Insight on TiO2-ZnO Photocomposite Competence Fabricated via Sonication Assisted with Gelatin for Rhodamine B Degradation

Herewith we report a facile synthesis of zinc oxide doped with (5, 10, 15, and 20 wt%) titanium oxide nanocomposites in gelatin under ultra-sonication. The X-ray diffraction (XRD) data revealed ZnO the formation in addition to a rutile phase TiO2. The ZnO phase size decreased, and the rutile TiO2 phase increased with a TiO2 loading increment. The scanning electron microscopy (SEM) displayed a combination of spherical and hexagonal particles with a 60 – 80 nm size distribution. The prepared nanostructures photocatalytic activity was assisted using Rhodamine B dye, where they showed enhanced photodegradation competence under visible light irradiation. The kinetics of photodegradation followed the first-order kinetics with the 20 % wt sample having the maximum activity. The mechanistic investigation revealed the dominance of h and •O2 species during the dye photodegradation. The results indicate the potential application of such gelatin stabilized nanostructures for dye illumination from aqueous solutions under sunlight.


Introduction
Polymer stabilized nanocomposites are recently becoming of prime research interests by virtue of their value added benefits such as improved stabilizing, fictionalization, magnetic, thermal and mechanical qualities [1][2][3]. Being abundant, hydrophilic, biocompatible and biodegradable natural biopolymers are favored over the synthetic [4]. Gelatin obtained by denaturation of collagen is formed of a single sequence of amino acids. It is warm water soluble and gels when its concentration exceeds 1wt% [2]. It has the ability to coordinate with metal ions due to the polar groups in its structure [5]. Gelatin was dissolved in the sol form and used as a structure-guiding agent to tailor the ZnO nanostructures morphology. For instance, ZnO nano-plates were developed from ZnO nanoparticles using gelatin [6], while Bauermann et al. fabricated hexagonal ZnO plates in a gelatin matrix [5]. Gelatin was employed to stabilize CdTe/CdS/ZnS (CSSG) core/double shell quantum dots (QDs) to advance their biocompatibility [7], copper NPs [8] and ZnO NPs [9].
The n-type and large band gap semiconductors like ZnO, TiO2, and SnO2 have triggered the researchers curiosity persuading them to explore their properties and applications [10]. To benefit from the ZnO flexibility in synthesis and morphologies and the stability of TiO2,acouplingof TiO2 and ZnO nanoparticles is a convenient approach to induce substantial effects on the morphologies, porosity, electronic, and photoelectrochemical characteristics [11]. The catalytic activity of TiO2 was enhanced by adding ZnO [12] where the electron and hole transfer between their conduction and valence bands leads to a well separation of photogenerated charge carriers [12,13].
Previous reported methods of ZnO-TiO2nanostructures synthesis included magnetron sputtering [14,15], powders thermal processing [16], sol-gel [17,18], chemical vapor deposition [19] and homogeneous hydrolysis [20]. 3 In this study different gelatin stabilized TiO2 doped ZnO nanocomposites were synthesized via ultrasonic energy. A titanium oxide rutile phase has immerged and the morphology has altered as a result of doping. The optical properties and the employment of the nanocomposites to decolorize the RhB dye were investigated and the kinetics and mechanism of the process were probed.

Preparation of TiO2-ZnO photocomposites
For the synthesis of TiO2-ZnO photocomposites at different TiO2 nanoparticles content (5, 10, 15 and 20 wt%), gelatin and ultra-sonication process is employed. The required amounts of TiO2 and fixed amount of ZnO nanoparticles were dispersed in 100ml of absolute ethanol and the mixture sonicated at room temperature for 30min until the milky solution formed. The mixture obtained was added gradually to the 100ml of a hot solution of gelatin under vigorous stirring. Next, the mixture is sonicated for 1h at 50 o C and dried in an oven at 120 o C for 6h. The dried samples were calcined at 650 C in air atmosphere for an hour using muffle furnace. The samples were labelled as (5%ZnO, 10%ZnO, 15%ZnO, and 20%ZnO).

