All chemicals used to prepare pure and Cu doped ZnO were purchased from Sigma-Aldrich and used without further purification. The Zn_1−xCu_xO nanopowder (x = 0.0, 0.01, 0.02 and 0.03) were prepared by co-precipitation method using zinc acetate dihydrate as the source of zinc and copper acetate were used as the source of dopants. In typical process zinc acetate dihydrate and copper acetate in their respective stoichiometry were dissolved in ethanol separately. Both the solutions were mixed thoroughly and to this mixture NaOH solution in ethanol were added by constant magnetic stirring for 2 hours at room temperature. The obtained precipitate was separated from the solution by filtration, washed several times with distilled water and ethanol then dried in air at 100˚C and calcined at 450˚C for 8 hours to obtain Cu doped ZnO nanocrystals (x = 0.01, 0.02 and 0.03). Undoped ZnO (x = 0.0) was synthesized by the similar process except with copper acetate.

The photocatalytic degradation of methylene blue was determined by using 25 ml aqueous solution (with initial concentration 20 ppm) and 20 mg of catalyst in a 100 ml quartz beaker. The solution was stirred in dark for 15 minute to allow the equilibrium to take place between methylene blue dye solution and the catalyst. The photodegradations were carried out using five UV tubes (6 W each of wavelength 365 nm) keeping the sample at horizontal position at a distance 15 cm in a batch reactor. At different time intervals 5 ml sample was collected and centrifuged to separate the catalyst from the solution to record the spectra on Shimadzu UV-visible Spectrophotometer (UV-1800). After recording spectra the sample was pour back to original dye solution. All the experiments were carried out at identical condition and at room temperature. The degradation of methylene blue dye solution was noticed by integrating area under the absorbance curve. The percentage degradation (removal) was calculated by using following equation;

where and are the concentration of the methylene blue dye solution at the initial and any other time, respectively.

The crystal structure of Zn_1−xCu_xO samples with concentration x = 0.0, 0.01, 0.02 and 0.03 shown in Figure 1 were determined by X-ray diffractometer (Bruker D8 Advance Diffractometer) with CuKα radiations (λ = 1.5416 Å) in the range of 20˚ to 80˚ at room temperature. The sharp intense peak obtained in all the samples at 2θ ≈ 30.80, 34.22, 35.38, 46.70, 54.88, 62.02, 67.12 and 68.23 corresponds to the lattice plane (100), (002), (101), (102), (110), (103), (112) and (201) respectively confirms that the prepared samples are good crystalline in nature with wurtzite hexagonal structure and are agree with the JCPDS data (01-075-1533).

The structural data obtained from XRD pattern of pure and Cu doped ZnO nanomaterial is tabulated in the Table 1. The plot of lattice parameters “a” and “c” versus Cu concentration is shown in Figure 2. From Figure 2, it is observed that the lattice parameter “c” increase rapidly upto 1%, then slowly increase upto 2% and then decreases rapidly. At the same time the lattice parameter “a” decrease rapidly upto 1%, then slowly decrease upto 2% and then increase rapidly. This may be discussed in details as; the Cu can exist in Cu⁺, Cu²⁺ and Cu³⁺ ions having ionic radii 0.77 Å, 0.73 Å & 0.54 Å respectively. This shows that the lattice parameter “c” increase rapidly upto 1% due to the substitution of Cu⁺ ions into the Zn²⁺ ions since the ionic radius of Cu²⁺ (0.73 Å) is greater than of Zn²⁺ (0.74 Å). The slow increment in lattice parameter “c” from concentration 1% to 2% shows that the substitution of Cu⁺ (0.77 Å) and Cu²⁺ (0.73A Å) ions in Zn²⁺ (0.74 Å) site indicating that the percentage of Cu⁺ (0.77 Å) ions is slightly more than Cu²⁺ (0.73 Å) ions. This means there is a transition phase of Cu⁺ and Cu²⁺ ions from 1% to 2% of Cu dopant. The lattice parameter “c” decreases rapidly after 2% of Cu, owing to substitution of Cu²⁺ (0.73 Å) and Cu³⁺ (0.54 Å) ions in Zn site.

The number of unit cell in particle is calculated by using formula [27] ,

It is observed that the number of unit cell per particle increases upto 2% and then decreases. This shows that the ionic radii of Cu ions leads to shrinking of unit cell per particle in such way that lattice parameter “c” increase as “a” decrease and vice versa.

The volume of unit cell can be determined by using well known formula,

The volume of unit cell versus Cu concentration is shown in Figure 3. It is observed that the volume of unit cell increases rapidly upto 1% of dopant concentration and then slightly decrease up to 2% and then suddenly decreases. This may lead due to the defect or vacancies formation in the transition phase of Cu⁺, Cu²⁺ and Cu³⁺ ions during sintering or diffusion process.

The X-ray density of ZnO sample was calculated by using the formula [28] ,

where, is X-ray density, n is the number of atoms per unit cell, M is the molecular weight of the sample, is Avagadro’s number and V is the volume of unit cell. It is observed that the X-ray density depend on molecular weight of the sample as well as volume of the unit cell.

It is observed from the Table 1 that the average atomic packing fraction (APF) of all samples is ≈0.73 which is in good agreement with the standard wurtzite hexagonal structure. The c/a ratio shows that the isotropic nature of the prepared materials.

