The Construction of p/n-Cu₂O Heterojunction Catalysts for Efficient CO₂ Photoelectric Reduction

Abstract

Cu₂O is a p-type direct bandgap semiconductor with a band gap of 2~2.2 eV, which has excellent visible light absorption and utilization. However, slow charge transfer and poor stability hinder its practical application. In this paper, a facile electrodeposition approach successfully synthesized the heterostructure of p-Cu₂O and n-Cu₂O. The protective layer of n-Cu₂O on the surface of p-Cu₂O nanoparticles forms a p/n heterojunction. Due to the p/n heterojunction, the PEC performance of p/n-Cu₂O is enhanced significantly. The charge separation efficiency of photogenerated electron/hole pairs in p/n-Cu₂O is greatly improved. Therefore, p/n-Cu₂O shows superior photoelectrochemical (PEC) CO₂ reduction reaction (CO₂RR) efficiency when used as a photocathode.

1. Introduction

The continuous consumption of fossil fuels to support human society’s development causes serious environmental issues and energy crises [1,2,3]. For instance, the concentration of carbon dioxide (CO₂) in the air has increased to 400 ppm from 275 ppm since the industrial revolution [4]. Therefore, developing and establishing sustainable CO₂-reduction technology has become one of the important ways to solve environmental problems and energy crises. Commonly used CO₂ reduction methods include photocatalytic (PC) reduction, electrochemical (EC) reduction, photoelectrochemical (PEC) reduction, and coupling reduction [5,6]. Among these methods, the photocatalytic reduction has the advantages of mild reaction conditions and of being environmentally friendly. However, it has the disadvantage of low photon utilization due to low catalyst activity and weak light absorption ability [7,8,9]. Electrochemical reduction is also characterized by mild reaction conditions. Still, it has the disadvantages of low efficiency and relatively high energy consumption, leading to the problem of secondary carbon emissions [10,11]. In addition, the coupled reduction has the advantages of controllable and high-value-added products. However, it often involves multi-step reactions and expensive catalysts, which inevitably lead to the shortcomings of a complex route, poor economy, and high energy consumption, which limit its large-scale application in the industrial field [12]. Similar to photothermal synergy [13,14], photoelectrochemical CO₂ reduction (CO₂RR), as a practical extension of photocatalytic reduction and electrocatalytic reduction, combines the advantages of photocatalytic reduction and electrocatalytic reduction and utilizes photoelectric coordination to effectively promote carrier separation, enhance surface catalytic activity, and improve the efficiency and selectivity of CO₂ conversion while reducing energy consumption [15,16]. As it is a CO₂-reduction technology with excellent development potential, photoelectrocatalysis received widespread attention as soon as it was proposed [17,18]. Photoelectrochemical (PEC) conversion of CO₂ reduction using solar energy is a promising technology for achieving carbon neutrality [19].

PEC systems contain p-type semiconductors as photocathodes for CO₂ reduction. However, the number of semiconductors absorbing visible light with high charge separation efficiency is minimal [20]. In addition, CO₂ with a linear and center-symmetric structure is highly stable, requiring about 750 kJ/mol to break the initial C=O bond [21]. The main challenges for photoelectrochemical reduction of CO₂ are the intrinsic stability of CO₂, the low potential of the CO₂ reduction reaction (CO₂RR), and the low selectivity for reduction products [22,23]. Electrons reduce the CO₂ to various carbon fuels, such as CO and CH₄, in a typical catalytic CO₂RR. During the reaction, the catalyst plays a crucial role in providing high energy to the charge carriers and breaking the C=O bond of CO₂. Regarding photocatalysts for CO₂RR, several issues are generally considered, such as the absorption efficiency of sunlight, a conduction band negative enough for CO₂RR, and photostability [24,25]. Therefore, photoelectrocatalysts for CO₂RR with high selectivity, good stability, and excellent activity are highly anticipated.

