Journal of Crystal Growth 375 (2013) 78–83

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Journal of Crystal Growth

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Hydrothermal synthesis of La1−XSrXMnO3 dendrites

Darko Makovec a,(n), Tanja Goršak a, Klementina Zupan b, Darja Lisjak a

a Department for Materials Synthesis, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Sloveniab Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškarčeva 5, SI-1000 Ljubljana, Slovenia

a r t i c l e i n f o

Article history:Received 5 March 2013Received in revised form2 April 2013Accepted 8 April 2013

Communicated by: T. Nishinaga

300 1C. The products were characterized using a combination of X-ray diffractometry (XRD) and

Available online 17 April 2013

Keywords:A1. Crystal growthA1. DendritesA1. Hydrothermal synthesisMagnetic perovskite

48/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.jcrysgro.2013.04.019

responding author. Tel.: +386 1 4773 579; faxail address: darko.makovec@ijs.si (D. Makovec

a b s t r a c t

Single-crystalline dendrites of La1−XSrXMnO3 (LSMO) perovskite were synthesized using a simplehydrothermal method without the use of surfactants. The Sr2+, La3+, and Mn2+ ions were co-precipitated with aqueous NaOH under a flow of Ar. The aqueous suspension of the precipitates washydrothermally treated in an autoclave filled with ambient air at temperatures ranging from 220 1C to

transmission electron microscopy (TEM, HREM, EDXS). The dendrites formed either in a “tree-like”shape, with the trunk and the branches extending along the ⟨111⟩ directions of the quasi-cubic structure,or in the hexagonal shape of a “snowflake”. The mechanism of the dendrite nucleation was proposed,based on phase development. During the hydrothermal treatment at lower temperatures the hexagonalplatelet crystals of Sr1−XLaXMnO3 with the hexagonal perovskite structure form first. At highertemperatures the LSMO nucleates epitaxially at the edges of the hexagonal crystals and grows outward,forming the dendrite. To the best of our knowledge, this is the first report on the synthesis of crystallinedendrites of La1−XSrXMnO3 perovskite.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, new functional materials have been synthesizedwith architectural control of their micro/nanostructures. For applica-tions demanding a large surface area of material, the synthesis ofhierarchically branched, fractal structures is of special interest.Dendrites are examples of fractal structures that form by crystalgrowth under specific, non-equilibrium conditions. Dendrites occurin the nature, for example, as snowflakes, and have been deliberatelysynthesized and tested as new materials. [1–17] The synthesis ofdendritic microstructures of materials with a simple compositionand structure, including simple metals (Ag, Cu, Co, and Ni) [1–5],alloys (CuNi, FeNi3) [6,7], transition-metal chalcogenides (CdS, Ag2Se,PbTe) [8–10] and simple oxides (ZnO, CeO2, α-Fe2O3, WO3) [11–14] iswell documented, whereas the dendrites of mixed oxides with amore complex structure and composition have been successfullysynthesized on fewer occasions. Starting from mixed oxides, spinelmagnetite Fe3O4 [15] and the perovskites BaTiO3 [16] and SrTiO3 [17]were synthesized in the form of dendrites.

Because of their special, strongly correlated transport and mag-netic properties, mixed-valence manganites with a perovskite struc-ture such us La1−XSrXMnO3 have received special attention in differentareas of modern technology. By increasing X, the La1−XSrXMnO3 pero-vskite changes from an insulator to a metallic conductor, while itsmagnetic properties change from antiferromagnetic to ferromagnetic.[18] It has beenwidely used for magnetic sensors in combinationwith

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: +386 1 2519 385.).

colossal magnetoresistance (CMR), as cathode materials in solid-oxidefuel cells (SOFCs), for magnetocaloric refrigeration, as oxidation catal-ysts, and for mediator nanoparticles in the treatment of cancer using aself-regulating magnetic hyperthermia, etc. [19–24].

