This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2015 New J. Chem.

Cite this:DOI: 10.1039/c4nj02255f

Facile synthesis of single crystalline n-/p-typeZnO nanorods by lithium substitution and theirphotoluminescence, electrochemical andphotocatalytic properties†

Indrani Thakur,a Sriparna Chatterjee,*ab Smrutirekha Swain,ab Arnab Ghosh,c

Swaroop K. Beherad and Yatendra S. Chaudharyab

Both n- and p-type single crystalline ZnO nanorods were successfully synthesized via a facile one step

solvothermal method by lithium substitution. It was observed that substitution of Li is critical for the

formation of n-type or p-type ZnO nanorods. The detailed formation mechanism indicates that the

transition from n-type behavior to p-type behavior is due to the presence of defect states and lithium

incorporation in the lattice. Using photoluminescence (PL) measurements, the role of defect states in

generating the p-type or n-type nature of ZnO nanorods via Li substitution has been analyzed.

Furthermore, the photocatalytic performances of the as-prepared ZnO nanorods have been investigated

towards the degradation of rhodamine B and methyl orange, and the results demonstrate that the

n-type ZnO nanorods behave as an excellent photocatalyst compared to the p-type ZnO nanorods

owing to their higher charge carrier density.

1. Introduction

Development and exploration of one-dimensional semiconductorphotocatalysts1,2 has become a fascinating subject of researchas the space charge region in one-dimensional nanostructuresis well constructed along the longitudinal direction of thecrystal and thus the photogenerated electrons can flow alongthe crystal length. The increased delocalisation of electrons inone-dimensional systems leads to a significant decrease in e�/h+ (electrons/holes) recombination probability, which in turn,promotes the existence of a larger number of e� and h+ in theactive sites of the one dimensional nanostructure surface whichresults in higher photocatalytic activity. Among various semi-conductors, zinc oxide (ZnO), was found to be an attractivematerial, since it has a wide and direct band gap of 3.2 eV and alarge exciton binding energy of 60 meV, which is manifested byits multifunctional character and its potential applications inthe field of multiferroics,3 piezoelectrics,4 sensing5 and catalysis,6–8

and ease of synthesis with a unique morphology like nanorods,9,10

nanoprisms,11 nanosprings,12 etc. The dimensional anisotropyof the one-dimensional ZnO has contributed significantly to itssuperior electrochemical and photocatalytic properties. In orderto modify the optoelectronic and improve the catalytic propertiesof ZnO, efforts have been made to use doped ZnO structures.Doping not only affects the band gap of the material13,14 but alsoalters the oxidation states as well as structural parameters of thecrystal. Furthermore, it may also lead to a change in the redoxpotential value, which plays a primary role in determining thephotocatalytic activity of these doped materials.15 In this study,the nanorods of both n- and p-type ZnO have been synthesized toevaluate their photocatalytic activities and to understand theircorrelation with the conductivity type of ZnO and carrier concen-tration. ZnO is intrinsically n-type16 and several techniques havebeen employed to make it p-type by doping with group Velements such as N, P, As and Sb.17,18 The synthesis of p-typeZnO materials with group V elements is a challenging task due tothe formation of deep acceptor levels and low solubility ofacceptor dopants.19 In a quest to overcome these difficulties,researchers have shown that p-type doping can also be achievedby the substitution of Zn with group I elements like Li and Na, asthese materials can form shallow acceptor levels in the ZnO hostlattice. In particular, the acceptor level of Li substituting for Zn(LiZn) is 0.09 eV,7 which is the lowest value, among the energylevels of acceptor dopants reported. Furthermore, Li has thesmallest ionic radius (0.76 Å) among group I elements, which isclose to that of Zn (0.74 Å), which in turn favours substitution of

a Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and

Materials Technology, Bhubaneswar-751013, India. E-mail: sriparna@immt.res.inb Academy of Scientific and Innovative Research (CSIR-AcSIR), New Delhi, Indiac Institute of Physics, Sachivalaya Marg, Bhubaneswar-751005, Indiad School of Materials Science and Technology, Indian Institute of Technology (B.H.U.),

