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Jul 23, 2023

Bandgap engineering approach for designing CuO/Mn3O4/CeO2 heterojunction as a novel photocatalyst for AOP

Scientific Reports volume 13, Article number: 3009 (2023) Cite this article 1336 Accesses 3 Citations Metrics details A ternary nanohybrid CuO/Mn3O4/CeO2 was developed in the present work using a

Scientific Reports volume 13, Article number: 3009 (2023) Cite this article

1336 Accesses

3 Citations

Metrics details

A ternary nanohybrid CuO/Mn3O4/CeO2 was developed in the present work using a co-precipitation-assisted hydrothermal method. The designed photocatalyst's structural, morphology, elemental composition, electronic states of elements, and optical properties were studied using corresponding analytical techniques. Results from PXRD, TEM/HRTEM, XPS, EDAX, and PL showed that the desired nanostructure had formed. Using Tauc's energy band gap plot, it was determined that the nanostructures band gap was ~ 2.44 eV, which showed the band margins of the various moieties, CeO2, Mn3O4, and CuO, had modified. Thus, improved redox conditions led to a substantial decrease in the recombination rate of electron–hole pairs, which was further explained by a PL study in that charge separation plays a key role. Under exposure to visible light irradiation for 60 min, it was revealed that the photocatalyst achieved 98.98% of photodegradation efficiency for malachite green (MG) dye. The process of photodegradation proceeded according to a pseudo-first-order reaction kinetic model with an excellent rate of reaction of 0.07295 min−1 with R2 = 0.99144. The impacts of different reaction variables, inorganic salts, and water matrices were investigated. This research seeks to create a ternary nanohybrid photocatalyst with high photostability, visible spectrum activity, and reusability up to four cycles.

Many professions and industries use organic dyes, including fabric, plastic sheets, leather, medical, skincare, and nourishment, to color their goods and release their effluents into the ecosystem without any primary care1,2. These colorful organic pollutants are dangerous to the aquatic environment and human health3,4,5. These coloring agents prevent sunlight from penetrating water streams, slow down the photosynthetic phenomenon in the water ecosystem, and may interact with ionic metals to form chelating complexes, all of which lead to the toxicity of living beings6. Malachite green (MG), a cationic dye, is used in many industries as a coloring agent for leather, textile, and woolen goods and in the fishing industry as a parasiticide (Fig. 1). However, MG is known to be a cancer-causing toxin and may seriously harm human health even at minor concentrations (1 mg L−1)7,8,9,10. Initiatives have been taken to keep the water free of these harmful organic dyes like MG dye. In this context, producing an effective nano-sized photocatalyst has garnered much interest11,12.

Malachite green (MG) dye chemical structure.

In contrast to other approaches to treating wastewater, like filtration, sedimentation, coagulation, adsorption, etc., photocatalytic degradation entirely breaks down the organic pollutants. It does not produce toxic by-products that have proven resistant to other water treatment approaches13,14. Further studies in the field of photocatalysis have resulted in developing a novel short of an oxidative method, which is regarded as the Advanced Oxidation Process (AOP). These techniques aim to completely break down hazardous organic pollutants such as Malachite green, generating reactive oxygen species15,16.

The elimination of recalcitrant organic pollutants from water bodies has been accomplished using a kind of AOP called heterogeneous photocatalysis17,18. CeO2 is a semiconductor with efficient electron mobility, a band gap of ~ 3.27 eV, excellent electrochemical stability, and a good isoelectric point of 9 to catalyst systems19,20,21. Despite multiple benefits, its wide band gap limits its applicability in UV- band region irradiation22. Additionally, it has been shown that its function as a photocatalyst is compromised by excessive electron–hole recombination23,24. Fine-tuning the band gap of CeO2 by combining different semiconductors (metal oxides) with lower band gaps and right band edges would be necessary to maintain a substantial separation of photogenerated electron–hole pairs25,26. CeO2, in its purest form, has a band gap of about 3.2 eV wide. Pure CeO2 nanoparticles cannot efficiently capture visible light because received irradiation lacks the necessary energy to produce charge carriers27.

Nevertheless, it has been reported that n–p or n–n heterojunctions having narrow band gap semiconductors such as CeO2/CdS28, Flower-like CeO2/Mn3O4 microspheres29, or CeO2/CuO30 has significantly improved the separation between the valence and conduction band. Additionally, pairing narrower band gap semiconductors enhances the visible light harvesting potential of the catalyst; as a result, the catalyst's efficiency increases when it is subjected to visible light from the LED. Lately, metal oxides with a ternary composition of materials, including CeO2/MgAl2O4/Mn3O422, CeO2/PAN-ZnO/PAN-Mn3O431, and CeO2/CuO/TiO232, have been extensively studied for synergistically boosted photocatalytic efficiency underneath the visible light exposure. Developing a metal oxide ternary heterojunction with a suitable valence-conduction band position can significantly enhance the final nanocomposite's photocatalytic performance. The present study intended to tune the band gap boundaries of CeO2 in a way that would be fiscally feasible. Authors of the present study accomplished this by composing CeO2 with low-priced nanomaterials such as Mn3O4 and CuO, each of which has a lower band gap compared to CeO2 and band configurations that are suitable for one another. These materials are promising contenders for capturing visible light. The specific band gap of CuO is around 1.79 eV, and coupled systems have shown CuO to have high photocatalytic efficiency. A band gap of 2.27 eV or less in Mn3O4 might further improve the segregation of photogenerated electrons and holes. Mn3O4-based photocatalysts have recently shown exceptional responsiveness to visible-light-driven photocatalysis.

