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Article

Light-Driven Enhancement of Oxygen Evolution for Clean Energy Conversion: Co3O4-TiO2/CNTs P-N Heterojunction Catalysts Enabling Efficient Carrier Separation and Reduced Overpotential

by
Weicheng Zhang
1,
Taotao Zeng
1,
Yi Yu
1,
Yuling Liu
1,
Hao He
1,*,
Ping Li
2,* and
Zeyan Zhou
3,*
1
College of Material Science and Engineering, Changsha University of Science and Technology, Changsha 410114, China
2
College of Science, Xinjiang Institute of Technology, Aksu 843100, China
3
College of Materials Science and Engineering, Hunan University, Changsha 410082, China
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(15), 4185; https://doi.org/10.3390/en18154185
Submission received: 20 June 2025 / Revised: 30 July 2025 / Accepted: 31 July 2025 / Published: 7 August 2025
(This article belongs to the Special Issue State-of-the-Art Research in Advanced Materials for Energy Storage Applications: 2nd Edition)
Browse Figures
Versions Notes

Abstract

In the renewable energy conversion system, water electrolysis technology is widely regarded as the core means to achieve clean hydrogen production. However, the anodic oxygen evolution reaction (OER) has become a key bottleneck limiting the overall water splitting efficiency due to its slow kinetic process and high overpotential. This study proposes a novel Co3O4-TiO2/CNTs p-n heterojunction catalyst, which was synthesized by hydrothermal method and significantly improved OER activity by combining heterojunction interface regulation and light field enhancement mechanism. Under illumination conditions, the catalyst achieved an overpotential of 390 mV at a current density of 10 mA cm−2, which is superior to the performance of the dark state (410 mV) and single component Co3O4-TiO2 catalysts. The material characterization results indicate that the p-n heterojunction structure effectively promotes the separation and migration of photogenerated carriers and enhances the visible light absorption capability. This work expands the design ideas of energy catalytic materials by constructing a collaborative electric light dual field regulation system, providing a new strategy for developing efficient and low-energy water splitting electrocatalysts, which is expected to play an important role in the future clean energy production and storage field.

