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Cerous phosphate gels: Synthesis, thermal decomposition and hydrothermal crystallization paths
Journal of Non-Crystalline Solids 447 (2016) 183–189
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Cerous phosphate gels: Synthesis, thermal decomposition and hydrothermal crystallization paths T.O. Shekunova a,b, A.E. Baranchikov b, O.S. Ivanova b, L.S. Skogareva b, N.P. Simonenko b, Yu.A. Karavanova b, V.A. Lebedev a, L.P. Borilo c, V.K. Ivanov b,c a b c
Lomonosov Moscow State University, Russia Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Russia National Research Tomsk State University, Russia
a r t i c l e
i n f o
Article history: Received 16 April 2016 Received in revised form 27 May 2016 Accepted 5 June 2016 Available online xxxx Keywords: Cerous phosphate gels Ceria Crystallization Hydrothermal treatment Rare earth phosphates
a b s t r a c t Since their discovery in the late 19th century, cerium (IV) phosphates have been a very poorly studied class of rare earth compounds. In this paper, a facile method of amorphous cerous phosphate gels preparation is proposed, starting from nanocrystalline ceria and allowing for large-scale synthesis of these compounds. The use of this method provides deeper insight into the chemical composition and structure of cerous phosphate gels, and enables the peculiarities of their crystallization under hydrothermal conditions in H2O and H3PO4 aqueous solutions to be traced. Only Ce+3 compounds crystallize in a neutral medium if they are present in the composition of the cerous phosphate gels. For Ce+4 compounds to crystallize, a significant amount of orthophosphoric acid should exist in a reaction mixture. These features are likely to be related to the fact that crystalline Ce+4 orthophosphates can only exist in the form of acid salts. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Rare earth elements (REE) compounds are widely used in various high-tech applications, such as the production of lasers, catalysts and permanent magnets [1]. Trivalent REE orthophosphates are among the most extensively studied REE compounds [2–7]. They are of interest as X-ray and gamma-ray scintillators, thermophosphors for remote temperature measurements, biomaterial hosts, and so on [8–11]. Оrthophosphates of REEs having another oxidation state (+ 4) are only known for cerium, and even these compounds are still poorly studied, despite the fact that cerium is one of the most widespread REEs. Tetravalent cerium is thought not to form individual neutral orthophosphates [12], and Ce+4 hydroorthophosphates are typically amorphous substances of variable composition. Nevertheless, the formation of crystalline Ce+4 hydroorthophosphates was recently reported by Nazaraly et al. [13,14]. Possible practical applications of such compounds were discussed earlier. For instance, Alberti et al. [15] have reported high ion exchangeability of cerous phosphates, Barboux et al. [16] have described their noticeable proton conductivity. Nazaraly et al. [13,14] have indicated the possible use of cerous phosphates as sorbents and heterogeneous catalysts. Ce+4 hydroorthophosphates were first synthesized as yellow, gellike substances by Hartley [17] as early as 1882. The synthetic procedure
E-mail address: [email protected] (A.E. Baranchikov).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.06.012 0022-3093/© 2016 Elsevier B.V. All rights reserved.
comprised the interaction of cerium (III) nitrate with sodium phosphate taken in various molar ratios. In 1968, Larsen and Cilley [18] synthesized the same compounds by the interaction of (NH4)2[Ce(NO3)6] with orthophosphoric acid. Larsen and Cilley also found that Ce+4 phosphates are cation-exchange materials that can exchange hydroxonium ions for alkali metal ions (Li+, Na+, K+). The systematic studies of PO34 −/ Ce+4 system [15] allowed the following four different Ce+4 orthophosphates to be separated: amorphous cerium orthophosphate with the molar ratio P:Ce ~ 1.7, cerium orthophosphate with P:Ce ~ 1.15 containing up to ~7% of sulfate ions, cerium orthophosphate with P:Ce = 1.5, and fibrous cerium orthophosphate (suggested composition is Ce(HPO4)2·H2O). The last compound was then investigated thoroughly by Barboux et al. [16]. Starting from cerium (+4) orthophosphate solutions, Herman and Clearfield [19] also prepared several gel-like fibrous substances with the following compositions: Ce(OH)x(PO4)x(HPO4)2 − 2x·yH2O, Ce(HPO4)2·хН2О, and Ce(OH)0.7(PO4)1.1. In turn, Rajesh et al. [20] synthesized cerium phosphate gel by the interaction of cerium (III) nitrate and orthophosphoric acid in an ammonia atmosphere. Using thermal analysis data only, they wrote the overall gel composition as CePO4·H2O. Based on the data presented, we can conclude that the composition and the structure of amorphous Ce+4 hydroorthophosphates are still poorly understood. Hydrothermal treatment of Ce+4 hydroorthophosphate gels can result in their crystallization. For example, Nazaraly et al. [13,14] were the first to succeed in the preparation of crystalline Ce+ 4 hydroorthophosphate, Ce(PO4)(HPO4)0.5(H2O)0.5, by hydrothermal
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treatment of a cerium dioxide solution in orthophosphoric acid. They also solved the structure of this compound: it has a monoclinic system and possesses a layered structure (space group С2/с; unit cell parameters а = 21.0142(3) Å, b = 6.55082(7) Å, с = 6.94382(6) Å, β = 91.983(1)°, V = 955.32(2) Å3). Note that a substance of the same composition was also synthesized by Brandel et al. [21]. In turn, Yang et al. [22] hydrothermally synthesized a crystalline Ce+ 4 hydroorthophosphate of the Ce(H2O)(PO4)3/2(H3O)1/2(H2O)1/2 composition (monoclinic system also, unit cell parameters а = 15.706(2) Å, b = 9.6261(9) Å, с = 10.1632(4) Å, β = 121.623(7)°). With the hydrothermal treatment duration increased, this compound transformed into rhabdophane (CePO4·xH2O). Yang et al. [22] attributed this unexpected change in the valence state of cerium (Ce+4 → Ce+3) to the presence of organic admixtures, which can act as reducing agents under hydrothermal conditions. This short review contains almost all experimental data on amorphous and crystalline Ce+4 orthophosphates published in more than a century. In this paper, we propose a facile and versatile method of cerous phosphate gels synthesis, starting from nanocrystalline ceria and allowing for large-scale synthesis of these compounds. The use of this method has enabled us to gain a deeper insight into the chemical composition and the structure of cerous phosphate gels, and to trace the peculiarities of their crystallization under hydrothermal conditions. 2. Experimental 2.1. Starting materials The following substances were used as received without further purification: Ce(NO3)·6H2O (99%, Aldrich #238538), orthophosphoric acid (85 wt.% aq., analytical grade, Khimmed Russia), Ce2(CO3)3·xH2O (Aldrich #325503), nitric acid (65 wt.% aq., extra-pure grade, Khimmed Russia), aqueous ammonia (25 wt.%, extra-pure grade, Khimmed Russia), isopropanol (extra-pure grade, Khimmed Russia), distilled or deionized (18 MΩ) water. 2.2. Materials synthesis To synthesize amorphous cerous phosphate gels, we developed a new method, based on the dissolution of nanocrystalline ceria (particle size of 5 nm, synthesized by precipitation from 0.08 M cerium(III) nitrate, according to Ivanov et al. [23]) in excess of hot (100 °C), concentrated orthophosphoric acid. In preliminary experiments, we also used coarser CeO2 samples (7 nm, synthesized as described by Baranchikov et al. [24]; 15 nm, prepared via thermolysis of Ce2(CO3)3·xH2O at 700 °С for 1 h). In a typical dissolution experiment, 0.10 g (on anhydrous basis) of CeO2 powder was added to the 5 ml of 85 wt.% H3PO4 solution preheated to 100 °C. Then the system was mixed for 20 min at 100 °C until the complete dissolution of CeO2 and then cooled to ambient temperature (the cerium concentration in the resulting solution was 0.12 M). Then deionized water was added to the solution dropwise under stirring. Upon the addition of water, gradual formation of gellike precipitates was observed. Thus to obtain gels water was added to the solutions up to 1:27 H3PO4:H2O molar ratio. To purify the gels from orthophosphoric acid, they were dialyzed against deionized water for 1–2 weeks, with periodic changes of water. Dialysis was performed using Pur-A-Lyzer™ Mega Dialysis Kit (3.5 kDa). The final electrical conductivity of the mother liquor was less than 0.01 mS. After the dialysis the gels were dried in air at 50 °C overnight. Hydrothermal treatment of the initial gels (containing residual orthophosphoric acid) and the dialyzed gels redispersed in deionized water or in a 1 wt.% H3PO4 aqueous solution was performed using a Berghof Speedwave MWS 4 microwave system. We used microwave heating to minimize the duration of the non-isothermal heating stage and to provide the most homogeneous heating of reaction mixtures. The initial or redispersed gels were placed in Berghof DAP-100 100 ml
