Oxidative peeling of carbon black nanoparticles

Abstract

We demonstrate that layered carbon black nanoparticles can be oxidatively peeled via the reaction with potassium permanganate in sulfuric acid. As a result of this reaction, outer layers of carbon nanoparticles “peel” off due to high levels of oxidation while the less oxidized inner cores, though they exhibit remarkable solubility in water, remain mostly intact.

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Historically, interesting materials have often been synthesized by the oxidation of carbon materials of various dimensionalities.^1–3 Solution oxidation of three-dimensional (3D) graphite, which was first reported in 1859 by Brodie⁴ and later advanced in numerous studies,^5–10 results in two-dimensional (2D) sheets of a heavily oxidized material known as graphene oxide (GO).^1–3 The variation of this method that is presently most commonly used for the synthesis of GO^7,10 was introduced in 1958 by Hummers and Offeman and relies on potassium permanganate dissolved in sulfuric acid as the oxidizing agent. Recent interest in graphene, which was stimulated by the 2004 report by Novoselov et al.,¹¹ revitalized interest in GO and other graphene-like materials. In 2009, Tour et al. demonstrated that the same KMnO₄/H₂SO₄ oxidation chemistry can be used to longitudinally “unzip” carbon nanotubes (CNTs) thus converting them to one-dimensional (1D) graphene oxide nanoribbons.^12–14

A natural next step to a more complete understanding of oxidized carbon species is to explore the effects of oxidation of zero-dimensional (0D) graphitic carbon nanoparticles. There are many varieties of graphitic carbon nanoparticles, ranging from highly ordered multilayered fullerenes, which are often referred to as carbon nano-onions (CNOs), to larger and more disordered carbon black nanoparticles (CBNPs).^15,16 Recently, oxidation of CNOs and CBNPs has been studied with regard to the synthesis of graphene quantum dots,^17,18 which means that similar to other carbon materials, oxidation of carbon nanoparticles can break them into smaller fragments. However, the mechanism of this process is not well understood. For example, it was shown that the oxidation of CNOs with nitric acid results in two distinct products with different morphologies and light emission properties.¹⁷ It was hypothesized that one product, which was heavily oxidized, was formed from the outer shells of CNOs, whereas another less oxidized product was formed from the inner cores of CNOs. Here we report on chemical oxidation of larger and more disordered CBNPs and demonstrate that there are many similarities between the chemical oxidation of CNOs and CBNPs. A study on the morphological changes to carbon black induced by chemical oxidation is relevant to further understanding of this ubiquitous material.

In a typical synthesis, 25 mg of CBNPs obtained by laser excitation in a combustion process¹⁹ were measured into 6.7 mL of concentrated sulfuric acid (Alfa Aesar) and 1.26 g of concentrated phosphoric acid (Sigma Aldrich). The reaction flask was placed into an ice bath and set to stir while 0.295 g potassium permanganate (Sigma Aldrich) was carefully added. The reaction flask was sonicated for 24 hours before being transferred to a stir plate and allowed to proceed at room temperature for 48 hours. The reaction was quenched with 3% hydrogen peroxide (Sigma Aldrich) and then centrifuged at 7000 rpm for 30 minutes. Two phases were separated upon initial centrifugation, a black solid precipitate, and an orange soluble phase, a color associated with oxidized graphene-like materials. The orange solution was decanted off and further separated by repeated centrifugation at 14 000 rpm for 1 hour before being resuspended in aqueous solution, or in ethanol for photoluminescence measurements.

Raman spectra were recorded using a Thermo Scientific DXR Raman Microscope with a 532 nm laser. Fourier transform infrared spectroscopy (FT-IR) was performed using a Nicolet Avatar 360 FT-IR spectrometer. X-ray diffraction (XRD) data was gathered using a Rigaku D-Max/B horizontal Q/2Q X-ray diffractometer. Photoluminescence spectra were recorded using a Horiba Fluoromax 4 fluorimeter. Transmission electron microscopy (TEM) was taken using a JEOL JEM-2200FS high resolution transmission electron microscope. The core level X-ray photoemission spectra were taken using hemispherical electron energy analyzer, and a SPECS X-ray source with a Mg anode (hν = 1253.6 eV).

