Examination of Fatty Acid Exchanged Layered Double Hydroxides as

Jan 24, 1996 - The incorporation of titanium oxide into the interlayers of a myristic acid exchanged lithium aluminum layered double hydroxide was don...
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Langmuir 1996, 12, 402-408

Examination of Fatty Acid Exchanged Layered Double Hydroxides as Supports for Photochemical Assemblies Daniel S. Robins and Prabir K. Dutta* Department of Chemistry, The Ohio State University, 120 W 18th Avenue, Columbus, Ohio 43210 Received June 7, 1995. In Final Form: September 13, 1995X The incorporation of titanium oxide into the interlayers of a myristic acid exchanged lithium aluminum layered double hydroxide was done by partitioning of titanium butoxide into the interlayer, followed by hydrolysis under ambient conditions. Upon excitation with near-ultraviolet radiation in the presence of viologens and thiocyanate ion, viologen radical is formed. Cationic viologen radicals were held onto the solid surface, indicating that the titanium oxide is negatively charged. [Tetrakis(4-carboxyphenyl)porphyrinato]zinc(II) (ZnTPPC) was ion exchanged into the interlayers. Electronic spectral changes upon incorporation suggest that the ZnTPPC is in the monomeric form. In the presence of ethylenediamine tetraacetic acid (EDTA) as a sacrificial electron donor, the interlayer ZnTPPC was capable of reducing viologen molecules in solution upon visible radiation. Yields of viologen radical were higher for neutral viologens than cationic viologens, presumably due to the restricted access of cations into the interlayer space. The sensitization of the titanium oxide by ZnTPPC was also possible, resulting in viologen radicals upon visible light excitation. The anionic nature of the titanium oxide was exploited to confirm that sensitization was indeed taking place.

Layered double hydroxides constitute a class of materials with ion-exchangeable anions in their interlayers.1 These compounds complement clays such as smectites, which contain cations in their interlayers.2 A rich chemistry has been developed over the past several decades focusing on the interlayer space in clays. This has resulted in the creation of novel materials for applications ranging in diversity from chromatography to heterogeneous catalysts.3 The idea of using the restricted geometry of the interlayers to study photoprocesses has been extensively explored with smectite clays.4 Studying chemistry in the interlayer space of layered double hydroxides is more recent, but it is becoming increasingly clear that many of the same applications as with clays can also be done with these materials.4,5 Two metal ions surrounded octahedrally by hydroxyl groups form the framework of layered double hydroxides (LDH).1 A typical composition can be represented as [Ml-x2+Mx3+(OH)2]Ax/nn-‚zH2O, where M2+ ) Mg, Zn, Fe, Co, Ni, or Cu, M3+ ) Al, Cr, or Fe, and An- is the exchangeable interlayer anion. In this paper, we focus on an analogue of this class of materials: LiAl2(OH)6+, whose ion-exchange properties have also been investigated.6 The major difference between different composition metal LDHs arises from varying charge densities on * Corresponding author. Telephone: 614 292 4532. FAX: 614 292 1685. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, December 15, 1995. (1) (a) Allmann, R. Acta Crystallogr. 1968, B24, 972. (b) Martin, K. J.; Pinnavaia, T. J. J. Am. Chem. Soc. 1986, 108, 541. (c) Kwon, T.; Tsigdinos, G. A.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988, 110, 3653. (d) Kwon, T.; Pinnavaia, T. J. Chem. Mater. 1989, 1, 381. (e) Reichle, W. T. J. Catal. 1985, 94, 547. (f) Reichle, W. T.; Kang, S. Y.; Everhardt, D. S. J. Catal. 1986, 101, 352. (g) Reichle, W. T. Chemtech 1986, 16, 58. (2) Van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd ed.; Wiley-Interscience: New York, 1977. (3) (a) Pinnavaia, T. J. Science 1983, 220, 365. (b) Boyd, S. A.; Lee, J.-F.; Morland, M. M. Nature 1988, 333, 345. (c) Srinivasan, K. R.; Fogler, H. S. Clays Clay Miner. 1990, 38, 277. (d) White, D. Nature (London) 1957, 179, 1075. (e) White, D.; Cowan, C. T. Trans. Faraday Soc. 1958, 54, 557. (f) Mortimer, J. V.; Gent, P. L. Nature (London) 1963, 197, 789. (g) Zlatkis, A.; Jiao, J. Chromatogr. 1991, 31, 457. (4) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (5) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173.

