Mechanism of the OH Radical Generation in Photocatalysis with TiO2

Apr 15, 2014 - This difference could be explained as follows: at the surface of anatase TiO2, trapped holes become adsorbed OH radicals, while at the ...
0 downloads 8 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Mechanism of the OH Radical Generation in Photocatalysis with TiO2 of Different Crystalline Types Jie Zhang and Yoshio Nosaka* Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, 940-2188 Japan ABSTRACT: The difference in the OH radical generation through photocatalytic reaction with distinct crystalline types of TiO2 in aqueous suspension was explored by means of a fluorescence probe method. By employing two kinds of probe molecules with different adsorptivities, that is, coumarin and coumarin-3-carboxylic acid, we could detect OH radicals located near the TiO2 surface and those in the bulk solution individually. The amount of the OH radicals near the TiO2 surface was much larger than that in the solution, which diffused from the TiO2 surface to the bulk solution. Rutile TiO2 produced a much smaller amount of OH radicals as compared to anatase and anatase-contained TiO2. On the addition of H2O2, the OH radical generation for pure anatase TiO2 decreased but increased for rutile and rutile-contained TiO2. This difference could be explained as follows: at the surface of anatase TiO2, trapped holes become adsorbed OH radicals, while at the rutile surface, Ti-peroxo (Ti-OO-Ti) produced by the combination of two trapped holes which correspond to the adsorbed H2O2, could work as a catalyst for the OH radical generation from water.

■

•

INTRODUCTION TiO2 is the promising photocatalyst owing to its high photocatalytic activity, chemical stability, no toxicity, and commercial availability. The photocatalytic reactions at the TiO2 surface attract much attention because it can be applied to water purification and environmental cleaning.1−3 These applications have been realized for self-cleaning tiles, glasses, and windows.4 In general, it has been reported that the photocatalytic reactions proceed mainly by the reactions with active oxygen species, such as OH radical (•OH),5−9 superoxide radical (•O2−),10,11 and H2O2.11,12 Among them, •OH is a significantly important species, being frequently assigned to the major reactant responsible for the photocatalytic oxidation of organic compounds.9 However, the detailed mechanism of the OH radical generation has not been experimentally clarified yet, because it is difficult to detect •OH due to its high reactivity and short lifetime. Up to now, several methods to detect •OH in photocatalysis have been proposed. Among them, an ESR spin trapping method with DMPO (5,5-dimethyl-1,1-pyrrolineN-oxide) has been often employed to detect OH radicals. The spin trapping reaction proceeds by the direct oxidation of the trapping reagent with holes, as proved previously.14 The generated OH radical adduct is not a molecule stable enough in aerated aqueous solution and can be easily oxidized.13 In our previous study about the comparison of the detection methods, the fluorescence probe method was shown to be a suitable method to detect • OH in photocatalysis of aqueous suspension.14 Thus, we applied the fluorescence probe method to detect the OH radicals generated for various kinds of visiblelight-responsive TiO2 photocatalysts and proposed plausible reaction mechanisms.15 Furthermore, it was suggested that the © 2014 American Chemical Society

OH generated through photocatalytic reactions could distribute both on the TiO2 surface and in the bulk solution. As is well-known, the TiO2 surface is amphiphilic. The surface comprises of hydrophilic and hydrophobic parts. Coumarin used as the fluorescence probe is a hydrophobic molecule, which may interact preferably with the hydrophobic part of the TiO2 surface. The hydrophobic interaction could be weak. Therefore, in order to detect the OH radicals located near the surface, some fluorescence probe molecule, which can be adsorbed on the hydrophilic part of the TiO2 surface, would be desirable. Then, in the present research, we employed coumarin-3carboxylic acid (CCA), which is expected to be adsorbed on the hydrophilic surface of TiO2 by its carboxyl group. By using CCA as the fluorescence probe, the •OH generation near the hydrophilic surface of various kinds of TiO2 photocatalysts could be investigated. Furthermore, we examined the effect of the reaction intermediate, H2O2, and found that the generation process of •OH at the TiO2 surface is different between rutile and anatase. In combination with the experimental results of our previous studies,12 the distinct mechanisms of OH radical generation for TiO2 with different crystalline types were proposed. Received: February 3, 2014 Revised: April 8, 2014 Published: April 15, 2014 10824

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832

The Journal of Physical Chemistry C

■

Article

EXPERIMENTAL SECTION Materials. All of the TiO2 powders used were a generous gift from each manufacturer and used without further modification. The properties and manufacturer of TiO2 powders used are listed in Table 1. ST-01 and MT-150A are

Scheme 1. Formation of Fluorescing Products by the Reaction of Probe Molecules with OH Radicals

Table 1. Properties of TiO2 Powders Used name

anatase content (%)

particle size (nm)

