X-Ray Diffraction studies
Figure 2 shows the XRD pattern obtained on metal aluminate-activated carbon based photocatalyst materials. Figure 2A represents the XRD pattern of activated carbon which reveals broad diffraction peaks. The lack of sharp peaks indicates the presence of amorphous structure [Wang, Lu 1997]. The broad diffraction peaks at 2θ = 24.1° and 43.8° were attributed to the (002) and (001) planes of the activated carbon. The peaks were in good accordance with the standard JCPDS data (82-1691) of activated carbon [Sulistianti, 2017].
The XRD spectrum of the NiAl2O4/AC nanocomposite are shown in Fig. 2B. The diffraction peaks appeared at 2θ = 19.4°, 24.1°, 37.3°, 43.6°, 43.8°, 63.1° and 65.9° which correspond to the (111), (002), (311), (400), (001), (511) and (440) planes of NiAl2O4 and activated carbon. The results were in good agreement with the standard JCPDS data (10–0339)[33] for NiAl2O4 and (82-1691) for activated carbon.
The XRD spectrum of the Zn0.80Mn0.20Al2O4−δ /AC nanocomposite is shown in Fig. 2C. The diffraction peaks at 2θ = 24.1°, 31.5°, 36.6°, 43.8°, 44.9°, 56.2°, 59.3° and 65.2° corresponds to the (002), (220), (311), (001), (400), (422), (511) and (440) planes of Mn doped ZnAl2O4 and activated carbon. The results were in good agreement with the standard JCPDS data of ZnAl2O4 (82-1043) [Anchieta et al. 2015] and activated carbon (82-1691).The XRD spectrum of the Co0.85Ni0.15Al2O4/AC nanocomposite is displayed in Fig. 2D. The diffraction peaks found at 2θ = 24.1°, 31.5°, 37.1°, 43.8°, 44.9°, 55.8°, 59.4° and 65.4° attributed to the (002), (220), (311), (001), (400), (422), (511) and (440) planes of Ni doped CoAl2O4 and activated carbon. The results were in good agreement with the standard JCPDS data for CoAl2O4 (82-2249) [Kumari et al. 2018] and activated carbon (82-1691).
Ftir Studies
Figure 3A shows the FTIR spectrum of metal aluminate-activated carbon based photocatalyst materials. The peak found at 3350–3450 cm− 1 reveals the existence of C-OH bond in the sample. The peak appeared at around 1635 cm− 1 may be due to the C = O group in carboxylic or carbonyl functions and the large peak found at 1425 cm− 1 can be due to carboxylic carbon [Pereira et al. 2003]. The broad peak found near 1050 cm− 1 can be assigned to the saturated aliphatic carbon. In the FTIR spectrum of NiAl2O4/AC (Fig. 3B), the presence of some new peaks indicate the existence of metal oxide groups. The peaks found at 734 cm− 1 and 540 cm− 1 are very significant that can be associated to the bonds of the tetrahedral and octahedral sites of the spinel [Ahangaran et al. 2013]. In Fig. 3C (Zn0.80Mn0.20Al2O4−δ/AC), the peaks appeared between 400–500 cm− 1 may be due to the metal-oxygen (Zn-O) vibrational frequency as reported [Bouzid et al. 2009]. In Fig. 3D (Co0.85Ni0.15Al2O4/AC), the bands found at to 670, 560, and 495 cm− 1 are ascribed to the vibration forms of M − O, Al − O, and M − O−Al in the spinel structure [Ouahdi et al. 2008; Salleh et al. 2005]. The peak found at 1618 cm− 1 can be assigned to the H–O–H bending vibration and the band appeared at 1013cm− 1 may be due to the M-O-M starching vibration [Li et al. 2020].
