Mesoporous Iron Oxide-Layered Titanate Nanohybrids: Soft-Chemical

Publication Date (Web): September 3, 2008. Copyright © 2008 American ... The Journal of Physical Chemistry C 2016 120 (16), 8451-8460 ... Hideya Hatt...
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J. Phys. Chem. C 2008, 112, 14853–14862

14853

Mesoporous Iron Oxide-Layered Titanate Nanohybrids: Soft-Chemical Synthesis, Characterization, and Photocatalyst Application Tae Woo Kim,†,‡ Hyung-Wook Ha,† Mi-Jeong Paek,† Sang-Hoon Hyun,‡ Il-Hyun Baek,§ Jin-Ho Choy,† and Seong-Ju Hwang*,† Center of Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans UniVersity, Seoul 120-750, Korea, School of AdVanced Materials Science and Engineering, College of Engineering, Yonsei UniVersity, Seoul 120-749, Korea, and Fossil Energy EnVironment Research DiVision, Korea Institute of Energy Research, Daejon 305-343, Korea ReceiVed: June 22, 2008; ReVised Manuscript ReceiVed: July 24, 2008

Mesoporous iron oxide-layered titanate nanohybrids have been synthesized through a reassembling reaction between exfoliated titanate nanosheets and iron hydroxide nanoclusters, in which an electrostatic attraction between both nanosized species could be achieved at low pH of 1.5. The formation of the layer-by-layer ordered heterostructure with the repeating unit of 1.33 nm was clearly evidenced by powder X-ray diffraction and transmission electron microscopic analysis. According to Fe K-edge X-ray absorption spectroscopy, the hybridized iron oxide crystallizes with loosely packed network of three edge-shared FeO6 octahedra units. N2 adsorption-desorption isotherm and diffuse reflectance UV-vis measurements clearly demonstrated that the present iron oxide-layered titanate nanohybrids showed greatly expanded surface areas with mesopores (∼190-230 m2g-1) and narrow bandgap energies (∼2.3 eV), which are of crucial importance in creating a visible light-active photocatalyst. The test of photocatalytic activity revealed that the present nanohybrids could induce the photodegradation of organic pollutant molecules under visible light illumination (λ > 420 nm). Of special interest is that the chemical stability of iron oxide became remarkably improved upon the hybridization, which could be understood as a result of the encapsulation with titanate layers. In this regard, we could conclude that the present hybridization route is very effective in synthesizing novel visible lightactive photocatalysts as well as in stabilizing chemically labile species like iron oxides. Introduction Over the last decades, iron oxides have received special attention because of their intriguing physicochemical properties such as ferromagnetism, catalytic activity, biocompatibility, and so on.1-4 In particular, the low price, nontoxicity, and rich abundance of iron element make its oxides very suitable for industrial applications. Furthermore, the iron oxides showed remarkable diversity in terms of the oxidation state and local crystal structure of iron, and hence, their properties could be tailored by controlling these electronic and structural factors.5-8 Recently it was reported that the synthesis of low-dimensional nanostructured iron oxides led to the creation of novel properties that are absent in the bulk counterpart.9-11 In one instance, the iron oxides could harvest efficiently UV-vis photons from solar radiation, since they have a narrow bandgap separation of ∼2.3 eV corresponding to the photonic energy of visible light. In this regard, the iron oxides have been used as electrode materials for solar cells and photoelectrochemical devices.12-16 Despite such usefulness of iron oxides, their application fields were limited by their low stability in acidic media. One can suppose that the encapsulation of iron oxides by the inert inorganic species would provide a useful solution for making possible their use in a corrosive environment. In addition, hybridization with layered metal oxides provides a valuable tool to create * To whom all correspondence should be addressed. Tel: +82-2-32774370. Fax: +82-2-3277-3419, E-mail: [email protected]. † Ewha Womans University. ‡ Yonsei University. § Korea Institute of Energy Research.

novel functionality via the synergetic combination of two preexisting properties. For example, visible light-driven photocatalytic activity is expected to generate from the coupling between narrow bandgap iron oxide and wide bandgap semiconductive layered metal oxide. Moreover, the formation of porous stacking structure of nanohybrids can improve the photocatalytic activity of the metal oxide through increase of the surface area. However, it is not easy to intercalate a large amount of iron oxides into the two-dimensional titanate lattice via a conventional ionexchange method for creating porous pillared materials. This difficulty is attributed to the limited mobility of the guest species in the confined interlayer space of layered titanium oxide.17,18 In fact, the iron oxide-intercalated titanoniobate prepared by ion exchange showed only a slight increase of surface area (i.e., less than 10 m2/g).19-21 In order to prepare a highly porous iron oxide-layered titanate nanohybrid with improved thermal stability, we have applied an exfoliation-reassembling method, in which the layered titanium oxides were exfoliated into individual monolayers and the exfoliated titanate nanosheets were reassembled with iron oxide nanocrystals. This synthetic strategy was very effective, not only in intercalating a large amount of guest species but also in synthesizing thermally stable mesoporous nanohybrid material.17,18,22-24 In the present study, we have synthesized mesoporous iron oxide-layered titanate nanohybrids to improve the chemical stability of iron oxide and to develop new efficient visible lightactive photocatalysts, as well. The hybridization between layered titanate and iron oxide could be achieved by an electrostatic interaction between negatively charged titanate nanosheets and a positively charged iron hydroxide nanocluster at low pH. The

10.1021/jp805488h CCC: $40.75  2008 American Chemical Society Published on Web 09/03/2008

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Figure 1. Powder XRD patterns of (a) protonated layered titanate, (b) as-prepared iron oxide-layered titanate, and its derivatives calcined at (c) 200, (d) 300, (e) 400, and (f) 500 °C.

