Article pubs.acs.org/JPCC
Mesoporosity in Photocatalytically Active Oxynitride Single Crystals Simone Pokrant,*,† Marie C. Cheynet,‡ Stephan Irsen,§ Alexandra E. Maegli,† and Rolf Erni∥ †
Laboratory of Solid State Chemistry and Catalysis, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland ‡ CNRS, SIMAP, Grenoble University, F-38000 Grenoble, France § Electron Microscopy and Analysis, CAESAR, Center of Advanced European Studies and Research, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany ∥ Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland S Supporting Information *
ABSTRACT: Mesoporosity in photocatalytically active oxynitride single crystals and single-crystalline zones has been investigated by transmission electron microscopy techniques including nanobeam diffraction, electron energy loss spectroscopy, electron tomography, and high-resolution imaging. Several particle morphologies of the perovskiterelated oxynitride LaTiO2N were synthesized by solid-state and polymer complex synthesis of the La2Ti2O7 precursor followed by thermal ammonolysis. A detailed analysis of pore sizes, pore shapes, and lattice defects and the local analysis of oxidation states allowed correlation between morphology, synthesis procedures, chemical and crystal defects, and photocatalytic activity. A pore formation mechanism via lattice condensation is proposed, which is simultaneously linked to lattice defect formation processes. On the basis of mechanistic understanding of the transformation from oxide to oxynitride, mesoporosity, and hence the photocatalytic or photoelectrochemical properties of the material, can be tuned.
1. INTRODUCTION Mesoporosity in single crystals has lately received much attention,1 since it is expected that this material class can contribute significantly to the improved design of highly efficient solar cell devices. Especially for photocatalytic or photoelectrochemical applications, high surface area and good charge-transport properties are key to enhanced device performance.2,3 Good conductivity is usually obtained in defect-free structures such as single crystals, where the surface area is small. High surface area, however, is obtained best by a porous agglomeration of nanoparticles. In consequence, the conductivity is low because of the multiple grain boundaries. One possibility to achieve performance improvement consists in the fabrication of nanostructured layers with high crystalline quality, where the surface is enhanced but the charge transport still good. The effects of different kinds of nanostructuring procedures on device efficiency, in comparison to single crystals or nanoparticles, have been studied extensively, for example, on Fe2O3 electrodes and have been reported in Kay et al.4 and references therein. Improving the nanostructure, or in other words finding a better trade-off between large surface and good charge transport, is still considered as one of the most important parameters to obtain better device performance.2,5 Very recently a new synthesis method has been proposed, leading to a new approach: fabrication of large mesoporous single crystals on the micrometer scale based on templates.6 The improved electron conductivity of mesoporous single © 2014 American Chemical Society
crystals in comparison to nanocrystalline material has been demonstrated as well as their large surface area. In this paper we would like to draw attention to an alternative route to obtain mesoporous structures in large single crystals without using a template. This route consists of a solid−gas reaction as carried out for the synthesis of oxynitrides. Oxynitrides, including the subgroup of perovskite-related oxynitrides, are known for their wide range of applications7 and are specifically considered as an interesting material class for optical applications.8 One of the synthesis strategies to transform an oxide powder into its oxynitride is thermal ammonolysis: a solid−gas reaction, where the precursor oxide is placed in an ammonia flow at elevated temperatures (>600 °C).9 Several groups have observed that pores are formed in some of the compounds during ammonolysis. Park and Kim10 estimated for LaTaON2 mesopores of up to 30% of the apparent particle volume, while for EuTi(O,N)311 and for TaON and Ta3N512 the pore formation was only mentioned. At present LaTiO2N (LTON) is the material that has been studied most intensively in terms of pore formation and its influence on photocatalytic properties, in addition to a thorough physical and chemical characterization.13−15 Interestingly, better photocatalytic performance was observed for LTON samples that Received: June 2, 2014 Revised: August 18, 2014 Published: August 18, 2014 20940
dx.doi.org/10.1021/jp506597h | J. Phys. Chem. C 2014, 118, 20940−20947
The Journal of Physical Chemistry C
Article
