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Journal of Alloys and Compounds 693 (2017) 719e729

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Journal of Alloys and Compounds

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Optical and X-ray induced luminescence from Eu3þ doped La2Zr2O7nanoparticles

Madhab Pokhrel a, Mataz Alcoutlabi b, Yuanbing Mao a, *

a Department of Chemistry, University of Texas Rio Grande Valley, 1201 West University Drive, Edinburg, TX, 78539, USAb Department of Mechanical Engineering, University of Texas Rio Grande Valley, 1201 West University Drive, Edinburg, TX, 78539, USA

a r t i c l e i n f o

Article history:Received 22 June 2016Received in revised form3 September 2016Accepted 20 September 2016Available online 21 September 2016

Keywords:ZirconatesEuropiumPyrochloreNanoparticlesNanoscintillators

* Corresponding author.E-mail address: yuanbing.mao@utrgv.edu (Y. Mao

http://dx.doi.org/10.1016/j.jallcom.2016.09.2180925-8388/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

Nanocrystalline complex oxide scintillators have emerged for use in X-ray and gamma-ray detection inrecent years. Here the doping concentration dependence of europium-doped lanthanum zirconate(La2Zr2O7:xmol%Eu3þ, x ¼ 1 to 35) nanoparticles (NPs) were investigated by using X-ray diffraction(XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy(SEM), and optically and X-ray excited luminescence. Spectroscopic methods revealed a gradual increasein disorder from the ordered pyrochlore structure to the defect fluorite structure as the Eu3þ concen-tration increases. They showed red luminescence under charge transfer band (258 nm), and Eu3þ 4f-4fbands (322, 394 and 465 nm) excitation with optical intensity dependent on the Eu3þ doping concen-tration and not quenching up to 5% Eu3þ doping concentration. The quantum yield of the red emissionwas found to depend on the Eu3þ doping concentration and excitation wavelength. Moreover, theseLa2Zr2O7:x%Eu3þ NPs demonstrated an atypical Eu3þ related scintillating response with a red emissionunder high-energy X-ray excitation with the scintillation intensity also depending on the Eu3þ dopingconcentration. Finally, the irradiation damage behavior induced by the excitation source was represen-tatively examined for the La2Zr2O7:5%Eu3þ NPs. It is expected that the current study will pave the way forfuture research of nanoscintillators.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

Rare earth (RE)-doped inorganic compounds have been widelyused as phosphors and scintillators [1e6]. In addition, selected RE-doped inorganic compounds have also been used in fiber gratingfor realizing optical switching [7]. The properties of these RE ele-ments and their multifaceted applications to science and industryare equally fascinating and have remained important as today [8].Recently, there has been a great amount of interest in the derivativecomplex oxide structures, such as A2M2O7 (A ¼ RE3þ, and M ¼ Ti,Sn, Zr, Hf) of dioxide MO2type compounds as a RE host materialsdue to their binary metal cation sites [9e15]. The oxygen-deficientA2M2O7-type compounds are achieved by the introduction of twooxygen vacancies during the charge-compensation for these dop-ants whenM4þ cations inMO2 are replacedwith trivalent cations ofA3þ [14,16]. These A2M2O7 structures can exist in either the defectcubic fluorite (Fm3m) or ordered pyrochlore (Fd3m) crystal

).

structure (Schematic 1). In general, the stability of the disorderedfluorite structure is related to the ionic radius ratio of the A and Mcations (rA/rM) and is favored for smaller lanthanides (<Gd) [11]. Inthe disordered fluorite structure, oxygen vacancies are randomlydistributed on the anion site (8c) (Scheme 1a) and also known asdefect or disordered fluorite structure. In ordered pyrochlorecrystal structure, the A-site cations are 8-fold coordinated withoxygen and the M-site is 6-fold coordinated with oxygen (Scheme1b).

Several synthesis routes have been used to prepare A2M2O7 typepowders, including the conventional solid-state mixing method(CSSM), hydrothermal synthesis, combustion synthesis, solegelsynthesis and coprecipitation-calcination method [17e25]. Inter-estingly, synthesis of A2M2O7 pyrochlore compositions by thecombination of co-precipitation and molten salt synthesis (MSS)have been much less extensively studied when compared to othersynthesis methods. Here, we confine our interests to trivalent La3þ

and tetravalent Zr4þ to find phase stable pyrochlore La2Zr2O7:x%Eu3þ nanoparticles (NPs) below 1000 �C. There are two-steps forthe co-precipitation-MSS method. First, a single-source complex

Scheme 1. Crystal structures of La2Zr2O7: (a) defect fluorite structure where La3þ and Zr4þ occupy the unique cation site with 7-fold coordination; (b) ordered pyrochlore structurewhere La3þ ion occupy the A-site with 8-fold co-ordination with oxygen. The smaller Zr4þ cation occupies the M-site (16c) and is a 6-fold co-ordination with oxygen. Green, yellow,and orange spheres represent the oxygen on the 8c, 48f, and 8b sites, respectively.

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729720

precursor of La(OH)3$x%Eu(OH)3$ZrO(OH)2$nH2O was synthesizedvia a co-precipitation route [26]. In the second step, La2Zr2O7:x%Eu3þ NPs were synthesized through a facile MSS process using thesingle-source precursors of hydroxides and nitrate salt mixture. Inthe MSS route, source precursors of hydroxides can be mixedthoroughly in the molten salt medium using a ball miller, facili-tating the mixing and diffusion of reactant species and thus thesubsequent reaction at a relatively low temperature. In addition,the synthesis generally can be carried out in regular crucibleswithout using an expensive setup.

