To improve the photocatalytic degradation efficiency of photocatalytic materials UIO-66 and La-MOFs under visible-light irradiation, a series of photocatalytic materials with La and Zr as metal centers and terephthalic acid (H2BDC) and 2-amino terephthalic acid (H2ATA) as organic ligands were prepared by solvothermal method. The photocatalytic materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible (UV-vis) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and Mott-Schottky test. The photocatalytic degradation performance to Rhodamine B of the catalysts was fully investigated. Results show that the H2ATA series had stronger visible-light absorption capacity and better photocatalytic performance. The 0.35 La/Zr-H2ATA composite showed the best photocatalytic degradation. The quenching experiments confirmed that the active species in the photocatalytic degradation were the holes and superoxide radicals. The possible mechanisms of the carrier migration paths in the energy level matching for La/Zr-H2BDC and La/Zr-H2ATA were also discussed in detail.

  • A heterojunction photocatalyst 0.35 La/Zr-H2ATA has been synthesized and it possesses excellent stability.

  • The 0.35 La/Zr-H2ATA photocatalysts exhibited the apparent visible-light absorption and the weakened recombination of photogenerated electron/hole pairs.

  • Two possible photocatalytic reaction mechanisms have been proposed.

Graphical Abstract

Graphical Abstract
Graphical Abstract

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 and instruments

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).

Preparation of UIO-66 photocatalyst (Han et al. 2017)

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.

Preparation of La-MOFs photocatalyst

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.

Preparation of La/Zr-H2BDC and La/Zr-H2ATA photocatalysts

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.

Characterization methods

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: Eg = 1,240/λg, where λg is the absorption wavelength (nm), and Eg is the band gap energy (eV).

The flat-band potentials (EFB 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.

Photocatalytic performance test

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.

Effects of quenching agents on the photocatalytic degradation of Rhodamine B

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).

X-ray diffraction analysis

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

(a) XRD patterns of Zr-H2BDC, La-H2BDC and La/Zr-H2BDC; (b) XRD patterns of Zr-H2ATA, La-H2ATA and La/Zr-H2ATA.

Figure 1

(a) XRD patterns of Zr-H2BDC, La-H2BDC and La/Zr-H2BDC; (b) XRD patterns of Zr-H2ATA, La-H2ATA and La/Zr-H2ATA.

Close modal

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.

Infrared spectrum

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

(a) Infrared spectra of Zr-H2BDC, La-H2BDC and La/Zr-H2BDC; (b) Infrared spectra of Zr-H2ATA, La-H2ATA and La/Zr-H2ATA.

Figure 2

(a) Infrared spectra of Zr-H2BDC, La-H2BDC and La/Zr-H2BDC; (b) Infrared spectra of Zr-H2ATA, La-H2ATA and La/Zr-H2ATA.

Close modal

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.

UV-visible diffuse reflection

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 Eg = 1,240/λg (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.

Figure 3

(a) UV diffuse reflection spectra of Zr-H2BDC, La-H2BDC and La/Zr-H2BDC; (b) UV diffuse reflection spectra of Zr-H2ATA, La-H2ATA and La/Zr-H2ATA.

Figure 3

(a) UV diffuse reflection spectra of Zr-H2BDC, La-H2BDC and La/Zr-H2BDC; (b) UV diffuse reflection spectra of Zr-H2ATA, La-H2ATA and La/Zr-H2ATA.

Close modal

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 Eg = 1,240/λg, 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.

Transmission electron microscopy analysis and particle size test

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.

Table 1

Elemental composition of 0.35 La/Zr-H2ATA by energy dispersive spectroscopy analysis

ElementLine TypeWeight %Weight % SigmaAtomic %
Zr L series 30.64 0.33 9.97 
La L series 13.58 0.31 2.92 
K series 39.10 0.32 72.93 
Cl K series 16.86 0.18 14.19 
Total  100.00  100.00 
ElementLine TypeWeight %Weight % SigmaAtomic %
Zr L series 30.64 0.33 9.97 
La L series 13.58 0.31 2.92 
K series 39.10 0.32 72.93 
Cl K series 16.86 0.18 14.19 
Total  100.00  100.00 
Figure 4

(a,b) TEM of 0.25 La/Zr-H2BDC and 0.35 La/Zr-H2ATA; (c) Energy dispersive spectroscopy of 0.35 La/Zr-H2ATA; (d–g) Particle size distribution of Zr-H2BDC, Zr-H2ATA, 0.25 La/Zr-H2BDC and 0.35 La/Zr-H2ATA.

