Synthesis and structural study of a self-organized MnTiO3–TiO2 eutectic

Journal of Alloys and Compounds 659 (2016) 152e158

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Synthesis and structural study of a self-organized MnTiO3eTiO2 eutectic Katarzyna Kolodziejak a, *, Marcin Gajc a, Jaroslaw Sar a, Ryszard Diduszko a, Krzysztof Rozniatowski b, Dorota A. Pawlak a, c, ** a b c

Institute of Electronic Materials Technology (ITME), ul. Wolczynska 133, 01-919 Warsaw, Poland Materials Science Department, Warsaw University of Technology, ul. Woloska 141, 02-507 Warsaw, Poland Centre of New Technologies, University of Warsaw, ul.Banacha 2C, 02-097 Warsaw, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 July 2015 Received in revised form 20 October 2015 Accepted 3 November 2015 Available online 6 November 2015

A microstructured manganese titanate e titanium dioxide, MnTiO3eTiO2, hybrid eutectic has been grown by the micro-pulling-down method from a TiO2 57 mol.% e MnO 43 mol.% composition. X-ray powder diffraction verifies that two phases have been formed: rhombohedral MnTiO3 and tetragonal TiO2. The TiO2 phase forms a three-dimensional network of micron-scale oval precipitates interconnected with c.a. 0.5 mm in thickness lamellas and the MnTiO3 is the matrix phase. The two TiO2 precipitate types grow in different crystallographic orientations, which results in this complex geometry made of faceted and non-faceted inclusions. The lamellas grow in the [010] orientation in which a 2-fold axis is present, while the oval precipitates grow in the [001] orientation in which the 4-fold screw axis is present. A qualitative and quantitative analysis of the eutectic microstructure is presented. The MnTiO3 eTiO2composite is interesting due to its potential application in photoelectrochemistry. © 2015 Elsevier B.V. All rights reserved.

Keywords: Self-organization and patterning Eutectic Microstructure Directional solidification MnTiO3eTiO2

1. Introduction Industrial development depends on novel materials, on their associated novel properties and the functionalities which they enable. Composite materials in particular are a fertile area for new ideas and new developments, since they result from combining two or more phases with a chosen structuring. One special type of self-organized composite materials, which can often exhibit well-aligned micro/nanostructures when solidified directionally, is eutectics. Eutectics are two or multiphase materials forming during cooling a mixable melt with a eutectic composition. The self-organized patterns created in eutectics due to the interplay between the diffusion, capillary forces and anisotropies [1e4] provide a relatively easy way of structuring solidstate materials. The possibility of considering versatile combinations of various component materials in eutectics provides a broad * Corresponding author. ** Corresponding author. Centre of New Technologies, University of Warsaw, ul.Banacha 2C, 02-097 Warsaw, Poland E-mail addresses: [email protected] (K. Kolodziejak), Marcin. [email protected] (M. Gajc), [email protected] (J. Sar), Ryszard.Diduszko@ itme.edu.pl (R. Diduszko), [email protected] (K. Rozniatowski), Dorota. [email protected] (D.A. Pawlak). http://dx.doi.org/10.1016/j.jallcom.2015.11.010 0925-8388/© 2015 Elsevier B.V. All rights reserved.

palette for many applications and one of them is to utilize eutectics as energy-generating materials. This possible application of eutectics has been preliminarily studied, including solid-oxide fuel cells [5e7], thermoelectric materials [8,9], and recently their use in photoelectrochemical cells (PEC) has been proposed [10]. In the case of photoelectrochemical cells, eutectics made of photoactive phases could be very attractive due to their multiphase character and potential broadband absorption, high crystallinity, and high fraction of interfaces (clean and atomically sharp) which may enable better charge transport. However to convert the solar energy to hydrogen other combinations of phases enabling absorption of both UV and visible wavelengths and not only UV (as TiO2, and SrTiO3) would be beneficial. MnTiO3 has been already considered as a material for photoelectrochemical applications [11e13] and its reported band gap of 2, 4e3.2 eV [14e17] should be sufficient to enable absorption in the visible range and potential water splitting. While combining this material with well-known and used in PEC TiO2 phase could provide us with potentially useful material for PEC anode. Here, we demonstrate the growth of the MnTiO3eTiO2 eutectic and the analysis of its microstructure. The MnTiO3 forms a matrix in which the TiO2 phase (rutile) is embedded, forming a fishnet-like structure made of two kinds of TiO2 precipitates with two