La2Zr2O7 is considered to be a highly dense material and pro-posed as a scintillator material in X-ray computed tomographicdetectors [27e29]. Previous studies show that high density andhigh atomic number (Z) NPs can act as a sensitizer to enhance ra-diation dose delivered to tumors [30,31]. Thus, the combination ofLa and Zr in La2Zr2O7 makes it a good host for X-ray attenuatingproperties and an excellent candidate for computed tomographycontrast imaging agents in NP form. Recently, we have also shownthat ordered pyrochlore based La2Zr2O7:5mol%Eu3þ NPs synthe-sized above 1000 �C possess lower photoluminescence (PL) quan-tum yield (QY) compared to that of disordered fluorite structureLa2Zr2O7:5mol%Eu3þ NPs [32]. Interestingly, complex oxides suchas La2Zr2O7 have been investigated thoroughly to find the bestradiation-tolerant materials that can be used as host materials fornuclear wastes or as advanced nuclear fuel in recent years[9,33e35]. Radiation tolerance refers to a material's ability to resistundesirable radiation-induced phenomena.

Eu3þ is one of the most studied rare earth ions because of itsunique Stokes emission characteristics [2,3,24,36e44]. In addition,it has a nearly monochromatic strong visible red emission around612 nm. Importantly, there are already sufficient literature reportsabout the local structure of the Eu3þ ion site that can be easilyobtained from the f-f transition spectra [3,37,39,45e47]. Theground state (7F0) and excited state (5D0) of the Eu3þ ion are non-degenerate and give information about the local symmetry andinhomogeneity of the surrounding ligands [38]. Therefore, Eu3þ

dopants can be used as probes to estimate the local structure of thehost's ions.

In order to enhance the excitation and emission efficiency inEu3þ doped La2Zr2O7 phosphors and develop its application as aphosphor and scintillator, it is necessary to find the optimized Eu3þ

concentration and understand the position of the broad chargetransfer (CT) band position for excitation. It is also important tounderstand the correlation between the CT band of Eu3þ doped

La2Zr2O7 and the crystal structure at different doping levels. To thebest of our knowledge, no comparison report on the effect ofdopant concentration on the PL and scintillation of defect fluoritestructured La2Zr2O7:x%Eu3þ NPs has been reported under opticaland X-ray excitation, respectively. Although observation of con-stituent elements has been carried out with X-ray photoelectronspectroscopy (XPS) for La2Zr2O7 in previous studies [48,49], dopantconcentration dependent XPS analysis has not been carried outbefore.

In this work, we studied La2Zr2O7:x%Eu3þ NPs (mole percentagex ¼ 0, 1, 3, 5, 10, 15, 25, 30, and 35) under optical and X-ray exci-tation. Additionally, structural and morphological characteristics ofthe La2Zr2O7:x%Eu3þ NPs were investigated by X-ray diffraction(XRD), XPS, Raman spectroscopy, and scanning electronmicroscopy(SEM). We then studied their downconversion (Stokes) emissionproperties in the range of 500e750 nm with ultraviolet (258, 324,and 394), visible (465 nm) and X-ray excitations. In the analysis ofthe Stokes emission properties and mechanisms, we have includedthe dependence of the red emission intensity and the energytransfer process on the excitement wavelength. The La2Zr2O7:x%Eu3þ NPs also exhibit Eu3þ related emissions under high energy X-ray excitation. We performed detailed studies on the effect of Eu3þ

doping concentration on the La2Zr2O7 NP host, their PL, QY, anddecay mechanisms including crystal structure. Finally, we exam-ined the photo-stability of La2Zr2O7:5%Eu3þ NPs under differentradiation exposures related by excitation sources.

2. Experimental section

2.1. Chemicals

The starting materials including lanthanum nitrate hexahydrate(La(NO3)3$6H2O, 99.0%), zirconium dinitrate oxide hydrate (ZrO(-NO3)2$xH2O, 99.9%), europium(III) nitrate hexahydrate(Eu(NO3)3$6H2O, 99.9%), potassium nitrate (KNO3, 99.9%), sodiumnitrate (NaNO3, 98%) and ammonium hydroxide (NH4OH,28.0e30.0%) were purchased from Sigma Aldrich. All chemicals areof analytical grade reagents and used directly without furtherpurification.

2.2. Synthesis of La2Zr2O7:x%Eu3þ NPs

The La2Zr2O7:x%Eu3þ NPs (x ¼ 0, 1, 3, 5, 10, 15, 25, 30, and 35)were prepared by a two-step co-precipitation-MSS process

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729 721

following our previous reports [26,50,51]. First, a single-sourcecomplex precursor of La(OH)3$x%Eu(OH)3$ZrO(OH)2$nH2O wassynthesized via a co-precipitation route. In the second step, size-controlled La2Zr2O7:x%Eu3þ NPs were synthesized through afacile MSS process using the single-source complex precursors ofLa(OH)3$x%Eu(OH)3$ZrO(OH)2$nH2O and nitrate mixture(NaNO3:KNO3 ¼ 1:1, molar ratio) at 650 �C for 6 h. Further detailscan be found in ESI-1.

