Direct sunlight photodegradation of congo red in aqueous solution by TiO₂/rGO binary system: Experimental and DFT study

2.1 Materials

All chemicals materials (graphite powder (99.9%), sodium nitrate (98%), hydrogen peroxide (30%), potassium permanganate (98%), sodium hydroxide (97%), hydrochloric acid (36%), ethanol (70%), titanium isopropoxide (99%) and congo red dye (C₃₂H₂₄N₆O₆S₂.2Na, λmax = 497 nm)) were supplied from sigma Aldrich company and used without any future purification. All solution prepared by using distilled water.

2.2 Preparation of titanium oxide (TiO₂)

The synthesis of TiO₂ was reported by Zaid et al., 2020 (Mahmoud et al., 2022). 10 ml of isopropoxide stock solution was diluted by 40 ml distilled water and irradiated using manual irradiation 15 W power system in ice bath (Fig. S1, supplementary information). The irradiation process was carried out until a white precipitate was obtained, which was subsequently separated from the solution through decantation and washed by acetone, ethanol and distilled water. Finally, it dried at 80 °C for 5 h and calcined at 500 °C for 2 h.

2.3 Preparation of graphene oxide (GO)

The preparation of GO was carried out by using Hemmer modified method (Khatmi Maab et al., 2016). Firstly, in 300 ml beaker, (1gm, 2gm and 36 ml) of graphite powder, sodium nitrate and 1 M H₂SO₄ were mixed together in ice bath for 4 h. then, 5gm of potassium permanganate was added to mixture slowly under strong stirring. Next, 200 ml of warm water was added to the mixture. While continuously stirring, 35 ml of 30% hydrogen peroxide was gradually added until the color changed to yellow. Finally, the suspension solution of graphene isolated by decantation and washed several times by distilled water and 10% HCl before dried at 60 °C for 3 h.

2.4 Preparation of graphene oxide/Titanium oxide (rGO/TiO₂)nanocomposite

The preparation of rGO/TiO₂ nanocomposite was carried out using hydrothermal method. Firstly, various weight ratios of rGO (1%, 3%, and 5%) were mixed with an equivalent amount of TiO₂ solution and sonicated for 45 min until a brown solution was obtained. Next, the rGO/TiO₂ suspension was transferred to stainless steel auto-cleave and heated to 200 °C for 6 h and allowed to cool down naturally at room temperature. Finally, the gray precipitate of rGO/TiO₂ was transferred and isolated by centrifuging process before washing it several times by ethanol and distilled water (Shen et al., 2011; Yue et al., 2019; Liu et al., 2016).

2.5 Study of photocatalytic activity

The photocatalytic activity of (1, 3 and 5%) of rGO/TiO₂ were investigated for degradation of CR dye solution. The solution reaction was prepared by adding 0.01gm of each prepared materials in 100 ml of 10 ppm dye solution. Firstly, the resulting suspension was then stirred in a dark environment for 30 min to achieve adsorption–desorption equilibrium. The subsequent step involved exposing the sample to direct sunlight for varying durations using a manual irradiation system with an intensity of 1200 W.cm⁻² (Fig. S2, supplementary information). To maintain a constant temperature during the reaction, the water was circulated around the reactor. At predetermined intervals, a 5 ml sample of the solution was collected and subjected to centrifugation to remove the catalyst. The remaining concentration was then analyzed using UV–Vis spectrum and LC-MS techniques.

