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Size control of the photo-electrochemical water splitting activity of SrTiO3 nano-cubes
Accepted Manuscript Size control of the photo-electrochemical water splitting activity of SrTiO3 nano-cubes Monika Klusáčková, Roman Nebel, Kateřina Minhová Macounová, Mariana Klementová, Petr Krtil PII:
S0013-4686(18)32669-0
DOI:
https://doi.org/10.1016/j.electacta.2018.11.185
Reference:
EA 33186
To appear in:
Electrochimica Acta
Received Date: 18 July 2018 Revised Date:
19 October 2018
Accepted Date: 26 November 2018
Please cite this article as: M. Klusáčková, R. Nebel, Kateř.Minhová. Macounová, M. Klementová, P. Krtil, Size control of the photo-electrochemical water splitting activity of SrTiO3 nano-cubes, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.11.185. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Size control of the photo-electrochemical water splitting activity of SrTiO3 nano-cubes Monika Klusáčkováa, Roman Nebela, Kateřina Minhová Macounováa, Mariana Klementováb
a.
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and Petr Krtila* J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejškova
3, 18223, Prague, Czech Republic. E-mail:[email protected]
Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Prague, Czech
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b
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Republic Abstract
Size control of the photo-electrochemical activity of a n-semicondutor oxide in water oxidation was demonstrated on size and shape controlled SrTiO3 nano-cubes featuring
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homogeneous {100} oriented surface. The carbonate free SrTiO3 perovskite nano-cubes were synthesized from titanium(IV) bis(ammonium lactato)dihydroxide solution and strontium nitrate in presence of the structure ordering gelatin. The gelatin content determines the size of
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the perovskite nano-cubes in the range 10-30 nm. SrTiO3 nano-cubes show band gap in the range 3.2-3.3 eV with slight blue shift of the absorption edge for nano-cubes smaller than 20
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nm. All materials are active in photo-electrochemical water splitting, their specific activity, however, decreases with decreasing particle size and drop exponentially for nano-cubes smaller than 20 nm.
ACCEPTED MANUSCRIPT Introduction Oxide based nano-crystalline photo-catalysts were vigorously developed to support the storage of the renewable solar energy into environmentally friendly hydrogen fuel [15]. Given the heterogeneous nature of the underlying water splitting reaction one
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understands the stress on the maximization of the specific surface area of the available photo-catalysts as well as on the optimization of their surface orientation. The progress in the field has been to a great extent hindered by a lack of convenient synthetic
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techniques allowing precise control not only of the particle size and degree of crystallinity, but also the nano-crystal shape. Particle size effects in photo-catalysis
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have been addressed on various materials including TiO2 [6], Cu2O [7], Si [8] or BiVO4 [9]. The presented results are, however contradictory. While in the case of reduction of Si quantum dots the resulting activity was reported to increase with decreasing particle size due to blue shift of the conduction band edge to higher
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energies [8] in the case of water oxidation on BiVO4 it was observed an opposite trend in the particle size vs. activity relationship reportedly due to an increase of hole life time at the grain boundaries of agglomerates of larger nanoparticles [9]. It has to be
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summarized that there is no general understanding of the phenomena nor there are any semi-quantitative criteria steering the rational design of the photo-catalysts in terms of
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particle size, shape or material morphology. Reaching general relationship linking the photocatalytic activity with particle size in the nanoscale is hindered either by a lack of convenient techniques controlling the particle size on the d~ 10 nm level (as one encounters for double oxide and other complex photo-catalysts) simultaneously with orientation of the surface and agglomeration of the nano-crystals. To bridge the gap in fundamental understanding of the nano- crystal size effects we present here a
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ACCEPTED MANUSCRIPT systematic comparison of the photo-electrochemical activity of the SrTiO3 nano-cubes in water oxidation reaction. SrTiO3 is one of the most important oxide materials due to its prospective applications not only in photocatalysis [10-16], but also in ferroelectrics [17-20], or
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photoluminescence [21]. The crystal size control (and consequent maximization of the specific surface area) of strontium titanate is not a trivial task. SrTiO3 crystallizes in cubic system and conforms to perovskite structural type (Pm3m). The conventional
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synthesis of the strontium titanate relies on solid state reaction of strontium carbonate and titanium dioxide at temperatures exceeding 700°C and fails to deliver sub-micron
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particles. Alternative routes based on sol-gel [10, 22], Pechini process [23], peroxo complex based routes [24] and/or on hydrothermal synthesis [12, 25] have been developed with various degree of success. Namely the hydrothermal synthesis at temperatures below 180 °C yields nano-crystalline SrTiO3 with particle sizes ranging
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between 25 and 200 nm [26-28]. Despite the high versatility of the hydrothermal synthesis its application in synthesis of SrTiO3 often leads to a contamination of the final products either with unreacted starting materials (if one uses TiO2 as titanium
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source) [29] or by strontium carbonate [11, 27]. It also does not allow for combined crystal size and shape control due to limited variability of the parameters affecting the
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synthesis.
Any study focused on size related investigations on SrTiO3¸ therefore, need to be supported by a novel low temperature approach to SrTiO3 synthesis. This study therefore, also introduces a novel SrTiO3 synthesis based on spray freezing freeze drying approach in presence of gelatin as room temperature structure ordering agent.
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ACCEPTED MANUSCRIPT 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.),
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absolute ethanol (99.8%, Lach Ner, p.a.), and gelatin (Aldrich) were used as received. All the solutions were prepared using deionized water (Milli Q Gradient, Millipore). The stock solution of strontium nitrate (0.04 M) was prepared by dissolving the exact amount of the
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substance. Synthesis of SrTiO3 nano-particles
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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
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deionized water and stirred for 30 min at 60 °C to form a clear solution. The starting solution containing Sr(NO3)2 and TBALD was added to cooled gelatin solution and the total volume
without heating.
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was adjusted to 100 mL with deionized water. The reaction mixture was stirred for 60 min
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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
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ACCEPTED MANUSCRIPT using powder X ray diffraction (Miniflex I - Rigaku) using Cu Kα radiation. The band gap energy of the prepared SrTiO3 materials was assessed by means of the UV-Vis-NIR spectroscopy. The UV-Vis spectra of the prepared materials were measured in diffuse reflectance mode using Perkin - Elmer Lambda 950 UV-Vis-NIR spectrometer.
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The morphology of the synthesized materials was assessed by means of the Scanning Electron Microscopy (SEM - Hitachi S4800 microscope). The SEM micrographs were used to determine the average particle size of the prepared materials. The average particle size data
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are based on analysis of 200+ randomly selected nano-particles. Transmission electron microscopy (TEM) was used to determine surface orientation of the prepared nanocrystals
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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
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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. Specific surface area of the prepared materials was assessed by means of the nitrogen
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adsorption at 77.4 K using ASAP 2020 (Micrometrics, USA) instrument.
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Photo-electrochemical experiments The photo-electrochemical activity of the SrTiO3 nano-cubes was studied on electrodes prepared on gold covered Ti mesh (1 cm2, Goodfellow). The gold layer on Ti mesh substrate was deposited from colloidal Au ink (Fraunhofer Institute für Keramische Technologien und Systeme, Dresden, Germany) and calcined at 200 °C for 15 minutes. The active SrTiO3 layer was deposited from an ethanol based suspension (10 g/L of SrTiO3). The SrTiO3 nanoparticles were dispersed in ethanol by 30 minutes sonication. The electrodes were prepared by drop casting of the suspension in 10 µL increments until the total mass of the active catalyst 5
ACCEPTED MANUSCRIPT reached ca 1 mg. Each increment of the active layer was stabilized by drying at 100 °C. The electrode was calcined at 400 °C for 4 h in air before use. Given the surface loading and projected electrode area one may estimate that the SrTiO3 layers at electrode surfaces were ca 1.1 µm thick.
