Green Synthesis ZnO/TiO2 for High Recyclability Rapid Sunlight Photodegradation Textile Dyes Applications


 Composite ZnO/TiO2 have been successfully synthesized by green synthesis method with various calcination temperature 500oC, 600oC, 700oC, and 800oC (TiO2 concentrations: 2.5 g and 5 g) for photocatalyst application. In this study, Calopogonium mucunoides leaf extract was used as reducing and stabilizing agent. The synthesized composites were characterized by using Fourier Transform Infra-Red (FTIR), X-Ray Diffraction (XRD), and UV-Visible spectroscopy. The XRD spectra shows the hexagonal phase with wurtzite structure of ZnO and anatase for TiO2. The best degradation performance is 98.26% (only 10 min) for ZnO/TiO2 (5 g) with calcination temperature is 800oC. This is due to the highest distance between two optical phonon mode Δ(LO-TO) and lowest attenuating and propagating constant. The composite ZnO/TiO2 shows high potentials photodegradation of organic dyes with the high stable recyclability up to 5 cycles (> 95%) only for every 15 minutes. High potentials for applicability with the concept environmentally friendly principles and stability for circular chemistry, and efficiency of use the energy and chemicals.


INTRODUCTION
Hazardous contaminants in wastewater are affected by the textile industries, cosmetics manufacturing, pharmaceutical, metals industries, paper making, and agricultural industries [1][2][3]. Azo dye is a hazardous pollutant that contains (-N=N-) bond and phenyl or naphthyl group [4][5][6]. They are producing aromatic amines during the decomposition process and potentially cause carcinogenic and mutagenic. On the other hand, the presence of azo dye in water causes severe effects on clean water availability and the environment [5][6][7][8]. The type of azo dyes is a mono-azo dye (methylene blue, methylene orange, AO7 (acid orange 7)), diazo dye (congo red, 6B (direct lake blue)), and poly azo dyes (direct black BN) [6]. One of the most widely used azo dye is Congo Red (CR). CR is used in processing textile, paper, cosmetic, and pharmaceutical industries [9,10]. Those dye are difficult to degrade to be a severe issue for environmental and human health. The increased contaminant compounds in wastewater are interested researchers in finding the polluted water treatment method to provide clean and healthy water [11].
Water purification methods were varied through a physical, biological, and chemical process [12,13]. The photocatalyst is one of the most effective and simple techniques used in water treatment because it removes organic contaminants easily [11,14,15]. Green synthesis is used as an alternative method to produce nanoparticles for photocatalyst applications. It has several advantages, such as eco-friendly, and it does not produce second contaminants [3]. This method used extracts of various plants as reducing or stabilizing agents. Plant extract contains phytochemicals that act as bioreduction materials for capping agents in particular nanoparticles' synthesis process [16].
The reported materials nanoparticles have been produced by using green synthesis are: Au, Ag, Pt, Pd, α-Fe2O3, CeO2-, ZnO, TiO2, ZrO2, and SnO2 [19][20][21][22]. TiO2 and ZnO are chemically stable semiconductors that can produce an active charge during irradiation by light with suitable wavelengths [12,23,24]. Zinc oxide is n-type semiconductor with high excitation binding energy, bandgap width (3,37eV for anatase phase), biocompatibility, and it is more active when irradiation by visible light. Irradiation process for ZnO will create electron-hole by gain energy of electron for exciting to the conduction band [1]. The electron at the conduction band moves easily to the valence band by the recombination processes. The heterostructure method is effective approach to control and minimize the recombination rate in ZnO consequently increase the charge carrier lifetime [25][26][27][28].
The TiO2 is an effective semiconductor that combines with ZnO nanoparticles displays improvement in photocatalytic activities [29][30][31][32][33][34][35]. TiO2 has higher efficiency as a photocatalytic because it is responsive to the visible spectrum with bandgap 3.2 eV [36][37][38][39][40][41]. It is using as a photocatalytic due to its good optical properties, low cost, and high chemical stability [40][41][42]. The catalyst was used for photodegradation of chemical or organic pollutants in wastewater under visible irradiation [43,44]. Doping TiO2 with metals or non-metal ions will increase the visible light absorbance capacity or reactivity in the UV wavelength [45,46]. The previous reports for ZnO/TiO2 as a photocatalysis with various synthesized methods are: ZnO/TiO2 nanohybrids by using a hydrothermal method for MB, R6G, OTC degradation [1], ZnO/TiO2 by using solid-state for quinoline degradation [29], ZnO/TiO2 by using sol-gel for dye degradation of methylene blue [13,35], ZnO and TiO2 commercial for phenothiazine decolorization [36], and ZnO/TiO2 thin film by using the hydrothermal method for orange G degradation [10]. However, the green, appropriate, and efficient photocatalysts materials that can be easily integrated into wastewater plants and reused for real applications is crucial. The principles process of synthesized these photocatalysts materials should be environmentally friendly, circular chemistry, and increasing the efficiency of the use of energy and chemicals.
For environmentally friendly materials, we synthesize ZnO/TiO2 using Calopogonium mucunoides leaf extract by green method. Calopogonium mucunoides is a legume and easily grow were impacting agricultural if growth uncontrolled and seminatural ecosystems which becoming an environmental problem mainly in Indonesia. By using this type of a legume for photocatalyst, means that contributes to the green environment, low-cost fabrication, efficient, recyclable, and applicability in short time to produce clean water from decontaminant. In this study, a facile, suitable for massproductive, green, and cost-effective method for the fabrication of nanohybrid ZnO/TiO2 photocatalyst for enhanced visible-light was developed. The process of photocatalyst materials potentials to be holistically integrates with environmentally friendly principles, circular chemistry, and efficiency of use the energy and chemicals [47][48][49].
The ZnO/TiO2 composites for concentration of TiO2 are 2.5 g and 5 g, and various calcination temperatures (500 o C, 600 o C, 700 o C, and 800 o C) characterized by using XRD, FTIR, and UV-Vis spectroscopy. The XRD spectra used for analysis the structural properties and FTIR spectra for the optical properties in the form of the refractive index (n) and extinction coefficient (k) by applying Kramers Kronig (KK) relation. The longitudinal and transversal optical phonon mode, the complex dielectric function (real part (1) and imaginary part (2)), energy loss function (Im (-1/ )), and the optical absorption coefficient were determined from the n and k. The photocatalyst efficiency for degradation of CR were analysis using UV-Vis spectrometer spectra.
High recyclability of the ZnO/TiO2 composites as catalyst in photodegradation processes are also reported in this study.
The CM leaves were washed several times by distilled water to remove debris and dirt, then dry them at room temperature for one week. The leaves were powdered using blender and sieved the powder with 100 Mesh for uniform size. The extract was prepared by adding 5 g of CM leaves powder in 100 ml distilled water and stirred at 80 o C for 20 minutes. It was filtered with Whatman No.1 filter paper. The filtrate was used for synthesis ZnO/TiO2, for more detail illustration of synthesized processes see

