National Institute Of Materials Physics - Romania

This study synthesized Mg-doped ZnO nanoparticles using the co-precipitation method with doping concentrations ranging from 2 % to 8 %. The structural, morphological, and optical properties of the synthesized nanoparticles were systematically characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and UV-Visible spectroscopy. XRD analysis confirmed the successful incorporation of Mg2+ ions into the ZnO lattice, evidenced by lattice parameter shifts and a significant reduction in crystallite size from 30.91 nm (pure ZnO) to 18.10 nm (6 % Mg doping). SEM images showed uniform morphology with reduced particle agglomeration at optimal doping levels, while FTIR analysis identified characteristic Zn-O and Mg-O bonding vibrations, confirming structural integrity. UV-Vis spectroscopy revealed strong absorbance in the UV region, with the band gap energy decreasing from 3.68 eV (pure ZnO) to 3.16 eV (6 % Mg doping), indicating enhanced optical properties conducive to improved photocatalytic performance. The photocatalytic activity of Mg-doped ZnO nanoparticles was evaluated by degrading Basic Fuchsin (BF) dye under UV light irradiation. The Mg-doped ZnO nanoparticles exhibited significantly enhanced photocatalytic performance compared to undoped ZnO, achieving a maximum degradation efficiency of 99.38 % at 6 % Mg doping within 100 min. Optimal photocatalytic conditions were observed at pH 6, using 0.1 g of catalyst and an initial dye concentration of 10 ppm. These enhancements were attributed to improved electron-hole pair separation and increased generation of reactive oxygen species (ROS), facilitated by the strategic incorporation of Mg. To complement the experimental findings, Density Functional Theory (DFT) simulations were performed, integrating the Conductor-like Screening Model for Realistic Solvation (COSMO-RS), Reduced Density Gradient (RDG), and Quantum Theory of Atoms in Molecules (QTAIM). The DFT analysis revealed enhanced charge separation, optimized electron transfer dynamics, and stronger adsorption interactions at Mg-doped sites, which promoted efficient ROS generation. The calculated valence band (VB) and conduction band (CB) edge potentials supported the formation of a Z-scheme heterojunction mechanism, enhancing charge separation and minimizing recombination. These theoretical insights aligned with the experimental observations, confirming that Mg doping effectively enhances photocatalytic efficiency by optimizing electronic interactions and promoting reactive surface dynamics. This integrated experimental and theoretical investigation demonstrates that Mgdoped ZnO nanoparticles exhibit superior photocatalytic properties, making them highly effective for environmental remediation applications, particularly in degrading organic pollutants in wastewater treatment. The study highlights the potential of Mg-doped ZnO as a promising photocatalyst for sustainable environmental solutions.

Following the approach of inducing structural constrains for the creation of polar nanodomains/nanoregions in the perovskite thin films, the photoelectrochemical properties of complex heterostructures based on LaFeO3 and BiFeO3 materials have been studied in correlation with the imposed structural strain and polarization orientation anisotropy conditions. The LFO/BFO thin film heterostructures have been tested using photoelectrochemical (PEC) technique. The recorded photocurrent values for the heterostructure with the LFO layer on top- LFO/BFO/STON- in the potentiodinamic regime are considerably higher (similar to 1.64 mA/cm(2) at 2 V vs RHE) than those obtained for BFO/LFO/STON heterostructure where the BFO layer is acting as the surface layer (similar to 0.32 mA/cm(2) at 2 V vs RHE). Moreover, the LFO/BFO/STON heterostructure exhibits both photocathodic and photoanodic behavior. The atomic resolution STEM data demonstrate the polarization orientation anisotropy in LFO/BFO/STON heterostructure, due to the cooperative effects of competing surface chemistry and polarization of the LFO/BFO interface over the magnitude and orientation of the Fe3+ cations displacement within the La3+ cages. For the BFO/LFO/STON heterostructure however, robust single-oriented domains with large Fe3+ cations displacement characteristics for long-range ferroelectric order, has been revealed also. Taking into account the possibility to control the PEC features such as charge distribution/transport characteristics for both photocathodic/photoanodic reactions, as well as the control of the polarization direction without a pre-poling process, this work is of relevance for applications where the use of a top electrode is not possible.

