structural and photocatalytic properties of iron- and europium-doped tio2

However, the best photocatalytic activity in the photodegradation reaction of phenol was evidenced for the hydrothermal sample, TiO2: 1 at.% Fe, 0.5 at.% Eu, in both UV and visible light

Contents lists available atScienceDirect Materials Chemistry and Physics

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / m a t c h e m p h y s

nanoparticles obtained under hydrothermal conditions

L Diamandescua,∗, F Vasiliua, D Tarabasanu-Mihailaa, M Federa, A.M Vlaicua,

C.M Teodorescua, D Macoveia, I Enculescua, V Parvulescub, E Vasilec

aNational Institute of Materials Physics, Atomistilor 105 bis, P.O Box MG-7, Bucharest, Romania

bInstitute of Physical Chemistry ‘I G Murgulescu’, Bucharest, Romania

cMETAV S.A., Bucharest, Romania

a r t i c l e i n f o

Article history:

Received 19 February 2008

Received in revised form 10 May 2008

Accepted 16 May 2008

Keywords:

Semiconductors

Hydrothermal synthesis

Doped TiO 2

Nanostructures

Photocatalysis

a b s t r a c t Iron- and europium-doped (≤1 at.%) TiO2nanoparticles powders have been synthesized by a hydrothermal route at 200◦C, starting with TiCl4, FeCl3·6H2O and EuCl3·6H2O The structure, morphology and optical peculiarities were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS), M ¨ossbauer spectroscopy and UV–vis mea-surements The photocatalytic performance was analysed in the photodegradation reaction of phenol Rietveld refinements of XRD patterns reveal that the as-prepared samples consist in iron- and europium-doped TiO2in the tetragonal anatase structural shape, with particle size as low as 15 nm By means of

M ¨ossbauer spectroscopy on both57Fe and151Eu isotopes as well as by EXAFS analyses, the presence of

Fe3+and/or Eu3+ions in the nanosized powders has been evidenced It was found that iron and europium ions can substitute for titanium in the anatase structure From the UV–vis reflection spectra, by using

the transformed Kubelka–Munk functions, the band gap energy (Eg) of the hydrothermal samples has

been determined in comparison with that of Degussa P-25 photocatalyst A decrease of Egfrom 2.9 eV found for Degussa photocatalyst to 2.8 eV for the titania doped with 1 at.% Fe has been evidenced, indi-cating a valuable absorption shift (∼20 nm) towards visible light region However, the best photocatalytic activity in the photodegradation reaction of phenol was evidenced for the hydrothermal sample, TiO2:

1 at.% Fe, 0.5 at.% Eu, in both UV and visible light regions The photocatalytic activities of iron-doped and iron–europium-codoped samples are high and practically the same only in visible light The photocat-alytic properties in correlation with the structural and optical peculiarities of the hydrothermal samples are discussed

© 2008 Elsevier B.V All rights reserved

1 Introduction

Titanium dioxide (titania) is a cheap, nontoxic and highly

effi-cient photocatalyst being extensively applied for the degradation of

organic pollutants, air purification, water splitting, and reduction of

nitrogen to ammonia[1–10] However, only a small UV fraction of

solar light (<5%) can be utilised because of large band gap (∼3 eV) of

titanium dioxide semiconductor structure Recently, a considerable

number of studies were devoted to the development of efficient

visible light sensitive photocatalysts and to photocatalytic

prop-erties improvement[11–15] Transition metal selective doping is

one of the common approaches to extend the spectral response

∗ Corresponding author Tel.: +40 21 3690170; fax: +40 21 3690177.

E-mail addresses:diamand@infim.ro , ldiamandescu@gmail.com

(L Diamandescu).

of titania to the visible light region In particular the iron doping was found to increase the photocatalytic activity up to 2.5 times [12] Investigation on chloroform photodegradation revealed a sig-nificant photocatalytic reactivity increase over nanocrystalline TiO2 codoped with Fe3+and Eu3+by sol–gel method, as compared with undoped or monodoped TiO2nanoparticles[16]

It is already established that material properties depend strongly on precursors and synthesis methods in correlation with the thermodynamic process parameters For the synthesis of nanoparticle systems the hydrothermal method was intensively utilised in the last decade[17–21] However, no reports on the hydrothermal synthesis of iron- and europium-codoped TiO2 mate-rials have been published, by our knowledge

It is the aim of this work to present the hydrothermal synthesis

of iron- and europium-doped and -codoped TiO2nanoparticle sys-tems, their microstructure, morphology and catalytic properties in the photodegradation of phenol, in both UV and visible light region 0254-0584/$ – see front matter © 2008 Elsevier B.V All rights reserved.

