1. INTRODUCTION:

Copper II oxide bulk nanoparticles generally has the characteristics of stable oxides of copper, but its nanoparticles has change its physical, chemical properties due to its large surface area to volume ratio. It has many spectrums of applications such as electro-optical properties, catalysis, sensors, solar cells and conducting film. Past decades reported the CuO nanoparticles as P-type semiconducting materials (1-2), but its nanoparticles embedded in polymer deviate from semiconducting properties to dielectric properties(9). The high surface to volume ratio of CuO nanoparticles enhances more grain boundaries. There are lot of fabrication techniques reported (3-8) that are available for the preparation of Copper II Oxide nanoparticles, such as, Sol-gel, hydrothermal, chemical precipitation methods, combustion method and refluxing method.

Among these methods, refluxing method has been chosen for the present study of synthesis of Cu/MnO₂nanoparticles, because this method regulates energy in chemical reaction and it can regulate constant temperature on fixing suitable solvent for synthesis of Cu/MnO₂ nanoparticles.

2. MATERIALS AND METHODS:

2.1 Chemical reagents:

All the chemicals were used in this experiment were AR grade. CuCl₂(96%), KmnO₄(99%), PVP(K.30), PVA(k10334) and ethanol were Purchased from Merck. All the chemicals and regents were used as without further purification.

2.2 Instrumentation:

The sample were characterized using analytical method such as the structure of Cu/MnO₂ nanoparticles was taken from XRD (Expert pro). The morphology of the nanoparticles were carried out by SEM (VEGA3 TESCAN).The UV/Vis (model λ35) absorbance spectrum was used to measure the optical properties and energy gap.

2.3 Synthesis of Cu/MnO₂ nanoparticles:

The preparation of copper II manganese dioxide [Cu/MnO₂] nanoparticles were carried out using refluxing method, in this method 0.5 g of CuCl₂ mixed with the solvent of ethanol (100 mL) and the mixer was heated at 78⁰C and then KMnO₄ (0.94 g) solution was added slowly to the parent solutions. The mixture was carried out for 5 hrs in refluxing process, after 5 hrs refluxing we got nanoparticles of black color which confirm the formation of Cu/MnO₂ nanoparticles with and without surfactant PVA and PVP, the synthesized nanoparticles were kept in micro oven for 24 hrs. The samples were taken out from the oven and then air dried.

2.4 Degradation of Methyl orange on Cu/MnO2 nanoparticles:

10mg of methyl orange dye was dissolved in 1000 ml of de-ionized water used as stock solution. 0.1 M of freshly prepared sodium borohydride100 ml of stock solution. 0.1ml of NaBH₄ solution was mixed with 1 mg of prepared Cu/MnO2nanoparticles with 10 ml of methyl orange dye solution. There after the mixture was monitored at using UV-visible spectrophotometer at different time intervals, with gradually decrease in the absorbance. A blank solution was maintained without catalyst, methyl orange dye was observed which indicates the absorbance around at 465 nm. Percentage of degradation calculated using by this eqn,

% Dye Degredation = C₀-C_t/c₀*100

c₀- Initial concentration of dye solution.

c_t- Illuminating dye concentration with respect time

L_n(c₀/c_t)=kt

t- Time

K- Rate constant

2.5 Effect of contact time:

The rate of absorption of dye decreased with the increase in contact time (02 to 10 min) and reached to an optimum value where the absorption-desorption equilibrium was achieved. The maximum absorption capacity of methyl orange on Cu/MnO₂ nanoparticles without surfactant was obtained at 6 min and for with PVA and PVP surfactant 35 and 50 min.This result shows that the removal process was very fast and this is an important advantage of separation method for without surfactant.

2.6 Characterization techniques:

The sample were characterized using analytical method such as the structural of copper II manganese dioxide nanoparticles was taken out XRD (Expert pro ) with and the crystalline nature of Cu/MnO₂ nanoparticles was also analyzed by X-Ray diffraction studies..The UV/Vis absorbance spectrum (model λ35) was taken to measure the energy gap of Cu/Mn_O2nanoparticles and surfactant assisted Cu/MnO₂ nanoparticles using this formula;

E=hC/λ eV (1 eV = 1.602176565x 10^-19J)

Photoluminescence spectrum for Cu/MnO₂ nanoparticles and surfactant assisted Cu/MnO₂ nanoparticles was also taken in order to confirm the intensity of emission behavior of Cu/MnO₂ nanoparticles. Surface morphological characteristics such as particle size, shape and element present have been characterized by Scanning Electron Microscopy (SEM) (Model VEGA3 TESCAN).

