Photocatalytic Degradation of Organic Dyes Contaminated Aqueous Solution Using Binary CdTiO2 and Ternary NiCdTiO2 Nanocomposites

Abstract

The synergistic effect of binary CdTiO₂ and ternary NiCdTiO₂ on the photocatalytic efficiency of TiO₂ nanoparticles was investigated. The SEM analysis demonstrates spherical TiO₂ NPs of different sizes present in agglomerated form. The structural analysis of the nanocomposites reveals a porous structure for TiO₂ with well deposited Cd and Ni NPs. TEM images show NiCdTiO₂ nanocomposites as highly crystalline particles having spherical and cubical geometry with an average particle size of 20 nm. The EDX and XRD analysis confirm the purity and anatase phase of TiO₂, respectively. Physical features of NiCdTiO₂ nanocomposite were determined via BET analysis which shows that the surface area, pore size and pore volume are 61.2 m²/g, 10.6 nm and 0.1 cm³/g, respectively. The absorbance wavelengths of the CdTiO₂ and NiCdTiO₂ nanocomposites have shown red shift as compared to the neat TiO₂ due to coupling with Ni and Cd that results in the enhanced photocatalytic activity. The photocatalytic activity demonstrated that TiO₂, CdTiO₂ and NiCdTiO₂ degrade methylene blue (MB) and methyl green (MG) about 76.59, 82, 86% and 63.5, 88, 97.5%, respectively, at optimum reaction conditions.

1. Introduction

Industrial effluents containing synthetic dyes is a formidable challenge for water remedy processes [1,2]. Dyes are used for the coloration of several materials such as textile fibers, cosmetics, paper, tannery, food, leather and pharmaceutical products [3]. These synthetic dyes are major water pollutants and cause serious environmental problems due to their high aromaticity, low biodegradability, toxicity, chemical stability and carcinogenic nature [4]. These dyes also reduce the light penetration which reduces the photosynthetic activity that causes a deficiency in dissolved O₂ content of the water [5]. Various approaches are applied for the remediation of these pollutants such as adsorption [6], nanofiltration [7], ozonation [8], coagulation [9], biodegradation [10] and phytoremediation [11] etc. These conventional approaches are expensive, destructive, difficult and transform pollutants into sludge [12].

Advanced Oxidation Processes (AOPs) generate and use powerful transitory species such as hydroxyl radicals [13] to eliminate the organic pollutants by final conversion into small and stable molecules such as H₂O and CO₂ [14]. Among the AOPs, photocatalytic degradation is believed to be the most appropriate low-cost approach to treat organic pollutants [15]. Photodegradation has advantages over other conventional approaches owing to its simplicity, complete pollutants mineralization, cost-effectiveness, no harmful byproducts formation, ambient pressure and temperature operation [16]. Various semiconducting photocatalysts are used for the photodegradation of dyes such as ZnO [17], Fe₃O₄ [12], SnO₂ [18], TiO₂ [19] etc. Among these photocatalysts, titanium (IV) oxide has been the most investigated material for the environmental photocatalysis owing to its abundance, high specific surface area, nontoxicity, photostability, strong oxidation capability, low price, high photoactivity and chemical stability [20,21,22]. Titanium dioxide (TiO₂) as an intrinsically n-type semiconductor material with a band gap of around 3 eV [23,24] is extensively suggested for diverse applications such as lithium-ion batteries [25], super-capacitors [26], solar cells [27], sensors [28] and photocatalysts [29,30,31]. However, TiO₂ as photocatalyst represents low photocatalytic activity due to its high electron–hole pair recombination rate, wide band gap and its excitation only under UV light [32]. In order to retard these deficiencies, various approaches are developed such as doping [33], sensitization [34], supporting on a medium [35] and coupling with semiconductors [36,37,38]. Among these, coupling of TiO₂ with other semiconducting material having lower band gap energy forming a heterojunction is a strategic option. The semiconductor having lower gap energy plays the role of sensitizer by being excited first, and then inducing the excitation of TiO₂ by passing photoelectrons from its conduction band to that of TiO₂ [39].

In the present work, TiO₂ nanoparticles were prepared by precipitation technique and then coupled with Cd and Ni to obtain CdTiO₂ and NiCdTiO₂ nanocomposites through co-precipitation method. The CdTiO₂ and NiCdTiO₂ nanocomposites are not reported in literature nor utilized as photocatalysts in the photodegradation of dyes to the best of our knowledge. These photocatalysts were prepared from economical materials and simple approach. The photocatalysts are very efficient toward the photodegradation of both dyes. The photocatalytic efficacy of the TiO₂, CdTiO₂ and NiCdTiO₂ was assessed by degradation of methylene blue (MB) and methyl green (MG) dyes in aqueous solution under UV-light irradiation. MB and MG are selected as model dyes because these are recalcitrant organic pollutants with carcinogenic and mutagenic nature with LD50 = 1180 mg/kg [40]. At higher concentration these dyes cause great damage to the human body and environment [41,42]. In the photodegradation of MB and MG dyes, the effect of irradiation time, catalyst dosage and pH were assessed.

2. Results and Discussion

2.1. Morphological and Elemental Analysis

The surface morphology of TiO₂, CdTiO₂ and NiCdTiO₂ was studied via SEM analysis and the images at different magnifications are shown in Figure 1, Figure 2 and Figure 3, respectively. The images show spherical TiO₂ NPs of different shapes and sizes and present mostly in agglomerated form. The particles are also present and dispersed when highly magnified. The SEM analysis of CdTiO₂ shows that Cd NPs are deposited on the surface and embedded in the porous structure of TiO₂. The morphology of CdTiO₂ displayed that TiO₂ are present in porous nanotubes form and Cd NPs are dispersed on its surface, and inserted in the nanochannels. The CdNiTiO₂ nanocomposites are mostly agglomerated, and the Cd and Ni NPs significantly cover the pores and surface of TiO₂. The particles have different shapes and morphology.

