Journal of Industrial and Engineering Chemistry 31 (2015) 173–184

Electrochemical synthesized copper oxide nanoparticles for enhancedphotocatalytic and antimicrobial activity

Rishu Katwal a, Harpreet Kaur b, Gaurav Sharma a, Mu. Naushad c,*, Deepak Pathania a,*a School of Chemistry, Shoolini University, Solan 173212, H.P., Indiab Department of Chemistry, Punjabi University, Patiala 147002, Punjab, Indiac Department of Chemistry, College of Science, Building#5, King Saud University, Riyadh, Saudi Arabia

A R T I C L E I N F O

Article history:

Received 8 April 2015

Received in revised form 4 June 2015

Accepted 28 June 2015

Available online 4 July 2015

Keywords:

Nanoparticles

Copper oxides

Electrochemical

Reaction parameters

Antibacterial activity

Photo degradation

A B S T R A C T

The electrochemical method has been investigated for the synthesis of copper oxide nanoparticles (CuO

NPs) under different reaction conditions. The CuO NPs were used as excellent photocatalyst for the

degradation of different organic dyes under the illumination of sunlight irradiation. The highest

degradation was 93% for methylene blue. The rate constant for MB, MR, and CR was found to be first-

order with values 0.02059, 0.02046, and 0.01749 min�1, respectively. The antimicrobial efficiency of CuO

NPs was investigated against bacterial strains (Escherichia coli and Staphylococcus aureus) and fungal

strains (Aspergillus nigres and Candida albicans).

� 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

Introduction

Nowadays nanomaterials are of immense interest due to widerange of applications in chemical, biological, and environmentalsciences [1–3]. The size and shape of the nanomaterials are keyfactors for shaping their properties such as, electrical, optical,magnetic, catalytic, and antimicrobial. Metal and metal oxidenanoparticles have found wide variety of uses, including hetero-geneous catalysts, colloid science, environmental remediation,electronic, optoelectronics, chemical sensing devices, medicinalapplications, separations, thin films, inks, disinfection, andantimicrobial activity [4–6]. The different applications of metaland metal oxide nanoparticles varied with morphology and size[7,8]. Among the metal oxide nanoparticles, copper oxidenanoparticles have been potentially used for the PN junctiondiodes, humidity sensing, lithium ion battery, organic synthesis,antimicrobial activity and biomedical field [9–11]. Copper(II) oxidewith a narrow bandgap (Eg) of 1.0–2.08 eV behaves as a p-typesemiconductor. The semiconducting nature of metal oxides makesthem important for solar energy conversion, photocatalysis,

* Corresponding authors. Tel.: +91 9805440648; fax: +96614674198.

E-mail addresses: shad81@rediffmail.com (M. Naushad),

dpathania74@gmail.com (D. Pathania).

http://dx.doi.org/10.1016/j.jiec.2015.06.021

1226-086X/� 2015 The Korean Society of Industrial and Engineering Chemistry. Publis

antimicrobial, and antifouling applications [12,13]. In the pastfew years, different methods such as wet chemistry route,sonochemical preparation, alkoxide-based preparation, hydrother-mal process, sol–gel, microemulsions, spray pyrolysis etc., havebeen reported for the preparation of the nanomaterials [14,15]. Thetoxicity and relatively high material cost of these methodsrestricted their use in a better way. So a simple, direct, and greenroute has been needed for the preparation of metal oxidenanoparticles. The electrochemical method, a cost effective andresourceful process has been reported for the synthesis of metaloxide in nanodomains [16]. This process is suitable for thesynthesis of high surface area and highly efficient for noble metaloxide. Environmental pollution has been caused some unfavorableeffects on the living organisms and breaks the balance of theecological unit. The presence of organic waste and microbialcontamination created a serious problem to human being. Theremoval of organic pollutants and microbes from wastewater hasbeen the matter of increasing attention in the recent years.According to United States Environmental Protection Agency, water-related diseases kill a child every eight seconds, and are responsiblefor 80 percent of all illnesses and deaths in the developing world.Photocatalysis has been attracting much attention recently due tothe wide range of applications in renewable energy self-sterilizingsurfaces, photocatalytic lithography, microchemical systems, greensynthesis of organic compounds, and the generation of hydrogen and

