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Photocatalytic degradation of organic dyes by mesoporous nanocrystalline anatase
Materials Chemistry and Physics 125 (2011) 474–478
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Photocatalytic degradation of organic dyes by mesoporous nanocrystalline anatase Maria L. Carreon a , Hector G. Carreon b , Jaime Espino-Valencia a , Moises A. Carreon c,∗ a b c
Facultad de Ingeniería Química UMSNH Edif. “M” C.U. Morelia, Michoacán, México 58000-888, Mexico Instituto de Investigaciones Metalúrgicas UMSNH Edif. “U” C.U. Morelia, Michoacán, México 58000-888, Mexico Department of Chemical Engineering, University of Louisville, Louisville, KY, 40292, USA
a r t i c l e
i n f o
Article history: Received 9 June 2010 Received in revised form 3 September 2010 Accepted 16 October 2010 Keywords: Mesoporous anatase Photocatalysis Organic dyes Self-assembly
a b s t r a c t Mesoporous nanocrystalline anatase was prepared via EISA employing P123 and CTAB as structure directing agents. The resultant mesoporous crystalline phases exhibited specific surface areas as high as ∼150 m2 g−1 , average unimodal pore sizes of ∼3 nm and ∼6 nm, and average crystallite size of ∼10 nm; and were used as photocatalysts for the UV degradation of methylene blue, methyl orange, methyl red and rhodamine 6G. The mesoporous anatase phases photodegraded MB, MO and MR ∼2–3 times faster than conventional nanocrystalline anatase and showed limited photocatalytic activity for rhodamine 6G.
1. Introduction Colored waste waters from industrial effluents containing organic dyes are of great environmental and aesthetic concern [1]. Because of the potential toxicity of dyes and their visibility in surface waters, its removal or degradation has been a matter of considerable interest [2]. TiO2 , in anatase crystalline form, has been recognized as the ideal photocatalyst for the destruction of common organic pollutants in textile industry because of its biological and chemical inertness, stability towards photochemical and chemical corrosion, an electronic band gap that upon photoexcitation creates highly oxidizing holes, and highly reducing electrons [3]. During the last decade, the modified surfactant assisted self assembly approach known as Evaporation Induced self-assembly (EISA) has emerged as a powerful synthesis method to design technologically relevant and functional oxides in the fiber, particle and film form at the nanoscale [4–6]. The method relies in using very dilute surfactant initial concentration from which a liquid crystalline mesophase is gradually developed upon solvent evaporation. The slow co-assembly between the inorganic network and the liquid crystalline phase leads to the formation of long- range order well defined mesostructures. Recently, Carreon and Guliants have reviewed the fundamentals of mesostructuring metal oxides by the co-assembly between inorganic entities and
∗ Corresponding author. E-mail address: [email protected] (M.A. Carreon). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.10.030
© 2010 Elsevier B.V. All rights reserved.
liquid crystalline phases upon evaporation (EISA) [7]. The preparation of mesoporous titania (amorphous and crystalline) particles by EISA has been studied by independent research groups [8–10]. In all these reports superior photocatalytic efficiency has been observed for the mesoporous nanocrystalline titania prepared via self- assembly and EISA approaches as compared to non- periodic non- porous nanocrystalline titania. In general, the superior photocatalytic activity of the mesoporous titania has been related to the periodic open ordered porous architectures with organized framework of nanocrystals which provide easy diffusion pathways and readily accessible pore-wall system for the guest molecules. Herein, we report the synthesis of mesoporous nanocrystalline anatase by induced self-assembly approach (EISA). This method led to the formation of mesoporous anatase with small nanocrystallite size, relatively high surface area and uniform pore size distribution. The resultant mesophases were used as photocatalysts for the UV photodegradation of methylene blue, methyl orange, methyl red and rhodamine 6G. 2. Experimental 2.1. Materials and synthesis Titanium (IV) ethoxide (C8 H20 O4 Ti, Aldrich) was used as the inorganic precursor. Pluronic P-123, denoted as EO20 PO70 EO20 (BASF) and Hexadecyltrimethylammonium bromide, denoted as CTAB (C19 H42 NBr, 99% Sigma) were used as the structure-directing agents (SDA). 1-Butanol (99% Sigma–Aldrich) was used as organic solvent. Hydrochloric acid (37%, Sigma–Aldrich) was used to prehydrolize the inorganic precursor. Methylene blue (C16 H18 ClN3 S, Sigma–Aldrich), methyl orange (C14 H14 N3 NaO3 S, Sigma–Aldrich), methyl red (C15 H15 N3 O2 , Sigma–Aldrich) and rhodamine 6G (C28 H31 N20 ·3Cl, Sigma) were used as probe molecules for the
M.L. Carreon et al. / Materials Chemistry and Physics 125 (2011) 474–478 Table 1 BET Specific surface area, average pore size, and average crystallite size of mesoporous titania samples and conventional nanocrystalline anatase. Specific surface area (m2 g−1 )
Average pore size (nm)
Average crystallite size (nm)
P123 CTAB CNCA
101 151 123
6 3 6–18 (bimodal)
10 10 25
photocatalytic studies. Titanium (IV) oxide nanopowder (99.7% anatase, Aldrich), denoted at CNCA was used as a reference material. The mesoporous phases were prepared by reacting an acidic solution of the inorganic precursor with an alcoholic solution of the SDA. In a typical synthesis, HCl was added to C8 H20 O4 Ti at room temperature under vigorous stirring. Separately, the SDA was dissolved in 1-butanol, and then added to the acidic inorganic precursor solution. The resultant solution was stirred at room temperature for 3 h and then transferred into a petri dish and aged at 20 ◦ C and 80% relative humidity (RH) for 2 d in a controlled humidity chamber (Laboratory Humidity Chamber LH-1.5, Associated Environmental Systems, Ayer, MA, USA). The as-synthesized powder was then calcined in air at 300 ◦ C for 1 h followed by 400 ◦ C for 4 h with controlled heating and cooling rates (1 ◦ C min−1 ) to remove the template. The gel molar compositions for the samples prepared with P123 and CTAB as SDA were: 1 Ti(OEt)4 :2 HCl:0.013 P123:9 1-butanol and 1 Ti(OEt)4 :1.4 HCl:0.12 CTAB:12.5 1-butanol respectively. 2.2. Characterization XRD patterns were collected on a Bruker D8 Discover diffractometer. BET surface areas and N2 adsorption–desorption isotherms were obtained in a Micromeritics Tristar-3000 porosimeter. Before the measurements, the samples were degassed at 300 ◦ C for 3 h. The morphology and particle size of the mesophases were inspected with a FE-SEM (FEI Nova 600) with an acceleration voltage of 6 kV. TEM images were taken on Technai F20 FEI TEM using a field emission gun, operating with an accelerating voltage of 200 kV. TGA analysis was performed in a TA Instruments Thermogravimetric Analyzer Model 2950. 2.3. Photocatalytic degradation studies In a typical experiment, in a 20 ml pyrex beaker, 0.07 g of the catalyst (mesoporous or CNCA) was dispersed in 15 ml of 1 × 10−5 M dye solution (methylene blue, methyl orange, methyl red or rhodamine 6G) solution. This suspension was stirred initially for 30 min in the dark to allow the equilibrium adsorption of the dye. Then, it was exposed to ultraviolet irradiation with continuous stirring under ambient conditions with an intensity of 1350 W cm−2 produced by an ENTELA long wave UV lamp (365 nm). Aliquots of ∼8 ml were taken for the UV–vis measurements because this volume was enough to fill the quartz cuvette sample holder. For the photocatalytic degradation studies a new solution was irradiated each time. The absorption spectra was recorded every 30 min using a Perkin Elmer UV/VIS/NIR spectrophotometer.
