pubs.acs.org/Langmuir © 2010 American Chemical Society
ZnO Nanoparticles Immobilized on Flaky Layered Double Hydroxides as Photocatalysts with Enhanced Adsorptivity for Removal of Acid Red G Yun Zhi,† Yaogang Li,‡ Qinghong Zhang,*,‡ and Hongzhi Wang*,† †
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials and ‡Engineering Research Center of Advanced Glasses Manufacturing Technology, MOE, Donghua University, Shanghai 201620, China Received May 14, 2010. Revised Manuscript Received August 20, 2010
Flaky layered double hydroxides (FLDH) composed of cross-linked nanoflakes were prepared by the reconstruction of their oxides in alkali solution. The effect of reconstruction temperatures on the physicochemical properties was investigated. FLDH with a specific surface area of as high as 217 m2/g was obtained at a reconstruction temperature of 6 °C, and its derived flaky mixed metal oxides (FMMO) had a specific surface area of 249 m2/g. The ZnO nanoparticles were homogeneously deposited on the surface of the FLDH by coprecipitation. After calcination at 500 °C for 2 h, the ZnO-coated FLDH was transformed into ZnO-coated flaky mixed metal oxides (FMMO). The powders were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscope, N2 adsorption-desorption isotherm, UV-vis diffuse reflectance spectroscopy, and Fourier transform infrared spectroscopy. In the presence of FLDH as a support, the ZnO nanoparticles were of about 10 nm in size and showed higher photocatalytic decomposition of acid red G than bare ZnO powder prepared under similar experimental conditions. It should be noted that the ZnO-coated FMMO combined excellent adsorption with photocatalytic activity. The flaky structure of mixed metal oxides appears to play important roles in the adsorption and photodecomposition process.
1. Introduction Dyes are important water pollutants which are generally present in the effluents of the textile, cosmetics, paper, leather, pharmaceutical, food, and other industries. Many methods, such as coagulating sedimentation, filtration, electro-coagulation, and adsorption by activated carbon, have been investigated to remove the dyes, but all of these methods just transfer the dyes from one medium to another. Consequently, it is crucial to develop environmentally benign routes combining effective adsorption with enhanced photocatalytic efficiency, which completely mineralizes the organic pollutants.1,2 ZnO, a compound semiconductor with a band gap of 3.37 eV, has attracted substantial attention in recent years due to advances in its synthesis and unique optoelectronic, catalytic, and photochemical properties. Although TiO2 has been widely used as the most active photocatalyst,3,4 ZnO could be a suitable alternative because of its lower cost, larger quantum yields, and better antibacterial effect. ZnO has been successfully used in photocatalytic degradation of pollutants5,6 and is more efficient in the decomposition of several organic contaminants than TiO2.7,8 However, ZnO nanoparticles are prone to aggregation especially after calcination above 400 °C. This results in a remarkably *Corresponding author. Tel: þ86-21-67792943; fax: þ86-21-67792855. E-mail address:
[email protected] (Q. H. Zhang),
[email protected] (H. Z. Wang). (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69–96. (2) Tryba, B.; Morawski, A. W.; Tsumura, T.; Toyoda, M.; Inagaki, M. J. Photochem. Photobiol., A 2004, 167, 127–135. (3) Li, Y. Z.; Song, J. S.; Lee, N.; Kim, S. Langmuir 2004, 20, 10838–10844. (4) Sun, B.; Smirniotis, P. G.; Boolchand, P. Langmuir 2005, 21, 11397–11403. (5) Sakthivel, S.; Neppolian, B.; Shankar, M. V.; Arabindoo, B.; Palanichamy, M.; Murugesan, V. Sol. Energy Mater. Sol. Cells 2003, 77, 65–82. (6) Khodja, A. A.; Sehili, T.; Pilichowski, J. F.; Boule, P. J. Photochem. Photobiol., A 2001, 141, 231–239. (7) Sun, J. H.; Dong, S. Y.; Wang, Y. K.; Sun, S. P. J. Hazard. Mater. 2009, 172, 1520–1526. (8) Yu, J. G.; Yu, X. X. Environ. Sci. Technol. 2008, 42, 4902–4907.
