Pure Tetragonal ZrO2 Nanoparticles Synthesized by Pulsed Plasma in

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Pure Tetragonal ZrO2 Nanoparticles Synthesized by Pulsed Plasma in Liquid Liliang Chen,† Tsutomo Mashimo,*,‡ Emil Omurzak,§ Hiroki Okudera,|| Chihiro Iwamoto,z and Akira Yoshiasa# †

Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan Shock Wave and Condensed Matter Research Center, Kumamoto University, Kumamoto 860-8555, Japan § Priority Organization for Innovation and Excellence, Kumamoto University, Kumamoto 860-8555, Japan Faculty of Natural System, Institute of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan z Faculty of Engineering, Kumamoto University, Kumamoto 860-8555, Japan # Faculty of Science, Kumamoto University, Kumamoto 860-8555, Japan

)

‡

ABSTRACT: Pure tetragonal phase ZrO2 (t-ZrO2) nanoparticles of sizes smaller than 5 nm were synthesized by a pulsed plasma in liquid (PPL) method, a one-step synthesis method: in an ammonia solution with no use of dopants or annealing for stabilization. Lattice parameters of the obtained samples refined by the Rietveld method revealed structural differences compared with the nanoparticles obtained by other methods such as the solgel method. The intrinsic properties of the pulsed plasma in liquid method and the alkaline environment (OH) may be responsible for the stabilization of tetragonal zirconia, while the oxygen deficiency level checked by X-ray Absorption Near Edge Structure (XANES) is smaller than 1%, leading to light gray color of samples and a limited stabilization effect on the tetragonal phase. The particles displayed a higher UV absorption property than a commercial stabilized zirconia sample. Also, the emission lines of Zr I atoms and Zr II ions were observed by an optical emission spectrum to gather information on the synthesis mechanism.

’ INTRODUCTION Zirconium dioxide (ZrO2) nanoparticles have been subject to a great deal of research because of their many useful applications, including as electrolyte materials for solid oxide fuel cells,1 hard materials,2 and thermal barrier coating materials.3 Tetragonal ZrO2 (t-ZrO2) holds promise for such materials for toughening and as a catalyst. Recently, much attention has also been drawn to their photocatalytic properties as the particles were found to be potential catalysts or catalyst supports for photodecomposition of water and photoreduction of CO2 due to their wide band gap and other specific properties.4,5 Pure zirconia exhibits three polymorphs: monoclinic (below 1170 C), tetragonal (between 1170 and 2370 C), and cubic (above 2370 C). Stabilization of the tetragonal phase, the most active catalyst among the zirconium oxide phases,6 is crucial since the transition from the tetragonal to the monoclinic phase results in deactivation of the catalyst. Thus, tetragonal phase particles are generally stabilized by doping with cubic oxides such as Y2O3, MgO, CaO, or CeO2.7 Pure t-ZrO2 is very difficult to synthesize without a stabilizer. However, some cases of pure tetragonal zirconia nanoparticle synthesis without dopant use have been reported.813 This is possible due to the surface free energy of particles.14 Some experimental r 2011 American Chemical Society

evidence shows that the occurrence of metastable tetragonal zirconia is due to the so-called “critical crystallite size”.15 In other words, when the synthesized particle size is subcritical, or between 10 and 50 nm, tetragonal phase zirconia may be stable at room temperature without dopants1619 and does not enter the monoclinic phase. However, these reported methods for production of pure tetragonal zirconia particles all required initially a precursor from solution and then obtaining tetragonal powder by annealing in oven, a process which results in the sintering and growth of nanoparticles, thus probably deactivating the catalyst property. To eliminate the need for both dopants and annealing, a onestep synthesis method, pulsed plasma in liquid,20,21 was utilized for synthesis of pure tetragonal zirconia nanoparticles. Pulsed plasma appears from interelectrode space breakdown in high potential difference between two electrodes. This energy can evaporate, melt, and activate many kinds of metals which compose the rods, enabling their reaction with liquids. The short duration of the pulsed plasma— about several microseconds in our system—produces little heat, leading to smaller-sized nanoparticles; also, the solution quenches the Received: November 30, 2010 Revised: March 28, 2011 Published: April 26, 2011 9370

dx.doi.org/10.1021/jp111367k | J. Phys. Chem. C 2011, 115, 9370–9375

The Journal of Physical Chemistry C

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Figure 2. XRD powder pattern for the samples collected from the upper part (milky form) of the liquid after synthesis by pulsed plasma (a) in distilled water and (b) in hydrogen peroxide (30.0%).

