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1.
J Phys Chem Lett ; 10(21): 6672-6678, 2019 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-31608645

RESUMO

Bismuth vanadate (BiVO4) has emerged as one of the most promising photoanode materials for solar fuel production. Oxygen vacancies play a pivotal role in the photoelectrochemical efficiency, yet their electronic nature and contribution to  n-type conductivity are still under debate. Using first-principles calculations, we show that oxygen vacancies in BiVO4 have two distinguishable geometric configurations characterized by either undercoordinated, reduced VIVO3 and BiIIO7 subunits or a VIV-O-VIV/V bridge (split vacancy), quenching the oxygen vacancy site. While both configurations have similar energies in the bulk, the (001) subsurface acts like an energetic sink that stabilizes the split oxygen vacancy by ∼1 eV. The barrierless creation of a bridging V2O7 unit allows for partial electron delocalization throughout the near-surface region, consistent with recent experimental observations indicating that BiVO4(001) is an electron-rich surface.

2.
Nano Lett ; 19(8): 5754-5761, 2019 08 14.
Artigo em Inglês | MEDLINE | ID: mdl-31348861

RESUMO

The relationship between composition and plasmonic properties in noble metal nanoalloys is still largely unexplored. Yet, nanoalloys of noble metals, such as gold, with transition elements, such as iron, have unique properties and a number of potential applications, ranging from nanomedicine to magneto-plasmonics and plasmon-enhanced catalysis. Here, we investigate the localized surface plasmon resonance at the level of the single Au-Fe nanoparticle by applying a strategy that combines experimental measurements using near field electron energy loss spectroscopy with theoretical studies via a full wave numerical analysis and density functional theory calculations of electronic structure. We show that, as the iron fraction increases, the plasmon resonance is blue-shifted and significantly damped, as a consequence of the changes in the electronic band structure of the alloy. This allows the identification of three relevant phenomena to be considered in the design and realization of any plasmonic nanoalloy, specifically: the appearance of new states around the Fermi level; the change in the free electron density of the metal; and the blue shift of interband transitions. Overall, this study provides new opportunities for the control of the optical response in Au-Fe and other plasmonic nanoalloys, which are useful for the realization of magneto-plasmonic devices for molecular sensing, thermo-plasmonics, bioimaging, photocatalysis, and the amplification of spectroscopic signals by local field enhancement.

3.
Nanoscale ; 9(32): 11694-11704, 2017 Aug 17.
Artigo em Inglês | MEDLINE | ID: mdl-28776050

RESUMO

We demonstrate the thermal stability up to 450 °C of a titanium(iv)-porphyrin monolayer grown on the rutile TiO2(110) surface. Starting from a film of metal-free tetra-phenyl-porphyrin, 2HTPP, deposited at room temperature, we show that, beyond the self-metalation reaction at 150°-200 °C, a second phase transition takes place at ∼350 °C. Using surface diffraction and microscopy, we observe a change of the phase symmetry from (2 × 4)-obliq to (2 × 6)-rect. Core level photoemission indicates that the chemical states of both the molecular tetrapyrrolic macrocycle and the substrate are unchanged. X-ray absorption spectroscopy reveals that the driving mechanism is a rotation of the phenyl terminations towards the substrate (flattening) that triggers a conformational change of the molecule through partial cyclo-dehydrogenation. From comparison with first principles calculations, we show that the common feature of these multiple phase transitions is the chemical nature of the porphyrin bonding atop the substrate oxygen rows: the coordination of the macrocycle central pocket to the oxygen atoms beneath is preserved throughout both the self-metalation and flattening reactions. The molecular orientation and arrangement are determined by steric constraints and intermolecular interactions, whereas the specific adsorption site is further stabilized by the interaction of the peripheral C-H network with the adjacent oxygen rows. Porphyrins are thus trapped at the TiO2(110) surface, where they demonstrate an exceptionally high thermal stability (up to ∼450 °C), which makes this interface potentially useful for sensors and photocatalysis applications in harsh environments.