3. Results and discussion The textural properties of the mesoporous titania phases synthesized with P123 and CTAB as SDA are summarized in Table 1. For comparison, the BET surface area, average pore size and average crystallite size of CNCA are also shown. The nature of the structure directing agent helped to fine tune the pore size of the resulting mesophase. For instance, CTAB led to the formation of mesophases
(101)
(004)
(200)
(105)
Intensity (a.u.)
Sample ID
475
c
b
a
20
30
40
50
60
2θ(degrees) Fig. 1. XRD patterns of (a) mesoporous anatase synthesized with P123 as SDA, (b) mesoporous anatase synthesized with CTAB as SDA, (c) CNCA.
with average pore size of ∼3 nm, while P123 led to ∼6 nm pore sizes. Due to its larger micellar size, the amphiphilic block copolymer P123 led to larger average pore size. The average crystallite size for all samples was estimated using Scherrer equation and the FWHM of anatase (1 0 1) reflection, and corresponded to ∼10 nm for the mesoporous phases, and ∼25 nm for the CNCA reference sample. As shown in Table 1, the mesoporous samples displayed unimodal pore sizes, while the CNCA sample showed bimodal pore size distribution. Surface areas were comparable for all three samples. The XRD patterns of all the mesophases exhibited only the characteristic reflections of anatase at 2 ∼25◦ , 38◦ , 48◦ and 54◦ corresponding to the (1 0 1), (0 0 4), (2 0 0) and (1 0 5) planes respectively, of tetragonal titania [11] as shown in Fig. 1. Fig. 2 shows the nitrogen adsorption–desorption isotherms and pore size distribution of mesoporous anatase phases synthesized employing P123 and CTAB as SDA. Type IV isotherms with H2 hysteresis loop confirmed the formation of the mesostructure [12]. The sample prepared with P123 as SDA showed a specific surface area of ∼100 m2 g−1 and an average unimodal pore diameter of ∼6 nm. The sample prepared with CTAB as surfactant showed the highest specific surface area ∼150 m2 g−1 with unimodal pore diameter of ∼3 nm. The reference sample, CNCA displayed a typical isotherm for macroporous solids and non-unimodal pore size distribution [13]. High-resolution transmission electron microscopy (HRTEM) images of the mesoporous anatase samples synthesized with P123 and CTAB as SDA are shown in Fig. 3. The sample synthesized with P123 as SDA shows a well defined ordered channel pore archi-
Fig. 2. N2 adsorption–desorption isotherms and pore size distribution of the mesoporous anatase phases synthesized with (a) P123 (b) CTAB as SDA.
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Fig. 3. (a) and (b) HRTEM image of mesoporous nanocrystalline anatase synthesized with P123 as SDA at different magnifications. (c) HRTEM image of mesoporous nanocrystalline titania synthesized with CTAB as SDA. (d) A magnified image of the lattice fringes of (c) coincide with the anatase form of TiO2 viewed along (1 1 1) zone axis.
tecture with ∼10 nm anatase crystals (Fig. 3a) and unimodal pore size of ∼6 nm (Fig. 3b). The sample synthesized with CTAB as SDA shows crystals of ∼9–11 nm (Fig. 3c). For both samples the crystallite size agrees reasonably well with that calculated from the FWHM of anatase (1 0 1) reflection. Two sets of well-resolved lattice fringes are visible from a magnified image of Fig. 3c. The spacing of 0.35 nm, measured for these two sets of fringes (Fig. 3d), coincides with 0.352 nm, i.e., with the d-spacing of (0 1 1) type planes in anatase form of titania, and confirmed by XRD data (Fig. 1). Fig. 4 shows the SEM images of the mesoporous anatase samples synthesized with P123 and CTAB as SDA. The mesophase synthesized using P123 as SDA exhibited uniform ∼0.1 m clusters while the mesophase synthesized with CTAB as SDA resulted in the formation of larger clusters of ∼0.5–1.0 m. The morphology for the reference sample corresponded to non-uniform ∼ 1 m agglomerates [13]. Regarding the degree of dye adsorption, we found that methylene blue is the only dye that strongly adsorbs on titania [13]. Carreon et al. [10] reported a detailed study on the adsorption behavior of methylene blue on mesoporous nanocrystalline titania. The strong adsorption of methylene blue is related to the sulfurcentered cation of this dye, which interacts with the negatively charged titania surface (solution pH ∼7). This is supported by the zeta potential measurements which show that titania surface is negatively charged above the isoelectric point ∼6.2–6.5 [13]. The values of IEP are in good agreement with earlier reports [14,15]. In
the case of methyl red and rhodamine, both dyes contain carboxylic groups, which interact much weaker with the titania surface. The initial pH of the methyl red and rhodamine solutions was 6.5 and 6.4 respectively which is in the vicinity of the isolectric point of titania, therefore the protonated carboxylic groups may weakly interact with the “overall neutral charged surface of titania” leading to weak adsorption [16]. Finally, in the case of methyl orange, also a negligible adsorption was observed. In this case, the absence of polar or charged groups in the structure of this dye may be responsible for its weak adsorption in the surface of titania. The calculated fractions of dye adsorbed for MB, MO, MR, and R6G for all the samples were in the ∼14–19%, ∼2–4%, ∼1–2% and ∼1–4% range respectively [13], supporting the fact that only MB is adsorbed strongly on the surface of titania. The mesoporous anatase phases were evaluated in the photodegradation of methylene blue (MB), methyl orange (MO), methyl red (MR) and rhodamine 6G (RH). The photoactivity of these samples was compared to the reference sample. CNAC was choosen as a reference sample because it has comparable surface area than that of the mesoporous samples and lies in the crystallite size regime (tens of nanometers) of the mesoporous samples. This will allow us to have a more direct and reliable comparison between the nanocrystalline anatase mesophases vs commercially available anatase nanocrystals. Fig. 5 shows the photocatalytic degradation profiles of MB, MO, MR and RH over the mesoporous titania samples and CNCA. ‘C0 ’ is the initial dye concentration and ‘C’ is
M.L. Carreon et al. / Materials Chemistry and Physics 125 (2011) 474–478
477
Fig. 4. SEM images of the synthesized mesoporous anatase phases synthesized using (a) P123 and (b) CTAB as SDA.
