Visible light activated photocatalytic behaviour of rare earth modified commercial TiO2

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Visible light activated photocatalytic behaviour of rare earth modifiedcommercial TiO2

D.M. Tobaldi a,*, R.C. Pullar a, A. Sever Skapin b, M.P. Seabra a, J.A. Labrincha a

a Department of Materials and Ceramic Engineering/CICECO, University of Aveiro, Campus Universitario de Santiago, 3810-193 Aveiro, Portugalb Slovenian National Building and Civil Engineering Institute, Dimiceva 12, SI-1000 Ljubljana, Slovenia

1. Introduction

The contamination of natural water reserves, air (indoor andoutdoor) and soil is currently one of the major global environmen-tal concerns. Therefore, to further the sustainable development ofmodern society, there is an urgent need for advances in greentechnologies to achieve environmental remediation.

Environmental catalysis is one of the most studied processes inorder, not only to reduce, but also to prevent the causes ofpollution. Amongst these catalytic processes, photocatalysis isexpected to be one playing an important role in the 21st century’sefforts in reducing environmental pollution [1]. Titanium dioxide(TiO2) is the most investigated photocatalyst; the great interest inTiO2 can be related to the work by Fujishima and Honda, publishedin 1972 [2]. This described the photo-assisted electrolysis of waterupon irradiation of a single-crystal TiO2 electrode (with a Ptcounter-electrode), with photons of energy greater than the band

gap of TiO2. However, this was not strictly catalysis, the reactionnot being thermodynamically feasible without the photons – itwas actually an energy storage reaction that can be termedphotogalvanic [3]. The first reported works rigorously dealing with‘‘photocatalysis’’ are those by Doerfler and Hauffe, published in1964 [4,5]. In any case, we are still experiencing today anenormous boom in this field of research, with a great number ofpublications concerning photocatalysis appearing over the past 20years. Photocatalysis can be described as that phenomenon inwhich a material (a semiconductor) modifies the rate of a reaction,via the action of light having a suitable wavelength [6,7]. When asemiconductor is irradiated with photons having energy higherthan, or equal to, its energy band gap (Eg), an electron (e�) is able tomigrate from the valence band to the conduction band, leaving ahole (h+) behind. Such a photo-generated couple (e�–h+) is able toreduce and/or oxidise a pollutant adsorbed on the photocatalystsurface [8].

As a photocatalytic material, TiO2 is chemically inert and non-toxic; the reactions take place at mild operating conditions (e.g. alow level of solar or artificial illumination, room temperature (RT)and atmospheric pressure); no chemical additive is necessary;

Materials Research Bulletin 50 (2014) 183–190

A R T I C L E I N F O

Article history:

Received 16 April 2013

Received in revised form 2 October 2013

Accepted 17 October 2013

Available online 26 October 2013

Keywords:

A. Oxides

B. Phase transitions

B. Optical properties

C. Raman spectroscopy

D. Catalytic properties

A B S T R A C T

A commercial TiO2 nanopowder, Degussa P25, was modified with several rare earth (RE) elements in

order to extend its photocatalytic activity into the visible range. The mixtures were prepared via solid-

state reaction of the precursor oxides, and thermally treated at high temperature (900 and 1000 8C), with

the aim of investigating the photocatalytic activity of the thermally treated samples. This thermal

treatment was chosen for a prospective application as a surface layer in materials that need to be

processed at high temperatures.

The photocatalytic activity (PCA) of the samples was assessed in gas–solid phase – monitoring the

degradation of isopropanol (IPA) – under visible-light irradiation. Results showed that the addition of the

REs lanthanum, europium and yttrium to TiO2 greatly improved its photocatalytic activity, despite the

thermal treatment, because of the presence of more surface hydroxyl groups attached to the

photocatalyst’s surface, together with a higher specific surface area (SSA) of the modified and thermally

treated samples, with regard to the unmodified and thermally treated Degussa P25. The samples doped

with La, Eu and Y all had excellent PCA under visible-light irradiation, even higher than the untreated

Degussa P25 reference sample, despite their thermal treatment at 900 8C, with lanthanum producing the

best results (i.e. the La-, Eu- and Y-TiO2 samples, thermally treated at 900 8C, had, respectively, a PCA

equal to 26, 27 and 18 ppm h�1 – in terms of acetone formation – versus 15 ppm h�1 for the 900 8Cthermally treated Degussa P25). On the other hand, Ce–TiO2s had no significant photocatalytic activity.

