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Nonaqueous Sol-Gel Synthesized Hierarchical CeO2 Nanocrystal Microspheres as Novel Adsorbents for Wastewater Treatment Haiyan Xiao, Zhihui Ai, and Lizhi Zhang* Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China ReceiVed: May 28, 2009; ReVised Manuscript ReceiVed: August 3, 2009
Hierarchical CeO2 nanocrystal microspheres were synthesized with a nonaqueous sol-gel method at a low temperature of 120 °C. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and nitrogen sorption measurements. The adsorption performances of hierarchical CeO2 nanocrystal microspheres were tested with the batch removals of typical pollutants of Cr(VI) and rhodamine B from simulated wastewater. It was found that the nanostructured CeO2 could effectively remove Cr(VI) without pH preadjustment. The Freundlich adsorption isotherm was applicable to describe the removal processes. Kinetics of the Cr(VI) removal was found to follow pseudosecond-order rate equation. Furthermore, the as-prepared and Cr(VI)-adsorbed hierarchical CeO2 nanocrystal microspheres were carefully analyzed by X-ray photoelectron spectroscopy (XPS). On the basis of the XPS results, a possible mechanism of Cr(VI) removal with hierarchical CeO2 nanocrystal microspheres was proposed. Moreover, these nonaqueous sol-gel synthesized hierarchical CeO2 nanocrystal microspheres also exhibited remarkable ability to remove rhodamine B, suggesting they are promising absorbents for wastewater treatment. 1. Introduction Inorganic heavy metal ions and organic dyes removal have attracted considerable attention because of their long-term environmental toxicity and short-term public health damage.1,2 The development of nanoscience and nanotechnology is expected to play an important role for the remediation of environmental problems by using nanosorbents, nanocatalysts, bioactive nanoparticles, catalytic membranes with nanostructures, and so on.3-5 Various nanomaterials have been adopted for the treatment of environmental pollutants. For example, semiconductor nanomaterials, such as ZnS, Fe2O3, TiO2, and Cu2O, have been utilized to remove organic contaminants from various contaminated sources due to their excellent photoactivity.6-10 Other materials, such as zerovalent iron (Fe0) and bimetallic nanoparticles, were also used as effective redox media to remove organic and inorganic pollutants from water. These materials showed higher removal capacities than bulk materials.11,12 One of the ultimate goals of nanoscience and nanotechnology for the pollutant removal from wastewater is to develop ideal nanomaterials with low cost and strong mechanical structures that can suffer from water flow for long time as well as high removal capacities and fast adsorption rates for heavy metal ions.13 Hierarchically structured materials fit these criteria well. Their novel structures result in a high surface to bulk ratio and a large surface area, contributing to high adsorption capacities and faster adsorption rates. Another remarkable advantage of hierarchical architectures is their facile mass transportation ability. As one of the most important rare earth metal oxides, ceria has been extensively studied and applied in the field of catalysts,14,15 oxygen sensor,16 electrolyte materials of solid oxide fuel cells,17 ultraviolet blocking materials,18 luminescence,19,20 and chemical mechanical planarization materials21because of its * To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn. Phone/Fax: +86-27-6786 7535.
outstanding physical and chemical properties. The main environmental applications of ceria focus on the removal of gaseous noxious compounds from automobiles exhaust.22 Recently, it was interesting to find that hierarchical micro/nanocomposite ceria showed excellent removal capacities of toxic heavy metal ions. For example, Wan et al. used tetrabutylammonium bromide (TBAB) as a template to synthesize ceria with a flowerlike hierarchical structure in two steps.1 In the first step, ceria precursor with a flowerlike morphology was prepared through an ethylene glycol mediated process in the presence of urea at 180 °C. Then, the precursor was calcined at 450 °C to obtain ceria with flowerlike morphology. These three-dimensional (3D) flowerlike ceria were able to remove toxic ions from water with higher removal capacity than bulk materials. However, their outstanding adsorption performance could only be exhibited in case of the low pH value (ca. pH ) 3). In recent years, Markus Niederberger and his co-workers developed nonaqueous sol-gel strategies to synthesize various crystalline metal oxide nanostructures.23,24 In contrast to hydrothermal and aqueous sol-gel techniques that are in general highly sensitive to the reaction conditions, these nonaqueous sol-gel procedures offer a robust synthetic methodology allowing the direct preparation of crystalline nanoparticles without subsequent calcinations. The reactions are performed at low temperature without the use of any surfactants. Recently, InNbO4 and BiOX (X ) Cl, Br, I) photocatalysts were prepared by utilizing nonaqueous sol-gel methods in our group.25,26 In this study, we explore a nonaqueous sol-gel method to synthesize hierarchical CeO2 nanocrystal microspheres by reacting cerium nitrate hydrate with benzyl alcohol at a temperature as low as 120 °C for the first time. The as-prepared product was well crystallized and subsequent calcination was no longer necessary. The resulting hierarchical CeO2 nanocrystal microspheres could effectively remove heavy metal ions (Cr(VI)) and organic dye (RhB) from simulated wastewater at neutral pH.
