Enhancement of Photocatalytic Water Oxidation by the Morphological

Oct 31, 2013 - The promotional effect was most pronounced for LaTiO2N with a skeletal morphology, because such LaTiO2N contain most unsaturated bonds ...
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Enhancement of Photocatalytic Water Oxidation by the Morphological Control of LaTiO2N and Cobalt Oxide Catalysts Alexandra E. Maegli,† Simone Pokrant,† Takashi Hisatomi,‡ Matthias Trottmann,† Kazunari Domen,‡ and Anke Weidenkaff*,† †

Empa - Swiss Federal Laboratories for Materials Science and Technology, Solid State Chemistry and Catalysis, Ueberlandstr. 129, CH-8600 Duebendorf, Switzerland ‡ The University of Tokyo, Department of Chemical System Engineering, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: Controlled morphological and structural variations of LaTiO2N and CoOx nanoparticles loaded as a cocatalyst increased the photocatalytic O2 evolution. Four different LaTiO2N samples were prepared from two varied La2Ti2O7 precursors by thermal ammonolysis with and without flux. The most effective transformation from La2Ti2O7 into LaTiO2N in terms of crystallinity and nitrogen contents was obtained in the following two cases: flux-assisted ammonolysis of La2Ti2O7 characterized by a comparatively small particle size and a large surface area and ammonolysis without flux of highly crystalline La2Ti2O7. In both cases, high activity for O2 evolution under visible-light illumination (λ ≥ 420 nm) (∼50 μmol h−1) was obtained. Loading LaTiO2N with CoOx enhanced the photocatalytic activity for O2 evolution, although the effect of CoOx depended on the morphologies of LaTiO2N as a support. The promotional effect was most pronounced for LaTiO2N with a skeletal morphology, because such LaTiO2N contain most unsaturated bonds that act as nucleation centers that in turn generate small and highly dispersed CoOx nanoparticles.

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improved crystallinity,18,22 which led to the higher activities of (oxy)nitride photocatalysts. Because the manifold requirements on a semiconductor (efficient light absorption, charge separation and transport, and surface redox reactions) are difficult to meet by a single-component material, heterostructures of photocatalysts and cocatalysts emerge as indispensable, and further work on both aspects is ongoing.23 Cocatalysts loaded on photocatalysts can enhance photocatalytic activity by providing active centers for the oxidation/ reduction reactions and by improving charge carrier separation and lifetime. For example, LTON exhibited a high quantum efficiency of 27.1% at 440 nm for water oxidation after loading with CoOx cocatalyst.18 In fact, a class of cobalt-based cocatalysts24,25 such as Co3O4,26 LaCoO3,27 and CoOx,28,29 was reported to improve the H2O oxidation reaction kinetics and hole lifetime. They showed comparable or even better activity than the state-of-the-art IrO2 while being more earthabundant and affordable.18,22 The above-mentioned examples certainly show the importance of improving properties of the individual components in heterostructure approaches. However, particle morphologies of cocatalysts and photocatalysts as

INTRODUCTION Scarcity and increasing demand of energy sources such as oil and gas and awareness of severe environmental harm due to their combustion are accelerating the search for renewable, environmental-friendly energies. One attractive alternative concept is the direct energy transfer from solar radiation into chemical energy by using a semiconductor that absorbs photons and releases the energy gain via photocatalytic or photoelectrochemical water splitting into H2 and O2.1−4 Oxynitrides with a bandgap 1000

0.0811

2

without flux with flux without flux with flux

PC-LTON PC-LTON-Flux SS-LTON SS-LTON-Flux

Polymerized-complex synthesis. bSolid-state synthesis with flux. cEstimated from TEM.

