Understanding Internal Mechanisms To Obtain Nanomanganites by

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Understanding internal mechanisms to obtain nanomanganites by hydrothermal synthesis: the particular case of 4H-SrMnO3 Irma N. González-Jiménez, Almudena Torres-Pardo, Mar GarciaHernandez, Jose M. Gonzalez-Calbet, Marina Parras, and Áurea Varela Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg501835p • Publication Date (Web): 26 Mar 2015 Downloaded from http://pubs.acs.org on April 9, 2015

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Crystal Growth & Design

Understanding internal mechanisms to obtain nanomanganites by hydrothermal synthesis: the particular case of 4H-SrMnO3 Irma N. González-Jiménez1, Almudena Torres-Pardo1, 2, Mar García-Hernández3, José M. González-Calbet1, 4, Marina Parras1, Áurea Varela1, *. 1

Universidad Complutense de Madrid, Facultad de CC. Químicas, Departamento de Química

Inorgánica I, 28040 Madrid, Spain 2

CEI Campus Moncloa, UCM-UPM, Madrid, Spain

3

Instituto de Ciencia de Materiales, CSIC, Cantoblanco, 28049 Madrid, Spain

4

Centro Nacional de Microscopía Electrónica CNME, 28040 Madrid, Spain

KEYWORDS. 4H-SrMnO3 • hydrothermal synthesis • Manganese hexagonal perovskites • Electron microscopy • textural characterization

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ABSTRACT. We have determined why hydrothermal synthesis does not turn out to be an adequate method to obtain nanoparticles in the Sr-Mn-O system. At high KOH concentration, the appropriate one to obtain nanoparticles, Sr is partially incorporated inside an intermediate (K, Sr)xMnO2·nH2O layered phase that prevents the 4H-nanoparticles formation.

Numerous experiments have been performed by screening the parameters involved in such process (precursors, pH, temperature, etc...) in order to understand their influence in the shape, size and purity of the 4H-final product. Analysing the experimental results by means of XRD, TEM, TGA and EDS among others, some peculiarities of the synthetic procedure have been detected; the appearance of side phases, SrMn3O6-x nanoribbons or the above mentioned birnessite-type (K, Sr)xMnO2·nH2O phase, and the insertion of K within the 4H-phase prepared at high KOH concentration. Magnetic features are also discussed.

INTRODUCTION With the recent global interest in energy efficiency, green synthesis techniques have been investigated in an attempt to limit the industrial energy usage and waste. Conventional solid state inorganic synthesis methods usually require high temperatures and long reaction times, consuming large amounts of energy. Accordingly, material scientists are seeking new strategies to create adequate functionalities including alternative soft chemistry methods that are being extensively explored. The functionality of a given material is, among other features, highly dependent on its texture; therefore a considerable activity has been devoted to investigate the morphology dependencies of catalytic, magnetic, electric…properties of the materials. Manganese related perovskites have attracted great interest because due to their outstanding functional properties 1, 2. Although the


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magnetic and electric properties of these materials have been deeply studied, manganites are still subject of research on account of their potential applications in electronic devices and, very recently, as promising materials for catalytic applications 3. These materials are commonly prepared using the conventional solid state reaction but, in the last years, soft chemistry methods have been developed and employed for the synthesis of micrometre or sub-micrometre particles 4-6

. Among these methods, hydrothermal synthesis has happened to become one of the most

important techniques because due to its ease of use, its cost-effectiveness, its relatively mild reaction conditions (low temperature, aqueous solution, reasonably non-toxic reagents) and its ability to obtain a wide assortment of materials in one single-step processes 7. There are numerous works in which both manganites and hydrothermal synthesis are blended to attain particles in the sub-micron or nanoscale. Among them, La0.5Ba0.5MnO3 well-formed cubeshaped crystallites of dimension ∼0.5 µm with a narrow particle size distribution were prepared by Spooren et al. 8, whereas by varying pH, temperature or autoclave filling volume, La0.5Ba0.5MnO3 flower-like particles can be obtained as well as the nano or microcubes mentioned before 9. Querejeta et al. 10 also reported an extended study on how hydrothermal conditions affect the size control of 2H-BaMnO3 particles and their magnetic properties in turn. Concerning the hydrothermal preparation of 4H-SrMnO3 in KOH medium two works have been previously reported. Spooren et al. 11 described the first hydrothermal synthesis of perovskite related manganites. From a reagent mixture of metal sulphates, KMnO4 and KOH, the successful hydrothermal synthesis of 4H-SrMnO3 was achieved at 240 ºC. The obtained product contains blade-shape crystallites of a large variety of sizes in the micrometre range. In a later work using hydrothermal conditions, J. S. Zhu et al. 12 prepared SrMnO3 microcrystallites with different morphologies by using different reagents as starting materials. Up to now, this is the only work


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concerning the influence of some experimental parameters in the morphology of hydrothermally prepared SrMnO3. The effect of the reaction time and the mineraliser concentration on the quality of the SrMnO3 sample was also analysed. Recently, J. Zhang et al. 13 report the hydrothermal synthesis of microcrystals of manganese oxides (RMnO3, R= Ca, Sr, Ba) with different morphologies by using NaOH to adjust the alkalinity and a small amount of HTAB used as shape controller. Although it has been possible to obtain nanoparticles by hydrothermal synthesis of 2H-BaMnO3 phase or even from more complex systems, with a larger number of cations, such as La1xBaxMnO3,

nanoparticles in the Sr-Mn-O system have never been hydrothermally prepared.

