CDs were synthesized by changing the precursor concentration and their structures were studied (Fig. 1). In the TEM images, CDs were individually separated and their diameters became larger as the precursor concentration increased (Fig. 1 (a)). The diameter increase was dependent on the synthesizing mechanism of carbon dot, especially nuclear growth procedure. Hydrogen bonding between the citric acid promoted the construction of the core of CD (Dong et al., 2012). The core grew by consuming the molecules in the system. The molecules in the thin solution (1 mmol of precursor) had very rare chance to contact with the other molecules. When the solution got thicker, the molecules had better chance to be grow, resulting in bigger size.
Regardless of the concentration, all the precursors were successfully reacted to each other into CDs, as seen in FT-IR result (Fig. 1 (b) & Fig. S1). All the CDs had peaks such as O-H, N-H bonding (3400 cm− 1), carboxyl (1714 cm− 1), amine (1670 cm− 1), amide (1108 cm− 1) and sulfonate/sulfide groups (1000–1400 cm− 1), as coincident to our previous work (Ahn, Pak, Song, & Kim, 2021). The functional groups of CDs had no significant difference. However, the peak for sulfonate group of 5CD was smaller than those of 1CD and 3CD (colored in orange). The decrease of sulfonate group in 5CD will be discussed further in XPS analysis (Fig. 1 (c) & S2).
Difference in the number of the molecules in unit volume led to difference in chemical structure of CD. The N1s and S2p speciations of CDs were shown in the figure. Most of the nitrogen was formed in amino N in all the CDs. Graphitic N was barely developed with 1 mmol of precursors. Nitrogen in the thin solution was hard to be conjugated into core structure since the contact between the molecules was hard. As the precursor amount increased as 3 mmol, nitrogen was combined into the graphitic structure. Nitrogen could be easily participated in core-growth owing to the increased contact chance as well as the similar atomic size with carbon. The graphitic N speciation of 5CD was slightly decreased compared to 3CD because of the competitive structure formation between nitrogen and sulfur. The big atomic size makes sulfur hard to be penetrated the core structure. Instead of penetration, sulfur was developed as sulfide bonding at the edge and sulfonate group on the surface. As the reacting solution got thicker, the content of sulfide bonding was gradually increased while sulfonate group was decreased. With more chance of contact, more sulfur was combined with carbon at the edge.
Based on the CD development mechanism, the structure of CDCNF composite was further studied. In Fig. 2 (a), XRD profiles were shown to study the effect of in-situ synthesis on the CNF physical structure. All the sampled had cellulose Ⅰ crystal structure, indicating the hydrothermal treatment condition was not severe to destruct the crystalline structure of CNF. However, the crystallinity of the CDCNFs were changed. The crystallinity of the CNF was obtained as 62%, calculated by peak height method (Park et al., 2010). After the hydrothermal treatment, the crystallinity was increased. The crystallinity of CDCNFs was 75, 86 and 86% with 1, 3 and 5 mmol of CD precursors. Amorphous region can be more rapidly degraded compared to crystalline region resulted in the crystallinity increase. Citric acid in the reactor helped to break down more cellulose chain. The crystallinity did not increase over 3 mmol of citric acid. The citric acid promoted the degradation up to a certain amount, but the excessive amount did not affect the degradation significantly.
The morphologies of the CNF and the CDCNF were observed using TEM and shown in Fig. 2 (b). The CNF was highly entangled because of its high aspect ratio. The CDCNFs became less entangled after the in-situ synthesis treatment. The degradation of amorphous region during the hydrothermal carbonization reduced the CNF length, resulted in lower entanglements. Still the process conditions were not severe to destroy the fibrous structure. As shown in the high magnification images, CD was developed on the CNF surface (Fig. S2). The CD on the CNF was individualized with spherical shape for the all-precursor concentrations. Via the in-situ synthesis, CDs were successfully attached to the CNFs, regardless of the precursor concentration.
