Research on Cu-Site Modification of g-C₃N₄/CeO₂-like Z-Scheme Heterojunction for Enhancing CO₂ Reduction and Mechanism Insight

Abstract

In this work, the successful synthesis of a Cu@g-C₃N₄/CeO₂-like Z-scheme heterojunction through hydrothermal and photo-deposition methods represents high CO₂ reduction activity with remarkable CO selectivity, as evidenced by the impressive CO yield of 33.8 μmol/g for Cu@g-C₃N₄/CeO₂, which is over 10 times higher than that of g-C₃N₄ and CeO₂ individually. The characterization and control experimental results indicate that the formation of heterojunctions and the introduction of Cu sites promote charge separation and the transfer of hot electrons, as well as the photothermal effect, which are the essential reasons for the improved CO₂ reduction activity. Remarkably, Cu@g-C₃N₄/CeO₂ still exhibits about 92% performance even after multiple cycles. In situ FTIR was utilized to confirm the production of COOH* at 1472 cm⁻¹ and to elucidate the mechanism behind the high selectivity for CO production. The study’s investigation into the wide-ranging applicability of the Cu@g-C₃N₄/CeO₂-like Z-scheme heterojunction catalysts is noteworthy, and the exploration of potential reaction mechanisms for CO2 reduction adds valuable insights to the field of catalysis.

1. Introduction

In the face of the escalating energy crisis and environmental degradation spurred by the rapid pace of global industrialization, the quest for sustainable and clean energy solutions has become more critical than ever [1,2,3]. Solar energy stands out as a beacon of hope, offering a pollution-free, inexhaustible, and renewable source of power that is poised to play a pivotal role in the future landscape of energy production [4]. The harnessing of this abundant energy through semiconductor photocatalytic technology reduction of carbon dioxide (CO₂) into useful chemicals or fuel presents a dual-edged sword in tackling not only energy scarcity but also the pressing issue of the environmental greenhouse effect [5].

Graphitic carbon nitride (g-C₃N₄) has emerged as a promising candidate due to its remarkable stability, non-toxic nature, ready availability from abundant precursors, and economic feasibility [6]. However, the inherent limitations of pure g-C₃N₄, such as its low solar energy utilization efficiency, modest surface area, and high recombination rates of photogenerated charge carriers, have hindered its full potential as an efficient photocatalyst [7]. To overcome these challenges, researchers have been exploring the construction of g-C₃N₄-based photocatalysts with enhanced performance [8,9,10,11,12]. One such approach involves the design and fabrication of g-C₃N₄-based heterojunctions [13], which, through bandgap engineering, can significantly boost the response to visible light [14,15,16], charge separation efficiency, and overall photocatalytic activity of g-C₃N₄ [17]. Especially, the Z-scheme heterojunction with high redox potential exhibits excellent photocatalytic performance, which can reduce CO₂ to high-value-added hydrocarbon fuels (It refers to hydrocarbons with high market value and application value, which are usually produced through specific chemical or biological processes and have high energy density and/or specific chemical properties, making them widely used in energy, chemical, transportation and other fields).

Cerium dioxide (CeO₂) has garnered significant attention due to its exceptional properties, such as low cost, high stability, high oxygen storage capacity (as shown in Figure 1a), rich array of oxygen vacancies, reversible Ce³⁺/Ce⁴⁺ redox couples, and robust resistance to photocorrosion [18,19,20]. The well-matched band structures to g-C₃N₄ to form CeO₂/g-C₃N₄ Z-scheme heterojunctions has attracted widespread attention [21,22,23]. As far as we know the CeO₂/g-C₃N₄ Z-scheme has been extensively reported on CO₂ reduction, but there are still some issues: 1. The synthesis process require multiple steps and high temperatures, which increases production costs and complicates scalability. 2. The selectivity towards a specific product such as methane, ethylene, or methanol is often not optimal. Unwanted side reactions or competing pathways can lead to a mix of products, reducing the overall efficiency and economic viability. 3. The catalytic activity of the CeO₂/g-C₃N₄ composite under visible light or solar irradiation is not ideal. Therefore, it is necessary to further modify the CeO₂/g-C₃N₄ Z-scheme and systematically study the CO₂ reduction performance and mechanism.