BET surface area (m2 g−1)

point of zero charge

ST-01

100

7

320

6.3

MT150A P25

0

15

88

6.6

80

32

49

6.6

F1

73

52

26

manufacturer Ishihara Sangyo TAYCA

area of 8 × 8 mm2. The suspension was stirred vigorously for 5 min before and during the light irradiation. After the irradiation for a given time (0−150 s), 0.5 g of KCl was added into the suspension, and then the suspension was kept in the dark to precipitate the powder. After 1 day holding, the clear solution was obtained and its fluorescence spectrum was measured with the excitation wavelength of 340 or 332 nm for OH-CCA or umbelliferone, respectively. The intensity of the each spectrum could be corrected by the fluorescence peak of CCA or coumarin at 390 or 392 nm, respectively. In the separate experiments, it was confirmed that KCl did not affect the fluorescence intensities. In order to convert the fluorescence intensity to the concentration of OH-CCA, solutions containing various concentrations of OH-CCA in 0.1 mM CCA were prepared and the fluorescence spectra were measured. We confirmed that the increment of the fluorescence intensities at 445 nm was proportional to the OH-CCA concentration, as shown later in Figure 1. By using this relationship, the concentration of OHCCA generated in photocatalysis was calculated from the fluorescence intensity of the solution. In the experiments with coumarin, the similar calibration method was used to calculate the concentration of umbelliferone as described previously.15 Calculation of •OH Concentration. The amount of •OH was calculated from the concentration of OH-CCA with the conversion factor obtained by the following derivation with some references. On the irradiation of a γ-ray in a CCA aqueous solution, several OH substitutes of CCA are produced due to the reaction with •OH.17 Among the products, only 7OH substitute (OH-CCA) emits strong fluorescence and the yield is reported to be 12.7 nmol/J for 0.1 mM CCA aqueous solution.17 The yield of •OH on the γ-ray irradiation in pure water is reported to be 2.2 molecules/100 eV,18 which corresponds to 228 nmol/J. On the basis of the comparison between the yields of OH-CCA and •OH, it is calculated that 4.7% of •OH in water can be probed as OH-CCA by using 0.1 mM CCA. For 0.1 mM coumarin, 6.1% of OH radicals can be probed by the formation of umbelliferone19 according to the derivation shown in the previous report.15 Experiments with Additives. Addition of I− was examined for investigating the reaction between CCA and •OH by using a competitive reaction. Into the 3.5 mL of 0.1 mM CCA aqueous TiO2 (P25) suspension, KI of the concentrations ranging from 5 to 20 nM was added. In this case, the UV irradiation time was fixed to 150 s. For comparison, coumarin was used in the same condition. Since H2O2 is the possible intermediate in photocatalysis,20−23 the effect of H2O2 on the •OH generation during the

Nippon Aerosil Showa Denko

fine powders consisting of anatase and rutile, respectively. P25 and F1 are mixture of anatase and rutile crystallites. The potential of zero charge (PZC or isoelectric point) was measured with an electrophoresis analyzer (Microtech, Zeecom 2000) and shown in the Table. Although usually PZC of rutile is smaller than anatase, since the rutile used in the present study (MT-150A) was manufactured without heat treatment, the PZC showed a similar value to the anatase (ST-01). Coumarin, umbelliferone (7-hydroxy coumarin; Tokyo Kasei Co.), coumarin-3-carboxylic acid (CCA), 7-hydroxy-coumain3-carboxylic acid (OH-CCA; Sigma-Aldrich Co.), potassium chloride, potassium iodide, and 30% hydrogen peroxide solution (Nacalai Tesque Co.) were used without further purification. The photo absorption of the TiO2 powders was characterized by an UV−vis−NIR spectrophotometer (Shimadzu, UV− 3150) equipped with an integration sphere assembly (Shimadzu, ISR-3100). Fluorescence spectra were measured with a fluorescence spectrophotometer (Hitachi, Model 850) equipped with a PC recorder. An LED lamp (Nichia, NCCU033) emitting UV (365 nm) light was used for the excitation through a quartz convex lens and an 8 × 8 mm2 aperture. The intensity of the excitation light was measured with a power meter (Advantest, TQ8210) to be 24 mW. Irradiance spectra of the LED light were measured with a fiber spectrometer (Avantes, AvaSpec-2048), which has been calibrated with a standard light source (Avantes, AvaLightHAL-CAL). Fluorescence Probe Method. In our previous reports, we used terephthalic acid as the probe molecule for the •OH detection in alkaline solution.11,12,14 Recently we have been employing coumarin for the detection of •OH,15,16 because the chemical structure of coumarin molecule carries no charge. It reacts with OH radicals to form several OH substituted coumarins. Among them only 7-OH coumarin (umbelliferone) emits strong fluorescence and the yields have been reported.15 3-Carboxylic acid derivative of coumarin (CCA) also reacts with OH radicals to form OH-CCA (see Scheme 1) similar to coumarin.17 Experimental procedure of the fluorescence probe method was described in detail elsewhere.15 TiO2 powder (15 mg) was suspended in a Pyrex cell (10 × 10 × 45 mm3) containing 3.5 mL of a 0.1 mM CCA or coumarin aqueous solution, and one side of the cell was irradiated with the UV light confined to the 10825

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832

The Journal of Physical Chemistry C

Article

Adsorption of Probe Molecules. Adsorption of the probe molecules on each TiO2 powders was measured by the fraction of non-adsorbed molecules as follows. TiO2 powder (15 mg) was added to 3.5 mL of aqueous solutions containing 0.1 mM CCA with various amounts of OH-CCA. The solution was vigorously stirred and placed for 1 h for adsorption. Then, into the solution, 0.5 g of KCl was added and kept in the dark for 1 day to measure the fluorescence spectra of the supernatant solution. The fluorescence intensity of OH-CCA at the peak wavelength was plotted as a function of the OH-CCA concentration. The fraction of the adsorbed molecules was calculated from the decrease of the slope caused by the addition of TiO2 powders. Similar experiments were performed for umbelliferone in 0.1 mM coumarin solution. Furthermore, in the case of pH change, the adsorption on TiO2 powders was also measured for both OH-CCA and umbelliferone.