1. Introduction

Since the dawn of the industrial era, fossil fuels—coal, oil, and natural gas—have served as the primary energy foundation underpinning global industry, agricultural mechanization, and modern living standards. While catalyzing unprecedented economic growth and technological advancement, this fossil fuel dependence has exacted a severe toll: it is the principal driver of anthropogenic climate change through massive greenhouse gas emissions (notably CO2 and methane), contributes significantly to air pollution (releasing particulate matter, SOx, and NOx), and drives ecosystem degradation through resource extraction and spills [1,2,3]. Critically, these finite resources face inevitable depletion within conceivable timescales, creating profound energy security vulnerabilities as accessible reserves dwindle and extraction costs rise. Consequently, a global imperative has emerged to transition towards clean, sustainable energy systems. Intense research, development, and policy efforts are now focused on harnessing and integrating diverse renewable sources like solar, wind, hydro, and geothermal power. These alternatives are widely recognized as essential pathways to drastically curtail reliance on fossil fuels, mitigate environmental damage, and establish a resilient, long-term energy infrastructure [4,5].
Hydrogen has emerged as a particularly compelling clean energy vector within this transition. Its fundamental appeal lies in its intrinsic properties: as a chemical energy carrier, it possesses zero carbon content, meaning its combustion or electrochemical conversion in fuel cells produces only water as a by-product, eliminating direct CO2 emissions at the point of use. Furthermore, hydrogen boasts the highest gravimetric energy density of any common fuel (~142 MJ/kg, nearly three times that of gasoline), offering significant advantages for weight-sensitive applications like transportation [6,7]. However, hydrogen’s environmental credentials depend entirely on its production method. In this context, electrolytic hydrogen production, driven by electricity to decompose water molecules (2H2O = 2H2 + O2), represents a cornerstone strategy for renewable energy conversion and storage [8,9]. When powered by renewable electricity (“green hydrogen”), this process offers a truly sustainable pathway. It effectively converts intermittent renewable electricity into a storable, transportable chemical fuel, enabling decarbonization of sectors difficult to electrify directly (e.g., heavy industry, shipping, aviation, and long-haul trucking) and providing crucial grid-balancing services by absorbing surplus renewable generation. Advances in electrolyser technologies (e.g., PEM, AWE, SOEC), coupled with declining renewable electricity costs, are rapidly enhancing the economic viability and scalability of this approach, positioning it as a pivotal element in the future global energy landscape.
Electrolysis of water involves a hydrogen evolution reaction (HER) at the cathode and an OER at the anode [10,11,12]. Theoretically, it only requires a potential difference of 1.23 V to be realized between the anode and cathode to drive the entire reaction [13,14,15]. In practice, however, higher voltages are required in actual water electrolyzers due to overpotentials at the two electrodes [16]. HER is a relatively simple two-electron transfer process involving the electrochemical adsorption and desorption of H+. In contrast, OER is inherently a more complex process and has slow oxygen evolution kinetics because it requires the transfer of four electrons through a multistep reaction with only a single electron transfer per step [17,18,19]. Thus, the energy accumulation at each step causes the OER to be kinetically hindered, requiring a large overpotential to overcome the kinetic barrier [20,21]. On the other hand, OER is an important half-reaction involved in rechargeable metal-air batteries and is also considered as an emerging sustainable energy conversion technology [22,23]. However, bottlenecks such as short lifetimes, low energy conversion efficiencies, and limited stability of metal-air batteries mainly stem from the inherent kinetic retardation of OER [24]. Therefore, the development of effective and stable OER electrocatalysts by improving oxygen electrodynamics can significantly increase the energy conversion efficiency.
Spinel structured Co3O4 is considered to be one of the most promising OER catalysts with high research value due to its good catalytic activity and relatively low price. Many studies have shown that the key OER active sites lie in the high valence state of CoIV [2,25,26]. Therefore, its OER catalytic performance can be effectively improved by means of decreasing the formation potential of CoIV and increasing the amount of CoIV active sites. And nano TiO2 has unique optical, electronic and structural properties, which are widely used in the fields of photocatalysis, photochemistry, biomedicine and electrochemistry [27,28,29,30]. The separation of photoexcited carriers is facilitated by the heterogeneous bonding between TiO2 and other materials, which promotes the generation, capture and transfer of charge carriers, thus facilitating the charge transfer between the electrode and the reactants and improving the catalytic performance of OER [31].
This experiment successfully enhanced the conductivity and carrier separation efficiency of CO3O4-TiO2 by introducing CNTs into the material. Under different light conditions, the overpotentials of Co-TiO2/CNTs are lower than those of Co3O4 and TiO2, and the overpotentials of Co-TiO2/CNTs under light conditions decrease by 20 mV compared with those under non-light conditions, suggesting that the p-n structure formed by the composite of Co3O4 and TiO2 has increased the reactive sites, and the OER properties of Co3O4/CNTs are improved again after the illumination of light. The OER performance was improved after illumination. Currently, there are fewer studies on Co3O4/TiO2. Therefore, we will investigate the modification of Co3O4 nanoparticles on the surface of TiO2 nanoparticles, and use the synergistic assistance of interfacial engineering and external field effect to construct Co3O4/TiO2 composite catalysts, and provides a new approach for developing efficient and low-energy water splitting electrocatalysts. This study not only expands the design concept of energy catalytic materials, but also provides important references for the future production and storage of clean energy.

