Chemical Physics Letters 528 (2012) 16–20

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Chemical Physics Letters

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Preparation and XRD analyses of Na-doped ZnO nanorod arrays basedon experiment and theory

X.P. Yang, J.G. Lu ⇑, H.H. Zhang, Y. Chen, B.T. Kan, J. Zhang, J. Huang, B. Lu, Y.Z. Zhang, Z.Z. Ye ⇑State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China

a r t i c l e i n f o

Article history:Received 18 November 2011In final form 9 January 2012Available online 18 January 2012

0009-2614/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.cplett.2012.01.019

⇑ Corresponding authors. Fax: +86 571 879 52625.E-mail addresses: lujianguo@zju.edu.cn (J.G. Lu), y

a b s t r a c t

ZnO nanorod arrays (NRAs) with different Na contents were prepared by thermal evaporation. Sodiumpyrophosphate was adopted as the Na source. The Na contents in NRAs were determined by X-ray pho-toelectron spectra to be 0, 6.1, and 9.4 at.%. X-ray diffraction (XRD) analyses of Na-doped ZnO NRAs wereperformed in experiment and by first-principle calculation with the assumption of Na substitutions. Acouple of typical changes were found in XRD patterns of Na-doped ZnO. The simulation results wellagreed with the experimental data, which revealed that Na mainly located at the substitutional sitesin Na-doped ZnO NRAs.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

ZnO is well known as an important semiconductor with a widedirect band gap of 3.37 eV and large exciton binding energy of60 meV at room temperature [1,2]. The study of nanostructuredZnO has attracted much interest in the last few decades due toits potential applications in such fields as photocatalysis [3], piezo-electricity [4], optoelectronics [5], photovoltaic conversion [6], andgas-sensing devices [7]. The synthesis of stable p-type ZnO nano-structures would be the key to open up substantial opportunitiesfor electronic and optoelectronic nanodevices. However, it is stilldifficult to make stable p-type conductivity in ZnO, since the dop-ant solubility is low and the defects compensate for the substitu-tional impurity levels through the formation of deep level trapsrather than operative shallow acceptors [8–10].

In previous literatures, p-type ZnO nanostructures and filmswere reported using N or P dopants [11–16]. Recently, Na-dopedp-type ZnO nanostructures have been claimed by Liu et al. [17].Among several p-type dopants of ZnO, Na, a group-I element, wastheoretically predicted to have a shallow acceptor level of170 meV, assuming that Na substitutes for Zn. In addition, the sol-ubility of Na in ZnO is high. In previous reports, the Na content maybe up to 20% in ZnO and it is still a p-type without additional phase[18,19]. In a physical picture, the valence-band maximum mainlyconsists of the anion p orbitals with some mixing of the cation pand d orbitals, which gives only small perturbations when Zn is re-placed by Na [10]. In other words, the strain around the dopants issmall, and the coupling between the anion and cation orbitals isweak. Therefore, the acceptor level of Na substitutions in ZnO is rel-atively shallow. However, in theory Na also has a tendency to

ll rights reserved.

ezz@zju.edu.cn (Z.Z. Ye).

occupy an interstitial acting as a donor. In view of this, it will be apressing problem to find a convenient way to judge the Na sitesin ZnO.

X-ray diffraction is a powerful technique for characterizing thestructural properties of materials. Indeed, the shift of XRD peakscan be used to identify the doping effect of ZnO. Due to the dopingeffect, the intrinsic lattice structure of ZnO could be changed moreor less. If Na replaces Zn effectively, the Na content in ZnO can bewell presented in XRD due to the significant difference of atomicradiuses. To date, however, there has been no systematic studyon XRD change of Na-doped ZnO (ZnO:Na). In this Letter, we pres-ent the growth and properties of ZnO:Na nanorod arrays (NRAs).The migration and relative migration of XRD peaks were studiedin detail experimentally and computationally. The ZnO:Na is singlecrystal for a nanorod, and thus the effect of grain boundary on XRDanalyses can be ruled out.

2. Experiments

ZnO NRAs were synthesized by thermal evaporation, which wascarried out in a quartz boat covered with a quartz plate in a low-pressure horizontal tube furnace, with a background pressure lessthan 2 Pa, as shown in Figure 1. For preparing ZnO:Na NRAs, ZnOpowder (0.5 g), graphite powder (0.5 g), and sodium pyrophos-phate powder (0, 0.05, and 0.1 g) were grinded together and trans-ferred to the left side of the quartz boat (length of 17 cm). Si (100)wafers, with the 200 nm thick polycrystalline ZnO films previouslydeposited by pulsed laser deposition (PLD) as the buffer layers,were used as the substrates and typically placed �3–5 cm awayfrom the other side of the boat. This quartz boat was then placedinside a furnace tube, with the center of the quartz boat locatedat the center of the furnace tube, and the open side of the boatwas placed downstream of the gas flow. Then the furnace was

Figure 1. Schematic diagram of the experimental setup.