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the frameworks, which leads to variations in the ionexchanging ability and packing of ions in the interlayers.7 Semiconductor-driven photocatalysis, especially using TiO2, has and continues to be an active area of research.8 Sensitization of semiconductors by dyes to respond to longer wavelength visible radiation is also of current interest.9 Considerable work has been done on sensitized TiO2 colloids.8,9 We have in previous publications investigated in detail the ion exchange and dynamics of long chain fatty acids in LiAl-LDH.10 This hydrophobic interlayer space was found to act as a partitioning medium toward nonpolar molecules from both liquid and gas phases. In this study, our goal has been to assemble porphyrin-TiOx particles in the hydrophobic interlayers of LiAl-LDH-myristate and show that the semiconductor particles can be sensitized. A potential application of such a system could be in photodegradation of pollutants.11 Hydrophobic molecules in aqueous streams could be partitioned into the LDH followed by photodegradation by acting as a sacrificial electron donor to the porphyrin cation. The electron injected by the excited porphyrin into TiO2 can be used for carrying out appropriate reduction chemistry in solution. It is also interesting to (6) (a) Serna, C. J.; White, J. L.; Hem, S. L. Clays Clay Miner. 1977, 25, 384. (b) Dutta, P. K.; Puri, M. J. Phys. Chem. 1989, 93, 376. (c) Twu, J.; Dutta, P. K. J. Phys. Chem. 1989, 93, 7863. (d) Twu, J.; Dutta, P. K. J. Catal. 1990, 124, 503. (e) Cooper, S.; Dutta, P. K. J. Phys. Chem. 1990, 94, 114. (f) Serna, C. J.; Rendon, J. L.; Iglesias, J. E. Clays Clay Miner. 1982, 30, 180. (g) Hernandez, M. J.; Ulibarri, M. A.; Rendon, J. L.; Serna, C. J. Phys. Chem. Miner. 1985, 12, 34. (h) Ulibarri, M. A.; Hernandez, M. J.; Cornejo, J. J. Mater. Sci. 1991, 26, 1512. (i) Sissoko, I.; Iyagba, E. T.; Sahai, R.; Biloen, P. J. Solid State Chem. 1985, 60, 283. (j) Schutz, A.; Biloen, P. J. Solid State Chem. 1987, 68, 360. (k) Chisen, I. C.; Jones, W. J. Mater. Chem. 1994, 4, 1737. (l) Thiel, J. P.; Chiang, C. K.; Poeppelmeier, K. R. Chem. Mater. 1993, 5, 297. (7) Vucelic, M.; Moggridge, G. D.; Jones, W. J. Phys. Chem. 1995, 99, 8328. (8) Kamat, P. V. Chem. Rev. 1993, 93, 267. (9) (a) Amadelli, R.; Argazzi, R.; Bignozzi, C. A.; Scandola, F. J. Am. Chem. Soc. 1990, 112, 7099. (b) O’Reagan, B.; Gratzel, M. Nature 1991, 353, 737. (10) (a) Borja, M.; Dutta, P. K. J. Phys. Chem. 1992, 96, 5434. (b) Dutta, P. K.; Robins, D. S. Langmuir 1994, 10, 1851. (c) Dutta, P. K.; Robins, D. S. Langmuir 1994, 10, 4681. (d) Jakupca, M.; Dutta, P. K. Chem. Mater. 1995, 7, 989. (11) Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Mater. 1993, 93, 587.

