Synthesis of ZrO 2 nanoparticles by hydrothermal treatmentSiti Machmudah, W. Widiyastuti, Okky Putri Prastuti, Tantular Nurtono, Sugeng Winardi, Wahyudiono, Hideki

Kanda, and Motonobu Goto Citation: AIP Conference Proceedings 1586, 166 (2014); doi: 10.1063/1.4866753 View online: http://dx.doi.org/10.1063/1.4866753 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1586?ver=pdfcov Published by the AIP Publishing

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Synthesis of ZrO2 Nanoparticles by Hydrothermal Treatment

Siti Machmudah1*, W. Widiyastuti1, Okky Putri Prastuti1, Tantular Nurtono1, Sugeng Winardi1, Wahyudiono2, Hideki Kanda2 and Motonobu Goto2

1Chemical Engineering Department, Sepuluh Nopember Institute of Technology, Surabaya 60111, INDONESIA 2Department of Chemical Engineering, Nagoya University, Nagoya 464-8603, JAPAN

*Email: machmudah@chem-eng.its.ac.id

Abstract. Zirconium oxide (zirconia, ZrO2) is the most common material used for electrolyte of solid oxide fuel cells (SOFCs). Zirconia has attracted attention for applications in optical coatings, buffer layers for growing superconductors, thermal-shield, corrosion resistant coatings, ionic conductors, and oxygen sensors, and for potential applications including transparent optical devices and electrochemical capacitor electrodes, fuel cells, catalysts, and advanced ceramics. In this work, zirconia particles were synthesized from ZrCl4 precursor with hydrothermal treatment in a batch reactor. Hydrothermal treatment may allow obtaining nanoparticles and sintered materials with controlled chemical and structural characteristics. Hydrothermal treatment was carried out at temperatures of 150 – 200oC with precursor concentration of 0.1 – 0.5 M. Zirconia particles obtained from this treatment were analyzed by using SEM, PSD and XRD to characterize the morphology, particle size distribution, and crystallinity, respectively. Based on the analysis, the size of zirconia particles were around 200 nm and it became smaller with decreasing precursor concentration. The increasing temperature caused the particles formed having uniform size. Zirconia particles formed by hydrothermal treatment were monoclinic, tetragonal and cubic crystal.

Keywords: Zirconia, Nanoparticles, Synthesis, Hydrothermal. PACS: 81.20.-n

INTRODUCTION

Solid oxide fuel cells (SOFCs), particularly zirconia fuel cell devices, are electrochemical conversion devices that produce electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFCs have solid oxide or ceramic, electrolyte. Advantages of this class of fuel cells include high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. Zirconium oxide (zirconia, ZrO2) is the most common material used for electrolyte of SOFCs.

Zirconia has attracted attention for applications in optical coatings, buffer layers for growing superconductors, thermal-shield, corrosion resistant coatings, ionic conductors, and oxygen sensors, and for potential applications including transparent optical devices and electrochemical capacitor electrodes, fuel cells, catalysts, and advanced ceramics. It is known that the use of nanostructured zirconia can improve some of these technologies, and stable colloidal nanoparticles of this oxide have been considered to be used in the production of polymer nanocomposites with strong influence on the properties of the final materials. It is a wideband semiconductor, with bandgap in the range of 5–7 eV, which depends on the

phase. Nano-sized zirconia has specific optical and electrical properties as well as prospective applications in transparent optical devices, electrochemical capacitor electrodes, oxygen sensors, fuel cells, catalyst and advanced ceramics. It has been recently investigated as an intermediate biomaterial coating. One important area is the coating of solid surfaces with nanostructured zirconia by surface modification which would improve the biocompatibility of ceramic coatings [1, 2].

In recent years, with the increasing awareness of both environmental safety and the need for optimal energy utilization, there is a case for the development of nonhazardous materials. These materials should not only be compatible with human life but also with other living forms or species. Also, processing methods such as fabrication, manipulation, treatment, reuse, and recycling of waste materials should be environmentally friendly. In this respect, the hydrothermal technique occupies a unique place in modern science and technology. In the last decade, the hydrothermal technique has offered several new advantages like homogeneous precipitation using metal chelates under hydrothermal conditions, decomposition of hazardous and/or refractory chemical substances, monomerization of high

5th Nanoscience and Nanotechnology Symposium (NNS2013)AIP Conf. Proc. 1586, 166-172 (2014); doi: 10.1063/1.4866753

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polymers like polyethylene terephthalate, and a host of other environmental engineering and chemical engineering issues dealing with recycling of rubbers and plastics (instead of burning), and so on. The solvation properties of supercritical solvents are being extensively used for detoxifying organic and pharmaceutical wastes and also to replace toxic solvents commonly used for chemical synthesis [3].

