National Institute Of Materials Physics - Romania

In this study, pristine TiO2 and Copper (Cu)-doped TiO2 nanopowders were synthesized using two sol-gel methods: conventional sol-gel (SG) and microwave-assisted sol-gel (MW). The aim was to investigate the effect of the synthesis route and Cu doping on the structural, optical, and photocatalytic properties of TiO2. X-ray diffraction (XRD) analysis confirmed the formation of anatase TiO2 in all samples, with a minor presence of rutile observed in the samples obtained by the MW method. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray analysis (EDX) showed that the nanopowders obtained by the MW method exhibited a greater tendency for particle aggregation compared to those obtained through the SG method. The presence of Cu2+/Cu+ species in the samples was confirmed by X-ray photoelectron spectroscopy (XPS). UV-Vis reflectance measurements revealed a shift in the absorption edge of the 5% Cu-doped samples toward the visible light range, significantly reducing the band gap. Evaluation of photocatalytic activity demonstrated that doping TiO2 with 5% Cu enhanced the degradation of Congo Red dye under visible light. [GRAPHICS] .

In this work a method in two steps for the preparation of the composites based on poly(ortho-toluidine) (POT) and the MoS2 and WS2 sheets was reported. In the first step, by ball-milling of mixtures of MoS2 and WS2 particles, the sheets of MoS2 and WS2 (MoS2: WS2) with weight ratio equal to 3:1, 1:1 and 1:3 were prepared. In the second step, the interaction in solid-state of the MoS2: WS2 samples with POT in emeraldine-base (POT-EB) and emeraldine-salt (POT-ES) was used to obtain composites of the type MoS2: WS2/POT-EB and MoS2: WS2/ POT-ES. Using X-ray diffraction (XRD), FTIR spectroscopy, Raman scattering and X-ray photoelectron spectroscopy (XPS), we demonstrate that: i) the ball-milling method can allow the preparation of the MoS2 and WS2 sheets with different stacking order, ii) the interaction of POT-EB with the MoS2: WS2 samples involves the transformation of some repeating units of the type EB into ES; and iii) the interaction of POT-ES with the MoS2: WS2 samples leads to the appearance of new positive charges onto macromolecular chains which are compensated by S2- ions. According to thermogravimetric analysis (TG) and differential scanning calorimetry (DSC), all samples are demonstrated to be stable up to 230 degrees C. Dielectric spectroscopy data reveal a complex dependence of DC electrical conductivity on frequency, temperature, and composite concentration. We use the apparent activation energy, defined as the derivative of the logarithm of conductivity with respect to the inverse temperature. The obtained results indicate that apparent activation energy is influenced by system composition via filling factors. The electrical properties of these heterogeneous materials are described using Lichtenecker's mixing laws. For components with similar electrical properties, the effective conductivity and apparent activation energy were determined as linear combinations of the individual conductivities and activation energies, respectively, weighted by the component concentrations. Our findings align with experimental data, offering a framework for understanding conductivity and activation energy in multi-component systems.

Pathogenic microorganisms such as bacteria, virus, and fungi present in water matrices are cause of infections and epidemic diseases worldwide. Photocatalytic deactivation is emerging as a promising, powerful method for water disinfection and microbial control. Transition metal oxide semiconductor photocatalysts has been widely investigated, being non-toxic, eco-friendly, and cost effective. However, their use in practical applications is limited due to their relatively wide band gap, reducing their optical absorption range to the UV domain of the solar radiation, and high recombination rate of photo-induced electron-hole pairs. The synthesised Mg doped TiO2 / graphene-like reduced graphene oxide nanocomposites are highly effective in deactivating nucleic acid molecules extracted from model Saccharomyces cerevisiae yeast cells as well as Saccharomyces cerevisiae yeast cells cultures, both under simulated sunlight and visible-light irradiation. The morphology, structure, and composition of the layers were thoroughly investigated by high resolution scanning electron microscopy, transmission electron microscopy, high-angle annular dark field scanning transmission microscopy, energy dispersive X-ray spectroscopy, as well as X-ray photoelectron spectroscopy. In accordance with the energy band structure of the synthesised nanocomposite catalysts, hydrogen peroxide and hydroxyl radicals are the major oxidative species (ROS) involved in the deactivation process. This study reveals a highly efficient approach for ROS generation for cell components decomposition and pathogen deactivation under visible light, and it can serve as an essential confirmation of the efficiency of visible-light-responsive photocatalysts in water disinfection.

