12748
J. Phys. Chem. B 2005, 109, 12748-12754
Synthesis and Photoluminescent Properties of Titanate Layered Oxides Intercalated with Lanthanide Cations by Electrostatic Self-Assembly Methods Yasumichi Matsumoto, Ugur Unal,* Yoshitaka Kimura, Shunsuke Ohashi, and Kazuyoshi Izawa Department of Applied Chemistry, Faculty of Engineering, Kumamoto UniVersity, Kurokami 2-39-1, Kumamoto 860-8555, Japan ReceiVed: April 4, 2005; In Final Form: May 16, 2005
Various lanthanide cations were intercalated into the interlayer of the exfoliated HxTi(2-x/4)0x/4O4‚H2O (HTO) by the electrostatic self-assembly deposition (ESD) and layer-by-layer self-assembly (LBL) methods. X-ray diffraction and thermal analysis data indicated that interlayer lanthanide cations existed as an aqua ion and were coordinated with 7-10 water molecules under ambient conditions. The interlayer distances were found to be in the range 6-7 Å for HTO layered oxide intercalated with a lanthanide cation. Intercalation of lanthanide cations into the interlayer by the LBL method was monitored by UV-vis spectrum and X-ray diffraction. Photoluminescence properties were also discussed in detail. Eu3+ intercalated layered oxide exhibited intense red emission at room temperature. The presence of interlayer water molecules was found to be inevitable for the emission with high intensity. The emission intensity was significantly higher for the films conditioned at 100% RH than those at 5% RH. The icelike behavior of the confined water molecules in the interlayer around lanthanide cations was believed to be contributing highly to the emission mechanism. The mechanism was illustrated and explained by data obtained under several conditions.
Introduction Layered oxide materials are composed of repeating twodimensional units of host oxide layers between which a guest cation exists to hold the layers together. The unique structure of the layered oxides gives rise to a variety of interesting properties. For instance, the superconducting properties of Cu1-6 and Co7 layered oxides are brought about mainly from the host oxide layer with a planar structure. Utilizing in water splitting as a photocatalysis with high quantum efficiency, the nanospace in the interlayer acts as a site for the oxidation of water by holes produced in the host nanosheet layer.8-11 Electrochemical intercalation for the positive electrode in the Li cell is a wellknown behavior for layered oxides with both properties of high electrical conductivity of the host nanosheet layer and high ionic conductivity of the guest cation in the interlayer.12 Recently, in our laboratory, some interesting electrochemical properties such as n-type semiconducting behavior of the host nanosheet layer with photoelectrochemical response,13,14 clear electrochemical redox reaction of the Ag+/Ag couple, and visible light response of the Ru(bpy)32+ in the interlayer to yield a significant amount of photocurrent15 for Ti and Nb layered oxides have been found. These electrochemical reactions are based on the unique two-dimensional structure of these types of layered oxides in principle. Generally, two main soft-solution processes are utilized for the intercalation of a desired guest cation into the interlayer of a layered oxide, which initially possesses an alkali cation in the interlayer. One is the ion-exchange technique carried out in an aqueous solution of a cation of interest, which is substituted for an alkaline cation of the starting layered oxide.16-21 The other one is the direct assembly of a host nanosheet and a cation of interest in a solution due to the electrostatic principles. The latter technique is superior to the former in terms of easy and * Corresponding author. E-mail:
[email protected]. Phone: +81-96-342-3659. Fax: +81-96-342-3679.
