National Institute Of Materials Physics - Romania

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1. A comparative overview of MXenes and metal oxides as cocatalysts in clean energy production through photocatalysis
Authors: Nair, MM; Iacoban, AC; Neatu, F; Florea, M; Neatu, S

Published: JUN 20 2023, JOURNAL OF MATERIALS CHEMISTRY A, 11, DOI: 10.1039/d2ta08983a

Photocatalytic conversion of H2O, CO2 and N-2 represents one promising approach to harvest and store solar energy, for which efficient visible light responsive semiconductors are inevitable. Often, the presence of a small amount of an additional component called a "cocatalyst", is required to synergistically enhance the performance of the photocatalyst. Tremendous efforts were made in the past to identify inexpensive materials to be used as cocatalysts, for which metal oxides (MOs) are one of the traditional choices. Among alternative categories of materials investigated, the recently discovered MXenes display enormous potential owing to their unique 2D layered structure, tuneable composition, abundant surface functionalities and superior electronic conductivity. Specifically, MOs and MXenes encompass a variety of distinct as well as analogous characteristics that allows them to be tailored to different extents. Unfortunately, a comprehensive overview covering the synthetic, structural and photocatalytic aspects of MOs and MXenes is not available as of now. Herein, we intend to summarize the progress achieved so far in these two families of materials to be used as cocatalysts for the photoconversion of H2O, CO2 and N-2. Followed by a general introduction, we briefly outline the fundamental principles and the role of cocatalysts in photocatalytic reactions. A discussion regarding the use of MOs and MXenes as cocatalysts for the conversion of H2O, CO2 and N-2 is then provided in separate sections. Critical assessment regarding structure and morphology control, surface properties and stability concerns can not only help to recognize the challenges that limit further advancement, but can also highlight the future research directions of these materials for the effective transformation of H2O, CO2 and N-2.
2. Partial replacement of Pb2+ in MAPbI(2.6)Cl(0.4) perovskite films and their photovoltaic performance
Authors: Derbali, S; Nouneh, K; Leonat, LN; Stancu, V; Tomulescu, AG; Galca, AC; Touhami, ME; Pintilie, I; Florea, M

Published: APR 2023, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 34, 903, DOI: 10.1007/s10854-023-10318-9

Replacing lead atoms in halide perovskite materials is of significant importance for the development of environmentally friendly perovskite solar cells. In this paper, we investigated the effect of doping the MAPbI(2.6)Cl(0.4) hybrid perovskite (MA-methyl ammonium) with non-toxic elements, such as alkaline earth metal ions (Mg2+) and transition metal ions (Zn2+). The structural, morphological, and optical properties of the prepared samples were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV-Vis. spectroscopy. Finally, the doped films were used as photoactive layers in solar devices in order to evaluate their photovoltaic performance. Zn proved to be more appropriate to replace partially Pb and films with higher quality were obtained. As a result, the MAPb1(-x)Zn(x)I(2.6)Cl(0.4) based solar cells have demonstrated a slight improvement of the photovoltaic performances, resulting in a uniform and narrower PCEs (power conversion efficiency) range, compared to pristine MAPbI(2.6)Cl(0.4) based devices.
3. Understanding the Photocatalytic Activity of Sodium Hexatitanate Nanoparticles for Pollutants Degradation: A Spectroscopic Insight
Authors: Teixeira, ARFA; de Oliveira, ALM; Neatu, F; Kuncser, AC; Galca, AC; Rostas, AM; dos Santos, IMG

Published: MAR 24 2023, ACS APPLIED NANO MATERIALS, 6, DOI: 10.1021/acsanm.2c05577

