Hierarchical CoFe-layered double hydroxide and g-C₃N₄ heterostructures with enhanced bifunctional photo/electrocatalytic activity towards overall water splitting

Abstract

To achieve sustainable and clean energy for the hydrogen economy, developing efficient earth-abundant and non-noble transition metal photo/electrocatalysts toward overall water splitting is highly desirable. In this work, layered double hydroxide (LDH)@g-C₃N₄ composites with hierarchical flower-like micro/nanosheets and a high surface area have been synthesized by a solvothermal method. HRTEM images exhibit that the surface of the g-C₃N₄ nanosheets is highly orientated with the main exposure of the (002) plane. Compared with pristine CoFe-LDH, the hierarchical nanocomposite presents an excellent and stable elecrocatalytic performance in 1.0 M KOH, with a small Tafel slope of 58 mV dec⁻¹ and an overpotential of about 275 mV at a current density of 10 mA cm⁻². Simultaneously, CoFe-LDH@g-C₃N₄ exhibits an exceptional performance for the HER in 1.0 M KOH electrolyte, with an overpotential of 417 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 77 mV dec⁻¹. Therefore, this work not only accomplishes improved catalytic activity of CoFe-LDH by the introduction of g-C₃N₄ nanosheets, but also provides an insight into the correlation between hierarchical flower-like morphologies and photo/electrochemical catalytic activity for overall water splitting.

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The simultaneous production of oxygen (O₂) and hydrogen (H₂) from photo/electrochemical water-splitting provides one of the most promising approaches to develop low-cost, environmentally friendly and sustainable energy conversion/storage.^1–6 However, the commercial applications of photo/electrochemical water splitting are usually limited because the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER) are greatly uphill with large overpotentials. Electrocatalysts are critically necessary to minimize the energy barrier and enhance the kinetics of both OER and HER to ensure the overall process is more energy-efficient.^7–15 To date, Ru/Ir and Pt based precious metals are the state-of-the-art photo/electrocatalysts for OER and HER, respectively, but their relatively low durability, high cost and scarcity usually limit their large-scale utilization for water splitting. A tremendous amount of research has therefore been conducted to develop low-cost, highly active and durable photo/electrocatalysts with the aim of substituting noble metal-based catalysts.^16–24

Recently, photo/electrocatalysts with tunable hierarchical micro/nanostructures have gained significant attention, due to their high surface area, more active sites and proficient light harvesting for enhancing photo/electrocatalytic activity in overall water splitting. Moreover, various micro/nanostructures greatly influence the formation of heterojunctions and the tuning of the band gap.^25–32 Non-noble transition metal-based systems, such as single-metal or mixed-metal oxides and hydro(-oxy)oxides, are propitious for maintaining water oxidation with low overpotential and good durability under alkaline conditions. Hence, the construction of new non-noble metal-based semiconductor photo/electrocatalysts with hierarchical morphologies could be an effective way to enhance efficiency in the water splitting process.^33–37

As a large type of two-dimensional (2D) semiconductor material, layered double hydroxides (LDHs) have been used as photo/electrocatalysts in water splitting due to their remarkable activity, low cost and easy preparation. Generally, LDHs are represented by the formula [M^II_1−xM^III_x(OH)₂]^z+(Aⁿ⁻)_z/n·[yH₂O], in which M^II, M^III and Aⁿ⁻ represent divalent and trivalent metallic cations and interlayer guest anions compensating for metal cations, respectively.^{1,2,5,12,38–47} Simultaneously, another low-cost and metal-free 2D polymeric system, graphitic carbon nitride (g-C₃N₄), has also been evaluated, mostly for electrochemical and photoelectrochemical water splitting due to its engaging electronic structure, visible-light absorption, high stability (thermal and chemical) and environmental friendliness.^11,48–56 Furthermore, g-C₃N₄ composites have also been investigated for broad energy conversion techniques (such as CO₂ reduction to hydrocarbon, environmental purification and the photodegradation of organic pollutants).^57–63 To date, low surface area, weak electrical conductivity and easy recombination of electron–hole pairs are major disadvantages associated with g-C₃N₄.^11,64–66 To overcome these aforementioned problems, tremendous efforts (doping with metals, coupling with other semiconductors, and/or copolymerization) have been made to improve the catalytic activity of g-C₃N₄.^49,65–69 In this sense, the heterojunctions formed between g-C₃N₄ and LDH semiconductors could be a splendid strategy for boosting their photo/electrocatalytic properties since their similar 2D-layered characteristics are helpful to their heterostructural coupling, full charge separation and electron transfer. Additionally, the hierarchical morphologies and tunable chemical compositions of LDHs can be elegantly modified, which facilities an enhancement in electrical conductivity and charge separation. Recently, Shakeel et al. reported Ni–Mn-LDH/g-C₃N₄ composite for electrocatalytic water splitting with an overpotential of 316 mV to achieve a current density of 10 mA cm⁻²; Tanmay et al. reported that Co_0.4Fe_0.6-LDH/g-CN_x required overpotentials of 280 mV and 377 mV to acquire current densities 10 mA cm⁻² and 100 mA cm⁻². However, there is still a gap for researchers to develop new catalysts with unique architectures and special morphologies towards high catalytic activity at low overpotential. From an elemental point of view, it is also documented that Co²⁺/Co³⁺, Fe²⁺/Fe³⁺ and Co–N are the active sites for boosting electrochemical overall water splitting.^{11,52,64,70–74} Additionally, the effect of visible light on electrocatalytic activities remains a subject of scientific interest.⁷⁰