Many different chemical methods have been developed for thesynthesis of La1−XSrXMnO3, including solid-state reaction, carbonateand oxalate co-precipitation, the sol–gel method, pyrolysis, the citratemethod, the polymeric precursor route, the molten salt route, etc.[23,25–29] All these methods involve high temperatures or requirehigh-temperature post-annealing (calcination) to obtain the finalcomposition and the structure of the material. In contrast, thehydrothermal method [30–36] enables the synthesis of the La1−XSrXMnO3 perovskite in situ, during a hydrothermal treatment of thecorresponding hydroxides in water at moderate temperatures (usuallybelow 300 1C.) and under the autogenous pressure. Usually, a mixtureof Mn2+ and Mn7+ is employed in order to obtain the desired valenceof the manganese in the product, usually in the form of cuboid,micron-sized particles. Nanoparticles of the La1−XSrXMnO3 perovskitewere also hydrothermally synthesized with the addition of surfactants.

In this work, the hydrothermal method was applied to synthe-size the La1−XSrXMnO3 perovskite in the form of single-crystallinedendrites.

2. Experimental procedure

In a typical synthesis experiment a solution of the metal ions wasprepared by dissolving 9.8 mmol of La(NO3)3 �6H2O, 4.2 mmol of Sr(NO3)2, and 10.0 mmol of Mn(NO3)2 �4H2O in 100 mL of distilledwater. The solution of metal ions was then admixed into 100 mL of

Fig. 1. XRD spectra of the samples hydrothermally treated under different experi-mental conditions (temperature/time noted in the images, L—La(OH)3, *—SrMn3O6,H—hexagonal perovskite Sr1−XLaXMnO37 δ (PDF card no. 84-1612), P—perovskiteLa1−XSrXMn1.00O37δ (PDF card no. 51-0409)).

D. Makovec et al. / Journal of Crystal Growth 375 (2013) 78–83 79

aqueous NaOH (4 mol/L). The preparation of the solutions and theirmixing was performed under a flow of Ar so as to prevent oxidationof the Mn2+. The resulting suspension of white precipitates wastransferred into an autoclave in the ambient air, sealed and heated totemperatures ranging from 220 1C to 300 1C for different timesranging from 2 h to 3 days. The heating rate was set to 2.5 1C/min.The temperature inside the vessel was controlled using an internalthermocouple to within 72 1C. For simplicity the samples obtainedby the hydrothermal treatment at different conditions are indicatedas “T/t”, where T stands for the temperature and t for the time. Finally,the powders were washed with deionised water and dried at 60 1C inambient air.

The Sr+La/Mn ratio in the starting solution was set to 1.4, i.e., thecations occupying the A-sites of the perovskite ABO3 structure (Sr andLa) were in excess, in accordance with the nominal formula La0.98Sr0.42Mn1.00O37δ. The excess of Sr and La favors the formation of theperovskite phase, as we established in the preliminary experiments. Toobtain the pure perovskite phase, the powder was thoroughly washedwith a diluted aqueous solution of acetic acid (10%). The acid dissolvesthe La-rich secondary phase La(OH)3 and any Sr-rich secondary phases(i.e., SrCO3) formed as a consequence of the La and Sr excess, whereasthe perovskite phase is completely insoluble.

The phase evolution with temperature/time of autoclaving wasfollowed with X-ray powder diffraction (XRD, PANalytical X'Pert PROdifftactometer, CuKα1 radiation) in combination with transmissionelectron microscopy (TEM) coupled with energy-dispersive X-rayspectroscopy (EDXS). The analysis of the XRD was performed withTopas software (Bruker, AXS) using the Pawley method [37]. For theTEM investigations the powders were deposited on a copper-grid-supported, perforated, transparent carbon foil. A field-emission elec-tron-source TEM (JEOL 2010F), equipped with the EDXS system (LINK,ISIS EDS 300), was operated at 200 kV. The EDXS spectra werequantified using Oxford ISIS software containing a library of virtualstandards. The relative error of the analysis was quite large, estimatedto be approximately 77%. The synthesized powders were also exam-ined using scanning electron microscopy (SEM). The specimens for theSEM investigations were prepared by drying the diluted aqueoussuspension of the material on a polished graphite specimen support.The surface of the specimen was coated with a ∼5-nm-thick amor-phous carbon layer using Model 682 PECS (Gatan). The SEM imageswere taken using a field-emission electron-source SEM JEOL 7600F.