2. Methods

2.1. Chemical and Materials

Tetrabutyl titanate (99%), Concentrated Hydrochloric Acid, CNTs (≥95% OD:8–15 nm, Length:~50 μm, SSA: >140 m2 g−1) were purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China); Anhydrous ethanol (95%), acetic acid were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China); Cobalt acetate, sodium hydroxide, potassium hydroxide, nickel foam, Super-p conductive carbon black, were purchased from Shanghai Aladdin Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Material Preparation

2.2.1. Acid Treatment of CNTs

1 g of carbon nanotubes was added to a beaker containing 100 mL of concentrated nitric acid and sonicated for 30 min. Subsequently, the carbon nanotubes were filtered and then washed three times with 5% sodium hydroxide solution and distilled water, and the washed carbon nanotubes were dried in a blower drying oven at 80 °C for 8 h.

2.2.2. Preparation of TiO2/CNTs

Disperse 0.2 g of carbon nanotubes soaked in concentrated nitric acid in 20 mL of an-hydrous ethanol, sonicate for 30 min, and then add 20 mL of acetic acid. Adjust the pH to 2–3 with concentrated nitric acid to form solution A. Add 2 mL of tetrabutyl titanate ester to 10 mL of ethanol form solution B. Slowly add solution B dropwise to solution A and stir continuously at 60 °C for 1 h. Then transfer to a hydrothermal reactor and react at 180 °C for 6 h. Filter and dry the obtained powder, and heat treat it at 500 °C for 1 h under a nitrogen atmosphere to obtain TiO2/CNTs.

2.2.3. Preparation of Co3O4-TiO2/CNTs

1 g of TiO2/CNTs was taken, dispersed in 40 mL of ethanol and dispersed by sonication. Then 0.3 g of cobalt acetate was dissolved in 20 mL of ethanol and stirred for 30 min, followed by slow dropwise addition to the ethanol solution containing 1 g of TiO2/CNTs. Then, a hydrothermal reaction was carried out at 150 °C for 12 h to form Co3O4-TiO2 heterojunction. The reaction was cooled and dried by filtration. The resulting solid was Co3O4-TiO2/CNTs.

2.3. Electrochemical Property Testing of Materials

2.3.1. Configuration of Inks

8 mg of Co3O4-TiO2/CNTs and 2 mg of conductive carbon black, grind it well and take 5 mg of the mixture into a centrifuge tube. Then 250 μL of ethanol solution, 750 μL of deionized water and 60 μL of Nafion solution were added to the centrifuge tube for sonication.

2.3.2. Processing of Nickel Foam

Place the nickel foam in a 10–20% dilute hydrochloric acid solution and soak it for 10 min. Afterward, remove it from the solution and rinse with deionized water and ethanol, each with ultrasonic treatment for 10 min. Once cleaned, remove the foam and dry it in an infrared oven for later use.

2.3.3. Preparation of the Working Electrode

Place 10 μL of ink evenly on 1 × 1 cm2 piece of nickel foam. Repeat this process five times for a total of 50 μL. Each ink sample should be applied to two pieces of nickel foam to facilitate comparative experiments. After applying the ink, place the nickel foam under an infrared lamp to dry for 2 h. Once dried, load the nickel foam into the electrode clamp for further use.

2.3.4. Testing Instruments

The instrument used in this test is the CHI-760E (Shanghai CH Instruments Co., Ltd., Shanghai, China) electrochemical workstation. A three electrode system was used, with carbon rod as the counter electrode, Ag/AgCl as the reference electrode, Co3O4/TiO2 catalyst loaded on foam nickel as the working electrode, and 1 M KOH solution as the electrolyte. In addition, O2 needs to be introduced for 15 min before testing, and oxygen should be introduced at a low rate during the testing process.