3.1 Structural characterization

The XRD of TiO₂ and (1, 3 and 5) wt.% rGO/TiO₂ are shown in Fig. 1a-d. The observed patterns exhibit peaks that can be attributed to the formation of TiO₂ with lattice parameter of a = b = 3.78 and c = 9.50 and the results are in agreement with JCPDS (04–0477). The diffraction pattern of pure TiO₂ (Fig. 3a) displayed peaks at angles of 25.35°, 37.78°, 48.07°, 53.93°, 55.11° and 62.72° corresponding to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) respectively, indicating the presence of a tetragonal anatase phase. In Fig. 1b-d, it can be observed that the anatase phase remains unchanged for all diffraction peaks, irrespective of the rGO mass ratio dopant. This observation indicates that the prepared TiO₂ is stabilized. The XRD patterns illustrated in Fig. 1b and 1c for TiO₂ doped with 1% and 3% GO respectively do not exhibit any peaks that can be attributed to rGO. This may be due to the high degree of isolation of rGO sheets by TiO₂ nanoparticles (Moustafa et al., 2022). However, the diffraction peak centered at 10.99°, which corresponds to (0 0 1), is observed and can be attributed to rGO. rGO/TiO₂ was estimated by using Scherrer equation (Holzwarth and Gibson, 2011):

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Fig. 1

XRD of pure and 1,3 and 5 wt% rGO doping TiO_2.

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Where 0.9: is shape factor, λ: X-ray source wavelength, $\beta :$ full width at half maximum of peak. Table 1 presents the findings that reveal a decrease in crystallite size as the dopant weight ratio increases. This is indicative of rGO segregation at the TiO₂ boundaries, thereby inhibiting crystal growth. On the other side, slight elongation of c axis is shown (Table. 1), while a and b axis remain constant, that the carbon of GO fcc and bcc is positioned at the center or occupied in the structure of TiO₂ (Ma et al., 2021).

Raman spectrum technique was used to investigate the chemical composition and the structure of prepared compounds through vibrational properties.The spectrum of GO, rGO and rGO/TiO₂ are shown in Fig. 2a-c, respectively. At Fig. 2a, two bands can be observed that are centered at 1350 cm⁻¹(D) and 1595 cm⁻¹ (G) assigned to sp³ defect and sp² in plane vibration carbon atoms. Additionally, at the Brillouin zone, the E2g doubly degenerated mode can be observed (Krishnamoorthy et al., 2012). The spectrum of rGO (Fig. 2b) illustrates a shift in the G band towards lower wavelengths (1595 to 1584 cm⁻¹) while the D band remains unshifted. This may be attributed to the self-healing properties of rGO, whereby carbon atoms are restored to the hexagonal network (Chen et al., 2021). This downshift in G band indicate the successful reduction to rGO. In Fig. 2c, the two bands of D and G exhibit broadening in the rGO/TiO₂ nanocomposite, which can be attributed to the stacking of rGO sheets induced by carbonyl groups attached to rGO. This leads to a hindrance in the accumulation of rGO sheets. The disorder structure of prepared compounds are measured based on the ID/IG ratio. The findings indicate that the ratio experienced an increase from 0.9 to 0.98 as a consequence of the reduction of GO to rGO. This outcome can be attributed to the defects and elimination of functional groups (carboxylic acid) present on the surface of GO (Ansari et al., 2022). On the other hand, slight reduce in the ratio value (0.85) is shown in rGO/TiO₂ nanocomposite due to the reduction of sp³ to sp² carbon atoms as well as, reducing in the domain size of sp² during hydrothermal process (Karthik et al., 2020). Fig. 4d illustrates the zoomed spectrum of pure and (1, 3 and 5)wt.% rGO/TiO₂ nanocomposite with range of 100–200 cm⁻¹. The results showed that the intensity of Eg band mode is increased with increasing the doped ratio. Additionally, this mode is shifted toward a lower wavenumber with broadening, which can be attributed to the impact of phonon confinement and surface pressure. Numerous studies (Singh et al., 2022) have addressed the shift in Raman peak position, which is attributed to two possible factors. The first posits that the intensity of TiO₂ peaks is affected by vacancy defects, which are amplified by the disordered structure of graphene. The second possibility is that the shift results from electronic transitions occurring between graphene sheets and TiO₂.

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Fig. 2

Raman spectrum of (A) GO, (B) rGO, rGO doped TiO₂ and (D) Eg intensity peak of TiO_2.

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Fig. 3

XPS of (A) C 1 s, (C) 2p of rGO before doping and (B) C 1 s, (D) 2p of rGO after doping.

