Scientific Papers in SCI


Title: Effect of diffuse light scattering designs on the efficiency of dye solar cells: An integral optical and electrical description
Author(s): Galvez, FE; Kemppainen, E; Miguez, H; Halme, J
Source: Journal of Physical Chemistry C, 116 (2012) 11426-11433

abstract | fulltext

Herein, we present an integral optical and electrical theoretical analysis of the effect of different diffuse light scattering designs on the performance of dye solar cells. Light harvesting efficiencies and electron generation functions extracted from optical numerical calculations based on a Monte Carlo approach are introduced in a standard electron diffusion model to obtain the steady-state characteristics of the different configurations considered. We demonstrate that there is a strong dependence of the incident photon to current conversion efficiency, and thus of the overall conversion efficiency, on the interplay between the value of the electron diffusion length considered and the type of light scattering design employed, which determines the spatial dependence of the electron generation function. Other effects, like the influence of increased photoelectron generation on the photovoltage, are also discussed. Optimized scattering designs for different combinations of electrode thickness and electron diffusion length are proposed.

May, 2012 | DOI: 10.1021/jp2092708

Title: Electrons in the Band Gap: Spectroscopic Characterization of Anatase TiO2 Nanocrystal Electrodes under Fermi Level Control
Author(s): Berger, T; Anta, JA; Morales-Florez, V
Source: Journal of Physical Chemistry C, 116 (2012) 11444-11455

abstract | fulltext

Macroscopic properties of semiconductor nanoparticle networks in functional devices strongly depend on the electronic structure of the material. Analytical methods allowing for the characterization of the electronic structure in situ, i.e., in the presence of an application-relevant medium, are therefore highly desirable. Here, we present the first spectral data obtained under Fermi level control of electrons accumulated in anatase TiO 2 electrodes in the energy range from the MIR to the UV (0.1-3.3 eV). Band gap states were electrochemically populated in mesoporous TiO 2 films in contact with an aqueous electrolyte. The combination of electrochemical and spectroscopic measurements allows us for the first time to determine both the energetic location of the electronic ground states as well as the energies of the associated optical transitions in the energetic range between the fundamental absorption threshold and the onset of lattice absorption. On the basis of our observations, we attribute spectral contributions in the vis/NIR to d-d transitions of Ti 3+ species and a broad MIR absorption, monotonically increasing toward lower wavenumbers, to a quasi-delocalization of electrons. Importantly, signal intensities in the vis/NIR and MIR are linearly correlated. Absorbance and extractable charge show the same exponential dependence on electrode potential. Our results demonstrate that signals in the vis/NIR and MIR are associated with an exponential distribution of band gap states.

May, 2012 | DOI: 10.1021/p212436b

Title: Nanoclay Nucleation Effect in the Thermal Stabilization of a Polymer Nanocomposite: A Kinetic Mechanism Change
Author(s): Sanchez-Jimenez, PE; Perez-Maqueda, LA; Perejon, A; Criado, JM
Source: Journal of Physical Chemistry C, 116 (2012) 11797-11807

abstract | fulltext

The enhanced thermal stability of polymer-clay nanocomposites over the original polymers is one of their most interesting features, and it has been profusely studied within the last decades. Here, a thorough kinetic analysis of polystyrene and a montmorillonite-polystyrene nanocomposite has been performed making use of state-of-the-art kinetic procedures. It has been found that the degradation mechanism changes from a chain scission process for the polymer to a complex two-step nucleation-driven reaction for the nanocomposite. This mechanism change can explain the delayed onset of degradation found in the nanocomposite. Moreover, observation by transmission electron microscopy (TEM) has shown that the clay platelets within the composite could act as nucleation centers for the decomposition.

May, 2012 | DOI: 10.1021/jp302466p

Title: Plasma deposition of perylene-adamantane nanocomposite thin films for NO 2 room-temperature optical sensing
Author(s): Aparicio, FJ; Blaszczyk-Lezak, I; Sanchez-Valencia, JR; Alcaire, M; Gonzalez, JC; Serra, C; Gonzalez-Elipe, AR; Barranco, A
Source: Journal of Physical Chemistry C, 116 (2012) 8731-8740

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This work reports the preparation, by a new remote assisted plasma deposition process, of luminescent nanocomposite thin films consisting of an insoluble organic matrix where photonically active perylene molecules are embedded. The films are obtained by the remote plasma deposition of adamantane and perylene precursor molecules. The results show that the adamantane precursor is very effective to improve the perylene–adamantane nanocomposite transparency in comparison with plasma deposited perylene films. The plasma deposited adamantane films have been characterized by secondary-ion mass spectrometry and FT-IR spectroscopy. These techniques and atomic force microscopy (AFM) have been also used for the characterization of the nanocomposite films. Their optical properties (UV–vis absorption, fluorescence, and refractive index) have been also determined and their sensing properties toward NO2 studied. It is found that samples with the perylene molecules embedded within the transparent plasma deposited matrix are highly sensitive toward this gas and that the sensitivity of the films can be adjusted by modifying the aggregation state of the perylene molecules, as determined by the analysis of their fluorescence spectra. By monitoring the fluorescence emission of these films, it has been possible to detect a NO2 concentration as low as 0.5 ppm in air at room temperature. Because of their chemical stability and transparency in the UV region, the remote plasma deposited adamantane thin films have revealed as an optimum host matrix for the development of photonically active composites for sensing applications.

