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Article pubs.acs.org/JPCC

Synergetic Surface Sensitivity of Photoelectrochemical Water Oxidation on TiO2 (Anatase) Electrodes Katerina Minhová Macounová,† Monika Klusácǩ ová,† Roman Nebel,† Markéta Zukalová,† Mariana Klementová,‡ Ivano E. Castelli,§ Mathias D. Spo,§ Jan Rossmeisl,*,§ Ladislav Kavan,*,† and Petr Krtil*,† †

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, 18223 Prague, Czech Republic ‡ New Technologies - Research Centre, University of West Bohemia, 306 14 Pilsen, Czech Republic § Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 København Ø, Denmark S Supporting Information *

ABSTRACT: The paper compares photoelectrocatalytic activity and selectivity of nanocrystalline anatase dominated by {110}, {101}, and {001} faces in photo(electro)catalytic water splitting. Although the anodic half-reaction of water splittingoxygen evolutiondominates the overall photoelectrochemical behavior of the photoexcited anatase, simultaneous reduction under photoelectrochemical conditions is also observed on some anatase faces. The activity of individual facets in anodic half-reaction of water splitting (oxygen evolution) increases in the order {101} < {110} < {001}. The increasing oxidation activity tracks the tendency of the surface to generate the OH• radical producing intermediates (H2O2, ozone) on the trapped hole states. The activity in reduction processes increases in the reversed order. Particularly, the reduction activity of the {101} oriented anatase can be attributed to pronounced hydrogen evolution by a charge transfer of photogenerated electrons. The observed trends agree with DFT-based models which confirm the possibility of a rational design of the photocatalysts.

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{101} face,5 the optical and X-ray photoelectron spectra provide just the opposite order.6 However, a simple energetic approach turns out to be oversimplified for the actual catalyst design since it disregards important aspects of the catalyst activity connected with e.g. recombination, charge transport to the catalyst surface, and intrinsic catalytic activity of the semiconductor’s surface. The charge transport to the catalyst’s surface was systematically treated in ref 7 where anisotropy facilitated hole self-trapping was introduced to address the charge carrier availability on various anatase surfaces and was theoretically conceived8 to explain the experimentally observed face-dependent photocatalytic behavior of the anatase photocatalysts.9−15 The observed confinement of oxidation products near the {001} oriented anatase faces while that of the reduction reaction products near the {101} oriented faces led to a shape optimization approach which predicted the {001} and {101} featuring crystals to be the best attainable catalyst.13−15 The EPR studies attributed the direct radical formation to the {001} faces while the {101} oriented surfaces supposed to contribute,

INTRODUCTION

The efficiency of photocatalytic processes is intensively investigated in connection with the water and air purification, self-cleaning, and superhydrophilic coatings. Furthermore, the photocatalysts are expected to drive sustainable fuel production using solar light.1,2 Despite the vigorous attention paid to the development of the advanced photocatalysts, there are several fundamental aspects of the photocatalysts’ activity that are not satisfactorily understood preventing the rational design of the catalytic materials namely for solar fuel production. It is generally accepted that the photocatalytic action of the semiconducting material is connected with formation of the photogenerated electrons and holes whichif present at the surfacemay enter complex multielectron redox reactions. The simplest approach toward assessing of the activity of various titania phases (and/or faces) for the photoredox reactions is based on the determination of the energies of the valence/ conduction bands edges. Although theoretical means for estimating the position of the bands have been suggested,3 the experimental results remain highly controversial for the major titania polymorphs (anatase/rutile),3,4 and the same applies for the comparison of the most abundant anatase faces, i.e., {101} and {001}. While electrochemical studies put the conduction band of the {001} face slightly above that of the © 2017 American Chemical Society

Received: September 14, 2016 Revised: February 2, 2017 Published: February 2, 2017 6024