a NH3 flow of 200 mL·min−1 at 950 °C for 16 h. The nitrided PC-LTO (PC-LTON) was ground in a mortar and dried at 200 °C for 2 h before subsequent analysis and measurements. This synthesis is similar to that of-PC-LTON, as reported in ref 14. SS-LTON was also synthesized from an LTO precursor fabricated via the solid state synthesis route (SS-LTO). The synthesis is equivalent to the one presented in ref 14. A transmission electron microscope with an acceleration voltage of 200 kV (JEOL JEM 2000FS TEM/STEM), equipped with a high-angle annular dark field (HAADF) detector and an in-column Omega-type energy filter, was used to assess the morphology and local crystallinity of the samples by lowmagnification imaging, electron tomography, and highresolution electron micrographs (HREM). HREM images were further processed by Wiener filtering in the Digital Micrograph routine. For electron tomography, a JEOL single-axis tomography holder was used. STEM/HAADF images were manually recorded at 2° tilt intervals over a range from −76° to 70°. The images were aligned by use of the StackReg plugin of the image-processing software ImageJ. The 3D volume reconstruction was computed by 80 cycles of the simultaneous iterative reconstruction technique (SIRT) implemented in TomoJ, which is also an ImageJ plugin. Finally, the 3D voxel projection and the orthoslices were generated with the Amira visualization program. The same software was used to determine the relative intrinsic pore volume by integrating over all closed holes in the particle volume versus the total particle volume. To link the volume information from tomography with crystal structure information, high-resolution scanning transmission electron microscopy (HRSTEM) images were acquired from different zones of the particles. Electron energy loss spectroscopy (EELS) experiments were carried out acquiring line scans in STEM mode with a nominal spot size of 0.7 nm, a convergence half angle of 10.8 mrad, and a collection semiangle of 15 mrad. The experimental energy resolution of 1.3 eV was measured at the full width at halfmaximum (fwhm) of the zero loss (ZL) peak recorded in the vacuum. Spectrum acquisition was performed with a 2K × 2K camera with 5 s exposure time and a dispersion of 0.11 eV/ channel. Each line scan was composed of 100 spectra acquired with a step size of 0.45 nm. Successive spectra (up to five spectra) were summed up to improve the signal-to-noise ratio. For each line-scan spectrum, the background was subtracted with the assumption of an exponential power law. The spectra were processed by use of the quantification procedure within the Digital Micrograph routine. The N concentration was evaluated with respect to Ti concentration by integrating the signal over the N K-edges and Ti L2,3-edges in an energy window of 25 eV and calculating the ratio. For N K-edges, Ti L2,3-edges, and O K-edges, electron energy loss near edge structure (ELNES) analysis, a CRISP (Zeiss) fitted with a CEOS Cs-probe corrector, an omega-type monochromator, and a second-order aberration corrected in-column omega-type energy filter was used. The microscope operated at 200 kV with a Schottky field emission gun equipped with a Kö hler illumination system. Experiments were performed in STEM mode by use of an electron probe of 0.5 nm and a ZL width of 0.54 eV measured under vacuum at fwhm. The acquisition time per spectrum was fixed to 4 s. Spectra were acquired on a 2K × 2K camera, a 0.05 eV/channel energy dispersion, a beam convergence semiangle of 16 mrad, and an effective spectrometer collection semiangle of 17 mrad. The TEM
contained less than the stoichiometric amount of nitrogen, typically only about 70−80%, although their crystalline quality was inferior in comparison to the stoichiometric compound.16,17 In all material systems mentioned above, the shape and size distribution of the precursor oxide particles were preserved during ammonolysis without flux, independent of the crystalline quality and average size range of the precursor.10,12,13 In the case of LTON, mainly two different precursor oxide morphologies have been used for transformation of La2Ti2O7 (LTO) into its oxynitride. First, large brick-shaped singlecrystalline LTO particles were prepared via a solid-state (SS) chemistry route with an average size larger than 1 μm, leading to highly porous single-crystalline SS-LTON particles.13,14 Second, small LTO particles (50−300 nm) with lower crystalline quality were synthesized via a polymerized complex (PC) route, resulting in highly porous PC-LTON particles of the same size range as the educt LTO with very high crystalline quality.14 However, if flux (NaCl/KCl) was used during thermal ammonolysis, pore formation was suppressed in both cases and the particle size distribution and/or shape changed.14 As expected, the micrometer-size single-crystalline SS-LTON achieved superior results in terms of photocatalytic and photoelectrochemical properties.13−15 Up to now, the main characterization techniques employed to estimate pore quantity have been bulk methods like Brunauer−Emmett−Teller (BET) surface and Barrett−Joyner−Halenda (BJH) pore size, giving information about open porosity. Pore morphology was studied by scanning electron microscopy (SEM) and by transmission electron microscopy (TEM). These studies suggested that open and closed pores are formed.10,13,14 However, little is known about the size and shape distribution, especially for closed porosity. Concerning the pore formation process, it was presumed that spontaneous pore formation takes place to compensate for the lattice shrinkage occurring during the transformation of oxide into oxynitride.10 In this paper we will focus on microscopic pore characterization with the aim to correlate it with already known bulk characterization results. On the basis of electron tomography, electron diffraction, high-resolution transmission electron microscopy, and electron energy loss spectroscopy, we will show which types of pores and pore morphologies are created during thermal ammonolysis and give an explanation for the formation mechanism of different pore types. We chose to perform this study mainly on LTON particles prepared by PC synthesis, since their size range is more adapted to analysis on the nanoscale in transmission mode. With improved understanding of the solid-state gas reaction, we would like to open a door to further tune the porosity in large oxynitride single crystals like SS-LTON and/or to apply it to other precursor oxides.