L Diamandescu et al / Materials Chemistry and Physics 112 (2008) 146–153 147

2 Experimental

2.1 Hydrothermal synthesis

Fe 3+ - and Eu 3+ -doped and -codoped nanocrystalline titania samples have been

synthesized by a hydrothermal route, starting with titanium (IV), iron (III) and

europium (III) chlorides in solution Titanium tetrachloride has been obtained by

air oxidation under vigorous stirring (30 h) from a 15% titanium trichloride in

hydrochloric acid solution Europium trichloride has been prepared by dissolving

the corresponding europium oxide amount in 2N hydrochloric acid A 25%

ammo-nium hydroxide solution was added to the acidic solution containing the metal

chlorides in the predetermined ratio All reagents were of analytical grade (Aldrich

99.99%) The total chloride concentration in solution was 1.10−2M The pH value

was adjusted to ∼10 to assure the complete cation precipitation After filtration and

washing with distilled water (until no chlorine anions were detected), the

amor-phous mixed hydroxide precipitate was dispersed again in double distilled water

and brought up to a volume of 30 cm 3 The obtained suspension was placed into a

50 cm 3 Teflon-lined autoclave and then heated at 200 ◦ C for 1 h The heating rate

was 10 ◦ C min −1 and the corresponding vapour pressure at 200 ◦ C was about 15 atm.

The autoclave was cooled to room temperature and the resulting colloidal

suspen-sion of iron/europium-doped titanium oxide was heated at 50◦C for several hours

to remove water The following hydrothermal sample codes are used in this paper:

undoped sample (TiO 2 ); TiO 2 : 1 at.% Fe (sample TF); TiO 2 : 0.5 at.% Eu (sample TE);

TiO 2 : 1 at.% Fe, 0.5 at.% Eu (sample TFE) Owing to the nature of the synthesis process,

the accurate doping level could not be consistently predicted and the percent values

represent the nominal values of Fe or Eu atomic concentrations.

2.2 Structural characterisation

X-ray diffraction (XRD) patterns obtained on DRON 2 X-ray

diffractome-ter (linked to a data acquisition and processing facility with CuK ␣ radiation

 = 1.540598 ˚A and a graphite monochromator) were used to determine the

iden-tity of any phase present and their crystallite size JEOL 200 CX electron microscope

operating at an accelerating voltage of 200 kV was utilised to obtain information

about the structure and morphology of mixed oxide nanoparticles Particle sizes

were measured from bright field (BF) and dark field (DF) images, whereas the

phase content was investigated by electron diffraction For the transmission

elec-tron microscopy (TEM) investigations the samples have been prepared by placing

a drop of oxide powder, ultrasonically dispersed in ethanol, on 3 mm holey

car-bon grid The HRTEM images were obtained with a CM120ST Philips transmission

electron microscope (resolution ∼2 ˚A) 57 Fe and 151 Eu transmission M ¨ossbauer

spec-troscopy was applied to prove the presence of iron and europium dopants in the

titania structure A standard constant acceleration transmission M ¨ossbauer

spec-trometer was used to record the room temperature spectra on both isotopes The

local structure around the dopant atoms was investigated by extended X-ray

absorp-tion fine structure (EXAFS) spectroscopy The X-ray absorpabsorp-tion measurements have

been carried out at the E4 HASYLAB beam line (Germany), by using a double crystal

Si (1 1 1) monochromator and a pre-focusing Au mirror EXAFS spectra have been

measured in transmission mode, at Fe K- and Eu L 2 -edges for the doped

photocata-lysts, ␣-Fe 2 O 3 and Eu 2 O 3 standards The TiO 2 (anatase) standard has been measured

correspondingly at the Ti K-edge Normalized EXAFS function(k) (k = photoelectron

wave number) was calculated after subtraction of pre-edge and post-edge smooth

backgrounds, (fitted by Victoreen formula and cubic splines, respectively) from the

absorption spectra k n-weighted(k) (n = 2, 3) was Fourier transformed over the

k-range 2.8–11.2 ˚A −1 (Fe K) or 1.3–10.2 ˚A −1 (Eu L 2 ), to acquire preliminary information

on the dopant environment Radial ranges of interest in the Fourier transforms (FT)

were further isolated by Hanning-function windows, backtransformed into k-space,

and non-linearly fitted by a least-square method The fit provided the interatomic

distances and coordination numbers in the close neighbouring shells of the

absorb-ing atoms (Fe, Eu) The photoelectron backscatterabsorb-ing amplitudes and phases, as

calculated by the FEFF6 code [29] , have been used in the fitting runs The UV–vis

measurements have been performed on the PerkinElmer Lambda 45

Spectrome-ter in the wavelength range of 200–800 nm, with 0.5 nm step at a scan speed of

60 nm s−1.