To combine the advantages of Co-based 2D LDHs and g-C₃N₄, in this work, hierarchical CoFe-LDH/g-C₃N₄ composites have been chosen as model systems, which are synthesized via a simple one-step and easily scalable in situ solvothermal method. Their electrocatalytic and photo/electrocatalytic activities are systematically investigated. The hierarchical features of a CoFe-LDH@g-C₃N₄ heterostructure could provide various reflection sites and cavities for the absorption of light and consequently exhibit excellent catalytic activity for overall water splitting. The electrical conductivity of a CoFe-LDH@g-C₃N₄ composite also increases due to electronic interactions and the unique morphology between CoFe-LDH and g-C₃N₄ layers at 10 wt% loading of g-C₃N₄ content. Thus, the CoFe-LDH@g-C₃N₄ composite exhibits improved photo/electrocatalytic OER and HER activity and durability compared with its individual components. The photocurrent density of pure CoFe-LDH and hierarchical CoFe-LDH@g-C₃N₄ composite are 2.1 and 3.2 times that of g-C₃N₄, respectively. Furthermore, a current density of 10 mA cm⁻² is achieved by applying just 1.55 V, while a current density of 50 mA cm⁻² is reached by applying just 1.76 V across a two-electrode electrolyzer with the hierarchical CoFe-LDH@g-C₃N₄ composite acting as both anode and cathode during the overall water splitting process. Therefore, this work not only affirms the active role of the hierarchical heterostructure of the CoFe-LDH@g-C₃N₄ composite as co-catalysts in photo/electrochemical water splitting but also provides further insights into similar 2D semiconductors for renewable energy applications.

Experimental details

Materials

Chemicals Co(NO₃)₂·6H₂O, Fe(NO₃)₃·9H₂O, KOH, anhydrous N,N-dimethylformamide (DMF), C₂H₅OH, and Al₂O₃ powder were purchased from Sinopharm Chemical Reagent (Beijing Co. Ltd) All reagents were analytical grade and were used without further purification. Deionized water was used throughout the experiments.

Preparation of g-C₃N₄

Bulk g-C₃N₄ was synthesized by the thermal treatment of dicyandiamide in a muffle furnace. Typically, 5.0 g of dicyandiamide was ground into powder and heated at 550 °C for 4 hours at a heating rate of 2.5 °C min⁻¹. After cooling, the yellow solid was collected and ground into powder until the yellow color of bulk g-C₃N₄ changed to light yellow. g-C₃N₄ nanosheets were synthesized by further thermal treatment of bulk g-C₃N₄. Light yellow powder was calcined at 600 °C for 2 hours at a heating rate of 5.0 °C min⁻¹, and was further modified by liquid exfoliation. The g-C₃N₄ sample was ultrasonicated in water for 10 hours and then separated by centrifugation. The resulting powder was washed three times with pure water and ethanol, collected and then dried at 60 °C overnight.

Fabrication of the hierarchical flower-like CoFe-LDH@g-C₃N₄ composite

Typically, a certain amount of the g-C₃N₄ nanosheets was dispersed in 6 mL of anhydrous N,N-dimethylformamide (DMF) assisted by ultrasonication for more than 1 hour. The hierarchical flower-like CoFe-LDH nanosheets were synthesized by a facile solvothermal method. After that, 600 μL of 0.2 M Co(NO₃)₂·6H₂O and 200 μL of 0.2 M Fe(NO₃)₃·9H₂O aqueous solutions were added into an ultrasonicated mixture containing g-C₃N₄. For the adsorption of metal cations through the electrostatic interaction on the surface of g-C₃N₄, the mixture of CoFe-LDH and g-C₃N₄ was treated ultrasonically for at least 1 hour. For a typical run, Co(NO₃)₂·6H₂O and Fe(NO₃)₃·9H₂O with a Co/Fe molar ratio of 3 : 1 was optimized for photo/electrocatalytic activity. Firstly, the mixture was strenuously stirred at 85 °C for 4 hours. Then, the product was obtained and dispersed in a mixture of DMF (6 mL) and water (12 mL). After being stirred vigorously for 4 hours, the obtained mixture was transferred to a 40 mL Teflon-lined stainless steel autoclave. The reaction mixture in the autoclave was treated at 120 °C and maintained for 24 hours. The resulting product was separated by centrifugation and washed thoroughly with deionized water and ethanol more than three times. At the end, the obtained product was dried in a vacuum oven at 50 °C for one night. Pure CoFe-LDH was synthesized by the same procedure as aforementioned but without adding g-C₃N₄ nanosheets and with no ultrasonication throughout the reaction.

Synthesis of the CoFe-LDH and CoFe-LDH@g-C₃N₄ nanosheets

As control samples, CoFe-LDH nanosheets were synthesized by a facile co-precipitation and hydrothermal method. For a typical process, Co(NO₃)₂·6H₂O and Fe(NO₃)₃·9H₂O were dissolved in deionized water, forming a clear solution with a Co/Fe molar ratio of 3, keeping the total metal-ion concentration at 1.2 mol L⁻¹. Two different solutions including NaOH (1.92 M) and Na₂CO₃ (0.80 M) were also prepared with equal volumes of basic and salt solutions, respectively. After that, both solutions were mixed together and added into a beaker containing the metal salt solution under vigorous stirring. The resulting slurry was kept in an oven at 80 °C for 48 h. After cooling naturally, the precipitate was washed several times with deionized water and ethanol by centrifugation, and dried at room temperature. For comparison, a CoFe-LDH@g-C₃N₄ composite was also prepared by the same procedure with the addition of different amounts of g-C₃N₄.