crystallite size and strain from the peak broadening, the XRD patterns were treated with a Le Bail fit using the FullProf software.30 A transmission electron microscope (TEM) in imaging mode with an acceleration voltage of 300 kV (Philips CM30) was used to assess the morphology and the local crystallinity of the samples. A JEM 2000FS TEM/STEM equipped with a highangle annular dark field (HAADF) detector and an in-column filter was used to acquire the electron energy-loss spectra (EELS) in scanning transmission microscopy (STEM) mode with an exposure time of 4 s, a convergence half angle of 10.8 mrad, and an acceptance half angle of 15 mrad. The energy resolution was 1.3 eV obtained by measuring the full width at half-maximum (fwhm) of the zero loss peak. For TEM sample preparation, the as-synthesized powders were suspended in ethanol and deposited on a copper grid with a holey carbon thin film. High-resolution TEM (HRTEM) micrographs and STEM/EELS were acquired only on particles suspended over a hole in the carbon film. The chemical composition of the CoOx nanoparticles was studied. First, EELS in the energy range of Ti L2,3 and O K-edge and energy-dispersive X-ray (EDX) spectra were acquired (spot illumination with exposure times of 4 and 30−40 s for EELS and EDX, respectively). The La M4,5 edge was acquired together with the Co L2,3 edge either in single spectrum mode or as line scans from the margin of LTON throughout the CoOx particle with a step size of ∼1.4 nm. Co spectra of at least two different particles for each sample were compared and summed up to improve the signalto-noise ratio. The L3/L2 ratio of the Co edge gave an insight into the oxidation state of the cobalt species.31,32 The methodology of determining the ratio was adopted from Tan et al.33 The Co oxidation state was estimated relative to Co2O3 and CoO as references; however, absolute oxidation states were not determined. A scanning electron microscope (SEM), type FEI NovaNanoSEM, was used to study the particle morphology and size. The samples were carbon-coated, and the beam conditions were a 10 kV acceleration voltage with 5 to 6 mm working distances. Two types of secondary electron detectors were used; that is, for the magnification of 2500× an Everhart Tornley detector (ETD) was used, and for the magnifications 10 000−50 000× (insets in Figure 2), a through lens detector (TLD) was used. The particle sizes were approximated by averaging over 40 particles for each sample. The Brunauer−Emmett−Teller (BET) surface area was assessed with a Micromeritics ASAP 2020 by adsorption of N2 at −196 °C. Thermogravimetric analysis (TGA) was done with a NETZSCH STA 409 CD thermobalance to extract the weight gain during thermal reoxidation of LTON to LTO. Aliquots of 50−60 mg were heated with 10 °C min−1 to 1500 °C in 40 mL min−1 synthetic air. From the weight change Δm, the nitrogen stoichiometry (y) in oxynitride (considered as

well as their influence on photocatalytic activity have not been in the center of many studies concerning oxynitrides until now. Here the morphological control of powdered LTON photocatalysts was systematically investigated. The morphology was varied by preparing the powders with or without flux during the oxide and oxynitride synthesis. The influence of the oxide precursor and the flux during ammonolysis on nitridation processes and photocatalytic activities was studied. Furthermore, CoOx cocatalysts were loaded on the different LTON samples, and the effectivity of the cocatalyst depending on the LTON as support is discussed.

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EXPERIMENTAL SECTION La 2 Ti 2 O 7 (LTO) precursors were prepared by either polymerized-complex (PC) synthesis or by solid-state (SS) synthesis with flux (NaCl/KCl). For the PC synthesis, Ti{OCH(CH3)2}4 (Sigma-Aldrich, ≥ 99.999%), La(NO3)3·6H2O (Merck, > 99%), C6H8O7 (citric acid, Alfa Aesar, ≥ 99%), and CH3OH (methanol, Sigma-Aldrich, puriss) were added to C2H6O2 (ethylene glycol, Merck, > 99.5%) in the molar ratio of 1/1/6/15/30, respectively. After the complexation under reflux at 80 °C for 4 h, the organic matrix was carbonized at 300 °C. LTO was obtained after calcination at 1000 °C for 6 h (PC-LTO). In the SS synthesis, La2O3 (Alfa Aesar, ≥ 99.99%) and TiO2 (Merck, > 99%) were dried for 4 h at 500 °C. They were weighed in stoichiometric ratios with a flux (NaCl/KCl/oxides 0.5/0.5/1 by weight) and mechanically mixed for 1 h in ethanol. After calcination at 1150 °C for 5 h, LTO was obtained (SS-LTO). Nitridation reactions of 2 g batches of LTO were performed in an alumina cavity reactor using a NH3 flow of 200 mL min−1 at 950 °C for 16 h. The samples were quenched to room temperature under a NH3 atmosphere. Certain reactions were carried out in the presence of flux (NaCl/KCl/LTO 0.5/0.5/1 by weight), leading to four different sample types PC-LTON, PC-LTON-Flux, SS-LTON, and SS-LTON-Flux, where the suffix “Flux” indicates a fluxassisted nitridation. The preparation scheme is summarized in Table 1. Powders synthesized with flux (oxide and oxynitride) were washed with deionized H2O until the complete removal of the flux was verified by testing for AgCl precipitation with aqueous AgNO3. All samples were ground in an agate mortar and dried at 200 °C for 2 h before subsequent analysis and measurements. For the cocatalyst deposition, the oxynitrides were impregnated with an aqueous solution of Co(NO3)2·6H2O corresponding to 2 wt % Co ions. Then, the sample was treated in 100 mL min−1 NH3 at 700 °C for 1 h and subsequently in air at 200 °C for 1 h.18 X-ray diffraction (XRD) patterns were collected from 20 to 80° (2θ) with an angular step interval of 0.017° and a counting time of 1.27 s per step with a PANanalytical X′Pert PRO θ-2θ scan system (Bragg−Brentano geometry) equipped with a Johansson monochromator (Cu Kα1 radiation, 1.540598 Å) and an X′Celerator linear detector. To determine the apparent B

