1 application of the reaxff reactive force fields to nanotechnology adri van duin, weiqiao deng,...
TRANSCRIPT
1
Application of the ReaxFF reactive force fields to nanotechnology
Adri van Duin, Weiqiao Deng, Hyon-Jee Lee, Kevin Nielson, Jonas Oxgaard and William Goddard III
Materials and Process Simulation Center, California Institute of Technology
2
Contents
- ReaxFF: background, rules and current development status
- Ni-catalyzed nanotube growth
- Validation of the all-carbon ReaxFF potential
- Building the Ni/NiC potential
- Testing the Ni-cluster description: magic number clusters
- Study of the initial stages of nanotube formation
3
Tim
e
DistanceÅngstrom Kilometres
10-15
years
QC
ab initio,DFT,HF
ElectronsBond formation
MD
Empiricalforce fields
AtomsMolecular
conformations
MESO
FEA
Design
Grains
Grids
Hierarchy of computational chemical methods
ReaxFF Simulate bond formationin larger molecular systems
Empirical methods:- Allow large systems- Rigid connectivity
QC methods:- Allow reactions- Expensive, only small systems
ReaxFF: background and rules
4
underover
torsvalCoulombvdWaalsbondsystem
EE
EEEEEE
++
++++=
System energy description
2-body
multibody
3-body 4-body
5
-To get a smooth transition from nonbonded to single, double and triple bonded systems ReaxFF employs a bond length/bond order relationship. Bond orders are updated every iteration.
- Nonbonded interactions (van der Waals, Coulomb) are calculated between every atom pair, irrespective of connectivity. Excessive close-range nonbonded interactions are avoided by shielding.
- All connectivity-dependent interactions (i.e. valence and torsion angles) are made bond-order dependent, ensuring that their energy contributions disappear upon bond dissociation.
- ReaxFF uses a geometry-dependent charge calculation scheme that accounts for polarization effects.
Key features
6
General rules
- MD-force field; no discontinuities in energy or forces even during reactions.
- User should not have to pre-define reactive sites or reactionpathways; potential functions should be able to automatically handlecoordination changes associated with reactions.
- Each element is represented by only 1 atom type in the force field;force field should be able to determine equilibrium bond lengths,valence angles etc. from chemical environment.
7
Current status
‘Finished’ ReaxFF force fields for:- Hydrocarbons (van Duin, Dasgupta, Lorant and Goddard, JPC-A 2001, 105, 9396)
(van Duin and Sinninghe Damste, Org. Geochem.2003, 34, 515
- Si/SiO2 (van Duin, Strachan, Stewman, Zhang, Xu and Goddard, JPC-A 2003, 107, 3803)
- Nitramines/RDX (Strachan, van Duin, Chakraborty, Dasupta and Goddard, PRL 2003,91,09301
- Al/Al2O3 (Zhang, Cagin, van Duin, Goddard, Qi and Hector, PRB in press)
Force fields in development for:- All-carbon materials- Transition metals, metal alloys and metals interacting with first row elements- Proteins- Magnesium hydrides
8
Ni-catalyzed nanotube growth
Longer nanotube
Concept: grow nanotubes from buckyball building blocks
- Exothermic reaction- Huge activation barrier- Probably needs catalyst
9
Validation of the ReaxFF all-carbon potential
QC-data taken from hydrocarbon training set:
- Single, double and triple bond dissociation
- C-C-C, C-C-H and H-C-H angle bending
- Rotational barriers around single, double and aromatic C-C bonds
- Conformation energy differences
- Methyl shift and H-shift barriers
- Heats of formation for a large set of strained and unstrained non-conjugated, conjugated and radical hydrocarbons
- Density and cohesive energies for diamond, graphite, cyclohexane and buckyball crystals