optimal intercalation of the guest cation. In the latter technique, two methods such as the electrostatic self-assembly deposition (ESD)15 and layer-by-layer assembly (LBL) techniques22-35 are available to prepare the desired layered oxides. Various kinds of intercalated layered oxides can be easily prepared under controlled pH in a solution by the ESD method, while the LBL method allows us to control the number of layers deposited on a substrate. Recently, it has been reported that several types of layered oxides and TiO2 nanoparticles doped with lanthanide cations show strong emission by an energy transfer process from the host matrix excited by band gap illumination to the in-matrix lanthanide cations.36-39 This behavior suggests that lanthanide cations in the interlayer of the Ti layered oxides might result in strong emission under the excitation of the host layer, because the host TiO6 nanosheet layer has a quantum size effect of a large band gap in comparison with the bulk TiO2. However, only a few papers have been reported for the photoluminescence property of the Ti and Nb layered oxides intercalated with lanthanide cations by the ion-exchange technique, and only weak emission has been observed at 77 K.40-42 Recently, we also have reported the photoluminescence of various lanthanide cations in the interlayer of Ti layered oxide.43 In the present work, we demonstrate a successful preparation of new Ti layered oxide films intercalated with various lanthanide cations by the ESD and LBL methods and investigate the photoluminescence behavior of these films under several thermal and humidity conditions in detail. Strong emission of Eu3+ intercalated layered oxide is reported at room temperature. In addition, this work reveals a substantial promotion of water molecules surrounding Eu3+ to the emission in contribution with the host layer band gap excitation, in contrast to the general knowledge of the diminishing effect of the presence of water on emission in similar cases.
10.1021/jp0517089 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/14/2005
Layered Oxides Intercalated with Lanthanide Cations Experimental Section Materials. The starting material CsxTi(2-x/4)0x/4O4 (CTO) was prepared by the complex polymerization method.13 Prior to the addition of Ti(OCH(CH3)2)4 (10.796 mL), Cs2CO3 (3.4983 g) was dissolved in a mixture of methanol (160 mL) and ethylene glycol (60 mL). During the addition of titanium isopropoxide, the mixture was stirred vigorously with a magnetic stirrer and the temperature was raised gradually to 50 °C. The resultant solution was clear. Next, anhydrous citric acid (28.8 g) was added and the temperature was raised to 150 °C, yielding a resinlike mass, which transformed into ash and then into white powder upon raising the temperature to 300 °C. Calcination of the powder at 800 °C yielded the final CTO (CsxTi(2-x/4)0x/4O4, where x ) 0.76, since Cs/Ti is measured to be 0.42 by XPS analysis) product. Consequently, all Ti cations in the present CTO oxide exist as Ti4+. The final product of the CTO powder was pulverized in a ball mill in various solutions such as acetone, methanol, and ethanol to form a powder with small particle sizes. The average particle size was about 4.0 µm after milling. Nitrate or acetate salts of lanthanide cations (Wako Chemicals) were used without any further treatment. Protonation and Exfoliation. Protonation of the starting materials resulted in HxTi(2-x/4)0x/4O4‚H2O (HTO). Protonation was performed by stirring 5 g of CTO powder in 200 mL of 1 M aqueous HCl solution for 24 h. In fact, this sample contained only a small amount of Cs (Cs/Ti atomic ratio ) 0.04). Subsequent vacuum filtration yielded the HTO powder. The HTO powders were exfoliated in an aqueous ethylamine solution. The reaction was allowed to take place for 24 h under continuous stirring to obtain a reasonable amount of exfoliation. Subsequent centrifugation under 2000 rpm for 30 min yielded colloidal suspensions having exfoliated Ti-O host nanosheets (TiO6 layers). Intercalation with the ESD Method. The intercalation of lanthanide cations into the Ti-O interlayer was carried out using two methods: electrostatic self-assembly deposition (ESD)15 and layer-by-layer self-assembly deposition (LBL).22-35 The ESD method was simply done by adding 10 mL of ethylamine colloidal solution having Ti-O nanosheets into the aqueous solution of a lanthanide cation of interest. The pH of the colloidal solution was carefully adjusted to 7.5-8 with 0.1 M HCl solution. The initial pH values of aqueous lanthanide cation solutions were in the range 5.5-7, and no pH adjustment was applied to the solutions. Mixing two solutions resulted in an immediate precipitation, which consists of a single phase of the lanthanide intercalated Ti-O layered oxides formed by electrostatic interaction between a negatively charged Ti-O nanosheet and positively charged lanthanide cations. The precipitation was rinsed with double distilled