On Oxygen Activation at Rutile- and Anatase-TiO2 - ACS Catalysis

Nov 5, 2015 - On the basis of literature examples and our own studies, we ..... intermediates, and products at the surface of photocatalysts and (ii) ...
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On oxygen activation at rutile- and anatase-TiO2 Marta Buchalska, Marcin Kobielusz, Anna Matuszek, Micha# Pacia, Szymon Wojty#a, and Wojciech Macyk ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01562 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 11, 2015

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ACS Catalysis

On oxygen activation at rutile- and anatase-TiO2 Marta Buchalska,† Marcin Kobielusz,† Anna Matuszek,†,‡ Michał Pacia,† Szymon Wojtyła† and Wojciech Macyk†* † ‡

Faculty of Chemistry, Jagiellonian University in Kraków, Ingardena 3, 30-060 Kraków, Poland Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand

ABSTRACT: Rutile form of titanium dioxide (r-TiO2) usually shows a lower photocatalytic activity when compared to anataseTiO2 (a-TiO2). Nevertheless, there are numerous examples of sometimes unexpectedly high activity of r-TiO2. This material may appear particularly useful when a non-complete and selective photocatalytic oxidation of organic substrates is required. Basing on literature examples and our own studies we compare the photocatalytic activities of r-TiO2, a-TiO2 and r-TiO2/a-TiO2 composites. Due to a significantly better oxygen adsorption at the surface of r-TiO2 and a lower redox potential of the excited electron, a more efficient O2•– production takes place at the surface of rutile. As a consequence, also generation of 1O2 (involving O2 reduction and the subsequent oxidation of superoxide) and reduction of H2O2 to OH– and HO• are favored at this material. Therefore r-TiO2 can be considered as a particularly good photocatalyst for activation of molecular oxygen. On the other hand, a-TiO2 appears a stronger oxidant. In its presence an efficient HO• generation (the result of water or surface hydroxyl groups oxidation) and H2O2 oxidation to O2•– are observed.

KEYWORDS: rutile · anatase · titanium dioxide · oxygen activation · photocatalysis

INTRODUCTION The differences between rutile and anatase structures of titanium dioxide have a strong influence on their physicochemical properties. Rutile-TiO2 (r-TiO2) is a more stable and usually less photoactive form of titanium dioxide. It exhibits a lower electron mobility, a higher dielectric constant and a higher density than anatase-TiO2 (a-TiO2).1 The point of zero charge of TiO2 increases with the content of rutile in the sample.2 The rates of charge recombination are lower in the case of r-TiO2 materials.3 The difference between a-TiO2 and r-TiO2 reflects also in interactions of these materials with dioxygen. Molecular oxygen interacts weakly with a stoichiometric TiO2 surface, but shows a high affinity to oxygen vacancies.4 Under cryogenic conditions three oxygen molecules may interact with one vacancy.5 Opposite to rutile, in anatase the oxygen vacancies are localized under the most stable (101) facets.6 In consequence, the interaction of oxygen molecules with the (101) surface of anatase is weaker than with the most stable (110) facet of rutile.6 Therefore significant differences in the concentration of adsorbed oxygen are observed in the case of both forms of TiO2. Adsorption of O2 is associated with an electron transfer from the surface defects and results in formation of superoxide anion.4, 7 This reactive oxygen species presents a relatively low reactivity towards organic compounds oxidation, due to its disproportionation to molecular oxygen and H2O2. However, O2•– plays an important role in the oxidation

processes carried out in acidic and neutral solutions.8 O2•– participates also in early stages of degradation of phenolic compounds, while hydroxyl radicals react with the formed intermediates leading to their complete mineralization.9

Generation of reactive oxygen species at TiO2 surface Oxygen is an oxidant characterized by its unique triplet ground state. A total reduction of 3O2 is a four-electrons process with water as an end-product (E° = 1.23 V vs. SHE). The multiplicity of electron donor should also amount 3. Due to the scarcity of such donors 3O2 remains relatively inactive in oxidation reactions. The processes of partial reduction of oxygen result in formation of reactive oxygen species (ROS). This group of reactive products includes superoxide anion (O2•–), its protonated form – hydroperoxyl radical (HOO•), hydrogen peroxide (H2O2) and hydroxyl radical (HO•) being one of the strongest oxidants. Also the non-radical form, singlet oxygen 1O2, belongs to the group of ROS.10 Several mechanisms lead to the formation of reactive oxygen species. Reduction of molecular oxygen by electrons, eCB–, with superoxide anion formation, and water oxidation by holes, hVB+, with generation of hydroxyl radicals, are the main primary redox processes ruling the photocatalysis at TiO2 in aqueous environments.11 Further reduction and protonation of superoxide anion leads to hydroperoxyl radical, hydrogen peroxide, hydroxyl radical and water. Therefore hydroxyl radicals can be generated in two different pathways, as the

