Article pubs.acs.org/crystal
Bifunctional, Monodisperse BiPO4‑Based Nanostars: Photocatalytic Activity and Luminescent Applications Ana Isabel Becerro,* Joaquín Criado, Lionel C. Gontard, Sergio Obregón, Asunción Fernández, Gerardo Colón, and Manuel Ocaña Instituto de Ciencia de Materiales de Sevilla, CSIC-University of Seville, c/Américo Vespucio, 49, 41092 Seville, Spain S Supporting Information *
ABSTRACT: Monodisperse, monoclinic BiPO4 nanostars have been synthesized by a homogeneous precipitation reaction at 120 °C through controlled release of Bi3+ cations from a Bi−citrate chelate, in a mixture of glycerol and ethylene glycol, using H3PO4 as the phosphate source. The set of experimental conditions necessary to obtain uniform nanoparticles is very restrictive, as the change in either the polyol ratio or the reactant concentrations led to ill-defined and/or aggregated particles. The morphology of the particles consists of a starlike, hierarchical structure formed by the ordered arrangement of nanorod bundles. Transmission electron tomography has revealed that the nanostars are not spherical but flattened particles. Likewise, Fourier transform infrared spectroscopy and thermogravimetry have shown that the synthesized nanostars are functionalized with citrate groups. The mechanism of formation of the nanostars has been analyzed to explain their morphological features. The as-synthesized BiPO4 nanostars exhibit an efficient photocatalytic performance for the degradation of Rhodamine B. Finally, it has been demonstrated that the stars can be Eu3+-doped up to 2 mol % without any change in the particle morphology or symmetry, and the doped samples show emission in the orange-red region of the visible spectrum after ultraviolet excitation. These experimental observations make this material a suitable phosphor for biotechnological applications.
1. INTRODUCTION Inorganic luminescent materials have found extensive applications in optoelectronics (displays, LEDs, and lasers)1 and biomedicine (fluorescent markers and phototherapy).2 Many of the inorganic luminescent materials are obtained after doping different hosts with luminescent lanthanide ions. Among these hosts, there has been an increase in the level of interest, in the past few years, in rare earth-based nanoparticles, including fluorides,3 oxides,4 hydroxides,5 vanadates,6 and phosphates,7 which show a number of interesting features such as their high thermal and chemical stability. In particular, rare earth phosphates (REPO4, where RE = Y3+, La3+, Gd3+, or Lu3+) show, in addition, a high degree of biocompatibility, which is particularly valuable for their use in biomedicine. In addition to the mentioned biocompatibility, luminescent inorganic particles to be used in biomedical applications must fulfill a series of requisites, namely, (i) a high degree of homogeneity, in both size and shape, which is necessary to ensure the reproducibility of their physicochemical properties, (ii) a nanometer size, which is specifically important for in vivo applications to facilitate their elimination from the body, (iii) colloidal stability in a physiological medium, and (iv) having a functional group on their surface that can act as an anchor site for the addition of functional ligands such as antibodies, peptides, proteins, and drugs,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 © 2014 American Chemical Society
the host material) is much more costly than obtaining main group elements like Sb and Bi, which can be easily purified in large quantities by techniques like zone refining.9 Because of the similar ionic radii of Bi3+ and RE3+ and the fact that BiPO4 is isostructural to REPO4,10 BiPO4 was proposed by Guan et al.11 as a cheaper alternative to REPO4. Since the publication of that report, a number of papers have appeared about the synthesis of luminescent Ln3+-doped BiPO4 particles with different morphologies (rods, spheroids, cocoons, urchins, etc.), ranging in size from nanometers to micrometers.12−16 However, most of the BiPO4 particles reported in the literature showed a high degree of aggregation or a heterogeneous size, which is detrimental for biomedical applications, as mentioned above. Only Pan and Zhu12 described a method for synthesizing monodisperse BiPO4 nanospheres using oleic acid (OA) as a surfactant. However, the OA remained adsorbed on the particle surface, making it hydrophobic and, therefore, difficult to disperse in physiological media. In addition, although surface modification of luminescent particles with functional groups has been reported in the literature for numerous RE-based nanoparticles,17,6 no report of functionalization of BiPO4 nanoparticles has been published so far, to the best of our knowledge. Received: February 11, 2014 Revised: April 8, 2014 Published: May 23, 2014 3319