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Photo-electrochemical behavior of the SrTiO3 electrodes was studied in 0.1 M HClO4 (pH=1) in a single-compartment quartz cell. A three-electrode arrangement with SrTiO3 working, Pt auxiliary, and Ag/AgCl reference electrode was used. All photo-electrochemical experiments
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were carried out in chronoamperometric mode using AUTOLAB (PGSTAT 30) potentiostat for potential control. The Bluepoint LED spot source (Hönle UV Technology) with the 365
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nm wavelength operating with light intensity between 1 and 6 mW/cm2 was employed as an illumination source. The absorption coefficient of the SrTiO3 at the 365 nm which equals to be 8302 cm-1 [30] therefore suggests that the entire SrTiO3 layer is accessible to the illumination. All potentials were recalculated and referred in the reversible hydrogen electrode
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(RHE) scale. The Mott Schotky plots were constructed from ac impedance data measured in the 20 kHz to 10 mHz interval with 10 mV (peak to peak) amplitude.
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Results and discussion
The spray freezing-freeze drying approach is a convenient approach in synthesis of
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multicomponent oxide materials. It utilizes atomic scale mixing of the oxide components in the initial solution and subsequently also in the ice precursor. Despite the versatility of the approach in synthesis of both stable as well as metastable phases its potential for crystal size or shape control has never been reported. The applicability of spray freezing freeze drying approach to synthesize Ti based materials is rather restricted due to relative lack of soluble Ti precursors decomposing without residual contaminants. In the above described synthesis the sublimation of the solvent water from the ice-like precursor leads to formation of white rather hygroscopic powders which need to be 6
ACCEPTED MANUSCRIPT immediately calcined to prevent a formation of gel-like substance. The hygroscopic nature of the dried precursor prevents its structural diffraction characterization. Calcination of the precursor to temperatures above 400 °C leads to a formation of white nano-crystalline material. The phase composition of final material is affected by the oxygen partial pressure
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(see the X-ray diffraction patterns summarized in Figure 1). The annealing of the dried precursor in air leads to a formation of multiphase system featuring cubic SrTiO3 perovskite along with significant amount of strontium carbonate (see pattern b in Fig. 1).
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The fraction of the strontium carbonate can be suppressed by calcination in pure oxygen, which suppresses the carbonate content below 10 %. The carbonate contamination cannot be
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removed by washing with acids indicating that the carbonates most likely form inclusions in the SrTiO3 matrix (see pattern c in Fig. 1). Carbonate contamination can be quantitatively suppressed by addition of gelatin (see curve d in Fig. 1). The addition of gelatin is further necessary to achieve effective control of the particle size as well morphology (see below).
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Increasing addition of gelatin affects the coherent domain size determined from the XRD patterns (see Fig. 2). The coherent domain size increases with increasing gelatin concentration in the initial reaction mixture and ranges between 10 and 30 nm. If the synthesis was carried
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out without gelatin addition no coherent domain (particle) size variation was observed
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regardless of the variation of the experimental conditions. The SEM based characteristic particle size increases with increasing gelatin content and ranges between 8 and 35 nm in the manner similar to coherent domain size (see Fig. 2). A good agreement between the coherent domain values and SEM based particle size suggests that each nano-particle in fact represents nano-sized single crystal with low number of defects. The SEM based particle size values are in good agreement with the BET based specific surface area values (see Table 1). The presence of the gelatin has fundamental effect on the shape of the synthesized SrTiO3. While in the absence of gelatin the resulting SrTiO3 nano-particles show isometric shape 7
ACCEPTED MANUSCRIPT without preferential surface orientation the addition of gelatin leads to a formation of regular cube-shaped nano-crystals (see Fig. 3). The nano-particle shape is not affected by the actual gelatin content (see Fig. 4). The surface orientation of the synthesized nanocrystals was assessed from HRTEM images of the synthesized SrTiO3 materials (see typical data in Figure
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5). The synthesized nanocrystals project as squares in the HRTEM. The diffractograms based on FFT of the lattice images clearly identify the surface of the nano-cubes to be composed solely of {100} orientated faces. The resulting SrTiO3 nano-cubes show Gaussian particle
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size distribution the width of which increases with increasing gelatin content in the reaction mixture.
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The effect of gelatin addition clearly suggests an organization of the ionic species in the solution containing Ti(IV) lactate and Sr(NO3)2. Although the exact coordination of the Sr as well as of Ti (IV) lactate in the solution (and in ice as well) facilitated by gelatin is still under investigation, it prevents the formation of the SrCO3 which is the main impurity in materials
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prepared in other low temperature processes. It also allows for fine tune of the particle size while controlling the nanocrystal’s shape.
The positive network forming effect of the gelatin can be tentatively attributed to zwitterionic
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nature of the amino acids which form the active component of gelatin. Due to the presence of
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negative charge on carboxylic group and simultaneously a positive charge on amino group the zwitterionic forms of amino acids may effectively crosslink the lactate with strontium cations in a broad range of pH. It needs to be noted that similar formation of chelating networks with amino acids or with polyvinyl alcohols was previously reported for TiO2 [31] or for aluminates [32]. Such a model of the crosslinked cation network is supported by the fact that the amount of the networking agent directly affects the particle size of the formed perovskite nano-crystals. The relation between the networking agent content and resulting particle size is, however, not straightforward proportionality since the nanocrystal size becomes practically
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ACCEPTED MANUSCRIPT independent of the gelatin content at concentrations near 5 g/L. This trend is difficult to rationalize in part due to the complex nature of the gelatin (see Appendix 1). The prepared perovskite nano-cubes, regardless of their actual size, are structurally identical. The electronic structure of the prepared SrTiO3 nano-cubes reflected in UV-Vis spectra
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corresponds to a wide band n-semiconductor with band gap ranging between 3.2 and 3.3 eV (see Fig. 6).
The apparent linear dependence of the band gap on the particle size has to be, however, taken
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with a great care given the fact that the observed band gap variation of 0.1 eV corresponds to a change of the absorption edge by ca. 20 nm. Slight blue shift of the absorption edge
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observed for the finer nanocrystals vanishes for nanocrystals bigger than 20 nm suggesting that the electronic behavior of bigger nanocrystals (d>20 nm) should be the same as in the case of micro or single crystals. On the other hand, the nano-cubes smaller than 15 nm may show a behavior affected by an onset of quantum confinement.
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All nano-cube perovskites are active as anodes in photo-electrochemical water splitting without need for further doping. The photo-electrochemical behavior of the prepared SrTiO3
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nano-cubes can be predicted based on the position of the flat band potential Efb (see Fig. 7), photo-potential behavior (see Fig. 8) and photo-current vs. potential behavior plotted in Fig. 9.
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As follows from the Mott Schottky plots presented in Figure 7 the Efb of SrTiO3 nano-cubes is practically independent of the nano-cube size and equals to ca. - 0.17 V vs. RHE. The slopes of the Mott Schottky plots indicate the charge carrier concentration in dark of the order of 1019 cm-3 - 1020 cm-3 which increases slightly with increasing particle size (see Table 1). Particle size effects show also in the photo-potential behavior reflected in Figure 8. In an ideal case the photo-potential is defined as a difference of the open circuit potentials (OCP) in dark and under illumination and it represents a response of the semiconductor to the to the photogeneration of charge carries in the space charge layer upon band gap illumination. The Fermi 9
ACCEPTED MANUSCRIPT level energy needs to be consequently increased (for an n-type semiconductor) to maintain the electro-neutrality. The shift of the OCP following illumination towards more negative potential effectively decreases band bending and facilitates the electron hole recombination. The electrode’s OCP after illumination shifts towards a steady state value via mixed potential
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like mechanism.