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result of either oxidation of water or a multistep reduction of oxygen.12

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amount ca. 0.4 eV, therefore the photogenerated electrons should move efficiently from rutile to anatase particles.

Singlet oxygen is generated either through a direct energy transfer from the excited state of a photosensitizer (usually an organic dye) to oxygen, or as a consequence of consecutive redox reactions of O2. The Nosaka’s group put forward the hypothesis, that 1O2 is formed in the presence of TiO2 according to the latter mechanism: molecular oxygen is reduced to superoxide radical anion, which is then oxidized by holes forming singlet oxygen.13 This process can also occur in the presence of sensitized TiO2 materials.14

Photoactivity of rutile/anatase composites Unusually high photocatalytic activities of TiO2 have been reported for anatase/rutile composites. A good example is the P25 material produced by Evonik consisting of 70-80% of anatase and 30-20% of rutile. Since the photoreactivity of this composite is, in general, higher than that observed for bare aTiO2 and r-TiO2, P25 is often used as a reference material, although homogeneity of P25 is disputable. A low homogeneity of P25 leads to considerable differences in photocatalytic activity of various samples of this reference material.15 The generally reported high photoactivity of P25 is very likely associated with charge transfer taking place between particles of different phases. At least two different pictures of these processes exist in literature.16 Ohno et al. showed that in a mixture of large rutile crystals and smaller anatase particles the rutile phase is photoexcited. Although the conduction band edge of a-TiO2 is considered to be characterized by ca. 0.2 eV higher energy than the corresponding CB edge of r-TiO2,16d, 17 electrons are transferred from r-TiO2 to surface states of aTiO2, from where they are consumed in oxygen reduction (Fig. 1). Photogenerated VB holes of rutile can oxidize an organic pollutant directly, or indirectly, through hydroxyl radicals.16a This theory is supported by EPR studies of Thurnauer and coworkers who suggested that electrons excited within rutile crystals are stabilized within lower energy states of anatase.16b, 16c On the other hand, there are evidences of the electron transfer in the opposite direction, from the conduction band of anatase to CB of rutile,16d-f in accordance with the relative energies of CB edges of a-TiO2 and r-TiO2 presented in Fig. 1. Trapping of excited electrons within the r-TiO2 component of the composite was supported by the results of EPR measurements.16e, 16f In both cases the charge separation would lead to an increased lifetime of the electron-hole pair and a higher photoactivity of the r-TiO2/a-TiO2 composite. Thurnauer points also at the distorted four-coordinated interfacial sites localized at the interface between a-TiO2 and r-TiO2, which might act as catalytic “hot spots” decreasing the activation barrier of the electron transfer processes.16b, 16c Recent calculations of Scanlon et al. revealed the lower potential of the CB edge of rutile than of anatase, opposite to the situation shown in Fig. 1.16a The band alignment should

Figure 1. Two possible directions of electron transfer between aTiO2 and r-TiO2 in composite materials: from rutile to anatase16a-c and from anatase to rutile.16d-f

Phase composition vs. photoactivity Numerous studies have been dedicated to the influence of the anatase/rutile ratio on the photocatalytic activity.16a, 18 Usually a high content of more reactive anatase does not guarantee the high activity of the whole composite. Ohno et al.16a studied the photocatalytic degradation of naphthalene in the presence of a-TiO2 calcined at different temperatures. The material annealed at 800°C, containing 90% of r-TiO2, appeared the most active sample. This material was slightly better than P25 and nearly 3 times more active than the starting a-TiO2.16a Similar results were observed by other researchers in the processes of various compounds photodegradation, e.g. methylene blue,18 Acid Red 1,19 phenol,20 or acetaldehyde21 (the gas phase oxidation in the last case). Photocatalysts calcined at various temperatures were photoactive until both a-TiO2 and r-TiO2 were present in the samples. Apparently, the optimal composition of the photocatalyst cannot be generalized since it depends inter alia on the substrate that is oxidized in the photocatalytic process.

