Photocatalytic microreactors based on TiO2-modified ...

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Photocatalytic microreactors based on TiO2-modified polyelectrolyte multilayer capsules Dmitry G. Shchukin,*a Elena Ustinovich,b Dmitry V. Sviridov,b Yuri M. Lvov a and Gleb B. Sukhorukov c

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a

Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272, USA. E-mail: [email protected] b Institute for Physico-Chemical Problems, Belarussian State University, 220050 Minsk, Belarus c Max-Planck Institute of Colloids and Interfaces, am Mühlenberg 1, 14476 Golm, Germany Received 2nd June 2003, Accepted 17th July 2003 First published as an Advance Article on the web 28th July 2003 Spatially confined photocatalytic microreactors for microheterogeneous photoreduction of metal ions from aqueous solutions have been developed. The microreactors consist of hollow micron-sized polyelectrolyte capsules with photoactive TiO2 nanoparticles incorporated in the walls. Additional improvements in metal photoreduction efficiency were achieved by filling the microcapsule cavity with a more effective electron donor. Semiconductor photocatalysts have attracted a great deal of attention in the last decade because of their potential applications for removing toxic organic and inorganic species from aquatic environments.1,2 Besides that, the other important challenge of heterogeneous photocatalysis is to elaborate methods for non-reagent recovery of traces of valuable chemical elements (e.g. noble metals such as Pd and Pt) from water effluents.3–6 For photocatalytic detoxification and extraction, aqueous suspensions and colloids of TiO2 are mostly employed due to both the high oxidizing power of photoholes produced in TiO2 particles under band-gap illumination and their high resistance to photocorrosion. However, such photocatalytic systems suffer from serious drawbacks: (i) relatively low quantum yields in absence of hole (or electron) scavengers in the solution and (ii) the necessity of additional separation steps to remove the semiconductor particles from the treated solution. Magnetic microheterostructures,7 templated porous titania,8,9 and other complicated photocatalytic systems have all been suggested as ways to facilitate the separation of dispersed semiconductor from the treated solution; however, the problem of the cost of the scavengers still remains. Immobilization of titania nanoparticles in flat polyelectrolyte films 10 and semipermeable polyelectrolyte capsules opens up fresh opportunities to improve the performance of photocatalytic systems and combines the advantage of high photocatalytic yield inherent in particulate semiconductor photocatalysts with the possibility of running the photoreductive reactions at high concentrations of photohole scavenger. Micron-scale polyelectrolyte capsules were first introduced in ref. 11 and 12 as an extension of flat polyelectrolyte multilayer assembly. They can be fabricated via layer-by-layer assembly on the surface of micron- and submicron-scale template particles, with the template being removed subsequently by dissolution in an appropriate solvent.13 Polyelectrolyte capsule walls are permeable towards ions and small organic molecules, while nanoparticles and macromolecules can be entrapped inside the hollow capsules by changing the solvent or the pH or ionic strength of the medium.14 Thus, nanoengineering of polyelectrolyte capsules to incorporate titania nanoparticles in their walls should enable such capsules to be employed as effective microreactors for photocatalytic synthesis or degradation reactions. Here, we report the first study of heterogeneous photocatalytic reactions on the surface of polyelectrolyte capsule microphotoreactors. The example we have used is the removal of different metals 975

(Pd2⫹, Cu2⫹, Ag⫹) from aqueous solutions with TiO2-modified poly(allylamine)/poly(styrene sulfonate) microcapsules. To accelerate the photoreduction reaction, poly(vinyl alcohol) (PVA), known to be an effective electron donor, was introduced into the inner space of the polyelectrolyte microcapsules. Poly(styrene sulfonate) (PSS, MW ∼70 000)/poly(allylamine hydrochloride) (PAH, MW ∼50 000) polyelectrolyte capsules were employed as TiO2 microcontainers. They were prepared by layer-by-layer assembly of PAH/PSS multilayers on the surface of MnCO3 particles of 4 µm diameter 15 with PAH as the first layer and PSS as the last one. The MnCO3 template particles were then dissolved in 0.1 M deaerated HCl solution. TiO2 nanoparticles were impregnated in the polyelectrolyte capsule walls from a TiO2 sol (anatase particles ca. 4 nm in size,8 concentration 6.5 g l ⫺1, pH 3.5) over a period of 48 h [Fig. 1(a) and (b)]. The average weight of a single capsule and the reduced metal precipitate was measured employing SC-7201 Universal Counter quartz crystal microbalance (QCM). For this purpose, 4 µl of capsule suspension was dropped onto a quartz resonator and dried. To obtain the average weight of the single capsule, the QCM data were divided by the number of capsules in 4 µl of capsule suspension. Every weight measurement was repeated three times.