Sodium hexatitanate (Na2Ti6O13) nanoparticles have been synthesized by the hydrothermal method with microwave and conven-tional heating, after which their photocatalytic properties toward an azo dye pollutant degradation have been investigated. Insights into the dynamics and reactivity of the species involved in the photocatalytic mechanism of the (Na2Ti6O13) samples were precisely investigated, for the first time, by X-band electron paramagnetic resonance (EPR) spectroscopy under different experimental conditions. X-ray diffraction structural analysis revealed that all samples crystallized in a monoclinic C2/m structure, with different short-range structural order according to the employed heating, as indicated by Raman. Field-emission scanning electron microscopy and transmission electron microscopy results revealed the formation of rod-and fiber-like nanoparticles with different diameters and lengths. EPR measurements indicated the presence of different Ti3+ point defects and F centers in the samples. X-ray photoelectron spectroscopy analysis proved the presence of oxygen-related defects, but no Ti3+ was detected on the surface. Spin trapping experiments monitored the generation of hydroxyl (OH center dot) radicals over UV-irradiation time. Various parameters contribute to the photocatalytic activity of the samples; however, the type of defect and particle morphology appeared as key factors for enhanced efficiency. Our study provides significant information about paramagnetic defects in Na2Ti6O13 materials and their role in photocatalysis to design other Ti-based photocatalysts.
4. Understanding the Photocatalytic Activity of Sodium Hexatitanate Nanoparticles for Pollutants Degradation: A Spectroscopic Insight
Authors: Teixeira, ARFA; de Oliveira, ALM; Neatu, F; Kuncser, AC; Galca, AC; Rostas, AM; dos Santos, IMG

Published: MAR 2023, ACS APPLIED NANO MATERIALS, DOI: 10.1021/acsanm.2c05577

Sodium hexatitanate (Na2Ti6O13) nanoparticles have been synthesized by the hydrothermal method with microwave and conven-tional heating, after which their photocatalytic properties toward an azo dye pollutant degradation have been investigated. Insights into the dynamics and reactivity of the species involved in the photocatalytic mechanism of the (Na2Ti6O13) samples were precisely investigated, for the first time, by X-band electron paramagnetic resonance (EPR) spectroscopy under different experimental conditions. X-ray diffraction structural analysis revealed that all samples crystallized in a monoclinic C2/m structure, with different short-range structural order according to the employed heating, as indicated by Raman. Field-emission scanning electron microscopy and transmission electron microscopy results revealed the formation of rod-and fiber-like nanoparticles with different diameters and lengths. EPR measurements indicated the presence of different Ti3+ point defects and F centers in the samples. X-ray photoelectron spectroscopy analysis proved the presence of oxygen-related defects, but no Ti3+ was detected on the surface. Spin trapping experiments monitored the generation of hydroxyl (OH center dot) radicals over UV-irradiation time. Various parameters contribute to the photocatalytic activity of the samples; however, the type of defect and particle morphology appeared as key factors for enhanced efficiency. Our study provides significant information about paramagnetic defects in Na2Ti6O13 materials and their role in photocatalysis to design other Ti-based photocatalysts.
5. Influence of Ferroelectric Filler Size and Clustering on the Electrical Properties of (Ag-BaTiO3)-PVDF Sub-Percolative Hybrid Composites
Authors: Padurariu, L; Horchidan, N; Ciomaga, CE; Curecheriu, LP; Lukacs, VA; Stirbu, RS; Stoian, G; Botea, M; Florea, M; Maraloiu, VA; Pintilie, L; Rotaru, A; Mitoseriu, L

Published: 2023 JAN 18 2023, ACS APPLIED MATERIALS & INTERFACES, DOI: 10.1021/acsami.2c15641

The paper presents a study concerning the role of ferroelectric filler size and clustering in the dielectric properties of 20%BaTiO3-80%PVDF and of 20% (2%Ag-98%BaTiO3)-PVDF hybrid nanocomposites. By finite element calculations, it was shown that using fillers with epsilon > 103 does not provide a permittivity rise in the composites and the effective dielectric constant tends to saturate to specific values determined by the filler size and agglomeration degree. Irrespective of the ferroelectric filler sizes, the addition of metallic ultrafine nanoparticles (Ag) results in permittivity intensification and the effect is even stronger if the metallic nanoparticles are connected to a higher degree with the ferroelectric particles' surfaces. When using coarse ferroelectric fillers, the probability of clustering is higher, thus favoring the permittivity increase by field concentration in small regions close to the interfaces separating dissimilar materials. The modeling results were validated by an experimental dielectric analysis performed in a series of PVDF-based thick films with the same amount of BaTiO3 fillers or with Ag-BaTiO3 hybrid fillers. Similar trends as predicted by simulations were found experimentally but with slightly higher permittivity values which were assigned to the modifications of the polymer phase composition due to the presence of nanofillers and the local sample inhomogeneity (the presence of clustering, in particular for coarse BaTiO3 grains), which create regions with enhanced local fields.
6. A comparative overview of MXenes and metal oxides as cocatalysts in clean energy production through photocatalysis
Authors: Nair, MM; Iacoban, AC; Neatu, F; Florea, M; Neatu, S