Figure 1. Schematics of the pulsed plasma in liquid system with the graph of one pulse duration and the liquid after plasma synthesis and 12 h standing.

metastable phase of the nanoparticles. In this paper, we examine the synthesis of tetragonal ZrO2 nanoparticles by pulsed plasma in liquid.

’ EXPERIMENTAL SECTION Figure 1 shows a schematic of the pulsed plasma in liquid system with a graph of one pulse duration and the liquid resulting from plasma synthesis. The figure illustrates the two main components of our experimental setup: a low voltage alternating current (AC) electric power supply and a reactor including an anode, cathode, and sonic wave generator. Both the anode and cathode were rod shaped, 5 mm in diameter, and of 99.5% zirconium purity (Rare Metallic Co., Ltd.). They were submerged into three different dielectric solutions: distilled water, hydrogen peroxide (30.0%), and ammonia solution (28.0%), respectively. The cathode was set to vibrate in a frequency of 5000 vibrations per minute (vpm) and within a 01 mm gap between tips during the experiment. Continuous vibration of the cathode allowed maintenance of the required distance between the rods regardless of consumption of the rods’ zirconium. With a frequency of 60 Hz, a single-pulse duration of about 20 μs and near 200 A current applied between the two zirconium metallic electrodes (see Figure 1), pulsed plasma was observed between the tips of two rods. During the entirety of the synthesis process, the ultrasonic wave generator worked continuously on the prepared liquid with samples. After 1 h of reaction and 12 h of standing, the liquid divided into three layers: clear liquid, milky form liquid, and black deposit (see Figure 1). The light gray nanoparticles were carefully separated by a centrifuge from the white part in milky liquid form of the above three kinds of liquid; the unwanted particles, which had sunk to the bottom, were left. Crystal phases were determined by powder X-ray diffraction (RD: Rigaku RINT2000/PC) using Cu KR radiation (40 kV, 200 mA) and a laser Raman microspectrometer (HORIBA Jobin Yvon HR 800) with a wavelength of 514.53 nm. Crystal parameters were calculated by Rietveld refinement. Microstructural characterization of the nano ZrO2 particles was performed by means of High-Resolution Transmission Electron Microscopy (HRTEM). Specific surface areas were calculated by the BrunauerEmmettTeller (BET) method from the N2 adsorption isotherm measured at 80 K. The measurements of Zr K-edge

Figure 3. XRD powder pattern for the sample collected from the upper part (milky form) of the liquid after synthesis and the deposit by pulsed plasma in ammonia solution (28.0%).

Figure 4. Raman spectra of ZrO2 nanocrystals collected from the upper part of the liquid (milky form) by pulsed plasma in ammonia solution.

X-ray absorption fine structure (XAFS) were carried out in the transmission mode at beamline BL-9C of the Photon Factory in KEK, Tsukuba, Japan (Proposal No. 2009G049). The ringoperating condition was 2.5 GeV, and the synchrotron radiation was monochromatized by a Si(111) double crystal monochromator. UVvis spectra of the samples were taken by a JASCO V-550 UV/vis spectrometer, and the results were compared with a commercial stabilized tetragonal sample (300 mesh, 13 wt % CaO stabilized, Kojundo, Co. Ltd.). The optical emission spectrum was obtained by an SEC2000 UVvis spectrometer.

’ RESULTS AND DISCUSSION 1. Characterization. XRD analysis results of samples prepared in distilled water and hydrogen peroxide (30.0%) are depicted in Figure 2. The XRD powder pattern (Figure 2a) for the sample synthesized in water has ZrO2 monoclinic phase peaks at 2θ = 28.2, 31.5, 34.2, and 55.6 and tetragonal phase peaks at 2θ = 30.2, 9371

dx.doi.org/10.1021/jp111367k |J. Phys. Chem. C 2011, 115, 9370–9375


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Table 1. Structure Parameters of Tetragonal Zirconia by Pulsed Plasma in Liquid Using Tetragonal P42/nmc Space Groupa liquid type

ammonia solution

water

hydrogen peroxide

P42/nmc

P42/nmc

P42/nmc

a (Å)

3.6048(2)

3.6026(3)

3.5990(1)

c (Å)

5.1654(5)

5.1745(6)

5.1622(3)

tetragonality, c/(2a)1/2 volume (Å3)

1.0132 67.12

1.0156 67.16

1.0154 66.86

space group lattices parameter

Zr4þ ions positions

2a

2a

2a

x

0

0

0

y

0

0

0

z

0

0

0

Uiso

0.0058

0.0324

0.0386

O

2

ions

positions

4d

4d

4d

x y

0 0.5

0 0.5

0 0.5

z

0.201(1)

0.204(11)

0.207(4)

Uiso

0.0058

0.0324

0.0386

GOF

1.39

1.20

2.05

Rp

10.82

11.03

9.12

Rwp

13.57

14.39

11.86

profile R factors

a

Uiso is the mean square atomic vibration amplitude.

