Cobalt ion redox and conductive polymers boosted the photocatalytic activity of the graphite carbon nitride–Co₃O₄ Z-scheme heterostructure

Abstract

The enhanced photocatalytic hydrogen evolution performance of g-C₃N₄–Co₃O₄ 2D–1D Z-scheme heterojunctions was achieved by employing the cobalt ion redox and conductive polymers (polyaniline, PANi) for the first time. Specifically, a Co₃O₄ 1D nanobelt acting as the Z-scheme heterostructure component could promote the separation of photo-excited charge carriers and increased the redox capacity, and the conductive PANi brought about the boosted transport efficiency of the charge carriers. Notably, cobalt ions were adopted as the intermediate of the electron transfer between two components for boosting the carrier transport and utilization efficiency. Consequently, the Co₃O₄ nanobelt, PANi and cobalt ions exhibited a synergistic effect on facilitating the photocatalytic HER performance of the g-C₃N₄-based heterostructure, and the optimal ternary heterostructure (g-C₃N₄–PANi–Co₃O₄) exhibited a photocatalytic H₂-evolution rate of 4.61 μmol h⁻¹ under visible light, which further increased to 9.95 μmol h⁻¹ by adding Co²⁺ solution owing to the further facilitated transport of charge carriers through the redox reaction of cobalt ions. Moreover, even in the absence of sacrificial agents, this Z-scheme system exhibited a photocatalytic hydrogen production activity of 3.35 μmol h⁻¹ in the Co²⁺ aqueous solution under UV-visible light.

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Introduction

Photocatalytic conversion of solar energy into clean hydrogen or carbon energy has been deemed as the most promising method to solve the global energy and environmental problems.^1–6 The earth-abundant graphitic carbon nitride seems to be one of the most promising candidate photocatalysts due to its high thermal and chemical stability, environmental friendliness, low cost, and suitable bandgap for visible light absorption.^7–10 The 2D layered structure of g-C₃N₄ could provide a large number of photocatalytic sites, which is beneficial to the surface-based photocatalytic hydrogen evolution reaction (HER).^3,11–13 Moreover, the g-C₃N₄-based photocatalyst has been proved to produce H₂O₂ in the absence of sacrificial agents.^14,15 However, the high carrier recombination rate, low transfer rate and utilization of the photo-generated charge carriers limited its application.^3,10,13,16

To overcome the shortcomings of g-C₃N₄, constructing heterojunctions is a feasible idea, where the semiconductors have different work functions, leading to electron transfer from the higher work function to the lower one and the opposite hole transfer process, whereas, in the normal type II heterojunction, the carrier transfer process brings about the weaker reductive and oxidizing capacity in the hydrogen and oxygen production reactions, respectively.^17,18 Satisfyingly, the Z-scheme heterostructure provided a new path to inject electrons from the conductive band of one semiconductor (stronger oxidative capacity) to the valence band of the other semiconductor with better reducing capacity, and therefore the electron transfer in the “Z” shape would provide many benefits for photocatalytic reduction owing to the strong redox capacity.^3,19–21 Many g-C₃N₄-based Z-scheme heterojunctions have been reported and selecting a suitable semiconductor to inform a Z-scheme heterojunction with g-C₃N₄ is a foremost question.^22–25

As a classic p-type semiconductor and a versatile transition metal oxide, Co₃O₄ has a suitable energy band structure and excellent electronic properties, which make it a widely used cocatalyst and electrode material for supercapacitors.²⁶ But the performance in the photocatalytic hydrogen evolution of g-C₃N₄–Co₃O₄, which has been proven to be a Z-scheme structure, was reported minimally.^27,28 Further investigation of this structure is feasible.

To enhance the transport of photo-generated charge carriers in a photocatalyst, some classic conductive components have been widely reported, such as noble metals, graphene, porous carbon, conductive polymers and even highly crystallized solid solutions, which have been adopted as mediators.^10,29–38 In particular, conductive polymers (CPs) have been extensively studied and are promising candidates for facilitating the transport of photo-excited charge carriers between components.^34,39 In addition, the insertion of CPs between two photocatalyst components could increase the photocatalytic activity by increasing the visible light absorption, separation and transport efficiency of the photo-excited charge carriers due to their narrow bandgap.^34,39,40 In particular, due to its simple synthesis, remarkable catalytic activity, good environmental stability and reversible redox characteristics, PANi is an attractive alternative to improve the carrier performance of the interface between two photocatalyst components.

