1 Introduction

Cellulose is the most abundant constituent of lignocellulosic biomass [1] and one of the most common utilized feedstocks for biorefinery platforms and processes [2, 3]. Cellulose nanocrystals (CNCs) are a very interesting material derived by cellulose chemical [4] or enzymatic [5] controlled hydrolysis of different sources [6, 7]. Pristine CNCs show remarkably high storage modulus together with a very attractive aspect ratio [8]. Additionally, CNCs are characterized by an interesting polymorphism induced by tailoring with inorganic or organic functionalities [9,10,11]. This behavior was particularly relevant in the related composites where polymorphic CNCs induced very different properties [12, 13]. Despite these usefully properties, pristine CNCs lack in conductivity and dispersibility in nonpolar media [14, 15]. Also, pristine CNCs show the tendency to self-assemble into aggregates driven by strong hydrogen bond network formation [16]. As reported by Beck et al. [17], in the dried solid state CNCs are aggregated as round-shaped structures with an average size ranging from 10 to 20 μm. These drawbacks slow down their application in polymer composite science requiring the use of plasticizers [18, 19] or chemical modifications [20] for an efficient use. Alternatively, thermochemical CNCs conversion routes have been studied using the CNCs structures as templates for the production of nano- and microstructured carbonaceous materials such as carbon nanodots [21]. The interest for micro- and nanostructured carbon has grown over the years since the discovery of allotropic carbon species (i.e., carbon nanotubes, fullerene) and carbon fibers [22]. Nano- and microstructured carbon materials have gained great attention for energy harvesting [23] and energy storage applications [24, 25]. Cellulose has been proved a very promising template for the production of carbonaceous materials [26,27,28]. CNCs are good candidates as feedstock in nanostructured carbon production as demonstrated by Wu et al. [29]. Authors used a CNCs/melamine resin precursor to produce nitrogen-enriched nanorods used for energy storage devices production, achieving remarkable performance. Shopsowitz et al. [30] described a similar methodology producing carbonized nematic material using CNCs and silica. Furthermore, the use of a CNCs/organic polymer precursor was used to produce micro-carbon fibers [31, 32]. Chou et al. [33] reported a multi-step protocol for the conversion of commercial microcrystalline cellulose into carbonized fibers on nanoscale using the freeze drying technique. Recently, Eom et al. [34] described the graphitization process of CNCs from 1000 to 2500 °C using spray pyrolysis methodology to obtain fiber structures. Moreover, rounded carbon particles have been successfully used for water purification and environmental remediation [35, 36], composites production [37, 38] and energy storage [39]. According to the great interest in the carbon shape tunability, we used CNCs as template for producing microstructured carbon species, ranging from spherical to needle-like structures. We investigated the shape tunability of carbonized CNCs using solid-state and dispersed CNCs using several temperatures (400 °C, 600 °C, 800 °C, 1000 °C). The carbonaceous materials produced were studied using field emission scanning electron microscopy, spectroscopic and thermal techniques to evaluate their main properties. Carbon tailoring tunability will represent the first step for further studies focused on the reinforced polymers. The nanosized tailoring could be a winning factor for the enhancement of mechanical properties improving the interactions between the polymeric matrix and the carbonaceous filler avoiding the use of additional inorganic tailoring [40, 41].

2 Materials and methods

2.1 Materials

CNCs were purchased from Alberta-Pacific Forest Industries (Batch COMP170823-H) and used as received without any purification. Ethanol (EtOH, 98%) and polyethylene glycol (PEG, molecular weight 200 g/mol) were purchased by Sigma-Aldrich and used without any purification.

2.2 Carbonization of CNCs

Pyrolysis experiments were carried out using 25 g of CNCs. CNCs were placed in a ceramic crucible and then introduced into a quartz reactor. The reactor was sealed and a flux of argon was used to remove the initial residual atmosphere. The argon flux (4 mL/min) was kept constant during the pyrolytic run to drag out the gas released by the thermal degradation reactions. The heating rate was set at 50 °C/min with a set high treatment temperature (HTT), respectively, of 400 °C, 600 °C, 800 °C and 1000 °C. The system was kept at the HTT for 30 min and fast cooled down at room temperature using refrigerated water in argon atmosphere.

