Bifunctional, Monodisperse BiPO4‑Based Nanostars: PhotocatalyticActivity and Luminescent ApplicationsAna Isabel Becerro,* Joaquín Criado, Lionel C. Gontard, Sergio Obregon, Asuncion Fernandez,Gerardo Colon, and Manuel Ocana

Instituto de Ciencia de Materiales de Sevilla, CSIC-University of Seville, c/Americo Vespucio, 49, 41092 Seville, Spain

*S Supporting Information

ABSTRACT: Monodisperse, monoclinic BiPO4 nanostarshave been synthesized by a homogeneous precipitationreaction at 120 °C through controlled release of Bi3+ cationsfrom a Bi−citrate chelate, in a mixture of glycerol and ethyleneglycol, using H3PO4 as the phosphate source. The set ofexperimental conditions necessary to obtain uniform nano-particles is very restrictive, as the change in either the polyolratio or the reactant concentrations led to ill-defined and/oraggregated particles. The morphology of the particles consistsof a starlike, hierarchical structure formed by the orderedarrangement of nanorod bundles. Transmission electron tomography has revealed that the nanostars are not spherical butflattened particles. Likewise, Fourier transform infrared spectroscopy and thermogravimetry have shown that the synthesizednanostars are functionalized with citrate groups. The mechanism of formation of the nanostars has been analyzed to explain theirmorphological features. The as-synthesized BiPO4 nanostars exhibit an efficient photocatalytic performance for the degradationof Rhodamine B. Finally, it has been demonstrated that the stars can be Eu3+-doped up to 2 mol % without any change in theparticle morphology or symmetry, and the doped samples show emission in the orange-red region of the visible spectrum afterultraviolet excitation. These experimental observations make this material a suitable phosphor for biotechnological applications.

1. INTRODUCTION

Inorganic luminescent materials have found extensive applica-tions in optoelectronics (displays, LEDs, and lasers)1 andbiomedicine (fluorescent markers and phototherapy).2 Many ofthe inorganic luminescent materials are obtained after dopingdifferent hosts with luminescent lanthanide ions. Among thesehosts, there has been an increase in the level of interest, in thepast few years, in rare earth-based nanoparticles, includingfluorides,3 oxides,4 hydroxides,5 vanadates,6 and phosphates,7

which show a number of interesting features such as their highthermal and chemical stability. In particular, rare earthphosphates (REPO4, where RE = Y3+, La3+, Gd3+, or Lu3+)show, in addition, a high degree of biocompatibility, which isparticularly valuable for their use in biomedicine. In addition tothe mentioned biocompatibility, luminescent inorganic particlesto be used in biomedical applications must fulfill a series ofrequisites, namely, (i) a high degree of homogeneity, in bothsize and shape, which is necessary to ensure the reproducibilityof their physicochemical properties, (ii) a nanometer size,which is specifically important for in vivo applications tofacilitate their elimination from the body, (iii) colloidal stabilityin a physiological medium, and (iv) having a functional groupon their surface that can act as an anchor site for the addition offunctional ligands such as antibodies, peptides, proteins, anddrugs,8 as mentioned above.Obtaining large quantities of REs, in particular lanthanides, in

a highly pure form (which is essential when REPO4 is used as

the host material) is much more costly than obtaining maingroup elements like Sb and Bi, which can be easily purified inlarge quantities by techniques like zone refining.9 Because ofthe similar ionic radii of Bi3+ and RE3+ and the fact that BiPO4is isostructural to REPO4,

10 BiPO4 was proposed by Guan etal.11 as a cheaper alternative to REPO4. Since the publication ofthat report, a number of papers have appeared about thesynthesis of luminescent Ln3+-doped BiPO4 particles withdifferent morphologies (rods, spheroids, cocoons, urchins, etc.),ranging in size from nanometers to micrometers.12−16

However, most of the BiPO4 particles reported in the literatureshowed a high degree of aggregation or a heterogeneous size,which is detrimental for biomedical applications, as mentionedabove. Only Pan and Zhu12 described a method forsynthesizing monodisperse BiPO4 nanospheres using oleicacid (OA) as a surfactant. However, the OA remained adsorbedon the particle surface, making it hydrophobic and, therefore,difficult to disperse in physiological media. In addition,although surface modification of luminescent particles withfunctional groups has been reported in the literature fornumerous RE-based nanoparticles,17,6 no report of functional-ization of BiPO4 nanoparticles has been published so far, to thebest of our knowledge.