Redox couples adopted for efficient Z-scheme water splitting should possess the following properties: (i) an appropriate redox potential stretching across the reduction and oxidation potentials of water, (ii) sufficient reversibility (also known as redox cyclability) under reaction conditions without deposition or precipitation, and (iii) good transparency to UV and visible light.^19,41 As previously reported, Fe³⁺/Fe²⁺, IO₃⁻/I⁻, [Co(bpy)₃]^3+/2+ redox couples were adopted as the intermediate of the electron transfer between two photocatalysts for boosting the carrier transport.^42–45 However, the occurrence of backward electron transfer to the redox couple and the low redox reaction rate became the downside of this system,⁴⁶ so further increasing the separation and transport efficiency of photoinduced charge carriers was of particular importance for improving the photocatalytic activity.^47,48 In addition, most of the indirect Z-scheme systems are usually faced with the challenge of reaction stability.⁴¹ So searching for new redox couples for solid state direct Z-scheme heterostructures is of particular importance for the practical application of photocatalysts for solar energy conversion.⁴⁹

Herein, a novel g-C₃N₄–PANi–Co₃O₄ Z-scheme heterostructure was designed and its photocatalytic performance was systematically investigated. In detail, the Z-scheme heterostructure formed between a Co₃O₄ nanobelt and g-C₃N₄ could promote the separation of photo-excited charge carriers and increase the redox capacity, the adoption of the conductive PANi brought about the boosted transport efficiency of the charge carriers, and cobalt ions as the intermediate of the electron transfer in the Z-scheme heterostructure could facilitate the carrier transport and utilization efficiency. Consequently, the photocatalytic HER performance of the optimal ternary heterostructure (g-C₃N₄–PANi–Co₃O₄) was 4.61 μmol h⁻¹ and further increased to 9.95 μmol h⁻¹ by adding a certain amount of Co²⁺ under visible light. Moreover, even in the absence of the sacrificial agent, this “Z-scheme” system exhibited a photocatalytic HER activity of 3.35 μmol h⁻¹ with considerable H₂O₂ production under UV-visible light irradiation.

Experimental

Synthesis of g-C₃N₄

The g-C₃N₄ nanosheet was prepared by a traditional thermal polymerization strategy as reported previously.^16,34,50 In a typical procedure, 5 g of C₃N₃(NH₂)₃ was placed in a crucible with a cover and then annealed at 650 °C for 2 h in argon. The final yellow powder was then collected.

Synthesis of g-C₃N₄–PANi

To synthesize g-C₃N₄–PANi, 200 mg g-C₃N₄ was added to 30 mL deionized water and sonicated in an ultrasonic washer. Then, a suitable amount of aniline was introduced into the aforementioned solution with ultrasonication. 5 mL of an aqueous solution containing 50 mg ammonium persulfate was slowly added dropwise into the above-mentioned solution. After magnetic stirring for 12 hours, the resulting grey-green suspension was centrifuged and washed with water and ethanol three times, respectively. Finally, the product was dried at 60 °C overnight.

Synthesis of g-C₃N₄–PANi–Co₃O₄

Typically, 200 mg g-C₃N₄–PANi was dispersed in a 30 mL solution containing suitable amounts of carbamide, NH₄F and Co(NO₃)₂·6H₂O. After ultrasonic dispersion, the homogeneous solution was transferred to a 50 mL Teflon-lined reactor and maintained for 10 h at 120 °C. The final grey-black products were rinsed several times with distilled water and ethanol, and dried at 60 °C overnight. The g-C₃N₄–Co₃O₄ and Co₃O₄ control samples were prepared with a similar experimental process using pure g-C₃N₄ or without the addition of g-C₃N₄.

Results and discussion

X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) were employed for confirming the formation of the g-C₃N₄–PANi–Co₃O₄ ternary heterostructure. As exhibited in Fig. 1a, the distinct characteristic peak located around 27.4° could be observed in all the g-C₃N₄-based heterostructures, which was indexed as the (002) crystal plane of g-C₃N₄.⁵⁰ After the adoption of Co₃O₄ and PANi, there was no obvious difference with g-C₃N₄, except for the slightly reduced peak intensity. The diffraction peaks of the as-prepared pure Co₃O₄ could be matched with the standard PDF index card of cubic Co₃O₄ (JCPDS no. 42-1467), which could also be observed in g-C₃N₄–Co₃O₄ and g-C₃N₄–PANi–Co₃O₄, confirming the formation of the g-C₃N₄–Co₃O₄ surface heterostructure.⁵¹ The weaker peaks of Co₃O₄ might imply poor crystallinity, which is probably due to the hydrothermal method and lower synthesis temperature.^52–57 In addition, the low content ratio and test conditions might also have caused a difference in XRD results. As displayed in Fig. 1b, four obvious peaks appearing at 810, 887, 2134, and 3000–3300 cm⁻¹ for g-C₃N₄ could also be found in g-C₃N₄–PANi and g-C₃N₄–PANi–Co₃O₄, with the peaks at about 810 and 887 cm⁻¹ corresponding to the out-of-plane bending vibrations of tri-s-triazine and the peak located at 2134 cm⁻¹ belonging to the C≡N vibration band.⁵⁸ Furthermore, the existence of PANi characteristic peaks (N–H stretching vibrations, C–N stretching of a secondary aromatic amine and the C–H benzenoid unit) in g-C₃N₄–PANi and g-C₃N₄–PANi–Co₃O₄ affirmed the formation of the g-C₃N₄–PANi heterostructure.⁵⁹

Fig. 1 (a) PXRD patterns and (b) FTIR spectra of g-C₃N₄-based samples; (c) UV-vis absorption spectra and (d) (αhγ)²versus hγ curve of g-C₃N₄-based samples; (e) Mott–Schottky plots of g-C₃N₄ and Co₃O₄; (f) possible mechanism for the separation and transfer of charge carriers over the optimal g-C₃N₄-PANi-Co₃O₄ ternary heterostructure with cobalt ions.

