Ultrathin Europium Oxide Nanoplatelets - American Chemical Society

Jan 26, 2015 - J. Hoy,. §,∥. M. Y. Sfeir,. ∥. E. A. Stach,. ∥ and J. H. Dickerson*. ,†,∥. †. Department of Physics, Brown University, Pro...
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Ultrathin Europium Oxide Nanoplatelets: “Hidden” Parameters and Controlled Synthesis, Unusual Crystal Structure, and Photoluminescence Properties D. Hudry,*,† A. M. M. Abeykoon,‡ J. Hoy,§,∥ M. Y. Sfeir,∥ E. A. Stach,∥ and J. H. Dickerson*,†,∥ †

Department of Physics, Brown University, Providence, Rhode Island 02912, United States Photon Sciences Directorate, §Condensed Matter Physics and Materials Science Department, and ∥Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States

‡‡

S Supporting Information *

ABSTRACT: A good understanding of the relationship between the atomic scale structure of ultrasmall europium oxide nanocrystals (NCs) and their photoluminescence properties is of major interest in the design and development of innovative europium-based nanophosphors. As a consequence, the preparation of reliable (controlled size and shape distributions) and structurally well characterized ultrasmall europium oxide NCs is an essential prerequisite to understand the size effects on their photoluminescence properties. First, we reveal that nonaqueous approaches used to synthesize ultrasmall europium oxide NCs are deeply affected by “hidden” parameters that are directly related to the preparation of the reactive mixture. Indeed, trace amounts of products of side reactions and byproducts, such as acetic acid and water, act as growth-directing agents. Second, the challenging problem related to the structural characterization of ultrasmall europium oxide NCs is addressed for the first time by coupling high-resolution transmission electron microscopy and X-ray atomic pair distribution function. The ultrasmall thickness of the as-prepared NCs apparently dictates the crystalline structure, which can no longer be described by the crystal phases of their bulk counterparts. The induced distortions due to the ultrasmall thickness as well as the bonding of the stabilizing organic ligand are strong enough to break down the symmetry and, hence, prevent the europium oxide NCs from accommodating the usual bulk crystal phase. Finally, the formation of such unusual polymorphs of europium oxide has a profound impact on the resulting crystal field, with direct effects on the photoluminescence properties.

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field.11−19 Additionally, the controlled synthesis (well-defined NCs with controlled size and shape distributions) of Ln2O3 NCs is still challenging and was reported by only a few authors. This is in contrast to Ln-based fluoride, oxyhalide, or phosphate NCs, for which a wide variety of size and shapes were reported. Indeed, the controlled synthesis of geometrically well-defined Ln2O3 NCs is still limited to the formation of ultrasmall (i.e., at least one dimension ≤1 nm) nanostructures.4,20−25 Interestingly, most photoluminescence properties of Ln2O3 NCs are not governed by quantum confinement effects (contrary to many semiconductor NCs) because of the strongly localized character of the 4f wave functions. However, some emission properties (efficiency and lifetime, for example) of Lnbased NCs can be influenced by size effects that originate from electron−phonon coupling, phonon density, local crystal field, and nanoscopic ion−ion interactions, among other phenom-

INTRODUCTION Lanthanide (Ln)-based luminescent nanocrystals (NCs) represent an interesting class of materials due to their upand down-conversion properties.1 They are currently under intense investigation due to their potential application in various important technological fields, such as solid-state lighting, display panels, and imaging agents. Over the past 2 decades, the synthesis and properties of various Ln-based NCs have been reported, ranging from fluorides (LnF3, NaLnF4)2,3 to oxides (Ln2O3, LnO2)4−6 via oxyhalides (LnOX with X = Cl, F)7,8 and phosphates (LnPO4).9 A detailed review recently addressed the synthesis, properties, and applications of these different compounds.10 As bulk materials, luminescent Ln sesquioxide (Ln2O3) has been considered for technological applications for more than a century. Nevertheless, within the field of nanoscience, Ln2O3 NCs have attracted much less interest compared, for example, to doped NaLnF4 NCs, which currently represent the most studied Ln-based NCs due to their interesting up-conversion properties and promising applications in the biomedical © 2015 American Chemical Society

Received: November 19, 2014 Revised: January 9, 2015 Published: January 26, 2015 965