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LaTiO3.5−1.5yNy)5 was calculated according to y = (2Δm/((3/2)M(O 2 ) − M(N 2 ))·(M(La 2 Ti 2 O 7 )/ m(La2Ti2O7)). M(O2) and M(N2) are the molar masses of molecular oxygen and nitrogen, and m(La2Ti2O7) and M(La2Ti2O7) are the weight and molar mass of La2Ti2O7. The assumptions for this calculation are that cations are present in their highest oxidation state before and after reoxidation, reactant and product are both stoichiometric, and Δm is solely attributable to the exchange of nitrogen with oxygen. The error in terms of y is ±0.01 determined by the error of the balance (0.03 mg, certified by the manufacturer). X-ray photoelectron spectra (XPS) were acquired with a PHI Quantum 2000 operating with a monochromatic Al Kα (1486.6 eV) X-ray source and an electron takeoff angle of 45°. Spectra were obtained with pass energies of 117.4 eV and referenced to C 1s at 284.8 eV of adventitious carbon species. Diffuse reflectance spectra (DRS) were obtained over a spectral range of 400−850 nm (3.1 to 1.5 eV) with a UV-3600 Shimadzu UV−vis-NIR spectrophotometer equipped with an integrating sphere. The bandgap Eg was extrapolated from Tauc plots with hν (x-axis) and (αhν)1/n (y-axis) with n = 1/2 for a direct allowed transition assumed for LTON.34,35 The point of intersection of the line tangent to the plotted curve with the hνaxis gives the value for the bandgap. The error of this method is estimated to be ±0.05 eV.34 The absorption coefficient α was approximated by the Kubelka−Munk equation F(R) = ((1 − R)2)/(2·R), where R is the absolute reflectance of the sample.36 Photocatalytic O2 evolution reactions under visible-light illumination (300 W Xe lamp, λ > 420 nm cutoff filter) were carried out in a glass vessel containing a 200 mL aqueous suspension of 0.1 g oxynitride powder, 0.2 g La2O3 for pH adjustment to 8 to 9,11 and 10 or 40 mM AgNO3 as an electron scavenger. For reactions with CoOx-loaded LTON, the AgNO3 concentration was increased to 40 mM to ensure that the presence of electron scavengers was not rate-limiting due to the expected higher activity of the cocatalyst loaded samples. Note that the concentration of AgNO3 had a minor effect on the O2 evolution activity. SS-LTON-Flux without cocatalyst showed comparable activities of 25 and 28 μmol h−1 at the AgNO3 concentrations of 10 and 40 mM, respectively. The reaction vessel was connected to a closed-circulation system equipped with a vacuum pump and a gas chromatograph with a TCD detector using Ar as a carrier gas. After repeated evacuation, 100 mbar Ar was injected, and the solution was continuously stirred and kept at a constant temperature by a cold-water jacket. After starting visible-light illumination from a Xe lamp, gas volumes were taken at given time intervals. O2 evolution rates were estimated from the first hour of the reaction because the particles are progressively shaded by the deposition of Ag during the reaction, leading to a decreased O2 evolution.8 The reproducibility of such reactions was previously estimated to be within 10%.18

Figure 1. TEM images of the precursor oxides (a) PC-LTO and (b) SS-LTO.

