Understanding Hydroxyl Radical (•OH) Generation Processes in Photocatalysis
P
activity should depend on the kinds of crystal polymorphs. It is noticed that for rutile polymorphs, the MB decomposition rate is correlated with •OH formation, as can be seen in Figure 1. In the figure, the activity of anatase is observed to be higher than that of rutile, as is well-known in the literature. With certain reactants, the photocatalytic activity might change depending on how firmly the reactant molecules are adsorbed on the photocatalyst surface because the •OH generated at TiO2 could diffuse to the bulk solution. Kim et al.3 suggested that the difference in the reactivity between anatase and rutile can be ascribed to the higher adsorption capability of the •OH for rutile and that only the adsorbed molecules are decomposed by rutile powders, as shown in Figure 2. However, the difference in the •OH formation process between anatase and rutile has not been addressed in sufficient detail.
hotocatalysts have been gathering much attention because they are recognized as useful for environmental cleanup because harmful organic pollutants and stains can be decomposed with the aid of photoenergy. Furthermore, they are expected to be utilized with solar energy to produce hydrogen fuel by decomposition of water.1 For efficient development of photocatalytic products, it is desirable to elucidate the actual mechanisms of photocatalysis. However, there are still a lot of controversial issues. In this Viewpoint, we will focus on one of the current most controversial issues about the formation process of OH radicals in TiO2 photocatalysis. We will show that the formation of OH radicals by the reduction of H2O2, which has been reported by many researchers including us, must not take place in TiO2 photocatalysis. Dif ference in the •OH Formation in Anatase and Rutile. In many reports, the OH radical is assumed to be the main reactant for oxidation processes in photocatalysis, which is deduced from the correlation between the amount of OH radical generation and the photocatalytic reactivity. For example, Figure 1 shows the decomposition rate of methylene
Figure 2. Illustration of photocatalysis on anatase (left) and rutile (right) exerted by •OH in solution (•OHf) and/or at the surface (•OHS). S represents a substrate molecule.3 Reprinted from ref 3, copyright 2014, with permission from John Wiley and Sons.
For quite a long time, we have been investigating the formation rate of OH radicals for various kinds of TiO2 powders along with the effect of small amounts of H2O2.4−7 The experimental results for 10 commercially available TiO2 powders, that is, three anatase, four rutile, and three mixedphase, showed dependence as illustrated in Figure 3.4 In addition, using the other different probe molecules, one of which could be adsorbed on a TiO2 surface,5 the results supporting Figure 3 could be also obtained. The decrease in •OH formation with an increase in H2O2 concentration observed for anatase might be explained by scavenging of •OH with H2O2. However, the increase of •OH formation with the increase of H2O2 concentration observed for rutile and mixed-phase (rutile-containing anatase) powders could not be explained by the reaction with H2O2 nor the
Figure 1. Dependence of the decomposition rate of MB on the formation rate of OH radicals. Closed marks correspond to the samples containing more than 95% anatase and open marks to the samples containing more than 5% rutile phase.2 Reprinted from ref 2, copyright 2007, with permission from Elsevier.
blue (MB) as a function of the formation rate of OH radicals for various heat-treated TiO2 photocatalysts.2 In the experiments, two series of photocatalysts were prepared by heat treatment starting with anatase crystallites. With increasing treatment temperatures up to 700 °C, the MB photodecomposition rate increased along with the formation of OH radicals. As the rutile component generated at highertemperature treatment increased, the MB decomposition rate trend was reversed. This fact suggests that the photocatalytic © XXXX American Chemical Society
Received: May 27, 2016 Accepted: June 30, 2016
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DOI: 10.1021/acsenergylett.6b00174 ACS Energy Lett. 2016, 1, 356−359
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ACS Energy Letters
anodic potentials. Figure 5 shows the electrode current in an electrochemical flow cell and the formation of •OH by a spin-
Figure 3. Dependence of the formation rate of •OH on the H2O2 concentration for TiO2 powders of various crystalline phases according to the experimental results in ref 4 and other reports.5,6
adsorption of •OH on the rutile surface. Hence, we should take into account the difference in the •OH formation process between anatase and rutile. • OH Formation f rom H2O2. To explain the increase of •OH with H2O2 observed for rutile and the mixed phase, the photocatalytic reduction of H2O2, eq 1, is usually adopted based on the possible redox potential of +0.73 V vs SHE at pH 7. H 2O2 + e− → •OH + OH−
Figure 5. (A) Stationary electrode current and (B) concentration of DMPO−•OH formed at various electrode potentials with a dispersed Pt particle electrode. The flowing electrolyte solution contains 90 mM H2O2, 10 mM DMPO, and 0.1 M H2SO4.11 Reprinted from ref 11, copyright 2009, with permission from Electrochemical Society Inc.
