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Original Article
:19;
10472025
doi:
10.25259/AJC_1047_2025

Effect of Cu2O on the photocatalytic efficiency of NiO/C/CuxO

, , , , ,
 Xingzhi College, College of Chemistry and Materials Science, Zhejiang Normal University, Jinhua, P. R. of China
† Equally contributed to this paper, are the co-first authors.

*Corresponding authors: E-mail addresses: sky54@zjnu.cn (S. Hao), 68732506@qq.com (Y. Xie)

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

Abstract

This study aims to elucidate the regulatory role of cuprous oxide (Cu2O) in the photocatalytic degradation of Rhodamine B (RhB) by NiO/C/CuxO composites, with a focus on innovative material design and mechanism clarification. Acid red 14 (AR14) was creatively employed as a carbon source to fabricate NiO/C/CuxO composites via a co-deposition-adsorption method followed by N₂ calcination. The composites were characterized by x-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS), and the effects of copper salt dosage and calcination temperature on Cu2O content and photocatalytic activity were investigated through RhB degradation assays. The results demonstrate that the NiO/0.5C/0.5CuxO composite synthesized at 600°C with 0.5 g Ni(OH)2, 30 mL 0.5 mmol·L-1 AR14, and 0.5 g Cu(NO3)2·3H2O exhibits the highest Cu2O content, achieving 98% RhB degradation within 2 h. A key innovation lies in the clarification that Cu2O enhances the photocatalytic performance by reducing the composite’s band gap to 1.19 eV (boosting visible light absorption) and minimizing photogenerated electron-hole recombination (exhibiting the lowest PL intensity). Additionally, active species capture experiments confirm that superoxide radicals (•O2) and hydroxyl radicals (•OH) are the dominant reactive species, and the catalyst maintains high degradation efficiency after four reuse cycles, demonstrating excellent stability. This work provides a novel strategy for using AR14 as a carbon source in fabricating metal oxide composites and deepens the understanding of Cu2O’s modulation mechanism in photocatalysis, offering valuable insights for the design of high-efficiency carbon-based metal oxide photocatalysts for organic pollutant degradation.

Keywords

Cu2O
Electron-hole pair
NiO/C/CuxO
Photocatalysis
Synthesis

1. Introduction

Water contaminants such as organic dyes, antibiotics, and heavy metals have sparked widespread concern owing to their high recalcitrance, toxicity, and carcinogenicity [1]. Organic dyes, such as methyl orange (MO) and rhodamine B (RhB), are extensively used in the textile, cosmetics, plastic, and printing sectors [2], causing significant environmental harm and endangering human health. RhB, as an artificially synthesized alkaline fluorescent dye, enters the environment through industrial wastewater, agricultural irrigation, and other channels, can cause multi-dimensional and continuous harm to the ecosystem and human health. In water bodies, its strong water solubility and dyeability can reduce light transmittance, hinder the photosynthesis of aquatic plants, directly inhibit the synthesis of chlorophyll in algae, damage the liver, kidneys, and reproductive systems of fish, and also bioaccumulate through the food chain, amplifying its toxicity to high-trophic organisms. After entering the soil, it will combine with soil colloids and remain, altering the physical and chemical properties of the soil, inhibiting the activity of microorganisms and the function of soil enzymes, reducing the soil’s ability to convert nutrients, and also hindering the germination of plant seeds and root absorption, leading to reduced crop yields and creating residual hazards. For humans, RhB is classified as a potential carcinogen [1,3], with teratogenicity and mutagenicity. After accumulating in the human body through the food chain, it can damage the metabolic functions of the liver and kidneys, induce DNA damage in cells, and increase the risk of liver cancer, bladder cancer, and other cancers. In addition, this pollutant is difficult to degrade in the natural environment [4]. Long-term residue can also interfere with the behavioral patterns of aquatic organisms, reduce the biodiversity of the ecosystem, and, at the same time, its strong fluorescence property can also interfere with the accuracy of environmental monitoring.

Lots of technologies for removing pollutants from wastewater have been developed including ion exchange, oxidation, biodegradation, adsorption, and so on [5-7]. Among them, photocatalysis has become a promising technology for treating water pollution due to its low cost, high efficiency, low energy consumption, abundant resources, recyclability, and so on [8]. Semiconductor photocatalysts have received widespread attention in the photocatalytic degradation of pollutants [9]. The first semiconductor catalyst utilized was TiO2 by Fujishima and Honda in 1972 for photocatalytic water splitting [10]. Researchers have altered TiO2 using a variety of techniques or investigated other semiconductors due to its high bandgap, which results in low quantum efficiency under visible light [11]. Metal oxides play a crucial role in the science of photocatalysis due to their potential light absorption capabilities, variable band gap, and advantageous qualities for the transport of photogenerated electrons and holes [12-14]. NiO is a commonly used photocatalytic material due to its wide light absorption range, high catalytic activity, and good stability. Therefore, it has excellent applications such as in the degradation of organic pollutants [15,16].

