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Article

Efficient Electron Transfer in g-C3N4/TiO2 Heterojunction for Enhanced Photocatalytic CO2 Reduction

by
Peng Jiang
1,*,
Yang Yu
2,*,
Kun Wang
1 and
Wenrui Liu
1
1
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
2
Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(6), 335; https://doi.org/10.3390/catal14060335
Submission received: 22 April 2024 / Revised: 15 May 2024 / Accepted: 20 May 2024 / Published: 22 May 2024
(This article belongs to the Special Issue Non-precious Metal Catalysts for Energy and Environment-Related Applications)
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Abstract

Excessive emissions of carbon dioxide have led to the greenhouse effect and global warming. Reducing carbon dioxide into high-value-added chemicals through solar energy is a promising approach. Herein, a g-C3N4/TiO2 heterojunction photocatalyst with efficient electron transfer is designed for photocatalytic CO2 reduction. The CH4 (18.32 µmol·h−1·g−1) and CO (25.35 µmol·h−1·g−1) evolution rates of g-C3N4/TiO2 are higher than those of g-C3N4 and TiO2. The enhanced photocatalytic CO2 reduction performance is attributed to the efficient charge carrier transfer in the g-C3N4/TiO2 heterojunction. The electron transfer route was verified by in situ irradiated X-ray photoelectron spectroscopy (XPS). The photocatalytic CO2 reduction mechanism on g-C3N4/TiO2 was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This work provides a strategy for designing a polymer/metallic oxide heterojunction with efficient electron transfer for enhanced photocatalytic CO2 reduction.