Teflon autoclaves (filling coefficient of 40%) and treated hydrothermally at 120 °С, 140 °С, 155 °С, 170 °С, 180 °С or 220 °С for 1 h. In each case, the heating rate was 30 °C/min, relative microwave power 60%. After hydrothermal-microwave treatment autoclaves were cooled in air, the precipitates were decanted, washed with distilled water until neutral pH and dried in air at 50 °C overnight. 2.3. Methods of analysis To determine the valence state of cerium in cerous phosphate solutions, differential spectrophotometry was used [25]. This direct method has a high sensitivity in determining Ce+3 and Ce+4 in aqueous media, and allows detecting ≥ 2 μg/l Ce+3 in the presence of 8-fold excess of Ce+4 [26]. UV-vis spectra were registered using a Cary 5000 UV-VisNIR spectrometer. All measurements were performed at ambient temperature. The recorded spectra were treated as proposed by Stoyanov et al. [26]. X-ray powder diffraction patterns were recorded with a Bruker D8 Advance or Rigaku D/MAX 2500 diffractometers using CuKα radiation in the 2θ range 3–120° at a 2θ step of 0.01–0.02° and a counting time of 0.3–0.5 s per step. An analysis of the initial wet gels containing orthophosphoric acid was performed using a Bruker D8 Advance diffractometer with a horizontal goniometer axis. The gels were placed into the deepening of a plastic sample holder and their surfaces were levelled using a glass slide. The excess liquid was carefully removed with filtering paper. Full-profile analysis of the X-ray diffraction patterns of crystalline substances was performed using JANA2006 [27] and TOPAS 4.2 software. Fourth-order Chebyshev polynomials were used to fit the background. The overall fitting was performed using the fundamental parameter approach. The weighted profile residuals (Rwp) were from 3 to 6. The structure (scanning electron microscopy, SEM) and the chemical composition (energy dispersive X-ray analysis, EDX) of the samples were analyzed on a Carl Zeiss NVision 40 high-resolution scanning electron microscope equipped with an Oxford Instruments X-MAX (80 mm2) detector, and operating at an accelerating voltage of 1– 20 kV. SEM images were taken with an Everhart-Thornley detector (SE2). The structure of the samples was additionally studied by means of transmission electron microscopy (TEM) with a Carl Zeiss Libra 200MC analytical transmission electron microscope with a field emission gun and a corrected Omega energy filter. TEM images were taken at an accelerating voltage of 200 kV in the bright-field mode. Thermal analysis was performed on a simultaneous TGA/DSC/DTA SDT Q-600 analyzer (TA Instruments) upon linear heating to 1200 °С (heating rate of 10 °C/min) or 1400 °С (20 °C/min) in a 100 ml/min airflow. Mass-spectroscopic analysis of the gases evolved during thermolysis was performed using a Netzsch TG 209 F1 thermal analyzer equipped with a Netzsch 403 C Aëolos quadrupole mass spectrometer upon linear heating to 950 °C in argon at a heating rate of 10 °C/min. 3. Results and discussion To obtain initial cerous phosphate solutions, nanodispersed cerium dioxide was dissolved in hot 85% orthophosphoric acid. It is known that, when orthophosphoric acid is heated, oligophosphoric acids can form [28]. To find out whether this actually takes place we have performed gravimetric analysis [29] (precipitation of orthophosphate with ammonium molybdate in the presence of nitric acid) of two solutions, solution 1 and solution 2 (both without adding CeO2). Solution 1 containing only 85% H3PO4 was heated at 100 °C for 1 h. Solution 2 was prepared by mixing 85% H3PO4 and 65% HNO3 (the H3PO4:HNO3 molar ratio was chosen to be equal to described in [29]) followed by heating at 100 °C for 1 h. Nitric acid was added to solution 2 to initiate the transformation of oligophosphoric acids, which could exist in the solution, into orthophosphoric acid. A comparative analysis of the orthophosphate
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anion contents in solutions 1 and 2 showed that solution 1 contained and solution 2 contained 82.4 ± 0.4 wt.% PO3− 77.5 ± 0.3 wt.% PO3− 4 4 (confidence intervals are given for P = 95%); the latter value content in pure 85% H3PO4 (82.4%). The difcorresponded to the PO3− 4 ference between these two solutions was 4.9%, which indicated the presence of oligophosphoric acids in the 85% solution of phosphoric acid heated to 100 °C for 1 h. These data allowed us to conclude that a comparatively low content of oligophosphate anions can form in hot orthophosphoric acid during the dissolution of cerium dioxide. Thus, cerium complexes mainly containing orthophosphate ligands are likely to form during the dissolution of cerium dioxide in orthophosphoric acid. Note that determining the composition of the cerium phosphate complexes that exist in aqueous concentrated solutions of phosphoric acid is quite a complex problem. Nevertheless, in contrast to earlier techniques [17,18], the method of forming cerous phosphate solutions presented in this paper can be used to completely exclude − the appearance of admixture ions (NH+ 4 , NO3 etc.). The duration of CeO2 dissolution in orthophosphoric acid was found to depend on the nanoparticle size. For example, cerium dioxide with a particle size of ~5 nm dissolved fully in ~15 min. Identical cerium dioxide samples with larger nanoparticles (7 and 15 nm) dissolved fully in 3 h and 48 h, respectively. In further experiments, cerium phosphate solutions prepared using CeO2 with a particle size of 5 nm were used. According to differential spectrophotometry data, the first derivative of the spectrum of the solution of cerium dioxide in orthophosphoric acid had a specific inflection point at 280–290 nm, indicating the coexistence of Ce+3 and Се+4 complexes in the solution (see Fig. 1) [26]. This result is nontrivial, since the formation of Ce+3 in the solution cannot be caused by the interaction of cerium dioxide with orthophosphoric acid, due to the fact that this acid has no reducing properties. Therefore, it is assumed that Ce+3 impurities were present in the initial cerium dioxide, which agrees with previously reported data [26,30–33] and indicates possible oxygen non-stoichiometry of nanocrystalline CeO2. A gel-like precipitate formed upon mixing of cerous phosphate solutions with deionized water. The gel began to form at a molar ratio H3PO4:H2O ≈ 1:1, and the quantity of gel, as estimated by visual observation, increased when water was added up to a ratio of ≈1:27. A further increase in water concentration did not change the quantity of gel; therefore, the quantity of water added to cerous phosphate solutions for the synthesis of gels in subsequent experiments was fixed at the ratio 1:27. Note that it was difficult to analyze the structure and the chemical properties of the as-synthesized cerous phosphate gels, because of a high orthophosphoric acid content in them. Therefore, it was proposed that they should be purified using dialysis. Note also that, in previous studies (see e.g. [21,22]), cerous phosphate gels contained uncontrolled
