Materials Research Bulletin 90 (2017) 224–231

Facile synthesis of CdO nanorods and exploiting its properties towardssupercapacitor electrode materials and low power UV irradiationdriven photocatalysis against methylene blue dye

Sumeet Kumara, Bilal Ahmedb, Animesh K. Ojhab, Jayanta Dasc, Ashok Kumara,*aDepartment of Physics, Tezpur University, Assam, 784028, IndiabDepartment of Physics, Motilal Nehru National Institute of Technology Allahabad, 211004, IndiacDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, 721302, India

A R T I C L E I N F O

Article history:Received 9 November 2016Received in revised form 4 February 2017Accepted 28 February 2017Available online 1 March 2017

Keywords:SynthesisCdO nanorodsCo-precipitationPhotocatalytic activityElectrochemical activity

A B S T R A C T

Cadmium oxide (CdO) nanorods (NRs) were synthesized by a facile co-precipitation method withpotential for large scale production. The structural and optical properties of synthesized CdO NRs wereinvestigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman, Fouriertransform infrared (FTIR) and ultraviolet-visible (UV–vis.) spectroscopy. The outstanding electrochemicalcapacitive performance using cyclic voltammetry (CV) test indicates that CdO NRs may be a capableelectrode material for stable and high performance supercapacitors. The photocatalytic activity of CdONRs for the photodegradation of methylene blue (MB) dye was carried out with a UV light sourceequipped with 8W tube. The high surface area and synergistic effect in the synthesized CdO NRs areresponsible for excellent photodegradation rate under low power UV irradiation which renders the entireprocess economically feasible. The development of such low cost and effective photocatalytic andelectrode materials is desirable for the next generation photocatalysts and supercapacitor electrodes.

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1. Introduction

Nowadays, world energy crisis and water contamination haveattracted significant attention among scientific communities. Theever increasing demand of energy insists the development of highperformance energy storage devices. The electrochemical capaci-tor, also known as supercapacitor, has become one of the mostpromising energy storage devices in the recent years, because of itshigh energy density, large power density, faster rechargecapabilities and longer cycle life compared to the commercialbatteries [1]. The electrochemical performance of supercapacitorsstrongly depends upon the properties of electrode materials.Transition metal oxides have qualified to be one of the electrodematerials which can enhance energy and power density ofsupercapacitors [2]. Among various transition metal oxides,ruthenium oxide (RuO2) has been considered as an excellentelectrode material due to its high specific capacitance andexcellent cycle-life stability, but its high cost is a major problem

* Corresponding author at: Department of Physics, Tezpur University, Tezpur,Napaam, 784028, Assam, India.

E-mail address: ask@tezu.ernet.in (A. Kumar).

http://dx.doi.org/10.1016/j.materresbull.2017.02.0440025-5408/© 2017 Elsevier Ltd. All rights reserved.

for commercial application [3]. In this context, cadmium oxide(CdO) could be an alternative low cost electrode material forsupercapacitor. The high intrinsic dopability of CdO coupled withexcellent Hall mobilities provides high electrical conductivity(even without doping) due to the presence of shallow donorsproduced by intrinsic interstitial cadmium atoms and oxygenvacancies [4]. CdO is an n-type semiconductor with a direct bandgap of 2.3 eV, and an indirect one of 1.36 eV [5]. The nanostructuredmaterials of CdO such as CdO nanorods (NRs) can play animportant role in order to get cost effective and high performancesupercapacitor electrode materials due to their large surface-to-volume ratio and conducting nature [6]. Unfortunately, manyattempts have not been made on CdO due to its toxic nature. Onlyfew reports on the use of cadmium oxide as a supercapacitorelectrode are available [2,3,6].

Industrial wastewater treatment has become one of the majorissues of social concern due to the direct release of industrialpollutants into nearby water resources. The contamination of thewater resources causes several harmful effects to human healthand ecological systems [7]. The specific electrical (large carrierconcentration and high electrical conductivity) [8] and opticalproperties (high optical transmittance and large refractive index)[9] of CdO make it a suitable candidate for next generation

Fig. 1. Schematic of synthesis steps used for the synthesis of CdO NRs.

Fig. 2. XRD pattern of CdO NRs.

S. Kumar et al. / Materials Research Bulletin 90 (2017) 224–231 225

photocatalyst. The nanostructured materials of CdO such as CdONRs based photocatalysts could achieve remarkable importance inorder to get complete removal of organic pollutants from industrialwastewater. However, in the literature, the photocatalytic activityof CdO nanostructures is rare and thus, extensive investigations onphotocatalytic activity of different size and shape of CdO nano-structures are still required to be done.