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E-mail addresses: [email protected], [email protected] (D.M. Tobaldi).

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volatile organic compounds (VOCs) [9,10], and even very recalci-trant and persistent pollutants, can be degraded [11]. Moreover,TiO2 heterogeneous photocatalysis is, at the same time, efficient ingreen chemistry, in fine chemicals, and in advanced oxidationprocesses (AOP) [12,13].

The photocatalytic reaction of TiO2 is activated by UVA light,although titania is transparent for most of the visible radiationregion, since it is a wide band gap semiconductor material (Eg = 3.2and 3.0 eV for anatase and rutile, respectively). This means that thephotocatalytic reaction is exploited by only 3–5% of the solarspectrum [14]. Several different paths were followed in attempts tocreate visible-light activated photocatalysts. With the aim ofextending the absorption edge of TiO2 into the visible region, thiswas using doped/modified with non-metal atoms [15,16], transi-tion metals [17], or, more recently, noble-metal atoms [18,19].Anionic doping often had a detrimental effect on the photocatalyticactivity (PCA) of TiO2 under UV-light irradiation, because of anenhancement of the charge recombination [20], and also becausesuch TiO2 has a low thermal and chemical stability [21]; moreover,an excessive nitridation treatment might remove the doped Natoms from the TiO2 matrix, leading to a decrease in the visible-light PCA [22].

In this paper we report the synthesis, via solid-state reaction, ofvisible-light activated photocatalysts, made from a commercialTiO2 powder (Degussa P25) modified with several rare earthelements (RE = cerium, lanthanum, europium and yttrium), andthermally treated at temperatures as high as 900 and 1000 8C, inorder to investigate the photocatalytic activity mechanism (PCA) ofthe RE-modified-TiO2. Degussa P25 has been previously modifiedwith REs, in order to investigate its PCA under UV-light exposure[23,24], but this is the first time that such a high temperaturethermal treatment (essential for co-processing with some materi-als), together with exclusively visible light irradiation, is assessedin this system. REs were chosen as modifying agents because, dueto the transitions of 4f electrons, the optical absorption of titania isincreased, thus improving its visible-light response, and promotingthe separation of photo-generated electron–hole pairs [25].

2. Experimental

2.1. Sample preparation and characterisation

Samples were prepared via solid-state reaction of the twocrystalline end-members, according to the stoichiometry: Ti1�x-

RExO2 where RE = Ce, Eu, La, and Y, and x = 0, 0.01, and 0.025 mol.Degussa P25 TiO2 powder was used as the titania source (hereafterdesignated as P25), while reagent-grade CeO2, Eu2O3, La2O3, andY2O3 (all supplied by Aldrich) were used as RE precursors. Powderswere admixed, and wet ground in a rotary ball mill (30 min withdeionised water and sintered zirconia balls). Mixtures were driedin an oven (105 8C for 12 h), ground in an agate mortar, and thenthermally treated; the thermal treatment was performed in anelectric oven with static air atmosphere, the heating rate was200 8C h�1, and two maximum temperatures were used (900 and1000 8C), with a soaking time at the maximum temperature of 4 h,followed by natural cooling. Pure TiO2 samples, used as a reference,were referred to as P25 followed by a number indicative of themaximum temperature reached (i.e. P25-900 for Degussa P25 TiO2

powder thermally treated at 900 8C). For TiO2–RE mixtures, thesymbol of the RE chemical element followed by the amount of REcation inserted – with the molar content expressed as a percentage– was used, followed by the temperature (i.e. sample with 0.01 molof Europium added to P25 titania, and calcined at 900 8C, is referredto as: Eu1-900).

