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. 2024 Nov 26;14:29398. doi: 10.1038/s41598-024-80399-x

Study of morphology, phase composition, optical properties, and thermal stability of hydrothermal zirconium dioxide synthesized at low temperatures

Yuriy Garanin 1,2,✉, Rafael Shakirzyanov 1,2, Daryn Borgekov 1,2, Artem Kozlovskiy 1,2, Natalia Volodina 1, Dmitriy Shlimas 1,2, Maxim Zdorovets 1,2
PMCID: PMC11599745  PMID: 39592719

Abstract

Oxide nanoparticles exhibit unique features such as high surface area, enhanced catalytic activity, and tunable optical and electrical properties, making them valuable to various industry applications as well as for the development of new research projects. Nowadays, ZrO2 nanoparticles are widely used as catalysts and precursors in ceramic technology. Hydrothermal synthesis with metal salts is one of the most common methods for producing stable tetragonal-phase zirconium dioxide nanoparticles. However, hydrothermal synthesis requires relatively high process temperatures (160–200 °C) and the use of advanced heat-resistant autoclaves capable of maintaining high pressure. This paper investigates how different precursors (ZrOCl₂·8H₂O and ZrO(NO₃)₂·2H₂O) and synthesis temperatures (110–160 °C) affect the phase composition, optical properties, size, and shape of ZrO₂ nanoparticles produced by hydrothermal synthesis without calcination. In addition, the effect of temperature exposure in the range of 100–1000 °C on the phase stability of the synthesized nanoparticles was studied. X-ray diffraction and Raman spectroscopy techniques were used to determine the structure and phase composition, while the optical properties were examined through the analysis of transmission and absorption spectra in the visible and UV ranges. It was found that the obtained particles at synthesis temperatures of 110–130 °C have predominantly cubic c-ZrO2 phase, which changes to monoclinic phase when heated above 500 °C. Analysis of visible and UV spectroscopy data reveals that the experimental samples have pronounced absorption in the middle UV range (200–260 nm) and have an energy band gap Eg varying from 4.8 to 5.1 eV. The hydrothermal powders synthesized in this study can be used as absorbers in the mid-UV range and as reinforcing additives in the preparation of technical ceramics.

Keywords: Hydrothermal synthesis, ZrO2, Phase transformation, Optical properties, Temperature stability

Subject terms: Materials science, Nanoscale materials

Introduction

Zirconium dioxide (ZrO2) has unique properties such as high values of mechanical strength, fire resistance, low chemical activity, high ionic conductivity, and low thermal conductivity at high temperatures. Due to these characteristics, ZrO2 is a promising material across various fields of science and industry1–3. In particular, ZrO2 is used as a material for oxidizing diesel soot4, in optoelectronics5, as electrodes and electrolytes in solid oxide fuel cells (SOFCs)6, in dentistry7, as catalysts for water splitting8, and as a material for ultraviolet (UV) radiation absorption9–12. ZrO2 at normal pressure, depending on the temperature, has three different crystal structures: (1) monoclinic (m-ZrO2), at temperatures up to 1175 ℃; (2) tetragonal (t-ZrO2), at temperatures from 1175 to 2370 ℃; (3) cubic (c-ZrO2), at temperatures from 2370 to 2680 ℃13. Most of the above-mentioned unique properties are characteristic of the tetragonal and cubic phases of zirconium dioxide. Thus, t-ZrO2 has high fracture toughness, catalytic activity and biocompatibility14,15, due to which t-ZrO2 is used as a reinforcing additive in Al2O3 ceramics. Also, stabilized tetragonal zirconia can be considered as a promising biomaterial16,17. At the same time, c-ZrO2 has high oxygen conductivity and chemical stability at high temperatures and pressures18,19, which allows it to be used as cathodes in solid fuel oxide elements and heat-resistant ceramics20,21. The stabilization of these phases is a key focus for materials scientists working with zirconia ceramics, refractory coatings, catalysts, and nanoparticles. High-temperature t-ZrO2 and c-ZrO2 polymorphs exist at room temperature if doped with additives such as Y2O3, CaO, CeO2, and MgO22–25. When the Zr4⁺ cation is substituted with a cation of a different ionic radius, such as Y3⁺, Mg2⁺, Ca2⁺, or Ce4⁺, the reverse martensitic transformation from t-ZrO₂ and c-ZrO₂ to m-ZrO₂ does not occur26. This substitution prevents the shear displacement of zirconium cations and oxygen anions, thereby preventing the phase transition during cooling. Another method to achieve high-temperature phases is to obtain ZrO2 particles with a size of 20 nm or less. The metastable t-ZrO₂ and c-ZrO₂ phases in the synthesized particles can be stabilized due to the high surface energy of the nanoparticles27–30.

