4.1 UV-Vis diffuse reflectance spectroscopy (UV-DRS)
UV-Vis-DRS was recorded for ZIF-8 (Fig. 2 (a), line 1) and C-TiO2/ZIF-8 (Fig. 2 (a), line 2) to study absorbance as well as decrease in band gap energy of C-TiO2/ZIF-8 (Fig. 2 (b), line 2) in contrast to pure ZIF-8 (Fig. 2 (b), line 1). The investigation of the band gap shift after carbon doping in TiO2 was done and reported by us earlier (Negi 2021). The band gap energy of TiO2 after doping with carbon shifted to 2.96 eV from 3.0 eV. The absorbance spectra of ZIF-8 and C-TiO2/ZIF-8 signify the bathochromic shift in C-TiO2/ZIF-8 which indicates the better activity of the composite as compared to pure ZIF-8 in the visible region. Figure 2 (b) represents the Tauc plots for pure ZIF-8 as well as for C-TiO2/ZIF-8 (equation S1, supplementary information). From the Tauc plots, the band gap energy obtained for ZIF-8 was 4.97 eV and that for C-TiO2/ZIF-8 was 4.80 eV which is appreciably less than pure ZIF-8. The lattice interaction of O, Ti and C atoms accounts for the reduction in the band gap. The structure and characteristics of ZIF-8 are influenced by the dopant C-TiO2 which in turn affects the band gap of the composite (Negi 2021).
The mechanism of the photocatalytic degradation of methylene blue is represented by (equations (i-v)). The photocatalyst C-TiO2/ZIF-8 is irradiated with visible radiation to start the redox reaction. This causes electrons to be excited out from the valence band to the photocatalyst's conduction band which results in the formation of electron-hole pairs. The reduction and oxidation processes are carried out at the surface of the photocatalyst via electrons and holes, respectively. Superoxide anion radicals (·O22−) are formed when the conduction band electrons combine with oxygen, whereas hydroxyl radicals (·OH) are formed when the holes in the valence band interact with molecules of water as shown in the band diagram in Fig. 3. The ·O22− and ·OH radicals generated are responsible for the degradation of methylene blue (Khan 2022).
C-TiO2/ZIF-8 + light (hυ) e− (conduction band) + h+ (Valence band) (i)
e− + O2 Superoxide anion radical (·O22−) (ii)
h+ + −OH Hydroxyl radical (·OH) (iii)
·OH + Methylene blue CO2 + H2O (iv)
·O22− + Methylene blue CO2 + H2O (v)
The reduction in band gap leads to an increase the rate of electron transfer from the valence band to the conduction band, the electrons and the holes generated can either recombine or can undergo a reaction with H2O and O2 available at the surface of the catalyst. Here, the ZIF-8 acts as the main catalyst while C-TiO2 helps in the reduction of band gap as well as electron-hole recombination. The transferred electrons have more affinity to oxidize hydroxide ions than combining with holes on the surface of the catalyst leading to the production of hydroxide radicals which are the primary active species for the degradation of methylene blue as investigated by Chandra et al. (2017) from fluorescence intensity measurements. The ligand-to-metal charge transfer (LMCT) pathway is responsible for ZIF-8's photocatalysis (Chandra 2017). Also, the doping of C-TiO2 with other wide band gap materials enhances its visible range behaviour due to electrons or holes photo generated in those components could be infused into TiO2, leading to better separation of charge in the illuminated photocatalyst by improving the lifetimes of the electron and hole pairs. Further, the high surface area of ZIF-8 provides better absorption of methylene blue. Therefore, C-TiO2/ZIF-8 was proven to have better photocatalytic activity in the visible range.
