Mn²⁺ Doped Cobalt Oxide and Its Composite with Carbon Nanotubes for Adsorption-Assisted Photocatalytic Applications

Abstract

In this study, cobalt oxide (Co₃O₄), Mn-doped Co₃O₄ (MDCO), and Mn-doped Co₃O₄-functionalized carbon nanotube (MDCO-CNTs) were synthesized via the co-precipitation method using cobalt nitrate and manganese nitrate as a cobalt and manganese precursor, respectively. Synthesized materials were assessed using different characterization techniques like scanning electron microscopy, X-ray diffraction, and UV-visible spectroscopy. Congo red in an aqueous solution was adopted as the model dye to estimate the adsorption-assisted photocatalytic efficiency of the synthesized materials. The samples studied for adsorpsstion-assisted photocatalysis were found to be highly effective and among all the samples, the best removal performance (80%) was obtained by treating the MDCO-CNTs composite for 50 min at 50 °C. Mathematical modeling shows that all of the samples followed a pseudo-second-order kinetic model and data best fitted to a Langmuir isotherm, implying that the process involved in the removal of Congo red dye is chemisorption.

1. Introduction

Water pollution, a global concern threatening the survival of humans, has been remarkably accelerated with the boom of industrialization and the resultant effluents [1,2,3] In response, a variety of strategies including sedimentation, adsorption, and biodegradation have been employed for wastewater treatment [4]. However, addressing this problem is complex due to the diverse compositions and physicochemical behavior of the contaminants which calls for efficient, low-energy consumption and eco-friendly remediation for industrial effluents. Photocatalysis has been a hot topic across the globe for a variety of applications, particularly environmental remediation. In a typical heterogeneous photocatalytic system, semiconductors act upon light irradiation, absorbing photons to stimulate the electrons to jump to the conduction band from the valence band, leaving holes behind. These excited electrons form superoxide (O₂) radical ions by reacting with oxygen; hydroxyl radicals (HO) are also produced in the presence of H₂O and OH-(electron donor species). Both radical ions, superoxide (O₂) and hydroxyl (HO), are strong oxidizing agents and have the potential to degrade water pollutants [5,6,7].

The most common heterogeneous photocatalytic systems are typically based on semiconductors, such as TiO₂, a key contributor in the field of photocatalysis with over 2,240,000 photocatalytic reports [8,9,10,11]. Although efforts have been dedicated to discovering low-cost potential materials to operate as sole photocatalysts, currently there is also an option to develop strategies for fabricating composite catalysts by incorporating a semiconductor with different functional components and/or metals [1]. To design efficient catalysts, the exploitation of metal oxide-carbon nanotubes (CNTs) composites is commonly observed [12]. CNTs, owing to the tunable structural, physical, chemical, and electrical properties are inspiring enough to be used in this regard [13], e.g., TiO₂-CNTs [14,15,16,17,18,19,20,21,22], ZnO-CNTs [23,24], Cu₂O-CNTs [25], SnO₂-CNTs [26], Fe₂O₃-CNTs [27], CoS₂-CNTs [28], NiFe₂O₄-CNTs [29], CdS-CNTs [30], and in many other binary and ternary compounds [31,32,33,34,35,36], etc.

Cobalt oxide (Co₃O₄)_, is widely applied and the most versatile transition-metal oxide, having stable spinel geometry with Co²⁺ and Co³⁺ ions at tetrahedral and octahedral coordination, respectively [37,38]. Furthermore, different morphologies of Co₃O₄ nanoparticles are reported to degrade water pollutants very effectively [39]. Although considerable development in photocatalyst designing has been achieved, there are still a few aspects, that need to be considered, mainly: (i) relatively complex synthesis methods, (ii) high synthesis cost, and (iii) their harmful impact on the environment. Herein, we aimed to develop the new composite photocatalysts i.e., manganese (Mn) doped Co₃O₄ and Mn-doped Co₃O₄-functionalized CNTs using a simple and cost-effective co-precipitation method for wastewater treatment by opting for Congo red as a model dye.

