Li-Yuan Zhang*abc,
Yan-Lin Hana,
Min Liuabc and
Sheng-Lian Denga
aCollege of Chemistry and Chemical Engineering, Neijiang Normal University, Neijiang 641112, China
bKey Laboratory of Fruit Waste Treatment and Resource Recycling of the Sichuan Provincial College, Neijiang 641112, China
cSpecial Agricultural Resources in Tuojiang River Basin Sharing and Service Platform of Sichuan Province, No. 1, Xingqiao Street, Neijiang 641112, Sichuan, China. E-mail: zhangliyuansir@126.com; Tel: +86 832 2341577
First published on 5th June 2023
Nickel aluminum layered double hydroxides (Ni–Al LDHs) and layered mesoporous titanium dioxide (LM-TiO2) were prepared via a simple precipitation process and novel precipitation–peptization method, respectively, and Ni–Al LDH-coupled LM-TiO2 (Ni–Al LDH/LM-TiO2) composites with dual adsorption and photodegradation properties were obtained via the hydrothermal approach. The adsorption and photocatalytic properties were investigated in detail with methyl orange as the target, and the coupling mechanism was systematically studied. The sample with the best performance was recovered after photocatalytic degradation, which was labeled as 11% Ni–Al LDH/LM TiO2(ST), and characterization and stability studies were carried out. The results showed that Ni–Al LDHs showed good adsorption for pollutants. Ni–Al LDH coupling enhanced the absorption of UV and visible light, and the transmission and separation of photogenerated carriers were also significantly promoted, which was conducive to improving the photocatalytic activity. After treatment in the dark for 30 min, the adsorption of methyl orange by 11% Ni–Al LDHs/LM-TiO2 reached 55.18%. Under illumination for 30 min, the decolorization rate of methyl orange solution reached 87.54%, and the composites also showed an excellent recycling performance and stability.
For the treatment of industrial wastewater, the traditionally used technologies are mainly chemical, physicochemical, and biological methods, including electrocatalytic degradation,1–4 ultrasonic treatment,5,6 biological flocculation,7,8 adsorption,9,10 and photocatalytic degradation.11–16 Photocatalytic degradation, namely, photocatalytic oxidation technology, which uses sunlight and simulated natural light as light sources to act on semiconductor catalysts to produce photogenerated electrons and holes, and reacts with O2, H2O, and other active substances to generate hydroxyl radicals, superoxide radicals, and other active substances, and eventually decomposes pollutants into harmless inorganic small molecules. The commonly used photocatalysts are mainly semiconductor materials, including TiO2,17–19 CdS,20 and ZnO.21
Photocatalytic oxidation technology has the advantages of simple operation, significant effect, non-toxicity, and no secondary pollution. Presently, it has become an important means to degrade organic compounds in wastewater, and the most frequently used catalyst is TiO2. However, the band gap of TiO2 is very wide, which means that only under ultraviolet light illumination with a wavelength of less than 387 nm, the electrons on the valence band can be excited to transition to the conduction band, and then photogenerated electrons and holes with high activity are produced. The holes capture electrons of hydroxyl ions or water molecules on the material surface to generate hydroxyl radicals with strong oxidation, and the electrons adsorb oxygen molecules on the specimen surface to produce active superoxide free radicals, ˙O2−, which further transform into free radicals, ˙HO2 and ˙OH. Subsequently, ˙OH can degrade most organic pollutants into small inorganic molecules, CO2 and H2O. However, the sunlight utilization rate of TiO2 is only 3–5%, and the generated electrons and holes easily recombine, which decreases the photocatalytic performance and limits the practical application of TiO2. To address the shortcomings of photocatalysts in practical applications, researchers mainly modify photocatalysts by expanding their light absorption range and improving the separation efficiency of photo-generated carriers. The common modification methods include morphology or size regulation,22,23 metal and non-metal ion doping,24–26 semiconductor recombination,27 precious metal deposition,28 etc.