Through literature research, introducing heteroatoms into the g-C₃N₄ matrix or CeO₂ lattice seems to be a very effective method. The structure of g-C₃N₄ is shown in Figure 1b. The doping of transition metals such as Cu, Co, Ce, etc. [24,25,26] within g-C₃N₄ or CeO₂ could adjust the electronic structure, band gap and augment the charge separation process of the composite catalyst, as it allows for a sustained supply of electrons and holes that participate in the reduction and oxidation reactions, respectively [27]. Among these transition metals, Cu has attracted much attention because Cu 2p orbitals can hold more electrons, effectively facilitating the charge carrier transfer and further improving the CO₂ photoreduction capability of the catalyst. Moreover, the Cu can also be excited and generate the heat to supply electrons, making it easier for these hot electrons to participate in the CO₂ reduction reaction [28]. Therefore, the addition of Cu not only has the potential to establish a more effective barrier against electron-hole recombination [29,30,31], but also offers a novel approach to CO₂ reduction by involving thermal electrons in the reaction, thereby further improving the photocatalytic efficiency under visible light irradiation [32].

Thus, a novel Cu@CeO₂/g-C₃N₄-like Z-scheme heterojunction photocatalyst was prepared; by adding Cu sites (as shown in Figure 1c) to increase the thermal reaction pathway, the separation efficiency of electrons and holes can be further improved to optimize CO₂ conversion. By combining materials science strategies (TEM, electrochemical) with advanced spectroscopic techniques (such as the in-situ FT-IR and FL), researchers strive to gain a deeper understanding of the complex reaction dynamics at play, with the ultimate goal of advancing sustainable technologies for CO₂ utilization and contributing to the global effort against climate change.

2. Results and Discussion

2.1. Photocatalyst Characterization

The microstructures of g-C₃N₄, CeO₂, and Cu@g-C₃N₄/CeO₂ were evaluated using SEM and TEM. The results show that the g-C₃N₄ exhibits a uniform two-dimensional micro-sheet morphology with a diameter ranging from approximately 1.2–1.5 μm, as depicted in Figure 2a,d. The CeO₂ also possess a two-dimensional ultra-thin micro-sheet structure with a smooth surface, as illustrated in Figure 2b,e. Upon the formation of the Cu@g-C₃N₄/CeO₂, the surface of the CeO₂ micro-sheets exhibited a noticeable increase in roughness and thickness. The SEM and TEM images in Figure 2c,f demonstrate the tight binding between the CeO₂ and g-C₃N₄, along with the successful loading copper (Cu) [33,34]. The integration of the two-dimensional g-C₃N₄ with the two-dimensional CeO₂ and the loading of Cu nanoparticles is anticipated to enhance the transfer and separation of photogenerated charges. This synergistic effect is expected to augment the number of active sites available for photocatalysis, thereby improving the overall catalytic activity of the material. Additionally, the elemental distribution and composition within Cu@g-C₃N₄/CeO₂ were investigated using EDX-mapping. As shown in Figure 2(g1–g5), the mapping reveals a uniform distribution of Cu, C, N, Ce, and O elements across the surface of Cu@g-C₃N₄/CeO₂. In summary, the comprehensive characterization of the photocatalyst using SEM, TEM, and EDX mapping analysis confirms the successful fabrication of the Cu@g-C₃N₄/CeO₂. The unique structural features and elemental composition of the composite are expected to contribute to its enhanced photocatalytic performance.

