3.1 Powder X-ray Diffraction Studies:
Figure. 2a exhibits the XRD model of all the synthesized samples. All the diffraction peaks are showing identical peak pattern. As the copper content is increased the intensity of the major peak decreased which evidences the full-fledge incorporation of Cu2+ ions in ZIF-8. All the samples exhibited characteristic peaks at 7.44˚, 10.5˚, 12.83˚, 14.81˚, 16.57˚ and 18.15˚ attributed to the (011), (002), (112), (022), (013) and (222) planes of the ZIF-8 phase, respectively (17). The diffraction spectrum observed closely matched the conventional JCPDS pattern of file no. 00-062-1030 (18). The crystal structure was unaffected by the Cu-doping (19). There are no additional peaks are noticed. Due to the identical ionic sizes of Cu2+ (0.71 Å) and Zn2+ (0.74 Å) ions, adding a Cu2+ ion to the reaction species during the development of ZIF-8 crystals had no effect on the framework structure of the ZIF-8 materials.
3.2 Surface Analysis:
Photocatalyst adsorption capacity is critical for photocatalytic performance. As a result, N2 adsorption–desorption isotherms at 77 K were used to investigate the porosity of ZIF-8 and CDZ-20 samples (Figure. 2b). The detailed surface properties, such as, pore size, pore volume and surface area of ZIF-8 were 1968.7 m2 g-1, 1.443 nm and 0.7102 cm3 g-1, respectively. The pore structure of CDZ-20, on the other hand, was dramatically altered after the Cu-doping. CDZ-20 has a large surface area of 1740 m2 g-1, similar to ZIF-8. The abundance of hierarchical micro–mesopores is affirmed in all materials, as indicated by the abrupt slopes of the isotherms in the low relative pressure range and the hysteresis loops in the high relative pressure range (i.e., 0.92–1.0). Furthermore, as compared to pristine ZIF-8, the pore size (1.74 nm) and pore volume (0.758 cm3 g-1) of Cu-ZIF-8 remained hardly changed. Cu2+ ions may partially replace Zn2+ ions during the reaction path of Cu-ZIF-8, resulting in a change in pore structure. The ZIF-8 and Cu-ZIF-8 were type I hysteresis loops, as shown in Figure. 2b of the adsorption-desorption isotherms, suggesting the abundance of numerous micropores in the samples.
3.3 Morphology Analysis:
FESEM examination was used to analyse the morphologies of ZIF-8 and CDZ-20 photocatalysts. The studied samples have a well-defined truncated rhombic dodecahedron structure, reflecting the morphology for ZIF-8, as shown in Figure. 3(a-b) (20). After, Cu2+ ion doping on ZIF-8, the composite Cu@ZIF-8 inherited ZIF-8's original rhombic dodecahedron structure. These findings indicate that the surface geometry and framework structure of ZIF-8 are unaffected by Cu2+ ion doping (Figure. 3d-e). Because of the presence of properly tailored surface area, both metal ions are homogeneously adorned on the framework skeleton, providing active sites for improved photocatalytic activity. The EDAX studies confirm the presence of different elements such as Zn, Cu, C, and N in ZIF-8 and CDZ-20 as shown in Figure. S3(j-k).
Figure. 4 depict the morphological examination of ZIF-8 and CDZ-20 photocatalysts using TEM. The synthesised ZIF-8 (Figure. 4a) showed detached polyhedrons with sizes ranging from 400 to 500 nm, indicating octahedral ZIF-8 crystals with flat exteriors. The presences of constituent elements were mapped in ZIF-8 which are given in Figure. S2. Alternatively, the CDZ-20 showed polyhedrons that were identical to pure ZIF-8 but had rougher surfaces, indicating that the immobilised Cu2+ amorphous species was present (Figure. 4d). The average diameters of the nanocrystals decreased with the Cu2+ doping. The organic connectors on the ZIF-8 surface were partially rearranged during the Cu-doped process, and the Cu2+ ions eventually penetrated the ZIF-8 composite and replaced Zn2+ to link with the organic ligand to produce Cu@ZIF-8 (21). The presence of constituent elements was characterized by elemental mapping of CDZ-20 are given in Figure. S3.
