Boron Nitride Nucleation Mechanism during Chemical Vapour

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C: Physical Processes in Nanomaterials and Nanostructures

Boron Nitride Nucleation Mechanism during Chemical Vapour Deposition Ben D McLean, Grant Bruce Webber, and Alister J. Page J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05785 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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The Journal of Physical Chemistry

Boron Nitride Nucleation Mechanism during Chemical Vapour Deposition Ben McLean,a Grant B. Webber,b Alister J. Pageb* a

School of Environmental & Life Sciences, The University of Newcastle, Callaghan NSW

2308 Australia b

Priority Research Centre for Advanced Particle Processing and Transport, The University of

Newcastle, Callaghan NSW 2308 Australia

Abstract We present nonequilibrium molecular dynamics simulations demonstrating how boron nitride (BN) nanomaterials nucleate during boron oxide chemical vapor deposition (CVD). Chemical reactions between gas-phase B2O2 and NH3 precursors leads to the nucleation and growth of BN nanostructures in the presence of a boron nanoparticle catalyst. BN nucleates during BOCVD such that the formation of BN rings is mediated by a boron catalyst and promoted by the formation of H2O. Gas-phase H2 is also produced during this process, however, we demonstrate that H2 and H2O formation serve two distinctly different roles during BN nucleation. H2 formation promotes the clustering of BxOx species to form catalytic B nanoparticles; H2O formation promotes BN bond formation and ultimately BN ring condensation, both in the gas phase and at the catalyst surface. Thermal annealing of amorphous BN networks formed via this reaction undergo defect healing over significant simulation times (~20 ns) to afford tube-like BN nanostructures.

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1. Introduction Low-dimensional nanostructured materials such as nanotubes and nanowires have occupied the forefront of materials science over the last few decades due to their remarkable properties and unique applications. Carbon nanotubes (CNTs) have attracted significant attention since their identification in 19911 and can now be synthesised on a commercial scale via chemical vapour deposition (CVD). Boron nitride nanotubes (BNNTs), first theorised in 19942 and realised in 1995,3 are structural analogues to CNTs. In contrast to CNTs however, BNNTs are wide-gap semiconductors irrespective of the tube chirality/helicity.2 BNNTs exhibit outstanding thermal conductivity4 and possess a very high Young’s modulus5 and for this reason are used in the reinforcement of polymeric films,6 bioactive ceramics,7 optoelectronics8 and sensing applications.9-10 Initial attempts at synthesising BNNTs employed plasma11 and arc12-13 discharge methods. CVD synthesis of BNNTs was first reported in 2000,14 and employed ammonia borane feedstock and a NiB solid phase catalyst. A wide range of alternative CVD strategies have since been shown to successfully produce BNNTs.15 Metal oxides (e.g.

MoO3, V2O5,

Ag2O,16-17 CuO, PbO18 and Fe2O319) are commonly used as growth promoters in these synthetic strategies, typically with a boron source such as BxOy. The boron oxide-assisted CVD method (BOCVD)20 is notable in this respect. BOCVD has been shown to produce narrow, defect-free BNNTs and multi-walled BNNTs (MWBNNTs) exhibiting near zigzag (ZZ) structures, as well as vertically aligned BNNTs. BOCVD combines solid boron and a metal oxide growth promoter such as MgO,20 FeO,21 GaO,22 SnO23 or Li2O24 at elevated temperatures (e.g. 1100 °C) to form boron oxide gas (B2O2) which is then mixed with NH3 to produce solid BNNTs, 2MgO(s) + 2B(s) → B2O2(g) + 2Mg(s)


(1)

2

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B2O2(g) + 2NH3(g) → 2BN(s) + 2H2O(g) + H2(g)

(2)

