Enhanced CO₂ photo-reduction under visible light irradiation by rGO incorporated Cu₂O nanocomposite

2.1 Chemicals and materials

All chemicals (copper(II) acetate, sodium hydroxide, glucose, graphite oxide, and ethanol) were purchased from Sigma-Aldrich and used without further purification. Material preparation of cuprous oxide was done by the following procedure. Copper acetate (2.9946 g) was added to deionized water (20 mL) and stirred continuously in a 50 mL beaker. The sodium hydroxide post-solution (5 g NaOH, 10 mL of deionized water) was added dropwise and the solution was heated to 70°C for 5 min. When the solution temperature reached 70°C, glucose (0.58 g) was immediately added and mixed thoroughly. Upon the addition of glucose, the solution gradually changed in color from black to dark red. The reaction mixture was maintained at 70°C for 1 h. After completion, the resulting product was washed three times each with deionized water and ethanol to remove impurities, then dried under vacuum at 60°C (Fig. S1 in the supplementary file). The rGO-Cu₂O preparation is as followings. At first, Cu₂O was added to 60 mL of ethanol. Then, dried rGO powder was added and the solution was stirred for 2.5 h. After the completion of stirring, the obtained black-red powder composite was filtered and placed into oven at 80°C. It was then crushed into fine powder (Fig. S2 in supplementary file).

2.2 Instrumentation and methods

2.2.3 Photocatalyst reactor

Approximately 0.05 g of the sample was weighed and placed in a weighing bottle, which was then transferred to the reaction tank. The tank was sealed and evacuated. Using a mass flow controller, CO₂ was passed through an Erlenmeyer flask containing 15 mL of water at a flow rate of 15 mL/min, introducing both CO₂ and water vapor into the reaction tank for 30 min before illumination (Fig. 1a). This process was repeated every hour for a total of six injections. After each injection, a 1 mL gas sample was extracted using a sealed syringe and analyzed by GC. A 15W green LED (wavelength = 427∼622 nm), 15W blue LED (wavelength = 412∼505 nm), and 15W red LED (wavelength = 640∼680 nm) were used as the visible light irradiation sources, which were set above the glass container. The spectra of green, blue, and red LED lights have been presented in Fig. 1(b).

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Fig. 1.

(a) CO₂ photocatalytic reaction system diagram (b) spectra of 15 W green LED, 15 W blue LED, and 15 W red LED.

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3.1 Characterization of photocatalysts

3.1.1. XRD and structural analysis

The elemental composition and crystal structure of the materials were investigated using XRD. The XRD pattern of the rGO-Cu₂O composite (Fig. 2) exhibited characteristic peaks for Cu₂O at 29.3°, 36.3°, 42.0°, 61.0°, 73.3°, and 77.2°, corresponding to the (110), (111), (200), (220), (311), and (222) crystal planes, respectively (Mohammed et al., 2021; Jiang et al., 2020). Additional peaks at 38.0° and 44.1° confirmed the successful formation of the composite (Méndez et al., 2019). Due to the non-uniform distribution of rGO on Cu₂O, some XRD peaks appeared less pronounced. The crystallite size of Cu₂O was assessed using Scherrer’s equation, revealing minimal variation in particle size.

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Fig. 2.

XRD patterns of single material, composite material, and optimal ratio.

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3.1.2. Morphological and elemental analysis

SEM revealed that the synthesized Cu₂O particles exhibited an octahedral morphology, with smaller particles adhering to their surfaces (Figs. 3a-d). While initial observations suggested the presence of rGO, further verification was conducted. Energy-dispersive spectroscopy (EDS) mapping (Fig. 4a-d) confirmed the presence of Cu and O, supporting the incorporation of rGO in the composite. This was further corroborated by the atomic number contrast principle, where elements with higher atomic numbers (e.g., Cu) appeared brighter in the micrographs.

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Fig. 3.

(a) SEM image of Cu₂O (octahedron), (b) SEM image of Cu₂O (tetrahedron), (c) Cu₂O EDX diagram, (d) Cu₂O mapping diagram.

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Fig. 4.

(a) SEM image of (octahedral) cuprous oxide/rGO in a ratio of 2:1 low magnification, (b) SEM image of (octahedral) cuprous oxide/rGO in a ratio of 2:1, (c) 2:1 ratio (octahedral) cuprous oxide/rGO EDS diagram, (d) 2:1 ratio cuprous oxide/rGO mapping diagram.

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3.2 Experimental results of photoreduction reaction

Once a sample component has completely eluted through the detector, the signal returns to the baseline. The time elapsed between sample injection and the appearance of a peak is termed the retention time (t_R). Peak areas were determined by integration software. These peak areas, representing the retention volume, were then compared to the peak areas obtained from 100 ppm methane and carbon monoxide standards. The cumulative production of each gas was calculated by integrating the peak areas from multiple injections.