Subsequently, 25 g of CNCs were dispersed into 250 mL of water, EtOH or PEG through 10-min mechanical mix. The reactor was sealed and a flux of argon was used to remove the initial residual atmosphere. The argon flux was kept constant during the pyrolytic run. The reactor was placed in a preheated oven at 800 °C or 1000 °C. After an initial temperature decrease, the system rapidly came back to the preset temperature and it was left at HTT for 30 min. After that, it was fast cooled down at room temperature using refrigerated water in argon atmosphere.

2.3 Methods

Morphology of all samples was investigated using a field emission scanning electrical microscope (FESEM, Zeiss Supra 40). Pristine CNCs were coated with 5 μm layer of chromium prior the analysis to enhance the resolution. The metallic film deposition was performed using sputtering technique.

Carbonized CNCs were analyzed using Fourier transform infrared (FTIR) spectroscope (FTIR Nicolet 5700, Thermo Scientific) equipped with a SmartOrbit (Thermo Scientific) operating in attenuated total reflectance (ATR) in a range between 500 and 4000 cm−1.

Furthermore, Raman spectroscopy was used to investigate the carbon produced using a Renishaw® Ramanscope InVia (H43662) model equipped with a green laser source. The Raman spectra in a range from 250 to 3300 cm−1 were fitted with homemade software developed using MATLAB™ in order to fit the component peaks.

Stability of carbonaceous materials was investigated through thermogravimetric analysis (TGA) using Netzsch TG 209F1 Libra in N2 flux with a temperature ramp of 10 °C/min from 30 to 900 °C.

Conductivity measurements were taken with the method described by Giorcelli et al. [42] using a hydraulic press (Specac Atlas Manual Hydraulic Press 15T) and multimeter (Agilent 34401A). (A briefly description is reported in Scheme 1 of supporting materials.) Resistance of carbonized CNCs was measured applying a pressure ranging from 1 to 1500 bar. Conductivity was calculated according to the following equation:

$$\sigma = R \left( {\frac{l}{A}} \right)$$
(1)

where σ is the conductivity of the carbonized material, R is the measured resistance, A is the bottom surface of the copper cylinder and l is the thickness of the carbonized material.

3 Results

3.1 Yields of CNCs pyrolytic run

CNCs were carbonized using temperatures ranging from 400 to 1000 °C. This temperature range covers all the main degradative stages of cellulose [43, 44] together with the main stages of biochar evolution [45]. As shown in Fig. 1, the increment of temperature depleted the carbonized CNCs yields from 26.3 ± 0.8 wt% at 400 °C to 14.1 ± 1.0 at 1000 °C as reported by many authors [45, 46].

Fig. 1
figure 1

Yields of pyrolysis of solid state form CNCs (a) and CNCs solvent dispersed (b) at different temperatures. Data were reported as average values of three runs with error bars representing the standard deviation. Data marked with same letters are not significantly different (p < 0.05)

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As reported by Buffa et al. [47], the rheology of the CNCs dispersion is very sensitive to the addition of additives and polarity of the medium chosen. Room-temperature dispersion of CNCs is commonly achieved using polar solvents such as water [48], alcohols [49] or polyethers as non-ion surfactants [50]. The use of solvents during pyrolysis under pressure is a common practice to promote more homogenous reaction conditions [51]. We selected water as the simplest polar solvent, and we studied how the increment of unpolar residues using EtOH and PEG affected the pyrolytic behavior of CNCs dispersions according to the interactions proposed in Fig. 2.

Fig. 2
figure 2

Hypothetical interaction of solvent-dispersed CNCs: a water, b EtOH and c PEG

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In this study, solvents were rapidly removed of degraded during pyrolysis run. In these experiments, an initial temperature decrease was observed reaching 850–800 °C and 650–630 °C using a preheated furnace at 1000 °C and 800 °C, respectively. The temperature was rapidly increased to the original value, while the solvent was removed.

The use of water as a solvent did not significantly affect the yield (21.2 ± 0.6 wt%) of recovered carbonized CNCs at 800 °C and promoted an appreciable yield increment at 1000 °C, achieving 18.1 ± 1.1 wt%. This was reasonably due to the partial suppression of levoglucosan formation due to the watery residual atmosphere with a decrease in formation of highly reactive species such as hydroxyfurfural. Using EtOH or PEG, an increase in the yields was observed at both 800 °C (24.2 ± 0.4 wt% and 25.2 ± 0.8 wt%) and 1000 °C (21.1 ± 0.6 wt% and 22.1 ± 1.1 wt%). This appreciable increment was probably due to the radical simultaneous degradation of organic solvent used as reaction medium together with pristine CNCs.

