Preparation of Salen – Metal Complexes ( Metal = Co or Ni ) Intercalated ZnCr-LDHs and Their Photocatalytic Degradation of Rhodamine B

Salen–metal complexes (SalenM) were successfully intercalated into ZnCr layered double hydroxides (LDHs) through coprecipitation method, then a series of novel organic–inorganic hybrid materials were obtained. The structure and properties of the materials were thoroughly characterized by inductively-coupled plasma atomic emission spectrometry (ICP-AES), powder X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), and ultraviolet visible diffuse reflectance spectroscopy (UV-Vis DRS). Meanwhile, with Rhodamine B (RhB) as a target contaminant, the photocatalytic activities of SalenM-intercalated ZnCr-LDHs were investigated and compared with the traditional LDHs (ZnCr-LDHs, ZnCoCr-LDHs, and ZnNiCr-LDHs). Furthermore, the effect of the intercalation amount of SalenM (M = Co or Ni) on the photocatalytic activity was studied. The results showed that when the molar ratio of SalenM to Cr was 0.75, SalenM-intercalated ZnCr-LDHs exhibited significantly higher photocatalytic activities than the traditional LDHs. The degradation rates of RhB reached about 90%, and all of them had good recycling rates. In addition, the kinetics of photocatalytic process and the mechanism of photocatalysis are discussed.


Introduction
Recently, with the transformation of fiber, textile, printing, dyeing, and garment industries, the global dyeing and printing industry has developed rapidly.However, due to the development of the dyeing and printing industry, wastewater discharge has also experienced a substantial increase.At present, the treatment methods of printing and dyeing wastewater are mainly adsorption and photocatalytic degradation [1][2][3].
Layered double hydroxides (LDHs) are a class of anionic clay materials with octahedral layer structure.Their general formula is [M 2+  1−x M 3+ x (OH) 2 ] x+ (A n− ) x/n •mH 2 O, where M 2+ and M 3+ are convertible metal cations in host layers and A n− are exchangeable anions in the channel between the host layers [4,5].LDHs are often used as host structures for the loading of various complex anions.These materials exhibit many unique characteristics, such as large number of active alkali sites, large specific surface area, and high anion exchange capacity.They have been widely used in catalytic reactions and catalyst supports [6,7].For the treatment of printing and dyeing wastewater, LDHs can be used as adsorbents and photocatalysts [8][9][10][11].Because of the dual effects of surface adsorption and the interlayer adsorption of anions, LDHs can efficiently adsorb anionic contaminants in dyeing and printing wastewater [12].However, the adsorption capacity of LDHs on cationic contaminants is relatively low-only a small amount of physical adsorption [13].Moreover, the adsorption method has some disadvantages; that is, it may easily cause secondary pollution and cannot completely degrade contaminants [14].Therefore, the photocatalytic degradation method has become a research hotspot for the treatment of cationic contaminants because of its advantages of being green non-toxic, having mild reaction conditions, high efficiency, less secondary pollution, and thorough degradation [15][16][17].Rhodamine B (RhB) is one of the cationic contaminants with a carcinogenic effect on humans.It was found that the adsorption capacity and photocatalytic degradation efficiency of RhB by traditional LDHs were relatively low.In our previous work, 3 g/L CuMgAl-LDHs were used as catalysts to degrade 5 mg/L RhB solution; the adsorption and photocatalytic degradation rates were about 30% and 50%, respectively [18].Therefore, how to introduce other highly photoactive complexes to improve the photocatalytic ability of LDHs has become our new concern.Salen-metal complexes (SalenM) with photoactive metalloporphyrin structure [19][20][21] can be excited by visible light, and then oxygen molecules are activated to oxidize organic contaminants [22].Taking advantage of the exchangeability of anions in LDHs, SalenM can be intercalated into LDHs to become a class of organic-inorganic hybrid composites with supramolecular structure [23].Such composites may become effective heterogeneous photocatalysts with synergistic effects of adsorption and photocatalysis achieving adsorption-photocatalysis-adsorption cycles.However, the study of the heterogeneous photocatalytic degradation of cationic contaminants by composite materials composed of SalenM and LDHs has so far been rarely reported [24].
In this paper, intercalated ZnCr-LDHs with different ratios of SalenM (M = Co or Ni) to Cr were prepared by coprecipitation method.Under simulated visible light, using the typical cationic dye RhB as the target contaminant, the photocatalytic activities of the intercalated LDHs were studied and compared with the traditional LDHs.Meanwhile, the kinetics of photocatalysis and photocatalytic mechanism were also discussed.