2.2. Surface Composition and Photoelectric Analysis

Figure 3a shows the XRD patterns of the as-prepared samples. For CeO₂, the diffraction peaks at 2θ = 28.5°, 33°, 47.4°, 56.3°, 59.3°, 69.3°, 76.6° and 79°; these peaks are consistent with the material’s cubic fluorite crystal structure, which belongs to the Fm-3m space group and can be indexed to the (111), (200), (220), (311), (222), (400), (331) and (420) planes [35]. g-C₃N₄ displays a diffraction peak at 2θ = 27.3°, corresponding to the (002) plane and reflecting the interlayer spacing [14]. The Cu@g-C₃N₄/CeO₂ exhibits diffraction peaks at 28.5°, 33.0°, 47.4°, and 56.3°, which coincide with the known peaks of CeO₂ [36]. This alignment suggests the successful formation of a hybrid material through the integration of CeO₂ with g-C₃N₄. The preservation of CeO₂’s crystallographic planes in the hybrid is evidenced by the correspondence of these peaks to the (111), (200), (220), and (311) planes of CeO₂. The characteristic peak of g-C₃N₄ at 27.30° may be either influenced or superimposed upon by the hybridization process with CeO₂ [37,38]. The minimal presence of Cu peaks should be attributed to the metal’s uniform dispersion and low concentration within Cu@g-C₃N₄/CeO₂. Figure 3b displays the survey spectrum of CeO₂, g-C₃N₄, Cu@g-C₃N₄/CeO₂, which proved Cu@g-C₃N₄/CeO₂ was made up of C, N, O, Ce and Cu. And C, N, O, Ce and Cu had atomic percentages of 26.12%, 8.11%, 34.24%, 30.33%, and 1.21%, respectively. The presence of C and N elements were found in g-C₃N₄, Ce and O elements in CeO₂, and C, N, Ce, and O elements in Cu@ g-C₃N₄/CeO_2; Cu was not effectively detected due to its low concentration. Figure 3c shows the C1s spectra of g-C₃N₄ and Cu@g-C₃N₄/CeO₂, featured peaks at 288.4 eV and 284.5 eV, corresponding to sp² hybridized carbon (C=N–C) and hydroxylated carbon species with C–O and C–OH, or specific out-of-plane sp3 C–N species [13,39,40]. Concurrently, the O1s high-resolution XPS spectrum of g-C₃N₄ is also displayed in Figure 3d, where the peaks at 529.5 eV and 532.2 eV are assigned to the O=C and O–C groups, respectively. For Cu@g-C₃N₄/CeO₂, these peaks exhibited a slight shift towards higher binding energy relative to pure g-C₃N₄ [41], suggesting a significant interaction between CeO₂ and g-C₃N₄. As depicted in Figure 3e, the N1s spectra of g-C₃N₄ and Cu@g-C₃N₄/CeO₂ featured peaks at 395.1 eV and 398.7 eV, corresponding to sp² hybridized carbon (C=N–C). Figure 3f presents the high-resolution XPS spectra of the Ce 3d levels for pure CeO₂ and Cu@g-C₃N₄/CeO₂. The Ce 3d energy level spectrum of the CeO₂ sample can be deconvoluted into six contributions at 882.5 eV, 888.9 eV, 898.4 eV, 901.0 eV, 907.9 eV, and 916.7 eV, labeled as ν, ν′, ν″, μ, μ′, and μ″, where ν and μ represent the spin-orbit split states of Ce 3d_5/2 and Ce 3d_3/2, respectively. The peaks labeled as ν, ν′, ν″, and μ are characteristic of Ce⁴⁺, while the peaks labeled as ν″ and μ″ indicate the presence of Ce³⁺ ions. The coexistence of Ce³⁺/Ce⁴⁺ on the surface of Cu@g-C₃N₄/CeO₂ suggests that under oxygen-deficient conditions, CeO₂ can release lattice oxygen, which has an active effect on surface adsorption [42]. Some Ce⁴⁺ ions are reduced to Ce³⁺, forming oxygen vacancies. Under oxygen-rich conditions, the adsorbed oxygen is stored in the form of lattice oxygen, allowing Ce³⁺ to be re-oxidized to Ce⁴⁺, which helps improve charge separation efficiency and enhances the photocatalytic activity. Additionally, the characteristic peaks of N 1s for Cu@g-C₃N₄/CeO₂ also show a slight shift towards higher binding energy compared to pure g-C₃N₄, further confirming the strong interaction between CeO₂ and g-C₃N₄.

The decrease in photoluminescence (PL) intensity and extended fluorescence (FL) lifetime of Cu@g-C₃N₄/CeO₂ compared to pure g-C₃N₄ under excitation light at 375 nm, as illustrated in Figure 4a,b, indicates a reduction in recombination of charge carriers within the heterojunction structure. This phenomenon suggests that the Cu@g-C₃N₄/CeO₂ Z-scheme catalyst effectively promotes the separation and migration of photoexcited electrons and holes, leading to enhanced photocatalytic performance, which are crucial factors in achieving high catalytic activity and selectivity in CO₂ reduction processes [43]. The UV–Vis diffuse reflectance spectra (DRS) analysis depicted in Figure 4c reveals that there is no significant change in the absorption edge and strength of CeO₂, g-C₃N₄, and Cu@g-C₃N₄/CeO₂ [44]. This observation suggests that the introduction of Cu into the g-C₃N₄/CeO₂ heterojunction structure does not notably alter the light-absorption properties of the materials. The consistent absorption characteristics across the different samples indicate that the enhanced photocatalytic performance of Cu@g-C₃N₄/CeO₂ is likely attributed to factors other than changes in light-absorption capacity. Figure 4d presents a detailed depiction of the photocurrent response characteristics of three distinct materials: pristine CeO₂, g-C₃N₄, and the novel Cu@g-C3N4/CeO2 composite. Observations indicate that both pristine CeO₂ and g-C₃N₄ exhibit relatively stable photocurrent responses under prolonged illumination, with no significant decay in photocurrent over time. This phenomenon reveals their superior photochemical stability, conducive to maintaining high efficiency under continuous light exposure [45,46]. Moreover, the Cu@g-C₃N₄/CeO₂ shows a higher photocurrent response compared to individual CeO₂ and g-C₃N₄. This finding not only confirms the significant enhancement in the photochemical activity but also implies Cu@g-C₃N₄/CeO₂ possesses superior charge carrier separation capabilities. Such improvements in performance are likely attributed to the synergistic effects between CeO₂ and g-C₃N₄ within the composite, as well as the optimization of charge transfer and transport properties brought about by the incorporation of Cu elements [13,14].

