Structures of CNCs and FCNCs
The obtained cellulose nanomaterials were characterized by microscopic techniques. Resembling to most of the CNCs prepared by acid hydrolysis, the obtained CNCs in our experiments were rod-like nanofibers (Fig. 2(a) and (b)). According to the TEM images, the mean diameter and length of the short fibers were measured to be about 11 nm and 142 nm, respectively. The FCNCs were obtained from the spent liquor. Figure 2(d) and (e) display the SEM and TEM micrographs of FCNCs. The special cellulose products also showed homogeneous appearance. The flake-like morphology of FCNCs was obviously distinct with the commonly seen CNCs and CNFs. Inset in Fig. 2(e) was a zoom-in image of the corner of a nanoflake, which clearly indicated that some of the flaky celluloses were composed of stacked thin layers. The size of the flakes was statistically measured and given as a histogram in Fig. 2(f). Most of the cellulose flakes were in the range of 550–850 nm in size. As described in Section 2.2, SCNCs were obtained by freeze-drying of the clear top layer in the dialysis tubing. Microscopy analysis was carried out on the product as well. The results are shown in Fig. S1. The diameter of the SCNCs was about 60 nm in average. Since SCNCs had been prepared from the spend liquor in a previous work, the FCNCs were mainly focused in the present study.
The thickness of the FCNCs was determined by AFM. As shown in Fig. 3, the cellulose nano-flakes were well dispersive. Directly measurement indicated the flakes were about 3 ~ 3.5 nm in thickness. Compared with the width scale (~ 700 nm), the flake could be deemed as a two-dimensional material.
XRD was carried out to study the crystallographic structures of the MCC and nanocelluloses (Fig. 4). Three cellulose Ⅰ characteristic peaks at 2θ = 14.8°, 16.4°, and 22.6° (Wada et al. 2004) are shown in the profile of the raw material MCC. The diffraction peaks can be indexed by Miller indices of (1\(\stackrel{-}{1}\)0), (110) and (200) planes belonged to a one-chain triclinic unit cell (Cellulose I structure). The nanocelluloses exhibited three distinct peaks at 12.1°, 20.0°, and 21.7°, corresponding to the (1\(\stackrel{-}{1}\)0), (110), and (020) crystallographic planes of Cellulose II (Yan et al. 2015), respectively. However, carefully inspection on the diffraction patterns of these nanocelluloses indicated that the (110) plane of FCNCs showed relatively strong diffraction, compared to the peak of CNCs. The results suggested that the self-assembled FCNCs had preferred crystalline orientation. In combination with the microscopy observations, it was clear that the cellulose nano-flakes should have exposed (110) surfaces. On the other hand, the (110) diffraction peak exhibited obvious broadening, suggesting the length scale of the material along this direction was quite small, which also confirmed the two-dimensional characteristic of the material.
The FTIR spectra of MCC and nanocelluloses are shown in Fig. 5. Both MCC and nanocelluloses exhibited C-H stretching vibrations peaks at 2900 cm− 1. The absorption peak of the O-H stretching vibration of microcrystalline cellulose was at 3346 cm− 1. In contrast, the FTIR spectra of CNCs and FCNCs showed an absorption peak of the O-H stretching vibration between 3425–3444 cm− 1, indicating the hydrogen bonding stretching of type II cellulose (Zhang et al. 2009). The shift reflected the weaker inter- and intrachain hydrogen bonds of the nanocelluloses. Furthermore, for microcrystalline cellulose, the 1430 cm− 1 band was relatively strong, whereas it weakened and shifted to 1416 cm− 1 for nanocelluloses. This suggested that the conformation of the primary alcohol hydroxyl CH2OH at the C6 position in cellulose changes from trans-gauche (tg) to gauche-trans (gt), indicating the transition from cellulose type I to type II. The above characteristics in FTIR spectra all confirmed the crystalline structures of MCC, CNCs and FCNCs determined by XRD analysis.
As shown in Fig. 6, XPS was used to further investigate chemical compositions of the surfaces of CNCs, FCNCs, and SCNCs. All samples were primarily composed of carbon and oxygen atoms. The XPS C 1s spectrums of the three nanocelluloses all exhibited two distinct peaks with binding energies of 286.6, and 287.9-288.3 eV, corresponding to cellulose C-O, and O-C-O, respectively. Besides cellulose, C 1s also showed a peak with binding energies of 284.8 eV, corresponding to aliphatic carbons C-C/C-H(Yan et al. 2015). However, the O/C ratios varied from sample to sample. Compared with CNCs and SCNCs, the O/C ratio of FCNCs was significantly reduced. This observation might indicate a less presence of adsorbed water on the surface of FCNCs (Koljonen et al. 2003).