Figure 4

(a,b) TEM of 0.25 La/Zr-H2BDC and 0.35 La/Zr-H2ATA; (c) Energy dispersive spectroscopy of 0.35 La/Zr-H2ATA; (d–g) Particle size distribution of Zr-H2BDC, Zr-H2ATA, 0.25 La/Zr-H2BDC and 0.35 La/Zr-H2ATA.

Close modal

Electrochemical impedance test

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.

Mott-Schottky test

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 (EFB 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 (Ef) 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.

Figure 5

(a) Mott-Schottky curves of Zr-H2BDC, La-H2BDC and La/Zr-H2BDC; (b) Mott-Schottky curves of Zr-H2ATA, La-H2ATA and La/Zr-H2ATA.

Figure 5

(a) Mott-Schottky curves of Zr-H2BDC, La-H2BDC and La/Zr-H2BDC; (b) Mott-Schottky curves of Zr-H2ATA, La-H2ATA and La/Zr-H2ATA.

Close modal

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.

Energy band calculation of photocatalyst

For the n-type semiconductor photocatalyst, the conduction band (CB) potential (ECB) 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) (EVB) and CB potentials of each photocatalyst relative to the vacuum level (Table 2).

Table 2

Potential of each energy level of photocatalyst (relative to the vacuum pole)

CatalystsEf/(eV vs. vacuum)ECB/(eV vs. vacuum)Eg/eVEVB/(eV vs. vacuum)
Zr-H2BDC −4.965 −4.765 3.49 −8.255 
La-H2BDC −4.979 −4.779 3.71 −8.489 
0.15 La/Zr-H2BDC −4.9985 −4.7985 3.43 −8.2285 
0.25 La/Zr-H2BDC −4.96 −4.76 2.80 −7.56 
0.35 La/Zr-H2BDC −4.9945 −4.7945 3.32 −8.1145 
0.50 La/Zr-H2BDC −5.0135 −4.8135 3.40 −8.2135 
Zr-H2ATA −4.867 −4.667 2.71 −7.377 
La-H2ATA −4.694 −4.494 2.51 −7.004 
0.15 La/Zr-H2ATA −4.9957 −4.7957 2.68 −7.4757 
0.25 La/Zr-H2ATA −5.0155 −4.8155 2.61 −7.4255 
0.35 La/Zr-H2ATA −4.858 −4.658 2.48 −7.138 
0.50 La/Zr-H2ATA −4.9899 −4.7899 2.69 −7.4799 
CatalystsEf/(eV vs. vacuum)ECB/(eV vs. vacuum)Eg/eVEVB/(eV vs. vacuum)
Zr-H2BDC −4.965 −4.765 3.49 −8.255 
La-H2BDC −4.979 −4.779 3.71 −8.489 
0.15 La/Zr-H2BDC −4.9985 −4.7985 3.43 −8.2285 
0.25 La/Zr-H2BDC −4.96 −4.76 2.80 −7.56 
0.35 La/Zr-H2BDC −4.9945 −4.7945 3.32 −8.1145 
0.50 La/Zr-H2BDC −5.0135 −4.8135 3.40 −8.2135 
Zr-H2ATA −4.867 −4.667 2.71 −7.377 
La-H2ATA −4.694 −4.494 2.51 −7.004 
0.15 La/Zr-H2ATA −4.9957 −4.7957 2.68 −7.4757 
0.25 La/Zr-H2ATA −5.0155 −4.8155 2.61 −7.4255 
0.35 La/Zr-H2ATA −4.858 −4.658 2.48 −7.138 
0.50 La/Zr-H2ATA −4.9899 −4.7899 2.69 −7.4799 

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.

