abstract10.1016/j.xcrp.2022.101188
  • Revealing the influence of Mo addition on interphase precipitation in Ti-bearing low carbon steels
    Dong, H. and Chen, H. and Riyahi khorasgani, A. and Zhang, B. and Zhang, Y. and Wang, Z. and Zhou, X. and Wang, W. and Wang, H. and Li, T. and Yang, Z. and van der Zwaag, S.
    Acta Materialia 223 (2022)
    Mo is widely used as an effective microalloying element to improve mechanical performance of interphase precipitation steels, but the precise role of Mo in interphase precipitation behavior is not fully understood. In this contribution, interphase precipitation behavior in a series of Ti-Mo-bearing low carbon steels is systematically studied, and the role of Mo in interphase precipitates and its coarsening behavior is revisited. It is found that (Ti, Mo)C precipitates instead of TiC are formed in the Mo-containing alloys, and the average site fraction of Mo in (Ti, Mo)C is almost independent of the bulk Mo content. Moreover, the number density of interphase precipitates can be substantially enhanced by a minor addition of Mo, albeit it does not further rise with increasing the bulk Mo content. This is because the Mo fraction in (Ti, Mo)C rather than the bulk Mo content governs the driving force for precipitation nucleation and the interfacial energy of the (Ti, Mo)C/α and (Ti, Mo)C/γ interfaces. In addition to the reduced interfacial energy, decrease of Ti trans-interface diffusivity has been identified as another key reason for the enhanced carbide coarsening resistance in Mo-containing alloys. © 2021 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2021.117475
  • Atom probe tomography
    Gault, B. and Chiaramonti, A. and Cojocaru-Mirédin, O. and Stender, P. and Dubosq, R. and Freysoldt, C. and Makineni, S.K. and Li, T. and Moody, M. and Cairney, J.M.
    Nature Reviews Methods Primers 1 (2021)
    Atom probe tomography (APT) provides three-dimensional compositional mapping with sub-nanometre resolution. The sensitivity of APT is in the range of parts per million for all elements, including light elements such as hydrogen, carbon or lithium, enabling unique insights into the composition of performance-enhancing or lifetime-limiting microstructural features and making APT ideally suited to complement electron-based or X-ray-based microscopies and spectroscopies. Here, we provide an introductory overview of APT ranging from its inception as an evolution of field ion microscopy to the most recent developments in specimen preparation, including for nanomaterials. We touch on data reconstruction, analysis and various applications, including in the geosciences and the burgeoning biological sciences. We review the underpinnings of APT performance and discuss both strengths and limitations of APT, including how the community can improve on current shortcomings. Finally, we look forwards to true atomic-scale tomography with the ability to measure the isotopic identity and spatial coordinates of every atom in an ever wider range of materials through new specimen preparation routes, novel laser pulsing and detector technologies, and full interoperability with complementary microscopy techniques. © 2021, Springer Nature Limited.
    view abstract10.1038/s43586-021-00047-w
  • Atomic-Precision Tailoring of Au–Ag Core–Shell Composite Nanoparticles for Direct Electrochemical-Plasmonic Hydrogen Evolution in Water Splitting
    Mo, J. and Barbosa, E.C.M. and Wu, S. and Li, Y. and Sun, Y. and Xiang, W. and Li, T. and Pu, S. and Robertson, A. and Wu, T.-S. and Soo, Y.-L. and Alves, T.V. and Camargo, P.H.C. and Kuo, W. and Tsang, S.C.E.
    Advanced Functional Materials (2021)
    Traditionally, bandgap materials are a prerequisite to photocatalysis since they can harness a reasonable range of the solar spectrum. However, the high impedance across the bandgap and the low concentration of intrinsic charge carriers have limited their energy conversion. By contrast, metallic nanoparticles possess a sea of free electrons that can effectively promote the transition to the excited state for reactions. Here, an atomic layer of a bimetallic concoction of silver–gold shells is precisely fabricated onto an Au core via a sonochemical dispersion approach to form a core–shell of Au–Ag that exploits the wide availability of excited states of Ag while maintaining an efficient localized surface plasmon resonance (LSPR) of Au. Catalytic results demonstrate that this mix of Ag and Au can convert solar energy to hydrogen at high efficiency with an increase of 112.5% at an optimized potential of −0.5 V when compared to light-off conditions under the electrochemical LSPR. This outperforms the commercial Pt catalysts by 62.1% with a hydrogen production rate of 1870 µmol g−1 h−1 at room temperature. This study opens a new route for tuning the range of light capture of hydrogen evolution reaction catalysts using fabricated core–shell material through the combination of LSPR with electrochemical means. © 2021 Wiley-VCH GmbH
    view abstract10.1002/adfm.202102517
  • Design of a new wrought CrCoNi-based medium-entropy superalloy C-264 for high-temperature applications
    Hunfeld, J. and Sommer, H. and Kiese, J. and Wang, H. and Riyahi khorasgani, A. and Li, T. and Somsen, C. and Kostka, A. and Laplanche, G.
    Materials and Design 211 (2021)
    A new wrought CrCoNi-based medium-entropy superalloy (MESA) was designed by changing the composition of a commercial superalloy of type C-263, which is used for stationary components in gas turbines. ∼5 at.% Cr and 0.85 at.% Ti + Al were added at the expense of Ni while the Ti/Al ratio was decreased. Owing to these modifications, the brittle η phase, which is stable in C-263 below 900 °C is no longer observed in C-264. Besides, the solvus temperature and volume fraction of the γ′ phase in the peak-aged state are larger in C-264 (∼935 °C, 13.5%) compared to C-263 (∼890 °C, 12.8%), resulting in superior tensile and creep properties. The stress and temperature dependencies of the creep rates were described by power-law and Arrhenius relationships. The stress exponents were between 4 and 5, while the apparent activation energies were 550 and 400 kJ/mol for C-264 and C-263, respectively. During creep at 880 °C in air, internal nitridation in both MESAs resulted in the formation of TiN precipitates, with C-264 being slightly more affected due to its higher nitrogen solubility. Due to its superior creep resistance, good malleability and machinability, the C-264 MESA is currently commercially available from VDM Metals International. © 2021 The Authors
    view abstract10.1016/j.matdes.2021.110174
  • Direct Detection of Surface Species Formed on Iridium Electrocatalysts during the Oxygen Evolution Reaction
    BalaKrishnan, A. and Blanc, N. and Hagemann, U. and Gemagami, P. and Wonner, K. and Tschulik, K. and Li, T.
    Angewandte Chemie - International Edition 60 (2021)
    The effect of surface orientations on the formation of iridium oxide species during the oxygen evolution reaction (OER) remains yet unknown. Herein, we use a needle-shaped iridium atom probe specimen as a nanosized working electrode to ascertain the role of the surface orientations in the formation of oxide species during OER. At the beginning of electrolysis, the top 2–3 nm of (024), (026), (113), and (115) planes are covered by IrO−OH, which activates all surfaces towards OER. A thick subsurface oxide layer consisting of sub-stoichiometric Ir−O species is formed on the open (024) planes as OER proceeds. Such metastable Ir−O species are thought to provide an additional contribution to the OER activity. Overall, this study sheds light on the importance of the morphological effects of iridium electrocatalysts for OER. It also provides an innovative approach that can directly reveal surface species on electrocatalysts at atomic scale. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202106790
  • Rapid Interchangeable Hydrogen, Hydride, and Proton Species at the Interface of Transition Metal Atom on Oxide Surface
    Wu, S. and Tseng, K.-Y. and Kato, R. and Wu, T.-S. and Large, A. and Peng, Y.-K. and Xiang, W. and Fang, H. and Mo, J. and Wilkinson, I. and Soo, Y.-L. and Held, G. and Suenaga, K. and Li, T. and Chen, H.-Y.T. and Tsang, S.C.E.