In this work, CdO NRs were synthesized by a facile co-precipitation method with potential for large scale production.During synthesis, the amount and rate of addition of ammoniasolution were adjusted properly in order to get desired CdOnanostructures. The structural and optical properties of thesynthesized CdO NRs were investigated by XRD, TEM, Raman,FTIR and UV–vis. spectroscopy. The electrochemical performanceof the synthesized CdO NRs was investigated in order to exploittheir suitability as supercapacitor electrode materials. The presentwork has demonstrated the excellent photocatalytic activity of thesynthesized CdO NRs against methylene blue (MB) dye under UVirradiation of a low power UV light source with a UV light sourceequipped with 8W tube (Philips TUV-8W C8T5). Such an intriguingproperty makes CdO NRs more versatile material in order todevelop an inexpensive and valuable photocatalyst. The develop-ment of such low cost and effective photocatalysts is desirable forfuture applications in the field of photocatalysis.

2. Experimental

2.1. Synthesis

All chemicals were of analytical grade procured from Merck,India and used without further purification. CdO NRs have beensynthesized at room temperature by co-precipitation methodusing Cd(CH3COO)2�2H2O and ammonia solution as startingmaterials. The step-wise synthesis mechanism is shown in Fig. 1.

In detailed, an aqueous solution (prepared using water from aMillipore Waters Milli Q purification unit) of appropriate amountof Cd(CH3COO)2�2H2O (0.5 M) was prepared. The prepared solutionwas stirred for an hour and subsequently ammonia solution wasadded at a constant rate of 0.5 ml/s with the help of a stepper-motor controlled syringe pump to the prepared solution drop wiseuntil pH of the solution is reached to 8. While adding the ammonia,a white precipitate was formed. The stirring of solution wascontinued for another 5 h at room temperature. The whiteprecipitate was kept to settle down for 5 h. Thereafter, the solution

was filtered and washed repeatedly with distilled water to removeun-reacted materials. The obtained precipitate was dried at 80 �Cand then grinded to fine white powder with the help of mortar andpestle. The resulting powder was calcined at 400 �C for 2 h forobtaining the final product.

2.2. Characterization

The XRD pattern of as synthesized samples was recorded usinga Rigaku Smart Lab X-ray diffractometer with CuKa radiation atl = 1.540 Å. Transmission electron microscopy (TEM) analysis wasperformed on a JEOL JEM-2100 TEM at 200 kV at an acceleratingvoltage of 200 kV with resolution 0.19 nm. The Raman spectrumwas recorded in the spectral range 200–1400 cm�1 using WI-TecRaman spectrometer. The presence of surface functional groups inthe prepared samples was analyzed by FTIR spectrum recordedusing Perkin Elmer FTIR spectrometer. UV–vis. Absorptionmeasurements were carried out using Perkin Elmer (Lambda35)UV–vis. spectrometer.

2.3. Electrochemical measurements

Electrochemical measurement like cyclic voltammetry (CV) ofsynthesized CdO NRs was performed using a three-electrodeelectrochemical workstation (AEW2 Syscopel Scientific Ltd. UK)consisting of working electrode, counter electrode and referenceelectrode. The working electrodes were fabricated by mixing theas-prepared sample, carbon black and N-methyl-2-pyrrolidone(NMP) in a mass ratio of 85:10:5 and dispersed in ethanol by usingultrasonication probe for half an hour. The resulting mixture wasthen dropped onto a 2 mm diameter platinum working electrodeand dried at 40 �C for 12 h. The mass loading of the sample on theplatinum electrode was 1.5 mg.

The CV test was carried out in the potential range of �1.2 V to1.2 V at different scan rates of 5, 10, 20 mV/s with 0.1 M KCl aselectrolyte. The specific capacitance (Cs) of CdO NRs was calculatedfrom the CV curves using the following equation [10]:

Cs ¼

ZIdV

smDV

Table 1Values of the position of XRD peaks (2u), interplanar spacing (d), lattice constant (a) and the diameter of the synthesized CdO NRs.

Sr. No. Peak position (2u) Interplanar spacing (d) (Å) Lattice constant (a)(Å)

Diameter of the NRs (nm) by XRD Diameter of the NRs (nm) by TEM

1. 32.98� 2.714 4.701 11.05 11.882. 37.30� 2.348 4.696 11.363. 55.31� 1.659 4.691 9.544. 65.91� 1.416 4.695 8.455. 69.37� 1.354 4.690 8.62Average value 4.695 9.81

226 S. Kumar et al. / Materials Research Bulletin 90 (2017) 224–231

whereZ

IdV is the area under CV curve, m is the total mass of

electroactive material on the electrode, DV is the voltage differenceand s is the scan rate.