the adsorbed dye concentration at different time intervals. The used molar extinction coefficients for MB [17], for MO [18], for MR [19] and for RH [20] were 55,000 M−1 cm−1 , 25,000 M−1 cm−1 , 19,000 M−1 cm−1 and 50,000 M−1 cm−1 respectively. The degradation constant for MB, MO, MR and rhodamine 6G, k, followed a first-order decay kinetics [13]. In all cases (except for the sample P123 with MO and MR) the linear correlation factor R2 values were closer to 1 when first order was assumed. Nevertheless, even for these two cases the R2 values for first and zero order kinetics were very close. Therefore, we can assume first order kinetics. The mesoporous samples degraded ∼85–95% of MB after 150 min whereas CNCA degraded only ∼60% of MB after the same time of exposure to UV irradiation (Fig. 5a). The mesoporous samples degraded ∼50% of MO, while the CNCA degraded nearly ∼34% after the same UV exposure time (Fig. 5b). The mesoporous anatase phase synthesized with P123 degraded ∼82% of the dye; the meso-
porous sample synthesized with CTAB degraded ∼65% of the dye, while the CNCA degraded only ∼52% of MR at the same UV exposure time (Fig. 5c). The CNCA and P123 samples degraded nearly ∼22% of MR after 150 min of exposure to UV irradiation. The photodegradation of RH over the mesophase synthesized with CTAB was negligible (Fig. 5d). The photocatalytic activity data for MB, MO, MR and RH, including photodegradation rate constants and percent of dye degradation is summarized in Table 2. Since all samples (mesoporous and CNCA) have comparable surface areas and have the same crystalline structure, the enhanced photocatalytic activity of the mesoporous nanocrystalline anatase samples when compared to conventional nanocrystalline anatase for the photodegradation of MB, MO and MR may be related to the smaller crystallite size, and relatively ordered pore structure of the mesophases. It is well known that as the crystal size decreases, the surface density of active sites available for substrate adsorption
Fig. 5. Photocatalytic degradation profiles of (a) methylene blue, (b) methyl orange, (c) methyl red and (d) rhodamine 6G adsorbed on mesoporous anatase and conventional crystalline anatase.
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Table 2 Rate constants k and percents of dye degradation of mesoporous nanocrystalline anatase and CNCA samples. Sample ID
MB ×10−2
MO ×10−2
MR ×10−2
Rhodamine 6G ×10−2
P123 CTAB CNCA
2.0 (95%) 1.3 (85%) 0.6 (60%)
0.4 (51%) 0.4 (49%) 0.2 (34%)
1.1 (82%) 0.7 (65%) 0.5 (52%)
0.1 (21%) 0.03 (4%) 0.1 (22%)
increases, thus increasing the overall photocatalytic rate [21,22]. Moreover, the ordered pore architecture of the mesoporous samples as compared to the randomly organized pore network of CNCA may result in high diffusion rates of the guest molecules, and therefore to improved photocatalytic rates. The benefits of having an ordered mesopore structure for photocatalytic applications have been demonstrated by independent research groups. [10,17,23–25]. When comparing the photocatalytic behavior of MB, MO and MR over the mesophases (Table 2) is evident that the mesophase prepared with P123 as SDA shows better performance. These results suggest that the larger pore size of the mesophase synthesized with P123 as compared to that synthesized with CTAB may enhance the mass transfer of the molecules, leading to improved photo-oxidation rates. Furthermore, TGA analysis [13] revealed that more carbon was retained in the pores of the sample synthesized with CTAB (∼1.5%) as compared to the sample synthesized with P123 (0.9%). The excess of pyrolized carbon in the sample synthesized with CTAB may have led to pore blockage, and therefore to a decrease in its overall photocatalytic performance. In the case of RH, the photocatalytic behavior was different and limited as compared to the other dyes (MB, MO, and MR). Only the mesophase synthesized with P123 as SDA and CNCA showed photocatalytic activity. The mesophase synthesized with CTAB as SDA exhibited negligible activity. This can be partially explained by the difference in the pore diameter. The pores of the former samples are larger than those of the sample synthesized with CTAB. Therefore, the large rhodamine molecules can reach more efficiently the surface of the CNCA and P123 samples, as compared to the sample synthesized with CTAB as SDA where the accessibility is more limited by the smaller pores. It is not unusual to observe a different photocatalytic behavior of TiO2 depending on the chemical nature of the dye [26,27] since the adsorption-desorption mechanism of the dyes depends on its spatial geometry and specific functional groups. For both samples, the mesophase synthesized with P123 as SDA, and CNCA the photocatalytic activity was similar (Table 2) suggesting that the diffusion towards the active sites and the accessibility of the active sites for adsorption due to the presence of large pores are key parameters for the photo degradation of RH. For this particular dye, the crystal size and the pore order seems not to play a critical role in the photocatalytic behavior. From the molecular dimensions of the dyes, it can be inferred that rhodamine 6G may have more difficulties to diffuse through smaller pores, and therefore its photocatalytic degradation on anatase will be limited. MB has a rectangular parallelepiped shape with 1.6 × 0.7 × 0.37 nm3 dimensions [28]. The length of the long axis of methyl orange is ∼1.3 nm [29], similar to that of methyl red. Rhodamine 6G is a larger molecule with length of the long axis ∼1.4 nm [30]. In principle, diffusion of all 4 dyes through the anatase porous structures is possible, however, it has been demonstrated that a single rhodamine 6G molecule can occupy an area close to 4 nm2 [31] which makes more difficult its diffusion through small pores. 4. Conclusions Mesoporous nanocrystalline anatase was synthesized via EISA employing P123 and CTAB as structure directing agents. The resul-
