Kexin Zhang,Rui Wang,Fubin Jiang
Fe-Doped Metal Complex (LaCo0.9Fe0.1O3) and g-C3N4 Formed a Nanoheterojunction for the Photocatalytic Decomposition of Water
Photocatalytic water decomposition provides an environmentally friendly method of hydrogen production similar to “photosynthesis”, while current research aims to develop affordable yet efficient photocatalysts. Oxygen vacancy is one of the most significant defects in metal oxide semiconductors, including perovskite, which substantially influences the semiconductor material’s efficiency. To enhance the oxygen vacancy in the perovskite, we worked on doping Fe. A perovskite oxide nanostructure of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) was prepared by the sol–gel method, and a series of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9)/g-C3N4 nanoheterojunction photocatalysts were synthesized using mechanical mixing and solvothermal methods for LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9). Fe was successfully doped into the perovskite (LaCoO3), and the formation of an oxygen vacancy was verified by various detection methods. In our photocatalytic water decomposition experiments, we observed that LaCo0.9Fe0.1O3 demonstrated a significant increase in its maximum hydrogen release rate, reaching 5249.21 μmol h–1 g–1, which was remarkably 17.60 times higher than that of LaCoO3-undoped Fe. Similarly, we also explored the photocatalytic activity of the nanoheterojunction complex LaCo0.9Fe0.1O3/g-C3N4, and it exhibited pronounced performance with an average hydrogen production of 7472.67 μmol h–1 g–1, which was 25.05 times that of LaCoO3. We confirmed that the oxygen vacancy plays a crucial role in photocatalysis.
Fe-Doped Metal Complex (LaCo0.9Fe0.1O3) and g-C3N4 Formed a Nanoheterojunction for the Photocatalytic Decomposition of Water

Photocatalytic water decomposition provides an environmentally friendly method of hydrogen production similar to “photosynthesis”, while current research aims to develop affordable yet efficient photocatalysts. Oxygen vacancy is one of the most significant defects in metal oxide semiconductors, including perovskite, which substantially influences the semiconductor material’s efficiency. To enhance the oxygen vacancy in the perovskite, we worked on doping Fe. A perovskite oxide nanostructure of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) was prepared by the sol–gel method, and a series of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9)/g-C3N4 nanoheterojunction photocatalysts were synthesized using mechanical mixing and solvothermal methods for LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9). Fe was successfully doped into the perovskite (LaCoO3), and the formation of an oxygen vacancy was verified by various detection methods. In our photocatalytic water decomposition experiments, we observed that LaCo0.9Fe0.1O3 demonstrated a significant increase in its maximum hydrogen release rate, reaching 5249.21 μmol h–1 g–1, which was remarkably 17.60 times higher than that of LaCoO3-undoped Fe. Similarly, we also explored the photocatalytic activity of the nanoheterojunction complex LaCo0.9Fe0.1O3/g-C3N4, and it exhibited pronounced performance with an average hydrogen production of 7472.67 μmol h–1 g–1, which was 25.05 times that of LaCoO3. We confirmed that the oxygen vacancy plays a crucial role in photocatalysis.