The optical absorption characteristics and band structures of the as-synthesized g-C₃N₄, g-C₃N₄–PANi, g-C₃N₄–Co₃O₄ and g-C₃N₄–PANi–Co₃O₄ were evaluated by UV-visible diffuse reflection spectroscopy (Fig. 1c). The loading of PANi and Co₃O₄ can both promote the visible light absorption, which can be further increased by the co-loading of PANi and Co₃O₄, whereas the bandgap of g-C₃N₄ was not affected by the adoption of PANi and Co₃O₄, indicating the successful construction of the heterostructure. The band structures of g-C₃N₄ and Co₃O₄ were also investigated through UV-visible spectroscopy, and the results in Fig. 1d show that their bandgaps were, respectively, 2.82 and 1.80 eV. Using Mott–Schottky plots, the separation and transport directions of electron–hole pairs in g-C₃N₄–PANi–Co₃O₄ were confirmed (Fig. 1e), and it could be observed that the flat band potentials of g-C₃N₄ and Co₃O₄ were calculated to be −1.52 V and 0.65 V, respectively. Based on the above results, the schematic diagram for the separation and transfer of charge carriers in the g-C₃N₄–PANi–Co₃O₄ ternary heterostructure with the participation of cobalt ions is shown in Fig. 1f. Under light irradiation, electrons were stimulated and jumped from the valence band (VB) of Co₃O₄ to the conduction band (CB) of g-C₃N₄ through the highly conductive PANi and Co²⁺/Co³⁺ pairs as the redox reaction between Co²⁺ and Co³⁺ constituted a cycle to deliver electrons. In total, the electrons and holes separately accumulated in the CB of g-C₃N₄ and the VB of Co₃O₄, possessing the reduction and oxidation capacity to provide H₂ and H₂O₂. Notably, such a Z-scheme heterostructure promoted the separation of photo-excited charge carriers and increased the redox capacity, which was beneficial for the photocatalytic water splitting reaction.^60,61

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to characterize the morphologies of the as-prepared g-C₃N₄-based heterostructures. The nanosheet morphology of pristine g-C₃N₄ could be clearly observed from the TEM image (Fig. 2a), whereas Co₃O₄ exhibited a single layer nanobelt morphology as depicted in the TEM (Fig. 2b) and SEM images of Co₃O₄ (inset of Fig. 2b), which is favorable for radial carrier transmission.⁶² It can be observed from Fig. 2c that PANi nanoparticles were uniformly dispersed in the skeleton of g-C₃N₄.³⁴ Moreover, it could be found from the TEM image of ternary g-C₃N₄–PANi–Co₃O₄ (Fig. 2d) that the Co₃O₄ nanobelt and PANi nanoparticles were uniformly distributed over the g-C₃N₄ nanosheet. High-resolution TEM (HRTEM) was employed for further investigating the morphology of Co₃O₄ in the ternary g-C₃N₄–PANi–Co₃O₄ heterostructure. As exhibited in Fig. 2e, the lattice fringe with a lattice constant of 0.24 nm could be indexed to the (311) plane of Co₃O₄. Moreover, g-C₃N₄–PANi–Co₃O₄ exhibited a similar morphology on SEM images (Fig. S4, ESI†). Furthermore, elemental mapping was adopted to investigate the distribution of different components. The results reflect that the C, N and the Co, Ni scattered according to the corresponding outline of g-C₃N₄–PANi and Co₃O₄ nanobelt.

Fig. 2 TEM images of (a) g-C₃N₄, (b) Co₃O₄ (inset showing the SEM image), (c) g-C₃N₄–PANi, (d) and g-C₃N₄–PANi–Co₃O₄, and (e) the HRTEM image of g-C₃N₄–PANi–Co₃O₄.

The chemical structure of g-C₃N₄–PANi–Co₃O₄ was investigated by X-ray photoelectron spectroscopy (XPS). The C 1s XPS spectra of g-C₃N₄–PANi–Co₃O₄ and pure g-C₃N₄ were comprehensively compared in Fig. 3a. Corresponding to the C–N bonding of g-C₃N₄, the main peak of g-C₃N₄–PANi–Co₃O₄ located at 288.4 eV shifted to a higher energy state than that of pure g-C₃N₄, which implies the stronger combination and electronic coupling between g-C₃N₄ and Co₃O₄.¹⁰ Moreover, the peak at 284.8 eV of g-C₃N₄–PANi–Co₃O₄ is ascribed to the C–C/C=C species of PANi. The N 1s spectrum in Fig. 3b reveals three peaks, including pyridinic N, pyrrolic N and graphitic N. As the majority species, pyridinic-N displays a stronger hydrogen evolution activity, which was beneficial for the hydrogen evolution reaction. Moreover, compared with pure g-C₃N₄, the above-mentioned peaks also showed the same trend as those in the C 1s spectrum.^16,63 The Co XPS spectrum could be fitted into two peaks. As displayed in Fig. 3c, the Co 2p XPS spectra indicate two main binding energy bands at 781.8 eV (Co 2p_3/2) and 797.6 eV (Co 2p_1/2) with two shakeup satellite peaks at 786.6 and 803.5.⁶⁴ As depicted in Fig. 3d, two mean different peaks at 532.3 and 533.4 eV correspond to lattice oxygen atoms in the Co₃O₄ and external water molecule, respectively. Moreover, there was no significant difference for the optimal ternary heterostructure after the photocatalytic reaction in cobalt ions as revealed in the XPS results (Fig. S1, ESI†), indicating that the cobalt ions only acted as redox mediators and are not doped into the catalyst.