At present, the general view is that the process of PEC CO₂ reduction can be divided into five main steps: (i) light absorption, (ii) charge separation, (iii) carbon dioxide adsorption on the catalyst surface, (iv) surface redox reaction and (v) product desorption [26,27]. Together, these five processes determine the efficiency and selectivity of the reaction. In light absorption, the width of the catalyst band gap affects the light response range and determines the energy strength of photogenerated electrons [28,29,30]. So, the catalyst should have a suitable band gap to ensure a sizeable light response range. At the same time, it is sufficient to provide the high-energy photoelectrons required for carbon dioxide reduction. Previous studies considered 2–3 eV the most suitable bandgap width for CO₂ photoelectroreduction [31,32]. Cuprous oxide (Cu₂O) is a p-type semiconductor with a direct band gap of 2–2.2 eV [33,34,35].

Moreover, Cu₂O also has high carrier mobility, sufficient Cu active sites for CO₂ activation, extensive absorption of visible light, low toxicity, low processing cost, and increased natural abundance [36,37,38]. All these virtues make it a promising candidate for PEC CO₂ reduction [39,40,41,42]. However, the slow charge transfer and poor stability of Cu₂O seriously hinder its practical application for PEC CO₂RR [43]. Therefore, it is of great research and industrial value to modify Cu₂O by appropriate methods to improve the photoelectrochemical performance of Cu₂O.

Constructing heterojunctions is an effective modification method. By creating a heterojunction, photogenerated electrons move from the built-in electric field to n-type semiconductors. In contrast, photogenerated holes move to p-type semiconductors, improving photo-corrosion and leading to a high electron/hole recombination rate [44,45]. Aside from the properties mentioned earlier, p-n heterojunctions have been shown to possess exceptional photoelectrochemical capabilities in CO₂ reduction [2]. These heterojunctions facilitate superior charge transfer and increased photocatalytic activity due to their direct contact with CO₂, reduced by the electron-enriched n-type semiconductor. n-Cu₂O covers the surface of p-Cu₂O to form a two-layer structure, avoiding direct exposure of p-Cu₂O to light and direct contact with the electrolyte, improving problems such as photo-corrosion and high electron/hole recombination. Mixing two materials does not achieve this effect. Thus, constructing p-n heterojunctions such as In₂O₃@InP/Cu₂O ternary heterojunction catalysts and Cu@Cu₂O core-shell catalysts [46,47] has been widely used to enhance CO₂ reduction efficiencies. However, using a complex multi-component catalyst strategy or a complex regulation of reaction conditions to generate specific structural heterojunction catalysts may result in a lack of stable reproducibility. Therefore, it is imperative to utilize uncomplicated and viable techniques for catalyst assembly to create p-n heterojunctions to achieve consistency and precision in the development of catalysts.

In this paper, we successfully synthesized p-Cu₂O nanoparticles by electrodeposition. By depositing a protective layer of n-Cu₂O nanoparticles on the surface of p-Cu₂O nanoparticles, the p/n heterojunction is formed. The structure and PEC performance for CO₂RR is comprehensively studied through a series of tests such as XRD, XPS, LSV, IPCE, etc. This work provides a simple and efficient approach for preparing photocathodes for high-yield PEC CO₂RR.

2. Results

A series of Cu₂O catalysts having different p/n heterojunction ratios were synthesized and subjected to characterization for morphology and preliminary PEC CO₂ reduction activity screening, as depicted in Figures S1 and S2. The p/n Cu₂O sample exhibiting the best PEC CO₂ reduction performance was selected for further experimentation.

2.1. Structure and Morphology

The crystal structures of as-prepared Cu₂O samples were studied by X-ray diffraction (XRD), as shown in Figure 1. The XRD peaks are mainly due to Cu₂O and SnO₂ substrates. The X-ray diffraction peaks of prepared p-Cu₂O and p/n-Cu₂O films can be well-indexed to Cu₂O (PDF#77-0199). No diffraction peaks of impurities were found according to the XRD pattern. Specifically, the diffraction peaks at 29.9°, 36.72°, 42.56°, and 73.8° are attributed to the (110), (111), (200), and (311) crystal planes of Cu₂O, respectively [48]. From the intensity of the diffraction peak, it is clear that p-Cu₂O and p/n-Cu₂O exhibit a predominant (111) orientation. The p-Cu₂O and p/n-Cu₂O samples have the crystal phase with no apparent peak position and intensity change, which indicates that the addition of the n-type layer did not significantly affect the crystal structure of Cu₂O. Overall, the XRD results confirm the formation of high-quality p-Cu₂O and p/n-Cu₂O thin films with good crystallinity and no detectable impurities.