Currently, ZrO2 nanoparticles are produced using such methods as the sol–gel method, solvothermal, spray pyrolysis, green method, chemical vapor deposition (CVD), and hydrothermal synthesis31,32. Among the listed methods, hydrothermal synthesis is most commonly used to synthesize nanoparticles33–35. Hydrothermal synthesis is highly promising due to its simplicity, cost-effectiveness, and the ability to vary process parameters over a wide range, including synthesis temperature, solution pH, and the type of mineralizer used. In the hydrothermal method, the size (from nanometers to micrometers), shape of the particles, and their phase composition can be controlled. Many studies have reported the preparation of ZrO₂ nanoparticles using the hydrothermal method, achieving sizes ranging from a few to hundreds of nanometers with a well-controlled size distribution27,36,37.

In the hydrothermal synthesis of ZrO2 nanoparticles, several types of precursors are used. For example, Moshen et al.33 used ZrOCl2-8H2O as the raw material and varied the pH of the suspension by changing the content of NaOH mineralizer, resulting in ZrO2 nanoparticles with partial t-ZrO2 content. In addition, Shizhong Wang et al.38 also obtained ZrO2 nanoparticles with a partial content of metastable t-ZrO2 by supercritical hydrothermal synthesis using ZrO(NO3)2-2H2O as a precursor. The fabrication of ZrO2 nanoparticles by hydrothermal synthesis is mainly conducted at temperatures in the range of 160–250 ℃28,33,36,39,40. Reducing the process temperature in hydrothermal synthesis could be beneficial due to the lower production costs of nanoparticles.

In this work, the structure, morphology, temperature stability, and optical properties of ZrO2 nanoparticles prepared by hydrothermal synthesis in the temperature range of 110–160 °C were studied. This range was chosen to investigate the possibility of lowering the synthesis temperatures of stabilized zirconium dioxide nanoparticles. A combination of X-ray diffraction and Raman spectroscopy techniques was used to study the crystal structure. The optical properties of the synthesized particles were studied for potential applications in ultraviolet absorption. The thermal stability of the particles was investigated to assess their potential use as a reinforcing additive in ceramics, such as zirconia toughened alumina (ZTA)27. Information on the temperature stability of the reinforcing particles is crucial to prevent issues such as unwanted phase transformations during the sintering of ceramic components. This is especially important for ZrO2 particles due to the martensitic transition from t-ZrO₂ to c-ZrO₂, which is accompanied by a significant change in the lattice volume. These changes induce critical mechanical stresses in the ceramic microstructure, leading even to cracking of the sintered compacts. Thus, this study aims to provide essential data for the future application of ZrO2 nanoparticles synthesized via the hydrothermal method.

Methods and materials

ZrO2 nanoparticles were prepared using a hydrothermal synthesis method in a steel autoclave with a Teflon liner. The autoclave reactor volume was 25 mL, and the reactor was 90% full during synthesis. Zirconyl chloride octahydrate (ZrOCl2·8H2O) (Sigma Aldrich, Saint Louis, USA) and zirconyl nitrate dihydrate (ZrO(NO3)2·2H2O) (Sigma Aldrich, Saint Louis, USA) were used as precursors. For each of the precursors, 17.5 mL of 0.1 mol/L zirconium salt solution in distilled water was prepared. To prevent the formation of particle conglomerates during synthesis, 0.14 g of polyethylene glycol 6000 (PEG 6000) was added to the solution. A 5 ml solution of 10 mol/L NaOH in distilled water was used as the mineralizer. The hydrothermal synthesis was conducted in the IKA OVEN 125 basic dry drying oven (IKA-Werke GmbH & CO. KG, Staufen, Germany) at temperatures ranging from 110 to 160 ℃, for 12 h under autogenous water vapor pressure. The reactor was cooled under atmospheric conditions in a turned-off drying oven. After synthesis, the particles were processed through centrifugation in distilled water and ethanol, followed by boiling in ethanol. Obtained particles were then filtered and dried in an oven at 50 °C.