Received: February 11, 2014Revised: April 8, 2014Published: May 23, 2014

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In addition to the advantages of BiPO4 as an alternative hostto REPO4 for luminescent applications, this material has beenproven to be an efficient photocatalyst (PC) for decomposingorganic pollutants.18−20 In general, the photocatalytic activity isclosely related to the size, shape, phase, and structure of thephotocatalyst.21 Therefore, it is highly desirable to develop newsynthetic strategies to allow the control of such morphologicalparameters.Here we report an original procedure for the synthesis of

monodisperse BiPO4 nanoparticles with a new morphologyconsisting of a starlike, hierarchical structure formed by theordered arrangement of nanorod bundles. The synthesismethod consists of a homogeneous precipitation of BiPO4,through controllable release of Bi3+ cations from Bi−citratecomplexes, in a mixture of polyols (glycerol and ethyleneglycol) using H3PO4 as the phosphate source. The use of boththe polyol admixture and the citrate salt has never beenreported for the synthesis of BiPO4 particles, and their role inthe mechanism of formation of the nanostars is discussedherein. The crystal structure and morphology of the nanostarsare characterized in detail by means of X-ray diffraction andseveral complementary transmission electron microscopytechniques, like electron tomography. Functionalization of theBiPO4 nanostars with citrate ions is investigated by infraredspectroscopy and thermogravimetry measurements. Thebifunctional character of the BiPO4 nanostars is finally provenvia their PC and luminescence properties: their PC activity ismeasured for the degradation of two different pollutants, whilethe ability of BiPO4 as a host for luminescent applications isanalyzed in Eu3+-doped BiPO4 nanostars.

2. EXPERIMENTAL METHODS2.1. Reagents. Bismuth nitrate pentahydrate [Bi(NO3)3·5H2O,

≥99.99%, Aldrich] and phosphoric acid (H3PO4, 85% weight in H2O,99.99%) were selected as bismuth and phosphate precursors,respectively. Sodium citrate tribasic dihydrate [HOC(COONa)-(CH2COONa)2·2H2O, ≥99.5% NT, Sigma] was used as a complexingagent. Finally, glycerol (PA-ACS-ISO, Panreac), ethylene glycol(puriss., Fluka), and their admixtures at different ratios were testedas solvents.2.2. Synthesis of Samples. The typical procedure for the

synthesis of samples was as follows. Weighted amounts of sodiumcitrate tribasic dihydrate (SCTB hereafter) were dissolved in theselected solvent, either glycerol (G), ethylene glycol (EG), or theiradmixtures at different G/EG volumetric ratios of 10/90, 50/50, and90/10 under magnetic stirring for 15 h at 75 °C. The desired amountof bismuth nitrate was dissolved in this solution that was beingmagnetically stirred and heated at 75 °C to favor dissolution. Finally,the desired amount of phosphoric acid was admixed at roomtemperature. After homogenization, the final solution (total volumeof 10 cm3) was aged for 20 h in tightly closed test tubes using an ovenpreheated at 120 °C. The resulting dispersions were cooled to roomtemperature, centrifuged to remove the supernatants, and washed,twice with ethanol and once with doubly distilled water. For someanalyses, the powders were dried at room temperature.2.3. Characterization. The shape of the particles was examined by

transmission electron microscopy (TEM) (Philips 200CM). Particlesize distributions were obtained from the micrographs by countingseveral hundred particles. To gain additional information about thestructural features of the synthesized nanoparticles, they were alsocharacterized by high-resolution transmission electron microscopy(HRTEM) using a FEGTEM Tecnai 20 instrument operated at 80,200, and 300 keV. TEM images used for the morphological andcrystallographic characterization of the particles were acquired with anUltrascan X100 camera from Gatan. High-angle annular dark-fieldtransmission electron (HAADF STEM) tomography was used for

