Phase-Tunable Synthesis of Monodisperse YPO4:Ln3+ (Ln = Ce, Eu

May 9, 2017 - A novel aqueous-based and phase-selected synthetic strategy toward YPO4:Ln3+ (Ln = Ce, Eu, Tb) micro/nanocrystals was developed by selec...
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Phase-Tunable Synthesis of Monodisperse YPO4:Ln3+ (Ln = Ce, Eu, Tb) Micro/Nanocrystals via Topotactic Transformation Route with Multicolor Luminescence Properties Baiqi Shao,† Yang Feng,†,‡ Shuang Zhao,†,‡ Senwen Yuan,† Jiansheng Huo,†,‡ Wei Lü,† and Hongpeng You*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of the Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: A novel aqueous-based and phase-selected synthetic strategy toward YPO4:Ln3+ (Ln = Ce, Eu, Tb) micro/nanocrystals was developed by selecting specific precursors whose structure topotactically matches with the target ones. It was found that layered yttrium hydroxide (LYH) induced the formation of hexagonal-phased h-YPO4·0.8H2O with the crystalline relationship of [001]LYH//[0001]h-YPO4·0.8H2O, while the amorphous Y(OH)CO3 favored the formation of tetragonal-phased t-YPO4. We also systematically investigated the influence of Na2CO3/NaH2PO4 feeding ratio on the evolutions of morphology and size of the h-YPO4·0.8H2O sample, and we also obtained a novel mesoporous nanostructure for t-YPO4 single crystalline with closed octahedron shape for the first time. Besides, the multicolor and phase-dependent luminescence properties of the as-obtained h-YPO4·0.8H2O and t-YPO4 micro/ nanocrystals were also investigated in detail. Our work may provide some new guidance in synthesis of nanocrystals with target phase structure by rational selection of precursor with topotactic structural matching.

1. INTRODUCTION In the past decade, a tremendous amount of progress has been made in the nanoscience and nanotechnology driven by the potential applications in many areas due to the unique size/ shape-dependent physics and chemistry for nanocystals.1−6 Meticulous engineering of nanocrystals on the geometrical factors including dimensionality, uniformity, size, and shape is a central theme in nanotechnology, which caters to the emerging stringent technical demands in the modern science and technology.7−10 Till now, various physical- and chemicalbased synthetic strategies have been developed toward inorganic nanocrystals.11 Among them, solution-based chemical methods have been proven to be the most efficient and general strategy for the preparation of nanostructures, because the nanocrystal growth is regulated strictly by complex kinetic and thermodynamic rules in the aqueous system, which could be effectively tuned in a mild manner.12 The past decades have witnessed the mushroom development of controllable synthesis of inorganic nanocrystals, and some typical growth mechanisms and size/shape-dependent relationships have been observed and summarized.13−15 The logic of the common solution-based nanosynthesis usually means capping agent and organic solvent, wherein stringent experimental conditions and tedious postpurification are also involved.16−18 Although high-quality nanoparticles with good dispersibility in the solvents can be © 2017 American Chemical Society

obtained, the intrinsic disadvantages deviate from the green chemistry concept and also restrict the practical generality; therefore, a general and robust aqueous-based green synthetic strategy is highly essential.19,20 It is well-established that the physical and chemical properties of nanocrystals are determined first by their crystal phase, so that phase control is of preferential significance in nanosynthesis. However, because of the nature of the crystal structure and complex kinetic/dynamic factors during nanosynthesis, it is a longstanding challenge to target a specific phase for the products with rich polymorphs. For example, hexagonalphased β-NaYF4 nanocrystal is difficult to directly form during the solution-based synthesis, wherein cubic-phased α-NaYF4 preferentially forms as a kinetically favorable phase.21−23 Evaluating various synthetic methodologies, the topotactic transformation route based on the topotactic structural matching between the precursor and target is an ideal and practicable strategy to selectly target an expected phase. The typical case is the ion-exchange reaction on the basis of identical crystal structure between two species. Toshiharu et al. fabricated high-temperature stable phase ZnS nanocrystal by cation exchange reaction using CuxS as precursor at ambient Received: January 11, 2017 Published: May 9, 2017 6114