X.P. Yang et al. / Chemical Physics Letters 528 (2012) 16–20 17

ramped to 920 �C at a rate of 20 �C/min and hold for 20 min under aconstant flow of nitrogen (99sccm) and oxygen (1sccm). Weadopted the quartz plate to ensure that Na element could be dopedinto the NRAs more efficiently by preventing excessive dispersionof Na out of the boat. Under abovementioned conditions, threesamples were prepared; they are undoped ZnO NRAs andNa-doped ZnO NRAs with 0.05 and 0.1 g sodium pyrophosphatepowders in source materials, named as sample 1, sample 2, andsample 3, respectively.

The structural analyses of samples were carried out by XRD(X’Pert PRO) using Cu Ka line (1.540598 nm for Cu Ka1 and1.544426 nm for Cu Ka2) at the excitation voltage of 40 kV andtube current of 40 mA. The morphologies of the NRAs were charac-terized by field emission scanning electron microscopy (FESEM,Hitachi S-4800). The contents of Na, Zn and O were measured byXPS (Thermo ESCALAB 250) with an Al Ka (=1486.6 eV) source.The microstructures of ZnO NRAs were characterized by transmis-sion electron microscope (TEM, FEI F20).

3. Calculation details

The theoretical calculations were performed with the CASTEPcode [20], which is based on density functional theory (DFT) usingpseudopotentials and plane wave expansion of wave functions[21]. The exchange and correlation potential is described by thegeneralized gradient approximation (GGA) in the scheme of thePerdew–Burke–Ernzerhof (PBE) functional [22]. The potentialsamong the ion core and valence and valence electrons were de-scribed by norm-conserving pseudopotentials (NCP) [23]. Theprimitive cell parameters of ZnO with a wurtzite structure area = b = 0.3249 nm, c = 0.5206 nm [24]. Based on the primitive cell,infinite crystal was constructed from a supercell (2 � 2 � 2 or3 � 3 � 2) containing 32 or 64 atoms per cell. In case of 6.25 at.%of Na content, one Zn atom is substituted with a Na atom (Zn15O16-

Na1), and three Zn atoms are replaced by Na when it comes to9.375 at.% (Zn29O32Na3). With this method, we optimized the crys-talline structures and calculated the total energy. The valence-elec-tron configurations for the elements discussed in this Letter are Zn:3d104s2, O: 2s22p4, Na:2s22p63s1, respectively. We used cutoff580 eV for the plane wave expansion and a 5 � 5 � 4 mesh of spe-cial k points for integrations over the Brillouin zone. The self-con-sistent accuracy is set to the degree that the energy of every atomconverges to 10�6 eV. The lowest energy structures were chosenfor the following discussion of Na-doping of ZnO.

The reflex module was used to calculate the XRD pattern. ForXRD calculation, the copper is selected for the X-ray source, k1 isspecified as 1.540562 Å without k2, the step size is 0.05�, and theBragg–brentano line shift correction is applied to the data.

4. Results and analysis

4.1. Samples characterization

Figure 2a–c show the SEM images of ZnO NRAs. The nominallyundoped ZnO NRAs were grown vertically from the substrate, witha rather uniform morphology, as shown in Figure 2a. The diameters

of the nanorods normally range from 45 to 155 nm. In contrast,when the 0.05 g sodium pyrophosphate was added into the sourcematerials, the shape of rod tip likes or nearly likes a flat tortilla, andthe diameters of nanorods become quite uniform ranging from 170to 182 nm, as seen in Figure 2b. However, when the sodium pyro-phosphate increased to 0.1 g, the nanorod tips start to becomepointed and the nanorods lean randomly, as shown in Figure 2c.In general, there are many reasons that can cause the nanorodshape variation. In our cases, the presence of Na atoms in the ZnOmatrix may play an important role for these morphologicalchanges, since the growth conditions are same for the threesamples.