Figure 6

(a) Energy level band diagram of H2BDC series photocatalyst; (b) Energy level band diagram of H2ATA series photocatalyst.

Figure 6

(a) Energy level band diagram of H2BDC series photocatalyst; (b) Energy level band diagram of H2ATA series photocatalyst.

Close modal
In Figure 6(b), the gap energy in the 0.35 La/Zr-H2ATA photocatalyst was lower than that in the main Zr-H2ATA photocatalyst, and the VB also had the highest potential. The HOMO orbital of Rhodamine B was easier to match. Therefore, this photocatalyst was more favorable for the photocatalytic degradation of Rhodamine B. The gap energy of the H2ATA photocatalyst was smaller than that of the H2BDC photocatalyst, which increased the response ability of the former to the visible-light region and greatly improved its photocatalytic efficiency.
formula
(1)
formula
(2)
formula
(3)
formula
(4)

Photocatalytic performance for Rhodamine B

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.

Table 3

Catalytic degradation efficiency of Rhodamine B by each photocatalyst at 8 h

Photocatalyst (Metal center)H2BDC (ligands)H2ATA (ligands)
Zr 59.53% 65.47% 
La 31.88% 73.87% 
0.15 La/Zr 67.58% 66.01% 
0.25 La/Zr 81.60% 73.19% 
0.35 La/Zr 57.62% 92.01% 
0.50 La/Zr 71.68% 69.03% 
Photocatalyst (Metal center)H2BDC (ligands)H2ATA (ligands)
Zr 59.53% 65.47% 
La 31.88% 73.87% 
0.15 La/Zr 67.58% 66.01% 
0.25 La/Zr 81.60% 73.19% 
0.35 La/Zr 57.62% 92.01% 
0.50 La/Zr 71.68% 69.03% 
Figure 7

(a) Photocatalytic degradation efficiency of Rhodamine B by H2BDC series photocatalyst; (b) Photocatalytic degradation efficiency of Rhodamine B by H2ATA photocatalyst.

Figure 7

(a) Photocatalytic degradation efficiency of Rhodamine B by H2BDC series photocatalyst; (b) Photocatalytic degradation efficiency of Rhodamine B by H2ATA photocatalyst.

Close modal

Kinetic analysis

Figure S2 shows the ln (C0/C)-T diagram of the kinetics of the photocatalytic degradation of Rhodamine B on each photocatalyst after logarithmic fitting. The linear correlation coefficient (R2) of the photocatalytic degradation of Rhodamine B was more than 0.95. Thus, the catalytic degradation of Rhodamine B by MOFs may conform to the characteristics of first-order reaction kinetics. According to the first-order reaction kinetics (Sha et al. 2015):
formula
(5)
where k represents the reaction rate constant (min−1), and t represents the time of illumination reaction. The reaction rate constants of each photocatalyst in the degradation of Rhodamine B were determined and are shown in Table 4.
Table 4

Reaction rate constants of rhodamine B degradation by each photocatalyst

Serial numberPhotocatalystsDegradation rate constant (k/min−1)
Zr-H2BDC 0.00373 
La-H2BDC 0.00144 
0.15 La/Zr-H2BDC 0.00413 
0.25 La/Zr-H2BDC 0.00734 
0.35 La/Zr-H2BDC 0.00387 
0.50 La/Zr-H2BDC 0.0048 
Zr-H2ATA 0.00494 
La-H2ATA 0.00533 
0.15 La/Zr-H2ATA 0.00509 
10 0.25 La/Zr-H2ATA 0.00547 
11 0.35 La/Zr-H2ATA 0.01062 
12 0.50 La/Zr-H2ATA 0.00467 
Serial numberPhotocatalystsDegradation rate constant (k/min−1)
Zr-H2BDC 0.00373 
La-H2BDC 0.00144 
0.15 La/Zr-H2BDC 0.00413 
0.25 La/Zr-H2BDC 0.00734 
0.35 La/Zr-H2BDC 0.00387 
0.50 La/Zr-H2BDC 0.0048 
Zr-H2ATA 0.00494 
La-H2ATA 0.00533 
0.15 La/Zr-H2ATA 0.00509 
10 0.25 La/Zr-H2ATA 0.00547 
11 0.35 La/Zr-H2ATA 0.01062 
12 0.50 La/Zr-H2ATA 0.00467 

Effect of initial concentration of Rhodamine B on photocatalytic oxidation

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.