Fig. 3 High-resolution XPS spectra of (a) C 1s and (b) N 1s of pristine g-C₃N₄ and (c) C 1s and (d) N 1s of the g-C₃N₄–PANi–Co₃O₄ ternary heterostructure, and (e) Co 2p and (f) O 1s of the optimal g-C₃N₄–PANi–Co₃O₄ ternary heterostructure.

The photocatalytic HER of the g-C₃N₄–PANi–Co₃O₄ heterostructure (20 mg) with various contents of Co₃O₄ under visible light irradiation is compared in Fig. 4a, and the optimal contents of PANi were investigated as shown in Fig. S2 (ESI†). The sample with 10 wt% Co₃O₄ exhibits optimal hydrogen production activity, and further exploration of the influence of cobalt salt was based on g-C₃N₄–PANi–Co₃O₄ with the optimal ratio. The optimal concentration of cobalt salt was 0.3 mmol L⁻¹. Notably, trace amounts of cobalt ions could dramatically improve the photocatalytic hydrogen evolution, and the optimal photocatalytic hydrogen production is 9.95 μmol h⁻¹ under visible light and 22.89 μmol h⁻¹ under UV-visible light (Fig. 4b and Fig. S3, ESI†), which was 2.16 and 5.65 times larger than that of g-C₃N₄–PANi–Co₃O₄ and g-C₃N₄–PANi, respectively. Noticeably, the optimal g-C₃N₄–PANi–Co₃O₄-Co system exhibited a photocatalytic hydrogen evolution rate that is 17.5 times larger than that of g-C₃N₄ and the g-C₃N₄–Co₃O₄ heterostructure, indicating the obvious synergistic effect between PANi and Co²⁺/Co³⁺ on improving the photocatalytic hydrogen evolution performance of the g-C₃N₄–Co₃O₄ heterostructure. As displayed in Table 1, the optimal g-C₃N₄–PANi–Co₃O₄ with Co²⁺/Co³⁺ exhibited competitive photocatalytic activity in comparison with those of g-C₃N₄-based heterostructures, g-C₃N₄ loaded with different types of cocatalysts, and the reported system of g-C₃N₄–Co₃O₄ based heterostructures.

Fig. 4 (a) Effect of the amount of Co₃O₄ and cobalt ion concentration on the photocatalytic HER activity of g-C₃N₄–PANi and g-C₃N₄–PANi–Co₃O₄ under visible light. (b) Comparison results of the photocatalytic activity of g-C₃N₄, g-C₃N₄–PANi, g-C₃N₄–Co₃O₄, g-C₃N₄–PANi–Co₃O₄ and g-C₃N₄–PANi–Co₃O₄ with and without cobalt ions. Photocatalytic (c) hydrogen and (d) H₂O₂ production activity of g-C₃N₄–PANi–Co₃O₄ in pure water and cobalt ion aqueous solution under visible light irradiation. (e) Apparent quantum efficiency (AQE) for g-C₃N₄–PANi–Co₃O₄ with Co²⁺/Co³⁺ redox under monochromatic light irradiation with different wavelengths. (f) Photocatalytic stability of g-C₃N₄–PANi–Co₃O₄ and g-C₃N₄–PANi–Co₃O₄ in the aqueous solution of cobalt ions.

Table 1 Comparison of the photocatalytic H₂ generation activity of g-C₃N₄-based composites and the reported system of g-C₃N₄–Co₃O₄

Photocatalyst	Dosage (mg)	H2-evolved (mmol h^−1)	Performance improvement multiple (compared with g-C3N4)	Ref.
Fe2O3/g-C3N4	20	7.96	13	65
Pt/g-C3N4/SrTiO	50	27.6	1.86	66
Co–Pi/g-C3N4	50	1.27	8.7	67
PCN/PANI/BTO	20	12.04	21	68
Ag3PO4/Ag/g-C3N4	50	0.205	4.88	69
g-C3N4/Ag/MoS2	100	10.40	3.51	70
MoO3/1T-MoS2/g-C3N4	10	5.13	12	71
Co3O4@g-C3N4/CNFs	5	0.34	—	27
Co3O4/g-C3N4	100	5	3.3	26
g-C 3 N 4 –PANI–Co 3 O 4 with cobalt ions	20	9.95	17.5	This work