tant mesoporous samples exhibited unimodal pore size of ∼3 nm and ∼6 nm, respectively and average crystallite size of ∼10 nm. The mesoporous anatase phases photodegraded MB, MO and MR ∼2–3 times faster than conventional nanocrystalline anatase and showed limited photocatalytic activity for rhodamine 6G. The enhanced photocatalytic activity of the mesoporous titania samples for the degradation of MB, MO and MR when compared to conventional nanocrystalline titania was related to smaller crystallite size, and ordered pore structure. The poor photoactivity behavior for RH was associated with the limited diffusion towards the active sites and the accessibility of the active sites for adsorption and the limited accessibility to the pores of the large rhodamine 6G molecules. Acknowledgments This work was partially supported by CONACYT-MEXICO under the project 80883. We thank Dr. Surendar R. Venna for his help with SEM and TEM experiments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2010.10.030. References [1] B. Gözmen, M. Turabik, A. Hesenov, J. Hazard. Mater. 164 (2009) 1487–1495. [2] F. Han, V. Kambala, M. Srinivasan, D. Rajarathnam, R. Naidu, Appl. Catal. A: Gen. 359 (2009) 25–40. [3] M.A. Fox, M.T. Dylay, Chem. Rev. 93 (1993) 341–357. [4] H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afara, G.A. Ozin, Nature 379 (1996) 703–705. [5] H. Yang, N. Coombs, I. Sokolov, G.A. Ozin, Nature 381 (1996) 589–592. [6] Y. Lu, R. Ganguli, C.A. Drewin, M.T. Anderson, C.J. Brinker, W. Gong, Y. Guo, H. Soyez, B. Dunn, M.H. Huang, J.I. Zink, Nature 389 (1997) 364–368. [7] M.A. Carreon, V.V. Guliants, Mesostructuring Metal Oxides through Evaporation Induced Self-Assembly: Fundamentals and Applications, Vol. 16, Nanoporous Solids, Recent Advances and Prospects, Elsevier, 2008, pp. 407–432. [8] G.J. de, A.A. Soler-Illia, A. Louis, C. Sanchez, Chem. Mater. 14 (2002) 750– 759. [9] K. De Witte, A.M. Busuioc, V. Meynen, M. Mertens, N. Bilba, G. Van Tendeloo, P. Cool, E.F. Vansant, Microporous Mesoporous Mater. 110 (2008) 100–110. [10] M.A. Carreon, S.Y. Choi, M. Mamak, N. Chopra, G.A. Ozin, J. Mater. Chem. 17 (2007) 82–89. [11] E.L. Crepaldi, G.J. de A.A. Soller-Illia, D. Grosso, F. Cagnol, F. Ribot, C. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770–9786. [12] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierrot, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619. [13] See Supplementary information. [14] M. Kosmulski, Adv. Colloid Interface Sci. 99 (2002) 255–264. [15] J. Gustafsson, P. Mikkola, M. Jokinen, J.B. Rosenholm, Colloids Surf. A: Physicochem. Eng. Aspects 175 (3) (2000) 349–359. [16] J. Tschirch, D. Bahnemann, M. Wark, J. Rathousky, J. Photochem. Photobiol. A: Chem. 194 (2008) 181–188. [17] A. Katti, S.R. Venna, M.A. Carreon, Catal. Commun. 10 (2009) 2036–2040. [18] J.-Z. Liu, T.-L. Wang, L.-N. Ji, J. Mol. Catal. B: Enzym. 41 (2006) 81–86. [19] J.P. Jadhav, G.K. Parshetti, S.D. Kalme, S.P. Govindwar, Chemosphere 68 (2007) 394–400. [20] K. Garai, R. Sureka, S. Maiti, Biophys. J. 92 (7) (2007) L55–L57. [21] J.W. Moon, C.Y. Yun, K.W. Chung, Catal. Today 87 (2003) 77–86. [22] E.P. Reddy, B. Sun, P.G. Smiriniotis, J. Phys. Chem. B 108 (2004) 17198– 17205. [23] Y. Sakatani, D. Grosso, L. Nicole, C. Boissiere, G.J. de A.A. Soller-Illia, C. Sanchez, J. Mater. Chem. 16 (2006) 77–82. [24] K. De Witte, S. Ribbens, V. Meynen, I. De Witte, L. Ruys, P. Cool, E.F. Vansant, Catal. Commun. 9 (2008) 1787–1792. [25] E. Beyers, P. Cool, E.F. Vansant, Microporous Mesoporous Mater. 99 (2007) 112–117. [26] R. Comporelli, E. Fannizza, M.L. Curri, P.D. Cozzoli, G. Mazcolo, R. Passino, A Agastiano, Appl. Catal., B: Environ. 55 (2005) 81–91. [27] A. Mills, P. Sawunyama, J. Photochem. Photobiol., A: Chem. 84 (1994) 305– 309. [28] R. Hoppe, G. Schultz-Ekloff, J. Rathousky, J. Starek, A. Sukal, Zeolites 14 (1994) 126–129. [29] M. Takahashi, K. Kobayashi, K. Takaoka, K. Tajima, Thin Solid Films 307 (1997) 274–279. [30] R. Sasai, T. Fujita, N. Iyi, H. Itoh, K. Takagi, Langmuir 18 (2002) 6578–6583. [31] A. Kudelski, Chem. Phys. Lett. 414 (2005) 271–275.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Photocatalytic degradation of organic dyes by mesoporous nanocrystalline anatase Maria L. Carreon a , Hector G. Carreon b , Jaime Espino-Valencia a , Moises A. Carreon c,∗ a b c
Facultad de Ingeniería Química UMSNH Edif. “M” C.U. Morelia, Michoacán, México 58000-888, Mexico Instituto de Investigaciones Metalúrgicas UMSNH Edif. “U” C.U. Morelia, Michoacán, México 58000-888, Mexico Department of Chemical Engineering, University of Louisville, Louisville, KY, 40292, USA
a r t i c l e
i n f o
Article history: Received 9 June 2010 Received in revised form 3 September 2010 Accepted 16 October 2010 Keywords: Mesoporous anatase Photocatalysis Organic dyes Self-assembly
a b s t r a c t Mesoporous nanocrystalline anatase was prepared via EISA employing P123 and CTAB as structure directing agents. The resultant mesoporous crystalline phases exhibited specific surface areas as high as ∼150 m2 g−1 , average unimodal pore sizes of ∼3 nm and ∼6 nm, and average crystallite size of ∼10 nm; and were used as photocatalysts for the UV degradation of methylene blue, methyl orange, methyl red and rhodamine 6G. The mesoporous anatase phases photodegraded MB, MO and MR ∼2–3 times faster than conventional nanocrystalline anatase and showed limited photocatalytic activity for rhodamine 6G.
