Structural Analysis, Adsorption Behavior, and DFT Study of Alizarin on CuO-αFe₂O₃ Nanocomposite

INTRODUCTION
Metal oxide nanostructures are crucial since they have key importance in various sectors. Among these nanostructures, copper(II) oxide, CuO is known to possess lowest energy gap of around 1.20 eV and possesses optimum potential. Due to the strong dependency of material performance on morphology, this highly tunable property has been a boon in various applications such as supercapacitors (SC), batteries, solar cells and catalysts [1-4].
A wide range of CuO nanostructures have been constructed using diverse techniques such as hydrothermal and solvothermal treatments. Furthermore, the interesting features of CuO (cost-effective, abundant materials with non-toxicity in combination with heat tolerance and availability) make it more attractive [5, 6]. Recent advances highlight the effectiveness of the nanocomposite/p-n junction method, providing great potential for photocatalytic and solar-related applications. Enhanced photochemical properties (e.g. light-driven splitting of electrons) is observed for composite metal oxide nanomaterials; thus, in the case of zinc oxide even with subsequent n-suapded hydrothermal growth can show the synergistic CoO effect with CuO [7-11].
Another inorganic compound that has bandgap of 2.10 eV is hematite (Fe2O3), which was used in nano-composites synthesized through several methods where formations at the nanoscale showed distinct properties, adding to its potential as a versatile material. Nanostructures made of different materials have attracted even more attention in the last few years for their possible applications in adsorption processes and have been designed as nanohybrids to treat pollutants Solvent separated multiple atoms (metal, metal oxides) [12-19]. In this study, alizarin dye (ALZ) was chosen as the targeted adsorbate because it is widely used in the textile and dyeing industries and a major source of environmental pollution. Alizarin is a common type of synthetic dye used in textile industries with an anthraquinone framework responsible for significant permanent color properties leading to difficulty in elimination from wastewater using traditional treatment approaches. This property makes them a perfect target for testing new material with high adsorption potential [20-22]. Chemists, chemical engineers, biochemists and physicists use the DFT calculation to predict energies, chemical structures, spectroscopic characteristics of materials, and perform detailed calculations based upon quantum mechanics. DFT calculations are suitable for theoretical studies of various chemical processes on the electron density-dependent parameters with adsorbents and corrosion that includes a large class of materials [23, 24].
This study is designed to synthesize a CuO-αFe2O3 nanocomposite with enhanced functionalization and explore its adsorption capacity for alizarin dye (ALZ) in aqueous solutions. The investigation employs a dual approach, utilizing both practical and theoretical methods by leveraging the predictive power of DFT calculations alongside the practical laboratory experiments.

MATERIALS AND METHODS
Chemicals and reagents
All reagents employed throughout this work were of analytical grade and obtained from Sigma-Aldrich. The chemicals were used as received without any additional purification steps.

Synthesis of CuO-αFe2O3 Nanocomposite
A 0.05 M aqueous solution of copper(II) acetate monohydrate (Cu(CH₃COO)₂·H₂O) was prepared by dissolving the salt in 100 mL of deionized water. Subsequently, 0.1 g of polyvinylpyrrolidone (PVP, (C₆H₉NO)ₙ) was added to the solution, followed by vigorous stirring for 10 minutes. Afterward, 0.3 mg of iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) was introduced into the mixture.
The obtained solution was then mixed with 100 mL of 0.05 M NaOH solution under continuous stirring for 50 minutes. The reaction mixture was maintained at 80 °C for 2 hours. The resulting precipitate was separated by centrifugation and dried at 100 °C for 2 hours. Finally, the dried product was calcined at 450 °C to obtain the CuO–αFe₂O₃ nanocomposite.

Adsorption Experiments
Batch adsorption experiments were conducted using deionized water containing 20 mg/L of alizarin dye (ALZ). The solution was appropriately diluted to achieve the desired concentration. All experiments were carried out in glass tubes with 50 ml of dye solution and 0.01 g of CuO-αFe2O3. The glass tubes were subjected to 60 minutes of shaking at various temperatures (20, 25, 30, 35, and 40 ℃) using a shaker (HZQ-C). The concentration of the dye in the liquid phase was measured using a UV-visible spectrophotometer. The adsorption capacity (Qe) was determined using Eq. 1 [25, 26].

                                            
 Here, Qe represents the equilibrium adsorption capacity (mg/g), C0 and Ce are the initial and equilibrium concentrations of ALZ (mg/L), respectively, Vsol is the solution volume (L), and M is the mass of the nanocomposite as the adsorbent (g).