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reduced surface area and much larger crystallite size. In addition, ZnO has a relatively low adsorptive capacity, and its photocatalytic efficiency is not high in very dilute solutions of organic pollutants. Consequently, enrichment of reactants by adsorption is required for a highly efficient photocatalytic performance.9-11 Also attracting attention of late is a class of anionic clays known as layered double hydroxides (LDH) or hydrotalcite-like compounds (HT). Their general formula can be expressed as [M2þ1-xM3þx(OH)2]xþAm-x/m 3 nH2O, where M2þ and M3þ are di- and trivalent metal cations, respectively, Am- denotes an organic or inorganic anion with negative charge n, and x (=[M3þ]/([M2þ] þ [M3þ])) is the value of the stoichiometric coefficient.12-14 The M2þ and M3þ cations are uniformly dispersed within the layers without the formation of “lakes”.15 After calcination at temperatures from 300 to 600 °C, LDH is converted to mixed metal oxides (MMO),16 which have high adsorption capacity. Their high adsorption capacity and high anion exchange capacity17 are comparable to those of anion exchange resins. This facilitates their application as adsorption materials, catalyst precursors,18,19 (9) Ooka, C.; Yoshida, H.; Suzuki, K.; Hattori, T. Microporous Mesoporous Mater. 2004, 67, 143–150. (10) Fukahori, S.; Ichiura, H.; Kitaoka, T.; Tanaka, H. Environ. Sci. Technol. 2003, 37, 1048–1051. (11) Zhang, G. K.; Ding, X. M.; He, F. S.; Yu, X. Y.; Zhou, J.; Hu, Y. J.; Xie, J. W. Langmuir 2008, 24, 1026–1030. (12) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173–301. (13) Rives, V., Eds. Layered Double Hydroxides: Present and Future; Nova Science Publishers, Inc.: New York, 2001. (14) Arai, Y.; Ogawa, M. Appl. Clay Sci. 2009, 42, 601–604. (15) Duan, X., Evans; D. G., Eds. Layered Double Hydroxides: Structure and Bonding (Berlin); Springer-Verlag: Berlin, 2006; Vol. 119. (16) Valente, J. S.; Cantu, M. S.; Lima, M.; Figueras, F. Chem. Mater. 2009, 21, 5809–5818. (17) Goh, K. H.; Lim, T. T.; Dong, Z. Water Res. 2008, 42, 1343–1368. (18) Feng, J. T.; Lin, Y. J.; Evans, D. G.; Duan, X.; Li, D. Q. J. Catal. 2009, 266, 351–358. (19) Hetterley, R. D.; Mackey, R.; Jones, J. T. A.; Khimyak, Y. Z.; Fogg, A. M.; Kozhevnikov, I. V. J. Catal. 2008, 258, 250–255. (20) Martı´ nez-Ortiz, M. J.; Tichit, D.; Gonzalez, P.; Coq, B. J. Mol. Catal. A: Chem. 2003, 201, 199–210.
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and catalyst supports.20-23 MMO are able to recover the original layered structure, a property known as “memory effect”.12,13 If MMO are put into aqueous solution, in the presence of anions, the layered structure is recovered with anions incorporated in the interlayer. A more irregular structure of agglomerated flake-like platelets has bas been observed after reconstruction.24 Layered clays have been used to develop photoresponsive solids, in which the photocatalytic properties are derived from the incorporated guest. TiO2 pillared montmorillonite synthesized by the impregnation method25-27 showed a mesoporous structure and an increased specific surface area, which could adsorb and enrich organic compounds.28 In comparison, LDH have received less attention as materials to incorporate photocatalytic guests. Porphyrin-TiOx particles were assembled in the hydrophobic interlayers of LiAl-LDH-myristate and applied to photodegradation of propyl viologen sulfonate.29 Zn-containing LDH with general formula of layers of [M2þ1-x-yZn2þxM3þy(OH)2]xþ, such as ZnAl LDH and MgZnAl LDH, have also been tested as photocatalysts for removal of pollutants, including methyl orange,30 sulforhodamine B, an azobenzene-containing polymer,31 phenol,32-34 cresol,33 and 2,4-dichlorophenoxiacetic acid.34 In our previous research, we synthesized a new type of MMOsupported catalyst which consisted of ZnO nanorods and MMO.35 The nanocomposite was prepared via a homogeneous precipitation process and had both adsorption capacity and photocatalytic ability. However, the dispersion of ZnO over the MMO was not uniform, and more efficient removal of acid red G could be achieved by improving ZnO dispersion and reducing ZnO crystallite size. The morphological features of the hybrid catalyst-clay are very important in the production of catalyst supports, and an impregnated method is usually used. In this work, flaky layered double hydroxides (FLDH), made of flaky sheets with slit-like pores, were prepared to support ZnO photocatalysts. ZnO nanoparticles of size ∼10 nm were well-distributed on the surface of flaky mixed metal oxides (FMMO) by the coprecipitation method.