34.8, 35.3, 50.4, 50.7, 59.6, and 60.2, which are in agreement with the 65-1022 and 50-1089 standard card, respectively, from the JCPDS. Results for the sample obtained in hydrogen peroxide (Figure 2 b) are similar: a mixture of monoclinic and tetragonal phases. Figure 3 shows XRD data of the sample collected from the upper part of the liquid and the deposit by pulsed plasma in ammonia solution. The XRD of the upper part of the sample shows no peaks of monoclinic zirconia, indicating that pure tetragonal zirconia was synthesized, while XRD peaks of the deposit tell the existence of monoclinic zirconia and metal zirconium. Raman spectra of nanoparticles from the upper part of the sample in Figure 4 also confirmed that pure tetragonal phase zirconia rather than the cubic or monoclinic phase can be produced by pulsed plasma in ammonia solution. The Raman peaks at 263.9, 330.3, 471.2, and 641.3 cm1 can be attributed to tetragonal zirconia.22,23 Typical peaks at 278 cm1 and 607617 cm1 assigned to characteristic bands of cubic zirconia cannot be found,24,25 which may be due to the fact that the low heat produced by pulsed plasma in ammonia solution prevents a temperature too high for cubic phase synthesis during the experiment. Compared to the other two kinds of liquid above, the PH value of the ammonia solution is higher, and there exists the extra element, nitrogen. According to a study by Tahir et al., a high PH value is favorable for the synthesis of tetragonal zirconia, which exhibits higher symmetry than its monoclinic phase, in good agreement with our results.25,26 However, the nitrogen content calculated by elemental analysis is only 0.25%, indicating only a limited effect on the stabilization of tetragonal zirconia.

Figure 5. Rietveld refinement plot of tetragonal zirconia by pulsed plasma in ammonia solution using the tetragonal P42/nmc space group. Observed data are indicated by dots, and the calculated profile is indicated by a solid line. Short vertical bars below the pattern represent the positions of all possible Bragg reflections, and the line below the short vertical bars represents the difference between the observed and calculated patterns.

Rietveld refinement parameters of tetragonal structure zirconia by pulsed plasma in liquid are listed in Table 1. The crystallographic structure of tetragonal zirconia by pulsed plasma in ammonia solution is nicely refined using the tetragonal P42/nmc space group (see Figure 5). Observed data are indicated by dots, and the calculated profile is indicated by a solid line. Short vertical bars below the pattern represent the positions of all possible Bragg reflections, and the line below the short vertical bars represents the difference between the observed and calculated patterns. In Table 2 we summarize some previous parameter results2732 for the crystal structure of nondoped tetragonal zirconia to compare with the present sample from ammonia solution. The values of parameter a and crystal volume (V) as shown in Table 1 for the sample synthesized from ammonia solution are larger than those of other reference data (see Table 2) at room temperature prepared by such methods as solgel.27 The quenching effect by the surrounding cool liquid during plasma synthesis may be able to explain this phenomenon, as it could inhibit the crystal growth resulting in lattice expansion of nanocrystals.33 This can be confirmed by the position of oxygen ions in the lattice unit. The magnitude of the oxygen ion shift toward Æ001æ is the same as that of pure tetragonal zirconia at a high temperature of about 773 K.30 The opposite smallest tetragonality (c/(2a)1/2) of our sample compared to other data in the normal state may be due to the small size of nanoparticles as shown in Table 2, as enough small sizes of crystals were already proved to be able to change the phase from tetragonal to cubic, in which the parameter c reduces to equal parameter a.34,35 Compared with the results obtained at high pressure by Bouvier et al.,32 the values of tetragonality and parameter c of our sample are near the data of the ZrO2 sample obtained at several GPa. High-resolution TEM characterization of our sample obtained from ammonia solution (Figure 6) clearly shows entire particles with an average diameter smaller than 5 nm, in good conformance with the XRD results as estimated by the Scherrer equation. The values of 0.2920.293 nm shown in the image correspond to the d-spacing of lattice plane in the crystals of tetragonal ZrO2. These values are consistent with the calculated d-spacing of the (011) lattice plane of 0.295 nm derived by the 9372



