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Selectivity control of the photocatalytic water oxidation on nano-cube SrTiO via surface dimensionality 3

Katerina Minhová Macounova, Roman Nebel, Monika Klusá#ková, Petr Krtil, and Mariana Klementova ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Materials & Interfaces

Selectivity Control of the Photocatalytic Water Oxidation on Nano-cube SrTiO3 via Surface Dimensionality

Kateřina Minhová Macounováa, Roman Nebela, Monika Klusáčkováa, Mariana Klementováb and Petr Krtila1*

aJ.

Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejskova 3, 18223 Prague, Czech Republic b

Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Prague, Czech Republic

Abstract: The role of the surface dimensionality in photo-electrochemical water oxidation was studied for differently sized SrTiO3 nano-cubes. The band gap illumination of strontium titanate electrodes results in anodic current; the photo-current appears at bias of ca. 220 mV with respect to flat band potential. The bias needed to record anodic photo-current increases with pH reflecting the change in the protonation of surface oxygen atoms. Photo-electrochemical activity of SrTiO3 nano-cubes is size dependent and increases with increasing particle size. Semi-quantitative analysis of the observed photo-currents combined with mass spectrometric detection of the reaction products shows that the contact of water with illuminated SrTiO3 nano-cubes leads to a formation of oxygen, hydrogen peroxide and ozone. Oxygen and ozone are the primary products of the water oxidation proceeding on {100} oriented SrTiO3 faces and their fractions increase with increasing particle size. The hydrogen peroxide is simultaneously produced via oxygen reduction at the low dimensionality sites (crystal edges, vertices) the abundance of which increases with decreasing particle size.

Keywords: SrTiO3, photo-electrochemistry, selectivity, DEMS, water oxidation, ozone formation

* Corresponding author, E-mail:[email protected]

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Introduction Direct photo-catalytic water splitting represents one of the primary ways of renewable solar energy storage in fuels and chemicals [1-5]. The activity of the so far employed photo-catalytic systems, however, lags behind the expected levels needed for large scale exploration. The improvement of the direct photo-catalytic water splitting feasibility requires improvement of the energy harvesting capabilities of the photo-catalysts to maximize the utilization of the solar spectrum [6]. Unfortunately, the materials with convenient band gap energies are rather unstable at the conditions relevant to water splitting, namely to the oxygen evolution and need to be either protected by oxide over-layer (e.g. TiO2 or WO3) showing sufficient stability at operando conditions [6, 7] or teamed up with sufficiently stable and active oxygen evolving cocatalyst [8]. The over-layer forming oxides are semiconductors and their band gaps are rather wide. These oxide over-layers in fact form actual catalytically active surfaces at which the desired reactions (i.e. hydrogen evolution, oxygen evolution) take place hence their surface activity and reactivity should be optimized to maximize the output of the resulting hybrid photo-catalysts. It needs to be stressed that as of now there is no systematic understanding of the catalytic behavior of the most common oxide protective layers in terms of activity and selectivity. Also the role of surface orientation essential for rational photo-catalyst design remains unknown, hence the catalysts development still relies mainly on trial and error approach. Given the heterogeneous nature of the energy storing processes the maximization of the specific surface area of the photo-catalysts represents the primary approach in maximization of the photo-catalysts performance. The maximization of the specific surface area as achieved for advanced nanomaterials may in general allow for a better control of the surface orientation. It, however, also introduces large number of the lower dimensionality sites at the surface (crystal edges, vertices) which may play significant role in the catalysts performance. The understanding of the effects specific to oxide based photo-catalytic nanomaterials is still rather limited. Particle size effects in photo-catalysis have been addressed on various materials including TiO2 [9], Cu2O [10], Si [11] or BiVO4 [12]. The presented results are, however contradictory. In the case of reduction on Si quantum dots the resulting activity reportedly increases with decreasing particle size due to blue shift of the conduction band edge to higher energies [11]. The water oxidation on size resolved BiVO4 gives, on the other hand, an opposite trend due to an increase of hole life time at the grain boundaries of larger nanoparticle agglomerates [12]. As of now there is no general understanding of the phenomena attributable to the particle size triggered variation of the semiconductors photo-electrochemical activity. The only report in this respect represents a study on the photo-electrochemical activity of the SrTiO3 nano-cubes in water oxidation reaction, which describes the size dependence of the photo-electrochemical behavior free of particle shape or agglomeration effects [113]. SrTiO3 is rather versatile double oxide material with applications foreseen in photo-catalysis [1420], but also in ferroelectrics [21-24], or photoluminescence [25]. The synthesis of the strontium titanate perovskite (Pm3m) relies conventionally on solid state reaction at temperatures exceeding 700°C and on its doping at temperatures exceeding 1000 °C [26]. These procedures – namely the flux mediated doping as a rule produce microcrystalline rather-than nano-particulate 2 ACS Paragon Plus Environment