water three times in order to remove excess lanthanide cations and other undesired ions. To prepare films of the layered oxides intercalated with lanthanide cations, precipitate was simply applied on a Pt substrate after extracting with a pipet and was allowed to dry at room temperature. The average film thickness was calculated to be about 10 µm from the total amount of Ti in the film. Intercalation with the LBL Method. Deposition of the films by the LBL method was carried out by the same procedure followed by other related studies.22-35 Platinum was used as a substrate in this study. Substrates were primed in aqueous polyethyleneimine (PEI) 2.5 g/L solution for 20 min to charge the surface of the substrate positively. Primed substrates were dipped into the colloidal solution having negatively charged Ti-O nanosheets and then 0.01 M aqueous lanthanide cation solution for 10 min each under ultrasonic treatment, repeatedly
J. Phys. Chem. B, Vol. 109, No. 26, 2005 12749 with rinsing and drying under a N2 stream. The sequence was carried out n times. Characterization and Equipment. The crystal structure and orientation of the films were analyzed from X-ray diffraction patterns (using Cu KR radiation, Rigaku RINT-2500VHF). The compositions of the deposited films were analyzed by an inductively coupled plasma (ICP) spectrophotometer (Seiko Instruments, SPS7800) and/or X-ray photoelectron spectrometer (XPS, VG Scientific Σ-probe). The ICP was carried out after dissolving the films in HCl solution. Fourier transformed infrared spectra (FT-IR Perkin-Elmer) of the films were obtained with a KBr technique. An appropriate amount of each sample was mixed with 0.3-0.5 g of KBr and pressed into a pellet. The samples were analyzed immediately after preparing the pellets. Raman spectra of the samples were taken by a JASCO NRS-2000 Raman spectrometer. The excitation source was an argon ion laser with a wavelength of 514.5 nm. UV-vis absorption spectra of the deposited oxides were measured by an UV-vis spectrometer (Jasco V-550). Thermogravimetric differential thermal analysis curves were obtained by thermal analysis (Seiko TG/DTA). Excitation and emission spectra were analyzed by a Jasco FP-6500 spectrofluorometer with a 150 W Xe lamp source. Excitation and emission spectra were taken at wavelengths of 280 and 614 nm, respectively, at room temperature. The digital photographs of the films were taken in a dark room under illumination with an UV lamp (ASONE SLUV-4, 254/365 nm). Results and Discussion Synthesis and Characterization. Intercalation of lanthanide cations into the Ti-O layered oxides was successfully obtained by the ESD method. The ESD method needs pH adjustment in order to prevent the possible formation of lanthanide hydroxide at high-pH regions and HTO layered oxide at low-pH regions, because the initial values of the TiO6 colloidal nanosheet and lanthanide cation solutions fall generally into quite alkali and acidic regions, respectively. Mixing without any pH adjustment may result in undesired lanthanide hydroxide formation, if the pH is in the high range. In the case of lanthanide solutions, however, the pH values are in a reasonable range so as not to result in any hydroxide formation, when the pH of the exfoliation solution is adjusted to around 7.5. After mixing both solutions, the final pH stood stable at 6.5 ( 0.3, which is equally acceptable for both solutions so as not to yield any lanthanide hydroxides and HTO layered oxide. Therefore, all intercalation reactions succeeded and this was also supported by ICP and XRD data. XRD patterns of the HTO starting material and its intercalated form with different lanthanide cations are given in Figure 1. As observed in the figure, the layer distance increases depending on the intercalated cation in comparison with the starting material. The interlayer distance is also determined by the amount of intercalated water and is tabulated in Table 1 for all types of intercalated compounds. The compositions and water content are also given. The water content of all samples was calculated from the thermal analysis data and indicates that the intercalated lanthanide ions exist as an aqua ion and are coordinated with 7-10 water molecules under ambient conditions. Calculation was done under the assumption that a significant portion of the intercalated water molecules surrounds lanthanide cations in the interlayer, because lanthanides have a high tendency of hydration in an aqua environment. In this case, an interlayer distance of 6.88 Å is wide enough to accommodate 7-10-coordinated aqua lanthanide ions with a Eu-O distance
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Figure 2. TGA and DTA curves of Eu3+ intercalated layered oxide.
Figure 1. XRD patterns of the deposited films from the HTO starting material, its ethylamine (EA) exfoliation form, and layered oxides intercalated with different lanthanide(III) cations by the ESD method, respectively from bottom to top.