In order to identify the composition of NRAs, we performed ele-mental tests using XPS. Figure 2d and e display the Na 1s XPS spec-tra of samples 2 and 3. The Na 1s peaks are centered at about1071.9 and 1072.0 eV for samples 2 and 3, respectively, whichwere attributed to Na-O bonding within ZnO phase [25]. Usingthe measured areas of Na 1s and Zn 2p3 peaks, the Na contentswere computed and thus determined to be 6.1 and 9.4 at.% forsamples 2 and 3, respectively. The atomic sensitivity factors usedhere are 2.3 for Na 1s and 4.8 for Zn 2p3. A wide range of Na con-tents may be helpful to clarify its behaviors in ZnO. In theseregards, we choose the highest Na content of 9.4 at.% for ourdiscussion.

Images shown in Figure 3 present the crystal morphology andmicroscopic structures. Figure 3a–c show the low-magnificationTEM images of samples 1, 2 and 3, respectively. The morphologiesof three samples are different from each other, which may be anindicative of different Na contents in ZnO nanorods [26]. Figure3d displays the high-resolution TEM (HRTEM) images at the mid-dle region of ZnO:Na nanorods of sample 2, revealing the atomi-cally resolved wurtzite ZnO single crystals with the atomic layerdistance of 0.278 nm, which is slightly larger than that in intrinsicZnO (0.26 nm) [27]. Similar observations were obtained for sample3. This indicates the lattice expansion due to the substitution of Nafor Zn, as reported recently [28]. The atomic layer repeating isalong the growth direction of ZnO nanorods. Figure 3e shows animage obtained by Fast Fourier Transform (TFT) from Figure 3d,exhibiting that the atomic layers repeat along the [002] direction.That is to say, ZnO nanorods grew along the [002] orientation.

Figure 4a shows the XRD patterns of samples 1, 2, and 3. Thestandard diffraction pattern of wurtzite ZnO is also illustrated inFigure 4a for comparison. The (002) peak is strong and dominantin the XRD patterns. Both undoped and Na-doped ZnO NRAs wereof single-phase wurtzite ZnO structure and quasialignment with[002] growth direction. As displayed in the microscopic views ofXRD patterns, one of the significant observations is that the peakpositions of ZnO:Na NRAs shift to lower angles as compared withthose of undoped ZnO NRAs. This shift becomes larger with moreNa content in ZnO. For one sample, the shift of (101) peak is largestand that of (002) smallest among the (002), (100), and (101) dif-fraction peaks. The ionic radiuses of Na and Zn are 1.02 and 0.74 Å,respectively. If Na substitutes Zn in ZnO, a lattice expansion wouldbe induced [20], which results in the shift of XRD peaks to lowerangles.

4.2. Calculation and analysis

Zinc oxide crystallizes in two main forms, hexagonal wurtz-ite and cubic zincblende. The wurtzite structure is most stable atambient conditions and thus most common. Here, the bond alongc-axis is defined as a-bond, and other three are called b-bond inwurtzite structure, as shown in Figure 4b. The bond-length maybe lengthened or shortened due to the dopant atoms. We definedtwo functions, Ex and Fx

!to provide a means to evaluate the overall

influence of dopant atoms on chemical bonds.

Figure 2. (a)–(c) are the top view SEM images of ZnO NRAs: (a) sample 1, (b) sample 2, and (c) sample 3. Scale bars for (a)–(c) are 150 nm. (d) Na 1s XPS spectra of ZnO:NaNRAs sample 1. (e) Na 1s XPS spectra of ZnO:Na NRAs sample 2.

Figure 3. Low-magnification TEM images of undoped ZnO nanorods of sample 1 (a), ZnO:Na nanorods of sample 2 (b), and ZnO:Na nanorods of sample 3 (c). (d) HRTEMimage of ZnO:Na nanorods of sample 2. (e) Image obtained by FFT from Figure 3d.

18 X.P. Yang et al. / Chemical Physics Letters 528 (2012) 16–20

Ex ¼P

Idx�!� Iox

�!������

nð1Þ

Fx!¼

PIdx�!� Iox

�!� �

nð2Þ

where x is a or b, Id and Io stand for the calculated bond-lengthsafter and before doping, respectively, and n stands for the numberof a- or b-bonds in a supercell. Ex reflects the sum of crystal latticedisorder and Fx