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different crystallographic orientations. The refinement of the structure can be controlled by the growth rate. 2. Experimental procedure Crystal Growth. A eutectic has been grown from a TiO2eMnO [18] system by the micro-pulling-down method (m-PD) [19,20]. The m-PD method was developed originally for the growth of single crystal fibers [21], it has been efficiently used for eutectic materials with good mechanical properties [22]. Recently it has been used for various novel photonic materials, as metamaterials [23], and eutectic-based bulk nanoplasmonics materials and bulk nanoplasmonics materials obtained by direct doping of dielectric matrices with nanoparticles [24]. The method utilises a crucible with a die at the bottom in which there is a centrally-placed nozzle. The raw materials are molten in the crucible; the melt passes through the nozzle, is touched with the seed crystal, and the crystal is then pulled down. The details of the thermal system used in our laboratory for micro-pulling down, as well as the growth conditions, have been described elsewhere [25]. The crystals were seeded with an yttrium-aluminum garnet single crystal. High purity oxide powders MnO (99,99%) and TiO2 (99,99%), were used as starting materials. The oxides were mixed in alumina mortar with ethanol and then dried. For further investigations some as-grown samples were etched in hydrochloric acid and water mixture (1:1), for 5 min. After the etching process, the surface of some of the samples was covered with a layer of silver about 60 nm thick deposited by evaporation. Phase identification. Powder X-ray diffraction measurements were performed with KaCu radiation (l ¼ 1,5418 Å) on the asgrown eutectic samples using a Siemens D500 diffractometer equipped with a semiconductor Si: Li detector cooled with liquid nitrogen and ICDD PDF4þ2014 database. The powder diffraction pattern was measured in q/2q scanning mode with a step of 0.02 and an integration time of 10 s/step. The experimental data were analyzed by the R. A. Young DBWS-9807 program package [26]. The chemical composition of particular phases of obtained eutectic bi-crystals was examined by Scanning Electron Microscopy (SEM) equipped with an Energy Dispersive X-ray Spectrometer (EDS, Thermo NORAN). The measurements were performed on a HITACHI S3000N Scanning Electron Microscope. For the SEM and EDS measurements the (non-conductive) samples were covered with thin layer of carbon, using BAL-TEC SCD 005 equipment at 102 mBar of argon. Crystallographic orientation. The macroscopic crystallographic growth orientations of the eutectic phases were examined from the cross-section surface of the eutectic rod, using a BraggeBrentano geometry on diffractometer Siemens D500, and additionally fourcircle KUMA-diffraction KM4 diffractometer with KaCu radiation. Measurements were also performed with a High Resolution Transmission Electron Microscope (HRTEM) to explain the crystallographic relation of TiO2 precipitates in two different orientations. For these measurements, the specimen was mechanically thinned and polished on diamond lapping films to 50 mm thickness. Next the sample was glued onto a copper ring and polished again using a dimple grinder (GATAN) to 10 mm thickness. After final polishing, the specimen was sputtered in an ion polishing system (GATAN) to improve electron transparency. HRTEM measurements were performed on a JEOL 3010 microscope at an operating voltage 300 kV. The image analysis utilised commercial software (GATAN). Quantitative analysis of the microstructure. All the geometrical parameters were calculated from the scanning electron microscope images by the MICROMETER program [27]. For each sample a few images were chosen: for the crystal grown with a 0.15 mm/min pulling rate (p.r.) eight images with 603 investigated TiO2 phase