2. Materials and Methods

For the synthesis of Co₃O₄, the co-precipitation method was adopted. First, a 0.1 molar cobalt nitrate (Co(NO₃)₂.6H₂O) solution was prepared in double distilled water. Then, 0.2 M solutions of sodium hydroxide (NaOH) were added dropwise into the reaction medium to attain pH 10 and stirred for 2 h continuously at 60 °C. After that, precipitates of cobalt hydroxide (Co(OH)₂) were filtered and washed well with double distilled water to eliminate nitrates and other contaminants. These precipitates were dehydrated in the oven at 105 °C for 5 h, and after crushing, annealing was performed at 400 °C for 2 h.

For the synthesis of Mn-doped Co₃O_4, equimolar, equi-volume solutions of manganese nitrate (Mn(NO₃)₂.6H₂O) and Co(NO₃)₂.6H₂O were mixed in a beaker and heated continuously at 60 °C with constant stirring. Then, a 0.2 M solution of NaOH was added dropwise into the reaction medium to attain pH 10. With the addition of NaOH, precipitates of metal hydroxide appeared in the reaction mixture. These precipitates were washed with double distilled water after filtration. They were dried at 60 °C for 5 h and crushed to make fine powders. The muffle furnace was used to anneal the solution at 400 °C for two hours.

For the synthesis of the composite material, CNTs were used as matrix and undoped, as well as doped metal oxides, were applied as reinforcements. CNTs were functionalized before the formation of the composite. For the functionalization, CNTs were treated with a mixture of concentrated H₂SO₄ and HNO₃ in a ratio of 3 to 1. Then, 10 mg of unfunctionalized CNTs were mixed with an 8 ml mixture of acids. The mixture was refluxed at 70 °C for 4 h. After refluxing, the functionalized CNTs were filtered using vacuum filtration with 0.2 µm pore size filter paper. The CNTs were dried in the oven at 60 °C for 24 h.

For the synthesis of the MDCO-CNT composite, the mixture of ethanol and distilled water was used as a solvent in the ratio of 3:1, respectively. Next, 100 mL of MDCO (0.2 mg/mL) solution was mixed with 100 mL of CNT (0.4 mg/mL). This mixture was continuously sonicated for 3 h. After sonication, the mixture was filtered and cleaned with distilled water. The resultant composite was then dried in the oven at 60 °C for 5 h. Synthesized materials were subjected to different characterization techniques. The materials’ crystallization and phase studies were assessed using the Rigaku (MiniFlex 600) X-ray powder diffraction system. To ascertain the topographical and morphological structure of the materials, scanning electron microscopy (SEM) pictures were gathered using a VEGA3 (TESCAN) scanning electron microscope. Degradation efficiency for Congo red dye was studied by UV-vis. spectrophotometer BMS (UV-1602) (Shimadzu, Tokyo, Japan).

2.1. Adsorption Study

The adsorption study was carried out by using a 10 mg catalyst in a 25 mL solution of each concentration of 5 ppm to 50 ppm. Then, 25 mL of each solution with 10 mg of catalyst was continuously stirred in the dark for 60 min to achieve an adsorption-desorption equilibrium. Every 20 min, an aliquot was taken, filtered, and studied under a UV-visible spectrophotometer.

2.2. Photocatalytic Study

For the band gap analysis of the synthesized materials, a graph was plotted between absorbance and energy (eV) and the band gap value was assumed at the cutoff point of the graph.

Photocatalytic studies were carried out under a UV lamp and a 100-watt bulb in a specified photoreactor. For this study, dye solutions were irradiated under photo reactors. In the UV reactor, an Hg lamp was used and for visible light, a tungsten lamp (100 watts) was used at a distance of 15 cm. For this study, 25 mL of each concentration (5 ppm to 35 ppm) with a 10 mg catalyst was first kept in dark for adsorption and then under a tungsten lamp for 30 min. After irradiating, a solution was filtered and used for studies under a UV-visible spectrophotometer. The same process is used under UV radiation.

2.3. Effect of Different Parameters on Adsorption

By adjusting several factors, including the dye concentration, adsorbent dose, duration, temperature, and solution pH, the variation in dye adsorption on the surface of manufactured adsorbent was examined.

2.4. Effect of Dye Concentration

Solutions of dye at various concentrations, from 5 ppm to 19 ppm, were employed to study the impact of dye concentration on adsorption. Then, 25 mL of each concentration was stirred in the dark for 30 min with 10 mg adsorbent, filtered, and analyzed by UV-visible spectrophotometer.

