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[Atsuki Sakata](https://orcid.org/0009-0006-6330-1718), Hideki Abe, Takeshi Fujita, [Akira Yamaguchi](https://orcid.org/0000-0002-3550-4239), [Shigenori Ueda](https://orcid.org/0000-0001-9425-0614), Boborahimov Azamat Boborahim Ugli, [Masahiro Miyauchi](https://orcid.org/0000-0001-8889-2645), [Shusaku Shoji](https://orcid.org/0000-0002-8481-2633)

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[Synthesis of helical carbon nanofiber for greenhouse gas fixation and mechanistic insight into CO driven carbon formation](https://mdr.nims.go.jp/datasets/5d272be6-e45b-42a8-acb0-8e7d6e54c247)

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Synthesis of helical carbon nanofiber for greenhouse gas fixation and mechanistic insight into CO driven carbon formationSynthesis of helical carbon nanofiber for greenhouse gas fixation and mechanistic insight into CO driven carbon formationAtsuki Sakata a , Hideki Abe b, Takeshi Fujita c, Akira Yamaguchi a , Shigenori Ueda b ,  Boborahimov Azamat Boborahim Ugli b,d, Masahiro Miyauchi a,* , Shusaku Shoji b,*a Department of Materials Science and Engineering, School of Materials and Chemical Technology, Institute of Science Tokyo, 2-12-1 Ookayama, Meguro-ku, Tokyo 152- 8552, Japanb National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 304-0044, Japanc School of Engineering Science, Kochi University of Technology, 185 Miyanokuchi, Tosayamada, Kami City, Kochi 782-8502, Japand Graduate School of Science and Engineering, Saitama University, Shimo-Okubo 255, Saitama 338-8570, JapanA B S T R A C TThe utilization of carbon monoxide (CO), a key intermediate generated in CO2 reduction and syngas-based processes, is a central challenge in carbon management. Although several studies have explored carbon growth from CO, conventional approaches often suffer from limited structural control and heterogeneous products, hindering the valorization of CO into advanced materials. Here, we report a robust and potentially scalable method to directly synthesize helical carbon nanofibers (hCNFs) from CO under ambient pressure, enabling a carbon-negative pathway when integrated with dry reforming of methane (DRM, CH4 + CO2 → 2 H2 + 2CO). Using an earth-abundant Fe–Mo nanocomposite catalyst (Fe#MoOx), we achieve uniform Fe nanoparticle stabilization during reduction and carburization, which is essential for controlled helical growth. The resulting hCNFs exhibit uniform morphology (≈100 nm diameter; several micrometers in length), a narrow diameter distribution (σ < 11.6 nm), high graphitic order, and a carbon yield of ~20% relative to carbon input. Notably, structural characterization reveals an exceptionally high density of edge-plane exposure, approaching the theoretical maximum attainable for CNFs. This feature endows the material with intrinsically high surface energy and provides a foundation for exploiting edge-rich carbon architectures in applications. In situ mass spectrometry and Raman spectroscopy further elucidate the distinct sequence of reduction, carburization, and spiral growth. By transforming CO into nanostructured carbons of unprecedented structural quality, this study provides a practical solution to the CO bottleneck in CO2 utilization and establishes a platform for advanced applications in electrochemical storage, photothermal conversion, and biointerfaces.1. IntroductionCarbon dioxide (CO₂) is considered a major greenhouse gas responsible for global warming. Consequently, substantial research has been devoted to reducing CO₂ emissions in key industrial sectors such as steelmaking, power generation, and cement manufacturing [1–3]. The Paris Agreement on climate change aims to limit the global temperature increase to well below 2 ◦C by imposing international obligations for net reductions in CO₂ emissions. Although technologies such as carbon capture and storage (CCS), which involve the direct underground sequestration of CO₂, are under active investigation to prevent its atmospheric release, numerous technical challenges remain, including issues related to long-term storage stability and potential leakage. [4,5].In recent decades, various approaches have been researched to convert CO2 into useful chemicals, including