Structural and surface features of highly dispersed copper catalysts supported on hierarchical porous alumina were evaluated for the ethanol dehydrogenation reaction. The catalysts were prepared by ...incipient wetness impregnation of alumina obtained by a dual template sol–gel procedure. Structural characterizations provided evidence that the copper was highly dispersed on the alumina in the form of monomeric octahedral Cu2+ species adsorbed into an incomplete layer. A two‐step reduction was observed for the catalysts in a temperature range of 50–250 °C under H2/He flow. The temperature for onset of reduction and the percentage of Cu+ species formed upon activation were dependent on the Cu loading. At lower Cu loadings, the amount of Cu+ increased, leading to a slower and less complete reduction. Based on the apparent activation energy and turnover frequency, it could be concluded that Cu+ containing a mixture of Cu+/Cu0 results in a more efficient catalyst than Cu0 for the ethanol dehydrogenation reaction.
A highly dispersed Cu catalyst: The hierarchical porous alumina promoted submonolayer coverage of copper and a large amount of intermediate Cu+ is observed during reduction of CuO to Cu0 at 250 °C under H2/He. Catalytic tests for the ethanol dehydrogenation reaction reveal that the Cu+/Cu0 mixture is more active than Cu0.
Among solution‐processed nanocrystals containing environmentally benign elements, bismuth sulfide (Bi2S3) is a very promising n‐type semiconductor for solar energy conversion. Despite the prompt ...success in the fabrication of optoelectronic devices deploying Bi2S3 nanocrystals, the limited understanding of electronic properties represents a hurdle for further materials developments. Here, two key materials science issues for light‐energy conversion are addressed: bandgap tunability via the quantum size effect, and photocarrier trapping. Nanocrystals are synthesized with controlled sizes varying from 3 to 30 nm. In this size range, bandgap tunability is found to be very small, a few tens of meV. First principles calculations show that a useful blueshift, in the range of hundreds of meV, is achieved in ultra‐small nanocrystals, below 1.5 nm in size. Similar conclusions are envisaged for the class of pnictide chalcogenides with a ribbon‐like structure Pn4Ch6n (Pn = Bi, Sb; Ch = S, Se). Time‐resolved differential transmission spectroscopy demonstrates that only photoexcited holes are quickly captured by intragap states. Photoexcitation dynamics are consistent with the scenario emerging in other metal–chalcogenide nanocrystals: traps are created in metal‐rich nanocrystal surfaces by incomplete passivation by long fatty acid ligands. In large nanocrystals, a lower bound to surface trap density of one trap every sixteen Bi2S3 units is found.
Bandgap tunability and density of midgap states in Bi2S3colloidal nanocrystals are investigated. Energy shifts of tens of meV are observed in nanocrystals with 3 nm size. Similar bandgap tunability is envisaged for pnictide chalcogenides with the same ribbon‐like structure Pn4Ch6n (Pn = Bi, Sb; Ch = S, Se). An intragap density of states in excess of 1020 cm−3 is found.
Ultrafast spin probe: A short‐lived transient species of light‐excited Fe(bpy)32+ (bpy=2,2′‐bipyridine) shows geometry variations that correspond to excitation to a high‐spin state, in which the spin ...momentum of FeII has now been directly determined to be S=2 by spin‐sensitive ultrafast X‐ray emission spectroscopy.
Latent fingerprints were successfully visualized using fluorescence lifetime imaging (FLIM) on paper which emits strong fluorescence with a lifetime close to that of fingerprints and thus from which ...it is difficult for time‐resolved spectroscopy to visualize fingerprints. Latent fingerprint samples on paper were excited using a 450 nm or 532 nm nanosecond pulsed‐laser, and time‐resolved fluorescence images were obtained at a delay time of 6–16 ns in intervals of 1 ns, to the excitation pulse. The excitation beam was expanded using a lens, and the fluorescence from the fingerprints was captured using an intensified CCD camera. Because of the large fluorescence intensity of the background paper of approximately two to four orders of magnitude larger than that of the fingerprint, the fingerprint was not visualized on each fluorescence image by time‐resolved spectroscopy. However, the fingerprint was visualized in a FLIM image constructed using a series of the fluorescence images for the case with the fluorescence intensity of the background paper being four orders of magnitude larger than that of the fingerprint. The difference in fluorescence lifetime in the FLIM image of the visualized fingerprint and background paper was in the order of 0.1 ns, which was an order of magnitude smaller than the inherent fluorescence lifetime of a few nanoseconds for the fingerprints and paper. It was demonstrated that, at a background fluorescence intensity with a certain order of magnitude larger than that of fingerprints, FLIM has the potential to visualize latent fingerprints which cannot be visualized by time‐resolved spectroscopy.
The excited‐state dynamics of pentacene derivatives with stable radical substituents were evaluated in detail through transient absorption measurements. The derivatives showed ultrafast formation of ...triplet excited state(s) in the pentacene moiety from a photoexcited singlet state through the contributions of enhanced intersystem crossing and singlet fission. Detailed kinetic analyses for the transient absorption data were conducted to quantify the excited‐state characteristics of the derivatives.
