<svg xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg" style="vertical-align:-4.5268pt" id="M1" height="12.3952pt" version="1.1" viewBox="-0.0657574 -7.8684 9.93214 12.3952" width="9.93214pt"><g transform="matrix(.017,0,0,-0.017,0,0)"><path id="g113-104" d="M546 430L539 434C529 434 505 438 495 440C473 444 450 448 430 448C352 448 265 412 213 366C145 306 96 203 96 103C96 22 135 -12 160 -12C190 -12 238 14 262 32C310 68 368 120 411 184H413C403 117 396 75 384 21C353 -118 325 -158 291 -184C270 -200 241 -205 208 -205C133 -205 90 -164 74 -110C70 -98 58 -100 49 -107C34 -119 23 -140 23 -155C23 -190 74 -261 166 -261C219 -261 280 -233 314 -208C383 -157 446 -79 470 81C491 223 529 388 546 430ZM456 386C452 357 433 283 420 252C402 216 366 174 325 129C288 88 239 56 212 56C192 56 182 77 182 120C182 165 199 242 226 292C256 348 281 377 311 389C327 395 353 402 375 402C408 402 436 394 456 386Z"/></g></svg>-C3N4–Co3O4 Z-Scheme Junction with Green-Synthesized ZnO Photocatalyst for Efficient Degradation of Methylene Blue in Aqueous Solution

Tamiru Mengistu, Mintesinot; Wondimu, Tadele Hunde; Andoshe, Dinsefa Mensur; Kim, Jung Yong; Zelekew, Osman Ahmed; Hone, Fekadu Gashaw; Tegene, Newaymedhin Aberra; Gultom, Noto Susanto; Jang, Ho Won

doi:https://doi.org/10.1155/2023/2948342

Bioinorganic Chemistry and Applications

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 2948342 | https://doi.org/10.1155/2023/2948342

-C₃N₄–Co₃O₄ Z-Scheme Junction with Green-Synthesized ZnO Photocatalyst for Efficient Degradation of Methylene Blue in Aqueous Solution

Mintesinot Tamiru Mengistu,¹Tadele Hunde Wondimu,^1,2Dinsefa Mensur Andoshe ,¹Jung Yong Kim,^1,2Osman Ahmed Zelekew,¹Fekadu Gashaw Hone,³Newaymedhin Aberra Tegene,³Noto Susanto Gultom,⁴and Ho Won Jang⁵

Academic Editor: Guillermo Mendoza-Diaz

Received03 Feb 2023

Revised31 Mar 2023

Accepted20 May 2023

Published05 Jun 2023

Abstract

A simple wet chemical ultrasonic-assisted synthesis method was employed to prepare visible light-driven g-C₃N₄-ZnO-Co₃O₄ (GZC) heterojunction photocatalysts. X-ray diffraction (XRD), scanning electromicroscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET), ultraviolet (UV), and electrochemical impedance spectroscopy (EIS) are used to characterize the prepared catalysts. XRD confirms the homogenous phase formation of g-C₃N₄, ZnO, and Co₃O₄, and the heterogeneous phase for the composites. The synthesized ZnO and Co₃O₄ by using cellulose as a template show a rod-like morphology. The specific surface area of the catalytic samples increases due to the cellulose template. The measurements of the energy band gap of a g-C₃N₄-ZnO-Co₃O₄ composite showed red-shifted optical absorption to the visible range. The photoluminescence (PL) intensity decreases due to the formation of heterojunction. The PL quenching and EIS result shows that the reduction of the recombination rate and interfacial resistance result in charge carrier kinetic improvement in the catalyst. The photocatalytic performance in the degradation of MB dye of the GZC-3 composite was about 8.2-, 3.3-, and 2.5-fold more than that of the g-C₃N₄, g-C₃N₄-ZnO, and g-C₃N₄-Co₃O₄ samples. The Mott–Schottky plots of the flat band edge position of g-C₃N₄, ZnO, Co₃O₄, and Z-scheme g-C₃N₄-ZnO-Co₃O₄ photocatalysts may be created. Based on the stability experiment, GZC-3 shows greater photocatalytic activity after four recycling cycles. As a result, the GZC composite is environmentally friendly and efficient photocatalyst and has the potential to consider in the treatment of dye-contaminated wastewater.

1. Introduction

Today’s society faces environmental challenges due to the ever-increasing demand for energy, the excessive use of fossil fuels, and related complications such as greenhouse gas emissions. Organic dyes are chemical pollutants that contaminate water. These contaminants in wastewater are very hazardous, can cause carcinogens, and are dangerous to humans, animals, and entire ecosystems [1]. There are approximately 10,000 varieties of commercially used dyes, with an annual production of around 0.7 billion tons [2]. About 20% of these colors are lost during dyeing processes and discharged as textile effluents [3]. Various methods, in particular, ion exchange [4], coagulation-flocculation [5], oxidation [6], electrochemical treatment [7], and membrane-based filters have been employed in the removal of organic contaminants from polluted solution [8].