As shown in Figure 4e, CeO₂ and g-C₃N₄ exhibit higher impedance values, and the Cu@g-C₃N₄/CeO₂ exhibits the lowest charge transfer impedance; the low impedance characteristic is likely due to the synergistic effect between CeO₂ and g-C₃N₄, as well as the modification effect of Cu, which collectively promote efficient charge transfer, thereby enhancing the electrochemical activity of the material [47,48]. Additionally, the Mott–Schottky (M-S) plots in Figure 4f, at different frequencies of 2000 Hz, 2500 Hz, and 3000 Hz, of Cu@g-C₃N₄/CeO₂ consistently show a positive slope, indicating that the material is an n-type semiconductor with electrons as the primary charge carriers. Moreover, the flat-band potential (E_fb) is determined to be −0.35 V and remains stable across different frequencies, indicating the absence of defect or trap states within Cu@g-C₃N₄/CeO₂, implying minimal loss of charge carriers during their transport.

2.3. Photocatalytic Activity

This experiment used gas chromatography to detect the performance of catalysts in CO₂ reduction under a xenon lamp (1000 mW/cm²), stirring and adsorption for 30 min to achieve the adsorption and desorption equilibria. CO is our only CO₂ reduction product in this system. As shown in Figure 5a, in the presence of condensed water (6 °C), the CO yields of g-C₃N₄, CeO₂ and Cu@g-C₃N₄/CeO₂ are 0.56, 1.68, 11.44 μmol/g; the CO yield of Cu@g-C₃N₄/CeO₂ has been significantly improved; the reason should be the formation of the like-Z-scheme and the introducing of Cu reaction sites, allowing charge carriers to be quickly transferred and participate in CO₂ reduction reactions. In order to verify the effect of hot electrons on CO₂ reduction, the CO₂ reduction reaction without temperature control was conducted and it was found that the CO yields of g-C₃N₄, CeO₂ and Cu@g-C₃N₄/CeO₂ were 1.96, 3.28 and 33.82 μmol/g, respectively [49]; the result is shown in Figure 5b. Through data comparison, it was found that the CO performance without condensate water was more than three times higher than that with condensate water; this is because the addition of Cu makes better use of heat, converting some photo-generated charge carriers into hot electrons. These hot electrons not only participate in the reduction reaction of CO₂, but also bombard g-C₃N₄ at high speed due to the large amount of energy they carry, causing more charge carriers to participate in the chain reaction. This explains why higher CO yields can be achieved without condensation. Cycling experiment without condensate (Figure 5c) was carried out and it was found that after 5 cycles, the performance of Cu@g-C₃N₄/CeO₂ slightly decreased. XRD (X-ray diffraction) before and after the cycles shows that the crystal of Cu@g-C₃N₄/CeO₂ has not changed (Figure 6a). In order to determine that CO comes from CO₂, the different blank control experiments results are shown in Figure 6b. (a) Under a N₂ atmosphere for 4 h of illumination, no significant CO generation was detected. (b) Under a CO₂ atmosphere but without light, no significant CO generation was detected after 4 h. (c) Under normal testing conditions. (d) Under a CO₂ atmosphere illumination but without a catalyst, no significant CO generation was detected. Therefore, the prepared Cu@g-C₃N₄/CeO₂ can be identified as a stable photocatalyst.

