Preparation and photocatalytic performance of UIO-66/La-MOF composite

Traditional photocatalytic degradation technology is difficult to apply in the industry because of the low efficiency of photocatalytic activities. Metal–organic framework materials (MOFs) are exciting novel photocatalysts. Due to their adjustable porosity and controllable diverse composition, MOFs have great potential to improve the efficiency of photocatalytic degradation by adjusting their energy band structures (Chen & Zhang 2014).

As a common MOF photocatalyst, UIO-66 and its derivatives have been extensively studied for their inherent water stability and adjustability (Dong et al. 2018). Yin et al. (2021) synthesized mesoporous Cu-doped UIO-66 with abundant oxygen vacancies by using a one-pot method. The resulting photocatalyst could remove 93% of ciprofloxacin within 60 min, and its photocatalytic degradation is 3.7 times that of UIO-66. UIO-66 has poor photocatalytic activity due to its wide band gap energy, which can be improved by modification. Studies have shown that aminoated UIO-66 has better photocatalytic performance and broadened response range under visible light. Hosseini et al. (2022) simply modified NH₂-UIO-66 by using iron (III) complex (i.e., Fe[ACAC]₃), and a novel and cost-effective photocatalyst was prepared. The experimental results showed that the photocatalyst was superior in the photocatalytic degradation of water pollutants (e.g., dyes), and 87% Rhodamine B could be removed within 160 min.

Modifying photocatalysts by using rare-earth metals has always been an important research direction (Gao et al. 2020). La-MOFs have been noted for their excellent thermal and chemical stability. In terms of the matching of energy levels, a heterojunction structure is formed by the recombination of La-MOFs and semiconductor matched with the band structure. Such structure can improve the utilization rate under visible light and the photocatalytic degradation efficiency. Wang et al. (2019) synthesized La(III) MOF with 5-amino-phthalic acid (H₂L) by using the water/solvothermal method. The product showed an 81.6% removal of Rhodamine B in 8 h under UV irradiation. However, few studies have been done on La-MOFs.

Therefore, to improve the photocatalytic degradation efficiency of UIO-66 under visible light, new La/Zr-H₂BDC and La/Zr-H₂ATA nanocomposites were synthesized using the solvent–thermal method with different ratios of La-MOFs and UIO-66. The photocatalytic degradation of Rhodamine B by photocatalysts under visible light was investigated. The structure, morphology, and photocatalytic performance were fully studied. The possible mechanisms of the main active species and carrier migration paths in the matching of energy levels of La/Zr-H₂BDC and La/Zr-H₂ATA were also discussed in detail.

Reagents: Rhodamine B (C₂₈H₃₁ClN₂O₃, Sinopharm Chemical Reagents Co, LTD); lanthanum nitrate (La(NO₃)₃, Shanghai Maclean Biochemical Technology Co, LTD); zirconium oxychloride (ZrOCl₂, Sinopharm Chemical Reagent Co, LTD); terephthalic acid (C₈H₈O₄, Sinopharm Chemical Reagent Co, LTD); 2-amino-terephthalic acid (C₈H₇NO₄, Sinopharm Chemical Reagents Co, LTD); anhydrous ethanol (C₂H₅OH, Nanjing Chemical Reagent Co, LTD); N, N-dimethylformamide (C₃H₇NO, Sinopharm Chemical Reagents Co, LTD), all reagents are AR and can be used without further purification. Deionized water is used throughout the experiment.

Instruments: X-ray diffractometer (ESCALAB 250Xi, Thermo Fisher Technologies); UV-visible spectrometer (TU-1901, Beijing General Analysis Instrument Co, LTD); infrared spectrometer (VERTEX-70, Beijing Boland Technology Co, LTD.); Electrochemical Workstation (CHI660E, Shanghai Chenhua Instrument Co, LTD); UV-visible spectrophotometer (Spectrumlab-752Pro, Hunan Xiangyi Laboratory Instrument Development Co, LTD); field emission scanning electron microscope (Hitachi SU8100, Hitachi Scientific Instruments (Beijing) Co., LTD); transmission electron microscope (FEI F20, American FEI Company); laser particle size analyzer (Zetasizer-Nano-ZS-90, Beijing Haifuda Technology Co., LTD.); high-performance simulated solar xenon lamp source (CEL-HXF300, Beijing Zhongjiao Jinyuan Technology Co, LTD).

