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Original Article
:19;
2172025
doi:
10.25259/AJC_217_2025

Preparation of conducting polymer polyaniline/g-C3N4 heterojunction for enhanced CO2 photoreduction into fuel

, , ,
 Intelligent Construction School, Zhengzhou Business University, Zhengzhou, Henan, China

*Corresponding author: E-mail address: xauat2025@163.com (K. Bi)

Received: , Accepted: ,
© 2026 Arabian Journal of Chemistry - Published by Scientific Scholar
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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

The g-C3N4/polymer polyaniline (g-C3N4/PANI) is synthesized using constructing heterojunction strategy for CH3OH production by CO2 photoreduction. The CO2 photoreduction performance for CH3OH production of g-C3N4/PANI was investigated by regulating PANI content. The construction of the heterojunction effectively promotes the separation of photogenerated carriers with a high charge transfer rate, contributing to CO2 photoreduction. Besides, the incorporation of the PANI layer enhances electron transfer efficiency and improves CO2 adsorption capacity. The g-C3N4/PANI also exhibits good selectivity for CH3OH production, with a selectivity of 95.26%. The g-C3N4/PANI still maintains excellent CO2 photoreduction activity after cycles. The mechanism of the CH3OH production from CO2 photoreduction is analyzed through the energy band structure.

Keywords

Graphite phase carbon nitride
Photocatalytic reduction of CO2
Polyaniline
Z scheme heterojunction

1. Introduction

With the advancement of industrialization and the growth of population, the demand for energy continues to grow, and fossil fuels such as coal, oil, and natural gas are consumed in large quantities [1]. The burning of fossil fuels will emit large amounts of CO2, triggering environmental problems such as global warming, leading to rising sea levels and melting of polar ice and snow, which has had a serious impact on the ecological environment and human production and living order [1]. Besides, the intensification of extreme weather events has become increasingly evident across the globe, with rising frequencies and severities of droughts and heavy precipitation episodes. Therefore, how to achieve efficient utilization of the greenhouse gas CO2 has become very important. Currently, CO2 is mainly utilized through mineralization storage, biological transformation, chemical adsorption, and catalytic reduction [2]. Through catalytic reduction, CO2 is reduced to products such as methane, formic acid, and methanol. These products can be directly used as fuel for fuel cells, realizing the conversion of chemical energy into electrical energy, which can not only effectively alleviate the energy crisis but also reduce environmental pollution [3]. The reduction of CO2 can be driven by light energy, electrical energy, and thermal energy. Photocatalysis has the characteristics of high efficiency and greenness and has good potential applications in the field of CO2 reduction. However, the bond energy of the C-O bond of CO2 reaches as high as 750 kJ·mol-1, which makes CO2 reduction still a certain challenge [4]. Consequently, preparing efficient and stable materials for photocatalytic CO2 reduction is of critical importance.

The g-C3N4, as an excellent semiconductor photocatalytic material, has good chemical stability, simple preparation, and good visible optical response capabilities [5,6]. The g-C3N4 has found extensive application in environmental remediation for the degradation of organic pollutants, nitrogen oxides, and formaldehyde. Besides, it is also used in the field of new energy for photodegradation of water to produce H2 or photocatalytic reduction of CO2 to generate methanol [7,8]. However, the photogenerated carriers of the g-C3N4 are seriously recombined, which greatly limits the CO2 photoreduction for the production of methanol [9]. Therefore, the g-C3N4 should be modified to restrain the recombination of photogenerated carriers to realize the efficient CO2 photoreduction.

