Concept of Component Seed Vastly Broadens the Understanding of

Article pubs.acs.org/JPCC

Concept of Component Seed Vastly Broadens the Understanding of Nanotube Synthesis and Characteristics S. Noor Mohammad*,‡ Sciencotech, 780 Girard Street NW, Washington, D.C. 20001, United States ABSTRACT: Fundamental physics and chemistry underlying nanotube synthesis and characteristics have not been fully understood. To facilitate this understanding, the concept of component seed, component droplet, and component nanowire for nanotube synthesis and characteristics has been introduced. This concept generalizes the shell model for nanotubes. It vastly broadens our ability to explain nanotube materials characteristics that could not otherwise be explained. Experiments widely corroborate with the present findings. They lend support to the concept of component seeds and component droplets. Size-dependent and solubilitydependent melting point depressions have been studied. They provide new insight and uncover the basic causes of melting (nonmelting) of the catalyst nanoparticles. They also elucidate nanotube growth, employing metal nanoparticles at temperatures lower than their melting points. The concept of component seed (droplet) also successfully explains nanotube branching. In light of this concept, growth mechanisms available in the literature have been modified.

1. INTRODUCTION Since the discovery1 of carbon nanotubes (CNTs) in 1991, the interest in CNTs and the research efforts2−5 on CNTs have been astonishing. This research has led to interesting findings6−18 in both single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). Parallel efforts have led to the exploration of semiconductor nanotubes19−32 (SNTs), including GaN nanotubes (GNNTs),22,23 ZnO nanotubes,24 BN nanotubes (BNNTs),25,26 and InP nanotubes.32 However, the fundamentals of nanotube synthesis still remain murky. Our objective in this investigation is to introduce the concept of component seeds (droplets) and to demonstrate that it may broaden the understanding of nanotube synthesis and enrich the basic science of this synthesis. The acceptability of this concept demands extensive experimental support. To try to present it is an essential element of this endeavor.

for GaN nanotube growth) referred to hereafter as the RS species and the precursors of these vapors (for example, CH4, C2H2 for RS≡C) play a central role in the CVD synthesis.3 Nanotubes are synthesized on FECA nanoparticles produced on the substrate. Seeds are different from nanoparticles. Seeds are the elements and/or components of nanoparticles that mediate nanotube growth. Depending on FECA melting temperature TM and FECA/X eutectic temperature TE, they may be (1) FECA/X eutectic alloy (at T ≥ TE), (2) FECA/X noneutectic alloy or solid solution (at T < TE), (3) oxides, (4) amorphous X materials, (4) FECA itself (at T ≥ TM), or (5) solid solution of FECA, X, O, and FECA/X alloy (at T < TE). Note that O atoms generally participate in the FECA-X reaction. They originate from the support material; they may even be part or contaminant of precursor (for example, CO, CH4, C2H2, etc., for CNT growth) of the RS species. With a very low concentration of FECA, FECA/X may behave essentially as amorphous X material. Unlike RS≡X species, RS≡Y species are generally very volatile. So, they may not produce any stable FECA/Y species. They may though be component of an amorphous solid solution. A few examples of FECA/X alloy are Ni3C for FECA≡Ni and X≡C, Mn5C2 for FECA≡Mn and X≡C, and Fe2B for FECA≡ Fe and X≡B. The seeds may be molten or semimolten. FECA is solid at T < TM, but the FECA/X alloy is solid with disturbed (disordered) lattice structure and/or molten (semimolten) nanopores (at T < TE, T < TM).33 Depending on growth conditions, FECA/X may as well be solid solution at T < TE, T < TM. The properties of some of the FECA/C alloys used for CNT growths are listed in Table 1. These properties include the FECA cohesive energy

2. SALIENT FEATURES OF NANOTUBE GROWTHS Nanotubes may, in general, be called XmYn nanotubes (X and Y are the nanotube elements; X may be metal and Y nonmetal; m and n are integers, and one of them may be zero). They include carbon (X = C, Y = 0, m = 1, n = 0) nanotubes, GaN (X = Ga, Y = N, m = n = 1) nanotubes, BN (X = B, Y = N, m = n = 1) nanotubes, ZnO (X = Zn, Y = O, m = n = 1) nanotubes, and InP (X = In, Y = P, m = n = 1) nanotubes. The syntheses of these nanotubes at a temperature T are performed by many different techniques5 and explained by many different mechanisms. While some of these mechanisms make use of foreign element catalytic agents (FECAs),9−13 some others make use of oxygenated materials.17 Some others do not make use of any FECA at all.23 Among various growth techniques, the chemical vapor deposition (CVD) is widely employed for nanotube synthesis. The nanotube-containing vapors (for instance, carbon vapor for CNT growth, Ga and N vapors © 2012 American Chemical Society

Received: September 12, 2011 Revised: January 12, 2012 Published: January 19, 2012 5312

dx.doi.org/10.1021/jp208801p | J. Phys. Chem. C 2012, 116, 5312−5326

The Journal of Physical Chemistry C

Article

Table 1. Various Properties [e.g., FECA/C Eutectic Temperature TE≡TEC, Cohesive Energy Ecohb, Melting Temperature TM, the Mole Fraction z of (FECA)zC1−z Alloy (Solid Solution), Latent Heat of Fusion HL, Molar Volume ΩFECA, and Surface Energy σLS] of Some Representative FECAs Used for the Present Studya FECA parameters

a

FECA

Ecohb (kJ/mol)

TEC (°C)

mole fraction z of (FECA)zC1−z

TM (°C)

HL (kJ/mol)

σLS (J/m2)

ΩFECA (cm3/mol)

Au Ag Al Cu Cr Ca Mg Mn Fe Ni Co Hf Pt Pd Re Ru Mo Pb W Nb Ti V Li Na K Rb Cs Sr a-Carbon Si Ge SiC SiO2 GeO2 CaO MgO TiO2

368 284 327 336 395 178 145 282 413 428 428 621 564 376 775 650 658 196 859 730 468 512 158 107 90 82 78 166

