Selected publications

Nanoscopic Materials: Size-Dependent Phenomena and Growth Principles, Edition 2

Description
In recent years there have been great advances in the development of new nanomaterials. To facilitate the progress of new materials it is essential to understand the underlying principles at the nanoscale.

Nanoscopic Materials provides an accessible overview of the physico-chemical and physical principles of nanomaterials including electronic structure, magnetic properties, thermodynamics of size dependence and phase transitions and dynamics of clusters and two-dimensional systems. This new edition has been fully revised and updated to reflect recent developments in new nanomaterials including graphene and core–shell structures, properties of nano-structured and intelligent surfaces as well as applications in catalysis and energy. Additional chapters cover the development of nucleation and crystal shape engineering; self-assembly and biomimetics for fabricating nanostructures.

With helpful illustrations and summaries of key points in every chapter, this advanced textbook is ideal for graduate students of chemistry and materials science and researchers new to the field of nanoscience and nanotechnology.

Optical Spectroscopy: Fundamentals and Advanced Applications

Description
Developments in optical spectroscopy have taken new directions in recent decades, with the focus shifting from understanding small gas phase molecules towards applications in materials and biological systems. This is due to significant interest in these topics, which has been facilitated by significant technological developments.

Absorption, luminescence and excited state energy transfer properties have become of crucial importance on a large scale in materials related to light harvesting in photosynthesis, organic and inorganic third generation solar cells, for solar water splitting, and in light emitting diodes, TV screens and many other applications. In addition, Förster resonance energy transfer can be used as a ruler for the characterization of the structure and dynamics of DNA, proteins and other biomolecules via labelling with fluorescing markers.

This advanced textbook covers a range of these applications as well as the basics of absorption, emission and energy transfer of molecular systems in the condensed phase, in addition to the corresponding behaviour of metal nanoparticles and semiconductor quantum dots. It intends to close the gap between standard teaching at universities and today’s need in research and industry. Technical experimental requirements, aspects to avoid interfering perturbations and methods of quantitative data analysis make this book accessible and ideal for students and researchers in physical chemistry, biophysics and nanomaterials.

Contents

• Introduction
• Fundamentals
• Aspects of Experimental Setup and Data Analysis
• Principles of Optical Spectroscopy Demonstrated for a Set of Rigid Merocyanine Dyes
• Absorption and Luminescence of Semiconductor Quantum Dots
• Energy Transfer Processes of Excited States
• Advanced Applications of Optical Spectroscopy

Size matters: why nanomaterials are different

Gold is known as a shiny, yellow noble metal that does not tarnish, has a face centred cubic structure, is non-magnetic and melts at 1336 K. However, a small sample of the same gold is quite different, providing it is tiny enough: 10 nm particles absorb green light and thus appear red. The melting temperature decreases dramatically as the size goes down. Moreover, gold ceases to be noble, and 2–3 nm nanoparticles are excellent catalysts which also exhibit considerable magnetism. At this size they are still metallic, but smaller ones turn into insulators. Their equilibrium structure changes to icosahedral symmetry, or they are even hollow or planar, depending on size. The present tutorial review intends to explain the origin of this special behaviour of nanomaterials.

 Understanding catalysis

The large majority of chemical compounds underwent at least one catalytic step during synthesis. While it is common knowledge that catalysts enhance reaction rates by lowering the activation energy it is often obscure how catalysts achieve this. This tutorial review explains some fundamental principles of catalysis and how the mechanisms are studied. The dissociation of formic acid into H₂ and CO₂ serves to demonstrate how a water molecule can open a new reaction path at lower energy, how immersion in liquid water can influence the charge distribution and energetics, and how catalysis at metal surfaces differs from that in the gas phase. The reversibility of catalytic reactions, the influence of an adsorption pre-equilibrium and the compensating effects of adsorption entropy and enthalpy on the Arrhenius parameters are discussed. It is shown that flexibility around the catalytic centre and residual substrate dynamics on the surface affect these parameters. Sabatier’s principle of optimum substrate dsorption, shape selectivity in the pores of molecular sieves and the polarisation effect at the metal–support interface are explained. Finally, it is shown that the application of a bias voltage in electrochemistry offers an additional parameter to promote or inhibit a reaction.

In Command of Non-Equilibrium

The second law of thermodynamics is well known for determining the direction of spontaneous processes in the laboratory, life and the universe. It is therefore often called the arrow of time. Less often discussed but just as important is the effect of kinetic barriers which intercept equilibration and preserve highly ordered, high energy non-equilibrium states. Examples of such states are many modern materials produced intentionally for technological applications. Furthermore, all living organisms fuelled directly by photosynthesis and those fuelled indirectly by living on high energy nutrition represent preserved non-equilibrium states. The formation of these states represents the local reversal of the arrow of time which only seemingly violates the second law. It has been known since the seminal work of Prigogine that the stabilisation of these states inevitably requires the dissipation of energy in the form of waste heat. It is this feature of waste heat dissipation following the input of energy that drives all processes occurring at a non-zero rate. Photosynthesis, replication of living organisms, self-assembly, crystal shape engineering and distillation have this principle in common with the well-known Carnot cycle in the heat engine. Drawing on this analogy, we subsume these essential and often sophisticated driven processes under the term machinery of life.

Highly efficient formic acid and carbon dioxide electro-reduction to alcohols on indium oxide electrodes  

Kayode Adesina Adegoke,¹ Shankara Gayathri Radhakrishnan,¹ Clarissa L. Gray,¹ Barbara Sowa,¹ Claudia Morais,² Paul Rayess,² Egmont, R. Rohwer,¹ Clément Comminges,²* K. Boniface Kokoh,² Emil Roduner¹*

¹ Department of Chemstry, University of Pretoria, Pretoria 0002, South Africa
² Université de Poitiers, 4 rue Michel Brunet, IC2MP UMR-CNRS 7285, TSA 51106,
  86073 Poitiers cedex 9, France
³ Institute of Physical Chemistry, University of Stuttgart, Pfaffenwaldring 55,
  D-70569 Stuttgart, Germany

Sustainable Energy & Fuels 4 (2020) 4030–4038.

Formic acid is often assumed to be the first intermediate of carbon dioxide reduction to alcohols or hydrocarbons. Here we use co-electrolysis of water and aqueous formic acid in a PEM electrolysis cell with Nafion® as a polymer electrolyte, a standard TaC-supported IrO₂ water-splitting catalyst at the anode, and nanosize In₂O₃ with a small amount of added polytetrafluoroethylene (PTFE) as the cathode. This results in a mixture of methanol, ethanol and iso-propanol with a maximum combined Faraday efficiency of 82.5%. In the absence of diffusion limitation, a current density up to 70 mA cm^-2 is reached, and the space-time-yield compares well with results from heterogeneous In₂O₃ catalysis. Reduction works more efficiently with dissolved CO₂ than with formic acid, but the product distribution is different, suggesting that CO₂ reduction occurs primarily via a competing pathway that bypasses formic acid as an intermediate.