A definite line broadening of the diffraction peak (101) of pure and Cu doped ZnO samples is an indication that the synthesized materials are in nanometer range. The crystallite size (D) was calculated from line broadening of the major XRD peak (101) using the Scherrer’s formula [29] .

where, K is the shape factor, which is a constant taken as 0.9, λ is the wavelength of the X-ray radiation (λ = 1.5416 Å), β is the full-width at half-maximum (FWHM) in radians, θ is the Bragg’s angle in degree. The crystallite size of the pure and Cu doped ZnO sample obtained from Equation (4) are listed in Table 1. It is found that the samples synthesized by co precipitation route have grain size between 23 - 29 nm.

In order to understand the peak broadening with lattice strains, various peaks appeared in the XRD pattern were used. The Stokes and Wilson [30] formula given in Equation (5) were used to calculate the strain induced broadening of the Bragg’s diffraction peak.

The W-H plot of versus for Zn_1−xCu_xO samples with concentration x = 0.0, 0.01, 0.02

and 0.03 are shown in Figures 4(a)-(d). It is well known that, in the absence of strain in broadening of peak, the versus plot is expected to be a horizontal line parallel to the axis and in the presence of strain in broadening of peak, it should have a non-zero slope. The obtained values of grain size and strain induced in the broadening of the peak are tabulated in Table 1. It is observed that the strain value increases up to doping 0.02 and then decreases. This may be due to the increment in number of unit cell per particle as a result of substitution of Cu⁺, Cu²⁺ and Cu³⁺ ions in Zn site.

The UV-vis spectra of pure and Cu doped ZnO samples recorded in the wavelength range 300 - 1100 nm at room temperature. The band gap was calculated by plotting the absorption plot (αhν)² versus (Energy, E) shown in Figure 5. The values of energy band gap calculated from Figure 5 are tabulated in Table 1 and it is found that the band gaps of pure and Cu doped ZnO samples are in the range 3.20 - 3.55 eV. It is observed that band gap decrease with increasing Cu concentration upto 0.2 and then increases. This may be due to decrease in strain as a result of increment in number of unit cell per particles.

FTIR spectra (absorption vs wave number) of pure and Cu doped ZnO nanoparticles are shown in the Figure 6. The broad peak in higher energy region at 3740 - 3000 cm⁻¹ is due to O-H stretching and peak in the lower range at 1524 - 1691 cm⁻¹ is due to O-H bending. All other peaks are attributed to the characteristic of the prepared

pure and Cu doped ZnO nanoparticles. The bands appeared near at 1900 - 2354 cm⁻¹ indicates the CO adsorption on the surface of oxide. Similarly the bands at 780 - 980 cm⁻¹ might be due to the peroxide formation (M-O-O-M). The FTIR spectrum of the main absorption band is due to Zn-O stretching of ZnO in the range of 552 - 417 cm⁻¹.

No measurable dye degradation was observed without catalyst under UV light (without catalyst) and in dark (with catalyst). The potential of ZnO nanoparticles towards degradation of dye solution was tested by adding different mass of ZnO (5, 10, 20, 50 and 100 mg) in 20 ppm 25 ml methylene blue solution at pH = 6 under atmospheric pressure and at room temperature. Figure 7 illustrates the effect of catalyst concentration towards the degradation of methylene blue solution. The degradation of methylene blue increases as the mass of ZnO increases from 5 to 100 mg but after 20 mg of ZnO, photocatalytic degradation does not improved significantly. All further catalytic photocatalytic activities were restricted to 20 mg (optimum concentration for 25 ml methylene blue solution). Optimum concentration of the catalyst depends on the experimental setup (working condition) and the incident radiation. Figure 8 shows the absorption spectra of methylene blue solution with pure and Cu doped ZnO respectively, under UV light irradiation. By observing percentage removal curve (Figure 9), it is clear that undoped ZnO is more effective photocatalyst as compared to Cu doped ZnO. The photocatalytic efficiency of ZnO nanoparticles decreases with increasing Cu concentration. This is in accordance with C. M. Teh and A. R. Mohamed [31] , the doped metal oxide had thermal instability and the metal ions doped into metal oxide have been verified as the main cause for the partial blockage of surface sites available for photocatalytic activity [32] .

In the photocatalytic oxidation process the reactive species are h⁺, ^.OH, and ^.O₂. In order to find out the main

reactive species resposible for the degradation of dyes, the scavenger study was performed. In this study ammonium oxalate, benzoquinone and isopropanol were used to remove h⁺, ^.OH, and ^.O₂ respectively [33] . By adding ammonium oxalate, methylene blue solution undergoes degradationin regular manner, while the addition of isopropanol and benzoquinone does not produce any kind of degradation. These results indiacate that ^.OH and ^.O₂ are the main reactive species for all samples in the photocatalytic degradation process.

The scanning electron microscopy (SEM) images of pure and Cu doped ZnO is shown in Figure 10. The SEM

micrograph indicates that the shape and morphology of ZnO nanoparticles changes with increasing Cu concentration. These images revealed that the individual particles were composed by the collection of particles of various shapes with increasing Cu concentration. This indicates that doping of Cu ions influences strongly on morphology of ZnO nanoparticles. These images also show that the agglomeration in nanoparticles increases with increasing Cu concentration and dispersivity, homogeneity of particles was not good. This may be owing to the calcinations temperature which was 450˚C and substitution of different Cu⁺, Cu²⁺ and Cu³⁺ ion in Zn site with increasing Cu concentration.