NEWS

8 MARCH 2020

A SUBNANOMETER SILVER CLUSTER HAS REVEALED A GENERAL ELECTRON POLARIZATION PHENOMENA OF SURFACE POLARONS

(A ``paper story" written by María Pilar de Lara-Castells)

The polaron concept was first proposed by Landau in 1933 [1] to characterize an electron moving in a dielectric crystal such as titanium dioxide, where the O2- ions depart from the Ti atom hosting the polaron to screen its self-trapped Ti3+ 3d1 electron.  This lattice distortion is known as the phonon cloud. The entity formed by the electron and its associated phonon cloud is the polaron. The effective mass of the polaron is larger than that of the free electron and, accordingly, it is also ``slower". From a very simplified picture, a surface polaron could be thought as a travelling "slow" electron onto the given surface since it "cannot escape its shadow".

In our latest work [2], we have found that there is also an electron polarization phenomena accompanying the polaron formation: The self-trapped Ti3+ 3d1 electron repels nearby oxygen ions and attracts nearby titanium cations, which, in turn, affects their electronic structure, causing the transfer of electronic charge from Ti4+ cations to O2- ions. The stabilization of the surface polaron was possible through decoration of the TiO2 nanoparticles with subnanometer silver clusters (Ag5). In this way,
we found three different evidences of the electron polarization phenomena: (1) via a direct first-principles analysis of Ti(3d) orbitals population; (2)  through X-ray spectroscopic measurements; (3) via first-principles simulations of the UV-Vis spectra of the composite Ag5/TiO2 system.



I am very happy of having led the work, and greatly thankful to all my co-authors: to my outstanding student, Patricia López-Caballero (AbinitSim Unit, Institute of Fundalmental Physics at CSIC), to Salvador Miret-Artés (Institute of Fundamental Physics at CSIC), to M. Arturo Lopez Quintela (co-funder of Nanogap) and David Buceta from the University of Santiago de Compostela (USC), and to José Martín Ramallo-López, Lisandro J. Giovanetti, and Félix Requejo from CONICET at Argentina. Also, I would like to remark that the development of the work was possible using the facilies of the CESGA SuperComputer Center (Galicia, Spain), where the first-principles simulations were performed, and of the Brazilian Synchroton Light Laboratory (LNSN), where the X-ray measurements were taken. Our work has been also possible thanks to the group of M. Arturo López-Quintela (USC) because they have developed the special technique rendering the synthesis of subnanometer metal clusters feasible.

If you want to know more, please read the paper, it is open-access:

https://doi.org/10.1039/D0TA00062K

[1] L. D. Landau and S. I. Pekar, Effective mass of a polaron, Zh. Eksp. Teor. Fiz. 18, 419–423 (1948) [in Russian], English translation: Ukr. J. Phys., Special Issue, 53, pp. 71–74 (2008).

[2] P. López-Caballero, J. M. Ramallo-López, L. J. Giovanetti, D. Buceta, S. Mirét-Artés, M. Arturo López-Quintela, F. Requejo, M. P. de Lara-Castells,*  Exploring the properties of Ag5-TiO2 interfaces: stable surface polarons formation, UV-Vis optical response, and CO2 photoactivation, J. Mat. Chem. A. (2020)  https://doi.org/10.1039/D0TA00062K

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10 SEPTEMBER 2019: Patricia López-Caballero has co-authored her first paper. Congratulations Patricia!!. Press here to see the paper.

10 JULY 2019: A new report about our recent reserach has been published at https://www.lnls.cnpem.br/enhancing-solar-energy-production/

3 MAY 2019: A new report about our recent research has been published at CESGA. Press here to see the report.

8 APRIL 2019: The Book Of Abstracts from the MOLIM WG3 meeting "Ab-Initio Modelling of Molecular Processes Under Confinement" is already available. Press here.

1 APRIL 2019: Today, Patricia López-Caballero, graduate student (in Chemistry) from the Universidad Autónoma de Madrid, has been incorporated into the AbinitSim group. Welcome Patricia!.

26 MARCH 2019: Within the framework of our collaboration with Prof. Andreas W. Hauser's group (Graz University of Technology, Austria), Prof. Arturo M. López-Quintela's group (University of Santiago de Compostela, Spain), and Prof. Félix Requejo's group (CONICET, Argentina), we have just published an article on the deposition of subnanometer-sized Cu5 clusters onto TiO2 surfaces. The work is entitled "Increasing the optical response of TiO2 and extending it into the visible through surface activation with highly stable Cu5 clusters" (de Lara-Castells, Hauser, Ramallo-López, Buceta, Giovanetti, López-Quintela, Requejo; Journal of Materials Chemistry A 7 (2019) 7489).

MOTIVATION

TiO2 is one of the most popular materials for applications such as solar energy conversion and photocatalysis. However, its large band-gap (3-3.2 eV) makes that ultraviolet irradiation is necessary to trigger photo-catalytic reactions. The UV part is less than 8% of the solar radiation so that the reaction rate divided by the photon flux is less than 10%. In our work, we demonstrate via ab-initio modelling that the optical properties of TiO2 are much improved upon deposition of the Cu5 clusters and our fi ndings are further supported by selected experiments. The copper cluster donates electronic charge to TiO2, loosing its magnetic moment and leading to the formation of a small polaron state, which explains X-ray absorption spectroscopy data. More importantly, a monolayer of highly stable copper clusters is formed, which is not only enhancing the overall absorption, but also extending it into the visible region via a direct photo-induced electron transfer and formation of a charge-separated state. The synthesis of these subnanometer-sized metal clusters was achieved using a novel technique that has opened the door to applications such a cancer-therapeutic drugs (www.nanogap.es).

Overall, our study illustrates the suitability of ab-initio modelling in predicting useful properties of subnanometer-sized metal clusters under confinement in the surface of technologically relevant materials. We are currently extending our work to specific photo-induced reactions. So Stay Tuned!.

 

SPANISH VERSION:

26 de Marzo de 2019: Hemos publicado un trabajo que muestra cómo pueden mejorarse las propiedades ópticas de superficies de TiO2 mediante el depósito de agregados subnanométricos de cobre (Cu5). El TiO2 es una de los materiales más populares en aplicaciones fotocatalíticas y de conversión de la energía solar debido a su abundancia, no toxicidad, y estabilidad química. Sin embargo, su gran ancho de banda  (entre 3 y 3.2 eV) hace necesaria la absorción de radiación ultravioleta para desancadenar reacciones fotoinducidas. Esta característica no deseada limita severamente las aplicaciones del TiO2 ya que la parte UV comprende solo el 5-8% de la energía solar. Como consecuencia, la tasa de formación de productos de reacción por fotón incidente es típicamente inferior al 10% en fotocatalizadores basados en TiO2. En nuesto trabajo, hemos demostrado cómo el depósito de agregados de Cu5 de tamaño subnanométrico mejora en órdenes de magnitud la absorción de fotones del material en el ultravioleta, logrando que la absorción se extienda a la región visible. Además, se muestra cómo los agregados Cu5 inducen la formación de pequeños polarones en la superficie de TiO2. El trabajo ilustra la capacidad predictiva de modelizaciones ab-initio multiescala de nuevos materiales nanoestructurados, habiendo sido los resultados teóricos corroborados mediante medidas experimentales.

El artículo es open access y puede ser descargado desde:

https://pubs.rsc.org/en/Content/ArticleLanding/2019/TA/C9TA00994A#!divAbstract