!indicates the change of average atomic layer dis-

tance in the x direction.Figure 4c shows the simulated XRD patterns of ZnO with Na

contents of 0, 6.25, and 9.375 at.%, respectively. The diffractionpeak shift to lower angles in Na-doped ZnO manifests itself bythe elongation of the lattice constant; that is to say, the inter-planedistance increases. As the Na contents increase from 0 to 6.25 to9.375 at.% in ZnO, the peak shift becomes larger and larger, whichagrees well with the experimental observations. Also, note that the(100) and (101) peaks become rather wide and split into twopeaks when they move to lower angles. The space between two

splitting peaks increases as the peak shift enlarges. Taking the(100) peak for example, the space increases from 0.225� to0.354� as the Na content is up from 6.25 to 9.375 at.%. The (002)peak shows no splitting. In our computational model, the absoluteperiodicity of dopant atoms in ZnO lattice is assumed, and thechemical bonds are different due to the different distances awayfrom the Na atoms. Consequently, two sets of periodic lattice struc-tures are formed, leading to the peak splitting in the calculatedXRD patterns.

Figure 5 shows the theoretical shifts of different XRD peaks ofZnO:Na with Na contents of 6.25 and 9.375 at.%, as compared withthose of undoped ZnO. This plot demonstrates that the shifts arelarger for all peaks of ZnO:Na as the Na contents increase. Also,the peak shifts become larger as 2h values increase, with an exceptthat the shift of the (002) peak is smallest.

Table 1 lists the calculated Ex and j FX�!j values, as well as the

experimental and calculated position shift of (100), (002), and(101) peaks of ZnO with different Na contents. Taking the influenceof peak splitting into account, the (002) peak shows the smaller

Figure 4. (a) XRD patterns of sample 1 (black line with triangle dots), sample 2 (blue line with rounded dots), sample 3 (red line with squarish dots), and the standarddiffraction pattern of wurtzite ZnO (vertical dash line). Insets are microscopic views of (100), (002), and (101) peaks. (b) The ball-and-stick model of ZnO crystal shows thea-bonds along c-axis and b-bonds. The central atom is Zn, the rest are O. (c) Simulated XRD patterns of ZnO with Na contents of 0 at.% (black line with triangle dots), 6.250 at.%(blue line with rounded dots), and 9.375 at.% (red line with squarish dots). (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)

Figure 5. Offset of XRD peaks for ZnO:Na NRAs with different Na contents:6.250 at.% (black line with squarish dots) and 9.375 at.% (red line with roundeddots). (For interpretation of the references to colour in this figure legend, the readeris referred to the web version of this article.)

Table 1Calculated Ex and jFx

!j values and experimental and calculated results of position shift(A) of (100), (002), and (101) peaks of ZnO with different Na contents.

Calculated Experimental

6.25 at.% 9.375 at.% 6.1 at.% 9.4 at.%Ea = 0.040 Ea = 0.109Eb = 0.033 Eb = 0.065

Fa�!

= 0.008 Fa�!

= 0.086

Fb�!

= 0.018 Fb�!

= 0.056A(1 0 0) = 0.194–

0.419�A(1 0 0) = 0.788–1.142�

A(1 0 0) = 0.086� A(1 0 0) = 0.205�

A(0 0 2) = 0.223� A(0 0 2) = 1.01� A(0 0 2) = 0.047� A(0 0 2) = 0.065�A(1 0 1) = 0.274–

0.483�A(1 0 1) = 0.997–1.287�

A(1 0 1) = 0.087� A(1 0 1) = 0.224�

X.P. Yang et al. / Chemical Physics Letters 528 (2012) 16–20 19

shift than the (100) and (101) peaks. The shift of the (101) peak islarger than the (100) peak by about 0.07� and 0.13� for ZnO:NaNRAs with 6.25 and 9.375 at.% Na contents, respectively. By com-bining experimental and calculated analyses, it is now possible to

discover how peak shift and splitting are formed. By the regularityof Ex in Table 1, we conclude that the crystal lattice becomes moreand more disordered with increasing Na contents. In other words,the more dopant atoms there are, the more serious bond-lengthchanges appear. The value of Ea is bigger than that of Eb whichmeans that the bonds are varied more greatly in the c-axis directionthan any other directions. It is reasonable since the distance be-tween atomic layers in the c-axis direction is larger than otherdirections. However, the larger disorder does not always causegreater changes of ZnO crystal lattice as a whole, which can be con-firmed by the regularity of j FX

�!j listed in Table 1 The most probablereason is that the disorders counterbalanced each other in differentdirections, so the change of crystal lattice is small overall. Note thatthe c-axis direction is the most influential one in this case. Thus, theweak force between atomic layers leads to the smallest change ofZn-O bands in the c-axis direction. The experiment results wellagree with the theoretical calculations.

