Solid State Communications 152 (2012) 375–380

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Enhanced photocatalytic activity of Cu-doped ZnO nanorodsRajneesh Mohan a, Karthikeyan Krishnamoorthy b, Sang-Jae Kim a,c,∗

a Department of Mechatronics Engineering, Jeju National University, Jeju, 690-756, Republic of Koreab Department of Mechanical Engineering, Jeju National University, Jeju, 690-756, Republic of Koreac Research Institute of Advanced Technology, Jeju National University, Jeju 690-756, Republic of Korea

a r t i c l e i n f o

Article history:Received 17 October 2011Received in revised form6 December 2011Accepted 7 December 2011by E.V. SampathkumaranAvailable online 13 December 2011

Keywords:A. ZnO nanorodsE. Vapor transport methodD. Optical propertiesD. Photocatalytic activity

a b s t r a c t

Cu-doped ZnO nanorods with different Cu concentrations were synthesized through the vapor transportmethod. The synthesized nanorods were characterized by X-ray diffraction (XRD), field emissionscanning electron microscopy (FE-SEM) and UV–vis spectroscopy. The XRD results revealed that Cu wassuccessfully doped into ZnO lattice. The FE-SEM images showed that the undoped ZnO has needle likemorphology whereas Cu-doped ZnO samples have rod like morphology with an average diameter andlength of 60–90 nm and 1.5–3 µm respectively. The red shift in band edge absorption peak in UV-vis absorbance spectrum with increasing Cu content also confirm the doping of Cu in ZnO nanorods.The photocatalytic activity of pure and Cu-doped ZnO samples was studied by the photodegradationof resazurin (Rz) dye. Both pure ZnO and the Cu-doped ZnO nanorods effectively removed the Rz in ashort time. This photodegradation of Rz followed the pseudo-first-order reaction kinetics. ZnO nanorodswith increasing Cu doping exhibit enhanced photocatalytic activity. The pseudo-first-order reaction rateconstant for 15 % Cu-doped ZnO is equal to 10.17 × 10−2min−1 about double of that with pure ZnO. Theincreased photocatalytic activity of Cu-doped ZnO is attributed to intrinsic oxygen vacancies due to highsurface to volume ratio in nanorods and extrinsic defect due to Cu doping.

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The huge interest in the preparation of one-dimensional (1D)ZnO nanostructures such as nanowires, nanobelts, and nanorodscontinued in the past few years due to its excellent electrical,optoelectronic properties for wide potential applications inluminescence [1], photodetection [2], gas sensors [3], solar cells [4],medicines [5] and photo catalysis for degradation and eliminationof environmental pollutants [6–8]. ZnO is a II–VI compoundsemiconductor with a wide direct band gap (3.37 eV) and a largeexciton binding energy (60meV) at room temperature. It is knownthat ZnOmaterial can bemadewith themost diverse and abundantconfigurations of nanostructures such as quantum dots, rods,wires, belts, springs, bows, helices and prisms. For photocatalysis,ZnO has also been considered as a suitable alternative for TiO2 dueto its similar band and lower cost [8]. Moreover, it exhibits betterperformance in the degradation of organic dye molecule in bothacidic and basic media. The intrinsic defects of ZnO are beneficialfor setting up catalytic systems, which are expected to degradethe environmental contaminants. The photocatalytic activity of

∗ Corresponding authors at: Department of Mechatronics Engineering, College ofEngineering, Jeju National University, 1 Ara 1-dong, Jeju, Jeju-do, 690-756, Republicof Korea. Tel.: +82 64 754 3715; fax: +82 64 756 3886.

E-mail address: kimsangj@jejunu.ac.kr (S.-J. Kim).

0038-1098/$ – see front matter© 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2011.12.008

nanostructured ZnO is expected to be enhancednot only because oftheir increased surface area but also because of changes of surfaceproperties such as surface defects [9–13]. It has beenwidely provedthat modifications of oxide semiconductors, including doping oftransition metals or rare earths, could enhance their properties.The photocatalytic properties of ZnO were greatly enhanced whenmodified with the incorporation of dopant ions [14–17]. Thedoping of metal ions in ZnO nanostructures can lead to effectssuch as enhancement/decrease in fluorescence and controllingconcentration of surface defects. The doping of Cu in ZnO isexpected to modify absorption, and other physical or chemicalproperties of ZnO because of the different structure of theelectronic shell and the similar size of Cu and Zn. Cu can enterthe ZnO lattice substitutionally as deep acceptors in combinationwith a neighboring O vacancy [18]. Therefore, in the present work,Cu doped ZnO nanorods were synthesized through the vaportransport method and their photocatalytic activities with differentdoping concentrations were evaluated for the degradation ofresazurin.