Published: JAN 2023, JOURNAL OF MATERIALS CHEMISTRY A, DOI: 10.1039/d2ta08983a

Photocatalytic conversion of H2O, CO2 and N-2 represents one promising approach to harvest and store solar energy, for which efficient visible light responsive semiconductors are inevitable. Often, the presence of a small amount of an additional component called a "cocatalyst", is required to synergistically enhance the performance of the photocatalyst. Tremendous efforts were made in the past to identify inexpensive materials to be used as cocatalysts, for which metal oxides (MOs) are one of the traditional choices. Among alternative categories of materials investigated, the recently discovered MXenes display enormous potential owing to their unique 2D layered structure, tuneable composition, abundant surface functionalities and superior electronic conductivity. Specifically, MOs and MXenes encompass a variety of distinct as well as analogous characteristics that allows them to be tailored to different extents. Unfortunately, a comprehensive overview covering the synthetic, structural and photocatalytic aspects of MOs and MXenes is not available as of now. Herein, we intend to summarize the progress achieved so far in these two families of materials to be used as cocatalysts for the photoconversion of H2O, CO2 and N-2. Followed by a general introduction, we briefly outline the fundamental principles and the role of cocatalysts in photocatalytic reactions. A discussion regarding the use of MOs and MXenes as cocatalysts for the conversion of H2O, CO2 and N-2 is then provided in separate sections. Critical assessment regarding structure and morphology control, surface properties and stability concerns can not only help to recognize the challenges that limit further advancement, but can also highlight the future research directions of these materials for the effective transformation of H2O, CO2 and N-2.
7. Influence of Ferroelectric Filler Size and Clustering on the Electrical Properties of (Ag-BaTiO3)-PVDF Sub-Percolative Hybrid Composites
Authors: Padurariu, L; Horchidan, N; Ciomaga, CE; Curecheriu, LP; Lukacs, VA; Stirbu, RS; Stoian, G; Botea, M; Florea, M; Maraloiu, VA; Pintilie, L; Rotaru, A; Mitoseriu, L

Published: JAN 2023, ACS APPLIED MATERIALS & INTERFACES, DOI: 10.1021/acsami.2c15641

The paper presents a study concerning the role of ferroelectric filler size and clustering in the dielectric properties of 20%BaTiO3-80%PVDF and of 20% (2%Ag-98%BaTiO3)-PVDF hybrid nanocomposites. By finite element calculations, it was shown that using fillers with epsilon > 103 does not provide a permittivity rise in the composites and the effective dielectric constant tends to saturate to specific values determined by the filler size and agglomeration degree. Irrespective of the ferroelectric filler sizes, the addition of metallic ultrafine nanoparticles (Ag) results in permittivity intensification and the effect is even stronger if the metallic nanoparticles are connected to a higher degree with the ferroelectric particles' surfaces. When using coarse ferroelectric fillers, the probability of clustering is higher, thus favoring the permittivity increase by field concentration in small regions close to the interfaces separating dissimilar materials. The modeling results were validated by an experimental dielectric analysis performed in a series of PVDF-based thick films with the same amount of BaTiO3 fillers or with Ag-BaTiO3 hybrid fillers. Similar trends as predicted by simulations were found experimentally but with slightly higher permittivity values which were assigned to the modifications of the polymer phase composition due to the presence of nanofillers and the local sample inhomogeneity (the presence of clustering, in particular for coarse BaTiO3 grains), which create regions with enhanced local fields.
8. A comparative overview of MXenes and metal oxides as cocatalysts in clean energy production through photocatalysis
Authors: Nair, MM; Iacoban, AC; Neatu, F; Florea, M; Neatu, S

Published: 2023 JAN 16 2023, JOURNAL OF MATERIALS CHEMISTRY A, DOI: 10.1039/d2ta08983a