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note that organic LDHs have been implicated in early membrane evolution and a host for prebiotic chemistry.12 Experimental Section TiOx Incorporation. Lithium aluminate hydroxide chloride [LiAl2(OH)6]+Cl- was synthesized and exchanged with myristic acid according to the procedure described in earlier papers.10 The LDH-myristate was subjected to a series of ethanol (Midwest Grain Products Co.) and hexane (Optima, Fisher Chemical) washes. Approximately 2 g of hexane-washed LDH-myristate was added to a solution of 15 mL of titanium butoxide (Aldrich) in 10 mL of hexane in a drybox. After a contact period of 24 h, the LDH-myristate/titanium alkoxide was centrifuged and washed with hexane. The sample was then placed in a desiccator alongside a 25 mL beaker of deionized water for controlled hydrolysis. Once the hydrolysis procedure was complete (2-4 days), the titanium loading was determined by X-ray fluorescence. Reference standards were made with commercially available titanium dioxide (Degussa P25), LDH-myristate, and Spectroblend (Complex Industries, Inc.). The X-ray fluorescence experiment was carried out on a Kevex 0700 XRF with an attached Microanalyst 7500 computer. A linear calibration curve plotting the counts/second versus weight percent of Ti for each standard was constructed. Scanning electron micrographs were taken with a JEOL JSM820. Samples were prepared by taking powdered samples and dispersing them homogeneously in ethanol with ultrasonication. The SEM samples were sputtered with gold/palladium. Sensitization with ZnTPPC. Zinc meso-tetrakis(4-carboxyphenyl)porphine (sodium salt) (ZnTPPC), obtained from Porphyrin Products, was incorporated into LDH-myristate. Approximately 200 mg of LDH was placed in 10-12 mL of the ZnTPPC stock solution in 50% ethanol/water and allowed to remain in contact for a minimum of 12 h. Occassionally, the tubes were centrifuged and the residual ZnTPPC solution was replaced with fresh stock solution. Once the LDH was saturated with ZnTPPC, the product was washed with ethanol/water and centrifuged several times. The ZnTPPC-loaded clay was allowed to dry in ambient conditions and stored in capped vials protected from light. Atomic absorption measurements were used to determine the Zn loading. A Perkin Elmer atomic absorption spectrometer Model 3100 equipped with a zinc hollow cathode and GEM Desktop and Perkin Elmer Benchtop software was employed. ZnTPPC-incorporated clay samples of 10.0 mg were weighed and dissolved in 10 mL of 5.0 M HCl. A linear calibration curve was constructed from a zinc solution made by dissolving metallic powder in 0.5 M HCl. Diffuse reflectance spectra were collected with a Shimadzu UV-visible spectrometer adapted with a Harrick Diffuse Reflectance Apparatus (DRA). All data were transformed to Kubelka-Munk values using BaSO4 as the reference. Photolysis. Samples of LDH-myristate-TiOx were exchanged with 0.1 M NaSCN in 50% ethanol/water solution. After exchange for 12 h, samples were washed with excess ethanol/ water and allowed to dry. At all times, samples were shielded from light. EDTA was present in solution at a concentration of 0.01 M for all photolysis experiments with LDH containing ZnTPPC. Electron acceptors heptyl viologen, HV2+ (N,N′-diheptyl-4,4′bipyridinium bromide, Aldrich), and propyl viologen sulfonate, PVS (N,N′-dipropyl-4,4′-bipyridinium disulfonate), were used in the photocatalytic reactions. PVS was synthesized according to the procedure described in the literature.13 All samples subjected to photolysis were deoxygenated. It is important to have an oxygen free environment since oxygen reacts readily with viologen radicals. Removal of oxygen was achieved on a greaseless, glass vacuum line system at a pressure of 4 × 10-5 Torr. All samples were degassed overnight (∼12 h) under a constant vacuum. (12) Kuma, K.; Palplawski, W.; Gedulin, B.; Arrhenius, G. Origins Life Evol. Biosphere 1989, 19, 573. (13) Degani, Y.; Willner, I. J. Am. Chem. Soc. 1987, 109, 3568.

Figure 1. Powder diffraction patterns of LiAl-LDH-MA at various stages of incorporation of titanium oxide. After degassing, vacuum tubes holding the pellets were transferred into a drybox. Inside the drybox, pellets and solution were added to an anaerobic UV-vis cell holder manufactured by NSG Precision Cells. The cell path length was 10 mm, and the walls were constructed of quartz. All photolysis experiments were conducted with a Photon Technology International (PTI) A1010 Lamp Housing powered with a PTI LPS-250. Illumination was provided by a Ushio UXL 151H Xenon short arc lamp. A water filter removed IR radiation while Oriel dichroic mirrors reflected two wavelength regions of light. For UV energy, an Oriel dichroic mirror reflected 350450 nm light at 90°. For visible wavelengths, an Oriel dichroic mirror reflecting 420-630 nm light at 90° was substituted. Analysis for photoelectron transfer products was done with a Shimadzu UV-visible spectrometer in either transmission or reflectance mode.