In the classical models [33] based on single crystal behavior the overall photo-potential increases with increasing intensity of the illumination and the steady state OCP at high
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illumination intensities converges to the value of the flat band potential Efb. As follows from the data summarized in Figure 8 these nano-crystalline photo-catalysts do not conform
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completely to the expected behavior. The overall photo-potential decreases with both increasing illumination intensity as well as with increasing particle size. It needs to be noted, however, that the observed OCP values (under illumination) decrease with the increasing illumination intensity and
converge (at illumination intensity of 6 mWcm-2) to
values
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between 50 mV and ca. -20 mV vs. RHE mV for the 9 and 30 nm sized nano-cubes, respectively. These values are slightly more positive than the Efb values resulting from impedance measurement. This type of behavior can be, however, expected given the mixed
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potential like nature of the OCP under illumination. The observed OCP values also indicate
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the ability of the biggest SrTiO3 nano-cubes to split photo-catalytically (i.e. without further bias) the water. The observed photo-potential trends are significantly affected by the surface states the occupation of which (at OCP conditions in dark) is heavily affected by the electrode’s history and is, consequently, poorly reproducible. The photo-current vs. potential curves of nano-cube based electrodes measured in 0.1 M HClO4 upon illumination with monochromatic radiation (λ=365 nm) are shown in Figure 9. These data indicate that the charge transfer energetics reflected, e.g. in the photocurrent onset, is, apparently, insensitive to the actual particle size. Such a result is in agreement with
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ACCEPTED MANUSCRIPT negligible differences in the flat band potential hence the presented photo-current values in fact reflect the kinetic differences of various SrTiO3 nano-cubes at comparable band bending. The particle size, has, however, a pronounced effect on the overall activity of the photocatalysts (see Fig. 9). The specific activity of the strontium titanate expressed as photocurrent
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density (regardless whether normalized to the actual physical catalyst area or the catalysts mass) increases with increasing particle size. The observed activity trend is counterintuitive given the heterogeneous nature of the photo-electrochemical water splitting. Also the hole
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trapping mechanism proposed by Kudo et al. [9] cannot be used to explain the observed experimental trend due to absence of the nanocrystal agglomeration. The obtained activity
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data, therefore, reflect the crystal size control of the nano-crystalline semiconductor behavior since the SrTiO3 nano-cubes included in the study are of the same morphology and their surface is composed only of {100} oriented faces. The actual photo-current density data shown in Figure 10 suggest that the activity of the SrTiO3 catalysts decreases exponentially
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with the decreasing particle size once the characteristic particle size drops below ca. 20 nm and remains particle size independent for bigger nano-cubes. Given the low to negligible agglomeration of the nano-crystals in the prepared electrodes one cannot assume that the
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favorable behavior of the bigger crystals results from hole trapping at the crystal boundaries [8], but mostly likely reflects enhanced surface based recombination on the small
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nanocrystals. The observed behavior outlines the nanocrystal size as an efficient tool in control of photocatalytic activity. The observed behavior is difficult to assign to a single factor and most likely originates from convolution of several effects. Local structure and band structure effects This discrepancy between expected and actual trend in the size dependence of the photo-electrochemical activity can be explained realizing that the sizes of the particles are close to the Debye length in the SrTiO3 assuming the donor concentration to be in 11
ACCEPTED MANUSCRIPT the range 1019-1020 cm-3. The resulting Debye length is approximately 6 nm. It can be expected that while Debye length remains comparable with the particle size (i.e. the effective crystal radius does not exceed significantly the Debye length) one should expect significant deviation from the conventional model of the potential distribution
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between the semiconductor and electrolyte. It may be assumed that a formation of space charge layer in the individual small nanocrystals may be gradually suppressed and the charge transfer at SrTiO3 electrode will follow the model formulated by
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Bisqueret [34-35] when the photocurrent observed under external polarization is driven by the difference between the applied potential (controlling the Fermi level) and the
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quasi Fermi level of the electrons in the conduction band [36]. The absence or suppression of the space charge layer formation, however, results in enhanced surface recombination which suppresses the observed photocurrent. The Debye length activity restriction is removed in nano-particles bigger than 20 nm when the photo-
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electrochemical activity becomes apparently size independent. It can be deemed that the particle size exceeding ca. 2-3 times the Debye length represents the optimum particle size maximizing the specific surface area while maintaining the band structure
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typical for semiconductors. Such an optimum particle size can be devised for any semiconductor system; given the range of the relative permittivity in common photo-
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catalytically active materials (10-400) one can expect that synthesis of nanocrystals smaller than 5-10 nm lacks rational justification. The observed size dependence of the photo-electrochemical activity may be further accentuated by non-homogeneities of the surface on the nanoscale. The sites at the surface of the prepared nanocrystals may differ from those present at {100} oriented single crystals. The nano-cubes feature relative high number of “low dimensionality sites” near the nano-cube edges and vertices. These sites feature less bonding partners
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ACCEPTED MANUSCRIPT than typical for an ideal {100} SrTiO3 single crystal surface. Drawing an analogy from the electro-catalysis one may expect the “low dimensionality” sites to be more reactive (i.e. to adsorb the reaction intermediates rather strongly) than the sites prevailing on {100} single crystals surface [37] which may result in altered activity. Since the
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fraction of the “low dimensionality” sites at the surface of the prepared materials varies between 60 and 90% (see Fig. 11), one has to conclude that the effect of the surface non-homogeneity cannot account for exponential change in the observed in the photo-
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electrochemical activity.
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Conclusions The photo-electrochemical activity of the nano-crystalline SrTiO3 in water oxidation is controlled exclusively by the particle size. The strontium titanate nano-cubes of controlled crystal size can be provided by the spray freezing - freeze drying is. This synthesis allows to fine tune the particle size in the range 10 - 30 nm. The shape and
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size control is achieved via a room temperature formation of a cation organizing network between Ti(IV) lactate and peptide based gelatin. The networking capabilities
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of the gelatin also quantitatively suppress the formation of carbonates which are usual contaminants of Sr containing oxides. The SrTiO3 nano-cubes are active in photo-
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electrochemical water splitting, their activity, however, strongly decreases with decreasing crystal size. The observed specific activity of the SrTiO3 nano-cubes decreases roughly exponentially with the particle size for the materials with crystal size below 20 nm. The specific activity of bigger nano-cubes is less sensitive to particle size and becomes size independent when the actual nano-cube diameter raises to ca. 30 nm. The presented results clearly demonstrate the crystal size control of the photoelectrochemical behaviour of oxide semiconductors, when an optimum particle size maximizes the positive effects of the electrode area increase outweighing enhanced 13
ACCEPTED MANUSCRIPT recombination generally encountered on small nano-particles. Such an optimum behaviour is encountered for materials where the characteristic particle size is ca. 4 times bigger than the Debye length.
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Acknowledgements Financial support of the Grant agency of the Czech Republic under contract 17-12800S is greatly appreciated.
The HRTEM measurement time was provided within the support of the project
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LM2015087 of the Czech Ministry of Education, Youth and Sports.
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Table 1 Specific surface are and dopant concentration of SrTiO3 nanocubes with various particle size. S [m2g-1]
Cdop [cm-3]
137.7
1.12 1019
14.5
65.1
8.64 1019
20.5
46.9
1.08 1020
30.0
38.8
2.72 1020
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8.5
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d [nm]
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ACCEPTED MANUSCRIPT Figure captions Figure 1 Powder XRD patterns of microcrystalline SrTiO3 standard (a), the product of the spray freezing freeze drying synthesis annealed at 450 °C in air (b), the product of the spray freezing freeze drying synthesis annealed at 450 °C in oxygen (c) and of the product of the
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spray freezing freeze drying synthesis in presence of 2.5 g/L of gelatin annealed at 450 °C in oxygen (d).
Figure 2 Characteristic particle size (solid squares) and XRD based coherent domain size
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(open squares) of the nano-particulated SrTiO3 prepared by spray freezing freeze drying
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procedure as a function of the gelatin content in the reaction mixture. The analysis of the diffraction pattern indicates nano-crystalline nature of the prepared SrTiO3 perovskite with coherent domain.
Figure 3 SEM images of SrTiO3 nanocrystals prepared by spray freezing freeze drying from titanium lactate and strontium nitrate in absence (a) and presence (b) of gelatin. The actual
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gelatin content in the reaction mixture of sample (b) amounted to 5.0 g/L. Figure 4 SEM images of SrTiO3 nanocrystals prepared by spray freezing freeze drying from
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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
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g/L (d). The insets show the actual particle size distribution. Figure 5 TEM observations. (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). Figure 6 Diffuse reflectance UV-Vis spectra of nano-cubeSrTiO3 nano-cubes prepared by spray freezing freeze drying synthesis with different amount of gelatin (top) and corresponding particle size dependence of the band gap energy (bottom).
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ACCEPTED MANUSCRIPT Figure 7 Mott Schottky plots of nano-crystalline SrTiO3 electrodes composed of nanocubes of variable size. The actual nano-cube size is shown in the Figure legend. The Mott Schottky plots were constructed from data obtained in ac impedance measurements in 0.1 M HClO4.
