Nobel metal nanoparticles has already shown the ability of catalysing the formation of carbon forms contain sp1/sp2 carbon as was recently shown by Kutrovskaya S. et al. under an effect of laser irradiation and electromagnetic fields using gold nanoparticles.33 A similar methodology was reported by Casari et al., noble metals particles have shown a high affinity to form carbon structure containing H-terminated polyyne.34 Okada et. al. have shown the possibility to produce a polymeric composite containing polyyne stabilised by silver nanoparticles.35 Carbon films enriched with carbyne species can be produced either via Chemical vapour deposition or Physical vapour deposition.15 It has also been shown that the dechlorination of Polyvinylidene chloride formed polyyne-polyene carbon films. Recently, a new generation of N-doped carbons have been found to be interesting materials because of their superior physical and chemical properties as compared to those of their undoped counterparts. Accordingly, wide ranges of applications are anticipated for these nitrogen-doped carbon materials such as anodes in high performance lithium ion batteries, high performance supercapacitors, as an adsorbent to be exploited for removing toxic pollutants from water. In addition, carbon nitride materials can provide large number of anchoring point sites adsorption for positive charged metal ions due to the electronegativity difference between N and C atoms in CN@HDS sp2 hybridization.12,15,23
The current literature report on the propensity of Nobel metals to form carbon forms containing sp1/sp2, the possibility that FeNPs are able to catalyse the formation of such carbon nitride forms has never been reported previously. During the temperature increase in our experiments, from 20°C to 850°C in vacuum, N(CH3)4OH is expected to undergo significant structural transformations, the N(CH3)4OH transforms to nitrile N(CH)3 and CH3OH (E2 Reaction), the alcohol evaporate and the nitriles begin its transformation forming cyano groups that reacts specifically with unsaturated carbon atoms, the support FLBN provide for a source of nitrogen atom that is required to form cyanopolyynes with the ratio of 1 carbon atom to 1 nitrogen atom (Figure S13). The unsaturated carbon formation can be explained, as C-H bonds at around 300°C, begin to dissociate, transforming the alkyl chains to alkene, polyene and polyyne and cumulenic chains, thus gradually converting sp3-hybridised carbon to sp1-hybridised carbon. However, as sp1 chains are predicted to react with each other,21 the close proximity of the molecules within half dome structures in our experiments, are likely to trigger cross-linking of the chains, leading to the formation of 2D material with CN sp2 regions interconnected by sp1-hybridised CN chains (Figure S9) stabilised by positive charge over the nitrogen atoms. Such cross-linking immobilises the carbon material around the metal core, preventing desorption of the molecules, whilst de-hydrogenation continues as the temperature rises to 850°C, thus forming CN@HDS around the FeNP.
High-resolution TEM imaging indicates that the CN@HDS are different qualitatively to other known forms of carbon nanoparticles, such as fullerenes, carbon onions, carbon black or nano-horns, showing clearly the overall hemispherical morphology of the half dome structures. We have observed that with small iron nanoparticles the CN@HDS present a single layer, conversely, when the iron particles up to 20 nm diameter the half dome multilayers structures could be observed surrounding the metal core (Fig. 1b and S14). It was interesting to see that during the particle migration and coalescence of the FeNPs the capping agent still remained attached to the NPs as the solely available source of carbon in our systems is the organic base, as we have grown those CN@HDS on FLBN (BN was heated in air to removed advantageous carbon). The small Fe clusters during their migration were momentarily trapped at the step edges of the FLBN which acts as anchoring point, reaction site, and source of nitrogen atom which were uptake during the combustion that allowed the formation of the half dome structure 1 : 1 ratio (Fig. 5, Video file 2). Conversely, for the big nanoparticles in which the particle migration was not shown post intense heating the carbon nitride was surrounding the nanoparticle (Figure S14).
The EELS of the half dome structures shows a peak at 284,5 eV which lies in the region of carbon chains.15,23,36–37 The unique broad K-edge above 300 eV is differing from graphitic carbon, diamond at 290 eV and amorphous carbon at 295 eV (Fig. 3c-4e and S6 and S15-16).36–37 The broadness of the carbon K-edge (Fig. 3b, 4b and S15-16) is unusual for pure sp2-hybridised carbon and is comparable to that observed for materials containing sp1 and or sp2 carbon and carbon nitride.36–37,49 The high energy EELS measurements suggest that the bonding state within the half dome structure does not match with well-known forms of carbon, but can be explained by the presence of sp1-hybridised CN intermixed within sp2-CN material.15,31,32,35–38
In addition, the Raman of half dome (Figure S8) data shows a peak at 2200 cm− 1 characteristic of CN sp1 (Figure S8).23,39 Generally, Raman spectra of carbon nitride material show a characteristic peak between 2100 and 2200 cm− 1.9,15,20,23,39 The vibration frequencies of solid carbon nitrides are expected to lie close to the modes of the analogous unsaturated CN molecules, which are 1500–1600 cm− 1 for chain like molecules and 1300–1600 cm− 1 for ring like molecules. 40–41 This means that there is little distinction in the G-D region between modes due to C or N atoms. For example, the frequency of bond stretching skeletal and ring modes is very similar in benzene, pyridine, and pyrrole, so it is difficult to assess the presence of N in an aromatic ring. The modes in amorphous carbon nitrides are also delocalized over both carbon and nitrogen sites because of nitrogen’s tendency to promote more clustered sp2 bonding. It is expected little difference between the Raman spectra of nitrogenised and N-free carbon films in the 1000–2000 cm− 1 region. It was found no shift in the Raman spectra of two sputtered CN samples, one with 26% at 14N and the other with the same content of 15N. On the other hand, we clearly have detected a direct contribution of CN sp1 bonds in the 2100–2200 cm− 1 regions.15,23,39
In a previous study, silver nanoparticles containing C12H25S were heated in a dedicated heating stage and post their intense heating it was shown that the sulfur contained in the organic capping agent was retained in the discorded carbon structure formed.42 We have also heated the Fe2O3 capped with N(CH3)4OH in a dedicated heating stage, post intense heating, we have also shown for the first time that nitrogen of the N(CH3)4OH originally forming the capping agent can be retained in our CN@HDS (Figure S4 and S9).