Currently, the most pressing issue we encounter is increasing energy demand. Among various sustainable energy sources, hydrogen is one of the most promising energy sources. Photocatalytic water splitting is a hydrogen production technology with high potential that makes use of photocatalysis to extract hydrogen from water, producing a clean fuel. To cope with the disadvantages of the high cost and scarcity of noble metal catalysts, the development of low-cost and efficient transition-metal oxide catalysts (such as perovskite materials) has become a research hotspot. Perovskite generally refers to oxides having the chemical formula ABO3, which holds tremendous potential for applications such as sensors, magnetic reluctance devices, and catalysis. At the same time, it is easy to form abundant oxygen vacancies, which is one of the most common defects in the list of semiconductor materials, especially in metal oxide semiconductors, and has an important effect on the performance of the list of semiconductor materials. For example, LaCoO3 perovskite has attracted wide attention due to its excellent properties and its invention applications including photocatalytic water splitting hydrogen production catalysts, sensors, electromagnetic wave absorption, solid oxide fuel cells, and so forth. However, its wide band gap, small specific surface area, and fast electron–hole complexation are unfavorable for hydrogen precipitation reactions. By modulating the metal cation or valence state at the B-site while increasing the number of oxygen defects, the catalytic activity of the catalyst can be modulated efficiently. One of the most studied methods is to replace part of the B-site “Co” with iron. Tang and Wan reported that Fe-doped nanostructures can be used for efficient electrochemical water splitting for hydrogen production with a hydrogen yield of 6.06 L min–1 g–1. Dholam tried to dope Fe and Cr into TiO2, and the rate of photosplitting water to hydrogen gas was greatly improved, proving that the doping of Fe inhibits the direct recombination of electron–hole pairs. Hu et al. observed that when Fe is doped, La2Ti2O7 exhibits strong absorption in the visible light region, that is, the energy band structure changes. Recently, we have reported the performance of LaCoO3/g-C3N4 in photocatalytic decomposition of water, and the rate of hydrogen generation under the full spectrum reaches 1046.15 μmol h–1 g–1. Additionally, we investigated the photocatalytic ability of LaCoO3/C3N5, and the hydrogen decomposition rate reached 956.11 μmol h–1 g–1. The two catalysts provide more choices for photocatalysis, but the decomposition rate of water may still be improved. We suggest that the oxygen vacancy may be responsible for the hindrance of water decomposition. It has been reported that the oxygen vacancy of the perovskite can effectively improve the recombination of electrons and holes and increase the photocatalytic performance. Fe(III), as an effective solute cation, is often used in various catalytic reactions due to its special electronic structure of d-orbital half-filling. With this in mind, we aim to dope LaCoO3 with Fe and adjust the amount of the sacrificial agent according to the test. Through a large number of tests, we can prove that Fe is successfully doped and increases the oxygen vacancy. Our study found that the band gap of LaCoxFe1–xO3 after Fe doping is smaller, the specific surface area is reduced, and the ratio of M2+/M3+ is increased by X-ray photoelectron spectroscopy (XPS) testing, with the introduction of oxygen defects. This reduces the direct recombination of electron–hole pairs and effectively suppresses photogenerated electron–hole pair recombination. However, even if we doped Fe, the problem of a small visible light range cannot be ignored; experiments show that C3N4 is an excellent visible light response catalyst, We suspect that the perovskite and C3N4-formed heterojunction will be conducive to the response of visible light, so we have on this basis to carry out the synthesis of both. In this context, we synthesized LaCoxFe1–xO3 using a simple sol–gel method and then used mechanical stirring and dissolution heat to synthesize LaCoxFe1–xO3/g-C3N4 nanoheterojunction materials to study their properties of water to hydrogen production by photolysis in the presence of a full-spectrum irradiation reactor. The results show that the hydrogen release rate of LaCo0.9Fe0.1O3 is the highest at 5249.21 μmol h–1 g–1, which is 17.60 times that of undoped LaCoO3. Meanwhile, the nanoheterojunction composite LaCo0.9Fe0.1O3/g-C3N4 also showed excellent photocatalytic activity, and the average hydrogen production per hour was 7472.67 μmol h–1 g–1, which was 25.05 times that of LaCoO3. The existence of the oxygen vacancy proves to play an essential role in photocatalysis. The apparent quantum yield (AQY) is superior at 300–800 nm and was at most 3.85% under visible light.

Lanthanum nitrate hexahydrate (AR), cobalt nitrate hexahydrate (AR), ferric nitrate hexahydrate (AR), citric acid (AR), urea (AR), anhydrous methanol (AR), and ethanol (AR) were obtained from Beijing Haoke Technology Co. , Ltd. All other reagents used in the synthesis were of analytical grade and used without further purification. Deionized water, with a resistivity of 18 MΩ cm, was used throughout the experiments.

(1) The mass of the reactant is calculated as the ratio of the amount of cobalt nitrate and ferric nitrate to the amount of lanthanum nitrate and citric acid, which is 1:1:2. The cobalt nitrate is 0.2910, 0.5821, 0.8731, 1.1641, and 1.3096 g; the contents of ferric nitrate were 0.1616, 1.2120, 0.8080, 0.4040, and 0.1010 g; lanthanum nitrate is 2.1651 g, and citric acid is 1.9210 g. Using 10 mL of secondary water and 5 mL of ethanol to dissolve the substance, a deep red transparent solution was obtained. The preparation of the solution must avoid other ions mixing with the solution and the instrument before using secondary water several times. (2) The deep red transparent solution was transferred to a round-bottom flask, heated in an 80 °C water bath, and stirred by magnetic force until the solution was in a deep red sol state. (3) The deep red sol was poured into a clean ceramic crucible and dried at 120 °C for 6 h in a blast dryer to obtain a dark red expanded dry gel, which was fully ground for use. (4) The ground powder was placed in a box muffle furnace and calcined from 5 °C min–1 to 400 °C for 4 h, then from 5 °C min–1 to 700 °C for 4 h, and at 700 °C for 4 h. After the machine cooled naturally, black LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) powders were collected.

g-C3N4 was prepared by a simple thermal oxidation method of directly heating urea in a box muffle furnace. First, urea (5 g) was put into an open ceramic crucible, dried in a blast oven at 80 °C for 10 h, and ground to obtain dry urea powder. The ground urea powder was placed in a crucible with a LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) lid, heated in a muffle furnace at a heating rate of 5 °C min–1 from 20 to 550 °C, and kept at this high temperature for 3 h. After cooling the product to 20 °C, a yellow powder was obtained and ground.

Various weight ratios of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9)/g-C3N4 were prepared by the solvothermal method (Figure S1). A certain weight of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) and of g-C3N4 were dispersed in anhydrous ethanol (10 mL) separately and sonicated for half an hour. The suspension of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) was slowly added to the suspension of g-C3N4. At room temperature, the mixed solution was sonicated for half an hour. During the process of ultrasonication, the system temperature was kept normal. The mixture was stirred at room temperature for 10 h and transferred to a clean hydrothermal kettle. The hydrothermal kettle was heated at 120 °C for 8 h. Finally, the mixture was centrifuged for half an hour and dried at 80 °C for 10 ho. LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9)/g-C3N4 nanoflakes with 20, 25, 30, 35, and 40 wt % LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) contents were prepared by the above method.