Several mechanistic aspects of this process remain contentious however. For instance, it has been concluded25 that during BOCVD B2O2, BO and B form clusters in the gas phase prior to their reaction with NH3 to produce solid-phase BN. On the other hand, alternative BOCVD experiments suggest a vapour-liquid-solid (VLS) mechanism, with BxOy species decomposing over a liquid phase metal catalyst before reacting with ambient NH3.19 More generally, the parameters that favour specific BNNT growth modes during CVD are presently unclear, with both root-growth and tip-growth of BNNTs being reported in the literature.14, 26 This reflects the state of the field more generally; the current understanding of how BN nanomaterials nucleate and grow during CVD is relatively poor, compared to related fields such as CNT and graphene synthesis.15 In this respect there is a distinct lack of insight from theoretical simulations, which have to date primarily focused on the structural and energetic properties of complete BNNTs,27-33 rather than the mechanism of nucleation. A notable investigation is that reported by Ohta,34 who examined the formation of fulborenes from boron clusters via non-equilibrium molecular dynamics (MD) simulations. More recently, Krstic et al.35 reported extensive simulations of high-temperature arc-discharge BN nanomaterial formation using non-equilibrium MD. Interestingly, no catalyst was required for nucleation and growth of BN network structures in these simulations, despite the presence of a catalyst in commonly-accepted growth models (e.g. root-growth and tip-growth models). Hydrogen was concluded to not greatly influence the formation mechanism in these simulations. However, H has been suggested to catalyse the conversion between cubic BN (cBN) and hexagonal BN (h-BN), analogues to diamond and graphene respectively.36 A recent report by Yao et al.37 demonstrate that MgB2 is an efficient catalyst for BNNT growth during BOCVD. With first principles calculations, they showed that B2O3 dissolves into the MgB2 catalyst and decomposes to BO and BO2 which can then react with NH3 to produce BNNTs.


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Despite these advances, there is no clear, atomistic understanding of the fundamental chemical processes taking place during CVD nucleation and growth of BN nanomaterials. In this work, we address this shortcoming by presenting non-equilibrium MD simulations that reveal how BN networks nucleate and grow during the BOCVD process. We also elucidate how pertinent experimental parameters, such as H2O and H2 partial pressures, and the presence of a solid-phase catalyst, influence this mechanism. We demonstrate the important role of a boron catalyst in the formation of BN networks and discuss the validity of the equation above typically used to describe BOCVD, particularly showing that H2O formation is required to form BNNTs whereas H2 formation is not; H2 formation is instead implicated in the formation of catalytic boron nanoparticles prior to BN nucleation itself.

2. Computational Methods 2.1. Non-Equilibrium Molecular Dynamics Simulations Non-equilibrium MD simulations were performed employing the ReaxFF method38 as implemented in the SCM software package.39 The ReaxFF energy is defined as a function of the bond order, where the bond orders between all atom pairs is calculated on-the-fly at each MD iteration based on interatomic distances. We use the ReaxFFHBN force field38, 40 for all simulations reported here, which has been shown to accurately describe chemical pathways associated with the formation of extended BN networks. MD simulations were performed with an NPT ensemble enforced on the entire system via the Berendsen barostat.41 The target temperature and pressure were respectively maintained at 1100 °C (damping constant 100 fs) and 1 atm (damping constant 500 fs) throughout all simulations. Newton’s equations of motion were iterated using the velocity-Verlet algorithm42 with a timestep of 0.25 fs. This timestep is sufficiently small to ensure that ensure that the charges and bond orders (and


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hence potential energy) varied continuously as the simulation proceeded. Each NPT-MD trajectory was first equilibrated with 10,000 non-reactive iterations prior to each reaction and ran for at least 10 ns. Multiple unique trajectories were run for each simulation to ensure that a representative chemical mechanism was obtained. All quantitative data presented here is calculated as the mean of five trajectories (unless otherwise stated). 2.2. Models of the BOCVD Process We systematically evaluate the reaction between gas-phase B2O2 and NH3 according to the stoichiometric equation (2). This is achieved by a set of seven distinct simulations, depicted in Figure 1. The parameters of each simulation are summarised in Table 1. All simulations employ periodic boundary conditions. Simulation 1 consists of 125 B2O2 gas-phase molecules at an initial density of 0.460 g cm-3, and enables us to characterise how B2O2 reacts and coalesces to form solid phase boron in the absence of NH3 (boron nanoparticles having been observed during CVD BNNT synthesis14, 25). Simulation 2 consists of 125 B2O2 and 250 NH3 molecules at an initial gas-phase density of 0.417 g cm-3. This simulation enables us to elucidate how NH3 influences the native aggregation of B2O2, and also characterises the fundamental mechanistic processes related to solid-phase BN nucleation during the CVD process. We note that the formal reactant stoichiometry of B2O2 and NH3 in equation (2) is conserved in Simulation 2. We examine the potential catalytic role of boron nanoparticles (observed experimentally)14, 25 during CVD BNNT growth in Simulation 3, via the addition of a B102 cluster (approximate dimensions of 1 × 1 × 1 nm3) in an ambient atmosphere of 125 B2O2 and 250 NH3 gas-phase species. We consider the influence of H2O formation on the kinetics of BN nucleation in Simulations 4 and 5 (B102 cluster present), by manually removing hydrogen and oxygen atoms in a 2:1 ratio periodically during the simulation after 1 ns. These hydrogen and oxygen atoms were selected at random from those within a sphere of radius 4.5 Å from a boron cluster comprised of four or more boron atoms. In the same vein,


