Porphyrin Nanotubes Research Articles

Ultra-low power carbon nanotube/porphyrin synaptic arrays for persistent photoconductivity and neuromorphic computing

Developing devices with a wide-temperature range persistent photoconductivity (PPC) and ultra-low power consumption remains a significant challenge for optical synaptic devices used in neuromorphic computing. By harnessing the PPC properties in materials, it can achieve optical storage and neuromorphic computing, surpassing the von Neuman architecture-based systems. However, previous research implemented PPC required additional gate voltages and low temperatures, which need additional energy consumption and PPC cannot be achieved across a wide temperature range. Here, we fabricated a simple heterojunctions using zinc(II)-meso-tetraphenyl porphyrin (ZnTPP) and single-walled carbon nanotubes (SWCNTs). By leveraging the strong binding energy at the heterojunction interface and the unique band structure, the heterojunction achieved PPC over an exceptionally wide temperature range (77 K-400 K). Remarkably, it demonstrated nonvolatile storage for up to 2×104 s, without additional gate voltage. The minimum energy consumption for each synaptic event is as low as 6.5 aJ. Furthermore, we successfully demonstrate the feasibility to manufacture a flexible wafer-scale array utilizing this heterojunction. We applied it to autonomous driving under extreme temperatures and achieved as a high impressive accuracy rate as 94.5%. This tunable and stable wide-temperature PPC capability holds promise for ultra-low-power neuromorphic computing.

Porphyrin nanotubes based on a hydrogen-bonded organic framework.

Tubular structures offer a wide variety of applications; therefore, designing such materials with distinct dimensions is highly desirable yet challenging. In the current report, we have demonstrated the synthesis of a one-dimensional (1D) tubular assembly comprising porphyrin nanoring subunits. The porphyrin nanoring (PNR) 2 bearing ester groups was synthesized via Pt-mediated cyclization and then hydrolyzed to obtain PNR 3 with carboxylic groups. Under optimized conditions, porphyrin nanotubes (PNTs) were formed through hydrogen bonding between the carboxylic groups of 3. The morphology investigated by both SEM and TEM displayed well-defined arrays of nanotube bundles up to several micrometers long. Small crystals of PNTs were obtained by heating a solution of 3 in DMSO. High-resolution transmission electron microscopy (HR-TEM) accompanied by selected-area electron diffraction (SAED) exhibited a line of diffractions with d-spacing values of 6.17, 3.08, 2.07, and 1.57 Å. The miller indices of these diffractions could be assigned as 300, 600, 900, and 1200, respectively, suggesting that an ordered structure of 1D PNTs has been formed.

Diameter-dependent structural and electronic property of fused porphyrin nanotubes: A density functional study

We have systematically carried out a density functional theory-based investigation to understand the structural and electronic properties of various fused metalloporphyrin nanotubes (MPNT; M = Sc and Ti) by varying their diameters ranging from 7.91 Å to 18.70 Å for ScPNT and 7.90 Å to 18.59 Å for TiPNT. Binding energies and curvature energies are calculated to access the binding strength and stability of the nanotubes (NTs). From band structure and density of states, it is observed that the ScPNTs are metallic in nature and TiPNTs are semiconductors with small band gaps. The energy gap increases with increasing tube diameter. Our study also indicates that the transition metal atoms play an important role in determining the electrical nature (metallic or semiconducting) of the NTs. Furthermore, work functions for the fused NTs are found to decrease with increasing tube diameter. These results may have direct relevance to the technological applications in terms of band gap engineering or controlled thermionic emission.

Surface Engineering of Porphyrin Coordination on a Carbon Nanotube for Efficient Hydrogen Evolution

AbstractFunctionalised porphyrins have attracted considerable attention owing to their ability to coordinate with numerous metal ions. We design and synthesize a novel composite system based on functionalised Pt(II)‐porphyrin and multiwalled carbon nanotubes for hydrogen evolution reaction (HER) electrocatalysis. Rather than qualitatively integrating porphyrin with carbon nanotubes, the optimized Pt(II)‐porphyrin loading and precisely morphology control could be fine‐tuned and quantitatively premeditated by using fluorescence and UV‐vis titration based on the unique π–π interactions behaviour of the porphyrin for the first time. The as‐prepared composites exhibit superior HER catalytic activity with an overpotential of 35 mV for an electrocatalytic current density of j=−10 mA cm−2 and a Tafel slope of 32.3 mV dec−1. Moreover, such catalysts do not exhibit observable activity decay after a 1000‐cycle stability test, thus demonstrating their excellent durability. To further reduce Pt loading to achieve cost efficiency, we develop a Pt−Co bimetallic HER catalyst with an ultra‐low platinum content through a straightforward synthesis pathway with high catalytic activity and stability. This study provides a new approach for preparing a highly active and stable HER catalyst, which could potentially be extended to other reactions to improve the utilization efficiency of precious metal catalysts.