2.4. Mechanism of CO₂ Photoreduction

The in situ FTIR analysis was used to detect the intermediate products, in an attempt to clarify the mechanism of the reaction process of CO₂ photoreduction. As shown in Figure 7, a peak at 1398 cm⁻¹ is b-CO₃²⁻, and the inverted peak at 1672 cm⁻¹ was attributed to the typical bending vibrations of molecular water. These peaks indicate the water molecules participate in the reaction process as the electron donors. With the LED illumination, two distinct positive peaks appeared, which belonged to the intermediates of the CO₂ photoreaction process; the peak at 1472 cm⁻¹ belonged to the COOH* [13], which is the critical intermediate from CO₂ to CO; moreover, the peaks at 1123 and 1063 cm⁻¹ belonged to HCO₃⁻, and the peaks at 1398 cm⁻¹ were considered to be the CO₃²⁻ from the hydrolysis of CO₂. Based on the in situ FTIR results, the CO₂ photoreduction process can be inferred as follows:

The CO₂ reduction mechanism of Cu@g-C₃N₄/CeO₂ is shown in Figure 8. The light energy generated under xenon lamp irradiation not only stimulates the generation of e⁻/h⁺ pairs but also causes intramolecular motion, resulting in an undeniable thermal effect. The thermal effect of this part further promotes the generation of electron hole pairs. At the same time, this part of electrons is generated by receiving the thermal energy of intramolecular motion, so we call them hot electrons. The addition of Cu reaction sites not only builds a bridge for high-speed electron conduction between the two, but also serves as a high-speed channel for heat conduction. This allows photo-generated electrons to be converted into hot electrons, which are always in a high-speed-motion state, making it difficult to recombine with holes. These electrons are difficult to recombine with holes, and the electron transport channel constructed by Cu@g-C₃N₄/CeO₂ makes it easier for them to be delivered to the catalyst surface and react with CO₂.

Cu@g-C₃N₄/CeO₂ + Photothermal → Cu@g-C₃N₄/CeO₂ (e⁻ + h⁺)

(1)

CO₂ → CO₂*

(2)

H₂O + h⁺ → ·OH + H⁺

(3)

H₂O + ·OH + 3h⁺ → O₂ + 3H⁺

(4)

2H⁺ + 2e⁻ → H₂

(5)

CO₂ + H₂O → H₂CO₃

(6)

H₂CO₃ → H⁺ + HCO₃⁻

(7)

HCO₃⁻ → H⁺ + CO₃²⁻

(8)

CO₂* + e⁻ + H⁺ → COOH*

(9)

COOH* + e⁻ + H⁺ → CO* + H₂O

(10)

CO* → CO

(11)

3. Experimental Section

3.1. Methods

3.1.1. Materials

The precursors used to prepare Cu@g-C₃N₄/CeO₂-like Z-scheme heterojunctions are Cupric Nitrate Trihydrate (Cu (NO₃)₂·3H₂O, ≥99.0%), bought from Macklin (Shanghai, China). Cerium nitrate hexahydrate (Ce (NO₃)₃·6H₂O ≥ 99.95%) was bought from Aladdin Bio-Chem Technology Co. (Shanghai, China). Ammonium bicarbonate (NH₄HCO₃ ≥ 97.0%), Urea (CO(NH₂)₂ ≥ 97.0%), Polyvinylpyrrolidone (PVP ≥ 97.0%) and Methyl alcohol (MeOH ≥ 97.0%) were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All experiments were performed with deionized water, and all reagents did not require further purification.

3.1.2. Synthesis of CeO₂ Micro-Sheets

Ce(NO₃)₃·6H₂O (0.013 mmol) and NH₄HCO₃ (0.037 mmol) were dissolved into 200 mL deionized water and stirred thoroughly. Then the mix was statically aged at 30 °C for 24 h and the white precipitate was obtained. After the sediment was separated by filtration, it was washed three times with deionized water and then dried under 60 °C for 8 h. Finally, the dried sample was transferred to a tube furnace and calcined at 500 °C for 4 h to obtain the CeO₂ sample.

3.1.3. Synthesis of g-C₃N₄ Micro-Sheets

The single-step pyrolysis method was employed to create g-C₃N₄ by using CO(NH₂)₂ as a precursor. In a muffle furnace, 10 g of urea were heated to 550 °C for 240 min to obtain g-C₃N₄ micro-sheets [13].

3.1.4. Preparation of g-C₃N₄/CeO₂ Photocatalysts

CeO₂ (0.2 g) and g-C₃N₄ (0.8 g) were dissolved in a 10 mL methanol solution, stirred thoroughly at 60 °C for 24 h, followed by drying to obtain the solid powder of g-C₃N₄/CeO₂ [14].

