Co₃O₄-rGO—Synthesis, Characterization, and Evaluation of Photocatalytic Activities

Abstract

Water contamination with synthetic dyes is an escalating problem worldwide. Herein, Co₃O₄-decorated reduced graphene oxide (Co₃O₄-rGO) is reported as an effective heterogeneous photocatalyst for the decomposition of organic dyes. The synthesis of Co₃O₄-rGO was confirmed via spectroscopic techniques including XRD, XPS, TEM, and FTIR. After characterization, the prepared Co₃O₄-rGO composite was tested as a photocatalyst for the degradation of methylene blue and methyl orange. The photocatalytic efficiency of Co₃O₄-rGO was >95% after 60 min, corresponding to 200 mg/L as the initial concentration of each dye. The photodegradation of MB and MO was confirmed by BOD and COD measurements. Experimental parameters like the re-usability of Co₃O₄-rGO, the effect of catalyst dosage, and the effect of dye concentration on photocatalytic activity were also investigated. The photocatalytic activity of Co₃O₄-rGO for the degradation of MB was 2.13 and 3.43 times higher than that of Co₃O₄ and rGO, respectively. Similarly, the photocatalytic activity of Co₃O₄-rGO for the degradation of MO was 2.36 and 3.56 times higher than that of Co₃O₄ and rGO, respectively. Hence, Co₃O₄-rGO was found to be an efficient and reusable photocatalyst for the decomposition of selected dyes in the aqueous medium.

1. Introduction

Recently, catalysis in general and photocatalysis employing semiconductors as heterogeneous catalysts and sunlight as a source of energy in particular have received considerable attention due to a wide range of applications [1,2,3,4,5]. The photocatalytic reactions are comprised a series of oxidation and reduction reactions mediated by positive holes and electrons formed in the valence band and conduction band of semiconductor catalysts under irradiation with ultraviolet or visible light [6,7,8,9]. Among the photocatalytic redox reactions, the UV–visible light-assisted decomposition reactions of organic pollutants like dyes are the most reported. The overexploitation of limited resources and rapid industrial development due to human activities have increased water pollution globally [10]. Although many traditional methods like membrane filtration, adsorption, biological and chemical coagulation, and precipitation are used for the abatement of aqueous pollution, these techniques are not successful in the complete removal of pollutants from the environment [11,12,13,14]. These methods transform the pollutants from one phase to another. Therefore, the abatement of pollution due to organic dyes via photocatalytic reactions has received much attention from researchers worldwide.

ZnO and TiO₂ have been mostly used as single-component semiconductor photocatalysts for the photodegradation of dyes and other organic pollutants [15,16]. However, the practical applications of these semiconductor metal oxides are limited due to certain characteristics. The characteristics that limit their practical applications include insufficient absorption of visible light, poor photostability, and lower separation efficiency of photo-induced charges. Thus, researchers have focused on the development of multi-component visible light-active photocatalysts with enhanced charge separation efficiency [17,18,19,20,21,22,23]. The literature has shown that dopants, co-catalysts, and carbonaceous materials have been widely used for the development of multi-component photocatalysts with enhanced photocatalytic activity [24,25]. Hence, the immobilization of cobalt oxide on reduced graphene is reported in this study for the development of an efficient Co₃O₄-rGO photocatalyst. Cobalt oxide is a suitable semiconductor for the development of an effective photocatalyst due to its narrow band gap (~2.7 eV), exciting magnetic properties, fascinating catalytic properties, high chemical and thermal stability, and ease of preparation. The photocatalytic activities of Co₃O₄ can be enhanced by supporting them on the surface of graphene oxide. The synergistic effects of graphene oxide suppress the recombination of photoinduced charges, resulting in the enhancement of photocatalytic performance. It has been reported by many researchers that linking semiconductor metal oxides to graphene oxide suppresses the recombination of charge carriers formed by the absorption of photons and subsequently results in a significant enhancement in the photocatalytic performance. The graphene oxide in the metal oxide-graphene oxide composite acts as an electron mediator. As a result, the photoinduced charge carriers (h⁺/e⁻) are effectively separated. The effective separation of these photon-induced charge carriers causes an enhancement in the photocatalytic performance of the composite [26,27]. Therefore, we immobilized the Co₃O₄ particles on the surface of rGO to obtain an efficient, visible light-driven photocatalyst for the decomposition of selected synthetic dyes. The contribution of synthetic dyes to water pollution is very significant. The wastewater contaminated with synthetic dyes originates from different industries like the paint, tannery, paper, printing, and textile industries. Wastewater contaminated with synthetic dyes is very toxic for life. Methylene blue and methyl orange are typical cationic synthetic dyes used in various industries [28,29]. In this study, we report the photodegradation of methylene blue and methyl orange in the presence of Co₃O₄-rGO under the irradiation of visible light.