The state of play: Enhanced intersystem crossing (ISC) and singlet fission have been shown to occur in pentacene (Pn) derivatives with stable radical substituents. These effects result in the ultrafast formation of a triplet excited state in the pentacene moiety from a photoexcited singlet state, even in dilute solution. The excited‐state dynamics of the derivatives were evaluated in detail through transient absorption measurements.
Colloidal metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high ...photoluminescence quantum efficiency, large spin–orbit coupling, and extensive tunability via composition and choice of organic ligands. However, achieving pronounced and controllable polarized light emission remains challenging. Here, we develop strategies to achieve high CPL responses from colloidal formamidinium lead bromide (FAPbBr3) NCs at room temperature using chiral surface ligands. First, we show that replacing a portion of typical ligands (oleylamine) with short chiral ligands ((R)-2-octylamine) during FAPbBr3 NC synthesis results in small and monodisperse NCs that yield high CPL with average luminescence dissymmetry g-factor, g lum = 6.8 × 10–2. To the best of our knowledge, this is the highest among reported perovskite materials at room temperature to date and represents around 10-fold improvement over the previously reported colloidal CsPbCl x Br y I3‑x‑y NCs. In order to incorporate NCs into any optoelectronic or spintronic application, the NCs necessitate purification, which removes a substantial amount of the chiral ligands and extinguishes the CPL signals. To circumvent this issue, we also developed a postsynthetic ligand treatment using a different chiral ligand, (R-/S-)methylbenzylammonium bromide, which also induces a CPL with an average g lum = ±1.18 × 10–2. This postsynthetic method is also amenable for long-range charge transport since methylbenzylammonium is quite compact in relation to other surface ligands. Our demonstrations of high CPL and g lum from both as-synthesized and purified perovskite NCs at room temperature suggest a route to demonstrate colloidal NC-based spintronics.
The quick-EXAFS (QEXAFS) method adds time resolution to X-ray absorption spectroscopy (XAS) and allows dynamic structural changes to be followed. A completely new QEXAFS setup consisting of ...monochromator, detectors and data acquisition system is presented, as installed at the SuperXAS bending-magnet beamline at the Swiss Light Source (Paul Scherrer Institute, Switzerland). The monochromator uses Si(111) and Si(311) channel-cut crystals mounted on one crystal stage, and remote exchange allows an energy range from 4.0keV to 32keV to be covered. The spectral scan range can be electronically adjusted up to several keV to cover multiple absorption edges in one scan. The determination of the Bragg angle close to the position of the crystals allows high-accuracy measurements. Absorption spectra can be acquired with fast gridded ionization chambers at oscillation frequencies of up to 50Hz resulting in a time resolution of 10ms, using both scan directions of each oscillation period. The carefully developed low-noise detector system yields high-quality absorption data. The unique setup allows both state-of-the-art QEXAFS and stable step-scan operation without the need to exchange whole monochromators. The long-term stability of the Bragg angle was investigated and absorption spectra of reference materials as well as of a fast chemical reaction demonstrate the overall capabilities of the new setup.
Using continuous‐wave and time‐resolved Raman spectroscopy, we investigate and characterize the mineralogy of the NWA 7533 meteorite, a unique Martian regolith breccia. This meteorite is a crucial ...sample, giving a rare access to the formation and evolution of the primitive Martian crust, with crystallization processes beginning up to ~4.4 Ga years ago. We provide an overview of Raman spectra for feldspar, plagioclase, pyroxene, olivine, apatite, merrillite, zircon, pyrite, and various Fe‐Cr‐Ti oxides like magnetite, hematite, ilmenite, and rutile. Most analytical points are paired with previous electron microprobe analyses for deeper interpretation of Raman data. The variability of the silicate minerals composition obtained through microanalysis is traced through variation of the Raman spectra characteristics. No trace of any shock event is observed in the mineral structures. Structural diversity of zircon is linked to various metamictization stages. The luminescence signal of minerals bearing rare earth elements (REE), mainly phosphate and zircon, is documented in detail and discussed. We discuss the presence of at least two generations of phosphate minerals with implications for magmatic petrology. Because NWA 7533 provides a unique sampling of various crustal lithologies of Mars, this spectral dataset could be a reference for Raman instruments presently working on Mars (e.g., NASA Mars2020 Perseverance) or going to this planet or its moon Phobos in the future (e.g., ESA ExoMars, JAXA MMX).
We present here a systematic Raman and luminescence investigation of the NWA 7533 meteorite which is a unique Martian regolith breccia. For this, we use continuous‐wave and time‐resolved Raman spectroscopy to document the mineralogical diversity present in the meteorite, covering pyroxene, feldspar, olivine, and numerous accessory mineral phases (zircon, phosphate, and oxides).
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