Generation and Detection of Reactive Oxygen Species in

Review pubs.acs.org/CR

Generation and Detection of Reactive Oxygen Species in Photocatalysis Yoshio Nosaka* and Atsuko Y. Nosaka Department of Materials Science and Technology, Nagaoka University of Technology Nagaoka 940-2188, Japan ABSTRACT: The detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), i.e., superoxide anion radical (•O2−), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH) in photocatalysis, were surveyed comprehensively. Consequently, the major photocatalyst used in heterogeneous photocatalytic systems was found to be TiO2. However, besides TiO2 some representative photocatalysts were also involved in the discussion. Among the various issues we focused on the detection methods and generation reactions of ROS in the aqueous suspensions of photocatalysts. On the careful account of the experimental results presented so far, we proposed the following apprehension: adsorbed •OH could be regarded as trapped holes, which are involved in a rapid adsorption−desorption equilibrium at the TiO2−solution interface. Because the equilibrium shifts to the adsorption side, trapped holes must be actually the dominant oxidation species whereas • OH in solution would exert the reactivity mainly for nonadsorbed reactants. The most probable routes of generating intrinsic ROS at the surfaces of two polymorphs of TiO2, anatase and rutile, were discussed along with some plausible rational reaction processes. In addition to the four major ROS, three ROS, that is organic peroxides, ozone, and nitric oxide, which are less common in photocatalysis are also briefly reviewed. 5.1.6. Other Reactions Specific to 1O2 5.2. Generation Process of 1O2 5.3. Photocatalytic Reaction with 1O2 6. Hydroxyl Radicals (•OH) 6.1. Detection Methods of •OH 6.1.1. Laser-Induced-Fluorescence (LIF) Method 6.1.2. Direct ESR Observation 6.1.3. Spin Trapping ESR Observation 6.1.4. ESR Observation of the Decay of Radicals 6.1.5. Fluorescent Products by •OH 6.1.6. Chemiluminescence Reaction with •OH 6.1.7. Analysis of •OH Reaction Product 6.2. Generation Processes of •OH 6.2.1. Surface Trapped Holes or Adsorbed •OH 6.2.2. Terminal OH and Bridged OH 6.2.3. Anatase and Rutile TiO2 6.2.4. Mixed-Phase TiO2 6.2.5. With Non-TiO2 Photocatalysts 7. Reaction Mechanisms with ROS in Photocatalysis 7.1. Rational Model of ROS Generation Processes for TiO2 7.2. Oxidation Mechanism in TiO2 Photocatalysis 7.3. Other Minor ROS (ROO•, O3, and •NO) 7.3.1. Formation of Organic Peroxide (ROO•) 7.3.2. Oxidation Process with Ozone

CONTENTS 1. Introduction 2. Reactive Oxygen Species and Redox Properties 2.1. Electronic States of Reactive Oxygen Species 2.2. Redox Properties of Reactive Oxygen Species 3. Superoxide Radical (•O2−) 3.1. Detection Methods of •O2− 3.1.1. Optical Absorption of •O2− 3.1.2. Direct Observation of •O2− by ESR 3.1.3. Detection with an ESR Spin Trapping Method 3.1.4. Color Reaction with Tetrazorium 3.1.5. Chemiluminescence Reaction with •O2− 3.2. Generation Process of •O2− 3.3. Reaction Process of •O2− 4. Hydrogen Peroxide (H2O2) 4.1. Detection Methods of H2O2 4.1.1. Direct Optical Absorption 4.1.2. Coloration Methods 4.1.3. Fluorescent Products by H2O2 4.1.4. Chemiluminescence Reaction with H2O2 4.2. Generation Process of H2O2 4.3. Reaction of H2O2 in Photocatalysis 5. Singlet Oxygen (1O2) 5.1. Detection Methods of 1O2 5.1.1. Direct Light Emission by 1O2 5.1.2. Direct ESR Detection 5.1.3. ESR Detection with Probe Reagents 5.1.4. Chemiluminescence Reaction with 1O2 5.1.5. Fluorescent Products by 1O2 © 2017 American Chemical Society

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Received: March 21, 2017 Published: August 4, 2017 11302

DOI: 10.1021/acs.chemrev.7b00161 Chem. Rev. 2017, 117, 11302−11336

Chemical Reviews 7.3.3. Generation and Decomposition of Nitric Oxide (•NO) 8. Conclusions and Perspective Author Information Corresponding Author ORCID Notes Biographies References

Review

detection methods on the use of the individual ROS and to elucidate their actual functions in photocatalysis are prerequisite. Though the importance of ROS involved in the photocatalytic reaction mechanism has been sometimes much emphasized in the literature, the actual contributions of each ROS have not been properly ascertained and the relevant references have not been always adequately referred. In most reports, the possible pathways involving ROS as reaction species have been provisionally proposed for the redox reactions in photocatalysis. However, one should keep in mind that the actual reactions which could take place at the surface must be restricted by the specific adsorption and electric charges. With this concept, we have been intensively studying the detection techniques along with the behaviors of the ROS in photocatalysis. In this review, we surveyed detection methods and generation mechanisms proposed for the individual ROS in photocatalysis, which appeared in the literature published in these two decades, and pointed out the specific problems on researching the heterogeneous photocatalytic systems. Most of the reports in this review concern ROS in aqueous suspensions of photocatalyst powders. However, in some cases, reactions under gaseous conditions which might be different in behaviors from those for aqueous suspensions were involved in the cited literature. The detection of ROS in the presence of the pollutant (reactant) must become a more important issue for practical applications. For •O2− and H2O2 detection after the photocatalytic reaction could be possible, while for 1O2 a direct phosphorescence detection during the photocatalytic reaction would be useful. For the detection of •OH, probing in solution could be distinguished from the surface reaction by perceiving the location of the probe molecules, though this analysis is still under argument. The generation mechanisms of the individual ROS in photocatalysis have not been clearly elucidated yet at the molecular level. Some of them have been presented but are still controversial. For instance, the generation processes of OH radicals which have been regarded as important reactants in the oxidation process in photocatalysis have been suggested. Based on the experimental results obtained with various detecting methods, we conceived that the difference in reactivity between two polymorphs of TiO2, anatase and rutile, could be ascribed to the difference in the adsorption of H2O2. Based on the present reviews and discussions, the most probable routes of generating individual ROS for the anatase and rutile surface were proposed along with some rational reaction processes.

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1. INTRODUCTION Photocatalysis has been recognized as useful owing to the prominent functions of environmental cleaning such as selfcleaning of building materials, antifogging, and antibacterial exertion. The applications of photocatalysis are rapidly growing so far in numerous practical fields, and basic research has also been extensively progressing.1−9 Especially the production of hydrogen fuel by the decomposition of water with solar light, i.e., artificial photosynthesis, is highly expected as a potential application.10−13 The photocatalytic effects are exerted by the redox reactions caused by photoinduced electrons (e−) and holes (h+) generated on the heterogeneous solid surfaces of photocatalysts. Several reactive species are generated through the reactions with the holes and electrons, which are considered to be involved in the actual oxidative and reductive reactions in photocatalysis. Because photocatalysts are practically used with water vapor under aerobic conditions, photocatalysis involving oxygen and water as reaction species is important. The species to which oxygen converts with high reactivity are generally called reactive oxygen species (ROS). Four major ROS are recognized, comprising superoxide anion radical (•O2−), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH). In photocatalysis, the efficiency of the photoinduced degradation varies over a wide range depending on the kinds of semiconductors (photocatalysts) used and the solution conditions employed. Since ROS are primary intermediates of photocatalytic reactions, the identification, the quantification, and the kinetics evaluation of ROS are important in terms of understanding the photodegradation mechanisms, improving the degradation efficiency, and utilizing the various technologies developed for practical applications. In future applications for artificial photosynthesis, the analysis of ROS is also important for understanding the mechanism and improving the efficiency since they are intermediate species in the decomposition of water as well. Evaluation of ROS is also prerequisite in the field of biology. Lipids, proteins, and DNA could be damaged by ROS,14 which causes oxidative stress linking to uncountable pathologies.15−17 ROS act also as signaling molecules in plants18 and in the maintenance of physiological functions.19 Cytotoxicity, genotoxicity, and ecotoxicity of nanomaterials are also caused by the formation of ROS.20−22 ROS are also important for advance oxidation processes in which, for example, organic pollutants in the wastewater are removed. Several comprehensive reviews on the detection techniques in biological systems23−28 and in the wastewater treatments with advanced oxidation processes29,30 have been presented. However, on the application of these detection methods for photocatalysis, some serious problems might arise on taking into account that the photocatalytic reactions proceed on the surface or near the semiconductor solid. Therefore, to ascertain the limitation of the respective

2. REACTIVE OXYGEN SPECIES AND REDOX PROPERTIES 2.1. Electronic States of Reactive Oxygen Species

Molecular oxygen O2 is a relatively stable molecule which occupies 20% of the air components. Among the 16 electrons of the O2 molecule, the electrons having the highest energy reflect the characteristics. Figure 1 shows the electron spins at the highest occupied molecular orbitals for O2. Since the triplet state (3Σg−) of O2 possesses two unpaired electrons on each of the two antibonding π orbitals (πx* and πy*) at the same energy,32 the reactivity is not low though it is the ground state. Among the major ROS, •O2− and H2O2 are formed successively by reducing O2 or by filling the two π* orbitals with an 11303


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Figure 2. Reactive oxygen species generated in the photocatalytic reduction and oxidation steps of oxygen and water. Figure 1. Molecular π* orbitals for O2, 1O2, •O2−, and O22− and the relationship between •O2− and H2O2.

electron, followed by the protonation to each orbital. On further reduction of H2O2, the O−O bond dissociates to form • OH. As shown in Figure 1, the singlet oxygen (1O2) of the 1Δg state, whose energy is higher than that of the 3Σg− state, may be generated by the oxidation of •O2−, though 1O2 is generally produced by the energy transfer from the excited state of photosensitizers to the ground state of O2.33 Among ROS, the chemical structures of •O2−, H2O2, and • OH alter with pH of aqueous solution due to the rapid acid− base equilibrium as shown in eqs 1−3.34 •

O2− + H+ ⇄ •O2 H −

pK a = 4.8 −

H 2O2 + OH ⇄ HO2 + H 2O •

−

• −

OH + OH ⇄ O + H 2O

(1)

pK a = 11.7 pK a = 11.9

(2) (3)

Figure 3. pH dependence of one-electron redox of H2O, H2O2, and O2. Dotted line shows two-electron (2e) process. Calculated from the data in ref 34.

The property of •O2− under neutral conditions is different from that under acidic conditions because the addition of a proton to •O2−, eq 1, occurs at pH 10) where some metal oxides such as SiO2 are dissolved. Lucigenin could be used as a chemiluminescence probe at moderate pH (pH 9).40 Though lucigenin has been used as a probe molecule for •O2− in biological systems,28 it also reacts with H2O2 to produce a dioxetane127 which decomposes to the excited state of N-methyl acridone (Figure 17B). An example of the measurements of H2O2 by chemiluminescence using lucigenin is shown in Figure 18A.40 After stopping UV

Figure 16. Reactions for detecting H2O2 with fluorescence probes (A) p-hydroxyphenylacetic acid (HPA) and (B) dihydrorhodamine 123.

acetic acid using horseradish peroxidase as a catalyst.69,124 The amount of the dimer is analyzed using a fluorescence spectrophotometer at the emission wavelength of 408.5 nm with the excitation at 316.5 nm.124 Another fluorescence probe reagent, dihydrorhodamine 123, was also reported to detect H2O2 in photocatalytic systems.125 Dihydrorhodamine 123 is oxidized to a fluorescent molecule, rhodamine 123, by the reaction with H2O2 and peroxidase as shown in Figure 16B. Since H2O2 is a stable compound, the H2O2 produced in solution can be separated from the photocatalysts and then selective analyses are possible. 4.1.4. Chemiluminescence Reaction with H2O2. As shown in Figure 8, oxidized luminol (L), which is 5-amino-2,3diaza-1,4-naphthoquinone, reacts with H2O2 to generate the excited state of 3-aminophthalate ion, AP*. The reaction for detecting H2O2 is represented in Figure 17A. In the

Figure 18. (A) Time profile of chemiluminescence upon addition of lucigenin after 60 s of UV irradiation. (B) H2O2 concentration evaluated from chemiluminescence intensity as a function of UV irradiation time for aqueous suspension of anatase and rutile TiO2 nanoparticles. Reproduced with correction from ref 40 with permission from the PCCP owner Societies. Copyright 2015 Royal Society of Chemistry.

irradiation on TiO2 powder, lucigenin solution was injected into the suspension. On the injection, chemiluminescence appears and decays in several seconds. The time integral of the chemiluminescence intensity is proportional to the amount of H2O2, which is used to estimate the H2O2 concentration with calibrations. Increase of H2O2 concentration with light irradiation is shown in Figure 18B, which can be measured by changing the irradiation duration.40 Because H2O2 is strongly adsorbed on the surface of the metal oxides,40,114 the quantitative measurements of the accurate amount of the produced H2O2 in TiO2 photocatalysis are considered difficult. 4.2. Generation Process of H2O2

For generation of H2O2, there are two pathways as shown in Figure 2. Those are the two-electron reduction of O2 and the two-hole oxidation of water. Because of the low reactivity of • O2−, the reduction route seems dominant in the H2O2 generation process. For the generation of H2O2 from •O2−, there are two reduction paths. One is the disproportionation of one-electron-reduced species •O2−, as stated above with eq 7, and another is the reduction of •O2− by photoinduced conduction-band electrons, eq 8. For the latter case, the nonadiabatic reaction barrier was estimated to be 0.3 eV based on the density functional calculation for the •O2− adsorbed on anatase TiO2(101).128

Figure 17. Reactions for chemiluminescence detecting H2O2 using (A) luminol with hemoglobin (Hem) and (B) lucigenin.