The ternary NiCdTiO₂ nanocomposites were also examined via TEM analysis and the results are as shown in Figure 4 at different magnifications which support the SEM analysis. Images shows that Cd and Ni NPs are uniformly mixed with TiO₂ NPs. Highly crystalline NiCdTiO₂ nanocomposites of cubical as well as spherical geometry with an average particle size of 20 nm are confirmed by TEM images.

The composition of the ternary NiCdTiO₂ nanocomposite was ascertained via EDX and the spectrum along with %composition in tabulated form is presented in Figure 5. The EDX spectrum shows signals for the constituent elements (Ti, Ni, Cd and O). The carbon and silver signals are present due to their coating on samples prior to EDX analysis for obtaining good quality images.

2.2. XRD Analysis

The XRD patterns of the photocatalysts are displayed in Figure 6. The observed signals can be related to the corresponding (101), (004), (200), (105), (211), (204) and (116) crystal planes. The identified diffraction signals can be allocated to the anatase TiO₂ (JCPDS-21-1272). Peaks for the Cd and Ni NPs are not observed owing to their minute quantity. However, (105) and (211) crystal plane peaks observed in the TiO₂ patterns are replaced by single broadened peaks in the CdTiO₂ and NiCdTiO₂ nanocomposites patterns.

2.3. BET Analysis

The optimum porosity and high specific surface area are considered as essential parameters for the efficiency of nanocomposite materials used in photocatalysis. Figure 7a presents the BET and adsorption-desorption plot for NiCdTiO₂ nanocomposite. The adsorption/desorption of N₂ is important for investigating the surface area, average pore size and pore volume of the NiCdTiO₂ photocatalyst. The study revealed that the NiCdTiO₂ nanocomposite exhibit type IV isotherm with a sharp increase of the adsorbed volume starting from P/P₀ = 0.84, confirming the mesoporous nanosized nature of the nanocomposite. When the relative pressure approaches 1, the hysteresis loop shifts higher and shows that the microporous particles are also present. Figure 7b shows BJH plot for the porosity investigation of NiCdTiO₂ nanocomposite. Different surface parameters like BET surface area, pore size, volume and BJH average pore width of NiCdTiO₂ nanocomposite are represented in the Table 1.

2.4. UV-Visible Analysis

The absorbance wavelength and band gap energy of the TiO₂, CdTiO₂ and NiCdTiO₂ were recorded using UV–VIS spectroscopy. Figure 8 displays the UV–VIS absorption spectra of TiO₂, CdTiO₂ and NiCdTiO₂. The UV–Visible absorption spectrum of TiO₂ shows absorption band at 265 nm. The maximum absorbance wavelengths of the CdTiO₂ and NiCdTiO₂ have shown slight red shifts. This shift can make nanocomposites better photocatalysts compared to pure TiO₂.

Figure 9a–c displays the Tauc plots: (αhv)² versus energy of TiO₂, CdTiO₂ and NiCdTiO₂, respectively. The band gap was calculated applying Tauc plots, which represents the relation between the sample absorption edge with the energy of the incident photon.

αhv = A(hv − E_g) ⁿ(1)

where α = molar extinction coefficient, h = Planck constant, v = photon’s frequency, A = constant, E_g = band gap energy, and n = parameter associated with the electronic transition (½ in the present case). The results demonstrate 2.7 eV band gap energy for TiO₂. The band gap energy values for CdTiO₂ and NiCdTiO₂ are 2.64 eV and 2.52 eV, respectively, as shown in the Figure 9c inset. The results clearly show the effect of Cd doping and Ni, Cd co-doping on the band gap energy of TiO₂ which can further be correlated with the photocatalytic activity of the catalysts.

2.5. Photodegradation of Methylene Blue (MB) Dye

2.5.1. Effect of Irradiation Time

The photocatalysts were utilized for the photodegradation of MB dye in aqueous medium under UV–light irradiation. Figure 10a–c displays the UV–VIS spectra of MB dye before reaction and after varying UV light-irradiation times in the presence of TiO₂, CdTiO₂ and NiCdTiO₂, respectively. The MB dye has shown maximum absorbance in the absence of catalysts. A sharp decrease in the absorbance was observed in the presence of catalysts due to photodegradation of the dye. The absorbance shows a regular decrease in the presence of catalysts with increasing irradiation time. The %degradation of MB dye by the synthesized catalysts as presented in Figure 10d shows higher photocatalytic activity for the ternary NiCdTiO₂ as compared to the binary CdTiO₂ and neat TiO₂. A 40, 48 and 65% degradation of the dye was observed within 20 min irradiation time with TiO₂, CdTiO₂ and CdNiTiO₂, respectively. The degradation was increased to 76.59, 82 and 86% in the presence of TiO₂, CdTiO₂ and CdNiTiO₂, respectively, by increasing irradiation time to 100 min. The increase in photodegradation of dye with an increase in irradiation time is due to the availability of more and more time to generate more hydroxyl radicals, which is a key species in dye degradation. The photocatalytic degradation experiments of MB using 30 ppm initial concentration were carried out under optimal reaction conditions and a pseudo first-order kinetics was observed. The results were in good agreement with the reported literature [42,43,44,45,46,47,48,49,50].

The photocatalytic degradation of dye depends upon the light-harvesting efficiency, the efficiency of the reaction of the photogenerated electron/hole charges and the reaction of photogenerated charges with substrate molecules. Photodegradation of the dye is achieved when UV-light interacts with the photocatalyst. Photons having energy equal to or greater than the band gap of the catalyst, excite the electrons from their valence band (VB) to the conduction band (CB) and produce positive holes (h⁺) in the VB. The h⁺ of the VB react with the water molecules and produce hydroxyl radicals (·OH) while the excited electrons present in the CB react with oxygen molecules and generate superoxide anion radicals (O₂^·−) [43,44]. These radicals are highly reactive species and effectively degrade dye molecules into simple and small species such as H₂O and CO₂. In the case of pristine TiO₂, a major portion of the separated electron–hole pairs recombine and reduces the photocatalytic activity. However, in CdTiO₂ and NiCdTiO₂ nanocomposites, the electrons present in the VB of TiO₂ get captured by the coupled Cd and Ni so the electron–hole pairs recombination rate decreases. This makes the nanocomposites more efficient photocatalysts compared to the neat TiO₂. The suggested mechanism for the photodegradation of MB dye by ternary NiCdTiO₂ nanocomposite is presented in Figure 11.