The energy density (Ed) can be estimated with the help of thefollowing equation [10]:

Ed ¼ 12� Cs � V2

where Cs is the specific capacitance and V is the potential vs. Ag/AgCl.

2.4. Photocatalytic measurements

The UV light driven photocatalytic activity of the synthesizedCdO NRs was evaluated in terms of the photodegradation of MBdye using a homemade reactor equipped with low power PhilipsTUV-8W C8T5 tube (l = 254 nm). In a typical experiment, 50 mg ofsynthesized CdO NRs was dispersed in 50 ml of an aqueoussolution of 10�5M MB dye. The mixed solution was continuouslystirred for 30 min at room temperature before exposing by UV light

Fig. 3. TEM images of CdO NRs.

irradiation. The concentration of MB dye prior to irradiation wasused as the initial value for the measurement of concentration ofMB dye degradation. After a defined time interval of irradiation(20 min), 2 ml amount was taken out by the syringe from theirradiated solution. Subsequently, the dispersed photocatalystswas separated through centrifugation from solution prior to test.Thereafter, UV–vis. absorption spectra of the irradiated solutionwere recorded to evaluate the concentration of photodegraded MBdye. The photodegradation efficiency% of MB was calculated usingthe following equation [11]:

Photodegradation efficiency % ¼ C0 � CC

� 100

where C0 is the initial concentration of MB dye and C is theconcentration of MB dye after irradiation.

3. Results and discussion

3.1. Structural characterization

3.1.1. XRDX-ray diffraction (XRD) analysis was performed to analyze the

crystalline phase and crystal structure of the synthesized samples.XRD pattern of the synthesized CdO NRs, shown in Fig. 2,corresponds to the rock salt cubic structure of CdO with value oflattice constant, ‘a’ = 4.6946 Å (JCPDS card no 05-0640). Thediffraction peaks observed at 32.98�, 37.30�, 55.31�, 65.91� and69.37� in the XRD pattern correspond to (111), (200), (220), (311)

Fig. 4. Raman spectrum of CdO NRs.

Fig. 5. FTIR spectrum of CdO NRs.

S. Kumar et al. / Materials Research Bulletin 90 (2017) 224–231 227

and (222) planes, respectively. The interplanar spacing (d) valuescalculated for (111), (200), (220), (311) and (222) planes are foundto be 2.714, 2.348, 1.659, 1.416, and 1.354 Å, respectively. Nodiffraction peaks corresponding to any impurity or other phasessuch as CdO2, Cd(OH)2 and CdCO3 are observed in the XRD pattern,which confirm that the synthesized CdO NRs are free from anyimpurity. The sharp intensity of each diffraction peak indicates thatthe synthesized CdO NRs possess good crystallinity [5].

The lattice constant (a) were calculated using the standardrelationship: d = ap

h2þk2þl2ð Þ, where, d is the interplanar spacing and

h, k, l are the Miller indices. The average diameter of thesynthesized CdO NRs was estimated to be �10 nm using givenexpression [4]: D = 0:9l

bcosu, where D is the crystallite size, l is the X-

ray wavelength, u is the Bragg diffraction angle and b is full widthat half maximum (FWHM). The values of the position of XRD peaks(2u), interplanar spacing (d), lattice constant (a) and diameter ofthe synthesized CdO NRs are given in Table 1.