    Journal of the American Chemical Society 143 (2021)
    Hydrogen spillover is the phenomenon where a hydrogen atom, generated from the dissociative chemisorption of dihydrogen on the surface of a metal species, migrates from the metal to the catalytic support. This phenomenon is regarded as a promising avenue for hydrogen storage, yet the atomic mechanism for how the hydrogen atom can be transferred to the support has remained controversial for decades. As a result, the development of catalytic support for such a purpose is only limited to typical reducible oxide materials. Herein, by using a combination of in situ spectroscopic and imaging technique, we are able to visualize and observe the atomic pathway for which hydrogen travels via a frustrated Lewis pair that has been constructed on a nonreducible metal oxide. The interchangeable status between the hydrogen, proton, and hydride is carefully characterized and demonstrated. It is envisaged that this study has opened up new design criteria for hydrogen storage material. © 2021 American Chemical Society.
    view abstract10.1021/jacs.1c02859
  • Revisiting ω phase embrittlement in metastable β titanium alloys: Role of elemental partitioning
    Lai, M.J. and Li, T. and Yan, F.K. and Li, J.S. and Raabe, D.
    Scripta Materialia 193 (2021)
    The role of elemental partitioning between β and ω phase in embrittling an originally ductile ω-containing Ti–12Mo (wt.%) model alloy was studied using transmission electron microscopy and atom probe tomography. It is revealed that the embrittlement of this alloy already occurs after aging at 400 °C for as short as 10 min, when the size, inter-particle spacing and volume fraction of the ω particles remain almost unchanged. The origin of the aging-induced embrittlement is attributed to the significant rejection of Mo (>5 at.%) from the ω particles during aging, which leads to remarkable increase in the shear modulus (>30 GPa) of the ω particles, promoting intense plastic flow localization and facilitating crack nucleation prior to macroscopic yielding. © 2020
    view abstract10.1016/j.scriptamat.2020.10.031
  • Sintering Activated Atomic Palladium Catalysts with High-Temperature Tolerance of ∼1,000°C
    Yang, N. and Zhao, Y. and Zhang, H. and Xiang, W. and Sun, Y. and Yang, S. and Sun, Y. and Zeng, G. and Kato, K. and Li, X. and Yamauchi, M. and Jiang, Z. and Li, T.
    Cell Reports Physical Science 2 (2021)
    Sintering-induced aggregation of active metals is a major cause of catalyst deactivation. Catalysts that can operate above 800°C are rare. Here, we report an unusual noble metal catalyst with sintering-induced activation at temperatures up to 1,000°C. The catalyst consists of atomically dispersed palladium embedded in a reducible SnO2 support designated for lean methane combustion. High temperature reaction simultaneously causes favorable changes of palladium ensemble state combining synergistically with lattice oxygen activation. Such changes lead to at least one order of magnitude improvement of the intrinsic reactivity, which compensates the surface area loss. Extensive characterizations such as atom probe tomography, X-ray absorption spectroscopy, and isotope tracking together with theoretical calculations illustrate the structure and surface chemistry changes and their impacts on the reaction mechanism. The catalyst also shows notable long-term stability and facile regeneration after poisoning. Our work may provide new insights into designing active and thermally stable catalysts. © 2020 The Author(s) Yang et al. report that atomic Pd catalysts on SnO2 undergo sintering of both Pd and support during high-temperature catalytic oxidation process. The sintered catalyst not only shows significant increase in activity for methane combustion under harsh reaction conditions, but also retains catalytic stability with high-temperature tolerance up to 1,000°C. © 2020 The Author(s)
    view abstract10.1016/j.xcrp.2020.100287
  • Stabilization of an iridium oxygen evolution catalyst by titanium oxides
    Kasian, O. and Li, T. and Mingers, A.M. and Schweinar, K. and Savan, A. and Ludwig, A. and Mayrhofer, K.
    JPhys Energy 3 (2021)
    The anodic oxygen evolution reaction (OER) has significant importance in many electrochemical technologies. In proton exchange membrane water electrolyzers it plays a pivotal role for electrochemical energy conversion, yet sluggish kinetics and the corrosive environment during operation still compel significant advances in electrode materials to enable a widespread application. Up-To-date Iridium is known as the best catalyst material for the OER in acidic media due to its relatively high activity and long-Term stability. However, scarcity of iridium drives the development of strategies for its efficient utilization. One promising way would be the formation of mixtures in which the noble catalyst element is dispersed in the non-noble matrix of more stable metals or metal oxides. A promising valve metal oxide is TiOx, yet the degree to which performance can be optimized by composition is still unresolved. Thus, using a scanning flow cell connected to an inductively coupled plasma mass spectrometer, we examined the activity and stability for the OER of an oxidized Ir Ti thin film material library covering the composition range from 20 70 at.% of Ir. We find that regardless of the composition the rate of Ir dissolution is observed to be lower than that of thermally prepared IrO2. Moreover, mixtures containing at least 50 at.% of Ir exhibit reactivity comparable to IrO2. Their superior performance is discussed with complementary information obtained from atomic scale and electronic structure analysis using atom probe tomography and x-ray photoelectron spectroscopy. Overall, our data shows that Ir Ti mixtures can be promising OER catalysts with both high activity and high stability. © 2021 JPhys Energy. All right reserved.
    view abstract10.1088/2515-7655/abbd34
  • Unveiling the interface characteristics and their influence on the heat transfer behavior of hot-forged Cu–Cr/Diamond composites
    Jia, S.Q. and Bolzoni, L. and Li, T. and Yang, F.
    Carbon 172 (2021)
    Cu–Cr/55 vol% diamond composites with 1 wt % (Cu–1Cr/55Dia), 2 wt % (Cu–2Cr/55Dia), and 3 wt % (Cu–3Cr/55Dia) Cr additives, respectively, are fabricated by a hot forging method. The diamond particle surfaces are nearly completely covered by the formed carbides interface for the Cu–3Cr/55Dia composite, and the Cu–3Cr/55Dia has the highest measured thermal conductivity among the three fabricated composites (433 Wm−1K−1). High Resolution Transmission Electron Microscopy analyses suggest that a double-layered interface structure is formed between the copper matrix and the diamond particle in the Cu–3Cr/55Dia, which is composed of 160 nm-thick Cr3C2 and 2 nm-thick Cr23C6. This interface structure, together with high relative density of the composite, high interface coverage of diamond, thin interface layer, and formation of coherent atomic boundaries, significantly contributes to obtaining high thermal conductivity for the Cu–3Cr/55Dia composite. The Diffuse Mismatch Model and Differential Effective Medium model are modified for establishing the quantitative relationship among interface characteristics, interface thermal conductance, and the composite's thermal conductivity, based on considering the composite's interface structure and phonon transmission theory. The predicted thermal conductivity value is well matched with the measured value for the Cu–3Cr/diamond (487 Wm−1K−1 vs 433 Wm−1K−1). This helps understand heat transfer behavior in the Cu–Cr/diamond composites. © 2020 Elsevier Ltd
    view abstract10.1016/j.carbon.2020.10.036
  • Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks
    Wang, Z. and Li, T. and Jiang, Y. and Lafon, O. and Liu, Z. and Trébosc, J. and Baiker, A. and Amoureux, J.-P. and Huang, J.