hed by Elsevier B.V. All rights reserved.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184174

environmental remediation [17,18]. Photocatalytic degradation oforganic dye such as methylene blue, congo red, and methyl red isbeing paid much attention due to environmental effect of variousindustrial and agriculture pollutants. Copper oxide nanoparticleshave been selected as photocatalyst and antimicrobial agent becauseof its low cost, high catalytic efficiency, and narrow bandgap. Theypossess optical, catalytic, mechanical, and electrical properties[19,20]. Copper oxide has been used for photoelectron chemicalhydrogen production, printing electrodes, possess antimicrobialactivity, and catalytic activity. The platelet-like CuO nanostructureshave been demonstrated efficiently as catalyst for the degradation ofmethylene blue dye from the aqueous solution [21]. Hong et al. hasreported that urchin-like CuO microspheres for the degradation ofpyronin [22]. Flower like Cu2O has been also reported as photo-catalyst for the brilliant red X-3B dye degradation in solar light[23]. Furthermore, it is essential to improve the photocatalyticactivities of copper oxide nanoparticles for the degradation oforganic impurities. It has been revealed from the literature surveythat only limited applications are available on photocatalysis in thepresence of sunlight irradiations for the degradation of organicimpurities. In the photocatalytic reactions, the semiconductingmaterials absorb energy more than or equal to energy gap andgenerates the hole and electrons, which further contributes to riseefficient oxidizers of organic dyes. The antibacterial activities of CuONPs were reported against E. coli, P. aeruginosa, B. subtilis and S. aureus

[24]. Copper nanoparticles were tested for antifungal activityperformed against Cryptococcus and Candida albicans [25]. A mildinhibitory zone effect of Candida albicans has been observed. Coppernanoparticles showed antimicrobial activity against Klebsiella

pneumonia, Enterococcus faecalis, Salmonellatyphimurium, Proteus

vulgaris, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus

aureus. In addition, E. coli and E. faecalis exhibited the highest activityto CuO nanoparticles compared to K .pneumonia [26].

In the present research work, the influence of reactionparameters was explored on the shape and size of CuO NPs,which were prepared using electrochemical method. The effec-tiveness of CuO NPs was studied for the photocatalytic degradationof methylene blue, congo red, and methyl red dyes from watersystem. Moreover, antimicrobial activity of copper oxide nano-particles against Escherichia coli, Staphylococcus aureus, Aspergil-

lum nigres, and Candida albicans was investigated.

Experimental

Chemicals and reagents

The main reagents used were sodium hydroxide(SH), sodiumnitrate (SN), sodium carbonate (SC), acetonitrile (ACN), methanol,methylene blue (MB), congo red (CR), methyl red (MR). Deionizedwater was used throughout the experiments. The main instrumentused in this study was electrophoresis (Instruments & ChemicalsPvt. Ltd.), Fourier transform infrared (Model RZX; Perkin Elmer), X-ray diffractometer (XRD; PAN alytical X’Pert), transmissionelectron microscopy coupled with energy-dispersive X-ray (FEITecnai F 20), scanning electron microscopy (Quanta 250, FEI MakeMode No. D9393).

Synthesis of copper oxide nanoparticles (CuO NPs)

Electrochemical deposition method was used for the synthesisof copper oxide nanoparticles (CuO NPs) under different reactionconditions. In typical procedure, 200 ml of supporting electrolytewas dissolved in 1.25 mM of sodium hydroxide, sodium carbonate,and sodium nitrate in water, water: acetonitrile (12:1), and water:methanol (12:1) solvents. The copper plate (3 � 2 cm2) and inertplatinum (1 � 1 cm2) were used as a sacrificial anode and cathode,

respectively. Before electrochemical deposition, both electrodeswere cleaned with hydrochloric acid and distilled water. Thedistance between both electrodes was fixed at 1 cm for all theexperiments. The electrolysis reaction was carried out in anundivided electrochemical cell at 50–100 V potential for 2 h withvigorous stirring at room temperature. The electrolysis wasperformed at currents of 20, 50, and 100 mA. After the electrolysis,the dark brown precipitates were centrifuged, washed withethanol, and finally with distilled water. The products were thendried at 60 8C in a hot air oven for 2 h. The materials obtained indifferent conditions were calcined at 300, 600, and 900 8C for 1 h tostudy the effect of temperature on particle size.

