Photochemical Synthesis of Monodispersed Ceria Nanocrystals in

Mar 18, 2011 - Simple Cerium Nitrate Solution without Additives. Kai Kamada,* Koji Horiguchi, Takeo Hyodo, and Yasuhiro Shimizu. Department of Materia...
0 downloads 0 Views 2MB Size
Download PDF
ARTICLE pubs.acs.org/crystal

Photochemical Synthesis of Monodispersed Ceria Nanocrystals in Simple Cerium Nitrate Solution without Additives Kai Kamada,* Koji Horiguchi, Takeo Hyodo, and Yasuhiro Shimizu Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan

bS Supporting Information ABSTRACT: We developed a novel photochemical technique for preparation of ceria (CeO2) nanocrystals in simple aqueous solution containing only Ce(NO3)3, where no additive and no heating are required. Under UV light illumination, photochemical reduction of NO3 resulted in formation of NO2 accompanied by hydroxyl radicals ( 3 OH) as intermediate species. The hydroxyl radicals could oxidize Ce3þ to tetravalent state (Ce(OH)22þ), then CeO2 nuclei were formed through deprotonation. The aqueous CeO2 sol was prepared via dialysis of the reacted solution against deionized water to remove the excess electrolyte. Since the photooxidation process homogeneously occurs over the entire solution due to high optical transmittance of the solvent, the particle size distribution becomes relatively narrow and the mean diameter can be easily controlled by adjusting the initial concentration of Ce(NO3)3. The prepared CeO2 nanoparticles were potent against oxidative polymerization of phenolic derivatives (guaiacol) at room temperature. This paper clarifies the photochemical formation mechanism of CeO2 nanocrystals in the Ce(NO3)3 solution, and the advantages of the proposed method are discussed in detail.

’ INTRODUCTION As well-known, cerium is abundant in nature among rare-earth elements, and its oxide (ceria, CeO2) and doped oxides have been widely employed for various applications due to their unique properties. For example, their nanocrystals (nanoparticles) have attracted attention as cocatalysts for noble metals,1 ultraviolet light screeners,2 inorganic pigments,3 and anode materials for lithium ion batteries,4 along with active starting powder for low temperature sintering of oxide ion conducting ceramics.5 Recently, biological applications of CeO2 nanoparticles have been reported by independent groups. Concretely, CeO2 nanoparticles with oxygen vacancies were effective for protection from radiation-induced cellular damage6 and for scavenging of toxic reactive oxygen species.7 In general, these latest applications utilize an aqueous sol, which is convenient to add a trace amount of CeO2 nanoparticles to the reaction system as compared with solid powders. Among numerous synthesis methods, it is believed that the solution technique, which includes a homogeneous precipitation,8 hydro(solvo)-thermal,9 and electrochemical method,10 is one of most convenient routes for preparing the sols containing CeO2 nanoparticles. However, some additives such as precipitators and surfactants are required to bring about deposition of CeO2 nuclei and to prevent undesired aggregation of nanoparticles, respectively. The process might be performed in organic solvent or under high temperature or pressure in some cases. Moreover, the existence of contamination remaining even after washing may induce unfavorable influences. The present study proposes a novel photochemical solution synthesis of the aqueous suspension of CeO2 nanoparticles that r 2011 American Chemical Society

can be applicable for various research and industrial fields. Light irradiation to Ce3þ salt solution brings about formation of CeO2 nanoparticles under presence of photo-oxidizer. As a result, Ce3þ is transformed into CeIV state by the light energy and subsequently forms into (hydrated) CeO2 nanoparticles. In this study, we employ nitrate ions as photo-oxidizing agent since ultraviolet (UV) light irradiation stimulates reduction to nitrite ions. That is, the UV light illumination to simple Ce(NO3)3 solution results in the formation of CeO2 nanoparticles and no additives are demanded in the proposed system. In addition, high transmittance of the UV light through the water solvent would cause homogeneous photochemical oxidation and generation of CeO2 nuclei over the entire solution. This presumably enhances monodispersivity and size-tunability of the CeO2 nanoparticles. In the past decades, there are a few previous papers dealt with the photochemical preparation of inorganic nanoparticles.11 Even though Takamura and co-workers recently employed UV laser for fabricating oxides or metal nanoparticles in a solution system, harmful methanol was necessary as a solvent.12 The other groups reported that the photochemical loading of oxide particles on surface of semiconductor photocatalysts dispersed in the metal salt solution.13 On the other hand, Das et al. described a photochemical precipitation of rare earths for separation from other ions coexisting in aqueous solution.14 In this case, the rare earth ions were removed by formation of insoluble salts with Received: October 20, 2010 Revised: February 12, 2011 Published: March 18, 2011 1202