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Chemistry of Materials Scheme 1. Ligand Exchange Reaction between Europium Acetate and Oleic Acida

a

R1 denotes the following linear alkyl chain: −(CH2)7−. Then, the reaction flask is backfilled with N2 and glacial acetic acid (anhydrous CH3COOH, 1.5 mmol) is added. The purified mixture is heated up (10 °C/min, static N2) to 310 °C for 60 min. Afterward, the heating mantle is removed and the flask is left to cool naturally to room temperature. Synthesis of Tapelike Europium Oxide NCs. The procedure is the same as the one previously described for square europium oxide NCs, except that 4.5 mmol of CH3COOH was injected. Control Experiments. To identify parameters that influence the output of the synthesis, a series of control experiments (CEs) was performed. The various CEs were performed as the one previously described for square europium oxide NCs except that the chemical composition of the starting reactive mixture was modified (Supporting Information, Table S1). For clarity and simplicity, the various CEs will be referred to in the main text as CE-x (with x ranging from 1 to 6) (Supporting Information, Table S1). Additionally and for consistency throughout the text, the previously synthesized square and tapelike europium oxide NCs are also referred to in the main text as CE-4 and CE-6, respectively (Supporting Information, Table S1). Structural Characterization. Low-resolution transmission electron microscopy (TEM) analyses of europium oxide NCs were conducted with a JEOL 1400 operating at 120 kV and equipped with a Gatan CCD camera for imaging. High-resolution TEM (HRTEM) analyses were conducted with a FEI Titan equipped with a spherical aberration corrector and operating at 80 kV. X-ray atomic pair distribution function (PDF) and wide angle X-ray scattering (WAXS) measurements were carried out at the X17A beamline (67 keV; λ = 0.186 Å) at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. Optical Spectroscopy. The attenuation spectra were recorded with the PerkinElmer Lambda25 spectrophotometer. Photoluminescence spectra were collected on a spectrometer that utilizes an SC 2450-PP Fianium supercontinuum laser and interference filters for excitation. Photoluminescence excitation spectra were collected on an ISS PC1 spectrofluorometer running in analog mode. All ensemble spectroscopy was performed on the europium oxide NCs (square and tapelike) in 1 mm path length quartz cuvettes suspended in hexane.

ena.26 Because size-dependent optical properties in Ln-based NCs cannot be directly explained by quantum confinement effects, one must consider the modification of the optical properties from another point of view. Due to the increasing surface-to-volume ratio at the nanoscale, surface effects can play a significant role, though “surface effects” is commonly used as a generic term in which different physical phenomena are consolidated (e.g., surface oscillator, structural defects, deviations from the average crystalline structure). Although many interesting observations have been made regarding the modification of the optical properties in Ln2O3 NCs at the nanoscale, a deep understanding as well as a quantitative description of the previously mentioned phenomena is still lacking. Such a description would be highly valuable to better understand the basics of Ln2O3 NCs photophysics and would help to improve the design and synthesis of functional Ln2O3 nanomaterials. In this paper, we report on the controlled synthesis of ultrathin (∼1 nm) square and tapelike europium oxide NCs and demonstrate that “hidden” parameters play a significant role in the output of the synthesis (primarily as shape-directing agents). The structural characterization of the as-prepared NCs was performed for the first time by combining high-resolution transmission electron microscopy (HRTEM) and X-ray total scattering experiments with the subsequent data analysis by atomic pair distribution function (PDF) analysis. PDF analysis clearly indicates that the as-prepared ultrathin europium oxide NCs can no longer be described by the bulk counterpart crystal phases (cubic or monoclinic). Finally, the photoluminescence (PL) properties of the as-prepared europium oxide NCs differ notably from those reported either for their corresponding bulk counterparts or other europium oxide NCs. Moreover, despite the ultrasmall thickness (∼1 nm) of the as-prepared NCs, no blue-shift quantum confinement effects of their optical properties were observed. On the basis of our experimental data, the PL differences were largely due to the modification of the crystal field, which is a direct consequence of the ultrasmall thickness. Interestingly, the measured quantum yield (QY) can reach a value as high as 3.75% in the better case, whereas the QY of the bulk counterpart is around 0.1%. Clearly, size effects in ultrathin europium oxide NCs can modify both energy migration processes and quenching mechanisms.

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RESULTS AND DISCUSSION Controlled Synthesis of Europium Oxide NCs and Hidden Parameters. Europium oxide NCs were synthesized by a nonhydrolytic approach in a highly coordinating organic medium. Such an approach was first introduced by Bawendi and co-workers27 to prepare CdE (E = S, Se, Te) semiconductor NCs and is currently known as the thermal decomposition (heating up and hot injection techniques) method.28,29 This method is probably the most powerful approach to get narrow size and shape distributions and is the most widely used to synthesize high-quality transition metal oxide,30 actinide oxide,31−33 metal chalcogenide,34 and Lnbased NCs. The molecular mechanisms involved in the formation of NCs in highly coordinating organic media are far from being understood and can be completely different from one compound to another one. Regarding the nonhydrolytic synthesis of Ln oxide NCs, the formation of the active monomers (which are usually described as the molecular species generated by thermal and/or chemical

EXPERIMENTAL SECTION

Synthesis of Square Europium Oxide NCs. The synthesis is performed using air-free techniques under purified (5.0) nitrogen (N2). First, europium acetate hydrate [Eu(CH3COO)3·xH2O, 0.6 mmol] is introduced in a mixture of trioctylamine [N(Oct)3, 6 mmol] and oleic acid (OA, 6 mmol). The mixture is heated up to 135 °C under vigorous N2 flow to dissolve Eu(CH3COO)3·xH2O. After 20 min, the temperature is decreased to 100 °C and oleylamine (OAm, 12 mmol) is added. The resulting mixture is purified under vacuum. The vacuum purification consists in five N2 ↔ vacuum (1.5 × 10−2 mbar) cycles followed by a dynamic vacuum step (5 × 10−3 mbar) for 10 min. 966

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Chemistry of Materials Scheme 2. Multistep Mechanism Used To Explain the Formation of Various Metal Oxide NCsa

a Europium was chosen as an example of the metal center. R1 denotes the following linear alkyl chain: −(CH2)7−. R2 denotes the two blue fragments highlighted in Scheme 1. R3 denotes the green fragment highlighted in step 1.

