Hierarchical Hollow Structure ZnO: Synthesis, Characterization, and

Article pubs.acs.org/IECR

Hierarchical Hollow Structure ZnO: Synthesis, Characterization, and Highly Efficient Adsorption/Photocatalysis toward Congo Red Shi Lan, Lu Liu, Ruiqing Li, Zhihua Leng, and Shucai Gan* College of Chemistry, Jilin University, Changchun 130026, People’s Republic of China ABSTRACT: Hierarchical hollow structure ZnO (CZ-400) was synthesized successfully by a facile homogeneous precipitation method. Morphology, structure, and optical properties of the as-prepared CZ-400 were characterized by different techniques. The mentioned product possessed hollow core and hierarchical shell morphology, and grew well-crystallinity with high surface area. The CZ-400 exhibited adsorption capacity and photocatalyst activity toward congo red (CR) higher than those of TiO2 P25 and commercial ZnO. This is attributed to the hierarchical structure of CZ-400, which provides the improved charge transport and the reduced recombination rate of photogenerated electron−hole pairs. In addition, the combinatorial effect of adsorption and photodegradation reflected the importance of adsorption in the enhanced photoreactivity. The results indicated that CZ-400 is a potential catalyst and adsorbent material for removal of CR from water samples. against aggregate and larger active surface area.13 To the best of our knowledge, few have reported the research about the adsorption/photocatalysis of ZnO with hierarchical hollow structure toward removal of organic dyes. Herein, we report the facile synthesis of hierarchical hollow structure ZnO through a facile homogeneous precipitation method at low temperature using carbon spheres as templates followed by annealing process. The resulting porous ZnO exhibits a surface area up to 31.0 m2 g−1 and a pore volume of 0.26 cm3 g−1. Importantly, this ZnO can not only strongly adsorb CR from aqueous solution but photodegrade the CR with high efficiency. The results indicated that CZ-400 is a combined adsorbent and catalyst for removal of CR from water samples.

1. INTRODUCTION Organic dyes were excessively used in the textile, plastics, printing, and cosmetic industries and are a severe environmental problem.1,2 Among various classes of dye compounds, the azo dyes have occupied 80% of the total amount of industrial organic dyes.3 These pollutants generally result from the discharge of harmful chemicals and colored effluents. Because of growing concern regarding environmental problems and human health in recent years, several wastewater treatment techniques have been used extensively. The most common of these include electrocoagulation, ion-exchange, adsorption, catalytic oxidation, filtration, etc.4,5 Nevertheless, some synthetic organic dye in dyeing wastewater is non biodegradable and hence cannot be effectively removed or decomposed by the above methods. The photocatalytic treatment is one of the green technologies to degrade the contaminants of water caused by the organic dyes and chemicals used in the textile industries. Therefore, a series of semiconductor photosensitizers with nanostructure have been powerfully explored in water treatment.6 Zinc oxide, with a wide band gap (3.37 eV) and large excitation binding energy (60 meV), has been considered as an important photocatalyst because of its high photosensitivity and nontoxicity.7 Previous studies concluded that the conventional ZnO exhibits low photodegradation activity due to the easy recombination of the photogenerated electrons and hole pairs in photocatalytic process.8,9 Although nanosized ZnO could be a typical semiconductor-based photosensitizer, they tend to aggregate easily due to their small size, leading to unwanted reduction in the active surface area and then present reduced photodegradation efficiency. Recently, considerable effects have been devoted to the synthesis of ZnO with different surface areas including nanoplates ZnO (15.52 m2/g),10 nanocomposites ZnO (14.1 m2/g),11 nanoneedles ZnO (24.2 m2/ g), nanospheres ZnO (20.8 m2/g), and polyhedral ZnO (7.5 m2/g).12 One key point of recent research is the development of experimental methods to selectively control the size, shape, and organization of materials. Hierarchical porous morphologies have attracted a lot of attention due to a high stability © 2014 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and urea (CO(NH2)2) were purchased from Tianjin Chemical Reagent Research Co. Glucose (C6H12O6) was purchased from Shanghai Chemical Reagent Plant. For batch adsorption experiments, congo red (CR) was purchased from Beijing Chemical Reagent Research Co. and used without further purification. Distilled water was used in all experiments. 2.2. Characterization. Morphology, structure, and particles size of the synthesized magnetic nanocomposite particles were observed by using a Hitachi 8100 transmission electron microscope (TEM, Hitachi, Tokyo, Japan). The infrared spectra of the nanocomposites were taken in KBr pressed pellets on a NEXUS 670 infrared Fourier transform spectrometer (Nicolet Thermo, Waltham, MA). X-ray diffraction (XRD) measurements were recorded on a Rigaku D/MAXIIA diffractometer using Cu Kα radiation. Scanning electron microscopy (SEM) was performed on a TESCAN Received: Revised: Accepted: Published: 3131