The products of oxidation exhibited remarkably high solubility in water, with the black product being soluble in excess of 2.5 mg mL⁻¹ as demonstrated in Fig. 1. This is easily accounted for by the presence of oxygen functionalities introduced by the oxidation reaction. However, the presence of a byproduct in the form of an orange-yellow solution required further investigation. TEM imaging revealed more about the nature of products of oxidation. Fig. 1b–d shows TEM images of the precursor CBNPs, oxidized CBNPs, and byproduct of oxidation; all three TEM images are shown at the same magnification to simplify their comparison. Fig. 1b shows that the precursor CBNPs are uniform nearly spherical particles with diameters typically ranging from 20 to 40 nm. The major product of oxidation (Fig. 1c) were aggregates of carbon nanoparticles that are also nearly spherical but visibly smaller than the original CBNPs. Fig. 1d shows that the byproduct extracted from an orange-yellow solution has different morphology. Instead of spherical nanoparticles, observed in TEM images are thin nanosheets with diameters comparable to the size of original CBNPs.

Fig. 1 (a) Optical photograph of the CBNPs and the reaction products dispersed in water. From left to right, the pristine CBNPs, oxidized CBNPs, and byproduct of oxidation. (b–d) TEM images of (b) pristine CBNPs, (c) oxidized CBNPs, and (d) byproduct of oxidation (nanosheets).

In order to explain different morphology of the main product of oxidation and the byproduct, we have performed high-resolution TEM imaging of the precursor CBNPs. Representative TEM images that are shown in Fig. 2 reveal an interesting structure of CBNPs that have a disordered core and a more ordered shell of concentric graphitic layers, although the outer sheets are still notably discontinuous as shown by TEM. The structure of CBNPs can be explained by the nature of the laser-assisted combustion process that was used to produce CBNPs from C₂H₄.¹⁹ The combustion of ethylene first results in the formation of small carbon clusters, which then coalesce producing larger particles that overall form a rather disordered assembly. This can be seen in Fig. 2a where the core of a CBNP, which is marked by a dotted line, consists of smaller spherical particles that are made by concentric graphitic layers and marked by dashed lines. Once this aggregate of smaller particles is formed, it can serve as a nucleus for further formation of more ordered though still discontinuous graphitic layers from C₂H₄ that conformally grow over the entire core. These concentric graphitic layers forming the outer shells of CBNPs are clearly visible in Fig. 2a and b. Conformal growth of graphitic layers with similar morphology from small hydrocarbons have also been reported for flat and cylindrical surfaces,^20,21 which further supports the proposed mechanism upon which the originally formed smaller CBNPs aggregate and serve as surfaces for the outer graphitic layers to nucleate around. Depending on the synthetic process, CBNPs may have various structures and morphologies.²² However, the TEM observation of different structures of cores and shells of CBNPs produced from C₂H₄ by a laser-assisted combustion process¹⁹ is overall consistent with the results of studies of other types of CBNPs.^23,24

Fig. 2 TEM images of CBNPs. The yellow dotted lines mark the boundaries between the conformal graphitic layers that form the outer shells of CBNPs and the more disordered cores of the CBNPs. Dashed yellow circles in panel (a) show small aggregated CBNPs that likely served as a nucleus for the bigger CBNP.

We then performed high-resolution TEM imaging of the major product of the oxidation reaction. Fig. 3a shows that the oxidized CBNP still contain multiple cores and look similar in morphology to the inner core layers of the pristine CBNPs. However, these cores of the oxidized CBNPs are wrapped by a visibly smaller number of outer layers compared to the pristine CBNPs (Fig. 2), and based on the original TEM observations (Fig 1b and c), the oxidized CBNPs are smaller than the pristine CBNPs. To quantify the size difference, we prepared their size distributions of both pristine and oxidized CBNPs based on their TEM images (Fig. 3b and c). The average size of the CBNPs after oxidation was decreased from 28 nm to 20 nm, which can be explained by stripping off outer layers of pristine CBNPs during their oxidation. These results are consistent with TEM images of the oxidation byproduct (Fig. 1d), where individual and aggregated graphitic layers are indeed observed.

Fig. 3 (a) TEM image of oxidized CBNPs. (b) Size distribution chart of pristine CBNPs and (c) oxidized CBNPs.

