J. Phys. Chem. C 2009, 113, 18259–18263 L-Lysine-Assisted
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Synthesis of ZrO2 Nanocrystals and Their Application in Photocatalysis
He Zheng,† Kaiyu Liu,*,† Huaqiang Cao,*,‡ and Xinrong Zhang‡ School of Chemistry and Chemical Engineering, Central South UniVersity, Changsha 410083, People’s Republic of China, and Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: June 18, 2009; ReVised Manuscript ReceiVed: August 28, 2009
Nanocrystalline ZrO2 with narrow size distribution and mean size ∼8 nm has been synthesized by the L-lysineassisted hydrothermal method. The structural and morphological characterizations were studied by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM), and physicochemical characterizations were carried out by using infrared spectra (IR), UV-vis spectra, and photoluminescence (PL) spectra. On the basis of the choice of synthesis parameters, monoclinic (m-) and tetragonal (t-) phases of ZrO2 coexisted with a size ∼8 nm. This is an important result with respect to application in catalytic properties of nanocryalline ZrO2. The possible nucleation and growth process is also discussed. 1. Introduction The controlled synthesis and characterization of nanocrystals is a major objective that runs through nanotechnology, materials science, physics, and chemistry.1 Semiconductor nanocrystals are of great interest because their electronic properties can be easily tailored, providing tremendous potential for applications in optoelectronic devices,2 energy scavenging,3 superconducting devices,4 imaging probes,5 lithium ion battery electrodes,6 lightemitting diodes,7 thin-film transistors,8 memory technologies,9 solar cells,10 and lasers.11 Zirconia has aroused great interest due to its many practical applications, including ZrO2 toughening ceramics used as the alternate materials of thighbone, oral planting materials,12 yttrium stabilized zirconia (YSZ) used in fuel cells,13 and oxygen sensors.14 To date, many methods have been developed to prepare nanostructured ZrO2. ZrO2 nanoshells and nanospheres are synthesized from the oxidation of ZrCl4 primarily with O2, exhibiting a greatly enhanced photoluminescence (PL) compared with ZrO2 nanospheres or irregularly shaped ZrO2 nanoparticles.15 High-quality ZrO2 nanocrystals have been prepared via a two-phase interface hydrolysis reaction under hydrothermal conditions.16 The catalyst activity of CO oxidation over Au supported on monoclinic and tetragonal ZrO2 can be significantly modified.17 Highly crystalline and monodisperse tetragonal ZrO2 nanoparticles with a particle size of 4 nm are prepared by the nonhydrolytic sol-gel reaction between isopropoxide and zirconium(IV) chloride.18 Tetragonal ZrO2 nanowires are generated by a sol-gel template technique.19 It is well-known that generating monodispersed semiconductor nanocrystals with size under 10 nm still remains a big challenge.16 However, to date there are very few reports of the synthesis of ZrO2 using biomolecule-assisted hydrothermal method. It has aroused our interest in preparing new oxide nanocrystals using biomolecules.20 Here, we demonstrate a new approach to obtain ZrO2 nanoparticles by using a biomolecule L-lysine. We show that small sizes of ZrO2 nanoparticles are * To whom correspondence should be addressed. E-mail: hqcao@ mail.tsinghua.edu.cn. † Central South University. ‡ Tsinghua University.
