Degradation of Cd-yellow pigment: an ab initio study of defects and adsorption of oxygen and water on CdS

The cadmium yellow paints used in impressionist and modernist paintings in early 1900s are undergoing several deterioration processes, including whitening and discoloration. Relevant e↵ects produced at the surface of modern paintings include the growth of discolored crusts, formed mainly by white globular hydrated cadmium sulfate CdSO4*nH2O and cadmium carbonate (CdCO3 ). In view of the fact that the pigment, cadmium sulfide, was historically synthesized by means of dry and wet processes and that CdCO3 and CdSO4 are reagents for this procedure, their identification alone does not constitute conclusive proof of photo-oxidation. The origins of such chemical and physical alterations are still under debate. Structural defects in CdS, among other possible causes like photo-oxidation processes, may play a role in the degradation process. Their presence in the pigment surface alters the electronic structure of cadmium sulfide by forming acceptor levels in the gap of the semiconductor. Such levels make the surface more reactive in the interaction with external agents (oxygen, water ...). To this end, we present a theoretical study of points defects, namely Cd- and S- vacancies, in the structural wurtzite structure (bulk) and [10¯10] CdS surface. In order to understand, at atomic level, the oxidation and hydration mechanisms of these whitish globules, we present the early stages of the interaction between the hexagonal clean and defective [10¯10] surface of CdS and O2 and H2O molecules to simulate the combined e↵ects of exposure to air and humidity. The geometrical and electronic structures as well as the vacancy formation and adsorption energies are determined with the use of a first principles method. All the calculations are performed within the framework of the Density Functional Theory (DFT) in the Generalized Gradient Approximation (GGA-PBE) with the use of ultrasoft pseudopotentials. Despite the standard DFT-GGA ensures a correct order of electronic states, the G0W0 calculations are strongly required to attribute an accurate position of the trap level. Considering the possibility to investigate in a broader spectral range, such theoretical method would be helpful in the interpretation of experimental evidences on fluorescence emissions produced from by yellow altered. This thesis highlights the key role that first-principles methods can play in the application of materials science to art conservation

First principles study of the optical emission of cadmium yellow: Role of cadmium vacancies

We study the role of structural defects in the CdS-based cadmium yellow paint to explain the origin of its deep trap states optical emission. To this end, we combine a first principles study of Cd- and S- vacancies in the wurtzite (1010) CdS surface with experimental photoluminescence spectroscopy of the commercial hexagonal CdS pigment. Computational results clearly state that the presence of cadmium vacancies in the pigment surface alters the electronic structure of cadmium sulfide by forming acceptor levels in the gap of the semiconductor. Such levels are consistent with the optical emission from trap state levels detected in the CdS pigment. This finding provides a first step towards the understanding of the photo-physical mechanisms behind the degradation of this modern pigment, widely used in impressionist and modernist paintings

Progettazione e sintesi di nuovi materiali per la protezione, il ripristino e la conservazione di manufatti lapidei carbonatici

In recent times, the historical and architectural heritage has been subject to increasing degradation processes, largely due to outdoor air pollution, in addition to physical and biological factors. This research aims at evaluating the effectiveness and compatibility of new consolidating treatments of inorganic composition applied to distinct carbonate lithologies, such as marble and biomicrite limestones. The new compounds belong to the general classes of monoesters and functionalized monoamides of ammonium oxalate, with general formulas ROC(O)COONH4 and RNHC(O)COONH4, respectively (R = alkyl or aryl substituent). These salts were synthesized, purified, and fully characterized by elemental microanalysis, melting point determination, and spectroscopic methods (FT-IR, NMR). A few of the synthesized salts or their derivatives were characterized by single crystal X-ray diffraction. Full characterization of treated and untreated authentic stone samples was carried out by means of SEM, powder XRD, synchrotron tts-microXRD measurements, mercury intrusion porosimetry, determination of water transport properties, and pull-off tests. Furthermore, the experimental measurements were accompanied by DFT computational studies, aimed at understanding the stability of oxalate and oxamate salts in water solution as well as the nature of the interaction between the anions and the lattice of carbonate stone substrates. While the ammonium salts of oxalate monoesters, prone to spontaneous hydrolysis in aqueous solutions, are a potential source for calcium oxalate treatments, more effective than ammonium oxalate due to their larger solubility, ammonium oxamates represent a novel class of compounds alternative to ammonium oxalate. In conclusion, this thesis represents a comprehensive investigation that demonstrates that the salts of both oxalate monoesters and functionalized monoamides can be successfully adopted as novel precursors for the conservative treatments of carbonate stones

Degradation of Cd-yellow Paints: ab initio Study of the adsorption of oxygen and water on {10.0} CdS surface

