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Structure of nanoparticles in the ZrO2-Y2O3 system, as obtained under hydrothermal conditions
A. I. Shuklina1, A. V. Smirnov2, B. A. Fedorov2, S.A. Kirillova1, O. V. Almjasheva3
1 Saint Petersburg State Electrotechnical University "LETI", ul. Professora Popova, 5, Saint Petersburg, 197376, Russia
2ITMO University, Kronverksky Pr. 49, bldg. A, Saint Petersburg, 197101, Russia 3Ioffe Institute, Politekhnicheskaya St. 26, Saint Petersburg, 194021, Russia
PACS 61.46.+w, 81.40.-z DOI 10.17586/2220-8054-2020-11-6-729-738
It is shown that monocrystalline nanoparticles with fluorite structure are formed in the ZrO2-Y2O3 system, under hydrothermal conditions. The limiting content of Y2O3 in the nanocrystals based on zirconium dioxide is 21.7-22.6 mol.%. Yttrium oxide not included in the structure forms an amorphous phase, which is stable even upon thermal treatment at 1000-1300 °C. It has been found that under hydrothermal conditions the structure of the nanocrystals based on ZrO2(Y2O3) solid solution includes water, its content depending on yttrium oxide concentration in the solid solution. Keywords: nanocrystals, hydrothermal synthesis, solid solution, zirconia. Received: 10 August 2020 Revised: 15 October 2020
1. Introduction
Due to the presence of several polymorphic modifications [1-4] zirconium dioxide is of interest as a model for studying the effect of the synthesis methods and parameters on the formation of nanocrystals with different structure [5-15]. Zirconium dioxide based nanomaterials with various structures, morphologies, and particle size parameters have a wide range of applications [16-27], which makes the above studies relevant from a practical point of view as well.
In particular, papers [20,21] show the perspective of using the photoelectrodes based on nanocrystals formed in the ZrO2-Y2O3 system in perovskite solar cells. At the same time, it is noted in these works that the change in the band gap in such materials may depend not only on their composition, but also on the features of the nanoparticle structure [20,21]. As known, the band gap and other properties of functional materials can be sensitive to the methods and parameters of synthesis [5-12,23,28-32]. As mentioned in papers [28-30], one of the specific features of formation of the nanoparticles based on solid solutions in the ZrO2-M2O3(M = Gd2O3, Y2O3, In2O3) systems is the possibility of forming the nanoparticles with a "core-shell" structure. Studying the possibility of forming the nanoparticles with a particular structure and particular dimensional parameters, depending on the method and conditions of synthesis, will make it possible to intentionally synthesize the nanoparticles with the required properties.
The perspectives of using the ZrO2(Y2O3) nanocrystals to convert solar energy necessitate a systematic study of the formation processes of nanomaterials with a certain structure, morphology and dimension parameters of the particles. The present study is aimed at solving these issues.
2. Experimental part
Zirconium dioxide based nanoparticles in the ZrO2-Y2O3 system were obtained, in a wide range of the components' ratios, by dehydration of co-precipitated hydroxides under hydrothermal conditions, according to the method described in [6].
The precipitation was carried out from a mixture of aqueous solutions of zirconium oxychloride (ZrOCl2 8H2O, chemically pure (TU 6-09-3677-74)) and yttrium chloride (YCb 6H2O, pure (TU 6-09-4773-79)), with 12 M ammonium hydroxide solution (NH4OH, reagent grade (GOST 3760-79)). The obtained precipitates were rinsed with distilled water and dried at a temperature of 100 °C. Hydrothermal treatment was carried out at a temperature T = 250 °C and pressure P = 70 MPa. Time of the hydrothermal treatment was recorded according to the duration of isothermal holding of autoclaves in the furnace. The duration of isothermal holding was 4 h.
The elemental composition was determined by energy-dispersive X-ray spectroscopy (EDS) with the use of a scanning electron microscope (Hitachi S-570) with energy-dispersive analyzer (Bruker Quantax 220).