dx.doi.org/10.1021/cg1014048 | Cryst. Growth Des. 2011, 11, 1202–1207

Crystal Growth & Design

ARTICLE

anions produced by photochemical reaction. The present study investigates the photochemical formation mechanism of CeO2 nanoparticles in the solution containing Ce(NO3)3 only, and demonstrated that the proposed method is useful for synthesis of aqueous CeO2 sol without additives under ambient conditions.

’ EXPERIMENTAL SECTION The photochemical synthesis of CeO2 nanoparticles was performed by UV light irradiation (300 W HgXe lamp) to aqueous Ce(NO3)3 solution (1100 mM) at 283 K under mild stirring. The water used in all processes was doubly deionized water. Prior to the irradiation, the Ce(NO3)3 solution was filtered (pore diameter: 0.2 μm) to remove tiny solid contaminants in the solution. After the UV illumination, size distribution and average diameter of the CeO2 nanoparticles formed in the solution were measured through the dynamic light scattering (DLS) technique. On the other hand, the solid particles were collected by centrifuging the solution (21880  g, 30 min) followed by washing with water several times and drying at room temperature. The surface area of powder was determined by N2 adsorptiondesorption isotherm measurement at the liquid nitrogen temperature, where the powder samples were evacuated at 393 K overnight prior to the measurement. The crystal structure and morphology of the dried product were analyzed and observed using X-ray diffractometry (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The room temperature oxidation activity of produced CeO2 nanoparticles was evaluated for oxidative polymerization of guaiacol (omethoxyphenol). The reaction was triggered by addition of the aqueous CeO2 sol to guaiacol solution. The formation rate of oxidation products was calculated by absorption change at 470 nm and 298 K using a UVvis spectrophotometer.

’ RESULTS AND DISCUSSION The UV light irradiation to colorless and transparent Ce(NO3)3 solution (10 mM) for 2 h did not cause any change in appearance. However, the solution clearly scattered a laser light as shown in Figure 1a, implying that the colloidal suspension was produced as a result of the irradiation. Needless to say, no laser light scattering was observed after dark reaction. Figure 1b illustrates the UVvis transmittance spectra of 10 mM Ce(NO3)3 solution during the UV light irradiation. The absorption peak appeared at 298 nm in the spectra means the existence of NO3. The absorption edge was shifted to red side after the irradiation for 1 min (340 f 400 nm). The depression at 298 nm was slightly decreased with irradiation time, suggesting the consumption of NO3 during the photochemical reaction. CeO2 which is expected to be produced by the proposed reaction, can absorb light shorter than 400 nm in wavelength (band gap: 3.1 eV) by indirect transition of electrons from the valence band (O2p) to the empty Ce4f states. This fact supports that the absorption shoulder at 400 nm is due to the CeO2 crystals in the solution. To assess the crystal structure of the solid product, the dried powder collected by the centrifugation was analyzed by XRD, Raman and XPS spectroscopies (Figure 2). The powder XRD pattern (Figure 2a) displayed several diffraction lines assigned to (111), (220), and (311) planes of fluorite structure of CeO2 (ICDD No. 000040593) together with halo (2040 deg.) originated from the glass sample holder. The peak broading suggests that the product is composed of tiny crystallites. On the other hand, the Raman spectrum exhibited two bands (Figure 2b). The larger peak at 460 cm1 corresponds to the

Figure 1. (a) Photograph of 10 mM Ce(NO3)3 solution after UV light irradiation for 2 h at 283 K, and (b) UVvis transmittance spectra of 10 mM Ce(NO3)3 solution during photochemical reaction at 283 K.

