Insights into different photocatalytic oxidation activities of anatase

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Insights into different photocatalytic oxidation activities of anatase, brookite, and rutile single crystal facets Carsten Günnemann, Christoph Haisch, Manuel Fleisch, Jenny Schneider, Alexei V. Emeline, and Detlef W. Bahnemann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04115 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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XRD pattern (a) and SEM image (b) of the rutile (001) surface. 558x208mm (300 x 300 DPI)

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AFM images of the anatase (100) surface (a), the anatase (101) surface (b) and the brookite (100) surface (c). 482x358mm (300 x 300 DPI)


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X-ray photoelectron spectra of titanium (a), oxygen (b), carbon (c), aluminum (d), iron (e), and niobium (f) of the anatase (100) surface. Additionally to oxygen (b), gold can be observed as well in the shown binding energy area. The gold peak is caused by the holder, which was sputtered with gold. Further chlorine can be detected at a similar binding energy as compared to niobium (f). 482x536mm (300 x 300 DPI)


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UV-vis absorption spectra of the anatase (100) surface and the brookite (100) surface. 272x208mm (300 x 300 DPI)


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Spectra of formaldehyde transferred into DDL (a) and of 2-HTA (b) produced at the anatase (001) surface after different illumination times. 558x208mm (300 x 300 DPI)


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Formation of formaldehyde during the photocatalytic oxidation of methanol obtained by the three anatase surfaces (a), the brookite surface and the rutile surfaces (b). The amounts of formaldehyde were calculated based on the areas of the fluorescence peak via a calbriation curve and normalized to a surface area of 0.25 cm2. The measured peak area after 0 minutes of illumination was set as zero value and subtracted from all further values (Figure S3). The solid lines indicate the course for the formaldhyde amount in dependence of the illumination time. 558x208mm (300 x 300 DPI)


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Quantum efficiencies for the production of formaldehyde of all investigated surfaces for the whole spectral area (280 - 1000 nm) and for every photon with an energy higher than the bandgap (λ ≤ λBandgap). 272x208mm (300 x 300 DPI)


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Photocatalytically produced amounts of 2-HTA for the three anatase surfaces (a), the brookite surface and the rutile surfaces (b). The amounts of 2-HTA were calculated based on the intensity of the fluorescence peak via a calibration curve and normalized to a surface area of 0.25 cm2. The measured intensity after 0 minutes of illumination was set as zero value and subtracted from all further values (Figure S4). The solid lines indicate the course for the 2-HTA amount in dependence of the illumination time. 558x208mm (300 x 300 DPI)


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Comparison of the amounts of DDL and of 2-HTA produced after 100 minutes of illumination for all investigated surfaces. 288x201mm (300 x 300 DPI)


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Normalized fluorescence intensity, respectively amount of 2-HTA formation of the three anatase surfaces comparing the results of the present work with those of Pan et al.33 and of Ye et al.34. The values are normalized to the value reached by the most active surface. The results of the present work have been obtained after 100 minutes of illumination, the results of Pan et al. after 25 minutes of illumination and the results of Ye et al. after 60 minutes of illumination. 288x201mm (300 x 300 DPI)


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Normalized amount of formaldehyde and of 2-HTA produced by the rutile (001) surface and the rutile (101) surface after 100 minutes of illumination (present work) and normalized rate constants for the methyl orange decomposition reported by Luttrell et al.14 for both rutile surfaces. The values are normalized to the highest amount formed, respectively to the highest rate constant. 288x201mm (300 x 300 DPI)


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Mechanisms of the photocatalytic methanol oxidation depending on the ratio between water and methanol. Methanol is oxidized via hydroxyl radicals if the H2O:CH3OH ratio is ≥ 300:1 (a) and directly via holes if the H2O:CH3OH ratio is < 300:1. 232x85mm (96 x 96 DPI)


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Coordination of titanium and oxygen ions at the investigated surfaces.40,42,46,47 285x413mm (96 x 96 DPI)


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78x45mm (150 x 150 DPI)


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Insights into different photocatalytic oxidation activities of anatase, brookite, and rutile single crystal facets

Carsten Günnemann a,*, Christoph Haisch a, Manuel Fleisch a, Jenny Schneider a, Alexei V. Emeline b, and Detlef W. Bahnemann a,b a

Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 5, D-30167

Hannover, Germany b

Laboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg State University,

Ulyanovskaya str. 1, Peterhof, Saint-Petersburg, 198504 Russia *

Corresponding author. E-mail address: [email protected]

Abstract For the understanding of the activity of TiO2 photocatalysts knowledge of the activities of different crystal facets is necessary. This information can be achieved by the investigation of well-defined single crystalline TiO2 surfaces. In this study, the photocatalytic activity of different anatase, brookite, and rutile single crystal wafers with only one exposed surface has been investigated via the oxidation of methanol and the hydroxylation of terephthalic acid, respectively. XRD and SEM measurements have shown that all surfaces are clearly defined and possess a smooth surface, which allows a reliable comparison of the photocatalytic activities. The investigated anatase surfaces show a higher activity than the rutile surfaces, while the brookite surface is interestingly the least active one. To the best of our knowledge, there are no other reports based on the investigation and comparison of well-defined TiO2 anatase (100), anatase (001), and brookite (100) single crystalline surfaces concerning their photocatalytic activity. Furthermore, the influence of the coordination of the titanium and the oxygen ions on the photocatalytic activity is discussed.