2. Results and Discussion

2.1. Characterization of Co₃O₄-rGO

The formation of Co₃O₄-rGO was confirmed via XRD as the initial characterization technique. XRD analyses of rGO and Co₃O₄ were also carried out for comparison. Figure 1 shows the XRD patterns of the samples. The XRD pattern of rGO comprised peaks at 2θ, 24.2°, and 42.8°, which are related to the (002) and (100) planes of rGO. Similarly, the XRD pattern of Co₃O₄ comprised diffraction peaks at 2θ, 36.34°, 38.56°, 44.8°, 59.35°, 65.23°, 77.34°, and 78.4°, which correspond to the (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1), (6 2 0), (5 1 1), (5 3 3), and (6 2 2) planes of Co₃O₄ (JCPDS Card No. 42-1467). The XRD pattern of Co₃O₄-rGO matches well with the XRD patterns of rGO and Co₃O₄, which confirms the successful formation of the Co₃O₄-rGO composite [30,31,32,33].

The average crystallite size of Co₃O₄-rGO was calculated at 43.5 nm using the Debye–Scherrer equation (Equation (1)). The hkl planes and corresponding angles of diffraction (2θ) in XRD patterns were used for the calculation of lattice parameters of the cubic structure of the unit cell using Equation (2) via the non-linear method of analysis. The “Solver” add-in software of MS Excel was used for this purpose. The lattice parameters for the cubic structure were calculated as a = b = c = 11.98 Å.

$$d=\frac{0.98\times 1.54}{\beta Cos \theta }$$

(1)

$$\frac{1}{d_{hkl}^2}=\frac{h^2+k^2+l^2}{a^2}$$

(2)

The formation of Co₃O₄-rGO was further confirmed using XPS analysis. Figure 2 shows the XPS spectra of Co, O, and C, which confirms the existence of Co, O, and C in the prepared sample. Figure 2a displays the XPS spectrum of Co(2p), in which two peaks can be observed. The first peak at 780.3 eV is ascribed to Co(2p_3/2), while the second peak at 795.8 eV is ascribed to Co(2p_1/2). These peaks confirm the +3 oxidation state of Co in the composite [30]. The high-resolution deconvluted XPS spectrum for the O 1s region given in Figure 2b showed three peaks at binding energies of 529.7 (OI), 531.0 (OII), and 532.0 (OIII) eV, which are assigned to typical metal–oxygen bonds on the Co₃O₄ surface with low oxygen coordination. The latter two signals are typical peaks of O-Co linkages. Figure 2c depicts the deconvoluted C1s XPS spectrum, indicating three signals at 284.0, 288.3, and 294.0 eV. The existence of three signals suggests three different electronic environments. The peak observed at 288.3 eV is a typical peak for graphene oxide [34,35,36].

The morphology of Co₃O₄-rGO was investigated by transmission electron microscopy. The transmission electron micrographs of the sample are given in Figure 3. It can be observed that the rGO in the Co₃O₄-rGO composite is a thin, flat flake with a crumpled morphology. The existence of crumpled sheets and thin multilayers is associated with the exfoliation process in sonification and reduction. A slight aggregation of Co₃O₄ nanoparticles can be observed, which confirms that Co₃O₄ nanoparticles have been successfully anchored to the surface of rGO. The attachment of Co₃O₄ to the rGO suppresses the aggregation of Co₃O₄ particles; hence, the rGO is a suitable substrate for the improvement of the dispersion, stability, and photocatalytic performance of Co₃O₄-rGO [37,38].