4.2 Fourier transform infrared spectroscopy (FTIR) and Raman Spectroscopy
The FTIR spectroscopy analysis of the pure ZIF-8 sample (Fig. 4(a), line 1) and the composite sample (Fig. 4 (a), line 2) is carried outtoestablishthe effective doping of C-TiO2 into the ZIF-8 matrix and also to get a better understanding of interaction. Peaks around 2925 cm− 1 were allocated to the stretching vibration of C-H bond (aliphatic) while the band obtained at 3135 cm− 1 was for the C-H stretch (aromatic and asymmetric). The stretching mode of C = C bond created a band at 1640 cm− 1 in the ZIF-8 sample, while the C = N stretching mode caused a band at 1586 cm− 1. The signals from 1300 cm− 1 to 1460 cm− 1 in ZIF-8 were for stretching of the whole imidazole ring, but the band at 1148 cm− 1 was for aromatic C–N stretch. Furthermore, the vibration modes of C–N bend and C–H bend may be ascribed to the maxima at 995 cm− 1 and 761 cm− 1, respectively. The band at 688 cm− 1 was caused by the bending vibration of HMIM ring (out of plane). The Zn–N stretching vibration band was seen at 423 cm− 1, confirming the chemical interaction of Zn2+ ions with N atoms of the methylimidazole to generate imidazolate.
On analyzing the FTIR graph of C-TiO2/ZIF-8 we observed that the Zn-N vibration is unaffected by the incorporation of C-TiO2 and its peak was obtained at 423 cm− 1. The other vibrational modes which remained unaffected after doping with C-TiO2 were bending C-N mode, C-N stretching (aromatic), ring stretching, stretching band of C = C and C-H stretch (aliphatic and aromatic). The 642 cm− 1 vibration mode corresponds to the most prominent O-Ti-O stretching while the vibration at 565 cm− 1 confirms the formation of the O-Ti-N bond. The O-Ti-N bond presence indicates that chemical bonding, as opposed to only physical integration of the two elements, is involved in the formation of C-TiO2/ZIF-8 (Wibowo 2015, Li 2020). The shift towards lower wavenumber was obtained in the vibration modes of bending of HMIM (680 cm− 1, out of plane), C-H bend (747 cm− 1) and the stretching mode of C = N bond (1563 cm− 1) due to the decrease in bond length which is the result of some interactions between Ti-O stretching bandand these mentioned bonds in the system after doping of C-TiO2. Also, the change in the wavenumber of C = N can be attributed to the H-bonding with the available OH functional group. Owing to the existence of the O-Ti-N band which did not affect the stretching band of Zn-N in ZIF-8, we concluded that the synthesis of the composite has been done successfully.
Raman spectroscopy is recorded for both ZIF-8 and C-TiO2/ZIF-8 to detect the Raman shift in the bonding modes and to ascertain vibrational and rotational states in both molecular systems. Strong bands that correspond to the vibrations of the CH3 group and imidazole ring are existing in the Raman spectra for ZIF-8 as shown in Fig. 4 (b), line (1). At 297 cm− 1, the intensified bands for Zn-N stretching have been observed.Strong bands that correspond to the C-N stretching and imidazole ring have been seen at 686 cm− 1 and 1148 cm− 1 respectively. Methyl bending is identified as the cause of the noticeable band at 1154 cm− 1. The ring extension and N-H wagging band are located at 1310 cm− 1. Bands at 1385 cm− 1 and 1462 cm− 1 are ascribed to CH3 and C-H bending modes, respectively, whereas the bands at 1506 cm− 1 are identified stretching mode of C = C. Bands for N-H bend mode and C = N (out of plane) bending mode are distinguished at 752 cm− 1. The band at 642 cm− 1 signifies imidazolium ring twisting (Tanaka 2015). The four important Raman active modes for the C-TiO2/ZIF-8 (Fig. 4(b), line (2)) are observed at 145 cm− 1 (Eg), 411 cm− 1 (B1g), 513 cm− 1 (A1g) and 635 cm− 1 (Eg), which are in accordance with the anatase structure of TiO2 as in previously reported study by Zhang et al. (2019). Eg mode mainly occurs due to symmetric stretching vibration of O-Ti-O in TiO2, the B1g mode is the consequence of symmetric bending vibration of O-Ti-O, and A1g mode is the result of anti-symmetric bending vibration of O-Ti-O (Ramasamy 2014). The most noticeable mode of TiO2 at 145 cm− 1 (Eg) is the low-frequency phonon band of the TiO2 inanatase phase, which attributes to the bending vibration of Ti-O.From the analysis of Raman spectra of both the system, we observed a blue shift in the Zn-N stretching band (311 cm− 1) which may be attributed to the decrease in the crystallite size of the composite and the increase in the rigidity of the framework after C-TiO2doping into the ZIF-8 matrix (Ramasamy 2014, Zhang2019, Song 2021).