Nevertheless, the practical application of NiO as a photocatalyst is hindered by critical drawbacks, including the rapid recombination of photogenerated electrons and holes, as well as notable resistance to charge transport. Numerous studies have demonstrated that the formation of heterojunctions between NiO and other functional materials can effectively suppress the recombination of photogenerated charge carriers in NiO, which in turn leads to a significant improvement in its photocatalytic performance [17,18]. CuO, as a commonly employed semiconductor, exhibits a strong propensity to construct heterojunctions with NiO, and this heterojunction formation can significantly boost the photocatalytic activity of NiO by modulating charge carrier dynamics [19]. Although forming heterojunctions between different semiconductors can improve the photocatalytic efficiency of the product, the interfaces formed between semiconductors can reduce the efficiency of photogenerated electron transfer. To overcome this issue, researchers often introduce electron mediating materials (such as carbon [20]) between different semiconductors as bridges to improve the efficiency of photogenerated electron transfer at the interface. During the synthesis of carbon-based materials incorporating CuO and NiO, carbon can partially reduce CuO to Cu2O. As a narrow-bandgap semiconductor material, Cu2O facilitates the efficient generation of photogenerated electrons under visible light irradiation [21]. Furthermore, Cu2O possesses a relatively low conduction band potential, thus exhibiting prominent reductive characteristics when constructing heterojunctions with other semiconductors [22]. Consequently, Cu2O can be deemed to play a pivotal role in the photocatalytic performance of CuO/NiO-containing carbon-based materials. Investigating the influence mechanism of Cu2O on the photocatalytic efficiency of these carbon-based composites is of great significance for expanding the application scope of Cu2O in the photocatalysis field. Compared with previous studies on the NiO/CuO system, this study introduces C as the electron transport medium to enhance the transport rate of photogenerated carriers [23]. Based on the aforementioned considerations, nickel nitrate hexahydrate, copper nitrate trihydrate, and acid red 14 (AR14) were employed as the starting materials in this study. Nickel nitrate hexahydrate and copper nitrate trihydrate underwent co-precipitation in an alkaline medium to form Ni(OH)2@AR14, followed by the introduction of copper nitrate trihydrate into the Ni(OH)2@AR14-containing system to yield the precursor (Ni(OH)2@AR14@Cu(OH)2). Calcination of the precursor under an inert atmosphere afforded CuO/NiO-containing carbon-based materials with varying Cu2O contents. The photocatalytic degradation of RhB was selected as the probe reaction to explore the influence of Cu2O and the Cu+/Cu2+ ratio on the photocatalytic degradation efficiency of these CuO/NiO-containing carbon-based materials. Photocatalytic tests revealed that the presence of Cu₂O enhanced the photocatalytic efficiency of the carbon-based materials, which was mainly attributed to the improved visible-light absorption capacity and the elevated separation efficiency of photogenerated electron-hole pairs.

2. Materials and Methods

2.1. Chemicals

Nickel nitrate hexahydrate ((Ni(NO3)2·6H2O, ≥98.0%), copper nitrate trihydrate (Cu(NO3)2·3H2O, 99.0%), potassium iodide (KI, ≥99.0%), absolute ethanol (C2H5OH, 99.5%), and sodium hydroxide (NaOH, ≥99.5%) were purchased from Sinopharm Chemical Reagent Co. Acid red 14 (AR14, 50%) was purchased from Nanjing Chemical Reagent Co, Ltd. All the chemical reagents were used without further purification. Deionized water was obtained from Millipore Milli-Q® ultrapure water purification systems.

2.2. Synthesis

2.2.1. Synthesis of NiO and NiO/C

Typically, 1 g of Ni(NO3)2·6H2O was dissolved in 30 mL of deionized water, and the resulting solution was magnetically stirred in a water bath at 25°C for 15 min. 5 mol·L-1 NaOH aqueous solution was slowly added dropwise to adjust the solution pH to 9, followed by continuous stirring for 1 h and subsequent static aging for an additional hour. The precipitate was collected via vacuum filtration to obtain Ni(OH)2, which was then dried in an oven at 60°C and calcined in a tube furnace under N2 atmosphere at 600°C for 2 h to yield NiO.

A total of 0.5 g of the as-synthesized Ni(OH)2 was mixed with 30 mL of 0.5 mmol·L-1 AR14 solution in a 50 mL beaker. The mixture was magnetically stirred in a water bath at 25°C for 15 min, after which a 5 mol·L-1 NaOH solution was slowly added dropwise to adjust the pH to 9. Following continuous stirring for 1 h and subsequent static aging for 1 h, the solid product was collected by vacuum filtration to obtain Ni(OH)2@AR14. The as-obtained Ni(OH)2@AR14 was dried in an oven at 60°C, ground into a fine powder, and then calcined at 600°C for 2 h in a tube furnace under N2 atmosphere, yielding NiO/C composites.

2.2.2. Synthesis of CuxO and CuxO/C

CuxO and CuxO/C are prepared using the same methods as NiO and NiO/C, respectively, except that Ni(NO3)2·6H2O was replaced by Cu(NO3)2·3H2O.

2.2.3. Synthesis of NiO/C/CuxO and NiO/CuxO

A total of 0.50 g of the pre-synthesized Ni(OH)2 was dispersed in 30 mL of 0.50 mmol·L-1 AR14 solution, and the mixture was magnetically stirred in a water bath at 25°C for 1 h. Subsequently, different amounts of Cu(NO3)2·3H2O (0.10 g, 0.25 g, 0.50 g, 0.75 g, 1.0 g) were added to the system, followed by an additional 30 min of stirring. 5 mol·L-1 NaOH solution was then added to adjust the system pH to 9; the mixture was continuously stirred for 1 h and aged statically for another 1 h. The resulting solid was collected by vacuum filtration, washed alternately with deionized water and ethanol three times, yielding Ni(OH)2@AR14@Cu(OH)2. The as-obtained Ni(OH)2@AR14@Cu(OH)2 was dried in a 60°C oven, ground into a fine powder, and calcined at 600°C for 2 h in a N2-filled tube furnace to produce NiO/C/CuxO composites. For the synthesis of NiO/CuxO, the identical procedure was adopted except that the AR14 solution was omitted.

