5582
Langmuir 2005, 21, 5582-5587
Urea Photosynthesis inside Polyelectrolyte Capsules: Effect of Confined Media Dmitry G. Shchukin* and Helmuth Mo¨hwald Max-Planck Institute of Colloids and Interfaces, D14424 Potsdam, Germany Received February 17, 2005. In Final Form: April 4, 2005 The influence of the restricted volume of poly(styrene sulfonate)/poly(allylamine hydrochloride) capsules of different size (2.2, 4.2, and 8.1 µm) on the TiO2-assisted photosynthesis of urea from inorganic precursors (CO2 and NO3-) in aqueous solution was demonstrated. Poly(vinyl alcohol) was employed as electron donor to facilitate the photosynthetic process. Decreasing the size of the confined microvolume of polyelectrolyte capsules accelerates the NO3- photoreduction, which is a limiting stage of the urea photosynthesis and, correspondingly, increases the efficiency of urea production. The highest yield of urea photosynthesis (37%) was achieved for Cu-modified TiO2 nanoparticles encapsulated inside 2.2 µm poly(styrene sulfonate)/ poly(allylamine hydrochloride) capsules.
Introduction Conversion of carbon dioxide into energetically more favorable compounds (e.g., methanol, formate, urea, methane) is one of the vital problems of civilization. At the same time, the energy source providing CO2 fixation should produce less CO2, which is the case for solar or nuclear energy. Various photocatalysts and photoelectrocatalysts have been explored to date for carbon dioxide reduction: metallophthalocyanines,1 CdS,2 ZnS,3 TiO2,4,5 MoS2,6 SiC.7 The ultimate goal of the research is to demonstrate the artificial photosynthesis via CO2 reduction to produce hydrocarbons. Despite the advances achieved recently in this field, the direct photocatalytic reduction of CO2 remains a difficult and challenging task. Most effective photocatalysts are composites of Q-sized semiconductors with metal nanoparticles.1,8-10 However, semiconductor nanoparticles are inclined to agglomerate during long-term CO2 photoreduction thus decreasing TiO2 photocatalytic activity. Spatially confined reaction microenvironments can be employed both for synthesis of complex materials with dimensional, structural, and morphological specificity and for mimicking physicochemical processes occurring in nature. Advantages of the spatial confinement are low overconcentration and overheating of the reaction area, a different structure of the medium (solution), controlled access of the reagents to the reaction area, and possibility to model biochemical processes in living cells. Mesoporous * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +49 (0)331-567-9257. Fax: +49 (0)331-567-9202. (1) Shibata, M.; Furuya, N. Electrochim. Acta 2003, 48, 3953-3958. (2) Liu, B.-J.; Torimoto, T.; Yoneyama, H. J. Photochem. Photobiol., A 1998, 113, 93-97. (3) Gutierrez, M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 474-478. (4) Dey, G. R.; Belapurkar, A. D.; Kishore, K. J. Photochem. Photobiol., A 2004, 163, 503-508. (5) Kuwabata, S.; Uchida, H.; Ogawa, A. Chem. Commun. 1995, 829830. (6) Khalil, L. B.; Youssef, N. S.; Rophael, M. W.; Moawad, M. M. J. Chem. Technol., Biotechnol. 1992, 55, 391-396. (7) Dzhabiev, T. S.; Tarasov, B. B.; Uskov, A. M Catal. Today 1992, 13, 695-696. (8) Kuwabata. S.; Yamauchi, H.; Yoneyama, H. Langmuir 1998, 14, 1899-1904. (9) Anpo, M.; Chiba, K. J. Mol. Catal. 1992, 74, 207-212. (10) Tseng, I. H.; Chang, W. C.; Wu, J. C. S. Appl. Catal., B 2002, 37, 37-48.