2. Experimental Section 2.1. Synthesis of FLDH. The LDH with Mg/Al molar ratio 2 was prepared by coprecipitation by a high supersaturation (21) Prinetto, F.; Manzoli, M.; Ghiotti, G.; Martı´ nez-Ortiz, M. J.; Tichit, D.; Coq, B. J. Catal. 2004, 222, 238–249. (22) Chen, Y. Z.; Liaw, C. W.; Lee, L. I. Appl. Catal., A 1999, 177, 1–8. (23) Rives, V.; Labajos, F. M.; Trjillano, R.; Romeo, E.; Royo, C.; Monzon, A. Appl. Clay Sci. 1998, 13, 363–379. (24) Winter, F.; Xia, X.; Hereijgers, B. P. C.; Bitter, J. H.; van Dillen, A. J.; Muhler, M.; de Jong, K. P. J. J. Phys. Chem. B 2006, 110, 9211–9218. (25) Kitayama, Y.; Kodama, T.; Abe, M.; Shimotsuma, H.; Matsuda, Y. J. Porous Mater. 1998, 5, 121–126. (26) Kaneko, T.; Fujii, M.; Kodama, T.; Kitayama., Y. J. Porous Mater. 2001, 8, 99–105. (27) Ding, Z.; Zhu, H. Y.; Lu, G. Q.; Greenfield, P. F. J. Colloid Interface Sci. 1999, 209, 193–199. (28) Ooka, C.; Yoshida, H.; Horio, M.; Suzuki, K.; Hattori, T. Appl. Catal., B 2003, 41, 313–321. (29) Robins, D. S.; Dutta, P. K. Langmuir 1996, 12, 402–408. (30) Seftel, E. M.; Popovici, E.; M.; Witte, K. D.; Tendeloo, G. V.; Cool, P.; Vansant, E. F. Microporous Mesoporous Mater. 2008, 113, 296–304. (31) Zhao, Y; Wei, M; Lu, J; Wang, Z. L.; Duan, X. ACS Nano 2009, 3, 4009– 4016. (32) Patzko, A.; Kun, R.; Hornok, V.; Dekany, I.; Engelhardt, Y.; Schall, N. Colloids Surf., A 2005, 265, 64–72. (33) Mantilla, A.; Tzompantzi, F.; Fernandez, J. L.; Gongora, J. D.; Gomez, R. Catal. Today 2010, 150, 353–357. (34) Valente, J. S.; Tzompantzi, F.; Prince, J.; Cortez, J. G.; Gomez, R. Appl. Catal., B 2009, 90, 330–338. (35) Yuan, S. J.; Li, Y. G.; Zhang, Q. H.; Wang, H. Z. Colloids Surf., A 2009, 348, 76–81.
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Figure 1. XRD patterns of layered double hydroxides (LDH), mixed metal oxides (MMO), and flaky layered double hydroxides (FLDH). Table 1. Unit Cell Parameters and Crystallite Sizes for Layered Double Hydroxides (LDH) and Flaky Layered Double Hydroxides (FLDH)a unit cell parameters (A˚) sample
a
c
crystallite sizec (A˚) interlayer distanceb (A˚)
L003
L110
LDH 3.050 23.025 2.875 225 242 FLDH-0 3.046 23.430 3.010 93 137 FLDH-6 3.045 23.025 2.875 89 126 FLDH-10 3.047 23.186 2.929 91 146 FLDH-25 3.052 23.186 2.929 92 150 a a=2d110, c=3d003. b Difference between d003 and 4.8 A˚ (the thickness of the brucite-like sheet). c Calculated using the Scherrer formula with the crystal planes of d003 and d110, respectively.42
method.12 Mg(NO3)2 3 6H2O (125 mmol) and Al(NO3)3 3 9H2O (62.5 mmol) were dissolved in 87.5 mL of deionized water (solution A). NaOH (437 mmol) and Na2CO3 (208 mmol) were dissolved in 125 mL of deionized water (solution B). Solution A was added to solution B in a three-neck round-bottom reaction flask and preheated to 70 °C. The suspension was stirred at 70 °C for 18 h. LDH were obtained after collection of the precipitates by filtration, washing several times with distilled water, and drying at 110 °C overnight. The dried LDH were further calcined in air at 500 °C for 4 h, with a heating rate of 4 °C/min, to transform the calcined product into MMO. The MMO powder (3 g) was placed into 200 mL of 1.0 M Na2CO3 solution. After stirring vigorously for 5 min at room temperature, the suspension was placed in a temperaturehumidity chamber, and stirred for 24 h at 0, 6, 10 and 25 °C (labeled FLDH-0, FLDH-6, FLDH-10, and FLDH-25, respectively). The FLDH was collected by filtration, washed several times with distilled water, and dried at 100 °C overnight. The dried FLDH-6 and FLDH-25 were further calcined in air at 500 °C for 4 h, with a heating rate of 4 °C/min, to transform the calcined product into FMMO-6 and FMMO-25, respectively.
2.2. Synthesis of ZnO-Coated FMMO with a ZnO/ FMMO Mass Ratio of 1:1. The ZnO-coated FMMO was prepared using Zn(CH3OO)2 as the zinc source. The FLDH-6 powder was dispersed into 125 mL of 0.32 M Zn(CH3OO)2 solution; 62.5 mL of 1.83 M NH3 3 H2O was subsequently added into the suspension. The suspension was vigorously stirred at room temperature for 24 h. The resulting precipitate was washed five times with distilled water and three times with ethanol, and then dried at 100 °C for 8 h. After calcination at 500 °C for 2 h in air (with a heating rate of 4 °C/min), ZnO-coated FMMO was obtained. As a comparison, ZnO-coated MMO was synthesized by the same method. In addition, bare ZnO powder was also prepared by the same method but without FMMO or MMO. DOI: 10.1021/la1019313
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Figure 2. SEM and TEM images of samples: (a) SEM image of layered double hydroxides (LDH), (b) SEM image of mixed metal oxides (MMO), (c) SEM image of flaky layered double hydroxides (FLDH-6), and (d) TEM image of the circled area in part c.