The FTIR spectra of Co₃O₄, rGO, and Co₃O₄-rGO are given in Figure 4. The observed peak at 3404 cm⁻¹ is due to the stretching vibration of O-H, and the peak at 1740 cm⁻¹ showed the stretching vibration of the (C=O) carboxyl group. A broad band in the range of 1475–1543 cm⁻¹ is indexed to C-O as a vibration corresponding to rGO and also surface-decorated Co₃O₄ via the C-O bond. The absorption band at 1023 cm⁻¹ is due to C-O stretching vibration, while at 1623 cm⁻¹, it corresponds to the C=C bond. The O-H band was moved to 3424 cm⁻¹ for the Co₃O₄-rGO composite, and the band of the skeletal vibration of C=C was shifted to 1627 cm⁻¹ [25,39]. The band appearing at 565 and 661 cm⁻¹ in the FTIR spectrum of Co₃O₄ has been indexed to the Co-O vibration mode. These peaks have been shifted to 600 and 678 cm⁻¹ in the presence of rGO [30].

The band gap energy of Co₃O₄-rGO was calculated using the UV–visible DR spectrum as given in Figure 5. Using 470 nm as the absorption edge and the equation given in Figure 5, the band gap energy of Co₃O₄-rGO was calculated at 2.63 eV, which is comparable to reported data [7].

2.2. Photocatalytic Efficiency of Co₃O₄-rGO

The photocatalytic efficiency of the as-prepared Co₃O₄-rGO photocatalyst was assessed through the investigation of the photodegradation of methylene blue (MB) and methyl orange (MO) dyes. The photocatalytic experiments were carried out using a dye solution of 50 mL with an optimum value of 0.1 g of the photocatalyst under irradiation with UV–visible light. Before the evaluation of photocatalytic activity, two blank experiments were performed.

In the first blank experiment, 50 mL of 200 mg/L of MB or MO solution was taken in a Pyrex glass beaker. A sample of 0.5 mL was taken from the beaker, and its absorbance was noted after dilution. The dye solution was then stirred under UV–visible light for 1 h to detect any degradation due to photolysis. After stirring for one hour, a sample was taken and analyzed. It was found that there was no change in the concentration of the dye due to stirring under irradiation. Hence, there was no degradation of either dye due to photolysis.

In the second blank experiment, 0.1 g of the catalyst (rGO, Co₃O₄, or Co₃O₄-rGO) was added to the MB or MO dye solution (50 mL, 200 mg/L), and the reaction mixture was stirred for half an hour under dark conditions. This blank experiment was performed for the estimation of the removal of dye due to adsorption. After half an hour, a sample was taken, and its absorbance was measured. The analyses of the data showed that there was about 14, 20, and 32% removal of each dye due to adsorption on rGO, Co₃O₄, and Co₃O₄-rGO, respectively. The elimination of MB and MO due to sorption is shown in Figure 6a.

After two blank experiments, the reaction mixture was stirred under UV–visible irradiation for the evaluation of the photocatalytic activity of each sample. The absorbance measured after stirring for 30 min in the dark was considered the initial absorbance for the evaluation of the photocatalytic activity. The photodegradation of MB and MO over rGO, Co₃O₄, and Co₃O₄-rGO is given in Figure 6b. About 98, 62, and 45% photodegradation of methylene blue was observed in 60 min of reaction using Co₃O₄-rGO, Co₃O_4, and rGO as photocatalysts, respectively. Similarly, the photodegradation of methyl orange was 92, 57, and 42% over Co₃O₄-rGO, Co₃O_4, and rGO as photocatalysts, respectively. The photodegradation data demonstrated that the incorporation of Co₃O₄ in reduced graphene oxide sheets increased the photocatalytic ability of the composite toward the degradation of MB and MO dyes.

The photocatalytic degradation of the selected dyes was confirmed via COD and BOD measurements as well. About 80% and 68% reductions in COD and BOD were detected in the 50 mL (200 mg/L) solution of MB/MO after treatment with Co₃O₄-rGO for 60 min. The decrease in COD and BOD of the dye solutions indicates that Co₃O₄-rGO can be employed as an efficient catalyst for the photodegradation of the dyes.