2.3.5. Linear Sweep Voltammetry (LSV) Test

In the electrical analysis technology of batteries, linear voltammetry scanning test is a testing method that applies a linearly transformed scanning potential, records the current passing through the electrode, and generates an i-E curve. Using a three electrode system and a xenon lamp, electrodes were inserted into a 1 M KOH solution. Two samples prepared simultaneously were treated with and without light and then subjected to linear voltammetry scanning tests. During the testing process, it is necessary to introduce O2 for 15 min in advance and maintain a certain amount of O2 flow to ensure the stability of the reaction environment. At the same time, the same sample was tested three times, and the sample was taken out at intervals between each test to remove the O2 generated on the surface of the sample and prevent inaccurate test results caused by curve shaking. The test parameters were set as follows: voltage window range of 0.2 V–0.8 V, scanning rate of 10 mV s−1, sampling time interval of 0.001 V, and sensitivity of 1 × 10−3 A V−1.

2.4. Material Characterization and Analysis Methods

In this study, scanning electron microscopy (SEM, Zeiss Sigma, Zeiss, Tokyo, Japan) and transmission electron microscopy were used to analyze the surface morphology of the prepared catalysts. The SEM images were acquired using the InLens mode with an accelerating voltage of 5 kV. X-ray diffraction (XRD, Bruker D8 Advance, Bruker, Shanghai, China) was performed to collect X-ray powder diffraction signals at a scanning rate of 5°·min−1. X-ray photoelectron spectroscopy (XPS) was conducted on an XPS instrument (ESCALAB 250Xi, Thermo Fisher, Waltham, MA, USA) to analyze the elemental composition and chemical valence states of the samples, with binding energies calibrated using the standard carbon reference.