2.5.2. Effect of Photocatalyst Dosage

As photocatalytic activity is greatly affected by the available active sites, the effect of catalyst dosage on the dye degradation was, therefore, also investigated. Different amounts of catalysts (0.010, 015, 0.020, 0.025 and 0.030 g) were taken with a fixed irradiation time (30 min) and dye amount (10 mL) and the results for the degradation process as monitored by UV-Vis spectroscopy are shown in Figure 12a–c for TiO₂, CdTiO₂ and NiCdTiO₂ NPs, respectively. The %degradation of MB dye by TiO₂, CdTiO₂ and NiCdTiO₂ is compared in the Figure 12d. With an increase in catalyst dosage, the %photodegradation was also increased. Maximum photodegradation was achieved with 0.030 g of the catalyst. With this amount, 76.5, 83.5 and 86% dye was degraded by TiO₂, CdTiO₂ and NiCdTiO₂, respectively. Further increases in catalyst dosage beyond the limit (0.030 g) had no significance on the enhancement of the photocatalytic activity for the degradation process. No further increase in the catalytic activity could be attributed to the agglomeration of photocatalysts beyond the optimum dosage [16].

2.5.3. Effect of pH of the Medium

pH has an important role in the production of hydroxyl radicals responsible for the photodegradation process. Therefore, the effect of pH variation (2, 4, 6, 8, and 10) for a constant photocatalyst dosage (0.02 g) and irradiation time (30 min) on the photodegradation process was investigated and the results are shown in Figure 13a–c. The %degradation of the MB dye in different pH media are compared for the catalysts in the Figure 13d. The degradation process was low in acidic media and at pH 2 only 15, 22.07 and 26% MB dye was degraded by TiO₂, CdTiO₂ and NiCdTiO₂, respectively. However, in basic media the degradation percentages of the MB dye were quite high and at pH 10 about 79, 80.1 and 85.71% dye was degraded by TiO₂, CdTiO₂ and NiCdTiO₂, respectively. The cationic MB dye favors a high pH value for the adsorption process, which leads to an improved photocatalytic degradation [16].

2.6. Photodegradation of Methyl Green (MG) Dye

2.6.1. Effect of Irradiation Time

The photodegradation efficacy of the synthesized catalysts was also investigated against methyl green (MG) dye in aqueous solutions in the presence of UV light. Figure 14a–c demonstrates the UV–VIS spectra of MG dye before reaction and after different UV light irradiation times in the presence of TiO₂, CdTiO₂ and NiCdTiO₂, respectively. Figure 14d represents the %degradation of MG dye at varying irradiation times in the presence of catalysts. The graph clearly demonstrates that MG photodegradation increases effectively with increasing UV irradiation time. The %degradation results show that about 28, 45 and 59.5% of the MG dye was photodegraded by TiO₂, NiTiO₂ and CdNiTiO₂, respectively, within 20 min. The %degradation was increased to 63.3, 88 and 97.5% by TiO₂, NiTiO₂ and CdNiTiO₂, respectively, when irradiation time was increased to 100 min. The results clearly demonstrate that an increase in irradiation time results in an increased dye degradation due to availability of more and more time for dye adsorption followed by photodegradation.

2.6.2. Effect of Photocatalyst Dosage and Medium pH

The effect of catalyst dosage was evaluated by applying different dosages of photocatalysts keeping another parameters constant. Figure 15a represents the %degradation of MG dye photodegraded by different dosages of the catalysts The results show increased percent degradation of the dye with increased photocatalyst dosage. A 68, 87 and 98% degradation was achieved by 0.030 g (maximum dosage) of the TiO₂, CdTiO₂ and NiCdTiO₂, respectively, within 30 min. The effect of pH on Mg dye degradation was also evaluated by degrading dye in different pH solutions keeping other parameters constant. Figure 15b represents the %photocatalytic degradation of MG dye in different pH media. At pH 2, the TiO₂, NiTiO₂ and CdNiTiO₂ degraded about 12, 20 and 26% MG dye, respectively. The efficiency of MG dye degradation increases and about 67, 85 and 96.5% dye degraded at pH 10 by TiO₂, NiTiO₂ and CdNiTiO₂, respectively, within 30 min. The increased degradation of MG dye at higher pH is due to the production of more hydroxyl radicals in the basic medium.

2.7. Photocatalytic Activity Comparison

The comparative photocatalytic efficiency of the catalysts in the photodegradation of MB and MG dyes is shown as %degradation in the Figure 16. The dyes were irradiated for 100 min in the presence of catalysts. The data shows that neat TiO₂ is more efficient in degrading MB compared to MG dye. However, the nanocomposites are more effective in degrading MG dye compared to the MB dye. The ternary NiCdTiO₂ has shown the highest photocatalytic efficiency and degrades about 97.5% and 86% of MG and MB dye, respectively. The present study shows supremacy over the reported results [45,46,47,48,49,50,51] due to the photodegradation capability in correlation with band gap energy as presented in Table 2.

2.8. Lifespan of the Catalyst

The catalyst was regenerated from the reaction medium by simple centrifugation process and washed and dried in an oven overnight. The regenerated catalysts were utilized subsequently for five experimental runs under the optimum reaction conditions with no significant loss of activity as shown in Figure 17. The extended lifespan of the catalysts revealed the industrial scale applicability of the catalysts.

