Biomimetic Synthesis of Highly Reusable MoO₃-based Catalysts for Fast Degradation of Azo Dyes

Irum Shaheen ?? ¹ , Khuram Shahzad Ahmad ?? ¹

1 Department of Environmental Sciences, Fatima Jinnah Women University, Rawalpindi, 46000, Pakistan

Abstract

Over the recent decades, unrelenting efforts are being devoted to the sustainable design and synthesis of transitional metal oxide-based photocatalysts with controlled morphology and structural complexity to enhance their catalytic properties. In this account, we have reported the bio-fuel-assisted hydrothermal synthesis of MoO₃, MoO₃:NiO, and MoO₃:PdO/Pd as catalysts to remove azo pollutants from an aqueous solution. Methyl orange was selected as the model dye to represent organic pollutants. This work presents a facile method for improving the visible-light-driven catalytic activity of MoO₃ by introducing NiO and PdO. When MoO₃:NiO and MoO₃:PdO/Pd were illuminated by solar light, emitted radiation originating from oxygen vacancies of NiO and PdO synergistically participated in catalytic reactions of MoO₃ giving 98% and 95 % degradation of methyl orange, respectively, in 15 min. To confirm the supporting role of NiO and PdO in the catalysis of MoO₃, catalytic experiments were carried out in dark ambient conditions, with only catalysts (without stimulants). Subsequently, the degradation efficiency of MoO₃:PdO, and MoO₃:NiO was increased to 73% and 84% respectively, from 62 % efficiency of MoO₃ suggesting that NiO and PdO greatly increased the efficiency of MoO₃ in dark conditions and nearly complete removal of methyl orange by photo-induced visible light degradation. Furthermore, the photocatalysts illustrated good reusability till four runs of experiments without loss in its degradation efficiency. Therefore, the overall catalytic results of the current study are highly proposing MoO₃:PdO and MoO₃:NiO as excellent photocatalysts for water remediation. 

Keywords

Phyto­template, Organic Pollutants, Photocatalysis, MoO3:PdO, MoO3:NiO, Water Remediator

INTRODUCTION

Environmental pollution, especially water pollution due to organic pollutants is becoming a lifelong environmental challenge for human survival.^1- 3 The major reason for water pollution is industrialization particularly industries manufacturing paper, fabrics, cosmetics, leather accessories, etc. Azo dyes are widely used in these industries.^4-5 Consequently, the removal of azo dye pollutants from water is extremely important to protect human health and also to protect the aquatic environment.^5-8 In the recent past photocatalysis of organic pollutants such as azo dyes is being reported as the most successful method to remove and degrade organic dyes from water bodies.^5-12 The photodegradation with help of solar light and semiconductor materials (such as metal oxides) is greatly investigated in dealing with environmental issues related to organic pollutants. ^{3-7, 10-12}

Among the different semiconductors materials (ZnO, TiO₂, Co₃O_4, etc), molybdenum trioxide (MoO₃) is the most competitive photocatalyst. It is n-type material having a floating direct band gap (2.3 to 3.6 eV), and is the most fascinating transition metal oxide due to its unique double-layered planar structure, higher chemical stability, and nontoxicity.¹³ MoO₃ has high photogenerated charge transfer efficiency due to its narrow bandgap and solar light absorption. Thus, MoO₃ has a high potential for photocatalysis and solar cell applications. Recently, MoO₃ has been extensively investigated as a photocatalyst for the degradation of dyes.^13-17 Hussain and Khalid, 2020 synthesized MoO₃ via a surfactant-assisted synthesis route for photo-degradation of different organic dyes (methylene blue, rhodamine B, and alizarin). The results of their study revealed 98%, 90%, and 74% degradation of methylene blue, rhodamine B, and alizarin respectively within 120 min by MoO₃.¹³ In another study, Al-Alotaibi et al., investigated the catalytic efficiency of MoO₃ synthesized by sol-gel and hydrothermal method for degradation of methylene blue. They demonstrated the complete degradation of methylene blue within 180 min.¹⁴ Moreover, MoO₃ film was prepared by Zhang et al., for the degradation of methylene blue and they reported its degradation efficiency of with150 min.¹⁵ Although effective degradation of dyes by MoO₃-based catalysts is well reported in literate. However, the time taken to remove organic dyes is equally important. In order to enhance the efficiency of MoO₃ to degrade organic dyes, MoO₃ has been synthesized in combination with other metal oxides and nano materials. Such as Kamalam et al., prepared nanocomposites of α-MoO₃/Graphene Oxide (GO) and studied it for degradation of Victoria blue dye in visible light irradiation. They showed 89% efficiency of GO/MoO₃ for the removal of organic dyes in 150 min.¹⁶ In another study, Huang et al., synthesized molybdenum oxide (MoO₃) nanorods decorated with molybdenum phosphide quantum dots and reported its 100% degradation efficiency within 40 min for Rhodamine B  dye and 97% efficiency for norfloxacin dye.¹⁷

