*Article* **Photocatalytic Degradation of Organic Dyes Contaminated Aqueous Solution Using Binary CdTiO<sup>2</sup> and Ternary NiCdTiO<sup>2</sup> Nanocomposites**

**Shakeel Khan <sup>1</sup> , Awal Noor 2,\* , Idrees Khan <sup>3</sup> , Mian Muhammad <sup>4</sup> , Muhammad Sadiq <sup>4</sup> and Niaz Muhammad 1,\***


**Abstract:** The synergistic effect of binary CdTiO<sup>2</sup> and ternary NiCdTiO<sup>2</sup> on the photocatalytic efficiency of TiO<sup>2</sup> nanoparticles was investigated. The SEM analysis demonstrates spherical TiO<sup>2</sup> NPs of different sizes present in agglomerated form. The structural analysis of the nanocomposites reveals a porous structure for TiO<sup>2</sup> with well deposited Cd and Ni NPs. TEM images show NiCdTiO<sup>2</sup> 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 TiO<sup>2</sup> , respectively. Physical features of NiCdTiO<sup>2</sup> 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 CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> nanocomposites have shown red shift as compared to the neat TiO<sup>2</sup> due to coupling with Ni and Cd that results in the enhanced photocatalytic activity. The photocatalytic activity demonstrated that TiO<sup>2</sup> , CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> degrade methylene blue (MB) and methyl green (MG) about 76.59, 82, 86% and 63.5, 88, 97.5%, respectively, at optimum reaction conditions.

**Keywords:** TiO<sup>2</sup> ; nanocomposites; photocatalysts; photodegradation; methylene blue; methyl green

#### **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 O<sup>2</sup> 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 CO<sup>2</sup> [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

**Citation:** Khan, S.; Noor, A.; Khan, I.; Muhammad, M.; Sadiq, M.; Muhammad, N. Photocatalytic Degradation of Organic Dyes Contaminated Aqueous Solution Using Binary CdTiO<sup>2</sup> and Ternary NiCdTiO<sup>2</sup> Nanocomposites. *Catalysts* **2023**, *13*, 44. https://doi.org/ 10.3390/catal13010044

#### Academic Editor: John Vakros

Received: 21 November 2022 Revised: 19 December 2022 Accepted: 21 December 2022 Published: 26 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

byproducts formation, ambient pressure and temperature operation [16]. Various semiconducting photocatalysts are used for the photodegradation of dyes such as ZnO [17], Fe3O<sup>4</sup> [12], SnO<sup>2</sup> [18], TiO<sup>2</sup> [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 [20–22]. 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], supercapacitors [26], solar cells [27], sensors [28] and photocatalysts [29–31]. However, TiO<sup>2</sup> 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 [36–38]. Among these, coupling of TiO<sup>2</sup> 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 TiO<sup>2</sup> by passing photoelectrons from its conduction band to that of TiO<sup>2</sup> [39].

In the present work, TiO<sup>2</sup> nanoparticles were prepared by precipitation technique and then coupled with Cd and Ni to obtain CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> nanocomposites through co-precipitation method. The CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> 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, CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> 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, CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> was studied via SEM analysis and the images at different magnifications are shown in Figures 1–3, respectively. The images show spherical TiO<sup>2</sup> 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 CdTiO<sup>2</sup> shows that Cd NPs are deposited on the surface and embedded in the porous structure of TiO2. The morphology of CdTiO<sup>2</sup> displayed that TiO<sup>2</sup> are present in porous nanotubes form and Cd NPs are dispersed on its surface, and inserted in the nanochannels. The CdNiTiO<sup>2</sup> 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 NiCdTiO<sup>2</sup> 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 TiO<sup>2</sup> NPs. Highly crystalline NiCdTiO<sup>2</sup> nanocomposites of cubical as well as spherical geometry with an average particle size of 20 nm are confirmed by TEM images.

**Figure 1.** (**a–d**) SEM images of TiO<sup>2</sup> nanoparticles. **Figure 1.** (**a–d**) SEM images of TiO<sup>2</sup> nanoparticles. **Figure 1.** (**a–d**) SEM images of TiO<sup>2</sup> nanoparticles.

**Figure 2.** (**a–c**) SEM images of CdTiO<sup>2</sup> nanocomposite. **Figure 2.** (**a–c**) SEM images of CdTiO<sup>2</sup> nanocomposite. **Figure 2.** (**a–c**) SEM images of CdTiO<sup>2</sup> nanocomposite.

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**Figure 3.** (**a–d**) SEM images of NiCdTiO<sup>2</sup> nanocomposite. **Figure 3.** (**a–d**) SEM images of NiCdTiO<sup>2</sup> nanocomposite. particle size of 20 nm are confirmed by TEM images.