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Fig. 4

PL of pure and 1,3 and 5 wt% rGO/TiO_2.

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The chemical analysis of GO and rGO/TiO₂ nanocompsoite was investigated by using X-ray Photoelectron Microscopy (XPS). The findings of the analysis are presented in Fig. 3a-d. Fig. 6a and 6b display multiple peaks centered at binding energies of 284.6, 286.3, 287.3, and 288.9 eV, corresponding to the core level spectra of carbon and indicating the presence of carbon–carbon and carbon–oxygen bonds (Wang et al., 2020). The findings demonstrate a reduction in the intensity peaks of carbon–oxygen in rGO/TiO₂ nanocomposite, indicating the reduction of GO via hydrothermal processing. Additionally, a peak is observed at 282.4 eV corresponding to the Titanium-carbon bond. The core level of Ti⁴⁺ in pure TiO₂ and rGO/TiO₂ nanocompsoite were shown in Fig. 3c,d. Fig. 3c exhibits the presence of two intensity peaks at 453.8 and 459.1 eV, which are attributed to Ti 2p_3/2 and Ti 2p_1/2, respectively. The difference in binding energy between these two peaks is measured to be 6.01 eV, aligning with previous findings in literature (Wang et al., 2020). The difference in binding energy of Ti⁴⁺ remains present in rGO/TiO₂ nanocompsoite as shown in Fig. 3d, but the binding energies of Ti 2p_3/2 and Ti 2p_1/2 increased because of the interaction between titanium and carbonyl groups of rGO (Dong et al., 2017).

The crystal lattice of TiO₂ was analyzed to examine the defect and vacancy of oxygen ions using PL spectrum. The results are depicted in Fig. 4, which displays a wide peak containing four sub-band emissions positioned at 420, 487, 520, and 680 nm correspondingly. The emission band at 420 nm assigned to the interaction of h⁺ - e^- or may be attributed to self-trapped exciton (Dong et al., 2017). A sub-band at 487 nm is observed and attributed to the transfer of electrons from Ti³⁺ to O^2– in octahedral complex (Dong et al., 2017). Additionally, two sub-bands located at 520 and 680 nm are linked to self-trapped oxygen vacancies known as F centers (Nasralla et al., 2020). The PL spectrums of (1, 3 and 5)wt.% rGO incorporated TiO₂ are demonstrated at Fig. 4.b-d. The results demonstrate that as the concentration of rGO increases, there is a gradual reduction in the intensity of emission peaks. This phenomenon is attributed to peak quenching via rGO incorporation, and becomes more apparent at 5 wt% rGO sheet doping. The reduction in intensity is contingent upon the interplay of ions. At a 1 wt% incorporation ratio, the interaction between ions is insufficient to significantly impact energy levels. Conversely, with a 5 wt% incorporation ratio, the interaction between ions is considerably stronger due to a decrease in defects and an increase in ion concentration (Chetibi et al., 2017), ultimately resulting in quenching (John et al., 2020). There are two factors that contribute to the quenching of emission peaks. The first factor is non-radiative energy loss, which arises from the transfer of energy from ions to defect sites. The second factor involves the scattering of electrons by oxygen ions at states of trapping, which results in an increase in the duration of h⁺-e^- recombination (Sang et al., 2019).

Fig. 5 shows the band gap of pure TiO₂ and (1, 3 and 5)wt.% GO incorporated TiO₂ calculated by using Tauc plot (Chen et al., 2017). The findings indicate that there was a slight decrease in energies as the Rgo ratio increased. This could be attributed to the influence of dielectric confinement (Al-Kandari et al., 2021). This confinement is a significant factor in energy alteration and its influence surpasses that of electron-hole space confinement, leading to the observed decrease in energy levels.

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Fig. 5

Band gap of pure and 1,3 and 5 wt% rGO/TiO_2.

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Fig. 6

TEM of 5 wt% rGO-TiO_2.