Results I. Preparation of Materials. The starting material in this study is a myristate-exchanged LiAl-LDH (LiAlLDH-MA). Several publications detailing properties of this material have already appeared in the literature.10 From the diffraction pattern (Figure 1a), the interlayer spacing is calculated to be 21 Å, which corresponds to a monolayer of myristic acid molecules (CH3(CH2)12COOH). The alkyl chains are packed in an all-trans fashion and exhibit thermally induced phase transitions.10a (a) Incorporation of TiOx in the Interlayers. We took advantage of the hydrophobic nature of the interlayer space in incorporating titanium. Recently, Pinnavaia and coworkers have reported a strategy for pillaring of smectite clays by SiO2 by ion exchanging long chain alkylammonium compounds, followed by sorption of Si(OR)4.14 In our case, titanium butoxide Ti(OBu)4 was partitioned into LiAl-LDH-myristate from a hexane solution. Figure 1b shows that treatment with hexane does not perturb the powder diffraction pattern. However, upon incorporation of Ti(OBu)4 (Figure 1c), there is an increase in interlayer spacing from 21 to ∼25 Å and an increase in width of the diffraction peaks, indicating a broader (14) Galarneau, A.; Darodawalla, A.; Pinnavaia, T. J. Nature 1995, 374, 529.

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distribution of interlayer spacings. The Ti(OBu)4-LiAlLDH-myristate was exposed to a controlled hydrated environment. The diffraction peaks (Figure 1d) sharpen up somewhat and are similar to spacing to the original LiAl-LDH-myristate. The SEM micrograph of the samples shows a platelet morphology typical of the starting material. Because of interference from the matrix, it was difficult to conclude anything about the size of the TiOx particles from the diffuse reflectance spectrum. (b) Incorporation of Porphyrin in the Interlayers. The uptake of ZnTPP was found to be minimal. For porphyrins to get incorporated into the LDH, it was necessary to choose one with negative charges on the periphery of the molecule. Zinc meso-tetrakis(4-carboxyphenyl)porphyrin (ZnTPPC) could readily be incorporated into the LiAl-LDHmyristate by replacing myristate ions. How many of the carboxylate side groups of the ZnTPPC remain deprotonated inside the LDH has been difficult to determine. The infrared spectrum, which provide evidence of the state of carboxylation,10a is primarily dominated by the myristate carboxylic groups. The loading levels were controlled by choice of the concentration of ZnTPPC in the ethanolic exchange solutions, which was kept around a level of 2.0 wt % Zn. The samples were extensively washed with ethanol to remove surface bound porphyrin. The diffraction pattern upon porphyrin incorporation was similar to that of the starting LiAl-LDH-MA. Considering that the porphyrin dimension of ∼18 Å is smaller than the interlayer spacing of 21 Å for LiAl-LDH-MA, the porphyrin molecule could be held with its plane perpendicular to the metal hydroxide layer. Phthalocyanines ion exchanged into MgAl-LDHs intercalate with their macrocyclic plane perpendicular to the metal hydroxide layer.15 (c) Preparation of Zn-TPPC-TiOx Samples. Upon partitioning Ti(OBu)4 into LiAl-LDH-MA-ZnTPPC and hydrolysis, the diffraction patterns were similar to that with LiAl-LDH-MA (Figure 1). Samples with loading upto 6 wt% Ti were made. II. Spectroscopic Studies. The environment around ZnTPPC and its interaction with the hydrolyzed titanium species were examined by fluorescence, electronic, and Raman spectroscopy. Figure 2 compares the emission spectrum of ZnTPPC (excitation at 406.7 nm) in various environments. The emission spectrum of the ZnTPPC solid with bands at 677 and 743 nm is distinct from that of the solubilized species in water and ethanol, in which bands are observed at ∼605-610, 652-656, and 710 nm. This difference can be explained due to aggregation. Progressing from a monomer porphyrin to a dimer to a trimer, 7-9 nm red shifts for each addition of a porphyrin ring along with considerable quenching of fluorescence has been reported.16 ZnTPPC, unlike the Cu, Ni, and parent base, does not aggregate in aqueous solution.17,19 Moreover, for porphyrins that do aggregate in aqueous solution, such as TMPyP, alcohols such as methanol will destroy the aggregates and form monomers.18 Thus, the spectra of ZnTPPC in water and alcohol represent (15) (a) Ukrainczyk, L.; Chibwe, M.; Pinnavaia, T. J.; Boyd, S. A. J. Phys. Chem. 1994, 98, 2668. (b) Carrado, K. A.; Forman, J. E.; Botto, R. E.; Winarrs, R. E. Chem. Mater. 1993, 5, 472. (16) (a) Dubowchik, G. M.; Hamilton, A. D. J. Chem. Soc., Chem. Commun. 1986, 665. (b) Hofstra, U.; Koehorst, R. B. M.; Schoafsma, T. J. Chem. Phys. Lett. 1986, 130, 555. (17) Pasternack, R. F.; Francesconi, L.; Raff, D.; Spiro, E. Inorg. Chem. 1973, 12, 2606. (18) Kano, K.; Miyake, T.; Iomato, K.; Sato, T.; Ogawa, T.; Haskimoto, S. Chem. Lett. 1983, 1867. (19) Kalyanasundaram, K.; Naumann-Spallart, M. J. Phys. Chem. 1982, 86, 5163.