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Simulations 6 and 7 (B102 nanoparticle present) investigates the effect of H2 formation on BN nucleation via the periodic manual removal of hydrogen atoms only during the simulation after 1 ns. The random removal of H2O and H2 adopted in Simulations 4 and 6 simply mimics and accelerates the natural H2O formation mechanism (via activation and cleavage of B-O by ambient H2/NH3) observed in our trajectories. In total, 250 H2O and 375 H2 molecules were removed from Simulations 4 and 5 and Simulations 6 and 7 respectively. We note that similar approaches have previously been used to examine H removal during the formation of CNTs,43-44 fullerenes45 and polyaromatic hydrocarbons.46 The dimensions of the periodic unit cell in Simulations 3, 5 and 7 were increased to 5 nm such that the gas-phase density remained comparable across all simulations reported here. Table 1. Parameters of NPT-MD Simulations of the BOCVD process according to the stoichiometric reaction B2O2(g) + 2NH3(g) → 2BN(s) + 2H2O(g) + H2(g) (equation (2)). All reactions are simulated at 1100 ºC and 1 atm. Initial Contents

H2O Removal Rate

H2 Removal Rate

Initial volume (Å3)

Initial density (g cm-3)

Simulation 1 (Figure 1(a))

125 B2O2

-

-

24.4

0.460

Simulation 2 (Figure 1(b))

125 B2O2 + 250 NH3

-

-

64

0.417

Simulation 3 (Figure 1(c))

125 B2O2 + 250 NH3 + B102(s)

-

-

125

0.446


125 B2O2 + 250 NH3

1 / 10 ps (> 1 ns)

-

64

0.417


125 B2O2 + 250 NH3 + B102(s)

1 / 10 ps (> 1 ns)

-

125

0.446


125 B2O2 + 250 NH3

-

1 / 20 ps (> 1 ns)

64

0.417


125 B2O2 + 250 NH3 + B102(s)

-

1 / 20 ps (> 1 ns)

125

0.446


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Figure 1. Initial periodic unit cells for a trajectory of (a) Simulation 1, (b) Simulations 2, 4 and 6 and (c) Simulation 3, 5 and 7. Orange, red, blue and white spheres represent boron, oxygen, nitrogen and hydrogen atoms, respectively. Initial volume of each simulation presented in Table 1. 3. Results and Discussion 3.1. BOCVD Reaction Mechanism We begin by examining the gas-phase interactions between B2O2 and NH3 that occur prior to BNNT growth, and establish how these interactions are influenced by the boron catalyst nanoparticle and reduction in H2/H2O partial pressures. 3.1.1. Initial Reactive Pathways Populations of principle reactive intermediates observed in the early stages of the simulated BOCVD reaction in Simulations 1-7 are shown in Figure 2. Figure 2(a) shows that the consumption of the B2O2 and NH3 reactants occurs rapidly, as anticipated; only ~10% (or less) of the initial B2O2 remains after 1 ns across Simulations 1-7. Nevertheless, the population of NH3 remains significant (~50%) in the absence of the boron catalyst nanoparticle. There is a dramatic reduction in the amount of free NH3 when the nanoparticle is present, indicating that NH3 actively adsorbs to the nanoparticle surface and its surfaceadsorbed species; we return to a more detailed discussion of the nanoparticle’s influence below. Similarly, the nanoparticle has a significant impact on the rate and extent of B2O2 consumption in the first 5 ns of the simulation, with no free B2O2 molecules present after 100 ps. Figure 2(a) also shows that the amounts of H2O and H2, i.e. the stoichiometric products of


