Electrochemically crafted monodispersed γ-Cu 5 Zn 8 bimetallic nanoalloy: characterization and its synergistic effect on photocatalysis and antibacterial efficacy

: The present work reports the investigation of monodispersed nano brass prepared by the facile electrochemical technique. The influence of reaction conditions on the surface morphology and size of the bimetallic nano alloy were investigated through high resolution scanning electron microscopy (HR-SEM) and powder X-ray diffraction (PXRD) technique. From the powder XRD analysis the nano alloy was identified as a γ − Cu 5 Zn 8 phase. The detailed investigation was carried out using transition electron microscope (TEM) for the textural features and γ-CuZn phase stoichiometry was revealed by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDAX) analyses. Optical responses were analysed using diffuse reflectance spectroscopy (DRS). The energy band gap obtained from the optical spectrum revealed it can be a good catalyst. Rate of photocatalytic reaction and mechanism of degradation were discussed in detail. Most importantly, electrodeposited γ-CuZn nano alloy reveals superior nano-catalytic activity on methyl orange dye degradation in the shorter irradiation time. Furthermore, the antimicrobial efficacy of the γ-CuZn nano alloy was examined using gram positive and gram negative bacterial strains.

The effect of applied current is the primary factor which determines the nature and size of the nano-alloy. Secondly the electrolyte concentration of the corresponding metal ions from which the γ − phase of nano-alloy is determined [10]. The preparation methods associated with chemical means, electrochemical route for the synthesis of alloy/bimetallic nanostructures have extended substantial research attention in recent times owing to its practical benefits, for instance easy, fast preparation, and no toxic reagents are used. Controlled synthesis of shape, size, and composition in addition to the thickness of bimetallic nanoparticles can be achieved through varying electrodeposition parameters [11,12].
Organic effluents from dye and food industries represent major contamination to the water resources. A major portion of commercial dyes are released out untreated as leachate that leads to environmental pollution to the large extent. Methyl orange is a cationic diazo compound from the azo dye family. It is commonly used in food industries as a colouring agent and in textile industry for dyeing the synthetic materials as it does not work on cotton. It is mutagenic in living organisms and can cause permanent damage to the lives. It normally causes skin and eye irritation, and it has hazard in the solid form that can damage the lung tissue which increases the heart rate as a consequence.
With the never-ending environmental alarms, the quest for active catalyst having good response to the visible light photo catalysis for effective environmental remediation is still continuing and materialized as an important insertion area of research [13,14]. Environmental friendly photocatalytic materials are preferred as an effective photocatalysts for dye degradation. Semiconductor materials were highly focused for the photocatalytic applications for their unique features [15,16]. Though these semiconductor materials fails to be an active material in absorption of visible light and are fast in recombination of photo generated positive hole and electron pair. Hence, hybrid nanomaterials with hierarchical structure with enhanced photo response offers very good photocatalytic activity [17,18].
Owing to enhanced catalytic activity nano alloys are gaining more importance compared to their metallic counterparts [19][20][21][22][23][24][25][26]. Owing to the unique properties of nano alloys, they have been used as the functional materials in sensors, optics, electronic devices, and catalysts. The nature of the bimetallic NPs can be a core-shell structure otherwise in alloy form based on the preparation conditions. Amongst a number of bimetallic NPs, remarkable attention has been devoted to bimetallic nanostructures of Copper for outstanding characteristics [21][22][23]. Few reports have been available for the preparation of Cu/Pd nano alloy using various techniques [24][25][26][27]. On the other hand, as far as our knowledge is concerned, there is no literature on the electrochemical synthesis of γ-CuZn bimetallic nano alloy with its photocatalytic and antibacterial activities. Cu/Zn nano alloys are cost-effective compared to Ag NPs and Au NPs. Hence, we prepared stable γ-CuZn bimetallic nano alloy without a chemical reducing agent by the facile electrochemical method. The photocatalytic activity of γ-CuZn nano alloy was studied using methyl orange as a model dye.
Further, the well-diffusion assay (microdilution process) technique was used to evaluate the antimicrobial activity of γ-CuZn nano alloy on gram-positive microorganism bacillus subtilis and staphylococcus aureus (MRSA) and gram-negative pathogens Klebsiella pneumoniae and E. coli.