Several techniques are available for producing zirconia nanoparticles, such as sol/gel method [4], vapor phase method [5, 6], pyrolysis [1], spray pyrolysis [7], hydrolysis [8], hydrothermal [9], and microwave plasma [10]. Hydrothermal synthesis was reported to be a soft chemical route with important advantages due to low crystallization temperatures enabling directly production of nanocrystalline powders without any firing step and high degree of precipitation reducing the content of metal ions in effluents with respect to environmental regulations. Industrial applications were restricted mainly due to the difficulties in controlling the chemical reactions during synthesis and avoid agglomeration.

In this research, hydrothermal synthesize was used to fabricate zirconia nanoparticles because its mild process may allow obtaining of nanoparticles and sintered materials with controlled chemical and structural characteristics. Formation of zirconia micro-porous particles was also investigated. The synthesis was carried out in a batch reactor at various temperatures, precursor concentrations and time of reactions. ZrO2 was used as model material.

EXPERIMENT

Materials

The starting material was commercial ZrCl4 (90%, Merck). Aquadest was used as solvent, and polystyrene was used as template to synthesize zirconia microparticles.

Hydrothermal Treatment

Zirconia particles were synthesized using hydrothermal process in a batch reactor. An autoclave reactor made of stainless steel with Teflon container and 10 mL volume was used. Precursor solution with concentration 0f 0.1 to 0.5 M was loaded into the Teflon container and placed in an autoclave reactor, sealed, and then heated up at certain temperature (150 and 180oC) for several hours (12 h). The reaction was stopped by quenching into cold water. The synthesized particles were then washed with distillated water or acetone for 3 times, dried at 60oC for 6 h, and

calcinated at 600oC for 6 h. The products were then stored prior to analysis.

In order to synthesize zirconia micro-porous particles, synthesized zirconia particles were mixed with polystyrene solution as template, and treated at hydrothermal condition (150 and 180oC) for 3 h. The colloid product was then dried at 60oC for 6 h and calcinated at 600oC for 6 h.

Characterization of Particles

Particle size and shape were determined from an acetone suspension, after ultrasonic treatment, by counting particles on scanning electron microscope (SEM) pictures using ImageJ. Calcinated particles were identified by X-ray diffraction (XRD) to identify crystalline phases. Detection and weight fraction of crystalline phases were determined from peak intensities at the following spacings (in nm) and angles respectively: monoclinic (0.316, 28), (0.220, 41), (0.202, 44.8); tetragonal and/or cubic (0.295, 30.2); tetragonal (0.213, 43).

RESULTS AND DISCUSSIONS

Synthesis of Zirconia Particles

Zirconia particles were synthesized at various concentrations of 0.1, 0.3, and 0.5 M, constant temperature of 150oC for 12 h. The synthesized particles were then washed with distillated water, dried and calcinated. SEM image of the particles are shown in Figure 1 (a), (b), and (c), respectively. Based on the SEM image in Figure 5.1, the shape of synthesized zirconia is spherical particles with the average sizes of 189, 269, and 292 nm for concentration of 0.1, 0.3, and 0.5 M, respectively. The particle size increased with an increase in concentration. The increasing precursor concentration might cause an increasing particle growth [9]. It also can be seen that at high concentration, particles can be separated. On the contrary, low concentration caused particles agglomeration.

In order to understand the effect of washing on the particles morphology, zirconia was synthesized at temperature of 150oC, concentrations 0.3 M for 12 h. Figures 2(a) and (b) show the SEM image of particles with and without washing, respectively.

The effect of washing on the morphology of particles was not clearly occurred. The particles sizes for both conditions were almost similar. For instance, the particles sizes with and without washing are 113 nm and 116 nm, respectively. However, particles could be separated each other with washing.

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(a)

(b)

(c)

FIGURE 1. SEM image of synthesized zirconia particles at temperature of 150oC and reaction time of 12 h for various concentrations. (a) 0.1 M; (b) 0.3 M; (c) 0.5 M.

The effect of calcination on the particles

morphology was also studied at temperature of 150oC and concentrations of 0.3 M for 12 h. Figures 3(a) and

(b) show SEM image of particles with and without calcination, respectively.

The calcination could assist the observation of particle morphology and avoid the agglomeration. The particles size was not affected by the calcination.

(a)

(b)

FIGURE 2. SEM image of synthesized zirconia particles at temperature of 150oC, concentration of 0.3 M, and reaction time of 12 h for the effect of washing. (a) with washing; (b) without washing.

