Improved photocatalytic activity of ZnO-TiO₂ nanocomposite catalysts by modulating TiO₂ thickness

Since the pioneer study on the photosensitization effect of a photoelectrochemical Pt-TiO₂ electrode for hydrogen and oxygen production from water in 1979 [1], the development and application of nano-sized metal-oxide semiconductors such as SnO₂, Cu₂O, ZnO and Fe₂O₃ have attracted significant attention of researchers in the field of oxidative degradation of environmental organic contaminants [2–5]. Several strategies and techniques e.g. band-gap engineering, co-catalyst incorporation and surface engineering have been investigated to circumvent the restrictive barriers of these catalysts for the industrial applications including high recombination rate of photo-excited electron-hole pairs [6–9]. For example, Xu et al described in-situ synthesis of a novel plate-like Co(OH)₂ decorated TiO₂ nano-sheets with efficient H₂ photocatalytic evolution activity [10]. Wang et al reported a co-catalyst for enhanced photocatalytic hydrogen evolution using TiO₂ [11]. Lin et al presented an effective surface of disordered metal oxide nanocrystals to improve their photocatalytic efficiency [12]. However, apart from intricate approaches, these features are indispensable for practical applications because the synthetic procedures often necessitate expensive engineering processes.

Among the conventional solutions to overcome the short lifetime problem of photo-exciting charge carriers, an effective method is coupling of nanomaterials with different band-gap energies in heterostructures [13]. The precise alignment of the energy bands in heterojunction nanomaterials can improve charge separation efficiency by prolonging the lifetime of photo-generated excitons. To achieve this objective, Xu et al investigated the CoMoS₂/rGO/C₃N₄ ternary heterojunction catalysts having high photocatalytic activity and stability for hydrogen evolution process under visible light irradiation [14]. In another study, Jiang et al introduced a two dimensional Z-Scheme AgCl/Ag/CaTiO₃ nano-heterojunctions for photocatalytic hydrogen production enhancement [15].

While variable nanostructures have been developed to dominate the aforementioned problems, several research groups attempted to improve the photocatalytic performance of metal-oxide semiconductor nanomaterials by synthesizing dual core-shell nanostructures [16]. For example, Kwiatkowski et al prepared ZnO/TiO₂ core/shell composites by sol-gel deposition of TiO₂ on ZnO nanorods hydrothermally grown on electrically conductive indium tin oxide substrate (ITO) for photodecomposition of methylene blue (MB) [17]. Nadrah et al synthesized core@shell TiO₂@SiO₂ nanoparticles with photocatalytic activity featuring a significantly faster preferential degradation of model pollutant (rhodamine B) in presence of abundant concentration natural organic matter compared to pure TiO₂ (P25) [16]. Abdel-Wahab et al presented the performance of photocatalytic degradation of paracetamol over magnetic flower-like TiO₂/Fe₂O₃ core-shell nanostructures [18]. In another study, Vasilaki et al synthesized ZnO/TiO₂ core-hell photocatalysts of a complex flower-like architecture to enhance photocatalytic performance and superhydrophilicity without UV irradiation [19]. In 2019, a stable Ta₃N₅@PANI core-shell photocatalyst was fabricated by Niu et al to investigate shell thickness effect on high-efficient photocatalytic performance and enhanced mechanism [20].

Liu et al prepared p-n heterojunction BiFeO₃-TiO₂ photocatalyst with and applied it for visible-light photocatalytic degradation [21]. According to positions of energy bands, the metal-oxide heterostructures can be defined as Type I (straddling gap), Type II (staggered gap), and Type III (broken gap) photocatalysts. Photo-excited charge separation can occur in staggered gap nano-sized heterojunctions in band-energy engineering, which can enhance the photocatalytic activity. The lowest energy states for photo-induced holes and electrons on opposite sides of the junction in staggered gap. Therefore, upon excitation, charge carrier pairs can migrate in the opposite direction and be enriched in both semiconductors [22].