November 30, 2013 January 23, 2014 February 4, 2014 February 4, 2014 dx.doi.org/10.1021/ie404053m | Ind. Eng. Chem. Res. 2014, 53, 3131−3139

Industrial & Engineering Chemistry Research

Article

Scheme 1. Schematic Illustration for the Formation of CZ-400 with Hierarchical Hollow Structure

2.4. Adsorption and Photocatalysis Activity Tests of Congo Red. The prepared CZ-400 can be utilized as efficient adsorbent/photocatalyst for CR removal. To evaluate the adsorption/catalysis efficiency of CZ-400, the removal experiments of CR with TiO2 P25 and commercial ZnO were also carried out under the identical reaction conditions. Typically, a stock solution of CR (1000 mg L−1) was prepared in deionized water, and then reaction solution with different initial concentrations (50−200 mg L−1) was obtained by successive dilutions. Batch experiments were conducted by mixing 20 mL of CR solution of different initial concentrations with 40 mg of adsorbent at a pH value of 7 (the pH value of the aqueous solution was adjusted to 7 for comparative studies). The mixtures were vibrated in the dark for 1 h to establish the adsorption equilibrium between the CR molecules and the adsorbents surface. The concentration of CR before and after treatment was calculated via a UV−vis spectrophotometer (UV-2550, Shimadzu, Japan). The adsorption capacity, qe (mg g−1), was calculated using the following equation:15,16

5136MMSEM at an accelerating voltage of 20 kV. The Brunauer−Emmett−Teller (BET) surface area of the powders was analyzed by nitrogen adsorption on a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.). Before nitrogen adsorption measurement, the sample was degassed in a vacuum at 150 °C for 7 h. The BET surface area was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.06−0.30. Adsorption branches of the isotherms were used to determine the pore size distributions for the samples studied via the Barrett− Joyner−Halenda (BJH) method. The volume of nitrogen adsorbed at the relative pressure (P/Po) of 0.99 was used to determine the pore volume. Differential scanning calorimetrythermogravimetric analyzer (DSC/TGA 1600 LF, METTLER TOLEDO, Switzerland) up to 800 °C was performed on the sample at a heating rate of 10 °C min−1, while the N2 gas flow rate was 60 mL min−1. 2.3. Preparation of Hierarchical Hollow Structure ZnO. All chemicals are analytical grade reagents and used without further purification. Carbon spheres were prepared according to the reported procedure.14 The hierarchical hollow structure ZnO was synthesized by a facile homogeneous precipitation method. Briefly, zinc nitrate hexahydrate (0.89 g, 3 mmol) was dissolved in 50 mL of deionized water at room temperature. Next, the aqueous solution of carbon (0.03 g) and urea (0.54 g, 9 mmol) was added into the solution under stirring at room temperature. The resultant mixture was heated to 90 °C in water bath and maintained at 90 °C for 9 h. The mole ratio between the precipitant agent urea to zinc precursor was maintained at 3:1. After being cooled, centrifuged, and washed, the brown sample was dried at 65 °C. To obtain the ZnO, the prepared sample was then calcined at 400 °C for 2 h under air. The products without and with calcination were denoted as CZ-90 and CZ-400, respectively. The schematic diagram for preparation of CZ-400 nanostructures has been illustrated in Scheme 1.

qe =

(C0 − Ce) × V W

where C0 is the initial concentration of CR (mg L−1), Ce is the equilibrium concentration of CR after adsorption (mg L−1), V is the volume of CR solution (mL), and W is the weight of the synthesized adsorbent (mg). The photocatalytic activity of the prepared samples was estimated by determining the decomposition of CR under UV light irradiation. Photocatalytic experiments were carried out in a thermostat under continuous stirring to obtain complete mixing of the reaction solution. A UV Osram lamp (λ = 365 nm, 125 W) was used as UV source and was fixed on the top of the reactor. Prior to illumination, the reaction suspension was stirred continuously in the dark for 40 min to establish the adsorption/desorption equilibrium of CR on the samples. The 3132

dx.doi.org/10.1021/ie404053m | Ind. Eng. Chem. Res. 2014, 53, 3131−3139


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Figure 1. XRD pattern (left) and FTIR spectra (right) of CZ-90 (a and c) and CZ-400 (b and d).

