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Title: Correlations between photocatalytic activity and chemical structure of Cu-modified TiO2-SiO2 nanoparticle composites Authors: ID Čižmar, T. (Copyright holder) ID Čižmar, T., UNG (Author) Files: Dissertation_Tihana_Cizmar.pdf (3,05 MB) MD5: B7885E27BC3E182977715A88ECEAE8DB   Language: English Work type: Doctoral dissertation Typology: 2.08 - Doctoral Dissertation Organization: FPŠ - Graduate School Abstract: The objective of this dissertation was to examine how copper modification can improve the photocatalytic activity of TiO2-SiO2 and to explain the correlation between Cu concentration and chemical state of Cu cations in the TiO2-SiO2 matrix, as well as the photocatalytic activity under the UV/solar irradiation. The Cu-modified TiO2-SiO2 photocatalysts were prepared by a low temperature sol-gel method based on organic copper, silicon and titanium precursors with varied Cu concentrations (from 0.05 to 3 mol%). The sol-gels were dried at 150 °C to obtain the photocatalysts in the powder form. To test thermal stability, additional set of photocatalysts was obtained by calcinating dried samples in air at 500 °C for 1 h. The photocatalytic activity was determined by a fluorescence-based method of terephthalic acid decomposition. Up to three times increase in photocatalytic activity of air-dried samples is obtained when TiO2-SiO2 matrix is modified with Cu in a narrow concentration range from 0.05 to 0.1 mol%. At higher Cu loadings the photocatalytic activity of Cu-modified photocatalyst is smaller than in the unmodified reference TiO2-SiO2 photocatalyst. Calcined samples showed significantly reduced photocatalytic activity compared to air-dried samples. XRD analysis showed that all Cu-modified TiO2-SiO2 nanocomposites with different Cu concentrations have the same crystalline structure as unmodified TiO2-SiO2 nanocomposites (air-dried or calcined). The addition of Cu does not change the relative ratio between the anatase and brookite phase or unit cell parameters of the two TiO2 crystalline structures. TEM analysis showed that the addition of Cu does not change the morphology of TiO2-SiO2 catalyst dried at 150 °C. The Cu K-edge XANES and EXAFS analysis were used to determine valence state and local structure of Cu cations in Cu-modified TiO2-SiO2 photocatalyst. The results elucidate the mechanism responsible for the improved or hindered photocatalytic activity. In the air-dried samples with low Cu content, which exhibit largest activity, Cu-O-Ti connections are formed, suggesting that the activity enhancement is due to Cu(II) cations attachment on the surface of the photocatalytically active TiO2 nanoparticles, so Cu(II) cations may act as free electron traps, reducing the intensity of recombination between electrons and holes at the TiO2 photocatalyst’s surface. At higher Cu loadings no additional Cu-O-Ti connections are formed, instead only Cu-O-Cu connections are established, indicating the formation of amorphous or nanocrystalline Cu(II) oxide, which hinders the photocatalytic activity of TiO2. Calcination of Cu-modified TiO2-SiO2 photocatalysts at 500 °C induces significant structural changes: Cu-O-Ti connections are lost, Cu partially incorporates into the SiO2 matrix and amorphous copper oxides, which again reduce the photocatalytic activity of the material, are formed. Keywords: titanium dioxide, Cu-modified TiO2-SiO2 photocatalyst, photocatalytic activity, Cu K-edge XANES, EXAFS. Publication status: Published PID: 20.500.12556/RUNG-4305-37d32d2b-4f2f-e041-16ad-d38498d72d39  COBISS.SI-ID: 5294331  NUK URN: URN:SI:UNG:REP:PRTWMEJY Publication date in RUNG: 17.12.2018 Views: 5746 Downloads: 159 Metadata:    Cite this work Plain text BibTeX EndNote XML EndNote/Refer RIS ABNT ACM Ref AMA APA Chicago 17th Author-Date Harvard IEEE ISO 690 MLA Vancouver : Copy citation