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Key Words: anatase, brookite, rutile, single crystalline surfaces, TiO2 surfaces, TiO2 modifications, photocatalytic methanol oxidation, terephthalic acid hydroxylation

1. Introduction For the first time in 1972 Fujishima and Honda have described the splitting of water with TiO2 as photocatalyst by help of an applied potential.1 After years of research TiO2 is still the most studied photocatalyst.2 Comparisons of the photocatalytic activity of the three main modifications of TiO2 (anatase, brookite, and rutile) have resulted in different reactivities depending upon the modification. For the brookite modification a higher photocatalytic activity compared to anatase and rutile was found.3 The comparison of the anatase and the rutile modification has concluded a higher activity of anatase in most reports.4 However, most studies have been performed employing powders exhibiting different morphologies, most likely affecting the photocatalytic properties.5–7 In particular, investigations of well-defined single crystalline TiO2 surfaces should provide information for a reliable comparison of the modifications under the same conditions. By investigating isolated facets not only the photocatalytic activity of the modifications, but also the activity of the oriented crystal surface itself can be determined. The photocatalytic and photoelectrochemical activity of single crystalline TiO2 surfaces has been mostly studied for rutile (001), rutile (100) and rutile (110). For example, Imanishi et al.8 have investigated the photocurrents of these three surfaces, while Ahmed et al.9 have shown different photocatalytic activities without applying an external potential. The only investigated well-defined single crystalline surface of the anatase modification is the (101) surface. Kavan et al.10 have compared the photocurrent of the anatase (101) surface to a rutile (001) surface and Ahmed et al.9 have investigated the photocatalytic activity of the anatase (101) surface. ACS Paragon Plus Environment

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Isolated brookite single crystalline surfaces have, to the best of our knowledge, not been investigated so far regarding their photocatalytic actvity. All reports have in common that the surfaces were n-type doped before the photocatalytic and the photoelectrochemical experiments have been carried out. Generally, this doping is necessary to achieve an electric conductivity of the surfaces and thus is indispensable for electrochemical experiments. The doping itself, however, leads to a change in the texture of the surfaces and to the formation of cavities.11,12 Consequently, it is unclear whether a high photocatalytic activity is caused by the single crystalline surface itself or by the larger surface area due to, e.g., these cavities as compared to other surfaces. There are only a few reports concerned with the photocatalytic activity of untreated single crystalline TiO2 surfaces. For example, different rutile surfaces were investigated by Hotsenpiller et al.13 for the reduction of Ag+ ions and by Luttrell et al.14 for the decomposition of methyl orange. Hence, there is an obvious need concerning the investigation of the photocatalytic activity of untreated and isolated single crystalline TiO2 surfaces to enable an unbiased comparison of the activities of the different modifications and surfaces. In this work, untreated single crystalline anatase, brookite, and rutile facets were investigated regarding their photocatalytic activity for the oxidation of methanol and for the hydroxylation of terephthalic acid, respectively.

2. Experimental Section 2.1 Chemical reagents The chemical reagents 2-hydroxyterephthalic acid (Sigma-Aldrich), acetylacetone (Carl Roth), ammonium acetate (Carl Roth), acetic acid (Carl Roth), methanol (Carl Roth), sodium hydroxide (1 N, Carl Roth), and terephthalic acid (Alfa Aesar) were used as received without further purification. 2.2 Preparation of the single crystals The anatase (001), anatase (100), anatase (101), brookite (100), rutile (001), rutile (101), and rutile (111) single crystal facets with one side polished were purchased from SurfaceNet GmbH (Germany) in sizes of 5 x 5 x 0.5 mm and were used for the experiments without further treatment. The anatase and brookite single crystal facets were cut from a natural crystal, while the rutile facets were synthesized via the 4-zone floating method. The position of the crystals was fixed in the photocatalytic cell with a copper wire glued to the backside of the crystals,