1. Introduction

With the rapid development of society and economy, the mass consumption of fossil fuels has caused severe environmental pollution and led to sharp declines in their reserves. The excessive emissions of carbon dioxide (CO2) in recent years have resulted in serious environmental issues, such as the greenhouse effect, global warming, sea ice melting, and sea-level rise [1,2,3,4]. It is urgent to eliminate CO2 in the atmosphere for sustainable development. The conversion of CO2 into high-value-added chemicals and fuels eliminates CO2 in the atmosphere, solves the energy crisis, and reduces environmental pollution. Converting CO2 into high-value-added chemicals by solar energy is a promising approach which is environmentally benign [5,6,7,8]. Solar energy being an abundant renewable resource is the powerful motivation to employ photocatalytic CO2 reduction to produce high-value-added chemicals and fuels.
The CO2 molecule is chemically stable and inert under normal conditions. Therefore, the activation of CO2 by solar energy usually requires a photocatalyst to overcome the thermodynamic energy barrier. Metal dioxide semiconductors, polymer semiconductors, and organic semiconductors are widely used as photocatalysts for photocatalytic CO2 reduction. However, it is necessary to address issues such as the rapid recombination of photogenerated electron–hole pairs, as well as the poor photoredox ability of charge carriers, to improve the photocatalytic performance of semiconductor photocatalysts. Designing semiconductor/semiconductor interfaces for heterojunction materials can improve the efficiency of spatial charge separation, thereby suppressing charge carrier recombination and enhancing photocatalytic performance [9,10,11,12]. A heterojunction is an interface region formed by the contact of two different semiconductors. According to the conductive types of the two materials, heterojunctions can be divided into homojunctions (P-p junctions or N-n junctions) and heterojunctions (P-n or p-N junctions). Multilayer heterojunctions are called heterostructures. Therefore, heterojunction materials have become a promising solution to address the limitations of traditional photocatalysis.
Single photocatalysts suffer from unsatisfactory photocatalytic activity due to the rapid recombination of the photogenerated electrons and holes. As is well known, constructing heterojunctions can realize improved photocatalytic activity due to the enhanced separation and transfer of the photogenerated electrons and holes. Specifically, the construction of heterojunctions is a promising strategy to enhance photocatalytic CO2 reduction performance. Generally, a heterojunction consists of a reduction photocatalyst (RP) and an oxidation photocatalyst (OP). Driven by band bending and internal electric field (IEF), the electrons transfer between the RP and OP, leading to the photogenerated electrons of the RP with strong reduction ability and the photogenerated holes of the OP with strong oxidation. Therefore, the heterojunction structure can promote charge carrier transfer and improve photocatalytic CO2 reduction performance.
Polymer semiconductor materials are organic electronic materials composed of polymer compounds that have semiconductor properties and can be used to make semiconductor devices and integrated circuits. As a promising polymer semiconductor, graphitic carbon nitride (g-C3N4) has attracted increasing attention in photocatalysis due to its low cost, high thermal and chemical stability, easy preparation process, semiconductivity, and appropriate band gap [13,14,15,16]. Among various semiconductor photocatalysts, titanium dioxide (TiO2) has been widely used as a photocatalyst due to its non-toxicity, low cost, excellent sunlight harvesting capability, and high chemical stability [17,18,19,20]. Thus, g-C3N4 and TiO2 are widely used as catalysts for photocatalytic CO2 reduction to produce high-value-added chemicals by solar energy. However, g-C3N4 and TiO2 suffer from low photocatalytic activity owing to their poor photoreduction ability and fast recombination of photogenerated electrons and holes. Constructing a g-C3N4/TiO2 heterojunction for enhancing photocatalytic CO2 reduction is a feasible and effective approach [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35]. Due to the different energy band potentials, the coupling of g-C3N4 and TiO2 is expected to form a g-C3N4/TiO2 heterojunction which can promote the separation and transfer of the photogenerated electrons and holes, thereby improving the photocatalytic CO2 reduction activity.
Herein, the g-C3N4/TiO2 heterojunction is designed for enhanced photocatalytic CO2 reduction. The g-C3N4/TiO2 heterojunction photocatalyst exhibits higher CH4 (18.32 μmol·h−1·g−1) and CO (25.35 µmol·h−1·g−1) evolution rates, compared with g-C3N4 and TiO2. Although the g-C3N4/TiO2 heterojunction has promising prospects for enhanced photocatalytic CO2 reduction activity, the underlying mechanisms of the charge transfer and CO2 reduction pathways remain unknown. In this regard, we applied in situ irradiated X-ray photoelectron spectroscopy (XPS) to investigate the electron transfer in the g-C3N4/TiO2 heterojunction and in suit diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to detect the pathway of CO2 reduction on the surface of the g-C3N4/TiO2 photocatalyst during photocatalytic CO2 reduction. The efficient electron transfer in the g-C3N4/TiO2 heterojunction improved the photocatalytic CO2 reduction performance, which provides a promising approach to designing polymers/metal dioxide photocatalysts with efficient electron transfer for enhanced photocatalytic CO2 reduction.