TABLE 1: Interlayer Distances and Compositions of the Intercalated Layered Oxide Films Prepared by the ESD Method intercalated Ln ion
interlayer distance (Å)
composition
Y3+ Ce3+ Sm3+ Eu3+ Gd3+ Tb3+
6.43 6.35 6.25 6.88 6.49 6.04
Y0.21Ti1.81O4‚2.1H2O Ce0.21Ti1.81O4‚1.8H2O Sm0.31Ti1.81O4‚2.2H2O Eu0.31Ti1.81O4‚2.1H2O Gd0.27Ti1.81O4‚2.5H2O Tb0.25Ti1.81O4‚2.0H2O
of 2.3-2.6 Å.44,45 Consequently, a large amount of water and/ or hydronium ions coexists in the interlayer and surrounds the lanthanide cations. Compositions of the intercalated compounds were estimated on the basis of ICP and TGA data as given in Table 1. The negative charge of the TiO6 layers may be neutralized only if 0.25 mol of trivalent cations exists in the interlayer of 1 mol of layered oxide according to the chemical composition of the original layered compound, Ln0.25Ti1.81O4‚nH2O. The estimated compositions from the experimental ICP data are in harmony with those based on the given neutrality theory. Small deviations in these values from the original composition may come from the excess amount of lanthanide adsorbed on the nanosheet surfaces and/or hydronium co-intercalation. On the TGA curve, the main weight loss appeared to be a result of removal of the intercalated water, as given in Figure 2. The weight loss takes place in two main steps. The first step between RT and 100 °C stands for the removal of relatively free water and is followed by the removal of hydrate water coordinating Eu3+, which occurs in the range 100-400 °C. Since the intercalated layered oxide does not have any ethylamine molecules according to FTIR and XPS data, the weight loss in the second region can be assigned to the loss of water only. All samples gave rise to a broad DTA peak in the weight loss range. The peak covers overlapped endothermic evaporation peaks and exothermic peaks ascribable to reordering in the crystal structure. This is a typical behavior of this type of layered oxides.13,15,46 The XRD patterns of heat-treated samples at different temperatures are given in Figure 3. Typically, heat treatment results in the shift of the (020) peak to higher degrees, which is an indication of contraction in the interlayer distance as a result
Figure 3. Effect of heat treatment on the XRD pattern of Eu3+ intercalated film prepared by the ESD method. The peaks at the lowest 2θ values are due to the diffraction from the (020) plane.
of the consecutive removal of free and hydrate water. As the temperature increases, the number of coordinated water molecules decreases, and finally, the layer distance represents a value close enough to accommodate a bare lanthanide cation, which has a radius of around 1.1 Å. In addition, decreasing intensity and broadening of the (020) diffraction peak point out the weakening in the crystallinity of the layered oxide. The layered structure is destroyed by 400 °C. Although we have observed the formation of the anatase phase by this temperature for other intercalated layered oxides,15,46 these samples did not go into any phase change in the temperature range. The reason is the existence of lanthanide cations in the interlayer bringing about higher thermal stability. UV-vis spectra of heat-treated samples are shown in Figure 4. Lanthanide cation intercalated samples show a red shift on the absorption edge relative to the starting material, which can be assigned to the host layer-guest ion interaction and resultant reduction in the quantum size effect of TiO6 nanosheets. On the other hand, the slight shift on the absorption edge as a result of heat treatment may be described by a phenomenon related to the loss of interlayer water and consequent improvement in the interaction of the host layers with each other at shorter interlayer distances. Intercalation of lanthanide cations into the interlayer in a sequential process was also succeeded by the LBL method. ICP results showed that the Ln and Ti compositions of the films prepared by the LBL method are approximately the same as those prepared by the ESD method (Table 1). XRD patterns of the films prepared by the LBL method are given in Figure 5.
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J. Phys. Chem. B, Vol. 109, No. 26, 2005 12751
Figure 6. UV-vis absorption spectra of Eu3+/Ti-O nanosheet multilayer films prepared by the LBL method. Figure 4. UV-vis absorbance spectra of films deposited from exfoliated Ti-O nanosheets and its Eu3+ intercalated form prepared by the ESD method at different temperatures.
Figure 5. XRD patterns of Eu3+/TiO6 nanosheet multilayer films prepared by the LBL method, composed of different numbers of layers.