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Secondary language

Language:	Slovenian
Title:	Korelacije med fotokatalitsko aktivnostjo in kemijsko strukturo kompozita nanodelcev TiO2-SiO2, modificiranih z bakrom
Abstract:	Cilj disertacije je raziskati, kako modifikacije z bakrom lahko izboljšajo fotokatalitsko aktivnost nanokompozita TiO2-SiO2, in pojasniti korelacije med koncentracijo, kemijskim stanjem Cu kationov v matrici TiO2-SiO2 ter fotokatalitsko aktivnostjo nanokompozita ob prisotnosti UV/sončnega sevanja. Nanokompozitni fotokatalizator TiO2-SiO2 modificiran z bakrom smo pripravili z nizkotemperaturno sol-gel metodo, zuporabo organskih bakrovih, silicijevih in titanovih prekurzorjev. Dodali smo različne koncentracije Cu (med 0.05 in 3 mol%). Sol-gele smo posušili na zraku pri 150 °C, da smo dobili fotokatalizator v obliki praška. Termično stabilnost fotokatalizatorjev smo preverili s kalcinacijo suhih vzorcev na zraku pri 500 °C za 1 h. Fotokatalitsko aktivnost katalizatorjev smo določili z metodo meritev fluorescence ob fotokatalitski razgradnji terftalatskih kislin. Fotokatalitska aktivnostvzorcev sušenih pri 150 °C se poveča do trikrat, ko je TiO2-SiO2 matrica modificirana z bakrom v ozkem razponu koncentracij med 0,05 in 0,1 mol%. Pri višjih koncentracijah bakra je bila aktivnost modificiranih vzorcev nižja od aktivnosti TiO2-SiO2 fotokatalizatorja brez bakra. Po kalcinaciji pri 500 °C se fotokatalitska aktivnost vzorcev bistveno zmanjša v primerjavi z vzorci sušenimi pri 150 °C. XRD analiza je pokazala, da imajo nanokompoziti TiO2-SiO2 modificirani z različnimi koncentracijami bakra enako kristalno strukturo kot nemodificiran TiO2-SiO2 nanokompozit (sušen na zraku ali kalciniran). Dodatek bakra ni spremenil relativnega razmerja med anatasom in brukitom in ni spremenil parametrov kristalne mreže obeh kristalnih strukturTiO2. TEM analiza pokaže, da modifikacija z bakrom ne spremeni morfologije TiO2-SiO2 nanokompozita. Za določanje valenčnega stanja in lokalne structure bakrovih kationov v modificiranih katalizatorjih smo uporabili metodi rentgenske absorpcijske spektroskopije XANES in EXAFS na bakrovem absorbcijskem robu K. Rezultati so razkrili dva mehanizma, ki vodita do izboljšav ali poslabšanj katalitske aktivnosti. V vzorcih, z nizko vsebnostjo bakra, sušenih pri 150 °C, ki so pokazali najvišjo aktivnost, so prisotne vezi Cu-O-Ti, kar nakazuje, da je povečana fotokatalitska aktivnost posledica Cu(II) kationskih povezav na površini fotokatalitsko aktivnih TiO2 nanodelcev. Cu(II) kationi tako delujejo kot proste elektronske pasti, kar zmanjšajo intenziteto rekombinacij med elektroni in vrzelmina površini TiO2 fotokatalizatorja. Pri višjih koncentracijah bakra nismo opazili dodatnih Cu-O-Ti vezi, prisotne so bile zgolj vezi Cu-O-Cu, kar nakazuje na nastanek amorfnih ali nanokristalnih Cu(II) oksidov, ki zmanjšajo fotokatalitsko aktivnost TiO2. Kalcinacija modificiranih katalizatorjev pri 500 °C povzroči znatne strukturne spremembe: Cu-O-Ti povezave izginejo, baker se delno veže v SiO2 matrico in oblikujejo se amorfni bakrovi oksidi, kar zmanjša fotokatalitsko aktivnost materiala.
Keywords:	titanov dioksid, TiO2-SiO2 fotokatalizator modificiran z bakrom, fotokatalitska aktivnost, Cu K-edge XANES, EXAFS.

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