Carbon Nanotubes: Development of Nanomaterials for Hydrogen Storage

Transcription

1 Carbon Nanotubes: Development of Nanomaterials for Hydrogen Storage Hongjie Dai Department of Chemistry & Laboratory for Advanced Materials Stanford University GCEP, September 19, 2006

2 Outline Can carbon nanotubes (CNTs) be used for efficient hydrogen storage? - Physisorption (non-covalent)? - Chemisorption (covalent)? Synthesis of CNTs for hydrogen storage Interaction of CNTs with reactive H-species Funding: GCEP; Collaborators: A. Nilson; K. Cho; B. Clemens.

3 Single-Walled Carbon Nanotubes (SWNT) diameter ~1-2 nm, molecular wire length: 10 nm to 10mm Chiral angle θ Sp2 C-C bonding: one of the strongest bond in nature Multi-walled CNTs: ~ nm in diameter Radius of curvature is important to physical/chemical properties

4 Synthesis by Chemical Vapor Deposition CVD PECVD C x H y catalyst Furnace C C x H y plasma ~600 ºC 600 C CH 4 C+ H 2 plasma 1-2 nm seed Base growth Yuegang Zhang, Yiming Li, J. Phys. Chem, 1999;

5 Previous: Patterned Growth of SWNTs by CVD Jing Kong, Hyongsok Soh, Calvin Quate, H. Dai Nature, 1998.

6 SWNTs Synthesis From Individual Nanoparticles pattern individual seeds grow 1 tube per seed CVD 2nm Fe seed CVD 100 nm 250 nm 300 nm Ali Javey, H. Dai, JACS, 2005

7 CVD Growth of Vertically Aligned Multi-Walled CNTs Shoushan Fan, Nathan Franklin, H. Dai., Science, 1999.

8 Synthesis of Single-Walled Nanotubes by PECVD CH 4 plasma 600 ºC 5 nm C CH 4 C+ H 2 SWNT 1% O 2 2 nm Fe particle as growth seed A quasi-remote plasma enhanced CVD approach

9 High Yield Vertical-SWNTs by PECVD (a) (b) Monolayer of ~2 nm Fe particles RBM G Grown by oxygen Assisted PECVD (c) Raman shift (cm -1 ) V-SWNT film (d) 1 inch 20 nm Exclusive (e) single-walled, no multi-walled; Highly repeatable SiO 2 (f) Guangyu Zhang, et al., PNAS, 2005; GCEP patent

10 Patterned Growth of Vertical SWNTs (a) (b) (c) (d) (e) 20 µm 20 µm Reproducible growth of vertical SWNTs

11 Vertical SWNTs (V-SWNT) Patterns

12 O 2 Addition (~1%) Drastically Enhances PECVD of SWNTs From CH 4 Plasma A B 5 µm With O2 Addition O 2 -free CH 4 growth Same catalyst, drastically different yield with and without oxygen! In plasma CVD, H species are detrimental to SWNT formation Oxygen scavenging of H: O + H OH favors SWNT formation

13 O Scavenging of H: O + H OH Favors SWNT Growth C H C-feeding to growth H attacking sp 2 C O Fe-C OH Optical Emission OH Wavelength (nm) Plasma of C x Hy: C x H y C + H (H is inevitable) C is needed for growth of SWNT. H does not favor SWNT formation, especially for small tubes Solution to the dilemma of PECVD: add oxygen (<4%)

14 Vertical SWNTs Transferred onto Various Substrates (Cu, polymer ) A V-SWNT film floating on H2O H 2 O bath Si substrate H 2 O bath B V-SWNT Cu Cu substrate Polymer binding layer

15 Vertical SWNTs on Various Substrates (Cu, polymer ) Cu Glass Plastic

16 An Artistic Presentation of V-SWNT Films

17 of SWNTs (Guangyu Zhang, et al., JACS, 2006) 200nm 2.5 nm SWNTs swell upon hydrogenation Swelling uniform along tube length

18 Theoretical Prediction Atomic structure change due to hydrogenation K.Park, et al., J. Phys. Chem B 109, 8967 (2005).

19 Raman Spectroscopy of Hydrogenation/dehydrogenation of SWNTs G As-grown Plasma Annealed D Raman Shift (cm -1 ) Hydrogenation decreases G / D band ratio. Complete reversal upon 500 ºC annealing (starts at ~200ºC).

20 Infrared Spectroscopy of Hydrogenated/dehydrogenated SWNTs 1.00 A IR Transmittance 0.99 CH/CH 2 RT Plasma 500 annealed CH wavelength (cm -1 ) CH x species observed on SWNTs, x=1, cm -1 : sp3 CH stretch/asymmetric stretch of sp3 CH 2 groups. 2850cm -1 : symmetric stretch of sp3 CH 2

21 Electrical Transport in a SWNT Through Hydrogenated/Dehydrogenation S (c) 10-7 D I ds (A) V gs (V) Hydrogenation decreases conductance by orders of magnitude Reversible upon 500ºC annealing.

22 Hydrogenation/Hydro-carbonation (Etching) Under Harsh high Plasma power, 3min 10 min

23 Hydrogenation/Hydro-carbonation (Etching) Under ºC 10min 200ºC plasma 300 ºC plasma 400 ºC plasma 1.5 Diameter (nm) T ( ο C) Smaller diameter SWNTs are more easily etched Diameter dependent chemical reactivity!

24 Conclusions Based on Microscopy, Spectroscopy & Electrical Data Reversible hydrogenation can be achieved with SWNTs under specific hydrogenation conditions SWNTs can retain structural integrity through hydrogenation/dehydrogenation processes A viable hydrogen storage approach Optimum diameter of may SWNTs exists for repeated reversible hydrogenation/dehydrogenation at low temperatures

25 Summary Role of hydrogen in PECVD growth identified. Highly efficient growth of SWNTs achieved. Reversible hydrogenation of SWNTs can be achieved with retained structures and properties. Hydrocarbonation/etching can occur. Diameter dependent reactivity elucidated. Next: Identify optimum diameter of SWNTs that allow for hydrogenation and reversal at low temperatures.

26 Acknowledgements Guangyu Zhang David Mann Pengfei Qi Yuerui Lu Xinran Wang Ali Javey James Gibbons Yoshio Nishi Anders Nilson K. J. Cho Bruce Clemens GCEP