Characterization of Materials
Inductively-coupled plasma atomic emission spectrometry (ICP-AES) analyses were performed to determine the metallic composition of the successfully-intercalated LDHs.As shown in Table 1, the experimental mole ratios of Zn:M:Cr (M = Co or Ni) in the successfully intercalated LDHs were close to the prospective mole ratios of Zn:M:Cr (M = Co or Ni) in the starting materials.Figure 1A shows the XRD patterns of ZnCr-LDHs (a), ZnCoCr-LDHs (b), and ZnNiCr-LDHs (c).The obvious diffraction peaks (003), (006), (009), and (110) of curves a, b, and c in Figure 1A indicate that the synthetic traditional LDHs had typical lamellar structure [25,26].The (003) lattice diffraction peaks of the traditional LDHs (a-c) appeared at 2θ = 9.99 • -11.18 • ; thus, the interlayer distances(d 003 ) were 0.79 nm to 0.88 nm, which were similar to the nitrate-intercalated LDHs [27] (see Table 2).
Figure 1B,C show the XRD patterns of ZnCr-nSalenCo-LDHs and ZnCr-nSalenNi-LDHs with different amounts of Salen-cobalt and Salen-nickel.As illustrated in Figure 1B,C, the characteristic diffraction peaks of LDHs and some other new peaks appeared.These new peaks were basically lined up with the peaks in the XRD patterns of SalenM (See Figure S1), indicating that they were derived from SalenM.Meanwhile, according to the results of ICP-AES (see Table 1), the contents of Co and Ni in the SalenM-intercalated LDHs were slightly larger than those of the prospective materials.Therefore, these new peaks in the XRD patterns of SalenM-intercalated LDHs may correspond to a small amount of SalenM which was not successfully intercalated.In addition, when the ratios of SalenM to Cr (n) were 0.1 and 0.25, the (003) and (006) diffraction peaks were basically the same as those of ZnCr-LDHs (a), indicating that SalenM was not successfully intercalated.Additionally, when n was greater than 0.25, the (003) and (006)-plane diffraction peaks of the intercalated LDHs had both shifted to lower angles.The intercalation of SalenM led to an increase in the interlayer distances (d 003 = 1.10-1.12nm), which were significantly larger than those of the traditional LDHs (see Table 2).Subtracting the thickness of the LDHs layer (0.48 nm) [28], the interlayer channel heights were from 0.62 nm to 0.64 nm.Then, to more clearly understand the intercalation of ZnCr-nSalenM-LDHs (n = 0.5, 0.75, 1), SalenCo and SalenNi were simulated from Materials Studio 5.5 software.The calculated sizes of SalenCo and SalenNi were 1.25 nm × 0.97 nm × 0.56 nm and 1.35 nm × 0.96 nm × 0.38 nm, respectively (see Figure S2).Thus, it can be inferred that the anions of SalenM are slightly inclined between the ZnCr-nSalenM-LDHs (n = 0.5, 0.75, 1) layers along the orientation of the short axis of SalenM.
Catalysts 2017, 7, 143 3 of 14 the SalenM-intercalated LDHs were slightly larger than those of the prospective materials.Therefore, these new peaks in the XRD patterns of SalenM-intercalated LDHs may correspond to a small amount of SalenM which was not successfully intercalated.In addition, when the ratios of SalenM to Cr (n) were 0.1 and 0.25, the (003) and (006) diffraction peaks were basically the same as those of ZnCr-LDHs (a), indicating that SalenM was not successfully intercalated.Additionally, when n was greater than 0.25, the (003) and (006)-plane diffraction peaks of the intercalated LDHs had both shifted to lower angles.The intercalation of SalenM led to an increase in the interlayer distances (d003 = 1.10-1.12nm), which were significantly larger than those of the traditional LDHs (see Table 2).Subtracting the thickness of the LDHs layer (0.48 nm) [28], the interlayer channel heights were from 0.62 nm to 0.64 nm.Then, to more clearly understand the intercalation of ZnCr-nSalenM-LDHs (n = 0.5, 0.75, 1), SalenCo and SalenNi were simulated from Materials Studio 5.5 software.The calculated sizes of SalenCo and SalenNi were 1.25 nm × 0.97 nm × 0.56 nm and 1.35 nm × 0.96 nm × 0.38 nm, respectively (see Figure S2).Thus, it can be inferred that the anions of SalenM are slightly inclined between the ZnCr-nSalenM-LDHs (n = 0.5, 0.75, 1) layers along the orientation of the short axis of SalenM.The infrared spectra of SalenCo (a), SalenNi (b), ZnCr-LDHs (c), ZnCr-0.75SalenCo-LDHs(d), and ZnCr-0.75SalenNi-LDHs(e) are shown in Figure 2. In the IR spectra of all LDHs, the broad absorption peaks appearing at about 3400 cm −1 can be attributed to the stretching vibration of hydroxyl groups in the LDH layers and interlayer water molecules.For SalenCo (a), the characteristic absorption at 1635 cm −1 was assigned to C=N stretching of imine bond [29]; the absorption peaks at 1400 cm −1 and 1545 cm −1 were assigned to the symmetric and asymmetric stretching vibrations of C=O bonds of -COO-groups [30]; the disappearance of the characteristic bands at 1700-1800 cm −1 indicated that -COOH groups were completely deprotonated [31].In the case of SalenNi (a), the IR spectrum was similar to that of SalenCo.For ZnCr-LDHs (c), the sharp strong absorption peak at 1380 cm −1 can be attributed to the NO 3 − stretching vibration [32], indicating that the main anion between the ZnCr-LDHs layers was NO 3 − .Then, for ZnCr-0.75SalenCo-LDHs(d) and ZnCr-0.75SalenNi-LDHs(e), the absence of the NO 3 − stretching vibration bands indicated that the intercalation processes had been completed.In addition, the characteristic absorption peaks of C=N bonds and -COO-groups in the IR spectra of ZnCr-0.75SalenCo-LDHs(d) and ZnCr-0.75SalenNi-LDHs(e) were similar to those of SalenM (e).Meanwhile, a series of absorption peaks appearing in the IR spectra of 400-800 cm −1 may correspond to the lattice vibrations of Zn-O, Cr-O bonds derived from LDHs and the stretching vibrations of Co-O, Ni-O, Co-N, Ni-N bonds derived from SalenM [33,34].Based on the above results, we can further infer that SalenM have successfully entered the LDHs interlayer.
