Time-resolved dynamics of granular matter by random laser emission

Because of the huge commercial importance of granular systems, the second-most used material in industry after water, intersecting the industry in multiple trades, like pharmacy and agriculture, fundamental research on grain-like materials has received an increasing amount of attention in the last decades. In photonics, the applications of granular materials have been only marginally investigated. We report the first phase-diagram of a granular as obtained by laser emission. The dynamics of vertically-oscillated granular in a liquid solution in a three-dimensional container is investigated by employing its random laser emission. The granular motion is function of the frequency and amplitude of the mechanical solicitation, we show how the laser emission allows to distinguish two phases in the granular and analyze its spectral distribution. This constitutes a fundamental step in the field of granulars and gives a clear evidence of the possible control on light-matter interaction achievable in grain-like system.Comment: 16 pages, 7 figure

Band structure and charge carrier dynamics in (W,N)-codoped TiO 2 resolved by electrochemical impedance spectroscopy combined with UV–vis and EPR spectroscopies

Semiconductor photocatalysis is on the verge of (probably) its most important deployment and boost since the pioneering paper of Fujishima and Honda in 1972. Photo-generation of unbound excitons, i.e. separated conduction band electrons and valence band positive holes, is the fundamental primary process triggering charge separation in solid semiconductors necessary to initiate their photocatalytic activity. Immediately after being generated, charge carriers can undergo processes like recombination, trapping in mid-band-gap states or, paramount for photocatalytic processes, transfer to species adsorbed on the solid semiconductor surface. In TiO2 and doped TiO2, interfacial charge transfers are the slowest amongst the primary processes; therefore, electron (and hole) transfer most likely occurs from single electron traps (i.e. involving radical species). We report here on an effective approach combining electrochemical impedance spectroscopy with other spectroscopic techniques such as UV–vis and electron paramagnetic resonance. This approach allows deriving important information about band structure and following electron dynamics triggered by photon absorption. The redox potentials of the band edges and the influence of the dopants on the band structure are elucidated by electrochemical impedance spectroscopy combined with UV–vis spectroscopy. Electron dynamics are then studied using electron paramagnetic resonance spectroscopy, to elucidate the photochemical reactions at the basis of the photo-generated electron–hole pairs, and subsequent trapping and/or recombination. Results of a TiO2 sample containing W and N as dopants (0.1 at.% of W) highlight a narrowing of the intrinsic band gap of about 0.12 eV. The semiconductor visible light photochemistry is driven by diamagnetic donor states [NiO]‑, and [NiO]w‑ (formally NO3‑), from which electrons can be excited to the conduction band, generating EPR active paramagnetic [NiO] and [NiO]w states (formally NO2‑). The formation of W5 + electron trapping states, energetically more favourable than Ti3 + electron trapping states, is also identified

Shaken Granular Lasers

Granular materials have been studied for decades, also driven by industrial and technological applications. These very simple systems, composed by agglomerations of mesoscopic particles, are characterized, in specific regimes, by a large number of metastable states and an extreme sensitivity (e.g., in sound transmission) on the arrangement of grains; they are not substantially affected by thermal phenomena, but can be controlled by mechanical solicitations. Laser emission from shaken granular matter is so far unexplored; here we provide experimental evidence that it can be affected and controlled by the status of motion of the granular, we also find that competitive random lasers can be observed. We hence demonstrate the potentialities of gravity affected moving disordered materials for optical applications, and open the road to a variety of novel interdisciplinary investigations, involving modern statistical mechanics and disordered photonics.Comment: 4 pages, 3 figures. To be published in Physical Review Letter

On the Underlying Mechanisms of the Low Observed Nitrate Selectivity in Photocatalytic NOx Abatement and the Importance of the Oxygen Reduction Reaction

The authors are grateful to the German Ministry of Economics for funding the AiF/IGF project 18152 N and to the European Commission for funding the European Project Light2CAT (grant agreement no. 283062) in which some of the results presented here were obtained.Peer reviewedPostprin

Effect of support acidity during selective hydrogenolysis of glycerol over supported palladium-ruthenium catalysts

We report the role of the acidity of support during the selectivity hydrogenolysis of glycerol over supported bimetallic palladium–ruthenium (PdRu) catalysts. The PdRu nanoparticles were supported on a series of metal oxides and zeolitic supports via the modified impregnation method and tested for the liquid-phase hydrogenolysis of glycerol using gaseous hydrogen. The relative acid site densities of selected catalysts were determined by ammonia temperature-programmed desorption and pyridine desorption experiments. Based on these studies, we report a direct correlation between the catalytic activity (conversion and 1,2 propane diol yield) and two different acid sites (strong acid sites and very strong acid sites). Besides zeolite-supported catalysts, TiO2 supported PdRu nanoparticles exhibit moderate catalytic activity; however, this catalyst shows high selectivity for the desired C–O bond cleavage to produce C3 products over the undesired C–C bond cleavage to produce < C3 products

Probing the structure of copper(II)-casiopeina type coordination complexes [Cu(O-O)(N-N)]+ by EPR and ENDOR spectroscopy