2.3. Characterization

Powder La2Zr2O7:x%Eu3þ NPs samples were characterized usingX-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS),Raman spectroscopy, scanning electron microscopy (SEM), andoptically and X-ray excited luminescence. Further details aboutinstruments used, characterizations setup and measurementtechniques can be found in ESI-2.

The excitation, kinetic scan, emission spectra and QY of the as-prepared La2Zr2O7:x%Eu3þ NPs were measured using an Edin-burgh Instruments FLS980 fluorometer system. The details can alsobe found in our previous publications [32].

3. Results and discussion

3.1. Summary of the structural (XRD, SEM, EDX, and XPS)characterization

Powder XRD, Raman, SEM, EDX and XPS were used to charac-terize the crystalline phase, chemical composition, size andmorphology of the La2Zr2O7:xmol%Eu NPs. The XRD patterns of theexamined La2Zr2O7:x%Eu NPs are shown in Fig. 1 and correspond todefect cubic fluorite structure with space group Fm3m (PDF CardNo. (JCPDS 73-0444). There are four prominent diffraction peaks.We refer to these reflections as parent fluorite peaks and indexthem as (222), (622), (440), and (400) assuming a structure with afluorite unit cell. These peaks are the only allowed reflections in theangular range between (20e60�) for an oxide with the fluoritecrystal structure, especially for those oxides close to the parentfluorite (MO2) structures [10]. In addition, the intense and sharpdiffraction peaks suggested well-crystallized samples. Within thedetection limit of the X-ray diffractometer, no additional peakswere observed for any of our La2Zr2O7:xmol%Eu NP samples. Spe-cifically, the absence of typical super-lattice peaks at 2q ¼ 27�, 36�,50�, etc. indicated that none of our products were found to haveordered pyrochlore type lattice based on XRD data [17]. The

Fig. 1. (a) XRD patterns of the La2Zr2O7:xmol%Eu3þ NPs with x ¼ 0, 1, 3, 5, 10, 15, 20, 25, 30~28.80� .

expanded view of the peak corresponding to (222) plane wasshown in Fig. 1b, which clearly showed the broadening of the XRDpeakwith the increase of Eu3þ dopant concentration. The peak shifttowards higher angle clearly indicated that its lattice-spacing isdecreasing (ESI-2) with increasing Eu3þ doping concentration inLa2Zr2O7. This could be due to the lattice contraction caused by thedifference in the ionic radii of La3þ (130 pm) and Eu3þ (118 pm). Thedoping of Eu3þ into La3þ sitewas further confirmed by EDX analysis(ESI-3).

SEM characterization was performed (Fig. S4) to assess the sizeand morphology of the La2Zr2O7:xmol%Eu3þ NPs. Morphologies ofthe as-prepared NPs were found to be regular and spherical. Theaverage crystallite size of these NPs calculated using the Scherrerequation from XRD data ranged from 28 to 13 nm (ESI-5) with anaverage size of 20 ± 2 nm. Size calculations from SEM imagesconfirmed these values of particle size from XRD calculations wereaccurate approximations (ESI-5).

3.2. Raman analysis

The absence of pyrochlore phase based on the XRD analysiscould be due to the inability of XRD to resolve those diffractionpeaks corresponding to pyrochlore phase. This is difficult sincefluorite and pyrochlore structures have the same parent patternand the emergence of minor reflections is the only indication of thepyrochlore structure, making accurate quantification by XRDanalysis challenging.

Raman spectroscopy has been widely utilized as a tool todistinguish between defect-fluorite and ordered pyrochlore struc-tured materials [22]. So we further investigated the compositionsand structure of these NPs using Raman spectroscopy (Fig. 2).

Based on the group theory, the disordered fluorite La2Zr2O7 withspace group Fm3m has only one Raman active mode T2g, whereordered pyrochlore with symmetry Fd3m possesses six peakswithin the range of 200e1000 cm�1 [52,53]. A detailed Ramanspectroscopic characterization has been carried out on the La2Z-r2O7:x%Eu NPs in the range of 170e1000 cm�1 (Fig. 2a and inset ofFig. 2b). In order to avoid the Eu luminescence interferencewith theRaman features of the La2Zr2O7 host, an excitation wavelength of785 nm was used. The spectrum from the La2Zr2O7 host (Fig. 2b)consists of two strong bands at approximately 302 cm�1 (1) and504 cm�1 (3) as well as four weak bands at about 386 cm�1 (2),633 cm�1 (4), 733 cm�1 (5) and 842 cm�1 (6). The strongest bandlocated at ~302 cm�1 can be assigned to the Eg internal La-Ostretching mode and the bands at 386 cm�1 and 504 cm�1 are

and (b) the enlarged view of the diffraction peak corresponding to the (222) plane at

Fig. 2. (a) Raman spectra of the La2Zr2O7:xmol%Eu3þ NPs with x ¼ 0, 1, 3, 5, 10, 15, 20, 25, 30 and 35. (b) Fitted Raman spectrum of the undoped La2Zr2O7 NPs. Inset shows theFWHM at 302 cm�1 of the La2Zr2O7:xmol%Eu3þ NPs with x ¼ 0, 1, 3, 5, 10, 15, 20, 25, 30 and 35.