2.3. Characterization

X-ray diffraction (XRD) patterns were collected on a Philips PW3040/60 powder diffractometer using Cu Kα radiation (λ = 0.154 nm). N2 adsorption isotherms were measured with a Micromeritics ASAP 2020 apparatus at -196°C, and the specific surface area of the investigated samples was calculated using the multi-point Brunauer-Emmett-Teller (BET) method. Solid-state UV-Vis diffuse reflectance spectra (UV-Vis DRS) were measured using a Cary5000 spectrophotometer, with a measurement range of 200-800 nm. Liquid-phase UV-Vis absorption experiments were conducted on a Nicolet Evolution 500 dual-beam spectrophotometer, with a scanning rate of 240 nm/min and a scanning range of 200-800 nm. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded by a Nicolet Nexus 670 spectrometer with a resolution of 4 cm-1 using the KBr pellet method. The photoluminescence (PL) spectra of the samples were obtained at room temperature by a spectrofluorometer (NanoLOG-TCSPC, Horiba Jobin Yvon, USA) with an excitation wavelength of 325 nm. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a RBO upgraded PHI-5000 C ESCA system (Perkin Elmer) using monochromated Al Kα X-ray radiation (E = 1486.6 eV) at 250 W. All binding energies were calibrated using carbon (C 1s = 284.6 eV) as a reference. Online sequential detection of Ni, O, C, and Cu species in the resulted sample was carried out using a commercial ICP-MS (iCAP RQ, Thermo Fisher Scientific) equipped with a homemade online sequential elution device. Total organic carbon (TOC) results were obtained from the Elementar vario TOC, in which the liquid organic carbon was converted into CO2 through high-temperature combustion, and the CO2 concentration was quantitatively measured using a non-dispersive infrared (NDIR) detector to calculate the TOC content.

2.4. Photocatalytic tests

The photocatalytic activity of the as-prepared samples was assessed via the photocatalytic degradation of RhB. Typically, 20 mg of the various catalysts were weighed and placed in a 50 mL beaker, followed by the addition of 30 mL of 0.06 mmol·L-1 RhB solution. The resulting suspension was magnetically stirred under dark conditions for 30 min to achieve adsorption-desorption equilibrium. Then, continue stirring the solution under the illumination of 300 W Hg lamp, the distance between the RhB solution and the light source is 20 cm, and at given time intervals, extract a small amount of the suspension and centrifuge to remove the photocatalyst. Record the absorbance at 554 nm (the maximum absorption peak of RhB) using a UV-Vis spectrophotometer (Evolution 500 LC) to analyze the concentration of residual RhB in the filtrate. Calculate the residual concentration of RhB using Eq. 1 [24] and then calculate the degradation efficiency (η) using Eq. 2 [25].

(1)
A = k b c

Where A is the absorbance, k is the absorption coefficient, b is the thickness of the absorbing layer, and c is the concentration of residual dye.

(2)
η = C 0 − C / C 0

Where C0 and C are the initial and post-irradiation concentrations of RhB, respectively.

3. Results and Discussion

3.1. Structural properties

Figure 1 shows the XRD patterns of NiO, NiO/C, NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO, and NiO/0.5C/1CuxO. According to Figure 1, all samples exhibit distinct cubic phase NiO diffraction peaks at planes (111), (200), (220), (311), and (222) (JCPDS card No. 47-1049) [26]. The XRD peak shapes and positions of NiO and NiO/C are identical. When carbon is introduced, the diffraction intensity decreases, and the full width at half maximum (FWHM) broadens, which may be related to lattice changes and defect formation caused by the introduction of carbon. However, no diffraction peaks of carbon were found in the XRD pattern of NiO/C, possibly because the carbon is amorphous. When Cu(NO3)2·3H2O was introduced into the reaction system, diffraction signals corresponding to Cu2O were detected at the (111), (200), and (220) planes (PDF#05-0667), with the intensity of these signals increasing with the amount of copper. After the addition of Cu(NO3)2·3H2O, the diffraction peaks corresponding to CuO were observed in all synthesized products, matching the CuO planes of (110), (11-1), (111), (20-2), (020), (202), (022), (310), (113), and (311) (JCPDS card No.48-1548) [27]. This structure matches well with the XRD patterns of other Cu-containing doped materials [28-30], proving the successful synthesis. The presence of both Cu2+ and Cu+ in the synthesized products was further confirmed by XPS results (detailed analysis provided in the section on XPS and its results). Moreover, it is evident from Figure 1 that the diffraction peak intensity of NiO/0.5C/0.5CuxO is significantly higher than that of other samples, indicating a higher crystallinity of NiO/0.5C/0.5CuxO compared to the others. The reason may be that the specific ratio of NiO/0.5C/0.5CuxO can promote the formation of a well-ordered crystalline structure.

XRD patterns of NiO, NiO/C, NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO and NiO/0.5C/1CuxO.
Figure 1.
XRD patterns of NiO, NiO/C, NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO and NiO/0.5C/1CuxO.

