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, including PhD projects for 2021, here.

Selected Publications

  • Single yttrium sites on carbon-coated TiO2 for efficient electrocatalytic N2 reduction.

    2020
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    Journal article
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    Chemical communications (Cambridge, England)
    We report single yttrium sites anchored on carbon-coated TiO2 for efficient and stable electrocatalytic N2 fixation, delivering an NH3 faradaic efficiency exceeding 11.0% and an NH3 yield rate as high as 6.3 μgNH3 h-1 mgcat.-1 at low overpotentials, thus surpassing many reported metal electrocatalysts.

  • Current-Density-Dependent Electroplating in Ca Electrolytes: From Globules to Dendrites

    2020
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    Journal article
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    ACS ENERGY LETTERS

  • Direct observation and catalytic role of mediator atom in 2D materials.

    2020
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    Journal article
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    Science advances
    The structural transformations of graphene defects have been extensively researched through aberration-corrected transmission electron microscopy (AC-TEM) and theoretical calculations. For a long time, a core concept in understanding the structural evolution of graphene defects has been the Stone-Thrower-Wales (STW)-type bond rotation. In this study, we show that undercoordinated atoms induce bond formation and breaking, with much lower energy barriers than the STW-type bond rotation. We refer to them as mediator atoms due to their mediating role in the breaking and forming of bonds. Here, we report the direct observation of mediator atoms in graphene defect structures using AC-TEM and annular dark-field scanning TEM (ADF-STEM) and explain their catalytic role by tight-binding molecular dynamics (TBMD) simulations and image simulations based on density functional theory (DFT) calculations. The study of mediator atoms will pave a new way for understanding not only defect transformation but also the growth mechanisms in two-dimensional materials.

  • 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.

  • 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

  • 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
    <jats:p> Transmission electron microscopy (TEM) has long been an essential tool for understanding the structure of materials. Over the past couple of decades, this venerable technique has undergone a number of revolutions, such as the development of aberration correction for atomic level imaging, the realization of cryogenic TEM for imaging biological specimens, and new instrumentation permitting the observation of dynamic systems <jats:italic>in situ</jats:italic> . Research in the latter has rapidly accelerated in recent years, based on a silicon-chip architecture that permits a versatile array of experiments to be performed under the high vacuum of the TEM. Of particular interest is using these silicon chips to enclose fluids safely inside the TEM, allowing us to observe liquid dynamics at the nanoscale. <jats:italic>In situ</jats:italic> imaging of liquid phase reactions under TEM can greatly enhance our understanding of fundamental processes in fields from electrochemistry to cell biology. Here, we review how <jats:italic>in situ</jats:italic> TEM experiments of liquids can be performed, with a particular focus on microchip-encapsulated liquid cell TEM. We will cover the basics of the technique, and its strengths and weaknesses with respect to related <jats:italic>in situ</jats:italic> TEM methods for characterizing liquid systems. We will show how this technique has provided unique insights into nanomaterial synthesis and manipulation, battery science and biological cells. A discussion on the main challenges of the technique, and potential means to mitigate and overcome them, will also be presented. </jats:p>

  • Biomass-Derived Nickel Phosphide Nanoparticles as a Robust Catalyst for Hydrogen Production by Catalytic Decomposition of C2H2 or Dry Reforming of CH4

    2019
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    Journal article
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    ACS Applied Energy Materials
    Copyright © 2019 American Chemical Society. Nickel is well recognized for its high catalytic activity in hydrogen production and hydrotreating, but it experiences severe deactivation upon coke formation. The development of a nickel-based catalyst that is resilient to deactivation while maintaining its signature high activity is therefore desirable. We demonstrate a simple but effective technique for the synthesis of nickel phosphide catalysts by impregnating nickel into carbonized natural cellulose fibers (NCFs) that naturally contain phosphorus. At a sufficient annealing temperature (1100 °C), the nickel particles react with phosphorus, forming Ni2P and Ni5P4 nanoparticles. Higher annealing temperatures (>1500 °C) promote the formation of single atom nickel, which greatly supplements the catalytic performance. Our nickel phosphide catalyst exhibits a markedly superior activity and stability in the synthesis of hydrogen by C2H2 decomposition and the dry reforming of methane (DRM) compared to the Ni/Al2O3 model catalyst. The sustained activity and stability exhibited by the developed catalyst suggest its promise for other hydrotreating reactions and the hydrogen evolution reaction.