Auto-combustion Fabrication and Optical Properties of Zinc Oxide Nanoparticles for Degradation of Reactive Red 195 and Methyl Orange Dyes

A new preparation method has been successfully utilized for the fabrication of zinc oxide nanoparticles using the auto-combustion technique and fuels (tartaric acid: TA, citric acid: CA and a mix between them) with molar ratio (Zn:TA; Zn:CA and Zn:CA:CA = 1:1:0, 1:0:0.55 and 1:0.5:0.275). The as-fabricated zinc oxide powder (ZTA, ZCA and ZCTA samples) annealed at 500 °C. The annealed zinc oxide powder was tested by different techniques like DRS, UV–Vis, FT-IR, HR-TEM and XRD. The Crystallite size of the annealed zinc oxide was estimated to be 24, 39 and 26 nm for ZTA, ZCA and ZCTA samples, respectively. The direct band gap, lattice parameters, unit cell volume (V), the dislocation density (D) and the Zn–O bond length (L) of the prepared zinc oxide powder (ZTA, ZCA and ZCTA samples) were determined. The synthesized zinc oxide nanoparticles are used as nanocatalyst for photodegradation of reactive red 195 and methyl orange as anionic dyes utilizing UV light irradiation. The degradation of reactive red 195 dye was 91–94% after 70 min over the synthesized zinc oxide and the values of degradation increased to be 99–99.8% in 50 min with H2O2 under UV light irradiation. Also, the degradation of methyl orange dye was 57.55–70.57% after 300 min over the synthesized zinc oxide (ZTA, ZCA and ZCTA samples) and the values of degradation increased to be 81–95% in 70 min with H2O2 under UV light irradiation. Finally, the appeared rate constant (Kapp) is determined and the mechanism of the photocatalysis process is suggested for the degradation of the dyes over the synthesized zinc oxide nanoparticles.


Introduction
As a result of global industrialization, various pollutants are produced and contaminated the water resources of our world. Wastewater produced from different industries such as printing, textile, painting, photography, drugs and leather industries and the existence of little quantity of dyes inside the water has large deleterious side effects on living things [1,2]. Textile dyes and pharmaceutical compounds are examples of organic pollutants which considered the toughest pollutants to remove from aqueous media as a result of the chemical bonding with the water molecules. Water pollution requests some noticeable attention because it is the primary reason for the diffuse of some diseases around the world. Methyl orange and reactive red dyes are usually described by the presence of azo group and aromatic rings [3,4]. They have complex aromatic molecular structures and they are considered very toxic to a living organism and resistant to biodegradation. In recent years, the synthesized inorganic nanomaterials, in the form of simple and composite oxides are employed in the inorganic and organic pollutants using photodegradation and adsorption processes [5][6][7].
Recently various synthesis techniques were utilized for the fabrication of inorganic materials such as thermal degradation method [8][9][10], hydrothermal method [11][12][13], sol-gel method [14][15][16][17], precipitation method [18][19][20], combustion synthesis [21][22][23], microwave method [24][25][26], chemical synthesis method [27] and sol-gel auto-combustion method [28,29]. Many inorganic oxides like titanium oxide, zinc oxide, zirconium oxide, stannic oxide and other oxides have utilized as attractive photo-catalysts because of their high catalytic performance in the degradation of organic materials like pesticides, dyes and detergents under sunlight and UV light irradiation [12,30,31]. Zinc oxide is an example of the wurtzite structure and is used as a photocatalyst because it has a comparable band-gap with a value of 3.2 eV which reflected the relatively large activity of zinc oxide [32][33][34][35]. It has great potential applications in room temperature such as ceramic, UV-lasers, sensors, electronics, catalysis, paint and other fields because of the chemical and physical properties like nontoxic, high chemical stabilization, low cost, good photo-electric, high surface area, small particle size, optical and electrical properties [1,2,[36][37][38][39]. Fire combustion synthesis is recognized as low solution combustion synthesis and it employed for the fabrication of simple and complex inorganic compounds [6,23,[40][41][42]. The auto-combustion fabrication technique is used due to the easy, rapid fabrication method, save time, and cut costs of energy. It also utilized to prepare pure, homogeneous, ultrafine and crystalline materials.
In the current work, we pointed to synthesize the zinc oxide nano-catalyst by utilizing auto-combustion synthesis and the prepared powder followed by the annealing to the improvement of the shape of the fabricated ZnO nanoparticles and the degree of crystallinity. The structure, optical and morphology properties of the fabricated zinc oxide are well investigated by different techniques such as DRS, XRD, UV-Vis, FT-IR, and TEM. Methyl orange and reactive red 195 dyes were chosen as organic hydrocarbon dyes according to their environmental importance and the degradation and mechanism of the photocatalysis were studied using zinc oxide nanoparticles.

