Shape tunability of carbonized cellulose nanocrystals
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Bioderived scaffolds are largely used as template for the synthesis of drugs and materials. Among biomass sources, cellulose has been used as platform for numerous conversions and applications. In this study, we report the use of crystalline microstructured cellulose as feedstock for microstructured carbon production. Different morphologies were produced according to pyrolytic conditions ranging from sphere, needle and carbon-on-carbon-decorated surfaces. All the materials were characterized using spectroscopic techniques (i.e., Raman, FTIR), FESEM and thermogravimetric analysis. Hypothetical reaction pathways were proposed for describing materials shape and properties. In particular, for cellulose nanocrystals, the thermal treatments induced noteworthy electrical properties that could be applied for the production of conductive composites.
KeywordsCellulose nanocrystals Carbon spheres Carbon needles Pyrolysis Carbon tailoring
Cellulose is the most abundant constituent of lignocellulosic biomass  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  or enzymatic  controlled hydrolysis of different sources [6, 7]. Pristine CNCs show remarkably high storage modulus together with a very attractive aspect ratio . 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 . As reported by Beck et al. , 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  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 . 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 . Nano- and microstructured carbon materials have gained great attention for energy harvesting  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. . Authors used a CNCs/melamine resin precursor to produce nitrogen-enriched nanorods used for energy storage devices production, achieving remarkable performance. Shopsowitz et al.  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.  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.  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 . 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
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.
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.
3.1 Yields of CNCs pyrolytic run
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 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 . Contrary, nanometric-sized CNCs were observed using dispersed material in very low concentration as described by many authors [5, 53, 54].
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).
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.
3.3 Spectroscopic analysis on carbonized CNCs
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.
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 . 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
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.  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 . According to Ferrari et al. , 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 , 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
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 . 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.
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.
The authors would like to express their acknowledgment and appreciation for the contribution of materials from Alberta-Pacific Forest Industries and to Salvatore Guastella for FESEM analysis.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 4.Tsao GT, Chou TY (1981) Process for recovering and utilizing cellulose using sulfuric acid. Google patentsGoogle Scholar
- 31.Attia AAM, Antonious MS, Shouman MAH, Nada AAA, Abas KM (2019) Processing and fundamental characterization of carbon fibers and cellulose nanocrystals derived from bagasse. Carbon Lett 29(2):145–154Google Scholar
- 35.Meng L, Xu X, Bai B, Ma M, Li S, Hu N, Wang H, Suo Y (2018) Surface carboxyl-activated polyester (PET) fibers decorated with glucose carbon microspheres and their enhanced selective adsorption for dyes. J Phys Chem Solids 123:378–388. https://doi.org/10.1016/j.jpcs.2018.08.005 CrossRefGoogle Scholar
- 49.Börjesson M, Westman G (2015) Crystalline nanocellulose—preparation, modification, and properties. In: Poletto M, Ornaghi HL (eds) Cellulose-fundamental aspects and current trends. InTech, London, pp 159–191Google Scholar
- 50.Góis GS, Nepomuceno NC, França CH, Almeida YM, Hernandéz EP, Oliveira JE, Oliveira MP, Medeiros ES, Santos AS (2019) Influence of morphology and dispersion stability of CNC modified with ethylene oxide derivatives on mechanical properties of PLA-based nanocomposites. Polym Compos 40(S1):E399–E408CrossRefGoogle Scholar
- 62.Downie A, Crosky A, Munroe P (2009) Physical properties of biochar. In: Lehmann J, Joseph S (eds) Biochar for environmental management: science and technology. Earthscan, London, pp 13–32Google Scholar