Figure 3. Nitrogen adsorption-desorption isotherm and the corresponding pore size distribution curve (inset) of layered double hydroxides (LDH), mixed metal oxides (MMO), and flaky layered double hydroxides (FLDH-6). Table 2. Corresponding Specific Surface Area, Pore Volume, and Pore Diameter for Layered Double Hydroxides (LDH), Mixed Metal Oxides (MMO), Flaky Layered Double Hydroxides (FLDH), and Flaky Mixed Metal Oxides (FMMO) sample
SBETa (m2/g)
Vb (cm3/g)
Dc (nm)
LDH 87 0.619 MMO 257 1.113 FLDH-0 177 0.592 FLDH-6 217 0.700 FLDH-10 192 0.693 FLDH-25 184 0.650 FMMO-6 249 1.238 FMMO-25 221 1.209 a BET surface area. b BJH pore volume. c BJH pore diameter.
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24.7 11.6 4.7 8.5 7.5 5.1 16.1 15.0
2.3. Adsorption Tests. Acid red G (ARG, C18H13N3Na2O8S2, M=509.43, Sigma-Aldrich) was applied as a model pollutant. The adsorption rates of ARG over MMO, FMMO-6, FMMO-25, and ZnO-coated FMMO were evaluated by placing 0.1 g of each sample in 500 mL of ARG solution. The initial concentration of ARG in the solution at the start of the adsorption was 50 ppm (0.01 mmol/L), and the initial pH value was 6.5. Adsorption was carried out in the dark, under magnetic stirring at 30 °C. At a defined time interval, the adsorption of ARG was monitored on a UV-vis spectrophotometer (model Lambda 35, Perkin-Elmer Co.) at 530 nm. After the adsorption measurements, the suspension of ZnO-coated FMMO was centrifuged, and the obtained precipitate was dried at 80 °C overnight for further Fourier transform infrared spectroscopy (FT-IR) analysis. Langmuir 2010, 26(19), 15546–15553
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Figure 4. Schematic representation of the flaky layered double hydroxides (FLDH) formation process.
Figure 5. Comparison of ARG adsorption rates of MMO and flaky mixed metal oxides (FMMO-6 and FMMO-25) at 30 °C. The adsorption capacities of MMO and FMMO-6 were determined in seven solutions of ARG in a volume of 500 mL with distilled water, those concentrations varying between 50 and 350 ppm (0.01 and 0.69 mmol/L). Calcined solid (0.1 g) was added to the solution with constant stirring at 30 °C for 2 h. The initial and residual amounts of ARG in the solution were determined using the same UV-vis spectrophotometer. 2.4. Photocatalytic Tests. The photocatalytic activities of ZnO-coated FMMO, ZnO-coated MMO, and bare ZnO for decomposition of ARG solution (50 ppm) were determined under UV light irradiation with a 300 W medium Hg lamp (the strongest emission at the wavelength of 365 nm). Each sample (0.1 g) was mixed into 500 mL of ARG solution. The photoreactor was the same as that described elsewhere.36,37 The absorbance of the ARG solution at defined time intervals was determined using the UV-vis spectrophotometer detailed earlier. After photocatalytic degradation, the suspension of ZnO-coated FMMO was centrifuged, and the obtained precipitate was dried at 80 °C overnight for further FT-IR analysis. Both adsorption and photocatalytic degradation contribute to bleaching of ARG with ZnO-coated FMMO under UV light. To distinguish among the respective contributions of these, another experiment was carried out with ZnO-coated FMMO (0.1 g) in 500 mL of ARG solution (50 ppm), where the solid particles in the suspensions were dissolved with 0.1 M HCl. After UV light illumination, a 5 mL aliquot of the reaction mixture was taken out at regular time intervals and diluted to 10 mL with 0.1 M HCl. This dissolved the ZnO-coated FMMO in the suspension to eliminate any scattering effect from the solid-state particles. The ARG anions adsorbed in the ZnO-coated FMMO were released after the nanocomposites reacted with HCl. At this point, removal of ARG was entirely due to photocatalysis of the ZnO on ZnO-coated FMMO.38 The ARG concentration in the solution was monitored by UV-vis spectrophotometry and used to distinguish the contribution of adsorption from the total removal of ARG. (36) Zhang, Q. H.; Gao, L. Langmuir 2004, 20, 9821–9827. (37) Zhang, Q. H.; Fan, W. G.; Gao, L. Appl. Catal., B 2007, 76, 168–173. (38) Yuan, S. J.; Li, Y. G.; Zhang, Q. H.; Wang, H. Z. Res. Chem. Intermed. 2009, 35, 685–692.