Characterizations

Powder X-ray diffraction (XRD) patterns of all of the samples were conducted using graphite-filtered Cu Kα radiation operating at 40 kV and 30 mA and λ = 0.15418 nm (Shimadzu XRD-6000 diffractometer). X-ray photoelectron spectrometry (XPS) was recorded using Al Kα radiation (Thermo VG ESCALAB MK II). The positions of all BEs were calibrated using the C 1s line at 284.8 eV. Scanning electron microscopy with an accelerating voltage of 20 kV (SEM, Zeiss SUPRA 55) was applied for detailed morphology analyses. Transmission electron microscope (TEM) and EDS mapping were taken using a microscope (JEOL JEM-2010F) combined with an EDX (OxfordX-MaxN 80-TLE) spectroscope. Photoluminescence (PL) experiments were performed on an Edinburgh FLS980 fluorescence spectrometer.

Electrode preparation

Typically, 5 mg of the as-synthesized catalyst was dispersed in 1000 μL of deionized H₂O; 250 μL of ethyl alcohol and 15 μL of Nafion solution were ultrasonically treated for at least 1 hour to form a homogeneous mixture. Then, 10 μL of the mixture was drop-caste onto the surface of a glassy carbon (GC) electrode by micro-pipette and the GC allowed to dry by evaporation of the solvent at room temperature.

Electrochemical and photoelectrochemical (PEC) measurements

All electrochemical and photoelectrochemical (PEC) measurements for OER and HER were conducted on an electrochemical workstation (CHI 660 C, CH Instrument Co. USA) using 1 M KOH solution as an electrolyte. All tests were carried out in a three-electrode electrochemical cell, i.e. Ag/AgCl (reference electrode) with saturated KCl filling solution, Pt wire (counter electrode) and a GC electrode with a diameter of 3 mm (working electrode). For activation, the GC electrode was polished with alumina powder on a nylon polishing pad. Then, the electrode was completely rinsed with deionized H₂O and cleaned in acetone by ultrasonication for 5 s. For 1 M KOH as an electrolyte, the applied potential changed from E vs. Ag/AgCl to overpotential and reversible hydrogen electrode (RHE), E vs. RHE = E vs. Ag/AgCl + 1.009 V, and the overpotential (η) for OER was η = E vs. RHE − 1.23 V = E vs. Ag/AgCl − 0.221 V. Cyclic voltammograms (CVs) were carried out between 0.2 and 0.8 V vs. Ag/AgCl at a scan rate of 100 mV s⁻¹ for 30 cycles to activate the electrode electrochemically in 1 M KOH. LSV was conducted between 0.2 and 0.8 V vs. Ag/AgCl at a scan rate of 10 mV s⁻¹; Tafel slopes and polarization curves were calculated from the LSV curves. Furthermore, an electrochemically active surface area (ECSA) was measured from the capacitive current associated with double-layer charging from CVs at different scan rates. For this purpose, small potential windows (0.2–0.3 V) of CVs were measured versus Ag/AgCl at different scan rates (10, 20, 40, 60, 80 and 100 mV s⁻¹). The double-layer capacitance (C_dl) was investigated by plotting the ΔJ = (J_a − J_c) at 0.25 V versus Ag/AgCl against the scan rate and the linear slope was found to be twice that of C_dl. Tafel slopes were calculated from the LSV curves by plotting log(J) on the x-axis and η on the y-axis. In a similar cell setup (three-electrode configuration), electrochemical impedance spectroscopy (EIS) evaluation was carried out to determine the electrical conductivity of the as-synthesized catalysts. The frequency range for EIS was from 100 kHz to 0.10 Hz with a modulation voltage amplitude of 5 mV at 0.5 V across Ag/AgCl potential. A durability (time and current density curves at a constant potential of 0.5 V across Ag/AgCl) test for the hierarchical CoFe-LDH@g-C₃N₄ composite was conducted on a three-electrode photo/electrochemical cell configuration using 1 M KOH solution as an electrolyte. PEC measurements were carried out using a CHI660C electrochemical workstation (CH Instruments Co. USA) with a three-electrode system, in which Pt wire, Ag/AgCl and Ni foam were used as the counter, reference and working electrodes, respectively. The working electrode was irradiated under a 300 W xenon arc lamp with an AM 1.5G filter (illumination intensity: 100 mW cm⁻²) using 1 M KOH aqueous solution as an electrolyte.

Photoluminescence spectroscopy (PL)

PL spectroscopy was conducted to examine the charge transfer behavior between g-C₃N₄ and CoFe-LDH.

Overall water splitting

The overall water splitting reaction was performed on a two-electrode system, in which hierarchical CoFe-LDH@g-C₃N₄ acts as anode and cathode in 1.0 M KOH electrolyte. The two electrodes were placed ∼2 cm apart from each other to avoid mixing of gases (H₂ and O₂) generated during the overall water splitting reaction, and the assembly design was opened to the air to release the gases during water splitting. The as-synthesized homogeneous ink of the hierarchical CoFe-LDH@g-C₃N₄ composite was assembled by drop-casting onto 1 × 1 cm² Ni foam and the LSV curves were measured at a scan rate of 10 mV s⁻¹ with iR-compensation.

Calculation method

The TOF value is determined from the equation: TOF = j × S/(4F × m). Here, j (mA cm⁻²) is current density at a given η (in our case at 350 mV), S (0.07 cm²) is the surface area of the GC electrode, the number 4 means 4 electrons per mole of O₂, F is Faraday's constant (96485.3 C mol⁻¹), and m is the number of moles of metal deposited on the GC electrode. In this case, we suppose that all the metal sites are actively participating in the PEC reaction.