10.1021/jp9050269 CCC: $40.75 2009 American Chemical Society Published on Web 08/27/2009
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2. Experimental Section 2.1. Chemicals. Ce(NO3)3 · 6H2O and benzyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification. All chemicals used in this study were of commercially available analytical grade. Deionized water was used in all experiments. 2.2. Preparation of Hierarchical CeO2 Nanocrystal Microspheres. In a typical synthesis, 1 mmol of Ce(NO3)3 · 6H2O was dissolved in 10 mL of benzyl alcohol. The resulting solution was reacted in a single-neck flask equipped with a condenser under stirring at 120 °C for 2 days. The resulting solid products were centrifuged, washed with anhydrous ethanol, and finally dried at 60 °C in air for use. 2.3. Characterization. X-ray powder diffraction (XRD) measurements of all the samples were performed in the reflection mode (Cu KR radiation, λ ) 1.54178) on a Rigaku Ultima III X-ray diffractometer. The morphology and particle sizes were determined by a scanning electron microscopy (SEM, JEOL 6700-F). The samples for transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were prepared by dispersing the final powders in ethanol and the dispersion was dropped on carbon-copper grids. Then, the obtained powders deposited on a copper grid were observed by a high-resolution transmission electron microscope (JEOL JSM-2010) operating at 200 kV. UV-vis absorption spectra were recorded on a Hitachi U-3310 UV-visible spectrophotometer. Nitrogen adsorption-desorption isotherms were collected on a Micromeritics Tristar-3000 surface area and porosity analyzer at 77 K after the sample had been degassed in the flow of N2 at 180 °C for 5 h. The BET surface area was calculated from the linear part of the BET plot. pH values were measured with an Orion model 920A pH/ISE meter. 2.4. Cr(VI) Adsorption. The stock solution containing 1000 mg L-1 of Cr(VI) was prepared by dissolving 1.4114 g of K2Cr2O7 in 500 mL of deionized water. Simulated wastewater with different Cr(VI) concentrations (2.0, 4.0, 8.0 mg L-1) were prepared by dilution of the stock K2Cr2O7 standard solution with deionized water. Typically, the Cr(VI) adsorption experiments were carried out in a 100 mL beaker containing 50 mL of simulated wastewater at room temperature (25 °C) and without the further pH adjustment. For comparison, the initial pH values of the Cr(VI) solution were adjusted to acidic or alkaline condition by addition of dilute hydrochloric acid and sodium hydroxide in some cases. After the adsorption processes, the samples were filtered through a 0.45 µm membrane, and the filtrates were immediately analyzed by atomic absorption spectrometry (WFX-1F2, China). All the adsorption experiments were carried out in duplicate. The relative deviations are in the range of (5.0%. The amount of Cr(VI) adsorbed per unit mass of the adsorbent was evaluated by using the mass balance equation (eq 1)
qt ) (C0 - Ct)V/W
(1)
where qt (mg g-1) is the amount adsorbed per gram of adsorbent at time t (min), C0 is the initial concentration of Cr(VI) in the solution (mg L-1), Ct is the concentration of Cr(VI) at time t of adsorption (mg L-1), W is the mass of the adsorbent used (g), and V (L) is the initial volume of the Cr(VI) solution. 2.5. RhB Adsorption. On hundred milligrams of hierarchical CeO2 nanocrystal microspheres was suspended in 100 mL of RhB aqueous solution with a concentration of 5 mg L-1 in a flask under stirring. The flask was covered with tin foil to
Figure 1. The XRD pattern of the as-prepared sample.
