Revealing hole trapping in zinc oxide nanoparticles by time-resolved X-ray spectroscopy

Abstract

Nanostructures of transition metal oxides, such as zinc oxide, have attracted considerable interest for solar-energy conversion and photocatalysis. Both applications are sensitive to the transport and trapping of photoexcited charge carriers. The probing of electron trapping has recently become possible using time-resolved element-sensitive methods, such as X-ray spectroscopy. However, valence-band-trapped holes have so far escaped observation. Herein we use X-ray absorption spectroscopy combined with a dispersive X-ray emission spectrometer to probe the charge carrier relaxation and trapping processes in zinc oxide nanoparticles after above band-gap photoexcitation. Our results, supported by simulations, demonstrate that within 80 ps, photoexcited holes are trapped at singly charged oxygen vacancies, which causes an outward displacement by ~15% of the four surrounding zinc atoms away from the doubly charged vacancy. This identification of the hole traps provides insight for future developments of transition metal oxide-based nanodevices.

Direct observation of the electronic states of photoexcited hematite with ultrafast 2p3d X-ray absorption spectroscopy and resonant inelastic X-ray scattering

“…5,6 In combination with calculations, XAS enables the determination of the detailed local structure and electronic states of materials. 7,8 There have been pioneering studies to investigate photoexcited states of metal oxides with the pump-probe XAS methodology. 7,[9][10][11][12][13][14][15][16] While synchrotron pump probe XAS experiments are limited by the pulse duration of X-rays (B100 ps) or by the reachable flux for slicing experiments, the birth of X-ray free electrons lasers (XFEL) has expanded the horizon of the time resolved studies based on the pump-probe XAS methodology.…”

“…7,8 There have been pioneering studies to investigate photoexcited states of metal oxides with the pump-probe XAS methodology. 7,[9][10][11][12][13][14][15][16] While synchrotron pump probe XAS experiments are limited by the pulse duration of X-rays (B100 ps) or by the reachable flux for slicing experiments, the birth of X-ray free electrons lasers (XFEL) has expanded the horizon of the time resolved studies based on the pump-probe XAS methodology. 1,9,11,[17][18][19][20] XFELs can provide ultrashort and intense X-ray pulses down to 10 fs.…”

Design of Heterostructured Hollow Photocatalysts for Solar‐to‐Chemical Energy Conversion

“…On the other hand, novel characterization methods such as in situ attenuated total reflectance infrared spectroscopy and X‐ray absorption spectroscopy can provide detailed information about the surface and bulk properties of the photocatalysts during the reactions . Besides, time‐resolved spectroscopies, such as time‐resolved transient absorption spectroscopy, time‐resolved photoluminescence and time‐resolved X‐ray spectroscopy, would be useful tools for the study of the charge trapping behaviors and reaction mechanisms . It is of great importance to experimentally investigate the enhancing mechanisms, which can guide further design and fabrication of highly active heterostructured hollow photocatalysts.…”

Resolving structures of transition metal complex reaction intermediates with femtosecond EXAFS

“…2 Furthermore, they play essential roles in catalytic applications; Transition Metal (TM) complexes are used as photosensitizers for hydrogen evolution, 3 for conversion of solar to electric energy 4 and TM oxide nanoparticles are exploited as photocatalysts. 5,6 Hard X-ray absorption spectroscopy (XAS) is ideally suited to investigate such TM centers due to its element specificity and sensitivity to changes in the geometric and electronic structures. The X-ray Absorption Near Edge Structure (XANES), i.e.…”

“…While the aforementioned XANES measurements of photoexcited TiO 2 revealed the timescale of the electron trapping, the extension to the EXAFS regime to study ZnO nanoparticles allowed the precise characterization of the hole trapping site as a singly charged oxygen vacancies. 5 A further advantage of time-resolved EXAFS is the high accuracy -on the order of 1 picometer -in resolving the TM-ligand bond distances and the changes therein, 27 as well as the sensitivity in resolving asymmetries in the first coordination shell around the TM. [28][29][30][31][32] Extending EXAFS of weakly concentrated solvated samples to the femtosecond regime with beam slicing at synchrotrons 33 or tabletop plasma source 34 is not viable due to the low photon flux and/or limited photon energy tunability of these source.…”