The phase composition of the starting titania powders, as wellas that of the TiO2–RE mixtures, was obtained via X-ray diffraction,

using a Rigaku Geigerflex (JP); the patterns were collected in the15–808 2u range (0.028 2u s�1 step-scan, and 5 s/step). A semi-quantitative estimate of the likely amount of anatase/rutile ratio,as well as the secondary phases – in the TiO2–RE mixtures – wasachieved by adopting the generalised RIR method [26,27],assuming the absence of amorphous phase in the examinedpowders. By contrast, the as-received P25 was assumed to be –after Ohtani et al. [28] – a mixture of anatase/rutile/amorphousphase, that is approximately: 78/14/8; having a specific surfacearea of �50 m2 g�1. The morphology of the samples wasinvestigated by FE-SEM (Hitachi, SU-70, JP), equipped with anenergy dispersive X-ray spectroscopy (EDS) attachment (BrukerQuantax 400, DE).

Optical spectra of the samples were acquired on a Shimadzu UV3100 (JP), in the UV–Vis range (200–800 nm), with 0.02 nm step-size, and using BaSO4 as reference. The diffuse reflectance (R1) wasconverted into the absorption coefficient a, using the Kubelka–Munk equation: a ¼ ð1 � R1Þ2 � 2R�1

1 [29]. The Eg of the sampleswas calculated using the Tauc plot. This method assumes that theabsorption coefficient, a, of a semiconductor can be expressed as:(ahn)

g= A(hn � Eg), where A is a material constant, h is Plank’s

constant, n is the frequency of the light, Eg is the energy band gap ofallowed transitions, and the power coefficient g is characteristic ofthe type of transition. For nanoscale semiconductor materials, thevalue of g is accepted to be equal to 0.5, since for such materials,the transition is assumed to be indirectly allowed [30]. Hence,plotting [F(R1)hn]0.5 against hn, one can obtain the energy bandgap of the semiconductor material, from the x-axis (a = 0)intercept of the line tangent to the inflection point of the curve.

In order to find out the possible occurrence of OH groups and/orwater adsorbed on photocatalyst’s surface, FT-IR analysis wasperformed using a Bruker Tensor 27 (DE). The measurements werecarried out over the wavenumber range of 4000–350 cm�1, and thepowders (2 mg) were mixed with KBr (200 mg, to give 1 wt% ofpowder in the KBr disks), and pressed into thin pellets. Prior tocarrying out the FT-IR analysis, the pellets were stored in an oven at120 8C for 30 min. Raman spectra of the samples were acquired ona HR 800 (Horiba Jovin Yvon, JP), equipped with a 532 nm laser asthe excitation source (DPSS Ventus laser). The specific surface area(SSA) of the prepared samples was evaluated by the Brunauer–Emmett–Teller method (BET) (Micromeritics Gemini 2380, US),using N2 as the adsorbate gas.

2.2. Evaluation of photocatalytic activity (PCA)

The PCA of the prepared powders was evaluated in gas-solidphase, by monitoring the degradation of isopropanol (IPA) and thesubsequent formation of acetone, with FT-IR spectroscopy [31,32].Dyes were not used because they can be excited by visible-lightirradiation, and consequently act as a sensitiser, with electroninjection from the photo-excited dye to the photocatalyst [33,34].Hence, this electron transfer may destroy the regular distributionof conjugated bonds within the dye molecule, causing itsdecolourisation, but not its mineralisation [35]. The deviceemployed for the gas-solid phase tests, operating in batchconditions, is shown in Fig. 1. It is composed of a cylindricalreactor (1.4 L in volume), covered by quartz glass and connected byTeflon tubes to the FT-IR spectrometer, the whole system washermetically sealed; the PCA experiments were assessed at23 � 2 8C. The light source was a 300 W Xenon lamp (Newport OrielInstruments, US); visible-light irradiation was achieved by way of afilter at 400 nm. Light intensity was approximately zero in the l rangeof 300–400 nm, and 160 W m�2 in the l range of 400–800 nm (insetin Fig. 1).