1027 953 1070 1534 1232 1153 1327 1320 2209 1738 2500 1953 2221 2720 2367 1643 1625

0.9993 0.9964 0.9999 0.858 0.971 0.818 0.89−0.92 0.974 0. 975 0.988 0.993 0.975 0.825 0.781 0.947 0.985 0.859

36.95

-

-

-

0.999 0.9995 -

1065 962 660 1084 1857 839 639 1246 1535 1453 1495 2227 1772 1552 3180 2250 2617 327 3407 2468 1660 1902 180 98 64 39 29 769 3675 1410 937 2830 1830 1115 2572 2852 2116

1.541 1.250 1.180 1.77 2.006 0.50 0.79 1.298 2.361 2.24 2.161 1.923 1.286 0.886 2.220 2.510 0.540 2.990 2.313 1.92 2.301 0.52 0.26 0.13 0.12 0.1 0.42 1.14 0.88 2.50 0.32 0.62 1.31 1.113 0.790

13.6 10.33 9.50 7.11 7.040 29.90 12.97 7.35 7.09 6.59 6.66 13.37 9.02 8.78 8.81 8.17 9.35 18.27 9.50 10.75 10.55 8.25 13.12 23.7 45.3 55.90 70.01 33.32 12.06 13.62 12.53 26.35 24.61 16.90 12.80 24.05

446 372 1227 405 637 1061 993 2078

10.71 13.26 16.90 8.54 9.04 12.05 17.81 19.48 16.80 24.05 19.70 17.60 33.21 24.00 32.00 4.79 35.40 26.40 15.45 20.91 2.33 2.60 2.40 2.19 2.09 8.31 50.21 33.75 37.28 11.06 51.12 67.33 29.16

Amorphous carbon has been denoted by a-Carbon.

Ecohb, the FECA melting temperature TM, the FECA/C eutectic temperature TE, the FECA atomic % in the FECA/C alloy, the molar volume ΩFECA of FECA, the liquid−solid surface energy σLS, and the enthalpy of melting HL. They were obtained from the public domain (e.g., worldwide web) and/or the literature.34−37 The FECAs include3,9 the commonly used transition metals (e.g., Fe, Ni, and Co); semiconductors (e.g., SiC, Si, and Ge); and oxides (e.g., SiO2, TiO2, Al2O3, etc). Table 1 shows that the carbon solubility in solid FECA is quite low at the eutectic temperature TE. It is, for example, only about 0.07 at. % for the Au/C eutectic alloy and 0.7 at. % for the Re/C eutectic alloy. The nanotube growth involves the catalytic decomposition of the precursor of the RS species on the FECA surface and the diffusion of the RS species primarily from the bulk to the FECA peripheral surface and/or into the FECA at (near) the FECA peripheral surface. Growth temperature T, chamber pressure Pchm, and FECA dimension generally determine the RS solubility in the FECA. Supersaturation of the RS species at

the L/S interface (e.g., the interface of the liquid dropet and the solid substrate, or the interface of liquid droplet and nanotube tip lying underneath the liquid droplet) results in the nanotube nucleation. During the past years, noneutectic CNT syntheses, employing the FECA/C alloy (for example, Fe3C, Ni3C, Co2C, TiC, Mn5C2, Mo2C, etc.) have been performed at T < TE. However, eutectic SNT synthesis32 has been achieved at T ≥ TE. Nanotube synthesis is though possible at T ≥ TM. FECAs for CNT synthesis have been metals38 (e.g., Fe, Co, Ni, Mn, Au, Cu, Pt, Pd, etc.); semiconductors39 (e.g., SiC, Si, Ge, SiGe, etc.); and oxides9,40 (e.g., SiO2, Al2O3, GeO2, MgO, CaO, V2O5, La2O3, TiO2, etc.). Some of the CNT syntheses mediated by FECA are listed39−57 in Table 2. FECA species, precursors of the RS vapor species, nanoparticle diameter Dnano, nanotube length LNT, nanotube diameter dNT, growth temperature T, and chamber pressure Pchm are presented in this table. Various entries of Table 1 and Table 2 indicate that FECA-mediated 5313



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Table 2. List of Growth Temperatures, Carbon Sources, and FECAs Used for the Growths of Some Representative Carbon Nanotubes by the Chemical Vapor Depositiona no.

carbon source precursor(s)

FECA

lowest growth temp. (°C)

1

ethanol vapor

SiC, Si, Ge

850

2

CH4

SiO2, Al2O3, TiO2, Er2O3 Fe/Mo Fe/Mo Fe/Pt Fe/Mo Fe/Mo Fe/Mo Ni Co, Mo Au, Ag, Pt, Pd Cu, Fe, Co, Ni Cu

900

3 4 5 6 7 8 9 10 11 12 13

900 900 900 900 900 900 800 700 850 850 825

14 15 16

CO/H2 CH4/H2 CO/H2 CO/H2 CH3OH/H2/Ar CO/H2 C2H4/NH3 CO ethanol vapor ethanol vapor CH4, ethanol, isopropanol CH4/H2 CH4, C2H2 CO