Synthesis of ZnO/TiO2
Sol gel method was used for green synthesis ZnO/TiO2. The solution prepared with 20 ml CM leaf extracts, 80 ml distilled water, and 5 g Zn(CH3COO)2•2H2O as a precursor. It was stirred at constant temperature 95 o C and speed 500 rpm. After 15 minutes heated, TiO2 was added to the zinc oxide solution. TiO2 solution was prepared 2.5 g and 5 g TiO2 powder by added 10 ml distilled water, then homogenized for 5 minutes by constant stirring at 200 rpm using magnetic stirrer. NaOH was added dropwise to the solution till pH 7. The paste was formed after continuous heating and stirring for overnight. The sample paste was evaporated at 80 o C for 10 hours and continue calcined for 2 hours at the temperature 500 o C, 600 o C, 700 o C, and 800 o C to obtain ZnO /TiO2 powder, the illustration procedure as shown in Figure 2.

Photocatalytic Mechanism
The photocatalytic degradation procedure consisted of adding 0.02 g of ZnO/TiO2 powder into 100 ml of CR solution (40 mg/L). The solution was stirred and exposed to light using 300W Osram Tungsten Halogen lamp as a light source. To analyze the CR concentration after degradation process, the solution was carried out every 5 minutes and filtered it. Concentration of the solution after the degradation process was determined by using UV-Vis spectrophotometer and the percentage of degradation determined by: where D (%) is the percentage of degradation, C0 is the initial concentration (at t is 0 min), and Ct is the concentration after time irradiation t min.
The recyclability of composite in photodegradation was studied by using ZnO/TiO2 (2.5 g) at temperature 500 o C. The 0.04 g of ZnO/TiO2 was added into 100 ml CR solution (40 mg/L). The solution was stirred at 150 rpm by using magnetic stirrer and it was irradiated for 15 minutes. The solution was filtered, and the concentration was analyzed by using UV-Vis spectrophotometer. The composite was washed with distilled water and centrifuged at 1500 rpm within 5 minutes. These steps were applied repeatedly for 5 times. The sediment was dried at 80 o C within 10 minutes. Dried composite was added back into 100 ml CR solution. It was repeated for five cycles.