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pulled through a glass tube and the crystals were subsequently attached with silicone to the tube, resembling the experimental set-up described by Ahmed et al.9. 2.3 Characterization of the single crystals The single crystals were characterized via XRD (X-ray diffraction), SEM (Scanning electron microscopy), AFM (Atomic force microscopy), XPS (X-ray photoelectron spectroscopy), and UV-vis measurements. For the XRD measurements a Bruker D8 Advance (USA) device was used and the crystals were irradiated with Cu Kα rays (λ = 1.54 Å). The SEM measurements were performed by means of a JEOL JSM-6700F electron microscope (Japan) with a secondary electron detector (SEI) at an accelerating voltage of 2 kV and a working distance of 8 cm. The AFM measurements were carried out using a Nanosurf Easycan 2 (Switzerland) device. For the XPS measurements a device fabricated by Leybold Heraeus GmbH (Germany) with a hemispheric analyzer, an EA 10/100 spectrometer and an X-ray source with Mg and Al anode (emitting Al Kα rays) was used. UV-vis diffuse reflectance spectroscopy measurements were performed using a Varian Cary 100 Bio (USA) device. 2.4 Photocatalytic activity tests For testing the photocatalytic activity 2.5 mL aqueous methanol solution (0.25 M) or basic (1 mM NaOH) terephthalic acid (0.4 mM) solution, respectively, were filled into a quartz glass cuvette (6030-UV, Hellma GmbH & Co. KG, Germany), following the procedure described by Ahmed et al.9. The isolated single crystal facets were also placed in the cuvette and were stored in the dark under continuous stirring. After 24 hours of dark adsorption the single crystals were transferred into fresh solutions and were illuminated in stand-alone experiments for 20, 40, 60, 80 and 100 minutes employing a solar simulator equipped with a xenon lamp (300 W, 678 W·m2

) and an AM-1.5g filter.

The amount of photogenerated formaldehyde was determined after the oxidation of methanol by taking 300 µL samples after every illumination time. After addition of 600 µL of the Nash reagent15 (0.02 M acetylacetone, 0.05 M acetic acid and, 2 M ammonium acetate) the yellowcolored diacetyldihydrolutidine (DDL) is formed16, which can be detected by fluorescence spectroscopy17:


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(1)

After excitation at a wavelength of 405 nm there is a characteristic emission of DDL at 510 nm. The fluorescence of the diacetyldihydrolutidine was measured in a wellplate (NunclonTM Delta Surface, Thermo Fisher Scientific Inc., USA) sample holder. Terephthalic acid reacts with photogenerated hydroxyl radicals yielding the fluorescent 2hydroxyterephthalic acid (2-HTA)18:

(2)

The amount of the thus produced 2-hydroxyterephthalic acid was analyzed via fluorescence spectroscopy after every illumination time in a fluorescence cuvette (117F-QS, Hellma GmbH & Co. KG, Germany). 2-hydroxyterephthalic acid shows a characteristic emission at a wavelength of 425 nm after excitation at 315 nm.18

3. Results 3.1 Characterization All surfaces were characterized via XRD and SEM measurements. The XRD pattern and the SEM image of the rutile (001) surface are shown as an example in Figure 1. Because of the single crystallinity and the orientation there is just one peak with a high intensity in the XRD pattern. The observed (002) signal is the reflex of the second lattice plane of the (001)-orientated surface. The SEM image of the surface shows that the surface itself is completely smooth and that there are no cavities present at all. This was also confirmed by the AFM measurements, where in Figure 2 the anatase (100) surface, the anatase (101) surface and the brookite (100) surface are represented as an example. The root mean square surface roughness is 1.062 nm for


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the anatase (100) surface, 1.737 nm for the anatase (101) surface and 1.584 nm for the brookite (100) surface. All employed single crystalline surfaces possess such a smooth surface. This should enable to a good comparability between the surfaces, because the same well-defined morphologies and conditions are given for every surface.

Figure 1: XRD pattern (a) and SEM image (b) of the rutile (001) surface.

Figure 2: AFM images of the anatase (100) surface (a), the anatase (101) surface (b) and the brookite (100) surface (c).

The single crystalline surfaces were further investigated by XPS measurements to detect impurities at the surfaces. As an example the detected elements for the anatase (100) surface are shown in Figure 3. The expected peaks for titanium and oxygen can be observed, while there are no impurity peaks by metals such as iron, aluminium or niobium. Therefore it can be assumed that the amount of metal impurities is very low. A high amount of carbon can be


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detected (high intensity of the C 1s peak), which can be mainly addressed to adsorbed adventitious carbon at the surface of the sample holder. Furthermore a peak corresponding to chlorine was observed on the sample with a low intensity. As further examples the results of the XPS measurements of the anatase (101) surface and the brookite (100) surface are given in the Supporting Information (Figures S1,S2). Only carbon could be observed for the brookite (100) surface in addition to titanium and oxygen, while also chlorine was present at the anatase (101) surface. All in one no significant impurity signals could be observed, ensuring a high purity of the samples.

Figure 3: X-ray photoelectron spectra of titanium (a), oxygen (b), carbon (c), aluminum (d), iron (e), and niobium (f) of the anatase (100) surface. Additionally to oxygen (b), gold can be observed as well in the shown binding energy area. The gold peak is caused by the holder, which was sputtered with gold. Further chlorine can be detected at a similar binding energy as compared to niobium (f).


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In Figure 4 the UV-vis spectra of the anatase (100) surface and the brookite (100) surface are given as example. It can be seen that both surfaces absorb a similar amount of light at the considered wavelength area. Thus there should be a good comparability of the surfaces. The relatively constant absorption of both surfaces could be caused by the expected small amount of impurities in natural single crystals.

Figure 4: UV-vis absorption spectra of the anatase (100) surface and the brookite (100) surface.

