Thin Solid Films 520 (2011) 1348–1352

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X-ray absorption fine structure investigations on heat-treated Cr-doped titaniathin films

Diana Mardare a,⁎, Valentin Nica a, Valentin Pohoata a, Dan Macovei b, Nicoleta Gheorghe b,Dumitru Luca a, Cristian-Mihail Teodorescu b

a Alexandru Ioan Cuza University, Faculty of Physics, 11, Carol I Blvd. 700506-Iaşi, Romaniab National Institute of Materials Physics, P.O.Box MG-7, 105 bis Atomistilor St. 077125-Magurele-Ilfov, Romania

⁎ Corresponding author. Tel.: +40 232 201169; fax:E-mail address: dianam@uaic.ro (D. Mardare).

0040-6090/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tsf.2011.04.124

a b s t r a c t

a r t i c l e i n f o

Available online 28 April 2011

Keywords:Thin filmsCr-doped titanium oxideRF sputteringXRDEXAFSXANESRefractive index

Chromium-doped titanium oxide thin films were investigated in the as-deposited state and after thermaltreatment (723 K for 3 h in air). X-ray diffraction data revealed an improvement in film crystallinity inducedby the thermal treatment. Extended X-ray absorption fine structure data revealed similar atomic neighboringaround Cr atoms in both as-deposited and annealed samples. A lattice contraction of ~2% is observed in theannealed samples. The 67% enhancement of the amplitude of the Cr 1 s X-ray absorption fine structure pre-edge peak after thermal treatment, which is a sign of “dipole-forbidden” 1 s→3 d transitions, suggests strongalteration in the number of Cr 3 d vacancies, in spite of similar Cr local environment in the two kinds ofinvestigated samples. We discuss here the Cr+→Cr4+ and Cr2+→Cr6+ changes induced by thermaltreatment, and/or the evolution in local structures without inversion center.Refractive index dispersion spectra in the visible wavelength domain allowed us to compute the values of thedispersion energy, the single-oscillator energy and the coordination number of Ti atoms in both as-depositedand annealed samples.

+40 232 201150.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Transition metal-doped titania is nowadays a class of materialswith promising applications in photocatalysis, gas sensing, dilutedmagnetic semiconductors etc. [1–6]. The technological interest hasbeen the main stimulating factor for the basic research of thesematerials. Important information on the local environment around alow-concentration element in a matrix can be derived, among others,from the non-destructive Extended X-ray Absorption Fine Structure(EXAFS) technique [7]. The X-ray absorption near-edge structure(XANES) and EXAFS techniques are based on the changes in theabsorption spectrum of the material obtained by gradually increasingthe incident X-ray photon energy in a synchrotron facility.

After complete absorption of a photon (whose energy range ischosen to include the absorption edge of the investigated element inthe material), an inner photoelectron is ejected from the absorbingatom, thus leading to occurrence of an inner hole in the atom. As aresult, a sharp increase in the absorbance of the studied material (theedge) is observed. The interaction between the photoelectron and thesurrounding atoms results in interference between the propagatedand reflected waves. The desired information on the coordination

environment of the central excited atom could be derived from theoscillating part of the absorption spectrum, following the edge.

The aim of this paper is to understand the structural modificationsin amorphous Cr-doped titania thin films, induced by thermaltreatment. A study on the alteration of the optical constants inducedby annealing was also performed, and the coordination number oftitanium atomswas evaluated by applying the single-oscillatormodel.

2. Experimental details

Chromium-doped titania thin films were obtained on glasssubstrates (microscope slides) by DC reactive sputtering, in an O2/Ar mixture, by using three sputtering targets (two Ti discs and one Crdisc, 50 mm in diameter each). The samples were prepared withoutadditional substrate heating (during sputter deposition, the substratetemperature reached about 100 °C).The total pressure has been set at0.35 N/m2, the partial pressure of the oxygen being of 0.10 Pa. The filmthickness (240 nm) was measured by using a step profilometer andchecked with cross-section SEM analysis.

The set of the as-deposited samples were heat treated in an openfurnace (Barnstead F21130-33). The heat treatment consisted of: (a)heating up in air with a temperature rate of 7.5 K/min; (b) annealingfor 3 h at 723 K; (c) cooling down to room temperature, in thefurnace, under no heating conditions. We denote here, by TiO2:Cr andTiO2:Cr/t, the as-deposited and the heat-treated samples, respectively.