The structural state of the samples and the parameters of the unit cells were determined from the data of X-ray powder diffraction obtained using Shimadzu XRD-7000 X-ray diffractometer. Diffractograms were taken using CuKa radiation in the range of angles 20 from 10 to 60 °C. When determining the unit cell parameters of compounds in the samples, silicon was introduced as an internal standard.
To study the structural changes during heating, the Shimadzu XRD-7000 diffractometer is equipped with a high temperature furnace (Anton Paar HTN 1200 K). The sample was air-heated from room temperature to 1100 ° C, with a stop after every 100 ° C. At each temperature step, the isothermal holding time was 10 min.
Dimensional parameters of the crystallites were determined from the data on the X-ray diffraction lines' profile. The crystallites' distribution was determined using software (SmartLabStudio II). Average dimensions of the crystallites were calculated using the Scherrer formula and also data on the crystallites' size distribution.
The dimensional characteristics of nanoparticles were determined from the data of small-angle X-ray scattering [33,34]. Two methods were applied for the calculation, i.e. the "Guinier method" [33] and the method of "standard curves" [34]. The small-angle measurements were made using Kratky block-camera. CuKa radiation was used. The absorption factor of the sample was determined using an attachment with a moving slit.
The pycnometric density was measured by helium pycnometry, using the gas pycnometer Ultra Pycnometer 1000 (Quanta Chrome). For measurements, all the samples were pre-dried at 100 °C for 30 minutes. The measurements were carried out in a 10 cm3 cell, with preliminary degassing of the sample for 5 min. in stream mode. The number of consecutive measurements was 10, the preset limit of measurement error was maximum 0.01 g/cm3.
Thermal transformations were studied by differential scanning calorimetry combined with thermogravimetric analysis, using the thermal analyzer NETZSCH (STA 429CD).
3. Results and discussion
The results of elemental analysis showed that, subject to the method sensitivity limits, there were no elements other than Zr, Y, O. According to EDS analysis, subject to error limits of the method, the ratio of elements, in terms of the oxides ZrO2 and Y2O3, corresponded to (nominal) composition of the samples specified for the synthesis, as shown in Table 1.
Table 1. Results of elemental analysis of the samples based on the system ZrO2-Y2O3
Content of YO15 in the system, mol.%
Nominal composition EDS
5 5.0
8 8.6
12 12.0
15 16.4
20 19.7
25 23.3
30 31.0
45 46.4
55 57.2
According to X-ray diffraction analysis, all the samples obtained by co-precipitation of the components were X-ray amorphous. After their hydrothermal treatment at 250 °C, for 4 h, at Y2O3 content in the samples of up to 46.4 mol.%, zirconium dioxide based phases with fluorite structure (Fig. 1a) were formed. For the sample containing 57.2 mol.% Y2O3, narrow peaks corresponding to Y(OH)3 were recorded (Fig. 1b) against the background of X-ray maxima corresponding to the zirconium dioxide based phase with fluorite structure.
Based on the X-ray diffraction data, parameter values of the unit cell of the zirconium dioxide based phase with fluorite structure were calculated, and the unit cell parameter was plotted against the amount of yttrium oxide in the system (Fig. 2). The obtained values correlate well with literature data, up to the Y2O3 content in the system corresponding to 19.7 mol.%, and can be described by a linear relationship:
a(x) = a0 + a ■ x, (1)
where a is a parameter of the cubic unit cell of the phase based on zirconium dioxide with fluorite structure; x is content of YO15 in the system ZrO2-YO15, expressed in mole fractions; a0 = 5.116 ± 0.004 A, a = 0.29 A/mole
25 30 35 40 45 50 55 60 65 20 30 40 50 60 70
20, degree 20, degree
Fig. 1. X-ray diffractograms of the samples in the system ZrO2-YO15: (a) samples obtained by hydrothermal treatment of co-precipitated hydroxides at T = 250 °C, P = 70 MPa, t = 4 h (Si -peaks of the internal standard); (b) the sample containing 31.0 mol.% Y2O3, after heat treatment during high temperature X-ray diffraction (Al2O3 - peaks of the high temperature cell); (c) the sample containing 46.4 mol.% Y2O3, after additional heat treatment by air at T = 1300°C
fractions. However, starting from YO15 content in the system of 19.7 mol.%, the dependence of a(x) greatly deviates from the straight line (1), actually taking the following form in the range 31.0-46.4 mol.% Y2O3:
a(x) = a„
where a„ is a constant, which in this case equals to the value aa
(2)
: 5.178 ±0.002.