2.3 Photocatalytic activity tests

The phenol photodegradation was investigated using a stationary quartz

reac-tor with UV (60 W, filter at  = 312 nm), and visible (60 W,  > 380 nm) lamps The

photocatalytic activity was evaluated from the decomposition of phenol in aqueous

solution at 2 M or 0.2 M concentrations The experiments were carried out in quartz

made cylindrical flask The pre-aerated reaction mixture was illuminated with UV

or visible light under continuous magnetic stirring The reactor was maintained at

room temperature for all experiments The distance between the light source and

the reaction tube was 11 cm After a given irradiation time, samples of 4 ml volume

were withdrawn and the catalysts were separated from the suspensions by filtration

through cellulose membranes The reaction products were filtered through Millipore

3 Results and discussion

3.1 X-ray diffraction

Primary structural information was given by XRD patterns of the hydrothermal samples (Fig 1a–d)

All spectra display the characteristic patterns of TiO2in tetrag-onal anatase phase (the anatase characteristic lines are indexed in Fig 1a) No relevant differentiation can be observed by changing the doping element (Fig 1b–d) except for small line intensity vari-ations and a slight increase of diffraction line widths from undoped (Fig 1a) to codoped (Fig 1d) samples Rietveld refinement of XRD patterns reveals a slight increase of anatase lattice parameters a (∼0.14%) and c (∼0.04%), suggesting the presence of higher radius doping ions in the TiO2lattice A particle mean size of about 15 nm (calculated with Scherrer equation) was found to characterise the hydrothermally doped anatase

3.2 Transmission electron microscopy

The electron diffraction and electron microscopy analysis evi-dence the presence of anatase like structure with particle mean size

as low as 15 nm and a strong morphology dependence on doping element Thus, a rectangular and quadratic morphology is

predom-Fig 1 XRD patterns of hydrothermally synthesized doped and undoped TiO.

Fig 2 TEM images of undoped TiO2 (a), Eu-doped TiO 2 (b) and Fe- and Eu-doped TiO 2 (c).

inant for undoped and Fe-doped TiO2samples (Fig 2a) In Eu-doped

TiO2many elongated particles can be seen (Fig 2b) whereas large

shape diversity is found in codoped Fe and Eu-TiO2 specimen

(Fig 2c) In many cases the lattice (1 0 1) planes (d1 0 1= 0.352 nm)

of anatase can be clearly solved (Fig 3a) and selected area

elec-tron diffraction patterns confirm the unique presence of the anatase

phase (Fig 3b)

3.3 M¨ ossbauer spectroscopy

The specific presence of both iron and europium ions in the

doped TiO2samples is supported by the M ¨ossbauer spectroscopy

measurements In Fig 4 the 151Eu M ¨ossbauer spectrum of the

TFE sample is presented In spite of the poor effect, the spectrum

indicates the presence of the Eu3+ions in the TiO2 anatase

lat-tice [22] The presence of iron ions together with their valence

states has been evidenced by M ¨ossbauer spectrum on the TF

sam-ple, 90% enriched in57Fe in order to improve the signal-to-noise

ratio (Fig 5) The best fit was obtained with the isotropic

elec-tronic relaxation model[23] The values of M ¨ossbauer hyperfine

parameters (isomer shift and quadrupole splitting) are

character-istics for Fe3+[22] In others words, the iron ions are present in the

TiO2lattice, the spin–spin electron interaction between the

neigh-bouring Fe3+ions giving rise to the relaxation M ¨ossbauer spectrum

[23]

3.4 EXAFS

A more detailed analysis on the dopant location and resulting

interactions has been carried out by EXAFS investigations The k3 -weighted Fe K-edge EXAFS spectra of the doped samples TF, TFE and

␣-Fe2O3(hematite) are shown together with the Ti K-edge EXAFS of the standard anatase inFig 6a Their Fourier transforms displayed

inFig 6b are the radial functions with maxima corresponding, until

a specific shift, to the neighbouring shells of the absorbing species (Fe, Ti)

