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Title:Bi2O3-BASED PYROCHLORE NANOSTRUCTURES AND THEIR CRYSTALLOGRAPHIC, OPTOELECTRONIC AND PHOTOCATALYTIC PROPERTIES
Authors:Univerza v Novi Gorici (Authorship owner)
Benčina, Metka (Author)
Valant, Matjaž (Mentor) More about this mentor... New window
Files:.pdf Thesis_Bencina_22.pdf (7,29 MB)
 
Language:English
Work type:Doctoral dissertation (mb31)
Tipology:2.08 - Doctoral Dissertation
Organization:FPŠ - Graduate School
Abstract:Bi2O3-based pyrochlore nanomaterials and their photocatalytic, optoelectronic and crystallographic properties are the research topics of this doctoral dissertation. We synthesized these materials at nanoscale, since they are expected to possess photoactivity in the visible-light spectrum, which is an important target for the high-performance photocatalysts. To prepare nanoparticles that are not agglomerated, different synthesis methods (coprecipitation, coprecipitation followed by the digestion, reverse micelle) and post-treatments (furnace or hydrothermal reactor) were examined. Micrometer size materials were prepared with the solid-state reaction and annealing at high temperature. Bi2Ti2O7 nanoparticles were synthesized with the coprecipitation reaction, followed by annealing at 570 °C in the furnace or in the hydrothermal reactor at 230 °C for 18h in NH4OH medium. UV-Vis diffuse reflectance spectra of the Bi2Ti2O7 samples showed that they exhibit the maximum absorption edge at ~420-440 nm. The UV-Vis diffuse reflectance measurements of Bi2Ti2O7 loaded with 10 wt. % of Ag as a co-catalyst showed an enhanced absorbance in the visible region, presumably due to a contribution of Local Surface Plasmon Resonance. The Bi2Ti2O7 nanoparticles are not photocatalytically active under the visible light irradiation without assistance of a sacrificial reagent. The photocatalytic activity does not increase even with Ag as a co-catalyst under visible light irradiation. All samples are active under UV light irradiation, especially the sample with Ag that is able to decolourize 7 mg/L of metyl orange in just 3 min of exposure to UV light. However, H2O2 as a sacrificial reagent increases the photocatalytic activity of Bi2Ti2O7 under visible light irradiation. Phase pure pyrochlore Bi1.647Nb1.118Fe1.157O7 (BNF) nanoparticles were synthesized with coprecipitation reaction and further annealing in the furnace at 570 °C for 7h or 15h and in the hydrothermal reactor at 160 °C for 18h in 3 M or 6 M NaOH medium. The BNF materials exhibit shift toward longer wavelengths in absorbance of visible light (up to ~550-650 nm). This result confirmed that incorporation of Fe in the pyrochlore structure decreases the band gap. The BNF nanoparticles annealed for 7h showed intense photoactivity under visible light irradiation in the presence of a sacrificial reagent. The improved photoactivity, much better than with BNF, was shown for the composite nanoparticles BiOCl/BNF. They decolourize 100 % of the 7 mg/L methyl orange within only 2h of the visible light irradiation in the presence of H2O2. Phase pure pyrochlore Bi1.9Te0.58Fe1.52O6.87 (BTF) nanoparticles were synthesised by coprecipitation reaction and further annealing in the furnace at 570 °C for 7h. Although the BTF nanoparticles include higher amount of Fe than BNF nanoparticles and exhibit narrower band gap, the photoactivity of these nanoparticles is a bit lower than that of BNF nanoparticles. We assume that Fe at some critical concentration causes formation of mid-band states, which act as recombination centers. The same as BiOCl/BNF, the BiOCl/BTF composite is able to decolourize 100 % of the 7 mg/L MO within 2h of the visible light irradiation in the presence of H2O2. Based on the band gap determination and estimation of valence and conduction band levels, the proposed photodecolourization mechanisms are discussed. The conduction band levels of all synthesized materials lie more positively than O2/O2● redox potential (vs. negative hydrogen electrode potential), but are more close to H2O2/●OH redox potential. Thus, we assume that the photogenerated electrons form ●OH radicals, the species that induce dye decolourization under visible light irradiation. We believe that better performance of composite materials (BiOCl/BNF, BiOCl/BTF) is due to the formation of heterojunction, which facilitates the electron transfer between semiconductors, and O2 vacancy states formation in the BiOCl material.
Keywords:nanoparticles, pyrochlore structure, photocatalysis, visible light
Year of publishing:2015
COBISS_ID:3943419 Link is opened in a new window
URN:URN:SI:UNG:REP:FH5H97TU
Views:4063
Downloads:97
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Secondary language