The organic ligand terephthalic acid (H₂BDC) (or 2-amino-terephthalic acid [H₂ATA], 10 mmol) was mixed with zirconium oxychloride (10 mmol) and 70 mL of N,N-dimethylformamide (DMF) at room temperature. The mixture was stirred for 1 h, ultrasonicated for 30 min, and poured into a polytetrafluoroethylene (PTFE)-lined stainless steel autoclave. The autoclave was placed in an oven and heated to 150 °C. After 24 h, the autoclave was taken out and left to cool to room temperature. The samples were washed with DMF and methanol and centrifuged at 6,000 rpm for 5 min. The precipitate was taken and vacuum-dried at 100 °C for 12 h. The photocatalyst Zr-H₂BDC (or Zr-H₂ATA) was obtained after grinding the precipitate.

At room temperature, H₂BDC (or H₂ATA, 10 mmol) was mixed evenly with lanthanum nitrate (10 mmol) and 70 mL of DMF, and the mixture was stirred for 1 h. After 30 min of ultrasonication, the mixture was poured into the PTFE-lined stainless steel autoclave, placed in an oven, and heated to 150 °C. After 24 h, the autoclave was taken out and left to cool to room temperature. The mixture was washed with DMF and methanol and centrifuged at 6,000 rpm for 5 min. The precipitate was taken (Li et al. 2018) and vacuum-dried at 100 °C for 12 h. The photocatalyst La-H₂BDC (or La-H₂ATA) was obtained after grinding the precipitate.

At room temperature, H₂BDC or H₂ATA (10 mmol) was mixed with zirconium oxychloride (10 mmol), lanthanum nitrate (in different proportions), and 70 mL of DMF. The mixture was stirred for 1 h. After 30 min of ultrasonication, the mixture was poured into a Teflon-lined stainless steel autoclave, placed in an oven, and heated to 150 °C. After 24 h, the autoclave was taken out and left to cool to room temperature. The mixture was washed with DMF and methanol and centrifuged at 6,000 rpm for 5 min. The precipitate was taken and vacuum-dried at 100 °C for 12 h. The photocatalyst was obtained after grinding the precipitate. The molar ratios of zirconium chloride and lanthanum nitrate were set as 1:0.15, 1:0.25, 1:0.35 and 1:0.50, respectively. A series of composite materials with varying molar ratios were prepared, namely, 0.15 La/Zr-H₂BDC, 0.25 La/Zr-H₂BDC, 0.35 La/Zr-H₂BDC, 0.50 La/Zr-H₂BDC, 0.15 La/Zr-H₂ATA, 0.25 La/Zr-H₂ATA, 0.35 La/Zr-H₂ATA, and 0.50 La/Zr-H₂ATA.

The crystal structure of the samples were characterized using an ESCALAB 250Xi diffractometer. The scanning velocity was 10°/min at 40 KV, 80 mA, and Cu Kα radiation (λ = 1.542 Å). The scanning angle range was 5°–80°.

The surface morphology of the samples was determined using a scanning electron microscope.

The IR spectra of the samples were recorded using the VERTEX-70 Fourier transform infrared (FT-IR) spectrometer with pure potassium bromide as the background. The weight ratio of potassium bromide to the sample was 50:1, and the scanning range was 4,000–500 cm⁻¹.

UV-vis diffuse reflectance spectroscopy (DRS) was performed using the TU-1901 UV-vis spectrophotometer as reference. The UV-vis light absorption characteristics of the photocatalysts were measured (Luo et al. 2019). The band gap energy was calculated as follows: E_g = 1,240/λ_g, where λ_g is the absorption wavelength (nm), and E_g is the band gap energy (eV).

The flat-band potentials (E_FB vs. Ag/AgCl) of the photocatalysts were obtained using the Mott-Schottky test analysis, in which the CHI660E electrochemical workstation was used. A three-electrode system was adopted: saturated AgCl was used as the reference electrode, a platinum wire as the auxiliary electrode, glassy carbon electrode as the working electrode, and 0.2 mol/L potassium ferricyanide solution as the electrolyte buffer (Mancuso et al. 2020). The test frequency, system temperature, sweep voltage step, and sinusoidal signal amplitude were 1 kHz, 1.2 °C, 100 mV/s, and 10 mV, respectively. The voltages shown in the system were those of the AgCl electrode.