thermochemical, electrochemical, photocatalytic, and biological methods [6–9]. Among these, CO₂ hydrogenation — in which CO₂ reacts with H₂ to produce fuels or value-added chemicals via pathways such as methanation (Sabatier reaction), methanol synthesis, and the reverse water-gas shift (RWGS) reaction — has attracted particular attention as a scalable thermochemical route [10,11]. A promising extension of this concept is the conversion of CO₂-derived gases into solid carbon nanomaterials, which offers permanent carbon fixation while yielding high-value functional carbon products. However, these methods have not yet been widely implemented due to low reaction efficiency and generation of CO2 during the process. To reduce the annual CO2 emission of 40Gt, it is necessary to establish efficient, large-scale, and CO2-neutral CO2 fixation methods. [12,13].Dry reforming of methane (DRM) is a highly promising method for CO2 reduction. This reaction (chemical Eq. 1) enables the conversion of methane and CO2 into mixture of hydrogen and carbon monoxide [14–19]. CH4 and CO2 are two of the top greenhouse gases, so the significance of converting them is immense.Our previous work established a catalyst capable of maintaining * Corresponding authors.E-mail addresses: mmiyauchi@ceram.titech.ac.jp (M. Miyauchi), SHOJI.Shusaku@nims.go.jp (S. Shoji). Contents lists available at ScienceDirectJournal of CO2 Utilizationjournal homepage: www.elsevier.com/locate/jcouhttps://doi.org/10.1016/j.jcou.2026.103471Received 29 September 2025; Received in revised form 3 May 2026; Accepted 20 May 2026  Journal of CO2 Utilization 109 (2026) 103471 Available online 26 May 2026 2212-9820/© 2026 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). https://orcid.org/0009-0006-6330-1718https://orcid.org/0009-0006-6330-1718https://orcid.org/0000-0002-3550-4239https://orcid.org/0000-0002-3550-4239https://orcid.org/0000-0001-9425-0614https://orcid.org/0000-0001-9425-0614https://orcid.org/0000-0001-8889-2645https://orcid.org/0000-0001-8889-2645https://orcid.org/0000-0002-8481-2633https://orcid.org/0000-0002-8481-2633mailto:mmiyauchi@ceram.titech.ac.jpmailto:SHOJI.Shusaku@nims.go.jpwww.sciencedirect.com/science/journal/22129820https://www.elsevier.com/locate/jcouhttps://doi.org/10.1016/j.jcou.2026.103471https://doi.org/10.1016/j.jcou.2026.103471http://creativecommons.org/licenses/by/4.0/stable activity for more than 1000 h, with no detectable side reactions such as coking. [20]. During this process, hydrogen is purified in high purity through a permeable membrane and is eliminated from the system. However, in systems represented by DRM, CO cannot be effectively utilized, leading to issues such as catalyst deactivation due to unexpected carbon deposition. If CO produced from greenhouse gases can be fixed in a useful form, it would have a significant impact on the environment, industry, and science.Here, we report a method for producing carbon nanofibers (CNFs) with a uniform diameter from iron(Fe) and molybdenum (Mo) nanocomposites (Fe#MoOx) using CO as raw material. These CNFs are helical in structure and feature highly exposed edge surface of carbon (hCNFs). Additionally, they exhibit strong hydrophilicity and disperse well in water. hCNFs are obtained by heating the Fe-Mo composite in a CO atmosphere. Typically, methods such as CVD process extracts carbon from CO to CNF(chemical Eqs. 2,3), but they are known to produce a lot of coarse and randomly formed carbon [21–23]. In contrast, our method utilizes a specific "Mo-pinning effect" to enforce a helical morphology even at mild temperatures (450 ◦C) under atmospheric pressure. Unlike conventional iron-catalyzed methods that often result in random carbon aggregates due to uncontrolled sintering, our Fe–Mo design regulates the nanoparticle size within a critical window, creating high-density active edges. This precise morphological control offers a distinct advantage over disordered carbons typically synthesized via the Boudouard reaction, providing a robust platform for solid carbon fixation. CH4 + CO2 → 2H2 + 2CO ΔH̊= +247 kJ/mol                                 (1)2CO → C + CO2 ΔH̊= -172kJ/mol                                                  (2)CO + H2 → C + H2O ΔH̊= -131kJ/mol                                           (3)Additionally, Reaction (1) is an endothermic reaction, while Reaction (2) is an exothermic reaction. Therefore, the most of reaction energy for (1) can be obtained by driving it with (2,3). By combining these two reactions, methane, a greenhouse gas, can be converted into high-value hCNF in a separate reaction environment without exhausting CO2. Furthermore, this process simultaneously enables the extraction of hydrogen, which is essential for society. This makes it promising for applications in electrode materials, photothermal conversion materials, and biological applications. Furthermore, by upgrading CO into valuable solids of hCNF derived from greenhouse gases such as CH4 and CO2, our approach is expected to contribute to addressing environmental issues.2. Experimental2.1. Catalyst synthesisA precursor plate of Fe-Mo alloy (Taiyo Koko Co.,LTD.) was sectioned into 6 mm × 6 mm squares and placed inside a quartz flow reactor, which was purged with argon gas. The gas was switched to a CO and O2 flow with a volumetric ratio of 2:1, and heat treatment was conducted at 600 ◦C for 12 h. The resulting plate (Fe#MoOx) was used as a catalyst for subsequent carbon production.2.2. Synthesis of hCNFsThe reactor was heated to 450 ◦C under the Ar atmosphere, after which a mixed gas containing 10% CO and 5% H₂ was introduced. The reaction was maintained for approximately 15 h. Preliminary tests with shorter (~5 h) and longer (20–25 h) durations revealed that ~15 h provided the optimum balance between yield and structural uniformity of the obtained hCNFs. The synthesis temperature of 450 ◦C was chosen based on preliminary screening in the 400–600 ◦C range, in which lower temperatures led to quite slow carbon growth rate and higher temperatures promoted random growth of carbon. The CO/H₂ composition (10%/5%) was selected to ensure both effective reduction/carburization of Fe to Fe₃C and suppression of excessive CO disproportionation, thereby yielding uniform helical CNFs. After the reaction, the reactor was cooled to room temperature under Ar atmosphere, and the sample was retrieved for subsequent analysis.2.3. Characterization and measurementA quadrupole mass spectrometer (QMS: Qulee BGM102, Ulvac) was used to continuously monitor the gas generated throughout the reaction in real time. The samples, both before and after the reaction, were characterized using X-ray diffraction (XRD: SmartLab, Rigaku), hard X- ray photoelectron spectroscopy (HAXPES), scanning electron microscopy (SEM: SU8230, Hitachi), scanning transmission electron microscopy (STEM: JEM-ARM200F NEOARM), and Raman spectroscopy (PR- 1w, JASCO) to analyze structural, chemical, and morphological properties. For morphology analysis, SEM images were used to measure fiber diameters at 20 points across different regions of the sample. The average diameter and standard deviation were calculated as 96.4 nm, and a narrow size distribution (σ < 11.6 nm) was confirmed. Raman spectra were collected with a 532 nm laser, and the ID/IG ratio was calculated by fitting the D (~1350 cm⁻¹) and G (~1580 cm⁻¹) bands using Lorentzian functions, providing a measure of the graphitization degree and defect density of the synthesized hCNFs.2.4. Performance evaluationThe adsorption performance was evaluated using Methylene Blue (MB) as a model pollutant. In a typical test, 20 mg of the adsorbent (hCNF, or benchmark materials including Multi Wall Carbon Nanotubes (MWCNTs), Activated Carbon, and Graphene) was dispersed in 10 mL of an aqueous MB solution with an initial concentration of 100 mg/L. The suspension was super sonicated then stirred continuously in the dark at room temperature to reach adsorption-desorption equilibrium. At predetermined time intervals, the carbon mixed solution were filtered to remove the adsorbent. The residual MB concentration was determined by measuring the absorbance at 664 nm using a UV–vis spectrophotometer. The adsorption capacity at 60 min (q60) and surface utilization efficiency (ηSU) were calculated based on the mass balance principle and the specific surface area (SBET) of each material.The photothermal conversion capability was assessed using a solar simulator(ASAHI SPECTRA, HAL-320) equipped with an AM 1.5 G filter. The light intensity was calibrated to 100 mW/cm2 (1 sun). The sample was then irradiated with simulated sunlight, and the surface temperature evolution was monitored in real-time using a radiative thermometer (JAPANSENSOR, FLHX-TNE). All measurements were conducted under ambient conditions.3. Result and discussion3.1. Morphology of hCNFsFig. 