triply degenerate Raman active F2g mode of the fluorite structure, which is detected as a symmetric breathing mode of the oxygen atoms surrounding cations.15 The small peak at the larger Raman shift (ca. 590 cm1) is related to the oxygen defects due to the partial reduction of Ce4þ, that is, CeO2x. This fact was also supported by the Ce3d XPS spectrum (Figure 2c) which displayed the characteristic peaks at 903.5 and 885.0 eV (indicated by arrow) assigned to Ce3þ in addition to the tetravalent state. Figure 3 shows the SEM image of powdered product prepared in 100 mM Ce(NO3)3 solution. This indicates the presence of aggregates consisting of nanoparticles with diameters below 10 nm. The TEM image (Supporting Information Figure 1S) also implied that the nanocrystals with lattice fringes were produced and partially agglomerated to form the multicrystals. Supporting Information Figure 2S depicts the nitrogen adsorptiondesorption isotherm of the identical powder with Figure 3. A marked hysteresis in the pressure region of P/P0 > 0.45 implies the existence of mesoporous structure among the nanoparticles in the aggregates. The isotherm can be classified as type I and IV in the BDDT (Brunauer, Deming, Deming, and Teller) classification along with H4-type hysteresis loop in the IUPAC classification.16 In the P/P0 region from 0.05 to 0.4, the adsorption isotherm gives a good fit with the BET equation, and the estimated BET specific surface area was 130 m2 g1. Consequently, it can be concluded that the facile method that is the UV light irradiation to simple Ce(NO3)3 solution causes the formation of crystalline CeO2 nanoparticles. The fundamental formation mechanism of CeO2 nanoparticles appears to be photooxidation of Ce3þ by excited NO3 followed by precipitation of CeO2. As shown in Figure 1b, NO3 absorbs photon energy around 298 nm. The previous paper 1203

dx.doi.org/10.1021/cg1014048 |Cryst. Growth Des. 2011, 11, 1202–1207

Crystal Growth & Design

ARTICLE

Figure 3. SEM image of CeO2 nanoparticles obtained by photochemical reaction of 100 mM for 18 h at 283 K.

Figure 4. Variations in nitrite ion concentration with time during UV light irradiation to 100 mM Ce(NO3)3 or Al(NO3)3 at 283 K.

The direct oxidation of Ce3þ to CeO2 and/or Ce(OH)4 may also occur as suggested by the previous paper.10d

Figure 2. (a) XRD pattern, (b) Raman, and (c) Ce3D X-ray photoelectron spectra of powdered product obtained by photochemical reaction of 100 mM Ce(NO3)2 solution for 18 h at 283 K.

described that NO3 in aqueous solution is reduced to NO2 by UV light irradiation.17 The process is initiated by photolysis of NO3 to 3 NO2 and hydroxyl radical (eq 1). NO3  þ Hþ þ hν f 3 NO2 þ 3 OH ð1Þ Subsequently, association of two 3 NO2 radicals (eq 2) and then N2O4 will decomposed into NO2 and NO3 (eq 3). 2 3 NO2 r f N2 O4

ð2Þ

N2 O4 þ H2 O f NO2  þ NO3  þ 2Hþ

ð3Þ

The hydroxyl radical formed by the eq 1, which behaves as a strong oxidizer, receives 4f1 electron of Ce3þ (eq 4), then produced Ce(OH)22þ with tetravalent state is transformed into CeO2 via dehydration of intermediate Ce(OH)4 (eq 5, 6).18

Ce3þ þ 3 OH þ H2 O f CeðOHÞ2 2þ þ Hþ

ð4Þ

CeðOHÞ2 2þ þ 2H2 O f CeðOHÞ4 þ 2Hþ

ð5Þ

CeðOHÞ4 f CeO2 þ 2H2 O

ð6Þ

Thus, the total reaction can be expressed as follows: 2Ce3þ þ NO3  þ 3H2 O f 2CeO2 þ NO2  þ 6Hþ

