effective optoelectronic and photocatalytic response of eu3+-doped tio2 nanoscale systems...

Effective optoelectronic and photocatalytic response of Eu31-dopedTiO2 nanoscale systems synthesized via a rapid condensation technique

Nibedita PaulDepartment of Physics, Nanoscience and Soft Matter Laboratory, Tezpur University, Tezpur 784 028, Assam, India

Dambarudhar Mohantaa)

Department of Physics, Nanoscience and Soft Matter Laboratory, Tezpur University, Tezpur 784 028, Assam,India; and Department of Physics and School of Engineering and Applied Sciences, Harvard University,Cambridge, Massachusetts 02138

(Received 3 January 2013; accepted 16 April 2013)

In this work, we report on the optoelectronic and photocatalytic features of europium (Eu31)-dopedTiO2 nanoscale particles synthesized via a sol-gel mediated rapid-condensation technique. X-raydiffraction studies have revealed the mixed phases of the synthesized systems. In particular, a mixtureof anatase, brookite, and rutile phases was found to coexist beyond a sintering temperature of 600 °Cwhile a pure anatase phase was witnessed below 500 °C. The photoluminescence spectra of ;7 nmsized anatase TiO2 nanoparticles have exhibited different intra 4f (Eu

31 ion related) transitions with themost intense red emission (5D0!7F2) peak located at ;613 nm. The emissions due to color centersand oxygen vacancies of TiO2 were also evident in the PL spectra. The Brunauer-Emmett-Tellersurface area analysis has revealed a significant increment of surface area and pore volume owing to theenhanced interfacial region introduced by point defects and dislocations due to Eu doping.The photocatalytic activity of the Eu31 doped TiO2 nanoscale system was found to be;12% strongerthan its un-doped counterpart, as assessed from the degradation of methyl orange (MO) solution underUV light irradiation. The percentage of degradation was found to be strongly dependent on theduration of the UV exposure and Eu doping concentration. As an efficient photosensitive candidate,rare earth sensitized TiO2 systems would bring new insights while displaying both optoelectronic andphotocatalytic characteristics through use of the localized states present in the band gap of the host.

I. INTRODUCTION

As an industrially relevant candidate, nanoscale tita-nium dioxide (TiO2) system is widely used in paints,1

pigments,2 ointments3 etc. In addition to its non-hazardousnature, TiO2 is regarded as one of the best photosensitive,wide band gap (e.g., Eg 5 3.1 eV at 300 K for anatasephase) semiconductor systems in the family of oxidessuch as, ZnO, WO3, and SnO2.

4 It was demonstrated that,as an efficient heterogeneous photocatalyst, nano-sizedTiO2 is very useful in degrading pollutants and dyes.5,6

Furthermore, owing to its good mechanical, optical, andthermal properties, TiO2 is believed to be the ideal hostsystem for incorporating rare-earth (RE) impurity ions asdopants. One of the major advantages offered by thesematerials is the ability to tailor size dependent opticalproperties and to achieve high luminescence responsethrough the sensitization by the host.7 Several reportshave demonstrated the intense RE31 emission responseof RE incorporated TiO2 nanoscale systems along with asuitable explanation of the energy transfer mechanism.8–10

Compared to other trivalent RE ions, Eu31 gives strongred emission due to intra 4f carrier transitions thus makingthe system more useful in technologically importantoptoelectronic and display elements.11,12 Besides that,Eu-doped TiO2 nanomaterials could exhibit better photo-catalytic activity as compared to their undoped counter-part. This is because of generation of adequate number ofelectron trap centers by Eu31 ions as a result of whichrecombination time of the charge carriers can be sup-pressed drastically.13 On the other hand, the RE ions canpromote the degradation process (photocatalysis) of manyhazardous pollutants and dyes through active interactionwith the organic molecules of the pollutants.14,15 The in-corporation of lanthanide ions into the TiO2 host may helporganic pollutants to concentrate on the semiconductorsurface. Earlier, the efficiency of photocatalysis was foundto be dependent on the crystal phase of the product and wasprominent when comprised of a single anatase phase.16

The effect of mixed phase and synergistic response initiatedby nanoscale titania on photocatalytic performancewas alsodemonstrated in the past.17,18

It is known that sol-gel routes are capable of synthesizinghigh surface area TiO2 nanoparticles.