In the present work, a simple hydrothermal method for the synthesis of CuO/Mn3O4/CeO2, abbreviated as 'CMCu' as an efficient photocatalyst nanostructure is addressed, and a thorough study is carried out to evaluate the photocatalytic degradation efficiency of the CMCu in the aqueous medium photodegradation of an organic pollutant, Malachite green (MG) dye. For photocatalytic degradation of the MG dye, CMCu, took 60 min, with a photodegradation efficiency of 98.98%. The pseudo-first-order reaction kinetic model with a rate constant of 0.07295 min−1 with R2 = 0.99144 was observed to be compatible with the photodegradation process. To better understand the application of the designed photocatalyst in the real world, a rudimentary investigation was conducted into the impacts of co-existing substances and real water specimens on the photodegradation of the Malachite green dye (pollutant). This work is innovative because it synthesizes a novel ternary heterojunction of metal oxides, i.e., CMCu, and then uses it to remove a pollutant, MG dye.

AR-grade reagents were collected from Sigma Aldrich and utilized without further purification. These reagents included cerium nitrate hexahydrate (Ce (NO3)3.6H2O), copper acetate monohydrate (C4H8CuO5), manganese chloride tetrahydrate (MnCl2.4H2O), sodium hydroxide, Malachite green (C23H25ClN2), and deionized water, etc.

The crystalline nature of CuO/Mn3O4/CeO2 or CMCu and CeO2 was evaluated using a Bruker D8 Advance X-ray diffractometer that was irradiated with Cu-Ka. TEM and SAED studies of CMCu were carried out with the assistance of a JEOL JEM 2100 apparatus. In order to carry out X-ray photoelectron spectroscopy of CMCu, a spectrometer PHI 5000 Versa Prob II was used. Using a JEOL Model JSM—6390LV, an EDAX spectrum and elemental mapping were produced for the CMCu nanocomposite. The Hitachi F4600 instrument was used to obtain data on the photoluminescence of CMCu, Mn3O4/CeO2 (MC), CeO2/CuO (CCu), CeO2, Mn3O4, and CuO. The GENESYS 10S UV–visible spectrophotometer was utilized to record the materials' UV–Vis absorbance responses.

Synthesis of nanomaterials was approached via the one-pot precipitation cum hydrothermal method and followed the previously published work33 in the field with some necessary modifications. A glacial acetic acid solution of 1 mL was added to an aqueous solution of 25 mmol of C4H8CuO5 (copper acetate monohydrate) and heated to 100 °C on a hot plate of magnetic stirrer. To this solution drop-by-drop, an aqueous solution of NaOH was added under magnetic agitation. The color of the mixture slowly shifted from blue to black, and a significant quantity of black precipitate of CuO developed. Again, 25 mmol of cerium nitrate hexahydrate (Ce (NO3)3.6H2O) was added to this solution, followed by an injection of dropwise aqueous NaOH solution, and stirred the whole mixture vigorously. Following the addition of 75 mmol of manganese chloride tetrahydrate (MnCl2.4H2O) and a dropwise injection of an aqueous sodium hydroxide solution to the reaction mixture. The reaction mixture was then transferred to a Teflon autoclave after being thoroughly agitated using a magnetic stirrer. The autoclave was then placed in a 180 °C oven for 18 h. Afterward, a brownish residue was obtained, collected, washed multiple with an ethanol solution, and air dried. A muffle furnace was maintained at 400 °C for 2 h for calcination of an air-dried sample. An identification tag of CuO/Mn3O4/CeO2 (also abbreviated as CMCu) was placed on the calcined sample. The exact process was used to synthesize samples of Mn3O4/CeO2 (MC), CeO2/CuO (CCu), CeO2, Mn3O4, and CuO from their respective precursor material. Figure 2 shows the schematic diagram for the synthesis of CuO/CeO2/Mn3O4 heterostructure.

Schematic representation of the synthesis of CuO/CeO2/Mn3O4 heterostructure.

The photocatalytic efficiency of the developed nanocomposite was determined by observing its capability to disintegrate the MG in its aqueous medium under an LED light. The experiment was conducted in a wooden chamber with a 25 W Philips LED bulb (white light). A lux meter was fixed in an enclosure to estimate light intensity, which was observed at 48.75 W m2 at 11,870 lx. The MG dye degradation process was carried out at room temperature. To reach adsorption–desorption equilibrium, the dye solution and photocatalyst were shaken mechanically for 30 min; the absorbance was then measured. The highest absorbance of Malachite green dye at 617 nm was recorded at 10-min increments for 60 min to evaluate photodegradation. The efficiency of degradation was calculated using the given Eq. 1:

where constants C0 and C signify the corresponding MG dye concentrations at t = 0 and t = t, respectively, and Eq. 2 below was employed to evaluate photodegradation kinetics: C0 and C denote MG concentrations at t = 0 and t = t, and k is the pseudo-first-order reaction rate constant (in min−1).