inorganic materials can be one of the perspective microenvironments for photocatalysis possible to overcome particle agglomeration due to their high porosity and specific surface area at large external size.11-14 Furthermore, the spatially restricted pore volume (∼0.1 µm3) can affect composition and properties of the products of the photochemical reaction performed inside pores. Ramamurthy et al.15 explored zeolites as a media for achieving chiral induction of optically active organic molecules during photochemical reactions. Zeolite confined space forces a reactant and the chiral inductor to interact intimately to yield enantiomerically enriched product as high as 90%.15-18 Porous TiO2 is also an effective photocatalyst for water decomposition,19 phenol,20 chlorophenol,21,22 and n-pentane23,24 oxidation, as a component of a photovoltaic cell.25 Photoinduced CO2 reduction was demonstrated using TiO2 nanocrystals embedded in a SiO2 matrix.26 Both formate and carbon monoxide were produced as the reduction products of CO2, ammonia was formed as the reduction product of nitrate, and in addition to these products, urea was formed as a major reduction product of both CO2 and NO3-. Although mesoporous materials are promising candidates for a spatially confined photocatalytic reactor due (11) Ozin, G. A.; Oliver, S. Adv. Mater. 1995, 7, 943-948. (12) Wark, M.; Wellmann, H.; Rathousky, J.; Zukal, A. Stud. Surf. Sci. Catal. 2002, 142, 1457-1464. (13) Forster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195-217. (14) Caruso, R. A. Top Curr. Chem. 2003, 226, 91-118. (15) Joy, A.; Ramamurthy, V. Chem. Eur. J. 2000, 6, 1287-1293. (16) Lakshminarasimhan, P.; Thomas, J. K.; Johnston, L. G.; Ramamurthy, V. Langmuir 2000, 16, 9360-9367. (17) Ramamurthy, V.; Shailaja, J.; Kaanumalle, L. S.; Sunoj, R. B.; Chandrasekhar J. Chem. Commun. 2003, 1987-1999. (18) Sivaguru, J.; Sunoj, R. B.; Wada, T.; Origane, Y.; Inoue, Y. J. Org. Chem. 2004, 69, 6533-6547. (19) Tang, J.; Wu, Y.; McFarland, E. W.; Stucky, G. D. Chem. Commun. 2004, 1670-1671. (20) Yan, X.; Evans, D. G.; Zhu, Y.; Duan, X. J. Porous Mater. 2004, 11, 131-139. (21) Shchukin, D. G.; Schattka, J. H.; Antonietti, M.; Caruso, R. A. J. Phys. Chem. B 2003, 107, 952-957. (22) Shchukin, D. G.; Caruso, R. A. Chem. Mater. 2004, 16, 22872292. (23) Yu, J. C.; Zhang, L. Z.; Yu, J. G. Chem. Mater. 2002, 14, 46474653. (24) Zhang, L.; Yu, J. C. Chem. Commun. 2003, 2078-2079. (25) Smarsly, B.; Grosso, D.; Brezesinski, T.; Pinna, N.; Boissiere, C.; Antonietti, M.; Sanchez, C. Chem. Mater. 2004, 16, 2948-2952. (26) Liu, B.-J.; Torimoto, T.; Yoneyama, H. J. Photochem. Photobiol., A 1998, 115, 227-230.
10.1021/la050429+ CCC: $30.25 © 2005 American Chemical Society Published on Web 05/04/2005
Photosynthesis inside Polyelectrolyte Capsules
Langmuir, Vol. 21, No. 12, 2005 5583
Figure 1. Schematic illustration of the assembly of a photocatalytic microreactor: (a, b) PVA encapsulation inside PAH/PSS capsules; (b, c) TiO2 immobilization inside PVA-PAH/PSS capsules.