Photocatalysis has emerged as an intriguing degradation mechanism among the existing potential methods of removing organic effluents from the solution due to its low cost, nontoxicity, safety, and renewable nature. TiO₂, which was used in 1972 by Fujishima for photoelectrochemical water splitting, is the most extensively used photocatalyst today [9]. The typical TiO₂ catalyst, on the other hand, is stimulated by UV light that takes up less than 5% of the whole solar spectrum. This has prompted researchers to create innovative materials with lower bandgap energy (E_g) to improve sensitivity to an increasingly plentiful visible light photon [10–15]. Therefore, the heterojunction of cobalt(II, III) oxide and bismuth oxyiodides efficiently remove nitrophenol from solutions due to the high specific surface area and density of the photogenerated charge carrier resulting in its visible light-sensitivity of the catalyst [16, 17]. Acid Blue 25 dyes were removed effectively by tetraphenylporphyrin/tungsten (VI) oxide/reduced graphene oxide photocatalyst [18]. Moreover, mesoporous dendritic silica supports TiO₂ to improve the catalytic performance for the degradation of carbamazepine due to the reduction of the charge recombination rate, charge kinetics improvement, and increased density of the active site for the analyte adsorption [19].

Graphitic carbon nitride (g-C₃N₄) has recently gained a lot of interest because it possesses ∏-conjugated planar layers akin to graphite, giving it great thermal and chemical stability as well as an attractive electrical structure [20]. As a result, it could be used as a direct semiconductor catalyst in sustainable chemistry. The g-C₃N₄ has distinct properties such as a good electrical and optical structure, as well as strong photochemical stability, and is considered a potential photocatalyst [21–23]. g-C₃N₄ has been shown to function well in photo-degrading various organic dyes when exposed to visible light [24, 25]. Nevertheless, due to its small surface area, limited visible light absorption, and the electron-hole recombination rate, the photocatalytic efficiency of g-C₃N₄ is far from optimal. Several ways have been used to overcome these limitations. Many methods, including chemical and physical exfoliation methods, have been used to enhance the surface area of g-C₃N₄. Exfoliation of g-C₃N₄ provides not only a high surface area but also a shorter diffusion length which might result in a low recombination rate [26–29]. Doping with metals and nonmetals has also been found to be an effective method for increasing visible light absorption using bandgap engineering [30]. Qiao’s group demonstrated that phosphorus doping porous g-C₃N₄ nanosheets can significantly lower the bandgap from 2.98 to 2.66 eV which creates more photo-excitation of electrons and holes [31–34]. Concerning the high electron-hole recombination rate of g-C₃N₄, it can be reduced by loading noble metals on its surface or creating junction with different other semiconductors such as TiO₂/g-C₃N₄ [35], graphene oxide/g-C₃N₄ [36], Au/g-C₃N₄ [37]/g-C₃N₄/α-Fe₂O₃ [38], g-C₃N₄/Ag₂O/TiO₂ [39], g-C₃N₄/Bi₂WO₆ [40], Ag/g-C₃N₄ [41], g-C₃N₄/Co₃O₄/V₂O₅ [42], Cu doped ZnO/g-C₃N₄ [43], g-C₃N₄/Co₃O₄ [44], BiOBr-NFs/g-C₃N₄-SAF [45], and Co₃O₄@CeO₂ [46], and ZnO is an inexpensive and nontoxic semiconductor material that has been utilized as a photocatalyst; however, due to its high band gap (3.2-3.3 eV), it is only active in the UV region of the solar spectrum [47–49]. Several literatures demonstrate that the ZnO-coupled g-C₃N₄ composite has shown to increase the oxidation potential and removal efficiency of inorganic and organic contaminants. However, due to reduced visible light absorption in the solar spectrum, the application of the g-C₃N₄-ZnO composite has restricted activity in the visible light area. To solve this issue, a p-type inorganic spinal cobalt oxide (Co₃O₄) semiconductor material with great thermal durability, nontoxicity, and excellent optical property energy bandgap (2.1 eV) can be used [50]. Few studies have combined all of the above-mentioned approaches for increasing the photocatalytic properties of g-C₃N₄. Herein, the incorporation of Co₃O₄ into g-C₃N₄ was used to increase absorbance in the visible portion of the solar spectrum. Chemical exfoliation of bulk g-C₃N₄ was used to reduce the number of stacked layers while increasing the active surface area. We used cellulose extracted from the local plants as a template for metal oxide synthesis because the hydroxyl groups on cellulose act as efficient hydrophilic substrates for metal oxide nucleation and growth, resulting in rod-like morphology which results in a high surface area. The Z-scheme system is designed to enhance the charge carrier density and collection efficiency of the photocatalyst [51]. As a result, the current article presents a thorough examination of the effects of combining all of the approaches on enhancing the performance of g-C₃N₄ regarding methylene blue degradation. Under visible light illumination, the composite outperformed pure g-C₃N₄ in photodegradation of methylene blue by over 8.2 times. Using various characterization techniques, the role of each approach in enhancing photocatalytic activity of g-C₃N₄-ZnO-Co₃O₄ compared to g-C₃N₄ is discussed.