Materials
Analytical grade sulphate salts of copper and zinc ( 4 . 5 2 ) and ( 4 . 7 2 ), were used as the precursors for the bimetallic CuZn nano alloy. Sodium sulphate (Na2SO4) was used in required proportion as a supporting electrolyte and the electrolyte pH is controlled by the reducing agent Hydrazine sulphate monohydrate ( 2 6 4 . 2 ), conductivity water (Merck) was used to prepare the electrolyte. The electrolyte was pre-treated with 2 6 4 for the inert atmosphere in the electrolyte. Carbon electrodes of 75 mm long with 12 mm diameter were used as working and counter electrodes. The carbon rods were degreased with acetone followed by washed with, deionized water before the electrolysis. The electrodeposition of CuZn bimetallic NPs was carried out at a constant current of 15 mA per second using a regulated power supply unit in the acidic condition.

Synthesis of CuZn bimetallic nano alloy
Electrodeposition was selected for the synthesis of CuZn bimetallic NPs because of its cost-effectiveness, simplicity, and ease of availability. Copper is more electropositive (0.34 eV) compared to zinc (-0.76 eV), consequently, copper will preferentially be deposited. However, the bimetallic NPs obtained contained predominantly Zn.
At lower current densities, copper ions have sufficient time to diffuse in from bulk solution and replace the ions being deposited. At a higher applied current, the molarity difference between copper and zinc becomes more significant at the working electrode. Copper is deposited at its limiting applied current, but zinc deposition is restricted. Copper ions slowly diffuse in from the bulk solution while sufficient Zn ions are available for deposition to occur. Zinc is preferentially deposited as the copper ion concentration is considerably reduced in the electrolyte. CuZn bimetallic deposited at the high-applied current was found to be rich in zinc while the deposition at the low-applied current was found to be rich in copper and the interface area between the two electrodes showed a smooth variation in composition. The formation of CuZn alloy is depicted in the following scheme.

Instrumentation and Analysis
Various physio-chemical characterization techniques have been used to characterize as synthesized nano-catalyst. In order to delineate the phase structure of the CuZn bimetallic NPs; Xray Diffractometer (XRD) Siemens-D-500 with Ni-filtered Cu Kα radiation operated at 30 kV with 10 mA emission current. Information on diffractions were picked up over the range of 2θ from 10° to 80 °C with a scan rate of 7°/min. High Resolution -Scanning Electron Microscope FEI Quanta FEG 200 SEM with EXS was used to observe the surface morphology and the stoichiometry the nano alloy. Diffuse reflectance spectrometer Shimadzu 2100 with BaSO4 as the reference material was used for optical characterisations. Functional group analysis was carried out with Fourier Transform Infra-Red (Perkin Elmer RX1 instrument) spectrophotometer. Transmission Electron Microscopic (TEM) studies were recorded by using JEOL 3010 UHER pole piece model instrument.

Photocatalysis experiment
The photo degradation process was optimized with 0.01g/L of nano-catalyst and 50 mg/L of dye concentration. Before the irradiation process, the dye solution is mixed with the nano-catalyst for 30 min in the dark in order to achieve the equilibrium and subsequently irradiated. A small portion of the aliquot was withdrawn and analysed for its absorbance measurements. The efficiency of degradation percentage is calculated using the formula.  High crystallinity is an advantageous feature that determines the photocatalytic efficacy of the bimetallic. The defects are performing the role of entrapping/recombination centres for photogenerated electron-hole pair, causing decrease in photocatalytic efficiency. Larger particles will possess higher recombination probability, which is an undesirable quality for photocatalytic applications. Hence, higher the crystallinity smaller is the defects in it. These findings clearly indicate that the body centre cubic (BCC) CuZn bimetallic nano alloy was synthesized from the electrodeposition technique. Especially the X-ray diffraction analysis show that the sample prepared in the optimized condition (in accordance with stoichiometry ratio) exhibited good crystalline nature.  fig. 3 (a-c). Table 1 summarizes the geometric parameters of the CuZn bimetallic synthesized from various reaction conditions.