amounts of orthophosphoric acid and inorganic anions and cations (e.g. 2− and NH+ NO− 3 , SO4 4 ), which inevitably affected the results of respective studies of their properties and crystallization behaviour. The synthesis and purification method described in this paper provided a means of specifying the composition of cerous phosphate gels in a more flexible manner. Fig. 2 shows the X-ray diffraction patterns of the initial and dialyzed cerous phosphate gels. Both X-ray diffraction patterns have a well pronounced broad maximum at 2θ ~ 7.5°. When analyzing the gel synthesized by the interaction of Ce+ 4 sulfate with orthophosphoric acid, Hayashi et al. [34] obtained a similar X-ray diffraction pattern, and attributed this maximum to a layered structure of the partly formed amorphous compound. A short-range ordering with a characteristic distance of ~12 Å is thought to occur in these gels. The existence of a broad peak in the range 2θ = 25–45° is likely to be related to strong intra-layer distortions: similar X-ray diffraction patterns are characteristic of strongly amorphized solids with a highly distorted short-range ordering, e.g. hydrated zirconium dioxide [35,36]. Note that, as evidenced by X-ray diffraction (XRD), dialysis purification of the gels only weakly changed their structure. The decrease in the relative intensity of the peak at 2θ ~ 7.5° can be related to further disordering of the layered structure. Using the method developed, we succeeded in analysis of the cerous phosphate gels by scanning (Fig. 3a) and transmission (Fig. 3b) electron microscopy. It is clearly visible that the dialyzed gel consisted of closely interwoven fibers (anisotropy factor reaches 75:1 or more). The fibers were continuous, with no voids or channels (Fig. 3b). The possibility of the formation of anisotropic cerous phosphate particles was also reported recently, by Tang et al. [37], who synthesized cerium phosphate nanotubes instead of fibers. The mechanism of formation of well-defined anisotropic particles of an almost amorphous compound is not clear, and definitely deserves further investigation. Thermal analysis of the as-formed gels is complicated by the presence of a significant amount of orthophosphoric acid which can react with alumina crucibles at high temperatures (~ 800 °C). The protocol of synthesis and purification used allows refining of the chemical composition of amorphous cerous phosphate gels by thermal analysis, including thermal analysis combined with mass-spectrometric detection of the gaseous products (see Fig. 4). Data presented in Fig. 4 show that thermal decomposition of the gel sample is accompanied by the evolution of gaseous products providing mass-to-charge signals corresponding to water (m/z = 18) and oxygen in atomic and molecular forms (m/z = 16 and m/z = 32, respectively). No signals corresponding to other species were detected. The evolving of H2O proceeds in two overlaying stages corresponding to removal of physically and chemically bound water and finishes at 500 °С. As
Fig. 1. Absorption spectrum of the initial cerous phosphate solution (85 wt.% orthophosphoric acid was used as a blank) and its first derivative.
Fig. 2. X-ray diffraction patterns of (1) initial and (2) dialyzed cerous phosphate gels.
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Fig. 3. Dialyzed cerous phosphate gel: (a) scanning and (b) transmission electron microscopy data.
water molecules are subjected to electron impact, m/z = 16 and m/z = 32 signals are also detected in this temperature range. At higher temperatures (namely, at 630 °C and 840 °C) two pronounced maxima in m/z = 16 and m/z = 32 ionic current curves are observed which obviously correspond to evolution of molecular oxygen from the sample. The overall composition of the gels, without regard for weakly bound water, which is removed at temperatures below ~ 300 °C, can be represented as CeP1.04 ± 0.02O4.60 ± 0.04·(0.030 ± 0.003)H2O. When analyzing the X-ray diffraction patterns of the products of annealing of the purified gels at various temperatures (see Figs. 5, S1), it was possible to describe the main stages of thermal decomposition of the cerous phosphate gels as follows: 500
fCeP1:04 O4:6 0:03H2 Og → 700
C
fCeP1:04 O4:6 g þ H2 O↑
C
fCeP1:04 O4:6 g → CePO4 þ CeP2 O7 þ O2 ↑
Fig. 4. (top) Results of thermogravimetric analysis of dialyzed cerous phosphate gel, and (bottom) the temperature dependences of the ionic currents for m/z = 16, 18 and 32.
860
C
2CeP2 O7 → CePO4 þ CeP3 O9 þ 0:5O2 ↑ CeP3 O9
1000
→
C
CePO4 þ 0:5P4 O10 ↑:
This scheme is consistent with the data presented by Hirai et al. [38] and Masui et al. [39]. Thus, using the proposed synthesis and purification method, the work described in this paper has been the first to analyze the thermal behaviour of cerous phosphate gels comprehensively. However, the composition of the cerous phosphate gels can change, to a certain extent, with respect to the initial state during dialysis (e.g. as a result of the removal of weakly coordinated orthophosphate groups and a change in the coordination environment of cerium ions). An analysis of such processes should be the subject of further investigation. In any case, dialysis opens up fresh opportunities for arbitrary setting of a reaction medium composition to perform further crystallization of cerous phosphate gels, or to modify their structure to adjust ion-exchange and other functional properties. To study the hydrothermal crystallization paths of the cerous phosphate gels, hydrothermal treatment of the initial gels (containing ~ 11 wt.% of orthophosphoric acid), dialyzed gels and dialyzed gels redispersed in 1 wt.% aqueous solution of orthophosphoric acid was performed. The initial gels did not undergo crystallization during hydrothermal treatment at 120 °С and 140 °С, and the X-ray diffraction patterns of the
Fig. 5. X-ray diffraction patterns of the samples prepared by annealing of a dialyzed cerous phosphate gel at 500, 700, and 860 °С. Sign (+) indicates the diffraction peaks of cerium pyrophosphate CeP2O7; sign (*), the peaks of cerium cyclo-triphosphate (CeP3O9); and sign (#), monazite CePO4.
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Fig. 6. X-ray diffraction patterns of the products of hydrothermal treatment (155–220 °С, 1 h) of the initial cerous phosphate gels (containing ~11 wt.% of orthophosphoric acid).
treated samples almost coincided with those presented in Fig. 2. The product formed upon hydrothermal treatment at 155 °С was a crystalline compound, while the peaks in its X-ray diffraction pattern were considerably broadened, and an amorphous phase is likely to exist along with the crystalline phase. Well crystallized substances were obtained only after hydrothermal treatment at 170 °С or above (Figs. 6, S2). The X-ray diffraction patterns shown in Figs. 6 and S2 correspond to the Ce(PO4)(HPO4)0.5(H2O)0.5 phase, the structure of which was solved by Nazaraly et al. [13]. The unit cell parameters refined for a well crystallized sample are а = 21.023(2) Å, b = 6.5586(8) Å, с = 6.9554(8) Å and
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β = 91.98(1)°, which agrees satisfactorily with the earlier data for this compound (Fig. S3) [13]. According to SEM data, the Ce(PO4)(HPO4)0.5(H2O)0.5 phase consisted of aggregated 30 nm-thick lamellar particles (see Fig. 7). The particle shape was almost consistent with that reported by Hayashi et al. [34]. Thus, the particle morphology changed from quasi-one-dimensional to quasi-two-dimensional during hydrothermal crystallization. According to the results of EDX measurements, the molar Ce:P ratio in Ce(PO4)(HPO4)0.5(H2O)0.5 samples was 1:1.5, which supports the chemical composition of the synthesized compound (see [14,21]). The hydrothermal crystallization of the cerous phosphate gels purified by dialysis proceeded along a completely different path. For example, a mixture of an amorphous phase and crystalline CePO4·xH2O (rhabdophane, PDF2 00-035-0614) formed during hydrothermal treatment of the dialyzed gel in a deionized water at 180 °C for 1 h (see Fig. 8a). A SEM image of this sample (Fig. 8b) showed that it consisted of both initial amorphous phase fibers, and columnar crystals with an average diameter of about 100 nm and the shape that is specific to rhabdophane (see e.g. [40–43]). Since the reaction mixture contained no reducing agents, trivalent cerium existed in the initial gel and, hence, in the initial cerous phosphate solution (see also the differential spectrophotometry data, Fig. 1). Thus, our results indicate that a Ce+3 impurity is actually present in nanocrystalline cerium dioxide. The possibility of the existence of trivalent cerium in nanocrystalline cerium dioxide is being actively discussed, since this can explain the mechanism of well pronounced antioxidant action of nanocrystalline CeO2 and its biological activity [24,44–48]. The crystalline phase content in the sample prepared by hydrothermal treatment of a dialyzed gel in deionized water at 180 °С for 1 h (see Fig. 8a) was estimated, by calculating the ratio of the integral intensities of rhabdophane peaks to the total integral intensities of the amorphous
Fig. 7. Lamellar particles of the Ce(PO4)(HPO4)0.5(H2O)0.5 phase synthesized from the initial cerous phosphate gels at (а) 170 °C and (b) 220 °С.