3. Discussion and Results

3.1. Physical Properties Analysis of Co3O4-TiO2/CNTs

In order to investigate the microstructure and crystal structure of the Co3O4-TiO2/CNTs, a focused high-energy electron beam was used to scan the Co3O4-TiO2/CNTs s surface, resulting in scanning electron microscope (SEM) images of the Co3O4-TiO2/CNTs
The characterization results of Co3O4-TiO2/CNTs at different magnifications are shown in Figure 1. From the SEM images at lower magnification (Figure 1a–c), it is evident that multiple nanowires tend to agglomerate irregularly, and significant aggregation is observed. This may be due to the small diameter of the nanowires, less than 10 nm, resulting in poor dispersion, and their sensitivity to intermolecular forces causing them to bind together. From the high magnification SEM image (Figure 1d), it can be observed that Co3O4-TiO2/CNTs synthesized by hydrothermal method form a nanowire like structure, and the Co3O4-TiO2/CNTs composite catalyst is uniformly loaded on CNTs.
The X-ray energy spectrum analysis was performed on the SEM images, with the selected regions shown in Figure 2. From the analysis, it can be observed that the Co3O4-TiO2/CNTs contains four elements: C, O, Ti, and Co. These elements are evenly distributed across the Co3O4-TiO2/CNTs without any obvious segregation, indicating that Co3O4 and TiO2 have formed a rich interface. This is beneficial for the construction of heterojunctions.
Figure 3 shows the XRD pattern of the Co3O4-TiO2/CNTs composite catalyst compared with the standard PDF cards of Co3O4 (PDF#42-1467) [32] and TiO2 (PDF#21-1272) [33]. As seen in the figure, the XRD peaks of Co3O4-TiO2/CNTs appear at 2θ values of 25.32°, 37.90°, 48.24°, 53.89°, 55.11°, 62.69°, 68.76°, 70.31°, and 75.03°, corresponding to the crystal planes (101), (004), (200), (105), (211), (204), (116), (220), and (215) of anatase TiO2 [34]. The XRD peaks at 2θ values of 36.84°, 44.85°, 55.65°, 59.35°, and 65.23° correspond to the crystal planes (311), (400), (422), (511), and (440) of the spinel structure of Co3O4 [35]. The XRD results indicate that the Co3O4-TiO2/CNTs, prepared by the hydrothermal method, is primarily composed of anatase TiO2, and Co3O4 has successfully combined with TiO2, forming a structurally stable composite catalyst.
X-ray photoelectron spectroscopy (XPS) was used to further analyze the chemical states and elemental composition of the surface of the Co3O4-TiO2/CNTs. The XPS spectrum of the Co3O4-TiO2/CNTs composite catalyst is shown in Figure 4a, where peaks corresponding to Co, O, Ti, and C elements are observed, which is consistent with the SEM results. Specific information on the Co element is shown in Figure 4b, where two characteristic peaks at 780.5 eV and 795.8 eV correspond to Co 2p3/2 and Co 2p1/2, respectively, confirming the presence of Co3O4 crystals in the Co3O4-TiO2/CNTs [36]. As shown in Figure 4c, the characteristic peaks at 458.9 eV and 464.7 eV correspond to Ti 2p1/2 and Ti 2p3/2, which are consistent with the TiO2 structure [37]. Additionally, the characteristic peak at 532.0 eV for O 1s indicates the oxygen adsorbed on the material’s surface. The characteristic peak at 284.5 eV for C 1s represents the carbon nanotubes (CNTs), the catalyst support material in the Co3O4-TiO2/CNTs.
UV-Visible spectroscopy was performed on the Co3O4-TiO2/CNTs, obtaining the UV-Vis spectra for the Co3O4-TiO2/CNTs composite catalyst, Co3O4, and TiO2. The band gaps of Co3O4-TiO2/CNTs, Co3O4, and TiO2 were calculated using the Tauc Plot method. As shown in Figure 5a, TiO2 exhibits light absorption in the 300–400 nm wavelength range, while Co3O4 absorbs light in the 380–550 nm wavelength range. Due to the interaction between TiO2 and Co3O4 forming a p-n junction, the Co3O4-TiO2/CNTs composite shows enhanced absorption in the visible light region. From Figure 5b,c, the calculated band gaps (Eg) for Co3O4 and TiO2 are 1.37 eV and 3.66 eV, respectively. Figure 5d shows that after the TiO2-Co3O4 composite is formed, its electronic structure is effectively adjusted, leading to a reduction in the band gap, which decreases the difficulty of electronic transition. This facilitates the generation of electron-hole pairs, increasing the active sites per unit area and enhancing the catalytic performance.
To further understand the energy band structure of the catalytic reaction materials, experiments were conducted in Na2SO4 solution, and Mott–Schottky curves for TiO2, Co3O4, and Co3O4-TiO2/CNTs were plotted. As shown in Figure 6a, the slope of the TiO2 curve is greater than 0, indicating that it is an n-type semiconductor. In contrast, as shown in Figure 6b, the slope of the Co3O4 curve is less than 0, indicating that it is a p-type semiconductor. However, in Figure 6c, the Co3O4-TiO2/CNTs composite catalyst material displays an “inverted V” shape, indicating that it has a p-n heterojunction structure
By converting the reference electrode Ag/AgCl to the reversible hydrogen electrode (RHE) and using Equation (1), the flat band potentials for TiO2 and Co3O4 are calculated to be −0.64 eV and 0.84 eV, respectively. In n-type semiconductors, due to their structural characteristics, the flat band potential is 0.1–0.3 eV more positive than the conduction band potential. In p-type semiconductors, the flat band potential is 0.1–0.3 eV more negative than the valence band potential. Using this approach, and combining the band gaps of Co3O4 and TiO2 with Equation (2), the conduction band and valence band positions can be roughly calculated. The conduction and valence band potentials for TiO2 are −0.54 V and 3.12 V vs. RHE, respectively, while for Co3O4, the conduction and valence band potentials are −0.23 V and 1.14 V vs. RHE, respectively.
ERHE = EAg/AgCl + 0.059 × pH + 0.1981
EVB = ECB + EEg