The figure shows diffraction from the repeating nanosheet/cation nanostructure, which is equivalent to the intercalated layered oxide films prepared by the ESD method with respect to its structure. As can be observed from the patterns, the interlayer distance of the monolayer film is lower than those of the intercalated samples prepared by the ESD method. On the other hand, assembling more layers increases the layer distance up to a value similar to the ESD intercalations. The narrower interlayer might be the consequence of intercalation of a lesser amount of water, that is, a smaller number of water molecules surrounding the lanthanide cation. In addition, the diffraction pattern is initially weak and obtained only from the monolayer of the intercalated nanosheets on the surface of the substrate. As the number of assembled layers increases, nanosheets cover the surface more homogeneously and the structure approaches the sample prepared by the ESD method under the assumption that ESD yields the formation of 10 piled up nanosheets.15 Water might penetrate into the preassembled intercalated nanosheets to increase the amount of water each time the film is dipped into an exfoliation or aqueous lanthanide cation solution. As will be explained in the following section, this is quite a reasonable approach, because it was observed that the interlayer distance is altered when the film prepared by the ESD method is treated under different humidity conditions. Films kept in a high-humidity environment had an interlayer distance larger than that of the film treated in about a 5% RH environment. Figure 6 shows UV-vis absorption spectra of the LBL films. The linear increase in absorbance in the range 200-300 nm with the number of layers, which stands for the absorption by TiO6 nanosheets, is evidence for sequential film growth by the LBL method. To the best of our knowledge, this is the first report for the intercalation of a lanthanide cation into the interlayer of a layered oxide prepared by the LBL self-assembly method. Photoluminescence Properties. A room-temperature emission spectrum of Eu3+ intercalated layered oxide prepared by
the ESD method is given in Figure 7a, where excitation was carried out at 280 nm. No emission from the host layer is observed in this figure. The peaks at 593.2 and 614.4 nm were assigned to the 5D0 f 7F1 and 5D0 f 7F2 transitions, respectively. The most intense peak was observed at 614.4 nm. The peak represents red emission, which also could be seen with the naked eye under illumination with a UV light source at a wavelength of 254 nm (See Supporting Information Figure S2). It is known that the local symmetry of the Eu3+ ion in the host matrix determines the relative dominance of the 5D0 f 7F1 or 5D f 7F transition over one another.47-51 The relatively strong 0 2 parity forbidden 5D0 f 7F2 electric dipole transition peak over the 5D0 f 7F1 magnetic dipole transition peak shows that Eu3+ cations in the interlayer occupy sites without inversion symmetry. The excitation spectrum of the Eu3+ intercalated Ti-O nanosheets at room temperature is obtained by monitoring at 614 nm with peaks belonging to the band gap excitation in Ti-O nanosheets, as seen in Figure 7b. The broad peak (270-370 nm) on the excitation spectrum of the intercalated layered oxide, which is the same as the UV absorption spectrum in Figure 4, clearly provides evidence that the observed emission is mainly contributed by band gap excitation in the host nanosheet layer. Thus, electrons and holes produced by the band gap excitation migrate in the host layer and easily move into the interlayer to be trapped by the Eu3+ cations. We have already reported that interlayer molecules for this type of layered oxide are photochemically active and produced electrons and holes in the host layer can take place in the redox reactions of interlayer molecules easily.13 The charge transfer bands in the spectra of the films show that the energy transfer effectively takes places between the host and guest, and the transfer of electrons and holes possibly takes place through the coordinating water molecules to the interlayer lanthanide cations, as stated in a later section. In addition to the broad peak in the UV region on the excitation spectra, three peaks in the range 350-400 nm are observed, with the most intense one due to the main absorption 7F0 f 5L6 band of the Eu3+ ion at 392 nm, which might be a consequence of the direct excitation of the surface adsorbed Eu3+ cations on the film. Consequently, Eu3+ exists in two forms: a water-coordinated one in the interlayer, which is in interaction with the host layer through the surrounding water molecules to lead to a high emission, and a surface adsorbed one, which acts as free Eu3+ species on the surface. On the other hand, heat treatment altered emission behavior significantly. Figure 8 shows the effects of heat treatment on the emission and excitation spectra of the Eu3+ intercalated layered compound prepared by the ESD method, respectively. The intensity of the excitation peak due to the host TiO6 nanosheets is dramatically diminished, as shown in the figure, which reveals that the contribution of the host band gap excitation to the emission decreases. In addition, heat treatment brings about a red shift in the host nanosheet layer excitation
12752 J. Phys. Chem. B, Vol. 109, No. 26, 2005
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Figure 7. (a) Emission (λex ) 280 nm) and (b) excitation (λem )614 nm) spectra of the as-prepared Eu3+ intercalated layered oxide by the ESD method.