**Figure 4. Figure 4.**  (**a–d** (**a–**) TEM images of NiCdTiO **d**) TEM images of NiCdTiO<sup>2</sup> nanocomposite at different magnifications. <sup>2</sup> nanocomposite at different magnifications.

**Figure 4.** (**a–d**) TEM images of NiCdTiO<sup>2</sup> nanocomposite at different magnifications.

The composition of the ternary NiCdTiO<sup>2</sup> 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. 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.

Figure 5. (a) EDX spectra and (b) mapping of the ternary NiCdTiO2 nanocomposite. **Figure 5.** (**a**) EDX spectra and (**b**) mapping of the ternary NiCdTiO<sup>2</sup> nanocomposite.

2.2. XRD Analysis

#### *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 TiO<sup>2</sup> (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 TiO<sup>2</sup> patterns are replaced by single broadened peaks in the CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> nanocomposites patterns. crystal planes. The identified diffraction signals can be allocated to the anatase TiO<sup>2</sup> (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 TiO<sup>2</sup> patterns are replaced by single broadened peaks in the CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> nanocomposites patterns.

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**Figure 6.** XRD patterns of the TiO2, CdTiO<sup>2</sup> and NiCdTiO2. **Figure 6.** XRD patterns of the TiO<sup>2</sup> , CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> .

#### *2.3. BET Analysis 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 NiCdTiO<sup>2</sup> nanocomposite. The adsorption/desorption of N<sup>2</sup> is important for investigating the surface area, average pore size and pore volume of the NiCdTiO<sup>2</sup> photocatalyst. The study revealed that the NiCdTiO<sup>2</sup> nanocomposite exhibit type IV isotherm with a sharp increase of the adsorbed volume starting from *P/P<sup>0</sup>* = 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 NiCdTiO<sup>2</sup> nanocomposite. Different surface parameters like BET surface area, pore size, volume and BJH average pore width of NiCdTiO<sup>2</sup> nanocomposite are represented in the Table 1. 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 NiCdTiO<sup>2</sup> nanocomposite. The adsorption/desorption of N<sup>2</sup> is important for investigating the surface area, average pore size and pore volume of the NiCdTiO<sup>2</sup> photocatalyst. The study revealed that the NiCdTiO<sup>2</sup> nanocomposite exhibit type IV isotherm with a sharp increase of the adsorbed volume starting from *P/P<sup>0</sup>* = 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 NiCdTiO<sup>2</sup> nanocomposite. Different surface parameters like BET surface area, pore size, volume and BJH average pore width of NiCdTiO<sup>2</sup> nanocomposite are represented in the Table 1.

**Table 1.** The specific surface area, pore size, pore volume and BJH average pore width of NiCdTiO<sup>2</sup> nanocomposite.


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**Figure 7.** (**a**) BET and adsorption desorption plot for NiCdTiO<sup>2</sup> (**b**) BJH plot showing porosity evaluation of NiCdTiO2. **Figure 7.** (**a**) BET and adsorption desorption plot for NiCdTiO<sup>2</sup> (**b**) BJH plot showing porosity evaluation of NiCdTiO<sup>2</sup> . 61.2 0.1 10.6 9.6/8.7

#### **Table 1.** The specific surface area, pore size, pore volume and BJH average pore width of NiCdTiO<sup>2</sup> nanocomposite. *2.4. UV-Visible Analysis 2.4. UV-Visible Analysis*

**BET (m<sup>2</sup> /g) Pore Volume (cm<sup>3</sup> /g) Pore Size (nm) BJH Average Pore Width Ads/Des (nm)** 61.2 0.1 10.6 9.6/8.7 The absorbance wavelength and band gap energy of the TiO2, CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> were recorded using UV–VIS spectroscopy. Figure 8 displays the UV–VIS absorption spectra of TiO2, CdTiO<sup>2</sup> and NiCdTiO2. The UV–Visible absorption spectrum of TiO<sup>2</sup> shows absorption band at 265 nm. The maximum absorbance wavelengths of the CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> have shown slight red shifts. This shift can make nanocomposites better photocatalysts compared to pure TiO2. The absorbance wavelength and band gap energy of the TiO2, CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> were recorded using UV–VIS spectroscopy. Figure 8 displays the UV–VIS absorption spectra of TiO2, CdTiO<sup>2</sup> and NiCdTiO2. The UV–Visible absorption spectrum of TiO<sup>2</sup> shows absorption band at 265 nm. The maximum absorbance wavelengths of the CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> have shown slight red shifts. This shift can make nanocomposites better photocatalysts compared to pure TiO2.

**Figure 8. Figure 8.**  UV–VIS absorption spectra of TiO UV–VIS absorption spectra of TiO2, CdTiO<sup>2</sup> and NiCdTiO2. 2 , CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> .