measuring the shape of the nanostars in three dimensions.22 Tilt seriesof HAADF STEM images were acquired for a range of angles from−70° to 70° with a tilt step of 1°. The size of the electron probe was∼0.2 nm and the beam current 1.4 nA. Alignment and tomographicreconstruction using a simultaneous iterative (SIRT) algorithm wereconducted with Inspect3D, and isosurface visualization was conductedwith Amira.23 The crystallinity and crystal uniformity of the particleswere confirmed using dark-field TEM. The symmetry of the unit cellof the nanorods was derived from the comparison of experimentalHRTEM images with computer simulations. Diffractograms wereobtained by Fourier transforming HRTEM images using DigitalMicrograph, and the symmetry and intensities of the peaks werecompared with those of calculated diffraction patterns of monoclinicand hexagonal crystal structures obtained with Eje-Z (University ofCadiz, Cadiz, Spain).24 We found a best match for the low-temperature monoclinic P21/n unit cell (a = 6.7626 Å, b = 6.9516Å, c = 6.4822 Å, α = 90.000°, β = 103.736°, and γ = 90.000°).10

Atomistic models of monoclinic BiPO4 were built and used tocompute multislice simulations of HRTEM images using Eje-Z andJEMS.25

The quantitative composition of the Eu3+-doped samples wasanalyzed by inductively coupled plasma atomic emission spectroscopy(ICP-AES, Horiba Jobin Yvon, Ultima 2).

Information about the colloidal stability of the nanostars in anaqueous suspension (0.5 mg/mL of solid) was obtained from theanalysis of the particle size by means of dynamic light scattering(DLS). The experiments were conducted using Malvern ZetasizerNano-ZS90 equipment, which was used as well to measure the Zpotential of the suspensions.

The crystalline structure of the prepared particles was assessed byX-ray diffraction (XRD) using a Panalytical, X’Pert Pro diffractometer(Cu Kα) with an X-Celerator detector over a 2θ angular range of 10−70°, a 2θ step width of 0.05°, and a counting time of 10 s. Crystallitesize was calculated using the Scherrer formula from the full width athalf-maximum of several single reflections. Unit cell parameters of theundoped and Eu3+-doped nanostars were calculated from the XRDpatterns using the Rietveld method with TOPAS. Refined parameterswere scale factor, zero error, background coefficients, and unit cellparameters. Starting crystallographic parameters were taken from thosereported for monoclinic BiPO4 by Romero et al.10

The presence of species adsorbed on the particles surface wasanalyzed by both infrared spectroscopy and thermogravimetry. Theinfrared spectra for the powders diluted in the KBr pellet wererecorded in a Nicolet 510 Fourier transform spectrometer. TGA wasperformed in air at a heating rate of 10 °C min−1 using a Q600 TAInstrument apparatus.

The photocatalytic activity was tested on two different pollutants:Methylene Blue (MB) and Rhodamine B (RhB). Their oxidationreactions were performed using a batch reactor (150 mL). The UVlight source was obtained by six UV germicidal lamps (λ = 254 nm, 4W each). In the oxidation tests, an air flow was employed to produce ahomogeneous suspension of the photocatalyst in the solution. Beforeeach experiment, the catalysts (1 g/L) were settled in a suspensionwith the reagent mixture for 15 min. The blank experiment wasperformed without a catalyst, and no dye degradation was observedafter 2 h. The evolution of the initial MB (∼10 ppm) and RhB (∼5ppm) concentrations was followed through the evolution of thecharacteristic 664 and 554 nm bands, respectively, using a centrifugedaliquot of ∼2 mL of the suspension (microcentrifuge Minispin,Eppendorf). The natural pH of the suspension was unchanged duringthe photodegradation tests (pH ∼6.5 and ∼5.5 for MB and RhB,respectively) for all BiPO4 samples. In both cases, for RhB as well asfor MB, dye discoloration proceeds by chromophore cleavage becauseno significant shift was observed in their characteristic UV−vis bandsfollowed by the photoactivity studies.

Finally, the excitation and emission spectra of the Eu3+-dopedBiPO4 samples, dispersed in water (2.5 mg mL−1), were recorded in aHoriba Jobin Yvon spectrofluorimeter (Fluorolog3). The emissionspectra were transformed to the CIE color coordinates system using a2A,A° observer.