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BOCVD (equation (2)), are remarkably small during the initial stages of the B2O2 + NH3 reaction in the absence of the nanoparticle (Simulations 2, 4 and 6). H2O is typically formed through H transfer between NHx and OH radical species, while H2 forms via NH3 dissociation. Figure 2(a) shows that the production of H2 is increased in the presence of the boron nanoparticle, while this is not the case for H2O formation. The nanoparticle activates NH3 and promotes the subsequent formation of B-N bonds. The low populations of H2O during the initial stages shows that additional side reactions, unrelated to equation (2), dominate during the initial phases of the BOCVD process. It is shown in Figure S1 that H2O is produced in greater number later in the BOCVD process, i.e. during the condensation of solid phase BN itself, however still not within the stoichiometry of equation (2) over the timescales employed here. The formation of H2 and H2O is also associated with the formation of trace quantities of N2 (Figure S2), despite the absence of this chemical product in equation (2).


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Figure 2. Populations of reactants, products and principle reactive intermediates in the first 5 ns of the simulated BOCVD reaction at 1100 ºC and 1 atm. (a) Consumption of B2O2 and NH3 (inset) reactants and production of expected products H2O and H2 (according to equation (2)). (b) Reactive BxOx intermediates produced by B2O2 self-interaction, B2O2 B-B bond scission and BO self-oligomerisation. (c) Reactive BxNyOxHz intermediates produced via reaction of B2O2 and NH3 – derived radicals. Dashed lines indicate the presence of a boron nanoparticle catalyst, while solid lines are “nanoparticle-free”. Red indicates no manual H2O/H2 removal; blue indicates H2O removal; green indicates H2 removal (Table 1). Data are only shown for “free” species, i.e. species not adsorbed to the nanoparticle surface. 3.1.2. Boron Catalyst Self-Assembly The initiator of these initial side reactions is B-B bond scission in individual B2O2 molecules, which drives the formation of BxOx species via BO self-oligomerisation. Figure 3(a) and (b) are direct evidence of this mechanism and show that the populations of B-B and B-O bonds decrease and increase, respectively, throughout this period of the reaction. Figures 2 and 3 shows that BO self-oligomerisation is most prevalent in the absence of NH3 (i.e. Simulation 1). Under such conditions significant amounts of small BxOx species (e.g.


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B3O3, B4O4), but also BxOx species as large as B15O15, are formed during the first 5 ns of decomposition (see Figure S3). It is conceivable that this B-B scission / BO selfoligomerisation process is the potential origin of solid-phase boron nanoparticles observed in CVD experiments.14, 25 Indeed, one such example is shown in Figure 4(a). Here, a capped B16 cluster forms following the oligomerisation of two distinct BxOx chains at 160 ps. This initial period of chain growth (~160-178 ps) ultimately yields a linear chain structure 20 boron atoms in length. The high thermal energy drives strong vibrational motions in the chain which lead to its structural isomerisation forming an enclosed cage structure by ~400 ps. This cage structure itself is structurally similar to the repeating icosahedral subunits comprising solid-phase boron. Such clusters are stable for the timescales employed here (~5 ns), indicating that they are likely persistent during the BOCVD process. Figure 4(b) also shows that aggregation of individual cage structures is favourable, and suggests a route to the formation of larger catalyst nanoparticles observed in CVD experiments.14, 25 However, we note that individual cage structures are themselves capable of catalysing BN network nucleation and growth. We discuss this process more fully in the next section.


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Figure 3. Populations of B-B, B-N, B-O and N-O bonds observed in the first 5 ns of the simulated BOCVD reaction at 1100 ºC and 1 atm. Dashed lines indicate the presence of a boron nanoparticle catalyst, while solid lines are “nanoparticle-free”. Red indicates no H2O/H2 removal; blue indicates H2O removal; green indicates H2 removal (Table 1). Populations of all other bond types are presented in Figure S4.

Remarkably, these cages formed are pure boron. Oxygen, nitrogen or hydrogen atoms are excluded from the cage structure during its formation, and instead cap its surface. It can be expected that, ultimately, this surface adsorbed oxygen and hydrogen is etched/reduced (in the form of H2O and H2), while nitrogen remains to form BN networks. We note that no boron cages are observed in Simulation 1, i.e. in the absence of NH3, despite the presence of extensive BO chain growth and large BxOx chains (Figure S3). This is because the formation of N-O bonds in the presence of NH3 promotes the formation of B-B bonds, rather than B-O bonds, during the chain growth process (Figure 3(b)). In turn, when the chain reaches a


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sufficient length, the higher population of B-B bonds in the presence of NH3 makes chain → cage isomerisation more favourable.