■

RESULTS AND DISCUSSION

Adsorption of OH-CCA and Umbelliferone. Figure 1a,b are the plots of the fluorescence intensities of various amounts of OH-CCA and umbelliferone, respectively, in the absence and presence of four kinds of TiO2 particles. On the addition of TiO2, the fluorescence intensity of OH-CCA (Figure 1a) decreased significantly, while that of umbelliferone (Figure 1b) was not changed much. Thus, Figure 1a,b clearly shows that OH-CCA could be adsorbed on the TiO2 surface, but coumarin could scarcely be adsorbed. The fraction of the adsorbed molecules could be calculated from the decrease of each observed slope. The numerical data of the adsorbed fraction will be shown later in Figure 4 and Table 2. From the fluorescence intensity of OH-CCA in the bulk solution, we could estimate the adsorption of OH-CCA but not of CCA, because CCA does not fluoresce. Since CCA is used to detect OH radicals, the adsorbability of CCA must be also taken in consideration. Figure 1b shows that umbelliferone could not adsorb on the surface of TiO2. That is, the presence of OH group at the seventh position of coumarin had no effect on the adsorption. On the other hand, OH-CCA showed strong adsorption, as shown in Figure 1a. These facts implied that only −COOH group in the structure of OH-CCA is participated in the adsorption. Therefore, it may be concluded that CCA can detect the OH radicals near the surface of TiO2, especially for P25 with a high adsorbed fraction. On the other hand, coumarin could dominantly detect the OH radicals in the bulk solution. The amount of generated OH-CCA was calculated by taking account of the adsorbed fraction, because the detected OHCCA was located only in the bulk solution.

Figure 1. Change in the fluorescence intensity for various amounts of (a) OH-CCA and (b) umbelliferone on the addition of four kinds of TiO2 (15 mg/3.5 mL) in 0.1 mM solutions of (a) CCA and (b) coumarin.

photocatalytic reaction was investigated. A total of 1 h before the irradiation, 50 μL of 10 mM H2O2 solution was added into the sample of 3.5 mL, which corresponds to the H2O2 concentration of 0.14 mM in the sample suspension. Then, the rate of OH-CCA formation with the UV irradiation was measured. The H2O2 concentration of 0.14 mM was selected because it was the most effective concentration in the previous research.12 For comparison, the procedure described above for CCA was similarly employed for the coumarin probing. The pH values of 0.1 mM CCA and coumarin solutions were 4.4 and 5.9, respectively. Though the experiments were usually performed without adjusting pH, in some experiments, the pH value was adjusted to explore the effect of pH difference. By adding H2SO4 or KOH solution, the pH of the CCA and coumarin solutions was adjusted to 4.4, 5.9, and 9.0. Due to the alternation of the surface charges with the pH change, the adsorption of the probe molecules on TiO2 surface as well as the •OH generation would be changed.

Table 2. Effect of pH on the Adsorption and the OH Radical Generation probe molecules fraction of adsorbed probe (%)

rate of •OH generation (nM/s)

quantum yield of •OH (×10−2 %)

CCA

coumarin

name of TiO2

pH 4.4

pH 5.9 (+KOH)

pH 9.0 (+KOH)

pH 4.4 (+H2SO4)

pH 5.9

pH 9.0 (+KOH)

ST-01 MT-150A P25 ST-01 MT-150A P25 ST-01 MT-150A P25

61 24 97 26.8 0.8 56.0 10.7 0.24 20.3

5 2 94 4.3 0.2 80.1 1.7 0.06 29.0

10 0 6 0.83 1.8 5.1 0.33 0.53 1.8

0 0 24 6.7 0.23 14.7 2.7 0.07 5.3

0 0 3 2.2 0.36 10.5 0.9 0.12 3.8

0 0 18 2.2 0.44 9.2 0.9 0.15 3.3

10826

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832

The Journal of Physical Chemistry C

Article

Quantum Yields of OH Radicals. Figure 2a,b show the amounts of OH-CCA and umbelliferone produced for each

Figure 3. Absorption spectra of TiO2 used in this study and irradiance spectrum of the UV (365 nm) LED used.

Figure 4 shows the quantum yields of •OH generation with coumarin and CCA together with the adsorbed fraction of

Figure 2. Concentrations of (a) OH-CCA and (b) umbelliferone formed in 3.5 mL of aqueous suspension of various TiO 2 photocatalysts (15 mg) containing 0.1 mM (a) CCA and (b) coumarin, respectively, were plotted as a function of the irradiation time with the 365 nm LED.