2. Materials and Methods

2.1. Material Characterization

The phase composition and crystallinity of the prepared photocatalyst were measured by using XRD, ShimadzuXRD-7000, with CuK α radiation. Fourier-transform infrared (FTIR) analysis was performed using Spectrum 65 FTIR (Perkin Elmer) in the range of 4000−400 cm⁻¹ using KBrpellets. The morphologies of the samples were examined by field-emission scanning electron microscopy (FESEM, JSM 6500F, and JEOL). The optical absorption spectra were measured using a Shimadzu 3600 UV–Vis-NIR spectrophotometer in the wavelength range of 200–800 nm using BaSO4 as a reference. Brunauer, Emmett, and Teller (BET), ASAP 2020 HD88 surface area analyzer, was used to measure the specific surface area of the samples by N₂ adsorption. A spectrophotometer, PE-LS55, USA, a xenon lamp light source, and an exciton wavelength of 326 nm were used to measure the photoluminescence spectra of the sample.

EIS and Mott–Schottky were measured using a three-electrode system with an Autolab PGSTAT 302 N electrochemical test system (Metrohm Autolab B.V.), with Pt wire and Ag/AgCl electrode as the counter and reference electrodes, respectively.

2.2. Materials

Enset (Ensete ventricosum) fibre was collected from Hawssa, South Ethiopia. Glacial acetic acid (CH₃COOH), sodium hydroxide (NaOH), formic acid (CH₂O₂), 30% hydrogen peroxide (H₂O₂), urea (CH₄N₂O), sulfuric acid (H₂SO₄), cobalt nitrate hexahydrate (Co (NO₃)₂•6H₂O), zinc nitrate hexahydrate (Zn (NO₃)₂•6H₂O), and methylene blue (C₁₆H₁₈N₃SCl) were purchased from Sigma Aldrich. All of the chemicals were utilized as purchased, with no further refining.

2.3. Synthesis of Cellulose

Cellulose from enset was synthesized, as shown in Figure S1 [40]. The plant fibre was washed and dried in an electric oven at 80°C to eliminate reaming dust. The raw material was boiled with a 2% NaOH (1 : 40) W/V ratio for 2 hr, then dried in an electric oven at 70°C, and chopped. After the first alkaline treatment, the second alkaline pretreatment was carried out on a hot plate at 90°C for 1.5 hr in 10% NaOH solutions with a 1 : 10 (W/V) solid-to-liquid ratio of the dry material. The resulting pulps were centrifuged, dried, and subsequently treated with 20% of CH₂O₂/20% of CH₃COOH/7.5% of H₂O₂ (2 : 1 : 2) solution at 90°C temperature of hotplate for 1.5 hr. The dignified pulps were filtered and washed with hot water to separate the cooking liquid (which contains lignin and hemicellulose) from the cellulose. Before the bleaching procedure, the pulps were centrifuged and dried. The pulps were bleached for 30 min at 70°C with 7.5% H₂O₂ and 10% NaOH solutions. The pulps were washed several times with distilled water to eliminate residuals before drying at 60°C in an oven. Finally, 5 g cellulose was added to a 250 ml 50% H₂SO₄ aqueous solution for 30 min at 60°C before being diluted with distilled water. After centrifuging and washing with distilled water until the pH was neutral, the cellulose gel was ultrasonically dispersed in 1 L of distilled water and stored for later use.

2.4. Synthesis of ZnO

A simple chemical precipitation synthesis method was used to prepare ZnO, as shown in Figure S2 [41]. In a nutshell, 0.1 M of zinc nitrate hexahydrate was dissolved in 200 ml of dispersed cellulose solution under vigorous stirring for 0.5 hrs, and then the pH of the solution was adjusted to 10 by adding 1 M NaOH drop wise with continuous stirring for precipitation. Then, the solution was stirred for 2 hr at 60°C and later aged for 24 hr. The precipitate was centrifuged and rinsed with distilled water and ethanol. After washing, the precipitate sample was recovered and dried in an electric oven at 100°C for 2 hrs. The dried sample was then calcined in a muffle furnace at 500°C for 2 hrs. After being ground to a powder, the calcined ZnO was recovered and used in future studies.