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More interestingly, without any sacrificial agents, the optimal “Z-scheme“ system possessed considerable HER performance of 0.62 μmol h⁻¹ under visible light irradiation and 1.44 μmol h⁻¹ under UV-visible light with the corresponding amount of H₂O₂ production (Fig. 4c and d). Even without Co²⁺/Co³⁺ ions, the g-C₃N₄–PANi–Co₃O₄ ternary heterostructure also possesses the photocatalytic hydrogen production capacity in pure water. In order to verify the electron-promoted photocatalytic hydrogen evolution reaction in the as-prepared g-C₃N₄–PANi–Co₃O₄ ternary heterostructure with the addition of cobalt ions, the apparent quantum efficiency (AQE) of the optimal sample was calculated under different specific wavelength illuminations in Table S1 (ESI†) and are compared in Fig. 4e. At the irradiation wavelengths of 365, 420, 475 and 550 (±8) nm, the corresponding apparent quantum efficiencies were 12.97, 5.56, 2.74 and 1.17%, respectively, indicating the fairly good consistency between the AQE and the UV-visible diffuse reflection spectra. Furthermore, both the ternary g-C₃N₄–PANi–Co₃O₄ heterostructure and that with an optimal Co²⁺ concentration exhibited excellent photocatalytic stability over 30 h (Fig. 4f), which proved the existence of the Co²⁺/Co³⁺ redox conducting carrier mechanism.

The formation of H₂O₂ was investigated by Electron Paramagnetic Resonance (EPR) spectroscopy in the photocatalytic reaction process, which was carried out with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin trapping agent. As depicted in Fig. 5a, under visible light irradiation, there appeared distinct characteristic signals of ˙OH, which proved the oxidation of H₂O to ˙OH.^16,36 Room temperature photoluminescence (PL) was adopted to prove the important role of PANi and Co₃O₄ loading in promoting the separation efficiency of the photo-generated charge carriers. As shown in Fig. 5b, the introduction of Co₃O₄ can significantly quench the strong PL intensity at about 450 nm in theoriginal g-C₃N₄, which may be related to the formation of g-C₃N₄–Co₃O₄ heterostructures to improve the separation efficiency of charge carriers. In particular, owing to the improvement of carrier transport between g-C₃N₄ and Co₃O₄, introducing the CPs into g-C₃N₄–Co₃O₄ further weakened the PL intensity, which indicates the inhibition of charge carrier combination. In addition, ternary g-C₃N₄–PANi–Co₃O₄ with 10 wt% Co₃O₄ exhibited the lowest carrier recombination rate, consistent with the photocatalytic H₂-evolution results.

Fig. 5 (a) PL emission spectra, (b) Mott–Schottky plots at 1000 Hz, electrocatalytic (c) HER and (d) OER activity, (e) transient photocurrent responses and (f) EIS spectra of g-C₃N₄, g-C₃N₄–PANi, g-C₃N₄–Co₃O₄, and g-C₃N₄–PANi–Co₃O₄ with different amounts of Co₃O₄.

The electrochemical performances were further evaluated to confirm the extremely important roles of Co₃O₄ and PANi in the transport and utilization efficiency of photo-generated charge carriers. Electrocatalytic HER and OER measurements were employed for evaluating the behavior of photocatalytic hydrogen and the results are depicted in Fig. 5c and d. By intercalating PANi or CO₃O₄, the g-C₃N₄–PANi and g-C₃N₄–Co₃O₄ heterostructures exhibited lower HER and OER overpotential than pristine g-C₃N₄, indicating the improved HER and OER activity. Furthermore, g-C₃N₄–PANi–Co₃O₄ showed the lowest HER and OER overpotential. The photocurrent responses in Fig. 5e indicate that the photocurrent density of g-C₃N₄–PANi–Co₃O₄ was about 4 times as high as that of the g-C₃N₄–Co₃O₄ heterostructure. Electrochemical impedance spectroscopy (EIS) characterization was performed to evaluate the internal resistance during the charge transfer process of the g-C₃N₄-based samples (Fig. 5f). It could be clearly found that the addition of PANi significantly reduced the semi-circular diameter and the smallest appears in the EIS spectrum of optimal C₃N₄–PANi–Co₃O₄ with 10 wt% Co₃O₄ added, which indicates the important role of PANi in reducing the interfacial charge transport resistance.

The effect of Co²⁺/Co³⁺ ions on the separation, transport and utilization efficiency of charge carriers is shown in Fig. 6. When the ternary g-C₃N₄–PANi–Co₃O₄ heterostructure is in a cobalt ionic environment, as shown in Fig. 6a, the intensity of the PL peak was further attenuated. Different degrees of attenuation can be observed, and all the PL intensities at different Co²⁺ concentrations were weaker than that of the optimal g-C₃N₄–PANi–Co₃O₄. Notably, C₃N₄–PANi–Co₃O₄ with 0.3 mM Co²⁺ presented the weakest PL intensity. As displayed in Fig. 6b, the Mott–Schottky plots of g-C₃N₄, g-C₃N₄–PANi and g-C₃N₄–Co₃O₄ indicated that Co₃O₄ dramatically improved the density of charge carriers, as evidenced by the increased slope of the Mott−Schottky plots. Notably, the slope of ternary g-C₃N₄–PANi–Co₃O₄ samples displayed the maximum enhancement, and the sample with 0.3 mmol L⁻¹ Co²⁺ showed a larger slope than the sample without cobalt, which indicated the charge carrier density increase. As depicted in Fig. 6c, the participation of cobalt ions also increased the photocurrent density, indicating the cooperation of PANi and Co₃O₄ for improving the separation efficiency of photogenerated charges. In the presence of the Co²⁺/Co³⁺ redox, the ternary heterostructure displayed the smallest interfacial charge transport resistance in the EIS spectrum, which further verified the extremely important role of PANi and cobalt ions in building the “highway” of carrier transport (Fig. 6d). The above results signify that the synergy of PANi and cobalt ions can effectively inhibit the carrier recombination and enhance the transport and utilization efficiency.

