Synthesis of Newly Discovered Carbon Nanoframes: A Self‐Assembly Strategy Based on DTAB @ NaCl

Carbon nanomaterials have attracted much attention in the field of science and technology for their excellent properties; however, developing a simple, environmentally friendly, and versatile synthesis strategy for the preparation of carbon nanomaterials with novel structures remains a great challenge. Herein, a surfactant @ salt (dodecyltrimethylammonium bromide DTAB @ NaCl) self‐assembly strategy for the synthesis of carbon nanomaterials with novel structures, i.e., carbon nanoframes is reported. The synthesis differs from the traditional template method in that it is characterized by the introduction of surfactants for separation and protection, and the salt can be recycled. In addition, the carbon frame size in this system can be adjusted on demand by simply adjusting the concentration of surfactant, thus realizing that the carbon nanoframe size is adjustable in the range of 232.58–322.51 nm. Impressively, the carbon nanoframe has purple, blue, and green PL emission behavior. It is anticipated that this research will provide new insights into the development of novel carbon nanomaterials.


Introduction
8][9][10][11] A particular area of interest is the synthesis of carbon nanomaterials with novel structures, which leaves great scope DOI: 10.1002/admi.202300832 for new areas and applications related to unforeseen properties and functions. [2]owever, the synthesis of carbon nanomaterials usually relies on the template method. [12]The synthetic route emphasizes that the shape and size of the nanospace can be reflected in the structure of the resulting carbon when the organic compound is carbonized in each nanospace. [12]While we appreciate the variety of carbon nanostructures produced using this technique.However, its operation is cumbersome, involving lengthy synthesis protocols as well as complex separation systems.In addition, the resulting product is of low purity, limiting its potential for widespread application.Therefore, it is necessary to explore some new synthesis strategies from a scientific, technological, and economic point of view.For example, there is no need for complex equipment; it is simple, green, versatile, and easy to scale up. [4,5,13,14]ased on these factors, our research focuses on the design and synthesis of carbon nanomaterials with novel structures.Here, we directly use surfactants as a carbon source and cheap salts as templates.No additional pre-treatment is needed.For the first time, a surfactant @ salt self-assembly strategy is proposed to construct carbon nanomaterials.On the one hand, since surfactants are composed of hydrophilic (polar) heads and hydrophobic (nonpolar) hydrocarbon chains, they are adsorbable on crystals. [15,16]The key feature of the method is that the surfactant provides a unique separation and protective environment for the carbon framework to guide the acquisition of well-dispersed carbon nanomaterials.On the other hand, the crystalline layer of the salt can provide the framework space as a template, which facilitates the replication of the surfactant.In addition, the salt is easily removed by simple water washing without contaminating the carbon nanomaterials, while the nanocarbon retains the original shape of the precursor.More importantly, the strategy inherits the advantages of the traditional method, namely, modulability.In more detail, this approach allows us to regulate the size distribution of the desired carbon nanomaterials by controlling the concentration of surfactant.This is mainly attributed to the fact that surfactants have the effect of inhibiting crystallization.Despite the conceptual straightforwardness of this approach, its key advantages over conventional methods are ease of operation, low sunk costs, and environmental friendliness.Due to the presence of various combinations of salts and surfactants, it is conceivable to synthesize novel carbon nanomaterials of various morphologies using the proposed strategy.
Under the above framework, we establish a DTAB (dodecyltrimethylammonium bromide) @ NaCl system and design a novel cubic carbon nanomaterial, which is named "carbon nanoframe".In the preparation process, cubic crystals of NaCl serve as a recyclable primitive backbone, and DTAB is adsorbed on its crystal edges.During the self-assembly process, the segregation and confinement effects of DTAB are utilized to guide the growth of highly homogeneously dispersed cubic precursors.With pyrolysis complete, carbon nanoframes can be synthesized.Impressively, the size of the carbon nanoframe can be tuned as desired by adjusting the concentration of DTAB.This results in a tunable size from 232.58 to 322.51 nm.In addition, carbon nanoframes have specific optical properties (purple, blue, and green PL emission).We envision that our research may provide new avenues for the further development of novel carbon nanomaterials for a variety of applications.