Fig. 2. TEM image of the sample hydrothermally treated for 24 h at 220 1C (L—La(OH)3, SM3—SrMn3O6, SM—SrMnO3).

3. Results and discussion

Fig. 1 shows the XRD spectra of the samples that were hydro-thermally treated at different conditions. In the sample hydrother-mally treated for 24 h at 220 1C. (sample 220/24), three crystallinephases can be detected: hexagonal La(OH)3 (PDF card no. 36-1481),SrMn3O6 [38] (SM3, PDF card no. 28-1233) and SrMnO3 [39,40] (SM,PDF card no. 84-1612). The XRD reflections of the SM phase wereslightly shifted to lower 2θ, suggesting the partial substitution of Srwith La. The TEM image in Fig. 2 shows the morphology of thephases present: La(OH)3 was in the form of several-micrometers-long and 10–50-nm-thick, needle-like crystallites (marked in Fig. 2with L), the SM3 was in the form of several-tens-of-nanometersthick, several-hundreds-of-nanometers wide, and several-tens-of-micrometers-long nanoribbons, while the hexagonal-perovskite SMphase was in the form of micrometer-sized hexagonal plateletcrystals. The EDXS showed that the SM3 and SM phases containedlow concentrations of La.

After 2 h of hydrothermal treatment at 240 1C. an additionalphase, i.e., the rhombohedral perovskite La1−XSrXMnO3−δ (LSMO),appeared in the XRD spectrum (Fig. 1), while the peaks of the SM3phase completely disappeared. With increasing time and/or tempera-ture of the hydrothermal treatment, the content of the perovskite

LSMO phase increased, while the content of the La(OH)3 and of thehexagonal-perovskite SM phase decreased. Here, it should be notedthat the La and Sr were added in excess with respect to the nominalLa1−XSrXMnO37δ perovskite composition (Sr+La/Mn¼1.4).

D. Makovec et al. / Journal of Crystal Growth 375 (2013) 78–8380

The changes in the content and the structure of the perovskite-related phases with the time and temperature of the treatment canbe qualitatively followed by observing the strongest XRD reflectionsof the phases: the (110)H reflection for the hexagonal-perovskite SM,and the (110)P, (104)P reflections for the rhombohedral perovskiteLSMO (Fig. 1(b)). The XRD data was always fitted with the twoperovskite-related structures (in addition to the La(OH)3 structure);the diffractograms of the samples 240/2, 240/24, and 260/2 werefitted with the hexagonal perovskite and the rhombohedral per-ovskite phases, whereas for the samples treated for longer times orat higher temperatures, two rhombohedral structures with differentcell parameters had to be used to achieve satisfactory fits. Moreover,the peaks corresponding to the two rhombohedral phases werealways broadened, most probably due to compositional inhomo-geneities, internal stresses or the small crystal size. Thus, the exactvalues of the cell parameters could not be measured. However, itwas evident that the cell parameters of both phases increased withthe temperature and/or time. In the samples treated at lowertemperatures the content of the rhombohedral perovskite with thesmaller cell volume was higher, whereas in sample 300/24 therhombohedral perovskite with the larger cell volume prevailed.

The TEM analysis showed that besides the La(OH)3 needle-likecrystallites and the perovskite-structure-related phases, SM3 nanor-ibbons are also present in all the samples. Most probably the SM3nanoribbons were not detected by the XRD due to poor crystallinityand preferential orientation.