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materials. Alternative routes based on sol-gel [14, 27], Pechini process [28], peroxo complex based routes [29] and/or on hydrothermal synthesis [16, 30] have been also developed with various degree of success in synthesis of nano-particulate materials [31-33]. Regardless of the adopted synthetic procedure nano-particulate SrTiO3 is often contaminated either with unreacted starting materials [34] or by strontium carbonate [15, 32]. Spray freezing freeze drying approach using titanium (IV) lactate and Sr(NO3)2 recently reported successful synthesis of particle size controlled SrTiO3 free of carbonates [13]. The photo-electrochemical behavior of SrTiO3 is dominated by the hole charge transfer [35, 36] and is reported to reach quantum yield of up to 30% on microcrystalline samples [26, 37]. In the case of nano-particulate SrTiO3 the photo-electrochemical activity was reported to drop for nanocrystals comparable with Debye length most likely because of the complications in formation of space charge layer [13]. The reported band gap energy of 3.2 eV combined with flat band potential of -0.3 V vs RHE [38] suggests high oxidative power of the holes generated by band gap illumination of SrTiO3. The hole energy corresponds to the potential close to 3.0 V vs. reversible hydrogen electrode (RHE) which greatly exceeds the stability of the water (E0=1.23 V). The illuminated SrTiO3 can in theory oxidize water to oxygen, hydrogen peroxide or OH radical. In contrast to, e.g. titanium dioxide based materials [39] there is no knowledge outlining possible competition of the thermodynamically allowed anodic reactions on strontium titanate. To compensate for this deficiency we present here the results of a systematic study combining photo-electrochemical investigations of water oxidation on illuminated SrTiO3 with mass spectroscopic detection and quantification of the reaction products. SrTiO3 nano-cubes with particle size ranging between 9 and 30 nm were used to outline the role of the different dimensionality sites (i.e. sites residing on {100} faces and crystal edges) in the photo-anodic water oxidation and their effect on the selectivity of the water oxidation process.

Experimental Materials and chemicals Strontium nitrate (Lachema, p.a.), titanium(IV) bis(ammonium lactato)dihydroxide (TBALD) solution (50wt.% in water, Sigma-Aldrich), perchloric acid (70%, Sigma-Aldrich, p.a.), absolute ethanol (99.8%, Lach Ner, p.a.), and gelatin (Aldrich) were used as received. The stock solutions of strontium nitrate and TBALD (0.04 M) were prepared using Milli Q quality deionized water (Millipore). Synthesis of SrTiO3 nano-particles Starting solution in the spray freezing freeze drying synthesis was prepared by mixing equal volumes (10 mL) of stock solutions of TBALD and strontium nitrate. The starting solution was complemented by addition of variable amount of gelatin to reach a pre-set gelatin content in the range 0.5 g to 10 g/L. Corresponding amount of gelatin was dissolved in 50 ml of deionized water and stirred for 30 min at 60 °C to form a clear solution. The starting solution containing Sr(NO3)2 3 ACS Paragon Plus Environment