Catalysts 2017, 7, 143 4 of 14 The infrared spectra of SalenCo (a), SalenNi (b), ZnCr-LDHs (c), ZnCr-0.75SalenCo-LDHs(d), and ZnCr-0.75SalenNi-LDHs(e) are shown in Figure 2. In the IR spectra of all LDHs, the broad absorption peaks appearing at about 3400 cm −1 can be attributed to the stretching vibration of hydroxyl groups in the LDH layers and interlayer water molecules.For SalenCo (a), the characteristic absorption at 1635 cm −1 was assigned to C=N stretching of imine bond [29]; the absorption peaks at 1400 cm −1 and 1545 cm −1 were assigned to the symmetric and asymmetric stretching vibrations of C=O bonds of -COO-groups [30]; the disappearance of the characteristic bands at 1700-1800 cm −1 indicated that -COOH groups were completely deprotonated [31].In the case of SalenNi (a), the IR spectrum was similar to that of SalenCo.For ZnCr-LDHs (c), the sharp strong absorption peak at 1380 cm −1 can be attributed to the NO3 − stretching vibration [32], indicating that the main anion between the ZnCr-LDHs layers was NO3 − .Then, for ZnCr-0.75SalenCo-LDHs(d) and ZnCr-0.75SalenNi-LDHs(e), the absence of the NO3 − stretching vibration bands indicated that the intercalation processes had been completed.In addition, the characteristic absorption peaks of C=N bonds and -COO-groups in the IR spectra of ZnCr-0.75SalenCo-LDHs(d) and ZnCr-0.75SalenNi-LDHs(e) were similar to those of SalenM (e).Meanwhile, a series of absorption peaks appearing in the IR spectra of 400-800 cm −1 may correspond to the lattice vibrations of Zn-O, Cr-O bonds derived from LDHs and the stretching vibrations of Co-O, Ni-O, Co-N, Ni-N bonds derived from SalenM [33,34].Based on the above results, we can further infer that SalenM have successfully entered the LDHs interlayer.The scanning electron microscopy (SEM) images in Figure 3 present the morphologies of ZnCr-LDHs, ZnCr-0.75SalenCo-LDHs, and ZnCr-0.75SalenNi-LDHs.From Figure 3a,b, it is clear that ZnCr-LDHs displayed a typical layered lamellar-shaped morphology of LDHs.ZnCr-0.75SalenCo-LDHs(See Figure 3c,d) and ZnCr-0.75SalenNi-LDHs(See Figure 3e,f) also displayed layered structures similar to the pristine ZnCr-LDHs, confirming that the processes of intercalation did not change the typical morphologies of LDHs.
As is well known, the light absorption and light-induced migration of electrons and holes are the key factors in controlling the photocatalytic process [35].The behavior of the materials in the UV-Vis diffuse reflectance indicates the photo-absorption ability.Previous studies showed that the narrowed-down band gap may lead to the enhancement of the responsiveness to visible light, as well as charge transfer and separation [36,37].According to the literature, the band gap of the material can The scanning electron microscopy (SEM) images in Figure 3 present the morphologies of ZnCr-LDHs, ZnCr-0.75SalenCo-LDHs, and ZnCr-0.75SalenNi-LDHs.From Figure 3a,b, it is clear that ZnCr-LDHs displayed a typical layered lamellar-shaped morphology of LDHs.ZnCr-0.75SalenCo-LDHs(See Figure 3c,d) and ZnCr-0.75SalenNi-LDHs(See Figure 3e,f) also displayed Catalysts 2017, 7, 143 5 of 15 layered structures similar to the pristine ZnCr-LDHs, confirming that the processes of intercalation did not change the typical morphologies of LDHs.
As is well known, the light absorption and light-induced migration of electrons and holes are the key factors in controlling the photocatalytic process [35].The behavior of the materials in the UV-Vis diffuse reflectance indicates the photo-absorption ability.Previous studies showed that the narrowed-down band gap may lead to the enhancement of the responsiveness to visible light, as well as charge transfer and separation [36,37].According to the literature, the band gap of the material can be calculated using the simplified Kubelka-Munk formula E g = 1240/λ (nm) [38], where λ is the wavelength corresponding to the absorption onset.The UV-Vis diffuse reflectance spectra of the traditional LDHs and SalenM-intercalated LDHs are shown in Figure 4.All of the LDHs presented a certain response in the visible region.The band gaps of ZnCr-LDHs, ZnCoCr-LDHs, and ZnNiCr-LDHs are 2.39 eV, 2.36 eV, and 2.21 eV.While, ZnCr-0.5SalenCo-LDHs,ZnCr-0.75SalenCo-LDHs,ZnCr-1SalenCo-LDHs, ZnCr-0.5SalenNi-LDHs,ZnCr-0.75SalenNi-LDHs, and ZnCr-1SalenNi-LDHs showed narrower band gaps of 2.01 eV, 1.94 eV, 2.04 eV, 1.93 eV, 1.91 eV, and 1.97 eV, which were also narrower than other traditional photocatalytic materials such as zinc oxide [39] and titanium dioxide [40].Therefore, the above results implied that the synergetic effect between SalenM and LDHs can narrow the band gaps and enhance the visible light harvesting of the materials, which contributed to improving their photocatalytic activities.
Catalysts 2017, 7, 143 5 of 14 also narrower than other traditional photocatalytic materials such as zinc oxide [39] and titanium dioxide [40].Therefore, the above results implied that the synergetic effect between SalenM and LDHs can narrow the band gaps and enhance the visible light harvesting of the materials, which contributed to improving their photocatalytic activities.