Substrate nature vs. photoactivity As often demonstrated, several photocatalytic reactions can be realized in the presence of r-TiO2, although usually the efficiencies of these processes are lower than those observed for a-TiO2. For example, the efficiency of 2-propanol oxidation in the presence of irradiated r-TiO2 was found to reach ca. 40% of that measured for a-TiO2.22 A significantly lower photoactivity of r-TiO2 is, however, not a general rule. Tayade et al.23 studied a photocatalytic degradation of various organic compounds (acetophenone and nitrobenzene among others) in the presence of both a-TiO2 and r-TiO2. Although a-TiO2 in most cases showed a higher photoactivity, r-TiO2 also appeared active. In particular, the degradation rate of acetophe-

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none in the presence of rutile was almost as high as at anatase. Similar rates were observed also for nitrobenzene, however, degradation of this compound proceeded generally much slower than the decomposition of acetophenone, due to a higher stability of the former compound. Considering structures of acetophenone and nitrobenzene it can be noticed, that oxidation of these compounds may also be initiated by a DielsAlder addition of singlet oxygen. A possible role of singlet oxygen in photocatalytic oxidation reactions will be discussed hereafter. Presented works confirm the varying photoactivity of r-TiO2. Depending on the substrate structure it may be almost inactive or more active than a-TiO2. This leads to a conclusion, that either adsorption of substrates at r-TiO2 and a-TiO2 differs dramatically or the photocatalytic oxidation in the presence of these two materials follows various pathways and, apparently, involves various reactive oxygen species.

The influence of H2O2 The efficiency of hydroxyl radicals and superoxide anions formation can be very different when comparing r-TiO2 and aTiO2 (vide infra).24 In addition, the presence of H2O2 can dramatically influence the generation of HO• and O2•–, differently at r-TiO2 and a-TiO2. For instance, the production of hydroxyl radicals at rutile surface is very inefficient, however, generation of this reactive species in the presence of H2O2 occurs with high yields at r-TiO2 and a,r-TiO2 composites (10-20% of rutile), but not at a-TiO2.24 Moreover, in aqueous solutions a higher stability of O2•– is offered by the rutile surface, whereas at a-TiO2 superoxide radical is efficiently consumed to produce hydrogen peroxide. The phenomenon of a significant difference in activity of r-TiO2 in the presence and absence of H2O2 was explained by different adsorption properties of both materials.24 Only in the case of r-TiO2 the η2-peroxide structure can be formed (both oxygen atoms of H2O2 are bound to one Ti site, Figure 2), leading to the formation of hydroxyl radicals from H2O2.24 Similar conclusions can be drawn from the results of research done by the Ohno’s group. Photooxidation of adamantine was achieved in the presence of both crystalline forms of TiO2.25 Oxidation of adamantine by hydroxyl radicals proceeded selectively leading to 1-adamantanol, 2-adamantanone and 2-adamantanol as main products.25a The most significant difference could be observed in the efficiency of 2adamantanol generation – it was the lowest in the presence of r-TiO2 and the highest in the presence of a-TiO2. A high yield of 2-adamantanol generation was observed when H2O2 photolysis was done in the presence of adamantine. At r-TiO2 the reaction strongly accelerated upon addition of hydrogen peroxide.25b These results are in accordance with the enhanced generation of hydroxyl radicals at r-TiO2 in the presence of H 2O 2.

Figure 2. Mechanism of hydroxyl radical generation at the surface of r-TiO2 involving η2-peroxide structure.24

Partial oxidation reactions Benzene oxidation in air showed that 60-80% of the product obtained in the presence of r-TiO2 material contains –OH groups originating from O2 molecules. In the case of anatase 70-90% of formed phenol contained –OH groups originating from water. The surface oxide and peroxide radicals could be the source of oxygen as proposed in Fig. 3. Further oxidation steps of benzene (to CO2 as the final product) were observed with a higher efficiency in the presence of a-TiO2, while at rTiO2 large amounts of phenol were formed.26

Figure 3. The mechanism of benzene hydroxylation in the presence of anatase and rutile with water (A) and O2 (B) as the oxygen source. The contribution of both pathways in phenol formation in the presence of r-TiO2 and a-TiO2 (C).26