Figure 2 shows the FT-IR spectra of NiO carbon-based composites with varying copper content. It can be seen from Figure 2 that all samples exhibit characteristic peaks at 500, 670, 1380, 1640, 2350, 2860, 2936, and 3460 cm-1. The bands near 400-600 cm-1 are attributed to the vibrations of Ni-O and Cu-O bonds in the samples [31]. The weak peak at 1380 cm-1 may be due to residual NO3-. The absorption band around 1640 cm-1 can be ascribed to the stretching mode of hydroxyl groups [32]. The weak band at 2350 cm-1 is due to CO2 absorbed from the atmosphere [33]. The absorption peaks near 2860 and 2936 cm-1 correspond to the antisymmetric and symmetric vibration modes of -CH3 groups in sp3 hybridization, respectively. The absorption peak around 3430 cm-1 is attributed to the stretching mode of hydroxyl groups in physically adsorbed water [2].

Infrared spectra of NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO and NiO/0.5C/1CuxO.
Figure 2.
Infrared spectra of NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO and NiO/0.5C/1CuxO.

Figure 3 presents the morphological characteristics, elemental distribution, and elemental content of the NiO/0.5C/0.5CuxO composite. As observed from Figures 3(a)–(d), the Ni, O, C, and Cu elements are uniformly distributed across the entire sample. Figure 3(e) reveals that the mass percentage and atomic percentage of Cu are 34.04% and 20.13%, respectively, which are consistent with the theoretical values; the corresponding values for C are 4.13% (mass) and 12.91% (atomic). This further confirms that AR14 and copper nitrate are converted into substantial amounts of activated carbon and copper oxide, which are homogeneously dispersed throughout the catalyst matrix. As shown in Figure 3(f), NiO/0.5C/0.5CuxO exhibits an irregularly stacked block-like morphology with numerous pores on its surface.

(a) Mapping of Ni, (b) O, (c) C, (d) Cu (e) EDS element content, and (f) SEM image of NiO/0.5C/0.5CuxO.
Figure 3.
(a) Mapping of Ni, (b) O, (c) C, (d) Cu (e) EDS element content, and (f) SEM image of NiO/0.5C/0.5CuxO.

3.2. Photocatalytic activity

The photocatalytic degradation efficiencies of different catalysts for 0.06 mmol·L-1 of RhB are presented in Figure 4(a). It is evident from the figure that the ternary NiO/C/CuₓO composite exhibits a significantly higher RhB degradation rate than pristine NiO and NiO/C, which indicates that the introduction of CuxO effectively facilitates the improvement of NiO’s photocatalytic activity. Specifically, NiO/0.5C/0.5CuxO delivers the best photocatalytic performance among all the synthesized catalysts, achieving a RhB removal efficiency of 98% within 2 h of visible-light irradiation. Compared with similar work (Table 1, [34-38]), it has a better ability to degrade dyes. In contrast, when the dosage of Cu(NO3)2•3H2O in the reaction system increases to 50%, the resultant product shows a decline in photocatalytic efficiency. This reduction is likely due to the excessive introduction of copper species, which may cause a decrease in the pore size of NiO/C and a reduction in the number of available active sites for the photocatalytic reaction. From the results of Table S1, it can be seen that the first order model is better to fit the photocatalytic data of different catalysts than that of second order model, and that the rate constant of K1 of NiO/0.5C/0.5CuxO is more negative than those of other catalysts, indicating more efficient removal efficiency for NiO/0.5C/0.5CuxO. To verify that the removal of RhB resulted from photocatalysis, infrared characterization was conducted on three samples: RhB powder, the freshly prepared NiO/0.5C/0.5CuxO composite, and the recovered catalyst. Detailed results of this characterization are presented in Figure 4(b). According to Figure 4(b), after photocatalytic degradation, the characteristic peaks of NiO/0.5C/0.5CuxO are almost identical to those of the freshly prepared NiO/0.5C/0.5CuxO, with no observable characteristic peaks of RhB, thus confirming that the discoloration of the RhB dye is indeed due to photocatalytic degradation. Furthermore, TOC of filtrate (not contain NiO/0.5C/0.5CuxO) after irradiation of 120 min was obtained, and the result showed that the value of TOC is almost zero, indicating that the removal of RhB is attributed to the photocatalysis. The corresponding reasons for the above experimental results are detailed in the following text.

Table S1
(a) Photocatalytic efficiency of RhB over different catalysts at different time and (b) Infrared spectra of NiO/0.5C/0.5CuxO before and after photocatalysis and RhB.
Figure 4.
(a) Photocatalytic efficiency of RhB over different catalysts at different time and (b) Infrared spectra of NiO/0.5C/0.5CuxO before and after photocatalysis and RhB.
Table 1. Comprehensive comparison based on the work of Cu, Ni, and Zn based photocatalysts.
Catalyst Types of dyes Illumination time/min Removal rate/% Reference
C1−x(Zn)xO RO-4 80 90.25 [34]
Cu0.95Cd0.025Ba0.025O DB-1 70 92 [35]
Cu0.9Mn0.05Ag0.05O MB 150 83.9 [36]
Silver-doped CuO AO 60 92.77 [37]
CuxNi1-xO MB 60 ∼95 [38]
NiO/0.5C/0.5CuxO RhB 120 98 This work

To investigate the reasons behind Figure 4 and understand the separation efficiency of photogenerated electrons and holes in different catalysts, this study analyzed NiO, NiO/C, NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO, and NiO/0.5C/1CuxO using PL spectroscopy, and the results are shown in Figure 5. According to Figure 5, all samples exhibit strong PL emission peaks at 289, 392, and 497 nm, which may be due to the recombination of conduction band electrons and holes [39]. After introducing C and CuxO, the peak positions of NiO/C and NiO/C/CuxO are almost consistent with those of pure NiO, but the PL intensities of the corresponding products are lower than that of NiO, with the ternary composite NiO/0.5C/0.5CuxO showing the lowest intensity. Generally, a lower PL intensity indicates a lower recombination rate of carriers and higher photocatalytic activity. Therefore, it can be inferred that NiO/0.5C/0.5CuxO has the best photocatalytic efficiency because it has the lowest PL intensity, which is consistent with the results in Figure 4. Furthermore, according to Figures 4 and 5, the trend of PL peak intensity changes of the corresponding samples is consistent with their photocatalytic performance trends.