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aqueous solution containing 1 mmoL of Y(NO3)3 under magnetic stirring, and white precipitation occurred immediately. After it was stirred for 5 min, aqueous solution (10 mL) containing certain amounts of NaH2PO4 were introduced into the above mixture. After it was stirred for another 10 min, the mixture was sealed in 50 mL Teflon-lined autoclave and maintained at 180 °C for 10 h. After the solution cooled to room temperature, the products were collected and washed with deionized water and absolute ethanol twice, in turn, and dried at 60 °C for 12 h. t-YPO4. The typical synthesis was same as that for h-YPO4·0.8H2O except that Na2CO3 was changed to NH4HCO3. The lanthanidedoped samples YPO4:Ln3+ (Ln = Eu, Tb, Ce,) were synthesized following the same procedure. 2.2. Characterization. The phase structure of the as-prepared products was characterized by powder X-ray diffraction (XRD) with a D8 Focus diffractometer (Bruker, with Cu Kα radiation, λ = 0.154 06 nm) at a scanning rate of 10° min−1. The morphology and size of the as-prepared samples were inspected by a field emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive spectrometer (EDS; S-4800, Hitachi, Japan). Transmission electron microscopy (TEM) image were obtained using a JEOL-2010 transmission electron microscopy operating at 200 kV. Photoluminescence spectra were recorded with a Hitachi F-7500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source.

temperature, and they also obtained CdS nanocages while maintaining the multiply twinned structures.24 Despite these successes, ion-exchange reaction is very limited in generality, because seeking for an isostructure precursor is difficult. Relatively speaking, the transformation strategy based on partial structural resemblance between precursor and target is more feasible, which is tolerant for seeking practicable precursor. Our group developed two novel strategies to synthesize β-NaYF4 nanocrystals bypassing the formation of intermediate α-NaYF4 using layered yttrium hydroxide Y2(OH)5NO3·nH2O (LYH) and NaY(CO3)F2 as precursors on the basis of the structural matching of [001]LYH//[0001]βNaYF4 and [001]NaY(CO3)F2//[0001]β-NaYF4, respectively.25,26 As an extension of our precious work, herein we select YPO4 as a platform to study the topotactic transformation strategy in phase tuning via aqueous-based route considering that YPO4 has rich polymorphs: monazite (monoclinic), rhabdophane (hexagonal), zircon (tetragonal), orthorhombic, etc.27 Lanthanide orthophosphate (LnPO4) is a series of important inorganic host matrixes, which have been widely used in phosphors, laser material, moisture sensor, and in biolabeling and phototherapy fields since the integration of nanotechnology into biology in recent years.28−32 To date, tremendous efforts have been paid to the solution-based synthesis of monodisperse LnPO4 nanocrystals with various phases, shapes, and size.33 Li’s34,35 and Yan’s36 groups have reported the systematic synthesis of high-quality and monodisperse LnPO4 nanocrystals by hydrothermal and ionexchange routes, respectively. Despite these considerable achievements, phase tuning for the current synthetic methods are mainly based on empirical results, which needs in-depth investigation. In addition, most solution-based synthetic routes inevitably suffer from the routine annoying matters, such as harsh condition, toxic organic solvent, and tedious purification process, etc. Therefore, it is very urgent to develop an aqueousbased green synthetic strategy considering the green chemistry trend. Herein, we developed a novel aqueous-based strategy to synthesize YPO4 micro/nanocrystals with target phases by selecting precursor with specific crystal structure that topotactically matches with the target ones. We studied the topotactic transformation mechanism from the crystal structural point of view, and we also systematically controlled that size and shape of the obtained products. Hexagonal phased YPO4·0.8H2O micro/nanocrystals (labeled as h-YPO4·0.8H2O) with two types of morphologies (hexagonal prism and hexagonal shuttle), and tetragonal phased YPO4 nanocrystal (labeled as t-YPO4) with octahedron morphology were obtained. To the best of our knowledge, it is the first time to get mesoporous nanostructure for t-YPO4 single crystalline with closed octahedron shape, which enriches the morphological typology for LnPO4 series. Besides, the multicolor and phase-dependent luminescence properties of the as-obtained h-YPO4·0.8H2O and t-YPO4 micro/nanocrystals were also investigated in detail.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Morphology. Figure 1 showed the XRD patterns of the two typical as-prepared

