Synthesis, structure, characterization and photoelectrochemical application of pseudobrookite Fe2TiO5, thick layers

[Z. Ž. Vasiljević]

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Tehnološko inženjerstvo - Inženjerstvo materijala / Technological Engineering - Materials Engineering

U poređenju sa fotonaponskim ćelijama, fotoelektrohemijske ćelije predstavljaju efikasan vid pretvaranja sunčeve energije u električnu, jer se pored energije dobija i vodonik koji se može iskoristiti kao gorivo. Fotoelektrohemijske ćelije sastoje se od poluprovodničke anode (p- ili n-tipa) koja apsorbuje svetlost, pri čemu se konverzija energije odigrava na granici faza poluprovodnik-elektrolit. U ovoj doktorskoj disertaciji primenjena su tri istraživačka pravca za dobijanje filmova u svojstvu fotoaktivnih anoda. Prvi istraživački pravac bio je priprema nanočestičnog poluprovodnika - pseudobrukita, Fe2TiO5, reakcijom u čvrstom stanju između anatasa, TiO2 i hematita, α-Fe2O3. Analiza dobijanja najpre je praćena na tabletama radi optimizacije sastava, temperature i vremena sinterovanja. Ispitan je uticaj promene temperature sinterovanja (750 – 1250 C) na fazni sastav, temperaturu faznog prelaza, morfologiju, kao i električna svojstva sinterovanih uzoraka. Izabrane su dve oksidne smeše  maseni odnos prahova 40%α-Fe2O3/60%TiO2 (4F6T) i 60%α-Fe2O3/40%TiO2 (6F4T) što približno odgovara molskom odnosu početnih prahova hematita i anatasa od 1:3 i 1,5:2, sa ciljem da se dobije pseudobrukit i analizira uticaj viška anatasa. Uzorci sinterovani na temperaturi 850 C sadržali su rutil i pseudobrukit sa rombičnom strukturom čime je potvrđeno da gvožđe ubrzava faznu transformaciju anatasa u rutil pri čemu se formira i pseudobrukitna faza. Dalje povišenje temperature uticalo je na povećanje gustine uzoraka, promene u veličini zrna i smanjenje električne otpornosti. Drugi istraživački pravac bio je priprema elektrodnih filmova sitoštampanjem pasti koje su dobijene mešanjem homogenizovanih prahova sa organskom smolom (butil celuloza), rastvaračem (terpinol) i malom količinom veziva – ROSiO2B2O3 staklo (RO-oksid retkih zemalja). Dobijene paste deponovane su sitoštampom na supstrat od alumine i potom sinterovane u hibridnoj peći 60 minuta u temperaturnom opsegu 800 – 950 C. Rendgenskom difrakcionom analizom utvrđeno je da se na 800 C formira monoklinični pseudobrukit i da je završena fazna transformacija anatasa u rutil, kao i da je dalje povišenje temperature utiče na prelaz monokliničnog u rombični pseudobrukit. Uzorci sinterovani na 850 C imali su malu veličinu zrna sa homogenom strukturom, dok je dalje povišenje temperature uticalo na formiranje pseudobrukita sa većim zrnima štapićastog oblika. Strujno-naponska analiza pokazala je da uzorak 60%α-Fe2O3/40%TiO2 sinterovan na 850 C ima mogućnost za primenu u fotoelektrohemijskim ćelijama...

Compared to photovoltaic cells, photoelectrochemical cells represent an efficient way of converting solar into electrical energy, because besides energy, hydrogen is also available that could be used as a fuel. A photoelectrochemical cell consists of a semiconductor anode (p- or n-type) that absorbs light, whereby energy conversion takes place at the boundary of the semiconductor-electrolyte phase. This doctoral dissertation is divided into three main research directions, related to the preparation of films in the form of photoactive anodes. The first one describes preparation of the nanoparticle semiconductor - pseudobrookite, Fe2TiO5, by a solid state reaction between anatase, TiO2, and hematite, α-Fe2O3. Pseudobrookite formation was first monitored on tablets to optimize the composition, temperature and time of sintering. The influence of the sintering temperature (750 –1250C) on the phase composition, phase transition temperature, morphology and electrical properties of bulk sintered powder mixtures composed of starting anatase and hematite nanopowders was investigated. Two oxide mixtures were first selected - nanopowders of -Fe2O3 (hematite) and TiO2 (anatase 99.7%) were mixed in the weight ratios 40:60 and 60:40, respectively which approximately corresponds to the molar ratios of starting hematite and anatase powders of 1:3 and 1.5:2, with the aim of obtaining pseudobrookite and analyzing the effect of excess anatase. Samples sintered at 850 °C contained rutile and pseudobrookite with an orthorhombic structure, which confirmed that iron accelerates the phase transformation of anatase to rutile, thereby forming a pseudobrookite phase. Higher sintering temperatures lead to increased sample density, changes in grain size and decreased electric resistivity. The second research direction was the preparation of films starting from prepared oxide mixtures with an organic vehicle (butyl cellulose), solvent (terpinol) and a small amount of binding lead boron silicone oxide glass frit (ROSiO2B2O3). Pastes were deposited onto alumina substrate using the screen printing technique and then sintered 60 minutes at 800  950 C. X-ray diffraction analysis determined formation of monoclinic pseudobrookite at 800 C and completion of the phase transformation of anatase into rutile. Further increase in temperature leads to the transition of monoclinic pseudobrookite into orthorhombic pseudobrookite. Samples sintered at 850 C still had a small grain size, with a relatively homogenous distribution, while further increase in temperature lead to the formation of larger rod-shaped grains. Analysis of current voltage measurements of thick film samples sintered at 850 C showed that the 60%α-Fe2O3/40%TiO2 sample had potential for application as a photoanode for light driven water splitting...

gvožđe(III)-oksid, titan(IV)-oksid, gvožđe(III)-titanat, fotoanoda, sitoštampanje, fotoelektrohemijska ćelija

iron(III) oxide, titanium(IV) oxide, iron(III) titanate, photoanode, screen printing, photoelectrochemical cell

66.017:544.6:544.52(043.3)

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