Results and Discussion
Scanning electron microscopy (SEM) shows that the carbon nanoframes are very uniform in size and shape (Figure 1a,b).
Impressively, the state of existence of the carbon nanoframes relies on the cubic-like crystals formed by NaCl.In more detail, DTAB replicates the morphology of NaCl crystals, and the dispersion and cubic structure of carbon nanomaterials are well maintained after high-temperature carbonization.Transmission electron microscopy (TEM) shows more details of individual carbon nanoframes (Figure 1c).DTAB is involved in the crystal growth process, but unlike previous reports in the literature, the surfactant is not concentrated on the surface of the crystals but adsorbed on the edges of the crystals. [17]This finding provides another unique perspective on understanding the effect of surfactants on salt crystallization.It is worth mentioning that carbon nanoframes did not undergo significant agglomeration and adhesion in all samples.This can be attributed to the fact that the presence of surfactants can form a hydrophobic film that acts as a protective and separating agent for the crystalline layer to maintain the cubic structure and prevent its aggregation.In addition, the size of the carbon nanoframe can be easily controlled by adjusting the concentration of DTAB from 15 to 25 CMC.This can be further confirmed by dynamic light scattering (DLS).The DLS results show that the obtained carbon nanoframes have a relatively uniform size, with an average particle size of 322.51 nm at 15 CMC.As the concentration increased to 20 CMC, the average particle size of the carbon nanoframes decreased to 287.45 nm.By further increasing the concentration to 25 CMC, the average particle size of the carbon nanoframes continues to decrease to 232.58 nm.It should be noted that the size reduction is actually due to the increasing concentration of surfactant, thus limiting the crystal growth.Therefore, it can be concluded that the variation in the concentration of DTAB plays a vital role in size control.
X-ray powder diffraction spectra (XRD) show a diffraction peak near 2 = 23°-26°, corresponding to the 002 diffraction pattern of typical graphitized carbon (Figure 2a). [6]Notably, the asymmetric peak shape of the (002) peak is a distinctive feature of carbon nanoframes undergoing partial graphitization.The d 002 values obtained from Bragg's law calculations are all less than the non-graphitic carbon layer spacing of 0.3440 nm.This means that all three sets of samples showed some degree of graphitization.This result is in general agreement with the results for the graphitization g-value.The Raman spectra of all the samples in Figure 2b further confirm the XRD results.The Raman spectra of all three samples show two bands centered at 1350 cm −1 (the D peak) and 1580 cm −1 (the G peak), which are attributed to disordered sp 3 carbon and graphitic sp 2 carbon, respectively. [11]A significant increase in the height difference between the D and G peaks of sample 25 CMC can be clearly seen when compared to samples 15 CMC and 20 CMC.This indicates a shift from a graphite-disordered layer structure to an ordered structure inside the carbon nanoframe and a gradual decrease in microcrystalline defects.In addition to spectral peaks, the use of I D /I G to reflect the degree of graphitization of a sample has been widely accepted by scholars studying carbon materials. [1]The I D /I G values of samples 15 CMC and 20 CMC are very close to each other, but the I D /I G of sample 25 CMC is smaller, indicating that the graphitization of the carbon skeleton is higher in this sample.In contrast, sample 25 CMC has the largest La value, indicating the least amount of disorder or defects.Overall, both XRD and Raman analyses confirmed that the prepared carbon nanoframes have a graphite-like structure with the presence of microcrystalline defects.