Figure 1. XRD patterns of typical (A) h-YPO4·0.8H2O synthesized with Na2CO3/NaH2PO4 feeding ratios of 4/1 and (B) corresponding standard data with JCPDS No. 42−0082; (C) t-YPO4 synthesized with NH4HCO3/NaH2PO4 feeding ratio of 12.5/1 and (D) corresponding standard data with JCPDS No. 11−0254.

samples. It can be seen that all the peaks of the two samples can be indexed to YPO4·0.8H2O (JCPDS No. 42−0082) with hexagonal phase and YPO4 (JCPDS No. 11−0254) with tetragonal phase, respectively. No other additional peaks were detected, indicating the high phase purity. Figure 2 exhibits the SEM images of the as-prepared YPO4 samples with hexagonal (Figure 2A−C) and tetragonal (Figure 2D) phases, respectively. All the samples reveal the phase-dependent morphology trend. The hexagonal-phased YPO4·0.8H2O features hexagonal contour, while the tetragonal-phased YPO4 samples features octahedron contour, indicating the self-limitation behavior in crystallography. The h-YPO4·0.8H2O samples prepared with

2. EXPERIMENTAL SECTION Chemicals. Ln(NO3)3 (Ln = Y, Ce, Eu, Tb) stock solution was obtained by dissolving the corresponding rare-earth oxides (99.99%) in dilute HNO3 under heating. Commercially available Na2CO3, NH4HCO3, and NaH2PO4 are of analytical grade and were used without further purification. 2.1. Preparation of h-YPO4·0.8H2O and t-YPO4 Micro/Nanocrystals. h-YPO4·0.8H2O. In a typical synthesis, aqueous solution (20 mL) containing certain amounts of Na2CO3 were added into 10 mL of 6115

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Figure 2. SEM images of the h-YPO4·0.8H2O samples prepared with Na2CO3/NaH2PO4 feeding ratios of (A) 4/1, (B) 6/2.5, (C) 4/2.5, and t-YPO4 sample prepared with NH4HCO3/NaH2PO4 feeding ratio of (D) 12.5/1. Note that, in our case, the Na2CO3/NaH2PO4 and NH4HCO3/NaH2PO4 feeding ratios mean the actual feeding amount of Na2CO3 (or NH4HCO3) and NaH2PO4, not an arithmetical ratio.

Figure 3. TEM and corresponding HRTEM images of (A, C) h-YPO4· 0.8H2O hexagonal prism and (B, D) t-YPO4 octahedron. (insets, C, D) Corresponding FFT patterns.

respective partial three-dimensional body surface to constitute the continuous crystal lattice, throughout the whole octahedron body, namely, the single crystalline, leading to the mesoporous structure and indicating an aggregation growth mechanism. Such special mesoporous structure has been observed in hYPO4·0.8H2O nanostructure;34,37 however, it is rare for t-YPO4 single crystalline with closed shape, which makes a great promise in some functional material area. 3.2. Topotactic Transformation Mechanism for hYPO4·0.8H2O and t-YPO4. In our synthesis, different additives (Na2CO3 vs NH4HCO3) lead to entirely different crystal phase (h vs t) for YPO4. Before introducing the PO43− ions, white precipitation emerges at the initial stage, which indicates that the preformed precursors may be responsible for the phase selection. Figure 4 shows the XRD patterns of the two