3.2 Photocatalytic activity The photocatalytic activity of the single crystal TiO2 surfaces was tested via the oxidation of methanol and via the hydroxylation of terephthalic acid, respectively. Methanol is a typical model compound for the qualitative, as well as for the quantitative, assessment of the photocatalytic activity of TiO2 photocatalysts and is oxidized to formaldehyde.19 Terephthalic acid reacts mainly with hydroxyl radicals, formed via trapping of the photogenerated holes and was therefore chosen as second model compound.18 Furthermore, it was reported that terephthalic acid is the most appropriate compound for the qualitative and quantitative detection of hydroxyl radicals.20 Figure 5 shows as an example the fluorescence spectra of produced formaldehyde after having been transferred into DDL (Diacetyldihydrolutidine) (a), and the spectra of 2-HTA (2-hydroxyterephthalic acid) (b) measured after different illumination times for the anatase (001) surface. At a wavelength of 510 nm DDL exhibits a peak which corresponds to the fluorescence signal of the molecule after excitation at 405 nm. The fluorescence peak of the 2-HTA appears at a wavelength of 425 nm after excitation at 315 nm. For both molecules the intensity of the fluorescence signal increases with increasing illumination time.


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Figure 5: Spectra of formaldehyde transferred into DDL (a) and of 2-HTA (b) produced at the anatase (001) surface after different illumination times.

3.2.1 Methanol oxidation The produced amounts of formaldehyde after different illumination times are shown for all investigated surfaces in Figures 6a and 6b. For all surfaces a linear increase of the amount of formaldehyde as a function of the illumination time can be observed. All investigated anatase surfaces show different photocatalytic activities for the production of formaldehyde increasing in the order anatase (100) > anatase (001) > anatase (101). The rutile (001) surface and the rutile (111) surface exhibit similar activities comparable to the activity of the anatase (100) surface. The activity of the rutile (101) surface is slightly lower as compared to the other rutile surfaces. Interestingly, the brookite (100) surface shows the lowest photocatalytic activity of all investigated single crystal TiO2 surfaces.

Figure 6: Formation of formaldehyde during the photocatalytic oxidation of methanol obtained by the three anatase surfaces (a), the brookite surface and the rutile surfaces (b). The amounts of formaldehyde were calculated based on the areas of the fluorescence peak via a calbriation curve and normalized to a surface area of 0.25 cm2. The measured peak area after 0 minutes of illumination was set as zero value and subtracted from all further values (Figure S3). The solid lines indicate the course for the formaldhyde amount in dependence of the illumination time.

The quantum efficiencies for the formation of formaldehyde of all investigated surfaces are given in Figure 7. All values were calculated according to the following formula (Eq. 3), where ACS Paragon Plus Environment

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𝜁𝜁 is the quantum efficiency, 𝑟𝑟 the rate of the formaldehyde production (determined from the slope of the formaldehyde amount in dependence of the illumination time in Figure 6) and 𝐼𝐼 the photon flux.

𝑟𝑟 𝜁𝜁 = · 100 𝐼𝐼

(3)

In Figure 7 the quantum efficiencies are shown for photons of the whole spectral area of the sunlight simulator, as well for photons with an energy similar or higher than the band gap of the investigated surfaces only. For the whole spectral area the photon flux is 0.5595 µmol·min1

, while the photon fluxes for photons with an energy similar or higher than the band gap depend

on the modification of the surface. For the anatase surfaces only photons with a wavelength similar or higher than 387.5 nm are considered, while for the brookite surface the limit wavelength is 400 nm and for the rutile surfaces 413 nm. The limit wavelengths were calculated according to the band gap of the modifications, which is 3.2 eV for anatase, 3.1 eV for brookite and 3.0 eV for rutile.21 Thus the photon fluxes are 5.30·10-3 µmol·min-1, 1.06·10-2 µmol·min-1, and 1.60·10-2 µmol·min-1 for the anatase, brookite, and rutile surfaces, respectively. According to the quantum efficiencies by considering only photons with a similar or higher energy than the band gap the anatase surfaces achieve the highest values. The brookite surface and the rutile surfaces show comparable lower values compared to the anatase surfaces and the rutile (101) surface achieves the lowest value. By considering the whole spectral area also the anatase surfaces show the highest values with exception of the anatase (100) surface, where a lower quantum efficiency compared to the rutile (001) surface and the rutile (111) surface can be observed. The lowest value is in this case achieved by the brookite (100) surface.

Figure 7: Quantum efficiencies for the production of formaldehyde of all investigated surfaces for the whole spectral area (280 - 1000 nm) and for every photon with an energy higher than the bandgap (λ ≤ λBandgap).


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3.2.2 Terephthalic acid hydroxylation Figures 8a and 8b show the produced amounts of 2-HTA for the anatase, brookite, and rutile surfaces as a function of the illumination time. For all surfaces the amounts of 2-HTA increase linearly with increasing illumination time. The highest photocatalytic activities for the generation of 2-HTA are once again achieved by the three anatase surfaces, with the anatase (100) surface being the most active and the anatase (001) surface being the least active one. All rutile surfaces show lower activities than the anatase surfaces. The activities of the rutile (001) surface and the rutile (101) surface are nearly the same, while the rutile (111) surface is significantly less active. Compared to all other single crystals the brookite (100) surface shows the lowest activity for the terephthalic acid hydroxylation.