The performance of photocatalysts was studied by CEL-HXF300 xenon lamp. The light source was composed of 300 W xenon lamp and 420 nm cut-off filter, and the illumination intensity was 100 mW/cm².

The performance of the photocatalytic degradation of Rhodamine B was evaluated with a 300 W xenon lamp as the simulated light source. The photocatalyst (10 mg) was added to 100 mL of Rhodamine B solution (20 mg/L) to form a photocatalyst suspension. The suspension was stirred for 60 min in the dark to ensure the adsorption-desorption balance between the photocatalyst and the solution. Photocatalytic degradation was carried out under a 300 W xenon lamp. Then, 5 mL of the reaction samples were collected at 60 min intervals, and 3 mL of the supernatant were collected after 5 min of high-speed centrifugation at 6,000 rpm. The absorbance was measured using a UV-vis spectrophotometer at absorption wavelength of 554 nm (Odoh et al. 2015) and was converted to concentration.

Based on the experiments on the photocatalytic degradation of Rhodamine B, 0.4 mL of isopropanol (IPA, a hydroxyl radical •OH quenching agent), 0.1 mL of triethanolamine (TEOA, a hole [h⁺] quenching agent), and 0.0050 g of benzoquinone (BQ, a superoxide radical [•O₂⁻] quenching agent) were added to the system to determine the contributions of the reactive intermediates (i.e., h⁺, •O₂⁻, and •OH) to the photocatalytic degradation of Rhodamine B (Ren et al. 2014).

Figure 1 shows the X-ray diffraction (XRD) patterns of the La/Zr-H₂BDC and La/Zr-H₂ATA series. As shown in Figure 1(a), the characteristic diffraction peaks of the synthesized Zr-H₂BDC are at 7.4°, 11.8°, 17.6° and 25.5° (Guan et al. 2017). The characteristic diffraction peaks of La-H₂BDC are at 9.7° and 19.6° (Shang et al. 2013). As the mole ratio of La increased, the diffraction peak of La-H₂BDC increased gradually. This indicated that the introduction of La-H₂BDC did not change the crystal structure of Zr-H₂BDC. The above results also proved that La-H₂BDC can be successfully loaded onto the surface of Zr-H₂BDC.

Figure 1(b) shows the characteristic diffraction peaks of the synthesized Zr-H₂ATA at 11.9°, 14.5°, 25.7° and 30.6° (Su et al. 2018). La-H₂ATA exhibited characteristic diffraction peaks at 12.3°, 17.2°, 19.7°, and 29.9° (Subudhi et al. 2019). As the molar ratio of La increased, the diffraction peak of La-H₂ATA increased gradually. This indicated that the introduction of La-H₂ATA did not change the crystal structure of Zr-H₂ATA. The above results also proved that La-H₂ATA can be successfully loaded onto the surface of Zr-H₂ATA.

Figure 2 shows the FT-IR spectra of the La/Zr-H₂BDC and La/Zr-H₂ATA series. Figure 2(a) illustrates the FT-IR spectra of the La/Zr-H₂BDC series. The vibrational peak of the metal-oxygen bond was located at 661–823 cm⁻¹ in the atlas. The vibration peaks of C–C–C were located at 1,021 cm⁻¹, and the symmetric stretching vibration peak of the carboxyl group (COO) was located at 1,387 cm⁻¹. The asymmetric stretching vibration peak of COO was located at 1,584 cm⁻¹. The C–H vibration peak of the DMF guest molecule was located at 2,786 cm⁻¹.

Figure 2(b) shows the FT-IR spectra of the La/Zr-H₂ATA series. The vibrational peak of the metal–oxygen bond was located at 661–823 cm⁻¹ in the atlas. The vibration peaks of C–C–C were located from 1,020 cm⁻¹ to 1,251 cm⁻¹. The symmetric stretching vibration peak of COO was located at 1,380 cm⁻¹, and the asymmetric stretching vibration peak of COO was located at 1,584 cm⁻¹. The peak at 2,786 cm⁻¹ corresponds to the C–H vibration of DMF. The peak between 3,471 and 3,356 cm⁻¹ corresponds to the stretching vibration of N–H.