ð7Þ

To confirm the validity of photoreaction mechanism proposed here, concentration change of NO2 in the Ce(NO3)3 solution was monitored during the UV light irradiation by means of the Griess method described elsewhere.19 As shown in Figure 4, the presence of NO2 was detected in the Ce(NO3)3 solution exposed to the UV radiation, and the NO2 concentration was tended to increase with increasing the irradiation time. Interestingly, the UV light irradiation to Al(NO3)3 solution composed of identical trivalent aluminum ion with Ce3þ produced a negligible amount of NO2. Minero et al. pointed out that the formation of NO2 by the photolysis of NO3 is dramatically promoted by the presence of hydroxyl radical ( 3 OH) scavengers such as HCHO and HCOO.19 In the Al(NO3)3 solution, Al3þ cannot 1204

dx.doi.org/10.1021/cg1014048 |Cryst. Growth Des. 2011, 11, 1202–1207

Crystal Growth & Design

Figure 5. (a) Particle size distribution curves of CeO2 nanoparticles produced in Ce(NO3)3 solutions after UV light irradiation and (b) effect of initial concentration of Ce(NO3)3 on mean diameter of CeO2 nanoparticles, where the reaction period and temperature were fixed to 2 h and 283 K, respectively.

interact with the hydroxyl radical because the ion cannot be oxidized to higher valence than þ3. In contrast, the 3 OH could oxidize Ce3þ to the tetravalent state (Ce(OH)22þ in eq 4), in other words, the Ce3þ acted as a 3 OH scavenger. Consequently, the photolysis of NO3 continuously proceeded, then the CeO2

ARTICLE

nanoparticles were formed in the irradiated solution. The pH value of the 10 mM Ce(NO3)3 solution after the irradiation for 2 h was lowered as compared with that of starting solution (5.6 f 3.7). This fact is not conflicted with the total reaction formula (eq 7, proton formation). From these results, it was revealed that the 3 OH formed by the photolysis of NO3 gave rise to the oxidative formation of CeO2 from Ce3þ. On the other hand, the photochemical reaction yield was estimated on the basis of inductively coupled plasma (ICP) quantitative analysis of collected CeO2 nanoparticles after dissolving them in a sulfuric acid solution. As a result, the efficiency obtained for the photoreaction of 10 mM Ce(NO3)3 (2 h) was extremely low (∼1.9%). It is considered that prolonged reaction time may improve the yield of CeO2. Figure 5 shows the particle size distribution of CeO2 nanoparticles produced in the Ce(NO3)3 solution after the photochemical reaction for 2 h and the effect of initial concentration of Ce(NO3)3 on the mean diameter of CeO2 nanoparticles, which were measured by the dynamic light scattering (DLS) method. When the CeO2 nanoparticles are exploited as a sol for practical applications, the diameter and its dispersion would be important characters. From the results of investigations under various experimental conditions, the reaction temperature (283313 K) and atmosphere (solution saturated with air, nitrogen, or oxygen) did not affect to the particle size of the CeO2 produced. In addition, the lowering of the UV irradiation intensity in half had little effect on the particle size distribution. Each particle size distribution curve in Figure 5a is composed of a single peak, indicating that the present photochemical technique provides the formation of monodispersed CeO2 colloidal solution with a few tens nm in diameter. Judging from the SEM and TEM observations (Figure 3 and Supporting Information Figure 1S), the crystallite size of CeO2 seems to be equivalent to or smaller than ca. 10 nm. That is, the mean diameters estimated by the DLS would be affected by the size of secondary particles composed of several CeO2 crystallites. As shown in Figure 5b, the mean diameter was linearly increased with the initial concentration of Ce(NO3)3. Given the high linearity between the diameter and the Ce(NO3)3 concentration, the size-tunable synthesis of CeO2 colloidal solution is achieved in the concentration range lower than 40 mM. On the other hand, the CeO2 sizes were spread over wide range at higher Ce(NO3)3 concentrations than 40 mM. The concentrated conditions would increase the formation density of CeO2 nucleus and then promote aggregation of the CeO2 nanoparticles. The large ionic strength in the concentrated solution might also reduce electrostatic repulsion among the particles. The influence of NO3 concentration on the mean diameter was also studied (Supporting Information Figure 3S), where [NO3] was adjusted by addition of NaNO3 to the 10 mM Ce(NO3)3 solution. Supporting Information Figure 3S clearly indicates that the particle size depends on [NO3] and has broad distribution at high [NO3] in contrast to narrow distribution at diluted conditions. Furthermore, the particle size was increased with increasing the reaction time especially at high [NO3]. The saturation of particle growth observed at longer reaction time may be reflected by photoreductive dissolution of Ce4þOH surface group of the CeO2 nanoparticles. Under the irradiation of photon energy significantly exceeding the band gap of CeO2, the photogenerated electrons trapped at the CeO2solution interface give rise to reduction of surface Ce4þ, followed by dissolution of reduced species (Ce3þ) showing higher solubility.20 Hence, it was confirmed that the particle size can be controlled 1205