- All-carbon ReaxFF should also work for hydrocarbons
10
Compound ERef (kcal/atom) EReaxFF
Graphite 0.00a 0.00
Diamond 0.8a 0.52
Graphene 1.3a 1.56
10_10 nanotube 2.8b 2.83
17_0 nanotube 2.84b 2.83
12_8 nanotube 2.78b 2.81
16_2 nanotube 2.82b 2.82
C60-buckyball 11.5a 11.3
a: Experimental data; b: data generated using graphite force field (Guo et al. Nature 1991)
- ReaxFF gives a good description of the relative stabilities of these structures
Relative energies for all-carbon phases
All-carbon data added to the hydrocarbon training set
11
Binding energies in all-carbon compounds relative to Graphite
0
20
40
60
80
100
120
Acyclic C2Acyclic C3Cyclic C3Acyclic C4Cyclic C4C4 pyramidAcyclic C5Cyclic C5Acyclic C62_C3Cyclic C6Acyclic C7Cyclic C7Acyclic C8C8 3ringC8 3ringIIC8 cubeCyclic C8Acyclic C9Cyclic C9Acyclic C10Cyclic C10Tricyclic C10Acyclic C12Acyclic C13Cyclic C13Tricyclic C13Acyclic C14Cyclic C15Cyclic C17Bicyclic C17Acyclic C20Hexacyclic C20C20-dodecaC60-buckyballDiamond
Relative binding energy (kcal/atom)
Reax
QC
- Even-carbon acyclic compounds are more stable in the triplet state; odd-carbon, mono and polycyclic compounds are singlet states- Small acyclic rings have low symmetry ground states (both QC and ReaxFF)- ReaxFF reproduces the relative energies well for the larger (>C6) compounds; bigger deviations (but right trends) for smaller compounds- Also tested for the entire hydrocarbon training set; ReaxFF can describe both hydro- and all-carbon compounds
12
0
50
100
1.5 2 2.5
DFTReaxFF
0
50
100
1.5 2 2.5
DFTReaxFF
C-C distance (Å)
Ene
rgy
(kca
l/m
ol)
Ene
rgy
(kca
l/m
ol)
- ReaxFF gives good energies for key structures in buckyball growth-Training set includes all hydrocarbon cases used for ReaxFFCH
13
Angle bending in C9
- ReaxFF properly describes angle bending, all the way towards the cyclization limit
14
0
0.05
0.1
0.15
0.2
10 15 20
c-axis (Å)
E (
eV/a
tom
)
diamond
graphite
Diamond to graphite conversionCalculated by expanding a 144 diamond supercell in the c-direction and relaxing
the a- and c axes
QC-data: barrier 0.165 eV/atom(LDA-DFT, Fahy et al., PRB 1986, Vol. 34, 1191)
-ReaxFF gives a good description of the diamond-to-graphite reaction path
15
Applications of all-carbon ReaxFF: buckyball+nanotube collisions
QuickTime™ and aCinepak decompressorare needed to see this picture.
QuickTime™ and aCinepak decompressorare needed to see this picture.
Impact velocity:6 km/sec(1500K)
Impact velocity:9 km/sec(2500K)
16
QuickTime™ and aCinepak decompressorare needed to see this picture.QuickTime™ and aCinepak decompressorare needed to see this picture.QuickTime™ and aCinepak decompressorare needed to see this picture.
Impact velocity: 6 km/sec
Impact velocity: 8 km/sec Impact velocity: 10 km/sec
Side impact
-Materials are too stable, extremely high impact velocities are required to start reaction
- Catalyst required to lower reaction barriers
17
Transition metal catalysis: Ni
1: ReaxFF and QC EOS for Ni bulk phasesQC
0
25
50
75
100
0 5 10 15 20 25
Energy (kcal/mol)
ReaxFF
0
25
50
75
100
0 5 10 15 20 25
FCC
BCC
A15
SC
Diamond
0
5
10
15
20
25
0 5 10 15 20 25
Volume/Atom (A 3)
0
5
10
15
20
25
0 5 10 15 20 25
Volume/Atom (A3)
FCC
BCC
A15
SC
Diamond
-ReaxFF gives a good fit to the EOS of the stable phases (FCC, BCC, A15)-ReaxFF properly predicts the instability of the low-coordination phases (SC, Diamond)
18
-85
-80
-75
0 400000 800000
Icosahedron
FCC
Liquid
Amorphoussolid
MD-iterations
Ene
rgy/
atom
(kc
al)
Testing the force fields for Ni magic number clusters
QuickTime™ and aGIF decompressorare needed to see this picture.
