Publications by authors named "Michele Pavone"

44 Publications

d-Glucose Adsorption on the TiO Anatase (100) Surface: A Direct Comparison Between Cluster-Based and Periodic Approaches.

Front Chem 2021 31;9:716329. Epub 2021 Aug 31.

CEITEC - Central European Institute of Technology Central European Institute of Technology, Brno University of Technology, Brno, Czech.

Titanium dioxide (TiO) has been extensively studied as a suitable material for a wide range of fields including catalysis and sensing. For example, TiO-based nanoparticles are active in the catalytic conversion of glucose into value-added chemicals, while the good biocompatibility of titania allows for its application in innovative biosensing devices for glucose detection. A key process for efficient and selective biosensors and catalysts is the interaction and binding mode between the analyte and the sensor/catalyst surface. The relevant features regard both the molecular recognition event and its effects on the nanoparticle electronic structure. In this work, we address both these features by combining two first-principles methods based on periodic boundary conditions and cluster approaches (CAs). While the former allows for the investigation of extended materials and surfaces, CAs focus only on a local region of the surface but allow for using hybrid functionals with low computational cost, leading to a highly accurate description of electronic properties. Moreover, the CA is suitable for the study of reaction mechanisms and charged systems, which can be cumbersome with PBC. Here, a direct and detailed comparison of the two computational methodologies is applied for the investigation of d-glucose on the TiO (100) anatase surface. As an alternative to the commonly used PBC calculations, the CA is successfully exploited to characterize the formation of surface and subsurface oxygen vacancies and to determine their decisive role in d-glucose adsorption. The results of such direct comparison allow for the selection of an efficient, finite-size structural model that is suitable for future investigations of biosensor electrocatalytic processes and biomass conversion catalysis.
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http://dx.doi.org/10.3389/fchem.2021.716329DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8438178PMC
August 2021

Charge Carrier Processes and Optical Properties in TiO and TiO-Based Heterojunction Photocatalysts: A Review.

Materials (Basel) 2021 Mar 27;14(7). Epub 2021 Mar 27.

Department of Physics "E. Pancini", University of Naples "Federico II", Complesso Universitario di Monte S. Angelo, Via Cupa Cintia 21, 80126 Napoli, Italy.

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H) production (e.g., via water splitting or photo-reforming of organic substrates), CO reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO) is a "benchmark" photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO. We describe the main characteristics and advantages of TiO as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO photoluminescence and on the effect of molecular oxygen (O) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO limitations and possible ways to overcome them by discussing some of the "hottest" research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO without any cocatalysts. Discussed topics are highly-reduced "black TiO", grey and colored TiO, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO composites or heterostructures with metals (e.g., Pt-TiO, Au-TiO), with other metal oxides (e.g., CuO, NiO, etc.), direct Z-scheme heterojunctions with g-CN (graphitic carbon nitride) and dye-sensitized TiO.
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http://dx.doi.org/10.3390/ma14071645DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8036967PMC
March 2021

First-Principles Study of Na Intercalation and Diffusion Mechanisms at 2D MoS/Graphene Interfaces.

J Phys Chem C Nanomater Interfaces 2021 Feb 21;125(4):2276-2286. Epub 2021 Jan 21.

Department of Chemical Sciences, University of Naples "Federico II", via Cintia 21, 80126 Naples, Italy.

Na-ion batteries (NIBs) are emerging as promising energy storage devices for large-scale applications. Great research efforts are devoted to design new effective NIB electrode materials, especially for the anode side. A hybrid 2D heterojunction with graphene and MoS has been recently proposed for this purpose: while MoS has shown good reversible capacity as a NIB anode, graphene is expected to improve conductivity and resistance to mechanical stress upon cycling. The most relevant processes for the anode are the intercalation and diffusion of the large Na ion, whose complex mechanisms are determined by the structural and electronic features of the MoS/graphene interface. Understanding these processes and mechanisms is crucial for developing new nanoscale anodes for NIBs with high performances. To this end, here we report a state-of-the-art DFT study to address (a) the structural and electronic properties of heterointerfaces between the MoS monolayers and graphene, (b) the most convenient insertion sites for Na, and (c) the possible diffusion paths along the interface and the corresponding energy barrier heights. We considered two MoS polymorphs: 1T and 3R. Our results show that 1T-MoS interacts more strongly with graphene than 3R-MoS. In both cases, the best Na host site is found at the MoS side of the interface, and the band structure reveals a proper n-type character of the graphene moiety, which is responsible for electronic conduction. Minimum-energy paths for Na diffusion show very low barrier heights for the 3R-MoS/graphene interface (<0.25 eV) and much higher values for its 1T counterpart (∼0.7 eV). Analysis of structural features along the diffusion transition states allows us to identify the strong coordination of Na with the exposed S atoms as the main feature hindering an effective diffusion in the 1T case. These results provide new hints on the physicochemical details of Na intercalation and diffusion mechanisms at complex 2D heterointerfaces and will help further development of advanced electrode materials for efficient NIBs.
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http://dx.doi.org/10.1021/acs.jpcc.0c10107DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7876776PMC
February 2021

Interfacial electronic features in methyl-ammonium lead iodide and p-type oxide heterostructures: new insights for inverted perovskite solar cells.

Phys Chem Chem Phys 2020 Dec;22(48):28401-28413

Department of Chemical Sciences, University of Naples Federico II, Comp. Univ. Monte Sant'Angelo, Via Cintia 21, 80126, Naples, Italy.