3.4 Optical Properties:
The photophysical properties of all the prepared photocatalysts have been studied using diffuse reflectance spectra to reaffirm the presence of Cu on the ZIF-8 framework. The rigorous modification observed in the UV profile is in line with the colour change of the crystals, as it is the results from Cu doping in the composite Figure S1. It has further been observed in the earlier reports that numerous MOFs have semiconducting in nature and depict photoredox activity resulting the creation of electron-hole pairs upon suitable light response. (22) The Cu+2 doping resulted in absorption bands at 350 and 590 nm, which grew in intensity with the amount of doped Cu+2. Kubelka−Munk function is used to illustrate the optical properties of absorption edges and band gap of the prepared nanostructures (23). From Figure. 5a, it can be seen that the variations occurred in the absorbance spectra of ZIF-8 and its analogues. Due to its band gap energy Eg = 4.89 eV, ZIF-8 only shows an absorption band in the ultraviolet spectral window (24). The light response adsorption edge of ZIF-8 crystals is shown to shift from the UV to the visible range after Cu doping. Cu-doped ZIF-8 crystals have a well-depicted band at 490 nm in their absorption spectra. A second broad band beginning at 600 nm and extends beyond visible spectral window (Infrared, IR region). Both of these bands can be attributed to Cu+2 ion d-d transitions and indicate the heterobimetallic character of the ZIF materials (25-26). The band shifting in ZIF-8 crystals caused by Cu+2 ion doping makes it possible to re-engineer the band gap and move absorption to the visible range. Furthermore, the Tauc plot displays the measured band gap energy for ZIF-8 and CDZ-20 as 4.89 and 4,42 respectively (insert Figure. 5b). As can be seen, the band gap of the ZIF-8 matrix is 4.89 eV, but two new energy leaps have emerged. One transition has an Eg of 3.27 eV and is centered at 350 nm, while the other has an Eg of 2.1 eV and is centered at 470 nm.
To further understand the optoelectronic properties of photocatalysts, photoluminescence (PL) spectra were recorded and used to investigate the e-/h+ pair lifetimes of ZIF-8, CDZ-6, CDZ-20 and CDZ-25 photocatalysts (Figure. 5c). PL spectra also gives the information about the separation efficiency of the photo-generated e–/h+ pairs (27). The combination of ZIF-8 and Cu+2 can significantly diminish PL intensity, indicating that the recombination of h+ pairs transported to the ZIF-8 surface is hampered. Because the optimum loading of Cu+2 on ZIF-8 reduced the recombination rate of the charge carriers by extending their lifetime, the peak intensity of CDZ-20 is lower than that of other photocatalysts. As a result, it can be confirmed that optimal doping reduces exciton recombination by interacting with ZIF-8 and Cu+2.
3.5 Structural Properties:
Figure. 5d. shows the FTIR spectra of all the prepared photocatalyst. The asymmetric stretching vibrations of aliphatic C-H and aromatic rings are represented by the peaks at 3132 and 2930 cm-1. The stretching vibration of the C=N bond is responsible for the absorption band at 1587 cm-1, whereas the bending signals of the imidazole ring are responsible for the peaks at 1146 and 1308 cm-1. The stretching vibrations of the imidazole ring are responsible for the peak at 1425 cm-1. The bending vibrations of C-N and C-H are also responsible for the peaks at 997 and 754 cm-1. Furthermore, the signal at 691 cm-1 is a variant of the imidazole ring bent out of plane (28). The presence of the ZIF-8 structure in all of the material samples is shown by these peaks.
3.6 XPS Analysis:
XPS was used to examine the elemental states of the particular component contained in the material. According to the XPS survey scan in Figure. 6a, the CDZ-20 surface contained C, N, Zn, and Cu components. Figure. 6(b-c) depicts the high-resolution deconvolution spectra of the elements Zn 2p and Cu 2p found in composite CDZ-20, with Zn 2p exhibiting two significant peaks at 1021.8 and 1044.85 eV, in respect of Zn 2p3/2 and Zn 2p1/2. The Cu 2p deconvolution spectra show two distinct peaks at 932.7 and 952.9 eV, which are ascribable to Cu 2p3/2 and Cu 2p1/2, respectively (29). The C 1s spectra can be separated into two peaks of 286.7 eV and 284.7 eV, which correspond to C-N and C-C & C-H bonds, respectively, as shown in Figure. 6d (30). The dimethylimidazole structure in CDZ-20 was confirmed by the C 1s peaks. In Figure. 6e, the N 1s spectra revealed two fitted peaks at 398.8 eV and 400.8 eV, standing to N-Zn & N-Cu and N-C bonds, respectively (31).