2. Results and Discussion

2.1. Microstructure and Analysis of Physical Properties

TiO2 was dispersed on g-C3N4 to form a g-C3N4/TiO2 heterojunction (Figure 1a). Field-emission scanning electron microscopy (FSEM) was applied to investigate the microstructure of the photocatalysts. The microstructures of g-C3N4 and TiO2 are shown in Figures S1 and S2. g-C3N4 shows a big bulk structure with a size of 5–10 µm, while TiO2 is a small bulk structure with a size of 0.5–1 µm. As Figure 1b,c show, TiO2 is uniformly dispersed on the surface of g-C3N4, which implies the successful loading of TiO2 onto g-C3N4. The high-resolution TEM (HRTEM) image suggests that the lattice–fringe spacing of 0.35 nm is indexed into the (101) plane of TiO2 (Figure 1d), indicating the existence of TiO2 in the g-C3N4/TiO2 heterojunction. The X-ray powder diffraction (XRD) patterns were employed to study the phase structures of the samples. As shown in Figure 1e, the XRD patterns correspond to typical anatase TiO2 and g-C3N4. The peaks of g-C3N4 and TiO2 can be observed in g-C3N4/TiO2, which implies that TiO2 was successfully loaded onto g-C3N4 [13,20]. According to the XRD patterns, the average crystallite size of TiO2 was 55 nm. The above results indicate the g-C3N4/TiO2 heterojunction photocatalyst was successfully prepared. The nitrogen adsorption–desorption isotherms of g-C3N4, TiO2, and g-C3N4/TiO2 display typical type IV isotherms with the type H3 hysteresis loop (Figure 1f). The pore size distribution curves of g-C3N4, TiO2, and g-C3N4/TiO2 indicate that the photocatalysts have abundant mesoporous structures (Figure S1). The Brunauer–Emmett–Teller surface areas (SBET) of g-C3N4, TiO2, and g-C3N4/TiO2 were 8.2, 11.2, and 10.1 m2·g−1, respectively (Table S1).
To determine the light absorption of g-C3N4, TiO2, and g-C3N4/TiO2, the UV–vis diffuse reflectance spectra (UV–vis DRS) were measured. From Figure 2a, compared with TiO2, the absorption edge of g-C3N4/TiO2 shows a red shift. The absorption of g-C3N4 in the 350–520 nm region is stronger, which leads to g-C3N4/TiO2 enhancing the response to visible light. Meanwhile, compared with g-C3N4, the absorption of TiO2 in the ultraviolet region is stronger than that of g-C3N4; thus, g-C3N4/TiO2 improves absorption in the ultraviolet region. The result of the UV–vis DRS indicates strong ultraviolet–visible light absorption ability for g-C3N4/TiO2, which is advantageous for photocatalytic CO2 reduction. According to the Tauc plots (Figure 2b), the band gaps (Eg) of g-C3N4 and TiO2 were 2.55 and 3.05 eV, respectively. From the Mott–Schottky (M-S) curves, the flat-band potentials (Efb) of g-C3N4 and TiO2 were −0.97 and −0.55 eV (V vs. Ag/AgCl), respectively (Figure 2c). With reference to normal hydrogen electrode (NHE), the flat-band potentials (Efb) of g-C3N4 and TiO2 were −0.77 and −0.35 eV, respectively [36,37,38,39]. Generally, for n-type semiconductors, the Efb roughly equals to the conduction band (CB) potential. Therefore, according to the formula Eg = Evb − Ecb, the valence band (VB) potentials of g-C3N4 and TiO2 were 1.78 and 2.70 eV, respectively [40,41,42,43]. The estimated band structures of g-C3N4 and TiO2 are shown in Figure 2d.