3.1.5. Preparation of Cu@g-C₃N₄/CeO₂ Photocatalysts

A total of 0.1 g of the obtained solid powder from the above, along with 0.01 g of Cu(NO₃)₂, was dispersed in 20 mL of deionized water and sonicated for 10 min. Subsequently, the dispersion was subjected to photo-deposition under UV light (365 nm) for 30 min. Finally, the resulting Cu@g-C₃N₄/CeO₂ was obtained after centrifugation and drying.

3.2. Characterizations

The crystal structure was examined by X-ray diffraction (XRD model, MAC Science, Yokohama, Japan) using Ni-filtered Cu-K radiation. At 2θ, the scanning range was 10–80° and the scanning speed 10°/min. The measurements of X-ray photoelectron spectroscopy (XPS) were analyzed on a PerkinElmer PHI 5300 (PerkinElmer, Shanghai, China) instrument to check the element’s status on the sample surface tested with a monochrome Mg K source. The High0 Resolution Transmission Electron Microscope (HRTEM) images were taken on a field-emission electron microscope (Tecnai G2 F20, Hitachi, Tokyo, Japan) with an acceleration voltage. The XRD patterns were performed on a Shimazu-6100 powder X-ray (Shimadzu, Tokyo, Japan) diffractometer with Cu Kα radiation at a scan rate of 6° min⁻¹. The XPS data were obtained using a Shimadzu/Krayos AXIS Ultra DLD (Hitachi, Tokyo, Japan). The scanning electron microscopy (SEM) images were characterized on a field-emission scanning electron microscope (FE-SEM, JSM-7001F, Hitachi, Tokyo, Japan). The ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) was carried out on a Shimadzu UV-3600 spectrometer. The photoluminescence (PL) spectra for solid power were investigated by an F4500 (Hitachi, Tokyo, Japan) and the xenon (Xe) lamp with an excitation wavelength of 375 nm. In situ Fourier transform infrared spectroscopy (in situ FTIR) was carried out to research the CO₂ photoreduction process (Thermo Fisher Nicolet iS-10, Thermo Fisher Scientific, Waltham, MA USA).

3.3. CO₂ Photoreduction Experiments

CO₂ photoreduction was carried out in a sealed self-made 50 mL quartz reactor with a 300 W Xe lamp (1000 mW/cm², Education Au-light Co., Ltd., Beijing, China) as the white-light source. In a typical procedure, 10 mg of catalyst was dispersed in 10 mL of deionized water. Then, it was subjected to ultrasound for 10 min to make it completely disperse. CO₂ was then introduced into the reactor for 20 min to completely remove air. Gas products were detected by a gas chromatography (GC 5890N, Education Au-light Co., Ltd., Beijing, China) equipped with hydrogen flame ionization detector (FID), and the carrier gas was N₂.

3.4. Photo-Electrochemical Measurements

The corresponding electrode was prepared as follows: 0.05 g of the prepared photocatalyst was dispersed in 3.0 mL of ethanol and 0.03 mL of oleic acid, and then 0.01 g of polyvinylpyrrolidone (PVP) was added. The above mixture was spin-coated on a 1 × 1 cm² fluorine-doped tin oxide glass electrode, and then dried at room temperature. The measurement system includes a standard three-electrode quartz cells with 0.5 M Na₂SO₄ electrolyte solution; a Pt wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrodes, respectively. The amplitude of applied sine wave potential in each case was 5 mV, which was carried out using the Chen Hua electrochemical workstation, and all electrochemical signals were recorded by a CHI660 B electrochemical analyzer (Chen Hua Instruments, Shanghai, China).

4. Conclusions

The Cu@g-C₃N₄/CeO₂-like Z-scheme heterojunction was meticulously fabricated through hydrothermal and photo-deposition techniques. It represents charming CO₂ reducing activity and product selectivity (CO selectivity 100%), attributed to the internal electric field between g-C₃N₄ and CeO₂, promoting the photogenerated charge transfer, as well as the introduction of Cu, which generates high-energy hot electrons, triggering a cascade of charge carrier generation. The findings expand the photocatalytic mechanism beyond the conventional photoreduction induced by photogenerated carriers to include photothermal reactions. The enhanced charge separation, the introduction of high-energy hot electrons, and the robust stability of the material are key factors that make this heterojunction a promising candidate for CO₂ photoreduction under white-light irradiation. This work not only advances the understanding of photocatalytic mechanisms but also opens new avenues for the development of efficient and sustainable photocatalytic systems.