Currently, common modification strategies include co-catalysts [10], morphology modulation [11], elemental doping [12], and construction of heterojunctions [13] for improving the CO2 photoreduction performance of the g-C3N4 for methanol production. Among these modification methods, the construction of heterojunctions, including g-C3N4/TiO2 [14], ZnO/g-C3N4 [15], and MoS2/g-C3N4 [16], is regarded as an efficient strategy for solving the photogenerated carrier recombination of the g-C3N4. Song et al. prepared a multifunctional g-C3N4-PDI heterojunction for enhanced CO2 reduction with excellent photocatalytic performance [17]. The formation of a heterojunction not only improves CO2 adsorption but also accelerates the migration of photogenerated electrons, playing a crucial role in CO2 photoreduction. Ma et al. synthesized Au/g-C3N4/CdS heterojunction for CO2 photoreduction, indicating that the formed heterojunction speeds ups the carriers’ separation efficiency [18]. Therefore, the construction of a heterojunction can improve the CO2 photoreduction performance of the g-C3N4.

Organic semiconductors, as a class of conductive polymers, exhibit several distinctive properties, including being metal-free, possessing high stability, electrical conductivity, biocompatibility, and environmental friendliness [19-20]. Common organic semiconductors encompass a range of conductive polymers, including polyaniline, polydopamine, polypyrrole, and polyacrylonitrile [21]. Among these organic semiconductors, the polyaniline (PANI) exhibits excellent electrical conductivity and photothermal conversion properties, facilitating free electron migration due to its extensive internal π-π conjugation system [22-23]. Miao et al. (2023) prepared the AuCu/PANI/g-C3N4 heterostructure for enhanced CO2 photoreduction, indicating that the PANI layer enhances CO2 adsorption and accelerates charge transfer with excellent photocatalytic reduction performance of CO2 [24]. However, the preparation process of AuCu/PANI/g-C3N4 heterostructure is complex and expensive, which limits large-scale preparation and application. Therefore, it is of great significance to develop low-cost and easy-to-prepare photocatalysts that reduce CO2 to methanol [25]. Relevant studies have demonstrated that PANI can improve the separation of photogenerated carriers and function as efficient energy band modulators [25]. Besides, PANI exhibits excellent photostability and thermal stability, ensuring the recyclability of photocatalysts [26]. Therefore, preparing the PANI/g-C3N4 heterostructure can improve its capacity for enhanced CO2 photoreduction by improving light utilization and CO2 adsorption, and accelerating charge transfer.

The coupling of the g-C3N4 with the conductive polymer PANI is achieved through the strategy of constructing a heterojunction with good selectivity for CH3OH production. The preparation process of the PANI/g-C3N4 is simple, which is conducive to industrial utilization. Physicochemical properties of PANI/g-C3N4 were analyzed. CO2 photoreduction performance of PANI/g-C3N4 was optimized by modulating PANI content under visible light irradiation. The formed heterojunction improves the light utilization and accelerates the charge separation. The PANI/g-C3N4 exhibited excellent CO2 photoreduction performance compared to g-C3N4. A possible mechanism was put forward to analyze the enhanced photocatalytic performance for CO2 reduction. This research provides a new approach for designing photocatalysts aimed at CO2 photoreduction.

2. Materials and Methods

2.1. Experimental material

Melamine (C3H6N6), potassium bromide (KBr), sodium sulphate (Na2SO4), barium sulphate (BaSO4), ammonium persulphate ((NH4)2S2O8), anhydrous ethanol (C2H5OH), aniline (C6H7N), sulphuric acid (H2SO4), are purchased from Aladdin Reagent Co. All the medicines are not further purified.

2.2. Preparation of PANI/g-C3N4

The g-C3N4 was prepared by in situ thermal polymerization [27]. For this, 5 g of melamine were placed in a porcelain boat and subsequently transferred to the muffle furnace. The sample was heated under a nitrogen atmosphere at 15°C·min-1 to 540°C for 3 h. Afterward, the residue in the muffle furnace was naturally cooled to room temperature; this residue was g-C3N4.