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oval objects were recorded; for p.r. ¼ 0.45 mm/min three images with 276 investigated oval objects of TiO2 phase were recorded; for p.r. ¼ 10 mm/min five images with 652 investigated TiO2 phase oval objects were recorded. The following parameters have been used for the quantitative analysis of the microstructure: dmax the maximum chord length (between the two points on perimeter of the particle projection) including the gravity center of this projection; dmin the minimum chord length (between the two points on perimeter of the particle projection) including the center of gravity of this projection; d2 the equivalent diameter e the diameter of the circle with the same area as the particle projection area. 3. Results and discussion In this work, the MnTiO3eTiO2 binary eutectic composite with 43 mol.% MnO and 57 mol.% TiO2 composition [18] was grown by the micro-pulling-down method. Four different pulling rates were applied: 0.15, 0.45, 5, and 10 mm/min. The black-colored as-grown MnTiO3eTiO2 eutectic rods and samples cut perpendicularly to the growth direction are shown in Fig. 1a and b. As investigated by X-ray powder diffraction, two phases have been formed in the eutectic: manganese titanate, MnTiO3, and titanium oxide, TiO2, in the form of rutile, Fig. 1c. Both phases grow in distinct crystallographic orientations within the eutectic rod. MnTiO3 phase grows in [11.0] and TiO2 phase grows in [001] orientation as determined by single crystal XRD measurements. High level of background on red diffraction pattern and small “artifact” peak at the angle of 65 , derived from the sample holder. Determining the orientation of TiO2 precipitates with XRD was only possible after etching the sample. Following etching with mixture of hydrochloric acid and water, the MnTiO3 matrix phase has been removed from the surface, while TiO2 precipitates were left, Fig. 2a, b. After 5 min etching in boiling HCleH2O (1:1) mixture the height of TiO2 precipitates was about 110 nm, while after 5 min etching in boiling H2SO4e H2O (1:1) mixture it was 300 nm (Fig. 2c, d). MnTiO3eTiO2 eutectic exhibits a three dimensional net made of TiO2 precipitates (black color, Fig. 1d, e), roughly ellipsoidal in the cross-section, interconnected by thin TiO2 lamellas, Fig. 3, and embedded in the MnTiO3 phase (grey color, Fig. 1d, e). As indicated by SEM images, TiO2 precipitates are elongated in the growth direction (longitudinal section). The refinement of the structure is higher for higher pulling rates, but this tendency is not as strong as that observed for eutectics exhibiting regular microstructures [28,29]. It is demonstrated in Fig. 3 with the microstructures of the eutectics grown with different pulling rates, 5 mm/min (Fig. 3a, b), 0.45 mm/min (Fig. 3c, d) and a comparison of two microstructures grown with p.r ¼ 10 (Fig. 3e) and 0.45 mm/min (Fig. 3f). The quantitative description of the microstructure was performed using the MICROMETER 0,99b program [27]. In Table 1 the average mean parameters of the eutectic microstructure are compared for the samples grown with different pulling rates cut perpendicular to the growth direction. This is illustrated in Fig. 4a, b, c, where the minimal/maximal intercept chords, dmin and dmax, are shown as a function of the equivalent diameter of the particle, d2, for the MnTiO3eTiO2 eutectic grown with three different pulling rates: 0.15, 0.45 and 10 mm/min. The minimal/maximal intercept chords are the minimal/maximal distances between the two points on perimeter of the particle projection, and the equivalent diameter of the particle is the diameter of the circle with the same area as the particle projection area. In Fig. 4a dmin is compared for three different pulling rates, and it can be seen that: (i) dmin increases slightly faster with the increase of d2 for the samples grown with lower p.r., and (ii) the dependence

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Fig. 1. Manganese titanate-titanium oxide, MnTiO3eTiO2, eutectic composite. a) As-grown eutectic rods obtained with different pulling rates: p.r. ¼ 0.15,0.45, 5 and 10 mm/min; b) Samples cut perpendicularly to the growth direction; c) Phase analysis by X-ray powder diffraction (blue line) with peaks for MnTiO3 (A), and TiO2 (B) phases; X-ray single crystal diffraction (red line) measured ⊥ to the growth direction indicating distinguished orientation of grown eutectic phases; d) SEM image, arrow indicates place and direction of the linear EDS analysis; e) Linear EDS analysis of oxygen, manganese and titanium. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 2. Microstructure obtained by etching the MnTiO3 phase out of MnTiO3eTiO2 eutectic, with hydrochloric acid, leaving the TiO2 precipitates on the surface. a) Optical microscope image e cross-section of the whole sample; b) AFM image; ced) SEM images observed at different magnifications: 500 and 3000, respectively.

of the parameters is linear. In Fig. 4b dmax is compared for three different pulling rates. The observations are as follows: (i) the dependence between dmax and d2 has a rather non-linear character, and (ii) in contrast to dmin, dmax increases faster with the increase of d2 for the samples grown with higher p.r. In Fig. 4c the dependence of dmin and dmax on d2 is compared for eutectics grown with

different pulling rates. It can be observed that dmax increases faster than dmin with the increase of the particle size represented by d2, so as the TiO2 precipitates increase their diameter, they increase their longer dimension more rapidly. The shape of the precipitates in the eutectic microstructure can additionally be described by two shape factors a and b. The shape

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Fig. 3. Fishnet-like microstructure of MnTiO3eTiO2 eutectic grown with different pulling rates (p.r.). Comparison of microstructure cross-section (a) and longitudinal section (b) of eutectic grown with p.r. ¼ 5 mm/min. Comparison of microstructure cross-section (c) and longitudinal section (d) of eutectic grown with p.r. ¼ 0.45 mm/min. And comparison of the microstructure cross-section of eutectic grown with two different pulling rates e p.r. ¼ 10 mm/min (e) and p.r. ¼ 0.45 mm/min (f).

