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C: Plasmonics; Optical, Magnetic, and Hybrid Materials 4
Phosphate Ion-Driven BiPO:Eu Phase Transition Peng Li, Taoli Yuan, Feng Li, and Yanpeng Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10410 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019
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The Journal of Physical Chemistry
Phosphate Ion-Driven BiPO4:Eu Phase Transition Peng Li,Ϯ,†,‡ Taoli Yuan,Ϯ,§ Feng Li,*,†,‡ and Yanpeng Zhang*,†,‡ †Key
Laboratory for Physical Electronics and Devices of the Ministry of Education and ‡School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China §School
of Electrical & Information Engineering, Shaanxi University of Science and Technology, Xi’an, 710021, China
ABSTRACT: Phase transition of crystals is one of the most important topics in condensed matter physics, chemistry and materials sciences in which the basically physical and chemical properties of the crystals themselves are first determined during atomic rearrangements. In general, achieving the phase transition needs high temperature, high pressure and heavy doping treatment. Here we describe a BiPO4:Eu crystal system where the phase transition from hexagonal to low temperature monoclinic structure can be effectively manipulated by a simple method of increasing the anion (PO43-) concentration under reaction solution. This is associated with the improvement of the free energy in the crystallographic system at which only the crystallographic phase having the energy lower than the system free energy is finally formed. The observed morphologies of BiPO4:Eu crystals within low temperature monoclinic structure are divided into four types and each of them stems from geometrical evolution of the standard monoclinic structure. The low temperature monoclinic structure exhibits superior performance in luminescence as compared to hexagonal one due to the extent of significant non-radiative process in lattices of the latter. The idea provided here could be extended to understanding the effects of reaction anion or cation on the phase transition of other important down/up-conversion luminescence materials such as YPO4, Na(Y, Gd)F4. A question about how many crystal systems can achieve phase transition through the method reported here needs and deserves to be explored.
The functional crystal of BiPO4 has three different crystal structures, namely, hexagonal phase (HP, space group: P3121), low temperature monoclinic phase (LTMP, space group: P21/n), and high temperature monoclinic phase (HTMP, space group: P21/m).10,11 The BiPO4 and Ln3+-doped BiPO4 with adopting the LTMP structure, as compared to that with the HTMP and HP, are regarded as promising photocatalyst and down/up-conversion luminescence material, due to the most distorted PO4 tetrahedron for effective generation and separation of the photo-excited electron-hole pairs and no lattice water molecules for luminescence quencher.12,13 The phase transition of BiPO4 from LTMP or HTMP to HP occurs by continuously doping Ln3+ ions, whereas the conventional approaches for realizing the inverse transition from HP to LTMP or HTMP usually require high temperature, high pressure and extending duration of heating.14-18 Considering the superior performances often exhibited in BiPO4 and Ln3+-doped BiPO4 with adopting LTMP structure, the efficient manipulation of the phase transition to LTMP remains attractive but challenging.
Introduction Phase transition of functional crystals has been one of the most intensively studied topics in condensed matter physics, chemistry and materials sciences.1-5 The efficient manipulation of the phase transition usually leads to great improvements in performances of novel materials, and the investigation into this process helps to understand and obtain new crystal structures via rearrangement at atomic scale. The physical nature of the phase transition was illustrated sufficiently well by Guggenheim, Landau and Lifshitz with an analysis of the behavior of the van der Waals (VDW) fluid, and as a result of which, the formation of new phase is possible when the corresponding minima of the free energy are less or at least equal to that of the old phase.6,7 Thus, the correct determination of the system free energy in each particular or experimental case is of great importance. However, the difficulties arise from 1) incomplete knowledge of the exact dependence of the free energy on the experimentally controllable parameters8 and 2) the existing expression forms of the free energy cannot cover all experimentally multivariate parameters such as reaction time and temperature, pH value, doping level, and using of chelating ligand et, al. which already induced the formation of the new phases.9
In this work, we realize the phase transition from HP to LTMP by just increasing phosphate ion (PO43-) concentration in BiPO4:Eu system. We find the underlying mechanism to be that the phosphate ion concentration modifies the free energy of the crystalline
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luminescent spectra and decay lifetimes were performed on using Hitachi F-4600 fluorescence spectrophotometer with a 150 W xenon lamp as the excitation source. For luminescent property comparison, all measurements were carried out under the same conditions including sample mass, slits for excitation and emission, room temperature, and so on.
system and selects our desired crystal phase. The corresponding changes for morphology and luminescence property closely interrelate with the phase transition. Such finding can be well applied to other material systems such as YPO4 and Na(Y, Gd)F4, which could make the study of interest for researchers in the field. Experimental Section
Results and Discussion
Materials. The analytical grade bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), europium oxide (Eu2O3), and ammonium dihydrogen phosphate (NH4H2PO4) were purchased from Sinopharm Chemical Reagent (Shanghai, China). All chemicals were used without further purification. Deionized water was used throughout the experiments.