Sem Studies
Figure 4 shows the SEM micrographs of the metal aluminate-activated carbon based photocatalyst materials. Fig. 4 A shows the absorbent characteristics of activated carbon. These pores afforded a decent opportunity for aluminate based nanoparticles to be confined into them. Figure 4B exhibits the surface morphology of NiAl2O4 assisted on the activated carbon. This micrograph revealed that NiAl2O4 nanoparticles were accumulated into uneven pores and also irregularly dispersed on the surface of activated carbon. Similarly the morphological images of Zn0.80Mn0.20Al2O4-δ and Co0.85Ni0.15Al2O4 nanoparticles supported on activated carbon aredisplayed on Fig. 4C and Fig. 4D respectively.
Edax Analysis
Figures 5A, B, C and D represent the EDAX spectra obtained on activated carbon (AC), NiAl2O4/AC, Zn0.80Mn0.20Al2O4−δ/AC and Co0.85Ni0.15Al2O4/AC based nanophotocatalysts respectively. Existence of appropriate elements in appropriate levels in the samples has been confirmed based on EDAX data. The data clearly inferred that the presence of appropriate metal aluminates on the surface of activated carbon.
Uv Spectroscopic Studies
The optical properties of the as- synthesized nanophotocatalysts were investigated using UV visible technique. The UV spectrum of the prepared nanophotocatalysts are displayed in Fig. 6. In Fig. 6A, represents the absorption spectrum of activated carbon. Then by improving the surface area of the dispersion, a larger absorption in the UVspectra is anticipated [Sharif et al. 2009]. The strong absorption band in carbon materials is correlated to electronic transitions among bonding and anti-bonding π orbital [Green et al. 1991]. Usually, the π –π* transitions emerge in between 180–260 nm in activated carbon materials, as reported [Jager et al. 1999]. Figure 6B explains the UV spectra of carbon based NiAl2O4 composite. NiAl2O4/AC nanophotocatalyst shows a strong peak at around 360 to 390nm, with the maximum value at 380 nm approximately. Figure 6C displays the UV spectra of Zn0.80Mn0.20Al2O4-δ /AC nanocomposites acheived between the wavelength range of 200 to 800 nm. The excitation wavelength of Zn0.80Mn0.20Al2O4-δ /AC was in the range of 340 to 370nm, with the maximum absorption around 365nm. Figure 6D shows the UV visible absorption spectra of Co0.85Ni0.15Al2O4-δ /AC nanocomposites. All these samples show a uniform feature, a strong and triple absorption band (540-650nm) and also a peak around 400nm is located at the visible region of the spectrum [Llusar et al. 2001].
From the UV results, we can calculate the direct band gap from Tauc plot [Cimino et al. 1971]. The Tauc equation is indicated below in Equation no. 2.
αhν = A(hν – Eg)n -- (2)
where, ‘α’ = absorption coefficient,
‘h’ = Planck’s constant,
‘ν’ = photon’s frequency,
‘A’ = constant,
‘Eg’ = Band gap energy,
‘n’ = ½ (for direct band gap).
An extrapolation of the linear region of a plot of (αhν)2 vs hν gave the value of the optical band gap (Eg) as shown in the Fig. 7. The band gap values were found to be 3.15, 2.21, 1.51 and 1.07eV for NiAl2O4/AC, Zn0.80Mn0.20Al2O4−δ/AC, Co0.85Ni0.15Al2O4−δ/AC and activated carbon respectively which are in line with the recent reported data [Karpinska, Kotowska, 2019; Somraksa et al. 2019; El Jabbar et al. 2019].