    Nature Communications 11 (2020)
    Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C–H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis. © 2020, The Author(s).
    view abstract10.1038/s41467-019-13907-7
  • Chemical boundary engineering: A new route toward lean, ultrastrong yet ductile steels
    Ding, R. and Yao, Y. and Sun, B. and Liu, G. and He, J. and Li, T. and Wan, X. and Dai, Z. and Ponge, D. and Raabe, D. and Zhang, C. and Godfrey, A. and Miyamoto, G. and Furuhara, T. and Yang, Z. and van der Zwaag, S. and Chen, H.
    Science Advances 6 (2020)
    For decades, grain boundary engineering has proven to be one of the most effective approaches for tailoring the mechanical properties of metallic materials, although there are limits to the fineness and types of microstructures achievable, due to the rapid increase in grain size once being exposed to thermal loads (low thermal stability of crystallographic boundaries). Here, we deploy a unique chemical boundary engineering (CBE) approach, augmenting the variety in available alloy design strategies, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after heating to high temperatures. When applied to plain steels with carbon content of only up to 0.2 weight %, this approach yields ultimate strength levels beyond 2.0 GPa in combination with good ductility (>20%). Although demonstrated here for plain carbon steels, the CBE design approach is, in principle, applicable also to other alloys. Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).
    view abstract10.1126/sciadv.aay1430
  • Defect Segregation and Its Effect on the Photoelectrochemical Properties of Ti-Doped Hematite Photoanodes for Solar Water Splitting
    Scherrer, B. and Li, T. and Tsyganok, A. and Döbeli, M. and Gupta, B. and Malviya, K.D. and Kasian, O. and Maman, N. and Gault, B. and Grave, D.A. and Mehlman, A. and Visoly-Fisher, I. and Raabe, D. and Rothschild, A.
    Chemistry of Materials 32 (2020)
    Optimizing the photoelectrochemical performance of hematite photoanodes for solar water splitting requires better understanding of the relationships between dopant distribution, structural defects, and photoelectrochemical properties. Here, we use complementary characterization techniques including electron microscopy, conductive atomic force microscopy (CAFM), Rutherford backscattering spectroscopy (RBS), atom probe tomography (APT), and intensity-modulated photocurrent spectroscopy (IMPS) to study this correlation in Ti-doped (1 cat. %) hematite films deposited by pulsed laser deposition (PLD) on F:SnO2 (FTO)-coated glass substrates. The deposition was carried out at 300 °C followed by annealing at 500 °C for 2 h. Upon annealing, Ti was observed by APT to segregate to the hematite/FTO interface and into some hematite grains. Since no other pronounced changes in microstructure and chemical composition were observed by electron microscopy and RBS after annealing, a nonuniform Ti redistribution seems to be the reason for reduced interfacial recombination in the annealed films, as observed by IMPS. This results in a lower onset potential, higher photocurrent, and larger fill factor with respect to the as-deposited state. This work provides atomic-scale insights into the microscopic inhomogeneity in Ti-doped hematite thin films and the role of defect segregation in their electrical and photoelectrochemical properties. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.chemmater.9b03704
  • Induced C–C coupling in CO2 photocatalytic reduction via carbothermally reduced nonstoichiometric tungsten oxide
    Li, T. and Dong, X. and Chen, W. and Zhao, X. and Li, G. and Feng, G. and Song, Y. and Wei, W. and Sun, Y.
    Applied Surface Science 526 (2020)
    Photocatalytic conversion of carbon dioxide (CO2) is a promising strategy for both renewable solar energy storage and carbon emission reduction. Forming multicarbon products in CO2 photocatalytic reduction remains very difficult, due to the kinetic barriers of C–C coupling. In this study, we introduce surface pentavalent tungstic (W5+) species in nonstoichiometric tungsten oxides via carbothermal reduction of commercial WO3 powder, achieving photocatalytic CO2 conversion with an aldehyde selectivity of 35% at a total normal CO2 conversion rate of 1.8 µmol·gcat−1·h−1. The as-prepared nonstoichiometric tungsten oxides exhibit improved band structures and electron transport properties, and the W5+ surface species with oxygen vacancies play a pivotal role in facilitating C–C coupling of key intermediates. In-depth carbothermal reduction at an elevated temperature (880 °C) delivers more tetravalent W4+ species, decreasing the CO and aldehyde production rates. This work provides fundamental information to facilitate C–C coupling in CO2 photoreduction via the introduction of surface active species. © 2020 Elsevier B.V.
    view abstract10.1016/j.apsusc.2020.146578
  • Insights into the Formation, Chemical Stability, and Activity of Transient NiyP@NiO x Core-Shell Heterostructures for the Oxygen Evolution Reaction
    Wilde, P. and Dieckhöfer, S. and Quast, T. and Xiang, W. and Bhatt, A. and Chen, Y.-T. and Seisel, S. and Barwe, S. and Andronescu, C. and Li, T. and Schuhmann, W. and Masa, J.
    ACS Applied Energy Materials 3 (2020)
    NiyP emerged as a highly active precatalyst for the alkaline oxygen evolution reaction where structural changes play a crucial role for its catalytic performance. We probed the chemical stability of NiyP in 1 M KOH at 80 °C and examined how exposure up to 168 h affects its structure and catalytic performance. We observed selective P-leaching and formation of NiyP/NiOx core-shell heterostructures, where shell thickness increases with ageing time, which is detrimental for the activity. By tuning the particle size, we demonstrate that prevention of complete catalyst oxidation is essential to preserve the outstanding electrochemical performance of NiyP in alkaline media. © 2020 American Chemical Society.
    view abstract10.1021/acsaem.9b02481
  • On the role of chemical heterogeneity in phase transformations and mechanical behavior of flash annealed quenching & partitioning steels
    Liu, G. and Li, T. and Yang, Z. and Zhang, C. and Li, J. and Chen, H.
    Acta Materialia 201 (2020)
    The microstructure of advanced high-strength steels (AHSSs) is usually designed via adjusting austenite decomposition behavior upon cooling, while relatively less attention was paid to austenite formation upon heating. Here we explore the potentials of flash heating in tuning the microstructure and mechanical behavior of Quenching & Partitioning (Q&P) steels with an emphasis on the role of chemical heterogeneity. Besides substantially refining intercritical austenite grains (e.g. austenite formed during intercritical annealing), it was interestingly found that flash heating can also allow intercritical austenite to inherit Mn heterogeneity in the original pearlite-ferrite microstructure due to the kinetic mismatch between the sluggish diffusion of Mn and the rapid austenite formation. Chemical heterogeneity can to a large extent alter the decomposition of intercritical austenite and carbon partitioning upon cooling, and plays a notable role in enhancing thermal stability of austenite. The role of chemical heterogeneity in austenite decomposition and carbon partitioning behavior was explained via phase field simulations. The flash treated Q&P (FQP) steels have a broad range of tensile strength (from 980 MPa to 1180 MPa) and good ductility, which outperforms the conventional Q&P (CQP) steels. The current study demonstrates that flash heating opens alternative routes to create unique microstructures and improve the mechanical performance of AHSSs. © 2020
    view abstract10.1016/j.actamat.2020.10.007
  • Sequencing of metals in multivariate metal-organic frameworks
    Ji, Z. and Li, T. and Yaghi, O.M.