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2.5. Characterization. X-ray diffraction (XRD) patterns of the powder phase compositions were identified by X-ray diffractometer (model D/Max-2550, Rigaku Co. Tokyo, Japan) using Cu KR radiation (λ = 1.5406 A˚) at 40 kV and 100 mA. The morphologies of the LDH and ZnO-coated FMMO were observed by transmission electron microscope (TEM) (model JEM2100F, JEOL, Tokyo, Japan) and field-emission scanning electron microscopy (FE-SEM) (model S-4800, Hitachi, Japan). N2 adsorption desorption isotherms at -196 °C were measured on a Quantachrome Autosorb 1-MP analyzer. Before analysis, the samples LDH and FLDH were outgassed in vacuum at 100 °C for 16 h, and calcined MMO were outgassed in vacuum at 300 °C for 16 h. The specific surface area was determined by a multipoint BET method, using the adsorption data in the relative pressure (P/P0) range 0.05-0.3. The adsorption branch of nitrogen adsorption-desorption isotherms was used to determine the pore size distribution by the Barrett-Joyner-Halenda (BJH) method,39 assuming a cylindrical pore model. The volume of nitrogen adsorbed at a relative pressure (P/P0) of 0.994 was used to determine the total pore volume. The UV-vis diffuse reflectance spectra of the powder samples were collected with a spectrophotometer (model Lambda 950, Perkin-Elmer Co.). The FT-IR (NEXUS670, Nicolet) spectra were recorded from 400 to 4000 cm-1 using KBr pellets.
3. Results and Discussion 3.1. FLDH Prepared by Reconstruction. Figure 1 illustrates the XRD patterns for the original LDH, MMO, and FLDH-0, -6, -10, and -25. The unit cell parameters and crystallite sizes for these samples were calculated, and the results are listed in Table 1. The typical X-ray pattern of the original LDH could be clearly identified (Figure 1), and included sharp (003), (006), and (009) planes and less intense and asymmetrical reflections for (012), (015), (018), (110), and (113) planes.40 After calcination of the LDH at 500 °C, MMO formed, and the XRD pattern of these (Figure 1) were of rock-salt like MgO. After the MMO were soaked in a Na2CO3 solution, the original layered LDH structures were recovered due to the memory effect. It is worth nothing that, although both the original LDH and the FLDH have a bare LDH structure, the differences in intensity and width of XRD peaks are obvious.41 Compared with the original LDH, the peaks of the FLDH were broader and lower in intensity. Among FLDH-0, -6, -10, and -25, FLDH-6 had broader peaks than FLDH-0, -10, and -25. Crystallite sizes calculated from the full width at half-maximum of the peaks reflected this trend (Table 1). In addition, the interlayer distances (Table 1) were very similar among the original LDH and the FLDH, indicating that they all have the same chargebalancing anions. Both the LDH and FLDH with Mg/Al molar ratio equaled to 2 as verified by the X-ray energy-dispersive spectroscopy equipped with TEM. (39) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619. (40) Gaini, L. E.; Lakraimi, M.; Sebbar, E. J. Hazard. Mater. 2009, 161, 627– 632. (41) Roelofs, J. C. A. A.; Lensveld, D. J.; Dillen, A. J.; Jong, K. P. J. Catal. 2001, 203, 184–191.
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Figure 6. Comparison of adsorptive capacity of MMO and FMMO-6 for the adsorption of ARG at 30 °C: adsorption isotherms of ARG (a) and Langmuir adsorption isotherms of ARG (b).
The FE-SEM image of LDH (Figure 2a) illustrates that the original LDH are made up of agglomerated platelet-shaped particles with sizes of about 20 nm. It is important to note that after calcination the lamellar LDH structure collapses and a solid solution is produced.43 The FE-SEM image of calcined MMO (Figure 2b) shows a house-of-cards structure,44 which has continuous and interconnecting platelets. When MMO undergoes reconstruction of the original LDH structure, the morphology of the sample is evolved further (Figure 2c). The reconstructed FLDH-6 shows an irregular stacking of flaky sheet with an average size of 100 nm. The nanoflakes coalesced irregularly with each other and formed a porous and coarse surface. The irregularly oriented sheets are consistent with the broader and lower intensity peaks observed by XRD. The thickness of FLDH-6 sheet was determined to be 10 nm from the TEM image (Figure 2d). Figure 3 shows the N2 adsorption-desorption isotherm and the corresponding pore size distribution curves for LDH, MMO, and FLDH-6; and the specific surface area, pore volume, and pore diameter of samples are summarized in Table 2. All samples exhibited type IV isotherms according to IUPAC classification, which corresponds to mesoporous solids. The LDH sample had an H3-type hysteresis loop, implying irregular pore size distribution. Moreover, this loop does not exhibit any limiting adsorption at higher P/P0, indicating the existence of macropores.45 This result is in agreement with the pore size distribution presented in the inset of Figure 3 (4-100 nm, maximum 24.7 nm). This is due to the aggregation of the platelet-shaped particles of the LDH. In comparison, MMO had a H1 type hysteresis loop, indicating uniform pores. This result is further confirmed by the corresponding rather narrow pore size distribution in Figure 3 (5-20 nm, maximum 11.6 nm). The narrowing of pore size distribution in MMO compared to LDH can be explained by the interconnection of particles after calcination. In addition, after calcination the BET surface area of MMO (257 m2/g) is much larger than LDH (87 m2/g) (Table 1). For FLDH-6, the hysteresis loop is type H4, which corresponds to slit-like pores with narrow distribution. Compared with LDH, the average FLDH-6 pore size is much smaller, and the pore size distribution is more homogeneous. The BET specific surface area of FLDH-6 is 217 m2/g, which is much larger than that of the parent LDH, suggesting that FLDH-6 has a relatively high surface-to-volume ratio. FLDH-6 shows higher (42) Radmilovic, V.; Gasteiger, H. A.; Ross, P. N. J. Catal. 1995, 154, 98–106. (43) Valente, J. S.; Cantu, M. S.; Cortez, J. G. H.; Montiel, R.; Bokhimi, X.; Lopez-Salinas, E. J. Phys. Chem. C 2007, 111, 642–651. (44) Gursky, J. A.; Blough, S. D.; Luna, C.; Gomez, C.; Luevano, A. N.; Gardner, E. A. J. Am. Chem. Soc. 2006, 128, 8376–8377. (45) Zhou, L.; Wang, W. Z.; Xu, H. L.; Sun, S. M.; Shang, M. Chem.;Eur. J. 2009, 15, 1776–1782.
