Istvan Szekely
Verified @yahoo.com
Faculty of Physics
Babeș-Bolyai University
RESEARCH INTERESTS
Heterogenous Photocatalysis, Semiconductors, Physical Chemistry, Water treatment, Polllutant removal, SERS material
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Endre-Zsolt Kedves, Claudiu Fodor, Ákos Fazekas, István Székely, Ákos Szamosvölgyi, András Sápi, Zoltán Kónya, Lucian Cristian Pop, Lucian Baia, and Zsolt Pap
Elsevier BV
István Székely, Zoltán Kovács, Mihai Rusu, Tamás Gyulavári, Milica Todea, Monica Focșan, Monica Baia, and Zsolt Pap
MDPI AG
Developing highly efficient Au/TiO2/WO3 heterostructures with applications in heterogeneous photocatalysis (photocatalytic degradation) and surface-enhanced Raman spectroscopy (dye detection) is currently of paramount significance. Au/TiO2/WO3 heterostructures were obtained via heat or time-assisted synthesis routes developed by slightly modifying the Turkevich–Frens synthesis methods and were investigated by TEM, SEM, XRD, Raman spectroscopy, XPS, photoluminescence, and UV–vis DRS techniques. Structural features, such as WO3 crystalline phases, TiO2 surface defects, as well as the WO3 (220) to TiO2-A (101) ratio, were the key parameters needed to obtain heterostructures with enhanced photocatalytic activity for removing oxalic acid, phenol, methyl orange, and aspirin. Photodegradation efficiencies of 95.9 and 96.9% for oxalic acid; above 96% (except one composite) for phenol; 90.1 and 97.9% for methyl orange; and 81.6 and 82.1% for aspirin were obtained. By employing the SERS technique, the detection limit of crystal violet dye, depending on the heterostructure, was found to be between 10−7–10−8 M. The most promising composite was Au/TiO2/WO3-HW-TA it yielded conversion rates of 82.1, 95.9 and 96.8% for aspirin, oxalic acid, and phenol, respectively, and its detection limit for crystal violet was 10−8 M. Au/TiO2/WO3-NWH-HA achieved 90.1, 96.6 and 99.0% degradation efficiency for methyl orange, oxalic acid, and phenol, respectively, whereas its limit of detection was 10−7 M. The Au/TiO2/WO3 heterojunctions exhibited excellent stability as SERS substrates, yielding strong-intensity Raman signals of the pollutant molecules even after a long period of time.
Bíborka Boga, Vasile-Mircea Cristea, István Székely, Felix Lorenz, Tamás Gyulavári, Lucian Cristian Pop, Lucian Baia, Zsolt Pap, Norbert Steinfeldt, and Jennifer Strunk
Elsevier BV
Alin Grig Mihis, Liviu Cosmin Cotet, Calin Cadar, Lucian Cristian Pop, Milica Todea, Mihai Marius Rusu, Adriana Vulpoi, István Székely, Cătălin Alexandru Sălăgean, Klara Magyari,et al.
Springer Science and Business Media LLC
Zsejke-Réka Tóth, Diána Debreczeni, Tamás Gyulavári, István Székely, Milica Todea, Gábor Kovács, Monica Focșan, Klara Magyari, Lucian Baia, Zsolt Pap,et al.
MDPI AG
The widespread use of Ag3PO4 is not surprising when considering its higher photostability compared to other silver-based materials. The present work deals with the facile precipitation method of silver phosphate. The effects of four different phosphate sources (H3PO4, NaH2PO4, Na2HPO4, Na3PO4·12 H2O) and two different initial concentrations (0.1 M and 0.2 M) were investigated. As the basicity of different phosphate sources influences the purity of Ag3PO4, different products were obtained. Using H3PO4 did not lead to the formation of Ag3PO4, while applying NaH2PO4 resulted in Ag3PO4 and a low amount of pyrophosphate. The morphological and structural properties of the obtained samples were studied by X-ray diffractometry, diffuse reflectance spectroscopy, scanning electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy. The photocatalytic activity of the materials and the corresponding reaction kinetics were evaluated by the degradation of methyl orange (MO) under visible light. Their stability was investigated by reusability tests, photoluminescence measurements, and the recharacterization after degradation. The effect of as-deposited Ag nanoparticles was also highlighted on the photostability and the reusability of Ag3PO4. Although the deposited Ag nanoparticles suppressed the formation of holes and reduced the degradation of methyl orange, they did not reduce the performance of the photocatalyst.
Klára Magyari, Alexandra Dreancă, István Székely, Andra Popescu, Alexandra Feraru, Emőke Páll, Tamás Gyulavári, Maria Suciu, Mihai Cenariu, Emma Bobu,et al.
