Diamond-Like Carbon Nanofoam from Low-Temperature Hydrothermal Carbonization of a Sucrose/Naphthalene Precursor Solution

Abstract

Unusual structure of low-density carbon nanofoam, different from the commonly observed micropearl morphology, was obtained by hydrothermal carbonization (HTC) of a sucrose solution where a specific small amount of naphthalene had been added. Helium-ion microscopy (HIM) was used to obtain images of the foam yielding micron-sized, but non-spherical particles as structural units with a smooth foam surface. Raman spectroscopy shows a predominant sp² peak, which results from the graphitic internal structure. A strong sp³ peak is seen in X-ray photoelectron spectroscopy (XPS). Electrons in XPS are emitted from the near surface region which implies that the graphitic microparticles have a diamond-like foam surface layer. The occurrence of separated sp² and sp³ regions is uncommon for carbon nanofoams and reveals an interesting bulk-surface structure of the compositional units.

1. Introduction

A large variety of nanomaterials can be formed from elemental carbon. Amorphous carbon materials with a high fraction of sp² bonds are named graphite-like carbons. When sp³ bonds are dominating, the materials are named diamond-like carbons. Their properties depend on the relative abundance of the sp²- and sp³- hybridized carbon atoms [1]. Carbon materials with both graphite- and diamond-like bonds were realized by various synthetic methods, e.g., laser ablation, chemical vapor deposition or hydrothermal carbonization [2]. For example, a material containing graphene sheets and diamond-like structures has been observed after catalytic carbonization of wood charcoal [3]. In this study, we focus on hydrothermal carbonization (HTC) for the synthesis of carbon nanofoam with separated regions of sp²- and sp³-hybridized carbon atoms.

Hydrothermal carbonization is a promising method for the synthesis of carbon nanomaterials from biomass [4] for a wide range of applications in fields such as energy storage environment protection [5]. It has been used in industry for the production of fuels and other energetic materials [6]. The carbonization of sucrose biomass usually leads to foam-structured materials consisting of spherical micron-sized particles [7]. It has been shown that a small amount of naphthalene added to a sucrose solution acts as a nucleation seed during the HTC process leading to low density, high-quality carbon foams consisting of micropearls with internal pores [8]. The surface of these materials has been modified by chemical functionalization, and the bulk structure by introducing additional porosity [9]. A different morphology has been obtained by adding graphene oxide to biomass, which results in carbon materials with higher degree of carbonization and higher conductivity [10]. Porous carbon structures have also been produced by HTC of crude plant material, which can be up-scaled to large quantities with low cost, and these materials may have industrial applications for catalytic support and for sorption purposes [11]. Other potential applications of carbon nanofoam include hydrogen storage [12], cathodes for batteries [13,14] or composites for electrodes [15,16]. It follows that the HTC method for the synthesis of functional carbon materials can be used for a broad range of applications [2].

For this study, we produced carbon nanofoam by naphthalene-assisted hydrothermal carbonization of sucrose. We find that the amount of added naphthalene critically determines the foam morphology. At low naphthalene concentrations, the foam consists of an agglomeration of porous micropearls [8]. At higher concentration, however, the carbon nanofoam consists of micron-sized irregularly shaped particles with sponge-like surfaces as visualized by helium ion microscopy (HIM). X-ray photoelectron spectroscopy (XPS) reveals a diamond-like carbon surface. The Raman spectrum shows the D and G bands of carbon with a distinctly higher G peak. These results characterize the foam as a material consisting of species with a graphite-like core and a diamond-like shell. The density of the analyzed sample is 0.21 g·cm⁻³, far below the density of graphite, due to internal pores.

2. Results

2.1. Microscopy

Figure 1 displays two helium ion microscopy images of the carbon foam sample. Figure 1a shows one side of a foam particle. The particle has a diameter of about 200 µm and a uniform rough surface structure.

A higher-magnification image of the foam particle is presented in Figure 1b. It depicts the upper right part of the particle shown in Figure 1a, and it can be seen that the surface has a sponge-like and frayed character. The sponge-like surface implies a porous foamy structure, which is also indicated by the low density of 0.21 g·cm⁻³ of the sample. The edge resolution of the image in Figure 1b is around 15 nm, which was determined by the distance between 25% and 75% of the maximum intensity of a sharp edge [17].