3. RESULTS AND DISCUSSIONS:

3.1 X-ray diffraction pattern:

X-ray diffraction study of Cu/MnO₂ nanoparticles synthesized by refluxing method was purely crystalline in nature. The XRD spectra showing the intense peak at 28.41 is having plane (020) which is the crystal plane of Cu/MnO₂ nanoparticles. The low intensity peak at 32.90˚, 35.69˚,35.78˚, 38.81˚,39.28˚,46.45˚, 52.08˚ and 66.55˚ which match well with the plane (110), (002), (111), (200), (111), (111), (112) and (310). This indicates that the prepared Cu/MnO₂ nanoparticles is highly crystalline characteristic of pure monoclinic crystals without having any peak due to the possible Cu/MnO₂ nanoparticles and Cu(OH)₂ impurity and well arrange in specificorientation.The low intensity peak of sample Cu/MnO₂ nanoparticles with PVP 32.05˚, 35.44˚, 35.53˚, 38.71˚, 38.96˚, 46.24˚, 48.74˚, 51.35˚, 56.66˚, 61.52˚ and 65.76˚ which is well-matched with the plane (9-14), (002), (110), (111), (202), (111), (202), (-311), (310). These xrd planes confirmed that Cu/MnO₂nanoparticles are formed. The sample Cu/MnO₂ nanoparticles with PVA of the XRD shown in amorphous nature of the nanoparticles. The crystallite size was estimated using the Scherrer’s equation.

D=Kλ/β cosθ

Where,

λ is the wavelength of copper k line (1.54056A^˚)

θ is the diffraction angle

β is the full width at half maximum of the peak

D is the average particle size,

Calculation.

The estimated sizes are 35.10 nm, 24.10, and 58.93 nm for Cu/MnO₂nanoparticles and surfactant assisted with PVA and PVP of Cu/MnO₂ nanoparticles, respectively. In addition, the values of lattice parameters, strain, volume and dislocation density of the products are calculated and the values are presented in the Table 3.1. The dislocation density and strain was calculated from the grain size using the following equation

δ =1/D².

The strain was calculated by the formula,

ε=βCosθ/4

Where, β is the half width full maximum

Fig:1 Powder XRD Spectra of Cu/MnO₂ nanoparticles

Table 4.1:Lattice parameters, crystallite size, Volume and Dislocation density of Cu/MnO₂ nanoparticles synthesized with and without surfactants

Sample	Crystallite Size (nm)	Strain %	Dislocation density(kg/m³)
Sample	Crystallite Size (nm)	Strain %
Cu/MnO₂ nanoparticles	35.10	0.0823378	0.000811
Cu/MnO₂ nanoparticles Assisted PVP	24.10	0.1418822	0.007123
Cu/MnO₂nanoparticles Assisted PVA	58.93	0.0588125	0.00288

3.2 Morphology Studies:

SEM image of pure Cu/MnO₂ nanoparticles was shown in Fig (a). From the SEM image it was observed nanoparticles were in spherical structure. High magnification image show that no of small crystals are agglomerated structures are grown into large crystal structures and the particle size of 48.33 nm

The SEM image of Cu/MnO₂ nanoparticles surfactant with PVA was shown in Fig (b) From the SEM image it was show that the small cauliflower structures are grown on the surface large crystal and particles size of 80.74 nm. SEM image of Cu/MnO₂ nanoparticles with surfactant PVP was shown in Fig (c) High magnification image show that the small earth worm structures are grown on the surface of the big crystal structure and particles size of 160.66 nm.

3.3 UV-Visible Absorbance Spectra Analysis:

The UV-Vis – NIR spectrum of Cu/MnO₂ nanoparticles with PVA, PVP was recorded in the range of 190 to 1100 nm. The band gap energy of PVA, PVP 2.53 eV and 2.75 eV using double beam spectrophotometer. Band gap of Cu/MnO₂ nanoparticles value is 3.26 eV.Thus ascertain the fact that the nanoparticles with PVA assisted changes from insulator to semiconductor and can be used for optoelectronic application. From the transmission spectrum,

E_g= 1240/λ eV

Where,

E_g - is the band gap energy

h- is the plank constant (6.626x10^-34JS

c- is the velocity of light (3x10⁸m/s),

λ- is the wavelength (nm).