The surface morphology and microstructure were surveyed by scanning electron microscopy (SEM, HITACHI S-4800). With the help of a Shimadzu Maxima X-ray diffractometer (XRD 7000), the samples were irradiated by Cu Kα at a current of 30 mA and a voltage of 40 kV, and the crystal structure and phase composition of the samples were determined. The specific surface area, total pore volume, and average pore size of calcined perovskites were measured by N2 physical adsorption at 77 K using a QUADRASORB Si (Quantachrome) adsorption analyzer. Before each surface area measurement, the 200 mg sample was degassed at 453 K for 3 h. The specific surface area of the material was calculated by the Brunauer–Emmett–Teller (BET) model, and the pore size distribution was calculated from the desorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) method. Transmission electron microscopy (TEM, FEI Talos F200S) was used to characterize the spacing, and the growth of the material and the elemental distribution were characterized by energy-dispersive spectroscopy (EDS). XPS analysis was performed with a Thermo Fisher electron spectrometer (XPS, ESCALAB 250XI) equipped with a monochromatic microfocused Al Kα X-ray source, with the C 1s peak binding energy determined to be 284.8 eV for carbon correction. Fourier transform infrared (FT-IR) spectroscopy was measured by using an infrared spectrometer (Shimadzu IRAffinity-1). The optical properties of the samples were measured by UV–vis diffuse reflectance spectroscopy (DRS) using a UV-2600 spectrophotometer (Shimadzu, Japan) (BaSO4 as the blank substrate). Photoluminescence (PL) spectral characterization was performed on an FS 980 photometer at room temperature using a 500 W xenon lamp source with an excitation wavelength of 300 nm. In a standard three-electrode system, a CHI1030C electrochemical workstation with a platinum mesh as the pair electrode, a silver/silver chloride electrode as the reference electrode, and 5 mg LaCo0.9Fe0.1O3 were used. g-C3N4 and LaCo0.9Fe0.1O3/g-C3N4-70 wt % were dissolved in a mixture of 800 μL distilled water, 200 μL isopropanol, and 30 μL Nafion. After ultrasonic treatment, it was dropped on a glassy carbon electrode employed as the working electrode. In addition, 0.1 mol/L KHCO3 solution was used as the electrolyte. The width of the excitation and emission slits was 5 nm. The resistance of carrier migration was further investigated by electrochemical impedance spectroscopy (EIS) using 5 mmol L–1 Fe(CN)63–/4– solution and 0.1 mol/L KCl solution as the electrolyte. The yield of H2 was detected by manual injection on a Shimadzu gas chromatograph (GC-2014C). The electron paramagnetic resonance spectrum was measured by an electron paramagnetic resonance spectrometer (Bruker E500).

The photocatalytic activity of the prepared samples was evaluated by photocatalytic hydrogen production from water decomposition. In this paper, all water decomposition experiments were carried out at a room temperature of 25 °C, and magnetic stirring was carried out during illumination. The light source was a 300 W xenon lamp (PLS-SXE300), and photocatalytic experiments were performed using full-spectrum (250 < λ < 1200 nm) simulated sunlight. The specific operation was as follows: first, 10 mg of catalyst was placed into a custom-made quartz round-bottom flask, and 80 mL of water was added for ultrasonic treatment, making it evenly dispersed in the water solution. Then, 20 mL of anhydrous methanol was added as a sacrificial agent. After adding magnetons, nitrogen was introduced for 60 min to remove dissolved oxygen from the solution in the condition of protecting from light. A full-spectrum xenon lamp was used to illuminate the container filled with nitrogen gas with a rubber plug. The irradiation duration of the xenon lamp was set to 5 h, and the irradiation current was set to 18.5 A. Then, the “RUN” button was pressed to operate the xenon lamp, cycling five times. It is worth mentioning that the process of photocatalysis always uses magnetic stirring so that the catalyst in the solution is evenly dispersed to prevent catalyst deposition at the bottom of the vessel, which can affect the catalytic effect. Due to the use of a full-spectrum irradiation reactor, light energy is higher. It is easy for this to result in a shorter-time temperature rise too quickly, which can increase the number of variables that affect the response. Therefore, we use the cooling circulating water system to maintain a relatively stable reaction temperature in the photocatalytic process. Generally, the temperature of the cooling circulating water is set to 20 °C, and the cooling circulating water cools the vessel through the outside of the reaction vessel so that the inside of the vessel maintains a reaction environment close to room temperature. The H2 production rate was monitored hourly by a gas chromatograph equipped with a molecular sieve column and a TCD detector. Each material underwent three photocatalytic experiments to eliminate unexpected factors and enhance the reliability of the experimental data. The optimum ratio of the photocatalyst was determined, and the stability of the photocatalyst was tested. The photocatalyst was tested five times in a cycle of 5 h for a total of 25 h. Apparent quantum efficiency was computed using the following equation

