Polymorphic nanocrystalline metal oxides

Thermodynamics And Applications

Shantanu SoodDepartment 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

Tetragonal(I36) Tetragonal(I41/amd)

Sood(2012)

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

Bulk state Nano state

Particle Size Micron-size 8nm

Transformation Temperature

1473K 873K

Ref. [1],[2]

Other Examples of polymorphs from literature – Bulk and Nano conditions

Phase Bulk Nano Ref.

γ-Fe2O3 to α-Fe2O3

933K 5-25nm 563-673K [1],[2]

Monoclinic to Tetragonal ZrO2

1143K 10nm Room T [3]

α-WO3 to ε-WO3

220K -- Room T [4]

γ-Al2O3 to α-Al2O3

773K 3.2nm Room T [5],[6]

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/nl010089m2. Ozden Ozdemir and Subir K. Banerjee, Geophysics research letters, Vol. 11, No. 3, 1983, Pages 161-164, DOI: /10.1029/GL011i003p001613. R. C. Garvie, M. F. Goss. J. Mater. Sc. 21, 1986, pp 1253-1257, DOI: /10.1007/BF005532594. L. Wang, A. Teleki, S. E. Pratsinis, and P. I. Gouma. Chem. Mater. , 20, 2008, 4794–4796, DOI: /10.1021/cm800761e5. 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

Surface atoms have high charge due to unfilled energy bands and broken bonds.

In nanometer dimensions, grain size is so small that most atoms are surfaceAtoms exerting very high pressure.

This cause internal pressure.

For equal mass in grams of material,Bulk volume = Nano volumeNano number of grains >>> Bulk number of grainsTotal Surface area = (number of grains).(4).(3.14).(r)2

This leads to a very high Surface area to volume ratio.

Expression for Bulk state phase transformation.

This causes an exponential increase in surface energy

Bulk Nano

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

Surface Area effect[1] Internal Pressure effect[1],[2]

From thermodynamics we know that at the point of equilibrium, free energy is zero,thus, solving for critical particle size, ‘r’,

ΔP for water drops of different radiiDroplet radius

1 mm 0.1mm 1μm 10nm

ΔP (atm) 0.0014 0.0144 1.436 143.6

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

Applications

Gas Sensing

1. 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.2004.09.045

2. 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.

ε-WO3 on acetone gas.[2]β-MoO3 on NH3 gas.[3] Anatase TiO2 on CO gas.[1]

Orthorhombic StructureGrain Size = 50nmTemperature = above 425K

Monoclinic StructureGrain Size = 20nmTemperature = Room Temp.

Tetragonal StructureGrain Size ~ 13.2nmTemperature = 773K

Catalysis – Solid Oxide Fuels Cells

Cubic Zirconia[2], as a Catalyst

Polymorphs of Bismuth Oxide[3], as catalyst

Mesopore size distribution and nanocrystalline channel walls lead to improvements[1] in: • fuel mass transport, • oxide ion mobility, • electronic conductivity, and • charge transfer

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

α-Bi2O3 β-Bi2O3 γ-Bi2O3 δ-Bi2O3 Ref

Ion Conductivities(Scm-1) 3X10-4 2X10-3 5X10-3 1 [3]

Bloom Energy

• Yttrium stabilized Nanocrystalline Cubic Zirconia• Benefits like, uniform intergranular pore size and greater

oxide ion conductivity due to yttrium stabilization

• Bismuth oxide based systems have higher ion conductivity than Zirconia based systems.

SOFCs are an oxygen ion conducting electrolyte through which the oxide ions migrate from the environment electrode (cathode) side to the fuel electrode (anode) side reacting with the fuel (H2, CO, etc.) thereby generating electrical voltage.

Electrochemical Cells and Batteries

Sood(2012)

Li Intercalation Capacity

Discharge capacity

Orthorhombic MoO3 1.5Li/MoO3 300mAh/g

Hexagonal MoO3 2.2Li/MoO3 400mAh/g

• Hexagonal MoO3 show better charge storage capacity than orthorhombic MoO3[1],[2]

• Similarly, hexagonal WO3 also readily form Tungsten oxide bronze(MxWO3), and has better intercalation capacity than orthorhombic WO3[3]

• 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

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)01035-93. 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

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

Thank You