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3. RESULTS3.1. Synthesis of BiPO4 Nanostars. It is well-documented

that the formation of uniform particles in solution requires ahomogeneous precipitation process, which can be achieved, forexample, through a slow and controlled release of thecorresponding precipitating anions or cations in the reactionmedium.26 To achieve such conditions in our system, we usedcitrate anions, which form Bi−citrate chelates that are stable atroom temperature.27 Heating the chelates produces theliberation of Bi cations, which precipitate as BiPO4 afterreaction with the phosphate anions present in the reactionmedium.Under the conditions described above, we found that aging a

solution containing bismuth nitrate (0.015 M), SCTB (0.15M), and H3PO4 (0.075 M) in a mixture of glycerol (G) andethylene glycol (EG) (G/EG volumetric ratio of 90/10) at 120°C for 20 h led to the formation of monodisperse, nanometer-sized particles (Figure 1a). The morphology of the particles can

be observed in detail in Figure 1b. Each particle consisted ofthree bunches of nanorods that crossed over at ∼60° one fromthe other so that the final morphology resembled that of a star.The mean diameter of the stars, from now on “nanostars”,obtained by counting several hundreds of them in TEMmicrographs, was 150 nm (standard deviation of 30), and thenanorods had a width of 10 nm. To have a better overview ofthe morphology of the stars, they were analyzed in threedimensions using electron tomography. Figure 1c shows anisosurface visualization of the morphology of the same starviewed from two orthogonal directions, demonstrating that thenanostars are flattened particles.The set of experimental conditions mentioned above is

essential for obtaining uniform nanostars. We found that using

lower or higher values of the G/EG ratio, and keeping the otherparameters constant, produced either irregular, aggregatednanoparticles or ill-defined micrometer-sized stars with anincrease in the G/EG ratio (Figure S1a,b of the SupportingInformation). Likewise, the SCTB content clearly controlledthe size and shape of the particles; therefore, decreasing theSCTB concentration to 0.10 M led to the formation ofaggregated, small, rounded nanoparticles, while increasing it to0.20 M produced ill-defined particles (Figure S1c,d of theSupporting Information). It was also determined that thechange in H3PO4 (Figure S1e,f of the Supporting Information)concentration produced either strongly aggregated nano-particles or ill-defined precipitates. Finally, decreasing theBi(NO3)3·5H2O concentration led to strongly aggregatedrounded nanoparticles, while increasing it produced largeaggregated stars (Figure S1g,h of the Supporting Information).

3.2. Crystalline Structure of the Nanostars. Figure 2shows the X-ray diffraction pattern of the nanostars synthesized

under the conditions described in section 3.1. The patternmatches the standard PDF ICDD 01-089-0287 correspondingto BiPO4 and indicates that the nanostars exhibit a monoclinicsymmetry with space group P21/n. The inset in Figure 2 is adark-field TEM image of one nanostar showing high intensityfor only crystalline areas with a similar orientation. This imageconfirms that the nanorods are single crystals with few localizeddefects.Further details of the crystalline structure of the nanostars

were obtained from HRTEM and computer simulations(Figure 3). Figure 3a is a HRTEM image of four spikesbelonging to the same particle. The four spikes displaycontinuous atomic crystal planes extending all along theirlength, which confirms their single-crystal nature. In particular,one of the spikes is suitable for crystallographic identification ofthe unit cell because it is oriented on-axis and displays highsymmetry. The diffractogram corresponding to the area insidethe white box in Figure 3a, which has been magnified androtated for display purposes in Figure 3d, is shown in Figure 3band matches the simulated diffractogram of a monoclinic BiPO4crystal oriented in zone axis [211] (Figure 3c). The growthdirection of the nanorod can be identified along the (0−11)direction. To further confirm the correctness of the identifiedsymmetry, we obtained the atomistic model of one unit cell of

Figure 1. (a and b) Bright-field TEM images at differentmagnifications of the starlike particles obtained after a solutioncontaining Bi(NO3)3 (0.015 M), SCTB (0.15 M), and H3PO4 (0.075M) in a mixture of glycerol and ethylene glycol (90/10 volumetricratio) had been aged at 120 °C for 20 h. (c) Isosurface visualization ofthe three-dimensional morphology of the one star viewed from twoorthogonal directions measured using transmission electron tomog-raphy.