Figure 10. Model of photoluminescence of the water-coordinated Eu3+ in the interlayer. Figure 8. Effect of heat treatment on the excitation (λem ) 614 nm) and emission spectra (λex ) 280 nm) of Eu3+ intercalated layered oxide prepared by the ESD method.
Figure 9. Excitation (λem ) 614 nm) and emission (λex ) 280 nm) spectra of as-prepared Eu3+ intercalated film under different humidity conditions at room temperature.
peak, being in harmony with the UV-vis absorption peak given in Figure 4. In fact, the intensity of the emission is decreased by heat treatment, as shown in Figure 8. In addition, it is observed on the excitation spectra that heat treatment resulted in an increase in the intensity of 7F0 f 5D2, 5L6, and 5Gj transition peaks. Probably, some adsorbed Eu3+ species react with the host layer by heat treatment and contribute significantly to the excitation spectrum. Since heat treatment, especially at low temperatures, helps in removing water from the interlayer only, it is obvious that coordinating water molecules to the intercalated Eu3+ cation play a supplementary role in the luminescent properties of the Eu3+ intercalated layered oxide. Humidity controlled experiments confirm this mechanism, as Figure 9 clearly shows. The as-deposited sample gave a highintensity peak on emission and excitation spectra at ∼100% RH, while the film conditioned at 5% RH produced much lowerintensity peaks. In addition, films conditioned in different humidity environments resulted in different XRD diffraction patterns. The layer distance of the film kept at ∼100% RH
decreased by about 2 nm at 5% RH as a result of the release of water in the interlayer. It is known that the intensity of the 5D0 f 7F2 electric dipole transition peak is strongly related to the environmental symmetry and covalency of Eu3+ with the surrounding molecules.47-52 The ratio of the 5D0 f 7F2 peak to 5D0 f 7F1 gives the extent of symmetry and polar and covalent character of the Eu3+ binding site. A higher ratio implies a lower symmetry and more covalency. We have observed the decrease in the ratio with heat treatment or dehumidification. Therefore, we can conclude that a higher intensity of the peak in the case of water-coordinated Eu3+ rather than that of the heat-treated or dehumidified samples suggests stronger bonding with the surrounding water molecules rather than that with O2- of the layer. When the interlayer Eu3+ cations are coordinated with water molecules, the crystal field strength applied on the Eu3+ cations is stronger. Thus, electrons and holes migrating in the host layer move simultaneously through the surrounding water molecules to interlayer lanthanide cations to yield emission rather than giving radiation-less quenching via energy transfer to OH vibration on water molecules. Probably, water molecules in the interlayer will be fixed via hydrogen bonding, as in ice, leading to a decrease the radiation-less quenching. This photoluminescence mechanism via surrounding water molecules to a Eu3+ cation is illustrated in Figure 10. The icelike behavior of the water molecules could be attributed to the strong hydrogen bonding that appears in confined water molecules as in montmorillonite or other layered or porous compounds.53-58 Figure 11 shows the Raman spectra in the OH stretching region of the Eu3+ intercalated layered oxide prepared by the ESD method. The peak at 3100 cm-1 is assigned to the open structure of water as in the tetrahedral structure of ice.59,60 The peak shifts slightly to a higher wavenumber, and two peaks around 3200 and 3300 cm-1 (represent close structure of water in the interlayer) disappear after heat treatment at 300 °C. Apparently, outer layer water molecules that bonded weakly around Eu3+ decompose after heat treatment, while the inner layer composed of water with strong hydrogen bonding remains intact despite a little structural
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Figure 11. Raman spectra of the as-deposited and heat-treated Eu3+ intercalated layered oxide prepared by the ESD method. Figure 13. Effect of Eu3+ concentration on the 614 nm emission intensity (λex ) 280 nm) for the films prepared by both the ESD and LBL methods. The square, diamond, and triangle symbols represent the intensities of one, three, and eight layer LBL films, respectively, and the circles represent the intensity of the films prepared by ESD deposition at various Eu3+ concentrations.