Nanostructured titanium dioxide (TiO₂) and zinc oxide (ZnO) are well-known n-type semiconductor materials with wide band-gaps (∼3.22 eV, and 3.37 eV, respectively) playing effective roles in electromagnetic energy absorption and photo-degradation of organic pollutants [18, 19]. Although, TiO₂ and ZnO have similar band-gap energy values, the conduction and valence bands of TiO₂ are located under the corresponding bands of ZnO (∼0.48 eV) [13]. Therefore, the difference of redox energy levels existing between the two metal oxides make them appropriate for the preparation of core-shell heterojunction nanostructures.

Although, there are several reports focusing on the synthesis of ZnO/TiO₂ heterostructures including physical vapor deposition [23], microwave-assisted hydrothermal technique [24] and radio frequency sputtering [25], it is still very challenging to achieve photocatalysts with tunable structures via low-cost fabrication procedures. Among the previously published synthesis methods, the co-precipitation technique is a facile chemical process widely used for industrial production due to its cost effective practice [26]. Accordingly, Banavatu et al prepared γ-Bismuth molybdate (γ-Bi₂MoO₆) catalyst using co-precipitation method to evaluate photocatalytic degradation of Rhodamine-B, Crystal Violet and Orange II dyes under visible light irradiation [27]. Ayodhya et al prepared ZnS nanoparticles by co-precipitation method using various capping agents for their photocatalytic application in degradation of XO dye [28]. A research by Lassoued et al presented the synthesis and characterization of Co-doped nano-TiO₂ through co-precipitation method for photocatalytic activity [29]. Moitra et al. reported a simple 'in situ' co-precipitation approach for preparation of multifunctional CoFe₂O₄-RGO nanocomposites suggesting an excellent microwave absorber and a highly efficient magnetically separable recyclable photocatalyst for dye degradation [30].

It is noteworthy that there are a few articles regarding the precise control of the shell thickness due to laborious synthesis requirements [31]. In the present work, we report the preparation of wurtzite ZnO/anatase TiO₂ nanocomposite catalysts by a simple and effective chemical co-precipitation technique utilizing the advantages of heterojunctions with core-shell nanostructures. In addition, the optimum condition in photocatalytic performance was found by controlling the thickness of TiO₂ layer. We also preformed precise studies to understand the role of charge carriers (electrons and holes) and the photocatalytic performance mechanism at heterojunctions.

2.1. Materials and characterization

2.2. Synthesis of ZnO/TiO₂ nanocomposite photocatalysts

Figure 1 shows the overall synthesis procedure for the preparation of the ZnO/TiO₂ photocatalyst. The process utilized to synthesize ZnO/TiO₂ core-shell heterojunctions consisted of ZnO NPs synthesis by chemical co-precipitation method followed by TiO₂ encapsulation. It is noteworthy that the thickness of the TiO₂ shell can be tuned by controlling the amount of the titanium precursor. The fabrication procedure was completed by preparing ZnO/TiO₂ nanostructures with different TiO₂ shell thicknesses. In the first stage to synthesize the ZnO nanostructures, 1.0 M and 0.5 M of zinc acetate and sodium hydroxide were separately dissolved in deionized water (200 ml) and stirred for 30 min at room temperature. The obtained solutions were added drop-wise into an empty beaker under vigorous stirring. The resultant mixture was stirred for 2 h at room temperature to yield a white precipitate. In order to remove the impurities, the precipitation was centrifuged (4000 rpm) for 10 min, washed with deionized water and ethanol, and dried in an oven at 80 °C for 24 h. The dried powder was finally calcined at 350 °C for 2 h under air atmosphere to complete the process of acquiring ZnO NPs.