The crystal structure of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) powder was characterized by X-ray diffraction (XRD), as shown in Figure 1. The distinct peaks of LaCoxFe1–xO3 are in line with the characteristic peak of JCPDS no. 44-0361, JCPDS no. 53-1211, JCPDS no. 40-0224. Upon analysis of the characteristic peaks, it can be observed that there are no discernible impurities present in the results. Additionally, the level of crystallinity exhibited in the analysis is of high quality. As Fe-to-Co partial substitution increases, the intensity of the characteristic peaks increases, and the splitting peaks merge into a single peak and move toward lower angles. This is because the covalent radius of Fe3+ (LS 132 pm, HS 152 pm) is larger than that of Co3+ (LS 126 pm, HS 150 pm). In addition, when the doping ratio of Fe is 0.8 and 0.6, the perovskite is orthorhombic. When the doping ratio is 0.4 and 0.2, the perovskite is a mixed structure of orthorhombic and triangular shapes, respectively. When the ratio is 0.1, its structure is a triangular system, which indicates that the doping of Fe changes the lattice structure. The crystal structures of g-C3N4, LaCo0.9Fe0.1O3, and LaCo0.9Fe0.1O3/g-C3N4 with various weight ratios were characterized by XRD, as shown in Figure 2. The XRD of g-C3N4 displays two main peaks at 13.1 and 27.7°. The stronger peak is at 27.7° from the (0 0 2) crystal plane of layered g-C3N4, while the weaker peak is at 13.1° from the in-plane (1 0 0) crystal plane (JCPDS no. 87-1526). The XRD peak of LaCo0.9Fe0.1O3 is highly consistent with the crystal phase of JCPDS no. 44-0361, and the characteristic peak at 32.8° is attributed to the (1 1 0) crystal plane. Behind the combination of LaCo0.9Fe0.1O3 and g-C3N4, the characteristic peak intensities of LaCo0.9Fe0.1O3 and g-C3N4 are significantly reduced, which once again proves that the nanoheterojunction composite was successfully prepared.

It can be seen from Figure 3a–d that LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) obtained by the sol–gel method is composed of many ellipsoids, among which the ellipsoid particles are LaCo0.9Fe0.1O3. The particle size is the most uniform, and the particles grow most closely together to form a two-dimensional nanolayered structure, as shown in Figure 4a. In Figure 4b, the synthesis of the g-C3N4 utilized urea as the precursor material through a process of calcination. The resulting structure of the g-C3N4 takes the form of a lamellar tremella, with each individual lamella possessing a flat surface ideal for use as a substrate material. From Figure 4c,d, we can observe that the LaCo0.9Fe0.1O3 nanoparticle layer is dispersed as particles and grows in clusters on g-C3N4. This staggered structure has a larger surface area, which is conducive to the absorption and utilization of light energy by the catalyst. Both the surface and the interior can receive light sources, which increases the number of active sites for photosplitting water to produce hydrogen.

After that, we verified the morphology, elemental composition, composition structure, particle size, and other characteristics of the catalyst by TEM and high-resolution TEM (HRTEM) to explore the influence on its properties. The LaCo0.9Fe0.1O3 in Figure 5a shows an obvious ellipsoid structure with an ellipsoid particle size distribution ranging from 60 to 80 nm. In Figure 5b,c, we can observe that the well-crystallized LaCo0.9Fe0.1O3 nanoparticles have obvious lattice spacing (d value), and the spacing between adjacent lattice fringes is fixed. The distance between adjacent lattice fringes, that is, the plane spacing, is about 0.272 nm, which is consistent with the crystal plane (1 1 0). As shown in Figure 5d,e, g-C3N4 exhibited a distinct layered structure, consistent with the above observations. In Figure 5f,g, lattice plane separation can be observed at the interface, and the plane spacing between adjacent lattice fringes is approximately 0.27 nm, which is consistent with (1 1 0). The crystal plane index (1 1 0) of LaCo0.9Fe0.1O3 obtained by XRD analysis is consistent with the obvious diffraction peaks in the SAED mode, as shown in Figure 5h–j. Figure 6 shows EDS, which verifies the composition of elements in LaCo0.9Fe0.1O3, and no other elements are found. The elemental composition and distribution of LaCo0.9Fe0.1O3/g-C3N4-70 wt % are shown in Figure 7, and La, Co, Fe, C, N, and O are all uniformly distributed.

According to the SEM image and TEM image of LaCo0.9Fe0.1O3, we can see that it is in the form of spherical particles. The specific area, average pore size, and average particle size of the samples were measured by the BET technique. Figure 8a,c,e shows that the nitrogen adsorption analytical isotherms of g-C3N4, LaCo0.9Fe0.1O3, and LaCo0.9Fe0.1O3/g-C3N4 are inconsistent with the desorption curves, and there is a hysteresis ring. In line with the characteristics of the type IV adsorption equilibrium isotherm, all are mesoporous materials.Table 1 shows that the pore size of the sample is concentrated in the range of 2–3 nm, which belongs to mesopores, which is also consistent with the above adsorption and desorption isotherm analysis results. The specific surface areas of LaCo0.9Fe0.1O3, g-C3N4 and LaCo0.9Fe0.1O3/g-C3N4-70 wt % are 64.99, 100.69, and 153.87 m2 g–1, respectively. The specific surface area of LaCo0.9Fe0.1O3/g-C3N4-70 wt % is 2.4 times that of LaCo0.9Fe0.1O3 and 1.5 times that of g-C3N4. That is, the combination of the two is beneficial for increasing the number of active sites, so as to improve the photocatalytic water splitting activity.