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measuring the shape of the nanostars in three dimensions.22 Tilt series of 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 tomographic reconstruction using a simultaneous iterative (SIRT) algorithm were conducted with Inspect3D, and isosurface visualization was conducted with Amira.23 The crystallinity and crystal uniformity of the particles were confirmed using dark-field TEM. The symmetry of the unit cell of the nanorods was derived from the comparison of experimental HRTEM images with computer simulations. Diffractograms were obtained by Fourier transforming HRTEM images using Digital Micrograph, and the symmetry and intensities of the peaks were compared with those of calculated diffraction patterns of monoclinic and hexagonal crystal structures obtained with Eje-Z (University of Cádiz, Cádiz, Spain).24 We found a best match for the lowtemperature monoclinic P21/n unit cell (a = 6.7626 Å, b = 6.9516 Å, c = 6.4822 Å, α = 90.000°, β = 103.736°, and γ = 90.000°).10 Atomistic models of monoclinic BiPO4 were built and used to compute multislice simulations of HRTEM images using Eje-Z and JEMS.25 The quantitative composition of the Eu3+-doped samples was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Horiba Jobin Yvon, Ultima 2). Information about the colloidal stability of the nanostars in an aqueous suspension (0.5 mg/mL of solid) was obtained from the analysis of the particle size by means of dynamic light scattering (DLS). The experiments were conducted using Malvern Zetasizer Nano-ZS90 equipment, which was used as well to measure the Z potential of the suspensions. The crystalline structure of the prepared particles was assessed by X-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. Crystallite size was calculated using the Scherrer formula from the full width at half-maximum of several single reflections. Unit cell parameters of the undoped and Eu3+-doped nanostars were calculated from the XRD patterns using the Rietveld method with TOPAS. Refined parameters were scale factor, zero error, background coefficients, and unit cell parameters. Starting crystallographic parameters were taken from those reported for monoclinic BiPO4 by Romero et al.10 The presence of species adsorbed on the particles surface was analyzed by both infrared spectroscopy and thermogravimetry. The infrared spectra for the powders diluted in the KBr pellet were recorded in a Nicolet 510 Fourier transform spectrometer. TGA was performed in air at a heating rate of 10 °C min−1 using a Q600 TA Instrument apparatus. The photocatalytic activity was tested on two different pollutants: Methylene Blue (MB) and Rhodamine B (RhB). Their oxidation reactions were performed using a batch reactor (150 mL). The UV light source was obtained by six UV germicidal lamps (λ = 254 nm, 4 W each). In the oxidation tests, an air flow was employed to produce a homogeneous suspension of the photocatalyst in the solution. Before each experiment, the catalysts (1 g/L) were settled in a suspension with the reagent mixture for 15 min. The blank experiment was performed without a catalyst, and no dye degradation was observed after 2 h. The evolution of the initial MB (∼10 ppm) and RhB (∼5 ppm) concentrations was followed through the evolution of the characteristic 664 and 554 nm bands, respectively, using a centrifuged aliquot of ∼2 mL of the suspension (microcentrifuge Minispin, Eppendorf). The natural pH of the suspension was unchanged during the photodegradation tests (pH ∼6.5 and ∼5.5 for MB and RhB, respectively) for all BiPO4 samples. In both cases, for RhB as well as for MB, dye discoloration proceeds by chromophore cleavage because no significant shift was observed in their characteristic UV−vis bands followed by the photoactivity studies. Finally, the excitation and emission spectra of the Eu3+-doped BiPO4 samples, dispersed in water (2.5 mg mL−1), were recorded in a Horiba Jobin Yvon spectrofluorimeter (Fluorolog3). The emission spectra were transformed to the CIE color coordinates system using a 2à , ° observer.
In addition to the advantages of BiPO4 as an alternative host to REPO4 for luminescent applications, this material has been proven to be an efficient photocatalyst (PC) for decomposing organic pollutants.18−20 In general, the photocatalytic activity is closely related to the size, shape, phase, and structure of the photocatalyst.21 Therefore, it is highly desirable to develop new synthetic strategies to allow the control of such morphological parameters. Here we report an original procedure for the synthesis of monodisperse BiPO4 nanoparticles with a new morphology consisting of a starlike, hierarchical structure formed by the ordered arrangement of nanorod bundles. The synthesis method consists of a homogeneous precipitation of BiPO4, through controllable release of Bi3+ cations from Bi−citrate complexes, in a mixture of polyols (glycerol and ethylene glycol) using H3PO4 as the phosphate source. The use of both the polyol admixture and the citrate salt has never been reported for the synthesis of BiPO4 particles, and their role in the mechanism of formation of the nanostars is discussed herein. The crystal structure and morphology of the nanostars are characterized in detail by means of X-ray diffraction and several complementary transmission electron microscopy techniques, like electron tomography. Functionalization of the BiPO4 nanostars with citrate ions is investigated by infrared spectroscopy and thermogravimetry measurements. The bifunctional character of the BiPO4 nanostars is finally proven via their PC and luminescence properties: their PC activity is measured for the degradation of two different pollutants, while the ability of BiPO4 as a host for luminescent applications is analyzed in Eu3+-doped BiPO4 nanostars.