Characterization of the photocomposites
The crystalline structure of the powders was investigated by X-ray powder diffraction (XRD) using Bruker high-resolution diffractometer equipped with Cu-Ka radiation (1.5418 Å), operating at 40 kV and 40 mA. Morphological images recorded by field emission scanning electron microscopy (FE-SEM) using high-resolution Jeol JSM 7600F.
The optical properties were determined by means of diffuse reflectance spectroscopy (DRS) using JASECO V-770 spectrophotometer in the wavelength range 300-800 nm. 4

Photocatalytic efficacy of fabricated photocatalysts
The photocatalytic competence of the as-fabricated photocomposites was assessed by monitoring the photodegradation of Rhodamine B dye under visible-light illumination. In a typical test, 50 mg the photocatalyst powder was dispersed in 100 ml aqueous solution of RhB dye concentration = 20 mg L -1 . Prior to the light illumination, the photocatalyst with the dye solution was strongly magnetically stirred (400 rotation/min) in full darkness for 30min under the ambient condition to achieve an adsorption/desorption equilibrium of dye on the photocatalyst surface. Then 5 ml of the mixture solution was withdrawn as an initial concentration (Co) and exposed to visible-light irradiation (OSRAM lamp 58 IM/W).
Following exposure to visible-light 5 ml suspension was withdrawn at different time intervals, centrifuged (5000 rpm for 10 min) to remove suspended and the absorbance was measured. Temporal concentration variations of RhB were measured via the change in the maximal spectra (λmax = 525 nm) of the dye.

Radicals scavenger experiment
To probe the photocatalytic mechanistic activity of the photocomposites, the effective reactive species (holes and radicals) were identified using radical scavenging tests method.
In this method, the valence band holes (h + ), conduction band electrons (e -), hydroxyl radical ( * OH), and super-oxide radical ( * O2 − ) were monitored by adding EDTA (h + ) [21], silver nitrate (e -) [22], isopropanol ( * OH) [21], and ascorbic acid ( * O2 − ) [22] to the reaction solution, individually, through the process of photocatalytic degradation of RhB dye. In a typical procedure, 50 mg of x% ZnO and 10 mM of radical scavengers were placed in 100 mL of 20 mg. L -1 RhB dye solution; then the mixture was illuminated utilizing the visible light at the same time after adsorption equilibrium. Ultimately, the degradation rate of the dye was calculated to highlight the main role of active species. 5

XRD analysis
The XRD patterns of the TiO2/ZnO composite are sketched in Fig. 1. reveal sharp and distinct diffraction peaks with relative intensity and broadening, as a clue of the development of nano-crystalline structure of high crystallinity [23]. The peaks at 2θ = 31,  [24][25][26]. It can be noticed that the intensities of the peaks corresponding to the rutile phase increase proportional to the TiO2 ratio in the sample (Fig. 1b) which is an expected consequence of the ZnO % lowering in the precursor powders [27]. A noticeable decrease in the FWHM with increase of Ti loading is an indication of improved crystallinity [28].
The Scherrer equation [29] was used to compute the crystallite size of the nanocomposites: (1) where β is the full width at half maximum (FWHM) of the XRD peak (in radians), the constant, k value of ≈ 0.90, θ is diffraction angle and λ is the wavelength of the Cu-Kα (1.5418 Å). The crystallite size for ZnO wurtzite was estimated taking the most intense peak (101), while the peak (110) was used for the TiO2 rutile phase. The crystallite size of the wurtzite ZnO (D(w)) exhibits a decrease with the addition of more TiO2 [30], while a reverse trend is shown by the rutile phase crystallite size (D*(R)). From the XRD findings, a successful synthesis of nanocomposites comprising of rutile TiO2 and wurtzite ZnO can be confirmed. The d-spacing, lattice parameters and the unit cell volumes were estimated by the expressions: 6 The micro-strain is estimated via the expression [31] and the stress σ(GPa) in the crystallite's plane [32] are computed where c and cbulk are the measured lattice parameter and the strain-free lattice parameter of ZnO (5.2061 Å) respectively. The Zn -O bond length (L) was calculated using the equation: where The tabulated values (Table 1) of the d-spacing, a, c, V and L reveal a decrease with the increment of titanium in the nanocomposites. This may attributed to the substitution of larger radius Zn 2+ (74 pm) by the smaller radius Ti 4+ (68 pm) [30]. The stress (ϭ) in the nanocomposites can be estimated by the strain model ϭ (Gpa) = -233*(c0c)/c0, where c and c0 are the lattice constants calculated from XRD data the lattice constant of the pure ZnO (0.5206 nm) respectively [33]. It is evident that the samples containing titania have a negative or very less stress that points to a compressive stress. Thus the minor shrinkage in the lattice parameters can be ascribed to a compressive stress as a result of the partial displacement of the large Zn 2+ by the small Ti 4+ in the structures [34,35]. Using the formula ε = β / 4 tan θ, the microstrain (ε) was calculated and tabulated ( Table 2). As can be noticed that the microstrain decreased monotonously with the increase in titnia loading 7 which may be attributed to the atom diffusion [36] during the Ti 4+ substitution of Zn 2+ during composites formation. From the table it is evident that the preferred orientation has changed from (002) to the (101) plane as titanium loading is increased as reflected by the increase in (I101/I002) ratio. This may be attributed effect of Zn and Ti atoms that reduce the surface free energy of the (101) which might have changed the plane of minimum surface free energy from (002) to (101) [37].
The crystallite is calculated from the intercept of lnβ versus ln (1/cosθ) graph. Fig. 2a shows