prevent the photodegradation of RhB. At different intervals, 5 mL of the suspensions was collected, filtered through a 0.45 µm membrane, and finally analyzed by a Hitachi U-3310 UV-visible spectrophotometer immediately. 3. Results and Discussion Characterization of the As-Prepared Product. The X-ray diffraction (XRD) was used to characterize the purity and crystallinity of the as-prepared sample (Figure 1). The XRD pattern could be indexed to cubic CeO2 (JCPDS, file No. 75120). The diffraction peaks at 2θ values of 28.49, 33.05, 47.37, and 56.18° can be assigned to the reflections of (111), (200), (220), and (311) planes of cubic CeO2, respectively. No other peaks were observed, revealing high purity of the sample. The broadening of the XRD peaks reflects the nanocrystalline nature of the resulting CeO2. Electron microscopy was utilized to study the microstructure of the as-prepared sample. Figure 2a shows a typical SEM image of the as-prepared CeO2 sample. The as-prepared CeO2 consists of plenty of highly aggregated spheres with rough surface. These spheres were not uniform and their sizes were in the range from several nanometers to micrometers. Some broken spheres were also observed. TEM analysis clearly revealed the microstructure of the CeO2 nanospheres (Figure 2b). These nanospheres are of porous structure and 50-100 nm in diameter. Each sphere contains numerous particles of several nanometers in size. These observations confirm that the asprepared CeO2 sample is of three-dimensional hierarchical structures. These hierarchical structured spheres were robust enough to tolerate ultrasound irradiation during TEM sample preparation without beimg destroyed. A representative highresolution TEM (HRTEM) image in Figure 2c shows lattice fringes with a spacing of 0.251 nm, corresponding to the spacing of the (200) planes of ceria.27,28 The porous structure of the as-prepared CeO2 sample was studied by N2 gas adsorption-desorption (Figure S1, Supporting Information). Figure S1 displays typical IV adsorption isotherms with the hysteresis loop H1, indicative of porous structure arisen from intraaggregated nanoparticles in the microspheres.4,26 This is consistent with electron microscopy results. The product exhibits Brunauer-Emmett-Teller (BET) specific surface areas of 65 m2/g and total pore volume of 0.037 cm3/g, significantly higher than those of flowerlike hierarchical ceria synthesized in ethylene glycol.1
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Figure 3. Time profile of Cr(VI) removal with hierarchical CeO2 nanocrystal microspheres. The concentration of hierarchical CeO2 nanocrystal microspheres adsorbent was 1.0 g L-1; the initial Cr(VI) concentrations ranged from 2.0 to 8.0 mg L-1.
TABLE 1: Comparisons of Cr(VI) Adsorption Capacities of Different Ceria Adsorbents
Figure 2. (a) SEM, (b) TEM, and (c) HRTEM images of the as-prepared CeO2 sample.
Removal of Cr(VI). Cr(VI) is considered as a highly toxic pollutant. In this study, the adsorption ability of the as-prepared hierarchical CeO2 nanocrystal microspheres was first tested by adsorptive removal of Cr(VI) from simulated wastewater. Figure 3 shows the time profile of Cr(VI) removal with 1.0 g L-1 of hierarchical CeO2 nanocrystal microspheres at different initial Cr(VI) concentrations and neutral pH. From Figure 3, two important conclusions could be drawn. First, the adsorption rates within the first 5 min were surprisingly fast under all the concentrations, nearly 75% of Cr(VI) were adsorbed, which may be attributed to the hierarchical structures of ceria microspheres. The adsorption process almost finished within 120 min, and no significant change was observed from 120 to 240 min. The removal efficiencies were found to be 77.6, 85.5, and 84.4% at the initial Cr(VI) concentrations of 2.0, 4.0, and 8.0 mg L-1, respectively. Therefore, the maximum Cr(VI) adsorption capacity of these hierarchical CeO2 nanocrystal microspheres was found to be 6.76 mg g-1 when the initial Cr(VI) concentration was 8.0 mg L-1. This reveals the high efficiency of the hierarchical CeO2 nanocrystal microspheres for the removal of
adsorbent
surface area (m2/g)
removal capacity (mg/g)
ceria microspheres 3D flowerlike ceria commercial ceria
65 34 2
6.76 5.8 0.37
pH
references
≈7 3
this work 13