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Figure 8. Photo-potential (top) and steady state OCP under illumination (bottom) as a function of the intensity illumination for SrTiO3 based electrodes composed of nano-cubes with variable particle size. The actual nano-cube size is given in the Figure legend. The data were
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obtained in 0.1 M HClO4 solution.
Figure 9 Electrode potential dependence of the photo-current density of oxygen evolution on
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SrTiO3 nano-cubes. The actual photo-currents were normalized to photo-catalysts’ mass (left) and physical surface area (right). The data were extracted from potentiostatic measurements in 0.1M HClO4. The individual particle sizes are shown in the Figure legend. The current density was calculated from the specific surface area and known active mass.
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Figure 10 Particle size dependence of the specific activity of the SrTiO3 nano-cubes in photoelectrochemical water oxidation. The presented data correspond to a limiting photocurrent density obtained in experiments carried out in 0.1 M HClO4 when the electrode was
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illuminated by monochromatic radiation of λ=365 nm.
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Figure 11 Relative fraction of the under-coordinated Ti sites in the edge vicinity as a function of SrTiO3 nano-cube size
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9 (2016) 1523. F. Rechberger, F. J. Heiligtag, M. J. Süess, M. Niederberger, Assembly of BaTiO3
Nanocrystals into Macroscopic Aerogel Monoliths with High Surface Area, Angew. Chemie Int. Ed. 53 (2014) 6823.
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S. Ouyang, H. Tong, N. Umezawa, J. Cao, P. Li, Y. Bi, Y. Zhang, J. Ye, Surface
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J. H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y. L. Li, S. Choudhury, W. Tian,
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M. E. Hawley, B. Craigo, A. K. Tagantsev, X. Q. Pan, S. K. Streiffer, L. Q. Chen, S. W. Kirchoefer, J. Levy, D. G. Schlom, Room Temperature Ferroelectricity in Strained SrTiO , Nature 430 (2004) 758.
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Nanocrystalline BaTiO3 Ceramics, Phys. Rev. B 70 (2004) 024107. C. Chao, Z. Ren, Y. Zhu, Z. Xiao, Z. Liu, G. Xu, J. Mai, X. Li, G. Shen, G. Han, Self
Templated Synthesis of Single Crystal and Single Domain Ferroelectric Nanoplates, Angew. Chemie Int. Ed. 51 (2012) 9283. [20]
J. F. Scott, Applications of Modern Ferroelectrics, Science 315 (2007) 954.
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Sizes in Photoluminescence of Nanocrystalline SrTiO3, J. Phys. Condens. Matter 11 (1999) 5655.
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H. Yu, S. Ouyang, S. Yan, Z. Li, T. Yu, Z. Zou, Sol gel Hydrothermal Synthesis of
Visible Light Driven Cr Doped SrTiO3 for Efficient Hydrogen Production, J. Mater. Chem. 21 (2011) 11347. [23]
P. Balaya, M. Ahrens, L. Kienle, J. Maier, B. Rahmati, S. B. Lee, W. Sigle, A. Pashkin,
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C. Kuntscher, M. Dressel, Synthesis and Characterization of Nanocrystalline SrTiO3, J. Am. Ceram. Soc. 89 (2006) 2804. [24]
M. Kakihana, T. Okubo, M. Arima, Y. Nakamura, M. Yashima, M. Yoshimura,
Polymerized Complex Route to the Synthesis of Pure SrTiO3 at Reduced Temperatures:
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Implication for Formation of Sr Ti Heterometallic Citric Acid Complex, J. Sol-Gel Sci. Technol. 12 (1998) 95–109.
R. Niishiro, S. Tanaka, A. Kudo, Hydrothermal Synthesized SrTiO3 photocatalyst
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Codoped with Rhodium and Antimony with Visible Light Response for Sacrificial H2 and O2 Evolution and Application to Overall Water Splitting, Appl. Catal. B Environ. 150 (2014) 187. [26]
S. M. Kobayashi, Y. Suzuki, T. Goto, S. H. Cho, T. Sekino, Y. Asakura, S. Yin,
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Low-Temperature Hydrothermal Synthesis and Characterization of SrTiO3 Photocatalysts for NOx Degradation, J. Ceram. Soc. Japan 126 (2018) 135. [27]
L. Mu, Y. Zhao, A. Li, S. Wang, Z. Wang, J. Yang, Y. Wang, T. Liu, R. Chen, J. Zhu, F.
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Fan, R. Li, C. Li, Enhancing Charge Separation on High Symmetry SrTiO3 Exposed with Anisotropic Facets for Photocatalytic Water Splitting, Energy Environ. Sci. 9 (2016) 2463. Y. Zhang, L. Zhong, D. Duan, A Single Step Direct Hydrothermal Synthesis of SrTiO3
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Nano-particles from Crystalline P25 TiO2 Powders, J. Mater. Sci. 51 (2016) 1142. [29]
T. Alammar, I. Hamm, M. Wark, A. V. Mudring, Low Temperature Route to Metal
Titanate Perovskite Nano-particles for Photocatalytic Applications, Appl. Catal. B Environ. 178 (2015), 20. [30]
Y. Kozuka, Y. Hikita, T. Susaki, and H. Y. Hwang: „Optically tuned dimensionality
crossover in photocarrier-doped SrTiO3: Onset of weak localization“, Phys. Rev. B, 76 (2007) 085129
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M. Kobayashi, V. Petrikyn, K. Tomita, M. Kakihana, New Water-Soluble Complexes of
Titanium with Amino Acids and Their Application for Synthesis of TiO2 Nanoparticles, J. Ceram. Soc. Japan, 116 (2008) 578. [32]
M. A. Gülgün, M. H. Nguyen, W. M. Kriven, Polymerized Organic-Inorganic Synthesis
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of Mixed Oxides, J. Am. Ceram. Soc., 82 (1999) 556. V.A. Myamlin, Y.V. Pleskov, in “Electrochemistry of semiconductors”, p. 103, Plenum
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J. Bisquert, G. Garcia-Belmonte, F. Fabregat-Santiago, Modelling the Electric Potential
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Distribution in the Dark in Nanoporous Semiconductor Electrodes, J. Solid State Electrochem. 3 (1999) 337.
J. Bisquert, Physical Electrochemistry of Nanostructured Devices, Phys. Chem. Chem.
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Phys. 10 (2008) 49. [36]
L. M. Peter, Energetics and Kinetics of Light-Driven Oxygen Evolution at Semiconductor
Electrodes: The Example of Hematite, J. Solid State Electrochem. 17 (2013) 315.
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[37] M. Nesselberger, S. Ashton, J. C. Meier, I. Katsounaros, K. J. J. Mayrhofer and M. Arenz, The Particle Size Effect on the Oxygen Reduction Reaction Activity of Pt Catalysts: Influence of Electrolyte and Relation to Single Crystal Models, J. Am. Chem. Soc. 133
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(2011) 17428.
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ACCEPTED MANUSCRIPT Appendix 1 Gelatin analysis
Alanine Glutamine and Glutamic acid
0.121 0.090
Arginine Aspargine and Aspargic Acid
0.050 0.054 0.024 0.036
Ammonia Serine
Phenylalanine Isoleucine
0.005 0.001
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Histidine Tyrosine
0.029 0.028 0.024 0.021 0.013 0.015
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Lysine Leucine Valine Threonine
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Glycine Proline
x AMA 0.299 0.140
AMA
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Table 1 Actual gelatin composition
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Gelatin composition was determined by ion exchange chromatography using Biochrom 30 (Biochrom, Cambridge UK) analyser. 0.01 g of the gelatin sample was placed into test tube containing 0.1 mL of 6M HCl and sealed. The sample in the sealed test tube was digested at 110 °C for 20 hours. Following the digestion the acid was evaporated and the pH adjusted using citrate buffer. The sample was transferred into the ion exchange column. Individual amino acids were reacted with ninhydrine and detected by UV-Vis spectrophotometry at λ = 550 nm (primary aminoacids) and λ = 440 nm (secondary amino acids).