Uniform deformation model (UDM):
According to Williamson-Hall approach [39], strain and crystallite size contribute significantly to the diffraction lines broadening (equation 9) [40,41]. In the uniform deformation model (UDM), it is assumed that a crystal is isotropic [42] and subsequently its properties are independent of the crystallographic direction along which the measurement is considered.  A plot of versus represents a linear graph [42], and the crystallite size (D) and microstrain ( ) are respectively calculated from the intercept and slope of the line (Fig. 2b).The data obtained are enumerated in table (2).

Uniform stress deformation model (USDM):
As isotropy and homogeneity conjecture is unachieved at all instances, a more realistic anisotropic model is therefore developed. Thus, Williamson-Hall formula is reformed by including an anisotropic term related to strain ( ) [43]. In the Uniform Stress Deformation Model (USDM), Hooke's law represents the strain in addition to a direct correlation between the stress (σ), anisotropic microstrain ( ) and Young's modulus (Yhkl) as represented by σ = εYhkl. For hexagonal ZnO structure (Yhkl) was given as 127GPa [40].
Consequently, the Williamson-Hall formula is remodeled to (10): By plotting as a function of , σ and D are obtained from the slope and intercept respectively, while ε is computed using Young's modulus, Yhkl, of ZnO nanoparticles hexagon. Fig. (2c) illustrates the USDM for the samples and the data obtained is shown (Table 2).

An additional form of Williamson-Hall methods known as the Uniform Deformation
Energy Density Model (UDEDM) is applied to deduce the crystal's energy density ( ).
For elastic systems that comply with Hooke's law, is inter related with the strain through the term = ( 2 Yhkl)/2 [40]. Equation (11) provides the UDEDM term: Where After Plotting versus , we can estimate the anisotropic energy density ( ) from the slope. While, the stress (σ) and microstrain (ε) are estimated from ( )and Yhkl, and the crystallite size ( D) is calculated from intercept [40].  Table 2.  can be ascribed to variation in particle size distribution averaging [39]. However, the data shows larger crystallite size for some of the samples by all Williamson-Hall models, still our data agree with previous investigations [44] that reported such convergence in the crystallite size results. All the ɛ and σ values obtained employing all the Williamson Hall models change proportional to the crystallite size in agreement with previously reported data [39]. When matching up the data obtained by the three models, it can be observed that there is a variation between the parameters obtained by UDM and the other two models confirming the anisotropic character of the nanoparticles [39]. This finding may support the change in the preferred orientation [37] as discussed earlier.