LDHs are a new class of inorganic materials, also known as ionic clay compounds,29–32 which have attracted extensive attention due to their unique physicochemical properties. The spatial structure of LDHs is similar to the octahedral structure of Mg(OH)2, and its molecular formula is [M2+1−xM3+x(OH)2]x+(An−)x/n·mH2O, where M2+ is a divalent metal ion (such as Zn2+, Fe2+, Mg2+, and Ni2+); M3+ is a trivalent metal ion (such as Al3+, Fe3+, Mn3+, and Cr3+), where the divalent metal ion is the main body of the layer; X is the molar ratio of M3+ and M2+ + M3+, which is generally between 0.2–0.33; and An− is a neutral molecule or anion (H2O, CO2, etc.) that can exist stably between the layers.33–35 The main laminates of LDHs are connected by covalent bonds, and the main forces between the main laminates and the interlayer objects are electrostatic interaction, hydrogen bond, van der Waals force, etc. The layered complex structure is formed between the subjects and objects through orderly arrangement. Due to the unique structure, green environmental protection and other properties of LDHs, they have attracted extensive attention.36–39 Their layered structure is not only controllable and recoverable, but also enables a good adsorption performance. However, their low photocatalytic performance limits their application. As is known, the photocatalytic efficiency of materials is mainly determined by the photon absorption and utilization efficiency, the photogenerated carrier separation efficiency and the surface catalytic reaction efficiency, and thus improving the photocatalytic efficiency requires the synergy of these three aspects. This also makes the coupling of LDHs and semiconductor materials an effective method to elevate their photocatalytic efficiency. Cai et al.40 combined the hydrothermal process and anion exchange method to prepare Co–Al/CdS-LDHs by embellishing bimetallic hydroxides with CdS nanoparticles, which effectively adjusted the photocatalytic activity of Co–Al LDHs under visible light. Wu et al.41 deposited ZnO nanoparticles on the surface of ZnMgAl–CO3-LDH microspheres via the coprecipitation method to form a ZnO/ZnMgAl–CO3-LDH heterojunction photocatalyst, which significantly enhanced the degradation of phenol under UV light, and pointed out that the formation of the heterojunction helped to reduce the recombination rate of excited electrons and holes and improved the photocatalytic performance.
In addition, LDHs are also widely used in the field of photocatalysis. Frogoso et al.42 doped blue light-emitting graphene quantum dots (GQD) in NiTi-based bimetallic hydroxide nanosheets (NiTi-LDH) through a simple impregnation method, and GQD/NiTi-LDH showed excellent photo removal activity for NO and NO2. Zhao et al.43 prepared ZnAl-LDH, which greatly improved the driving force and selectivity for photocatalytic CO2 reduction to CO. Simultaneously, ZnAl-LDH reduced the charge transfer resistance and had a higher photogenerated current density, achieving the reduction of CO2.
The lamellar structure and interlayer anion exchangeability of LDHs endow them with a strong adsorption performance, especially for azo dyes and heavy metal ions in wastewater. However, LDH materials have a low reuse rate after adsorption, cannot be adsorbed under the specific conditions, and are easily affected by other substances. Thus, to improve the property of LDHs, researchers have attempted to modify them in various ways. Deng et al.44 prepared AM/SDS-LDH by inserting sodium dodecyl sulfate between layers, and then grafting acrylamide on LDHs, and tested the adsorption of Congo red by the as-prepared samples. The results showed that the adsorption performance of LDHs after modification was improved, which is mainly due to the electrostatic interaction between the anionic dyes and surface charges. Huang et al.45 synthesized flower-like cobalt aluminum layered double hydroxide hollow microspheres via a one-step solvothermal process. The hollow microspheres possessed a large specific surface area and an excellent adsorption performance. The limited space between the LDH layers provided size selectivity for the adsorbate.
Herein, Ni–Al LDHs and LM-TiO2 were prepared via a simple precipitation method and novel precipitation–peptization approach, respectively, and Ni–Al LDHs/LM-TiO2 composites with adsorption and photodegradation properties were prepared via the hydrothermal process. Furthermore, the modification mechanism was systematically studied, and the adsorption and photocatalytic properties of the samples were investigated.
(1) |
The adsorption kinetics of LDHs/LM-TiO2 was studied using the pseudo-first-order kinetic eqn (2) and pseudo-second-order kinetic eqn (3), and the related constants were also determined.