different Na2CO3/NaH2PO4 feeding ratios are all monodisperse hexagonal prisms (Figure 2A,C) or hexagonal shuttles (Figure 2D) with well-defined crystal facets. The Na2CO3/ NaH2PO4 feeding ratio has a profound influence on the morphology and size evolution for h-YPO4·0.8H2O, which will be discussed in the latter section. The sizes (length × diameter) for the hexagonal prisms prepared with Na2CO3/NaH2PO4 of 4/1 and 4/2.5 are ∼500 × 120 nm (corresponding size distribution histogram, Figure S1A) and 150 × 30 nm, and the size (length × diameter, diameter is measured at the middle part) hexagonal shuttle prepared with Na2CO3/ NaH2PO4 of 6/2.5 is ∼750 × 200 nm. The t-YPO4 samples prepared with NH4HCO3/NaH2PO4 of 12.5/1 are monodisperse flattened octahedron nanocrystals with somewhat rough crystal surface. The edge-length is ∼110 nm, and the length of the short axis is ∼80 nm (corresponding size distribution histogram, Figure S1B). Such octahedron morphology is rare for YPO4 with tetragonal phase, which enriches the morphological species for YPO4 and may bring new shape/function relationship. TEM images provide further morphological and structural information for h-YPO4·0.8H2O hexagonal prism and t-YPO4 octahedron. The distinct bright/dark contrast in the individual particles confirms the prototypes hexagonal prism (Figure 3A) and octahedron (Figure 3B). The high-resolution (HR) TEM images exhibit well-resolved two-dimensional lattice fringes for both samples, indicating the high crystallinity. For h-YPO4· 0.8H2O hexagonal prism, the adjacent lattice planes with d spacing of 0.43 and 0.34 nm can be indexed to (101̅1) and (1120̅ ). The (1120̅ ) plane is parallel to the principle axis of the hexagonal prism, revealing the [0001] anisotropic direction based on the structural geometry of hexagonal phase. For tYPO4 octahedron, the orthogonal lattice planes with adjacent d spacing of 0.345 nm can be assigned to the equivalent (200) planes. The distinct dot array in the fast Fourier transfer (FFT) patterns correspond to the HRTEM images and further imply the single crystalline nature for both samples. A close observation shows that there are many pores with an average diameter of 4 nm within the t-YPO4 octahedron body, indicating the mesoporous structure. The HRTEM suggests that the subunits mutually linked together by integration of

Figure 4. XRD patterns of the precursors (a) Y2(OH)5NO3·nH2O prepared with Na2CO3 as additive and (b) amorphous prepared with NH4HCO3 as additive.

precursors collected with Na2CO3 and NH4HCO3 as additives, respectively. When Na2CO3 is used as additive, the peaks of the precursor can be indexed to Y2(OH) 5NO3·nH2O (the corresponding thermogravimetric analysis (TGA) is in the Supporting Information, Figure S2),38 while the counterpart is amorphous of Y(OH)CO3·nH2O when NH4HCO3 is used as additive. Such difference in precursor crystal structures should lead to a different growth behavior of YPO4. In our previous work, β-NaYF4 nanocrystals were synthesized bypassing the formation of intermediate α-NaYF4 using LYH as precursor via 6116

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three kinds of Y3+ site; two are nine-coordinated to oxygen, forming YO9 monocapped square antiprism, and the third one is eight-coordinated to oxygen, forming a distorted YO8 dodecahedron. The YO8 polyhedra in both two crystal lattices are distorted dodecahedron, while the YO9 monocapped square antiprism in LYH can be intuitively viewed as YO8 distorted dodecahedron in the h-YPO4·0.8H2O (Figure 5). Therefore, the microcosmic resemblances further greatly reduce the energy barrier in the topotactic transformation process. To sum up, the macroscopical structural matching in atomic arrangement and microcosmic one in atomic coordination environment facilitate the topotactic transformation. Following this way of thinking, the amorphous precursor leading to t-YPO4 can also be well-explained. In the amorphous crystal structure, the disordered atomic arrangement in threedimensional space is close to those in cubic, tetragonal, orthorhombic phase structures, rather than in hexagonal phase with strong structural anisotropy. In this way, the amorphous crystal structure results in the transformation of t-YPO4. In summary, the topotactic structural matching between the precursor and target causes the structure-induced phase selection, LYH→h-YPO4·0.8H2O and amorphous→t-YPO4, as illustrated in Figure 6. On the basis of the above discussion, it