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Figure 2. Emission spectra of ZnTPPC in various environments (Excitation wavelength, 406.7 nm). Since solution and solid spectra are presented, the y-axis scale is in arbitrary units.

monomeric species and not surprisingly are different from the red-shifted bands in the aggregated solid form. Since the spectrum of ZnTPPC in LiAl-LDH-MA has band positions typical of that of unaggregated species (604, 657, and 718 nm), it suggests that the ZnTPPC is “solubilized” in the LiAl-LDH-MA in a dispersed fashion and not present as aggregates. Second, the trend of the relative intensity of the vibronic bands within a sample shows that in changing solvent from water to ethanol, the ∼605 nm band decreased in intensity relative to the ∼652 nm band. The intensity of the 604 nm band decreases further in LiAl-LDH-MA, because of the increased hydrophobicity of the interlayer space. Figures 3 and 4 show the electronic spectra of ZnTPPC in the Soret and Q band regions in various environments. In changing from water to ethanol, there is a slight broadening of the Soret band and a pronounced vibronic component at 515 nm. Upon incorporation of the ZnTPPC into the LDH, there is a significant broadening of the Soret band and a slight blue shift in peak position. Such changes are reported for aggregates of porphyrins and arise from exciton coupling,20 and we also observe this broadening in the absorption spectrum of the ZnTPPC crystallites (Soret, 410 nm; Q band, 563 and 604 nm). But, as we noted above, the fluorescence measurements indicate that there are no aggregates in the LDH. Moreover, upon aggregation there is not only a blue shift in the Soret band but also a red shift in the Q band region.16,20b Thus, the difference in the Soret region, i.e., (20) (a) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (b) Hunter, C. A.; Leighton, P.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1989, 547.

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Figure 3. Electronic spectra of ZnTPPC in various environments.

Figure 5. Emission quenching of LiAl-LDH-MA-ZnTPPCTiOx as a function of increasing Ti loading (a) 1.8 wt % Ti, (b) 3 wt % Ti, and (c) 6 wt % Ti (y-axis scale was same for all spectra).

Figure 4. Electronic spectra, with emphasis in the Q band region, for ZnTPPC in various environments.

the considerable broadening of the band and slight blue shift from 425 to 420 nm upon incorporation into the LDH must be arising due to other interactions. These include the highly nonpolar environment surrounding the porphyrin. Also, the orientation of the benzoic acid rings relative to the porphyrin plane could be quite different in the LDH because of the constraining environment. It has been shown that cationic porphyrins, such as CoTMPyP, will exhibit shifts in Soret bands upon incorporation into clays, and this has been attributed to the orientation of the pyridinium ring.15a In addition, with TPPC in cyclodextrins, it was found that hydrogen bonding of the carboxylic groups with the hydroxyl groups of the cyclodextrin led to red shifts in the Soret band.21 In the LDH, the carboxylic groups of the porphyrin can hydrogen bond to framework hydroxyl groups and can contribute to the broadening and the intensity in the 430 nm region. (21) Zhao, S.; Luong, J. H. T. J. Chem. Soc., Chem. Commun. 1994, 2307.