The electrolytic cell can be represented as follow:

Cu þð Þ Solvent þ SEj jPt �ð Þ;

where SE represents the supporting electrolyte.

Characterization

Fourier transform infrared spectra of CuO NPs were recordedusing KBr disk method. Crystal structure of the copper oxidenanoparticles were determined by powder X-ray diffractometeremploying Cu–Ka radiation (l = 1.5418 A) at 50 kV and 200 mA.Transmission electron microscopy coupled with energy-disper-sive X-ray studies were performed by dropping diluted solutionof nanoparticles on copper grids covered with a thin amorphouscarbon film at 200 kV. Scanning electron microscopy studies ofCuO NPs were recorded at different magnifications.

Photocatalytic activity

Photocatalytic activities of CuO NPs were investigated for thedegradation of methylene blue (MB), congo red (CR), and methylred (MR) dyes under sunlight irradiation. Photocatalytic activitiesof CuO NPs obtained in water–ACN solvent, NaOH electrolyte, andsample which was sintered at 600 8C were studied. For thereaction, 0.1 g of CuO NPs was dispersed in 2 � 10�5mol/L dyesolutions in pyrex beaker, which was continuously stirred toestablish the adsorption/desorption equilibrium. The dyes and CuONPs suspension was then irradiated under sunlight with constantstirring for 120 min. Then, 5 ml of solution was withdrawn atspecific interval of time. The absorbance maxima for MB, CR, andMR were determined at 664, 497, and 410 nm, respectively usingUV–visible spectrophotometer. The % degradation of dyes wascalculated using following formula [27]:

% degradation ¼ Co � Ct

Co� 100:

The photodegradation kinetics of dyes was fitted in pseudofirst-order kinetic model [28]:

ln Co � lnCt ¼ kappt;

where C0 is the concentrations of dye before illumination and Ct isthe concentration of dye at time t, and kappt is the apparent rateconstant.

Antibacterial activity

Antimicrobial activity of CuO NPs was carried out by growthcurve method against bacterial strains (Escherichia coli andStaphylococcus aureus) and fungal strains (Aspergillus nigres andCandida albicans) [29]. The bacteria were cultured overnight innutrient broth and fungus was cultured in potato dextrose broth(PDB) solution at 37 8C incubator shaker. The microbial culture wasexposed to CuO NPs with different concentration (25 mg/ml and

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184 175

50 mg/ml) and the microbial cell concentration was adjustedapproximately 107 CFU/ml. The control was used without thenanoparticles. The cultures were shaken at 150 rpm at 37 8C. After1 hr, 3 ml of suspension was taken out to determine the microbialgrowth rates by measuring optical density at 600 nm (OD600) usinga UV–visible spectrophotometer [30]. The same procedure wasrepeated after every 1 h for 16 h to obtain the growth curve of thebacteria and fungi. The efficiency of nanoparticles for inhibiting thegrowth of microorganisms was determined by the differences inpercentage of microbes before and after treatment as:

Inhibition rate ¼1 � ODsample

ODcontrol� 100:

Results and discussion

The electrochemical preparation, a soft chemical technique wasused to produce copper oxide nanoparticles in different reactionmedia. It has been observed that the shape, size, and yield ofnanoparticles were highly affected by reaction parameters, (e.g.,electrode, electrolyte, temperature, electrolysis time, currentsolvent, solution, and dimension or shape of the cell) which arelisted in Table 1.