Fig .3: UV–Vis-NIR optical transmittance spectrum of Cu/MnO2 nanoparticles surfactant with PVA, PVP and Cu/MnO2 nanoparticles nanoparticles

3.4 Photoluminescence (PL)

Photoluminescence (PL) emission spectra were recorded for Cu/MnO₂ nanoparticles (a) , surfactant with PVP (b) and PVA (c) in the wave length range of 400 -700 nm with the excitation wavelength of 402 nm and the spectra are shown in the Fig1. The PL spectrum for copper oxide ( a ), surfactant with PVP (b) and PVA (c) has emission bands at 407.21, 407.79, 408.44 nm a strong violet band at 440 nm , and a violet band 421.36, 421.94, 421.36 nm at 440 nm. A weak blue band at 486.00, 486.01, 485.35 nm at 500 and a green band 519.23, 518.58, 518.00 at 570 nm. The PL spectrum of PVA, PVP nanoparticles consists of only two emission bands at 402 nm and strong violet band at 440 nm. It is confirmed that the nanoparticles emit UV light, green , blue and red fluorescence light when they with Cu/MnO₂ nanoparticles (a) , surfactant with PVP (b) and PVA (c)

Fig .4 : PL spectrum of Cu/MnO₂ nanoparticles surfactant with PVA, PVP and Cu/MnO₂ nanoparticles

In another way, strong emission can be assigned for smaller Cu/MnO₂ nanoparticles with energy gap 3.41eV resulting in UV- shift, Where as weak emission is assigned towards agglomeration of small nanoparticles to from bigger ones resulting in dark band gap energy of 2.37 eV.

3.5 Catalytic dye degradation

The dye degradation yield of Cu/MnO₂ nanoparticles of methyl orange (MO) to colorless turned into exploited inside the presence of NaBH₄. UV/visible spectra (fig-5) of a dye confirmed absorbance top at 463 nm. The degradation system had been absorbed UV/vis spectrophotometrically, lower of absorbance at 463nm with time respectively. The decolorization of dye solution acquired inside 8min time duration for Cu/MnO₂without surfactant and for PVA, PVP surfactants were 35 min,50min respectively. Which is indicate complete dye removal of electron- hole pairs of amount of dye taken.

The degradation of MO, decrease slightly with catalyst indicating very slow reaction rate, the Cu/MnO₂ nanoparticles brought about degradation process are quicker reaction charge. The degradation manner because of the absorption and desorption have been obtained the dyes solution turns into equilbrium inside the favorable manner, the electrons are switch from sodium borohydride (SB) to MO dye inside the Cu/MnO₂ nanoparticles, which act as properly catalyst. The system have been in faster response rate.

4. CONCLUSIONS:

Copper/ manganese dioxide Cu/MnO₂ nanoparticles were synthesized with and without surfactants PVA, PVP by hydrothermal technique. Grain size, dislocation density and micro strain of Cu/MnO₂, assisted with PVA and PVP nanoparticles were calculated from the powder XRD. The UV-Visible absorption spectra shows that Cu/MnO₂ nanoparticles is transparent in the entire visible and NIR regions with lower cut-off wavelength at 400 nm. The band gap energy (Eg) for the Cu/MnO₂ nanoparticles, with PVA and PVP were found to be ( a ) 3.26 eV, ( b ) 2.53 eV, and ( c ) 2.75 eV which shown to be the change of electronic properties of PVA assisted Cu/MnO2 nanoparticles from insulator to semiconductor. The dye removal capacity of Cu/MnO₂ nanoparticles was found to be 8 minutes with sodium borohydride and with PVA, PVP surfactants were 35 min and 50 min. The microstructure of Cu/MnO₂ nanoparticles was analyzed using SEM studies and all the synthesized nanoparticles were falls in the nanoscale range (24-58nm). Photoluminescence spectrum shows that the Cu/MnO2 nanoparticles has violet emission at 440 nm. Thus the synthesized Cu/MnO₂ nanoparticles without surfactants (3.26 eV) has high optical properties with wide band gap energy that enhance the catalytic activity of Cu/MnO₂ on methyl orange dye removal along with NaBH₄ catalyst.

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Received on 15.09.2017 Modified on 05.01.2018

Asian J. Research Chem. 2018; 11(2):247-252.

DOI:10.5958/0974-4150.2018.00047.0