DOI: 10.1021/acs.jpcc.6b09289 J. Phys. Chem. C 2017, 121, 6024−6032

Article

The Journal of Physical Chemistry C

copious amounts of Milli-Q water and dried at 100 °C for 5 h. The as-received material contained between 6.5 and 20 wt % of F as determined by energy-dispersive X-ray spectroscopy (EDS) analysis. After calcination (500 °C, 1 h) the F content dropped practically to zero. The structural characterization of synthesized samples was based on powder X-ray diffraction performed using Rigaku Miniflex powder X-ray diffractometer with Cu Kα radiation. The particle shape was evaluated by scanning electron microscopy (SEM) (Hitachi S4800). The surface orientation of the prepared photocatalysts was determined by analysis of high-resolution transmission electron micrographs. Transmission electron microscopy (TEM) and scanning TEM (STEM) were carried out on a JEOL JEM 2200FS microscope operating at 200 kV (autoemission Schottky gun, point resolution 0.19 nm) with an in-column energy filter, a HAADF detector, and an EDX silicon drift detector Oxford Instruments X-Max attached. Images were recorded on a Gatan CCD camera with resolution 2048 × 2048 pixels using the Digital Micrograph software package. Powder samples were dispersed in ethanol, and the suspension was treated in ultrasound for 5 min. A drop of very dilute suspension was placed on a holey-carbon-coated copper grid and allowed to dry by evaporation at ambient temperature. The Brunauer− Emmett−Teller (BET) surface areas of the prepared materials were determined from nitrogen adsorption isotherms at 77 K (ASAP 2010, Micromeritics). The X-ray photoelectron spectroscopy (XPS) spectra were recorded using Omicron Nanotechnology instrument equipped with a monochromatized Al Kα source (1486.7 eV) and a hemispherical analyzer operating in constant analyzer energy mode with a multichannel detector. All materials included in photoelectrochemical experiments were according the energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis found to be free of impurities (Figure S1, Supporting Information). The anatase electrodes used in photoelectrochemical experiments were drop casted on gold-covered Ti mesh (Goodfellow) (1 cm2). The gold interlayer was deposited from colloidal Au ink (Fraunhofer Institute fü r Keramische Technologien und Systeme, Dresden, Germany) and cured at 200 °C. Anatase suspension (10 g/L) in absolute ethanol was dropped on the substrate in 15 μL increments. The electrode was dried at 80 °C after each increment. The whole procedure was repeated until the mass of the photocatalyst on the electrode surface was in the range 1−2 mg. Mechanically stable electrodes were obtained by calcination at 400 °C in air for 4 h. Photoelectrochemical Experiments. Photoelectrochemical behavior of the prepared samples was studied in 0.1 M HClO4 (Aldrich p.a.) solution, using a single-compartment cell made from quartz. A three-electrode arrangement with anatase working, Pt auxiliary, and Ag/AgCl reference electrode (sat. KCl) was used. All photoelectrochemical experiments were carried out in chronoamperometric mode using AUTOLAB (PGSTAT 30) potentiostat for potential control. The measured potentials were recalculated and are quoted in reversible hydrogen electrode (RHE) scale. The Bluepoint LED spot source (Hönle UV Technology) with the 365 nm wavelength was employed as an illumination source. The nature of the reaction products was assessed by differential electrochemical mass spectrometry (DEMS) approach combined with photoelectrochemical measurements. The DEMS apparatus consisted of a Prisma QMS200 quadrupole mass spectrometer (Balzers)

aside of the reduction activities also by indirect oxidation via formation of surface confined peroxides.16 This was later rationalized by theoretical prediction of hole trapping near the valence band edge showing a pronounced anisotropy face selectivity.7 The trapping of holes in the band gap provides an additional stabilization with respect to the edge of the valence band in the range between 0.5 eV for the {101} oriented surfaces and of approximately 1.3 eV for {001} oriented surfaces. Although this mechanism seems to explain and support the existing models of the photocatalytic activity allowing sufficient population of the charge carriers at the catalyst surface even in the absence of the band bending, it counters, however, the observed high photocatalytic activity of the nanoparticulate catalysts (namely, those featuring {001} oriented facets), which is conventionally attributed to the catalyst induced photogeneration of the OH• radicals. It needs to be stressed that the hole trapping in the band gap effectively lowers their oxidation ability since the average surface hole energy does not allow generation of radical product(s).16 It needs to be stressed that the holes enter the charge transfer reaction as electron acceptors. Henceforth, their activity increases with decreasing energy. Unfortunately, the existing experimental data reflecting the face-specific effects in the photocatalytic behavior of oxides do not allow linking the observed activity neither with the population nor with the typical energy of the charge carriers. To bridge the gap of this fundamental knowledge, we present here, for the first time, results of a systematic investigations of the photocatalytic activity of differently oriented anatase catalysts in electrochemical oxidation of water. To address the fluxes of both majority (electrons) and minority (holes) carriers, we present the results obtained in photoelectrochemical experiments with various anatase catalyst deposited on a gold substrate. We use an external control of the Fermi level in the catalyst and the photocurrent measured upon UV illumination quantifies the overall flux of the charge carriers. The efficiency of the water splitting is further checked by an independent mass spectroscopic detection of the envisaged reaction productsnamely, of oxygen and hydrogen. To reflect the behavior relevant to the solar technologies, we restrict our analysis to excitation photon energies around 3.4 eV which are near the band gap.