X-ray diffraction analysis was performed using X-ray diffraction on a Bruker D8-advance diffractometer (Bruker GmbH, Mannheim, Germany) with Cu-Kα radiation in Bragg–Brentano geometry. The size of the coherent scattering region (CSR) was calculated for the (111) reflections of the monoclinic and cubic (or (101) for tetragonal) phases of ZrO2. These reflections were initially approximated using the Pseudo–Voigt function with the Scherrer equation41:

graphic file with name M1.gif 1

where Inline graphic is the width at half maximum (FWHM) for each of the reflections and 0.9 is a coefficient accounting for particle shape.

The fraction of monoclinic phase in the experimental samples was calculated using42:

graphic file with name M3.gif 2

where

graphic file with name M4.gif 3

where Inline graphic and Inline graphic are intensities of the corresponding reflections on the diffractogram.

The phase composition of the synthesized particles was also examined using Raman spectroscopy on an Enspectr M532 spectrometer (Spectr-M LLC, Chernogolovka, Russia) with a laser wavelength of 532 nm. The size and morphology of the obtained ZrO2 particles were investigated using transmission electron microscopy (TEM) with a Jeol JEM—1400Plus transmission electron microscope (Jeol, Tokyo, Japan). Mass loss thermograms during heating were recorded using thermogravimetric analysis (TGA) with a Themys One (Setaram KEP Technology, Caluire, France). TGA was conducted in an argon atmosphere with a partial pressure of 0.5 Pa. The samples were heated from room temperature to 1000 ℃ at a heating rate of 10 ℃/min for each sample. The optical properties of the synthesized ZrO2 nanoparticles were investigated by UV–visible spectroscopy on a SPECORD 200/210/250 PLUS (Analytik Jena, Jena, Germany). The energy band gap (Eg) values were estimated by generating Tauc plots43:

graphic file with name M7.gif 4

where C is a constant, Inline graphic is absorbance, and Inline graphic is photon energy.

The study of phase stability of ZrO2 particles was performed by annealing at temperatures in the range of 100–1000 ℃ in the Nabertherm LHT 08/18 (Nabertherm GmbH, Lilienthal, Germany) furnace for 2 h for each cycle with a heating rate of 10 ℃/min with subsequent evaluation of diffractograms using expressions (2) and (3).

Results and discussion

Figure 1 shows X-ray diffraction patterns for ZrO2 particles obtained at different temperatures by hydrothermal synthesis. The presented data indicate that, regardless of the precursor used, ZrO₂ particles synthesized at temperatures of 110–130 ℃ contain exclusively the c-ZrO₂ phase (PDF 01-071-6425) with space group Fm-3m (225) or the t-ZrO₂ phase (PDF 00-050-1089) with space group P42/nmc (137). Due to the structural similarities and the close proximity of the main reflections in the X-ray diffraction patterns, it is challenging to accurately distinguish between the t-ZrO₂ and c-ZrO₂ phases in the samples. With further increase of the synthesis temperature, peaks characteristic of m-ZrO2 (PDF 00-065-0687) space group P21/a(14) appear on the diffractograms. As the synthesis temperature rises, the activity within the reactor intensifies, leading to the formation of m-ZrO₂. This phase transition is driven by a reduction in surface energy, which occurs as the average particle size exceeds the critical range of 10–20 nm.

Fig. 1.

Fig. 1

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X-ray diffractograms of ZrO2 particles obtained using hydrothermal synthesis from (a) ZrOCl2·8H2O and (b) ZrO(NO3)2·2H2O precursors at different temperatures.