The surface morphology of the p-Cu₂O and p/n Cu₂O was observed using the scanning electron microscope (SEM). Figure 2a,b shows that the p-Cu₂O nanoparticles are uniformly and densely deposited on a relatively smooth FTO surface. The p-Cu₂O film is composed of tiny crystals in the shape of sharp pyramids. The morphology of Cu₂O nanoparticles can be affected by the pH of the electrolyte and the sedimentary matrix, etc. It can be seen from Figure 2d,e that an ultra-thin protective layer of n-Cu₂O on the surface of p-Cu₂O makes the surface of the p-Cu₂O film smoother. The cross-sectional diagram Figure 2c,f shows that the average thickness of p-type and n-type Cu₂O is 1.93 μm and 0.22 μm, respectively, achieved by controlling the amount of deposited charge. In addition, the grain size of p/n-Cu₂O is smaller than that of pristine p-Cu₂O, which exposes more active crystal planes and retains sufficient CO₂ adsorption sites for CO₂RR.

The examination of the internal microstructures of both p-Cu₂O and p/n-Cu₂O was conducted through the utilization of sophisticated techniques and equipment such as a transmission electron microscope (TEM), a high-resolution transmission electron microscope (HRTEM), selected area electron diffraction (SAED), and energy dispersive spectrometer (EDS) mapping, as illustrated in Figure 3 and Figure 4. Figure 3a,b and Figure 4a show the HRTEM images of p-Cu₂O and p/n-Cu₂O, respectively. A lattice spacing of 0.308 nm corresponds to the (110) crystal plane of Cu₂O, as shown in Figure 3b. The HRTEM results for p/n-Cu₂O (Figure 4a) show a lattice spacing of 0.301 nm, corresponding to the (110) crystal plane of Cu₂O. The EDS mapping results for p-Cu₂O (see Figure 3c–e) and p/n-Cu₂O (see Figure 4b–d) also confirm the uniform element distribution of Cu and O in p/n-Cu₂O. The TEM, HRTEM, SAED, and EDS mapping results provide a better understanding of the structural and elemental properties of p-Cu₂O and p/n-Cu₂O, essential for optimizing their photoelectrochemical applications.

2.2. Chemical Composition and Surface Information

Since the XRD could not detect the small content of Cu (II) and Cu (0) impurities in the as-prepared Cu₂O film, X-ray photoelectron spectroscopy (XPS) measurements were conducted to obtain the surface chemical composition and valence state of the p-Cu₂O and p/n Cu₂O samples.

As shown in Figure 5a, in the survey spectra, Cu and O can be attributed to the basic components in p-Cu₂O and p/n-Cu₂O, as verified by the XRD and EDS mapping results. In the Cu 2p spectra, as shown in Figure 5b, the p-Cu₂O sample has two peaks at 952.2 eV and 932.4 eV with a binding difference of 19.8 eV, corresponding to the Cu⁺ 2p_1/2 and the Cu⁺ 2p_3/2 [49], respectively. In addition, two satellite peaks at 954.6 eV and 935.1 eV are attributed to Cu²⁺ 2p_1/2 and Cu²⁺ 2p_3/2, respectively. The sample of p/n-Cu₂O has two peaks at 952.4 eV and 932.5 eV with a binding difference of 19.9 eV, which originate from Cu⁺ 2p_1/2 and Cu⁺ 2p_3/2, respectively. In addition, two satellite peaks at 954.8 eV and 934.6 eV are attributed to Cu²⁺ 2p_3/2 and Cu²⁺ 2p_3/2, respectively. Clearly, p-Cu₂O and p/n-Cu₂O contain the primary state of Cu⁺ and a small amount of Cu²⁺. In the O 1s spectra (Figure 5c), the p-Cu₂O has two peaks at 535.5 eV and 531.7 eV, which are attributed to chemically adsorbed oxygen and lattice oxygen, respectively. However, the sample p/n-Cu₂O has three peaks of 535.5 eV, 531.4 eV, and 530.5 eV, which are attributed to chemically adsorbed oxygen, Cu₂O lattice oxygen, and CuO lattice oxygen, respectively. The presence of CuO impurities might be due to the reaction of Cu⁺ ions with oxygen in the air during the preparation or storage of the sample. Overall, the XPS results indicate that both p-Cu₂O and p/n-Cu₂O samples are predominantly comprised of Cu⁺ ions with a small amount of Cu²⁺ and impurities.