Therefore, the present study is also an attempt to overcome the challenge of faster degradation of organic pollutants. For this purpose, in the current study, NiO and PdO were synthesized in MoO₃ semiconducting material to degrade methyl orange (hereafter called “MO”) in water. NiO is a p-type semiconducting material having a wide band gap which is making it apt as a photocatalyst application. Due to the higher hole mobility of NiO, it can degrade organic dyes in a shorter time under solar irradiation.¹⁸ Pd and PdO exhibit higher catalytic activity at lower temperatures. It is reported that incorporation of PdO with other transitional metal oxides (TMOs) results in superior photocatalytic behavior of TMOs. ¹⁹ Such a study is reported by Zhou et al., where they synthesized PdO–TiO₂ nanocatalyst which exhibited tremendously high photocatalytic potential for degradation of methylene and rhodamine B.²⁰ Therefore, in the current study MoO₃, MoO₃:NiO and MoO₃:PdO were synthesized using phytochemicals of A.pindrow leaves as template and fuel. Phytochemicals, not only as reducing but also as stabilizing agents, have been critically investigated.^21-23 In our previous study, we have successfully synthesized Co₃O₄ based photocatalyst by A.pindrow plant leaves for degradation of azo dye.²⁴ Therefore, the current research was the first attempt to synthesize MoO₃, MoO₃:NiO, and MoO₃:PdO/Pd using phytochemicals of A.pindrow. In the current experimentation, the hydrothermal method was used employing Phyto reducing and stabilizing agents. In the current experiment, hydrothermal reactions were initially carried out at 70 ^oC and final MoO₃ was obtained by calcination in air at ~ 400 ^oC. Moreover, the cost-effective and efficient hydrothermal synthesis route was adopted in the current experiment employing green reducing cum stabilizing agents to replace costly and hazardous chemical reagents and solvents. These organic compounds of A.pindrow, as stabilizing agents, incorporate into synthesized material increasing O and C atoms in the mediated materials to enhance their photocatalytic activity. In the present work, MO has been degraded using MoO₃, MoO₃:NiO and MoO₃:PdO/Pd with maximum degradation efficiency in 15 min. To the best of our knowledge, we have demonstrated the maximum degradation of MO using MoO₃-based catalysts in a shorter time of 15 min due to the addition of NiO and PdO as well as by increased O and C atoms of bioactive compounds. 

MATERIALS AND METHODS

Chemicals and Materials

The precursors used in the current study were Molybdenum acetate (Mo?(O?CCH?)?), nickel(II) acetate tetrahydrate (Ni(CH?CO?)?·4 H?O), Palladium acetate ((Pd(CH₃COO)₂). For catalytic experiments, methyl orange (MO) (C₁₄H₁₄N₃NaO₃S) was used. All chemicals were purchased from Merck chemicals Ltd. The solvent used in the present study was deionized water. Leaves extract of A.pindrow was used as synthesizing agent. 