The g-C3N4/PANI (3%) heterostructure was prepared as follows. Firstly, 0.5 g g-C3N4 was homogeneously added into 10 mL H2SO4 (0.05 mol·L-1). Secondly, 0.015 g of aniline and 0.10 g of ammonium persulfate were added at a temperature of -5-0°C, which was stirred for 12 h and then filtered. Finally, the g-C3N4/PANI (3%) heterostructure was washed using deionized water and anhydrous ethanol, and then dried at 60°C for 24 h. According to the above-mentioned method, the g-C3N4/PANI (1% or 5%) heterostructure was prepared by adjusting the dosage of aniline, and PANI was prepared without adding g-C3N4.

2.3. Characterization

The physical phase and crystal of the sample were characterized by a Bruker D8 QUEST ECO X-ray diffraction (XRD) instrument (Kα Cu target, tube voltage 40 kV, current 40 mA, 2θ =10-40°). The composition and structure of the sample were characterized by a Thermo Nicolet IS50 infrared spectrum using the KBr compression method. The morphology and elemental composition of the sample were analyzed using Zeiss GeminiSEM 460 scanning electron microscopy (SEM), Hitachi HT-7700 transmission electron microscopy (TEM), and a high-angle ring hidden field projection electron microscope. The element composition of the sample was characterized using the Shimadzu Axis Nova X-ray optoelectronics spectrum (the Kα Al source, the voltage of 5.0 kV, the power of 600 W). The spectrum response of the sample was characterized by the Agilent Cary 3500 UV visible reflectance spectrometer (BaSO4 reference, the scanning range 220-800 nm). The photogenerated carrier recombination and lifetimes of the sample were characterized by a Thermo ARL 9900 photoluminescence spectrometer. The photocurrent intensity and AC impedance of the sample were determined by the Zahner Zennium pro electrochemical workstation. The test system was composed of a Pt electrode, an AgCl/Ag electrode, and an FTO electrode. The electrolyte was Na2SO4 solution with 0.1 mol·L-1.

2.4. CO2 photoreduction for production of CH3OH

The photocatalytic experimental device is the Zhongke Micro Energy CME-PC6 photochemical reactor. The light source is a 500 W Xe lamp (λ>420 nm) to simulate visible light. The light source is 20 cm away from the liquid surface, and the average illumination intensity of the liquid surface is 60μW·cm-2. The reaction temperature is 25°C, and the visible light photocatalytic reduction performance of CO2 by the g-C3N4/PANI composite is evaluated. Firstly, 50 mg of the photocatalyst is evenly dispersed into a cylindrical stainless steel photocatalytic reactor containing 20 mL of deionized water, stirred magnetically, bubbled with high-purity CO2 (99.999%) gas for 10 min to discharge the air, and the outlet valve is closed. Secondly, high-purity CO2 is introduced and the flow rate is controlled at 50 mL·min-1. When the CO2 pressure in the reactor reaches about 1.0 atm, CO2 is stopped, the Xe lamp is turned on, and the reaction time is 4 h. Finally, the gas-liquid products are analyzed by an Agilent 7890B high-performance gas chromatograph equipped with an FID detector (Porapak Q column, TCD detector) to determine product composition and yield. The yield (product formation rate) is calculated according to Eq. (1).

(1)
R Product = n Product m × t

RProduct : the product generation rate, μmol·g-1·h-1;

nProduct : the amount of product substance, μmol;

m: the amount of catalyst, g;