Analysis of the average crystallite sizes for c (or t)—ZrO2 phases (Fig. 2), calculated using the Scherrer equation, shows that the average value dc (or t) does not change when varying the synthesis parameters and ranges from 5 to 6 nm. At the same time, the dm values for m-ZrO2 are in the range of 17–32 nm, surpassing the critical size threshold for the existence of the cubic (or tetragonal) phase of ZrO2. The data obtained suggest that the monoclinic phase forms when the critical particle size of ZrO₂ is exceeded, triggering the c (t) → m phase transition.It is worth noting that for the samples produced from ZrOCl2·8H2O, the formation of m-ZrO2 occurs at a lower synthesis temperature, which can be explained by the fact that zirconyl chloride has a lower degree of hydrolysis, which is correlated with a higher degree of dissociation of hydrochloric compounds, compared to nitrate compounds44. As a result, ZrO₂ particles synthesized from ZrOCl₂·8H₂O at temperatures above 120 °C undergo sufficient growth, leading to a partial phase transition from c (or t)-ZrO₂ to m-ZrO₂.

Fig. 2.

Fig. 2

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Dependence of m-ZrO2 phase content and d(11–1) for m-ZrO2 nanoparticles, prepared from (a) ZrOCl2·8H2O and (b) ZrO(NO3)2·2H2O on the hydrothermal synthesis temperature.

Figures 3 and 4 show TEM images of synthesized ZrO2 samples, from which it is clear that the obtained samples are spherical-shaped nanoparticles of 5–10 nm in size, forming larger conglomerates. The analysis of particle size distribution histograms reveals that as the synthesis temperature increases, the proportion of particles larger than 10 nm rises, with some particles eventually exceeding 100 nm (not shown in Figs. 3 and 4). This observation is consistent with the results calculated using the Debye–Scherrer equation.

Fig. 3.

Fig. 3

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TEM images of ZrO2 particles obtained using ZrOCl2·8H2O by hydrothermal synthesis at temperatures (a–f) 110–160 °C.

Fig. 4.

Fig. 4

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TEM images of ZrO2 particles obtained using ZrO(NO3)2·2H2O by hydrothermal synthesis at temperatures (a–f) 110–160 °C.

Figure 5 demonstrates the change in particle shape from spherical to elongated "rod-like" for powders synthesized at 160 °C. The presence of cubic (or tetragonal) phase at high synthesis temperature is attributed to the preservation of some fraction of particles smaller than the critical size, which is confirmed by TEM images (Fig. 5b,d).

Fig. 5.

Fig. 5

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TEM images of ZrO2 particles, obtained using (a, b) ZrOCl2·8H2O and (c, d) ZrO(NO3)2·2H2O at T = 160 °C.

Raman spectroscopy is capable of distinguishing between the cubic and tetragonal phases of ZrO2. Figure 6 illustrates the Raman spectra of the samples obtained using ZrOCl₂·8H₂O (6a) and ZrO(NO₃)₂·2H₂O (6b) as precursors at different temperatures. Samples synthesized at temperatures below 150 °C exhibit a broad peak around 600 cm⁻1, which corresponds to the F₂g vibrational mode of cubic ZrO₂45. The modes at 273, 716, and 807 cm-1 characteristic of t-ZrO224 are observed only for the sample synthesized at 140 ℃ for ZrOCl2·8H2O precursor. The presence of modes characteristic of the tetragonal phase may indicate the co-existence of t- and c-phases in the synthesized powders. From the analysis of Raman spectroscopy results, it can be stated that the particles contain predominantly c-ZrO2 phase with a minor content of t-ZrO2. When the synthesis temperature is above 130 ℃, the spectra show modes at 178, 190, 221, 308, 332, 346, 382, 477, 503, 538, 558, 612, and 630 cм−1 indicative of m-ZrO245, which is consistent with the X-ray diffraction results.

Fig. 6.

Fig. 6

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Raman spectra of ZrO2 particles prepared using (a) ZrOCl2·8H2O and (b) ZrO(NO3)2·2H2O by hydrothermal synthesis at different temperatures.

Table 1 presents the data on the chemical composition of the samples obtained from different starting materials. As can be seen from the presented data, in addition to zirconium and oxygen, the chemical composition contains residues of synthesis products such as sodium, chlorine and nitrogen, which indicates the presence of residues of synthesis products.

Table 1.

Comparison of chemical compositions of obtained samples after hydrothermal synthesis.