Fig. 8. (а) X-ray diffraction pattern and (b) SEM image of dialyzed cerous phosphate gel subjected to hydrothermal treatment (180 °С, 1 h). The indexed peaks correspond to rhabdophane CePO4·xH2O.
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and crystalline phases. This ratio was found to be ~ 4%. Rhabdophane was assumed to form during the crystallization of a Ce+3-containing compound, and the Ce+ 4-containing phosphate compound was thought to be retained in the form of an amorphous phase. With allowance for the estimation of the rhabdophane content given above, this assumption made it possible to estimate the formal oxidation state of cerium in the initial gel and, hence, in the initial cerium dioxide. It was found to be +3.96. This value agrees well with our previous independent estimation of the formal oxidation state of cerium in nanocrystalline (~5 nm) cerium dioxide in terms of Kim's model [24,49]. Thus, hydrothermal treatment of cerous phosphate gel in 11 wt.% H3PO4 and deionized water, at 180 °С for 1 h, resulted in the crystallization of Ce(PO4)(HPO4)0.5(H2O)0.5 and Ce(PO4)·xH2O, respectively. In addition, hydrothermal treatment of the preliminarily dialyzed gel redispersed in 1 wt.% H3PO4 under the same hydrothermal conditions was performed. According to XRD data (see Fig. 9), a mixture of Ce(PO4)(HPO4)0.5(H2O)0.5, rhabdophane CePO4·xH2O and a third unknown phase (with the main peaks at 2θ = 17.3, 20.9, 21.7 and 26.1°) formed, in this case. An analysis of the experimental data enabled a number of assumptions regarding the peculiarities of hydrothermal crystallization of cerous phosphate gels. Only Ce+3 compounds crystallize in a neutral medium if they are present in the composition of the initial gel. For Ce+4 compounds to crystallize, a significant amount of orthophosphoric acid should exist in a reaction mixture. These features are likely to be related to the fact that crystalline Ce+4 orthophosphates can only exist in the form of acid salts [12]. During crystallization in the presence of a small amount of orthophosphoric acid, Ce+ 3 forms Ce+3 orthophosphate hydrate with a rhabdophane structure, while in media with a higher H3PO4 concentration it is likely to be incorporated into the structure of crystalline acid Ce+4 orthophosphates. 4. Conclusions A new facile method to synthesize admixture-free amorphous cerous phosphate gels has been developed allowing for precise determination of the chemical composition, structure, and morphology of cerous phosphate gels. The gels were found to consist of monolithic interwoven fibers possessing a short-range ordering with a characteristic distance of ~12 Å. The chemical composition of the gels without regard for weakly bound water was estimated as CeP1.04O4.6·0.03H2O. Hydrothermal crystallization paths of cerous phosphate gels strongly depend on the presence of H3PO4 in reaction medium. Hydrothermal treatment of the gels in the presence of orthophosphoric acid
Fig. 9. X-ray diffraction pattern of the sample prepared by hydrothermal treatment (180 °С, 1 h) of a dialyzed cerous phosphate gel redispersed in 1 wt.% H3PO4. Sign (*) indicates the peaks corresponding to Ce(PO4)(HPO4)0.5(H2O)0.5, sign (+) indicates the peaks corresponding to CePO4·xH2O and symbol (#) indicates the unidentified peaks.
results in the formation of crystalline cerous hydroorthophosphate Ce(PO4)(HPO4)0.5(H2O)0.5, while treatment in pure H2O promotes crystallization of cerium orthophosphate with a rhabdophane structure. Acknowledgements The work was supported by a grant of the President of the Russian Federation, project no. MK-8977.2016.3. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jnoncrysol.2016.06.012. References [1] D.A. Atwood (Ed.), The Rare Earth Elements: Fundamentals and Applications, Wiley, 2012. [2] S. Lucas, E. Champion, D. Bregiroux, D. Bernache-Assollant, F. Audubert, J. Solid State Chem. 177 (2004) 1302. [3] S. Lucas, E. Champion, D. Bernache-Assollant, G. Leroy, J. Solid State Chem. 177 (2004) 1312. [4] M.R. Rafiuddin, E. Mueller, A.P. Grosvenor, J. Phys. Chem. C 118 (2014) 18000. [5] J.P. Pepin, E.R. Vance, J. Inorg. Nucl. Chem. 43 (1981) 2807. [6] X. Liu, R.H. Byrne, Geochim. Cosmochim. Acta 61 (1997) 1625. [7] G.M. Begun, G.W. Beall, L.A. Boatner, W.J. Gregor, J. Raman Spectrosc. 11 (1981) 273. [8] L.A. Boatner, L.A. Keefer, J.M. Farmer, D. Wisniewski, A.J. Wojtowicz, Hard X-ray and Gamma-ray Detector Physics VI, 2004, 73. [9] S.W. Allison, M.R. Cates, L.A. Boatner, G.T. Gillies, U.S. Patent 5,885,484, March 23, 1999. [10] S. Gai, C. Li, P. Yang, J. Lin, Chem. Rev. 114 (2014) 2343. [11] J. Shen, L.-D. Suna, C.-H. Yan, Dalton Trans. 42 (2008) 5687. [12] A. Borhan, B. Apetrăchioaei, K. Popa, Rev. Roum. Chim. 55 (2010) 389. [13] M. Nazaraly, G. Wallez, C. Chaneac, E. Tronc, F. Ribot, M. Quarton, J.-P. Jolivet, Angew. Chem. Int. Ed. 44 (2005) 5691. [14] M. Nazaraly, C. Chanéac, F. Ribot, G. Wallez, M. Quarton, J.-P. Jolivet, J. Phys. Chem. Solids 68 (2007) 795. [15] G. Alberti, U. Constantino, F.D. Gregorio, P. Galli, E. Torracca, J. Inorg. Nucl. Chem. 30 (1968) 295. [16] P. Barboux, R. Morineau, J. Livage, Solid State Ionics 27 (1988) 221. [17] W.N. Hartley, J. Chem. Soc. 41 (1882) 202. [18] E. Larsen, W.A. Cilley, J. Inorg. Nucl. Chem. 30 (1968) 287. [19] R.G. Herman, A. Clearfield, J. Inorg. Nucl. Chem. 37 (1975) 1697. [20] K. Rajesh, P. Mukundan, P.K. Pillai, V.R. Nair, K.G.K. Warrier, Chem. Mater. 