dx.doi.org/10.1021/cg1014048 |Cryst. Growth Des. 2011, 11, 1202–1207

Crystal Growth & Design

Figure 6. (a) UVvis absorption spectra of 18 mM guaiacol solution after addition of CeO2 colloidal solution (0.22 mM) in 0.02 M potassium acetate buffer solution (pH = 4.0) at 298 K and (b) influence of guaiacol and CeO2 concentration on oxidation rate at pH = 4.0 and 298 K.

by the initial concentration of Ce(NO3)3. In the present photochemical method, the 3 OH radicals would be homogeneously produced over the entire solution due to high optical transmittance of water solvent as similar to microwave-21 or ultrasonicassisted fabrication.22 The homogeneous evolution of numerous CeO2 nuclei in the solution would lead to the sharp particle size distribution. Conventional homogeneous precipitation method can also cause a uniform reaction.8 However, it requires an addition of precipitator (ex. hexamethylenetetramine (HMT) or urea) besides metal salt and heating for activation of precipitator. CeO2 nanoparticles with high specific surface area are frequently employed as oxidation catalysts at high temperature. Recently, CeO2 nanoparticles have attracted much attention as reactive oxygen scavengers (antioxidants) at ambient temperature range as stated already.7 Therefore, to investigate the oxidation activity of CeO2 nanoparticles at room temperature, the prepared CeO2 sol was applied for oxidation of phenol derivative (guaiacol: o-methoxyphenol). The oxidative removal of phenol derivatives is significant for wastewater treatment in dye and textile industries.23 In general, the oxidation is often undertaken by using expensive enzymes (ex. horseradish peroxidase (HRP), manganese peroxidase, etc.) in the presence of hydrogen peroxide. The high molecular weight products with low solubility are easily separated from the wastewater. On the other hand, it has been reported that the polymer products by the guaiacol oxidation have useful properties containing antimicrobial activities.24 Since the synthesis of CeO2 nanoparticles is