TiO2 photocatalyst as a function of the irradiation time. For all TiO2 photocatalysts studied, almost linear increase was observed. Since the OH-CCA was measured in the solution, the fact that the amount of OH-CCA is proportional to the irradiation time indicates that equilibrium of OH-CCA in the adsorbed state to the free state in solution is rapidly attained. Thus, the rate of •OH generation can be calculated from the OH-CCA formation rate derived from the slope in Figure 2a with the corrections for the adsorption fraction (Table 2) and then for the probing factor of 0.047. In the case of coumarin, as is the case of CCA, the •OH generation rate was calculated from the rate of umbelliferone formation (Figure 2b) with the correction for the adsorption (though almost null; Table 2) and then divided by 0.061 of the probing factor. To calculate the quantum efficiency of the •OH generation, the amount of the absorbed light for each TiO2 powder was estimated from the optical absorption spectra shown in Figure 3, where the absorption was given by the compliment of the diffuse reflectance (1-R) of the powder samples. The irradiance spectrum of the LED used in the present study is also shown in Figure 3. The power of the light absorbed in the photocatalysts was calculated by integrating the irradiance spectrum of LED over the wavelength after multiplying the absorption at each wavelength. Thus, the absorbed light intensities were calculated. The quantum yield was calculated from the •OH generation rate divided by the amount of photons absorbed per unit time which was calculated from the absorbed light intensity and the photon energy of the 365 nm light.

Figure 4. Quantum yields of OH radicals using the different probe molecules, coumarin (blue) and CCA (red), and the fraction of adsorbed CCA (green) for four kinds of TiO2 powders (P25, F1, ST01, and MT-150A).

CCA. The numerical data are shown in Table 2 with the data for the pH-change experiments. As shown in Figure 4, anatase and anatase-contained TiO2 (ST-01, P25, and F1) generated OH radicals in the substantial yields. Since the quantum yield for CCA was much larger than that for umbelliferone, the amount of OH radicals located near the surface was larger than that in the bulk solution. As for rutile type TiO2 (MT-150A), the •OH yields were extremely small for both on the surface and in the bulk solution, indicating that OH radicals are scarcely generated for rutile through photocatalytic reactions. Therefore, in photocatalysis with mixed phase TiO2, most of the OH radicals are suspected to be generated on the surface of anatase TiO2. These observations are well consistent with our previous results.12 Kinetic Analysis of OH Radicals. In order to analyze the reaction rate of OH radicals with the probe molecules, we investigated the competing reaction with I−. The probe reactions with CCA and coumarin can be expressed by eqs 1 and 2, with rate constants kc and ku, respectively. Iodide ions, I−, react with OH radicals with the rate constant of kI (eq 3). •OH + CCA → OH − CCA 10827

kc

(1)

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832

The Journal of Physical Chemistry C •OH + coumarin → umbelliferone •OH + I− → HOI−

Article

ku

concentrations of the probed molecules have been corrected for the adsorption fraction. The linear line in Figure 5a for the CCA experiment has a slope of 0.811 nM−1 mM−1 with the intersection of 0.002 nM−1 or [OH-CCA]0 = 500 nM. By adopting the values of [CCA] = 0.1 mM and kI = 1.1 × 1010 M−1 s−1,24,25 kc was calculated to be kc = 2.7 × 108 M−1 s−1. In the same way, from the plot for coumarin in Figure 5b, ku was calculated to be ku = 1.3 × 109 M−1 s−1 with [umbelliferone]0 = 60 nM. The value of ku obtained in the present study is consistent with that (1.5 × 109 M−1 s−1) obtained for visiblelight-responsive photocatalysts.15 The obtained rate constant of kc was smaller than ku by a factor of about five. This difference is attributable to the high local concentration of CCA at TiO2 surface. We could assume that I− ions are homogeneously distributed in solution without distinct adsorption on TiO2 and that the distribution of OH radicals was not changed by replacing coumarin with CCA. The CCA concentration in the bulk solution decreased by the adsorption as shown in Figure 1a. In the competitive reaction with I−, the effective concentration of CCA was significantly decreased from 0.1 mM. Therefore, actual value for kc must be larger than that estimated above and compatible to ku. The high concentration of [OH-CCA]0, as compared to [umbelliferone]0, indicated that the •OH concentration near the TiO2 surface was higher than that in the bulk solution, as discussed above. Without reactant, OH radicals decay by dimolecular reaction to form H2O2 (eq 6) with the rate constant kd of 5.5 × 109 M−1 s−1 (=5.5 nM−1 s−1).24

(2)

kI

(3)

Therefore, when CCA was used as a probe molecule, reaction 1 competes with reaction 3 on the addition of I−. Since the reaction of OH radicals obeys pseudo-first-order reaction, the OH-CCA formation rate (r) in the presence of I− is given by the following equation: r = r0k C[CCA]/(k C[CCA] + kI[I−])

(4)

where r0 represents the OH-CCA formation rate in the absence of I−, and [CCA] and [I−] express the concentrations of CCA and I− in the solution. The concentration of the product [OHCCA] was increased linearly with the irradiation time even in the presence of I−. It means that the concentration [OH-CCA] at a given irradiation time is proportional to the reaction rate r. Therefore, r and r0 in eq 4 can be replaced by [OH-CCA] and [OH-CCA]0, respectively, where [OH-CCA]0 represents the concentration of OH-CCA produced in the absence of I− at the given irradiation time. Then, by taking the reciprocal of both terms, eq 4 can be converted to the following equation (eq 5). 1/[OH‐CCA] = 1/[OH‐CCA]0 + (kI/k C[CCA][OH‐CCA]0 )[I−]