crystal structure, band structure, chemical composition, and pore structure of the nanohybrids have been investigated with powder X-ray diffraction (XRD), high resolution-transmission electron microscopy (HR-TEM), diffuse reflectance UV-vis spectroscopy, elemental analysis, and N2 adsorption-desorption isotherm measurement, respectively. The electronic configuration and local atomic arrangement of host and guest in the nanohybrids have been studied using Fe K-edge and Ti K-edge X-ray absorption spectroscopy (XAS). The photocatalytic activity of the nanohybrids was tested against the photodegradation of organic molecules, together with the chemical stability of hybridized iron oxide against acidic corrosion. Experimental Section Sample Preparation. Layered cesium titanium oxide, Cs0.67Ti1.8300.17O4, with lepidocrocite-type structure and its protonated derivative were obtained by solid-state reaction and the subsequent HCl treatment, respectively.25 As reported previously,17,18,25 the colloidal suspension of exfoliated titanate was prepared by reacting the protonated titanate with tetrabutylammonium hydroxide (TBA · OH). The hybridization between layered titanate nanosheets and iron oxide was done by the dropwise addition of 20 mL of 0.25 M aqueous iron(II) acetate solution into the colloidal suspension of exfoliated titanium oxide nanosheets (0.1 g L-1, 100 mL) under vigorous stirring. The reaction was carried out at 60 °C for 5 days at pH 1.5. The maintenance of low pH is important in forming positively charged iron hydroxide trimer [Fe3Or(OH)s]9-(2r+s) or dimer [Fe2(OH)4]2+ clusters.26 Under this pH condition, the iron oxide species could be flocculated with negatively charged titanate nanosheets. While the reaction proceeded, the pH of the reactant solution increased. For this reason, we have added 0.1 M aqueous HCl solution continuously to maintain the pH of 1.5. The resultant products were centrifuged, washed thoroughly with distilled water, and dried in oven. Sample Characterization. The crystal structures of the protonated layered titanate and the iron oxide-layered titanate nanohybrids were studied by powder XRD (Rigaku, λ ) 1.5418 Å, 298 K). The chemical compositions and thermal behaviors of the iron oxide-layered titanate nanohybrids were estimated by performing induced coupled plasma (ICP) spectrometry (Shimazu ICPS-5000), elemental CHN analysis (CE-Instruments-EA-1110), and thermogravimetric analysis (Rigaku TAS-

Kim et al. 100), respectively. The stacking and in-plane structures of the nanohybrids were examined using high-resolution transmission electron microscopy (HR-TEM, Philips-CM200 microscope, 200 kV) with electron diffraction (ED) analysis. To obtain crosssectional images of the heterostructure, the nanohybrid sample was blended with acrylic resin (methyl methacrylate:n-butyl methacrylate ) 4:6, 1.5% benzoyl peroxide) in a polyethylene capsule and then the sample was sliced using the ultramicrotome with a diamond knife. The surface area and porosity of the nanohybrids were examined by measuring volumetrically nitrogen adsorption-desorption isotherms at liquid nitrogen temperature. The calcined samples were degassed at 150 °C for 2 h under vacuum before the adsorption measurement. Diffuse reflectance UV-vis spectra were obtained on a PerkinElmer Lambda 35 spectrometer equipped with an integrating sphere, using BaSO4 as a reference. XAS experiments were carried out at the Fe K-edge and Ti K-edge with the extended X-ray absorption fine structure (EXAFS) facility installed at the beam line 7C at the Pohang Accelerator Laboratory (PAL) in Korea. The present XAS data were collected from the thin layer of powder samples deposited on transparent adhesive tapes in a transmission mode using gas ionization detectors. The measurements were carried out at room temperature with a Si(111) single-crystal monochromator. No focusing mirror was used. All the present spectra were carefully calibrated by measuring titanium or iron metal foil simultaneously. In the course of Fe K-edge EXAFS fitting analysis, all the bond distances (R), Debye-Waller factors (σ2), and energy shifts (∆E) were allowed to vary, under the constraint that the energy shifts were kept the same value for adjacent (Fe-O) shells and (Fe-Fe) shells, respectively. Such constraints can be rationalized from the fact that the adjacent shells consisting of the same types of atoms at very close distances would possess nearly the same degree of energy shift.27 For the case of reference R-Fe2O3, the coordination number (CN) was fixed to the crystallographic values to determine the amplitude reduction factor (S02). Using the obtained S02 factor, we have quantitatively determined the coordination number of iron ions in the nanohybrid. Chemical Stability Test. The chemical stability of iron oxide hybridized with layered titanate was tested by monitoring the leaching of iron ions in the acidic media with pH 2, 4, and 6, in comparison with bare iron oxide. After the reaction with aqueous HCl solution for 5 h, the concentration of iron ions in the supernant solution was analyzed with ICP analysis. Photocatalytic Reactivity Test. The photocatalytic activity of the nanohybrids was examined by adopting methylene blue (MB) and dichloroacetic acid (DCA) as target substrates. The substrate was added to an aqueous photocatalyst suspension in the glass reactor (100 mL) with a quartz window and then equilibrated for 30 min with stirring in the dark before illumination. All the tests of catalytic reactivity were carried out in aerobic conditions. The light from a 400-W Xe arc lamp (Oriel) was passed through a 10-cm IR water filter and a cutoff filter (λ > 420 nm) and then focused onto the reactor. Sample aliquots were withdrawn intermittently with a 1-mL syringe during the illumination and filtered through a 0.45-µm PTFE filter (Millipore) to remove catalyst particles. The concentration change of MB was monitored spectrophotometrically by measuring the absorbance at λ ) 665 nm with an UV-vis spectrophotometer. The degradation of DCA and the concurrent production of chlorides were determined by using an ion chromatography (IC, Dionex DX-500) equipped with a Dionex

Mesoporous Iron Oxide-Layered Titanate Nanohybrids

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TABLE 1: Lattice Parameters, Surface Areas (SBET), and CHN Contents of Cs0.67Ti1.8300.17O4, H0.67Ti1.8300.17O4 · H2O, Iron Oxide-Layered Titanate Nanohybrid, and Its Calcined Derivatives sample

a (Å)

b (Å)

c (Å)

