How the Anatase-to-Rutile Ratio Influences the Photoreactivity of TiO2

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How the Anatase-to-Rutile Ratio Influences the Photoreactivity of TiO2 Ren Su,† Ralf Bechstein,*,† Lasse Sø,† Ronnie T. Vang,† Michael Sillassen,† Bj€orn Esbj€ornsson,‡ Anders Palmqvist,‡ and Flemming Besenbacher*,† †

Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, Ny Munkegade, DK-8000 Aarhus C, Denmark ‡ Department of Chemical and Biological Engineering and Competence Centre for Catalysis, Chalmers University of Technology, SE-41296 G€oteborg, Sweden ABSTRACT: In 1991, Bickley et al. proposed a synergetic effect between anatase and rutile in Degussa P25. Since then, there has been an intensive debate about the correctness of this proposal, the origin of the synergism, and the right polymorph composition. However, a comparison of pure titanium dioxide samples with various anatase-to-rutile ratios, but otherwise identical properties, is missing. In this paper, we report about a series of utterly pure, highly porous titanium dioxide films with identical grain sizes, surface areas, and crystallinity, but varying polymorph compositions. Photocatalytic oxidation of methylene blue was utilized to investigate the influence of the anataseto-rutile ratio on the photoreactivity. We clearly observe the synergetic effect within a well-defined range of anatase-to-rutile ratios. A film with ∼60% anatase and ∼40% rutile exhibits optimal performance at a 50% improved activity compared with pure anatase.

1. INTRODUCTION Titanium dioxide (TiO2) is widely applied as a heterogeneous catalyst,1 photocatalyst,2 and gas sensor3 owing to its high efficiency, stability, nontoxicity, and abundance.4,5 Several physical parameters are known to have a dramatic impact on the reactivity of TiO2.4
7 Engineering crystallinity, grain size, and impurity concentration have proven to influence the band structure of TiO2 and may generate visible light photocatalytic activity.8
10 On the other hand, recombination kinetics, charge separation, and charge trapping efficiencies are modified as well if these parameters are changed. Another promising route to enhance the photocatalytic potential of TiO2 is to engineer the polymorph composition. The three most stable polymorphs of TiO2, that is, rutile, anatase, and brookite, usually show different photocatalytic reactivity.11
14 Often, anatase is considered to be the most active due to both a higher adsorption affinity for organic molecules15 and a lower recombination rate.12 There are, however, studies that show a similar or even higher photocatalytic activity for rutile.16
18 Surprisingly, one of the most extensively used commercial TiO2 photocatalyst powder materials, Degussa P25, consists of a mixture of anatase and rutile. Bickley et al. were the first ones to investigate that fact and proposed a synergetic effect between anatase and rutile to be responsible for the fairly high photoreactivity of Degussa P25.19 Ohno et al. identified the close contact between rutile and anatase in P25 to be prerequisite for the synergetic effect.7 According to Hurum et.al., it is the rutile that plays the key role in separating the electrons from the r 2011 American Chemical Society

holes.12 Electrons that are photoexcited in rutile can migrate to the conduction band of anatase; the holes remain in the rutile. Thereby, the recombination is effectively suppressed. This finding was later confirmed by Kho et.al.20 and Li et al.21,22 using their own TiO2 nanoparticles. However, things are not as settled as they might appear at first glance. Astonishingly, Ohtani et al. concluded in a recent study that the components of P25, that is, ∼80% anatase, ∼15% rutile, and ∼5% amorphous titania, behaved independently without any interactions and that the synergetic effect of the copresence of anatase and rutile on the photocatalytic activity of P25 was a myth without any scientific proof.23 Moreover, it appears unclear what role amorphous titania plays in P25. A number of groups have tried to unravel the mystery of the synergetic effect between anatase and rutile by comparing samples with varying polymorph compositions.11,24
39 Several of these reports observed the phenomenon and the anatase content where synergetic effects are observed ranging from ∼20% to ∼90%.22,26
32 There have been approaches mixing rutile and anatase nanoparticles in order to systematically vary the anatase-to-rutile ratio.11,24
26,31 It is very difficult to interpret these results, since mixing does not guarantee that the polymorphs do really get into contact with each other as expected to be necessary.7 As a matter of fact, the synergetic effect is often not observed when separately Received: September 8, 2011 Revised: October 28, 2011 Published: November 02, 2011 24287

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Table 1. Parameters for TiO2 Film Synthesis with Various Polymorph Compositions label

PEO1

PEO2

PEO3

PEO4

PEO5

PEO6

voltage (V)

250

250

250

235

225

210

current limit (A) powerb (W)