3.2. The Electrochemical Performance Analysis of Co3O4-TiO2/CNTs

In order to evaluate the electrochemical performance of the prepared Co3O4-TiO2/CNTs, LSV analysis was performed on the Co3O4, TiO2, and Co-TiO2/CNTs samples, with the results shown in Figure 7. Under different light conditions, the current density of Co3O4, TiO2, and Co-TiO2/CNTs increased as the applied potential increased. The overpotentials corresponding to the current density of 10 mA cm−2 are shown in Figure 7e. Under dark conditions, the overpotentials for TiO2, Co3O4, and Co-TiO2/CNTs were 430 mV, 440 mV, and 410 mV, respectively. Under light conditions, the overpotentials for TiO2, Co3O4, and Co-TiO2/CNTs were 430 mV, 420 mV, and 390 mV, respectively. Compared with the materials in the literature, Co-TiO2/CNTs demonstrated significantly superior OER catalytic activity (Table S1).
As shown in Figure 7a, under different light conditions, the overpotentials corresponding to 10 mA cm−2 for Co-TiO2/CNTs are lower than those of Co3O4 and TiO2. This indicates that the p-n structure formed by the composite of Co3O4 and TiO2 increases the active sites for the reaction, which is beneficial for enhancing the light-assisted OER performance. As shown in Figure 7c–e, under light conditions, compared to the dark state, the overpotential of Co3O4 decreased by 20 mV, there was almost no change in TiO2, and the overpotential of Co-TiO2/CNTs also decreased by 20 mV. Therefore, photoaugmentation the OER performance of Co-TiO2/CNTs, Co3O4, and TiO2.
Based on the OER kinetics of Co-TiO2/CNTs, a limited exploration was also conducted on its oxygen reduction reaction (ORR) kinetics performance. Figure 8 compares the LSV curves of the samples under different light conditions at six rotation speeds: 400 rpm, 625 rpm, 900 rpm, 1225 rpm, 1600 rpm, and 2025 rpm. As shown in Figure 8a–f, at the same rotation speed, the limiting current densities vary for different samples, and the limiting current densities also differ under different light conditions for the same sample. The limiting current density increases with the rotation speed, which may be due to the faster elimination of oxygen gas bubbles produced on the surface of the samples as the rotation speed increases, accelerating the ORR rate and increasing the current.
To further investigate the catalytic performance of the samples, Figure 9 presents a comparison of the LSV curves of TiO2, Co3O4, and Co-TiO2/CNTs under both light and dark conditions. As shown in Figure 9a, under dark conditions, the limiting current densities of the samples TiO2, Co3O4, and Co-TiO2/CNTs are 3.12, 4.50, and 4.82 mA cm−2, respectively, with corresponding half-wave potentials of 0.68, 0.72, and 0.76 V. Under light conditions, the limiting current densities for TiO2, Co3O4, and Co-TiO2/CNTs are 3.68, 4.25, and 5.38 mA cm−2, respectively, with corresponding half-wave potentials of 0.70, 0.74, and 0.78 V. The composite Co-TiO2/CNTs shows an increase in both limiting current density and half-wave potential compared to pure TiO2 and Co3O4. Additionally, under light con ditions, the limiting current density of Co-TiO2/CNTs increased by 0.56 mA cm−2, and the half-wave potential increased by 0.02 V compared to the dark conditions. This suggests that the number of active sites per unit area for Co-TiO2/CNTs has increased, enhancing its OER catalytic performance, under photoaugmentation conditions, the performance of Co-TiO2/CNTs is superior to that under dark conditions.
Based on the calculations above, a rough simulation of the catalytic mechanism of Co3O4-TiO2/CNTs under photoaugmentation conditions is shown in Figure 10. When Co3O4 and TiO2 come into contact, a p-n heterojunction is formed. At the thermodynamic equilibrium of the p-n junction, there is an internal field directed from the n-type TiO2 to the p-type Co3O4. According to the positions of the conduction band (CB) and valence band (VB), the excited electrons on the CB of p-type Co3O4 transfer to the n-type TiO2, while the holes move from n-type TiO2 to p-type Co3O4 [38,39]. Therefore, the p-n junction formed between Co3O4 and TiO2 can effectively separate the photogenerated electron-hole pairs, reduce the recombination of the electron-hole pairs, and increase the number of active sites, which is beneficial for improving the adsorption properties of intermediates. The p-n junction structure plays a crucial role in the effective separation of light-induced electrons and holes. The separated electrons and holes then react freely with the adsorbed reactants on the surface, thereby enhancing the catalytic activity. As a result, the Co3O4/TiO2 heterostructure exhibits high photoaugmentation OER catalytic activity [23,40,41].