A reusability test was performed, indicating excellent repeatability until four study repeats under the same parameters. Trapping experiments with different scavengers were performed to clarify the impact of reactive species throughout the photodegradation of MG dye.

To identify the crystalline phases of the synthesized heterostructure CMCu, an XRD examination was conducted. The XRD results of synthesized nanocomposite CuO/Mn3O4/CeO2 (CMCu) and pristine CeO2 are shown in Figs. 3a, b. The CuO/Mn3O4/CeO2, ternary nanocomposite's XRD data showed peaks of the CeO2, Mn3O4, and CuO phases, labeled in Fig. 3a. In the CeO2 phase (JCPDS 65–2975), diffraction from planes (111), (200), (220), (311), (400), (331), and (442) produced peaks at 28.54°, 33.08°, 47.48°, 56.34°, 69.42°, 76.70°, and 88.43°, respectively34,35. These peaks matched well with the previously reported JCPDS file number 65–2975 and were compatible with the cubic system with a face-centered lattice structure of CeO2 nanomaterial. The cell dimensions of the cubic CeO2 with the space group of Fm3̅m [225] are, a = 5.411 Å and α = β = γ = 90°36. Peaks were identified at 2Ɵ values of 32.53°, 35.55°, 38.75°, 48.70°, 58.33°, 65.80°, 66.48°, 68.15°, and 75.28°, respectively, correlating to the crystallographic planes (110), (1̅11), (111), (2̅02), (202), (022), (310), (220), and (2̅22) of CuO nanomaterial37. These peaks were coherent with the monoclinic system with an end-centered lattice structure of CuO nanomaterial, as well as the results matched with the previously reported literature (JCPDS 89–5899) presenting the cell parameters of the monoclinic CuO with the space group of Cc9, are a = 4.689 Å, b = 3.420 Å, c = 5.130 Å, and β = 99.57°38,39. Peaks were observed at 2Ɵ values of 18.01°, 28.9°, 31.0°, 32.3°, 36.1°, 44.4°, 58.5°, and 63.2° corresponding to diffraction planes of (101), (112), (200), (103), (211), (220), (321), and (116), respectively and these peaks were consistent with the tetragonal system with body-centered lattice structure of Mn3O4 nanomaterial, and the data best fit with the earlier reported JCPDS file number 89–483740,41. With the space group of I41/amd [141], the cell parameters of the tetragonal Mn3O4 are a = 5.763 Å and c = 9. 456 Å42. In Fig. 3b, the XRD pattern of the base material of nanocomposite, i.e., pristine CeO2, is given for the best matching of the peaks in the final nanocomposite CuO/Mn3O4/CeO2, and the peaks well matched with the 2Ɵ values of 28.54°, 33.08°, 47.48°, 56.34°, 69.42°, 76.70°, and 88.43°, and they could be attributed to the CeO2 crystalline planes at (111), (200), (220), (311), (400), (331), and (422), respectively, matching with JCPDS 65–2975. JCPDS 65–2975 suggested the face-centered cubic lattice structure of CeO2 nanoparticles with cell parameters a = b = c = 5.411 and α = β = γ = 90°. Adition1ay, the crystallinity of the fabricated material was found to be 91.04%, and the average crystallite size was found to be 5.69 nm.

PXRD spectrums of (a) CuO/Mn3O4/CeO2 heterostructure and (b) pristine CeO2.

The Debye- Scherrer's equation (Eq. 3) was used to determine the average crystallite size of the fabricated nanoparticles43,44. D is the crystallite size (nm), λ (wavelength of CuK radiation) = 1.54056 A, k (shape factor) = 0.89, β (full width at half maximum of the individual peak), and θ is the Bragg's diffraction angle.

The TEM (Transmission Electron Spectroscopy) method was used so that the morphological features of the developed nanohybrid could be investigated and more information on its structure could be gathered (Fig. 4a–c)). The transmission electron microscopy shown in Fig. 4a displayed images of dispersed nanoparticles of CeO2, CuO, and Mn3O4 in interaction with one another. It is quite likely that inter-facial heterojunctions have indeed been established by these nanoparticles. The nanoparticles were able to accurately identify based on the spacing of the lattice fringes that were visible there in micrographs produced by HRTEM (Fig. 4d). The three distinct lattice fringes had inter-planar spacings of 0.321 nm, 0.248 nm, and 0.232 nm, respectively, for CeO2, Mn3O4, and CuO nanoparticles. These interplanar spacings might be related to the (111) facet of the CeO2 phase (JCPDS 65–2975)45, the (211) crystallographic plane of the Mn3O4 phase (JCPDS 89–4837)46, and the (111) facet of the CuO phase (JCPDS 89–5899)47. The presence of concentric rings in the SAED patterns (Fig. 4e) was suggestive of the polycrystalline nature of the material48. The lattice planes of all three phases were correctly located and marked properly. It was determined that 24.8 nm was the average size of the particles in the final nanocomposite by plotting the histogram (Fig. 4f).

(a–c) TEM images of CMCu heterostructure at (a) 100 nm, (b) 50 nm, (c)10 nm, (d) HRTEM image of CMCu heterostructure at 2 nm, (e) SAED pattern of CMCu heterostructure, and (f) histogram of CMCu heterostructure for average particle size calculation.