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and TBALD was added to cooled gelatin solution and the total volume was adjusted to 100 mL with deionized water. The reaction mixture was stirred for 60 min without heating. The reaction mixture was subsequently sprayed into liquid nitrogen forming an ice precursor which was subject of freeze drying. The excessive solvent was dried at reduced pressure using a Labconco FreeZone Triad freeze-dryer. The freeze drying process proceeded at pressure of 0.05 mBar according to the following protocol: −30 °C for 2 h, −25 °C for 5 h, −20 °C for 6 h, −15 °C for 5 h, and 30 °C for 4 h. The dried precursor was transferred into a tube furnace (Nabertherm™ Tube Furnace with B180 Controller) and heated up to 450 °C in oxygen flow at heating rate 15 ° per minute; the precursor was then calcined at 450 °C for 1 h to obtain nano-crystalline SrTiO3. Crystallinity and phase purity of the resulting SrTiO3 was checked using powder X ray diffraction (Miniflex I - Rigaku) using Cu Kα radiation. The particle shape was evaluated by scanning electron microscopy (SEM) Hitachi S4800. The surface orientation of the prepared photo-catalysts was determined by analysis of high resolution transmission electron micrographs. The Brunauer-Emmett-Teller (BET) surface areas of the prepared materials were determined from nitrogen adsorption isotherms at 77 K (ASAP 2010,Micromeritics). Transmission electron microscopy (TEM) was used to determine surface orientation of the prepared nanocrystals and was carried out on an FEI Tecnai TF20 X-twin microscope operated at 200 kV (FEG, 1.9Å point resolution) with an EDAX Energy Dispersive X-ray (EDX) detector attached. Images were recorded on a Gatan CCD camera with resolution of 2048x2048 pixels using the Digital Micrograph software package. Powder samples were dispersed in isopropanol and the suspension was treated in ultrasound for 5 minutes. A drop of dilute suspension was placed on a holey-carbon-coated copper grid and allowed to dry by evaporation at ambient temperature. The SrTiO3 electrodes used in photo-electrochemical experiments were drop casted on gold covered Ti mesh (Goodfellow) (1cm2). The gold interlayer was deposited from colloidal Au ink (Fraunhofer Institute fur Keramische Technologien und Systeme, Dresden, Germany) and cured at 200 °C. SrTiO3 suspension (10g/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 photo-catalyst on the electrode surface was in the range 1-2mg. Mechanically stable electrodes were obtained by calcination at 400°C in air for 4 hours. Photo-electrochemical experiments Photo-electrochemical behavior of the SrTiO3 nano-cube based electrodes was studied in the pH range 1-13. The desired pH of the electrolyte solution for the experiments was obtained by mixing 0.1M solutions of HClO4 (Sigma Aldrich, p.a.) and NaOH (Lach-Ner, p.a.). The exact pH value of the electrolyte solutions was checked using a glass electrode and Radelkis OK-104 pHmeter. All photo-electrochemical experiments were performed in quartz cuvette (5 cm) in a three electrode setup with SrTiO3 nano-cube working electrode complemented with Ag/AgCl (sat. KCl) and Pt mesh as reference and auxiliary electrodes, respectively. The electrode potentials were recalculated to the reversible hydrogen electrode (RHE) scale to maintain the same pH dependence as the flat-band potentials. The illumination was realized using monochromatic UV 4 ACS Paragon Plus Environment

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radiation of λ = 365 nm (Bluepoint LED lamp, Hönle UV Technology) with intensity of 6 W/cm2. The photo-electrochemical behavior of the SrTiO3 nano-cubes was tested under potentiostatic conditions using Autolab, PGSTAT 30 (Metrohm). Electrodes were first equilibrated at pre-set potential for 150 seconds in dark and then illuminated for 30 seconds. The photo-current vs. potential curves were constructed using the steady state currents achieved 25 s after illumination. Quantitative comparison of the photo-electrochemical activity of individual SrTiO3 nano-cube materials was based on photo-current densities based on known photo-catalysts mass and specific surface area obtained in BET measurements. The cell was purged with either oxygen or argon during the experiments. The Mott Schottky plots were constructed from ac impedance data measured in the 20 kHz to 10 mHz interval with 10 mV (peak to peak) modulation amplitude. The nature of the reaction products was assessed by differential electrochemical mass spectrometry (DEMS) approach combined with photo-electrochemical measurements. The DEMS apparatus consisted of a Prisma QMS200 quadrupole mass spectrometer (Balzers) connected to SU071 turbomolecular drag pumping station (Blazers). The DEMS experiments were carried out in a single –compartment Kel-F cell with three electrode arrangement [35] under automated pressure control to ensure comparability of the measurements.

Results

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As follows from comparison of Xray diffraction patterns of the prepared materials with that listed in ICSD database (see Figure 1) all recorded reflections can be indexed within the structural perovskite model. The prepared nanocrystalline catalysts are phase pure and apparently free of strontium carbonate contamination. The obtained diffraction patterns show regular relative intensity of the peak intensities which indicates the prepared nanocrystals do not feature anomalous growth in any direction.



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The Figure 2A SEM images of SrTiO3 nanocrystals prepared by spray freezing freeze drying from titanium lactate and strontium nitrate in presence of various amount of gelatin. The actual gelatin content in the reaction mixture amounted to 0.5 g/L (a), 2.5 g/L ( b), 5.0 g/L (c) and 10 g/L (d).

Figure 2B Particle size distribution based on the analysis of the SEM images shown in Figure 2a. The assignment of samples is the same as in Figure 2a.