Figure 8: Photocatalytically produced amounts of 2-HTA for the three anatase surfaces (a), the brookite surface and the rutile surfaces (b). The amounts of 2-HTA were calculated based on the intensity of the fluorescence peak via a calibration curve and normalized to a surface area of 0.25 cm2. The measured intensity after 0 minutes of illumination was set as zero value and subtracted from all further values (Figure S4). The solid lines indicate the course for the 2HTA amount in dependence of the illumination time.

4. Discussion 4.1 Photocatalytic activity The anatase single crystal facets and the brookite single crystal facet were cut from natural single crystals, while all rutile surfaces were synthesized via the 4-zone floating method. This has to be taken into account before discussing the photocatalytic activity itself, because the crystals from a natural source may contain some impurities. For example, a natural grown anatase single crystal contains Fe and Nb impurities.22 These impurities might possibly effect the photocatalytic activity of the respective single crystal surfaces. However, no obvious impurity signals for any single crystal wafers were obtained in the XRD investigations (cf. ACS Paragon Plus Environment

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Figure 1). Also no metal impurity signals could be detected during the XPS measurements (cf. Figure 3). Further it is important to notice that TiO2 surfaces can show reconstruction under certain conditions. Rutile (001) doesn’t show reconstruction or faceting for cleaving at room temperature, while annealing at 900 K leads to a structure with {011} facets and annealing at 1300 K results in a structure with mostly {114} facets.23,24 It was reported as well that a {011}facetted surface shows a higher photocatalytic activity than the {114}-facetted surface.25 In general, the surface stability is determined by the coordination number of the titanium ions at the surface. The rutile (110) surface doesn’t show any reconstruction (five- and sixfold coordinated titanium ions), the rutile (011) surface reconstructs (fivefold coordinated titanium ions) and the rutile (001) surface (fourfold coordinated titanium ions) forms a structure with planes with a higher coordination of the titanium ions.24 Besides annealing the chemical environment also influences the reconstruction of a TiO2 surface. For the TiO2 (110) surface it was observed that the adsorption of formic acid, acetic acid and benzoic acids causes a reconstruction of the surface.26 A solution of carboxylic acid causes as well for the reconstructed anatase (001) surface a reorganization which remains after rinsing.27 The rutile (011) surface (which exhibits reconstruction after vacuum preparation) shows after the contact with water a restructuring of the surface.28 The following discussion will be based on the structure of the surfaces at ideal conditions, but it has to be taken into account, that the surfaces could reconstruct or facet, while a closer look to this would be beyond the scope of this study. The illumination of TiO2 with UV light leads to the generation of electron-hole pairs (Eq. 3), which can participate in chemical reactions. Methanol molecules can be directly oxidized via these photogenerated holes (Eq. 7) or indirectly via hydroxyl radicals (Eq. 8). The latter result from the reaction of bridging oxygen ions >O2- with holes (Eq. 5) or from the intermediate oxidation of terminal hydroxyl groups >(-OH) (Eq. 6).19,29 In both reactions the α-hydroxyradical ∙CH2OH is formed, which reacts in a further step with molecular oxygen to formaldehyde CH2O (Eq. 9). The photogenerated electrons e- react with dissolved oxygen molecules to superoxide radicals O2-∙ (Eq. 10). These radicals can react in further reactions to hydrogen peroxide (H2O2) and hydroxyl radicals as described in the literature.19 TiO2 + hν → h+ + e-

(4)

>O2- + h+ → -O-∙

(5)


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>(-OH) + h+ → -(∙OH)

(6)

CH3OH (ads) + h+ → ∙CH2OH + H+

(7)

CH3OH (aq) + -(∙OH) → ∙CH2OH + H+

(8)

∙CH2OH + O2 → CH2O + H+ + O2-∙

(9)

e- + O2 → O2-∙

(10)

The only oxidation product, the quantity of which is considered for the determination of the photocatalytic activity for the oxidation of methanol, is formaldehyde. It should be considered that a reaction at different surfaces can lead to different products, as it was shown by Apno et al.30 for the reduction of CO2 at the rutile (110) surface and the rutile (100) surface. Nevertheless, the first stable oxidation product can be assumed to be formaldehyde for all surfaces. The anatase and the rutile modification possess a tetragonal crystal structure.31 Consequently, the (010) facet and the (100) facet of the anatase modification are equal. The same is true for the (011) facet and the (101) facet of the rutile modification. For a better comparison with the literature data the anatase (010) facet and the rutile (011) facet are discussed here as well. The investigations of the anatase surfaces have shown that the anatase (101) surface is the most active surface for the oxidation of methanol, while for the generation of hydroxyl radicals the anatase (100) surface shows the highest activity (cf. Figures 6,7,8,9). The activity ranking of the three surfaces for the hydroxyl radical generation is: anatase (100) > anatase (101) > anatase (001). Hengerer et al.32 reported for anatase electrodes that the (001) surface shows a slightly earlier onset potential than the (101) surface in aqueous media, while the observed photocurrents are similar. While the anatase (101) surface is slightly more active for the generation of hydroxyl radicals than the anatase (001) surface in this study, a similar activity can be confirmed. The activity ranking in this study corresponds to the results of Pan et al.33 who investigated nanoparticles with these anatase surfaces exposed (Figure 10). Ye et al.34 observed by the investigation of nanoparticles the increasing order anatase (001) > anatase (101) > anatase (100) concerning their photocatalytic activity for the generation of hydroxyl radicals. These results do not agree with the results obtained in the present work. However, it should be noted that the morphology of a nanoparticle, even with a predominant surface, can ACS Paragon Plus Environment