Photocatalytic Activity
The photocatalytic activities of the materials were investigated by the degradation of RhB (5 mg/L).Figure 5 shows the results of photocatalytic activities.It can be seen that the degradation rate of blank experiment (without catalyst) is very low, showing that RhB is relatively stable.As shown in Figure 5, the photocatalytic activities of SalenM-intercalated LDHs were greatly improved compared with the pristine ZnCr-LDHs.Additionally, with the increasing amount of SalenM intercalation, the degradation rates of RhB first increased and then decreased.The optimal ratio of intercalation was n = 0.75 (M:Cr); after 7 h photodegradation, the degradation rates of ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHs to RhB reached 87.9% and 91.3%, respectively.When the intercalation ratio was n = 1 (M:Cr), the photocatalytic activities of the materials decreased.This may be because the metal complexes easily form a dimer with low activity when the ratio of intercalation is too large [41].In addition, the metal complexes may occupy the adsorption sites on the surface of the carrier when their amount is too large [42].This will affect the adsorption of oxygen and RhB molecules by the LDHs, thus hindering the photocatalytic reaction.At the same time, we also conducted controlled photocatalytic degradation experiments with other traditional LDHs as catalysts.When the ZnCr-LDHs layer was doped with photosensitive metal (Co or Ni), the photocatalytic activities were not significantly improved.After 7 h photodegradation reaction, the degradation rates of ZnCoCr-LDHs and ZnNiCr-LDHs to RhB were only 42.10% and 45.74%.Meanwhile, in similar experimental conditions, ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHsalso exhibited better photocatalytic activities of RhB degradation than some other types of photocatalytic materials such as TiO 2 -diatomite composite [43], CdMoO 4 @CdS [44], and BiVO 4 /TiO 2 [45].

