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J. Phys. Chem. B 1999, 103, 4672-4677
Surface Modification of TiO2 Nanoparticles with Carotenoids. EPR Study Tatyana A. Konovalova† and Lowell D. Kispert* Department of Chemistry, Box 870336, UniVersity of Alabama, Tuscaloosa, Alabama 35487
Valery V. Konovalov‡ The Institute of Chemical Kinetics and Combustion SB RAS, 630090 NoVosibirsk, Russia ReceiVed: January 5, 1999; In Final Form: March 16, 1999
EPR measurements demonstrate efficient charge separation on carotenoid-modified titanium dioxide nanoparticles (7 nm). Strong complexation of carotenoids containing terminal carboxy groups (-CO2H) with the TiO2 surface leads to electron transfer from the adsorbed carotenoid molecule to the surface trapping site. For these systems, EPR signals of the carotenoid radical cations Car•+ and the electrons trapped on the TiO2 are observed before irradiation (77 K). Their UV-visible spectra show an absorption band with a maximum near 650 nm that is characteristic of the trapped electrons. Surface modification of the TiO2 by other carotenoids results in the formation of a complex with an optical absorption band near 545 nm. These systems form charge-separated pairs [Car•+‚‚‚TiO2(e-tr)surf. TiO2(e-tr)latt.] only upon 365-600 nm illumination at 77 K. Complexation of the TiO2 colloids with carotenoids enhances spatial charge separation, shifts the absorption threshold into the visible region, and thus greatly improves the reducing ability of the semiconductor. Photoreduction of acceptor molecules such as 2,5-dichloro-1,4-benzoquinone, nitrobenzene, and oxygen is demonstrated.
Introduction Among the semiconductors, titanium dioxide is the most suitable for many environmental applications. TiO2 is biologically and chemically inert, and its photocatalytic properties are favorable for oxidation of numerous hazardous chemicals,1 reduction of heavy metal ions,2 and photodestruction of bacteria and viruses in water.3 The redox processes on the TiO2 surface can be initiated by electrons and holes photogenerated upon band gap excitation.4,5 However, two principal drawbacks of most semiconductors can inhibit these processes. One is very fast electron-hole recombination on semiconductor materials (less than 10 ns for TiO2). In this case adsorption of molecules that coordinate strongly to the surface and can be hole or electron scavengers allows efficient charge separation. It has been shown by EPR spectroscopy at cryogenic temperatures that cysteine6 and R-mercapto carboxylic acid7 molecules, strongly bound to the TiO2 surface, can trap the holes generated and greatly facilitate reduction processes. Another important defect of wide band gap semiconductors such as TiO2 (Eg ) 3.2 eV) is that their photoactivity is limited to the visible light region. Therefore, photosensitization of TiO2 using adsorption of dye molecules with high extinction coefficients in the visible region is of great practical interest. The electron-transfer product in this process was first identified as a radical cation of the sensitizer (S) by nanosecond laser flash photolysis (Gra¨tzel et * To whom correspondence should be addressed. † On leave from the Institute of Catalysis SB RAS, 630090 Novosibirsk, Russia. ‡ Present address: Center for Materials for Information Technology, University of Alabama, POB 870209, Room 205 Bevill Build., Tuscaloosa, AL 35487.
al.)8 and then confirmed by time-resolved resonance Raman spectroscopy:9 hν
TiO2
S 98 1S* 98 S•+ + TiO2(e-)
(1)
Since surface complexation of TiO2 with molecules that serve as charge scavengers or sensitizers results in the formation of paramagnetic species, EPR spectroscopy can be very useful. Although many organic dyes have been employed extensively in the surface modification of TiO2,10-15 only a few naturally occurring substances have been used.16 In the present work we studied surface modification of TiO2 by carotenoids. Carotenoids are natural colored pigments occurring in all photosynthetic organisms that evolve O2. They assume a dual role as photoprotective agents17 and as a major component of the light-harvesting antenna.18 The function of carotenoids as accessory antenna pigments is due to the properties of their excited singlet states, which allow efficient light energy absorption and transfer.19 High extinction coefficients of carotenoids (>105 M-1 cm-1) in the spectral region 420-550 nm make them potentially attractive sensitizers. On the other hand, adsorption of long-chain carotenoid molecules having special functional groups can block surface trapping sites of photogenerated charges and/or act as charge scavengers. Recently, considerable interest has been shown in the use of nanometer-sized semiconductor particles. In addition to the unique catalytic behavior due to their large surface area, they can yield transparent solutions, allowing direct optical analysis.20,21 To understand the role of carotenoids in photoinduced processes at the carotenoid/TiO2 interface, we carried out EPR
10.1021/jp9900638 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/07/1999
Surface Modification of TiO2 Nanoparticles
J. Phys. Chem. B, Vol. 103, No. 22, 1999 4673
SCHEME 1
Figure 1. EPR spectra of 7′-apo-7′-(4-carboxyphenyl)-β-carotenemodified TiO2 in CH2Cl2 measured at 77 K before irradiation (microwave frequency ν ) 9.186 Ghz): (a) microwave power P ) 30 mW; (b) P ) 15 mW; (c) P ) 2 mW.