    Science 369 (2020)
    We mapped the metal sequences within crystals of metal-oxide rods in multivariate metal-organic framework-74 containing mixed combinations of cobalt (Co), cadmium (Cd), lead (Pb), and manganese (Mn). Atom probe tomography of these crystals revealed the presence of heterogeneous spatial sequences of metal ions that we describe, depending on the metal and synthesis temperature used, as random (Co, Cd, 120°C), short duplicates (Co, Cd, 85°C), long duplicates (Co, Pb, 85°C), and insertions (Co, Mn, 85°C). Three crystals were examined for each sequence type, and the molar fraction of Co among all 12 samples was observed to vary from 0.4 to 0.9, without changing the sequence type. Compared with metal oxides, metal-organic frameworks have high tolerance for coexistence of different metal sizes in their rods and therefore assume various metal sequences. © 2020 American Association for the Advancement of Science. All rights reserved.
    view abstract10.1126/science.aaz4304
  • Sintering and biocompatibility of blended elemental Ti-xNb alloys
    Chen, Y. and Han, P. and Dehghan-Manshadi, A. and Kent, D. and Ehtemam-Haghighi, S. and Jowers, C. and Bermingham, M. and Li, T. and Cooper-White, J. and Dargusch, M.S.
    Journal of the Mechanical Behavior of Biomedical Materials 104 (2020)
    Titanium-niobium (Ti–Nb) alloys have great potential for biomedical applications due to their superior biocompatibility and mechanical properties that match closely to human bone. Powder metallurgy is an ideal technology for efficient manufacture of titanium alloys to generate net-shape, intricately featured and porous components. This work reports on the effects of Nb concentrations on sintered Ti-xNb alloys with the aim to establish an optimal composition in respect to mechanical and biological performances. Ti-xNb alloys with 33, 40, 56 and 66 wt% Nb were fabricated from elemental powders and the sintering response, mechanical properties, microstructures and biocompatibility assessed and compared to conventional commercial purity titanium (CPTi). The sintered densities for all Ti-xNb compositions were around 95%, reducing slightly with increasing Nb due to increasing open porosity. Higher Nb levels retarded sintering leading to more inhomogeneous phase and pore distributions. The compressive strength decreased with increasing Nb, while all Ti-xNb alloys displayed higher strengths than CPTi except the Ti–66Nb alloy. The Young's moduli of the Ti-xNb alloys with ≥40 wt% Nb were substantially lower (30–50%) than CPTi. In-vitro cell culture testing revealed excellent biocompatibility for all Ti-xNb alloys comparable or better than tissue culture plate and CPTi controls, with the Ti–40Nb alloy exhibiting superior cell-material interactions. In view of its mechanical and biological performance, the Ti–40Nb composition is most promising for hard tissue engineering applications. © 2020
    view abstract10.1016/j.jmbbm.2020.103691
  • Composition of the nanosized orthorhombic O′ phase and its direct transformation to fine α during ageing in metastable β-Ti alloys
    Li, T. and Lai, M. and Kostka, A. and Salomon, S. and Zhang, S. and Somsen, C. and Dargusch, M.S. and Kent, D.
    Scripta Materialia 170 (2019)
    The structure and chemistry of the orthorhombic O′ phase after quenching of a Ti-23 at.%Nb-2 at.%O was measured using aberration-corrected transmission electron microscopy and atom probe tomography. The nanosized O′ phase, formed in the vicinity of ω, is enriched with oxygen and slightly depleted in Nb. Upon annealing, ω dissolves and the O′ phase develops in β up to 350 °C, above which temperature it transforms to colonies of fine α phase. Another needle-like form of α with lower Nb content is thought to nucleate from Nb-lean regions related to spinodal decomposition of β. © 2019 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2019.06.008
  • Degradation of iridium oxides via oxygen evolution from the lattice: Correlating atomic scale structure with reaction mechanisms
    Kasian, O. and Geiger, S. and Li, T. and Grote, J.-P. and Schweinar, K. and Zhang, S. and Scheu, C. and Raabe, D. and Cherevko, S. and Gault, B. and Mayrhofer, K.J.J.
    Energy and Environmental Science 12 (2019)
    Understanding the fundamentals of iridium degradation during the oxygen evolution reaction is of importance for the development of efficient and durable water electrolysis systems. The degradation mechanism is complex and it is under intense discussion whether the oxygen molecule can be directly released from the oxide lattice. Here, we define the extent of lattice oxygen participation in the oxygen evolution and associated degradation of rutile and hydrous iridium oxide catalysts, and correlate this mechanism with the atomic-scale structures of the catalytic surfaces. We combine isotope labelling with atom probe tomography, online electrochemical and inductively coupled plasma mass spectrometry. Our data reveal that, unlike rutile IrO2, Ir hydrous oxide contains -IrIIIOOH species which directly contribute to the oxygen evolution from the lattice. This oxygen evolution mechanism results in faster degradation and dissolution of Ir. In addition, near surface bulk regions of hydrous oxide are involved in the oxygen catalysis and dissolution, while only the topmost atomic layers of rutile IrO2 participate in both reactions. Overall our data provide a contribution to the fundamental understanding of the exceptional stability of Ir-oxides towards the oxygen evolution reaction. The proposed approach to a quantitative assessment of the degree of lattice oxygen participation in the oxygen evolution reaction can be further applied to other oxide catalyst systems. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9ee01872g
  • Enhanced propylene oxide selectivity for gas phase direct propylene epoxidation by lattice expansion of silver atoms on nickel nanoparticles
    Yu, B. and Ayvalı, T. and Raine, E. and Li, T. and Li, M.M.-J. and Zheng, J. and Wu, S. and Bagabas, A.A. and Tsang, S.C.E.
    Applied Catalysis B: Environmental 243 (2019)
    A series of surfactant-free nickel-core and silver-shell (Ni@Ag) nanoparticles encapsulated within the mesopores of SBA-15 were synthesized and tested as catalysts for direct propylene oxidation by molecular oxygen. The influences of temperature, Gas Hour Space Velocity (GHSV) and Ni/Ag ratio on catalytic activity were systematically investigated. Among the prepared samples, Ni1Ag0.4/SBA-15 exhibited the best catalytic performance with selectivity of 70.7% and PO production rate of 4.4 nmol/g/s under 1 bar at 220 °C with GHSV of 192 h−1. High selectivity was attributed to longer Ag-Ag interatomic distance obtained by careful engineering the thickness of Ag shell over preformed Ni nanoparticles. In addition, all prepared new Ni@Ag core-shell catalysts presented excellent stability, which could maintain the conversion and selectivity for at least 10 h. These results suggest that new designs based on Ag surface atoms tailoring might pave the way to highly efficient and robust Ag catalysts for direct propylene oxidation using molecular oxygen as sole oxidant. © 2018 Elsevier B.V.
    view abstract10.1016/j.apcatb.2018.10.061
  • Sensitive and selective detection of Cu2+ ions based on fluorescent Ag nanoparticles synthesized by R-phycoerythrin from marine algae Porphyra yezoensis
    Xu, Y. and Hou, Y. and Wang, Y. and Wang, Y. and Li, T. and Song, C. and Wei, N. and Wang, Q.