3.7 Photoelectrochemical (PEC) studies:
Under the simulated sun light of AM 1G illuminator (100 mWcm2), a PEC study was conducted to investigate linear sweep voltammetry (LSV), transient photocurrent, and electrochemical impedance studies (EIS) for the produced catalysts ZIF-8 and CDZ-20. The LSV experiments were performed at a scan rate of 2 mV/s from -1 V to 1.5 V vs. RHE. The photocurrent density of CDZ-20 is significantly greater than that of the other photocatalysts, as shown in Figure. 7a. This increased current density is the result of efficient charge separation at optimum doping of Cu+2 ions on ZIF-8. These findings suggest that doping ZIF-8 with Cu+2 ions improved photocatalytic performance. At 1.5 V vs. calomel electrode, CDZ-20 had a high current density of 0.034 mA/cm2 and ZIF-8 had a low current density of 0.001 mA/cm2. The photocurrent response of ZIF-8, CDZ-6, and CDZ-20 has been tested transiently, and it is clear that the photocurrent response of CDZ-20 is larger than that of ZIF-8 and CDZ-6, signifying a significant improvement in photoexciton recombination quenching (Figure. 7b). The photoexcited electron/hole transfer process inside the suggested system has been disclosed by the electrochemical impedance spectroscopy (EIS) Nyquist plots which in turn correlate with the photocurrent results. CDZ-20 has a lower arc radius than pure ZIF-8, indicating that CDZ-20 has improved photo-induced charge carrier separation and migration during hydrogen production (Figure. 7c) (32). The flat-band potential is calculated using the Mott–Schottky equation (33). The open space dielectric constant is ε and the film electrode is ε0, with the remaining variables being ND for donor density, C for space charge capacitance, E for applied voltage, T for temperature, kB for Boltzmann's constant, and q for the electronic charge. The flat potential (Efb) of the screened photocatalyst under the various frequencies is shown by the intersection points in the Mott–Schottky plots (Figure. 7d). The Efb of ZIF-8 and CDZ-20 are 0.66 and 0.57 V, respectively (34).
3.8 Photocatalytic hydrogen production activity:
Under light illumination, the photocatalytic water reduction of all pre-prepared photocatalysts is examined by dispersing 10 mg of catalyst in aqueous mixture of 0.35 M Na2S and 0.25 M Na2SO3 sacrificial electron donors. In theory, the conduction band edge position of ZIF-8 is higher than 0 eV compared to NHE, favouring hydrogen production. When the photocatalyst is exposed to light, photoexcitons are created and transported to their respective band locations, resulting in a series of redox reactions. Figure. 8a depicts the hydrogen evolution under continuous irradiation, revealing that ZIF-8 and Cu doped ZIF-8 produced hydrogen production over time in the sequence: CDZ-20> CDZ-6> CDZ-2> CDZ-25>ZIF-8. Figure. 8b depicts the rate of hydrogen production, which shows that the CDZ-20 (13992 µmol. h-1 gcat-1) performed better. Table 2 demonstrates similar works done for the generation of hydrogen. Based on the findings, it is clear that optimum doping of Cu+2 plays a critical role in the photocatalytic H2 creation process. Excessive loading, on the other hand, is not favourable to improving photocatalytic performance. The photocatalytic activity decreases as the Cu+2 loading amount is raised further. In any case, the addition of Cu+2 can dramatically boost ZIF-8's ability to respond to visible light. In Table 1 apparent quantum efficiency (AQY) of all photocatalyst was calculated and CDZ-20 have found 9.08%, which is higher than other composites (S3). The photocatalytic stability of the CDZ-20 sample for photocatalytic hydrogen production is shown in Figure.8c, with a 2% decrease in activity after 4 cycles. The decline in activity could be attributed to the deterioration of SEDs. After the photocatalytic activity the catalyst was given for XRD analysis where we have noticed that ZnS formation with strong peaks at 29.1˚, 48.2˚ and 56.9˚. It is due the in situ formation of ZnS in the presence of SEDs Na2S and Na2SO3 during hydrogen production. The morphology was also studied for the catalyst after application. The shapes are retained but catalyst is agglomerated. The structure of the CDZ-20 changes very little after the photocatalytic reaction in Fig. S4.