XRD analysis
The XRD spectra were used to identify the crystalline phase of composites ZnO/TiO2 extracted by green synthesis mediated by Calopogonium mucunoides leaf. [51] as can be seen in Figure 3 (a) and for TiO2 is 5 g in Figure 3 (b).  TiO2 concentration (2.5 g and 5 g) The intensity (101) crystal plane of TiO2 clearly can be seen in composite at the temperature ≤ 600 o C but for > 600 o C the intensity peak reduces drastically or almost disappears may due to the phase change from TiO2 to ZnTiO3. In addition, the (102)  From the XRD spectra in Figure 3 clearly shows relationship between calcination temperatures with the peak intensities. At low calcination temperature (500 o C) the intensity of diffraction peaks are sharp and decrease with increasing the calcination temperature, consequently decrease the crystallite size as can be seen in Table 1 calculated from the Debye Scherrer's equation [3,60,61]: where Γ is the crystallite size, θ is the Bragg diffraction angle, K is the constant about 0.9, λ is the wavelength of X-ray (for Cu is 1.5406 Å), and  is full width at half-length maximum (FWHM). The average of crystallite size, dislocation density, and strain were shown in the Table 1.  [63]. The function group of C-H band at the wave number 1000-1100 cm -1 [14] and at the wavenumber 1447 cm -1 and 1665 cm -1 , there are chemical bonds of C=C-C and C=C tensile vibration, respectively [21,55]. O-H bond appears at the wavenumber 2504 cm -1 and 3455 cm -1 corresponding to vibration band from suspension of hydroxyl group in adsorbed water [3,64]. A weak infrared peak at 2338 cm -1 probably due to the CO2 vibration bond absorbed during calcination process. The peak at 2968 cm -1 is mainly due to the stretching vibration of C-H bond from the absorption of the alkane groups [14].

Optical properties
The optical properties (refractive index (n) and extinction coefficient (k)) were determined from the quantitative analysis of FTIR spectra by applying K-K relation [65][66][67]. For the analysis optical properties, we have used only wavenumber in the range from 860 cm -1 to 1115 cm -1 due to the C=C-C bonding which probably come from the green synthesis methods. The FTIR spectra form the equipment is in the form of transmittance (T(ω)), where need to be converted to the reflectance (R(ω)) [68] by the relations: The reflectance R(ω) as a function of the wavenumber is substitute in the following equations for determining the optical properties (refractive index for real part n(ω) and extinction coefficient for imaginary part k(ω)) [68]: (ω) is the phase change from the incident photon beam bombardment to the sample and traveling at the surface down to few atoms at the sample and then reflected photon beam out which calculated by the following equation: For easily computational of by (ω), the K-K (Kramers-Kronig) relation was used and the new equation of phase change (ω): where ∆ = +1 − and j is series of wavenumber, if j is an odd number, so then i parts is 2,4,6,8,…,j-1, j+1 and while wavenumber j is an even, i parts is 1,3,5,7,…,j-1, j+1.   The real part (ε 1 ) and imaginary part (ε 2 ) of dielectric function functions were calculated from the relation between n and k as follows [66]: The main peak position of the imaginary part (ε 2 ) and the energy loss function Im (-1/1(ω)) were used for confirmation of the TO and LO phonon vibration modes from the intersection point between n and k, respectively. These finding shows consistency result as can be seen in the Figure 5, indicated that the effectiveness of the FTIR spectra for determining optical properties and for identification optical phonon modes. The energy loss function also identified as a plasma frequency as reported in Ref. [65,66,72] from the quantitative analysis of electron spectroscopy [66,73]. The (LO-TO) is important parameters to identified stability of the covalent bond and the lattice match in the ZnO/TiO2 composite [70].
where µ0 is a constant related to the permeability and ε0 is also constant for permittivity in a free space. (right) and opposite trend for 5 gr of TiO2 in ZnO/TiO2 composite as can be seen in Figure 6. The best composite for photocatalyst is for low attenuating and propagating constant, means that the structure will easily suppressing recombination of the charge particle due to the higher strain between the atoms [52,54]. In this study shows 5 gr of TiO2 in ZnO/TiO2 composite high potentials compared than that of 2.5 gr TiO2 in ZnO/TiO2 composite for the same calcination temperature [64]. Band gap of ZnO/TiO2 composites indicated by the arrows at the absorbance spectra in Figure 7. Band gap values were obtained by using Touch Plot method and the corresponding results are presented in Table 4, which determined by the relation as follows:

Bandgap and Photocatalytic Degradation
where α is absorption coefficient, h refers to photon energy, C is band form parameter, m=1/2 for the direct allowed transition, and Eg is band gap energy.
The calcination temperature from 500 o C to 700 o C, the band gap of composite ZnO/TiO2 for 5 g TiO2 is same but for composite with 2.5 g TiO2 is fluctuates due to the effect of cohesion force between the atoms of Zn and Ti. For calcination temperature from 700 o C to 800 o C, the sample for the different concentration of TiO2 shows increases the band gap may due to the effect of particle size and crystal phase in the composite is increase [14,74].
The ability of ZnO/TiO2 as photocatalytic materials for degradation of CR was analyzed using UV-Vis spectrophotometer. Figure 8 shows the absorbance spectrum of CR with ZnO/TiO2 during irradiation, for the solution can be identified visually or by the absorbance spectra at the wavelength 400-600 nm. The time for every sample to degrade CR was different, it influenced by the ability of charge particle to produce hydroxyl and superoxide radical which play the important roles in photodegradation.
The relation between the percentages of degradation with irradiation times are shown in Figure 9 and the corresponding results of percentage degradation for all sample with various calcination temperature and various TiO2 concentration were shown in Table 3. The efficiency of photocatalyst ZnO/TiO2 mediated by Calopogonium mucunoides leaves extract is higher than 94%. The highest efficiency is 98,26% that obtained by ZnO/TiO2 (5 g) with calcination temperature is 800 o C. It indicates the synthesis composites ZnO/TiO2 have a great potential for photocatalytic materials.
Photocatalytic activity was affected by some factors: type of photocatalyst material, crystallite size, and agglomeration level. It means that, the photodegradation process may related to the different competing factors [14]. Based on the previous reported which published by Selvi et.al, a diminution of the crystallite size led to an increase in the specific surface area that could enhance the active reaction of the photocatalyst materials [75].     (kr) in the photocatalytic reaction was calculated by the following equation [60,64]: where C0 is the initial concentration and Ct is the concentration at the time t and corresponding results for kr and R 2 for composite ZnO/TiO2 are shown in Table 4. The kr and R 2 were increased with increasing the calcination temperature for ZnO/TiO2 (5 g) compared to ZnO/TiO2 (2.5 g) and the highest rate constant is for composite ZnO/TiO2 (5 g) at 800 o C which faster degradation only for 10 min irradiation.

Photocatalytic activity
The degradation of wastewater in the photocatalytic process by demineralization of pollutant targets with final molecules are CO2, H2O, and N2 [29]. During irradiation processes, there is interaction between photon with the charge particle in composite increases which consequently the degradation process is faster [1,29]. The schematic of degradation process of CR from the incoming photon, transfer charge (electron and hole) to the final product as shown in Figure 11 for composite ZnO/TiO2. were used to mineralize the organic and inorganic contaminants in wastewater [1,21]. Figure 12 shows the active radicals degraded CR dye start by the breaking the bond of molecules CR and continue with intermediates reaction to mineralize into carbon dioxide, hydrogen dioxide (water), ammonium, and nitrate ion [76].

Recycle
In this work, five cycles successfully show the ability of composite ZnO/TiO2 as a new and high potentials photocatalyst to be holistically integrates with environmentally friendly principles and circular chemistry, minimizing wastewater and efficiency of use the energy, water, and chemicals. Figure 13 shows the excellent performance > 95.02% degradation of CR dye with high stability up to 5 times for every 15 min irradiation. To show the novelty in this study, we have compared the performance of green synthesized composite ZnO/TiO2 in this study with the previous reported references as shown in Table 5. The treatment time and the degradation performance in this study shows high potentially with faster and high stability. It's indicated that the high applicability for the concept holistically integrates with environmentally friendly principles and stability for circular chemistry, minimizing wastewater and efficiency of use the energy and chemicals.

Conclusion
The composite ZnO/TiO2 was synthesized from Calopogonium mucunoides leaf extract by using green synthesis method. The composites were prepared for various calcination temperature (500 o C, 600 o C, 700 o C, and 800 o C) and different concentration of TiO2 (2.5 g and 5 g). From XRD analysis, the crystallite size of composites was influenced by the concentration of TiO2 and calcination temperature during the synthesis. FTIR spectra observed the Zn-O band at the peak 446 cm -1 and O-Ti-O band at the peak 723 cm -1 . The optical properties were analyzed by using K-K relation from the infra-red spectra, it shows the highest energy loss function and distance between optical phonon vibration modes (LO-TO) is in the ZnO/TiO2 (5 g) at temperature 800 o C. It indicates the stable and strong bonding formation and the lattice match in the composite which has effect to enhance photocatalytic activity. According to UV-Vis study, the synthesis of ZnO/TiO2 shows high effectivity in photodegradation of CR dye.
The materials presented the excellent photocatalytic performance with highest degradation efficiency for composite ZnO/TiO2 (5 g) at 800 o C. In this study shows high potentially with faster and high stability, indicated that high applicability concept for holistically integrates with environmentally friendly principles and high stability for circular chemistry, and efficiency of use the energy and chemicals.