Isotherm of Adsorption
The isotherm of adsorption plays a crucial role in characterizing the adsorption process of a material. In our study, adsorption isotherms were employed to monitor the release of unwanted chemicals from aqueous media by solid-phase adsorption at a constant temperature. One of the models used in this study is Langmuir isotherm which represented in the Eq. 2 [27,28].

Where Ce is the adsorbate concentration at equilibrium (mg/L), Qe is the adsorption capacity at equilibrium (mg/g), Qmax is the maximum adsorption capacity (mg/g), and KL is Langmuir constant (L/mg).
Eq. 3 represents the Freundlich isotherm [29, 30].

  
n and kf are representing the Freundlich constants as the strength and ability of adsorption, respectively. In addition, Eq. 4 was used to determine the thermodynamic parameters such as entropy changes (ΔS), free energy (ΔG), and enthalpy (ΔH) [31, 32]. 

Here, T represents the Temperature in (K), R represents the gas constant, and xm represents the max adsorbed amount in (mg/g). 

DFT Calculations
The density functional theory (DFT) calculations were done using B3LYP basis set with a size of 311++G(d, p) to estimate the optimized molecular structures of the studied molecules in a vacuum medium. This basis set was selected due to the accuracy with high computational performance according to similar systems [33, 34]. The frontier molecular orbitals, including the highest unoccupied molecular orbital (HOMO), lowest occupied molecular orbital (LUMO), energy gap, and other energy parameters were obtained based on Koopman’s theorem [35].  The calculations were conducted using Gaussian 09 package and visualized through GaussView 5.0 [36, 37]. A monomer of CuO-αFe2O3 was selected and optimized in order to simplify the calculations and investigate its interaction with alizarin molecule.

RESULTS AND DISCUSSION
X-ray Diffraction and SEM Analysis
Fig. 1 illustrates the X-ray diffraction (XRD) pattern of the CuO-αFe2O3 binary metal oxide. The diffraction peaks observed at (2θ) values of 38.83˚, 52.10˚, 59.45˚, 69.81˚, and 72.83˚ correspond to Miller indices (111), (020), (202), (220), and (311), respectively. These results align with the established diffraction data for CuO (JCPDS NO.01-1117). Additionally, peaks at (2θ) = 33.28˚ and 63.13˚, corresponding to the Miller indices (104) and (214) of αFe2O3, are consistent with the standard diffraction data (JCPDS NO.33-0664).
The incorporation of Fe2O3 into CuO induces discernible alterations in the XRD mixed pattern, suggesting a modification in the structural arrangement of CuO. The average crystallite size (D) was determined to be 13.96 nm using the standard Scherrer formula (Eq. 5) [20,22, 6].

Here, λ represents the XRD wavelength in nanometres, β is the mean width of the diffraction peak features at half maximum height due to small crystallite size in radians, and k is the crystallite form constant, typically set at 0.9.
In Fig. 2 FE-SEM images of synthesized CuO-αFe2O3 nanocomposite with surface morphology, shape and particle size. 30 The CuO-αFe2O3 at nanostructures form has a particle shape with diameter 13 to 23 nm. This size range correlates quite well with average crystal size (D), which had previously been obtained from XRD pattern data. The FE-SEM images provide important details on the microstructural properties of CuO-αFe2O3 nanocomposite and its dimensional characteristics.

Adsorption Experiments
Fig. 3 illustrates the impact of adsorbent concentration (ppm) on the removal efficiency of ALZ. The removal rate increases as the number of nanocomposites approaches the equilibrium limit, reflecting the increased availability of sorption sites until saturation occurs.
To optimize alizarin dye adsorption, the influence of contact duration on percent removal was investigated while keeping other parameters constant. As shown in Fig. 4, the adsorption of ALZ rises during the initial 90 minutes before reaching equilibrium. The rapid diffusion of ALZ to the external surface of CuO-αFe2O3 NPs within the first hour can be attributed to the nanoparticles’ small size, facilitating efficient transfer of adsorbate species. As sorption sites become progressively filled, the adsorbed ALZ is transferred from the bulk solution to the occupied sorption sites. This delayed diffusion leads to a reduction in the rate of subsequent dye absorption.
 When it comes to the adsorption isotherms, the Langmuir equation did not fit the adsorption curve of alizarin red dye from its solution, as shown in Fig. 5a. The Freundlich equation, however, provided a suitable model for the desorption curve of alizarin red dye, as observed in the relationship between lnQe and lnCe in Fig. 5b.