Effect of electrolytes

In the electrochemical route, the bulk metal oxidized to metalcations, migrate to the cathode, which was site for reduction. Theeffect of ion-solvent interaction on the conductivity of anelectrolyte has been well studied in the literature [31]. Theelectrical conductivity of an electrolyte in solution gives manyimportant qualitative insights [32]. The effect of differentsupporting electrolytes such as sodium hydroxide, sodium nitrate,and sodium carbonate has been studied at constant current andsolvent system for the formation of copper oxide NPs. Thesupporting electrolytes (participate in an electrode process byattacking intermediate species, or alter product distribution bychanging the acid-base character of the solution) stabilized thegrowth of the solution and also increased the rate of reaction[33]. The effect of the supporting electrolyte on controlling themorphologies of CuO NPs suggested that the ion diffusion ofelectrolyte played a key role for growth mechanism. In case of NaOHelectrolyte, the solution change gradually from colorless liquid tolight yellow suspension and finally dark bluish brown precipitates.It was due to transformation at anode copper into CuOparticles through several reactions as Cu ! Cuþ! Cu OHð Þ2!CuO þ Cu2O. The formation of CuO and Cu2O was further confirmedby XRD and TEM as shown in Figs. 1 and 2. While, in the case ofNaNO3 and Na2CO3, the color changes from colorless to bluish brownsuspension and finally dark bluish brown. The yield of product was

Table 1Summary of as prepared samples at different experimental conditions Captions.

Reaction parameters Solvent Electrolytes Current

Effect of electrolyte Water–ACN NaOH 100 mA

Water–ACN NaNO3 100 mA

Water–ACN Na2CO3 100 mA

Effect of solvent Water NaOH 100 mA

Water–methanol NaOH 100 mA

Water–ACN NaOH 100 mA

Effect of current Water–ACN NaOH 20 mA

Water–ACN NaOH 50 mA

Effect of electrolysis time Water–ACN NaOH 100 mA

Water–ACN NaOH 100 mA

Water–ACN NaOH 100 mA

observed higher in sodium carbonate as compared to otherelectrolytes.

Effect of solvent

The effect of different solvent such as distilled water, water–methanol, and water–acetonitrile was investigated on the shapeand size of CuO NPs. All the reactions were performed at100 mAcurrent in a NaOH (1.25 mM) supporting electrolyte. Acetonitrile isa polar aprotic solvent, due to its high conductivity, solvatingpower, high dielectric constant (e = 37), and nontoxic nature; it hasbeen used as solvent in fabrication of metals and alloys [34]. Indeionized water solvent, light greenish blue suspension wasformed after 20 min of reaction time. The solution turned darkyellow suspension in the presence of water–methanol solvent. Inwater–ACN solvent, solution changes from colorless to light yellowsuspension and finally dark bluish brown precipitates was formed.It was revealed that the yield of the product was recorded higher inwater–ACN solvent followed by water–methanol and watersolvents. The change in shape and size of CuO NPs prepared indifferent solvent media is shown in Fig. 5.

Effect of current

The effect of current on the shape and size of CuO NPs wasstudied in NaOH (1.25 mM) electrolyte and water–ACN (12:1)solvent systems (Fig. 3). To study the effect of current onto theshape and size of copper oxide nanoparticle, a series ofpreparations was carried out by varying the current (20, 50, and100 mA). During the electrolysis, a change in current affected theyield of product and color of the solution. It was observed that theyield of product was increased with the increases in current. It wasdue to the increase in the rate of hydrogen evolution with current,which increased the penetration and distribution of hydrogenbubbles through the bed of copper at cathode surface [35]. At20 mA, solution took 10 min to turn into light yellow color andfinally to light bluish suspension after 2 h. When, 50 mA currentwas applied to electrolyte solution, it took few minutes to turn intolight blue color suspension. At 100 mA current, the color ofsolution immediately changed to dark bluish brown suspension. Itwas revealed that the particle size decreased with the increase incurrent. It was due to the correlation between the current andparticle size, obtained from the free energy of formation ofnanoparticles [35]. The TEM results clearly showed the effects ofcurrent on the particle size of CuO NPs as shown in Fig. 3.