Cr(VI) ions in aqueous solutions with low Cr(VI) concentration. Second, it reveals that the total amount of Cr(VI) adsorbed with the increase of the initial Cr(VI) concentrations. This is because more Cr(VI) could provide higher driving force for the ions from the solution to the hierarchical CeO2 nanocrystal microspheres and more collisions between Cr(VI) ions and active sites of the hierarchical CeO2 nanocrystal microspheres. Similar phenomena have also been observed in the literature.12 After the adsorption, the solid/liquid separation in suspension was rather easy by centrifugation as the as-prepared ceria spheres were several micrometers in size. We also interestingly found that the high adsorptive removal efficiency of these nonaqueous sol-gel synthesized hierarchical CeO2 nanocrystal microspheres was not affected by the pH value of aqueous solutions (Figure S2, Supporting Information), suggesting the preadjusting of pH value of wastewater is no longer necessary. This advantage is very important for practical applications. On the contrast, the TBAB-template synthesized flowerlike hierarchical ceria in ethylene glycol could only possess high adsorptive removal efficiency of Cr(VI) at low pH value (ca. pH ) 3).1 Table 1 summaries the comparisons between the adsorption capacity of the nonaqueous sol-gel synthesized hierarchical CeO2 nanocrystal microspheres in this study and those of ceria synthesized in previous literatures. Obviously, the surface area and adsorption capacity of our nonaqueous sol-gel synthesized hierarchical CeO2 nanocrystal microspheres are higher than those of other ceria products. Adsorption Isotherms. Both Langmuir and Freundlich isotherms tried to describe the adsorption behavior of Cr(VI) on the hierarchical CeO2 nanocrystal microspheres29,30 respectively. The Langmuir isotherm is often applicable to a homogeneous adsorption surface with all the adsorption sites having equal adsorbate affinity, while the Freundlich isotherm model assumes heterogeneity of adsorption surfaces, expressed by the Freundlich equation (eq 2), where qe and Ce are the amount of
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Figure 4. Representative Freundlich isotherm of Cr(VI) adsorption. The initial Cr(VI) concentrations were from 2.0 to 8.0 mg L-1; the hierarchical CeO2 nanocrystal microspheres adsorbent dose was 1.0 g L-1, and the contact time was 4 h.
Cr(VI) adsorbed (mg g-1) and Cr(VI) concentration at equilibrium, respectively. K and n are the Freundlich isotherm constants; K value indicates the adsorption capacity, and n is related to the energetic heterogeneity (average energy of sites).
qe ) KC1/n e
(2)
Figure 4 reveals that the adsorption data of Cr(VI) on the hierarchical CeO2 nanocrystal microspheres fit well to Freundlich isotherm model with a correlation coefficient R2 value of 0.97 and n value of 1.78 (between 1 and 10), representing beneficial adsorption.31 For comparison, the Langmuir model was also used to fit the adsorption data. However, the resulting correlation coefficient of 0.94 was not as high as that of Freundlich model (Figure S3, Supporting Information). The better fitting of the Freundlich isotherm model may be attributed to the heterogeneous distribution of active sites on the hierarchical CeO2 nanocrystal microspheres surface.32 Kinetics Study. The kinetics of Cr(VI) removal on hierarchical CeO2 nanocrystal microspheres was further investigated at various initial Cr(VI) concentrations in the presence of 1.0 g L-1 of CeO2. The pseudosecond-order rate constants (k2) and the amount of Cr(VI) adsorbed at equilibrium (qe) were calculated from the slope and intercept of the plots of t/qt versus t according to eq 3.
t 1 1 ) + t 2 qt q k2qe e
(3)
Where k2 (g/mg-1 g-1) is the pseudosecond-order rate constant, qe is the amount of Cr(VI) adsorbed (mg g-1) at equilibrium, and qt is the amount of the adsorption (mg g-1) at any time t. The kinetics of the removal process was shown in Figure 5. Table 2 summarizes the theoretical and calculated qe values, pseudosecond order rate constants k2, and correlation coefficient values (R2). The calculated qe is calculated from the slope and intercept of the plots of t/qt versus t according to the eq 3. The theoretical qe values are the equilibrium concentrations of Cr(VI) in the adsorbed hierarchical CeO2 nanocrystal
Figure 5. Removal kinetics of 2.0, 4.0, and 8.0 mg L-1 of Cr(VI) in the presence of 1.0 g L-1 hierarchical CeO2 nanocrystal microspheres. The contact time was 4 h.