Varanasi-221005, India

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nj02255f

Received (in Montpellier, France)9th December 2014,Accepted 14th January 2015

DOI: 10.1039/c4nj02255f

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Li in Zn sites. The optical,20 ferroelectric,21 dielectric, infraredreflectrometry,22 FET23 properties of lithium doped ZnO thinfilms are well studied by several research groups. But there havebeen few studies on Li-doped ZnO nanorods until now. From2009 onwards, few interesting reports on properties like roomtemperature ferromagnetism,24 and stimulated emission25 oflithium doped zinc oxide nanorods have appeared. An enhancedferroelectric and dielectric property of lithium doped ZnOnanorods was reported by Gupta and Kumar.26 Photovoltaicsmade of lithium doped ZnO nanorod arrays have been fabricatedby Ruankham et al.27 and Lee et al., explained the p-type conduc-tion in lithium doped ZnO nanorods.28 Recently two other groupsreported the optical and magnetic properties of lithium doped ZnOnanorods29,30 in detail. Furthermore, there are handful of reportson the photocatalytic activity of lithium doped (/substituted) ZnOnanostructures. For instance, Xie et al. reported the catalytic activityfor biodiesel preparation31 and Ganesh et al. reported the influenceof lithium doping on the photocatalytic activity of a lithium dopedZnO nanopowder system.32 In this work, we have investigated thephotocatalytic activity of Li substituted ZnO nanorods, which to thebest of our knowledge has not been reported so far. Differentresearch groups have followed several synthesis techniques togrow lithum doped ZnO nanorods, which includes hydrothermalsynthesis,28 solid-state synthesis,24 the sol–gel method27 andthe pyro-hydrolysis route.32 In this study, we report a facile andone-step solvothermal synthesis method to grow single crystal-line lithium substituted ZnO nanorods. In contrast to earlierreports,33 we have observed that upon Li incorporation then-type behavior of ZnO transforms into p-type, and that upto1 at.% Li substitution, n-type behavior was maintained. Whenthe substitution was increased to 2 at.% then p-type conductivity isobserved and with further increase in lithium substitution (5 at.%)n-type conductivity reverts back again. We have addressed thisobservation taking into account the effect of substitution alongwith interstitial incorporation of Li into the ZnO lattice. Earlier ithas been shown that the photocatalytic activity of lithium dopedzinc oxide increases with increasing lithium concentration. How-ever, we find that the photocatalytic ability mostly depends on thepresence of the defect states and the charge carrier density ratherthan on the dopant concentration. The detailed results on thephotocatalytic activities and their correlation with the structural,electrical and photoluminescence properties of Li substituted ZnOnanorods have been presented.

2. Experimental2.1 Synthesis of Li substituted ZnO nanorods

Typically 1 g of commercially available zinc acetate ((C2H3O2)2Zn�2H2O) (Sigma-Aldrich 99.9%) was used as the precursor materialfor the preparation of ZnO nanorods. For the preparation of Lisubstituted ZnO samples, lithium acetate (C2H3LiO2�2H2O)(Sigma-Aldrich 99.9%) was added in accordance to the requiredatomic percentages (1, 2 and 5). The precursor materials weremixed thoroughly and kept in an oven at 110 1C overnight to ensurecomplete drying. Trioctylamine (TOA) (C24H51N) (Sigma-Aldrich

98%) was added as the solvent and the mixture was refluxedat 320 1C for 6 hours. After the reaction time was over, thesolution was allowed to cool down to room temperature auto-matically. The obtained powder was washed with ethanolseveral times to ensure complete removal of TOA. The washedpowder was air dried. Henceforth, the unsubstituted samplewill be represented as ZnO, while 1 at.%, 2 at.% and 5 at.% Lisubstituted samples will be represented as 1LiZO, 2LiZO and5LiZO, respectively.