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hardly be compared to a smooth single crystalline surface. Every nanoparticle consists of more than one crystalline facet which will also effect its photocatatlytic activity. This is why the present study has been focused on the investigation of single crystal wafers with only one light exposed surface being available for the photocatalytic process. All three rutile surfaces show a similar activity for the photocatalytic oxidation of methanol with a slightly lower activity of the rutile (101) surface, while for the generation of hydroxyl radicals the rutile (101) and the rutile (001) surface are found to be more active than the rutile (111) surface (cf. Figures 6,7,8,9). Ahmed et al.9 have previously investigated the photocatalytic activity of n-doped single crystalline rutile facets. The authors reported a higher activity of the n-doped rutile (001) surface compared to the rutile (110) and the rutile (100) surface. In comparison to the rutile (111) surface a high photocatalytic activity of the rutile (001) surface can be confirmed for the generation of hydroxyl radicals by the results of this work. It should, however, be taken into account that the doping of single crystalline TiO2 facets leads to the formation of cavities11,12, which complicates the comparison of results obtained with doped and undoped facets in general. For example, Luttrell et al.14 have reported a change in the photocatalytic activity for four different rutile surfaces after etching with HF. During their investigation of a rutile (101) single crystal Quah et al.35 have shown that there is no special photocatalytic activity of this face compared to other rutile surfaces. The results obtained here for the photocatalytic oxidation of methanol are in good agreement with these results of Quah et al.35. According to the results of Lowekamp et al.36 the {001}, {101} and {111} planes show a higher reactivity for the reduction of Ag+ compared to the {110} and the {100} planes, while the {101} plane is the most active one. A higher photocatalytic activity of the rutile (101) surface compared to the other surfaces can be confirmed for the hydroxylation of terephthalic acid, but not for the photocatalytic oxidation of methanol. It has to be taken into account that Lowekamp et al.36 have investigated a reduction reaction, while in this study oxidation reactions were investigated, which could explain the differences. Ohno et al.37 have reported that the {011} face of rutile is more oxidative than the {110} face and that the {001} face of anatase is more oxidative than the {011} face. Thus it is rather complicated to compare the oxidation and reduction activities of surfaces to each other. Luttrell et al.14 observed a higher photocatalytic activity of a rutile (101) single crystal compared to a rutile (001) single crystal for the decomposition of methyl orange (Figure 11). The present work shows the same result for the generation of hydroxyl radicals, but not for the oxidation of methanol. Wang et al.38 have suggested that methanol is oxidized directly via holes, provided that the concentration of methanol is higher than 0.184 M (with the molar ratio between water and methanol being 300). ACS Paragon Plus Environment

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Thus, two different mechanisms can be assumed for the photocatalytic methanol oxidation. If the molar ratio between water and methanol is higher or equal to 300:1 primarily water is adsorbed at the TiO2 surface and methanol molecules are oxidized via hydroxyl radicals (Figure 12a, Eq. 8). At a molar ratio between water and methanol lower than 300:1 not only water but also some methanol molecules are adsorbed to the TiO2 surface. Therefore, methanol can be directly oxidized via photogenerated holes (Figure 12b, Eq. 7). Methanol molecules should hence be mainly oxidized via holes at the concentration of 0.25 M employed here. The photocatalytic decomposition of methyl orange can be initiated directly via holes or indirectly via hydroxyl radicals depending upon the concentration of the dye.39 The differences between the present work and the results of Luttrell et al.14 could thus be explained provided that the methyl orange molecules are mainly oxidized via hydroxyl radicals.

Figure 9: Comparison of the amounts of DDL and of 2-HTA produced after 100 minutes of illumination for all investigated surfaces.

Figure 10: Normalized fluorescence intensity, respectively amount of 2-HTA formation of the three anatase surfaces comparing the results of the present work with those of Pan et al.33 and of Ye et al.34. The values are normalized to the value reached by the most active surface. The results of the present work have been obtained after 100 minutes of illumination, the results of Pan et al. after 25 minutes of illumination and the results of Ye et al. after 60 minutes of illumination.


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Figure 11: Normalized amount of formaldehyde and of 2-HTA produced by the rutile (001) surface and the rutile (101) surface after 100 minutes of illumination (present work) and normalized rate constants for the methyl orange decomposition reported by Luttrell et al.14 for both rutile surfaces. The values are normalized to the highest amount formed, respectively to the highest rate constant.