2). Unraveling the various molecular pathways involved in the formation of NCs is highly challenging. Such an attempt has been reported by Mezailles and co-workers in the case of a very simple system (thermal decomposition of nickel acetylacetonate in pure OAm).40 Even in that particular case, drastic experimental simplifications were necessary. In the case of europium oxide NCs, the molecular mechanism given in Scheme 2 is strengthened by the fact that the replacement of the primary amine (OAm) by a tertiary amine [N(Oct)3] (CE-3; SI, Table S1) gave results that are identical to those reported for CE-1. Nevertheless, although CE-2 clearly indicates that OAm is necessary to trigger the formation of europium oxide, the as-prepared NCs are not monodisperse either in terms of size or shape distributions (SI, Figure S3). This lack of agreement with the results reported by Yan and co-workers24 is a good indication of the existence of hidden parameters that might be explained by taking into account side reactions. Whereas such reactions are always neglected when dealing with the formation of Ln oxide NCs, several authors reported the influence of the nature of the starting metal precursor or even the grade of the starting organics.41−43 As previously described, the first step involved in the synthesis of europium oxide NCs is the dissolution of europium acetate hydrate. Hence, the ligand exchange reaction generates CH3COOH (Scheme 1) and H2O (water molecules from the starting acetate precursor; SI, Figure S4). Both CH3COOH and H2O can be partially or entirely eliminated from the reactive mixture depending on the temperature, gas flow, and pressure (highly dependent on the experimental setup). CE-4 (SI, Table S1) was performed exactly as CE-2, but in the former, CH3COOH was intentionally added after the final vacuum purification step. The quantity of CH3COOH was calculated on the basis of the reaction given in Scheme 1 and taking a value for x equal to 3 (formation of pure europium oleate). The freshly introduced glacial acetic acid (anhydrous CH3COOH) can react with OAm according to the acid−base reaction given in Scheme 3. Direct experimental evidence of

processes) and the nucleation and growth steps are widely accepted to be dependent on the chemical composition of the reactive mixture.35 The approach developed in this paper is similar to the one reported by Yan and co-workers24 and is based on the use of (Eu(OAc)3·xH2O) as the europium source along with OA, N(Oct)3, and OAm (a detailed description of the experimental setup and procedure is given in the Supporting Information (SI) (Figure S1). In a first step, the dissolution of Eu(CH3COOH)·xH2O within a mixture of OA and N(Oct)3 is most likely facilitated by a ligand exchange reaction between the acetate and oleate anions (Scheme 1). Depending on the starting ratio between OA and Eu(OAc)3· xH2O and the temperature, the ligand exchange will generate either a mixed europium acetate−oleate complex (i.e., 0 < x < 3, Scheme 1) or a pure europium oleate complex (i.e., x = 3, Scheme 1). The resulting mixture is highly stable; a heat treatment at high temperature (i.e., 310 °C for 60 min) under nitrogen did not trigger the formation of europium oxide NCs (CE-1; SI, Table S1). Such a reaction was found to end with a waxy europium oleate intermediate (no NCs were observed by TEM). These results are in good agreement with those reported by Yan and co-workers.4 The use of a primary amine, such as OAm, is necessary to trigger the formation of europium oxide NCs (CE-2; SI, Table S1). Several authors reported that OAm (or others primary amines) can act as an activator in the formation of various NCs.36−39 Such a mechanism, although direct experimental evidence is still missing for many compounds, has been widely used to explain the formation of various NCs. The mechanism typically involves the nucleophilic attack of the carbonyl group (step 1, Scheme 2) by OAm generating an unstable intermediate. The latter, after internal rearrangement, induces the formation of a hydroxyl group bonded to the metal center and amide elimination as well (step 2, Scheme 2). Finally, hydroxyl groups bonded to different metal centers undergo condensation reactions, generating metal−oxygen− metal bonds (i.e., the fundamental building blocks of oxide networks) while eliminating water molecules (step 3, Scheme 967

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Scheme 3. Acid−Base Reaction between Acetic Acid and Oleylamine Inducing the Formation of an Acid−Base Complex