experiments, the luminol solution is added to the reaction mixture after keeping it for 30 min in the dark to eliminate • O2−.75,92 After 10 min stirring of the mixture, a hemoglobin (Hem) solution is added. Because L is unstable in water as suggested in the reaction scheme (Figure 8), the oxidant Hem must be mixed in at the end. On the addition of Hem, the chemiluminescence is observed. The detection limit of H2O2 by the chemiluminescence method is about 1 nM.75 In place of hemoglobin, K3Fe(CN)6 could be used as an oxidant, and chemiluminescence was detected in a continuous flow system for irradiated TiO2 suspensions.126 The application of luminol

•

O2− + e− + 2H+ → H 2O2

(8)

As shown in Figure 18B, H2O2 generation with anatase is larger than that with rutile in the batch photocatalytic reaction.40 The fact that the •O2− generation for anatase quickly reached a steady state40 suggests that H2O2 should be generated from •O2−. The faster transfer to H2O2 for anatase may be another reason for the observed smaller generation of • O2− for anatase. In the case of the continuous flow 11310


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10−7.131 The quantum yields of H2O2 for anatase TiO2 films in water under the illuminations of 15 mW cm−2 and 50 μW cm−2 were reported to be about 2.8 × 10−4 and 2.1 × 10−3, respectively.125 These values are considerably low compared to the quantum yield of •O2−. Therefore, most of the generated • O2− would be deactivated125 and the major amount of the produced H2O2 could be adsorbed on the surface or consumed by further reduction and oxidation.74 In the photocatalytic oxidation of 2-propanol, where trapped holes are effectively consumed by the reactant, the formation of H2O2 from O2 was nearly stoichiometric against the formation of the oxidation product, acetone.64 The photocatalytic formation of H2O2 with oxidation of alcohol gathers attention as a safe synthetic method of H2O2 production.109 The information on the degradation potential, optimum operation parameters, and generation methods of H2O2 could be provided by the recent review paper132 which describes in situ production of H2O2 in textile-wastewater treatment including photocatalysis.

detection,126 the order of H2O2 generation was reported to be rutile > P25 > anatase, while the •O2− generation was essentially the same on these three kinds of nanoparticles.126 This observation suggests the existence of the H2O2 generation process other than that via •O2− stated above. Namely, the oxidation of H2O to generate H2O2 was suggested. The molecular mechanism of water photooxidation reaction at atomically flat n-TiO2 (rutile) surfaces was investigated in aqueous solutions of various pH values, using photoluminescence (PL) measurements.129 Combining with the IR observation stated above,110 the oxidation process was elucidated as shown in Figure 19. Though in Figure 19 the

4.3. Reaction of H2O2 in Photocatalysis

The generated H2O2 reverts to H2O and O2 by the following disproportionation, which corresponds to the simultaneous two-electron redox reaction. 2H 2O2 → O2 + 2H 2O Figure 19. Plausible oxidation processes at TiO2 surface and adsorption of H2O2 based on nucleophilic water attacking. Reprinted from ref 129. Copyright 2007 American Chemical Society.

H 2O + h+ → •OH + H+ •

2 OH → H 2O2

+

Because the relevant redox potentials are E (H2O2,2H /2H2O) = 1.763 V and E0(O2,2H+/H2O2) = 0.695 V,34 the potential difference of eq 12 is ΔE0(12) = 1.068 V. By doubling the potential difference, the Gibbs energy change is calculated to be ΔG(12) = −2.136 eV = −206 kJ mol−1, which shows a considerably exoenergetic reaction. Because reaction 12 takes place spontaneously on the Pt surface, it may also be the case on the surface of various photocatalysts. Although H2O2 is a stable intermediate in the oxidation of H2O to O2, one-step oxidation of H2O2 generates •O2− as indicated by eq 6. In fact, with the addition of a small amount of H2O2, the generation of •O2− was significantly increased in the TiO2 photocatalytic reaction as shown in Figure 20A40 and our previous study.92 It is noticed that the increase was more remarkable for anatase TiO2.40,92

nucleophilic attack of water to Ti−O−Ti is associated with the [Ti−O• HO−Ti] formation,110 an alternative model was proposed in which the bridging oxygen radicals [Ti−OH• Ti] are photogenerated with the intrinsic band-gap surface state.111 Details of the latter model will be described in section 6.2.2 of • OH generation. In both models, the surface-trapped holes become bridged peroxo (Ti−O−O−Ti) by combining with a hole secondarily generated in the same crystallite.130 The bridged peroxo structure (Figure 14B) is regarded as the intermediate step of the water photooxidation. Thus, the species generated by two-hole oxidation is equivalent to the adsorbed H2O2 (Figure 15B) as evidenced by the FT-IR spectra under the UV irradiation on rutile TiO2 powders.40 Though the two-hole oxidation of H2O generates H2O2 as in eq 9, this oxidation could be separated into two steps: the oxidation of water to generate •OH, eq 10, and the dimerization of the •OH, eq 11. The latter process has been experimentally suggested by detecting the decrease of H2O2 on the addition of organic •OH scavenger under anaerobic condition.112 In this case, however, the radical scavenger may be possibly oxidized by holes and suppress the generation of • OH. Thus, the presence of •OH is not necessarily evidenced. Since the concentration of •OH may be very low due to the low photon density in the common photocatalytic reactions, •OH in eqs 10 and 11 is likely in the state of adsorbed •OH on the TiO2 surface which probably corresponds to the trapped holes as will be discussed later. 2H 2O + 2h+ → H 2O2 + 2H+

(12) 0

Figure 20. Effect of H2O2 on (A) •O2− and (B) •OH generation measured after 60 s of UV irradiation on aqueous suspension of anatase and rutile TiO2 nanoparticles. Reproduced with permission from ref 40. Copyright 2015 Royal Society of Chemistry and PCCP owner Societies.

(9)

On the other hand, since H2O2 is also a stable intermediate in the reduction of O2 to H2O, one-step reduction of H2O2 may generate OH radical as indicated by eq 13.

(10)

H 2O2 + e− → •OH + OH−

(11)

For the H2O2 released from the TiO2 powder to the air, the quantum yields were estimated to be 1.4 × 10−7119 and 1.8 ×

(13)

From the thermodynamic point of view, reaction 13 must be a favorable process for •OH generation because ΔE0(13) = +0.73 11311


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V at pH 7 as shown in Figure 3. In our early report,92 •OH generation was increased with H2O2, only for pure rutile and rutile-containing anatase TiO2 powders. On the other hand, • OH generation was decreased for pure anatase powders with the addition of H2O2. This peculiar phenomenon has been recently confirmed as shown in Figure 20B.40 A similar phenomenon was also reported by other researchers,59 in which the amount of •OH detected by a spin trapping ESR method was increased with H2O2 for the rutile coated hydroxyapatite but it was opposite for the anatase counterpart. Our previous explanation for Figure 20B is as follows.92 In the case of anatase, the decrease of •OH with H2O2 was the result of the oxidation of H2O2 in place of H2O. On the other hand, the fact that the reduction of H2O2 did not generate •OH for anatase may be explained as follows. The reduction of H2O2 generates trapped holes htr+ as in eq 14 and they may rapidly react with trapped electrons as in eq 15 before an htr+ is detected as •OH. Since only the rutile surface takes side-on peroxo structure (Figure 14C),108 reactions 14 and 15 which do not take place for rutile should be specific to end-on and bridged structures of adsorbed H2O2 (Figure 14A,B). H 2O2 + e− → h tr + + 2OH−

(14)

h tr + + e− → TiO2

(15)

Sahel et al.114 showed that, when the photocatalytic degradation rates of H2O2 are compared among several kinds of TiO2, the decomposition rate for the rutile polymorph is faster than that for the anatase. The faster degradation of H2O2 for rutile could be explained by the adsorbed structure which presents yellow color originated from the bridged structure.114 Furthermore, they showed that, on rutile, the decomposition of H2O2 occurs mainly due to its reduction, while on anatase H2O2 is mainly degraded by its oxidation.114 Thus, H2O2 becomes •OH on rutile, while it becomes •O2− on anatase, which is in good agreement with the above perspective. However, the experimental evidence of the higher adsorption of H2O2 on the rutile surface was not supported by the study of theoretical calculation,136 in which the adsorption of H2O2 on TiO2 anatase (101) and rutile (110) surfaces was investigated by first-principle calculations with a density functional theory. In the calculation, the adsorption energy at the surface Ti5C for Ti−OOH is larger for the rutile, while that at the surface O2C− O2C for peroxo is slightly larger for the anatase. This result in the theoretical calculation was supported by scanning tunneling microscopy (STM) assignments, where the peroxo formation on the anatase (101) surface through the adsorption of O2 was reported,137 though the assignments in this report have been corrected by the same research group.138 On the other hand, in the investigation with the hybrid density functional based energy calculations and the first-principle molecular dynamic simulations, a surface-bridging peroxo dimer is composed of one water and one surface lattice oxygen atom, which is consistent with the surface peroxo intermediates revealed by “in situ” measurements on rutile.139 There are numerous arguments about the effect of H2O2 addition on TiO2 photocatalytic reactions. In the photocatalytic oxidation of benzene vapor over TiO2 (P25) in a flow reactor, the addition of gaseous H2O2 into the reactor feed provided the enhanced and sustained oxidation of benzene vapor.140 In the case of photocatalytic oxidation with acetone vapor, the rate of the complete oxidation decreased with an increase in the inlet concentration of gaseous H2O2.140 In the photocatalyzed oxidation of adamantane in butyronitrile/acetonitrile mixture, on the addition of H2O2 to the solution, the activity of rutile powders is remarkably enhanced and becomes much higher than that of anatase powders.141 The enhancement was explained by the availability of side-on adsorption for rutile TiO2. In the case of the dye degradation in aqueous solution there is no evidence on any positive synergy associated with the TiO2−H2O2 combination.142 In the case of the photocatalytic degradation of phenol in the aqueous TiO2 dispersion system, the increase in the decomposition rate with H2O2 was observed for both anatase143 and rutile.144 On the addition of H2O2 in the preparation process, the surface peroxo structure of rutile was produced, which showed high catalytic efficiency in the visible light region.145 Even for amorphous TiO2, the photocatalytic activity was enhanced with H2O2.146 This indicates that the increase in the light absorption with the formation of bridged peroxo (Figure 14B) accelerates photocatalytic reactions in some cases. In biological systems, ROS can not only actually damage cells but also promote fundamental processes including growth differentiation and migration. 147 In terms of possible biomedical applications, the versatile ways in which H2O2 can be activated by metal ions and metal compounds have been reviewed recently.148 It is demonstrated in the review that the direct contact between the bacterial cells and the photocatalyst

Thus, only side-on peroxo (Figure 14C) exerts reaction 13 to generate •OH at the rutile surface. However, the triangle sideon peroxo structure may not be present in aqueous media because of the hydration. Thus, the difference in the reaction of H2O2 for different crystal phases could not be explained by the side-on structure on rutile. In the recent literature, the presence of side-on adsorption in water was not supported, but bridged peroxo (Figure 14B) was considered specific to the rutile surface.114 Then, an alternative explanation will be discussed in section 6.2.3 of •OH generation. H2O2 reacts with •OH as in eq 16 to form •O2−,133 in which ΔG(16) = −0.66 eV = −63.7 kJ mol−1 calculated from ΔE(16) = (2.38 − 1.72) V because E0(•O2−,2H+/H2O2) = 1.72 V. H 2O2 + •OH → •O2− + H 2O + H+

(16)

In this case, the rate constant of eq 16 is k(16) = 3 × 10 M−1 s−1.54 Then, reaction 16 presumably takes place when •OH is formed from H2O2. The produced •O2− may also react with H2O2, which is known as the Haber−Weiss reaction, eq 17. Numerous reports in the literature have proposed that •OH generation in photocatalysis could be attributed to this reaction. 7

H 2O2 + •O2− → •OH + O2 + OH−

(17)

The Gibbs energy change of reaction 17 calculated from E0(H2O2/•OH,OH−) = 0.31 V and E0(O2/•O2−) = −0.33 V is negative as ΔG(17) = −0.64 eV = −62 kJ mol−1. However, in the absence of a redox catalyst such as Fe3+, the bimolecular reaction rate constant is as small as k(17) = 0.13 M−1 s−1.37 Actually, reaction 17 proceeds by two steps with a redox catalyst:134 (i) the formation of Fe2+ by the reduction of Fe3+ with •O2−, and (ii) the Fenton reaction135 of H2O2 with Fe2+ to form •OH. However, the surface ≡Ti4+ of TiO2 could not function as a redox couple like Fe3+ ions in solution. In other words, since •O2− could not form trapped electrons (≡Ti3+) at the surface, reaction 17 proceeds usually too slow in TiO2 photocatalysis. 11312


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from the data accumulation. In the case of the continuous photocurrent mode, the noise in which signal is not contained must be accumulated together. Furthermore, the time-resolved detection allows the temporal discrimination against undesired photoemissions.31 The detection limit could be estimated to be about 2 nM.156 For the photocatalysis with TiO2 powder suspension, quite a few researchers31,91,156−161 reported the detection of 1O2 phosphorescence at 1270 nm. Figure 21 shows the wavelength dependence of the emission intensity accumulated from 1.5 to 6.5 μs after the laser light

is not important because the major reactive species is H2O2, which has a relatively long lifetime and can diffuse into the bulk solution to inactivate bacterial cells.149 In the antibacterial tests against Staphylococcus epidermidis with a layer of bacterial suspension on the resin−TiO2 surfaces, H2O2 was found to be the most efficient ROS component contributing to the antibacterial effect.150 On the other hand, in the antibacterial tests with suspended bacteria and photocatalytic TiO2 nanoparticles, both •OH and H2O2 showed equally significant effects on the bacterial inactivation having a typical Chick−Watson disinfection kinetics behavior with a steady disinfection rate.150 However, in photocatalysis, the generation of ROS components is localized around the surface of the tissues because the excitation light does not invade the inside. Therefore, the effective species among ROS in photocatalytic reactions were not usually specified.151 The inactivation of Escherichia coli is strongly influenced by the presence of curli or the adsorption properties on bacterial membrane, which are affected by solution pH and the particle size.152 Therefore, the role of H2O2 in photokilling mechanism is still under discussion.152

5. SINGLET OXYGEN (1O2) 5.1. Detection Methods of 1O2

Figure 21. Emission spectra observed after laser pulse irradiation at 355 nm for TiO2 (Degussa P25) suspension in water (◇), ethanol (○), and water−ethanol mixture (△). Delay time, gate width, and number of scans at each wavelength were 1.5 μs, 5 μs, and 300, respectively. Reproduced with permission from ref 156. Copyright 2004 Royal Society of Chemistry and PCCP owner Societies.