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Table 3. Langmuir Parameters Qm, K, and Linearization Coefficient r2 for the Adsorption of ARG by Mixed Metal Oxides (MMO) and Flaky Mixed Metal Oxides (FMMO) sample
Qm (mmol/g)
K (L/mmol)
r2
MMO FMMO-6
1.171 1.156
193.402 574.738
0.997 0.999
specific surface area than FLDH-0, -10 and -25, which is consistent with the broadest peaks in XRD investigation. From these data, it can be concluded that the temperature of LDH reconstruction has an impact on the specific surface area, and LDH reconstructed at 6 °C reveals the moderate crystal growth for obtaining the mesoporous structure and LDH in a finer crystallite size. The possible formation process of FLDH is shown in Figure 4. The original LDH are platelet-shaped particles with a layered structure. When LDH is calcined, dehydroxylation and interlayer carbonate decomposition occur, water and carbon dioxide are released, and the layered structure collapses.46 The nanocrystals transform to mixed oxides, which cross-link to form the continuous and porous platelets. This structure plays a vital role in the final growth of FLDH. After reconstruction in the Na2CO3 solution, with intercalation of carbonate anions in the layered host structure, the LDH structure is reconstructed. A more irregular structure of the agglomerated platelets is observed after reconstruction than in the original LDH. The newly formed FLDH has a coarse surface (Figure 4), which is similar to the morphology reported,41 due to the irregular interconnected nanosheets (Figure 2c) and the remaining slit-like mesopores (Figure 2d). The coarse surface is highly accessible for N2 adsorption, and the newly formed FLDH-6 has a high surface area (217 m2/g). Owing to the special structure and the large surface area, FLDH are more suitable for the support of catalysts than LDH. 3.2. Adsorption Capacity of FMMO. The adsorption rates of ARG by MMO, FMMO-6, and FMMO-25 are plotted in Figure 5, where Ct is the concentration of aqueous ARG at time t, and C0 is the initial concentration of aqueous ARG (50 ppm). The adsorption equilibrium time of FMMO is shorter than that of MMO, which may attributed to the unique flaky microstrutures of FMMO, and there, the mass transportation path is much shortened. The pores between sheets and slit-like pores within the sheets can be considered as pathways for transport of reactants (46) Valente, J. S.; Lima, E.; Toledo, J. A.; Cortes, M. A.; Lartundo, L.; Montiel, R.; Prince, J. J. Phys. Chem. C 2010, 114, 2089–2099. (47) Yu, X.; Yu, J.; Cheng, B.; Jaroniec, M. J. Phys. Chem. C 2009, 113, 17527– 7535.
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Figure 7. SEM and TEM images of samples: SEM image of ZnO-coated flaky mixed metal oxides (FMMO) (a); low-magnification TEM image of ZnO-coated FMMO (b); high-magnification TEM image of ZnO-coated FMMO (c); and TEM image of bare ZnO (d).