17

CH4

Fe

600

18 19 20

CH4 CH4 C2H2/H2

Re Fe, Co Fe3C

950 800 600

21 22

CO ethanol

Fe/Mo Fe2O3, Fe, Ni

900 950

23 24 25

ethylene CO C2H5OH or C6H12

m-ferritin Fe Fe

900 880 650

Ni, Fe/Ni/Cr Co, Mo Mo

650 900 1200

comments

ref

CG = Ar/H, NP = Ge, Dnano = 5 nm, growth rate higher with NP = Ge than with NP = Si, SiC. Long, oriented SWCNTs, dNT = 0.8−1.4 nm, substrate = Si, substrate thermally annealed before growth. SWCNTs, Dnano ≈ 4.2 nm, tdur = 10 min. SWCNTs, LNT = 2.1 mm, tdur = 10 min. straight SWCNTs, LNT = 3.9 mm, tdur = 20 min. SWCNT arrays, tdur = 10 min. long SWCNTs, tdur = 10 min. Randomly oriented SWCNTs, tdur = 10 min. Vertically aligned MWCNTs, LNT = 29 μm, tdur = 10 min, and Pchm = 1 atm. Bundles of SWCNTs, tdur = 3−60 min, dNT = 1 nm, substrate = SiO2 (gel). SWCNTs grown on Al-hydroxide films; NP = Au, Ag, Pt, Pd, Dnano = 3 nm. SWCNTs grown on Al-hydroxide films; NP = Cu, Fe, Co, or Ni; Dnano = 3 nm. Random SWCNTs, substrate = Si, CG =A r and/or H2, Dnano ≈ 10 nm, Cu derived from CuCl2. SWCNTs, PECVD growth, Pchm = 250−300 Pa, tdur = 30−60 min. SWCNTs and DWCNTs, tdur = 10−60 min, SiO2 (oxidized silicon) environment. CVD growth for 60 min produced bundles of SWCNTs, 1−1.7 nm in diameter, on Al2O3 substrate. SWCNTs, tdur = 5 min, dNT = 1−5 nm, LNT = several micrometers, substrate = Al2O3. SWCNTs, DWCNTs, and MWCNTs, dNT = 5−8 nm. SWCNTs, substrate = Si, Dnano = 30 nm, T = 900 °C for Fe, but T = 800 °C for Co. SWCNTs and MWCNTs, substrate = SiO2/Si, NP structural fluctuations promoted nanotube growths. Substrate = Si/SiO2, CG = H2, Dnano = 4.2 nm Substrate = Si(001), T ≥ 950 °C for SWCNTs, T < 900 °C for MWCNTs, Dnano ≈ 10 nm. Substrate = sapphire; dNT = 1.97 ± 0.83 nm; Dnano = 2.9 ± 1.2 nm SWCNTs, grown on SiO2 film, dNT = 3−6 nm, LNT = 30 μm, Dnano = 4.5−8.0 nm. MWCNTs, outer diameter distribution 19.5 ± 2.5 nm, Dnano= 10−30 nm.

39 40 41 41 41 41 41 41 42 43 44 44 45 46 47 48 49 50 51 52 53 54 55 56 57

a Nanoparticle is denoted by NP; carrier gas by CG; duration of growth by tdur, nanoparticle diameter by Dnano, nanotube length by LNT, nanotube diameter by dNT, growth temperature by T, and chamber pressure by Pchm.

CNT growths are indeed possible at T ≪ TE. We cite two examples, TE ≈ 1320 °C for the Ni/C alloy and TM = 1453 °C for Ni. Yet, using large-sized FECA≡Ni (Dnano = 70−80 nm), CNTs were produced46 at T ≈ 650 °C. TE > 2500 °C for the Re/C alloy, but TM = 3180 °C for Re. Yet, using FECA≡Re (Dnano = 6−8 nm), CNTs were produced50 at T ≈ 950 °C. Two reasons generally put forth to justify growths at T < TE are the size-dependent melting point depression58,59 of the FECA nanoparticle and the solubility of RS≡X≡C in the FECA nanoparticle. This depression60−62 may have dependency on thermodynamic imbalance and the type of seeds. The seeds may be classified as NSA, NSB, and NSC type seeds,33 shown schematically in Figure 1. They may have dipole moment.63

vital role in the nanotube growths. Due to thermodynamic imbalance,60 FECA nanoparticles, and seeds from these nanoparticles experience fluctuations. They may also be affected by nonuniform, nonstationary growth temperature and chamber pressure. The surface energy of the FECA nanoparticle60 may not, as a result, be uniform throughout the FECA peripheral surface. The segregation of the RS species from the FECA core to the FECA surface may not be uniform, as well. This segregation may be affected differently by scattering at different locations of the FECA surface. Because of these, the shelled seed (for example, the NSA seed),33 or in other words, the seed of the shell, resulting species primarily from the segregation of the RS species from the nanopaticle core to the nanoparticle surface periphery may comprise one, a few, or a large number of component seeds (CSDs). If the shelled seed is very small and thin, and has small diameter, it may just have one CSD. If it is quite large and thick, and has large diameter, it may have many CSDs. CSD, made of X material, may comprise one or more X atoms; CSD, made of oxygenated X material, may comprise one or more oxygenated X atoms (molecules); and CSD of FECA/X material may comprise one or more FECA/X dimers or FECA/X clusters. Some of them may have traces of some or all of FECA, X, Y, and O. A FECA/X cluster may be made of several FECA/X dimers. A CSD may appear as (1) a grain with a grain boundary, (2) a cluster with a sharp edge, or

3. CONCEPTUAL ELUCIDATION OF COMPONENT SEEDS (CSDS) AND COMPONENT DROPLETS (CODS) Figure 1 shows that the NSA seed has the shape of a shell, but the NSB and the NSC seeds have the shapes of a cap. They may be made of disordered (e.g., polycrystalline, amorphous, etc.) X material for the self-catalytic growth, oxygenated X material (or a combination of oxygenated X material and FECA) for the oxide-assisted growth, and FECA/X material (with possible traces of FECA, X, Y, O) for the FECA-mediated growth.33 They are central to nanotube growth. They play a 5314



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Figure 1. Schematic diagrams of three possible droplet (seed) structures, namely, NSA (nanoseed type-A), NSB (nanoseed type-B), and NSC (nanoseed type-C) structures of nanotube. NSA seed (droplet) is a shell, but NSC seed (droplet) is a FECA/X hemispherical cap. NSB seed is also a hemispherical cap but has a FECA/X alloy at the periphery and a FECA at the central core. NSC seed is different from the hemispherical cap of carbon pentagons and heptagons that close the nanotube tip. NSC seed is composed of FECA/X material, but the hemispherical caps of carbon pentagons and heptagons have primarily carbon atoms and no FECA.

between/among CSDs. These may cause realignment of CSDs, annihilation of some old CSDs, and creation of some other new CSDs. These may also lead to a change in the shelled droplet diameters. These may interfere in even the nucleation process in CODs or in some domains of a droplet. As a result, the number of CSDs, CODs from these CSDs, and even thin concentric droplets resulting from CODs may increase or decrease during growth. Gohier et al.64 grew CNTs by PECVD at 700 °C employing FECA≡Co. They observed three walls near the base but two walls near the tip of the CNTs. This could be possible only if CODs merge together to create concentric droplets, and due to axial and radial deformations, they (e.g., CODs) merge together to create a different number of concentric droplets at different times during growth. These authors observed triangular-shaped enlargement in the wall of SWCNTs, which took place only in a certain location of the tube wall. It could be possible by the axial deformation of only some CODs, but not all CODs, yielding the droplet, and the nanotube produced with mediation by this droplet.

