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J. Phys. Chem. C 2010, 114, 14722–14728
TiO2/ZnLDH as a Self-Assembled Nanocomposite with Photoresponsive Properties Gabriela Carja,*,† Akira Nakajima,‡ Sofronia Dranca,† Cristian Dranca,† and Kiyoshi Okada§ Department of Chemical Engineering, Faculty of Chemical Engineering and EnVironmental Protection, Technical UniVersity “Gh. Asachi” of Iasi, Bd. D. Mangeron, Iasi 700554, Romania, Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan, and Materials and Structures Laboratory, Tokyo Institute of Technology, 4259, Nagatsuta, Midori, Yokohama 226-8503, Japan ReceiVed: April 9, 2010; ReVised Manuscript ReceiVed: June 17, 2010
A novel bicomponent photoresponsive nanocomposite consisting of zinc-based anionic clay (ZnLDH)-supported anatase TiO2 nanoparticles was obtained by the structural reconstruction of the zinc-containing clay in a TiOSO4 aqueous solution and was characterized by X-ray diffraction, IR spectroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and N2 adsorption at 77 K. The reconstruction process of the Zn-based clay in a TiOSO4 aqueous medium at room temperature produced significant effects on the structure, texture, surface, and morphology features as opposed to the parent ZnLDH, giving rise to a complex nanoarchitecture consisting of nanoparticles of TiO2 (average size ) 7 nm) deposited on the larger nanoparticles (∼100 nm) of the zinc-substituted anionic clay. The photoresponsive properties of TiO2/ZnLDH were analyzed by the evaluation of the optical characteristics and the catalytic behavior in the process of phenol photodegradation. The photocatalytic performances might arise from the specific nanotexture of the obtained titania-zinc-based nanocomposite self-assembly that can enhance the light harvesting and the ability of generating photoinduced electron-hole pairs of the active sites and the favorable electron-transfer properties of the heterojunctions of the TiO2/ZnLDH semiconductor. 1. Introduction Using the clean energy of the sun to decompose the contaminants from air and water, photocatalysis can solve many environmental and pollution challenges. As the most promising photocatalyst,TiO2 (titania) has been and continues to be the subject of considerable research for wastewater detoxification, drinking water treatment and indoor air purification.1-4 TiO2 is a wide band-gap semiconductor (3.03 eV for rutile and 3.18 eV for anatase form) with a great ability to produce powerful oxidants (holes in the valence band) and reductants (electrons in the conduction band) by absorbing photons.5 The major drawback of TiO2 photocatalysts lies in the fact that it can absorb only 5% of sunlight in the ultraviolet region, which greatly limits their practical applications.6 The coupled TiO2-zinc formulations can overcome this limitation, showing superior photocatalytic performances in comparison with TiO2 alone (as was reported for phenol or 2-chlorophenol degradation, decomposition of salicylic acid, or decolorization of organic dyes7-10). The superior photocatalytic activity of bicomponent semiconductors containing titania and zinc has been attributed to a decrease in the electron-hole pair recombination rate and an increase in lifespan.11,12 Moreover, high interest exists to develop nanosized semiconductor formulations in the condition that the high surface-to-volume ratio of nanoranged materials is able to develop specific transport properties related to photons.12 Layered double hydroxides (LDHs) are a class of anionic clays * To whom correspondence should be addressed. E-mail: carja@ uaic.ro. Tel/Fax: +40232201231. † Technical University “Gh. Asachi” of Iasi. ‡ Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology. § Materials and Structures Laboratory, Tokyo Institute of Technology.
that have received much attention in the last decades due to their multifunctional properties, thus giving rise to applications in areas, such as catalysis,13 separation,14 nanotechnology,15 and biotechnology.16,17 The basic structure of LDHs is derived from a partial isomorphous substitution of divalent cations by trivalent cations in a brucite lattice (Mg(OH)2) such that the positively charged cationic layers are bound together by the interlayer counteranions, as well as by water molecules. LDHs are representedbythegeneralformula[M(II)1-xM(III)x · (OH)2]x+(An-)x/ n · mH2O. The large variety of compositions that can be developed by altering the nature of the divalent and trivalent cations in the layers, the type of interlayer anions (An-), and the stoichiometric coefficient (x) gives rise to a large compositional diversity of these layered materials and specific textural properties.18 Zinc-based anionic clay (ZnLDH) is a member of the layered double hydroxides group, containing Zn2+ as M(II) cations