The cadmium yellow paints (CdS) used in impressionist and modernist paintings in early 1900s are undergoing several deterioration processes including whitening and discoloration. A relevant effect produced at the surface of the paintings is the growth of discolored crusts formed mainly by white globular hydrated cadmium sulfate (CdSO4*nH2O) and cadmium carbonate (CdCO3). Recent studies [1,2] ascribe to an initial photo-oxidation process of CdS the input for the formation of such whitish compounds. In order to understand the oxidation and hydration mechanisms, at atomic level, of these whitish globules, we present the early stages of a theoretical study on the interaction between the hexagonal (10-10) surface of CdS and O2 and H2O molecules to simulate the combined effects of exposure to air and humidity. More specifically, we determined the favorite adsorption sites and calculated the adsorption energies of the different molecules on top of the surface. The details of the electronic structure of the interactions are given via the bonding charge analysis along with a thorough description of the geometry

The solvation Structure of Ca (II), Pb (II), Cd (II) in dilute aqueous solution: A first principles study

When an ion is inserted into a network of water molecules, the structure of the hydrogen bonds changes. Water is a polar molecule. It tends to orientate so to face its opposite charge to the ion. The group of water molecules bound to the ion is called “first hydration shell”. The orientation of the molecules in the hydration shell results in a net charge on the outside of this shell, a charge of the same sign as that of the ion in the center. The charge on the outside of the hydration shell tends to orient water molecules in the vicinity, leading to a second hydration shell. The purpose of this work is to conduct a systematic study of ion solvation, comparing positive ions of different size, namely the divalent Ca2+, Pb2+ and Cd2+. In particular, the analysis focuses on the characterization of the structural reorganization of the solvent due to the presence of the ion. To this aim, we use first principles calculations within the framework of the Density Functional Theory (DFT), i.e. an investigation at an electronic and atomistic scale, accounting for electronic polarization as well as geometrical conformations. A metal ion in aqueous solution (aqua-ion) is a cation, dissolved in water, of chemical formula [M(H2O)n]z+. The n water molecules directly bonded to the metal ion are meant to belong to the first coordination sphere. The sistems considered are small clusters, from one water molecule to the cluster containing a number of molecules equal to the coordination number. This stepwise analysis allows for an accurate detection of the structural and energy changes due to each additional water in the first hydration shell. There are many experimental and theoretical reports on the hydration of ions. More specifically, calcium raises great interest due to its role in many biological fuctions, several industrial applications (paper, rubber, plastics, paint production, and the wide occurrence in works of art)

Density Functional Theory modelling of protection agents for carbonate stones: case study of oxalate and oxamate inorganic salts

Sulphur and nitrogen oxide pollutants cause acid rain that can eventually lead to the dissolution of calcite in marble and limestones. Calcium oxalate is an inorganic protective agent, which is obtained by treatment with ammonium oxalate. The functionalization of oxalic acid to give monoesters and monoamides (oxamates) allows tailoring the solubility of the relevant ammonium and calcium salts. In this context, theoretical calculations carried out at the Density Functional Theory (DFT) level were exploited to investigate the capability of oxalate, methyloxalate, phenyloxalate, oxamate, methyloxamate, and phenyloxamate to interact with the calcium carbonate lattice. An in-depth validation based on the structural data showed that DFT calculations with the PBE0 functional along with a single or triple-zeta def2 basis set allow understanding the different reactivity of the oxalate and oxamate derivatives and their efficiency in interacting with stones containing calcium carbonate, such as Carrara marble and biomicritic limestones

Oxalate and oxamate derivatives: novel synthetic strategies in the design of materials for the restoration of marble and biomicrite limestone substrates

Air pollution, and in particular the presence of sulphur and nitrogen oxides, results in acidic rain determining the dissolution of calcite in marble and limestones. This degradation process induces evident roughness of the stone surfaces and the partial loss of carved details in artefacts. The formation of an artificial coating of calcium oxalate, obtained by treatment with ammonium oxalate, was proved to be a very promising technique for the protection of stone items. The functionalization of oxalic acid to give monoesters and monoamides (oxamates) allows tailoring the solubility of the relevant ammonium and calcium salts. In this context, theoretical calculations carried out at Density Functional Theory (DFT) level can help predicting the capability of the investigated compounds in interacting with the calcium carbonate lattice. We summarize here the experimental (X-ray diffraction, Mercury Intrusion Porosimetry, FT-MIR and FT-Raman spectroscopy, SEM) and theoretical (DFT PES and NBO analysis) investigations on the effects induced by the variation of the functionalized anions in oxalate and oxamate salts on the efficiency of the treatment of marble from the Cimitero Monumentale di Bonaria in Cagliari (Italy) and limestone samples from Cava Flore (Santa Caterina di Pittinuri, Oristano, Italy)