t: the reaction time, h

3. Results and Discussion

3.1. X-ray diffraction and Fourier transform infrared analysis

The physical phase and crystal of PANI, g-C3N4, and g-C3N4/PANI are characterized (Figure 1a). As displayed in Figure 1(a), diffraction peaks at 12.9° and 27.6° correspond to the (100) and (002) crystal planes of g-C3N4, respectively [28]. The existence of the (100) plane is associated with the s-triazine ring. The (002) plane is attributed to the lamellar stacking of carbon-nitrogen aromatic heterocyclic rings [29]. The (011), (020), and (200) crystal surfaces of PANI appear at 2θ=15.4°, 20.4°, and 24.9°[30]. The XRD pattern of g-C3N4/PANI exhibits near consistency with the g-C3N4, which is mainly due to the weak crystallinity and low content of PANI. Therefore, the incorporation of the PANI doesn’t modify the crystalline structure of g-C3N4. With increasing in PANI content, the intensity of the diffraction peak at 2θ=12.9° and 27.6° gradually weakens. This result indicates that the g-C3N4 is wrapped by the conductive polymer PANI. The average grain sizes of g-C3N4, g-C3N4/PANI (1%), g-C3N4/PANI (3%), and g-C3N4/PANI (5%) composites are calculated by the Debye-Scherrer equation to be 8.92, 8.11, 7.97, and 7.89 nm, respectively. This result indicates that the grain size of g-C3N4 is reduced after PANI wrapping. The reason for this result may be the formation of chemical bonding between the PANI and g-C3N4 interface, thereby forming a heterojunction, resulting in lattice distortion and inhibiting the growth of grains [31].

(a) XRD and (b) FTIR spectra of PANI, g-C3N4, and g-C3N4/PANI.
Figure 1.
(a) XRD and (b) FTIR spectra of PANI, g-C3N4, and g-C3N4/PANI.

The surface functional group of PANI, g-C3N4, and g-C3N4/PANI is characterized by Fourier transform infrared (FTIR) (Figure 1b). The absorption bands of g-C3N4 at 3000-3500 cm-1, attributed to -NH2 and -OH groups, are associated with the incomplete polymerization of the melamine precursor and adsorbed H2O. The absorption bands at 1200-1650 cm-1 correspond to stretching vibrations of C=N and C-N bonds within the carbon-nitrogen rings. While the deformation vibration of the s-triazine ring appears at 809 cm-1 [32]. The stretching vibrations of aromatic rings and benzene ring C=C of the PANI appear at 1498 and 1582 cm-1, respectively. The absorption band at 1304 cm-1 is a C-N stretching vibration. The absorption bands at 1156 cm-1 and 827 cm-1 correspond to the =NH+ and the C-H stretching vibration outside the benzene ring face, respectively [33]. The g-C3N4/PANI exhibits an FTIR pattern nearly identical to the pristine g-C3N4. However, the absorption band intensity gradually weakens as PANI content increases due to the fact that g-C3N4 is encapsulated by the PANI. This result is consistent with XRD analysis.

3.2. Micro-morphological analysis

As Figure 2(a) shows, g-C3N4 has a typical layer-stacked 2D nanosheet structure with a relatively rough surface. PANI is polymerized in situ onto g-C3N4 nanosheets, resulting in a smooth surface (Figure 2b). The white arrows of the Figure 2(b) denote the PAN and g-C3N5, respectively. Besides, the conductive polymer PANI uniformly covers the g-C3N4 surface. Morphology of g-C3N4/PANI is further observed and analyzed by TEM and HAADF-STEM (Figure 2c,d). As Figures 2(c,d) shown, the conductive polymer PANI also uniformly coats the g-C3N4 surface. The “dark” part of the HAADF-STEM image is g-C3N4, and the “bright” part proves that g-C3N4 is uniformly wrapped by the conductive polymer PANI (Figure 2d). Figures 2(e,f) show the EDS images of the g-C3N4/PANI. As Figure 2(e,f) shows, the C and N elements in g-C3N4/PANI exhibit a dense distribution.

SEM (a) of g-C3N4 and (b) g-C3N4/PANI(3%), The white arrows denote the PAN and g-C3N5, respectively. (c) TEM, (d) HAADF-STEM of g-C3N4/PANI(3%), and (e-f) EDS images of g-C3N4/PANI (3%) (The white arrows of the
Figure 2.
SEM (a) of g-C3N4 and (b) g-C3N4/PANI(3%), The white arrows denote the PAN and g-C3N5, respectively. (c) TEM, (d) HAADF-STEM of g-C3N4/PANI(3%), and (e-f) EDS images of g-C3N4/PANI (3%) (The white arrows of the