Based on the TEM data, a reaction mechanism, whereby the outer layers are “peeled” away leaving the inner layers more or less intact, is proposed and shown in Fig. 4. In such a case, the two products of this reaction can be identified. The black solid phase is assigned to be the mostly intact cores of the CBNPs while the orange soluble portion is assigned to be the peeled outer layers. The identification of these products is supported by the TEM images of the respective products, which show that the major product of oxidation has a similar morphology to that of the inner cores of the pristine CBNPs and that the byproduct appears sheet-like in morphology, like a “peeled” outer layer of the CBNP. The unfurling of the outer shells is likely driven by steric forces between the sheets caused by the swelling of the intersheet distances incurred upon addition of oxygen functionalities to the edges and basal planes of the outer shells. This causes the individual graphene oxide-like nanosheets in the outer shells to expand and thereby release the inner cores of the CBNPs. The more disordered inner cores, though oxidized on the surface (which explains their high solubility in water) retain their spherical morphology.

Fig. 4 A simplified scheme of the proposed mechanism of the oxidative peeling of CBNPs resulting in peeled graphene-oxide-like sheets from the outer shells and lightly oxidized cores. Note that irregularities in the structure of the CBNPs and the products, such as discontinuity of the shells and presence of the oxygen functional groups, are omitted.

It is interesting to point out that the morphology of the products will likely depend on the degree/duration of oxidation, as a longer reaction should cause more oxidative damage to CBNPs. For example, Kamegawa et al., reported an experiment in which acetylene-derived CBNPs were treated by concentrated nitric acid at 100 °C for 1000 h.²⁴ After such long oxidative treatment the disordered cores of CBNPs were completely eliminated, resulting in hollow particles of more stable outer shells of CBNPs.²⁴ Extensive oxidation of CBNPs was also reported to yield water-soluble functionalized polyaromatic species.^25,26

Because of the low concentration of peeled-off CBNP shells in the orange-yellow solution (Fig. 1a) we have not been able to collect a sufficient amount of the byproduct for bulk characterization. However, the results of the spectroscopic measurements of the pristine and oxidized CBNPs are overall consistent with the TEM data and the proposed mechanism (Fig. 4). Fig. 5a shows C 1s XPS spectra of the pristine and oxidized CBNPs. The only visible feature in the C 1s XPS spectrum of pristine CBNPs is the peak at 284.7 eV that corresponds to C–C peak. In addition to this peak, observed in C 1s XPS spectrum of oxidized CBNPs are a small shoulder at 286.7 eV that corresponds to C–O bonds, and a small peak at 288.9 eV that represents carbons to the carboxyl groups. In the C 1s XPS spectra of the samples of graphene oxide and unzipping graphene nanoribbons, both of which are prepared through similar KMnO₄/H₂SO₄ oxidative chemistries, C–C and C–O peaks usually have comparable intensities, and the carboxyl peak is more pronounced,^10,27 while the C–O and O–C=O features are barely visible in the C 1s XPS spectrum of oxidized CBNPs. This result is in agreement with previous conclusion that oxidized CBNPs, while bearing some oxygen functionalities that are responsible for water solubility, are mostly intact cores of the precursor CBNPs.

Fig. 5 Spectroscopic characterization of pristine and oxidized CBNPs. (a) C 1s XPS spectra of pristine and oxidized CBNPs. (b) XRD patterns of pristine and oxidized CBNPs. (c) Raman spectra of pristine and oxidized CBNPs. (d) FT-IR spectra of pristine and oxidized CBNPs. In all panels spectra of pristine CBNPs are shown in black, while spectra of oxidized CBNPs are shown in red.

Similarly, only little difference can be observed in the XRD (Fig. 5b) and Raman spectra (Fig. 5c) of the pristine and oxidized CBNPs. Both materials show XRD signals at around 24° and 43° that correspond to the (002) and (100) planes of graphite, respectively.²⁸ Graphite and multiwalled carbon nanotubes also show an XRD peak at ∼25.8° that corresponds to the distance, d between π–π stacked graphene layers of 3.4 Å.²⁹ By comparison, the peak at 24.3° corresponds to a interlayer spacing of about 3.7 Å. This is consistent with previously reported interplanar distances of CBNP layers and is likely due to the mismatched stacking of the π–π systems imposed by the breaking of hexagonal symmetry necessitated by the curvature and disorder.²⁸ When nanotubes and graphite are oxidized using KMnO₄/H₂SO₄, the introduction of oxygen functionalities between the layers results in the increase in d up to 8.3 Å, and as a result the XRD peak has a large shift to ∼10.6°.¹² Therefore, nearly same positions of the peaks in the XRD spectra of pristine and oxidized CBNPs again suggest only mild surface functionalization of the latter.