2.2. XPS

The chemical composition of the carbon nanofoam surface was determined by XPS. Figure 2a shows the C1s and Figure 2b the O1s photoelectron signals of the foam sample. The de-convoluted carbon curve in Figure 2a consists of five peaks, labeled as C1–C5. It has been shown that the sp³-hybridized carbon peak is shifted around 1 eV from the sp²-hybridized carbon peak [18]. Therefore, the two main peaks at 284.4 eV (C1) and 285.5 eV (C2) are assigned to aromatic and aliphatic carbons, respectively. The C1 peak has an area of 28%, whereas the C2 peak has an area of 39%. Since XPS is a surface-sensitive method, it follows that the foam surface has a diamond-like composition, because the aliphatic carbons dominate in the spectrum. The remaining three peaks at 286.4 eV (19%), 287.3 eV (10%) and 289.1 eV (4%) are assigned to carbon–oxygen bonds; to hydroxyl, carbonyl and carboxylic groups, respectively [19]. Further surface groups such as carboxylic anhydride, lactone or lactol may also contribute to the carbon–oxygen peaks [20]. The feature at 289.1 eV, assigned to O=C–OH, could in part be due to π–π* shape-up satellites associated with electron transitions between the carbon HOMO–LUMO gap. Such satellite peaks have been reported in the literature at ~290 eV [21], ~290.5 eV [22], and ~291 eV [23].

The O1s curve shows the presence of the oxygenated groups as seen in the C1s spectrum and can be divided into two components, C–O bonds at 533.4 eV and C=O bonds at 532.1 eV, with areas of 62% and 38%, respectively. We note that no feature is observed at about 536.0 eV, where adsorbed water would give a peak in the XPS O1s spectrum [22]. This shows that the water from the aqueous solution has entirely been removed from the samples after extracting from the autoclave and their subsequent thermal treatment. From these various results, we conclude that the surface structure of the nanofoam can be described as a diamond-like open framework decorated with oxygenated functional groups.

2.3. Raman Spectroscopy

The Raman spectrum shown in Figure 3 exhibits a pattern which is common for carbonized materials [24]. We used a Gaussian two-curve fitting to decompose the main Raman signal into two peaks. The deconvoluted G peak arises from graphite carbon bond-stretching motion and is located at around 1582 cm⁻¹. The second peak at around 1360 cm⁻¹ is known as the defect-derived D peak, and is assigned to ring breathing vibrations in benzene [19]. The peak locations are in good agreement with literature values for disordered graphite, where the G peak is usually located at around 1580 cm⁻¹ and the D peak at around 1350 cm⁻¹ [25]. The observed G peak differs from the sharp Raman peak at 1575 cm⁻¹ for perfect graphite [26]. This reveals that the material has a mixed content of aromatic and aliphatic carbons. The measured intensity of the D band relative to that of the G band is 0.52 corresponding to the values for disordered graphite, which typically lie between 0.51 and 1.1 [27]. Since the G band is broader than 45 cm⁻¹ (the value for graphite), the core of the foam appears to be dominated by small aromatic clusters with sizes less than 10 Å [28].

3. Discussion

We have confirmed that hydrothermal carbonization is an excellent method for the production of high-quality carbon nanomaterials. It has generally been used in the past in industrial applications such as, for example, the charcoal production from raw cellulosic materials [29], mesoporous carbons from crude plant material [11], or ethanol from biomass [30]. If using natural precursor materials, excessive purification however is usually required. No such purification is necessary if synthetic hydrocarbons are used. Saccharides are among the precursor materials which are non-toxic and easily carbonized without catalysts [7,19,31]. Among these, glucose [32,33] and sucrose [8] are convenient precursors.

Many carbonization studies were performed at high temperatures and high pressures (HTHP) [34]. Serp et al. [35], for example, produced carbon nanospheres (CNS) by decomposition of methane at 1100 °C, Wang et al. [36] by the decomposition of benzene at 1000 °C. With the support of catalysts, pentane has been decomposed at 900–1000 °C [37], anthracene at 900–1000 °C [38] and camphor at 1000 °C [39], in all these cases resulting in the synthesis of carbon nanospheres.

With solid-state precursors, process temperatures above and below the critical point of water have been used. For example, the hydrothermal carbonization was performed for cellulose in the 220–250 °C range [40], for glucose between 170 and 500 °C [19,41,42,43,44], for sucrose at 190 °C [45] and for starch at 600 °C [46]. Process times were typically between 4 and 12 h. For some of the materials, the graphitic nature has been improved by annealing the sample after synthesis. This typically requires temperatures of up to about 2000 °C [47]. In this paper, we have shown for naphthalene-assisted sucrose carbonization that low process temperature of only 130 °C combined with a long process time of 72 h can be used. Even at this low temperature, one obtains complete carbonization, with a high-quality product of uniform morphology and a quite high degree of graphitization.