Figure 4. (a) Formation of a capped B16 cluster (Simulation 3) via B-B bond scission / BO oligomerisation mechanisms following reaction between B2O2 and NH3 at 1100 ºC and 1 atm. (b) Example of boron cluster aggregation forming larger boron nanoparticles (Simulation 3). (c) Catalytic generation of boron cages via an existing boron nanoparticle (Simulation 3). Atom colours as per Figure 1.

Figure 2 indicates that this boron cluster growth mechanism occurs independently to the formation of H2O and H2; the manual removal of these product species from the simulation does not influence the population of BxOx (Figure 2(b)). However, there is a noticeable difference in B-B bond scission rates when H2 or H2O are manually removed (Figure 3(a)); H2O removal promotes B-B bond formation, while H2 removal promotes B-B bond scission. The rate of B-B bond scission is also accelerated by a boron nanoparticle (Figure 3(a)), while essentially no BO molecules are observed during the first 5 ns of the reaction (Figure 2(b)). This means that the B2O2 B-B bond is activated by the nanoparticle surface. The boron nanoparticle is observed to promote BxOx self-oligomerisation according to the ensemble of


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adsorbed species on the nanoparticle surface during this period. These observations, paired with the relatively high concentration of co-adsorbed NH3, noted above, result in the acceleration of B-N bond formation compared to catalyst-free conditions (Figure 3(c)). The boron nanoparticle is also observed to catalytically form new boron cage structures akin to that shown in Figure 4(a). Figure 4(c) illustrates one such autocatalytic process, in which the BO chain growth mechanism is effectively tethered by the catalyst surface (up to ~343 ps), before it isomerises into the capped boron cage structure (within ~2.7 ns). This mechanism can either result in the autocatalytic production of new, smaller, catalyst particles (as shown in Figure S6), but also lead to surface aggregation of boron yielding fewer, larger, catalyst particles (as shown in Figure 4(c)). In either case, the structure of the boron cage is consistent with those formed in gas-phase, i.e. without a nanoparticle surface (Figure 4(a)). Figures 2 and 3 demonstrate that the chain growth of BxOx is disrupted by the formation of reactive intermediate BxNyOxHz species, viz. BONH2, BONH3 and BONH4 (Figure 2(c)). Such species are typically formed via B-N (Figure 3(c)) and B-H bond formation between the BO moieties and ambient NH3, and NH3-derived radicals, following initial B2O2 B-B bond scission. For instance, BNOH3 is typically formed simply via direct addition of BO and NH3 or the dissociation of B2O2NH3 via B-B bond scission. BNOH2 forms following H abstraction from BNOH3. NH3 hydrogen is often abstracted directly by a BO moiety, leading to BOH. BNOH4 is primarily formed through BNOH3 abstracting H from a nearby NH3 group to form an O-H bond. Intramolecular H transfer from the N atom to B is often observed within these species. Figure 2(c) shows that BNOH2 and BNOH3 formation is impeded by the boron nanoparticle. As noted above, B-B bond scission is also impeded, which in turn impacts the observed populations of BxNyOxHz intermediates subsequently formed. The single exception here is BNOH4, the population of which is comparable with and without the boron nanoparticle present (i.e. Simulations 1 and 3) over the timescales reported here.