Perovskite solar cells (PSCs) represent a promising technology for highly efficient sunlight harvesting and its conversion to electricity at convenient costs. However, a few flaws of current devices undermine the long-term stability of PSCs. Some of them concern the interface between the photoactive perovskite and the hole transport layer (HTL), e.g. undesired charge recombination, polarization barriers and oxidation processes. A strategy to solve this problem is to replace the standard organic HTL (e.g. Spiro-OMeTAD) with a solid-state inorganic layer. Being extensively used in p-type dye sensitized solar cells (DSSCs), nickel oxide (NiO) has been the first choice as an inorganic HTL. Despite the great interests in the application of NiO and other p-type oxides in PSCs, there is no available atomistic model of their interface with a halide perovskite. Here, we address this knowledge gap via a thorough first-principles study of the prototypical PSC perovskite methyl-ammonium lead iodide (MAPI) and two inorganic p-type oxides: NiO and CuGaO2. This copper-gallium delafossite oxide is one of the most promising alternatives to NiO in p-type DSSCs, thanks to its wide optical bandgap and low valence band edge. Here, we characterize the properties of both isolated surface slabs and MAPI/HTL heterostructure models. Besides considering MAPI/NiO and MAPI/CuGaO2 interfaces from the pristine materials, we also address the effects of intrinsic and extrinsic p-type defects in both NiO (Ni vacancy, Ni vacancy with Li and Ag doping) and CuGaO2 (Cu vacancy) using more realistic models. Our study reveals the most convenient interfaces in terms of structural affinities and adhesion energies. From the electronic perspective, we present a detailed analysis on band edge alignments, with direct insights into the key functional parameters of PSCs: hole injection driving force and open circuit potential. Our data show how the presence of defects/dopants is crucial for a convenient hole injection in NiO and CuGaO2. These results provide new science-based design principles for further development of p-type oxides in PSC devices.
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http://dx.doi.org/10.1039/d0cp05328gDOI Listing
December 2020

Sodium diffusion in ionic liquid-based electrolytes for Na-ion batteries: the effect of polarizable force fields.

Phys Chem Chem Phys 2020 Sep 2;22(35):20114-20122. Epub 2020 Sep 2.

Laboratoire de Chimie, ENS de Lyon, CNRS, Université de Lyon, 69364 Lyon, France.

Understanding the transport of sodium ions in ionic liquids is key to designing novel electrolyte materials for sodium-ion batteries. In this work, we combine molecular dynamics simulation and experiments to study how molecular interactions and local ordering affect relevant physico-chemical properties. Ionic transport and local solvation environments are investigated in electrolytes composed of sodium bis(fluorosulfonyl)imide, (Na[FSI]), in N,N-methylpropylpyrrolidinium bis(fluorosulfonyl)imide, [CCpyr][FSI], at different salt concentrations. The electrolyte systems are modelled by means of molecular dynamic simulations using a polarizable force field. We show that including polarization effects explicitly in the molecular simulations is required in order to attain a reliable description of the transport properties of sodium in the [CCpyr][FSI] electrolyte. The validation of the computational results upon comparison with experimental data allows us to assess the suitability of polarizable force fields in describing and interpreting the structure and dynamics of the sodium salt-ionic liquid system, which is essential to enable the application of IL-based electrolytes in novel energy-storage technologies.
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http://dx.doi.org/10.1039/d0cp02760jDOI Listing
September 2020

Structural and electronic properties of defective 2D transition metal dichalcogenide heterostructures.

J Comput Chem 2020 Aug 16;41(22):1946-1955. Epub 2020 Jun 16.

Department of Chemical Sciences, University of Naples Federico II, Naples, Italy.

We present a first-principles study on the structure-property relationships in MoS and WS monolayers and their vertically stacked hetero-bilayer, with and without Sulfur vacancies, in order to dissect the electronic features behind their photocatalytic water splitting capabilities. We also benchmark the accuracy of three different exchange-correlation density functionals for both minimum-energy geometries and electronic structure. The best compromise between computational cost and qualitative accuracy is achieved with the HSE06 density functional on top of Perdew-Burke-Ernzerhof minima, including dispersion with Grimme's D3 scheme. This computational approach predicts the presence of mid-gap states for defective monolayers, in accordance with the present literature. For the heterojunction, we find unexpected vacancy-position dependent electronic features: the location of the defects leads either to mid-gap trap states, detrimental for photocatalyst or to a modification of characteristic type II band alignment behavior, responsible for interlayer charge separation and low recombination rates.
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http://dx.doi.org/10.1002/jcc.26364DOI Listing
August 2020

Luminescent -Iridium(III) Complex Based on a Bis(6,7-dimethoxy-3,4-dihydroisoquinoline) Platform Featuring an Unusual cis Orientation of the CN Ligands: From a Theoretical Approach to a Deep Red LEEC Device.

ACS Omega 2019 Jan 25;4(1):2009-2018. Epub 2019 Jan 25.

Department of Chemical Sciences and Department of Physics "E. Pancini", University of Naples Federico II, via Cintia 4, I-80126 Napoli, Italy.

By pursuing the strategy of manipulating natural compounds to obtain functional materials, in this work, we report on the synthesis and characterization of a luminescent cationic iridium complex (-), designed starting from the catecholic neurotransmitter dopamine, exhibiting the unusual cis arrangement of the CN ligands. Through an integrated experimental and theoretical approach, it was possible to delineate the optoelectronic properties of -. In detail, (a) a series of absorption maxima in the range 300-400 nm was assigned to metal-to-ligand charge transfer and weak and broad absorption maxima at longer wavelengths (400-500 nm) were ascribable to spin-forbidden transitions with a mixed character; (b) there was an intense red phosphorescence with emission set in the range 580-710 nm; and (c) a highest occupied molecular orbital was mainly localized on the metal and the 2-phenylpiridine ligand and a lowest unoccupied molecular orbital was localized on the NN ligand, with a Δ set at 2.20 eV. This investigation allowed the design of light-emitting electrochemical cell (LEEC) devices endowed with good performance. The poor literature reporting on the use of -iridium(III) complexes in LEECs prompted us to investigate the role played by the selected cathode and the thickness of the emitting layer, as well as the doping effect exerted by ionic liquids on the performance of the devices. All the devices exhibited a deep red emission, in some cases, quite near the pure color (devices #1, #4, and #8), expanding the panorama of the iridium-based red-to-near-infrared LEEC devices. The characteristics of the devices, such as the brightness reaching values of 162 cd/m for device #7, suggested that the performances of - are comparable to those of trans isomers, opening new perspective toward designing a new set of luminescent materials for optoelectronic devices.
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http://dx.doi.org/10.1021/acsomega.8b02859DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6648618PMC
January 2019