The EDAX analysis of three different selected areas (Fig. 5) of the CuO/Mn3O4/CeO2 nanocomposite was performed to gather information about the elements present and their ratios in the synthesized nanocomposite CMCu. EDAX spectrum of all three selected areas exhibited signals that corresponded to Ce, Cu, Mn, and O. The peaks that appear at around 4.8 keV, 8.0 keV, 5.9 keV, and 0.5 keV might be related to Ce, Cu, Mn, and O, respectively, and all of these peaks match to the K-series emissions except Ce peak which matched with L-series emission. Table 1 lists these elements' atomic percentages, implying that a successful synthesis of CeO2, CuO, and Mn3O4 nanomaterials with strong physical coherence among the separate moieties has been placed. In addition, the atomic percentages point to the possible inclusion of CeO2, CuO, and Mn3O4 at the nanocomposites in a ratio of around 1:1:1. Additionally, the absence of contaminant signals (unwanted peaks) in the EDAX spectrum was somewhat compatible with the clear XRD pattern that was produced for the final nanocomposite. The existence of Ce, Cu, Mn, and O in the final nanocomposite was verified yet again through elemental mapping, which can be seen in Fig. 6.

EDAX spectrum of three selected area of CMCu.

Elemental mapping of CMCu.

The UV–Vis absorbance spectrum (Fig. 7a) of CMCu, CeO2, Mn3O4, and CuO were measured so that the optical characteristics of these materials could be evaluated. It was observed that the CeO2 in its pristine form has the highest absorption at a wavelength of less than ~ 345 nm. The absorbance spectrum of Mn3O4 showed a hump that was centered at about ~ 435 nm, while the highest absorbance of pure CuO was recorded at ~ 367 nm. In a similar manner, CMCu reacted over the whole of the visible spectrum, with the highest absorbance occurring at a wavelength of ~ 380 nm. Further, the Tauc's plots of CMCu with pristine metal oxides were plotted and given inset of Fig. 7a to determine their corresponding direct energy band gaps and, that indicated pristine CeO2 nanomaterials, Mn3O4 nanomaterials, and CuO nanomaterials had energy band gaps measuring ~ 3.27 eV, ~ 2.21 eV, and ~ 1.79 eV, respectively. The band gap of the final nanohybrid CMCu was around ~ 2.44 eV. The 'red shift' in the absorbance edges of the nanocomposites, which occurred while the CMCu (ternary photocatalyst) was experiencing the highest shift, provided additional confirmation of the creation of integrated photocatalysts with strong interfacial contacts. A plausible orbital intermixing in the valence shells of Ce, Cu, and Mn species could also be predicted, resulting in the formation of a conduction band (CB) at a lower magnitude of energy in the nanocomposites which was shown in the experiment49.

(a) UV- absorbance spectrum with their insect Tauc's energy band gap plots of different nano-sizes samples with CuO/Mn3O4/CeO2 heterostructure at (b–d) photoluminescence plots of various samples for comparative study.

The principal cause of Photoluminescence (PL) spectra in a semiconductor material is the downward electronic transition from the conduction band (CB) to the valence band (VB), and the intensity of the PL spectra may be used as a measurement of the rate of recombination of electron–hole pairs. Therefore, the binary and final nanocomposites were evaluated alongside the unadulterated, pristine samples. The PL spectra of CeO2 were acquired by excitation of the material at 345 nm and at the same wavelength, CeO2/CuO, Mn3O4/CeO2, and CMCu were excited as well, and the spectra of these four systems were compared (Fig. 7b). While emissions that occurred at ~ 450 nm may be related to oxygen defects50, the PL-emission signal at 370 nm could be attributable to 'exciton–exciton collision'51. These PL emissions exhibited lower intensities in the binary metal oxide nanohybrids and the least intensity in the CMCu (ternary nanohybrid). Similarly, the spectra of CuO, CeO2/CuO, and CMCu were obtained after being stimulated at a wavelength of 367 nm (Fig. 7c). The very first emission peak in the CuO PL-emission spectra, located at approximately 397 nm, might be attributed to the 'radiative exciton annihilation,' and some other light humps, located at higher wavelengths, could be the result of defect levels in the metal oxides52. Again, it was observed that emission intensities of nanohybrids were significantly decreased, whereas the ternary nanocomposite CMCu had shown the least emission intensity. In order to make a comparison with Mn3O4, Mn3O4/CeO2 and CMCu were stimulated at 435 nm (Fig. 7d)53. Radiative recombination was at its strongest in the pristine sample, then decreased in the binary nanocomposites, and finally reached its lowest point in the CMCu nanocomposite. According to all of these findings, it seems that the final ternary nanocomposites may be capable of achieving a significant separation between the photogenerated pairs of electrons and holes.