We analyzed the XPS spectrum for obtaining the chemical state and the interaction, as shown in Figure 9a, with carbon correction for the C 1s peak at 284.8 eV. The LaCo0.9Fe0.1O3/g-C3N4-70 wt % composite is composed of La, Co, Fe, O, C, and N. The high-resolution C 1s spectrum of pure g-C3N4 can be separated into two major peaks located at nearly 284.8 and 288.2 eV. The C 1s peak at 284.8 eV is attributed to the C–C bond of carbon, and the peak at 288.2 eV is attributed to the sp2-type C=N bond. The high-resolution N 1s spectrum of the g-C3N4 sample shown in Figure 9c can be divided into three major peaks. We assigned the N 1s peak corresponding to the lowest binding energy (398.8 eV) for the C–N bond and a central peak of 400.2 eV for the sp2-type C=N bond. The peak with the largest binding energy is at 401.5 eV, which arises from nitrogen surrounded by three carbons in the amorphous CN network. Note that the characteristic peak of the N–(C)3 bond in the LaCo0.9Fe0.1O3/g-C3N4-70 wt % spectrum is redshifted by 0.2 eV. This indicates that the electron density increases after LaCo0.9Fe0.1O3 is combined with g-C3N4. The high-resolution O 1s XPS spectrum has two main features at approximately 529.1 and 531.9 eV. The spectrum of the pure LaCo0.9Fe0.1O3 sample can be divided into three peaks: the peak with the lowest binding energy (approximately 529.1 eV) is due to the lattice oxygen atoms on the surface; the next peak (approximately 531.3 eV) is attributed to surface-adsorbed oxygen, and the peak with the highest binding energy (approximately 532.9 eV) is attributed to adsorbed water. The high-resolution O 1s spectrum of LaCo0.9Fe0.1O3/g-C3N4 exhibits two main features at 529.1 and 531.9 eV, which are attributed to the surface lattice oxygen atoms and the surface adsorbed oxygen, respectively. Among them, the peak position of surface-adsorbed oxygen bonds in the spectrum of LaCo0.9Fe0.1O3/g-C3N4-70 wt % increased by 0.6 eV, and the occupied area increased, which indicated that after combining with LaCo0.9Fe0.1O3, the adsorbed oxygen content in LaCo0.9Fe0.1O3/g-C3N4 increased significantly. The typical high-resolution XPS La 3d spectrum of the LaCo0.9Fe0.1O3/g-C3N4-70 wt % sample shows two shoulder peaks with vibrational features at 830–840 and 850–857 eV, which can be split into two distinct peaks located at 834.2 and 837.8 and 850.8 and 854.8 eV, corresponding to the binding energies of La 3d5/2 and La 3d3/2, respectively, confirming the existence of La3+ in the crystal structure. For the high-resolution Co 2p spectrum of the LaCo0.9Fe0.1O3/g-C3N4-70 wt % sample (Figure 9f), there are two main peaks located at 780.3 and 795.6 eV, which are attributed to the typical Co3+ structure at 787.2 eV typical of shaking. In addition, there are two satellite peaks at 782.8 and 797.9 eV. There are two shoulder peaks in the high-resolution Fe 2p spectrum (Figure 9g), which are at 710.9 and 712.9, 723.8, and 724.8 eV, corresponding to the binding energies of Fe 2p3/2 and Fe 2p5/2, respectively. There is also a satellite peak at 717.9 eV. From Table 2, we can see that with Fe doping, the ratio of M2+/M3+ increases, enriching oxygen vacancies (OVs) and suppressing direct electron–hole recombination. In short, the XPS results further demonstrate that LaCo0.9Fe0.1O3/g-C3N4 composites were successfully obtained and that the main electron transfer pathway was from LaCo0.9Fe0.1O3 to g-C3N4.

The FT-IR spectrum of LaCoxFe1–xO3 (x = 0.2, 0.4, 0.6, 0.8, and 0.9) is shown on the left side of Figure 10. The absorption band at 600 cm–1 is attributed to the Co–O stretching vibration, and there is a strong vibrational band at 574 cm–1, which may be derived from the characteristic absorption bands of the Fe–O stretching vibration and the O–Fe–O bending vibration. The FT-IR spectra of LaCo0.9Fe0.1O3, g-C3N4, and 25, 30, 35, and 40 wt % LaCo0.9Fe0.1O3/g-C3N4 are shown on the right side of Figure 10. For pure g-C3N4, the characteristic peaks at 1508.4, 1541.2, and 1558.6 cm–1 are consistent with aromatic-CN stretching vibrational modes. In addition, the breathing vibration of g-C3N4 appears as a characteristic absorption peak at 810 cm–1, corresponding to the breathing vibration peak in g-C3N4 (tri-triazine unit) and the deformation vibration peak of surface N–H. The characteristic peaks of LaCo0.9Fe0.1O3 at 600 and 560 cm–1 are related to the bending and stretching vibrations of Co–O, corresponding to the perovskite structure. As the content of g-C3N4 in the LaCo0.9Fe0.1O3 sample increases, the characteristic peaks of the composites gradually become sharper. Furthermore, the spectrum of the LaCo0.9Fe0.1O3/g-C3N4 sample shows a blueshift at 810 cm–1 (tris-triazine unit) compared to the spectrum of g-C3N4. This indicates that the number of hydrogen bonds in the g-C3N4 structure is reduced, which will facilitate the transfer of charge carriers in the carbon nitride layer.

