Excellent charge separation over NiCo₂S₄/CoTiO₃ nanocomposites improved photocatalytic hydrogen production

Abstract

The rapid migration and separation of photoinduced carriers is a key factor influencing photocatalytic efficiency. Constructing an S-scheme heterojunction is a strategic technique to enhance the separation of photogenerated carriers and boost overall catalytic activity. Herein, a simple physical stirring technique was adopted to successfully fabricate a novel NiCo₂S₄/CoTiO₃ S-scheme heterojunction photocatalyst. Upon exposure to light, the NiCo₂S₄/CoTiO₃-10 specimen demonstrated an outstanding hydrogen evolution rate of 2037.76 μmol·g^–1·h^–1, exceeding twice the rate observed for the pristine NiCo₂S₄ (833.72 μmol·g^–1·h^–1). The experimental outcomes reveal that the incorporation of CoTiO₃ significantly enhances the charge separation and transfer within the system. Concurrently, the formation of the S-scheme mechanism facilitates the separation of carriers while maintaining high redox capabilities. This work introduces an innovative approach to forming S-scheme heterojunctions based on bimetallic sulfides, thereby offering new prospects for the efficient utilization of solar energy.

Graphical abstract

Keywords

NiCo₂S₄ / CoTiO₃ / in situ X-ray photoelectron spectroscopy / S-scheme heterojunction

1 Introduction

Growth in the global population and the drive for higher living standards have led to a sharp rise in the demand for energy, raising the possibility of an energy crisis [1,2]. In response to environmental challenges, there is an exigent need to supplant conventional fossil fuels with cleaner, low-carbon energy alternatives. Photocatalytic water splitting hydrogen production technology is a technology that converts light energy into hydrogen energy, and solar energy is inexhaustible. Using it to solve the energy crisis problem is a reliable method [3−5]. Hydrogen energy has emerged as a promising and environmentally friendly source of clean energy owing to its remarkable high calorific value and notable efficiency in heat energy conversion [6−8]. Nevertheless, the proficiency of solar energy conversion and utilization remains to be enhanced. The optimization of hydrogen production kinetics hinges on augmenting the efficiency of light utilization and the efficacy of carrier separation and transfer [9]. Therefore, it is necessary to develop new photocatalyst with high charge separation and transfer rate [10].

Currently, bimetallic sulfides ZnCdS [11], CdIn₂S₄ [12], MnCdS [13] and ZnIn₂S₄ [14] have emerged as a focal point of research in the realm of photocatalytic hydrogen production due to their superior light absorption properties and chemical stability. In addition to inheriting the advantages of monometallic sulfides, bimetallic sulfides also modulate the electronic structure through metal-to-metal interactions, thereby expanding the range of light absorption and enhancing the efficiency of carrier separation [15,16]. Due to the potential exchange of valence or charge hopping between Co and Ni ions, NiCo₂S₄ exhibits excellent electrochemical performance and stability, which has led to extensive research in the field of electrocatalysis [17]. Li et al. [18] prepared NiCo₂S₄ hollow nanotubes (NCT-NiCo₂S₄) coated with N-doped carbon by solvothermal method. This special design maximizes the catalytic active site and improves the electron transfer efficiency, thus effectively enhancing its performance. Meanwhile, NiCo₂S₄ has superior light absorption properties and has an appropriate band gap, indicating its potential as a viable material for photocatalysis. However, Coulombic gravitational force existing between the photogenerated electron-hole can cause the carriers to complex, leading to difficulties in maximizing the redox capacity of the photocatalyst [19]. Wu et al. [20] employed a low-temperature solvent-thermal method to fabricate unique hierarchical hollow heterojunctions through the in situ growth of ZnIn₂S₄ on porous NiCo₂S₄, thereby providing efficient pathways for charge transfer and abundant active sites for H₂ production.