    Ecotoxicology and Environmental Safety 168 (2019)
    In this study, using a natural and green protein R-phycoerythrin (R-PE) extracted from marine Porphyra yezoensis as the stabilizer and reducer, silver nanoparticles (AgNPs) were synthesized. Based on this, a highly sensitive and selective method for the detection of Cu2+ ions was developed using R-PE-AgNPs as fluorescent probe. The interactions between R-PE-AgNPs and Cu2+ ions were systematically characterized by fluorescence spectroscopy, transmission electron microscopy (TEM), elemental mapping and Fourier transform infrared (FTIR). It was found that Cu2+ ions could cause aggregation of the R-PE-AgNPs, accompanied by the greatly increased particle size. Importantly, the method offered a wide linear detection range from 0 μM to 100.0 μM with a detection limit of 0.0190 μM. Moreover, the proposed method was successfully applied to analyze Cu2+ ions in tap water and lake water samples, acquiring satisfactory recovery between 91.6% and 102.2%. Such a green, fast and cost-effective fluorimetric method of the R-PE-AgNPs probe has great potential for tracing Cu2+ ions in diverse aqueous media. © 2018 Elsevier Inc.
    view abstract10.1016/j.ecoenv.2018.10.102
  • Effect of tool wear evolution on chip formation during dry machining of Ti-6Al-4V alloy
    Dargusch, M.S. and Sun, S. and Kim, J.W. and Li, T. and Trimby, P. and Cairney, J.
    International Journal of Machine Tools and Manufacture 126 (2018)
    The complex microstructure of segmented chips and the changing deformation mechanisms during the machining of the Ti-6Al-4V alloy for a given cutting tool have been explored. Chip geometry and microstructure were investigated for increasing volumes of material removed at a cutting speed at which the tool characteristically develops gradual flank wear. The degree of chip segmentation and deformation mode changed significantly as machining progressed from using a new tool towards a worn tool. Chip formation processes when machining near the end of the cutting tool life is characterised by increasing amounts of twinning formed through both tension and compression. © 2017 Elsevier Ltd
    view abstract10.1016/j.ijmachtools.2017.12.003
  • Nucleation driving force for ω-assisted formation of α and associated ω morphology in β-Ti alloys
    Li, T. and Kent, D. and Sha, G. and Liu, H. and Fries, S.G. and Ceguerra, A.V. and Dargusch, M.S. and Cairney, J.M.
    Scripta Materialia 155 (2018)
    The structural and chemical changes at ω/β interfaces and the evolution of the morphology of ω in a near-β alloy during isothermal ageing at 573 K were investigated by atom probe tomography and aberration-corrected high-resolution transmission electron microscopy. Ledges and local O enrichment at semi-coherent isothermal ω interfaces are proposed to provide the key driving force for nucleation of ω-assisted α. Following nucleation of α, the morphology of ω evolves from ellipsoidal to rod-like, induced by rapid consumption of ω by α. © 2018 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2018.06.039
  • Elastically confined martensitic transformation at the nano-scale in a multifunctional titanium alloy
    Wang, H.L. and Hao, Y.L. and He, S.Y. and Li, T. and Cairney, J.M. and Wang, Y.D. and Wang, Y. and Obbard, E.G. and Prima, F. and Du, K. and Li, S.J. and Yang, R.
    Acta Materialia 135 (2017)
    A martensitic transformation (MT) is a typical first-order diffusionless crystal structural change with strong autocatalysis like avalanche at a speed of sound propagation. This unique characteristic, however, is undetectable in some multifunctional titanium alloys. Recently, a nano-scale elastically confined MT mechanism was proposed because a nano-scale Nb modulation in a Ti-Nb based alloy was observed. Here we analyze the elastic confinement in details and its induced novel properties in a wide temperature range. The statistical analyses of atom probe tomography (APT) data confirm the existence of the nano-scale Nb concentration modulation. The synchrotron X-ray diffraction (SXRD) profiles demonstrate that the nano-scale Nb modulation causes weak diffuse scattering, as evidenced by the extreme broad diffraction bands. The tensile tests find a critical temperature of ∼150 K, where the critical stress to induce the MT and Young's modulus reach the minimum and the superelastic strain reaches the maximum (∼4.5%) and keeps constant as the temperature decreases further to <4.2 K. To reveal these abnormal behaviors of the MT, the Born criterion governing the elastic stability of cubic crystal is modified by introducing an elastic confinement term and a new Clausius-Clapeyron relationship is established to evaluate the elastically confined MT. The results are consistent with the experimental findings, including the solely stress-induced (no thermally induced) reversibility. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.06.040
  • Tracing the coupled atomic shear and shuffle for a cubic to a hexagonal crystal transition
    Wang, H.L. and Hao, Y.L. and He, S.Y. and Du, K. and Li, T. and Obbard, E.G. and Hudspeth, J. and Wang, J.G. and Wang, Y.D. and Wang, Y. and Prima, F. and Lu, N. and Kim, M.J. and Cairney, J.M. and Li, S.J. and Yang, R.
    Scripta Materialia 133 (2017)
    Tracing the rearranged atoms in the first-order phase transformation is unrealistic due to the discrete structure change. Here we report that, by tuning a nano-scale decomposition in a titanium alloy, the bcc crystal distorts successively toward the hcp crystal by keeping an orthorhombic symmetry. Thus, the shear-shuffle relationship is traced experimentally to enrich the well-known Burgers mechanism. Our results reveal also that the successive tuning on crystal structure at the atomic level leads to some novel properties which are unexpected from the discrete phase transformations. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2017.02.024
  • Elemental distributions within multiphase quaternary Pb chalcogenide thermoelectric materials determined through three-dimensional atom probe tomography
    Yamini, A. and Li, T. and Mitchell, D.R.G. and Cairney, J.M.
    Nano Energy 26 (2016)
    Nanostructured multiphase p-type lead chalcogenides have shown the highest efficiencies amongst thermoelectric materials. However, their electronic transport properties have been described assuming homogenous distribution of dopants between phases. Here, we have analyzed elemental distributions in precipitates and matrices of nanostructured multiphase quaternary Pb chalcogenides doped to levels below and above the solubility limit of the matrix, using three-dimensional atom probe tomography. We demonstrate that partitioning of sodium and selenium occur between the matrix and secondary phase in both lightly- and heavily-doped compounds and that the concentrations of sodium and selenium in precipitates are higher than those in the matrices. This can contribute to the transport properties of such multiphase compounds The sodium concentration reached ~3 at% in sulfur-rich (PbS) precipitates and no nano precipitates of Na-rich phases were observed within either phase, a result that is supported by high resolution TEM analysis, indicating that the solubility limit of sodium in PbS is much higher than previously thought. However, non-equilibrium segregation of sodium is identified at the precipitates/matrix interfaces. These findings can lead to further advances in designing and characterizing multiphase thermoelectric materials. © 2016 Elsevier Ltd.
    view abstract10.1016/j.nanoen.2016.05.019
  • New insights into the phase transformations to isothermal ω and ω-assisted α in near β-Ti alloys
    Li, T. and Kent, D. and Sha, G. and Stephenson, L.T. and Ceguerra, A.V. and Ringer, S.P. and Dargusch, M.S. and Cairney, J.M.