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729722

attributed to T2g modes of pyrochlore, respectively [52,54]. Peaksabove 600 cm�1 (633 cm�1, 733 cm�1 and the other band at842 cm�1) could be due to the distortion of the La site, which iscoordinated in octahedral structure [55]. Peak shifts towards lowerwavenumbers have been noticed after doping, especially at orabove La2Zr2O7:1%Eu compositions. The appearance of more thanone internal stretching mode in the present samples ensures thatthese La2Zr2O7:x%Eu NPs are not crystalized in the disorderedfluorite structure as demonstrated from the XRD analysis.Comparing with other pyrochlore zirconates and hafnates, it couldbe seen clearly that Raman bands are sharp and ordered [55,56].These Raman results clearly indicated that the La2Zr2O7:x%Eu NPssynthesized using the coprecipitation-MSS method do not attain apure defect fluorite structure contrary to results reported for lowtemperature synthesis by Kiel Holliday and et al. [57], but insteadhave a tendency to form a ordered pyrochlore structure. It has beenwell established that if the cations radius ratio {La3þ (130 pm)/Zr4þ

(86 pm) ¼ 1.51} is larger than 1.46, A2M2O7 oxide becomes pyro-chlore structure [17]. Therefore, it is not surprising that the zirco-nates with large La3þ ions prepared in the present investigationappear to adopt an ordered pyrochlore lattice based on Raman data.As the radius ratio is decreased, one gets clear defect fluoritestructure. In general, Eu3þ doping ions are assumed to replace theLa3þ sites because of the same valence and small difference in La3þ

(130 pm) and Eu3þ (118 pm) ionic radii. The broadening (Fig. 2b)and shift of this band (302 cm�1) is evident for the doped samples,indicating the doping of Eu ion into the La sites. However, broad-ening is insignificantly small at lower Eu3þ doping (inset of Fig. 2b),indicating that Eu3þ ions are distributed uniformly replacing theLa3þ cation sites as expected. However, significant broadening(inset of Fig. 2b) and a decrease in the number of Raman peaks(Fig. 2a) indicated that there was an increase in localized disorderacross the entire La2Zr2O7 structure with higher Eu3þ doping.Furthermore, we noticed that the ordered pyrochlore structure isgradually pushed towards disordered fluorite structure with higherEu3þ doping. This kind of structural change with Eu3þ doping wasunprecedented. However, this structural change with higher Eu3þ

doping becomes possible since the radius ratio of Eu3þ/Zr4þ (1.37) issmaller than that of La3þ/Zr4þ (1.51). Our observations showedsome deviations from this range of stability of defect fluorite(<1.46), however, we believe that this theoretically evaluatedlimiting radius ratio should be used only as a broad guideline, orpossibly only for undoped samples or bulk samples [17]. In sum-mary, Raman analysis indicated that our La2Zr2O7:x%Eu3þ NPspossessed a pyrochlore structure. With the exception of un-doped

and up to 5mol%Eu3þ doped La2Zr2O7 NPs, which showed all sixRaman peaks, there was an evidence of anion disorder at higherEu3þ doping. The disorder of the anions, especially in the pyro-chlore structure, resulted in the presence of broad Raman bands. Itis worth to mention here that there is no extra crystal phaseobserved at doping up to 35% based on the XRD study. These ob-servations are also consistent with the particle size, XPS analysis,excitation; emission and PL yield measurements discussed in latersections.

3.3. XPS analysis

It is well known that oxygen ions and oxygen vacancies are or-dered across all anion sublattice sites in pyrochlore La2Zr2O7 andone could anticipate that oxygen ions and vacancies are rearrangedor destroyed with the addition of Eu3þ ions [33]. Here, the possi-bility of Eu3þ ion replacing the Zr4þ regular sites cannot becompletely discarded although they have very different ionic radii(rEu3þ ¼ 118 pm and rZr4þ ¼ 86 pm) and oxidation states. However,the substitution of Eu3þ into the O sites is not feasible. Therefore,the changes in O 1s XPS spectra (Fig. 3) of the La2Zr2O7:x%Eu3þ NPsas a function of the Eu/La ratio could be a dopant ion effect. Thephotoelectron peaks of these samples are highly sensitive to dopantconcentration. The O 1s profile is, in general, more complicated dueto the overlapping contribution of oxygen from La and Zr in the caseof La2Zr2O7. The broad O 1s profile for low Eu3þ doped sample couldbe due to random distribution of oxygen ions over many crystal-lographic equivalent sites [56]. This also points towards the exis-tence of a high volume of oxygen defect states and disorders inundoped and 1% Eu doped La2Zr2O7 NPs compared to those of thehighly doped La2Zr2O7:x%Eu3þ NPs. This observation is consistentwith the PL and QYanalysis discussed below. However, two distinctoxygen peaks were observed for higher Eu doped samples indi-cating an ordered La2Zr2O7 structure with oxygen ion redistribu-tion. The spectral fitting demonstrated the O 1s binding energy (BE)peaks at approximately 529.6 eV and 532.0 eV. In the La2Zr2O7

pyrochlore crystal structure, there are two oxygen sites at 48f and8b (Scheme 1b). Six out of every seven oxygen ions occupy the 48fsites and each is coordinated by two lanthanum and two zirconiumions, and the seventh oxygen ion occupies the 8b site and is sur-rounded by four zirconium ions (Scheme 1b). This means that theelectron density on oxygen for the LaeO bond is higher than that ofthe ZreO bond and could be the most possible reason for havingtwo distinct binding energy peaks for O 1s in the La2Zr2O7 crystalstructure. So, this can be proposed as the combined effect of two

Fig. 3. O 1s XPS spectra of the as-prepared La2Zr2O7:xmol%Eu3þ NPs with x ¼ 0, 1, 3, 5, 10, 15, 20, 25, 30 and 35.