to their intrinsic activity and high specific surface area, surface modification of as-prepared mesoporous materials as well as lending them additional functionalities to increase quantum yield is quite difficult and requires further efforts, which evokes the search for other types of confined photocatalytic systems of high efficiency and selectivity of the photocatalytic process. Polyelectrolyte capsules can act as one of the promising spatially confined photocatalytic reactors. They are fabricated by the layer-by-layer assembly technique on the surface of micrometer and submicrometer scale template particles (MnCO3, SiO2, methylformaldehyde, etc.) followed by dissolution of the template in appropriate solution.27,28 The main advantage of polyelectrolyte capsules is controlled permeability of the polyelectrolyte shell, which enables one to entrap macromolecules and inorganic nanoparticles by changing solvent, pH, or ionic strength of the medium29 while ions and small organic molecules can freely diffuse through the shell and to protect the activity of the encapsulated material (catalyst) against oxidation or poisoning.30 The feasibility of the shell modification by different materials imparts various functionalities to the polyelectrolyte capsule.31 Hence, the combination of the properties of polyelectrolyte capsules can provide favorable conditions to use them as an effective spatially confined microreactor for urea photosynthesis, i.e., shell-restricted reactor microvolume and high concentration of photohole scavenger, stabilization of semiconductor nanoparticles inside the capsule lumen resulting in their high specific surface area, and controlled diffusion of the reagents and products through the capsule shell. Previously conducted research32,33 showed perspectives to employ titania-immobilized semipermeable polyelectrolyte capsules for metal (Ag, Pd, Pt, and Ni) reduction combining high photocatalytic yield inherent to nanoparticulated photocatalysts with running the photoreductive reactions inside the spatially confined micrometer and submicrometer volume. In the presented paper we investigate the influence of the restricted volume of polyelectrolyte capsules of different size (2.2, 4.2, and 8.1 µm) on the semiconductorassisted photosynthesis of urea from inorganic precursors (CO2 and NO3-) in water solution. Nanosized TiO2, which does not undergo photocorrosion, was taken as model (27) Sukhorukov, G. B.; Donath, E.; Lichtenfeld, H.; Knippel, M.; Budde, A.; Mo¨hwald, H. Colloids Surf., A 1998, 137, 253-266. (28) Voigt, A.; Lichtenfeld, H.; Sukhorukov, G. B.; Zastrow, H.; Donath, E.; Baumler, H.; Mo¨hwald, H. Ind. Eng. Chem. Res. 1999, 38, 4037-4043. (29) Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Mo¨hwald, H. J. Phys. Chem. B 2001, 105, 2281-2284. (30) Shchukin, D. G.; Shutava, T.; Shchukina, E.; Sukhorukov, G. B.; Lvov, Y. M. Chem. Mater. 2004, 16, 3446-3451. (31) Gaponik, N.; Radtchenko, I. L.; Sukhorukov, G. B.; Rogach, A. L. Langmuir 2004, 20, 1449-1452. (32) Shchukin, D. G.; Ustinovich, E.; Sviridov, D. V.; Lvov, Y. M.; Sukhorukov, G. B. Photochem. Photobiol. Sci. 2003, 2, 975-977. (33) Shchukin, D. G.; Ustinovich, E.; Sukhorukov, G. B.; Mo¨hwald, H.; Sviridov, D. V. Adv. Mater. 2005, 17, 420-423.
semiconductor encapsulated inside poly(allylamine hydrochloride)/poly(styrene sulfonate) polyelectrolyte capsules. Poly(vinyl alcohol) was employed as electron donor to facilitate photosynthetic processes. The reaction was monitored by measuring the concentration of three main products of the simultaneous CO2 and NO3- photoreduction in aqueous media: urea, NH4+, and formate.26 Experimental Methods Materials. Sodium poly(styrene sulfonate) (PSS, MW ∼ 70000), poly(allylamine hydrochloride) (PAH, MW ∼ 50000), CuSO4, AgNO3, NaNO3, poly(vinyl alcohol) (PVA, MW ∼ 15000), fluorescein isothiocyanate (FITC), and 8-anilinonaphthalene1-sulfonate were purchased from Aldrich and used without additional purification. Synthetic protocol to produce monodisperse MnCO3 template particles of 2.2, 4.2, and 8.1 µm diameter was adapted from ref 34 (synthesis was carried out at elevated temperatures). The water was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18 MΩ‚cm. To visualize PVA encapsulated inside polyelectrolyte capsules by confocal fluorescence microscopy, PVA macromolecules were labeled with FITC according to the procedure described in ref 35. First, PVA was activated by 2-fluoro-1-methylpyridinium p-toluene sulfonate in dimethyl sulfoxide and purified by dialysis against deionized water. Subsequently, FITC was coupled in dimethyl sulfoxide catalyzed by triethylamine. The resulting PVA-FITC contains approximately 5-10 fluorescein molecules attached to one PVA