Fig. 6. UV–vis. spectrum (a) and direct ba

3.1.2. TEMThe TEM images of the synthesized CdO NRs are shown in Fig. 3.

The average diameter of CdO NRs appears to be �11.88 nm. It is ingood agreement with diameter values calculated using XRDresults. In the present study, ammonia solution was used asstabilizer for the formation of well defined anisotropic CdO NRs.The appropriate amount and proper rate of addition of ammoniasolution helps to control the parameters of the precipitationprocess, leading to the formation of well defined anisotropicnanostructures of desired size and shape. The size and morphologyof the synthesized NRs were controlled by achieving desired pHvalue of Cd salt solution by drop-wise addition of ammoniasolution at a constant rate. The drop-wise addition of ammoniasolution involves addition of a single drop of ammonia solutiononce at a time repeatedly until the desirable pH level is achieved. Itis expected that when ammonia solution is added drop-wise atconstant rate, the intrinsic anisotropy in CdO nanostructures canbe influenced by adsorption of stabilizing molecules on specificcrystal facets. As a result, the growth rate along longitudinaldirection is faster. It is reported that the shape of transition metalnanostructures is easily controlled with the help of stabilizingreagent which prevents the uncontrolled growth and agglomera-tion of the nanostructures [12,13]. Generally, the control of shapeand size of nanostructures depends on the nucleation process.During the synthesis, nuclei are formed through self nucleationprocess with a continuous supply of atoms via precursordecomposition. Instantly, the formed nuclei will start to grow inthe form of nanocrystals. The shape of nuclei can have a strongeffect on the final structural morphology of the nanostructures.The high monomer concentration determines high chemicalpotential surroundings which provide additional environmentfor the configuration of anisotropic shapes and other elongatednanostructures [14]. Usually, the manipulation between thermo-dynamically and kinetically controlled growth regimes is a criticalfactor in determining the shape of nanostructures [15].

3.2. Spectroscopy characterization

3.2.1. RamanThe Raman spectrum of the synthesized CdO NRs showing the

Raman shifts in the spectral range 200–1400 cm�1 is shown inFig. 4. Raman spectrum of CdO NRs was recorded using 633 nm lineof the solid state laser as the excitation wavelength. The Raman

nd gap measurement (b) of CdO NRs.

Fig. 7. Cyclic voltammetry (CV) profiles of CdO NRs. CV profiles of CdO NRs at different scan rates (a), CV profiles of CdO NRs at scan rate of 20 mV/s for hundred cycles (b),specific capacitance vs. scan rate (c) and energy density vs. scan rate (d).

228 S. Kumar et al. / Materials Research Bulletin 90 (2017) 224–231

spectrum of CdO NRs contains one broad intense band positionedat �279 cm�1 spanning from 200 to 500 cm�1 and a weaker bandappeared at �941 cm�1. The Raman band centered at �279 cm�1 isassigned to 2TA(L) mode while the band at �941 cm�1 correspondsto 2LO mode [16]. In a recent study [17], a similar kind of featurehas been observed in Raman spectrum of CdO nanoparticles. Theauthors found a broad feature centered at �268 cm�1 and anintense peak at �940 cm�1. Cusco et al. [18] has also observed theRaman bands at �265 cm�1 and �940 cm�1. They have reportedthat due to the principle of selection rules for the rocksalt structureof CdO, only second order Raman scattering is expected and both,TO and LO modes are dipole forbidden [18].

3.2.2. FTIRThe FTIR spectrum of CdO NRs is recorded in the spectral range

of 4000–400 cm�1 and it is shown in Fig. 5. The strong broadabsorption band centered at �3433 cm�1 is assigned to the OH-stretching vibrations of hydroxyl groups [19] while the band at�1448 cm�1 is assigned to the asymmetric stretching vibrations ofwater molecules associated with synthesized CdO NRs [20]. Theabsorption peak at �951 cm�1 corresponds to metal-oxygenstretching of CdO NRs [21]. The formation of absorption band�508 cm�1 is attributed to Cd-O stretching vibrations [22]. It hasbeen reported that the intense IR bands around �500, �1000 and�1400 cm�1 are the characteristic bands of CdO [23–25]. Hence,the FTIR spectrum also confirms the formation of CdO because ofthe existence of the characteristic bands of CdO.

3.2.3. UV–visThe UV–vis. absorption spectrum of the synthesized CdO NRs is

shown in Fig. 6(a). The UV–vis. absorption spectrum was recordedat room temperature for the spectral range 200–700 nm. In orderto perform the absorption measurements, the samples weredispersed in distilled water using ultrasonication. The absorptionspectrum exhibits an absorption band at �298 nm, which isassigned to the excitonic absorption feature of CdO. The followingrelation between the absorption coefficient (a) and the incidentphoton energy (hn) absorption spectrum was used to determinethe nature and value of optical band gap of CdO NRs: (ahn)1/n = A(hn-Eg), where A is a constant and Eg is the optical band gap ofmaterial and the value of n corresponds to 1/2 and 2 for direct andindirect optical transition, respectively [26]. The direct band gap ofCdO NRs was calculated from (ahn)2 vs. hn plot by extrapolatingthe straight portion of the graph on hn axis (see Fig. 6b). The valueof direct band gap turns out to be 3.29 eV which is higher than thatof the value of bulk CdO. The higher value of optical band gap ofCdO NRs is the direct consequence of quantum confinement effectat nanoscale. The energy bandgap of semiconducting nano-structures as a function of particle size is explained by quantumconfinement effect using the following equation based on Brusmodel [27]:

Eg = Eg0 + [h2/8 mR2] � [1.8e2/4peR]

where Eg0 is the energy band gap for the bulk material, h is the

Planck’s constant, R is the radius of nanoparticles,1/m = 1/me + 1/mh

Fig. 8. Absorption spectra of aqueous solution of MB dye exposed under UV lightirradiation at different irradiation times (a) photodegradation rate of pure MB dyesolution and MB dye solution mixed with CdO NRs as photocatalyst under UV lightirradiation (b).

S. Kumar et al. / Materials Research Bulletin 90 (2017) 224–231 229

(as me and mh are the electron and hole effective masses,respectively), e is the dielectric constant and e is the electroniccharge. The different terms like first, second and last in aboveequation represent the energy gap of the bulk CdO, the electron–hole pair confinement kinetic energy and, the Coulomb interactionenergy between the electron and hole, respectively.

3.3. Electrochemical activity

Cyclic voltammetry (CV) is a suitable electrochemical mea-surement to characterize the capacitive behavior of the electrodematerials. The rectangular CV curve, high current and symmetry incathodic and anodic peak directions in any CV profile are the maincharacteristics of ideal capacitive electrode materials. Fig. 7 showsthe electrochemical performance of the synthesized CdO NRs.Fig. 7a shows the CV curves of CdO NRs in a potential range from�1.2 V to 1.2 V (vs. Ag/AgCl) at various scan rates of 5,10 and 20 mV/s. As seen in Fig. 7a, the area of CV curve is increased with theincrease of scan rate and the CV curve at different scan ratesmaintains the symmetrical shape, exhibiting an ideal capacitiveperformance of the synthesized CdO NRs. The symmetrical shapeof the CV curves over a broad range of scan rates is an extremelyimportant feature for its applications in making supercapacitors[6]. In addition, the observable increase of current with theincrease of scan rates in the CV curve of Fig. 7a indicates a good ratecapability of CdO NRs. The large surface area of CdO NRs createsmore contact interface between CdO NRs and the electrolyte forelectrochemical energy conversion [3].

Fig. 7b shows the cyclic performance of CdO NRs electrodes atthe scan rate of 20 mV/s for 100 cycles. The CV curves of Fig. 7bshow a minor decrease in the value of peak position current in thesubsequent cycle, which refers to little current loss. The cyclicperformance of CdO NRs electrodes demonstrates good cyclestability and reversibility of CdO NRs. Cyclic stability of electrodematerials is an important parameter for practical applications. Allthe CV curves exhibit a quasi-rectangular shape and aresymmetrical in nature, which indicates the ideal supercapacitorcharacteristics of CdO NRs. The quasi-rectangular shape also showssuperior charge propagation within the electrodes. The CV profilesfound in the present case are very similar to those previouslyreported for CdO nanostructures [2,3,6].

The value of specific capacitance (Cs in F/g) of CdO NRselectrodes decreases with increasing scan rate, as shown in Fig. 7c.It is clear from Fig. 7c that the value of Cs decreases from 654 to183 F/g as the scan rate is increased from 5 to 20 mV/s. The decreasein the value of Cs is attributed to diffusion limits of electrolyte ions.It also suggests that only the outer surface of CdO NRs electrodes isactive for charge storage, while the inner surface cannot maintainthe redox transitions. Therefore, the value of Cs at the slowest scanrate is the actual value of the CdO NRs electrodes. The dependenceof the energy density of CdO NRs electrodes on the scan rate isshown in Fig. 7d. The outstanding energy density of CdO NRs atdifferent scan-rates may originate from the high electricalconductivity and high surface area of CdO NRs [6]. Hence, theexcellent electrochemical performance of CdO NRs indicates thatCdO NRs may be a capable electrode material for high performanceenergy storage supercapacitors.

3.4. Photocatalytic activity

To investigate the photocatalytic activity of the synthesized CdONRs, the photodegradation of MB dye was carried out under thefollowing conditions (i) MB dye solution is kept under the UV lightirradiation in the absence of CdO NRs photocatalyst (ii) MB dye

solution, mixed with CdO NRs photocatalyst, is exposed under UVlight irradiation.