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Table 2. Structural Parameters of Various Tetragonal Zirconia c/(2a)1/2

V (Å3)

5.1685(8)

1.016

66.85

5.1804(3)

1.019

66.92

5.1882(8)

1.020

method

a (Å)

c (Å)

zirconia annealed at 673 K, then test at room temperature

3.5964(3)


3.5941(3)


3.5954(3)

O2position

size (nm)

ref

-

12.5(6)

27

-

22.0(5)

27

67.07

-

22(3)

27

zirconia prepared by the alkoxide method

3.591(1)

5.169(1)

1.018

66.66

0.204(4)

16

28

zirconia (weight ratio for tetragonal to monoclinic 93:7)

3.5970(3)

5.1804(9)

1.018

67.03

-

-

29

zirconia test at 773 K

3.612(1)

5.212(1)

1.020

68.00

0.201(4)

-

30


3.626(2)

5.235(4)

1.021

68.83

0.199(10)

-

30

zirconia test at 1273 K zirconia test at 1473 K

3.642(1) 3.646(1)

5.275(1) 5.285(1)

1.024 1.025

69.97 70.26

0.198(5) 0.196(3)

-

30 30


3.64

5.27

1.024

69.83

0.185

-

31

zirconia test at 1 GPa

3.5948(2)

5.1824(7)

1.0194(2)

66.97

-

-

32

zirconia test at 10.9 GPa

3.565(2)

5.037(13)

0.999(4)

64.02

-

-

32


3.512(3)

4.988(8)

1.004(2)

61.52

-

-

32


3.495(1)

4.952(3)

1.002

60.49

-

-

32


3.484(1)

4.930(4)

1.000

59.84

-

-

32


3.478(2)

4.921(5)

1.000

59.53

-

-

32

Figure 8. Experimental Zr K-edge XANES spectra of pure standard monoclinic ZrO2, our ZrO2 nanoparticles from ammonia solution, and water by pulsed plasma, respectively.

Figure 6. High-resolution TEM image of ZrO2 nanocrystals grown in ammonia solution (28.0%) by pulsed plasma.

Figure 7. Schematic free-energy diagram of monoclinic and tetragonal ZrO2 nanoparticles.

Rietveld refinement results of XRD showed above. The studies conducted by Michael W. Pitcher36 and Alexandra Navrotsky37 note that enthalpy crossovers correspond sufficiently such that monoclinic zirconia, the stable bulk polymorph, is readily synthesized with particle size greater than 50 nm; tetragonal zirconia, with intermediate particle size; and amorphous material, with particles smaller than about 48 nm. Our sample size of less than 5 nm thus eases the stabilization of the tetragonal phase. Moreover, the surface area of particles obtained from ammonia

solution by the BET method, which was 108.15 m2/g, conforms the small size of particles and leads to high surface energy which can stabilize metastable phase zirconia without doping, a theory first put forth by Garvie and illustrated in Figure 7.14 Previous work has shown that normal pure ZrO2 samples, such as those synthesized by the solgel method, are white, whereas our plasma samples are light gray. This color is typical of oxygendeficient zirconia;38 since plasma in liquid may create a nearvacuum zone devoid of oxygen during synthesis, one can conclude that oxygen vacancies are created in the sample. Also, some literature has shown that oxygen vacancies proved able to stabilize the high-temperature phase.39 However, the X-ray Absorption Near Edge Structure (XANES) spectra for pure standard monoclinic ZrO2, ZrO2 nanoparticles from ammonia solution, and water, shown in Figure 8, indicated almost no comparable chemical shift at the Zr K-edge. The thresholds presented in this figure are 18.0039 keV for the standard monoclinic sample and 18.0036 and 18.0038 keV for our samples from water and ammonia solution, respectively. The calculated content of Zr3þ was smaller than 1% according to the usual chemical shift of 2050 eV between different valence states of elements. Because of the charge balance, the quantity of oxygen vacancies is the same, below 1%. Therefore, the effect of oxygen vacancy on the stabilization of tetragonal can be considered to be limited. 9373



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Figure 9. UVvis absorption spectra of ZrO2 nanocrystals grown in ammonia solution (28.0%) by pulsed plasma and commercial sample.

