Alexander Robertson

Dr Alex Robertson
Royal Society University Research Fellow
alex.robertson@materials.ox.ac.uk

The rapid advancements over the past couple decades in transmission electron microscopy (TEM) - aberration correction, cold field emission electron sources, monochromation, direct electron detection, sophisticated sample holders, environmental chambers, and many others - has massively expanded the range of materials science questions that can be addressed with this venerable technique.

My interest is in applying these new techniques to explore the fields that have been opened up to TEM investigation. The atomic structure of many nanomaterials, including graphene and carbon nanotubes, are perhaps best realised with TEM. Understanding the atomic structure and defect behaviour of these new materials has been a major focus of my research. Using custom-built in-situ sample holders I have explored the atomic level behaviour of 2D materials in electronic devices, imaging the changes in atomic structure they undergo while under bias.

More recently I have been interested in the emerging area of in-situ liquid characterisation. This permits the characterisation of liquid chemistry by TEM at the nanoscale, permitting the nanoscale investigation of important materials systems, including battery electrodes, hydrogen fuel cell cathodes, and inorganic catalysts.

Please also see my group website here.

Selected Publications

  • Trace metals dramatically boost oxygen electrocatalysis of N-doped coal-derived carbon for zinc-air batteries.

    2020
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    Journal article
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    Nanoscale
    The commercialization of metal-air batteries requires efficient, low-cost, and stable bifunctional electrocatalysts for reversible electrocatalysis of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). The modification of natural coal by heteroatoms such as N and S, or metal oxide species, has been demonstrated to form very promising electrocatalysts for the ORR and OER. However, it remains elusive and underexplored as to how the impurity elements in coal may impact the electrocatalytic properties of coal-derived catalysts. Herein, we explore the influence of the presence of various trace metals that are notable impurities in coal, including Al, Si, Ca, K, Fe, Mg, Co, Mn, Ni, and Cu, on the electrochemical performance of the prepared catalysts. The constructed Zn-air batteries are further shown to be able to power green LED lights for more than 80 h. The charge-discharge polarization curves exhibited excellent and durable rechargeability over 500 (ca. 84 h) continuous cycles. The promotional effect of the trace elements is believed to accrue from a combination of electronic structure modification of the active sites, enhancement of the active site density, and formation of a conductive 3-dimensional hierarchical network of carbon nanotubes.

  • Achieving Highly Selective Electrocatalytic CO2 Reduction by Tuning CuO-Sb2O3 Nanocomposites

    2020
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    Journal article
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    ACS Sustainable Chemistry & Engineering

  • Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes.

    2020
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    Journal article
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    Nature materials
    The application of transition metal fluorides as energy-dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities and limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single-crystalline, monodisperse iron(II) fluoride nanorods. Near theoretical capacity (570 mA h g-1) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr1,3FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 °C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface-specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semicoherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF2.

  • Metal-Tuned W18O49 for Efficient Electrocatalytic N2 Reduction

    2020
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    Journal article
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    ACS Sustainable Chemistry & Engineering

  • Reduced graphene oxides with engineered defects enable efficient electrochemical reduction of dinitrogen to ammonia in wide pH range

    2020
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    Journal article
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    Nano Energy
    © 2019 Elsevier Ltd Electrochemical nitrogen fixation under mild conditions is highly demanded yet remains a grand challenge. Herein we report metal-free electrocatalysis of aqueous N2 reduction to produce NH3 over reduced graphene oxide (DrGO) with tuned defects. The defect sites in DrGO, consisting of unsaturated carbon [single vacancy (SV), double vacancy (DV), and –COOH], were examined, and showed an improved NH3 selectivity due to the strong binding of N2 instead of H. In addition to improved selectivity, the calculated free energies for N2 reduction reaction at DrGO-COOH and DrGO-DV sites suggest that the thermodynamic overpotentials of these metal-free catalysts are comparable to the most efficient transition metal-based catalysts reported thus far. Our nonmetallic and dopant-free catalysts can convert N2 to NH3 at a faradaic efficiency of up to 22.0% at −0.116 V (versus the reversible hydrogen electrode vs. RHE) in 0.1 M HCl and 10.8% at −0.166 V (vs. RHE) in 0.1 M KOH, surpassing most earlier reported catalysts. An NH3 formation rate exceeding 7.3 μg h−1 mg−1 was achieved at low overpotentials in both acidic and alkaline environments, comparable to the values shown by metal electrocatalysts under similar conditions.

  • Liquid cell transmission electron microscopy and its applications

    2020
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    Journal article
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    Royal Society Open Science

  • Magnetism and transport in transparent high-mobility BaSnO3 films doped with La, Pr, Nd, and Gd

    2019
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    Journal article
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    PHYSICAL REVIEW MATERIALS

  • Contiguous and Atomically Thin Pt Film with Supra-Bulk Behavior Through Graphene-Imposed Epitaxy

    2019
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    Journal article
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    ADVANCED FUNCTIONAL MATERIALS
    band structure, dissociation energy, epitaxy, graphene, strain

  • The interface between Li6.5La3Zr1.5Ta0.5O12 and liquid electrolyte

    2019
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    Journal article
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    Joule
    An advantageous solid electrolyte/liquid electrolyte interface is crucial for the implementation of a protected lithium anode in liquid electrolyte cells. Li6.5La3Zr1.5Ta0.5O12 (LLZTO) garnet electrolytes are among the few solid electrolytes that are stable in contact with lithium metal. We show LLZTO is unstable in contact with the organic carbonate-based Li+ liquid electrolyte used in conventional Li-ion cells. The interfacial resistance between LLZTO and LiPF6 in (CH2O)2CO: OC(OCH3)2 (1:1 v/v) increases with time due to the growth of a lithium-ion-conducting solid electrolyte interphase (SEI) at the surface of the ceramic electrolyte. The interphase is composed of Li2CO3, LiF, Li2O, and organic carbonates. Even at a rate of 5 mA cm−2, a 3 V potential drop occurs across the LLZTO/liquid electrolyte interface. A practical LLZTO membrane (thickness ∼10 μm), in contact with a lithium anode, gives a potential loss of ∼16 mV, less than 1% of the resistance of the SEI.
    FFR

  • Atomic Structure and Dynamics of Epitaxial Platinum Bilayers on Graphene.

    2019
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    Journal article
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    ACS nano
    Platinum atomic layers grown on graphene were investigated by atomic resolution transmission electron microscopy (TEM). These TEM images reveal the epitaxial relationship between the atomically thin platinum layers and graphene, with two optimal epitaxies observed. The energetics of these epitaxies influences the grain structure of the platinum film, facilitating grain growth via in-plane rotation and assimilation of neighbor grains, rather than grain coarsening from the movement of grain boundaries. This growth process was enabled due to the availability of several possible low-energy intermediate states for the rotating grains, the Pt-Gr epitaxies, which are minima in surface energy, and coincident site lattice grain boundaries, which are minima in grain boundary energy. Density functional theory calculations reveal a complex interplay of considerations for minimizing the platinum grain energy, with free platinum edges also having an effect on the relative energetics. We thus find that the platinum atomic layer grains undergo significant reorientation to minimize interface energy (via epitaxy), grain boundary energy (via low-energy orientations), and free edge energy. These results will be important for the design of two-dimensional graphene-supported platinum catalysts and obtaining large-area uniform platinum atomic layer films and also provide fundamental experimental insight into the growth of heteroepitaxial thin films.