Nano-materials and Nano-structures - Introduction

1. Nano-materials and Nano-structures - Introduction Economy of promises! Hype? GETTY Most people learn about developments in science and medicine from the mass media. Can nanotechnology cure cancer by 2015? Or Real?

2. Nano-particles Sol-gel; high-energy ball milling, hydrothermal Nano-tube, nano-wire, nano-rod and nano-belt Evaporation(Science 291 (2001) p 1947) Laser ablation(Science 279 (1998) p 208) Anodization(J. Mater. Res. 16 (2001) p 3331) Electro-spinning(Nano lett. 3 (2003) p 555) Surface modification (Adv. Matls., 16[3] (2004) p 260, in print) Nano-porous structures Emulsion templating(J. Mater. Res. 18 (2003) p 156) Photo-electrochemical etching(Electrochem. Solid-State. Lett. 1 (1998) p 175) Fabrication Techniques

3. Xerogel film Dense film Heat Coating Metal Alkoxide Solution Dense ceramics Wet gel Xerogel Coating Hydrolysis Polymerization Evaporation Heat Gelling Extraction of solvent Precipitating Aerogel Sol Uniform particles Sol-gel Technologies and Their products Source: http://www.chemat.com/html/solgel.html

4. Process Parameters Thin film & particle properties Sol-gel synthesis : Overview Mechanistic step Process step gelation Pore elimination pyrolysis dehydration organometallics in solution viscous gel Spin/dip coat Xerogel film • Heterogeneous nucleation • Homogeneous nucleation • Growth • Phase transformation heat Substrate crystallization heat Nano-particles Crystalline film

5. Typical synthesis conditions • Ti precursors - Titanium isopropoxide - Titanium butoxide - Titanium methoxyethoxide - Ti diisopropoxide bisacetylacetonate - Titanium methoxypropoxide • Solvent : Various alcohol - 2-methoxy ethanol - Methanol - n-butanol • Complexing / chelating agents - Acetyl acetone - Acetic acid • Crystallization temperatures - 100 ~ 1000 °C • Sn precursors - Tin Tetrachloride - Tin ethoxide - Tin ethylhexanoate - di-n-butyl-tin-bis-acetylacetonate

6. Sol-gel synthesis of metal oxides • Hexavalent oxides, WO3 and MoO3 • Tungsten ethoxide W(OEt)6 solutions stabilized by acetic acid have been used to form WO3 expensive and high sensitivity toward water • WOCl4 (Chloro-alkoxides) readily dissolves in all kinds of alcohols forming stable solutions of oxychloroalkoxides  cheap and stable for several months • WOCl4 + xROH  WOCl4-x(OR)x +xHCl • MoO3 can be deposited from alkoxides, chlorides, chloro-alkoxides or molybidic acid • 2. Tetravalent oxides, TiO2 and SnO2 • TiO2 can be easily deposited from alkoxide solutions. The best precursors seemsto be Ti(OBun)4 • SnO2 can be obtained from the organic tin oxide precursor di-n-butyl-tin-bis-acetylacetonate ((C4H9)2Sn(acac)2)

7. Sol-gel sub-micron and nano-particles Sol-gel SnO2 Sol-gel silica spheres

8. Sol-gel for BaTiO3 Ferroelectrics J. Sol-gel Sci. Tech., 15, 263-270 (1999)

9. TiO2 by Sol – gel for sensors La-modified TiO2 by a sol–gel [La/Ti] atomic ratio of 5% and 10% Response transients to 100 ppm ethanol at 500 °C for the La–TiO2 films calcined at 900 °C Addition of La stabilized the anatase phase and the grain size At 600°C, 27 nm for 5% La/Ti atomic ratio and 11 nm for 10% Sensors and Actuators B 111–112 (2005) 7–12

10. SnO2 by Sol - gel for sensors SnO2, In2O3 and In2O3–SnO2 thin films by sol–gel technique Fine and uniform SnO2 film Sensors and Actuators B 114 (2006) 646–655

11. Application of nanoparticles as gas sensor: SnO2 Diameter 30 nm Diameter 18 nm O2 O2 CO Time dependence reflectance at 620 nm during exposure at CO and O2 at 400 C for samples Representative TEM micrograph of sample calcined at 550 C Formation of oxygen vacancies in a reducing gas leads to a decrease in the reflectance of the SnO2 particles. In the smaller nanoparticles, oxygen vacancies can be formed to a higher extent leading to a higher sensitivity.

12. General Conclusion of Sol - gel • Versatile powders, films, fibers, monoliths, aerogels, xerogels • High purity controlled by purity of starting solution • Homogeneity controlled by hydrolysis, condensation and polymerization reactions • Limited mass-scale production particles, coatings, thin-films

13. High-energy ball milling xSnO2-(1-x)αFe2O3 are fabricated by mechanical alloying Sens. Actuators B 65 (2000) 361

14. VS (Vapor-Solid) • No metal catalysts used • Vapor phase chemical species adsorbs on the surface of the substrate due to DT leading to 1-D nucleation and growth Continuation of adsorption of Si SiH4(g)  Si (g) + 2H2 substrate Low temperature zone Mechanism is not well established Screw dislocation (Sears), Defect (twins or stacking fault) for nucleation sites TEM image showing the silicon nanowires prepared via VS2 2P.P. Yu, Synthesis of nano-scale silicon wires by excimer laser ablation at high temperature, Solid state communication, 105(1998) 403-407.