April, 2012 | DOI: 10.1021/jp209272s

Title: Electrostatic Induced Molecular Tilting in Self-Assembled Monolayers of n-Octadecylamine on Mica
Author(s): Oviedo, J; San-Miguel, MA; Heredia-Guerrero, JA; Benitez, JJ
Source: Journal of Physical Chemistry C, 116 (2012) 7099-7105

abstract | fulltext

Self-assembled monolayers of n-octadecylamine on mica (ODA/mica SAMs) have been investigated by atomic force microscopy (AFM) and by attenuated total reflectance infrared (ATR-FTIR) and X-ray photoelectron (XPS) spectroscopies. Topographic data characterizes a stable configuration with the alkyl skeleton tilted approximate to 46 degrees from the surface normal that is rationalized according to a well established structural alkyl chain packing model. Extended contact with air increases molecular tilting up to approximate to 58 degrees. ATR-FTIR and XPS reveal the presence of protonated amino groups within the monolayer and its increment upon exposure to air. The transition between both tilted states is explained assuming the protonation reaction as the driving force and introducing a model to evaluate an electrostatic repulsions term in the overall cohesive energy balance of the system. ODA molecules in the self-assembled monolayer respond to their spontaneous protonation by atmospheric water by tilting as a mechanism to relax the repulsions between -NH3+ heads.

March, 2012 | DOI: 10.1021/jp300829g

Title: CO-Induced Morphology Changes in Zn-Modified Ceria: A FTIR Spectroscopic Study
Author(s): Penkova, A; Laguna, OH; Centeno, MA; Odriozola, JA
Source: Journal of Physical Chemistry C, 116 (2012) 5747-5756

abstract | fulltext

A FTIR study of the CO adsorption on a Zn-modified ceria is presented. The results indicate that at lower activation temperatures only Ce 4+ carbonyls were detected, which were reduced with the increase of the activation temperature. At higher activation temperatures, up to three Zn 2+ carbonyls were also identified according to the arrangement of the Zn 2+ cations. The consecutive CO adsorptions demonstrated an irreversible modification of the surface, resulting in an agglomeration of the zinc cations. A stepwise conversion of the isolated Zn 2+ carbonyls into carbonyls of the closely situated zinc cations followed by formation of bigger zinc oxide clusters was observed. The carbon monoxide coordinated on the isolated Zn 2+ cations at the interface with ceria reacts with the lattice oxygen leading to formation of oxygen vacancies. An insight into the origin of the activation during the CO oxidation process is proposed.

March, 2012 | DOI: 10.1021/jp210996b

Title: In Situ XAS Study of Synergic Effects on Ni-Co/ZrO2 Methane Reforming Catalysts
Author(s): Gonzalez-delaCruz, VM; Pereniguez, R; Ternero, F; Holgado, JP; Caballero, A
Source: Journal of Physical Chemistry C, 116 (2012) 2919-2926

abstract | fulltext

Four different mono and bimetallic Ni–Co/ZrO2 catalysts have been studied by means of in situ XAS, X-ray diffraction, TPR, and measurements of the catalytic activity in the dry reforming reaction of methane (DRM). Even though the cobalt monometallic system has no activity for the methane reforming reaction, both bimetallic catalysts (with 1:1 and 1:2 Ni/Co ratio, respectively), showed a better activity and stability than the nickel monometallic system. The XRD data indicate that a mixed cobalt–nickel spinel is formed by calcination of the precursor solids, leading to the formation of an alloy of both metals after reduction in hydrogen. In situ XAS experiments showed a much better resistance of metals in the bimetallic systems to be oxidized under reaction conditions at temperatures until 750 °C. After these results, we proposed the formation in the bimetallic systems of a more reducible nickel–cobalt alloy phase, which remains completely metallic in contact with the CO2/CH4 reaction mixture at any temperature. The presence of adjacent nickel and cobalt sites seems to avoid the deactivation of cobalt in the DRM reaction. In the case of cobalt sites, the presence of adjacent nickel atoms seems to prevent the deposition of carbon over the cobalt sites, now showing its higher activity in the dry reforming reaction. Simultaneously, this higher activity of the cobalt sites in the bimetallic system produces more hydrogen as a product, maintaining the nickel atoms completely reduced under reaction conditions. This synergic effect accounts for the better performance of the bimetallic systems and points at both, the oxidation state of nickel particles under reaction conditions and the carbon deposition processes, as important factors responsible for differences in catalytic activities and stabilities in this hydrocarbon reaction.