4. Conclusions

This study successfully synthesized a Co3O4/TiO2/CNTs composite catalyst through a hydrothermal method, resulting in stable and uniformly distributed nanowire structures. The material’s key feature is the well-constructed p–n heterojunction between p-type Co3O4 and n-type TiO2, which effectively enhances charge separation by efficiently separating photogenerated electron–hole pairs. This photoaugmentation interface design significantly improves the performance of light assisted oxygen evolution reaction (OER), which is a key process for sustainable hydrogen production in water splitting.
Electrochemical characterization through linear sweep voltammetry (LSV) showed that at 10 mA cm−2, the overpotentials for TiO2, Co3O4, and Co3O4/TiO2/CNTs were 430 mV, 440 mV, and 410 mV, respectively, in dark conditions. Notably, under light illumination, the Co3O4/TiO2/CNTs composite demonstrated superior performance with a reduced overpotential of 390 mV, representing both a 20 mV improvement over its dark-state performance and better catalytic activity than its individual components. These results clearly demonstrate that the designed p-n heterojunction not only provides more active sites but also enables light-enhanced catalytic efficiency.
The synergistic combination of interfacial charge modulation and photoactivation makes this nanohybrid system particularly promising for solar-powered water splitting applications. The Co3O4/TiO2/CNTs catalyst, with its improved OER activity and structural stability, shows great potential for clean energy technologies, including large-scale hydrogen production and energy-saving water treatment. This research provides important insights for developing advanced photocatalysts suitable for combined renewable energy generation and environmental cleanup applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en18154185/s1, Table S1 Comparison of electrocatalytic performance in the literature. References [42,43,44,45,46,47] are cited in the supplementary materials.