(3) simply a region surrounding the nanopipe and having sharp edges. CSDs in different locations of a seed may have different shapes, sizes, compositions, and surface energies. These shapes, sizes, compositions, and surface energies may change with time during growth. Some of them (e.g., the CSDs) may merge together, while some others may split into smaller ones during growth. Some of them may be defective due to the presence of foreign elements. Some of them, shown in Figure 2(a),(b), may be large enough to cover almost the entire width of the shelled seed, but some others, shown in Figure 2(c),(d), may be quite small. They may or may not overlap. At a certain growth temperature, CSDs may be molten. Even when solid, the CSDs may have disturbed (disordered) lattice structure and weakened interatomic interactions. They may be solid while exhibiting molten (semimolten) nanopores. The RS species may diffuse through the molten (semimolten) nanopores of them (e.g., CSDs). The CSDs suitable for the diffusion of the RS species may be called component droplets (CODs). Various scenarios may be envisioned during growth. First, large CODs, from large CSDs (see Figure 2(a)), may merge together to create a droplet (see Figure 2(e)) thick enough to cover the entire width (thickness) of the seed wall. Second, depending on the number of CSDs in the shelled seed, small CODs from small CSDs (see Figure 2(b)) may merge together and align to create one or more thin concentric droplets (see Figure 2(f)) in the shelled seed. Third, if the CODs do not merge together, they may retain their individual characteristics. Fourth, while most of the small CODs may merge together to create one or more concentric droplets, some other of these small CODs may retain their individual identity and characteristics. The van der Waals interaction between CSDs, and continuous fluctuations of the CSDs during growth often under nonstationary and nonuniform growth conditions, may facilitate substantial axial and radial deformations of the CSD shape, size, and alignment. Strain energy may develop within the seed due to bond bending and bond stretching of the bonds

4. IMPACT OF CSDS AND CODS ON NANOTUBE GROWTH The concept of CSDs, and CODs from these CSDs, paves the explanation of many different nanotube characteristics observed by experiments. During nanotube growth, the CSDs may experience at least four situations Λ1 to Λ4. The situations Λ1 and Λ2 correspond to large CSDs (see Figure 2(a)), but the situations Λ3 and Λ4 correspond to small CSDs (see Figure 2(b)). In situation Λ1 and under appropriate growth conditions, all of the large CSDs (see Figure 2(a)) of a seed become CODs, have identical characteristics, and are very close to one another. They may merge together to create a thick, shelled droplet (see Figure 2(e)), which covers the entire width (thickness) of the seed. The RS≡X and RS≡Y species diffuse through this droplet and react, during this diffusion, together as mX + nY → XmYn to yield XmYn molecules. These molecules, 5315



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As indicated earlier, CSDs may have distinct compositions and surface energies. Even when a droplet is created from the coalescence of the CODs from CSDs, this droplet may have different domains of different surface energies. Because of this, the nanotube grown from the droplet may have a different stacking sequence in different domains, and it may suffer from stacking faults as shown for the NT-1 nanotube in Figure 3.

Figure 2. Schematic diagrams of the cross-section of shelled seeds comprising CSDs: (a) seed exhibiting large FECA/X CSDs of different shapes and sizes; (b) seed exhibiting RS/oxide CSDs of different shapes and sizes; (c) seed comprising small FECA/X CSDs; and (d) seed comprising small FECA CSDs. The small FECA CSDs are CSDs in the sense that they may be molten, yielding CODs due to sizedependent melting point depression.

Figure 3. Schematic diagrams of the cross sections of nine different (NT-1 to NT-9) nanotubes of various possible shapes, compositions, and purity. Among them, NT-1 has stacking faults; NT-2 has nonuniform thickness; NT-3 has foreign element contamination; NT-4 has one thin, narrow shell; NT-5 has two thin narrow shells; NT-6 has several thin narrow shells; NT-7 has a different number of walls in different locations of the crosssection; NT-8 has a missing layer; and NT-9 has stacking faults.

upon supersaturation at the L/S interface, yield a XmYn nanotube. In situation Λ2 and under appropriate growth conditions, the large CSDs (see Figure 2(a)) may be close to one another but may not merge. They may rather retain their independent and individual characteristics and may also be converted into CODs. Most of these CODs may be large enough to cover the entire width of the seed. The RS≡X and RS≡Y species diffuse through these CODs. They are influenced by fast heat treatment. As a result, while diffusing through the CODs, the RS≡X and RS≡Y species have preferably large concentration gradient Crs and motive fore Fmot; they react together as mX + nY → XmYn to yield XmYn molecules. These molecules, upon supersaturation at the L/S interface, yield very thin nanowires, called component nanowires (CNWs). Any barrier to smooth and effective supersaturation is reduced, or even eliminated by a significantly large motive for Fmot. A large number of CNWs may be created in a single seed. These CNWs may be very close to one another and parallel featuring identical growth direction. An assemblage nanotube may be created from the close, ordered, uniform, and coaxial assemblage of these CNWs.