and product molecules moving in or out of the material, which means chemical reactions occur more easily.47 Therefore, the flaky sheet is expected to have a faster adsorption rate. Figure 6a shows the adsorption isotherms for ARG binding to MMO and FMMO-6, where Ce is the equilibrium concentration of ARG in the solution (mmol/L) and Q is the amount of ARG adsorbed per gram of solid (mmol/g). The adsorption isotherms for ARG can be considered L-type, which is similar to the adsorption behavior observed for other adsorbates.48,49 The data obtained from the adsorption process were described by the Langmuir isotherm, which in its linearized form is Ce Ce 1 þ ¼ Q Qm K
ð1Þ
where Ce and Q are as defined above, Qm is the maximum amount adsorbed per unit mass of the solid (mmol/g), and K is the affinity constant. Langmuir adsorption curves are presented in Figure 6b. The calculated Langmuir model parameters (Qm and K) and linearization coefficient r2 are reported in Table 3. The theoretical Langmuir maximum adsorption of ARG (Qm) on MMO is slightly larger than that on FMMO-6, while the affinity of ARG (K) on MMO is lower than that on FMMO-6. The fact that MMO and FMMO-6 present nearly the same Qm may be (48) Auxilio, A. R.; Andrews, P. C.; Junk, P. C.; Spiccia, L. Dyes Pigments 2009, 81, 103–112. (49) You, Y.; Vance, G. F.; Zhao, H. Appl. Clay Sci. 2001, 20, 13–25.
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explained by their nearly identical BET surface areas. In addition, the higher K affinity constant in FMMO-6 (compared to that in MMO) is due to special sheets with slit-like pores. 3.3. ZnO Coating on FLDH. Among the FLDHs, FLDH-6 had the highest surface area, and after calcination, it had the fastest adsorption rate. Therefore, FLDH-6 is the optimum sample for catalyst support. The FLDH-6 powder, with positively charged surface, was placed into the Zn(CH3OO)2 solution. After this, NH3 3 H2O was added dropwise into the Zn2þ solution, and ZnO formed (Figure 8a). At an alkali solution, the ZnO clusters are negatively charged and may be electrostatically attracted to the positively charged LDH in a wider pH value range.50 After calcinations at 500 °C, ZnO-coated FMMO was formed and the ultrafine crystallite size of ZnO was kept. The SEM image (Figure 7a) shows that ZnO-coated FMMO still retains the flake superstructure of the FLDH. The TEM image of ZnO-coated FMMO reveals that ZnO nanoparticles are well-distributed on the surface of nanosheets, with a diameter of about 10 nm. A high-resolution TEM image of ZnO-coated FMMO (Figure 7c) shows the lattice structure of the ZnO nanoparticles. The spacing between adjacent lattice fringes is 0.25 nm, which is close to the basal spacing of the plane (101) of ZnO. As a comparison, the TEM image of bare ZnO calcined at 500 °C for 2 h (Figure 7d) shows agglomerated particles with diameters between 40 and 300 nm. From these results, it can be (50) Liu, J.; Huang, X.; Li, Y.; Sulieman, K. M.; He, X.; Sun, F. J. Phys. Chem. B 2006, 110, 21865–21872.
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Figure 8. XRD patterns of samples: ZnO-coated flaky layered double hydroxides (FLDH) (a), ZnO-coated flaky mixed metal oxides (FMMO) (b), and bare ZnO (c).
concluded that ZnO particles tend to be much smaller in size in the presence of FMMO as a support. In the XRD patterns of the ZnO-coated FMMO (Figure 8b) and ZnO (Figure 8c), most of the diffraction peaks could be attributed to the wurtzite structure of ZnO. Meanwhile, the extra diffraction peaks in the ranges 40-46° and 60-65° in 2θ illustrate that the FMMO has a MgO phase. The crystallite size can be calculated from the broadening of corresponding XRD peaks according to the Scherrer formula. The crystallite size of ZnO in the nanocomposite determined by the Scherrer formula is 9.5 nm, which is in agreement with the TEM results, while the crystallite size of bare ZnO is much larger, 74.7 nm. This difference may occur because the FLDH play a vital role in formation of ZnO nuclei and inhibition of ZnO growth during the subsequent calcination. Figure 9 displays the N2 adsorption-desorption isotherm and the corresponding pore size distribution curves for ZnO-coated FMMO. The shape of the hysteresis loop is very similar to that of MMO, indicating that the pore structure was preserved after the deposition of ZnO nanoparticles. The pore size distribution curves of MMO and ZnO-coated FMMO are different at 10-20 nm, indicating that ZnO nanoparticles occupy pore volumes between flaky sheets of FMMO. The BET specific surface area of ZnO-coated FMMO is 204 m2 /g, slightly lower than that of FMMO-6. The UV-vis diffuse reflectance spectra of the bare ZnO, ZnOcoated FMMO, and FMMO are shown in Figure 10. There was no obvious adsorption of the FMMO in the UV-vis region due to the fact that it is an isolator. A prominent absorption was observed for ZnO-coated FMMO, and the absorption onset is located at around 390 nm. The band gap energies of bare ZnO and ZnOcoated FMMO were determined by an established method.34,51 ZnO-coated FMMO has band gap energy of 3.18 eV, blue-shifted ∼0.06 eV compared to that of bare ZnO (3.12 eV). 3.4. Removal ARG of ZnO-Coated FMMO. The ZnOcoated FMMO combines the adsorption of FMMO and the photocatalytic ability of ZnO to efficiently remove ARG from aqueous solution. Figure 11 shows changes in the absorption spectra of an ARG aqueous solution exposed to UV light for various times in the presence of ZnO-coated FMMO. The absorption spectrum of the original solution shows three distinctive peaks in the ultraviolet band. The peak at 238 nm is ascribed to the benzene ring, the peak around 311 nm is attributed to the naphthalene ring, and the peak at 505 nm arises from the nitrogen-to-nitrogen double bond. The decomposition rate of (51) Sanchez, E.; Lopez, T. Mater. Lett. 1995, 25, 271–275.