3.3. XPS analysis

The elemental compositions of g-C3N4 and g-C3N4/PANI(3%) were characterized using XPS spectra. As Figure 3(a) shows, the binding energies at 284.4, 287.1, and 287.6 correspond to the C-C, C-N, and N=C-N groups, respectively. Additionally, the peak near 292.9 eV corresponds to the characteristic peak of π-π* conjugation [34]. The binding energy at 283.9 eV corresponds to the characteristic peak of sp3-hybridized heterocyclic carbon (C-C or C-H) in the conductive polymer PANI. The binding energy at 285.7 eV corresponds to the sp3-hybridized heterocyclic carbon (C-N) in the conductive polymer PANI [35]. The C 1s characteristic peak of g-C3N4/PANI exhibits a shift toward higher binding energy compared to pristine g-C3N4. This result indicates that it exists a strong interaction between PANI and g-C3N4. This interaction serves as the foundation for charge transfer.

(a) XPS spectrum of C 1S and (b) N 1s of g-C3N4 and g-C3N4/PANI(3%).
Figure 3.
(a) XPS spectrum of C 1S and (b) N 1s of g-C3N4 and g-C3N4/PANI(3%).

As Figure 3(b) shows, the binding energies of 398.3, 398.7, and 400.8 eV correspond to the C=N-C, N-(C)3, and N-H group, respectively. The g-C3N4 is wrapped by the conductive polymer PANI. A new characteristic peak appears near the binding energy of 404.3 eV, attributed to the presence of π-π* conjugation [36]. N 1s characteristic peak position of g-C3N4/PANI exhibits a shift toward higher binding energy compared to g-C3N4. This result indicates the presence of an interaction between the PANI and g-C3N4. The above results indicate that the heterojunction is formed in the g-C3N4/PANI.

3.4. Pore structure analysis

Table 1 presents the pore structure data of PANI, g-C3N4, and g-C3N4/PANI. As shown, the specific surface area of PANI and g-C3N4 is 80.21 and 21.31 m2·g-1, respectively (Table 1). This result indicates that the surface area of g-C3N4/PANI decreases compared to PANI after incorporating PANI with g-C3N4. The pore volume and average pore diameter of g-C3N4 are 0.20 cm3·g-1 and 16.22 nm, respectively. After the coupling between g-C3N4 and PANI to form a heterojunction, the pore volume and average pore diameter significantly improved. The increase in pore volume and average pore diameter is beneficial to the material’s adsorption of CO2 and the diffusion of CO2 in the pore structure. The reason for this result may be that the coupling between PANI and g-C3N4 forms a heterojunction, which leads to the local reconstruction of the layered structure of g-C3N4, thus achieving an increase in pore volume and pore diameter.

Table 1. Pore structure data of the PANI, g-C3N4, and g-C3N4/PANI.
Sample Specific surface area (m2·g-1) Pore volume (cm3·g-1) Average pore size (nm)
PANI 80.21 0.35 21.75
g-C3N4 21.31 0.20 16.22
g-C3N4/PANI (1%) 68.48 0.46 34.33
g-C3N4/PANI (3%) 66.13 0.46 35.01
g-C3N4/PANI (5%) 60.34 0.45 35.14

3.5. UV-visible diffuse reflectance spectrum analysis

The UV-vis diffuse reflectance spectrum (DRS) was used to investigate the spectral response of g-C3N4, PANI, and g-C3N4/PANI (3%). As displayed in Figure 4(a), the g-C3N4, PANI, and g-C3N4/PANI exhibit absorption responses in both the ultraviolet and visible light regions. Cutoff wavelengths of g-C3N4, PANI, and g-C3N4/PANI are 478, 585, and 548 nm, respectively. The absorption boundary of g-C3N4/PANI is significantly red-shifted, and the spectral absorption range is broadened, contributing to improving its photocatalytic performance. This phenomenon is closely associated with heterojunction interaction between PANI and g-C3N4 [37]. From the Tauc curve in Figure 4(b), it can be obtained that the band gap energies (Eg) of g-C3N4, PANI, and g-C3N4/PANI (3%) were 2.61, 2.13, and 2.50 eV, respectively. This result indicates that the band gap energy reduced after coupling between g-C3N4 and PANI to form a heterojunction, which broadened the visible light spectrum absorption range [38]. Therefore, the photocatalytic performance improved.

