We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge, and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes is observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. This first, time-resolved element-specific study of perovskites helps understand the rather modest charge carrier mobilities in these materials.

Vanadium-doped TiO2 nanoparticles (V-TiO2 NPs) with a V/Ti ratio of 3.0 at. % were prepared by gas-phase condensation and subsequent oxidation at elevated temperature. Both photocatalytic activity for -NO2 reduction and photoelectrochemical water splitting were induced by V-doping in the visible spectral range > 450 nm, where undoped TiO2 NPs are completely inactive. The photocatalytic properties were correlated with the ultrafast dynamics of the photoexcited charge carriers studied by femtosecond transient absorption (TA) spectroscopy with three different excitation wavelengths, i.e. e = 330, 400, and 530 nm. Only in V-doped NPs, the photoexcitation of electrons into the conduction band by sub-bandgap irradiation (e = 530 nm) was detected by TA spectroscopy. This observation was associated with electronic transitions from an intra-gap level localized on V 4+ cations. The photoexcited electrons subsequently relaxed, with characteristic times of 200-500 ps depending on e, into Ti-related surface traps that possessed suitable energy to promote -NO2 reduction. The photoexcited holes migrated to long-lived surface traps with sufficient overpotential for the oxidization of both 2-propanol and water. On the basis of TA spectroscopy and photocurrent measurements, the position of the dopant-induced intra-gap level was estimated as 2.2 eV below the conduction band minimum.

We report a detailed study of the K-edge X-ray absorption spectra of four transition metal phthalocyanines (MPc, M = Fe, Co, Cu and Zn). We identify the important single and multiple scattering contributions to the spectra in the extended energy range and provide a robust treatment of thermal damping; thus, a generally applicable model for the interpretation of X-ray absorption fine structure spectra is proposed. Consistent variations of bond lengths and Debye Waller factors are found as a function of atomic number of the metal ion, indicating a variation of the metal-ligand bond strength which correlates with the spatial arrangement and occupation of molecular orbitals. We also provide an interpretation of the near edge spectral features in the framework of a full potential real space multiple scattering approach and provide a connection to the local electronic structure.

Doping with transition metals is an effective method to enhance visible light absorption in TiO2 nanoparticles and to improve the efficiency of many photocatalytic processes under solar radiation. A determination of the incorporation site of the dopant and an understanding of the local bonding arrangement and electronic structure is a necessary step for knowledge -based materials design. In this paper, we report an in-depth X-ray Absorption Spectroscopy study of V dopants in TiO2 nanoparticles deposited by gas phase condensation with a local structure similar to anatase, rutile or intermediate. The combination of K and L edge spectra in the pre-edge, edge, and extended energy regions with full potential ab-initio spectral simulations shows that V ions occupy substitutional cationic sites in the TiO2 structure, irrespective of whether it is similar to rutile, anatase or mixed.

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We report an X-ray absorption near edge structure (XANES) study of vanadium (V) and nitrogen (N) dopants in anatase TiO thin films deposited by radio-frequency magnetron sputtering. Measurements at the Ti K and V K edges were combined with soft X-ray experiments at the Ti L, O K and N K edges. Full potential ab initio spectral simulations of the V, O and N K-edges were carried out for different possible configurations of substitutional and interstitial dopant-related point defects in the anatase structure. The comparison between experiments and simulations demonstrates that V occupies substitutional cationic sites (replacing Ti) irrespective of the film structure and dopant concentration (up to 4.5 at%). On the other hand, N is found both in substitutional anionic sites (replacing O) and as N dimers within TiO interstices. The dopants' local structures are discussed with reference to the enhanced optical absorption and photocatalytic activity achieved by (co)doping.

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Refined X-ray spectroscopies can be crucial in elucidating charge transfer phenomena which play a key-role in photo-catalysis and other processes relevant for clean energy production. A deep understanding of electron photo-dynamics is, in fact, essential to develop efficient knowledge-based devices. We developed a differential illumination RIXS and HERFD-XAS [1], method on ID26 @ ESRF to investigate charge transfer phenomena with chemical sensitivity; specifically, we studied Vdoped TiO2 nanoparticles, a promising materials system for photo-catalysis, performing measurements around both the V Kα and Ti Kβ emissions. We found that visible light absorption induces the transfer of electrons from the V dopants to the host matrix cations in defective sites. With a steady state model, it was also possible to estimate the lifetime of the excited state. The value we obtained (around 1 ms) suggests that dopant-injected electrons can remain trapped near Ti atoms for a very long time. The procedure we used is completely general and can be successfully applied to detect any kind of long-living charge transfer phenomena in a wide range of possible devices [2].