2.2. Electron Transfer Route

X-ray photoelectron spectroscopy (XPS) was employed to identify the surface elements and electronic chemical states of the photocatalysts. As presented in Figure S3, XPS survey spectra show that C, N, Ti, and O elements can be detected in g-C3N4/TiO2, indicating that the g-C3N4/TiO2 heterojunction was successfully prepared. In Figure 3a, the peaks centered at 458.7 and 464.4 eV can be assigned to the 2p3/2 and 2p1/2 states of Ti4+ in TiO2, respectively. Additionally, the peaks located at 530.8 and 531.9 eV are attributed to the Ti-O bond and −OH group, respectively (Figure 3b). For g-C3N4, the peaks centered at 399.3, 400.0, and 401.7 eV can be ascribed to C-N=C, N-(C)3, and C-N-H, respectively. In general, the change in electron binding energy reflects the change in electron density. Therefore, the change in electron binding energy can verify the direction of charge carrier transfer in heterojunction photocatalysts [44,45,46]. As shown in Figure 3a,b, the binding energies of Ti 2p and O 1s in g-C3N4/TiO2 shift toward higher energy levels, indicating loss of electrons for TiO2. Compared with g-C3N4, the binding energy of N 1s in g-C3N4/TiO2 shifts to a lower energy level, implying that g-C3N4 gains electrons (Figure 3c). The electrons migrate from TiO2 to g-C3N4 in the g-C3N4/TiO2 heterojunction. The binding energies of Ti 2p and O 1s in g-C3N4/TiO2 shift toward lower energy levels under light irradiation, while the binding energy of N 1s shift to a higher energy level. The in situ high-resolution XPS data indicate the photogenerated electron transfer from g-C3N4 to TiO2.
On account of the results of in situ irradiated XPS, the electron transfer route between g-C3N4 and TiO2 is proposed. The work functions (Φ) of g-C3N4 and TiO2 were calculated by density functional theory (DFT). Figure 3d,e show that the work function of g-C3N4 (0.174 Ha) is larger than that of TiO2 (0.165 Ha), which indicates that the Fermi level of TiO2 is higher than that of g-C3N4. As shown in Figure 3f, before the formation of the g-C3N4/TiO2 heterojunction, g-C3N4 has larger W1 and lower Ef1, while TiO2 has smaller W2 and higher Ef2. The difference in work function will lead to band bending and internal electric field (IEF) at the interface between g-C3N4 and TiO2. After the contact of g-C3N4 with TiO2, the g-C3N4/TiO2 heterojunction forms, with the electron transfer from TiO2 to g-C3N4 being driven by the IEF. When the g-C3N4/TiO2 heterojunction is under light irradiation, the electrons are excited from the VB to the CB of g-C3N4 and TiO2, respectively. Then, the photogenerated electrons migrate from the CB of g-C3N4 to the CB of TiO2 and the holes transfer from the VB of TiO2 to the VB of g-C3N4, leading to the efficient separation and transfer of the photogenerated electron–hole pairs. The efficient separation and transfer of the photogenerated electron–hole pairs in the heterojunction is crucial to the improvement in photocatalytic CO2 reduction performance. The photogenerated electrons of the RP with strong reduction ability in TiO2 and the photogenerated holes of the OP with strong oxidation ability in g-C3N4 are preserved.
Photocurrent in semiconductors is caused by light irradiation. When materials are under light irradiation, the photon excites the electrons in the valence band of the semiconductor to the conduction band and generates an electric current. Electrochemical impedance is the electrical resistance during charge carrier transport in photocatalysts. Transient photocurrent spectra (TPC) and electrochemical impedance spectroscopy (EIS) data can be used to examine charge carrier separation and transfer behavior in a photocatalyst. As can be seen in Figure 4a, the photocurrent density of g-C3N4/TiO2 is larger than those of g-C3N4 and TiO2. g-C3N4/TiO2 shows a smaller electrochemical impedance spectroscopy radius than g-C3N4 and TiO2 (Figure 4b). The results of TPC and EIS illustrate efficient photogenerated charge carrier separation and transfer in the g-C3N4/TiO2 heterojunction. The fluorescence lifetimes of g-C3N4, TiO2, and g-C3N4/TiO2 are shown in Figure S5. The average lifetimes (τave) of the photocarriers in g-C3N4, TiO2, and g-C3N4/TiO2 are 2.85, 1.85, and 1.15 ns, respectively. The results of fluorescence lifetime indicate enhanced electron separation and transfer process in the g-C3N4/TiO2 heterojunction.