19 In the present work,we used an easy to use sol-gel mediated rapid condensationtechnique to synthesize un-doped and Eu31 doped TiO2

a)Address all correspondence to this author.e-mail: [email protected], [email protected]

DOI: 10.1557/jmr.2013.122

J. Mater. Res., Vol. 28, No. 11, Jun 14, 2013 �Materials Research Society 2013 1471

nanostructured systems. Apart from the nature and origin ofradiative emission characteristics, photocatalytic activity onmethyl orange (MO) dye was assessed for varying Eu con-tent. The incorporation of Eu31 ions into the TiO2 host hasa 2-fold aim: to enhance the catalytic performance and toyield a system that is capable of displaying tunable opto-electronic transitions along with select emission response.

II. EXPERIMENTAL DETAILS

The synthesis protocol and techniques used for studyingvarious properties are as detailed below.

A. Growth of Eu31doped TiO2 nanoscale powder

We have synthesized un-doped and Eu31 doped TiO2

nanoparticles by hydrolyzing titanium isopropoxide(TTIP, Aldrich, 99.9% pure, St. Louis, MO) in an alco-holic medium followed by a rapid condensation process.7

In a typical reaction, 3 mL of TTIP was transferred to aconical flask containing 15 mL of propanol and then, the solwas subjected to stirring (;250 rpm) for 30 min. A hydro-lyzer solution of 1 mL of HCl in 5 mL of H2O was addedto it followed by stirring at moderate temperature (;75 °C)for 3 h. The precursor solution was then kept in the darkovernight for homogenousgrowthofTiO2 nanoscale colloids.The gel formed was separated through filtration and high-speed centrifugation. Finally, the as-received gel was driedat room temperature and subsequently, annealed at differenttemperatures to yield fine powders of TiO2. For dopingpurpose, we took a suitable amount of europium acetate(EuAc, 99.9% pure, CDH) in the hydrolysis step. In the host,the doping level of Eu31 was varied as 1, 2.5, and 5%.

B. Analytical tools used for structural andoptical studies

The crystallographic information such as, diffractionplane and structural phase of the un-doped and dopedsamples was revealed by x-ray diffraction (XRD) studiesby using a Rigaku MiniFlex diffractometer (Shibuya-Ku,Tokyo, Japan) equipped with an intense Cu Ka radiation(k 5 1.54 Å). The diffraction signal was monitored forthe Bragg’s diffraction angle (2h) in the range of 20–60°.The particle size andmorphology of the un-doped and dopednanoparticles were assessed using a JEOL JEM-2100 trans-mission electronmicroscope (TEM;Akishima, Tokyo, Japan)operating at an accelerating voltage of 200 kV. The energydispersive x-ray (EDX) analysis was performed on aJEOL, 6390 LV (Singapore), scanning electron microscope.The optical absorption features and photoluminescence (PL)characteristics of the synthesized samples were examinedusing a Shimadzu-2450 UV–Vis spectrophotometer (Japan)and PerkinElmer LS 55 Fluorescence spectrophotometer(Singapore), respectively. Finally, the nanostructure-surfacearea estimation was performed through a vivid analysis of

N2 adsorption/desorption isotherms obtained at 77 K(Quantachrome Nova 1000e, Boynton Beach, FL).

