Nature of the Oxidizing Species Formed upon UV Photolysis of C-TiO2

J. Phys. Chem. C 2009, 113, 12489–12494

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Nature of the Oxidizing Species Formed upon UV Photolysis of C-TiO2 Aqueous Suspensions Sara Goldstein,* David Behar, and Joseph Rabani Institute of Chemistry and the Accelerator Laboratory, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: March 4, 2009; ReVised Manuscript ReceiVed: May 26, 2009

UV photolysis of aerated aqueous suspensions of C-TiO2 (KRONOS vlp 7000) or TiO2 (KRONOS uvlp 7500, Degussa P25) containing CH3OH produces HCHO and H2O2, and the latter accumulates only in the case of C-TiO2. The UV photolysis of C-TiO2 in aerated suspensions produces holes, which oxidize CH3OH to •CH2OH leading to the formation of HCHO in the presence of oxygen. H2O2 produced via the oxidation of CH3OH to HCHO and through the reduction of O2 by the conduction band or trapped electrons is adsorbed by all titania. Catalase, which catalyzes CH3OH oxidation by H2O2, substantially increases the rate of HCHO formation only in the case of C-TiO2. This observation indicates that the electrons produced upon UV excitation of C-TiO2 differ from those of unmodified TiO2 with respect to their reactivity toward the photolytically generated H2O2. The holes produced in P25 behave differently from those formed in C-TiO2 and uvlp 7000 with respect to their reaction with added H2O2. This difference is, at least in part, due to the relatively much lower saturation level of H2O2 at the P25 surface rather than different types of holes. Comparison between UV and visible excitation of C-TiO2 reveals qualitative differences with respect to the nature of the oxidizing and reducing species. The different nature of the holes obtained upon UV and visible irradiation suggests that localization of the holes at the surface and subsequent oxidation of CH3OH is the predominant route as is the case with undoped titania. Introduction It is well-known that TiO2 in general produces conduction band electrons and valence band holes upon photolysis at λ < 388 nm (reaction 1), which are quickly trapped at the surface (reactions 2 and 3).1-3

hν

+ TiO2 98 hVB + eCB

+ hVB + {Ti-O-Ti} + H2O f {Ti-O•HO-Ti} + H+ (5)

Recently, however, Salvador11 identified the reactive holes in water suspensions of TiO2 particles as surface -O•- (or -•OH depending on pH) covalently linked to Ti atoms and suggested that they are produced according to reaction 6.

(1) + hVB + {Ti-O2--Ti} f {Ti-•O--Ti}

eCB f eT-

(2)

+ hVB f hT+

(3)

The mobile and trapped species can recombine (reaction 7) or react with solutes at the surface. In the following equations, h+ and e- stand for all the forms of holes and electrons, irrespective of whether mobile or localized.

h+ + e- f TiO2 It had been argued that in the case of TiO2 the valence band holes produce adsorbed •OH radicals by oxidation of surface water molecules (reaction 4)4-7 or give rise to lower energy surface sTi-O• radicals, for example, according to reaction 5, which involves a nucleophilic attack of an H2O molecule accompanied by bond breaking.8-10 + hVB + Ti-OH2 f Ti-•OH + H+

(4)

* To whom correspondence should be addressed. Telephone: 972-26586478. Fax: 972-2-6586925. E-mail: [email protected].

(6)

(7)

Most organic compounds are oxidized by the holes as demonstrated for CH3OH (reaction 8). In the presence of oxygen, the organic radical is converted to the respective peroxyl radical (reaction 9). The peroxyl radical may decompose bimolecularly or unimolecularly.12 In the case of CH3OH, the respective peroxyl radicals decompose unimolecularly (reaction 10) and this reaction is base-catalyzed.13

h+ + CH3OH f •CH2OH + H+

10.1021/jp9019969 CCC: $40.75  2009 American Chemical Society Published on Web 06/22/2009

(8)

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•

Goldstein et al.