    Acta Materialia 106 (2016)
    For multicomponent near-β alloys, we have investigated the mechanisms responsible for the β-to-ω and ω-to-α phase transformations upon isothermal ageing at 573 K. Experimental evidence from atom probe tomography and aberration-corrected high-resolution transmission electron microscopy indicates that the formation of isothermal ω involves a structural reconstruction assisted by nanoscale spinodal decomposition of the β matrix, prior to the specific chemistry change required to form ω, rather than a mixed-mode process with structure and chemistry changes occurring simultaneously as has been previously suggested. First, incommensurate embryonic ω evolve via a displacive mechanism within Mo-lean regions created by second-order coherent spinodal decomposition of the β matrix. The subtle spinodal decomposition in β and chemistry of embryonic ω are carefully analysed by an advanced atom probe data analysis algorithm. When the size of embryonic É·exceeds a critical value, commensurate isothermal É·forms through the exit of the other alloying solutes. O-rich regions present at the isothermal ω/β interface provide potent sites for the formation of α. The concurrent compositional partitioning of solutes in É·and α indicates the transformation from ω to α involves both a rapid lattice reconstruction at the ω/α interface and a slow Al diffusion at the α/β, therefore a mixed-mode displacive-diffusive process. This study provides novel experimental evidence to understand the much-disputed transformation processes and elucidate the mechanisms responsible for these important phase transformations. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.12.046
  • Superelasticity and Tunable Thermal Expansion across a Wide Temperature Range
    Hao, Y. L. and Wang, H. L. and Li, T. and Cairney, J. M. and Ceguerra, A. V. and Wang, Y. D. and Wang, Y. and Wang, D. and Obbard, E. G. and Li, S. J. and Yang, R.
    Journal of Materials Science & Technology 32 (2016)
    Materials that undergo a reversible change of crystal structure through martensitic transformation (MT) possess unusual functionalities including shape memory, superelasticity, and low/negative thermal expansion. These properties have many advanced applications, such as actuators, sensors, and energy conversion, but are limited typically in a narrow temperature range of tens of Kelvin. Here we report that, by creating a nano-scale concentration modulation via phase separation, the MT can be rendered continuous by an in-situ elastic confinement mechanism. Through a model titanium alloy, we demonstrate that the elastically confined continuous MT has unprecedented properties, such as superelasticity from below 4.2 K to 500 K, fully tunable and stable thermal expansion, from positive, through zero, to negative, from below 4.2 K to 573 K, and high strength-to-modulus ratio across a wide temperature range. The elastic tuning on the MT, together with a significant extension of the crystal stability limit, provides new opportunities to explore advanced materials. Copyright (C) 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
    view abstract10.1016/j.jmst.2016.06.017
  • The role of ω in the precipitation of α in near-β Ti alloys
    Li, T. and Kent, D. and Sha, G. and Cairney, J.M. and Dargusch, M.S.
    Scripta Materialia 117 (2016)
    To identify the conditions under which ω assists α formation in a near-β Ti alloy, we employed transmission electron microscopy and atom probe tomography to study α precipitation in alloys designed to contain two different types of ω. Coherent incommensurate embryonic ω formed upon isothermal ageing, does not directly assist α precipitation. When this incommensurate embryonic ω grows to a critical size, it transforms into commensurate isothermal ω, during which stress is thought to be the dominant factor. Regions of O enrichment at the semi-coherent isothermal ω/β interfaces are observed, which is thought to promote α formation. © 2016 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2016.02.026
  • New approaches to nanoparticle sample fabrication for atom probe tomography
    Felfer, P. and Li, T. and Eder, K. and Galinski, H. and Magyar, A.P. and Bell, D.C. and Smith, G.D.W. and Kruse, N. and Ringer, S.P. and Cairney, J.M.
    Ultramicroscopy 159 (2015)
    Due to their unique properties, nano-sized materials such as nanoparticles and nanowires are receiving considerable attention. However, little data is available about their chemical makeup at the atomic scale, especially in three dimensions (3D). Atom probe tomography is able to answer many important questions about these materials if the challenge of producing a suitable sample can be overcome. In order to achieve this, the nanomaterial needs to be positioned within the end of a tip and fixed there so the sample possesses sufficient structural integrity for analysis. Here we provide a detailed description of various techniques that have been used to position nanoparticles on substrates for atom probe analysis. In some of the approaches, this is combined with deposition techniques to incorporate the particles into a solid matrix, and focused ion beam processing is then used to fabricate atom probe samples from this composite. Using these approaches, data has been achieved from 10-20 nm core-shell nanoparticles that were extracted directly from suspension (i.e. with no chemical modification) with a resolution of better than ±1 nm. © 2015 Elsevier B.V..
    view abstract10.1016/j.ultramic.2015.04.014
  • The evolution of microstructure and mechanical properties of Ti-5Al-5Mo-5V-2Cr-1Fe during ageing
    Ahmed, M. and Li, T. and Casillas, G. and Cairney, J.M. and Wexler, D. and Pereloma, E.V.
    Journal of Alloys and Compounds 629 (2015)
    The phase transformations and compositional changes occurring during thermo-mechanical processing and subsequent high temperature ageing of Ti-5Al-5Mo-5V-2Cr-1Fe (wt.%) were investigated using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). High resolution STEM revealed nano-sized α (< 10 nm) and athermal ω (∼1-3 nm) formed during accelerated cooling from 800°C and slow heating to an ageing temperature of 650°C. Nuclei of α were found to form heterogeneously in the β matrix as well as at the ω phase. APT revealed pronounced Mo compositional fluctuations in the β matrix. No direct connection was established between Mo-rich or Mo-lean regions and α or ω nuclei. APT also failed to detect the ω phase, which supports theories that it forms by a shuffle mechanism, without any compositional difference from the β phase. Very small α particles, after initial ageing, showed only a minute change in composition with respect to the β matrix, indicative of a displacive-diffusional transformation. With further ageing, growth of the α lamellae was accompanied by compositional changes according to the diffusion rates of β-stabilising elements. Pile-up of the slowest diffusing solutes (Mo, V) at the α/β interface were pronounced in the initial stages of ageing. The best combination of mechanical properties (1200 MPa ultimate tensile strength with 15% total elongation) was recorded after 3.6 ks of ageing. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2015.01.005
  • The influence of partitioning on the growth of intragranular α in near-β Ti alloys
    Li, T. and Ahmed, M. and Sha, G. and Shi, R. and Casillas, G. and Yen, H.-W. and Wang, Y. and Pereloma, E.V. and Cairney, J.M.
    Journal of Alloys and Compounds 643 (2015)
    Abstract We report on partitioning of alloying elements during the formation of fine intragranular α plates in a Ti-55521 alloy after thermo-mechanical processing (TMP) and isothermal ageing at 923 K. The microstructures were characterised using atom probe tomography and high-resolution transmission electron microscopy. The partitioning of Mo, V and Al are strongly affected by their diffusivities and their mutual interaction. This leads to a deviation of the measured contents of alloying elements in the two phases from the predicted equilibrium values. The alloying elements at the broad faces and tips of α plates were found to exhibit different pile-up and segregation behaviours, which is thought to affect the lengthening and thickening kinetics of the α plates. As a result, the aspect ratio of α plates decreased rapidly with increasing ageing time. This study suggests that careful selection of alloying elements could be an effective way in controlling the growth anisotropy of α plates and thus α + β microstructures in near-β Ti alloys. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2015.04.143
  • The mechanism of ω-assisted α phase formation in near β-Ti alloys
    Li, T. and Kent, D. and Sha, G. and Dargusch, M.S. and Cairney, J.M.