Calcium titanate has broad application prospects in fields such as photocatalytic hydrogen production and degradation due to its unique crystal structure. As one of the typical ABO₃ perovskites, cobalt titanate (CoTiO₃) exhibits avorable visible light absorption properties, along with adjustable electronic characteristics and robust structural stability [21]. However, the hydrogen production performance of CoTiO₃ is constrained by rapid carrier recombination. To augment its photocatalytic activity, its electronic structure and band alignment can be modulated through the introduction of defects [22], doping [23], or the construction of heterojunctions [24], thereby enhancing carrier separation efficiency. Generally speaking, the heterojunction can be successfully constructed by two photocatalysts that satisfy the energy band interleaving, which can be used to improve the catalytic performance of a single catalyst. The S-scheme heterojunction is unique because of the built-in electric field at the two catalysts’ interface, which is caused by the difference in Fermi energy levels. This energy level difference drives the efficient migration of electrons from the higher energy level to the lower energy level, resulting in the bending of band edges until equilibrium is established between the Fermi levels of the two catalysts [25,26]. The existence of heterojunctions accelerates the effective transfer of charge carriers, thereby achieving spatial separation of charge carriers and maximizing the redox performance of catalysts. Meng et al. [27] designed a g-C₃N₄/CoTiO₃ S-scheme heterojunction photocatalyst, which successfully achieved the decomposition of pure water under visible light to produce H₂. This excellent hydrogen production rate is attributed to the enhanced light absorption and visible light response, as well as the improved charge separation efficiency.

Based on the considerations outlined, a convenient and gentle physical mixing approach was employed in this study to tightly integrate the non-precious metal NiCo₂S₄ with CoTiO₃, resulting in the successful production of a novel S-scheme heterojunction photocatalyst. The NCS/CTO-10 composite material demonstrated superior hydrogen evolution performance compared to NiCo₂S₄. The hydrogen production rate of the NCS/CTO-10 composite material could reach up to 2037.76 μmol·g^–1·h^–1 (101.89 μmol), exceeding twice the hydrogen evolution rate of NiCo₂S₄ alone (833.72 μmol·g^–1·h^–1). Numerous tests, including in situ X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), time-resolved photoluminescence (TRPL), and linear sweep voltammetry (LSV), have validated the successful separation and transfer of charge carriers. The improved hydrogen evolution rate can be attributed to the significant improvement in the utilization of photogenerated electrons facilitated by the incorporation of CoTiO₃. The heterojunction effectively suppressed the recombination of electrons and holes, thereby boosting the photocatalytic activity.

2 Experimental

2.1 Chemicals and reagents

The main chemicals and reagents used in this work are shown in Tab.1.

Tab.1 Reagent source and purity standard

Reagent	Chemical formula	Purity
Titanium butoxide	C16H36O4Ti	≥ 99%
Ethylene glycol	C2H6O2	99%
Cobalt acetate	(CH3COO)2Co	98%
Thiourea	CH4N2S	98%
Polyethylene glycol 600	H(OCH2CH2)nOH	CP
Cobalt(II) nitrate hexahydrate	Co(NO3)2·6H2O	98%
Eosin Y(water soluble)	C20H6Br4Na2O5	Indicator
Triethanolamine	C6H15NO3	≥ 99.5%

2.2 Preparation of photocatalyst

The synthesis of nanoflower-shaped NiCo₂S₄ was conducted using a solvothermal approach, following a previously reported method [28]. Initially, a mixed solution was prepared by dissolving 4 g of thiourea in polyethylene glycol. Then, a specific quantity of Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O were dissolved in the mixed solution. The resulting solution was heated at 140 °C in an oven for 16 h. Upon completion of the reaction, the precipitate was obtained through centrifugation and subsequently washed alternatively with deionized water and ethanol. Finally, the obtained precipitate was dried in an oven to yield the desired nanoflower-shaped NiCo₂S₄ catalyst.

The synthesis of CoTiO₃ was conducted based on a combination of previously reported methods [29]. Initially, Co(CH₃COO)₂·4H₂O and C₁₆H₃₆O₄Ti were added to 70 mL of C₂H₆O₂ and stirred at room temperature for 24 h. The resulting precipitate was then washed with deionized water and ethanol. Following this, the washed precipitate was dried in an oven, after which the obtained precursor was collected and ground into a fine powder. Finally, the precursor was placed into a muffle furnace and roasted at 700 °C for 4 h to obtain green CoTiO₃.

A series of NiCo₂S₄/CoTiO₃ with different contents of CoTiO₃ were synthesized and designated as NCS/CTO-X (X = 5%, 10%, 15%, 20%), where X represents the mass ratio of CoTiO₃ to NiCo₂S₄. Initially, 20 mg of NiCo₂S₄ was added to a 20 mL ethanol liquid, along with varying amounts of CoTiO₃. Subsequently, the mixture was subjected to ultrasonication and stirred overnight to form a uniform solution. Finally, the solution was heated in a water bath at 80 °C until dry (Fig.1).

Fig.1 Preparation of the catalyst.