2. EXPERIMENTAL METHODS 2.1. 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 complexing agent. Finally, glycerol (PA-ACS-ISO, Panreac), ethylene glycol (puriss., Fluka), and their admixtures at different ratios were tested as solvents. 2.2. Synthesis of Samples. The typical procedure for the synthesis of samples was as follows. Weighted amounts of sodium citrate tribasic dihydrate (SCTB hereafter) were dissolved in the selected solvent, either glycerol (G), ethylene glycol (EG), or their admixtures at different G/EG volumetric ratios of 10/90, 50/50, and 90/10 under magnetic stirring for 15 h at 75 °C. The desired amount of bismuth nitrate was dissolved in this solution that was being magnetically stirred and heated at 75 °C to favor dissolution. Finally, the desired amount of phosphoric acid was admixed at room temperature. After homogenization, the final solution (total volume of 10 cm3) was aged for 20 h in tightly closed test tubes using an oven preheated at 120 °C. The resulting dispersions were cooled to room temperature, centrifuged to remove the supernatants, and washed, twice with ethanol and once with doubly distilled water. For some analyses, the powders were dried at room temperature. 2.3. Characterization. The shape of the particles was examined by transmission electron microscopy (TEM) (Philips 200CM). Particle size distributions were obtained from the micrographs by counting several hundred particles. To gain additional information about the structural features of the synthesized nanoparticles, they were also characterized 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 and crystallographic characterization of the particles were acquired with an Ultrascan X100 camera from Gatan. High-angle annular dark-field transmission electron (HAADF STEM) tomography was used for 3320
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3. RESULTS 3.1. Synthesis of BiPO4 Nanostars. It is well-documented that the formation of uniform particles in solution requires a homogeneous precipitation process, which can be achieved, for example, through a slow and controlled release of the corresponding precipitating anions or cations in the reaction medium.26 To achieve such conditions in our system, we used citrate anions, which form Bi−citrate chelates that are stable at room temperature.27 Heating the chelates produces the liberation of Bi cations, which precipitate as BiPO4 after reaction with the phosphate anions present in the reaction medium. Under the conditions described above, we found that aging a solution containing bismuth nitrate (0.015 M), SCTB (0.15 M), and H3PO4 (0.075 M) in a mixture of glycerol (G) and ethylene glycol (EG) (G/EG volumetric ratio of 90/10) at 120 °C for 20 h led to the formation of monodisperse, nanometersized particles (Figure 1a). The morphology of the particles can
lower or higher values of the G/EG ratio, and keeping the other parameters constant, produced either irregular, aggregated nanoparticles or ill-defined micrometer-sized stars with an increase in the G/EG ratio (Figure S1a,b of the Supporting Information). Likewise, the SCTB content clearly controlled the size and shape of the particles; therefore, decreasing the SCTB concentration to 0.10 M led to the formation of aggregated, small, rounded nanoparticles, while increasing it to 0.20 M produced ill-defined particles (Figure S1c,d of the Supporting Information). It was also determined that the change in H3PO4 (Figure S1e,f of the Supporting Information) concentration produced either strongly aggregated nanoparticles or ill-defined precipitates. Finally, decreasing the Bi(NO3)3·5H2O concentration led to strongly aggregated rounded nanoparticles, while increasing it produced large aggregated stars (Figure S1g,h of the Supporting Information). 3.2. Crystalline Structure of the Nanostars. Figure 2 shows the X-ray diffraction pattern of the nanostars synthesized
Figure 2. XRD pattern of the nanostars together with reflections of PDF ICDD 01-089-0287 corresponding to monoclinic BiPO4. The inset is a dark-field TEM image of one nanostar showing high intensity in only crystalline areas with the same orientation.