Figure 3 shows the UV-vis diffuse reflection absorption spectra of the prepared samples. In Figure 3(a), the La/Zr-H₂BDC series of photocatalysts had good absorption in the UV region of 300–500 nm. In the range of 400–500 nm, the absorption capacity of the 0.25 La/Zr-H₂BDC photocatalyst was the highest. The maximum absorption wavelengths of Zr-H₂BDC, La-H₂BDC, 0.15 La/Zr-H₂BDC, 0.25 La/Zr-H₂BDC, 0.35 La/Zr-H₂BDC, and 0.50 La/Zr-H₂BDC were 355, 334, 361, 443, 373, and 365 nm, respectively. According to E_g = 1,240/λ_g (Xiong et al. 2021), the gap widths of Zr-H₂BDC, La-H₂BDC, 0.15 La/Zr-H₂BDC, 0.25 La/Zr-H₂BDC, 0.35 La/Zr-H₂BDC, and 0.50 La/Zr-H₂BDC were 3.49, 3.71, 3.43, 2.80, 3.32, and 3.40 eV, respectively.

In Figure 3(b), after modification with the amino group, the response abilities of the La/Zr-H₂ATA series of photocatalysts were enhanced in the visible-light region. In the visible light range of 400–600 nm, the 0.35 La/Zr-H₂ATA composite showed the strongest absorption and response. The maximum absorption wavelengths of Zr-H₂ATA, La-H₂ATA, 0.15 La/Zr-H₂ATA, 0.25 La/Zr-H₂ATA, 0.35 La/Zr-H₂ATA, and 0.50 La/Zr-H₂ATA were 456, 495, 462, 475, 499, and 461 nm, respectively. According to E_g = 1,240/λ_g, the gap widths of Zr-H₂ATA, La-H₂ATA, 0.15 La/Zr-H₂ATA, 0.25 La/Zr-H₂ATA, 0.35 La/Zr-H₂ATA, and 0.50 La/Zr-H₂ATA were 2.71, 2.51, 2.68, 2.61, 2.48, and 2.69 eV, respectively. Compared with the La/Zr-H₂BDC series of photocatalysts, the band gap energy of the La/Zr-H₂ATA series of photocatalysts became smaller, which was conducive to the photocatalytic degradation of pollutants driven by visible light.

Figure 4 shows the transmission electron microscopy (TEM) images and particle size distribution of the prepared La/Zr-H₂BDC and La/Zr-H₂ATA series. The TEM results showed that the composite materials were clumped together, which may have been caused by inhomogeneity during preparation. However, a certain distance existed between the particles, which could provide reaction sites for photocatalysis and improve the efficiency of the photocatalytic oxidation. The distribution curve of the average particle size showed a normal distribution. The average particle sizes of Zr-H₂BDC, Zr-H₂ATA, 0.25 La/Zr-H₂BDC, and 0.35 La/Zr-H₂ATA were 90.67, 309.1, 267.6, and 476.3 nm, respectively. The energy dispersive spectroscopic analysis of 0.35 La/Zr-H₂ATA showed the elemental composition of the heterostructure. Zr and La elements were detected in the structure, confirming the successful construction of the composite material. Table 1 shows the elemental composition of 0.35 La/Zr-H₂ATA energy spectrum analysis.

Electrochemical impedance (EIS) experiments were performed to further explore the photo-generated carrier migration. The arc radius of the EIS Nyquist curve reflected the resistance of the charge transfer at the interface between the electrode and the electrolyte solution. The greater the radius, the greater the resistance (Tong 2008). Figures S1(a) and (b) show that 0.25 La/Zr-H₂BDC and 0.35 La/Zr-H₂ATA possessed lower emission peaks than monomeric compounds. Such characteristic represented a faster electron transfer process and lower electron transfer resistance on the composite surface. Thus, the compound could reduce the recombination of photo-generated electrons and holes. The emission peak of 0.35 La/Zr-H₂ATA was significantly lower than that of 0.25 La/Zr-H₂BDC, and the photoelectric separation efficiency of the former was higher than that of the latter.