Effect of electrolysis time

The effect of electrolysis times (30, 60, and 120 min) onsynthesis of CuO NPs was investigated. It was observed that with

Reaction time Stirring rate (r/s) TEM (Average particle size (nm))

2 h 5 20

2 h 5 200

2 h 5 25

2 h 5 60

2 h 5 59

2 h 5 30

2 h 5 60

2 h 5 50

30 min 5 Incomplete formation of particle

60 min 5 3

120 min 5 20

Fig. 1. XRD spectrum of CuO NPs prepared in presence of (a) sodium hydroxide, (b) sodium nitrate, and (c) sodium carbonate electrolytes.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184176

increase in the electrolysis time from 30 to 120 min the yield ofproduct and size of particle also increased [36]. It was due to theincrease in the number of nuclei formation at cathode at higherelectrolysis time. The influence of electrolysis times on the size ofparticle is shown in Fig. 4. The CuO NPs synthesized in differentoptimized conditions of NaOH electrolyte, water-ACN solvent,100 mA current, and 120 min time was studied for antimicrobialand photo catalytic applications.

Based on the above results, the possible mechanisms for thesynthesis of CuO NPs have been presented as follow:

In presence of NaOH electrolyte

Anode:

Cu ! Cu2þ þ 2e�

Cathode:

OH� þ 2H2O þ 2e� ! 3OH� þ H2

Electrolyte solution:

Cu2þ þ OH� ! CuO þ Cu2O

In presence of NaNO3 electrolyte

Anode:

Cu ! Cu2þ þ 2e�

Cathode:

NO3� þ H2O þ 2e� ! 2OH� þ NO2

�

Electrolyte solution:

Cu2þ þ OH� ! CuO þ Cu2O

In presences of Na2CO3 electrolyte

Anode:

Cu ! Cu2þ þ 2e�

Cathode:

CO32� þ H2O þ 2e� ! 2OH� þ CO2

Electrolyte solution:

Cu2þ þ OH� ! CuO þ Cu2O

In the presence of current, OH� ions and Cu2+ ions weregenrated on the surface of cathode and anode at a same time.Cu2+ ion reacts with OH� ions in the reaction mixture to produceCuO and CuO2. It was observed that the nature of electrolyteaffected the shape and size of particles due to presence ofdifferent anions.

Characterization techniques

XRD studies

Fig. 1 shows the XRD spectra of CuO NPs synthesized indifferent electrolyte at different temperature. Fig. 1a–c showed theXRD pattern of CuO NPs prepared in SH, SN, and SC electrolyte. Thediffracted lines were broad and less intense. The diffraction peaksappeared at 2u angles 32.5, 35.6, 36.6, 38.7, 48.8, 50.5, 58.0, 61.6,66.1, 68.6, and 74.18 corresponds to CuO and Cu2O NPs. The broadand less intense diffraction peaks at 2u values of 32.6, 35.7, 36.7,38.8, 42.6, 48.9, 53.6, 58.0, 61.7, 66.2, 68.0, and 72.28 was recordedfor the nanoparticles synthesized in SN electrolyte as shown inFig. 1b. Fig. 1c shows the CuO NPs prepared in SC electrolyte with2u angle of 29.8, 33.1, 35.6, 36.7, 38.6, 42.7, 43.4, 49.1, 50.5, 53.3,57.9, 61.8, 66.7, 68.0, and 74.18. The size of CuO particles was

Fig. 2. TEM micrographs of CuO NPs prepared in presences of different supporting electrolytes (a) sodium hydroxide, (b) sodium nitrate, and (c) sodium carbonate.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184 177

calculated from XRD data using Debye Scherrer equation. Themixed phase of CuO and Cu2O was observed in all the cases at roomtemperature [37]. X-ray diffraction peaks are indexed with latticeplanes and compared to the International Center for DiffractionData (ICDD) Card No: 41-0254 [38]. The growth mechanism ofelectrodeposited CuO NPs was reported in the literature [39]. XRDpatterns of sample calcined at higher temperature corresponds tobroad diffraction peaks, indicated the formation of CuO withsmaller particle.