1 shows the SEM and STEM images of synthesized hCNFs. As shown in Fig. 1a, the carbon produced from Fe#MoOx exhibits a helical structure. The synthesized hCNFs had a diameter of approximately 100 nm, a length of several micrometers, and a helical pitch of 250 nm, forming a continuous helical structure. All hCNFs obtained were uniform and exhibited consistent diameters(Fig. 1a). Furthermore, High resolution STEM analysis revealed that the hCNFs had a high exposure of edge planes(Fig. 1c, e and Figure S1). In contrast, carbon generated from iron metal, which is a typical catalyst for carbon production, under the same conditions was not observed to form fibers but instead grew in a random manner(Fig. 1b).Typically, in carbon nanotubes (CNTs), the basal planes of carbon are rolled up, leaving the edges unexposed [24–26]. In contrast, for graphene, the edges of its basal planes are exposed, representing high-energy sites where functional groups such as A. Sakata et al.                                                                                                                                                                                                                                  Journal of CO2 Utilization 109 (2026) 103471 2 Figure. 1. SEM, TEM, and STEM-EDX images of synthesized carbon. (a)SEM image of hCNF from Fe#MoOx precursor. (b)SEM image of amorphous carbon from Fe precursor. (c) TEM image of the tip of an hCNF and selected-area electron diffraction pattern. (d) STEM-EDX elemental maps of Fe, Mo and C acquired from the same region as panel (c), showing Fe in the particle core and Mo distributed at the periphery. (e) High-resolution STEM images of an hCNF, in which the exposed graphitic edges are clearly visible.Figure. 2. Schematic diagrams of graphene(a) and commercially available CNFs with highly exposed edges (Pyrograf® from Aldrich), along with the STEM image and schematic diagram of hCNFs(c).A. Sakata et al.                                                                                                                                                                                                                                  Journal of CO2 Utilization 109 (2026) 103471 3 hydroxyl (OH) or carboxyl (COOH) groups are more likely to form [22, 27,28](Fig. 2a). In our hCNFs, the helical structure facilitates significant exposure of these edge planes (0001), as evidenced by the analysis. Fig. 2b illustrates a schematic diagram of a commercially available CNF (Pyrograf®) that is generally recognized for having highly exposed edges. The angle θ between the outer peripheral surface and the edge planes of graphite is reported to be approximately 25◦. Based on this, the number of edge planes per 10 nm, or the density of exposed edges, is calculated to be 14–20 edges per 10 nm. Theoretically, the edge density reaches 29.4 edges/10 nm when θ is 90◦, as in the case of graphite.In contrast, for hCNFs, θ is significantly larger at 60◦, resulting in a calculated edge density of ~25 edges/10 nm(Fig. 2c). This corresponds to approximately 87% of the theoretical maximum edge density of ideal graphene edges. This geometric feature is critical for the enhanced surface activity discussed later. This indicates that the hCNFs exhibit an edge density several tens of percent higher than that of CNFs, which are generally considered to have highly exposed edges.It is known that the size of CNFs varies depending on the size of the catalyst, when the size is a few nanometers, carbon nanotubes (CNTs) are formed, while at around 100 nm, helical CNFs are produced, and when the size exceeds that, random and coarse carbon fibers are more likely to form [29,30]. Fig. 3 shows the surface structure of Fe#MoOx catalyst during the CO driven carbon formation reaction over time. By heating in a CO atmosphere, Fe-based particles are formed on the surface in the sequence of a → b → c, and after 4 h, it