4.
Rapid Commun Mass Spectrom ; 31(2): 179-192, 2017 Jan 30.
Artigo em Inglês | MEDLINE | ID: mdl-27806439

RESUMO

RATIONALE: [Cu(P)4 ][BF4 ]-type complexes (P = tertiary phosphine) have shown significant antitumor activity. This biological property appears to be activated via formation of coordinative unsaturated [Cu(P)n ]+ species (n < 4), that may interact with various molecules starting from the solvent(s) in which they are dissolved. Aim of our study was to investigate the interaction of these species with different solvent mixtures. METHODS: The interaction has been investigated by electrospray ionization mass spectrometry, and the interaction products have been characterized by multiple collisional experiments, using an ion trap mass instrument. Density functional theory (DFT) calculation studies, using a meta-hybrid exchange correlation (xc) functional and an implicit solvent model, were employed to investigate the equilibrium distribution of species in solution. RESULTS: Depending on the nature of the solvent mixture and coordinated phosphine, three [Cu(P)4 ][BF4 ]-type complexes undergo dissociation with formation of [Cu(P)2 ]+ , [Cu(P)(solv)]+ and [Cu(solv)2 ]+ species (solv = solvent). Preferred collisional-induced fragmentation pathways provide qualitative information on the selectivity of [Cu(P)n ]+ for specific solvents and donor atoms. Formation free energies and equilibrium constants pertaining to [CuI (PTA)n ]+ , [CuI/II (solv)n ]m+ (n ≤ 4; m = 1, 2) and [CuI (PTA)2-k (sol)k ]+ (k = 1, 2) provide a comprehensive picture of equilibria in solution. CONCLUSIONS: Dimethyl sulfoxide (DMSO) and acetonitrile (MeCN) strongly affect [Cu(P)n ]+ assemblies producing mixed-ligand [Cu(P)(DMSO)]+ and [Cu(P)(MeCN)]+ species. Excess of both DMSO and MeCN solvents are able to fully displace coordinated phosphines giving [Cu(solv)2 ]+ -type adducts. The presence of phosphines in the native complex is mandatory to retain the reduced oxidation state of copper. Instead, the more labile [CuI (MeCN)4 ]+ complex dissolved in DMSO and MeCN displays a combination of Cu(I) and Cu(II) adducts. Copyright © 2016 John Wiley & Sons, Ltd.

5.
Phys Chem Chem Phys ; 17(44): 30119-24, 2015 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-26496858

RESUMO

Metal-free porphyrin molecules adsorb on the rutile TiO2(110) surface with their pyrrolic nitrogen atoms atop the O-bridge rows, whereas the iminic nitrogen atoms capture two additional hydrogen atoms. Hydrogenation occurs spontaneously at room temperature, irrespective of the distance of the polypyrrolic macrocycle from the surface, as varied by changing the porphyrin functionalization.

6.
Chemistry ; 20(44): 14296-304, 2014 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-25200655

RESUMO

We explore a photochemical approach to achieve an ordered polymeric structure at the sub-monolayer level on a metal substrate. In particular, a tetraphenylporphyrin derivative carrying para-amino-phenyl functional groups is used to obtain extended and highly ordered molecular wires on Ag(110). Scanning tunneling microscopy and density functional theory calculations reveal that porphyrin building blocks are joined through azo bridges, mainly as cis isomers. The observed highly stereoselective growth is the result of adsorbate/surface interactions, as indicated by X-ray photoelectron spectroscopy. At variance with previous studies, we tailor the formation of long-range ordered structures by the separate control of the surface molecular diffusion through sample heating, and of the reaction initiation through light absorption. This previously unreported approach shows that the photo-induced covalent stabilization of self-assembled molecular monolayers to obtain highly ordered surface covalent organic frameworks is viable by a careful choice of the precursors and reaction conditions.