3. Experimental Work

3.1. Materials

Titanium (IV) isopropoxide (C₁₂H₂₈O₄Ti) (97%), cadmium (II) chloride hemi(pentahydrate) (CdCl₂·2.5H₂O) (98%), and nickel (II) chloride hexahydrate (NiCl₂·6H₂O) (98%) were purchased from Riedel-de Haen, Germany. Analytical grade sodium hydroxide (NaOH) (98%), methylene blue (98%), and methyl green (97.5%) were obtained from Sigma Aldrich, USA.

3.2. Synthesis of TiO₂ Nanoparticles

The precipitation procedure was utilized for the preparation of TiO₂ NPs by titanium (IV) isopropoxide precursor. A careful addition of 5 mL of titanium (IV) isopropoxide to 8 mL deionized water at 45 °C was followed by constant stirring for 1 h and resulted in white precipitate formation. The precipitates were centrifuged and washed three times with deionized water followed by methanol washings. The resultant powder was dried in an oven at 80 °C for 8 h followed by calcination at about 450 °C for 5 h.

3.3. Synthesis of Binary CdTiO₂ Nanocomposites

CdTiO₂ nanocomposites were synthesized via co-precipitation method. Cadmium (II) chloride hemi(pentahydrate) (0.030 mol, 6.85 g) was added to the deionized water (250 mL) and the mixture was stirred for 30 min at room temperature. The pH (9–10) of the mixture was adjusted by dropwise addition of 1 M NaOH solution. The precipitates formed during the process were centrifuged, separated and washed with deionized water followed by ethanol washing. The precipitates were dried in an oven at 80 °C for 5 h and annealed at 500 °C for 4 h. The resultant CdO NPs (0.01 M) were dispersed in methanol (5 mL) via sonication for 30 min. The dispersion was added to the TiO₂ (1 M) dispersion made in 5 mL methanol. The mixture was stirred at 80 °C for 30 min and the precipitates formed were separated via centrifuge. The CdTiO₂ binary nanocomposites obtained were kept overnight and oven dried at 100 °C for 2 h.

3.4. Synthesis of Ternary NiCdTiO₂ Nanocomposites

Ternary NiCdTiO₂ nanocomposites were prepared via co-precipitation method. Nickel (II) chloride hexahydrate (0.030 mol, 7.13 g) and cadmium (II) chloride hemi(pentahydrate) (0.030 mol, 6.85 g) were dissolved separately in deionized water (250 mL) and stirred for 30 min at ambient temperature. The pH (9–10) of the solutions were adjusted via dropwise addition of 1 M NaOH solution. The precipitates formed during the process were centrifuged and washed with deionized water followed by methanol three times. The obtained precipitates were oven dried at 80 °C for 6 h and annealed at 550 °C for 4 h. The synthesized Ni (0.001 M) and Cd (0.01 M) NPs were dispersed separately in 5 mL aqueous methanol via sonication for 30 min. The Ni and Cd dispersions were mixed with TiO₂ (1 M) solution made in aqueous methanol. The mixture was sonicated continuously at 80 °C for 30 min. The precipitates were centrifuged and the product NiCdTiO₂ was kept overnight and then dried at 100 °C for 5 h.

3.5. Characterization

The morphology and surface analysis were achieved through SEM (Model No. JEOL-5910; company Japan) and TEM (Tecnai F-20 FEI, USA). The elemental analysis was carried out via EDX (model INCA 200/oxford instrument, UK), while the size and phase of crystals were evaluated through XRD measurements (Model JEOL-300). The surface area and the porosity of the materials were evaluated applying a N₂ adsorption instrument (Micrometrics ASAP 2020). The photocatalytic degradation of the dyes was monitored using UV-VIS spectrophotometer (UV-1800, Shimadzu, Japan).

3.6. Dyes Photodegradation

The photocatalytic activity of the prepared catalysts was evaluated for the photodegradation (PD) of methylene blue (MB) and methyl green (MG) dyes in aqueous media under UV-light. The photocatalysts were individually added to 5 mL deionized water and sonicated constantly for 30 min, followed by an addition of 10 mL dye solutions (30 ppm) to each beaker. The beakers were covered with transparent plastic sheets. The mixtures were again sonicated for 30 min, kept in the dark to attain the adsorption/desorption equilibria and then kept under UV light (UV lamp (254 nm, 15 W)) with constant stirring for specific times. The catalysts were carefully removed through centrifugation (1200 rpm, 10 min). The study was completed by evaluating the effect of irradiation time (20, 40, 60, 80 and 100 min with a 0.03 g of catalyst), photocatalyst dosage (0.010, 0.015, 0.020, 0.025 and 0.03 g at 30 min irradiation time) and medium pH (2, 4, 6, 8 and 10 with 0.03 g catalyst and 30 min irradiation time) on dyes degradation. The degradation study was followed by UV-Vis spectrophotometer and the %degradation was found by applying the equation:

(2)$\mathrm{Degradation}\mathrm{rate}(\%)=\left(\frac{A_o-A}{A_o}\right)\times 100$

where A_o and A show dye absorbance before and after UV light irradiation, respectively. While for photocatalytic degradation, a pseudo first-order kinetic model was proposed.

(3)$-\frac{\mathrm{dC}}{\mathrm{dt}}=k_{ap}\mathrm{C}$

(4)$\ln \left(\frac{\mathrm{C}}{{\mathrm{C}}_0}\right)=k_{ap}\mathrm{t}$

where C₀: initial concentration, C: final concentration, k_ap: apparent rate constant and t: time in minutes.