fixed amount of the photocatalyst was mixed with the aqueous suspension containing CR (100 mg L−1), and then was irradiated by UV light for regular intervals of time under constant shaking. After the samples were collected from the suspension at regular intervals of time, the absorbance measurements were analyzed via a UV−visible spectrophotometer at a wavelength of 498 nm.

was shown in both samples. FTIR results indicated that ZnO was successfully fabricated through a homogeneous precipitation method followed by the calcination process.25,26 3.2. Morphological and Structural Analysis. Figure 2a and b presents the typical TEM and SEM images of the as-

3. RESULTS AND DISCUSSION 3.1. XRD Patterns and FTIR Spectra Analysis. XRD pattern combined with FTIR analysis were employed to indicate the crystal structure and the chemical composition of the prepared samples. The corresponding XRD pattern of the CZ-90 was shown in Figure 1a. The position and relative intensity of CZ-90 were assigned to monoclinic hydrozincite Zn5(OH)6(CO3)2 phase (JCPDS card no. 72-1100) with lattice parameters of a = 13.580 Å, b = 6.280 Å, and c = 5.410 Å.17,18 Figure 1b shows the typical XRD pattern of sample CZ-400. The characteristic diffraction peaks at 2θ = 31.8°, 34.6°, 36.3°, 47.6°, 56.7°, 62.9°, 66.5°, 67.8°, 69.2°, and 77.0° represent the corresponding indexes (100), (002), (101), (102), (110), (103), (200), (112), (201), and (202), respectively. These characteristic diffraction peaks were in good agreement with the standard data for the ZnO (JCPDS card no. 65−3411) with hexagonal structure.19,20 No other peaks resulting from reactants or intermediate phases were observed, indicating that CZ-90 was transformed to ZnO with high crystallinity and purity. The crystallite size of CZ-400 can be estimated using Scherrer’s equation:21,22

Figure 2. TEM image (a) and SEM image (b) of CZ-400.

prepared CZ-400. TEM observation reveals that the CZ-400 possesses a hollow core and rough shell composed of a large amount of thin nanoplates. SEM shows that the hierarchical architecture CZ-400 consists of nanoplates with the size range of 100−200 nm and the thickness of 10−15 nm, and the pores are dispersed randomly in the smooth planar surface. Energy dispersive spectroscopy (EDS) analysis was also carried out to further investigate the chemical compositions of the materials as shown in Figure 3. Besides oxygen and zinc elements from hydrozincite phase, carbon element was detected for CZ-90 as well. As for CZ-400, the corresponding signal of carbon element almost completely disappeared after calcination treatment, reflecting the successful removal of carbon template as shown in Figure 3b. 3.3. BET and TGA-DSC Analysis. The nitrogen adsorption/desorption analyzed by the Brunauer−Emmett− Teller (BET) method was employed to study the surface properties of the CZ-400. As shown in Figure 4, the isotherm of the CZ-400 exhibits type-IV curve, which is the typical characteristic of mesoporous and macroporous materials.27,28 It was also observed that the synthesized CZ-400 showed a BET surface area of 31.0 m2 g−1 and pore volume of 0.26 cm3 g−1. The corresponding pore size distribution of CZ-400 was studied using the BJH method as well, showing a sharp peak in the range of 3−120 nm with a maximum at 14 nm. The thermal behaviors of the fabricated CZ-90 and CZ-400 were estimated quantitatively by TGA-DSC profiles under N2 gas. When the as-prepared CZ-90 was heated in N2 gas flow,