(a) UV-vis DRS spectrum and (b) Tauc plot of g-C3N4, PANI, and g-C3N4/PANI(3%).
Figure 4.
(a) UV-vis DRS spectrum and (b) Tauc plot of g-C3N4, PANI, and g-C3N4/PANI(3%).

3.6. Fluorescence spectrum analysis

The photogenerated carrier separation, recombination, and lifetime of g-C3N4 and g-C3N4/PANI(3%) were analyzed using photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy (Figure 5). As Figure 5(a) shows, the g-C3N4 has a large typical emission peak intensity, indicating that g-C3N4 exhibits high photogenerated carrier recombination. This phenomenon is the primary factor limiting the photocatalytic performance of g-C3N4. When the conductive polymer PANI wraps g-C3N4, the typical emission peak intensity of the g-C3N4/PANI reduces. This reduction effectively suppresses photogenerated carrier recombination in the g-C3N4/PANI, which can be explained by the interaction at the heterojunction between PANI and g-C3N4. As Figure 5(b) shows, the average photogenerated carrier lifetime of g-C3N4/PANI and g-C3N4 is 6.18 and 4.97 ns, respectively. This phenomenon can be attributed to the excellent electrical conductivity of the conductive polymer PANI, which facilitates the transfer of excited-state photoelectrons to the interface and effectively inhibits photogenerated carriers’ recombination [39]. The effective separation and prolonged lifetime of photogenerated carriers are attributed to the heterogeneous interactions.

(a) PL and (b) TRPL spectrum of g-C3N4 and g-C3N4/PANI (3%).
Figure 5.
(a) PL and (b) TRPL spectrum of g-C3N4 and g-C3N4/PANI (3%).

3.7. Electrochemical property analysis

The transient photocurrent and electrochemical impedance spectroscopy (EIS) of g-C3N4 and g-C3N4/PANI(3%) were characterized using transient photocurrent (TPC) measurements and EIS, with the results presented in Figure 6. As Figure 6(a) shows, g-C3N4 and the g-C3N4/PANI exhibit stable photocurrents during three cycling periods. Notably, the transient photocurrent intensity is significantly enhanced after encapsulation of g-C3N4 with the conductive polymer PANI. This result indicates that the charge transfer rate significant increases due to the rapid migration of photoelectrons generated by light-driven charge separation in g-C3N4, following its encapsulation by the conductive polymer PANI. These photoelectrons efficiently migrate to the conductive interface and subsequently to the surface of g-C3N4 [40]. The g-C3N4/PANI exhibits significantly smaller AC impedance radius than pristine g-C3N4 (Figure 6b). The charge transfer resistance Rct of g-C3N4/PANI and g-C3N4 is 45.1 and 199.2 Ω in the inset of Figure 6(b), respectively. This result indicates that the conductivity of the conductive polymer PANI encapsulating the g-C3N4 significantly increases. Therefore, the separation rate of photogenerated carriers is effectively improved due to the formed heterojunction between PANI and g-C3N4 [41]. The excellent electrochemical properties of g-C3N4/PANI are conducive to enhancing its photocatalytic performance.

(a) TPC and (b) EIS spectrum of g-C3N4 and g-C3N4/PANI(3%).
Figure 6.
(a) TPC and (b) EIS spectrum of g-C3N4 and g-C3N4/PANI(3%).