Photocatalytic Activity
The photocatalytic activities of the materials were investigated by the degradation of RhB (5 mg/L).Figure 5 shows the results of photocatalytic activities.It can be seen that the degradation rate of blank experiment (without catalyst) is very low, showing that RhB is relatively stable.As shown in Figure 5, the photocatalytic activities of SalenM-intercalated LDHs were greatly improved compared with the pristine ZnCr-LDHs.Additionally, with the increasing amount of SalenM intercalation, the degradation rates of RhB first increased and then decreased.The optimal ratio of intercalation was n = 0.75 (M:Cr); after 7 h photodegradation, the degradation rates of ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHs to RhB reached 87.9% and 91.3%, respectively.When the intercalation ratio was n = 1 (M:Cr), the photocatalytic activities of the materials decreased.This may be because the metal complexes easily form a dimer with low activity when the ratio of intercalation is too large [41].In addition, the metal complexes may occupy the adsorption sites on the surface of the carrier when their amount is too large [42].This will affect the adsorption of oxygen and RhB molecules by the LDHs, thus hindering the photocatalytic reaction.At the same time, we also conducted controlled photocatalytic degradation experiments with other traditional LDHs as catalysts.When the ZnCr-LDHs layer was doped with photosensitive metal (Co or Ni), the photocatalytic activities were not significantly improved.After 7 h photodegradation reaction, the degradation rates of ZnCoCr-LDHs and ZnNiCr-LDHs to RhB were only 42.10% and 45.74%.Meanwhile, in similar experimental conditions, ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHsalso exhibited better photocatalytic activities of RhB degradation than some other types of photocatalytic materials such as TiO2-diatomite composite [43], CdMoO4@CdS [44], and BiVO4/TiO2 [45].The kinetic parameters of RhB photocatalytic degradation by ZnCr-0.75SBCo-LDHs,ZnCr-0.75SBNi-LDHs, and traditional LDHs were investigated based on the Langmuir-Hinshelwood model [46,47].When the molecular adsorption reaches equilibrium, the reaction kinetic equation can be described as: The kinetic parameters of RhB photocatalytic degradation by ZnCr-0.75SBCo-LDHs,ZnCr-0.75SBNi-LDHs, and traditional LDHs were investigated based on the Langmuir-Hinshelwood model [46,47].When the molecular adsorption reaches equilibrium, the reaction kinetic equation can be described as: where k r is the reaction rate constant, K ad is the adsorption equilibrium constant, and C is the concentration of RhB at time t.Since the concentration of RhB in the solution is very low and the adsorption of the material is weak (K ad C << 1), Equation ( 1) can be simplified as: Integrating ( 2), the following equation is obtained: where C 0 is the concentration of RhB solution after adsorption-desorption equilibrium and K app is the pseudo-first-order apparent rate constant.Figure 6 is the photodegradation fitted linear plot of RhB, using ln (C 0 /C) as the ordinate and light time t for the abscissa.It can be seen that all of the linearly-dependent coefficients are over 0.95, so the processes of RhB degradation by ZnCr-0.75SalenCo-LDHs,ZnCr-0.75SalenNi-LDHs, and traditional LDHs are consistent with pseudo-first-order kinetic model.The apparent rate constant (K app ) can be calculated from the gradient of fitted lines.In addition, the values of the K app and the corresponding half-life t 1/2 are listed in Table 3.
The results showed that the reaction rates of RhB photocatalytic degradation by SalenM-intercalated LDHs were much higher than traditional LDHs, and they were considerably efficient.
Catalysts 2017, 7, 143 7 of 14 Integrating (2), the following equation is obtained: where C0 is the concentration of RhB solution after adsorption-desorption equilibrium and Kapp is the pseudo-first-order apparent rate constant.Figure 6 is the photodegradation fitted linear plot of RhB, using ln (C0/C) as the ordinate and light time t for the abscissa.It can be seen that all of the linearlydependent coefficients are over 0.95, so the processes of RhB degradation by ZnCr-0.75SalenCo-LDHs,ZnCr-0.75SalenNi-LDHs, and traditional LDHs are consistent with pseudo-first-order kinetic model.The apparent rate constant (Kapp) can be calculated from the gradient of fitted lines.In addition, the values of the Kapp and the corresponding half-life t1/2 are listed in Table 3.The results showed that the reaction rates of RhB photocatalytic degradation by SalenM-intercalated LDHs were much higher than traditional LDHs, and they were considerably efficient.Then, the activation energies of the photocatalytic reactions with ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHsas catalysts were investigated based on the results of RhB photodegradation at different temperatures (288 K, 298 K, 308 K).As shown in Figure S3 and S4, all of the linearly dependent coefficients were over 0.95, indicating that the processes of RhB degradation by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHs at different temperatures were consistent with a pseudo-first-order kinetic model.Therefore, the values of activation energies can be calculated by Arrhenius equation:  Then, the activation energies of the photocatalytic reactions with ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHsas catalysts were investigated based on the results of RhB photodegradation at different temperatures (288 K, 298 K, 308 K).As shown in Figures S3 and S4, all of the linearly dependent coefficients were over 0.95, indicating that the processes of RhB degradation by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHs at different temperatures were consistent with a pseudo-first-order kinetic model.Therefore, the values of activation energies can be calculated by Arrhenius equation: where A is the pre-exponential factor, E a is the apparent activation energy, R is the universal gas constant, T is the absolute temperature.Taking the natural logarithm of the Arrhenius equation, the following equation is obtained: lnK app = −E a /RT + lnA (5) Figure 7 shows the Arrhenius plots for RhB photodegradation by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHs, in which lnK app demonstrates a good linear dependence relation with T −1 .Based on the slopes of the Arrhenius plots, the apparent activation energies (E a ) were 17.32 kJ/mol and 15.63 kJ/mol, respectively.It was apparent that the RhB photodegradation tests by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHshad relatively low apparent activation energies similar to other photocatalytic reactions [48].The low apparent activation energies also showed that the temperature had little effect on the reaction rate.
Catalysts 2017, 7, 143 8 of 14 Based on the slopes of the Arrhenius plots, the apparent activation energies (Ea) were 17.32 kJ/mol and 15.63 kJ/mol, respectively.It was apparent that the RhB photodegradation tests by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHshad relatively low apparent activation energies similar to other photocatalytic reactions [48].The low apparent activation energies also showed that the temperature had little effect on the reaction rate.