In this study, different light sources were used to irradiate the photocatalytic materials to diminish carbon dioxide to carbon monoxide and methane. The cumulative methane production was 24.24 μmol g⁻¹ under red light, 21.82 μmol g⁻¹ under green light, and 20.59 μmol g⁻¹ under blue light. The corresponding cumulative carbon monoxide yields were 26.05 μmol g⁻¹, 25.76 μmol g⁻¹, and 24.65 μmol g⁻¹, respectively (Figs. 5a-d). These results suggest that the optimal performance of Cu₂O occurs under red light irradiation. Additional experiments were conducted using tetrahedral cuprous oxide. The octahedral Cu₂O structure demonstrated improved CO and CH₄ production compared to the tetrahedral structure, likely due to differences in the reduction potential of the different crystal planes.

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Fig. 5.

(a) Cu₂O (octahedron) cumulative CH₄ production under different light sources, (b) Cu₂O (octahedron) cumulative CO production under irradiation with different light sources, (c) Cu₂O (tetrahedral) CH₄ cumulative production chart under different light sources, (d) Cu₂O (tetrahedral) CO cumulative production chart under different light sources.

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The yield graphs from different experiments have been listed above. We can find that in the different ratios of cuprous oxide and graphene composites, the best ratio is 2:1 (octahedral) cuprous oxide/graphene and the best light source red light, so that the accumulative yield of methane and carbon monoxide is 25.36 μmol g^-1 and 43.32 μmol g^-1, respectively, which are the best yields (Figs. 6a-c).

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Fig. 6.

(a) Cu₂O (octahedral) blue light hourly cumulative methane production chart, (b) Cu₂O (octahedral) green light cumulative methane production per hour chart, (c) Cu₂O (octahedron) red light hourly cumulative methane production chart.

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Using red, green, and blue light sources, we conducted long-term illumination experiments with varying proportions of the composite material. Methane and carbon monoxide production was monitored. The 15W red LED proved to be the most effective of the three light sources, yielding the highest production of both methane and carbon monoxide. Furthermore, the 2:1 Cu₂O:rGO ratio demonstrated optimal performance under red light irradiation (Figs. 7a-c).

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Fig. 7.

(a) Cu₂O (octahedral) blue light cumulative carbon monoxide production per hour chart, (b) Cu₂O (octahedral) green light cumulative carbon monoxide production per hour chart, (c) Cu₂O (octahedron) red light cumulative carbon monoxide production per hour chart.

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3.2.1 Quantum yields

Quantum yield quantifies the efficiency of photon utilization in photochemical reactions. It represents the number of product molecules formed per photon absorbed. This metric is commonly used in studies investigating the interaction of light sources of varying wavelengths with photocatalytic materials (Zhu et al., 2025). The calculated quantum yields for the photocatalysts studied, compared against the selected light sources, are presented below:

$\begin{array}{c}n=\frac{P\cdot \Delta t}{\left(hc\text{/}\lambda \right)\cdot N_A}\\ \varphi =\frac{n_{C{H}_4}}{n}=\frac{n{N}_{A}hc}{\lambda \cdot P\cdot \Delta t}\end{array}$

ϕ: quantum yield

n: Number of molecules generated

NA: Avogadro constant

c: speed of light

ℎ: Planck constant

l: wavelength of light excitation

P: Light power density

r: Light receiving radius (m)

Δt: unit reaction time

$P=140\text{\ }W\text{/}{m}^2=140J\text{/}s\times \pi r^2\left(m^2\right)=2.47275\text{\ }J{s}^{-1}$

The following is a single material (ortho octahedron) oxide copper oxide with different light sources to test the quantum production rate

Methane green LED: CO₂ + 8e- + 8 H+ → CH₄ + 2H₂O

$\begin{array}{c}\Phi =\frac{ \left(21.82\times {10}^{-6}\right)\times \left(6.022\times {10}^{23}\right)\times \left(6.626\times {10}^{-34}\right)\times \left(2.997\times {10}^8 \right) }{\left(4\times {10}^{-7} \right)\times 1.05975\times 3600\times 6 }\times 8\times 100\%\\ =0.22\%\end{array}$

Carbon monoxide green LED: CO₂ + 2e- + 2H⁺ → CO + H₂O

$\begin{array}{c}\Phi =\frac{\left(25.76\times {10}^{-6} \right)\times \left(6.022\times {10}^{23}\right)\times \left(6.626\times {10}^{-34}\right)\times \left(2.997\times {10}^8 \right)}{\left(4\times {10}^{-7} \right)\times 1.05975\times 3600\times 6 }\times 2\times 100\%\\ =0.06\%\end{array}$

Methane blue light LED: CO₂ + 8 e- + 8 H⁺ → CH₄ + 2H₂O

$\begin{array}{c}\Phi =\frac{ \left(20.59\times {10}^{-6} \right)\times \left(6.022\times {10}^{23}\right)\times \left(6.626\times {10}^{-34}\right)\times \left(2.997\times {10}^8 \right)}{\left(4\times {10}^{-7} \right)\times 1.23637\times 3600\times 6}\times 8\times 100\%\\ =0.18\%\end{array}$