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Figure 3 Typical TEM observations of SrTiO3 nano-cube sample:. (a) low magnification bright-field image, (b) HRTEM image of a single SrTiO3 nano-cube viewed down [001], (c) indexed FFT of the nano-cube shown in (b).

Characteristic particle size which can be obtained by analysis of the SEM images of the prepared SrTiO3 materials falls between 9 and 30 nm (see Figure 2B). It needs to be noted that all included samples show well defined cubic habitus. The cubic shape of the prepared nanocrystals is in agreement with the absence of preferential growth suggested by XRD. The HRTEM based surface orientation of the prepared nanocrystals (see Figure 3) implies that the prepared electrodes despite of being polycrystalline nature show in fact high homogeneity of the surface which is composed solely by {100} oriented facets. In fact, the only structural difference between prepared SrTiO3 samples can be found in the number of the low dimensionality sites, i.e. sites residing at the edges between individual {100} oriented faces. The prepared SrTiO3 samples are of semiconducting nature with band gap ranging between 3.1 and 3.2 eV [13]. This value corresponds well with the band gap energies known in the literature for similar systems [40].



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Figure 4 pH dependence of the photo-current vs potential curves recorded for SrTiO3 nano-cubes of variable size. The actual particle sizes as well as pH are indicated in the Figure legends. All curves were constructed using steady state photo-current values obtained at potentiostatic experiments using monochromatic illumination with λ=365 nm. The definite pH of the used electrolyte solutions was achieved by careful mixing of 0.1M HClO4 and 0.1 M NaOH solutions.

Photo-electrochemical behavior of nano-cube based SrTiO3 electrodes in acidic and alkaline media is summarized in Figure 4. The photo-current observed upon band gap illumination increases (at constant pH) with applied potential and reaches eventually a steady state value at sufficiently positive potentials. The observed photo-current densities decrease with decreasing particle size as reported previously [13]. The measured photo-current vs. E curves, however, shift to more positive potentials with increasing pH of the electrolyte solution. The incident photon to electron conversion (IPEC), which allows for a comparison of nano-cube activity with that of, e.g., microcrystalline materials [37], reflects higher intrinsic activity of bigger nano-cubes (see Figure 5). The observed IPCE values range between 6 and 8 % in acid media and between 1 and 4 % in alkaline media. These values are comparable with those reported for the SrTiO3 doped by flux assisted method [37].


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The observed photo-electrochemical behavior of the SrTiO3 nano-cubes can be - as in the case of any nsemiconductor - analysed using flat band potential (Efb), photo-current onset potential and steady incident photon to electron conversion at bias sufficiently departed from the Efb, which reflects the effective anodic activity of the catalyst.

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Figure 6 Mott Schottky plots constructed from the ac impedance measurements on nano-cube SrTiO3 based electrodes in acid and alkaline media. The measurements at pH 1 were done in 0.1M HClO4, for pH 13 in 0.1M NaOH. The actual nano-cube size is indicated in the Figure legend.

There are several methods of assessing the Efb for n- semiconductors including. These approaches are based on ac impedance (Mott Schottky plots see Figure 6) or on measurement of the photo-current onset (see Fig. 7). The Mott Schottky plots identify the Efb as independent of the particle size appearing at ca. - 170 mV vs RHE. The impedance based flat band potential values show conventional dependence on the pH of the solution in contact with the SrTiO3 electrode, i.e. the change of the pH by 1 shifts Efb by 59 mV negatively in the standard hydrogen electrode (SHE) scale. This fact stresses the convenience of the reversible hydrogen electrode scale in which the Efb remains constant. The observed flat band potentials are negative to the standard potential of the hydrogen evolution and agree with the values reported in the literature [41] , they are, however, slightly negative to those reported for SrTiO3 materials nottreated in reducing atmosphere [42].

The photo-current onset values which can be extracted from photo-current vs. bias measurements presented in Figure 4 (at constant illumination intensity) show rather anomalous behavior. The nanocube SrTiO3 based electrodes show particle size independent photo-current onset in acid media. The anodic photo-current can be detected at bias of ca 250 mV (i.e. 250 mV positive of the Efb). Such a 9 ACS Paragon Plus Environment


difference can be generally expected given the fact that the measured photo-current inherently integrates also surface recombination, which may be rather pronounced for nano-crystalline electrodes and which causes a systematic deviation of the photo-current onset values from the Efb. The photocurrent onset, in contrast to Efb, does not remain invariable in the reversible hydrogen scale but shifts by ca. 59 mV per unit of pH once the pH rises above 4 (see Figure 7). The photo-current onset potential remains particle size independent and shifts up to ca. 600 mV vs RHE (at pH of 10-11) before encountering a slight photo-current onset decrease to ca. 500 mV vs. RHE at pH of 12 - 13. It needs to be noted that the photo-current onset potential behavior becomes particle size dependent in alkaline media showing higher photo-current onset values for small nanoparticles than for bigger nanoparticles. Data presented in Figs 4 and 7 clearly show that the flat band potential determination 700 should not be based on photo-current 9nm 15nm measurements. 600 20nm Eonset [mV vs RHE]