15. Experimental setup Nanobelts (Evaporation Technique) Furnace Alumina tube Ar Ar +SnO2 deposition evaporate Ar SnO2 powder Alumina substrate Cooling water • Powders are placed at the center of the tube • Alumina substrate is placed downstream inside the tube • Evacuate tube to around 210-3 torr • Evaporation is conducted at 1350 °C in Ar atmosphere

16. Nano-belts (Evaporation Technique) Source: Science, 291 (2001) 1947 • Typical widths of nano-belts are in the range of 50-200 nm • The length is several hundred micrometer or more. • It has rectangular cross-section • Each nano-belt is a single crystal without dislocations • Nano-belts of ZnO, SnO2, In2O3, CdO, Ga2O3, PbO2 can be easily produced by evaporation

17. VLS (Vapor-Liquid-Solid) Liquid metal droplet forms Droplet captures X from gas leading to saturation and nucleation X nanowire forms by diffusion of X X: SiH4 Si + 2H2 FeSix metal liquid droplet Metal catalyst cap Substrate (Si) substrate substrate Typical Si nanowires1 1S.Q. Feng, The growth mechanism of silicon nanowires and their quantum confinement effect, Journal of crystal growth, 209(2000) 513-517.

20. Nanowire fabrication by electrospinning • A polymer solution or melt is injected from a small nozzle under the influence of an electric field. • The build up of electrostatic charges on the surface of a liquid droplet induces the formation of a continuous ultrathin fiber. • Various engineering plastics, biopolymers, electrically conductive polymers, and oxide nanowires have been produced by the technique. Nanowire source materials Schematic view of the setup for electrospinning Controlling variables: Electric field strength, polymer molecular weight and deposition distance

21. TiO2 nanowires by electrospinning TEM image of the same sample after it was calcined in air at 500 C for 3 hrs TEM image of TiO2/PVP composite nanowires fabricated by electrospinning For the inorganic nanowires like TiO2, inorganic precursors are required to be added in liquid polymer solutions. For the above results, titanium tetraisopropoxide (Ti(OiPr)4), poly vinyl pyrrolidone (PVP, a liquid polymer) and ethanol were used. Added acetic acid to stabilize the solution and control the hydrolysis. As-spun nanowires in air transformed to TiO2 by the hydrolysis of Ti(OiPr)4. Finally, PVP is removed by calcining at 500 C in air. D. Li, Y. Xia, Fabrication of titania nanofibers by electrospinning, Nano Letters 3 (2003) 555-560.

22. Co-axial process helps fabricate hollow tubes. Mineral oil is extracted (etched) in octane and calcined in air at 500 C to remove PVP. Titania nano-tube fabricated by electrospinning. D. Li, Y. Xia, Direct Fabrication of Composite and Ceramic Hollow Nanofibers by Electrospinning, Nano Lett. 4 (2004) 933-938.

23. MoO3 nanowires by electrospinning TEM image of MoO3/PVP composite nanowires before calcination HRTEM image of MoO3 nanowires after calcination at 500 C in air 0.5 M molybdenum isopropoxide sol was prepared and mixed with 0.1mM polyvinylpyrrolidone. The solution was electrospun in air using a DC voltage power supply at 20 kV.

26. Experimental setup Nanotubes (Anodization) Electrolyte TiO2 I Electrolyte Ti OH-, O2- dissolution TiO2 oxidation Ti4+ Ti

27. Nano-tubes (Anodization) Source: Adv. Mater. 15 (2003) 624 • TiO2 nano-tubes fabricated by anodization • Diameter of nano-tubes: 10 – 80 nm • Nano-tubes are oriented and perpendicular to the surface • Nano-tube length increases with anodization time (400 nm in 20 min) • As-received nano-tubes are amorphous and oxygen deficient Annealing in oxygen atmosphere is required

28. Photo-electrochemical Etching UV light mirror Ec EF hν UV supply e- Ev Current TiO2 H2SO4 Eg power supply Sintering H2 heat treatment PEC etching Counter electrode TiO2 H2SO4 h+ 1300 °C for 6hrs 700°C for 4hrs in 10% H2/N2 Nano-honeycomb structure TiO2 + SO42- + 2h+ → TiO·SO4 + 1/2O2

29. 1 μm 1 μm Invention of Titania Nano-fibers • Process Adv. Matls., 16[3], 260 (2004) Patent appl. 2003-678772 Sintering H2 heat treatment 1200 °C for 6hrs 700°C for 8hrs in 5% H2/N2 heat-treatment - Nano-fibers are parallel, oriented in the same direction - Diameter of nano-fibers: 15 – 50 nm Length of nano-fibers: up to 5 μm

30. Ceramic Nano-machining? Wide variety of possibilities by gas-phase reaction - CISM Nano-fibers Nano-channels (different sintering) Nano-whiskers (doped with Fe2O3) Nano-lamellar (two-phase)

31. <001> <110> Patterned TiO2 Nanowires Planar view Tilted view

32. 2 m Gas-Phase Growth in SnO2 • Experimental Conditions: • Sintered 1300 C for 24 hours • Heat treat in a humid 5% H2 – 95% N2 atmosphere at 700 C for 4 hours • 1 L/min gas flow • Au-coated SnO2

33. TiO2 nano-wire on Ti64 alloy in Ar Nano-wire diameter : ~ 50-100 nm Length of nano-wire : ~ 3-5 m Magnified image for the circled area

34. Self-assembled Nano-islands 200 nm (Gd,Ce)O2 thin film on YSZ substrate breaks up into a psuedo-periodic array of single crystal islands with average size of 200 nm upon annealing at 1150oC.

35. Challenges and Opportunities Mass-scale synthesis and fabrication Materials R&D, manufacturing Assembling into devices and structures Nano-packaging, bonding, adhesion Stability in hostile environments Surface and interface chemistry and physics Fundamental understanding of properties Mechanism and modeling Potential applications Sensing, catalysis, bio-medical, nano-electronics nano-composites, TBC, corrosion

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