2.3. Photocatalytic CO2 Reduction Performance

The photocatalytic CO2 reduction performances of different photocatalysts were measured under UV–vis light. As presented in Figure 5a, CH4 and CO are the major photoreduction products in the photocatalytic CO2 reduction processes of g-C3N4, TiO2, and g-C3N4/TiO2. The CH4 and CO evolution rates of g-C3N4/TiO2 are up to 18.32 and 25.35 µmol·h−1·g−1, respectively, while the CH4 evolution rates of g-C3N4 and TiO2 are 7.32 and 2.07 µmol·h−1·g−1, respectively. The CH4 evolution rates of g-C3N4/TiO2 are 2.5 and 8.9 times larger than those of g-C3N4 and TiO2, respectively. The CO evolution rates of g-C3N4 and TiO2 are 1.63 and 16.34 µmol·h−1·g−1, respectively. The CO evolution rates of g-C3N4/TiO2 are 15.6 and 1.6 times larger than those of g-C3N4 and TiO2, respectively. The CH4 and CO evolution rates of g-C3N4/TiO2 grow nearly linearly over time (Figure 5b,c) and are much higher than those of g-C3N4 and TiO2 during photocatalytic CO2 reduction. Compared with g-C3N4 and TiO2, g-C3N4/TiO2 exhibits excellent photocatalytic CO2 reduction activity. Moreover, the photocatalytic CO2 reduction performance of g-C3N4/TiO2 is higher than those of many other photocatalysts under similar conditions (Table S2, Supporting Information). The CH4 and CO yields of g-C3N4/TiO2 exhibited minor changes during the cycled stability test (Figure 5d), and the crystal structure of g-C3N4/TiO2 remained stable after the cycled stability test (Figures S6 and S7), indicating that g-C3N4/TiO2 remains stable during photocatalytic CO2 reduction. The CH4 and CO evolution rates of g-C3N4/TiO2 indicate that the efficient electron transfer in the g-C3N4/TiO2 heterojunction leads to strong oxidation and reduction abilities, thereby improving the photocatalytic CO2 reduction activity.
In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed to detect the CO2 reduction pathway and possible reaction intermediates on the surface of the g-C3N4/TiO2 photocatalyst during photocatalytic CO2 reduction. The DRIFTS data of g-C3N4/TiO2 in the dark and under light irradiation were collected during photocatalytic CO2 reduction (Figure 6). Initially, no absorption peaks could be detected on the surface of the g-C3N4/TiO2 photocatalyst in the dark (0 min). With the introduction of CO2 and H2 (10–30 min), monodentate carbonate species (m-CO32−; 1303, 1424, 1489, and 1558 cm−1) and bidentate carbonate species (b-CO32−; 1520 and 1650 cm−1) could be detected [40,43,47,48]. The existence of m-CO32− and b-CO32− indicates that CO2 was successfully adsorbed and activated on the surface of g-C3N4/TiO2. New peaks appeared in the spectroscopy under light irradiation (10–40 min). Carboxylate species (COO−; 1339 and 1363 cm−1), formaldehyde species (HCHO−; 1507 and 1749 cm−1), and methoxy groups (CH3O−; 1681, 1715 and 1731 cm−1) were detected on the surface of the g-C3N4/TiO2 photocatalyst during photocatalytic CO2 reduction [43,47,49]. COO−, HCHO−, and CH3O− are the crucial reaction intermediates in the formation of CH4 and CO. Thus, the proposed pathway for photocatalytic CO2 reduction over g-C3N4/TiO2 can be briefly expressed as follows: CO2 → COO− → HCHO− → CH3O− → CH4 and CO2 → COO− →CO. The g-C3N4/TiO2 photocatalyst-induced process of the photocatalytic reduction of CO2 to CH4 and CO involves electron transfer steps that can be expressed as follows: CO2 + 8H+ + 8e− → CH4 + 2H2O and CO2 + 2H+ + 2e− → CO + H2O.
The mechanism of the N4/TiO2 heterojunction for photocatalytic CO2 reduction can be summarized as follows (Figure 7): Firstly, when TiO2 comes into contact with g-C3N4, electrons transfer from TiO2 to g-C3N4 because of the different work function and Fermi level. Subsequently, g-C3N4/TiO2 is exposed to light irradiation, and the photogenerated electrons are excited from the VB to the CB of g-C3N4 and TiO2, respectively. Then, the photogenerated electrons of the CB of g-C3N4 migrate to the CB of TiO2; the holes transfer from the VB of TiO2 to the VB of g-C3N4, improving the separation and transfer of the photogenerated electron–hole pairs. The efficient electron transfer in the g-C3N4/TiO2 heterojunction results in the photogenerated electrons of the RP with strong reduction ability in TiO2 and the photogenerated holes of the OP with strong oxidation ability in g-C3N4. Meanwhile, H2 is oxidized to H+ on the VB of g-C3N4, and CO2 is reduced to CH4 and CO on the CB of TiO2. The efficient charge carrier transfer in the g-C3N4/TiO2 heterojunction enhances the photocatalytic CO2 reduction performance.