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The ZnO/TiO₂ nanocomposite fabrication was pursued by a co-precipitation route utilizing a previously reported technique [32]. In a typical synthesis, 1 g of the prepared ZnO NPs was added into a solution including 15 g of titanium isopropoxide and isopropyl alcohol with a mole ratio of 1:4 and stirred for 30 min to fabricate core-shell nanostructures. The obtained mixture was sonicated for 30 min to disperse the ZnO NPs in alkoxide solution. A solution of isopropyl alcohol and deionized water with a mole ratio of 1:1 was added to the mixture under vigorous stirring to achieve a final molar ratio of 1:8:4 between titanium isopropoxide, isopropyl alcohol and deionized water. An acidic condition was achieved at pH ∼3 by adding HCl into the reaction mixture. It should be noted that optimizing an acidic solution is essential for the stability of TiO₂ nanostructures [33]. The final mixture was aged by stirring smoothly for 24 h at room temperature. The then sample was centrifuged and dried in an oven at 90 °C for 12 h. The resultant solids were finally calcined at 350 ^oC in air for 3 h and to produce ZnO/TiO₂ core-shell nanostructures. To tune the thickness of TiO₂ shell, various weight portion of TiO₂ to ZnO was applied. The nanocomposite catalysts could be prepared in weight ratios of 1:20 and 1:25 and were labeled as ZnO/TiO₂-1/20 and 1/25.

2.3. Photocatalytic reactions

The photocatalytic activity of ZnO/TiO₂ nanocomposite catlysts was evaluated by studying the degradation of methylene blue (MB) as a model dye in aqueous solutions under UV light irradiation. The utilized photocatalytic reactor consisted of two UV lamps with the radiance power of 18 W and the light intensity of 115 mW cm⁻² (Philips) as light sources with wavelength of 320 nm illuminating aqueous solutions from the top. The photocatalytic reactor was kept inside a box made of medium-density fiberboard to eliminate the effect of the UV light background. In addition, the reactor was equipped with two fans to circulate the air and maintain the temperature. In a typical photocatalytic reaction, 0.04 g of the nanostructured catalyst was added into 250 ml of MB solution with an initial concentration of 10 ppm. To perform photocatalytic tests, alkaline solution with pH 8 was provided using NaOH. All samples were stirred under dark conditions for 20 min to ensure the establishment of an adsorption-desorption equilibrium between the surface of the prepared catalysts and MB. The MB de-colorization was evaluated using UV-Vis spectrophotometer (UV-Vis, Hach DR 5000). For this purpose, first at given time intervals (each 20 min), catalysts were centrifuged with a speed of 3000 rpm for 10 min and then filtered by a syringe filter in order to remove particles of used catalysts from the solution. The characteristic absorption peak of MB was monitored by UV-Vis spectrophotometer at λ_max = 633 nm. The photo-degradation percentage was calculated by the following equation [34]:

$$\begin{eqnarray}&&{\rm{Degradation}}\,{\rm{efficiency}}\left( \% \right)=\displaystyle \frac{({C}_{0}-{C}_{t})}{{C}_{0}}\times 100\end{eqnarray} \tag{ 1 }$$

Where ${C}_{0}$ is the initial concentration of MB in mgl⁻¹, and ${C}_{t}$ is final concentration at the irradiation time t (min). The kinetic of MB removal by ZnO/TiO₂ nanocomoosites was studied by fitting the experimental data to Langmuir-Hinshelwood kinetics expressions given as [35]:

$$\begin{eqnarray}&&\displaystyle \frac{d[C(t)]}{dt}={k}_{n}{[C(t)]}^{n}\end{eqnarray} \tag{ 2 }$$

Where $C(t)$ is the MB concentration in mgL⁻¹ at given time t (min). The parameter n indicates kinetic order, and ${k}_{n}$ is the decolorization rate constant for n-order. By substitution of n = 0, 1, and 2 into the equation (2), and the integration for boundary conditions, $C(t)={C}_{0}$ at $t=0,$ and $C(t)={C}_{t}$ at $t=t,$ the following equations can be derived:

$$\begin{eqnarray}&&({C}_{t}-{C}_{0})={k}_{0}t\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&Ln\left(\displaystyle \frac{{C}_{t}}{{C}_{0}}\right)={k}_{1}t\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&\left(\displaystyle \frac{1}{{C}_{t}}-\displaystyle \frac{1}{{C}_{0}}\right)={k}_{2}t\end{eqnarray} \tag{ 5 }$$