2.2 Characterizations

The X-ray diffraction (XRD) data of the as prepared sampleswere recorded using a PANalytical X’pert Pro diffractometer.The surface area was obtained using the Brauner–Emmet–Teller (BET) from N2 physisorption data at 77 K, recorded ona Micromeritics ASAP 2020. About 100 mg of each sample wasdegassed under vacuum of 10�6 torr at 100 1C prior to N2

adsorption. Li ion detection was carried out using a flamephotometer with a detection limit of 50 ppm (Elico FlamePhotometer CL 378). The microstructure was analysed using aCarl-Zeiss field emission scanning electron microscope(FESEM). The morphology and crystal structure of individualnanorods were studied using a JEOL 2010 transmission electronmicroscope (TEM), equipped with a LaB6 filament, operated at200 kV. Samples for TEM were prepared by dispersing a verysmall amount of powder in ethanol and placing a drop of thedispersion on a carbon coated Cu grid. The capacitancemeasurement of the samples was carried out using a three-electrode IVIUM electrochemical workstation. Ag/AgCl and Ptwere used as a reference and counter electrode, respectively.Pellets of both ZnO and Li substituted ZnO samples wereconverted into electrodes by making ohmic contact with a Cuwire using silver paint and were covered by epoxy. Diffusereflectance UV-Vis spectra (Shimadzu UV-2450, Japan) of theunsubstituted and Li substituted ZnO samples were recordedusing BaSO4 as reference. Photoluminescence measurements wereconducted using a FLS980 Combined Steady State and LifetimeSpectrometer. The steady state photoluminescence (PL) was carriedout using a Xe lamp of power 450 W.

2.3 Photocatalysis experiments

The photocatalytic measurements were carried out at roomtemperature in a 80 mL beaker placed under a UV lamp ofpower 16 W. The photocatalytic performance of the nanorodswere examined via the photo-assisted decolorization of a 10�6

M aqueous solution of Rhodium B (RhB) and similar concen-tration of methyl orange (MO) under UV irradiation. Thepowder samples were dispersed well in the dye solution byconstant stirring. Evaporative losses were minimized by tightlycovering the beaker. The dye solution was stirred in dark priorto UV irradiation for 2 hours to ensure adsorption–desorptionequilibrium of the dye on the catalyst surface and to correctlyaccount for the loss of the dye due to surface adsorption. Thecharacteristic absorption peak of RhB at 553 nm (and at 454 nmin case of MO) was chosen to monitor the photo-induceddegradation and the peak intensity was analyzed over a period

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of 90 minutes, at regular intervals using a UV-Vis spectrophotometer(Shimadzu UV-2450, Japan). To ensure the reproducibility of theresult, three different experiments were done varying the amount ofcatalyst and volume of dye. Control experiments i.e. photo-assisteddegradation of dye was carried out once in the absence of the catalystand once in dye solution with the catalyst in the dark.

3. Results and discussion

The unsubstituted and lithium substituted ZnO nanorods aresynthesized using a facile one step solvothermal method usingzinc acetate, lithium acetate as a source of zinc and lithium,and trioctyl amine as solvent. As described in the experimentalsection, the unsubstituted sample is represented as ZnO, while1 at.%, 2 at.% and 5 at.% Li substituted samples are representedas 1LiZO, 2LiZO and 5LiZO, respectively.

3.1. Structural and morphological studies

The XRD patterns of the as-prepared ZnO and Li substitutedZnO powders are shown in Fig. 1. All samples are crystallineand the diffraction peaks indicate the formation of hexagonalwurtzite structure (JCPDS card No. 36-1451).

As evident from Fig. 1, the peak positions do not show anysignificant variation upon Li substitution as compared to theunsubstituted ZnO nanorods. The calculated lattice constantsof ZnO and Li substituted samples are in good agreement withthe reported values of pure ZnO34 (Table 1).