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729 723

metals with different ionicities in one binary oxide (i.e. La and Zr inthe case of La2Zr2O7) generating small BE shifts due to a specificredistribution of electron density during atomic ordering andcrystal lattice formation [56,58]. Furthermore, the O 1s peak for theLa2Zr2O7:x%Eu3þ NPs with higher Eu3þ concentration shift to lowerBE which can be simply related to the change in the broadening ofthe peak. The shift to lower BE was also observed for La 3d and Zr3d (Figs. 4 and 5). In conclusion, the presence of two oxygen peakson O 1s XPS spectra further supports the fact that these as-synthesized La2Zr2O7:x%Eu3þ NPs crystallizes in pyrochlore phase,

Fig. 4. La 3d XPS spectra of the as-prepared La2Zr2O7:xmol%

which is consistent with the Raman analysis.Fig. 4 shows the binding energies of the La 3d photoelectron

peaks at 834.4, 838.3 and 851.1 eV for La 3d5/2 and La 3d3/2 lines,respectively. Small changes in BE indicate that the chemical state oflanthanum has changed slightly. However, the majority remainedalmost the same irrespective of dopant concentration. In addition,the absence of extra peaks and the shoulder peak ensures the pu-rity of the compositions.

Fig. 5 shows the XPS spectra of Zr 3d from the as-preparedLa2Zr2O7:xmol%Eu3þ NPs. The Zr4þ 3d Gaussian-fitted profile

Eu3þ NPs with x ¼ 0, 1, 3, 5, 10, 15, 20, 25, 30 and 35.

Fig. 5. Zr 3d XPS spectra of the as-prepared La2Zr2O7:xmol%Eu3þ NPs with x ¼ 0, 1, 3, 5, 10, 15, 20, 25, 30 and 35.

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729724

consisted of two main peaks of 2d5/2 and 2d3/2 levels at 181.9 and184.2 eV, respectively, resulting in a spineorbit coupling (D) of2.4 eV, which is comparable to that observed in related systems[48,59]. The undoped and 1% Eu3þ doped La2Zr2O7:x%Eu3þ NPsshowed broad XPS features of Zr 3d. This could be due to the non-uniform distribution of the Zr4þ in the undoped and low Eu3þ

containing compositions in all available sites [56]. More distinctand narrowed Zr 3d peak with larger x in the La2Zr2O7:x%Eu3þ NPsindicated the more ordered nature of the zirconium chemical statein the La2Zr2O7 NPs at higher Eu3þ doping. Comparing with defectfluorite structure [23], it is truly ordered as expected from pyro-chlore structure. To sum, these XPS results (Figs. 3e5) are consis-tent with the Raman data (Fig. 2).

The XPS spectra of Eu3þ 3d (Fig. 6) were broad and could be dueto either the exchange interaction between the 3d holes andpartially filled 4f shell of La3þ ion or the oxidized nature of Eu3þ

ions on the surface of the La2Zr2O7:xmol%Eu NPs [60,61]. Here, noEu3þ 3d peak was detected for the La2Zr2O7:xmol%Eu NPs withx ¼ 1 and 5. This phenomenon was probably seen because XPS issurface sensitive technique and the measured signal is exponen-tially surface-weighted. Even the amount of Eu3þ ions on the sur-face of the La2Zr2O7:xmol%Eu NPs with x ¼ 1 and 5 wasundetectable by XPS, Eu3þ was certainly present based on the EDXspectra (Fig. S3) and PL spectra (Figs. 8 and 9 shown below) of thecorresponding La2Zr2O7:1mol%Eu and La2Zr2O7:5mol%Eu NPs. Athigher Eu3þ doping levels, the characteristic peak of Eu3þ 3d5/2(~1134.6 eV) was detected from the as-synthesized compositions(La2Zr2O7:x%Eu3þwith x¼ 10 to 35). This valuewas consistent withthat reported on the literature, indicating that the oxidation statesof europium ions of all the La2Zr2O7:x%Eu3þ NPs wereþ3 [62,63]. Italso indicated more uniform nature of the europium dopant, whichcould be further related to the increase in surface area of La2Zr2O7NPs with the decrease in particle size induced by increased dopingconcentration of Eu3þ (Table S5). No extra Eu phase has beendetected on the surface of the La2Zr2O7:xmol%Eu NPs through XPSanalysis even for the highest Eu3þ doped sample. This furtherensured the doping of Eu3þ ions at the La sites, irrespective of

doping concentration in this particular La2Zr2O7 host, and invali-dated the assumption that Eu3þ ion agglomeration at the surface ofthe NPs at higher doping Eu3þ ions.

In addition, survey XPS spectra (ESI-6 and Fig. 7) were used toestimate the relative atomic percentage on the surface of theLa2Zr2O7:xmol%Eu NPs based on the areas under the La 3d, Zr 3dand Eu 3d peaks with the atomic sensitivity factors (ASF) providedby the XPS company (Thermo Scientific). It can be concluded thatthe ratio of the La: Zr: Eu is in close agreement with the initialprecursors added formost of the compositions studied except 1 and5 mol%Eu doped compositions. For example, the calculated ratio ofLa: Zr: Eu ¼ 0.71: 1.0: 0.28 was in close agreement with the initialratio of added precursors for the synthesis of the La2Zr2O7:35%Eu3þ

NPs (La: Zr: Eu ¼ 0.65: 1.0: 0.35). Although no Eu3þ 3d peak wasdetected by XPS from the La2Zr2O7:xmol%Eu NPs where x¼ 1 and 5,it can be anticipated the existence of Eu3þ ions in these two com-positions like in other samples as proved with the photo-luminescence study discussed in later sections.