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9 nm 18 nm 25 nm 30 nm
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OCPlight vs. RHE [V]
-0.25
0.00
1
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IPC /m [ A/mg]
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Photoelctrochemical activity of SrTiO3 is size dependent for particles below 20 nm
S0013-4686(18)32669-0
DOI:
https://doi.org/10.1016/j.electacta.2018.11.185
Reference:
EA 33186
To appear in:
Electrochimica Acta
Received Date: 18 July 2018 Revised Date:
19 October 2018
Accepted Date: 26 November 2018
Please cite this article as: M. Klusáčková, R. Nebel, Kateř.Minhová. Macounová, M. Klementová, P. Krtil, Size control of the photo-electrochemical water splitting activity of SrTiO3 nano-cubes, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.11.185. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Size control of the photo-electrochemical water splitting activity of SrTiO3 nano-cubes Monika Klusáčkováa, Roman Nebela, Kateřina Minhová Macounováa, Mariana Klementováb
a.
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and Petr Krtila* J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejškova
3, 18223, Prague, Czech Republic. E-mail:[email protected]
Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Prague, Czech
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b
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Republic Abstract
Size control of the photo-electrochemical activity of a n-semicondutor oxide in water oxidation was demonstrated on size and shape controlled SrTiO3 nano-cubes featuring
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homogeneous {100} oriented surface. The carbonate free SrTiO3 perovskite nano-cubes were synthesized from titanium(IV) bis(ammonium lactato)dihydroxide solution and strontium nitrate in presence of the structure ordering gelatin. The gelatin content determines the size of
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the perovskite nano-cubes in the range 10-30 nm. SrTiO3 nano-cubes show band gap in the range 3.2-3.3 eV with slight blue shift of the absorption edge for nano-cubes smaller than 20
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nm. All materials are active in photo-electrochemical water splitting, their specific activity, however, decreases with decreasing particle size and drop exponentially for nano-cubes smaller than 20 nm.
ACCEPTED MANUSCRIPT Introduction Oxide based nano-crystalline photo-catalysts were vigorously developed to support the storage of the renewable solar energy into environmentally friendly hydrogen fuel [15]. Given the heterogeneous nature of the underlying water splitting reaction one
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understands the stress on the maximization of the specific surface area of the available photo-catalysts as well as on the optimization of their surface orientation. The progress in the field has been to a great extent hindered by a lack of convenient synthetic
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techniques allowing precise control not only of the particle size and degree of crystallinity, but also the nano-crystal shape. Particle size effects in photo-catalysis
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have been addressed on various materials including TiO2 [6], Cu2O [7], Si [8] or BiVO4 [9]. The presented results are, however contradictory. While in the case of reduction of Si quantum dots the resulting activity was reported to increase with decreasing particle size due to blue shift of the conduction band edge to higher
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energies [8] in the case of water oxidation on BiVO4 it was observed an opposite trend in the particle size vs. activity relationship reportedly due to an increase of hole life time at the grain boundaries of agglomerates of larger nanoparticles [9]. It has to be
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summarized that there is no general understanding of the phenomena nor there are any semi-quantitative criteria steering the rational design of the photo-catalysts in terms of
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particle size, shape or material morphology. Reaching general relationship linking the photocatalytic activity with particle size in the nanoscale is hindered either by a lack of convenient techniques controlling the particle size on the d~ 10 nm level (as one encounters for double oxide and other complex photo-catalysts) simultaneously with orientation of the surface and agglomeration of the nano-crystals. To bridge the gap in fundamental understanding of the nano- crystal size effects we present here a
2
ACCEPTED MANUSCRIPT systematic comparison of the photo-electrochemical activity of the SrTiO3 nano-cubes in water oxidation reaction. SrTiO3 is one of the most important oxide materials due to its prospective applications not only in photocatalysis [10-16], but also in ferroelectrics [17-20], or
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photoluminescence [21]. The crystal size control (and consequent maximization of the specific surface area) of strontium titanate is not a trivial task. SrTiO3 crystallizes in cubic system and conforms to perovskite structural type (Pm3m). The conventional
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synthesis of the strontium titanate relies on solid state reaction of strontium carbonate and titanium dioxide at temperatures exceeding 700°C and fails to deliver sub-micron
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particles. Alternative routes based on sol-gel [10, 22], Pechini process [23], peroxo complex based routes [24] and/or on hydrothermal synthesis [12, 25] have been developed with various degree of success. Namely the hydrothermal synthesis at temperatures below 180 °C yields nano-crystalline SrTiO3 with particle sizes ranging
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between 25 and 200 nm [26-28]. Despite the high versatility of the hydrothermal synthesis its application in synthesis of SrTiO3 often leads to a contamination of the final products either with unreacted starting materials (if one uses TiO2 as titanium
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source) [29] or by strontium carbonate [11, 27]. It also does not allow for combined crystal size and shape control due to limited variability of the parameters affecting the
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synthesis.
Any study focused on size related investigations on SrTiO3¸ therefore, need to be supported by a novel low temperature approach to SrTiO3 synthesis. This study therefore, also introduces a novel SrTiO3 synthesis based on spray freezing freeze drying approach in presence of gelatin as room temperature structure ordering agent.
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ACCEPTED MANUSCRIPT 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.),
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absolute ethanol (99.8%, Lach Ner, p.a.), and gelatin (Aldrich) were used as received. All the solutions were prepared using deionized water (Milli Q Gradient, Millipore). The stock solution of strontium nitrate (0.04 M) was prepared by dissolving the exact amount of the
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substance. Synthesis of SrTiO3 nano-particles
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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
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deionized water and stirred for 30 min at 60 °C to form a clear solution. The starting solution containing Sr(NO3)2 and TBALD was added to cooled gelatin solution and the total volume
without heating.
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was adjusted to 100 mL with deionized water. The reaction mixture was stirred for 60 min
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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
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ACCEPTED MANUSCRIPT using powder X ray diffraction (Miniflex I - Rigaku) using Cu Kα radiation. The band gap energy of the prepared SrTiO3 materials was assessed by means of the UV-Vis-NIR spectroscopy. The UV-Vis spectra of the prepared materials were measured in diffuse reflectance mode using Perkin - Elmer Lambda 950 UV-Vis-NIR spectrometer.
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The morphology of the synthesized materials was assessed by means of the Scanning Electron Microscopy (SEM - Hitachi S4800 microscope). The SEM micrographs were used to determine the average particle size of the prepared materials. The average particle size data
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are based on analysis of 200+ randomly selected nano-particles. Transmission electron microscopy (TEM) was used to determine surface orientation of the prepared nanocrystals
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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
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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. Specific surface area of the prepared materials was assessed by means of the nitrogen
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adsorption at 77.4 K using ASAP 2020 (Micrometrics, USA) instrument.
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Photo-electrochemical experiments The photo-electrochemical activity of the SrTiO3 nano-cubes was studied on electrodes prepared on gold covered Ti mesh (1 cm2, Goodfellow). The gold layer on Ti mesh substrate was deposited from colloidal Au ink (Fraunhofer Institute für Keramische Technologien und Systeme, Dresden, Germany) and calcined at 200 °C for 15 minutes. The active SrTiO3 layer was deposited from an ethanol based suspension (10 g/L of SrTiO3). The SrTiO3 nanoparticles were dispersed in ethanol by 30 minutes sonication. The electrodes were prepared by drop casting of the suspension in 10 µL increments until the total mass of the active catalyst 5
ACCEPTED MANUSCRIPT reached ca 1 mg. Each increment of the active layer was stabilized by drying at 100 °C. The electrode was calcined at 400 °C for 4 h in air before use. Given the surface loading and projected electrode area one may estimate that the SrTiO3 layers at electrode surfaces were ca 1.1 µm thick.
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Photo-electrochemical behavior of the SrTiO3 electrodes was studied in 0.1 M HClO4 (pH=1) in a single-compartment quartz cell. A three-electrode arrangement with SrTiO3 working, Pt auxiliary, and Ag/AgCl reference electrode was used. All photo-electrochemical experiments
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were carried out in chronoamperometric mode using AUTOLAB (PGSTAT 30) potentiostat for potential control. The Bluepoint LED spot source (Hönle UV Technology) with the 365
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nm wavelength operating with light intensity between 1 and 6 mW/cm2 was employed as an illumination source. The absorption coefficient of the SrTiO3 at the 365 nm which equals to be 8302 cm-1 [30] therefore suggests that the entire SrTiO3 layer is accessible to the illumination. All potentials were recalculated and referred in the reversible hydrogen electrode
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(RHE) scale. The Mott Schotky plots were constructed from ac impedance data measured in the 20 kHz to 10 mHz interval with 10 mV (peak to peak) amplitude.