1. Introduction Colored waste waters from industrial effluents containing organic dyes are of great environmental and aesthetic concern [1]. Because of the potential toxicity of dyes and their visibility in surface waters, its removal or degradation has been a matter of considerable interest [2]. TiO2 , in anatase crystalline form, has been recognized as the ideal photocatalyst for the destruction of common organic pollutants in textile industry because of its biological and chemical inertness, stability towards photochemical and chemical corrosion, an electronic band gap that upon photoexcitation creates highly oxidizing holes, and highly reducing electrons [3]. During the last decade, the modified surfactant assisted self assembly approach known as Evaporation Induced self-assembly (EISA) has emerged as a powerful synthesis method to design technologically relevant and functional oxides in the fiber, particle and film form at the nanoscale [4–6]. The method relies in using very dilute surfactant initial concentration from which a liquid crystalline mesophase is gradually developed upon solvent evaporation. The slow co-assembly between the inorganic network and the liquid crystalline phase leads to the formation of long- range order well defined mesostructures. Recently, Carreon and Guliants have reviewed the fundamentals of mesostructuring metal oxides by the co-assembly between inorganic entities and
∗ Corresponding author. E-mail address: [email protected] (M.A. Carreon). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.10.030
© 2010 Elsevier B.V. All rights reserved.
liquid crystalline phases upon evaporation (EISA) [7]. The preparation of mesoporous titania (amorphous and crystalline) particles by EISA has been studied by independent research groups [8–10]. In all these reports superior photocatalytic efficiency has been observed for the mesoporous nanocrystalline titania prepared via self- assembly and EISA approaches as compared to non- periodic non- porous nanocrystalline titania. In general, the superior photocatalytic activity of the mesoporous titania has been related to the periodic open ordered porous architectures with organized framework of nanocrystals which provide easy diffusion pathways and readily accessible pore-wall system for the guest molecules. Herein, we report the synthesis of mesoporous nanocrystalline anatase by induced self-assembly approach (EISA). This method led to the formation of mesoporous anatase with small nanocrystallite size, relatively high surface area and uniform pore size distribution. The resultant mesophases were used as photocatalysts for the UV photodegradation of methylene blue, methyl orange, methyl red and rhodamine 6G. 2. Experimental 2.1. Materials and synthesis Titanium (IV) ethoxide (C8 H20 O4 Ti, Aldrich) was used as the inorganic precursor. Pluronic P-123, denoted as EO20 PO70 EO20 (BASF) and Hexadecyltrimethylammonium bromide, denoted as CTAB (C19 H42 NBr, 99% Sigma) were used as the structure-directing agents (SDA). 1-Butanol (99% Sigma–Aldrich) was used as organic solvent. Hydrochloric acid (37%, Sigma–Aldrich) was used to prehydrolize the inorganic precursor. Methylene blue (C16 H18 ClN3 S, Sigma–Aldrich), methyl orange (C14 H14 N3 NaO3 S, Sigma–Aldrich), methyl red (C15 H15 N3 O2 , Sigma–Aldrich) and rhodamine 6G (C28 H31 N20 ·3Cl, Sigma) were used as probe molecules for the
M.L. Carreon et al. / Materials Chemistry and Physics 125 (2011) 474–478 Table 1 BET Specific surface area, average pore size, and average crystallite size of mesoporous titania samples and conventional nanocrystalline anatase. Specific surface area (m2 g−1 )
Average pore size (nm)
Average crystallite size (nm)
P123 CTAB CNCA
101 151 123
6 3 6–18 (bimodal)
10 10 25
photocatalytic studies. Titanium (IV) oxide nanopowder (99.7% anatase, Aldrich), denoted at CNCA was used as a reference material. The mesoporous phases were prepared by reacting an acidic solution of the inorganic precursor with an alcoholic solution of the SDA. In a typical synthesis, HCl was added to C8 H20 O4 Ti at room temperature under vigorous stirring. Separately, the SDA was dissolved in 1-butanol, and then added to the acidic inorganic precursor solution. The resultant solution was stirred at room temperature for 3 h and then transferred into a petri dish and aged at 20 ◦ C and 80% relative humidity (RH) for 2 d in a controlled humidity chamber (Laboratory Humidity Chamber LH-1.5, Associated Environmental Systems, Ayer, MA, USA). The as-synthesized powder was then calcined in air at 300 ◦ C for 1 h followed by 400 ◦ C for 4 h with controlled heating and cooling rates (1 ◦ C min−1 ) to remove the template. The gel molar compositions for the samples prepared with P123 and CTAB as SDA were: 1 Ti(OEt)4 :2 HCl:0.013 P123:9 1-butanol and 1 Ti(OEt)4 :1.4 HCl:0.12 CTAB:12.5 1-butanol respectively. 2.2. Characterization XRD patterns were collected on a Bruker D8 Discover diffractometer. BET surface areas and N2 adsorption–desorption isotherms were obtained in a Micromeritics Tristar-3000 porosimeter. Before the measurements, the samples were degassed at 300 ◦ C for 3 h. The morphology and particle size of the mesophases were inspected with a FE-SEM (FEI Nova 600) with an acceleration voltage of 6 kV. TEM images were taken on Technai F20 FEI TEM using a field emission gun, operating with an accelerating voltage of 200 kV. TGA analysis was performed in a TA Instruments Thermogravimetric Analyzer Model 2950. 2.3. Photocatalytic degradation studies In a typical experiment, in a 20 ml pyrex beaker, 0.07 g of the catalyst (mesoporous or CNCA) was dispersed in 15 ml of 1 × 10−5 M dye solution (methylene blue, methyl orange, methyl red or rhodamine 6G) solution. This suspension was stirred initially for 30 min in the dark to allow the equilibrium adsorption of the dye. Then, it was exposed to ultraviolet irradiation with continuous stirring under ambient conditions with an intensity of 1350 W cm−2 produced by an ENTELA long wave UV lamp (365 nm). Aliquots of ∼8 ml were taken for the UV–vis measurements because this volume was enough to fill the quartz cuvette sample holder. For the photocatalytic degradation studies a new solution was irradiated each time. The absorption spectra was recorded every 30 min using a Perkin Elmer UV/VIS/NIR spectrophotometer.