4.3 X-Ray diffraction
To understand the lattice properties, parameters, interplanar distance, average crystallite size and the microstrain in the synthesized composite as well as ZIF-8, XRD analysis has been done. Figure 5 (a) shows the diffraction pattern of cubic ZIF-8 which exhibited peaks at 2θ angles of 10.4⁰, 12.7⁰, 14.7⁰, 16.4⁰, 18.0⁰, 22.1⁰, 29.7⁰ corresponding to the (022), (112), (022), (013), (222), (114), (044) crystallographic planes, respectively, and are in close accordance with the reported literature (Zhang 2019). Further, the fundamental diffraction peaks for C-TiO2/ZIF-8 in Fig. 5 (b) were observed at 2θ angles of 10.4⁰, 12.7⁰, 14.7⁰, 16.5⁰, 18.0⁰, 22.2⁰, 25.6⁰, 29.7⁰, 37.4⁰, 38.3⁰, 48.5⁰ for (022), (112), (022), (013), (222), (114), (101), (044), (004), (112) and (200), respectively. The peaks at 2θ 25.6⁰, 37.4⁰, 38.3⁰ and 48.5⁰ are attributed to the peaks of the anatase phase (A) of TiO2 in C-TiO2 (Negi 2021, Zhang 2019). The average interplanar distance or the d-spacing in the ZIF-8 lattice calculated by Bragg’s equation was 0.545 nm and that for the C-TiO2/ZIF-8 was 0.293 nm (equation S2, supplementary information). This decrease in d-spacing signifies the increase in the crystallinity of composite in contrast to ZIF-8. In addition, the ionic radii of Ti+ 4 cation is smaller than Zn+ 2 cation so the incorporation of Ti+ 4 into the lattice of ZIF-8 will decrease the d spacing of the sample. The average crystallite size was determined to be 98.73 nm and 38.18 nm for ZIF-8 and C-TiO2/ZIF-8 utilizing Debye Scherrer’s equation. This decrease in average crystallite size can be ascribed to the introduction of the dopant into the ZIF-8 matrix which results in the decrease in the agglomeration rate of composite particles because of prolonged magnetic agitation (equation S3, supplementary information). As depicted in Table 1, the reduction in the volume of composite crystal lattice after doping with C-TiO2 was observed which confirms the shrinkage of the composite matrix (Negi 2021).
Further, the microstrain present in the ZIF-8 and C-TiO2/ZIF-8 lattices were analyzed by the Uniform Deformation Model of the William Hall technique (equation S4, supplementary information). The strain inside the matrix is supposed to be consistent throughout all crystallographic planes as per UDM. Moreover, it assumes the crystal's property to be isotropic, meaning that all characteristics of the compound are equivalent independent of measurement direction. The microstrain in the samples was determined by the slope of βcosθ versus 4sinθ plots and was observed to be positive for the ZIF-8 lattice and all the components of C-TiO2/ZIF-8. The positive microstrain in ZIF-8 can be assigned to the cubic morphology of the crystal lattice whereas thepositive slope in the case of C-TiO2/ZIF-8 can be attributed to the presence of tensile strain in the lattice caused by the anatase phase of TiO2 (Negi 2021, Tanaka 2015) (Fig. S1, supplementary information).
Table 1
XRD analysis of ZIF-8, C-TiO2 and C-TiO2/ZIF-8.