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Results and discussion
The spray freezing-freeze drying approach is a convenient approach in synthesis of
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multicomponent oxide materials. It utilizes atomic scale mixing of the oxide components in the initial solution and subsequently also in the ice precursor. Despite the versatility of the approach in synthesis of both stable as well as metastable phases its potential for crystal size or shape control has never been reported. The applicability of spray freezing freeze drying approach to synthesize Ti based materials is rather restricted due to relative lack of soluble Ti precursors decomposing without residual contaminants. In the above described synthesis the sublimation of the solvent water from the ice-like precursor leads to formation of white rather hygroscopic powders which need to be 6
ACCEPTED MANUSCRIPT immediately calcined to prevent a formation of gel-like substance. The hygroscopic nature of the dried precursor prevents its structural diffraction characterization. Calcination of the precursor to temperatures above 400 °C leads to a formation of white nano-crystalline material. The phase composition of final material is affected by the oxygen partial pressure
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(see the X-ray diffraction patterns summarized in Figure 1). The annealing of the dried precursor in air leads to a formation of multiphase system featuring cubic SrTiO3 perovskite along with significant amount of strontium carbonate (see pattern b in Fig. 1).
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The fraction of the strontium carbonate can be suppressed by calcination in pure oxygen, which suppresses the carbonate content below 10 %. The carbonate contamination cannot be
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removed by washing with acids indicating that the carbonates most likely form inclusions in the SrTiO3 matrix (see pattern c in Fig. 1). Carbonate contamination can be quantitatively suppressed by addition of gelatin (see curve d in Fig. 1). The addition of gelatin is further necessary to achieve effective control of the particle size as well morphology (see below).
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Increasing addition of gelatin affects the coherent domain size determined from the XRD patterns (see Fig. 2). The coherent domain size increases with increasing gelatin concentration in the initial reaction mixture and ranges between 10 and 30 nm. If the synthesis was carried
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out without gelatin addition no coherent domain (particle) size variation was observed
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regardless of the variation of the experimental conditions. The SEM based characteristic particle size increases with increasing gelatin content and ranges between 8 and 35 nm in the manner similar to coherent domain size (see Fig. 2). A good agreement between the coherent domain values and SEM based particle size suggests that each nano-particle in fact represents nano-sized single crystal with low number of defects. The SEM based particle size values are in good agreement with the BET based specific surface area values (see Table 1). The presence of the gelatin has fundamental effect on the shape of the synthesized SrTiO3. While in the absence of gelatin the resulting SrTiO3 nano-particles show isometric shape 7
ACCEPTED MANUSCRIPT without preferential surface orientation the addition of gelatin leads to a formation of regular cube-shaped nano-crystals (see Fig. 3). The nano-particle shape is not affected by the actual gelatin content (see Fig. 4). The surface orientation of the synthesized nanocrystals was assessed from HRTEM images of the synthesized SrTiO3 materials (see typical data in Figure
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5). The synthesized nanocrystals project as squares in the HRTEM. The diffractograms based on FFT of the lattice images clearly identify the surface of the nano-cubes to be composed solely of {100} orientated faces. The resulting SrTiO3 nano-cubes show Gaussian particle
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size distribution the width of which increases with increasing gelatin content in the reaction mixture.
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The effect of gelatin addition clearly suggests an organization of the ionic species in the solution containing Ti(IV) lactate and Sr(NO3)2. Although the exact coordination of the Sr as well as of Ti (IV) lactate in the solution (and in ice as well) facilitated by gelatin is still under investigation, it prevents the formation of the SrCO3 which is the main impurity in materials
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prepared in other low temperature processes. It also allows for fine tune of the particle size while controlling the nanocrystal’s shape.
The positive network forming effect of the gelatin can be tentatively attributed to zwitterionic
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nature of the amino acids which form the active component of gelatin. Due to the presence of
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negative charge on carboxylic group and simultaneously a positive charge on amino group the zwitterionic forms of amino acids may effectively crosslink the lactate with strontium cations in a broad range of pH. It needs to be noted that similar formation of chelating networks with amino acids or with polyvinyl alcohols was previously reported for TiO2 [31] or for aluminates [32]. Such a model of the crosslinked cation network is supported by the fact that the amount of the networking agent directly affects the particle size of the formed perovskite nano-crystals. The relation between the networking agent content and resulting particle size is, however, not straightforward proportionality since the nanocrystal size becomes practically
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ACCEPTED MANUSCRIPT independent of the gelatin content at concentrations near 5 g/L. This trend is difficult to rationalize in part due to the complex nature of the gelatin (see Appendix 1). The prepared perovskite nano-cubes, regardless of their actual size, are structurally identical. The electronic structure of the prepared SrTiO3 nano-cubes reflected in UV-Vis spectra
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corresponds to a wide band n-semiconductor with band gap ranging between 3.2 and 3.3 eV (see Fig. 6).
The apparent linear dependence of the band gap on the particle size has to be, however, taken
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with a great care given the fact that the observed band gap variation of 0.1 eV corresponds to a change of the absorption edge by ca. 20 nm. Slight blue shift of the absorption edge
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observed for the finer nanocrystals vanishes for nanocrystals bigger than 20 nm suggesting that the electronic behavior of bigger nanocrystals (d>20 nm) should be the same as in the case of micro or single crystals. On the other hand, the nano-cubes smaller than 15 nm may show a behavior affected by an onset of quantum confinement.
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All nano-cube perovskites are active as anodes in photo-electrochemical water splitting without need for further doping. The photo-electrochemical behavior of the prepared SrTiO3
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nano-cubes can be predicted based on the position of the flat band potential Efb (see Fig. 7), photo-potential behavior (see Fig. 8) and photo-current vs. potential behavior plotted in Fig. 9.
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As follows from the Mott Schottky plots presented in Figure 7 the Efb of SrTiO3 nano-cubes is practically independent of the nano-cube size and equals to ca. - 0.17 V vs. RHE. The slopes of the Mott Schottky plots indicate the charge carrier concentration in dark of the order of 1019 cm-3 - 1020 cm-3 which increases slightly with increasing particle size (see Table 1). Particle size effects show also in the photo-potential behavior reflected in Figure 8. In an ideal case the photo-potential is defined as a difference of the open circuit potentials (OCP) in dark and under illumination and it represents a response of the semiconductor to the to the photogeneration of charge carries in the space charge layer upon band gap illumination. The Fermi 9
ACCEPTED MANUSCRIPT level energy needs to be consequently increased (for an n-type semiconductor) to maintain the electro-neutrality. The shift of the OCP following illumination towards more negative potential effectively decreases band bending and facilitates the electron hole recombination. The electrode’s OCP after illumination shifts towards a steady state value via mixed potential
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like mechanism.