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The data summarized in Fig. 7, on the other hand, exemplify the role of the oxide surface chemistry on the overall photoelectrochemical activity of the SrTiO3. It may be envisaged that the {100} oriented SrTiO3 surface features two types of surface oxygens - a coordination unsaturated (cus) oxygen residing on top of a Ti atom, bridging (bridge) oxygen bridging two adjacent Ti

atoms (see Figure S1). It needs to be noted that each of these surface oxygen atoms is in principle coordination unsaturated and enters protonation/deprotonation equilibria which result in overall surface charge - positive at pH below the point of zero charge (pzc) and negative above the pzc. One may, therefore attribute the observed shift of the photo-current onset to a change of the overall positive surface charge (encountered below pzc) to negative overall surface charge (encountered above pzc). In this model the inflection point on the photo-current onset vs. pH plot should coincide with pzc. This is, apparently, not completely met in the case for SrTiO3 nano-cubes. The literature reports the pzc of SrTiO3 to range between 8 and 9 [38]. These values are significantly higher than those of, e.g. TiO2 polymorphs [38] which can be attributed to the alkaline earth nature of surface confined Sr. The experimental data presented in Figure 7 generally agree with this assumption. It needs to be noted, however, that they show sensitivity of the photo-current onset to pH already in the range of pH between 3 and 4 i.e. 4 – 5 units of pH below the literature based pzc when the anticipated change in surface protonation ought to be rather small. The suppression of the photo-electrochemical activity of the SrTiO3 nano-cubes in alkaline media might be attributed to an enhancement of the surface recombination processes which are

Figure 7

pH dependence of the photo-current onset potential Eonset differently size SrTiO3 nano-cubes. The Eonset data obtained from photocurrent vs bias curves for illumination with monochromatic radiation of λ=365 nm. The electrolyte solutions were deaerated before experiments and the cell was purged with Ar during measurement.


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specifically promoted by deprotonated surface oxygen atoms regardless of their actual local environment. Possible role of the recombination processes can be further visualized by comparing the photoelectrochemical behavior of the SrTiO3 nano-cube based electrodes in absence and presence of reducible species, e.g. oxygen (see Figure 8).

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Figure8 Photo-current vs. bias dependence of the photo-electrochemical water splitting in 0.1M HClO4 (left) and 0.1 M NaOH (right) on SrTiO3 nano-cubes of variable particle size. Red symbols represent the measurement in oxygen saturated solutions; blue symbols represent the measurements in Ar saturated (i.e. oxygen free) solutions.

In oxygen free solutions the change of pH affects only the potential of the photo-current onset but leaves the limiting photo-current unchanged. In oxygen saturated solutions, however, one encounters a behavior when the SrTiO3 electrodes respond to illumination with a cathodic current if the applied bias falls between the flat band potential and potential of the photo-current onset observed in oxygen free solutions. This cathodic current is independent of the applied bias, but its magnitude (related to the photo-current observed in oxygen free solutions) decreases with increasing particle size. Aside of it, the presence of oxygen increases the anodic photo-currents observed at high potentials. This photo-current increase is more pronounced for bigger nanoparticles, but remains notable also for small SrTiO3 nanocubes. 11 ACS Paragon Plus Environment


The measured photo-current can be attributed to the hole assisted water oxidation and is usually taken as measure of the activity of the catalyst. It is conventionally assumed that water is quantitatively oxidized to oxygen. This assumption, however, is not fully justified given the actual energy of the holes entering the anodic reaction(s) at the SrTiO3 surface. The position of the valence band edge in SrTiO3 suggests the photo-generated holes can thermodynamically drive four anodic processes involving water as of the reactants:

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Figure 9 Photo-current (blue) and typical concomitant water oxidation related DEMS signals depicting the photo-electrochemical behavior of 9 nm SrTiO3 nano-cubes in 0.1 M HClO4.at 200 mV vs. RHE. The vertical arrows mark the onset and offset of the illumination of the SrTiO3 with UV radiation of λ=365 nm.