Dhkl = Kλ /β cos θ

where λ is the X-ray wavelength (1.5405 Å), β is the full width at half-maximum of the XRD peaks, θ is the diffraction angle, K is the Scherrer constant (0.89), and Dhkl means the size along the (hkl) direction. Herein, we select the strongest peak (101) at 2θ = 36.3° to calculate the crystallites size. The estimated crystallite size of CZ-400 is calculated to be 13.9 nm. Figure 1c and d shows the FTIR spectra of CZ-90 and CZ400, respectively. A broad absorption band at about 3419 cm−1 was observed in both of these spectra that corresponded to the stretching vibration of O−H of adsorbed water on samples.23 The band at 1640 cm−1 was assigned to the bending vibration of O−H.24 As can be seen in the Figure 1c, the two bands at 1394 and 1049 cm−1 were attributed to the stretching vibration of C−C and C−O−C framework, respectively, reflecting the existence of carbonaceous spheres. Moreover, the absorption band at 489 cm−1 attributed to the stretching mode of Zn−O 3133



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Figure 3. EDS spectra of (a) CZ-90 and (b) CZ-400.

calcination of carbonaceous materials.32 As for CZ-400, the weight loss across the whole temperature range is relatively small, suggesting that ZnO is stable enough within the current TG measured temperature range. 3.4. Formation of Hierarchical Hollow Architecture ZnO. As shown in Scheme 1, zinc ions were easily attached to the surface of the aforementioned carbon spheres, and the monoclinic hydrozincite was initially formed on the surface of carbon core via a homogeneous precipitation process. Many short nanorods appeared initially, and further grew with the extended reaction time, then evolved into nanosheets and intersected with each other. Finally, these nanosheets assembled into a hierarchical structure to reduce the interfacial activation energy. A series of samples obtained from the different time-dependent experiment were analyzed via XRD. The corresponding XRD patterns of the products formed under different reaction times were shown in Figure 6. There is no significant difference between the intensity of the diffraction peak of CZ-90 formed under different reaction times. As can be seen in Figure 6b, all diffraction peaks can be indexed to ZnO with hexagonal structure, and no characteristic impurity peaks were observed, indicating the corresponding CZ-90 was transformed to ZnO with high crystallinity and purity.33−35 3.5. Adsorption of CR from Aqueous Solution. The prepared CZ-400 can be utilized as efficient adsorbent for CR removal. As can be seen in Figure 7a, when the initial dye concentration changed from 50 to 200 mg/L, the amount of dye adsorbed per unit weight of adsorbent (qe) at equilibrium

Figure 4. N2 adsorption−desorption isotherm of CZ-400. The inset indicates the pore size distribution of CZ-400 calculated from the adsorption branch by the BJH model.

the distinct weight loss was observed in the Figure 5a curve. A tiny weight loss was detected upon the decomposition of CZ90 in the range of 50−200 °C, which is attributed to the loss of residual water adsorbed physically on the sample.29 The significant weight loss between 200 and 300 °C was ascribed to the decomposition of monoclinic hydrozincite Zn5(OH)6(CO3)2 and the release of carbon dioxide and water.30 It is obvious that the decomposition temperature of hydrozincite Zn5(OH)6(CO3)2 is about 290 °C.31 The weight loss within the range of 300−750 °C was caused by the

Figure 5. TGA-DSC curves of CZ-90 (a) and CZ-400 (b). 3134



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Figure 6. XRD patterns of the CZ-90 (a) and CZ-400 (b) for different reaction times.

Figure 7. (a) Effect of the initial dye concentration on the adsorption capacity of CZ-400, commercial ZnO, and TiO2 P25 for CR. (b) The photograph of adsorbents before and after the adsorption of CR dye.

where qe is the equilibrium adsorption capacity of adsorbent (mg g−1), ce is the equilibrium concentration of dyes (mg L−1), qmax is the maximum amount of dye adsorbed (mg g−1), KL is the constant that refers to the bonding energy of adsorption (L mg−1), KF is the constant related to the adsorption capacity of the adsorbent (mg1−n Ln g−1), and n is the constant related to the adsorption intensity and adsorption capacity. The maximum sorption capacities (qm) by applying the Langmuir equation were shown in Table 1. The dye adsorption

time increased from 10.2 to 97 mg/g for CZ-400, from 7.4 to 41.2 mg/g for commercial ZnO, and from 2.4 to 7.2 mg/g for TiO2 P25. The enhanced adsorption activity with increasing initial concentration of CR was attributed to the fact that more organic molecules were adsorbed on the surface of adsorbents to overcome all mass transfer resistances of the pollutant between the aqueous and solid phases. As expected, the CZ-400 had higher adsorptive preference for CR than did commercial ZnO and TiO2 P25. This is in agreement with the fact that the adsorption performance of materials was not only determined by the specific surface area but correlated with the materials’ pore size and the molecular size of adsorbate.36 Accordingly, the as-obtained CZ-400 with hierarchical hollow porous structure is beneficial for enhancing adsorption intensity relatively large organic molecule. As shown in the Figure 7b, the color difference between adsorbents before and after the adsorption of CR dye further verified the adsorption behavior of CZ-400. 3.5.1. Isotherms of CR Adsorption. To describe how adsorbate molecules interact with adsorbent, the Langmuir and Freundlich isotherm equations were used to interpret the adsorption experimental data. The linear form of Langmuir and Freundlich isotherms can be described as the equation:37,38