C. Analytical techniques used forphotocatalytic studies

The photocatalytic activity of un-doped and Eu31

doped TiO2 nanoparticles were evaluated by examiningthe degradation of MO dye under UV light. Methyl orange(MO, C14H14N3NaO3S), a common target is known to becarcinogenic and mutagenic, was chosen in our photo-catalytic study. A suspension of 0.25 g/L of TiO2 catalystwas added toMO solution (10 mg/L) and then subjected toultrasonic agitation and stirring (in the dark) for 15 min,and 1 h; respectively. The doped and un-doped sampleswere kept independently under a UV cabinet having a UVsource power of 0.75 lW/cm2 and k 5 365 nm. The cat-alyst loaded dye solution was irradiated for 15, 30, 45, and60 min and later subjected to centrifugation (;5000 rpm)to separate out residue and supernatant free from the cata-lyst. To examine the product, 5 mL of the test specimenwas taken for the optical absorption study knowing thatthe MO solution, without a catalyst, gives an absorptionpeak (C0) at 464 nm.20,21 The decrease in concentra-tion (or degradation) of MO from its initial value wasanalyzed from the suppressed nature of the absorptionpeak maxima (Ct) of the UV-exposed specimen.

III. RESULTS AND DISCUSSION

The crystallographic, morphological, optoelectronic andphotocatalytic responses of the synthesized titania nano-structured systems are as discussed below.

A. Structure, phase, and morphological analysis

Figure 1(a) depicts a set of XRD patterns of the un-doped TiO2 nanopowder samples synthesized at differentannealing temperatures of 200, 300, 400, 500, and 600 °Cwhich are labeled as T1, T2, T3, T4, and T5; respectively.As can be noticed for samples T1 to T4, the diffractionpeaks (101), (004), (200), and (105) correspond primarilyto the tetragonal anatase phase of TiO2 (a5 b5 3.785 Å,c 5 9.514 Å; space group: D194h).20 But T5 sampleexhibited a mixed phase of tetragonal anatase, rutile(a 5 b 5 4.593 Å, c 5 2.959 Å; space group: D14

4h) andorthorhombic brookite (a 5 5.143 Å, b 5 5.456 Å,c 5 9.182 Å; space group: D15

2h).22 For T5 case, in

addition to the (101) diffraction peak corresponding to theanatase phase, there exist three prominent peaks emanat-ing from (121) and (131) planes of brookite and (220) ofrutile phases and are consistent to others work reportedelsewhere (JCPDS 36-1451).23 A significant phase transfor-mation is thus realized at a comparatively higher annealingtemperature (600 °C) with the intense diffraction signal at2h; 31.38°which corresponded to (121) crystal plane of thebrookite phase.

N. Paul et al.: Effective optoelectronic and photocatalytic response of Eu31-doped TiO2 nanoscale systems

J. Mater. Res., Vol. 28, No. 11, Jun 14, 20131472

It was shown earlier that, the activation energy of nucle-ation has a size dependency along with the other param-eters.24 Essentially, the anatase phase is thermodynamicallystable for crystallite sizes smaller than;11 nm. Whereas, astable brookite phase is evident for an average size in therange of 11–35 nm.24 Using the popular Williamson-Hallmethod,25 the average crystallite size is estimated as ;5,5.6, 6, 9, and 23 nm for T1, T2, T3, T4, and T5 samples,respectively. The phase transformation (occurred for T5sample) is also accompanied by an increment in the crys-tallite size, which is in agreement with the earlier works.26

The phase contents of the nano-TiO2 samples were calcu-lated from the integrated intensities using the followingformulae27:

Wb ¼ KbAb=KaAa þ Ar þ KbAb ;

Wa ¼ KaAa=KaAa þ Ar þ KbAb ;