In the case of␣-Fe2O3, the first maximum of FT corresponds

to the nearest oxygen neighbours, at a mean distance of 2.03 ˚A (Table 1) The second split maximum, more intense and broadened,

is due to the superposed contributions of closely related shells, at the mean distances of 2.95 ˚A (4 Fe), 3.38 ˚A (3 Fe, 3 O) and 3.69 ˚A (6 Fe,

6 O) The FT looks differently for anatase, with a less compact tita-nium surrounding The first maximum, contributed by six oxygen neighbours in a distorted octahedral configuration, is dominant, while the second maximum, corresponding to the next-nearest neighbours (4 Ti) at 3.04 ˚A, has the amplitude about 2 times smaller The amplitude ratio between the first two maxima in the Fourier transforms rules out the iron segregation to an oxidized Fe2O3 phase The photocatalyst FTs closely resemble that of TiO2, sug-gesting similar Fe and Ti surroundings in the doped and undoped samples In order to verify this statement, the filtered EXAFS of

L Diamandescu et al / Materials Chemistry and Physics 112 (2008) 146–153 149

Fig 3 (a) HRTEM image showing (1 0 1) anatase planes in a TiO2 nanocrystal

belong-ing to (Fe, Eu)-doped specimen; (b) SAED pattern of the same specimen showbelong-ing the

main diffraction rings of polycrystalline anatase.

Fig 4.151 Eu M ¨ossbauer spectrum of the sample TiO 2 : 1 at.% Fe, 0.5 at.% Eu showing

3+

Fig 5.57 Fe M ¨ossbauer spectrum of the sample TiO 2 : 1 at.% 57 Fe and the fit with the isotropic relaxation model (continuous lines).

the photocatalysts corresponding to the first two radial maxima

(r = 0.9–3.1 ˚A) was fitted with two neighbouring shells The best fit

has been obtained considering for iron and titanium ions the same local configuration (seeTable 1) This result clearly indicates that

Fe3+ions substitute for Ti4+in the anatase tetragonal structure The substitution of Ti4+by the larger Fe3+ions (Ti4+: 0.605 ˚A;

Fe3+: 0.645 ˚A[11]) expands the metal–oxygen distances by∼0.1 ˚A,

up to values close to the Fe O bond length in␣-Fe2O3, while the metal–metal distances remain unchanged (sample TF) or shorten (TFE) with respect to these distances in anatase structure[24] A similar effect was recently reported by Zhu et al.[25]in Fe-doped anatase samples, being explained by changes of the Ti O Ti bond angles after the Ti substitution by Fe

Europium environment in the sample TFE was investigated by EXAFS at the Eu L2-edge (7617 eV) Since Eu L3-edge EXAFS has stronger oscillations it could not be analysed, due to the superposi-tion of the Fe K-edge (7112 eV) at 135 eV above Eu L3(6977 eV) The

k2-weighted EXAFS of the sample is shown inFig 7a, together with the spectra of Eu2O3 (cubic) and anatase, measured at the Eu L2 -and Ti K-edges, respectively The corresponding Fourier transforms are illustrated inFig 7b

In the cubic structure of Eu2O3, the Eu atoms are surrounded by six nearest oxygen neighbours at distances ranging between 2.30 ˚A and 2.38 ˚A, with an average Eu–O distance of 2.34 ˚A (Table 1) Their contribution to EXAFS is described by the main maximum of FT, at 1.82 ˚A The further maxima correspond to more distant Eu shells at 3.60 ˚A and 4.11 ˚A, with six atoms on each shell

Although the FT of the sample TFE manifests a certain similar-ity with that of Eu2O3, its maximum at 2.83 ˚A seems closer related

Table 1

Fe and Eu local environments (interatomic distances R, coordination numbers N) in

the doped samples (TF, TFE), as inferred by the fit of EXAFS

3.38/3 Fe, 3 O 3.69/6 Fe, 6 O

The structural parameters (R, N) were compared with the characteristic of␣-Fe 2 O 3 ,

Eu O (cubic) and TiO (anatase, rutile), calculated from crystallographic data.