1

Because singlet oxygen O2 is an excited state of O2, it can be deactivated to the original stable O2 without being involved in chemical reactions or electron transfer. The lifetime of 1O2 (1Δg state) is several tens of milliseconds in air, while it becomes so short as 3 μs in H2O. Because the lifetime is determined by the energy transfer to the vibrational energy levels of the surrounding molecules, it becomes longer in D2O as 68 μs.153 To confirm the presence of 1O2, D2O is sometimes used in place of H2O. The detection methods of 1O2 are categorized as (1) direct emissions (section 5.1.1), (2, 3) electron magnetic resonance (sections 5.1.2 and 5.1.3 ), (4) chemiluminescence (section 5.1.4), and (5) fluorescence probe methods (section 5.1.5). Other reactions specific to 1O2 are discussed in section 5.1.6. 5.1.1. Direct Light Emission by 1O2. Two kinds of emission are known for the observation of 1O2. They are the phosphorescence (1Δg → 3Σg) at 1270 nm in the near-IR region and the dimol emission (21Δg → 23Σg−) occurring at 634 nm.154 The former measurement in the near-IR region is recognized to be the most reliable detection method for 1O2.153 On the other hand, despite the rather long history of the dimol emission, the chemistry behind this phenomenon has not been yet well understood.154 The two types of luminescence from 1 O2 have been observed simultaneously in the reaction of NaOCl with H2O2.155 The quenching properties were different between two kinds of luminescence. Since the intensity of the phosphorescence at 1270 nm is proportional to the amount of 1 O2, it is difficult to regard that the intensity of the dimol emission directly reflects the amount of 1O2. For the phosphorescence at 1270 nm, since the radiation lifetime of the 1Δg → 3Σg transition is as long as 5 s,153 the emission yield in the steady state measurements is at most on the order of 10−7 (≈3 μs/5 s). Therefore, the detection of 1O2 under continuous-light excitation is not easy. On the other hand, in the time-resolved detection after pulsed laser excitation, signals in the time period of the phosphorescence could only be accumulated by using a photon-counting technique. This means that noises at the time period which are free from the desired signal could be completely eliminated

excitation at 355 nm for TiO2 powder suspension.156 Though the fringe of the emission from photocatalyst is also observed near 1270 nm, a distinct broad emission that peaked at 1270 nm was observed. The intensity decreased in ethanol in spite of the longer phosphorescence lifetime (14 μs) than that in H2O (3 μs). The decrease in the 1O2 generation in ethanol solution indicates that the generation process involves photocatalytic oxidation because the photoinduced holes are consumed by ethanol.157 5.1.2. Direct ESR Detection. Though 1O2 possesses no electron spin, it exhibits paramagnetic properties caused by the orbital angular momentum.162 Hence, an ESR spectrometer with a microwave frequency of about 9 GHz (X-band) could be used to observe the quartet signal of 1O2 in the gas phase at 950 mT.162 However, this detection method has not been applied yet for 1O2 produced in photocatalysis. 5.1.3. ESR Detection with Probe Reagents. Sterically hindered cyclic amines are known to generate corresponding 1oxyl radical by the reaction with 1O2. Then, they are utilized for the ESR detection of 1O2.163 In the case of 4-hydroxy-2,2,6,6tetramethylpiperidine (HTMP), a well-known stable nitroxide radical (4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, TEMPOL) is formed as shown in Figure 22A. For the detection of 1 O2 in photocatalysis, besides HTMP,164−168 4-oxo-tetramethylpiperidine62,169−173 and tetramethylpiperidine174−177 have been reported. However, 1O2 is not the exclusive reactant for the radical production because •O2−, •OH, and trapped holes could be also the oxidant to produce the 1-oxyl radicals.178 Careful analysis on the addition of several reactants, such as SCN−, I−, methanol, DABCO (1,4-diaza-bicyclo[2.2.2]octane, or triethylene diamine), and H2O2, suggested that TEMPOL should be produced from HTMP by the oxidation with trapped holes in photocatalysis.178 As has been pointed out by 11313


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response to •OH or •O2−.182 However, when it was used under light illumination, significant problems in photosensitized 1 O2 generation and photodecomposition were clearly demonstrated.183 Since most probe reagents shown in the literature83 are fluorescein-based molecules similar to SOSG, these problems are common drawbacks for the photocatalytic application.183 5.1.6. Other Reactions Specific to 1O2. The formations of endoperoxides by the reaction with furans,184 9,10diphenylanthracene,185 and α-terpinenes186 were employed to detect 1O2. Decrease of furfuryl alcohol, a kind of furan, was used as an indicator to estimate the concentration of 1O2 generated in photocatalysis.67 In some cases, the role of 1O2 in photocatalytic reactions was elucidated by the addition of sodium azide (NaN3) which is expected to quench 1O2.187−191 However, azido ion is easily oxidized to N2 and N2O by photocatalysts.192 Hence, the generation of ROS might be suppressed. Though the quenchers, such as azido ion in the case of 1O2, are added aiming to react with the produced ROS, the decrease may not be caused by the reaction with the quencher, but the decrease would actually occur in the generation of ROS on the addition of the quencher. In this case the quencher becomes unspecific to the desired ROS. Thus, though several detection methods for 1O2 in photocatalysis have been reported, the validity of the methods seems not to be clarified yet except for phosphorescence measurements at 1270 nm.153

Figure 22. (A) Photocatalytic reactions of hindered cyclic amine (HTMP) to nitroxide radical (TEMPOL)178 and (B) fluorescence detection of 1O2 with TDI. Reprinted with permission from ref 180. Copyright 2007 Springer Science + Business Media.

Dimitrijevic et al.,179 the detection of 1O2 in photocatalysis by the production of nitroxide radical should be carefully performed. In D2O the rate of TEMPOL formation from HTMP is expected to increase because the lifetime of 1O2 becomes longer by 20-fold than that in H2O. However, the formation rate was actually decreased by the factor of 3,178 which is consistent with the observation by Konaka et al.170 These observations clearly show that the TEMPOL radical was not produced by the reaction with 1O2. Some of the literature165,166 reported that the signal intensity of TEMPOL measured after 16 min irradiation with HTMP was larger for the reaction in D2O. Since the generated TEMPOL radicals decompose by the further photocatalytic reaction,178 the larger TEMPOL signal observed in D2O165,166 would be explained by the slower photocatalytic decomposition of TEMPOL in D2O than in H2O. 5.1.4. Chemiluminescence Reaction with 1O2. The reaction of chemiluminescence probe reagents with 1O2 generates the fluorescent molecules of the excited states. Thus, 1O2 can be sensitively detected by the light emission in the visible light region. In the past, luminol and MCLA were reported as probe reagents for the chemiluminescence detection of 1O2. However, afterward the chemiluminescence reaction has been ascribed to that with •O2− but not with 1O2.83 Hence, novel detecting reagents specific to 1O2 have been developed.83 However, because of the short lifetime, it may be difficult to utilize them after the photocatalytic reaction as chemiluminescence probes for 1O2. 5.1.5. Fluorescent Products by 1O2. 1O2 generated in air has been detected with the terrylene diimide (TDI) derivative which produces fluorescent diepoxide (Figure 22B). TDI was coated on a glass plate and faced to the photocatalyst with an air gap through which 1O2 was diffused. Using a microscope, the detection of single-molecule fluorescence has been reported.180,181 For powder suspension, a singlet oxygen sensor green (SOSG) was employed,177,179 which produces an endoperoxide that fluoresces at 528 nm by the excitation at 488 nm. According to the supplier, SOSG shows no appreciable

5.2. Generation Process of 1O2

Singlet oxygen is usually generated through the energy transfer from the excited dye molecules to the triplet ground state of O2.33 The 1O2 generation mechanism of the energy transfer from the excited photocatalysts was reported for the photocatalysis with modified TiO2.161,189,193 However, the energy transfer process may not contribute to the formation of 1O2 for unmodified TiO2 because the lifetime of the excited state is very short. Namely, the trapping processes of electrons and holes in semiconductor photocatalysts are usually very rapid.194 However, it is not denied that, in the individual photocatalysis, the reactant or reaction intermediates may modify the surface of TiO2 to act as a photosensitizer to produce 1O2.193 Instead of the energy transfer, to produce 1O2 the electron transfer, or the oxidation of •O2−, has been reported.195 It was reported in 1973 that no experimental support could be provided to the suggested use of the •O2− as a source of 1O2.196 However, it was also reported that 1O2 could be produced by the oxidation of •O2− with electrochemically generated ferricenium ions.197 The procedure of the 1O2 generation by the oxidation of •O2− is shown in Figure 1. Because three equivalent electrons in the antibonding π orbitals are present in • O2−, there are three ways for the electrons to be removed on the oxidation, which generate three different electronic states of molecular oxygen.31 Namely, the three kinds of O2 states in the order of the energy are 3Σg−, 1Δg, and 1Σg+. In the oxidation, one of the five states among three triplet states and two singlet states will be selected with the same probability. Since the 1Σg+ state with the highest energy immediately transfers to the 1Δg state,153 the distribution to the singlet state, 1O2(1Δg), is 2 against 3 for that in the triplet state. Thus, on the oxidation of • O2−, the probability of 1O2 generation is 2/5 and that of the triplet ground state (3Σg−) becomes 3/5. For the mechanism of 1O2 generation from •O2−, the Haber−Weiss reaction with H2O2 stated above, eq 17, was 11314


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proposed.198 However, the Gibbs energy change in reaction 17 was ΔG(17) = −0.64 eV as described above, which is much smaller than the excitation energy for 1O2 of 0.98 eV (=1270 nm in wavelength). Therefore, 1O2 could not be generated by the Haber−Weiss reaction, even if the reaction occurred. When •O2− is oxidized with •O2H as in eq 7, the change of Gibbs energy is ΔG(7) = −1.27 eV, because E0(O2/•O2−) = −0.33 V and E0(•O2−,2H+/H2O2) = +0.94 V at pH 7. Thus, the 1 O2 generation reaction given by eq 18 takes place with ΔG(18) = (−1.27 + 0.98) eV = −0.29 eV = −28 kJ mol−1. •

O2− + •O2 H + H+ → 1O2 + H 2O2

anatase and rutile in ethanol was reported to be 0.02.159 According to a recent study, the quantum yields for unmodified and modified TiO2 in the aqueous suspension were reported to be 0.003 and 0.012, respectively.161 Naito et al. reported photocatalytic efficiencies of about 10−8 and 10−9 for 1O2 desorbed into air from photoirradiated films of pure TiO2 and N-doped TiO2, respectively.200 For gold-nanoparticle-deposited TiO2 (AuNP/TiO2), the continuous wave (CW) laser excitation at 532 nm causes the phosphorescence of 1O2 as shown in Figure 23. Though AuNP

(18)

In reaction 18, an intermediate molecule, HOOOOH, has been assumed before generating 1O2.169,199 Though reaction 18 with excess energy is allowed to produce 1O2, the reaction rate constant k(18) = (2/5)k(7) = 2.4 × 1012−pH M−1 s−1 is not high at neutral and alkaline pH. For the oxidation of •O2− with •OH as in eq 19, a large energy difference of ΔG(19) = −1.31 eV = −127 kJ mol−1 is expected from E0(•OH/OH−) = 1.97 V at pH 7. •

O2− + •OH → 1O2 + OH−

(19)

•

Though the concentration of OH may be smaller than that of • O2−, reaction 19 seems a more probable process than eq 18 because the reaction occurs at the diffusion limit, i.e., k(19) = 1.0 × 1010 M−1 s−1.37 Since both •O2− and •OH are generated at the surface of photocatalyst, •OH in eq 19 would be the •OH adsorbed on the surface. The surface adsorbed •OH is regarded as the trapped holes as will be described later. Therefore, •O2− is most probably oxidized by valence band holes, or trapped holes, to generate 1O2 as in eq 20. The analysis of the •O2− decay profile has suggested that the deactivation of •O2− should be mainly caused by the holes at the photocatalyst surface.74 •

O2− + h+ → 1O2

Figure 23. Emission spectra for gold-nanoparticle-deposited anatase TiO2 powder suspension in D2O under continuous-laser irradiation at 532 nm: AuNP/TiO2 (■), bare TiO2 (◆), and AuNP (▲). Reprinted from ref 31. Copyright 2014 American Chemical Society.

solely presents the absorption band at 550 nm, no luminescence peak was observed in the near-IR region. Furthermore, without AuNP deposition, bare TiO2 did not show the 1270 nm emission either. Though the detection of 1 O2 by two-photon excitation at 532 nm with a pulsed laser for bare TiO2 has been reported,160 as shown in Figure 23 the fluorescence of 1O2 was not observed by using a CW laser. With several kinds of TiO2 powders of distinct sizes and crystal phases, the linear correlation between the formations of •O2− and 1O2 was observed for AuNP/TiO2.31 In this system, the excitation of AuNP plasmon band with a 532 nm laser generates conduction band electrons201 which reduce O2 to • O2−. Then, the •O2− is oxidized with the hole remaining in AuNP to form 1O2. The apparent quantum yield is estimated as small as 2 × 10−6.31,202 In the estimation process of the apparent quantum yield of 1 O2, many errors may be accompanied. Those are the evaluation of absorbed photon for both the sample and the reference dye, the difference in the lifetimes of 1O2, scattering of the detected phosphorescence light by powders, and the separation of the phosphorescence signal from that from photocatalyst luminescence or scattered light. Thus, the values of 1O2 quantum yield may largely vary depending on the characteristics of photocatalysts. In the case of Cu(II) modified AuNP/TiO2, the photoinduced charge separation was increased by the modification, resulting in increased yields of both •O2− and 1O2.203 If 1O2 is generated by the dimerization of •O2− as in eq 18, the amount of 1 O2 might be significantly increased at higher •O2 − concentrations. Thus, the fact203 that the amount of 1O2 is linearly correlated with that of •O2− supports the oxidation mechanism with holes, eq 20. When •O2− is formed on TiO2 by the dye sensitization, the trapped holes in eq 20 are absent. Therefore, the dimerization