(2) |
(3) |
Mineralization degree = (1 − TOC30min/TOC0) × 100% | (4) |
Fig. 2 SEM of simple TiO2, LM-TiO2, LDHs and LDHs/LM-TiO2 (a) simple TiO2, (b) LM-TiO2, (c) LDHs, and (d) LDHs/LM-TiO2, and (e) histogram of the particle size distribution of LDHs/LM-TiO2. |
Fig. 3 presents the XRD patterns of various LDHs/LM-TiO2 and LDHs calcined at 550 °C. Based on anatase TiO2 (ICDD PDF #21-1272), the diffraction angle at 2θ = 25.3° is the characteristic peak of the (101) lattice plane of anatase titanium dioxide and no diffraction peak of rutile was detected, which indicates that the titanium dioxide prepared at this temperature is anatase. In addition, the diffraction peak position of each diffraction angle in curve f is basically consistent with the standard card (Ni–Al LDHs ICDD PDF#22-0452), indicating that the crystal phase is nickel aluminum bimetallic hydroxides, and it can also be analyzed that it has the characteristics of a layered structure, because the peaks at 11.6° and 22.3° correspond to (003) and (006), respectively. They are generated by the diffraction of the plane base, which can be found in the hexagonal structure of Rm rhombic symmetry.50 According to the curve of simple titanium dioxide in Fig. 3a, the (101) characteristic peak intensity at the 2θ value of 25.3° is quite high with a small peak width, demonstrating that the TiO2 prepared via the precipitation–peptization method has a good crystallinity. Fig. 3b–e show the patterns of CTAB-modified titanium dioxide with a loading of 0%, 9%, 11% and 13% LDHs, respectively. It can be seen that CTAB-modified TiO2 does not have the diffraction peak of CTAB, and the intensity of the characteristic peak decreased and its width increased, illustrating that the crystallinity decreased and the growth of titanium dioxide grains was inhibited. This is because CTAB has a low melting point, and the residues were at a quite low level after calcination at 550 °C. As a surfactant, CTAB has solubilization and other functions, which weaken the crystallization degree of titanium dioxide. After loading LDHs, the peak intensity of LM-TiO2 increased. However, with an increase in the LDH level, the characteristic peak intensity of the specimen showed a weakening trend, indicating that an appropriate amount of LDH-coupled TiO2 promotes crystallization. LDHs have a special layered morphology. Thus, the coupling of LDHs with titanium dioxide results in a certain preferred orientation of the grains, which is reflected by the variation in the intensity of the characteristic peak in the XRD spectrum. In addition, there is no characteristic peak of LDHs in the curves of the LDH/LM-TiO2 composites with various ratios. This is because the loading content of LDHs on the surface of TiO2 is less than the detection limit of the XRD instrument. Besides, according to curve f, it also can be observed that the crystallinity of LDHs synthesized by the precipitation process is not sufficient, which also affects the XRD detection.
Fig. 3 XRD patterns of TiO2 with various coupling ratios and LDHs (a) simple TiO2, (b) 0%, (c) 9%, (d) 11%, (e) 13%, (f) LDHs, and (g) 11% (ST). |
As is known, the diffraction intensity and sharpness of the peaks are not only affected by the sample crystallinity, but also related to the grain size. According to the Scherrer formula (5), the smaller the crystal grain, the greater the diffraction peak width, which further indicates that the grain size of CTAB-modified titanium dioxide is reduced, which is also consistent with the SEM results. The grain size of titanium dioxide was calculated using the Scherrer formula (5), and the results are shown in Table 1.
D = Kλ/(βcosθ) (nm) | (5) |
Sample | Crystallite size/nm |
---|---|
TiO2 | 22.30 |
LM-TiO2 | 9.96 |
9% LDHs/LM-TiO2 | 11.85 |
11% LDHs/LM-TiO2 | 11.38 |
13% LDHs/LM-TiO2 | 11.05 |
11% LDHs/LM-TiO2(ST) | 11.54 |
Obviously, the grain size of simple TiO2 is large; however, the introduction of CTAB reduced the grain size, which improved the specific surface area of the specimen and promoted the photoreaction. After coupling with LDHs, the grain size and the diffraction peak intensity both increased; however, with an increasing content of LDHs, the grain size was almost maintained, which illustrates that the loading amount of LDHs does not obviously affect the crystallinity and grain size of the composites.
Fig. 4 shows the N2 adsorption–desorption isothermal curve and pore size distribution of LM-TiO2, 11% LDHs/LM-TiO2 and Ni–Al LDHs. It can be seen in Fig. 4a and b that the curves of LM-TiO2 and 11% LDHs/LM-TiO2 are basically the same; however, there is a convex hysteresis loop at the relative pressure (p/p0) of 0.36 in the second half of the curve, which conforms to type IV H2(b). This phenomenon is often observed in three-dimensional mesoporous materials, illustrating that the product has a complicated pore structure, which includes typical “ink bottle” pores and tubular pores with uneven pore size, and the pore size distribution is quite wide. In addition, the N2 adsorption desorption isotherm curve of Ni–Al LDHs also shows a hysteresis loop, which is mainly related to the capillary condensation of porous materials. This curve belonging to type IV H2(a) has a steeper desorption branch because the pores are blocked at the pore diameter or the hole volatilization effect occurs.
Fig. 4 Adsorption–desorption isotherms and pore size distribution of (a) LM-TiO2, (b) 11% LDHs/LM-TiO2 and (c) Ni–Al LDHs. |
By analyzing the pore size distribution of the various samples, it can be found in Fig. 4a that LM-TiO2 has a single high and narrow peak, and the distribution is also uniform, mostly in the range 2–16.56 nm, where the average pore size is 6.36 nm. In Fig. 4b, compared with LM-TiO2, the pore size distribution of 11% LDHs/LM-TiO2 is similar; however, the number of mesopores is reduced. Additionally, according to the pore size distribution diagram of Ni–Al LDHs in Fig. 4c, it can be obtained that the LDH sample also forms a mesoporous structure, with an uniform pore size distribution of 1.8–16.57 nm, and the maximum number of pores with an aperture of 6.47 nm is observed. Overall, the pore size distribution of LM-TiO2 (a) and Ni–Al LDHs (c) is very close, indicating that the pore size distribution slightly changed after coupling. The detailed structure data of the samples is shown in Table 2. Ni–Al LDHs have a large specific surface, which is attributed to its special structure. The specific surface area and pore volume of the 11% LDHs/LM-TiO2 composites decreased to a certain extent, which is due to the fact that some pores were blocked during the coupling of LM-TiO2 with LDHs, resulting in a reduction in specific surface area.
