Magnetic photocatalysts of the core-shell type

Dmitry G. Shchukin,*a Anatoly I. Kulak b and Dmitry V. Sviridov b

a Max-Planck Institute of Colloids and Interfaces, Golm, 14476, Germany.E-mail: Dmitry.Shchukin@mpikg-golm.mpg.de

b Institute for Physico-Chemical Problems, Belarusian State University, 220050, Minsk, Belarus

Received 31st July 2002, Accepted 28th August 2002First published as an Advance Article on the web 6th September 2002

Two magnetic core-shell type photocatalytic systems arepresented: first, Zn0.35Ni0.65Fe2O4–SiO2–TiO2, showed highactivity in the photooxidation of oxalate, and the other,Fe3O4–Fe2O3–polyaniline: P2W18O62

6�, was tested in thereaction of SO2 photooxidation.

Semiconductor photocatalysts have attracted much attentionin the last decade because of their potential application in theremoval of toxic organic and inorganic species from aquaticenvironments.1,2 For photocatalytic detoxification, the aqueoussuspensions and particular films of semiconductor oxides(e.g., TiO2, etc.) are mostly employed due to the high oxidizingpower of photoholes produced in semiconductor particlesunder supra-bandgap illumination. A major drawback of suchphotocatalytic systems in the photodegradations is that thesuspensions of semiconductor oxides require additionalseparation steps to remove the semiconductor particles fromthe treated solution, whereas the immobilized photocatalystssuffer from relatively low “surface area-to-volume” ratios andinefficiencies introduced by light absorption in the contactingsolution.1 These problems can, however, be overcome by usingmagnetic photocatalysts comprising magnetic particles andphotoreactive oxides. In this communication we show that theeffective magnetically-controllable photocatalysts, which donot suffer from photodissolution,3 can be obtained throughthe chemical deposition of photosensitive, insulating, and, ifneeded, corrosion-protective coatings on the magnetic cores.

Spinel-like ferrite Zn0.35Ni0.65Fe2O4, chosen in this work as amaterial for magnetic cores, is stable against calcination in air atmore than 400 �C and can be obtained in dispersed form bysimple hydrolysis of corresponding salts [0.35 M ZnCl2 � 0.65M NiCl2 � 1 M Fe2(SO4)3] followed by heating of the residueat 450 �C. The resulting magnetic particles had an averagediameter of around 1.2 µm and a saturation magnetization of20.2 G cm3 g�1. However, the magnetic core can behave as asink for the charge carriers generated in the non-continuoustitania shell that should result in a drastic enhancement of therecombination losses or in the photodissolution of the core.3 Toprevent charge exchange between the magnetic support and thephotocatalyst deposited onto its surface, the core was encap-sulated in a uniform silica shell which separates the magneticferrite from the TiO2 catalyst and also diminishes the possibleinfluence of the magnetic core on the optoelectronic propertiesof TiO2. The silica shell also precludes undesirable doping oftitania during the annealing of the catalyst that could result ina decline of photoreactivity.4 The precipitation of a thin filmcoating of silica onto the magnetic cores involved thedeposition of an adhesive layer from an aqueous solution ofsilicic acid 5 followed by a further silica coating via the Stöbermethod.6 The estimated thickness of the formed SiO2 layer isapproximately 25 nm. The titania coating was then depositedby stirring the suspension of Zn0.35Ni0.65Fe2O4–SiO2 particles(8 g l�1) for 1.5 hours in an ethanolic solution containing water(∼ 4 volume percent) and titanium n-butoxide (10�2 M). Heat-ing to 450 �C transforms the obtained coating into the poly-crystalline TiO2 (anatase modification, as has been evidencedby wide angle X-ray scattering (WAXS) analysis).

The photocatalyst Zn0.35Ni0.65Fe2O4–SiO2–TiO2 shows highphotoreactivity towards the oxalic acid oxidation (Fig. 1) anddoes not exhibit noticeable degradation during the photolysis.The initial rate of disappearance of oxalate (0.72 mM min�1

under the experimental conditions used) is comparable withthat observed in the case of a suspension of the highly effectivecommercial TiO2 photocatalyst Degussa P25 (1.02 mM min�1).The magnetic photocatalyst remains active after completeoxidation of oxalate and can be reused (Fig. 1).

The X-ray photoelectron spectrum of the resulting magneticphotocatalyst Zn0.35Ni0.65Fe2O4–SiO2–TiO2 shows only photo-

Fig. 1 (a) Time dependence of photodegradation of oxalate in thesuspension (pH=6) of Degussa P25 (�); Zn0.35Ni0.65Fe2O4–SiO2–TiO2

(�); Zn0.35Ni0.65Fe2O4–SiO2–TiO2 reused after complete oxidation ofoxalate (�), and the schematic cross-section of the magneticphotocatalyst. Irradiation conditions: 120 W high-pressure Hg lamp(365 nm line), the light flux incident upon the quartz window of thephotoreactor was completely absorbed in the magnetically stirredsuspension of Zn0.35Ni0.65Fe2O4–SiO2–TiO2 photocatalyst (6 g l�1) orDegussa P25 (2 g l�1). The oxalate concentration was determinedelectrochemically.7 (b) X-Ray photoelectron spectrum for Zn0.35Ni0.65-Fe2O4–SiO2–TiO2.