Figure 2. XRD pattern of the nanostars together with reflections ofPDF ICDD 01-089-0287 corresponding to monoclinic BiPO4. Theinset is a dark-field TEM image of one nanostar showing high intensityin only crystalline areas with the same orientation.

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the P21/n BiPO4 crystal oriented in zone axis [211]. Thismodel was then used as the starting structure to performmultislice simulations of HRTEM images for different imagingconditions.25 In can be observed that the simulation in Figure3e successfully fits the positions and relative intensities of Biand P atoms (note that for non-aberration-corrected HRTEMimages, O atoms are unresolved). These observations confirm,therefore, that the nanorods forming the stars display amonoclinic symmetry and grow along the (0−11) direction.3.3. Analysis of the BiPO4 Nanostar Surface. Surface

functionalization of luminescent nanoparticles is required fortheir use as biological luminescent labels, as mentioned in theIntroduction. We used both infrared spectroscopy andthermogravimetry to analyze the presence, on the nanostarssurface, of citrate species that could act as functionalizingagents. Panels a and b of Figure 4 show the Fourier transforminfrared (FT-IR) spectra of pure SCTB and BiPO4 nanostars,respectively. The FT-IR spectrum of the nanostars shows thecharacteristic bands of PO4

3− ions at <1200 cm−1.28 The mainabsorption band in the region of the hydroxyl stretching,centered at 3435 cm−1, is assigned to water adsorbed on thesurface of the nanostars because the monoclinic BiPO4structure does not contain structural water. On the otherhand, two broad bands can be observed in the 1800−1200cm−1 region (magnified in the inset), the one at lower energiesshowing, in turn, two maxima centered at 1631 and 1566 cm−1.The former corresponds to the bending vibration of the water

molecules on the particle surface,28 while the latter, togetherwith the band on the high-energy side of this region (centeredat 1393 cm−1), can be assigned to vibrations of the carboxylateanion in the citrate species. Their position in the spectrum,slightly shifted from the spectrum of pure citrate (Figure 4a),indicates that the citrate species are attached to the heavy Bi3+

ions of the particles and proves the surface functionalization ofthe BiPO4 nanostars.

29

This result was further confirmed by thermogravimetry,which allowed as well the determination of the amount ofcitrate species on the nanostar surface. The thermogravimetriccurve (Figure 5) shows a mass loss over a wide temperature

range, from room temperature to 700 °C, which could bedivided into two terms. The first one (∼3.3 wt %) is due tosurface water release, while the second one (∼2 wt %) mustcorrespond to the decomposition of the citrate ions adsorbedon the nanostar surface. This assignation was further confirmedby the FT-IR spectrum of the sample after calcination at 400 °Cfor 2 h (Figure 4c), which clearly lacks the bands associatedwith the citrate species.The results shown above demonstrate that the use of SBTC

provides a one-pot method for the synthesis and functionaliza-tion of BiPO4 nanostars.

Figure 3. (a) HRTEM image showing the crystal planes of four spikesof one particle. (b) Experimental diffractogram corresponding to thearea inside the white box in panel a and magnified and rotated in paneld. (c) Simulated diffractogram of the monoclinic BiPO4 structure(space group P21/n) oriented along zone axis [211]. (e) Unit cell ofthe BiPO4 crystal (Bi, red atoms; P, yellow; O, gray) and multislicesimulation of the HRTEM image. The simulation fits very well thepositions of Bi and P atoms (O atoms are unresolved in conventionalTEM).

Figure 4. IR spectra of (a) sodium citrate tribasic, (b) as-synthesizedBiPO4 nanostars, and (c) BiPO4 nanostars calcined at 400 °C for 2 h.The inset is a magnification of the 1800−1200 cm−1 region.

Figure 5. Thermogravimetric curve of the BiPO4 nanostar powdersample.