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2.3 Characterization

The crystal structure of the material was characterized using X-ray diffraction (XRD). The morphology and microstructure of the catalyst were examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The elements states of the samples were investigated by XPS. The optical absorption properties of the samples were evaluated using ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), with BaSO₄ powder as the background substrate. PL and TRPL spectra were obtained using a FLUORO-MAX-4 spectrofluorometer. The catalysts were subjected to electrochemical analysis using an electrochemical workstation (VersaStat 4400), including photocurrent response, linear sweep voltammetry (LSV), Mott-Schottky (MS), and cyclic voltammetry (CV).

2.4 Photocatalytic activity experiment

Photocatalytic activity experiments were analyzed by gas chromatography (Tianmei GC7900). In a typical experiment, 10 mg of catalyst and a certain amount of EY were dispersed in 30 mL of a 15% TEOA solution. The gas inside the reaction bottle was then replaced with N₂. Subsequently, the reaction bottle was transferred to a nine-channel photocatalytic reaction system illuminated by a 5-W light emitting diode (LED) light. During the experiment, 0.5 mL of H₂ was extracted from the reaction flask every hour and analyzed using gas chromatography. To assess the stability of the NCS/CTO-10 photocatalyst, a series of five 15-h cycle experiments were conducted. Before each cycle begins, 10 mg of Eosin Y (EY) needs to be added to the reaction bottle and N₂ was utilized to exhaust the gas in the hydrogen-producing bottle.

2.5 Electrochemical test

Electrochemical testing of catalysts was done using an electrochemical analyzer. The electrolyte solution was a 0.2 mol·L^–1 Na₂SO₄ aqueous solution. The photocurrent response curve of the sample was measured under 0.2 V bias and 300 W Xe lamp irradiation. The overpotential of the catalyst was measured using LSV at a scanning rate of 0.01 V·s^–1. The MS analysis of the catalyst was performed at 500 Hz to ascertain the flat band potential and determine the semiconductor type. The electrochemical energy storage performance of the catalyst was obtained through CV testing.

3 Results and discussion

3.1 Morphology and structure analysis

The crystal structure of the sample was determined by XRD. In Fig.2(a), peaks were observed at 31.6°, 38.1°, and 54.9°, corresponding to the (311), (400), and (440) crystallographic planes of NiCo₂S₄ (PDF#20-782). Additionally, the low crystallinity of NiCo₂S₄ can be deduced from the observed diffraction peaks, which suggest potential damage to the crystal structure. This alteration may be attributable to the distinctive nanoflower-like morphology. Peaks with high intensity at 32.6°, 35.3°, 40.3°, 48.8°, and 53.3° correspond to the (104), (110), (113), (202), and (116) crystal planes of CoTiO₃ (PDF#15-866) respectively, as shown in Fig.2(b). According to the XRD patterns shown in Fig.2(c), all composite materials exhibit similar characteristics to NiCo₂S₄. It is noteworthy that the lower proportion of CoTiO₃ in the NCS/CTO composite catalyst led to the non-detection of distinct CoTiO₃ peaks. Moreover, no additional diffraction peaks were observed in NiCo₂S₄, CoTiO₃, and all composite catalysts NSC/CTO-X (X = 5%, 10%, 15%, 20%), indicating successful preparation and high purity of all catalytic agent.

Fig.2 XRD spectra of (a) NiCo2S4, (b) CoTiO3 and (c) NSC/CTO-X (X = 5%, 10%, 15%, 20%).

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The morphologies of NiCo₂S₄, CoTiO₃ and the composite catalyst NCS/CTO-10 were characterized by SEM (Fig.3). In Fig.3(a), NiCo₂S₄ exhibits a nanoflower-like structure consisting of many nanosheets of different sizes interconnected with each other. The interstices between those nanosheets afford increased internal space, which can serve as efficacious active sites. The SEM image of CoTiO₃ shows a rectangular structure formed by many irregular nanoparticles clumped together (Fig.3(b)). And Fig.3(c) is the SEM of NCS/CTO-10, showing the tight binding between NiCo₂S₄ and CoTiO₃. Fig.3(d) shows the elemental mapping of NCS/CTO-10, and the scatter of Co, Ni, S and Ti elements proved the successful preparation of NCS/CTO-10. To further observe the microstructure of the catalyst, we performed TEM characterization. In the TEM image of NCS/CTO-10 (Fig.3(e)), the voids of nanoflowers in NiCo₂S₄ and irregular nanoparticles in CoTiO₃ are distinctly illustrated. Moreover, the intimate connection between the two constituents is evident, thereby further substantiating the successful fabrication of NCS/CTO-10. In addition, the high-resolution TEM (HRTEM) images of CoTiO₃ and NiCo₂S₄ are displayed in Fig.3(f) and Fig.3(g), respectively. The lattice stripes of CoTiO₃ and NiCo₂S₄ correspond to the (104) and (400) crystallographic planes, respectively, at 0.271 and 0.239 nm.