The XRD pattern of CuO NPs, prepared in different electrolytesafter calcinations is shown in Fig. 1a–c. The two main peaks of CuOat35.63 and 38.778were observed in SH, SN, and SC electrolyte[40]. The crystallite average grain size of copper oxide at 300 8C inSH, SN, and SC electrolyte were noted as 13, 16, and 20 nm,respectively. At high calcination temperature (600 8C), a character-istic peak of CuO was appeared in all the samples. The averageparticles size obtained at 600 8C were 16, 15, and 19 nmrespectively, in SH, SN, and SC electrolytes. With further increasein calcinations temperature from 600 8C to 900 8C, the characteristic

CuO peaks became sharper and crystalline. The average particlessize obtained in SH, SN, and SC electrolytes were 26, 27, and 30 nm,respectively.

TEM studies

TEM micrographs of CuO NPs, prepared in different reactionconditions are shown in Figs. 2–5. The results indicated that allreaction conditions not only affected the particle size but alsoinfluenced the shape of particles. The well dispersed round shapeparticles was found in SH electrolyte (Fig. 2a). Whereas needle andagglomeration of particle was observed in presence of SN and SCelectrolytes, respectively [36,39]. The good agreement betweenparticle size and crystal size indicated the highly crystalline natureof the particles. TEM images of CuO NPs prepared in differentelectrolytes at various calcination temperatures of 300 and 900 8Care shown in Fig. 3. The round shaped particles with averageparticle size (20 and 25 nm at 900 8C) were obtained in SH and SC.The needle shaped particles were obtained (200–500 nm at 900 8C)in SN as revealed from Fig. 3d–e. The selected area electron

Fig. 3. TEM micrographs of CuO NPs in presences of sodium hydroxide, sodium nitrate, and sodium carbonate supporting electrolytes calcined at (a) (a–d) 300 8C, (b) (e–h)

900 8C, and (c) (f–i). Electron diffraction pattern and particle size distributions of CuO NPs.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184178

diffraction (SAED) pattern of CuO NPs indicated the crystalline andorderedorientations(Fig.3c).TheSAEDpatternshowedtheringswithdiffraction points ascribed to (

—111), (111), (

—202), (020), (

—113), (

—310),

and (220), which confirmed the monoclinic crystalline structure withlattice parametersa = 4.685 A, b = 3.423 A,c = 5.132 A,b = 91.52,andV = 78.70 A [38]. The micrographs showed that calcination tempera-tures and electrolytes strongly influenced the morphology and size ofas prepared nanoparticles.

Fig. 4a, b and Fig. 3a, b shows TEM micrographs of the effect ofcurrent on the size of CuO NPs in SH electrolyte and water–ACNsolvents. The results revealed that the average particle size of CuONPs was obtained 60, 50, and 20 nm at currents of 20, 50, and100 mA, respectively. The TEM histogram showed that the particle

size decreased with the increase in current. The obtained resultswere in well agreement with reported literature [39].

Fig. 5a–c shows the TEM micrograph of CuO NPs (SH and water–ACN at 100 mA current) for the influence of electrolysis time. Theimages clearly indicated that incomplete formation of particleprepared at 30 min and as the reaction time increased 60 to120 min, particle size was also increased.

SEM studies

Fig. 6 displays the SEM images of nanoparticles synthesized indifferent solvents such as water, water–methanol, and water–acetonitrile. The granular spherical shape particle was obtainedwhen synthesis was carried out in the presence of water. Whereas

Fig. 4. TEM micrographs of CuO NPs at current (a) 20 mA (b) 50 mA.