Regarding this fact and in order to understand the dependence on the particle size with the nature of the alkaline-earth cation (Sr2+), we have systematically analysed the parameters involving the synthesis pathway of 4H-SrMnO3 under hydrothermal conditions. From that, we have been able to understand the external process that prevents nanoparticles formation in the Sr-Mn-O system. Besides, an assessment of the variables affecting the shape and size of the prepared products has been investigated. EXPERIMENTAL SECTION Previous results in the BaMnO3-y system show that the optimal metallic salts concentration to achieve isolated nanoparticles of BaMnO3 (ca. 20 nm) is 50 mM for an autoclave filling volume of 40% 10. According to that, we have fixed at 50 mM the metal salts concentration and the samples were prepared from solutions filling 42% of reactor volume. Maintaining these parameters, different hydrothermal synthesis of different metallic salts by screening reaction temperature (175-240 ºC), reaction time (5-36 h) and KOH concentration (2-26 M) have been carried out.


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Independently on the metal precursor (sulphates or chlorides), the general experimental procedure is the following: solutions of each starting material were prepared by dissolving the required amounts of Sr2+, Mn2+ and MnO4- salts and KOH in a total volume of 85 mL of distilled water. In order to homogenise the mixture, sonication was performed with a BRANSON 1510 sonicator. The final solution was introduced in a Teflon-lined vessel and this, in turn, into a PARR P-ST-FS stainless-steel autoclave. After reaction completion, the solution was cooled to room temperature and the resulting suspensions were centrifuged in order to separate the precipitate from the mother liquid. Excess of alkali medium were removed by dialysis and the final powder was dried at 60-80 ºC overnight. The detailed experimental conditions for the synthesis of SrMnO3 from sulphates and chlorides and corresponding brief results are gathered in Tables I and II, respectively. Characterisation Powder X-ray diffraction (XRD) patterns were collected using CuKα monochromatic radiation (λ=1.54056 Å) at room temperature on a Panalytical X´PERT PRO MPD diffractometer equipped with a germanium 111 primary beam monochromator and X'Celerator fast detector. The oxygen stoichiometry was determined by thermogravimetric analysis (TGA) in a thermobalance CAHN D-200 heating the 4H-oxide to 1173 K in 0.3 bar H2/ 0.2 bar He atmosphere. The green final products were identified by XRD as SrO and MnO. The morphological study was carried out on a JEOL JSM6335-FEG Scanning Electron Microscope (SEM) operating at an acceleration voltage of 5 kV. Selected Area Electron Diffraction (SAED) and High Resolution Transmission Electron Microscopy (HRTEM) were performed on a JEOL 300FEG electron microscope. Cationic composition of the crystals was determined by energy-dispersive X-ray spectroscopy (EDS) in a JEOL 300FEG transmission


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electron microscope and with a JEOL Superprobe JXA-8900M connected to a JSM8600 scanning electron microscope. EELS spectra were acquired on a JEOL JEM-ARM200cF electron microscope (Cold Emission Gun) operating at 200 kV provided with a spherical aberration corrector in probe (current emission density ~1.4×10-9 A and probe size ~0.08 nm), a GIFQuantumERTM spectrometer and an Oxford INCA-350 detector. Magnetization measurements on fresh samples were obtained by SQUID magnetometry using a Quantum Design magnetometer equipped with a 5 Tesla superconducting coil. The temperature range explored is (4-400 K) at 50 Oe. The field dependence of the magnetization at 5 K was carried out in the range of 0 – 50 kOe. RESULTS AND DISCUSSION In order to perform a systematic study of the parameters affecting the hydrothermal process, the metal precursor, KOH concentration, temperature and reaction time were screened whereas the autoclave filling was fixed at 42% and the metal salts concentration at 50 mM. Table I summarises the detailed hydrothermal reaction conditions and the XRD results from metallic sulphates (set S samples) and chlorides (set C samples) as reagents. HIGH KOH CONCENTRATIONS: 26M ≥ [KOH] ≥ 15M Sulphate precursors The first set of experiences (set S1) shows the results obtained for the highest [KOH] concentration. The reaction temperature was fixed at 200 ºC whereas the influence of the reaction time in the final product was evaluated. The XRD results (See Figure S1 in supporting information) show that, under these conditions, SrMnO3 single phase is never attained.


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Table I. Reaction conditions and XRD identification of experiments carried out at high KOH concentrations. Metal Set precursor

Entrant

sulphates

[KOH]

T

t

p

(M)

(ºC)

(h)

(bar)

1

26

200

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Understanding Internal Mechanisms To Obtain Nanomanganites by

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