is observed that the Fe particles are being pushed up. It is revealed from the subsequent HAXPES measurements, described in the next section, that the Fe particles are in the form of cementite (Fe3C)(FigureS2). During this process, the catalyst surface becomes homogenized in the range of 50–100 nm, and once the size of Fe3C reaches this range, hCNFs are rapidly formed (Fig. 3d). In other words, the Fe#MoOx catalyst promotes the formation of homogeneous Fe based nano-particles on the surface, and once these particles reach the optimal size, it functions as a substrate catalyst to generate uniform hCNFs. Furthermore, STEM-EDX analysis in Figure 1d reveals that Fe is concentrated in the nanoparticle core (consistent with the Fe₃C identified by XRD), while Mo is distributed mainly at the periphery of the particle and in the surrounding matrix, rather than at the apex from which carbon is extruded. This spatial segregation, combined with HAXPES analysis showing that Mo persists predominantly in a Mo⁴⁺ (MoO₂) oxidic state with only a minor surface-carbide contribution throughout the reaction (Figure S2) — in contrast to Fe, which is predominantly transformed to Fe3C and Fe0 as the operative phases for carbon growth (Figure S2; Fig. 4b) — supports the interpretation that Mo acts as a structural stabilizer at the Fe₃C surface rather than a direct catalytic site for CO dissociation, ensuring a narrow size distribution (50–100 nm) critical for hCNF growth. This size control prevents random carbon deposition and enables consistent helical morphology. [31] Our time-resolved analysis confirms that without Mo, Fe particles rapidly sinter into large aggregates (>200 nm), leading to amorphous carbon formation (Figure 1b). Thus, Mo acts as a critical structural stabilizer ("pinning agent") that confines Fe particles to the 50–100 nm window essential for helical growth. Under our standard conditions (10% CO + 5% H₂ in Ar, 50 sccm total flow, ~10 g Fe#MoOₓ substrate, 450 ◦C, 15 h), approximately 20% of the supplied CO is converted to solid carbon — estimated by integration of the in-situ QMS CO and CO₂ traces (Fig. 5a) — corresponding to a carbon productivity of ≈ 24 mg_C per gram of bulk substrate over 15 h (≈ 1.6 mg_C g⁻¹ h⁻¹). Because the catalytically active region is confined to the surface of the substrate, the per-gram-of-Fe productivity is considerably higher than this bulk-substrate value.Fig. 3. SEM image of the substrate surface during the reaction(a)0 h (b)2 h (c)4 h (d)6 h.A. Sakata et al.                                                                                                                                                                                                                                  Journal of CO2 Utilization 109 (2026) 103471 4 3.2. Spectroscopic analysis of Fe#MoOx and hCNFThe HAXPES spectra(Figure S3) indicates that synthesized carbon exhibits peaks distinct from those of carbon nanotubes(CNTs), revealing the presence of a higher proportion of graphitic bonds [32,33]. This suggests that the synthesized carbon is closer to carbon nanofibers (CNFs) rather than CNTs. This result is good agreement with the STEM image, where the (0001) planes of graphite are stacked and exposed. HAXPES analysis of the Mo 3d region (Figure S2) confirms that Mo exists predominantly as Mo⁴⁺ (MoO₂; 3d₅/₂ ≈ 229.5 eV [34,35]) with a minor Mo⁶⁺ (MoO₃) contribution before the reaction, consistent with a mixed-valence surface-oxide environment. After the CO/H₂ treatment, the Mo 3d spectrum broadens, but Mo⁴⁺ remains the dominant component, with additional minor contributions from residual Mo⁶⁺ and a weak low-binding-energy shoulder near ~228 eV consistent with a small fraction of surface Mo²⁺ carbide [36]. Importantly, Mo does not undergo a clean transformation to a single reduced or carbide phase — in marked contrast to Fe, which is predominantly transformed to Fe₃C and Fe⁰ as the operative phases for carbon growth confirmed by XRD of the separated hCNFs (Fig. 4b) and HAXPES(Figure S2). This chemical contrast supports the role of Mo as a structural stabilizer rather than an active carbon-growth site.The XRD analysis prior to the reaction revealed that the main phase of Fe#MoOx is a mixture of Fe2O3 and Fe (Fig. 