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3.4. Mechanism of Formation of the BiPO4 Nanostars.The morphology of the starlike particles was monitored afterdifferent reaction times aimed at understanding their complexhierarchical structure. Figure 6 shows the TEM micrographs

and XRD patterns of the samples obtained after they had beenaged for 3.5, 5, 10, and 20 h at 120 °C. Before 3.5 h, clearsolutions were obtained. Ovoid nanoparticles (∼80 nm × ∼100nm) were observed in the first stages of the reaction (3.5 h).Their XRD pattern matched that of PDF ICDD 00-015-0766,which corresponds to hexagonal BiPO4. The crystalline domainsize, calculated using the Scherrer formula on single reflections(located at 2θ values of 20.03°, 31.24°, 37.7°, and 41.78°) wasbetween 10 and 15 nm. This observation indicates that theovoid particles are formed by the aggregation of smallerprimary units. In fact, if one observes in detail thecorresponding TEM micrograph, small rounded particles canbe seen especially when the contour of the ovoid particles isobserved. After 5 h, small needlelike particles grew radially fromthe surface of the ovoid nanoparticles, which also showed

hexagonal symmetry, although broader reflections wereobserved in their XRD pattern. Upon prolonged aging (10h), the needles grew along their long axis to form nanorods andthe XRD pattern of the sample showed monoclinic BiPO4reflections. At the same time, the size of the particles coreclearly decreased, giving rise, eventually (after 20 h), to the finalnanostars consisting of bunches of nanorods with monoclinicsymmetry.The hexagonal to monoclinic phase transformation of BiPO4

with reaction time, already reported in the literature for othertypes of BiPO4 particles,

15 suggests that monoclinic BiPO4 isthe thermodynamically stable phase. However, hexagonalBiPO4 shows faster kinetics, which allows its observation atthe first stages of the reaction. The morphology changesobserved with an increasing reaction time suggest that themetastable, hexagonal, ovoid nanoparticles progressivelyrecrystallized, forming the nanorods that constitute the starspikes. However, dissolution of the ovoid particles followed bya diffusion-controlled growth process could also contribute tothe growth of the nanorods.30 The observed reactionmechanism has been summarized in Figure 7.

The proposed reaction mechanism can be used to under-stand the change in the morphological characteristics of theBiPO4 particles precipitated under different experimentalconditions, as described at the beginning of the Results. Theionic strength (given by the reactant concentrations) and thedielectric constant of the solvent (42.5 and 37 for G and EG,respectively, at 25 °C) determine the equilibrium betweenattractive and repulsive forces in a colloidal system, thuscontrolling the aggregation process.31 Likewise, the viscosity ofthe solvent controls the ionic diffusion rate in the system, sothat the higher the viscosity, the slower the diffusion rate(viscosity values of G and EG at 20 °C of 1412 and 20.2 mPa s,respectively).32,33 These facts explain the different shape, size,and aggregation of the particles obtained at different G/EGratios and reactant concentrations.

3.5. Colloidal Stability of the BiPO4 Nanostars.Although the TEM micrographs shown in Figure 1demonstrated that the BiPO4 nanostars consisted of mono-disperse and nonaggregated particles, this aspect was furtherconfirmed by dynamic light scattering (DLS) of an aqueoussuspension of the sample (pH 6.71). The average hydro-dynamic diameter obtained by DLS was 166 (60) nm, which isvery close to that calculated from the TEM micrographs. Thisresult indicates the absence of particle aggregation and reveals

Figure 6. TEM micrographs (top) of the particles that precipitatedunder the experimental conditions described in the legend of Figure 1after different aging times. Corresponding XRD patterns (bottom).Top marks: PDF ICDD 00-015-0766 (hexagonal BiPO4). Bottommarks: PDF ICDD 01-089-0287 (monoclinic BiPO4).

Figure 7. Scheme showing the reaction mechanism for the formationof BiPO4 nanostars.

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Figure 8. Concentration of Methylene Blue (C0 = 10 ppm) (a) and Rhodamine B (C0 = 5 ppm) (b) as a function of photodegradation time. (c)Conversion plot for the photocatalytic degradation of Methylene Blue and Rhodamine B.

Figure 9. (a) Excitation spectrum (λem = 593 nm) of the 2% Eu3+-doped BiPO4 sample. (b) Emission spectra (λex = 250 nm) of Eu3+-doped BiPO4samples with different doping levels. (c) CIE diagram with the coordinates obtained from the emission spectrum of the 2% Eu3+-doped BiPO4sample.