In the classical models [33] based on single crystal behavior the overall photo-potential increases with increasing intensity of the illumination and the steady state OCP at high
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illumination intensities converges to the value of the flat band potential Efb. As follows from the data summarized in Figure 8 these nano-crystalline photo-catalysts do not conform
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completely to the expected behavior. The overall photo-potential decreases with both increasing illumination intensity as well as with increasing particle size. It needs to be noted, however, that the observed OCP values (under illumination) decrease with the increasing illumination intensity and
converge (at illumination intensity of 6 mWcm-2) to
values
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between 50 mV and ca. -20 mV vs. RHE mV for the 9 and 30 nm sized nano-cubes, respectively. These values are slightly more positive than the Efb values resulting from impedance measurement. This type of behavior can be, however, expected given the mixed
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potential like nature of the OCP under illumination. The observed OCP values also indicate
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the ability of the biggest SrTiO3 nano-cubes to split photo-catalytically (i.e. without further bias) the water. The observed photo-potential trends are significantly affected by the surface states the occupation of which (at OCP conditions in dark) is heavily affected by the electrode’s history and is, consequently, poorly reproducible. The photo-current vs. potential curves of nano-cube based electrodes measured in 0.1 M HClO4 upon illumination with monochromatic radiation (λ=365 nm) are shown in Figure 9. These data indicate that the charge transfer energetics reflected, e.g. in the photocurrent onset, is, apparently, insensitive to the actual particle size. Such a result is in agreement with
10
ACCEPTED MANUSCRIPT negligible differences in the flat band potential hence the presented photo-current values in fact reflect the kinetic differences of various SrTiO3 nano-cubes at comparable band bending. The particle size, has, however, a pronounced effect on the overall activity of the photocatalysts (see Fig. 9). The specific activity of the strontium titanate expressed as photocurrent
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density (regardless whether normalized to the actual physical catalyst area or the catalysts mass) increases with increasing particle size. The observed activity trend is counterintuitive given the heterogeneous nature of the photo-electrochemical water splitting. Also the hole
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trapping mechanism proposed by Kudo et al. [9] cannot be used to explain the observed experimental trend due to absence of the nanocrystal agglomeration. The obtained activity
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data, therefore, reflect the crystal size control of the nano-crystalline semiconductor behavior since the SrTiO3 nano-cubes included in the study are of the same morphology and their surface is composed only of {100} oriented faces. The actual photo-current density data shown in Figure 10 suggest that the activity of the SrTiO3 catalysts decreases exponentially
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with the decreasing particle size once the characteristic particle size drops below ca. 20 nm and remains particle size independent for bigger nano-cubes. Given the low to negligible agglomeration of the nano-crystals in the prepared electrodes one cannot assume that the
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favorable behavior of the bigger crystals results from hole trapping at the crystal boundaries [8], but mostly likely reflects enhanced surface based recombination on the small
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nanocrystals. The observed behavior outlines the nanocrystal size as an efficient tool in control of photocatalytic activity. The observed behavior is difficult to assign to a single factor and most likely originates from convolution of several effects. Local structure and band structure effects This discrepancy between expected and actual trend in the size dependence of the photo-electrochemical activity can be explained realizing that the sizes of the particles are close to the Debye length in the SrTiO3 assuming the donor concentration to be in 11
ACCEPTED MANUSCRIPT the range 1019-1020 cm-3. The resulting Debye length is approximately 6 nm. It can be expected that while Debye length remains comparable with the particle size (i.e. the effective crystal radius does not exceed significantly the Debye length) one should expect significant deviation from the conventional model of the potential distribution
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between the semiconductor and electrolyte. It may be assumed that a formation of space charge layer in the individual small nanocrystals may be gradually suppressed and the charge transfer at SrTiO3 electrode will follow the model formulated by
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Bisqueret [34-35] when the photocurrent observed under external polarization is driven by the difference between the applied potential (controlling the Fermi level) and the
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quasi Fermi level of the electrons in the conduction band [36]. The absence or suppression of the space charge layer formation, however, results in enhanced surface recombination which suppresses the observed photocurrent. The Debye length activity restriction is removed in nano-particles bigger than 20 nm when the photo-
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electrochemical activity becomes apparently size independent. It can be deemed that the particle size exceeding ca. 2-3 times the Debye length represents the optimum particle size maximizing the specific surface area while maintaining the band structure
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typical for semiconductors. Such an optimum particle size can be devised for any semiconductor system; given the range of the relative permittivity in common photo-
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catalytically active materials (10-400) one can expect that synthesis of nanocrystals smaller than 5-10 nm lacks rational justification. The observed size dependence of the photo-electrochemical activity may be further accentuated by non-homogeneities of the surface on the nanoscale. The sites at the surface of the prepared nanocrystals may differ from those present at {100} oriented single crystals. The nano-cubes feature relative high number of “low dimensionality sites” near the nano-cube edges and vertices. These sites feature less bonding partners
12
ACCEPTED MANUSCRIPT than typical for an ideal {100} SrTiO3 single crystal surface. Drawing an analogy from the electro-catalysis one may expect the “low dimensionality” sites to be more reactive (i.e. to adsorb the reaction intermediates rather strongly) than the sites prevailing on {100} single crystals surface [37] which may result in altered activity. Since the
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fraction of the “low dimensionality” sites at the surface of the prepared materials varies between 60 and 90% (see Fig. 11), one has to conclude that the effect of the surface non-homogeneity cannot account for exponential change in the observed in the photo-
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electrochemical activity.
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Conclusions The photo-electrochemical activity of the nano-crystalline SrTiO3 in water oxidation is controlled exclusively by the particle size. The strontium titanate nano-cubes of controlled crystal size can be provided by the spray freezing - freeze drying is. This synthesis allows to fine tune the particle size in the range 10 - 30 nm. The shape and
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size control is achieved via a room temperature formation of a cation organizing network between Ti(IV) lactate and peptide based gelatin. The networking capabilities
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of the gelatin also quantitatively suppress the formation of carbonates which are usual contaminants of Sr containing oxides. The SrTiO3 nano-cubes are active in photo-
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electrochemical water splitting, their activity, however, strongly decreases with decreasing crystal size. The observed specific activity of the SrTiO3 nano-cubes decreases roughly exponentially with the particle size for the materials with crystal size below 20 nm. The specific activity of bigger nano-cubes is less sensitive to particle size and becomes size independent when the actual nano-cube diameter raises to ca. 30 nm. The presented results clearly demonstrate the crystal size control of the photoelectrochemical behaviour of oxide semiconductors, when an optimum particle size maximizes the positive effects of the electrode area increase outweighing enhanced 13
ACCEPTED MANUSCRIPT recombination generally encountered on small nano-particles. Such an optimum behaviour is encountered for materials where the characteristic particle size is ca. 4 times bigger than the Debye length.
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Acknowledgements Financial support of the Grant agency of the Czech Republic under contract 17-12800S is greatly appreciated.
The HRTEM measurement time was provided within the support of the project
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LM2015087 of the Czech Ministry of Education, Youth and Sports.
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Table 1 Specific surface are and dopant concentration of SrTiO3 nanocubes with various particle size. S [m2g-1]
Cdop [cm-3]
137.7
1.12 1019
14.5
65.1
8.64 1019
20.5
46.9
1.08 1020
30.0
38.8
2.72 1020
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8.5
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d [nm]
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ACCEPTED MANUSCRIPT Figure captions Figure 1 Powder XRD patterns of microcrystalline SrTiO3 standard (a), the product of the spray freezing freeze drying synthesis annealed at 450 °C in air (b), the product of the spray freezing freeze drying synthesis annealed at 450 °C in oxygen (c) and of the product of the
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spray freezing freeze drying synthesis in presence of 2.5 g/L of gelatin annealed at 450 °C in oxygen (d).
Figure 2 Characteristic particle size (solid squares) and XRD based coherent domain size
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(open squares) of the nano-particulated SrTiO3 prepared by spray freezing freeze drying
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procedure as a function of the gelatin content in the reaction mixture. The analysis of the diffraction pattern indicates nano-crystalline nature of the prepared SrTiO3 perovskite with coherent domain.
Figure 3 SEM images of SrTiO3 nanocrystals prepared by spray freezing freeze drying from titanium lactate and strontium nitrate in absence (a) and presence (b) of gelatin. The actual
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gelatin content in the reaction mixture of sample (b) amounted to 5.0 g/L. Figure 4 SEM images of SrTiO3 nanocrystals prepared by spray freezing freeze drying from
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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
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g/L (d). The insets show the actual particle size distribution. Figure 5 TEM observations. (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). Figure 6 Diffuse reflectance UV-Vis spectra of nano-cubeSrTiO3 nano-cubes prepared by spray freezing freeze drying synthesis with different amount of gelatin (top) and corresponding particle size dependence of the band gap energy (bottom).
16
ACCEPTED MANUSCRIPT Figure 7 Mott Schottky plots of nano-crystalline SrTiO3 electrodes composed of nanocubes of variable size. The actual nano-cube size is shown in the Figure legend. The Mott Schottky plots were constructed from data obtained in ac impedance measurements in 0.1 M HClO4.
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Figure 8. Photo-potential (top) and steady state OCP under illumination (bottom) as a function of the intensity illumination for SrTiO3 based electrodes composed of nano-cubes with variable particle size. The actual nano-cube size is given in the Figure legend. The data were
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obtained in 0.1 M HClO4 solution.