X-ray photoelectron spectroscopy (XPS) of the three sets of carbon nanoframes shows two peaks at ≈284.6 and 530.3 eV, corresponding to C 1s and O 1s, respectively (Figure 3a). [11]Notably, the absence of Na and Cl signals in the XPS indicates that NaCl has been completely removed.The water-washed NaCl can be easily recovered through the recrystallization process for recyclability.It should be mentioned that the elements N and Br are present in the precursor, and a large amount of N and Br is lost as the carbonization temperature increases.By using the DTAB @ NaCl self-assembly strategy, the sample structures have more C atoms (97.02-99.01%),indicating that the proposed carbon nanoframes based on this strategy are composed of abundant carbonaceous matter.High-resolution C 1s XPS spectra reveal the presence of C─C and C─O functional groups (Figure 3b). [18,19]It should also be noted that the C─C sp 2 contents of the three sets of samples are 77.77,85.63, and 86.84 at%, respectively, and these data further confirm the presence of graphitic carbon. [3]This is consistent with our previous findings.It can also be concluded that a small amount of oxygen is introduced into the structure, which can be attributed to the charring conditions.This further demonstrates the two groups, C─O and C═O, in the O 1s peak of the carbon nanoframe (Figure 3c). [20]Interestingly, the presence of these oxygen-containing groups can give rise to certain defects in carbon nanoframes. [21]Based on the above observations, we propose a dynamic assembly mechanism of surfactant @ salt to account for the formation of carbon nanoframes with tunable sizes (Figure 4).We believe that NaCl spontaneously forms cubic crystals when it reaches a certain level of saturation.And the adsorption of surfactants on the NaCl crystal edges contributes to the formation of carbon nanoframes by template effect during the self-assembly process.In addition, the surfactant also facilitates the segregation of carbon nanoframes and prevents them from aggregating during pyrolysis.Specifically, after the surfactant is adsorbed on the crystal edge, its polar group end faces toward the crystal, while the non-polar group end faces outward, forming a hydrophobic film encapsulated on the crystal edge, thus serving to protect the original skeleton.In addition, this hydrophobic film encapsulating the edges of the crystals also acts as a separator between the grains.Most of the studies reported that surfactant is involved in the crystal growth process and that its presence inhibits crystallization. [15]In more detail, the force field is larger in the protruding part due to the crystal morphology, and the amount of adsorbed surfactant is larger but not easily desorbed.Here, the hydrophobic groups of the surfactant are oriented outwardly, thus preventing further growth there.Thus, the particle size of carbon nanoframes decreases with increasing concentrations of surfactant.
The photoluminescence spectra of all synthesized samples are shown in Figure 5. Three main characteristic fluorescence peaks appeared in all three groups of samples: the ultraviolet emission peak at 380-450 nm, the blue emission peak at 450-475 nm, and the green emission peak at 495-570 nm.[24] It should be noted that the PL spectrum is relatively broad, indicating the existence of a certain size distribution of the prepared carbon nanoframes, which is consistent with the results of the DLS analysis.When the excitation wavelength is increased from 300 to 360 nm, all three groups of samples show typical excitation-dependent luminescence characteristics, accompanied by an obvious red shift of the emission spectra.In most cases, carbon nanomaterials with excitation-dependent PL properties may be attributed to multiple energy levels within their structures. [25]In fact, it has been shown in the literature that self-assembled aggregates occurring at high concentrations also have the potential to lead to redshifted emission from carbon nanomaterials. [26]When the excitation wavelength exceeds 380 nm, the obtained emission spectra exhibit typical excitation-independent PL behavior.Based on the above analysis, this novel carbon nanoframe maintains the excellent photoluminescence properties of carbon nanomaterials.The above results predict that carbon nanoframes synthesized based on the DTAB @ NaCl self-assembly strategy may have important applications in displays and solid-state lighting in the future. [27,28]