The Langmuir-Hinshelwood models linearized as equations (3)–(5) are best-known as zero, first and second-order kinetic models, respectively. Where ${C}_{0}$ is considered as the MB concentration immediately after the adsorption-desorption equilibrium. ${k}_{0}$ is the rate constant of zero-order model in mg l⁻¹ min⁻¹,${k}_{1}$ is the rate constant of first-order model in min⁻¹ and ${k}_{2}$ is the rate constant of second-order model in L mg⁻¹ min⁻¹.

XRD patterns of the pristine ZnO and ZnO/TiO₂ nanocomposites are depicted in figure 2. The Miller indices related to TiO₂ and ZnO planes are given by T and Z letters, respectively. The diffraction peaks related to ZnO nanostructure appeared in the XRD pattern are corresponding to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes, which are well indexed to hexagonal wurtzite structure of ZnO (JCPDS Card No. 36-1451). No impurity was observed indicating the high purity of ZnO NPs before TiO₂ encapsulation. The XRD pattern of TiO₂ coated ZnO NPs shows new phases related to ZnO/TiO₂ nanostructure, which can be assigned as anatase TiO₂ (JCPDS Card No. 21-1272). The peak intensities of TiO₂ anatase phase are increased with increasing the weight portion of TiO₂ to ZnO. The average crystallite size of ZnO NPs and ZnO/TiO₂ nanocomposite photocatalysts was estimated by XRD patterns using the Debye-Sherrer's equation [32]. The mean crystal size of ZnO NPs was calculated to be ∼23 nm by the full width at half-maximum (FWHM) of the (101) peak in figure 2(a). The average crystallite size of ZnO/TiO₂-1/15, ZnO/TiO₂-1/20, and ZnO/TiO₂-1/25 was obtained as ∼25 nm, ∼32 nm, and ∼35 nm, respectively, by the same method using dominant peak of (101) plane of anatase TiO₂ (figures 2(b)–(d)). The results show an increase in the average crystalline size with increasing the weight portion of TiO₂ to ZnO. Actually, the increase of the titanium precursor amount leads to the formation of thicker TiO₂ nanostructures with bigger crystalline sizes.

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The EDX mapping of the prepared nanocomposite confirmed the uniform distribution and presence of the elements in the ZnO/TiO₂ nanomaterials. The FESEM image (figure 3(A)) and EDX maps for the nanocomposite are exhibited in figure 3. The EDX maps illustrate the presence of the elements titanium (Ti), zinc (Zn), and oxygen (O) with a homogeneous distribution (figures 3(B)–(D)).

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Surface of the ZnO NPs and ZnO/TiO₂-1/15 nanostructures were studied in the wavenumber range of 400-4000 cm⁻¹ by FT-IR spectroscopy. As indicated in figure 4(a), FT-IR analysis of the ZnO NPs illustrates several well-defined absorption peaks at 422.06 cm⁻¹, 573.81 cm⁻¹, 1447.71 cm⁻¹, 1590 cm⁻¹, 2358.15 cm⁻¹, and 3435.90 cm⁻¹. The characteristics peaks observed in range of 400–900 cm⁻¹ belonging to typical Zn–O–Zn vibration caused by stretching vibration in ZnO lattice [36]. A strong absorption peak at 1447.71 cm⁻¹ suggests the presence of the astatine functional groups in ZnO NPs [37]. The weak peak in the vicinity of 1590 cm⁻¹ is also observed for the ZnO sample, which can be ascribed to H–O–H bending vibration mode due to surface–absorbed water. The water molecules can be absorbed on the ZnO surface appearing as –OH stretching vibration peak [24]. In addition, the bands observed at 2358.15 cm⁻¹ and 3435.90 cm⁻¹ can be attributed to O=C=O bonds, and interactions between hydroxyl groups and the surface of ZnO [37].