The X-ray photoelectron spectroscopy (XPS) survey spectra of CuO/Mn3O4/CeO2 (Fig. 8a) exhibited signals that may be attributed to Cu, Mn, Ce, and O. The peaks identified at 932.34 eV and 952.09 eV in the XPS spectra of Cu 2p as shown in Fig. 8b, matched to the Cu 2p3/2 and Cu 2p1/2 with the existence of two prominent satellite signals at 944.86 eV and 944.58 eV verified the existence of Cu+2 in the CMCu54,55. The spectra of Mn 2p reveal the peaks owing to 'spin–orbit splitting' at 641.45 eV and 653.21 eV, which resemble Mn 2p3/2 and Mn 2p1/2, respectively, as shown in Fig. 8c 56. The difference in energy between them of 11.76 eV further validated the creation of the Mn3O4 phase in the nanocomposite. Both Mn 2p3/2 and Mn 2p1/2 signals were deconvoluted into four different signals at ~ 641.45 eV, 653.21 eV, 643.38 eV, and 657.05 eV corresponding to Mn (III) 2p3/2, Mn (III) 2p1/2, Mn (II) 2p3/2, and Mn (II) 2p1/257. The bands designated Ce1, Ce2, Ce3, Ce4, and Ce5 with CeI, CeII, CeIII, and CeIV in the high-resolution XPS spectra of Ce3d indicate satellite characteristics emerging from corresponding Ce3d5/2 and Ce3d3/2 states58. The major spikes at Ce3 (884.8 eV), Ce4 (888.6 eV), Ce5 (898.2 eV), and CeIII (901.6 eV), Ce1V (907.2 eV) are also indicative of Ce4+ electronic states. The detection of spikes at places denoted as Ce1, Ce2, and CeI, CeII provided conclusive evidence for the existence of Ce3+ electronic states. As a result, the Ce3d spectra are made up of a combination of Ce ions with the charge states + 3 and + 4 (Fig. 8d). The XPS spectra of O1s reveal a signal at 529.5 eV that may be attributed to the Ce–O bond in the CeO2 moiety59. The peak at 532.06 eV is most likely owing to different oxygen vacancies and surface-adsorbed oxygen species (Fig. 8e)60,61. The modest variations seen in the values of metal oxide binding energies (BE) compared to those published in earlier research, revealed the 'interfacial interactions' between the different moieties (metal oxides), so confirming that the planned linked photocatalyst system was successfully fabricated.

(a) XPS survey spectrum of CuO/Mn3O4/CeO2 heterostructure, XPS spectrum of (b) Cu2p, (c) Mn2p, (d) Ce3d, and (e) O1s for electronic state investigation in CuO/Mn3O4/CeO2 heterostructure.

In order to prevent the unnecessary application of the developed photocatalyst CMCu, it was necessary to conduct the screening to evaluate its ideal loading amount. Consequently, the developed photocatalyst could be used to its full potential and achieve the highest possible efficiency level. Figure 9a displays the rate of Malachite green photodegradation that occurs in the presence of varying dosages of the nanocomposite CMCu, at an initial pH of 7, and a 50 mL solution containing 50 mgL−1 of Malachite green was used. The catalyst intake was varied in the range of 0.04–0.24 gL−1 so that the optimal quantity of the developed photocatalyst necessary for Malachite green degradation could be determined. A dose concentration of 0.16 gL−1 of the CMCu yielded the highest photodegradation efficiency of malachite green, and beyond this, there was a little drop in efficiency. Even though further deployments of the photocatalyst would mean adding even more 'active sites' to its surface, there is significant 'solution opacity' that causes the photocatalytic efficiency to decrease. At 0.16 gL−1 of photocatalyst dose for 50 mgL−1 of Malachite green dye concentration, the velocity constant achieved its highest value of 0.0599 min−1 (with R2 = 0.99603) (Fig. 9b and Table 2), and with the photodegradation efficiency of 97.12%.

(a) Degradation profile and (b) kinetics plot for CMCu photocatalyst dose optimization.

Studies were conducted using the optimum dose of the catalyst, i.e., 0.16 gL−1, and at a pH of 7 with varying concentrations of Malachite green dye 25–125 mgL−1 to evaluate how the initial concentration of the dye impacts the photodegradation activity of the catalyst CMCu. At 100 mgL−1 of Malachite green dye concentration, maximum efficiency of MG dye degradation was observed (Fig. 10a), which was 98.06%. A modest drop in the efficiency of the MG dye degradation was noticed at dye concentrations higher than its optimum value. Higher MG dye concentrations may be responsible for the decrease in photon path length62. Moreover, higher concentrations of MG dye would demand more surface area of CMCu photocatalyst for further photodegradation, which could only be accomplished by adding an extra amount of CMCu, which would inevitably increase the opacity of the solution63. In order to get the highest yield of photodegradation, the MG dye dosage was optimized. At the optimum concentration of the MG dye, the magnitude of the velocity constant was recorded, which was 0.07259 min−1 with R2 = 0.99357 (Fig. 10b and Table 3).

(a) Degradation profile and (b) kinetics plot for MG dye concentration optimization.

The influence of pH on the photodegradation of the aqueous MG dye solution using a CMCu photocatalyst was examined. In this regard, the dose of the photocatalyst was kept constant at 0.16 gL−1, and the MG dye concentration was kept constant at 100 mgL−1 throughout the studies. On the other side, the initial pH of the dye solution was adjusted within the range of 4 to 10 by adding the corresponding amount of concentrated HCl and NaOH solutions. A slight rise in photodegradation efficiency was observed up to pH > 7, while for pH < 7, a slight drop was observed (Fig. 11a). It was determined using the mass titration method that the pHZPC (point of zero charges) of the photocatalyst is approx. 6.4. Therefore, a high concentration of hydroxide ions could be generated on the photocatalyst surface when the solution pH rises beyond the value of pHZPC64. In an environment with a higher pH, the accumulation of hydroxide ions on the surface of the photocatalyst could cause the catalyst to attract the electron-deficient cationic MG dye molecules, while at pH lower than pHZPC, the photodegradation of MG dye declined because of the accumulation of H+ ion concentration over the catalyst surface which repels the cationic MG dye molecules1. As can be observed from pseudo-first-order kinetics (Fig. 11b), the velocity constant reached its highest value at a pH of 10, which is 0.07295 min−1 (Table 4).