The samples were prepared in the form of a thin layer of powder,with a constant mass (about 50 mg), and thus approximately

D.M. Tobaldi et al. / Materials Research Bulletin 50 (2014) 183–190184


constant thickness, in a 6 cm diameter Petri dish; the workingdistance between the Petri dish and the lamp was 6 cm. Therelative humidity in the reacting system was kept constant in therange 25–30% by means of a flow of air passing through molecularsieves, until a pre-defined humidity was attained. Each experi-ment was performed by injecting 5 mL of IPA (�1100 ppm in gasphase) into the reacting system through a septum; the totalreaction time was set at 24 h. At the beginning of eachphotocatalytic experiment, when IPA had been injected, the lampwas kept switched off, so as to achieve an IPA adsorption–desorption equilibrium with that powder. The lamp was thenswitched on after that adsorption–desorption equilibrium hadbeen accomplished. The photocatalytic activity of the powderswas assessed according to the value of the rate constantrepresenting the oxidation of IPA into acetone. The concentrationsof IPA and acetone in the reacting system, before and afterswitching on the lamp, were continuously checked by monitoringthe calculated areas of their characteristic peaks, at 951 cm�1 and1207 cm�1, respectively, using a FT-IR spectrometer (Perkin ElmerSpectrum BX, US) in situ. The analysis of the data was establishedon a two-step oxidation mechanism: in the first step, IPA degradesinto acetone; in the second step, acetone transforms into furtheroxidation products. The first step is usually considered a zero-order reaction – with a rate constant (k1) – while the second stepcan be assumed to be a first-order reaction [31,32]. In the presentcontext we are merely interested in the relative change of the rateconstant k1, which is an estimate of the photocatalytic activity.Therefore, the PCA was evaluated as the rate constant of the initialacetone formation, because at RT, the photocatalytic oxidation ofIPA to form acetone is fast, whereas the subsequent oxidation ofacetone to CO2 and H2O is slower [36]. Over very short times, theslope is proportional to the formation rate constant. Being the firstintermediate of IPA degradation, the formation of acetone is aprocess that can easily be distinguished from the subsequentdegradation process occurring during IPA photo-oxidation [37].The PCA measurements of P25-REs were repeated in duplicate inall cases, and the values reported were calculated as an average

value. The experimental error of the method employed was foundto be in the range of 0.5 ppm h�1.

3. Results and discussion

3.1. X-ray diffraction

The addition of RE did not delay the anatase-to-rutile phasetransition (ART) in all cases, at the thermal treatment temperaturesused. Notwithstanding, a delaying effect by REs on the ART withP25 TiO2 has actually been reported by Lin and Yu (CeO2, La2O3, andY2O3, at a concentration of 0.5 wt%, with calcination temperaturesof 650 and 700 8C) [23], and by Du et al. (CeO2 and La2O3, at aconcentration of 2 wt%, thermally treated at 800 8C) [24]. The XRDpatterns all showed only the rutile phase of TiO2, the onlyexception being sample La1 thermally treated at 900 8C, which stillcontained a small amount of anatase (1.8 wt%) amongst 98.2 wt%rutile (Table 1, inset in Fig. 2). As recently reported by Ermokhinaet al. [38], as lanthanum is more electropositive than titanium,causing a partial transfer of electrons from the La–O bonds to theTi–O bonds, leading to a strengthening of the Ti–O bonding, in theanatase structure. Here, we believe that La3+ ions occupiedinterstitial positions in titania lattice – because of ionic radiiissues, see below – thus favouring the slow grain growth rate ofanatase [39]. This was due to the segregation of La3+ at the anatasegrain boundaries, thus increasing the diffusion barrier at thetitania–titania grain interface, hindering the grain growth process.

Apart from with cerium, which does not form a stable mixedtitanate compound in air, La1-900 was also the only sample whichdid not contain evidence of a reaction between the RE and TiO2

forming a RE-titanate compound (Fig. 2). The absence of a solidsolution between RE and TiO2 at the low temperature of 900 8C isreasonable, due to the significant difference between the effectiveionic radii of REs and Ti4+ ([6]Ce4+ = 0.87 A, [6]Eu3+ = 0.95 A,[6]La3+ = 1.03 A, [6]Y3+ = 0.90 A; [6]Ti4+ = 0.61 A [40]). At 1000 8C,or with 2.5 mol% La3+ at 900 and 1000 8C, the lanthanum titanateLa4Ti9O24 was detected in significant amounts. The pyrochlore

Fig. 1. Scheme of the reactor utilised for the photocatalytic tests; in the inset is reported the spectral irradiance of the Xenon lamp, at the working distance, with filter at

400 nm.

