LSM Materials Synthesis in Supercritical Water

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Copyright copy 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Nanoscience andNanotechnology LettersVol 3 324ndash327 2011

Hydrothermal Synthesis of Strontium DopedLanthanum Manganite Nanoparticles by a

Supercritical Flow Reaction System

Nazrul M Islamlowast dagger Takio Noguchi Yukiya Hakuta and Hiromichi HayashiResearch Centre for Compact Chemical System National Institute of Advanced Industrial Science and

Technology (AIST) 4-2-1 Nigatake Miyagino-ku Sendai 983-8551 Japan

We report first ever the rapid-continuous and one-pot synthesis of nanocrystalline perovskite stron-tium doped lanthanum manganite La1minusxSrxMnO3 (where x = 01sim03) nanomaterials for solid oxidefuel cells (SOFCs) cathode applications by using environmentally benign supercritical water (SCW)The synthesis was performed from the starting solution of the nitrates of La Sr and Mn metals ina flow reactor and the strontium doped lanthanum manganite (LSM) materials could be obtainedat 390sim410 C and 30MPa without any aid of organic solvent within as short as less than a sec-ond of time The XRD patterns revealed that the LSM material could be obtained only under thesupercritical conditions of water The TEM and BET analysis proved shape morphology and sizeof the materials Interestingly the LSM materials were obtained around 6 nm in size with a verylarge specific surface area of 142 cm2g with certainly a big hope of higher triple-phase-boundary(TPB) of solid oxide fuel cell (SOFC) cathode materials Inductively coupled plasma (ICP) analysiswas performed for the structural composition of the synthesized materials

Keywords One-Pot Hydrothermal Synthesis Supercritical Water Flow Reactor LSMNanoparticles SOFC Cathode Materials High Surface Area

Solid oxide fuel cells (SOFCs) are promising efficientenergy-saving and environmentally-friendly devices con-verting chemical energy to electrical energy and heatresulting in low emissions of pollutants Strontium dopedlanthanum manganite (LSM) is one of the preferred mate-rials for SOFCs cathode applications because of its highelectrochemical activity for oxygen reduction reaction(ORR) and good stability and compatibility with yttria sta-bilized zirconia (YSZ) electrolyte1ndash3 Conventional SOFCsare operated at high temperature up to sim1000 C andare based on well-established component systems4 Thehigh temperature operation limits the durability of all thecomponents including catalysts and membrane materialsand eventually the fuel cell One of the critical chal-lenges for the operation of SOFCs at intermediate and lowtemperature is to reduce the interface polarization resis-tance In addition the high temperature operation limitsthe options for the microstructure of the cathode There-fore the development of a synthesis method for LSM

lowastAuthor to whom correspondence should be addresseddaggerPresent address Department of Organic and Polymeric MaterialsTokyo Institute of Technology 2-12-1-5S-20 Ookayama Meguro-kuTokyo 152-8552 Japan

cathodes is essential to create not only a macroscopicallyhomogenous material but also nano-structured materialsNanoparticles and well designed structures with dimen-sions of 20 nm or smaller would serve to substantiallyincrease the triple phase boundary (TPB) catalytic activityand also to increase the thermal stabilityUntil now a large variety of methods have been

used to prepare both powder and films of perovskiteLSM such as a modified nitrate-citrate-Pechini methodutilizing gold nanoparticles induced by ultrasound-driven synthesis3 microwave-assisted solndashgel or solndashgel5

co-precipitation6 spray pyrolysis7 a self propagating hightemperature system8 a ball milling process9 grinding10

and a hydrothermal process11ndash15 It is worth mentioningthat the hydrothermal synthesis process draws lots of atten-tion for its green solvent nature Although several workshave been published on hydrothermal synthesis processesall of those were in a batch-type reaction systemHere for the first time we have synthesized perovskite

LSM nanomaterials by a continuous hydrothermal processfrom La(NO33 Sr(NO32 and Mn(NO32 precursors inthe presence of KOH solution The reaction time was ofthe order of milliseconds in environmentally benign super-critical water which does not require any organic solvent

324 Nanosci Nanotechnol Lett 2011 Vol 3 No 3 1941-490020113324004 doi101166nnl20111184

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Islam et al Hydrothermal Synthesis of LSM Nanoparticles by a Supercritical Flow Reaction System

The particles obtained at 400 C were 6ndash20 nm in sizewith crystalline structure and a high surface area and it isexpected that high TPB for good ORR at intermediate andlow operating temperature can be achievedA detailed description of the hydrothermal flow reac-

tor system is presented elsewhere16 Two kinds of startingsolutions are used in the synthesis of LSM nanoparticlesin the present study The first solution was a mixture of thenitrates of Sr (0014 M) La (0048 M) and Mn (006 M)which were dissolved in distilled water (solution A) Thesecond solution was of KOH (02 M) (solution B) Thesesolutions were pressurized and fed forward to a reactor bya high-pressure pump at a flow rate of 4 g minminus1 eachand these two streams were premixed before entering thehigh temperature reaction zone Meanwhile distilled waterwas fed by another high-pressure pump at a flow rate of22 g minminus1 (effluent C) and heated to an appropriate tem-perature by an electric furnace The two streams meet ata mixing point and then solution A and B were rapidlyheated to higher temperature (300ndash410 C) The reactantswere passed through a SUS 316 (159 mm id) tube reac-tor of 400 mm in length in around 017 s (87 and 1740 mmin length for 0037 and 017 s respectively) The desiredtemperature of reactants was maintained by another elec-tric furnace The pressure of the flow reaction system wasmaintained with back pressure regulator (Tescom Co Ltd26-1761) After the prescribed reaction time period thehydrothermal reaction was quenched with a cooling waterheat exchanger at the end of the reactor The product wasdepressurized through a back-pressure regulator and thecooled product solution including the particles was recov-ered at the exit of the systems by a membrane filter (poresize 0025 m) followed by washing with distilled waterand then dried in an electric oven at 60 C overnight

The possible way of occurring reaction for the synthesisof perovskite LSM materials as follows in supercriticalwater flow reaction system

MXn+nH2O sub andor supercritical=MOn+2nHX(1)

Where M=SrLa Mnso on X=NOminus3 CH3COO

minus(2)

The crystal structure of the products was detected by pow-der X-ray diffraction (XRD) (Rigaku Co Ltd ModelLint 2000) using Cu K radiation (40 kV and 30 mA)and a scan speed of 2 degree minminus1 and the synthesizedLSM materials was confirmed by a very close JCPDScard No 54-1195 shown in Figure 1 The ICP analysisresults showed that the composition of the material wasLa1minusxSrxMnO3 (where x = 01sim03) from the precursorscomposition of La08Sr02MnO3 Here it is to be noted thatthere is a broader peak around 27ndash29 in Figure 1 Thispeak might come from amorphous phase of SrO La2O3

and Mn2O3 Here it is noted that the LSM nanoparti-cles were not formed at subcritical water of 300ndash350 C

10 20 30 40 50 60 70 80 90

Inte

nsity

(ar

b u

nit) 01

2

104

111

202 02

4

116 21

4

208

LSM 400 ordmC

R at 350 ordmC

R at 300 ordmC

2θdeg

Fig 1 XRD pattern of hydrothermally synthesized LSM materialsunder supercritical conditions of water Here R Reaction

and 30 MPa pressure during the short reaction time Thesolubility features of the subcritical water are essentiallyresponsible for this result because the solubility of theintermediate species is expected to be high under theseconditions The cloudlike products observed only in sub-critical water experiments are probably due to the aggre-gation of intermediate species LSM nanoparticles wereformed in supercritical conditions (from 390 C to 410 C)which suggest that the solvent power of supercritical wateris much less than that of subcritical water The solubilityof intermediates is very low for the lower density differentdielectric constant and other specific features that controlthe hydrothermal reactions which might be factors respon-sible for the nanoparticle formation process16 Therefore avery high degree of supersaturation exists and rapid nucle-ation occurs to produce the desired products of LSM Thedielectric constant of the solvent (water) under such hightemperature is very low at around 2ndash10 Therefore the rateof the hydrothermal reaction is extremely high due to thehigh temperature and low dielectric constant of supercriti-cal waterThe size and morphology of the nanoparticles were

determined to be around 20 nm by TEM (Fig 2) whereFigure 2(b) revealed the crystallinity of the particles andthe selected area electron diffraction (SAED) pattern indi-cates that the LSM nanoparticles are single crystals TheBET analysis showed the particle size was 6 nm andthe representative figure was shown of 400 C sample Thecalculation of the mean particle size [D] from the BETsurface area is described in Eq (3)

D = 6s (3)

Where s is the BET surface area (m2 gminus1 and is thedensity of LSM (kg mminus3 The density of LSM is con-sidered to be 673times 103 kg mminus3 according to the crystallattice density in JCPDS card No 54-1195It is well known that one of the utmost attractive prop-

erties for catalytic activity of LSM materials in practi-cal SOFC applications is high surface area This study

Nanosci Nanotechnol Lett 3 324ndash327 2011 325

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Hydrothermal Synthesis of LSM Nanoparticles by a Supercritical Flow Reaction System Islam et al

(a)

(b)

Fig 2 (a) TEM image of hydrothermally synthesized LSM materialsat 400 C (b) HRTEM of the LSM materials inset shows the SAEDpattern

shows a significantly high surface area of 142 m2 gminus1

from the BET analysis which definitely resulted from thesmaller particles size Thus the present green syntheticprocess might become a key route in the development offuture cathode materials for SOFC technology In contrastwith batch-type hydrothermal synthesis the nanoparticlesobtained in our study are much smaller in size more-over in most of the batch-type synthesis processes thesize of the material was thousand times bigger than thepresent synthetic results For example Spooren et al11

showed synthesis of La05Sr05MnO3 from the precursors ofKMnO4 MnCl2 SrSO4 La(NO33 and KOH) by heatingfor a period of 24 h at 240 C obtaining cube shaped par-ticles 1ndash10 m in size Zhu12 reported hydrothermal syn-tesis of nanowires of La05Sr05MnO3 with typical lengthof 15 to 15 m and width 50 to 400 nm by heating at

280 C for 50 h from KMnO4 MnCl2 middot4H2O La(NO33 middot6H2O and Sr(NO32 precursors where KOH was used asmineralizer Liu13 synthesized La05Sr05MnO3 nanowiresfrom very different type of precusors than Zhu12 they usedKMnO4 Mn(CH2COO)2 middot4H2O Sr(CH2COO)2 middot12 middotH2OLa(CH2COO)3 middot 15H2O and KOH as precursors at rela-tively lower temperature of 200 C while heated for 72 hobtaining an average width and length of the nanowires of40 nm and 4 m respectively Wang14 prepared cubic sin-gle crystal of La05Sr05MnO3 with average particles size2ndash5 m from KMnO4 MnCl2 middot 4H2O La(NO33 middot 4H2Oand Sr(OH)2 middot8H2O and KOH precursors while heated at240 C for 1ndash3 days under autogenous pressure Lianget al15 synthesized smaller La05Sr05MnO3 particles com-pared to above reports They reported cubic particles of40ndash80 nm grain size in closed hyderthermal system fromKMnO4 MnCl2 middot4H2O La(NO33 middot6H2O and SrCl2 middot6H2Oand KOH precursors while reacted at 240 C for 24 hHowever till to date our synthesized LSM nanoparticlessmallest in size compared to the above studies whereclosed hydrothermal systems were used and precursorsalso were different than us In our study contineous reac-tion system has been used that can produce nanoparticlesin milliseconds of reaction time So our system till uniqueto present such a small LSM nanoparticles in hydrother-mal system with a draw back of morphological control Inshort the particles obtained in our study are much smallercompared to the batch technique till to date due to the shortprocess time which avoids Oswald ripening A heatingtime of more than 017 s such as 074 s shows impuritypeaks of SrMnO3 at 28

SrO at 39 and Mn2O3 at 58 as

shown in Figure 3 Here it is also concluded that 0037 sreaction also was not sufficient for particle formation bythe continuous hydrothermal processIn summary the LSM nanomaterials of 6ndash20 nm size

with high surface area (142 cm2 gminus1 were successfullysynthesized by a one step continuous hydrothermal syn-thetic process in supercritical water for the first timeNanoparticles of LMS were formed from nitrates of La Sr

10 20 30 40 50 60 70 80 90

2θdeg

SrMnO3

SrO Mn2O3

RT 074 s

RT 017 s

RT 0037 s

Inte

nsity

(ar

b u

nit)

Fig 3 Hydrothermally synthesized LSM materials using three differentreaction time at 400 C Here diams LSM peak RT reaction time

326 Nanosci Nanotechnol Lett 3 324ndash327 2011

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Islam et al Hydrothermal Synthesis of LSM Nanoparticles by a Supercritical Flow Reaction System

and Mn metals at 390ndash410 C and 30 MPa pressure Theparticle formation time was as short as 017 s The presentsynthetic route might be one of the key systems for furtherdevelopment of the materials controlling size and morphol-ogy that can be one of the potential candidates for SOFCcathode catalyst for future renewable energy generationHere it should be noted that in general the nanoparti-

cle synthesis by means of supercritical flow reaction sys-tem limits the morphological control of the particles Thepresent particles are also limited in this way To over comethis problem suitable organic material(s) (surface modifier)should be introduced on the surface of the particles There-fore our future target is to control the shape and size of thenanoparticles by the addition of suitable organic modifierWe hope that our results for a simple and environ-

mentally sustainable green system will stimulate furtherresearch to over come morphological shortcomings insupercritical water nanoparticle synthesis for future cath-ode catalysts for SOFC and renewable energy as well

Acknowledgment The authors are thankful to the e8group (wwwe8org) for the postdoctoral scholarship ofNazrul M Islam on the education for sustainable energydevelopment (ESED) programs and authors are alsothankful to Dr Stephen M Lyth for useful discussion

References and Notes

1 P Singh and N Q Minh Int J Appl Ceram Technol 1 5 (2004)2 P Decorse G Caboche and L-C Dufour Solid State Ionics

117 161 (1999)

3 S Barison M Battagliarin S Daolio M Fabrizio E Miorin P LAntonucci S Candamano V Modafferi E M Bauer C Bellittoand G Righini Solid State Ionics 177 3473 (2007)

4 A Weber and E Ivers-Tiffee J Power Sources 127 273 (2004)5 (a) Udayakumar and A Kodihalli Eur Pat 1796191A1 (2007)

(b) P Lenormand A Lecomte C Laberty-Robert F Ansart andA Boulle J Mater Sci 42 4581 (2007)

6 (a) M Marinsek K Zupan T Razpotnik and J Macek Materialsand Technology 41 85 (2007) (b) M Marinsek Materials and Tech-nology 43 79 (2009) (c) S Ramanathan P K Singh M B Kakadeand P K De J Mater Sci 39 3207 (2004) (d) A Ghosh A KSahu A K Gulnar and A K Suri Scripta Materialia 52 1305(2005)

7 H A Hamedani K-H Dahmen D Li H Peydaye-SaheliH Garmestani and M Khaleel Mat Sci and Eng B 153 1(2008)

8 Q Ming M D Nersesyan J T Richardson D Luss and A AShiryaev J Mater Sci 35 3599 (2000)

9 M M Seabaugh and S L Swartz US Pat 2003007033 A1 (2003)10 Q Zhang T Nakagawa and F Saito Journal of Alloy and Com-

pounds 308 121 (2000)11 (a) J Spooren A Rumplecker F Millange and R I Walton

Chem Maters 15 1401 (2003) (b) J Spooren R I Walton andF Millange J Maters Chem 15 1542 (2005)

12 (a) D Zhu H Zhu and Y Zhang J Phy Condensed Mater14 L519 (2002) (b) D Zhu H Zhu and Y Zhang J Cryst Growth249 172 (2003)

13 J Liu H Wang M Zhu B Wang and H Yan Mater Res Bull38 817 (2003)

14 D Wang R Yu S Feng W Zheng R Xu Y Matsumura andM Takano Chem Lett 32 74 (2003)

15 S Liang F Teng G Bulgan and Y Zhu J Phys Chem C111 16742 (2007)

16 (a) Y Hakuta H Ura H Hayashi and K Arai Mater Lett 59 1387(2005) (b) Y Hakuta H Ura H Hayashi and K Arai MaterChem Phys 93 466 (2005)

Received 20 December 2010 Accepted 1 February 2011

Nanosci Nanotechnol Lett 3 324ndash327 2011 327