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TEM images were utilized to investigate the morphology of both prepared TiO₂ and rGO/TiO₂ nanocomposite. The results, as depicted in Fig. 6, reveal a uniform distribution of TiO₂ nanospheres on rGO sheets, with an interlayer distance in the low nanometer range. These findings are consistent with the (1 0 1) crystal plane of anatase, indicating good agreement.

3.2 UV–Vis and TOC

The photodegradation of congo red (CR) dye over pure TiO₂ and 5 wt% rGO/TiO₂ was studied by analyzing band gap, EIS, UV–Vis spectrum, and total organic carbon (TOC) results. The UV–Vis spectrum of CR photodegraded under sunlight over 5 wt% rGO/TiO₂ is depicted in Fig. 7a. The UV–Vis spectrum of CR shows strong adsorption band with range of 330–500 nm, making a part of sunlight photons unattainable to the pure TiO₂ and 5 wt% rGO/TiO₂ catalysts. In Fig. 7b, the percentage decolorization of CR over pure TiO₂ and (1, 3 and 5)wt.% rGO/TiO₂ is shown. The findings indicate that the integration of TiO₂ is more effective in the degradation of dye when exposed to sunlight. Within the initial 15 min, the decolorization efficiency is 35% for 5 wt% rGO/TiO₂ in contrast to only 7% for the pure catalyst. The efficiency of degradation increases in proportion to the duration of sunlight exposure, ultimately reaching 99% after a time span of 100 min. The rate constant for the degradation of CR dye was calculated using Langmuir-Hinshelwood kinetic for pure TiO₂ and (1, 3 and 5)wt.% rGO/TiO₂ nanocomposites. The values of K are displayed in Fig. 7c. The findings demonstrate that the value of K for 5 wt% rGO/TiO₂ is greater than that of pure TiO₂. This implies that the inclusion of rGO into the TiO₂ matrix results in a heightened efficiency of photo-excited electrons. Additionally, it leads to a reduction in charge carriers during the recombination process for pure TiO₂ (Movahedi et al., 2009). As the studies mentioned (Ali et al., 2020); the CR dye undergoes a decolorization process rather than mineralization, as it loses its yellow color and forms intermediate compounds. This process occurs through an initial reaction with oxidizing agents such as hydroxyl free radicals and oxygen super anions, which also cause adsorption edge damage to the conjunction. To recognize between decolorization and mineralization, the Total Organic Carbon (TOC) measurements for 5 wt% rGO/TiO₂ and pure TiO₂ was carried out and the results are summarized in Fig. 7d. The findings demonstrate that the utilization of 5 wt% rGO/TiO₂ leads to a mineralization of the CR dye by a factor of 10. The comparisonn between decolorization and mineralization of CR dye under direct sunlight by using UV–Vis and TOC are shown in Fig. 7e. During the initial 15 min of exposure to sunlight, the decolorization of CR dye was observed to be 45%, while only 17% of organic matter was removed. This suggests that oxidative species are reacting with the dye to form intermediate compounds, resulting in fragmentation and damage to its conjugate structure, ultimately leading to its decolorization. The study showed that decreasing the concentration of CR led to an increase in the removal of organic carbon due to the oxidative species attack. It was noted earlier that TiO₂ has a favorable value for reducing oxygen, which is equal to −4.03 V. This indicates that the formation of super oxygen anions is unavoidable (Ali et al., 2020).

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Fig. 7

(A) CR decolorization UV–Vis spectrum at different time over 5 wt% rGO-TiO₂, (B) comparison CR decolorization over pure and wt.% rGO-TiO₂, (C) investigation plot of decolorization rate, (D) TOC removal of CR dye over pure and 5 wt% rGO-TiO₂, (E) comparison of decolorization and mineralization of CR dye over 5 wt% rGO-TiO_2.