and UV-visible studies of TiO2 particles modified with several carotenoids, including two containing terminal -CO2H groups. Experimental Section Materials and Sample Preparation. Titanium oxide (anatase) nanoparticles (7 nm, ST-1) were obtained from the Ishihara Co. (Japan) and were used as received. Carotenoids 7′-apo-7′(4-carboxyphenyl)-β-carotene (1), 8′-apo-β-caroten-8′-oic acid (2), and 8′-apo-β-caroten-8′-aldoxime (3) were prepared from 8′-apo-β-caroten-8′-al (4) (Roche Vitamins and Fine Chemicals): 1 by base hydrolysis of the corresponding methyl ester, which was obtained by a Wittig reaction with the in situ generated ylide of triphenyl(4-methoxycarbonylbenzyl)phosphonium bromide; 2 by Ag2O oxidation; 3 by treatment with H2NOH.22 Canthaxanthin (5) was supplied by Fluka. Purity of the carotenoids was checked by 1H NMR (360 MHz, CDCl3) and TLC analyses. The carotenoids were stored in the dark at -14 °C in a desiccator containing activated CaSO4 and were allowed to warm to room temperature just before use. Methylene chloride and toluene (Aldrich, anhydrous), ethanol (100%), and acetonitrile (Fisher, HPLC grade) were used without further purification. Nitrobenzene (Fisher, ASC grade) and 2,5-dichloro1,4-benzoquinone (Aldrich, 98%) were employed as acceptor molecules. Nitrous oxide (N2O) (Matheson) served as electron scavenger. Appropriate volumes of 10-3 M solutions of carotenoid were added to a powder of TiO2 (≈50 mg). The mixture was stirred for several minutes, allowed to stand 2-5 h, and then subjected to centrifugation. The bright red composite was thoroughly washed with CH2Cl2 and dried in air at room temperature. The samples were prepared by adding 0.5 mL of solvent to 20-30 mg of modified TiO2 powder in a quartz EPR tube. The samples were then degassed by three freeze-pumpthaw cycles or oxygenated by passing a stream of O2 through the solution. The samples were then shaken to produce colloids and immediately cooled to 77 K. Apparatus and Measurements. The samples were irradiated for 2-3 min at 77 K by a Xe/Hg lamp (1000 W) with a Kratos monochromator (365-578 nm, 5-30 mW) or an excimer
Questek-2540 laser (308 nm, 40 Hz, 10 mJ/pulse). UV-visible absorption spectra were recorded with a Shimadzu UV-1610 spectrophotometer. EPR measurements were carried out with a Bruker ESP 300-10/7 spectrometer at 77 K (microwave frequency 9.14 GHz) in a quartz Dewar mounted in the EPR cavity. Results and Discussion Several carotenoids with different functional groups, 7′-apo7′-(4-carboxyphenyl)-β-carotene (1), 8′-apo-β-caroten-8′-oic acid (2), 8′-apo-β-caroten-8′-aldoxime (3), 8′-apo-β-caroten-8′-al (4), and canthaxanthin (5) (Scheme 1) were selected for modification of TiO2 nanoparticles. Infrared measurements6a,7,23 demonstrated that carboxylate groups can replace OH groups at the TiO2 surface and chemically bind to surface Ti atoms. Thus, carotenoids containing terminal -CO2H groups were expected to interact strongly with the semiconductor surface. In our study frozen (77 K) colloid solutions (solvents: CH2Cl2, ethanol, toluene, acetonitrile) of TiO2 nanoparticles modified by -CO2H containing carotenoids (1, 2) give EPR signals in the absence of irradiation. These EPR spectra (Figure 1) exhibit a signal with g ) 2.0028 ( 0.0002, ∆Hpp ) 13.0 ( 0.5 G, which is characteristic of carotenoid radical cations (Car•+)24-26 (Figure 1, I) and a line with g < 2 (Figure 1, II). The intensity of the signal II increases with enhancement of microwave power (Figure 1a-c). The UV-visible spectra of transparent suspensions of TiO2 modified by 1 or 2 in CH2Cl2 and ethanol (concentration of TiO2 nanoparticles about 10-5 M) show a broad absorption band with a maximum near 650 nm (Figure 2, solid line). It is known that bare TiO2 has a maximum absorption in the region