Combined Structural, Chemometric, and Electrochemical Investigation of Vertically Aligned TiO Nanotubes for Na-ion Batteries.

ACS Omega 2018 Jul 31;3(7):8440-8450. Epub 2018 Jul 31.

GAME Lab, Department of Applied Science and Technology-DISAT, and MPMNT Group, Department of Applied Science and Technology-DISAT, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy.

In the challenging scenario of anode materials for sodium-ion batteries, TiO nanotubes could represent a winning choice in terms of cost, scalability of the preparation procedure, and long-term stability upon reversible operation in electrochemical cells. In this work, a detailed physicochemical, computational, and electrochemical characterization is carried out on TiO nanotubes synthesized by varying growth time and heat treatment, viz. the two most significant experimental parameters during preparation. A chemometric approach is proposed to obtain a concrete and solid multivariate analysis of sodium battery electrode materials. Such a statistical approach, combined with prolonged galvanostatic cycling and density functional theory analysis, allows identifying anatase at high growth time as the TiO polymorph of choice as an anode material, thus creating a benchmark for sodium-ion batteries, which currently took the center stage of the research in the field of energy storage systems from renewables.
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http://dx.doi.org/10.1021/acsomega.8b01117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6644502PMC
July 2018

Study of Anchoring Groups for CuGaO Delafossite-Based p-Type Dye Sensitized Solar Cells.

Front Chem 2019 29;7:158. Epub 2019 Mar 29.

Department of Chemical Sciences, University of Naples "Federico II", Comp. Univ. Monte Sant'Angelo, Naples, Italy.

Here we report the first theoretical characterization of the interface between the CuGaO delafossite oxide and the carboxylic (-COOH) and phosphonic acid (-POH) anchoring groups. The promising use of delafossites as effective alternative to nickel oxide in p-type DSSC is still limited by practical difficulties in sensitizing the delafossite surface. Thus, this work provides atomistic insights on the structure and energetics of all the possible interactions between the anchoring functional groups and the CuGaO surface species, including the effects of the Mg doping and of the solvent medium. Our results highlight the presence of a strong selectivity toward the monodentate binding mode on surface Ga atoms for both the carboxylic and phosphonic acid groups. Since the binding modes have a strong influence on the hole injection thermodynamics, these findings have direct implications for further development of delafossite based p-type DSSCs.
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http://dx.doi.org/10.3389/fchem.2019.00158DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6449920PMC
March 2019

An ab initio study of Cu-based delafossites as an alternative to nickel oxide in photocathodes: effects of Mg-doping and surface electronic features.

Phys Chem Chem Phys 2018 May;20(20):14082-14089

Department of Chemical Sciences, University of Naples Federico II, Comp. Univ. Monte Sant'Angelo Via Cintia 21, 80126 Naples, Italy.

CuMO2 delafossites (M = Al, Ga, and Cr) are p-type semiconductor oxides that have been recently proposed as the electrode in p-type dye-sensitized solar cells (p-DSSC) which is an alternative to the standard, low-performing nickel oxide. To assess this potential application of delafossites, we report here a DFT-based investigation of the structural and electronic properties of CuAlO2, CuGaO2 and CuCrO2. In particular, we address the role of Mg doping to obtain the p-type semiconducting character: the substitution of an M3+ cation with Mg2+ is easier with Ga than with Al and Cr, and, in all cases, the hole introduced by Mg2+ leads to the formation of Cu2+ species. Moreover, we address surface electronic features in order to characterize the most exposed delafossite surface termination and, more importantly, to predict the valence band maximum energy value, which determines the p-DSSC open circuit potential. From analysis of all our results, CuGaO2 emerges as the most promising system that can boost the development of new photocathodes for p-DSSCs.
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http://dx.doi.org/10.1039/c8cp00848eDOI Listing
May 2018

Effective scheme for partitioning covalent bonds in density-functional embedding theory: From molecules to extended covalent systems.

J Chem Phys 2016 Dec;145(24):244103

Department of Chemical Sciences, University of Naples Federico II, Complesso Universitario M. S. Angelo Via Cintia 21, 80126 Naples, Italy.

Density-functional embedding theory provides a general way to perform multi-physics quantum mechanics simulations of large-scale materials by dividing the total system's electron density into a cluster's density and its environment's density. It is then possible to compute the accurate local electronic structures and energetics of the embedded cluster with high-level methods, meanwhile retaining a low-level description of the environment. The prerequisite step in the density-functional embedding theory is the cluster definition. In covalent systems, cutting across the covalent bonds that connect the cluster and its environment leads to dangling bonds (unpaired electrons). These represent a major obstacle for the application of density-functional embedding theory to study extended covalent systems. In this work, we developed a simple scheme to define the cluster in covalent systems. Instead of cutting covalent bonds, we directly split the boundary atoms for maintaining the valency of the cluster. With this new covalent embedding scheme, we compute the dehydrogenation energies of several different molecules, as well as the binding energy of a cobalt atom on graphene. Well localized cluster densities are observed, which can facilitate the use of localized basis sets in high-level calculations. The results are found to converge faster with the embedding method than the other multi-physics approach ONIOM. This work paves the way to perform the density-functional embedding simulations of heterogeneous systems in which different types of chemical bonds are present.
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http://dx.doi.org/10.1063/1.4972012DOI Listing
December 2016

Copper Bipyridyl Redox Mediators for Dye-Sensitized Solar Cells with High Photovoltage.