2.5. Effect of Catalyst Dosage

To study the effect of adsorbent dosage, 25 mL of 15 ppm solution was put into a small beaker and various amounts of adsorbent were added: 10 mg, 20 mg, 30 mg, 40 mg, 50 mg, and 60 mg. Each solution was continuously stirred in the dark for 30 min, filtered, and analyzed by a UV-visible spectrophotometer.

2.6. Effect of Time

The effect of time studied was by varying the contact time for adsorption. In total, 50 mL of dye solution was used in this work, and it was constantly agitated in the presence of 20 mg of adsorbent. An aliquot was obtained every 5 min, and it was then examined using a UV-visible spectrophotometer. The duration ranged from 5 to 60 min.

2.7. Point of Zero Charges (PZC) of Photocatalyst

The pH value at which a material’s surface becomes neutral is known as the point of zero charges (PZC). Material surfaces change from being positively charged above the PZC value to being negatively charged below.

To find the charge on the surface, 15 mL of electrolyte solution (0.1 M NaCl) was taken, and the pH was adjusted using buffer solutions. After adjusting the pH, 10 mg of the catalyst was added to the electrolyte solution and continuously stirred for 1 h. The final pH of the solution was taken using the pH meter [40].

2.8. Effect of pH

For 25 mL of a 15-ppm dye solution, the impact of pH was investigated. By employing a buffer solution with a different pH, the pH of the solution was adjusted. After setting the pH of each solution, 10 mg of catalyst was added and stirred for 30 min.

2.9. Effect of Temperature

The temperature of the fluid was regulated for this investigation using a water bath with flowing water. Then, 50 mL of 15 ppm solution of Congo red dye was taken and continuously stirred in the presence of 20 mg of the catalyst. The effect was examined by varying the temperature of the solution from 20 °C to 60 °C.

2.10. Quantitative Analysis of Data

The effect of each parameter on adsorption and removal was quantized by using some mathematical relations. The following equation was used to compute the adsorption capacity, q_t (mg/g):

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{C}}_{°}-{\mathrm{C}}_{\mathrm{t}}\mathrm{V}}{\mathrm{W}}$$

(1)

where “q_e” is adsorbing capacity (mg/g), “ ${\mathrm{C}}_{°}$” is the initial concentration of dye in solution, C_t is the concentration of dye in solution after time t, V is the volume of the solution in liters (L), W is mass of adsorbent in gram (g).

The percentage (%) removal of dye from the solution was evaluated by using the following equation:

$$\Phi =\frac{{\mathrm{C}}_{°}-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_{°}}\times 100$$

(2)

where $\Phi$ is removal efficiency, ${\mathrm{C}}_{°}$ is the initial concentration of dye, and C_t is the concentration of dye after time.

3. Results and Discussion

In this work, Co₃O₄ and MDCO were synthesized using the co-precipitation method. MDCO-CNT composite was synthesized by combining CNTs with MDCO using the solution method. The photocatalytic performance of the synthesized samples was assessed in a UV reactor using an Hg lamp, while visible light was produced using a tungsten lamp (100 watts). System temperature was maintained at 25 °C by a circulating water bath and the distance between light sources and reaction solution was 15 cm. The Congo red solutions and synthesized powders were agitated for three hours in the dark before illumination to achieve the adsorption equilibrium. The dye removal process was evaluated and monitored by the UV-visible absorption spectrum and the concentration of the solution was analyzed quantitatively by measuring the maximum absorption at 497 nm.

XRD behavior of the synthesized materials was evaluated by matching with the standard reference pattern of Co₃O₄ (ICSD code-00-009-0418) and is given in Figure 1. According to the standard pattern, the intense peak was indexed as (311) at 36°. The absence of Mn or any other impurity shows the purity of the sample with good homogeneous dispersion of Mn in the Co₃O₄ crystals. Co₃O₄ showed a cubic structure. CNTs showed a characteristic peak (002) at 26°. The calculated value of lattice parameters is given in

Table 1. XRD analysis of Co₃O₄, Mn_0.8Co_2.2O_4, and Mn_0.8Co_2.2O_4-CNT.