Materials and Reagents
All chemicals used in this work were purchased and utilized as received without any further purification. Zinc nitrate hexahydrate (Zn(NO 3 ) 3 ·6H 2 O; 98%), reactive Red 195 dye (RR195) and methyl orange (MO) dye were purchased from Sigma-Aldrich company. Citric acid (CA: C 6 H 8 O 7 ; 99.5%), tartaric acid (TA: C 4 H 6 O 6 ; 99.5%) was purchased from El Nasr pharmaceutical chemicals company. The chemical structure and characteristics of reactive Red 195 (RR195) and methyl orange (MO) dyes are outlined in Scheme 1.

Preparation of Zinc Oxide Nanoparticles via Auto-combustion Method
0.02 mol of zinc nitrate and estimated amount of fuels were dissolved in 50 mL distilled water in which zinc nitrate to fuel molar ratios: 1:1, 1:0.55 and 1:0.5:0.275 for tartaric acid, citric acid, and a mix of them (TA and CA), respectively. The oxidizing power of zinc nitrate estimated to be (-10) and the reducing power of citric and tartaric acid detected to be (18) and (10), respectively. The zinc nitrate and fuel ratio estimated for the rules of the combustion method (oxidizing and reducing powers) and Eqs. (1, 2 and 3). The as-prepared solutions were heated with stirring at 80 °C for 10 min to complete the solubility. After homogenization, the solutions were heated at 120 °C to transform into the gel. The viscous gels were ignited on the hotplate at 250 °C until auto-ignition was finished in a few minutes, producing the black ashes and gases.

Scheme 1
The chemical structure and characteristics of the testing dyes: methyl orange (MO) and reactive red 195 dyes (RR195)

3
The as-fabricated ashes were annealed at 500 °C for two hours and the pure zinc oxide nanoparticles fabricated (ZTA, ZCA, ZCTA samples). The names of products, the composition and the ratio of the starting chemicals are summarized in Table 1.

Evaluation of Photocatalytic Studies
50 mg of the fabricated zinc oxide samples (ZTA, ZCA and ZCTA samples) was added to 25 mL of dye at pH 4.2 for reactive red and pH 5.1 for methyl orange at ambient temperature with an initial concentration of 30 mg/L. Philips UV mercury lambs (4 × 20 W at 365 nm) were used in the photodegradation test as the irradiation source of UV-light. The obtained suspension was magnetically stirred for a specific time. Before UVlight irradiation, the dyes solutions with catalyst were stirred in dark for one hour to attain the equilibrium between the adsorption of the dye on the ZnO catalyst surface and the desorption of the dye molecules on the bulk of the solution. Aliquots were withdrawn at regular duration time and centrifuged to separate zinc oxide catalyst from the dye solutions. The separated filtrates of the dye solutions were analyzed by using a UV-visible spectrophotometer (Jasco; model V670). The degradation yield (DY) of organic dyes over zinc oxide catalyst was given by using Eq. (4). Kinetic studies of the photodegradation of the testing dyes over the fabricated zinc oxide nanocatalyst (ZTA, ZCA, ZCTA samples) were tested using first-order models as manifested in Eq. (5). (1) where K F is the appeared first-order constant. S 0 and S t are the initial concentration of the dye and the concentration of the dye after illumination for time t, respectively.