2. EXPERIMENTAL SECTION The LTO precursor was prepared by polymerized complex (PC) synthesis. Ti[OCH(CH 3 ) 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 1/1/6/15/30, respectively. After complexation under reflux at 80 °C for 4 h, the organic matrix was carbonized at 300 °C. PC-LTO was obtained after calcination at 1000 °C for 6 h. The nitridation reaction of a 2 g batch of LTO was performed in an alumina cavity reactor with 20941
dx.doi.org/10.1021/jp506597h | J. Phys. Chem. C 2014, 118, 20940−20947
The Journal of Physical Chemistry C
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samples were tilted out of zone axis to prevent possible channeling and crystal field effects in the spectra. Diffraction patterns were acquired in nanobeam mode (20− 100 nm diameter probe size) on a conventional LaB6 TEM (JEOL 3010). The experimental diffraction patterns recorded from a set of LTON nanoparticles were indexed by use of ACOM (automated crystal orientation mapping), a plugin attached to the ASTAR software.18,19 This software tool allows indexing out-of-zone axis patterns as well as in-zone axis patterns. The procedure consists of comparison of the experimental diffraction patterns with a template data bank corresponding to several thousands of possible orientations calculated from the parameters of different expected structures. For this study, the data banks corresponding to the orthorhombic structure with Imma space group reported by Yashima et al.20 and to the triclinic structure with I1̅ space group reported by Logvinovich et al.21 were used. Each experimental pattern is fully described by three angles (the Euler angles). Finding the best fit between experimental patterns and calculated templates corresponding to known Euler angles allowed us to index the diffraction patterns and identify the crystal structure of the particle. The small difference between the reported orthorhombic and triclinic structure required the matching of several different diffraction patterns in order to decide between these two possible structures. Similarly, the zone axis indexation was difficult in some cases due to the small difference between the a and the c parameter. Therefore, it was often necessary to take into account the relative intensities of the diffraction spots to discriminate between two zone axes. In the case of experimental diffraction patterns that were very close to a zone axis, comparison with diffraction patterns simulated by the JEMS software22 were also performed with success and validated the results obtained with ACOM/ASTAR. For TEM sample preparation, the as-synthesized powders were suspended in ethanol and deposited on a copper grid with a holey carbon thin film (Plano). HREM micrographs and STEM/EELS spectra were acquired only on particles suspended over a hole in the carbon foil. For electron tomography, the same suspension was deposited on a copper grid with a thin carbon foil.
Figure 1. Experimental electron nanobeam diffraction patterns indexed by use of the ACOM-TEM routine running with ASTAR: (a) [101] zone axis and (b) [113] zone axis. For both patterns, the best fits between experiment and simulation are achieved for zone axes belonging to the orthorhombic structure of Imma space group.
analysis and the fit at high order indices allowed the iterative ACOM procedure to select the [101] zone axis. All analyzed diffraction patterns confirmed that the structure of the investigated samples was orthorhombic with the space group Imma. This result correlated very well with powder X-ray diffraction (XRD) data as published in ref 14. HREM studies performed on more than 10 PC-LTON particles showed that there were hardly any zones without crystal defects. The HREM image of a typical sample area is displayed in Figure 2a. Although this area did not show any apparent disorder, detailed analysis of part of this image (corresponding to the black square), displayed at higher magnification, proved the existence of numerous crystal defects (Figure 2b). The fast Fourier transform (FFT) image corresponding to this area (see insets in Figure 2a,b) showed that the main diffraction spots are surrounded by diffuse scattering streaks and some small satellite spots. Several origins of these diffuse scattering streaks are possible. In this context, it was generally associated either with substitutional disorder and the resulting crystal relaxation or with a displacement disorder.24,25 Diffuse scattering coming from displacement disorder was often related to distortion of the octahedra and creation of a one-dimensional defect.24 Regarding the satellite spots, it is clear from the FFT associated with the HREM image in Figure 2b that the diffraction spots form “doublets”. This indicates that there are two sets of dislocations in the area. They are organized such that the first dislocation network is roughly orthogonal to the second. This is indeed what was observed in the HREM image in Figure 2b. Dislocations parallel to the image plane (indicated by arrows in the upper part of the image) and emerging in the image plane (arrow at the bottom of the image) are clearly identified. In other sample areas, more defects have been identified, either directly in HREM images or by analysis of their associated FFT. The HREM image displayed in Figure 2c shows three different types