Cs0.67Ti1.8300.17O4 H0.67Ti1.8300.17O4 · H2O FeOx-Ti1.8300.17O4 FeOx-Ti1.8300.17O4-200 °C FeOx-Ti1.8300.17O4-300 °C FeOx-Ti1.8300.17O4-400 °C FeOx-Ti1.8300.17O4-500 °Cb

3.823 3.752 3.731 3.724 3.719 3.714 3.738

17.215 18.175 13.33 11.08 10.57 8.16 3.738

2.955 2.955

9.366

SBET (m2g-1)

C (%)a

H (%)a

N (%)a

230 204 203 190 103

1.02 0.61 0.53 0.37 0.0

1.53 0.92 0.51 0.28 0.0

0.0 0.0 0.0 0.0 0.0

The error limits of CHN analysis are 0.3% for C, 0.01% for H, and 0.2% for N. b The lattice parameters of the FeOx-Ti1.8300.17O4-500 °C correspond to those of anatase TiO2. a

Figure 2. Cross-sectional HR-TEM image of the as-prepared iron oxide-layered titanate nanohybrid.

IonPac AS 16 (2 mm × 2 mm) column and a conductivity detector. The eluent solution was 3.5 mM Na2CO3/1 mM NaHCO3. Results and Discussion Powder XRD and HR-TEM Analyses. The powder XRD patterns of as-prepared iron oxide-layered titanate nanohybrid and its calcined derivatives are plotted in Figure 1, together with that of the protonated titanate. The as-prepared nanohybrid showed equally spaced (0k0) reflections at lower angle compared to the protonated titanate, indicating the formation of an intercalation structure accommodating nanosized iron oxide. In addition, the as-prepared nanohybrid displayed the (200) peak of the titanate layer at ∼49°, and a broad and diffuse peak appeared in the 2θ range of 22-26°. The observation of the latter peak could be interpreted as evidence for the turbostratic stacking of lamellar titanate nanosheets.28 The as-prepared nanohybrid did not show any peaks corresponding to anatase TiO2 in the 2θ range of 26-55°; Although the (101) and (200) peaks of the anatase TiO2 might be concealed by the neighboring reflections of the layered titanate, no observation of (105) and (211) reflections at 53° and 55° in the XRD pattern of the nanohybrid clearly demonstrated the absence of impurity anatase titania in this sample. This finding confirmed the single-phase nature of the present intercalative nanohybrid. From the least-squares fitting analysis, the basal spacing b was determined to be 1.33 nm for the as-prepared iron oxide-layered titanate. From the thickness of titanate layer, the gallery height was calculated to be 0.58 nm (Table 1), strongly suggesting the subnanometer-level size of the hybridized iron oxide species. Although the lattice parameter a could

be determined from the position of the (200) peak, the absence of the (hkl) Bragg reflections with l * 0 prevented us from estimating the lattice parameter c. The HR-TEM analysis provided further evidence for the layer-by-layer ordering of titanate layers and iron oxides. The cross-sectional HR-TEM image of Figure 2 exhibited an assembly of parallel dark lines corresponding to the titanium oxide layers, confirming the formation of an interstratified structure composed of titanate layers and iron oxide nanoparticles. The distance between parallel dark lines estimated from the present HR-TEM image (∼1.33 nm) was well consistent with the b-axis lattice parameter (Table 1). A slight deformation of the layered lattice in the present image can be attributed to tensions caused by the use of ultramicrotome. Due to the high structural anisotropy of the iron oxide-layered titanate nanohybrid, it is quite difficult to obviously probe its in-plane structure with the powder XRD technique. In this regard, we have carried out ED analysis. As illustrated in Figure 3, all the observed diffraction rings could be indexed as (hl) reflections for the two-dimensional orthogonal lattice, which is in good agreement with the in-plane lattice parameters of the host layered titanate. This finding could be regarded as proof for the maintenance of individual titanate layer after the hybridization. In addition, the ring-type patterns of the ED spots indicated the disordered turbostratic stacking of titanate nanosheets along the in-plane ac direction, as demonstrated in the structural model of Figure 3. Also we have studied structural variation of the iron oxidelayered titanate nanohybrid upon the postcalcination under N2 flow. As illustrated in the powder XRD patterns of Figure 1, distinct (010) reflection is still observable after the calcinations at 200-400 °C. Except the nanohybrid calcined at 500 °C, all the calcined samples did not display any peaks corresponding to anatase TiO2 in the 2θ range of 26-55°. This underscored the high thermal stability of the present nanohybrid. In comparison with the as-prepared nanohybrid, the calcined derivatives display the (010) peak at higher angle, indicating the decrease of basal spacing upon the postcalcination (Table 1). In the case of nanohybrid calcined at 500 °C, the (010) peak of the heterostructure completely suppressed whereas the (101), (004), (200), (105), and (211) reflections of anatase TiO2 appeared at 2θ ) ∼26°, ∼37°, ∼48°, ∼53°, and ∼55°, respectively, showing the collapse of intercalation structure and the phase transformation to anatase titania, as well. Elemental Analysis and TG-DTA. The chemical composition of the nanohybrids was estimated with elemental and thermal analyses. According to ICP analysis, the ratio of Fe/Ti in the present nanohybrid was determined to be 0.29, confirming the hybridization of ion oxide with the titanium oxide. As summarized in Table 1, the as-prepared nanohybrid did not contain any nitrogen element, highlighting the complete replace-

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Figure 3. ED pattern and structural model of the as-prepared iron oxide-layered titanate nanohybrid.

