Irina V. Chernyshova

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition

Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (alpha-Fe(2)O(3)) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (gamma-Fe(2)O(3))-like defects in the near surface regions, to which a vibrational mode at 690 cm(-1), active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced alpha-Fe(2)O(3)<-->gamma-Fe(2)O(3) phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates-concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the alpha-->gamma phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the E(u) band at about 460 cm(-1) and the spinel band at 690 cm(-1), instead of the A(2u)/E(u) band at about 550 cm(-1) used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.

An in situ FTIR study of galena and pyrite oxidation in aqueous solution

Abstract FTIR spectroelectrochemical measurements demonstrated that oxidation of galena starts with the reaction PbS+2 xh + =Pb 1− x S+ x Pb 2+ , which is followed by the reaction PbS+2 h + =Pb 2+ +S 0 . Pb(OH) 2 is formed by the precipitation mechanism. Lead sulfite and thiosulfate and polythionate ions are formed from the elementary sulfur formed in the first oxidation stage. Dissolved oxygen participates in both the cathodic and anodic half-reactions for the galena oxidation. The anodic oxidation of pyrite occurs in two stages. At the first stage, the S deficient clusters or FeS defects degrade by reaction the FeS=Fe 2+ +S 2− , followed by hydrolysis and (electro)chemical oxidation–precipitation of the products (elemental sulfur, and iron hydroxide). The second stage starts with depopulation of the negatively charged iron acceptor surface states located at the valence band edge. After that, the bulk pyrite oxidation takes place through the thiosulfate pathway. It was shown that the sulfur formed on sulfides has a different origin: on pyrite, sulfur precipitates from solution, while on galena it is residual sulfur left after metal ion leaching from sulfide. The pyrite sulfur is oxidized mainly to polythionates and sulfate, as opposed to galena, for which the main oxysulfur compound is thiosulfate. The sequences of the reactions were explained based on the electronic band structures and correlated with the floatability of the sulfides. A new model of pyrite oxidation at the open circuit potential was suggested, based on the lateral electrochemical inhomogeneity of the pyrite surface.

Mechanisms of amine–feldspar interaction in the absence and presence of alcohols studied by spectroscopic methods

Abstract The adsorption of long-chain primary amines on feldspars at neutral pH 6–7 was investigated using Hallimond flotation, zeta-potential, FT-IR and XPS studies. Two-dimensional (2D) followed by three-dimensional (3D) precipitation mechanism of amine adsorption on quartz reported earlier (Langmuir 16 (2000) 8071) was further substantiated. The orientation and packing of dodecyl- and hexadecylammonium acetate and chloride adsorbed on albite in the different regions of the adsorption isotherm were determined. It was shown that these characteristics depend strongly on the substrate. The influence of long-chain alcohols on the adsorption of amines in mixed amine/alcohol on feldspars, i.e. albite (NaAlSi3O8) and microcline (KAlSi3O8), was also examined. Coadsorption of the counterion was not revealed, but the counterion was found to affect indirectly the adsorption at concentrations above the concentration of the bulk amine precipitation. It was proved spectroscopically that co-adsorption of long-chain alcohols along with amine cations leads to formation of a closed packed surface layer as compared to the case of adsorption of pure amine alone at the same concentration. The highest order and packing at the surface were observed when the alkyl chain length of mixed amine and alcohol were the same. The condition of same chain length of amines and alcohols adsorbing at the surface corresponded to maximum flotation recovery. The results also confirmed the synergistic enhancement of amine adsorption in the presence of alcohols. A mechanism of mixed long-chain amine and alcohol adsorption onto silicates (albite and quartz) consistent with the primary adsorption species of alkylammonium–water–alcohol complex, where deprotonation of ammonium groups in the adsorbed layer leading to 2D precipitation of molecular amine was illustrated.

Adsorption of fatty acids on iron (hydr)oxides from aqueous solutions.

The interaction of iron (hydr)oxides with fatty acids is related to many industrial and natural processes. To resolve current controversies about the adsorption configurations of fatty acids and the conditions of the maximum hydrophobicity of the minerals, we perform a detailed study of the adsorption of sodium laurate (dodecanoate) on 150 nm hematite (α-Fe(2)O(3)) particles as a model system. The methods used include in situ FTIR spectroscopy, ex situ X-ray photoelectron spectroscopy (XPS), measurements of the adsorption isotherm and contact angle, as well as the density functional theory (DFT) calculations. We found that the laurate adlayer is present as a mixture of inner-sphere monodentate mononuclear (ISMM) and outer-sphere (OS) hydration shared complexes independent of the solution pH. Protonation of the OS complexes does not influence the conformational order of the surfactant tails. One monolayer, which is filled through the growth of domains and is reached at the micellization/precipitation edge of laurate, makes the particles superhydrophobic. These results contradict previous models of the fatty acid adsorption and suggest new interpretation of literature data. Finally, we discovered that the fractions of both the OS laurate and its molecular form increase in D(2)O, which can be used for interpreting complex spectra. We discuss shortcomings of vibrational spectroscopy in determining the interfacial coordination of carboxylate groups. This work advances the current understanding of the oxide-carboxylate interactions and the research toward improving performance of fatty acids as surfactants, dispersants, lubricants, and anticorrosion reagents.