2. Experimental

2.1. Synthesis of Cu-doped ZnO nanorods

Vapor transport techniquewas used for the growth of Cu dopedZnO nanorods. The schematic of the experimental setup for the

376 R. Mohan et al. / Solid State Communications 152 (2012) 375–380

Ar + O2 gas inlet

Gas outlet

Heater

Quartz tube

Alumina Boat

Fig. 1. Experimental setup for the growth of Cu doped ZnO nanorods.

growth of Cu doped ZnO is shown in Fig. 1. In this technique,a horizontal quartz tube furnace was used. A mixture of ZnO,graphite, Zn, and CuCl2 in 1:1:0.5: x (x = 0, 0.05, 0.1 and 0.15)ratio (by weight) was used as source material. It was loaded inan alumina boat and placed in the center of a 1 m long quartztube. High purity argon gas was introduced through one side ofthe quartz tube, while the other side was connected to a waterbubbler. The sourcematerialwas heated to 1100 °Cat 360 °C/h rateunder a constant argon flow of 250 sccm. When the temperaturereaches 800 °C, oxygen gas is also introduced with a flow rate of35 sccm. The furnace was maintained under these conditions for 1h and then cooled to room temperature at a rate of 6 °C/min. Thenanorodswere collected 20 cm away from the source in an up flowdirection of gas flow.

2.2. Characterization

X-raydiffractionpatterns of synthesized sampleswere recordedwith a Rikagu D/MAX 2200H diffractometer system with CuKα ir-radiation (λ = 1.5418 Å). Morphologies of the synthesized ZnOnanostructures were observed by a field emission scanning elec-tron microscope (FE-SEM, JEOL, JSM-6700F). UV-vis absorbancespectra were obtained using a HP 8453 spectrophotometer in thewavelength range 250–700 nm.

2.3. Photocatalytic experiment

The photocatalytic activities of the synthesized Cu-doped ZnOnanorods were evaluated by the degradation of resazurin (Rz)aqueous solution under UV light. The experiments were carriedout as follows: 1 mL of 0.1 g/L ZnO NW solution was added to10 mL solution of 1.5 mg/L Rz in a 20 mL glass reactor. Priorto illumination, these suspensions were thoroughly stirred in thedark to ensure the establishment of absorption equilibrium ofRz on the sample surfaces. Subsequently, the suspension wasirradiated under a 350 W UV lamp with a peak wavelength of350 nm. UV-vis adsorption spectra (HP 8453) were recorded atdifferent time intervals to monitor the process.

3. Results and discussion

3.1. Characterization of Cu-doped ZnO nanorods

The XRD patterns of the pure and Cu-doped ZnO nanorods areillustrated in Fig. 2. The diffraction peaks were indexed with thePowder Diffraction Standards data (JCPDS 3-1457), showing thatthe main structure of the samples is the wurtzite structure of ZnO.Only peaks corresponding to the ZnO wurtzite phase were foundin the XRD patterns of the undoped and Cu doped samples. Theabsence of Cu related peaks rules out the existence of Cu-basedclusters within the detection limit of XRD. There is a slight shift inXRD peaks towards the higher angle with the Cu doping comparedto the undoped ZnO. This shift is attributed to the shrinkage of ZnOcrystal lattice due to the substitution of Zn2+ (0.06 nm) by smaller

25 30 35 40 45 50 55 60 65

Cu = 5%

Inte

nsi

ty (

a.u

.)

Cu = 0%

Cu = 15%

Cu = 10%

100

002

101

102 11

0

103

2

Fig. 2. The XRD patterns of undoped and Cu doped ZnO nanorods showing slightshift towards higher angle with increasing Cu content.

Cu2+ (0.057 nm) [19]. This observation shows the doping of Cu inZnO crystal lattice.

The FESEM images of the prepared undoped and Cu dopedsample are shown in Fig. 3. These images show that the undopedZnO have a nanoneedle like morphology. As the doping of Cuincreases the morphologies of ZnO changes to nanorods. The Cudoped nanorods have a diameter of ∼60–90 nm and a length of1.5–3 µm respectively.