Synthesis of Materials

The detailed methodology has been reported in our recent study²⁴ and was adopted to synthesize MoO₃, MoO₃:NiO and MoO₃:PdO materials with little modifications. In brief, 1 g of A.pindrow powdered leaves were treated in 50 mL of deionized water at 60 ^oC and 600 rpm for 30 min and then filtered. 10 mL of filtrate was added as fuel into 100 mL of 20 mM Mo?(O?CCH?)? aqueous solution on magnetic stirring. The reaction mixture was kept at 70 ^oC for 2 h at 550 rpm and then retained at room temperature in dark conditions for 24 h. Afterward, the solution was evaporated at 95 ^oC to get dry powder which was annealed at 460 ^oC for 4.5 hours to obtain MoO₃. The calcinated MoO₃ was suspended in deionized water in two different beakers. In one beaker of prepared MoO₃ suspension, the aqueous solution of Ni(CH₃CO₂)₂·4H₂O was added, and in other beakers, Pd(CH₃COO)₂ aqueous solution was added. The ratio of MoO₃:NiO and MoO₃:PdO/Pd were carefully controlled in 8:2. Therefore, 20 % Ni(CH₃CO₂)₂·4 H₂O and 20 % Pd(CH₃COO)₂ were added in the suspension of MoO₃. In each reaction, 5 mL of A.pindrow extracted phyto-reagents was added and was kept at constant stirring for 2 h at 80 ^oC. Then, reactions were evaporated completely at 96 ^oC before calcinating at 450 ^oC for 3.5 h to obtain MoO₃:NiO and MoO₃:PdO. 

Catalytic degradation of methyl orange (MO)

In the present catalytic experiment, MO was taken as an organic pollutant. Solar radiations were used to provide visible light. The measured quantity of 2 mg of each prepared material was added to 15 mL of MO solution (1 mg/mL), on magnetic stirring for 60 min to achieve reaction equilibrium. After then the solution was subjected to visible light radiation for 15 min. During the process, a small amount of solution was taken regularly to monitor MO concentration by UV Vis spectrophotometer at 464 nm. To study the effects of visible light on the degradation of MO, a dark experiment was also run, for that three centrifuge tubes were taken and covered entirely by Aluminium foil to avoid interference of any type of stimulant. In one tube 2 mg MoO₃ was added and in the other two tubes, the same amount of MoO₃:NiO and MoO₃:PdO/Pd were added. In each tube, 15 mL of MO solution was added and then left at room conditions for 15 min. The degradation of MO in dark mild conditions was also observed at equal time intervals as for light experiments. Moreover, an aliquot from the stock solution of MO was taken and subjected to the same light and dark experimentation as a blank sample to appraise the meticulous efficiency of the catalysts. After completion of each experiment (i.e. after every 15 min) catalysts were recovered, dried, and tested for another run of the experiment to investigate the reusability potential of the bio-synthesized materials. 

Characterization

Each synthesized material was keenly investigated for its structural, compositional, and morphological properties by numerous analytical instruments and techniques as described in our recently published work.²⁴ The catalytic experiment was monitored by ultraviolet-visible spectrophotometry 1602-BS, Spain (UV-Vis). The gas chromatography coupled with mass spectroscopy (instrument model: GC-MS-QP5050) (GCMS) and FTIR-8400S was used to study the organic constituents of synthesized materials. PANalytical X'Pert Pro-XRD5 (X-ray powder Diffractometer (XRD), Quanta FEG-250 SEM (an environmental Scanning Electron Microscopy (FE-SEM)) coupled with energy-dispersive X-ray microanalysis (EDX) and Renishaw's Raman systems (Raman spectroscopy) were employed to study phase, morphology, and composition of mediated nanomaterials. 

