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Dynamics of Photo-Induced Surface Oxygen Vacancies in Metal-Oxide Semiconductors Studied Under Ambient Conditions.

ABSTRACT: Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique commonly used in the detection of traces of organic molecules. The mechanism of SERS is of a dual nature, with Raman scattering enhancements due to a combination of electromagnetic (EM) and chemical contributions. In conventional SERS, the EM component is largely responsible for the enhancement, with the chemical contribution playing a less significant role. An alternative technique, called photo-induced enhanced Raman spectroscopy (PIERS) has been recently developed, using a photo-activated semiconductor substrate to give additional chemical enhancement of Raman bands over traditional SERS. This enhancement is assigned to surface oxygen vacancies (V _o) formed upon pre-irradiation of the substrate. In this work, the exceptional chemical contribution in PIERS allows for the evaluation of atomic V _o dynamics in metal oxide surfaces. This technique is applied to study the formation and healing rates of surface-active V _o in archetypical metal-oxide semiconductors, namely, TiO₂, WO₃, and ZnO. Contrary to conventional analytical tools, PIERS provides intuitive and valuable information about surface stability of atomic defects at ambient pressure and under operando conditions, which has important implications in a wide range of applications including catalysis and energy storage materials.

SUBMITTER: Glass D

PROVIDER: S-EPMC6864511 | biostudies-literature | 2019 Nov

REPOSITORIES: biostudies-literature

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Dynamics of Photo-Induced Surface Oxygen Vacancies in Metal-Oxide Semiconductors Studied Under Ambient Conditions.

Glass Daniel D Cortés Emiliano E Ben-Jaber Sultan S Brick Thomas T Peveler William J WJ Blackman Christopher S CS Howle Christopher R CR Quesada-Cabrera Raul R Parkin Ivan P IP Maier Stefan A SA

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 20190930 22

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique commonly used in the detection of traces of organic molecules. The mechanism of SERS is of a dual nature, with Raman scattering enhancements due to a combination of electromagnetic (EM) and chemical contributions. In conventional SERS, the EM component is largely responsible for the enhancement, with the chemical contribution playing a less significant role. An alternative technique, called photo-induced enhanced Raman ...[more]

PMID: 31763155

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Project description:ConspectusThe liquid-microjet technique combined with soft X-ray photoelectron spectroscopy (PES) has become an exceptionally powerful experimental tool to investigate the electronic structure of liquid water and nonaqueous solvents and solutes, including nanoparticle (NP) suspensions, since its first implementation at the BESSY II synchrotron radiation facility 20 years ago. This Account focuses on NPs dispersed in water, offering a unique opportunity to access the solid-electrolyte interface for identifying interfacial species by their characteristic photoelectron spectral fingerprints. Generally, the applicability of PES to a solid-water interface is hampered due to the small mean free path of the photoelectrons in solution. Several approaches have been developed for the electrode-water system and will be reviewed briefly. The situation is different for the NP-water system. Our experiments imply that the transition-metal oxide (TMO) NPs used in our studies reside close enough to the solution-vacuum interface that electrons emitted from the NP-solution interface (and from the NP interior) can be detected.We were specifically exploring aqueous-phase TMO NPs that have a high potential for (photo)electrocatalytic applications, e.g., for solar fuel generation. The central question we address here is how H2O molecules interact with the respective TMO NP surface. Liquid-microjet PES experiments, performed from hematite (α-Fe2O3, iron(III) oxide) and anatase (TiO2, titanium(IV) oxide) NPs dispersed in aqueous solutions, exhibit sufficient sensitity to distinguish between free bulk-solution water molecules and those adsorbed at the NP surface. Moreover, hydroxyl species resulting from dissociative water adsorption can be identified in the photoemission spectra. An important aspect is that in the NP(aq) system the TMO surface is in contact with a true extended bulk electrolyte solution rather than with a few monolayers of water, as is the case in experiments using single-crystal samples. This has a decisive effect on the interfacial processes that can occur since NP-water interactions can be uniquely investigated as a function of pH and provides an environment allowing for unhindered proton migration. Our studies confirm that water is dissociatively adsorbed at the hematite surface and molecularly adsorbed at the TiO2 NP surface at low pH. In contrast, at near-basic pH the water interaction is dissociative at the TiO2 NP surface.The liquid-microjet measurements presented here also highlight the multiple aspects of photoemission necessary for a full characterization of TMO nanoparticle surfaces in aqueous environments. For instance, we exploit the ability to increase species-specific electron signals via resonant photoemission, so-called partial electron yield X-ray absorption (PEY-XA) spectra, and from valence photoelectron and resonant Auger-electron spectra. We also address the potential of these resonance processes and the associated ultrafast electronic relaxations for determining charge transfer or electron delocalization times, e.g., from Fe3+ located at the hematite nanoparticle interface into the aqueous-solution environment.

Dataset Information

Dynamics of Photo-Induced Surface Oxygen Vacancies in Metal-Oxide Semiconductors Studied Under Ambient Conditions.

Publications

Dynamics of Photo-Induced Surface Oxygen Vacancies in Metal-Oxide Semiconductors Studied Under Ambient Conditions.

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