SS-LTO were larger compared with PC-LTO, as evidenced by the smaller full width at half-maximum (fwhm) values of the (211) reflections in the corresponding XRD patterns (Table 1). The oxide precursors were transformed into LaTiO2N (LTON) via thermal ammonolysis, as evidenced by the corresponding diffractograms (SI Figure S1), and the synthesis details are summarized in Table 1. Both precursors, PC-LTO and SS-LTO, were nitrided with and without a flux (NaCl/KCl). The four resulting samples are referred to hereafter as PC-LTON, PC-LTON-Flux, SS-LTON, and SSLTON-Flux. No reflection shift in the diffractograms of oxides or oxynitrides due to a possible incorporation of Na+ or K+ was observed. XPS did not provide any evidence of surface contamination by flux more than the detection limit of 1% after washing the powders (SI Figure S2). Attributes of the four different LTONs including crystallite size, BET surface area, nitrogen content, bandgap value, and relative strain are listed in Table 2. For the ammonolysis Table 2. Crystallite Size, BET Surface Area, Nitrogen Content, Bandgap, and Relative Strain of the Oxynitrides sample

crystallite size (nm)a

BET surface area (m2 g−1)

nitrogen stoichiometry y (−)

relative strain (−)a

PC-LTON SS-LTON PC-LTON-Flux SS-LTON-Flux

36 40 44 38

11 15 8 9

0.73 0.75 0.79 0.69

15 23 14 21

a

Crystallite size and strain were extracted from the microstructural analysis within the software FullProf after a LeBail fit of the XRD patterns.

without flux, the less crystalline PC-LTO precursor with small particle size led to an oxynitride (PC-LTON) consisting of strongly aggregated particles with irregular shape (Figure 2a). The size of discriminable units within the agglomerates ranged from 50 to 400 nm with an average of 200 nm (measured over 40 particles in SEM images), and the crystallite size determined with the Scherrer equation was detected to be lowest (36 nm) among all samples. The discrepancy between crystallite and particle size was certainly caused by the fact that a particle consists of several crystallites. The TEM image in Figure 3a further illustrates pore formation as it has been previously reported for LTON.19,37 The conversion of SS-LTO to SSLTON by ammonolysis without flux was pseudomorphic because the size and contour of the precursor oxide particles were predominantly preserved. SEM shows cuboid particles with the short sides measuring ∼300 nm and the plain

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RESULTS AND DISCUSSION The particles of the oxides prepared by the polymerizedcomplex synthesis (PC-LTO) were irregularly shaped and agglomerated with a broad size distribution in the range of 50 to 300 nm (TEM images in Figure 1). With the flux-assisted solid-state synthesis, well-dispersed, relatively large (micrometer range) and faceted La2Ti2O7 particles (SS-LTO) were obtained. Correspondingly, the surface area of SS-LTO was only half as large as the one of PC-LTO. The crystallite sizes of C