CH2OH + O2 f •OOCH2OH

(9)

OOCH2OH f HCHO + H+ + O2•-

(10)

The TiO2 electrons are known to react with O2 according to reaction 11, which eventually produces H2O2 through reactions 12 and 13.

e- + O2 f O2•-

(11)

e- + O2•- + 2H+ f H2O2

(12)

O2•- + O2•- + 2H+ f H2O2 + O2

(13)

Subsequently, H2O2, if it accumulates to sufficient levels, is reduced by e- to form •OH (reaction 14), generating an additional organic radical (reaction 15).

•

e- + H2O2 + H+ f H2O + •OH

(14)

OH + CH3OH f •CH2OH + H2O

(15)

The discovery that doping of TiO2 leads to extension of the photoactive region from UV to visible light has remarkably increased the interest in such doped TiO2 because of the potential application for visible light driven solar conversion14-22 and photocatalytic oxidations of organic pollutants.7 The visible photoactivity is attributed either to introduction of intragap localized states by the dopants17,23-25 or to narrowing of the band gap.26-30 These issues have been recently addressed in our laboratory.31 We have demonstrated that visible photolysis of aqueous suspensions of C-TiO2 (KRONOS vlp 7000) aqueous suspensions induces CH3OH oxidation to HCHO with a rather low yield presumably by the relatively low energy holes, hC+. However, Zabek et al.32 have recently suggested that the commercial visible light photocatalyst (KRONOS vlp) contains an organic sensitizer tentatively assigned to an aryl carboxylate group, which is responsible for the visible light activity. This observation challenges the involvement of substitutional or interstitial carbon in visible light induced charge separation. The reported square root dependence of ΦHCHO on the visible light density using the vlp material31 cannot be attributed to direct oxidation of CH3OH by the excited organic sensitizer but through some kind of electron and hole produced from the excited organic sensitizer. Since the UV photon energy is considerably higher compared to the visible photon, the question arises as to whether the UV excitation of C-TiO2 eventually leads to the same oxidizing species as observed upon visible light excitation. The present Article concerns a comparative study of the kinetic nature of the electrons and holes produced upon UV photolysis of C-TiO2 and unmodified TiO2. Experimental Section All materials were of analytical grade and were used as received. C-TiO2 (vlp 7000) and TiO2 (uvlp 7500) (surface area > 250 m2 g-1 and crystallite size ca. 15 nm) were obtained as a gift from KRONOS Titan GmbH and will be denoted as C-TiO2 and uvlp 7500, respectively. TiO2 was obtained from