Table 1. Adsorption Parameters of the Langmuir and Freundlich Models for the Adsorption of CR onto the CZ400, Commercial ZnO, and TiO2 P25 Langmuir model CZ-400 commercial ZnO TiO2 P25

Freundlich model

qmax

KL

R2

KF

1/n

R2

500 100 35.7

0.029 0.008 0.001

0.984 0.992 0.919

10.1 1.67 8.49

1.08 0.70 0.77

0.933 0.996 0.980

results revealed that only a small amount of CR was adsorbed on both the commercially available ZnO and the TiO2 P25. However, the as-prepared CZ-400 showed powerful adsorption of CR. Isotherm results of the adsorption process of CR by using different adsorbents were shown in Figure 8. On the basis of the squared correlation coefficient values, the adsorption data of the CZ-400 were well simulated with the Langmuir model, whereas for those of the commercial ZnO and TiO2

ce/qe = 1/KLqm + ce/qm

log qe = log KF + log ce/n 3135



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Figure 8. Adsorption Langmuir and Freundlich isotherms of CR onto different adsorbents: (a and b) CZ-400, (c and d) the commercial ZnO and TiO2 P25.

P25, simulation with the Freundlich model was better than those with the Langmuir model. 3.6. Photocatalytic Activity of CZ-400. 3.6.1. Effect of CZ-400 Dosage. Effect of catalysts concentration on the photodegradation efficiency of CR was investigated. Typically, the catalysts dosage within the range of 0.5−2.0 g/L was used for a CR initial concentration of 100 mg/L in aqueous solutions. Figure 9 shows the effect of catalysts on the removal of CR. From Figure 9, the photodegradation of CR in the absence of CZ-400 also did not change within 120 min UV irradiation, whereas a higher photodegradation of the CR dye

was observed with the presence of CZ-400 photocatalyst. The degradation percentage of CR increased with increasing CZ400 concentration because the increase in the catalyst doses increased the number of active sites on the photocatalyst surface, thus resulting in the increase of formation of hydroxyl radicals. In the following experiments, a lower loading of CZ400 was selected for the removal of CR to reduce the amount of used catalysts. 3.6.2. Effect of Reaction Temperature. Reaction temperature also plays a significant role in CR photodegradation process. The reaction was tested at 25, 35, and 45 °C, and the data were shown in Figure 10. The results revealed that the percentage of dye degradation increases with increasing temperature and reached up to 85% at 25 °C, to 88% at 35 °C, and then to 98% at 45 °C within 120 min irradiation time. As can be seen from Table 2, the pseudo-first-order rate constants (K) of CR degradation were found to be 0.015 min−1 (R2 = 0.980) at 25 °C, 0.017 min−1 (R2 = 0.978) at 35 °C, and 0.031 min−1 (R2 = 0.980) at 45 °C, which suggested that raising the reaction temperature could significantly increase the CR removal rate. The Arrhenius equation was used to understand the relationship between CR degradation rate and temperature. The equation form was expressed as follows:39,40 ln K = ln A − Ea /RT

where A is the frequency of collisions between two molecules in the proper orientation for reaction to occur, R is the gas constant (8.314 J/mol K), T is the absolute temperature, and Ea is the activation energy. The activation energy was 19.7 kJ/mol obtained by plotting ln K versus 1/T.

Figure 9. Effect of CZ-400 dosage on photodegradation efficiency of CR. Reaction conditions: C0(CR) = 100 mg/L, pH = 7, V = 20 mL, T = 25 °C. 3136



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photocatalysis because the oxygen defects could effectively separate the electron−hole pairs in the photocatalytic process.43−45 The optical properties of the CZ-400 and commercial ZnO were measured by DRS absorption spectroscopy, which was used to investigate the effects of oxygen vacancies on the band gap energy of samples. As shown in the inset of Figure 12, the band gap energy was 3.08 eV for CZ-400

Figure 10. Effect of reaction temperature on degradation efficiency of CR by CZ-400. The inset indicates the kinetics of CR degradation versus time. Reaction conditions: C0(CR) = 100 mg/L, C(CZ-400) = 1.0 g/L, pH = 7, V = 20 mL.