Photoresponsive Porphyrin Nanotubes of Meso-tetra(4-Sulfonatophenyl)Porphyrin and Sn(IV) meso-tetra(4-pyridyl)porphyrin.

Porphyrin macrocycles and their supramolecular nanoassemblies are being widely explored in energy harvesting, sensor development, catalysis, and medicine because of a good tunability of their light-induced charge separation and electron/energy transfer properties. In the present work, we prepared and studied photoresponsive porphyrin nanotubes formed by the self-assembly of meso-tetrakis(4-sulfonatophenyl)porphyrin and Sn(IV) meso-tetra(4-pyridyl)porphyrin. Scanning electron microscopy and transmission electron microscopy showed that these tubular nanostructures were hollow with open ends and their length was 0.4–0.8 μm, the inner diameter was 7–15 nm, and the outer diameter was 30–70 nm. Porphyrin tectons, H4 : Sn(IV)TPyP4+, self-assemble into the nanotubes in a ratio of 2:1, respectively, as determined by the elemental analysis. The photoconductivity of the porphyrin nanotubes was determined to be as high as 3.1 × 10−4 S m−1, and the dependence of the photoconductance on distance and temperature was investigated. Excitation of the Q-band region with a Q-band of SnTPyP4+ (550–552 nm) and the band at 714 nm, which is associated with J-aggregation, was responsible for about 34 % of the photoconductive activity of the H4-Sn(IV)TPyP4+ porphyrin nanotubes. The sensor properties of the H4- Sn(IV)TPyP4+ nanotubes in the presence of iodine vapor and salicylate anions down to millimolar range were examined in a chemiresistor sensing mode. We have shown that the porphyrin nanotubes advantageously combine the characteristics of a sensor and a transducer, thus demonstrating their great potential as efficient functional layers for sensing devices and biomimetic nanoarchitectures.

Invertion and methylation of pyrrole ring in tetrasulfophenylporphyrin: basicity, aggregation properties, chirality

Equilibria of methylated N-confused platform of 2-N-methyl-5,10,15,20-tetrakis-(4′-sulfophenyl)-2-aza-21-carbaporphyrin tetraanion HMeIP(PhSO3H)4 platform diprotonation by perchloric acid in water, H– and J-aggregates self-assembly were investigated using synchronous UV–Vis-fluorescence-pH titration and DFT/B3LYP/6-31++G(d,p) calculations. Methylation of inverted pyrrole ring in H2IP(PhSO3−)4 allows to fix the tautomer HMeIP(PhSO3−)4 with external NH-proton. Intramolecular tightness causes formation of two bifurcated NHN IMHB between pyrrole hydrogen and pyrrolenine nitrogen of platform HMeIP(PhSO3−)4. IMHB protect intramolecular centers of hydrogen bonding from intermolecular interactions. First stage of HМеIP(PhSO3−)4 protonation is accompanied by bifurcated IMHB type change from NHN to HNH. On the second protonation stage IMHB are broken and intermolecular hydrogen bonds with solvent molecules are formed. Diprotonated H3МеIP2+(PhSO3−)4 platform is a flexible 1,3-alternate, possessed molecular and anion receptor properties. Equilibrium in the second stage of protonation in water is completely shifted to the aquacomplex, which is formed due to hydrogen and electrostatic binding of the solvent molecules at the both receptor sites. Methylation and invertion of pyrrole ring leads to HMeIP(PhSO3−)4 proton affinity increasing and eliminated the phenomena of synchronous H2P(PhSO3−)4 porphyrin platform diprotonation, which is due to more stable aquacomplex [H4P2+(PhSO3−)4](H2O)2 formation. Initial tetraanion HMeIP(PhSO3−)4 and aquacomplex [H4IP2+(PhSO3−)4](H2O)2 are monomers of H- and J-aggregates self-assembly. Driving force of H-aggregates formation is the ionic self-assembly between sulfonate groups of monomers and spacer cations. J-aggregates self-assembly is due to formation of more stable anion complexes as a result of water molecules intermolecular substitution in [H3MeIP2+(PhSO3−)4](H2O)2 by monomers sulfonate groups. Protonated chitosan was used as a inducer and chiral scaffold of CW-H- and tubular CW-J-aggregates (porphyrin nanotubes) self-assembly. Parameters of CW-H- and CW-J-dimers geometry were calculated and 3D models of H-aggregates and porphyrin nanotubes were developed on their basis.

