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Photocatalytic H2 Evolution over TiO2 Nanoparticles. The Synergistic Effect of Anatase and Rutile Yung Kent Kho,† Akihide Iwase,† Wey Yang Teoh,† Lutz Ma¨dler,‡ Akihiko Kudo,§ and Rose Amal*,† ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Sciences and Engineering, The UniVersity of New South Wales, NSW 2032, Australia, Foundation Institute of Materials Science (IWT), Department of Production Engineering, UniVersity of Bremen, 28359 Bremen, Germany, and Department of Applied Chemistry, Faculty of Science, Tokyo UniVersity of Science, Shinjuku-ku, Tokyo 162-8601, Japan ReceiVed: NoVember 13, 2009; ReVised Manuscript ReceiVed: January 6, 2010
Photocatalytic H2 evolution over aqueous TiO2 suspension, with methanol as holes scavenger, is systematically studied as a function of anatase and rutile phase compositions. The highly crystalline, flame-synthesized TiO2 nanoparticles (22-36 m2 g-1) were designed to contain 4-95 mol % anatase, with the remaining being rutile. Although the amount of photocurrent generated under applied potential bias increases with increasing anatase content, a different trend was observed during photocatalytic H2 evolution in suspension form; that is, without potential bias. Here, synergistic effects in terms of H2 evolution were observed for a wide range of anatase contents, from 13 to 79 mol %. At the optimal 39 mol % anatase, the photocatalytic activity was enhanced by more than a factor of 2 with respect to the anatase- and rutile-rich phases. The synergistic effect in these mixed anatase-rutile phases was thought to originate from the efficient charge separation across phase junctions. No synergistic effect was observed for the physically mixed anatase and rutile particles due to insufficient physical contact. Here, we also identify the formation of highly reducing hydroxymethyl radicals during the simultaneous oxidation of methanol, which efficiently inject additional electrons into the TiO2 conduction band, that is, current-doubling, for heterogeneous (instead of homogeneous) H2 evolution. 1. Introduction 1-4
5,6
Photocatalytic and photoelectrochemical production of H2 has attracted wide interest, in line with the search for sustainable photo (solar) harnessing and energy conversion processes. Despite the large number of photocatalysts developed for H2 evolution via water splitting,7-9 titanium dioxide (TiO2) is still one of the most active, stable, low cost and yet nontoxic photocatalysts. Photocatalysis is also considered in biomass decomposition (such as methanol), for the enhancement of the overall rate of H2 production.10-13 In the near future, integrated systems of simultaneous TiO2-mediated H2 evolution and water purification may be possible.14-16 Establishing direct relationships between the structure and photocatalytic activity of TiO2 is rarely straightforward due to the intervening effect of various physicochemical properties, such as particle size, surface area, crystallinity, and morphology. In particular, understanding the relationship between the crystallite-phase compositions and the photocatalytic activity of TiO2 nanoparticles is of great significance but remains rather ambiguous. In its thermodynamically stable form, nanoscale TiO2 could exist as anatase and rutile, or in some cases as metastable brookite.17-20 Attempts to relate TiO2 photocatalytic activity solely to the crystallite phase of the nanoparticles, that is, without incorporating the aforementioned physicochemical factors, is inherently complex from the viewpoints of materials synthesis, electronic interactions, substrate specificity, as well as reaction * Corresponding author. Fax: +61 2 9385 5966. E-mail: r.amal@ unsw.edu.au. † The University of New South Wales. ‡ University of Bremen. § Tokyo University of Science.