Scheme 4. Thermal Decomposition of the Acid−Base Complex (cf. Scheme 3) Inducing the Formation of an Amide and Water Elimination

synthesis. It is worth pointing out that the generated byproducts (i) are produced in trace quantities and (ii) their chemical nature is dependent on the nature of the starting europium source [e.g., Eu(CH3COO)3, Eu(NO3)3, EuCl3, Eu(C5H7O2)3]. As a consequence, the method used (europium precursor, temperature, inert gas flow, and pressure) for the preparation of the initial mixture can dramatically modify the output of the synthesis and is most likely to be one of the main sources responsible for the lack of reproducibility. All our attempts to grow larger europium oxide NCs were unsuccessful. Such a behavior is particularly puzzling, and investigations are currently ongoing to understand why all reported Ln oxide NCs, prepared by colloidal processes, can only be synthesized as ultrasmall 2D nanostructures. Unusual Crystal Phase Due to the Ultrasmall Thickness of the Square and Tapelike Europium Oxide Nanoplatelets. The accurate atomic-scale structural characterization is highly challenging when dealing with ultrasmall nanostructures, mainly because of the limitations of “traditional” crystallography.46 Crystallography assumes infinite and ideal periodicity and uses two main parameters (atomic displacement parameters and partial occupation of lattice sites) to take into consideration some deviations from ideal periodicity. On the other hand, when the size of the coherent domains of a given material approaches the size of the lattice parameters, as is the case with ultrasmall NCs, the assumption of infinite periodicity is no longer verified and strong deviations from the average bulk counterpart crystal structure are expected. This problem, known as the nanostructure problem,47 was described by several authors48−52 and is currently under intense investigation. Because of the ultrasmall thickness of the as-prepared square and tapelike europium oxide nanoplatelets, whether significant deviations from the average bulk crystal structure exist must be known because this may impact the interpretation of the relationship between crystal structure and optical properties. Nevertheless, traditional X-ray powder diffraction (XPD), which only takes into account Bragg scattering, cannot reveal such deviations. Instead, total scattering (i.e., Bragg and diffuse scattering) experiments are required. The experimental PDFs [G(r)] (see SI and Figure S2 for details of the PDF measurements) obtained for the as-prepared square and tapelike nanoplatelets are given in Figure 2 (blue and green curve, respectively) and are compared to the calculated PDFs

this reaction is supported by the sharp increase of the temperature upon CH3COOH injection (SI, Figure S5 and corresponding explanations). The in situ formed acid−base complex is unstable at high temperature and decomposes, generating an amide and eliminating water (Scheme 4) as explained by Whiting and co-workers.44 Interestingly, ultrathin (0.9 ± 0.3 nm; SI, Figure S6) and nearly monodisperse square nanoplatelets (Figure 1a) were produced when CH3COOH was intentionally added. Note that a similar effect was reported for the synthesis of PbSe NCs.45 Similarly, the effect of H2O was investigated by performing CE-5, in which CH3COOH was replaced by H2O (SI, Table S1). The quantity of H2O was calculated on the basis of the thermogravimetric analysis of the starting europium acetate (SI, Figure S4) and refers to four water molecules bonded to one europium acetate. Surprisingly, the shape of the as-prepared NCs switched from square nanoplatelets to ultrasmall nanowires (SI, Figure S7). This first set of experiments clearly indicates that trace quantities of either CH3COOH (86 μL, 1% of the total volume of the reactive mixture) or H2O (36 μL, 0.4% of the total volume of the reactive mixture) play a significant role as growth-directing agents (by inducing side reactions that are not entirely understood) in the synthesis of europium oxide NCs. Although slightly different, such a similar effect was recently reported by Murray and co-workers, who controlled the shape of TiO2 NCs by changing the nature of the starting titanium precursor (TiCl4 vs TiF4).43 Similarly, Hudry et al. were able to modify the shape of ThO2 and UO2 NCs by using different thorium and uranium precursors.41 The effect of the starting CH3COOH content was further investigated. Increasing the starting CH3COOH content by a factor of 3 (CE-6; SI, Table S1) dramatically changes the shape of the NCs from square (Figure 1a) to tapelike nanoplatelets (Figure 1b). Whereas the side dimensions of the latter are very similar to those observed for square nanoplatelets (i.e., 12−17 nm), the mean length is around 125−140 nm and can even reach 200−250 nm. Additionally, based on the different TEM contrast observed for the square and tapelike nanoplatelets, the thickness of the latter is likely smaller than 0.9 nm. The presented results clearly show that the chemical species, which can be released in the organic medium when the starting metal precursor undergoes chemical modifications during the dissolution step, can drastically modify the output of the 968