742 Photochem. Photobiol. Sci., 2002, 1, 742–744 DOI: 10.1039/b207477j

This journal is © The Royal Society of Chemistry and Owner Societies 2002

Publ

ishe

d on

06

Sept

embe

r 20

02. D

ownl

oade

d by

Uni

vers

ity o

f Sy

dney

on

04/0

9/20

13 1

6:17

:37.

View Article Online / Journal Homepage / Table of Contents for this issue

electron and Auger peaks characteristic of TiO2 and SiO2

(Fig. 1), suggesting that the magnetic cores are encapsulated bythe silica and titania shells. Taking into account that the averagetitania loading, obtained by means of atomic emission analysis,corresponds to a titania shell 5–7 nm in thickness, the relativelyhigh intensity of the Si 2p peak can be attributed to the fact thatthe titania layer undergoes some aggregation upon annealing;similar coalescence effects were observed previously for crystal-lization of the titania coatings deposited onto the Stöber silicaspheres.8 Such photocatalysts, which comprise the photo-reactive semiconductor and sorbent (e.g. SiO2), allow thephotooxidation of many species which hardly adsorb on thetitania surface.9

The Fe2O3 photocatalyst, obtained through chemicaldeposition, exhibits high photocatalytic activity without anyadditional annealing 10,11 and can be, therefore, effectivelycombined with the magnetite cores prepared by coprecipitationof iron() and iron() salts (0.3 M FeSO4 � 0.33 M FeCl3) inthe presence of ammonia. The resulting Fe3O4 particles had anaverage diameter of 1 µm and a saturation magnetization of56.6 G cm3 g�1. Fe2O3 was deposited onto the Fe3O4 surface byadding 0.01 M FeCl3 solution under mechanical stirring to thesuspension of Fe3O4 (8 g l�1) heated to 80 �C. However, Fe2O3

suffers from photocorrosion resulting from oxide reduction bytrapped photoelectrons.11 To prevent the parasitic dissolutionof Fe2O3 during the course of SO2 photolysis, the polyaniline(PAni) shell capable of capturing the non-equilibrium chargecarriers was grown via photocatalytic oxidation of aniline at thesurface of Fe3O4–Fe2O3 particles suspended in an acetonitrilesolution containing 3.6 × 10�4 M aniline and 3.6 × 10�2 MNaClO4 or heteropolytungstate acids (HPT). The HPT loadingonto the particles of Fe3O4–Fe2O3–PAni:HPT was estimatedwith the use of atomic emission spectroscopy to be 2.8 × 10�4

mol g�1 of photocatalyst. The modification of the photo-catalyst with PAni, which is known to be an effective catalyst forSO2 oxidation in acid media,12 results in a substantial increaseof the SO2 oxidation rate (Fig. 2) accompanied by a pronouncedsuppression of Fe2O3 photocorrosion and enhanced long-termstability of the Fe3O4–Fe2O3–PAni photocatalytic system.Notwithstanding the fact that the charge carriers of both signsare injected into the polyaniline shell simultaneously, therecombination losses appear to be low because of rapid con-sumption of the photoproduced charge carriers in the inter-facial reactions together with the phase segregation resulting inthe formation of electrically-connected conductive domains(serving as channels for the unimpeded electron transport) inthe polyaniline shell.14

Further enhancement of the performance of the Fe3O4–Fe2O3–PAni catalyst calls for the improvement of catalyticactivity of the PAni shell towards oxygen reduction, which canbe attained by doping polyaniline with heteropolytungstateswhich are known to be effective catalysts for O2 reduction.15

It has been shown previously that heteropolytungstatePW12O40

3� of the Keggin structure (denoted briefly as PW12)reduces O2 to H2O, whereas the Dawson-type heteropolytung-state P2W18O62

6� (PW18) provides the two-electron reduction ofmolecular oxygen yielding H2O2.

16 The latter possibility is ofspecific interest because the generated peroxide is in a positionto participate in the oxidation of SO2 that can result in thedoubling of the sulphate yield. The combination of the hetero-geneous catalytic oxidation with the homogeneous catalyticoxidation is responsible for the much higher rate of SO2 photo-conversion observed for the magnetic photocatalyst with thepolyaniline shell doped with P2W18 when compared to theFe3O4–Fe2O3–PAni:PW12 photocatalyst (Fig. 2) as evidenced bythe fact that for both photocatalysts the emission of Fe() isalmost the same (Fig. 2).

In conclusion, we have developed the procedures for theformation of dispersed magnetic photocatalysts of the core-shell type adapted to the reactions of SO2 photooxidation andorganic photodegradation. The magnetic properties impartedto the semiconductor photocatalysts permit the separation ofdispersed catalyst from the treated solution without invokingthe procedures of filtration or centrifugation and offer strong

possibilities for controlling slurry-based photocatalytic reactorsystems.