Figure 12: Mechanisms of the photocatalytic methanol oxidation depending on the ratio between water and methanol. Methanol is oxidized via hydroxyl radicals if the H2O:CH3OH ratio is ≥ 300:1 (a) and directly via holes if the H2O:CH3OH ratio is < 300:1.

According to most reports anatase photocatalysts exhibit higher photocatalytic activities than rutile photocatalysts.4 Comparing the anatase and rutile surfaces such a higher reactivity of the anatase modification is also observed herein for both, the methanol oxidation and the generation of hydroxyl radicals (cf. Figures 6,7,8,9). The only exception appears to be the activity of the anatase (100) surface towards the oxidation of methanol. Consequently, the higher photocatalytic activity of most anatase nanoparticles can be explained by the high activity of the thermodynamically most stable anatase surface (101)40, which should be the dominant facet in anatase photocatalyst particles. Consistently, the thermodynamically most stable rutile surface (110) has a relatively lower photocatalytic activity, in good agreement for that reported for most rutile nanoparticles.9,41


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Both, for the oxidation of methanol and for the generation of hydroxyl radicals the brookite (100) surface shows the lowest activity of all investigated single crystal surfaces in the present work (cf. Figures 6,7,8,9). This is a very different result compared to what has been reported for brookite nanoparticles and powders in the literature. For example, Ohtani et al.3 have shown a higher photocatalytic activity of brookite powder as compared to anatase and rutile for the dehydrogenation of 2-propanol. However, such a higher photocatalytic activity of the brookite modification could not be confirmed by our investigation of single crystal brookite (100) facets. This might be explained by the fact that the investigated brookite (100) facet is not the photocatalytically most active brookite surface. The investigated brookite (100) surface is the predominant facet in naturally grown brookite single crystals, however, it has a relatively high value for the surface energy. It can therefore be expected that a synthesized brookite nanoparticle mainly consists of surfaces with lower values for the surface energy such as (001), (111) and (210).42 Accordingly, Lin et al.43 found a very high photocatalytic activity for brookite nanosheets with the (210), (101), and (201) facets exposed. Also Kandiel et al.44 synthesized brookite nanorods with similar facets that produced higher amounts of molecular hydrogen upon Pt loading as compared to other TiO2 modifications including anatase. Recently, Vequizo et al.45 reported mechanistic studies on the brookite modification in comparison to anatase and rutile. They found that the brookite powder has a different energetic depth of the trapped electron which can contribute to an enhancement of the photocatalytic activity of the powder. Generally, a higher activity of brookite photocatalysts cannot be ruled out by the present investigation, but it can be concluded from the results that the high photocatalytic activity of the brookite nanoparticles is not caused by the brookite (100) facet. 4.2 Influence of the coordination of the titanium and oxygen ions In bulk TiO2 material the titanium ions are sixfold coordinated and the oxygen ions are threefold coordinated. The coordination of both types of ions on the different surfaces differs from the bulk material. Figure 13 shows the coordination numbers of titanium and oxygen ions for the different TiO2 surfaces investigated here.


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Figure 13: Coordination of titanium and oxygen ions at the investigated surfaces. The illustrations of the surfaces were created in analogy to the calculated surfaces shown in the references 40, 42, 46, and 47.

The 3d-orbitals of undercoordinated titanium ions act as lewis acids, while the 2p-orbitals of undercoordinated oxygen ions act as lewis bases.48 Hence, the adsorption of methanol molecules takes primarily place at titanium ions. Hydroxyl radicals are mainly formed at bridging oxygen ions, because their energy level lies above the valence band of TiO2.29 Generally, it can be expected that a surface with a high number of undercoordinated titanium and oxygen ions also exhibits a high photocatalytic activity. The highest amount of hydroxyl radicals is generated at the three anatase surfaces where the oxygen ions are twofold and threefold coordinated. From this point of view a similar activity can be expected for the generation of hydroxyl radicals. Such a similar activity was only observed for the anatase (001) and the anatase (101) surface while the anatase (100) surface exhibits a higher activity for the generation of hydroxyl radicals (cf. Figures 8,9). The anatase (001) surface and the anatase (100) surface possess only fivefold coordinated titanium ions in the upper layer, while the titanium ions at the anatase (101) surface are fivefold and sixfold coordinated. Lazzeri et al.40 have calculated the surface density of fivefold coordinated titanium