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Figure 9. Nitrogen adsorption-desorption isotherm and the corresponding pore size distribution curve (inset) for the ZnO-coated flaky mixed metal oxides (FMMO).
Figure 10. UV-vis diffuse reflectance spectra of samples.
Figure 11. Changes in the UV-vis spectra during photocatalytic decomposition of 50 mg/L ARG by ZnO-coated flaky mixed metal oxides (FMMO) at reaction times of 0, 10, 20, 30, 40, 50, and 60 min.
ARG was very fast at the beginning and then slowed. The sharp decrease indicates that ZnO-coated FMMO provided excellent removal of ARG. Figure 12 presents the results for removal of ARG by ZnOcoated FMMO, ZnO-coated MMO, and bare ZnO. It is obvious that the removal of ARG by ZnO-coated FMMO under UV light is due to two factors: adsorption of FMMO in ZnO-coated FMMO and photocatalysis of ZnO on ZnO-coated FMMO. ZnO-coated FMMO exhibited a high adsorption rate, which indicates that the coated ZnO has little impact on the adsorption rate of FMMO. Moreover, the photocatalytic ability of ZnO on ZnO-coated FMMO is much higher than that of bare ZnO. This Langmuir 2010, 26(19), 15546–15553
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Figure 12. Effect of contact time on removal of ARG.
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that of ZnO-coated MMO (192 m2/g) and quicker adsorption of ARG by FMMO compared with MMO (Figure 5). Figure 13 shows the FT-IR spectra of ARG, ZnO-coated FMMO after adsorption of ARG, and ZnO-coated FMMO after photodecomposition of ARG. The ZnO-coated FMMO after adsorption of ARG presents the peaks characteristic of ARG: -NdN- band at 1498 cm-1, O;SdO band at 1216 cm-1, and a symmetric stretching vibration at 1050 cm-1 that arises from adsorption of ARG by MMO. These three peaks are not present in the spectrum of ZnO-coated FMMO after photodecomposition of ARG, but a SO42- peak is observed at 1114 cm-1. We may assume that the ARG was photodecomposed to inorganic molecules and ions by ZnO, and among them SO42- was absorbed by FMMO. In the light of these results, we can deduce that there are three steps involved in the decomposition process: First, the ARG molecule is absorbed on the surface of the ZnO-coated FMMO. Second, the ZnO photocatalytically decomposes ARG into CO2, H2O, SO42-, and so on, when irradiated by UV light. And third, anions such as NO3- and SO42- resulting from the photocatalysis are absorbed by FMMO.
4. Conclusions
Figure 13. FTIR spectra of ARG and Zn-coated FMMO.
can be interpreted by three factors. First, the much smaller ZnO nanoparticles on ZnO-coated FMMO ensure its higher photocatalytic ability. Second, there is a good dispersion of the small ZnO nanoparticles on the surface of MMO nanoflakes, which results in a larger surface area accessible to light and ARG and higher activity of the catalytic sites.47,52 Third, ZnO-coated FMMO has a high adsorption capability, which is very crucial to photocatalytic reactions.11 Because ZnO-coated FMMO combines adsorption and photodecomposition, ZnO-coated FMMO removes ARG very rapidly (Figure 12). In addition, this removal rate is faster than that of ZnO-coated MMO, due to the higher BET specific surface area of ZnO-coated FMMO (204 m2/g) than (52) Li, Y. Z.; Kim, S. J. Phys. Chem. B 2005, 109, 12309–12315.
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FLDH with a high specific surface area were prepared by reconstruction of their oxides in alkali solution. The large specific surface areas of the FLDH had slit-like mesopores in irregular interconnected flaky sheets. The specific surface area of FLDH was as high as 217 m2/g when the reconstruction temperature was 6 °C. In the presence of FMMO as support, the ZnO nanoparticles were much smaller in size (about 10 nm) than that of bare ZnO (74.7 nm) and well-distributed. The FMMO supported nanoparticles enhanced photocatalytic decolorization of ARG compared to bare ZnO powers prepared under similar experimental conditions. The structure of the ZnO-coated FMMO composite facilitated high ARG decomposition, as it was easily accessible to light and reactants. There was no residual ARG in the recovered sample, and this could be attributed to the combination of adsorption by FMMO and photocatalytic activity of ZnO. Acknowledgment. This work was supported by National Key Technology R&D Program (No. 2006BAA04B02-01), National Natural Science Foundation of China (No. 50772127 and No. 50925312), and Shanghai Leading Academic Discipline Project (B603).
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