We also like to note that our use of sucrose as precursor material, without the addition of a catalyst, is a sustainable, bio-based method for the “green” production of carbon materials. Together with the low-temperature and low-pressure process conditions, no toxic byproducts and no necessary pre- or post-purification, it provides an easy and clean way for the production of nanocarbons with promising future applications.

The morphology of the carbon nanofoam obtained in this study is unique. Carbon nanofoams usually consist of agglomerations of porous nano- or micron-sized spheres. Such so-called nano- or micro-pearls have been produced by various methods such as laser ablation [48,49,50], chemical vapor deposition (CVD) [39,51,52,53,54,55,56,57,58,59,60] or hydrothermal carbonization (HTC) [30,31,32,33,46,61,62,63,64,65,66,67]. In this paper, we show that morphologies other than micropearls can be synthesized just by changing the process parameters. In our previous study, carbon microspheres were formed by using hydrothermal carbonization of a sucrose solution with a trace amount of naphthalene [8]. In the present work, we have increased the naphthalene concentration and the process time which resulted in an entirely different foam morphology. This shows that variations in the process conditions can have a major effect on the outcome of the synthesis.

The diamond-like property of the foams in this study is unique since usually nanoporous carbon materials are rather graphitic [68] or graphenic [69] in nature. This applies to various types of materials such as nanoporous hard carbon microspheres [70], fiber-based nanoporous carbon [71], nanoporous carbon tubes [72], nanoporous carbon films [73] or activated carbon [74]. The graphitic nature, given by sp² hybridization, provides the large surface area in the interior of the foam scaffold. Also, the sp²-hybridization allows for a large variety of structures typically composed of planar aromatic sixfold rings. In addition, inclusion of pentagons leads to ring structures with positive curvature, inclusion of heptagons to areas with negative curvature. Furthermore, the aromatic graphene sheets can be curved similar to those forming the carbon nanotubes. This can lead to a large variety of graphite-like morphologies with many possible applications. For example, it is responsible for the uptake of molecules such as methane [75], CO₂ [76], proteins [77], or heavy metals [78]. Also, catalytic properties of the foam [79], as well as gas- [80] and glucose-sensing properties [81] or photoluminescence [82] are related to a graphitic or graphene-like interior of the foams. In particular, hydrogen storage, long proposed for graphitic carbon materials [83], has recently been considered for nanoporous carbons [84,85,86]. We note that a strong sp³ contribution, as observed for the nanofoam in this work, will affect the general properties of these foams as well as their possible use in future technologies. New applications may be possible for such diamond-like carbon foams.

Diamond-like features of a carbon nanomaterial in Raman spectra can arise mainly from two different sources; either from sp³-related vibrations due to small regions of crystalline diamond (usually nanodiamonds) [87], or from regions with amorphous sp³-related structures [88]. It has been well documented that diamond films in Raman spectroscopy show a sharp feature at about 1332 cm⁻¹ [88]. These phonon vibrations involve the sp³-hybridized carbon atoms in diamond. Very close to it, a band centered at about 1350 cm⁻¹ is generally associated with sp³ amorphous carbon sites and is called D-band (D standing for “disorder”). The sharp peak of crystalline diamond may be superimposed on the broader amorphous carbon band. However, for a material which contains both, amorphous and nanodiamond regions, or a hybrid combination of both, the two bands usually cannot be distinguished since the difference in location is only a few cm⁻¹. In addition, upon shrinking of the crystal to the nanoscale, major changes are observed in the Raman response of nanodiamonds: compared to bulk diamond, the nanodiamond peak is shifted to lower frequencies and appears broadened, sometimes with an asymmetry [89,90,91]. The phonon confinement effect is mainly considered as the physical origin for these changes [92]. In addition, the position and line shape of the Raman bands may also be affected by lattice defects in nanocrystalline areas resulting in phonon scattering as well as by strain in the sample. For example, Shin et al. [93] found that the G peak of the Raman spectrum shifts to higher wave numbers with the increase of compressive stress. In our studies, only one band is observed in the wavenumber region where either diamond structures and/or amorphous structures would occur. This is not surprising, since both the close proximity of the sp³ bands for diamond and of the amorphous carbon, as well as the confinement effect for small diamond clusters, leads to the observation of just one band. We conclude that the D peak in the diamond-like foam of our studies, being separated from the G peak and relatively narrow, may be due to an amorphous/nanodiamond hybrid structure.