2.5. Synthesis of Co₃O₄

Co₃O₄ was prepared by a simple chemical precipitation method, as shown in Figure S3 [42]. In a nutshell, 0.1 M of cobalt nitrate hexahydrate was dissolved in 200 ml of dispersed cellulose solution under vigorous stirring for 0.5 hrs, and then the pH of the prepared solution was raised to 10 by adding 1 M NaOH drop wise with continuous stirring for precipitation. Then, the solution was stirred for 2 hrs at 60°C and then aged for 24 hrs. The precipitate was centrifuged and washed with distilled water and ethanol. After washing, the precipitate sample was recovered and dried in an electric oven at 100°C for 2 hrs. The dried sample was then calcined in a muffle furnace at 400°C for 2 hrs. The calcined Co₃O₄ was collected and ground to powder and used in future studies.

2.6. Synthesis of g-C₃N₄

To synthesize g-C₃N₄, a thermal pyrolysis method was used, as shown in Figure S4 [31, 32]. 10 g of urea flake was placed in a crucible tightly wrapped in aluminum foil and heated in a muffle furnace at the rate of 2°C min^-1 up to 550°C and held for 4 hrs. After naturally cooling to room temperature, a pale-yellow sponge-like powder product was collected. 300 mg of as-prepared powder was mixed in 15 ml of H₂SO₄ and vigorously stirred at room temperature for 24 hrs to break the bonds between layered stacked structures of g-C₃N₄. This solution was diluted with 400 ml of distilled water and ultrasonicated for 6 hrs to exfoliate the g-C₃N₄ layers. The suspension was filtered from the residual solution and washed multiple times with distilled water and ethanol to clean any residual acid from the sample. Finally, at 80°C, the mixture was dried to obtain a g-C₃N₄ powder.

2.7. Synthesis of the g-C₃N₄-ZnO-Co₃O₄ Composite

A simple wet chemical ultrasonic-assisted synthesis method was employed to prepare the g-C₃N₄-ZnO-Co₃O₄ composite, as shown in Figure S5. Typically, different weight ratios of Co₃O₄ and ZnO catalyst samples were added to g-C₃N₄ in 30 ml of ethanol solution, 5% ZnO and 5% Co₃O₄ (GZC-1), 10% ZnO and 10% Co₃O₄ (GZC-2), 15% ZnO and 15% Co₃O₄ (GZC-3), 20% ZnO and 20% Co₃O₄ (GZC-4), and 25% ZnO and 25% Co₃O₄ (GZC-5). The mixture was sonicated for 30 min and vigorously stirred for 2 hrs, and the resulting solid was dried for 6 hrs at 70°C. The binary composites, g-C₃N₄-ZnO (GZ) and g-C₃N₄-Co₃O₄ (GC), were prepared following a similar process.

3. Results and Discussion

3.1. X-Ray Diffraction Analysis

A simple wet chemical ultrasonic-assisted synthesis method was employed to prepare the -C₃N₄-ZnO-Co₃O₄ composite, as shown in Figure S5. XRD was used to examine the phase purity and the crystalline structure of samples. Figure 1 (A-K) depicts XRD results of bulk g-C₃N₄, exfoliated g-C₃N₄, Co₃O₄, ZnO, g-C₃N₄-Co₃O₄, g-C₃N₄-ZnO, and g-C₃N₄-ZnO-Co₃O₄. Figure S6 shows that the XRD result shows peaks at 16°, 22°, and 34° at (2θ), respectively, with the assigned crystallographic planes of (110), (200), and (400), indicating the formation of cellulose I, as well as no doublet in the intensity of the peaks, indicating the absence of cellulose II [52]. There are two distinct peaks for bulk g-C₃N₄ with a strong signal at 27.5° and 13.2°, which correspond to the (002) and (100) planes of g-C₃N₄ (#JCPDS-87-1526) [53] (see Figure 1 A). Tri-s-triazine units and aromatic interlayer stacking are the indication of (002) and (100) crystal planes, respectively [52]. The peak intensity of plane (002) is weak for exfoliated g-C₃N₄, as shown in Figure 1 (B), which implies a reduction in the number of stacked layers in g-C₃N₄ due to exfoliation using sulfuric acid [41]. The metal oxides XRD result is also depicted in Figure 1 (C and D) (#JCPDS-36-1451) [54]. The diffraction peaks at 31.68°, 34.4°, 36.17°, 47.53°, 56.6°, 62.88°, 64.39°, 68.03°, 69.14°, and 77.03° of 2θ belong to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) diffraction planes of the wurtzite hexagonal phase of ZnO as shown in Figure 1 (C). The diffraction peaks at 18.9°, 31.6°, 36.8°, 44.8°, 59.3°, and 65.2° of 2θ match with the cubic face-centered Co₃O₄ crystal planes of (111), (220) (211) (222) (400), (422), (511), (440), and (533), respectively (Figure 1 (D) (#JCPDS-43-1003) [55]. The XRD patterns of g-C₃N₄-ZnO, g-C₃N₄-Co₃O₄, and prepared g-C₃N₄-ZnO-Co₃O₄ composite catalysts with different weight percent contents of zinc oxide and cobalt oxide to graphitic carbon nitride were illustrated in Figure 1(E-K). The g-C₃N₄-ZnO-Co₃O₄ composites have matched with original diffraction peaks of g-C₃N₄, the wurtzite hexagonal ZnO phase, and the cubic Co₃O₄ phase, with no additional peaks present, indicating that the composite has successfully formed. In the composites, a similar peak shape and position as seen in g-C₃N₄, ZnO, and Co₃O₄ were observed that indicates that the interaction of ZnO and Co₃O₄ with g-C₃N₄ does not affect the original lattice structure of g-C₃N₄.