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EXPERIMENTAL SECTION Materials and Chemicals. Anatase samples were prepared by solution-based procedures as follows. Samples A and B were synthesized by hydrothermal synthesis from water-soluble Ti peroxocomplex by the procedure developed by Kobayashi et al.17,18 Ti powder (Goodfellow, 99.9%) was dissolved in concentrated hydrogen peroxide (30%) and NH3 (25%) (both Aldrich). Citric acid (Aldrich p.a.) in equimolar amount to Ti was added to the solution as a complexing agent. The solvent was evaporated at 80 °C until a formation of a yellow gel-like substance. The gel-like substance was dissolved in Milli-Q water (sample A) or in in 60% (m/m) solution of ethylenediamine (sample B). Both transparent solutions were hydrothermally treated in a PTFE-lined stainless steel autoclave at 200 °C for 24 h. The resulting precipitate was filtered, washed with water, dried in air, and calcined at 400 °C for 4 h. Sample C was prepared by hydrolysis of 20 mL of titanium(IV) butoxide (purum, ≥97.0%, Sigma-Aldrich) with 2.4 mL of hydrofluoric acid (48%; Sigma-Aldrich) at vigorous stirring. The mixture was autoclaved at 200 °C for 24 h. The product was washed with 6025


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shape can be derived from the variation in the relative intensities of the {004} and {200} reflections which reveal the tendency of the nanocrystal to grow along the z-axis and in the x−y plane, respectively. While in the case of the sample C (platelet-like nanocrystals) the growth along the z-axis is clearly suppressed (with respect to the growth along (101) direction), the growth in the x−y plane is not hindered in any of the included samples. Overall surface orientation can be obtained with help of the high-resolution TEM micrograph analysis (see Figures S2−S8). The HRTEM results clearly indicate that the sample A reflects combined behavior of the {110} faces (with a contribution from the {101} faces). Closer examination also reveals a presence of {001} and {100} oriented surface; their representation is, however, low. Sample B surface, despite a highly defect inner structure of the material, is dominated by {101} faces, while in the sample C the {001} faces dominate at the surface with small contributions from {101} oriented facets forming the sides of the prepared platelets. It needs to be noted that the presence of {110} oriented surface is somewhat surprising since this orientation is not frequently found at polycrystalline materials.12 A quantitative assessment of the surface orientation is given in the Supporting Information (see Table S1). Given the n-semiconductor nature of the anatase, one expects its photoelectrochemical behavior to be controlled by the hole charge transfer at the semiconductor−electrolyte solution interface. From the energy of the valence band edge in anatase one anticipates the photogenerated holes to be involved in four conceivable anodic processes as follows: four electron water oxidation producing oxygen and protons (E0 = 1.23 V)

connected to SU071 turbomolecular drag pumping station (Balzers). The DEMS experiments were carried out in a singlecompartment Kel-F cell with three-electrode arrangement19 under automated pressure control to ensure comparability of the measurements. DFT Calculations. The activity of titanium dioxide−anatase surfaces in reaction of hydrogen evolution was studied by DFT modeling using the GPAW code.20,21 The adsorption energies of a hydrogen were investigated under periodical conditions on the four layers slabs, where atomic possessions were constrained in the two bottom layers of the slab to reproduce the structure of the bulk and relaxed in the two top layers and the adsorbate. The RPBE was used as the exchange-correlation functional.22 To reproduce the n-character of the anatase, we replaced one of the Ti atoms in the bulk with V to create an excess of the electron density. Details of the computations are given in the Supporting Information.

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RESULTS The behavior of the individual surface orientations in relation to the practical photo(electro)catalytic applications can be best demonstrated on selectively synthesized nanocrystalline catalysts of distinct diffraction patterns and crystal shapes (see Figure 1). As follows from Figure 1, the present study compares

2H 2O → O2 + 4H+ + 4e−

(1)

electrocatalytic formation of hydrogen peroxide (E0 = 1.77 V) 2H 2O → H 2O2 + 2H+ + 2e−

(2)

electrocatalytic ozone generation (E0 = 2.05 V) O2 + H 2O → O3 + 2H+ + 2e−

(3)

direct electrochemical OH• radical formation (E0 = 2.6 V) H 2O → OH′ + H+ + e−

(4)