3. Results and discussion The textural properties of the mesoporous titania phases synthesized with P123 and CTAB as SDA are summarized in Table 1. For comparison, the BET surface area, average pore size and average crystallite size of CNCA are also shown. The nature of the structure directing agent helped to fine tune the pore size of the resulting mesophase. For instance, CTAB led to the formation of mesophases
(101)
(004)
(200)
(105)
Intensity (a.u.)
Sample ID
475
c
b
a
20
30
40
50
60
2θ(degrees) Fig. 1. XRD patterns of (a) mesoporous anatase synthesized with P123 as SDA, (b) mesoporous anatase synthesized with CTAB as SDA, (c) CNCA.
with average pore size of ∼3 nm, while P123 led to ∼6 nm pore sizes. Due to its larger micellar size, the amphiphilic block copolymer P123 led to larger average pore size. The average crystallite size for all samples was estimated using Scherrer equation and the FWHM of anatase (1 0 1) reflection, and corresponded to ∼10 nm for the mesoporous phases, and ∼25 nm for the CNCA reference sample. As shown in Table 1, the mesoporous samples displayed unimodal pore sizes, while the CNCA sample showed bimodal pore size distribution. Surface areas were comparable for all three samples. The XRD patterns of all the mesophases exhibited only the characteristic reflections of anatase at 2 ∼25◦ , 38◦ , 48◦ and 54◦ corresponding to the (1 0 1), (0 0 4), (2 0 0) and (1 0 5) planes respectively, of tetragonal titania [11] as shown in Fig. 1. Fig. 2 shows the nitrogen adsorption–desorption isotherms and pore size distribution of mesoporous anatase phases synthesized employing P123 and CTAB as SDA. Type IV isotherms with H2 hysteresis loop confirmed the formation of the mesostructure [12]. The sample prepared with P123 as SDA showed a specific surface area of ∼100 m2 g−1 and an average unimodal pore diameter of ∼6 nm. The sample prepared with CTAB as surfactant showed the highest specific surface area ∼150 m2 g−1 with unimodal pore diameter of ∼3 nm. The reference sample, CNCA displayed a typical isotherm for macroporous solids and non-unimodal pore size distribution [13]. High-resolution transmission electron microscopy (HRTEM) images of the mesoporous anatase samples synthesized with P123 and CTAB as SDA are shown in Fig. 3. The sample synthesized with P123 as SDA shows a well defined ordered channel pore archi-
Fig. 2. N2 adsorption–desorption isotherms and pore size distribution of the mesoporous anatase phases synthesized with (a) P123 (b) CTAB as SDA.
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Fig. 3. (a) and (b) HRTEM image of mesoporous nanocrystalline anatase synthesized with P123 as SDA at different magnifications. (c) HRTEM image of mesoporous nanocrystalline titania synthesized with CTAB as SDA. (d) A magnified image of the lattice fringes of (c) coincide with the anatase form of TiO2 viewed along (1 1 1) zone axis.
tecture with ∼10 nm anatase crystals (Fig. 3a) and unimodal pore size of ∼6 nm (Fig. 3b). The sample synthesized with CTAB as SDA shows crystals of ∼9–11 nm (Fig. 3c). For both samples the crystallite size agrees reasonably well with that calculated from the FWHM of anatase (1 0 1) reflection. Two sets of well-resolved lattice fringes are visible from a magnified image of Fig. 3c. The spacing of 0.35 nm, measured for these two sets of fringes (Fig. 3d), coincides with 0.352 nm, i.e., with the d-spacing of (0 1 1) type planes in anatase form of titania, and confirmed by XRD data (Fig. 1). Fig. 4 shows the SEM images of the mesoporous anatase samples synthesized with P123 and CTAB as SDA. The mesophase synthesized using P123 as SDA exhibited uniform ∼0.1 m clusters while the mesophase synthesized with CTAB as SDA resulted in the formation of larger clusters of ∼0.5–1.0 m. The morphology for the reference sample corresponded to non-uniform ∼ 1 m agglomerates [13]. Regarding the degree of dye adsorption, we found that methylene blue is the only dye that strongly adsorbs on titania [13]. Carreon et al. [10] reported a detailed study on the adsorption behavior of methylene blue on mesoporous nanocrystalline titania. The strong adsorption of methylene blue is related to the sulfurcentered cation of this dye, which interacts with the negatively charged titania surface (solution pH ∼7). This is supported by the zeta potential measurements which show that titania surface is negatively charged above the isoelectric point ∼6.2–6.5 [13]. The values of IEP are in good agreement with earlier reports [14,15]. In
the case of methyl red and rhodamine, both dyes contain carboxylic groups, which interact much weaker with the titania surface. The initial pH of the methyl red and rhodamine solutions was 6.5 and 6.4 respectively which is in the vicinity of the isolectric point of titania, therefore the protonated carboxylic groups may weakly interact with the “overall neutral charged surface of titania” leading to weak adsorption [16]. Finally, in the case of methyl orange, also a negligible adsorption was observed. In this case, the absence of polar or charged groups in the structure of this dye may be responsible for its weak adsorption in the surface of titania. The calculated fractions of dye adsorbed for MB, MO, MR, and R6G for all the samples were in the ∼14–19%, ∼2–4%, ∼1–2% and ∼1–4% range respectively [13], supporting the fact that only MB is adsorbed strongly on the surface of titania. The mesoporous anatase phases were evaluated in the photodegradation of methylene blue (MB), methyl orange (MO), methyl red (MR) and rhodamine 6G (RH). The photoactivity of these samples was compared to the reference sample. CNAC was choosen as a reference sample because it has comparable surface area than that of the mesoporous samples and lies in the crystallite size regime (tens of nanometers) of the mesoporous samples. This will allow us to have a more direct and reliable comparison between the nanocrystalline anatase mesophases vs commercially available anatase nanocrystals. Fig. 5 shows the photocatalytic degradation profiles of MB, MO, MR and RH over the mesoporous titania samples and CNCA. ‘C0 ’ is the initial dye concentration and ‘C’ is
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Fig. 4. SEM images of the synthesized mesoporous anatase phases synthesized using (a) P123 and (b) CTAB as SDA.