16 (2004) 2700. [21] V. Brandel, N. Clavier, N. Dacheux, J. Solid State Chem. 178 (2005) 1054. [22] R. Yang, J. Qin, M. Li, Y. Liu, F. Li, CrystEngComm 13 (2011) 7284. [23] V.K. Ivanov, A.E. Baranchikov, O.S. Polezhaeva, G.P. Kopitsa, Y.D. Tretyakov, Russ. J. Inorg. Chem. 55 (2010) 325. [24] A.E. Baranchikov, O.S. Polezhaeva, V.K. Ivanov, Y.D. Tretyakov, CrystEngComm 12 (2010) 3531. [25] A.A. Bunaciu, V.D. Hoang, H.Y. Aboul-Enein, Crit. Rev. Anal. Chem. 43 (2013) 125. [26] A.O. Stoyanov, V.K. Ivanov, A.B. Shcherbakov, I.V. Stoyanova, N.A. Chivireva, V.P. Antonovich, Russ. J. Inorg. Chem. 59 (2014) 139. [27] V. Petříček, M. Dušek, L. Palatinus, Z. Kristallogr.-Cryst. Mater. 229 (2014) 345. [28] N. Wiberg, A.F. Holleman, E. Wiberg (Eds.), Inorganic Chemistry, Academic Press 2001, p. 727. [29] G. Charlot, Les Méthodes de la Chimie Analytique. Analyse quantitative minérale, Masson & Cie, Paris, 1961 845. [30] R.K. Hailstone, A.G. DiFrancesco, J.G. Leong, T.D. Allston, K.J. Reed, J. Phys. Chem. C 113 (2009) 15155. [31] C. Paun, O.V. Safonova, J. Szlachetko, P.M. Abdala, M. Nachtegaal, J. Sa, E. Kleymenov, A. Cervellino, F. Krumeich, J.A. van Bokhoven, J. Phys. Chem. C 116 (2012) 7312. [32] S. Tsunekawa, R. Sivamohan, S. Ito, A. Kasuya, T. Fukuda, Nanostruct. Mater. 11 (1999) 141. [33] S. Deshpande, S. Patil, S.V.N.T. Kuchibhatla, S. Seal, Appl. Phys. 87 (2005) 133113. [34] H. Hayashi, K. Torii, S. Nakata, J. Mater. Chem. 7 (1997) 557. [35] D.A. Zyuzin, E.M. Moroz, A.S. Ivanova, A.N. Shmakov, Crystallogr. Rep. 48 (2003) 413. [36] V.K. Ivanov, A.Y. Baranchikov, G.P. Kopitsa, S.A. Lermontov, L.L. Yurkova, N.N. Gubanova, O.S. Ivanova, A.S. Lermontov, M.N. Rumyantseva, L.P. Vasilyeva, M. Sharp, P.K. Pranzas, Y.D. Tretyakov, J. Solid State Chem. 198 (2013) 496. [37] C. Tang, Y. Bando, D. Golberg, R. Ma, Angew. Chem. Int. Ed. 44 (2005) 576. [38] H. Hirai, T. Masui, N. Imanaka, G. Adachi, J. Alloys Compd. 374 (2004) 84. [39] T. Masui, H. Hirai, N. Imanaka, G. Adachi, Phys. Status Solidi A 198 (2003) 364. [40] C.R. Patra, A. Gabashvili, S. Patra, D.S. Jacob, A. Gedanken, A. Landau, Y. Gofer, New J. Chem. 29 (2005) 733. [41] X. Wang, M. Gao, J. Mater. Chem. 16 (2006) 1360. [42] Y. Zhang, H. Guan, J. Cryst. Growth 256 (2003) 156. [43] Z.-G. Yan, Y.-W. Zhang, L.-P. You, R. Si, C.-H. Yan, J. Cryst. Growth 262 (2004) 408. [44] A.E. Sokolov, A.E. Baranchikov, D.O. Gil, V.K. Ivanov, G.P. Kopitsa, A.A. Petrunin, E.V. Fomin, J. Surf. Invest. 8 (2014) 997.
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Cerous phosphate gels: Synthesis, thermal decomposition and hydrothermal crystallization paths T.O. Shekunova a,b, A.E. Baranchikov b, O.S. Ivanova b, L.S. Skogareva b, N.P. Simonenko b, Yu.A. Karavanova b, V.A. Lebedev a, L.P. Borilo c, V.K. Ivanov b,c a b c
Lomonosov Moscow State University, Russia Kurnakov Institute of General and Inorganic Chemistry of the Russian Academy of Sciences, Russia National Research Tomsk State University, Russia
a r t i c l e
i n f o
Article history: Received 16 April 2016 Received in revised form 27 May 2016 Accepted 5 June 2016 Available online xxxx Keywords: Cerous phosphate gels Ceria Crystallization Hydrothermal treatment Rare earth phosphates
a b s t r a c t Since their discovery in the late 19th century, cerium (IV) phosphates have been a very poorly studied class of rare earth compounds. In this paper, a facile method of amorphous cerous phosphate gels preparation is proposed, starting from nanocrystalline ceria and allowing for large-scale synthesis of these compounds. The use of this method provides deeper insight into the chemical composition and structure of cerous phosphate gels, and enables the peculiarities of their crystallization under hydrothermal conditions in H2O and H3PO4 aqueous solutions to be traced. Only Ce+3 compounds crystallize in a neutral medium if they are present in the composition of the cerous phosphate gels. For Ce+4 compounds to crystallize, a significant amount of orthophosphoric acid should exist in a reaction mixture. These features are likely to be related to the fact that crystalline Ce+4 orthophosphates can only exist in the form of acid salts. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Rare earth elements (REE) compounds are widely used in various high-tech applications, such as the production of lasers, catalysts and permanent magnets [1]. Trivalent REE orthophosphates are among the most extensively studied REE compounds [2–7]. They are of interest as X-ray and gamma-ray scintillators, thermophosphors for remote temperature measurements, biomaterial hosts, and so on [8–11]. Оrthophosphates of REEs having another oxidation state (+ 4) are only known for cerium, and even these compounds are still poorly studied, despite the fact that cerium is one of the most widespread REEs. Tetravalent cerium is thought not to form individual neutral orthophosphates [12], and Ce+4 hydroorthophosphates are typically amorphous substances of variable composition. Nevertheless, the formation of crystalline Ce+4 hydroorthophosphates was recently reported by Nazaraly et al. [13,14]. Possible practical applications of such compounds were discussed earlier. For instance, Alberti et al. [15] have reported high ion exchangeability of cerous phosphates, Barboux et al. [16] have described their noticeable proton conductivity. Nazaraly et al. [13,14] have indicated the possible use of cerous phosphates as sorbents and heterogeneous catalysts. Ce+4 hydroorthophosphates were first synthesized as yellow, gellike substances by Hartley [17] as early as 1882. The synthetic procedure
E-mail address: [email protected] (A.E. Baranchikov).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.06.012 0022-3093/© 2016 Elsevier B.V. All rights reserved.