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Other studies on partial oxidation of methoxybenzyl alcohol to p-anisaldehyde in the presence of various forms of TiO2 showed that the selectivity of the process differs for rutile, anatase and their composites.27 R-TiO2 (both, prepared by authors and a commercial material) appeared nearly twice as efficient in the aldehyde generation as a-TiO2. Moreover, the rate of this reaction in the presence of P25 was lower than in the case of r-TiO2 or a-TiO2. Rutile materials showed the highest selectivity of the aldehyde formation. A-TiO2 and a,rTiO2 photocatalyzed efficiently a complete oxidation of alcohol. This effect can be explained by a weaker p-anisaldehyde bonding to the rutile surface than to anatase or P25, therefore a further oxidation of p-anisaldehyde was hindered due to its diffusion from the surface of the catalyst. Also a selective oxidation of benzyl alcohol to benzaldehyde was achieved in the presence of rutile nanorods.28 The selectivity of the process was very high – benzaldehyde was produced with a selectivity of 99%. A high density of surface hydroxyl groups was the key issue to reach a high selectivity of the photocatalyst. Desorption of aldehyde was facilitated by a high surface hydrophilicity (the consequence of a high density of surface hydroxyl groups). In the case of a-TiO2 adsorption of benzaldehyde was better and in consequence its further oxidation to benzoic acid took place.28 Very interesting results of L-alanine photooxidation were presented by Matsushita et al.29 The distribution of products of this reaction (pyruvic acid and acetamide), as well as the reaction yield, depended strictly on the crystal structure of the photocatalyst. R-TiO2 photocatalyzed a direct oxidation of Lalanine to pyruvic acid. In the presence of a-TiO2 acetamide was formed through condensation of acetaldehyde with ammonia, the products of L-alanine decomposition. The products distribution in the presence of P25 was similar to that observed for a-TiO2.29 Irradiation of the naphthalene solution (in water/acetonitrile 99:1) in the presence of TiO2 also led to formation of various products, depending on crystal structure of the applied photocatalyst. In the first step of reaction a partial oxidation of aromatic ring took place, similarly to the process of benzene oxidation,26a with generation of hydroxynaphthalene in the first step. In the presence of hydroxyl radicals the substitution of the next hydrogen atom with hydroxyl group occurred at the neighboring position. As the result of further oxidation steps the ring opening with generation of e.g. phthalic anhydrate was observed, in particular in the presence of r-TiO2. A further oxidation to CO2 as the final product was more efficient at aTiO2. The first steps of the photocatalytic oxidation of naphthalene were similar to those valid for benzene oxidation (Fig. 3).16a, 30 The presented state of art shows, that the differences in photoactivity of r-TiO2 and a-TiO2 can be attributed to two main factors: i) different adsorption of substrates, intermediates and

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products at the surface of photocatalysts, and ii) various mechanisms of reactive oxygen species generation and their further transformations. In our studies we focus on the differences in oxygen activation at r-TiO2 and a-TiO2. For our studies we have selected four commercial TiO2 materials: r-TiO2 (TR, Tronox), nano-r-TiO2 (n-r-TiO2, Sachtleben), a-TiO2 (AK-1, Tronox) and P25 (Evonik).