Sample	Lattice Constant(Å) 8.084 *	Crystallite Size (nm)	Vcell (Å3) 528.30 *	X-ray Density (g/cm3) 6.05 *
Co3O4	8.10	29	532.26	6.11
MDCO	8.19	27	549.10	5.92
MDCO-CNT	8.06	50	523.30	5.93

*: Standard values matched with JCPDS (00-009-0418).

. No distortion in crystal structure after the addition of doping is observed [41] while a decrease in crystallinity by the addition of doping is observed, evident by a decrease in peak intensity.

Unit cell parameters were calculated by using this formula:

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

(3)

Unit cell volume was determined from data of unit cell parameters as:

$$V_{cell} = a^3$$

(4)

The average crystallite size, D was calculated using Scherrer’s equation:

$$D=\frac{k\lambda }{\beta \cos {\theta }_B}$$

(5)

X-ray density (gcm⁻³) was determined by the following mathematical equation:

$${\rho }_x{}_{-ray}=\frac{ZM}{V_{cell}{N}_A}$$

(6)

where (d) is the d-spacing of the XRD pattern, (hkl) are corresponding Miller indices, (β) is the full width at half maximum of intensity, (λ) is the wavelength of Cu Kα X-rays, equal to 1.542 Å, (θ) is the Bragg’s angle (k) is a constant value = 0. 0.9 for the cubic system, (Z) is the number of molecules per formula unit (for cubic crystal structures it is 8), (M) is the molar mass of the synthesized compound, and (N_A): 6.02 × 10²³/mole is the Avogadro’s number. Values of different XRD parameters are given in Table 1

In undoped Co₃O₄, Co²⁺ (0.65 Å) the tetrahedral sites reside, while Co³⁺ (0.61 Å) occupies the octahedral sites. When dopant content is incorporated, Mn²⁺ replaces Co³⁺ at the tetrahedral site making spinel with a random distribution of cations at tetrahedral/octahedral sites causing expansion of the lattice. The general formula for random spinel can be written as below:

(Mn²⁺Co²⁺ Co³⁺) _tetra [Mn²⁺Co³⁺ Co²⁺] _OctaO₄

(7)

X-ray density of parent material is greater compared to its doped derivatives because of reduced molecular masses.

Table 1 discusses the crystallographic aspects of the selected synthesized samples. Samples subjected to XRD and further analysis were coded thus: Undoped Co₃O₄ as Co₃O₄, MDCO as Mn_0.8Co_2.2O_4, and Mn_0.8Co_2.2O_4-CNT carbon nanotubes as MDCO-CNT composite.

SEM images of Co₃O₄, MDCO, and MDCO-CNT composite are given in Figure 2 and results show that parent Co₃O₄ has a heterogeneous surface while MDCO has flower-like morphology. MDCO-CNT composite micrograph shows that beads like metal oxides are distributed on threads like carbon nanotubes. These metal oxides are homogeneously distributed on the CNTs’ surface with very few agglomerations.

Band gap analysis of the synthesized materials showed a decrease in the band gap of Co₃O_{4 by} incorporating Mn was observed due to the replacement of Co³⁺ ions by Mn²⁺. As low-charge Mn²⁺ replaces the highly charged Co³⁺, it forms an acceptor level above the valence band, hence shrinking the band gap and making the electronic transition easier. Low Mn²⁺ doped Co₃O₄ showed a band gap value of 2.0 eV while highly doped Mn²⁺ has 1.9 eV.