chemistry. Perhaps one of the most intriguing but often encountered phenomena in TiO2 photocatalysis is the crystallite phase-dependent synergistic effect, in which mixed phases of anatase and rutile have been reported to exhibit higher activity than their pristine compositions.21-26 Although not yet fully understood,27 the effect was believed to involve photoexcited charge migration between the two phases that in turn enhances charge separation.25,28,29 In the present work, we describe a systematic study of the effect of anatase to rutile compositions on the photocatalytic H2 evolution. The TiO2 nanoparticles were prepared by flame spray pyrolysis,30-33 allowing for the fine-tuning of anatase/ rutile content without affecting significantly the specific surface areas. Herein, we report a synergistic effect for mixed anatase/ rutile particles during the photocatalytic H2 evolution from aqueous methanol. This effect is, however, absent during photoelectrochemical measurements. To the best of our knowledge, the study is the first to report a synergistic effect on photocatalytic H2 evolution and over a broad range of anatase-rutile composition. 2. Experimental Section 2.1. Synthesis and Characterization of TiO2. Titanium dioxide nanoparticles were prepared by flame spray pyrolysis in an enclosed quartz tube (Schott, OD ) 70 mm, l ) 500 mm).34,35 The liquid precursor consisted of 0.65 M Ti by mixing titanium tetraisopropoxide (TTIP, Aldrich, 97%), xylene (Riedel de Haen, 98%), and acetonitrile (Ajax, HPLC grade) in the volume ratio 20:55:25, respectively. During synthesis, liquid precursor was delivered to the FSP nozzle at 3-12 mL min-1 using a syringe pump (Inotech) and dispersed by 5 L min-1 of O2 (1.5 bar). Combustion of the dispersed spray was ignited by
10.1021/jp910810r 2010 American Chemical Society Published on Web 01/22/2010
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a surrounding oxymethane flame (1.5 L min-1 CH4/3.2 L min-1 O2). An additional 40 L min-1 of sheath gas (O2, N2, or both) was provided through the outermost metal ring. The nanoparticles leaving the flame were collected on a glassfibre filter (Whatmann GF/D), with the aid of a vacuum pump (Alcatel SD series). The TiO2 particles were calcined at 550 °C in air for 2 h to remove any carbon impurities present prior to use. X-ray diffraction (XRD) of the TiO2 powders was carried out on Phillips X’pert MPD using Cu KR radiation operating at 40 kV, 40 mA and scanning from 20 to 60° with a step size of 0.02°. The XRD patterns were analyzed by reference intensity ratio and Rietveld refinement techniques using the commercial PANalytical X’pert HighScore Plus software. Specific surface areas (SSA) of the particles were measured on Micromeritics Tristar 3000 by nitrogen adsorption at 77 K according to the Brunauer-Emmett-Teller method. The powder samples were pretreated in a Micromeritics VacPrep unit at 150 °C for at least 1 h before measurement. High-resolution transmission electron microscopy (HRTEM) images were obtained on a Phillips CM200 operating at 200 kV. UV-vis diffuse reflection spectra were obtained on a Varian Cary 5 UV-vis-NIR spectrophotometer and converted from reflectance to absorbance by the Kubelka-Munk method. 2.2. Photoelectrochemical Measurements. The TiO2 photoelectrodes were prepared by a standard doctor-blading procedure36 onto transparent tin-doped indium oxide conductive glass (ITO, Delta Technologies) and calcined at 450 °C in air for 30 min. Photoelectrochemical measurements were carried out in a standard three-electrode system, with Pt and silver (Ag/ AgCl) as counter and reference electrodes,37 respectively, and a 300 W Xe lamp was used as the illumination source. The electrolyte medium was made up of 0.1 M NaClO4 aqueous solution and continuously purged with N2 before and during the measurements. The photocurrents were recorded on an Autolab PGTAT12 potentiostat. 2.3. Photocatalytic H2 Evolution and Detection of Hydroxymethyl Radicals. Photocatalytic hydrogen evolution was carried out in a closed gas circulation system.8 TiO2 powders (0.1 g) were dispersed and magnetically stirred in 150 mL of aqueous solution containing 10% v/v methanol and hexachloroplatinic acid (equivalent to 0.4 wt % Pt with respect to TiO2). Prior to photocatalytic H2 evolution, the suspension was evacuated for 30 min to remove dissolved O2. Irradiation with a 300 W Xe lamp was delivered from the top of the cell through a Pyrex window. The amount of hydrogen evolved was measured using an online gas chromatograph (Shimadzu, GC8A, TCD). For in situ spin-trapping of hydroxymethyl radicals ( · CH2OH), 1 mM of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO, Aldrich) was added into the TiO2 suspension prior to evacuation and H2 evolution. The DMPO-CH2OH spin adduct was separated from the TiO2 by centrifugation, and the supernatant was kept at -36 °C and analyzed within 24 h on an electron paramagnetic resonance spectrometer (EPR) (Bruker EMX). The EPR spectrometer was operated at 0.79 mW, 9.47 GHz, 100 kHz field modulation and center field 3480 G. The supernatant was also analyzed by LC-MS on a Thermo LCQ Deca XP Plus, using acetonitrile and 0.1% formic acid in water as the mobile phase. Samples were injected (10 µL injection loop) into a reversed-phase liquid chromatography column (Symmetry C18 5 µm, 150 × 3.9 mm, Waters). Two micropumps in series were programmed to deliver a gradient based on water and acetonitrile at a total flow rate of 500 µL min-1. For the first 2 min, the acetonitrile concentration was kept at 15%, followed by a linear
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Figure 1. XRD patterns of TiO2 powder samples with different amounts of anatase-rutile compositions: (a) 4, (b) 8, (c) 13, (d) 39, (e) 58, (f) 79, and (g) 95 mol % anatase. Also shown are the reference peaks of anatase (PDF: 021-1272) and rutile (PDF: 021-1276).