Sample | Specific surface area (m2 g−1) | Pore volume (cm3 g−1) | Pore diameter (nm) |
---|---|---|---|
LM-TiO2 | 76.65 | 0.13 | 5.34 |
11% LDHs/LM-TiO2 | 56.88 | 0.11 | 5.42 |
Ni–Al LDHs | 130.42 | 0.16 | 3.77 |
After coupling titanium dioxide with LDHs, the UV-Vis absorption band edge was red shifted, which is mainly due to the fact that Ni–Al LDH specimens show strong absorption at about 380 nm and 640 nm in the UV-Vis absorption spectrum. Therefore, after coupling LDHs with titanium dioxide, the absorption intensity in the corresponding UV and visible regions improved.50 Obviously, the absorption enhancement of LDHs/LM-TiO2 in the ultraviolet region can be attributed to the charge transfer from the O 2p to Ni 3d orbital, while the increased absorption in the visible region is due to the feature of Ni2+ in the octahedral configuration, resulting in the d–d transition.52 Therefore, the increase in the absorption intensity of the prepared composites in the UV visible region is mainly due to the action of the small amount of LDHs, and LDHs can provide more photogenerated carriers for photocatalytic reactions, resulting in an improvement in photocatalytic activity.
Fig. 7 illustrates the Fourier transform infrared spectra (FT-IR) of LM-TiO2, LDHs/LM-TiO2 and Ni–Al LDHs. In the infrared spectrum of the Ni–Al LDHs composite, the wide absorption band at about 3800–3400 cm−1 is due to the O–H bond stretching vibration and hydroxyl group of the interlayer water molecules,53 the characteristic peak at about 1635 cm−1 is attributed to the peak generated by the water of crystallization in the layered bimetallic, and the sharp strong peaks at about 1348 cm−1 and 844 cm−1 are caused by the asymmetric stretching of interlayer anion CO32−.53 In the spectra of LM-TiO2 and LDHs/LM-TiO2, the wide absorption peaks at 3800–3400 cm−1 are caused by the stretching of the hydroxyl group on the surface of TiO2 or the O–H bond of the adsorbed water molecules. However, the peak at about 3450 cm−1 is attributed to the bending vibration of H2O molecules. The strong and wide absorption peak detected at approximately 500 cm−1 is attributed to titanium dioxide, and the shape of the two curves is basically the same. The peak observed at about 2360 cm−1 corresponding to the stretching vibration of C–O. With the coupling of LDHs, the peak intensity increased, which may be caused by the increment in the content of CO32− in the interlayer of LDHs. There are many absorption peaks of LDHs in the range of 800–500 cm−1 with high intensity, which may be caused by the stretching vibration of the Ni–O and Al–O bonds formed by metal ions and oxygen atoms or the bonding of metal atom–oxygen atom–metal atoms (Ni–O–Al).54 Through the superposition of LM-TiO2 and LDHs/LM-TiO2 infrared spectra, it can be found that the absorption intensity of 11% LDHs/LM-TiO2 is higher and wider than that of LM-TiO2 at a wavenumber of less than 800 cm−1, indicating that the wave peak in this band contains the absorption peak of Ni–O, Al–O or Ni–O–Al in LDHs, and LDHs were well loaded on the surface of LM-TiO2.