The UV-visible absorption spectra of undoped and Cu-dopedZnO nanorods are shown in Fig. 4. It can be seen in Fig. 4 that thereis a strong excitonic absorption peak in all samples. This peak isattributed to the large exciton binding energy and the good opticalquality of the nanowires [20].With the increasing Cu content thereis slight red shift in band edge absorption peak. The calculatedbandgap of the Cu-doped ZnO nanorods were 3.297, 3.28 and3.263, for 5%, 10% and 15% doping respectively. The decrease ofband gapwith Cu doping is in agreement with the earlier works onCu doping in ZnO [21–23]. There are two primary causes that maycontribute to variations in band gap energies, namely, quantumsize effect and electronic structure modifications. Considering thefact that the Bohr radius of ZnO is in the range 0.9–2.4 nm [24–27],the decrease of band gap in Fig. 4 is not likely due to the quantumsize effect because the as-synthesized samples have the diametersabove 60 nm. It is reasonable to expect the slight band gap (Eg),decrease with increase in concentrations of Cu as possibly due todoping induced band edge bending.

3.2. Photocatalytic properties

The photocatalytic properties of the Cu-doped ZnO nanorodswere investigated by the degradation of a model dye Rz. Rz is aphenoxazin-3-one dye which is widely used for testing variousbiological materials [28].

Since Rz is a redox indicator, it can be reduced into resorufin(RF) indicated by changing its color from blue (Rz) to pink (RF).The UV-vis absorption spectrum of Rz in aqueous solution consistsof an intense absorption band at 602 nm and a weak band at380 nm. The bands are assigned to the π–π∗ transition of thephenoxazin-3-one and to the weak n–π∗ transitions of the N-oxide. The UV-vis absorption spectrum of RF is characterizedby an intense band centered at 574 nm. The deoxygenation of

R. Mohan et al. / Solid State Communications 152 (2012) 375–380 377

Fig. 3. The FESEM images of the undoped and Cu doped ZnO nanostructures. The FESEM images of the undoped and Cu doped ZnO nanostructures; (a) Cu = 0%; (b) Cu = 5%;(c) Cu = 10%; (d) Cu = 15%.

heterocyclic N-oxides like Rz has been considered as amechanisticmodel for oxygen atom transfer in enzymatic processes. Thephotophysics of this dye in the presence of ZnO photocatalysthas not been greatly explored. It is well known that ZnO hasbeen used as a semiconductor photo catalyst for the photocatalyticdegradation of water pollutants, and the photocatalytic reductionof toxic metal ions [8]. Therefore, Rz was chosen as a modeldye to evaluate the photocatalytic activity of the Cu doped ZnOin the present study. The photocatalytic activity evaluation forthe photocatalytic decolorization of Rz aqueous solution wasperformed at ambient temperature in air. No obvious variationoccurred to the concentration of resazurin for the measurementusing undoped ZnO and Cu-doped ZnO samples under darkconditions. At the same time, the photocatalytic decolorizationof Rz was not achieved under ultraviolet (UV) irradiation in theabsence of undoped ZnO and Cu-doped ZnO samples. These resultsalso revealed that the photocatalytic reactions were induced bythe catalyst in combination with UV irradiation which led todegradation decolorization of Rz aqueous solution. The UV-visabsorbance spectra of ZnO nanorod solution, Rz solution, and ZnO+ Rz solution with and without UV irradiation are shown in Fig. 5.ZnO shows a characteristic peak around 372 nm correspondingto its band gap while Rz shows peaks at 602 and 374 nm. Thepeaks around ∼374 nm are similar in the two species (ZnO andRz). Therefore the characteristic absorption peak (λ = 602 nm)of Rz was used to track the photocatalytic degradation process.Fig. 6(a)–(d), present the variation of absorption spectra of aqueousRz solution in the presence of the catalyst of 0%, 5%, 10% and 15%Cu-doped ZnO nanorods. From these figures, it could be found thatthe intensity of the characteristic adsorption peak (λ = 602 nm)decreased with the UV irradiation. A new absorption peak at 574nm occurred in the degradation process. This peak corresponds toRf. This photoproduct of the Rz deoxygenation, RF, was also foundto undergo photodecomposition in the presence of Cu-doped ZnO

Fig. 4. The UV-visible absorption spectra of undoped and Cu-doped ZnO nanorods.The vertical dotted line shows the trend of shift in the excitonic absorption peakwith increasing Cu content.