The bond lengths of the samples are calculated usingthe following formula. The ratio a/c of the as-prepared nanorodsare related to the positional parameter u in the wurtzitestructure by26

u ¼ a2

3c2þ 0:25 (1)

and the Zn–O bond length l is given by

l ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2

3þ 1

2� u

� �2

c2

s(2)

The Zn–O bond length of Li substituted ZnO samples are close to thevalue for the unsubstituted sample35 (Table 1), which indicates thatthe substituted Li atoms are homogenously incorporated intothe ZnO lattice. The average crystallite sizes of the as-synthesizedsamples were calculated using Scherrer formula and found tobe in the range of 120–150 nm. No other characteristic peakswere detected, indicating the absence of any impurity in theas-synthesized powder samples. The plausible sequence of chemicalreactions leading to the formation of both unsubstituted and lithiumsubstituted ZnO nanostructures is represented as follows:10,36

Zn CH3COOð Þ2�2H2Oþ Li CH3COOð Þ � 2H2O

þ CH3 CH2ð Þ7� �

3N �!D Zn1�xLix CH3 CH2ð Þ7

� �þ CH3OHþH2OþNH3 "

(3)

Zn1�xLix[CH3(CH2)7] + CH3OH - Zn1�xLix(OH)2

+ CH3(CH2)7CH3 (4)

Zn1�xLix(OH)2 - {Zn1�xLix}2+ + OH� (5)

{Zn1�xLix}2+ + OH� - [{Zn1�xLix}2+ (OH)4]2� (6)

[{Zn1�xLix}2+(OH)4]2� - Zn1�xLixO + H2O + OH� (7)

Here, x = 0, 0.01, 0.02, and 0.05 for unsubstituted, 1 at% Lisubstituted, 2 at% Li substituted, and 5 at% Li substituted ZnOsamples respectively.

The FESEM image (Fig. 2a) of the as-prepared ZnO nanorodsshows the presence of uniform bundles of nanorods. Thetransmission electron microscopy (TEM) analysis yields detailsabout the morphology and crystal structure of the as-synthesizedsamples. The TEM image of the ZnO sample (Fig. 2b) clearlyindicates the formation of well-defined nanorods with a dia-meter of about 25–35 nm. It is observed that the nanorodmorphology is retained even after Li incorporation upto 5 at.%(Fig. S1, ESI†). Fig. 2c represents the TEM image of 1LiZO, whichagain shows presence of nanorods with the diameter of the orderof 20–30 nm. High Resolution TEM images in Fig. 2d and e,corresponding to a single nanorod of ZnO and 1LiZO respectively,shows well ordered crystalline planes with a lattice spacingFig. 1 XRD patterns of the as-prepared ZnO and Li doped ZnO nanorods.

Table 1 Table giving an comparative account of the structural, optical and electrochemical properties of the as-synthesized ZnO, 1LiZO, 2LiZO and5LiZO nanorod samples

Sample

Lattice parameters (Å)

Zn–O bond length (Å) Surface area (m2 g�1) Eg (eV) Nd (cm3) t1/2 (min) k (min�1)a b

ZnO 3.251 5.210 1.979 19.3 3.17 1.9 � 1025 6.2 0.1121LiZO 3.250 5.206 1.978 20.9 3.16 5.9 � 1023 11.5 0.0602LiZO 3.250 5.208 1.978 16.7 3.18 1.4 � 1017 12.5 0.0555LiZO 3.250 5.206 1.978 18.5 3.19 8.2 � 1022 11.7 0.059

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of approximately 2.6 Å, which coincides closely with thed-spacing of (0002) set of planes of ZnO. In the HRTEM images,it is further observed that Li atoms are homogenously incorpo-rated into ZnO without forming any Li cluster. The SAED patternshows the single crystalline nature of the nanorods (Fig. 2f). Thelattice constant c for 1LiZO nanorods was found to be about5.204 Å, which is in good agreement with the value calculatedfrom the XRD data. The TOA used in the synthesis acts both assolvent as well as a structure directing agent.13 TOA is basicallyan organic amine containing non-polar amine groups. There-fore, TOA is expected to have a strong tendency to attach to thenon-polar facet of ZnO. As a result, the (002) plane is the only oneexposed plane for epitaxial growth, which results in the prefer-ential growth of nanorods along the (002) axis.‘10

The surface area of the as-synthesized nanorods obtainedusing the Brauner–Emmet–Teller (BET) surface area analyzer isin the range of 16–21 m2 g�1. Fig. S2 (ESI†) represents the BETsurface area plot of 1LiZO.