(1)
However, with this process, Figure 3 could reasonably be explained only for the case of the rutile surface.4 On the other hand, for the mixed phase, it is well-known that electrons transfer from the rutile to the anatase part,8 but •OH formation is increased with H2O2. Therefore, reaction 1 would be inconsistent. Previously we considered only the reduction process.4 However, the oxidation process should also be taken into account because in the actual photocatalysis, both processes must involve the same number of electrons and holes. To distinguish between both processes, namely, to make a distinction between the reduction and the oxidation, an electrochemical analysis would be useful. By using photoelectrodes of rutile single crystals, we found that in the oxidation process, the addition of H2O2 increased •OH formation (Figure 4).9 The photooxidation current in a
trapping ESR (electron spin resonance) method. The current− potential relationship (Figure 5A) shows that the H2O2 was reduced at potentials below 0.7 V (vs RHE), while at the higher potential, it was oxidized. The conversion potential of 0.7 V corresponds to the two-electron redox potential of [O2,2H+/ H2O2]. In the oxidation process of H2O2, OH radicals were formed similarly to the reduction process, as shown in Figure 5B. Thus, it was confirmed that •OH can be formed in the electrochemical oxidation process of H2O2. As indicated in Figure 3, for anatase TiO2, •OH formation decreased in the presence of H2O2, whereas it increased for rutile and rutile−anatase mixed crystals. This suggests that the formation process of •OH is different for anatase and rutile. Because the formation of •O2−, which is the reduction product of O2, significantly increased with H2O2 for any crystalline form,4,6,12 the reduction of H2O2 to •OH is not the proper • OH formation path. Hence, the different formation processes of •OH should be discerned. • OH Formation at the Surface of Bridged O Sites. •OH may be produced by the oxidation of H2O at the metal oxide surface, as summarized by reaction 2 OH− + h+ → •OH
(2) •
However, the details of the OH formation process seem important to understand the low activity of rutile. At the surface of TiO2, the OH group could be classified in two different coordination sites, namely, terminal OH and bridge OH. These two are in equilibrium with proton release at the aqueous interface, as represented by reactions 3 and 4, respectively.13,14
Figure 4. Effect of the addition of 0.1 mM H2O2 on the amounts of • OH produced in the oxidation of water at anodically polarized TiO2 single crystals of different orientations.9 Reproduced from ref 9, copyright 2015, by permission of the PCCP Owner Societies.
photoelectrolysis cell represents production of O2 from H2O with H2O2 as an intermediate.10 Under these experimental conditions, the •OH is produced as a byproduct in O2 formation from H2O via H2O2. Formation of •OH during electrochemical oxidation of H2O2 was observed even when a Pt powder electrode was used.11 It is known that the Pt surface carries a platinum oxide layer at
Ti−OH + H+ ⇄ Ti−OH 2+ −
+
(3)
[Ti−O −Ti] + H ⇄ [Ti−OH−T]
(4)
Reaction 3 is equivalent to reaction 3′ by taking into account the dissociation of surface-adsorbed water. 357
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Figure 6. Plausible photocatalytic reaction paths at a bridge OH site of TiO2. Solid arrows are for anatase, and dotted arrows for rutile.
Figure 7. Plausible photocatalytic reaction paths at the terminal OH site of TiO2.