Fig. 2. Unit cell parameter vs. amount of yttrium oxide in the system
The independence of the unit cell parameter on the phase composition indicates that increasing of yttrium oxide content in the system does not lead to increase in its concentration in the phase of variable composition -Zr(1-x)YxO(2—0.5x). The fact that in this range of compositions the X-ray diffractogram does not show any other crystalline phases leads one to conclude that yttrium oxide not included in the structure of the variable composition phase based on zirconium dioxide is in X-ray amorphous state. In papers [28, 29] this state was classified as the state of yttrium oxide in the X-ray amorphous shell surrounding nanocrystalline particles. Thus, in the range of Y2O3 content of 23.3 to 46.4 mol.%, a noticeable amount of amorphous phase can be expected in the system, since the concentration Y2O3 in these samples, as calculated from the data on the unit cell parameter values, varies in the range 21.7-22.6 mol.%, i.e. differs from the system composition.
After high temperature treatment, the X-ray diffraction data (Fig. 1) showed that the system comprising 31.0 and 46.4 mol.% Y2O3 did not contain any other crystalline phases except for the phase of variable composition with fluorite structure. Meanwhile, based on the data on the unit cell parameter values of the variable composition phase Zr(1-x) YxO(2_0.5x) with fluorite structure for these samples (Fig. 2.), we can conclude that yttrium oxide concentration therein equals to x « 0.272 mole fractions (« 27.2 mol.% Y2O3), i.e. significantly less than the content of yttrium oxide in the system. Thus, even after thermal treatment a significant portion of yttrium oxide remains in the X-ray amorphous state. However, based on the state diagram of the system ZrO2-Y2O3 [1-4] we could expect yttrium oxide entering the solid solution at a treatment temperature of 1000-1300 °C. Apparently, insufficient thermal treatment time does not allow the formation of Zr(1-x) YxO(2-05x) phase with fluorite structure of equilibrium composition, and spatial constraints, presented as grains of the zirconium dioxide based phase with fluorite structure, do not allow the transformation of the amorphous phase based on yttrium oxide into crystalline one. The possibility of realizing such an effect was noted in [38] and was theoretically analyzed in [39].
As shown in [40], in the course of heat treatment nanocrystalline zirconium dioxide loses water in several stages, at different temperatures. However, the weight loss is associated not only with the removal of the surface adsorbed water, but also with composition change and with the corresponding structural transitions in the nanocrystals based on zirconium dioxide, in the temperature range 200-500 °C [40,41]:
A similar situation, as Fig. 3 shows, is observed in the samples based on the system ZrO2-Y2O3. When studying the samples behavior by means of synchronous thermal analysis, prominent endothermic effect is observed in the range of temperatures of ~ 25-150°C, which is accompanied by weight loss and corresponds to the removal of water adsorbed on the nanoparticles surface. At higher temperatures, two more steps of water loss in the samples are observed: in the range of temperatures of ~ 150-570 °C and ~ 570-1120 °C, respectively.
ZrOi.965(OH)0.070 • 0.19H2O ^ ZrOi.965(OH)0.070 + 0.№O, ZrOi.965(OH)o.070 ^ ZrO2 + 0.035H2O.