ARTICLE

carried out in the solution containing only Ce(NO3)3, the purified aqueous CeO2 sol can be obtained after dialysis against pure water to remove the electrolytes remained. Though there are few studies on the ability of CeO2 nanoparticles for wet oxidation of organic compounds except for alcohols or azo dyes,25 the aqueous CeO2 sols obtained by the present technique may be used for liquid reaction system because of facile handling. Figure 6a depicts the UVvis absorption spectra during the guaiacol oxidation by the CeO2 nanoparticles at ambient temperature (298 K). The colorless mixture turned red-brown after the addition of the CeO2 sol at pH = 4, implying the formation of products, that is, dimeric and trimeric polymers of guaiacol molecules as similar to the catalytic oxidation by the HRP and the other peroxidase in the presence of H2O2.26 As shown in Figure 6a, the absorbance assigned to the polymeric products at 470 nm rose with progress of the reaction. The light scattering by CeO2 colloids had little effect on the spectra in visible light range (400600 nm) because bulk CeO2 can absorb a photon energy with wavelength shorter than ∼400 nm. This is agreed with the existence of peak at 297 nm with an edge around 400 nm. The oxidation rate of guaiacol was calculated by molar absorption coefficient of the products (ε470 = 26.6 mM1 cm1) and the slope of the absorbance versus reaction time. Figure 6b summarizes the influence of CeO2 (closed circle) or guaiacol (open circle) concentrations on the oxidation rate. The oxidation was accelerated at concentrated conditions of CeO2 or guaiacol, where the measured oxidation rates are extremely slower than the HRP-catalyzed reaction (ex. 0.045 mM s1 at [guaiacol] = 10 mM, [HRP] = 1.5 μM, and [H2O2] = 1 mM).27 Figure 6b clearly indicates that the effect of [CeO2] was more sensitive than that of [guaiacol]. It appears that, accordingly, the total surface area of CeO2 nanoparticles providing reaction sites determines the oxidation rate under the presence of excess guaiacol molecules because the commercial CeO2 submicrometer powder (2 m2 g1) showed no activity. According to the pH dependence of the oxidation rate (pH = 3.27.4 shown in Supporting Information Figure 4S), the acidic solution is preferable for promoting the reaction. The oxidation of guaiacol would be initiated by abstraction of hydrogen atoms in phenolic OH group,28 where the Ce4þ at CeO2 surface would behave as an electron acceptor, that is, oxidizer (reduction to Ce3þ). The redox potential of Ce4þ/3þ is linearly shifted to positive side with increasing acidity, which means that the Ce4þ possesses stronger oxidation performance in the acidic solution than the neutral one.29 This fact is consistent with the pH dependence of the reaction rate as shown in Supporting Information Figure 4S. On the other hand, it can be clearly seen that the absorbance assigned to the CeO2 below 400 nm was decreased by the reaction (Figure 6b), indicating that the dissolving of reduced form (Ce3þ) to the solution (reductive dissolution). Thus, it was revealed that the photochemically synthesized CeO2 nanoparticles can oxidize phenolic compounds. The oxidation ability might be due to the relative high specific surface area (130 m2 g1) and the low crystallinity of nanoparticles.

’ CONCLUSIONS The present study reported that the UV light irradiation to Ce(NO3)3 solution realized the formation of CeO2 colloidal solution. The hydroxyl radicals, which were produced during the photolysis of nitrate ion, oxidized Ce3þ to Ce(OH)22þ, then CeO2 nanoparticles were produced by deprotonation. The 1206

dx.doi.org/10.1021/cg1014048 |Cryst. Growth Des. 2011, 11, 1202–1207

Crystal Growth & Design particle size in the solution was linearly increased with increasing the initial concentration of Ce(NO3)3. The obtained CeO2 nanoparticles possessed the oxidative polymerization performance of phenolic derivative (guaiacol) at ambient temperature as similar to peroxidase. In the present technique, no additive, no heating, and only single reactant (Ce(NO3)3) were required for the synthesis of CeO2 colloidal solution. In the near future, various metal ions showing insolubility at higher valence (Ti, Pb, Fe, etc.) may be targeted to expand the usefulness of the present method.

’ ASSOCIATED CONTENT

bS

Supporting Information. TEM image and N2 adsorptiondesorption isotherm of powdered CeO2, effect of nitrate concentration of mean diameter of CeO2 nanoparticles, and pH dependence on oxidation rate of guaiacol induced by addition of CeO2 sol. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Address: 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan. Phone/fax: þ81-95-819-2667. E-mail: [email protected]. jp.