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The sharp peaks of stretching vibration at 400–900 cm⁻¹ disappear in the FT-IR spectrum of ZnO encapsulated with TiO₂ shell and a new wide absorption in the same range appears compared to the FT-IR spectrum of the ZnO NPs. In addition, a similar occurrence can be observed between 1400 cm⁻¹ and 3500 cm⁻¹ [38]. A broad peak related to the stretching vibration of O–H bond, and different interactions between hydroxyl groups and the surface of TiO₂ at 3411.89 are also emerged [39].

FESEM studies determined the surface morphology of the ZnO NPs and ZnO/TiO₂ nanocomposites. A typical FESEM image captured from the ZnO NPs is displayed in figure 5(a). The morphology shown in figure 5(a) does not exhibit uniform NPs with a high degree of homogeneity. The sheet-like shapes formed in FESEM image shown in figure 5(a) are due to the agglomerated spheres. Accordingly, the size of particles can be estimated with measuring the thickness of sheet-like structures. The diagram illustrated in figure 5(b), ascertain the particle size of 18 nm to 50 nm. However, morphological observations of ZnO/TiO₂ core-shell structures as illustrated in figure 5(c) indicate relatively uniform of spheres particles with an average size particle about 30 nm (figure 5(d)).

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The ZnO/TiO₂ nanocomposites with different shell thicknesses were precisely investigated using TEM analysis (figures 6–8). According to TEM images in, the core-shell structure can be clearly identified for all samples in which ZnO cores are encapsulated inside TiO₂ shells (figures 6(a)–8(a)). The particle size histograms (figures 6(b)–8(b)) estimate the average particle size of ∼24 nm, 39 nm, and 42 nm for ZnO/TiO₂-1/15, ZnO/TiO₂-1/20, and ZnO/TiO₂-1/25 core-shell nanostructures, respectively.

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The TiO₂ shell thickness of the nanocomposites was also calculated from distribution patterns of shell thickness. The values were measured to be about 13 nm, 15 nm, and 18 nm for ZnO/TiO₂-1/15, ZnO/TiO₂-1/20, and ZnO/TiO₂-1/25 nanocomposites, respectively (figures 6(c)–8(c)). The thickness of outer shell gradually grows with increasing the TiO₂ to ZnO weight ratio. The shell thickness can be controlled by changing TiO₂ concentration to reach the optimal thickness. In addition, the quantitative characterization results obtained from FESEM and TEM histogram analysis can be found in table 1.

The UV-Vis observations and the corresponding Tauc plots can be employed to understand the optical characteristics of the pristine ZnO NPs and ZnO/TiO₂ nanocomposite photocatalysts. The light absorbance characteristics of the all synthesized samples are plotted in a wavelength range of 200–800 nm and the results are displayed in figure 9(a). The optical absorbance data can specify the near band-edge optical absorption of the prepared samples. The absorbance of the ZnO NPs exhibits a broad and strong absorption profile in the UV region of electromagnetic spectrum with a steep edge in visible region. As it can be seen from figure 9(a), optical characteristic of the pristine ZnO NPs shows a sharp absorption peak at ∼365 nm. It is clear from the absorbance spectrum (figure 9(a)) that ZnO/TiO₂ nanocomposites reveal no particular peak inferring the absorption peaks. As shown in figure 9(a), after coating ZnO with TiO₂ layers, the overall absorbance of ZnO/TiO₂ core-shell nanostructures becomes weaker. This can be mainly attributed to the scattering effect of the TiO₂ shell according to Mie's theory for spherical structures [40]. Li et al reported a similar behavior with studying the effect of SiO₂ shell thickness on optical absorption characteristics of Ag@SiO₂@Ag sandwich nanostructures [41]. Zhang et al also presented another similar trend for Ag@SiO₂@TiO₂ core-shell nanoparticles, which happened with direct coating TiO₂ on Ag core without a SiO₂ interlayer and also in the presence of SiO₂ interlayer [42].