macromolecule. Capsule Preparation and Loading. Hollow PAH/PSS capsules were prepared by alternating deposition of PAH, PSS monolayers on the MnCO3 template particles from 2 mg/mL PAH and 2 mg/mL PSS aqueous solutions in the presence of 0.5 M NaCl. Each deposition step was accompanied by excessive washing of MnCO3/PAH/PSS particles in deionized water. After formation of four PAH/PSS bilayers, the MnCO3 core was dissolved in 1 M HCl. As a result, PAH/PSS capsules of three different diameters (2.2, 4.2, and 8.1 µm) were obtained. More detailed information on the capsule fabrication can be found elsewhere.27,28 PVA was introduced inside the capsule volume by pH-changing protocol.29 A 2 mg/mL PVA solution was mixed with capsule suspension at pH ) 1, at which the shell of the capsule is permeable for macromolecules.29 Then, PVA-containing capsules were transferred into the water at pH ) 6.5 where the shell is not permeable and the excess of nontrapped PVA presenting is solution was washed out with distilled water by centrifugation. For encapsulation of colloidal TiO2, PVA-PAH/ PSS capsules were kept in 7 mg/mL TiO2 colloid solution at pH ) 3.5, which is low enough to let TiO2 nanoparticles be entrapped also retaining already encapsulated PVA, and then washed at pH ) 6.5 (for schematic illustration see Figure 1). Synthesis of TiO2 Nanoparticles. TiO2 nanoparticles were fabricated following a previously published procedure.36 TiCl4 was hydrolyzed by the addition of a 12.5 wt % aqueous ammonia solution at 0 °C. The precipitate was centrifuged, washed with (34) Hamada, S.; Kudo, Y.; Okada, J.; Kano, H. J. Colloid Interface Sci. 1987, 118, 356-365. (35) Song, L.; van Gijlswijk, R. P. M.; Young, I. T.; Tanke H. J. Cytometry 1997, 27, 213-223. (36) Poznyak, S. K.; Kokorin, A. I.; Kulak, A. I. J. Electroanal. Chem. 1998, 442, 99-105.
5584
Langmuir, Vol. 21, No. 12, 2005
Shchukin and Mo¨ hwald
Figure 2. Fluorescence confocal microscopy images of initial 8.1 (A), 4.2 (B), and 2.2 (C) µm PAH/PSS polyelectrolyte capsules (the outermost PAH layer was labeled with Rhodamine 6G) and of PAH/PSS polyelectrolyte capsules loaded with poly(vinyl alcohol) with diameters of 8.1 (D), 4.2 (E), and 2.2 µm (F). water, and, after adding nitric acid as a stabilizer, redispersed by sonication. The resulting TiO2 colloid (concentration 7 mg/ mL, the average size of TiO2 particles is 4 nm) does not exhibit any flocculation on standing for several months. The deposition of nanosized copper onto TiO2 particles was performed in two stages: At first, a small amount of silver was deposited via photocatalytic reduction from 0.01 M AgNO3 solution, then copper was deposited at 80 °C from a standard copper electroless deposition bath.37 Irradiation Conditions. A cylindrical flask of ca. 5 mL equipped with a bottom optical window of ca. 1 cm in diameter was used to illuminate the magnetically stirred capsule suspension (∼108 mL-1) with UV light. Illumination was provided by a Hamamatsu E7536 Hg-Xe lamp through a 2.2-cm-thick circulating water cell and a cutoff filter (λ > 340 nm, Corning 0.52). The light flux entering the capsule suspension was 20 mW cm-2; the corresponding number of photons per second potentially absorbable by the TiO2 was 1.3 × 1017. Characterization. To view the nanoparticles composing the interior of the capsules, the samples were embedded in poly(methyl metacrylate) holders and ultrathin sections (30-100 nm in thickness) were obtained using a Leica ultracut UCT ultramicrotome. Carbon-coated copper grids were used to support the thin sections, and a Zeiss EM 912 Omega transmission electron microscope (TEM) was employed for analysis. Confocal microscopy images of polyelectrolyte capsules in solution were obtained on a Leica TCS SP scanning system equipped with a 100× oil immersion objective and operating in fluorescence mode. Energy dispersive X-ray (EDX) analysis was made on a Zeiss DSM 940 scanning electron microscope. Fluorescence spectra of polyelectrolyte capsules filled with 8-anilinonaphthalene-1-sulfonate were recorded using a Spex Fluorolog 212 spectrofluorimeter (355 nm excitation line). The concentration of photoproduced urea in the aquatic phase was measured colorimetrically with biacetyl oxime thiosemicarbazone in the presence of Fe3+.38 The formate concentration was asserted photometrically using the procedure described by Higgs et al.39 The concentration of photoproduced NH4+ was measured by an Orion ammonium ion selective electrode. (37) Electroless Metal Deposition from Aqueous Solutions; Sviridov, V. V., Ed.; Belar. State University Publ.: Minsk, 1987. (38) Douglas, L. A.; Bremner, J. M. Anal. Lett. 1970, 3, 79-87. (39) Higgs, D. G.; Charles, A. F. Analyst 1971, 96, 502-514.