S.No. | Sample | Average d-spacing (nm) | Average Crystallite size (nm) | Strain (ε × 10 − 3) | Lattice parameters (nm) | Volume (nm3) | References |
1. | ZIF-8 | 0.545 | 98.73 | 0.836 | 1.212 (a = b = c) | 1.780 | Present study |
2. | C-TiO2 | 0.348 | 18.76 | 1.38 (A) | 0.457 (a = b) 0.537 (c) | 0.112 | (Negi 2021) |
3. | C-TiO2/ZIF-8 | 0.293 | 38.18 | 4.89 (ZIF-8) 1.73(A) | 0.802 (a = b = c) | 0.516 | Present study |
4.4 X-ray photoelectron spectroscopy
The C 1s spectrafor ZIF-8 is represented in Fig. 6 (a) to investigate the carbon states present in the synthesized sample. The spectra revealed three peaks at 282.94 eV, 284.69 eV, and 288.66 eV. Figure 6 (b) represents the spectra of O1s for ZIF-8. Two peaks were fitted for ZIF-8 which appeared at 530.12 eV and 531.60 eV. The intensity peak at 530.12 eV can be ascertained to the O atom in the Zn-OH bond whereas the 531.60 eV can be ascertained to the O atom of Zn-O. N 1s spectrum is represented in Fig. 6 (c) and two peaks were observed at 399.5 eV and 398.99 eV which are assigned to the N atom in C-NH and C = N bonds, respectively. Further, the Zn 2p spectrum is shown in Fig. 6 (d) with the intensity peaks at 1126.11 eV and 1048.5 eV given to the Zn-atom in 2p3/2 and 2p1/2, respectively.
4.5 UV-Visible Spectroscopy
The photocatalytic activity of ZIF-8 and C-TiO2/ZIF-8 has been studied for methylene blue (50 µM) degradation (supplementary information). The samples were taken out of the visible light at 10 minutes intervals for both the samples C-TiO2/ZIF-8 and ZIF-8. ZIF-8 took more prolonged visible light irradiation to change the color of methylene blue from dark blue to colorless. This can be attributed to the narrow band gap energy of C-TiO2/ZIF-8 (4.80 eV) as compared to ZIF-8 (4.97 eV). Another aspect that favors the better performance of C-TiO2/ZIF-8 is the excellent surface area provided by the ZIF-8structure enhancing the surface adsorption of pollutants on the sample. It took 70 minutes for C-TiO2@ZIF-8 to degrade 50 µM methylene blue whereas it took almost 120 minutes as shown in Fig. 7, depicting the better performance of the composite than pure ZIF-8 (Fig. S2, supplementary information).
The photocatalytic degradation of methylene blue is observed to be a pseudo first-order reaction. Therefore, the initial concentration of methylene blue will have an impact on the rate of the reaction. We have seen that due to the enhanced adsorption of the visible light as a result of less band gap energy, C-TiO2/ZIF-8 has better photocatalytic performance against methylene blue. Another confirmation of the enhanced performance of C-TiO2/ZIF-8 in this reaction is the calculation of the rate constants (equation S5, supplementary information). The calculated rate constant for C-TiO2/ZIF-8 was 4.9 × 10− 2 min− 1and that for ZIF-8 was 2.4 × 10− 2 min− 1. These values signify that the rate of the reaction for the composite is greater than ZIF-8 and thus, it takes much lesser time to degrade methylene blue than ZIF-8. The dependence of the initial concentration of methylene blue on the rate of the reaction is found to be an inverse relationship. At a lower concentration of dye, the number of hydroxyl radicals generated is more due to uninhibited light interaction on the surface of the catalyst. On increasing the concentration of the dye, the time of degradation will increase which will decrease the rate constant and eventually the reaction rate. This decline in the rate of reaction can be described by the limited number of reactive superoxide of oxygen formed on the catalyst’s surface due to the hindrance to the visible light created by dye molecules of the composite as the methylene blue concentration rises.
Table 2
Kinetic study of the solar light harvested degradation of methylene blue by C-TiO2, ZIF-8 andC-TiO2/ZIF-8.
S. No. | Photocatalyst | Rate Constant (min− 1) | Methylene Blue Concentration (µM) | Time (mins.) | Radiation | References |
1. | C-TiO2 | 29.39 × 10− 2 | 10 | 45 | Visible | (Negi 2021) |
2. | ZIF-8 | 1.7 × 10− 2 | 31.26 | 120 | Ultraviolet | (Jing 2014) |
3. | ZIF-8 | 2.4 × 10− 2 | 50 | 120 | Visible | Present study |
4. | C-TiO2/ZIF-8 | 4.9 × 10− 2 | 50 | 70 | Visible | Present study |