generated, which can be prepared by adjusting the L-lysine concentration, reaction temperature, and time. 2. Experimental Section 2.1. Synthesis. In a typical procedure, ZrOCl2 · 8H2O (analytical reagent, AR, 2 mmol) was dissolved in 20 mL of deionized water with stirring for 10 min to form solution A. L-Lysine (C6H14N2O2, >99% purity, 4.32 mmol) was dissolved in 20 mL of deionized water with stirring for 10 min to form solution B, and then was added dropwise into solution A, with stirring for 13 h at room temperature. The mixture was sealed into a 50 mL Teflon-line autoclave, heated to a selected temperature (ranging from 170 to 260 °C), and maintained at this temperature for a selected time (ranging from 10 to 48 h). After the autoclave was cooled down to room temperature naturally, the products were collected and washed with deionized water and then absolute alcohol. The cycle was repeated three times, followed by drying at 50 °C for 3 h. 2.2. Characterization. The phase structure of as-prepared products were characterized with X-ray diffraction (XRD, Bruker D8 advance) with Cu KR (λ ) 1.5418 Å). The morphology of as-prepared products was studied by using highresolution transmission electron morphology (HRTEM, JEOL JEM-2010F electron microscope, operating at 200 kV). Infrared spectra (IR) measurements were carried out on a NICOLET 560 Fourier transform infrared spectrophotometer. UV-vis measurement was carried out in a UV-vis spectrophotometer (Shimadzu, UV-2100S). Photoluminescence (PL) spectra were recorded by using a fluorescence spectrophotometer (Perkin-Elmer LS55). 2.3. Photocatalytic Activity Test. The photocatalytic activities of the as-synthesized ZrO2 nanoparticles were evaluated in terms of the degradation of rohdamine B (RhB) in an aqueous solution. A 250 W high-pressure mercury lamp (λ > 365 nm, Beijing Huiyixin Electric Forces Technology Development Co. LTD) was positioned inside a cylindrical vessel and surrounded by circulating water jacket for cooling. A 50 mg sample Z-10 was suspended in 50 mL of an aqueous solution of 10-5 M RhB. The solution was continuously stirred for about 30 min at room temperature to ensure the establishment of an adsorption-desorption equilibrium among the photocatalyst,
10.1021/jp9057324 CCC: $40.75 2009 American Chemical Society Published on Web 09/23/2009
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J. Phys. Chem. C, Vol. 113, No. 42, 2009
Zheng et al.
TABLE 1: Experimental Parameters of the As-Synthesized Products reaction ZrOCl2 · 8H2O/L-lysine temperature size r(101) n sample (nm) (molar ratio) (L-lysine) (°C)/time (h) Z-1 Z-2 Z-3 Z-4 Z-5 Z-6 Z-7 Z-8 Z-9 Z-10 Z-11 Z-12
1:2.16 1:2.16 1:2.16 1:2.16 1:2.16 1:2.16 1:2.16 1:2.16 1:2.16 1:2.16 1:2.16 1:2.16
4.32 4.32 4.32 4.32 4.32 4.32 4.32 4.32 4.32 4.32 12.96 21.60
140 °C/10 h 170 °C/10 h 200 °C/10 h 230 °C/10 h 260 °C/10 h 140 °C/16 h 170 °C/16 h 200 °C/16 h 230 °C/16 h 170 °C/24 h 170 °C/48 h 260 °C/20 h
7 7 7 6 6 6 8 6 7 8 9 8
RhB, and water before irradiation with UV light from the highpressure mercury lamp. The distance between the light source and the bottom of the solution was about 10 cm. The concentration of RhB was monitored by using a UV-vis spectrometer (UNICO Corp. UV-2102PC). The pH values of RhB solutions were adjusted by adding HCl or NaOH solutions. 3. Results and Discussion A series of as-synthesized products with different concentrations of reactants, ZrOCl2 · 8H2O/L-lysine molar ratios, reaction
Figure 1. XRD patterns of as-prepared samples with a reaction ratio of ZrOCl2 · 8H2O/L-lysine of 1:2.16, at (a)140 °C/10 h (denoted as Z-1), (b) 170 °C/10 h (denoted as Z-2), (c) 200 °C/10 h (denoted as Z-3), (d) 230 °C/10 h (denoted as Z-4), and (e) 260 °C/10 h (denoted as Z-5), respectively. M indicates monoclinic phase of ZrO2, and T indicates tetragonal phase of ZrO2.