Figure 4. TG (solid lines) and DTA (dashed lines) curves of the asprepared iron oxide-layered titanate nanohybrid. The thermal analysis was carried out at the rate of 10 °C/min the under N2 flow.

ment of TBA with iron oxide. In addition, CHN analysis revealed that there is only a small amount of carbon in the asprepared nanohybrid,18 suggesting that negligible amounts of acetates were intercalated into the layered titanate lattice. The residual carbon species could be removed by postcalcination at elevated temperature. As shown in the TG-DTA data of Figure 4, the iron oxide-layered titanate nanohybrid exhibited three steps of weight loss below 600 °C. The first weight decrease in the temperature range of 50-200 °C could be attributed to the water evaporation from the nanohybrid, which was supported by the observation of an endothermic DTA peak. The second weight loss around 200-450 °C with a broad endothermic DTA peak was assigned as the dehydroxylation of the hybridized guest species. A weight loss occurring beyond 450 °C would be related to the decomposition of the residual organic acetate group in the interlayer space of the nanohybrid, which was supported by the appearance of a sharp exothermic DTA peak. N2 Adsorption-Desorption Isotherm Analyses. We have investigated the evolutions of surface area and pore structure upon hybridization and postcalcination processes using N2 adsorption-desorption isotherm measurements. As presented in Figure 5, the Brunauer-Deming-Deming-Teller type I and IV shape of isotherms appeared commonly for the as-prepared iron oxide-layered titanate nanohybrid and its calcined derivatives at 200-400 °C, together with H3-type hysteresis loop in the IUPAC classification.29 This result provides strong evidence for the presence of open slit-shaped capillaries with very wide bodies and narrow short necks, which is due to the house-of-

Figure 5. Nitrogen adsorption-desorption isotherms for (a) the asprepared iron oxide-layered titanate nanohybrid and its derivatives calcined at (b) 200, (c) 300, (d) 400, and (e) 500 °C.

cards-type stacking structure of the nanohybrid crystallites.30 The fitting analysis with the BET equation revealed that the as-prepared nanohybrid had a remarkably expanded surface area of ∼230 m2 g-1, indicating the effectiveness of hybridization in increasing surface area. The postcalcination at 200-400 °C caused only a slight decrease in the surface area of the iron oxide-layered titanate nanohybrid (Table 1), underscoring the high thermal stability of its pore structure. In contrast, the calcined nanohybrid at 500 °C exhibited different H2-type hysteresis loop in the relative pressure range (p/p0) of 0.70-0.97, which is characteristic of mesoporous materials with assemblies of the capillaries of one of the shape groups for type A analyzed by de Boer.31 From the fitting analysis to the BET equation, the surface area of the sample was notably decreased to ∼100 m2 g-1 upon heat treatment at 500 °C, reflecting the destruction of intercalation structure. This is in good agreement with the powder XRD results (Figure 1). As plotted in Figure 5, all the nanohybrids under investigation displayed only a weak adsorption of N2 molecules in a low-pressure (p/p0) region, implying that micropores make little contribution to the nitrogen adsorption by the present materials. Such a negligible adsorption of

Mesoporous Iron Oxide-Layered Titanate Nanohybrids

Figure 6. Pore size distribution curve for the as-prepared iron oxidelayered titanate nanohybrid (circles) and its derivatives calcined at 200 (squares), 300 (triangles), 400 (diamonds) and 500 °C (inverse triangles) analyzed by BJH method.

Figure 7. Diffuse reflectance UV-vis spectra for (a) the pristine cesium titanium oxide, (b) the as-prepared iron oxide-layered titanate nanohybrid, and its derivatives calcined at (c) 200, (d) 300, and (e) 400 °C.

N2 molecules into micropores was interpreted as a result of the presence of water molecules or residual carbon species blocking the micropores. The pore size analysis for desorption branch data, based on the Barrett-Joyner-Halenda (BJH) method, clearly demonstrated that the calcined nanohybrids at 200-400 °C possessed two kinds of mesopores with an average diameter of ∼3.7 and ∼4.5-5.5 nm. Judging from pore sizes and basal spacing of the nanohybrids, the observed mesopores should originate from the stacking structure of nanohybrid crystallites, not from the intercalation structure. As plotted in Figure 6, the heat treatment at 500 °C suppressed these smaller mesopores but created bigger mesopores with a broad size distribution of ∼6-12 nm, which was ascribed to the destruction of pillared structure and the formation of porous stacking structure of iron oxide and titanium oxide particles, respectively. This coincides well with the powder XRD results (Figure 1f). Diffuse Reflectance UV-Vis Spectroscopy. The optical property and band structure of the nanohybrids were probed with diffuse reflectance UV-vis spectroscopy. As plotted in Figure 7, the hybridization with iron oxide created distinct

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Figure 8. Ti K-edge XANES spectra of (a) rutile TiO2, (b) anatase TiO2, (c) layered cesium titanate, (d) the as-prepared iron oxide-layered titanate nanohybrid, and its derivatives calcined at (e) 200, (f) 300, (g) 400, and (h) 500 °C.