topotactic transformation on the basis of the structural matching of [001]LYH//[0001]β-NaYF4. Further analysis exhibits that h-YPO4·0.8H2O and β-NaYF4 share similar hexagonal-phase structural anisotropy and hexagonal channel structure along c axis. Taken together, these discussions make a clear indication that the structural matching between LYH and β-NaYF4 can also be applicable to LYH and h-YPO4·0.8H2O with the crystalline relationship of [001]LYH//[0001]h-YPO4· 0.8H2O, which leads to the topotactic transformation from LYH to h-YPO4·0.8H2O. To further verify the topotactic transformation mechanism, we monitored the time-dependent phase evolution of YPO4 during the synthesis with and without Na2CO3 as additive, namely, with and without LYH precursor (Supporting Information, Figure S3). We found that within 30 min, the LYH precursor entirely transformed into h-YPO4·0.8H2O, while only t-YPO4 could be detected during the synthesis without Na2CO3 additive. Such contrastive results exclude the possible dissolving−recrystallization mechanism, thus indicating the topotactic transformation of LYH→h-YPO4·0.8H2O. Layered rare-earth hydroxides with chemical formula Ln2(OH)5A·nH2O (Ln = lanthanide cations, A= NO3−, Cl−, etc., n = 0−2) are a series of newly emerged layered inorganic compounds.39 Structurally, they are built from alternate stacking of [Ln2(OH)5(H2O)n]+ infinite sheets along the c axis and the charge-balancing anions in the interlayer galleries. The unique layered structure can be viewed as a superlattice consisting of a fundamental hexagonal cell and a rectangular supercell.40 The fundamental hexagonal cell in pseudohexagonal symmetry indicates a hexagonal arrangement of rareearth atoms within the unit layers, while, in the h-YPO4·0.8H2O (0001) lattice planes, they exhibit the similar atomic arrangement with hexagonal symmetry along c axis. Thus, the similar atomic arrangement in LYH and h-YPO4·0.8H2O favors the topotactic transformation from LYH to h-YPO4·0.8H2O with the crystalline relationship of [001]LYH//[0001]h-YPO4· 0.8H2O, as depicted in Figure 5. Besides the resemblance in atomic arrangement, the similarity in Y3+ coordination environment in the two crystal structures is also worth mentioning. In the [Ln2(OH)5(H2O)n]+ unit layer, there are

Figure 6. Schematic illustration of topotactic transformation mechanism from LYH to h-YPO4·0.8H2O and from amorphous to tYPO4.

makes an indication that the macroscopical similar atomic arrangement is the precondition for topotactic transformation, the amorphous structure preferentially induces the transformation of phase structure with isotropy to a certain extent, such as cubic, tetragonal, and orthorhombic phase structures. Some researchers reported that amorphous Y(OH)CO3 led to the formation of cubic α-NaYF4 nanocrystals,41,42 which further favors our discussion. Our work may provide a guidance of aqueous synthesis of nanocrystals with target phase structure by rational selection of precursor with topotactic structural matching. 3.3. Effects of the Experimental Variables. As abovementioned, the Na2CO3/NaH2PO4 (NH4HCO3) feeding ratio exhibits significant influence on the morphology and size evolution for YPO4. To systematically study the effects of the experimental variables on the growth of nanocrystals in aqueous system, herein we choose h-YPO4·0.8H2O as a representative to show the empirical relationship between Na2CO3/NaH2PO4 feeding ratio and morphology and size

Figure 5. Schematic illustration of the topotactic structural relationship between the Y2(OH)5NO3·nH2O (LYH) and h-YPO4·0.8H2O. The H2O molecules inserted in the hexagonal are omitted for clarity. 6117

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Figure 7. SEM images of the h-YPO4·0.8H2O samples with different Na2CO3/NaH2PO4 feeding ratios. Na2CO3 and NaH2PO4 feeding amounts are fixed as constant in column and row, respectively. Column A: (A1) 4/1, (A2) 4/2.5, and (A3) 4/4. Column B: (B1) 5/1, (B2) 5/2.5, and (B3) 5/4. Column C: (C1) 6/1, (C2) 6/2.5, and (C3) 6/4.

evolution. Figure 7 shows the morphology and size evolution of the as-prepared h-YPO4·0.8H2O with orthogonal experimental Na2CO3/NaH2PO4 feeding ratios. The detailed Na2CO3/ NaH2PO4 feeding ratios and crystal sizes of the as-prepared samples are listed in Table 1. All the samples feature uniform Table 1. Na2CO3/NaH2PO4a Feeding Ratios and Corresponding Crystal Sizes (nm) of the As-Prepared Samples row 1 row 2 row 3

column A

column B

column C

4/1−500 × 120 4/2.5−150 × 30 4/4−110 × 30

5/1−1700 × 600 5/2.5−400 × 90 5/4−130 × 30

6/1−2200 × 800 6/2.5−750 × 200 6/4−400 × 100

The abbreviation is denoted as Na2CO3/NaH2PO4-length × diameter.