RESULTS AND DISCUSSION

Phyto-synthesized materials

The annealed powders of MoO₃, MoO₃:PdO, and MoO₃:NiO were explored by FTIR and subsequent spectra are annotated in Figure 1(a-c). In all synthesized materials, vibrational peaks corresponding to O-H stretch (phenol, alcohol) were found at 3448.84 cm^-1, 3457.47 cm^-1, and 3490.06 cm^-1 for MoO₃, MoO₃:PdO, and MoO₃:NiO respectively. This revealed the incorporation of phenolic compounds in the synthesized materials. However, MoO₃ NPs (Figure 1(a)) revealed the presence of aromatics (C-C stretch in ring and C-H “oop) at 1648.14 cm^-1 and 1437.8 cm^-1, 855.7 cm^-1, and 791.8 cm^-1 respectively. In Figure 2(b) vibrational frequencies of MoO₃:PdO at 1648.14 cm-¹, 1028.9 cm^-1, 786.71 cm^-1, 560.89, and 439.9 cm^-1 corresponding to C-C stretch of aromatics, N-H stretching relating to aliphatic amines, C-H “oop” of aromatics and M-O bond of metal oxides (M = Mo, Ni, Pd) respectively.^25-26 M-O bonds were also demonstrated in MoO₃:NiO (Figure 1c) at 567.8cm^-1, whereas in MoO₃:NiO aromatics groups were also observed at 1622.13 cm^-1, 989.5 cm^-1, and 871.44 cm^-1(C-H “oop”) while the minor peak at 1377.52 cm^-1 was indicating the presences of nitro compounds as it depicts the presences of N-O groups. 

Figure 1: MoO, (b) MoO:PdO and (c) MoO(d) MoO, (e) MoO:PdO and (f) MoO:NiO.

From Figure 1(a-c) it is shown that aromatic functional groups of phenol compounds are present in the synthesized materials. However, to confirm these belong to bio-organic compounds of the plant leaves, MoO₃, MoO₃:PdO, and MoO₃:NiO were studied by GC-MS and the resulting spectra have been annotated in Figure 1(d-f). According to NIST library of GC-MS-QP5050, the peaks of figure 1d are illustrating the cyclobutanol (C₄H₈O) in MoO₃ nanoparticles. While Benzeneethanamine (C₈H₁₁N), Benzenemethanol (C₉H₁₃NO) Benzeneethanamine (C₈H₁₁N), and cyclobutanol were predicted in MoO₃:PdO (Figure 1(e)) at 4.7 min, 4.9 min, 5.4 min and 20.4 min retention times respectively. MoO₃:NiO in Figure 1(f) depicts the incorporated compounds at 28.4 min retention time, corresponding to cyclobutanol. Therefore, GCMS has validated the FTIR peaks interpretation that the aromatic organic phenolic compounds are present in the synthesized materials. The proposed compounds by GC-MS are allied with A.pindrow phytochemicals which has been reported in our recent study.²⁴ The carbon/oxygen functional groups will in turn enhance the visible-light-induced degradation of the catalysts due to their hydrophilic nature and the higher negative charge densities. These features of carbon/oxygen functional groups can facilitate the degradation of azo dye by augmenting the separation between electrons and holes during photocatalysis.^36-37

Figure 2: Raman spectra of organic compounds derived nanomaterials: (a) MoO₃, (b) MoO₃:NiO and (c) MoO₃:PdO.

Figure 3: X-rays diffraction patterns of A.pindrow assisted (a) MoO₃, (b) MoO₃:PdO, and (c) MoO₃:NiO. 

Further, MoO₃, MoO₃:PdO, and MoO₃:NiO were studied by Raman spectroscopy (Figure 2). All the images in Figure 2 are suggesting MoO₃ as key constituents of the phytosynthesized materials, as Raman scattering vibration of Figure 2 are well matched with reported Raman spectra of MoO₃ by Krishna et al.,²⁷ Joya et al.,²⁸ Liu et al.²⁹ The presence of NiO and PdO in figure 2b and Figure 2c is determined by comparing the spectral vibrations with reported Raman scattering vibrations of NiO and PdO by Ren et al.,³⁵ and Korifi, et al.,³⁹ respectively. Nevertheless, the increased concentration of organic compounds in the MoO₃:PdO can be speculated in the Raman scatterings of MoO₃:PdO in Figure 2(c) as demonstrated by the GC-MS spectrum in Figure 1e. GC-MS analysis of MoO₃:PdO illustrated more than one organic compound. Moreover, Figure 1e revealed the higher peak intensities of organic species as compounds to figure 1d and Figure 1f of MoO₃ and MoO₃:NiO respectively. Therefore, MoO₃:PdO showed a noticeably changed spectrum of Raman over 300–3000 cm^-1 due to different organic compounds of A.pindrow  template. However, determined phases from Raman scatterings vibrational patterns of synthesized materials were further endorsed by X-ray diffraction patterns as given in Figure 3. 