Carbon monoxide blue light LED: CO₂ + 2e- + 2H+ → CO + H₂O

$\begin{array}{c}\Phi =\frac{ \left(24.65\times {10}^{-6} \right)\times \left(6.022\times {10}^{23}\right)\times \left(6.626\times {10}^{-34}\right)\times \left(2.997\times {10}^8 \right)}{\left(4\times {10}^{-7} \right)\times 1.23637\times 3600\times 6 }\times 2\times 100\%\\ =0.05\%\end{array}$

Methane red light LED: CO₂ + 8 e- + 8 H⁺ → CH₄ + 2H₂O

$\begin{array}{c}\Phi =\frac{\left(24.24\times {10}^{-6}\right)\times \left(6.022\times {10}^{23}\right)\times \left(6.626\times {10}^{-34}\right)\times \left(2.997\times {10}^8 \right)}{\left(4\times {10}^{-7} \right)\times 0.90836\times 3600\times 6} \times 8\times 100\%\\ =0.30\%\end{array}$

Carbon monoxide red light LED: CO₂ + 2e- + 2H⁺ → CO + H₂O

$\begin{array}{c}\Phi =\frac{ \left(26.05\times {10}^{-6} \right)\times \left(6.022\times {10}^{23}\right)\times \left(6.626\times {10}^{-34}\right)\times \left(2.997\times {10}^8\right)}{\left(4\times {10}^{-7}\right)\times 0.90836\times 3600\times 6} \times 2\times 100\%\\ =0.08\%\end{array}$

The quantum yield calculations suggest that red light provides optimal performance. Key factors influencing quantum yield include light intensity and photon scattering. Calculations reveal that the full width at half maximum (FWHM) of the red light is significantly greater than that of the green and blue light sources, indicating a broader spectral range. The enhanced illumination intensity of the red light also contributes to better photon quantum efficiency. Furthermore, the UV-Vis absorption range of Cu₂O aligns closely with red light. Consequently, blue light generates fewer effective electrons. It was also observed that other wavelength bands experienced significant light interference, negatively impacting their effectiveness. In conclusion, red LED light was identified as the optimal light source. Calculations further revealed that the 2:1 Cu₂O:rGO material under the 15W red LED yielded a CH₄ quantum efficiency 1.125 times greater than that under the 15W green LED, and 1.134 times greater than that under the 15W blue LED. Similarly, the CO quantum efficiency under the 15W red LED was 0.930 times that under the 15W green LED, and 0.938 times that under the 15W blue LED.

The photocatalytic reduction of CO₂ using rGO-Cu₂O nanocomposites presents a promising strategy to combat rising atmospheric CO₂ levels while simultaneously producing valuable fuels such as methane (CH₄), methanol (CH₃OH), or carbon monoxide (CO). The synergistic effects between Cu₂O (a p-type semiconductor with visible-light absorption) and rGO (a conductive support with high electron mobility) enhance charge separation and suppress electron-hole recombination, leading to superior photocatalytic activity. This research not only contributes to sustainable energy solutions but also aligns with the UN’s Sustainable Development Goals (SDGs) by addressing clean energy (SDG 7) and climate action (SDG 13).”

3.3 Photoreduction reaction mechanism

The photocatalytic reaction of pure cuprous oxide (Cu₂O) under visible light (570–670 nm) was analyzed. The incorporation of graphene extended the absorption range, enhancing visible light utilization. When irradiated, valence band electrons in Cu₂O are excited to the conduction band, generating electron-hole pairs. The holes oxidize water, while the electrons reduce CO₂. However, Cu₂O suffers from rapid electron-hole recombination, lowering efficiency. Under high-energy blue light, electrons are excited to higher energy levels but are often lost as heat or through relaxation, reducing yield. In contrast, red and green light—with intensities exceeding the reduction potentials of CO and CH₄—exhibit higher quantum yields (Fig. 8). Graphene modification also alters product selectivity. In pure Cu₂O, the energy band favors CO production over CH₄. However, the Cu₂O/rGO heterojunction shifts the reduction potential, suppressing CO formation while promoting CH₄, a more economically valuable product.

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Fig. 8.

Photoreduction reaction mechanism of rGo-Cu₂O composite material.

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The photoreduction reaction mechanism proceeds via the following Eqs. (1–5) (Zhu et al., 2025; Wang et al., 2021; Kumar et al., 2019). Under visible light irradiation, valence electrons are excited and transferred to the conduction band of the as-prepared Cu₂O/rGO heterojunction, leading to the separation of photogenerated electron-hole pairs (Eq. 1). Water (H₂O) molecules then react with photoinduced holes (h⁺) to produce oxygen (O₂) and protons (H⁺) (Eq. 2). Subsequently, carbon dioxide (CO₂) reacts with these protons and electrons to form carbon monoxide (CO) and water (Eq. 3). The generated CO can further react with additional electrons and protons to yield methane (CH₄) and water (Eq. 4). Alternatively, CO₂ may directly undergo reduction with electrons and protons to produce CH₄ and H₂O (Eq. 5).