Only few reports are available on the photocatalytic activity ofCdO nanostructures and its composites [7,28–31]. Li et al. [29] hasreported a comparative photocatalytic study of pure CdO NRs andCdO NRs mixed with CdS NRs under visible light irradiation for thephotodegradation of MB dye. They have reported that pure CdONRs showed only 39.7% of photodegradation rate of MB dye whileCdO-CdS mixed NRs exhibited higher photocatalytic activity with71.1% of photodegradation rate of MB dye.

In the present study, an excellent photocatalytic activity of CdONRs was observed for photodegradation of MB dye under UV lightirradiation. The photodegradation rate of MB dye with CdO NRs isfound to be 55% even under the irradiation of low power (8W) UVlight source. Generally, it is seen that a high power UV light sourceof �200-500W is used to perform photocatalytic activity for thephotodegradation of organic contaminants and dyes [32–34]. Themost important drawback of using such a high power UV source isthe need of high voltage to operate that makes entire processcostly. To the best of our knowledge, no one has reported such an

Fig. 9. Mechanism for the photodegradation of MB dye with the help of CdO NRs.

230 S. Kumar et al. / Materials Research Bulletin 90 (2017) 224–231

excellent photodegradation rate especially with low power UVlight source. Hence, the main objective of this work is to explorethe possibility of such an inexpensive and effective photocatalystwhich, in fact, reduces the entire cost of the process of wastewatertreatment.

Fig. 8a shows the change in the absorption spectra of MB dyesolution exposed under the UV light for different irradiation times.The decrease in the intensity of characteristic absorbance bandappeared at �664 nm is selected to monitor the degradation of MBdye. Fig. 8b compares the photodegradation rate of pure MB dyesolution and MB dye solution mixed with CdO NRs as photocatalystunder UV light irradiation. The self degradation rate of pure MB dyeunder UV light irradiation is negligible but the degradation rate hasbecome excellent when CdO NRs are mixed as photocatalyst intothe MB dye solution. The large surface area of CdO NRs providesmore interaction sites to MB dye molecules and as a result, highphotodegradation rate is achieved for the degradation of MB dye.

The schematic diagram of photocatalytic mechanism with CdONRs is shown in Fig. 9. When UV light is incident on MB dyesolution, the electron (e�)-hole (h+) pairs are generated due to theexcitation of valence band (VB) electrons to conduction band (CB)by creating a hole in the VB. These produced holes are allowed toreact with OH� ions present in the aqueous solution of MB dye andcreates �OH radicals while electrons may react with dissolved O2

into the solution and creates �O2� radicals. These radicals then

degrade the MB dye molecules into small fragments [35].The complete mechanism for the photodegradation of MB dye

can be understood with the help of the following chemicalreactions:

CdO NRs photocatalysts + photon (hn, 254 nm) ! (e�) CB + (h+) VB

H2O + h+! OH� + H+

OH� + h+!�OH (radical)

O2 + e�!�O2� (radical)

MB dye + (CdO NRs) + �O2�! degraded products

MB dye + (CdO NRs) + �OH ! degraded products

4. Conclusion

Using a simple, mild and cheap co-precipitation method, highyield CdO NRs were synthesized by adjusting the concentration ofammonia solution during the synthesis. The present syntheticapproach could be useful for the synthesis of a variety of metaloxide and mixed metal oxide nanostructures with desired shapeand size. The observed XRD peaks with sharp intensity confirm theformation of impurity free and highly crystalline CdO NRs. TEMimages suggest that the synthesized NRs are of well definedanisotropic structures. The Raman spectrum shows second orderRaman scattering at 279 cm�1 and 941 cm�1 due to the principle ofselection rules for the rocksalt structure of CdO. The presence ofcharacteristic functional groups in FTIR spectrum indicates theformation of CdO. The observed absorption band at 298 nmsuggests that the synthesized CdO NRs exhibit quantum confine-ment effect. The higher value of direct band gap (3.29 eV) of thesynthesized NRs than that of the value of bulk CdO is the directconsequence of quantum confinement effect at nanoscale. Theoutstanding electrochemical capacitive behavior makes thesynthesized CdO NRs a promising electrode material for highperformance supercapacitors. The excellent photocatalytic activityagainst MB dye in aqueous solution under low power UV lightirradiation demonstrates that the synthesized CdO NRs would beprospective photocatalysts in wastewater treatment.

Acknowledgements

SK is thankful to University Grants Commission (UGC), India forproviding the financial support through UGC-Dr. D. S. Kothari PostDoctoral Fellowship Scheme (No.F.4-2/2006(BSR)/PH/15-16/0067).

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