The chemical structure of CDCNFs were characterized using FT-IR and XPS (Fig. 3 & Fig. S2.) The CNF had fingerprint region of cellulose as well as carboxylate group peak at 1606 cm− 1, which are typical in tempo-oxidized cellulose nanofiber (Fig. 3 (a)). In the process of tempo-oxidization, the glucose unit is partially changed into glucuronate group, containing carboxylate group on the C6 position. All the CDCNFs had the distinct fingerprint region of cellulose in FT-IR spectra as like to the CNF. The detailed peaks from CDs such as sulfonate groups were not clearly observed due to its small amount. However, the carboxylate group of the CNF was remarkably changed after the in-situ synthesis. The peak at 1606 cm− 1 was decreased while the peak at 1719 cm− 1 (colored in orange) was increased in all the samples. The increased peak was originated from the functional groups of CDs, such as carboxyl groups and amino groups. Glucuronate groups can be decomposed easily by mild hydrothermal carbonization because it is less stable than glucose group. During the in-situ synthesis, the decomposed glucuronate fragments were participated in CD-core growth and attached to the CNF surface. The peak at 1719 cm− 1 in 1CDCNF was smaller than 3CDCNF and 5CDCNF. As discussed above, more cellulose (including glucuronate group) were degraded with higher precursor concentration. More CDs were attached to the CNF surface in 3CDCNF and 5CDCNF. The shoulder peak at 1604 cm− 1 (colored in green) was increased as well. The peak was driven from C = O bonding of amide bond, originated from the linkage between the CD and the CNF (Ahn, Pak, Song, & Kim, 2021). It was clear that CD was attached to the CNF by sacrificing the carboxylate group and linked by amide bonding.
XPS analysis was conducted to study the chemical structure of CD@CDCNFs (Fig. 3 (b) & Fig. S2). Interestingly, the N1s and S2p speciations of CD@CDCNFs showed similar results to CDs except for CD@1-CDCNF. In the in-situ synthesis, the presence of CNF certainly affected the CD structure development with 1 mmol of precursor. Graphitic N structure was more developed and sulfur was barely functionalized in the CD@1-CDCNF. Bonding formation between sulfur and the carbon core was disturbed by the CNF. Without competing to sulfur, nitrogen penetrated the core structure easily and developed graphitic N structure. With equal or more than 3 mmol of precursors, the N1s and S2p speciations of CD@CDCNF followed the tendency of CD: the graphitic N contents was highest in CD@3-CDCNF and the formation of sulfide bond predominated over sulfonate groups in CD@5-CDCNF, as coincident to CD. In turn, the number of molecules per volume majorly influenced the chemical structure of CDs and the presence of CNF minorly affected the CD formation. The chemical structures of the CD@CDCNF were considered similar to that of CDs, except for CD@1-CDCNF.
Figure 4 shows the optical properties of each sample, the UV absorption curves and the PL emission spectra. In Fig. 4 (a), all the UV absorption curve did not exhibit the absorption peak related to π-π* transition because of the low graphitization of CDs (Ahn et al., 2019). In 1CD curve, small shoulder absorption around 350 nm was observed. As the precursor concentration increased, the absorption of CDs at 350 nm became more distinct. The absorption was from n- π* transition arouse by the functional groups such as carboxylate, amine and amino groups. As shown in Fig. S3 (a), absolute nitrogen content of 1CD was much lower than the others, resulted in weak absorption. Higher n- π* absorption of 3CD and 5CD was achieved from nitrogen both in the core and surface. More amino N in 5CD led the stronger absorption at 350 nm. Figure 4 (b) shows the UV absorption of CDCNFs. Unlike CDs, all the curves had a smooth decreasing absorption without any apparent peak. The diverse functional groups of the CD@CDCNFs split the energy level under the π * level, leading to broad absorption (Woo, Song, Ahn, & Kim, 2020). As well the long-tailed absorption was observed originated from the geometrical feature of the CNF (Ahn, Pak, Song, & Kim, 2021). The fibrous structure of CNF scattered the light. CD@1-CDCNF couldn’t strongly absorb light because of their small amount and the incident light was scattered by the CNF. With the increase of precursor amount, stronger absorption curves were observed, as coincident to CDs. Incident light was absorbed by the CDs on the surface more instead of being scattered by CNF.