Photocatalytic Studies
The photocatalytic efficiency of activated carbon based metal aluminate nanocrystalline photocatalyst materials was examined by the removal of malachite green dye under visible light irradiation for 120 minutes. The results are shown in Fig. 8. To determine the photocatalytic activity of the prepared nanophotocatalysts on the elimination of malachite green dye under visible light, a series of experiments were carried out. The adsorption of the MG solution was carried out for 60 minutes in the absence of light to attain the adsorption - desorption equilibrium of the photocatalysts before exposing to the visible light. Blank test was performed under visible light irradiation without addition of any photocatalyst in which the removal of MG dye is very less. From Fig. 8, it was revealed that the photocatalytic activity enhances with the addition of carbon amount. The activated carbon based composites shows greater efficiency in degrading the malachite green dye than the metal aluminate based nanoparticles alone [Arunkumar, Nesaraj 2020; Arunkumar, Nesaraj 2021]. The carbon based photocatalyst absorbs large photons, creating large electron–hole pairs in the carbon based photocatalyst surface, which enhances the hydroxyl ions liberation and correspondingly enhances the degradation process [Ahmed et al. 2011]. This is because of the fact that activated carbon has larger surface area and also increases the adsorption sites and active centers [Li et a. 2012]. The best photocatalytic activity was observed in Co0.85Ni0.15Al2O4-δ /AC (0.50g) photocatalyst, which degraded all the MG dye present in the solution within 90 minutes of visible light irradiation.
In Fig. 9, the first order rate constant of carbon based composites was presented. A plot drawn between ln Ct/C0 against time is shown in Fig. 9. The slope of the straight line gave the value of first order rate constant, k. The photocatalytic reaction obeys pseudo first order reaction mechanism explained by Langmuire and Hinshelwood kinetic model [Pan, Liu, 2012].
lnCt/C0 = -kt (3)
Where ‘C0’ is the initial concentration, ‘Ct’ is the concentration after time t, and k is the first order rate constant. The rate constant values were found to be 0.0199 min− 1 (NiAl2O4/AC), 0.0196 min− 1 (Zn0.80Mn0.20Al2O4−δ /AC), 0.0278 Co0.85Ni0.15Al2O4−δ /AC) and 0.0085 min− 1 (AC).
The efficient photocatalyst (Co0.85Ni0.15Al2O4−δ /AC) has been employed to investigate its reusability performance. Reusability of the photocatalyst is one of the significant requirement of any photocatalyzed reaction, but very few tests are reported so far on reusability investigations [Sridewi et al. 2011; Markovic et al. 2013]. The degradation time for every recycling process has been set to 2 hours. After 2 hours of photocatalytic degradation, the photocatalysts were removed from the photoreactor, filtered off, dried in an oven at 80 oC for 2 hours and calcined at 600 oC for 2 hours to eliminate any kind of impurities. The reusablity characteristics of Co0.85Ni0.15Al2O4−δ/AC after 4 recycles are presented in Fig. 10. As the cycle enhances, the degradation efficiencies are going downward slowly from the first time (100%) to the fourth time (84%). This may be due to the reduction of active centers of the photocatalysts after recycling process. These results show that Co0.85Ni0.15Al2O4−δ /AC photocatalyst has outstanding photocatalytic properties as well as an admirable recycling characteristics under visible light. The reusability performance of Co0.85Ni0.15Al2O4−δ /AC on the degradation of malachite green under visible irradiation is reported in Fig. 10.
The photocatalysts with lower band gap exhibits better photocatalytic efficiency. Because electrons in the valence bands could easily excited to conduction bands, holes (h+) were remain in the valence bands. Then the photoexcited electrons reacted with adsorbed O2 and the holes reacted with H2O at the surface of the photocatalysts to generate superoxide (●O2−) radicals and hydroxyl (OH●) radicals, respectively. The above reactive species (●O2−, OH● and h+) were the reason for the degradation of MG in solution [Somraksa et al. 2020]. From the results, it was understand that the photocatalysts with the highest carbon concentration, displays better photocatalytic activity than the other samples. The photocatalytic mechanism suggested for the degradation of MG dye present in water sample with carbon based composites in presence of visible light can be indicated as shown in Equations 3–7 [Wilhelm, Stephen, 2007; Mills et al. 1993].
Step 1. AAC composite + hγ → AAC composite (e− +h+) -- (3)
Step 2. e− + O2→ ●O2− -- (4)
Step 3. 2e− + O2 + 2H+→ H2O2 -- (5)
Step 4. H2O2 + ●O2− → OH− + OH● -- (6)
Step 5. OH● /h+ / ●O2− + MG → Degraded product -- (7)