In sample 240/2, the LSMO perovskite phase appears in the formof dendrites growing from the edges of the hexagonal platelets(Fig. 3). The dendrites have a specific “tree-like” shape with a distincttrunk and branches. The selected-area-electron-diffraction (SAED)patterns recorded for any of the dendrites growing from the hexa-gonal platelet show equal patterns corresponding to the perovskitestructure, proving that the dendrites are single-crystalline and theirorientation is related to the structure of the hexagonal platelet. Forsimplicity, the diffraction patterns were indexed according to a basic,quasi-cubic perovskite structure. The SAED pattern in the inset ofFig. 3(a) corresponds to the [111] direction of the quasi-cubicperovskite. High-resolution imaging (HREM) (Fig. 3(b)) showed thatthe dendrites are actually composed of nanosized sub-crystallites.The EDXS analysis showed the presence of Mn, La, Sr and O, how-ever; with significant fluctuations in the La/Sr ratio, i.e., with a lowerconcentration of La in the hexagonal platelet ((La/Sr<0.5) and ahigher concentration in the dendrites (La/Sr ∼0.6–1.1).

When the temperature and/or time of hydrothermal treatmentincreased, the dendrites growing from the hexagonal platelets were

Fig. 3. TEM (a) and HREM (b) images of dendrites of the LSMO perovskite phase growingrecorded for the dendrites corresponds to the [111] direction of the quasi-cubic perovsk

shorter, thus the dendrite got the specific shape of hexagonal“snowflake” (Fig. 4(a) and (b)). The SAED pattern taken from the“body” of the hexagonal dendrite (Fig. 4(b)) corresponded to theperovskite structure. The majority of the dendrites, however, app-eared to be detached from the hexagonal platelets and had a “tree-like” shape. Fig. 4(c)–(f) shows such single-crystalline, tree-likedendrites in the sample 260/2. The trunk of the dendrites is from1 to 3 mm long and 100 to 200 nm thick, whereas the branches ex-tend up to 1 mm long, with further branching into shorter sec-ondary branches. Fig. 4(d) and (e) shows the same dendrite orientedalong the [011] and [010] directions of the quasi-cubic perovskitestructure, respectively. When oriented along [011], the trunk of thedendrite extends along [1̄11̄], whereas the longer branches extendalong [111̄] (directions marked with arrows in Fig. 4(d)). When thedendrite was tilted into the [0 1 0] orientation, the trunk and thebranches seem to extend in the [1̄01] and [101] directions. However,further tilting experiments in the TEM showed that the trunk and thebranches of all the dendrites actually always extend along the ⟨111⟩directions of their quasi-cubic structure.

Besides the well-crystallised dendrites, like the one shown inFig. 4(d) and (e), the dendrites that were in the initial state of thecrystallization process were also present in the same sample (Fig. 4(f)). They were composed of smaller sub-crystallites than the better-crystallised dendrites formed in the earlier stages of the process. Thereflections in the SAED pattern taken from the whole dendriteshown in the inset of Fig. 4(f) are streaked due to the slight structuralmismatch in the orientation of the sub-crystallites.

The EDXS analysis showed that the composition of an individualdendrite was relatively homogeneous; it never fluctuated above theestimated uncertainty of the method. However, the compositionvaried considerably between the dendrites. The well-crystallisedhexagonal and tree-like dendrites contained higher concentrationsof Sr than the dendrites that were in the initial state of growth. TheLa/Sr ratio was between 0.3 and 0.6 in the well-crystallised dendritesand from 0.9 to 1.3 in the dendrites in the initial state.

When the material was hydrothermally treated for a longer time(72 h) at 240 1C, or at higher temperatures, the LSMO fractal structurewas less pronounced since the dendrites were composed of largersub-crystallites. Fig. 5(a) and (b) shows the dendrites from the sam-ples 240/72 and 280/24, respectively. The EDXS analysis showed asimilar situation for both samples: considerable fluctuations in thecomposition between the dendrites, with the La/Sr ratio ranging fromapproximately 0.7 to over 3.