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formation was explained by the transformation from a layered perovskite-type oxide into the simple perovskite-type oxynitride accompanied by a lattice condensation due to the exchange of 3 O2− with 2 N3−. Also, the formation of porous Ta3N5 from Ta2O5 was attributed to lattice shrinkage.17,39 A similar mechanism was expectable for LTON. Like the study by Park and Kim, the pore development was more pronounced for SSLTON with the large precursor oxide than for PC-LTON. The lattice strain was highest for SS-LTON, as evidenced by the XRD patterns, which corroborates well with the idea of structural transformation in large and rigid crystallites. Flux used during ammonolysis of PC-LTO resulted in the oxynitride PC-LTON-Flux with faceted particles of homogeneous appearance ranging from 100 to 400 nm with an average of 200 nm (Figure 2c). The particles were dense and highly crystalline (Figure 3c), and the crystallite size of 44 nm was the largest among the four samples. In line with that, the surface area was with 8 m2 g−1, the lowest among all of the samples. The absence of pores could be explained by the presence of the flux evoking dissolution and recrystallization steps and facilitating ionic diffusion during the ammonolysis. This has already been observed for porous Ta3N5 particles that were transformed into nonporous particles when post-treated in a flux.17 The lattice strain of PC-LTON-Flux was found to be the lowest. It seemed reasonable because the strain accumulation upon structural transformation in small and less rigid oxide particles was expected to be lower; furthermore, during dissolution and recrystallization, the lattice strain could be partially released. The sample SS-LTO ammonolyzed in the presence of flux, SS-LTON-Flux, displayed a distinct skeletal morphology (Figure 2d). The outline of the perforated particles was taken as an estimate for particle sizes, and it was found that they are in the same range as the ones obtained for SS-LTON. Also, TEM (Figure 3d) confirms that the size of the precursor oxides was preserved during the ammonolysis similar to SSLTON. The crystallite sizes were found to be rather small (38 nm). In contrast with PC-LTON-Flux, the particles were seemingly too large and rigid for a complete restructuration, leading to the formation of a nonporous, skeletal structure. All oxynitride samples were nitrogen-deficient (0.69 ≤ y ≤ 0.79). This is typical for the gradual transformation from the layered perovskite-type phase of the general formula AnBnO3n+2 with n = 4 (La2Ti2O7) into the 3D perovskite-type phase with n = ∞ (LaTiO2N).20,40,41 On the atomic scale and without periodic ordering, thus not detectable in XRD patterns, oxygen-rich intermediate phases with 4 < n < ∞ are expected to be present. Such phases richer in oxygen compared with the n = ∞ phase are considered to be responsible for nitrogen deficiency.41 It was not intended to synthesize a nearstoichiometric composition because extended nitridation times would lead to partially reduced titanium species at the surface.21,40 Moreover, it was found that a not fully transformed sample showed higher photocatalytic activity than a sample close to stoichiometric LTON.40 Hence, with prolonged ammonolysis time, there seems to be a trade-off between diminishing the defects related to oxygen-rich phases and the increasing Ti3+ defects. In a previous study, 16 h of ammonolysis was identified as the ideal treatment period regarding the photocatalytic activity with the setup used in this work.42 The highest nitrogen content was detected for PCLTON-Flux (y = 0.79) and the lowest was detected for SSLTON-Flux (y = 0.69), while the other two had intermediate nitrogen contents. The different nitrogen contents of PC-

Figure 2. SEM images of (a) PC-LTON, (b) SS-LTON, (c) PCLTON-Flux, and (d) SS-LTON-Flux.

Figure 3. TEM overview images of the oxynitrides. (a) PC-LTON: The inset represents the fast Fourier transform (FFT) of an HRTEM micrograph acquired on the particle edge illustrating high crystallinity. (b) SS-LTON: The inset zooms on a particle edge displaying the high porosity. (c) PC-LTON-Flux: The insets show a magnified zone in high-resolution mode and the corresponding FFT. (d) SS-LTONFlux.

measuring ∼1500 nm on average (Figure 2b). TEM indicates that the particles were pervaded by a highly porous network (Figure 3b) leading to the largest surface area of 15 m2 g−1. A similar morphology for LTON obtained from a relatively large and highly crystalline LTO precursor was reported by Zhang et al.18 Park and Kim38 studied pore formation in LaTaON2 prepared from La2Ta2O7 particles with different sizes, and the extent of pore formation was observed to be highest for the largest (micrometer range) precursor oxides. The pore D

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on the photocatalytic activity because charge-carrier recombination is reduced.8,44 Hence, the O2 evolution activity could be reasonably explained by the bulk characteristics of the different LTONs in accordance with previous observations.18,22,40 The LTON samples were loaded with 2 wt % Co, which was transformed after post-annealing in NH3 followed by air into CoOx.18 Figure 4b shows the photocatalytic activity of the four LTON samples loaded with CoO x nanoparticles (LTON/CoOx). In all cases, loading of CoOx as an O2 evolution catalyst improved the photocatalytic O2 evolution activity. SS-LTON-Flux with the lowest O2 evolution rate before the cocatalyst loading exhibited the highest activity among all samples (260 μmol h−1) after CoOx loading (SSLTON-Flux/CoOx). The activity was enhanced by a factor of 10. PC-LTON-Flux with a high O2 evolution rate resulted in the smallest improvement (by a factor of 1.2). Note that only a two-fold enhancement was obtained for SS-LTON/CoOx, which is in contrast with the previously reported 30-fold improvement obtained with a similar sample by Zhang et al.18 To gain deeper understanding on the activity of the cocatalyst on different LTON supports, we chose SS-LTONFlux/CoOx and PC-LTON-Flux/CoOx, the most and the least effective cases, for further microscopic studies. TEM (Figure 5)