3.4. Photoluminescence properties of the La2Zr2O7:xmol%Eu3þ NPpowders

Excitation spectra of the as-prepared La2Zr2O7:xmol%Eu3þ NPswith x ¼ 1, 3, 5, 10, 15, 20, 25, 30 and 35 were collected by moni-toring the Eu3þ emission at 612 nm corresponding to 7F2 / 5D0transition (Fig. 8a). These excitation spectra all present a wideexcitation band from 230 to 300 nm that corresponds to the chargetransfer band (CT) transition and is attributed to O2�(2p)/Eu3þ(4f)ligand-to-metal charge transfer. The 322 nm, 390 nm and 470 nmexcitation bands correspond to the Eu3þ (fef) transitions. Therelative intensity of the O2�/Eu3þ CTB peak (~258 nm) was foundto increase with increasing Eu3þ doping concentration up to 5mol%Eu3þ (Fig. 8b). For doping concentration higher than 5mol% Eu3þ,the peak intensity was found to decreasing slowly up to 15mol% Eu(Fig. 8b). However, a sharper decrease in excitation intensity cor-responding to CTB was observed for the samples with doping levelshigher than 20mol% Eu3þ. This may be due to an increase in

Fig. 6. Eu 3d XPS spectra of the as-prepared La2Zr2O7:xmol%Eu3þ NPs with x ¼ 0, 1, 3, 5, 10, 15, 20, 25, 30 and 35.

Fig. 7. The atomic percentage based on the XPS spectra of La 3d, O 1s, Zr 3d, and Eu 3dpeaks from the as-prepared La2Zr2O7:xmol%Eu3þ NPs with x ¼ 1, 3, 5, 10, 15, 20, 25, 30and 35.

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729 725

nonradiative ion-ion energy transfer and increased localized dis-order at higher doping levels, which may cause energy migrationand emission quenching [45].

Additionally, the charge transfer peak was red shifted at higherEu3þ concentrations and the maximum shift was found to be 6 nmfor La2Zr2O7:35%Eu3þ as shown in Fig. 8a. It was obvious to observesuch a shift in the CTB peak based on the metal to ligand envi-ronmental changes that occur with the change in the Eu:La ratioamong the La2Zr2O7:x%Eu3þ NP samples [64]. Here, we proposethat the red shift in the CTB is partially governed by the decrease inthe lattice parameter due to the change in cation size as Eu3þ re-places La3þ, especially for higher Eu3þ doped samples compared tothat of low doped samples (Fig. 8b). In addition, broadening couldhave minor effect on CTB shifting as observed in the similar study[65]. Importantly, the broadening pattern of the CTB band from the

La2Zr2O7:x%Eu3þ NPs (Fig. 8b) was also consistent with the fact thatthere was a decrease of particle size as the doping concentrationincreased (Fig. S4 and Table S5).

Fig. 9 shows the emission spectra of La2Zr2O7:xmol%Eu3þ

nanoparticles when excited with 258 nm corresponding to thecharge-transfer band and the Eu3þ (fef) transition bands at 322 nm,394 nm, and 465 nm. As can be seen in Fig. 9 and Fig. S10, thespectra consist of various sharp emission lines due to the directexcitation of the Eu3þ ions from the ground level to higher levels ofthe 4f-manifold. All the as-prepared La2Zr2O7:xmol%Eu3þ NPsamples showed the characteristic Eu3þ emissions from the5D2/

7F3, 5D1/7Fi (where i runs from 0, to 2), and 5D0/

7Fj (wherej runs from 0 to 4) transitions. The signals at 512, 527, 534, 552,579e592, 612, 656, and 711 nm correspond to the 5D2/

7F3,5D1/

7F0, 5D1/7F1, 5D1/

7F2, 5D0/7F1, 5D0/

7F1, 5D0/7F2,

5D0/7F3 and 5D0/

7F4 transitions of Eu3þ, respectively (See detailsESI-7a). The 5D0/

7F1 and 5D0/7F3 states are magnetic dipole

transitions independent of the surrounding ions. The strongestemission corresponds to the hypersensitive electric dipole transi-tion from 5D0/

7F2. Comparing to our previous studies, the sitesymmetry of Eu3þ in La2Zr2O7 has been identified as D2d with twoEu3þ crystallographic sites [32]. The first site could be due to Eu3þ

ion replacing Zr4þ antisites and second site could be due to the Eu3þ

ions replacing the regular RE3þ sites. In general, Eu3þ ions occupy atregular La3þ sites due to matching ionic radius and oxidation state.However, there have been studies suggesting the high volume ofcation antisite defects are due to difference in ionic radius ratio,lattice parameters, and the oxygen positional parameters in twometal cations based oxides such as La2Zr2O7 [16,66]. In an idealsituation, we anticipate that initially Eu3þ ions replace the Zr4þ

antisites, and upon reaching a threshold concentration most, if notall, of the antisites are filled. Above this critical concentration, im-purities can replace regular La sites.

By comparing the PL spectra of the La2Zr2O7:x%Eu3þ samples,one could reveal how sensitive the Eu3þ PL spectra were to thedoping concentration. In addition, emission peak intensity wasstrongly dependent on dopant concentration as observed in Fig. 9.