Tsynthesis (°C) Starting material Zr, at. % O, at. % Na, at. % Cl, at. % N, at. %
110 ZrOCl2·8H2O 24.9 65.7 5.5 3.9 –
ZrO(NO3)2·2H2O 26.4 64.1 5.4 – 5.1
120 ZrOCl2·8H2O 29.7 59.9 5.8 4.6 –
ZrO(NO3)2·2H2O 29.6 61.8 4.1 – 4.5
130 ZrOCl2·8H2O 28.2 62.8 5.2 3.8 –
ZrO(NO3)2·2H2O 29.2 60.3 4.7 – 5.8
140 ZrOCl2·8H2O 28.6 61.5 4.4 5.5 –
ZrO(NO3)2·2H2O 27.8 62.0 4.8 – 5.4
150 ZrOCl2·8H2O 26.6 63.7 6.5 3.2 –
ZrO(NO3)2·2H2O 30.4 61.3 4.5 – 3.8
160 ZrOCl2·8H2O 29.6 61.7 4.4 4.3 –
ZrO(NO3)2·2H2O 30.9 60.2 5.2 – 3.7
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When using ZrO2 nanoparticles produced by hydrothermal synthesis as a precursor in the synthesis of high-density ceramics or as a reinforcing component, it is necessary to first remove physiosorbed and chemisorbed water as well as residual synthesis products (such as PEG and Na, Cl, N-containing compounds). Since the residual byproducts will be vaporized after pressing the green powder and subsequent sintering, the resulting volatile gases can create pores in the ceramics. TGA tests were conducted to investigate the mass loss and processes occurring to the particles during temperature exposure. The results for the sample obtained at 130 ℃ using ZrOCl2·8H2O and ZrO(NO3)2·2H2O are shown in Fig. 7 (these curves were chosen as representative; similar curve trends were observed for the other samples). The curve can be divided into four distinct regions: (1) 100–200 °C, where the evaporation of physisorbed water from the particle surface occurs and decomposition of some residual synthesis products (such as NO3) is possible; (2) 200–400 °C, which corresponds to the removal of chemisorbed water; (3) 400–600 °C, associated with the decomposition of hydroxyl (–OH) bonds; and (4) temperatures above 600 °C, where the decomposition of chemical compounds containing sodium, such as NaCl or NaNO₃, is observed.29,33,46. For all experimental samples, heating the particles to 1000 °C results in a mass loss of 14–17% of the initial weight, which is attributed to the removal of physisorbed and chemisorbed water, as well as –OH groups.

Fig. 7.

Fig. 7

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TGA curves for samples synthesized at 130 ℃ using (a) ZrOCl2·8H2O and (b) ZrO(NO3)2·2H2O.

Figure 8 demonstrates the absorption spectra and Tauc plots of the energy band gap (Eg). For ZrO2 nanoparticles obtained using ZrOCl2·8H2O and synthesis temperatures of 110–130 °C, a more pronounced absorption ability in the mid-UV range of 200–300 nm can be observed compared to other temperatures, as can be seen from the spectra of Fig. 8a. Table 2 presents the calculated energy bandgap Eg for ZrO2 nanoparticles using Eq. (3). The Eg values for all samples are in the range of 4.89–5.09, which is in good agreement with the literature data47. No clear dependences between Eg values and the parameters of hydrothermal synthesis were found. A possible explanation for this fact could be the presence of chemisorbed and physisorbed water in the powders produced by hydrothermal synthesis. In general, an analysis of the obtained absorption and transmission spectra (insets to Fig. 8a,c) in the visible and UV range reveals that the synthesized particles exhibit significant absorption at wavelengths between 200 and 260 nm.

Fig. 8.

Fig. 8

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Absorbance spectra and Tauc plots for samples obtained at different temperatures by hydrothermal synthesis using (a, b) ZrOCl2·8H2O and (c, d) ZrO(NO3)2·2H2O, respectively. Insets display transmission spectra in the wavelength range of 200–300 nm.

Table 2.

Eg values for ZrO2 particles obtained using different precursors.