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.

This study explores the development of highly sensitive hydrogen sulfide (H2S) gas sensors employing hierarchical aluminum-doped zinc oxide (AZO) nanostructures. Vertically oriented AZO nanoplatelets with Al/ZnO molar ratios of 4% and 6% were successfully synthesized using an automated successive ionic layer adsorption and reaction (SILAR) technique. The morphological features of the AZO films significantly changed with the Al content. The AZO thin films exhibited a polycrystalline wurtzite structure and an increase in crystallite size with increasing Al concentrations. This work demonstrates that our AZO sensor structures achieved a maximum response at 150 ppm H2S and 573 K of 23.3%, being characterized by fast response and recovery times of 28 and 464 s, respectively. Notably, the 6% AZO samples exhibited an augmented selective sensitivity to H2S, demonstrating stable detection performance. Additionally, the significant improvement in detection capabilities can be attributed to the synergistic effects of electronic and chemical sensitization. These effects enhance the formation of active sites and create doping-induced defects while providing shorter and more efficient diffusion paths for the electrons, significantly improving the sensor's sensitivity and response speed.

Most photocatalytic processes involve physicochemical phenomena occurring at the semiconductor-water interface. The interfacial charge transfer strongly depends on the charge carrier self-trapping or defect-based trapping mechanisms active in the crystal lattice of the photocatalyst. Thus, the crystal lattice distortion is expected to influence the photocatalytic efficiency during polymorphic phase transformations (PPT). A simple synthesis method involving the ultrasound-assisted excess hydrolysis of titanium tetra-isopropoxide (TTIP) (hydrolysis ratio (number of moles of water/number of moles of TTIP) r = 245) was used to obtain multiphase titanium dioxide (TiO2) nanomaterials with complex defect structures. Electron paramagnetic resonance (EPR) spectroscopy was employed to characterize the paramagnetic centers in the synthesized TiO2 and their behavior during incipient PPT. The calcined samples showed a complex defect structure comprising three types of paramagnetic centers: F+-centers (an electron trapped in an oxygen vacancy (Ov)), V-centers (oxygen ions with trapped holes) and paramagnetic centers involving Ti3+ such as Ti3+- Ov. The sample obtained at 600 degrees C, temperature marking the onset of a massive mixed transformation of anatase into rutile and brookite, composed of approximately 81 % anatase, 10 % brookite, and 9 % rutile, exhibited an intense and broadened EPR signal and enhanced photocatalytic activity for hydroxyl radical generation and hydrogen production by water splitting, despite its rather low specific surface area of 34 m2/g. The results revealed the synergistic effects of charge carrier trapping mechanisms in the early stages of PPT, boosting the photocatalytic performance of TiO2. The present study supports the design of facile synthesis methods for better TiO2 photocatalysts and promotes the development of further studies regarding lattice defect engineering during phase transformations in nanomaterials.

When used as the active layers-either as a light absorber in photovoltaic devices or as an electroluminescent material in light-emitting devices-lead-free perovskites significantly impact the performance of optoelectronic devices. This study focuses on antimony-based perovskites, which are promising for lighting applications. These types of perovskites enable the formation of self-trapped excitons (STEs) with higher dissociation energy than lead-based perovskites, which generate excitons with lower dissociation energy. The (CH3NH3)3Sb2I9 crystals are synthesized using two methods, resulting in distinct spatial configurations - dimer and dimer/layered mixtures, each exhibiting unique structural and spectroscopic properties, as revealed by comprehensive multi-parametric complementary analyses. Their emissive properties underscore the efficiency of the STE photoluminescence, driven by electron-phonon interactions and influenced by Sb-Sb distances in (CH3NH3)3Sb2I9 powder, whether dispersed in polymethyl-methacrylate or solution. The phase transition from monoclinic to hexagonal (dimer) and trigonal (layered) structures enabled the tuning of the optical properties in direct correlation with the structural and electrical features. The photoluminescence behavior of the STEs, analyzed in conjunction with the Raman spectroscopy, elucidates the dynamic process of the electron-phonon coupling effects in the dimer (face-capping Sb-I octahedra) and layered (corner-sharing Sb-I octahedra) crystallographic structures.