(a) Degradation profile and (b) kinetics plot for MG dye degradation at various pH environments.

By monitoring the degradation output over time at various intervals while performing under the various optimal parameters of photocatalyst dose, initial concentration of dye, and pH, the influence of LED exposure time was examined. The photocatalyst CMCu loading utilized in this study was 0.16 gL−1, with an initial MG dye concentration of 100 mgL−1 and a pH > 7. After 60 min, the maximum MG dye photodegradation of 98.98% was recorded (Fig. 12a, b). The kinetic for the contact time is also provided in Fig. 12c, which results in the velocity rate constant value of 0.07352 min−1 with R2 = 0.9956. After 60 min, the light-driven photodegradation reaction was stopped because of photocatalyst's active sites had been nearly exhausted.

(a) Contact time degradation profile, (b) Degradation profile, and (c) kinetics plot for MG dye degradation at a different time interval.

The heterostructure photocatalyst that had been developed was recovered, and it was found that it could be utilized up to four runs in a row (Fig. 13a, b). The recovered catalyst was separated using centrifugation at 1000 rpm. After that, it was cleaned many times with distilled water and ethanol before being dried at a temperature of 90 °C for 2 h. The processed photocatalyst was used once again in the subsequent experimental run. Both the degradation efficiency and the pseudo-first-order rate constant revealed a little drop (Table 5), which could be explained by the fact that the coupled unit of photocatalyst had been somewhat dislodged. Further, the XRD data that was produced for the regenerated photocatalyst indicated that the typical crystallographic planes that were present in CuO/Mn3O4/CeO2 before usage were also present in the regenerated photocatalyst material (Fig. 14a). This demonstrated that the developed photocatalyst has a high degree of overall durability. Moreover, it was further validated by the outcomes of the EDAX spectra of the regenerated photocatalyst (Fig. 14b), which revealed very little variation from the information derived for the photocatalyst before use. These observations showed that the developed ternary nano-scaled composite was stable, which indicated that there was extensive interfacial coupling among the components, and they provided great proof of low leaching of metal ions throughout the photocatalytic activity. The synthetic method that was used to develop the hierarchical nanostructure may have been a factor in achieving excellent elemental compatibility with excellent dispersion quality of the photocatalyst CMCu, as evidenced by EDAX, Transmission electron micrographs, optical data, and X-ray photoelectron spectrum, and this resulted in lowering of accumulation of any particular metal oxide within ternary photocatalyst CMCu. Additionally, some articles suggest that semiconductors with a hierarchical system of nanostructure have more excellent resistance to accumulation65. As a result, the hierarchical design will make it more difficult for metal ions to leach out of aggregates, which is caused by accumulation.

(a) Degradation profile and (b) kinetics plot up to four cycles of reused photocatalyst for MG dye degradation.

(a) PXRD and (b) EDAX spectrum of photocatalyst CuO/Mn3O4/CeO2 after the run of the fourth cycle for MG dye degradation.

Photolysis, along with the pristine CeO2 nanomaterial, scarcely accomplished any appreciable photocatalytic activity (Fig. 15a, b). CuO/Mn3O4/CeO2 displayed a photocatalytic degradation efficiency of 98.98 ± 1.5% within 60 min, whereas Mn3O4/CeO2 and CuO/CeO2, Mn3O4, and CuO could achieve photocatalytic degradation efficiencies of 62.61%, 51.42%, 37.64%, and 32.16% under the equal time duration and identical set of circumstances (Fig. 15a, b). During photocatalytic degradation over CuO/Mn3O4/CeO2, an exceptional rate constant of 0.07295 min−1 was recorded. In terms of the percentage of photocatalytic degradation, the ternary nanocomposite performed 1.58-fold higher than the Mn3O4/CeO2 combination and 1.92-fold higher than the CuO/CeO2 combination. Additionally, the ternary photocatalyst was shown to be astonishingly more effective than CuO-Gd2Ti2O7, which required 90 min to produce 88.60% of Malachite green dye degradation at a rate constant of 0.0198 min−166 and SnO2/ZnO flower like composite which requires 150 min for 98% of Malachite green dye degradation with the velocity constant of 0.00168 min−167. Table 6 presents the results of a comparison between the photodegradation performance of the newly synthesized photocatalyst and that of the catalysts that have been reported in the past.

(a) Degradation profile and (b) kinetics plot for comparative study of different samples of the catalyst with photocatalyst CMCu for MG dye degradation.