Fig. 1 Schematic illustration of the photoinduced formation of silver nanoparticles inside PAH/PSS polyelectrolyte capsules. (a b) TiO2 immobilization in polyelectrolyte shell from TiO2 sol (6.5 g l⫺1, pH 3.5). n⫹ (b c) Metal (M = Pd, Ag, Cu) photoreduction from M solution. Irradiation conditions: 120 W high pressure Hg lamp (365 nm line), the light flux incident upon the quartz window of the photoreactor was completely absorbed in the composite photocatalyst.

To obtain the aqueous sol of titanium dioxide, 2.5 M TiCl4 ⫹ 0.65 M HCl aqueous solution cooled to 0 ⬚C was slowly titrated with 12.5% NH4OH under continuous mechanical stirring up to a final pH of ca. 5. The precipitate was centrifuged, washed out with distilled water and, after adding HNO3 as a stabilizer (TiO2 : HNO3 molar ratio 5 : 1), exposed to ultrasound (22 kHz). Prior to use, the sol was dialyzed. The TiO2 sol was highly photocatalytically active: photocatalytic experiments using 2-chlorophenol oxidation as a test reaction evidenced that the activity of the TiO2 sol was only 10% less than that of the highly effective commercial TiO2 photocatalyst Sachtleben Hombikat UV100 (also pure anatase). A more detailed description of the procedure for TiO2 sol synthesis can be found elsewhere.8

Photochem. Photobiol. Sci., 2003, 2, 975–977 This journal is © The Royal Society of Chemistry and Owner Societies 2003

DOI: 10.1039/b306197c


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After immobilization of the TiO2, the PAH/PSS capsules were washed out using deionized water (pH 7). The presence of TiO2 in capsule shells was confirmed by transmission electron microscopy. According to the weight measurements, the weight of a single TiO2-containing capsule is approximately 16 pg (for comparison, the weight of a pure PAH/PSS capsule is ∼10 pg). Poly(vinyl alcohol) (MW ∼15 000), used as a photohole scavenger, was encapsulated before impregnation with TiO2 from 1.5 mg ml⫺1 PVA solution at pH 3 (adjusted with HCl). PVA-loaded capsules were then rinsed with water at pH 9 in order to prevent PVA diffusion from the interior.14 The weight of a PVA-loaded PAH/PSS capsule is around 12 pg. Photocatalytic reduction of different metals was performed by irradiating 1% v/v TiO2-containing capsules ⫹ 5 × 10 ⫺3 M MCl2 (or AgNO3) aqueous solutions with a 120 W high pressure Hg lamp (365 nm line, light intensity 10 mW cm ⫺2) for 5 min. Photoelectrons generated under band-gap irradiation are involved in the reduction of the metal ions, while the photoholes are captured by the polyelectrolyte shell constituents, i.e. irreversible oxidation occurs during the course of photocatalyst operation. As a result, the metal is reduced and the resulting metal-modified polyelectrolyte capsules, which tend to undergo sedimentation, can be easily separated from the solution. Fig. 2 shows confocal microscope images (Leica TSP SP; taken in bright field mode) of TiO2-modified capsules in an aqueous solution containing dissolved AgNO3 before [Fig. 2(A)] and after [Fig. 2(B)] 5 min of UV irradiation. The capsules become dark after the irradiation, indicating the precipitation of reduced metal in the capsule shell. Depending on the metal, the amount of metal reduced as a result of 5 min irradiation can vary from 2.6 (Pd) to 15.4 pg (Ag) per capsule. The presence of metals in elemental form on the surfaces of the polyelectrolyte capsules was confirmed by wide angle X-ray analysis (Enraf-Nonius PDS-120). The crystallite size, estimated from the broadening of the XRD peaks, varies within the range 10–16 nm. The XRD patterns also evidence traces of metal hydroxides.