Photocatalytic conversion of H2O, CO2 and N-2 represents one promising approach to harvest and store solar energy, for which efficient visible light responsive semiconductors are inevitable. Often, the presence of a small amount of an additional component called a "cocatalyst", is required to synergistically enhance the performance of the photocatalyst. Tremendous efforts were made in the past to identify inexpensive materials to be used as cocatalysts, for which metal oxides (MOs) are one of the traditional choices. Among alternative categories of materials investigated, the recently discovered MXenes display enormous potential owing to their unique 2D layered structure, tuneable composition, abundant surface functionalities and superior electronic conductivity. Specifically, MOs and MXenes encompass a variety of distinct as well as analogous characteristics that allows them to be tailored to different extents. Unfortunately, a comprehensive overview covering the synthetic, structural and photocatalytic aspects of MOs and MXenes is not available as of now. Herein, we intend to summarize the progress achieved so far in these two families of materials to be used as cocatalysts for the photoconversion of H2O, CO2 and N-2. Followed by a general introduction, we briefly outline the fundamental principles and the role of cocatalysts in photocatalytic reactions. A discussion regarding the use of MOs and MXenes as cocatalysts for the conversion of H2O, CO2 and N-2 is then provided in separate sections. Critical assessment regarding structure and morphology control, surface properties and stability concerns can not only help to recognize the challenges that limit further advancement, but can also highlight the future research directions of these materials for the effective transformation of H2O, CO2 and N-2.
9. Tuning the acidity by addition of transition metal to Mn modified hollow silica spheres and their catalytic activity in ethanol dehydration to ethylene
Authors: Florea, M; Bocirnea, A; Neatu, S; Kuncser, AM; Trandafir, MM; Neatu, F

Published: SEP 25 2022, APPLIED CATALYSIS A-GENERAL, 646, 118860, DOI: 10.1016/j.apcata.2022.118860

Due to the currently worldwide petrochemical feedstock shortage, the ethylene synthesis from renewable non-oil sources becomes of high interest. The catalysts were prepared in two steps: (i) formation of spheres containing the carbon-coated Mn core by hydrothermal method, (ii) formation of the Si-Zr oxide shell by sol-gel method. The prepared catalysts were characterized by N2 physisorption, SEM-EDX, XRD, XPS, and NH3-TPD. The catalytic results have shown that Fe, Zn or Ni modified Mn core exhibited superior activity compared to the catalysts containing only Mn in the core. With 75% yield and 98% ethanol conversion at 350 degrees C and WHSV of 1.4 h-1, MnNi@SiZr was the best catalyst. These results are due to an increased number of acid sites compared to the other materials and an optimal ratio of weak/medium acid sites. Our findings suggest new lines for developing active and stable catalysts for ethylene synthesis from ethanol.
10. Electrocatalytic Properties of Mixed-Oxide-Containing Composite-Supported Platinum for Polymer Electrolyte Membrane (PEM) Fuel Cells
Authors: Ayyubov, I; Talas, E; Salmanzade, K; Kuncser, A; Paszti, Z; Neatu, S; Mirea, AG; Florea, M; Tompos, A; Borbath, I

Published: MAY 2022, MATERIALS, 15, 3671, DOI: 10.3390/ma15103671

TiO2-based mixed oxide-carbon composite supports have been suggested to provide enhanced stability for platinum (Pt) electrocatalysts in polymer electrolyte membrane (PEM) fuel cells. The addition of molybdenum (Mo) to the mixed oxide is known to increase the CO tolerance of the electrocatalyst. In this work Pt catalysts, supported on Ti1-xMoxO2-C composites with a 25/75 oxide/carbon mass ratio and prepared from different carbon materials (C: Vulcan XC-72, unmodified and functionalized Black Pearls 2000), were compared in the hydrogen oxidation reaction (HOR) and in the oxygen reduction reaction (ORR) with a commercial Pt/C reference catalyst in order to assess the influence of the support on the electrocatalytic behavior. Our aim was to perform electrochemical studies in preparation for fuel cell tests. The ORR kinetic parameters from the Koutecky-Levich plot suggested a four-electron transfer per oxygen molecule, resulting in H2O. The similarity between the Tafel slopes suggested the same reaction mechanism for electrocatalysts supported by these composites. The HOR activity of the composite-supported electrocatalysts was independent of the type of carbonaceous material. A noticeable difference in the stability of the catalysts appeared only after 5000 polarization cycles; the Black Pearl-containing sample showed the highest stability.

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