Figure 9a–c displays the Tauc plots: (αhv)<sup>2</sup> versus energy of TiO2, CdTiO<sup>2</sup> 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.

$$
\alpha \mathbf{h} \mathbf{v} = \mathbf{A} (\mathbf{h} \mathbf{v} - \mathbf{E}\_{\mathbf{g}})^{\mathbf{n}} \tag{1}
$$

**Figure 8.** UV–VIS absorption spectra of TiO2, CdTiO<sup>2</sup> and NiCdTiO2.

photon.

where α = molar extinction coefficient, h = Planck constant, v = photon's frequency, A = constant, E<sup>g</sup> = band gap energy, and n = parameter associated with the electronic transition ( <sup>1</sup> 2 in the present case). The results demonstrate 2.7 eV band gap energy for TiO2. The band gap energy values for CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> 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 TiO<sup>2</sup> which can further be correlated with the photocatalytic activity of the catalysts. where α= molar extinction coefficient, h = Planck constant, v = photon's frequency, A = constant, E<sup>g</sup> = 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 CdTiO<sup>2</sup> 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 TiO<sup>2</sup> which can further be correlated with the photocatalytic activity of the catalysts.

αhv = A(hv − Eg)

<sup>n</sup> (1)

Figure 9a–c displays the Tauc plots: (αһv)<sup>2</sup> 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

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**Figure 9.** Tauc plots: (αhv)<sup>2</sup> versus energy for the (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> , inset; comparison of band gap energy.

#### *2.5. Photodegradation of Methylene Blue (MB) Dye 2.5. Photodegradation of Methylene Blue (MB) Dye* 2.5.1. Effect of Irradiation Time

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#### 2.5.1. Effect of Irradiation Time The photocatalysts were utilized for the photodegradation of MB dye in aqueous me-

son of band gap energy.

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, CdTiO<sup>2</sup> 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 NiCdTiO<sup>2</sup> as compared to the binary CdTiO<sup>2</sup> and neat TiO2. A 40, 48 and 65% degradation of the dye was observed within 20 min irradiation time with TiO2, CdTiO<sup>2</sup> and CdNiTiO2, respectively. The degradation was increased to 76.59, 82 and 86% in the presence of TiO2, CdTiO<sup>2</sup> 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 [42–50]. dium 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, CdTiO<sup>2</sup> 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 NiCdTiO<sup>2</sup> as compared to the binary CdTiO<sup>2</sup> and neat TiO2. A 40, 48 and 65% degradation of the dye was observed within 20 min irradiation time with TiO2, CdTiO<sup>2</sup> and CdNiTiO2, respectively. The degradation was increased to 76.59, 82 and 86% in the presence of TiO2, CdTiO<sup>2</sup> 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 [42–50].

**Figure 9.** Tauc plots: (αһv)<sup>2</sup> versus energy for the (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO2, inset; compari-

**Figure 10.** UV–Visible absorption spectra of MB photodegraded by (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> (**d**) Comparison of %degradation of MB dye inset kinetic model of degradation. **Figure 10.** UV–Visible absorption spectra of MB photodegraded by (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> (**d**) Comparison of %degradation of MB dye inset kinetic model of degradation.

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 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<sup>+</sup> ) in the VB. The h<sup>+</sup> 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 (O<sup>2</sup> ·−) [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 CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> nanocomposites, the electrons present in the VB of TiO<sup>2</sup> 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 NiCdTiO<sup>2</sup> nanocomposite is presented in Figure 11. (VB) to the conduction band (CB) and produce positive holes (h<sup>+</sup> ) in the VB. The h<sup>+</sup> 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 (O<sup>2</sup> ·−) [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 CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> nanocomposites, the electrons present in the VB of TiO<sup>2</sup> 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 NiCdTiO<sup>2</sup> nanocomposite is presented in Figure 11.

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

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**Figure 11.** Proposed mechanism of photodegradation of MB dye by NiCdTiO2. **Figure 11.** Proposed mechanism of photodegradation of MB dye by NiCdTiO<sup>2</sup> .

2.5.2. Effect of Photocatalyst Dosage 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, CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> NPs, respectively. The %degradation of MB dye by TiO2, CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> 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, CdTiO<sup>2</sup> and NiCdTiO2, respectively. Further increases in catalyst dosage beyond the limit (0.030 g) had 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, CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> NPs, respectively. The %degradation of MB dye by TiO2, CdTiO<sup>2</sup> and NiCdTiO<sup>2</sup> 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, CdTiO<sup>2</sup> 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].

of photocatalysts beyond the optimum dosage [16].

**Figure 12.** UV-VIS absorption spectra of MB dye before and after different dosage of (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> (**d**) Comparison of %degradation of MB dye photodegraded by different dosage of photocatalysts. **Figure 12.** UV-VIS absorption spectra of MB dye before and after different dosage of (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> (**d**) Comparison of %degradation of MB dye photodegraded by different dosage of photocatalysts.