absorption in the visible light region, a result of electronic coupling between wide bandgap titanate and narrow bandgap iron oxide. A linear extrapolation of the UV-vis data revealed that the as-prepared nanohybrid had the Eg value of ∼2.3 eV, which was much smaller than that of the pristine layered titanate (3.6 eV). Before and after the postcalcination, there was no marked change in Eg value. Along with the red shift of absorption edge, the hybridization with iron oxide generated a shoulder peak at ∼2.5 eV corresponding to the d–d transition of iron ions. Ti K-Edge XANES Analysis. The chemical bonding nature of titanium ions in the as-prepared nanohybrid and its calcined derivatives has been studied with Ti K-edge XANES analyses. Figure 8 represents the Ti K-edge XANES spectra of the asprepared FeOx-Ti1.8300.17O4 and its calcined derivatives, together with those of rutile TiO2, anatase TiO2, and the layered cesium titanate. All the nanohybrids are quite similar to the present Ti4+-containing references in terms of edge position, indicating the tetravalent oxidation state of titanium ions in the nanohybrids. In the pre-edge region, there were three pre-edge peaks P1, P2, and P3 corresponding to the transitions from core 1s level to unoccupied 3d states.32 It has been well-known that the overall spectral features of these pre-edge peaks reflect sensitively the crystal structure of titanium oxides.32 As can be seen clearly from Figure 8, the as-prepared nanohybrid and its calcined derivatives at 200-400 °C commonly showed nearly the same pre-edge spectral features as the layered titanate, confirming the maintenance of lepidocrocite-structured titanate lattice upon hybridization and postcalcination processes. This result is consistent with the above-mentioned ED results (Figure 3). After heating at 500 °C, a significant spectral change to anatase TiO2like features occurred, cross-confirming the present XRD results (Figure 1f). In the main-edge region, all the compounds under investigation displayed several peaks A, B, and C corresponding to the dipole-allowed 1s f 4p transitions.32 Like the pre-edge region, overall main-edge features of the as-prepared nanohybrid and its calcined derivatives at 200-400 °C were nearly identical to those of the layered cesium titanate, cross-confirming the maintenance of lepidocrocite titanate lattice upon hybridization and postcalcination. On the other hand, after the heat treatment at 500 °C, the main-edge spectral feature of the nanohybrid became similar to that of the anatase TiO2. This observation

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Figure 9. (Left) Fe K-edge XANES spectra and (right) expanded view for pre-edge spectra for (a) FeO (diamonds), (b) Fe3O4 (triangles), (c) R-Fe2O3 (squares), (d) the as-prepared iron oxide-layered titanate nanohybrid (solid lines), and its derivatives calcined at (e) 200 (dashed lines), (f) 300 (dot-dashed lines), (g) 400 (dot-dot-dashed lines), and (h) 500 °C (circles).

Figure 11. Fourier-filtered Fe K-edge EXAFS spectra for (a) the asprepared iron oxide-layered titanate nanohybrid and (b) the reference R-Fe2O3. The circles and solid lines represent the experimental and fitted data, respectively.

Figure 10. (Top) k3-weighted Fe K-edge EXAFS spectra and (bottom) FT data for (a) the as-prepared iron oxide-layered titanate nanohybrid and its calcined derivative at (b) 200, (c) 300, (d) 400, and (e) 500 °C, in comparison with reference (f) R-Fe2O3, (g) γ-Fe2O3, and (h) Fe3O4.

could provide strong support for the local structural change from layered titanate to anatase TiO2 upon the calcination at 500 °C, as suggested by XRD analysis. Fe K-Edge XANES/EXAFS Analyses. We have investigated the electronic configuration and local symmetry of iron ions in

the nanohybrids with Fe K-edge XANES analysis. The Fe K-edge XANES spectra for the as-prepared iron oxide-layered titanate nanohybrid and its calcined derivatives are plotted in Figure 9, together with those for several iron oxides. The edge position of the as-prepared nanohybrid was found to be nearly the same as that of the reference R-Fe2O3, indicating the trivalent oxidation state of iron ions in this compound. Before and after the postcalcination process, there was no marked change in edge position, indicative of little change of Fe oxidation state. As can be seen clearly from the right panel of Figure 9, all of the present nanohybrids exhibited a pre-edge peak P at ∼7112-7115 eV, which corresponds to the 1s f 3d transition. Although this transition with ∆l ) 2 is forbidden by the electronic dipolar selection rule, it is partially allowed for the tetrahedral symmetry due to the mixing of 4p and 3d states.19 Among the materials under investigation, this peak showed the largest spectral weight for the reference Fe3O4 with the spinel structure, in which one-third of iron ions exist in tetrahedral sites and the rest of them are located in octahedral sites. On the contrary, only a weak pre-edge peak P appeared for the asprepared nanohybrid, clearly demonstrating the octahedral

Mesoporous Iron Oxide-Layered Titanate Nanohybrids TABLE 2: Results of Nonlinear Least-Squares Curve Fittings for the Fe K-Edge EXAFS Spectra of the As-Prepared Iron Oxide-Layered Titanate Nanohybrid and the Reference r-Fe2O3 sample

bond

CN

R (Å)

σ2 (10-3 × Å2)

FeOx-Ti1.8300.17O4a

(Fe-O) (Fe-O) (Fe-Fe) (Fe-Fe) (Fe-O) (Fe-O) (Fe-Fe)face (Fe-Fe)edge (Fe-Fe)corner (Fe-Fe)corner

2.8 2.8 2.0 7.4 3 3 1 3 3 6

1.89 1.99 2.92 3.14 1.92 2.07 2.88 2.96 3.39 3.69

4.25 4.25 3.60 12.30 4.36 4.42 3.10 3.10 2.47 8.38

Fe2O3b

The curve-fitting analysis was performed for the range of R ) 0.706-3.191 Å and k ) 3.55-11.95 Å-1. b The curve-fitting analysis was performed for the range of R ) 0.675-3.805 Å and k ) 3.60-11.65 Å-1. a

Figure 12. pH-dependent variation of iron concentration dissolved from the iron oxide-layered titanate nanohybrid calcined at 300 °C (open circles) and bare R-Fe2O3 (closed circles). The dissolution experiments were performed in acidic media with pH 2, 4, and 6 for 5 h.

