Subnanometer engineering: Clean energy sources and new materials properties

Professor María Pilar de Lara-Castells from the Institute of Fundamental Physics at the Spanish National Research Council is leading research in order to uncover the special properties of a new generation of materials: subnanometer-sized metal clusters, which could push the next generation of photocatalysts to a new level.

Read about the research in these three papers:

  1. https://doi.org/10.1039/C9TA00994A
  2. https://doi.org/10.1039/D0TA00062K
  3. https://doi.org/10.1021/acs.jpcc.9b06620

Hello and welcome to Research Pod. Thank you for listening and joining us today. In this episode we will be looking at the work coordinated by Professor María Pilar de Lara-Castells, Group Leader of the Ab-Initio Simulation and Modelling Unit at the Institute of Fundamental Physics at the Spanish National Research Council. Her group is conducting top research using first-principles computational modelling in order to uncover the special properties of a new generation of materials: subnanometer-sized metal clusters, which could push the energy absorbing potential of solar panels to a new level.

The quest for clean and renewable energy sources has intensified in recent years due to the increase in atmospheric concentration of greenhouse gases and the consequent increase in the average temperature of the planet. One alternative energy source is the conversion of sunlight into electricity through photo-voltaic panels. The efficiency of these depends on the intrinsic properties of the materials used in the manufacturing of the panels, and year by year, that efficiency is increasing with the discovery of new and better materials.

Titanium dioxide is an extraordinary material bearing numerous applications. It is abundant, nontoxic, biologically inert and chemically stable, known primarily as a white pigment used in paints, cosmetics and even toothpastes.

Titanium dioxide is also often used in sunscreens since it is especially capable of absorbing radiation in the ultraviolet region. This special property is used to preserve drugs and food from the damage caused by this harmful light. However, the same property severely limits the employment of TiO2 for solar energy conversion, since ultraviolet emissions comprise only 5 to 8% of the total energy from the sun.

The question is: Can this be extended to visible light? To answer this question, Professor de Lara-Castells and her collaborators conducted research in which they investigated how a special treatment can change the optical properties of titanium dioxide.

For this purpose, the researchers used the smallest copper nanoparticles, atomic clusters of sub-nanometer size composed by just five copper atoms. Links to the three papers in which their findings are published can be found in the show notes for this episode.

When deposited on the surface of titanium dioxide, these copper clusters can shift the absorption of sunlight from the high energy range, i.e. from the ultraviolet region, towards visible light, where the sun emits most of its energy. Therefore, much more energy can be collected from sunlight. This energy is harvested and stored temporarily in the material in the form of charge pairs which is a perfect prerequisite for follow-up chemistry.

Moreover, the researchers also demonstrated that the same strategy leads to an enhancement of the absorption of the material in the ultraviolet region. Finally, they also showed that the copper clusters induce a separation of photo-generated electron and holes, hindering their recombination rate, and improving the catalytic efficiency of titanium dioxide.

What is it about those copper nanoparticles that makes them so special? It turns out, when the size of metal clusters is reduced to a very small number of atoms (e.g., less than 10 of them), the continuous band of the metal splits into a sub nanometer sized network of molecular orbitals, with the interconnections having the length of a chemical bond.

Molecular orbitals are mathematical functions describing the location and wave-like behavior of the electrons in a molecule. The special spatial structures of these molecular orbitals in the smallest metal clusters make all their atoms cooperatively active and accessible, leading to the appearance of their novel properties.

Motivated by the enhanced absorption efficiencies acquired by titanium dioxide when modified with copper nanoparticles, the researchers planned to apply the outcome to some relevant photocatalytic reactions. By mimicking the processes of photosynthesis triggered by sunlight, this new photocatalyst material could also offer new strategies, for example, to eliminate carbon dioxide and other common pollutants in the atmosphere through solar-activated oxidation processes.

This research is especially timely as the increase of the greenhouse effect, causing a climate change worldwide, is a major concern. This increase is produced by an excessive accumulation in the atmosphere of certain gases, such as carbon dioxide (CO2), emitted by cars, heaters, and industry.

Carbon dioxide is a very stable molecule. The energy required to break the carbon-oxygen bond of the molecule is very high, about 7.3 electron volts. The deposition of the copper nanocluster on a Titanium dioxide surface catalyses the bond breaking, causing the energy to drop to 1.3 eV and the spontaneous decomposition of the CO2 molecule. This is possible thanks to the “flexibility” of the copper clusters: by means of concerted elongation and contractions of the copper bonds, the cluster adopts a structure lying flat over the titanium dioxide surface easing the bond breaking in CO2.

Moreover, the researchers also demonstrated that, in presence of sunlight, the activation of CO2 occurs at the interface weakening the bonds of the CO2 molecule and facilitating its reduction.

Seeking further applications, Professor de Lara-Castells and their collaborators recently modified the titanium dioxide surface with an atomic silver cluster, in collaboration with Salvador Miret-Artés, Director of the Institute of Fundamental Physics at the Spanish National Research Council.

They found that silver-modified surfaces work as photoactive materials for energy generation from visible light, and as potential photo-catalysts for CO2 reduction. Most importantly, the researchers uncovered new insights into a general property of surface polarons. The polaron concept, first proposed by the Novel Laureate Lev Davidovich Landau, characterizes an electron moving in a dielectric crystal such as titanium dioxide.

Defects on titanium dioxide surfaces, such as oxygen vacancies, lead to an excess of electrons which become self-trapped in specific sites. In order to screen an electron trapped in titanium sites, the neighboring oxygen anions depart from their equilibrium positions.

This lattice distortion is known as the phonon cloud, and the entity formed by the electron and its associated phonon cloud is the polaron. Furthermore, the trapped electron also carries a polarization cloud which modifies the electronic structure in its vicinity, characterizing a “polarization phenomenon” associated with the formation of a surface polaron.

This polarization phenomenon is in fact the discovery from the research coordinated and conceptualized by Professor de Lara- Castells. Think of it as a slow electron projecting a shadow on its travelling way, causing not only nuclear motion (i.e., a structural distortion of the crystal lattice) but also electronic rearrangements which are manifested in a fascinating electron polarization phenomenon.

In their work the team also demonstrated modification with silver clusters served to stabilize surface polarons, building polaronic 2D materials which could bear a number of applications, such as easing the water splitting process to make hydrogen, a very promising “green” fuel.

All these discoveries have been possible thanks to the application and further development of first-principles tools at the Ab-Initio Simulation research Unit led by Prof de Lara-Castells, in collaboration with Andreas Hauser from the Graz University of Technology, as well as the facilities of the Brazilian Synchrotron Light Laboratory.

The team also give their thanks to the research techniques developed by Félix Requejo at CONICET in Argentina, and Manuel Arturo López Quintela from the University of Santiago de Compostela.

In conclusion, the properties of subnanometer-sized metal clusters are pushing our understanding of these, more “molecular” than “metallic”, systems. The recent research by Prof. de Lara-Castells and collaborators is helping to confirm the great potential that lies in this new class of materials, and that first principles modelling is a powerful tool to shape the modern field of subnanometer science.

Thanks very much for listening. Don’t forget to check out the links to the original research papers, linked in the notes for this episode, and stay subscribed to ResearchPod for all the latest in research news. See you again soon.

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