The diffraction peaks of ZnO and Co₃O₄ were steadily strengthened at the cost of the g-C₃N₄ peak intensity while the ZnO and Co₃O₄ concentrations increased. Furthermore, no additional peaks appeared in the samples for any possible impurities or phases.

3.2. FTIR Analysis

In Figure 2, three separate bands in the FTIR spectra of the g-C₃N₄, ZnO, Co₃O₄, and GZC-3 samples were observed. The heterocyclic aromatic bonds, C-N, are associated with the stretching vibration of C=N, C-N, and C-N-C. The stretching vibration peak is predicted at the wavenumber range of 1200–1650 cm⁻¹, along with 1238, 1318, 1410, 1574, and 1640 cm⁻¹ [56–59]. The broad peak in the vicinity of 3000–3500 cm⁻¹ is attributed to the N-H stretching of residual amine groups (-NH₂ and -NH), which might be a residue of the precursor urea. This broad peak might also attribute to the O-H stretching band due to moisture absorption of the sample from the environment [60]. The typical characteristic peaks at around 808 cm⁻¹ indicate the s-triazine ring structure of g-C₃N₄, which is evidence of proper phase formation [61]. In Figure 2, the FTIR spectra of the Co₃O₄ absorption spectra band at 3416 cm⁻¹ and 1635 cm⁻¹ show O-H stretching and bending vibrations, respectively. It might be due to moisture absorption of the sample from the environment [62, 63]. Furthermore, the absorbance bands from the FTIR spectra at 654 cm⁻¹ and 558 cm⁻¹ indicate vibrations of Co(III)-O bonds and Co-O stretching, confirming the formation of Co₃O₄ [60, 64, 65]. In Figure 2, the FTIR spectra of the ZnO absorption spectra band at 3400 cm⁻¹ and 1380 cm⁻¹ show O-H stretching and bending vibrations, respectively. It might be due to moisture absorption of the sample from the environment [66]. The absorbance bands from the FTIR spectra at 1110 cm⁻¹ indicate vibrations of the Zn–O bond, confirming the formation of ZnO [67]. The FTIR results support the g-C₃N₄, ZnO and Co₃O₄ phase formation as claimed in XRD analysis. The FTIR spectra of the GZC-3 composite include bands associated with g-C₃N₄, Co₃O₄, and ZnO functional groups, respectively, confirming the successful synthesis of the g-C₃N₄-ZnO-Co₃O₄ composite as illustrated in Figure 2.

3.3. SEM Analysis

Scanning electron microscopy was used to examine the morphology of synthesized samples. Cellulose templates can bind metal cations and regulate the particle size of metal oxide during the synthesis process. When cobalt nitrate and zinc nitrate are mixed in a cellulose solution, the cobalt nitrate and zinc nitrate hydrolyze and interact to generate a strong adhesion on the cellulose surface because cellulose is rich in hydrogen bonds and a hydroxyl macromolecule. The physical space from cellulose makes it difficult for the formation of a large aggregation during the growth of these metal oxides. After calcination, the template is removed, resulting in dispersed ZnO and Co₃O₄. Figures 3(a) and 3(b) show the image of the SEM for the ZnO and Co₃O₄ grown using cellulose as a template, which has a characteristic of rod-like morphology. The rod-like morphology allows for faster kinetics of charge carriers to the catalyst’s surface, which may reduce electron and hole recombination rates [68]. Figure 3(c) displays a sheet-like morphology for g-C₃N₄; it is typical for g-C₃N₄ synthesized by thermal polymerization followed by chemical exfoliation. Figure 3(d) shows the image of the scanning electron microscope of the GZC-3 composite and illustrates the typical morphology of ZnO and Co₃O₄ across the matrix of g-C₃N₄. The chemical composition of the GZC-3 composite was analyzed using EDX spectroscopy, with the findings shown in Figure 3(e). The EDX spectra of the GZC-3 composite contained peaks corresponding to the Zn, Co, O, C, and N elements. These results clearly show the successful formation of the GZC composite.