4. Conclusions

In the present study, neat TiO₂ nanoparticles and its binary CdTiO₂ and ternary NiCdTiO₂ nanocomposites were synthesized by precipitation method. The synthesis of the TiO₂ and their binary and ternary nanocomposites were confirmed through various instrumental techniques. The synthesized materials have shown significant photocatalytic degradation of MB and MG dyes. The photodegradation results demonstrate that the dyes degradation increases with increasing irradiation time, catalyst dosage and pH of the medium. The increase in the degradation of the dyes with increasing photocatalyst dosage is due to more available active sites for dye adsorption followed by photodegradation. Similarly, the rapid photodegradation of the dyes at higher pH values could be a result of more reactive hydroxyl radicals formation. The coupled CdTiO₂ and NiCdTiO₂ nanocomposites result in the synergistic effect and have shown better photocatalytic efficiency compared to the neat TiO₂ nanoparticles. The NiCdTiO₂ nanocomposite is the most efficient photocatalyst for the degradation of MG (97.5% degraded in 100 min) as compared to MB (86% degraded in 100 min).

Author Contributions

Conceptualization and methodology, S.K., I.K.; Initial manuscript preparation, M.M.; Review and editing of the manuscript, M.S., A.N.; Investigation and supervision, N.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

The authors are grateful to Abdul Wali Khan University Mardan, for supporting the work.

Conflicts of Interest

The authors of this work have no conflict of interest to declare.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. (a–d) SEM images of TiO2 nanoparticles.

Figure 2. (a–c) SEM images of CdTiO2 nanocomposite.

Figure 3. (a–d) SEM images of NiCdTiO2 nanocomposite.

Figure 4. (a–d) TEM images of NiCdTiO2 nanocomposite at different magnifications.

Figure 5. (a) EDX spectra and (b) mapping of the ternary NiCdTiO2 nanocomposite.

Figure 6. XRD patterns of the TiO2, CdTiO2 and NiCdTiO2.

Figure 7. (a) BET and adsorption desorption plot for NiCdTiO2 (b) BJH plot showing porosity evaluation of NiCdTiO2.

Figure 8. UV–VIS absorption spectra of TiO2, CdTiO2 and NiCdTiO2.

Figure 9. Tauc plots: (αhv)2 versus energy for the (a) TiO2 (b) CdTiO2 (c) NiCdTiO2, inset; comparison of band gap energy.

Figure 10. UV–Visible absorption spectra of MB photodegraded by (a) TiO2 (b) CdTiO2 (c) NiCdTiO2 (d) Comparison of %degradation of MB dye inset kinetic model of degradation.

Figure 11. Proposed mechanism of photodegradation of MB dye by NiCdTiO2.

Figure 12. UV-VIS absorption spectra of MB dye before and after different dosage of (a) TiO2 (b) CdTiO2 (c) NiCdTiO2 (d) Comparison of %degradation of MB dye photodegraded by different dosage of photocatalysts.

Figure 13. UV–VIS spectra of MB dye photodegraded in different pH medium in the presence of (a) TiO2 (b) CdTiO2 (c) NiCdTiO2 (d) Comparison of %degradation of MB dye.

Figure 14. UV–VIS absorption spectra of MG degraded in the presence of (a) TiO2 (b) CdTiO2 (c) NiCdTiO2 (d) Comparison of %degradation of MG dye.

Figure 15. %degradation of MG dye photodegradation (a) by different dosage of photocatalysts and (b) at different pH.

Figure 16. Comparison of %degradation of MB and MG dyes photodegraded by all the photocatalysts.

Figure 17. Recycling of catalyst for five experimental runs.

Table 1

The specific surface area, pore size, pore volume and BJH average pore width of NiCdTiO₂ nanocomposite.

BET (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)	BJH Average Pore Width Ads/Des (nm)
61.2	0.1	10.6	9.6/8.7

Table 2

Comparative analysis of the synthesized catalysts with the reported studies.

No.	Catalysts	Dye	Band Gap (eV)	Irradiation(nm)	Time(min)	Conversion(%)	Ref
1	Ni-Cr/TiO2	methylene B	2.45	Sunlight	90	95.6	1
2	B-La/TiO2	methyl O	-	Sunlight	90	98	2
3	Ag-MoO3/TiO2	methyl O	2.89	UV	330	95.6	3
4	Sn-La/TiO2	rhodamine B	3.17	UV	180	98	4
5	Ce-B/TiO2	malachite G	3.42	UV	120	90	5
6	CdS/TiO2	Acid B	2.55	UV	90	84	6
7	Ni/TiO2	methylene B	2.92	UV	240	99	7
8	Ni-Cd/TiO2	methylene G	2.52	UV	100	97.5	Work

References

1. Saeed, M.; Muneer, M.; ul Haq, A.; Akram, N. Photocatalysis: An effective tool for photodegradation of dyes—A review. Environ. Sci. Pollut. Res.; 2022; 29, pp. 293-311. [DOI: https://dx.doi.org/10.1007/s11356-021-16389-7] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/34523090]

2. Köktürk, M.; Altindağ, F.; Ozhan, G.; Çalimli, M.H.; Nas, M.S. Textile dyes Maxilon blue 5G and Reactive blue 203 induce acute toxicity and DNA damage during embryonic development of Danio rerio. Comp. Biochem. Physiol. Part C Toxicol. Pharmacol.; 2021; 242, 108947. [DOI: https://dx.doi.org/10.1016/j.cbpc.2020.108947] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33285322]

3. Slama, H.B.; Bouket, A.C.; Pourhassan, Z.; Alenezi, F.N.; Silini, A.; Cherif-Silini, H.; Oszako, T.; Luptakova, L.; Golińska, P.; Belbahri, L. Diversity of Synthetic Dyes from Textile Industries, Discharge Impacts and Treatment Methods. Appl. Sci.; 2021; 11, 6255. [DOI: https://dx.doi.org/10.3390/app11146255]

4. Khan, I.; Sadiq, M.; Khan, I.; Saeed, K. Manganese dioxide nanoparticles/activated carbon composite as efficient UV and visible-light photocatalyst. Environ. Sci. Pollut. Res.; 2019; 26, pp. 5140-5154. [DOI: https://dx.doi.org/10.1007/s11356-018-4055-y] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30607840]