3. Photocatalytic Activity for CO₂RR

3.1. Photoelectrochemical Performance

The UV–Vis absorption spectra of Cu₂O photocathodes in Figure 5d show that p-Cu₂O and p/n-Cu₂O have a visible response. Moreover, the light absorption of p/n-Cu₂O is stronger than that of p-Cu₂O in the wavelength range of 350–600 nm. Obviously, the p/n junction of Cu₂O enhances light utilization. It extends the absorption edge to 641 nm, which suggests that forming a p/n junction in Cu₂O can significantly improve its photoconversion efficiency and expand its spectral response. Additionally, the p/n junction enables efficient charge separation and collection, reducing recombination losses and enhancing photocurrent generation.

A series of photoelectric tests of the p-Cu₂O and p/n-Cu₂O samples was performed to further study their photoelectric properties. Under chopping irradiation, linear sweep voltammetry (LSV) measurement was carried out under AM 1.5 G light, as presented in Figure 6a. The photocurrent density of p/n-Cu₂O is significantly higher than that of p-Cu₂O. The maximum photocurrent density of p/n-Cu₂O is −0.35 mA/cm², which is 1.75 times that of p-Cu₂O (−0.2 mA/cm²) at 0.15 V vs. RHE. The significantly augmented photocurrent exhibited by p/n-Cu₂O can be attributed to its effective charge separation mechanism. As illustrated in Figure S4, the n-Cu₂O sample shows a comparatively low photocurrent density when contrasted with the p-Cu₂O and p/n-Cu₂O. The primary objective of utilizing n-Cu₂O is to enable the effective separation of photogenerated carriers by constructing a heterojunction with p-Cu₂O, which ultimately leads to the strong photocurrent response of p/n-Cu₂O. This mechanism significantly enhances the efficiency of CO₂ reduction.

The chronoamperometry (I-t) measurement was used to investigate the chemical stability of the samples at 0.65 V vs. RHE, as presented in Figure 6b. The p/n-Cu₂O showed a higher photocurrent density than the p-Cu₂O, which resulted from the heterojunction between the p-Cu₂O and the n-Cu₂O. The p/n-Cu₂O sample exhibited steady photocurrent densities during the long cycling, implying its photostability improved significantly.

Figure 7 shows the Mott–Schottky curve for each sample. The Mott–Schottky curves provide essential information about the flat band potential of the semiconductor material. The flat band potentials of p-Cu₂O and p/n-Cu₂O can be fitted using the Mott–Schottky equation of 0.802 V and 0.882 V vs. RHE, respectively. The flat band potential is infinitely close to the bottom of the semiconductor material’s valence band, indicating the valence band position of p-Cu₂O and p/n-Cu₂O.

