Herein, we report the synthesis of copper nanoparticles by simple, cost–effective, eco–friendly wet chemical reduction method using L–ascorbic acid as reducing agent. The synthesized nanoparticles were characterized by X–ray diffraction, electron microscopy (SEM and TEM) and BET surface area analyzer. The surface area of Cu nanoparticles was found to be 10 times higher than the previous reports. The as–synthesized Cu nanoparticles were used as catalyst for the degradation of Rhodamine B (RhB) under both dark and light conditions. The Cu nanoparticles showed noteworthy enhancement in the degradation of RhB organic dye.

Keywords: cu nanoparticles, synthesis, surface area, catalytic degradation, rhodamine B

The fabrication, designing and manipulation of materials within the dimensions of 1–100 nm could be termed as nanomaterials. Nowadays nanomaterials are being used in various fields of physics, chemistry, biology, engineering etc.^1,2 The major advantage of nanomaterials is that their properties changes drastically from bulk material of the same composition. The properties of nanoparticles could be easily altered by varying their size, shape, and chemical environment.³ The earth–abundant and inexpensive metal and metal oxide nanomaterials have acquired significant attention of research community due to their potential applications in catalysis,^4–6 sensors,⁷ antimicrobial activity,^7–11 contrast agents etc.¹² Among the inexpensive metal nanoparticles, copper nanoparticles have attained attentions because of their interesting and high ultraviolet–visible sensitivity, thermal and electrical conductivity, and catalytic properties as compared to metallic gold and silver nanoparticles.

Several chemical routes viz. reverse micelles,¹³ solvothermal,¹⁴ hydrothermal,¹⁵ citrate precursor¹⁶ methods etc. were reported in literature for the fabrication of numerous nanostructures. Copper nanoparticles could be synthesized through different techniques such as metal vapour synthesis,¹⁷ exploding wire method,¹⁸ sonochemical reduction,¹⁹ chemical reduction^20–22 and microemulsion techniques.²³ Among all these methods the chemical methods are of foremost importance because during the reaction copper nanoparticles oxidize to form thermodynamically stable copper oxide and to avoid oxidation, the reactions were carried out in inert media such as argon, nitrogen and also by using reducing, capping or protecting agents for the reduction of copper salt used.^24–26 In the chemical reduction method conventional expensive and toxic reducing agents have been used for the reduction of metal ions to zero valent metal.²⁷ To avoid the toxic effects we have used ascorbic acid as a reducing agent which also acts as a protecting agent to cease the oxidation reaction.

Rhodamine B (RhB) is a heterocyclic fluorescent dye used as a colorant in textile and food industries. It is also known for water tracer fluorescent.²⁸ Being carcinogenic and toxic in nature, it adversely affects the humans and animals.²⁹ The textile and food industries were constantly discharging this toxic dye in the form of effluents in water and increasing the problem of water contamination.³⁰ In the present work, we have used the as–synthesized Cu nanoparticles for the catalytic degradation of RhB dye in dark as well as light conditions and it was found that the Cu nanoparticles were highly efficient for the degradation of organic pollutant.

Materials

Analytical grade chemicals were used throughout the experiment unless otherwise stated. All the chemicals were used as received without further purification. Copper (II) acetate monohydrate (98%), L–Ascorbic acid (99%) and Ethanol (99.9%) were procured from Merck India Ltd. Double distilled water was used throughout the experiments. All the glass wares were washed thoroughly with nitric acid followed by distilled water.

Synthesis of copper nanoparticles

In a typical procedure 0.05M Copper (II) acetate was taken in a beaker and heated the solution at 90 °C on an oil bath with constant stirring. The color of the solution changes from royal blue to colorless (at 90 °C). At this moment 0.6 M L–Ascorbic acid solution was added dropwise to the above solution with vigorous stirring. The mixture was kept at 90 °C until a dark brown colored precipitate was obtained. To collect the precipitates, the dispersion was centrifuged at 8000 rpm for 20 min and washed them 2–4 times with double distilled water and finally with ethanol. The collected precipitate was dried in an oven at 90 °C and ground to the powder. The detailed procedure for the synthesis of copper nanoparticles is shown in Figure 1.

Figure 1 Schematic representation of synthesis of Cu nanoparticles.