3.2 FESEM analysis of carbonized CNCs

Pristine CNCs were analyzed using FESEM (Fig. 3) after chromium metallization.

Fig. 3
figure 3

FE-SEM capture of a pristine CNCs and b surface details after metallization

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Pristine CNCs were aggregated as defective spheres with average diameters ranging from 2 to 10 μm. Spheroidal particles formed an interconnected network (Fig. 2b) kept together by hydrogen bonds [52]. Contrary, nanometric-sized CNCs were observed using dispersed material in very low concentration as described by many authors [5, 53, 54].

The FESEM images collected after pyrolysis of pristine CNCs at different temperatures are shown in Fig. 4.

Fig. 4
figure 4

FE-SEM images of carbonized CNCs at 400 °C (a), 600 °C (b), 800 °C (c) and 1000 °C (d)

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All the images collected show a partial retention of the initial structure with some defects on the surfaces (magnified in Fig. 5) due to the release of volatile organic matter during pyrolysis. In the early stages of the thermal degradation of the CNCs, pyrolysis of the sphere surface took place simultaneously with the release of the high-boiling compounds (levoglucosan, glucose dimers and trimers [55]) from the inner particle cores. After reaching the surface, these compounds started to boil and to decompose at the same time. The results of this process are the tiny bubbles clearly visible in Fig. 4 and magnified in Fig. 5.

Fig. 5
figure 5

Surface details of CNCs pyrolyzed at 400 °C (a), 600 °C (b), 800 °C (c) and 1000 °C (d)

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Other noteworthy needle structures were observed using HTT up to 600 °C in Fig. 4; they are more clearly visible in Fig. 5b–d.

The needle structures showed a remarkable aspect ratio considering the length of 1–3 μm, a width around 200 nm and a negligible thickness.

Focusing the probe of the FESEM, the needles degradation was observed proving their carbon-based structure (Supporting Information Figure 1).

The carbonized CNCs lost their spherical shape after water was used as a solvent medium (Fig. 6).

Fig. 6
figure 6

FE-SEM images of carbonized CNCs watery solution at 800 °C (a and b) and 1000 °C (c and d)

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Needle structures were detected in large amounts in the shape of carbon needles (Fig. 6a) as either isolated needles or regrouped into clusters (Fig. 6b). This could be explained by the presence of water that could induce the pyrolytic degradation of the partially unpacked CNCs in the early stages of the process.

An increment of pyrolytic temperature to 1000 °C leads to decrement of the amount of carbon needles (Fig. 6d). Furthermore, the increased temperature promoted an excessively fast evaporation of the solvent avoiding the formation of clearly separated needles.

A different behavior was observed using EtOH as a solvent medium (Fig. 7).

Fig. 7
figure 7

FE-SEM images of carbonized CNCs EtOH solution at 800 °C (a and b) and 1000 °C (c and d)

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Applying a pyrolytic temperature of 800 °C, EtOH-dispersed CNCs retained the spherical structure of packed pristine CNCs (Fig. 7a) even when the surfaces were massively decorated with hemispheres (Fig. 7b). This behavior could be ascribed to the very fast evaporation and simultaneous degradation of EtOH that competed with pyrolytic degradation of dispersed CNCs. Further temperature increase to 1000 °C induced the formation of needle clusters as shown in Fig. 7c, d. This could be explained with the same arguments above mentioned considering watery medium. The high temperature promoted a very fast evaporation of EtOH, but at 1000 °C the temperature of the system was high enough to induce simultaneously the degradation of CNCs, leading to the formation of carbon needles. The use of PEG induced the collapse of any structures to amorphous materials (Fig. 8a, c). This was the degradation of PEG during the pyrolysis that substantially enriched the radical concentration. Adopting a processing temperature of 800 °C, PEG could be used to decorate the surface of carbonized CNCs with pyramidal structures with an average size of 100 nm (Fig. 8b), while a smooth surface was obtained at 1000 °C (Fig. 8d).