temperatures, and reaction times are listed in Table 1. Figure 1 shows the XRD patterns of the as-synthesized samples. The diffraction peaks shown in Figure 1 demonstrate that the samples belong to tetragonal (t-) (JCPDS card: 79-1769) and monoclinic (m-) (JCPDS card: 78-0047) phases of ZrO2. In these mixtures, t-ZrO2 is the dominant phase, while the m-ZrO2 is produced by increasing the temperature from 140 to 260 °C. We also studied the samples synthesized at 16 h with different temperatures from 140 to 230 °C. Figure S1, Supporting Information, shows the XRD patterns of these samples. All these samples belong to tetragonal and monoclinic phases of ZrO2. It also demonstrates that t-ZrO2 is the dominant phase, while m-ZrO2 is produced by increasing the reaction temperature. These data suggest that low temperature favors the generation of tetragonal ZrO2 under the L-lysine-assisted hydrothermal condition. The broad XRD peaks are attributed to the very small particle size, which is also demonstrated by the TEM observation. This research provides an interesting case for size-dependent ZrO2 phase transition from tetragonal to monoclinic nanocrystals at high temperature. Figure 2 shows the TEM images of the samples (Z-1, Z-2, Z-5, Z-10, Z-11, and Z-12). According to the observation, we find that the crystalline sizes of all these nanoparticles are ∼8 nm in diameter. That means the reaction temperature (comparing samples Z-1, Z-2, and Z-5), reaction time (comparing sample Z-2 and Z-10), and reaction concentration (comparing samples Z-10 to Z-11, and Z-5 to Z-12) have little effect on the size of as-prepared ZrO2 nanoparticles. It suggests that the lysine can control the growth of ZrO2 after the nucleation process. In order to understand the effect of L-lysine, we also carried out the comparison experiment, i.e., the same synthesis conditions but without any L-lysine. The diffraction peaks shown in Figure S2a demonstrate that the samples belong to tetragonal (t-) (JCPDS card: 79-1769) and monoclinic (m-) (JCPDS card: 78-0047) phases of ZrO2. However, no dominant phase can be found in the mixture. The corresponding TEM image (Figure S2b) shows that the product is composed of aggregated nanofibers ca. 50 nm in length and 5 nm in width. Obviously, the amino-acid L-lysine, with functional groups -NH2 and -COOH, has a great influence on the size and shape of the final nanocrystals of ZrO2. A similar phenomenon has been observed in our previous research work.21 Figure 3 shows the FT-IR spectrum of the Z-10 nanoparticle of ZrO2. The bands at ∼510 and 750 cm-1 can be attributed to the Zr-O vibrations.22 The bands at 3410 and 1620 cm-1 can be ascribed to the O-H vibrations of H2O absorbed in the
Figure 2. TEM images of (a) Z-1, (b) Z-2, (c) Z-5, (d) Z-10, (e) Z-11, and (f) Z-12, respectively.
ZrO2 Nanocrystals and Their Photocatalysis
Figure 3. FT-IR spectrum of the as-synthesized Z-10 sample.
samples. The band at 1360 cm-1 can be ascribed to the δC-H of the carboxylate group.22 This result suggests that the carbonrelated impurities are present in the sample, which may come from L-lysine. Absorption spectroscopy allowed us to characterize the quantum confinement effects in the nanocrystals with size 365 nm).