J Am Chem Soc 2016 11 3;138(45):15087-15096. Epub 2016 Nov 3.

Department of Chemistry, Ångström Laboratory, Uppsala University , 751 20 Uppsala, Sweden.

Redox mediators play a major role determining the photocurrent and the photovoltage in dye-sensitized solar cells (DSCs). To maintain the photocurrent, the reduction of oxidized dye by the redox mediator should be significantly faster than the electron back transfer between TiO and the oxidized dye. The driving force for dye regeneration with the redox mediator should be sufficiently low to provide high photovoltages. With the introduction of our new copper complexes as promising redox mediators in DSCs both criteria are satisfied to enhance power conversion efficiencies. In this study, two copper bipyridyl complexes, Cu(dmby)TFSI (0.97 V vs SHE, dmby = 6,6'-dimethyl-2,2'-bipyridine) and Cu(tmby)TFSI (0.87 V vs SHE, tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine), are presented as new redox couples for DSCs. They are compared to previously reported Cu(dmp)TFSI (0.93 V vs SHE, dmp = bis(2,9-dimethyl-1,10-phenanthroline). Due to the small reorganization energy between Cu(I) and Cu(II) species, these copper complexes can sufficiently regenerate the oxidized dye molecules with close to unity yield at driving force potentials as low as 0.1 V. The high photovoltages of over 1.0 V were achieved by the series of copper complex based redox mediators without compromising photocurrent densities. Despite the small driving forces for dye regeneration, fast and efficient dye regeneration (2-3 μs) was observed for both complexes. As another advantage, the electron back transfer (recombination) rates were slower with Cu(tmby)TFSI as evidenced by longer lifetimes. The solar-to-electrical power conversion efficiencies for [Cu(tmby)], [Cu(dmby)], and [Cu(dmp)] based electrolytes were 10.3%, 10.0%, and 10.3%, respectively, using the organic Y123 dye under 1000 W m AM1.5G illumination. The high photovoltaic performance of Cu-based redox mediators underlines the significant potential of the new redox mediators and points to a new research and development direction for DSCs.
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http://dx.doi.org/10.1021/jacs.6b10721DOI Listing
November 2016

Promoting oxygen vacancy formation and p-type conductivity in SrTiOvia alkali metal doping: a first principles study.

Phys Chem Chem Phys 2016 Oct;18(41):28951-28959

Department of Chemical Sciences, University of Naples Federico II, via Cintia 21, 80126 Naples, Italy.

Strontium titanate (SrTiO, STO) is a prototypical perovskite oxide, widely exploited in many technological applications, from catalysis to energy conversion devices. In the context of solid-oxide fuel cells, STO has been recently applied as an epitaxial substrate for nano-sized layers of mixed ion-electron conductive catalysts with enhanced electrochemical performances. To extend the applications of such heterogeneous nano-cathodes in real devices, also the STO support should be active for both electron transport and oxide diffusion. To this end, we explored using first-principles calculations the strategy of doping of STO at the Sr site with sodium and potassium. These two ions fit in the perovskite structure and induce holes in the STO valence band, so as to obtain the desired p-type electronic conduction. At the same time, the doping with alkali ions also promotes the formation of oxygen vacancies in STO, a prerequisite for effective oxide diffusion. Analysis of electron density rearrangements upon defect formation allows relating the favorable vacancy formation energies to an improved electronic delocalization over the oxide sub-lattice, as observed in closely related materials (e.g. SrFeMoO). Overall, our results suggest the alkali-doped STO as a new potential substrate material in nanoscale heterogeneous electrodes for solid oxide electrochemical cells.
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http://dx.doi.org/10.1039/c6cp05089aDOI Listing
October 2016

Stability of melamine-exfoliated graphene in aqueous media: quantum-mechanical insights at the nanoscale.

Phys Chem Chem Phys 2016 Aug;18(32):22203-9

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Comp. Univ. Monte Sant'Angelo Via Cintia 21, 80126 Naples, Italy.

In recent experiments, melamine (1,3,5-triazine-2,4,6-triamine) has been proposed as an effective exfoliating agent to obtain high quality graphene from graphite. After washing out the melamine in excess, small amounts (ppm) are still needed to stabilize the dispersion of graphene flakes in aqueous media. To understand the origin of this behaviour, we investigated the melamine-graphene-water system and the fundamental interactions that determine its structure and energetics. To disentangle the subtle interplay of hydrogen-bonding and dispersive forces we used state-of-the-art ab initio calculations based on density functional theory. First, we focused on the case of water molecules interacting with melamine-graphene assemblies at different melamine coverages. We found that water-melamine interactions provide the driving force for washing off the melamine from graphene. Then, we addressed the interaction of single and double layers of water molecules with the graphene surface in the presence of an adsorbed melamine molecule. We found that this melamine acts as a non-covalent anchor for keeping a number of water molecules conveniently close to the graphene surface, thus helping its stabilization in aqueous media. Our analysis helps understanding how competing weak forces can lead to a stable graphene water suspension thanks to small amounts of adsorbed melamine. From our results, we derive simple indications on how the water-graphene interfacial properties can be tuned via non-covalent adsorption of small functional molecules with H-bond donor/acceptor groups. These new hints can be helpful to prepare stable graphene dispersions in water and so to unlock graphene potential in aqueous environments.
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http://dx.doi.org/10.1039/c6cp04213aDOI Listing
August 2016

Design and synthesis of novel organometallic dyes for NiO sensitization and photo-electrochemical applications.