4a, bottom). After the reaction, the main phase was identified as iron carbide (cementite, Fe3C) (Fig. 4a, top). the strongest Fe peak observed at this stage originated from the bulk substrate, which did not participate in the reaction. In the hCNF grew on the surface, a strong peak attributed to carbon was observed near 26◦, along with peaks corresponding to Fe3C, whereas no peaks corresponding to metallic Fe were detected (Fig. 4b). This result is consistent with the STEM image and electron diffraction pattern of Fe3C incorporated within the hCNF, as shown in Figure 1c.The HAXPES analysis further confirmed the presence of Fe3C even in the separated carbon generated on the surface of Fe#MoOx, indicating that Fe3C generated from Fe#MoOx served as the active site for hCNF growth(Figure S2). Previous studies have shown the same phenomenon, for example, hematite (iron oxide) being reduced to iron and subsequently carburized into cementite under a CO and H2 atmosphere to produce carbon fibers [37–39]. Therefore, it can be inferred that a similar reaction is occurring in this case. It is worth noting that the residual Fe3C cores impart magnetic properties to the hCNFs, allowing for easy magnetic separation in slurry-type applications such as water treatment. For applications where high carbon purity is strictly required (e.g., electrode materials), we confirmed that a simple acid treatment (e. g., aqua regia) can effectively remove the catalyst residues.3.3. Time resolved catalytic changes and hCNF growthFig. 5 shows the temporal evolution of in-situ QMS signals throughout the reaction. From these data, qualitative changes in the concentrations of CO and CO₂ are clearly observed over time (Fig. 5a). In particular, during the initial stage of the reaction (Fig. 5b), a substantial decrease in the CO signal indicates that CO is rapidly consumed at the onset. Complementary in-situ Raman spectroscopy results (Figure S4) further reveal that carbon formation does not occur immediately but appears after a certain induction period following the introduction of CO and H₂ gases. Notably, the Raman spectra of the hCNFs show characteristic D and G bands at ~1350 and ~1580 cm⁻¹ , respectively, with an intensity ratio ID/IG ≈ 2.96. This relatively high ratio indicates that, while the carbon possesses graphitic layers, it also contains a large number of edge sites and structural defects that strongly contribute to the D band intensity. This interpretation is consistent with the STEM observations, which clearly reveal graphitic stacking accompanied by abundant exposed edges. Thus, the carbon formed can be regarded as graphitic in nature but with a high density of edge-related defects, in line with the structural features observed in the microscopic analysis [40].These observations suggest that, in the initial stage of the reaction, the reduction of iron oxide and its carburization into cementite take Fig. 4. (a) XRD patterns of Fe#MoOx before the reaction (orange, bottom) and after the reaction (blue, top), showing the transition from Fe₂O₃/Fe to Fe₃C. (b) XRD pattern of the generated hCNFs separated from the Fe#MoOx substrate, displaying peaks of graphitic carbon and Fe₃C.Fig. 5. (a) Temporal evolution of QMass signals of CO and CO₂ during the reaction. (b) Magnified view of the CO signal at the initial stage, highlighting its rapid consumption.A. Sakata et al.                                                                                                                                                                                                                                  Journal of CO2 Utilization 109 (2026) 103471 5 place. This reaction is consistent with those observed in direct reduction processes of iron under CO and H₂ atmospheres [37,38,41], with prior examples reported in the literature. The subsequent formation of carbon, following carburization, is corroborated by the changes observed in Raman spectra. Therefore, it can be concluded that the initial stage of the reaction involves the reduction and carburization of iron, while carbon formation occurs only after carburization has progressed to a certain extent.When the gas was changed to CO only instead of CO + H2, no carbon was generated, and XRD showed peaks