comprised the interaction of cerium (III) nitrate with sodium phosphate taken in various molar ratios. In 1968, Larsen and Cilley [18] synthesized the same compounds by the interaction of (NH4)2[Ce(NO3)6] with orthophosphoric acid. Larsen and Cilley also found that Ce+4 phosphates are cation-exchange materials that can exchange hydroxonium ions for alkali metal ions (Li+, Na+, K+). The systematic studies of PO34 −/ Ce+4 system [15] allowed the following four different Ce+4 orthophosphates to be separated: amorphous cerium orthophosphate with the molar ratio P:Ce ~ 1.7, cerium orthophosphate with P:Ce ~ 1.15 containing up to ~7% of sulfate ions, cerium orthophosphate with P:Ce = 1.5, and fibrous cerium orthophosphate (suggested composition is Ce(HPO4)2·H2O). The last compound was then investigated thoroughly by Barboux et al. [16]. Starting from cerium (+4) orthophosphate solutions, Herman and Clearfield [19] also prepared several gel-like fibrous substances with the following compositions: Ce(OH)x(PO4)x(HPO4)2 − 2x·yH2O, Ce(HPO4)2·хН2О, and Ce(OH)0.7(PO4)1.1. In turn, Rajesh et al. [20] synthesized cerium phosphate gel by the interaction of cerium (III) nitrate and orthophosphoric acid in an ammonia atmosphere. Using thermal analysis data only, they wrote the overall gel composition as CePO4·H2O. Based on the data presented, we can conclude that the composition and the structure of amorphous Ce+4 hydroorthophosphates are still poorly understood. Hydrothermal treatment of Ce+4 hydroorthophosphate gels can result in their crystallization. For example, Nazaraly et al. [13,14] were the first to succeed in the preparation of crystalline Ce+ 4 hydroorthophosphate, Ce(PO4)(HPO4)0.5(H2O)0.5, by hydrothermal
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treatment of a cerium dioxide solution in orthophosphoric acid. They also solved the structure of this compound: it has a monoclinic system and possesses a layered structure (space group С2/с; unit cell parameters а = 21.0142(3) Å, b = 6.55082(7) Å, с = 6.94382(6) Å, β = 91.983(1)°, V = 955.32(2) Å3). Note that a substance of the same composition was also synthesized by Brandel et al. [21]. In turn, Yang et al. [22] hydrothermally synthesized a crystalline Ce+ 4 hydroorthophosphate of the Ce(H2O)(PO4)3/2(H3O)1/2(H2O)1/2 composition (monoclinic system also, unit cell parameters а = 15.706(2) Å, b = 9.6261(9) Å, с = 10.1632(4) Å, β = 121.623(7)°). With the hydrothermal treatment duration increased, this compound transformed into rhabdophane (CePO4·xH2O). Yang et al. [22] attributed this unexpected change in the valence state of cerium (Ce+4 → Ce+3) to the presence of organic admixtures, which can act as reducing agents under hydrothermal conditions. This short review contains almost all experimental data on amorphous and crystalline Ce+4 orthophosphates published in more than a century. In this paper, we propose a facile and versatile method of cerous phosphate gels synthesis, starting from nanocrystalline ceria and allowing for large-scale synthesis of these compounds. The use of this method has enabled us to gain a deeper insight into the chemical composition and the structure of cerous phosphate gels, and to trace the peculiarities of their crystallization under hydrothermal conditions. 2. Experimental 2.1. Starting materials The following substances were used as received without further purification: Ce(NO3)·6H2O (99%, Aldrich #238538), orthophosphoric acid (85 wt.% aq., analytical grade, Khimmed Russia), Ce2(CO3)3·xH2O (Aldrich #325503), nitric acid (65 wt.% aq., extra-pure grade, Khimmed Russia), aqueous ammonia (25 wt.%, extra-pure grade, Khimmed Russia), isopropanol (extra-pure grade, Khimmed Russia), distilled or deionized (18 MΩ) water. 2.2. Materials synthesis To synthesize amorphous cerous phosphate gels, we developed a new method, based on the dissolution of nanocrystalline ceria (particle size of 5 nm, synthesized by precipitation from 0.08 M cerium(III) nitrate, according to Ivanov et al. [23]) in excess of hot (100 °C), concentrated orthophosphoric acid. In preliminary experiments, we also used coarser CeO2 samples (7 nm, synthesized as described by Baranchikov et al. [24]; 15 nm, prepared via thermolysis of Ce2(CO3)3·xH2O at 700 °С for 1 h). In a typical dissolution experiment, 0.10 g (on anhydrous basis) of CeO2 powder was added to the 5 ml of 85 wt.% H3PO4 solution preheated to 100 °C. Then the system was mixed for 20 min at 100 °C until the complete dissolution of CeO2 and then cooled to ambient temperature (the cerium concentration in the resulting solution was 0.12 M). Then deionized water was added to the solution dropwise under stirring. Upon the addition of water, gradual formation of gellike precipitates was observed. Thus to obtain gels water was added to the solutions up to 1:27 H3PO4:H2O molar ratio. To purify the gels from orthophosphoric acid, they were dialyzed against deionized water for 1–2 weeks, with periodic changes of water. Dialysis was performed using Pur-A-Lyzer™ Mega Dialysis Kit (3.5 kDa). The final electrical conductivity of the mother liquor was less than 0.01 mS. After the dialysis the gels were dried in air at 50 °C overnight. Hydrothermal treatment of the initial gels (containing residual orthophosphoric acid) and the dialyzed gels redispersed in deionized water or in a 1 wt.% H3PO4 aqueous solution was performed using a Berghof Speedwave MWS 4 microwave system. We used microwave heating to minimize the duration of the non-isothermal heating stage and to provide the most homogeneous heating of reaction mixtures. The initial or redispersed gels were placed in Berghof DAP-100 100 ml
Teflon autoclaves (filling coefficient of 40%) and treated hydrothermally at 120 °С, 140 °С, 155 °С, 170 °С, 180 °С or 220 °С for 1 h. In each case, the heating rate was 30 °C/min, relative microwave power 60%. After hydrothermal-microwave treatment autoclaves were cooled in air, the precipitates were decanted, washed with distilled water until neutral pH and dried in air at 50 °C overnight. 2.3. Methods of analysis To determine the valence state of cerium in cerous phosphate solutions, differential spectrophotometry was used [25]. This direct method has a high sensitivity in determining Ce+3 and Ce+4 in aqueous media, and allows detecting ≥ 2 μg/l Ce+3 in the presence of 8-fold excess of Ce+4 [26]. UV-vis spectra were registered using a Cary 5000 UV-VisNIR spectrometer. All measurements were performed at ambient temperature. The recorded spectra were treated as proposed by Stoyanov et al. [26]. X-ray powder diffraction patterns were recorded with a Bruker D8 Advance or Rigaku D/MAX 2500 diffractometers using CuKα radiation in the 2θ range 3–120° at a 2θ step of 0.01–0.02° and a counting time of 0.3–0.5 s per step. An analysis of the initial wet gels containing orthophosphoric acid was performed using a Bruker D8 Advance diffractometer with a horizontal goniometer axis. The gels were placed into the deepening of a plastic sample holder and their surfaces were levelled using a glass slide. The excess liquid was carefully removed with filtering paper. Full-profile analysis of the X-ray diffraction patterns of crystalline substances was performed using JANA2006 [27] and TOPAS 4.2 software. Fourth-order Chebyshev polynomials were used to fit the background. The overall fitting was performed using the fundamental parameter approach. The weighted profile residuals (Rwp) were from 3 to 6. The structure (scanning electron microscopy, SEM) and the chemical composition (energy dispersive X-ray analysis, EDX) of the samples were analyzed on a Carl Zeiss NVision 40 high-resolution scanning electron microscope equipped with an Oxford Instruments X-MAX (80 mm2) detector, and operating at an accelerating voltage of 1– 20 kV. SEM images were taken with an Everhart-Thornley detector (SE2). The structure of the samples was additionally studied by means of transmission electron microscopy (TEM) with a Carl Zeiss Libra 200MC analytical transmission electron microscope with a field emission gun and a corrected Omega energy filter. TEM images were taken at an accelerating voltage of 200 kV in the bright-field mode. Thermal analysis was performed on a simultaneous TGA/DSC/DTA SDT Q-600 analyzer (TA Instruments) upon linear heating to 1200 °С (heating rate of 10 °C/min) or 1400 °С (20 °C/min) in a 100 ml/min airflow. Mass-spectroscopic analysis of the gases evolved during thermolysis was performed using a Netzsch TG 209 F1 thermal analyzer equipped with a Netzsch 403 C Aëolos quadrupole mass spectrometer upon linear heating to 950 °C in argon at a heating rate of 10 °C/min. 3. Results and discussion To obtain initial cerous phosphate solutions, nanodispersed cerium dioxide was dissolved in hot 85% orthophosphoric acid. It is known that, when orthophosphoric acid is heated, oligophosphoric acids can form [28]. To find out whether this actually takes place we have performed gravimetric analysis [29] (precipitation of orthophosphate with ammonium molybdate in the presence of nitric acid) of two solutions, solution 1 and solution 2 (both without adding CeO2). Solution 1 containing only 85% H3PO4 was heated at 100 °C for 1 h. Solution 2 was prepared by mixing 85% H3PO4 and 65% HNO3 (the H3PO4:HNO3 molar ratio was chosen to be equal to described in [29]) followed by heating at 100 °C for 1 h. Nitric acid was added to solution 2 to initiate the transformation of oligophosphoric acids, which could exist in the solution, into orthophosphoric acid. A comparative analysis of the orthophosphate