Figure 13. Time profiles of photocatalytic degradation of MB in visible light-illuminated (λ > 420 nm) suspensions of the as-prepared iron oxide-layered titanate nanohybrid (open circles) and its derivatives calcined at 300 (diamonds) and 400 °C (open triangles), the pristine Cs0.67Ti1.8300.17O4 (close circles), and R-Fe2O3 (close triangles).

symmetry of iron ion in this compound. The intensity of this feature remained nearly unchanged after the postcalcination at 200-300 °C, suggesting good thermal stability of this metal oxide nanohybrid. After the heat treatment at above 400 °C,

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14859 the pre-edge peak P became slightly enhanced, reflecting the increased structural disorder around iron ions. In the main-edge region, there were several peaks denoted as A, B, and C corresponding to the 1s f 4p transitions. As plotted in the left panel of Figure 9, these peaks possessed much weaker intensity and broad shape for the nanohybrids than for the bulk R-Fe2O3, reflecting the nanocrystalline nature of hybridized iron oxide species. Such a weak intensity and broadening of main-edge features are typical of nanocrystalline metal oxides, a result of their short structural coherence length.19,33 The local atomic arrangement of hybridized iron oxide has been quantitatively investigated with Fe K-edge EXAFS analysis. The k3-weighted Fe K-edge EXAFS spectra for the iron oxide-layered titanate nanohybrid and its calcined derivatives are shown in the top panel of Figure 10, together with those for the references R-Fe2O3, γ-Fe2O3, and Fe3O4. The overall EXAFS oscillations of the nanohybrids were quite different from those of three reference compounds, reflecting their different local structures. In comparison with the present bulk iron oxide references, the nanohybrids showed simpler and weaker fine features in the oscillations, which could be attributed to their short structural coherence units. As shown in the top panel of Figure 10, there was close similarity in EXAFS oscillations for the as-prepared nanohybrid and its derivatives calcined at e300 °C, underscoring the weak influence of postcalcination on the local structure of hybridized iron oxide. In comparison, the heat treatment at above 400 °C gave rise to slight modifications of EXAFS oscillation like the broadening of oscillation peaks around k ) 7-8 and 11. The corresponding Fourier-transformed (FT) spectra are illustrated in the bottom panel of Figure 10. The first prominent FT peak at ∼1.5 Å (phase shift-uncorrected) was assigned to the (Fe-O) coordination shells, which is followed by the peaks at beyond 2 Å (phase shift-uncorrected) corresponding to the (Fe-Fe) pairs in the face-/edge-/corner-shared FeO6 octahedra. In comparison with the bulk iron oxides, the iron oxide-layered titanate nanohybrids showed much weaker intensity of the distant FT peaks corresponding to (Fe-Fe) shells, indicating their nanocrystalline nature. The overall FT features of the asprepared nanohybrid remained little changed after the calcinations below 300 °C, confirming the excellent thermal stability of the local crystal structure of hybridized iron oxide. A notable change in FT like peak splitting occurred after the heat-treatment at 500 °C. In order to determine the structural parameters of hybridized iron oxide species, first we have carried out quantitative fitting analysis for the reference R-Fe2O3. Among the present several references, the R-Fe2O3 compound was selected for the quantitative curve fitting to determine the amplitude reduction factor (S02). This is due to the simpler crystal structure of R-Fe2O3 phase, where there is only one type of crystallographic site for iron ions, which is in contrast with more complex structures of the other references containing several crystallographic sites for iron ions. The FT peaks related to (Fe-O) and (Fe-Fe) coordination shells in the FT spectrum of R-Fe2O3 were isolated by inverse Fourier transformation (i.e., Fourier-filtering) to k space for curve-fitting analysis. As plotted in Figure 11, the Fourier-filtered spectrum of the reference R-Fe2O3 was quite reproducible with the hematite structure (i.e., R-Fe2O3 structure).34 As summarized in Table 2, this compound possesses six (Fe-O) bonds with two difference bond distances of ∼1.92 and ∼2.07 Å, one (Fe-Fe) bond with the distance of ∼2.88 Å corresponding to face-shared FeO6 octahedra, three (Fe-Fe) bonds with the distance of ∼2.96 Å originating from edge-

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Figure 14. Time-dependent profiles of (left) DCA and (right) the evolved chloride ion in visible light-illuminated (λ > 420 nm) suspensions of the as-prepared iron oxide-layered titanate nanohybrid (open circles) and its derivatives calcined at 300 (diamonds) and 400 °C (open triangles), the pristine Cs0.67Ti1.8300.17O4 (close circles), and R-Fe2O3 (close triangles).

shared FeO6 octahedra, and nine (Fe-Fe) bonds with two different distances of ∼3.39 and ∼3.69 Å related to cornershared FeO6 octahedra, as reported previously.34 In contrast, the EXAFS spectrum of the as-prepared iron oxide-layered titanate nanohybrid could be reproduced with a simpler structural model with two types of (Fe-O) bonds at ∼1.89 and ∼1.99 Å and two types of (Fe-Fe) bonds at ∼2.92 and ∼3.14 Å, as listed in Table 2. In comparison with the reference R-Fe2O3, the present nanohybrid showed somewhat shorter (Fe-O) bond distances. Moreover, this compound exhibited smaller CN of 2.8 for both (Fe-O) shells compared to the R-Fe2O3. This fitting result could be understood as a result of the limited growth of iron oxide nanoparticles in the confined space of nanohybrid lattice, leading to the reduction in CN. In comparison with the bulk R-Fe2O3, the imperfect coordination of iron ions with lower CN in the nanohybrid led to the enhancement of (Fe-O) bond covalency, which is responsible for the observed shortening of the (Fe-O) bond distances in the nanohybrid. On the other hand, judging from the bond distance of the first (Fe-Fe) shell (i.e., 2.92 Å) in the nanohybrid, this shell with the CN of 2.0 is attributable to the edge-shared FeO6 octahedra. This local atomic arrangement is similar to that of reactant iron hydroxide [Fe3(OH)4]5+ clusters.26 On the other hand, the second shell of (Fe-Fe) had a bond distance of 3.14 Å, which is shorter than those for the corner-shared (Fe-O-Fe) shells in the reference R-Fe2O3 (Table 2). This finding suggested that this second shell would be related to the bent (Fe-O-Fe) bridging bonds in the imperfectly packed assembly of three FeO6 octahedra units. Therefore, it was concluded that the hybridized iron oxide crystallizes with the loosely packed network of three edge-shared FeO6 octahedra units. This conclusion was further supported from the fact that the gallery height of the nanohybrid (5.8 Å) is shorter than the c-axis lattice parameters of R-Fe2O3 (13.77 Å). Hence, the interlayer space of the nanohybrid cannot accommodate a single unit cell of R-Fe2O3 structure. In light of this, we could verify the fact that the intercalated iron oxide in the present nanohybrid is composed of an imperfectly packed network of three edge-shared FeO6 octahedra clusters, instead of a complete unit cell of R-Fe2O3 structure. On the other hand, in spite of the close spectral similarity between the as-prepared and calcined nanohybrids, the EXAFS spectra of the calcined derivatives at higher temperature were not well fitted with the same structural model, suggesting the significant increase of structural disorder or the minute local structural modification around iron ions upon the calcination process. Chemical Stability Measurements. The chemical stability of the iron oxide incorporated into the nanohybrid was evaluated