(20)

1

This O2 generation mechanism is supported by the experiments for various kinds of TiO2 powders, in which the small powders giving a higher yield of •O2− generated a higher amount of 1O2.157 This is also supported by the fact that 1O2 generation was decreased in ethanol157 because of the consumption of holes as suggested above. For CdS and ZnS photocatalysts, the electron transfer from •O2− to valence band holes is also suggested as the 1O2 generation mechanism.62 The quantum yield of 1O2 generation immediately after the excitation was roughly estimated to be 0.2 for TiO2 (Degussa P25)156 from the comparison with the experiment of rose bengal solution after the correction of the estimated excitation intensity. For rutile TiO2 of 2−4 nm particle size in chloroform, the quantum yield with two-photon excitation by the excitation at 532 nm was reported to be 0.23, while that with one-photon excitation at 355 nm was reported to be 0.22.160 The similar quantum yields for both excitations indicate that the simple excitation from the valence band to the conduction band is essential and that the possible interband state does not contribute to the 1O2 generation. Since the quantum yield of • O2− has been reported79 to be 0.8 as described above, when 2/ 5 of the •O2− transfers to 1O2 by the oxidation, the quantum yield of 1O2 could become possibly 0.32. In the comparison of anatase with rutile TiO2, the generation rate of 1O2 for anatase was reported 15-fold faster than that of rutile though they were detected with 9,10-diphenylanthracene.185 On the other hand, the quantum yield of 1O2 for both 11315


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process of eq 18 may contribute to the generation of 1O2,179 or the oxidized dye could react with •O2−. In this case, however, one should be cautious that the excited dye could generate 1O2 by the usual mechanism of the energy transfer from the dye to the triplet state of O2, independently from the photocatalyst. The generated •O2− decays by the bimolecular reaction for N- and S-doped TiO275 while it decays by the reaction with trapped holes for bare TiO2 74 as described above. For N-doped TiO2 under visible light irradiation, the release of 1O2 from the surface was reported.200 In this case the 1O2 generation mechanism may be the dimerization as given by reaction 18, though the oxidation of •O2− with trapped holes was suggested as the formation mechanism of 1O2.200 Since the •O2− in solution was detected but the adsorbed •O2− was not, the 1O2 generated at the surface could not be correlated with the observed decay mechanism of •O2−.

electrons and holes through the surface adsorbed O2, which has been suggested by Du et al.206 with the kinetic analysis. Besides photocatalytic systems, in biological research, 1O2 is recognized to be especially important in the reaction with unsaturated lipids in plasma membranes, resulting in compromised functions often associated with cell necrosis.199 In addition, bacteria inactivation,165,167,177 direct damage in plasmid DNA, and cytotoxicity172 have been also reported to be caused by 1O2. Furthermore, 1O2 also takes part in photoyellowing of high-lignin-content materials.171 However, the 1O2 generated in photocatalysis could not oxidize nicotinamide adenine dinucleotide which has no affinity to the TiO2 surface.159 The photocatalytic generation of 1O2 was enhanced in the phospholipid membrane, while some water-soluble protein effectively scavenged the generated 1O2. These findings suggest that 1O2 generated by TiO2 should contribute to the phototoxic effect through the oxidation of the membrane protein.159 The diffusion length of 1O2 within the lifetime (τ = 3 μs) was calculated to be 0.11 μm by using the equation (2Dτ)1/2 with the diffusion constant D of O2 (i.e., D = 2 × 10−5 cm2 s−1) in water.207 Since the long-lived ROS other than 1O2 are concurrently produced in photocatalysis, it was suggested that 1O2 should not necessarily react with the biological unit, due to the short diffusion length.153

5.3. Photocatalytic Reaction with 1O2

Singlet oxygen exhibits high reactivity to some organic compounds such as olefin and amine.204 However, because it is an excited state, the reactivity might quench by the energy transfer without being involved in any chemical reaction. Though the intrinsic lifetime of 1O2 is more than seconds, in aqueous solution the actual lifetime becomes several microseconds. This means that, when singlet oxygen cannot react within microseconds, the singlet oxygen cannot take part in reactions in solution. On the other hand, the other ROS, which are •OH, •O2−, and H2O2, decay actually by some reactions including reduction or oxidation. When 1O2 is generated from •O2− by oxidation with holes, the produced 1O2 locates on the surface of the photocatalyst. Therefore, the solid photocatalyst may quench the 1O2. Quenching of 1O2 by TiO2 particles was investigated for dye sensitized systems. With TiO2 nanoparticles of 2−4 nm size, Li et al.160 reported the quenching rate constant to be 2.4 × 109 M−1 s−1, indicating that 1O2 quenches at an almost diffusioncontrolled rate. On the other hand, we reported205 that the quenching rate constant was increased with the size of TiO2 and that the intrinsic rate constant extrapolated to the size of zero was 1.0 × 108 M−1 s−1. Since the bimolecular quenching rate constant is about 1/100 of the diffusion limit rate constant, 1 O2 generated at the surface of TiO2 may be effectively quenched by TiO2 as shown in Figure 24.158 This formation and quenching of 1O2 on the photocatalyst surface may be regarded as the recombination between the photoinduced

6. HYDROXYL RADICALS (•OH) 6.1. Detection Methods of •OH

Among various active oxygen species, the reactivity of •OH is considerably high as represented in Figure 12, where the reaction rate constants of •OH for various compounds are almost those of the diffusion limit. Therefore, OH radical is often regarded as the most effective reactant for photocatalytic decomposition. The detection methods utilized for •OH in photocatalysis are (1) direct fluorescence (section 6.1.1), (2−4) various ESR observations (sections 6.1.2, 6.1.3, and 6.1.4), (5) fluorescence probe (section 6.1.5), (6) chemiluminescence (section 6.1.6), and (7) other methods (section 6.1.7). 6.1.1. Laser-Induced-Fluorescence (LIF) Method. The LIF method is a method of high sensitivity and is utilized to detect a very small amount of OH radicals in the atmosphere. The experimental outline to measure the OH radicals released from irradiated TiO2 is shown in Figure 25A. The intensity of the fluorescence emitted from OH radicals appearing at 310 nm was measured as a function of the excitation wavelength by using a dye laser. The presence of OH radicals was confirmed from the obtained LIF spectrum (Figure 25B) with the characteristic rotational structure of the transition energies.208 Thus, it was indicated that OH radicals are dispersed from the photoexcited TiO2 surface to the gas phase. Since pulsed laser irradiation is performed within a very short time period (shorter than 5 ns), the light absorption generates condensed electron−hole pairs on the solid surface. The number of photons for a 3 mJ laser pulse of the wavelength of 355 nm is calculated to be 5 × 1015 photons per pulse. To understand the mechanism for the •OH generation at the TiO2 surface, the power dependence has been investigated for the OH-LIF intensities immediately (40 μs) and at 180 μs after the 355 nm laser excitation. The fast •OH generation could be identified to the direct two-photon excitation of H2O, and the slower one is the photocatalytic reduction of the surface H2O2 which is formed by the photocatalytic oxidation.208 The yield of

Figure 24. Photocatalytic processes of O2 on TiO2 surface showing generation and deactivation of 1O2 at the surface. Reproduced from ref 158. Copyright 2007 American Chemical Society. 11316


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of photocatalysis, where DMPO (5,5-dimethyl-1-pyrroline Noxide)49,57,214−216 is often utilized as a spin trapping reagent. Unstable OH radicals produced during the photocatalysis react with DMPO to convert to stable DMPO−OH radicals which could be detected by ESR spectroscopy. However, before the formation of OH radicals, valence-band holes may alternatively oxidize spin trapping reagents. Thus, three processes of DMPO−OH radical formation are possible in photocatalysis as shown in Figure 26. These are (A) free OH radicals react in

Figure 25. (A) Experimental outline for laser-induced fluorescence (LIF) detection of •OH released from irradiated TiO2 surface. Reproduced with permission from ref 209. Copyright 2010 Springer. (B) Obtained excitation spectrum of OH radicals. •

Figure 26. Three possible reaction routes for detection of OH radicals with a DMPO spin trapping reagent. Reactions with OH radicals (A) in solution and (B) at the surface, and (C) indirect reaction via oxidation of DMPO.

OH was larger for mixed-phase (P25) and rutile TiO2 with 0.5 Torr H2O than for anatase TiO2. The quantum yield of the • OH which was generated photocatalytically and released from the TiO2 surface was estimated to be 5 × 10−5.208 Though this LIF method is one of a few methods with which OH radicals can be directly detected, its application is limited to the gas phase. Since the fluorescence of OH radicals quenches with water molecules even in the gas phase, LIF method cannot be applied in aqueous solution. Gaseous OH radicals released from the anatase TiO2(101) surface by the photoinduced water dissociation have also been detected recently by temperature-programmed desorption and time-offlight methods.210 This report also indicated that the water dissociation on rutile TiO2(110) is largely inhibited by the strong hydrogen bonds at high water coverage.210 6.1.2. Direct ESR Observation. The direct observation of OH radicals by ESR spectroscopy was reported in the gas phase,211 but it was actually difficult to observe them in a condensed phase at room temperature because of the high reactivity.43 Therefore, ESR measurements in photocatalysis were performed at a low temperature of 77 or 4 K aiming at freezing the reaction. However, the ESR signal of OH radicals, which is characterized by the hyperfine splitting of the H atom, was not observed, but instead, that of trapped holes was observed for hydrated TiO2.212 The signal of trapped holes remained up to 150 K and disappeared at 240 K. In the presence of hole scavengers such as methanol, at higher temperature the signal converts into the hole adduct signal. Analyses of g and A parameters with 17O-labeled H2O revealed that the oxidized was the O atom at the surface and the Ti−O bond remained. Thus, due to the larger redox potential, water was not oxidized to •OH under the hydrated condition.212 6.1.3. Spin Trapping ESR Observation. A spin trapping method is a conventional method often utilized to detect the • OH generated in biological systems as described in a review article.213 This method has also been exploited in the research

solution, (B) surface trapped holes or adsorbed OH radicals react with DMPO, and (C) the trapping reagent DMPO itself is oxidized to a radical cation, DMPO+, and it becomes an OH radical adduct DMPO−OH by hydrolysis.57,217,218 The yield of DMPO−OH was studied as the effect of dichlorobenzene which showed reactivity similar to DMPO toward some of the photogenerated carriers.219 As a result, it was revealed that DMPO reacts with photogenerated holes and/or OH radicals on the surface of TiO2 particles rather than in a bulk solution.219 In a recent review paper,57 the surface oxidation (Figure 26B) was denied based on the decrease in the DMPO−OH radical adduct in the mixed solvent of DMSO (dimethyl sulfoxide) and water. In this report, however, the oxidation of DMSO by OH radicals alone in solution was assumed but not by trapped holes in the mixed solvent. Since the oxidation of DMSO at the surface under the high concentration condition was not excluded, the conclusion of this report is still under debate. The direct oxidation of the spin trapping reagent DMPO to a cation radical DMPO+ and the successive hydrolysis to form DMPO−OH (Figure 26C) were suggested in the radiationchemical experiments at low temperature in chlorofluorocarbon.220 However, in aqueous solution, DMPO+ radical probably decomposes to stable diamagnetic products since it seems difficult to keep the radicals during hydrolysis. Therefore, the reaction with trapped holes (Figure 26B) would be the most probable for the DMPO spin trapping. The quantum yield of OH radical generation was estimated to be about 0.046 for the anatase TiO2 by the detection with the DMPO spin trapping method.221 The high yield of •OH obtained by the spin trapping method suggests that •OH should be trapped at 11317


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the surface because the quantum yield of free •OH in solution was on the order of 10−4, which was measured by the fluorescence probe method described later. The reaction of surface adsorbed •OH could not probably be distinguished from the trapped holes as will be described below. The further problem in using DMPO would be that it can be also oxidized by photosensitized reactions. Because DMPO absorbs ultraviolet light and the concentration is not usually low (>0.1 M), it is excited and an electron transfers to the conduction band of photocatalyst and generates •O2−. When DMPO molecules trap •O2−, the adduct becomes the stable DMPO−OH radical as described above. Therefore, one should be cautious to conclude that OH radicals are involved in photocatalytic reactions on the basis of the experimental results with the DMPO spin trapping method.222,223 Besides DMPO, BMPO62 and CPYPMPO59 (Figure 5) have been used in photocatalysis to detect •OH as well as •O2− described above. These trapping reagents are commercially available but very expensive. Since DMPO and the derivatives are reactive, less reactive nitrone spin-trapping reagents such as α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) have been also used to detect OH radicals in photocatalysis.179,217 6.1.4. ESR Observation of the Decay of Radicals. The decay of stable nitroxyl radicals in aqueous suspension is utilized to detect OH radicals.224 One should be careful of the quantitative detection of OH radicals by measuring the decay because the stable nitroxyl radical itself can be oxidized and/or reduced by the electrode reaction.225 Thus, the oxidation by valence band holes or trapped holes is sometimes incorrectly regarded as the oxidation by OH radicals.222 6.1.5. Fluorescent Products by •OH. In biological systems, reagents with which fluorescein is formed by the reaction with OH radical have been demonstrated.26,28,29,226 Among the reagents, 3′-(p-hydroxyphenyl) fluorescein (HPF)200 and 3′-(p-aminophenyl) fluorescein (APF)227 have been used in photocatalytic reactions. HPF selectively reacts with OH radicals to form a strongly emissive fluorescein molecule as shown in Figure 27A but does not react with the other ROS, such as •O2−, 1O2, and H2O2.200 However, on using in photocalytic systems, the photosensitized oxidation is likely effective for these fluorescein-based probe molecules as described above.183 HPF molecules with a silanol group could combine a glass plate, which is separated from a TiO2 coating glass plate with a spacer as shown in Figure 27B.228 The distance between the two glass plates was controlled using polyimide films, and the space was filled with air-saturated water. In this way, it was confirmed that the •OH generated on the photocatalysts could diffuse to the HPF coated glass. In the research field of radiation chemistry, other simple molecules such as terephthalic acid229 and coumarin230 were employed to generate the fluorescent product for detecting OH radicals in solution. Different from the detection methods for • O2− and H2O2, the probe molecules were present during the photocatalytic reaction, and after the reaction the photocatalyst powders were removed from the suspension to improve the precision of the fluorescence spectrophotometry. The reactions of probing OH radicals with terephthalic acid and coumarin are shown in Figure 28, parts A and B, respectively. Since terephthalic acid66,91,185,231−234 and coumarin145,234−240 have no absorption at the excitation wavelength for photocatalysts, these molecules were adopted to detect OH radicals in photocatalysis. A carboxyl derivative of coumarin, coumarin-3carboxylic acid (CCA), which has been used in radical

Figure 27. (A) Detection of OH radicals with fluorescence probe HPF (hydroxyphenyl fluorescein). (B) Illustration of the experimental setup for single molecule detection of photogenerated •OH in water. Reprinted with permission from ref 228. Copyright 2014 Wiley-VCH Verlag GmbH & Co.