Fig. 8
figure 8

FE-SEM images of carbonized CNCs PEG solution at 800 °C (a and b) and 1000 °C (c and d)

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3.3 Spectroscopic analysis on carbonized CNCs

Raman analysis is a very powerful tool to analyze carbonaceous materials [56]. As reported by Ferrari et al. [57], Raman spectroscopy can be used to evaluate the disorder of amorphous carbon using the area ratio of D and G peaks. Raman spectra of carbonized CNCs are reported in Fig. 9 and show D and G peaks centered, respectively, at 1346–1371 cm−1 and 1590 cm−1.

Fig. 9
figure 9

Raman spectra of carbonized CNCs at HTT of a 400 °C, b 600 °C, c 800 °C and d 1000 °C in the region between 700 and 2300 cm−1

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Based on ID/IG ratio, pyrolyzed CNCs without the addition of solvents show an increase in disorder moving from HTT of 400–600 °C. Further increments of HTT (800 °C and 1000 °C) induced a carbon structure reorganization due to the increase in and enlargement of aromatic domains.

As shown in Fig. 10, CNCs pyrolyzed using solvents showed an appreciable increment of ID/IG with a more disordered structure produced when PEG was used. For a HTT of 1000 °C h, trend was less relevant remaining only an appreciable difference in the saddle of the carbonized PEG-dispersed CNCs.

Fig. 10
figure 10

 Raman spectra in the region between 700 and 2300 cm−1 of carbonized CNCs solutions at HTT of 800 °C (A) using a water, b EtOH, c PEG and at HTT of 1000 °C (B) using d water, e EtOH and f PEG

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Pristine and carbonized CNCs were analyzed through FTIR (ATR mode), and representative spectra are reported in Fig. 11.

Fig. 11
figure 11

FT-IR (ATR mode) spectra of carbonized CNCs at a 600 °C, b 400 °C and c solid state form CNCs

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Pristine CNCs spectrum (Fig. 11c) showed a broad band between 3600 and 3300 cm−1 due to the presence of hydroxylic groups (νO–H), signals of saturated νC–H at 2900–2850 cm−1, νCH2 at 1426 cm−1 due to the C6 of glucose structure and a band envelopment between 900 and 1100 cm−1 due to the δCH2 and νC–O. As a consequence of the first acidic hydrolysis, CNCs also showed a low intense peak at 1205 cm−1 (νS=O) due to sulfate residual groups. The literature lacks of study about sulfonated cellulose pyrolysis, but several authors described the effect of sulfonated additives able to enhance dehydration pathways to anhydrosugars and furans [58, 59]. According to these studies, it is possible to assume that the sulfate functionalities affected more biooils than biochar production. Nonetheless, they play a relevant role in a solvent-assisted pyrolytic process inducing a better dispersibility.

The spectrum of carbonized CNCs at 400 °C (Fig. 11b) did not show any bands at 3600 or 2850 cm−1 according to the pyrolytic degradation of cellulose chains [60]. Some residual groups could be detected such as ketonic (1580 cm−1, νC=O) and carboxylic (νC=O at 1700 cm−1 and νO–H at 1436 cm−1) functionalities together with ethers (νC–O at 1228 cm−1 and δC–O at 1119 cm−1). A further increment of HTT to 600 °C (Fig. 11 a) leads to the total loss of surface functionalities caused by the advanced thermal degradation reactions. Similar results were observed at higher HTT (800 °C, 1000 °C) with or without the use of a solvent medium. The absence of relevant IR signals proved the complete absence of EtOH or PEG residues.

3.4 Conductivity measurements of carbonized CNCs

Conductivity measurements on CNCs were performed using a hydraulic press that trough two copper cylinders measure the resistance variation during the compression (Supporting Information Scheme 1), and results are shown in Fig. 12.