under the conditions described in section 3.1. The pattern matches the standard PDF ICDD 01-089-0287 corresponding to BiPO4 and indicates that the nanostars exhibit a monoclinic symmetry with space group P21/n. The inset in Figure 2 is a dark-field TEM image of one nanostar showing high intensity for only crystalline areas with a similar orientation. This image confirms that the nanorods are single crystals with few localized defects. 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 spikes belonging to the same particle. The four spikes display continuous atomic crystal planes extending all along their length, which confirms their single-crystal nature. In particular, one of the spikes is suitable for crystallographic identification of the unit cell because it is oriented on-axis and displays high symmetry. The diffractogram corresponding to the area inside the white box in Figure 3a, which has been magnified and rotated for display purposes in Figure 3d, is shown in Figure 3b and matches the simulated diffractogram of a monoclinic BiPO4 crystal oriented in zone axis [211] (Figure 3c). The growth direction of the nanorod can be identified along the (0−11) direction. To further confirm the correctness of the identified symmetry, we obtained the atomistic model of one unit cell of
Figure 1. (a and b) Bright-field TEM images at different magnifications of the starlike particles obtained after a solution containing Bi(NO3)3 (0.015 M), SCTB (0.15 M), and H3PO4 (0.075 M) in a mixture of glycerol and ethylene glycol (90/10 volumetric ratio) had been aged at 120 °C for 20 h. (c) Isosurface visualization of the three-dimensional morphology of the one star viewed from two orthogonal directions measured using transmission electron tomography.
be observed in detail in Figure 1b. Each particle consisted of three bunches of nanorods that crossed over at ∼60° one from the 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 TEM micrographs, was 150 nm (standard deviation of 30), and the nanorods had a width of 10 nm. To have a better overview of the morphology of the stars, they were analyzed in three dimensions using electron tomography. Figure 1c shows an isosurface visualization of the morphology of the same star viewed from two orthogonal directions, demonstrating that the nanostars are flattened particles. The set of experimental conditions mentioned above is essential for obtaining uniform nanostars. We found that using 3321
dx.doi.org/10.1021/cg500208h | Cryst. Growth Des. 2014, 14, 3319−3326
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Figure 4. IR spectra of (a) sodium citrate tribasic, (b) as-synthesized BiPO4 nanostars, and (c) BiPO4 nanostars calcined at 400 °C for 2 h. The inset is a magnification of the 1800−1200 cm−1 region.
molecules on the particle surface,28 while the latter, together with the band on the high-energy side of this region (centered at 1393 cm−1), can be assigned to vibrations of the carboxylate anion 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 of the BiPO4 nanostars.29 This result was further confirmed by thermogravimetry, which allowed as well the determination of the amount of citrate species on the nanostar surface. The thermogravimetric curve (Figure 5) shows a mass loss over a wide temperature
Figure 3. (a) HRTEM image showing the crystal planes of four spikes of one particle. (b) Experimental diffractogram corresponding to the area inside the white box in panel a and magnified and rotated in panel d. (c) Simulated diffractogram of the monoclinic BiPO4 structure (space group P21/n) oriented along zone axis [211]. (e) Unit cell of the BiPO4 crystal (Bi, red atoms; P, yellow; O, gray) and multislice simulation of the HRTEM image. The simulation fits very well the positions of Bi and P atoms (O atoms are unresolved in conventional TEM).
the P21/n BiPO4 crystal oriented in zone axis [211]. This model was then used as the starting structure to perform multislice simulations of HRTEM images for different imaging conditions.25 In can be observed that the simulation in Figure 3e successfully fits the positions and relative intensities of Bi and P atoms (note that for non-aberration-corrected HRTEM images, O atoms are unresolved). These observations confirm, therefore, that the nanorods forming the stars display a monoclinic symmetry and grow along the (0−11) direction. 3.3. Analysis of the BiPO4 Nanostar Surface. Surface functionalization of luminescent nanoparticles is required for their use as biological luminescent labels, as mentioned in the Introduction. We used both infrared spectroscopy and thermogravimetry to analyze the presence, on the nanostars surface, of citrate species that could act as functionalizing agents. Panels a and b of Figure 4 show the Fourier transform infrared (FT-IR) spectra of pure SCTB and BiPO4 nanostars, respectively. The FT-IR spectrum of the nanostars shows the characteristic bands of PO43− ions at