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that the BiPO4 nanostars are stable in an aqueous medium atthe mentioned, close to neutral pH.On the other hand, the Z potential calculated on the same

suspension was −36.2 mV, which suggests that the colloidalstability of the suspension is mainly due to the electrostaticrepulsion among negatively charged particles. This result isinteresting for the photocatalytic study that will be describedbelow.3.6. Photocatalytic Activity. The photocatalytic perform-

ance of the as-prepared BiPO4 nanostars was evaluated byfollowing the photodegradation of MB and RhB under UV−vislight irradiation (panels a and b of Figure 8, respectively) of anaqueous suspension of the sample. From the evolution of theconcentration with irradiation time (Figure 8c), it can beobserved, first, that an important dark adsorption process istaking place in the case of MB (∼50%), while less than 10%adsorption is observed for RhB. This fact could be explained onthe basis of the different ionic character of MB and RhBmolecules. Thus, while MB shows a cationic configuration, RhBcan be a cationic or neutral dye molecule depending onwhether the carboxylic group is dissociated.34 The carboxylgroups of RhB are dissociated under these experimentalconditions (neutral pH), thus providing the RhB with anegative charge.35 As mentioned above, the BiPO4 nanostarsexhibit a negative Z potential of −36.2 mV at neutral pH, whichmeans that their surface is negatively charged. Therefore, MBadsorption is favored with respect to RhB, and thisphenomenon is detrimental for the photocatalityc activity ofthe BiPO4 particles, as shown in Figure 8c, which are only ableto degrade <30% of MB after illumination for 3 h. A similareffect has been previously observed for BiVO4 particles.

34

Second, Figure 8c shows that the BiPO4 nanostars are able todegrade >80% of RhB after illumination for 3 h. Moreover, theslight displacement observed in the main absorption band ofRhB at 550 nm (Figure 8b) indicates that the degradationproceeds by a simultaneous deethylation and chromophorecleavage mechanism.36 Therefore, the BiPO4 nanostars exhibitan efficient photocatalytic performance for the degradation ofRhB.3.7. Synthesis, Characterization, and Luminescence

Properties of Eu-Doped BiPO4 Nanostars. Eu3+ doping was

used, as a proof of concept, to analyze the luminescenceproperties of the resultant BiPO4-based phosphors. Thesynthesis method used to obtain BiPO4 nanostars was alsosuccessfully applied to obtain the Eu3+-doped BiPO4 nanostars.Different doping levels were used to evaluate the effect of theEu3+ content on the luminescence properties of the nano-phosphors. Nominal concentrations were 0.5, 1.0, 1.5, 2.0, and2.5 mol % Eu3+ in BiPO4. It was found that the same particlemorphology, size, and crystalline symmetry that were found forthe undoped material were reproduced for the doped samplesfor doping levels of ≤2%, while higher Eu3+ contents led to theformation of a heterogeneous sample consisting of irregularstars and rods (Figure S2 of the Supporting Information). TheEu contents of Bi1−xEuxPO4, calculated by ICP, were in goodagreement with the nominal compositions [x (ICP) = 0.0047,0.0083, 0.0131, and 0.0201 for nominal x = 0.005, 0.010, 0.015,and 0.020, respectively). To confirm the presence of Eu3+ in theBiPO4 crystal structure, that is, replacing Bi3+ in theircrystallographic sites, the corresponding unit cell volumeswere calculated by Rietveld analysis of their XRD patterns. Thevalues obtained for the undoped and the 2% Eu-doped materialwere 299.09 (0.29) and 298.70 (0.24), respectively, which is in

agreement with the ionic radius of Eu3+ (1.066 Å) being smallerthan that of Bi3+ (1.17 Å, in VIII coordination).To evaluate the luminescence properties of the Eu3+-doped

BiPO4 nanostars, their excitation spectra were recorded bymonitoring the emission of the 5D0−7F1 transition in Eu3+ at593 nm. Figure 9a shows such a spectrum for the 2% Eu3+-doped BiPO4 sample. The rest of the compositions showedspectra very similar to this one. A broad band can be observedbelow 300 nm, with its maximum lying below 250 nm,according to previous reports.37 This band has been assigned totwo different processes, namely, the Eu3+−O2− charge transfer38

and the 1S0 →3P1 transition of Bi3+.9 On the other hand, the

bands in the range of 300−410 nm are due to the directexcitation of Eu3+ from its ground state to higher levels of the 4fmanifold,39 which are labeled in the figure.The emission spectra of the Eu3+-doped BiPO4 samples

