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. 2025 Aug 11;10(33):37398–37407. doi: 10.1021/acsomega.5c03191

Morphology Tuning for Hard Carbon Nanomaterials by Manipulation of PTCDA Volatilization in the Chemical Vapor Deposition Process

Chaohui Wang 1, Xun Wang 1, Chenchen Fan 1, Xinlei Yao 1,*, Biyun Shi 1, Qiaojun Cao 1, Weidong Dou 1,*
PMCID: PMC12392011  PMID: 40893244

Abstract

A facile and low-cost synthesis method for carbon nanomaterials using an organic molecule perylene tetracarboxylic dianhydride (PTCDA) as a precursor is presented. The resulting products exhibit a combination of floccular and fibrous morphologies, along with a low-crystallinity graphitic nanostructure characteristic of hard carbon. The synthesis is carried out by using a low-pressure chemical vapor deposition (CVD) system, during which the pressure changes in the CVD chamber are continuously monitored. These pressure variations provide critical insights into the material transformations at specific temperatures. A range of characterization techniques are employed to elucidate the structural and chemical changes occurring at temperatures corresponding to the pressure shifts. It is demonstrated that the disruption of PTCDA crystallinity promoted by amorphous PTCDA volatilization at relatively low temperatures (≤450 °C) is important for the formation of floccular and fibrous nanomaterials with an enlarged specific surface area reaching 359 m2 g–1 and hierarchical porous structures. Consequently, controlling the volatilization and crystallinity of PTCDA is essential to tuning the structures of the produced nanomaterials. This study presents a novel template-free approach for the controllable synthesis of hard carbon materials, demonstrating that the investigation of the material formation process and mechanism is significant for optimizing the preparation of hard carbon materials with diverse morphologies and nanostructures.

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1. Introduction

Carbon, one of the most abundant elements on Earth, has played a vital role in human history. It exists naturally in various allotropic forms, such as graphite, diamond, and coal. Though carbon is extensively utilized, significant efforts have been devoted to expanding the applications and enhancing the properties of carbon-based materials in recent decades. − Particularly, carbon nanomaterials have attracted substantial attention due to their unique and tunable properties arising from nanoscale size effects. − Since the discovery of carbon nanotubes in 1991, the field has witnessed remarkable progress, leading to the development of a comprehensive family of carbon allotropes. This includes zero-dimensional fullerenes and quantum dots, one-dimensional carbon nanotubes and nanofibers, two-dimensional graphene, and three-dimensional nanodiamonds, etc. − These nanomaterials exhibit characteristics such as superior electrical and thermal conductivity, remarkable mechanical strength, high surface activity, and lightweight properties, making them valuable in numerous research fields including electronics, membrane technology, energy storage systems (batteries and capacitors), heterogeneous catalysis, and biological sciences. − Various synthesis techniques have been developed for carbon nanomaterials, including laser ablation, − arc discharge and chemical vapor deposition (CVD). , Among them, CVD stands out as a versatile method, involving the decomposition of gas-phase molecules into reactive species that facilitate film or particle growth. The primary advantage of CVD is the capability to precisely control the synthesis of nanomaterials with diverse stoichiometries. Numerous studies have reported the successful synthesis of various carbon nanomaterials, including fullerenes, carbon nanotubes, carbon nanofibers, and graphene, using CVD techniques. − However, the large-scale production of high-quality carbon nanomaterials remains a significant challenge. It is worthwhile exploring facile, controllable, and economical methods to synthesize carbon nanomaterials with desired properties.