3. Materials and Methods

3.1. Materials

Melamine (C3H6N6) and titanium butoxide (C16H36O4Ti) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Absolute alcohol was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used without further purification.

3.2. Synthesis of g-C3N4/TiO2

A total of 2 g of melamine was put into the crucible and then calcinated at 550 °C for 5 h with a ramp rate of 2.5 °C min−1 in a muffle furnace. After cooling to room temperature, bulk g-C3N4 was obtained. Bulk g-C3N4 was ground with a mortar to obtain g-C3N4 powder. Next, 0.5 g of g-C3N4 powder was dispersed into 100 mL of absolute alcohol under magnetic stirring for 2 h. Then, 5 mL of titanium butoxide was added into the above obtained suspension under magnetic stirring for 6 h. Finally, the mixture was calcinated at 550 °C for 5 h with a ramp rate of 2.5 °C min−1 in a muffle furnace to obtain g-C3N4/TiO2. TiO2 was prepared in the same way without the addition of g-C3N4 powder.

3.3. Characterization

The microstructures of g-C3N4, TiO2 and g-C3N4/TiO2 were observed by FSEM (Hitach S-4800, Tokyo, Japan) and TEM (Talos F200S, New York, NY, USA). The XRD patterns of g-C3N4, TiO2, and g-C3N4/TiO2 were obtained by an X-ray diffraction machine (Bruker D8 Advance, Billerica, MA, USA) using Cu Kα (λ = 0.15418 nm) radiation. The X-ray photoelectron spectroscopy (XPS; Omicron Sphera II, Taunusstein, Germany) data of g-C3N4, TiO2, and g-C3N4/TiO2 were obtained on a mono-chromated Al Kα X-ray source (hv = 1486.6 eV) at 15 kV/150 W to detect the chemical states of the elements in the samples. Nitrogen adsorption isotherms and the Brunauer–Emmett–Teller (BET) surface areas of g-C3N4, TiO2, and g-C3N4/TiO2 were obtained with a nitrogen adsorption apparatus (TriStar II 2020, Micromeritics, Norcross, GA, USA). The UV–vis diffuse reflectance spectra (UV–vis DRS) of g-C3N4, TiO2, and g-C3N4/TiO2 were acquired by a UV–VIS-NIR spectrometer (Lambda 750S; PerkinElmer, Freehold, NJ, USA) over a range of 200–800 nm. Transient photocurrent spectra (TPC), electrochemical impedance spectroscopy (EIS), and the Mott–Schottky (M-S) curve of g-C3N4, TiO2, and g-C3N4/TiO2 were measured with an electrochemical workstation (CS2350H; CorrTest, Wuhan, China) in 0.5 M Na2SO4 solution at room temperature by using a 300 W xenon lamp (PLS-SXE300; PerfectLight, Beijing, China). A Pt plate and Ag/AgCl were employed as the counter electrode and the reference electrode, respectively. In situ time-resolved DRIFT spectra of g-C3N4/TiO2 were obtained by a Fourier transform infrared spectrometer (VERTEX V80; Bruker, Hong Kong, China). The work functions of g-C3N4 and TiO2 were calculated by density functional theory (DFT) in Materials Studio.