PL spectra of NiO, NiO/C, NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO and NiO/0.5C/1CuxO.
Figure 5.
PL spectra of NiO, NiO/C, NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO and NiO/0.5C/1CuxO.

It is well known that UV-Vis DRS spectroscopy is an important method to reveal the energy structure and optical properties of semiconductor crystals. Figure S1 shows the UV-Vis absorption spectra of NiO/0.1C/0.5CuxO, NiO/0.5C/0.5CuxO and NiO/1C/0.5CuxO. It can be seen from the figure that with the increase of AR14 concentration, the photocatalytic performance of the material first increases and then decreases, indicating that the loaded carbon is related to the material’s absorption of visible light. The bandgap of the catalyst can be counted by the following formula: (αhv)n = k(hv - Eg). Where α is the absorption coefficient, k is the effective mass parameter related to the valence state and conduction band, parameter n = 1/2, hv is the absorption energy, Eg is the bandgap energy. Using the data in the graph, a correlation between (αhv)1/2 and hv can be plotted, and Eg can be obtained by extrapolating the intercept. According to Figure S1(b), the optical bandgap energies of NiO/0.1C/0.5CuxO, NiO/0.5C/0.5CuxO, and NiO/1C/0.5CuxO are 1.36, 1.19, and 1.41 eV, respectively, which are lower than NiO (3.5 eV). The reduction of bandgap can improve the absorption of visible light of the catalyst. Thus, the photocatalytic performance of the material is enhanced.

Figure S1

To further understand the above photocatalytic results, XPS tests were performed for NiO/0.1C/0.5CuxO, NiO/0.5C/0.5CuxO and NiO/1C/0.5CuxO. O1s XPS was used to evaluate the chemical state of oxygen elements in the above three materials, and the results were shown in Figure S2. The peaks at 531.4 eV and 529.3 eV in Figure S2 can be attributed to oxygen vacancy (OV) and lattice oxygen (OL), respectively. The oxygen vacancy content can be calculated by the formula. COV= SOVST, where SOV and ST represent the peak area of OV and the total peak area of OV and OL, respectively. The calculated results are shown in Figure S2. The content of oxygen vacancies in the three materials is 45.54%, 44.87% and 40.78%, respectively, indicating that a large number of oxygen vacancies were formed in NiO/0.5C/0.5CuxO. The presence of oxygen vacancy can introduce intermediate defect states, promote the extended absorption of visible light region, and accelerating the separation of photocarriers, resulting in a higher photocatalytic performance. However, too much oxygen vacancy can distort the crystal structure and reduce the mobility of the surface redox reaction, which may be one of the reasons for the decreased photocatalytic effect of Ni/0.1C/0.5CuxO.

Figure S2

Figure S3 shows the C1s XPS spectra of NiO/0.1C/0.5CuxO, NiO/0.5C/0.5CuxO, and NiO/1C/0.5CuxO. The peaks at 286.37, 286.77, 287.10, 287.86, and 290.05 eV correspond to the C=C, C-C, C-OH, C=O, and O-C=O bonds, respectively. As can be seen from Table S1, with the increase of AR14 concentration, the peak area of carbon bonds in the synthetic material increases, and the formation of these carbon bonds is conducive to speed up the transmission and transfer of photogenerated carriers, improving the separation efficiency of photogenerated electron-hole pairs. Therefore, the catalyst NiO/0.5C/0.5CuxO has the highest photocatalytic efficiency. This is consistent with the photocatalytic effect data in Figure 4(a). According to Table S2, the content of hydroxyl in NiO/0.5C/0.5CuxO is higher than that of other materials. Generally, the hydroxyl group can be combined with the protonated sulfonic acid group to improve the adsorption effect of dyes on the catalyst, thereby improving the photocatalytic efficiency of the synthetic material. Therefore, NiO/0.5C/0.5CuxO has the best photocatalytic efficiency.