While the processes 1−3 represent multielectron charge transfers which are likely to be electrocatalytic in nature and henceforth surface-sensitive, the last process per se represents a single electron transfer and may be therefore surface independent. The concomitantly formed electrons can be trapped in localized states near the conduction band edge and subsequently used for photocatalytic reductions. In photoelectrochemical experiments, these electrons are mostly transferred to the external circuit and finally enter dark cathodic reactions at the counter electrode. Their reaction at the TiO2 electrode−electrolyte interface, however, cannot be discounted. The most likely electron-related reactions in this respect represent oxygen reduction or hydrogen evolution. It needs to be noted that both these processes are catalytic in nature and therefore surface-sensitive. The effect of the surface orientation of the anatase in the photoelectrochemical oxidation/reduction of water is summarized in Figures 2 and 3 which present the photocurrent

Figure 1. Powder X-ray diffraction patterns (left), SEM images (middle), and HRTEM images (right) of anatase samples differing in the nanocrystal shape. The sample assignment is given in the figure annotation. The SEM images were taken on the catalysts attached to the electrodes; the HRTEM images correspond to as prepared catalysts after calcination.

samples composed of isometric particles containing {110} and {101} parts (sample A) (BET specific surface area of 110 m2/ g), spindle-like-shaped nanoparticles (sample B) (BET specific surface area of 97 m2/g), and platelet-like nanoparticles (sample C) (BET specific surface area 75 m2/g). The diffraction patterns of all studied catalysts reproduce all expected anatase-characteristic features; a closer examination, however, reveals differences related to the particular crystal growth in the individual samples. In general, the most significant parameter relevant to the observed nanocrystal 6026


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electrochemical mass spectroscopy (DEMS). The results of the mass spectrometric experiments are summarized in Figure 4.

Figure 2. Typical photocurrent vs time curves observed for anatasebased catalysts in acidic (pH = 1) electrolyte solution saturated with either Ar (green) or O2 (black). Illumination by UV lamp (365 nm). The applied potential in all cases equaled to 680 mV vs RHE. Panels A−C relate to samples A−C, respectively.

Figure 4. DEMS-based average number of electrons needed to evolve one molecule of oxygen as a function of the applied potential on different anatase photocatalysts illuminated in acid (pH = 1) electrolyte solution. The sample color coding is the same as in Figure 1. The error bars represent standard deviations of three independent measurements.

To compensate for the variations of the actual surface area of different samples, we present the detected amounts of oxygen normalized to the measured photocurrent yielding the apparent number of electrons needed to evolve one molecule of oxygen (z). The procedure of the z calculation is in detail described in the Supporting Information. The DEMS data in Figure 4 reflect the difference in photoelectrochemical behavior of {001}, {110}, and {101} faces. The catalyst dominated by {001} faces exhibits the z values between 2.0 and 4.0; the catalysts featuring {110} faces show systematically higher values of z in the range 3.0−4.5. The catalysts dominated by {101} faces show the values of z between 4.0 and 6.5 − the highest value of all studied samples. Regardless of the prevailing surface orientation, the z value changes with electrode potential. While in the case of the {001} and {110} faces the z gradually increases with increasing potential, in the case of the {101} faces the z gradually decreases with electrode potential. It needs to be stressed that the apparent number of electrons, z, observed for all samples usually deviates from the value of 4, which is expected for the water oxidation according to eq 1 forming O2. Such a behavior outlines that the photoelectrolysis of water represents a complex reaction sequence which combines all anodic processes outlined above. The z values significantly lower than four (namely, observed on catalyst dominated by {001} faces) indicate that the detected oxygen is not originating solely from a heterogeneous charge transfer process but also from homogeneous processes most likely involving radicals23 which may be produced by a decomposition of the products of the reactions 2 and 3. Low values of z, therefore, indicate a suitability of the catalysts for the photocatalytic oxidation processes intended, e.g., for wastewater treatment. Although the DEMS selectivity measurements might be in principle affected by the actual electrode morphology (catalyst layer thickness, porosity, etc.), these effects are in this particular case of minor importance since one can anticipate similarity in the layer thickness and porosity resulting from very similar specific surface area and surface loadings of the included catalysts. In this respect the data summarized in Figure 3 can be rationalized primarily in terms of the surface orientation of the

Figure 3. Left panel: photocurrent vs potential curves of the anatasebased catalysts in acidic (pH = 1) electrolyte solution. Illumination by UV lamp (365 nm). Right panel: a ratio of the photocurrent recorded in argon- or oxygen-saturated solutions as a function of the applied potential. The solid symbols reflect the behavior in solution saturated with oxygen; the open symbols correspond to behavior in solution saturated with argon. The color coding of samples is the same as in Figure 1. The presented photocurrents represent steady values obtained at potentiostatic conditions 30 s after illumination.