by monitoring the time-dependent dissolution of iron ions in acidic solutions with pH 2, 4, and 6. As shown in Figure 12, the concentration of iron ions dissolved from the bare iron oxide became greater as the pH was lowered. On the contrary, for all pH regions presented here, the dissolution of iron ions was found to be much weaker for the nanohybrid than for the bare iron oxide. Also, a weaker pH dependence of iron dissolution was observed, confirming the enhanced chemical stability of iron oxide through the hybridization. As found from the Fe K-edge EXAFS analysis, the hybridized iron species crystallize with the loosely packed network of [Fe3(OH)4]5+ clusters. This structure is much less densely packed than the reference R-Fe2O3, implying the weaker stabilization energy of the guest iron species. In light of this, we could reasonably suppose that, compared with the pure iron oxide, the hybridized iron species is weaker for chemical corrosion. In this regard, the observed higher stability of the iron oxide-layered titanate nanohybrid than the reference R-Fe2O3 clearly demonstrated that hybridization with the titanate layers greatly improved the chemical stability of the guest iron oxide nanocrystals. Considering that the poor stability of nanocrystalline iron oxide in the face of chemical corrosion prohibits the use of Fe2O3 in a solution at low pH, we could conclude that the present protection strategy through hybridization can provide a very valuable method to widen the application fields of iron oxide. Photocatalytic Activity Tests. We have examined the photocatalytic activity of the iron oxide-layered titanate nanohybrids against the photodegradation of MB in an illuminated catalyst suspension, in comparison with those of the pristine cesium titanate and R-Fe2O3. In order to separate the effect of adsorption of organic molecules, we soaked the photocatalyst in substrate solution for 30 min in dark conditions. As plotted in Figure 13, the present nanohybrids and the pristine layered titanate caused marked decrease of initial MB concentration due to the adsorption of dye on the pores or surfaces of these materials. In contrast, no notable change in MB concentration occurred for the reference iron oxide. Under illumination of visible radiation (λ > 420 nm), the calcined and as-prepared nanohybrids could induce rapid degradation of MB, showing the efficient photocatalytic activity of these materials. In contrast, the pristine cesium titanate and the reference iron oxide did not lead to any marked decrease of MB concentration, strongly suggesting the effectiveness of the iron oxide hybridization in deriving the visible light-induced photocatalytic activity from the wide bandgap semiconducting layered titanate. It is worthwhile to note here that the observed degradation of MB would occur via a dye-sensitized path, which does not

Mesoporous Iron Oxide-Layered Titanate Nanohybrids require the bandgap excitation of the nanohybrid catalyst. This is due to the fact that the MB molecule, one of the most representative substrates for the photocatalyst test, is apt to strongly adsorb on the surface of catalysts and can act as a selfsensitizer absorbing visible light.35 To rule out this possibility, the degradation of DCA under visible light irradiation was also tested; see Figure 14. The DCA was selected as another substrate for the photocatalyst test, since this molecule cannot absorb visible light by itself and is a relevant industrial pollutant with high toxicity.36,37 As plotted in the left panel of Figure 14, the as-prepared iron oxide-layered titanate nanohybrid and its calcined derivatives were commonly quite active for the visible lightinduced photodegradation of DCA, which was contrasted with the inactivity of the pristine cesium titanate and R-Fe2O3. This finding provided straightforward evidence for the usefulness of the iron oxide hybridization in providing the wide bandgap semiconducting titanate with visible light photocatalytic activity. Taking into account the fact that the photocatalytic destruction of DCA causes the formation of one proton and two chloride ions,38 we have tried to confirm the photolysis of the DCA by monitoring the evolution of chloride ions using IC. In the right panel of Figure 14, the production of chloride ions was clearly observed for the cases of the nanohybrids, confirming their visible light photocatalytic activity. Also, the inactivity of the pristine cesium titanate and R-Fe2O3 was cross-confirmed by the chloride evolution test. It is worthwhile to mention that the iron species hybridized with layered titanate is quite different from the reference R-Fe2O3 in terms of atomic arrangement and particle size. The smaller cluster size and less packed porous arrangement of the hybridized iron species are expected to be advantageous in achieving the visible light photocatalytic activity of the iron oxide-layered titanate nanohybrid. Summarizing the present results of the organic photodegradation test, the hybridization of wide bandgap layered titanates with visible-sensitized iron oxide nanoparticles could provide an effective route to develop visible light-active photocatalysts. The photocatalytic activity of the as-prepared nanohybrid could be further improved by a postcalcination process, as shown in Figures 13 and 14. Taking into account the fact that the postcalcination would enhance electrical connections between host and guest, the positive effect of postcalcination can be attributed to the increased lifetimes of holes and excited electrons via their spatial separation. Also, the improvement of crystallinity upon heat treatment would make a partial contribution to the enhancement of photocatalytic reactivity. Conclusion In this work, we were successful in synthesizing a mesoporous iron oxide-layered titanate nanohybrid through the soft-chemical resembling reaction between the positively charged iron hydroxide clusters and the exfoliated titanate nanosheets with negative surface charge. According to Fe K-edge and Ti K-edge XANES/EXAFS analyses and HRTEM analysis, the trivalent iron oxide nanoparticles are stabilized in-between lepidocrocite-type titanate layers and the crystal structure of hybridized iron oxide is composed of the loosely packed network of three edge-shared FeO6 octahedra units. It was also found that the obtained nanohybrids had a large amount of mesopores with a highly expanded surface area of ∼190-230 m2/g and a narrow bandgap of ∼2.3 eV, which are favorable for deriving visible