1349D. Mardare et al. / Thin Solid Films 520 (2011) 1348–1352

The phase composition of the films was studied with a Bruker D8Advantage diffractometer, using the Cu Kα radiation (λ=1.540591 Å)at 4° grazing incidence. The values for the full width at half maximum(FWHM) and for the crystallite sizes have been obtained by the curvefitting procedure (Igor Pro software), by using the sum of two Lorentzprofiles. Deconvolution errors were estimated by repeating the fittingprocedure many times (≥100) with random (but physically consis-tent) input parameters. The average crystallite size, corresponding toeach phase, was derived from the width of the anatase A(101) andrutile R(110) peaks, by using the Scherrer's formula [8]:

D =0:9λ

B1=2cosθð1Þ

where B1/2 is the peak FWHM (in radians), θ— the Bragg angle and λ—the X-ray wavelength.

The XANES and EXAFSmeasurements were performed at the Dorisstorage ring facility in Hasylab synchrotron facility in Hamburg. Themeasurements were performed using the beamline E4 (EXAFS II) ofthis research facility, with a double crystal Si(111) monochromator,preceded by a focusing mirror for intensity enhancement influorescence. Absorption spectra were acquired at the Ti and Cr K-edges, by using a 7-pixel SiGe:Li detector. Channel discriminationallowed us to select only the Ti or Cr Kα emission lines, exclusively,thus improving the signal/noise ratio. For setup calibration, trans-mission spectra were acquired using Ti and Cr foils as standards andionization chambers as detectors.

The transmittance of the films was measured in the visible regionusing a double-beam spectrophotometer UV-VIS Lambda 3 PerkinElmer.

3. Results and discussion

From the XRD patterns (Fig. 1), very weak signals corresponding toanatase A(101) and rutile R(110) peaks for an as-deposited samplecan be noticed. The heat treatment led to a visible increase in thedegree of crystallinity of chromium-doped sample, favoring thedevelopment of larger anatase crystallites (11.33±0.27 nm) incomparison with the rutile ones (5.76±0.51 nm). Crystallite sizeswere calculated by using Eq. (1), where the maxima in the XRD plotswere deconvoluted with two Lorentzian peaks, featuring FWHM of0.719° for the A(101) and 1.420° for the R(110). This deconvolution,for the case of the treated sample, is also presented as an inset in Fig. 1.However, the overall amplitude of the crystalline peaks is still low,

Fig. 1. X-ray diffraction patterns corresponding to a Cr-doped TiO2 sample as-deposited,and after the thermal treatment. In the inset, the background-subtracted X-raydiffraction of the heat-treated sample is presented.

when compared to the background, i.e. a large amount of the samplematerial is in amorphous state. Consequently, X-ray Absorption FineStructure (XAFS) measurements were conducted to gain more insightinto the atomic arrangements in the samples.

Fig. 2 shows the XANES spectra at the Cr K-edge for the as-prepared and annealed sample, together with a spectrum of metal Cr.The first noticeable result consists in the increase of the pre-edge peakby a factor of about 1.7 after the thermal treatment. This maximumoriginates from the dipole forbidden transitions 1 s→3d in the atomiccase [9]. Such transitions may, however, exist either as quadrupoletransitions, or as dipole-allowed transitions, by considering that theone-electron 3d states are just the zero order approximation.Therefore, owing to any perturbation without centre of inversion,the real corresponding “3d” wave functions contain also waves of pcharacter on which one-electron transitions originating from 1 sstates are dipole allowed [10]. In any case, the amplitude of this pre-edge peak may be seen as a measure of the density of 3d allowedstates for the inner-shell transition (i.e., 3d vacancies). This strongmodification of this pre-edge peak suggests a strong increase of theaverage number of the Cr 3d vacancies, despite the fact that, as it willbe further demonstrated by the EXAFS analysis, the local environmentof Cr in the sample is similar - before and after thermal treatment.