Figure 4: Elemental composition of A. pindrow assisted (a) MoO₃, (b) MoO₃:PdO, and (c) MoO₃:NiO Via Energy Dispersive Spectroscopy.

X-rays diffraction patterns are indicating the phyto-synthesis of MoO₃, MoO₃:PdO, and MoO₃:NiO materials by scrutinizing their phase purity dimensions and by estimating their crystallite sizes. The diagnosed patterns in XRD spectra in Figure 3(a) are depicting the MoO₃ phase of bio-mediated nanoparticles according to JCPDS:00-005-0508. Standards patterns further revealed that MoO₃ particles were crystallized with Orthorhombic, molybdite, syn (with lattice parameters a= 3.962, b= 13.858, and c = 3.697). Figure 3b shows diffraction patterns of MoO₃:PdO nanomaterial. MoO₃produce the peaks in figure 3b (*) at 12.6329^o (020), 23.2225^o (110), 25.5116 ^o (040), 27.225o (021)^{, 33.138o (101), 33.569o (111), 35.4385 o (041), 38.8161o (060), 45.0736o (200), 46.2775o (210), 49.2687o (002), 52.3683o (211), 55.1159o (112), 56.3964 o (042), 57.4644o (171), 58.5905o (081), 64.7541o (062), and 69.5093o (202) with respective hkl planes Moreover the X-rays diffractogram in Figure 3(b) also revealed the presence of tetragonal PdO (♦) at 29.317 (100), 33.55 (002), 33.84 (101), 41.93 (110), 45.139 (102), 60.22 (103), 60.75 (200), and 71.46 (211) (ICSD:00-041-1107) with P42/mmc space group and unit cell parameters of a = 3.0456 Å, b = 3.0456 Å and c = 5.3387 Å. Figure 3(c) shows the XRD patterns of bio-organic compounds derived MoO₃:NiO. The diffraction patterns in Figure 3(c) show well-defined prominent peaks of molybdenum oxide-MoO₃ (ICSD 00-005-0508) as well as NiO oxides (ICSD:00-047-1049). The X-ray diffractogram has shown that MoO₃ peaks (*) at 2theta (o) = 12.775, 23.41, 25.67, 27.38, 33.81, 35.48, 38.95, 46.39, 49.36, 52.75, 54.19, 56.44, 57.75, and 58.79 corresponding to hkl planes of (020), (110), (040), (021), (111), (041), (060), (210), (002), (211), (112), (042), (171), (081), (062), and (190) respectively. While the presence of NiO (?) in Figure 3c was indicated at 2 37.3o (111 hkl), 43.2 o(200 hkl), 62.8 o(220 hkl). The crystallite sizes of all synthesized samples were calculated via Scherrer's equation as described in our earlier study.25 The calculated sizes of MoO₃ was 40-43 nm, MoO₃:PdO had 31-33 nm while MoO₃:NiO had 19-21 nm crystallite size. XRD certified the phytosynthesis of MoO₃, MoO₃:PdO, and MoO₃:NiO material while the elemental composition of synthesized material was investigated by Energy Dispersive Spectroscopy (EDX) as given in Figure 4.} 

In Figure 4(a), EDX revealed the presence of O and Mo with atomic % of 74.11 and 21.39 respectively along with 4.3% carbon. Figure 4(b-c) also presents the presence of C in both MoO₃:PdO and MoO₃:NiO. The presence of C is due to the phytochemicals of A.pindrow extract proposing the phyto stabilization of MoO₃, MoO₃:PdO, and MoO₃:NiO. These results of EDX are well in agreement with Figure 1 where organic functional groups were described.