Figure S3

Table S2

Figure 6 shows the XPS spectra of Cu 2p for NiO/0.5C/0.1CuxO, NiO/0.5C/0.5CuxO, and NiO/0.5C/1CuxO. From Figure 6, it can be seen that there are obvious peaks in the range of 970.00-928.00 eV, indicating the presence of copper species in the samples. The peaks in the regions of 958.00-949.00 eV and 949.00-925.00 eV are attributed to Cu 2p1/2 and Cu 2p3/2, respectively. The peaks near 934.87 and 954.67 eV can be attributed to Cu+ [40], while the peak around 944 eV is attributed to Cu2+ [41]. The XPS results confirm the presence of both +1 and +2 valence states of copper ions in the products, which is consistent with the XRD results. The formation of Cu+ can be attributed to the reduction of a portion of Cu2+ by the carbon formed from the decomposition of dyes during the high-temperature calcination. According to the peak fitting results (detailed in Table 2), the peak area of Cu+ in NiO/0.5C/0.5CuxO (41821.49) is higher than that in NiO/0.5C/0.1CuxO (17188.1) and NiO/0.5C/1CuxO (39454.18), indicating that the content of Cu+ increases with the increase of Cu2+ in the reaction solution. According to the results of Table 2 and Figure 4, the photocatalytic efficiency of the synthesized products increases with the increase of Cu2O content, possibly due to the following reasons: firstly, the bandgap of Cu2O is relatively low (about 1.92 eV), making it easy to absorb visible light and generate more photogenerated electrons and holes; secondly, when combined Cu2O with NiO and CuO, a heterojunction can be formed, improving the separation efficiency of photogenerated electrons and holes [42]. It can be seen from Table S3 that the first order model is suitable for fitting the photocatalytic data of NiO/0.5C/0.5CuxO synthesized at different temperatures, and that the rate constant of K1 of NiO/0.5C/0.5CuxO synthesized at 600°C is more negative than those of other catalysts, indicating more efficient removal efficiency for this sample.

Table S3
(a) Cu 2p XPS spectrum of NiO/0.5C/0.1CuxO, (b) NiO/0.5C/0.5CuxO and (c) NiO/0.5C/1CuxO obtained at calcination temperature of 600°C.
Figure 6.
(a) Cu 2p XPS spectrum of NiO/0.5C/0.1CuxO, (b) NiO/0.5C/0.5CuxO and (c) NiO/0.5C/1CuxO obtained at calcination temperature of 600°C.
Table 2. The peak area and molar ratio of Cu(I)/Cu(II) in different samples.
Sample Area of Cu(I) located at 934.80 eV Area of Cu(I) located at 954.60 eV Total area of Cu(I) Total area of Cu(II) The molar ratio of Cu(I)/Cu(II)
NiO/0.5C/0.1CuxO 15475.04 6057.07 21532.11 29239.39 0.74
NiO/0.5C/0.5CuxO 30693.86 20218.92 50912.78 59171.72 0.86
NiO/0.5C/1CuxO 17050.47 14772.71 31823.18 57405.96 0.55

Based on the above analysis, it can be concluded that the Cu2O content in the as-synthesized materials is correlated with the Cu2+ content in the reaction raw materials, which in turn exerts a significant influence on the photocatalytic efficiency of the resulting products. Theoretical analysis reveals that the efficiency of CuO reduction to Cu2O is also dependent on the synthesis temperature. Therefore, to elucidate the impact of synthesis temperature on Cu2O formation efficiency and, in turn, on the photocatalytic performance of the as-synthesized products, this work prepared a series of products at different temperatures and evaluated their RhB degradation efficiency. The corresponding results are presented in Figure 7(a). As shown in Figure 7(a), when the calcination temperature is below 600°C, the RhB degradation efficiency of NiO/0.5C/0.5CuxO increases with rising temperature; above 600°C, the RhB degradation efficiency of the as-synthesized products declines. Specifically, the NiO/0.5C/0.5CuxO sample prepared at 600°C exhibits the maximum RhB degradation efficiency, achieving 98% RhB removal within 2 h of light irradiation.

Photocatalytic efficiency of RhB (a) and XRD patterns (b) of NiO/0.5C/0.5CuxO synthesized at different temperatures.
Figure 7.
Photocatalytic efficiency of RhB (a) and XRD patterns (b) of NiO/0.5C/0.5CuxO synthesized at different temperatures.

To elucidate the effect of calcination temperature on the formation efficiency of Cu2O, this study conducted XRD analysis on samples synthesized at different calcination temperatures, with the results detailed in Figure 7(b). According to Figure 7(b), the diffraction intensity corresponding to specific crystal planes in all samples increases with the calcination temperature, indicating higher crystallinity in the corresponding samples. As the calcination temperature rises, the diffraction peak intensities of Cu2O crystal planes ((221), (220)) in the corresponding synthesized products continuously increases, suggesting an elevated content of Cu2O in the products. This is conducive to the absorption of visible light and the separation of photogenerated electrons and holes, thereby enhancing the photocatalytic efficiency of the products.

To further validate the inferences drawn from XRD, this study conducted Cu 2p XPS analysis on NiO/0.5C/0.5CuxO samples obtained at different calcination temperatures, as detailed in Figure 6(b) and Figure 8. According to these figures, Cu+ is present in all samples. The peak fitting results show that the areas of the Cu+ peaks for products obtained at calcination temperatures of 350, 500, and 600°C are 33408.08, 43648.14, and 50912.78, respectively. This confirms that the content of Cu+ indeed increases with the calcination temperature (detailed in Table 3), consistent with the XRD inferences.