Fig. 7 FT-IR spectra of the different materials (a) LM-TiO2, (b) 11% LDHs/LM-TiO2, (c) 11% LDHs/LM-TiO2(ST), and (d) Ni–Al LDHs. |
The element composition and chemical bonding state of the LDH/LM-TiO2 surface were characterized by X-ray photoelectron spectroscopy (XPS), and the results are presented in Fig. 8, where Fig. 8a is the XPS full spectrum, while Fig. 8b–f are the element spectra of Ti, O, Ni, Al and C elements, respectively. The charge correction was carried out based on the external pollution carbon (284.8 eV). Simultaneously, the curve of each element was fitted to determine its chemical state or change in surface electronic structure. It can be obtained from Fig. 8a that the main components in LM-TiO2 and LDHs/LM-TiO2 are Ti, O and C, and C elements according to the XPS test. The C element in LDHs/LM-TiO2 has a relatively high content, some of which may be introduced during the sample test, and the other is due to the fact that the interlayer anion of the LDH material is CO32−, and the total amount of C element increased after coupling with LM-TiO2. Besides, some Ni and Al elements were detected in LDHs/LM-TiO2. Fig. 8b shows the XPS spectrum of the Ti element, where the binding energies of spin orbits Ti 2p3/2 and Ti 2p1/2 in LDHs/LM-TiO2 are 458.44 eV and 464.13 eV, respectively. The peak at 458.44 eV belongs to Ti3+ and that at 464.13 eV corresponds to Ti4+,55 in which Ti3+ is attributed to the existence of oxygen vacancy in TiO2.56 Also, the binding energy of Ti element is greater than that of the standard value, demonstrating that Ti mainly existed in the form of Ti4+.55 In comparison with LM-TiO2, the binding energies of the Ti 2p3/2 and Ti 2p1/2 spin orbits of the LDHs/LM-TiO2 composites decreased by 0.45 eV and 0.46 eV, respectively. The change in chemical shift indicates that the modification of LDHs greatly changed the chemical environment around Ti4+, which may be because some Ni atoms and Al atoms entered the LM-TiO2 lattice to form Ti–O–Ni bonds and Ti–O–Al bonds, resulting in lattice distortion and an increase in the electron cloud density of the Ti atom. Consequently, the peak of Ti element moved towards the low energy direction after coupling. This may also be because the Ti element obtains electrons and performs reduction, which shifts the binding energy towards the low field, which is also consistent with the FTIR analysis. The XPS spectrum of O element is shown in Fig. 8c. The binding energy of LDHs/LM-TiO2 at the O 1s orbit is 0.44 eV lower than that of LM-TiO2. Specifically, after coupling LDHs with titanium dioxide, the electron cloud density around the O atom increased, leading to a decrease in the binding energy of the Ti atom. After fitting, the peak of the hydroxyl oxygen (˙OH) appeared at about 530.79 eV,57 and the peak intensity of LDHs/LM-TiO2 hydroxyl oxygen (˙OH) is weaker than that of LM-TiO2, which may be attributed to the loading of LDHs on the surface of titanium dioxide during the coupling process. Therefore, the peak intensity of the hydroxyl oxygen (˙OH) on the sample surface during the XPS test decreased. In addition, this may also be caused by the combination of oxygen atoms with aluminum and nickel atoms to form Al–O bond and Ni–O bond.
Fig. 8 XPS spectra of LM-TiO2, 11% LDHs/LM-TiO2, 11% LDHs/LM-TiO2(ST) and LDHs (a) survey, (b) Ti 2p peaks, (c) O 1s and O 2s peaks, (d) Ni 2p peaks, (e) Al 2p peaks and (f) C 1s peaks. |
As observed in Fig. 8d, the peaks at the binding energies of 856.34 eV and 873.99 eV belong to the Ni 2p3/2 and Ni 2p1/2 spin orbits, respectively, which are attributed to Ni–OH.58 Alternatively, the peaks at 862.35 eV and 879.89 eV are the characteristic peaks of Ni 2p3/2 and Ni 2p1/2, respectively, which proves that the nickel in the sample existed in the form Ni3+, and the peaks are caused by the charge transfer and multi-electron transition in the atom.59,60 In addition, it can be observed that the binding energy of Ni 2p in LDHs/LM-TiO2 increased compared with that in Ni–Al LDHs because the binding energy is relevant to the surface electron density. The electrons transferred from the surface of LDHs to TiO2, increasing the surface electron density of titanium dioxide.32 As shown Fig. 8e, the peak at 75.37 eV belongs to the characteristic peak of Al 2p, indicating that aluminum exists in the form of Al3+.61 In the peak fitting diagram of C element in Fig. 8f, the peaks at about 284.8 eV, 286.21 eV and 288.78 eV are the peaks of extraneous C 1s caused in the test. However, according to the comparison of the two curves, it was found that only the peak at 284.8 eV in the LDHs/LM-TiO2 sample is much stronger than that in LM-TiO2, which is due to the increase in the total amount of C element and the intensity of the peak caused by the C source contained in the LDH interlayer anion.
The XPS quantitative results show that the contents of Ti, O, and C in LM-TiO2 are 28.14 at%, 62.21 at%, and 9.65 at%, respectively. The contents of the constituent elements Ti, O, C, Ni and Al in LDHs/LM-TiO2 are 26.09 at%, 54.79 at%, 15.4 at%, 0.95 at%, and 2.77 at%, respectively. It can be obtained that the atomic ratio of O/Ti in LM-TiO2 and LDHs/LM-TiO2 is greater than the stoichiometric ratio of the standard TiO2 (2:1). Due to the modification by CTAB in LM-TiO2, the specific surface area of the material increased, making it easier to adsorb water and hydroxyl (˙OH), which resulted in an increase in oxygen content on the material surface. In addition, when LDHs were loaded on the surface of titanium dioxide, the number of interlayer water molecules or surface hydroxyl decreased (Al–OH and Ni–OH), and the oxygen atoms entered the lattice and combined with Al or Ni atoms to form Al–O and Ni–O bonds, which reduced the oxygen content on the specimen surface. Therefore, the ratio of O/Ti in the sample was greater than the standard value.