Figure 12. Excitation (λem ) 614 nm) and emission (λex ) 280 nm) spectra of Eu3+/TiO6 multilayer film. The number of layers is 1, 3, 6, 9, and 12, respectively, from bottom to top, and the peak with the highest intensity is that of the film prepared by the ESD method.
change. To our knowledge, the characteristic of the water in the state of confinement in the interlayer of titanate layered oxides is being reported for the first time in this paper. Progressive deposition of the repeating nanosheets and Eu3+ units on a substrate by the LBL method resulted in an increase in the emission and excitation intensities on the spectra, as given in Figure 12. The trend of increase in both emission and excitation spectra is similar to that of the UV-vis spectra in Figure 6. The spectra indicate that the photoluminescence of the LBL films is also based on the energy transfer from the TiO6 band gap to the interlayer Eu3+. The 5D0 f 7F1 and 5D0 f 7F2 transition peaks are identical to those of the film prepared by the ESD method, although the intensity is much lower and the peaks are broader. Since the amount of Eu3+ cations in the interlayer affects the intensity, the peak intensity is enhanced as the number of layers increases. Nonobservance of the transition peaks on the excitation spectra in the region 350-500 nm indicates that there is no surface adsorbed Eu3+ species on the films prepared by the LBL method but only intercalated Eu3+ cations in the interlayer. The effect of the amount of Eu3+ in the film on the emission intensity is shown in Figure 13, where the 614 nm emission intensities for the films prepared by both the ESD and LBL methods are shown. The amount of Eu3+ is controlled by the number of assembled layers for LBL film and the Eu3+ concentration of the solution for ESD film. Consequently, a relatively small intensity of the films prepared by the LBL method is based on the small amount of Eu3+ in the films. From comparison of the spectra of the excitation and the UV-vis absorption, it is clear that the excitation for the photoluminescence is a TiO6 nanosheet band gap process, as stated already. No nF0 f 5L6 excitation of Eu3+ in the LBL films is observed, as Figure 12 shows. This indicates that only
Figure 14. Effect of heat treatment and humidity on the excitation (λem ) 614 nm) and emission (λex ) 280 nm) spectra of the film consisting of 15 layers prepared by the LBL method.
a band gap excitation process via surrounding H2O molecules contributes to the emission of Eu3+ in the interlayer for the LBL film. Figure 14 shows the humidity and heat treatment effect on the excitation and emission intensities of the LBL film intercalated with Eu3+. Both excitation and emission intensities are clearly diminished with heat treatment and low humidity. This is based on the same mechanism as that of the ESD films, as stated already (Figures 8 and 9). Thus, the surrounding water molecules around Eu3+ in the interlayer significantly affect the energy transfer process in the Eu3+ emission in the LBL films. Conclusion The attempt of introducing lanthanide cations into the interlayer of the titanate layered oxide was successful with the ESD method, where a TiO6 nanosheet colloidal solution was mixed with an aqueous solution of a lanthanide cation solution at a certain pH value. XRD and TGA data revealed that water was also co-intercalated and coordinated interlayer lanthanide cations. The coordination number was found to be 7-10 for the fully hydrated interlayer cations. Intercalation was also succeeded by the LBL method. Sequential deposition of TiO6 nanosheets and lanthanide cations on PEI-treated substrate resulted in the formation of thin layered oxide films intercalated with lanthanide cations. Eu3+ intercalated films prepared by both the ESD and LBL methods showed strong red emission of this cation. There is an obvious energy transfer between the TiO6 host and the intercalated Eu3+. The electron-hole pair produced in the host layer under illumination drifts into the interlayer