(a) Cu 2p XPS spectrum of NiO/0.5C/0.5CuxO-350 and (b) NiO/0.5C/0.5CuxO-500.
Figure 8.
(a) Cu 2p XPS spectrum of NiO/0.5C/0.5CuxO-350 and (b) NiO/0.5C/0.5CuxO-500.
Table 3. The peak area and molar ratio of Cu(I)/Cu(II) in NiO/0.5C/0.5CuxO prepared at different calcination temperatures.
Calcination temperature/°C Area of Cu(I) located at 934.80 eV Area of Cu(I) located at 954.60 eV Total Area of Cu(I) Total Area of Cu(II) The molar ratio of Cu(I)/Cu(II)
350 31240.98 12407.16 43648.14 136751.67 0.32
500 27272.67 6135.41 33408.08 98473.45 0.34
600 30693.86 20218.92 50912.78 59171.72 0.86

The XRD peak intensities of NiO/0.5C/0.5CuxO synthesized at 700°C are similar to those of the products synthesized at 600°C (not shown). It can be inferred that the Cu2O contents in these two products are similar. However, as seen in Figure 7, the photocatalytic efficiency of the product synthesized at 700°C is lower than that of the product synthesized at 600°C. To investigate the reason, this study conducted specific surface area tests on the products calcined at different temperatures, with the results detailed in Figure 9. According to Figure 9, it can be found that when the relative pressure rises above 0.9, there is a noticeable increase in the volume of adsorbed gas. The isotherms for all the samples correspond to a type-IV isotherm with a distinct H3 hysteresis loop, indicating the mesoporous nature of the materials [43]. The corresponding materials exhibit an increased N2 adsorption capacity as the calcination temperature increases, indicating a gradual increase of specific surface area. However, when the calcination temperature reaches 700°C, the synthesized material shows a slight decrease in N2 adsorption capacity, which may be related to partial pore collapse due to the higher calcination temperature. Typically, a larger specific surface area of a material is beneficial for the adsorption of pollutants. Based on the results in Figure 9, it can be deduced that the specific surface area of the product synthesized at 700°C is smaller than that of the product synthesized at 600°C. Therefore, it can be inferred that the product synthesized at 700°C has a lower adsorption capacity for RhB compared to the product synthesized at 600°C, leading to its lower photocatalytic efficiency towards RhB than the product synthesized at 600°C. Compared with other temperatures, the increased surface area of the samples synthesized at 600°C helps the photocatalyst interact better with the reactant molecules, thereby enhancing the adsorption capacity of the photocatalyst and ultimately strengthening its photocatalytic activity [44].

N2 adsorption and desorption isotherms of NiO/0.5C/0.5CuxO synthesized by different temperatures.
Figure 9.
N2 adsorption and desorption isotherms of NiO/0.5C/0.5CuxO synthesized by different temperatures.

The O1s XPS is shown in Figure S4, where the peaks at 531.4 and 529.3 eV can be attributed to oxygen vacancies (Ov) and lattice oxygen (OL), respectively. According to the oxygen vacancy calculation formula (see Figure S2), the oxygen vacancy content of NiO/0.5C/0.5CuxO catalysts at different calcination temperatures is 41.25%, 37.41% and 44.87%, respectively, indicating that the calcination temperature of 600°C is helpful for the formation of oxygen vacancies. It is well known that the electronegativity of oxygen atoms is higher than that of other components in the oxide material, which is good for the formation of positively charged vacancies. Moreover, the presence of oxygen vacancies can introduce intermediate defect states, thus promote the extended absorption of visible light regions, and accelerate the separation of photocarriers, resulting in a high photocatalytic performance. Therefore, it can be interfered that the NiO/0.5C/0.5CuxO synthesized at the calcination temperature of 600°C possesses the highest photocatalytic efficiency due to its largest number of oxygen vacancies.

Figure S4

Figure S5 shows the C1s XPS spectra of NiO/0.5C/0.5CuxO synthesis at different temperatures. The peaks at 286.37, 286.77, 287.10, 287.86, and 290.05 eV correspond to the C=C, C-C, C-OH, C-O, and O-C=O bonds, respectively. It can be seen from Table S4 that when the calcination temperature is 600°C, the hydroxyl content in NiO/0.5C/0.5CuxO is the highest, which is conducive to the improvement of its photocatalytic efficiency. NiO/0.5C/0.5CuxO synthesized at different temperatures has abundant carbon bond content, which is conducive to the separation of photogenerated electrons and holes. Moreover, in the products calcined at 600°C, the total area of corresponding carbon bond peaks is the largest, so it can be inferred that its photogenerated electrons and holes are separated in the highest efficiency, resulting in the best photocatalytic efficiency.

Figure S5

Table S4

From the viewpoint of practical application and environmental friendliness, the stability and re-use of the catalyst are very important. Therefore, this study conducted repetitive experiments on catalyst NiO/C/CuxO. It can be seen from Figure S6(a and b) that, after repeated use for 4 times, NiO/C/CuxO still showed high degradation efficiency on RhB and there was no obvious change in the crystal structure, indicating that the catalyst has high stability and can be reused for many times. As is known, superoxide free radicals (•O2-), hydroxyl free radicals (•OH), and holes (h+) play an important role in the process of photocatalytic degradation. Therefore, we conducted experiments to capture active species, and the results are shown in Figure S6(c). It can be seen from Figure S6(c) that the photocatalytic efficiency is basically unchanged when KI (hole catcher) was added, indicating that KI does not affect the photocatalytic efficiency. However, the photocatalytic efficiency is reduced after addition of benzoquinone (•O2- catcher) and terephthalic acid (•OH catcher), implying that •O2- and •OH play major roles in the photocatalytic degradation process.