February, 2012 | DOI: 10.1021/jp2092048

Title: Revealing Structural Detail in the High Temperature La2Si2O7–Y2Si2O7 Phase Diagram by Synchrotron Powder Diffraction and Nuclear Magnetic Resonance Spectroscopy
Author(s): Fernandez-Carrion, AJ; Allix, M; Florian, P; Suchomel, MR; Becerro, AI
Source: Journal of Physical Chemistry C, 116 (2012) 21523-21535

abstract | fulltext

High resolution synchrotron powder XRD, 89Y CPMG NMR, and 139La MAS NMR spectroscopy have been applied to eventually draw the phase diagram of the La2Si2O7–Y2Si2O7 system. The diagram presents a solid solubility region of G-(La,Y)2Si2O7, which extends to the La0.9Y1.1Si2O7 composition at any temperature of this study. Compositions richer in Y show two-phase domains, with G + α at T < 1450 °C and G + δ at T > 1450 °C. The Y-rich extreme is more complex, showing two solid solution regions of δ- and γ-(La,Y)2Si2O7 polymorphs which appear with increasing Y content, respectively. It is interesting to note that the La for Y substitution mechanism in the G-(La,Y)2Si2O7 polymorph is not homogeneous, but a preferential occupation of Y for the RE2 site is observed. Finally, the 89Y and 139La isotropic chemical shift values in G-(La,Y)2Si2O7 have been described here for the first time and assigned to the different RE crystallographic sites of the unit cell.

October, 2012 | DOI: 10.1021/jp305777m

Title: Making Photo-selective TiO2 Materials by Cation–Anion Codoping: From Structure and Electronic Properties to Photoactivity
Author(s): Marquez, AM; Plata, JJ; Ortega, Y; Sanz, JF; Colon, G; Kubacka, A; Fernandez-Garcia, M
Source: Journal of Physical Chemistry C, 116 (2012) 18759-18767

abstract | fulltext

Photoselective oxidation yielding high-added value chemicals appears as a green novel process with potential to be explored. In this study we combine spectroscopic XPS (N 1s and O 1s) and multiwavelength Raman data with density functional theory calculations to explore the structural and electronic properties of W,N-codoped TiO2 anatase surfaces and interpret the outstanding photocatalytic properties of such a system in partial oxidation reactions. Theoretical calculations allow us to examine several substitutional and N-interstitial configurations at different concentrations of the W,N dopants (similar to those experimentally found), as well as their interaction with structural point defects: Ti cation vacant sites and surface wolframyl species that are required to compensate the extra charge of the W6+ and N-containing anions. The joint use of theoretical and experimental XPS and Raman tools renders key structural information of W,N-codoped microcrystalline TiO2 solids. The incorporation of N at substitutional positions of anatase with the concomitant presence of W═O species introduces localized states in the band gap, a result that is critical in interpreting the chemical behavior of the solids. The combination of the electronic and geometric structural information leads to a simple mechanism that rationalizes the experimentally observed photoactivity and selectivity in partial oxidation reactions.

September, 2012 | DOI: 10.1021/jp3045143

Title: Adsorption Geometry Determines Catalytic Selectivity in Highly Chemoselective Hydrogenation of Crotonaldehyde on Ag(111)
Author(s): Brandt, Katrin; Chiu, May E.; Watson, David J.; Tikhov, Mintcho S.; Lambert, Richard M.
Source: Journal of Physical Chemistry C, 116 (2012) 4605-4611

abstract | fulltext

The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature-programmed desorption/reaction, high-resolution XPS, and NEXAFS. The organic molecule adsorbed without decomposition, all three possible hydrogenation products were formed and desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were found under TPR conditions. The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO2 catalysts, and the present findings permit a rationalization of the catalytic performance in terms of pronounced coverage-dependent changes in adsorption geometries of the reactant and the products. Thus, at low coverages, the C═O bond in crotonaldehyde lies almost parallel to the metal surface, whereas the C═C was appreciably tilted, favoring hydrogenation of the former and disfavoring hydrogenation of the latter. With increasing coverage of reactants, the C═C bond was forced almost parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing selectivity toward formation of crotyl alcohol. Butanol formation was the result of an overall two-step process: crotonaldehyde → crotyl alcohol → butanol, further hydrogenation of the desired product crotyl alcohol being promoted at high hydrogen coverage due to the C═C bond in the unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to C═O π bond, as suggested by C 1s XPS binding energies.

January, 2012 | DOI: 10.1021/jp208831h

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