Fig. 9 illustrates the fluorescence spectra, in which the linearity of the two curves is virtually identical. The maximum fluorescence emission wavelength of the two materials is 418.23 nm, and the fluorescence intensity of 11% LDH-modified LM-TiO2 was weakened in the measured wavelength range. As is known from theory, the recombination of electrons and holes produces photoluminescence; however, it is also related to fluorescence luminescence. In the fluorescence spectra, the fluorescence intensity of the maximum absorption peak of 11% LDHs/LM-TiO2 is lower than that of LM-TiO2, which indicates that the photogenerated electrons and holes are not easy to recombine, and the number of photons produced is reduced. Thus, the recombination rate of photogenerated electrons and holes decreased, and the life of electrons and holes was prolonged, which is conducive to the generation of a large number of strong oxidizing groups, thus promoting the photocatalytic reaction.
Fig. 10 Photocurrent curves of different materials (a) LM-TiO2, (b) 11% LDHs/LM-TiO2, and (c) Ni–Al LDHs. |
Fig. 11 Electrochemical impedance spectroscopy of various materials (a) LM-TiO2, (b) 11% LDHs/LM-TiO2, and (c) Ni–Al LDHs. |
Fig. 11 shows the electrochemical impedance spectra of the different samples, which can be used to explore the difficulty of charge transfer on the surface of materials. Generally, the semicircle of the Nyquist plot from the impedance spectrum is regarded as the charge transfer resistance of the electrode material, and its radius is related to the charge transfer efficiency between the electrode and the electrolyte. The smaller the semicircle diameter, the smaller the obstacles encountered by charge transfer at the interface, and the higher the transfer efficiency, which is conducive to the improvement of catalytic activity. It can be observed in Fig. 11 that the radius of LM-TiO2 is larger than that of LDHs; however, the arc radius of the 11% LDHs/LM-TiO2 composite material is smaller than that of LM-TiO2. This indicates that the modified LDHs/LM-TiO2 has a smaller impedance and better conductivity compared to LM-TiO2, illustrating an increase in the mobility and separation rate of the photo-generated electron hole pairs. Based on the above-mentioned results, it can be concluded that LDHs and titanium dioxide were successfully coupled, and the construction of 11% LDHs/LM-TiO2 composites is conducive to the transfer and separation of photogenerated carriers, which is consistent with the PL fluorescence test and following photocatalysis evaluation results.
The adsorption isotherms of the 11% LDHs/LM-TiO2 samples were studied. Firstly, methyl orange solution with different concentrations was prepared, and the adsorption tests were carried out with a solid–liquid ratio of 20 mg: 30 mL for 30 min. Subsequently, the solution was taken out and centrifuged, and the absorbance of the supernatant was measured using a UV-vis spectrophotometer, according to which the content of residual methyl orange in the solution after adsorption was determined from the standard curve. The adsorption capacity of LDHs/LM-TiO2 in the different solutions was calculated using formula (1), and the adsorption isotherms of methyl orange on the 11% LDHs/LM-TiO2 samples were fitted by the Langmuir (6) and Freundlich (7) adsorption isotherm equations.
(6) |
(7) |
Langmuir | Freundlich | ||||
---|---|---|---|---|---|
Qm (mg g−1) | KL (L mg−1) | R2 | n | KF (L g−1) | R2 |
__ | __ | __ | 1.047 | 2.045 | 0.9807 |
It can be seen from the linear relationship in Fig. 13 and Table 3 that the adsorption of methyl orange by LDHs/LM-TiO2 is better described by the Freundlich adsorption isotherm equation compared with the Langmuir isotherm adsorption model, and the linear correlation coefficient reaches 0.9807. The adsorption of methyl orange on the modified LDHs/LM-TiO2 is bilayer adsorption. As is known, the Freundlich adsorption isotherm model assumes that the adsorption process exists on a heterogeneous surface and is used to describe the reversible adsorption, not only monolayer adsorption.62
To understand the reaction process and mechanism, the adsorption kinetics of LDHs/LM-TiO2 was studied using the pseudo-first-order (2) and pseudo-second-order (3) adsorption kinetic equations, and the adsorption rate constants were also determined. The fitting results are presented in Fig. 14, and the calculated data are listed in Tables 4 and 5. It can be observed from Fig. 14, Tables 4 and 5 that the mean value of the linear correlation coefficient R2 obtained by fitting the different LDHs/LM-TiO2 samples with the pseudo-first-order and pseudo-second-order kinetic equations is 0.9514 and 0.9986, respectively, illustrating that the adsorption of methyl orange by LDHs/LM-TiO2 is more suitably described by the second-order kinetic equation.