5. Gita, S.; Shukla, S.P.; Deshmukhe, G.; Choudhury, T.G.; Saharan, N.; Singh, A.K. Toxicity Evaluation of Six Textile Dyes on Growth, Metabolism and Elemental Composition (C, H, N, S) of Microalgae Spirulina platensis: The Environmental Consequences. Bull. Environ. Contam. Toxicol.; 2021; 106, pp. 302-309. [DOI: https://dx.doi.org/10.1007/s00128-020-03074-7]

6. Sharma, S.; Sharma, G.; Kumar, A.; AlGarni, T.S.; Naushad, M.; ALOthman, Z.A.; Stadler, F.J. Adsorption of cationic dyes onto carrageenan and itaconic acid-based superabsorbent hydrogel: Synthesis, characterization and isotherm analysis. J. Hazard. Mater.; 2022; 421, 126729. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2021.126729]

7. Abdelhamid, A.E.; El-Sayed, A.A.; Khalil, A.M. Polysulfone nanofiltration membranes enriched with functionalized graphene oxide for dye removal from wastewater. J. Polym. Eng.; 2020; 40, pp. 833-841. [DOI: https://dx.doi.org/10.1515/polyeng-2020-0141]

8. Li, M.; He, Z.; Xu, J. A comparative study of ozonation on aqueous reactive dyes and reactive-dyed cotton. Color. Technol.; 2021; 137, pp. 376-388. [DOI: https://dx.doi.org/10.1111/cote.12534]

9. Mcyotto, F.; Wei, Q.; Macharia, D.K.; Huang, M.; Shen, C.; Chow, C.W.K. Effect of dye structure on color removal efficiency by coagulation. Chem. Eng. J.; 2021; 405, 126674. [DOI: https://dx.doi.org/10.1016/j.cej.2020.126674]

10. Srinivasan, S.; Sadasivam, S.K. Biodegradation of textile azo dyes by textile effluent non-adapted and adapted Aeromonas hydrophila. Environ. Res.; 2021; 194, 110643. [DOI: https://dx.doi.org/10.1016/j.envres.2020.110643]

11. Ahila, K.G.; Ravindran, B.; Muthunarayanan, V.; Nguyen, D.D.; Nguyen, X.C.; Chang, S.W.; Nguyen, V.K.; Thamaraiselvi, C. Phytoremediation Potential of Freshwater Macrophytes for Treating Dye-Containing Wastewater. Sustainability; 2020; 13, 329. [DOI: https://dx.doi.org/10.3390/su13010329]

12. Khan, I.; Zada, N.; Khan, I.; Sadiq, M.; Saeed, K. Enhancement of photocatalytic potential and recoverability of Fe₃O₄ nanoparticles by decorating over monoclinic zirconia. J. Environ. Health Sci. Eng.; 2020; 18, pp. 1473-1489. [DOI: https://dx.doi.org/10.1007/s40201-020-00563-z] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33312656]

13. Kurian, M. Advanced oxidation processes and nanomaterials—A review. Clean. Eng. Technol.; 2021; 2, 100090. [DOI: https://dx.doi.org/10.1016/j.clet.2021.100090]

14. Izghri, Z.; Enaime, G.; Eouarrat, M.; Chahid, L.; El Gaini, L.; Baçaoui, A.; Yaacoubi, A. Hydroxide Sludge/Hydrochar-Fe Composite Catalysts for Photo-Fenton Degradation of Dyes. J. Chem.; 2021; 2021, 5588176. [DOI: https://dx.doi.org/10.1155/2021/5588176]

15. Khan, I.; Khan, A.A.; Khan, I.; Usman, M.; Sadiq, M.; Ali, F.; Saeed, K. Investigation of the photocatalytic potential enhancement of silica monolith decorated tin oxide nanoparticles through experimental and theoretical studies. New J. Chem.; 2020; 44, pp. 13330-13343. [DOI: https://dx.doi.org/10.1039/D0NJ00996B]

16. Khan, I.; Saeed, K.; Ali, N.; Khan, I.; Zhang, B.; Sadiq, M. Heterogeneous photodegradation of industrial dyes: An insight to different mechanisms and rate affecting parameters. J. Environ. Chem. Eng.; 2020; 8, 104364. [DOI: https://dx.doi.org/10.1016/j.jece.2020.104364]

17. Dodoo-Arhin, D.; Asiedu, T.; Agyei-Tuffour, B.; Nyankson, E.; Obada, D.; Mwabora, J.M. Photocatalytic degradation of Rhodamine dyes using zinc oxide nanoparticles. Mater. Today Proc.; 2021; 38, pp. 809-815. [DOI: https://dx.doi.org/10.1016/j.matpr.2020.04.597]

18. Najjar, M.; Hosseini, H.A.; Masoudi, A.; Sabouri, Z.; Mostafapour, A.; Khatami, M.; Darroudi, M. Green chemical approach for the synthesis of SnO₂ nanoparticles and its application in photocatalytic degradation of Eriochrome Black T dye. Optik; 2021; 242, 167152. [DOI: https://dx.doi.org/10.1016/j.ijleo.2021.167152]

19. Chairungsri, W.; Subkomkaew, A.; Kijjanapanich, P.; Chimupala, Y. Direct dye wastewater photocatalysis using immobilized titanium dioxide on fixed substrate. Chemosphere; 2022; 286, 131762. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2021.131762]

20. Lee, S.Y.; Kang, D.; Jeong, S.; Do, H.T.; Kim, J.H. Photocatalytic Degradation of Rhodamine B Dye by TiO₂ and Gold Nanoparticles Supported on a Floating Porous Polydimethylsiloxane Sponge under Ultraviolet and Visible Light Irradiation. ACS Omega; 2020; 5, pp. 4233-4241. [DOI: https://dx.doi.org/10.1021/acsomega.9b04127]