Author Contributions

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

Funding

This research was funded by Shandong Natural Science Foundation Youth Project grant numbers ZR2024QE312 and ZR2024QE307.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. SEM Image of Co3O4-TiO2/CNTs Composite Catalyst. (a–c) SEM image at low resolution. (d) SEM image at high resolution.
Figure 1. SEM Image of Co3O4-TiO2/CNTs Composite Catalyst. (a–c) SEM image at low resolution. (d) SEM image at high resolution.
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Figure 2. (a) EDAX Image of Co3O4-TiO2/CNTs Composite Catalyst. (b) Distribution map of C element. (c) Distribution map of O element. (d) Distribution map of Ti element. (e) Distribution map of Co element.
Figure 2. (a) EDAX Image of Co3O4-TiO2/CNTs Composite Catalyst. (b) Distribution map of C element. (c) Distribution map of O element. (d) Distribution map of Ti element. (e) Distribution map of Co element.
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Figure 3. XRD Patterns of Co3O4-TiO2/CNTs Composite Catalyst, Pure Co3O4, and Pure TiO2.
Figure 3. XRD Patterns of Co3O4-TiO2/CNTs Composite Catalyst, Pure Co3O4, and Pure TiO2.
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Figure 4. (a) XPS spectrum of all catalysts. (b) XPS spectrum of pure Co3O4. (c) XPS spectrum of pure TiO2.
Figure 4. (a) XPS spectrum of all catalysts. (b) XPS spectrum of pure Co3O4. (c) XPS spectrum of pure TiO2.
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Figure 5. (a) UV-Visible spectra of Co3O4-TiO2/CNTs composite catalyst, Co3O4, and TiO2. (b) Band gap diagram of Co3O4. (c) Band gap diagram of TiO2. (d) Band gap diagram of Co3O4-TiO2/CNTs.
Figure 5. (a) UV-Visible spectra of Co3O4-TiO2/CNTs composite catalyst, Co3O4, and TiO2. (b) Band gap diagram of Co3O4. (c) Band gap diagram of TiO2. (d) Band gap diagram of Co3O4-TiO2/CNTs.
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Figure 6. Mott–Schottky curve of (a) TiO2, (b) Co3O4, and (c) Co3O4-TiO2/CNTs.
Figure 6. Mott–Schottky curve of (a) TiO2, (b) Co3O4, and (c) Co3O4-TiO2/CNTs.
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Figure 7. LSV plots of the samples: (a) Co3O4, TiO2, and Co-TiO2/CNTs under dark conditions; (b) Co3O4, TiO2, and Co3O4-TiO2 under light conditions; (c) pure Co3O4; (d) pure TiO2; (e) pure Co-TiO2/CNTs; (f) Overpotential and Tafel comparison table.
Figure 7. LSV plots of the samples: (a) Co3O4, TiO2, and Co-TiO2/CNTs under dark conditions; (b) Co3O4, TiO2, and Co3O4-TiO2 under light conditions; (c) pure Co3O4; (d) pure TiO2; (e) pure Co-TiO2/CNTs; (f) Overpotential and Tafel comparison table.
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Figure 8. LSV curves at different rotation speeds: (a) Co3O4, (c) TiO2, (e) Co-TiO2/CNTs, (b) light-Co3O4, (d) light-TiO2, (f) light-Co-TiO2/CNTs.
Figure 8. LSV curves at different rotation speeds: (a) Co3O4, (c) TiO2, (e) Co-TiO2/CNTs, (b) light-Co3O4, (d) light-TiO2, (f) light-Co-TiO2/CNTs.
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Figure 9. Comparison of LSV curves of different samples at 1600 rpm: (a) Dark conditions, (b) Light conditions.
Figure 9. Comparison of LSV curves of different samples at 1600 rpm: (a) Dark conditions, (b) Light conditions.
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Figure 10. Catalytic mechanism of Co3O4/TiO2 under xenon lamp irradiation.
Figure 10. Catalytic mechanism of Co3O4/TiO2 under xenon lamp irradiation.
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Zhang, W.; Zeng, T.; Yu, Y.; Liu, Y.; He, H.; Li, P.; Zhou, Z. Light-Driven Enhancement of Oxygen Evolution for Clean Energy Conversion: Co3O4-TiO2/CNTs P-N Heterojunction Catalysts Enabling Efficient Carrier Separation and Reduced Overpotential. Energies 2025, 18, 4185. https://doi.org/10.3390/en18154185

AMA Style

Zhang W, Zeng T, Yu Y, Liu Y, He H, Li P, Zhou Z. Light-Driven Enhancement of Oxygen Evolution for Clean Energy Conversion: Co3O4-TiO2/CNTs P-N Heterojunction Catalysts Enabling Efficient Carrier Separation and Reduced Overpotential. Energies. 2025; 18(15):4185. https://doi.org/10.3390/en18154185

Chicago/Turabian Style

Zhang, Weicheng, Taotao Zeng, Yi Yu, Yuling Liu, Hao He, Ping Li, and Zeyan Zhou. 2025. "Light-Driven Enhancement of Oxygen Evolution for Clean Energy Conversion: Co3O4-TiO2/CNTs P-N Heterojunction Catalysts Enabling Efficient Carrier Separation and Reduced Overpotential" Energies 18, no. 15: 4185. https://doi.org/10.3390/en18154185

APA Style

Zhang, W., Zeng, T., Yu, Y., Liu, Y., He, H., Li, P., & Zhou, Z. (2025). Light-Driven Enhancement of Oxygen Evolution for Clean Energy Conversion: Co3O4-TiO2/CNTs P-N Heterojunction Catalysts Enabling Efficient Carrier Separation and Reduced Overpotential. Energies, 18(15), 4185. https://doi.org/10.3390/en18154185

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