Fig. 14 Pseudo-first-order (a–c) and pseudo-second-order (d–f) kinetic models of various LDHs/LM-TiO2 samples. |
LDHs/LM-TiO2 | ρ (mg L−1) | qe (mg g−1) | Pseudo-first-order | ||
---|---|---|---|---|---|
kad (min) | qe (mg g−1) | R2 | |||
9% | 20.44 | 11.82 | 0.084 | 12.75 | 0.9576 |
11% | 17.72 | 0.086 | 12.31 | 0.9409 | |
13% | 11.26 | 0.074 | 8.57 | 0.9557 |
LDHs/LM-TiO2 | ρ (mg L−1) | qe (mg g−1) | Pseudo-second-order | ||
---|---|---|---|---|---|
k (g mg−1 min−1) | qe (mg g−1) | R2 | |||
9% | 20.44 | 11.82 | 0.0055 | 14.79 | 0.9977 |
11% | 17.72 | 0.0143 | 18.94 | 0.9996 | |
13% | 11.26 | 0.0142 | 12.40 | 0.9984 |
The adsorption rate constant and equilibrium adsorption capacity of LDHs/LM-TiO2 with different coupled ratios were calculated according to the pseudo-second-order kinetic equation. For 11% LDHs/LM-TiO2, the adsorption rate constant (k) and the equilibrium adsorption capacity (qe) are 0.0143 g mg−1 min−1 and 18.94 mg g−1, respectively. In addition, the adsorption rate constant and the equilibrium adsorption capacity of 9% LDHs/LM-TiO2 are 0.0055 g mg−1 min−1 and 14.79 mg g−1, and that of 13% LDHs/LM-TiO2 are 0.0142 g mg−1 min−1 and 12.40 mg g−1, respectively. The adsorption rate constant is positively correlated with the adsorption rate, which means that the adsorption rate and adsorption capacity of 11% LDHs/LM-TiO2 for methyl orange are greater than the other two. The adsorption process conforms to the pseudo second-order kinetic model, and the way is mainly chemical adsorption.
Fig. 15 shows the photocatalytic performance of the LDH/LM-TiO2 samples with various coupling ratios. Firstly, the samples were treated under the same dark conditions for 30 min, and then the absorbance of the supernatant was tested after centrifugation, as shown in Table 6. It can be observed that the absorbance of the titanium dioxide group did not change significantly; however, that of the LDHs/LM-TiO2 group changed obviously, showing a good adsorption performance. Adsorption equilibrium was basically reached after treatment in the dark for 30 min, and thus the photodegradation process could be carried out continuously. Under the same illumination, the decolorization rate of methyl orange by the different LDHs/LM-TiO2 followed the order 11% > 13% > 9% > LM-TiO2 > TiO2 > blank. The absorbance of methyl orange solution in the blank group was basically maintained, indicating that only the photocatalysts had a photocatalytic degradation effect on methyl orange solution. Maintaining the other conditions, the photocatalytic performance of LM-TiO2 was better than that of simple TiO2 when illuminated for 25 min due to the formation of a layered mesoporous structure via the induction of CTAB in the former. Thus, the specific surface area and the number of reactive sites increased, which are helpful for the degradation performance of the photocatalyst.
Fig. 15 Decolorization rates of methyl orange by simple TiO2 and LDHs/LM-TiO2 with various coupled ratios (a) blank group, (b) TiO2, (c) 0%, (d) 9%, (e) 11%, and (f) 13%. |
Sample | Blank group | TiO2 | 0% | 9% | 11% | 13% |
---|---|---|---|---|---|---|
Absorbance after 30 min in dark | 1.440 | 1.448 | 1.380 | 0.876 | 0.797 | 0.798 |
After treatment in the dark, the adsorption rate of LDHs/LM-TiO2 for methyl orange reached about 50%, showing a good adsorption performance. As is known, the degradation of the dye by the photocatalyst is mainly due to the oxidation of hydroxyl radicals and superoxide radicals generated on its surface and the adsorption of methyl orange dye on the photocatalyst surface helps to improve the photocatalytic reaction rate, elevating the photocatalytic performance. Under the same adsorption and illumination, the decolorization rate of LM-TiO2 was 54.72% after 30 min illumination, while that of 11% LDHs/LM-TiO2 was up to 87.54%, and the decolorization rates of 9% and 13% LDHs/LM-TiO2 were 75.64% and 82.56%, respectively. Thus, it can be concluded that the modification of titanium dioxide with an appropriate amount of LDHs significantly improved the photocatalytic degradation efficiency. With an increase in the loading of LDHs, they agglomerated and reduced the number of active sites for the photocatalytic reaction, thus reducing the number of photogenerated electrons and holes and the generated hydroxyl radical (˙OH), which is not beneficial to the photocatalytic reaction.