TABLE 1: TiO2 Anaste/Rutile Compositions and the Corresponding Specific Surface Areas (SSA) and Crystallite Sizes (danatase and drutile) anatasea (mol %)
SSA (m2 g-1)
danataseb (nm)
drutileb (nm)
4 8 13 39 58 79 95
24 36 22 28 30 35 25
29 18 34 33 25 26 35
36 21 31 32 26 24 28
Rutile composition (%) ) 100% - anatase (%). b Calculated by Debye-Scherrer equation. a
increment to 95% over a period of 10 min. The mobile phase composition was then linearly brought back to 15% acetonitrile in 2 min. The system was equilibrated for 5 min between injections. Full scan spectra or product ion scan spectra were acquired continuously. A splitter was introduced between the LC column and the mass spectrometer to reduce the flow to approximately 50 µL min-1. 3. Results and Discussion 3.1. Physical Characteristics of TiO2 Nanoparticles. TiO2 nanoparticles were synthesized by flame spray pyrolysis (FSP)30-33 resulting in well-crystallined nanoparticles as depicted by the XRD patterns shown in Figure 1. The crystallite compositions of anatase and rutile were carefully controlled by varying the oxygen partial pressure of the sheath environment during FSP (see the Supporting Information for detailed particle synthesis conditions), which, in turn, allowed the gradual tuning of the anatase content from 4 to 95 mol % (with the remainder being rutile).33 Here, the specific surface area of the powders was kept constant within a relatively small variation of 22-36 m2 g-1 (Table 1). Furthermore, the crystallite sizes of anatase and rutile are very similar (Table 1), which is quite different from that of standard P25 TiO2 (in which rutile exhibits larger crystals than anatase).30,32 These are among the most important features in this work because it allows a unique study of the influence of TiO2 crystallite compositions on photocatalytic activity without having a significant influence of SSA and crystallite size differences. High-resolution transmission electron microscopy (HRTEM) images in Figure 2 show faceted TiO2 nanoparticles
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Figure 2. TEM images of different TiO2 samples with different amount of anatase compositions: (a, e) 4, (b) 39, (c) 58, and (d, f) 95 mol % anatase.
with lattice fringes extending uninterrupted to the edges of the particle (Figure 2e, f). In other words, the rutile (d spacing 3.25 Å for the (110) face, 2.49 Å for the (101) face) and anatase (3.52 Å for the (101) face) phases exist as discrete crystallite nanoparticles. In terms of morphology, these particles are aggregated, as would be expected, given their continuous Brownian collisions during flame synthesis.30 As
has been shown by other researchers38-41 and also in the current work (to be discussed below), the intimate contact of anatase and rutile crystals (i.e., aggregation) is a crucial parameter for the interparticle charge migration. In fact, it was recently shown that the creation of a surface-phase junction between anatase and rutile could improve the intrinsic photocatalytic H2 evolution over TiO2.42
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Figure 3. Typical UV-vis diffuse reflectance spectra of TiO2 samples shown here for 4, 39, and 95 mol % anatase. Inset shows the Tauc plot for estimation of optical bandgaps, shown here for 4% (3.0 eV) and 95% (3.2 eV) anatase.