As can be seen in Figure 3, the MDCO-CNT composite’s chemical composition and the electronic states of each of its constituent elements were assessed by employing XPS analysis. As shown in Figure 3a, the high-resolution XPS curve fitting of Co 2p shows distinctive peaks at 778.72 and 793.73 eV that are comparable to the Co³⁺ state’s Co 2p_3/2 and Co 2p_1/2 spin orbits. The coexistence of Co²⁺ and Co³⁺ states is indicated by the peaks at 780.93 and 796.61 eV, which were also attributed to the unpaired electrons present in the 3D state and the shake-up satellite peaks. Peaks at 529.96 and 531.45 eV are visible in Figure 3b’s high-resolution XPS spectra of the O1s curve fitting and are due to potent interfacial interactions with Co-O and C-O/-C. Figure 3c illustrates the Mn 2p_3/2 and Mn 2p_1/2 pattern’s contribution to the two distinct peaks at 641.51 and 653.37 eV in the high-resolution XPS curve fitting of Mn 2p. Figure 3d illustrates how the high-resolution XPS spectra of the C1s curve fitting revealed unique peaks at 284.71, 286.11, 288.81, and 289.57 eV, attributed to C-C, C-O, and C = O (O), respectively. The XPS study also showed that the MDCO-CNTs had Co₃O₄, Mn²⁺, and CNTs, which gave more weight to the SEM results. On the other hand, the BET surface area was assessed to be 21.84, 31.72, and 89.93 m²/g for all formulations: Co₃O₄, MDCO, and MDCO-CNTs (Figure 4).

UV-vis. spectra of Congo red before and after adsorption on MDCO and MDCO-CNT composite at different Concentrations are given in Figure 5a–c. These spectra show that both the materials have the tendency to adsorb dye, but MDCO-CNT composite has more efficiency than bare MDCO due to its large surface area, owing to the incorporation of CNTs.

The removal efficiency of the synthesized samples for dye is shown in Figure 6a,b. The removal efficiency of MDCO-CNT composite is greater than bare MDCO due to the high surface area of the composite than bare MDCO. Additionally, adsorption capacity increases with an increase in concentration up to a certain level because, after occupying all vacant sites, the adsorption process slows. Functionalized CNTs have π-bonds, carboxylic and hydroxyl groups which interact with dye via Van der Waal’s forces and π-π stacking. An increase in adsorption capacity (mg/g) of both materials with the increase in dye concentration is because of the concentration gradient and this leads to a reduction in the resistance in the mass transfer of dye molecules between liquid and solid phase [42].

In addition to the effects of dye concentration, the influence of a sample dose on dye removal performance (percent removal and adsorption capacity) has also been examined. The results are shown in Figure 6c,d. The efficiency is shown to grow linearly with the sample dose, as anticipated, and reaches up to 80% as a result of the improvement in adsorption sites with greater dosage [43].

The adsorption process depends on the nature of the dye as well as the nature of the adsorbent. Another crucial factor for dye adsorption research is the sample’s pH. For this adsorption experiment, manufactured samples were utilized to assess pH selectivity. Both dye molecules and adsorbents behave differently in acidic and basic mediums. Congo red is an anionic dye while in the acidic medium it becomes cationic because its pKa value is 4.1. To find the charge on the surface, 15 mL of electrolyte solution (0.1 M NaCl) was used, and pH was adjusted via buffer solutions. After adjusting the pH, 10 mg of catalyst was added to the electrolyte solution and continuously stirred for 1 h and the final pH of the solution was measured. PZC values for bare MDCO and MDCO-CNT composites are 8.1 and 7.2, which indicates that, below these values, both adsorbent materials carried a positive charge while above this point, the surface of the material becomes negatively charged. This charge variation is dependent on the pH of the medium and the structure of the material. In an acidic medium, both materials show cationic behavior because the oxygen of metal oxide captures proton from the solution and becomes positively charged (M-OH₂⁺) as shown in the equation below:

nCo₃O₄ + nH₂O → (Co₃O₄)_n[OH^─]_n + H⁺ → (Co₃O₄)_n(OH₂⁺)_n

(8)

At the same time, in a basic medium, the oxygen of cobalt oxide becomes negative after losing a proton [44].

nCo₃O₄ + nH₂O → (Co₃O₄)_n[OH]_n + ^─OH → (Co₃O₄)_n[O^─]_n + H₂O

(9)

In the case of the composite, the hydroxyl group on CNTs captures protons from the solution, and oxygen become positively charged in form of C-OH₂⁺. On the other hand, in the basic medium hydroxyl and carboxylic group present in CNTs loses the proton and appear negative in form of (C-O^─ and C-COO^─). Additionally, due to this charge variation, bare MDCO and MDCO-CNT composite have interactions with anionic and cationic dyes [45].

