Polymorphic nanocrystalline metal oxides Thermodynamics And Applications Shantanu Sood Department of Materials science and engineering
Layout of the presentation• Nanocrystalline Metal oxides Elaborate the importance of nanoscale for polymorphic metal oxides• Thermodynamics of polymorphic transitions Explanation of a thermodynamic model to explain the differing transitions due to nanoscale• Applications One material many structures, differing behavior
Ceramic Materials analysis• Binary metal oxides are some of the most useful materials and the modifications serve as the basis of our civilization.• Nanocrystalline metal oxides are of current research interest.• Synthesis Techniques: – Sol-Gel – Electrospinning – Flame Spray Pyrolysis• Characterization Techniques: – XRD – Electron Microscopy – Differential Scanning Calorimetry
Polymorphs due to Phase transition• Various polymorphs of metal oxides occur due to phase transitions.• In bulk size(micrometer or higher grain size), temperature and pressure are the factors that affect phase transition.• In nano size(100nm or less), temperature and particle size are the two factors that contribute to phase transition• It is observed that there is a lowering of external energy required for phase transition at nano scale, this helps lower the temperature and pressure conditions
Example Sood(2012) Bulk state Nano state Particle Size Micron-size 8nm Transformation 1473K 873K Temperature Ref. ,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 , 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
Other Examples of polymorphs from literature – Bulk and Nano conditions Phase Bulk Nano Ref. γ-Fe2O3 to α- 933K 5-25nm 563-673K , Fe2O3 Monoclinic to 1143K 10nm Room T Tetragonal ZrO2 α-WO3 to ε- 220K -- Room T  WO3 γ-Al2O3 to α- 773K 3.2nm Room T , 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, 4794–4796, 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 603–613, DOI: /10.1021/jp9627584
Thermodynamic model for explanation Bulk Nano For equal mass in grams of material, In nanometer Bulk volume = Nano volume dimensions, grain size is so Nano number of grains >>> small that most atoms are Bulk number of grains surface Total Surface area = Atoms exerting very high (number of grains).(4).(3.14).(r)2 pressure. Surface atoms have high This leads to a very high charge due to unfilled energy Surface area to volume ratio. bands and broken bonds.Expression for Bulk state phase transformation. This causes an exponential increase This cause internal in surface energy pressure.
Surface Area effect Internal Pressure effect, ΔP for water drops of different radii Droplet 1 mm 0.1mm 1μm 10nm radius ΔP (atm) 0.0014 0.0144 1.436 143.6 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 258–261, DOI: /10.1006/jcis.1999.6105
Lowering of activation barrier due to particle size• In bulk, external pressure is required to overcome the barrier for phase transition.• But at nano size, the internal pressure and surface effects contribute and lower the barrier making available the high pressure phases at ambient conditions.• Thus increasing the spectrum of phases that are available for each material
Gas Sensing β-MoO3 on NH3 gas. ε-WO3 on acetone gas. Anatase TiO2 on CO gas. Orthorhombic Structure Monoclinic Structure Tetragonal Structure Grain Size = 50nm Grain Size = 20nm Grain 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, 4794–4796, DOI: /10.1021/cm800761e3. Arun K. Prasad’s. Phd thesis, Stony brook university, May 2005.
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 in:• fuel mass transport,• oxide ion mobility,• electronic conductivity, and• charge transferCubic Zirconia, as a Catalyst• Yttrium stabilized Nanocrystalline Cubic Zirconia• Benefits like, uniform intergranular pore size and greater oxide ion conductivity due to yttrium stabilization Bloom EnergyPolymorphs of Bismuth Oxide, as catalyst• Bismuth oxide based systems have higher ion conductivity than Zirconia based systems. α-Bi2O3 β-Bi2O3 γ-Bi2O3 δ-Bi2O3 Ref Ion Conductivities(Scm-1) 3X10-4 2X10-3 5X10-3 1 1. Marc Mamak, Neil Coombs, and Geoffrey Ozin. J. Am. Chem. Soc., 122 (37), 2000, pp 8932–8939, DOI: /10.1021/ja00136772. S.C Singhal. Solid State Ionics. Vol 135, Iss 1–4, November 2000, Pages 305–313, 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
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 capacity than orthorhombic MoO3, Li Intercalation Discharge Capacity capacityOrthorhombic MoO3 1.5Li/MoO3 300mAh/gHexagonal MoO3 2.2Li/MoO3 400mAh/g• Similarly, hexagonal WO3 also readily form Tungsten oxide bronze(MxWO3), and has better intercalation Sood(2012) capacity than orthorhombic WO3 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 1751–1756, DOI: /10.1016/j.materresbull.2005.05.007 2. 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)01035-9 3. 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
Conclusion• Nano scale makes available polymorphs of metal oxides that were hitherto unavailable due to conditions of high pressure and temperature involved• The internal pressure and surface energy due to nano dimensions helps compensate for high pressure needed externally in bulk state• Some polymorphs which have better properties can now be used in applications like as sensing, catalysis etc, as no high pressure synthesis is required