Andrea Giacomo Marrani

Probing the redox states at the surface of electroactive nanoporous nio thin films

Nanoporous NiO thin film electrodes were obtained via plasma-assisted microwave sintering and characterized by means of a combination of electrochemical techniques and X-ray photoelectron spectroscopy (XPS). The aim of this study is the elucidation of the nature of the surface changes introduced by the redox processes of this nanostructured material. NiO undergoes two distinct electrochemical processes of oxidation in aqueous electrolyte with the progress of NiO anodic polarization. These findings are consistent with the sequential formation of oxyhydroxide species at the surface, the chemical nature of which was assessed by XPS. Electronic relaxation effects in the Ni 2p spectra clearly indicated that the superficial oxyhydroxide species resulted to be β-NiOOH and γ-NiOOH. We also show for the first time spectral evidence of an electrochemically generated Ni(IV) species. This study has direct relevance for those applications in which NiO electrodes are utilized in aqueous electrolyte, namely catalytic water splitting or electrochromism, and may constitute a starting point for the comprehension of electronic phenomena at the NiO/organic electrolyte interface of cathodic dye-sensitized solar cells (p-DSCs).

Copper protection by self-assembled monolayers of aromatic thiols in alkaline solutions

Copper corrosion in alkaline solutions is inhibited by the formation of self-assembled monolayers of aromatic thiols, made of either benzenethiol or 2-naphthalenethiol or 4-acetamidothiophenol. Electrochemical experiments, based on voltammetry and impedance spectroscopy, point out the much lower reactivity of copper surfaces towards oxidation, when covered by compact adlayers of the above molecules bonded through the S atom. The peculiar shape and peak position in the voltammetric reduction of residual oxides grown on modified metal surfaces suggest that they are due to Cu(I) suboxides, probably grown on reactive metal defects. XPS experiments have confirmed that the aromatic adlayers are still covering most of the Cu surface even after 1 h immersion in 0.5 M NaOH. The main changes in Auger and XP spectra indicate the formation of much less Cu(2)O in the protected samples than in the corresponding bare Cu aged in NaOH. From the experimental data the presence of defective copper oxides on modified Cu has been deduced.

Electrochemically deposited ZnO films: an XPS study on the evolution of their surface hydroxide and defect composition upon thermal annealing

Electrodeposition from ZnCl2 aqueous solution was performed to grow ZnO thin films on the surface of polycrystalline copper plates. Electrochemical parameters for deposition were optimized by means of cyclic voltammetry (CV). The morphology of the deposits was studied via scanning electron microscopy (SEM), and their chemical composition was ascertained by means of X-ray photoelectron spectroscopy (XPS). The effects of changing the deposition bath temperature (Tbath) and the role played by post-deposition treatments, such as temperature and time of annealing in air, were studied. SEM images of freshly deposited vs. annealed samples have shown that in the former case the films display a rough morphology with mixed grain/hexagonal platelets structures and in the latter smaller but more uniformly dispersed cubic grains. Tbath is found to be the key parameter to induce the different morphology in the deposited films, which reflects in a different chemical reactivity of surface species, as found on the basis of the binding energies and relative quantitative ratios between Zn 2p and O 1s peaks. In fact, a higher Tbath favours a more efficient desorption of OH groups upon annealing, the O 1s peak resulting to much more drastically modified oxide/hydroxide intensity ratio with respect to the case of the sample deposited at lower Tbath.

Metal-free and transition-metal tetraferrocenylporphyrins part 1: synthesis, characterization, electronic structure, and conformational flexibility of neutral compounds

H(2)TFcP [TFcP = 5,10,15,20-tetraferrocenyl porphyrin(2-)] was prepared by a direct tetramerization reaction between pyrrole and ferrocene carbaldehyde in the presence of a BF(3) catalyst, while the series of MTFcP (M = Zn, Ni, Co and Cu) were prepared by a metallation reaction between H(2)TFcP and respective metal acetates. All compounds were characterized by UV-vis and MCD spectroscopy, APCI MS and MS/MS methods, high-resolution ESI MS and XPS spectroscopy. Diamagnetic compounds were additionally characterized using (1)H and (13)C NMR methods, while the presence of low-spin iron(ii) centers in the neutral compounds was confirmed by Mössbauer spectroscopy and by analysis of the XPS Fe 2p peaks, revealing equivalent Fe sites. XPS additionally showed the influence on Fe 2p binding energies exerted by the distinct central metal ions. The conformational flexibility of ferrocene substituents in H(2)TFcP and MTFcP, was confirmed using variable-temperature NMR and computational methods. Density functional theory predicts that alpha,beta,alpha,beta atropisomers with ruffled porphyrin cores represent minima on the potential energy surfaces of both H(2)TFcP and MTFcP. The degree of non-planarity is central-metal dependent and follows the trend: ZnTFcP < H(2)TFcP approximately CuTFcP < CoTFcP < NiTFcP. In all cases, a set of occupied, predominantly ferrocene-based molecular orbitals were found between the highest occupied and the lowest unoccupied, predominantly porphyrin-based molecular orbitals. The vertical excitation energies of H(2)TFcP were calculated at the TDDFT level and confirm the presence of numerous predominantly metal-to-ligand charge-transfer bands coupled via configurational interaction with expected intra-ligand pi-pi* transitions.