Degussa (Titandioxid P25, surface area 50 m2 g-1, crystallite size 21 nm) and will be denoted as P25. Catalase solution (EC 1.11.1.6, 2 mg mL-1, 130 000 U mL-1) was purchased from Boehringer-Mannheim. The concentration of HCHO was determined using the Nash reagent (2 M ammonium acetate, 0.05 M acetic acid, 0.02 M acetylacetone).33 The reagent was mixed with an equal volume of the suspension, and after 10 min incubation at 60 °C the titanium dioxide was removed by centrifugation and the concentration of formaldehyde determined from its absorption using ε412 ) 7600 M-1 cm-1 against the same suspension kept in the dark. The molar extinction coefficient obtained in aqueous suspensions of titania after its removal by centrifugation was similar to that in water. The concentration of H2O2 was determined by the molybdateactivated iodide method.34 The reagent was added either before or after the removal of the titania. In the latter case, a correction was required for the adsorption of I3- to the titania. This correction ranged from 6% (0.5 g L-1 P25) to 14% (0.5 g L-1 KRONOS materials). Aqueous suspensions of the titania were prepared daily and were used after 20 min of sonication. Photolysis was carried out using the SX-17MV Applied Photophysics setup composed of an Osram 150 W ozone free Xe lamp and a monochromator. The monochromator was set at 320 ( 7 nm. The incident light intensity was measured with a calibrated Si photosensor (Hamamatsu S2281) coupled with a Keithley 617 programmable electrometer. Photolysis was carried out in a cylindrical Suprasil quartz cell (2 cm i.d., 2 cm length, V ) 6.4 mL) with flat windows under magnetic stirring at room temperature. The illumination cell was shielded from room-light. In some experiments, the light density was varied by introducing appropriate neutral density filters. Each reported result represents an average of at least three measurements showing a scatter of less than 10%. Spectral measurements of the titania aqueous suspensions were carried out in an integrating sphere “Labsphere” with 15 cm diameter. The suspension in a 1 cm four window cuvette was placed inside the integrating sphere at the center, and the nonabsorbed light intensity was measured with the calibrated photosensor. The monochromator was set at selected wavelengths using narrow slits ((2.3 nm). The absorbance measurements were occasionally repeated in order to verify that no precipitation occurred during the measurements since no stirring was possible inside the integrating sphere. Results The absorption spectra of aqueous suspensions of 1 g L-1 C-TiO2 and uvlp 7500 in the absence and presence of 1 mM H2O2 are presented in Figure 1. The absorbance increases with the concentration of the titania as shown in Figure 2 for C-TiO2. The deviations from Beer’s law are due to light scattering. The absorbance at 1.5 g L-1 C-TiO2 (A320 ) 1.37) corresponds to 250 m g ca. 15 nm yes (RH2O2 < RHCHO) doubles RHCHO affects the absorption spectrum (5.3 ( 0.8) × 104 (1.5 ( 0.1) × 10-4 suppresses RHCHO 0.5 2

-1

>250 m g ca. 15 nm no no effect on RHCHO affects the absorption spectrum (5.3 ( 0.8) × 104 (2.6 ( 0.2) × 10-4 suppresses RHCHO ndc 2

TiO2 (P25) 2

-1

50 m g ca. 21 nm no no effect on RHCHO hardly affects the absorption spectrum (4.5 ( 0.9) × 104 (2.6 ( 0.2) × 10-5 enhances RHCHO ndc

In the presence of 4-5 M CH3OH. b 1 mM H2O2 added to 1 g L-1 titania. c nd: not determined.

In the presence of catalase, the reactions of h+ and e- with H2O2 are suppressed and both h+ and e- contribute to the formation of HCHO (eq 21), namely, RHCHO ) Rh+ + Re-. catalase

h+ + e- + 2 CH3OH + O2 98 2 HCHO + 2 H2O

(21) It may be argued that catalase adsorbs to C-TiO2 and it competes with CH3OH for the holes and/or with O2 for the electrons. In the latter case, the formation of H2O2 is suppressed and RHCHO should decrease. If catalase competes with CH3OH for the holes, its oxidation to compound I (OdFeV•+) is unlikely because the active site of the protein is well-shielded by many reactive groups, which can easily become oxidized by the holes. Even if compound I is formed by subsequent oxidation, such a competition cannot explain the doubling effect of RHCHO. In addition, under our experimental conditions, the concentration of catalase is of the order of 10 nM. Even if it is totally adsorbed, most C-TiO2 nanocrystallites (ca. 90%) are free of adsorbed catalase. In the absence of catalase RHCHO is determined also by the competition between CH3OH and H2O2 for the holes and by the competition between O2 and H2O2 for the electrons. The adsorption of H2O2 to C-TiO2 is unaffected by the presence of up to at least 4 M CH3OH, indicating that the adsorption of CH3OH is relatively weak. The effect of catalase depends on [CH3OH] because CH3OH competes with H2O2 for the holes. At low [CH3OH], part of the holes oxidize H2O2 rather than CH3OH and do not contribute to the formation of HCHO. Doubling of RHCHO upon addition of catalase is expected only under the conditions where CH3OH competes efficiently with the photolytic H2O2 for the holes, while the H2O2 does not react with the electrons. The plateau values in Figure 5, both in the presence and in the absence of catalase, represent practically a complete scavenging of the surface holes, which are available for CH3OH oxidation. The limiting value ΦHCHO ) 1 in the presence of catalase (Figure 7) corresponds to Φh+ + Φe-. This value is considerably lower than the maximum limit of 2, indicating that reaction 3 does not completely suppress electron-hole recombination. The relatively high ΦHCHO suggests that the carbon sites do not act as very efficient recombination centers. The above mechanism predicts that RHCHO should approach Re- at high added [H2O2] so that it competes efficiently with CH3OH for the holes and with O2 for the electrons. However, the results given in Figure 6 demonstrate the suppression of RHCHO upon the addition of H2O2 to values below Re-. This unexpected result is discussed in detail in the next section.