Figure S6

The photocatalytic degradation of RhB by NiO/C/CuxO composites is a synergistic process driven by the NiO/CuO/Cu2O ternary isotype heterojunction and AR14-derived carbon layer, integrating enhanced visible light absorption, efficient charge separation, and synergistic oxidation by reactive species. Cu2O forms an energy level-matched system with NiO and CuO, narrowing the composite’s bandgap and expanding the visible light response. Under irradiation, all three oxides generate abundant e--h+ pairs, while the in-situ pyrolyzed carbon layer, rich in C=C and C-OH groups, further boosts photon capture and suppresses carrier aggregation, an advantage over traditional carbon sources like rGO. The carbon layer acts as an electron-mediating bridge, addressing interface charge transfer resistance. Leveraging energy level matching (NiO: VB = 2.51 eV, CB = -1.51 eV; CuO: VB = 3.52 eV, CB = -0.52 eV; Cu2O: VB = 0.8 eV, CB = -1.3 eV [45,46], electrons from CuO’s CB migrate via carbon to NiO’s VB, while NiO’s VB holes diffuse to Cu2O’s VB, forming directional transfer. NiO’s CB electrons are also rapidly transferred to Cu2O’s CB via carbon, minimizing e⁻-h⁺ recombination (evidenced by the lowest PL intensity in NiO/0.5C/0.5CuxO). Optimal oxygen vacancies (44.87% at 600°C, O1s XPS) introduce defect states to accelerate separation, while excess vacancies distort the crystal structure. Dominant reactive species •O2- and •OH are generated synergistically: Cu2O’s CB electrons (-1.3 eV) reduce O2 to •O2- [47,48], promoted by an optimal Cu+/Cu2+ ratio of 0.86; NiO/CuO’s VB holes oxidize H2O to •OH, enhanced by the carbon layer’s surface hydroxyl groups. Trapping experiments confirm •O2- and •OH reduce degradation efficiency by ∼60% and ∼50%, respectively. These species synergistically degrade RhB: •O2- attacks conjugated double bonds, while •OH oxidizes amino/carboxyl groups, disrupting the chromophore [49]. RhB is stepwise decomposed into small molecules and mineralized to CO2/H2O. The composite’s mesoporous structure enhances RhB adsorption, shortening reaction distances. Excellent stability stems from the carbon layer inhibiting nanoparticle aggregation/oxidation and the dynamic Cu+/Cu2+ equilibrium maintaining active sites, with ≥90% degradation efficiency retained after 4 cycles. Key factors optimizing performance include the 0.86 Cu+/Cu2+ ratio, 44.87% oxygen vacancies, 600°C calcination, and the carbon layer’s electron-mediating role, providing a theoretical basis for high-efficiency organic pollutant degradation catalysts. Therefore, the possible photocatalytic mechanism of RhB over NiO/C/CuxO can be illustrated by the schematic diagram of Eq. 3-7 [50,51] and Figure 10.

(3)
catalyst + hv → e − + h +

(4)
O 2 + e → · O 2 −

(5)
h + + H 2 O → · OH + H +

(6)
O 2 − + · OH + RhB → H 2 O + CO 2

(7)
h + + RhB → oxidation product

Proposed photocatalytic mechanism over the heterojunction. (h+: hole, VB: valence band, CB: conduction band).
Figure 10.
Proposed photocatalytic mechanism over the heterojunction. (h+: hole, VB: valence band, CB: conduction band).

4. Conclusions

This study prepared NiO/C/CuxO composites using AR14 as the carbon source and elucidated the intrinsic mechanisms by which Cu2O, the Cu+/Cu2+ molar ratio, and calcination temperature regulate the photocatalytic degradation of RhB. As a narrow-bandgap semiconductor, Cu2O reduces the composite’s bandgap to 1.19 eV, drastically enhancing visible light absorption, and forms a ternary isotype heterojunction with NiO and CuO. With the carbon layer acting as an electron bridge, Cu₂O effectively suppresses photogenerated electron-hole recombination and accelerates the generation of •O2- and •OH, the dominant reactive species for RhB degradation. The Cu+/Cu2+ molar ratio is a key regulatory factor: NiO/0.5C/0.5CuxO, with a Cu+/Cu2+ ratio of 0.86, achieves the highest Cu2O content, accompanied by an optimal oxygen vacancy content (44.87%) and surface hydroxyl content. These factors synergistically boost dye adsorption and carrier separation efficiency, while an imbalanced ratio leads to reduced Cu2O formation and impaired photocatalytic performance. Calcination at 600°C is optimal, as it maximizes Cu2O yield via efficient carbon reduction of CuO, generates a mesoporous structure with a large specific surface area for mass transfer, and produces abundant carbon bonds and oxygen vacancies to optimize carrier transport. Temperatures below 600°C limit Cu2O synthesis, while temperatures above 600°C cause pore collapse and reduced adsorption capacity. NiO/0.5C/0.5CuxO maintains high activity after four reuse cycles, demonstrating excellent stability. This work deepens the understanding of Cu2O’s modulation mechanism in carbon-based metal oxide photocatalysts and provides theoretical support for designing high-efficiency photocatalysts for organic pollutant degradation.

Acknowledgment

The financial support by the National Natural Science Foundation of China (21876158), Key Projects of Jinhua Science and Technology Bureau (2022-1-077), and Zhejiang Province New Talent Plan (2023R404052) is gratefully acknowledged.

CRediT authorship contribution statement

Shiyi Chen: Writing – original draft, Writing – review & editing. Yujing Zhou: Conceptualization, Data curation, Formal analysis. Yunlong Xie: Resources, Software, Supervision. Jialong Fu: Validation. Guoju Chang: Resources. Shiyou Hao: Supervision, Formal analysis, Investigation, Project administration, Resources, Writing – review & editing.

Declaration of competing interest

There are no conflicts of interest.

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

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

Supplementary data

Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_1047_2025.

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