12754 J. Phys. Chem. B, Vol. 109, No. 26, 2005 Eu3+ to produce its emission, and this process takes place via water molecules surrounding Eu3+. The presence of interlayer water molecules was found to be inevitable for the emission with high intensity. Removal of interlayer water molecules by heating or dehydrating resulted in less contribution of the host layer to the emission. Consequently, it should be noted that the interlayer water molecules strongly promote the photoluminescence of the interlayer lanthanide cation by the energy transfer. The icelike strong hydrogen bonding in the water molecules was suggested to be contributing highly for the high emission from the interlayer Eu3+ cations. This state of water molecules in the interlayer of titanate layered oxides was reported for the first time. Acknowledgment. This work was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Co. (JST) and a Grant-in-Aid for Scientific Research (no. 440, Panoscopic Assembling and High Ordered Functions for Rare Earth Materials, and no. 15350123) from the Ministry of Education, Culture, Sports, Science, and Technology. We would like to thank Prof. Dr. Junichi Hojo and Dr. Kai Kamada at Kyushu University, Japan, for helping us analyze the Raman spectra. Supporting Information Available: Figures showing FTIR spectra of titanate layered oxides intercalated with various lanthanide cations and a photograph of Eu3+ intercalated titanate layered oxide film prepared by the ESD method. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chu, C. W.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang, Y. Q. Phys. ReV. Lett. 1987, 58, 405-407. (2) Hor, P. H.; Meng, R. L.; Wang, Y. Q.; Gao, L.; Huang, Z. J.; Bechtold, J.; Forster, K.; Chu, C. W. Phys. ReV. Lett. 1987, 58, 18911894. (3) Maeda, H.; Tanaka, Y.; Fukutomi, M.; Asano, T. Jpn. J. Appl. Phys. 1988, 27, L209-L210. (4) Matsui, Y.; Maeda, H.; Tanaka, Y.; Horiuchi, S. Jpn. J. Appl. Phys. 1988, 27, L372-L375. (5) Ihara, H.; Sugise, R.; Hirabayashi, M.; Terada, N.; Jo, M.; Hayashi, K.; Negishi, A.; Tokumoto, M.; Kimura, Y.; Shimomura, T. Nature 1988, 334, 510-511. (6) Sugise, R.; Hirabayashi, M.; Terada, N.; Jo, M.; Shimomura, T.; Ihara, H. Jpn. J. Appl. Phys. 1988, 27, L1709-L1711. (7) Takada, K.; Sakurai, H.; Muromachi, E. T.; Izumi, F.; Dilanian, R. A.; Sasaki, T. Nature 2003, 422, 53-55. (8) Choy, J. H.; Lee, H. C.; Jung, H.; Hwang, S. J. J. Mater. Chem. 2001, 11, 2232-2234. (9) Choy, J. H.; Lee, H. C.; Jung, H.; Kim, H.; Boo, H. Chem. Mater. 2002, 14, 2486-2491. (10) Takata, T.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. J. Photochem. Photobiol., A 1997, 106, 45-49. (11) Takata, T.; Furumi, Y.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. Chem. Mater. 1997, 9, 1063-1064. (12) Bruce, P. G. Chem. Commun. 1997, 1817-1824. (13) Matsumoto, Y.; Funatsu, A.; Matsuo, D.; Unal, U.; Ozawa, K. J. Phys. Chem. B 2001, 105, 10893-10899. (14) Koinuma, M.; Seki, H.; Matsumoto, Y. J. Electroanal. Chem. 2002, 531, 81-85. (15) Unal, U.; Matsumoto, Y.; Tanaka, N.; Kimura, Y.; Tamoto, N. J. Phys. Chem. B 2003, 107, 12680-12689. (16) Sasaki, T.; Watanabe, M.; Komatsu, Y.; Fujiki, Y. Inorg. Chem. 1985, 24, 2265-2271. (17) Sasaki, T.; Komatsu, Y.; Fujiki, Y. Inorg. Chem. 1989, 28, 27762779. (18) Abe, R.; Kondo, J. N.; Hara, M.; Domen, K. Supramol. Sci. 1998, 5, 229-233. (19) Abe, R.; Hara, M.; Kondo, J. N.; Domen, K.; Shinohara, K.; Tanaka, A. Chem. Mater. 1998, 10, 1647-1651. (20) Abe, R.; Ikeda, S.; Kondo, J. N.; Hara, M.; Domen, K. Thin Solid Films 1999, 343-344, 156-159. (21) Nunes, L. M.; de Souza, A. G.; de Farias, R. F. J. Alloys Compd. 2001, 319, 94-99.
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