Synthesis and Properties of Porphyrin Nanotubes

AbstractDiscrete π‐conjugated zinc porphyrin nanotubes are investigated as molecular analogues of carbon nanotubes. These porphyrin nanotubes have a diameter of 2.4 nm (Zn–Zn distance) and lengths of up to 3.6 nm, measured to the van der Waals surfaces of the outer β‐pyrrole hydrogen atoms, or 4.5 nm measured to the para hydrogen atoms of the aryl groups. We explore three different strategies for synthesizing these nanotubes. The first two strategies use a template to achieve direct or sequential stave‐joining, respectively, and proceed via linear oligomers that pre‐define the length of the nanotube. These strategies are applied to synthesize porphyrin nanotubes containing 12‐ or 18‐porphyrin subunits, with ethynylene (C2) or butadiynylene (C4) links between the 6‐porphyrin nanorings. The third strategy involves the covalent stacking of pre‐formed 6‐porphyrin nanorings to form a 12‐porphyrin nanotube, without using a template to guide this coupling reaction. The nanotubes show strongly red‐shifted absorption spectra and low fluorescence quantum yields, indicating structural rigidity and extensive π‐conjugation.

Charge stabilizing tris(triphenylamine)-zinc porphyrin-carbon nanotube hybrids: synthesis, characterization and excited state charge transfer studies.

Functionalization of single-walled (SWCNTs) and double-walled carbon nanotubes (DWCNTs) with a charge stabilizing zinc porphyrin functionalized with triphenylamine entities has been accomplished. The synthetic approach involved the reaction of tris-(triphenylamine)porphyrinato zinc(ii) with iodobenzene functionalized nanotubes through a Sonogashira C-C cross coupling reaction under microwave irradiation conditions. Evidence of covalent functionalization and the extent of functionalization was obtained from systematic studies carried out by AFM, TGA, XPS and Raman spectroscopy techniques. The porphyrin-nanotube interactions in the SWCNT-porphyrin hybrid were probed by studies involving optical absorbance, Raman spectroscopy, steady-state and time resolved emission and electrochemical studies. The fluorescence of porphyrin in this hybrid was found to be quenched due to interactions with the CNTs. Femtosecond transient absorption spectral studies covering both the visible and near-infrared regions were supportive of excited state charge transfer interactions in the zinc porphyrin-SWCNT. The charge separated state was persistent for about 1 ns. Electron pooling experiments suggested that this donor-acceptor nanohybrid could be a useful photocatalyst.

Comparative NO2-sensing in cobalt and metal-free porphyrin nanotubes

In the present study, the nanotubes of 5-(4-hydroxyphenyl)-10, 15, 20-tri(4-chlorophenyl) porphyrin (p-HTClPP) (1) and 5-(4-hydroxyphenyl)-10, 15, 20-tri(4-chlorophenyl) porphyrin cobalt (p-HTClPPCo) (2) were successfully prepared by using anodize alumina oxide (AAO) template method. The p-HTClPP and p-HTClPPCo nanotubes have been confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electronic absorption spectra, fluorescence spectroscopy, fourier transform infrared spectroscopy (FT-IR), low-angle X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) techniques. Both p-HTClPPCo and p-HTClPP nanotubes showed excellent sensitivity, reproducibility and selectivity toward NO2. Especially the prepared sensor of p-HTClPPCo nanotubes exhibited faster response/recovery characteristics and lower detection limit of NO2 (up to 500ppb) than that of p-HTClPP nanotubes, which pave a new avenue in the gas sensitive field.