Within the carbon nanomaterial family, hard carbon has emerged as a promising material due to its properties, including large surface area, high porosity, and good electrical conductivity. These characteristics have made it a focus of research in fields such as energy storage and catalysis. − Hard carbon can be derived from various precursors, including resin carbon, pyrolyzed organic polymers, carbon black, and biomass-derived carbon. , Recent studies have focused on tuning the nanostructures of hard carbon to optimize its performance. Notably, organic semiconductors such as perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and perylene tetracarboxylic diimide (PTCDI) have shown promise as precursors for hard carbon synthesis. For example, Liu et al. demonstrated the fabrication of carbon nanofibers from self-assembled perylene diimide derivative gels, which exhibited superior performance as supercapacitor electrode materials. Similarly, Turner et al. developed a versatile synthesis method for hard carbon nanofiber aerogels using PTCDI, where carbon nanostructures were generated through pyrolysis of self-assembled gel fibers under inert gas. In their work, the carbon nanostructure is generated by pyrolysis of self-assembled gel fibers of PTCDI under inert gas. These materials maintained the nanofiber aerogel structure, offering large surface areas and porosity. However, these methods often involve multiple steps and are relatively time-consuming.

Luo et al. presented a simplified approach involving the calcination of PTCDA at 1100 °C in a N2-protected tube furnace, resulting in disordered porous nanostructures with a specific surface area of 21.61 m2 g–1 and enhanced lithium storage capabilities compared to conventional graphite. While this method offers a more straightforward synthesis route, it lacks precise control over the molecular transformation and morphological evolution during pyrolysis. Furthermore, while three-dimensional porous morphologies have been extensively studied, there remains significant potential for exploring two-dimensional and one-dimensional hard carbon nanostructures for specific applications.

Therefore, in this Letter, we present a novel, one-step synthesis method for carbon nanomaterials using PTCDA as a precursor in a low-pressure CVD system. This cost-effective and facile approach yields nanostructured materials with floccule and fiber morphologies. The presence of graphene sheets with small crystallite sizes within the material domains confirms the formation of hard carbon. Compared with that reported in the literature, there is no need to pretreat PTCDA precursors with ammonium hydroxide or aminoisophthalic acid solution to get PTCDI derivatives, or several following steps, such as freeze-drying, annealing, pyrolysis, etc., to obtain and turn aerogels into carbon nanofibers. , The carbon nanomaterials with floccular and fibrous morphologies are formed by direct calcination of PTCDA without sophisticated treatments. Thus, the yield for carbon nanomaterials is presumed to be improved and beneficial for a larger-scale production. The controllability of carbon nanostructure is realized by real-time monitoring of the pressure changes in the CVD chamber during the heating process, which is a distinctive feature of our approach and is rarely presented in publications. Through comprehensive characterization techniques, the morphology and composition of materials at critical temperature points corresponding to observed pressure changes were analyzed in detail, giving insights into material transformation during calcination. Additionally, we have explored the manipulation of material morphology through pretreatment methods, such as mixing with urea in a certain ratio before calcination, that adjust PTCDA volatilization and disrupt its crystallinity. It proves that mixing with urea facilitates the volatilization of amorphous PTCDA at a relatively low temperature, which prompts the disruption of PTCDA crystallinity, which is essential for the formation of floccular and fibrous carbon nanomaterials. The obtained hard carbon with floccular and fibrous morphologies has an enlarged specific surface area reaching 359 m2 g–1 and a porous structure containing both mesopores and micropores. This study establishes a morphology-regulated synthesis strategy for hard carbon architectures by investigating the hard carbon formation process and mechanism in detail, which provides potential for controllable synthesis of hard carbon materials with applications such as energy storage, catalysis, etc.

2. Experimental Section

2.1. Materials and Reagents

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA, 97%) was purchased from Sigma-Aldrich, urea (AR, ≥99%) was purchased from Macklin Reagent Co., Ltd., and ethanol (AR) was purchased from Sinopharm Chemical Reagent Co., Ltd. and used without further purification.

2.2. Synthesis of Carbon Nanomaterials by CVD

To obtain the carbon nanomaterials, PTCDA samples (3 g) were put inside a crucible with a size of 10 cm × 3 cm × 2 cm and were heated in the chamber of the plasma-enhanced CVD system (model: G-CVD-50). The equipment was initially pre-evacuated to approximately 1 Pa to establish a low-pressure environment, and the pressure variations were continuously monitored throughout the entire heating process. The temperature was programmed to rise from 25 to 1050 °C at a rate of 10 °C·min–1, completing the ramp in approximately 2 h. After reaching 1050 °C, the temperature was maintained for about 1 h, followed by a natural cooling of the system back to room temperature. For the pretreatment, the PTCDA was mixed with urea in a certain mass ratio, and the mixture was placed in a grinding mortar and thoroughly ground. The heating procedure of the PTCDA/urea mixture is similar to that mentioned above.

