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NANOSYSTEMS: PHYSICS, CHEMISTRY, MATHEMATICS, 2018, 9 (4), P. 568-572
Formation mechanism of core-shell nanocrystals obtained via dehydration of coprecipitated hydroxides at hydrothermal conditions
O. V. Almjasheva1'2, A. A. Krasilin2, V. V. Gusarov2 1 Saint-Petersburg Electrotechnical University "LETI", Saint-Petersburg, 197376 Russia 2Ioffe Institute, Saint-Petersburg, 194021, Russia almjasheva@mail.ru, ikrasilin@mail.ioffe.ru, victor.v.gusarov@gmail.com
DOI 10.17586/2220-8054-2018-9-4-568-572
Here, we propose a formation mechanism for core-shell nanoparticles by self-organization in coprecipitated mixed hydroxides under hydrothermal conditions. A thermodynamic reason for this process is because of a decrease in the components' solubilities together with an increase of structure's dimension. As a particular example of such type of behavior, we investigate core-shell nanoparticle formation in the ZrO2-Y2O3-H2O system.
Keywords: core-shell nanoparticles, hydrothermal synthesis, multi-component systems, solid solution, zirconia, formation mechanism.
Received: 16 August 2018
1. Introduction
Core-shell nanoparticles have garnered substantial interest due to the potential of their application in the field of biomedicine [1-3], catalysis [4,5], sensorics [6,7], luminophores [8], and materials for optics [9,10]. A large number of papers [1-12], that have been published recently, cover aspects of chemical synthesis, structure and properties investigation of the nanoparticles.
There are two means of core-shell nanoparticle synthesis. The first one includes various types of application of the nanosized layers on the initial nanoparticle surface. This can be done by precipitation from a gaseous phase, for example, by CVD, PVD, ALD (ML) methods [12-17]; or by precipitation from a liquid phase by SILD and LbL methods [18,19]. Layers and shells can be obtained by transport from one particle to another [20-23] as a result of non-autonomous (surface) phase movement [23]. However, this group of methods is not limited to the aforementioned techniques.
Another group relies on the effect of substance segregation inside the particle with redistribution of the components between the volume and the surface [24-26]. Despite the fact that the thermodynamic theory describing the component redistribution between autonomous (volume) and non-autonomous (surface) phase is well-developed [27], in certain cases there is a dramatic discrepancy between experimentally observed effects and their thermodynamic predictions. Among these problems, there is a case of substantial difference between the measured shell thickness, obtained after the segregation, and the equilibrium thickness of the non-autonomous phase [28]. In particular, this feature was observed for oxide nanoparticles in ZrO2-Y2O3-H2O (M=Y, In, Gd) system [25,26], produced by the hydrothermal treatment of coprecipitated hydroxides.
The aim of this work is to model processes which lead to the formation of core-shell structures through decomposition of initial substances and formation of nanoparticles - nuclei for the new nanostructured phase.
2. Model
Let us consider the case, in which initial substance forms quasi-2D layers of variable composition situated some substantial distance from each other and randomly oriented. In such a manner, chemical composition, structure, and properties do not depend on the layers' relative position. Distancing of quasi-2D layers owes to adsorption of molecules on their surface and their immersion into an isotropic medium, for example, water. Removal of adsorbates or liquid environment between the layers initiates distance decrease in between them. In turn, this leads to the layers' rearrangement, correlation in their structural changes and, as a consequence, their properties. In such a manner, 3D particles can be formed involving oriented attachment mechanism [29-32]. Fig. 1 shows the transformations described above. Let us assume that quasi-2D layers are of nanometer or sub-nanometer thick, and the width of layers is at least in order of magnitude larger that the thickness. At the same time, the width stays in nanometer scale, so the particles formed in oriented attachment process can be considered as nanoparticles. For the sake of simplicity, but without loss of generality of principle derivations, present research focuses on transformations occurring in binary solid solutions.
Fig. 1. Scheme of formation of c-ZrO2(Y2O3) nanoparticles during dehydration of coprecipitated hydroxides under hydrothermal conditions
For the case of two-component phases of variable composition and different dimension, the Gibbs energy of mixing is:
G(v)M = G(v)EX(xi,X2; {at}) — TS(v)ID(xi,X2), (1)
where x1? x2 = 1 — x1 are molar contents of components 1 and 2 in phases of variable composition; upper index v = 1, 2,3 shows dimension of corresponding phases: v = 1 is for quasi-lD, v = 2 is for quasi-2D, and v = 3 is for 3D one (in general, v can be non-integer denoting structures with certain fractal dimension); T is temperature, K; S(v)ID is configuration entropy of ideal state [33]; G(v)EX (x1,x2; {a^}) is the excess Gibbs energy of mixing determined by configuration entropy of ideal state; and {a^} is a number of parameters included in model equations [33]. For the most basic case of ideal mixture of components 1 and 2 SID = SI = —R(x1 lnx1 + x2 lnx2). For the case SID = S1 GEX = GE, i.e. the equation (1) coincides with the commonly used equation for the excess Gibbs energy of mixing: GE = GM + TS1.