Alterations in the electronic spectra upon incorporation of porphyrins into other layered materials, such as hectorite and tetratitanic acid, have also been noted.22 Upon incorporation of TiOx, the Soret band broadens even further and the peak shifts to 410 nm, demonstrating that the ZnTPPC is interacting with the TiOx, presumably by hydrogen bonding with the oxide interface. This also leads to considerable enhancement in the vibronic intensities in the Q band region. The interaction of ZnTPPC with TiO2 colloids has been reported and typically results in small red shifts of the Soret (8 nm) and Q bands (2 nm) and slight broadening of the Soret band.23 In this case of the LiAl-LDH-MA, the spectral perturbations upon interaction of ZnTPPC with TiOx are considerably more significant than in the colloidal systems, in that the Soret band is blue shifted by about 15 nm and is approximately 5-6 times as broad and the vibronic intensities in the Q band region are enhanced. There also appears to be a mirror image relationship between the absorption and emission spectra in the 500-800 nm region. Thus, the change in profile of the Frank-Condon envelope in both the absorption and emission spectra has significant contributions from the nonpolar environment in the LDH, leading to alterations in the equilibrium conformation in the excited state. To further examine the nature of the interaction between ZnTPPC and TiOx, we investigated the emission quenching of the porphyrin as a function of Ti loading. These spectra are shown in Figure 5 (the fluorescence intensity is on the same scale). With increasing Ti, there is increased quenching, but even at 6 wt % Ti, there is some residual fluorescence. Comparison of the Raman spectra obtained after excitation in the Soret band for ZnTPPC in LiAl-LDHMA in the absence and presence of TiOx is shown in Figure (22) (a) Abdo, S.; Cruz, M. I.; Fripiat, J. J. Clays Clay Miner. 1980, 28, 125. (b) Nakato, T.; Iwata, Y.; Kuroda, K.; Kanedo, M.; Kato, C. J. Chem. Soc., Dalton Trans. 1993, 1405. (23) Kalyanasundaram, K.; Vlachopoulos, N.; Krishnan, V.; Monnier, A.; Gratzel, M. J. Phys. Chem. 1987, 91, 2342.

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Figure 6. Raman spectra of (a) LiAl-LDH-MA-ZnTPPC and (b) LiAl-LDH-MA-ZnTPPC-TiOx.

6 and shows minor shifts in two of the Raman bands at 1244 and 1365 to 1239 and 1360 cm-1. On the basis of the assignment of metallotetraphenylporphyrins,24 the band at 1244 cm-1 is assigned to the Cmeso-C6H5COOH stretch and its shifting upon interaction with the TiOx surface is not surprising and could be a result of H-bonding interactions with the hydrolyzed TiOx interface. The 1365 cm-1 band is assigned to the Ca-N stretch of the pyrrole ring and can be considered a porphyrin vibration. The spectral signal to noise ratio was poor and made it difficult to pinpoint the shifts of other porphyrin bands. Also, spectra could not be obtained in the Q-band region due to increased fluorescence. III. Photochemical Studies. To examine the nature of the TiOx that we are forming in the interlayers, we utilized the well-known property of semiconductors such as TiO2 that photoexcitation into the bandgap leads to electron-hole pair formation. It has been reported that the hole can oxidize molecules such as SCN-, thereby inhibiting the hole-electron recombination.25 Thus the electron can be transferred to molecules such as viologen. Since the viologen radical is readily detected by its characteristic electronic spectra, this system provided a good test of examining whether upon hydrolysis of Ti(OBu)4 in the interlayers, the TiOx formed exhibited semiconductor-like properties. The LiAl-LDH-MA-TiOx was partially ion exchanged with SCN- and suspended in an aqueous solution containing viologen. Two viologens were examined, a cationic viologen, heptyl viologen (HV2+), and a neutral zwitterionic viologen, propyl viologen sulfonate (PVS). Figure 7 shows the absorption spectrum of the solution after photolysis of LiAl-LDH-MA-TiOx-SCN with 350-450 nm radiation in the presence of PVS. PVS•- is observed in solution, indicating that the hydrolyzed Ti(OBu)4 is indeed behaving like a semiconductor. Upon photolysis with HV2+ in solution, no viologen radical species is observed in solution, though the photolyzed solid turns a deep blue. Figure 8a shows the diffuse reflectance spectrum of the blue powder in the 500-700 nm region. Clearly, the peaks due to the viologen radical are observed. The vibronic structure of the viologen radical is enhanced due to the hydrophobic environment. There was no photochemical activity observed upon illuminating these samples with 420-650 nm radiation. (24) Stein, P.; Illman, A.; Spiro, T. G. J. Phys. Chem. 1984, 88, 369. (25) (a) Duonghong, D.; Ramsden, J.; Gratzel, M. J. Am. Chem. Soc. 1982, 104, 2977. (b) Draper, R. B.; Fox, M. A. J. Phys. Chem. 1990, 94, 4628.