D.M. Tobaldi et al. / Materials Research Bulletin 50 (2014) 183–190 185


phases Eu2Ti2O7 and Y2Ti2O7 existed in all Eu and Y doped samples.This observed behaviour is also in good agreement with therespective phase diagrams. According to Iwasaki [41], Mizutaniet al. [42], and Skapin et al. [43], Eu2O3, Y2O3, and La2O3 reactedwith TiO2 to form the RE-titanium oxides Eu2Ti2O7, Y2Ti2O7 andLa4Ti9O24, respectively. In the TiO2–Y2O3 system observed here,together with rutile and the Y2Ti2O7 intermediate, there was also avery small amount (1.4–0.6 wt%) of the other end-member/starting material, Y2O3, which did not react with TiO2. At a highertemperature, 1000 8C, the amount of Y2O3 decreased, with greaterY2Ti2O7 formation. Neither the Eu nor La doped samples retainedany of the RE oxide starting materials. As expected, the TiO2–CeO2

system shows no reactions between the two end-members, at boththe temperatures used here.

3.2. FT-IR and Raman analysis

The FT-IR spectra of the samples are depicted in Fig. 3a–c. TheTi–O–Ti vibration band, in the range of 400–600 cm�1 [44], is

observed in all the FT-IR spectra. The peak at �1620 cm�1

corresponds to the bending vibrations of O–H, and the broad bandat around 3450 cm�1 was attributed to the surface adsorbedhydroxyl groups [45]. The sharp bands that are in the region 2750–3000 cm�1 are attributable to the surface adsorbed water [31].The thermal treatment at 900 8C of the undoped P25 led to adecrease in the intensity of the bands at 3450, and 2750–3000 cm�1, suggesting that the terminal hydroxyls are removedfrom the surface of TiO2. On the other hand, the TiO2–RE mixtures,thermally treated at 900 8C (Fig. 3b and c), still showed thepresence of those bands (more intense in the La-, and Eu-dopedsamples, not present in Ce-doped samples), indicating that the REmodification resulted in more surface hydroxyl groups on thephotocatalysts, even after the thermal treatment. It has beensuggested that RE metal ions with a 3+ oxidation state, create acharge imbalance in TiO2. Hence, more hydroxide ions would beadsorbed on the surface, for a charge balance. These hydroxideions can accept holes to form hydroxyl radicals, oxidising theadsorbed molecules [23]. CeO2–TiO2, not having that charge

Table 1Phase composition, energy band gap (Eg), and specific surface area (SSA) of the samples.

Sample Phase composition (wt%) Eg (eV) SSABET (m2 g�1)

Anatase Rutile RE–TiO2 RExOy

P25-900 – 100 – – 3.01 2.0

Ce1-900 – 98.0 – 2.0 2.91 2.1

Ce2.5-900 – 94.1 – 5.9 2.90 3.3

Eu1-900 – 98.7 1.3 – 3.03 8.6

Eu2.5-900 – 91.7 8.3 – 3.04 9.1

La1-900 1.8 98.2 – – 3.04 11.5

La2.5-900 – 88.5 11.5 – 3.04 12.0

Y1-900 – 98.2 1.8 – 3.03 8.1

Y2.5-900 – 94.8 3.9 1.4 3.03 8.1

P25-1000 – 100 – – 2.98 0.5

Ce1-1000 – 98.0 – 2.0 2.84 <0.1

Ce2.5-1000 – 94.8 – 5.2 2.89 0.4

Eu1-1000 – 96.6 3.4 – 3.01 2.8

Eu2.5-1000 – 91.2 8.8 – 3.01 3.1

La1-1000 – 94.8 5.2 – 3.02 4.6

La2.5-1000 – 88.4 11.6 – 3.03 4.4

Y1-1000 – 97.6 2.4 – 3.00 2.2

Y2.5-1000 – 93.8 5.6 0.6 3.01 2.3

Fig. 2. XRD patterns of the RE-modified samples thermally treated at 900 8C. The inset represents a magnification of the La1-900 sample (framed area), in the 24.5–29.58 2urange, in order to highlight the anatase (1 0 1) reflection. A and R are symbols, standing for: anatase and rutile, respectively; *, y, z, � symbols indicate: CeO2, Eu2Ti2O7, Y2O3,

and Y2Ti2O7, respectively.



imbalance, has considerably fewer hydroxyl groups adsorbed onthe surface (cf Fig. 3b and c).