2.3. Characterization Methods

The structure of materials in our experiments was observed by a ZEISS-SIGMA300 scanning electron microscopy (SEM) instrument. They were adhered to a stud using a double-sided carbon tape and were imaged directly. A JEM-2100F high-resolution field emission transmission electron microscope was used to characterize the graphite structure of carbon nanomaterials. X-ray diffraction (XRD) patterns were collected on a DX-2700BH multipurpose diffractometer. The Cu anode was operated at 40 kV and 40 mA. XRD patterns were measured over 10–90° 2θ with a scan time of minutes. Raman signals for the materials were observed in a HORIBA LabRAM. HR evolution spectrometer. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Scientific K-Alpha + instrument equipped with an Al Ka source (hν = 1486.6 eV). The Brunauer–Emmett–Teller (BET) surface area and pore volume by N2 sorption at 77 K were determined using an Autosorb-iQ gas adsorption analyzer. Samples were degassed for 10 h at 150 °C before N2 sorption analysis.

3. Results and Discussion

Figure a shows a schematic of the CVD heating apparatus. The PTCDA precursor, an organic dye molecule consisting of a perylene core to which two anhydride groups have been attached, one at either side, is depicted in the molecular structure diagram in the upper right corner of Figure d. It is in the form of a red solid, visible in the lower-right corner of Figure b. The pristine PTCDA sample is characterized by SEM and XRD. Large pieces of PTCDA crystals as well as small particles are observed according to the SEM image in Figure b. The XRD analysis further proved the crystalline nature of the PTCDA sample. As shown in Figure d, a prominent diffraction peak at 27.4° is displayed, indicating a d-spacing measuring 3.22 Å, which aligns with the diffraction pattern of the (102) plane within the β-phase configuration of a single-crystal PTCDA. ,

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(a) Scheme of tube-furnace CVD equipment; (b) SEM image of PTCDA crystals and photograph of PTCDA samples inside a crucible (bottom-right corner); (c) SEM image of obtained products and the photograph of products inside the crucible (bottom right corner) after CVD heating process; X-ray diffraction (XRD) patterns of PTCDA samples: (d) before and (e) after CVD heating treatment.

Upon completion of the CVD heating process, a transformation is observed as the initial red color of the PTCDA sample changes to a deep black, shown in the lower-right corner of Figure c. There is no red solid in the resulting products because of the full reaction of pristine PTCDA. The nanostructure of the produced material is also studied by SEM and XRD. The SEM image in Figure c reveals a mixed nanostructure with intertwined and convoluted floccules and fibers, contrasting with the crystalline order of PTCDA illustrated in Figure b. The XRD patterns lose signals of the sharp peaks as in Figure e; instead, a broad peak located at around 24° and a smaller one at 43° are recorded. The wide and broad diffraction peak corresponds to the (002) crystal plane, indicating that the products are composed of amorphous carbon. ,, The smaller diffraction peak is assigned to a lattice plane of (100). Notably, the diffraction peak position of the (002) crystal plane shifts to a lower angle compared with that of pure graphite (2θ = 26.5°). From the calculation of the Bragg formula (2d sinθ = nλ), the interlayer spacing within the graphite is deduced to be approximately 0.37 nm. It is evident from these results that after CVD heating treatment, the PTCDA crystals transform to carbon nanomaterials, with nanofibers and floccules as the major morphologies. Thus, our experimental method directly yields a three-dimensional porous carbon nanostructure alongside one-dimensional nanofibers under low-pressure CVD heating conditions without using any templates. It is interesting and worthwhile to discover the emergence of diverse carbon nanomaterial morphologies and further explore the underlying mechanism forming these varied nanostructures.