The photocatalytic behavior of C-TiO2 upon UV excitation is in several respects different from that observed upon visible excitation:31 (i) The limiting ΦHCHO ) 0.5 is about 1 order of magnitude higher than that observed in the visible region. (ii) H2O2 is accumulated upon UV but not upon visible photolysis, although the rate of its formation is considerably slower than RHCHO (Figure 4). (iii) RHCHO increases in the presence of catalase, although upon UV irradiation the increase depends on [CH3OH] (Figure 5) while in the visible excitation RHCHO is doubled independently of [CH3OH]. (iv) The addition of H2O2 suppresses RHCHO (Figure 6) upon UV excitation, while it doubles RHCHO upon visible excitation. Upon visible excitation, catalase doubles RHCHO independent of [CH3OH] because H2O2 does not react with the holes.31 Thus, both the effect of catalase and the time profiles of HCHO and H2O2 suggest that the oxidizing species obtained upon UV and visible excitations are different. The observation that the relatively low concentrations of the accumulated H2O2 react with the electrons produced upon UV excitation, but not upon visible illumination,31 suggests a different nature of the electrons produced upon UV and visible irradiation. These conclusions are further supported by the large difference between the limiting ΦHCHO values observed upon UV and visible excitations, which is attributed to different oxidizing and reducing species. To summarize, the results show that the active species produced upon UV and visible excitations are different. Although our results are unable to discriminate between the different possibilities concerning the nature of the active species formed upon visible irradiation, namely, hC+ versus excited organic compound,31 a direct oxidation of CH3OH by an organic excited state cannot be accommodated with the square root dependency of ΦHCHO on the visible light density.31 The square root dependency implies a competition between second and first order processes, namely, a competition between electron-hole recombination and conversion of the hole to the species reacting with the substrate, for example, CH3OH. It is difficult to see how an analogous mechanism operates in the case of an excited sensitizer unless the latter is a precursor of a different transient reacting with the substrate, for example, the excited sensitizer transfers an electron to the conduction band of the C-TiO2. In such a case, the system can be still discussed in terms of electrons and holes where the oxidized sensitizer replaces hC+ although it is difficult to envision how an aryl carboxylate group is regenerated from the respective cation radical in aerated aqueous medium. It is well-known that under such conditions the aryl cation radical is quickly converted into the respective stable hydroxy-aryl.43-45 The valence band holes produced upon UV irradiation have sufficient energy to oxidize the carbon state or the aromatic carbon compound.31 The different nature of the holes obtained upon UV and visible irradiation suggests that localization of