The flat-band potential of each photocatalyst was measured by the Mott-Schottky electrochemical experiment. The Fermi energy levels were calculated by comparing the potential and band potential of the reference electrode. The flat band-potential of n-type semiconductors is more negative than the hydrogen electrode potential, while the p-type semiconductor has a more positive potential than the oxygen electrode (Zhang et al. 2018). Figure 5 shows that all photocatalysts were n-type semiconductor photocatalysts. A tangent line to each curve was plotted, and the truncated value was the flat-band potential (E_FB vs. AgCl/Ag) of each catalyst. As shown in Figure 5(a), the flat-band potentials of Zr-H₂BDC, La-H₂BDC, 0.15 La/Zr-H₂BDC, 0.25 La/Zr-H₂BDC, 0.35 La/Zr-H₂BDC, and 0.50 La/Zr-H₂BDC were 0.225, 0.239, 0.2585, 0.220, 0.2545, and 0.2735 V (vs. AgCl/Ag), respectively. The potential of the AgCl/Ag reference electrode at the reaction system temperature was 0.24 V (Chen et al. 2020). The Fermi level potentials (E_f) of the H₂BDC series of photocatalysts relative to the standard hydrogen electrode potentials were 0.465, 0.479, 0.4985, 0.46, 0.4945, and 0.5135 eV, respectively. According to the formula 0 eV(vs. vacuum) = −4.5 V (vs. normal hydrogen electrode, NHE), the Fermi energy level potentials of the H₂BDC series photocatalysts were −4.965, −4.979, −4.9985, −4.96, −4.9945, and −5.0135 eV relative to the vacuum energy levels.

According to Figure 5(b), the flat-band potentials of Zr-H₂ATA, La-H₂ATA, 0.15 La/Zr-H₂ATA, 0.25 La/Zr-H₂ATA, 0.35 La/Zr-H₂ATA, and 0.50 La/Zr-H₂ATA were 0.127, −0.046, 0.2557, 0.2755, 0.118, and 0.2499 V (vs. AgCl/Ag), respectively. The corresponding Fermi level potentials relative to the standard hydrogen electrode potentials were 0.367, 0.194, 0.4957, 0.5155, 0.358, and 0.4899 eV. The corresponding Fermi level potentials of the H₂ATA series photocatalysts were −4.867, −4.694, −4.9957, −5.0155, −4.858, and −4.9899 eV, respectively.

For the n-type semiconductor photocatalyst, the conduction band (CB) potential (E_CB) of the photocatalyst was slightly higher than the Fermi level potential by 0.2 eV. In the calculation of the energy band of the photocatalyst, the difference value was selected as 0.2 eV, and Equations (1)–(4) were used to calculate the valence band (VB) (E_VB) and CB potentials of each photocatalyst relative to the vacuum level (Table 2).

The energy band level diagram of each photocatalyst is shown in Figure 6, with the vacuum level as reference. The HOMO orbital energy level potential (0.95 eV) of Rhodamine B was higher than the VB potential of the semiconductor photocatalyst. Rhodamine B could be oxidized by the photocatalyst (Shi et al. 2022). In Figure 6(a), the gap energy in the 0.25 La/Zr-H₂BDC photocatalyst was lower than that in the main Zr-H₂BDC photocatalyst, and VB also had the highest potential. The HOMO orbital of Rhodamine B was easier to match. Therefore, the photocatalytic degradation of Rhodamine B was more favorable.

As shown in Figure 7(a), the photocatalytic degradation of 0.25 La/Zr-H₂BDC was obviously better than those of Zr-H₂BDC and La-H₂BDC. In Figure 7(b), the photocatalytic performance of 0.35 La/Zr-H₂ATA was obviously better than those of Zr-H₂ATA and La-H₂ATA. As the mole ratio of La increased, the number of active free radicals produced by photocatalyst under photoexcitation increased, so the photocatalytic oxidation and degradation effect of Rhodamine B solution was enhanced. However, when the molar ratio exceeded 0.25 La/Zr-H₂BDC and 0.35 La/Zr-H₂ATA, the aggregation reaction of catalyst particles would occur, which would reflect and scatter visible light, thus the photocatalytic degradation rate was reduced. Therefore, when the molar ratio was 0.25 La/Zr-H₂BDC/0.35 La/Zr-H₂ATA, the photocatalytic activity was the highest and the degradation effect was the best, so they were the best ratios. Table 3 shows that, under the irradiation of a xenon lamp, the catalytic efficiency of the H₂ATA series was higher than that of the H₂BDC series. Amino modified MOFs increased the visible light absorption range. Thus, the photocatalytic degradation efficiency of the photocatalyst in the visible-light region could be improved.