Table 2. Kinetic Rate Constants of Degradation of CR by CZ-400 T (°C)

K (min−1)

R2

standard error

25 35 45

0.015 0.017 0.031

0.980 0.978 0.980

9.70 × 10−4 1.12 × 10−3 1.92 × 10−3

Figure 12. UV−visible diffuse reflectance spectra of the CZ-400. The inset shows the plots of (αhν)2 versus the energy of absorbed light of the CZ-400 and commercial ZnO.

and 3.15 eV for commercial ZnO, which clearly indicated that the band gap of CZ-400 decreased as compared to commercial ZnO due to oxygen vacancies resulting from the elimination of the hydroxyl groups during the thermal treatment of CZ-400. Accordingly, the as-prepared CZ-400 could be a promising photacatalyst. 3.6.4. Kinetic Analysis. In kinetics, the photodegradation of CR can be considered as a pseudo-first-order reaction, and the rate expression is given by the equation:46,47

3.6.3. Comparison of the Photocatalytic Activity. For the evaluation of the photocatalytic activity of CZ-400 on the removal of CR, TiO2 P25 and commercial ZnO were also selected as the reference under the same reaction conditions. The photocatalytic result of CR was shown in Figure 11. When

ln Co/Ct = kt

where k is the degradation rate constant (min−1), and Co and Ct represent the initial CR concentration measured after 40 min adsorption and the reaction concentration (mg/L) of CR at time, respectively. The values of the rate constants have been determined from the slope of ln Co/Ct versus t plots. As shown in Figure 13, the as-synthesized CZ-400 shows high photocatalytic activity with rate constant (k) of 0.016 min−1 about 3 and 5 times higher than that of TiO2 P25 (0.005 min−1) and Figure 11. Photodegradation activity of CR by the CZ-400, commercial ZnO, and TiO2 P25 under UV light irradiation. Reaction condition: Co(CR) = 100 mg/L, C (catalyst) = 0.5 g/L, pH = 7, V = 20 mL, T = 25 °C.

the photocatalyst concentration was 0.5 g/L, the degradation percentiles of each sample were 65% for CZ-400, 41% for TiO2 P25, and 31% for commercial ZnO within 120 min irradiation time. The results of the photocatalytic experiments indicated that CZ-400 increases significantly the degradation of the CR. This distinguished behavior may be attributed to the large surface area and porous structure, which significantly facilitate diffusion and mass transportation of CR molecules and delay the recombination time of photogenerated electron−hole pairs in the photocatalysis process.41,42 According to the previous studies, defects in catalysts structure play a significant role in

Figure 13. The corresponding kinetics of degradation of CR by the CZ-400, commercial ZnO, and TiO2 P25 under UV light irradiation. 3137



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commercial ZnO (0.003 min−1), respectively. The higher photocatalytic activity of the CZ-400 is mainly due to its larger surface area, which can provide more specific surface sites for the reaction between catalysts and CR molecules.

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4. CONCLUSION Hierarchical hollow structure ZnO (CZ-400) with hollow structure and porous feature have been successfully prepared via a simple homogeneous precipitation method. The photocatalyst activity and adsorption capacity of CZ-400 were compared to TiO2 P25 and commercial ZnO. The results demonstrated that the CZ-400 exhibits higher adsorption capacity and photocatalyst activity than that of the other two catalysts. The combinatorial effect of adsorption and photodegradation reflected the importance of adsorption in the enhanced photoreactivity. This excellent behavior may be attributed to the large surface area and porous structure, which significantly facilitates diffusion and mass transportation of CR molecules and delays the recombination time of photogenerated electron−hole pairs in the photocatalysis process. On the basis of the results, CZ-400 is a combined adsorbent and catalyst material for removal of CR from water samples.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 431 88502259. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the key technology and equipment of efficient utilization of oil shale resources, no. OSR-05, and the national science and technology major projects, no. 2008ZX05018.

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Hierarchical Hollow Structure ZnO: Synthesis, Characterization, and

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