31.0 mol. %
0 200 400 600 800 1000 1200
Arn, %
z, °c
86"
200 400 600 800 1000 1200
L°C
Fig. 3. Results of synchronous thermal analysis
Analysis of the thermogravimetric study results allowed us to conclude that the composition of the zirconium dioxide based nanoparticles obtained by hydrothermal treatment can be presented as a formula comprising a certain amount of chemically bound water: Zr(1-x)YxO(2-0.5x) • nH2O. The results of estimating the amount of chemically bound water, depending on the content of yttrium oxide in the system, are shown in Fig. 4. Considering the results of [41] relating to water state in the structure of nanoparticles based on zirconium dioxide, we can conclude that in the nanoparticles, where zirconium dioxide is partially substituted by yttrium oxide, water is presented both in the form of crystallization water with dehydration temperature range of ~ 150-570 °C, and in the form of constitution water -the temperature range of removal of ~ 570-1120 °C.
n, mole fraction
■ -----
0 I-*-*-.-.-*-,
0 0.05 0.1 0.15 0.2 0.25 0.30 0.35
x(YOi.5), mole fraction
Fig. 4. Amount of water n1(H2O) and n2(H2O) (n2(OH)= 2 • n2(H2O) in the nanoparticles of variable composition (n1(H2O) + n2(H2O) = n(H2O) vs. yttrium oxide content in the system
Considering the results of [41] relating to the hydrated state in the nanocrystals based on zirconium dioxide and the relationships shown in Fig. 4, or in the analytical form:
m (H2O) = 0.07 +1.022 • x,
n2(H2O) = 0.012 + 0.325 • x, n2 (OH) = 2 • n2 (H2O) = 0.024 + 0.675 • x,
where n1 is the amount of crystallization water, mole fractions; n2 is the amount of constitution water, mole fractions, (n(H2O = n1(H2O) + n2(H2O)), the formula of the solid solution comprising yttrium oxide and water can be recorded as follows: Zr1_xYxO2_0.5x-0.5(0.024+0.675 x)OH(0.024+0.675 x) • (0.07 + 1.022 • x)H2O. The values obtained are in qualitative compliance with data presented in [41], wherein comprehensive studies by means of proton magnetic resonance, X-ray phase analysis and comprehensive thermal analysis were used to determine the composition of nanoparticles based on zirconium dioxide, expressed by the formula ZrO1 965(OH)007 0.19H2O. Some quantitative differences between the chemical composition of a nanocrystalline zirconium dioxide based compound obtained for nanocrystals not containing yttrium oxide in the structure - ZrO1 965(OH)007 0.19H2O [41], and the composition obtained by extrapolation x ^ 0 of the expression Zr1-xYxO1.988-0.838xOH(0.024+0.675 x) • (0.07 + 1.022 • x)H2O, i.e. ZrO1.988OH0.024-0.07H2O, are apparently due to their structural differences. In the case described in [41], the nanocrystals have a structure close to tetragonal one, which is stabilized by water entry into the lattice. The paper discusses the case of nanocrystals of solid solutions based on zirconium dioxide, whose structure is described as a cubic one, stabilized by the substitution of zirconium ions by yttrium ions in the crystal lattice, with charge compensation in the anion sublattice.
The density of the samples was determined by the method of helium pycnometry, in comparison with the X-ray density shown in Fig. 5. The X-ray density of the crystalline samples was determined from data on the volume of the unit cells and their elemental composition, with and without regard for water included in their structure. Based on the analysis of the relationship in Fig. 5, it can be concluded that as the yttrium oxide amount in the system increases, the values of pycnometric density of the samples somewhat decrease as compared with the X-ray density, calculated both with and without regard for water in the structure. This is most likely due to the presence in the samples enriched with yttrium oxide, especially at a content of x > 19.7 mol.%, where, under hydrothermal synthesis conditions, an amorphous phase is produced that is enriched with hydrated forms of yttrium oxide having significantly lower density than zirconium dioxide based crystalline phases of interest. Since the substance density decreases
Fig. 5. Samples density vs. amount of Y2O3
during the transition to the amorphous state [42,43], the density of hydrated forms of yttrium oxide can be estimated as: p(YOOH) < 4.48 -4.63 g/cm3 [44-46], p(Y(OH)3) < 3.80 - 3.90 g/cm3 [44,47-49].