Figure S3 shows the influence of different initial concentrations of Rhodamine B on the photocatalytic oxidation reaction. The initial concentration of Rhodamine B was adjusted to 10 mg/L, 15 mg/L, 20 mg/L, 25 mg/L and 30 mg/L, respectively. Under the same experimental conditions, the influence of the initial concentration of Rhodamine B on the photocatalytic oxidation reaction was explored. Figure S3(a) shows that 0.25 La/Zr-H₂BDC was used as the experimental photocatalyst. It can be seen from Figure S3(b) that as the concentration of Rhodamine B solution to be degraded did not increase, the oxidative degradation of Rhodamine B by 0.25 La/Zr-H₂BDC photocatalyst first increased and then decreased. Figure S3(c) takes 0.35 La/Zr-H₂ATA as the experimental photocatalyst. It can be seen from Figure S3(d) that as the concentration of Rhodamine B solution to be degraded no longer increases, the oxidative degradation of Rhodamine B by 0.35 La/Zr-H₂ATA photocatalyst first showed an increasing trend and then decreases. In contrast, the degradation rate of H₂ATA series was higher than that of H₂BDC series.

Under the condition of light irradiation, the number of active free radicals produced by photocatalyst under light excitation was limited and fixed in a certain period of time. In the case of low concentration of Rhodamine B solution, it could still meet the degradation requirements. However, with the continuous increase of Rhodamine B solution concentration, the generated active free radicals were not enough to meet the degradation requirements, so the degradation efficiency of the photocatalyst begins to decline.

According to the experimental results in Table 5, when the initial concentration of Rhodamine B solution was 20 mg/L, 0.25 La/Zr-H₂BDC and 0.35 La/Zr-H₂ATA had the best photocatalytic degradation effect on Rhodamine B solution.

Figure S4 shows the influence of different pH on photocatalytic oxidation reaction. pH was adjusted to 3, 5, 6, 7, 9 respectively under the same experimental conditions to explore the influence of pH on the photocatalytic oxidation reaction. The degradation rate of Rhodamine B showed great difference under different pH values, and the photocatalytic degradation reaction of Rhodamine B was better under acidic conditions. With the increase of pH value to alkaline, the degradation rate decreased and the photocatalytic effect weakens obviously. In contrast, the degradation rate of H₂ATA series was higher than that of H₂BDC series.

According to the experimental results in Table 6, when pH value was 3, 0.25 La/Zr-H₂BDC and 0.35 La/Zr-H₂ATA had the best photocatalytic degradation effect on Rhodamine B solution.

To further study the reaction mechanism, the role of free radicals and holes in the photocatalytic degradation was determined using free-radical and hole quenching experiments. With all other factors being equal, TEOA (0.1 mL), IPA (0.4 mL), and BQ (0.0050 g) were added to the 0.25 La/Zr-H₂BDC system. As shown in Figure S5(a), with the addition of the three quenching agents, varying degrees of inhibition of photocatalytic reactions were observed. After 8 h of photocatalytic degradation, the photocatalytic degradation efficiencies of TEOA, IPA, and BQ to Rhodamine B were 2.47%, 57.72%, and 79.9%, respectively. Hence, during the photocatalytic degradation of Rhodamine B, •O₂⁻ had little effect on the degradation, but h⁺ played a major role, followed by •OH.

With all other factors being equal, TEOA (0.1 mL), IPA (0.4 mL), and BQ (0.0050 g) were added to the 0.35 La/Zr-H₂ATA system. The results are shown in Figure S5(b), in which addition of the three quenching agents led to varying degrees of inhibition of the photocatalytic reactions. After 8 h of photocatalytic degradation, the photocatalytic degradation efficiencies of TEOA, IPA, and BQ to Rhodamine B were 4.43%, 87.76%, and 77.98%, respectively. Hence, during the photocatalytic degradation of Rhodamine B, •OH had little effect on the degradation, but h⁺ played a major role, followed by •O₂⁻.