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14861 light-induced photocatalytic activity. In fact, the iron oxidelayered titanate nanohybrids exhibited high photocatalytic activity under visible light irradiation (λ > 420 nm), which was contrasted with the inactivity of the pristine layered titanate and the guest iron oxide. This finding underscored the usefulness of hybridization in developing new visible light active photocatalysts. Of special importance is that the chemical stability of iron oxide could be improved through the encapsulation with layered titanate. Currently we are trying to apply the iron oxide-layered titanate nanohybrids as heterogeneous redox catalysts for various organic reactions as well as electrodes for lithium ion batteries. Acknowledgment. This work is the outcome of a Manpower Development Program for Energy & Resources supported by the Ministry of Knowledge and Economy (MKE) and partly supported by the SRC/ERC program of the MOST/KOSEF (grant R11-2005-008-03002-0). The experiments at Pohang Accelerator Laboratory (PAL) were supported in part by MOST and POSTECH. References and Notes (1) Shinagawa, T.; Izaki, M.; Inui, H.; Murase, K.; Awakura, Y. Chem. Mater. 2006, 18, 763. (2) Redl, F. X.; Black, C. T.; Papaefthymiou, G. C.; Sandstrom, R. L.; Yin, M.; Zeng, H.; Murray, C. B.; O’Brien, S. P. J. Am. Chem. Soc. 2004, 126, 14583. (3) Foster, T. L.; Caradonna, J. P. J. Am. Chem. Soc. 2003, 125, 3678. (4) Li, Z.; Wei, L.; Gao, M. Y.; Lei, H. AdV. Mater. 2005, 17, 1001. (5) Wen, X. G.; Wang, S. H.; Ding, Y.; Wang, Z. L.; Yang, S. H. J. Phys. Chem. B 2005, 109, 215. (6) Vayssieres, L.; Sathe, C.; Butorin, S. M.; Shuh, D. K.; Nordgren, J.; Guo, J. H. AdV. Mater. 2005, 17, 2320. (7) Cao, M. H.; Liu, T. F.; Gao, S.; Sun, G. B.; Wu, X. L.; Hu, C. W.; Wang, Z. L. Angew. Chem. Int. Ed. 2005, 44, 4197. (8) Titirici, M. M.; Antonietti, M.; Thomas, A. Chem. Mater. 2006, 18, 3808. (9) Rondinelli, F.; Russo, N.; Toscano, M. Inorg. Chem. 2007, 46, 7489. (10) Cao, S.-W.; Zhu, Y.-J.; Ma, M.-Y.; Li, L.; Zhang, L. J. Phys. Chem. B 2008, 112, 1851. (11) Insin, N.; Tracy, J. B.; Lee, H.; Zimmer, J. P.; Westervelt, R. M.; Bawendi, M. G. ACS Nano 2008, 2, 197. (12) Lai, J.; Shafi, K. V. P. M.; Ulman, A.; Loos, K.; Yang, N. -L.; Cui, M. -H.; Vogt, T.; Estournes, C.; Locke, D. C. J. Phys. Chem. B 2004, 108, 14876. (13) Long, J. W.; Logan, M. S.; Rhodes, C. P.; Carpenter, E. E.; Stroud, R. M.; Rolison, D. R. J. Am. Chem. Soc. 2004, 126, 16879. (14) Ingler, W. B.; Baltrus, J. P.; Khan, S. U. M. J. Am. Chem. Soc. 2004, 126, 10238. (15) Khader, M. M.; Vurens, G. H.; Kim, I. K.; Salmeron, M.; Somorjai, G. A. J. Am. Chem. Soc. 1987, 109, 3581. (16) Srivastava, D. N.; Perkas, N.; Gedanken, A.; Felner, I. J. Phys. Chem. B 2002, 106, 1878. (17) Kim, T. W.; Hur, S. G.; Hwang, S. -J.; Park, H.; Choi, W.; Choy, J. -H. AdV. Funct. Mater. 2007, 17, 307. (18) Kim, T. W.; Hur, S. G.; Hwang, S.-J.; Choy, J.-H. Chem. Commum. 2006, 220. (19) Jang, J. S.; Kim, H. G.; Reddy, V. R.; Bae, S. W.; Ji, S. M.; Lee, J. S. J. Catal. 2005, 231, 213. (20) Yanagisawa, M.; Sato, T. Solid State Ionics 2001, 141, 575. (21) Yanagisawa, M.; Yamamoto, T.; Sato, T. Solid State Ionics 2002, 151, 371. (22) Kim, T. W.; Hwang, S.-J.; Jhung, S. H.; Chang, J.-S.; Park, H.; Choi, W.; Choy, J.-H. AdV. Mater. 2008, 20, 539. (23) Kim, T. W.; Hwang, S.-J.; Park, Y.; Choi, W.; Choy, J.-H. J. Phys. Chem. C 2007, 111, 1658. (24) Kim, T. W.; Han, A. R.; Hwang, S.-J.; Choy, J.-H. J. Phys. Chem. C 2007, 111, 16774. (25) Sasaki, T.; Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4628. (26) Baes., Jr C. F.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons: New York, 1982. (27) Teo, B. -K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986. (28) Sasaki, T.; Nakano, S.; Yamauchi, S.; Watanabe, M. Chem. Mater. 1997, 9, 602.

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