In order to confirm the presence of Li in substitutedsamples, flame photometric analysis was performed. Theamount of Li detected in flame photometric analysis was foundto increase with increasing lithium concentration.

3.2. Optical properties

The diffuse reflectance UV-Vis spectra of the as-prepared samples(Fig. 3a) show the predominance of typical fundamental absorption

of both ZnO and Li substituted ZnO nanorods in the UV region.The absorption edge doesn’t show any significant variation withincrease in dopant concentration as compared to the ZnOnanorods. The optical band gaps for all samples were calculatedusing the following equation,

Eband-gap ¼hc

lint(8)

where, ‘h’ is the Plank’s constant (4.135 � 10�15 eV s), ‘c’ thevelocity of light (3 � 108 m s�1), and ‘lint’ the wavelength (m)corresponding to the intersection of the extension of the linear-part of the spectrum and the x-axis. The optical band gaps of allsamples are almost comparable, as shown in the inset of Fig. 3a.This implies that Li substituted ZnO nanorods are photo-activein the UV region of the spectrum.

Room-temperature photoluminescence of pristine ZnO and Lisubstituted ZnO nanorods were studied at an excitation wavelengthof 325 nm and the respective emission spectra are shown in Fig. 3b.For the better understanding of the charge transfer behavior, wehave put an eye guide to resolve the several sub-emission peaks(Fig. 3c–f). It is well known that the photoluminescence occurs dueto the radiative recombination of photo-generated excitons.37 In allfour samples, a peak at B396 nm is observed which shows theband-edge emission of ZnO38,39 and it is the characteristic emissionof ZnO arising due to free excitonic transitions.

A broad peak in the range of 450–800 nm is observed whichmay be attributed to defect state emissions37 originating fromtransitions between photoexcited holes and singly-ionized oxy-gen vacancies (VO).40,38,41 The broad emission peak can beresolved into several sub-emission peaks which includes thegreen emission bands at 480–520 nm, yellow emission bands at520–600 nm, and orange-red emission bands at 600–750 nm.The green emission occurs due to singly ionized oxygen vacanciesand neutral oxygen vacancies located at the surface. The yellowemission bands and orange-red emission bands are characteristicsof oxygen interstitials (Oi) located at the subsurface.37 ZnO and1LiZO nanorod samples exhibit similar photoluminescence char-acteristics (Fig. 3c and d). In the case of ZnO and 1LiZO, green andyellow emission bands predominate along with a red emissionpeak near 700 nm. A closer look in the individual photolumines-cence spectra reveals that the intensity of the green emission band(at 480–520 nm) is highest in the case of 1LiZO, which indicates thepresence of higher concentration of oxygen vacancies in 1LiZO. Thegreen emission bands for 2LiZO and 5LiZO are comparatively lessintense than other two ZnO and 1LiZO, which signifies thepresence of less oxygen vacancy states in these samples. It is alsoseen that the yellow emission band at 560 nm is less intense in2LiZO and totally absent in 5LiZO. Interestingly, a broad andintense orange-red emission band at 610 nm is observed in 2LiZO(which was absent in ZnO and 1LiZO). This band originates frompronounced oxygen interstitial contribution.37 The contributionfrom orange-red emission is further enhanced in 5LiZO. The resultsindicate that upto 1 at.% of lithium substitution, oxygen vacancyplays a major role and oxygen vacancies are known to control redoxreactions, which occur at the surface of nanorods, and thus controlthe efficiency of photocatalysts.6 With further increase in lithium

Fig. 2 (a) FESEM image of ZnO nanorods, TEM micrographs of (b) ZnOand (c) 1LiZO samples, (d) and (e) represents the HRTEM image of ZnO and1LiZO nanorods, respectively, (f) SAED pattern of 1LiZO nanorods.

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concentration, the interstitial oxygen defects predominate overoxygen vacancies.