Thermodynamic Characteristics
The thermal influence on the adsorption process of alizarin red dye on the nanocomposite surface was investigated at various temperatures (20, 25, 30, 35, 40 °C). The results indicate an increase in the quantity of adsorbed alizarin red dye with rising temperatures, suggesting an endothermic process with a positive average value of enthalpy (ΔH). This supports the presence of an absorption system in relation to the adsorption process. The plot in Fig. 6, showing the relationship between logxe and 1/T for ALZ adsorption, allowed for the calculation of entropy (ΔS) using the intercept method and maximum adsorbed species (xm) using the slope of the van Hoff plot [34].
The calculated values for enthalpy (ΔH) and entropy (ΔS) were 20.7 kJ.mol-1 and -71.9 J.mol-1K-1, respectively. The adsorption isotherm at 313 K yielded a computed value of 21.1 kJ.mol-1.K-1, indicating that the adsorption occurs spontaneously without external force.

Theoretical Calculations
The equilibrium geometry of ALZ, as determined by the most efficient measurement (DFT), is presented in Fig. 7a. Table 1 presents the HOMO and LUMO energies along with the energy gaps for Alizarin, CuO-αFe2O3, and the Alizarin-CuO-αFe2O3 combination. The energy gap represents the difference between the energy values of the HOMO and LUMO orbitals, which serves as a valuable clue in determining the chemical reactivity and kinetic stability of the studied molecules.31 An examination of the results reveals a significant alteration in the HOMO and LUMO energies attributed to the adsorption of the Alizarin molecule (Fig. 8). This impact suggests the involvement of chemisorption adsorption.32 Furthermore, the energy gap expanded from 1.49 eV (CuO-αFe2O3) to 2.49 eV (Alizarin-CuO-αFe2O3 complex), indicating that the generated Alizarin-CuO-αFe2O3 complex is considerably more stable than isolated CuO-αFe2O3. 
Moreover, to assess nucleophilic and electrophilic attack sites, electron density distribution on the optimal geometries of adsorbate and adsorbent molecules was computed. The molecular electrostatic potential (MEP) of Alizarin and CuO-αFe2O3, as depicted in Fig. 7b and Fig. 7c, revealed that oxygenated functional groups of Alizarin exhibited a large negative electron density (red colour), indicating nucleophilic attacking sites. In contrast, the metal atoms of CuO-αFe2O3 displayed a large positive electron density (blue colour), suggesting sites susceptible to electrophilic assault. The adsorption mechanism involves interactions between oxygen-containing nucleophilic groups and electrophilic atoms of the adsorbent [38, 39].
Comparing energies (E values) for a proposed linkage between the surface and the inhibitor facilitated the anticipation of adsorption types. Optimization operations using the DFT approach were performed for the surface (CuO-αFe2O3), Alizarin, and (Alizarin-CuO-αFe2O3). The covalent link in Alizarin-CuO-αFe2O3 was considered a chemical bond in the first state, while other states were assumed to be physical interactions as visualized in Fig. 9. Based on total electron density (TED) and electrostatic surface potential (ESP) calculations, the oxygen atom emerged as the active site in these interactions.
The adsorption type was deduced as physical, with ΔE values indicating physisorption and chemisorption. In chemisorption, ΔE equalled 75.99 H, while in physisorption, it was equal to 38.08 H (Table 2). The linkage between the dye and the surface, illustrated in Fig. 8, supported the theoretical understanding of the adsorption process.

CONCLUSION
The physical characterization of the synthesized CuO-αFe2O3 nanocomposite using XRD and SEM shows its nanoscale diameter. The adsorption of Alizarin red studies fitted the model by Freundlich’s isotherm, and this research demonstrated that free-standing layer-thin nanocomposite has a great efficiency as an adsorbent. Thermodynamic parameters support favourable adsorption and DFT calculations show a chemical state as well as physical interactions of the Alizarin–CuO-αFe2O3 link in two subsequent states. Importantly, in these interactions the oxygen achingly emerges as the active site. The high value of energy from the investigation indicates that physical adsorption is still the dominant type. The integration of experimentation and theory provides valuable insights into the adsorption of alizarin red over CuO-αFe2O3, thus advancing the knowledge on adsorption through a nanocomposite-powered approach.

ACKNOWLEDGEMENT
We would like to thank Mustansiriyah University, Baghdad, Iraq. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.