Raman spectra of the samples thermally treated at 900 8C areshown in Fig. 4a and b. The band at around 144 cm�1,corresponding to the Eg anatase mode [46], is found to be inevery spectrum (Fig. 4a). This is in contrast to the XRD data, whichonly showed anatase to be present in sample La1-900. In thissample the anatase XRD peak was extremely small (Fig. 3),estimated as <2 wt%, and if present in lower amounts in othersamples, anatase could be undetectable by XRD – the Ramantechnique having a lower detection limit [47]. Although the Ramantechnique cannot be used for quantitative analysis, a comparisonbetween different samples can still be performed [48]. It can beseen that the anatase peak is much more intense for the La1-900

sample, indicating a higher anatase concentration in this samplethan the others, explaining the absence of anatase peaks in theother XRD data. The peaks at around 447 cm�1 (Eg), 612 cm�1 (A1g),and 822 cm�1 (B2g), are assigned to rutile [49]. The broad band,centred at around 250 cm�1, which is a specific feature of rutile,has a complex nature. Gotic et al. and Frank et al. [50,51], suggestedthat second-order scattering, as well as disorder effects, areinvolved in its formation. Furthermore, in the RE-modifiedsamples, the Raman peak corresponding to the Eg mode of anataseis only very slightly blue-shifted compared to the unmodifiedsample (P25-900), whilst that corresponding to the Eg rutile modehad a very weak red-shift (Table 2). This slight shifting in the Eg

modes of anatase and rutile are barely noticeable, and are notconsidered significant enough to show any increase in oxygenvacancies or deficiency [52], that could be induced by RE

Fig. 3. FT-IR spectra of a) the undoped P25 at RT, and thermally treated at 900 8C; b)

FT-IR spectra of the Ce1, Eu1, La1, and Y1 samples thermally treated at 900 8C; c)

magnification, in the wavenumber range 4000–2800 cm�1, of the samples modified

with 2.5 mol% of REs and thermally treated at 900 8C, in order to emphasise the

surface adsorbed hydroxyl groups.

Fig. 4. a) Raman spectra of the samples, in the wavenumber range of 100–

1000 cm�1. A and R are symbols, standing for anatase and rutile, respectively; b)

Raman spectrum of the Eu1-900 sample, in the wavenumber range of 1000–

3500 cm�1.

Table 2Raman peak positions of the anatase and rutile Eg vibrational modes.

Sample Raman peak (cm�1)

Anatase (Eg) Rutile (Eg)

P25-900 144.0 445.8

Eu1-900 144.8 445.3

La1-900 144.4 445.6

Y1-900 144.3 445.5



substitution [53,54]. Therefore, we can say that the oxygenstoichiometry of the titania is preserved with the addition ofthe RE dopants.

The Raman spectrum of sample Eu1-900, in the wavelengthrange 1000–3500 cm�1, is depicted in Fig. 4b. It is interesting to notehere the luminescence emissions of the Eu-modified TiO2, consistentwith Frindell et al. [55], who found these emissions on a titania filmdoped with Eu3+, but only when this was excited with a wavelengthabove the titania band gap. The emission lines shown are associatedwith Eu3+ ions, and they correspond to radiative relaxations from thelevel 5D0 to its low-lying multiplets 7FJ (J = 0, 1, 2, 3) [56], indicatingthat the Eu3+ emission is associated with a non-radiative energytransfer process from TiO2 to Eu3+ crystal field states [57]. Thefollowing emissions were found: 5D0! 7F0, 5D0! 7F1, 5D0! 7F2,5D0! 7F3 [56,58], with the strongest emission band – located at�2500 cm�1 – associated with 5D0! 7F2 transitions. These lattertransitions have been reported to be possible only if Eu3+ ions occupya site without an inverse symmetry [55].