Photocatalytic degradation studies were carried out with quenchers to evaluate the important characteristics performed by various reactive species, such as \({\mathrm{O}}_{2}^{2-}\), ·OH, e − , and h+ towards the photodegradation of MG dye. For scavenging the \({\mathrm{O}}_{2}^{2-}\), ·OH, e − , and h+, the corresponding scavengers (trapping agents), TEMPOL (4-hydroxy-2,2, 6,6- tetramethyl piperidinyl oxy), C7H6O2 (Benzoic acid), K2S2O8 (Potassium persulfate), and AgNO3 (silver nitrate), respectively, were used. The photodegradation experienced an apparent retardation in the vicinity of TEMPOL68 and BA69 (Fig. 16a, b), and it was accompanied by a decline in efficiencies, which went from 98.98% with CuO/Mn3O4/CeO2 photocatalyst to 16.42% and 34.28%, respectively. Interestingly, the drop in photodegradation efficiencies was not as significant when K2S2O870 and AgNO371 were present. Based on the information mentioned above, substantial roles in the photodegradation of Malachite green dye are likely played by reactive oxygen species (ROS), \({\mathrm{O}}_{2}^{2-}\) and ·OH radicals72,73.

(a) Degradation profile and (b) kinetics plot for scavenger experiment for MG dye degradation with photocatalyst CMCu.

The spectacular photodegradation efficiency of the fabricated nano-heterostructure as a photocatalyst provided an efficient demonstration of the photocatalytic activity of the integrated photocatalyst. Furthermore, PL results clearly demonstrated the segregation of photo-induced electron–hole pairs. Moreover, the nano-sized photocatalyst exhibited a level of efficiency that was quite constant up to the fourth cycle. The above observations are consequently suggestive of the establishment of strong interfacial reactions among the participating components. The plausible mechanism of CuO/Mn3O4/CeO2 photocatalysis is shown in Fig. 17. With reference to a normal hydrogen electrode (NHE), the potential edge value of the CB (conduction band) for CuO is located at − 1.07 eV74, while for Mn3O4, it is located at − 0.80 eV75, and CeO2 has a CB edge potential at − 0.57 eV76. CB of CuO will always have a greater negative edge potential value than the CB edge potential value of Mn3O4, which will, in response, have a greater negative CB edge potential value than CeO2. Consequently, as a response to the photon absorption from the source of visible light, electrons (e−) are moved from the VB (valence band) of CuO towards its own CB These electrons are then moved towards the conduction band of semiconductor Mn3O4, and subsequently, they concentrate at the Conduction band of nanomaterial CeO2. At the same time, holes (h+) follow the opposite direction movement from the Valence band of CeO2 to the Valence band of semiconductor Mn3O4, with a further move towards the valence band of semiconductor CuO. Afterward, the electrons concentrated at the Conduction band of CeO2 interacted with the oxygen molecules (O2) that had been adsorbed over the surface of the reaction system, which resulted in the generation of superoxide anion radicals (. O2−). This is due to the fact that the potential edge value of CB of CeO2 is lower than the typical reduction potential of O2/.O2-, which is − 0.33 eV77. The holes, on the other side, generated hydroxyl radicals (·OH) by reacting with water molecules (H2O). It's possible that the pathways indicated below will lead to the formation of ·OH radicals. Equations 4–13 below provide the best description of the synergistic effect that exists among the various components that make up the integrated photocatalyst, which ultimately results in the superior photocatalytic activities of the integrated photocatalyst22, 78,79,80

(a) Schematic presentation of plausible mechanism for photodegradation of Malachite green dye using designed heterostructure CuO /Mn3O4/ CeO2.

There is currently a dearth of knowledge on how inorganic ions affect the photodegradation efficiency of environmental pollutants. In light of the potential practical uses, this work also examined the impact of inorganic ions on the photodegradation of MG Unfortunately, because of the intricate impacts of co-existing inorganic ions, photocatalytic degradation is currently restricted in terms of practical applicability81. For instance, several inorganic ions, such as carbonate, chloride, fluoride, calcium, aluminum, and sodium ions, are present in water. Therefore, it is important to investigate these ions' effect on the prepared material's photocatalytic activity for its real-world practical applications as a photocatalyst.

To study these ions' effect, 0.1 M of Na2SO4, CaSO4, Al2(SO4)3, NaF, NaCl, and Na2CO3 are added into the reaction mixture at optimum conditions before irradiating the solution. The cations Na+, Ca2+, and Al3+ being in their stable oxidation states, are known to cause a negligible effect on photocatalysis82. As illustrated in Figs. 18a, b, the degradation of MG is inhibited to varying extents in the presence of these cations. This decrease in the photocatalytic activity might be due to the presence of sulfate anion in the salts. However, the Al3+ greatly inhibited the degradation of MG because of its higher charge density and the tendency to adsorb on the surface of the photocatalyst83. Aluminum ions blocked the active sites on the surface, thereby inhibiting the generation of reactive oxygen species (ROS). Ca2+ was found to slow down the degradation process a bit more than Na+ ions.

(a) Degradation profile and (b) kinetics plot in an environment of selected cations, and (c) Degradation profile and (d) kinetics plot in an environment of selected anions for MG dye degradation using photocatalyst CMCu.

The wastewater also contains a number of inorganic anions, which are known to obstruct photogenerated ROS and influence the total removal of organic contaminants. As illustrated in Figs. 18c, d, the degradation efficiency is significantly hindered in the presence of anions. The negative effect of these anions on the degradation of MG follows the order CO32− > Cl− > F−. This decrease could be due to two main reasons, firstly, due to the blockage of active sites of the photocatalyst inhibiting the generation of ROS, and secondly, due to the quenching effect of these anions.