Fig.3 SEM images of (a) NiCo2S4, (b) CoTiO3 and (c) NCS/CTO-10; (d) SEM mapping images of NCS/CTO-10; (e) TEM images of NCS/CTO-10; HRTEM images of (f) CoTiO3 and (g) NiCo2S4.

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The elemental chemical valence states of CoTiO₃, NiCo₂S₄, and NCS/CTO-10 were assessed via XPS characterization. For the NiCo₂S₄ catalyst, its Ni 2p XPS spectrum showed two spin-orbit double peaks corresponding to the characteristics of Ni²⁺ and Ni³⁺, respectively (Fig.4(a)). In particular, Ni 2p_3/2 and Ni 2p_1/2 of Ni²⁺ are responsible for the peaks at 853.45 and 870.78 eV, and Ni 2p_3/2 and Ni 2p_1/2 of Ni³⁺ are responsible for the peaks at 855.89 and 873.86 eV [30]. The satellite peaks at 861.21 and 879.40 eV, on the other hand, show that the Ni²⁺ state predominates in the Ni 2p spectrum due to their prominence. In Fig.4(b), the S 2p XPS spectrum of NiCo₂S₄ shows peaks at 162.13 and 163.40 eV corresponding to S 2p_3/2 and S 2p_1/2, respectively, while the peak at 168.40 eV is identified as a satellite peak. In particular, the peak at 163.40 eV could be attributed to S 2p_1/2 of the metal-sulfur bond (Co–S) [31]. The Co 2p XPS spectrum of NiCo₂S₄ that in Fig.4(c), the peaks at binding energies of 779.30 and 781.42 eV were due to Co 2p_3/2, while the peaks at binding energies of 794.02 and 797.17 eV correspond to Co 2p_1/2. This implies that both Co³⁺ and Co²⁺ are present simultaneously, and the two peaks at 785.81 and 802.56 eV subsequently match up with satellite peaks. In addition, the characteristic peaks at 780.52 and 783.06 eV are due to Co 2p_3/2 of CoTiO₃, while the characteristic peaks at 796.00 and 797.32 eV are due to Co 2p_1/2 of CoTiO₃, thus providing further evidence for the coexistence of both Co²⁺ and Co³⁺ in CoTiO₃ [32,33]. The peaks with binding energies of 787.24 and 803.39 eV belong to satellite peaks. As for the Ti 2p XPS spectra of CoTiO₃, Fig.4(d) displays that the peaks at 463.84 and 458.08 eV correspond to Ti 2p_1/2 and Ti 2p_3/2, respectively [34]. In addition, the characteristic peaks of Ni 2p, Co 2p and S 2p in NCS/CTO-10 exhibit a higher binding energy compared to those in NiCo₂S₄. Furthermore, the characteristic peaks of Co 2p and Ti 2p in NCS/CTO-10 are shifted in the negative direction relative to those of Co 2p and Ti 2p in CoTiO₃. These observations further support the existence of significant interaction between NiCo₂S₄ and CoTiO₃ in the composites.

Fig.4 XPS of (a) Ni 2p and (b) S 2p spectra for NiCo2S4 and NCS/CTO-10; (c) Co 2p spectra for NiCo2S4, CoTiO3 and NCS/CTO-10; (d) Ti 2p spectra for CoTiO3 and NCS/CTO-10.

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3.2 Photocatalytic activity analysis