Figure 7. Photolysis of LiAl-LDH-MA-TiOx-SCN with 350-450 nm radiation, in 0.01 M PVS solution. The spectra shown are those of the surrounding solution after photolysis for (a) 0 h and (b) 2 h.

Figure 8. Diffuse reflectance spectra of a photolyzed pellet of LiAl-LDH-MA-TiOx-SCN in 0.01 M HV2+ solution (a) spectra taken after 15 min exposure to ambient (b) 2 h exposure to ambient.

Porphyrins are well-known to act as sensitizers for photochemical reduction of viologen in the presence of sacrificial electron donors.26 We examined the ability of the intercalated ZnTPPC to act as sensitizer toward HV2+ and PVS in the presence of EDTA. Figure 9 shows the characteristic UV-visible data for HV2+ showing the HV•+ bands in solution. It is clear that the intercalated porphyrin upon excitation can indeed promote viologen reduction. These solution spectra also establish that there is no ZnTPPC leaching out from the LDH into the solution or being exchanged via the EDTA. Thus, it is ZnTPPC in the LDH that is responsible for the photochemistry. Similar results were also obtained with PVS. However, the rates of radical generation are quite different for the two viologens, with the neutral viologen reacting at a higher rate. These rates are compared in Figure 10. Since the LDH is an anion-exchange material, the positively charged heptyl viologen is excluded, which is reflected in the slower growth rate of the HV•+ radical. Ideally, we would like to express the differences in the photochemistry by quantum yields. However, because of the scattering (26) Darwent, J.; Douglas, P.; Harriman, A.; Porter, G.; Richoux, M. Coord. Chem. Rev. 1982, 44, 83.

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Figure 9. Photolysis of LiAl-LDH-MA-ZnTPPC in 0.01 M HV2+ and 0.01 M EDTA solution. The spectra shown are those of the surrounding solution as a function of photolysis time (a) 0 h, (b) 30 min, (c) 1 h, and (d) 1 h 30 min.

Figure 10. Comparison of the yields of viologen radical as a function of photolysis of LiAl-LDH-MA-ZnTPPC with 0.01 M EDTA and (4) 0.01 M PVS or (+) 0.01 M HV2+.

from the micrometer-sized LDH particles, this is difficult to do. We have instead kept all conditions similar between all photolyses, i.e., the sample size, light intensity, sacrificial electron donor, and viologen concentrations. So, the plots of the amounts of viologen radical with time are an appropriate measure of the relative photochemical efficiency of the systems. The photolysis studies of the LiAl-LDH-MA-ZnTPPC-TiOx system with both viologens and EDTA as a sacrificial electron donors were also examined. Photolysis was carried out by visible radiation (420-650 nm), so no direct excitation of the TiOx semiconductor was occurring. With PVS, the yields of the radical and the rate of its generation were slightly lower in the TiOx-sensitized material than the porphyrin alone, as shown in Figure 11a. With HV2+, the LDH pellet turned a deep blue color upon visible photoexcitation similar to the unsensitized case with UV excitation, and the rate of generation of HV•+ was 5 times lower and is compared in Figure 11b. Discussion There are several facts that emerge from the above results, which are hereby used to understand how the sensitization may be occurring. 1. The hydrolyzed TiOx that is being generated behaves as a semiconductor. The uncertainty is whether there is TiOx both inside the interlayer and on the LDH surface. Even though the samples were extensively washed to remove surface bound Ti(OBut)4 prior to hydrolysis, it is

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(b)

Figure 11. Comparison of the yields of viologen radical generated with (a) PVS, (4) LiAl-LDH-MA-ZnTPPC and (+) LiAl-LDH-MA-ZnTPPC-TiOx and (b) HV2+, (+) LiAl-LDHMA-ZnTPPC and (4) LiAl-LDH-MA-ZnTPPC-TiOx.