Phase Transition. Figure 1 shows the XRD patterns of the BiPO4:Eu samples prepared at 1:1 molar ratio of PO43- : [Bi3++Eu3+] in different pH values ranging from 0.5 to 9. For the samples achieved at 0.5 and 1, the pure low temperature monoclinic phase (LTMP, JCPDS card No. 150767) can be obtained. When the pH values are increased to 3 and 5, two phases, i.e., LTMP and hexagonal phase (HP, JCPDS card No. 15-0766), coexist and the new HP dominates on a large scale in addition to the LTMP (marked with symbol “m”). At pH value higher than 7, intensity of HP peaks further increases and the LTMP is completely converted to the HP. The process of improving pH value can put forward the phase transition from LTMP to HP.
Synthesis of Eu3+ (5 at%)-doped BiPO4 samples (written as BiPO4:Eu) with different crystal phases by adjusting the pH values. By dissolving the Eu2O3 and Bi(NO3)3·5H2O in HNO3 solution with agitation and the NH4H2PO4 in deionized water, Bi0.95Eu0.05(NO3)3 (0.1 M) and NH4H2PO4 (0.1 M) aqueous solution were obtained, respectively. Then, 25 mL of NH4H2PO4 solution (0.1 M) was added dropwise into the 25 mL of Bi0.95Eu0.05(NO3)3 solution (0.1 M) under continuous stirring. The pH value of the mixture was adjusted to be about 0.5 using the addition of HNO3 solution. Under vigorous stirring for 1 h, the resulting suspension was poured into the Teflon-lined stainless steel autoclave and then heated at 180 °C for 9 h. After the autoclave cooled to room temperature naturally, to collect the final BiPO4:Eu (1:1 molar ratio of PO43- : [Bi3++Eu3+]) we washed the white precipitates with deionized water and absolute ethanol by centrifugation, and then dried them for 12 h at 70 °C. In this way, the LTMP BiPO4:Eu was obtained. All the other samples with different crystal phases were prepared with a similar method except for changing the pH value of the precursor solution to be about 1, 3, 5, 7, and 9, respectively.
Figure 1. XRD patterns of BiPO4:Eu samples prepared at 1:1 molar ratio of PO43- : [Bi3++Eu3+] in different pH values ranging from 0.5 to 9. (“m” stands for LTMP BiPO4:Eu.)
Synthesis of Eu3+ (5 at%)-doped BiPO4 samples (written as BiPO4:Eu) with different crystal phases by adjusting the molar ratio of PO43-: [Bi3++Eu3+]. For a given pH value of the final precursor solution, the same procedure was employed while preparing except for changing the molar ratio of PO43-: [Bi3++Eu3+] from mixing the NH4H2PO4 and Bi0.95Eu0.05(NO3)3 solution.
Figure 2(a) shows the XRD patterns of the samples obtained at different molar ratios of PO43- : [Bi3++Eu3+] while keeping the pH value of the precursor solution at about 1. In the case of BiPO4:Eu sample prepared in 0.5:1, the pure HP is obtained. However, the 1:1 has led to a complete phase conversion to LTMP from HP, above which, the sample also show a well-crystalline LTMP system constantly. This experiment indicates that the PO43- concentration takes responsibility for the tendency of phase transition. A similar trend is also observed in pH=3 experiments. As shown in Figure 2(b), the sample prepared in 1:1 of PO43- : [Bi3++Eu3+] shows a HP system with extra h-phase diffraction peaks. With the increase in the PO43- concentration, the peaks of the HP disappear gradually. The fully phase transition from mixture of HP and LTMP to pure LTMP occurs in 50:1, much later than that finished only in 1:1 at pH=1. The phase transition becomes harder to be induced into LTMP in higher pH
Characterization. The crystal structures of the asprepared samples were identified by X-ray powder diffraction using Rigaku D/max-2200 with Cu Kα (λ = 1.5406 Å) radiation under 40 kV, 50 mA. The samples were scanned at a scanning rate of 8°/min in the twotheta ranging from 10° to 50°. The scanning electron microscopy (SEM) images were recorded by a field emission scanning electron microscopy (Hitachi S-4800) at an accelerating voltage of 3 kV. The infrared (IR) spectra were obtained on a FT-IR spectrometer (VECTOR-22) over the range of 400–4000 cm-1. In IR measurements, the powder samples were studied by making thin pellets with dry KBr powder. The
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The Journal of Physical Chemistry BiPO4:Eu being preferential than that of LTMP along with the increasing of 𝜇𝐿 arising from the improvement of PO43-, the free energy of HP (𝜇𝐻) should be less than that of LTMP ( 𝜇𝑀).
value, mainly due to the sample having higher proportion of HP at the initial stage (Fig. 1).
Based on our experimental results, combined with the classical nucleation theory, the phase transition process of BiPO4:Eu under energetic driving force is described as shown in Figure 3. From Figure 3(a), there is no formation of any BiPO4:Eu crystal phases because of 𝜇𝐿