Figure 11. Schematic of the synthesis mechanism for zirconia particles synthesized by pulsed plasma in liquid.

Figure 10. Optical emission spectrum from pulsed plasma in ammonia solution (28.0%) with zirconium electrodes.

According to the above discussion, we can conclude that the small size due to the quenching effect by pulsed plasma in the liquid method and high PH value allow pure tetragonal ZrO2 samples to be synthesized by plasma in ammonia solution, while the stabilization effect by the extra element of nitrogen in ammonia solution and the oxygen vacancy may be negligible. Figure 9 shows UVvis absorption spectra of as-prepared tetragonal samples synthesized in ammonia solution by pulsed plasma compared with commercial tetragonal ZrO2 particles stabilized by CaO, confirming that the former exhibit higher absorption properties, especially in the visible light range (400800 nm), than the latter. As the quantity of Zr3þ and element nitrogen is very low, we can exclude the impacts of these on the apparent absorption in the visible region. Thus, only the high surface area of small-size particles and the defects induced during the pulsed plasma synthesis in liquid40 may explain the high absorption of the plasma sample obtained from ammonia solution and may increase its potential application for photocatalyst or photocatalyst support. 2. Optical Emission Spectrum and Synthesis Mechanism. To discuss the synthesis mechanism of prepared zirconia nanoparticles, the optical emission spectrum from the plasma discharge was collected using an optical probe placed adjacent to the beaker and transmitting via an optical fiber to a spectrometer, as shown in Figure 10. The spectral lines were identified using reference data.41 Previous works have shown that the major ion components in water during the plasma process are superoxides (O2) and further formed H2O2 and hydroxyl (OH) created by the reaction between plasma and water molecules.42 These strong oxides are considered to be the oxygen sources for the formation of the oxidized product, zirconia oxide in our case. The coexistence of atoms Zr I and ions Zr II, apparent from the optical emission spectrum, allows explanation of the zirconia particle

formation mechanism, as shown in Figure 11. It is assumed that zirconium is first evaporated instantly by the plasma to form very active zirconium atoms Zr I. After emitting electrons, the atoms transform into ions Zr II, which then react with strong oxides decomposed from water molecules by the plasma discharge, such as superoxides, H2O2, and hydroxyl. Hot zirconia clusters appear in close proximity to the plasma discharge zone, accumulate in size, and are then immediately quenched by the surrounding liquid, leading to the formation of nanocrystals (III). The ultrasonic wave plays a very important role here as it prevents aggregation of clusters forming in the plasma discharge zone, keeping particles small in size. Without the ultrasonication, one of the layers, the milky form liquid from which we collected pure tetragonal nanoparticles, would not have appeared. Water, hydrogen peroxide, and ammonia solution all produce similar major ions and molecules by plasma, thus explaining the virtual phase compositions of the samples synthesized in these solutions.

’ CONCLUSION ZrO2 nanoparticles were prepared in a one-step synthesis process by pulsed plasma in three liquid types. The ammonia solution allowed preparation of pure tetragonal zirconia by plasma due to the small particle size due to quenching effect by pulsed plasma in the liquid method and the alkaline environment (OH), excluding the impacts from element nitrogen in ammonia solution and the oxygen vacancy. HRTEM confirmed the particle size to be smaller than 5 nm. The synthesis mechanism of the particles was analyzed using the emission spectrum. This sample has a different lattice structure compared to zirconia samples synthesized by other methods and shows higher absorption, especially in the visible light range, than commercial tetragonal phase samples. As mentioned above, zirconia nanoparticles hold potential for photocatalytic decomposition of pure water4 and photocatalytic reduction of carbon dioxide5 and photocatalyst supports. TiO2, the most common photocatalyst currently used, by itself only works under UV light due to its bandgap, limiting its use to only about 4% of the incoming solar energy.43 With visible light accounting for 43% of all solar 9374