0.6 60

0.45 45

0.35 35

0.27 25.4

0.22 19.8

0.2 16.8

a

a

The set point of the maximum applied voltage. b The maximum power calculated from the voltage, current, and duty cycle.

synthesized anatase and rutile nanopowders are simply mixed together.11,24,25 Another approach to vary the anatase-to-rutile ratio is annealing.26,34
38 When anatase nanoparticles are gradually annealed, a certain fraction of them transforms into thermodynamically more stable rutile. Unfortunately, particles usually sinter at the same time, which means that, besides the anatase-torutile ratio, also crystallinity, surface area, and particle size change.33
37 This sintering effect renders any conclusion about the influence of the anatase-to-rutile ratio alone impossible. A much more feasible approach is to synthesize titanium dioxide samples with variable polymorph compositions and compare them. Several groups have tried that approach using various synthesis techniques.27
30,32,39 Additives to the preparation precursor, such as NO3
, SO42
, and Cl
, have been successfully used to effectively lower the sintering temperature, thus producing mixed-phase TiO2 nanoparticles.27
29 Unfortunately, these additives constitute undesired impurities at varying concentrations that often increase the defect density and influence the recombination rate. Despite the large number of studies published on the topic, the authors could not find a single example where pure titanium dioxide samples with various anatase-to-rutile ratios, but otherwise identical properties, are compared for their photocatalytic activity. Either the synergetic effect is demonstrated based on a particular mixture or more than just the anatase-to-rutile ratio is changed at the same time. In this study, we have synthesized a series of utterly pure, highly porous TiO2 films that contain only anatase and rutile. The anatase content ranges from 0% to 86%, but otherwise, the films have identical physical properties. Thus, we are able to directly correlate the photocatalytic performance of the TiO2 films to the anatase and rutile content. We do observe the synergetic effect between rutile and anatase in a well-defined range of anatase-to-rutile ratios, and we are able to determine the mixture of optimum activity.

2. MATERIALS AND METHODS 2.1. Sample Preparation. Porous TiO2 films were produced by advanced plasma electrolytic oxidation (PEO) of pure titanium (99.6%, Advent, U.K.) sheets (0.5 mm) and foils (0.025 mm) punched into disks with a diameter of 12 mm. Prior to PEO, both sides of the specimens were polished mechanically with silicon carbide abrasive paper (#500 f #1200 f #2400) and thereafter chemically using a 20 nm silicate suspension with 2% H2O2 and 2% NH3 3 H2O. The polished specimens were degreased using acetone and dried out in air. As the electrolyte, a 0.15 M H2SO4 aqueous solution was used. A computerized power supply (SM400-AR-4, Delta) controls the high voltage and current density limit throughout the PEO process. The high

Figure 1. GIXRD patterns of TiO2 films with various anatase contents (a). The incidence angle was 2°. The anatase content as a function of the preparation power maximum is shown in (b).

voltage was applied in a square-wave fashion for 0.5 s per cycle with a repetition rate of 0.8 Hz (duty cycle ∼ 40%). The applied voltage maximum, the preset current limit, and the resulting power maximum for synthesizing porous films with variable polymorph compositions are exemplarily shown in Table 1. It turns out that anatase and rutile contents of the films are strictly monotonic functions of the maximum power, as depicted in Figure 1b. A pure rutile film was prepared by annealing the asprepared film (PEO3) at 800 °C for 2 h in a N2 atmosphere. 2.2. Characterization of Physical Properties. Grazing incidence X-ray diffraction (GIXRD) was employed to examine the crystallite structure of the films using an X-ray diffractometer (XRD, D8 Discover, Bruker) with Cu Kα radiation. By varying the incidence angle, the GIXRD technique allows for depth profiling the polymorph composition and crystallite size. As the probing depth, we use 3 times the absorption length according to the Beer
Lambert Law, with an absorption coefficient of 532 cm
1 for TiO2 at a wavelength of 1.54 Å.19 To compensate for peak broadening caused by the equipment and microstrain, Diffracplus TOPAS 2.1 based refinements were applied to extract the grain size and the integrated area for anatase volume fraction calculations. The chemical composition of the samples and the oxidation states of the containing elements were analyzed using X-ray photoelectron emission spectroscopy (AXIS Ultra, Kratos, U.K.). The surface analysis chamber contains a monochromatic aluminum Kα source that was operated at a 15 mA emission current at an accelerating voltage of 15 kV. Survey scans were collected using an X-ray spot size of 700  300 μm2, a pass energy of 160 eV, a step size of 1 eV, and 0.1 s per step. High-resolution spectra were acquired in the energy region of interest using a pass energy of 40 eV, a step size of 0.1 eV, and 0.5 s per step. The binding energy scale was calibrated using the C 1s binding energy of 285 eV 24288