The EELS and Raman measurements shows the presence of Nitrogen from the sp1/sp2 CN formed during the heating process of the capping agent, the N(CH3)4OH due to intense heating transforms to nitrile N(CH)3 and CH3OH (E2 Reaction), the alcohol evaporate and the nitrile begin its transformation forming cyano groups that reacts with unsaturated carbon atoms to form cyanopolyynes chains. The cyano radicals can react with a variety of molecules in combustion chemistry, amongst which are simple unsaturated hydrocarbons.43–46
The synthesis of monocyanopolyyens and dicyanopolyynes could be easily produced either by laser ablation or with arch discharge techniques.15,47–48 For instance; two graphite electrodes were connected at the pole of the d.c. power supply and submerged in a Pyrex three-necked round bottom flask filled with acetonitrile to yield cyanopolyyens.15 In recent years, a renewed interest in the reaction of CN radicals with simple unsaturated hydrocarbons has risen because of their alleged role in some extra-terrestrial environments, namely the atmosphere of Saturn’s moon Titan and in cold molecular clouds in the interstellar medium (ISM).15 In addition cyanopolyyne have been extensively identified in cold molecular clouds and outflow of late M-type AGB carbon stars and their production routes have been widely investigated. The interstellar medium products can be arranged in four groups cyanopolyynes, methyl substituted cyanopolyynes, cyanopolyynes radicals and olefin nitriles containing a double carbon bond and a CN group.15
In a previous study, A. La Torre et al has reported on the growth and formation of single boron nitride half dome structure BN@HDS mediated by small naked metal particle that reacted with the dangling bonds of the FLBN to form BN protrusion which display sharp corner and edges, the shape of the iron nanoparticle rearranged to a rectangular shape to promote the formation of BN@HDS (Fig. 4c, S12). The formation mechanism was already presented; two mechanisms can be accepted. One possibility is the dissolution-precipitation mechanism, whereby BN dissociates on the surface of the Fe clusters (noting B and N atoms are soluble in Fe)29–31, followed by dissolution, diffusion and precipitation of BN in a form of dome-shaped nanostructures in advance of loss of the Fe at elevated temperature. This is similar to the case of carbon nanostructures, where the nucleation of protrusions on the surfaces of metal atom clusters can occur.29 The second possibility is the "scooter" mechanism, as proposed to explain the growth of carbon nanotubes.29 Here, mobile Fe atoms could continue to etch the BN layers in such a way that small strips remain that subsequently curl and close to form dome-shaped nanostructures. After nanostructure closure, the Fe atoms may either remain trapped inside the cage or escape, depending on the heating conditions or on the experimental conditions.29
This new CN@HDS discovered in our study, might combines the features of graphene and carbyne as they can also be grown on graphene, and offers significant potential for a wide variety of applications. These CN@HDS are found to be very stable under ambient conditions in air or at high temperature in vacuum, exhibiting no measurable structural deterioration in our experiments, and thus being as robust as graphene and or diamond. Recently, we have reported on a new form of nanoscale carbon in the shape of hemispherical matrices with the bonding characteristics of either sp1 or sp2 carbon.49 The carbon matrices were formed spontaneously at high temperature from organic molecules adsorbed on the surface of silver nanoparticles, with the metal core acting simultaneously as catalyst and template for the carbon matrices growth. Significantly, this new form of carbon exhibits photo luminescent activity in the visible and near-IR ranges, with the wavelength of emitted light determined by the length of sp1 chains which link sp2 domains.49 The optical and electron properties of carbon chains and Nano ribbons are well known, the optical properties of sp2 carbon nitride has already been reported in literature showing an upper limit at c.a. 680 nm,23 hence, we speculate that the half dome structure could potentially present interesting photoluminescence properties ranging from the NIR range to the UV depending on the cyanopolyyne chain length and the size of the sp2 carbon nitride domains.23,49
Last but not least, we should be taking into consideration the formation of a ternary domain by exploring the heating rate condition of Fe2O3@FLBN hybrid structure as the Iron particle is able to solubilise carbon, boron and nitrogen, hence it would also be possible to form pentagonal BCN monolayers or even BN chains as it was also reported by Cretu et al.,21 perhaps linear chains of BCN would be a possibility in the near future.