Dalton Trans 2016 Aug;45(31):12539-47

Institut des Sciences Moléculaires, Université de Bordeaux, CNRS UMR 5255, Talence, France.

Two metallo-organic dyes were synthesized and used for NiO sensitization in view of their photoelectrochemical applications. The new dyes present an original π-conjugated structure containing the [Ru(dppe)2] metal fragment with a highly delocalized allenylidene ligand on one side and a σ-alkynyl ligand bearing an electron-rich group, i.e. a thiophene or triphenylamine unit, and one or two anchoring functions on the other side. The optoelectronic, electrochemical and photoelectrochemical properties of the dyes were systematically investigated. A broad photoresponse was observed with the absorption maximum at 600 nm. The X-ray crystal structure of one precursor was obtained to elucidate the structural conformation of the organometallic complexes and theoretical calculations were performed in order to address the photophysical properties of the new dyes. These photosensitizers were further implemented in NiO-based photocathodes and tested as photocurrent generators under pertinent aqueous conditions in association with [Co(NH3)5Cl]Cl2 as an irreversible electron acceptor. The dye-sensitized photocathodes provided good photocurrent densities (40 to 60 μA cm(-2)) at neutral pH in phosphate buffer and a high stability was observed for the two dyes.
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http://dx.doi.org/10.1039/c6dt02177hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5495160PMC
August 2016

study of PbCrS O solid solution: an inside look at Van Gogh Yellow degradation.

Chem Sci 2016 Jul 10;7(7):4197-4203. Epub 2016 Mar 10.

Department of Chemical Sciences , University of Naples Federico II , Comp. Univ. Monte Sant'Angelo Via Cintia 21 , 80126 Naples , Italy . Email:

Van Gogh Yellow refers to a family of lead chromate pigments widely used in the 19 century and often mixed with lead sulfate to obtain different yellow hues. Unfortunately, some paintings, such as the famous series, suffered degradation problems due to photoactivated darkening of once bright yellow areas, especially when irradiated with UV light. Recent advanced spectroscopic analyses have proved that this process occurs mostly where the pigment presents a sulfur-rich orthorhombic phase of a PbCrS O solid solution, while chromium-rich monoclinic phases are lightfast. However, the question of whether degradation is purely a surface phenomenon or if the bulk properties of sulfur-rich pigments trigger the process is still open. Here, we use first-principles calculations to unveil the role of sulfur in determining important bulk features such as structure, stability, and optical properties. From our findings, we suggest that degradation occurs an initial local segregation of lead sulfate that absorbs at UV light wavelengths and provides the necessary energy for subsequent reduction of chromate ions into the greenish chromic oxide detected in experiments. In perspective, our results set reliable scientific foundations for further studies on surface browning phenomena and can help to chose the best strategy for the proper conservation of art masterpieces.
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http://dx.doi.org/10.1039/c5sc04362jDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6014092PMC
July 2016

Origin and Electronic Features of Reactive Oxygen Species at Hybrid Zirconia-Acetylacetonate Interfaces.

ACS Appl Mater Interfaces 2015 Oct 25;7(39):21662-7. Epub 2015 Sep 25.

Dipartimento di Scienze Chimiche, ‡Dipartimento di Agraria and §Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli "Federico II" , Napoli, Italia.

The hybrid sol-gel zirconia-acetylacetonate amorphous material (HSGZ) shows high catalytic activity in oxidative degradation reactions without light or thermal pretreatment. This peculiar HSGZ ability derives from the generation of highly reactive oxygen radical species (ROS) upon exposure to air at room conditions. We disclose the origin of such unique feature by combining EPR and DRUV measurements with first-principles calculations. The organic ligand acetylacetonate (acac) plays a pivotal role in generating and stabilizing the superoxide radical species at the HSGZ-air interfaces. Our results lead the path toward further development of HSGZ and related hybrid materials for ROS-based energy and environmental applications.
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http://dx.doi.org/10.1021/acsami.5b06988DOI Listing
October 2015

Structure and energy level alignment at the dye-electrode interface in p-type DSSCs: new hints on the role of anchoring modes from ab initio calculations.

Phys Chem Chem Phys 2015 May;17(18):12238-46

Department of Chemical Sciences, University of Naples Federico II, Complesso Universitario Monte Sant'Angelo Via Cintia, 80126 Naples, Italy.

p-type dye-sensitized solar cells (DSSCs) represent the complementary photocathodes to the well-studied n-type DSSCs (Grätzel cells), but their low performances have hindered the development of convenient tandem solar cells based on cost-effective n- and p-type DSSCs. Because of their low efficiencies, experimental investigations highlighted the role of hole-electron transport processes at the dye-electrode interface. However, the effects of the dye anchoring groups on interfacial electronic features are still unclear. We report here a first principles study of a benchmark p-type DSSC model, namely the widely used Coumarin-based dye C343 adsorbed on the p-NiO surface. Together with the original carboxylic acid, we test the alternative phosphonic acid as the anchoring group. We investigate binding energies, structural features and electronic energy level alignments: our results highlight that these properties are highly sensitive to the binding modes. In particular, both the chemical nature of the anchoring group and the binding mode strongly affect the thermodynamic driving force for the dye-electrode hole injection process. From analysis of the electronic densities, we find that favorable driving forces are correlated with small values of the interfacial electrostatic dipole that is formed upon dye adsorption. From our results, we derive new hints for improving open circuit potential and the hole injection process in p-type DSSCs based on NiO electrodes.
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http://dx.doi.org/10.1039/c5cp01020aDOI Listing
May 2015

Size-extensivity-corrected multireference configuration interaction schemes to accurately predict bond dissociation energies of oxygenated hydrocarbons.