Furthermore, a detailed examination of the carbon nanostructure, particularly the nanofibers, was conducted using high-magnification scanning electron microscopy (SEM) (Figure a) and transmission electron microscopy (TEM) (Figure b–e). It is observed in Figure a and b that carbon nanofibers intersect and aggregate into intricate networks. A single nanofiber, approximately 120 nm in diameter, is distinctly visible in Figure c, showing a smooth surface morphology. High-resolution transmission electron microscopy (HRTEM) was employed to “look into” the atomistic structures of these carbon nanomaterials. As shown in Figure d,e, the material consists of turbostratic nanodomains with a short-range graphited lamellar order, confirming that the synthesized carbon nanomaterials are nongraphitizable hard carbons in the long-range order. , The HRTEM image in Figure e reveals that the short-range graphite layers have an interlayer spacing of about 0.4 nm (Figure f), which aligns with the value derived from the XRD diffraction peak in Figure g. The structural order of the carbon nanomaterials was further analyzed by means of Raman spectroscopy. Only the D and G peaks can be observed, showing the values of 1341.1 cm–1 and 1588.6 cm–1 in Figure g. The 2D band at around 2685 cm–1 for graphite disappears completely for hard carbons, resulting from a reduced stacking number of graphite crystallites. The pronounced D band suggests a higher defect density in this turbostratic hard carbon nanomaterial compared to graphite. The intensity ratio of the D band to the G band (I D/I G) can quantify the overall disorder degree of hard carbons. , Furthermore, the size of the nanocrystals along the ab plane (La) can be determined by the following equation

La(nm)=(2.4×10−10)λnm4(IGID)

Therefore, the degree of structural disorder in the carbon material is quantitatively characterized by an I D/I G ratio of 1.0 and a crystallite size (La) of 19.2 nm. For further illustration, additional SEM and HRTEM images depicting the synthesized carbon nanomaterials with mixed morphological features are presented in Figure S1.

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(a) Magnified SEM image and (b) TEM image of carbon nanofiber networks; (c) TEM image of a single carbon nanofiber; (d,e) HRTEM images of carbon nanomaterials; (f) graph of spacing measurement for graphite layers; and (g) Raman spectrum of carbon nanomaterials.

In order to elucidate the formation mechanism of carbon nanostructures with diverse morphologies, we systematically monitored and recorded three key parameters during the pyrolysis process: material transformation, morphological evolution, and pressure variations within the CVD tube-furnace chamber. The pressure variations were continuously tracked with heating time. Figure e presents a comprehensive diagram with both the heating profile (black curve) and the corresponding pressure changes (gray curve). The initial temperature (T 0) represents the room temperature at which no thermal treatment had been applied to the PTCDA sample. Throughout the heating process, we observed four distinct pressure peaks, indicating significant pressure increases at specific temperature points designated as T 1, T 2, T 3, and T 4. These characteristic temperatures were identified at approximately 300 °C, 450 °C, 650 °C, and 1050 °C, respectively. The pronounced pressure increases are noteworthy and are supposed to be correlated with material transformation events occurring during the pyrolysis process.

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(a–d) SEM images and photographs of products inside the crucible (bottom-right corner) for the PTCDA sample calcined at temperatures T 1–T 4, respectively; (e) thermal treatment program (temperature vs time) and pressure changing vs time traces for the PTCDA sample; (f) XRD patterns and (g) Raman spectra for the PTCDA sample at different temperatures T 0–T 2; (h) XRD patterns and (i) Raman spectra for obtained carbon products at different temperatures T 3 and T 4.