The photocatalytic hydrogen production ability of the composite catalyst NCS/CTO-10 was investigated under 5 W LED radiation. As shown in Fig.5(a), NiCo₂S₄ was able to produce hydrogen under sunlight irradiation, but its yield was low, with a H₂ yield of 41.69 μmol after 3 h of reaction. And CoTiO₃ displayed minimal hydrogen-evolving activity, with a yield of merely 1.63 μmol. Whereas, the NCS/CTO-10 composite consisting of NiCo₂S₄ and CoTiO₃ demonstrated a higher photocatalytic hydrogen production performance than that of NiCo₂S₄ alone. The hydrogen production rate of NCS/CTO-10 composite material is as high as 2037.76 μmol·g^–1·h^–1 (101.89 μmol), more than twice that of NiCo₂S₄ alone (833.72 μmol·g^–1·h^–1). This enhancement could be induced by the S-scheme heterojunction formed in the composites, which helps to inhibit the combination of carriers, thus enhancing the photocatalytic reaction efficiency. More specifically, under visible light irradiation, with the increase of CoTiO₃ content, the amount of hydrogen produced by the composite catalyst NCS/CTO-X (X = 5%, 10%, 15%, 20%) after 3 h of reaction was 63.45, 101.89, 50.81 and 44.21 μmol, respectively (Fig.5(b)). It is clearly demonstrated that the composite catalyst shows the highest rate of hydrogen production when the mass ratio of CoTiO₃ to NiCo₂S₄ reaches 10%. This is because at this ratio, the effective interfacial contact between CoTiO₃ and NiCo₂S₄ contributes to the construction of the S-scheme heterojunction, and helps to inhibit the combination of carriers, thereby allowing more photogenerated electrons to participate in the hydrogen production process. When the load of CoTiO₃ exceeds 10%, the hydrogen production activity decreases instead of increase. This is because CoTiO₃ is a green substance, and a large amount of load will affect light absorption, which is not conducive to hydrogen production. Therefore, the composite sample of NCS/CTO-10 with appropriate ratio showed excellent hydrogen production performance. Fig.5(c) presents the hydrogen production outcomes of NCS/CTO-10 across varying pH levels. It is evident that the photocatalytic performance of NCS/CTO-10 initially improves with increasing pH, followed by a subsequent decline. At a pH of 9, the photocatalytic hydrogen production efficiency of NCS/CTO-10 reaches its peak. This phenomenon may be due to the influence of pH value on the existence state of electron donors. In weakly alkaline environments, triethanolamine is protonated, which weakens its ability to act as an electron donor, reduces the excitation of EY, and thus affects the efficiency of hydrogen production in the system. In a strong alkaline environment, an excess of OH^– in the system will neutralize the required H⁺ in the hydrogen production reaction, thereby reducing the progress of the reaction and hindering hydrogen production. Fig.5(d) displays the findings from the experiment on the hydrogen production cycle, which is carried out every three hours. After five cycles, the hydrogen production of NCS/CTO-10 was basically stable. Fig.5(e) shows the comparison of the XRD patterns before and after the hydrogen production test of the composite catalyst NCS/CTO-10. It could be found that the positions of the characteristic peaks after hydrogen production remain largely unchanged from those before the reaction. No new diffraction peaks were observed, while only a slight decrease in the intensity of the diffraction peaks was observed. This indicates that the structure of the composite NCS/CTO-10 was minimally affected by the photocatalytic reaction, with only minor surface alteration. This result fully demonstrates that NCS/CTO-10 has good photocatalytic hydrogen precipitation activity and stability. Tab.2 presents a comparison of hydrogen evolution rates for sulfide-based photocatalysts, further demonstrating the superior hydrogen production performance of NCS/CTO-10.

Tab.2 Comparison of photocatalytic hydrogen production rates based on sulfides

Photocatalysts	Light source	Sacrificial reagent	Hydrogen production rate/(μmol·g–1·h–1)	Ref.
NCS/CTO-10	5 W LED	TEOA	2037.76	This work
ZnIn2S4@ NiCo2S4	300 W Xe lamp	TEOA	1950	[35]
NiCo2S4/g-C3N4	300 W Xe lamp	TEOA	293.4	[36]
CoS/g-C3N4/NiS	300 W Xe lamp	TEOA	1930	[37]
CoSx-ZnIn2S4	300 W Xe lamp	TEOA	1538.4	[38]

Fig.5 (a) H2 evolution of catalyst NiCo2S4, CoTiO3 and NCS/CTO-10; (b) hydrogen production rate of NCS-CTO-X (X = 5%, 10%, 15%, 20%); (c) hydrogen production of NCS/CTO-10 at different pH levels; (d) stability test of NCS/CTO-10; (e) XRD before and after the reaction of NCS/CTO-10.

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3.3 Photoelectric characteristic analysis

The optical properties of the catalysts NiCo₂S₄, CoTiO₃ and the composite sample NCS/CTO-10 were researched by UV-vis DRS (Fig.6(a)). Given that NiCo₂S₄ is a black substance, it exhibits a notably stable absorption spectrum within the range of 300 to 800 nm. Specifically, the absorption fluctuations observed in the 300–500 nm range for CoTiO₃ were attributed to the O^2–→ Ti⁴⁺ charge transfer interactions occurring inside the material. The two absorption peaks observed at 537 and 604 nm are due to the Co²⁺ to Ti⁴⁺ charge transfer in CoTiO₃, likely arising from the crystal field splitting within the material [39]. Concurrently, the composite catalyst NCS/CTO-X (X = 5%, 10%, 15%, 20%) exhibits similar optical properties to NiCo₂S₄. The coupling of NiCo₂S₄ and CoTiO₃ helps to excite photogenerated electrons and holes for redox reactions. The E_g values of the NiCo₂S₄ and CoTiO₃ are calculated using the following formula (Eq. (1)) [40].