In our studies, the sp³-related D-band in the Raman spectrum is measured with relatively low intensity but the C1s XPS spectrum shows a strong sp³-contribution. We relate these two results to the different depth of probing the sample. While Raman spectroscopy yields results for the bulk of the sample, XPS is strongly surface-sensitive. Therefore, we conclude that the bulk of the nanofoam of this study is graphitic in nature while the surface is diamond-like. In addition, both the macroscopic appearance and the HIM images reveal the foam structure of surface. In addition, we note that the low value for the mass density indicates that the core of the foam has also porous structure. The combination of the two different properties of core and surface regions may lead to exciting and valuable new applications of this material.

In the XPS O1s spectrum, various functional groups can generally be assigned to the deconvolution peaks [94,95,96]. In Figure 2, the peak at around 531.3 eV corresponds to C=O bonds, and the peak at around 533.5 eV corresponds to C–OH and C–O bonds. The C=O peak dominates in the de-convoluted O1s distribution. At binding energies below the C=O peak we observe a small peak which may be assigned to quinone, which is expected to give a feature at 530.7 eV [97]. We note that diamond films have been oxidized showing a strong O1s peak in the XPS [98]. The nature of the oxygen surface groups was found to depend on the oxidation process [99]. Surface morphology was found to change upon oxidation leading to lattice reconstruction at the oxygen-terminated diamond surface [100].

For diamond crystal surfaces, adsorbed oxygen exists in several functional surface groups [101]. The carbon–oxygen interaction of the diamond surface has been investigated and the oxidized diamond surface has been found to be hydrophilic [102]. We note that in our sample the C=O peak is the strongest peak in the O1s envelope. In fact, the C=O group has also been found as the dominant functional group in oxidized diamond-like carbon films [103].

At about 535.8 eV, a peak would be observed for adsorbed water. However, no feature is seen at this energy which shows that the water from the sucrose solution has successfully been removed after synthesis. For foam structures with closed cavities the water would remain in the sample. However, since no indication for water adsorption is seen in the O1s spectrum, we conclude that the foam consists of an open scaffold able to release enclosed water by heating the sample after removing from the autoclave. Also, this suggests that this material may effectively be used in adsorption applications, since the pores are open to the environment.

We conclude that the carbon nanofoams, synthesized in these studies, have a new morphology with a porous graphitic core and a diamond-like foam surface. These materials may have new and unexpected applications in various fields of technology.

4. Instruments and Methods

4.1. Hydrothermal Synthesis

A 2M sucrose solution and 9 mg of naphthalene were filled into a 130 mL stainless steel autoclave with a head space of 7 mm and heated at 130 °C. After 72 h, the formed foam was removed from the autoclave, filtered with hot water and dried at room temperature. The resulting mass density of the foam is 0.21 g·cm⁻³, calculated using a high-precision balance and a pre-defined volume container. We note that control experiments, where no naphthalene was added, did not result in foam formation. This suggests that naphthalene acts as a nucleation seed for starting the growth of carbon foam. The aromatic ring structure of naphthalene induces the formation of the graphitic carbon morphology. Hydrothermal carbonization, however, is a very complex thermodynamic and chemical process with a large variety of intermediate products. From our experimental results, we find that these intermediates may also lead to diamond-like carbon formation.

Hydrothermal carbonization is a thermochemical process where hydrocarbon precursors are transformed into all-carbon materials [2,9,61]. Variables such as precursor composition, as well as autoclave temperature, pressure, and process time, critically determine the synthesized carbon morphologies and their functionalities [104]. The reaction mechanisms are complex, and the operational parameters are interconnected [105]. In addition, with higher temperature and pressure, dramatic changes in the properties of water are known to occur. The dielectric constant of water for example can be considerably lower compared to ambient conditions. The water density increases with pressure and becomes an important variable in the hydrothermal reaction process [106]. At temperatures below 300 °C, the carbon materials are mainly produced by dehydration and polymerization [2]. The mild temperature of 130 °C, applied in this work, is far below the critical point of T = 373 °C, and therefore provides subcritical conditions. At this mild temperature, water is considered as a reacting dehydrating agent. We note that the catalyst-free carbonization under such subcritical water conditions is more environmentally friendly than most of the other production methods for nanocarbon materials [107].