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

#### 2.5.3. Effect of pH of the Medium 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, CdTiO<sup>2</sup> 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, CdTiO<sup>2</sup> and NiCdTiO2, respectively. The cationic MB dye favors a high pH value for the adsorption process, which leads to an improved photocatalytic degradation [16]. 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, CdTiO<sup>2</sup> 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, CdTiO<sup>2</sup> and NiCdTiO2, respectively. The cationic MB dye favors a high pH value for the adsorption process, which leads to an improved photocatalytic degradation [16].

**Figure 13.** UV–VIS spectra of MB dye photodegraded in different pH medium in the presence of (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> (**d**) Comparison of %degradation of MB dye. **Figure 13.** UV–VIS spectra of MB dye photodegraded in different pH medium in the presence of (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> (**d**) Comparison of %degradation of MB dye.

#### *2.6. Photodegradation of Methyl Green (MG) Dye 2.6. Photodegradation of Methyl Green (MG) Dye*

#### 2.6.1. Effect of Irradiation Time 2.6.1. Effect of Irradiation Time

photodegradation.

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, CdTiO<sup>2</sup> 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, NiTiO<sup>2</sup> and CdNiTiO2, respectively, within 20 min. The %degradation was increased to 63.3, 88 and 97.5% by TiO2, Ni-TiO<sup>2</sup> 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 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, CdTiO<sup>2</sup> 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, NiTiO<sup>2</sup> and CdNiTiO2, respectively, within 20 min. The %degradation was increased to 63.3, 88 and 97.5% by TiO2, NiTiO<sup>2</sup> 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.

**Figure 14.** UV–VIS absorption spectra of MG degraded in the presence of (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> (**d**) Comparison of %degradation of MG dye. **Figure 14.** UV–VIS absorption spectra of MG degraded in the presence of (**a**) TiO<sup>2</sup> (**b**) CdTiO<sup>2</sup> (**c**) NiCdTiO<sup>2</sup> (**d**) Comparison of %degradation of MG dye.

#### 2.6.2. Effect of Photocatalyst Dosage and Medium pH 2.6.2. Effect of Photocatalyst Dosage and Medium pH

radicals in the basic medium.

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, CdTiO<sup>2</sup> 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, NiTiO<sup>2</sup> and CdNiTiO<sup>2</sup> 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, NiTiO<sup>2</sup> and CdNiTiO2, respectively, within 30 min. The increased degradation of MG dye at higher pH is due to the production of more hydroxyl 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, CdTiO<sup>2</sup> 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, NiTiO<sup>2</sup> and CdNiTiO<sup>2</sup> 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, NiTiO<sup>2</sup> 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.

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 14 of 20

**Figure 15.** %degradation of MG dye photodegradation (**a**) by different dosage of photocatalysts and (**b**) at different pH. **Figure 15.** %degradation of MG dye photodegradation (**a**) by different dosage of photocatalysts and (**b**) at different pH. *2.7. Photocatalytic Activity Comparison* The comparative photocatalytic efficiency of the catalysts in the photodegradation of

#### *2.7. Photocatalytic Activity Comparison 2.7. Photocatalytic Activity Comparison*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 TiO<sup>2</sup> is more efficient in

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 TiO<sup>2</sup> 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 [45–51] due to the photodegradation capability in correlation with band gap energy as presented in Table 2. 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 TiO<sup>2</sup> 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 NiCdTiO<sup>2</sup> 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 [45–51] due to the photodegradation capability in correlation with band gap energy as presented in Table 2. 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 [45–51] due to the photodegradation capability in correlation with band gap energy as presented in Table 2.

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

**Figure 16.** Comparison of %degradation of MB and MG dyes photodegraded by all the photocata-

lysts.

lysts.


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**Table 2.** Comparative analysis of the synthesized catalysts with the reported studies. **Table 2.** Comparative analysis of the synthesized catalysts with the reported studies.

#### *2.8. Lifespan of the Catalyst* process and washed and dried in an oven overnight. The regenerated catalysts were uti-

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. lized 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.

The catalyst was regenerated from the reaction medium by simple centrifugation

**Figure 17.** Recycling of catalyst for five experimental runs. **Figure 17.** Recycling of catalyst for five experimental runs.

#### **3. Experimental Work**

#### **3. Experimental Work** *3.1. Materials*

*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 Al-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.

#### drich, USA. *3.2. Synthesis of TiO<sup>2</sup> Nanoparticles*

*3.2. Synthesis of TiO<sup>2</sup> Nanoparticles* The precipitation procedure was utilized for the preparation of TiO<sup>2</sup> 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.