Spectroscopic study of galena surface oxidation in aqueous solutions I. Identification of surface species by XPS and ATR/FTIR spectroscopy

Abstract The measurements by ATR/FTIR and X-ray photoelectron spectroscopy were performed in order to identify galena surface compounds formed in air-saturated solutions under different experimental conditions. Wet polishing under distilled water was found to produce lead- or sulphur-rich galena surface depending on the semiconducting nature of the sample. The probable mechanisms of this phenomenon are discussed. After cathodic polarization at −0.5 V (SHE) in borate buffer (pH 9.2), the surface appears to be lead-rich regardless of the origin of galena. Bulk lead hydroxide Pb(OH) 2 was established to be the major product of the galena oxidation at +0.4 V in the air-saturated buffer. The assignment of IR bands and XPS peaks of bulk lead hydroxide was made. It was shown with the ATR/FTIR spectroscopy that galena, when being oxidized with hydrogen peroxide, is covered by lead hydroxide, lead thiosulphate and adsorbed oxygen in addition to elemental sulphur and basic lead sulphate (detected earlier by XPS). The relative quantity of each compound as well as the outermost layer composition was found to depend strongly on the oxidation kinetics.

On the origin of an unusual dependence of (bio)chemical reactivity of ferric hydroxides on nanoparticle size

Application of in situ UV-Vis absorption spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS) makes it possible to resolve the controversies about the electronic properties of hematite (α-Fe(2)O(3)) nanoparticles (NPs) and, on this basis, to rationalize the unusual dependence of aquatic (bio)chemistry of these NPs on NP size. 2-Line ferrihydrite (FH) is also included in the study as the end polymorph of the size-driven phase transformation of hematite NPs in aqueous media. It is shown that the absorption edge of all NPs studied is due to the direct O 2p-Fe 3d charge transfer (CT) process, while a manifold of weak bands superimposed onto two main p-d CT bands is attributed to the d-d ligand field transitions. The band gap decreases from 2.95 to 2.18 eV with increasing NP size from 7 nm to 120 nm. This effect is attributed to restoration of hematite lattice structure, which ultimately results in an increase in the O 2p-Fe 3d hybridization, stabilization of the valence band, and delocalization of valence electrons, as confirmed by XPS. Finally, we show that the optical effects such as the Mie resonance significantly distort absorption spectra of hematite NPs larger than ∼120 nm. Possible impacts of these findings on (photo)catalytic and biochemical properties of ferric (hydr)oxide NPs are discussed.

Rational design of interfacial properties of ferric (hydr)oxide nanoparticles by adsorption of fatty acids from aqueous solutions.

Notwithstanding the great practical importance, still open are the questions how, why, and to what extent the size, morphology, and surface charge of metal (hydr)oxide nanoparticles (NPs) affect the adsorption form, adsorption strength, surface density, and packing order of organic (bio)molecules containing carboxylic groups. In this article, we conclusively answer these questions for a model system of ferric (hydr)oxide NPs and demonstrate applicability of the established relationships to manipulating their hydrophobicity and dispersibility. Employing in situ Fourier transform infrared (FTIR) spectroscopy and adsorption isotherm measurements, we study the interaction of 150, 38, and 9 nm hematite (α-Fe(2)O(3)) and ∼4 nm 2-line ferrihydrite with sodium laurate (dodecanoate) in water. We discover that, independent of morphology, an increase in size of the ferric (hydr)oxide NPs significantly improves their adsorption capacity and affinity toward fatty acids. This effect favors the formation of bilayers, which in turn promotes dispersibility of the larger NPs in water. At the same time, the local order in self-assembled monolayer (SAM) strongly depends on the morphological compatibility of the NP facets with the geometry-driven well-packed arrangements of the hydrocarbon chains as well as on the ratio of the chemisorbed to the physically adsorbed carboxylate groups. Surprisingly, the geometrical constraints can be removed, and adsorption capacity can be increased by negatively polarizing the NPs due to promotion of the outer-sphere complexes of the fatty acid. We interpret these findings and discuss their implications for the nanotechnological applications of surface-functionalized metal (hydr)oxide NPs.

Spectroscopic study of structure and intermolecular interactions of diphenylformamidine and diphenylacetamidine in solution

Abstract The conformational structures of diphenylformamidine (DPFA) and diphenylacetamidine (DPAA) have been studied by measuring the IR and NMR spectra in solution. The structures of self-associates and hydrogen-bonded complexes with acetic acid were determined. The formation of cyclic dimers of DPFA and open associates of DPAA was proved. Complexes of both amidines with acetic acid were found to be cyclic. Spectral and thermodynamic parameters of cyclic association were obtained.

Investigation of the products of oxidation of lead selenide by IR spectroscopy

The optical properties of lead selenide powders are investigated by Fourier-transform IR spectroscopy at room temperature, as well as at temperatures of 500 and 550°C. The diffuse reflection spectra of the initial and heated powders of the PbSe compound are measured. It is revealed for the first time that the surface of the initial and heated samples of the PbSe compound contains a layer of amorphous biselenite Pb(HSeO3)2 · n H2O. This makes it possible to explain the previously observed local minimum at temperatures close to 100°C in the temperature dependence of the electrical resistivity of the powders.

Tailoring (bio)chemical activity of semiconducting nanoparticles: critical role of deposition and aggregation.

The impact of deposition and aggregation on (bio)chemical properties of semiconducting nanoparticles (NPs) is perhaps among the least studied aspects of aquatic chemistry of solids. Employing a combination of in situ FTIR and ex situ X-ray photoelectron spectroscopy (XPS) and using the Mn(II) oxygenation on hematite (α-Fe(2)O(3)) and anatase (TiO(2)) NPs as a model catalytic reaction, we discovered that the catalytic and sorption performance of the semiconducting NPs in the dark can be manipulated by depositing them on different supports or mixing them with other NPs. We introduce the electrochemical concept of the catalytic redox activity to explain the findings and to predict the effects of (co)aggregation and deposition on the catalytic and corrosion properties of ferric (hydr)oxides. These results offer new possibilities for rationally tailoring the technological performance of semiconducting metal oxide NPs, provide a new framework for modeling their fate and transport in the environment and living organisms, and can be helpful in discriminating between weakly and strongly adsorbed species in spectra.