Recycling of Catalytic Materials
In practical application, the photocatalytic stability of catalytic materials is of great significance.Traditional photocatalytic materials such as some TiO2-based materials are restricted in their application because they are easily deactivated and are difficult to recover effectively during photocatalysis [49].To study the stability and recyclability of the catalytic materials, ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHswere collected and reused under the same experimental condition.The XRD patterns of ZnCr-0.75SalenM-LDHs(M = Co or Ni) before and after four cycles of photodegradation tests are shown in Figure 8.It was obvious that the characteristic diffraction peaks of the recycled materials differ negligibly from the corresponding peaks of the fresh materials.Therefore, the well-ordered layered structures of ZnCr-0.75SalenM-LDHs were retained after four cycles of photocatalytic tests, suggesting that the materials were stable.Figure 9 shows the photocatalytic results of four successive cycles of the materials.The photodegradation rates of RhB by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHsdecreased 13.1% and 13.4%, respectively.It was obvious that the photocatalytic activities of ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHswere not significantly reduced after four cycles.Therefore, these two materials will be potential photocatalysts with good performance, strong stability, and high reusability.Arrhenius plots for RhB photodegradation by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHs.

Recycling of Catalytic Materials
In practical application, the photocatalytic stability of catalytic materials is of great significance.Traditional photocatalytic materials such as some TiO 2 -based materials are restricted in their application because they are easily deactivated and are difficult to recover effectively during photocatalysis [49].To study the stability and recyclability of the catalytic materials, ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHswere collected and reused under the same experimental condition.The XRD patterns of ZnCr-0.75SalenM-LDHs(M = Co or Ni) before and after four cycles of photodegradation tests are shown in Figure 8.It was obvious that the characteristic diffraction peaks of the recycled materials differ negligibly from the corresponding peaks of the fresh materials.Therefore, the well-ordered layered structures of ZnCr-0.75SalenM-LDHs were retained after four cycles of photocatalytic tests, suggesting that the materials were stable.Figure 9 shows the photocatalytic results of four successive cycles of the materials.The photodegradation rates of RhB by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHsdecreased 13.1% and 13.4%, respectively.It was obvious that the photocatalytic activities of ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHswere not significantly reduced after four cycles.Therefore, these two materials will be potential photocatalysts with good performance, strong stability, and high reusability.
diffraction peaks of the recycled materials differ negligibly from the corresponding peaks of the fresh materials.Therefore, the well-ordered layered structures of ZnCr-0.75SalenM-LDHs were retained after four cycles of photocatalytic tests, suggesting that the materials were stable.Figure 9 shows the photocatalytic results of four successive cycles of the materials.The photodegradation rates of RhB by ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHsdecreased 13.1% and 13.4%, respectively.It was obvious that the photocatalytic activities of ZnCr-0.75SalenCo-LDHs and ZnCr-0.75SalenNi-LDHswere not significantly reduced after four cycles.Therefore, these two materials will be potential photocatalysts with good performance, strong stability, and high reusability.