3.4. Photocatalytic CO2 Reduction

A total of 100 mg of catalyst was placed into a 200 mL stainless steel reactor with an optical quartz window at the top. The reactor was firstly vacuumed; then, CO2 and H2 were introduced into the stainless steel reactor in a volume ratio of 1:4 for half an hour to blow out the air in the reactor. A 300 W xenon lamp (PLS-SXE300; Beijing PerfectLight, Beijing, China) with a filter (AM 1.5 G; Ceaulight Technology Co., Ltd., Beijing, China) was employed to simulate solar illumination at about 100 mW·cm−2. The reactor was irradiated by a xenon lamp for the desired time. During the photocatalytic reaction process, the gaseous mixture was periodically sampled from the stainless steel reactor every 0.5 h and analyzed by gas chromatography.

4. Conclusions

In summary, the g-C3N4/TiO2 heterojunction was successfully prepared by growing TiO2 on g-C3N4 for enhanced photocatalytic CO2 reduction. Compared with g-C3N4 and TiO2, the photocatalytic CO2 reduction performance of g-C3N4/TiO2 was significantly enhanced due to the efficient charge carrier transfer in the heterojunction. The electron transfer mechanism in the g-C3N4/TiO2 heterojunction was verified by in situ irradiated XPS. The photogenerated electrons migrate from g-C3N4 to TiO2, while holes transfer from TiO2 to g-C3N4. The photogenerated electrons of the reduction photocatalyst (RP) with strong reduction ability in TiO2 and the photogenerated holes of the oxidation photocatalyst (OP) with strong oxidation ability in g-C3N4 are preserved for photocatalytic CO2 reduction. The pathway of the photocatalytic reduction of CO2 to CH4 and CO on g-C3N4/TiO2 was revealed by in situ DRIFTS. The pathway of the photocatalytic reduction of CO2 to CH4 and CO over g-C3N4/TiO2 is briefly expressed as CO2 → COO− → HCHO− → CH3O− → CH4 and CO2 → COO− → CO. Therefore, this work reveals the electron transfer mechanism in polymer/metallic oxide heterojunctions and provides a feasible approach to design polymer/metallic oxide heterojunctions with efficient electron transfer for enhanced photocatalytic CO2 reduction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14060335/s1, Figure S1. Pore size distribution curves of g-C3N4, TiO2, and g-C3N4/TiO2, Figure S2: FESEM images of g-C3N4, Figure S3: FESEM images of TiO2, Figure S4: XPS survey spectra of g-C3N4, TiO2, and g-C3N4/TiO2, Figure S5. Time-resolved PL spectra of g-C3N4, TiO2, and g-C3N4/TiO2, Figure S6. SEM images after the stability tests of g-C3N4/TiO2, Figure S7. XRD patterns before and after the stability tests of g-C3N4/TiO2, Table S1: Brunauer–Emmett–Teller surface areas (SBET) of samples, Table S2. Performance comparison of TiO2-based and g-C3N4-based photocatalytic materials for CO2 reduction, Table S3. Different mixing rations of g-C3N4 and TiO2 photocatalysts for CO2 reduction. Refs. [45,50,51,52,53] are cited in Supplementary Materials.