We reiterate that different CSDs, and CODs from these CSDs, may have different dimensions. The droplet created from these CODs may have varied width; this droplet may not be circular. Nanotubes grown from this droplet may not have perfectly cylindrical structure (see NT-2, Figure 3). Some of the CSDs, and CODs from these CSDs, may be contaminated with foreign elements. Nanotubes from these CODs may, due to the presence of foreign elements (see NT-3, Figure 3), suffer from defects. They may have kinks and nanopipes. In situation Λ3, and depending on the number of CSDs in the shell, small CODs from small CSDs (see Figure 2(c)) may merge together and align in one or more thin concentric, droplets (see Figure 2(f)). The RS≡X and the RS≡Y species diffuse through these concentric droplets. While diffusing through these concentric droplets, they react together as mX + nY → XmYn to yield XmYn molecules. These molecules, upon supersaturation at the L/S interface, yield thin, concentric XmYn nanotubes. There may be one (see NT-4, Figure 3), two (see NT-5, Figure 3), or several 5316



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(see NT-6, Figure 3) thin, concentric XmYn nanotubes. These are the single-walled, double-walled, and multiwalled nanotubes. In situation Λ4, small CODs from small CSDs (see Figure 2(d)) may not merge together. They may rather retain their individual characteristics. The RS≡X and RS≡Y species diffuse through the molten (semimolten) nanopores of them. While diffusing through these nanopores, they react together as mX + nY → XmYn to yield XmYn molecules. These molecules, upon supersaturation at the L/S interface, yield thin XmYn CNWs. Depending on the number of these CNWs, they may align into one nanotube or a number of concentric nanotubes. If merged together, they create thin concentric assemblage-nanotubes. There may be one (see NT-4, Figure 3), two (see NT-5, Figure 3), or several (see NT-6, Figure 3) thin, concentric Xm Yn nanotubes. These are the single-walled, double-walled, and multiwalled assemblage-nanotubes. The assemblage-nanotubes may suffer from dislocations. If the number of small CODs in a seed is higher than the one required for creating, for example, four thin (narrow) droplets but smaller than that required for creating five thin (narrow) droplets, the multiwalled nanotube thus created would have some regions of four walls but some other regions of five walls (see NT-7, Figure 3). Due to some CSDs not participating in the nucleation and growth, the nanotube may have a missing layer (see NT-8, Figure 3). Due to differences in the surface energy, the innermost shell of a multiwalled nanotube may have one polytype (suppose, wurtzite), but another shell of the same multiwalled nanotube may have a different polytype (suppose, zincblende) (see NT-9, Figure 3).

Table 3. Size-Dependent and Solubility-Dependent Reduced Melting Temperature of Various Nanoparticles aize-dependent reduced melting point TMR (°C)

aolubility-dependent reduced melting point TMS (°C)

nanoparticle

TM (°C)

rNP = 2 nm

rNP = 5 nm

rNP = 10 nm

ϑS = 5%

ϑS = 10%

ϑS = 15%

Al Au Cu Mn Ni Co Fe Pd Pt Cr Hf Nb Mo Re W

660 1065 1084 1246 1454 1495 1535 1552 1772 1857 2227 2468 2617 3180 3407

22 461 55 259 352 214 92 866 729 305 143 698 1307 673

384 823 672 851 1012 983 958 1278 1355 1236 1274 1538 1849 2431 2313

522 944 878 1049 1232 1239 1246 1415 1563 1547 1751 2003 2233 2805 2860

636 1049 1181 1399 1429 1457 1485 1695 1760 2131 2361 2518 3042 3260

588 1017 1050 1346 1364 1340 1418 1619 1565 1938 2148 2322 2767 2965

540 985 919 1292 1299 1211 1351 1542 1370 1746 1935 2126 2493 2671

5. CATALYTIC AND SIZE-DEPENDENT EFFECTS The catalytic and the size-dependent effects of seeds, and CSDs from these seeds, are central to the nanotube growth. To investigate it, we assume that seeds (CSDs) are spherical. The size-dependent reduction ΔTM of the melting temperature TM of these seeds (CSDs) of radius rNP may be given by ΔTM =

2σLSΩLSTM r NPHL

(1)

We employed eq 1 to calculate the value of ΔTM as a function of rNP (Dnano = 2rNP) for more than a dozen FECA nanoparticles. We also employed the equation (2)

Figure 4. Variation of reduced size-dependent melting temperature depression ΔTM/TM with the nanoparticle radius rNP for Fe, Mo, and Pd nanoparticles, respectively.

to calculate the size-dependent reduction TMR in the melting temperature TM for these nanoparticles. Various parameters used for the calculations were taken from Table 1. The results of these calculations for seeds (CSDs) of three different radii rNP = 2, 5, and 10 nm, respectively, are listed in Table 3. These results for the Fe, Mo, and Pd nanoparticles of radii 1 nm ≤ rNP ≤ 15 nm are depicted in also Figure 4. Both Table 3 and Figure 4 indicate that the melting point depression is very high for very small FECA nanoparticles. However, it decreases rapidly with increasing FECA radius rNP. Seidel et al.65 investigated the impacts of Fe, Co, and Ni nanoparticles on the behaviors of CNT growths. They observed that the order of the lowest growth temperatures agrees with the order of the bulk melting points of these transition metals (e.g., Ni, 1450 °C; Co, 1490 °C; and Fe, 1540 °C). A close look of Table 3 indicates that the observation of Siedel et al.65 is surprisingly consistent with the calculated results for seeds of radius rNP = 10 nm, but it is exactly opposite to the calculated results for seeds of radius rNP = 2 nm. Figures 5(a),(b) show TMR versus TM plots for all the FECA