Conclusion
In summary, we have successfully prepared a new type of carbon nanomaterials, i.e., carbon nanomaterials with cubic-like structures, using our proposed surfactant @ salt self-assembly strategy.The synthesis process involves the creation of a DTAB @ NaCl system that is driven by shear flow to direct polymerization and then self-assembly into carbon nanoframes.Key factors in the evolution of carbon nanoframes have been elucidated.In this case, the hydrophobic film of the surfactant acts as an isolation and confinement of the carbon-pristine framework.This reflects the novelty of the strategy.Notably, our strategy is simpler, more environmentally friendly, and less costly than previously reported methods for synthesizing carbon nanomaterials.This is mainly based on the separating and limiting effects of DTAB and the water solubility and recoverability of NaCl.Detailed mechanistic studies show that the size of carbon nanoframes in the system can be well regulated by changing the concentration of surfactant.We found that carbon nanoframes are not only well dispersed with clearly tunable sizes (232.58-322.51nm), but also have specific optical properties (violet, blue, and green PL emission).This makes them promising candidates for lighting devices such as displays and solid-state lighting.Overall, this simple, lowcost, and novel synthesis strategy promises to be applicable to a wide range of fields requiring a variety of carbon nanostructure designs.

Experimental Section
Materials: All commercially available reagents were of analytical grade and ready for use without further purification.Ultrapure water was used in all experiments.NaCl and dodecyltrimethylammonium bromide (DTAB) were purchased from Aladdin Biochemical Technology (Shanghai) Co.
Synthesis of Carbon Nanoframes: The simplicity of the synthesis strategy and the tunability of the size distribution ensured the formation of a range of carbon nanoframes.The focus was on utilizing the separating and protecting effects of surfactants (DTAB).Briefly, 100 mL of DTAB of different concentrations (15, 20, and 25 CMC) were self-assembled with an excess of NaCl (100 g), respectively, at 120 °C driven by a shear flow (300 rpm).Eventually, DTAB was made to concentrate on the edges of the NaCl framework crystals to form DTAB @ NaCl carbon nanoframe precursors.It should mentioned that the entire self-assembly process was done using a magnetic stirrer.Then, the essentially evaporated precursors were then placed in a muffle furnace (SX-8-10) with a heating rate of 5 °C min −1 .After carbonization at 700 °C for 2 h, it was removed by natural cooling.After carbonization was completed, NaCl was removed with large amounts of deionized water.Finally, a series of carbon nanoframes with different size distributions can be formed by drying the samples in a vacuum drying oven at 60 °C.The samples obtained were labeled as 15, 20, and 25 CMC.In addition, NaCl was easily recovered through the recrystallization process.
Characterization of Carbon Nanoframes: The morphology of carbon nanomaterials was observed by SEM (ZEISS GeminiSEM 500, Germany).TEM images were obtained with a JEM-2100F transmission electron microscope (JEOL, Japan) at 200 KV.The particle size distribution of the samples was measured by the DLS method (OTSUKA FRAR-1000, Japan).The crystalline structure parameters of the samples were determined using a D/MAX-B type X-ray diffractometer from RIKEN, Japan.Cu target radiation ( = 0.154056 nm) and scanned the sample at a scanning speed of 4°min −1 in the range of 2 = 15°-60°.Raman spectra were measured by an InVia Reflex laser microscope confocal Raman spectrometer (Renishaw, UK).An argon-ion laser was used as the excitation light source, and the excitation wavelength was set to  = 532 nm.The spectral range of the acquisition was from 500 to 2250 cm −1 .X-ray photoelectron spectroscopy (XPS) was measured on a Thermo ESCALAB 250XI (Thermo Fisher Scientific, US) system.The fluorescence spectra were recorded by an F-4600 fluorescence spectrometer (Hitachi, Japan).The emission spectra were recorded in the range of 300-800 nm at the excitation wavelengths of 300, 320, 340, 360, 380, 400, and 420 nm, respectively.

Figure 1 .
Figure 1.Synthesis of carbon nanoframes by the DTAB @ NaCl self-assembly strategy.a) Carbon nanoframes shown in representative SEM images at DTAB concentrations of 15, 20, and 25 CMC.b) Carbon nanoframes shown in representative TEM images at DTAB concentrations of 15, 20, and 25 CMC.c) DLS curves of carbon nanoframes at different DTAB concentrations obtained after pyrolysis.

Figure 3 .
Figure 3. Elemental composition and surface chemical states of DTAB @ NaCl carbon nanoframes.a) XPS spectra and elemental percentages of DTAB @ NaCl carbon nanoframes.b) High-resolution XPS mapping of C 1s in carbon nanoframes.c) High-resolution XPS mapping of O 1s in carbon nanoframes.

Figure 4 .
Figure 4. Schematic diagram of the formation mechanism of DTAB @ NaCl carbon nanoframes.