in the brucite-like layers. Calcination of ZnLDH destroys the layered structure of the anionic clay. It progressively eliminates the physisorbed and interlamellar water, dehydroxylates the layers, removes the charge compensating anions, and gives rise to a solid solution of Zn-based mixed oxides.19 The solutions of mixed oxides obtained by thermal decomposition of LDHs have the remarkable capacity of reconstructing the original structure of the layered anionic clay upon adsorption of anions and water.20 Recent results have shown that the calcined LDHs are able not only to take anions from an aqueous solution to rebuild their interlayers but also to adsorb cations from the solutions.21 We have recently reported that the conjugation of the intercalation process of anions with the adsorption process of cationsswhen an aqueous solution of metal salt (Mex+Yy-) is used during the structural reconstruction of the claysleads to nanostructured assemblies of nanoparticles
10.1021/jp103210m 2010 American Chemical Society Published on Web 08/17/2010
TiO2/ZnLDH as a Self-Assembled Nanocomposite of metal or metal oxides deposited on the anionic clay matrix.22 These materials are able to combine the versatile composition of the LDHs’ mesoporous matrix and the induced characteristics of the nanosized metal and/or metal oxide into one single material. Very recently, Gomes Silva and co-workers have introduced the new concept of ZnLDH-based semiconductors by reporting the behavior of Me(III) or Me(IV) (e.g., Cr, Ce, Ti) partially substituted ZnLDHs as zinc oxide semiconductors doped with the tri- or tetravalent metal ions.23 Based on the above information, this work presents the self-assembly of nanoparticles of titania-Zn-based anionic clay (TiO2/ZnLDH) as a new bicomponent semiconductor based on ZnLDH. The growth and organization of nanoparticles of TiO2 on the ZnLDH clay matrix are done in one single step during the structural reconstruction of the calcined ZnLDH clay in TiOSO4 solution at room temperature. No organic compounds are used during the fabrication procedure of the semiconductor assembly. The structural, surface, and textural properties of TiO2/ZnLDH and its photoresponsive properties (optical characteristics and photocatalytic activity in the process of phenol photodegradation) are presented. 2. Experimental Section Materials Synthesis. The zinc-substituted hydrotalcite-like clay, denoted as ZnLDH, was obtained by the coprecipitation method following the procedure described elsewhere.22 The calcined clays, denoted as ZnLDH550 and ZnLDH800: ZnLDH parent clays, were calcined at 550 and 800 °C, respectively, for 10 h. The nanosized titania on zinc-substituted clay, denoted as TiO2/ZnLDH: TiOSO4 · xH2O (98%), were obtained from Fluka and used as received. The procedure previously described in ref 24 was used to prepare a dilute aqueous solution of TiOSO4 (250 mL, 0.01 mol/L). In this solution, 1.5 g of the “freshly” calcined clay was added with stirring, under nitrogen atmosphere. SO42- from the aqueous solution was used as an anion source for the structural reconstruction of the clay interlayer. The obtained sample was aged at ambient temperature, washed, centrifuged, and dried under vacuum. After calcination at 800 °C, this sample was denoted as TiO2/ZnLDH800. Instruments and Techniques. Structural characteristics, crystallinity, and purity information were recorded by X-ray diffraction (XRD) using a Shimadzu XRD 6100 diffractometer with monochromatic Cu KR radiation (λ ) 0.1541 nm), operating at 40 kV and 30 mA over a 2θ range from 4 to 70°. Elemental analyses were performed by ICP spectroscopy (Prodigy JEOL) using solutions prepared by dissolving the samples in dilute H2SO4. Fourier transform infrared (FTIR) spectra were collected on a PerkinElmer Spectrum 100 spectrophotometer in the wavenumber range of 450-4000 cm-1, at a resolution of 4 cm-1, using KBr pellets. X-ray photoelectron spectroscopy (XPS) spectra were recorded using a PerkinElmer model 5500-MT spectrometer equipped with Mg KR radiation (1253.6 eV), operating at 15 kV and 20 mA; the binding energies (BEs) were corrected by referencing the C 1s peak to 284.8 eV. Transmission electron microscopy (TEM) observation was performed on a Hitachi H-900 instrument operating at an accelerating voltage of 200 kV, coupled with an energydispersive X-ray (EDX) spectrometer. The texture of the sample was also studied by a Mira II LMU Tescan field emission scanning electron microscope (FESEM). Photoresponsive Properties by UV-vis Analysis and Photocatalytic Activity in Photodegradation of Phenol. UV-vis absorption spectra were recorded on a Jasco V550 spectrophotometer.
J. Phys. Chem. C, Vol. 114, No. 35, 2010 14723
Figure 1. XRD patterns of the (a) as-synthesized, (b) calcined, and (c, d) reconstructed clays.