3.3. Electrochemical properties

The Mott–Schottky analysis for the different nanorod sampleswas used to determine the flat band potential (VFB) andestimate the dopant density (ND) of the electrodes, usingfollowing Mott–Schottky equations.42

1

C2¼ 2

ee0A2eNDV � VFB �

kBT

e

� �(9)

Here C and A are the interfacial capacitance and area,respectively, ND the number of donors, V is the applied voltage,kB is the Boltzmann’s constant, T is the absolute temperatureand e is the electronic charge and the

Slope ¼ 2

ee0A2eND(10)

Fig. 4 represents the plot of inverse square values of area-normalized capacitance against the applied bias potential of allthe samples. The slope of the linear part of the curve is positive

for ZnO indicating its n-type nature (Fig. 4a). For 1LiZO, theslope of the linear part is again positive and therefore thissample is also n-type (Fig. 4b). When the lithium concentrationis increased to 2 at.%, the slope of the M–S plot becomesnegative which indicates the p-type nature of the sample(Fig. 4a). Interestingly, for sample 5LiZO, the slope is againpositive indicating the n-type nature of the sample (Fig. 4b). InZnO, the n-type behavior is attributed to the native pointdefects, which arises due to the presence of oxygen vacanciesas evident in the photoluminescence study (Fig. 3c). If Li+

substitutes the Zn2+ ions (as they have comparable ionic radii)and behave as a shallow acceptor24 then p-type conductivityshould be generated. But, in the case of 1LiZO, the amount ofoxygen defects is significantly large as observed in photolumi-nescence analysis (Fig. 3d). Therefore, it may be assumed thatin the case of 1LiZO, partial substitution of Li is not enough tocompensate all the oxygen vacancies and as a result excesselectrons predominate as majority charge carriers in 1LiZO,which induces n-type behavior. In the case of 2LiZO nanorodsamples, flame photometric analysis suggests the higheramount of Li incorporation than 1LiZO. Further, from the

Fig. 3 (a) DR-UV Vis spectra of ZnO and Li substituted ZnO nanorods. Inset shows the comparison of band gap values with variation in Li concentration.(b) Steady state room temperature photoluminescence (PL) emission spectra of the as prepared powder samples. The original spectra (black) were fittedinto several sub-bands (color dotted lines), (c) ZnO, (d) 1LiZO, (e) 2LiZO and (f) 5LiZO.

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photoluminescence study, it was seen that contribution ofoxygen vacancies (a green emission band at 500 nm) is quitesuppressed in 2LiZO (Fig. 3e). Therefore, it may be assumedthat these two factors, i.e. (i) higher amount of Li+ incorporation

in the ZnO lattice and (ii) suppressed contribution of oxygenvacancies, together may contribute to the p-type conductivity of2LiZO (Fig. 4a). Now, when the Li concentration was furtherincreased to 5 at.%, i.e. 5LiZO sample, again we observe n-typeconductivity (Fig. 4b). Probably here, along with substitution, Li+

ions also occupy the interstitial position in the lattice43 or theyform electrically inactive LiZn–Lii pairs,44 which generate n-typeconductivity. It is also to be noted here that the contribution ofoxygen vacancy (the green emission band at 500 nm) (Fig. 3f) isalso higher than 2LiZO (Fig. 3e) and comparable to ZnO (Fig. 3c).The larger amount of oxygen vacancy can also contribute ton-type conductivity. The carrier density ND was determined fromthe slope (eqn (10)) of M–S plots. The calculated carrier densityfor ZnO nanorods was found to be highest among all foursamples.

3.4. Photocatalysis

The liquid phase photocatalytic activity of ZnO and Li substitutedZnO nanorods was investigated under ambient conditions. Twocontrol experiments show that no degradation occurs in theabsence of either a nanorod photocatalyst or UV irradiation(Fig. S3, ESI†). Fig. 5a depicts the time dependent photodegradationof Rh-B in the presence of ZnO and Li substituted ZnOphotocatalysts.