3. RESULTS For LTON, two structure types have been already proposed: the orthorhombic Imma20 and the triclinic I1̅.21,23 Nanobeam diffraction patterns recorded from several crystalline zones of various PC-LTON particles were indexed by comparing the experimental patterns with calculated zone axes of these two structures, by use of the ACOM-TEM routine. For each pattern, the intensity of the spots and the fact that there might be a tiny off-axis tilt were taken into account. In Figure 1 panels a and b, two experimental electron diffraction patterns and the simulated [101] and [113] zone axis diffraction pattern corresponding to the orthorhombic structure are displayed. These figures illustrate that the simulated patterns perfectly fit each of the experimental patterns, even at high order indices. For crystalline areas of the particles oriented perfectly in zone axis, the same orthorhombic structure was confirmed by simulations using the JEMS software. However, it is worth noting that a doubt about the [101] zone axis determination shown in Figure 1a could exist a priori, since the [0-10] and [101] zone axes of the LTON orthorhombic structure are almost identical. In the case presented here, the spot intensity 20942
dx.doi.org/10.1021/jp506597h | J. Phys. Chem. C 2014, 118, 20940−20947
The Journal of Physical Chemistry C
Article
scale contrast variation does not shift. Here the origin of the contrast could be identified as oxygen-rich planes followed by a dense perovskite block. The precursor oxide LTO can be expected to follow this pattern, since it belongs to the [110] phases with n = 4.27 Therefore, it is reasonable to interpret these defects as remaining LTO blocks or oxygen-rich planes, where the lattice condensation associated with transformation from LTO to LTON did not take place. In Figure 3, a representative PC-LTON particle is displayed. The particle size of PC-LTON determined by TEM was
Figure 2. (a) HREM image recorded in an apparently little disordered region of the sample. (Inset) FTT calculated on the whole area, showing that the main spots are surrounded by diffuse scattering and additional satellite spots. (b) Magnified zone, corresponding to the black square in panel a. It shows the presence of numerous dislocations in the image plane as well as perpendicular to the image plane. The corresponding FFT (see inset) clearly resolved diffraction spots as doublets. The latter are the signature of a double set of orthogonally organized dislocations. (c) HREM image showing the main types of defects observed in the samples: area A, stacking fault; area B, coherent low-angle domain boundary; area C, knitting pattern. (d) Magnified HREM image of the knitting pattern, showing a sample region with contrast changes perpendicular to the plane direction. The overlaid profile indicates gray-scale intensity versus position. The grayscale intensity was obtained by integrating over the area enclosed by the gray bars. The red dotted arrow indicates a row of atom columns with changing contrast.
Figure 3. Bright-field STEM image of a PC-LTON particle. Dashed lines indicate the intercrystallite boundaries as determined by HRSTEM in several zones of the particle. For proof, the fast Fourier transforms (FFT) of the indicated zones are displayed beside the particle. The arrows mark elongated mesopores that form a 90° angle.
between 81 and 364 nm, with 206 nm average over 14 particles. It is interesting to note that the mean size of the PC-LTON particles was found to be very close to the mean PC-LTO particles size with 200 nm on average over 10 particles. We conclude that during the nitridation reaction the average particle size was not changed, as observed for SS-LTON.14 On the basis of HREM studies of different zones on several particles, we found that a particle consists of several singlecrystalline zones or grains with a size range between 50 and 200 nm and an average of 125 nm measured over seven crystallites. In the single-crystalline grains, numerous pores were formed as displayed in Figures 3, 4, and 5. Their size distribution is rather wide, varying between few and several tens of nanometers. In the reconstructed 3D volume representation and in the orthoslices (Figure 4a−c), a first kind of pore (macropore) was easily identified. It consists of large, mainly irregularly shaped pores (several tens of nanometers) and often with an outlet to the surface, that is, mostly open pores. Several smaller pore shapes (mesopores) can be distinguished: (i) circular faceted mesopores, with an average size range of 4.6 nm and a size range of 2−8 nm, and (ii) and elongated faceted mesopores, with an average length of 20 nm in the size range 6−74 nm and an average width of 4.3 nm in the size range 3−7 nm. The elongated mesopores appear in projections either nearly parallel or rectangularly aligned to each other with respect to their long axes (Figure 4b,c), which suggests that the elongation direction is linked to the crystal lattice. The elongated mesopore shapes can be distinguished in both the tomogram and the orthoslices (see Figure 4), while the circular mesopore shapes (4.6 nm diameter in average) were represented by roundish spots. It cannot be expected that the smallest mesopores (