Results and Discussion The initial polyelectrolyte capsules were obtained using the layer-by-layer approach for assembling alternating poly(styrene sulfonate) and poly(allylamine) monolayers on MnCO3 template particles having 2.2, 4.2, and 8.1 µm diameters. Panels a-c of Figure 2 represent confocal fluorescence microscopy images of the resulting capsules with the shell labeled by adsorbed fluorescent dyes Rhodamine 6G. The capsules are slightly aggregated (approximately 15% of dimers and trimers were found) and have an irregular spherical shape. The size of the capsules corresponds to the size of the chosen template with small deviations (2-4%) and good monodispersity. The next step for fabricating a photocatalytic microreactor is to introduce an appropriate photohole scavenger in the capsule volume. As shown before,2,6,8 secondary alcohols (e.g., 2-propanol) can be effective electron donors increasing urea photoreduction from CO2 and NO3- on particulate TiO2 up to 10 times. On the other hand, electron donors employed for performing the photoreaction in the inner volume of polyelectrolyte capsules should be uptaken by them and retained inside during the reaction. Another feature of the donor material is the possibility to stabilize semiconductor nanoparticles against aggregation in the microreactor media. Poly(vinyl alcohol) conforms with these requirements enhancing, as shown in refs 32 and 33, the rate of TiO2-assisted photoreduction of metals inside PAH/PSS capsules by 150-220% depending on the metal nature, so it was chosen as electron donor for our photocatalytic microreactors (see encapsulation procedure in the experimental part). Complete filling of the capsule volume by PVA macromolecules was confirmed by confocal fluorescence microscopy for all three types of capsules employing FITC-labeled PVA for impregnation (Figure 2d-f). The as-prepared TiO2 nanoparticles were embedded inside capsules on the last stage of the photocatalytic microreactor fabrication (see Figure 1, experimental part). The resulting polyelectrolyte capsules contain both com-
Photosynthesis inside Polyelectrolyte Capsules
Langmuir, Vol. 21, No. 12, 2005 5585
Figure 3. Transmission electron microscopy images of ultramicrotomed 8.1 (A), 4.2 (B), and 2.2 µm (C) PAH/PSS polyelectrolyte capsules containing TiO2 nanoparticles inside.
ponents necessary to perform urea photosynthesis from inorganic reagents: PVA as electron donor and nano-TiO2 as photocatalyst. Here, it is important to emphasize that the concentration of PVA and TiO2 is the same for all capsule microreactors because the impregnation of PVA and TiO2 was carried out from the solutions identical for all three sizes of the capsules. This statement was proved by quartz crystal microbalance measurements, which showed equal increment of the capsule weight (for all diameters) normalized by capsule volume after PVA and TiO2 encapsulation. Figure 3 presents TEM images of ultramicrotomed TiO2-PVA-PAH/PSS capsules. TiO2 nanoparticles can be found in the whole inner volume of polyelectrolyte capsules with a small excess near the capsule shell, which can be explained by a higher PVA content closer to the shell (Figure 3) caused by adsorption of PVA on the polyelectrolytes. Aggregation of TiO2 nanoparticles, small deviation of their number, and a curved shape of the capsules shown on TEM images are caused by preparation of the sample for TEM analysis (embedding inside poly(methyl metacrylate) holders and ultramicrotoming). Urea photosynthesis was carried out at the conditions standard for all three sizes of capsule microreactor. A suspension of TiO2-PVA-PAH/PSS capsules (∼108 mL-1) was immersed in CO2-saturated 0.1 M NaNO3 aqueous solution at pH ) 5.5. Gaseous CO2 was purged through the solution during photosynthesis to keep a permanent saturation level (≈20 mM CO2). The mixture was stirred in the dark for 90 min before