Rhodium-doped SrTiO3 perovskite as a monophasic titanate-based catalyst (SrTi1-xRhxO3) showed photocatalytic activity for oxygen evolution reaction (OER) from water under solar light irradiation with an instant induction period, although Rh4+ in SrTiO3 introduces deep trap states thereby diminishing the efficiency of the hydrogen evolution reaction (HER). Despite its potential, the exact crystal structure of Rh:SrTiO3 has not been yet completely investigated. Overcoming these challenges, here, we synthesized a monophasic SrTi0.95Rh0.05O3 (RSTO) perovskite oxide with a precisely determined crystal structure and highlighted an unconsidered pivotal role of the Rh iso-aliovalency reversibility that enables excellent photocatalytic water oxidation. With structural, morphological, optical, and electronic insights from XRD, FE-SEM, HR-TEM, XPS, and advanced XAS measurements in both total electron yield (TEY) and fluorescence yield (TFY), the oxygen evolution reaction (OER) process is attributed to the redox dynamics of Rh4+ <-> Rh3+ synergistic interplay.

This study presents annealing time variation as an alternative approach for tuning structural and ionic defects in SnO2 nanoparticles (NPs) synthesized by a modified Pechini method, to modulate charge transfer and the formation of reactive oxygen species (ROS), enabling a correlation of these with photocatalytic activity. Structural refinements of the X-ray diffraction (XRD) data, combined with Raman and infrared (IR) spectroscopy, revealed that the samples adopted a tetragonal P42/mnm symmetry, with a distinct short-range structural order (associated with intraoctahedral [SnO6] distortions), which strongly depends on the annealing time. Field emission scanning electron microscopy (FE-SEM), scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM/EDX), high-resolution transmission electron microscopy (HRTEM), and Brunauer-Emmett-Teller (BET) analyses showed the formation of homogeneous NPs smaller than 50 nm in the SnO2 samples, with a surface area between 12.072 and 14.102 m2 g-1. The presence of unusual reduced Sn3+ species associated with oxygen vacancies (VO) was evidenced by electron paramagnetic resonance (EPR) spectroscopy, and Sn2+ was characterized by Sn MNN Auger electron emission. The amount of these defects also depends on the annealing time. The synergistic effect and the photocatalytic mechanism were then elucidated. The degree of intraoctahedral [SnO6] distortions and the amounts of Sn3+:VO defects play a fundamental role in the charge transfer mechanism to modulate ROS generation during photoexcitation, as indicated by the photohydroxylation of terephthalic acid (TA) and in situ spin-trapping EPR measurements. Specifically, we show the formation of two main ROS, center dot OOH and center dot OH, which leads to a different photocatalytic pathway in SnO2 NPs. In conclusion, our study enlightens and uncovers the importance of choosing appropriate conditions for materials processing to fine-tune structural and electronic properties to design other functional materials.

We investigate the second-order Floquet topological (SOFT) phase transitions, from the perspective of spatial symmetries. In this respect, we consider a generic honeycomb lattice Floquet topological insulator (FTI), realized by circularly polarized light irradiation, in the presence of spin-orbit coupling (Kane-Mele model). We find that our studied FTI presents chiral symmetry on a preferential direction in Fourier space, the same property that protects the topological phases in the Su-Schrieffer-Heeger (SSH) model. Thus we were allowed to characterize the SOFT phases in terms of mirror-graded winding numbers (Zak phase). Moreover, our model exhibits C2 and C3 symmetry in Fourier space, a property which lead us to investigate two finite structures having the aforementioned symmetries, namely, rhombic and triangular shapes. Indeed, we find that both of them undergo SOFT phase transitions, characterized by the appearance of 0D corner states symmetrically localized over the whole sheet. Finally, we investigate the C2 and C3 symmetry breaking. Interestingly, we reveal that the corner states are not destroyed, but localize at preferential corners instead, giving rise to a corner polarization.

The catalytic hydrodeoxygenation (HDO) of lignocellulose-derived pyrolysis oil is a critical process for producing high-quality biofuels. This study investigates the effect of the Mg/Al molar ratio on the catalytic performance of CuMg(Al)O mixed oxide catalysts in the HDO reaction of benzyl alcohol as a model oxygenated compound. They were synthesized by coprecipitation with a fixed Cu content of 15 at. %, with respect to cations, and different Mg/Al molar ratios (0/1, 1/1, 3/1, 5/1, and 1/0). The catalysts were characterized using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), UV-vis spectroscopy, nitrogen adsorption-desorption isotherms, temperature-programmed reduction with hydrogen (H2-TPR), and temperature-programmed desorption (TPD) of CO2 and NH3. It has been shown that the Mg/Al molar ratio strongly influences the physicochemical characteristics of the CuMg(Al)O mixed oxides and, hence, their catalytic performance. Catalytic tests were conducted in a stainless-steel autoclave reactor and the obtained results indicated that the systems with Mg/Al molar ratios of 3/1 and 5/1, issued from layered double hydroxide precursors, exhibited the highest activity, with yields to toluene higher than 85%. This superior performance is attributed to the well-dispersed copper species on the catalyst surface combined with appropriate acid-base properties. As the CuMg(Al)O system with Mg/Al molar ratio of 5/1 was the best in terms of benzyl alcohol conversion, i.e., ca. 98% at 230 degrees C, under 5 atm of H2, for 3 h of reaction time, with high selectivity to toluene of ca. 87%, the influence of the reaction time, temperature and reusability over multiple reaction cycles on its performance were investigated.