a

and well-defined one-dimensional micro/nanocrystals with hexagonal contour. In the h-YPO4·0.8H2O crystal structure, the YO8 dodecahedra link together by edge-sharing, forming an infinite hexagonal tunnel structure along the c axis. The hexagonal tunnels can obstruct the stacking of YO 8 dodecahedra along a and b axes, which kinetically results in a preferential growth along the c axis, forming the onedimensional structure. A whole scan of the samples shows that the Na2CO3/NaH2PO4 feeding ratio leads to distinct but regular trend of the morphology and size evolution, while the uniformity and monodispersity are well-kept. As the Na2CO3 feeding amount increases, the crystal sizes increase remarkably, while the morphology gradually evolves from hexagonal prism into hexagonal shuttle. Such morphology evolution seems more evident at higher NaH2PO4 feeding amount. The angle at the vertex is measured as 40° when an individual shuttle lies at {101̅0} facet; thus, it can be inferred that the shuttles are enclosed by six equivalent {1010̅ } facets and 12 equivalent {303̅1} facets on the basis of the structural symmetry (Figure 8). When the NaH2PO4 feeding amount increases, an inverse tendency of the crystal sizes can be observed, while the

Figure 8. Schematic depiction of the shuttle crystal shape and the morphology and size evolution vs the increasing Na2CO3/NaH2PO4 feeding amounts.

morphology maintains well. It is well-established that the critical radius rc of nuclei is dependent on the saturation ratio S, which can be expressed as rc = 2γVm/RT ln S.43 γ is the surface free energy per unit area (γ > 0), and Vm is the molar volume of bulk crystal. Apparently, an increase of S leads to a decrease of rc, and smaller rc would give rise to smaller size of the final products under the same growth process. So that, increasing NaH2PO4 feeding amount means increasing S, which results in a decrease in crystal size. On the basis of the above analysis, both Na2CO3 and NaH2PO4 play two crucial roles in morphology and size evolution, as illustrated in Figure 8. For Na2CO3, it acts as an additive to induce the formation of LYH precursor. Simultaneously, it also plays as a capping agent for the final products; namely, besides tuning the crystal size, excess Na2CO3 favors the exposure of {303̅1} facets of h-YPO4· 0.8H2O, while, on the one hand, for NaH2PO4, the H+ ions can 6118

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Inorganic Chemistry accelerate the LYH→h-YPO4·0.8H2O topotactic transformation by protonating the LYH; on the other hand, the excess feeding amount kinetically decreases the crystal size. 3.4. Multicolor Luminescence Properties. As abovementioned, LnPO4 is a highly efficient host matrix for phosphors. Herein we investigated the multicolor luminescence of the as-prepared YPO4 nanocrystals by Ln3+ (Ln = Ce, Eu, Tb3+) doping. First, we chose h-YPO4·0.8H2O:xEu3+ (x = 0.05, 0.1, 0.15, 0.2) to study the influence of the doping ratio of the activator on the phase and morphology of the final products. We found that with increasing doping ratios from 0.05 to 0.2, the products were still nearly monodisperse hexagonal prisms, and the corresponding XRD patterns revealed the pure hexagonal phase (Supporting Information, Figure S4). The UV-absorption feature of the as-prepared t-YPO4:0.05Ln3+ (Ln = Ce, Eu, Tb) was investigated. As shown in Figure S5 (Supporting Information), all the spectra exhibited a broad band in the UV region. For t-YPO4:0.05Ce3+, the absorption band in the range of 200−350 nm can be attributed to the Ce3+ 4f−5d transition, while the band in the range of 200−300 nm for t-YPO4:0.05Eu3+ is due to the O−Eu charge transfer, and the band in the range of 200−260 nm for t-YPO4:0.05Tb3+ corresponds to the Tb3+ 4f−5d transition. Figure 9 exhibits the