To investigate the correlation between pressure variations and material transformations, we conducted a systematic analysis of the morphology and composition at four characteristic pyrolysis temperatures corresponding to the observed pressure peaks. SEM characterization, as presented in Figure a–d, reveals distinct morphological evolution at each temperature stage. At T 1, as seen from Figure a, while large PTCDA crystal fragments persist, some smaller particles disappear compared with the initial morphology shown in Figure c. This observation, coupled with the moderate pressure increase, suggests the evaporation of amorphous PTCDA components. Upon reaching T 2, it is observed in Figure b that large crystals break into smaller fragments, accompanied by the emergence of flocculent structures. These morphological changes at 300 and 450 °C are relatively modest, corresponding to the smaller pressure peaks at T 1 and T 2. The persistent presence of a PTCDA characteristic red color, shown in the bottom-right corners of Figure a,b, confirms that carbonization has not yet been initiated at these temperatures. A distinct transformation occurs at higher temperatures. Figure c,d reveals a complete morphological change at T 3 and T 4, characterized by the formation of twisted nanofiber aggregates and flocculent networks. This structural evolution coincides with substantial pressure increases, particularly at T 3, reaching the maximum intensity at T 4, as clearly demonstrated by the pressure profile in Figure e. According to the literature, these significant pressure changes are explained by two distinct chemical processes: at 650 °C, the decomposition of anhydride groups in PTCDA generates CO2, producing perylene free radicals that subsequently polymerize into polynaphthalene structures. Further heating to 1050 °C induces the C–H bond cleavage in the naphthene rings, resulting in hydrogen evolution and consequent pressure increase. Overall, the pressure variations observed during pyrolysis, coupled with corresponding morphological transformations at four specific temperatures, provide insights into the PTCDA carbonization process. The visual evidence from SEM images and the accompanying product photographs in Figure a–d evidence these transformations. The distinct color transition from red to black between 450 and 650 °C, as shown in the bottom-right corners of Figure b,c, marks the initiation of PTCDA carbonization.

Furthermore, we conducted composition and crystallinity characterizations of these samples calcined at T 1–T 4 through XRD and Raman spectroscopy, as presented in Figure f–i. In Figure f, the diffraction peak at 27.4° intensifies at T 1 compared to that at the initial state (T 0), indicating enhanced crystallinity of the PTCDA nanostructure. This observation suggests a potential recrystallization process occurring at this temperature stage. As the temperature progresses to T 2, a decrease in the 27.4° diffraction peak intensity is observed, signaling a partial loss of crystallinity. This structural change, along with the physicochemical properties of PTCDA, supports the hypothesis that the pressure increase at 450 °C primarily results from PTCDA sublimation. This interpretation is consistent with the morphological evolution observed in the corresponding SEM image (Figure b), where large PTCDA crystals are merged into smaller fragments and the emergence of flocculent structures is evident. Based on these results, we can conclude that the material composition remains essentially unchanged at T 1 and T 2, with only minor morphological alterations. It suggests that the transformations occurring below 450 °C are predominantly physical processes, including recrystallization and sublimation, rather than chemical decomposition.

A significant structural transformation occurs at T 3, as evidenced by the XRD pattern in Figure g. The characteristic crystalline peaks of PTCDA completely disappear, replaced by a broad diffraction feature centered at 24°, corresponding to the (002) plane of the amorphous carbon. This structural transition results from the thermal decomposition of the anhydride groups and subsequent dehydrogenation of aromatic rings in the PTCDA molecular structure. The release of gaseous byproducts during this process, primarily CO2 from anhydride decomposition, accounts for the substantial pressure increase observed at 650 °C. This structural transformation is directly correlated with the morphological changes observed in SEM images, where the PTCDA crystal carbon breaks down and nanofibers and flocculent structures form. The diffraction peak of amorphous carbon is also detected after heating at 1050 °C. The maximum pressure increase at this temperature suggests ongoing pyrolysis reactions. These observations collectively demonstrate that the carbonization process extends beyond the initial transformation at 650 °C with additional gas evolution and mass loss at higher temperatures.