Figure 3 shows the diffuse reflectance spectra of the TiO2 nanoparticles. The bandgap of anatase and rutile particles could be estimated from the Tauc plot, [F(R∞)hV]n vs hV, where n ) 0.5 for an indirect bandgap (Figure 3 inset), by extending the tangential lines to the abscissa.29,43 The measured bandgaps for a rutile-rich sample (4 mol % anatase) were 3.0 and 3.2 eV for anatase-rich ones (95 mol % anatase), in good approximation to the theoretical bandgap of TiO2 (rutile, 3.02 eV; anatase, 3.23 eV).44 The apparent intermediate bandgap values for other mixed compositions were simply spectroscopic averaging of the anatase and rutile bandgaps, rather than the actual bandgap of the materials. In all cases, the absorption intensities above the bandgap edge were similar, implying a similar number of electrons being excited across all the anatase-rutile compositions. 3.2. Photoelectrochemical Measurements of TiO2 Nanoparticles. To characterize the photoelectrochemical properties of the anatase-rutile compositions, the TiO2 particles were fabricated as photoelectrodes in a standard doctor-blading procedure.36 Although not shown, the procedure of doctorblading followed by calcination at 450 °C did not impart significant changes to the physical characteristics; that is, specific surface areas, crystallite phase, and compositions of the parent TiO2 nanoparticles. Figure 4a shows the I-V curve of the films in the presence of 10 vol % methanol and UV irradiation in a chopping mode. The TiO2 electrode gave a cathodic dark current and a large anodic photocurrent, typical of an n-type semiconductor. In both cases, with and without addition of methanol, the net saturated photocurrent of TiO2 increases with the increase in anatase fraction (Figure 4b). Since the TiO2 samples have similar aboveedge absorption intensities across all anatase-rutile compositions, implying similar numbers of electrons are being excited under irradiation (Section 3.1), the lower photocurrent with increasing rutile content would imply higher recombination rates. In the presence of 10% v/v methanol, a significant ∼5-fold increase in photocurrent density was measured. This increase is attributed to two phenomena: (1) suppression of charge recombination and enhancement of surface oxidation as a result of continuous scavenging of photoholes by organic a sacrificial agent (i.e., methanol and its derivatives) and (2) the current doubling effect because the highly reducing hydroxymethyl radical, a derivative of methanol oxidation, contributes an additional electron, which is directly injected into the TiO2 conduction band.45-47 The presence of hydroxymethyl radicals has even been demonstrated for the reduction of metal cations, which reduction potentials are above that of the TiO2 conduction band edge.45 Given the important role of hydroxymethyl radicals
Figure 4. (a) Typical I-V curve of TiO2 samples, shown here for 95 mol % anatase, deposited onto a transparent, ITO, conductive glass electrode, measured under the irradiation of a 300 W Xe lamp; in chopping mode (repeated light on-off, inset); 0.1 mol L-1 NaClO4 and 10% v/v methanol; reference and counter electrodes, Ag/AgCl and Pt electrodes. (b) The saturated anodic photocurrent for different TiO2 samples as a function of anatase compositions, with (solid circle) and without (open circle) the presence of methanol.
in the photoreduction process, we have detected its presence during photocatalytic H2 evolution and identified its reduction path, as discussed in detail in a later section. The onset potentials of the TiO2 photoelectrodes, which can be taken as a measure of quasi-Fermi levels of the anodic photocurrent, were plotted as a function of anatase content (Figure 5). Below 60% anatase, only a slight increase in onset potential was measured as compared to that of the highest rutile content; that is, 96% rutile (or 4% anatase). This suggests that most of the photoexcited electrons are transported to the ITO through the conduction band of the rutile. The migration of excited electrons from the anatase conduction band to that of rutile is similar to that postulated by many38,48,49 and is energetically possible given the lower conduction band level of the latter. As the anatase fraction increases beyond 60%, the strong electronic interactions with rutile becomes significant, resulting in a rapid increase in the onset potential, reaching that of the anatase conduction band at 95% anatase. The intermediate onset potential between that of pristine rutile and anatase TiO2 implies some mechanism of Fermi level band-bending, which required intimate physical interaction between the two phases.
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Figure 5. The estimated onset potential (vs Ag/AgCl) of TiO2 samples as a function of anatase composition.
Figure 6. (a) Kinetics of photocatalytic H2 evolution over TiO2 nanoparticles (suspension), shown here for 4, 39, and 95 mol % anatase and that of 39 mol % prepared by physical mixing of the 4 and 95% anatase. Catalyst: 0.1 g TiO2. Solution: 10% v/v methanol aqueous solution, 120 mL, H2PtCl6 (equivalent to 0.4 wt % Pt with respect to TiO2). Light source: 300 W Xe lamp. Reaction cell: top irradiation cell with a Pyrex window. (b) The rate of H2 evolution over as-prepared TiO2 samples (solid circles) as a function of anatase content and that prepared by physical mixing (open circle).