3.4. UV Analysis

The optical absorbance of synthesized samples g-C₃N₄, ZnO, Co₃O₄, and GZC composite samples were measured by using a UV-vis spectrometer, as shown in Figure 4(a). ZnO absorbs more in the UV range of the solar spectrum, less than 400 nm wavelength, as shown in the spectra designated by the black line in Figure 4(a) [69]. The Co₃O₄ exhibited a strong absorption tail at a wavelength longer than 500 nm, as shown in the spectra designated by the red line in Figure 4(a) [70]. The -C₃N₄ displayed high absorption capabilities between 250 and 380 nm, with an absorption edge at 460 nm, which is still short for absorbing the visible portion of solar radiation and leaving the large part of the visible light spectrum unexplored. The addition of Co₃O₄ and ZnO samples in the g-C₃N₄ matrix increased absorption intensity while also red-shifting the wavelength towards the visible light area (shifted to lower energy). The appropriate form of a junction formed between g-C₃N₄, ZnO, and Co₃O₄ might be the reason for the better absorbance to the visible area of the composites. The Tauc equation was employed to determine the energy bandgap of the synthesized samples from UV-visible absorbance spectra, as shown in Figure 4(b). The calculated optical band gaps of the prepared photocatalyst sample use Tauc’s plots 3.1, 1.9, 2.85, 2.95, 2.3, 2.62, 2.55, 2.5, 2.4, and 2.35 eV for ZnO, Co₃O₄, g-C₃N₄, GZ, GC, GZC-1, GZC-2, GZC-3, GZC-4, and GZC-5 composites, respectively. The energy band gap of the composite samples seems to obey the superposition principle, as shown in Figure 4(b).

(a)

(b)

3.5. BET Surface Area and Pore Size Analysis

The pore volume and the specific surface area of metal oxide prepared by cellulose templated or not are summarized in Figure 5. The specific surface area of ZnO prepared without cellulose measured 484.707 m²/g compared to cellulose templated 538.563 m²/g. The surface area of cobalt oxide prepared without cellulose is 48.673 m²/g, and with cellulose templated, it is 520.102 m²/g. For cellulose-templated ZnO and Co₃O₄, the total pore volume was 0.15 cm³/g and 0.16 cm³/g, respectively, whereas they were 0.11 cm³/g and 0.10 cm³/g for ZnO and Co₃O₄ without cellulose.

The results show that using cellulose as a template to prepare ZnO and Co₃O₄ result in higher total pore volume and specific surface area. The pore volume and specific surface area increment attributed to high-temperature cellulose decomposition and the creation of a porous structure of ZnO and Co₃O₄. The bulk g-C₃N₄ synthesized without the exfoliation process typically shows a poor surface area (<10 m²/g⁻¹) [71]. The sulfuric acid-treated g-C₃N₄ surface area and pore volumes show 47.136 m²/g⁻¹ and 0.09 cm³/g, respectively. The surface area of the GZC-3 composite (60.578 m²/g) and the pore volume (0.12 cm³/g) increased from bulk g-C₃N₄, as shown in Figure 5.

3.6. Photocatalytic Performance

The photocatalytic MB dye degradation process takes place in the photoreactor with a 250 Watts halogen lamp as the light source. Typically, around 30 mg of the prepared sample is added to a 100 ml (10 ppm) aqueous solution of MB dye. The catalyst-containing solution was stirred for 30 min in the dark before the photocatalytic degradation test commenced. That leads to achieving equilibrium for the adsorption and desorption of dye molecules on the photocatalyst surface, a dark reaction. To separate the solution from the photocatalyst powder, a 4 ml dye solution was taken and centrifuged to separate the catalyst and solution at each time interval. A UV-Vis spectrophotometer was used to measure the concentration of MB dye at each time interval. The distance between the light source and the solution remained constant at 5 cm. The MB dye degradation efficiency (η) was determined using the following formula: η = ((C_o − C_t)/Co) ∗ 100%. The C_o and C_t are the MB dye concentrations at the initial and each time interval of light irradiation, respectively. All tests have been conducted again to validate the findings.

3.7. Photoluminescence (PL) Analysis

To observe the recombination of photogenerated charge carriers in the catalyst room temperature, PL spectra were measured at 326 nm excitation wavelength, as shown in Figure 6(a). The g-C₃N₄ sample has the maximum PL peak intensity and exhibits wider peak width emission at 419 [43], demonstrating a high rate of electron and hole recombination relative to the synthesized sample. The formation of GZC composites results in photoluminescence quenching; it might boost photocatalytic activity. Figure 6 illustrates the PL emission spectra of the photocatalysts g-C₃N₄, showing the highest peak and the composite GZC’s lowest peak. The peak intensity is from highest to lowest in the order -C₃N₄, GC, GZ, GZC-5, GZC-4, GZC-1, GZC-2, and GZC-3, as shown in Figure 6(a). The GZC-3 composite shows the lowest PL emission peak intensity among all synthesized catalysts, implying the effective suppression of photogenerated charge carriers from recombination. This phenomenon will favor having a large number of electrons and holes engage in the redox process.