The incident photon current efficiency (IPCE) of p-Cu₂O and p/n-Cu₂O was further evaluated. The samples of p-Cu₂O and p/n-Cu₂O have a photocurrent response in the 300 nm–600 nm range. The photocurrent density of p-Cu₂O and p/n-Cu₂O increases with the increase to applied bias, as shown in Figure 8a. In addition, the photocurrent density of p/n-Cu₂O is significantly larger than that of p/n-Cu₂O. Due to the n-Cu₂O protective layer on the p-Cu₂O, the charge separation efficiency of photogenerated electron/hole pairs of p/n-Cu₂O is enhanced. The IPCE values of p-Cu₂O and p/n-Cu₂O gradually increase as the voltage increases, see Figure 8b. The IPCE value of p/n-Cu₂O is greater than that of p-Cu₂O. The bandgap of p-Cu₂O and p/n-Cu₂O can be evaluated by the Tauc plotting method, as shown in Figure 8c. The sample of p-Cu₂O has a band gap of 2.24 eV, while p/n-Cu₂O has a narrower band gap of 2.2 eV. The narrower bandgap of p/n-Cu₂O facilitates visible light absorption, potentially useful for practical applications in solar energy conversion.

Moreover, the electrochemical impedance spectroscopy (EIS) shown in Figure 8d demonstrates the decreased charge transfer resistance in p/n-Cu₂O due to the formed p/n heterojunction. Both holes and electrons move away from the interface, and the space charge region is widened, strengthening the internal electric field. Thus, the resistance of the internal electric field to the diffusion of electrons is enhanced, while the diffusion current is significantly reduced. At this time, the drift current of the minority carriers is formed in the p/n junction area derived from the action of the internal electric field. The drift current is greater than the diffusion current, which can be ignored. Therefore, the p/n junction presents low resistance.

3.2. CO₂ Photoelectric Reduction

As shown in Figure 9, the PEC CO₂RR of p-Cu₂O and p/n-Cu₂O was further evaluated with an automated online trace gas analysis system (Labsolar-6A) and a gas chromatograph (GC9790). The test system was assisted with an electrochemical workstation (CHI 760E) under a 300W Xenon lamp light source with an optical power of 1.9 W cm^–2 (PLS-SXE300D). The electrolyte used for the measurement was prepared with acetonitrile (CH₃CN), tetrabutylammonium hexafluorophosphate (C₁₆H₃₆F₆NP), and triethanolamine (C₆H₁₅NO₃) to obtain a solution with a pH of 9.3. The photoelectrochemical CO₂ reduction was carried out at 0.148 V vs. RHE. The reaction temperature was 5 °C.

The active surface area was 2 × 3 cm², the deposition charge of p-Cu₂O was 3 mAh, and the deposition charge of n-Cu₂O was 0.18 mAh. If Faraday efficiency is 100%, we can calculate that the masses of p-Cu₂O and n-Cu₂O are 7.11 mg and 0.462 mg, respectively. The reaction chamber was 500 mL, and 1 mL was sampled from the Labsolar-6A each time and sent to gas chromatography for product analysis. Figure 9a shows that during the test, the CO₂ reduction rate of the p/n-Cu₂O samples increased cumulatively with the products of CO and CH₄. The main product is CO. By generating more effective charge separation and transfer and increasing the CO₂ adsorption active site, the sample p/n-Cu₂O had better PEC CO₂ reduction efficiency, which is essential for converting CO₂ to valuable chemical compounds. Figure 9b shows that with light illumination, no apparent product was observed when no bias voltage was applied. Therefore, we believe that electrons and photons play a synergistic role in the current system. Electrons or photons alone will not achieve the effect we want. Figure 9c shows no apparent product is produced when only bias voltage was applied (0.75 V vs. RHE, 0.45 V vs. RHE, 0.15 V vs. RHE) without light illumination. Figure 9d shows that the p/n-Cu₂O exhibited excellent reduction performance within 6 h. At the fourth hour, a bias voltage of 0.15 V vs. RHE was maintained, and with the addition of a light source, there was significant product production. Figure 9d shows that the results showed that during the first 11 h, the product yield continued to increase as the reaction progressed. When the reaction time reached 11 h, the sample was inactivated, and there was almost no product regeneration. In addition, we confirmed that CO and CH₄ are produced from CO₂ reduction and not from other contaminations, see Figure S3.

