Ambrus, Zoltán

Bajnóczi, Éva Gabriella

Balázs, Nándor

Canton, Sophie E.

Homonnay, Zoltán

Kuzmann, Ernő

Mogyorósi, Károly

Norén, Katarina

Oszkó, Albert

Pálinkó, István

Sipos, Pál Miklós

Srankó, Dávid Ferenc

Vértes, Attila

Bajnóczi Éva Gabriella

Balázs Nándor

Mogyorósi Károly

Srankó Dávid Ferenc

Ambrus Zoltán

Canton Sophie E.

Norén Katarina

Kuzmann Ernő

Vértes Attila

Homonnay Zoltán

Oszkó Albert

Pálinkó István

Sipos Pál Miklós

SZTE Publicatio Repozitórium - SZTE - Repository of Publications

il$a*iil,x3+a;;.lll1rjr:+:.,:;tj; .:ls.s sR *ffiffi*ffi 'wwAcnvtrY RrpoRr7.009Eolrro ey["r. JoHANssoN, A. Nygrne, R. [\!ynounSvivcHRornou RnotnloN - BtaulLrrue 181 1The local structure of Fe(III) in doped TiO2 photocatalysts tudied with X-Rayab sorption spectroscopyEva Bajn 6czit, Nfndor Bal6zst, K6roly Mogyorosir, D6vid F. Srank6l, Zoltitn Ambrusl,Sophie E. Canton',Kuturina Nor6n', E.no Kuzmann3, Attila Vdrtes3, ZoltdnHomonnay3,Albert Oszk6a, Istv6n P6linkos, PAI Siposr*t (Jniversity of Szeged, Department of Inorganic and Analytical Chemistry, Szeged, Hungary2 Chemical Physics Department, Chemical Centre, Lund {Jniversity, Lund, Swedent MTA-ELTE Research Group on the Application of Nuclear Techniques in Structural Chemistry, Lorand Ec)tvdsUniversist, Budapes t, Hungarya (Jniversity of Szeged, Department of Physical Chentistryand Materials Science, Szeged, HungaryslJniversity of Szeged, Department of Organic Chemistry, Szeged, Hungary* corresponding autltor, email : s ipos@lchem.tt-szegecl.furSince the discovery of photocatalytic water splitting with sunlightr, the study of titania(TiOz) based semiconductor photocatalysts is one of the most active areas of materialsscience, both on fundamental and on applied levels2-4. A common way of photosensitizingTiOz photocatalyst is doping with metals (".9., transition metals like Fe(III), V(V), Cr(III),Cu(II), Mn(II), Co(II), etc.) or nonmetals (r.9., I in various oxidation states, P(V), N(III),S(VD, etc.). Photocatalyic activity of transition metal doped TiOz catalysts usually passesthrough a maximum with increasing dopant concentration, but excessive amount of dopantusually causes a decrease in the photocatalytic activity. In other cases, metal doping wasreported to cause detrimental effects on the photocatalytic activity even at the smallest dopantlevels. The dopant atoms can be present in various forms: in some cases in separated"islands", either in crystalline or in amorphous forms or dispersed in the atomic level andsubstitute Ti(IV), either on the surface layer or in the bulk of the semiconductor. The actualform is likely to be related to the photoactivity and associated with the variations of the localstructure of the dopant.Recently, a kind of flame-hydrolytic technique has been established in our laboratoriesfor the preparationTiOz photocatalysts with tailor made crystallinity, anatase-to-rutil ratio andsurface properties.' It is based on the introduction of the vapor of an appropriately chosenvolatile Ti(IV) compound into HzlOz flame. The synthesis has been optimized in terms ofmechanical settings and we managed to prepare undoped TiO2 catalysts that are almost twiceas active in photocatalfiic degradation phenol and methanol, as Degussa P-25, which isusually considered to be the rnost efficient commercial photocatalyst.' Fe-doped TiO2 sampleswere also prepared by flame-hydrolytic technique' (F-series hereafter). We found that dopingwith iron has a detrimental effect on the photoactivity of flame-synthesized samples.Cxidative hydrolysis of TiCl:6 was also utilized for preparing undoped or Fe-dopednanocrystalline TiO2 (S-series hereafter). Doping enhanced the photoactivity of the sampleswithin the S-series and it was found tcl pass through a maximum with increasing Fe-content.First, the bulk properties of the products were characterized by standard analyicaltechniques which are conventionally used in photocatalysis (i.u.,powder X-ray diffraction,various microscopic techniques, UV-Vis diffuse reflectance spectroscopy, Nz adsorptionisotherms). From these measurements no difference was found in the