nanoparticles of Table 3. While Figure 5(a) is for nanoparticles of radii rNP = 10 nm, Figure 5(b) is for nanoparticles of radii rNP = 5 nm. It is interesting that they yield straight lines. However, the calculated data in Figure 5(b) are more scattered than those in Figure 5(a). Due to increased sensitivity of ΔTM to rNP for smaller nanoparticles, the TMR versus TM plot for the FECA nanoparticles of radius rNP = 2 nm of Table 3 is not very precise. The calculated results are just too scattered to yield any meaningful variational relationship between the size-dependent melting temperature TMR and the bulk melting temperature TM. Our results for rNP = 2, 5, and 10 nm may thus set a limit for the validity of Seidel’s observation, and this limit may be quite general. It suggests that the order of low growth temperature may follow the order of bulk melting point until rNP 302.8 K. However, it has relatively high surface energy (∼700 mJ/m2) and dangling bonds, and it attracted ammonia molecules. These ammonia molecules therefore landed on the molten Ga and reacted with this Ga producing GaN : 2Ga + 2NH3 → 2GaN + 3H2. The GaN molecules then diffused into molten Ga yielding tiny, hollow GaN spheres, which are actually CSDs. A large number of CSDs was formed all around the nanoparticle surface. At about 700 °C, the tiny GaN CSDs were converted into GaN CODs. When merged together, these CODs created a GaN shelled droplet, about 20− 25 nm in diameter. The thickness of the tiny GaN spheres was though 3.5−4.5 nm. It is also possible that RS≡Ga and RS≡N from the vapor phase diffused through the shelled droplet and reacted together to yield GaN molecules. These molecules, upon supersaturation at the L/S interface, were nucleated into the GaN nanotube. These are indeed true. X-ray diffraction studies confirmed that the GaN droplet was made of polycrystalline GaN with a wurtzite structure. The energy-dispersive X-ray (EDX) spectrum obtained for the shelled droplet showed that it was composed of many tiny entities, which we call CODs (molten CSDs). This spectrum thus lends support to the concept of CODs made from CSDs. HRTEM images obtained by Yin et al.82 confirm that the RS species do segregate to the periphery of a nanoparticle in the form of a shell. The HRTEM images obtained for the wall of the GaN nanotube demonstrated that the nanotube wall was composed of small nanocrystals, 3.0−3.5 nm in diameter. These are the CNWs. So, the observation by Yin et al.82 validates the idea of CNWs and that nanotube may comprise CNWs. The X-ray analysis of the seed by Yin et al. yielded results in complete agreement with our model. It showed that seed structure was disordered; it was polycrystalline, amorphous, or semiamorphous. The concept of CNWs resulting from CSDs via CODs is supported by Zhao et al.,83 who detected extraordinarily long one-dimensional linear C chains consisting of more than 100 carbon atoms inserted inside MWCNTs. Carbon chains have been observed before, but never inside a nanotube wall. We called them CNWs (component nanowires). These CNWs were grown in cathode deposits from the dc arc discharge evaporation of pure carbon electrodes in hydrogen gas. As observed by HRTEM, the CNWs were all thinner than 10 nm in diameter, possessed a perfect tubular lattice, and had

nanotube is created by the coalescence of a large number of very thin nanowires, e.g., CNWs. Obviously, each of these CNWs was grown from a COD. Transmission electron microscopy (TEM) analysis of the multiwalled boron nitride nanotubes by Jaffrennou et al.75 indicated that the nanotubes had facets, stacking faults, and dislocations in their walls. The CdSe nanotubes produced by Shen et al.76 were not either perfectly single-crystalline. These nanotube had many stacking faults and missing layers. These stacking faults resulted from the difference in stacking sequences in two adjacent domains of the nanotube seed, and this difference in stacking sequences stems from differences in compositions and surface energies of these adjacent domains, which are actually CSDs. The missing layers in nanotubes, as observed by experiments, might have resulted from simply the presence of gaps between two CSDs (see Figure 2(b)). As noted by Shen et al.,76 there were some half-tubes, which are almost similar to the GaN nanotubes produced in our laboratory (see Figure 8), and these may be

Figure 8. GaN nanotubes grown in our laboratory by the self-catalytic growth mechanism and exhibiting incomplete (broken) nanotube walls.

explained only with the concept of CSDsthat CSDs were formed only in half of the shell and were either absent or inactive in the other half of the shell. Ugarte et al.77 (see Figure 2 by these authors) produced a thick graphitic nanotube of nonuniform thickness. They had exactly NT-2 type structure (see Figure 3). Bera et al.78 observed different numbers of walls on the two sides of MWCNTs. They also observed incomplete and bent layers at the inner concentric wall of the MWCNTs. These findings are completely in line with our demonstration (see NT-7, Figure 3) that, depending on the availability of small CSDs and the alignment of these CSDs into concentric shells, the number of the shells in one side of a nanotube may indeed be different from that on the other side of the nanotube. The carbon nanotube grown by the microwave-plasmaenhanced CVD technique by Okai et al.46 had nonuniform and noncylindrical structure. The carbon nanotube grown by SenGupta et al.79 employing FECA≡Ni had multiple walls. The diameter of this nanotube was not uniform; it decreased with an increase in nanotube length. The diameter of the innermost wall was larger at the nanotube tip than at the nanotube base (see Figure 5(b) by these authors). An electron micrograph image of thick MWCNT produced by Qin80 had distorted graphitic lattice fringes. Although the wall thickness (about 15 nm) was almost constant along the length of the tubule, the inner diameter varied along the tubule axis, ranging 5321



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have different domains with different surface energies. The RS≡X and RS≡Y species diffuse through the droplet to yield XmYn nanotubes. The droplet remains at the nanotube tip (see Figure 1) as the nanotube grows. The nanotube growth takes place at the liquid/solid (L/S) interface. The solid of the L/S interface is the substrate at the initiation of the growth but is the nanotube tip (underneath the droplet) during the subsequent stages of the growth. The liquid of the L/S interface is always the droplet. The growth process is vastly simplified for CNTs for which RS≡Y = 0, and RS≡X = C has very small surface energy (∼17 mJ/m2) as compared to that of the FECA nanoparticle. The absence of RS≡Y eliminates requirement of the reaction mX + nY → XmYn to yield XmYn molecules during diffusion through the droplet.

cumulenic (...CCCCC...) or polynic (...−CC− CC−C...) structure. A low-magnification SEM image of the tubular ZnO whiskers by Hu et al.84 is reproduced in Figure 9.