(representing the actual net reaction rate) as a function of the electrode potential in completely deaerated or oxygen saturated acidic electrolyte solution. The data summarized in Figures 2 and 3 reflect pronounced sensitivity of the photoelectrochemical activity of anatase to the presence/absence of oxygen in the electrolyte solution. The catalyst featuring {110} faces apparently shows no sensitivity toward oxygen presence in acidic media. In the case of the catalysts dominated by {101} faces the presence of oxygen suppresses the observed photocurrent (Figure 3). The suppression of the photocurrent decreases with increasing electrode potential (Figure 3). In contrast, the catalysts dominated with {001} faces show enhanced photoelectrochemical activity in the presence of oxygen (Figures 2 and 3). The surface sensitivity of the photoelectrochemical water oxidation on anatase catalysts dominated by {110}, {101}, and {001} faces can be followed by the quantification of the photoelectrochemically generated oxygen by online differential 6027


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It needs to be noted that the model of mesoscopic semiconductor behavior cannot be applied to nanocrystals the size of which at least in one direction exceeds the expectable Debye’s length (at usual doping levels).25 Such material’s feature provides notable band bending in the direction perpendicular to the dimension exceeding the Debye length; hence, their behavior is in fact addressed by the conventional model. The latter describes photoelectrochemical behavior of the semiconductors assuming a formation of a space charge layer at the semiconductor−electrolyte interface and formation of a Schottky barrier at the metal−semiconductor interface. The space charge layer is responsible for the population of the charge carriers at the semiconductor−electrolyte interface and can be affected by the applied electrode potential (which controls the position of the Fermi level in the semiconductor). Following the analysis of the particle shape of the anatase nanocrystals employed in this study (see Figure 1), one has to conclude that the mesoscopic electrode model holds for the sample A. Keeping in mind that the nanocrystals composing the samples B and C show at least one characteristic dimension of the order of 100 nm (which exceeds the expectable Debye length), one can describe their behavior also using the conventional band bending model. Comparing the water oxidation selectivity of the materials conforming to the mesoscopic electrode model (sample A) with that of the materials showing the band bending (samples B and C), one finds a striking difference. In the case of truly mesoscopic anatase (sample A), the selectivity shows rather a weak dependence on the applied potential. Conversely, samples B and C (which may form the space charge layer) show pronounced dependence of the selectivity of water oxidation on the electrode potential as well as sensitivity of the photocurrent to presence of oxygen. Given the fact that the surface-statecatalyzed reductions represent the only mechanism causing potential dependence of the selectivity in the case of sample A, one can conclude that the nanocrystals featuring {110} and {101} oriented faces provide just limited number of surface states. On the other hand, the pronounced potential dependence encountered in the case of the samples B and C suggests major role of the electrode potential on the band structure in the case of the anatase materials following the Gärtner’s model as it is explained below. Effect of Dissolved Oxygen. Given the opposite charge of the majority and minority charge carriers, the overall photocurrent in fact represents a difference between the charge transfer processes involving electrons in the conduction band and/or trap states (i.e., reductions) and the charge transfer processes involving the holes in the valence band (i.e., oxidations). The presence of oxygen in the electrolyte solution may affect both types of charge transfer processes. One needs to keep in mind that oxygen acts as a reactant in the charge transfer of the electrons Je (oxygen reduction). Oxygen may also be involved in the anodic hole charge transfer Jh forming both reaction product (water oxidation) as well as reactant (ozone production). One has, therefore, to anticipate that the actual surface orientation may determine if an increase in oxygen concentration enhances the Je or Jh. If the oxygen presence enhances Je, an increase of the bulk oxygen concentration should cause drop of the observed photocurrent. In a similar manner one can attribute the photocurrent increase in the presence of oxygen to a selective surface related promotion of the Jh. In this respect one may conclude that the {001} faces favor the holes’ reactivity, while the {101} faces

individual nanocrystals. One can, therefore, identify the catalyst dominated with {001} faces as the most favoring for photocatalytic oxidation processes; the material with {110} faces, showing the z closest to 4, on the other hand, seems to be the catalyst of choice for the conventional oxygen evolution. This observation apparently contradicts theoretical analysis of the {110} activity, suggesting a formation of adsorbed OH radical in the water oxidation process.24 Conversely, the z values exceeding 4, observed for catalyst dominated with {101} surface, indicate a presence of an additional anodic process (complementing oxygen evolution) which decreases the apparent yield of oxygen.