Figures 4 (c) ~ (e) shows the emission spectra of the CDs. All the CDs had emission maximum at 425 nm and showed weak emission dependency on the excitation wavelength. This was driven from the sulfur-containing surface state. The sulfur functionalized surface can create electron trap site with high density, weakening the excitation wavelength dependency on PL property (Dong et al., 2013). The emission wavelength of the CDs was red-shifted when the excitation wavelength shifted from 320 nm to 380 nm. As explained in the previous reports of our groups, the emission wavelength changes at different excitation wavelengths depends on the chemical structure of carbon dot. The well-graphitized carbon core of carbon dot has strong influence on the recombination of the excited electrons (Ahn et al., 2019). When carbon dot has highly developed graphitic core, the emission wavelength is firstly blue-shifted with the increasing excitation wavelength until it reaches the emission maximum, and then red-shifted as excitation wavelength increases. In this study, no blue-shift was observed with increasing excitation wavelength. The core of CDs was not fully developed and had not significantly affected the emission behavior. In turn, the radiative recombination of electron at functional groups, especially the sulfur containing group, dominantly managed the PL property of CDs. The emission behavior of the CDs were affected by anchoring to CNF (Fig. 4 (f) ~(h)). The emission spectra of the CDCNFs became slightly broader and showed stronger emission wavelength dependency on the excitation wavelength. The anchor between the CD and the CNF led to energy level splitting. Still the emission wavelength was red-shifted along the excitation wavelength, showing that the functional groups managed the emission behavior. The changes in emission pathway will be further studied in lifetime analysis.
The effect of anchorage to CNF on the electron pathways of CD were investigated using lifetime decay. The obtained lifetime and the quantum yield (QY) of each sample were summarized in Table. 1. The average lifetimes were 7.47, 7.75 and 7.74 ns for 1CD, 3CD and 5CD, respectively (upper row). All the decay curves of the CDs were fitted with the short lifetime component τ1, and the long lifetime component τ2. τ1 for all the CDs had similar value to each other, as the core structure did not have significant impact on PL emission (Ahn et al., 2019). N-doped structure did not significantly affect the core state-emission pathway although the number of electrons in π system increased. τ2 were driven from the functional group emission (Ahn et al., 2019). Comparing to 1CD, 3CD and 5CD had significantly long τ2. The increase of τ2 was driven from the increase of sulfide bonding contents. As many researchers described, oxidized sulfuric functional groups such as sulfonate group has strong electron withdrawn property (Xu et al., 2015). It effectively recombines the electron-hole via light emitting pathway. The abundant sulfonate group in 1CD rapidly recombined the electrons. On the contrary in 3CD and 5CD, un-oxidized sulfide bonding held the electrons in the excited state of CD and prolonged the lifetime τ2.
The chemical structure managed the QYs of the CDs as well. 3CD had high QY of 42% compared to 1CD (35%) and 5CD (32%). The contents of sulfonate group and sulfide bonding served the major effect on QY changes. The sulfonate group contents gradually decreased as the precursor concentration increased while sulfide bonding was increased. The electrons in the excited state of 1CD could effectively recombined via light-emitting pathway owing to the large sulfonate group content. Despite of the decreased sulfonate group contents, 3CD exceptionally showed the highest QY value among CDs. Large number of electrons could be excited to π* owing to the developed graphitic N structure, resulted in increased QY (Ahn et al., 2019). 5CD had the lowest QY value as it had small sulfonate group content among the CDs. Instead of fast radiative recombination, the excited electrons in the π system moved to energy level endowed by functional groups via vibrational relaxation.