As shown in Figure 11b, the solid UV diffuse reflectances of pure LaCo0.9Fe0.1O3, g-C3N4, and LaCo0.9Fe0.1O3/g-C3N4-70 wt % were measured by solid UV diffuse reflectance spectroscopy in the range of 220–800 nm. Obviously, g-C3N4 has the strongest absorption for ultraviolet light, and its visible spectral absorption edge is approximately 470 nm, which is consistent with the known reports. The LaCo0.9Fe0.1O3/g-C3N4 composite has similar absorption properties to g-C3N4. Almost all ranges of light can be absorbed by pure LaCo0.9Fe0.1O3, which shows the excellent optoelectronic properties of LaCo0.9Fe0.1O3. Note that the absorption edge of the LaCo0.9Fe0.1O3/g-C3N4 composite is redshifted compared with that of pure g-C3N4, which indicates that the LaCo0.9Fe0.1O3/g-C3N4 composite can absorb more visible light. The band gap energy (Eg) of a semiconductor is determined according to the Kubelka–Munk equation as followsHere, α, h, ν, A, Eg, and n represent the absorption coefficient, Planck constant, incident light frequency, constant, band gap, and integer, respectively. Figure 11a shows the band gap energy (Eg) of LaCoxFe1–xO3 (x = 0.20, 0.40, 0.60, 0.80, and 0.90); Figure 11a,c,d shows the calculated band gap Eg values of g-C3N4, LaCo0.9Fe0.1O3, and LaCo0.9Fe0.1O3/g-C3N4, which are approximately 3.10, 2.24, and 1.96 eV, respectively. The Eg values obtained from the composites are smaller. Alternatively, the VB and CB potentials of semiconductors can also be obtained from the following empirical formulasECB and EVB represent the CB and VB edge potentials of the semiconductor, respectively. χ is the electronegativity of a semiconductor. According to ref (47), the χ values of pure LaCo0.9Fe0.1O3 and g-C3N4 can be obtained and are approximately 5.60 and 4.63 eV, respectively. Eθ is the energy of a free electron with a hydrogen scale (∼4.5 eV relative to NHE), and Eg is the band gap energy of the semiconductor. The values of χ, ECB, and EVB are calculated and displayed in Table 3. The results show that the CB edge potential of g-C3N4 (−1.42 eV) is more negative than that of LaCo0.9Fe0.1O3 (−0.02 eV), while the VB edge potential of LaCo0.9Fe0.1O3 (2.22 eV) is more positive than that of g-C3N4 (1.68 eV). The M–S curves of LaCo0.9Fe0.1O3 and g-C3N4 exhibit positive slopes, indicating their n-type semiconductor nature (Figure 11e,f), which is in agreement with the reported results. Furthermore, the flat-band potentials (Ef) of g-C3N4 (−1.32 V vs Ag/AgCl) and LaCo0.9Fe0.1O3 (0.08 V vs Ag/AgCl) were obtained from the M–S plots (Figure 11e,f). Considering that the voltage discrepancy between the ECB and Ef of an n-type semiconductor is approximately 0.1 V, the ECB of g-C3N4 and LaCo0.9Fe0.1O3 is calculated to be −1.42 and −0.02 eV (vs NHE), respectively. Therefore, this is consistent with empirical formulas used for the calculation of VB and CB potentials. Distinctly, the well-matched band structures of LaCo0.9Fe0.1O3 and g-C3N4 for Z-scheme charge transfer can be established after the rational construction of the LaCo0.9Fe0.1O3/g-C3N4 heterojunction.