the adsorbed dye concentration at different time intervals. The used molar extinction coefficients for MB [17], for MO [18], for MR [19] and for RH [20] were 55,000 M−1 cm−1 , 25,000 M−1 cm−1 , 19,000 M−1 cm−1 and 50,000 M−1 cm−1 respectively. The degradation constant for MB, MO, MR and rhodamine 6G, k, followed a first-order decay kinetics [13]. In all cases (except for the sample P123 with MO and MR) the linear correlation factor R2 values were closer to 1 when first order was assumed. Nevertheless, even for these two cases the R2 values for first and zero order kinetics were very close. Therefore, we can assume first order kinetics. The mesoporous samples degraded ∼85–95% of MB after 150 min whereas CNCA degraded only ∼60% of MB after the same time of exposure to UV irradiation (Fig. 5a). The mesoporous samples degraded ∼50% of MO, while the CNCA degraded nearly ∼34% after the same UV exposure time (Fig. 5b). The mesoporous anatase phase synthesized with P123 degraded ∼82% of the dye; the meso-
porous sample synthesized with CTAB degraded ∼65% of the dye, while the CNCA degraded only ∼52% of MR at the same UV exposure time (Fig. 5c). The CNCA and P123 samples degraded nearly ∼22% of MR after 150 min of exposure to UV irradiation. The photodegradation of RH over the mesophase synthesized with CTAB was negligible (Fig. 5d). The photocatalytic activity data for MB, MO, MR and RH, including photodegradation rate constants and percent of dye degradation is summarized in Table 2. Since all samples (mesoporous and CNCA) have comparable surface areas and have the same crystalline structure, the enhanced photocatalytic activity of the mesoporous nanocrystalline anatase samples when compared to conventional nanocrystalline anatase for the photodegradation of MB, MO and MR may be related to the smaller crystallite size, and relatively ordered pore structure of the mesophases. It is well known that as the crystal size decreases, the surface density of active sites available for substrate adsorption
Fig. 5. Photocatalytic degradation profiles of (a) methylene blue, (b) methyl orange, (c) methyl red and (d) rhodamine 6G adsorbed on mesoporous anatase and conventional crystalline anatase.
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Table 2 Rate constants k and percents of dye degradation of mesoporous nanocrystalline anatase and CNCA samples. Sample ID
MB ×10−2
MO ×10−2
MR ×10−2
Rhodamine 6G ×10−2
P123 CTAB CNCA
2.0 (95%) 1.3 (85%) 0.6 (60%)
0.4 (51%) 0.4 (49%) 0.2 (34%)
1.1 (82%) 0.7 (65%) 0.5 (52%)
0.1 (21%) 0.03 (4%) 0.1 (22%)
increases, thus increasing the overall photocatalytic rate [21,22]. Moreover, the ordered pore architecture of the mesoporous samples as compared to the randomly organized pore network of CNCA may result in high diffusion rates of the guest molecules, and therefore to improved photocatalytic rates. The benefits of having an ordered mesopore structure for photocatalytic applications have been demonstrated by independent research groups. [10,17,23–25]. When comparing the photocatalytic behavior of MB, MO and MR over the mesophases (Table 2) is evident that the mesophase prepared with P123 as SDA shows better performance. These results suggest that the larger pore size of the mesophase synthesized with P123 as compared to that synthesized with CTAB may enhance the mass transfer of the molecules, leading to improved photo-oxidation rates. Furthermore, TGA analysis [13] revealed that more carbon was retained in the pores of the sample synthesized with CTAB (∼1.5%) as compared to the sample synthesized with P123 (0.9%). The excess of pyrolized carbon in the sample synthesized with CTAB may have led to pore blockage, and therefore to a decrease in its overall photocatalytic performance. In the case of RH, the photocatalytic behavior was different and limited as compared to the other dyes (MB, MO, and MR). Only the mesophase synthesized with P123 as SDA and CNCA showed photocatalytic activity. The mesophase synthesized with CTAB as SDA exhibited negligible activity. This can be partially explained by the difference in the pore diameter. The pores of the former samples are larger than those of the sample synthesized with CTAB. Therefore, the large rhodamine molecules can reach more efficiently the surface of the CNCA and P123 samples, as compared to the sample synthesized with CTAB as SDA where the accessibility is more limited by the smaller pores. It is not unusual to observe a different photocatalytic behavior of TiO2 depending on the chemical nature of the dye [26,27] since the adsorption-desorption mechanism of the dyes depends on its spatial geometry and specific functional groups. For both samples, the mesophase synthesized with P123 as SDA, and CNCA the photocatalytic activity was similar (Table 2) suggesting that the diffusion towards the active sites and the accessibility of the active sites for adsorption due to the presence of large pores are key parameters for the photo degradation of RH. For this particular dye, the crystal size and