Functionalized Carbon Nanotubes a key to nanotechnology?

Transcription

1 1 27th Max Born Symposium Multiscale Modeling of Real Materials Wroclaw, Sep 19, 2010 Functionalized Carbon Nanotubes a key to nanotechnology? Karolina Milowska, Magda Birowska & Jacek A. Majewski Faculty of Physics, University of Warsaw

2 2 Carbon nanotubes (CNTs) S. Iijima, Nature 354, 56 (1991) Carbon Nanotubes (CNTs) S. Iijima, Nature 354, 56 (1991) Flag ship of the nanotechnology Electrical properties High conductivity FET Mechanical properties Very high Young s modulus (9,0) CNT- polymer composite

3 3 CNTs Growth Methods chemical vapor deposition laser ablation arc discharge methods Why to functionalize CNTs? Pristine nanotubes are unfortunately insoluble in many liquids such as water, polymer resins, and most solvents. Thus they are difficult to evenly disperse in a liquid matrix (for example, epoxies and other polymers). This complicates efforts to utilize the nanotubes' outstanding physical properties in the manufacture of composite materials, as well as in other practical applications (biological, optical, magnetic, etc.) that require preparation of uniform mixtures of CNTs with many different organic, inorganic, and polymeric materials.

4 4 Various possibilities of CNTs Functionalization Non-covalent functionalization Defect functionalization Side wall functionalization π-stacking Endohedral functionalization Example of π-stacking functionalization pi-stacking of 17-(1-pyrenyl)-13-oxa-heptadecanethiol (PHT) with SWCNTs and self-assembling of gold nanoparticles from a colloid gold

5 5 Example of π-stacking functionalization 1-Pyrenebutanoic acid succinimidyl ester irreversibly adsorbed onto the sidewall of a SWCNT via p-stacking. Amino groups of a protein react with the anchored succinimidyl ester to form amide bonds for protein immobilization. Chen RJ et al. (2001), J. Am. Chem. Soc. Functionalization of CNTs Thus, functionalization of nanotubes becomes an essential prerequisite for potential nanotechnological applications and can be seen as a key to nanotechnology.

6 6 This talk Functionalization of CNTs for composite materials Theoretical study Covalent Functionalization of CNTs + = sp 2 rehybridization sp 3 CNT + molecules (groups, radicals) = Functionalized CNT

7 7 Functionalization of CNTs for composite materials The production of robust composite materials requires strong covalent chemical bonding between the filler particles and the polymer matrix, rather than the much weaker van der Waals physical bonds (which occur if the CNTs are not properly functionalized). Sidewall chemical (covalent) functionalization of single wall carbon nanotubes (SWNTs) with molecules OH, COOH, NH n, CH n (n = 1,2,3,4) Task: To establish possible density of doping with molecules Theoretical study of functionalized CNTs Infinite CNTs with superlattice constant a equal to 2 periods determined by (n, m) Attach a number of molecules to side walls of CNTs a Perform first-principles calculation to determine optimized geometry stability of the functionalized SWNT ( ) Calculation of the energy E { R i}

8 8 Stability of Functionalized CNTs Binding energy (per atom) 1 Nat Ebind = Etot ( CNT + Groups) Etot ( Atomα ) Nat α Total energy of functionalized CNT Total energy of the free atom α Adsorption energy (interaction energy) Heat of formation Nβ Eads = Etot ( CNT + Groups) Etot ( CNT) Etot ( Group β ) Total energy of CNT β Total energy of the group β Density Functional Theory (DFT) Generalized Gradient Approximation (GGA) to the exchange & correlation TM Pseudopotentials used to account for electron - ion interaction SIESTA SIESTA code to solve Kohn-Sham Eqs.

9 9 Results: SWNTs functionalized with OH groups SWNTs functionalized with OH groups Binding energy: generally increases (i.e., weaker bonding) with the density of attached groups, is weakly dependent on the diameter of the CNTs and their metallicity There is a critical density of attached groups, above which the distribution of OH groups over the surface is not uniform

10 10 SWNTs functionalized with OH groups H 2 O molecules a chain of OH groups O atom bound to two carbon atoms (π bonds) Results: SWNTs functionalized with NH n groups

11 SWNTs functionalized with NH n Binding energy: generally increases (i.e., weaker bonding) with the density of attached groups, The strongest bonding for NH -NH & -NH 2 uniformly distributed over the CNT surface At higher densities of NH 4 dissociation of the molecule into NH 2 (bound to the surface) and H 2 (unbound) SWNTs functionalized with NH n SWNT functionalized with NH 2 and with NH 4 C-N bond reminds C-C bond in diamond local sp3 rehybridization For critical number (7) of NH 4 molecules, C atom from the wall is substituted by N atom. 11