EXPERIMENTAL SECTION Materials: Evonik P25 (70% anatase/30% rutile; specific surface area of 58 m2/g), anatase a-TiO2 – Tronox AK-1 (>97% anatase; 90 m2/g), rutile r-TiO2 – Tronox TR (>99.5% rutile; 5.5 m2/g), nanocrystalline rutile, n-r-TiO2 – Sachtleben (“Nano-Rutil” E3-692-011-009, >99% rutile; 122 m2/g). Brunauer-Emmett-Teller (BET) measurements were performed using an ASAP 2010 (Micromeritics) instrument. The samples were heated at 250°C for 3 h under vacuum prior to the measurements. The specific surface area of the samples was estimated using the BET equation (nitrogen adsorption at 77 K). Measurements of hydrodynamic diameter were performed using a Malvern Zetasizer NanoZS instrument. Surface conditioning: All materials were heated under vacuum conditions at 150°C for 5 h. Afterwards the samples were stored in argon atmosphere. Oxygen adsorption: PyroScience FireStingO2 oxygen meter was used to analyze dissolved oxygen concentration in all measurements. In order to determine the amount of oxygen adsorbed at tested materials the previously reported procedure was applied.31 Equal volumes of air saturated water (100 µL) was injected (11 times) into argon saturated titanium dioxide suspension (5.0 mL, 1 g/dm3). The injections were done in 5 min intervals. After injection of the last air saturated water dose the oxygen concentration was monitored for 15 minutes, until a stable reading from the sensor was achieved. The difference in dissolved oxygen concentration measured for distilled water and sample suspensions was used to calculate the amount of oxygen adsorbed at the surface of particles. Photoelectrochemical measurements: Photoelectrochemical measurements were carried out using Autolab PGSTAT302N potentiostat and XBO150 xenon lamp with a monochromator (Instytut Fotonowy). Measurements have been done in the three-electrode cell using platinum wire and Ag/AgCl as a counter and reference electrodes, respectively. A thin layer of material on ITO foil (resistivity: 60 Ω/sq) was used as a working electrode. The electrodes were placed in a quartz cuvette filled with 0.1 mol/dm3 KNO3 (≥99.0%, SigmaAldrich) solution (pH = 6.1) in pure water (Milli-Q water) as an electrolyte. The measurements have been done after saturation of the electrolyte with oxygen or argon (oxygen free conditions). Irradiation was done at 380 nm. Photocurrent generation was measured in the potential range of –0.2-1 V vs. Ag/AgCl.

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Redox properties of the materials: Redox properties of the materials were determined using spectroelectrochemical method which is based on electrochemical measurements combined with UV-vis diffuse reflectance spectroscopy.32 The changes in reflectance were recorded at λ = 780 nm by Perkin Elmer UV-Vis Lambda 12 spectrometer equipped with a 5 cm dia. integrating sphere. The electrochemical measurements were carried out in three-electrode cell, with platinum wire and Ag/AgCl as a counter and reference electrodes, respectively. Titanium dioxide materials were previously ground in a mortar and suspended in acetonitrile. Working electrodes were prepared by casting of tested materials at the surface of platinum foil (ca. 2 cm × 1 cm). Afterwards the working electrodes were dried at approx. 100°C. In this way opaque films of the materials were formed on Pt plate. The electrodes were placed in a cuvette with a quartz window filled with 0.1 mol/dm3 LiClO4 (99.99% trace metals basis, Aldrich) solution in acetonitrile. Oxygen was thoroughly removed from the electrolyte by purging with argon before (15 min) and during the experiments. The cuvette was placed in front of the sphere, facing the working electrode (platinum foil with deposited TiO2) towards the light beam. The potential control was provided by the electrochemical analyzer Bio-Logic SP-150 (scan rate 0.5 mV/s).

cuvette was placed in the spectrofluorometer Fluorolog-3 (Horiba Jobin Yvon). The suspension was stirred with a magnetic stirrer during the measurement and irradiated continuously at 290 nm. The phosphorescence of singlet oxygen was recorded by a NIR photomultiplier tube with InP/InGaAsP photocathode (Hamamatsu) equipped with an iHR320 imaging spectrometer. Singlet oxygen phosphorescence spectra were collected within 1000 cycles and then averaged.

Photocatalytic activity: The photocatalytic activity of the materials was examined by irradiation of the suspensions of TiO2 (20 mL, 1 g/dm3) in the solution of terephthalic acid (Aldrich, 98%) (6×10–3 mol/dm3 in aqueous solution of 0.02 mol/dm3 NaOH, pH = 11) with an XBO-150 xenon lamp (Instytut Fotonowy). A water filter (10 cm) with 0.1 mol/dm3 solution of CuSO4 was used to absorb NIR and IR radiation, together with a 320 nm cut-off filter. Air was slowly bubbled through the suspension to avoid sedimentation as well as to ensure a constant access of oxygen. Samples of 2 mL were collected during irradiation and filtered through CME filters (pore size of 0.22 µm). The applied TiO2 concentration enabled a direct comparison of photocatalytic activity of tested sample (a further increase of TiO2 concentration did not result in the reaction acceleration).