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The optical band-gap energy of the pristine ZnO NPs and ZnO NPs encapsulated with TiO₂ shell can be estimated by Kubelka-Munk equation as$(\alpha h\nu )=A{(h\nu -{E}_{g})}^{2}$ [43], where α is the absorption coefficient, $h\nu$ is the photon energy, A is the proportionality constant and ${E}_{g}$ is the optical band-gap energy. The band-gap energy values can be calculated by plotting ${(\alpha h\nu )}^{1/2}$against photon energy$(h\nu ).$ The plots converted from the absorbance spectra shown in figure 9(a), are exhibited in figure 9(b). The band-gap energy of the samples are obtained from intercept of the tangent to $h\nu$-axis. Table 1 shows the characteristics of the pristine ZnO NPs and ZnO/TiO₂ nanocomposites.

The band-gap energy of the pristine ZnO NPs and ZnO encapsulated with TiO₂ shell having the weight portion of 1/15, 1/20 and 1/25 are found to be ∼3.4 eV, 3.2 eV, 3.1 eV, and 3.05 eV, respectively. The results indicate a decrease for the all prepared ZnO/TiO₂ nanocomposites. The band-gap engineering for ZnO/TiO₂ nanocomposites can occur with being encapsulated ZnO core NPs into TiO₂ shells. Indeed, the absorbance edges of all ZnO/TiO₂ core-shell nanostructures generate a red shift towards bigger wavelengths in UV light region. The absorption edges obtained for ZnO/TiO₂ samples suggest an improvement in charge separation, which can be due to less photon energy required to excite electrons and transfer charge-carriers involving in photocatalytic processes. Figure 9(b) shows a red shift ∼22 nm, ∼35 nm, and ∼41 nm for ZnO/TiO₂-1/15, ZnO/TiO₂-1/20 and ZnO/TiO₂-1/25 core-shell nanostructures in contrast to the pristine ZnO NPs, respectively. In general, the resultant red shift can be described as the decrease of electron-transition energy, which can be achieved by narrowing the band-gap size. In addition, it is expected that ZnO/TiO₂-1/25 nanocomposite would present a better photocatalytic activity due to the smaller band-gap energy. This is because smaller band-gaps cause photo-excited excitons, which need less energetic photons and thereby the bigger photocatalytic rate [44]. According to the discussion presented in the literature, fundamental parameters of photocatalytic activity are influenced by the wavelength of the exposed light and band-gap of employed semiconductor material. It was reported that lower band-gap played an active role to reach superior efficiency [45]. It is worth mentioning that a considerable amount of literature has been published on reducing the band-gap of semiconductors using various strategies such as doping non-metal ions (e.g., N and C) [46–49] and transition-metal cations (e.g., Cr and V) [50, 51] to improve photocatalytic activity. Therefore, it is necessary to optimize the shell thickness of the nanostructures to achieve appropriate ban-gap energies for the practical applications.

The photocatalytic performance of the pristine ZnO NPs and ZnO/TiO₂ nanocomposites were evaluated by the de-colorization of MB aqueous solutions. The validity of UV light irradiation time on the de-colorization efficiency was investigated in solutions with 10 ppm dye concentration and pH 8 containing 0.04 g photocatalyst. Optical absorption spectra were periodically recorded in different time intervals (20 min) up to 120 min at room temperature. The resultant data are presented in figure 10. Figures 10(a)–(d) illustrates changes in UV-visible absorption spectra of MB solutions in terms of UV light exposure time in the presence of the pristine ZnO NPs, ZnO/TiO₂-1/15, 1/20, and 1/25 nanocomposite photocatalysts, respectively. The synthesized photocatalysts exhibited moderate to good photocatalytic activity to remove MB from aqueous solutions.