nanorods. This is evident from the reduction of RF peak shownin Fig. 6(a)–(d). This indicates that the photoreaction involves therupture of the dye. When the UV irradiation time reached 25 min,the characteristic peak of RF significantly reduced in all samples.It can also be observed from Fig. 6(a)–(d) that with increasing Cucontent, conversion of Rz to RF increased more. The produced RFthen degrades with the subsequent irradiation time. On the basisof all these findings it is clear that Rz under UV irradiation in thepresence of Cu-doped ZnO undergo two types of photoreactions. Itconverts to RF and also undergoes photodecomposition. The totalconversion of Rz to RF can also be ruled out, since the RF peakwas always found to be very much less than the original Rz peak.The percentage of photo degradation of Rzwas calculated from thefollowing Eq. (1):

%Degradation =

1 −

Ct

C0

× 100 (1)

378 R. Mohan et al. / Solid State Communications 152 (2012) 375–380

Fig. 5. Comparison of the UV-visible absorption spectra for the control experiment.

where Ct is the absorbance after time t and C0 is the dyeinitial concentration before degradation. The percentage of photodegradation of Rz with irradiation time is plotted in Fig. 7.As depicted in Fig. 7, both pure ZnO and the Cu-doped ZnOnanorods can effectively remove the Rz in a short time, butZnO nanorods with increasing Cu doping exhibit enhancedphotocatalytic activity. It can also be seen that the Cu-dopedZnO catalysts with a Cu doping of 15% show the highestphotocatalytic activity. The photocatalytic activity of the Cu-dopedZnO samples increases with an increase of the Cu doping level.The influence of the initial concentration of the solution on thephotocatalytic degradation rate of the most organic compounds isdescribed by the pseudo-first order kinetics, which is rationalizedin terms of the Langmuir–Hinshelwood model, modified toaccommodate reactions occurring at a solid–liquid interface [29].The photocatalytic activity of Cu doped ZnO for the degradation ofRz, obeys the pseudo-first-order reaction kinetics and is given by

Eq. (2) as

Ct = C0e−kt . (2)

This equation can be written in a form given in Eq. (3) as

lnC0

Ct

= kt (3)

where k is the rate constant of pseudo-first-order reaction. Aplot of ln(C0/Ct) versus the UV irradiation time for the Rzphotodegradation catalyzed by Cu-doped ZnO is shown in Fig. 8. Anear linear relation between ln(C0/Ct) and the irradiation time hasbeen observed, which implies the photodegradation of resazurinfollows the pseudo-first order kinetics. The nonlinearity for highCu doping and for more irradiation time is due to the fact thatincreased surface to volume ratio in Cu doped ZnO results in theincreased production of electron and holes, but after a certaintime the number of adsorbed resazurin (Rz) molecules (due toits photodegradation) will not increase proportionally [30]. Thisresulted in the nonlinearity of photocatalytic activity. The obtainedfirst-order rate constants (k) are, 0.0412, 0.0544, 0.0717 and0.1017 min−1 for 0%, 5%, 10% and 15% Cu doped ZnO samplesrespectively, which reveal a significant effect of Cu-doped ZnO onthe photocatalytic degradation of Rz. The higher the first order rateconstant, the more outstanding the photocatalytic performance.The k for 15% Cu-doped ZnO, showing the excellent photocatalyticactivity, is equal to 10.17×10−2 min−1 which is about double thatwith pure ZnO.

3.3. Mechanism of photocatalysis

The photocatalytic reaction in contact with a semiconductorsurface is schematically represented in the Fig. 9. When light withenergy higher or equal to the band gap energy is irradiated to thesemiconductor surface, a photo excited valence band electron is

Fig. 6. The variation of UV-vis absorption spectra of Rz aqueous solution in the presence Cu-doped ZnO nanorods with different Cu content recorded every 10 min uponexposure to UV light; (a) Cu = 0%; (b) Cu = 5%; (c) Cu = 10%; (d) Cu = 15%. With increasing irradiation time the absorbance of the peak due to Rz (602 nm) decreases(marked with dotted blue line) and that due to the reduced form of the dye, Rf (574 nm) decreases (marked with dotted red line) also formed but decreases upon subsequentirradiation time (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

R. Mohan et al. / Solid State Communications 152 (2012) 375–380 379

%

Ph

oto

gra

dat

ion

Time (min)

Fig. 7. The comparison of % degradation of resazurin versus time using pure ZnOand Cu-doped ZnO photocatalyst samples with different Cu doping concentrations.