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DISCUSSION

The outlined photoelectrochemical data reflect pronounced variability of the catalytic behavior of anatase in the water photoelectrolysis. Despite the apparent contradictions in the selectivity and activity of different samples, the observed behavior can be generalized if simple corollaries controlling the observed photocurrent can be introduced. (1) The experimentally observed photocurrent JPC integrates all conceivable charge transfer processes at the anatase− electrolyte interface and involves both majority and minority charge carriers. The JPC is therefore composed of two partial current densities Jh and Je: Jh represents the flow of photoexcited holes from the valence band of TiO2 to the electrolyte solution (anodic contribution to the net photocurrent), and Je represents the flow of electrons from the conduction band and/or localized trap states to the electrolyte solution (cathodic contribution to the net photocurrent). The relation between JPC, Je, and Jh governs eq 5:

JPC = Je + Jh

(5)

(2) The electron transfer from the localized trap states is possible only if their energy is lower than the Fermi level in the semiconductor. (3) Each of the partial current densities is controlled by the position of the quasi-Fermi levels of both electrons and holes as well as by the actual population of the charge carriers and deviate in this sense from the conventional Butler−Volmer formalism commonly used for metals but not for semiconductors.30 It needs to be stressed that while these corollaries hold universally, the actual population of the charge carriers may be affected by the electrode potential, the effect of which can be deduced from relevant band structure model. The band structure of nanocrystalline semiconductors addresses a model introduced by Bisquert and Gomez et al.26−28 The model applies to materials composed of crystals, which characteristic dimensions fall below the Debye length.25−27 In such a casein contrast to the conventional Gärtner’s modelone does not anticipate a formation of the space charge layer in the interphase and the photocurrent observed under external polarization is driven by the difference between the applied potential (controlling the Fermi level) and the quasi-Fermi level of the electrons in the conduction band.29 As a result, the positions of the quasi-Fermi levels of electrons and holes do not change with external polarization. Hence, the only processes that may change the selectivity of the photoelectrochemical water oxidation are the reactions proceeding via electron surface states which may be selectively suppressed by the applied electrode potential. 6028


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Je Enhancement. Face-specific catalytic behavior is also reflected in anisotropy of Je enhancement. It may involve electrons present in the conduction band or the electrons localized in band gap confined electron trap states. The difference in the energy between the CB states and localized trap states may result in different selectivity of both types of the electron transfer processes. Keeping in mind the energy of the band edges of the anatase, one may anticipate the surface state catalyzed reductions to involve the oxygen as the only reactant in our experiments. The electron transfer process involving conduction band electrons may enteraside from the oxygen reduction mentioned previouslyalso hydrogen evolution. The demonstrated catalytic nature of both considered electron transfer processes suggests a pronounced face specific reactivity of anatase which should project also in the experimental trends of Je. It needs to be stressed that the actual position of the conduction band in anatase in principle allows for the direct hydrogen evolution on semiconductor.3 The actual extent of the hydrogen evolution reaction, which competes with oxygen reduction at the given conditions, is controlled by a specific catalytic activity of the anatase surface. The extent of these processes will decrease with increasing electrode potential. A similar electrode potential dependence is to be, however, expected also for an electron charge transfer via surface confined electron traps, which, on the other hand, are unlikely to produce hydrogen. The energy of the band gap electron trap then controls the product distribution of the oxygen reduction processes representing the only possible electron charge transfer process. The shallow electron traps (i.e., surface states close to the conduction band edge) may produce water and hydrogen peroxide. It needs to be noted that the oxygen reduction on the shallow trap states has apparently the same effect as the oxidation at photogenerated holes (see above). The specific catalytic activity of various anatase surfaces is accessible theoretically and can be used to deconvolute the contributions of the electron transfer through conduction band states and trapped states. The trends in Je specific catalytic activity can be visualized by following the bonding energy of the key intermediate of the corresponding electrode process in a manner similar to conventional (electro)catalysis.32,33 The catalytic properties of the anatase with respect to the hydrogen evolution (as summarized in Figure 5) are instrumental in elucidation of the extent of different reduction processes. The key intermediate in the hydrogen evolution is a hydrogen adatom which can be located either on the Ti atom or on the nonequivalent oxygen atoms (see Figure S12). The hydrogen evolution intermediate will be directed to the most stable surface position, i.e., into the position showing the most negative adsorption energy. Regardless of the actual surface orientation the possible hydrogen evolution is directed to surface oxygen atoms. The {110} and {001} oriented anatase surfaces bind hydrogen more strongly than an ideal catalyst; the {101} oriented surface binds hydrogen more weakly than the ideal catalyst. The hydrogen adsorption energies can be converted to theoretical overpotential34 representing the thermodynamic limit of the hydrogen evolution activity31 (see Figure 5). The theoretical overpotentials favor the hydrogen evolution on {101} oriented anatase. The theoretical overpotentials differ by up to ca. 0.4 eV, which corresponds to exchange current