(5)

−

Therefore, the plot of 1/[OH-CCA] against [I ] could be fit with a linear line and the values of kI/kc and [OH-CCA]0 can be obtained from the slope and intersection of the plot. Figure 5 shows the plot of data for the experiments of 150 s irradiation with (a) CCA and (b) coumarin, in which the

•OH + •OH → H 2O2

kd

(6)

By assuming that the decay rate is equal to the generation rate in the steady-state, the •OH concentration under the UV irradiation in the absence of probe molecules can be estimated. From the •OH generation rate for P25 TiO2 shown in Table 2, the •OH concentrations are calculated to be 3.2 nM (=(56 nM s−1/5.5 nM−1 s−1)1/2) and 1.3 nM (=(10.2 nM s−1/5.5 nM−1 s−1)1/2) near the surface and in the bulk solution, respectively. Then, the half-life in the dimolecular decay of •OH can be estimated by (kd [•OH])−1 to range from 57 to 134 ms, provided that •OH is free from any reaction with other chemical species including the probe molecules. Effect of H2O2. In the generation of OH radicals in photocatalytic reaction, H2O2 is likely the reaction intermediate.11 Therefore, the effect of H2O2 on the OH radical generation was investigated. Figure 6 shows the effect of the addition of H2O2 on the formation rate for (a) OH-CCA, (b) umbelliferone, and (c) the ratio of (a) to (b). As concerns the ratio of the •OH concentration, the values in (c) should be reduced with the factor of 0.75 (=4.7/6.1%) for the difference of the probing factor. As shown in Figure 6a,b, with the addition of H2O2, the generation rates of OH radicals detected by CCA and coumarin were increased for all the TiO2 except for pure anatase TiO2 (ST-01). For rutile TiO2 (MT-150A), even though the •OH generation rate was very low, it increased in the presence of H2O2. On the other hand, for pure anatase TiO2 (ST-01), though a significant amount of OH radicals was produced in the absence of H2O2, the addition of H2O2 depressed the •OH generation. This considerable observation could be seen for other pure anatase TiO2 powders, as reported in the previous study. 12 This difference indicates that the generation

Figure 5. Relationships between (a) 1/[OH-CCA] and [I−] and (b) 1/[coumarin] and [I−] for P25 TiO2 suspensions irradiated with the 365 nm LED for 150 s. The suspensions contain 0.1 mM of (a) CCA and (b) coumarin, respectively. 10828

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832

The Journal of Physical Chemistry C

Article

mM)), which is shorter by 3 orders of magnitude than the free half-time of OH radicals estimated above. Therefore, the small concentration of H2O2 is effective to deactivate OH radicals in solution. This consideration suggests that, though the addition of H2O2 may increase the •OH concentration, a higher amount of H2O2 decreases the •OH concentration in bulk solution by contraries and then photocatalytic activity may also be decreased. Effect of pH. The adsorption of CCA and the generation of OH radicals may be changed in different pH environments, because the surface charge of TiO2 is known to be changed with pH of the solution. Since the pH values of aqueous solutions of 0.1 mM CCA and 0.1 mM coumarin were 4.4 and 5.9, respectively, at first the pH values for the CCA and coumarin solutions were adjusted to 5.9 and 4.4, respectively. Furthermore, pH 9.0 was also selected to investigate the effect of the negative surface charge because the isoelectric point of these TiO2 was around 6.3−6.6 (Table 1). The fraction of adsorbed probe molecules and the generation rate of OH radicals were measured at these pH values and shown in Table 2. Table 2 shows that the adsorption property of CCA and coumarin depends on the kinds of TiO2. With the addition of KOH to CCA solution to change the pH from 4.4 to 5.9, the adsorptivity for ST-01 and MT-150A decreased, while for P25 high adsorptivity was retained. Since, at pH 9.0, the surfaces of all TiO2 and CCA are negatively charged, the adsorptivity must decrease due to the electrostatic repulsion. It is noted that, by adding H2SO4 and KOH, the surface of P25 TiO2 became to adsorb coumarin as shown in Table2. P25 TiO2 may have a unique adsorptivity. As for the •OH yield of anatase TiO2 (ST-01), at acidic solution, the higher OH radical yield was obtained. OH radicals are considered to be in an equilibrium with the holes trapped at the TiO2 surface.15 At the surface of anatase TiO2, holes might be trapped in the form of Ti−O•, which could interact with proton to generate the adsorbed OH radicals Ti+(•OH), as represented by eq 8.

Figure 6. Effect of the addition of 0.14 mM H2O2 on the formation rates of (a) OH-CCA, (b) umbelliferone, and (c) the ratio of the formation rate of umbelliferone (b) to that of OH-CCA (a) with the effect of 0.14 mM H2O2. In the experiment, 3.5 mL of aqueous solutions containing 15 mg of various TiO2 photocatalysts with 0.1 mM of (a) CCA and (b) coumarin were irradiated with the UV light.