Fig.6 (a) UV-vis DRS spectra of NiCo2S4, CoTiO3 and NCS/CTO-X (X = 5%, 10%, 15%, 20%), (b) and (c) band gap of CoTiO3 and NiCo2S4.

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(1)$\alpha hv={A\left(hv-E_{\mathrm{g}}\right)}^{n/2}.$

where α, v, A and E_g are the absorption coefficient, optical frequency, constant and band gap, respectively. The value of n is determined by the nature of the optical transition in a semiconductor. CoTiO₃, classified as a semiconductor with indirect transitions, possesses a value of n equal to 4. NiCo₂S₄, which is a semiconductor with direct transitions, has a value of n equal to 1. Based on the above equation, the E_g of CoTiO₃ is 2.02 eV and the E_g value of NiCo₂S₄ is 1.50 eV (Fig.6(b, c)).

In the evaluation of the transfer of photo-generated electrons and holes in the photocatalysts, electrochemical tests were conducted on NiCo₂S₄, CoTiO₃ and the composite sample NCS/CTO-10. A stronger photocurrent response generally indicates a higher separation efficiency of photogenerated electrons and holes [41,42]. Among the three catalysts tested, CoTiO₃ exhibited the weakest photocurrent response, while the composite sample NCS/CTO-10 demonstrated the highest photocurrent response (Fig.7(a)). This difference in photocurrent response could be due to the successful establishment of an S-scheme heterojunction through the combination of NiCo₂S₄ and CoTiO₃. This heterojunction facilitates efficient separation of electron-hole, thereby effectively suppressing their combination processes. The LSV curves of NiCo₂S₄, CoTiO₃ and the sample NCS/CTO-10 are depicted in Fig.7(b). Generally speaking, the lower the hydrogen production overpotential, the easier it is to perform photocatalytic hydrogen production and the better the photocatalytic performance [43]. In the context of the LSV curves, it was observed that NCS/CTO-10 demonstrated a decreased overpotential in comparison to both NiCo₂S₄ and CoTiO₃. This reduction in overpotential suggests the superior hydrogen production activity of NCS/CTO-10. Next, we tested the cyclic voltammetry curve of NiCo₂S₄, CoTiO₃ and the composite sample NCS/CTO-10. In general, the region of the CV curve is related to the amount of charge storage and release in the hydrogen production reaction. The large area indicates that the catalyst has higher efficiency and charge storage capacity in the hydrogen production process. In the same Fig.7(c), by comparing the CV curves of NiCo₂S₄, CoTiO₃ and the composite sample NCS/CTO-10, it is observed that the area under the NCS/CTO-10 CV curve is slightly larger than that of NiCo₂S₄ and CoTiO₃. This also indirectly shows that the catalyst NCS/CTO-10 can maintain excellent hydrogen production performance. Fig.7(d) and Fig.7(e) are MS of NiCo₂S₄ and CoTiO₃. The flat-band potentials (E_fb) of CoTiO₃ and NiCo₂S₄ relative to the saturated calomel electrode (SCE) are –0.32 and –0.5 V, respectively. Since E_NHE = E_SCE + 0.24 V (E_NHE is the potential relative to the standard hydrogen electrode), the E_fb of CoTiO₃ and NiCo₂S₄ relative to NHE are –0.08 and –0.26 V, respectively. Both have positive MS curve slopes, indicating that they are n-type semiconductors [44]. The E_CB of the n-type semiconducting material in the water environment is 0.1–0.2 eV negative than its E_fb, and the E_CB of CoTiO₃ and NiCo₂S₄ relative to NHE are –0.28 and –0.46 eV, respectively. According to UV-Vis DRS testing and the relationship E_VB = E_g + E_CB, the E_VB of CoTiO₃ and NiCo₂S₄ were determined to be 1.74 and 1.04 eV, respectively [45]. By measuring the zeta potential, we can determine the surface charge characteristics of the photocatalyst. As demonstrated in Fig.7(f), the zeta potentials of NiCo₂S₄, CoTiO₃ and the composite sample NCS/CTO-10 are –32.9, –21.7, and –35.92 mV, respectively. The zeta potential of NCS/CTO-10 is the most negative, indicating that the combination of NiCo₂S₄ and CoTiO₃ enhances the negative charge on the surface. This negative potential attracts positively charged free electrons in water molecules, facilitating their migration to the catalyst’s surface and enhancing the efficiency of photocatalytic hydrogen production.