Table 1 Quantitative analysis of size and shape of the TiO2 oval precipitates in the MnTiO3eTiO2 eutectic. p.r.

[mm1]

[mm2]

[mm]

[mm]

[mm]

[mm]

0.15 0.45 10

0.1 0.1 0.1

188.5 155.2 66.2

14.0 12.3 7.9

11.1 8.4 5.3

22.3 21.4 16.1

60.5 54.9 40.6

p.r. e crystal pulling rate in [mm/min],  specific surface of TiO2 oval precipitates phase boundaries,  mean area of the cross-section of TiO2 oval precipitates,  mean equivalent diameter of the oval TiO2 precipitates,  mean minimal chord intercept of the TiO2 precipitates, e mean maximal chord intercept of the TiO2 precipitates,

 mean parameter of the cross-section and longitudinal e section of the TiO2 precipitates. All samples were oriented ⊥ to the growth direction.

factor a describes the elongation of the ‘particle’ cross-section and factor b represents the development of boundaries between the ‘particle’ e the TiO2 phase and the MnTiO3 matrix phase; a ¼ (dmax)/ d2, b ¼ p/(pd2), where p is the TiO2 particle perimeter. In Fig. 4d, e, f the shape factors a and b are shown for the oval TiO2 particles for samples grown with three different pulling rates: 0.15, 0.45 and 10 mm/min, and cut perpendicularly to the growth direction. In all cases, independent of the pulling rate, a increases slightly faster than b with the increase of the particle size. This shows that with the increase of the particle size, the particles tend

rather to elongate than to become more complicated in shape which increases the boundary between the particle and the surrounding matrix. Both a and b increase faster with the increase of d2 for higher pulling rates. On carefully taken BSE SEM images one can notice slightly different coloration tones of the two kinds of TiO2 precipitates (Fig. 5a), and also that thin TiO2 lamellas often are incorporated into the oval precipitates. This is also very clear on specially prepared samples, etched and then covered with a 60 nm thin layer of silver, Fig. 5b. Neither X-ray diffraction nor scanning electron microscopy

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Fig. 4. Quantitative analysis of the MnTiO3eTiO2 microstructure: aec) Minimal/maximal intercept chord, dmin and dmax, as a function of equivalent diameter of the particle, d2, for the MnTiO3eTiO2 eutectic grown with three different pulling rates: 0.15, 0.45 and 10 mm/min, d-f) Shape factors a and b as a function of the equivalent diameter, d2, for the MnTiO3eTiO2 eutectic grown with three different pulling rates: 0.15, 0.45 and 10 mm/min.

show any phase other than MnTiO3 and TiO2 in this eutectic, so the two coloration tones belong to the same material e TiO2 in the form of rutile as indicated also by diffraction patterns of high resolution transmission electron microscopy (HRTEM). The HRTEM measurements revealed that the two kinds of TiO2 precipitates, which correspond to two colorations on SEM images, have different crystallographic orientations. These are: (i) the TiO2 faceted lamellas, which grow in [010] direction with atomically smooth interfaces, and (ii) the oval TiO2 precipitates, which grow in [001] orientation. The HRTEM images have been obtained from the TiO2 phase forming oval inclusions and the thin TiO2 lamella interconnecting them (Fig. 5c, d). In order to see it clearly in the image, a short interconnecting lamella (‘bridge’) of TiO2 has been chosen for investigations (Fig. 5c). In Fig. 5d the area where the TEM measurements have been performed is shown. The TiO2 ‘bridge’, TiO2 oval precipitates and MnTiO3 matrix are indicated. MnTiO3 matrix material was etched by ion polishing process during TEM sample preparation, which explains why this phase has not been observed by TEM at this particular place. Only one kind of boundary was observed in MnTiO3eTiO2 eutectic, where the two phases are in contact and no amorphous layer was observed between them (boundary between MnTiO3 phase and TiO2 phase). The HRTEM images of (01 0)TiO2//(1 01)TiO2 interfaces were taken along the [001] TiO2 zone axis (Fig. 5g), and along [010] TiO2 zone axis (Fig. 5h). These images clearly show that the TiO2 [010] lamellas grow in perpendicular orientations to the oval precipitates of the TiO2 [001] phase. According to Hunt and Jackson's theory [30], which has been developed on the basis of metalemetal eutectics, the kind of the microstructure obtained strongly depends on the entropy of melting of both phases. As a reference, the entropy of melting in dimensionless units is often used, c ¼ DS/Rg (DS e entropy of melting [molKJ1], Rg ¼ 8.314472 (15) [Jmol1K1] e gas constant). For c < 2 for both eutectic phases the non-faceted/non-