Fig. 6 Effect of cobalt ion concentration on the (a) PL emission spectra, (b) Mott–Schottky plots (1000 Hz), (c) transient photocurrent responses and (d) EIS spectra of g-C₃N₄, g-C₃N₄–Co₃O₄, g-C₃N₄–PANi–Co₃O₄, and g-C₃N₄–PANi–Co₃O₄.

Conclusions

Novel g-C₃N₄–PANi–Co₃O₄ Z-scheme heterostructures were fabricated and Co²⁺/Co³⁺ redox couples were used to achieve efficient photocatalytic hydrogen production activity with and without any sacrificial reagent. The main merits of the ternary g-C₃N₄–PANi–Co₃O₄ heterostructure could be attributed to the following aspects: (i) the Co₃O₄ 1D nanobelt acting as the Z-scheme heterostructure component could promote the separation of photo-excited charge carriers and increase the redox capacity; (ii) the conductive PANi brought about the boosted transport efficiency of the charge carriers; (iii) cobalt ions were adopted as the intermediate of the electron transfer between two components for boosting the carrier transport and utilization efficiency. Consequently, the photocatalytic HER performance of the optimal ternary heterostructure (g-C₃N₄–PANi–Co₃O₄) was 4.61 μmol h⁻¹ and could be further enhanced to 9.95 μmol h⁻¹ after adding a certain amount of Co²⁺ under visible light. More significantly, even in the absence of the sacrificial agent, this Z-scheme system exhibited a photocatalytic HER activity of 1.44 μmol h⁻¹ with considerable H₂O₂ production under visible light and 3.35 μmol h⁻¹ under UV-visible light irradiation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 22071081, 21902061 and 21601063), the Opening Project of Key Laboratory of Inorganic Functional Materials and Devices, Chinese Academy of Sciences (Grant No. KLIFMD202007), and the Special Research Project of Nanyang Normal University (ZX2016014).