The separation and transfer efficiency of photogenerated charge carriers can be evaluated by PL spectroscopy. The higher the PL signal intensity, the higher the recombination rate. The lower the PL signal intensity, the lower the recombination rate. The peak height of the fluorescence spectrum is directly related to the recombination rate of the hole pair, and the stronger the intensity, the higher the recombination rate. Figure 12a shows the results of excitation of LaCo0.9Fe0.1O3, g-C3N4, and LaCo0.9Fe0.1O3/g-C3N4 composites with various weight ratios by light at 300 nm. We can clearly see that pure g-C3N4 has a strong emission peak, but under excitation at 300 nm, pure LaCo0.9Fe0.1O3 has almost no fluorescence response signal. The fluorescence response signal of the composite of LaCo0.9Fe0.1O3 and g-C3N4 was significantly lower than that of pure g-C3N4, indicating that the composite ratio of the composite decreased, and the fluorescence response signal of LaCo0.9Fe0.1O3/g-C3N4-70 wt % was the lowest. The results show that it has the lowest carrier recombination rate and improves the activity of photocatalytic decomposition of water. Furthermore, the edge of the fluorescent band of the LaCo0.9Fe0.1O3/g-C3N4 composite was blue-shifted compared with that of pure g-C3N4, and it was clear that there was an interaction between LaCo0.9Fe0.1O3 and g-C3N4. The PL spectra show that the recombination of carriers in LaCo0.9Fe0.1O3/g-C3N4 composites is greatly inhibited, which is mainly due to the photogenerated electrons in the LaCo0.9Fe0.1O3 conduction band tending to migrate down to the valence band. g-C3N4 is irradiated by full-wavelength light and forms a Z-type heterostructure, which effectively inhibits the direct recombination of electron–hole pairs. Light-induced electron transfer analysis, shown in Figure 12b, revealed a current response under light illumination with pure LaCo0.9Fe0.1O3, g-C3N4, and LaCo0.9Fe0.1O3/g-C3N4-70 wt %. Pure LaCo0.9Fe0.1O3 and g-C3N4 revealed a small current response under light illumination; however, LaCo0.9Fe0.1O3/g-C3N4-70 wt % showed a very large, rapidly rising, and stable current response, and the current response remained unchanged after multiple cycles without attenuation or time delay. Among them, the LaCo0.9Fe0.1O3/g-C3N4-70 wt % composites had a photocurrent density of 0.066 mA cm–2, which was higher than that of LaCo0.9Fe0.1O3 (0.047 mA cm–2) and g-C3N4 (0.028 mA cm–2). It showed the strongest transient photocurrent response. Photocurrent is formed by the diffusion of photogenerated electrons to the back contacts, and the holes are absorbed by the hole receptors in the electrolyte. Therefore, the LaCo0.9Fe0.1O3/g-C3N4-70 wt % composite has a higher photogenerated electron–hole separation efficiency. The resistance of carrier migration can be further studied by EIS. On this basis, the electronic impedance (EIS) responses of LaCo0.9Fe0.1O3/g-C3N4-70 wt % and pure LaCo0.9Fe0.1O3, g-C3N4 were further measured. The intersection of the semicircle with the X-axis in the pure g-C3N4 and pure LaCo0.9Fe0.1O3 maps is larger than that in the composite photocatalyst, which means that charge carrier migration will encounter greater resistance, as shown in Figure 11c. That is, the charge transfer resistance of LaCo0.9Fe0.1O3/g-C3N4-70 wt % is lower than that of LaCo0.9Fe0.1O3 and g-C3N4, and LaCo0.9Fe0.1O3/g-C3N4-70 wt % has the lowest impedance. It was proven that the LaCo0.9Fe0.1O3/g-C3N4-70 wt % carrier migration resistance was lower, and photogenerated carrier migration was more likely to occur. This result proved that the LaCo0.9Fe0.1O3/g-C3N4-70 wt % composite photocatalyst had a higher hydrogen production efficiency and could inhibit the recombination of photogenerated electron–hole pairs more effectively.

First of all, we conducted a blank control test, carrying it out without full-spectrum light and photocatalyst to evaluate the effect of light and photocatalyst on the experiment. Under this premise, the photolysis efficiency is not worth considering, which shows that TES water containing 20% methanol is stable under visible light irradiation. Then, anhydrous control experiments were conducted to eliminate the possibility of methanol reforming in the full-spectrum range. 20 mL of methanol and 10 mg of the LaCo0.9Fe0.1O3 catalyst were added to the photocatalytic experiment. The results showed hydrogen generation is not worth considering, which could be ignored. This shows that methanol just consumes holes to promote photocatalytic hydrogen production. Then, the photocatalytic properties of LaCoxFe1–xO3 (x = 0.20, 0.40, 0.60, 0.80, and 0.90) nanoparticles were measured in aqueous solution with 20% (volume) methanol as the sacrificial reagent under full-spectrum (250–1200 nm) irradiation of a 300 W xenon lamp. H2 emissions were detected using gas chromatography, as shown in Figure 13a,b. We performed three parallel tests on all samples, as shown in Figure 13c–g, and calculated the average hydrogen production rate, which could eliminate unexpected factors and obtain more convincing data. Obviously, we can see that the materials have various hydrogen production rates with various Fe doping ratios. The hydrogen release rates of LaCoxFe1–xO3 (x = 0.20, 0.40, 0.60, 0.80, and 0.90) are 4813.21, 2431.54, 2828.36, 3776.70, and 5249.21 μmol h–1 g–1, respectively. LaCo0.9Fe0.1O3 has the highest photocatalytic activity, with an average hydrogen production of 5249.21 μmol h–1 g–1, which is 17.60 times higher than that of LaCoO3 without Fe doping. As shown in Figure 13c, the photocatalytic efficiency did not show a significant loss even after five consecutive cycles, which suggests that under the irradiation of a xenon lamp, the catalyst is highly stable in the process of photocatalytic hydrolysis. The photocatalytic properties of LaCo0.9Fe0.1O3/g-C3N4 nanoparticles with various mass ratios were tested. H2 emissions were detected using gas chromatography, as shown in Figure 14a,b. We performed three parallel tests on all samples and calculated the average hydrogen production rate, which eliminates the element of surprise and yields more convincing data. By calculating the errors, it was confirmed that the repeatability and reproducibility of the data were maintained at a level of 0.02% or less. Clearly, the prepared g-C3N4, LaCo0.9Fe0.1O3, and LaCo0.9Fe0.1O3/g-C3N4-60, 65, 70, 75, and 80 wt % composites have various levels of hydrogen production by photolysis. The hydrogen rates of LaCo0.9Fe0.1O3 and g-C3N4 were 5249.21 and 3657.75 μmol h–1 g–1. LaCo0.9Fe0.1O3/g-C3N4-70 wt % composites had the highest photocatalytic activity, with an average hourly hydrogen production of 7472.67 μmol h–1 g–1, which was 1.5 and 2.0 times higher than that of LaCo0.9Fe0.1O3 and g-C3N4, respectively, and 7.14 times that of LaCo0.9Fe0.1O3/g-C3N4 without Fe doping. As shown in Figure 14c, there was no apparent loss of photocatalytic efficiency even after five consecutive cycles, which suggests that under full-spectrum irradiation of the Xenon lamp, the heterojunction photocatalyst is highly stable in the process of photocatalytic hydrolysis. The AQY of LaCo0.9Fe0.1O3/g-C3N4-70 wt % is 3.9%, 2.4, 1.2, 0.5, and 0.4% at 300, 420, 450, 500, and 600 nm, respectively, as shown in Figure 15.