TABLE 1: Hydrogen production results and AQY values of all the synthesized photocatalysts.
Photocatalyst
|
Rate of H2 mmol. g-1h-1
|
AQE %
|
ZIF-8
|
0.80
|
0.52
|
CDZ-2
|
7.42
|
4.80
|
CDZ-6
|
10.61
|
6.90
|
CDZ-20
|
13.99
|
9.08
|
CDZ-25
|
6.33
|
4.10
|
TABLE 2: Comparison of H2 evolution activity for different photocatalysts.
Photocatalyst
|
Light source
|
Sacrificial
agent
|
Activity
(mmol-g-1h-1)
|
Ref.
|
MoS2-RGO/ZnO
|
300W Xe
|
Na2S/Na2SO3
|
0.288
|
40
|
CoP/Co@NPC
|
300W Xe (>420 nm)
|
TEOA
|
0.004
|
41
|
CoSx/g-C3N4
|
350W Xe (>400 nm)
|
TEOA
|
0.629
|
42
|
CuxO/ZnO@Au
|
300W Xe
|
Na2S/Na2SO3
|
0.012
|
43
|
g-C3N4/ZnO
|
300W Xe (>420 nm)
|
TEOA
|
0.322
|
44
|
Au/ZnO
|
500W Xe (>420 nm)
|
Na2S/Na2SO3
|
0.029
|
45
|
ZnO/ZIF‑8/rGO/Carbon
|
300W Xe (>420 nm)
|
MeOH
|
0.731
|
46
|
TiO2@ZIF-8
|
300W Xe (>420 nm)
|
MeOH
|
0.261
|
47
|
C-ZIF/g-C3N4
|
300W Xe (>420 nm)
|
TEOA
|
0.326
|
48
|
ZnCdS QDs
|
300W Xe (>420 nm)
|
Na2S/Na2SO3
|
0.370
|
49
|
Cu@ZIF-67
|
420 W Xe
|
Na2S/Na2SO3
|
13.9
|
Present Work
|
3.9 Photocatalytic Mechanism:
The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in MOFs (LUMO) typically takes part in the photocatalysis process (35). The organic linkers (HOMO) can function as VB while the metal clusters (LUMO) can function as CB. Moreover, it was noted earlier that the e- in the CB and h+ in the VB of MOF-5 is the first example of an effective photocatalyst (36) application. The structure of ZIF-8 is similar to MOF-5 in that it contains tetrahedra of Zn4N, with the HOMO owing primarily to the N 2p bonding orbitals (VB) that form the imidazole linker, and the LUMO owing primarily to empty Zn orbitals (CB) (37). Based on the findings, a plausible mechanism for H2 generation over Cu+2 doped ZIF-8 can be proposed. ZIF-8 can expeditiously harvest visible light from the up-conversion action of Cu+2 to make photogenerated electron hole pairs because Cu+2 absorbs long wavelength light and emits short wavelength light (38). In ZIF-8, electrons are stimulated to the conduction band (CB) and a hole is created in the valence band (VB). Cu+2 acts as both an electron sink and a proton reduction site. Apart from H+ ions, sacrificial sulphate ions engage in the oxidation reaction and generate peroxy sulphates in the presence of H2O in the valance band (eq. 3). As illustrated in eq. 4-6, the direct participation of holes in sulphate ion oxidation into sulphide ions results in the formation of various intermediates, including dithionate and peroxy disulphide ions. By utilising the electrons in the conduction band, the formed/adsorbed H+ ions at catalytic sites undergo a reduction reaction to generate hydrogen (eq. 7). The oxidation of SO3-2 and S-2 ions occurred in the valance band due to the trapping of holes by inorganic scavengers, while the production of hydrogen occurred in the conduction band due to the reduction of H+ ions (39). The mechanism is shown in Figure 9.
The reactions involved in the Cu+2 doped ZIF-8 photocatalytic procedure are defined below.
Reaction mechanism:
Oxidation Reaction at Valence Band
Reduction Reaction at Conduction Band
Hence, from the above studies, the significant decrease of the charge-transfer resistance by introductions of Cu NP’s, clearly corroborates the superior charge transport ability of the frame work composite with desirable optoelectronic properties and stability when compared to its other metalorganic hybrid counterparts.