Language:Slovenian
Title:PIROKLORNE NANOSTRUKTURE NA OSNOVI Bi2O3 IN NJIHOVE KRISTALOGRAFSKE, OPTOELEKTRONSKE IN FOTOKATALITIČNE LASTNOSTI
Abstract:V disertaciji so predstavljene fotokatalitske, optoelektronske in kristalografske lastnosti piroklornih nanomaterialov na osnovi Bi2O3. Te materiale smo sintetizirali na nanonivoju, saj se pričakuje, da so nanodelci fotoaktivni v vidnem spektru svetlobe, kar je pomemben cilj za visoko zmogljive fotokatalizatorje. Z namenom priprave neaglomeriranih nanodelcev smo uporabili različne metode sinteze (soobarjanje, soobarjanje z refluksom in metoda reverznih micel) in termične obdelave (peč ali hidrotermalni reaktor). Materiale mikrometerskih velikosti smo pripravili z reakcijo v trdnem in sintranju pri visokih temperaturah. Bi2Ti2O7 nanodelce smo pripravili z reakcijo soobarjanja, čemur je sledilo žganje pri temperaturi 570 °C v peči ali pri temperaturi 230 °C v hidrotermalnem reaktorju v NH4OH mediju. Analiza z UV-Vis difuzivno reflektančno spektrometrijo je pokazala, da imajo vzorci rob maksimalne absorpcije pri ~420-440 nm. Vzorec Bi2Ti2O7 z naloženim Ag (10 ut. %), ki deluje kot ko-katalizator, je pokazal povečano absorpcijo v vidnem delu spektra. Sklepamo, da zaradi lokalizirane površinske plazmonske resonance. Ugotovili smo, da Bi2Ti2O7 nanodelci niso fotoaktivni pod vidno svetlobo brez uporabe H2O2. Tudi ob uporabi Ag kot ko-katalizatorja se fotokatalitska aktivnost delcev ne poveča. Nasprotno so vsi Bi2Ti2O7 vzorci fotoaktivni pod UV svetlobo, posebej vzorec z Ag na površini, ki razbarva metil oranž s koncentracijo 7 mg/L v samo treh minutah. Pod vidno svetlobo je Bi2Ti2O7 material fotoaktiven samo ob dodatku H2O2. Čisto-fazne Bi1.647Nb1.118Fe1.157O7 (BNF) nanodelce smo sintetizirali z reakcijo soobarjanja in žganjem na 570 °C - 7h ali 15h in v hidrotermalnem reaktorju na 160 °C - 18h v 3 M ali 6 M NaOH mediju. Za vse BNF materiale smo opazili rdeči premik absorbance (absorpcijski rob pri ~550-650 nm). Ta rezultat je potrdil vpliv Fe na zmanjšanje prepovedanega pasu. BNF nanodelci, žgani 7h, so pokazali intenzivno fotokatalitsko aktivnost pod vidno svetlobo v prisotnosti H2O2. Še večjo učinkovitost fotoaktivnosti smo dosegli s kompozitnim materialom BiOCl/BNF. Ta material razbarva metil oranž s koncentracijo 7 mg/L v dveh urah osvetljevanja z vidno svetlobo v prisotnosti H2O2. Čisto-fazne Bi1.9Te0.58Fe1.52O6.87 (BTF) nanodelce smo sintetizirali z reakcijo soobarjanja in žganjem na 570 °C - 7h. Čeprav BTF nanodelci vsebujejo večjo koncentracijo Fe kot BNF nanodelci in imajo zato manjši prepovedan pas, je fotoaktivnost teh delcev nižja. Predvidevamo, da Fe povzroči nastanek vmesnih stanj, med valenčnim in prevodnim pasom, ki delujejo kot rekombinacijski centri. Tako kot BiOCl/BNF, tudi BiOCl/BTF kompozit razbarva metil oranž s koncentracijo 7 mg/L v dveh urah osvetljevanja z vidno svetlobo v prisotnosti H2O2. Na podlagi določenega prepovedanega pasu in izračunanih nivojev valenčnih in prevodnih pasov v polprevodnikih smo predlagali mehanizme razbarvanja barvila. Prevodni pasovi sintetiziranih materialov ležijo bolj pozitivno kot nivo O2/O2● redoks potenciala, vendar bliže nivoju H2O2/●OH redoks potenciala. Torej predvidevamo, da foto-generirani elektroni povzročijo nastanek ●OH radikalov, ki so odgovorni za razbarvanje barvila pod vidno svetlobo. Menimo, da sta kompozitna materiala (BiOCl/BNF in BiOCl/BTF) fotokatalitsko učinkovitejša zaradi nastanka p-n spoja, ki pospeši prenos elektronov med polprevodnikoma in nastanka kisikovih vrzeli v BiOCl materialu.
Keywords:nanodelci, piroklorna struktura, fotokataliza, vidna svetloba


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