Wr ¼ Ar=KaAa þ Ar þ KbAb ;

where Wa andWb represent the respective weight fractionsof anatase and brookite phases, and Ab the integratedintensity of the brookite peak. The authors introduced theoptimized correction coefficients Ka and Kb with values of0.886 and 2.721, respectively.27 The wt% of differentphases of T5 sample was calculated to be 73.68, 14.89, and11.43% for brookite, rutile, and anatase phases, respec-tively. The diffractograms of un-doped (T2) and Eu-doped(EuT1, EuT2, and EuT3 synthesized from independentprecursors of 1, 2.5, and 5% of Eu) nano-titania sampleshave exclusively revealed the existence of TiO2 anatasephase [Fig. 1(b)]. Previously, a dominant anatase phasewas also observed in the case of Ag-doped TiO2 particlesobtained from a rapid quenching technique.28 Since theanatase phase is superior to other phases of TiO2 in termsof photocatalytic response,19 we took interest in obtaining

the anatase phase. As the diffraction peak positions re-mained intact for un-doped and Eu-doped TiO2 nano-powder samples, it is possible that Eu31 has undergonesubstitutional doping into the titania host lattice. As canbe found, the intensity of the diffraction peaks gets sup-pressed with the increase in dopant concentration thussignifying loss of crystallinity due to lattice distortion.29

The average crystallite size was calculated to be 5.6, 5, 4,3.4 nm for T2, EuT1, EuT2, and EuT3, respectively. Anapparent suppression of the diffraction signal with the in-crease in Eu content indicates increased micro-strain in thelattice.

Both un-doped and doped TiO2 nanoscale specimenswere also viewed under the electron microscope (HRTEM)and the micrographs are presented in Fig. 2. As revealedfrom the micrograph, the un-doped system is comprisingapparently spherical particles of ;7 nm size [Fig. 2(a)].The system was believed to be highly crystalline owing tothe existence of distinct lattice fringes and having an in-terplanar spacing of ;3.52 Å [Fig. 2(a)]. The predictedvalue of the interplanar distance corresponds to the spac-ing between the consecutive (101) crystal planes of thetetragonal anatase phase of the TiO2 system, which is alsoevident from one of the major diffraction peaks of theXRDpattern. TheHRTEM image of EuT2 specimen, shownin Fig. 2(b), depicts a number of crystalline particles withwell-resolved crystal planes. A dotted red circle signifiesa ;7 nm sized particle with an interplanar spacing of;3.52 Å. A number of nanoscale defects were also evidentfrom the closer look of the micrograph. Several vacancies,missing planes as well as edge-dislocations are denoted byopen-red arrows. Although different defects might havetheir own origin but the chief reason is the difference inionic radii of Ti and Eu (ionic radii: Ti41 5 64 pm andEu31 5 94 pm). It may be noted that a particle maycomprise several crystallites and that the crystallite andparticle sizes are generally not the same in polycrystallinesystems. That is why we observed a difference in the

FIG. 1. XRD patterns of (a) un-doped TiO2 nanopowders synthesized by annealing at different temperatures of 200 °C(T1), 300 °C(T2), 400 °C(T3),500 °C(T4), and 600 °C(T5); (b) The corresponding diffractograms of un-doped (T2) and Eu-doped TiO2 samples with 1% (EuT1), 2.5%(EuT2), and5%(EuT3) doping concentrations.


J. Mater. Res., Vol. 28, No. 11, Jun 14, 2013 1473

average size predicted through XRD and TEM analysis.The selected area electron diffraction (SAED) pattern ofEuT2 sample is shown in Fig. 2(c). Though the diffractionrings are distinguishable they are not sharp owing to theintroduction of appreciable amount of polycrystallinity.The rings were indexed as (101), (004), (200), and (105)and are concomitant with the XRD patterns [Fig. 1(b)].20

The compositional analysis was performed by EDX studies.The EDX pattern of T2 specimen is characterized bystrong diffraction signals arising from Ti and O elements[Fig. 2(d)]. In contrast, for EuT1 specimen, in addition to Ti,O, the signal from Eu is also clearly observable [Fig. 2(e)].