In spite of the similarity in chemical structure, the electron of in-situ synthesized CDs had different electron recombination pathways when attached to the CNF. The average lifetimes of 1, 3 and 5CDCNF were 2.82, 2.88 and 3.50 ns, respectively. The decay curves were fitted with two components. CD and CDCNF did not have difference in τ1, because the CNF did not influence on the core state emission of the CD@CDCNF. It is noteworthy that τ2 of CDCNFs had huge decrease as CNF was introduced. Regardless of the surface states (especially the sulfur speciation) differences, the τ2 values of CDCNFs were similar to each other. The electron recombination of the CD@CDCNFs was not controlled by the functional groups. As well, the QY of CDCNFs were dramatically decreased compared to CDs: The excited electrons were mostly recombined via non-radiative pathway. It is reasonable to assume that the decrease of τ2 was attributed to the electron transfer from the CD to the CNF. τ2 of CDCNFs were slightly increased with increasing precursor concentration originated from the different surface state. As studied in the XPS, more precursors produced more sulfide bonding in the CDs, i.e., the electrons in the π system remained for longer time in the excited state and had more chance to be transferred to the CNF. CD@5-CDCNF had abundant sulfide groups resulted in more electrons transferred to the CNF. In other words, the electron-hole pairs of CD@5-CDCNF could be efficiently separated.
Table 1
The quantum yield (QY), photoluminescence lifetime (τav, τ1 and τ2) of the samples
1CD | τave | (ns) | QY (%) | 3CD | τave | (ns) | QY (%) | 5CD | τave | (ns) | QY (%) |
7.47 | τ1 | 5.14 | 35 | 7.75 | τ1 | 5.06 | 42 | 7.74 | τ1 | 4.93 | 32 |
τ2 | 9.31 | τ2 | 12.06 | τ2 | 11.71 |
1CD CNF | τave | (ns) | QY (%) | 3CD CNF | τave | (ns) | QY (%) | 5CD CNF | τave | (ns) | QY (%) |
2.82 | τ1 | 5.69 | 2.58 | 2.88 | τ1 | 5.31 | 10.09 | 3.50 | τ1 | 5.98 | 8.76 |
τ2 | 1.14 | τ2 | 1.21 | τ2 | 1.36 |
The lifetime decay results indicates that the excited electrons from CD@CDCNFs were transferred to the CNF, owing to its electron withdrawing property. To confirm this, the changes of PL intensities were studied as the strong electron donor N,N-diethyl aniline (DEA, 0.88 V vs. NHE) was added to the CDCNF suspension (Fig. 5). Unlike common carbon dot, the emission intensities were barely changed as the concentration of DEA was increased (LeCroy et al., 2019; Wu et al., 2018). As an electron donor, DEA injected more electrons to the ground state of CD@CDCNFs (black dotted arrow) and excited more electrons to π* (Fig. 5 (d),). Via vibrational relaxation, the electrons were trapped to the CNF (blue dotted arrow) and non-radiatively recombined, resulted in negligible change in PL emission. Coincident to the lifetime decay results, CNF played an essential role in electron relaxation process under UV irradiation. The trap suppressed the electron radiative recombination, hence believed to enhance photocatalytic effect.
Figure 6 shows the catalytic performance of the samples. For the test, Methylene blue (MB) was used as the model pollutant. The MB degradation rate along time were plotted in Fig. 6 (a), in the absence (blank) and presence of the catalysts. All the CNFs had catalytic effects against MB degradation. The CNF could degrade the dye without help of CD due to the radical generation by sacrificing itself under UV irradiation, which will be further discussed in Fig. 7. With the CD attached on the CNF surface, MB was degraded more effectively by photocatalytic reaction of CD@CDCNF. Among the CDCNFs in this study, 5CDCNF showed the highest photocatalytic power. Most of the MB molecules (~ 98%) were degraded within 25 min in 5CDCNF suspension. For the clear comparison, the photocatalytic kinetics were studied (Fig. 6 (b)). The degradation performance of MB with each catalyst were plotted with the pseudo first-order degradation equation where C0 and C are the initial concentration and the concentration of MB at time t, respectively. The apparent rate constants (kapp) of MB degradation were calculated as expressed in the figure. k5CDCNF was almost 3 times higher than kCNF, exhibiting the effect of CDs on the CNF surface.