Results and discussion

The crystallographic structure and phase purity of the as-prepared samples were firstly evaluated by powder X-ray diffraction (XRD), as shown in Fig. 1. For g-C₃N₄, two peaks appeared at ca. 13.0° and 27.8°, in which the sharp one at 27.8° confirms the high degree of graphitization of the as-prepared composite. The weaker graphitization intensity of the product at 13.0° is due to an in-planar structural packing motif at the (100) plane. Stacking of the conjugated aromatic systems in the product leads to a strong peak at 27.8° that is assigned to the (002) plane in the (00l) direction. For CoFe-LDH, diffraction peaks at 11.4°, 23.2°, 34.0°, 38.4°, 46.1°, 59.3° and 60.4° can be ascribed to the (003), (006), (012), (015), (018), (110) and (113) diffraction planes, respectively, for typical 2D LDH systems. The XRD pattern of hierarchical CoFe-LDH@g-C₃N₄ confirms the presence of both g-C₃N₄ and CoFe-LDH individual compounds in the resulting composite. Additionally, the peak intensity of g-C₃N₄ at 27.8° increases without disturbing the peak positions of CoFe-LDH by increasing the weight percentage loading of g-C₃N₄ (3–15 wt%) on CoFe-LDH. This means that the formation of g-C₃N₄ nanosheets does not hinder the crystal growth of CoFe-LDH. Hence it could be inferred that the crystal structures of g-C₃N₄ and CoFe-LDH remain unaffected during the formation of the CoFe-LDH@g-C₃N₄ heterostructure.^64,70

Fig. 1 XRD patterns of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ composite materials.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed to evaluate the morphological and structural features of the g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ systems. For pure g-C₃N₄, SEM exhibits curved and irregular 2D small nanosheets, which form the aggregation structure (Fig. 2a). While for pure CoFe-LDH, the SEM images exhibit a hierarchical flower-like morphology with an amount of cavities, with a thickness of 10–15 nm and length of 100–150 nm (Fig. 2b and Fig. S1a, b, ESI†). This hierarchical flower-like morphology is totally different from the traditional hexagonal micro/nanosheet morphology of LDHs, which may be related to the formation of multiple nucleation centers during the solvothermal process (Scheme 1).

Fig. 2 SEM images of g-C₃N₄ (a), CoFe-LDH (b) and CoFe-LDH@g-C₃N₄ composites (c and d); EDS mapping of CoFe-LDH@g-C₃N₄ composite (e).

Scheme 1 Schematic illustration of the fabrication of a CoFe-LDH@g-C₃N₄ hybrid semiconductor for photoelectrochemical overall water splitting.

Upon formation of CoFe-LDH@g-C₃N₄ (Fig. 2c and d), the morphologies of the composites vary from those of their individual components. Taking CoFe-LDH@g-C₃N₄ (10%) as an example, the g-C₃N₄ nanosheet arrays are elegantly dispersed within the hierarchical flower-like CoFe-LDH. Flower and petal-like nanosheets of CoFe-LDH become more corrugated and curlier after combination with g-C₃N₄ during the in situ solvothermal process (Fig. S1c and d, ESI†). Such a CoFe-LDH@g-C₃N₄ architecture may lead to a greater electrochemically active surface area due to corrugation of the flower-like structure, optimal pore-size and reduced ion transport distance. Energy dispersive spectra (EDS) of CoFe-LDH@g-C₃N₄ (10%) obviously prove the overlapping of all corresponding elements of Co, Fe, C, N and O, authenticating the uniform dispersion of CoFe-LDH and g-C₃N₄ in the composite (Fig. 2e and Fig. S2, ESI†). Furthermore, TEM was characterized to evaluate the detailed nanostructures of pure CoFe-LDH and CoFe-LDH@g-C₃N₄ composites. The CoFe-LDH exhibits obvious petal-like structures composed of ultrathin nanosheets and some cavities (Fig. 3a and Fig. S3a, b, ESI†). CoFe-LDH@g-C₃N₄ exhibits a high dispersion of g-C₃N₄, and the corrugated LDHs nanosheets are interlaced at the edges of the nanocomposites (Fig. 3b and c), which is consistent with the SEM images. The lattice fringe spacing values of 0.20, 0.23 and 0.32 nm can be attributed to the interplanar spacings of the (018), (015) and (002) planes for the CoFe-LDH and g-C₃N₄ nanosheets, respectively (Fig. 3d–f), which are consistent with their XRD patterns. The criss-crossing between the CoFe-LDH and g-C₃N₄ lattices also indicates the formation of heterostructures (Fig. S3c and d, ESI†). The selected-area electron diffraction (SAED) pattern provides an indication of the polycrystalline state and homogeneous distribution of g-C₃N₄ on the CoFe-LDH layers through the appearance of diffraction rings (Fig. 3d inset). Such interfacial characteristics may facilitate an increase in ECSA and the separation of the interfacial charge between CoFe-LDH and g-C₃N₄.

Fig. 3 TEM images of CoFe-LDH (a) CoFe-LDH@g-C₃N₄ composite (b–d) (inset in figure d: photography of CoFe-LDH@g-C₃N₄ hybrid and SAED pattern) and the profile of the distance between lattice fringes of hierarchical LDH in the CoFe-LDH@g-C₃N₄ hybrid (e and f).