Figure 28. Reactions for detection of OH radical with fluorescence probes, (A) terephthalic acid, (B) coumarin, and (C) coumarin-3carboxylic acid (CCA), which become corresponding fluorescent molecules to be measured with a common spectrofluorophotometer.

dosimetry (Figure 28C),241 was also employed in order to see the effect of the adsorption of probe molecules on TiO2 surface.242 The efficiencies of probing •OH could be calculated from the radiation-chemical yields of OH radical (228 nmol/J) and the • OH adducts reported in the literature.229,230,241 They are 45, 6, 11318


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and 4.7% by using 1 mM terephthalic acid,243 0.1 mM coumarin,244 and 0.1 mM CCA,242 respectively, which did not much depend on the concentrations.229,230 Since these values are calculated for homogeneous solution, in photocatalytic systems the probing efficiency may vary to a certain extent. Fluorescence photometry is sensitive to measure a low concentration of molecules. For the case of coumarin, by subtracting the fluorescence spectrum of the unreacted coumarin, 0.3 nM umbelliferone could be detected, indicating that the detection limit for •OH is 5 nM.245 The adsorption of probe molecules for CCA and coumarin is shown in Figure 29A. The ratio of the adsorption of coumarin

electrodes. In the catalytic ozonation, it was reported that coumarin was not oxidized at the catalyst surface and able to detect the •OH in solution.247 In the photocatalytic system, the amount of the products is saturated at a higher concentration of terephthalic acid, which indicates the absence of the direct oxidation at the TiO2 surface.74 Furthermore, the investigation by Czili et al. under anaerobic conditions has clearly demonstrated that OH radicals are responsible for the umbelliferone formation from coumarin.235 As for the direct oxidation issue of fluorescence probe molecules, some discussion will be provided in section 7.2. The problem with using coumarin is that it self-decomposes gradually in alkaline solutions.243 On the other hand, terephthalic acid is effective in alkaline solutions.74 When phthalic acid was used instead of terephthalic acid at pH below 4, it was not oxidized by OH radicals but oxidized by holes with a yield of 8%.249 The quantum yields of OH radicals in solution detected by the use of the fluorescence probes are extremely small, on the order of 10−5−10−4 for TiO2 photocatalysts.133,242,245,250 6.1.6. Chemiluminescence Reaction with •OH. OH radicals react with phthalhydrazide to generate hydroxyphthalhydrazide. It becomes the excited state of hydroxyphthalate by oxidation as shown in Figure 30. Then the emission of

Figure 29. (A) Ratio of adsorbed probe molecules coumarin and CCA on three kinds of TiO2 powders. (B) Apparent quantum yields (Q.Y.) of •OH generation. Data were taken from ref 242.

and CCA in solution (0.1 mM) by the addition of various TiO2 powders (15 mg/3.5 mL) was measured from the fluorescence intensities of the added umbelliferone and OH-CCA (5−50 nM), respectively, where the attached OH group of the probe molecule was not considered to influence the adsorption properties. The adsorption of coumarin is as small as 3%, while that of CCA is 97%.242 On UV irradiation, the initial rates of • OH generation were measured and the apparent quantum yields are shown in Figure 29B. The quantum yield measured with CCA was 12-fold larger than that with coumarin, indicating that CCA detects the •OH near the TiO2 surface before the diffusion to the solution, while coumarin detects mainly the •OH in solution.246 The quantum yield of OH radicals measured with surface adsorbed probe molecule (CCA) was on the order of 0.001, and they were smaller than 1 order of magnitude against the spin trapping ESR method stated above (section 6.1.3). The intrinsic bimolecular reaction rate constants of •OH for CCA241 and DMPO54 are 5.6 × 109 and 4.3 × 109 M−1 s−1, respectively. Therefore, the difference in the •OH yields indicates that, in the case of DMPO spin trapping, the •OH adduct was produced by the reaction with surface trapped holes, while for coumarin, the adduct could not be produced from trapped holes but only from •OH desorbed from the surface. The validity of using the probe molecules was recently discussed based on the oxidation of the molcules.248 In this report it was concluded that coumarin was not suitable to detect OH radicals due to a less anodic potential to the •OH formation because the oxidation of the probe takes place at the potential as low as 2.1 V (vs NHE). However, as will be shown in section 7.2, OH radical would be produced from trapped holes whose redox potential is 1.5 V, and the oxidation of coumarin may not take place. It was confirmed that no direct oxidation for coumarin244 or terephthalic acid (in alkaline solution)243 at the TiO2 surface takes place because no increase in the photooxidation current was observed for TiO 2

Figure 30. Reaction scheme for detection of OH radicals with a chemiluminescence method with phthalhydrazide. Adapted with permission from ref 251. Copyright 2013 by the authors; licensee MDPI.

hydroxyphthalate could be observed. Thus, the chemiluminescence method could be also used to detect OH radicals.126,251 However, for this method, it is assumed that the product, hydroxyphthalhydrazide whose chemical structure is similar to that of luminol, should not be oxidized during the irradiation in photocatalysis. Actually, when it was used in a nonflow system,126 the oxidation took place to a certain extent and the quantitativeness in the analysis was decreased. Therefore, this method may be useful to detect OH radicals by using a flow system. 6.1.7. Analysis of •OH Reaction Product. The reaction of OH radical with DMSO (dimethyl sulfoxide) is used in the ISO 10676:2010 standard test of photocatalysts,4 that is, Test method for water purif ication performance of semiconducting photocatalytic materials by the measurement of forming ability of active oxygen. By using aqueous solution of 10 mg/L DMSO, the product, MSA (methanesulfonic acid, CH3SO3H), is analyzed by using chromatography. Besides DMSO, benzene,252 4-chlorophenol,253 N,N′-(5-nitro-1,3-phenylene)bisglutaramide,254 and 4chlorobenzoic acid255 were used to detect OH radicals by analyzing each reaction product. Decoloration of p-nitrosodimethylaniline (molar extinction coefficient at 440 nm is 11319


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3.3 × 104 M−1 cm−1) has been also used to detect OH radical.70 The oxidation of 1,5-diphenyl carbazide to 1,5-diphenyl carbazone with the absorption at 563 nm was also reported as the photocatalytic reaction with OH radicals.256 In some cases the detection of OH radical was verified by the quenching of the OH radical with tert-butanol.255 Though the reactions of OH radicals with these molecules are well-known, the possibility of oxidation with the trapped holes could not be excluded in each reaction. Methanol was also employed to evaluate •OH by the analysis of formaldehyde as the reaction product in some of the literature.257,258 However, unfortunately, methanol was found to be oxidized by trapped holes without the contribution of OH radicals on the basis of the results of ESR measurements at low temperature259 and the transient absorption measurements of trapped holes generated in TiO2 films at room temperature.260 FT-IR analysis of the intermediate species of methanol oxidation suggests the conversion to CO2 via the adsorbed intermediate states.261 Kinetics analysis of OH radicals in solution suggested that methanol in aqueous solution should be oxidized by surface trapped holes without the contribution of OH radicals in solution.246 Thus, though numerous methods with which OH radicals could be detected have been proposed so far, one should be very cautious about the experimental conditions employed on the applications as well as the interpretations of the results.

adsorption energy for •OH seems reasonable. The effective pH of the intrinsic surface of TiO2 should be the isoelectric point, i.e., pH 6. At this pH, the redox potential calculated from Figure 3 for free •OH is +2.03 V. Thus, the shift of the redox potential from +2.03 to +1.5 corresponds to the change of the free energy of the adsorption of •OH on TiO2, which is compatible to the shift of pKa stated above. From these thermodynamical viewpoints, the reactivity of trapped holes may be smaller than free OH radicals owing to the stabilization by the adsorption. The reactions of phenol have been compared between the radiation-chemically produced •OH and the trapped holes in photocatalysis, and it was indicated that localized TiO2 holes were relatively long-lived and capable of reacting with nonadsorbed molecules.267 Thus, the surface radical, or the surface trapped hole (htr+), could be regarded as the adsorbed OH radical194,262,268 represented by eq 22. h tr + + OH− ⇄ (•O−)ads + H+

In low temperature ESR study, OH radical could not be observed as stated above (section 6.1.2); on the other hand, free OH radicals could be observed by the fluorescence probe method in aqueous solution (section 6.1.5) or by LIF method in the gas phase (section 6.1.1) though the yields were very small. This experimental evidence supports the assumption that OH radicals should be equivalent to the trapped holes. Based on the following discussion, (•O−)ads in eq 22 should not be terminal ≡Ti−O• but the bridged [Ti−O•−Ti] or [Ti− O• HO−Ti]. 6.2.2. Terminal OH and Bridged OH. As stated above, OH radicals in photocatalysis are generated from the surface OH groups of the metal oxide. For the surface of metal oxides such as TiO2, the location of O atoms could be classified into two different coordination types, i.e., terminal OH (OHt−) and bridged OH (OHbr−), as shown in Figure 31.111 Terminal OH

6.2. Generation Processes of •OH

Generation processes of OH radicals are most important to discussing the photocalytic oxidation reaction. Therefore, this section will be described by dividing it as follows: (1) the relationship of OH radicals with surface trapped holes (section 6.2.1), (2) characteristics of the surface O species as terminal OH and bridged OH (section 6.2.2), (3) the effect of crystal phases, anatase and rutile, in the case of TiO2 (section 6.2.3), (4) the synergy effect of the mixed-phase TiO2 (section 6.2.4), and (5) •OH generation with non-TiO2 photocatalysts (section 6.2.5). 6.2.1. Surface Trapped Holes or Adsorbed •OH. When OH radicals are produced at the metal oxide surface by the photocatalytic oxidation of H2O with the valence band holes (hvb+), the •OH generation reaction may be simply described by eq 21.

OH− + h vb+ → •OH

(22)

Figure 31. Two types of surface OH groups, bridged (br) and terminal (t), resulting from dissociative adsorption of water on the TiO2 crystal. Reproduced with permission from ref 111. Copyright 2011 Elsevier.

(21)

When OH− is oxidized at the surface, a catalytic effect is expected to involve dissociatively adsorbed water. Therefore, the surface OH groups on the photocatalyst are likely oxidized,262 but not the hydrated OH− ions in solution. When OH radicals are radiation-chemically generated by the ionization radiation in solution, they are adsorbed on the TiO2 surface.263 The redox potential of the adsorbed •OH is reported to be 1.5263 or 1.6 V,264 which corresponds to that of the trapped holes generated in TiO2 electrodes.130,265 Since the signal of OH radicals could not be detected even by a low temperature ESR method212 as stated above, the adsorbed •OH might become •O− at the surface. Though at higher pH, •OH becomes •O− in homogeneous solution (pKa = 11.9) as shown by eq 3, the adsorption on the TiO2 surface might take place in the form of •O− in neutral pH solution. Since pKa of the •OH adsorbed on TiO2 was reported to be 2.8,263 the free energy change by the adsorption could be roughly estimated to be ΔG = (2.8 − 11.9) × 0.059 eV = −0.54 eV = −52 kJ mol−1. This

is a 1-fold coordinated hydroxyl group (Brønsted base sites), and bridged OH is a 2-fold coordinated bridging hydroxyl group (Brønsted acid sites) resulting from the protonation of bridging oxygen ions. With the release of a proton, terminal OH and bridged OH are in the equilibrium at the aqueous interface as represented by eqs 23 and 24, respectively.269 ≡Ti−OH + H+ ⇄ ≡Ti−OH 2+

(23)

[Ti−O−−Ti] + H+ ⇄ [Ti−OH−Ti]

(24)

Equation 23 is equivalent to eq 23′ on considering the dissociation of the surface adsorbed water. ≡Ti−OH ⇄ ≡Ti+ + OH− 11320

(23′) DOI: 10.1021/acs.chemrev.7b00161 Chem. Rev. 2017, 117, 11302−11336

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Note that the proton dissociation of ≡Ti−OH for ≡Ti−O− takes place only at extremely high pH (pH ≥13),130 as it has been incorrectly assumed even in the neutral pH solution. For the formation process of •OH on the TiO2 surface, the following simple equation has been sometimes proposed based on eq 21 in the literature. ≡Ti−OH + h+ → ≡Ti+ + •OH