Fig 6 k3 -Weighted Fe K-edge EXAFS spectra (a) and their Fourier transforms (b) for the doped photocatalysts (TF, TFE) and ␣-Fe 2 O 3 Ti K-edge EXAFS of TiO 2 (anatase) and the corresponding FT are also shown, for a comparison.

to that describing the Ti–Ti pairs in the TiO2structure The fit of

the filtered EXAFS in the range 1.4–3.2 ˚A resulted in∼6 O and 2 Ti

atoms around Eu, at distances of 2.33 ˚A and 3.37 ˚A, respectively (see

Table 1) The presence of titanium in the europium neighbourhood

emphasizes the Eu accommodation on Ti sites in the TiO2lattice,

similarly with the Fe case The large Eu3+ions (0.947 ˚A) locally

expand the host structure, with elongations of the interatomic

dis-tances almost equalling with the difference between Eu3+and Ti4+

ionic radii (0.34 ˚A)

A peculiar effect of the Eu incorporation into the TiO2 lattice

is the change of the local symmetry around Eu, from anatase to

rutile structure This is indicated by the lowering of the number of

the next-nearest Ti neighbours from four, specific to anatase

struc-ture, to only two, as in rutile lattice This effect was also found for neodymium-doped anatase nanoparticles,[26]suggesting that it could be specific to the rare-earth ions embodied in the TiO2lattice

3.5 UV–vis

The UV–vis absorption edge and band gap energy have been

determined from the room temperature reflectance (R)

spec-tra The reflectance spectrum of TiO2 Degussa P-25 commercial photocatalyst shows the absorption onset at 335 nm while for hydrothermal samples the absorption settles at 355 nm, which is

20 nm shifted toward the visible range (Fig 8) Further, the opti-cal absorbance of the samples was deduced by the Kubelka–Munk

Fig 7 k2 -Weighted Eu L -edge EXAFS (a) and corresponding Fourier transforms (b) for the sample TFE and cubic Eu O Ti K-edge EXAFS of anatase and its FT are also shown.

L Diamandescu et al / Materials Chemistry and Physics 112 (2008) 146–153 151

Fig 8 UV–vis reflectance curves for the hydrothermal TiO2 doped samples.

formula F(R) = (1 − R )2/(2R),[27] Using the Tauc plot (F(R) h)nvs

h)[28], where h  is the photon energy and n = ½ for direct band

gap semiconductors, the band gap energies were deduced from

the intersection of the Tauc’s region extrapolation with the

pho-ton energy axis (Fig 9) The calculated band gap energy values are

given inTable 2 For the hydrothermal TiO2undoped sample and

TE sample, the band gap energy is about 3.0(3) eV, which is higher

than 2.9(7) eV obtained for the TiO2Degussa P-25 sample The

low-est band gap energy of 2.84 eV was observed for TF sample, while

for TFE sample the energy was 2.92 eV

Fig 9 Transformed Kubelka functions [F(R)h] 1/2 for undoped and doped titania

samples to estimate the band gap energies.

Table 2

Band gap energies from UV–vis data on hydrothermal samples as compared with Degussa P-25

Photocatalyst Band gap energy (eV) TiO 2 Degussa P-25 2.9 (7)

TiO 2 : 1 at.% Fe 2.8 (4) TiO 2 : 0.5 at.% Eu 3.0 (3) TiO 2 : 1 at.% Fe, 0.5 at.% Eu 2.9 (2)

3.6 Phenol photodegradation

In the present study, the photocatalytic activity of the modi-fied titania samples was investigated in the phenol degradation reaction The interest was focused on the effects of composi-tional modifications in titania synthesized by a hydrothermal route, on water decontamination through photocatalytic degra-dation/mineralization of organic pollutants Fig 10 shows the

conversion degree CPh(%), in the phenol photodegradation reac-tion at 312 nm, catalyzed by hydrothermally synthesized titania samples, for two initial phenol concentrations The undoped tita-nia gave the lowest phenol conversion, while the codoped Fe/Eu sample gave the highest activity For separate doping, the conver-sion degree can become double or triple as compared to undoped TiO2, while for codoped sample the conversion degree is nearly 6 times greater In the case of codoped TiO2 (TFE) sample, conver-sion degrees up to 30% at low initial phenol concentrations and up

to 15% at higher phenol concentrations were obtained The higher degradation degree and no detectable organic compounds for TFE sample indicate the superiority of this catalyst A little quantity of organic compounds was identified after reactions in the presence

of TF and TE catalysts Hydroquinone, p-benzoquinone and

cate-chol were detected as main reaction intermediates The presence

of the organic intermediates was attributed to the reduced rate of hydroxyl radical generation and phenol hydroxylation The degra-dation mechanism proposed is based on hydroxylated steps[29] Full mineralization of organic compounds (as phenols and other aromatics) on the surface of illuminated titania proceeds via many steps, which make one-electron oxidation or reduction reactions possible[30] In the first step hydroxy radicals are generated by the reaction between holes, resulted after photo-excitation of the