The mechanism of the dendrite formation can be discussedconsidering the phase development during the hydrothermal

from the edges of the hexagonal platelet crystal. Inset of Fig. 3(a): the SAED patternite.

Fig. 4. Dendrites present in the sample hydrothermally treated for 2 h at 260 1C. (260/2). The LSMO dendrites growing from a hexagonal platelet crystal are shown in theSEM image (a) and in the TEM image (b) (SAED pattern in the inset of (b) corresponds to the LSMO along the [111] direction of the quasi-cubic perovskite). Tree-like LSMOdendrites are shown in the SEM image (c) and in the TEM images (d)–(f). (d) and (e) shows the same dendrite oriented along the [011] and [010] directions of its quasi-cubicperovskite structure, respectively.

D. Makovec et al. / Journal of Crystal Growth 375 (2013) 78–83 81

treatment. From a combination of XRD and TEM/EDXS analyses wecan conclude that the partially La-substituted hexagonal-perovs-kite Sr1−XLaXMnO3−δ (SM) already forms below 220 1C, while atapproximately 240 1C. the rhombohedral perovskite La1−XSrXMnO3−δ

(LSMO) starts to form. In the initial stage of dendrite formation theLSMO nucleates at the surfaces of the structurally-similar SM crystalsand grows epitaxially, forming a (0001)SM||(111)LSMO interface. Thetemplate crystals were always too thick for a reliable SAED analysisthat could confirm this hypothesis beyond any doubt. However, theEDXS in the TEM and SEM showed that the hexagonal-crystaltemplate always contained a much higher concentration of Sr thanthe perovskite dendrites growing from it. This fact strongly supportsthe hypothesis that the LSMO perovskite dendrites grow out of thehexagonal-perovskite template, and with an increasing time and/ortemperature the new LSMO gradually forms. The content of La in theLSMO increases with the time and the temperature of the treatment.The newly formed perovskite dendrites contain a larger concentra-tion of La than the well-crystallised dendrites formed in the initial

stages of the process. Obviously, the diffusion of La into the already-formed LSMO perovskite is relatively slow, causing differences in thecomposition between individual dendrites. While the LSMO perovs-kite forms, the SM hexagonal-perovskite dissolves. In this process theSM templates where the LSMO dendrites first nucleated, are dis-solved, leaving self-standing dendrites.

4. Conclusions

Single-crystalline dendrites of La1−XSrXMnO3 (LSMO) perovskitewere synthesized for the first time using a simple hydrothermalmethod. The Sr2+, La3+, and Mn2+ ions were co-precipitated withaqueous NaOH under a flow of Ar and the aqueous suspension of theprecipitates was hydrothermally treated in an autoclave filled withambient air at temperatures ranging from 220 1C to 300 1C. The La1−XSrXMnO3 dendrites formed either in a “tree-like” shape or in thehexagonal shape of a “snowflake”. The trunk and the branches of the

Fig. 5. TEM images of the dendrites from the samples 240/72 (a) and 280/24 (b).

D. Makovec et al. / Journal of Crystal Growth 375 (2013) 78–8382

dendrite extend along the ⟨111⟩ directions of their quasi-cubicperovskite structure. The mechanism of the dendrite nucleationwas proposed based on phase development during the hydrothermaltreatment. At temperatures below 220 1C a Sr1−XLaXMnO3 phase withthe hexagonal perovskite structure forms in the shape of hexagonalplatelet crystals. At approximately 240 1C the La1−XSrXMnO3 formswith epitaxial nucleation at the edges of the hexagonal plateletcrystals and grows outward in the form of “tree-like” dendrites.

Acknowledgments

The support of the Ministry of Higher Education, Science andTechnology of the Republic of Slovenia within the National ResearchProgram is gratefully acknowledged.

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