Fig.7 (a) Photocurrent response, (b) LSV curves and (c) CV curve of NiCo2S4, CoTiO3 and NCS/CTO-10; MS plots of (d) NiCo2S4 and (e) CoTiO3; (f) Zeta potential of NiCo2S4, CoTiO3 and NCS/CTO-10.

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To elucidate the underlying mechanism of the superior hydrogen production performance exhibited by NCS/CTO-10, we performed PL and TRPL measurements on the catalyst, employing an EY solution as the background. Fig.8(a) illustrates the PL emission spectra of NiCo₂S₄, CoTiO₃ and the composite sample NCS/CTO-10 NCS/CTO-X (X = 5%, 10%, 15%, 20%). The fluorescence signal of single catalyst NiCo₂S₄ and CoTiO₃ is stronger than that of composite catalyst NCS/CTO-X (X = 5%, 10%, 15%, 20%), and the fluorescence intensity of NCS/CTO-10 is the lowest among all composite catalysts. This suggests that CoTiO₃’s loading effect significantly inhibits the radiative recombination of charge carriers, leading to a significant carrier separation effect and allowing more photogenerated electrons to be involved in the H⁺ reduction reaction [46]. All catalytic fluorescence intensities are consistent with the kinetic data of photocatalytic hydrogen. Next, we conducted transient fluorescence spectroscopy on the individual catalysts NiCo₂S₄ and CoTiO₃, as well as the composite catalyst NCS/CTO-10 (Fig.8(b)). The decay curves were fitted to a second-exponential kinetic equation, and the results obtained are shown in Tab.3. The experiments show that the average carrier lifetime of NiCo₂S₄, CoTiO₃ and NCS/CTO-10 are 0.39, 0.31 and 0.59 ns, respectively. It is evident that the strong interaction between NiCo₂S₄ and CoTiO₃ greatly extends the lifetime of charge carriers in NCS/CTO-10. This interaction drives an increase in the effective utilization of photogenerated electrons, which in turn is more favorable for their participation in the reduction reaction.

Tab.3 Secondary fitting attenuation parameters for the average life time of CoTiO3, NiCo2S4 and NCS/CTO-10

Sample	Preelectronic exponential factor A	Lifetime, τ/ns	Average lifetime, τ/ns	χ2
CoTiO3	A1 = 96.23A2 = 3.77	τ1 = 0.29τ2 = 3.52	0.31	1.09
NiCo2S4	A1 = 4.77A2 = 95.23	τ1 = 4.14τ2 = 0.37	0.39	1.32
NCS/CTO-10	A1 = 93.07A2 = 6.93	τ1 = 0.56τ2 = 2.69	0.59	1.36

Fig.8 (a) CoTiO3, NiCo2S4 and NCS/CTO-X (X = 5%, 10%, 15%, 20%) of PL spectra; (b) CoTiO3, NiCo2S4 and NCS/CTO-10 of TRPL.

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3.4 Mechanism analysis

In situ XPS measurements can provide deeper insights into the reaction mechanism of the NCS/CTO-10 composites. According to the previously mentioned XPS analysis (Fig.4), the characteristic peaks of Ni 2p, S 2p, and Co 2p in NCS/CTO-10 exhibit a shift toward higher binding energy compared to those in NiCo₂S₄. Conversely, in comparison to CoTiO₃, the characteristic peaks of Ti 2p and Co 2p in NCS/CTO-10 show a shift toward lower binding energy. This indicates that when NCS/CTO-10 is in the dark, the construction of the heterojunction between NiCo₂S₄ and CoTiO₃ changes its chemical state. In comparison to NCS/CTO-10 under dark conditions, the S 2p and Ni 2p characteristic peaks of NCS/CTO-10 shift toward lower binding energy under illumination, indicating that electrons are transferred into NiCo₂S₄. The characteristic peak of Ti 2p moves toward a higher binding energy, indicating that some electrons flow out of CoTiO₃ (Fig.9). In situ XPS measurements confirmed that electrons are transferred from CoTiO₃ to NiCo₂S₄ when NCS/CTO-10 is exposed to light.

Fig.9 In situ XPS spectra of (a) Ni 2p, (b) S 2p, and (c) Ti 2p of NCS/CTO-10.