Characterization techniques

The Rigaku Ultima IV with Ni–filter (for Cu–Kα, λ = 1.5416 Å) powder X–ray diffractometer was used at the step size of 0.05° and step time of 1 sec, from 10° to 80° (2θ). The Kα2 reflections were removed by a normal stripping procedure to obtain the accurate lattice constants. The morphology of the synthesized nanoparticles was determined by JSM 6390 LV scanning electron microscope (SEM; JEOL, Tokyo, Japan). FEI–TECHNAI G² 20 Transmission electron microscope (TEM) was used to determine the shape and size of as–acquired nanoparticles at the accelerating voltage 200 kV. The TEM measurements were done by placing a drop of sonicated sample on a carbon coated copper grid. The surface area and the pore size of as–synthesized nanoparticles were determined using BET surface area analyzer (Model: Nova 2000e, Quantachrome Instruments Limited, USA) at liquid nitrogen temperature (77K). Approximately 0.06g of the samples was degassed at 90 °C for 3 h in a vacuum degassing mode to remove the surface contaminants. The degassed sample was then subjected for the analysis and the data was recorded by admitting known quantities of adsorbing N₂ gas into the powder sample. As the adsorption occurs, the pressure in the cell changes until the equilibrium was attained. The specific surface area was calculated using the multipoint BET equation. The pore size distributions were determined from the N₂ desorption isotherms at 77 K, using the Dubinin–Astakhov (DA) method.

Catalytic measurements

The catalytic efficiency of copper nanoparticles was measured against the degradation of Rhodamine B (RhB) dye under the dark and light conditions. The degradation process was studied by UV–Vis spectrophotometer (Agilent 8453) by analysing the change in absorption maxima λ_max=553 nm of RhB. 0.5 mg of Cu nanoparticles was dispersed in 20 mL RhB (1x10^–5 M) and 0.5 mL aqueous solution of NaBH₄ (0.1 M) was added with constant stirring. The degradation process was carried out at room temperature at pH 7. Thereafter, the catalytic process was checked in dark as well as in light conditions. In every 5 min interval an aliquot was taken to study the absorption spectra. The catalytic degradation reaction of RhB dye was also in checked in the presence of NaBH₄ only. All the experiments were repeated three times with excellent agreement.

The phase crystallinity and structural analysis of the as–synthesized copper nanoparticles was done by powder X–Ray diffraction technique. Figure 2 represents the diffraction pattern of Cu nanoparticles in which the peaks at 2θ value of 43.28°, 50.40° and 74.81° correspond to (111), (200) and (220) planes respectively, confirms the cubic lattice of copper. All the diffraction peaks are in good agreement with the standard pattern for pure face centred cubic phase of copper nanoparticles (JCPDS No. 040836). No impurity peaks of CuO or Cu₂O were observed. The very intense peaks indicating the highly crystalline nature of sample and the significant broadening of peaks attributed to the nanocrystalline nature of particles.

Figure 2 x-Ray diffraction patterns of copper nanoparticles.

The surface morphology of the as–synthesized nanoparticles was estimated by Scanning electron microscopic studies, as shown in Figure 3. The SEM image clearly indicates that the particles were very small in size and spherical in shape. The SEM image shows slight agglomeration of nanoparticles which could be attributed to the electrostatic attraction between the as–synthesized nanoparticles.

Figure 3 SEM image of copper nanoparticles.

The particle size of the as synthesized copper nanoparticles was examined by transmission electron microscopy (TEM). The TEM micrograph of as–synthesized copper nanoparticles is shown in Figure 4a. The TEM micrograph confirms the formation of monodispersed spherical Cu nanoparticles. The size of the as–synthesized nanoparticles was found to be in the range of 1–5 nm with an average grain size of 3 nm. Figure 4b shows the size distribution histogram of the as–synthesized Cu nanoparticles.

Figure 4 (a) TEM micrograph and (b) size distribution histogram of copper nanoparticles.

Surface area plays an important role to determine different properties of the material, for example with increase in surface area the catalytic properties of nanoparticles are enhanced to the large extent due the availability of more surface–active sites. Therefore, to find the application of as–synthesized Cu nanoparticles as a catalyst it becomes imperative to analyse the surface area of nanoparticles. The surface area and the pore radius of as–synthesized Cu nanoparticles was determined by multipoint BET equation with relative pressure (P/P_o) in the range from 0.05 to 0.35.³¹ The specific surface area of these nanoparticles was found to be 365m²g^–1 as shown in Figure 5a. The as–synthesized Cu nanoparticles show 10–time higher surface area as compared to the previous studies of Cu nanoparticles.³² Figure 5b shows the DA plot, which determines the pore radius of Cu nanoparticles and it was found to be ~15 nm. The average particle size of nanoparticles is also estimated using surface area by assuming the particles have spherical shape and smooth surface using the equation:

$D_{BET}=6000/\left(\rho .S\right)$

where, D_BET is the average diameter of the particles in nm,  is the density in gcm^–3 and S is the measured surface area of nanoparticle in m²g^–1. The average particle size was found to be 3 nm which is in support with the TEM results.

Figure 5 (a) BET plot and (b) DA plot of Cu nanoparticles.