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or d) direct electrochemical OH· radical formation (E0 = 2.6 V) H2O  OH·+ H++ e(4)

The measured photo-current in principle integrates contributions from each of the above listed anodic processes and the elucidation of their contributions is rather complicated tasks which requires independent assessment of the reaction products. This assessment can be done , e.g. by spectroscopic determination of the reaction production by differential electrochemical mass spectrometry (DEMS). Typical course of mass spectrometric signal characterizing the water oxidation on illuminated SrTiO3 – oxygen (m/z=32) - is shown in Figure 9. Data presented in Fig. 9 demonstrate that the passage of the anodic photo-current results in an increase of the DEMS signals reflecting the abundance of the possible water oxidation products namely of oxygen (m/z=32), although a weaker signal of m/z of 34 is also simultaneously observed. The intensity of the signal attributable to oxygen (m/z of 32) decreases with increasing bias (and concomitantly also with increasing photo-current). This decrease was observed for all samples regardless of the nano-cube size (see Figure 10).


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To eliminate the variations induced by variation of the electrode surface area differences encountered

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Figure 10 Photo-current dependence of the DEMS based signal of the evolved oxygen (m/z=32) (left) and corresponding apparent number of electrons needed to evolve an oxygen molecule on SrTiO3 nano-cubes of variable particle size. Data were extracted from experiments carried out in 0.1 M HClO4 (pH 1 and 0.1M NaOH (pH 13). Actual particle sizes are shown in Figure legend.

for different samples, we normalize the detected amounts of oxygen by the charge integrated from the measured photo-current. This normalization yields an apparent number of electrons needed to evolve one molecule of oxygen (z). The procedure of the z calculation was introduced in [39] and is in detail described in Supporting Information. The apparent number of electrons needed to evolve oxygen molecule z depends on the nano-cube size and ranges between 1 and 6. In general, the z increases with applied bias as well as with particle size. The z values observed in acid media are significantly higher than those in alkaline media. It also needs to be noted that the obtained z values significantly deviate from the value expected for water oxidation (four) based on the stoichiometry of the reaction (1). However, it needs to be stressed that z based on DEMS data may agree with the stoichiometric one only if i) the signal of the m/z=32 fragment originates solely in the charge transfer reaction (and does not result from another molecule’s fragmentation) and ii) the measured photo-current is a quantitative measure of the hole transfer at the electrode electrolyte interface. Discussion 13 ACS Paragon Plus Environment


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The photo-electrochemical behavior of the nano-cube SrTiO3 as represented by the photocurrent in Figures 4-10 may be summarized as follows: a) overall activity (in anodic reactions) decreases with decreasing particle size b) the activity in anodic processes is relatively facile in acid media, but decreases with increasing pH c) The photo-electrochemical activity of the SrTiO3 nano-cubes increases in presence of the oxygen d) The selectivity of the SrTiO3 can be described by the apparent number of electrons needed to evolve an oxygen molecule z. This descriptor shows significant deviation from the value expected for simple water to oxygen oxidation. The observed z changes with particle size as well with applied bias All these features of the SrTiO3 behavior can be rationalized if the nature of the measured photo-current and its individual components are taken in account. The photo-current JPC observed in all experiments in fact integrates all conceivable charge transfer processes at the electrode – electrolyte interface. These processes may in principle involve both types of photo-generated charge carriers - electrons (majority charge carriers) and holes (minority charge carriers). The JPC can be, therefore, expressed as a sum of two partial current densities: JPC=Je+Jh

(5)

where each of the partial current densities Je and Jh for electrons and holes, respectively, is controlled by the number of the charge carriers at the electrode surface, concentration of the reactant and intrinsic kinetics of the underlying charge transfer reactions. Each of the partial current densities may from thermodynamic point of view integrate several electrode reactions. For instance the Jh integrates processes (1)-(4) while the Je in principle integrates (in the case of the described experiments) oxygen or proton reduction: O2 + 2H+ +2e-  H2O2

(6)

O2 + 4H+ + 4 e-  2H2O

(7)

2H+ + 2 e-  H2

(8)

As follows from equation (5) any occurrence of processes (6)-(8) will decrease the measured photocurrent. It needs to be also noted that in the experiments where the z drops below four one also observes notable mass spectroscopic signal with m/z of 34. The electrode potential dependence of the signal with m/z of 34 is plotted in Figure 11. This signal can be in principle attributed either to an oxygen molecule with 16O18O isotopic distribution (i.e. to the product of water oxidation with holes) or to a molecular ion of hydrogen peroxide (which can be produced either oxidatively according to reaction (2) or reductively according to reaction (6)). The signal with m/z=34 decreases with increasing electrode potential which 14 ACS Paragon Plus Environment