Characterization
X-ray diffraction (XRD) of the sample was obtained using a diffractometer (Bruker; model D8 advance) with monochromatic Cu-Ka radiation, 1.54178 (˚A) in the 2θ range of 15°-80°. FT-IR spectra were measured using FT-IR spectrometer (Bomem; model MB 157S) from 4000 to 400 cm −1 at room temperature. The morphology of the calcined sample was studied using a high transmission electron microscope (HR-TEM, JEOL; model 1200 EX) at an electron voltage of 200 kV and Field emission scanning electron microscope (FE-SEM, JEOL: model JSM-6500F). Diffuse reflectance of the calcined sample studied in the ultraviolet-visible (200-800 nm) using Jasco-V670 spectrophotometer and integrating sphere calibrated with the white standard as barium sulfate.  [20]. The lattice parameters (a and c) were determined as shown in Eq. (7). The lattice constant 'a' is determined from Eq. (7) after reducing the Eq. (6). Also, the lattice constant 'c' is determined from Eq. (8) after reducing the Eq. (6). The crystal sizes (S) of zinc oxide determined by using the Debye-Scherrer Eq. (9). Also, The unit cell volume (V), the dislocation density (D) and the Zn-O bond length (L) [43,44] determined from equations No. (10), (11) and (12), respectively. where λ is the wavelength (1.5406 A for CuKα), θ is the diffraction angle and W is the x-ray full width at a half-maximum height of the diffraction peak.(h), (k) and (l) are miller indices, a and b are lattice parameters, V is unit cell volume, D is dislocation density, S is the particle size in (nm), L is the Zn-O bond length and the value of atom displacement (R) can be calculated from R = 0.25 + a 2 ∕ 3c 2 . Table 2 outlined the lattice parameters (a and c) wurtzite crystal structure of the synthesized zinc oxide (ZTA,

X-Ray Diffraction (XRD) and Structural Studies
ZCA and ZCTA samples) using Eqs. No. 6-8. The value of the lattice parameter (a) was determined according to Eq. (7) and the miller indices of the peak take the form of The average crystal sizes of the calcined zinc oxide nanoparticles (ZTA, ZCA and ZCTA samples) estimated using Eq. (9) as shown in Table 3. The crystalline sizes (S, nm) of zinc oxide nanoparticles synthesized by tartaric acid (sample ZTA) and a mixture from tartaric acid and citric acid as fuels (sample ZCTA), are less than the obtained using citric acid fuel (sample ZCA). Table 3 summarized the values of the average lattice parameter (a and c), unit cell volume (V), dislocation density (D), the Zn-O bond length (L) and average crystallite size of the synthesized zinc oxide. According to the average lattice constants of zinc oxide in Table 3, the ratio between c and a: (c/a) = 1.60251-1.60423 and the values are near to the standard value for the hexagonal cell

FTIR Analysis
The FT-IR spectra of the calcined zinc oxide (ZTA, ZCA and ZCTA samples) in the wavenumber range of 400-4000 cm −1 are shown in Fig. 1b

The Morphology Studies
The morphology of the calcined zinc oxide in the form of Wurtzite nanoparticles was examined by high-resolution transmission electron microscopy (HR-TEM) as shown in Fig. 2a-e. As shown in the HR-TEM, the morphology of the calcined zinc oxide sample reveals spherical and hexagonal shape with hard agglomeration. Figure 3f displays the histogram graph for TEM images and the average particle size calculated from TEM-image is about 52 nm for the ZTA sample. Figure 3a manifests the UV-Vis and NIR diffuse reflectance spectra of the calcined zinc oxide at 500 °C and spectra show the reflectance edge at 375 nm, 373 nm and 377 nm for ZTA, ZCA and ZCTA samples, respectively. Kubelka-Munk   where RE is the reflectance data of the calcined sample and F(RE) is Kubelka Munk function. The direct and indirect band gap of the prepared powder can be determined by using the Eq. (14) [23,45].