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To discuss the effect of shell thickness in the synthesized catalysts on the photocatalytic activity, the characteristic absorbance peak at λ_max = 663 nm was detected. The photoactivity of the examined samples is illustrated in figure 11. It can be concluded from photocatalytic experiments that the highest photo-degradation efficiency is achieved in the presence of ZnO/TiO₂-1/25 nanocomposite photocatalyst in which the photocatalytic activity trend can be represented as ZnO/TiO₂-1/25 (59.56%) > ZnO/TiO₂-1/20 (35.55%) > ZnO/TiO₂-1/15 (23.87%) > Pristine ZnO (20.1%) after 120 min of UV irradiation time. Such photocatalytic activity can be explained by the factor relevant to the decrease of band-gap sizes due to the significant presence of the TiO₂-related crystalline structure around the ZnO core. The band-gap energy effect can be associated with differences created among the band-gap structures during synthesis process. As shown in figure 9(b), the band-gap engineering technique by encapsulation process can provid smaller band-gap energies equivalent to 3.2 eV, 3.1 eV, and 3.05 eV for ZnO/TiO₂-1/15, ZnO/TiO₂-1/20 and ZnO/TiO₂-1/25 nanostructures compared with the value of 3.4 eV for the pristine ZnO NPs. Although no huge changes are seen for band-gaps of nanocomposites in comparison with band-gap of ZnO NPs, the resultant values have led to more efficient separation of photoexcited charge carriers and better improvement of photocatalytic performance.

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As previously mentioned according to the band-gap theory, semiconductors with narrower band-gaps can extend the light absorption range resulting improvement the photocatalytic performance [52]. Therefore, possessing the least band-gap value might be a reason for ZnO/TiO₂-1/25 nanocomposite catalyst with enhanced photocatalytic activity. In addition to the band-gap effect, the improvement of photocatalytic efficiency in nanocomposite catalysts can be attributed to reduction of recombination occurring in their structures, which can be further explained by photodegradation mechanism. These results suggest the importance of the controlling shell thickness to achieve the optimum conditions for the photocatalytic performance.

Figure 12 presents the schematic depiction of the photocatalytic mechanism in the presence of ZnO/TiO₂ core-shell heterojunction nanostructures. The possible charge transfer process suggests a charge exchange through inverse type II heterojunctions [53] between ZnO and TiO₂ components in which both ZnO and TiO₂ can absorb the illuminated UV photons in a core-shell structure. In inverse type II heterojunction illustrated in figure 12, the valence band of ZnO (VB_Z) is located at a more positive potential compared to TiO₂ (VB_T), while the conduction band of TiO₂ (CB_T) is more negative rather than that of ZnO (CB_Z) [33]. Charge-transfer pathways exhibit the opposite migration directions for photoexcited electrons and holes in both ZnO core and TiO₂ shell regions enriching both semiconductors. Photogenerated electrons migrate from CB_T to CB_Z to reach a less negative conduction band and holes passing from VB_Z to the corresponding band in TiO₂ decreasing the positive valance band [54].

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The band alignment estimated in inverse type II heterojunction of ZnO/TiO₂ nanocomposite catalysts causes photogenerated electrons diffusing into the core region. Therefore, the corresponding holes take part in the photocatalytic activity at the valance band of TiO₂ shell. Consequently, the photoexcited electrons are effectively isolated in CB_Z and only holes placed in VB_T are allowed to participate in the photocatalytic process. It is concluded that the photocatalytic performance improvement resulted for nanocomposite catalysts in comparison with the pristine ZnO catalysts can be due to the charge separation mechanism, which occurs at the interface and amplifies the lifetime enhancement of the photoexcited carriers.