difficult to state with certainty that there is no hydrolyzed titanium species on the LDH surface. The interlayer spacing is on the order of 21 Å, which sets an upper bound for the TiOx particles. The extent of penetration of TiOx into the layer is unknown. It is also possible that TiOx is sticking out from the interlayers and connecting with the TiOx in other layers to form a particle covering the edge of the LDH crystallite. 2. Upon UV photolysis of the LiAl-LDH-MA-TiOx, HV•+ is not released into solution and slowly decays upon exposure to air (Figure 8b), whereas PVS•- is released into solution. HV•+ is positively charged and should not be retained by the LDH framework, which is also positively charged. There is a strong parallel to this in the work of Frank et al. who have reported on the photolysis of negatively charged TiO2/SiO2 colloids in the presence of viologens.27 They discovered that the quantum yield for electron transfer to PVS was higher than that of MV2+, because the MV•+ cation was retained at the negatively charged colloid surface. This parallel suggests that the TiOx generated in this study is negatively charged and is responsible for retaining the HV•+. 3. The porphyrin is present in the interlayers, as evidenced from its spectroscopic properties which suggest that its environment is hydrophobic. Moreover, neutral porphyrins such as ZnTPP could not be incorporated into LDH. 4. The porphyrin and TiOx interact with each other, on the basis of fluorescence quenching. Could the porphyrin be primarily associated with TiOx on the outside surface of the LDH crystallite? There are two reasons why this cannot be so. First, the synthesis method involved incorporation of TiOx into the ZnTPPC ion-exchanged LDH. There is no obvious mechanism by which the ionexchanged porphyrin would come out from the interlayer onto the surface during TiOx formation. We of course cannot exclude the possibility that TiOx alone is on the (27) Frank, A. J.; Willner, I.; Groren, Z.; Degani, Y. J. Am. Chem. Soc. 1987, 109, 3568.

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outside. Second, more compelling reason is that the electronic spectra of the porphyrin suggest that it is in a hydrophobic enviroment. Adsorption of ZnTPPC on colloidal TiO2 has been reported in the literature23 and is different from what is seen in Figure 3d. 5. Finally, the most important issue: are we sensitizing the TiOx with the internal porphyrin, or is the photochemistry arising via direct electron transfer from the porphyrin to viologen? Upon visible excitation of LiAlLDH-MA-ZnTPPC-TiOx, the rate of yield of PVS•- is comparable to that of LiAl-LDH-MA-ZnTPPC. From the fluorescence emission, we estimate that about 10% of the emission is not quenched at the 6 wt % Ti loading sample used for the photolysis studies. If these unassociated porphyrins are the sole cause of viologen radical formation, then the yields in the sensitized case should be considerably lower. The comparable rates of radical generation for the porphyrin alone and the sensitized sample indicate that this is not the case and that PVS•must also be formed by electron transfer from sensitized TiOx. This is confirmed in the case of photolysis with HV2+, since the rate of yield of HV•+ is about 5 times lower in the TiOx-sensitized case as compared to only the porphyrin-containing sample. This is consistent with the observations in the unsensitized TiOx sample and arises because the HV•+ is being retained by the negatively charged TiOx, similar to the observations with negatively charged SiO2-TiO2 colloids.27 This strongly supports that indeed the sensitization is working. But then, we need to address why any of the HV•+ is being released in solution, since in the unsensitized case, there was no release of viologen in solution. This is because all the ZnTPPC is not quenched by the TiOx at the loadings of 6 wt % Ti used for the photolysis studies. Thus, the yield of free HV•+ observed in solution is arising primarily from the unquenched ZnTPPC directly reacting with HV2+, instead of through TiOx.

Robins and Dutta

Figure 12. Schematic of the LDH photochemical assembly.

The purpose of this study was to establish that it is possible to create TiOx in LDH which interacts with ionexchanged anionic porphyrins in the interlayers as exemplified by the scheme shown in Figure 12. The scheme, as depicted, implies that the photochemical reactions take place near the edge of the LDH particle. This is not unreasonable, since the TiOx is formed by the hydrolysis of the dissolved Ti(OBu)4 by ambient moisture and water is not expected to penetrate deep into the myristic acid containing LDH interlayer. This is also consistent with the reaction of positively charged species such as HV2+, which will not enter deep into the interlayer. Acknowledgment. We acknowledge funding from the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences. LA9504467