The optical properties of the g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ composites were evaluated using diffuse reflectance UV-Vis spectroscopy (Fig. S7, ESI†). The spectra with absorbance less than 500 nm or higher than 500 nm can be attributed to the metal-to-metal charge transfer (MMCT) and metal-to-ligand charge transfer (MLCT), respectively.^75–77 MLCT arises from the 2p orbital of oxygen to the 3d orbitals of Co²⁺/Fe³⁺ ions (i.e. O_2p/Co_3d-t2g, O_2p/Fe_3d-t2g).^78,79 The g-C₃N₄ exhibits an intrinsic semiconductor-like absorption spectrum with a band edge of 450 nm (blue region of the visible spectrum) and can be assigned to the n–π* direct transition of lone pairs on the edge of the N atoms.^70,80 Thus, the modification of CoFe-LDH with g-C₃N₄ comprehensively changes the optical properties in the resultant hierarchical CoFe-LDH@g-C₃N₄ heterostructure. As displayed in Fig. S7 (ESI†), all the modified composites (CoFe-LDH@g-C₃N₄) with different weight loadings of g-C₃N₄ exhibit obvious absorption over the whole visible range of wavelengths. With an increase in loading of g-C₃N₄, an optimal change in the absorption edges is observed in the spectra, and this change is attributed to the strong interaction between g-C₃N₄ nanosheets and CoFe-LDH. An extremely high visible light absorption ability of the CoFe-LDH@g-C₃N₄ heterostructures is attributed to the dual roles of the delocalized conjugated structure for g-C₃N₄ and CoFe-LDH (1s/π* and 1s/o′* transitions, and LMCT and d–d transitions, respectively).⁶⁴ The optimized optical absorption spectrum of the CoFe-LDH@g-C₃N₄ composite occurs at a content of 10 wt% of g-C₃N₄. The maximum absorption of the composites is highly related to the unique morphology, exposed active sites and high surface area. However, further increasing the content of g-C₃N₄ (>10 wt %) may lead to a shielding effect of the active sites, disturbing the layered structure and blocking the active sites on the surface of CoFe-LDH, resulting in decreased electronic excitation under irradiation. The band gap energy of a crystalline semiconductor can be calculated from the following expression:

(αhν)^1/n = A(hν − E_g)

where α (absorption coefficient), h (Planck's constant), ν (light frequency), E_g (band gap energy) and A (constant) are fundamental constants. The n is determined by the type of semiconductor optical transition: its value is n = 1/2 for a direct transition.⁷⁰ Therefore, a plot of (αhν)²vs. hν (Kubelka–Munk function as a function of light energy) gives values of E_g = 2.15, 2.11, 1.98, 1.90, 1.95 and 2.70 eV corresponding to CoFe-LDH, CoFe-LDH@g-C₃N₄ (3%), CoFe-LDH@g-C₃N₄ (5%), CoFe-LDH@g-C₃N₄ (10%), CoFe-LDH@g-C₃N₄ (12%) and g-C₃N₄, respectively. Hence the smallest value of E_g under optimized conditions indicates the maximum visible light absorption. In addition to this, the small band gap between the valance band and the conduction band facilitates the transportation of charge carriers.

The separation and transfer efficiencies of the photogenerated hole and electron pairs in various semiconductor materials are evaluated by PL. The PL spectra of g-C₃N₄, CoFe-LDH, and CoFe-LDH@g-C₃N₄ (10%) heterostructures are shown in Fig. S4 (ESI†). The main peak of g-C₃N₄ is centered at approximately 440–450 nm, which is associated with the typical photoemission of g-C₃N₄, close to the band-gap energy of 2.7 eV and is totally consistent with the UV-Vis and E_g results. The sharp decrease in the PL intensity for the CoFe-LDH@g-C₃N₄ (10%) composite proves that the recombination of photogenerated electron–hole pairs is obviously suppressed with the maximum photoelectrocatalytic activity towards overall water splitting upon the formation of the CoFe-LDH@g-C₃N₄ (10%) composite due to the synergistic coupling of CoFe-LDH and g-C₃N₄ nanosheets. This is correlated with the dynamic and fast charge transfer from g-C₃N₄ to CoFe-LDH, which can be favorable for enhancing the PEC water oxidation reaction. In addition, the diffuse reflectance UV-Vis results of CoFe-LDH@g-C₃N₄ (10%) support the PL spectra. These results suggest that the strong electronic coupling of the g-C₃N₄ nanosheets with the flower-like CoFe-LDH matrix facilitates the efficient movement of photogenerated excitons of the g-C₃N₄ layers during energy transfer. Additionally, the hierarchical flower-like surface of CoFe-LDH could act as trapping sites for photogenerated electrons, which may reduce the rate of electron–hole recombination for enhanced photoelectrocatalytic activity during water splitting.