and 226 nm, the emission spectra of t-YPO4:Eu3+ and tYPO4:Tb3+ exhibit the characteristic electronic transitions of the Eu3+5D0−7FJ (J = 1, 2, 3, and 4) and Tb3+5D4−7FJ (J = 1, 2, 3, and 4), yielding orange-red and green emissions, respectively. In the t-YPO4:Eu3+ emission spectrum, the higher-energy emissions of 5Di−7Fj are very weak due to the multiphonon relaxation on the basis of the vibration of the phosphate groups. The emission of the t-YPO4:Eu3+ is dominated by 5D0−7F1 magnetic dipole transition, generally indicating that the Eu3+ ion occupies a site with inversion symmetry. t-YPO4 belongs to I41/amd space group, the Y3+ ion is eight-coordinated to O2− ions with two sets of Y−O band distance, forming a unique distorted dodecahedron with D2d point symmetry. However, D2d point symmetry has no inversion center, which should favor the 5D0−7F2 forced electric dipole transition. Actually, such dodecahedron is evolved from a distorted cube visualized to give its point group by considering it as two perpendicular trapezoids; as a result, the D2d point symmetry here is endowed with some centrosymmetry to some extent. In addition, the quantum efficiencies of as-prepared t-YPO4:0.05Ln3+ (Ln = Ce, Eu, Tb) samples were determined to be 14.7% (Ce), 27.3% (Eu), and 30.1% (Tb), respectively. It is well-established that the crystal phase plays a predominant role in determining the physical and chemical properties of nanocrystals. Here, we investigated the phasedependent luminescence properties of the as-prepared YPO4:Ln3+ (Ln = Eu, Tb) samples. Figure 10 shows the

Figure 9. Excitation and emission spectra of the as-prepared tYPO4:0.05Ce3+ (A), t-YPO4:0.05Eu3+ (B), and t-YPO4:0.05Tb3+ (C).

excitation and emission spectra of the as-prepared tYPO4:0.05Ln3+ (Ln = Ce, Eu, Tb). For t-YPO4:Ce3+, an intense broad band centered at 280 nm is observed in the excitation spectrum, which can be assigned to the Ce3+ 4f−5d transitions. Under excitation at 280 nm, the sample shows strong near-UV emission at 350 nm, corresponding to the characteristic Ce3+ 5d−4f emission. For t-YPO4:Eu3+and tYPO4:Tb3+, both the excitation spectra consist of a broad band and a series of weak lines. The band centered at 228 nm for tYPO4:Eu3+ owing to the O2−−Eu3+ charge transfer band, and the weak lines stem from the electric dipolar forbidden 4f−4f intraconfiguration transition of the Eu3+ ions within the 4f6 configuration. Meanwhile, the band peaked at 226 nm for tYPO4:Tb3+ is due to the spin-allowed 7F6−7DJ (ΔS = 0) components of the Tb3+ 4f8−4f75d transitions, and the spinforbidden 7F6−9DJ (ΔS = 1) one in located at 268 nm. The band at ∼210 nm is ascribed to the host absorption, and the weak lines are due to the characteristic 4f−4f intraconfiguration transition within the 4f8 configuration. Upon excitation at 228

Figure 10. Comparison of the luminescence intensity of Eu3+ (A) and Tb3+ (B) doped h-YPO4·0.8H2O (red dot line) and t-YPO4 samples (green solid line).

comparison of the luminescence spectra of h-YPO 4 · 0.8H2O:Ln3+ (red dot line) and t-YPO4: Ln3+ (green solid line) samples. It can be seen that both the two exhibit the same luminescence behavior; however, there is a big difference in the luminescence intensity. The difference between the two phase structures should be responsible for the difference in the luminescence intensity. As described above, h-YPO4·0.8H2O has hexagonal tunnels structure along the c axis, wherein the H2O molecules are inserted. It is reported that the confined H2O molecules in the tunnels are closer to the YO8 units and far from the PO4 groups;44 thus, the H2O molecule acts as an efficient quencher leading to severe luminescence quenching in hexagonal phase by nonradiative relaxation of the excited states caused by high-energy vibrational modes of OH groups (3200− 6119