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We have preliminarily proposed two possible methods of electron transfer, namely type-II heterojunction and S-scheme heterojunction, by utilizing the energy band structures of NiCo₂S₄ and CoTiO₃ (Fig.10). Both NiCo₂S₄ and CoTiO₃ can attain excited states upon exposure to visible light excitation, resulting in electron-hole pairs and the excitation of electrons in the VB to transition to the CB. The CB of NiCo₂S₄ is positioned more negatively than that of CoTiO₃ in a conventional type-II heterojunction. Consequently, the holes will go from CoTiO₃ to NiCo₂S₄, and the electrons in NiCo₂S₄ will flow in the direction of the CB of CoTiO₃. Eventually, the electrons will accumulate at the CB of CoTiO₃ to facilitate hydrogen production via reduction reactions. At the same time, the holes will congregate at NiCo₂S₄ of VB to take part in oxidation processes, and sacrificial reagents will consume these holes [47]. However, the in situ XPS characterization results differ from the direction of electron transfer in the heterojunction, and it is considered that the type-II heterojunction mechanism is not suitable for this study.

Fig.10 Hydrogen production mechanism of S-scheme heterojunction with NCS/CTO-10 under visible light.

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Therefore, considering the aforementioned analysis, an S-scheme electron transfer mechanism is proposed. Because of the difference in Fermi levels between the two materials, electrons will migrate from NiCo₂S₄ to CoTiO₃ when they are close to one another until the Fermi level reaches equilibrium. At the contact interface, NiCo₂S₄ exhibits positive charge accumulation, while CoTiO₃ exhibits negative charge accumulation. The band edges of CoTiO₃ and NiCo₂S₄ bend downward and upward, respectively, as a result of this charge redistribution. In the end, an internal electric field is created at the interface between NiCo₂S₄ and CoTiO₃ [48,49]. Under illumination, the Coulomb force and band bending interaction facilitate the movement of photoexcited electrons from the CB of CoTiO₃ to the VB of NiCo₂S₄, where they recombine with their corresponding photoexcited holes. The electrons that are still in the NiCo₂S₄ CB are involved in the hydrogen generation reaction and exhibit excellent reduction capabilities. Meanwhile, the holes trapped in the VB of CoTiO₃ demonstrate enhanced oxidation abilities, resulting in their consumption by sacrificial reagents [50]. In addition, EY, as a photosensitizer, can absorb energy and form an excited state EY^1* under visible light irradiation. EY^1* briefly exists and then transitions between systems to a more stable triplet excited state (EY^3*). When TEOA exists, EY^3* is reduced and quenched to form EY^−*. In this system, EY can transfer its excess electrons to the CB of the catalyst. These electrons not only engage in diverse redox reactions but also contribute to enhancing the photocatalytic activity of the catalyst. To sum up, the charge transfer process observed in this work is in excellent agreement with the electron transfer mechanism of the S-scheme heterojunction. The effective electron hole pair migration and separation, as well as strong redox ability of the heterojunction, jointly promote photocatalytic hydrogen production.

4 Conclusions

In short, a novel NCS/CTO S-scheme heterojunction was successfully created using a straightforward physical stirring method, enabling efficient photocatalytic hydrogen production by maximizing the spatial separation of redox centers. Through assembly of NCS/CTO photocatalysts with various mass ratios, it was discovered that NCS/CTO-10 exhibited superior catalytic performance in hydrogen production experiments. The photocatalytic activity of NCS/CTO-10 surpasses that of NiCo₂S₄, with a hydrogen evolution rate reaching up to 2037.76 μmol·g^–1·h^–1, which is more than twice that of NiCo₂S₄ (833.72 μmol·g^–1·h^–1). This indicates that the introduction of CoTiO₃ has substantially improved charge transfer and separation within the system. Furthermore, the charge transfer mechanism present in the S-scheme heterojunction was firmly validated by the in situ XPS data, which markedly diminished carrier recombination while concurrently sustaining the high redox capacity of the NCS/CTO-10 composite. This study introduces a novel approach to constructing an S-scheme heterojunction using bimetallic sulfides, offering promising prospects for efficient photocatalytic applications.

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

This work was financially supported by the Fundamental Research Funds for the Central Universities of the North Minzu University (Grant No. 2023ZRLG20); Innovative Team for Transforming Waste Cooking Oil into Clean Energy and High Value-Added Chemicals; Ningxia Low-Grade Resource High Value Utilization and Environmental Chemical Integration Technology Innovation Team Project. We would also like to express our gratitude to the 2023 Ningxia Hui Autonomous Region Young Scientific and Technological Talents Promotion Project for the financial support.

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