B. Optical and optoelectronic characteristics

The UV-visible absorption spectroscopy is an importanttool to study carrier transition aspects of dopants in the host.

Figure 3(a) shows the UV-visible absorption spectra ofT1-T5 titania nanopowder samples. As can be seen fromthe spectra, the absorption edge of the samples prepared ata higher annealing temperature is broadened and extend-ing upto the visible region. The feature is more prominentin the case of T5 sample, which comprised of the mixedphase, with a proportionately large amount of the brookitephase and a small amount of anatase and rutile contents.30

The absorption spectra of Eu-doped TiO2 samples withdifferent Eu concentrations are presented in Fig. 3(b).There exists a broad absorption edge in the visible region,for all the cases (EuT1, EuT2, and EuT3). For the un-dopedsample, the sharp absorption edge was found to be locatedat ;380 nm. The extent of broadening in the spectra in-creases with increasing Eu concentration. The optical bandgap, as predicted from the Tauc’s plot31 (not shown), shows

FIG. 2. HRTEM image of synthesized TiO2 nanopowders: (a) un-doped (T2), (b) doped (EuT2) nano-titania samples, (c) The corresponding SAEDpattern of EuT2 sample. The EDX spectra highlighting constituent elements present in the samples of (d) T2 and (e) EuT2.

FIG. 3. UV-Vis absorption spectra of (a) un-doped (T1 to T5) and (b) Eu-doped TiO2 (EuT1, EuT2, and EuT3) samples. For comparison sake,response of T2 is also depicted in (b). The inset of (b) shows important electronic transitions of Eu.



a decreasing trend with respective values of 3.2, 3.16, 3.06,2.93 eV for T2, EuT1, EuT2, and EuT3 samples, respec-tively. The variation in the optical band edge could occurdue to the incorporation of localized impurity states owingto Eu31 doping. Apart from the band edge absorption, wenoticed two distinct absorption peaks (for doped samples ofEuT1, EuT2, and EuT3) corresponding to 7F0!5L6 and7F0!5D2 excitations which were positioned at ;394 and464 nm, respectively.32 However, such transitions did notexist for the un-doped sample (T2).

The interaction of photons with matter can also bevisualized from the light emission characteristics throughPL spectroscopy. The radiative/nonradiative nature alongwith the intra- and inter-band transitions can be exploredfrom the PL measurements. Figure 4(a) illustrates the roomtemperature PL spectra of the un-doped and Eu31 dopedTiO2 systems excited at a wave length of 300 nm.The asymmetrically broadened features have indicatedthat, the overall spectrum, in each case, is due to the super-imposition of independent peaks of different origin. To infera clear idea about the peak positions, we have introducedthe normalized Gaussian fitting on each of the experimentalcurve and the results are presented in Figs. 4(b)–4(e).Essentially, five peaks could be extracted which are locatedat;392, 430, 460, 490, and 540 nm. As a general trend, allthe spectra exhibited a strongUVemission peak (;392 nm)due to phonon assisted indirect transition from the edge (X)to the zone center (C) of the Brillouin zone, i.e., X1b to C3.

As predicted earlier, the emission band at ;430 nm is as-signed to the intrinsic self-trap excitons (STE) localized onTiO6 octrahedra.33 The peaks at ;460 and 540 nm are

due to color centers associated with the oxygen vacancyof two-trap electron (F center) and single-trap electron(F1 center), respectively.34 The peak positioned at;490 nm is ascribed to the charge transfer from Ti41 tooxygen ions in the TiO6 octahedron associated with theoxygen vacancies.35 The variation of the intensity ratio ofthe band edge emission to the STE and oxygen defect-related emissions is highlighted in Fig. 4(f). For a suffi-ciently large Eu concentration, the number of ionizedoxygen defects tends to increase, owing to the introduc-tion of charge imbalance created by Ti41 and Eu31 ions.Consequently, charge compensation would influence theradiative emission events, without affecting phase trans-formation and crystal growth. However, RE-doping relatedhigher order transitions (and hence, emission) originated vialuminescent centers of Eu31 ions were found to be absent.