Although copper based complexes have been widely used in homogeneous catalysis, more recently they are attracting considerable attention as pharmaceutical therapeutic agents. Of paramount importance in their efficacy of use is their structure and electronic properties, which can be thoroughly probed using advanced EPR techniques. In this study, a series of [Cu(acac)(N-N)]+ Casiopeina type complexes were investigated, bearing a series of diimine N-N ligands (including bipy, phen, Py-bipy and dppz). All complexes displayed rhombic g and CuA tensors, although the extent of rhombicity was dependent on the N-N ligand. Greater Cu(II)-N2 in-plane distortion, away from the square planar arrangement, was detected by CW W-band EPR for the smaller bipy and phen ligands compared to the larger Py-bipy and dppz ligands. Changes in ligand spin density distributions (over the 1H and 14N nuclei) were revealed by CW Q-band ENDOR. The largest components of the 1H imine and 14N hyperfine coupling decreased as the ligand size increased, following the trend bipy > phen > Py-bipy > dppz. These results indicate how even small structural and electronic (spin density) perturbations within the Casiopeina family of Cu(II) complexes can be probed by advanced EPR methods

Tuning the reactivity of nitriles using Cu(ii) catalysis - potentially prebiotic activation of nucleotides

During the transition from prebiotic chemistry to biology, a period of solution-phase, non-enzymatic activation of (oligo)nucleotides must have occurred, and accordingly, a mechanism for phosphate activation must have existed. Herein, we detail results of an investigation into prebiotic phosphate activation chemistry using simple, prebiotically available nitriles whose reactivity is increased by Cu2+ ions. Furthermore, although Cu2+ ions are known to catalyse the hydrolysis of phosphodiester bonds, we found this deleterious activity to be almost completely suppressed by inclusion of amino acids or dipeptides, which may suggest a productive relationship between protein and RNA from the outset

A Combination of EPR, Microscopy, Electrophoresis and Theory to Elucidate the Chemistry of W- and N-Doped TiO2 Nanoparticle/Water Interface

Funding: The authors thank the European Commission (FP7-ENV-2011-ECO-INNO-TwoStage 283062) for funding. C.J.K. gratefully acknowledges funding from the National Science Foundation Major Research Instrumentation program (GR# MRI/DMR-1040229).Peer reviewedPublisher PD

Monitoring the substrate-induced spin-state distribution in a Cobalt(II)-Salen complex by EPR and DFT

Ground state changes of (R,R’)-N,N’-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino Co(II), following coordination of various pyridyl substrate has been examined by CW EPR, pulsed relaxation measurements and DFT. The solution-based Co(II) complex possesses a low spin (LS) state urn:x-wiley:14341948:media:ejic202101071:ejic202101071-math-0001 (with g-values of 1.96, 1.895, 3.14). Upon coordination of the pyridyl substrate, the resulting bound adduct reveals a distribution of LS ‘base-on’ species, possessing a urn:x-wiley:14341948:media:ejic202101071:ejic202101071-math-0002 electronic ground state (with g-values of 2.008, 2.2145, 2.46) and a high spin (HS) species (with geff = 4.6). DFT indicated that the energy gap between the LS and HS state is dramatically lowered (ΔE < 25 kJmol−1) following substrate coordination. DFT suggests the main geometrical difference between the LS and HS systems is the severe puckering of the N2O2 ligand backbone. The results revealed a tentative dependency on the pKa−H of the substrates for the spin distribution where, in most cases, the higher pKa−H substrate values favoured the HS species

Understanding the coordination modes of [Cu(acac)2(imidazole)n=1,2] adducts by EPR, ENDOR, HYSCORE, and DFT analysis

The interaction of imidazole with a [Cu(acac)2] complex was studied using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), hyperfine sublevel correlation spectroscopy (HYSCORE), and density functional theory (DFT). At low Im ratios (Cu:Im 1:10), a 5-coordinate [Cu(acac)2Imn=1] monoadduct is formed in frozen solution with the spin Hamiltonian parameters g1 = 2.063, g2 = 2.063, g3 = 2.307, A1 = 26, A2 = 15, and A3 = 472 MHz with Im coordinating along the axial direction. At higher Im concentrations (Cu:Im 1:50), a 6- coordinate [Cu(acac)2Imn=2] bis-adduct is formed with the spin Hamiltonian parameters g1 = 2.059, g2 = 2.059, g3 = 2.288, A1 = 30, A2 = 30, and A3 = 498 MHz with a poorly resolved 14N superhyperfine pattern. The isotropic EPR spectra revealed a distribution of species ([Cu(acac)2], [Cu(acac)2Imn=1], and [Cu(acac)2Imn=2]) at Cu:Im ratios of 1:0, 1:10, and 1:50. The superhyperfine pattern originates from two strongly coordinating N3 imino nitrogens of the Im ring. Angular selective 14N ENDOR analysis revealed the NA tensor of [34.8, 43.5, 34.0] MHz, with e2qQ/h = 2.2 MHz and η = 0.2 for N3. The hyperfine and quadrupole values for the remote N1 amine nitrogens (from HYSCORE) were found to be [1.5, 1.4, 2.5] MHz with e2qQ/h = 1.4 MHz and η = 0.9. 1H ENDOR also revealed three sets of HA tensors corresponding to the nearly equivalent H2/H4 protons in addition to the H5 and H1 protons of the Im ring. The spin Hamiltonian parameters for the geometry optimized structures of [Cu(acac)2Imn=2], including cis-mixed plane, trans-axial, and trans-equatorial, were calculated. The best agreement between theory and experiment indicated the preferred coordination is trans-equatorial [Cu(acac)2Imn=2]. A number of other Im derivatives were also investigated. 4(5)-methyl-imidazole forms a [Cu(acac)2(Im-3)n=2] trans-equatorial adduct, whereas the bulkier 2-methyl-imidazole (Im-2) and benzimidazole (Im-4) form the [Cu(acac)2(Im-2,4)n=1] monoadduct only. Our data therefore show that subtle changes in the substrate structure lead to controllable changes in coordination behavior, which could in turn lead to rational design of complexes for use in catalysis, imaging, and medicine