Thermal properties of CNCs and FCNCs
The thermos gravimetric and derivative thermos gravimetric curves of microcrystalline cellulose and nanocelluloses are shown in Fig. 7. These cellulose nano-materials exhibited significantly different thermal characteristics relative to the MCC. Decomposition of the MCC initiated at the temperature of 257 ℃. The maximum weight loss appeared at 327 ℃. The onset decomposition temperatures and maximum weight loss temperatures of CNCs and FCNCs were lower than those of MCC. Specifically, CNCs had an onset decomposition temperature of 124°C and a maximum weight loss temperature of 180°C. The two characteristic temperatures for FCNCs were 167°C and 298°C, respectively. On the other hand, the nanocelluloses showed more gradual thermal transition. MCC lost nearly 82% of its mass between 300–400°C, leaving only 7.4% ash at 600°C. In contrast, FCNCs lost 53% of their mass in the range of 167–300°C, followed by approximately 24% of their mass between 300–600°C, leaving 15% of their mass behind. CNCs lost 36% of their mass in the initial decomposition temperature range up to 300°C, and 30% of their mass between 300–600°C. Meanwhile, CNCs retained more residue, close to about 30%. These major differences in thermal behaviors between the CNCs and the FCNCs might be attributed to variations in surface area, morphology, and particle size. The high surface area of nanocelluloses is a significant factor in reducing their thermal stability, as a consequence of the increased surface area exposed to heat (Lu and Hsieh 2010). Furthermore, it was reported that the thermal degradation of one nanofiber could lead to degradation in neighboring nanofibers (Quiévy et al. 2010). It could be observed from SEM and TEM that, CNCs were smaller than FCNCs in size and had more contact with each other, which resulted in enhanced thermal conductivity. Moreover, due to the large-diameter flaky structure of FCNCs, fewer readily decomposable free end chains were formed on the surface of FCNCs relative to that of CNCs (Zhao et al. 2019). Consequently, FCNCs began to decompose at relatively high temperatures. Compared to CNCs, FCNCs were more thermally stable but had a lower carbon residual rate. The results suggested that FCNCs possess relatively better thermal stability. Additionally, both CNCs and FCNCs exhibited lower weight loss, which could potentially enhance the amorphous carbon yield.
Surface property of FCNCs
The above profile and structural features of nanocelluloses revealed that FCNCs had (110) exposed surfaces. The formation mechanism is analyzed as shown in Fig. 8. Cellulose is composed of alternating crystalline and amorphous regions. During hydrolysis, amorphous regions were preferentially hydrolyzed, whereas the crystalline domains were more predictable to be preserved to form CNCs (Habibi et al. 2010). Nevertheless, hydrolysis produced small-sized cellulose fragments that were dispersed in acid solutions due to the repulsive force of their surface negative charges. During dialysis, as the acidity of the solution diminishing, the H-bonding and van der Waals forces between the cellulose molecules gradually overcame the repulsive forces of the negative charges on their surfaces, resulting in aggregation and stacking (Lu and Hsieh 2010). It has been demonstrated that cellulose type II structure is the most thermodynamic stability of molecular chains stacking. The H-bonding between the molecular chains precisely aligned the cellulose molecular chains along their crystallographic [1\(\stackrel{-}{1}\)0] directions, resulting in a two-dimensional structure with (110) surface exposed. Then, the molecules between the (110) surfaces were stacked by van der Waals forces to form two-dimensional cellulose nanocrystals with a certain thickness (3 ~ 3.5 nm in the present case).
According to the atomic projection in the middle part in Fig. 8, each dehydrated glucose unit of the cellulose adopts a 1C4 chair conformation, with the all alcohol substituents in the ring plane, while the hydrogen atom in the vertical position. Therefore, in a manner analogous to the (200) surface of cellulose type Iβ structure (Mazeau and Rivet 2008; Zhang et al. 2020), the (110) surface of cellulose type II structure exhibited a relatively hydrophobic characteristic, with the hydrophobic C-H moieties exposed to the surrounding medium. To confirm this deduction, contanct angle tests were used to evaluated the hydrophobicity of the FCNCs (Fig. 9). It was observed that the water contact angle on FCNC film was considerably larger than that on CNCs. The contact angle value for CNCs was found to be 43.5°, while that for FCNCs reached 72.0°. This finding indicated that FCNCs possess apparently hydrophobic surface. The results of the contact angle experiments reversely verified that the (110) surface was actually the main exposed surface of FCNCs.