Fig. 5. TEM micrographs of CuO NPs at different electrolysis time (a) 30 min (b) 60 min, and (c) 120 min.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184 179

Fig. 6. SEM micrographs of CuO NPs prepared in the presence of (a) water, (b) water–methanol, (c) water–ACN and (d) EDS of CuO NPs solvents.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184180

the surface become rough when synthesis was done in water–methanol solvent. Water–ACN solvent resulted small granularspherical shape particles with aggregation.

EDX studies

To determine the composition of the CuO NPs, energydispersive X-ray spectroscopy was used. EDX spectrum of CuONPs indicated the existence of carbon, copper, and oxygen asshown in Fig. 6d with 60.64% copper and 32.13% oxygen.

UV-visible studies

The CuO NPs bandgap was determined by the UV spectra in therange from 200 to 900 nm as shown in Fig. 7. The absorption peakat 260 nm confirmed the formation of copper oxide nanoparticles.The optical bandgap of CuO NPs was calculated using the Tauc’srelation as follow [41]:

ahv ¼ Aðhv � EgÞn;

where A is constant (independent of n),a is absorption coefficientand n is the exponent that depends upon the quantum selectionrules.

A plot of (ahn)2 against photon energy (hn) (Fig. 7b) showedstraight line due to direct allowed transition (n = 1). The interceptof the straight line correspond to optical bandgap (Eg) of CuO NPswhich was calculated to be 3.57 eV. This may be due to thequantum size effect in case of CuO NPs [39].

FTIR studies

Fig. 7c shows the FTIR spectra of CuO NPs prepared in SH andwater–ACN at 100 mA current. A wear band at around 2318 cm�1

can be due to the vibration of C–O [42,43]. The peaks at 587 and536 cm�1 might be due Cu–O bond stretching [44–46]. Thepresence of peaks in the region between 500 and 600 nm indicatedthe formation of CuO NPs.

TGA studies

Fig. 8 shows the thermogravimetric analysis of CuO NPs. Theinitial weight loss (2%) was observed up to 2008 C, might be due tothe dehydration. The weight loss of only 3.3% was recorded inbetween 200–4008 C, which was due to complete oxidation ofcompound [47]. It was found that no weight loss was observedfrom 400 to 6008 C. The results inferred the high stability of CuONPs as only 5.3% weight loss occurred upto 6008 C.

Applications of CuO NPs

CuO NPs prepared in water–ACN solvent, SH electrolyte,100 mA current, and 120 min at 300 8C has been selective forphotocatalytic and antimicrobial activity. The photo catalyticdegradation of different dyes (methylene blue (MB), methyl red(MR), and congo red(CR)) has been attempted. The structures oforganic dyes are shown (Scheme 1) below:

Photo catalytic degradation of dyes

Photo degradation of cationic dyes such as MB, MR, and CRusing CuO NPs under sunlight irradiation were studied. Thedecreases in absorbance band intensities in presences of CuO NPswith irradiation time, were observed for MB, MR, and CR asindicated in Fig. 9. It clearly revealed the efficient degradation ofthese dyes. The photo degradation of dyes in the absence of CuONPs was observed 2–2.28% as shown in Fig. 9b, which indicated

Fig. 7. (a) UV–vis spectra of CuO NPs, (b) Plot of (ahn)2vshn of CuO NPs, and (c) FTIR spectra.

Fig. 8. TG/DTA of CuO nanoparticles.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184 181

Scheme 1. The chemical structure of MB, MR, and CR dyes.

Fig. 9. (a) Photo degaradation of MB, CR, and MR with CuO NPs, (b) without CuO NPs unde

(f) MR dye.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184182

very low self-sensitized photolysis of dyes. The color of dyessolution faded within 120 min of sunlight irradiation in presence ofCuO NPs. The absorbance decreased gradually with exposure timefor all dyes in presence of CuO NPs. The degradation was 81, 81%,and 67% for MB, MR, and CR dyes, respectively within 80 min ofphoto irradiation. While it was 93%, 90%, and 85% for MB, MR, andCR dyes, respectively within120 min of photo irradiation as shownin Fig. 9c. The degradation of MB dye was observed higher ascompared to other two organic dyes. When dyes solution wasexposed to light, the aggregate species disappeared first followedby the monomeric ones. It showed that photo degradationdestroyed not only the conjugate system but also the intermediateproducts [48]. It was also revealed that the chemical structureof MB and MR was found more susceptible to oxidation ascompared to CR. The rate of photo catalytic degradation for CR(0.01749 min�1) was lower than MB (0.02059 min�1) and MR(0.02046 min�1). It may due to the large steric hindrance arisingfrom biphenyl group and naphthenic groups [49].