Fig. 8. Kinetics of the degradation of resazurin using pure ZnO and Cu-doped ZnOphotocatalyst samples with different Cu doping concentrations.

promoted to the conduction band, leaving behind a hole in thevalence band. Thus creating electron-hole pairs (1). Migration ofthe pairs to the semiconductor surface (2) allows the occurrenceof redox reactions with adsorbates with suitable redox potentials(3). Thermodynamically, oxidationwill occur if the redox potentialof the valence band is more positive than that of the adsorbates.Similarly, conduction band electrons can reduce adsorbed speciesif their redox potential ismore negative than that of the adsorbates.

Clearly, the most likely outcome of the formation of electron-holepairs is their simple recombination, with subsequent release ofthermal energy and/or light (4). Recombination proceeding withinthe same timescale as the redox processes being promoted reducesphotocatalytic activity considerably. Therefore, for electron-holepairs, whose lifetime can normally be considered to be of the orderof femtoseconds, separation is a vital parameterwhen determiningactivity. Before any reaction or recombination step can take placethe charge carriers must be trapped, and defects play an importantrole in this process. Defects can be either intrinsic, for exampleoxygen vacancies in nanostructured reducible oxides, or extrinsic,such as dopants or impurities. Minimizing the likelihood ofrecombination has led to the preparation of nanostructured oxidematerials together with the development of materials capable ofenhanced photocatalytic activity through doping. In the presentstudy, the doping of Cu in ZnO results in a lower band gap andthe increase of defect sites. It has also been believed that thehigher surface-to-volume ratio for the nanorods results in moresurface oxygen vacancies [31–33]. More oxygen vacancies willtherefore increase surface activity and thus the photocatalyticproperty. The photoelectron can easily be captured by the Rzmolecule, to produce an Rf molecule, whereas the photo inducedholes can be easily trapped by electronic donors, such as OH ororganic pollutants, to further degrade Rz and Rf. The possiblephotocatalytic reaction process can be proposed as follows:

ZnOhv>Eg−−−→ e−

cb + h+

vb

Rz + e−

cb → Rf

h+

vb + H2Oads → OH•

ads + H+

Rz + OH•

ads → decompositionRf + OH•

ads → decomposition.

As observed in the Fig. 6, after UV irradiation the RF absorptionpeak is higher than that of Rz in Cu-doped ZnO sample with higherCu concentration. This suggests that Cu-doped samples offer morehole trapping center and therefore the photoelectron can be easilycaptured by Rz molecules to form RF.

The comparison of the rate constant and band gap energy withCu doping content is shown in the Fig. 10. This figure clearlyshows that with Cu doping there is a gradual decrease in bandgap energy and a gradual increase in the rate constant value.The lowering of band gap energy in Cu-doped ZnO resulted inenhanced photocatalytic activity.Moreover,whenCu2+ was dopedin ZnO, more surface defects could be produced and a space chargelayer could be formed on the surface, which is beneficial forhindering the recombination of photo induced electron-hole pairs.This contributed to the improvement of the photocatalytic activity

Fig. 9. Schematic representation of photocatalytic reaction in contact with a semiconductor surface.

380 R. Mohan et al. / Solid State Communications 152 (2012) 375–380

Fig. 10. The comparison of the rate constant and band gap energy with Cu dopingcontent.

of Cu doped ZnO nanorods, compared to that of pure ZnO. It shouldalso be stated that further understanding of the photocatalyticproperties of Cu-doped ZnO nanorods demands more in-depthexperimental and theoretical work.

4. Conclusions

Cu-doped ZnO nanowires with an average diameter of 60–90nm have been fabricated by a vapor transport method. Thephotocatalytic activity of pure and Cu-doped ZnO samples wasstudied by the degradation of resazurin. The result showedthat Cu doping concentration had a remarkable effect on theefficiency of photocatalytic activity. The surface defects caused byCu doping could serve as favorable trap sites of the electrons orholes to reduce their recombination and consequently increasethe photocatalytic activities. Furthermore, a bigger surface tovolume ratio in nanorods results in more surface oxygen vacanciesand thus increased surface activity. The Cu-doped ZnO nanorodsprepared here exhibited good photocatalytic activity for organicpollutants in water and showed their potential use in wastewatertreatment.

Acknowledgments

This researchwas supported by a National Research Foundationof Korea Grant under contract numbers 2009-0087091 and2011-0015829 through the Human Resource Training Project for

Regional Innovation. A Part of this research was also supportedby the 2011 Jeju Sea Grant College Program funded by Ministry ofLand, Transport and Maritime Affairs (ML TM), Republic of Korea.A Part of this work was carried out at Research Instrument Center(RIC), Jeju National University, Jeju, Republic of Korea.

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