Photocatalytic activity

The catalytic activity of as–synthesised nanoparticles was demonstrated by monitoring the degradation of RhB in the dark and light for 25 min and 15 min, respectively. Figure 6a shows the UV–visible absorption spectra of catalytic degradation of RhB dye with Cu nanoparticles and NaBH₄ in dark. A strong characteristic peak at 555 nm corresponds to the λ_max of RhB dye. In this report the degradation of RhB in presence of NaBH₄ was studied in dark as well as in light using as–synthesized Cu nanoparticles as catalysts. From the studies it was observed that Cu nanoparticles take 25 min to completely degrade the RhB dye. Similar experiment was carried out in presence of sunlight and it was observed that in presence of sunlight the catalytic activity of the Cu nanoparticles is further enhanced as compared to dark conditions. In light conditions it was observed that Cu nanoparticles take much lesser time (15 min) to completely degrade the RhB dye. The UV–visible spectra of degradation reaction of RhB dye with Cu nanoparticles and NaBH₄ in dark conditions and in presence of light is shown in Figure 6a & Figure 6b respectively. Figure 6a reveals that the characteristic peak of RhB appeared at 555 nm and in dark reaction conditions the peak intensity decreased with the time (till 25 min). Similarly, figure 6b represents the UV–visible spectra of RhB dye under sunlight, Cu nanoparticles and NaBH₄, it was observed that intensity of characteristic peak at 555 nm decreases with increase in time (till 15 min). The similar experiments were done for 60 min without Cu nanoparticles (i.e., with NaBH₄ only) under dark and light conditions and no change in the intensity of absorption maxima of RhB was observed. So, it could be assumed that the degradation reaction takes place on the surface of the Cu nanoparticles which is further enhanced in the presence of sunlight. Since the synthesized nanoparticles were very small in size therefore the surface to volume ratio will be high providing more surface to enhance the catalytic property of the nanoparticles.

Figure 6 UV-visible absorption spectra of Cu nanoparticles (a) in dark and (b) in presence of sunlight.

The possible mechanism for photo–catalytic degradation of RhB dye by the Cu nanoparticles could be described by following equations: In

$Cu+h\upsilon \to Cu\left(e^{–}+h^{+}\right)$

$Cu\left(h^{+}\right)+H_{2}O\to Cu+O{H}^{•}+H^{+}$

$Cu\left(h^{+}\right)+H{O}^{–}\to Cu+O{H}^{•}$

The electrons (e^–) of Cu nanoparticles reduce the molecular oxygen (O₂) to hydroperoxyl radicals (i.e. protonated form of superoxide):

$Cu+O_2+e^{–}\to Cu+O_2{{}^{•}}^{–}$  

So, the degradation of RhB dye could takes place either by the direct oxidation on the surface of photocatalysts or by hydroxyl radicals:

$O{H}^{•}+RhB\to C{O}_2+H_{2}O$ (oxidized products)

Figure 7a shows the catalytic efficiency of as–synthesized Cu nanoparticles, which was calculated by plotting the change in relative concentration i.e. C/C₀ verses time for RhB dye under dark and light conditions. Further the percentage removal of dye was calculated by using equation:

$D\left(\%\right)=\left(C_0-C/C_0\right)\times 100$

The percentage removal of RhB dye from aqueous dye solution was shown in Figure 7b and it was observed that Cu nanoparticles were able to removes ≈98% of dye within 15 mins when exposed to sunlight and in 25 mins in dark. There is found to be remarkable enhancement in the catalytic efficiency of Cu nanoparticles compared to the previous reports.^33,34 The enhanced catalytic activity of as–synthesized Cu nanoparticles could be attributed to the high surface area of nanoparticles as discussed in BET surface area studies.

Figure 7 (a) Catalytic efficiency and (b) percentage dye removal of RhB dye with time using Cu nanoparticles in light and dark conditions.

We have successfully synthesized copper nanoparticles via simple wet chemical reduction route. The as–synthesized nanoparticles have cubic structure with an average particle size of 3 nm. The synthesized nanoparticles have 10 times higher surface area as compared to the literature. The catalytic degradation of organic pollutant (RhB dye) was investigated on the surface of the Cu nanoparticles under both light and dark conditions and it was observed that the as–synthesized nanoparticles degrade ≈98% of dye. The acquired results suggest that the as–synthesized Cu nanoparticles could be used as effective catalyst for the degradation of organic pollutants in waste water treatment or environmental remediation.

TA thanks DST & CSIR, New Delhi, Govt. of India for financial support. Authors are thankful to CIF, Jamia Millia Islamia for X–ray diffraction studies, AIIMS, New Delhi for electron microscopy studies. UF and RP thanks to UGC for research fellowships.

Author declares there is no conflict of interest.

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