Based on the analysis of data obtained by small-angle X-ray scattering, size distribution of the particles was determined, and mean values of the particle sizes were calculated (Table 2). Data in Fig. 6 show that as yttrium oxide amount in the system increases, both mean dimensions of crystallites and mean dimensions of particles decrease remarkably. Almost complete matching, within error limits, of the mean dimensions of crystallites and particles (except for the point at 46.4 mol.% of Y2O3) indicates that the obtained nanoparticles are predominantly monocrystalline. Comparison of the size distribution curves of crystallites and particles (Fig. 7) shows their similar nature. In both cases, there is a tendency towards narrowing of the sizes distribution region and shifting of the size distribution maxima towards smaller sizes both of particles and crystallites, as yttrium oxide content in the system increases, and this is another proof of the above conclusion about monocrystalline nature of the nanoparticles, which is irrespective of the size thereof.
Table 2. Results of the study by means of small-angle X-ray analysis
Y2O3 content in the sample Particle diameter, nm Particle surface per unit volume, m2/cm3 Density*) g/cm3 Specific surface area, m2/g
standard curve method D Guinier method Dr (S/V ) particle P Sspecific
5.0 12.0±0.2 11.98±0.05 917±39 6.21 151±9
8.6 8.90±0.04 8.77±0.06 1224±3 6.19 197±5
16.4 8.26±0.06 8.29±0.04 1400±2 6.17 225±3
23.3 7.92±0.8 7.84±0.06 1539±8 6.14 248±2
31.0 5.9±0.2 5.7±0.1 1777±47 6.12 293±7
46.4 9.7±0.6 9.4±0.1 1571±12 6.11 254±5
57.2 5.12±0.06 5.08±0.04 1887±32 6.06 309±3
*) X-ray density with regard for water included in the structure
Based on the data on the particle size and their X-ray density with regard for water included in the structure, specific surface areas of the samples were calculated (Fig. 8).
Note the inverse relationship between the samples' specific surface areas on the one hand and the size of nanopar-ticles and the content of yttrium oxide in the system on the other. This is due to the fact that the nanoparticle sizes themselves steadily decrease as the amount of yttrium oxide in the samples increases. The nature of the relationship between water adsorbed on the nanoparticles' surface on the one hand and their specific surface areas and Y2O3 content in the system on the other (Fig. 9) is explained similarly.
Fig. 6. Crystallite (a) and particles (b) size in the system ZrO2-Y2O3 vs. amount of yttrium oxide
Fig. 7. Size distribution of crystallites (a) and particles (b)
Fig. 8. Specific surface areas of the samples vs composition (a) and size (b) of the particles
Fig. 9. Amount of water adsorbed on the surface of nanoparticles vs. their specific surface areas and amount of yttrium oxide in the system
4. Conclusion
The study showed that nanocrystalline particles are formed in the ZrO2-Y2O3 system under hydrothermal conditions, with the particle size decreasing as yttrium oxide content in the system increases. The limiting content of Y2O3 in the nanocrystals based on zirconium dioxide with fluorite-like structure is 21.7-22.6 mol.%, when they are formed by hydrothermal treatment of co-precipitated hydroxides at T = 250 °C, pressure P = 70 MPa, for 4 h. Yttrium oxide not included in the structure forms an amorphous phase, stable even upon high-temperature treatment of the samples. It is shown that under hydrothermal conditions the structure of nanocrystals based on ZrO2(Y2O3) solid solution includes water, its content depending on yttrium oxide concentration in the solid solution. Comparison of the data on the crystallites' and particles' size distribution showed that the nanoparticles formed under hydrothermal conditions are monocrystalline structures.
Acknowledgments
The authors express their gratitude to V. V. Gusarov for the attention he paid to the work. X-ray diffraction and SEM/EDS was performed using the equipment of the Engineering Center Saint Petersburg State Institute of Technology.
This study was financially supported by the Russian Science Foundation (project no. 20-63-47016).
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