XPS studies were conducted to analyze the oxidation state and chemical bonding of the as-prepared heterostructures (Fig. 4). According to full-scan XPS spectra, composite materials include all C, N, O, Co, and Fe elements from g-C₃N₄ and CoFe-LDH (Fig. 4a). The fitting analysis of the C 1s XPS spectra of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ is shown in Fig. 4b, which exhibits the presence of graphitic C=C (∼284.82 eV), and C–N=C bonds (∼288.22 eV) for both g-C₃N₄ and CoFe-LDH@g-C₃N₄, while extra peaks at ∼283.77 (C–H), 285.12 (C=N), 287.27 (carbonyl carbon, C=O), 287.87 (N–C=O) and 288.52 eV (carboxyl groups, O–C=O) appear only for CoFe-LDH@g-C₃N₄. Furthermore, the N 1s XPS spectra of CoFe-LDH@g-C₃N₄ and g-C₃N₄ (Fig. 4c) present three almost similar peaks with binding energies of 400.87, 398.92 and 398.42 eV, corresponding to quaternary-N, N–C and pyridinic-N, respectively. In the N 1s spectra of hierarchical CoFe-LDH@g-C₃N₄, the composite exhibits one additional peak to be found at ∼399.42 eV (Co–N) relative to pure g-C₃N₄. These nitrogen centers are favorable to OER, and in particular the appearance of the Co–N bond will benefit a speedy redox process.^11,81–83 The presence of additional bonds (C–O, C=O, O–C=O, and Co–N) and an additional peak around 781.8 eV signify that CoFe-LDH@g-C₃N₄ is not a simple physical mixture, but has a strong chemical interaction between the heterostructures. Such a structure could accelerate the electronic coupling between two components and generate extra active sites (i.e. interfaces) for OER and HER during the overall water splitting process.^11,16,66,67 The XPS spectrum also proves the presence of Fe in the as-prepared composite material, as shown in Fig. 4d. The Fe species in the CoFe-LDH@g-C₃N₄ composite can be deconvoluted into five peaks, which are dominated by the Fe³⁺ oxidation state. The peak at 711.2 eV assigned to Fe 2p_3/2 and the peak at 725.1 eV assigned to Fe 2p_1/2 are consistent with the occurrence of Fe(OH)₃ units in CoFe-LDH. Furthermore, the fitting result of the Co 2p peak indicates two oxidation states recognized as Co²⁺ and Co³⁺ (Fig. 4e). In the Co 2p XPS spectra, the peaks at 780.17 and 796.6 eV are assigned to Co³⁺ and the peaks at 786.2 and 802.9 eV are attributed to Co²⁺. The electrocatalytic activities of hierarchical CoFe-LDH@g-C₃N₄ were explored for both OER and HER studies with a three-electrode electrochemical cell in 1 M KOH solution (Fig. 5). Fig. 5a exhibits the polarization curve after iR correction (scan rate: 10 mV s⁻¹). The linear sweep voltammetry (LSV) curves reveal overpotentials (to achieve a current density of 10 mA cm⁻²) of 525, 314, 268 and 275 mV for g-C₃N₄, CoFe-LDH, RuO₂ and CoFe-LDH@g-C₃N₄ (10%), respectively (Fig. S5, ESI†). To achieve a current density of 60 mA cm⁻², g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%) show overpotentials of 548, 408 and 322 mV, respectively. These observations affirm that the individual g-C₃N₄ and CoFe-LDH deliver very high overpotential and lower current densities, but when they are combined into hierarchical heterostructures, the catalytic activity is tremendously enhanced. Moreover, the photoelectrochemical performances for water splitting were also studied. Typically, the anodic and cathodic reactions (OER and HER) are accompanied by the transfer of a majority of carriers (holes and electrons, respectively). The photoexcitation process changes the amount of carriers at the interface and the current density is therefore enhanced by illumination. Hence, the photoelectrochemical results show a decrease in overpotential. In the presence of light, the hierarchical CoFe-LDH@g-C₃N₄ composite exhibits an obviously decreased overpotential at 270 mV (Fig. 5a). Furthermore, an obvious decrease in overpotential from 380 mV to 346 mV is also noted to achieve a current density of 100 mA cm⁻² for CoFe-LDH@g-C₃N₄ (10%) under visible light illumination, confirming the high-efficiency PEC OER effect (Fig. S6, ESI†). The optimized photoelectrochemical activity of the composites occurs at a content of 10 wt%. In our opinion, the photo/electrochemical performance of the composites is highly related to their unique morphology, having good light harvesting capability, with a high surface area and exposed active sites to facilitate charge transportation in the heterostructure. The turnover frequency (TOF) is also an important kinetic parameter for OER, illustrating the intrinsic properties of the electrocatalytic activity. Here, it is assumed that all metal sites in the CoFe-LDH@g-C₃N₄ composite are electrochemically active in the water oxidation reaction, and their turnover frequencies at an overpotential of 350 mV can be calculated. The results give values of TOF = 0.32 s⁻¹, 0.237 s⁻¹, 0.213 s⁻¹, 0.123 s⁻¹, 0.094 s⁻¹ and 0.046 s⁻¹, corresponding to CoFe-LDH@g-C₃N₄ (10%) with light, without light, CoFe-LDH@g-C₃N₄ (12%), CoFe-LDH@g-C₃N₄ (5%), CoFe-LDH@g-C₃N₄ (3%) and CoFe-LDH, respectively. It is worth mentioning that the above TOFs are obviously underestimated because some metal sites may be electrochemically inaccessible.

Fig. 4 (a) Full-scan XPS spectra of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%); (b) C 1s of g-C₃N₄, LDH and CoFe-LDH@g-C₃N₄; (c) N 1s of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%); (d) Fe 2p spectra of CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%); (e) Co 2p spectra of Co.

Fig. 5 (a) Linear scan voltammogram (LSV) OER curves of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (with different wt%). (b) Corresponding Tafel slopes for OER. (c) Chronoamperometry of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%) in light and dark applying E = +0.20 V vs. Ag/AgCl for 400 s in 1 M KOH solution. (d) EIS of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%). (e) LSV HER curves of Pt/C, g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%). (f) Corresponding Tafel slopes for HER.