Figure 5: FE-SEM images A.pindrow synthesized materials at different magnifications (a) MoO₃ at 15KX , (b) MoO₃ at 31KX , (c) MoO₃ at 52KX, (d) MoO₃:PdO at 20KX, (e) MoO₃:PdO at 50KX, (f) MoO₃:PdO at 100KX, (g) MoO₃:NiO at 30 KX, (h) MoO₃:NiO at 103KX, and (i) FESEM images MoO₃:NiO at 202 KX.

Figure 5 shows the uniform arrangements and regular structures of particles at different magnifications by FE-SEM. The uniform pattern and regular structures of MoO₃, MoO₃:PdO, and MoO_3:NiO are particularly observable at lower magnifications in Figure 5a, Figure 5(d), and Figure 5(g) respectively. From Figure 5a-c, rod-like structures of MoO₃ particles can be speculated at 5 µm to 1 µm scales. Figures 5(d-e) are presenting 5 µm, 2 µm, and 1 µm structures of MoO₃:PdO with non-homogeneous shapes but Figure 5d-e is correspondingly showing some porosity in these particle’s structures due to embed PdO. Upon the introduction of NiO and by re-thermal annealing (Figure 5(g-i)), MoO₃ morphology has greatly changed into sheet-like structures with spherical-shaped NiO. Agglomeration is generally occurred in doped materials especially at the nanoscale, however, from Figure 5 it can found that synthesized MoO₃:PdO (Figure 5f) and MoO_3:NiO (Figure 5(i)) materials are showing less agglomeration at 1µm and 500 nm, respectively. This is suggesting the role of stabilizing agents of the plant leaves biochemical as reported extensively in the literature.^22-26

Degradation study of MO by synthesized materials

The catalytic activity of the biosynthesized MoO_3, MoO₃:NiO, and MoO₃:PdO catalysts was scrutinized by MO degradation in the water under solar light radiations at the room temperature. The concentrations of MO were taken at 1 mg/mL and catalysts loading was controlled at 2 mg/15 mL. The catalytic behavior of MoO₃:NiO and MoO₃:PdO was investigated in comparison with MoO₃. The λ _max of MO was reported at 464 nm,^{30, 31, 37, 38} thus, absorbance intensity at 464 nm was observed critically to determine the amount of MO degraded by the fabricated catalysts using the equation reported previously.²⁴

Figure 6: UV/Vis spectra of MO photo-degradation by (a) MoO₃, (b) MoO₃,:PdO, (c) MoO₃:NiO, and MO degradation without stimulant under dark conditions of (d) MoO₃, (e) MoO₃:PdO, and (f) MoO₃:NiO.

Figure 7: (a) Percentage degradation of MO without catalysts at different time intervals (insert: absorption spectra of MO without catalysts in light and in dark), the calculated degradation percentages of MO by synthesized nanomaterials: (b) MoO₃, (c) MoO₃:PdO, (d) MoO₃:NiO.

As shown in Figure 6, absorption intensity decreases with the increment of time. The UV/Vis measurements of synthesized catalysts were taken at a range of time intervals from 0 to 20 min, and subsequently, absorption curves were delineated at 2, 5, 10, and 15 min. Reduction in absorption is due to the breakage of azo bonds by synthesized nanomaterials in the presence of light (Figure 6(a-c)) and dark conditions (Figure 6(d-f). The absorption spectra are depicting that MO peaks are significantly reduced with solar light stimulants than without solar light stimulants (under dark conditions). It can be seen in Figure 6c that the absorbance peak completely disappeared till 15 min by MoO₃:NiO where the degradation was simulated by Visible light while MoO₃:PdO also revealed a great reduction in absorption as compared to MoO₃. It is worth describing that in Figure 6d the absorption of the MO (without catalysts) is considerably higher in both light and dark as compared to Figure 6. Thus, the reduction in absorption intensities in Figure 6 is associated with the catalysis potential of the synthesized materials. However, higher reduction in Figure 6 (b, c) and in Figure 6 (e and f) is indicating that introduction of Ni and Pd in MoO₃ has shifted the electronic structure of MoO₃ by the formation of NiO and PdO in MoO₃ which have enhanced the efficiency of MoO₃ to degrade MO, as also revealed in Figure 7 and Table 1. According to table 1, the self-photodegradation of MO was only 5 % till 15 min and almost no degradation was observed under ambient conditions, this has further supported the role of mediated materials in degrading MO. 