Ammonium monoethyloxalate (AmEtOx): a new agent for the conservation of carbonate stone substrates

The ammonium salt of monoethyloxalate (AmEtOx) was investigated as a novel precursor for the conservation of carbonate stone substrates, such as biomicritic limestone and marble. Full characterization of treated and untreated authentic stone samples was carried out by means of SEM, X-ray powder diffraction, synchrotron tts-mXRD measurements, mercury intrusion porosimetry, determination of water transport properties, and pull-off tests. The improved solubility (1.49 M, 20.1% w/w) of AmEtOx as compared to that of ammonium oxalate (AmOx; 0.4 M, 5% w/w) results in the formation of microcrystalline phases 30–50 and 200–500 mm thick of calcium oxalate mono-(whewellite) or dihydrate (weddellite), on marble and biomicrite samples, respectively, after treatment with AmEtOx 5% and 12% w/w aqueous solutions. As a result, a reduction in the porosity of the stone samples and an enhancement of their cohesion are observed. DFT calculations, carried out to investigate the hydrolysis reaction leading from AmEtOx to AmOx, showed that the localization of the Lowest Unoccupied Molecular Orbital (LUMO) and the natural charge distribution account nicely for the tendency to hydrolyse observed experimentally, eventually leading to the formation of whewellite and weddellite on the stone surface

Ammonium monoethyloxalate (AmEtOx): a new agent for the conservation of carbonate stone substrates

The ammonium salt of monoethyloxalate (AmEtOx) was investigated as a novel precursor for the conservation of carbonate stone substrates, such as biomicritic limestone and marble. Full characterization of treated and untreated authentic stone samples was carried out by means of SEM, X-ray powder diffraction, synchrotron tts-μXRD measurements, mercury intrusion porosimetry, determination of water transport properties, and pull-off tests. The improved solubility (1.49 M, 20.1% w/w) of AmEtOx as compared to that of ammonium oxalate (AmOx; 0.4 M, 5% w/w) results in the formation of microcrystalline phases 30–50 and 200–500 μm thick of calcium oxalate mono-(whewellite) or dihydrate (weddellite), on marble and biomicrite samples, respectively, after treatment with AmEtOx 5% and 12% w/w aqueous solutions. As a result, a reduction in the porosity of the stone samples and an enhancement of their cohesion are observed. DFT calculations, carried out to investigate the hydrolysis reaction leading from AmEtOx to AmOx, showed that the localization of the Lowest Unoccupied Molecular Orbital (LUMO) and the natural charge distribution account nicely for the tendency to hydrolyse observed experimentally, eventually leading to the formation of whewellite and weddellite on the stone surface.M. A., M. C. A., F. I., and V. L. thank the Fondazione di Sardegna (FdS) and Regione Autonoma della Sardegna (RAS) (Progetti Biennali di Ateneo FdS/RAS annualità 2016) for financial support. A. P. acknowledges RAS for the funding in the context of the POR FSE 2014–2020 (CUP F24J17000190009). Comune di Cagliari, Segretariato Regionale per i Beni e le attività culturali per la Sardegna, and Soprintendenza Archeologia, Belle Arti e Paesaggio per la città metropolitana di Cagliari e le province di Oristano e Sud Sardegna are kindly acknowledged for supporting the Cultural Heritage Conservation Laboratory “Colle di Bonaria” in their research and teaching activities.Peer reviewe

Ammonium N-(pyridin-2-ylmethyl)oxamate (AmPicOxam): a novel precursor of calcium oxalate coating for carbonate stone substrates

Ammonium N-(pyridin-2-ylmethyl)oxamate (AmPicOxam), synthesized from O-methyl-N-(pyridin-2-ylmethyl)oxamate, was spectroscopically and structurally characterized and assayed as a novel precursor for the protection and consolidation of carbonate stone substrates. An in-depth characterization of treated and untreated biomicritic limestone and white Carrara marble samples was carried out by means of SEM microscopy, X-ray powder diffraction, helium pycnometry, determination of water transport properties, and pull-off tests. The improved solubility (1.00 M, 16.5% w/w) of the title compound with respect to ammonium oxalate (0.4 M, 5% w/w) results in the formation of a thicker protective coating of calcium oxalate (CaOx) dihydrate (weddellite) on marble and biomicrite samples after the treatment with 5% and 12% w/w water solutions, producing a reduction in the stone porosity and increased cohesion. Theoretical calculations were carried out at the DFT level to investigate both the electronic structure of the N-(pyridin-2-ylmethyl)oxamate anion and the hydrolysis reaction leading from AmPicOxam to CaOx.</p