Complementary to the XRD analysis, Raman spectroscopy with a 532 nm excitation wavelength was employed to further investigate the compositional evolution of materials calcined at different temperatures. The Raman spectra in Figure g reveal that PTCDA samples treated at T 1 and T 2 maintain vibrational characteristics similar to the untreated sample (T 0). Specifically, the characteristic modes at 1306 cm–1 and 1387 cm–1 are attributed to the C–H in-plane bending and stretching vibrations, respectively, while the prominent peak at 1575 cm–1 corresponds to C–C/CC vibrations within the perylene core. − These spectral features corroborate the XRD-derived conclusion that only physical transformations occur below 450 °C. A distinct transition in vibrational characteristics appears at higher temperatures, as evidenced in Figure i. At T 3, the Raman spectrum exhibits characteristic D and G bands at 1365 cm–1 and 1582 cm–1, respectively, which shift to 1333 cm–1 and 1589 cm–1 at T4. These spectral changes confirm the formation of turbostable carbon structures. Comparative analysis reveals an increased intensity ratio of the D band to G band (I D/I G) at T 4 (0.9) relative to T 3 (1.0), indicating enhanced structural disorder and defect concentration in the carbon nanomaterials at higher pyrolysis temperatures.

In conclusion, the characterization through both Raman spectroscopy and XRD analysis consistently demonstrates the progressive material transformation during the thermal treatment process. The structural evolution, evidenced by crystallinity and chemical composition changes, correlates with the pressure variations observed in the CVD chamber. The characteristic pressure profiles, featuring distinct sharp peaks at critical temperatures T 1 (300 °C), T 2 (450 °C), and T 3 (650 °C), are presented in Figures S2–S4. Supplementary SEM images and XRD spectra, corresponding to the results in Figure , provide a complete picture of the temperature-dependent transformation mechanism.

Through analyzing pressure changes and morphological transformations during heating, we identified key factors influencing material evolution. Our findings suggest the following: (1) the volatilization of amorphous PTCDA is crucial for forming carbon nanomaterials with flocculent and fibrous morphologies. (2) Reduced crystallinity of PTCDA molecules promotes the development of aerogel-like structures with mixed floccules and fibers. To validate these hypotheses, we conducted comparative experiments using a new batch of PTCDA with different crystallinity characteristics. While the SEM image reveals larger crystal sizes and fewer small particles in the new batch of PTCDA at the macroscopic scale (Figure b), XRD spectra show peak patterns comparable to those of the original sample (inset of Figure b). However, the degree of crystallinity is hard to distinguish on an atomic scale for these two PTCDA samples. Pressure monitoring during heating (Figure a) shows only two significant pressure increases at T 3 (∼700 °C) and T 4 (∼1000 °C), contrasting with the four peaks observed in the original sample. This absence of T 1 and T 2 peaks suggests inhibited amorphous PTCDA volatilization due to enhanced crystallinity. As a result, the obtained carbon materials from the new batch (Figure c) exhibited different morphologies comprising rough rods, blocks, and flaky structures with granular surfaces. The XRD pattern shown as the inset of Figure c presents the typical character of amorphous carbon, with a broad peak located at around 24° and a smaller one at 43°. These results highlight the critical role of pressure increases at T 1 and T 2, induced by PTCDA volatilization and crystal breakdown, in shaping carbon nanomaterial morphologies. This understanding enables morphology control through PTCDA volatilization and crystallinity manipulation, as exemplified by mixing PTCDA with volatile urea. The SEM image of the PTCDA/urea mixture with a mass ratio of 3:1 is shown in Figure e. The XRD pattern detected for the PTCDA/urea mixture (inset of Figure e) has the characteristic diffraction peaks for both PTCDA and urea. The PTCDA/urea mixture (Figure e) restored the four-pressure-peak profile during heating (Figure d), with enhanced T 1 peak intensity due to urea volatilization. The resulting carbon nanomaterials (Figure f) successfully replicated the desired nanofiber and flocculent morphologies, demonstrating the effectiveness of our approach in controlling carbon nanostructures through crystallinity modification and volatile additive incorporation. Supporting Information (Figure S5) further confirmed the reproducibility of these morphological features across different urea ratios and heating rates, validating our hypothesis that amorphous PTCDA volatilization (T 1) and crystal breakdown (T 2) are important in determining carbon nanomaterial morphologies. This discovery provides a reliable strategy for nanostructure control in carbon material synthesis.