3.8. CO2 photoreduction performance for production of CH3OH

Figure 7 shows the product yields of the CO2 photoreduction process using g-C3N4 and g-C3N4/PANI, and the CH3OH yield of g-C3N4/PANI(3%) after five cycles. As Figure 7(a) shows, the pure g-C3N4 exhibits low yields of 0.73 and 0.06 μmol·g-1·h-1 for CH3OH and CO during the CO2 photoreduction process, respectively. It may be attributed to the relatively fast recombination rate of photogenerated carriers. The CH3OH and CO yield of g-C3N4/PANI significantly increases. The reason is that the PANI layer enhances CO2 adsorption and electron delivery. Besides, the light utilization and charge separation rate of the g-C3N4/PANI improved. The formed heterojunction in g-C3N4/PANI also effectively promotes photogenerated carriers separation. The CH3OH yields of g-C3N4/PANI(1%), g-C3N4/PANI(3%), and g-C3N4/PANI(5%) were 1.26, 3.12, and 2.01 μmol·g-1·h-1, respectively. While CO yields of g-C3N4/PANI(1%), g-C3N4/PANI(3%), and g-C3N4/PANI(5%) were 0.21, 0.19, and 0.23 μmol·g-1·h-1, respectively. Above results indicate that g-C3N4/PANI(3%) exhibits excellent CO2 photoreduction performance compared to other PANI contents. The CH3OH yield of the g-C3N4/PANI(3%) is 4.23 times larger than g-C3N4. When conductive polymer PANI content is low, the heterojunction is not easy to be formed in g-C3N4/PANI, which consequently hinders effective separation of photogenerated carriers. When PANI content is high, the g-C3N4 is sufficiently wrapped, and the effective contact between the conductive polymer PANI and the g-C3N4 is significantly restricted. Therefore, the CO2 photoreduction performance is poor. These results also indicate that the formed heterojunction can effectively promote photogenerated carriers separation and improve CO2 photoreduction performance for production of CH3OH. Besides, the CH3OH yield of the g-C3N4/PANI is higher than that of the CO. This result indicates that g-C3N4/PANI exhibits higher selectivity towards CH3OH production compared to CO production.

The yields of (a) CO2 photoreduction products and (b) CH3OH after five cycles (g-C3N4/PANI (3%: 50 mg, CO2: 1.0 atm).
Figure 7.
The yields of (a) CO2 photoreduction products and (b) CH3OH after five cycles (g-C3N4/PANI (3%: 50 mg, CO2: 1.0 atm).

Furthermore, the stability of photocatalytic materials is another critical factor influencing their practical applications. The CH3OH yield of the g-C3N4/PANI(3%) is 2.97 μmol·g-1·h-1 after five cycles, maintaining 95% CO2 photoreduction performance. This analysis indicates that g-C3N4/PANI(3%) exhibits excellent stability, contributing to actual application.

3.9. Analysis of CO2 photoreduction mechanism

The energy band potential is very important for the analysis of the charge transfer mechanism and the photocatalytic mechanism. The Tauc curve in Figure 4(b) shows that the band gap energies (Eg) of g-C3N4 and PANI were 2.61 and 2.13 eV, respectively. From the VB-XPS spectrum in Figure 8, it can be obtained that the valence band potentials (EVB) of g-C3N4 and PANI were 1.54 and 1.61 eV, respectively. Based on the calculation method of Eg=EVB-ECB, the conduction band potential (ECB) of g-C3N4 and PANI were determined to be -1.07 and -0.52 eV, respectively.

VB-XPS spectrum of g-C3N4 and PANI.
Figure 8.
VB-XPS spectrum of g-C3N4 and PANI.