The modifications that may be taken into account to explain theincrease of the pre-edge peak are related to the Cr+→Cr4+ transitions(like when going from Cr2O to CrO2) and/or Cr2+→Cr6+ (like whengoing fromCrO toCrO3) inducedby the thermal treatment. In the formercase, the ratio between the number of 3d vacancies is 8/5=1.60, in thesecond case it is 10/6≈1.67. This important increase of the pre-edgepeak may also be caused by the evolution towards structures withoutinversion centers as, for example, distorted octahedral environments,where increases of the probability of 'dipole forbidden' transition arereported to date [11]. Briefly, the first hypothesis implies an increase ofthe number of available states (unoccupied states, where Cr 1 selectrons may be promoted by inner-shell excitations), while thesecond one is an effect on the transition amplitude. Moreover, if weretain the first hypothesis, this implies the possibility to control the(average) oxidation state of Cr by thermal treatment, which may serveto an eventual tuning of the material's properties, with applications inthe case of using thismaterial as a dilutedmagnetic semiconductor or asa photocatalyst. Finally, the Ti K-edge spectra (not shown here) aresimilar to previously reported ones for the anatase or rutile polymorphs[12], with no noticeable differences.

The EXAFS spectra obtained after the atomic absorption back-ground subtraction, μ0, and transformed into the wave vector space,

Fig. 2. X-ray absorption near-edge structure (XANES) at the Cr K-edge for the studiedsamples, compared with the XANES spectrum of a Cr thin foil.

Fig. 4. Fourier transforms of the EXAFS functions of the spectra represented in Fig. 3.

1350 D. Mardare et al. / Thin Solid Films 520 (2011) 1348–1352

are represented in Fig. 3. The EXAFS function and the photoelectronwave vectors are defined as:

χ kð Þ = μ kð Þ−μ0 kð Þμ0 kð Þ ; k =

1ℏ

2m hν−E Kð Þ0

� �h i 1=2 ð2Þ

where E0(K) is the K-edge energy, determined as the first inflection

point of the absorption spectrum, hν is the photon energy, m is themass of the electron, and μ0(k) is the atomic (unstructured)absorption coefficient, obtained by an arctangent fit of the absorptionedge. Fig. 3 shows the EXAFS spectra of the as-deposited and heat-treated samples at the Cr K-edge, a spectrum obtained at the K-edgefor a Cr metal foil, as well as a spectrum acquired at the Ti K-edge ofboth samples (we already stated that these two spectra are identicalat the Ti K-edge).

A first straightforward finding is that no Cr metal particles oraggregates are formed in the investigated materials. At the same time,the Cr K-edge spectra are quite close to the Ti K-edge spectrum,though not identical. This implies that maybe not the totality of the Crimpurities goes in Ti substitutional sites, and one cannot excludeinterstitial Cr or formation of local intermediate oxides. On the otherhand, the spectra corresponding to the both samples, untreated andthermal treated, are quite similar (except for the second oscillation in2-nd case, which is more pronounced). This behavior may beconnected with the X-ray diffraction observation of a highlyamorphous degree and, consequently, the recorded EXAFS spectraought to be attributed to the prevalence of the Cr neighboring in theamorphous phase.

A high degree of amorphisation is exhibited also by the Fouriertransforms (FT) of the EXAFS functions (Fig. 4), as long as the firstmaximum in the FT is much enhanced as compared to the other ones.Note that, in the case of a crystalline structure, a typical FT resemblesthat of metal Cr, also represented in Fig. 4. At the same time, thedegree of amorphisation around Ti atoms seems more elevated thanaround Cr. Also, the Cr FT of the EXAFS signals features minordifferences between the untreated and annealed samples. Themaxima of the FT of the Cr K-edge are given in Table 1. The errorsin the FT maxima positions are determined by performing multipleintegrations with slightly different integration windows (kmin, kmax).Here, kmin varied between 2.8 and 3.4 Å−1, whereas kmax variedbetween 12.6 and 13.6 Å−1.

Fig. 3. EXAFS spectra of the as-deposited and heat-treated samples at the Cr K-edge,along with a spectrum obtained at the K-edge for a Cr metal foil, and a spectrumacquired at the Ti K-edge of both samples.

Systematically, the FT's of the treated samples occur at lowerinteratomic distances. It looks like, by thermal treatment, the sample“shrinks” instead of expanding (by around 2%). At the same time, thenoticeable increase of the pre-edge peak cannot exclude a strongincrease of the Cr ionization degree (and only for Cr, the Ti pre-edgepeak remains unchanged). The “shrinkage” of the crystalline structureresults in strong ionization processes of chromium.

Another plausible explanation would be the presence of oxygenvacancies around Cr in the as-deposited sample, which may explainsuch local stoichiometries as Cr2O or CrO. Relaxing the network, byannealing, implies vanishing of oxygen vacancies, thus the averageionization degree of chromium seriously increases.