often found.33 The size effect is attributed to the lower surface energy of the tetragonal or cubic phase of zirconia as compared to that of the monoclinic phase. Semiconductor photocatalytic activity has attracted great interest due to potential applications in the degradation of environmental pollutants and organic pollutant transformation. Dye pollutants in effluents from the textile and paper industries are regarded as major environmental pollutants, because of their nonbiodegradability and toxicity.34 The degradation of RhB, an organic dye, in aqueous suspension was used as a probe reaction to evaluate the catalytic activity of semiconductor photocatalytic performance.35 The photocatalytic activities of the ZrO2 nanoparticles were evaluated in terms of the degradation of RhB in aqueous solution. Figure 5 shows the degradation of the RhB solution with 250 W high-pressure mercury lamp irradiation (λ > 365 nm) under different pH conditions. After 5 h of different pH values (pH ) 9, 6, and 2), the degradation of RhB reaches 21.40%, 30.90%, and 63.39%, correspondingly. In comparison, we also carried out a study of the degradation of RhB in aqueous solution but without ZrO2 nanocrystals (Figure S3), which demonstrates that self-degradation of RhB solution over a period of 5 h was negligible in the absence of ZrO2 nanocrystals. A similar phenomenon was observed by other groups.36 These results suggest the acid condition favors the degradation of RhB. Oxygen vacancy defects on the surface of ZrO2 nanoparticles are generated due to the hydrothermal treatment. The oxygen vacancies can induce the formation of new energy levels in the bandgap. The photocatalytic behavior of semiconductors is mainly dependent upon the separation of photogenerated electron-hole pairs and transfer the separated electrons from photocatalyst to the organic pollutants through the oxygen vacancy defects on the surface of photocatalyst.37 4. Conclusion In summary, the synthesis of nanocrystalline ZrO2 has been carried out by using biomolecule-assisted (L-lysine-assisted) hydrothermal method. Detailed characterization using XRD and TEM revealed all samples to be mixtures of t-ZrO2 and m-ZrO2. It was also demonstrated that t-ZrO2 was the dominant phase, while the m-ZrO2 was produced by increasing the temperature. These data suggest that low temperature favors the generation of t-ZrO2 under the L-lysine-assisted hydrothermal conditions.
However, the reaction parameters, including the concentration of biomolecule or the ratio of ZrOCl2/Lys, reaction temperature, and time, have little effect on the size and shape of the assynthesized ZrO2 nanocrystals. This synthesis route gives almost monodisperse products with mean particle sizes below 10 nm. The as-synthesized nanocrystalline ZrO2 own photocatalytic activities of RhB under UV irradiation which is attributed to the more oxygen vacancies on the surface of the nanocrystalline ZrO2. Details need further investigation. Potential applications of the as-synthesized ZrO2 nanocrystals have been considered. This is an important result with respect to obtaining small nanocrystalline ZrO2 below 10 nm in size and to degrading the RhB. Acknowledgment. The authors gratefully thank the financial support from the National Natural Science Foundation of China (No. 20671056 and 20535020), the Innovation Method Fund of China (No. 20081885189) and the National High Technology ResearchandDevelopmentProgramofChina(No.2009AA03Z321). Supporting Information Available: XRD pattern showing the samples synthesized at different temperatures ranging from 140 to 230 °C for 16 h, XRD pattern and corresponding TEM image of ZrO2 without using L-lysine, as well as the selfdegradation of RhB in the absence of ZrO2 nanocrystals [the UV-vis spectral changes of the RhB aqueous solutions in the absence of ZrO2 nanocrystals under visible-light irradiation (λ> 365 nm)].This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55. (b) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (c) Shim, M.; Guyot-Sionnest, P. Nature 2000, 407, 981. (d) Ishii, D.; Kinbara, K.; Ishida, Y.; Ishii, N.; Okochi, M.; Yohda, M.; Aida, T. Nature 2003, 423, 628. (e) Tenne, R. Nat. Nanotechnol. 2006, 1, 103. (f) Lauritsen, J. V.; Kibsgaard, J.; Helveg, S.; Topsøe, H.; Clausen, B. S.; Lægsgaard, E.; Besenbacher, F. Nat. Nanotechnol. 2007, 2, 53. (g) Gao, L. Z.; Zhuang, J.; Nie, L.; Zhang, J. B.; Zhang, Y.; Gu, N.; Wang, T. H.; Feng, J.; Yang, D. L.; Perrett, S.; Yan, X. Y. Nat. Nanotechnol. 2007, 2, 577. (h) Berti, L.; Burley, G. A. Nat. Nanotechnol. 2008, 3, 81. (i) Satoh, N.; Nakashima, T.; Kamikura, K.; Yamamoto, K. Nat. Nanotechnol. 2008, 3, 106. (j) Pauzauskie, P. J.; Radenovic, A.; Trepagnier, E.; Shroff, H.; Yang, P. D.; Liphardt, J. Nat. Mater. 2006, 5, 97. (k) Kan, S.; Mokari,
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