The photocatalytic mechanism of the CDCNFs can be explained based on the active centers of photocatalytic reaction. The photocatalytic mechanism was schematically expressed in Fig. 6 (c). Under the UV irradiation, the electrons in CD@CDCNFs were excited and electron-hole pairs were generated (Eq. (1)). The excited electrons were radiatively recombined with the holes or transferred to the energy trap induced by the CNF (Eq. (2)). The trapped electrons were retarded to be recombined, which enhanced the photocatalytic efficiency. The separated electron and hole favored to form the active radicals: the electron generated superoxide radicals (•O2−) with molecule oxygen (Eq. (3)). The hole reacted with the water absorbed on the CDCNF surface to form hydroxyl radicals (• OH, Eq. (4)). The ROS interacted with the MB molecules and degraded MB into water and carbon dioxide (Eqs. (5) and (6)).
Two types of active-species-trapping experiments were carried out to elucidate the role of the major active species in the photocatalytic process (Fig. 6 (d)). Two radical scavengers (1 mM of tert-butanol (t-BA, as •OH scavenger) and para-benzoquinone (p-BZQ, as •O2− scavenger)) were separately added to each suspension before the UV irradiation. Figure 6 (d) exhibits the results of photocatalytic effect in the absence (control) and presence of the radical scavengers. In CNF and 1CDCNF suspension, the effect of the scavengers was less than ~ 5% indicating the role of •OH and •O2− were not significant in the MB degradation. In 3CDCNF and 5CDCNF suspension, the addition of t-BA had minor effect on the photocatalytic power. On the other hand, the addition of p-BZQ in 3CDCNF and 5CDCNF suspension suppressed the degradation efficiencies to ~ 94% and ~ 76%, respectively, indicating that •O2− radical made the major contributions to the photocatalytic reaction. The retarded electron recombination in 5CDCNF led active production of •O2− radical under UV light. As discussed in the lifetime analysis, electron relaxation pathway was prolonged with increase of precursor concentration. Therefore, more electrons reacted with molecule oxygens and produced •O2− radical in 5CDCNF, resulted in effective degradation of MB (Fig. 6 (c), Eq. (5)).
For easy handling, the composites were formed into aerogel via freeze-drying process. The photocatalytic reaction of the aerogels was tested (Fig. 7 (a)). The images of MB/aerogel solution along the UV irradiation time were displayed in Fig. S4. With all the aerogels, MB degradation occurred faster than the suspensions. The movement of MB molecules in the suspensions were restricted as the entangled fibrous structure of CNF caused viscosity. On the other hand, MB in the solution could be dispersed without restriction and continuously absorbed to the surface of the aerogels. As well known, the negative charged carboxylate group of CNF can highly interact with positive charged MB. Even after the CD attachment on the surface of CNF, the MB absorption-ability was maintained, leading to the faster degradation (Ahn, Pak, Song, & Kim, 2021).
The degradation results of the CNF and 1CDCNF aerogels after 10 min were hard to be obtained while 3CDCNF and 5CDCNF aerogels degraded most of MB within 25 min as coincident to the suspensions. As shown in the images (Fig. 7 (b) & S4), the morphology of CNF and 1CDCNF aerogels were severely damaged after 10 min of reaction. As mentioned, CNF could be decomposed by radicals under UV irradiation (Ma et al., 2021). During the decomposition, ROS such as hydrogen peroxide was generated as side reactant, leading to the sequential decomposition of CNF. The continuous ROS generation helped to degrade MB as well in the first 10 min. However, the further decomposition of the CNF made aerogel lost its shape. The self-decomposition of CNF was greatly suppressed in 3CDCNF and 5CDCNF aerogel, owing to the CD on the surface. The UV light was absorbed by CD@CDCNF and the decomposition of the CNF was inhibited. As shown in the images, 3CDCNF and 5CDCNF aerogel retained its shape even after the reaction for 25 min. With simple wash with ethanol, the 3CDCNF and 5CDCNF aerogel was reusable while maintaining its photocatalytic performance (Fig. 7 (c)).