The re-usability of Co₃O₄-rGO in the photodegradation of MB and MO was also investigated. For this purpose, the spent Co₃O₄-rGO was washed with ethanol and distilled, followed by drying at 80 °C overnight. The regenerated Co₃O₄-rGO was employed as a catalyst in separate photodegradation experiments of MB and MO for 30 min. The spent Co₃O₄-rGO was recycled three times. The results indicated that there was no significant loss in the photocatalytic activity of Co₃O₄-rGO. The obtained results confirmed that Co₃O₄-rGO can be re-used in the photodegradation of MB and MO.

2.3. Optimization of Catalyst Dose

The unnecessary use of a catalyst can be avoided by employing the optimum dosage of the catalyst. The optimization of catalyst dosage was accomplished by performing photodegradation experiments using a 50 mL (200 mg/L) solution of MB or MO with different catalyst dosages in the range of 0.05–0.2 g Co₃O₄-rGO under similar experimental conditions. The obtained results are given in Figure 7. The data given in Figure 7 show that photocatalytic activity initially increased with catalyst dosage and then decreased. Initially, the photocatalytic activity increased with catalyst dosage because the number of active sites of the catalyst increased with catalyst dosage. However, higher catalyst dosages caused a decrease in photocatalytic activity. This is because photons cannot penetrate effectively through the scattering of photons and the turbidity of the reaction mixture. Hence, 0.1 g was selected as the optimum catalyst dose in this study [40,41].

2.4. Mechanism and Kinetics of Photodegradation

Figure 8 explains the mechanism of photodegradation. Photodegradation is based on the excitation of both the dye and the photocatalyst. The irradiation of the dye solution results in the excitation of the dye molecules. The excited electrons then flow to the conduction band of Co₃O₄ via rGO sheets. Similarly, the excitation of Co₃O₄ results in the formation of positive holes and electrons in the conduction band and valence band, respectively. The intermediate work function of rGO (−4.42 eV) between the conduction band (−3.40 eV) and valence band (−6.65 eV) of Co₃O₄ causes a flow of electrons to the rGO. Hence, the positive holes and electrons are separated, resulting in a decrease in the rate of their recombination. These separated positive holes and electrons then undergo a series of reactions and produce OH radicals [42,43,44,45].

The reactions are given below (CG: Co₃O₄-rGO).

$\mathrm{C}\mathrm{G}+\mathrm{h}\mathsf{\nu }\to \mathrm{C}\mathrm{G}\left({\mathrm{h}}^{+}\right)+\mathrm{C}\mathrm{G}\left({\mathrm{e}}^{-}\right)$	(Reaction 1)
${\mathrm{H}}_2\mathrm{O}+\mathrm{C}\mathrm{G}\left({\mathrm{h}}^{+}\right)\to {\mathrm{H}}^{+}+\mathrm{O}\mathrm{H}$	(Reaction 2)
${\mathrm{C}\mathrm{G}(\mathrm{e}}^{-})+{\mathrm{O}}_2\to {\mathrm{O}}_2^{-}$	(Reaction 3)
${\mathrm{H}}^{+}+{\mathrm{O}}_2^{-}\to {\mathrm{H}\mathrm{O}}_2$	(Reaction 4)
${\mathrm{H}\mathrm{O}}_2\to {\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$	(Reaction 5)
${\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{O}\mathrm{H}$	(Reaction 6)
$\mathrm{O}\mathrm{H}+\mathrm{D}\mathrm{y}\mathrm{e}\to \mathrm{D}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}$	(Reaction 7)

As indicated in the above-mentioned reactions, positive holes, electrons, and hydroxyl radicals are the main species responsible for the photodegradation of dyes; therefore, the formation of these charge carriers was confirmed via scavenging experiments. For this purpose, separate photodegradation experiments were performed in the presence of EDTA, BQ, and t-BuOH. These species were added to a reaction mixture to arrest positive holes, superoxide anion radicals, and hydroxyl radicals, respectively. The photocatalytic activity of Co₃O₄-rGO towards the degradation of MB decreased from 98% to 67, 61, and 49% in the presence of EDTA, BQ, and t-BuOH, respectively. The decline in photocatalytic performance in the presence of EDTA is due to the non-availability of positive holes for the photodegradation reaction. Similarly, the decrease in photocatalytic performance in the presence of BQ and t-BuOH is due to the non-availability of superoxide anion and hydroxyl radicals, respectively [46,47].