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(a) Thermal treatment program (temperature vs time) and pressure changing vs time traces for a new batch of PTCDA sample; (b) SEM image and the XRD pattern (inset) for a new batch of PTCDA sample before heat treatment; (c) SEM image and the XRD pattern (inset) for a new batch of PTCDA sample after heating at 1050 °C; (d) thermal treatment program (temperature vs time) and pressure changing vs time traces for PTCDA/urea mixture; (e) SEM image and the XRD pattern (inset) for PTCDA/urea mixture before heat treatment; and (f) SEM image and the XRD pattern (inset) for PTCDA/urea mixture after heating at 1050 °C.

Furthermore, the critical properties of the synthesized hard carbon with varying morphologies, including the degree of ordering (defects), surface area, and porosity, were examined by using Raman spectroscopy, N2 adsorption/desorption isotherms, and XPS. The materials derived from PTCDA and PTCDA/urea after carbonization at 1050 °C, with the SEM image in Figure c,f, were designated as PTCDA-1050 and PTCDA/urea-1050, respectively. In Figure a, the Raman spectra reveal two characteristic peaks: the D band at 1350.2 cm–1, indicative of sp3 hybridization associated with disordered states, and the G band at 1586.6 cm–1, corresponding to sp2 hybridization of ordered graphite-like structures, present in both PTCDA/urea-1050 and PTCDA-1050 samples. The intensity ratios of the D to G peaks are 1.047 and 1.029, respectively, suggesting a predominance of disordered states and an increase in the number of carbon defects with the incorporation of urea in the PTCDA precursor. The N2 adsorption/desorption isotherms, depicted in Figure b, exhibit characteristic hysteresis loops, signifying the presence of H3-type mesoporous structures. This indicates that mesopores constitute the primary components of these materials. Additionally, the BET surface area calculations detailed in Table confirm the presence of micropores. Notably, the hard carbon synthesized from the PTCDA/urea mixture exhibits a significantly larger specific surface area (359.05 m2 g–1) compared to that derived from PTCDA alone (118.43 m2 g–1). The total pore volume of the former is 0.72 cm3 g–1, approximately 2.5 times greater than that of the latter (0.29 cm3 g–1). However, the average pore diameters of the two materials are comparable, at 10.9 and 10.3 nm, respectively. These findings are consistent with the SEM image analysis, which shows that the nanofiber and floccule morphologies of the hard carbons contribute to their large surface area and porosity, potentially enhancing the storage and transport of chemical products. XPS analysis was conducted to further investigate the functional group compositions of these materials. XPS full spectra of PTCDA-1050 and PTCDA/urea-1050 are shown in Figure S6. For both samples, the strength of C 1s is very high, and the strength of O 1s is weak compared with that of C 1s, indicating C as the main component. And the peak strength of C 1s is of little difference; however, the peak intensity of O 1s increased for PTCDA/urea-1050. As illustrated in Figure c, the most intense peak at 284.8 eV corresponds to C–C bonds in PTCDA-1050. In the O 1s XPS spectra (Figure e), the peak intensity of the C–O bond at 532.5 eV is minimal, resembling a weak impurity peak, indicating the near-complete disappearance of CO and C–O functional groups from PTCDA, leaving behind a predominantly hard carbon structure. Conversely, the C 1s XPS spectra for the hard carbon derived from the PTCDA/urea mixture (Figure d) show a reduced intensity of the C–C peak compared to the PTCDA-derived material, along with the presence of the O–CO bond at 288.5 eV. Furthermore, the O 1s peaks at 533.4 and 531.9 eV in Figure f confirm the existence of a small quantity of CO and C–O bonds, which are more pronounced than those in the PTCDA-derived hard carbons. These functional groups may offer additional active sites, which could potentially enhance the catalytic performance.