The Journal of Physical Chemistry C energy,44 the high absorption of visible light of our samples indicates their potential in the field of solar energy.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by the Global Center of Excellence Program of Japan and the China Scholarship Council. ’ REFERENCES (1) Yan, R.; Ding, D.; Lin, B.; Liu, M.; Meng, G.; Liu, X. J. Power Sources 2007, 164, 567–571. (2) Jimenez, S.; Rodríguez, R.; Lima, R.; Fuentes, R.; Rubio, E.; Herrera, A.; Casta~no, V.; Rubio, E. Mater. Res. Innovations 2000, 4, 42–44. (3) Lugscheider, E.; Rass, I. J. Therm. Spray Technol. 1992, 1, 49–55. (4) Sayama, K.; Arakawa, H. J. Phys. Chem. 1993, 97, 531–533. (5) Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S. Chem. Commun. 1997, 841–842. (6) Morterra, C.; Cerrato, G.; Pinna, F.; Signoretto, M. J. Catal. 1995, 157, 109–123. (7) Gravie, R. C.; Hanink, R. H. J.; Pascoe, R. J. Nature 1975, 258, 703–704. (8) Murakami, H.; Ohno, H. J. Ceram. Soc. Jpn. 1991, 99, 1197–1202. (9) Wang, H.; Qiu, X.; Li, L.; Li, G. Chem. Lett. 2007, 36, 1132–1133. (10) Lamsa, D. G.; Rosso, A.; Anzorena, M.; Fernandez, A.; Bellino, M.; Cabezas, M.; Walsoe de Reca, N.; Craievich, A. Scr. Mater. 2006, 55, 553–556. (11) Chen, C.; Yang, X.; Chiang, A. J. Taiwan Inst. Chem. Eng. 2009, 40, 296–301. (12) Rezaei, M.; Alavi, S. M.; Sahebdelfar, S.; Yan, Z.; Teunissen, H.; Jacobsen, J. H.; Sehested, J. J. Mater. Sci. 2007, 42, 1228–1237. (13) Wang, L.; Cai, K. F.; Wang, Y. Y.; Yin, J. L.; Li, H.; Zhou, C. W. Ceram. Int. 2009, 35, 2499–2501. (14) Garvie, R. C. J. Phys. Chem. 1965, 69, 1238–1243. (15) Garvie, R. C. J. Phys. Chem. 1978, 82, 218–224. (16) Shukla, S.; Seal, S. J. Phys. Chem. B 2004, 108, 3395–3399. (17) Xie, S.; Iglesia, E.; Bell, A. T. Chem. Mater. 2000, 12, 2442–2447. (18) Becker, J.; Hald, P.; Bremholm, M.; Pedersen, J. S.; Chevallier, J.; Iversen, S. B.; Iversen, B. B. ACS Nano 2008, 2, 1058–1068. (19) Rong, Y.; Meng, Q.; Zhang, Y.; Hsu, T. Y. Mater. Sci. Eng., A 2006, 438, 414–419. (20) Omurzak, E.; Jasnakunov, J.; Mairykova, N.; Abdykerimova, A.; Maatkasymova, A.; Sulaimankulova, S.; Matsuda, M.; Nishida, M.; Ihara, H.; Mashimo, T. J. Nanosci. Nanotechnol. 2007, 7, 3157–3159. (21) Omurzak, E.; Mashimo, T.; Iwamoto, C.; Matsumoto, Y.; Sulaimankulova, S. J. Nanosci. Nanotechnol. 2009, 9, 6372–6375. (22) Djurado, E.; Bouvier, P.; Lucazeau, G. J. Solid State Chem. 2000, 149, 399–407. (23) Naumenko, A. P.; Berezovska, N. I.; Biliy, M. M.; Shevchenko, O. V. Phys. Chem. Solid State 2008, 9, 121–125. (24) Gazzoli, D.; Mattei, G.; Valigi, M. J. Raman Spectrosc. 2007, 38, 824–831. (25) Tan, D.; Lin, G.; Liu, Y.; Teng, Y.; Zhuang, Y.; Zhu, B.; Zhao, Q.; Qiu, J. J. Nanopart. Res. 2010, 1–8. (26) Tahir, M. N.; Gorgishvili, L.; Li, J.; Gorelik, T.; Kolb, U.; Nasdala, L.; Tremel, W. Solid State Sci. 2007, 9, 1105–1109. (27) Bokhimi, X.; Morales, A.; Novaro, O.; Lopez, T.; Gomez, R.; Garcia-Ruiz, A. Adv. X-Ray Anal. 2000, 42, 245–250. (28) Igawa, N.; Ishii, Y.; Nagasaki, T.; Morii, Y.; Funahashi, S.; Ohno, H. J. Am. Ceram. Soc. 1993, 76, 2673–2676.

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Pure Tetragonal ZrO2 Nanoparticles Synthesized by Pulsed Plasma in

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