TABLE 2: Theoretical and Calculated qe Values, Pseudo-Second-Order Rate Constants, k2, and Correlation Coefficient Values (R2)a theoretical qe (mg g-1)
calculated qe (mg g-1)
k2 (g/mg-1min-1)
R2
2.0 4.0 8.0
1.5621 3.4616 6.8101
0.6402 0.2889 0.1468
0.9999 0.9998 0.9995
a The dosage of hierarchical CeO2 nanocrystal microspheres adsorbent was 1.0 g L-1.
microspheres assuming 100% of Cr(VI) is removed. The calculated qe values were very close to the theoretical ones, showing quite good linearity with R2 above 0.999. Therefore, the adsorption kinetics follows the pseudosecond-order model,33 suggesting a chemisorption process.34 XPS Spectra of the As-Prepared and Cr(VI)-Adsorbed Hierarchical CeO2 Nanocrystal Microspheres. X-ray photoelectron spectroscopy was further used to study the surface chemical compositions of the as-prepared and Cr(VI)-adsorbed hierarchical CeO2 nanocrystal microspheres (Figure 6). No obvious changes were observed on two survey spectra of the two samples (Figure 6a). This may due to tiny amount of Cr(VI) absorbed on the CeO2. However, the high resolution XPS spectrum of the Cr 2p region clearly reveals the presence of Cr(VI) (Figure 6b). The two broad peaks attributed to Cr 2p1/2 and Cr 2p3/2 could be fitted to two peaks, respectively. The Cr 2p1/2 line peak split into two peaks at binding energies of 587.6 and 586.2 eV, which are the characteristics of Cr(VI) and Cr(III), respectively.35,36 While the broad peak of Cr 2p3/2 could also be fitted to two peaks located at 578.6 and 576.7 eV, confirming that both Cr(VI) and Cr(III) coexist on the surface of Cr(VI)adsorbed hierarchical CeO2 nanocrystal microspheres.12 Obviously, the reduction of Cr(VI) to Cr(III) on ceria took place during the adsorption. In order to find how Cr(VI) was reduced, we further investigated the Ce 3d spectra of hierarchical CeO2 nanocrystal microspheres before and after Cr(VI) was adsorbed (Figure 6c). The series of V and U peaks of cerium are observed on the 3d5/2 and 3d3/2 spectra, respectively. Since the two valleys between V and V′ as well as U and U′ arise from the photoemission from Ce3+ cations, a qualitative estimation of the degree of reduction of Ce4+ in CeO2 could be made on the
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Figure 7. (a) Photographs of and (b) absorption spectra of 100 mL of 5 mg L-1 RhB aqueous solution in the presence of 100 mg of hierarchical CeO2 nanocrystal microspheres at different times.
Figure 6. (a) Survey, (b) Cr 2p high resolution XPS spectra of the Cr(VI)-adsorbed sample, (c) Ce 3d high resolution XPS spectra of the as-prepared and Cr(VI)-adsorbed hierarchical CeO2 nanocrystal microspheres.