Notes and references

1. M. Lu, J. Liu, Q. Li, M. Zhang, M. Liu, J.-L. Wang, D.-Q. Yuan and Y.-Q. Lan, Angew. Chem., Int. Ed., 2019, 58, 12392–12397.
2. B. Zhao, Y. Huang, D. Liu, Y. Yu and B. Zhang, Sci. China: Chem., 2020, 63, 28–34.
3. Y. Yang, J. Wu, T. Xiao, Z. Tang, J. Shen, H. Li, Y. Zhou and Z. Zou, Appl. Catal., B, 2019, 255, 117771.
4. X. Xu, L. Pan, Q. Han, C. Wang, P. Ding, J. Pan, J. Hu, H. Zeng and Y. Zhou, J. Catal., 2019, 374, 237–245.
5. X.-B. Meng, J.-L. Sheng, H.-L. Tang, X.-J. Sun, H. Dong and F.-M. Zhang, Appl. Catal., B, 2019, 244, 340–346.
6. C. Yang, Z.-D. Yang, H. Dong, N. Sun, Y. Lu, F.-M. Zhang and G. Zhang, ACS Energy Lett., 2019, 4, 2251–2258.
7. T. Su, Z. D. Hood, M. Naguib, L. Bai, S. Luo, C. M. Rouleau, I. N. Ivanov, H. Ji, Z. Qin and Z. Wu, Nanoscale, 2019, 11, 8138–8149.
8. Y. Tian, L. Zhou, Q. Zhu, J. Lei, L. Wang, J. Zhang and Y. Liu, Nanoscale, 2019, 11, 20638–20647.
9. K. Li, Y.-Z. Lin, Y. Zhang, M.-L. Xu, L.-W. Liu and F.-T. Liu, J. Mater. Chem. C, 2019, 7, 13211–13217.
10. K. Li, Y. Zhang, Y. Z. Lin, K. Wang and F. T. Liu, ACS Appl. Mater. Interfaces, 2019, 11, 28918–28927.
11. L. Lin, Z. Yu and X. Wang, Angew. Chem., Int. Ed., 2019, 58, 6164–6175.
12. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76–80.
13. Y. Yang, Z. Tang, B. Zhou, J. Shen, H. He, A. Ali, Q. Zhong, Y. Xiong, C. Gao, A. Alsaedi, T. Hayat, X. Wang, Y. Zhou and Z. Zou, Appl. Catal., B, 2020, 264, 118470.
14. C. Zhu, M. Zhu, Y. Sun, Y. Zhou, J. Gao, H. Huang, Y. Liu and Z. Kang, ACS Appl. Energy Mater., 2019, 2, 8737–8746.
15. R. R. Wang, K. C. Pan, D. D. Han, J. J. Jiang, C. X. Xiang, Z. Q. Huang, L. Zhang and X. Xiang, ChemSusChem, 2016, 9, 2470–2479.
16. L.-Z. Qin, Y.-Z. Lin, Y.-C. Dou, Y.-J. Yang, K. Li, T. Li and F.-T. Liu, Nanoscale, 2020, 12, 13829–13837.
17. F. Al Marzouqi, Y. Kim and R. Selvaraj, New J. Chem., 2019, 43, 9784–9792.
18. X. Liu, Z. Ni, Y. He, N. Su, R. Guo, Q. Wang and T. Yi, New J. Chem., 2019, 43, 8711–8721.
19. Y. P. Zhang, H. L. Tang, H. Dong, M. Y. Gao, C. C. Li, X. J. Sun, J. Z. Wei, Y. Qu, Z. J. Li and F. M. Zhang, J. Mater. Chem. A, 2020, 8, 4334–4340.
20. M. Zhang, M. Lu, Z.-L. Lang, J. Liu, M. Liu, J.-N. Chang, L.-Y. Li, L.-J. Shang, M. Wang, S.-L. Li and Y.-Q. Lan, Angew. Chem., Int. Ed., 2020, 59, 6500–6506.
21. D. Liu, Y. Xu, M. Sun, Y. Huang, Y. Yu and B. Zhang, J. Mater. Chem. A, 2020, 8, 1077–1083.
22. C. Zhao, L. Tian, Z. Zou, Z. Chen, H. Tang, Q. Liu, Z. Lin and X. Yang, Appl. Catal., B, 2020, 268, 118445.
23. X. Yang, L. Tian, X. Zhao, H. Tang, Q. Liu and G. Li, Appl. Catal., B, 2019, 244, 240–249.
24. L. Tian, X. Yang, X. Cui, Q. Liu and H. Tang, Appl. Surf. Sci., 2019, 463, 9–17.
25. W. Liu, J. Shen, X. Yang, Q. Liu and H. Tang, Appl. Surf. Sci., 2018, 456, 369–378.
26. L. Yang, J. Liu, L. Yang, M. Zhang, H. Zhu, F. Wang and J. Yin, Renewable Energy, 2020, 145, 691–698.
27. R. F. He, H. O. Liang, C. P. Li and J. Bai, Colloids Surf., A, 2020, 586, 124200.
28. X. X. Zhao, Z. Y. Lu, R. Ji, M. H. Zhang, C. W. Yi and Y. S. Yan, Catal. Commun., 2018, 112, 49–52.
29. H. Tada, T. Mitsui, T. Kiyonaga, T. Akita and K. Tanaka, Nat. Mater., 2006, 5, 782–786.
30. K. Li, R. Chen, S.-L. Li, M. Han, S.-L. Xie, J.-C. Bao, Z.-H. Dai and Y.-Q. Lan, Chem. Sci., 2015, 6, 5263–5268.
31. K. Chang, Z. Mei, T. Wang, Q. Kang, S. Ouyang and J. Ye, ACS Nano, 2014, 8, 7078–7087.
32. Q. Xiang, J. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575–6578.
33. M. D. A. Khan, A. Akhtar and S. A. Nabi, New J. Chem., 2015, 39, 3728–3735.
34. K. Li, Y.-Z. Lin, K. Wang, Y. Wang, Y. Zhang, Y. Zhang and F.-T. Liu, Appl. Catal., B, 2020, 268, 118402.
35. M. O. Ansari, M. M. Khan, S. A. Ansari and M. H. Cho, New J. Chem., 2015, 39, 8381–8388.