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of adventitious carbon. Depth profiles of the porous films were obtained using argon (Ar+) sputtering at an emission current of 5 mA and an acceleration voltage of 5 kV. Microscopic features of the films were characterized using a scanning electron microscope (SEM, NOVA600, FEI) equipped with an energy-dispersive X-ray (EDX) spectroscope. The crosssectional microstructure and the thickness of the TiO2 films were characterized using cut foil samples. The surface area was determined by the Brunauer
Emmett
Teller (BET) method using a Micromeritics ASAP2010 (Micromeritics, U.S.) with Kr as the adsorbate.40 For that, the samples were degassed for 12 h at 350 K in a vacuum oven and mounted into a glass tube before measurements. The Kr adsorption isotherm was measured at 77 K. The vapor pressure of Kr at 77 K was measured prior to each sample measurement. 2.3. Photoreactivity Measurement. Photooxidation of methylene blue (MB) was carried out under conditions that favor the oxidation of MB. That is, the pH of the MB solution was adjusted to 7 using NaOH and HCl, and the whole degradation process was executed under strong stirring. The initial concentration of MB was adjusted around 4 μM to ensure that the dissolved oxygen (∼250 μM) is more than sufficient to fully oxidize the MB. Note that it is possible that MB is not entirely converted to CO2 and water in our activity test. Therefore, we will compare our samples based on the temporal decay of MB concentration. For this purpose, a continuously analyzing setup has been designed to evaluate the photocatalytic performance of the porous TiO2 films precisely. A 30 mL MB solution was continuously irradiated by an ultraviolet (UV) light source (365 nm LED diode, Optima 365) and circulated by a peristaltic pump (C/L, Masterflex, U.S.) through a flow cell cuvette (Hellma, Germany) placed inside a UV
vis spectrometer (UV-1800, Shimadzu, Japan). Prior to the activity test, each film was cleaned by UV irradiation for 4 h in deionized water, then transferred into MB solution and kept in the dark for 2 h in order to establish an adsorption equilibrium.

3. RESULTS AND DISCUSSION 3.1. Physical Properties. To investigate the influence of the anatase-to-rutile ratio on the photocatalytic activity, we have synthesized a series of porous titanium dioxide films with variable polymorph compositions. The synthesis parameters have been varied, as indicated in Table 1, for selected samples. Figure 1a depicts the GIXRD patterns of these as-prepared TiO2 films. Please note that the pure rutile film (PEO3-800) was prepared by annealing the as-prepared film PEO3. The data show the main diffraction peaks of anatase and rutile measured at an incidence angle of 2°. As we did not observe any brookite peaks in the fullrange diffraction patterns (not shown) and there was no indication of amorphous titania, we conclude that our TiO2 films consist exclusively of anatase and rutile. Furthermore, Figure 1a depicts that the intensity of anatase (101) increases from PEO1 to PEO6, whereas the intensity of rutile (110) decreases. The polymorph composition of the films is characterized by the anatase content [A], which is derived from

½A=% ¼

100  IA IA þ 1:265  IR

ð1Þ

where IA and IR correspond to the areas of the anatase (101) and rutile (110) XRD peaks, respectively.41 Hence, the rutile content equals [R] = 100
[A]. Figure 1b depicts the resulting anatase

Figure 2. Depth profiles of anatase content (a), anatase grain size (b), and rutile grain size (c) as derived from the Scherrer equation. The probing depth corresponds to 3 times the effective penetration depth of the X-rays.

content as a function of the maximum power. The anatase and rutile contents of the films are strictly monotonic functions of the maximum power, which can be explained by the thermal stability of anatase and rutile. A lower power generates plasma with less energy and thus lower sample temperature. These conditions favor the growth of anatase, the thermodynamically metastable phase. The strictly monotonic trend, furthermore, implies that parameters for synthesizing films with any desired polymorph composition are predictable. By varying the incidence angle of the X-rays, we have derived depth profiles of the anatase content and the size of anatase and rutile grains for all films, as depicted in Figure 2a
c. It is clear from Figure 2a that all samples exhibit a very homogeneous anatase content throughout the film. Therefore, the anatase-torutile ratio is a well-defined parameter for each film. The grain sizes for anatase and rutile were calculated from the corresponding XRD peak widths according to the Scherrer equation.42 The grain size is almost the same for anatase and rutile and for all films. The average grain size is ∼25 nm in all cases. This allows for a fair comparison of the photoreactivity of anatase and rutile phases.43 The chemical composition of the as-prepared films was characterized using XPS. Exemplary results are shown in Figure 3. The survey spectra in Figure 3a reveal Ti, O, carbon, and sulfur. From a quantitative analysis of Ti 2p and O 1s peak areas, it is clear that the stoichiometry of the samples equates to TiO2. Moreover, the oxidation state of Ti is found to be 4+, indicating a fully oxidized TiO2 film. The presence of carbon on the surfaces can be ascribed to adventitious contamination, as there is no carbon source in the 24289