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gray curve) and monoclinic (Figure 2b, gray curve) counterparts. All peaks (i.e., interatomic distances) are observed at the same position and the only difference is the damping of the PDF (SI, Figure S8), which is faster for the truncated structures due to the finite size of the latter compared to the bulk. Such a result is expected because the truncation did not take into consideration any constraints due to the size effect. As a consequence, bond distances, angles, and atomic positions do not show any deviation compared to the bulk counterpart; the only difference is due to the lack of correlation at high interatomic distances because of the finite size of the truncated models. On the other hand, Figure 2 clearly confirms that neither the bulk cubic (Figure 2a, gray curve) nor monoclinic (Figure 2b, gray curve) crystal structures can explain the experimental PDFs of the as-prepared square (Figure 2, blue curves) and tapelike (Figure 2, green curves) nanoplatelets. While the experimental PDFs show a sharp first peak, in agreement with the models, the second, third, and fourth peaks are heavily distorted when compared to the one exhibited both for the bulk cubic (Figure 2a, gray curve) and monoclinic (Figure 2b, gray curve) Eu2O3. The lack of agreement is even more noticeable when considering longer interatomic distances (i.e., >5 Å), while, as previously noted, it is not the case for perfectly truncated bulk structures with similar sizes (Figure 2, red curves). Such a discrepancy is clear evidence of a significant modification of the crystalline structure of the as-prepared europium oxide nanoplatelets compared to their known bulk counterparts (cubic and monoclinic). Note that the observed discrepancies between the experimental PDFs of the asprepared europium oxide nanoplatelets and those calculated from the bulk cubic and monoclinic structures of Eu2O3 cannot be attributed to a lack of crystallinity. Indeed, as thin as they are, the nanoplatelets are very well crystallized, as revealed by the HRTEM images and their corresponding fast Fourier transforms (FFTs) (Figures 1). Another important point is that the experimental PDFs of the square and tapelike nanoplatelets are different. Such differences might be due to additional distortions related to the different thicknesses. Note that the HRTEM images of an individual square nanoplatelet (Figure 1c,g) unambiguously show the particle as being a single domain. On the other hand, the HRTEM images of a single tapelike nanoplatelet (Figures 1d,h) reveal both very well crystallized and amorphous or highly disordered areas within a single particle. Nevertheless, the different domains that constitute the tapelike nanoplatelet shown in Figure 1d,h are not randomly oriented, and coherence throughout the width and the length is clearly visible. This observation suggests either an oriented attachment growth mechanism or electron beam damage (such damage was observed both on the square and tapelike nanoplatelets at 300 kV). Because XPD is the current reference technique to characterize the crystal structure of NCs, high-resolution XPD patterns were also acquired (Figure 3). As with the PDFs, the XPD patterns clearly show that both the bulk cubic (Figure 3a, gray curve) and monoclinic (Figure 3b, gray curve) structural models fail in explaining the crystalline structure of the as-prepared square (Figure 3, blue curves) and tapelike (Figure 3, green curves) nanoplatelets. The insets in Figure 3 show also the same experimental high-resolution XPD patterns but within the Q range that is commonly accessible (with a sufficient resolution when dealing with NCs) with a classical laboratory powder X-ray diffractometer (Cu Kα radiation). Our

Figure 1. Transmission electron microscopy (a, b) and high-resolution transmission electron microscopy (c, d) images as well as the corresponding FFTs (e, f) of the as-prepared square (left-hand side) and elongated (right-hand side) europium oxide nanoplatelets. Parts g and h overlay the original HRTEM images of parts c and d overtop of an inverse FFT of the indicated spots on the diffraction patterns. The overlay is at 20% transparency.

using the bulk cubic (space group Ia3̅, Figure 2a, gray curve) and monoclinic (space group C2/m, Figure 2b, gray curve) structures of Eu2O3. Additionally, the experimental PDFs are compared to the calculated one from the ideal truncated bulk cubic (Figure 2a, red curve) and monoclinic (Figure 2b, red curve) structures of Eu 2 O 3 without any surface reconstruction or specific constraints. The objective of the calculated PDFs based on the truncated bulk structures is to show that all distances in the truncated PDFs (Figure 2, red curves) are very similar to the distances in the PDFs calculated for the bulk cubic (Figure 2a, 969

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Figure 2. Experimental pair distribution functions (PDFs, G(r)) of the as-prepared square (blue) and tapelike (green) europium oxide nanoplatelets with the calculated PDFs from (a) the bulk cubic (gray) and truncated cubic (10 × 10 × 1 unit cells, red) structures and (b) the bulk monoclinic (gray) and truncated monoclinic (15 × 2 × 17 unit cells, red) structures. Extended interatomic distances are given in the SI (Figure S8).

Figure 3. High resolution X-ray powder diffraction (XPD) patterns of the as-prepared square (blue) and tapelike (green) europium oxide nanoplatelets compared with the XPD patterns of bulk (a) cubic (gray) and (b) monoclinic (gray) Eu2O3. The insets show the same XPD patterns (experimental and bulk) within the Q range that is commonly accessible (with a sufficient resolution when dealing with NCs) with a classical powder X-ray diffractometer (Cu Kα radiation).