(25)

For example, for the low temperature STM study, the photocatalytic dissociation of water on terminal Ti sites has been reported.270 However, on the basis of the electronic structure of the surface-bound water obtained from the data of photoelectron spectroscopy, Ti of the terminal OH cannot be photooxidized under the UV illumination.262 Thus, reaction 25 does not actually take place as discussed in the literature.110,130,265 A simple explanation follows. As suggested in eq 23′, the Ti atom of the terminal OH carries positive charge, which forms the cationic site of the TiO2 surface.269 Hence, positive holes hardly attack the Ti−O bond of the terminal OH. On the other hand, O of the bridged OH in eq 24 carries negative charge which likely attracts positive holes. Thus, trapped holes are generated at the bridged O site.262 This prospect is supported with the ATR-IR spectrum change in the adsorbed water. In the O−H stretching mode, 3200 cm−1 absorption decreased under UV irradiation;40 on the other hand, the fluorination which takes place at the terminal OH causes the decrease of IR absorption near 3400 cm−1.271 The formation process of trapped holes at the surface could be expressed as the water oxidation process in Figure 19.110,129,130 The photoinduced valence-band hole migrates on the TiO2 surface and attacks the bridged O in the step, the kink, or the terrace, along with the concerted nucleophilic attack of water on a neighboring Ti, resulting in the formations of a [Ti−O• HO−Ti] structure.130 The isotope effect on the photocatalytic activity170,178 described above in section 5.1.3 suggests that the cleavage of the O−H bond may be involved in this key step of the oxidation. An alternative process for water oxidation is shown in Figure 32,111 where the difference from the nucleophilic attack mechanism (Figure 19) is that water does not coordinate to the surface as shown in Figure 32A on the formation of trapped holes. Subsequently, surface O vacancies are formed (Figure 32B), and then water molecules adsorb dissociatively at the photoinduced bridging O vacancies to form bridged peroxide (Figure 32C). In both mechanisms, the bridged O becomes the surface trapped hole at first and then generates the bridged peroxide (Figure 14B) by the second hole. Thus, the bridged peroxide is probably released into the solution as a H2O2, besides the further oxidation to generate O2 (Figure 32D). In the water oxidation mechanism by density functional theory (DFT) calculation, however, instead of peroxo, the formation of HOO− has been suggested.272 6.2.3. Anatase and Rutile TiO2. The generation rate of OH radical for anatase is significantly high while that for rutile is very low in the absence of H2O2, as shown in Figure 20B, our previous studies,92,242 and the other research.94,139,185,228,273,274 This observation is consistent with that the anatase phase is often regarded as the most active in TiO2 photocatalysis.95,194,232,273 This difference may be caused by the difference in the development of the trapped hole shown in Figures 19 and 32. Though this concept is one of the explanations based on the adsorption of H2O2 on rutile surface, for rutile the bridged peroxo (Ti−OO−Ti) is easily formed owing to the

Figure 32. Sequence of reaction steps of radical pair mechanism for water oxidation at TiO2 surface under acidic condition. Subscripts indicate the coordination number of Ti atoms. (A) Trapping of first hole, (B) trapping of second hole, (C) formation of surface peroxo, (D) generation of O2 with the addition of two holes, and (E) recovery of the original surface by hydration. Reproduced with permission from ref 111. Copyright 2011 Elsevier.

proper Ti−Ti alignment at the surface, while for anatase the trapped hole releases OH radical because of the inconvenient surface structure to form the bridged peroxo structure.40,242 By using terephthalic acid, Ahmed et al.232 investigated the generations of OH radicals for anatase powder and rutile single crystals and showed the dependence of the •OH generation on the crystal surfaces. On the comparison of rutile TiO2 singlecrystal (001), (100), and (110) surfaces, the (110) surface showed the minimum •OH generation activity, though the rutile (001) surface exhibits •OH generation activity comparable to that on the anatase (101) surface. Because the (101) and the (110) are the thermodynamically most stable surfaces for anatase and rutile, respectively, it is expected that most synthesized anatase and rutile nanomaterials possess a large percentage of these surfaces. Thus, they explained why anatase usually exhibits higher photocatalytic activity than rutile.232 In their experiments, the irradiation time seems too long to generate •OH for the reduction products of O2 besides the oxidation of H2O. Kim et al.228 investigated the diffusion of •OH from the illuminated TiO2 surface to the bulk solution using an HPF probe modified with a silylated group as an anchor (Figure 27B). They confirmed the presence of free •OH only for anatase and attributed the difference of the reactivity of two polymorphs to the difference of the adsorbability of •OH. That is, for rutile, owing to the higher adsorbability, •OH radicals are not released to the solution but remain on the surface as illustrated in Figure 33. The higher activity for anatase in the photocatalytic decomposition was attributable to the generation of free •OH in soution.228,274 However, as shown in Figure 29, when the probe molecule CCA was adsorbed on the TiO2 surface, the amount of the detected •OH for rutile was also smaller than that for anatase. Accordingly, the difference in the • OH adsorption between anatase and rutile as shown in Figure 33 could be replaced by the difference in the generation of •OH 11321


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tase.273,277−279 Despite numerous collected evidence, a thoroughly convincing explanation of why mixed-phase samples outperform the individual polymorphs has remained unclear.280 One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO2.280,281 The optical absorption band of the interfacial charge transfer from the valence band to the deposited Cu(II) shows that the valence-band top of anatase is lower (more positive in the potential) than that of rutile.282 ESR experiments showed the electron transfer from the rutile part to the anatase part in the mixed-phase TiO2.283 By spectroelectrochemical measurements, the potential of the conduction band edge of rutile was shown to be more negative than that of anatase.91 A direct measurement of the surface potential profile by Kelvin probe force microscopy over the interface of the phase junction revealed the presence of the electric field from rutile to anatase nanoparticle.284 However, there are some reports so far which were referred to as reverse charge transfer. One of them is the hole transfer from the rutile part to the anatase part on the time scale of milliseconds by means of transient absorption spectroscopy,266 and the other is the electron transfer from the anatase part to the rutile part in the mixed-phase TiO2, which was also shown by ESR experiments.285 In these experiments, the charge transfer via the trapped states may affect the conclusion. Since the electron is trapped at 0.9 eV below the conduction band for rutile and 0.1 eV below for anatase (Figure 10),88 the conduction-band bottom of anatase could be lower than that of rutile to allow the electron transfer from the trap of anatase to the trap of rutile.281,286 By taking into account the observation that electrons are deeply trapped on the rutile surface on the time scale of picoseconds (Figure 10),88 the electron transfer from the rutile part to the anatase part should be accomplished within picoseconds. Thus, the electric connection between the two phases must be important to effectively attain the charge transfer between each band in the mixed-phase TiO2. This is one of the reasons why the mixed-phase synergy effect has not been clarified yet. One study in the literature suggested the electron transfer from the anatase part to the rutile part reportedly by the deposition of Ag metal on the rutile film near anatase belts.287 In the discussion of this report, however, consideration of the difference in the number of absorbed photons was missing. Because of the lager volume and higher absorption coefficient of the rutile part, the generation of electron−hole pairs predominantly takes place at the rutile part. Therefore, the electron transfer takes place from the rutile to the anatase part, and Ag was deposited on the anatase and a part of rutile only locates near the anatase belts as shown in the report.287 As for the photocatalytic •OH generation in the mixed-phase TiO2 such as P25 (Degussa), as shown in Figure 34, •OH was produced similarly to the case of anatase in the absence of H2O2, while in the presence of H2O2 the rate of •OH generation was increased similarly to the case of rutile.74,92,242 If holes transfer from the rutile part to the anatase part in the mixed-phase particle, •OH generation in the absence of H2O2 at the anatase part could be explained. However, on the addition of H2O2 the increase in the •OH generation could not be explained because the •OH generation on the pure anatase particle was decreased with the addition of H2O2. This contradiction could be overcome when the energy alignment shown in Figure 35 is employed,282 where the reduction and the oxidation take place at the anatase and rutile parts,

Figure 33. Illustration of OH-radical-mediated photocatalysis on anatase (left) and rutile (right). Reprinted with permission from ref 228. Copyright 2014 Wiley-VCH Verlag GmbH & Co.

or the trapped hole at the surface. Therefore, this reaction model to explain the activity difference between anatase and rutile may be still unclear. As described in section 4.3 of the photocatalytic reactions of H2O2, the addition of H2O2 increased the photocatalytic generation of •OH for rutile but decreased it for anatase TiO2.40 This crystal-structure dependence of the effect of H2O2 on the •OH generation has been also reported in our previous studies92 and recent studies242 as schematically illustrated in Figure 34. Besides the photocatalytic reduction of H2O2 as

Figure 34. Schematic illustration for effect of H2O2 concentration on • OH generation rate for mixed-phase TiO2 and other polymorphs based on the data in refs 40, 92, and 242.

described above, •OH could be generated by the oxidation of H2O on the rutile when the surface adsorbed H2O2.243,275 Though this process is still under debate, the •OH generation with H2O2 in the water oxidation process was also reported with a DMPO spin trapping flow ESR method for Nafionmodified Pt electrode.276 The difference in the surface reaction paths between anatase and rutile is discussed in detail in section 7. 6.2.4. Mixed-Phase TiO2. It is well-known that the photocatalytic activity of anatase in the decomposition of organic compounds is higher than that of rutile, while mixedphase crystals show further higher activity than ana11322


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smaller than that for WO3. Though the valence-band top of WO3 has higher oxidation potential than that of BiVO4, a higher photocatalytic O2 evolution was known for BiVO4.10 Therefore, the smaller •OH generation for BiVO4 suggests that the surface should be favorable to generate the peroxo structure [Bi−O−O−V] similarly to the case of rutile TiO2. Actually, by the addition of H2O2, a significant increase in the •OH generation was observed for BiVO4.288 As for sulfides, ZnIn2S4 generated OH radicals under the photoirradiation,289 while CdIn2S4, AgInS2,289 CdS, Ag2S,290 and ZnS173 could not. The difference could be explained by the difference in the potential of the valence-band top of these semiconductors. However, CdSe/CdS nanorods have been reported to produce OH radicals,291 indicating the energy shift by the size quantization effect in this case.

7. REACTION MECHANISMS WITH ROS IN PHOTOCATALYSIS

Figure 35. Plausible mechanism of •OH generation at the rutile part of mixed-phase TiO2 without the addition of H2O2.

7.1. Rational Model of ROS Generation Processes for TiO2

Based on the above perspective, the photocatalytic surface processes to generate ROS from H2O and O2 could be illustrated at a molecular level in Figures 37 and 38, by dividing the processes into the reactions at anionic bridged OH site and those at cationic terminal OH site, respectively.

respectively. Though the rutile part could not generate •OH without H2O2, the H2O2 formed at the anatase part by the reduction of O2 via •O2− migrates to the rutile part as shown in Figure 35 and the rutile surface adsorbs H2O2. Thus, the rutile part of the mixed-phase TiO2 could generate •OH to lead a high photocatalytic activity without the addition of H2O2. This explanation of the effect of H2O2 may be still under debate, because the energy band alignments of anatase and rutile have not reached a consensus of researchers yet. 6.2.5. With Non-TiO2 Photocatalysts. For photocatalysts other than TiO2, such as WO3 and BiVO4 which are common metal oxides for water oxidation, the generation of OH radical was investigated in aqueous suspension.288 The band energy levels for WO3 and BiVO4 are shown in Figure 36. On the

Figure 37. Plausible photocatalytic reaction paths at bridged OH site of TiO2. Broken lines represent adsorption or desorption. Double lines may be restricted to anatase. Modified from ref 275. Copyright 2016 American Chemical Society.

Figure 37 shows the plausible processes at the bridged OH site of the TiO2 surface based on the oxidation model of nucleophilic water attack,130 where the reaction paths specific to anatase are distinguished from those to rutile by different arrow lines. A photoinduced hole attacks the bridged O(2−) (a)

Figure 36. Difference in generation processes for •OH and •O2− between WO3 and BiVO4 photocatalysts in relation to electric potentials of energy bands and additives. Adapted with permission from ref 288. Copyright 2015 Elsevier.

irradiation of visible light (470 nm LED), the apparent quantum yield of OH radical for WO3 was 3.4 × 10−2%288 and the addition of Fe3+ ions to the WO3 suspension increased the •OH yield. This is explained by that the conduction-band bottom of WO3 is lower than the O2 reduction potential to make the O2 reduction difficult and that the addition of Fe3+ causes the consumption of photoinduced electrons to prevent electron−hole recombination. The yield of •OH for BiVO4 was

Figure 38. Plausible photocatalytic reaction paths at terminal OH site of TiO2. Broken lines represent adsorption or desorption. Double lines may be preferable for anatase. Modified from ref 275. Copyright 2016 American Chemical Society. 11323