corresponding to Fe3O4. This suggests that hydrogen plays a crucial role in the reduction of iron. Specifically, it indicates that it is necessary to reduce iron as a precursor to the transformation into Fe3C. furthermore, after carbon formation, when the supply of H2 gas was stopped and only CO was flowed, no CO2 generation was observed. Similarly, this suggests that the presence of hydrogen is essential for the growth of carbon as well.3.4. Adsorption performance and benchmarkingTo evaluate the surface activity and industrial potential, we conducted Methylene Blue (MB) adsorption tests and compared the results with three representative benchmarks: Activated Carbon (AC), commercial Multi-Walled Carbon Nanotubes (MWCNTs), and Graphene Nanoplatelets. As summarized in Table 1, hCNF exhibited an equilibrium adsorption capacity (q60) of 32.3 mg/g. Although this value is comparable to commercial MWCNTs (30.3 mg/g), the specific surface area of hCNF (46.1 m2/g) is less than half that of MWCNTs (119.6 m2/ g). Consequently, the Surface Utilization Efficiency (ηSU) of hCNF is calculated to be 0.70 mg/m2, which is ~2.8 times higher than that of MWCNTs (0.25 mg/m2) and ~13 times higher than Activated Carbon (0.05 mg/m2). This confirms that the hCNF surface is qualitatively superior (higher site density) compared to the basal-plane dominated walls of CNTs. Furthermore, in terms of kinetics (Figure S5), hCNF showed rapid adsorption comparable to AC in the initial stage, proving the excellent accessibility of the exposed helical edges. Notably, the experimental adsorption capacity of hCNF (38.4 mg/g) is approximately 82% of that of Graphene Nanoplatelets (46.6 mg/g, edge-rich reference). This ratio is in reasonable agreement with the geometric prediction derived in Section 3.1, where the edge density of hCNF was calculated to be ~87% of the theoretical maximum. This quantitative consistency supports the validity of our simplified helical model.Additionally, regarding energy applications, we demonstrated the photothermal capability (Figure S6). The hCNF film exhibited a rapid temperature rise of 20 ◦C within 25 min under simulated sunlight, outperforming Graphene Nanoplatelets and MWCNTs due to enhanced light trapping within the helical structure. The superior photothermal heating of hCNFs can be well explained by their specific geometry, consistent with established light-trapping mechanisms. Similar to the multiple scattering effects reported in helical graphitic carbon nitrides, the coiled structure likely induces internal reflections that trap incident photons [42]. Additionally, the ~100 nm diameter suggests enhanced light absorption, likely due to multiple scattering and internal reflections within the helical structure, similar to light-trapping effects observed in other structured carbons. These precedents strongly indicate that the helical architecture effectively functions as a "light trap," resulting in the observed enhancement in photothermal conversion [43].4. ConclusionCO-based CNF synthesis typically results in random growth modes, yielding only coarse carbon. However, by using the Fe-Mo composite Fe#MoOx, we successfully controlled carbon formation and synthesize high-quality hCNF. Through in-situ observations combining QMS gas analysis and Raman spectroscopy, we elucidated the reaction mechanism and identified the cause of carbon growth mode. In a H2 and CO atmosphere, iron is reduced and carburized into iron carbide (cementite, Fe3C), serving as the carbon source. The Fe–Mo nanocomposite (Fe#MoOx) enabled uniform hCNF growth by controlling Fe nanoparticle size through Mo-assisted structural stabilization during reduction and carburization. This size control is essential because Fe particles outside the 50–100 nm range lead to CNT or amorphous carbon formation, whereas optimized Fe particle sizes favor helical CNF growth. Under standard conditions, the carbon productivity is ≈ 24 mg_C per gram of bulk Fe#MoOₓ substrate over 15 h (CO conversion ≈ 20%, estimated from in-situ QMS). We note that the catalyst is single-use in its present form, since the active Fe nanoparticles become encapsulated into the growing hCNFs through a tip-growth mechanism; scalable industrial deployment will therefore require a regeneration