Fig. 5c shows the comparison of Raman spectra of pristine and oxidized CBNPs. Both exhibit the characteristic D band around 1350 cm⁻¹ and G band around 1600 cm⁻¹. The most notable difference between the two spectra is the change in the ratio between the intensities of the G band and the D band.³⁰ This decrease in the I_D/I_G ratio is due to the shrinking of the hexagonal sp² hybridized domains which comes about from two different factors. The first contributing factor is the formation of oxidized regions of carbon which break the sp² hybridization. The second factor could be a result of the tearing of the outer layers of the CBNPs due to the curvature induced strain also seen in the oxidation of carbon nanotubes.

The FT-IR spectra shown in Fig. 5d are consistent with previous reports on the oxidation of carbon nanotubes. The peaks emerging around 1690 and 1710 cm⁻¹ are indicative of the addition of C=O in carboxylic acid as has been previously reported in the oxidative unzipping of carbon nanotubes.²⁷ This result is consistent with the XPS data presented in Fig. 5a.

The byproduct of oxidation exhibits strong photoluminescence as shown in Fig. 6, whereas the untreated CBNPs exhibited no photoluminescence. Previous studies have linked the incorporation of functional groups on the carbon nanoparticles to the quantum yield.³¹ The inset in Fig. 6 shows an optical photograph of the vial with an aqueous solution of the byproduct of oxidation under a 365 nm UV lamp. This suggests that oxidation of CBNPs may be a useful reaction for the synthesis of graphene-like quantum dots (GQDs), a material that has attracted significant attention and have been fabricated by a wide variety of techniques.^32–37 Recent interest in photoluminescent GQDs in particular for biomedical applications is due the fact that GQDs are inexpensive, chemically stable and less toxic than their CdS or CdSe counterparts.³⁸ Potentially GQDs may find applications in bioimaging, sensing, and as antioxidants and inhibitors of tumor cells.^39–41

Fig. 6 Photoluminescence spectrum recorded for the aqueous solution of byproduct material with a 350 nm excitation. The inset shows photograph of photoluminescence from the same solution under a 365 nm UV lamp.

In summary, in this study we extended the well-known KMnO₄/H₂SO₄ oxidation chemistry from 3D, 2D and 1D carbon materials to 0D CBNPs. The CBNPs that were used in this study had an interesting structure with a disordered core of aggregated CBNPs of smaller sizes, and a more ordered shell of concentric graphitic layers. We demonstrate that oxidation of CBNPs peels off the outer layers of the nanoparticles due to high levels of oxidation while the less oxidized inner cores retain a nearly spherical shape. Similar oxidation-induced morphological changes were recently discussed with regard to CNOs,¹⁷ so it is possible that this peeling mechanism is relevant to a large class of graphitic nanoparticles and may be potentially applied to other complexly structured carbon nanomaterials. Two products of oxidation, the main product of oxidized CBNPs and the byproduct of peeled-off shells can be separated by centrifugation. The byproduct exhibits blue photoluminescence and, due to the ease of fabrication of carbon black, could potentially be a cheaply available product to be used for biomedical applications, as well as photovoltaics and optoelectronics. Overall, this study has made an addition to the increasing repertoire of fabrication procedures that sculpt carbon species into a specific nanoscopic morphology^16,42–45 and elucidated a mechanism whereby two products consisting of peeled outer shells and the nearly intact cores of the CBNPs may be formed by the same process.

Acknowledgements

This work was supported by the National Science Foundation (NSF) through CHE-1455330. The materials characterization was performed in part in Central Facilities of the Nebraska Center for Materials and Nanoscience (NCMN), which is supported by the Nebraska Research Initiative, and in MISIS where the work was carried out with the financial support from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISIS” (No. K3-2015-012). The Nanoscopy Facility, an electron microscopy facility at UPR, was supported by the NSF through EPS-0701525.

Notes and references

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