’ ACKNOWLEDGMENT This study was partly supported by Special Coordination Funds for Promoting Science and Technology, MEXT, Japan, “The Nagasaki University Strategy for Fostering Young Scientists”. The authors also thank to the JGC-S Scholarship Foundation for financial assistance. ’ REFERENCES (1) (a) Yu, T; Zeng, J.; Lim, B.; Xia, Y. Adv. Mater. 2010, 22, 5188. (b) Zhou, H.-P.; Wu, H.-S.; Shen, J.; Yin, A.-X.; Sun, L.-D.; Ya, C.-H. J. Am. Chem. Soc. 2010, 132, 4998. (c) Miedziak, P. J.; Tang, Z.; Davies, T. E.; Enache, D. I.; Bartley, J. K.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Taylor, S. H.; Hutchings, G. J. J. Mater. Chem. 2009, 19, 8619. (2) (a) Masui, T.; Hirai, H.; Hamada, R.; Imanaka, N.; Adachi, G.-y.; Sakata, T.; Mori, H. J. Mater. Chem. 2003, 13, 622. (b) Sato, T.; Katakura, T.; Yin, S.; Fujimoto, T.; Yabe, S. Solid State Ionics 2004, 172, 377. (3) Masui, T.; Tategaki, H.; Imanaka, N. J. Mater. Sci. 2004, 39, 4909. (4) Zhou, F.; Zhao, X.; Xu, H.; Yuan, C. J. Phys. Chem. C 2007, 111, 1651. (5) Moure, A.; Tartaj, J.; Moure, C. J. Am. Ceram. Soc. 2009, 92, 2197. (6) (a) Tarnuzzer, R. W.; Colon, J.; Patil, S.; Seal, S. Nano Lett. 2005, 5, 2573. (b) Colon, J.; Herrera, L.; Smith, J.; Patil, S.; Komanski, C.; Kupelian, P.; Seal, S.; Jenkins, D. W.; Baker, C. H. Nanomedicine 2009, 5, 225. (7) (a) Chen, J.; Patil, S.; Seal, S.; McGinnis, J. F. Nat. Nanotechnol. 2006, 1, 142. (b) Tsai, Y.-Y.; Oca-Cossio, J.; Agering, K.; Simpson, N. E.; Atkinson, M. A.; Wasserfall, C. H.; Constantinidis, I.; Sigmund, W. Future Med. 2007, 2, 325. (8) (a) Li, X.; Chen, Z.; Chen, J.; Chen, Y.; Ni, C. J. Rare Earths 2005, 23, 321. (b) Ozawa, M.; Onoe, R.; Kato, H. J. Alloys. Compd. 2006, 408412, 556. (c) Uekawa, N.; Ueta, M.; Wu, Y. J.; Kakegawa, K. Chem. Lett. 2002, 854. (9) (a) Zhang, J.; Ohara, S.; Umetsu, M.; Naka, T.; Hatakeyama, Y.; Adschiri, T. Adv. Mater. 2007, 19, 203. (b) Zhang, F.; Chan, S.-W.; Spanier, J. E.; Apak, E.; Jin, Q.; Robinson, R. D.; Herman, I. P. Appl. Phys.