irradiation to reach adsorption equilibrium between encapsulated TiO2 nanoparticles and initial reagents. After photocatalytic reduction, polyelectrolyte capsules were washed out by centrifugation and urea, formate, and NH4+ concentrations were determined in the supernatant, which is reliable due to the unimpeded diffusion of the ions and small molecules through the polyelectrolyte shell.29 Figure 4 shows the irradiation time dependence for TiO2mediated urea photoreduction inside TiO2-PVA-PAH/ PSS microphotoreactors of 2.2, 4.2, and 8.1 µm diameter. Urea photosynthesis in TiO2 bulk solution containing the same amount of TiO2, PVA, NaNO3, and CO2 was also monitored to compare the efficiency of the confined photocatalytic microreactors. Decreasing the diameter of the polyelectrolyte capsule leads to an increase of the urea yield by 1.5 times for 8.1 µm capsules, 2.3 times for 4.2 µm capsules, and more than a factor of 3 for 2.2 µm capsules. The amount of the product grows linearly with the time indicating that the activity of the TiO2 does not deteriorate during the experiment. Besides urea, NH4+ and formate are also obtained as reduction products of CO2 and nitrate, respectively. The ammonia quantity gradually increases during the irradiation time; however,
Figure 4. Time course of the photoproduction of urea in CO2saturated TiO2 suspension (1) and inside TiO2-loaded polyelectrolyte capsules of 8.1 (2), 4.2 (3), and 2.2 µm (4) diameter.
Figure 5. Concentration of urea, ammonia, and formate after 5 h of irradiation of TiO2 suspension and TiO2-loaded polyelectrolyte capsules of 8.1, 4.2, and 2.2 µm diameter.
the formate/urea and ammonia/urea ratios are different for each photocatalytic reactor (Figure 5). The quantity of the formate in the reaction products is approximately 30% for TiO2 nanoparticles in bulk solution and has a tendency to decrease inside spatially confined microphotoreactors with the decrease of their diameter achieving 4% of the urea content for 2.2 µm TiO2-PVA-PAH/PSS capsules. On the contrary, the part of ammonia increases
5586
Langmuir, Vol. 21, No. 12, 2005
Shchukin and Mo¨ hwald
together with the overall efficiency of the urea photoproduction from 16% for TiO2 in the bulk to 35% for TiO2 in 2.2 µm capsules. These proportions remain unchanged during the irradiation. In general, TiO2-mediated urea photosynthesis in water can be described by the following equations:26 hν
TiO2 98 e- + h+ Reduction reaction
CO2 + H+ + 2e- f HCOONO3- + 10H+ + 8e- f NH4+ + 3H2O 2NO3- + CO2 + 18H+ + 16e- f (NH2)2CO + 7H2O Oxidation reaction
The main product of the TiO2-assisted simultaneous photoreduction of NO3- and CO2 in water is urea while formate and ammonia are byproducts, whose yield, however, affects the rate of urea formation. Yoneyama et al.26 found that the reduction of NO3- is the ratedetermining step in urea photosynthesis in the presence of saturated CO2 solution. Furthermore, Liu et al.40 observed an increase of ammonia production in solvents with the hydrogen-bond network. Accordingly, it may be said that the formation of urea is preferred in solvents with a more organized hydrogen-bond network in which ammonia formation occurs easily. There are a number of papers describing the influence of the confined micro (submicro-)environment on the solvent structure inside and, as well, on the properties of the substances resulting from the physicochemical processes performed in the confined microenvironment. Studies of the water structure in restricted geometries revealed an organized hydrogen-bond network of water molecules in micelles,41 microemulsions,42 and lipid nanotubes.43 Gold nanoparticles of unique structure and conductivity were grown inside mesoporous titania,44 and a one-dimensional BaI2 chain was obtained in singlewalled carbon nanotubes.45 Similar phenomena were observed by us while performing synthesis of inorganic nanomaterials in the inner lumen of polyelectrolyte capsules containing dissolved polymers. Rare earth phosphates,46 hydroxyapatite,47 and calcium carbonate48 are more ordered or have metastable (40) Liu, B.