3.3. UV–vis spectroscopy

The DRS spectra of the doped powders, thermally treated at900 8C and 1000 8C, are shown in Fig. 5a and c. All the spectra, at

both the thermal treatment temperatures, consisted of a singleabsorption band below approximately 410 nm, ascribed to themetal-ligand charge transfer in titania [59]. At 900 8C, all the dopedpowders showed a slight shift of their absorption edge into thevisible region, due to the presence of rutile (Fig. 5a). Moreover, theCe–TiO2 sample showed a tail in the visible region of the spectrum,that extends up to �450 nm, due to the contribution of the freeCeO2 in that sample [60]. At 1000 8C (Fig. 5c), as before, only theCe–TiO2 showed a noticeable shift of its absorption edge into thevisible region, keeping the tail, at around 450 nm.

The Egs of the powders were estimated by the Tauc procedure(Table 1, and Fig. 5b and d): the curve – resulting from the plot ofthe transformed Kubelka–Munk equation [F(R1)hn]0.5 versus thephoton energy (hn) – was fitted adopting a sigmoidal Boltzmannfunction. Afterwards, the Eg value was obtained by the x-axisintercept of the line tangent to the inflection point of that curve.The resulting ‘‘apparent’’ band gaps of the samples are reported inTable 1, and were assigned to the rutile phase, since they areconsistent with the expected rutile Eg value, that is 3.02 eV. Theonly exceptions, consistent with the DRS spectra, were the Ce–TiO2

samples, whose Egs were shifted into the visible region (2.84–2.91 eV, cf Table 1), due to the contribution of free CeO2 in thesamples, as detected by the tail in the DRS spectra.

Fig. 5. a) UV–vis spectra of the RE-modified samples, thermally treated at 900 8C; b) Kubelka–Munk elaboration for the same samples. The dotted line represents the x-axis

intercept of the line tangent to the inflection point of the sample La1-900, e.g. its apparent Eg, calculated with the Tauc procedure; c) UV–vis spectra of the RE-modified

samples, thermally treated at 1000 8C; d) Kubelka–Munk elaboration for the same samples. The dotted line represents the x-axis intercept of the line tangent to the inflection

point of the sample La1-1000, calculated with the Tauc procedure.



4. Photocatalytic activity

The PCA results in gas–solid phase, under visible lightirradiation, are shown in Fig. 6. The untreated P25 powder hada PCA – in terms of acetone formation – of 23 ppm h�1. Hurum et al.[61], using electron paramagnetic resonance (EPR) spectroscopy,claimed that the visible-light response of P25 is due to the presenceof small rutile crystallites amongst the anatase, their smaller bandgap extended the useful range of PCA into the visible region.Moreover, the points of contact between these anatase and rutilecrystals allow for rapid electron transfer from rutile to anatase.Thus, rutile acts as an ‘‘antenna’’ to extend the PCA into visiblewavelengths. Quite obviously, the thermal treatment of theundoped P25 powder, led to a decrease of its PCA (acetoneformation equal to 15 and 4 ppm h�1, at 900 and 1000 8C,respectively). This is due to the concomitant size growth of thetitania particles, and the decrease of their SSA. On the contrary, at900 8C, the RE addition to TiO2 improved the PCA of thesethermally treated samples. The best performing sample was La2.5-900, with an activity of 32 ppm h�1 of acetone formation. The threesamples La2.5-900, Eu1-900 and La1-900 all had excellent PCAunder visible-light irradiation, even higher than the untreatedreference sample, with these two latter samples having a PCA of 27and 26 ppm h�1, respectively, in terms of acetone formation –despite the thermal treatment at a temperature as high as 900 8C.The only sample that had no PCA was the CeO2–TiO2 one. Theincrease of the thermal treatment temperature to 1000 8C led to anobvious decrease of the overall PCA. At this temperature, samplespossessing the best PCA were La1-1000 and La2.5-1000, with anactivity, estimated as acetone formation, of 8 and 13 ppm h�1,respectively. Such behaviour can be explained by the high SSAvalues (and consequently by their reduced particle size), as shownin Table 1, and by the FE-SEM images in Fig. 7a–c, comparing theP25-900, La1-900 and Eu1-900 samples after heating at 900 8C.The samples with the largest SSA values also had the highest PCA(cf Table 1 and Fig. 6). As a matter of fact, the samples with thelowest SSA, based on Ce–TiO2, were also the samples with thelowest PCA, at both the thermal treatment temperatures, and theincrease of the thermal treatment temperature at 1000 8C led to adecrease of the SSA, and consequently, to a decrease in the PCA. It isalso believed that a high thermal treatment can induce desorptionof surface oxygen, this leaving an oxygen vacancy and Ti3+ sites ontitania. It was hence supposed that electrons may be excited fromthe valence band to the oxygen vacancy states, even with theenergy of visible light [62,63]. Yin et al., proposed that rutile withsmall particle size, together with its small band gap value, can be a