Ti − O • + H+ ↔ Ti+(•OH)

If this is the actual case for the OH radical generation, a higher pH with low H+ concentration would not be favorable to produce OH radicals. Then, the higher yield at the acidic condition could be explained. For rutile TiO2 (MT-150A), the •OH yield increased at pH 9.0, though the yield was low. Different from anatase, the conduction band potential of rutile TiO2 is close to the reduction potential of O2, which is independent from pH change.26 Thus, the increase in OH radical formation with pH could be explained by the shift of the conduction band potential, at which O2 is reduced to form •O2− (eq 9).

mechanism of OH radicals is different at different crystalline surfaces, as will be discussed later. Figure 6c shows the effect of 0.14 mM H2O2 on the ratio of the formation rates of umbelliferone to OH-CCA. Since OH radicals are formed at the TiO2 surface and diffuse to the bulk solution with some decrease, the ratio was less than 1.0 for all kinds of TiO2 powders with and without H2O2. Provided that the OH radicals are deactivated by dimolecular reaction forming H2O2 (eq 6), the deactivation becomes faster with the higher concentration of OH radicals. Therefore, with the addition of 0.14 mM H2O2, the ratio of the rates may be decreased by the increase of the •OH concentration. However, since the ratio of the rates for anatase TiO2 also decreased with H2O2, even though the •OH concentration was decreased, another explanation must be introduced. The observation in Figure 6c is attributable to the reaction of OH radicals with H2O2 (eq 7) in solution, whose rate constant kH is 2.7 × 107 M−1 s−1.24 •OH + H 2O2 → •HO2 + H 2O

kH

(8)

O2 + e−(TiO2 ) → •O2−

(9)

Thus, if electrons on conduction band are consumed in this way, electron−hole recombination would be reduced, and the oxidation of water by holes is promoted to generate OH radicals. Mechanism of •OH Generation. According to the investigation on the photoelectrochemical oxidation process of water with rutile single crystals,16 the OH radical is generated via Ti-peroxo (Ti-OO-Ti) as a byproduct of the O2 generation. The Ti-peroxo is produced from two trapped holes and equivalent to the adsorbed H2O2.27 Therefore, this mechanism

(7)

The half-life of OH radicals in the reaction with 0.14 mM H2O2 is calculated to be 0.18 ms (=0.693/(2.7 × 107 M−1 s−1 × 0.14 10829

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832

The Journal of Physical Chemistry C

Article

of •OH generation can explain the present observation that the addition of H2O2 increased the •OH generation rate for rutile (MT-150A) and rutile-contained TiO2 (P25 and F1). On the other hand, for anatase TiO2, the •OH generation could not be explained by this reaction mechanism, because the addition of H2O2 decreased the •OH generation, as shown in Figure 6 for ST-01 TiO2 and in our previous studies for ST01,28 ST-21, and AMT-600 TiO2.12 In the previous research,12 the amount of •O2−, which is formed by the oxidation of H2O2 or the reduction of O2, was measured for several kinds of TiO2 powders. Then, we found that the amount of •O2− was significantly increased for anatase than rutile TiO2 with the addition of H2O2. This explains that, with the addition of H2O2, the generation of OH radicals at the anatase surface was replaced by the oxidation of H2O2 to form •O2−, as illustrated in Figure 7.

increase in the oxidation of H2O2 at V.B., reduction of O2 at C.B. to form •O2− was increased.12 For rutile TiO2, because the surface is preferable for the O2 generation, H2O2 was easily oxidized to O2 and then OH radical was produced as a byproduct in the generation process of O2.16 This increase in the oxidation of H2O2 to form O2 would also accelerate the reduction of O 2 for rutile TiO 2 .12 However, the • O 2− generation was smaller than anatase TiO2,12 because •O2− was produced preferably by the oxidation of H2O2 at anatase surface as stated above. In our previous report,11 the •OH generation by the reduction of H2O2 (eq 10) with conduction band (C.B.) electrons has been proposed for rutile TiO2 based on the reduction potential of H2O2. H 2O2 + e−(TiO2 ) → •OH + OH−

(10)

However, the increase of •O2− with H2O2 was significantly larger than that of •OH generation,12 indicating that H2O2 is predominantly oxidized by valence band holes to promote the reduction of O2 to generate •O2− by C.B. electrons. The •OH generation rate became constant at higher H2O2 concentrations,12 which does not fit the reaction of eq 10. Therefore, the increase of the OH radicals with H2O2 for rutile TiO2 may not be attributable to the reduction of H2O2. The increase must be attributed to the catalytic effect of adsorbed H2O2 for •OH generation.16 Thus, the photocatalytic mechanism previously proposed by our research group11 should be replaced by Figure 7 in which the conduction band electrons reduce only O2 but not H2O2. Based on the above discussions, the detailed generation mechanism of OH radicals on anatase and rutile TiO2 surfaces can be proposed as shown in Figure 8. On the anatase surface,

Figure 7. Photocatalytic processes at the conduction band (C.B.) and the valence band (V.B.) of TiO2 with anatase and rutile crystalline types in the absence and the presence of H2O2. The thickness of arrows expresses the degree of the reaction rate.