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is typical for surface state facilitated reductions with photo-generated electrons [39]. This supports an assignment of the observed signal with m/z of 34 to hydrogen peroxide produced according to reaction (6). The hydrogen peroxide related signal also decreases with increasing particle size (see Fig. 11) indicating that reduction processes are facilitated by specific features found in a higher abundance on smaller nano-cubes (e.g. crystal edges or vertices). It also indicates that Jh and Je have a different particle size dependence. Such a trend also projects particle size and potential dependence of z. The z 9 nm 7x10 15 nm values lower than 4 are observed namely 30 nm for the smallest SrTiO3 nano-cubes. This is 6x10 consistent with extensive surface assisted oxygen reduction producing either 5x10 hydrogen peroxide or water. Reductive 4x10 formation of hydrogen peroxide or water, however, would mean that the photo3x10 generated electrons are not quantitatively transferred into external circuit and 400 600 800 1000 1200 400 600 800 1000 1200 E vs. RHE [mV] E vs. RHE [mV] consequently the measured photo-current does not provide accurate reading of the Jh. In addition, it needs to be noted that Figure 11 Potential dependence of the DEMS based signal of hydrogen formed hydrogen peroxide also affects the peroxide formed during photo-electrochemical water splitting on DEMS reading of the produced oxygen, i.e. SrTiO3 nano-cubes in 0.1 M HClO4.(left) and in 0,1M NaOH (right). The actual particle size is shown in the Figure legend. signal with m/z of 32, since the hydrogen peroxide fragmentation produces oxygen in the vacuum part of the DEMS equipment (see Figure S4 of the supporting information). As a result the observed z values are likely to be lower than 4 as long as the hydrogen peroxide is formed. -14

-14

m/z=34 [A]

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


-14

-14

-14

The quantification of the water oxidation selectivity of the illuminated SrTiO3 (based solely on DEMS) is further complicated by the possible ozone formation (see reaction (3)). The ozone formation cannot be qualitatively proven in DEMS data, but can show itself in the increase of the measured photo-current if one saturates the electrolyte solution with oxygen (see Figure 8). Given that the number of photogenerated charge carriers (at constant illumination and constant band bending) is independent of the solution composition one has to attribute the increase in the observed (anodic) photo-current to a specific anodic reaction which features oxygen as a reactant. The reaction (3) is the only conceivable scenario of such a process. This reaction seems to be significantly more pronounced on big nano-cubes (see Figure 8) suggesting the ozone formation is more likely to be confined to sites present on welldeveloped {100} oriented SrTiO3 faces. Based on the photo-current measurement the ozone formation is more pronounced in acid than in alkaline media. It needs to be noted, however, that the water oxidation to oxygen (reaction (1)) and possible ozone formation (reaction (3)) are interdependent processes (i.e. reaction (3) consumes the product of the reaction (1). As a result of this interdependence the relationship between DEMS detected amount of oxygen and passed charge no longer corresponds to 15 ACS Paragon Plus Environment


the stoichiometry of any of the reactions (1)-(4) and the calculated z should exceed 4 as long as the reactions (1) and (3) proceed in parallel. This behavior is, in fact, observed for the biggest nanocrystals in acid media (see Figure 10) †.

Although the DEMS data (z values, resolved signal of the fragment with m/z of 34) give a strong indication of the hydrogen peroxide and ozone are produced at illuminated SrTiO3 surface, the actual proof of their formation needs to be further corroborated from additional experiments. Both reaction products (hydrogen peroxide as well as ozone) are strong oxidizers, 0.07 Ar their presence can be determined O 0.06 via reaction with solution of indigo 0.05 blue solution in 0.1M HClO4, where the presence of ozone or hydrogen 0.04 peroxide leads to discoloration of 0.03 the solution[44]8 (see Figure S4 in Supporting Information). The 0.02 absorbance recorded at 600 nm 0.01 clearly shows more pronounced discoloration in the experiments 0.00 5 10 15 20 25 30 with electrolyte solution saturated d [nm] with oxygen. The decrease in absorbance measured by UV-Vis Figure 12 Particle size dependence of charge fraction of the advanced oxidation spectroscopy recalculated to the products (i.e. hydrogen peroxide and ozone) Xaop formed during photo-electrolysis of ozone/hydrogen of 0.1M HClO4 saturated with Ar (open symbols) and O2 (solid symbols) on SrTiO3 amount nano-cubes. peroxide (assuming the known 1:1 stoichiometry of the reaction) and normalized with respect to the passed charge. The resulting parameter Xaop represents the fraction of the total charge realized in formation compounds which can act in so called advanced oxidation processes (aop) and is presented in Figure 12. 2