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the holes at the surface and subsequent oxidation of CH3OH is the predominant route. UV Photolysis of C-TiO2 Compared to TiO2. The physical properties and the results obtained upon UV excitation of aerated suspensions of C-TiO2, uvlp 7500, or P25 are summarized in Table 1. The results obtained in the absence and presence of catalase clearly indicate that the nature of the reducing species in C-TiO2 differs from that of undoped TiO2, namely the photolytic H2O2 in C-TiO2 does not compete effectively with O2 for the electrons as opposed to the case in the undoped TiO2. Suppression of RHCHO upon the addition of H2O2 at concentrations much higher than the photolytically produced H2O2 (Figure 6) is expected to lead to a plateau value of RHCHO when saturation of the TiO2 surface is achieved. The concentrations of H2O2, which are required to reach the plateau values in the three titania, are in reasonable agreement with the respective adsorption data (Table 1). As demonstrated in Figure 6, both C-TiO2 and uvlp 7500 show similar profiles for RHCHO dependency on added H2O2, suggesting a similar oxidation mechanism. The substantial suppression of RHCHO may be related to the oxidation of H2O2 by the holes and to the change in the absorption spectrum of both C-TiO2 and uvlp 7500 in the presence of H2O2. The spectral changes become pronounced above 350 nm (Figure 1). If the new state obtained by the interaction of the titania with H2O2 contributes to the optical absorption at 320 nm and the excitation light is shared between the titania and the chemisorbed H2O2, the overall RHCHO should depend on the contribution of the new absorption to the formation of HCHO. In such a case the lower than expected RHCHO would imply that the absorption of the UV light by the adsorbed H2O2 gives rise to lower yields of oxidation than the excitation of the titania. However, catalysis of electron-hole recombination by the titania-H2O2 complex seems more reasonable, namely formation of adsorbed HO2 by h+ oxidation and subsequent reduction of the latter by e-. Note that enhancement of RHCHO is observed in the case of P25, which does not show a new absorption associated with added H2O2, suggesting that the adsorbed H2O2 does not react with the holes but competes for the electrons with the electron-hole recombination. In this case, the plateau value is reached at relatively low [H2O2] compared to the other titania due to the lower H2O2 surface saturation (Table 1). Conclusions The electrons produced upon UV excitation of C-TiO2 (vlp 7500) and unmodified TiO2 (uvlp 7000 and P25) are different, as the former react relatively slowly with the photolytically generated H2O2. Although the band gap excitation supposedly leads to the same kind of conduction band electrons, the surface electrons are different, implying that C-TiO2 has different electron traps. The holes in P25 behave differently from those in C-TiO2 and uvlp 7000 with respect to their reaction with added H2O2. This difference is, at least in part, due to the relatively much lower saturation level of H2O2 at the P25 surface rather than different types of holes. Comparison between UV and visible excitation of C-TiO2 reveals qualitative differences with respect to the nature of the oxidizing and reducing species. The valence band holes produced upon UV irradiation have sufficient energy to oxidize the carbon state or the aromatic carbon compound.31 The different nature of the holes obtained upon UV and visible irradiation suggests that such oxidation, which could lead to the same type of holes, is not important. Localization of the holes at the surface and subsequent oxidation of CH3OH is the predominant route as is the case with undoped titania. The behavior of both C-TiO2 and undoped titania seems to be more complicated than anticipated.