-J.; Torimoto, T.; Matsumoto, H.; Yoneyama, H. J. Photochem. Photobiol., A 1997, 108, 187-192. (41) Gonzalez-Blanco, C.; Rodriguez, L. J.; Velazquez, M. M. Langmuir 1997, 13, 1938-1945 (42) Willard, D. M.; Levinger, N. E.; J. Phys. Chem. B 2000, 104, 11075-11080. (43) Yui, H.; Guo, Y.; Koyama, K.; Sawada, T.; John, G.; Yang, B.; Masuda, M.; Shimizu, T. Langmuir 2005, 21, 721-727. (44) Perez, M. A.; Otal, E.; Bilmes, S. A.; Soler-Illia, J. A. A.; Crepaldi, E. L.; Grosso, D.; Sanchez, C. Langmuir 2004, 20, 6879-6886. (45) Sloan, J.; Grosvenor, S. J.; Friedrichs, S.; Kirkland, A. I.; Hutchison, J. L.; Green, M. L. H. Angew. Chem., Int. Ed. 2002, 41, 1156-1159. (46) Shchukin, D. G.; Sukhorukov, G. B.; Mo¨hwald, H. J. Phys. Chem. B 2004, 108, 19109-19113. (47) Shchukin, D. G.; Sukhorukov, G. B.; Mo¨hwald, H. Chem. Mater. 2003, 15, 3947-3950. (48) Antipov, A. A.; Shchukin, D. G.; Fedutik, Y.; Zanaveskina, I.; Klechkovskaya, V.; Sukhorukov, G. B.; Mo¨hwald, H. Macromol. Rapid Commun. 2003, 24, 274-277.
Figure 6. Fluorescence spectra of 8-anilinonaphthalene-1sulfonate in bulk water (solid line) and in suspension of 2.2 µm TiO2-loaded PVA-PAH/PSS capsules (dashed line). Excitation wavelength is 355 nm.
crystal modifications as compared to the nanomaterials obtained in bulk solution at the same conditions but without spatial restriction by the polyelectrolyte shell. Figure 6 compares the fluorescence spectra of 8-anilinonaphthalene-1-sulfonate, which is often used for probing the microscopic polarity and viscosity of solvents,49 in bulk water and in the suspension of 2.2 µm TiO2-loaded PVAPAH/PSS capsules. In bulk water the peak wavelength is around 510 nm, which well correlated to previously reported values.50 There are two peaks observed in the capsule suspension: a small peak at ∼410 nm and a broad peak at ∼480 nm. The remarkable blue shift as compared to that in bulk water indicated at least two local environments with the water properties different from the bulk. According to observations made in ref 51 on micelles and lipid nanotubes, a shorter 410 nm peak is derived from 8-anilinonaphthalene-1-sulfonate molecules adsorbed within polyelectrolyte multilayers while a longer 480 nm peak can be assigned to the 8-anilinonaphthalene-1sulfonate molecules surrounded by confined water inside the lumen of polyelectrolyte capsule. A blue shift of 8-anilinonaphthalene-1-sulfonate fluorescence is due to the decreasing polarity of the surrounding water,50 which, in its turn, is determined by the mobility of the water molecules.51 Hence, reduced mobility and polarity of the water inside polyelectrolyte capsules, which contain dissolved PVA macromolecules and TiO2 nanoparticles, is caused by a more organized, strong hydrogen-bond network. Now one can arrive at a possible explanation of the observed increase of urea photoreduction efficiency along with the decrease of the capsule diameter. The interior of the smaller capsules has a more organized hydrogen-bond network of water, which augments, according to refs 4143, the rate of nitrate photoreduction. The latter is a limiting stage for urea photosynthesis because intermediates of NO3- reduction to NH4+ react with CO•- ion radical producing urea.26 This is also confirmed by the decreased formate yield, which is the final product of further reduction of the CO•- ion radical not involved in urea synthesis, by decreasing the diameter of the capsule. (49) Kosower, E. M. Acc. Chem. Res. 1982, 15, 259-263. (50) Upadhyay, A.; Bhatt, T.; Tripathi, H. B.; Pant, D. D. J. Photochem. Photobiol., A 1995, 89, 201-210. (51) Yui, H.; Guo, Y.; Koyama, K.; Sawada, T.; John, G.; Yang, B.; Masuda, M.; Shimizu, T. Langmuir 2005, 21, 721-727.