In Figure 7, the reaction process proposed in the present study is summarized. On the surface of anatase, photoinduced valence band (V.B.) holes were trapped to form a surface radical, Ti−O•, which can be assumed to a kind of adsorbed • OH (eq 8) and equilibrated with free •OH in bulk solution.15 Thus, the significant amount of •OH at anatase TiO2 surface was observed. However, the surface of rutile TiO2 is different from anatase TiO2, because water could be oxidized to O2 only for rutile TiO2.29 At the surface of rutile TiO2, water was oxidized to H2O2 and then further oxidized to O2.27 In the generation process of O2, a certain amount of OH radicals could be produced as reported previously.16 On the other hand, in the reduction process at C.B., O2 is reduced to •O2− and it may become H2O2 by disproportionation.12 When H2O2 was added into these reaction systems, H2O2 was favored to be adsorbed on the surface, especially for rutile TiO2. As shown in the right side in Figure 7, for anatase TiO2, the formation of surface Ti−O• was suppressed by the adsorption of H2O2, or H2O2 reacted with Ti−O• to form • O2−. Therefore, the •OH generation was suppressed with 0.14 mM H2O2, as shown in Figure 6a,b. Associated with the

Figure 8. Plausible mechanisms of •OH generation at anatase TiO2 (a−d) and rutile TiO2 (e−h). Surface Ti-peroxo (g) corresponds to H2O2 adsorbed on the TiO2 surface.

photogenerated valence band holes, h+, are trapped at the surface oxygen to form (b) trapped holes (Ti−O•) that can be regarded as the adsorbed •OH in the deprotonated form (•O−),15 then an OH radical is released (c). On the other hand, for rutile TiO2, since the crystalline structure is packed more tightly than that for anatase, the stability of the surface trapped holes may be different. According to the reaction mechanism of water oxidation,27 Ti-peroxo (g) is formed by trapping of h+ near the trapped hole (f). As discussed previously16 for the 10830

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832

The Journal of Physical Chemistry C

Article

(11) Hirakawa, T.; Nosaka, Y. Properties of O2•− and OH• Formed in TiO2 Aqueous Suspensions by Photocatalytic Reaction and the Influence of H2O2 and Some Ions. Langmuir 2002, 18, 3247−3254. (12) Hirakawa, T.; Yawata, K.; Nosaka, Y. Photocatalytic Reactivity for O2− and OH• Radical Formation in Anatase and Rutile TiO2 Suspension as the Effect of H2O2 Addition. Appl. Catal., A 2007, 325, 105−111. (13) Grela, M. A.; Coronel, M. E. J.; Colussi, A. J. Quantitative SpinTrapping Studies of Weakly Illuminated Titanium Dioxide Sols. Implications for the Mechanism of Photocatalysis. J. Phys. Chem. 1996, 100, 16940−16946. (14) Nosaka, Y.; Komori, S.; Yawata, K.; Hirakawa, T.; Nosaka, A. Y. Photocatalytic •OH Radical Formation in TiO2 Aqueous Suspension Studied by Several Detection Methods. Phys. Chem. Chem. Phys. 2003, 5, 4731−4735. (15) Zhang, J.; Nosaka, Y. Quantitative Detection of OH Radicals for Investigating the Reaction Mechanism of Various Visible-Light TiO2 Photocatalysts in Aqueous Suspension. J. Phys. Chem. C 2013, 117, 1383−1391. (16) Nakabayashi, Y.; Nosaka, Y. OH Radical Formation at Distinct Faces of Rutile TiO2 Crystal in the Procedure of Photoelectrochemical Water Oxidation. J. Phys. Chem. C 2013, 117, 23832−23839. (17) Gerald, L. N.; Jamie, R. M. Fluorescence Detection of Hydroxyl Radicals. Radiat. Phys. Chem. 2006, 75, 473−478. (18) Dainton, F. S.; Watt, W. S. The Effect of pH on the Radical Yields in the γ-Radiolysis of Aqueous Systems. Nature 1962, 195, 1294−1296. (19) Louit, G.; Foley, S.; Cabillac, J.; Coffigny, H.; Taran, F.; Valleix, A.; Renault, J. P.; Pin, S. The Reaction of Coumarin with the OH Radical Revisited: Hydroxylation Product Analysis Determined by Fluorescence and Chromatography. Radiat. Phys. Chem. 2005, 72, 119−124. (20) Doong, R.-A.; Chang, W.-H. Photoassisted Titanium Dioxide Mediated Degradation of Organophosphorus Pesticides by Hydrogen Peroxide. J. Photochem. Photobiol., A 1997, 107, 239−244. (21) Li, X.; Chen, C.; Zhao, J. Mechanism of Photodecomposition of H2O2 on TiO2 Surfaces under Visible Light Irradiation. Langmuir 2001, 17, 4118−4122. (22) Vorostov, A. V.; Sainov, E. V.; Davydov, L.; Smirniotis, P. G. Photocatalytic Destruction of Gaseous Diethyl Sulfide over TiO2. Appl. Catal., B 2001, 32, 11−24. (23) San, N.; Hatipogle, A.; Kocturk, G.; Cinar, Z. Prediction of Primary Intermediates and the Photodegradation Kinetics of 3Aminophenol in Aqueous TiO2 Suspensions. J. Photochem. Photobiol., A 2001, 139, 225−232. (24) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical Review of Rate Constant for Reaction of Hydrated Electrons, Hydrogen Atoms and OH Radicals (•OH/•O−) in Aqueous Solution. J. Phys. Chem. Ref. Data 1988, 17, 513−886. (25) Jayson, G. G.; Parsons, B. J.; Swallow, A. J. Some Simple, Highly Reactive, Inorganic Chlorine Derivatives in Aqueous Solution. Their Formation Using Pulses of Radiation and Their Role in the Mechanism of the Fricke Dosimeter. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1597−1607. (26) Nosaka, Y.; Takahashi, S.; Sakamoto, H.; Nosaka, A. Reaction Mechanism of Cu(II)-Grafted Visible-Light Responsive TiO2 and WO3 Photocatalysts Studied by Means of ESR Spectroscopy and Chemiluminescence Photometry. J. Phys. Chem. C 2011, 115, 21283− 21290. (27) Imanishi, A.; Okamura, T.; Ohashi, N.; Nakamura, R.; Nakato, Y. Mechanism of Water Photooxidation Reaction at Atomically Flat TiO2 (Rutile) (110) and (100) Surfaces: Dependence on Solution pH. J. Am. Chem. Soc. 2007, 129, 11569−11578. (28) Oguma, J.; Kakuma, Y.; Murayama, S.; Nosaka, Y. Effects of Silica Coating on Photocatalytic Reactions of Anatase Titanium Dioxide Studied by Quantitative Detection of Reactive Oxygen Species. Appl. Catal., B 2013, 129, 282−286. (29) Ohno, T.; Sarukawa, K.; Matsumura, M. Photocatalytic Activities of Pure Rutile Particles Isolated from TiO2 Powder by