Figure 9 Electrode potential dependence of the photo-current density of oxygen evolution on
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SrTiO3 nano-cubes. The actual photo-currents were normalized to photo-catalysts’ mass (left) and physical surface area (right). The data were extracted from potentiostatic measurements in 0.1M HClO4. The individual particle sizes are shown in the Figure legend. The current density was calculated from the specific surface area and known active mass.
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Figure 10 Particle size dependence of the specific activity of the SrTiO3 nano-cubes in photoelectrochemical water oxidation. The presented data correspond to a limiting photocurrent density obtained in experiments carried out in 0.1 M HClO4 when the electrode was
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illuminated by monochromatic radiation of λ=365 nm.
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Figure 11 Relative fraction of the under-coordinated Ti sites in the edge vicinity as a function of SrTiO3 nano-cube size
…
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ACCEPTED MANUSCRIPT References [1] M. R. Wasielewski, Self-Assembly Strategies for Integrating Light Harvesting and Charge Separation in Artificial Photosynthetic Systems, Acc. Chem. Res. 42 (2009) 1910. [2] A. Kudo, Y. Miseki, Heterogeneous Photocatalyst Materials For Water Splitting, Chem. Soc.
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[7] H. Xu, W. Wang, W. Zhu, Shape Evolution and Size-Controllable Synthesis of Cu2O Octahedra and Their Morphology-Dependent Photocatalytic Properties, J. Phys. Chem. B, 110 (2006) 13829.
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[8] T. Kojima, H. Sugimoto, M. Fujii, Size-Dependent Photocatalytic Activity of Colloidal Silicon Quantum Dot, J. Phys. Chem. C 122 (2018) 1874.
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T. K. Townsend, N. D. Browning, F. E. Osterloh, Nanoscale Strontium Titanate
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9 (2016) 1523. F. Rechberger, F. J. Heiligtag, M. J. Süess, M. Niederberger, Assembly of BaTiO3
Nanocrystals into Macroscopic Aerogel Monoliths with High Surface Area, Angew. Chemie Int. Ed. 53 (2014) 6823.
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J. H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y. L. Li, S. Choudhury, W. Tian,
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M. E. Hawley, B. Craigo, A. K. Tagantsev, X. Q. Pan, S. K. Streiffer, L. Q. Chen, S. W. Kirchoefer, J. Levy, D. G. Schlom, Room Temperature Ferroelectricity in Strained SrTiO , Nature 430 (2004) 758.
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Nanocrystalline BaTiO3 Ceramics, Phys. Rev. B 70 (2004) 024107. C. Chao, Z. Ren, Y. Zhu, Z. Xiao, Z. Liu, G. Xu, J. Mai, X. Li, G. Shen, G. Han, Self
Templated Synthesis of Single Crystal and Single Domain Ferroelectric Nanoplates, Angew. Chemie Int. Ed. 51 (2012) 9283. [20]
J. F. Scott, Applications of Modern Ferroelectrics, Science 315 (2007) 954.
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C. Kuntscher, M. Dressel, Synthesis and Characterization of Nanocrystalline SrTiO3, J. Am. Ceram. Soc. 89 (2006) 2804. [24]
M. Kakihana, T. Okubo, M. Arima, Y. Nakamura, M. Yashima, M. Yoshimura,
Polymerized Complex Route to the Synthesis of Pure SrTiO3 at Reduced Temperatures:
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Implication for Formation of Sr Ti Heterometallic Citric Acid Complex, J. Sol-Gel Sci. Technol. 12 (1998) 95–109.
R. Niishiro, S. Tanaka, A. Kudo, Hydrothermal Synthesized SrTiO3 photocatalyst
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Codoped with Rhodium and Antimony with Visible Light Response for Sacrificial H2 and O2 Evolution and Application to Overall Water Splitting, Appl. Catal. B Environ. 150 (2014) 187. [26]
S. M. Kobayashi, Y. Suzuki, T. Goto, S. H. Cho, T. Sekino, Y. Asakura, S. Yin,
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Low-Temperature Hydrothermal Synthesis and Characterization of SrTiO3 Photocatalysts for NOx Degradation, J. Ceram. Soc. Japan 126 (2018) 135. [27]
L. Mu, Y. Zhao, A. Li, S. Wang, Z. Wang, J. Yang, Y. Wang, T. Liu, R. Chen, J. Zhu, F.
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Fan, R. Li, C. Li, Enhancing Charge Separation on High Symmetry SrTiO3 Exposed with Anisotropic Facets for Photocatalytic Water Splitting, Energy Environ. Sci. 9 (2016) 2463. Y. Zhang, L. Zhong, D. Duan, A Single Step Direct Hydrothermal Synthesis of SrTiO3
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T. Alammar, I. Hamm, M. Wark, A. V. Mudring, Low Temperature Route to Metal
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Y. Kozuka, Y. Hikita, T. Susaki, and H. Y. Hwang: „Optically tuned dimensionality
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[37] M. Nesselberger, S. Ashton, J. C. Meier, I. Katsounaros, K. J. J. Mayrhofer and M. Arenz, The Particle Size Effect on the Oxygen Reduction Reaction Activity of Pt Catalysts: Influence of Electrolyte and Relation to Single Crystal Models, J. Am. Chem. Soc. 133
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(2011) 17428.
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ACCEPTED MANUSCRIPT Appendix 1 Gelatin analysis
Alanine Glutamine and Glutamic acid
0.121 0.090
Arginine Aspargine and Aspargic Acid
0.050 0.054 0.024 0.036
Ammonia Serine
Phenylalanine Isoleucine
0.005 0.001
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Histidine Tyrosine
0.029 0.028 0.024 0.021 0.013 0.015
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Lysine Leucine Valine Threonine
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Glycine Proline
x AMA 0.299 0.140
AMA
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Table 1 Actual gelatin composition
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Gelatin composition was determined by ion exchange chromatography using Biochrom 30 (Biochrom, Cambridge UK) analyser. 0.01 g of the gelatin sample was placed into test tube containing 0.1 mL of 6M HCl and sealed. The sample in the sealed test tube was digested at 110 °C for 20 hours. Following the digestion the acid was evaporated and the pH adjusted using citrate buffer. The sample was transferred into the ion exchange column. Individual amino acids were reacted with ninhydrine and detected by UV-Vis spectrophotometry at λ = 550 nm (primary aminoacids) and λ = 440 nm (secondary amino acids).
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Jmax [A/cm ]
10
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15
20
particle size [nm]
25
30
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1.0
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0.4
0.0
5
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0.2
10
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0.8
15
20
particle size [nm]
25
30
35
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c
b
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0.5 0
10
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a 20
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30
40
50
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60
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5
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particle size [nm]
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cgel. [g/L]
10
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80 70
40
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Counts
50
20
5
10
diameter [nm]
15
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15
10
5
0
10
20
30
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0
20
20
0
30
10
0
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0
40
20
10
40
diameter [nm]
50
60
0
10
20
30
diameter [nm]
40
50
30 25 20
Counts
Counts
30
15 10 5 0
0
10
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40
diameter [nm]
50
60
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6 5
9 nm 18 nm 25 nm 30 nm
4
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K/M
3 1
3.4
200
Eg [eV]
3.2
5
10
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3.1
[nm]
600
800
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3.3
400
EP
0
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15
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d [nm]
25
30
35
4x10
10
2x10
10
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6x10
20 nm 9 nm 25 nm 30 nm
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0
0.0
0.4
E vs. RHE [V]
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-0.4
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-0.8
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8x10
1/C
2
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0.8
1.2
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-0.15
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Ephoto [V]
-0.10
-0.20
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9nm 20nm 25nm 30nm
0.05
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OCPlight vs. RHE [V]
-0.25
0.00
1
2
3
4
5
Light intensity [mW/cm2]
6
3600 3200
UV ON
UV OFF
UV ON
30 nm 25 nm 18 nm 9 nm
UV OFF
2400
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2500
IPC /m [ A/mg]
2800 2000 1600 1200
0 20
40
60
80
100
120
Time [sec]
140
1500
500 0
-200
0
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1000
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400
200 400 600 800 1000 1200
E vs. RHE [mV]
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800
6
JPC/A [A/cm2]
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E vs. RHE [mV]
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Photoelctrochemical activity of SrTiO3 is size dependent for particles below 20 nm