promote the charge transfer of the photogenerated electrons either via direct transfer from the conduction band or via surface confined trap states. The {110} faces seem to have no strong enhancement either for anodic or for cathodic processes. The behavior of the {110} oriented surface may be accentuated by the absence of the band bending. Jh Enhancement. The anodic behavior of the n-semiconductor surface is energetically controlled by the valence band edge while its bending near the surface (if any, see the discussion of mesoscopic model above) affects the hole population. The anodic behavior of anatase should show a negligible anisotropy since the energy of the valence band edge is assumed to be constant for various surface orientations.6 The observed anodic behavior is therefore primarily controlled by the holes trapped in band gap which dominate the anodic behavior at the most negative potentials. The energy of the trapped holes is controlled by the surface structure and chemistry7,8 while the activity of the trapped holes is proportional to their number (related to their stabilization energy with respect to the valence band edge). Their selectivity in the anodic processes is determined by their actual energy which selects the anodic processes they may enter. The energy of the band gap trapped holes for {001} surfaces is significantly higher than for any other surface orientation, and the potential of the trapped holes corresponds to the 1.5−1.7 V range (in RHE scale). The energy of these holes restricts their activity to four electron water oxidation or for anodic hydrogen peroxide formation. The extended formation of the hydrogen peroxide can causeaside of the homogeneous radical formationalso enhanced catalyst corrosion in the form of a complex Ti(IV) peroxo cation. Formation of the peroxo complexes as well hydrogen peroxide was observed on {001} dominated catalyst by means of the rotating ring disc electrode (RRDE) (see Figure S10). The apparent increase of the Jh recorded on {001} dominated surfaces can be thus attributed to a self-healing of the partially corroded catalysts surface. A rather low value of z, observed on {001} oriented surface (see Figure 4), suggests a formation of hydrogen peroxide on {001} oriented surface given the fact the fragment of m/z = 32 (conventionally representing the molecular ion of oxygen) is also the most abundant product of hydrogen peroxide fragmentation. The hydrogen peroxide formation is in agreement with theoretically predicted energy of the trapped holes and agrees well with superior activity of {001} oriented surface in photocatalytic oxidation of organic species.13 The band gap trapped holes in the case of {110} and {101} oriented surfaces should show sufficiently low energy to form radical yielding intermediates/products. The formation of these species on {101} and {110} dominated surfaces, although thermodynamically possible, is however suppressed. This tendency can be explained by a low population of the surface trapped holes on these anatase surface orientations due to most likely kinetic hindrances of the trapped hole formation. The fraction of the formed radicals apparently decreases with increasing electrode potential. This behavior is difficult to reconcile with the holes’ reactivity. Keeping in mind that the hole’s energy at the surface remains pinned while the holes’ population increases with the increase of the electrode potential, one should in fact anticipate an increase in the radical formation. The tendency observed particularly on {001} oriented surfaces contradicts the expected behavior. This apparent conflict can be reconciled assuming a synergetic effect of the photogenerated electrons (vide inf ra). 6029


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The oxygen reduction on shallow electron trap states, i.e., on surface trap states with energy higher than ca. 0.5 V vs RHE,33 may in principle produce both water and hydrogen peroxide. It needs to be noted that the effect of the shallow trap states vanishes once the Fermi level attains the energy value lower than that of the surface state. The surface state confined reductive production of hydrogen peroxide leads to a decrease of the z. This effect can be clearly tracked in the behavior of both {101} and {001} dominated surfaces showing unusually low z values at the electrode potentials below 0.5 V (see Figure 4). It has to be therefore stressed that the superiority of the {001} anatase facets in radical formation is in fact an effect of pronounced synergy between the reactivity of the photogenerated holes and surface state catalyzed oxygen reduction.