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Inorganic Chemistry 3600 cm−1). To further evaluate the luminescence properties of the t-YPO4:Ln3+ (Ln = Eu, Tb) samples, we made a comparison of the photoluminescence decay time between the t-YPO4:Ln3+ (Ln = Eu, Tb) octahedron and the counterparts prepared with solid-state reaction (the synthesis details and the XRD patterns are in the Supporting Information, Figure S6), as shown in Figure 11. It can be

properties of the as-obtained h-YPO4·0.8H2O and t-YPO4 micro/nanocrystals were also investigated in detail. Our aqueous-based green synthetic strategy may provide some new guidance for the synthesis of nanocrystals with target phase structure by rational selection of precursor with topotactic structural matching.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00083. The size distribution histograms of h-YPO4·0.8H2O hexagonal prism and t-YPO4 octahedron; TGA curve of the Y2(OH)5NO3·nH2O precursor; time-dependent XRD patterns of the samples prepared with Na2CO3 as additive and that without Na2CO3 as additive; SEM images of h-YPO4:xEu3+ (x = 0.05, 0.1, 0.15, and 0.2) and the corresponding XRD patterns; diffuse reflection spectra of the as-prepared t-YPO 4 :0.05Ce 3+ , tYPO4:0.05Eu3+, and t-YPO4:0.05Tb3+; XRD patterns of the t-YPO4:0.05Eu3+and t-YPO4:0.05Tb3+ samples prepared with solid-state reaction (PDF)

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Figure 11. Decay curves of the as-prepared (A) t-YPO4:0.05Eu3+ (excited at 230 nm, monitored at 596 nm) and (C) t-YPO4:0.05Tb3+ (excited at 230 nm, monitored at 544 nm); and the counterparts (B) tYPO4:0.05Eu3+ and (D) t-YPO4:0.05 Tb3+prepared with solid-state reaction. (○) Experimental data; red or green solid line: fitting results.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongpeng You: 0000-0003-2683-6896 Notes

seen that all the decay curves can be well-fitted into a single exponential function as I = I0 exp(−t/τ), wherein τ is the decay time. The decay time of t-YPO4:Ln3+ (Ln = Eu, Tb) octahedron can be determined to be 1.92 and 1.66 ms, respectively, while those of the t-YPO4:Ln3+ (Ln = Eu, Tb) counterparts can be determined to be 2.26 and 1.85 ms, respectively, which are longer than those of t-YPO4:Ln3+ (Ln = Eu, Tb) octahedron. Compared with the bulk counterparts, nanocrystal possesses much larger specific surface area and relatively lower crystallinity, which means more structural defects and thus increases the nonradiative transition rate. Small nanocrystals come at the expense of weaker performance compared with the bulk one; however, the controllable small size, morphology, and monodispersity make it possible to explore novel physical/chemistry properties, and it also opens a new access to design advanced functional nanomaterials for the stringent technical demands in the modern science and technology.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is financially supported by the National Basic Research Program of China (973 Program, Grant No. 2014CB643803), the National Natural Science Foundation of China (Grant No. 51472236), the Fund for Creative Research Groups (Grant No. 21521092), and Key Program of the Frontier Science of the Chinese Academy of Sciences (Grant No. YZDY-SSW-JSC018).

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4. CONCLUSIONS In conclusion, we developed a novel aqueous-based and phaseselected synthetic strategy toward YPO4:Ln3+(Ln = Ce, Eu, Tb) micro/nanocrystals via topotactic transformation process. By selecting specific precursors that structural toptactically match with the target phase structure, the structure-induced phase selection of LYH→h-YPO4·0.8H2O and amorphous→t-YPO4 were obtained. We systematically investigated the influence of Na 2CO 3/NaH2PO4 feeding ratio on the evolutions of morphology and size of the final products, and we also obtained a novel mesoporous nanostructure for t-YPO4 single crystalline with closed octahedron shape for the first time. Besides, the multicolor and phase-dependent luminescence 6120

DOI: 10.1021/acs.inorgchem.7b00083 Inorg. Chem. 2017, 56, 6114−6121

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DOI: 10.1021/acs.inorgchem.7b00083 Inorg. Chem. 2017, 56, 6114−6121