Table 1: Degradation of azo dye calculated for each synthesized catalyst and for blank sample.

Figure 8: Degradation Reaction kinetics of investigated catalysts under solar light: (a) MoO₃ (b) MoO₃:PdO, (c) MoO₃:NiO and Degradation Reaction kinetics of investigated catalysts under dark conditions (d) MoO_3 , (e) MoO₃:PdO (f) MoO₃:NiO. 

Table 1 and Figure 7(b) show 80% and 85% degradation of MoO₃ with the help of solar light in 10 and 15 min whereas without visible light (in dark room conditions) only 48 % and 62 % MO were degraded in the same time. However, by doping of PdO in MoO₃ photodegradation of MO was increased to 66 % and 95 % in 10 and 15 min respectively as presented in Figure 7(c). Under dark conditions, 73 % degradation was observed after 15 min by MoO₃-PdO. The highest degradation (98 %) was observed when NiO was incorporated in MoO₃ with solar light stimulation. Even after 2 min, 72.23 % degradation of MO was found assisted by visible light while after 5 min 86 % removal of MO was observed by MoO₃:NiO which is higher than 85 % degradation of MO by only MoO₃ sample after 15 min. Not only by solar irradiation, MoO₃:NiO revealed the highest degradation of ~ 85 % in dark conditions. Thus, MoO₃:NiO has huge potential as a catalyst (particularity as a photocatalyst) for the degradation of MO due to the combination of p-type NiO and n-type MoO₃ as well as due to nanoscopic features of MoO₃:NiO as described in Figure 5(g-i). The Fermi equilibrium (at the p-n junction) is inhibited the recombination of photogenerated charge carriers (e^_/h⁺) to prolong their separation, consequently increasing the photocatalysis.^35, 37-38 The experiments of degradation of MO under solar irradiation showed superior catalytic efficiency of MoO₃:PdO and MoO₃:NiO (with 2 mg catalyst in 15 mL of dye solution of 100g/L) than the photocatalytic activity of molybdenum doped TiO₂ nanostructure (dye 0.02M and 1.36% catalysts)³³ MoO₃ (having 250 mg per 250 mL catalyst and dye amount),¹⁶ ZnO/MoO₃ nanotubes (0.3 g/L dye and 1 g catalyst),¹⁵ and Mo-doped TiO₂ (20 mg/L MO solution and 0.5 g catalyst).¹⁷ The enhanced photocatalytic activity can be due to carbon phytochemicals which retard the recombination of holes and electrons in the presence of light.^38-39 Thus, it can be concluded that the carbonaceous functional groups of Phyto template may have a significant role in improving the catalytic activity in the presence of light radiation. 

Figure 9: Different cycles of photodegradation experiments of MO by (a) MoO₃, (b) MoO₃:PdO and (c) MoO₃:NiO.

From Figure 8, it can be notable illustrated that ln(C₀/C) varies linearly with the reaction time accordingly, the kinetics of MO degradation is consistent with the pseudo first-order reaction model. The regression values (R²) for MoO₃, MoO₃:PdO, MoO₃:NiO in light irradiation were 0.88, 0.89 and 0.8 respectively while R² = 0.65, 0.96, and 0.74 were found for MoO₃, MoO₃:PdO, MoO₃:NiO respectively for the dark catalytic reaction. The results of regression analysis of MoO₃:PdO (Figure 8(e-f)) were depicted. MoO₃:PdO was stable catalysis with and without solar light stimulating conditions while all catalysts showed primary stability with the application of solar light.