Ti−OH ⇄ Ti+ + OH−
concerted nucleophilic attack of water,16 resulting in the formation of Ti−O• and Ti−OH (b). Structure (b) represents a surface-trapped hole, and naturally, a reverse process takes place by conduction-band electrons, which is carrier recombination. At the anatase surface, a certain fraction of trapped holes (b) probably release •OH into bulk solution.23,24 On the other hand, at the rutile surface, when a second hole is generated in the particle having the trapped hole (b), the hole migrates to combine with the existing hole to form peroxo (Ti−O−O−Ti) species at the surface (c). The difference of the reaction paths, that is, •OH release for anatase and dimer formation for rutile, may be caused by the difference of the Ti− Ti distance at the surface.6 The peroxo (c) in Figure 6 is the intermediate of water oxidation25 and probably equivalent to the adsorbed hydrogen peroxide as studied by measuring ATR FT-IR spectra,6 which is specific for rutile and colored yellow.26 In further oxidation of the peroxo (c), when the Ti−O bond was cleaved, Ti−OO• (d) was formed.10,16 This radical could release •O2− or be further oxidized to release O2 and then return to the original [Ti−O− Ti] (a). When the O−O bond of the peroxo (c) was cleaved by a third hole, the radical of Ti−O• (e) was formed to return to the peroxo (c) by releasing •OH similarly to the case of the trapped hole (b) for anatase. On the other hand, at the Ti(4+) or terminal OH sites of the surface, the photocatalytic reduction proceeds, as shown in Figure 7. A photoinduced conduction-band electron is trapped at the surface Ti(4+) to become Ti(3+) (b).15 The Ti(3+) could reduce the molecular oxygen to form the adsorbed •O2− (d), which would become an adsorbed H2O2 (c) by further reduction. The produced or added H2O2 are in the adsorbed
(3′)
For the formation process of •OH based on eq 3, the following simple equation is provided sometimes in the literatures15 Ti−OH + h+ → Ti+ + •OH
(5)
However, as suggested in eq 3′, the excess charge of the Ti atom of terminal OH is positive, and the positive hole prevents the attacking of the Ti−O bond of the terminal OH.16,17 Therefore, the mechanism of reaction 5 at the terminal OH must not take place.17−19 On the other hand, the O in the bridge OH in eq 4 has negative charge, suggesting that this O atom should be energetically favorable to attract positive holes. Adsorbed •OH and Trapped Holes. It was reported that when the •OH generated by ionization radiation is adsorbed on the TiO2 surface, the redox potential is 1.520 or 1.6 V,21 which corresponds to the potential of the trapped hole estimated from the measurements of photoluminescence.16 Because the signal of the •OH could not be detected even by the low-temperature ESR method,22 the •OH may become an •O− because the pKa of 11.9 (in homogeneous solution) probably shifts by the adsorption on TiO2.24 It is not strange that the adsorption on a cationic site of the TiO2 surface takes place in the form of •O− in neutral pH solution. Thus, the surface-trapped holes can be regarded as adsorbed •OH. Rational Model of •OH Formation in TiO2 Photocatalysis. On the basis of the above discussions, a plausible process at O(2−) (or bridged OH) sites of the TiO2 surface5,6,10 is shown in Figure 6 where the processes specific for anatase and rutile are distinguished with different arrow lines. A photoinduced valence-band hole attacks the bridged O(2−) site (a) with 358
DOI: 10.1021/acsenergylett.6b00174 ACS Energy Lett. 2016, 1, 356−359
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ACS Energy Letters state on the Ti(4+) site (c) exclusively for anatase TiO2.6,26 The adsorbed H2O2 (c) could be oxidized to become an adsorbed • O2− (d) and return to the original surface (a) with release of • O2− or be reduced to become a trapped electron (b) with release of H2O2. In the reduction of the adsorbed H2O2 (c), the formation of OH radical is unlikely because the O−O bond should be dissociated. In conclusion, though many possible pathways involving H2O, •OH, H2O2, •O2−, and O2 as reactive species have been proposed for the redox reactions in TiO2 photocatalysis, the actual reactions that could take place at the surface may be restricted by specific adsorption and electric charges. We have detected •O2− and H2O2 as well as •OH in TiO2 photocatalysis.4,6,12 On the basis of the results, the most probable reaction routes for anatase and rutile surface are as shown in Figures 6 and 7. The difference in the process for anatase and rutile shown in Figure 6 is basically consistent with that suggested by Kim et al. (Figure 2) as long as the adsorbed •OH and H2O2 are regarded as a trapped hole and a surface peroxo group, respectively. As for •OH formation, the adsorbed H2O2 of peroxo (e) in Figure 6 may be reduced to trapped holes (b), but •OH radical is not released in the case of rutile. Furthermore, the reduction of adsorbed H2O2 (c) in Figure 7 cannot lead to form •OH but desorb H2O2 and cause a trapped electron (b). Thus, it was concluded that OH radical in solution could not be generated by the reduction of H2O2 nor O2. In this Viewpoint, we would like to draw one’s attention to photocatalyst preparation with desirable properties. It is important to establish the reaction pathways while designing new photocatalysts because surface reactions dictate the overall course of chemical transformations.
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Yoshio Nosaka* Atsuko Nosaka
Department of Materials Science and Technology, Nagaoka University of Technology, Nagaoka 940-2188, Japan
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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