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with the attack of water resulting in the formation of a Ti−O• and Ti−OH pair (b).130 The structure b represents a surface trapped hole, which reverts to structure a by the recombination with a conduction-band electron. At the surface of anatase, a certain fraction of the trapped holes (b) desorbs as OH radicals into the solution.228,246 On the other hand, at the surface of rutile, when a second hole is generated in the particle, it migrates to combine with the existing hole to form bridged peroxo species [Ti−O−O−Ti] at the surface (c).130,194 The structure c may be specific to rutile,40,114 which might be because the separation of adjacent surface Ti atoms is favorable for the bridging coordination to form the surface peroxo (Figure 14B).40 The surface peroxo is the intermediate of the water oxidation110,130 and is probably equivalent to the adsorbed H2O2 as suggested by Nakamura et al.110 with ATR FT-IR measurements. Therefore, depending on the situation, the formed peroxo could release H2O2 from the surface and return to the original structure (Figure 37a). By the reduction of peroxo (c) O−O bond cleaves and •OH is released with a return to the original surface structure (a). On the other hand, when the peroxo (c) is further oxidized with a third hole, following two paths may be possible.244 When the Ti−O bond of [Ti−O−O−Ti] is cleaved, Ti−OO• (d) is formed.130 This radical could release •O2− or be further oxidized to release O2 and then return to the original bridged O (a). The generation of •O2− on the addition of H2O2 for rutile was supported by the experiment40 shown in Figure 20A. On the other hand, when the O−O bond of [Ti−O−O−Ti] is cleaved by a third hole, Ti−O• (e) is formed. This radical could return to the peroxo (c) by releasing •OH similarly to the case of the trapped hole (b) for anatase. The generation of •OH in the oxidation process in the presence of H2O2 has been suggested by the experiments using rutile single-crystal photoelectrodes,243 though this reaction pass is still under debate. These processes proceed at the bridged OH site by starting from the hole attack. On the other hand, at the terminal OH sites, the photocatalytic reaction proceeds as depicted in Figure 38. In contrast to the bridged OH sites, positively charged holes could not react with the original terminal OH or Ti(4+) of a cationic character. An electron photoinduced at the conduction band is trapped at the surface ≡Ti+ to form ≡Ti• (b).194,273 The trapped electron, ≡Ti•, could reduce O2 to form an adsorbed • O2− (d).48,72 The adsorbed •O2− is released to return to the original (a) or oxidized to generate 1O2.157 On the other hand, by further reduction the adsorbed •O2− (d) becomes an adsorbed H2O2 to form end-on peroxide (c). The adsorbed H2O2 at the terminal Ti site in Figure 38c, could be reduced to become ≡Ti−O− with releasing •OH, and then the ≡Ti−O− structure is rapidly protonated to the original structure ≡Ti− OH. This reduction process may not take place at the anatase surface since the addition of H2O2 did not increase •OH generation (see Figures 20B and 34). Another reduction path of end-on peroxide (Figure 38c), which occurs likely on the anatase surface, is the release of H2O2 with a return to the trapped electron (b) which could generate •O2−. The photocatalytic oxidation of adsorbed H2O2 (c) generates adsorbed •O2− (d) and then releases •O2− by returning to the original terminal ≡Ti+ (a). On the addition of H2O2 the generation rate of •O2− was increased as evidenced by the experiment shown in Figure 20A, where the increase was especially remarkable for anatase. This oxidation of H2O2 on anatase suppresses the oxidation of water at the bridged OH

site, resulting in the decrease of •OH generation rate as shown in Figures 20B and 34. Thus, Figures 37 and 38 can describe in detail the generations of •O2−, H2O2, 1O2, and •OH at the TiO2 surface, which are schematically illustrated in Figure 2 and consequently give a rational explanation of the effect of the H2O2 adsorption on the generation processes. The difference in the behaviors between the anatase and the rutile surfaces may be ascribed to the difference of the dominant routes which are indicated in the figures by double arrows for anatase. 7.2. Oxidation Mechanism in TiO2 Photocatalysis

OH radicals are generally believed to be the most important species among ROS in photocatalysis, which is suggested by various experimental results.273,292,293 For example, the correlation of the decomposition of methylene blue with the • OH generation has been reported.294 In the photoelectrochemical oxidation at the TiO2 surface, by single-molecule fluorescence microscopy, it was demonstrated that Amplex Red is oxidized indirectly with adsorbed OH radicals.295 Although many reports in the literature easily opt to ascribe the photocatalytic oxidation to the contribution of OH radicals, it is still controversial whether OH radicals are actually involved in photocatalytic reactions or not. In other words, whether the decomposition takes place via the direct oxidation with valenceband holes or the indirect oxidation with OH radicals has not been still clarified yet.273 Then, let us take a glance over the contribution of OH radicals to the oxidation decomposition reactions. Plausible oxidative decomposition processes in photocatalysis would be classified to the four behaviors as shown in Figure 39.4

Figure 39. Possible processes in oxidation of a molecule by a photogenerated valence band hole. Adapted with permission from ref 4. Copyright 2016 Royal Society of Chemistry.

In Figure 39, ①, the photoinduced valence band holes (hvb+) directly extract electrons from the adsorbed molecules and oxidize them. In ② the photoinduced valence band holes are stabilized on the TiO2 surface to become trapped holes (htr+) which oxidize the molecules at the surface. In ③ the photoinduced valence band holes oxidize the surface hydroxyl groups or adsorbed water to generate adsorbed OH radicals (•OHad), which successively oxidize the molecules at the surface. Finally, in ④ the generated OH radicals are released from the surface to the solution, which can oxidize the molecules distant from the surface. Behaviors of trapped holes could be examined by means of transient absorption measurements for TiO2 film to investigate the kinetics under various circumstances.260 In the absence of reactants, holes were trapped very rapidly and decayed on the 11324


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order of microseconds, while in the presence of alcohols the trapped holes were observed with the absorption but they decayed on the order of nanoseconds.260 By the ESR analysis for the photocatalytic reaction with methanol at low temperature, the above observation has been confirmed. The ESR signal of trapped holes changed to that of •CH2OH, by the hydrogen subtraction from the methoxy group, while •OH and CH3O• which was formed by cleaving the Ti−OCH3 bond were not observed.259 Therefore, the process ① (Figure 39), in which valence-band holes directly react with the adsorbed molecule before the hole trapping, seems not to be a dominant process at least for the alcohol decomposition. By the ESR measurements for methanol decomposition in aqueous solution at room temperature,297 •CH2OH could be observed but not CH3O• which is observed in the oxidation with the •OH generated in solution by ionization radiation.54 In the IR kinetic analysis of gas phase photooxidation of methanol over TiO2, it was suggested that the defects and TiOH sites were important to stabilize the low-active formate intermediate on the surface.298 Intensity-modulated photocurrent spectroscopy has been used to analyze the oxidations of methanol at the rutile single-crystal electrodes of (001) and (100) facets.299 The higher reactivity of the (100) surface toward the photooxidation of methanol and its lower reactivity toward the photooxidation of water were confirmed by the stronger interaction of the polar (100) surface with methanol and the longer distance between radicals at the bridged OH groups. According to the reports, trapped holes could be described by [Ti−O•−Ti] instead of [Ti−O• HO−Ti] of the nucleophilic water attack model in Figure 37. In the photocatalytic decomposition of acetaldehyde which could be adsorbed on the TiO2 surface, the generation rate of the final product, CO2, was higher by 3 orders of magnitude than that of the •OH in solution.245 The difference in the generation rates clearly indicates that •OH in solution is not the key reactant but the trapped hole is the major reactant in the photocatalytic decomposition. These observations suggest that the oxidation process should be via trapped holes, path ② (Figure 39). As described above, the trapped holes are regarded as the adsorbed • − O formed at the bridged OH site on the TiO2 surface. Therefore, process ③ in Figure 39 could not be distinguished from process ②. On the basis of the •OH detection in solution, the photocatalytic reactions via OH radicals may be explored by adding several kinds of additives (reactants), such as alcohols (methanol and ethanol) and inorganic ions (I−, Br−, and SCN−).246 In the experiments, OH radicals competitively react with coumarin and additives. The generation rate of OH-Cou (7-hydroxycoumarin or umbelliferone) was measured by changing the concentration of additives. When alcohols were used as additives, the effect of the additives on OH-Cou formation rate was significantly small, suggesting that alcohols should not react with the •OH in solution but react with trapped holes at the surface. On the other hand, when halide ions were used as additives, the reaction rate constants kA elucidated from the competitive reactions were compatible with those obtained in the homogeneous solution.54 The fact that alcohols are adsorbed on TiO2 was confirmed by the desorption of CCA on the addition of alcohols,246 while on the addition of I− ions the adsorbed CCA remained unchanged. Then, the OH-Cou formation rates were analyzed based on the reaction model of Figure 40. As a result, the equilibrium constant of free •OH against trapped holes, or the ratio of the

Figure 40. General scheme for oxidization process of adsorbed methanol (MeOH) and nonadsorbed I− reactants deduced from detection of OH radical in solution by fluorescence probe method with coumarin. Modified with permission from ref 246. Copyright 2014 Elsevier.

rate constants, was elucidated to be k0/kh = 0.01, suggesting that only about 1% of OH radicals should exist in the free form in the solution.246 This observation agrees with the radiationchemical study by Lawless et al.263 but seems to contradict the recent report in which •OH in the solution was not much scavenged on the addition of TiO2 powder.300 However, in the latter report, •OH was detected by the production of formaldehyde and the scavenging of •OH by TiO2 was compared only qualitatively with those by ZrO2 and Y2O3 photocatalysts.300 Therefore, the strong adsorption of •OH on TiO2 suggested above could not be denied by this literature. The photocatalytic reactions have been analyzed with a Langmuir−Hinshelwood (L−H) model, in which Langmuir adsorption equilibrium is assumed and only adsorbed molecules can react at the surface of the photocatalyst. Since this L−H model cannot tell details about photocatalytic kinetics, a direct−indirect model (Figure 41) has been

Figure 41. Energy level diagram showing photocatalytic reduction of O2 and oxidation of RH2. There are two hole transfer paths, direct transfer (DT) and indirect transfer (IT) via a trapped hole. Reprinted from ref 303. Copyright 2014 American Chemical Society.

proposed by Salvador and co-workers.296,301 In this model the following reaction steps have been involved: (i) electron reduction of dissolved O2 molecules with CB electrons, (ii) photooxidation of physisorbed RH2 species via an adiabatic indirect transfer of surface trapped holes (hs+), and (iii) direct photooxidation of chemisorbed substrate (RH2)ads with valence-band free holes (hf+) via an inelastic direct transfer. In this model, λ is the reorganization energy, which was 11325


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introduced by Gerischer302 to analyze the electron transfer at the semiconductor−electrolyte interface. Therefore, the direct transfer (vDT) corresponds to path ① in Figure 39, while the indirect transfer (vIT) may be related to the paths ②. Since the presence of •OH has been denied,301 paths ③ and ④ in Figure 39 are not considered in this kinetic model. Experimental data of photooxidation rates as functions of the photon flux and the reactant concentration besides in situ ATR FT-IR measurements were analyzed based on the direct− indirect model303 for three model organic molecules: formic acid (FA) dissolved in water, benzene (BZ) dissolved in acetonitrile, and phenol (PhOH) dissolved in either water or acetonitrile. As a result, FA dissolved in water is preferentially photooxidized via a direct mechanism. In contrast, BZ dissolved in acetonitrile is exclusively physisorbed on the TiO2 surface, and consequently photooxidized via a pure indirect mechanism. PhOH dissolved in water is photooxidized via a combination of both direct and indirect mechanisms. In this analysis, the trapped holes locate at 0.2 eV above the valence-band top and the reorganization energy, λ, is about 0.6 eV. The experimental data of the •OH detection with additives, which have been analyzed by the model in Figure 40, may also be analyzed with the direct−indirect model in Figure 41. By using this model, CCA could become OH-CCA with the direct transfer (vDT OX) because the effect of alcohols on the OH-CCA generation was very small.242 Furthermore, coumarin and all additives react with trapped holes depending on the distinctive adsorption ratios and different oxidation potentials. In this model, the effect of H2O2 addition on the OH-Cou generation (Figure 20B) may be explained by the increase and decrease in the adsorption of coumarin on the addition of H2O2 for rutile and anatase, respectively. However, the increase in the OHCCA generation on the addition of H2O2242 could not be explained by the increase of CCA adsorption, because 97% of CCA has been adsorbed without H2O2. Thus, the direct− indirect model could not fully explain the results of the fluorescence probe experiments with coumarin and CCA. Consequently, concerning the detailed oxidation mechanism, all processes in Figure 39 may be possible depending on the strength of the interaction of the reactant molecules with the photocatalyst surface. For chemisorbed reactants which can modify the surface electronic states of photocatalyst, direct oxidation ① takes place without passing trapped holes.194,303 The physisorbed reactant which could be concentrated near the photocatalyst surface can be oxidized with trapped holes ② or adsorbed •OH ③. In the case that the reactant molecules are scarcely adsorbed on the photocatalyst surface, OH radicals which are proved to diffuse into solution228 oxidize molecules in solution ④.