In Fig. 5(a), we demonstrate the room temperature PLspectra of the un-doped and Eu31 doped TiO2 nano-structured systems excited at a wave length of ;405 nm.Each of the spectra is characterized by four important peaksof Eu31, which are located at;579, 591, 615, and 699 nmand corresponded to 5D0!7F0,

5D0!7F1,5D0!7F2, and

5D0!7F4 transitions.32 Note that, the overall-intensity of

the PL emission increases with the increase in Eu contentof the host. The most striking feature was the enhancementof electrically allowed 5D0!7F2 transition, which is sen-sitive to the surrounding environment of Eu31 ions, whilemagnetically allowed 5D0!7F1 transition is affected onlyweakly.36 To uncover superimposed peaks, the PL spec-trum of the EuT1 specimen was subjected to the normal-ized Gaussian fitting, as shown in Fig. 5(b). In reality, the

FIG. 4. (a) Room temperature PL spectra (kex 5 300 nm) of un-doped and Eu-doped TiO2 nanopowders; (b–e) respective Gaussian fittedcurves for T2, EuT1, EuT2, and EuT3 samples. The intensity ratio of NBE to STE and defect emission peaks versus Eu-content are shown in (f).



relative intensity ratio (R) of 5D0!7F2 to5D0!7F1 tran-

sitions would describe the breaking of centro-symmetryand degree of disorderness around the Eu31 ions.36 The Rvalue follows an increasing trend with the increase inEu- level with representative values as ;1.32, 2.44, and2.56 for EuT1, EuT2, and EuT3, respectively [Fig. 5(c)].In other words, electrically driven transitions (5D0!7F2)are more favorable than spin-exchange mediated magnet-ically allowed ones (5D0!7F1). The R value has a satura-tion trend at a higher Eu31 doping concentration.

All the possible radiative transitional events are high-lighted in the scheme of Fig. 5(d). We invoke that thesubstitutional doping that leads to a local charge imbalance

and facilitating thereby electrically driven transition is notan indefinite process. As the R value for both the EuT2 andEuT3 samples is greater than 2, this type of phosphor ishighly desirable for LED applications and can be compa-rable with the other reported oxide phosphors.37

C. BET surface area and pore size analysis

Figure 6(a) shows a set of N2 adsorption/desorptionisotherms of un-doped (T2) and Eu-doped TiO2 (EuT2)nanostructured systems. The adsorption is rather slowup to a relative pressure of magnitude ;0.5. Afterward,the isotherms exhibited a hysteresis pattern, in the relativepressure range of 0.5–0.9. The hysteresis loop is more

FIG. 5. (a) Room temperature PL spectra (kex 5 405 nm) of Eu-doped TiO2 nanopowders, (b) Gaussian fitted spectra of EuT2, and (c) the intensityratio of 5D0!7F2 and

5D0!7F1 transitions with Eu-doping level.

FIG. 6. BET surface area analysis: (a) N2 adsorption/desorption isotherms and (b) pore size distribution of T2 and EuT2 nanosystems.



steep and open for EuT2 specimen, as compared to the T2one. In fact, a hysteretic feature is the indication of dif-ferent adsorption and desorption rates of N2 moleculeswithin the specimen under study. This is chiefly because ofadequate pore-size distribution in the mesostructure region.From the pore-size distribution plot, shown in Fig. 6(b), thepore size (diameter5 2r) is estimated to be;5.2 and 4 nmfor T2 and EuT2 samples, respectively. Owing to thelarger ionic size, the introduction of Eu31 into the hostTiO2 lattice might have caused a higher space filling effect.Essentially, the pore filling (adsorption) ismediated throughcapillary condensation while pore emptying (desorption)occurs through the abrupt evaporation as per Kelvin’s ex-pression38: ln (P/P0) 1 2 cs Vm/(rRT) 5 0. Here, P and P0