Based on reaction 7, the rate of reaction can be written as

$$Rate=-\frac{d\left[Dy\right]}{dt}=k_r[\mathrm{O}\mathrm{H}]{\left[Dy\right]}^n$$

(3)

For the same catalyst dosage and continuous irradiation of the reaction mixture, the rate of formation of OH radicals or the concentration of OH radicals will remain the same; hence, the rate expression is modified as follows (where k is the apparent rate constant).

$$Rate=-\frac{d\left[Dy\right]}{dt}=k{\left[Dy\right]}^n$$

(4)

Considering the first order reaction (n = 1), the rate expression is written as

$$\ln \frac{{\left[Dy\right]}_o}{{\left[Dy\right]}_t}=k t$$

(5)

$${\left[Dy\right]}_t={\left[Dy\right]}_o e^{-k t}$$

(6)

The photodegradation data of MB and MO, as given in Figure 6b, were analyzed using a non-linear method of analysis according to the kinetics Equation (6). For non-linear analyses, the Excel “Solver” add-in optimization software was used. The non-linear analyses of the photodegradation data are given in Figure 9. The rate constants and correlation coefficients are given in

Table 1. Rate constants determined using non-linear analyses.

Photocatalyst	MB		MO
	k	R2	k	R2
rGO	0.0094	0.995	0.0085	0.995
Co3O4	0.0152	0.992	0.0128	0.994
Co3O4-rGO	0.0323	0.983	0.0303	0.985

. The rate constants given in Table 1 show that the photocatalytic activity of Co₃O₄-rGO for degradation of MB is 2.13 and 3.43 times higher than that of Co₃O₄ and rGO, respectively. Similarly, the photocatalytic activity of Co₃O₄-rGO for the degradation of MO is 2.36 and 3.56 times higher than that of Co₃O₄ and rGO, respectively.

2.5. Effect of Dye Concentration

In wastewater treatment, the concentration of the dye is a very vital parameter because the concentration significantly affects the efficiency of the method used for its removal. To study the effect of dye concentration, photodegradation experiments involving methylene blue and methyl orange were carried out with various dye concentrations such as 100, 200, and 300 mg/L in the presence of a Co₃O₄-rGO photocatalyst. The dependence of photocatalytic efficiency on the concentration of dyes was examined for the photo-degradation of MB and MO separately using 0.1 g of the Co₃O₄-rGO photocatalyst. The results obtained from the degradation of MB and MO are shown in Figure 10. The rate constants determined using kinetics analyses are given in

Table 2. Effect of dye concentration on the rate constant of Co₃O₄-rGO-catalyzed photodegradation of dyes.

Concentration (mg/L)	MB		MO
	k	R2	k	R2
100	0.0432	0.991	0.0363	0.991
200	0.0323	0.983	0.0303	0.985
300	0.0206	0.987	0.0151	0.988

. The degradation data given in Figure 10 and the rate constants given in Table 2 show that the photocatalytic efficiency of the catalyst decreased with an increase in dye concentration. The decrease in photocatalytic activity with an increase in the concentration of the dyes is due to the following reasons [48,49,50]:

(1) As dyes impart an intense color to the solution, the penetration of photons is hindered due to the intense color.

(2) The ratio of dye molecules to OH radicals increases with an increase in the concentration of dye.

Finally, the photocatalytic activity of Co₃O₄-rGO reported in this study was compared with the photocatalytic activities of various graphene-based photocatalysts used for the degradation of various dyes reported in the previous literature.

Table 3. Graphene-based photocatalysts reported in the literature for degradation of various dyes.