Optical Studies
where F(RE) is Kubelka Munk function, F(hυ) is energy function, R is the reflectance of the samples, K is constant and M is the value between 1/2 and 2 based on the direct and indirect allowed electronic transitions, respectively [14]. The direct and indirect band gaps calculate by using the relation between (F(RE)hυ) 1/M and (hυ) as shown in Fig. 4a-f. The values of direct (see Fig. 5a-c and indirect (see Fig. 4d-f) band gaps are found to be 3.13-3.18 and 2.95-3.05 eV, respectively [12].
The photodegradation activity is controlled by the formation of the hydroxyl and superoxide anions radicals, obtained   (a, b), photocatalytic degradation (c, d) and kinetic (e, f) of the reactive red (a, c and e) and methyl orange (b, d and f) dyes over the synthesized zinc oxide nanoparticles (ZTA, ZCA and ZCTA samples) from the photogeneration action of photocatalysts for the starting of the photocatalysis reaction. The energy level of the synthesized zinc oxide can be controlled using the determined bandgap energy and the electronegativity. The conduction and valence bands of the fabricated zinc oxide nanoparticles at the point of zero charge can be determined utilizing the following Eqs. (15)(16).
where E CB , E VB , E g , X and E o are conduction band, valence band, band gap energy, the absolute electronegativity of zinc oxide, the reference electrode redox potential for the standard hydrogen electrode (− 4.5 eV), respectively [7,46,47]. The values of band gap, conduction band and valence band are determined and summarized in Table 4.

Photocatalytic Studies
Photocatalytic activities of the synthesized ZnO nanoparticles (ZTA, ZCA and ZCTA samples) have been studied by testing the degradation of methyl orange (MO) and reactive red (RR195) dyes under UV-light. The intensity of the absorption bands of methyl orange (MO) and reactive red 195 (RR 195) dyes was recorded at 462 nm and 542 nm, respectively. After one hour in dark, the adsorption percentage of methyl orange dye over the ZTA, ZCA, ZCTA is 14.5%, 17.6% and 11.9% respectively. Also, the adsorption capacity of reactive red dye over the ZTA, ZCA and ZCTA is 17.94%, 22.15% and 29.9%, respectively. Figure 5a, b displays the spectra data of the degradation of the reactive red (Fig. 5a) and methyl orange (Fig. 5b) with irradiation time under the influence of UV illumination using ZTA, ZCA and ZCTA samples. The degradation of reactive red 195 (RR195) dye was 94%, 93%, and 91% after 70 min using ZTA, ZCA and ZCTA samples, respectively as shown in Fig. 4c. From Fig. 5d, the degradation of methyl orange (MO) dye was 67.8, 57.55, and 70.57% after 300 min using ZTA, ZCA and ZCTA samples, respectively. According to Eq. (5), Fig. 5e, f materialized the kinetic model in the form of the First order model of the degradation of methyl orange (MO) and reactive red 195 (RR195) dyes under UVlight over the synthesized ZTA, ZCA and ZCTA samples. Table 5 outlined the degradation percentage, R 2 values and rate constant (K, min −1 ) of methyl orange (MO) and reactive red 195 (RR195) dyes under UV-light over the synthesized nanoparticles. The rate of degradation of RR195 dye over ZTA sample is faster than the other samples and photolysis of RR195 dye in 70 min (ZTA > ZCA > ZTCA). Besides, the degradation of MO dye on ZCTA sample is also faster than the other samples and photolysis of MO dye in 300 min (ZCTA > ZTA > ZCA). Figure 6a, b displays the photodegradation of RR195 and MO dyes on the ZTA sample and photolysis of the texting dyes in the existence of the hydrogen peroxide. The RR195 dye degradation on the ZTA sample increased from 94% to 99.8% and the degradation time reduced from 70 to 50 min in the existence of the hydrogen peroxide as manifested in Fig. 6a. The rate degradation of RR195 dye on the produced ZTA sample in the presence of hydrogen peroxide is three times faster than that observed on the fabricated ZTA sample without hydrogen peroxide as displayed in Fig. 6b. Also, the degradation of MO dye over the ZTA sample increased from 68.8% to 95% and the degradation time reduced from 300 to 70 min with H 2 O 2 as manifested in Fig. 6a. The rate degradation of Mo dye on the fabricated ZTA sample with H 2 O 2 is ~ 13 times faster than that observed over the fabricated ZTA sample without H 2 O 2 as displayed in Fig. 6b. Figure 7a

Mechanism of Photocatalysis
The

Conclusions
A new fabrication method for the preparation of zinc oxide nanoparticles (ZTA, ZCA, ZCTA samples) was done using auto-combustion method using zinc nitrate as an oxidizer and fuels (citric acid, tartaric acid, and a mixture of them) as reducing power.  Also, the mechanism of the photodegradation was successfully suggested.