12 12 SWNTs functionalized with NH 4 Damage to the CNT done by 9 NH 4 molecules Creation of the capped nanotubes SWNTs functionalized with NH n Heat of formation Extremely weak bonding of NH 3 to CNT

13 13 Results: SWNTs functionalized with CH n groups Qualitatively very similar to the NH 4 However, CH 2 molecules defect the CNTs SWNTs functionalized with CH n (9,0) SWNT (a) CH 2 fragments strongly defect CNT (b) sp 3 rehybridization in the case of CH 3 (c) Dissociation of CH 4 into CH 2 and H 2 in the presence of CNT (d) characteristic heptagon/pentagon defects in the case of CH 2

14 14 Elastic properties of functionalized CNTs 2r Parameters characterizing elastic properties of a pipe t V o = 2 π r l t l Young s modulus 2 1 E Y = Vo ε σ zz Y = ε strain 2 zz Poisson coefficient ν = zz r r l l E strain Volume of the tube = El+ l E l Y Bulk modulus K = 3(1 2ν ) Kirchoff s modulus (shear modulus) ε zz = l l Y G = 2(1 + ν )

15 15 Elastic properties of functionalized CNTs - computational details Ab initio calculations: Ansatz about CNT volume: Thickness t of bare CNT double Van der Waals radius of Carbon (3.4 Å). l ε zz = l l Volume of attached molecules = sum of volumes of Van der Waals spheres of atoms that built the attached molecule. l+ l V 0 E strain = El+ l E l Y, ν, K, G Elastic modules of bare Nanotubes Young modulus 1.03 TPa Experiment 1.25 TPa (±50%) Other theoretical works TPa Poisson coefficient (exp.); (theory) Kirchoff s module 0.58 TPa Ca. 0.5 TPa (exp. & theory), Bulk modulus 0.79 TPa 0.74 TPa (theory) Excellent agreement between theory and experiment How the functionalization of CNTs changes their elastic modules?

16 16 Young s modulus as a function of the diameter of (n,0) CNTs Young's modulus [TPa] 1,11 1,08 1,05 1,02 0,99 0,96 0,93 0,90 Bare (n,0) CNT Diameter of (n,0) CNT [A] Young s modulus saturates for CNTs of larger diameter Equlibrium length of functionalized CNTs Equilibrium length of CNT cell [A] 8,74 8,72 8,70 8,68 8,66 Functionalized (9,0) CNT CH 2 OH 8,64 8,62 8, Number of molecules, per CNT cell

17 17 Young s modulus of functionalized CNTs Young's modulus (TPa) 1,1 1,0 0,9 0,8 0,7 0,6 Functionalized (9,0) CNT CH 2 OH 0, Number of molecules, per CNT cell Functionalization of CNTs diminishes their Young s modulus, but not to the extent that could hamper strong enforcement of the composite materials. CNT Functionalization with metals Fe, Ni, Cu Pd Al, Zn Ferromagnetic structures?

18 18 SWNT with attached Ni atoms Two Ni atoms per cell make characteristic chain on the CNT surface Substitution of Ni atoms into the backbone of CNT followed by strong deformation More than 2 atoms per CNT period cause the total destruction of the CNT SWNT with atomami Fe Only structures with one Fe atom per CNT unit cell are stable Two & more Fe atoms per CNT unit cell cause, respectively, very strong deformation and further total destruction of CNT

19 SWNT Functionalization with Cu Cu atoms bind with CNT Two atoms per unit cell form a chain similar to Ni case, however with longer bond length Further substitution of Cu atoms leads to the formation of Cu clusters and deformation of CNTs SWNT functionalized with Pd atoms Pd atoms bind with CNT They build chains (alike Ni & Cu) Small density of Pd atoms elongation of CNTs Higher density of Pd atoms a regular network of Pd chains is formed without relevant deformation of CNT A metallic Pd shell bound to the CNT is formed 19

20 20 SWNT functionalized with Al, Zn Atoms surround tubes causing NO DEFORMATION CNT + Al, Zn Lack of covalent bonds between carbon and metal atoms Metal layers are formed around the CNTs CNT + Al, Zn Ferromagnetic ordering of Fe chains on CNTs One Fe atom on the CNT unit cell Magnetic moment per cell µ= 2.2 Note that two Fe atoms per cell are unstable How it is with Ni chains, where 2 Ni per cell are stable?

21 21 Anti-Ferromagnetic ordering of Ni chains on CNTs Two Ni atoms on the CNT unit cell Magnetic moment per cell µ= 0 Summary Modeling based on ab initio calculations provide insight into energetics of CNTs functionalized with simple molecules (OH, NH n, CH n, COOH), determine critical densities of the adsorbed molecules, functionalized CNTs at critical densities are strong enough to be utilized as an enforcement in new composite materials.

22 22 Thank you! ACKNOWLEDGMENT Polish Council for Science, Development Grants for the years (NR /2008, NR. KB/72/13447/IT1-B/U/08).