XRD, XPS, optical, and Raman investigations of structural changes of nanoCo-doped ZnO

Journal of Molecular Structure 1022 (2012) 167–171

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XRD, XPS, optical, and Raman investigations of structural changes of nanoCo-doped ZnO Z.K. Heiba a,b, L. Arda c,⇑ a

Taif University, Faculty of Science, Physics Department, Saudi Arabia Ain shams University, Faculty of Science, Physics Department, Cairo, Egypt c Bahcesehir University, Faculty of Arts and Sciences, Mathematics & Computer Sciences Department, Besiktas 34100, Istanbul, Turkey b

h i g h l i g h t s " The mixed oxides Zn1xCoxO (ZCO) as nanopolycrystalline powders and thin films. " x 6 0.12, Co replaces Zn substitutionally yielding ZCO single phase; x P 0.15 two phases. +2

" Replacing Zn

by Co+2 increases the lattice parameter a and decreases parameter c.

" The IR analysis shows red shift entirely dependent on Co concentration. low

" With increased concentration of cobalt, E2

a r t i c l e

i n f o

Article history: Received 19 July 2011 Received in revised form 25 April 2012 Accepted 30 April 2012 Available online 10 May 2012 Keywords: ZnCoO nanomaterial Sol–gel chemistry X-ray diffraction Crystal structure

shows a blueshift due to reduced mass effect.

a b s t r a c t The mixed oxides Zn1xCoxO (ZCO) (0.0 6 x 6 3.0) were prepared as nanopolycrystalline powders and thin films by a simple sol–gel process and dip coating method. Structural and microstructural analysis was carried out applying X-ray diffraction (XRD) and Rietveld method. Analysis showed that for x < 0.12, Co2+ replaces Zn2+ substitutionally yielding ZCO single phase, while for x P 0.15 two phases are identified; ZCO and Co3O4. Replacing Zn+2 by Co+2 affects the lattice parameters in opposite ways, the parameter c decreases while a increases with an overall decrease in the ratio c/a of the wurtzite ZCO, which deviates the lattice gradually from the hexagonal structure as Co+2 increases. The IR analysis shows red shift entirely dependent on Co concentration. With increased concentration of cobalt the Ehigh 2 . Raman mode exhibits a redshift attributed to the phonon softening caused by the ab-plane lattice expansion, while the Elow mode shows a blueshift due to the reduced mass effect caused by cation replacement. 2 Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Recently, there has been a great deal of interest in the physical properties of diluted magnetic semiconductors (DMSs) for their potential technological applications to optoelectronics, magnetoelectronics, and microwave devices. The substitution of the cations of III–V or II–VI nonmagnetic semiconductors by magnetic transition-metal ions (TM) such as Mn, Fe, and Co allows the existence of charge and spin degree of freedom in a single substance [1], which leads to a number of magnetic, optical, and magneto-transport phenomena. Recently, ZnO alloying with the 3d TM has attracted much attention as a dilute magnetic semiconductor (DMS), with room temperature ferromagnetism, for spintronic applications [2,3]. There are several theoretical arguments as well as experimental

⇑ Corresponding author. Tel.: +90 2123810323; fax: +90 2123810300. E-mail address: lutfi[email protected] (L. Arda). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.04.091

reports which predict that ZCO may be the most promising DMS material for room temperature ferromagnetism [4,5]. Many studies on the magnetic properties of ZCO exist, often with ambiguous and even contradicting results [6–11]. One of the key questions is whether the resulting materials is indeed a single phase alloy of ZCO; and if it is so, are the Co ions incorporate substitutionally for Zn2+ or interstitially with uniform distribution or clusters keeping the ZnO structure. The ionic radius of Co2+ (0.58 Å) is similar to that of Zn2+ (0.60 Å), so replacement of Zn by Co should not cause a significant change in lattice constants. However, the large crystal structure dissimilarity between wurtzite-hexgonal ZnO and rock-salt-cubic CoO can cause unstable phase mixing. The aims of the present work are: to prepare the mixed oxides Zn1xCoxO as nanopolycrystalline powders and thin films by a simple sol–gel process and dip coating method; to investigate the changes in the structure and microstructure of ZCO upon increasing Co content and discuss the limited miscibility of Co in ZnO keeping its hexagonal wertzeit