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3.3 LC-Ms

The study investigated the liquid chromatography-mass spectroscopy analysis of photodegradation of CR dye under sunlight exposure for two different periods of time, namely 60 and 120 min, using 5%rGO/TiO₂ (Fig. 8a,b) depict LC-MS spectra which exhibit distinct peaks at varying (m/z) values that correspond to intermediate compounds, thus providing confirmation of the photodegradation of CR dye. The suggest pathway of CR dye photodegradation by using 5%rGO/TiO₂ is shown in Fig. 7. Firstly, the mechanism suggests that the cleaving of amine group and oxygenation of dye leads to the formation of a new compounds with m/z 679 and 680 (Bhat et al., 2020) as shown in Fig. 8a. The results show two intensity peaks with m/z ratio of 244 and 207, which assign to hydroxnapthalane −1-sulfonic acid result from cleavage of C₆H₆ ring, N⚌N and C-NFurthermore, the sustained activity of the OH free radical leads to the formation of 4-carboxybutanoate and naphthalene with respective molecular weights of 138 and 122 (Bhat et al., 2020). ROS, or reactive oxidative species, can cause the cleavage or reduction of sulfonic, amine nitro, and hydroxyl groups that are bonded with a C₆H₆ ring. This process results in the production of 3-carboxyprppanoate, malonic acid, and malonate with m/z ratio of 120, 108 and 101 respectively (Shaban et al., 2017). The intermediate compounds of the thesis were effectively mineralized to CO₂ and H₂O. The LC-MS findings indicated that complete degradation of CR under sunlight was achieved after 120 min using 5%rGO/TiO₂. The proposed mechanism for CR decolorization is visually presented in Fig. 9.

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Fig. 8

LC-MS of CR dye (A) before and (B) after decolorization.

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Fig. 9

Proposed decolorization path of CR dye.

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3.4 The photocatalytic activity

The study examined the photocatalytic capabilities of synthesized compounds in the degradation of CR dye under direct sunlight. The photodegradation outcomes are presented in Fig. 10, which illustrates the decline of CR dye in pure and rGO-integrated TiO₂ nanocomposites with varying rGO concentrations (1%, 3%, and 5%). The concentration of CR dye prior to and after degradation under direct sunlight is represented by Co and Ci, respectively. The findings demonstrate that a mere 8% of CR dye underwent self-degradation when exposed to sunlight. However, the other photocatalysts that were prepared exhibited outstanding activity, with a 99% success rate achieved by 5%rGO-TiO₂. The results displayed in Fig. 10 demonstrate that samples incorporating rGO exhibit a greater capacity for CR dye compared to pure TiO₂. This can be attributed to the increased surface area and π–π system present in the rGO structure, which enhances the rate of reaction (Movahedi et al., 2009). The incorporation of rGO played a significant role in promoting the generation of hydroxyl free radicals, which ultimately results in the enhancement of congo red dye molecule breakdown through direct attack.

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Fig. 10

Photodegradation of CR over catalyst.

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3.5 Electrochemical impedance (EIS) measurements

In order to predict the behavior of photo-excited carriers in the future, the electrochemical impedance spectrum (EIS) was utilized to study the transport and recombination characteristics of electron-hole carriers. The prepared samples were subjected to an applied bias of 0–0.5 V, and their Nyquist plot is depicted in Fig. 11. The typical plot demonstrating the photodegradation of congo red dye comprises two semicircles, which correspond to high and low frequency and represent charge transport resistance (Rct) and charge recombination process (Rrec), respectively. The presence of two semicircles on the plot indicates that the degradation process involves the transfer or recombination of electrons-holes, which is an important step in determining the rate of degradation. The values of resistances (Rs, Rct and Rrec) of prepared samples are summarized in Table. 2. The Rs values exhibit a decline as the rGO doping level rises. Conversely, the Rrec values experience an increase as the doping ratio increases, signifying the efficient separation of e-h with minimal recombination. The findings suggest that the sample doped with 5 wt% rGO exhibits activity in the photodegradation of CR dye, while also indicating that the easy inclusion of rGO can enhance carrier transportation. It is possible that the recombination process reduction can be attributed to two factors related to rGO. Firstly, the rGO may capture photoinduced excitons, which would lead to an increase in carrier concentration. Secondly, the rGO may serve to increase the surface area of TiO₂, thus promoting rapid transfer of photoinduced electrons to adsorbed oxygen on the surface of TiO₂ (Raya et al., 2022).