Curvature-Dependent Selectivity of CO2 Electrocatalytic Reduction on Cobalt Porphyrin Nanotubes

The selective electrocatalytic conversion of CO2 into useful products is a major challenge in facilitating a closed carbon cycle. Here, on the basis of first-principles calculations combined with computational hydrogen electrode model, we report a curvature-dependent selectivity of CO2 reduction on cobalt–porphyrin nanotubes which are thermodynamically stable, displaying tunable geometric and electronic properties with tube radius. We have found that CO production is preferred on nanotubes with larger diameter, and the predicted current density from microkinetics is larger than that on Au, the best metal catalyst for CO production from CO2 electroreduction. In contrast, highly curved nanotubes with small radii tend to further catalyze CO reduction to CH4 gas and the overpotential is much lower in comparison with the cases on Cu surfaces. The selectivity and the feasibility of synthesis make cobalt–porphyrin nanotubes very promising for CO2 conversion.

Out-of-Plane Coordinated Porphyrin Nanotubes with Enhanced Singlet Oxygen Generation Efficiency.

A supramolecular porphyrin nanotube displaying J-aggregation feature was constructed by out-of-plane coordinated bismuth-porphyrin. Significantly, compared to traditional J-aggregated porphyrin suffering from fluorescence and singlet oxygen quenching, the nanotube exhibits excellent bio-imaging ability and enhanced production efficiency of singlet oxygen. The out-of-plane structure of bismuth to porphyrin makes the aggregation an appropriate material for theranostics. Furthermore, it is also a potential radio-therapeutic drug owing to the presence of radio-active bismuth. Thus, the self-assembly of out-of-plane coordinated porphyrin can be a facile approach toward effective therapy of tumors and other diseases.

Unusual Bandgap Oscillations in Template-Directed π-Conjugated Porphyrin Nanotubes.

Using large-scale DFT calculations (up to 1476 atoms and 18 432 orbitals), we present the first detailed analysis on the unusual electronic properties of recently synthesized porphyrin nanotubes. We surprisingly observe extremely large oscillations in the bandgap of these nanostructures as a function of size, in contradiction to typical quantum confinement effects (i.e., the bandgap increases with size in several of these nanotubes). In particular, we find that these intriguing electronic oscillations arise from a size-dependent alternation of aromatic/nonaromatic characteristics in these porphyrin nanotubes. Our analyses of band structures and orbital diagrams indicate that the electronic transitions in these nanostructures are direct-bandgap, optically active "bright" states that can be readily observed in photoelectron spectroscopic experiments. Most importantly due to their unusual bandgap oscillations, we find that both type I and type II donor-acceptor p-n heterojunctions are possible in these template-directed, "bottom-up synthesized" porphyrin nanotubes-a unique property that is not present in conventional carbon nanotubes.

Surface-Enhanced Raman Spectra of Tetra(4-Sulfonatophenyl)Porphyrin on the Surface of Plasmonic Silver Films

Absorption and Raman spectra of the tetra(4-sulfonatophenyl)porphyrin (TSPP) zwitterion in aqueous solutions under conditions at which porphyrin nanotubes (PNT) form (pH 1) are presented. TSPP was immobilized on the surface of plasmonic silver films (PSF) via quick transfer of a suspension of the molecules into a solution at pH 5 and onto the film surface in order to avoid degrading the film. Images of PNT and spheroidal TSPP aggregates on the PSF surface were visualized using confocal microscopy. Spatially resolved surface-enhanced Raman spectra (SERS) of these objects were recorded. Differences in SERS of PNT and TSPP globular aggregates are discussed based on quantum-chemical calculations of TSPP vibrational spectra. Vibrational bands sensitive to the tube–spherulite transition are found.

Room temperature NO2 sensor based on highly ordered porphyrin nanotubes

Highly ordered nanotubes of 5, 10, 15, 20-tetrakis(4-aminophenyl)porphyrin zinc (ZnTAP) are fabricated by using nanoporous anodized aluminum oxide (AAO) membrane as the template. Electronic absorption spectra, fluorescence spectra, transmission electron microscope (TEM), scanning electronic microscopy (SEM), low-angle X-ray diffraction (XRD) techniques are adopted to characterize these nanotubes. The highly ordered nanotubes of ZnTAP show good conductivity and present an efficient gas sensor platform for the ultrasensitive detection of NO2 under room temperature. The proposed sensor shows high sensitivity, reproducibility and fast response/recovery behavior, and provides a promising avenue for improving the sensing performance.