In this work, three methods for the synthesis of composites based on poly(ortho-toluidine) (POT) and WS2 are reported: (a) the solid-state interaction (SSI) of POT with WS2 nanoparticles (NPs); (b) the in situ chemical polymerization (ICP) of ortho-toluidine (OT); and (c) the electrochemical polymerization (ECP) of OT. The preparation of WS2 sheets was performed by the ball milling of the WS2 NPs followed by ultrasonication in the solvent N,N'-dimethyl formamide. During the synthesis of the POT/WS2 composites by SSI and ICP, an additional exfoliation of the WS2 NPs was reported. In this work, we demonstrated the following: (a) the ICP method leads to POT/WS2 composites, which contain repeating units of POT in the leucoemeraldine salt (LS) state, while (b) the ECP method leads to POT/WS2 composites, which contain repeating units of POT in the emeraldine salt (ES) state. Capacitances equal to 123.5, 465.76, and 751.6 mF cm-2 in the cases of POT-ES/WS2 composites, synthesized by SSI, ICP, and ECP, respectively, were reported.

Laser thin layer deposition technologies were applied to develop organic heterostructures on flexible transparent conductive electrode (TCE). Flexible substrates such as flexible glass (FG), polyethersulfone (PES), amorphous polyethylene terephthalate (PET-A) and biaxially-oriented polyethylene terephthalate (PET-B) were employed to assess the influence of the substrate type on the optical and electrical characteristics of the organic devices. For comparison reason, the organic heterostructures were fabricated on rigid glass substrate and commercially available indium tin oxide (ITO)-coated PET. Hence, flexible and rigid glass substrates were coated with ITO film by pulsed laser deposition (PLD) at low fluence, subsequently a blend layer based on zinc phthalocyanine (ZnPc) and N, N '-bis-(1-dodecyl)perylene-3,4,9,10 tetracarboxylic diimide (AMC14) being deposited by matrix assisted pulsed laser evaporation (MAPLE) on the TCE film. The investigations evidenced that the roughness and the substrate type can strongly influence the properties of the ITO layer deposited by PLD as well as the optical and electrical characteristics of the organic heterostructures based on the blend layer deposited by MAPLE. Thus, the lowest roughness (0.8 nm) and the best Hall mobility (41.9 cm2/V center dot s) were achieved for ITO coatings deposited on flexible glass substrate. Also, the highest current density value (9.3 x 10- 4 A/cm2 at 0.5 V) was reached for the organic heterostructures fabricated on this type of flexible substrate.

Capitalizing on invasive plant species and stopping their aggressive spread might be achieved by using them as a renewable source of useful products such as cellulose. The study aimed to develop new cellulose-based food packaging materials with antioxidant and antimicrobial activity. The cellulose was extracted from the invasive plant species Robinia pseudoacacia pods, crosslinked with citric acid, used as reinforcement for polyvinyl alcohol (PVA) and functionalized with ferulic acid (FA). The obtained materials were characterized by XRD, ATR-FTIR, contact angle and SEM. The materials exhibited low solubility in water and the swelling degree was proportional to the FA content. The FA release from the matrix was assessed by HPLC and the antioxidant profile by CUPRAC, FRAP, and TEAC methods. The obtained materials inhibited the growth of bacteria, yeasts and molds, being especially active on Gram-positive bacteria and yeasts. Overall, the most promising formulation for further developing new packaging materials for products with water activity less than 0.95 was the one with the highest FA content.

Interest in self-cleaning coatings is rising due to their potential to enhance comfort and quality of life in polluted urban environments, driving the search for materials with optimal physical properties. Convergent with this goal, this study investigates the wetting properties and photo-catalytic efficiency of reticulated TiO2 layers. It shows that these properties are significantly influenced by the topographical characteristics of the TiO2 surface, which can be precisely controlled through variations in pulverization pressure and low-temperature post-annealing treatments. Post-deposition annealing of the TiO2 layers achieves 100 % self-cleaning efficiency for both thick and thin films, with optical transmission ranging from approximately 60 %-80 % in the visible spectrum. Additionally, the TiO2 layers exhibited promising capabilities for eliminating pathogenic microorganisms and disinfecting surfaces. The underlying causal factors of these remarkable and technologically promising surface features are explored and discussed.