Based on the energy band structure, the Z-type charge transfer mechanism and g-C3N4/PANI photocatalytic reduction of CO2 mechanism in Figure 9 were obtained. Under the irradiation of visible light (λ>420 nm), the photocatalytic material g-C3N4 and the conductive polymer PANI undergoes charge separation, generating photoelectrons (e-) and holes (h+). The e- transition occurs and migrates from the valence band (VB) to the conduction band (CB), causing CB to generate a large amount of h+. Because of the difference in the energy band positions between g-C3N4 and PANI, the CB position e- of g-C3N4 will transition to the CB position of PANI, and the VB position h+ of PANI will transition to the VB position of g-C3N4. The CB potential of PANI is more negative than the standard hydrogen electrode potential (SHE) of CO2/CH3OH. There is a significant thermodynamic potential difference, enabling electrons to readily reduce CO2 to CH3OH. However, the test results show that the products of g-C3N4/PANI heterojunction catalytic reduction of CO2 are CH3OH and CO. Therefore, the charge transfer path of g-C3N4/PANI heterojunction does not conform to the type II charge transfer mechanism [42]. Therefore, a Z-type charge transfer mechanism is proposed. The e- at the CB position of PANI migrates to the VB position of g-C3N4. A large amount of h+ remains at the VB position of PANI, and a large amount of e- remains at the CB position of g-C3N4, achieving efficient photogenerated carrier separation [43]. The CB potential of g-C3N4 is more negative than the SHE potential of O2/·O2-. Thermodynamically, electrons at the conduction band (CB) position can reduce O2 to form highly reactive superoxide radicals (·O₂⁻). The VB potential of PANI is more negative than the SHE potential of H2O/·OH and the SHE potential of H2O/O2. Thermodynamically, the h+ at the VB position cannot oxidize H2O to hydroxyl radicals (·OH). While, it can oxidize H2O to O2 and H+. The CB potential of g-C3N4 is more negative than the SHE potential of CO2/CO and CO2/CH3OH. With the cooperation of the active free radical ·O2-, the e- at the CB position reduces CO2 to the main product CH3OH and the by-product CO. Besides, this reaction shows high selectivity. The Z-type charge transfer mechanism of g-C3N4/PANI effectively promotes the separation of photogenerated carriers and broadens the visible light spectrum absorption range. The CB position of g-C3N4 retains highly reducing active e-, achieving efficient reduction of CO2 [44].

Z-scheme charge transfer mechanism and photocatalytic reduction of CO2 mechanism of g-C3N4/PANI heterojunction.
Figure 9.
Z-scheme charge transfer mechanism and photocatalytic reduction of CO2 mechanism of g-C3N4/PANI heterojunction.

4. Conclusions

The g-C3N4/PANI was synthesized by constructing heterojunction strategy using aniline and g-C3N4 as precursors. The g-C3N4 was wrapped by the conductive polymer PANI to form a heterojunction, realizing efficient photoelectron-hole separation and rapid charge transfer. The photocatalytic reduction of CO2 by g-C3N4/PANI was superior to that of g-C3N4. CH3OH and CO yields of the g-C3N4/PANI (3%) were 3.12 and 0.19 μmol·g-1·h-1, respectively, showing high selectivity of CH3OH production. The selectivity of CH3OH production was as high as 95.26%. A meticulously engineered g-C3N4/PANI heterojunction can significantly enhance charge separation and expedite the migration of photogenerated electrons. CO2 photoreduction mechanism of g-C3N4/PANI is investigated. The prepared g-C3N4/PANI has potential application in greenhouse gas CO2 reduction for production of CH3OH as energy fuel.

Acknowledgment

The authors would like to express their gratitude to the Provincial Science and Technology Research and Development Program of Henan Province in 2025 Project: Research on the Failure Mechanism and Deformation Optimization Improvement Design of Expansion Joints in Prefabricated Highway Bridges (Project Number: 252102241016) for financial support.

CRediT authorship contribution statement

Yujuan Liu: Writing - review & editing, Methodology, Investigation.Suping Wang: Resources, Formal analysis, Visualization. Kejun Bi: Writing-original draft, Resources, Investigation, Supervision. Yapo Sun: Resources, Formal analysis.

Data availability

Data will be made available on request.

Declaration of competing interest

There are no conflicts of interest.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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