11. GROWTH MECHANISM Formation of CSDs and the melting/nonmelting of these CSDs (see Sections 7 and 8) follow some distinct mechanisms. Formation of FECA/X eutectic CSDs, and the melting of these CSDs into CODs at T ≥ TE, follow the VLS mechanism (referred to as the ζ1 mechanism). If not merged into the droplet, FECA/X CODs mediate the CNW growth. However, if merged into the droplet, they mediate the nanotube growth. The droplet stays at the nanotube tip, as the nanotube grows. At the end of the growth, the FECA/X eutectic alloy solidifies at the nanotube tip. The InP nanotube by Bakkers and Verheijen32 grown at 515 °C employing FECA≡Au is a good example of the VLS mechanism for nanotube growth. The temperature T = 515 °C for this growth corresponds to AuIn alloy in the Au/In binary phase diagram.68 Formation of FECA CSDs and the melting of these CSDs at an appropriate temperature also yield CODs. These CODs may merge together into a droplet. The growth by this mechanism (called the ζ2 mechanism) is different from the ζ1 mechanism. CODs in the ζ1 mechanism are created by the FECA/X eutectic effect and are composed of a FECA/X eutectic alloy. However, the CODs, in the ζ2 mechanism, are created by the size-dependent and solubility-dependent melting point depression solely of the fractured FECA particles; they are composed of FECA material. Following the ζ2 mechanism, Andrews et al.86 produced MWCNTs employing FECA≡Fe at 650 °C. The temperature for this growth was far lower than the Fe/C eutectic temperature TE (see Table 1). Yet, MWCNTs were produced simply by Fe, as apparent from pure Fe particles observed after nanotube growth at the nanotube tip. Formation of noneutectic FECA/X CSDs, the creation of noneutectic FECA/X CODs from these CSDs, and the creation of droplet from these CODs lead to nanotube growth at temperature T < TE by the VQS mechanism87 (called the ζ3 mechanism). The ζ3 mechanism is different from the ζ1 and the ζ2 mechanisms. CODs in the ζ3 mechanism are created from the noneutectic CSDs. The mixture of FECA, X, O, and FECA/X alloys of these CSDs is not molten but has molten (semimolten) nanopores. This is evident from an experiment by Pan et al.,88 who produced MWCNTs employing tetraethoxysilane, iron nitrate, and acetylene at 600 °C. There was no NSB or NSC cap (see Figures 1(b), (c)) on the nanotube tip, but there was a mixture of C, Fe, Si, and O in the shelled seed (e.g., nanowall edge of the nanotube). The nanotube growth may be caused by the migration of the RS species through CODs (or the droplet from these CODs) to the L/S interface. These CODs are not molten but have disturbed (disordered) lattice structure. The nanotube growth is therefore very slow. The mechanism for this growth, called the ζ4 mechanism, is apparent from an experiment by Mensah et al.89

Figure 9. ZnO nanotubes showing nanorods at the tip, incomplete sidewalls, and nonuniform wall thickness. These nanotubes by Hu and Bando have been reproduced from ref 84.

For inspection, four of these tubular whiskers are marked as W1, W2, W3, and W4, respectively. A close look of W1−W4 indicates that (1) the growth of these whiskers was catalyzed by CODs lying at the whisker tips, (2) these CODs had distinctly individual characteristics, and (3) some of the CODs were wider than some other CODs. One may note that W1 ended up with several nanorods. It was possible simply because the W1 wall was made of nanorods. The nanorods emerged from the nanotube tip because some CODs functioned, while others did not function, during the final stage of the growth. W2 had an incomplete wall near the whisker edge. A possible reason for this is again dysfunction of some CODs during the final stages of the growth. Both W3 and W4 have nonuniform wall thickness, and it resulted from some of the CODs being thicker than some other CODs of the shelled droplet. In summary, the characteristics of W1 to W4 may be best explained in terms of CODs mediating nanotube growth. Nanotube growth always accompanies fluctuations in growth conditions. These fluctuations might have caused some of the CODs to become dysfunctional during growth.

10. GROWTH MODEL REVISITED The shell model for nanotube synthesis put forth in the previous investigations33,60 may be refined, extended, and generalized in light of new knowledge derived from this investigation. For this, we restate that FECA may be metal, semiconductor, oxide, cluster, polymer, etc. The droplets (seeds) shown in Figures 1 and 2 are integral elements of nanotube growth. They experience fluctuation even when at the nanotube tip, and because of this fluctuation, the material species of the seed (droplet) may undergo compositional changes with time during growth. Unless otherwise stated, the seeds, and the droplets from these seeds, are the same as defined earlier.33,60,85 These seeds comprise CSDs, while droplets comprise CODs. RS species play key roles in nanotube growth. Both the adatom-induced process and the diffusion-induced process contribute33 to nanotube growth. CODs from CSDs may merge together to create a droplet.33,85 This droplet may be molten (semimolten) in the whole area or in certain parts of it, solid with numerous molten (semimolten) nanopores inside it, or just solid with disturbed (disordered) lattice structure. It may 5322



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who produce ZnO nanotubes (diameter: 80−500 nm) from a mixture of ZnO and graphite powder. The noneutectic CSDs (see Section 7) mediated the growth. The FECA/X (FECA≡SiO2, Al2O3, TiO2, ZnO, Er2O3, carbon black, etc.) noneutectic material mediating nanotube growth is not also molten (see Section 9) but creates molten (semimolten) nanopores for the diffusion of the RS species. Nanotubes are grown by them at temperature T < TE by the VQS mechanism.87 It is called the ζ5 mechanism. The FECA/X material stays at the nanotube base or the nanotube tip as the nanotube grows. This is evident from SWCNT growth by Liu et al.90 employing a 30 nm thick SiO2 sputtering-deposited Si or Si/SiO2 wafer as substrate and CH4 as a carbon source. SiO2 has a melting point of 1610 °C, yet nanotubes were grown at a temperature (∼900 °C) at which SiO2 was solid. The nanotube growth by nonmolten seeds (see Section 8) is different from that by molten seeds (see Section 7). This growth does not rely on diffusion of the RS species through the CODs or droplets. It rather relies on the binding of the RS species to the shelled seed surface. A graphene layer of finite size has many dangling bonds.91 These dangling bonds are reduced by the formation of closed tubular carbon shell(s). The dangling bonds are almost completely eliminated by the RS species binding to the shelled seed (droplet) surface and causing the formation, for example, of CNTs.