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Figure 3. XPS spectra of the porous titania films prepared by PEO using 0.15 M H2SO4. Survey spectra (a) and high-resolution spectra of the S 2p peak (b) of as-prepared as well as mildly sputtered TiO2 films. Solid lines represent 3-point averages of the raw data (dots). The binding energy of sulfur dopants in TiO2 is indicated by a dotted line in (b).45,46

Figure 4. Top-view (a, b) and cross-sectional (c, d) SEM images of the porous TiO2 film PEO2 with 25% anatase.

electrolyte recipe. On as-prepared samples, high-resolution XPS data (Figure 3b) typically indicate 0.8 at. % sulfur. The position of the S 2p peak allows one to conclude that the sulfur represents adsorbed sulfur dioxide44 that remains from the electrolyte (H2SO4) rather than sulfur incorporated into the TiO2, which would be expected at a much lower binding energy.45,46 To clarify that sulfur is only a surface contamination, we have mildly sputtered the samples. Figure 3b shows the resulting depth profile of S 2p. Obviously, the sulfur can be completely removed

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Figure 5. Kr-BET plots of selected porous films (a). The derived specific surface area (squares) and the film thickness (circles) measured in SEM images are shown in (b). Solid lines represent linear leastsquares fits to the data.

within 3 min of mild argon sputtering. Meanwhile, no Ti 3+ was formed upon sputtering (not shown), which indicates that the sample surface was indeed only gently treated to remove all surface contamination. Therefore, we conclude that the films consist of utterly pure, stoichiometric titanium dioxide. The morphology of the films is shown in Figure 4. SEM images of the as-prepared sample PEO2 with [A] = 25% illustrate the typical microstructure of our films. The top-view SEM images (Figure 4a,b) exhibit a porous structure with a homogeneous pore distribution and an average pore size of ∼300 nm. Cross sections of the film (Figure 4c,d) show the TiO2 film with a thickness of ∼10 μm nicely distinguishable from the dense Ti support. The particle size, pore size, and pore distribution appear very homogeneous throughout the whole film. The openings in the surface are actually connected to pores below the sample surface that run through the whole film, creating a very open structure that presumably results in a large effective surface area. All films exhibit a comparable porous structure and similar pore sizes. The film thickness, however, decreases linearly with increasing anatase content (see Figure 5b). That is a result of the lower voltage and current limit settings needed to create higher anatase contents. One could argue that the differences in film thickness might be reflected in the measured photoreactivity, but that is not the case. The film of highest anatase content (PEO6 with [A] = 86%) still has a thickness of more than 6 μm, which is very large compared with the penetration depth of the UV light used in the photocatalysis test. At λ = 365 nm, the absorption length in TiO2 is 250 nm,47 which means that the upper 1 μm of the film absorbs the UV light completely. Therefore, the differences in film thickness are irrelevant for the photocatalytic activity measurement. The results of the Kr-adsorption measurements are shown in Figure 5a. The BET plots of the Kr-adsorption isotherms give a 24290

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discussion, we have normalized all rate constant values to the rate constant, kR, of the pure rutile film. The values for the normalized rate constant, K = k/kR, are listed as well in Figure 6a and are plotted in Figure 6b as a function of the anatase content. Pure rutile exhibits the lowest photoreactivity of all films. Because both anatase and rutile crystallites have identical grain sizes, it is fair to conclude that anatase is much more reactive than rutile. The maximum activity, however, is obtained with sample PEO4, a film with [A] = 55%, that is, an anatase-to-rutile ratio of approximately 1:1. An even higher anatase content, surprisingly, leads to a drop in performance, and the activity reaches a medium level for high anatase content. This suggests that pure anatase would not be the most active TiO2 sample. To qualitatively discuss the dependence of photocatalytic activity on the anatase-to-rutile ratio, we fitted the normalized rate constant curve in Figure 6b with the following function K ¼ KA
ðKA
1Þ  e
½A=τ þ KS  e
2