structural data show discrepancies compared to data already reported in the literature regarding ultrasmall Ln oxide NCs. All structural characterizations reported in the literature and dealing with ultrasmall europium oxide NCs are based on low-resolution XPD experiments, and the obtained data are solely interpreted by indirect comparison of Bragg peaks positions (i.e., 2θ values of the main peaks only) of a reference pattern found in the ICSD database23,24,53,54 and comparing those theoretical values with the experimental one. According to these references, the as-prepared ultrasmall europium oxide NCs (nanoplatelets, nanowires, and nanodots) crystallize within the bulk body-centered cubic (bcc) structure (space group Ia3̅). Nevertheless, such an interpretation faces major inconsistencies. For example, one of the main conclusions published by Yan and co-workers24 is that Eu3+ cations within the nanodisks occupy a site of lower symmetry similar to the one existing in the bulk monoclinic Eu2O3. Additionally, although more than one-third (according to the authors) of the Eu3+ cations occupy such a site, no consequence on the crystal structure of such ultrathin nanodisks would be realized. Important differences also are obvious when comparing the XPD patterns of ultrasmall bcc (space group Ia3̅) isotropic europium oxide (2.4 nm)54 or europium-doped gadolinium oxide (2.5 nm)55 nanodots and gadolinium oxide nanoplates.56 Finally, additional features in the XPD patterns of samarium oxide nanoplates and nanowires are irrelevant to the formation of the bulk bcc structure, as stated by the authors.57

Unfortunately, the published data related to the structural characterization of ultrasmall Ln oxide NCs suffer from a lack of resolution (very broad peaks due to the small size of the coherent domains, strong asymmetry, and very often poor counting statistics) or interpretation based on Rietveld refinements. Note at this point that all structural identification based on XPD measurements must be ultimately validated by a full Rietveld refinement to ensure that both Bragg peaks positions and the relative intensities of the experimental XPD pattern agree well with the structural model. Most importantly, a Rietveld refinement is also invaluable to extract physically interesting data (e.g., cell parameters, isotropic or anisotropic atomic displacement parameters, site occupancy, and residual electronic densities) and hence to detect potential problems with the structural model. Hudry and co-workers demonstrated such a possibility with the Rietveld refinement of the crystal structure of ultrasmall thorium, uranium, and plutonium oxide NCs based on the bulk face-centered cubic (fcc) structure (space group Fm3̅m).31,58 The results showed that these ultrasmall NCs adopt a crystal structure that is very close to the bulk one. Nevertheless, large atomic displacement parameters of actinide atoms (unusual for such heavy atoms) suggest deviations from the average bulk structure, which were not quantified. Our PDF results combined with HRTEM images and careful examination of the published data support the formation of new europium oxide polymorphs whose exact crystal structures are 970

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Figure 4. (a) Attenuation spectra, (b, c) room temperature photoluminescence spectra (λex. = 280 nm), and (d) photoluminescence excitation (λem. = 614 nm) spectra of the as-prepared square (gray) and tapelike (red) europium oxide nanoplatelets. The attenuation spectrum of the stabilizing ligand (oleic acid) is also given (a, blue).

and spectral positions that are independent of the embedding matrix. In principle, this could lead to comparable emission and absorption spectra for a given active lanthanide ion in a range of different hosts. In reality, because of the crystal field, intensities and fine structure could possibly experience dramatic changes. The emission of light in lanthanide-based materials is due to f−f transitions. These are promoted by electric dipole (ED), magnetic dipole (MD), or electric quadrupole (EQ) “operators”; the allowed transitions are determined by selection rules60 in which f−f transitions are parity allowed via both the MD and EQ operators but forbidden by the ED operator (Laporte’s parity selection rule). Nevertheless, under the influence of a crystal field, the parity selection rule can relax and ED transitions become partially allowed (induced or forced ED transitions). As a consequence, the crystal structure in which the lanthanide ion is introduced and its corresponding site symmetry are of prime importance to understand the corresponding PL properties. The room temperature PL spectra of the as prepared square and tapelike europium oxide nanoplatelets (Figure 4b) are very similar. The observed spectral lines are due to 5D0 → 7FJ (J = 0, 1, 2, 3, 4) transitions. Several authors have reported on the PL properties of europium oxide NCs, and depending on the experimental approach used for the synthesis, the PL spectra can be different.24,54,61−63 Indeed, although the spectral lines are at the same energy (i.e., same electronic structure of the 4f6 configuration of the Eu3+ ion), relative intensities and fine structure are different. Similarly, large differences in relative PL peak intensities were observed between nanocrystalline and bulk Eu2O3 (either cubic or monoclinic).24,64 Here, we observed that the 5D0 → 7F2 and 5 D0 → 7F4 spectral emission lines are strongly enhanced (compared to the bulk cubic or monoclinic) in the case of the