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structure. IR and Raman measurements were performed to study the Co ion position in the host ZnO lattice and the influence of Co doping on the lattice dynamic properties. 2. Sample preparation and characterization The mixed oxides Zn1xCoxO were prepared as solutions and polycrystalline nanoparticle powders with various compositions (0.0 6 x 6 3.0) using sol–gel technique. Zinc acetate dihydrate (C4H6O4Zn2H2O) and cobalt acetate tetrahydrate (CoC4H6O4 4H2O) were used as precursor materials and methanol (CH3OH) and acetyl acetone (C5H8O2) were as solvents and chelating agent. The appropriate weighing amount of the constituents were put all together in a Pyrex container and mixed with a magnetic stirrer at room temperature until a transparent solution was obtained. The total cation concentration of Zn1xCoxO was 0.1 mol/l. Powder samples were prepared by gelling and drying of sol–gel derived precursors solutions in a beaker. The obtained powders were ground and annealed individually in Ar/H2 atmosphere at temperatures in steps of 100 °C from 500 to 900 °C. For thin films, triethanolamin (C6H15NO3) was added to precursor solution to improve the adhesion of ZCO film on glass substrates. The surfaces of the glass substrates were ultrasonically cleaned in propanol, methanol and distilled water. The cleaned glass substrates were dipped into the ZCO solutions and then pulled through the vertical furnace at various temperatures between 250 °C and 350 °C. The film thicknesses were controlled by the withdrawal speed, the number of dipping and the dilution of the solution. Films were then annealed at 600 °C for 30 min under air in a box furnace same as the annealing of powder samples. XRD scans were recorded using a Rigaku diffractmeter with Cu Ka radiation. Microstructure properties of prepared samples were observed using scanning electron microscope (SEM) (JEOL, JSM-5910LV). Raman spectra were acquired at room temperature using a Jobin-Yvon T64000 Triple-mate instrument. The spectra were recorded using micro-Raman sampling in air with 514.5 nm from a coherent argon ion laser. The spectra reported here were averages of five acquisitions of 20 s integrations of a Nitrogen cooled CCD camera as detector. Optical transmittance spectra of the ZCO films were analyzed at room temperature by using a UV–NIR (Shimadzu-2101 PC).

Fig. 1. XRD patterns of the system Zn1xCoxO with x = 0.0, 0.05, 0.08, 0.10 and 0.15 arranged from bottom to top.

Fig. 2. SEM micrographs of Zn0.99Co0.01O (a and b) and Zn0.95Co0.05O (c and d) films at different magnifications.

3. Sample preparation and characterization 3.1. XRD structural analysis The XRD patterns of the system Zn1xCoxO (x = 0.01, 0.05, 0.08, 0.1, 0.15) are shown in Fig. 1. All prepared samples are polycrystalline and single phase up to x = 0.1, with ZnO wurtzite hexagonal structure of space group P63mc. This means that Co ions are incorporated substitutionally in the ZnO lattice replacing the Zn ions in the position 2b. Applying the MAUD program [12], Rietveld analysis showed that the occupation number of the Co ions in the position 2b is equal to the intended value during the preparation for each sample of x 6 0.1. Smooth, crack-free and pinhole-free thin films could be grown on glass substrate by sol–gel dip coating process. Fig. 2 shows the SEM surface morphology of ZCO nanofilms. The films depict homogeneous surface with clear individual grains of uniformly distributed size and shape. The surface morphology of the ZnCoO films depend on substrate nature and sol–gel parameters such as withdrawal speed, drying, heat treatment, deep number (film thickness) and annealing condition. Fig. 3 shows a cross-section SEM micrograph from the Zn0.95Co0.05O sample deposited on glass substrate. The film thickness of 110 nm is determined from the clear image contrast.

Fig. 3. SEM micrographs of a longitudinal cross-section of ZCO film deposited on glass substrate.