The materials selected for our studies are both nanocrystalline a-TiO2 (particle size 30-50 nm) and n-r-TiO2 (30x70 nm), as well as microcrystalline r-TiO2 (120-150 nm). A-TiO2 and n-r-TiO2 are characterized by similar specific surface areas (90 and 122 m2/g, respectively), whereas r-TiO2 shows a much less developed surface (5.5 m2/g). N-r-TiO2 forms spherical aggregates with the diameter of ca. 10 µm, as confirmed by SEM measurements (data not shown). Other materials form less ordered aggregates. In water, at pH ca. 6, a-TiO2 and n-rTiO2 form aggregates of a similar hydrodynamic diameter of (0.5-0.6 µm), while aggregates of r-TiO2 and P25 are smaller (ca. 100 nm; data not shown). Photocatalytic activity of TiO2 can be influenced by various contribution of different facets in the overall surface area or by different terrace/step configurations.34 All selected materials have irregular shapes and therefore present in some sense “average” anatase and rutile samples. To avoid the influence of adsorbed species on the studied properties all materials were heated at 150°C under vacuum for 5 h and stored in argon atmosphere.

The process of terephthalic acid (TA) conversion to hydroxyterephthalic acid (TAOH) was used to compare efficiencies of hydroxyl radicals generation (Fig. 4).33 Photocatalysts were irradiated in TA solution (3 mM TA in 10 mM NaOH) for 30 min. Samples were collected in 5 min intervals. In the reaction of non-fluorescent TA with hydroxyl radicals the formation of TAOH can be monitored by emission spectra measurements. TAOH shows a broad emission band at λmax = 425 nm when excited at λexc = 315 nm. Fluorescence spectra were measured using FluoroLog-3 (Horiba JobinYvon) spectrofluorometer in 1 cm quartz cuvette. Singlet oxygen detection: The materials (0.1 g dm–3) were suspended in CD3OD (2.5 mL, Aldrich) in a quartz cuvette equipped with a rubber septum. Oxygen was bubbled through the suspension for 5 minutes prior to the measurement. The

Synthetic air, oxygen and argon (99.9999%) were purchased from Air Products.

Figure 4. The process of TAOH formation in the reaction of terephthalic acid (TA) with hydroxyl radicals.33

RESULTS AND DISCUSSION Materials

Oxygen adsorption The amounts of oxygen adsorbed at r-TiO2 and n-r-TiO2 are 1.5 and 2.5 times higher than at a-TiO2, respectively (red bars in Fig. 5). The results of measurements were recalculated to the amount of oxygen adsorbed at 1 g and 1 m2 of the photocatalyst. The surface concentrations of adsorbed oxygen are significantly higher for rutile samples than for anatase (Table 1). These differences are in agreement with previous reports on oxygen adsorption at TiO2 materials.5b, 35

Photoelectrochemistry

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ACS Catalysis Photocurrents generated at the electrodes covered with rTiO2 and a-TiO2 were measured as a function of wavelength and potential (Fig. 6). For both r-TiO2 and a-TiO2 anodic and cathodic photocurrents were recorded. The anodic photocurrent is generated when valence band holes oxidize water, while electrons are transferred to the ITO electrode through the TiO2 film. The cathodic photocurrent is generated when electrons reduce an adsorbed electron acceptor (e.g. molecular oxygen), while holes migrate to the electrode.36 adsorbed oxygen 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 water

r-TiO2

a-TiO2

Adsorbed oxygen / mg dm-3

dissolved oxygen

Dissolved oxygen / mg dm-3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

n-r-TiO2

Figure 5. Oxygen adsorption at the surface of anatase and rutile materials.

Table 1. Amounts of oxygen adsorbed at a-TiO2, r-TiO2 and n-r-TiO2 surfaces (±5%). dissolved oxygen concentration [µmol/dm3]

specific surface area [m2/g]

[nmol/m2]

[µmol/g]

r-TiO2 n-r-TiO2

3.72 3.12

5.5 122

5280 390

29 48

a-TiO2

4.04

90

210

19

Material

adsorbed oxygen concentration

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The net photocurrent is the sum of anodic and cathodic photocurrents. In the absence of any electron acceptor (O2) the anodic photocurrents prevail, while cathodic photocurrents dominate in the presence of oxygen. Particularly high cathodic photocurrents can be observed for r-TiO2 at negative potentials (Fig. 6). When the electrode is covered with a-TiO2 the influence of oxygen is insignificant. This observation points at a much more efficient oxygen reduction at r-TiO2 than at aTiO2. It is also consistent with the higher oxygen availability at the surface of rutile, as confirmed in the study of oxygen adsorption (Fig. 5). Recently, using spin trapping and EPR spectroscopy Li et al. confirmed more efficient O2•− photogeneration at r-TiO2 compared to a-TiO2.37