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(a) Raman spectra of PTCDA-1050 and PTCDA/urea-1050; (b) N2 adsorption/desorption isotherms of PTCDA-1050 and PTCDA/urea-1050; XPS spectra of PTCDA-1050 and PTCDA/urea-1050: (c) C 1s of PTCDA-1050; (d) C 1s of PTCDA/urea-1050; (e) O 1s of PTCDA-1050; and (f) O 1s of PTCDA/urea-1050.

1. BET Adsorption Parameters of the PTCDA-1050 and PTCDA/Urea-1050 Samples.

samples SBET/(m2 g–1) Smic/(m2 g–1) Smes/(m2 g–1) Vtotol/(cm3 g–1) pore diameter/(À)
PTCDA-1050 118.43 18.30 100.64 0.290 109.51
PTCDA/urea-1050 359.05 49.36 309.69 0.721 103.13
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The linking of structural and morphological properties of hard carbons with the electrochemical behavior for lithium and sodium cells has been proposed in the literature. Hard carbon nanomaterials possessing disordered porous nanostructures, large specific surface area, and more exposed active points are proven to provide channels for the rapid transport of lithium ions. As a result, they show superior rate cycle behaviors compared with conventional commercial graphite materials when used as a negative material for lithium-ion batteries, which provides new insights into developing novel carbon negative materials for fast charging/discharging lithium-ion batteries. Besides, hard carbon nanofibers are a type of 1D material with drastically different electronic properties than bulk materials and possess excellent thermal and mechanical properties, as well as high electrical conductivity. By exposing active sites and facilitating mass transfer in the solid state, hard carbon incorporating 1D nanofibers has the potential to significantly improve the performance of these materials for a variety of applications such as supercapacitor electrodes, lithium-ion battery anodes, catalytic supports, etc. Overall, the applications of the synthesized hard carbon, characterized by its floccular and fibrous morphologies, hierarchical porous structures, and active sites, in the fields of energy storage and catalysis are promising to explore in future studies.

4. Conclusions

In this study, we present a one-step, cost-effective, and straightforward method for synthesizing carbon nanomaterials, utilizing PTCDA as a precursor and a low-pressure chemical vapor deposition (CVD) heating apparatus. Our findings reveal that the synthesized carbon nanomaterials are classified as hard carbon materials, exhibiting nanostructured morphologies of floccules and fibers. By monitoring the pressure within the CVD chamber, we identified several peaks of pressure increase. These observations, combined with other characterization techniques, allowed us to examine the morphology and chemical composition of the materials at temperatures corresponding to these pressure increases. And this offers an unprecedentedly detailed analysis of the material transformation throughout the heating process. Our results underline the critical role of PTCDA volatilization at T 1 (300 °C) and PTCDA decomposition at T 2 (450 °C) in shaping the diverse morphologies of the carbon nanomaterials. The whole research provides a method to manipulate the morphology of nanomaterials by altering the crystallinity of the PTCDA sample and controlling the volatilization of molecules during the heating process. This work demonstrates the significance of investigation for the hard carbon formation process and mechanism and explores the approach for the controlled synthesis of hard carbon nanomaterials with diverse dimensional morphologies. By fine-tuning the structural properties in the future, the hard carbon products are promising to achieve significant surface areas, high porosity, and enhanced electrical conductivity, making them good candidates for applications in energy storage, catalysis, etc.

Supplementary Material

ao5c03191_si_001.pdf (1.1MB, pdf)

Acknowledgments

The work described in this paper was supported by grants from the Natural Science Foundation of Zhejiang Province (No. LY19F040005).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c03191.

  • Additional SEM and HRTEM images depicting the synthesized carbon nanomaterials; characteristic pressure profiles, featuring distinct sharp peaks at critical temperatures T 1 (300 °C), T 2 (450 °C), and T 3 (650 °C), and supplementary SEM images and XRD spectra of samples calcined at these temperatures; details of morphological features of hard carbon produced from PTCDA/urea mixtures with different urea ratios and heating rates, including SEM images, XRD patterns, and Raman spectra; and XPS full spectra of PTCDA-1050 and PTCDA/urea-1050 (PDF)

The authors declare no competing financial interest.

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