Fig. 12
figure 12

 Relation between pressure and conductivity of carbonized CNCs at a 400 °C, b 600 °C and c copper reference

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Carbonized CNCs at 400 °C showed an insulator behavior up to a pressure of 300 bar, while carbonized CNCs at 600 °C showed higher values reaching a value around 7 S/m at 400 bar. It was impossible to measure a reliable conductivity values for all samples obtained at 800 °C and 1000 °C because they were indistinguishable from the reference. Relation between conductivity and temperature was described by Giorcelli et al. [42] using pristine and thermally annealed biochar applied for composite production. The increment of conductivity was due to the formation and the turbostratic reorientation of graphitic domains in the carbon matrix [61, 62]. Formation of small graphitic domains was also supported by the disappearing of the IR signals after treatments up to 600 °C (Fig. 11a). At higher temperatures, these domains become bigger and ordered with layered graphitic structures [34]. According to Ferrari et al. [57], the transition phase from amorphous to nanocrystalline graphite carbon could be monitored through the shift of IG peak position from low to higher Raman shift. CNCs pyrolyzed at 400 °C showed an IG peak at 1580 cm−1, while the CNCs peak showed an IG peak at 1593 cm−1 close to the full nanocrystalline graphite carbon at 1600 cm−1. Furthermore, this process was also associated with a disorganization of the material in good agreement with the increment of ID/IG ratio observed in the spectra reported in Fig. 9.

The monolithic biochar showed conductivity around 10 S/m [63], and consequently, the well-packed materials above described showed a good improvement of up one or two orders of magnitude.

3.5 Thermogravimetric analysis of carbonized CNCs

All carbonized CNCs were analyzed using thermogravimetric analysis (TGA). In Fig. 13 are reported the results of pristine and carbonized CNCs.

Fig. 13
figure 13

TGA analysis of a pristine CNCs and carbonized CNCs without solvent addition at b 400 °C, c 600 °C, d 800 °C, e 1000 °C and using water at f 800 °C, g 1000 °C, EtOH at h 800 °C, i 1000 °C and PEG at j 800 °C and k 1000 °C

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Thermal stability of carbonized CNCs was strongly affected by thermal history of the material. Pristine CNCs show a 5 wt% loss at 101 °C due to water loss and a main degradation process around 250 °C primarily due to the breakage of β-1,4-glycosidic bonds of the cellulosic structures [64]. Material recovered after pyrolysis at 400 °C showed a particular behavior with a 5 wt% loss at 403 °C and a broad curve. This is reasonably due to the high amount of residual functional groups on the surface of carbonaceous materials, as proved by the great fluorescence detected during Raman analysis and bands observed in the FTIR spectrum (Fig. 11b). According to these observations, during the TGA analysis carbonized CNCs completely underwent through pyrolytic degradation forming stable carbon materials in the crucible. Other carbonized CNCs materials showed main degradation process around 500 °C with a decrease in stability observed for those pyrolyzed at 1000 °C. This was in agreement with FESEM analysis that showed a lot of defects onto those structures.

Carbonized solutions of CNCs showed a main degradation stage between 500 and 600 °C. Needle structures obtained using CNCs suspended in water at 800 °C showed an increased stability compared to those produced at 1000 °C. This could be attributable to the defects introduced into the amorphous carbon layers that act as support for the bidimensional carbon structures. Using EtOH solutions, the thermal stability of carbonized CNCs produced at 1000 °C was comparable with material recovered from watery dispersion obtained at 800°. This was in good agreement with FESEM data that showed similar morphologies (Figs. 6c, d, 7c, d). Compared to those produced at 1000 °C, carbonized ethanol CNCs dispersions produced at 800 °C were less stable with a broad curve due the thermal decomposition of carbon-on-carbon growth on the surface of CNCs.

A different behavior was observed using CNCs suspended in PEG. In these experiments, the radical-rich environment leads to the formation of more tightly compacted structures (Fig. 8a–d) with an enhanced thermal stability.

4 Conclusions

CNCs were pyrolyzed under different pyrolytic regimes, and carbon-derived materials were analyzed. Increasing temperature from 400 to 1000 °C leads to a drastic increment of conductivity together with the comparison of structural defects such as irregular carbon structures on the surface and the appearance of carbon needles. The use of solvents as dispersing medium induced noteworthy properties to carbonized CNCs. Water promoted the massive formation of carbon needles on amorphous carbon structures at both 800 °C and 1000 °C. The use of EtOH made it possible to switch from carbon-decorated spheres at 800 °C to thin carbon needles at 1000 °C. The use of PEG leads to a richer carbon radical pyrolytic environment inducing the lost of CNCs microstructure. At 800 °C, PEG-dispersed carbonized CNCs were formed by amorphous carbon with tailored nanosized pyramidal structures replaced by smooth surfaces at 1000 °C. The conductivity of carbonized CNCs increased drastically with the temperature, and it was undistinguishable from copper reference passed 800 °C.