(a)

(b)

3.8. The Nyquist Plots

Electrochemical impedance spectroscopy (EIS) is a powerful technique for understanding the transport of interface charges [72]. We examined the charge transfer resistances and interfacial charge separation efficiency of -C₃N₄ and GZC composites by EIS, as shown in Figure 6(b). During interfacial transport, the composites possess the smallest EIS semicircle radius compared to g-C₃N₄, implying the lowest impedance, which permits the rapid transfer of charges. Based on the EIS data, the GZC-3 composite catalyst has shown a small radius compared to other samples in this experiment. Small radii correspond to small charge transfer resistance, thereby increasing the efficiency of photocatalysts, which complement the PL spectra somewhere above.

3.9. Photocatalytic Activity of the Samples

The photocatalytic MB dye degradation performance of the synthesized samples was measured using the beer lambert law. The catalyst-dye-containing solution was irradiated, and the dye concentration was measured by UV-visible spectroscopy. The absorbance vs. wavelength graph of the GZC-3 composite catalyst change is shown in Figure 7(a), and the concentration of MB dye over time changed in the presence of the catalyst.

(a)

(b)

(c)

(d)

The blank test indicates that MB dye is only weakly degraded in the absence of a catalyst, noting that photolysis is not an option to degrade the MB dye, as shown in Figure 7(b) gray line. The g-C₃N₄, ZnO, and Co₃O₄ exhibited photocatalytic degradation efficiency for the degradation of MB dye 41.92, 45.10, and 50.40% of MB, respectively. The binary and ternary composites of g-C₃N₄, ZnO, and Co₃O₄ exhibit improved photodegradation activity for MB dye degradation. In particular, the ternary composites g-C₃N₄/ZnO/Co₃O₄ show promising performance for the photodegradation of MB. The GZC-3 composite had the maximum photodegradation activity showing 97.4% MB dye degradation after 60 min of photoreaction.

The degradation kinetic was studied to highlight the photodegradation activity of the materials, as shown in Figure 7(c). The pseudo-first-order kinetic equation -ln (C_t/C₀) = k_app was used to fit the degradation kinetic plots, where k_app and t represent the apparent first-order reaction rate constant and irradiation time, respectively. Table 1 shows the corresponding correlation coefficient values, which are all close to one, showing that the photocatalytic degradation process is a first-order reaction. On the other hand, observing the results of the kinetic plot in Figure 7(d), the GZC-3 composite has the highest value of k_app (0.001136 min⁻¹); in comparison, the k_app values for g-C₃N₄, ZnO, and Co₃O₄ are, respectively, 0.000137, 0.000154, and 0.0002025 min⁻¹. It implies that the photodegradation activity of the GZC-3 composite is approximately 8.29, 7.38, and 5.60 times greater than pure g-C₃N₄, ZnO, and Co₃O₄, respectively.

3.10. Recyclability

A catalyst for practical use has to examine its photostability in the effluent. The GZC-3 composite catalyst sample measured the photocatalytic MB dye degradation activity for 60 min. Subsequently, 4.0 ml of the degraded solution has taken to measure the dye concentration, and the remaining degraded solution was centrifuged and separated the catalyst from the solution. The separated catalyst was washed and cleaned with distilled water and dried in an electric oven at 60°C for 2 hrs. The recyclability and photostability of the catalyst were measured for four cycles following the procedure.

The catalyst shows excellent stability in degrading MB dye in an aqueous solution, as shown in Figure 8 and Figure S7. However, after the four cycles of the degradation process, negligible fluctuation in the degradation performance was observed, which might be attributed to the loss of catalyst amount while collecting from the centrifuge tube, since the amount of the catalyst added in the dye solution is small.

3.11. Reaction Mechanism

The flat band potential of the composite materials was measured using the Mott–Schottky technique at 800 Hz in the dark, as shown in Figures 9(a)–9(c). The Mott–Schottky plot for the g-C₃N₄ and ZnO shows a positive slope, which indicates n-type conductivities. The Co₃O₄ has a negative slope indicating p-type conductivities.

(a)

(b)

(c)

(d)

Extrapolating the linear section of the plot to the x-axis shows the flat band potentials (V_FB) of g-C₃N₄, ZnO, and Co₃O₄ as −1.05 V, −0.83 V, and 1.6 V vs. RHE, respectively. The conduction band edge potential of g-C₃N₄, ZnO, and Co₃O₄ was approximately the same as the V_FB [73]. The valence band edge position of g-C₃N₄, ZnO, and Co₃O₄ is determined to be ∼1.7 V, 2.3 V, and 3.4 V vs. RHE; therefore, the composite may create a junction band structure shown in Figure 9(d).