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ions for the investigated anatase surfaces and reported a density of 0.07 Å-² for the (001) surface, of 0.054 Å-² for the (100) surface, and of 0.051 Å-² for the (101) surface. Consequently, it can be expected that the anatase (001) surface shows the highest activity while the anatase (101) surface should be the least active one. The opposite behaviour was observed for the photocatalytic methanol oxidation with the highest activity being exhibited by the anatase (101) surface (cf. Figures 6,7,9). Apparently, the different photocatalytic activities of the anatase surfaces do not only depend on the coordination of the titanium and the oxygen ions. The brookite (100) surface possesses twofold and threefold coordinated oxygen ions and shows the lowest activity for the generation of hydroxyl radicals (cf. Figures 8,9). Compared to the three rutile surfaces, at which all oxygen ions are twofold coordinated and more active, an influence can be awaited by the coordination of the oxygen ions. Because of the twofold coordinated oxygen ions at the rutile surfaces, a similar activity can be expected for the generation of hydroxyl radicals at all three surfaces. This assumption can be supported by comparing the rutile (101) surface and the rutile (001) surface with each other, which show a similar photocatalytic activity (cf. Figures 8,9). The rutile (111) surface is less active than the other two rutile surfaces, which cannot be explained by the coordination of the oxygen ions. The brookite (100) surface is also the least active surface for the oxidation of methanol (cf. Figures 6,7,9). At the brookite (100) surface the titanium ions are fivefold and sixfold coordinated, while the ions at all rutile surfaces have a lower coordination number. However, the brookite (100) surface has the same coordination for titanium and oxygen ions as the most active anatase surface in this study. The low activity for the photocatalytic oxidation of methanol molecules in solution can be explained by the low amount of generated hydroxyl radicals at the brookite surface. At the rutile (001) surface the titanium ions are fourfold and sixfold coordinated, while the titanium ions at the rutile (111) surface are fivefold and sixfold coordinated. Both surfaces show a similar activity for the oxidation of methanol (cf. Figures 6,7,9). This cannot simply be explained by the coordination of the titanium ions itself. It has also to be taken into account that methanol can not only be oxidized via holes, but also via hydroxyl radicals in solution. A surface with fourfold and sixfold coordinated titanium ions should adsorb more methanol molecules than a surface with fivefold and sixfold coordinated titanium ions. Because of this we can assume that more methanol molecules are directly oxidized via holes at the rutile (001) surface compared to the rutile (111) surface. Despite of only fivefold coordinated titanium ions on the rutile (101) surface, this surface is slightly less reactive than the other investigated rutile facets (cf. Figures 6,7,9).


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Generally, the different activities of the investigated surfaces cannot only be explained by the coordination numbers of the titanium and oxygen ions. This was also concluded by Ma et al.49 after DFT calculations for different anatase surfaces, where it was shown that even with a lower number of active adsorption sites a higher photocatalytic activity can be achieved (e.g. by a higher accommodation of charge carriers and a higher oxidation potential). It is also important to take into account the strong influence of the type of adsorption of water and methanol molecules to the surfaces. For example, Valentin et al.50 have shown with DFT calculations that methanol molecules can be adsorbed undissociated and dissociated to the anatase (101) surface, but a direct hole transfer is only possible for the dissociated form. On the other hand, Gong et al.51 have indicated a favorable dissociative adsorption of methanol molecules to the anatase (001) surface. The different preferred dissociative and undissociative adsorption to the surfaces is besides the coordination numbers of the ions necessary for the explanation of the different activities of the surfaces. Further the recombination kinetics of photogenerated charge carriers play an important role for the photocatalytic activity of a surface. Maity et al.52 have reported slower recombination rates for an anatase (101) single crystal compared to a rutile (110) single crystal. They observed also a higher number of bulk defects in the rutile single crystal compared to the anatase single crystal and dedicated this to the lower recombination rate observed for anatase. Further studies of the bulk properties of the single crystals would be beyond the scope of this study, but it should be taken into account that the bulk of the single crystals also can have an influence to the photocatalytic activity.

5. Conclusions The photocatalytic activity of anatase, brookite, and rutile isolated single crystalline facets was investigated via the oxidation of methanol and the hydroxylation of terephthalic acid. Except the activity of the anatase (100) surface towards the methanol oxidation, the anatase surfaces are photocatalytically more active than the rutile and brookite surfaces for both experiments. It was found that the brookite (100) facet is the least active surface for both, the oxidation of methanol and the hydroxyl radical generation in the present study. Thus, it can be concluded that the main photocatalytic activity of the TiO2 brookite modification is derived from other single crystal facets rather than the (100) facet. The different photocatalytic activities of the various surfaces cannot only be explained by the coordination of the titanium and the oxygen ions. The obtained results show that there also has to be an influence by the type of adsorption of methanol or water molecules to the surfaces ACS Paragon Plus Environment

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itself. Nevertheless, the investigation of untreated TiO2 single crystal wafers with only one facet exposed avoids the influence of other surfaces to the photocatalytic activity. For the evaluation of the photocatalytic activity of different single crystal facets, the observed activities should only be caused by the oriented crystal surface itself.

Conflicts of interest There are no conflicts to declare.

Supporting Information. XPS spectra of the anatase (101) surface and the brookite (100) surface, Figures showing the determination of the concentration of the reaction products. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors thank M. Sc. Stephanie Melchers, M. Sc. Julian Koch, and Dipl.-Chem. Verena Becker for performing the SEM measurements, the XPS measurements and the AFM measurements, respectively. Moreover the authors gratefully acknowledge financial support from the Federal Ministry of Education and Research BMBF (Project “DuaSol” No. 03SF0482C). A.V.E. and D.W.B. acknowledge the support by a Mega-grant of the Government of the Russian Federation within the Project “Establishment of the Laboratory ‘Photoactive Nanocomposite Materials’” No.14.Z50.31.0016.

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