The mechanism of the photocatalytic degradation in the presence of ZnO/TiO₂ nanocomposite photocatalyst can be described by the formation of the hydroxyl radical (^·OH), which is performed through the holes transferred into the valence band of TiO₂ shell region. The ^·OH radicals are mainly formed from ${h}_{V{B}_{T}}^{+}$ in the presence of either the c adsorbed H₂O according to equation (6) or the OH⁻ groups on the surface based on equation (7) [55]. Repeated attacks of ^·OH radicals on dye molecules finally lead to the degraded products as described in equation (8).

$$\begin{eqnarray}&&{h}_{V{B}_{T}}^{+}+{{\rm{H}}}_{2}{\rm{O}}\to {}^{\cdot }{\rm{O}}{\rm{H}}+{{\rm{H}}}^{+}\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{h}_{V{B}_{T}}^{+}+{{\rm{HO}}}^{-}\to {}^{\cdot }{\rm{O}}{\rm{H}}\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&{}^{\cdot }{\rm{H}}{\rm{O}}+dye\to {\rm{Degraded}}\,{\rm{product}}\end{eqnarray} \tag{ 8 }$$

It is necessary to study the rate-governing steps for time dependent catalysts to modulate the efficiency of the photo-degradation systems. To investigate the photocatalytic de-colorization process of MB, three Langmuir-Hinshelwood kinetic models including zero, first and second-order models are used by plotting the values of $({C}_{t}-{C}_{0}),$ $Ln\left(\tfrac{{C}_{t}}{{C}_{0}}\right)$and $\left(\tfrac{1}{{C}_{t}}-\tfrac{1}{{C}_{0}}\right)$ versus t for the pristine ZnO NPs and ZnO/TiO₂ photocatalysts. Figures 13(a)–(c) exhibits the kinetic curves accompanied by the corresponding theoretical fits for zero, first and second-order models, respectively. The reaction rate constants (k) are determined from the slope of theoretically fitted lines given by equations (3)–(5). The best-fitting kinetic model is chosen using the linear regression method based on the correlation coefficients (R²). Table 2 shows the photo-degradation rate constants (${k}_{0},$ ${k}_{1}$ and ${k}_{2}$) and the corresponding determination coefficients (${R}_{0}^{2},$ ${R}_{1}^{2}$ and ${R}_{2}^{2}$) for the synthesized photocatalysts.

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The R² values are obtained to determine the most appropriate model for the applied photocatalysts. According to results presented in table 2 and based on the highest R² values, the pristine ZnO NPs and ZnO/TiO₂-1/15 nanostructures follow zero-order kinetics, while ZnO/TiO₂-1/20 and ZnO/TiO₂-1/25 nanostructures pursue second-order and first-order models, respectively. The higher k values generally show the faster degradations and thereby the better photocatalytic activities.

The dimensionless parameter ${k}_{{\rm{ZnO}}-{{\rm{TiO}}}_{2}}/{k}_{{\rm{ZnO}}}$ is defined as an indicator of an improvement order for synthesized ZnO/TiO₂ nanocomposites in comparison to the pristine ZnO catalyst. The obtained comparative data are summarized in table 3. All applied models indicate similar trends as ZnO/TiO₂-1/25 > ZnO/TiO₂-1/20 > ZnO/TiO₂-1/15 > pristine ZnO, which are in agreement with the sequence resulted from figure 11.

ZnO/TiO₂ nanocomposite catalysts were synthesized using a simple chemical co-precipitation method. The TiO₂ shell thickness could successfully be controlled by tuning the weight portion of TiO₂ to ZnO. The synthesized photocatalysts showed pure wurtzite ZnO crystal structure and anatase TiO₂ phases for the core and shell structures, respectively. The prepared ZnO/TiO₂ catalysts revealed broader absorption ranges of UV-Vis spectra compared with the pristine ZnO nanoparticles because of red-shifts obtained from diffuse reflectance spectrum. The influences of TiO₂ shell thickness on the photocatalytic activity were explored by measuring the degradation rate of methylene blue. The de-colorization rate of the ZnO/TiO₂ photocatalysts was higher compared with the pristine ZnO photocatalyst, which is in agreement with the corresponding band-gap energies. The optical and photocatalytic properties of the prepared nanostructures indicated the importance of finding the optimal shell thickness for the practical applications. The photo-degradation mechanisms suggested to be initiated by holes transferred into the valence band of TiO₂ shell region.