Eg (eV)
110, °C 120, °C 130, °C 140, °C 150, °C 160, °C
ZrOCl2·8H2O 5.08 5.07 5.09 4.91 4.89 5.03
ZrO(NO3)2·2H2O 5.07 4.97 4.82 4.99 4.97 5.09
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To study the phase stability of the ZrO2 particles, all samples were annealed at temperatures from 100 to 1000 ℃ for 2 h. Figure 9 depicts the cubic phase content percentage as a function of temperature. The data indicate that the phase composition of all synthesized particles remains stable between 100 and 500 °C. Beyond this range, however, there is a marked decrease in the cubic phase content and a corresponding increase in the monoclinic phase content. As the temperature rises above 500 ℃, particle growth and sintering of conglomerates occur, leading to the following phase transformation c-ZrO2 → m-ZrO2. The average crystallite size calculated using the Scherrer equation (Fig. 2) confirms the presence of particle size growth processes driven by surface energy minimization and the sintering of nanoparticle conglomerates.

Fig. 9.

Fig. 9

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Percentage of cubic phase content obtained from (a) ZrOCl2-8H2O and (b) ZrO(NO3)2-2H2O at different temperatures as a function of annealing temperature.

Thus, the high-temperature c-ZrO₂ phase remains stable up to 600 °C. When temperature exceeds 600 ℃, a rapid phase transition from c-ZrO2 to m-ZrO2 occurs. The mechanism behind this transition is associated with the sintering of contacting nanoparticles and the reduction of surface energy. In the fabrication of ZrO2-c reinforced bulk ceramics, the temperature treatment of the precursor powders should not be higher than 600 °C and good mixing of the components should be ensured to avoid aggregation of zirconia particles. Significant particle aggregation is likely to trigger the c → m transition, which diminishes the hardening effect and undermines the improvement of mechanical properties. It was also determined that the synthesized nanoparticles can be used as materials for absorbing mid-UV light (wavelength 200–260 nm).

Conclusions

In this study, ZrO₂ nanoparticles were synthesized using the hydrothermal method with various precursors and at different temperatures. X-ray diffraction revealed that at synthesis temperatures of 110–130 ℃ the obtained particles consisted exclusively of the c (t)-ZrO2 phase with an average crystallite size of 5 nm. Further increase in the synthesis temperature resulted in larger crystallite sizes, ranging from 10 to 30 nm, which consequently induced the phase transition from c-ZrO₂ to m-ZrO₂. Investigations performed using TEM confirmed that the particle size for samples containing only c (t)-ZrO2 phase was in the range of 4–10 nm and increased up to 10–200 nm along with the increase in the fraction of m-ZrO2 phase in the sample. Raman spectroscopy determined that at synthesis temperatures of 110–130 °C, the nanoparticles contained the c-ZrO₂ phase, with its fraction decreasing as the synthesis temperature increased, which is consistent with the X-ray diffraction results. TGA tests indicated that the obtained ZrO2 nanoparticles contained 14–17% of water, –OH groups and residual synthesis products (PEG and Na-containing compounds). Optical spectroscopy data demonstrated that ZrO₂ nanoparticles are highly effective at absorbing radiation in the mid-UV range (200–260 nm) and have an energy band gap of 4.8–5.1 eV. Thermal exposure tests confirmed that all samples remain stable within the temperature range of 100–500 °C. When the temperature exceeds 500 °C, the fraction of the monoclinic phase in the samples increases rapidly, which is associated with particle growth due to sintering. The synthesized nanopowders can be used as UV radiation absorbers and as hardening additives in technical ceramics.

Author contributions

Conceptualization, Y.G, R.S., N.V., D.S., M.Z., D.B. and A.K.; methodology, Y.G., R.S., N.V., D.S., M.Z., D.B. and A.K.; formal analysis, Y.G., R.S., N.V., D.S., M.Z., D.B. and A.K.; investigation, Y.G., R.S., N.V., D.S., M.Z., D.B. and A.K.; resources, A.K.; writing—original draft preparation, review, and editing, Y.G, R.S. and N.V.; visualization, A.K. and Y.G.; supervision, A.K.

Funding

This research was funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (No. BR21882390).

Data availability

Data sets generated during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Associated Data

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Data Availability Statement

Data sets generated during the current study are available from the corresponding author on reasonable request.

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