Table 1 Quantities of various metals reduced by a single microphotoreactor capsule Capsule

Metal

Weight per capsule/pg

Hollow

Pd Ag Cu Pd Ag Cu

2.6 15.4 — 56 a 312.2 a 11.4

PVA loaded

a

Agglomerated metal particles were found in solution.

Fig. 3(A). Silver nuclei deposited onto the surface of capsule shell do not stabilize the spherical shape of the capsule in the dried form, unlike metal oxide nanoparticles,16 possibly due to there being an insufficient amount of silver deposited to form a rigid inorganic scaffold. In order to investigate the inner structure of metal-loaded polyelectrolyte capsules, they were ultramicrotomed and analyzed using transmission electron microscopy (Zeiss EM 912 Omega). It can be seen in Fig. 3(B) that deposition of the photoreduced metal mostly occurs inside the capsule wall, where the TiO2 particles are adsorbed, leading to the formation of hollow metal microspheres composed of metal nanoparticles. The average particle size, estimated by transmission electron microscopy, was found to be around 20 nm, which agrees well with the crystallite size derived from the XRD peak broadening.

Fig. 3 Scanning (A) and transmission (B) electron microscope images of PVA-loaded (TiO2)PAH/PSS microcapsules after Ag reduction from 5 × 10⫺3 M AgNO3.

Fig. 2 Confocal microscope images (in bright field mode) of (TiO2)PAH/PSS polyelectrolyte capsules in aqueous 5 × 10⫺3 M AgNO3 solution before (A) and after (B) UV irradiation for 5 min.

In order to further enhance the photocatalytic performance of the microphotoreactors, we needed to improve the consumption of photogenerated holes. This was achieved by embedding PVA into the polyelectrolyte capsules. In the presence of a more effective (as compared to the capsule material) electron donor, a dramatic enhancement in photoreduction efficiency occurs, resulting in a ∼20-fold increase in the amount of metal deposited over the same irradiation time (see Table 1). Moreover, the use of PVA-loaded microcapsules makes possible the photoinduced deposition of copper in the capsule wall, a process which does not occur in the absence of poly(vinyl alcohol), perhaps due to participation of PVA photooxidation intermediates in homogeneous Cu2⫹ reduction. However, not all the metal particles deposit directly on the capsule walls, and considerable amounts of Ag particle agglomerates were found freely suspended in the solution. The TiO2-modified polyelectrolyte capsules loaded with silver tend to collapse on drying and form flat overlapping structures with a number of folds and creases, as can be seen in the scanning electron microscope (Gemini Leo 1550) image in

In conclusion, we have developed a new type of photocatalytic microreactor—TiO2-containing poly(allylamine)/ poly(styrene sulfonate) polyelectrolyte microcapsules—for photoinduced reduction of metal salts from aqueous solutions. The TiO2 nanoparticles adsorbed in the capsule walls act as microheterogeneous photocatalysts, while the polyelectrolyte layers are good irreversible electron donors to fill the photogenerated holes in the TiO2. Further improvements in the photocatalytic activity of the microreactors was achieved by impregnating a more effective electron donor [poly(vinyl alcohol)] in internal space and walls of the capsules. Reduced metal nanoparticles precipitate inside the capsule shell, leading to the formation of hollow metal microspheres. Significant amounts of silver metal aggregates were observed in solution during AgNO3 photoreduction. The use of polyelectrolyte capsules as microphotoreactors enables the dispersed nanosized photocatalyst to be separated from the treated solution without the need for filtration or centrifugation, and offers the possibility of controlling the composition of the inner and outer parts of the microphotoreactor in order to achieve the highest quantum yield.

Acknowledgements This work is supported by NSF-NIRT, USA, “Bioengineered nanocapsules” grant no. 0210298 and the Sofja Kovalevskaja Program of the Alexander von Humboldt Foundation, Germany. D. V. S. acknowledges support from the Basic Research Foundation of Belarus. Photochem. Photobiol. Sci., 2003, 2, 975–977

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