Figure 6. Photooxidation of methylene blue (MB). The MB concentration as a function of the irradiation time is shown in (a). The measured rate constants as well as the normalized rate constants for all TiO2 films are listed. In (b), these normalized rate constants are shown as a function of the anatase content. The solid line represents the fit according to the depicted function. It consists of an exponentially growing part (dotted line) and a peak-shaped offset due to synergism between anatase and rutile.

linear behavior with respect to the relative pressure (P/P0) for all samples. From the intercept and slope of the linear least-squares fits to the data, the heat of condensation for krypton and the specific surface area of the films can be derived, as explained elsewhere.48 The calculated values for the heat of condensation are very similar for all measurements (∼2 kJ/mol), which indicates that the results are very reliable. The obtained values for the specific surface area are shown in Figure 5b, together with the film thickness measured from SEM cross-sectional images. Obviously, the specific surface area decreases linearly with increasing anatase content, but so does the film thickness. Apparently, the decrease in surface area is solely caused by the decrease in film thickness. For the purpose of our photocatalytic activity measurement where only the upper 1 μm of the film absorbs the UV light completely, all films have the same effective surface area. 3.2. Photocatalytic Activity. Photooxidation of methylene blue is used to characterize the photocatalytic activity of the TiO2 films with varying anatase-to-rutile ratios. In Figure 6a, the evolution of MB concentration as a function of irradiation time is exemplarily shown for five selected films. It is observed that all films exhibit pseudo-first-order photooxidation. Only the pure rutile film shows a gradual decrease in activity after ∼50 min. The rate constant values, k, of all analyzed films derived from linear least-squares fits are listed in Figure 6a. For the following

½A
½Amax w


2

ð2Þ

Here, KA denotes the normalized rate constant of pure anatase. The first part of the equation describes that the activity grows exponentially against the activity of pure anatase when the anatase content gradually increases from 0 to 100%. The parameter τ
1 describes how quickly the activity grows upon polymorph composition change. In other words, τ gives rise to an estimate of an anatase content threshold that still corresponds to reasonable activity. The second part of the equation describes a peak-shaped addition to the rate constant of the order of KS. The maximum is located at the anatase content [A]max. The effect decays within the anatase content range [A]max ( w. We ascribe this part to the synergetic effect between rutile and anatase. The anatase content is, accordingly, divided into three regions: In region I (0
40%), the photoreactivity is limited by the insufficient amount of anatase. The activity, however, increases quickly as the anatase content increases. That suggests that anatase is much more active than rutile. That is probably due to both a higher adsorption affinity for organic molecules15 and a lower recombination rate.12 At an anatase content of ∼2τ ≈ 40%, the rate constant is already enhanced by a factor of ∼6, compared with that of pure rutile, and has almost reached the activity of anatase. Region II (40
80%) is dominated by the peak function we assigned to be the synergetic effect. The activity in that region is truly larger than that of either pure polymorph. The rate constant is up to ∼50% higher compared with that of anatase and ∼11 times higher than that of pure rutile. This constitutes a significant improvement. Apparently, more than 40% anatase and at least 20% rutile are necessary for a sufficient synergetic effect. The optimum is reached at ∼60% anatase and ∼40% rutile. Although the measurement does not unravel what exactly causes the synergism, it seems plausible to assume that electrons that are photoexcited in rutile can migrate to the conduction band of anatase, thereby effectively suppressing recombination.12 However, the reaction, in fact, takes place preferably at anatase sites. Thus, both polymorphs are comparably important for the synergism. This explains why the optimum mixture is close to 1:1. It is, of course, possible, if not likely, that the range and value of the synergetic effect depend crucially on the specific catalytic reaction that is used for testing the photoreactivity. That would explain the conflicting reports about the synergism, for example, in P25 that can be found in the existing literature.23 24291

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The Journal of Physical Chemistry C In region III (>80%), the amount of rutile is insufficient to assist electron
hole pair separation. Thus, there is no significant effect on the activity. The region is dominated by the anatase phase that proves to have a fairly high activity for this reaction compared with pure rutile.

4. CONCLUSIONS We have synthesized pure, porous TiO2 films with differing anatase and rutile contents, but otherwise identical physical properties. We are able to correlate the photocatalytic activity to the anatase-to-rutile ratio, unambiguously. We do observe a significant synergetic effect between rutile and anatase that gives rise to a higher activity than that of either pure polymorph. Moreover, we identified a well-defined anatase content range of 40% < [A] < 80% where the synergetic effect is distinct. The optimum mixture is found to be 60% anatase and 40% rutile. The rate constant at this mixture is increased by ∼50% compared with that of anatase alone. We are currently exploring the implications of these interesting findings on other relevant photochemical reactions. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (R.B.), [email protected] (F.B.).

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