are the actual and saturated vapor pressures and cs is thesurface tension of liquid N2. The parameters Vm, r, R,and T representing molar volume, radius of the N2droplet, the universal gas constant (8.31 SI units) andworking temperature (77 K), respectively. Interestingly,the Brunauer-Emmett-Teller (BET) surface area was foundto be 311.24 m2/g for the doped specimen (EuT2), which isabout 3.3 times larger than that of the un-doped (T2) sample(93.98 m2/g) (Table I). It is possible that, smaller crystallitesize of the EuT2 specimen (as revealed from XRD) mayexperience a larger surface area, as compared to the T2specimen. Again, corresponding to the optimal pressurevariation, the amount of nitrogen adsorption was found toget enhanced drastically from a value of 90.2 to 187.4 cc/gfor EuT2 and T2 systems, respectively. In contrast, earlierwe have observed adequate reduction in the BET surfacearea as well as molar volume due to PbS molecular adsorp-tion in TiO2 pores.

39 In the present case, a substantial in-crease in the surface area (SBET) and the pore volume (VP),despite suppression of the pore size (r) in EuT2 specimen, isexpected to be due to plentiful interfacial regions associatedwith the point defects and dislocations that were also evi-dent in TEM studies. Consequently, a significant reductionin the N2 desorption rate can be found wrt the adsorptionprocess, resulting in a more open hysteresis trend of theEuT2 specimen.

D. Free radical generation andphotocatalytic activity

The photocatalytic activity of un-doped and Eu31

doped TiO2 nanoparticles was studied considering MOdye as the targeting agent. The optical absorption responseof MO targeted un-doped and doped samples, with and

without UV-exposure, are depicted in Fig. 7. As shown inFig. 7(a), in the presence of a catalyzing medium (un-dopedTiO2), the absorbance of theMO-dye decreases only slowlywith the UV-exposure time. In contrast, the absorbance getssuppressed by substantial amounts in the case of the useof Eu-doped TiO2 catalysts [Fig. 7(b)]. Essentially, uponUV light irradiation, the host TiO2 semiconductor releaseselectrons (e�CB) into the conduction band and holes (h

1VB)

to the valence band and the charge carriers can either betrapped as Ti31 and O� defects or recombine to dissipateenergy.40 Alternatively, the charge carriers can migrate tothe catalyst surface and initiate redox reaction withadsorbed species (organic molecules) of the surroundingenvironment and convert into CO2 and H2O. The holes,upon trapped by the surface adsorbed H2O, release H

1 andOH� radicals, which are regarded as extremely powerfuloxidizing agents. The electrons in the conduction band arerapidly trapped by the surface adsorbed molecular oxygenand are reduced to O2

�� radicals that may further reactwith H1 to generate �OOH and H2O2.

41,42 The generationof free radicals by the photocatalyst is schematically shownin Fig. 7(c). The recombination events, however, could beobstructed by incorporation of new energy levels (due to theimpurity ions) in the main energy band (of the titania host).TiO2 nanosystem being abundant with oxygen vacancies,the unoccupied energy states would act as photogeneratedelectron scavengers with the surface adsorbed OH groupacting as the hole trap.43 The oxygen vacancy centers canhelp transferring electrons to the adsorbed oxygen pro-ducing thereby superoxide radicals. During the process ofphotocatalytic reaction, oxygen vacancies and defects cap-ture photogenerated electrons and consequently, recom-bination of e and h gets inhibited. Moreover, oxygenvacancies could promote the adsorption of O2 moleculesand the photoinduced electrons bound to the oxygen vacan-cies couldmake easy adsorption of O2 while generating freeradicals. Thus, oxygen vacancies and defects were in favorof photocatalytic reactions (provided that UV excitationenergy is higher than the band gap of the TiO2 system) inthe sense that, O2 is an active agent to promote the oxidationof organic substances.41 In the photocatalytic degradationcurve, shown in Fig. 7(d), it is quite evident that, theEu-doped TiO2 specimen is about 10–12 times more ef-ficient than the un-doped counterpart. The photocatalyticdegradation experienced a saturation trend beyond 45 minof the UV-exposure. As can be seen from the absorptionspectra, the absorption edge of the TiO2 nanosystem, uponEu-doping, gets red-shifted owing to the band gap narrow-ing enabling enhanced photodegradation. The reason forthe increased photodegradation efficiency is ascribed to theintroduction of Eu31 doping states, which helped inhibitinge-h pair recombination while enhancing interfacial chargetransfer reaction.