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Fig. 11

EIS of pure and %rGO-TiO₂ under illumination conditions.

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3.6 Charge electronic transference process and band diagram

In order to examine the electronic charge transfer process, we conducted PL spectrum and cyclic voltammetry (CV) analysis on both pure and 5 wt% rGO-TiO₂. The PL spectrum of both prepared samples were showed and discussed in Fig. 4. The results demonstrate that both samples exhibit a similar spectrum, indicating comparable defects and electronic structure. The PL data confirms that the inclusion of TiO₂ in rGO effectively prevents charge recombination more than pure TiO₂. However, the rate constant of standard heterogeneous, which determines the rate of electron transport in each material, was calculated using the Nicholson method (Martín-Yerga et al., 2016) and obtained through CV techniques under both dark and illuminated conditions. Fig. 12 presents the findings of an investigation on CVs utilizing thin-film electrodes composed of pure and 5 wt% rGO-TiO₂. It seems that the cyclic voltagram for illumination did not exhibit frequent modulation with the dark voltagram for each of the electrodes that were prepared. The study found that compared to dark conditions, illuminating a pure TiO₂ electrode resulted in an increase in ΔE of 0.460 V vs. SCE. In contrast, the 5 wt% rGO-TiO₂ electrode showed a decrease in ΔE under illumination (0.468 V vs. SCE) compared to dark conditions (0.487 vs. SCE), as reported in Table 3.Under illumination conditions, the expansion of ΔE values for both electrodes and a decrease in the current of reduction and oxidation density are induced, as shown in Table. 3. For the standard heterogeneous rate constant (k_o) calculation, the coefficient of diffusion utilized were D_R = D_o = 0.726 * 10^-5cm²/s and the ko values are displayed in Table. 3 (Soares et al., 2016). The Ko values exhibited a degree of proximity, and the presence of a 5 wt% rGO-TiO₂ heterostructure resulted in decreased electron transfer. The process of illumination was observed to decrease electron transport in the electrode, indicating a possible increase in recombination. The 5 wt% rGO-TiO₂ heterostructure demonstrates that the absorbed light by doped rGO induces a noteworthy photothermal effect in the photocatalyst, thereby facilitating an increase in charge transfer (Low et al., 2017; Di et al., 2018; Merrad et al., 2023). The findings reveal that TiO₂ nanoparticles exhibit slightly higher Ko and recombination rates compared to the 5 wt% rGO-TiO₂ heterostructure. This suggests the importance of achieving a balance between recombination process and electron transport in the heterostructure sample, which ultimately enhances the photocatalytic efficiency.

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Fig. 12

Cyclic Voltammograms of pure and 5 wt% rGO-TiO2 electrodes in dark and lighting conditions.

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3.7 Reusability of studies

To investigate the reusability of 5 wt% rGO-TiO₂ binary nanocomposite, a cycling experiment was carried out on CR dye up to four cycles and the results are shown in Fig. 13. After each cycle, 5 wt% rGO-TiO₂ was separated and washed to remove the CR dye and then dried at 80 °C for 10 h to utilize it for adsorption and photodegradation. The ratio of nanocomposite sample to dye concentration employed in the first cycle was maintained throughout subsequent cycles. The results indicate that the sample exhibits high capability with increasing cycle number, which suggests a high level of stability and potential suitability for the photodegradation of dyes.

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Fig. 13

The reusability of 5wt.%rGO-TiO₂ for up to four cycles.