Photodegradation of phenol was carried out by using solutions of 0.425 mmol/L in distilled water. Four aliquots of 250 mL were taken, and 0.5 g of TiO2/ZnLDH was added to each one. The temperature was kept constant at 298 K, and the solutions were stirred in a batch reactor for 40 min in order to reach the adsorption equilibrium. Afterward, the solutions were irradiated with UV-light for 1-5 h by using a UV Pen-Ray Power Supply (UVP Products) with λ ) 254 nm and an intensity of 4400 mW/ cm2, placed in a quartz tube, which was immersed in the solution. After defined irradiation times, 20 mL of the dispersion was filtered, prior to the analysis, to remove the catalyst. The recovered catalyst was reused for a second and a third catalytic run using the initial experimental conditions of the first run. The catalyst photoefficiency was monitorized by UV-vis spectroscopy analysis following the absorbance (A) at 269 nm characteristic to phenol (with A ) A0 at the irradiation time t ) 0). Assuming a similar photodegradation mechanism for all the materials, the results are expressed as the variation of A/A0 as a function of time. 3. Results and Discussion Phases, Structural, and Surface Properties by XRD, FTIR, and XPS Analyses. Figure 1 presents the XRD patterns of the as-synthesized, calcined, and reconstructed clays. The synthesized ZnLDH (Figure 1, pattern a) crystallized in a hydrotalcitelike phase with a series of sharp and symmetric basal reflections of the (00l, l ) 3,6) planes and broad, less intense reflections for the nonbasal (01l, l ) 2,5,8) planes. The (110) and (113) reflections are sharp and can be clearly distinguished around 60° 2θ, pointing out to a low intralayer disorder of the clay network.26 Calcination for 12 h at 550 °C destroyed the layered structure of the zinc-based clay and well-shaped peaks ascribed to the crystalline ZnO (wurtzite JCPDS file no. 36-1451; 2θ ) 31.8, 34.4, 36.3, 47.6, 56.7, 62.9 and 68.2, corresponding to the basal reflections of (100), (002), (101), (102, (110), (103), and (112) planes, respectively) can be clearly observed in the corresponding XRD pattern (Figure 1, pattern b). When the “freshly” calcined clay is shortly introduced (1 min) in the aqueous solution of TiOSO4, the resulted XRD pattern (Figure 1, pattern c) joins the characteristic reflections of the layered clay and its calcined form, thus indicating that the clay reconstruction process is not complete at this time. When the contact time between the calcined clay and the TiOSO4 solution was increased by 15 min, the XRD pattern of TiO2/ZnLDH (shown
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Figure 2. EDX characteristic pattern of TiO2/ZnLDH; the signal of Cu is generated from the Cu grids.
in Figure 1, pattern d) displays some sets of reflections. The peaks marked with “/” can be indexed to the regained LDH structure, whereas the peaks marked with “O” (at 2θ ) 25.28, 37.8, 36.3, 47.6, 56.7, 62.9 and 68.2) are characteristic to the reflections of the anatase phase (JCPDS file no. 21-127). The EDX pattern of TiO2/ZnLDH is presented in Figure 2. It shows major peaks assigned to Ti, Zn, O, and Al, whereas Cu peaks came from C-coated Cu TEM grids. The EDX Ti/Zn atomic ratio was ∼0.7. The XRD reflections of the reconstructed and the parent clay were indexed using a hexagonal cell with rhombohedral symmetry (R3m) and calculated from d(110) and d(003) reflections. Table 1 presents the EDX and ICP composition, the calculated lattice parameters, and the interlayer free spaces (IFSs)20 for ZnLDH and TiO2/ZnLDH. The increase in the c parameter and IFS values after clay reconstruction in the TiOSO4 solution indicates altered layer-interlayer interactions.25 This results show good agreement with the results previously reported for the LDH anionic clays reconstructed in solutions containing SO32- and/or SO42- anions.26 To get more information about the structural evolution of the reconstructed clay, we compared the XRD patterns of TiO2/ZnLDH and ZnLDH, after calcination at 800 °C. The XRD pattern of ZnLDH800 (Figure 3, pattern a) joined the characteristic reflections of ZnO (wurtzite JCPDS file no. 36-1451; 2θ ) 31.8, 34.4, 36.3, 47.6, 56.7, 62.9 and 68.2, corresponding to the basal reflections of (100), (002), (101), (102, (110), (103), and (112) planes) and ZnAl2O4 (JCPDS file no. 5-0669; 2θ = 31.26, 36.8, 49.1, 55.656 59.342 and 65.233, corresponding to the basal reflections of (220), (311), (331), (422), (511), and (440) planes).27 The XRD pattern of TiO2/ZnLDH800 (Figure 3, pattern b) is more complex; it joins not only the characteristic reflections of ZnO and ZnAl2O4 but also the reflections of Zn2TiO4 (JCPDS file no. 25-1164; 2θ ) 29.827, 35.136, 42.717, 56.52 and 62.0, corresponding to the basal reflections of (220), (311), (400), (511), and (440), marked with “/” in Figure 3, pattern b) can be identified. This result also points to more complex structural features after the reconstruction process of ZnLDH clay in TiOSO4 solution. In the case of LDH matrices, FTIR analysis gives information about the nature of the anions from the interlayers. Figure 4 presents the FTIR patterns of ZnLDH, ZnLDH550, and TiO2/ ZnLDH. The broad absorption bands in the region of 2800-3600 and 1560 cm-1 are attributed to the O-H symmetric stretching mode and the bending modes in the metal hydroxide layers or interlayer water molecules. The band corresponding to the bending mode δOH appears between 1644 and 1648 cm-1 and may be assigned to the adsorbed interlayer water.20 For ZnLDH, an intense absorption band between 1370 and 1383 cm-1 is attributed to the ν3 vibration mode of CO32-. In the same range, the ν3 vibration mode of the nitrate could appear, if this anion still exists in the interlayer.27 This band almost disappears for
Carja et al. the TiO2/ZnLDH, whereas a new band is observed at 1122 cm-1. The new band is assigned as the ν3 vibration mode of SO42and shows that SO42- is present as an anion in the reconstructed clay interlayers. In the low wavenumber region (