Synthesis of Zirconia Micro-porous Particles

In order to increase surface area of zirconia particles, synthesis of zirconia micro-porous particles was carried out at 150 and 180oC with addition of polystyrene as a template. In general the mechanism of zirconia micro-porous particles formation can be explained as follow. Synthesized zirconia particles explained in the previous section are added in the polystyrene template. Zirconia particles may be

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arranged on the surface of polystyrene particles, where the size of polystyrene particles is bigger than the size of zirconia particles. During evaporation, the solvent of zirconia particles is removed; and then polystyrene is removed under the calcination process. As the result, zirconia micro-porous particles are formed.

(a)

(b)

FIGURE 3. SEM image of synthesized zirconia particles at temperature of 150oC, concentration of 0.3 M, and reaction time of 12 h for the effect of calcination. (a) with calcination; (b) without calcination.

Figures 4 and 5 show the effect of concentration on

the particles morphology at 150 and 180oC, respectively. For 150oC, the increasing concentration could promote micro-porous particles formation. However, the homogeneous micro-porous particles have not been formed yet. At 0.1 M precusor concentration, micro-porous particles could not be formed. On the other hand, for 180oC, decreasing precursor concentration could form the micro-porous particles. At 0.5 M precursor concentration, zirconia particles formed became spherical one.

(a)

(b)

(c)

FIGURE 4. SEM image of synthesized zirconia micro-porous particles at temperature of 150oC and various concentrations of (a) 0.1 M, (b) 0.3 M, and (c) 0.5 M.

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(a)

(b)

(c)

FIGURE 5. SEM image of synthesized zirconia micro-porous particles at temperature of 180oC and various concentrations of (a) 0.1 M, (b) 0.3 M, and (c) 0.5 M.

(a)

(b)

(c)

FIGURE 6. SEM image of synthesized zirconia micro-porous particles at temperature of 150oC and concentration of 0.3 M for various reaction times of (a) 12 h, (b) 18 h, and (c) 24 h.

The effect of reaction time on the morphology of

zirconia micro-porous particles was studied at 150 and

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0.3 M for 12, 18, and 24 h. The SEM image of synthesized zirconia particles is shown in Figure 6. As shown in Figure 6 the shorter reaction time caused the broken micro-porous particles. On the contrary, longer reaction time promoted faster particles growth.

FIGURE 7. XRD patterns of synthesized zirconia micro-porous particles at various temperatures and reaction times. (a) 12 h, 150oC; (b) 12 h, 180oC; (c) 12 h, 200oC; (d) 18 h, 150oC; (e) 18 h, 180oC; (f) 24 h, 150oC; (g) 24 h, 180oC; (h) 24 h, 200oC.

Figure 7 shows XRD patterns of synthesized

zirconia micro-porous particles at various temperatures

of 150, 180, and 200oC for reaction times of 12, 18 and 24 h. As shown in the figure, crystals were formed at all reaction conditions. The crystals formed were in monoclinic, cubic, and tetragonal phase.

The amount of crystal phases and the diameter size of crystal at various temperatures and reaction times are listed in Table 1. As shown in the table, the optimum crystallinity is obtained at 180oC and 12 h reaction time. It can be seen in Figure 7 that the highest intensity was observed at that condition. Moreover, the biggest crystal diameter was also obtained at that condition (Table 1).

In Table 1, the amount of crystal phase was influenced by the temperature. The higher the temperature, the fewer the monoclinic phase was formed. Conversely, the higher the temperature, the larger the tetragonal and cubic phases were formed.

CONCLUSION

Zirconia particles with micro-porous morphology were successfully synthesized using hydrothermal treatment at various precursor concentrations, temperatures and reaction times. The increasing precursor concentration caused an increasing particle growth, and resulted in the increasing particle size. The calcination could assist the observation of particle morphology and avoid the agglomeration. The particles size was not affected by the washing and calcination. For the formation of zirconia micro-porous particles, the shorter reaction time caused the broken micro-porous particles. On the contrary, longer reaction time promoted particles growth. The optimum crystallinity was obtained at 180oC and 12 h of reaction time. The crystals formed were in monoclinic, tetragonal, and cubic phase.

TABLE 1. Crystal phases and crystal diameter of zirconia micro-porous particles

Variable Monoclinic (%) Tetragonal (%) Cubic (%) Diameter (nm) 12 h, 150oC 67 17 16 48.96 12 h, 180oC 59 19 22 48.97 12 h, 200oC 52 29 19 18.84 18 h, 150oC 66 20 14 34.98 18 h, 180oC 93.9 6.1 0 40.81 24 h, 150oC 71.3 14.9 13.8 15.30 24 h, 180oC 63 13 24 48.95 24 h, 200oC 54 29 17 30.61

ACKNOWLEDGMENTS

This work was financially supported by Directorate General of Higher Education, Ministry of Education

and Art of Indonesia through a research grant Desentralisasi – Penelitian Unggulan Perguruan Tinggi contract no. 013674.178/IT2.7/PN.08.01/2013.

Monoclinic

Tetragonal

Cubic

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

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