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3.8 Mechanism of CR photodegradation

Before proposing the mechanism of degradation, it is crucial to consider the scavenger effect in the study. This entails taking into account the presence of ROS, including hydroxyl free radicals, superoxide anions, as well as holes and electrons, which are all highly significant (Baruah et al., 2021). Four compounds of sliver nitrate (e^-), mannitol (OH*), benzoquinone (h⁺) and triethanolamine O₂^–* were used as a scavenger for examining the effect of radical (Fig. 14) on the photodegradation of CR under sunlight. The findings indicate that scavenger compounds significantly reduce photodegradation efficiency by more than 50%. However, the use of AgNO₃ does not appear to have an effect on the photodegradation mechanism. The application of benzoquinone as a scavenger is substantiated by its crucial role in the mechanism pathway (Ali et al., 2022), which signifies its primary involvement in the generation of OH* from water and consequent degradation of CR dye. The addition of benzoquinone to the reaction system resulted in the inhibition of CR degradation.

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Fig. 14

photodegradation of CR under different scavenger.

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The integration of rGO with TiO₂ is considered a viable option for facilitating the transfer and acceptance of excited electrons from the TiO₂ CB. This is primarily due to the planar structure, as depicted in Fig. 15. The rapid separation of carriers, achieved through the incorporation of a substance, has been observed to contribute to the restraint of electron-hole recombination process and achieve high rates. Additionally, this incorporation has been shown to improve the visible light adsorption of TiO₂ and expand it in comparison with pure TiO₂. The rGO/TiO₂ photo catalyst exhibits a high level of efficiency in the adsorption of the CR dye due to its planar structure, which facilitates a platform for the dye to be adsorbed.

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Fig. 15

Proposed mechanism CR photodegradation at optimum condition over 5 wt% rGO-TiO_2.

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3.9 Computational details

Theoretical analysis was conducted on various pH levels using the Gaussian 09 W software. The optimization of geometry and reactivity of congo red dye towards catalyst were investigated and carried out via density function theory (DFT) with Backers parameter of exchange functional (Ali et al., 2022). Additionally, a split-valance double zeta basis set 6.31G(d) and Lee-Yang-Parr (B3LYP) correlation function was employed in the calculation (Becke, 1993). Upon optimization, several theoretical properties were examined, including HOMO, LUMO, ionization energy, electron affinity, softness and hardness. The DFT calculations indicate a relationship and interaction between the 5wt.%rGO-TiO₂ catalyst and both anionic and cationic congo red dyes (Koch and Holthausen, 2015). Theoretical reactivity parameters such as E HOMO, E LUMO, softness, hardness, electrophilicity and electronic chemical potential were determined using a reaction catalyst and Congo Red dye species (CR—N⚌NH, CR⚌N—NH and CR—N⚌N). The obtained results are presented in Fig. 16 and Table. 4.

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Fig. 16

Electron density mapping of (A) CR-N = N, (B) CR-N = NH and (C) CR = N-NH.

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The chemical hardness, softness and electronic chemical potential calculated from the following equations (Domingo et al., 2016; Parr and Yang, 1984):

Generally, The exchange capacity of electron density among congo red species dye is contingent upon the potential of chemical electronic (μ) (Parr et al., 1999). When the molecules exhibit high values of μ, it indicates the presence of potent electron-acceptor molecules. Conversely, a low value of η signifies increased reactivity (Bredas, 2014). The high value of $\overline{\omega }$ , has an inverse effect on the ability of dye species to accept electron load. According to the data presented in Table 3, the smallest value of parameter η is associated with CR—N⚌N species, which makes it more likely to interact with 5wt.%rGO-TiO₂ compared to other species. Furthermore, this species has the smallest energy gap when compared to other species, which accelerates photodegradation by promoting recombination of hole-electron pairs (Valadi et al., 2020). At low pH, the CR—N⚌NH species exhibits electrophilicity of 3.49, which is lower than that of CR—N⚌N and CR⚌N-NH, which have electrophilicities of 5.36 and 4.11, respectively. This makes it easier for the CRN⚌NH species to be oxidized by OH free radicals and O super anions. Additionally, in an acidic medium, TiO₂ forms TiOH₂ which can adsorb more anionic species of CR-N⚌NH and CR⚌N-NH, thus improving its degrading capabilities (Thomas et al., 2016; Soares et al., 2021).