the pore order seems not to play a critical role in the photocatalytic behavior. From the molecular dimensions of the dyes, it can be inferred that rhodamine 6G may have more difficulties to diffuse through smaller pores, and therefore its photocatalytic degradation on anatase will be limited. MB has a rectangular parallelepiped shape with 1.6 × 0.7 × 0.37 nm3 dimensions [28]. The length of the long axis of methyl orange is ∼1.3 nm [29], similar to that of methyl red. Rhodamine 6G is a larger molecule with length of the long axis ∼1.4 nm [30]. In principle, diffusion of all 4 dyes through the anatase porous structures is possible, however, it has been demonstrated that a single rhodamine 6G molecule can occupy an area close to 4 nm2 [31] which makes more difficult its diffusion through small pores. 4. Conclusions Mesoporous nanocrystalline anatase was synthesized via EISA employing P123 and CTAB as structure directing agents. The resul-
tant mesoporous samples exhibited unimodal pore size of ∼3 nm and ∼6 nm, respectively and average crystallite size of ∼10 nm. The mesoporous anatase phases photodegraded MB, MO and MR ∼2–3 times faster than conventional nanocrystalline anatase and showed limited photocatalytic activity for rhodamine 6G. The enhanced photocatalytic activity of the mesoporous titania samples for the degradation of MB, MO and MR when compared to conventional nanocrystalline titania was related to smaller crystallite size, and ordered pore structure. The poor photoactivity behavior for RH was associated with the limited diffusion towards the active sites and the accessibility of the active sites for adsorption and the limited accessibility to the pores of the large rhodamine 6G molecules. Acknowledgments This work was partially supported by CONACYT-MEXICO under the project 80883. We thank Dr. Surendar R. Venna for his help with SEM and TEM experiments. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2010.10.030. References [1] B. Gözmen, M. Turabik, A. Hesenov, J. Hazard. Mater. 164 (2009) 1487–1495. [2] F. Han, V. Kambala, M. Srinivasan, D. Rajarathnam, R. Naidu, Appl. Catal. A: Gen. 359 (2009) 25–40. [3] M.A. Fox, M.T. Dylay, Chem. Rev. 93 (1993) 341–357. [4] H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afara, G.A. Ozin, Nature 379 (1996) 703–705. [5] H. Yang, N. Coombs, I. Sokolov, G.A. Ozin, Nature 381 (1996) 589–592. [6] Y. Lu, R. Ganguli, C.A. Drewin, M.T. Anderson, C.J. Brinker, W. Gong, Y. Guo, H. Soyez, B. Dunn, M.H. Huang, J.I. Zink, Nature 389 (1997) 364–368. [7] M.A. Carreon, V.V. Guliants, Mesostructuring Metal Oxides through Evaporation Induced Self-Assembly: Fundamentals and Applications, Vol. 16, Nanoporous Solids, Recent Advances and Prospects, Elsevier, 2008, pp. 407–432. [8] G.J. de, A.A. Soler-Illia, A. Louis, C. Sanchez, Chem. Mater. 14 (2002) 750– 759. [9] K. De Witte, A.M. Busuioc, V. Meynen, M. Mertens, N. Bilba, G. Van Tendeloo, P. Cool, E.F. Vansant, Microporous Mesoporous Mater. 110 (2008) 100–110. [10] M.A. Carreon, S.Y. Choi, M. Mamak, N. Chopra, G.A. Ozin, J. Mater. Chem. 17 (2007) 82–89. [11] E.L. Crepaldi, G.J. de A.A. Soller-Illia, D. Grosso, F. Cagnol, F. Ribot, C. Sanchez, J. Am. Chem. Soc. 125 (2003) 9770–9786. [12] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierrot, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603–619. [13] See Supplementary information. [14] M. Kosmulski, Adv. Colloid Interface Sci. 99 (2002) 255–264. [15] J. Gustafsson, P. Mikkola, M. Jokinen, J.B. Rosenholm, Colloids Surf. A: Physicochem. Eng. Aspects 175 (3) (2000) 349–359. [16] J. Tschirch, D. Bahnemann, M. Wark, J. Rathousky, J. Photochem. Photobiol. A: Chem. 194 (2008) 181–188. [17] A. Katti, S.R. Venna, M.A. Carreon, Catal. Commun. 10 (2009) 2036–2040. [18] J.-Z. Liu, T.-L. Wang, L.-N. Ji, J. Mol. Catal. B: Enzym. 41 (2006) 81–86. [19] J.P. Jadhav, G.K. Parshetti, S.D. Kalme, S.P. Govindwar, Chemosphere 68 (2007) 394–400. [20] K. Garai, R. Sureka, S. Maiti, Biophys. J. 92 (7) (2007) L55–L57. [21] J.W. Moon, C.Y. Yun, K.W. Chung, Catal. Today 87 (2003) 77–86. [22] E.P. Reddy, B. Sun, P.G. Smiriniotis, J. Phys. Chem. B 108 (2004) 17198– 17205. [23] Y. Sakatani, D. Grosso, L. Nicole, C. Boissiere, G.J. de A.A. Soller-Illia, C. Sanchez, J. Mater. Chem. 16 (2006) 77–82. [24] K. De Witte, S. Ribbens, V. Meynen, I. De Witte, L. Ruys, P. Cool, E.F. Vansant, Catal. Commun. 9 (2008) 1787–1792. [25] E. Beyers, P. Cool, E.F. Vansant, Microporous Mesoporous Mater. 99 (2007) 112–117. [26] R. Comporelli, E. Fannizza, M.L. Curri, P.D. Cozzoli, G. Mazcolo, R. Passino, A Agastiano, Appl. Catal., B: Environ. 55 (2005) 81–91. [27] A. Mills, P. Sawunyama, J. Photochem. Photobiol., A: Chem. 84 (1994) 305– 309. [28] R. Hoppe, G. Schultz-Ekloff, J. Rathousky, J. Starek, A. Sukal, Zeolites 14 (1994) 126–129. [29] M. Takahashi, K. Kobayashi, K. Takaoka, K. Tajima, Thin Solid Films 307 (1997) 274–279. [30] R. Sasai, T. Fujita, N. Iyi, H. Itoh, K. Takagi, Langmuir 18 (2002) 6578–6583. [31] A. Kudelski, Chem. Phys. Lett. 414 (2005) 271–275.