Photoanodes based Ce-doped ZnO nanorods arrays were prepared by hydrothermal method in order to improve photoelectrochemical efficiency of ZnO photoanodes in water splitting process. Scanning electron microscopy investigation showed ZnO based nanorods with length of around 500 nm and different thicknesses and growth directions. Some morphological changes were noted following the thermal treatment. Energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy measurements proved the presence of cerium species both in bulk and on the surface of ZnO nanorods. A current density of 2.44 mA/cm(2) at 1.23 V against the reversible hydrogen electrode (0.265 V vs. Ag/AgCl) was obtained for Ce-doped ZnO sample, which is >162% increase over that of ZnO sample. The increased photocurrent value obtained for this sample was correlated with the passivation of surface defects evidenced by photoluminescence study and the increased concentration of Ce3+ on the surface. Also, the electrochemical impedance spectroscopy measurements suggested that Ce doping improves the charge transfer in bulk.

Herein, alpha-MoO3 micro- and nanoparticles were synthesized by a modified Pechini method, and the impact of the crystal structure and crystal growth orientation on the formation of ionic defects and, consequently, on the catalytic performance of the materials in the ethylic transesterification reaction for biodiesel production was investigated. Structural refinements from X-ray diffraction data and Raman spectra revealed the formation of alpha-MoO3 in a Pbnm orthorhombic phase, with nanoplate-like morphology at 500 degrees C (thickness between 100 and 260 nm) or ribbon-like morphology at 700 degrees C (thickness between 400 and 900 nm). An anisotropic crystal orientation along the [010] direction was observed with an increase of the calcination temperature. We emphasize the dependence of the orientation change with the elimination of ionic-type defects (oxygen vacancies and reduced Mo5+ centers) by the temperature using complementary techniques such as X-ray photoelectron and electron paramagnetic resonance spectroscopies. The catalytic activity of the samples depends on the orientation process and the presence of defects that act as acid-active sites on the catalyst surface and therefore play an important role in biodiesel production. This effect was confirmed by surface stability and reactivity simulated by density functional theory calculations, suggesting that the Mo and O surface terminals greatly impacted the interface catalytic reaction. The highest catalytic performance toward the biodiesel conversion (89% of conversion at 150 degrees C for 2 h) was achieved for the polycrystalline catalyst calcined at 500 degrees C, which was correlated with random crystal orientation and the presence of reduced Mo5+ and oxygen vacancy centers on the different facets exposed on the surface. The biodiesel production was confirmed by H-1 and C-13 NMR spectroscopy and gas chromatography analysis.

In photocatalysis, a common challenge is the rapid recombination of photogenerated electrons and holes, coupled with low surface reaction efficiency in aqueous environments. This study addresses these issues by improving the photocatalytic performance of Al3+-doped SrTiO3 perovskite through the strategic loading of CoOOH onto its surface. We successfully demonstrated that the combined approach of Al3+ doping and CoOOH co-catalyst functionalization significantly enhances the photocatalytic performance of SrTiO3. The Al3%:SrTiO3 was functionalized with CoOOH using a two-step process. This process involved the oxidation of Co2+ ions to Co3+ ions, followed by the precipitation of cobalt oxyhydroxide (CoOOH) onto the surface of Al3%:SrTiO3, resulting in the formation of an Al3%:SrTiO3@CoOOH composite heterostructure. The UV-Vis data shows an enhanced light absorption capabilities into the visible spectrum with a direct band gap of 1.73 eV in contrast to 3.24 eV for the pristine perovskite. XPS analysis confirms the surface functionalization with CoOOH co-catalyst and the determined 1:1 Sr:Ti stoichiometry, the reduced state of Al, and the absence of oxygen vacancies were identified as beneficial properties for photocatalytic applications, as shown in DFT calculations. The oxacillin photodegradation was tested at three different concentrations of Al3%:SrTiO3@CoOOH photocatalyst (0.25 g/L; 0.5 g/L and 1 g/L) and we observed that the removal efficiency significantly varies from 99 % for 1 g/L Al3%: SrTiO3@CoOOH to 78 % and 44 % for 0.5 g/L and, respectively 0.25 g/L. Additionally, under visible light irradiation, the Al3%:SrTiO3@CoOOH composite achieved an exceptional degradation rate of the (3-lactam antibiotic oxacillin of up to 99 %, with holes identified as key players in the photocatalytic process. This study highlights that effective surface modification using a well-chosen co-catalyst can substantially boost the photocatalytic efficiency of semiconductor-based materials, offering a promising strategy for developing advanced photocatalysts for environmental remediation applications.