LTON-Flux and SS-LTON-Flux could be explained by the two following arguments. The particle size of the precursor PCLTO was significantly smaller than the one of SS-LTO, which facilitated nitrogen diffusion in the former sample. It is known that amorphous precursor oxides with small particle sizes are more reactive and more easily transformed into oxynitrides.5,43 Furthermore, as the nitridation is a solid−gas exchange reaction, an excess of molten phase (flux) can hinder exposure of the solid to the gas phase.17 Because the surface area of SSLTO was half as large as that of PC-LTO, the molten flux might have covered up the surface of SS-LTO too much, thereby lowering the contact with the gas phase. Extracted from Tauc plots, the bandgaps varied between 2.10 (SS-LTON) and 2.18 eV (SS-LTON-Flux), which is within the measurement error. The photocatalytic O2 evolution activity under visible light (λ > 420 nm) of the different LTON samples in the presence of AgNO3 as an electron acceptor is summarized in Figure 4a. PC-

Figure 4. Photocatalytic O2 evolution rates of 0.1 g catalyst in aqueous solution (200 mL) containing 0.2 g La2O3 (pH buffer) under illumination of a 300 W xenon lamp (λ > 420 nm). (a) LTON and 10 mM AgNO3 as electron scavenger. (b) LTON loaded with 2 wt % CoOx cocatalyst and 40 mM AgNO3. Numbers represent the factor of improvement with respect to the O2 evolution rate without cocatalyst.

Figure 5. Micrographs of SS-LTON-Flux/CoOx (a) in STEM mode where the darker gray spheres at the edges of LTON particles are CoOx (marked with circles) and (b) zoom on a representative CoOx particle in HRTEM mode. Micrographs of PC-LTON-Flux/CoOx (c) in TEM mode illustrating strong CoOx aggregation and (d) zoom in HRTEM mode of a representative, strongly aggregated CoOx particle.

clearly illustrates morphological differences of CoOx nanoparticles depending on the supports. On SS-LTON-Flux/ CoOx, the CoOx cocatalyst particles were regularly dispersed with a size of 13 ± 1.2 nm (measured over six CoOx particles). On PC-LTON-Flux/CoOx, CoOx particles were irregularly dispersed with accumulations on the smaller PC-LTON-Flux particles (Figure 5c). Furthermore, they were present in two size distributions; that is, around two-thirds had a size of 35 ± 4.2 nm (measured over 18 particles) and one-third had a size of 12 ± 1.5 nm (measured over six particles). In both SS-LTON-

LTON-Flux and SS-LTON exhibited a similar activity for photocatalytic O2 evolution (48 and 53 μmol h−1, respectively) that was larger compared with PC-LTON (37 μmol h−1) and SS-LTON-Flux (25 μmol h−1). Both PC-LTON-Flux and SSLTON were characterized by the largest crystallite sizes and highest nitrogen contents among the four LTON samples. A higher nitrogen content may indicate fewer bulk defects associated with intermediate phases (4 < n < ∞), which was partially reflected in the larger crystallite sizes. High crystallinity and fewer bulk defects are known to have a positive influence E

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reported.27,48,49 Harriman et al.26 asserted that an effective oxide catalyst should be stable below its maximal attainable oxidation state to accumulate oxidizing equivalents. The lower initial oxidation state as found for SS-LTON-Flux/CoOx might have been beneficial in this regard. In addition, the bandgap alignment at the junction between LTON and CoOx would be dependent on the cobalt oxidation states, while on the basis of the presented data it was impossible to judge its influence on the hole transfer across the heterojunction. In summary, the results regarding particle size and dispersion emphasized that the effectivity of CoOx was not only determined by the amounts of cocatalyst and the loading step (synthesis) itself but also by the nature of LTON as a support.