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anion contents in solutions 1 and 2 showed that solution 1 contained and solution 2 contained 82.4 ± 0.4 wt.% PO3− 77.5 ± 0.3 wt.% PO3− 4 4 (confidence intervals are given for P = 95%); the latter value content in pure 85% H3PO4 (82.4%). The difcorresponded to the PO3− 4 ference between these two solutions was 4.9%, which indicated the presence of oligophosphoric acids in the 85% solution of phosphoric acid heated to 100 °C for 1 h. These data allowed us to conclude that a comparatively low content of oligophosphate anions can form in hot orthophosphoric acid during the dissolution of cerium dioxide. Thus, cerium complexes mainly containing orthophosphate ligands are likely to form during the dissolution of cerium dioxide in orthophosphoric acid. Note that determining the composition of the cerium phosphate complexes that exist in aqueous concentrated solutions of phosphoric acid is quite a complex problem. Nevertheless, in contrast to earlier techniques [17,18], the method of forming cerous phosphate solutions presented in this paper can be used to completely exclude − the appearance of admixture ions (NH+ 4 , NO3 etc.). The duration of CeO2 dissolution in orthophosphoric acid was found to depend on the nanoparticle size. For example, cerium dioxide with a particle size of ~5 nm dissolved fully in ~15 min. Identical cerium dioxide samples with larger nanoparticles (7 and 15 nm) dissolved fully in 3 h and 48 h, respectively. In further experiments, cerium phosphate solutions prepared using CeO2 with a particle size of 5 nm were used. According to differential spectrophotometry data, the first derivative of the spectrum of the solution of cerium dioxide in orthophosphoric acid had a specific inflection point at 280–290 nm, indicating the coexistence of Ce+3 and Се+4 complexes in the solution (see Fig. 1) [26]. This result is nontrivial, since the formation of Ce+3 in the solution cannot be caused by the interaction of cerium dioxide with orthophosphoric acid, due to the fact that this acid has no reducing properties. Therefore, it is assumed that Ce+3 impurities were present in the initial cerium dioxide, which agrees with previously reported data [26,30–33] and indicates possible oxygen non-stoichiometry of nanocrystalline CeO2. A gel-like precipitate formed upon mixing of cerous phosphate solutions with deionized water. The gel began to form at a molar ratio H3PO4:H2O ≈ 1:1, and the quantity of gel, as estimated by visual observation, increased when water was added up to a ratio of ≈1:27. A further increase in water concentration did not change the quantity of gel; therefore, the quantity of water added to cerous phosphate solutions for the synthesis of gels in subsequent experiments was fixed at the ratio 1:27. Note that it was difficult to analyze the structure and the chemical properties of the as-synthesized cerous phosphate gels, because of a high orthophosphoric acid content in them. Therefore, it was proposed that they should be purified using dialysis. Note also that, in previous studies (see e.g. [21,22]), cerous phosphate gels contained uncontrolled
amounts of orthophosphoric acid and inorganic anions and cations (e.g. 2− and NH+ NO− 3 , SO4 4 ), which inevitably affected the results of respective studies of their properties and crystallization behaviour. The synthesis and purification method described in this paper provided a means of specifying the composition of cerous phosphate gels in a more flexible manner. Fig. 2 shows the X-ray diffraction patterns of the initial and dialyzed cerous phosphate gels. Both X-ray diffraction patterns have a well pronounced broad maximum at 2θ ~ 7.5°. When analyzing the gel synthesized by the interaction of Ce+ 4 sulfate with orthophosphoric acid, Hayashi et al. [34] obtained a similar X-ray diffraction pattern, and attributed this maximum to a layered structure of the partly formed amorphous compound. A short-range ordering with a characteristic distance of ~12 Å is thought to occur in these gels. The existence of a broad peak in the range 2θ = 25–45° is likely to be related to strong intra-layer distortions: similar X-ray diffraction patterns are characteristic of strongly amorphized solids with a highly distorted short-range ordering, e.g. hydrated zirconium dioxide [35,36]. Note that, as evidenced by X-ray diffraction (XRD), dialysis purification of the gels only weakly changed their structure. The decrease in the relative intensity of the peak at 2θ ~ 7.5° can be related to further disordering of the layered structure. Using the method developed, we succeeded in analysis of the cerous phosphate gels by scanning (Fig. 3a) and transmission (Fig. 3b) electron microscopy. It is clearly visible that the dialyzed gel consisted of closely interwoven fibers (anisotropy factor reaches 75:1 or more). The fibers were continuous, with no voids or channels (Fig. 3b). The possibility of the formation of anisotropic cerous phosphate particles was also reported recently, by Tang et al. [37], who synthesized cerium phosphate nanotubes instead of fibers. The mechanism of formation of well-defined anisotropic particles of an almost amorphous compound is not clear, and definitely deserves further investigation. Thermal analysis of the as-formed gels is complicated by the presence of a significant amount of orthophosphoric acid which can react with alumina crucibles at high temperatures (~ 800 °C). The protocol of synthesis and purification used allows refining of the chemical composition of amorphous cerous phosphate gels by thermal analysis, including thermal analysis combined with mass-spectrometric detection of the gaseous products (see Fig. 4). Data presented in Fig. 4 show that thermal decomposition of the gel sample is accompanied by the evolution of gaseous products providing mass-to-charge signals corresponding to water (m/z = 18) and oxygen in atomic and molecular forms (m/z = 16 and m/z = 32, respectively). No signals corresponding to other species were detected. The evolving of H2O proceeds in two overlaying stages corresponding to removal of physically and chemically bound water and finishes at 500 °С. As
Fig. 1. Absorption spectrum of the initial cerous phosphate solution (85 wt.% orthophosphoric acid was used as a blank) and its first derivative.
Fig. 2. X-ray diffraction patterns of (1) initial and (2) dialyzed cerous phosphate gels.
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Fig. 3. Dialyzed cerous phosphate gel: (a) scanning and (b) transmission electron microscopy data.
water molecules are subjected to electron impact, m/z = 16 and m/z = 32 signals are also detected in this temperature range. At higher temperatures (namely, at 630 °C and 840 °C) two pronounced maxima in m/z = 16 and m/z = 32 ionic current curves are observed which obviously correspond to evolution of molecular oxygen from the sample. The overall composition of the gels, without regard for weakly bound water, which is removed at temperatures below ~ 300 °C, can be represented as CeP1.04 ± 0.02O4.60 ± 0.04·(0.030 ± 0.003)H2O. When analyzing the X-ray diffraction patterns of the products of annealing of the purified gels at various temperatures (see Figs. 5, S1), it was possible to describe the main stages of thermal decomposition of the cerous phosphate gels as follows: 500
fCeP1:04 O4:6 0:03H2 Og → 700
C
fCeP1:04 O4:6 g þ H2 O↑
C
fCeP1:04 O4:6 g → CePO4 þ CeP2 O7 þ O2 ↑
Fig. 4. (top) Results of thermogravimetric analysis of dialyzed cerous phosphate gel, and (bottom) the temperature dependences of the ionic currents for m/z = 16, 18 and 32.