The chemical formula of the as-deposited Cr-doped titania films,derived from the energy dispersive X-ray (EDX) microanalysis, wasapproximated as Ti0.98Cr0.02O2[13]. Taking into account the plausibleexistence of oxygen vacancies, this formula might be better written asTi0.98Cr0.02O2− δ.

It is well-known that TiO2 thin films are transparent in the visibleregion and its transparency exhibits a sharp decrease in the UV region[14]. The values for the optical band gap, reported in the literature, are3.2 eV for the anatase phase [15] and 3.00 to 3.11 eV for the rutilephase, respectively [16]. Here, chromium doping determines adecrease in the transmittance in the wavelengths range shorter than550 nm (Fig. 5a). The optical absorption spectra of the studiedsamples, presented in Fig. 5b, give the values of the optical band gap,Eg, for the indirect allowed transitions, which dominates over theoptical absorption [14].

By extrapolation the straight lines down to (αhν)1/2=0, a value of(2.52±0.01) eVwas derived for the as-deposited film, while the samequantity increased to (2.56±0.01) eV for the annealed sample (withcorrelation coefficients for linear fits larger than 0.985). Unlike thisannealing-related increase in the optical band gap value (Fig. 5b), wenoticed a more significant decrease in the band gap due to Cr doping,in agreement with other results [17]. This observed Cr-dopinginduced red shift of the fundamental absorption edge, could beattributed to the local energy levels introduced in the TiO2 band gap,as demonstrated in [17].

Titanium dioxide is known to be one of the best photocatalyticmaterials, but unfortunately activated with UV radiation exclusively.Therefore, any doping-induced absorbtion range widening towardsvisible range could increase the efficiency of the photocatalyticactivity.

A slight decrease in the optical transmittance, along with a slightdisplacement of the interference maxima toward smaller wave-lengths, can be observed from Fig. 5, for the heat treated sample. This

Table 1The FT maxima position, the coordination number of the cation (Nc) for the studied films, the intercept with the vertical axis (E0/Ed), the slope [(E0Ed)−1], the correlation coefficients(R) and the standard deviation (SD) for the linear fits presented in the inset of Fig. 6.

Sample FT maxima position (Å) Nc E0/Ed (E0Ed)−1 (eV−2) SD R

TiO2:Cr 1.63±0.1 4.69±0.05 0.334±0.0042 0.0077±7.71×10−4 0.003 0.9812.74±0.23.36±0.1

TiO2:Cr/t 1.60±0.1 4.71±0.05 0.331±0.0037 0.0077±6.82×10−4 0.004 0.9852.72±0.23.27±0.1

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correlates well with the improvement in the crystallinity ofchromium-doped sample, induced by the heat treatment. Havingthe same thickness, this means that we may qualitatively considerthat the “optical density” of the film, characterized by the refractiveindex, is higher for the TiO2:Cr/t sample [18].

To verify the above-mentioned observation, the Swanepoelprocedure [19] was applied to calculate the dispersion of therefractive index, n, for the transparent range of TiO2 thin films(Fig. 6). A slight increase in the refractive index of the material,induced by heat treatment, due to the improvement in the filmscrystallinity is evident from the data. A comparison between the valuen=2.08 for a heat-treated film, and the values of both rutile(n=2.75) and anatase (n=2.52) single crystals [20]) at the samewavelength (633 nm), indicates a lower “optical density” of the

300 400 500 600 700 800

0

20

40

60

80

100

TiO2:Cr

TiO2:Cr/t

T (

%)

a

b(nm)

0

200

400

600

800

Eg=2.52eV

(TiO2:Cr)

Eg=2.56eV

(TiO2:Cr/t)

Photon energy (eV)

(h)1/

2 (cm

-1eV

)1/2

Fig. 5. a) Transmittance spectra for the as-deposited and heat-treated chromium dopedTiO2 films; b) The optical absorption spectra, plotted as (αhν)1/2= f(hν) for the studiedsamples.

investigated films, related to high void content in the investigatedfilms.