Fig. 8. (a) Excitation spectra of the as-prepared La2Zr2O7:xmol%Eu3þ NPs with x ¼ 1, 3, 5, 10, 15, 20, 25, 30 and 35 by monitoring the 5D0/7F2 (612 nm) emission. (b) Relative

excitation intensity and FWHM variation of the O2�/Eu3þ CTB peak (~258 nm) at different Eu3þ concentrations.

Fig. 9. Emission spectra of the as-prepared La2Zr2O7:xmol%Eu3þ NPs with x ¼ 1, 3, 5, 10, 15, 20, 25, 30 and 35 at four different excitation wavelengths: 258 nm, 322 nm, 394 nm, and465 nm.

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729726

For example, the intensity of the emission peak at 612 nm wasfound to decrease in the following excitation wavelength order:258 nm > 394 nm > 465 nm > 322 nm (ESI-7b). From these results,it could be inferred that the energy transfer extents from the host toEu3þ ions and Eu3þ ions to Eu3þ ions depended on the concentra-tion of Eu3þ and the excitation wavelengths.

In conclusion, the large ionic size mismatch between Zr4þ andEu3þ leads to distortion of the original lattice and breaks theinversion symmetry. The strongest emission corresponds to thehypersensitive electric dipole transition from 5D0/

7F2. Theobserved PL intensity differences between the samples clearlyindicated that Eu3þ doping concentration and excitation wave-lengths plays an important role in getting highest photo-luminescence emission from La2Zr2O7:xmol%Eu3þ NPs. At higherdoping, the La2Zr2O7:x%Eu3þ samples exhibited weaker emissions

compared to that of lower Eu3þ doped counterparts. In addition, theintensity of the 5D0/

7F2 electric dipole transition was found to bestronger compared to that of 5D0/

7F1 magnetic dipole transitionfor all samples at all the excitations as shown in Fig. 9.

To quantify the dopant concentration effect, we also measuredthe absolute photoluminescence quantum yield (PLQY) as a func-tion of the Eu3þ doping percentage under 258 nm excitation(Table 1). It was observed that PLQY increased as Eu3þ concentra-tion increased from 1mol% to 5mol%. For higher Eu3þ concentra-tions up to 35mol%, there was a decrease in PLQY. The decrease inthe PLQY and associated decrease in PL intensity (ESI-7b) with theincrease in Eu3þ concentration could be attributed to the increasednumber of defects and concentration quenching mechanism. In thepresent study, quenching was dominating at or above 15%Eu3þ

doped NPs (ESI-7b), which was higher than the experimental value

Table 1PLQYof the La2Zr2O7:xmol%Eu3þ NPs (x¼ 1, 3, 5, 10, 15, 20,25, 30 and 35) at room temperature under 258 nm exci-tation after integrating the emission wavelength rangefrom 500 to 750 nm.

Samples Average PLQY (%)

1% Eu 3.44 ± 0.343% Eu 6.71 ± 0.675% Eu 9.89 ± 0.9810%Eu 9.42 ± 0.9415%Eu 7.84 ± 0.7820%Eu 5.09 ± 0.7025%Eu 3.35 ± 0.3330%Eu 2.08 ± 0.2035%Eu 0.88 ± 0.088

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729 727

obtained for Eu3þ:Gd2O3 [45]. This quenching effect can most likelybe explained by an optimal distance of Eu3þ-Eu3þ being reached at5mol%, and higher concentrations leading to nonradiative energydissipations [20]. Additionally, it is known that an increased dopantconcentration can introduce defect states in nanoparticle hostsleading to defect based quenching, which may play a role in oursamples as well [20].

3.5. Decay analysis

To quantify the effect of dopant Eu3þ concentration on theluminescent dynamics of the La2Zr2O7:x%Eu3þ NPs, we calculatedthe effective lifetime (teff) by fitting the experimental decay curves(Fig. 10a) with a multi exponential function (ESI-8 and Table S9).Effective lifetime teff as a function of the dopant concentrationshown in Fig. 10b consisted of two concentration dependent pat-terns: (a) lifetime increases up to doping level of 10%Eu3þ in theLa2Zr2O7:x%Eu3þ NPs and (b) lifetime decreases rapidly abovedoping level of 15%Eu3þ in the La2Zr2O7:x%Eu3þ NPs. These decaypatterns clearly indicate that energy transfer and decay processesdepended on the dopant concentration [40]. Furthermore, the fit ofthe kinetics for the La2Zr2O7:x%Eu3þNPs at 612 nm showed average28% contribution from the short (t1) and 72% contribution from thelong (t2) component (Table S9) for samples with doping level lowerthan 15mol% of Eu3þ. At higher doping, lifetime contribution wasreversed (Table S9). Based on the concept of phonon contribution,long component of the decay arises from Eu3þ ions occupying 8-fold coordinated La3þ site whereas short component can beascribed to Eu3þ ions occupying at Zr4þ antisites. Lower photo-luminescence QY with the increased contribution from shortcomponent of lifetime at higher Eu3þ doping justifies our

Fig. 10. (a) Fluorescent decay curves at lex ¼ 258 nm and lem ¼ 612 nm, and (b) average lifet5, 10, 15, 20, 25, 30 and 35.

discussion on order-disorder (anion-disorder) transition at higherEu3þ doping in La2Zr2O7. In addition, it justifies our previousassumption that initially Eu3þ ion replaces the Zr4þ antisites. Thepossible excitation and decaymechanisms are further supported bythe proposed energy level diagram (Fig. S10).