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Table 1 gives the refined structural parameters obtained from Rietveld analysis. For x P 0.15 two phases were identified in the diffraction patterns, ZCO and Co3O4, indicating the limited miscibility of Co+2 ions in the ZnO lattice. Obviously the distinct coordination preferences of Co and Zn prevent the formation of a continuous solid solution across the entire composition range, x. While Co has a point symmetry of Oh in its binary oxide CoO, wurtzite ZnO possesses only a C3v environment. Quantitative phase analysis applying Rietveld method showed that for x = 0.15 part of the Co+2 ions incorporated substitutionally in the ZnO lattice, and the other part segregates forming a separate Co3O4 spinel phase. Fig. 4 depicts the pattern fitting resulting from Rietveld quantitative phase analysis for x = 0.15. During refinements, the site 2b occupancy shared by Zn+2 and Co+2 was set free to refine giving values of 0.883 for Zn+2 and 0.117 for Co+2. Meanwhile, the resulting weight percentages are 97.9% for ZnO and 2.1% for Co3O4. Correlating these values together, we can conclude that the upper limit of Co+2-miscibility in the ZnO lattice is around 12%, for the present procedure. Other values were reported; 25% solubility of Co ions into the ZnO structure was achieved using a standard solid state reaction [13]. Using pulsed laser deposition (PLD), thin films with high Co concentration (30%) were grown on sapphire and mica substrates [14]. The different values reported for the percentage of Co ions miscible in the hexagonal wurtzeit ZnO may be attributed to the different preparation methods which may yield different structural defects. As shown in Table 1, anisotropic crystallite size and microstrain were found for all samples during Rietveld refinement with size  0i direction and values ranging from 45 to 89 nm along the h1 0 1 from 35 to 74 nm along the h0 0 0 1i direction. No systematic variation is observed for the crystallite size or lattice microstrain upon increasing the Co content. As a general remark for all samples, the microstrain increases with Co content and along the direction  0i. This h0 0 0 1i it is much less than along the basal directions h1 0 1 can be attributed to the high density of Zn population in the basal ab-plane (0 0 0 1) which when replaced by Co ions produces Frankel defects which results in high microstrain along this plane. Inspection of Table 1 indicates that replacing Zn+2 (0.60 Å) ions by Co+2 (0.58 Å) ones results in an increase in the cell parameter a and a decrease in parameter c with an overall decrease in the c/a ratio of the hexagonal, which results in deviating the structure gradually from the wurtzite structure and leads to limited miscibility of Co in ZnO lattice. The corresponding changes in the Zn/Co tetrahedron dimensions are shown in Table 1, where a slight increase in the Zn/ CoAO bond length is observed. The Zn/Co tetrahedrons have a base in the ab-plane and apex along the c-direction. Replacing Zn by Co decreases the average basal bond angles hObAZnAObi, from 109.74° for x = 0.0 to 109.43° for x = 0.1, and increases the average

Fig. 4. The XRD profile fitting resulting from Rietveld analysis of Zn0.85Co0.15O showing two phases, ZCO and Co3O4.

base-apex angles hOaAZnAObi, from 109.20° for x = 0.0 to 109.52° for x = 0.1. The internal tetrahedral distortion and the spontaneous polarization in Zn1xCoxO can be assessed using the atomic position parameter u. The four nearest cation-anion pairs are equidistant if
2 u ¼ 13 ac þ 14, whereas the charge separation in each tetrahedral unit will vanish if u = 3/8 (0.375). The Rietveld refined u-fractional coordinate monotonously increased from 0.3707 for x = 0.0 to 0.3726 for x = 0.1. This indicates that the tetrahedral distortion in ZnO is gradually relieved by alloying with CoO, and that ionic polarization of ZCO should decrease with the Co concentration. Similar results were obtained for Zn1xMgxO applying pair-distribution-function studies [15]. 3.2. Transmittance spectra The ZCO films are known to be transparent with slight green color which overwhelmed as the Co concentration increases with transparency faded away [16]. This darkening of the color is assigned as typical d–d transitions of Co ions [17]. Fig. 5 depicts the optical transmittance spectra of ZCO films with Co concentration 5%, 10% and 15%. All films exhibit a transmittance higher than 90% in the visible range; however, it falls very sharply in the UV region due to the onset of fundamental absorption. The internal d–d transition of high spin states Co2+ 3d7 (4F) in tetrahedral oxygen coordination are manifested as three absorption bands appear at approximately 570, 620 and 660 nm corresponding to 2.18, 2.00 and 1.88 eV respectively [18,19]. These absorptions are attributed