faceted microstructure is expected; for c < 2 for only one of the two phases the non-faceted/faceted microstructure is predicted, while for c > 2 for both phases growth of independent crystals of both phases should be observed. In the case of the TiO2 phase c ¼ 3.7 [23], which suggests faceted growth of this phase. In the case of an SrTiO3eTiO2 eutectic which has been reported previously [23], faceted TiO2 precipitates have been observed. In that eutectic TiO2 inclusions grew in the [001] crystallographic orientation in which TiO2 (rutile, space group P42/mnm) has a 4-fold screw axis. The precipitates had shapes strongly resembling tetragonal symmetry. The volume fraction of TiO2 phase there was much larger. In the case of MnTiO3eTiO2 eutectic, clearly faceted TiO2 lamellas are observed as well as the oval precipitates which at the interface with the MnTiO3 phase seem to be non-faceted, Fig. 5i. This indicates that, for such complex compounds as oxides which often have c > 2, the theory has to be further developed in order to take into account other parameters. Hopefully numerical methods will play an increasing role in enabling prediction of the eutectic microstructure in the future [31e33]. The lamellas grow in the [010] orientation in which a 2-fold axis is present, while the oval precipitates grow in the [001] orientation in which the 4-fold screw axis is present. No clear influence of the symmetry of the atomic lattice on the formed precipitates is observed in the MnTiO3eTiO2 eutectic. In order to investigate interface coherency Inverse Fast Fourier Transformation (IFFT) analysis was performed (Fig. 5e). Bragg images were created using parallel frequencies gTiO2(101) and gTiO2(010) to the boundary plane. The interface (010)TiO2(bridge)// (101)TiO2(oval shapes) is incoherent as confirmed by the presence of dense misfit dislocations, Fig. 5e. The calculated lattice mismatch for this interface is 45%. Interface between TiO2 (oval shapes) and MnTiO3 phases is shown in Fig. 5f. IFFT analysis of (111)TiO2// (115)MnTiO3 interface shows semi-coherency with angle boundary

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Fig. 5. Two crystallographic orientations of two types of TiO2 precipitates in the MnTiO3eTiO2 eutectic indicated by SEM and HRTEM: a) Back-scattered electron SEM image; b) Back-scattered-electron SEM image of a eutectic sample, partially etched by hydrochloric acid and covered with a 60 nm layer of silver, which enabled a clearer view of the two kinds of precipitates; c) SEM image of the MnTiO3eTiO2 eutectic microstructure, ⊥ growth direction, presenting two kinds of TiO2 precipitates e oval precipitates and layers; d) TEM image revealing a ‘bridge’ (layer/lamella) and two oval precipitates area further investigated with HRTEM, e) and f) Bragg image created using parallel frequency gTiO2(101) and gTiO2(010) to the boundary plane; e) The array of misfit dislocations (white arrows) with the average spacing of TiO2 1d(101); d) Angle boundary of 17 at TiO2//MnTiO3 interface; g) and h) HRTEM images of the (01 0)TiO2//(1 01)TiO2 interfaces between the oval precipitate and the ‘bridge’; i) HRTEM images of the (111)TiO2//(115)MnTiO3 interface observed in another place of the sample.

of 17 The calculated lattice mismatch for this interface is 12%.

demonstrated elsewhere.

4. Conclusions

Acknowledgment

A eutectic from a MnOeTiO2 system has been grown by the micro-pulling-down method. Four different pulling rates were applied: 0.15, 0.45, 5, and 10 mm/min. As investigated by X-ray powder diffraction, two phases have been formed: MnTiO3 and TiO2 in the form of rutile. The TiO2 phase forms a 3-D network of interconnected oval precipitates interconnected with each other by thin lamellas and this interconnected structure is embedded in the MnTiO3 phase. The two different TiO2 precipitates grow in two crystallographic orientations perpendicular to each other: oval precipitates [001] and lamellas [010]. The results obtained in the present study demonstrate a new hybrid composite material obtained by the self-organization mechanism, which is easily available in millimeter-scale pieces, and potentially scalable. The MnTiO3eTiO2 mixed composite material made of two semiconducting phases with bandgaps enabling absorption of UVeVis wavelengths and with both phases extending in a connected way across the whole sample may be a promising material for application in photoelectrochemical cells. There is therefore more scope for further investigation here, and the analysis of photoelectrochemical properties of this composite will be

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