Mechanism of Photocatalytic Degradation
Generally speaking, light absorption by the catalytic materials and migration of the lightinduced electrons and holes are the key factors in the process of photocatalytic degradation.In the process of the photocatalytic degradation of RhB, when ZnCr-nSalenM-LDHs adsorbed photons with energy larger than or equal to its band gap, the excited valance band electrons transitioned to conduction band-forming electrons (eCB − ) and holes (hVB + ) (Equation 6).The photogenerated electrons (eCB − ) reacted with dissolved oxygen to produce •O2 − radicals (Equation 7).Meanwhile, the photogenerated holes (hVB + ) further reacted with H2O or OH − to produce •OH radicals (Equation 8).Then, these active species (•O2 − and •OH) interacted with RhB to carry out a redox reaction (Equation 9).In addition, part of the photogenerated holes (hVB+) can also directly oxidize RhB (Equation 10) [50][51][52].

Mechanism of Photocatalytic Degradation
Generally speaking, light absorption by the catalytic and migration of the light-induced electrons and holes are the key factors in the process of photocatalytic degradation.In the process of the photocatalytic degradation of RhB, when ZnCr-nSalenM-LDHs adsorbed photons with energy larger than or equal to its band gap, the excited valance band electrons transitioned to conduction band-forming electrons (e CB − ) and holes (h VB + ) (Equation 6).The photogenerated electrons (e CB − ) reacted with dissolved oxygen to produce •O 2 − radicals (Equation 7).Meanwhile, the photogenerated holes (h VB + ) further reacted with H 2 O or OH − to produce •OH radicals (Equation 8).Then, these active species (•O 2 − and •OH) interacted with RhB to carry out a redox reaction (Equation 9).In addition, part of the photogenerated holes (h VB +) can also directly oxidize RhB (Equation 10) [50][51][52].
LDHs It was recently reported that SalenM in visible light can be excited to transition to the excited state, and would lose electrons involved in photocatalytic degradation reaction [53].Thus, we believed that the SalenM between the LDHs layers also played a corresponding role in the photocatalytic process.Under the excitation of visible light, SalenM turned to the excited state SalenM* and then lost electrons, forming SalenM + (Equation 11).The lost electrons entered the conduction band of ZnCr-nSalenM-LDHs (Equation 12), thus effectively suppressing the recombination of electrons and holes, and improving the photocatalytic ability.Meanwhile, SalenM + can take electrons from RhB to degrade it and return to SalenM (Equation 13), thereby achieving a catalytic degradation cycle [54,55].Therefore, the photocatalytic activities of SalenM-intercalated LDHs were greatly improved compared with the traditional LDHs.

Synthesis of SalenM
The synthesis process of SalenM is shown in Figure 11.Ethanol solution (50 mL) containing 50 mmol salicylaldehyde was slowly added dropwise to the ethanol solution (100 mL) containing 50 mmol p-aminobenzoic acid.After dropwise addition with continuous stirring, the reaction system was heated to reflux for 3 h.Then, the resultant slurry was cooled, filtered, washed, and dried at 85 °C for 12 h to obtain a bright yellow needle-like Salen.Then, 6 mmol Salen was dissolved in 100 mL ethanol and the pH was adjusted to 7.0 with 0.2 mol/L NaOH ethanol solution.After that, 10 mL aqueous solution containing 3 mmol M(CH3COO)2•4H2O was slowly added dropwise to the above solution.The resulting turbid solution was heated to reflux for 3 h.Finally, the slurry was cooled, filtered, washed, and dried at 85 °C for 12 h to get SalenM (M = Co or Ni).