Table 5

Photocatalytic oxidation and degradation effects of Rhodamine B solutions with different initial concentrations

Initial concentration of Rhodamine B (mg/L)1015202530
Degradation rate of Rhodamine B by 0.25 La/Zr-H2BDC 61.6% 66.4% 81.6% 77.9% 69.6% 
Degradation rate of Rhodamine B by 0.35 La/Zr-H2ATA 69.6% 90.4% 92.0% 82.5% 80.1% 
Initial concentration of Rhodamine B (mg/L)1015202530
Degradation rate of Rhodamine B by 0.25 La/Zr-H2BDC 61.6% 66.4% 81.6% 77.9% 69.6% 
Degradation rate of Rhodamine B by 0.35 La/Zr-H2ATA 69.6% 90.4% 92.0% 82.5% 80.1% 

Influence of photocatalyst dosage on photocatalytic oxidation reaction

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.

Table 6

Photocatalytic oxidation and degradation effect of Rhodamine B with different pH

Different pH35679
Degradation rate of Rhodamine B by 0.25 La/Zr-H2BDC 78.3% 77.1% 75.0% 71.8% 61.4% 
Degradation rate of Rhodamine B by 0.35 La/Zr-H2ATA 84.3% 80.0% 74.2% 72.6% 68.3% 
Different pH35679
Degradation rate of Rhodamine B by 0.25 La/Zr-H2BDC 78.3% 77.1% 75.0% 71.8% 61.4% 
Degradation rate of Rhodamine B by 0.35 La/Zr-H2ATA 84.3% 80.0% 74.2% 72.6% 68.3% 

The reaction mechanism of photocatalytic degradation

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.

The photocatalytic properties of semiconductor materials are closely related to the transport mechanism of the electron–hole pairs. For the n-type semiconductor photocatalyst, the CB potential of the photocatalyst is slightly higher than the Fermi level potential by 0.2 eV. In the calculation of the energy band of the photocatalyst, the difference was selected to be 0.2 eV. The possible photocatalytic mechanism was further discussed according to the band structure of the MOFs. The VB and CB potentials of Zr-H2BDC, La-H2BDC, Zr-H2ATA, and La-H2ATA were calculated using Equations (6) and (7) (Li et al. 2020). The results are shown in Table 7.
formula
(6)
formula
(7)
Table 7

Potential of each energy level of photocatalyst (relative to the normal hydrogen electrode)

CatalystsEf/(eV vs. NHE)ECB/(eV vs. NHE)Eg/eVEVB/(eV vs. NHE)
Zr-H2BDC 0.225 0.025 3.49 3.515 
La-H2BDC 0.239 0.039 3.71 3.749 
Zr-H2ATA 0.127 −0.073 2.71 2.637 
La-H2ATA −0.046 −0.246 2.51 2.264 
CatalystsEf/(eV vs. NHE)ECB/(eV vs. NHE)Eg/eVEVB/(eV vs. NHE)
Zr-H2BDC 0.225 0.025 3.49 3.515 
La-H2BDC 0.239 0.039 3.71 3.749 
Zr-H2ATA 0.127 −0.073 2.71 2.637 
La-H2ATA −0.046 −0.246 2.51 2.264 

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.

Figure 8

(a) Schematic diagram of electron-hole pair separation and possible photocatalytic mechanism of Zr-H2BDC and La-H2BDC; (b) Schematic diagram of electron-hole pair separation and possible photocatalytic mechanism of Zr-H2ATA and La-H2ATA.

Figure 8

(a) Schematic diagram of electron-hole pair separation and possible photocatalytic mechanism of Zr-H2BDC and La-H2BDC; (b) Schematic diagram of electron-hole pair separation and possible photocatalytic mechanism of Zr-H2ATA and La-H2ATA.

Close modal

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.

Cycle experiment

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.

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Supplementary data