The superior performance of CoFe-LDH@g-C₃N₄ (10 wt%) can be associated with its hierarchical flower-like morphology and abundant active sites (such as quaternary N, Co–N, pyridinic N, Co²⁺ and Co³⁺, as confirmed by XPS). However, a further increase in the content of g-C₃N₄ (>10 wt%) may lead to the shielding effect of active sites and electron channels in LDHs, as well as the penetration of free electrolyte, which are not beneficial to an enhancement in catalytic activities. Remarkably, the low overpotential to achieve a current density (10 mA cm⁻²) value for CoFe-LDH@g-C₃N₄ is comparable to some other state-of-the-art OER catalysts (Table S1, ESI†). The efficient photo/electrocatalytic activity of the hierarchical heterostructure is further confirmed by the low Tafel slope (Fig. 5b). The CoFe-LDH@g-C₃N₄ composite shows a much lower Tafel slope value (58 mV dec⁻¹) relative to its individual components. In contrast, the pristine g-C₃N₄, CoFe-LDH and RuO₂ exhibit higher Tafel slope values of 114, 82, and 78 mV dec⁻¹, respectively. It is believed that a lower Tafel slope correlates with a speedy catalytic reaction, and thus encourages high OER activity. Under visible light, the Tafel slope of hierarchical CoFe-LDH@g-C₃N₄ further decreases to 51 mV dec⁻¹, which confirms the higher PEC catalytic activity, and is consistent with the trend in LSV. To further investigate the conductivity, electrochemical impedance spectroscopies (EIS) of hierarchical CoFe-LDH@g-C₃N₄, CoFe-LDH and g-C₃N₄ were performed in a three-electrode system. Values of charge transfer resistance (R_ct) and solution resistance (R_s) were measured and compared according to the corresponding equivalent circuit values from the Nyquist plots. Both R_s and R_ct are resistance across the electrode/electrolyte interface and CPE is the constant phase component (inset in Fig. 5d). Various semi-circles in the Nyquist plots in the high-frequency range can be correlated with the R_ct of the as-synthesized electrocatalysts. The diameter of the semicircle illustrates the charge transfer resistance (R_ct) and a smaller value of R_ct signifies high conductivity as well as faster charge-transfer ability. Therefore, the EIS results (Fig. 5d) prove that the hierarchical CoFe-LDH@g-C₃N₄ shows a much lower charge transfer resistance (97.02 Ω) relative to CoFe-LDH (157.0 Ω) or g-C₃N₄ (498.6 Ω), symbolic of its miraculous electron-transfer behavior during the electrochemical reaction. Under visible light irradiation, the EIS of the hierarchical CoFe-LDH@g-C₃N₄ composite decreases from 97.02 Ω to 76.00 Ω, confirming its high PEC catalytic activity. It is known that typical LDHs undergo low conductivity, and the enhanced electronic coupling in the hierarchical CoFe-LDH@g-C₃N₄ composite would accelerate the charge transfer and electron conductivity, which agrees well with the LSV and Tafel slope results (Fig. 5b). As shown above, the Tafel slope and overpotential are simultaneously decreased under visible light irradiation. The hierarchical CoFe-LDH@g-C₃N₄ (10%) heterostructures could play a vital role in the excellent electrochemical activities due to the synergistic effects between the flower-like CoFe-LDH and g-C₃N₄. To detect the possible PEC mechanism, the transient photocurrent responses of g-C₃N₄, CoFe-LDH and the hierarchical CoFe-LDH@g-C₃N₄ composite were also investigated. Fig. 5c exhibits the photocurrent curves of g-C₃N₄, CoFe-LDH and hierarchical CoFe-LDH@g-C₃N₄ for certain on–off cycles under visible light. The photocurrent density will go down immediately once the light is turned off, while the photocurrent density will return to a high constant value when the light is turned on. The photocurrent density of CoFe-LDH@g-C₃N₄ (10%) is calculated by subtracting the dark current from the total current. The photocurrent density of CoFe-LDH@g-C₃N₄ is 0.196 mA cm⁻², which is 2.0 and 3.2 times higher than those of CoFe-LDH and g-C₃N₄, respectively. The results reveal that the hierarchical CoFe-LDH@g-C₃N₄ composite shows a higher photocurrent response than its individual parts, indicating that the hierarchical composites dramatically suppress electron–hole recombination and enhance the separation of interfacial charge transfer between CoFe-LDH and g-C₃N₄ under visible light, which favors an improvement in PEC activity in overall water splitting.

Next, the HER catalytic capabilities of g-C₃N₄, commercial Pt/C, CoFe-LDH and hierarchical CoFe-LDH@g-C₃N₄ catalysts were further detected in 1 M basic solution for the probability of overall water splitting (Fig. 5e). The current density (−10 mA cm⁻²) of hierarchical CoFe-LDH@g-C₃N₄ appears at −417 mV (η10 = −417 mV). In contrast, CoFe-LDH and g-C₃N₄ require η10 of −538 mV and −670 mV. To reach a current density of −50 mA cm⁻², CoFe-LDH@g-C₃N₄, CoFe-LDH and g-C₃N₄ require η50 of −472 mV, −592 mV and >−700 mV, respectively. The overpotential of hierarchical CoFe-LDH@g-C₃N₄ (η50 = −472 mV) is very close to that of Pt/C (η50 = −416 mV). The HER catalytic kinetics for all the synthesized samples were also evaluated by the Tafel plots, as shown in Fig. 5f. It is believed that there are two mechanisms ((1) Volmer–Tafel or (2) Volmer–Heyrovsky) involving the HER process under basic conditions.^23,27 Both mechanisms involve the adsorption of a water molecule, the dissociation of adsorbed water to an adsorbed H atom and OH⁻, desorption of OH⁻, and the transformation of adsorbed H into H₂, as shown below:

H₂O + e⁻ → H_ads + OH⁻(Volmer);

H_ads + H_ads → H₂(Tafel);

H₂O + e⁻ → H_ads + OH⁻(Volmer);

H_ads + H₂O + e⁻ → H₂ + OH⁻(Heyrovsky)