The normalized absorption curve shows exponential decaywith time, thereby implying that the photodecomposition is afirst order reaction (Fig. S4, ESI†). In the presence of ZnOnanorod, complete degradation of Rh-B occurs within 30 minutesof irradiation. On the other hand, when 2LiZO was used as acatalyst, then it took about 60 minutes to completely degrade Rh-B.The value of t1/2 (time required for the 50% degradation of Rh-B) is

Fig. 4 Mott–Schottky plots comparing the semiconducting nature of (a)ZnO and 2LiZO nanorods and (b) 1LiZO and 5LiZO nanorods.

Fig. 5 (a) Liquid phase photocatalytic degradation of Rh-B by ZnO and Li substituted ZnO nanorods, (b) plot of t1/2 with varying concentration of the Lidopant, (c) plot of t1/2 vs. donor density (Nd) as a function of Li dopant concentration, and, (d) liquid phase photocatalytic degradation of MO by ZnO and2LiZO samples.

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a measure of the catalyst performance and was found to be 6.2, 11.5,12.5 and 11.7 minutes for ZnO, 1LiZO, 2LiZO and 5LiZO, respectively(Table 1). It is interesting to note that though surface areas of all foursamples are comparable, the unsubstituted ZnO nanorod sample isphotocatalytically more active than Li substituted ZnO nanorods(Fig. 5b). It is known that photodegradation of the dye molecule isprocessed through redox reactions and is greatly influenced bythe separation of the e�–h+ pair. Under UV irradiation, the photo-generated holes are trapped by the surface defects and thus decreasethe rate of a radiative recombination process. As a result, the surfacedefects act as active sites to facilitate the catalytic reaction.6 Asrevealed in PL analyses, ZnO nanorods are having larger amount ofsurface defect states than 2LiZO and 5LiZO samples. Thereforephotocatalytic activity of ZnO nanorods is highest. Although thesurface defect states of 1LiZO is higher than that of in ZnO, still it’sphotocatalytic activity is poor. This can be understood in terms ofcharge carrier concentration or donor density. A higher donordensity will lead to faster charge carrier transfer and also shiftsthe Fermi level of ZnO towards the conduction band, which in turnfacilitates the charge separation by increasing the degree of bandbending at the semiconductor surface and thus is expected toshow better photocatalytic performance.45 Carrier density of ZnOnanorods as calculated from M–S plot was found to be higher(1.9 � 1025 cm�3) than that of all Li substituted ZnO nanorods(5.9 � 1023 cm�3 for 1LiZO, 1.4 � 1017 cm�3 for 2LiZO and 8.2 �1022 cm�3 for 5LiZO) and thus shows better photocatalyticperformance (t1/2 = 6.2 minutes) compared to all lithium sub-stituted ZnO nanorod samples.

We have also studied the effect of surface charge of dyemolecules on photocatalytic degradation rates for all four typesof nanorods. Methyl orange (an anionic dye) was used as aprobe molecule instead of rhodamine B (a cationic dye) undersimilar experimental conditions. But it was observed that thesurface charge of the dye does not affect the overall efficiency ofthe photocatalyst. In this case also, ZnO nanorods showed betterphotocatalytic activity than Li substituted ZnO nanorod (Fig. 5d).

4. Conclusions

Here, we present a facile solvothermal method to grow singlecrystalline Li substituted ZnO nanorods. With the help of photo-luminescence and Mott–Schottkey analyses, we find that the n-typeor p-type behavior of Li substituted ZnO nanorods depends on thepresence of defect states and site of lithium incorporation. We havefound that n-type unsubstituted ZnO nanorods show better photo-catalytic activity as compared to lithium substituted ZnO nanorodswhich establishes that mere lithium substitution does not alwaysimprove the photocatalytic performance, rather it mostly dependson the presence of the defect states and the charge carrier density.

Acknowledgements

Research funding for this work was supported by the Depart-ment of Science and Technology, India (Grant no. IFA12-CH-65).The authors are grateful to Dr Deepa Kushalani, TIFR, Mumbai,

for carrying out the BET measurements and Prof. B. K. Mishra,Director, IMMT Bhubaneswar, for providing the support toundertake this work.

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