No	Graphene-Catalyst	Dye	Activity
1	Ag [51]	MB RhB	Almost complete degradation in 70 min using 10 mg/L of dye solution
2	BiOBr [52]	RhB	Almost complete degradation in 5 h using 20 mg/L dye solution
3	MoS2 [53]	MB	A 99% degradation in 60 min using 10 mg/L dye solution
4	CeO2 [54]	RhB	An 85% degradation in 2 h using 10 mg/L dye solution
5	ZnO [55]	MO	A 92% degradation in 3 h using 5 mM dye solution
6	Cu2O [56]	MO	A 70% degradation in 5 h using 5 mg/L dye solution
7	TiO2 [57]	MO	A 90% degradation in 5 h using 10 mg/L dye solution
8	CoFe2O4 [58]	MO	A 78% degradation in 30 min using 10 mg/L dye solution
9	Au [59]	MB (10 mg/L)	Almost complete degradation in 10 min using 10 mg/L dye solution
10	Co3O4 (this work)	MB MO	Complete degradation in 60 min using 100 mg/L dye solution

shows a comparison of the photocatalytic activities of various graphene-based catalysts for different dyes. It can be concluded that the graphene-based catalyst reported in this study is effective for the photodegradation of methylene blue and methyl orange dyes.

3. Experimental Process

3.1. Reagents

The chemicals used in this study include sodium nitrate (Merck, Rahway, NJ, USA), graphite powder (Merck), sulfuric acid (Scharlau, Hamburg, Germany), potassium permanganate (Merck), hydrogen peroxide (30%, commercial grade), cobalt nitrate (Merck), sodium borohydride (Merck), sodium hydroxide (Merck), methylene blue (Sigma-Adrich, Saint Louis, MO, USA), and methyl orange (Sigma-Adrich).

3.2. Synthesis of Co₃O₄-rGO

In the first step, graphene oxide (GO) was synthesized. For this purpose, 2 g of sodium nitrate and 4 g of graphite were added to 100 mL of concentrated sulfuric acid. After stirring for 30 min in an ice-cold bath, 14 g of potassium permanganate was slowly added while stirring, keeping the temperature below 20 °C. After 4 h of stirring, a thick paste was formed. Then, 200 mL of distilled water was added while stirring. As a result, the temperature rose to 98 °C, and a yellowish suspension was formed. The reaction was stopped through the addition of hydrogen peroxide, and the obtained product, designated as graphene oxide (GO), was washed and dried.

In the second step, Co₃O₄-rGO was synthesized. For this purpose, a 60 mL solution containing 0.043 g of cobalt nitrate was added to a 60 mL suspension containing 0.08 g of previously prepared graphene oxide, followed by ultrasonication. Then, 0.28 g of sodium borohydride was added to the above mixture. A total of 0.1 M sodium hydroxide solution was added slowly until pH reached 11. After 60 min, black precipitates were obtained via centrifugation and washed multiple times with triply distilled water and ethanol. The precipitates were desiccated in a vacuum oven at 98 °C.

Reduced graphene oxide (rGO) was also synthesized for comparison. For this purpose, 70 mg of sodium borohydride was added to a suspension of already-prepared graphene oxide. After sonication and stirring, the rGO was collected, washed, and dried.

3.3. Characterization of Co₃O₄-rGO

The successful synthesis of Co₃O₄-rGO was verified using various characterization techniques, such as XRD, XPS, TEM, and FTIR. The XRD analyses were carried out with a Bruker X-ray diffractometer (Bruker D8, Bremen, Germany). A ESCALAB 250XI (Thermo Fisher Scientific, Waltham, MA, USA) photoelectron spectrophotometer was employed for XPS analyses. A JEM 2100 JEOL (Tokyo, Japan) electron microscope was employed for the TEM studies of the prepared samples. A Bruker VRTEX70 (Bremen, Germany) model was used for FTIR analyses.

3.4. Photocatalytic Activity of Co₃O₄-rGO

The photocatalytic performance of synthesized Co₃O₄-rGO was assessed via the decomposition of methylene blue and methyl orange dyes. A UV–visible photometer was used to estimate the extent of the decomposition of selected dyes. To acquire the adsorption/desorption equilibrium in the photocatalytic experiment, a 50 mL solution of dye was stirred in the dark at a speed of 400 revolutions per minute after adding 0.1 g of Co₃O₄-rGO. A sample of the reaction mixture was taken and analyzed. The reaction mixture was then stirred under solar irradiation. A total of 0.5 mL of the sample was removed from the mixture after a regular interval of 15 min and examined. For further analysis and to confirm the degradation of dye, COD and BOD measurements were employed. COD and BOD measurements were carried out on two samples, namely an untreated dye solution and a dye solution treated with Co₃O₄-rGO, for 60 min.