basis of the features of these two valleys on the spectrum according to the literatures.37,38 The two valleys would be very obvious in case of a small amount of Ce3+ in CeO2. On the contrast, if the degree of reduction of Ce4+ to Ce3+ is high and Ce3+ becomes more concentrated, the two valleys between V and V’ as well as U and U’ start to vanish.38 It is clearly that
the valleys between V and V′ as well as U and U′ are much less obvious for the as-prepared hierarchical CeO2 nanocrystal microspheres, suggesting they have more concentrated Ce3+ and thus more oxygen vacancies. It was reported that cerium would be present as Ce3+ at the oxygen vacancy site in CeO2 and thus has the ability to reduce Cr(VI) to Cr(III).39 This explains the reduction of Cr(VI) to Cr(III) in this study. Previous studies also found that oxygen vacancies located at the surface of ceria were the site of catalysis and allowed for redox reactions with oxygen containing compound.39-41 Therefore, in this study the adsorption and the partial reduction of Cr(VI) on hierarchical CeO2 nanocrystal microspheres took place simultaneously, similar to that on Fe@Fe2O3 core-shell nanowires reported in our previous study.12 Removal of RhB. The removal of dye pollutants are also of great importance for wastewater treatment. RhB, a common cationic dye used in textile industry, is one of the most notorious contaminants in aquatic environments because of it huge amounts, slow biodegradation, and toxicity. In this study, the nonaqueous sol-gel synthesized hierarchical CeO2 nanocrystal microspheres are further used to remove RhB. The absorption spectrum of RhB solution was characterized by its characteristic absorption at 553 nm, which was attributed to the chromophorecontaining azo linkage (conjugated xanthene ring) of the dye molecules.42 Figure 7a shows that 100 mg of hierarchical CeO2 nanocrystal microspheres could completely decolorize 100 mL of 5 mg L-1 RhB aqueous solution in 60 min. UV-vis absorption spectra (Figure 7b) confirms that over 95.0% of RhB in 100 mL of 5 mg L-1 RhB aqueous solution could be removed with 100 mg of the as-prepared hierarchical CeO2 nanocrystalline microspheres within 60 min. Two reasons may account for the efficient removal of Cr(VI) and RhB with hierarchical CeO2 nanocrystal microspheres. The first reason should be their hierarchical architectures to facilitate the diffusion and adsorption of Cr(VI) and RhB.1,13 The second one is attributed to the oxygen vacancies on the surface of hierarchical CeO2 nanocrystal microspheres, as revealed by XPS analysis. These abundant surface oxygen vacancies could reduce Cr(VI) to Cr(III) and produce strong electrostatic attraction with
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the cationic groups of RhB as well as hydrogen bonding with the nitrogen atoms of RhB. 4. Conclusions We have successfully fabricated hierarchical CeO2 nanocrystal microspheres with a benzyl alcohol nonaqueous sol-gel method at a low temperature for the first time. These hierarchical CeO2 nanocrystal microspheres exhibited excellent Cr(VI) removal ability at neutral pH. The Freundlich adsorption isotherm was applicable to describe the removal processes. Kinetics of the Cr(VI) removal was found to follow pseudosecond-order rate equation. We also proposed the possible mechanism of removal of Cr(VI) with the hierarchical CeO2 nanocrystal microspheres, which involved the dominant Cr(VI) adsorption, followed by the partial reduction of Cr(VI) to Cr(III) on the surface of adsorbents. These hierarchical CeO2 nanocrystal microspheres could also adsorb RhB from stimulated wastewater efficiently. This study reveals that these hierarchical CeO2 nanocrystal microspheres are novel adsorbents for wastewater treatment. Acknowledgment. This work was supported by National Science Foundation of China (Grants 20673041 and 20777026), National Basic Research Program of China (973 Program) (Grant 2007CB613301), Program for New Century Excellent Talents in University (Grant NCET-07-0352), the Key Project of Ministry of Education of China (Grant 108097), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and Postdoctors Foundation of China (Grant 20070410935). Supporting Information Available: The N2 adsorptiondesorption isotherms of the as-prepared sample; the influence of the initial pH on the removal of Cr(VI) with hierarchical CeO2 nanocrystal microspheres; the Langmuir isotherm of Cr(VI) adsorbed on the hierarchical CeO2 nanocrystal microspheres. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhong, L. S.; Hu, J. S.; Cao, A. M.; Liu, Q.; Song, W. G.; Wan, L. J. Chem. Mater. 2007, 19, 1648–1655. (2) Fei, J. B.; Cui, Y.; Yan, X. H.; Yang, Y.; Wang, K. W.; He, Q.; Li, J. B. AdV. Mater. 2008, 20, 452–456. (3) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729–7744. (4) Chen, H. M.; He, J. H. J. Phys. Chem. C 2008, 112, 17540–17545. (5) Cao, S. W.; Zhu, Y. J. J. Phys. Chem. C 2008, 112, 6253–6257. (6) Georgekutty, R.; Seery, M. K.; Pillai, S. C. J. Phys. Chem. C 2008, 112, 13563–13570. (7) Zhong, Z. Y.; Ho, J.; Teo, J.; Shen, S. C.; Gedanken, A. Chem. Mater. 2007, 19, 4776–4782. (8) Xu, H.; Jia, F. L.; Ai, Z. H.; Zhang, L. Z. Cryst. Growth Des. 2007, 7, 1216–1219.
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