21. Pang, S.; Pang, S.; Lu, Y.; Lu, Y.; Cheng, L.; Cheng, L.; Liu, J.; Liu, J.; Liu, J.; Ma, H. et al. Facile synthesis of oxygen-deficient nano-TiO₂ coordinated by acetate ligands for enhanced visible-light photocatalytic performance. Catal. Sci. Technol.; 2020; 10, pp. 3875-3889. [DOI: https://dx.doi.org/10.1039/D0CY00592D]

22. Padmanabhan, N.T.; Thomas, N.; Louis, J.; Mathew, D.T.; Ganguly, P.; John, H.; Pillai, S.C. Graphene coupled TiO₂ photocatalysts for environmental applications: A review. Chemosphere; 2021; 271, 129506. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2020.129506] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33445017]

23. Barzegar, M.H.; Sabzehmeidani, M.M.; Ghaedi, M.; Avargani, V.M.; Moradi, Z.; Roy, V.A.L.; Heidari, H. S-scheme heterojunction g-C₃N₄/TiO₂ with enhanced photocatalytic activity for degradation of a binary mixture of cationic dyes using solar parabolic trough reactor. Chem. Eng. Res. Des.; 2021; 174, pp. 307-318. [DOI: https://dx.doi.org/10.1016/j.cherd.2021.08.015]

24. Tian, X.; Cui, X.; Lai, T.; Ren, J.; Yang, Z.; Xiao, M.; Wang, B.; Xiao, X.; Wang, Y. Gas sensors based on TiO₂ nanostructured materials for the detection of hazardous gases: A review. Nano Mater. Sci.; 2021; 3, pp. 390-403. [DOI: https://dx.doi.org/10.1016/j.nanoms.2021.05.011]

25. Heo, B.; Ha, J.; Kim, Y.T.; Choi, J. 10 μm-thick MoO₃-coated TiO₂ nanotubes as a volume expansion regulated binder-free anode for lithium ion batteries. J. Ind. Eng. Chem.; 2021; 96, pp. 364-370. [DOI: https://dx.doi.org/10.1016/j.jiec.2021.01.048]

26. Kumar, R.; Kumar, R.; Singh, B.K.; Soam, A.; Parida, S.; Sahajwalla, V.; Bhargava, P. In situ carbon-supported titanium dioxide (ICS-TiO₂) as an electrode material for high performance supercapacitors. Nanoscale Adv.; 2020; 2, pp. 2376-2386. [DOI: https://dx.doi.org/10.1039/D0NA00014K]

27. Ziaeifar, F.; Alizadeh, A.; Shariatinia, Z. Dye sensitized solar cells fabricated based on nanocomposite photoanodes of TiO₂ and AlMo_0.5O₃ perovskite nanoparticles. Sol. Energy; 2021; 218, pp. 435-444. [DOI: https://dx.doi.org/10.1016/j.solener.2021.03.024]

28. Kwon, S.H.; Kim, T.H.; Kim, S.M.; Oh, S.; Kim, K.K. Ultraviolet light-emitting diode-assisted highly sensitive room temperature NO₂ gas sensors based on low-temperature solution-processed ZnO/TiO₂ nanorods decorated with plasmonic Au nanoparticles. Nanoscale; 2021; 13, pp. 12177-12184. [DOI: https://dx.doi.org/10.1039/D1NR01001H]

29. Badvi, K.; Javanbakht, V. Enhanced photocatalytic degradation of dye contaminants with TiO₂ immobilized on ZSM-5 zeolite modified with nickel nanoparticles. J. Clean. Prod.; 2021; 280, 124518. [DOI: https://dx.doi.org/10.1016/j.jclepro.2020.124518]

30. Veziroglu, S.; Obermann, A.L.; Ullrich, M.; Hussain, M.; Kamp, M.; Kienle, L.; Leißner, T.; Rubahn, H.G.; Polonskyi, O.; Strunskus, T. et al. Photodeposition of Au Nanoclusters for Enhanced Photocatalytic Dye Degradation over TiO₂ Thin Film. ACS Appl. Mater. Interfaces; 2020; 12, pp. 14983-14992. [DOI: https://dx.doi.org/10.1021/acsami.9b18817]

31. Bellè, U.; Pelizzari, F.; Lucotti, A.; Castiglioni, C.; Ormellese, M.; Pedeferri, M.; Diamanti, M.V. Immobilized Nano-TiO₂ Photocatalysts for the Degradation of Three Organic Dyes in Single and Multi-Dye Solutions. Coatings; 2020; 10, 919. [DOI: https://dx.doi.org/10.3390/coatings10100919]

32. Zada, N.; Saeed, K.; Khan, I. Decolorization of Rhodamine B dye by using multiwalled carbon nanotubes/Co–Ti oxides nanocomposite and Co–Ti oxides as photocatalysts. Appl. Water Sci.; 2020; 10, 40. [DOI: https://dx.doi.org/10.1007/s13201-019-1124-4]

33. Khlyustova, A.; Sirotkin, N.; Kusova, T.; Kraev, A.; Titov, V.; Agafonov, A. Doped TiO₂: The effect of doping elements on photocatalytic activity. Mater. Adv.; 2020; 1, pp. 1193-1201. [DOI: https://dx.doi.org/10.1039/D0MA00171F]

34. Samuel, J.J.; Yam, F.K. Photocatalytic degradation of methylene blue under visible light by dye sensitized titania. Mater. Res. Express; 2020; 7, 015051. [DOI: https://dx.doi.org/10.1088/2053-1591/ab6409]

35. Pino, E.; Calderón, C.; Herrera, F.; Cifuentes, G.; Arteaga, G. Photocatalytic Degradation of Aqueous Rhodamine 6G Using Supported TiO₂ Catalysts. A Model for the Removal of Organic Contaminants From Aqueous Samples. Front. Chem.; 2020; 8, 365. [DOI: https://dx.doi.org/10.3389/fchem.2020.00365]