As predicted earlier, the presence of intermediate trapcenters such as, impurity states and defect states are

TABLE I. Textural parameters of various samples obtained from theBET surface analysis.

Samples SBET (m2/g) VP (cm3/g) r (nm)

T2 93.976 0.169 2.62EuT2 311.245 0.322 2.0



capable of reducing the recombination time thus enhancingphotocatalytic activity.44 Being Eu-doped samples moreeffective under UV irradiation, the MO degradation ac-tivity of each of the samples increases rapidly [Fig. 7(d)].But increasing the Eu doping concentration from 1 to 5%has showed only weak photocatalytic activity. From theabove discussion, it is now apparent that, Eu-doped TiO2

nanoparticles are a better choice over the un-doped ones,owing to the effective free radical generation via partici-pation of new energy levels and lower rate of e� and h1

recombination thatwould facilitate photocatalytic reactions.With relatively small crystallite size and higher surface area,an enhanced photo response in the UV region could makethe specimen a more efficient photocatalyst. Moreover,doped specimens would ensure improved photocatalysingpower even for a short exposure time.

IV. CONCLUSIONS

The sol-gel assisted rapid condensation route was usedto produce un-doped and Eu31 doped nanoscale titaniaparticles. The co-existence of the mixed phases (brookite,rutile, and anatase) was observed when the sample wasprepared at higher annealing temperature. The overall PLspectra were found to be dominated by the emission due toself-trap exciton as well as oxygen defect related emis-sions. The Eu31 related D–F transitions were revealedfrom the PL spectra acquired at a higher excitation wavelength (kex 5 405 nm). With the increase in Eu-doping

level, the electrically driven transition was found to bedominant over magnetically driven ones. The nanoporesize, owing to Eu-doping into titania host, gets reducedfrom a value of ;5.2–4 nm while the BET surface areawas enhanced by a factor of 3.3. Furthermore, Eu-dopedTiO2 nanopowder showed an enhanced photocatalyticresponse wrt degradation of MO dye under the UV illu-mination. Further investigation is necessary to make outa quantitative correlation among the optoelectronic andphotocatalytic performances based on appropriate energylevel schemes.

ACKNOWLEDGMENTS

One of the authors (NP) acknowledges DST, NewDelhi for providing fellowship through INSPIRE scheme.We thank SAIF, NEHU, Shillong for extending TEM facil-ity. NP would like to thank Ms. Giti Das for her assistanceduring synthesis steps. The authors also thank Prof. R.C.Deka, Dr. G.A. Ahmed, and Ms. Rasna Devi for their helpand support in BET surface analysis through N2 adsorption/desorption study. A part of the work was carried out throughproject no.UFR-50307/2011 supportedby IUAC,NewDelhi.

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FIG. 7. UV-Vis absorption spectra ofMO loaded (a) un-doped TiO2 (T2) and (b) Eu-doped TiO2 (EuT2) catalysts under UV irradiation; (c) a scheme offree radical generation upon UV exposure and MO degradation. The effective MO degradation by the photocatalyst is shown in (d).



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effective optoelectronic and photocatalytic response of eu3+-doped tio2 nanoscale systems...

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