detailed mechanism, OH radical is produced by h+ from H2O (h) with Ti-peroxo, which plays the role of a catalyst. Since Tiperoxo, Ti-OO-Ti, (g) is equivalent to the adsorbed H2O2,30 the increase of the •OH generation with H2O2 for rutile TiO2 can be explained.

■

CONCLUSIONS In the present research, we could conclude the following items: 1 Coumarin-3-carboxylic acid (CCA) was successfully employed to detect OH radicals located near the surface of TiO2 photocatalysts under UV light irradiation. 2 Effect of the addition of H2O2 revealed that the OH radical generation mechanism was different between anatase TiO2 and rutile-contained TiO2 powders. 3 In combined with the results of our previous investigations for •O2− generation, we could conclude that OH radicals are formed from the trapped holes for anatase TiO2, while for rutile TiO2 the Ti-peroxo site plays the role of catalyst to generate OH radicals from water. 4 In the generation process of OH radicals in TiO2 photocatalysis, reduction of H2O2 by conduction band electrons should be excluded.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +81-258-47-9315. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dr. Atsuko Y. Nosaka for the valuable comments on the manuscript preparation and Miss Wenjing Yang for the measurement of isoelectric point of the TiO2 powders.

■

REFERENCES

(1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (2) Henderson, M. A. A Surface Science Perspective on TiO2 Photocatalysis. Surf. Sci. Rep. 2011, 66, 185−297. (3) Pichat, P., Eds. Photocatalysis and Water Purification; Wiley-VCH: Weinheim, Germany, 2013. (4) Fujishima, A.; Zhang, X.; Tryk, D. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (5) Carp, O.; Huisman, C. L.; Reller, A. Photoinduced Reactivity of Titanium Dioxide. Prog. Solid State Chem. 2004, 32, 33−177. (6) Ryu, J.; Choi, W. Substrate-Specific Photocatalytic Activities of TiO2 and Multiactivity Test for Water Treatment Application. Environ. Sci. Technol. 2008, 42, 294−300. (7) Herrmann, J.- M. Fundamentals and Misconceptions in Photocatalysis. J. Photochem. Photobiol., A 2010, 216, 85−93. (8) Teoh, W. Y.; Scott, J. A.; Amal, R. Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors. J. Phys. Chem. Lett. 2012, 3, 629− 639. (9) Turchi, C. S.; Ollis, D. F. Photocatalytic Degradation of Organic Water Contaminants: Mechanisms Involving Hydroxyl Radical Attack. J. Catal. 1990, 122, 178−192. (10) Gerischer, H.; Heller, A. The Role of Oxygen in Photooxidation of Organic Molecules on Semiconductor Particles. J. Phys. Chem. 1991, 95, 5261−5267. 10831

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832

The Journal of Physical Chemistry C

Article

Dissolving the Anatase Component in HF Solution. J. Phys. Chem. B 2001, 105, 2417−2420. (30) Nakamura, R.; Nakato, Y. Primary Intermediates of Oxygen Photoevolution Reaction on TiO2 (Rutile) Particles, Revealed by in Situ FTIR Absorption and Photoluminescence Measurements. J. Am. Chem. Soc. 2004, 126, 1290−1298.

10832

dx.doi.org/10.1021/jp501214m | J. Phys. Chem. C 2014, 118, 10824−10832