Springer Science and Business Media LLC
Bíborka Boga, István Székely, Monica Focșan, Monica Baia, Tibor Szabó, László Nagy, and Zsolt Pap
Elsevier BV
Nikita Sharma, Zsolt Pap, István Székely, Monica Focsan, Gábor Karacs, Zoltan Nemeth, Seema Garg, and Klara Hernadi
Elsevier BV
István Székely, Endre-Zsolt Kedves, Zsolt Pap, and Monica Baia
MDPI AG
The design of a semiconductor or a composite semiconductor system—with applications in materials science—is complex because its morphology and structure depend on several parameters. These parameters are the precursor type, solvent, pH of the solution, synthesis approach, or shaping agents. This study gives meaningful insight regarding the synthesis design of such WO3 materials. By systematically alternating the precursor (sodium tungstate dihydrate—NWH, or ammonium tungstate hydrate—AMT), subsequently shaping the agents (halide salts—NaX, KX, or hydrohalic acids—HX; X = F−, Cl−, Br−, I−), we have obtained WO3 semiconductors by hydrothermal treatment, which in composite systems can enhance the commercial TiO2 photocatalytic activity. We investigated three sample series: WO3-NWH-NaX/WO3-NWH-KX and, subsequently, WO3-AMT-HX. The presence of W+5 centers was evidenced by Raman and X-ray photoelectron spectroscopy. W+5 and W+6 species affected the band gap values of the NaX and KX series; a higher percentage of W+5 and, subsequently, W+6 caused a redshift, while, regarding the HX series, it led to a blue shift. Increased electronegativity of the halide anions has an unfavorable effect on the composites’ photoactivity. In contrast, in the case of hydrohalic acids, it had a positive impact.
István Székely, Monica Baia, Klára Magyari, Bíborka Boga, and Zsolt Pap
Elsevier BV
Abstract In the present work novel, ternary semiconductor composites were prepared by applying the pH adjustment method (the WO3 based components were obtained separately, while TiO2 was a commercial one), where the pH was fine-tuned to 1, 4, 7 and 10. The composition of the prepared samples was in each case 76% commercial TiO2 (Evonik Aeroxide P25), 12% monoclinic (WO3) and 12% hexagonal partial hydrate (WO3·0.33H2O). All the composites showed improved UV light-driven photocatalytic activity compared to P25 in the degradation of methyl orange, oxalic acid and activity inhibition was observed in the case of phenol degradation. SEM micrographs and the XRD patterns of the samples proved that all three components were successfully integrated into the composite system. DRS spectra of the samples provided evidence concerning a mutual interaction of the composite components modifying their electronic structure, while the effect of the synthesis pH was also visible in the IR spectra. Raman Spectroscopy proved the presence of surface defects, manifested through the presence of (W5+ centers). Moreover, interesting correlations were found between several Raman peak intensity ratios (I324/I714; I943/I805) and the photoactivity pointing the importance of the W5+ centers and amorphous WO3 formed under basic conditions.
Biborka Boga, István Székely, Zsolt Pap, Lucian Baia, and Monica Baia
Hindawi Limited
WO3-TiO2 composite materials were obtained using commercial titania (Evonik Aeroxide P25) and hydrothermally crystallized WO3. Different ratios of TiO2/WO3 were investigated, starting at 1 wt.% of WO3 to 50 wt.%. The morphology of WO3 was of the star-like type, and its structure is basically composed of monoclinic crystalline phase. All spectroscopic characteristics of the composites and their derived data (band-gap energy value, light absorption threshold, and IR specific bands) directly varied with the increase of the WO3 content. However, the oxalic acid photodegradation achieved under UV light reached the highest yield for 24 wt.% WO3 content, a result that was attributed to the charge separation efficiency and the surface hydrophilicity. The latter mentioned reason points out the crucial importance of the surface quality of the investigated structure in photocatalytic tests.
Lucian Baia, Eszter Orbán, Szilvia Fodor, Boglárka Hampel, Endre Zsolt Kedves, Kata Saszet, István Székely, Éva Karácsonyi, Balázs Réti, Péter Berki,et al.
Elsevier BV
Abstract The present research focused on the preparation of TiO2/WO3 (76/24 wt. %) composite photocatalysts for the photodegradation of methyl orange under UV light. The composites were prepared by the adjustment of the surface charge of the individual semiconductors, achieving a homogeneous distribution of the components within the composite. This resulted in a homogeneous distribution of the components within the composite. The individual WO3 and TiO2 nanocrystals were obtained either by sol–gel or by hydrothermal synthesis. The produced nanomaterials were characterized by using TEM, SEM, XRD and DRS. Major differences in their photocatalytic performance depended on the nature of the precursors and the synthesis pathway.
István Székely, Gábor Kovács, Lucian Baia, Virginia Danciu, and Zsolt Pap
MDPI AG
A traditional semiconductor (WO3) was synthesized from different precursors via hydrothermal crystallization targeting the achievement of three different crystal shapes (nanoplates, nanorods and nanostars). The obtained WO3 microcrystals were analyzed by the means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS). These methods contributed to the detailed analysis of the crystal morphology and structural features. The synthesized bare WO3 photocatalysts were totally inactive, while the P25/WO3 composites were efficient under UV light radiation. Furthermore, the maximum achieved activity was even higher than the bare P25’s photocatalytic performance. A correlation was established between the shape of the WO3 crystallites and the observed photocatalytic activity registered during the degradation of different substrates by using P25/WO3 composites.