Based on the above results, the mechanism of p/n-Cu₂O for PEC CO₂ reduction performance was proposed, and the energy band diagram of p-Cu₂O and p/n-Cu₂O is also illustrated in Figure 10. Owing to the deposition of n-Cu₂O on the p-Cu₂O, an internal electric field was formed in the p/n-Cu₂O heterojunction under visible light irradiation. Thus, an internal electric field could effectively separate the photogenerated carriers. Photogenerated electrons move from the built-in electric field to the n-Cu₂O, while holes move to the p-Cu₂O. Photogenerated electrons are used for CO₂ reduction. Furthermore, the p/n heterojunction narrows the band gap of Cu₂O and significantly enhances the charge separation efficiency of the photogenerated electron/hole pairs. Therefore, p/n-Cu₂O exhibits a high performance for improved PC/PEC CO₂ reduction.

4. Materials and Methods

4.1. Materials

Fluorine-Doped Tin Oxide- (FTO) coated glass (2.2 mm thick, South China Xiang’s Science & Technology, Yiyang, Hunan, China) was used as the cathode substrate. In the experiment, acetone (C₃H₆O, AR ≥ 99.5%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), absolute ethanol (C₂H₆O, AR ≥ 99.7%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), lactic acid (C₃H₆O₃, ACS ≥ 85%, Aladdin Holdings Group Co., Ltd., Beijing, China), copper sulfate pentahydrate (Cu₂SO₄·5H₂O, AR ≥ 99%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), sodium hydroxide (NaOH, AR, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), copper acetate (C₄H₆CuO₄, AR ≥ 98%, Shanghai Titan Scientific Co., Ltd., Shanghai, China), acetic acid (C₂H₄O₂, AR, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), acetonitrile (CH₃CN, AR ≥ 99.9%, water ≤ 50 ppm, Adamas-Beta, Shanghai Titan Scientific Co., Ltd., Shanghai, China), tetrabutylammonium hexafluorophosphate (C₁₆H₃₆F₆NP, AR ≥ 97%, Bide Pharmatech Co., Ltd., Shanghai, China), and triethanolamine (C₆H₁₅NO₃, AR ≥ 99.0%, Shanghai Titan Scientific Co., Ltd., Shanghai, China) were all purchased from commercial sources. Deionized water was used in all the experiments (prepared by a laboratory ultrapure water machine, GZY-P10-W, Hunan Kertone Water Treatment Co., Ltd., Changsha, China).

4.2. Synthesis Methods

The process for preparing p-Cu₂O and p/n-Cu₂O is illustrated in Figure 11. The FTO substrate was cleaned by sonication in acetone, deionized water, and absolute ethanol for 30 min, then dried in a nitrogen stream. The p/n-Cu₂O was then deposited onto the FTO substrate using a simple two-step electrodeposition method. The FTO substrate served as the working electrode, while Pt and Ag/AgCl were used as counter and reference electrodes, respectively. The electrodeposition was carried out using an electrochemical workstation (CHI-760E, Shanghai Chenhua, Shanghai, China).

In the first step, we prepared an aqueous solution of 1.5 M lactic acid and 0.2 M copper sulfate pentahydrate (1:1 in volume). A total of 4.0 M sodium hydroxide solution was added to adjust the pH to 10. The electrodeposition was performed at −0.5 V vs. Ag/AgCl in a water bath at 60 °C, with a deposited charge of 0.5 mAh/cm². After deposition, the sample was rinsed with deionized water and dried at room temperature.

The second step used an aqueous solution containing 0.02 mol/L copper acetate and 0.08 mol/L acetic acid. A total of 4.0 M sodium hydroxide solution was added to adjust the pH to 4.9. The electrodeposition was performed at 0.02 V vs. Ag/AgCl in a water bath at 70 °C, with a deposited charge of 0.03 mAh/cm². After deposition, p/n-Cu₂O nanoparticles were obtained by rinsing the sample with deionized water and drying it at room temperature [50].