For COD measurement, a 10 mL sample was taken in a reflux flask, and 1 mL of mercury sulphate solution (0.1 g HgSO₄ in 5 mL of concentrated sulfuric acid) was added to it. Then, 5 mL of potassium dichromate solution (6.13 g/L) was added, followed by the addition of 15 mL of silver sulfate–sulfuric acid solution (10 g/L sulfuric acid). The content was digested by refluxing for 2 h. After cooling to room temperature, a few drops of ferroin indicator were added to the mixture, which was titrated against the ferrous ammonium sulfate solution (9.8 g/20 mL sulfuric acid and 100 mL distilled water). The same procedure was repeated for the blank experiment, using distilled water instead of the sample. COD was calculated using the following equation:

$$\mathrm{C}\mathrm{O}\mathrm{D}=\frac{8\times 1000\times \mathrm{M}({\mathrm{V}}_{\mathrm{b}}-{\mathrm{V}}_{\mathrm{s}})}{{\mathrm{V}}_{\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}} (\mathrm{m}\mathrm{L})}$$

(7)

M: Molarity of ferrous ammonium sulfate solution;

V_b: Volume of ferrous ammonium sulfate solution consumed in the blank experiment;

V_b: Volume of ferrous ammonium sulfate solution consumed in the blank experiment;

V_s: Volume of ferrous ammonium sulfate solution consumed in a sample experiment.

The BOD was estimated by measuring dissolved oxygen. Specialized BOD bottles were filled with samples. The dissolved oxygen concentration was measured using a DO meter. The BOD bottles were incubated at 20 °C in the dark for 5 days. After incubation for 5 days, the dissolved oxygen concentration was measured again using a DO meter. The BOD was calculated by dividing the difference between the initial and final dissolved oxygen concentrations.

The BOD was estimated by measuring dissolved oxygen. Specialized BOD bottles were filled with samples. The dissolved oxygen concentration was measured using a DO meter. The BOD bottles were incubated at 20 °C in the dark for 5 days. After incubation for 5 days, the dissolved oxygen concentration was measured again using a DO meter. The BOD was calculated by the difference between the initial and final dissolved oxygen concentration.

4. Conclusions

The results and observations reported in this study indicate the importance and significance of the photocatalyst developed for the degradation of dyes. The general conclusions are extracted as follows:

• The successful synthesis of Co₃O₄-rGO was confirmed through characterization with XRD, XPS, TEM, and FTIR analyses.
• The fabricated Co₃O₄-rGO composite showed efficient catalytic performance in the photodegradation of MB and MO. The photocatalytic activity of Co₃O₄-rGO was higher than the two-fold and three-fold catalytic activity of Co₃O₄ and rGO, respectively. The increase in the initial concentration of MB and MO resulted in a decrease in the photocatalytic performance.
• About 98, 62, and 45% photodegradation of methylene blue was observed in 60 min of reaction using Co₃O₄-rGO, Co₃O₄, and rGO as photocatalysts, respectively. Similarly, the photodegradation of methyl orange was 92, 57, and 42% over Co₃O₄-rGO, Co₃O₄, and rGO as photocatalysts, respectively.
• About 80% and 68% reductions in COD and BOD were detected in the 50 mL (200 mg/L) solution of MB/MO after treatment with Co₃O₄-rGO for 60 min. The decrease in COD and BOD of the dye solutions indicates that Co₃O₄-rGO can be employed as an efficient catalyst for the photodegradation of the dyes.
• The enhanced photocatalytic efficiency of Co₃O₄-rGO was attributed to the behavior of rGO as an electron reservoir that captures the photoexcited electron from Co₃O₄.
• The Co₃O₄-rGO was found to be an efficient and reusable photocatalyst for the degradation of methylene blue and methyl orange.