S. Santangelo

Effect of Nature and Location of Defects on Bandgap Narrowing in Black TiO2 Nanoparticles

The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO(2) nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO(2) nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications.

Vibrational properties and microstructure of reactively sputtered hydrogenated carbon nitrides

The present study is focused on the vibrational and structural characterization of a set of disordered hydrogenated carbon nitride (a-CN:H) thin films grown by reactive sputtering. A comparative analysis of the experimental results as achieved by Raman and infrared (IR) spectroscopies is made. The disorder-induced features of IR and Raman spectra are discussed as well in light of the current assessment on the vibrational properties of carbon-based materials. Some differences between a-CN:H and nonnitrogenated a-C:H materials are evidenced and attributed to the effects of charge redistribution and bond polarization due to the presence of nitrogen. In order to justify such a hypothesis, the dielectric constant, the dynamic effective charge, and the IR cross section determined by the charge transfer effects are calculated and found to be in agreement with the corresponding experimental values.

Nature of non-D and non-G bands in Raman spectra of a-C:H(N) films grown by reactive sputtering

The Raman spectra of sputter-grown a-C:H(N) films are analyzed giving particular emphasis to the regions below and above that dominated by the well-known D and G bands, in order to deduce complementary information about the film physical properties and eventually clarify the nature of the features there detected. The conventionally studied evolution of the D and G bands evidences a sharp tendency toward graphitization, accompanied by an increasing level of structural disorder. The weak feature observed at about 700 cm−1 is assigned to the disorder-induced L band, originating from the formation, within the films, of graphitic microcrystallites, whose number and size are shown to change when the partial pressure of argon is varied. Thanks to the complementary infrared film characterization, indicating no appreciable hydrogen incorporation into the investigated samples, the broad asymmetrical Raman band centered at about 3000 cm−1 is suggested to represent the second order of the D and G bands, whose compone...

Low-frequency Raman study of hollow multiwalled nanotubes grown by Fe-catalyzed chemical vapor deposition

In this work, it is shown that some Raman-active modes may be detected, below 500cm−1, in the spectrum of nanotubes synthesized by iron catalyzed chemical vapor deposition. By comparatively discussing results of Raman, high-resolution transmission electron microscopy, and thermogravimetric analyses, demonstration is given that these spectral features originate from scattering by nanoparticles of iron catalyst encapsulated within the tubes under nonstationary growth regime. Their intensity progressively weakens with increasing carbon supply rate until bands disappear as stationary conditions are reached.

Raman gain in niobium-phosphate glasses

In this paper, niobium-phosphate glasses doped with rare earths (Er and Sm) are investigated by Raman scattering. The goal of Raman characterization is twofold: (a) to perform a fine structural characterization of the synthesized glasses and (b) to measure the Raman gain coefficient of the samples and to compare it with fused silica. The results reveal the presence of NbO6 octahedra and Nb–O–P–Nb–O mixed chains. A broadening of bandwidth and a significant enhancement (∼24 times) in gain coefficient G with respect to conventional silica glasses are also demonstrated.

Soft x-ray absorption of FePS3 and NiPS3

Abstract With the partial yield technique we measured the absorption spectra of several core levels in FePS 3 and NiPS 3 . The M 2, 3 spectra of Fe and Ni are interpreted as localized transitions 3p 6 3d m− 3p 5 3d m+1 of the transition metal ion, split into a multiplet by final state multiconfiguration interaction. The P L 2, 3 , S L 2, 3 , and S L 1 spectra are similar to each other and are interpreted in terms of the projected density of states of the conduction bands derived from the states of the (P 2 S 6 ) 4− cluster.

A comparison of the ethanol sensing properties of α-iron oxide nanostructures prepared via the sol–gel and electrospinning techniques

Haematite (α-Fe2O3) nanostructures were synthesized via a Pechini sol-gel method (PSG) and an electrospinning (ES) technique. Their texture and morphology were investigated by scanning and transmission electron microscopy. α-Fe2O3 nanoparticles were obtained by the PSG method, whereas fibrous structures consisting of interconnected particles were synthesized through the ES technique. The crystallinity of the α-Fe2O3 nanostructures was also studied by means of x-ray diffraction and Raman spectroscopy. Gas-sensing devices were fabricated by printing the synthesized samples on ceramic substrates provided with interdigitated Pt electrodes. The sensors were tested towards low concentrations of ethanol in air in the temperature range (200-400 °C). The results show that the α-Fe2O3 nanostructures exhibit somewhat different gas-sensing properties and, interestingly, their sensing behaviour is strongly temperature-dependent. The availability of active sites for oxygen chemisorption and the diffusion of the analyte gas within the sensing layer structure are hypothesized to be the key factors responsible for the different sensing behaviour observed.

Electronic conduction in the layered semiconductor MnPS3

Thermopower, conductivity and photoconductivity measurements as a function of temperature, from 130 to 320 degrees C, have been carried out on manganese thiophosphate, MnPS3. The transport mechanisms involved in distinct temperature ranges and under different illumination conditions have been identified. At 130 degrees C a dark conductivity value of 8.6*10-13 Omega -1 cm-1 has been measured, while photoconductivity values ranged from 8.8*10-13 to 6.1*10-8 Omega -1 cm-1. In particular, the dark conductivity process has been attributed to holes in the phosphorus 3pz valence band. The results have been interpreted on the basis of a model, already used for NiPS3, that assumes a weak, ionic, interaction between the transition-metal and the sulphur atoms. A possible energy distribution of both valence and conduction bands, together with Mn 3d levels, is also provided.

Optical absorption spectra of some transition metal thiophosphates

Abstract Near-infrared and visible absorption spectra at room temperature of MnPS 3 , CoPS 3 , CoPS 3 are reported. These materials show weak structures below their fundamental absorption thresholds, due to the 3d-3d transitions occurring on the transition metal ion. By comparison with other transition metal compounds, such excitations are interpreted on the basis of the ligand field theory. The appearance of well developed crystal field spectra is a further confirmation that these materials are considerably ionic.

Chemical Modification of Graphene Oxide through Diazonium Chemistry and Its Influence on the Structure–Property Relationships of Graphene Oxide–Iron Oxide Nanocomposites

4-Carboxyphenyl groups are covalently grafted onto graphene oxide via diazonium chemistry for studying their role on the adsorption of iron oxide nanoparticles. The nanoparticles are deposited via a novel phase-transfer approach involving specific interactions at the interface between two immiscible solvents. The increased density and the homogeneous distribution of surface carboxyl moieties enable the preparation of a nanocomposite with improved iron oxide distribution and loading. Structure-properties relationships are investigated by analysing the electrochemical properties of the nanocomposites, which are regarded as promising active materials for application in supercapacitors. It is demonstrated that the nature of the interactions between the components similarly affects the overall electrochemical performances of the nanocomposites and the structure of the materials.