currently unknown. Our current hypothesis is that the ultrasmall thickness (≤1 nm) of the as-prepared nanoplatelets dictates the final crystal structure. Although the nanoplatelets are characterized by a relatively large size in two dimensions (12−17 nm for the square nanoplatelets and up to 200 nm for the tapelike nanoplatelets), the structural constraints on the third dimension (i.e., the thickness) directly impact the arrangement of the atoms in the other two dimensions. We assume that decreasing the size in one dimension initially induces deviations from the average bulk structure. When at least one dimension approaches the size of the unit cell, the induced distortions can be strong enough to break down the symmetry and, hence, prevent the NCs from accommodating the usual bulk structure. Additionally, in such ultrasmall nanostructures the stabilizing organic ligand cannot be neglected (SI, Figures S9 and S10) and most likely becomes part of the overall structure. As a consequence, describing such ultrasmall nanostructures in terms of a hybrid inorganic/ organic system in which the organic part, contrary to bigger NCs, can no longer be ignored (in terms of crystal structure determination) would be appropriate. A similar result was recently reported with ultrasmall (1.3 nm) CdSe NCs, for which the mixed-phase model (i.e., mixing of zinc blende and wurtzite structures with stacking faults) failed to describe their crystal structure. However, the said mixed phase worked reasonably well for slightly bigger (2.4, 2.8, and 3.4 nm) CdSe NCs.59 To the best of our knowledge, the crystal structure of those ultrasmall CdSe NCs has not yet been solved. Crystal Field and Photoluminescence Properties of Square and Tapelike Europium Oxide Nanoplatelets. The effective shielding of the 4f electrons in lanthanide-based materials (comprised of lanthanides with 3+ ionization state) produces luminescence spectra characterized by sharp peaks 971

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Chemistry of Materials

→ 7F2 transition, while in a lower symmetry environment without inversion center the latter becomes stronger. Finally, the 5D0 → 7F0 is a nondegenerate transition and cannot be split by a crystal field. As a consequence, the number of lines observed for the 5D0 → 7F0 transition gives an indication of the number of nonequivalent crystallographic sites occupied by Eu3+ cations.59 From the measured PL spectra, both square and elongated europium oxide nanoplatelets have a single unique site (Figure 4c). The photoluminescence excitation (PLE) spectra of the square and tapelike nanoplatelets (Figure 4d) are very similar and characterized by two main structures. At wavelengths shorter than 300 nm, the Eu3+ cations are excited by the charge transfer state (CTS) due to the transfer of an electron from the oxygen 2p state to a 4f orbital. Note that the CTS band is blueshifted (260 nm) compared to the one reported by Yan and coworkers24 and Kim and co-workers (∼280 nm). Although the origin of this blue shift has not been carefully examined, it has been shown that variations in degrees of confinement can result in spectral shifts of CTS in Ln-based nanomaterials. More importantly, as is noted in the paper by Kim and co-workers, a spectral correction for the instrument should be applied to avoid discrepancies. A spectral response was applied to both our PL and PLE data. The origin of the peak at 216 nm is not fully understood at the moment. The features that lie to longer wavelengths compared to the CTS transition have been assigned in the literature25,64,68 as follows: 7F0,1 → 5D2 (466 nm), 7F0,1 → 5G2 (382 nm), 7F0,1 → 5L6 (394 nm), 7F0,1 → 5F7 and 7F0 → 5G2−6 (363 nm), 7F0,1 → 5D3 (417 nm). The PL quantum yield (QY) was calculated by measuring the ratio of photons emitted and absorbed in the europium oxide nanoplatelets versus a standard dye (see the SI). The relative number of photons absorbed is determined from the attenuation value (Figure 4a), whereas the number of photons emitted is determined by integrating the intensity of the emission from the 5D0 → 7F1−4 transitions. The PL QYs for the square and elongated nanoplatelets upon excitation with 250 nm photons are 0.3% and 0.15%, respectively. Of the few references that discuss PL QYs of lanthanide oxide NCs, typical excitation wavelengths extend from the far-UV to the visible range. The ambiguous part of these discussions lies in the determination of the number of photons absorbed. In these excitation regions, a significant portion of the light is scattered or transmitted. Here we choose to estimate PL QYs of the square nanoplatelets at other excitation wavelengths by using a ratio of the PLE versus the attenuation. This method has been shown to replicate PL QY trends well for semiconductor quantum dots.69 For the square nanoplatelets, the PL QY increases to 0.75% and 3.75% at 363 and 383 nm, respectively. Surprisingly, these values are comparable to those reported for europium-doped (15 mol %) yttrium oxide nanodisks25 using visible light to excite the samples. Kim et al.64 discussed relative quantum efficiencies with respect to a bulk Eu-doped Y2O3 (instead of a dye), and the trend to increasing emission efficiency at longer wavelengths was also observed in bulk Eu2O3 samples. It has been proposed that the decreased PL QY at shorter wavelengths is due to additional fast nonradiative decay processes out of the CT manifold. Here, using the assumption that the increase would be the same factor for the elongated plates, the PL QY would increase from 0.15% (250 nm), 0.39% (363 nm), and 1.97% (383 nm). A more rigorous study is required to properly characterize the CTS states of Eu3+.