J Chem Phys 2014 Jan;140(4):044317

Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, USA.

Oxygenated hydrocarbons play important roles in combustion science as renewable fuels and additives, but many details about their combustion chemistry remain poorly understood. Although many methods exist for computing accurate electronic energies of molecules at equilibrium geometries, a consistent description of entire combustion reaction potential energy surfaces (PESs) requires multireference correlated wavefunction theories. Here we use bond dissociation energies (BDEs) as a foundational metric to benchmark methods based on multireference configuration interaction (MRCI) for several classes of oxygenated compounds (alcohols, aldehydes, carboxylic acids, and methyl esters). We compare results from multireference singles and doubles configuration interaction to those utilizing a posteriori and a priori size-extensivity corrections, benchmarked against experiment and coupled cluster theory. We demonstrate that size-extensivity corrections are necessary for chemically accurate BDE predictions even in relatively small molecules and furnish examples of unphysical BDE predictions resulting from using too-small orbital active spaces. We also outline the specific challenges in using MRCI methods for carbonyl-containing compounds. The resulting complete basis set extrapolated, size-extensivity-corrected MRCI scheme produces BDEs generally accurate to within 1 kcal/mol, laying the foundation for this scheme's use on larger molecules and for more complex regions of combustion PESs.
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http://dx.doi.org/10.1063/1.4862159DOI Listing
January 2014

Oxygen transport in perovskite-type solid oxide fuel cell materials: insights from quantum mechanics.

Acc Chem Res 2014 Nov 27;47(11):3340-8. Epub 2014 Jun 27.

Department of Chemical Sciences, University of Naples Federico II , Naples 80126, Italy.

CONSPECTUS: Global advances in industrialization are precipitating increasingly rapid consumption of fossil fuel resources and heightened levels of atmospheric CO2. World sustainability requires viable sources of renewable energy and its efficient use. First-principles quantum mechanics (QM) studies can help guide developments in energy technologies by characterizing complex material properties and predicting reaction mechanisms at the atomic scale. QM can provide unbiased, qualitative guidelines for experimentally tailoring materials for energy applications. This Account primarily reviews our recent QM studies of electrode materials for solid oxide fuel cells (SOFCs), a promising technology for clean, efficient power generation. SOFCs presently must operate at very high temperatures to allow transport of oxygen ions and electrons through solid-state electrolytes and electrodes. High temperatures, however, engender slow startup times and accelerate material degradation. SOFC technologies need cathode and anode materials that function well at lower temperatures, which have been realized with mixed ion-electron conductor (MIEC) materials. Unfortunately, the complexity of MIECs has inhibited the rational tailoring of improved SOFC materials. Here, we gather theoretically obtained insights into oxygen ion conductivity in two classes of perovskite-type materials for SOFC applications: the conventional La1-xSrxMO3 family (M = Cr, Mn, Fe, Co) and the new, promising class of Sr2Fe2-xMoxO6 materials. Using density functional theory + U (DFT+U) with U-J values obtained from ab initio theory, we have characterized the accompanying electronic structures for the two processes that govern ionic diffusion in these materials: (i) oxygen vacancy formation and (ii) vacancy-mediated oxygen migration. We show how the corresponding macroscopic oxygen diffusion coefficient can be accurately obtained in terms of microscopic quantities calculated with first-principles QM. We find that the oxygen vacancy formation energy is a robust descriptor for evaluating oxide ion transport properties. We also find it has a direct relationship with (i) the transition metal-oxygen bond strength and (ii) the extent to which electrons left behind by the departing oxygen delocalize onto the oxygen sublattice. Design principles from our QM results may guide further development of perovskite-based MIEC materials for SOFC applications.
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http://dx.doi.org/10.1021/ar4003174DOI Listing
November 2014

Density functional theory study of the interaction of vinyl radical, ethyne, and ethene with benzene, aimed to define an affordable computational level to investigate stability trends in large van der Waals complexes.

J Chem Phys 2013 Dec;139(24):244306

Dipartimento di Scienze Chimiche, Università di Napoli "Federico II," Complesso Universitario di Monte Sant'Angelo, Via Cintia, I-80126 Napoli, Italy.

Our purpose is to identify a computational level sufficiently dependable and affordable to assess trends in the interaction of a variety of radical or closed shell unsaturated hydro-carbons A adsorbed on soot platelet models B. These systems, of environmental interest, would unavoidably have rather large sizes, thus prompting to explore in this paper the performances of relatively low-level computational methods and compare them with higher-level reference results. To this end, the interaction of three complexes between non-polar species, vinyl radical, ethyne, or ethene (A) with benzene (B) is studied, since these species, involved themselves in growth processes of polycyclic aromatic hydrocarbons (PAHs) and soot particles, are small enough to allow high-level reference calculations of the interaction energy ΔEAB. Counterpoise-corrected interaction energies ΔEAB are used at all stages. (1) Density Functional Theory (DFT) unconstrained optimizations of the A-B complexes are carried out, using the B3LYP-D, ωB97X-D, and M06-2X functionals, with six basis sets: 6-31G(d), 6-311 (2d,p), and 6-311++G(3df,3pd); aug-cc-pVDZ and aug-cc-pVTZ; N07T. (2) Then, unconstrained optimizations by Møller-Plesset second order Perturbation Theory (MP2), with each basis set, allow subsequent single point Coupled Cluster Singles Doubles and perturbative estimate of the Triples energy computations with the same basis sets [CCSD(T)//MP2]. (3) Based on an additivity assumption of (i) the estimated MP2 energy at the complete basis set limit [EMP2/CBS] and (ii) the higher-order correlation energy effects in passing from MP2 to CCSD(T) at the aug-cc-pVTZ basis set, ΔECC-MP, a CCSD(T)/CBS estimate is obtained and taken as a computational energy reference. At DFT, variations in ΔEAB with basis set are not large for the title molecules, and the three functionals perform rather satisfactorily even with rather small basis sets [6-31G(d) and N07T], exhibiting deviation from the computational reference of less than 1 kcal mol(-1). The zero-point vibrational energy corrected estimates Δ(EAB+ZPE), obtained with the three functionals and the 6-31G(d) and N07T basis sets, are compared with experimental D0 measures, when available. In particular, this comparison is finally extended to the naphthalene and coronene dimers and to three π-π associations of different PAHs (R, made by 10, 16, or 24 C atoms) and P (80 C atoms).
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http://dx.doi.org/10.1063/1.4846295DOI Listing
December 2013