The Tafel slope of hierarchical CoFe-LDH@g-C₃N₄ is 71 mV dec⁻¹, which is smaller than that of CoFe-LDH (95 mV dec⁻¹), g-C₃N₄ (134 mV dec⁻¹), or Pt/C (136 mV dec⁻¹), demonstrating that hierarchical CoFe-LDH@g-C₃N₄ has more efficient electrocatalytic kinetics for HER. To determine whether the Tafel, Heyrovsky, or Volmer component is the rate-limiting step, slopes of 30, 40, or 120 mV dec⁻¹ are inspected, respectively. As shown in Fig. 5f, the Tafel value of the hierarchical CoFe-LDH@g-C₃N₄ composite is 71 mV dec⁻¹, which suggests that the HER over the hierarchical CoFe-LDH@g-C₃N₄ composite progresses by a Volmer–Heyrovsky mechanism and that the Heyrovsky reaction is the rate-limiting step. The electrochemically active surface area (ECSA) is an immensely vital parameter for catalysts used in the overall water splitting reaction, as it is admitted that an increase in ECSA generally leads to an enhancement in the photo/electrocatalytic activity of the as-prepared catalyst. To gain insight into the improved photo/electrochemical performance of hierarchical CoFe-LDH@g-C₃N₄ in comparison with the controls, the ECSA were calculated and predicted based on the electrochemical double-layer capacitance (C_dl) derived from different CV curves at different scan rates (Fig. 6). It is obvious that the C_dl value of hierarchical CoFe-LDH@g-C₃N₄ reaches 36.9 mF cm⁻², 2.17 times and 126.3 times higher than those of CoFe-LDH (17 mF cm⁻²) and g-C₃N₄ (0.292 g-C₃N₄), respectively. The higher ECSA indicates that the 2D hierarchically porous configuration could promote gas penetration and release during overall water splitting. Such an improved ECSA at the same mass loading will lead to the exposure of more active sites and maintain an adequate electrochemical reaction between hierarchical CoFe-LDH@g-C₃N₄ and electrolyte ions, thus delivering high photo/electrocatalytic activity.

Fig. 6 Typical cyclic voltammetry curves (a–c) of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%) electrodes in 1 M KOH with different scan rates; (d) charging current density differences (ΔJ = J_a − J_c) of g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ (10%).

To detect the advantages of the hierarchical flower-like structure of LDH, we also prepared CoFe-LDH@g-C₃N₄ using a combined co-precipitation and hydrothermal method for comparative analysis. The PXRD patterns, SEM images and electrochemical properties of the as-synthesized g-C₃N₄, CoFe-LDH and CoFe-LDH@g-C₃N₄ composites are discussed and displayed in Fig. S8–S10 (ESI†), respectively. Moreover, for additional comparative analysis, a physical mixture (CoFe-LDH and g-C₃N₄ physically mixed together) of CoFe-LDH and g-C₃N₄ was also prepared. SEM images and electrochemical properties of the as-prepared physical mixture are discussed and shown in Fig. S11 and S12 (ESI†). The results show that the hierarchical flower-like structure presents a higher PEC electrocatalytical performance than the sample with the other morphology or a purely physical mixture.

The overall splitting of water was performed in a two-electrode system in which hierarchical CoFe-LDH@g-C₃N₄ acts as both the anode and the cathode in 1 M basic solution (Fig. 7). The LSV curve for overall water splitting shows that it only requires an applied potential of ca. 1.55 V to achieve a current density of 10 mA cm⁻² for the hierarchical CoFe-LDH@g-C₃N₄ electrocatalyst. To deliver a current density of 100 mA cm⁻², hierarchical CoFe-LDH@g-C₃N₄ only requires a cell voltage of ca. 1.82 V for the overall water splitting process. Such a low potential of the CoFe-LDH@g-C₃N₄ composite for overall water splitting is remarkable and comparable to recently reported bifunctional electrocatalysts (Table S2, ESI†).

Fig. 7 Overall water splitting using CoFe-LDH@g-C₃N₄ (10%) as OER and HER catalyst.

The magnificent photo/electrocatalytic performance for overall water splitting of the hierarchical flower-like CoFe-LDH@g-C₃N₄ composite might be attributed to its unique hierarchical flower-like structure and ultrahigh active surface area, as discussed above. Stability is a very important parameter to evaluate the electrocatalytic properties of the as-prepared catalyst for practical application. A stability test was performed with and without light irradiation by continuous galvanostatic measurement for 12 hours.

Impressively, the hierarchical CoFe-LDH@g-C₃N₄ composite electrocatalyst exhibits good stability and durability in 1 M KOH basic solution in the presence and absence of light irradiation, as shown in Fig. 8. Furthermore, by applying 0.5 V potential, the current density maintained at ∼16.5 mA cm⁻² for a period of 12 hours is an indication of good durability. The high stability performance illustrates that the hierarchical flower-like CoFe-LDH@g-C₃N₄ composite structure is stable and suitable as a practical electrocatalyst for water splitting.

Fig. 8 Durability test of CoFe-LDH@g-C₃N₄ (10%) in 1 M KOH solution (time-dependent current density curve under constant potential of 0.5 V versus Ag/AgCl) with and without a light source.

Conclusions

In summary, we report for the first time, the synthesis of hierarchical flower-like CoFe-LDH@g-C₃N₄ as a 2D porous composite through a facile solvothermal method. Our study reveals the opportunities provided by these unique hierarchical structures to function as extremely active and highly stable micro/nanostructures for high-performance bifunctional OER and HER. The hierarchical CoFe-LDH@g-C₃N₄ electrode shows magnificent activity toward overall water splitting, with 10 mA cm⁻² current density achieved by only 1.55 V and with 100 mA cm⁻² current density by 1.82 V across a two-electrode system. The magnificent HER and OER activities are associated with the strong electronic interactions between combined components, as proved by the XPS (such as graphitic N and Co–N), SEM and HRTEM evaluations, which show the increased charge transfer and conductivity. The CoFe-LDH@g-C₃N₄ composite exhibits enhanced current density at a lower overpotential and high stability, indicating its potential practical applications for the overall water splitting process. Therefore, this work provides an efficient way to achieve a new type of hierarchical flower-like CoFe-LDH@g-C₃N₄ system towards full water splitting in energy applications, and supplies a detailed understanding of the PEC OER and HER processes of the 2D hybrid semiconductors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21473013, 21771021, and 21822501), the Beijing Nova Program (xx2018115), the Fundamental Research Funds for the Central Universities, and Analytical and Measurements Fund of Beijing Normal University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00677f

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