3.3. Photocatalytic H2 Evolution over TiO2 Nanoparticles. Photocatalytic evolution of H2 was carried out by UV-irradiating TiO2 in its suspension form in deaerated aqueous solution containing 10% v/v methanol and H2PtCl6 (equivalent to 0.4 wt % Pt). Figure 6a shows the typical H2 evolution kinetics, where the amount of H2 evolved increases linearly with
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2825 irradiation time. The evolution kinetics is further represented by the rates of H2 evolution (µmol h-1 H2) over all the anatase compositions (Figure 6b). It can be seen that the dependency of H2 evolution rates is clearly different from that previously measured in the photoelectrochemical system. Instead of increasing evolution rates with increasing anatase content, as would be predicted from the photocurrent measurements, here, a synergistic effect, in which the mixed anatase-rutile phase exhibits higher photocatalytic activity than that of rutile- or anatase-rich compositions, is clearly observed across a wide range of anatase fractions from 13 to 79%. At the optimal 39% anatase, the H2 evolution activity (425 µmol h-1 of H2) was more than twice that of the anatase- and rutilerich samples. It should be mentioned here that the dependency of H2 evolution rates on Pt loadings is ruled out, since the samples were found to be optimal at 0.4 wt % Pt. Similarly, statistical Pt particle size counting (average ∼ 2 nm) found little difference between those deposited on 39% anatase and that of anataseand rutile-rich samples (Figure 7). In a recent multivariable analysis incorporating a range of commercial TiO2 samples, Ohtani et al.50 suggested the likely strong influence of Pt size and morphologies on H2 evolution rates. This effect is clearly absent in the current studies, given the strong similarities in Pt deposits across different TiO2 samples. Detailed analysis by HRTEM on the TiO2 samples did not find any preferential phase deposition of Pt (see Suppporting Information). This further excludes the scenario of synergistic photocatalytic activity as originating from the Pt-induced anisotropic charge separation at either anatase or rutile particles. One of the main differences between the photoelectrochemical and that of the suspension photocatalytic systems lies in the external potential bias applied in the former, which lowers the photoelectrode Fermi level for direct channeling of photoelectrons to the external circuit (Scheme 1a). The separation of photogenerated charges across the TiO2 anode and counter electrode further allows the discrimination of redox reactions; that is, photocatalytic methanol oxidation at the photoanode and H2 evolution at counter electrode. Obviously, these mechanisms do not exist for the suspension-type photocatalytic system. However, the absence of applied potential bias during the photocatalytic H2 evolution renders the migration or percolation of photoelectrons to different sites within the individual aggregates13,51 and limited by the removal of photogenerated charges by surface redox reactions (Scheme 1b). In particular, the synergistic H2 evolution rates for mixed anatase-rutile in the presence of methanol was thought to arise from the efficient trapping and separation of photogenerated charges at the phase junction39,42 or the separation of the charges across different crystallite phases.21-26,38,40 For either of these two cases to occur, a significant amount of anatase and rutile phase mix is a prerequisite (e.g., 13-79% anatase), thereby providing a possible explanation on the observation of synergistic effects under mixed phase and only in the absence of external potential bias. In addition to phase compositions, it is further envisaged that the intimate short-range physical interactions between anatase and rutile are inevitable for efficient transport to or across phase junction. To exemplify this effect, a physically mixed sample of 39% anatase and 61% rutile was prepared by manual grinding of anatase- and rutile-rich samples. In this case, no synergistic effect could be observed, since the mixing is only limited at the interaggregate level, as opposed to the interparticle mixing for the as-prepared mixed phases.
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Figure 7. TEM images of used TiO2 after photocatalytic H2 evolution reaction, showing the dispersion of Pt cocatalysts (from the in situ photocatalytic reduction of H2PtCl6) for (a) 4, (c) 39, and (e) 95 mol % anatase. Also shown are the corresponding statistical distribution (b, d, f, respectively) of Pt size, average Pt size (dTEM), and geometric standard deviation (σg), calculated from sample size (N) of at least 180.