36. Y.-Z. Lin, K. Wang, Y. Zhang, Y.-C. Dou, Y.-J. Yang, M.-L. Xu, Y. Wang, F. Liu and K. Li, J. Mater. Chem. C, 2020, 8, 10071–10077.
37. S. Li, T. Zhu, L. Dong and M. Dong, New J. Chem., 2018, 42, 17644–17651.
38. S. Nilforoushan, M. Ghiaci, S. M. Hosseini, S. Laurent and R. N. Muller, New J. Chem., 2019, 43, 6921–6931.
39. Q. Zhou and G. Shi, J. Am. Chem. Soc., 2016, 138, 2868–2876.
40. Y.-H. Yao, J. Li, H. Zhang, H.-L. Tang, L. Fang, G.-D. Niu, X.-J. Sun and F.-M. Zhang, J. Mater. Chem. A, 2020, 8, 8949–8956.
41. Y. O. Wang, H. Suzuki, J. J. Xie, O. Tomita, D. J. Martin, M. Higashi, D. Kong, R. Abe and J. W. Tang, Chem. Rev., 2018, 118, 5201–5241.
42. K. Sayama, K. Mukasa, R. Abe, Y. Abe and H. Arakawa, Chem. Commun., 2001, 2416–2417.
43. R. Abe, K. Shinmei, K. Hara and B. Ohtani, Chem. Commun., 2009, 3577–3579.
44. Y. Sasaki, H. Kato and A. Kudo, J. Am. Chem. Soc., 2013, 135, 5441–5449.
45. Y. Miseki, S. Fujiyoshi, T. Gunji and K. Sayama, Catal.: Sci. Technol., 2013, 3, 1750.
46. O. Tomita, S. Nitta, Y. Matsuta, S. Hosokawa, M. Higashi and R. Abe, Chem. Lett., 2017, 46, 221–224.
47. H. Dong, X.-B. Meng, X. Zhang, H.-L. Tang, J.-W. Liu, J.-H. Wang, J.-Z. Wei, F.-M. Zhang, L.-L. Bai and X.-J. Sun, Chem. Eng. J., 2020, 379, 122342.
48. B. Wang, S. He, L. L. Zhang, X. Y. Huang, F. Gao, W. H. Feng and P. Liu, Appl. Catal., B, 2019, 243, 229–235.
49. H. Dong, X. Zhang, Y. Lu, Y. Yang, Y.-P. Zhang, H.-L. Tang, F.-M. Zhang, Z.-D. Yang, X. Sun and Y. Feng, Appl. Catal., B, 2020, 276, 119173.
50. G. Zhang, S. Zang and X. Wang, ACS Catal., 2015, 5, 941–947.
51. M. Kang, H. Zhou, N. Zhao and B. L. Lv, CrystEngComm, 2020, 22, 35–43.
52. Q. Xu, P. Zhao, Y. K. Shi, J. S. Li, W. S. You, L. C. Zhang and X. J. Sang, New J. Chem., 2020, 44, 6261–6268.
53. Y. Zhu, T. Wan, X. Wen, D. Chu and Y. Jiang, Appl. Catal., B, 2019, 244, 814–822.
54. J. Ganesamurthi, M. Keerthi, S. M. Chen and R. Shanmugam, Ecotoxicol. Environ. Saf., 2020, 189, 110035.
55. M. Y. Zhu, S. J. Yu, R. Y. Ge, L. Y. Feng, Y. X. Yu, Y. Li and W. X. Li, ACS Appl. Energy Mater., 2019, 2, 4718–4729.
56. C. Qian, X. Guo, W. Zhang, H. Yang, Y. Qian, F. Xu, S. Qian, S. Lin and T. Fan, Microporous Mesoporous Mater., 2019, 277, 45–51.
57. Q. Guo, C. W. Zhang, C. F. Zhang, S. Xin, P. C. Zhang, Q. F. Shi, D. W. Zhang and Y. You, J. Energy Chem., 2020, 41, 185–193.
58. F. He, G. Chen, Y. Yu, S. Hao, Y. Zhou and Y. Zheng, ACS Appl. Mater. Interfaces, 2014, 6, 7171–7179.
59. L. Ge, C. Han and J. Liu, J. Mater. Chem., 2012, 22, 11843–11850.
60. X. Zhao, Z. Lu, R. Ji, M. Zhang, C. Yi and Y. Yan, Catal. Commun., 2018, 112, 49–52.
61. H. Wu, C. Li, H. Che, H. Hu, W. Hu, C. Liu, J. Ai and H. Dong, Appl. Surf. Sci., 2018, 440, 308–319.
62. K. Li, M. Han, R. Chen, S.-L. Li, S.-L. Xie, C. Mao, X. Bu, X.-L. Cao, L.-Z. Dong, P. Feng and Y.-Q. Lan, Adv. Mater., 2016, 28, 8906–8911.
63. Y. Sun, J. Jiang, Y. Liu, S. Wu and J. Zou, Appl. Surf. Sci., 2018, 430, 362–370.
64. H.-L. Zhu and Y.-Q. Zheng, Electrochim. Acta, 2018, 265, 372–378.
65. Q. Xu, B. Zhu, C. Jiang, B. Cheng and J. Yu, Sol. RRL, 2018, 2, 1800006.
66. J.-T. Lee, Y.-J. Chen, E.-C. Su and M.-Y. Wey, Int. J. Hydrogen Energy, 2019, 44, 21413–21423.
67. L. Ge, C. Han, X. Xiao and L. Guo, Appl. Catal., B, 2013, 142–143, 414–422.
68. Q. N. Li, Y. G. Xia, K. L. Wei, X. T. Ding, S. Dong, X. L. Jiao and D. R. Chen, New J. Chem., 2019, 43, 6753–6764.
69. M. You, J. Pan, C. Chi, B. Wang, W. Zhao, C. Song, Y. Zheng and C. Li, J. Mater. Sci., 2017, 53, 1978–1986.
70. D. Lu, H. Wang, X. Zhao, K. K. Kondamareddy, J. Ding, C. Li and P. Fang, ACS Sustainable Chem. Eng., 2017, 5, 1436–1445.
71. J.-W. Shi, Y. Zou, D. Ma, Z. Fan, L. Cheng, D. Sun, Z. Wang, C. Niu and L. Wang, Nanoscale, 2018, 10, 9292–9303.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj04322b

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