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CONCLUSIONS Experimental and theoretical studies of three different anatase nanocrystals exposing various faces rationalize fundamental aspects of the photocatalytic performance of anatase in practical applications. The overall photoelectrochemical activity and selectivity result from competition between the reduction with photogenerated electrons and oxidation with the photogenerated holes. This competition is pronounced on the materials featuring at least one dimension out of the mesoscopic range. The purely mesoscopic materials seem to be primarily controlled by the activity of the holes. This competition is of thermodynamic as well as kinetic origin36 and is controlled by the charge carrier availability and by the catalytic behavior of the illuminated surface. The catalytic behavior then reflects the surface orientation in a manner usual in the conventional catalysis. The {101} oriented surfaces in this respect represent the primary site for reduction. The {101} orientation of the surface seems to be the only one forming hydrogen upon illumination, and its presence is therefore of paramount importance for direct photocatalytic water splitting in the nonelectrochemical variant of the reaction (e.g., with anatase particles in aqueous suspension). In turn, the water oxidation is favored on the {110} and {001} oriented faces. The {001} oriented surface shows particularly enhanced tendency to form radical species necessary for advanced oxidation processes (e.g., in wastewater treatment). The {110} oriented surface shows, on the other hand, strong preference for the conventional four-electron water to oxygen oxidation. The potential dependence of the peroxide formation as observed on {001} oriented surfaces apparently reflects overall band bending which selectively enables the reactivity of the holes trapped in the band gap. This fact stresses the importance of the actual Fermi-level position which can be controlled by external electric field as well as by doping in the synthetic procedure. The observed trends in anodic processes at the illuminated anatase surfaces reach beyond the shape tailoring of photocatalysts suggested previously.12−14 Particularly in the processes involving water splitting, e.g., photoelectrolysis of water on crystals featuring {101}/{110} facets ought to be employed. Similarly, the synergy of {101}/{001} surface orientations in peroxide formation (two-electron reduction at {101} oriented surfaces and direct peroxide formation at {001} facets) is indispensable in oxidation of water. The found selectivity of the individual anatase surface orientations and its dependence on the Fermi level position along with applicability of the DFT models establish a fundament for rational photocatalyst design

Figure 5. Theoretically constructed volcano plot of the hydrogen evolution activity as a function of the hydrogen adsorption energy on various surfaces. The arrows mark the activity of the anatase surfaces involved in the present study. Schematic representations of the most likely active sites are shown in the figure insets. The general trends marked by dashed lines along with theoretical activities of metal reference materials (green triangles) are taken from ref 34.

density difference of about 3 orders of magnitude between {101} and {110} surface orientations.35 The latter is therefore unlikely to reduce the protons to hydrogen unless a significant excitation energy is used. The theoretical prediction of facilitated hydrogen production on the {101} oriented surface is confirmed both spectroscopically (DEMS detects a formation of hydrogen triggered by anatase illumination) and electrochemically (RRDE confirms formation of hydrogen amperometrically). No hydrogen formation was observed for other prevailing surface orientations. Detection of hydrogen is visualized in Figure S11. The amount of formed hydrogen cannot be, however, precisely quantified since it can be simultaneously oxidized at other anatase facets as well at the surface of the gold substrate. Hydrogen oxidation proceeding in parallel with the oxygen evolution can explain the z values exceeding 4 shown for {101} dominated materials at potentials higher than 600 mV vs RHE (see Figure 3). It needs to be noted that the hydrogen formation in experiments is confined to the potentials negative to 0.6 V (vs RHE), i.e., at potentials significantly higher than the standard potential of the hydrogen evolution. This fact strongly indicates that the electron transfer forming hydrogen is not catalyzed by the surface state(s). It also supports the adequacy of the Gärtner’s model to describe the anatase behavior in the case of the sample B. The electron transfer at energies exceeding the conduction band edge fails, however, to explain the complex potential dependence of the Je enhancement observed for {101} oriented surface (see Figure 3, sample B) as well as the potential dependence of z observed for {001} surfaces (sample C). These experimental trends can be rationalized assuming a surface state catalyzed oxygen reduction proceeding in parallel. Following the theoretical investigations of the oxygen reduction, we have to expect the oxygen reduction on deep electron trap states to proceed exclusively by the four-electron mechanism to water.33 Such a process will cause a photocurrent suppression in the presence of oxygen, but its effect on the holes reactivity (expressed in z) should be negligible. 6030


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extending the particle shape based concepts expressed previously.7,10 Such a concept brings the photocatalysis on a common ground with conventional heterogeneous catalysis and enables rational design of photocatalysts targeted for particular applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09289. HRTEM-based orientation of the anatase surfaces, calibration graphs for the DEMS quantification, XPS data, the DEMS and RRDE detection of the produced hydrogen and details of the DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(J.R.) E-mail [email protected]. *(L.K.) E-mail [email protected]. *(P.K.) E-mail [email protected], Phone +420266053826. ORCID

Ladislav Kavan: 0000-0003-3342-4603 Petr Krtil: 0000-0001-8447-1333 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.K. and M.Z. acknowledge the support from the Grant Agency of the Czech Republic (Contract No. 13-07724S). REFERENCES

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