It can be further simplified by assuming that SID = S1. In making this assumption, the difference between the ideal entropy (SI) and configuration entropy of the proposed idealized state, which would, probably, better reflect the state of variable composition (SID), transfers to the value GE = GEX + T(SI — SID). One should expect to see the highest difference between the SI and SID (GE and GEX, correspondingly) values in case the phase of variable composition is formed by components with different oxidation states [33,34], because this leads to formation of defects like vacancies or ions in the interstitial space of the crystal lattice.
The 1D solid solution (v = 1), as it was shown in [35], withholds from segregation at any temperature: this means that GE = 0. Urusov [34] noted that the G(2)E value for quasi-2D structures is several times smaller than the G(3)E value for the 3D ones. One can conclude that the solid solution components solubility decreases with increase of the structure dimension (1D to 3D). There is a probability that, during the nanoparticle growth, the substance separated out of solid solution will form the shell on the particle surface. Next, we consider the possibility of the core-shell structure formation involving the proposed mechanism during dehydration in ZrO2-Y2O3-H2 O system.
3. Results
Degtyarev and Voronin [36] calculated the thermodynamic parameters of coexisting phases in the ZrO2-Y2O3 system. Dehydration of coprecipitated zirconium and yttrium hydroxides yields oxide nanoparticles with fluorite structure, which is characteristic for the high-temperature area of the ZrO2-Y2O3 phase diagram. Thus, we used high-temperature data for thermodynamic analysis of low-temperature area of the phase diagram according to adopted equations proposed by Degtyarev and Voronin in [36]:
Gex = x1 (1 — x1)C (Ao + A1T + Bo x1), (2)
SID = —R [2(1 — x1) ln(1 — x1) + 3x1 lnx1 + (4 — x^ ln(4 — x^ — 4(1 — x1) ln4 — 3x1 lnx3], (3)
where x1 is molar content of YO1.5; R = 8.31 is the gas constant, J/(mol-K); A0 = —13273R, A1 = 5.324R, and B0 = 12383R are numerical parameters; C (v) = 0.. 1 is a structure dimension depended coefficient.
Extrapolation of these data to the low-temperature area allows one to predict phase separation limits in cubic metastable phases in the ZrO2-Y2O3 system (Fig. 2a, curves 1). An over-estimation of the limits in comparison to the experimental results [25] can be related with the extrapolation error. Since the Zr1-xYxO2-0.5x nanocrystals grow from quisi-2D fluorite-like layers [29] during the dehydration, the yttrium content in the initial quasi-2D layer should be substantially higher than that in the 3D phase. Fig. 2b shows curves of ZrO2-Y2O3 solid solution decomposition calculated in accordance with the assumption of the Gibbs energy decrease with decrease of the phase dimension (v = 3 ^ v = 2), which is equal to decrease of the coefficient C(v) in (2).
Fig. 2. a) Part of the ZrO2-YOi.s calculated phase diagram. Points denote experimental data adopted from the paper [36]. Curve 1 was calculated at Cv = 1, curve 2 was calculated at Cv = 0.6 (see equations (2) and (3)). b) The Gibbs energy of mixing GM at 1 - C= 1,2-C(v) = 0.6, 3 - C(v) = 0.2
Fig. 3. Dependence of the parameters of the unit cell of the phase based on c-ZrO2 with the fluorite structure on the content of YO1.5 in the system. The inset: the dependence of the crystallite size of c-ZrO2 on the YO1.5 content
Summarizing the results for the thermodynamic analysis of the dimensions of the regions where solid solutions exist in quasi-2D and 3D phases, we conclude that, in the formation of nanocrystals based on phases of variable composition, by fusion of quasi-2D layers of variable composition, the solid solution can split into two coexisting phases. If the amount of matter released is not sufficient to form a crystalline nucleus of a new phase, it is possible to form core-shell structures, as shown on Fig. 1. Almjasheva et al. [25] showed that the "crystal c-ZrO2/YOi.5 fluorite solid solution core - amorphous Y2O3 shell" particles were formed in the ZrO2-Y2O3-H2O system under hydrothermal conditions. The results of the nanoparticles' composition and size parameters analysis (Fig. 3,4) demonstrate qualitative correlation with the thermodynamic data on the change in the solubility of components during the transition from quasi-2D to 3D structures. It should be noted that this explanation is, most likely, also applicable for the case of core-shell nanoparticles formation in another ZrO2-M2O3-H2O systems M - In, Gd) [25,26].
Fig. 4. Dependence of the thickness of the amorphous shell on the content of YO1.s in the system
Thus, the mechanism for the formation of core-shell nanostructures (Fig. 1) by decreasing the mutual solubility of the components with increase of the structure dimension can be considered as one of the more promising means of such nanoparticle design.
Acknowledgement
The research was supported by Russian Science Foundation grant 16-13-10252. References
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