Images of the methyl orange solution before and after degradation are presented in Fig. 16. The mineralization degree of methyl orange solution after photocatalytic degradation by 11% LDHs/LM-TiO2 was determined by testing the change in mass concentration of total organic carbon during the degradation process. According to Table 7, it can be seen that the mineralization degree of methyl orange was 16.0% after photocatalytic degradation. However, according to the change in absorbance, the degradation rate was 87.54%, as illustrated in Fig. 15. The main reason for the significant difference between the two test results is that when 11% LDHs/LM-TiO2 degraded methyl orange solution, most of the chromogenic groups of methyl orange were destroyed and methyl orange changed into colorless substances. This stage involves multiple reaction processes, and only some of the methyl orange eventually degraded into CO2.63
Sample | TOC (mg L−1) | Mineralization degree (%) |
---|---|---|
Initial solution | 12.25 | — |
Degradation by 11% LDHs/LM-TiO2 | 10.29 | 16.00 |
In this work, LDHs were coupled with LM-TiO2 to prepare adsorption-photocatalysis integrated LDHs/LM-TiO2 composites, which displayed good adsorption and photocatalytic properties. The main reasons for this are as follows: (1) compared with simple TiO2, LDHs/LM-TiO2 has a particular layered mesoporous structure, which increases the specific surface area, and more active sites for the photoreaction are provided on the material surface, which are helpful for the photocatalytic reaction. (2) LDHs have a good adsorption, but weak photocatalytic ability. After coupling with the semiconductor photocatalyst TiO2, the product possessed dual properties of adsorption and photocatalysis. Thus, it can quickly adsorb dyes, such as methyl orange, on its surface, increase the contact area, and effectively elevate the degradation of pollutants under irradiation. (3) Based on simple titanium dioxide, the threshold wavelength of the absorption spectrum of LDHs/LM-TiO2 is red shifted, and the absorption intensity in the ultraviolet and visible regions is obviously increased, which widens the light response range and effectively improves the photocatalytic activity. (4). According to the fluorescence analysis, it can be observed that the fluorescence intensity of LDHs/LM-TiO2 is lower than that of LM-TiO2 to a certain extent, indicating that modification with LDHs reduces the recombination rate of electrons and holes generated by titanium dioxide, thus affecting the photocatalytic reaction. In summary, the LDH-coupled layered mesoporous TiO2 composites improve the photocatalytic activity and also show a good adsorption performance.
In addition, the first-order reaction kinetics was studied, and the kinetic curve of photodegradation of methyl orange solution by the different materials was plotted and fitted, as shown in Fig. 17. As illustrated, each kinetic curve shows a good linear relationship between lnC0/Ct ∼ t, and the reaction rate constant was calculated according to the formula lnCt/C0 = kt. The results demonstrated that the maximum reaction rate constant k was 0.046 min−1 (11% LDHs/LM-TiO2) and the minimum constant k was 0.013 min−1 (TiO2). The photocatalytic degradation of methyl orange by TiO2 and 11% LDHs/LM-TiO2 followed the Langmuir Hinshelwood first-order kinetic model, and the photocatalytic activity of LDH-coupled TiO2 was significantly enhanced.
Fig. 17 Dynamic curves of degradation of methyl orange by simple TiO2 and LDHs/LM-TiO2 with various coupling ratios. |
To demonstrate the stability of the catalyst, 11% LDHs/LM-TiO2 was recovered after photocatalytic degradation and characterized by XRD, FT-IR, and XPS. By comparing curve g in the XRD patterns, curve c in the FTIR spectra, and XPS data of 11% LDHs/LM-TiO2 (ST) with the data before the photocatalytic reaction, it can be seen that the various data for the sample after the reaction are basically consistent with that before the reaction, indicating that 11% LDHs/LM-TiO2 has good stability.
The photocatalytic degradation mechanism of LDHs/LM-TiO2 on dyes is mainly that LDHs/LM-TiO2 absorbs greater photon energy than the bandgap energy, and the electrons in the valence band are excited to transition to the conduction band, while creating holes in the valence band. The electrons and holes generated during this process migrate to the surface of LDHs/LM-TiO2 under the action of an electric field and diffusion. The holes oxidize H2O and HO− adsorbed on the surface of the material into hydroxyl radicals (HO˙) with strong oxidizing activity. Simultaneously, the photogenerated electrons undergo a reduction reaction with the oxygen molecules adsorbed on the material surface to generate superoxide radicals (˙O2−), which further react with H+ to produce H2O2. Finally, the strong oxidizing free radicals such as HO˙ and ˙O2− can specifically mineralize the organic dye molecules adsorbed on the surface of LDHs/LM-TiO2 without breaking the special structure of LDHs/LM-TiO2.
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