It is evident from Figs. 18a, b the chloride ions g inhibited the degradation of MG because of their strong affinity towards the holes and hydroxyl radicals84. This quenching effect of Cl− was the main reason for the suppression of the degradation efficiency, as illustrated below (Eq. 14–16) 85:

After being adsorbed on the surface of the photocatalyst, the chloride ions interact with the holes and hydroxyl radicals, forming the \({\text{Cl}}^{ \cdot }\) free radicals, which further get converted to Cl− ions upon the reaction with the electrons 86. Furthermore, chloride ions are resistant to oxidation, rendering them good inhibitors of the photodegradation reaction.

The carbonate ions are greatly known for their quenching effect of hydroxyl radicals according to the following Eq. 1787 following second-order-kinetics:

The carbonate free radicals thus formed could also bring about the degradation of MG However, due to their low oxidation potential compared to hydroxyl radicals, they only hindered the photodegradation process.

The F− ions are highly stable, and due to their non-oxidizable nature and small size, they cover the surface of the photocatalyst, inhibiting the generation of hydroxyl radicals 88,89. Therefore, it could be confirmed that hydroxyl radicals are important species for the degradation of organic pollutants in the presence of F− ions 84.

To further authenticate the photodegradation process for real wastewater treatment, the degradation of MG was also investigated in three different water samples. Mineral water, lake water, and tap water have a pH of ~ 7.21, ~ 8.36, and ~ 7.86, respectively, and are employed to investigate the effect of water matrices on the degradation of malachite green. The degradation profiles and kinetics for MG dye photodegradation in three different water samples are shown in Fig. 19a, b. All the water samples showed a significant retarding effect on the degradation of MG dye. Mineral water, the cleanest among all, expectedly showed a considerable degradation efficiency of 62.36%, which is better than the other two samples. Lake water contains various other co-existing ions and organic compounds that showed minimum efficiency with only 14.28% degradation of MG, while 36.12% degradation could be achieved with tap water. Light refraction and the presence of different photocatalysis-inhibiting inorganic ions and organic molecules to variable degrees in the aforementioned water matrices are two main causes of the general decrease in the photodegradation activity of the photocatalyst in these aquatic environments. The fact that the photocatalyst performed poorly in the former matrix may be due to tap water's higher mineral content than lake water.

(a) Degradation profile and (b) kinetics plot for photodegradation of prepared MG dye solution in different water samples employing photocatalyst CMCu.

In this study, a ternary nano-heterostructure CuO/Mn3O4/CeO2 as a photocatalyst was designed employing simple hydrothermal methods, and the photocatalytic activity of the developed nano-heterostructure was thoroughly investigated. For determining the structural, morphological, composition, and optical characteristics, numerous characterization and analytical methods were used. Images from TEM clearly demonstrated the growth of CeO2 nanoparticles with cubic lattice structure in the composite. HRTEM images provide further insight into the successful fabrication of the desired ternary heterojunction and the coupling of CuO and Mn3O4 nanoparticles with CeO2 nanoparticles. The CMCu heterostructure has a band gap of 2.44 eV, which was evaluated according to Tauc's plot using UV-DRS data. The robust formation of a staggered type II heterojunction with a significant gap between light-induced electron–hole pairs was further supported by PL investigations. The nanostructure was shown to be photo-catalytically active in the UV–visible region of the spectrum of light, and it was capable of achieving a 98.98% photodegradation of Malachite green dye within 60 min with excellent pseudo-first-order rate constant value of 0.07295 min−1. Additionally, the recovered photocatalyst was repeatedly utilized and showed ~ 88.72% of photodegradation of Malachite green dye up to the fourth run. Scavengers' experiments, inorganic salt analysis, and experiments in different water matrices are briefly discussed to understand the species which influence the reaction by assisting or interfering in the reaction. Finally, a plausible mechanistic explanation is discussed to explain the path involved in the photodegradation of MG dye using CMCu photocatalyst.

The authors declare that all data supporting the findings of this study are available within the article.

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We acknowledge the IIC IIT Roorkee, STIC Cochin, and SAIF IIT Madras.

Department of Chemistry, National Institute of Technology, Silchar, Assam, 788010, India

Shaswat Vikram Gupta & Md. Ahmaruzzaman

Department of Civil Engineering, National Institute of Technology, Silchar, Assam, 788010, India

Vihangraj Vijaykumar Kulkarni

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S.V.G wrote the whole manuscript text and figures. V.V.K. edited the whole manuscript and M.A. supervised, edited and corrected the whole manuscript. All authors reviewed and approved the final manuscript.

Correspondence to Md. Ahmaruzzaman.

The authors declare no competing interests.

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Gupta, S.V., Kulkarni, V.V. & Ahmaruzzaman, M. Bandgap engineering approach for designing CuO/Mn3O4/CeO2 heterojunction as a novel photocatalyst for AOP-assisted degradation of Malachite green dye. Sci Rep 13, 3009 (2023). https://doi.org/10.1038/s41598-023-30096-y

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Received: 01 January 2023

Accepted: 15 February 2023

Published: 21 February 2023

DOI: https://doi.org/10.1038/s41598-023-30096-y

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