Xaop

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

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Figure 12 shows that aop are formed even in Ar saturated solutions. The extent of the reaction is, however small. The saturation of the solution with oxygen leads to an enhancement of aop formation. Analysing the data shown in Fig. 12 one needs to bear in mind that aop products (ozone and hydrogen peroxide) can be produced in anodic reaction utilizing photo-generated holes (ozone) or cathodically via surface catalysed reaction of electrons (hydrogen peroxide). Although the reaction of indigo blue with both aop products leads to the same discoloration of the solution one can still distinguish the formation of the peroxide from that of ozone from the trends in the measured photo-current. Comparing the increase in aop production in water oxidation on illuminated SrTiO3 nano-cubes in Ar and O2 saturated It needs to be stressed that the DEMS experiments principally do not allow experiments in oxygen saturated solutions. †


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solutions with the trends in measured photo- currents (see Fig. 8) one finds a good agreement for nanocubes bigger than 10 nm. The increase in Xaop roughly matches the increase in photo-current and relative increase of aop formation increases with particle size. Such a behavior corresponds well with a situation when the aop formation is dominated by ozone formation. The ozone formation is then additive to the conventional four electron water oxidation producing molecular oxygen. Anomalous behavior is observed for the smallest nano-cubes (9 nm) which show the highest increase in the aop formation (see Figure 10) while recording no significant change of the observed photo-current. This behavior contradicts possible dominance of the ozone in the aop formation but apparently agrees well with a situation when cathodic formation of hydrogen peroxide significantly contributes to aop formations as suggested by DEMS data. The pronounced formation of hydrogen peroxide is likely facilitated by the Schottky barrier between gold and SrTiO3 present at the back contact of the electrodes which could be in principle suppressed by a selection of another substrate [45]. The concept expressed by Equation (5) can be instrumental also in rationalization of the observed pH dependence of the photo-electrochemical behavior of SrTiO3 nano-cubes. The photo-current onset potential can be in this context perceived as the potential at which the Jh prevails over Je. The shift of the photo-current onset potential observed in alkaline media can be, therefore, interpreted as selective suppression of the reactions contributing to Jh. It needs to be noted that the processes contributing to Je remain unhindered and their prevalence demonstrates itself, e.g. in cathodic response to the electrode illumination observed in experiments in oxygen saturated solutions (see Fig. 8). The selective suppression of the Jh in alkaline media can be related to protonation/deprotonation of the surface oxygen atoms. Given the fact that anodic as well as cathodic processes integrated in Jh and Je are in fact multiple electron/proton charge transfers processes, hence the proton transfer from/to the electrode may play significant role in the control of the process kinetics. The facility of the proton transfer can be related to orientation of the water molecules in the solution part of the electrode – electrolyte interface [46]. It needs to be noted that the Jh is significantly suppressed in alkaline media when the deprotonated oxygens prevail at the surface and consequently the SrTiO3 surface bears a negative charge. The negative surface charge directs the water molecules with hydrogen atoms towards electrode. Such a water orientation is convenient for the proton transfer towards the electrode (utilized in Je) but appears to be countering the proton transfer from the electrode (needed in Jh). The proton transfer can be seen as the main cause for the positive shift of the photo-currentphoto-current onset potential as presented in Figure 4. Conclusions The photo-electrochemical activity of SrTiO3 in water oxidation is significantly affected by particle size as well as pH. The overall activity expressed by the measured photo-current decreases with decreasing nano-cube size. This trend is accompanied also in the distribution of the reaction products when one observes significant formation of hydrogen peroxide. The hydrogen peroxide is formed preferentially via oxygen reduction on the surface states, which are located on the nano-cube edges and vertices. Sites at the {100} faces are, on the other hand, selective for hole transfer yielding oxygen and consequently also ozone. The activity and selectivity of SrTiO3 nano-cubes are affected by the proton transfer either 17 ACS Paragon Plus Environment


Page 18 of 25

towards or from the electrode. The deprotonation of the SrTiO3 surface experienced in alkaline media apparently suppresses the anodic activity of the catalyst. The cathodic activity at the crystal edges remains unaffected.

Acknowledgements Financial support of the Grant Aagency of the Czech Republic under contract 17-12800S is greatly appreciated.


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