Demonstration of Plasmonic-mediated Charge Transfer in Hybrid Semiconductor-Plasmonic Photocatalyst

Nowadays, the use of solar light to drive chemical reactions is becoming a potential green alternative to traditional thermally driven heterogeneous catalysis (1). In this field, the most investigated system involves the coupling of transition metal oxides (TMOs) with plasmonic nanostructures. 
These structures exhibit a strong interaction with light which induces the collective oscillations of electrons. The excited electrons (holes) can be transferred to the catalytic semiconductor enhancing its reactivity. 
In the past 10 years researchers attempted understanding which is the principal mechanism involved in this electronic (hole) transfer using UV-Vis ultrafast spectroscopies. However, the insufficient chemical sensitivity of the ultrafast spectroscopies in the ultraviolet, visible and infrared range have imposed limitations to the quantitative interpretation of the results. X-ray absorption spectroscopy (XAS) represents a valuable tool to obtain element-specific information with very high sensitivity to the electronic structure of the individual elements present in the system. The unique performance of FERMI FEL combining the possibility of using ultrashort light pulses with a fine-tuneable photon energy within 20-300 eV and remarkable spectral stability and purity has allowed us to investigate the response of hybrid semiconductor-plasmonic photocatalyst using time-resolved XAS (TR-XAS). 
In our work, published in the ACS journal Nano Letters, we have demonstrated the capability of FELs to explore dynamic processes in functional materials. We observed how silver nanoparticles embedded in CeO₂catalyst transfer electrons to the oxide through an ultrafast (less than 200 fs) and high-efficient mechanism. The pump-probe TR-XAS measurements were performed at the EIS-TIMEX end-station operating in a single-shot laser pump-FEL probe mode selecting probe energies across the Ce N4,5-edge. The observed charge transfer is associated with the ultrafast changes (< 200 fs) of the Ce N_4,5 absorption edge (Figure 1a and b), compatible with the cerium oxidation state variation, Ce⁴⁺→Ce³⁺, as represented in Figure 1a) triggered by the selective excitation of the silver nanoparticles with a visible pump. The experiments have been carried out at the beamline EIS-TIMEX of FERMI in March 2020 during the first days of Covid-19 lockdown in Italy.
Our findings result from the collaboration between the EIS-TIMEX staff and the colleagues at the University of Modena where the samples were grown and pre-characterised. This research has been supported by the PRIN project “NEWLI”.
 

Figure 1. a) Ce N_4,5 XAS absorption spectra measured in transmission mode for CeO₂ (solid red line) and Ag@CeO₂ (solid black line) samples. The reference spectra of Ce⁴⁺ (dashed green line) and Ce³⁺ (dashed blue line) samples are also reported. The inset reports the relative variation of the absorption during Ce reduction (Ce⁴⁺→ Ce³⁺). Purple points, and the black points in the inset, indicate the selected FEL energies used to probe the variations of absorption. b. Relative variation of absorption at 122 eV (top) and 130 eV (bottom) as a function of pump-probe delay time and corresponding fit (red curve) 

This research was conducted by the following research team:

Jacopo Stefano Pelli Cresi¹, Emiliano Principi¹, Eleonora Spurio^2,3, Daniele Catone⁴, Patrick O'Keeffe⁵, Stefano Turchini⁴, Stefania Benedetti³, Avinash Vikatakavi^2,3, Sergio D’Addato^2,3, Riccardo Mincigrucci¹, Laura Foglia¹, Gabor Kurdi¹, Ivaylo P. Nikolov¹, Giovanni de Ninno¹, Claudio Masciovecchio¹, Jagadesh Kopula Kesavan⁶, Federico Boscherini⁶, Stefano Nannarone⁷and Paola Luches³

¹ Elettra-Sincrotrone Trieste, Trieste, Italy 
² Dipartimento FIM, Università degli Studi di Modena e Reggio Emilia, Modena, Italy. 
³ CNR-NANO, Centro di Ricerca S3, Modena, Italy. 
⁴ CNR-ISM, Division of Ultrafast Processes in Materials (FLASHit), Area della Ricerca di Roma 2 Tor Vergata, Rome, Italy. 
⁵ CNR-ISM, Division of Ultrafast Processes in Materials (FLASHit), Area della Ricerca di Roma 1, Monterotondo Scalo, Italy 
⁶ Dipartimento di Fisica e Astronomia, Alma Mater Studiorum – Università di Bologna, Bologna, Italy
⁷ IOM, CNR, Trieste, Italy

Contact persons:

Jacopo Stefano Pelli Cresi, email: