Polymorphic Nanocrystalline Metal Oxides

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<ul><li> 1. Polymorphic nanocrystallinemetal oxides Thermodynamics And ApplicationsShantanu Sood Department of Materials science and engineering</li></ul> <p> 2. Layout of the presentation Nanocrystalline Metal oxidesElaborate the importance of nanoscale for polymorphicmetal oxides Thermodynamics of polymorphic transitionsExplanation of a thermodynamic model to explain thediffering transitions due to nanoscale Applications One material many structures, differing behavior 3. Ceramic Materials analysis Binary metal oxides are some of the most usefulmaterials and the modifications serve as the basis ofour civilization. Nanocrystalline metal oxides are of current researchinterest. Synthesis Techniques: Sol-Gel Electrospinning Flame Spray Pyrolysis Characterization Techniques: XRD Electron Microscopy Differential Scanning Calorimetry 4. Polymorphs due to Phase transition Various polymorphs of metal oxides occur dueto phase transitions. In bulk size(micrometer or higher grainsize), temperature and pressure are the factorsthat affect phase transition. In nano size(100nm or less), temperature andparticle size are the two factors that contributeto phase transition It is observed that there is a lowering of externalenergy required for phase transition at nanoscale, this helps lower the temperature andpressure conditions 5. Example Sood(2012)Bulk stateNano stateParticle SizeMicron-size 8nmTransformation1473K 873KTemperature Ref. [1],[2]Tetragonal(I36)Tetragonal(I41/amd)1. P. I. Gouma and M. J. Mills, "Anatase to Rutile Transformation in Titania Powders", J. Am. Ceram. Soc., 84 [3], pp. 619-622, 2001.2. M. R. Ranade, A. Navrotsky, H. Z. Zhang, J. F. Banfield, S. H. Elder, A. Zaban, P. H. Borse, S. K. Kulkarni, G. S. Doran , and H. J. Whitfield. National Acad Sci., vol. 99 no. Suppl 2, 2002, 6476-6481, DOI: /10.1073/pnas.251534898PNAS 6. Other Examples of polymorphs fromliterature Bulk and Nano conditions Phase Bulk Nano Ref. -Fe2O3 to -933K 5-25nm 563-673K [1],[2] Fe2O3 Monoclinic to1143K10nm Room T [3]Tetragonal ZrO2-WO3 to - 220K -- Room T [4]WO3 -Al2O3 to -773K 3.2nmRoom T [5],[6] Al2O3 1. Fu Su Yen, Wei Chien Chen, Janne Min Yang, and Chen Tsung Hong. Nano Letters, Vol. 2, No. 3, 2002, 245-252, DOI: /10.1021/nl010089m 2. Ozden Ozdemir and Subir K. Banerjee, Geophysics research letters, Vol. 11, No. 3, 1983, Pages 161-164, DOI: /10.1029/GL011i003p00161 3. R. C. Garvie, M. F. Goss. J. Mater. Sc. 21, 1986, pp 1253-1257, DOI: /10.1007/BF00553259 4. L. Wang, A. Teleki, S. E. Pratsinis, and P. I. Gouma. Chem. Mater. , 20, 2008, 47944796, DOI: /10.1021/cm800761e 5. Shuxue Zhou, Markus Antonietti, and Markus Niederberger. Small 3(5), 763(2007). 6. .J. M. McHale, A. Navrotsky, A. J. Perrotta, J. Phys. Chem. B, 101 (4), 1997, pp 603613, DOI: /10.1021/jp9627584 7. Thermodynamic model for explanation Bulk NanoFor equal mass in grams of material, In nanometerBulk volume = Nano volumedimensions, grain size is soNano number of grains &gt;&gt;&gt;small that most atoms areBulk number of grainssurfaceTotal Surface area = Atoms exerting very high(number of grains).(4).(3.14).(r)2 pressure. Surface atoms have highThis leads to a very highcharge due to unfilled energySurface area to volume ratio.bands and broken bonds.Expression for Bulk state phase transformation.This causes an exponential increaseThis cause internalin surface energypressure. 8. Surface Area effect[1] Internal Pressure effect[1],[2] P for water drops of different radiiDroplet 1 mm0.1mm1m10nmradius P (atm)0.0014 0.01441.436 143.6 From thermodynamics we know that at the point of equilibrium, free energy is zero, thus, solving for critical particle size, r,1. Jiang, Q. Yang, C. C. Current Nanoscience Vol. 4 Issue 2, May 2008, , pp179-200, DOI: /10.2174/1573413087843409492. Sheryl H. Ehrman, Journal of Colloid and Interface Science. Volume 213, Issue 1, May 1999, Pages 258261, DOI: /10.1006/jcis.1999.6105 9. Lowering of activation barrier due to particle size In bulk, external pressureis required to overcomethe barrier for phasetransition. But at nano size, theinternal pressure andsurface effects contributeand lower the barriermaking available the highpressure phases atambient conditions. Thus increasing thespectrum of phases thatare available for eachmaterial 10. Applications 11. Gas Sensing -MoO3 on NH3 gas.[3] -WO3 on acetone gas.[2] Anatase TiO2 on CO gas.[1] Orthorhombic Structure Monoclinic Structure Tetragonal Structure Grain Size = 50nmGrain Size = 20nmGrain Size ~ 13.2nm Temperature = above 425K Temperature = Room Temp. Temperature = 773K1.Ana M. Ruiz, Albert Cornet, Kengo Shimanoe, Joan R. Morante, Noboru Yamazoe. Sensors and Actuators B: Chemical. Vol 108, Iss 1-2, July 2005, Pages 34-40, DOI:/10.1016/j.snb.2004.09.0452.L. Wang, A. Teleki, S. E. Pratsinis, and P. I. Gouma. Chem. Mater. , 20, 2008, 47944796, DOI: /10.1021/cm800761e3.Arun K. Prasads. Phd thesis, Stony brook university, May 2005. 12. Catalysis Solid Oxide Fuels CellsSOFCs are an oxygen ion conducting electrolyte through which the oxide ions migrate from theenvironment electrode (cathode) side to the fuel electrode (anode) side reacting with the fuel(H2, CO, etc.) thereby generating electrical voltage.Mesopore size distribution and nanocrystalline channel walls lead to improvements[1] in: fuel mass transport, oxide ion mobility, electronic conductivity, and charge transferCubic Zirconia[2], as a Catalyst Yttrium stabilized Nanocrystalline Cubic Zirconia Benefits like, uniform intergranular pore size and greateroxide ion conductivity due to yttrium stabilization Bloom EnergyPolymorphs of Bismuth Oxide[3], as catalyst Bismuth oxide based systems have higher ion conductivity than Zirconia based systems.-Bi2O3 -Bi2O3 -Bi2O3 -Bi2O3 Ref Ion Conductivities(Scm-1) 3X10-4 2X10-3 5X10-3 1 [3]1. Marc Mamak, Neil Coombs, and Geoffrey Ozin. J. Am. Chem. Soc., 122 (37), 2000, pp 89328939, DOI: /10.1021/ja00136772. S.C Singhal. Solid State Ionics. Vol 135, Iss 14, November 2000, Pages 305313, DOI: /10.1016/S0167-2738(00)00452-53. Laarif, A. and Theobald, F. Solid State Ionics, 21, 1986, 183-193, DOI: /10.1016/0167-2738(86)90071-8 13. Electrochemical Cells and Batteries Ions like H+, Li+, Na+, K+ etc intercalate in to the lattice of polymorphic metal oxides Some structures have a better intercalation capacity and charge discharge capacities than others making them better for charge storage applications Example Hexagonal MoO3 show better charge storage capacitythan orthorhombic MoO3[1],[2]Li IntercalationDischargeCapacitycapacityOrthorhombic MoO3 1.5Li/MoO3300mAh/gHexagonal MoO32.2Li/MoO3400mAh/g Similarly, hexagonal WO3 also readily form Tungstenoxide bronze(MxWO3), and has better intercalationSood(2012)capacity than orthorhombic WO3[3]1. Jimei Song, Xiong Wang, Xiaomin Ni, Huagui Zheng, Zude Zhang, Mingrong Ji, Tao Shen, Xingwei Wang. Materials Research Bulletin. Vol 40, Iss 10, October 2005, Pages 17511756, DOI: /10.1016/j.materresbull.2005.05.0072. S.H. Lee, M.J. Seong, C.E. Tracy, A. Mascarenhas, J.R. Pitts, S.K. Deb. Solid State Ionics, 147, 2002, p. 129, DOI: /10.1016/S0167-2738(01)01035-93. K.P. Reis, A. Ramanan, M.S. Whittingham, J. Solid State Chem. 96, 1992, pp 31-47, DOI: /10.1016/S0022-4596(05)80294-4 14. Conclusion Nano scale makes available polymorphs of metaloxides that were hitherto unavailable due toconditions of high pressure and temperatureinvolved The internal pressure and surface energy due tonano dimensions helps compensate for highpressure needed externally in bulk state Some polymorphs which have better propertiescan now be used in applications like assensing, catalysis etc, as no high pressuresynthesis is required 15. Thank You</p>


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