4.3. Electrochemistry (EC) and Photoelectrochemical (PEC) Testing

The PEC performance of p-Cu₂O and p/n-Cu₂O was measured using a standard three-electrode configuration with a side quartz window. The solution used for the measurement was prepared with acetonitrile, tetrabutylammonium hexafluorophosphate (0.1 mol/L), and triethanolamine (0.1 mol/L) to obtain a solution with a pH of 9.3. Before PEC measurements, the solution was deoxygenated by bubbling high-purity CO₂ for 20 min. Platinum and Ag/AgCl electrodes (saturated with KCl) were used as working and reference electrodes. The working electrode was fabricated on a fluorine-doped tin oxide (FTO) glass substrate (1 × 2 cm²) and utilized for PEC measurements.

For testing linear sweep voltammetry (LSV) and chronoamperometry (I-t), a three-electrode side window electrolytic cell was used to form the PEC reaction system, along with a high uniformity integrated Xenon light source (PLS-FX300HU, Beijing Perfectlight Technology Co., Ltd., Beijing, China) and an AM 1.5G filter (100 mW/cm²). The PEC reaction system was connected to an electrochemical workstation (CHI-760E).

LSV measurements were conducted by scanning the potential over a range of 0 to −0.6 V (relative to Ag/AgCl) at a scanning speed of 5 mV/s, with the light being cut off by a shutter at a frequency of 5 s⁻¹. Chronoamperometry (I-t) was performed under alternating illumination at −0.1 V (relative to Ag/AgCl) for a test duration of 950 s with a 30 s interval. During the test, the working electrode was fully submerged in the electrolyte and had a guaranteed illuminated area of 1 cm². The voltage measurements were converted to the Reversible Hydrogen Electrode (RHE) scale using the Nernst equation:

$${\mathrm{E}}_{\mathrm{R}\mathrm{H}\mathrm{E}}={\mathrm{E}}_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}+0.0592\times \mathrm{p}\mathrm{H}+{\mathrm{E}}_{\mathrm{A}\mathrm{g}/\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}}^0$$

where ${\mathrm{E}}_{\mathrm{Ag}/\mathrm{AgCl}}^0$ = 0.1976 V vs. Ag/AgCl at room temperature.

A photo-electrochemical test system (IPCE1000, Beijing Perfectlight Technology Co., Ltd., Beijing, China) was used for testing incident photon-to-current efficiency (IPCE). This system consisted of a 300 W Xenon lamp light source (PLS-SXE300D, Beijing Perfectlight Technology Co., Ltd., Beijing, China), a grating monochromator (7ISU, SOFN instruments co., Ltd., Beijing, China) with filters to eliminate higher order diffraction, and an electrochemical workstation (CS350H, Wuhan Corrtest Instrument Corp., Ltd., Wuhan, China). The IPCE was calculated using the following equation:

$$\mathit{I}\mathit{P}\mathit{C}\mathit{E}\left(\%\right)=\frac{1240\times \mathit{J}}{\mathit{\lambda }\times {\mathit{I}}_0}\times 100\%$$

where J is the photocurrent density measured at a specific wavelength, λ is the specific wavelength, and I₀ is the intensity of the incident light.

The EC performance of p-Cu₂O and p/n-Cu₂O was evaluated using a standard three-electrode configuration with a side quartz window. The electrolyte solution consisted of acetonitrile and tetrabutylammonium hexafluorophosphate (0.1 mol/L), producing a pH of 7. Before measurements, the solution was deaerated by bubbling high-purity Ar for 20 min.

An electrochemical workstation (Squidstat Plus, Admiral Instruments, Tempe, AZ, USA) was used to conduct electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M-S) curves. The fitted circuit can be obtained from the EIS curve, which calculates the CPE element. The impedance of the CPE in an AC circuit is:

$$\mathrm{C}\mathrm{P}\mathrm{E}=\mathsf{\sigma }{\mathsf{\omega }}^{-\mathrm{m}}\left[\cos \left(\frac{\mathrm{m}\mathsf{\pi }}{2}\right)-\mathrm{j}\mathrm{s}\mathrm{i}\mathrm{n}\left(\frac{\mathrm{m}\mathsf{\pi }}{2}\right)\right]$$

where σ is the prefactor of the CPE, ω is the angular frequency, m is the CPE index (0 ≤ m ≤ 1), and j is an imaginary number $\left(\mathrm{j}=\sqrt{-1}\right)$; if m = 1, then the CPE denotes the ideal capacitor C.