Author Contributions

Conceptualization, P.J. and K.W.; methodology, W.L.; validation, P.J., K.W. and W.L.; formal analysis, P.J.; investigation, Y.Y.; resources, K.W.; data curation, W.L.; writing—original draft preparation, P.J.; writing—review and editing, Y.Y.; visualization, Y.Y.; supervision, Y.Y.; project administration, P.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be made available upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Scheme of g-C3N4/TiO2; (b,c) FSEM images of g-C3N4/TiO2; (d) high-resolution TEM (HRTEM) image of g-C3N4/TiO2; (e) XRD patterns of g-C3N4, TiO2, and g-C3N4/TiO2; (f) nitrogen adsorption–desorption isotherms of g-C3N4, TiO2, and g-C3N4/TiO2.
Figure 1. (a) Scheme of g-C3N4/TiO2; (b,c) FSEM images of g-C3N4/TiO2; (d) high-resolution TEM (HRTEM) image of g-C3N4/TiO2; (e) XRD patterns of g-C3N4, TiO2, and g-C3N4/TiO2; (f) nitrogen adsorption–desorption isotherms of g-C3N4, TiO2, and g-C3N4/TiO2.
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Figure 2. (a) UV–vis DRS of g-C3N4, TiO2, and g-C3N4/TiO2; (b) Tauc plots of g-C3N4, TiO2, and g-C3N4/TiO2; (c) M-S plots of g-C3N4, and TiO2; (d) estimated band structures of g-C3N4 and TiO2.
Figure 2. (a) UV–vis DRS of g-C3N4, TiO2, and g-C3N4/TiO2; (b) Tauc plots of g-C3N4, TiO2, and g-C3N4/TiO2; (c) M-S plots of g-C3N4, and TiO2; (d) estimated band structures of g-C3N4 and TiO2.
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Figure 3. In situ high-resolution XPS results of (a) Ti 2p, (b) O 1s, and (c) N 1s of g-C3N4, TiO2, and g-C3N4/TiO2; work functions of (d) TiO2 and (e) g-C3N4; (f) schematic illustrations of electron transfer mechanism between g-C3N4 and TiO2 before contact, after contact, and after contact under light irradiation.
Figure 3. In situ high-resolution XPS results of (a) Ti 2p, (b) O 1s, and (c) N 1s of g-C3N4, TiO2, and g-C3N4/TiO2; work functions of (d) TiO2 and (e) g-C3N4; (f) schematic illustrations of electron transfer mechanism between g-C3N4 and TiO2 before contact, after contact, and after contact under light irradiation.
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Figure 4. (a) Transient photocurrent spectra (TPC) and (b) electrochemical impedance spectroscopy (EIS) results of g-C3N4, TiO2, and g-C3N4/TiO2.
Figure 4. (a) Transient photocurrent spectra (TPC) and (b) electrochemical impedance spectroscopy (EIS) results of g-C3N4, TiO2, and g-C3N4/TiO2.
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Figure 5. (a) Product evolution rate via photocatalytic CO2 reduction reaction; (b) time-dependent CH4 evolution; (c) time-dependent CO evolution of g-C3N4, TiO2, and g-C3N4/TiO2; (d) photocatalytic stability test of g-C3N4/TiO2.
Figure 5. (a) Product evolution rate via photocatalytic CO2 reduction reaction; (b) time-dependent CH4 evolution; (c) time-dependent CO evolution of g-C3N4, TiO2, and g-C3N4/TiO2; (d) photocatalytic stability test of g-C3N4/TiO2.
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Figure 6. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results of g-C3N4/TiO2.
Figure 6. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results of g-C3N4/TiO2.
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Figure 7. Schematic diagram of photocatalytic CO2 reduction over g-C3N4/TiO2.
Figure 7. Schematic diagram of photocatalytic CO2 reduction over g-C3N4/TiO2.
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Jiang, P.; Yu, Y.; Wang, K.; Liu, W. Efficient Electron Transfer in g-C3N4/TiO2 Heterojunction for Enhanced Photocatalytic CO2 Reduction. Catalysts 2024, 14, 335. https://doi.org/10.3390/catal14060335

AMA Style

Jiang P, Yu Y, Wang K, Liu W. Efficient Electron Transfer in g-C3N4/TiO2 Heterojunction for Enhanced Photocatalytic CO2 Reduction. Catalysts. 2024; 14(6):335. https://doi.org/10.3390/catal14060335

Chicago/Turabian Style

Jiang, Peng, Yang Yu, Kun Wang, and Wenrui Liu. 2024. "Efficient Electron Transfer in g-C3N4/TiO2 Heterojunction for Enhanced Photocatalytic CO2 Reduction" Catalysts 14, no. 6: 335. https://doi.org/10.3390/catal14060335

APA Style

Jiang, P., Yu, Y., Wang, K., & Liu, W. (2024). Efficient Electron Transfer in g-C3N4/TiO2 Heterojunction for Enhanced Photocatalytic CO2 Reduction. Catalysts, 14(6), 335. https://doi.org/10.3390/catal14060335

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