Synthesis of SalenM
The synthesis process of SalenM is shown in Figure 11.Ethanol solution (50 mL) containing 50 mmol salicylaldehyde was slowly added dropwise to the ethanol solution (100 mL) containing 50 mmol p-aminobenzoic acid.After dropwise addition with continuous stirring, the reaction system was heated to reflux for 3 h.Then, the resultant slurry was cooled, filtered, washed, and dried at 85 • C for 12 h to obtain a bright yellow needle-like Salen.Then, 6 mmol Salen was dissolved in 100 mL ethanol and the pH was adjusted to 7.0 with 0.2 mol/L NaOH ethanol solution.After that, 10 mL aqueous solution containing 3 mmol M(CH 3 COO) 2 •4H 2 O was slowly added dropwise to the above solution.The resulting turbid solution was heated to reflux for 3 h.Finally, the slurry was cooled, filtered, washed, and dried at 85 • C for 12 h to get SalenM (M = Co or Ni).
The synthesis process of SalenM is shown in Figure 11.Ethanol solution (50 mL) containing 50 mmol salicylaldehyde was slowly added dropwise to the ethanol solution (100 mL) containing 50 mmol p-aminobenzoic acid.After dropwise addition with continuous stirring, the reaction system was heated to reflux for 3 h.Then, the resultant slurry was cooled, filtered, washed, and dried at 85 °C for 12 h to obtain a bright yellow needle-like Salen.Then, 6 mmol Salen was dissolved in 100 mL ethanol and the pH was adjusted to 7.0 with 0.2 mol/L NaOH ethanol solution.After that, 10 mL aqueous solution containing 3 mmol M(CH3COO)2•4H2O was slowly added dropwise to the above solution.The resulting turbid solution was heated to reflux for 3 h.Finally, the slurry was cooled, filtered, washed, and dried at 85 °C for 12 h to get SalenM (M = Co or Ni).

Characterizations
The contents of Zn, Co, Ni, and Cr elements in the successfully-intercalated LDHs were determined by inductively-coupled plasma atomic emission spectrometry (ICP-AES) on an IRIS Intrepid II XSP instrument (Thermo Electron, Boston, MA, USA).Powder X-ray diffraction (PXRD) data of the prepared materials were collected on XRD-6 type diffractometer (Persee, Beijing, China) using Cu Ka radiation (λ = 0.15418 nm) at 6 kV, 20 mA and the scanning rate was 4 • /min in 2θ from 5 • to 70 • .Fourier transform infrared (FT-IR) spectra of the materials were obtained using a NICOLET Bruker Vector 22 (Bruker, Karlsruhe, Germany) spectrophotometer in the wavenumber range of 4000-400 cm −1 .The surface morphologies of the materials were characterized by Hitachi S-4700 scanning electron microscope with an accelerating voltage of 15 kV.UV-Vis diffuse reflectance spectra (UV-Vis DRS) were measured by UV-2660 spectrophotometer (Shimadzu, Kyoto, Japan) with an integrating sphere attachment using BaSO 4 as background at room temperature in air.

Photocatalytic Degradation Performance Test
The photocatalytic degradation performances of the materials were tested by degradation of RhB under simulated visible light using a 300 W Xenon lamp (380 nm < λ < 760 nm, light source is 25 cm from the reaction solution).Briefly, 50 mg of catalytic material was added to a double-layer quartz reaction tube containing 50 mL of 5 mg/L RhB solution at room temperature.Firstly, the adsorption-desorption equilibrium of RhB was achieved by stirring for 30 min in the dark.Then,

Figure 6 .
Figure 6.Pseudo-first-order kinetics degradation for RhB by different materials.

Figure 6 .
Figure 6.Pseudo-first-order kinetics degradation for RhB by different materials.

Table 1 .
The metallic composition of the successfully intercalated layered double hydroxides (LDHs).
M = Co or Ni.

Table 3 .
Pseudo-first-order kinetic parameters for RhB photodegradation by different materials.

Table 3 .
Pseudo-first-order kinetic parameters for RhB photodegradation by different materials.