(Eu3+ from 0.5 to 2.0 mol %) recorded after excitation at 250nm are shown in Figure 9b. All of them show the emission linescorresponding to the well-known 5D0−7FJ (J = 0, 1, 2, 3, or 4)transitions of Eu3+. The 5D0−7F1 transition is due to themagnetic dipole transition of Eu3+ ions and is independent ofthe symmetry of the Eu3+ site. However, the 5D0−7F2 transitionis a forced electric dipole transition, hypersensible to the sitesymmetry of the Eu3+ ions. When the Eu3+ ions are located at asite with no inversion center, the intensity of the latter isnormally greater than that of the former.40 We have calculatedthe intensity ratio of the 5D0−7F2 and 5D0−7F1 transitions(called asymmetry ratio) from the spectra of Figure 9b, and avalue of ∼1.1 has been obtained for all three compositions. Thisresult suggests that Eu3+ occupies the noncentrosymmetric Bi3+

sites in the monoclinic P21/n structure of BiPO4.Although the emission spectra of all compositions are very

similar to each other from a qualitative point of view, it can beobserved in Figure 9b that their absolute intensities increasewith an increase in Eu3+ content. The composition showing thegreatest emission is the BiPO4:2% Eu3+ form. The increase inluminescence intensity is clearly due to the progressive increaseof emission centers as the doping concentration increases. Ifany concentration quenching effect would exist in theconcentration range analyzed (0.5−2.0% Eu), such an effectis overcome by the increase in luminescence centers as the Eucontent increases.Finally, the emission bands of the Eu-doped BiPO4

nanophosphors, located in the spectral range of 575−710 nm,are responsible for the orange-red luminescence of the samples,as indicated by their CIE coordinates (x = 0.62, y = 0.38)(Figure 9c).

4. CONCLUSIONSMonoclinic, monodisperse BiPO4 nanostars (120−160 nm indiameter) can be synthesized after a solution containing 0.015M bismuth nitrate, 0.15 M sodium citrate tribasic, and 0.075 Mphosphoric acid in a mixture of glycerol and ethylene glycol(volumetric ratio of 90/10) is aged at 120 °C for 20 h. This setof experimental conditions is essential to obtain uniformnanostars, because the variation of either the Bi(NO3)3 or theH3PO4 concentration, the polyol ratio, or the SCTBconcentration led either to gel-like precipitates, to aggregated,irregular particles, or to micrometer-sized particles. The use ofSBTC provides a one-pot method for the synthesis andfunctionalization of BiPO4 nanostars, which exhibit an efficientphotocatalytic performance for the degradation of RhB. Finally,it has been demonstrated that the stars can be Eu3+-doped up to

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a 2% doping level without changing the particle morphologyand symmetry. The doped samples show emission in theorange-red region of the visible spectrum after UV excitation,demonstrating that they can be used as phosphors forbiotechnological applications.

■ ASSOCIATED CONTENT

*S Supporting InformationTEM micrographs showing the effect of the G/EG ratio andthe concentration of sodium citrate, H3PO4, and Bi(NO3)3 onthe morphology of the particles precipitated using theexperimental conditions required for the synthesis of theBiPO4 nanostars and changing the value of one of thementioned parameters, TEM micrographs of the 2 and 2.5%Eu3+-doped BiPO4 samples, and XRD pattern and EDXspectrum of the 2.0% Eu3+-doped BiPO4 sample. This materialis available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Instituto de Ciencia de Materiales de Sevilla, CSIC-Universityof Seville, c/Americo Vespucio, 49, 41092 Seville, Spain.Phone: +34 954489545. Fax: +34 954460165. E-mail: anieto@icmse.csic.es.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Supported by MEC (Project MAT2012-34919), Junta deAndalucia (JA FQM-6090 and FQM-4570), EU 7FP (ProjectAl-NanoFunc CT-REGPOT-2011-1-285895), and CSIC (Proj-ect 201460E08). S.O. thanks CSIC for the concession of a JAE-Pre grant.

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