as-prepared square and elongated europium oxide nanoplatelets. These transitions are pure ED transitions and as such are strongly influenced by the Eu site symmetry and its surrounding environment. All together, the 5D0 → 7F2 and 5D0 → 7F4 emissions lines represent nearly 90% of the total integrated intensity for both square and elongated nanoplatelets although the ratio is slightly different (56.9%−31.8% for the square nanoplatelets vs 52.6%−37% for the elongated nanoplatelets, for 5D0 → 7F2 and 5D0 → 7F4 respectively). The strong intensity enhancement of the 5D0 → 7F2 and 5D0 → 7F4 transitions relative to the bulk materials constitutes an additional clue in favor of the modification of the crystal structure of the as-prepared nanoplatelets. The discrepancies between the bulk and nanocrystalline PL spectra observed in the literature were explained in terms of surface sites without any impact on the crystal structure, which has been described as the bulk bcc (Ia3̅) for all reported ultrasmall Eu2O3 NCs. On the other hand, in this study, the structural results (see PDF and high resolution WAXS data) regarding the square and tapelike europium oxide nanoplatelets are in good agreement with the experimental PL data (Figure 4b), which clearly indicate that the crystal field is strongly modified compared to the bulk cubic or monoclinic counterparts. The modification of the crystal structure of the europium oxide nanoplatelets is also supported by recently reported results on Eu-doped yttrium oxide NCs. Indeed, the 5D0 → 7F2 transition of various PL spectra is strongly enhanced in the case of ultasmall NCs,4 whereas a very good agreement with the PL spectrum of bulk Eu-doped yttrium sesquioxide is obtained when the NCs are annealed at high temperature.65,66 In the latter case, the XPD patterns are unambiguously due to the formation of the cubic structure (supported by a Rietveld refinement65). Additionally, Antonietti and co-workers reported also on the formation of ultrathin yttria-based crystalline and lamellar nanostructures and also observed a strong enhancement of the 5D0 → 7F2 transition. In this case, the authors identified the crystal structure of the ultrathin europium-doped yttrium oxide NCs as the bulk monoclinic phase. Nevertheless and according to the authors, the XPD patterns of the asprepared ultrathin NCs, although similar to the monoclinic phase, show shifted peaks and splitting as well67 (in agreement with our assumption regarding the formation of an unknown polymorph of Ln oxide). Equally important, when Antonietti and co-workers annealed their samples at relatively high temperature (550 °C), a clear phase transition was observed, and the bulk cubic structure was ultimately formed. Such observations clearly indicate that the organic ligand (benzoate anions in this case) is necessary to maintain the integrity of the metastable polymorph, which collapses when the benzoate stabilizing ligands are removed, inducing the formation of the bulk structure. A similar behavior is expected for the oleatestabilized europium oxide nanoplatelets. The 5D0 → 7F1 emission line is a pure MD transition whose probability is independent of the surrounding matrix. The direct examination of its splitting gives information about the site symmetry. When the maximum splitting of three appears, the Eu3+ ion is located in a low-symmetry environment. In the case of the square and elongated nanoplatelets, two of the three transitions are not entirely resolved (Figure 4c), but the best fit is obtained when fitting to a sum of three Gaussians instead of two. Additionally, when the Eu3+ cation is positioned in a higher symmetry environment containing an inversion center, the 5D0 → 7F1 transition is predominant compared to the 5D0 972

DOI: 10.1021/cm504255y Chem. Mater. 2015, 27, 965−974

Chemistry of Materials

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CONCLUSION Investigations into the synthesis of ultrasmall lanthanide nanoparticles revealed that side reactions cannot be neglected when considering the formation of europium oxide NCs and even play a significant role regarding the output of the synthesis and the quality of the as-prepared NCs. These hidden parameters have not been previously reported in the literature. Additional experiments remain necessary to clarify the role of side reactions and to understand the reason why only ultrasmall lanthanide oxide NCs can be prepared by the nonaqueous approach. Second, the use of X-ray total scattering experiments coupled with the PDF analysis shed light on the crystal structure of these ultrathin europium oxide nanoplatelets. For the first time, experimental data clearly indicate that the atomic-scale structure of as-prepared ultrathin europium oxide nanoplatelets cannot be merely described on the basis of the bulk models. Nevertheless, an accurate structural determination solely based on the experimental PDF is very challenging and computationally time-consuming. A trial and error approach based on potential structural models is currently the standard procedure in use, as recently reported.70 On the other hand, the Eu3+ cation was used as a good structural probe in various compounds. Hence, combining the PDF results with specific spectroscopic data (polarized and low-temperature measurements, selective excitation) might be of interest to solve the structure. Finally, whereas our preliminary results clearly indicate that the down-conversion properties of ultrathin europium oxide are modified most likely because of their unusual atomic-scale structure, details regarding the energy transfer processes and quenching mechanisms are needed. Such data increase our understanding of the role the CTS plays and can lead to improvement in the strategy and design of highly efficient luminescent Ln oxide NCs.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Details about the synthesis and characterization of europium oxide nanoplatelets and additional TEM images. This material is available free of charge via the Internet at http://pubs.acs. org/.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.H.). *E-mail: [email protected] (J.H.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation (NSF) Award CHE-1402298. Research was carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Use of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. D.H. would like to thank Kim Kisslinger for TEM training. 973

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