4.2. Helium Ion Microscopy

Helium ion microscopy (HIM) imaging [108] was performed with a Carl Zeiss Orion Plus [109,110]. The ion source of the HIM consists of an atomically sharp tip in helium gas, kept at high positive voltage. Field ionization at the tip of the needle combined with an electrostatic ion optics leads to a very bright He ion beam. The instrument was operated at an acceleration voltage of 34.9 keV, a beam current of 0.3 pA and a working distance of 10 mm. The images were obtained by detecting secondary electrons [111] with an Everhart–Thornley detector in the line averaging mode. Since we have used an uncoated sample, an electron flood gun was applied during imaging to stabilize charging. Prior to imaging, the foam was mounted on the sample holder with conductive carbon pads.

Compared to the scanning electron microscope (SEM), the helium ion microscope (HIM) can be applied to a smaller probe size and has a larger secondary electron yield [108]. This leads to images with stronger topographic contrast showing finer structures at the surface of the material [112]. HIM imaging is performed by a fine helium ion beam, and either the backscattered He ions or secondary electrons are detected. The exceptional contrast in the secondary electron (SE) images is due to the small surface interaction volume [113]. The escape depth for the emitted secondary electrons is typically in the range of 5–15 nm for most materials [114]. The HIM provides a stronger surface sensitivity compared to the electron microscope. The HIM also yields images with a better signal-to-noise ratio [115]. Detailed modeling of the HIM image formation with secondary electron analysis has been provided [116]. A focused spot size smaller than 0.25 nm [117] or 0.3 nm [118] has been determined for imaging with the HIM in the SE mode.

4.3. Raman Spectroscopy

Raman spectra were obtained with a micro Raman spectrometer (Labram Aramis) operated in the backscattering mode. Spectra at 473 nm were acquired with a 10× objective and a thermoelectrically-cooled charge-coupled device detector. The foam was mounted on the sample holder with conductive carbon pads.

Oscillating molecules and functional groups have vibrations which correspond to the molecular structure and the microscopic environment. Such spectroscopic fingerprints, provided by Raman spectroscopy, can be used to identify bonding configurations. Bulk crystalline, nanocrystalline, and amorphous structures can usually well be distinguished. For carbon and carbonaceous materials [119], Raman spectroscopy is a valuable method for the distinction between sp²- and sp³-related bonds as well as sp²/sp³ hybrid structures. It can even differentiate between different sp² carbon nanostructures such as carbon nanotubes and graphene [120].

4.4. X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) was performed in a multi-chamber ultra-high vacuum system (Omicron) using a monochromatic X-ray source (Al Kα) and a Sphera electron analyzer with a spectral resolution of 0.9 eV. The spectra of the foam were measured at a chamber pressure below 10⁻⁹ mbar and an emission angle of 20°. The foam was mounted on the sample holder with conductive carbon pads. The spectra curves were fitted with a Shirley background for C1s, a linear background for O1s and symmetrical Voigt functions for both spectra.

XPS has been successfully applied to many different carbon materials [121] such as activated carbon [122], carbon spheres [123], amorphous carbon, nanodiamonds [124], diamond-like carbon films [125], carbon nanotubes [126], or nanoporous carbons [127]. Also, it can be used to study processes such as heat and chemical treatment [128], amorphization [129], electrochemical oxidation [130], or structural deformation [131]. The use of XPS for chemical analysis [128] and bonding [127] is of particular importance for solid-state research.

A characteristic feature of XPS measurements is that the instrument provides near-surface information about a material. The mean escape depth (MED) of photoelectrons is a complex result of various factors [132]. Both elastic [133,134] and inelastic [135] scattering of the photoelectrons determine the surface sensitivity in XPS. The MEDs vary slightly for different materials and obtain values such as 2.3 nm for polycrystalline silicon [136], 1.2 nm for germanium [137], 1–3 nm for various carbon materials [138], or 1–2 nm for amorphous carbon [139]. In our studies, we detect O1s in addition to C1s photoelectrons. It has been pointed out that the escape depth of O1s electrons in photoemission is of the same order as that of C1s [140]. This suggests that the MED in our XPS experiment is 1–2 nm, for photoelectrons from both the carbon and the oxygen core levels.