Oxide ion transport in Sr2Fe1.5Mo0.5O(6-δ), a mixed ion-electron conductor: new insights from first principles modeling.

Phys Chem Chem Phys 2013 May;15(17):6250-9

Department of Chemical Sciences, University of Naples Federico II, Naples 80126, Italy.

We use ab initio density functional theory + U calculations to characterize the oxide ion diffusion process in bulk Sr2Fe1.5Mo0.5O(6-δ) (SFMO) by analyzing the formation and migration of oxygen vacancies. We show that SFMO's remarkable ionic conductivity arises from its intrinsic content of oxygen vacancies and a predicted very low migration barrier of such vacancies. Theoretical analysis of the electronic structure reveals a crucial role played by strongly hybridized Fe 3d/O 2p states to achieve the attendant mixed ion-electron conductor character so important for intermediate temperature fuel cell operation. We predict a next-nearest-neighbor-type migration pathway for the O(2-) ion should dominate. The low energy barrier of this pathway is mainly related to electrostatic interactions with homogeneously distributed Mo in the SFMO sublattice. We identify the reasons why Fe-rich perovskites, with the key addition of a certain concentration of Mo, produce excellent electronic and ionic transport properties so crucial for efficient operation of intermediate temperature solid oxide fuel cells.
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http://dx.doi.org/10.1039/c3cp50995hDOI Listing
May 2013

Origin of the energy barrier to chemical reactions of O2 on Al(111): evidence for charge transfer, not spin selection.

Phys Rev Lett 2012 Nov 8;109(19):198303. Epub 2012 Nov 8.

Department of Mechanical and Aerospace Engineering and Chemistry, Program in Applied and Computational Mathematics, and Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, USA.

Dissociative adsorption of molecular oxygen on the Al(111) surface exhibits mechanistic complexity that remains surprisingly poorly understood in terms of the underlying physics. Experiments clearly indicate substantial energy barriers and a mysteriously large number of adsorbed single oxygen atoms instead of pairs. Conventional first principles quantum mechanics (density functional theory) predicts no energy barrier at all; instead, spin selection rules have been invoked to explain the barrier. In this Letter, we show that correct barriers arise naturally when embedded correlated electron wave functions are used to capture the physics of the interaction of O(2) with the metal surface. The barrier originates from an abrupt charge transfer (from metal to oxygen), which is properly treated within correlated wave function theory but not within conventional density functional theory. Our potential energy surfaces also identify oxygen atom abstraction as the dominant reaction pathway at low incident energies, consistent with measurements, and show that charge transfer occurs in a stepwise fashion.
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http://dx.doi.org/10.1103/PhysRevLett.109.198303DOI Listing
November 2012

Ab initio reaction kinetics of hydrogen abstraction from methyl formate by hydrogen, methyl, oxygen, hydroxyl, and hydroperoxy radicals.

J Phys Chem A 2012 Aug 10;116(33):8431-43. Epub 2012 Aug 10.

Department of Chemistry, Princeton University, Princeton, New Jersey 08544-5263, USA.

Combustion of renewable biofuels, including energy-dense biodiesel, is expected to contribute significantly toward meeting future energy demands in the transportation sector. Elucidating detailed reaction mechanisms will be crucial to understanding biodiesel combustion, and hydrogen abstraction reactions are expected to dominate biodiesel combustion during ignition. In this work, we investigate hydrogen abstraction by the radicals H·, CH(3)·, O·, HO(2)·, and OH· from methyl formate, the simplest surrogate for complex biodiesels. We evaluate the H abstraction barrier heights and reaction enthalpies, using multireference correlated wave function methods including size-extensivity corrections and extrapolation to the complete basis set limit. The barrier heights predicted for abstraction by H·, CH(3)·, and O· are in excellent agreement with derived experimental values, with errors ≤1 kcal/mol. We also predict the reaction energetics for forming reactant complexes, transition states, and product complexes for reactions involving HO(2)· and OH·. High-pressure-limit rate constants are computed using transition state theory within the separable-hindered-rotor approximation for torsions and the harmonic oscillator approximation for other vibrational modes. The predicted rate constants differ significantly from those appearing in the latest combustion kinetics models of these reactions.
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http://dx.doi.org/10.1021/jp304811zDOI Listing
August 2012

Unveiling structure-property relationships in Sr2Fe(1.5)Mo(0.5)O(6-δ), an electrode material for symmetric solid oxide fuel cells.

J Am Chem Soc 2012 Apr 9;134(15):6826-33. Epub 2012 Apr 9.

Department of Mechanical and Aerospace Engineering, Program in Applied and Computational Mathematics, and Gerhard R. Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, USA.