ARTICLE

Lett. 2002, 80, 127. (c) Inoue, M.; Kimura, M.; Inui, T. Chem. Commun. 1999, 957. (d) Kobayashi, T.; Hosokawa, S.; Iwamoto, S.; Inoue, M. J. Am. Ceram. Soc. 2006, 89, 1205. (e) Hayashi, Y.; Hosokawa, S.; Inoue, M. Microporous Mesoporous Mater. 2010, 128, 115. (10) (a) Lu, X.-H.; Huang, X.; Xie, S.-L.; Zheng, D.-Z.; Liu, Z.-Q.; Liang, C.-L.; Tong, Y.-X. Langmuir 2010, 26, 7569. (b) Li, G.-R.; Qu, D.-L.; Arurault, L.; Tong, Y.-X. J. Phys. Chem. C 2009, 113, 1235. (c) Wang, A. Q.; D’Souza, N.; Golden, T. D. J. Mater. Chem. 2006, 16, 481. (d) Kulp, E. A.; Limmer, S. J.; Bohannan, E. W.; Switzer, J. A. Solid State Ionics 2007, 178, 749. (11) (a) Wang, H.; Sun, F.; Zhang, Y.; Li, L.; Chen, H.; Wu, Q.; Yu, J. C. J. Mater. Chem. 2010, 20, 5641. (b) Ohtaki, M.; Toshima, N. Chem. Lett. 1990, 489. (c) Azuma, M.; Ichimura, M. Mater. Res. Bull. 2008, 43, 3537. (d) Stroyuk, A. L.; Shvalagin, V. V.; Raevskaya, A. E.; Kryukov, A. I.; Kuchmii, S. Y. Theor. Exp. Chem. 2008, 44, 205. (12) (a) Takamura, H.; Takahashi, N. Solid State Ionics 2010, 181, 100. (b) Watanabe, M.; Takamura, H.; Sugai, H. Nanoscale Res. Lett. 2009, 4, 565. (13) (a) Iwase, A.; Kato, H.; Kudo, A. Chem. Lett. 2005, 34, 946. (b) Maeda, K.; Lu, D.; Teramura, K.; Domen, K. J. Mater. Chem. 2008, 18, 3539. (c) Matsumoto, Y.; Ida, S.; Inoue, T. J. Phys. Chem. C 2008, 112, 11614. (14) Das, M.; Heyn, A. H. A.; Hoffman, M. Z.; Agarwal, R. P. Talanta 1970, 17, 925. (15) (a) Luo, M.-F.; Yan, Z.-L.; Jin, L.-Y.; He, M. J. Phys. Chem. B 2006, 110, 13068. (b) Kanakaraju, S.; Mohan, S.; Sood, A. K. Thin Solid Films 1997, 305, 191. (16) Gregg, S.J.; Shing, K.S.W. Adsorption, Surface Area, and Porosity, 2nd ed.; Academic: London, 1983. (17) (a) Mark, G.; Korth, H.-G.; Schuchmann, H.-P.; von Sonntag, C. J. Photochem. Photobiol. A; Chem. 1996, 101, 89. (b) Dubowski, Y.; Colussi, A. J.; Boxe, C.; Hoffmann, M. R. J. Phys. Chem. A 2002, 106, 6967. (18) Kamada, K.; Higashikawa, K.; Inada, M.; Enomoto, N.; Hojo, J. J. Phys. Chem. C 2007, 111, 14508. (19) Minero, C.; Maurino, V.; Bono, F.; Pelizzetti, E.; Marinoni, A.; Mailhot, G.; Carlotti, M. E.; Vione, D. Chemosphere 2007, 68, 2111. (20) (a) Kamada, K.; Hyodo, T.; Shimizu, Y. J. Phys. Chem. C 2010, 114, 3707. (b) Litter, M. I.; Blesa, M. A. J. Colloid Interface Sci. 1988, 125, 679. (c) Gr€atzel, M.; Kiwi, J.; Morrison, C. L.; Davidson, R. S.; Tseung, A. C. C. J. Chem. Soc. Faraday Trans. 1985, 81, 1883. (21) Cao, C.-Y.; Cui, Z.-M.; Chen, C.-Q.; Song, W.-G.; Cai, W. J. Phys. Chem. C 2010, 114, 9865. (22) Deshpande, A. S.; Pinna, N.; Beato, P.; Antonietti, M.; Niederberger, M. Chem. Mater. 2004, 16, 2599. (23) Nazari, K.; Esmaeili, N.; Mahmoudi, A.; Rahimi, H.; MoosaviMovahedi, A. A. Enzyme Microb. Technol. 2007, 41, 226. (24) Kobayashi, A.; Koguchi, Y.; Kanzaki, H.; Kajiyama, S.; Kawazu, K. Biosci., Biotechnol., Biochem. 1994, 58, 133. (25) (a) Anderson, J.; Karakoti, A.; Daz, D. J.; Seal, S. J. Phys. Chem. C 2010, 114, 4595. (b) Liu, Y.; Sun, D. Appl. Catal., B 2007, 72, 205. (26) (a) Tonami, H.; Uyama, H.; Nagahata, R.; Kobayashi, S. Chem. Lett. 2004, 33, 796. (b) Hwang, S.; Lee, C.-H.; Ahn, I.-S. J. Ind. Eng. Chem. 2008, 14, 487. (27) Kamada, K.; Tsukahara, S.; Soh, N. J. Mater. Chem. 2010, 20, 5646. (28) Samet, Y.; Elaoud, S. C.; Ammar, S.; Abdelhedi, R. J. Hazard. Mater. B 2006, 138, 614. (29) (a) Wang, A. Q.; Golden, T. D. J. Electrochem. Soc. 2003, 150, C616. (b) Golden, T. D.; Wang, A. Q. J. Electrochem. Soc. 2003, 150, C621.

1207

dx.doi.org/10.1021/cg1014048 |Cryst. Growth Des. 2011, 11, 1202–1207