3.4. Proposed Mechanism of Photocatalytic H2 Evolution. As mentioned earlier, the photocatalytic oxidation of methanol may lead to the formation of hydroxymethyl radicals,45-47,52 which are capable of a current doubling effect. The conversion of methanol to hydroxymethyl radicals proceeds rapidly with a rate constant, k ) 9.7 × 108 M-1 s-1, and also with high selectivity of 93%.53,54 Its high reduction potential E( · CH2OH/ CH2O) ) -0.95 V vs NHE55 renders the direct injection to TiO2 conduction band energetically possible. Here, we have attempted to detect the radical species during the photocatalytic H2 evolution by an in situ spin radical-trapping technique. Trapping of hydroxymethyl radicals by DMPO is a relatively efficient process (k ) 2.2 × 107 M-1 s-1).56 Figure 8a shows the characteristic electron paramagnetic resonance (EPR) sextet peaks of a DMPO-CH2OH adduct57 formed during the photocatalytic H2 evolution. The spin-trap
adduct was confirmed by LC-MS to have mass number, m/z of 144 (Figure 8b). It is worth noting that despite the rapid capturing of hydroxymethyl radicals by DMPO, the EPR signal of DMPO-CH2OH adduct was visible only after 2 h of H2 evolution, suggesting the competitive process of electron injection into the TiO2 conduction band and that of DMPO trapping. To further verify the efficiency of the former, photocatalytic H2 evolution was carried out over a TiO2 electrode in a sealed dual-compartment electrochemical cell with a relatively low applied voltage (+0.1 V vs NHE) to direct the flow of photoelectrons to the Pt counter electrode. The aim was to separate the photocatalytic oxidation and reduction reactions in the anode and cathode cells, respectively, further allowing identification of the reductive path by hydroxymethyl radicals (Scheme 2). In this instance, no H2 was detected at the anode cell, ruling out the possibility of direct water reduction, despite
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SCHEME 1: Differerence in the Charge Transport in (a) a Photoelectrochemical System, in Which Photocurrent Was Collected under Applied Potential Bias, and That of (b) a Suspension-Type Photocatalytic System, in Which Photogenerated Charges Percolate to Different Sites within an Aggregate
the favorable reduction potential of hydroxymethyl radicals. On the other hand, 98-100% of the generated photocurrent was converted to H2 at the cathode side; that is, [2 × amount of evolved H2]/[amount of electrons flowing through external circuit]. The results imply that the electrons from the hydroxym-
ethyl radicals were injected into the TiO2 conduction band. Direct reduction of water by hydroxymethyl radicals, simulated by the in situ Fenton reaction (Fe2+/H2O2) with methanol,58 is, indeed, significantly slower ( thin solid arrow > unfilled arrow. Note: For simplicity, the formation of hydroxymethyl radicals ( · CH2OH) by hydroxyl radicals ( · OH) is represented by the hole oxidation step.
in Scheme 3. Upon UV illumination of TiO2, the typical electron-hole pair is generated at the conduction band and valence band, respectively. The photogenerated holes are capable of methanol oxidation, either directly or through the formation of hydroxyl radicals, to form hydroxymethyl radicals. In the absence of dissolved oxygen,46,59 the hydroxymethyl radical efficiently injects an additional electron into the TiO2 conduction band. Although it may not be possible at this stage to distinguish between the conduction bands (anatase, -0.57 V vs NHE;60 rutile, -0.34 V vs NHE61) at which the electron is injected, both are in principle thermodynamically possible. In the case of injected electron and the earlier intrinsically excited electron at the anatase side, they may migrate across the phase junction to the rutile conduction band, given the latter lower band potential. Depending on the aggregate size, it may even be possible for the electrons to percolate through the grain boundaries of the intimately connected TiO2 particles, in an antenna-like mechanism.62,63 Herein, the aggregate size of the optimal 39% anatase (0.28 µm) was not significantly different from that of the 4% (0.22 µm) and 95% anatase (0.34 µm). Although the antenna mechanism has been postulated around the pristine phase (i.e., anatase) and was beneficial for charge separation,13,62,63 it is envisaged that such migration of electrons across phase junctions further enhances their separation efficiencies. The difference in the energy levels of anatase and rutile conduction bands may act as barrier separation, which at this juncture appears to be the most likely explanation for the synergistic behavior. It would appear also that the electron migration negates the otherwise lower activity of rutile. Such synergistic behavior may be absent during photoelectrochemical water reduction, since the applied potential renders the interparticle charge separating mechanism less important. 4. Conclusions The work presents a unique study of the effect of TiO2 anatase and rutile compositions on the aqueous photocatalytic H2 evolution (with methanol as the hole scavenger). Synergistic effects, in which a mixed anatase-rutile phase exhibited a much higher activity than that of the anatase- or rutile-rich phase, were observed in the slurry form of TiO2 over 13-79 mol % anatase. This is true despite the increase in photocurrent (as measured under an applied potential bias), with increasing anatase content, implying that photogenerated charges were less efficiently
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