Structure vs. excitonic transitions in self-assembled porphyrin nanotubes and their effect on light absorption and scattering.

The optical properties of diprotonated meso-tetrakis(4-sulphonatophenyl)porphyrin (TPPS(4)) J-aggregates of elongated thin particles (nanotubes in solution and ribbons when deposited on solid interfaces) are studied by different polarimetric techniques. The selective light extinction in these structures, which depends on the alignment of the nanoparticle with respect to the polarization of light, is contributed by excitonic absorption bands and by resonance light scattering. The optical response as a function of the polarization of light is complex because, although the quasi-one-dimensional structure confines the local fields along the nanotube axis, there are two orthogonal excitonic bands, of H- and J-character, that can work in favor of or against the field confinement. Results suggest that resonance light scattering is the dominant effect in solid state preparations, i.e. in collective groups (bundles) of ribbons but in diluted solutions, i.e. with isolated nanotubes, the absorption at the excitonic transitions remains dominant and linear dichroism spectra can be a direct probe of the exciton orientations. Therefore, by analyzing scattering and absorption data we can determine the alignment of the excitonic bands within the nanoparticle, i.e. of the orientation of the basic 2D porphyrin architecture in the nanoparticle. This is a necessary first step for understanding the directions of energy transport, charge polarization and non-linear optical properties in these materials.

Exploration of Giant Functional Porphyrin Arrays

Abstract Directly linked linear porphyrin arrays were synthesized and their photophysical properties depending on the length of the arrays were investigated. Ag(I)-promoted oxidative coupling of 5,15-diaryl Zn(II) porphyrins has proven to be effective for preparations of very long meso–meso-linked Zn(II) porphyrin arrays (Zn). A stepwise doubling strategy with strict purification at each doubling step allowed for preparation of gigantic but discrete linear arrays in an unprecedented manner. Large cyclic porphyrin wheels were also synthesized by final meso–meso coupling intramolecular cyclization. Crucial to this success are high reactivity and high regioselectivity of the Ag(I)-promoted oxidative coupling. Equally important are high solubilities of Zn despite their large molecular weights, which arise from their perpendicularly linked structures. Another advantage of this doubling strategy is a large difference in the molecular weights among reactants and products that increases as the reactant array becomes larger, allowing separation of coupling products by preparative GPC-HPLC. As an alternative protocol, Pd-catalyzed Suzuki–Miyaura cross-coupling has been extensively used for synthesis of cyclic porphyrin arrays. This strategy gave novel linear and cyclic porphyrin arrays from β-borylated porphyrins that were prepared by Ir-catalyzed borylation of meso-free porphyrins. As an interesting example, a porphyrin nanotube consisting of four porphyrins doubly bridged by 2,6-pyridinylene spacers was synthesized and revealed to possess a C60-encapsulation capability. Exciton coupling, effective energy hopping, coherent length, and excitation energy transfer of meso–meso-linked linear and cyclic porphyrin arrays are discussed. STM and AFM detections of the large porphyrin arrays are also presented. We believe that these studies expand the range of organic molecules we can manipulate and open a door to a new area of nanoscaled and sub-microscaled functional porphyrin materials.

Phenothiazine-Bridged Cyclic Porphyrin Dimers as High-Affinity Hosts for Fullerenes and Linear Array of C60in Self-Assembled Porphyrin Nanotube

Free-bases and a nickel(II) complex of phenothiazine-bridged cyclic porphyrin dimers bearing self-assembling 4-pyridyl groups (M2-Ptz-CPDPy(OCn); M = H2 or Ni, OCn = OC6 or OC3) at opposite meso-positions have been prepared as host molecules for fullerenes. The free-base dimer (H4-Ptz-CPDPy(OC6)) includes fullerenes with remarkably high association constants such as 3.9 ± 0.7 × 10(6) M(-1) for C60 and 7.4 ± 0.8 × 10(7) M(-1) for C70 in toluene. This C60 affinity is the highest value ever among reported receptors composed of free-base porphyrins. The nickel dimer (Ni2-Ptz-CPDPy(OC6)) also shows high affinities for C60 (1.3 ± 0.2 × 10(6) M(-1)) and C70 (over 10(7) M(-1)). In the crystal structure of the inclusion complex of C60 within H4-Ptz-CPDpy(OC3), the C60 molecule is located just above the centers of the porphyrins. The two porphyrin planes are almost parallel to each other and the center-to-center distance (12.454 Å) is close to the optimal separation (∼12.5 Å) for C60 inclusion. The cyclic porphyrin dimer forms a nanotube through its self-assembly induced by C-H···N hydrogen bonds between porphyrin β-CH groups and pyridyl nitrogens as well as π-π interactions of the pyridyl groups. The C60 molecules are linearly arranged in the inner channel of this nanotube.