bulk properties of themembers of the F- and S-series: their anatase-to-rutile ratio, particle size, band-gap energy,specific surface area and particle molphologies were practically identical. From this weassurned, that the difference in the photoactivity stems from the differences in the chemicalstate of the doping metal. Therefore we embarked on measurements (including XAFS) to304 MAX-lab Acrrvrry Riponr 2009SvncHRornoru Rnorarroru - B:arvrur.rr lB1 1elucidate the local structure of iron in these samples, X-Ray Absorption Spectra were taken atthe beamline IB 1 I,MAX II, Maxlab, Lund. The measurements were taken in fluorescencemode, on the iron K-edge.The pre-edge spectra of the samples in both series show a small octahedral distortionaround the iron. The XANES spectra show a significant variation with the increasing ironcontent in the S series. The main peak at ca. 7130 eV gradually sharpens and a shoulderappears at ca. 7140 eV, and another shoulder disappear at ca. 7200 eV with the increasingiron content. The shoulder at 1140 eV belongs to a forbidden transition in symmetricalgeometries, so we can say that the distortion increases with the increasing iron-content.Within the L series, no such variation is seen. therefore there is no chanse in the distortion.Figurel. Pseudo-radial distribution function of the S series0 1 2 3 4 5R (Angsti im)Figure 2. Pseudo-radial distribution functions of the L seriesThe evaluation of the EXAFS data was made by the EXAFSPAK programme. Duringthe analysis spline Victoreen constants were used and the Fourier-transformation was takenbetween 2.5-14 A-t without phase correction. Figures 1 and 2 show the pseudo-radialdistribution functions for the two series. The second and the third coordination shells arequalitatively different in the two series. Moreover, in the S series the second coordinationshell systematically changes with the Fe-content. This can explain why the photocatalyticactivity go through a maximum with the increasing iron-content. After modelling thestructures we can say that the local structure of the L series is much more regular, than thestructure of the S one. In the S series there is a remarkable difference between the structure ofthe highly concentrated samples and the low concentrated ones. Mossbauer and X-rayphotoelectron spectroscopic measurements funher confirm the differences existing betweenthe microenvironment of iron within the L- and S-series" corroboratine the results from theXAFS measurements.Based on these results, a clear relationship exists between the local structure of theFe(III) and the photocatalytic activity of TiOz photocatlysts. These results make it possible tooptimize the preparative ways of photocatalysts that are able to efficiently utilize the visibleregion of the solar light for heterogeneous photocatalysis.'  A. Fujishima, K. Honda, Nature, 238,37 -38 (1912)t A. Fujishima, T. N. Rao, D. Tryk, -/. Photochern. Photobiol. C; Photochem. Rew., l,l-27 (2000)'B.  Oregan, M. Gretzel ,  Nature,353,  737-139 (1991)" W. Wang, B. Gu, L. Liang, W. A. Hamilton, D. J. Weselowskr, J. Phys. Chem.,8., 108, 14789-14192 (2004)t Z. Ambrus, N. Bal6zs, T. Alapi, G. Wittman, P. Sipos, A. Dombi, K. Mogyor6si, Appl. Catal. B: Environmental, 81,27-36 (2008)u a.) N. Bal6zs, D. Srank6, K. Mogyor6si, T. Alapi, A. Pallagi, A. Dombi, A. Oszk6, P. Sipos, Appl. Catat. B: Environmental, 84,356-362(2008); b.) N. Bal6zs, D. F. Srank6, A. Dombi, P. Sipos, K. Mogyor6si, Appl. Catal. B: Envirownental, in press, DOI:10.1016/j.apcatb.2010.03.006; c.) K. Mogyortisi, N. Baliias, D.F- Srank6, E. Tombiicz, I. Ddk6ny, A. Oszk6, P. Sipos, A. Dombi, Appl.Catql. B : Environmenfa/, in press, DOI : I 0. 1 0 I 6/j.apcatb.20l 0.03.007f F.-.on,"n,AIJ - \n 1----r-, \,/\\^ryoT'.=trct)oEFl!-]--i5MAX-lab Acrrvny Reponr 2009 305

The local structure of Fe(III) in doped TiO2 photocatalysts studied with X-Ray absorption spectroscopy

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The local structure of Fe(III) in doped TiO2 photocatalysts studied with X-Ray absorption spectroscopy

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