or continuous- extraction strategy, which is a subject of ongoing work. Combining DRM with CO fixation is effective as it reduces greenhouse gases like methane and carbon dioxide while producing hydrogen. This study aims to achieve negative emissions of greenhouse gases and demonstrates the utility of greenhouse gas fixation technology through the synthesis of functional carbon.CRediT authorship contribution statementBoborahimov Azamat Boborahim Ugli: Data curation. Masahiro Miyauchi: Writing – review & editing, Supervision, Conceptualization. Shusaku Shoji: Writing – review & editing, Supervision, Conceptualization. Hideki Abe: Project administration, Conceptualization. Takeshi Fujita: Investigation, Formal analysis, Data curation. Akira Yamaguchi: Conceptualization. Shigenori Ueda: Investigation, Data curation. Atsuki Sakata: Writing – original draft, Investigation, Data curation.Declaration of Competing InterestThe authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Hideki Abe reports financial support was provided by New Energy and Industrial Technology Development Organization. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.AcknowledgementsThis work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Feasibility Study Program #23810887 and The Iketani Science and Technology Table 1 Comparison of adsorption performance and surface characteristics of hCNF and commercial carbon benchmarks.Materials Absorption Capacity q60(mgMB/gcarbons) BET surface area(m2/g) Surface Utilization Efficiency(mgMB/m2surface)hCNF 32.3 46.05 0.701MWCNTs 30.3 119.6 0.253Activated Carbon 48.2 910.6 0.053Graphene 45.1 8.16[a] 5.527[a]Note: a The low surface area of graphene is attributed to sheet aggregation.A. Sakata et al.                                                                                                                                                                                                                                  Journal of CO2 Utilization 109 (2026) 103471 6 Foundation (Grant No. 0371200-A). We thank Taiyo Koko CO.,LTD. for providing us the samples. The synchrotron radiation experiments were performed at the BL09XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2024A1536).Content of Interest• Demonstrates a robust and scalable method to directly synthesize high-quality helical carbon nanofibers (hCNFs) from CO, a key intermediate in CO₂ utilization and syngas chemistry.• Integrates catalytic CO fixation with dry reforming of methane (DRM), providing a carbon-negative route to durable nanostructured carbon materials.• Highlights a unique strategy to valorize CO₂-derived intermediates into high-value products, directly addressing the journal’s focus on sustainable CO₂ conversion and utilization technologies.• Offers industrial relevance by coupling greenhouse-gas mitigation with the scalable production of advanced carbon nanomaterials.Appendix A. 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Journal of CO2 Utilization 109 (2026) 103471 8 https://doi.org/10.1002/srin.202200043https://doi.org/10.1007/BF02654354https://doi.org/10.1007/s40831-022-00601-0https://doi.org/10.1007/s40831-022-00601-0https://doi.org/10.1021/nl8032697https://doi.org/10.1021/nl8032697https://doi.org/10.1007/BF02814970https://doi.org/10.1007/BF02814970http://refhub.elsevier.com/S2212-9820(26)00160-5/sbref42http://refhub.elsevier.com/S2212-9820(26)00160-5/sbref42http://refhub.elsevier.com/S2212-9820(26)00160-5/sbref42http://refhub.elsevier.com/S2212-9820(26)00160-5/sbref43http://refhub.elsevier.com/S2212-9820(26)00160-5/sbref43http://refhub.elsevier.com/S2212-9820(26)00160-5/sbref43 Synthesis of helical carbon nanofiber for greenhouse gas fixation and mechanistic insight into CO driven carbon formation 1 Introduction 2 Experimental 2.1 Catalyst synthesis 2.2 Synthesis of hCNFs 2.3 Characterization and measurement 2.4 Performance evaluation 3 Result and discussion 3.1 Morphology of hCNFs 3.2 Spectroscopic analysis of Fe#MoOx and hCNF 3.3 Time resolved catalytic changes and hCNF growth 3.4 Adsorption performance and benchmarking 4 Conclusion CRediT authorship contribution statement Declaration of Competing Interest Acknowledgements Content of Interest Appendix A Supporting information Data availability References