Figure 7b,c shows that maximum adsorption of the Congo red dye was observed at pH 6 which is 88 and 92%, respectively. At lower pH, Congo red dye becomes cationic and its pKa value is 4.1 whereas both MDCO and MDCO-CNT composites are positively charged below their PZC value, which is 8.2 and 7.1, respectively. Below this point, adsorbent and adsorbate cannot attract each other due to the same surface charges. Above pH 6, adsorption starts decreasing because above PZC value, the surface of the adsorbent becomes negatively charged and it repels the anionic dye molecules [46,47]. The result shows that both adsorbents almost have the same tendency to adsorb pollutant dye in an acidic medium, but in the basic medium at pH 10, MDCO-CNT composite shows very low adsorption efficiency than bare MDCO. This irregularity shows that the negative charge density on the surface of bare MDCO is less than that of the MDCO-CNT composite. Additionally, this negative charge density on the surface of the material in the basic medium repels the acidic dye and retards the adsorption process [48].

Contact time is an important factor for the photocatalytic removal study of dye on photocatalysts. Calculated data show that the removal efficiency of Congo red dye on MDCO and MDCO-CNT composites increases with an increase in time, as with time, more and more radicals were generated because light interacts with the photocatalysts and degrades the dye molecules [49]. Figure 8a,b illustrates how contact time affects dye adsorption. Since more dye molecules may interact with the adsorbent over time, the adsorption of dye rises with time. Because there are more accessible unoccupied sites at first, the rate of adsorption is greater. Later, the rate of adsorption decreases because an adsorbate layer forms on the surface of the adsorbent, and there are fewer open sites for subsequent interactions [50].

In addition to contact duration, adsorption tests were conducted at five different temperatures to examine the impact of temperature on the adsorption process. The rate of adsorption rises with temperature owing to an increase in Congo red solubility, as seen in Figure 8c,d. Additionally, when the temperature rises, the number of active sites on the adsorbent’s surface and the mobility of the dye molecules in the solution both increase. Maximum adsorption was observed at 323 K. After 323 K, the further increase in temperature led to a decrease in removal efficiency because at higher temperatures dye molecules start to detach from the surface due to the breaking of weak bonds between dye and composite. MDCO-CNT composite is a more efficient adsorbent than bare MDCO because of the larger surface area [50,51].

Data fitted to Temkin isotherm show that MDCO-CNT and MDCO do not follow the pattern which is a clue that there is non-uniformity in the distribution of bonding energies on the surface of the material. The Freundlich isotherm is shown in Figure 9c,d for MDCO-CNT composite and bare MDCO, respectively. The various functional groups attached to the surfaces of the adsorbent are connected to the heterogeneity on those surfaces. Calculated values of regression coefficient R² and n value shows that both MDCO and MDCO-CNT composite does not follow the Freundlich isotherm showing that the synthesized materials followed the homogenous adsorption.

According to Figure 9d,e, both bare MDCO and MDCO-CNT composites follow the Langmuir isotherm which gives an idea about the mechanism of adsorption. Dye molecules are chemically adsorbed on the surface of the material. Metal cations in MDCO interact with negatively charged sulfate ions of Congo red dye through electrostatic force of attraction and in the case of MDCO-CNT, both MDCO and CNT are involved in the adsorption of dye. Carbon nanotubes capture Congo red dye through π-π stacking. The calculated value of R_L for bare MDCO and MDCO-CNT composites given in

Table 2. Langmuir isotherm study.

Material Name	R2	qm (mg/g)	B (L/mg)	RL
MDCO-CNT	0.957	42.194	0.156	0.236
MDCO	0.904	30.30	0.209	0.195

are less than 1 and greater than 0 which is 0.1955 and 0.2364, respectively. Therefore, the adsorption process is favorable for both materials [52].

Both MDCO and MDCO-CNT composites follow the pseudo-second-order kinetic model as shown in Figure 9a,b. Parameters of pseudo-second order given in

Table 3. Kinetics study (pseudo 2nd order) for adsorption of CR on MDCO and MDCO-CNT.