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3.1.3. H2 vs H2O Formation: A Revised BOCVD Reaction Mechanism Formation of H2 and H2O during the initial stages of BOCVD (mimicked here via manual H2 and H2O removal) influences the population and lifetimes of the principal reactive BxNyOxHz intermediates in subtly different ways. For instance, the populations of BONH2, BONH3 and BONH4 all decrease upon periodic removal of both H2 and H2O (Figure 2(c)). However, this effect is significantly more pronounced when H2 is removed. On the other hand, H2 removal does little to accelerate B-N bond formation, whereas B-N bond formation is markedly accelerated upon H2O removal; this affect is observed both with and without the boron catalyst nanoparticle. While H2 removal does not influence the rate of B-O bond formation, Figure 3(d) shows that N-O bond formation increases markedly upon H2 removal, and this is the opposite effect to H2O removal (again, this trend is independent of the boron catalyst nanoparticle). The strength of the N-O bond evidently prevents further B-N bond formation (Figure 3(c)). Collectively, these trends indicate that H2O formation (necessitating B-O bond scission) is the principle mechanism by which extended B-N networks form during the BOCVD process via Le Chatelier - like behaviour. By contrast, H2 formation is not directly related to B-N network formation, despite the formal stoichiometry of equation (2). Instead, H2 formation is the key driver of earlier chemical processes in the BOCVD reaction. Firstly, it drives N-O bond formation, which in turn increases the likelihood of BxOx chain → cage isomerisation, and hence boron nanoparticle formation. Secondly, H2 formation is a prerequisite for the formation of BxNyOxHz intermediates, as the dissociation of NH3 frees NHx species for B-N bonds to form. The production of H2O, and not the production of H2, is the rate limiting step in the formation of BN. The formation of BxNyOxHz intermediates occur via varying mechanisms, all of which preclude the prerequisite of H2O formation being required for BN condensation. Considering these conclusions we propose a revised mechanism for BOCVD


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BN nanostructure growth in Scheme 1. In the next section we detail explicitly the nucleation mechanism of BN networks observed in our simulated BOCVD reaction.

-H2 NH3 B2O2

linear BxNyOxHz

-H2O BN

capped Bx cluster

aggregated B nanoparticle -H2O

-H2

Scheme 1. Proposed mechanism of BN nucleation during BOCVD. Non-equilibrium MD simulations show that the formation of H2 and H2O during BOCVD play distinct and independent roles; H2 formation is associated with the boron catalyst self-assembly, while H2O formation is associated with BN network condensation.

3.2. BN Nucleation Mechanism We now consider the formation mechanism of extended BN networks during the reaction between B2O2 and NH3. BN nanomaterials are structural analogues of carbon nanomaterials, and the nucleation and growth of carbon nanomaterials (e.g. graphene,47-51 SWCNTs52-56) is defined by the formation of polygonal carbon rings, principally pentagons and hexagons, and to a lesser extent heptagons. We therefore present the populations of equivalent BN ring structures observed in Simulations 2-7 in Figure 5.

Figure 5. Populations of polygonal five- (left), six- (middle) and seven-membered (right) BN rings observed during non-equilibrium NPT-MD simulations of BOCVD at 1100 ºC and 1 atm.


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Ring populations shown in Figure 5 are consistent with the populations of B-N bonds shown in Figure 3(c), and show that BN ring formation is accelerated in the presence of a boron nanoparticle, and by the removal of H2O from the reaction. Similarly, the removal of H2 from the reaction decreases the populations of BN rings. We also note that BN ring formation in the absence of the boron nanoparticle is essentially not observed (i.e. in Simulation 2). Supplementary simulations (not shown) demonstrate that this is also the case over longer timescales, e.g. up to 10 ns. The boron catalyst (Simulation 3) promotes formation of hexagonal, and interestingly, heptagonal BN rings, while pentagonal rings are not as common. In contrast, heptagonal carbon rings are rarely observed during carbon nanostructure nucleation and growth because they are relatively unstable and susceptible to defect healing processes.56 The increased ring formation observed in the presence of a nanoparticle catalyst is attributed to the relative rates of surface-mediated and gas-phase ring isomerisation. For instance, Figure 6(a) shows an illustrative example of how the boron catalyst surface mediates the formation of BN hexagonal rings. In this case, the catalyst shown is not the original nanoparticle, but one that is catalytically formed from it via the mechanism discussed above (Figure 4(c)). In effect, surface boron atoms in this catalyst tether a small BxNyOxHz moiety to the catalyst surface via B-N bonds and enable coordination with gas-phase fragments (~2448 ps). Subsequent structural rearrangement (~2458 ps) yields a complete BN ring structure within ~10 ps. Figure 6(b) shows that an almost identical mechanism is observed in gas-phase, i.e. independently of a catalyst surface, but at a substantially reduced rate of reaction. In this case, two intermediate BxNyOxHz species oligomerise at ~679 ps, but without a solid-phase support, excessive vibrational and rotational motion (due to the high thermal energy available in the simulation) impedes ring isomerisation, which is not observed for another ~500 ps.