Surface Reactivity of a Carbonaceous Cathode in a Lithium Triflate/Ether Electrolyte-Based Li–O2 Cell

Li-O2 batteries are currently one of the most advanced and challenging electrochemical systems with the potential to largely overcome the performances of any existing technology for energy storage and conversion. However, these optimistic expectations are frustrated by the still inadequate understanding of the fundamentals of the electrochemical/chemical reactions occurring at the cathode side, as well as within the electrolyte and at the three-phase interface. In this work, we illustrate the evolution of the morphology and composition of a carbonaceous cathode in the first discharge/charge in a Li-O2 cell with an ether-based electrolyte by X-ray photoemission spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy. Experiments have been carried out ex situ on electrodes recuperated from electrochemical cells stopped at various stages of galvanostatic discharge and charge. Apparently, a reversible accumulation and decomposition of organic and inorganic precipitates occurs upon discharge and charge, respectively. These precipitations and decompositions are likely driven by electrochemical and chemical parasitic processes due to the reactivity of the cathode carbonaceous matrix.

Assembly of Gold Nanoparticles on Functionalized Si(100) Surfaces through Pseudorotaxane Formation

The assembly of gold nanoparticles (AuNPs) on a hydrogenated Si(100) surface, mediated by a series of hierarchical and reversible complexation processes, is reported. The proposed multi-step sequence involves a redox-active ditopic guest and suitable calix[n]arene-based hosts, used as functional organic monolayers of the two inorganic components. Surface reactions and controlled release of AuNPs have been monitored by application of XPS, atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM) and electrochemistry.

1,2-Dimethoxyethane Degradation Thermodynamics in Li−O2 Redox Environments

The reaction thermodynamics of the 1,2-dimethoxyethane (DME), a model solvent molecule commonly used in electrolytes for Li-O2 rechargeable batteries, has been studied by first-principles methods to predict its degradation processes in highly oxidizing environments. In particular, the reactivity of DME towards the superoxide anion O2- in oxygen-poor or oxygen-rich environments is studied by density functional calculations. Solvation effects are considered by employing a self-consistent reaction field in a continuum solvation model. The degradation of DME occurs through competitive thermodynamically driven reaction paths that end with the formation of partially oxidized final products such as formaldehyde and methoxyethene in oxygen-poor environments and methyl oxalate, methyl formate, 1-formate methyl acetate, methoxy ethanoic methanoic anhydride, and ethylene glycol diformate in oxygen-rich environments. This chemical reactivity indirectly behaves as an electroactive parasitic process and therefore wastes part of the charge exchanged in Li-O2 cells upon discharge. This study is the first complete rationale to be reported about the degradation chemistry of DME due to direct interaction with O2- /O2 molecules. These findings pave the way for a rational development of new solvent molecules for Li-O2 electrolytes.

C–C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the σ/π hyperconjugation in EtFC and the π-conjugation in VFC and EFC.

Calix[4]arene-Functionalised Silver Nanoparticles as Hosts for Pyridinium-Loaded Gold Nanoparticles as Guests

A series of lipophilic gold nanoparticles (AuNPs) circa 5 nm in diameter and having a mixed organic layer consisting of 1-dodecanethiol and 1-(11-mercaptoundecyl) pyridinium bromide was synthesised by reacting tetraoctylammonium bromide stabilised AuNPs in toluene with different mixtures of the two thiolate ligands. A bidentate ω-alkylthiolate calix[4]arene derivative was instead used as a functional protecting layer on AgNPs of approximately 3 nm. The functionalised nanoparticles were characterised by transmission electron microscopy (TEM), and by UV/Vis and X-ray photoelectron spectroscopy (XPS). Recognition of the pyridinium moieties loaded on the AuNPs by the calix[4]arene units immobilised on the AgNPs was demonstrated in solution of weakly polar solvents by UV/Vis titrations and DLS measurements. The extent of Au-AgNPs aggregation, shown through the low-energy shift of their surface plasmon bands (SPB), was strongly dependent on the loading of the pyridinium moieties present in the organic layer of the AuNPs. Extensive aggregation between dodecanethiol-capped AuNPs and the Ag calix[4]arene-functionalised NPs was also promoted by the action of a simple N-octyl pyridinium difunctional supramolecular linker. This linker can interdigitate through its long fatty tail in the organic layer of the dodecanethiol-capped AuNPs, and simultaneously interact through its pyridinium moiety with the calix[4]arene units at the surface of the modified AgNPs.

Noticeable role of TFSI- anion in the carbon cathode degradation of Li-O2 cells

In this work we address the phenomena at the basis of the performance loss in a Li-O2 cell operating in the presence of a lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/tetraethylene glycol dimethyl ether (TEGDME) salt/solvent couple and a porous carbonaceous cathode. The cell was discharged/charged applying both voltage and capacity limits, and the effects of repeated galvanostatic cycling were addressed. The ex situ characterization of carbonaceous cathodes corresponding to different cutoff voltages was based on vibrational spectroscopies, transmission electron microscopy, and X-ray photoelectron spectroscopy. The reversible precipitation/decomposition of undesired products deriving from degradation of both carbon cathode and ethereal solvent is pointed out within a single voltage limited (2.0-4.6 V) discharge/charge cycle, whereas their irreversible accumulation on the surface of the electrode results after 100 capacity limited cycles. At the same time, the presence of polar degradation products (carbonates and carboxylates) at the cathode surface is accompanied by the buildup of a surface electric potential gradient, as revealed by differential binding energy shifts resulting from C 1s photoelectron spectra. This effect, seldom reported for Li-ion batteries, is for the first time put in evidence for a Li-O2 cell. Furthermore, the use of TFSI- anion is shown to lead to carbonate-based degradation products not involving the formation of Li2CO3. The peculiar occurrence of such degradation phenomena are attributed to the intrinsic low-donor number characteristic of the TFSI- anion.