Photosynthesis inside Polyelectrolyte Capsules
Figure 7. Time course of the photoproduction of urea in CO2saturated TiO2/Cu suspension (1) and inside TiO2/Cu-loaded polyelectrolyte capsules of 8.1 (2), 4.2 (3), and 2.2 µm (4) diameter.
However, stability of TiO2 nanoparticles against aggregation in the presence of PVA in capsule volume, increased probability of radical encounter and multielectron reduction process due to confined geometry (cage effect), and structure and properties of the polyelectrolyte shell could also affect the yield of urea photoproduction. Depositing on the TiO2 nanoparticles so-called “dark” photocatalysts promoting electron transfer from the TiO2 conduction band to the reagent in solution can increase urea photoreduction efficiency. The most pronounced catalytic activity for the urea reduction in water solutions was observed for Cu nanoparticles catalyzing reduction of both CO2 and NO3-,52 so TiO2 nanoparticles were modified by copper deposition. The amount of copper loading on TiO2 nanoparticles performed by standard electroless deposition was 2.0 wt %, which is the optimum amount to achieve the highest urea yield for TiO2 colloids.10 EDX analysis confirmed the formation of the copper clusters on the TiO2 surface. Impregnation of TiO2/Cu nanoparticles inside PVAPAH/PSS capsules results in further enhancement of urea photoproduction efficiency for all photocatalytic microreactors (Figure 7), with the urea photoproduction rate gradually increasing with decreasing capsule diameter and reaching its maximum for 2.2. µm TiO2/Cu-PVAPAH/PSS capsules. No depletion of TiO2/Cu photoactivity was observed during the reaction. Copper deposition not only leads to the increase of the TiO2 activity but also preserves the tendency of increasing the ammonia-to-urea photoproduction rate ratio going to capsules of smaller diameter (Figure 8). This is evidence of the identical effect of the confined environment of microphotoreactor on both bare TiO2 and Cu-modified TiO2 photocatalyst. However, the yield of formate slightly increases (approximately 110%) inside 2.2 µm capsules as compared to 8.1 µm ones. This contradicts the decrease of formate yield observed (52) Inoue, H.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. Chem. Lett. 1990, 1483-1486.
Langmuir, Vol. 21, No. 12, 2005 5587
Figure 8. Concentration of urea, ammonia, and formate after 5 h of irradiation of TiO2/Cu suspension and TiO2/Cu-loaded polyelectrolyte capsules of 8.1, 4.2, and 2.2 µm diameter.
on bare TiO2 nanoparticles inside polyelectrolyte capsules. As shown in ref 10, Cu nanodeposit simultaneously catalyzes electron transfer to NO3- and CO2 thereby enhancing NO3- reduction, which limits urea photosynthesis, so urea formation from CO2 and NO3- reduction intermediates can become diffusion-limited and we observe an increasing yield of all three reduction products. In conclusion, the reaction of urea photosynthesis in a confined microenvironment was demonstrated on nanosized TiO2 and TiO2/Cu particles introduced inside PVAloaded poly(styrene sulfonate)/poly(allylamine hydrochloride) capsules of 2.2, 4.2, and 8.1 µm diameter. The efficiency of urea photoreduction from CO2 and NO3- grows with the decrease of the capsule size. Decreasing the size of the confined microvolume of polyelectrolyte capsule accelerates the NO3- photoreduction, which is a limiting stage of overall urea photosynthesis. This could be caused by a more organized hydrogen-bond network and lower mobility of water inside the spatially restricted micro (submicro-)volume for smaller polyelectrolyte capsules; however, low aggregation of TiO2 nanoparticles in the presence of PVA in the capsule volume and increased probability of radical encounter and multielectron reduction process due to confined geometry should be taken into consideration. The highest yield of urea photosynthesis (37%) was achieved for Cu-modified TiO2 nanoparticles encapsulated inside 2.2 µm PVA-PAH/PSS capsules. Acknowledgment. D.S. acknowledges the Alexander von Humboldt Foundation (Germany) for an individual research fellowship. We thank R. Pitschke and Dr. J. Hartmann for electron microscopy analysis and Professor Dr. Dmitry Sviridov (Minsk, Belarus) for providing us with TiO2 nanoparticles. Professor Dr. Gleb Sukhorukov is gratefully acknowledged for continuous support and stimulating discussions. LA050429+