5. Conclusions

In summary, ZnO NRAs were synthesized by thermal evapora-tion, with the Na contents of 0, 6.1, and 9.4 at.%. ZnO nanorods aresingle-crystal growing along with the [002] direction. XRD analysesof ZnO:Na NRAs were carried out experimentally and theoretically.In experiment, the XRD peaks shifted to lower angles as the Na con-tent increased in ZnO NRAs, and the shift of (002) peak was smallestamong the (002), (100), and (101) peaks. In theory, by assumingthat Na substituted for Zn in ZnO, all the XRD peaks shifted to lowerangles with increasing Na contents, and the peaks were broadenedand even splitted with the smallest change of (002) peak. Theywere in good accordance between experiments and calculations.The results revealed that Na mainly located at the substitutive sitesin Na-doped ZnO NRAs, which may result in p-type conductivity ofZnO. The XRD method was a convenient technique to judge the exis-tence and doping effect of Na in ZnO crystal.

Acknowledgments

This work was supported by Specialized Research Fund for theDoctoral Program of Higher Education under Grant No.200803351004 and National Natural Science Foundation of Chinaunder Grant Nos. 51172204 and 51002131.

20 X.P. Yang et al. / Chemical Physics Letters 528 (2012) 16–20

References

[1] P. Yang et al., Adv. Funct. Mater. 12 (2002) 323.[2] M.H. Huang et al., Science 292 (2001) 1897.[3] J.L. Yang, S.J. An, W.I. Park, G.-C. Yi, W. Choi, Adv. Mater. 16 (2004) 1661.[4] Z.L. Wang, Appl. Phys. A-Mater. 88 (2007) 7.[5] X.D. Wang, C.J. Sumers, Z.L. Wang, Nano Lett. 4 (2004) 423.[6] Z.H. Chen et al., J. Phys. Chem. C 113 (2009) 13433.[7] H.-W. Ra, R. Khan, J.T. Kim, B.R. Kang, Y.H. Im, Nanotechnology 21 (2010)

085502.[8] F. Oba, S.R. Nishitani, S. Isotani, H. Adachi, I. Tananka, J. Appl. Phys. 90 (2001)

824.[9] S.B. Zhang, S.-H. Wei, A. Zunger, Phys. Rev. B 63 (2001) 075205.

[10] Y. Yan, S.B. Zhang, S.T. Pantelides, Phys. Rev. Lett. 86 (2002) 5723.[11] B. Xiang et al., Nano Lett. 7 (2007) 323.[12] G.D. Yuan et al., Nano Lett. 8 (2008) 2591.[13] P.J. Li et al., Nano Lett. 9 (2009) 2513.[14] B.Q. Cao, M. Lorenz, M. Brandt, H. von Wenckstern, J. Lenzner, G. Biehne, M.

Grundmann, Phys. Status Solidi RRL 2 (2008) 37.

[15] J.G. Lu, S. Fujita, T. Kawaharamura, H. Nishinaka, Chem. Phys. Lett. 441 (2007)68.

[16] G.D. Yuan, Z.Z. Ye, L.P. Zhu, Y.J. Zeng, J.Y. Huang, Q. Qian, J.G. Lu, Mater. Lett. 58(2004) 3741.

[17] W. Liu et al., J. Am. Chem. Soc. 132 (2010) 2498.[18] J.J. Lai et al., J. Appl. Phys. 110 (2011) 013704.[19] J.G. Lv, K. Huang, J.B. Zhu, X.M. Chen, X.P. Song, Z.Q. Sun, Phys. B 405 (2010)

3167.[20] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C.

Payne, J. Phys. Condens. Mater. 14 (2002) 2717.[21] M.C. Payne, M.P. Teter, D.C. Allan, T.A. Arias, J.D. Jornnopoulos, Rev. Mod. Phys.

64 (1992) 1045.[22] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.[23] D. Vanderbilt, Phys. Rev. B 41 (1990) 7892.[24] R.D. Vispute et al., Appl. Phys. Lett. 73 (1998) 348.[25] J.P. Lukaszewicz, M. Panas, J. Siedlewski, Sens. Actuators, B 32 (1996) 221.[26] W. Lee, M.C. Jeong, S.W. Joo, J.M. Myoung, Nanotechnology 16 (2005) 764.[27] Ü. Özgür et al., J. Appl. Phys. 98 (2005) 041301.[28] S.S. Lin et al., J. Phys. D Appl. Phys. 41 (2008) 155114.