    Scripta Materialia 104 (2015)
    Partitioning of alloying elements during the ω-to-α phase transformation in a near-β alloy after isothermal ageing at 573 K was measured using atom probe tomography and high-resolution transmission electron microscopy. O-rich regions associated with ω precipitates were observed for the first time, and likely serve as nucleation sites for the α phase. The partitioning behaviours of Al and O, unlike other elements, are different for α and ω, suggesting a mixed-mode mechanism for the ω-to-α phase transformation. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.scriptamat.2015.04.007
  • Atomic imaging of carbon-supported Pt, Pt/Co, and Ir@Pt nanocatalysts by atom-probe tomography
    Li, T. and Bagot, P.A.J. and Christian, E. and Theobald, B.R.C. and Sharman, J.D.B. and Ozkaya, D. and Moody, M.P. and Tsang, S.C.E. and Smith, G.D.W.
    ACS Catalysis 4 (2014)
    Atom probe tomography (APT) has been used to characterize commercially prepared Pt, Pt/Co alloy, and Ir@Pt core-shell nanoparticles supported on high-surface-area carbon black. Concentration profiles and 3D atom maps revealing the detailed internal structures and compositions of Pt, Pt/Co alloy, and Ir@Pt core-shell particles have been generated, and the distribution of trace impurity elements, including Na and Cl, has been examined. The observation of retained Na on the support, especially in the Pt nanoparticle system, indicates a more rigorous washing procedure is required. In the Pt/Co alloyed carbon-supported nanoparticle system, a marked variation in both compositions and particle sizes is observed. In the case of Ir@Pt, significant intermixing of the Ir core and Pt shell atoms takes place, which would be very difficult to measure by other techniques. All such observations will likely impact the catalytic performance of these materials. We envisage that the single nanoparticle analysis capability of APT, providing atomic-scale structures and chemical mapping, can also act as a means of quality control, identifying differences in the final product compared with the intended specification. Although the catalytic activity of these nanoparticles was not part of current study, the detailed information offered by such studies will permit knowledge-based improvements in nanoscale catalyst preparation methods and will also provide new ways of investigating structure and activity relationships at the nanometer scale. © 2014 American Chemical Society.
    view abstract10.1021/cs401117e
  • Nanojunction-mediated photocatalytic enhancement in heterostructured CdS/ZnO, CdSe/ZnO, and CdTe/ZnO nanocrystals
    Eley, C. and Li, T. and Liao, F. and Fairclough, S.M. and Smith, J.M. and Smith, G. and Tsang, S.C.E.
    Angewandte Chemie - International Edition 53 (2014)
    A series of highly efficient semiconductor nanocrystal (NC) photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots (QDs). The resulting contact between two small but high-quality crystals creates novel CdX/ZnO heterostructured semiconductor nanocrystals (HSNCs) with extensive type-II nanojunctions that exhibit more efficient photocatalytic decomposition of aqueous organic molecules under UV irradiation. Catalytic testing and characterization indicate that catalytic activity increases as a result of a combination of both the intrinsic chemistry of the chalcogenide anions and the heterojunction structure. Atomic probe tomography (APT) is employed for the first time to probe the spatial characteristics of the nanojunction between cadmium chalcogenide and ZnO crystalline phases, which reveals various degrees of ion exchange between the two crystals to relax large lattice mismatches. In the most extreme case, total encapsulation of CdTe by ZnO as a result of interfacial alloying is observed, with the expected advantage of facilitating hole transport for enhanced exciton separation during catalysis. That's a (quantum dot) wrap! A series of highly active semiconductor photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots. The resulting heterostructured CdX/ZnO nanocrystals with extensive type-II nanojunctions exhibit rapid photocatalytic decomposition of organic molecules in aqueous media. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201404481
  • Point-by-point compositional analysis for atom probe tomography
    Stephenson, L.T. and Ceguerra, A.V. and Li, T. and Rojhirunsakool, T. and Nag, S. and Banerjee, R. and Cairney, J.M. and Ringer, S.P.
    MethodsX 1 (2014)
    This new alternate approach to data processing for analyses that traditionally employed grid-based counting methods is necessary because it removes a user-imposed coordinate system that not only limits an analysis but also may introduce errors. We have modified the widely used "binomial" analysis for APT data by replacing grid-based counting with coordinate-independent nearest neighbour identification, improving the measurements and the statistics obtained, allowing quantitative analysis of smaller datasets, and datasets from non-dilute solid solutions. It also allows better visualisation of compositional fluctuations in the data. Our modifications include:.using spherical k-atom blocks identified by each detected atom's first k nearest neighbours.3D data visualisation of block composition and nearest neighbour anisotropy.using z-statistics to directly compare experimental and expected composition curves. Similar modifications may be made to other grid-based counting analyses (contingency table, Langer-Bar-on-Miller, sinusoidal model) and could be instrumental in developing novel data visualisation options. Crown Copyright © 2014 Published by Elsevier B.V.
    view abstract10.1016/j.mex.2014.02.001
  • Precipitation of the α-phase in an ultrafine grained beta-titanium alloy processed by severe plastic deformation
    Li, T. and Kent, D. and Sha, G. and Dargusch, M.S. and Cairney, J.M.
    Materials Science and Engineering A 605 (2014)
    A fine and uniform distribution of α phase at grain boundaries is expected to improve the mechanical properties and thermal stability of beta Ti alloys. To design high strength alloys, a key factor is the volume fraction of α, which is related to the concentration of the α phase. In this study, α-phase precipitates were characterized in an ultrafine-grained Ti-15Nb-2Mo-2Zr-1Sn (at%) alloy processed by severe plastic deformation in two different ways (hot drawing and cold rolling in conjunction with annealing). A combination of transmission Kikuchi diffraction, transmission electron microscopy and atom-probe tomography revealed that ultra-fine α precipitates precipitate at grain boundaries in hot-drawn material or at sub-grain boundaries in the cold-rolled samples. The Nb concentrations of α phases formed were not those expected for an equilibrium state, which highlights the importance of understanding the chemistry of the α precipitates for engineering microstructures in advanced Ti alloys. © 2014 Elsevier B.V.
    view abstract10.1016/j.msea.2014.03.044
  • Atomic engineering of platinum alloy surfaces
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Edman Tsang, S.C. and Smith, G.D.W.