The spectral behavior of the films refractive index can be fitted inthe visible domain (where the films are transparent) by the one-oscillator Sellmeier dispersion law (Fig. 6). In the frame of the single-oscillator model (Wemple-DiDomenico), the equation [21]:

1n2−1

=E0Ed

− hνð Þ2E0Ed

; ð3Þ

can provide the coordination number of the nearest-neighbor Tications to an O anion, Nc, if we also consider the empiricalrelation [21]:

Ed = βNcNeZa: ð4Þ

where E0, is the oscillator energy, Ed is the dispersion energy, Ne is theeffective number of the valence electrons per anion (a value of 8 hasbeen taken here [22]), Za is the formal chemical valence of the anion(here it is 2) and β is a constant, which amounts to 0.26±0.04 eV formost oxides (including TiO2), By representing (n2−1)−1 vs. (hν)2, alinear behavior was obtained (the inset in Fig. 6), having the slope(E0Ed)−1, and the intercept with the vertical axis, E0/Ed (see values inTable 1). The corresponding correlation coefficients R, and thestandard deviations SD of the straight-line fits are summarized inTable 1.

From Table 1, one can see an increase in the coordination number,due to the occurrence of the anatase and rutile phases, by annealing.The calculated values are much smaller than 6, the value theoreticallypredicted for TiO2. By taking into account the high degree ofamorphisation, the results agree well with the reported ones for

400 500 600 700 8002.00

2.05

2.10

2.15

2.20

2 3 4 5 6 7 8 9 100.26

0.28

0.30

0.32

1/(n

2 -1)

(h)2(eV)2

TiO 2:Cr

TiO 2:Cr/t

TiO 2:Cr

TiO 2:Cr/t

(nm)

Fig. 6. The n(λ) plots in the visible range, fitted by the dash line for the as-depositedsample, and by the solid line for the heat-treated sample. In inset, the lineardependences, (n2−1)−1 vs. (hν)2 are additionally presented.

1352 D. Mardare et al. / Thin Solid Films 520 (2011) 1348–1352

other amorphous TiO2 films [22]. One could see that, the differences inthe coordination numbers calculated for the as-deposited and theheat-treated films are within the experimental uncertainties comingfrom the slight decrease in the transmittance. The increase tendency isevident from the experimental data, along with a slight displacementof the interference maxima toward smaller wavelengths.

4. Conclusions

Chromium-doped titanium oxide thin films, deposited by DCreactive sputtering, on unheated glass substrates, present anamorphous structure. After heat treatment, a mixed anatase/rutilestructure developed (crystallites size under 12 nm), a large amount ofthematerial still remaining in the amorphous state. XANES and EXAFStechniques, used to derive information on the local structure aroundchromium atoms present in trace concentration (2 at.%) in the as-deposited and annealed sputtered TiO2 thin films, suggest theexistence of a high degree of amorphisation in both kind of samples.The strong enhancement (by a factor of 1.67) of the amplitude of theCr 1 s pre-edge peak after thermal treatment may suggest an increaseof the number of available states and thus, the possibility to controlthe (average) oxidation state of Cr by thermal treatment; this willcertainly may serve to trim the material properties in order to be usedas dilutedmagnetic semiconductor, or as photocatalyst. EXAFS spectrareveal similar atomic neighboring around Cr, and no Crmetal particlesor aggregates in the investigated material. By heat treatment, thesamples crystalline structure “shrinks” instead of expanding (byaround 2%), and this “shrinkage” results in strong ionization processesof chromium.

The optical transmittance measurements performed on the samesamples reveal a higher “optical density” of the heat treated film,which correlates well with the improvement in the crystallinity byheat treatment. The dispersion of the refractive index below theinterband absorption edge supports this hypothesis. The coordinationnumber increases from 4.69 for the as-deposited amorphous sample,to 4.71, for the polycrystalline sample. While the data obtained fromthe optical measurements were not very much satisfactory for theinterpretation of the annealing-induced effects on Cr-doped films, theEXAFS study has conducted to significant straightforward conclusions,demonstrating its usefulness in such investigations.

Acknowledgements

One of the authors (D. Mardare) is grateful to Professor F. Levyfrom the Institute of Applied Physics of the Polytechnic Federal Schoolof Lausanne, Switzerland for providing the necessary laboratoryfacilities to carry out a part of this investigation. We acknowledgequalified support from Dr. EdmundWelter and Dr. Dariusz Zajac fromthe Hasylab staff.

This work was supported by CNCSIS Contract PCCE-ID_76/2010and by grant POSDRU/89/1.5/S/49944.

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