Through the above experimental results and theoretical calculation, the possible photocatalytic reaction mechanism of La/Zr-H₂BDC and La/Zr-H₂ATA was deduced. As shown in Figure 8(a), the band gap energies of Zr-H₂BDC and La-H₂BDC obtained by UV-vis DRS were 3.49 eV and 3.71 eV, respectively. After visible-light irradiation, Zr-H₂BDC captured the photon energy from visible light, and the electrons in the VB were excited to transition to the CB. The corresponding holes in the position of the original valence band were retained. From the energy band matching of the two bands, the CB and VB potentials of Zr-H₂BDC were higher than those of La-H₂BDC. Therefore, the heterostructure of La/Zr-H₂BDC belonged to the typical type II heterojunction. When Zr-H₂BDC and La-H₂BDC came into contact to form a heterojunction, the energy band at the interface would be bent due to their different work functions. This phenomenon caused the photo-generated electrons to migrate from Zr-H₂BDC to the CB of La-H₂BDC, while the holes in the VB of La-H₂BDC migrated to the VB of Zr-H₂BDC until their Fermi energy levels reached equilibrium. In addition, the holes in the VB of Zr-H₂BDC reacted with the H₂O adsorbed on the surface of the catalyst to generate •OH radicals with strong oxidability. This process promoted the photocatalytic degradation of Rhodamine B. Therefore, the heterojunction interface between Zr-H₂BDC and La-H₂BDC could effectively separate the photo-generated electrons and holes, improve the utilization efficiency of visible light, and enhance the photocatalytic activity of visible light.

In Figure 8(b), the band gap energies of Zr-H₂ATA and La-H₂ATA calculated by UV-vis DRS were 2.71 eV and 2.51 eV, respectively. After visible-light irradiation, the energy of the photons in the visible light was captured by La-H₂ATA, and the electrons in the VB were excited to transition to the CB. This process left corresponding holes in the original VB position. In terms of the matching of the energy bands, when Zr-H₂ATA and La-H₂ATA were in contact to form a heterojunction, the energy band at the interface was bent due to their different work functions. This phenomenon promoted the migration of the photo-generated electrons from the La-H₂ATA to the Zr-H₂ATA CB. However, the holes in the upper VB of Zr-H₂ATA migrated to the VB of La-H₂ATA until the Fermi energy levels reached equilibrium. In addition, the photo-generated electrons in the Zr-H₂ATA CB reacted with O₂ to form •O₂⁻, which promoted the photocatalytic degradation of Rhodamine B. The efficiencies of the photo-generated electron and hole migration and separation were improved. Thus, the recombination of the photo-generated electrons and holes in the La-H₂ATA particles was significantly reduced. The photocatalytic activity of the heterojunction was improved.

The reusability and stability of the photocatalysts have always been the focus of industrialization. To assess the reusability and stability of photocatalysts, the used 0.25 La/Zr-H₂BDC and 0.35 La/Zr-H₂ATA photocatalysts were recovered, and the desorbed Rhodamine B on the photocatalyst was washed by deionized water and anhydrous ethanol. After drying, the dried catalyst was reused for the photocatalytic degradation of Rhodamine B solution under the same experimental conditions. The photocatalytic degradation effect is shown in Figure S6. The photocatalytic degradation efficiencies of Rhodamine B were 81.60%, 80.32%, 80.29%, 79.43%, and 92.01%, 91.20%, 90.90%, and 90.51% for 0.25 La/Zr-H₂BDC and 0.35 La/Zr-H₂ATA respectively. After recycling, the reusable photocatalytic degradation effect showed a slight decline. During recycling, the catalyst within the molecule Rhodamine B was probably not washed off completely. The photocatalyst experienced losses, decreasing the photocatalytic effect. However, after four times of repeated use in the assessment of photocatalytic degradation, the photocatalyst still showed a good stability and could be reused.

Twelve MOF photocatalytic materials with La and Zr as metal centers, with H₂BDC and H₂ATA as organic ligands were prepared by solvothermal method. Compared with the H₂BDC series, the H₂ATA series had stronger visible-light absorption. The 0.35 La/Zr-H₂ATA composite was the best photocatalyst for the photocatalytic degradation of Rhodamine B and showed photocatalytic degradation reaction rate constant at 0.01062 min⁻¹. The active species that played an important role in the photocatalytic degradation were h⁺ and •O₂⁻. Good stability of the photocatalyst was also observed in the cyclic experiments. In the photocatalytic reactions, compared with H₂BDC, the ligand H₂ATA was more conducive for the construction of the heterojunction system. This characteristic enhanced the absorption of visible light, promoted the transfer and separation of photocharges, and improved the photocatalytic activity of the composite catalyst.

This work was supported by the National Natural Science Foundation of China (22073011), Hunan Province Strategic New Major 14 Project (2019GK4041), Foundation of Hunan Educational Committee (21B0283) and Changsha Science and Technology Plan Project 15 (kq1907095).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.