12. NANOTUBE BRANCHING Nanotube branching is very common. If produced in a controlled manner, it may have important technological applications. Two of the branched nanotubes are reproduced in Figure 10(a),(b). While Figure 10(a) is by Huang et al.,92

Figure 11. Schematic diagram showing the possible mechanism, in the framework of CODs, of branching of a nanotube into several nanotubes. N-CODs (e.g., N1, N2, N3, and N4 CODs) resulted from the merger of smaller CODs. L-CODs are leftover CODs that did not merge with others to create larger N-CODS. These L-CODs remained at the junction probably as defects. Under some optimized growth conditions, the junction may though be free from L-CODs.

the branching of the AB stem into BC, BD, BE, and BE branches, respectively, but the nanotube of Figure 10(b) resulted from the coalescence of AC and BC nanotubes into the

Figure 10. Branched nanotubes resulting from the (a) branching of a nanotube into several nanotubes and (b) coalescence of two nanotubes into one nanotube. These are by Huang et al. (ref 92) and Luo et al. (ref 93). These figures are reproduced with copyright permission from the authors and publishers.

Figure 12. Schematic diagram showing the possible mechanism, in the framework of CODs, of coalescence of two nanotubes into one nanotube. Some CODs (shown as red circles) not participating in the growth of YT3 may remain at the junction.

Figure 10(b) is by Luo et al.93 These are created just by opposite means. The nanotube of Figure 10(a) resulted from 5323



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Table 5. List of Some Additional Experimental Evidence Supporting Various Features of the Text Presented above characteristic feature molten (semimolten) COD restructuring of FECA nanoparticle segregation of the RS species textured FECA oxygenated FECA nonmelting of seed (CSDs) oxide serving as FECA amorphicity, porosity nanotube branching nanoparticle fluctuations

experimeental evidence

ref

The FECA≡Ni appeared to be in a liquid or liquid-like state during certain phases of CNT nucleation and growth, even at growth temperatures much lower than the melting point of Ni.

6

Environmental transmission electron microscopy showed the restructuring of solid Ni nanoparticles during CNT growth. Frequent elongation/contraction of the nanoparticles was observed on a time scale of about 500 ms.

7

TEM micrographs, together with in situ heating experiments, for Fe-filled MWCNTs, showed a carbon shell gradually formed at the FECA periphery. While the thickness of the shell wall gradually increased, the carbon content in the FECA core gradually decreased. FECA≡Fe2(SO4)3/Al2O3, upon heating at 900 °C in Ar, had dramatically increased texture and pore volume. CVD synthesis of DWCNTs on silicon substrate employing FeSi2 yielded nanotubes only after the addition of a short oxidation step before the nanotube growth. CVD in the confinement of pores of mesoporous silica appears to have yielded highly aligned CNTs without diffusion of the RS species through droplet. Porous Al2O3 served as FECA yielding CNTs.

10

SEM images of the carbon black substrate prior to MWCNT growth demonstrated that the surface structure of this substrate was amorphous and porous. It had grains and grain boundaries. Emergence of thin nanowires from the tips of ZnO nanotube walls confirms the presence of CODs at the nanotube tip and the role of CODs in the formation of nanotubes and nanotube branches. Atomic-scale in situ observation of acetylene decomposition on FECA≡Fe at T = 600 °C and Pchm = 10−2 Torr indicated that the metal’s shape fluctuates during nanotube growth.

CD nanotube. Many suggestions94,95 have so far been made to describe nanotube branching. We argue that this branching is caused by CSDs, and CODs from CSDs. The formation mechanism of the branched nanotube of Figure 10(a) is described in Figure 11. It has four stages denoted by VT1, VT2, VT3, and VT4, respectively. In the VT1 stage, the NSA seed (droplet) is at the nanowall edge of the nanotube tip and is made of CODs. Due to change in growth conditions and fluctuations of the CODs, some of the CODs merged together into larger N-CODs (e.g., N1, N2, N3, and N4 CODs); others were left as L-CODs, as shown in the VT2 stage. There occurred segregation of various species in the stage VT3 inside the larger N-CODs. Because of this segregation, the central cores of the large N-CODs were made of low-surfaceenergy species, and the peripheral shells of the large N-CODs were made of high-surface-energy species. Each of the NCODS (eg., N1, N2, N3, and N4 CODs) of the droplet was consequently transformed into a tiny component NSA seed (droplet, see Figure 1), which produced, in stage VT4, branched nanotubes T1, T2, T3, and T4, respectively. However, L-CODs remained stuck at the junction. The formation mechanism of the branched nanotube of Figure 10(b) is described in Figure 12. It has four stages denoted by ST1, ST2, ST3, and ST4, respectively. In stage ST1, the NSA seeds (droplets) of YT1 and YT2 nanotubes came in contact with each other. Some CODs of YT1 and YT2 could thus interact with one another. These, together with a change in growth conditions and COD fluctuations, led to reorganization of the CODs. As a result, the two NSA seeds (droplets) merged together in stage ST3, and a new and relatively large NSA seed (droplet) was thus created. In stage ST4, this new seed (droplet) mediated the growth of a relatively larger-diameter YT3 nanotube on the top of YT1 and YT2 nanotubes. Some of the CODs not participating in the YT3 nanotube growth remained at the junction. These are consistent with the nanotube presented in Figure 10(b), where the CD nanotube resulting from the AC and BC nanotubes has a larger diameter than the diameters of both of them.

11 12 14 16 18 27 52

13. CONCLUSIONS An in-depth investigation has been carried out to describe the concept of component seed (CSD), component droplet (COD), and component nanowire (CNW) and to demonstrate how this concept vastly broadens the understanding of nanotube synthesis. This concept generalizes also the shell model and explains many different issues pertaining to nanotube synthesis. Additional experiments listed in Table 5 lend support to quantify some of the findings in the present investigation.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. ‡ Associated also with the US Naval Research Laboratory, Washington, DC 20375, USA.

■ ■

ACKNOWLEDGMENTS The author wishes to thank Maoqi He, Arif Khan, David Hermandez, and Albert Davydov for help. REFERENCES

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