Two-dimensional (2D) metallic states induced by oxygen vacancies at oxide surfaces and interfaces provide new opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used Angle Resolved Photoelectron Spectroscopy combined with density functional theory to study the reactivity of states induced by the oxygen vacancies at the (001)(14) surface of anatase TiO2, where both 2D metallic and deeper lying in-gap states (IGs) are observed. Remarkably, the two states exhibit very different evolution when the surface is exposed to molecular O2: while IGs are almost completely quenched, the metallic states are only weakly affected. The energy scale analysis for the vacancy migration and recombination resulting from the DFT calculations confirms indeed that only the IGs originate from and remain localized at the surface, whereas the metallic states originate from subsurface vacancies, whose migration and recombination at the surface is energetically less favorable rendering them therefore insensitive to oxygen dosing.
ARTICLE TEXTI.
The NiOOH electrode is commonly used in electrochemical
alcohol
oxidations. Yet understanding the reaction mechanism is far from trivial.
In many cases, the difficulty lies in the decoupling of the overlapping
influence of chemical and electrochemical factors that not only govern
the reaction pathway but also the crystal structure of the in situ formed oxyhydroxide. Here, we use a different approach
to understand this system: we start with synthesizing pure forms of
the two oxyhydroxides, β-NiOOH and γ-NiOOH. Then, using
the oxidative dehydrogenation of three typical alcohols as the model
reactions, we examine the reactivity and selectivity of each oxyhydroxide.
While solvent has a clear effect on the reaction rate of β-NiOOH,
the observed selectivity was found to be unaffected and remained over
95% for the dehydrogenation of both primary and secondary alcohols
to aldehydes and ketones, respectively. Yet, high concentration of
OH– in aqueous solvent promoted the preferential
conversion of benzyl alcohol to benzoic acid. Thus, the formation
of carboxylic compounds in the electrochemical oxidation without alkaline
electrolyte is more likely to follow the direct electrochemical oxidation
pathway. Overoxidation of NiOOH from the β- to γ-phase
will affect the selectivity but not the reactivity with a sustained
>95% conversion. The mechanistic examinations comprising kinetic
isotope
effects, Hammett analysis, and spin trapping studies reveal that benzyl
alcohol is oxidatively dehydrogenated to benzaldehyde via two consecutive hydrogen atom transfer steps. This work offers the
unique oxidative and catalytic properties of NiOOH in alcohol oxidation
reactions, shedding light on the mechanistic understanding of the
electrochemical alcohol conversion using NiOOH-based electrodes.
The semiconductor industry plans to keep fabricating integrated circuits, progressively decreasing there features size, by employing extreme ultraviolet lithography (EUVL). With this method, new designs and concepts for photoresist materials need to be conceived. In this work, we explore an alternative concept to the classic photoresist material by using an organic self-assembled monolayer (SAM) on a gold substrate. The monolayer, composed of a richly fluorinated thiol sensitive to low-energy electrons, is adsorbed on the Au substrate which acts as main EUV-absorber and as the source of photoelectrons and secondary electrons. We investigate the stability of the SAM adsorbed on gold towards EUV radiation by means of in-situ photoelectron spectroscopy. The photoelectron spectra indicate that the monolayer attenuates a significant amount of primary electrons generated in the gold layer. The spectral evolution upon EUV irradiation indicates that the SAM loses a significant amount of its initial fluorine content (ca. 40% at 200 mJ/cm 2 ). We attribute these chemical changes mostly to the interaction with the electrons generated in the thiol/Au system.
The formation and the evolution of electronic metallic states localized at the surface, commonly termed 2D electron gas (2DEG), represents a peculiar phenomenon occurring at the surface and interface of many transition metal oxides (TMO). Among TMO, titanium dioxide (TiO 2 ), particularly in its anatase polymorph, stands as a prototypical system for the development of novel applications related to renewable energy, devices and sensors, where understanding the carrier dynamics is of utmost importance. In this study, angle-resolved photo-electron spectroscopy (ARPES) and X-ray absorption spectroscopy (XAS) are used, supported by density functional theory (DFT), to follow the formation and the evolution of the 2DEG in TiO 2 thin films. Unlike other TMO systems, it is revealed that, once the anatase fingerprint is present, the 2DEG in TiO 2 is robust and stable down to a single-unit-cell, and that the electron filling of the 2DEG increases with thickness and eventually saturates. These results prove that no critical thickness triggers the occurrence of the 2DEG in anatase TiO 2 and give insight in formation mechanism of electronic states at the surface of TMO.
Silicon nanoparticles
(SiNPs) have been explored intensively for
their use in applications requiring efficient fluorescence for LEDs,
lasers, displays, photovoltaic spectral-shifting filters, and biomedical
applications. High radiative rates are essential for such applications,
and theoretically these could be achieved via quantum confinement
and/or straining. Wet-chemical methods used to synthesize SiNPs are
under scrutiny because of reported contamination by fluorescent carbon
species. To develop a cleaner method, we utilize a specially designed
attritor type high-energy ball-mill and use a high-purity (99.999%)
Si microparticle precursor. The mechanochemical process is used under
a continuous nitrogen gas atmosphere to avoid oxidation of the particles.
We confirm the presence of quantum-confined NPs (<5 nm) using atomic
force microscopy (AFM). Microphotoluminescence (PL) spectroscopy
coupled to AFM confirms quantum-confined tunable red/near-infrared
PL emission in SiNPs capped with an organic ligand (1-octene). Using
micro-Raman-PL spectroscopy, we confirm SiNPs as the origin of the
emission. These results demonstrate a facile and potentially scalable
mechanochemical method of synthesis for contamination-free SiNPs.
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