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Figure 5 shows that H2O removal promotes formation of pentagonal, hexagonal and heptagonal BN ring formation, irrespective of the presence of a B nanoparticle (Simulations 4 and 5). This is attributed to the extensive hydrogen and oxygen passivation of boron/nitrogen atoms in BxNyOxHz species and the ring structures subsequently formed from these species. Such passivation is illustrated by the two examples shown in Figure 6(a) and (b). The degree of passivation here is not influenced by whether or not they are surface adsorbed, or free in gas-phase. Following the removal of H2O, B-O bonds can no longer impede B-N bond formation (Figure 3(b)) and the condensation of extended ring networks over longer (nanosecond) timescales. An illustrative example of the formation of one such network is shown in Figure 6(c). Here, the formation mechanism is essentially the same as that shown in Figure 6(b), i.e. oligomerisation of free gas-phase BxNyOxHz species followed by structural rearrangement to form ring structures. The timescale of this process (i.e. ~5 ns) is again evidence of the slow kinetics of ring condensation in the absence of a catalyst surface. Figure 6(d) demonstrates that the same ring condensation process is accelerated substantially when the nascent BN network is tethered to a nanoparticle surface. In this case, H2O removal from the system leads to the formation of a well-defined, albeit defective, BN nanotube structure within ~20 ns. This structure is comparable to those observed following high-temperature quantum chemical MD simulations of catalyst-free plasma BN synthesis, despite the difference in the physicochemical conditions in our simulations (i.e. lower temperature catalytic CVD of B2O2 + NH3, as opposed to high-temperature (e.g. ~6000 K) catalyst-free plasma of BN monomer precursors and borazine precursors).35 Interestingly, the mechanism by which this structure emerges bears resemblance to the formation mechanism of SWCNTs. Notably, healing of structural defects (e.g. non-hexagonal rings, chains, homogeneous B-B/N-N bonds) is prevalent during the formation of the structure and gradually increases the population of BN bonds and hexagonal rings in the tube


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structure as it emerges, which are both initially low. However, there are stark contrasts between the BN nucleation mechanism shown in Figure 6(d) and the nucleation of SWCNTs. For instance, the tubelike structure shown in Figure 6(d) at ~20 ns was originally formed via BN chain oligomerisation and was not necessarily surface-mediated. This leads to an initial structure comprised of a much more amorphous network (~5-11 ns) with a more disperse distribution of non-hexagonal rings (and, indeed linear defect structures as well). By contrast, SWCNTs are known to nucleate on many catalysts52, 54, 57-58 via a significantly more ordered ‘yarmulke’ cap structure54, 59 adhered to the catalyst surface, composed entirely of pentagonal and hexagonal rings. However, we note that SWCNT nucleation observed on solid phase Al2O3 nanoparticle catalysts is more reminiscent of the BN nucleation mechanism reported here.44

Figure 6. (a) Catalytic nucleation of a BN hexagon by a B12 nanoparticle (Simulation 3). (b) Nucleation mechanism of a hexagonal BN ring in the gas phase (Simulation 3). (c) Gas-phase condensation of extended BN ring networks is accelerated by H2O removal (Simulation 4). (d) Nucleation of an extended BN network from the surface of a boron nanoparticle catalyst following H2O removal. (d) is a continuation of the trajectory shown in Figure 4(c). Atom colours as per Figure 1.


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4. Conclusions We have presented nonequilibrium molecular dynamics simulations revealing the nucleation mechanism of BN networks during BOCVD. These simulations detail for the first time the role of B2O2 and NH3 precursors, the influence of a B nanoparticle catalyst and the partial pressures of H2 and H2O during the BOCVD reaction. H2 and H2O formation during the BOCVD process are shown to play distinct and independent roles; H2O formation drives BN bond formation and the condensation of extended BN ring networks. On the other hand, H2 formation is unrelated to BN ring formation, but instead drives the formation of boron clusters prior to BN network nucleation. We show that such clusters can aggregate and grow via several pathways, and hence propose that this process explains the presence of catalytic boron nanoparticles observed experimentally during CVD BNNT synthesis.14, 25 On this basis we present a revised reaction mechanism for BN nucleation beyond that currently accepted in the literature (equations (1), (2)). Ultimately H2O formation affords amorphous BN networks which subsequently “heal” to tube-like structures via defect healing processes over longer time scales (~20 ns), akin to the healing observed during CVD growth of CNTs and graphene. We believe that the results presented here regarding the BOCVD reaction mechanism may assist in the optimisation of the experimental CVD synthesis of low dimensional BN nanostructures, in a similar fashion to previous improvements in the production of low dimensional carbon nanostructures.15