Material	R2	K2 (g.mg−1.min−1)	qe (g.mg−1)
MDCO	0.994	0.044	21.92
MDCO-CNT	0.990	0.021	19.88

and calculated data suggest that dye molecules are adsorbed through chemisorption. Both adsorbent and adsorbate show chemical interaction through the exchange or sharing of electrons. Positive charge metal ions in MDCO and Congo red comprehend sulfate ions with negative oxygen. During the process of adsorption adsorbent and adsorbate, dye interacts with each through the electrostatic force of attraction. On the other hand, MDCO-CNT composite contains metal oxide and functionalized CNTs. These CNT can adsorb Congo red dye through π-π stacking and p-type metal oxide captures dye molecules electrostatically [53].

Van’t Hoff plots for the elimination of Congo red dye on MDCO and MDCO-CNT composite are shown in Figure 10a,b. The values of Gibb’s free energy, enthalpy, and entropy that were determined using these plots are shown in

Table 4. Thermodynamic analysis for adsorption of CR dye on bare MDCO and MDCO-CNT composite.

Temperature (K)	MDCO			MDCO-CNT
	∆G° (kJmol−1)	∆H° (kJmol−1)	∆S° (Jmol−1 K−1)	∆G° (kJmol−1)	∆H° (kJmol−1)	∆S° (Jmol−1 K−1)
293	−9.116			−9.336
303	−9.614			−10.10
313	−10.724	−15.340	83.157	−11.124	−24.857	119.42
323	−11.830			−12.04
333	−12.157			−11.81

below. According to the data, the Gibbs free energy (ΔG°) and enthalpy (ΔH°) values for both materials are negative, suggesting an exothermic and spontaneous reaction. A drop in temperature favors accelerating the photodegradation of Congo red dye. In the case of MDCO value of entropy is positive indicating an increase in the degree of randomness and for the composite, its value is negative because randomness decreases with the degradation of the dye [54].

Thermodynamic parameters i.e., enthalpy (∆H°), Gibb’s free energy (∆G°), and entropy (∆S°) were calculated by using the Van’t Hoff’s plot which is given in Figure 8a,b. Values of these parameters mentioned in Table 4 express that the adsorption process is spontaneous and exothermic because the value of Gibb’s free energy and enthalpy is negative [55]. Therefore, by increasing the temperature up to a certain level, the rate of adsorption increases as the solubility of dye increases with increasing temperature but at higher temperatures, dye molecules start disengaging from the surface of the adsorbent. Thermodynamic data depict as well that dye molecules chemically adsorbed on MDCO and MDCO-CNT. In chemisorption a strong interaction between dye molecules and adsorbent is present and this interaction is stable at lower and higher temperatures as well.

Studies were conducted at five different temperatures, 293, 303, 313, 323, and 333 K, as shown in Figure 11, to investigate the impact of temperature (a,b). Results show that an increase in temperature reduces the removal capacity of photocatalysts because this process is exothermic and favorable at low temperatures. Maximum adsorption for both MDCO and MDCO-CNT composite is seen at 50 min owing to the formation of equilibrium after 60 min, at which point adsorption capacity becomes constant, as shown in Figure 11c,d. This is because the rate of adsorption rises with increasing contact time.

4. Conclusions

Cobalt oxide (Co₃O₄), Mn-doped cobalt oxide (MDCO), and Mn-doped cobalt oxide functionalized carbon nanotube (MDCO-CNTs) composites were successfully synthesized using soft chemistry and Congo red dye was chosen as a dye model to evaluate the removal efficiency of the synthesized materials. Adsorption-assisted photocatalytic performances of the synthesized samples for the removal of Congo red dye were evaluated by UV–Vis. spectrophotometry. The best removal performance (95%) was obtained by MDCO-CNTs (Mn_0.8Co_2.2O₄-CNT) nanocomposites. This improved photocatalytic activity was attributed to the synergistic effect of CNTs and Mn incorporation in cobalt oxide, in which Co₃O₄ absorbs photons, whereas Mn incorporation contributed to enhancing the photon absorbance and CNTs assisted in the adsorption procedure. The experimental results showed correlations with models for traditional, Freundlich, and Temkin adsorption. Additionally, analysis was conducted utilizing both the pseudo-first-order and pseudo-second-order kinetic models. According to the correlation coefficient, the pseudo-first-order model is more likely to match the kinetic data. This study can be helpful for the synthesis of novel nanocomposites and facilitates their application by the synergistic (adsorption + catalysis) removal of dye in the field of water treatment.