Preferred Orientation (Texture Coefficient)
The texture (preferred orientation) of the γ-CuZn was quantitatively analysed using Harris method of determining both the degree of preferred orientation and texture coefficient as shown in the fig. 4. Where I(hkl) is the measured peak intensity of the (hkl) plane for the i th peak, I 0 (hkl) the reference (JCPDS) intensity for the corresponding(hkl), and N reference the total diffraction peaks in the diffractogram. For arbitrarily oriented crystallites, the texture coefficient is always unity ( (ℎ ) = 1) however deviation from unity point toward the preferred growth of the nano alloy.
The TC calculations revealed the (222) plane as the preferred angle for its random orientation in the γ-CuZn bimetallic alloy and the results are presented in table 2.  that Cu is in the reduced form. Fig. 6 shows the normalized and background corrected images of the alloy and the Auger spectrum meant for Cu/Zn is shown in fig. 6 (bc). This information evidently indicates that Cu:Zn nano brass is present in the reduced form in the optimized reaction conditions. However, more pronounced Zn/Cu signals are observed which can be attributed to the reduced form of these metals in the alloy or lower amount of oxidized form which in turn intensify the sensitivity to bimetallic γ-CuZn recognition. This stresses the advantages of adopting this method for the nano alloy synthesis. Furthermore, it is perceived that Zn L 3 M 4 , the 5M4,5 stroke showed a shift towards the lower binding energies as a consequence the reduction of metal ions is carried out at room temperature. high resolution XPS 2p spectrum showing Zn 2p3/2 peak at 443 and 2p1/2 peak at 465 eV, (c) spectrum for Cu 2p3/2 peak at 535 and 2p1/2 peak at 555 eV

FT-IR spectral analysis
The FTIR study has been carried out to identify the elements and their phase, which is expected to be adsorbed on γ-CuZn nano surface during sample preparation. The infrared spectrum of the CuZn bimetallic nanoparticle synthesized from electrodeposition technique is shown in Fig.   7. The IR spectrum showed a broad and low intense peak near mid-IR region. In the case of metal, the adsorbed impurities on the surface of the particles are normally found near the mid-IR region.
This can be ascribed to the CO 2 concentration from the atmosphere on the surface of the metal and it can be understood from the spectrum that, the high surface area of the nano alloy facilitated the ease of adsorption of more CO 2 on the surface and the major frequency of absorption is seen near 2036 cm -1 [27].

EDAX spectrum of γ-CuZn bimetallic alloy
The EDAX spectrum mapping is presented in Fig. 8 clearly established the uniform distribution of copper and zinc in the γ-CuZn bimetallic alloy. This result is in good agreement with TEM observations. It was also observed that the atomic percentage of both the metals in the nano alloy synthesized from varied reaction parameters showed appreciable change. Surprisingly the atomic composition of Cu and Zn in the nano alloy did not show much difference as the applied current and pH of the electrolyte are varied. However, the EDX analysis displayed CuZn ratio of 28:35 with little oxide formation indicating that the electrodeposition is highly influenced by the concentration of metal ions in the electrolyte at low current densities as shown in the Fig. 8. These findings indicate that Cu-ions being nobler than Zn have gotten reduced at the cathode first since the migration of Cu 2+ ions faster at higher current densities and higher molar concentrations. The EDX results show that the alloy contains on average about 60 mole % of copper and 40 mole % Zn. It is also concluded that the large amount of Cu in the alloy apparently makes the CuZn nano alloy highly stable. Figure 8. EDX analysis of CuZn alloy

Diffuse reflectance spectroscopy analysis
Diffuse reflectance spectroscopy (DRS) was employed to disclose the optical properties and electrical band gap of γ-CuZn nano brass. Optical absorption is the characteristic property of the metals in particular for the bimetallic nano alloy systems. According to Mie theory, isotropic nanomaterials alone show single absorption band in the UV-Visible spectrum, while dispersed anisotropic particles with different surface morphologies exhibit two or even three absorption bands when the particle size falls below 10 nm. Fig. 9 a) demonstrates the absorption spectrum converted from the diffuse reflectance measurements for CuZn bimetallic alloy obtained from optimum reaction conditions. A quadrupole resonance band can be seen at shorter wavelength indicating the small particle size of the CuZn bimetallic nano alloy. The sharp peak with high intensity confirmed the formation of CuZn alloy and the Shift in peak position is observed for the CuZn bimetallic towards the shorter wavelength. On the basis of the observed surface plasmon band, it is observed that structure of the formed CuZn is cubic. The copper predominant alloy absorbed around 458 nm, while the alloy with Zn-rich composition is observed in the low wavelength.    Fig.11.b shows the effect of catalytic load on the photocatalytic degradation reaction. In order to evaluate the optimal dosage amount of the prepared catalysts; the catalytic weight was varied and the optimized amount was found to be 10 mg/L. Moreover, below this optimum load, leads to increasing surface active site which results rapid degradation. Above the optimum load, increases the scullery turbidity, as well the light penetration decreases and thus, availability of OH and O2becomes minimal.