Corresponding Author *[email protected] Supporting Information


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Populations of H2O and N2 formed during BOCVD; example of B15O15 structure formed in Simulation 1; populations of B-H, O-H, H-H, N-N and N-H; examples of gas-phase and catalyst-mediated boron cluster formation observed in Simulation 3. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements AJP acknowledges support from the Australian Research Council (INTERSECT, LE170100032) and Dr. S. J. A. van Gisbergen (Software for Chemistry & Materials BV). BM acknowledges the Australian Government for a Research Training Program (RTP) Scholarship. This research was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University and INTERSECT systems, through the National Computational Merit Allocation Scheme supported by the Australian Government.

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Graphical Abstract


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Figure 1. Initial periodic unit cells for a trajectory of (a) Simulation 1, (b) Simulations 2, 4 and 6 and (c) Simulation 3, 5 and 7. Orange, red, blue and white spheres represent boron, oxygen, nitrogen and hydrogen atoms, respectively. Initial volume of each simulation presented in Table 1. 90x35mm (300 x 300 DPI)



Figure 2. Populations of reactants, products and principle reactive intermediates in the first 5 ns of the simulated BOCVD reaction at 1100 ºC and 1 atm. (a) Consumption of B2O2 and NH3 (inset) reactants and production of expected products H2O and H2 (according to equation (2)). (b) Reactive BxOx intermediates produced by B2O2 self-interaction, B2O2 B-B bond scission and BO self-oligomerisation. (c) Reactive BxNyOxHz intermediates produced via reaction of B2O2 and NH3 - derived radicals. Dashed lines indicate the presence of a boron nanoparticle catalyst, while solid lines are "nanoparticle-free". Red indicates no manual H2O/H2 removal; blue indicates H2O removal; green indicates H2 removal (Table 1). Data are only shown for "free" species, i.e. species not adsorbed to the nanoparticle surface. 119x95mm (300 x 300 DPI)


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Figure 3. Populations of B-B, B-N, B-O and N-O bonds observed in the first 5 ns of the simulated BOCVD reaction at 1100 ºC and 1 atm. Dashed lines indicate the presence of a boron nanoparticle catalyst, while solid lines are "nanoparticle-free". Red indicates no H2O/H2 removal; blue indicates H2O removal; green indicates H2 removal (Table 1). Populations of all other bond types are presented in Figure S4. 112x143mm (169 x 169 DPI)



Figure 4. (a) Formation of a capped B16 cluster (Simulation 3) via B-B bond scission / BO oligomerisation mechanisms following reaction between B2O2 and NH3 at 1100 ºC and 1 atm. (b) Example of boron cluster aggregation forming larger boron nanoparticles. (c) Catalytic generation of boron cages via an existing boron nanoparticle. Atom colours as per Figure 1. 92x72mm (300 x 300 DPI)


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Figure 5. Populations of polygonal five- (left), six- (middle) and seven-membered (right) BN rings observed during non-equilibrium NPT-MD simulations of BOCVD at 1100 ºC and 1 atm. 150x42mm (300 x 300 DPI)



Figure 6. (a) Catalytic nucleation of a BN hexagon by a B12 nanoparticle (Simulation 3). (b) Nucleation mechanism of a hexagonal BN ring in the gas phase (Simulation 3). (c) Gas-phase condensation of extended BN ring networks is accelerated by H2O removal (Simulation 4). (d) Nucleation of an extended BN network from the surface of a boron nanoparticle catalyst following H2O removal. (d) is a continuation of the trajectory shown in Figure 4(c). Atom colors as per Figure 1. 95x91mm (300 x 300 DPI)


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Scheme 1. Proposed mechanism of BN nucleation during BOCVD. Non-equilibrium MD simulations show that the formation of H2 and H2O during BOCVD play distinct and independent roles; H2 formation is associated with the boron catalyst self-assembly, while H2O formation is associated with BN network condensation. 90x29mm (300 x 300 DPI)


Boron Nitride Nucleation Mechanism during Chemical Vapour

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