7.
Inorg Chem ; 49(9): 4099-108, 2010 May 03.
Artigo em Inglês | MEDLINE | ID: mdl-20356058

RESUMO

A Zn(II) 2-mercaptonicotinate coordination polymer (Zn1), with Zn(II) ions chelated by both sulfur and oxygen in a distorted square pyramidal environment, and a molecular Zn(II) 2-hydroxynicotinate complex (Zn2) were synthesized by the reaction of zinc acetylacetonate with 2-mercaptonicotinic (Zn1) and 2-hydroxynicotinc (Zn2) acid, respectively, under hydrothermal conditions. The crystal structures of Zn1 and Zn2 were determined by single crystal X-ray diffraction measurements. Dispersion-corrected density functional theory (DFT) calculations reproduce very well the experimental structures and show that Zn1 is stable against hydration, whereas Zn2 is stable against dehydration over wide ranges of temperature and pressure, in agreement with thermogravimetric analysis results. The electronic structure of the two compounds is computed with the DFT+U method. The theoretical valence band agrees well with the X-ray photoelectron spectroscopy experiments. Furthermore, the band gap of Zn1 is found to be narrower than that of Zn1 and is characterized by the presence of sulfur lone pairs at the edge of the valence band.


Assuntos
Simulação por Computador , Elétrons , Modelos Químicos , Ácidos Nicotínicos/química , Compostos Organometálicos/síntese química , Compostos de Sulfidrila/química , Zinco/química , Cristalografia por Raios X , Modelos Moleculares , Estrutura Molecular , Compostos Organometálicos/química , Espectroscopia Fotoeletrônica , Teoria Quântica
8.
Inorg Chem ; 48(9): 4044-51, 2009 May 04.
Artigo em Inglês | MEDLINE | ID: mdl-19348437

RESUMO

Dispersion-corrected density functional theory (DFT-D) calculations, Electron Spin Resonance spectroscopy (EPR), and variable temperature magnetic moment measurements were used to investigate the structure and the electronic/magnetic properties of bispyrazolato-copper(II) coordination polymer and of its hydration product. The Cu(II) ions are antiferromagnetically coupled through the sigma system of the pyrazolate rings in both compounds. Theoretical electron density maps reveal that water molecules interact simultaneously and to a comparable extent with two Cu(II) centers (through the electronegative O end) and two pyrazolate rings (through the partly positively charged H atoms), which is compatible with the observed internuclear distances. DFT-D calculations indicate that low kinetic barriers are involved in the rearrangement of the host structure.

9.
J Comput Chem ; 30(6): 934-9, 2009 Apr 30.
Artigo em Inglês | MEDLINE | ID: mdl-18785153

RESUMO

A semiempirical addition of dispersive forces to conventional density functionals (DFT-D) has been implemented into a pseudopotential plane-wave code. Test calculations on the benzene dimer reproduced the results obtained by using localized basis set, provided that the latter are corrected for the basis set superposition error. By applying the DFT-D/plane-wave approach a substantial agreement with experiments is found for the structure and energetics of polyethylene and graphite, two typical solids that are badly described by standard local and semilocal density functionals.

10.
J Phys Chem A ; 112(29): 6723-31, 2008 Jul 24.
Artigo em Inglês | MEDLINE | ID: mdl-18593104

RESUMO

Density functional theory (DFT) has been used to look into the electronic structure of [M(tpm)]+ molecular ion conformers (M = Cu, Ag; tpm = tris(pyrazol-1-yl)methane) and to study the energetics of their interconversion. Theoretical data pertaining to the free tpm state the intrinsic instability of its kappa3-like conformation, thus indicating that, even though frequently observed, the kappa3-tripodal coordinative mode is unlikely to be directly achieved through the interaction of M(I) with the kappa3-like tpm conformer. It is also found that the energy barrier for the kappa2-[M(tpm)]+ --> kappa3-[M(tpm)]+ conversion is negligible. As far as the bonding scheme is concerned, the tpm --> M(I) donation, both sigma and pi in character, is the main source of the M(I)-tpm bonding, whereas back-donation from completely occupied M(I) d orbitals into tpm-based pi* levels plays a negligible role.

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