36. Habibi-Yangjeh, A.; Feizpoor, S.; Seifzadeh, D.; Ghosh, S. Improving visible-light-induced photocatalytic ability of TiO₂ through coupling with Bi₃O₄Cl and carbon dot nanoparticles. Sep. Purif. Technol.; 2020; 238, 116404. [DOI: https://dx.doi.org/10.1016/j.seppur.2019.116404]

37. Rani, M.; Keshu,; Shanker, U. Efficient degradation of organic pollutants by novel titanium dioxide coupled bismuth oxide nanocomposite: Green synthesis, kinetics and photoactivity. J. Environ. Manag.; 2021; 300, 113777. [DOI: https://dx.doi.org/10.1016/j.jenvman.2021.113777]

38. Nyankson, E.; Efavi, J.K.; Agyei-Tuffour, B.; Manu, G. Synthesis of TiO₂-Ag₃PO₄ photocatalyst material with high adsorption capacity and photocatalytic activity: Application in the removal of dyes and pesticides. RSC Adv.; 2021; 11, pp. 17032-17045. [DOI: https://dx.doi.org/10.1039/D1RA02128A]

39. Chakhtouna, H.; Benzeid, H.; Zari, N.; Qaiss, A.e.k.; Bouhfid, R. Recent progress on Ag/TiO₂ photocatalysts: Photocatalytic and bactericidal behaviors. Environ. Sci. Pollut. Res.; 2021; 28, pp. 44638-44666. [DOI: https://dx.doi.org/10.1007/s11356-021-14996-y]

40. Radoor, S.; Karayil, J.; Jayakumar, A.; Parameswaranpillai, J.; Siengchin, S. Release of toxic methylene blue from water by mesoporous silicalite-1: Characterization, kinetics and isotherm studies. Appl. Water Sci.; 2021; 11, 110. [DOI: https://dx.doi.org/10.1007/s13201-021-01435-z]

41. Sun, Z.; Feng, T.; Zhou, Z.; Wu, H. Removal of methylene blue in water by electrospun PAN/β-CD nanofibre membrane. E-Polymers; 2021; 21, pp. 398-410. [DOI: https://dx.doi.org/10.1515/epoly-2021-0041]

42. Khan, I.; Saeed, K.; Zekker, I.; Zhang, B.; Hendi, A.H.; Ahmad, A.; Ahmad, S.; Zada, N.; Ahmad, H.; Shah, L.A. et al. Review on Methylene Blue: Its Properties, Uses, Toxicity and Photodegradation. Water; 2022; 14, 242. [DOI: https://dx.doi.org/10.3390/w14020242]

43. Sultana, S.; Rafiuddin,; Khan, M.Z.; Umar, K.; Ahmed, A.S.; Shahadat, M. SnO₂–SrO based nanocomposites and their photocatalytic activity for the treatment of organic pollutants. J. Mol. Struct.; 2015; 1098, pp. 393-399. [DOI: https://dx.doi.org/10.1016/j.molstruc.2015.06.032]

44. Haque, M.M.; Khan, A.; Umar, K.; Mir, N.A.; Muneer, M.; Harada, T.; Matsumura, M. Synthesis, Characterization and Photocatalytic Activity of Visible Light Induced Ni-Doped TiO₂. Energy Environ. Focus; 2013; 2, pp. 73-78. [DOI: https://dx.doi.org/10.1166/eef.2013.1029]

45. Shaban, M.; Ahmed, A.M.; Shehata, N.; Betiha, M.A.; Rabie, A.M. Ni-doped and Ni/Cr co-doped TiO₂ nanotubes for enhancement of photocatalytic degradation of methylene blue. J. Colloid Interface Sci.; 2019; 555, pp. 31-41. [DOI: https://dx.doi.org/10.1016/j.jcis.2019.07.070]

46. Zhang, W.; Li, X.; Jia, G.; Gao, Y.; Wang, H.; Cao, Z.; Liu, J. Preparation, characterization, and photocatalytic activity of boron and lanthanum co-doped TiO₂. Catal. Commun.; 2014; 45, pp. 144-147. [DOI: https://dx.doi.org/10.1016/j.catcom.2013.11.008]

47. Kader, S.; Al-Mamun, M.R.; Suhan, M.B.K.; Shuchi, S.B.; Islam, M.S. Enhanced photodegradation of methyl orange dye under UV irradiation using MoO₃ and Ag doped TiO₂ photocatalysts. Environ. Technol. Innov.; 2022; 27, 102476. [DOI: https://dx.doi.org/10.1016/j.eti.2022.102476]

48. Zhu, X.; Pei, L.; Zhu, R.; Jiao, Y.; Tang, R.; Feng, W. Preparation and characterization of Sn/La co-doped TiO₂ nanomaterials and their phase transformation and photocatalytic activity. Sci. Rep.; 2018; 8, 12387. [DOI: https://dx.doi.org/10.1038/s41598-018-30050-3]

49. Stoyanova, A.; Ivanova, N.; Bachvarova-Nedelcheva, A.; Christov, C. Synthesis and photocatalytic activity of cerium-doped and cerium-boron co-doped TiO₂ nanoparticles. J. Chem. Technol. Metall.; 2021; 56, pp. 1294-1302.

50. Qutub, N.; Singh, P.; Sabir, S.; Sagadevan, S.; Oh, W.C. Enhanced photocatalytic degradation of Acid Blue dye using CdS/TiO₂ nanocomposite. Sci. Rep.; 2022; 12, 5759. [DOI: https://dx.doi.org/10.1038/s41598-022-09479-0]

51. Ganesh, I.; Gupta, A.K.; Kumar, P.P.; Sekhar, P.S.C.; Radha, K.; Padmanabham, G.; Sundararajan, G. Preparation and Characterization of Ni-Doped TiO₂ Materials for photocurrent and photocatalytic applications. Sci. World J.; 2012; 2012, 127326. [DOI: https://dx.doi.org/10.1100/2012/127326] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/22619580]