The frequency range of the electrochemical analyzer was 0.01 to 100,000 Hz with voltage increments of 0.005 V and an AC amplitude of 10 mV. The working electrodes were tested at 1000 Hz, 1500 Hz, and 2000 Hz.

4.4. Photoelectrochemical (PEC) CO₂ Reduction Reaction Performances Testing

The PEC CO₂ reduction reaction test was conducted using an automated online trace gas analysis system (Labsolar-6A, Beijing Perfectlight Technology Co., Ltd., Beijing, China) and a gas chromatograph (GC9790, Fuli Instruments, Wenling, Zhejiang, China). A 300 W Xenon lamp (PLS-SXE300D, Beijing Perfectlight Technology Co., Ltd., Beijing, China) was used with an optical power of 1.9 W cm^–2. The electrolyte used for the measurement was prepared with acetonitrile, tetrabutylammonium hexafluorophosphate (0.1 mol/L), and triethanolamine (0.1 mol/L) to obtain a solution with a pH of 9.3. The photoelectrochemical CO₂ reduction was carried out at –0.6 V vs. Ag/AgCl (0.148 V vs. RHE) at the Labsolar-6A, which was assisted by an electrochemical workstation (CHI-760E). The reaction temperature was maintained at 5 °C using a water-cooling system that circulated condensed water throughout the reaction process. Samples were taken with a sampling needle every 1 h and analyzed using the gas chromatograph to obtain the peak area. The content of the gas phase product was then calculated using the standard external method. The following equation was used to calculate the yield for CO₂ reduction products:

$$Y=\frac{n\times V}{S}$$

where S is the sample area, S = 6 cm²; V is the volume of the reaction chamber in the Labsolar-6A, V= 500 mL; and n is the molar amount of the product.

4.5. Materials Characterization

The morphology of the samples was observed using a Scanning Electron Microscope (SEM, Apreo S LoV ac, Thermo Fisher Scientific, Waltham, MA, USA) with an operating voltage of 10 kV.

X-ray diffraction (XRD) data were collected using an X-ray diffractometer (Miniflex 600, Akishima, Rigaku, Tokyo, Japan) with CuK radiation within a measurement range of 20–80°.

Transmission Electron Microscopy (TEM), selected area electron diffraction (SAED), and High-Resolution Transmission Electron Microscopy (HRTEM) images were performed on a TF20 (FEI) operating at 200 kV (Thermo Fisher Scientific, Waltham, MA, USA).

The samples’ atomic composition and surface state were characterized using X-ray photoelectron spectroscopy using Al Ka rays as the excitation source (XPS, Thermo Scientific K–Alpha, Waltham, MA, USA)

The UV–vis absorption spectra were obtained using a Cary 5000 spectrophotometer (Agilent, Santa Clara, CA, USA).

5. Conclusions

In summary, by a simple electrochemical deposition method, the heterostructure of p/n-Cu₂O was successfully synthesized by depositing n-type Cu₂O onto p-type Cu₂O on an FTO substrate. The as-prepared p-Cu₂O and p/n-Cu₂O were employed for PEC CO₂RR. SEM and TEM images showed the ordered nanofilms of n-Cu₂O on the p-Cu₂O. XRD and XPS demonstrated the crystal structure and chemical states of p-Cu₂O and p/n-Cu₂O. The catalysts with a band gap of 2~2.2 eV can be excited by visible light with a wavelength of less than 600 nm resulting in excellent PEC performance. Moreover, linear voltammetry and IPCE tests show that the photocurrent response p/n-Cu₂O is improved significantly after modification. The modified catalyst of p/n-Cu₂O exhibits much higher activity than pristine p-Cu₂O. Therefore, the sample of p/n-Cu₂O behaves with superior PEC performance for CO₂RR, with the main products being CO and CH₄. This work provides an insightful and effective design of catalysts for achieving promising CO₂ photoelectric reduction.