visible light activated photocatalyst [64]. Also Orlikowski et al.,monitoring the oxidation of phenol under visible light exposure,attributed the small particle size as being a key factor for rutile asan active photocatalyst [65]. Moreover, as detected by the FT-IRanalysis, these RE modifications resulted in more water/hydroxylgroups attached to the surface of the thermally treated photo-catalyst. This enhances the PCA of titania, because it helps generatechemical oxidative species, such as hydroxyl radicals, that ‘‘switchon’’ the photocatalytic oxidation [66] (with the only exception ofCe–TiO2 samples, that basically had no PCA – also consistent withthe work by Lin and Yu [23]). Furthermore, as was reported by Niuet al. [67] and Shi et al. [68], REs such as Eu, La, and Y can favour theseparation of the photo-generated couple e––h+, and hence preventtheir recombination, increasing TiO2’s PCA. This statement was

Fig. 6. Photocatalytic activity of the samples, in gas-solid phase, under visible-light

irradiation.

Fig. 7. FE-SEM micrographs of samples: a) P25-900; b) La1-900; c) Eu1-900; in the

inset is reported an EDX spectrum of this sample (the signal of Al belongs to the

sample-holder).



further confirmed by Stengl et al. [25]: they claimed that the 4f

electron transitions of REs strengthened the optical adsorption ofthe catalysts, supporting the separation of photo-generated pairs.A combination of all these features resulted in a marked increase ofthe photocatalytic ability of the La, Eu or Y doped titania afterheating to 900 8C, particularly in the case of La. The only inactivesamples were those based on Ce–TiO2.

5. Conclusions

A commercial TiO2 powder, Degussa P25, was modified with aseries of REs in order to investigate the PCA mechanism, undervisible-light irradiation, of samples treated at high temperature.The mixtures studied were prepared via solid-state reaction of theprecursor oxides, and the products of the synthesis were thermallytreated at 900 8C and 1000 8C. From a mineralogical point of view,the addition of RE accelerated the ART, at both thermal treatmenttemperatures, because of the formation of oxygen vacancies. At thesame time, the RE modification led to more surface hydroxylgroups on the photocatalysts, even after the thermal treatment.This, together with an increase of the SSA – compared to theunmodified sample – greatly contributed to enhance the visible-light PCA of the Eu-, La-, and Y-TiO2s (all in the rutile phase, theonly exception being sample La1-900, containing a small amountof anatase), thermally treated at 900 8C. At this temperature, theonly inactive sample was the Ce-modified one. The increase of thethermal treatment temperature, 1000 8C, led to a dramaticdecrease of the PCA for all the samples.

Acknowledgements

Authors wish to acknowledge PEst-C/CTM/LA0011/2013 pro-gramme. M.P. Seabra and R.C. Pullar wish to thank the FCTCiencia2008 programme for supporting this work. Maria CelesteCoimbra de Azevedo (Chemistry Department, University of Aveiro)is gratefully acknowledged for the FT-IR measurements.

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Visible light activated photocatalytic behaviour of rare earth modified commercial TiO2

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