Redox properties of the materials To study redox properties of TiO2 we have applied spectroelectrochemical measurements developed recently in our laboratory.32 The electrochemical reduction of TiO2 generates TiIII centers absorbing at 780 nm, localized beneath the bottom of the conduction band. Reflectance changes measured at 780 nm as a function of the electrode potential (TiO2 deposited at platinum electrode) and transformed to Kubelka-Munk (KM) function are presented in Fig. 7. The deflection points on recorded curves correspond to the onset reduction potentials at which trapping of electrons occurs. The last deflection points correspond to potentials of the conduction band edges (–1.14, –1.24 and –1.53 V for a-TiO2, n-r-TiO2 and r-TiO2, respectively). The CB edge of r-TiO2 is therefore localized at lower potentials than the CB of a-TiO2, what is in contradiction to previous reports,16b, 16c but in accordance with calculations of Scanlon et al. 16h In the case of P25 the last deflection point is close to that of n-r-TiO2. Other values of potentials marked with arrows in Fig. 7 can be attributed to electronic states acting as traps. Our results show that the density of states close to the conduction band is higher for a-TiO2 than for r-TiO2. Similar results were reported by Jankulovska et al.38 Moreover, anatase is characterized be remarkable deeper traps. Electrons trapped within the bandgap of a-TiO2 offer much worse reduction properties than electrons trapped within r-TiO2 and n-r-TiO2. The results of the measurements for all materials are summarized in Figure 8. The values of CB potentials and electronic states measured in acetonitrile differ from those measured in water. The nonaqueous electrolytes containing alkali or alkaline earth cations shift the flat band potential.39 However, the measurements should be done in non-aqueous electrolytes to avoid water reduction and to lower the noise/signal ratio.39-40

Figure 6. Photocurrent generation (λ = 380 nm) in the presence of a-TiO2 and r-TiO2 electrodes. The measurements were carried out in aqueous 0.1 M KNO3 solution saturated with oxygen or argon.

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P25 a-TiO 2 r-TiO 2 n-r-TiO2

120

r-TiO 2 100

[TAOH] / µM

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0.2 a.u.

80 60 40 20

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

0

Potential / V vs. Ag/AgCl

0

Figure 7. Changes of KM measured at 780 nm as a function of the applied potential for a-TiO2, r-TiO2, n-r-TiO2 and P25 deposited at the surface of platinum plate. The measurements were carried out in deoxygenated 0.1 mol/dm3 LiClO4 solution in acetonitrile.

Figure 8. The energy diagram close to the conduction band edges of a-TiO2, r-TiO2, n-r-TiO2 and P25. Potential values vs. Ag/AgCl electrode. CB of r-TiO2 is marked in blue, CB of a-TiO2 – in green.

5

10

15

20

25

30

Irradiation time / min

Figure 9. Photogeneration of TAOH in the process of photocatalytic oxidation of TA in the presence of r-TiO2, n-r-TiO2, a-TiO2 and P25. Irradiation > 320 nm.

Singlet oxygen generation was followed by monitoring its phosphorescence (Fig. 10). Under the applied conditions no phosphorescence could be observed in the presence of a-TiO2, while all other samples appeared photoactive in 1O2 generation. A low intensity observed for n-r-TiO2 results to some extent from a fast sedimentation of this material (nanoparticles form aggregates of ca. 10 µm of diameter). A better evolved signal was observed for r-TiO2, while the strongest one was recorded for P25 material. A higher yield of singlet oxygen formation at r-TiO2 compared to a-TiO2 is justified. Our results are in accordance with the mechanism of 1O2 generation proposed by Nosaka, which involves formation of superoxide anion and its reoxidation to 1O2 by holes.13a The yield of singlet oxygen generation is influenced by several factors, including the concentration of adsorbed oxygen. a-TiO 2 n-r-TiO 2