We characterize experimentally and theoretically the promising new solid oxide fuel cell electrode material Sr(2)Fe(1.5)Mo(0.5)O(6-δ) (SFMO). Rietveld refinement of powder neutron diffraction data has determined that the crystal structure of this material is distorted from the ideal cubic simple perovskite, instead belonging to the orthorhombic space group Pnma. The refinement revealed the presence of oxygen vacancies in the as-synthesized material, resulting in a composition of Sr(2)Fe(1.5)Mo(0.5)O(5.90(2)) (δ = 0.10(2)). DFT+U theory predicts essentially the same concentration of oxygen vacancies. Theoretical analysis of the electronic structure allows us to elucidate the origin of this nonstoichiometry and the attendant mixed ion-electron conductor character so important for intermediate temperature fuel cell operation. The ease with which SFMO forms oxygen vacancies and allows for facile bulk oxide ion diffusivity is directly related to a strong hybridization of the Fe d and O p states, which is also responsible for its impressive electronic conductivity.
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http://dx.doi.org/10.1021/ja300831kDOI Listing
April 2012

Insufficient Hartree-Fock Exchange in Hybrid DFT Functionals Produces Bent Alkynyl Radical Structures.

J Phys Chem Lett 2012 Feb 11;3(3):289-93. Epub 2012 Jan 11.

Departments of †Chemical and Biological Engineering and ‡Mechanical and Aerospace Engineering, §Program in Applied and Computational Mathematics, and ∥Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey, 08544-5263, United States.

Density functional theory (DFT) is often used to determine the electronic and geometric structures of molecules. While studying alkynyl radicals, we discovered that DFT exchange-correlation (XC) functionals containing less than ∼22% Hartree-Fock (HF) exchange led to qualitatively different structures than those predicted from ab initio HF and post-HF calculations or DFT XCs containing 25% or more HF exchange. We attribute this discrepancy to rehybridization at the radical center due to electron delocalization across the triple bonds of the alkynyl groups, which itself is an artifact of self-interaction and delocalization errors. Inclusion of sufficient exact exchange reduces these errors and suppresses this erroneous delocalization; we find that a threshold amount is needed for accurate structure determinations. Below this threshold, significant errors in predicted alkyne thermochemistry emerge as a consequence.
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http://dx.doi.org/10.1021/jz201564gDOI Listing
February 2012

Accurate bond energies of hydrocarbons from complete basis set extrapolated multi-reference singles and doubles configuration interaction.

Chemphyschem 2011 Dec 3;12(17):3354-64. Epub 2011 Nov 3.

Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, 08544-5263, USA.

Quantum chemistry has become one of the most reliable tools for characterizing the thermochemical underpinnings of reactions, such as bond dissociation energies (BDEs). The accurate prediction of these particular properties (BDEs) are challenging for ab initio methods based on perturbative corrections or coupled cluster expansions of the single-determinant Hartree-Fock wave function: the processes of bond breaking and forming are inherently multi-configurational and require an accurate description of non-dynamical electron correlation. To this end, we present a systematic ab initio approach for computing BDEs that is based on three components: 1) multi-reference single and double excitation configuration interaction (MRSDCI) for the electronic energies; 2) a two-parameter scheme for extrapolating MRSDCI energies to the complete basis set limit; and 3) DFT-B3LYP calculations of minimum-energy structures and vibrational frequencies to account for zero point energy and thermal corrections. We validated our methodology against a set of reliable experimental BDE values of CC and CH bonds of hydrocarbons. The goal of chemical accuracy is achieved, on average, without applying any empirical corrections to the MRSDCI electronic energies. We then use this composite scheme to make predictions of BDEs in a large number of hydrocarbon molecules for which there are no experimental data, so as to provide needed thermochemical estimates for fuel molecules.
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http://dx.doi.org/10.1002/cphc.201100447DOI Listing
December 2011

Quantum mechanical embedding theory based on a unique embedding potential.

J Chem Phys 2011 Apr;134(15):154110

Department of Physics, Princeton University, Princeton, New Jersey 08544, USA.

We remove the nonuniqueness of the embedding potential that exists in most previous quantum mechanical embedding schemes by letting the environment and embedded region share a common embedding (interaction) potential. To efficiently solve for the embedding potential, an optimized effective potential method is derived. This embedding potential, which eschews use of approximate kinetic energy density functionals, is then used to describe the environment while a correlated wavefunction (CW) treatment of the embedded region is employed. We first demonstrate the accuracy of this new embedded CW (ECW) method by calculating the van der Waals binding energy curve between a hydrogen molecule and a hydrogen chain. We then examine the prototypical adsorption of CO on a metal surface, here the Cu(111) surface. In addition to obtaining proper site ordering (top site most stable) and binding energies within this theory, the ECW exhibits dramatic changes in the p-character of the CO 4σ and 5σ orbitals upon adsorption that agree very well with x-ray emission spectra, providing further validation of the theory. Finally, we generalize our embedding theory to spin-polarized quantum systems and discuss the connection between our theory and partition density functional theory.
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http://dx.doi.org/10.1063/1.3577516DOI Listing
April 2011

Magnetic properties of nitroxide spin probes: reliable account of molecular motions and nonspecific solvent effects by time-dependent and time-independent approaches.

J Phys Chem B 2010 Sep;114(35):11509-14

Department of Chemistry P. Corradini, University of Napoli Federico II and CR-INSTM Village, Complesso Universitario Monte Sant'Angelo, Via Cintia 80126, Napoli, Italy.

Application of a new integrated computational approach for two widely used nitroxide spin probes allows to show unequivocally that proper account of stereoelectronic, environmental, and dynamical effects leads to magnetic properties in quantitative agreement with experimental results without the need of any empirical parameter. Together with their specific interest, our results point out, in our opinion, the importance of developing and validating computational approaches able to switch on and off different effects, including environmental and dynamical ones, in order to evaluate their specific role in determining the overall experimental outcome.
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http://dx.doi.org/10.1021/jp102232cDOI Listing
September 2010
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