    Ultramicroscopy 132 (2013)
    A major practical challenge in heterogeneous catalysis is to minimize the loading of expensive platinum group metals (PGMs) without degrading the overall catalytic efficiency. Gaining a thorough atomic-scale understanding of the chemical/structural changes occurring during catalyst manufacture/operation could potentially enable the design and production of "nano-engineered" catalysts, optimized for cost, stability and performance. In the present study, the oxidation behavior of a Pt-31 at% Pd alloy between 673-1073. K is investigated using atom probe tomography (APT). Over this range of temperatures, three markedly different chemical structures are observed near the surface of the alloy. At 673. K, the surface oxide formed is enriched with Pd, the concentration of which rises further following oxidation at 773. K. During oxidation at 873. K, a thick, stable oxide layer is formed on the surface with a stoichiometry of PdO, beneath which a Pd-depleted (Pt-rich) layer exists. Above 873. K, the surface composition switches to enrichment in Pt, with the Pt content increasing further with increasing oxidation temperature. This treatment suggests a route for tuning the surfaces of Pt-Pd nanoparticles to be either Pd-rich or Pt-rich, simply by adjusting the oxidation temperatures in order to form two different types of core-shell structures. In addition, comparison of the oxidation behavior of Pt-Pd with Pt-Rh and Pd-Rh alloys demonstrates markedly different trends under the same conditions for these three binary alloys. © 2012.
    view abstract10.1016/j.ultramic.2012.10.012
  • Characterization of oxidation and reduction of a Palladium-Rhodium alloy by atom-probe tomography
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Tsang, S.C.E. and Smith, G.D.W.
    Journal of Physical Chemistry C 116 (2012)
    Platinum group metals (PGMs) are used in numerous catalyst applications, including conversion of engine exhaust gases and hydrocarbon reforming. Reducing the loading of PGMs without diminishing the overall catalyst activity is a major challenge. Fundamental studies of PGMs under reactive conditions can assist the design/synthesis of "nanoengineered" catalysts, tunable and optimized for cost, stability, and performance. In the present study, the oxidation and reduction behavior of a Pd-6.4 at. % Rh alloy is investigated following treatment at 873 K for various exposure times using atom-probe tomography. For short oxidation times (10 min), an oxide layer with PdO stoichiometry grows on the surface. As the oxidation time increases, two phases with stoichiometries of (Rh 1Pd 1)O 2 and (Pd 2O) evolve. When the alloy is subsequently reduced in hydrogen, a nanoscale dispersion of Rh-rich metallic regions remains. This provides a route for the synthesis of multifunctional catalysts with different nanosurface regions in close proximity to one another. © 2012 American Chemical Society.
    view abstract10.1021/jp211687m
  • Characterization of oxidation and reduction of Pt-Ru and Pt-Rh-Ru alloys by atom probe tomography and comparison with Pt-Rh
    Li, T. and Bagot, P.A.J. and Marquis, E.A. and Tsang, S.C.E. and Smith, G.D.W.
    Journal of Physical Chemistry C 116 (2012)
    Pt-based alloys containing Rh and Ru are effective catalysts in a range of applications, including pollution control and low-temperature fuel cells. As the Pt group metals are generally rare and expensive, minimizing the loading of them while also increasing the efficiency of catalyst materials is a continual challenge in heterogeneous catalysis. A smart method to "nanoengineer" the surface of the nanocatalyst particles would greatly aid this goal. In our study, the oxidation of a Pt-8.9 at. % Ru alloy between 773 and 973 K and the oxidation and oxidation/reduction behavior of a Pt-23.9 at. % Rh-9.7 at. % Ru alloy at 873 K for various exposure times were studied using atom probe tomography. The surface of the Pt-Ru alloy is enriched with Ru after oxidation at 773 K, whereas it is depleted in Ru at 873 K, and at 973 K. The surface oxide layer vanishes at higher temperatures, leaving behind a Pt-rich surface. In the case of the Pt-Rh-Ru alloy, oxidation initiates from the grain boundaries, forming an oxide with a stoichiometry of MO 2. As the oxidation time increases, this oxide evolves into a twophase nanostructure, involving a Rh-rich oxide phase (Rh, Ru) 2O 3 and a Ru-rich oxide phase (Ru, Rh)O 2. When this two-phase oxide is reduced in hydrogen at low temperatures, separate Rh-rich and Ru-rich nanoscale regions remain. This process could, therefore, be useful for synthesizing complex island structures on Pt-Rh-Ru nanoparticle catalysts. © 2012 American Chemical Society.
    view abstract10.1021/jp304359m
  • Non-syngas direct steam reforming of methanol to hydrogen and carbon dioxide at low temperature
    Yu, K.M.K. and Tong, W. and West, A. and Cheung, K. and Li, T. and Smith, G. and Guo, Y. and Tsang, S.C.E.
    Nature Communications 3 (2012)
    A non-syngas direct steam reforming route is investigated for the conversion of methanol to hydrogen and carbon dioxide over a CuZnGaOx catalyst at 150-200 °C. This route is in marked contrast with the conventional complex route involving steam reformation to syngas (CO/H 2) at high temperature, followed by water gas shift and CO cleanup stages for hydrogen production. Here we report that high quality hydrogen and carbon dioxide can be produced in a single-step reaction over the catalyst, with no detectable CO (below detection limit of 1 ppm). This can be used to supply proton exchange membrane fuel cells for mobile applications without invoking any CO shift and cleanup stages. The working catalyst contains, on average, 3-4 nm copper particles, alongside extremely small size of copper clusters stabilized on a defective ZnGa2O4 spinel oxide surface, providing hydrogen productivity of 393.6 ml g-1-cat h-1 at 150 °C. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms2242
  • Characterization of oxidation and reduction of a platinum-rhodium alloy by atom-probe tomography
    Li, T. and Marquis, E.A. and Bagot, P.A.J. and Tsang, S.C. and Smith, G.D.W.
    Catalysis Today 175 (2011)
    An active challenge in heterogeneous catalysis is to minimize the quantities of the expensive platinum group metals used without causing degradation of the overall catalytic efficiency in a chemical reaction. To achieve this goal, a thorough atomic-scale understanding of these materials under reactive conditions is required. This will enable the design and production of "nano-engineered" catalysts, optimised for cost, stability and performance. In this study, the oxidation and reduction behaviour of a Pt-Rh alloy between 873 and 1073K was investigated by atom-probe tomography (APT). Detailed observations of the concentration profiles at the oxide/metal interfaces show that the growth of Rh2O3 oxide is limited by diffusion of Rh in the alloy. By varying the oxidation conditions, it was possible to calculate the activation energy for Rh diffusion in Pt-Rh as 236 ± 41 kJ/mol, together with diffusion coefficients for Rh for a range of temperatures. Reduction of the oxide phase left a thin, almost pure, layer of the most reactive (and expensive) element, Rh, on the surface of the specimen, suggesting a simple route for engineering the formation of the core-shell structure Pt-Rh nanoparticles. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cattod.2011.03.046
  • Hydrogen production from formic acid decomposition at room temperature using a Ag-Pd core-shell nanocatalyst
    Tedsree, K. and Li, T. and Jones, S. and Chan, C.W.A. and Yu, K.M.K. and Bagot, P.A.J. and Marquis, E.A. and Smith, G.D.W. and Tsang, S.C.E.
    Nature Nanotechnology 6 (2011)
    Formic acid (HCOOH) has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy density, is non-toxic and can be safely handled in aqueous solution. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen production from formic acid at room temperature. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the production of H 2 from formic acid at ambient temperature. Atom probe tomography confirmed that the nanoparticles have a core-shell configuration, with the shell containing between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions. © 2011 Macmillan Publishers. All rights reserved.
    view abstract10.1038/nnano.2011.42
  • alloys

  • atom probe tomography

  • catalysis

  • phase stability

  • transmission electron microscopy

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