To understand the mechanism of the reaction, we carried out electron paramagnetic resonance (EPR) spectroscopy. As shown in Figure 16a, after 10 min of exposure to xenon lamp full-spectrum light, •OH free radical signals of LaCo0.9Fe0.1O3 and LaCo0.9Fe0.1O3/g-C3N4-70 wt % can be observed; no •OH free radical signal was observed for pure g-C3N4. The absence of a DMPO-•OH signal in g-C3N4 is because •OH cannot be generated from a hole in the VB in g-C3N4 (EVB = 1.68 eV, = 1.99 eV vs NHE). Observations of the DMPO-•OH signal of the LaCo0.9Fe0.1O3/g-C3N4-70 wt % complex indicate that the photogenerated hole remains in the VB of LaCo0.9Fe0.1O3 and does not transfer to the VB of g-C3N4. In Figure 16b, DMPO-•O2– signals of g-C3N4 and LaCo0.9Fe0.1O3/g-C3N4-70 wt % composite samples could be observed after 10 min of exposure to xenon lamp full-spectrum light, and •O2– signals of LaCo0.9Fe0.1O3 could not be observed. As Figure 16c shows, LaCo0.9Fe0.1O3, g-C3N4 and LaCo0.9Fe0.1O3/g-C3N4-70 wt % possess no ESR signals in the dark, which proves that both •O2– and •OH are excited by visible light. The results show that the photogenerated electrons in the g-C3N4 and LaCo0.9Fe0.1O3/g-C3N4-70 wt % composite samples have sufficient reduction ability to reduce O2 to form superoxide radical anions (O2–) (EO2θ/+O2– = −0.33 eV vs NHE). By calculation, we derive a g value of 2.002 for the LaCo0.9Fe0.1O3/g-C3N4-70 wt % complex, as evidenced by the presence of an oxygen vacancy in the complex in Figure 16d. On the basis of the above characterizations and analyses, the Z-scheme charge transfer mechanism for LaCo0.9Fe0.1O3/g-C3N4 in the photocatalytic hydrogen evolution (Figure 17). The EPR can prove that photogenerated electrons and holes exist in the CB and VB of LaCo0.9Fe0.1O3 and g-C3N4, respectively, and at the same time, charge transfer does not match the conventional type II heterojunction mechanism. It can be inferred that the charge transfer path of this reaction is a Z-type mechanism. Both LaCo0.9Fe0.1O3 and g-C3N4 have band gap widths, and when the photon energy is equal to or greater than the band gap width, the photogenerated electrons transition from the VB of g-C3N4 and LaCo0.9Fe0.1O3 to the CB and create holes in the valence band [eq 5]. Then, electrons on LaCo0.9Fe0.1O3 rapidly recombine with holes on g-C3N4 via a heterostructure [eq 6]. Water and holes combine to produce hydrogen ions and oxygen [eq 7], and hydrogen ions further combine with electrons in the g-C3N4 conduction band to produce hydrogen [eq 8]. The specific process of photocatalysis is as follows

Fe-doped LaCoxFe1–xO3 nanocomposites were successfully prepared by the sol–gel method. A LaCo0.9Fe0.1O3/g-C3N4 nanoheterojunction photocatalyst was successfully prepared to improve the semiconductor performance, and the photocatalytic activity was significantly increased. The existence of doped “Fe” was confirmed by XRD and EDS spectra. The chemical oxidation states of various elements in the photocatalysts were determined by XPS. A series of characterization results showed that LaCo0.9Fe0.1O3 was in the form of 60–90 nm nanospheres, and g-C3N4 had a layered structure with a good appearance. The results showed that LaCo0.9Fe0.1O3 nanoparticles were successfully compounded with g-C3N4 nanosheets. Under photocatalysis conditions, the highest photocatalytic activity of LaCo0.9Fe0.1O3/g-C3N4-70 wt % was 7472.67 μmol h–1 g–1, which was 7.14 times higher than that of LaCo3/g-C3N4 without Fe doping. The photocatalytic activity of LaCo0.9Fe0.1O3/g-C3N4 was significantly higher than that of the other catalysts. This study not only designed a low-cost and high-efficiency LaCo0.9Fe0.1O3/g-C3N4 photocatalytic system for water cracking or other photocatalytic applications but also provided a new idea for the construction of new materials for photocatalysis.