Finely designed arrangements and photoelectronic properties of supramolecular nanocarbons composed of fullerenes and cyclic porphyrin dimers

We have studied the supramolecular structures and photoelectronic properties of inclusion complexes of cyclic porphyrin dimers (M2-CPDPy, M = Ni, 2H) and fullerenes (C60, C70, PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) and Li+@C60). The porphyrin dimers have self-assembling 4-pyridyl groups at the meso positions and include fullerenes in their cavities with high affinities (Kassoc ≈ 105M−1). In crystals, C60 ⊂ Ni2-CPDPy and PCBM ⊂ Ni2-CPDPy give self-assembled porphyrin nanotubes containing the linear arrays of fullerene molecules. These self-assemblies are induced mainly by the non-classical hydrogen bonds between pyridyl nitrogens and porphyin β C–H groups. On the other hand, in the crystals of C60 ⊂ H4-CPDPy and C70 ⊂ H4-CPDPy, the included fullerene molecules show zigzag chains through van der Waals contacts with each other. Due to these well-ordered arrays of C60, both crystals of C60⊂ Ni2-CPDPy and C60 ⊂ H4-CPDPy have high charge mobilities (Σμ > 0.1 cm2 V−1 s−1). C60 ⊂ H4-CPDPy shows photoinduced electron transfer from the porphyrin moieties to C60 and photovoltaic activity with IPCEmax = 17% in photoelectrochemical cells. Moreover, the inclusion complexes of lithium-ion-encapsulated C60(Li+@C60) and M2-CPDPy exhibit photoinduced charge separation with very long lifetimes reaching submillisecond order. [TANSO 2013 (No. 260) 284–91.]

Photoactive Zn(II)Porphyrin–multi-walled carbon nanotubes nanohybrids through covalent β-linkages

Donor–acceptor nanohybrids by a covalent linkage between the β-position of a Zn(II)Porphyrin and multi-walled carbon nanotubes are reported for the first time, in a closer analogy to the natural light harvesting systems, which are based on β-substituted porphyrinoid structures, the chlorophylls. An unique and direct connection was established through the immobilization of the Zn(II)(β-NH2-tetraphenylporphyrin), using diazonium chemistry, in order to afford i) a short and conjugated linkage between the two aromatic systems and ii) an amide bond resulting from a three-step functionalization synthesis. Electronic and steady-state fluorescence spectroscopies confirmed high photoinduced electron communication through the β-linkage when compared to analogous meso-phenyl linkers, stating its positive effect. The procedure involving the amide linkage allowed higher chromophore loadings; however, the direct conjugated bond showed improved photoinduced activity and a different emission pattern that can be associated with intense communication within the expanded π-system MWCNT–metalloporphyrin.

Electrodes modified with iron porphyrin and carbon nanotubes: application to CO2 reduction and mechanism of synergistic electrocatalysis

Electrodes modified with iron porphyrin and carbon nanotubes (FeP–CNTs) were prepared and used for CO2 electroreduction. The adsorption of iron porphyrin onto the multiwalled carbon nanotubes was characterized by scanning electron microscopy and ultraviolet and visible spectroscopy. The electrochemical properties of the modified electrodes for CO2 reduction were investigated by cyclic voltammetry and CO2 electrolysis. The FeP–CNT electrodes exhibited less negative cathode potential and higher reaction rate than the electrodes modified only with iron porphyrin or carbon nanotubes. A mechanism of the synergistic catalysis was proposed and studied by electrochemical impedance spectroscopy and electron paramagnetic resonance. The direct electron transfer between iron porphyrin and carbon nanotubes was examined. The current study shed light on the mechanism of synergistic catalysis between CNTs and metalloporphyrin, and the iron porphyrin–CNT-modified electrodes showed great potential in the efficient CO2 electroreduction.