Morphological investigations:
The SEM images of the prepared nanocomposites are depicted in Fig. 3 (ad). The 5 %ZnO sample (Fig. 3 a)

Optical properties and band gap determination:
To optical properties the TiO2/ZnO nanocomposites with varying Ti percentage were probed by UV-visible and their respective spectra are shown in Fig. 5. The graph reveals a   maximum absorption at 373, 373, 369 and 370 nm for the 5%ZnO, 10%ZnO, 15%ZnO and 20%ZnO respectively. The band gap energy (Eg)was estimated from the(αhv) 2 versus hv graph (Fig. 6) using the Tauc equation [46]: (12) where h, ν, α and Eg stand for; the Planck's constant, frequency, absorption coefficient and the band gap energy respectively, whereas A is a proportionality constant, and n designates the nature of electron transition (for directly allowed transitions, n = 1/2). The energy band gap values of were found to be 3.218, 3.212, 3.227 and 3.237eV the 5 %ZnO, 10 %ZnO, 15 %ZnO and 20%ZnO samples respectively. The Eg of the nanocomposites make them a promising candidates for different photo-applications [47,48]. During the doping process and the Ti 4+ ions replacement of Zn 2+ ions at substitution sites [49] an increase in the number of the free charge carriers prevails which considerably distresses low energy excitations. Such a situation may lead to the semiconductor's conduction band Fermi level expansion and a consequent optical band gap widening, in accordance with the Burstein-Moss shift [50,51]. Analogous blue shift phenomenon of optical band gaps in Tidoped ZnO films [34,52] and Ti-doped ZnO nanoparticles [53] have been reported earlier.  (Fig. 7b). With increasing TiO2 content, the photodegradation competence of the nanocomposites has markedly improved [55]. Among them, 20% doped ZnO exemplified the best performance with a dye photo-degradation rate of 97% within 180 min (Fig. 8a). The kinetics of the photodegradation was greatly fits the first order kinetics as indicated by the linear ln C0/Ct vs t plots (Fig. 8b) and the regression values (R 2 ≈ 1.0) [56]. The rate constant and half-life values (Table 3) [59]. Fig. 9 is a graphical description for a photodegradation scheme.

Influence of radicals' scavengers on the photocatalytic activity
Oxygen species are reactive radicals are usually involved in the photodegradation process.
In order to verify the operative entities, the scavengers EDTA, isopropanol, silver nitrate and ascorbic acid are employed to monitor the effects of positive holes, hydroxyl radicals, electrons and superoxide radicals respectively on TiO2/ZnO photo-composite. The photodegradation (Ct/C0) percentage variation with scavengers is displayed in Fig. 10. It can be seen that photodegradation yields are 10%, 5%, 15%, and 30% due to the addition of EDTA (h + scavenger) [60] and ASC (•O2scavenger) [61] as compared to IPA (•OH scavenger) [61] and AgNO3 (escavenger) respectively. This implies that the h + [62] and •O2 - [63] radicals have more contribution to the dye photodegradation compared to •OH and ethat show less impact on the photodegradation process by the TiO2/ZnO photocatalyst.  Table 4 shows a comparison of the photocatalytic competence of 20%ZnO with other photocatalysts from literature. The data undoubtedly exhibit the higher performance of the photocatalyst to decolorize RhB in aqueous solutions.

Conclusion
The doping of ZnO with various percentage of TiO2 has dramatically improved it is competent for the photocatalytic degradation of RhB dye under visible light illumination.
The nanocomposites prepared via a facile ultra-sonication exhibited crystallite size reduction with the rutile TiO2 phase's development as perceived from the XRD data. The optical properties revealed higher absorbance for the 20 %ZnO sample with a band gap energy of 3.23 eV. The 20 %ZnO showed enhanced photodegradation activity that followed the pseudo-first-order kinetics with 0.01132 min -1 rate constant. The scavengers tests indicated that the h + and •O2are the most effective species involved in the dye degradation. Based on the results presented in this research, ZnO doped with TiO2 has the potential for use as an efficient photocatalyst for polluting organic dyes in aqueous discharges.