The stability and reusability of catalysts are a key important factor for the evaluation of their practical application.^31,39 Thus, the synthesized catalysts were tested for different cycles of photocatalytic performance. The catalytic performance of MoO₃, MoO₃:PdO, and MoO₃:NiO till four runs is shown in Figure 9(a-c) respectively in the presence of solar irradiation. The photocatalytic efficiency of the fabricated catalysts was remarkably maintained till the fourth run, as no change in the absorbance values was recorded in Figure 8. Thus, reusability experiments were demonstrating that tested catalysts retain excellent stability in terms of catalytic performance.

Degradation mechanism

Figure 10: Photocatalysis mechanism of MoO₃:NiO assisted dye degradation.

In the present experimentation, the possible process for degradation of MO over semiconducting MoO₃ in the presence of visible light radiation is a photocatalytic process, in which light-induced electron and hole existing over MoO₃ with O₂and OH^_ to form oxidants •O₂ and •OH^-, respectively, which then react with organic compounds and degrade them. In the MoO₃:PdO and MoO₃:NiO increased oxygen valances are accountable for the degradation process by the mechanism of chemical wet oxidation (CWO). This mechanism is described previously by Huang et al.,¹⁵ Li et al.,³⁶ and Srikhaow and Smith.³⁷

In the present study, the process of photocatalysis was accelerated by the carbon of phytochemicals. As Reddy et al.,³⁵ described that carbon is an efficient electron acceptor while semiconducting materials like MoO₃, are strong electron donors. The incorporated carbon-based phytochemicals (revealed by GC-MS) eased the flow of the charge carrier in opposite direction in the presence of photo-irradiation for efficient utilization of light for the degradation of azo dyes. Thus, depending on holes as well as electrons of MOs along with inserted carbon comprising materials, the mechanism of MO degradation can be described in reaction scheme 1.⁵

As shown in above scheme 1, electrons were produced by solar light and by the oxidation process. Freely transferred from the conduction bands to the surface of catalysts which is composed of mixed metal oxides and carbon and oxygen-containing atoms which then bind with conjugated MO molecules. At the same time, holes produced by photons as well as the CWO process persisted in the valence band and then get trapped by hydroxyl groups present on the catalyst surface to contribute OH• radicals. After this, dissolved oxygen reacted to electrons of carbon (e⁻) and gave O₂•⁻ (superoxide radical anion). O₂•⁻ after protonation produced hydroperoxy radicals (HO₂•) that were robust OAs in MO degrading, accordingly, improving catalytic action of the synthesized material. Consequently, the generated holes, hydroxyl free radicals as well as superoxides of •OH and •O₂ are responsible for the efficient degradation of MO.

CONCLUSIONS

In this work, the visible-light supported the catalytic activity of MoO₃, MoO₃:PdO and MoO₃:NiO, which was investigated to degrade MO. MoO₃ was synthesized by low-cost and green template of A. pindrow and then NiO and PdO were separately synthesized in MoO₃ in the presence of biofuel. The introduction of PdO and NiO greatly improved the catalytic activity of MoO₃. The photocatalytic investigation along with the structural, compositional, and morphological analysis supported the outstanding visible light-induced catalytic behavior of phyto-synthesized nanomaterials. Moreover, the effects of visible light on degradation were confirmed by degrading MO in dark conditions which demonstrated that degradation of MO by synthesized catalysts was greatly enhanced in the presence of solar radiation. The reasons for this enhancement of photodegradation were increased oxygen valances of mixed metal oxides, carbon of the phyto-stabling agents, and n-p-type semiconducting materials. These factors tailored the electronic structures of the synthesized materials to enhance their catalytic activity for azo dyes degradation. Finally, the excellent reusability till four cycles of experiments revealed the huge potential of MoO₃:PdO and MoO₃:NiO as photocatalysts for industrial scale. 

ACKNOWLEDGEMENTS

The authors acknowledge the Higher Education Commission of Pakistan, and Department of Environmental Sciences, Fatima Jinnah Women University Rawalpindi Pakistan. Authors profoundly acknowledge The University of Manchester UK for the providence of technical facilities. Authors declare no conflict of interest. 

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