Photocatalytic activity Interesting conclusions on photoactivity of a-TiO2 and rTiO2 can be drawn from the experiments of terephthalic acid (TA) oxidation and singlet oxygen generation. The efficiency of hydroxyl radicals generation is reflected by the progress of TAOH formation (Fig. 9). As expected, rutile samples show the lowest photoactivity in this test due to inefficient hydroxyl radical generation in the process of water oxidation at the most stable (110) facet of r-TiO2.41 The alternative mechanism of hydroxyl radicals generation at r-TiO2 involves three-step reduction of oxygen molecule. Oxidation of TA to TAOH, and thus generation of hydroxyl radicals, is much faster when aTiO2 or the r-TiO2/a-TiO2 composite are applied.

r-TiO2

P25

1220 1240 1260 1280 1300 1320

Wavelength / nm

Figure 10. Phosphorescence of singlet oxygen photogenerated in the presence of r-TiO2, n-r-TiO2, a-TiO2 and P25. Irradiation > 320 nm.

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CONCLUSIONS As often proved, r-TiO2 cannot be considered as an inactive material in photocatalytic processes. The activity of this crystalline form of TiO2 was described in several types of chemical reactions. R-TiO2 may appear more suitable for generation of the products of non-complete substrate oxidation and can be considered as a photocatalyst of reactions requiring a higher selectivity. In particular, the reactions involving superoxide radicals or singlet oxygen are favored at r-TiO2. Spectroelectrochemical studies enabled redox characterization of r-TiO2 and a-TiO2. The results are in contradiction with the commonly accepted picture presented in Fig. 1. Apparently, the reduction of a-TiO2 is easier than the reduction of rTiO2, therefore the electron transfer from the conduction band of r-TiO2 to a-TiO2 should be expected rather than the opposite process. Our results are in accordance with the latest theoretical studies of the band structures of TiO2 crystalline forms.16h, 38 A comparison of redox properties of r-TiO2 and a-TiO2 helps to understand the differences in yields of O2•− and HO• generation at these materials. Since excited r-TiO2 is a significantly stronger reducer and a weaker oxidant than a-TiO2, superoxide ions are easily formed at r-TiO2 as the result of O2 reduction and at a-TiO2 as the result of H2O2 oxidation. On the other hand, HO• radicals are produced efficiently at r-TiO2 through H2O2 reduction to OH– and HO• and at a-TiO2 through H2O oxidation. Redox properties of both polymorphs of titanium dioxide may, in addition to other factors (like differences in charge mobility, lifetimes of photogenerated charges, etc.), account for their various photocatalytic properties. The results of our studies confirm a considerable better oxygen adsorption at the surface of rutile (Fig. 5). This favors reduction of oxygen to O2•− in the presence of this form of TiO2 (evidenced by high cathodic photocurrents recorded for rTiO2 in the presence of oxygen, Fig. 6). Phosphorescence measurements show that the efficiency of singlet oxygen generation is higher in the presence of r-TiO2 than of a-TiO2. This observation is understandable taking into account a better O2 adsorption and its more efficient reduction at r-TiO2. In consequence, r-TiO2 can be recommended as a photocatalyst of dioxygen activation, i.e. when generation of superoxide radicals or singlet oxygen is required. Excitation of r-TiO2/a-TiO2 composites leads to a desirable charge separation, involving the electron transfer from r-TiO2 to a-TiO2 (confirmed by the studies of Ohno, compare Introduction). Due to the presence of r-TiO2 the amount of oxygen adsorbed at the composite is improved when compared to aTiO2. On the other hand, the oxidation properties of r-TiO2/aTiO2 are better than of bare r-TiO2. The overall photocatalytic activity of the composite can be optimized by tuning the anatase/rutile ratio. The analysis presented in this paper sheds light on the differences in photocatalytic activity of two most common forms of titanium dioxide and their composites. More detailed studies should take into account various features of specific crystal

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facets and terrace/step configurations, however, already a comparison of the macroscopic properties of r-TiO2 and aTiO2, in particular their redox characteristics, helps to understand the differences between r-TiO2, a-TiO2 and r-TiO2/aTiO2.

AUTHOR INFORMATION Corresponding Author * e-mail: [email protected]

ACKNOWLEDGEMENTS The authors thank Prof. Horst Kisch and Prof. Bunsho Ohtani for valuable discussions. The studies were realized within the „Activation of small molecules in photocatalytic systems” project of TEAM program (TEAM/2012-9/4) supported by the Foundation for Polish Science, co-financed by European Union, Regional Development Fund. The part of the work was supported by Polish Ministry of Science and Higher Education (grant no N N204 016739). The equipment was purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08).

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