860
C
2CeP2 O7 → CePO4 þ CeP3 O9 þ 0:5O2 ↑ CeP3 O9
1000
→
C
CePO4 þ 0:5P4 O10 ↑:
This scheme is consistent with the data presented by Hirai et al. [38] and Masui et al. [39]. Thus, using the proposed synthesis and purification method, the work described in this paper has been the first to analyze the thermal behaviour of cerous phosphate gels comprehensively. However, the composition of the cerous phosphate gels can change, to a certain extent, with respect to the initial state during dialysis (e.g. as a result of the removal of weakly coordinated orthophosphate groups and a change in the coordination environment of cerium ions). An analysis of such processes should be the subject of further investigation. In any case, dialysis opens up fresh opportunities for arbitrary setting of a reaction medium composition to perform further crystallization of cerous phosphate gels, or to modify their structure to adjust ion-exchange and other functional properties. To study the hydrothermal crystallization paths of the cerous phosphate gels, hydrothermal treatment of the initial gels (containing ~ 11 wt.% of orthophosphoric acid), dialyzed gels and dialyzed gels redispersed in 1 wt.% aqueous solution of orthophosphoric acid was performed. The initial gels did not undergo crystallization during hydrothermal treatment at 120 °С and 140 °С, and the X-ray diffraction patterns of the
Fig. 5. X-ray diffraction patterns of the samples prepared by annealing of a dialyzed cerous phosphate gel at 500, 700, and 860 °С. Sign (+) indicates the diffraction peaks of cerium pyrophosphate CeP2O7; sign (*), the peaks of cerium cyclo-triphosphate (CeP3O9); and sign (#), monazite CePO4.
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Fig. 6. X-ray diffraction patterns of the products of hydrothermal treatment (155–220 °С, 1 h) of the initial cerous phosphate gels (containing ~11 wt.% of orthophosphoric acid).
treated samples almost coincided with those presented in Fig. 2. The product formed upon hydrothermal treatment at 155 °С was a crystalline compound, while the peaks in its X-ray diffraction pattern were considerably broadened, and an amorphous phase is likely to exist along with the crystalline phase. Well crystallized substances were obtained only after hydrothermal treatment at 170 °С or above (Figs. 6, S2). The X-ray diffraction patterns shown in Figs. 6 and S2 correspond to the Ce(PO4)(HPO4)0.5(H2O)0.5 phase, the structure of which was solved by Nazaraly et al. [13]. The unit cell parameters refined for a well crystallized sample are а = 21.023(2) Å, b = 6.5586(8) Å, с = 6.9554(8) Å and
187
β = 91.98(1)°, which agrees satisfactorily with the earlier data for this compound (Fig. S3) [13]. According to SEM data, the Ce(PO4)(HPO4)0.5(H2O)0.5 phase consisted of aggregated 30 nm-thick lamellar particles (see Fig. 7). The particle shape was almost consistent with that reported by Hayashi et al. [34]. Thus, the particle morphology changed from quasi-one-dimensional to quasi-two-dimensional during hydrothermal crystallization. According to the results of EDX measurements, the molar Ce:P ratio in Ce(PO4)(HPO4)0.5(H2O)0.5 samples was 1:1.5, which supports the chemical composition of the synthesized compound (see [14,21]). The hydrothermal crystallization of the cerous phosphate gels purified by dialysis proceeded along a completely different path. For example, a mixture of an amorphous phase and crystalline CePO4·xH2O (rhabdophane, PDF2 00-035-0614) formed during hydrothermal treatment of the dialyzed gel in a deionized water at 180 °C for 1 h (see Fig. 8a). A SEM image of this sample (Fig. 8b) showed that it consisted of both initial amorphous phase fibers, and columnar crystals with an average diameter of about 100 nm and the shape that is specific to rhabdophane (see e.g. [40–43]). Since the reaction mixture contained no reducing agents, trivalent cerium existed in the initial gel and, hence, in the initial cerous phosphate solution (see also the differential spectrophotometry data, Fig. 1). Thus, our results indicate that a Ce+3 impurity is actually present in nanocrystalline cerium dioxide. The possibility of the existence of trivalent cerium in nanocrystalline cerium dioxide is being actively discussed, since this can explain the mechanism of well pronounced antioxidant action of nanocrystalline CeO2 and its biological activity [24,44–48]. The crystalline phase content in the sample prepared by hydrothermal treatment of a dialyzed gel in deionized water at 180 °С for 1 h (see Fig. 8a) was estimated, by calculating the ratio of the integral intensities of rhabdophane peaks to the total integral intensities of the amorphous
Fig. 7. Lamellar particles of the Ce(PO4)(HPO4)0.5(H2O)0.5 phase synthesized from the initial cerous phosphate gels at (а) 170 °C and (b) 220 °С.
Fig. 8. (а) X-ray diffraction pattern and (b) SEM image of dialyzed cerous phosphate gel subjected to hydrothermal treatment (180 °С, 1 h). The indexed peaks correspond to rhabdophane CePO4·xH2O.
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and crystalline phases. This ratio was found to be ~ 4%. Rhabdophane was assumed to form during the crystallization of a Ce+3-containing compound, and the Ce+ 4-containing phosphate compound was thought to be retained in the form of an amorphous phase. With allowance for the estimation of the rhabdophane content given above, this assumption made it possible to estimate the formal oxidation state of cerium in the initial gel and, hence, in the initial cerium dioxide. It was found to be +3.96. This value agrees well with our previous independent estimation of the formal oxidation state of cerium in nanocrystalline (~5 nm) cerium dioxide in terms of Kim's model [24,49]. Thus, hydrothermal treatment of cerous phosphate gel in 11 wt.% H3PO4 and deionized water, at 180 °С for 1 h, resulted in the crystallization of Ce(PO4)(HPO4)0.5(H2O)0.5 and Ce(PO4)·xH2O, respectively. In addition, hydrothermal treatment of the preliminarily dialyzed gel redispersed in 1 wt.% H3PO4 under the same hydrothermal conditions was performed. According to XRD data (see Fig. 9), a mixture of Ce(PO4)(HPO4)0.5(H2O)0.5, rhabdophane CePO4·xH2O and a third unknown phase (with the main peaks at 2θ = 17.3, 20.9, 21.7 and 26.1°) formed, in this case. An analysis of the experimental data enabled a number of assumptions regarding the peculiarities of hydrothermal crystallization of cerous phosphate gels. Only Ce+3 compounds crystallize in a neutral medium if they are present in the composition of the initial gel. For Ce+4 compounds to crystallize, a significant amount of orthophosphoric acid should exist in a reaction mixture. These features are likely to be related to the fact that crystalline Ce+4 orthophosphates can only exist in the form of acid salts [12]. During crystallization in the presence of a small amount of orthophosphoric acid, Ce+ 3 forms Ce+3 orthophosphate hydrate with a rhabdophane structure, while in media with a higher H3PO4 concentration it is likely to be incorporated into the structure of crystalline acid Ce+4 orthophosphates. 4. Conclusions A new facile method to synthesize admixture-free amorphous cerous phosphate gels has been developed allowing for precise determination of the chemical composition, structure, and morphology of cerous phosphate gels. The gels were found to consist of monolithic interwoven fibers possessing a short-range ordering with a characteristic distance of ~12 Å. The chemical composition of the gels without regard for weakly bound water was estimated as CeP1.04O4.6·0.03H2O. Hydrothermal crystallization paths of cerous phosphate gels strongly depend on the presence of H3PO4 in reaction medium. Hydrothermal treatment of the gels in the presence of orthophosphoric acid
Fig. 9. X-ray diffraction pattern of the sample prepared by hydrothermal treatment (180 °С, 1 h) of a dialyzed cerous phosphate gel redispersed in 1 wt.% H3PO4. Sign (*) indicates the peaks corresponding to Ce(PO4)(HPO4)0.5(H2O)0.5, sign (+) indicates the peaks corresponding to CePO4·xH2O and symbol (#) indicates the unidentified peaks.
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