Flux and PC-LTON-Flux, CoOx nanoparticles were polycrystalline with domain sizes of ∼3 nm. As it is known that the support has an influence on crystallization and dispersion of cocatalyst nanoparticles,26 the observed differences in CoOx dispersion were likely related to the supporting LTON. It should be noted that the surface area of both LTON samples was comparable and thus would not influence the dispersion of CoOx. Considering the complex and skeletal morphology of SS-LTON-Flux, this sample should contain more dangling bonds compared with the faceted and smooth surfaces of PCLTON-Flux. Thus, the density of nucleation centers in the form of dangling bonds was higher for SS-LTON-Flux, which presumably led to well-dispersed CoOx nanoparticles. EELS line scans throughout particles and EDX analysis in a single spectrum mode were recorded to verify the chemical composition of the different CoOx particles. They evidenced that CoOx did not contain Ti within the sensitivity of the experiment. However traces of La were found in almost all particles (SI Figure S4). The Co L2,3 edges were similar within one particle and independent of the presence of La, as it was confirmed on several (3−6) CoOx nanoparticles for each sample by STEM/EELS line scan analysis. For SS-LTON-Flux, they were also very similar comparing different CoO x nanoparticles, while small differences between the nanoparticles were found in the case of PC-LTON-Flux. The L3/L2 ratio of the Co edge obtained from sum spectra over several CoOx particles showed that the cobalt species had an average oxidation state between Co2+ and Co2.7+. Differences in the Co L3/L2 ratios among the LTON supports were found for PCLTON-Flux compared with SS-LTON-Flux (SI Figure S5). They translated into a small difference of the average oxidation state; that is, the Co2+ content of SS-LTON-Flux/CoOx was higher compared with PC-LTON-Flux/CoOx. In conclusion, CoOx particle size and dispersion varied between SS-LTONFlux/CoOx and PC-LTON-Flux/CoOx, while the average crystallite size of CoOx, the impurity levels, and the oxidation state of cobalt were very similar. High dispersion of the cocatalyst is essential because it allows for a larger surface area. In fact, highly dispersed cocatalyst nanoparticles were found to improve the photocatalytic and photoelectrochemical activity, as reported for GaN:ZnO and TaON.45−47 The influence of the Co3O4 particle size on the electrochemical activity was thoroughly investigated, and it was determined that the activity increased proportionally with the surface area of the particles.48 In our study, the difference in surface areas estimated from the sizes of CoOx nanoparticles varied by a factor of ∼3 between SS-LTON-Flux/CoOx and PC-LTON-Flux/CoOx. It did not match the difference in the activity enhancement by the cocatalyst loading that was a factor of 8. Therefore, surface areas of CoOx cannot have been the only activity-determining factor of the catalyst on a support. It was considered that well-dispersed CoOx nanoparticles would allow covering the support more homogeneously. Hence, on average, a shorter hole diffusion path from the excitation site to the cocatalyst was expected. Assuming constant crystallite sizes, small CoOx particles indicated that photoexcited holes injected from LTON had to migrate through fewer crystallite boundaries within CoOx to reach the outermost surface. As previously described, the dispersion of the cocatalyst was better for SS-LTON-Flux/CoOx, which certainly explained its higher photocatalytic activity compared with PC-LTON-Flux/CoOx. Furthermore, the cobalt oxidation state was expected to influence the O2 evolution activity, as it has been previously

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CONCLUSIONS

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ASSOCIATED CONTENT

For La2Ti2O7 with small particle sizes, using a flux during nitridation resulted in higher crystallinity and higher nitrogen contents (PC-LTON-Flux) compared with LaTiO2N prepared without flux (PC-LTON). As for the large and highly crystalline La2Ti2O7, the flux was not effective enough to completely dissolve the particles, leading to a skeletal morphology of LaTiO2N (SS-LTON-Flux), while nitridation of the same precursor without flux evoked a highly porous LaTiO2N (SSLTON). The O2 evolution activity was positively influenced by high crystallinity and high nitrogen contents presumably because the electron−hole recombination rate at defective sites was suppressed. The catalytic effect of CoOx cocatalyst depended largely on its dispersion. The dispersion of CoOx nanoparticles was better on supporting LaTiO2N with an irregular, skeletal morphology (SS-LTON-Flux) compared with a LaTiO2N with dense and homogeneous particles (PCLTON-Flux). It was considered that this peculiar skeletal and edgy morphology contained more unsaturated bonds, providing a higher density of nucleation centers for CoOx nanoparticles improving the dispersion of the cocatalyst.

S Supporting Information *

XRD patterns of the oxide and oxynitride samples, XPS spectra of the sample ammonolyzed with flux, Tauc plots of the oxynitrides, HAADF overview image of CoOx particles with STEM/EELS line scan, and EELS sum spectra of CoOx particles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +41-58-765-4212. Fax: +41-58-765-4019. E-mail: Anke. Weidenkaff@empa.ch. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the European Commission (Project NanoPEC, contract number 227179) and Grant-in-Aids for Specially Promoted Research (no. 23000009) and for Young Scientists (B) (no. 25810112) of the Japan Society for the Promotion of Science (JSPS). We gratefully acknowledge the technical support by R. Erni and D. Schreier from the EMPA-EM center. F

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