C3H7OH, and CH3COOH. These radical species could not be detected in the presence of O2, suggesting the possibility of subsequent rapid reactions.305 With low temperature ESR measurements in the presence of O2, the radical species CH3COCH2OO•,306 CH3CO(OO•),307 and PhCH2OO•308 for acetone, acetaldehyde, and toluene have been identified, respectively. Thus, peroxide radicals ROO• for organic compounds RH are usually generated by photocatalytic oxidation via carbon radical •R as given in eqs 26 and 27. Reaction 27 proceeds usually very rapidly as expected by the large bimolecular rate constants (>109 M−1 s−1) reported.309 RH + h tr + → •R + H+ •

R + O2 → ROO•

(26) (27)

For the oxidation of benzene to produce phenol, peroxy species are known to be involved only on the rutile surface as shown in Figure 42.310 Based on the isotope analysis for O2,

Figure 42. Possible mechanism for production of phenol from benzene (A) by an oxygen transfer process using water as the oxygen source and (B) by a hole transfer process using O2 as the oxygen source through peroxyl radical. Reprinted from ref 310. Copyright 2010 American Chemical Society.

benzene is oxidized to phenol efficiently on anatase particles with an oxygen transfer process using water as the oxygen source (Figure 42A). The atomic configuration on the surface of the anatase particles is favorable for their efficient production using water as the oxygen source. On the other hand, in the case of rutile particles, the contribution of the oxygen transfer process is small and the hole transfer process becomes dominant. As shown in Figure 42B, in the hole transfer process, benzene is found to be oxidized to the cationic radical, which further becomes organic peroxide ions or radicals and then finally is oxidized to phenol.310 7.3.2. Oxidation Process with Ozone. Ozone (O3) is poisonous gas with solubility of about 1.05 g/L (=22 mM). In aqueous solution it decomposes to O2 (half-life is 20 min at 20 °C).311 Since the reaction can be easily controlled due to selfextinction with leaving the inert molecule of O2, O3 is used as ozonation in advance oxidation processes.293,312 The concentration of O3 was nearly constant (7.5 × 10−6 M) during ozonation.312 On the ozonation, organic compounds (RH) react with O3, and then organic radical •R and •OH could be produced as in eq 28.293,313

7.3. Other Minor ROS (ROO•, O3, and •NO)

Besides the four major ROS described above, organic peroxide ROO•, ozone O3, and nitric oxide •NO are also accounted as reactive oxygen species. Although they do not often appear in common photocatalysis, reactions with these minor ROS in photocatalysis are briefly reviewed in sections 7.3.1, 7.3.2, and 7.3.3. 7.3.1. Formation of Organic Peroxide (ROO•). In the absence of O2, carbon radicals were detected by ESR spectroscopy in TiO2 photocatalysis of organic compounds in aqueous suspension. They are •CH2OH, •CH(OH)CH3, •C(CH3)2OH,297 •CH3, and •CH2COOH297,304 resulting from the corresponding oxidation of CH3OH, C2H5OH, iso11326


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RH + O3 → •R + •OH + O2

On the other hand, the reduction of HNO2 to •NO in TiO2 photocatalysis, eq 35, has been reported.324

(28)

Though O3 is usually manufactured in gas phase, it could also be produced by the oxidation in water. However, O3 could not be generated by the photocatalytic reaction from either O2 or H2O, because the standard potential for eq 29 is as high as E0(O3,2H+/O2,H2O) = 2.075 V. In other words, O3 has a large oxidation potential. O3 + 2H+ + 2e− → O2 + H 2O

HNO2 + H+ + e− → •NO + H 2O 0

Then, O3 could be reduced more easily than O2 because the reduction potential for O3 is E0(O3/•O3−) = +1.01 V.35 Therefore, in the photocatalytic reaction, O3 can be reduced as in eq 30 in place of O2. The reduction product •O3− decomposes rapidly to generate •OH314 with the rate constant of k(31) = 9 × 1010 M−1 s−1.309 •

O3−

+

+ H → OH + O2

(31)

For wastewater treatments, photocatalysis is used together with ozonation. Combining photocatalytic reactions eqs 30 and 31 with the ozonation eq 28, the synergy effect has been anticipated. The synergy index, which is defined by the ratio of the removal efficiency for the combined system against the sum of the removal efficiency of each system, became up to 4.3 for some cases by using metal-ion doped TiO2 as a photocatalyst.315 By using a nanocomposite of TiO2/montmorillonite, the synergy factor, which is defined with decomposition rates, was 1.6.316 Using porous g-C3N4 as an ozone photocatalyst for hydroxybenzoic acid degradation, the synergy factor of 6.5 was reported.314 7.3.3. Generation and Decomposition of Nitric Oxide (•NO). Nitric oxide, •NO, is one of the nitrogen oxides, NOx, which is a kind of air pollutant. In air it is slowly oxidized with O2 to generate NO2 as in eq 32. 2•NO + O2 → 2NO2

Table 1. Recommended Detection Methods for Each ROS in Photocatalysis

(32)

Removal of •NO from air by means of photocatalysis is one of the important issues for environmental cleaning and the reaction was well elucidated as the standard test method of ISO 22197.317,318 In the removal process of •NO, it is oxidized stepwise through HNO2 and NO2 to HNO3.317 In the photooxidation of •NO with a graphitic carbon nitride (gC3N4) photocatalyst, •O2− plays an important role.319 Furthermore, the photocatalytic processes for the removal of NOx have been reviewed recently.320,321 In the biological field, • NO plays important roles as a cell-signaling molecule, an antiinfective agent, and an antioxidant, though there is some debate about the exact role of •NO in its antimicrobial effects.21 • NO is slightly soluble in water with a solubility of 5.6 mg/ 100 mL (=1.9 mM). In solution the generation of •NO by the oxidation of N2 may not be easy because the standard potential for eq 33 is E0(2•NO,4H+/N2,2H2O) = 1.678 V. 2•NO + 4H+ + 4e− → N2 + 2H 2O

detection method •

O2−

H2O2 1

O2

•

OH

(33)

On the other hand, the reduction of NO3 , eq 34, could easily generate •NO because of the positive reduction potential, E0(NO3−,4H+/•NO,2H2O) = 0.957 V.34 −

comments

MCLA Luminol could be used for alkaline chemiluminescence40 solutions. lucigenin Luminol could be used for alkaline chemiluminescence40 solutions. 1270 nm emission31 For every methods, the increase in D2O is essential. coumarin fluorescence DMPO spin trapping ESR method would probing242 be used for trapped holes.

Though there are some debates on the processes, the plausible generation processes of ROS in TiO2 photocatalysis are illustrated in Figures 37 and 38 for the bridged O site and the terminal OH site on the surface, respectively. In our perspective, the adsorbed •OH could be regarded as the trapped holes, suggesting the rapid exchange between the surface trapped holes and the •OH in solution, or the adsorption−desorption equilibrium for •OH.245,246,263,264 Since the equilibrium shifts to the adsorption, the photocatalytic oxidation mainly takes place at the solid surface with trapped holes or adsorbed OH, but in the case of less adsorption of reactants it may take place in the solution as well. On the rutile surface, a pair of trapped holes can combine to form bridged peroxo species which could be regarded as the adsorbed H2O2.110,111,130 OH radicals are also produced by the

−

NO3− + 4H+ + 3e− → •NO + 2H 2O

(35) 34

8. CONCLUSIONS AND PERSPECTIVE It is worth noting that the surface reactions govern the overall course of the photocatalytic chemical transformations. Since the surface functions as catalyst, the surface adsorption aspects should be taken into account on considering the surface redox reaction in photocatalysis. Thus, “perspective” reactions should be taken into account on discussing the detailed reactions though most reports on photocatalysis just accept easily the “accomplished” reactions which are regarded as seemingly correct. In this sense the detection and the quantitative analysis of ROS as primary intermediate species in photocatalysis are inevitable for understanding the surface redox reactions. In this review, detection methods of major ROS (•O2−, H2O2,1O2, and •OH) in photocatalysis were thoroughly surveyed and the generation mechanism for each ROS and each reaction between major ROS have been discussed for photocatalysis. Based on the survey, the most suitable detection methods for each ROS are listed in Table 1 with some comments.

(30)

•

+

Since E (NO3 ,3H /HNO2,H2O) = 0.950 V, the reduction potential for eq 35 is calculated with eq 34 to be E0(HNO2,H+/•NO,H2O) = 0.007 V, which suggests easier reduction than O2. However, the generation of •NO was transient, because it was further reduced to NH4+.322 On the other hand, •NO could be selectively reduced to N2 by using TiO2 photocatalysts in the prescence of carbon black which is oxidized to CO2.325 Complete reduction of •NO to N2 has been reported with a TiO2 photocatalyst by using bioelectrons from microbial metabolism as the hole scavenger.326

(29)

O3 + e− → •O3−

−

(34) •

Actually, in TiO2 photocatalysis, NO3 is reduced to NO in aqueous solution by using formic acid as a hole scavenger.322,323 11327


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(5) Photocatalysis: Fundamentals and Perspectives; Schneider, J., Bahnemann, D., Ye, J., Puma, G. L., Dionysiou, D. D., Eds.; Royal Society of Chemistry: Cambridge, 2016. (6) Photocatalysis: Fundamentals, Materials and Potential; Pichat, P., Ed.; MDPI AG: Basel, 2016. (7) Heterogeneous Photocatalysis: From Fundamentals to Green Applications (Green Chemistry and Sustainable Technology); Colmenares, J. C., Xu, Y.-J., Eds.; Springer: Heidelberg, 2016. (8) Ameta, R.; Ameta, R. C. Photocatalysis: Principles and Applications; CRC Press: Boca Raton, FL, 2017. (9) Spasiano, D.; Marotta, R.; Malato, S.; Fernandez-Ibãnez, P.; Di Somma, I. Solar photocatalysis: Materials, reactors, some commercial, and pre-industrialized applications. A comprehensive approach. Appl. Catal., B 2015, 170−171, 90−123. (10) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253−278. (11) Abe, R. Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation. J. Photochem. Photobiol., C 2010, 11, 179−209. (12) Takata, T.; Pan, C.; Domen, K. Recent progress in oxynitride photocatalysts for visible-light-driven water splitting. Sci. Technol. Adv. Mater. 2015, 16, 033506. (13) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Titanium dioxide-based nanomaterials for photocatalytic fuel generations. Chem. Rev. 2014, 114, 9987−10043. (14) Dasuri, K.; Zhang, L.; Keller, J. N. Oxidative stress, neurodegeneration, and the balance of protein degradation and protein synthesis. Free Radical Biol. Med. 2013, 62, 170−185. (15) Schieber, M.; Chandel, N. S. ROS function in redox signaling and oxidative stress. Curr. Biol. 2014, 24, R453−R462. (16) Wang, X.; Wang, W.; Li, L.; Perry, G.; Lee, H.; Zhu, X. Oxidative stress and mitochondrial dysfunction in Alzheimer’s disease. Biochim. Biophys. Acta, Mol. Basis Dis. 2014, 1842, 1240−1247. (17) Sosa, V.; Moliné, T.; Somoza, R.; Paciucci, R.; Kondoh, H.; LLeonart, M. E. Oxidative stress and cancer: An overview. Ageing Res. Rev. 2013, 12, 376−390. (18) Wrzaczek, M.; Brosche, M.; Kangasjarvi, J. ROS signaling loops  production, perception, regulation. Curr. Opin. Plant Biol. 2013, 16, 575−582. (19) Ray, P. D.; Huang, B.-W.; Tsuji, Y. Reactive oxygen species (ROS) homeostasis and redox regulation in cellular signaling. Cell. Signalling 2012, 24, 981−990. (20) Fu, P. P.; Xia, Q.; Hwang, H.-M.; Ray, R. C.; Yu, H. Mechanism of nanotoxicity: Generation of reactive oxygen species. J. Food Drug Anal. 2014, 22, 64−75. (21) Vatansever, F.; de Melo, W. C. M. A.; Avci, P.; Vecchio, D.; Sadasivam, M.; Gupta, A.; Chandran, R.; Karimi, M.; Parizotto, N. A.; Yin, R.; Tegos, G. P.; Hamblin, M. R. Antimicrobial strategies centered around reactive oxygen species - bactericidal antibiotics, photodynamic therapy and beyond. FEMS Microbiol. Rev. 2013, 37, 955−989. (22) Ma, H.; Williams, P. L.; Diamond, S. A. Ecotoxicity of manufactured ZnO nanoparticles - A review. Environ. Pollut. 2013, 172, 76−85. (23) Buettner, G. R. Spin trapping: ESR parameters of spin adducts. Free Radical Biol. Med. 1987, 3, 259−303. (24) Maghzal, G. J.; Krause, K.-H.; Stocker, R.; Jaquet, V. Detection of reactive oxygen species derived from the family of NOX NADPH oxidases. Free Radical Biol. Med. 2012, 53, 1903−1918. (25) Mahé, E.; Bornoz, P.; Briot, E.; Chevalet, J.; Comninellis, C.; Devilliers, D. A Selective chemiluminescence detection method for reactive oxygen species involved in oxygen reduction reaction on electrocatalytic materials. Electrochim. Acta 2013, 102, 259−273. (26) Woolley, J. F.; Stanicka, J.; Cotter, T. G. Recent advances in reactive oxygen species measurement in biological systems. Trends Biochem. Sci. 2013, 38, 556−565. (27) Hawkins, C. L.; Davies, M. J. Detection and characterization of radicals in biological materials using EPR methodology. Biochim. Biophys. Acta, Gen. Subj. 2014, 1840, 708−721.

reduction of H2O2 maybe mainly on the rutile surface but not on the anatase surface. In this review various photocatalytic reactions of TiO2 were mainly described, since it is the only commercially applied photocatalyst, but the reaction mechanisms are still under debate. As practical future applications, TiO2 photocatalysts must be modified for environmental cleaning exerting under visible light, and narrow band gap semiconductor photocatalysts should be developed for water photosplitting. For these new photocatalysts, the research in chemical reaction mechanisms by detecting ROS would be prerequisite in terms of designing and modifying efficient photocatalysts.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Yoshio Nosaka: 0000-0003-1305-5724 Notes

The authors declare no competing financial interest. Biographies Yoshio Nosaka finished his doctoral course in chemistry at the Graduate School of Science, Kyoto University, in 1977, majoring in magnetic resonance spectroscopy and radiation chemistry and became an Encouraged Researcher of JSPS (Kyoto University), a postdoctoral research fellow (1978−1980) at the RIKEN Institute, Wako, dedicated to research for radiation chemistry and photochemistry, and then joined the faculty of Nagaoka University of Technology (1980−2015), meanwhile becoming a visiting researcher (1985−1986) at The University of Texas, Austin. He retired from the position at the university in 2015 and became an emeritus professor. His main research area is physical chemistry of advanced materials, such as semiconductor nanoparticles and photoelectrodes. http:// researchmap.jp/ynos/?lang=english. Atsuko Yamada Nosaka completed her Ph.D. from the Graduate School of Science of Kyoto University in 1977. She worked as a postdoctoral research fellow of the Alexander von Humboldt Foundation at the Max-Planck-Institute in Heidelberg (1978−1979), a research associate at The University of Texas, Austin (1985−1986), a senior research associate at GRI-NIH, Baltimore (1986−1987), a research associate of Tokushima University (1987−1989), and a research group leader at the International Research Laboratory of Ciba-Geigy AG (1989−1998). Afterward, she joined Prof. Yoshio Nosaka’s research group and was engaged in research on TiO2 photocatalysis and on the characterization of water contained in the membranes used for fuel cells as a researcher of a project conducted by NEDO, Japan. Her main research areas are (1) characterization of water molecules adsorbed on solid and membrane surfaces and (2) structural characterization of biomolecules mainly by means of NMR spectroscopy.

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