Fig 10 Phenol conversion degree CPh (%) after 5 h of UV illumination ( = 312 nm) for the hydrothermally synthesized TiO 2 samples; 2 M and 0.2 M are the initial phenol

Fig 11 Phenol conversion degree CPh (%) under visible irradiation ( > 380 nm)

cat-alyzed by Fe- and Eu-doped and codoped TiO 2

semiconductor, and the surface hydroxyl species of the catalyst

The next step is the hydroxylation of the phenyl ring, followed by

further oxidation of the hydroxylated phenol intermediates

A remarkable photocatalytic activity increase was obtained in

visible light (>380 nm) for all samples (Fig 11) The best activity

was obtained for TF and TFE samples where the obtained

conver-sion degree of phenol is as high as 65% These results are almost

in agreement with UV–vis results revealing a shift of absorption to

visible range for the TF and TFE samples After 3 h of reaction no

detectable organic intermediates were evidenced

The best photocatalytic activities of the hydrothermally doped

titania samples can be attributed to the cationic species present in

the doped and codoped titania which avoid possible electron–hole

recombination, stabilizing the holes in the valence band and the

electrons in the conduction band Moreover, the hydrothermal

catalysts have high surface areas (∼100 m2g−1) and probably a

large amount of O-H surface groups which seem to stabilize the

electron–hole pairs At this point, we cannot elucidate the

relation-ship between the band gap, shift of the absorption onset toward

the visible range, and the results of the photocatalytic tests The

TFE specimen revealed a maximum photocatalytic activity in

phe-nol degradation reaction, while its UV–vis spectrum revealed an

intermediate value of the band gap It seems that surface effects

play a crucial role in determining the photocatalytic properties of

doped titania structures Probably the europium ions at the surface

of titania nanoparticles increase the life time of electron–hole pairs

improving the photocatalytic properties

In summary, the undoped titania gives the lowest phenol

con-version, and the transformation of phenol increases for the samples

with Fe or Eu, while the codoped Fe/Eu sample gives the highest

activity

4 Conclusions

Iron- and europium-doped TiO2nanoparticles were obtained by

a hydrothermal route, at mild temperature and pressure (∼200◦C

and ∼15 atm, for 1 h) Rietveld refinements of the XRD patterns

reveal the exclusive presence of iron- and europium-doped anatase

phase in hydrothermally synthesized samples; the particle mean

size was less than 15 nm and the morphology was found to depend

on doping element EXAFS analysis strongly support that both

Fe3+and Eu3+ions enter the TiO lattice, by substituting the Ti4+

ions Ti4+ replacement by the larger Fe3+ and Eu3+ions distorts the host lattice around doping atoms For iron-doped anatase, the metal–oxygen distances increase, while the metal–metal distances slightly shorten or remain unchanged, with respect to the undoped anatase structure Europium doping locally dilates the TiO2 lat-tice, by lengthening both metal–oxygen and metal–metal distances Additionally, the symmetry around Eu3+ changes from anatase towards rutile structure; this behaviour could be a specific effect

of the rare-earth doping.57Fe M ¨ossbauer spectroscopy reveals the presence of isotropic electronic relaxation effects as a result of spin–spin interaction between the electronic spins of neighbour-ing Fe3+ions Measurements on both57Fe and151Eu M ¨ossbauer isotopes reveal the presence of Fe3+and Eu3+in the TiO2host lat-tice

The photocatalytic activity of all hydrothermal samples in the degradation reaction of phenol is much higher in the visible light than in UV region An important UV→ visible absorption shift (∼20 nm) has been evidenced for the sample TF However, the best photocatalytic activity in the photodegradation reaction of phe-nol was evidenced for the hydrothermal sample TFE, in both UV and visible light regions The remarkable conversion degree of phe-nol recommends the codoped specimen TFE, obtained by a simple hydrothermal route at moderate temperature, as a promise visible photocatalyst for the degradation of harmful organic compounds in water We hope that these results will stimulate further theoretical and experimental works for a better understanding of mechanisms and doping effects in photocatalytically active materials

Acknowledgements

This work was supported by the Romanian Ministry of Educa-tion and Research through Contract CEEX-Matnantech no 23/2005

We gratefully acknowledge the valuable assistance of Dr Edmund Welter and Dr Dariusz Zajac (HASYLAB) during the EXAFS exper-iments, as well as the whole scientific support of Prof Eberhardt Burkel and Dr Radu Nicula (University of Rostock)

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