When CuO NPs were irradiated by sunlight light, the (e�CB) and(h+

VB) were generated on NPs surface. The holes counter withwater adhered to the surfaces of CuO NPs to form highly reactivehydroxyl radicals (�OH). In the intervening time, oxygen acted asan electron acceptor by forming a superoxide radical anion (O2

�).The dyes were destroyed through direct oxidation by the OHradicals and O2

� radicals. The mechanism for catalytic degradationof dyes using CuO NPs in the presence of sunlight irradiation wasexplained as follow [50]:

CuO þ hv ! e�CBþ hþVB

hþVBþ H2O ! Hþ þ �OH

e�CBþ O2 ! O2��

O2�� þ Hþ ! HO2

� þ ! O2�� ! HO2

� þ O�

HO2� ! H2O2þ O2

r sunlight, (c) % of degradation of dyes, (d) Plot of lnA versus time for MB, (e) CR, and

Fig. 10. Growth curves of (a) E. coli, (b) S. aureus, (c) C. albicans, and (d) A. nigres exposed to 25 and 50 mg concentration (mg/ml) of CuO NPs, and (e) Inhibition rate (%) CuO NPs

against microbes.

R. Katwal et al. / Journal of Industrial and Engineering Chemistry 31 (2015) 173–184 183

H2O2þ O2�� ! HO� þ OH� þ O2

H2O2þ e� ! HO� þ OH�

H2O2þ hv ! 2HO�

O��2þ�OH þ dye ! degraded product:

Fig. 10(d–f) shows the plot of lnA vs irradiation time for MB, MR,and CR. The linear correlation with good precision was observed.This indicated the pseudo first-order rate order kinetics for dyesdegradation. The constant rate value clearly specified the fast andeffective photo degradation of MB as compared to other dyes [43].

Antimicrobial activity

The antimicrobial activity of CuO NPs was studied against E. coli,S. aureus, A. nigres, and C.albicans strain using growth curvemethod. Fig. 10 shows the growth curves of microbesat alteredconcentration of CuO NPs. The maximum growth of microbes wasinhibited at 50 mg/ml�1after 16 h period of incubation in all thecases. The 94% inhabitation of A. nigres was recorded usingnanoparticles. It may be due to the binding of NPs to the outermembrane of microbes, which resulted in inhabitation of activetransport and nucleic acids synthesis etc. [51,52].

Conclusions

In this study, CuO NPs were prepared by simple and cost-effective method. The effects of different parameters wereinvestigated for the synthesis of nanoparticles. The particles sizeand shape of CuO NPs were highly affected by electrolytes,solvents, electrolysis time, and current. The nanoparticles werecharacterized by different instrumentals techniques. The photocatalytic activities of CuO NPs for degradation of different organicdyes (MB, MR, and CR) have been investigated under sunlightirradiation. The highest degradation of MB was recorded 93% ascompared to other dyes. The antimicrobial activity of CuO NPsagainst E. coli, S. aureus, A. nigres, and C. albicans were studied andcompared. Thus, the synthesized CuO NPs may be suitably used fornew photocatalyst and antimicrobial material for pharmaceuticaland biomedical applications.

Acknowledgments

The authors are thankful to the Vice Chancellor, ShooliniUniversity, Solan, India for providing all necessary researchfacilities. One of the authors (Mu. Naushad) would like to extendtheir sincere appreciation to the Deanship of Scientific Research atKing Saud University for funding this work through the ResearchGroup NO. RG-1436-034.

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