Acknowledgements

D. G. S. acknowledges the support from INTAS (Grant YSF00-161).

References1 M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann,

Environmental application of semiconductor photocatalysts, Chem.Rev., 1995, 95, 69–96.

2 Photocatalytic Purification and Treatment of Water and Air, eds.D. Ollis and H. Al-Ekabi, Elsevier, Amsterdam, 1993.

3 D. Beydoun, R. Amal, G.K.-C. Low and S. McEvoy, Novel photo-catalysts: titania-coated magnetite. Activity and photodissolution,J. Phys. Chem. B, 2000, 104, 4387–4396.

4 W. Choi, A. Termin and M. R. Hoffmann, Effects of metal-iondopants on the photocatalytic reactivity of quantum-sized TiO2

particles, J. Phys. Chem., 1994, 98, 13669–13679.5 A. P. Philipse, M. P. B. van Bruggen and C. Pathmamanoharan,

Magnetic silica dispersions – preparation and stability of surface-modified silica particles with a magnetic core, Langmuir, 1994, 10,92–99.

6 W. Stöber, A. Fink and E. Bohn, Controlled growth of monodis-persed silica spheres in micron-sized range, J. Colloid Interface Sci.,1968, 26, 62–68.

7 K. V. Thrivikraman, R. W. Keller, S. K. Wolfson, S. J. Yao and J .C.Morgenlander, Cyclic voltametric studies on the application of car-bon electrodes in the determination of oxalic acid, Bioelectrochem.Bioenerg., 1982, 9, 357–364.

8 A. Hanprasopwattana, S. Srinivasan, A. G. Sault and A. K. Datye,Titania coating on monodisperse silica spheres (characterizationusing 2-propanol dehydration and TEM), Langmuir, 1996, 12,3173–3179.

Fig. 2 Concentration vs. time profiles for the production of (a) SO42�

and (b) Fe2� during the course of SO2 photooxidation (5 × 10�2 M SO2,2 g l�1 of photocatalyst, pH=1.5) in the suspensions of the magneticphotocatalysts: Fe3O4–Fe2O3 (�); Fe3O4–Fe2O3–PAni (�); Fe3O4–Fe2O3–PAni: PW12 (�); Fe3O4–Fe2O3–PAni: PW18 (�). The figure alsoshows a schematic representation of the processes occurring in thesuspension during the course of SO2 photolysis. Irradiation conditions:1000 W tungsten halogen lamp accompanied by glass filters selectingthe wavelength range of 300–600 nm. The Fe2� concentration wasasserted using the procedure described by Tamura et al.;13 the SO4

2�

concentration was measured gravimetrically with Ba(NO3)2.

Photochem. Photobiol. Sci., 2002, 1, 742–744 743

Publ

ishe

d on

06

Sept

embe

r 20

02. D

ownl

oade

d by

Uni

vers

ity o

f Sy

dney

on

04/0

9/20

13 1

6:17

:37.

View Article Online

9 C. Anderson and A. J. Bard, Improved photocatalysts ofTiO2/SiO2 prepared by sol-gel synthesis, J. Phys. Chem., 1995, 99,9882–9885.

10 S. N. Frank and A. J. Bard, Heterogeneous photocatalytic oxidationof cyanide and sulfite in aqueous solutions at semiconductorpowders, J. Phys. Chem., 1977, 81, 1484–1488.

11 B. C. Faust, M. R. Hoffmann and D. W. Bahnemann, Photocatalyticoxidation of sulfur dioxide in aqueous suspensions of α-Fe2O3,J. Phys. Chem., 1989, 93, 6371–6381.

12 D.V. Sviridov and A.I. Kulak, Photoelectrochemical oxidation ofsulfur dioxide on a polyaniline-modified n-Si/ITO electrode, Sol.Energy Mater. Sol. Cells, 1995, 39, 49–53.

13 H. Tamura, K. Goto, T. Yotsuyanagi and T. Nagayama, Spectro-

photometric determination of iron() with 1,10-phenanthroline inpresence of large amounts of iron(), Talanta, 1974, 21, 314–318.

14 K. Aoki and M. Kawase, Introduction of a percolation-thresholdpotential at polyaniline-coated electrodes, J. Electroanal. Chem.,1994, 377, 125–129.

15 G. Bidan, E. M. Genies and M. Lapkowski, Polypyrrole andpoly(n-methyl)pyrrole films doped with Keggin-type heteropoly-anions – preparation and properties, J. Chem. Soc., Chem. Com-mun., 1988, 533–535.

16 D. G. Shchukin and D. V. Sviridov, Highly-efficient generationof H2O2 at composite polyaniline/heteropolyanion electrodes: effectof heteropolyanion structure on H2O2 yield, Electrochem. Commun.,2002, 4, 402–405.

744 Photochem. Photobiol. Sci., 2002, 1, 742–744

Publ

ishe

d on

06

Sept

embe

r 20

02. D

ownl

oade

d by

Uni

vers

ity o

f Sy

dney

on

04/0

9/20

13 1

6:17

:37.

View Article Online