In conclusion, PL spectra and decay analysis of the La2Zr2O7:x%Eu3þ samples indicated the presence of two Eu3þ crystallographicsites. Increased contribution from short component of lifetime athigher Eu3þ doping as shown in Table S9 clearly indicates the in-crease number of defects with higher Eu3þ doping; the proposedconcept is consistent with Raman analysis and PL analysis.

3.6. Emission properties under X-ray excitation

X-ray excited luminescence (XEL, Fig.11a) wasmonitored for theselected La2Zr2O7:x%Eu3þ (x ¼ 0, 1, 3, 5, 10, 15, 20, and 25) NPsamples. In the X-ray excitation mode, it was the La2Zr2O7 host thatwas excited, with the subsequent formation of a great number ofelectron-hole pairs that migrate within its structure [67]. Themigration stage is strongly affected by defects, surfaces and in-terfaces that can introduce energy levels into the band gap of thehost material [67]. The scintillation luminescence occurs whenthese electron-hole pairs recombine at the activator Eu3þ sites withthe net scintillation intensity being determined by the competitionbetween radiative recombination at the Eu sites versus non-radiative recombination at quenching centers and trapping of thecarriers. It was apparent from the evaluation of the optically and X-ray excited spectra that the XEL originated from the same Eu3þ

levels as in the optically excited PL emission. The XEL intensityvaried with the Eu3þ concentration (ESI-11) and the maximumluminescence intensity was observed for the La2Zr2O7:5mol%Eu3þ

NPs, similar to the trend of the 612 emissions under 258 nm exci-tation (Fig. 9a).

Furthermore, ionizing radiation stability of the brightest XELcomposition, i.e. La2Zr2O7:5mol%Eu3þ NPs, was tested under X-rayirradiationwith a dose at the rate of 2.28 Gy/min for 1 h. SignificantXEL intensity change (~45%) over time (1 h) indicated that La2Zr2O7is not a stable host under the X-ray radiation exposure (Fig. 11b).This is contrary to the stability observed for the same La2Z-r2O7:5mol%Eu3þ NPs under UV exposure (inset of Fig. 11b). Weanticipated that these ordered zirconate pyrochlore compositionsbecome disordered (amorphous) due to disordering of the A and Msite cations, i.e. by losing the long range order [68,69]. Similaramorphization behavior was observed for other pyrochlore com-pounds [70,71]. Although this preliminary study provided a basisfor predicting the durability of zirconate pyrochlore as a

ime as a function of dopant concentration of the La2Zr2O7:xmol%Eu3þ NPs with x ¼ 1, 3,

Fig. 11. (a) X-ray induced luminescence of the La2Zr2O7:xmol%Eu3þ NPs with x ¼ 1, 3, 5, 10, 15, 20, and 25 excited with an X-ray source at ~130 kVp and 5 mA. (b) Variations of XELintensity over exposure time for the red emission at 612 nm from the La2Zr2O7:5mol%Eu3þ NPs at an X-ray tube voltage of 130 KV and current of 5 mA. Inset shows the kinetic scanfor the same La2Zr2O7:5mol%Eu3þ NPs under 258 nm excitation from a Xenon lamp.

M. Pokhrel et al. / Journal of Alloys and Compounds 693 (2017) 719e729728

scintillation host, it requires additional detailed experiments toexplain these effects in the future.

4. Conclusions

In this paper, X-ray diffraction and Raman spectroscopy wereused as complementary techniques for the study of the disorderedfluorite and ordered pyrochlore structure of La2Zr2O7:xmol%Eu NPsto provide information on short-range and long-range ordering,respectively. Raman analysis showed that the pyrochlore structurewas favored for the La2Zr2O7:xmol%Eu NPs at low Eu3þ dopinglevels. Furthermore, Raman analysis demonstrated a gradual in-crease in localized anion disorder across the entire La2Zr2O7structure with higher Eu3þ doping levels. We also reported thedoping concentration dependence of the optical properties of theLa2Zr2O7:x%Eu NPs under both optical and X-ray excitations. Themeasured PL, PLQY, and fluorescence lifetimes of these Eu3þ dopedLa2Zr2O7 NPs with different doping levels demonstrated that therewas fluorescence quenching for more than 5% Eu3þ doping.Although the energy transfer process and amorphization under X-ray excitation have not been fully understood yet, our observationshowed that these unique La2Zr2O7:x%Eu3þ NP systems deservefurther investigation. These NPs also deserve a synthetic optimi-zation in order to further understand the photoluminescence andscintillation relationship. To sum, the Eu3þ related luminescenceinduced by multiple optical excitation combined with La2Zr2O7host properties, such as strong X-ray absorption, indicated thatthese high density La2Zr2O7:xmol%Eu3þ NPs are promising nano-materials for various applications.

Acknowledgments

The authors thank the support from the Defense ThreatReduction Agency (DTRA) of the U.S. Department of Defense (award#HDTRA1-10-1-0114) and the National Science Foundation underDMR grant #1523577 (PREM: UTRGV-UMN Partnership forFostering Innovation by Bridging Excellence in Research and Stu-dent Success). The authors acknowledge the startup support fromthe STARs Program at the University of Texas System (M.Alcoutlabi).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://

dx.doi.org/10.1016/j.jallcom.2016.09.218.

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