19
-85
-80
-75
-70
0 400 800 1200 1600
55 atoms
147 atoms
309 atoms
561 atoms
Temperature (K)
Ene
rgy/
atom
(kc
al)
IcosahedronFCC
Liquid
Amorphoussolid
MD-heatup/cooldown simulations
heatup
cooldown
- ReaxFF gets the right trend for fcc/icosahedron transition
- ReaxFF heat of melting converges on Ni bulk melting temperature (1720K)
200
50
100
150
1 2 3 4
DFT singletDFT tripletReaxFF
0
50
100
150
1 2 3 4
DFT singletDFT tripletReaxFF
NiCH3CH3
NiCH3CH3
Ni-C bond breaking in H3C-Ni-CH3
Bond length (Å)
Ene
rgy
(kca
l/mol
)E
nerg
y (k
cal/m
ol)
Ni-C bond breaking in Ni=CH2
Ni CH2
Ni CH2
2. Results for Ni-C interactions
21
0
50
100
150
1 2 3 4
DFT singletDFT tripletReaxFF
Ni-C bond breaking in Ni(CH3)4
Bond length (Å)
Ene
rgy
(kca
l/mol
)
NiCH3
CH3
CH3
CH3
NiCH3
CH3
CH3CH3
Ni dissociation from 5-ring compound
Ene
rgy
(kca
l/mol
)
Ni
HH
HH
Ni
HH
HH
22
0
25
50
2 3 4
DFT singletDFT tripletReaxFF
0
50
100
150
1 2 3 4
DFT singletDFT tripletReaxFF
Bond length (Å)
Ni dissociation from 6-ring compound
Ene
rgy
(kca
l/mol
)
Ni
HH
H
H H
Ni
HH
H
H H
Ni dissociation from benzene
Ni
Ni
Ene
rgy
(kca
l/mol
)
23
0
50
100
1 2 3 4
DFT singletDFT tripletReaxFF
Ni dissociation from benzyne
Ene
rgy
(kca
l/mol
)
Bond length (Å)
Ni
Ni
C-Ni-C angle bending in benzyne/Ni complex
Angle (degrees)
Ene
rgy
(kca
l/mol
)
Ni
Ni
24
0
10
20
60 90 120 150
DFT singletReaxFF
Angle (degrees)
Ene
rgy
(kca
l/mol
)Ni
CH3CH3
Ni
CH3
CH3Ni CH3
CH3
C-Ni-C angle bending in H3C-Ni-CH3
25
Ni-assisted C2-incorporation in C 20-ring
-250
-200
-150
-100
-50
0
Incorporation pathway
Relative energy (kcal/mol)
QC
ReaxFF
Ni-assisted C2-incorporation on nanotube edge
-400
-350
-300
-250
-200
-150
-100
-50
0
50
100
Growth pathway
Relative energy (kcal/mol)
QM/MM
ReaxFF
(A)
(B)
(C)
(D)
(E)
(F)
(G)
Ni-assisted C2-incorporation reactions
- ReaxFFNi can describe the binding between Ni and C- A similar strategy has been used to make ReaxFF descriptions for Co/C and Cu/C, allowing us to compare their catalytic properties
26
R12= 1.45 Å
21
21
R12= 1.49 Å
Influence adsorbed Ni on buckyball reactions
- ReaxFF predicts that buckyball C-C bonds get substantially weakened by adsorbed Ni-atoms- Might lower buckyball coalescence reaction activation barrier
ReaxFF-minimized buckyball
ReaxFF-minimized buckyball+2 Ni
27
QuickTime™ and aGIF decompressorare needed to see this picture.QuickTime™ and aGIF decompressorare needed to see this picture.-150
-100
-50
0
50
100
150
1.522.533.544.555.5
No Ni
Ni
En
erg
y (k
cal/m
ol)
Reaction coordinate
Low-T ReaxFF restraint MD-simulation
Influence adsorbed Ni on reaction barrier
- Ni-atoms lower reaction barrier- Overall reaction becomes exothermic due to formation of Ni-Ni bonds- May explain Ni catalytic activity
28
Influence Ni on initial stages of buckyball growth
MD NVT-simulation (1500K); 5 C20-rings, 10 C4-chains (blank experiment)
t=0 ps. t=125 ps.
- C4 reacts with rings to form long acyclic chains- No branching
29
MD NVT-simulation (1500K); 5 C20-rings, 10 C4-chains and 15 Ni-atoms
t=0 to t=125 ps. t=125 to t=750 ps.
QuickTime™ and aGIF decompressorare needed to see this picture.QuickTime™ and aGIF decompressorare needed to see this picture.
30
QuickTime™ and aGIF decompressorare needed to see this picture.
A closer look at the 750 ps. product
- Ni-atoms help create cage-structures
- 750 ps. product has no internal C-C bonds
- Ni-atoms leave ‘finished’ material alone and move away to defect and edge sites
- Total simulation time: 4 days on 1 processor
- Future work: Co, Fe
31
Metal-catalyzed nanotube growth
- Start configuration: 20 C6-rings, 5 metal atoms on edge
- NVT simulation at 1500K
- Add C2-molecule every 100,000 iterations
Inital configuration
32
QuickTime™ and aGIF decompressorare needed to see this picture.- Ni-atoms can grab C2-monomers and fuse them as new 6-membered rings
33
Metal-catalyzed nanotube growth
Results after 2,000,000 iterations
Metal=Ni
Metal=Co
Metal=CuNo metal
-Ni and Co lead to greatly enhanced ring formation. Cu is far less active.
34
Conclusions
- ReaxFF has proven to be transferable to transition metals and can handle both complex chemistry and chemical diversity
- The low computational cost of ReaxFF (compared to QC) makes the method highly suitable for screening heterogeneous and homogeneous transition metal catalysts