Goldstein et al. Acknowledgment. This research was partially supported by a grant from the Israel Science Foundation of the Israel Academy of Sciences. We are indebted to KRONOS Titan GmbH for providing the TiO2 samples and to Degussa for the Titandioxid P25. References and Notes (1) Hashimoto, K.; Irie, H.; Fujishima, A. Jpn. J. Appl. Phys. 2005, 44, 8269. (2) Thompson, T. L.; Yates, J. T., Jr. Chem. ReV. 2006, 106, 4428. (3) Chen, D.; Jiang, Z.; Geng, J.; Wang, Q.; Yang, D. Ind. Eng. Chem. Res. 2007, 46, 2741. (4) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979, 83, 3146. (5) Goldstein, S.; Czapski, G.; Rabani, J. J. Phys. Chem. 1994, 98, 6586. (6) Szczepankiewicz, S. H.; Colussi, A. J.; Hoffmann, M. R. J. Phys. Chem. B 2000, 104, 9842. (7) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (8) Micic, O. I.; Zhang, Y. N.; Cromack, K. R.; Trifunac, A. D.; Thurnauer, M. C. J. Phys. Chem. 1993, 97, 7277. (9) Nakamura, R.; Nakato, Y. J. Am. Chem. Soc. 2004, 126, 1290. (10) Nakamura, R.; Okamura, T.; Ohashi, N.; Imanishi, A.; Nakato, Y. J. Am. Chem. Soc. 2005, 127, 12975. (11) Salvador, P. J. Phys. Chem. C 2007, 111, 17038. (12) von Sonntag, C.; Schuchmann, H. P. Peroxyl radicals in aqueous solution. In Peroxyl Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, 1997. (13) Rabani, J.; Klug-Roth, D.; Henglein, A. J. Phys. Chem. 1974, 78, 2089. (14) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (15) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943. (16) Tachikawa, T.; Tojo, S.; Kawai, K.; Endo, M.; Fujitsuka, M.; Ohno, T.; Nishijima, K.; Miyamoto, Z.; Majima, T. J. Phys. Chem. B 2004, 108, 19299. (17) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617. (18) Sun, B.; Smirniotis, P. G.; Boolchand, P. Langmuir 2005, 21, 11397. (19) Sharma, S. D.; Singh, D.; Saini, K. K.; Kant, C.; Sharma, V.; Jain, S. C.; Sharma, C. P. Appl. Catal., A 2006, 314, 40. (20) Serpone, N. J. Phys. Chem. B 2006, 110, 24287. (21) Behar, D.; Rabani, J. J. Phys. Chem. B 2006, 110, 8750. (22) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2005, 127, 8286. (23) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (24) Kuznetsov, V. N.; Serpone, N. J. Phys. Chem. B 2006, 110, 25203. (25) Kuznetsov, V. N.; Serpone, N. J. Phys. Chem. C 2007, 111, 15277. (26) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (27) Umebayashi, T. Y. T.; Yamamoto, S.; Miyashita, A.; Tanaka, S.; Sumita, T.; Asai, K. J. Appl. Phys. 2003, 93, 5156. (28) Belver, C. B., R.; Stewart, S. J.; Requejo, F. G.; Fernandez-Garcia, M. Appl. Catal., B 2006, 65, 309. (29) Kisch, H.; Sakthivel, S.; Janczarek, M.; Mitoraj, D. J. Phys. Chem. C 2007, 111, 11445. (30) Cong, Y.; Zhang, J. L.; Chen, F.; Anpo, M.; He, D. N. J. Phys. Chem. C 2007, 111, 10618. (31) Goldstein, S.; Behar, D.; Rabani, J. J. Phys. Chem. C 2008, 112, 15134. (32) Zabek, P.; Eberl, J.; Kisch, H. Photochem. Photobiol. Sci. 2009, 8, 264. (33) Nash, T. Biochem. J. 1953, 55, 416. (34) Klassen, N. V.; Marchington, D.; McGowan, H. C. E. Anal. Chem. 1994, 66, 2921. (35) Boonstra, A. H.; Mustsaers, C. A. H. A. J. Phys. Chem. 1975, 79, 1940. (36) Li, X.; Chen, C.; Zhao, J. Langmuir 2001, 17, 4118. (37) Chen, H. Y. Z. O.; Bouchy, M. J. Photochem. Photobiol., A 1997, 108, 37. (38) Connor, P. A.; McQuillan, A. J. Langmuir 1999, 15, 2916. (39) Du, Y.; Rabani, J. J. Phys. Chem. B 2003, 107, 11970. (40) Keilin, D.; Hartree, E. F. Biochem. J. 1945, 39, 293. (41) Kobayashi, K.; Kawai, S. J. Chromatogr. 1982, 245, 339. (42) Johansson, L. H.; Borg, H. L. A. Anal. Biochem. 1988, 174, 331. (43) Walling, C. Acc. Chem. Res. 1975, 8, 125. (44) Walling, C.; Camaioni, D. M.; Kim, S. S. J. Am. Chem. Soc. 1978, 100, 4814. (45) Goldstein, S.; Czapski, G.; Rabani, J. J. Phys. Chem. 1994, 98, 6586.

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