Up on irradiation to the catalyst in the dye solution, electrons from the valance bands of g-C₃N₄, ZnO, and Co₃O₄ are excited to the conduction band of g-C₃N₄, ZnO, and Co₃O₄, which creates a large number of charge carriers in the catalyst valance and conduction band. The electron on the conduction band of cobalt oxides will fall to the valance band of g-C₃N₄ due to the potential variance; this could result in a Z-scheme electron transfer process [74, 75], as shown in Figure 9(d).

The excited electron on the conduction band of g-C₃N₄ will transfer to the conduction band of the ZnO. The Z-scheme improves the photogenerated charge carriers' transfer and separation efficiency while maintaining the strong reduction capability of electrons in g-C₃N₄ transferred to the ZnO and oxidation capability of holes in the ZnO and Co₃O₄ for effective MB dye degradation.

The electron in the conduction band of ZnO will participate in the reduction process to produce superoxide (O^•−) radicals, and the holes in the valance bands of ZnO and Co₃O₄ will primarily participate in the oxidation reaction to form hydroxyl (OH^•) radicals.

ZnO is thermodynamically more feasible to undergo a reduction reaction to produce superoxide (O^•−) radicals due to high negative conduction band potential (−0.8 vs. NHE) compared to (O^•−/O₂), whereas the hole in the ZnO and Co₃O₄ valance band are thermodynamically more feasible to undergo oxidation process to hydroxyl (OH^•) radicals due to sufficient positive valance band potentials compared to H⁺/H₂O and OH⁻/OH^• [76–78]. This active species (O^•−) and OH• reacts with MB dye to break down the bond of the dye into nonpolluting molecules such as water, carbon dioxide, and others.

4. Conclusions

The Z-scheme ZnO-g-C₃N₄-Co₃O₄ composite was prepared by wet chemical and ultrasonicate-assisted synthesis methods. It has been observed that utilizing cellulose as a template to prepare metal oxide increases the specific surface area with a distinct morphology that might enhance the density of the active site. The photocatalytic activity of the GZC composite towards MB dye degradation was better than that of the pure and binary composites. The GZC-3 composite demonstrated the highest degradation efficiency in degrading MB dye, with 97.4% degradation efficiency in 60 min under visible light irradiation. All the synthesized composite photocatalysts exhibit improved photocatalytic activity compared to the binary composite and pure ones due to the proper band alignment, the higher specific surface area, better visible light absorbance, and low charge transfer resistance. The GZC-3 composite demonstrates excellent stability and degradation efficiency, suggesting that GZC is a feasible candidate photocatalytic material for MB dye degradation in an aqueous solution.

Data Availability

The data used to support the findings of this study are included within the article, and the supporting information/supplementary file is available from the Hindawi Online Library.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Acknowledgments

The authors extend their appreciation to the Department of Materials Science and Engineering, Adama Science and Technology University, Ethiopia, for technical expertise support in doing experiments and Google inclusion research support.

Supplementary Materials

Figure S1: schematic illustration of the celluloses synthesis process. Figure S2: schematic illustration of the ZnO synthesis process. Figure S3: schematic illustrations of the Co₃O₄ synthesis process. Figure S4: schematic illustration of the g-C₃N₄ synthesis process. Figure S5: schematic illustration of the g-C₃N₄/ZnO/Co₃O₄ composite synthesis process. Figure S6: XRD of prepared cellulose. (Supplementary Materials)

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Copyright © 2023 Mintesinot Tamiru Mengistu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Bioinorganic Chemistry and Applications

-C3N4–Co3O4 Z-Scheme Junction with Green-Synthesized ZnO Photocatalyst for Efficient Degradation of Methylene Blue in Aqueous Solution

Abstract

1. Introduction

2. Materials and Methods

2.1. Material Characterization

2.2. Materials

2.3. Synthesis of Cellulose

2.4. Synthesis of ZnO

2.5. Synthesis of Co3O4

2.6. Synthesis of g-C3N4

2.7. Synthesis of the g-C3N4-ZnO-Co3O4 Composite

3. Results and Discussion

3.1. X-Ray Diffraction Analysis

3.2. FTIR Analysis

3.3. SEM Analysis

3.4. UV Analysis

3.5. BET Surface Area and Pore Size Analysis

3.6. Photocatalytic Performance

3.7. Photoluminescence (PL) Analysis

3.8. The Nyquist Plots

3.9. Photocatalytic Activity of the Samples

3.10. Recyclability

3.11. Reaction Mechanism

4. Conclusions

Data Availability

Conflicts of Interest

Acknowledgments

Supplementary Materials

References

Copyright

-C₃N₄–Co₃O₄ Z-Scheme Junction with Green-Synthesized ZnO Photocatalyst for Efficient Degradation of Methylene Blue in Aqueous Solution

2.5. Synthesis of Co₃O₄

2.6. Synthesis of g-C₃N₄

2.7. Synthesis of the g-C₃N₄-ZnO-Co₃O₄ Composite