Table 1 The refined lattice parameters (a and c) (Å), u – fractional coordinate of Zn/Co, average Zn/Co–O bond lengths hdZn–Oi (Å), average bond angle hOAZnAOi (°), anisotropic crystallite size D (nm), microstrain e, and reliability factors: Rwp and Rp (%) obtained from Rietveld analysis of the powder XRD patterns of Zn1xCoxO (x = 0.0, 0.05, 0.08, 0.1, 0.15) systems. x

0.00

0.05

0.08

0.10

0.15

a c c/a u(Zn) hdZn–Oi hOb–Zn–Obi hOa–Zn–Obi D(100) e(100) D(001) e(001) Rwp % Rp %

3.24907 (4) 5.20500 (9) 1.601997 0.37066 (55) 1.97704 (18) 109.74 109.20 74 135 85 105 9.9 7.2

3.25094 (3) 5.20194 (7) 1.600134 0.37180 (55) 1.97747 (18) 109.56 109.38 51 325 52 355 10.0 7.4

3.25208 (3) 5.20014 (5) 1.599020 0.37191 (55) 1.97762 (18) 110.53 109.41 35 446 45 696 9.9 7.6

3.25288 (10) 5.19886 (20) 1.598233 0.37262 (96) 1.97791 (19) 109.43 109.52 45 635 54 745 14.7 12.9

3.25338 (3) 5.19816 (9) 1.597772 0.37263 (79) 1.97778 (20) 109.43 109.52 72 665 89 775 12.16 8.13

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Fig. 5. Room temperature optical transmittance spectra of ZCO thin films deposited on glass substrate.

Fig. 6. Room temperature Raman spectra of Zn1xCoxO targets for x = 0.0, 0.05 and 0.10.

to the transitions 4A2(F) ? 2E(G), 4A2(F) ? 2T1(P) and 4A2(F) ? 2 A1(G) respectively [20]. They are ascribed to be charge-transfer transitions between donor and acceptor ionization levels presumably located within the band gap of the host ZnO as verified by results of ZnCoS and ZnCoSe from both experiments and hypothetical calculations [21]. A clear decrease in average transmission is observed with the increase of Co2+ concentration indicating stronger d–d crystal field transitions. In its neutral charge state, the Co2+ ions have an [Ar]3d7 electron configuration. The atomic 4F ground state splits under the influence of the tetrahedral component of the crystal field into a 4A2 ground state and 4T2 + 4T1 excited states. The smaller trigonal distortion and spin–orbit interaction split the ground 4A2 state into E1/2 + E3/2 [22]. The observation of these characteristic absorption bands and the direct proportionality of its intensities to the relative concentration of Co2+ indicate that the Zn2+ ions are replaced by Co2+ ions. In other words, Co ions exist in a tetrahedral crystal field in the +2 state without destroying the wurzite crystal structure of ZnO; a result corroborates what we have got from the previous analysis of the crystal structure. Inspecting Fig. 5, the absorption peaks by d–d transitions as well as the band edge shifted to the lower energy side. The red shift entirely depends on Co concentration and is explained as mainly due to sp–d exchange interactions between the band electrons and the localized d electrons of the Co2+ ions substituting Zn2+ ions [23]. The exchange interactions between s–d and p–d orbital give rise to a negative and a positive correction to the conduction-band and valence-band edges, respectively, resulting to a decrease in band gap [24].

high E1(TO), Ehigh and 2 , and E1(LO), respectively, with dominated E2 low E2 modes. For all cases of x = 0–0.10, the measured Raman spectra agree well with the wurtzite ZnO vibration modes, without any new bands arising from Co-substitution. Raman scattering efficiencies of individual modes in ZnO are known to vary with the excitation energy [21]. With 514.5 nm (2.41 eV) excitation, the highest Raman efficiencies are observed from Elow and Ehigh modes. However, the 2 2 polar LO modes exhibit a strong resonance effect as the excitation energy approaches the electronic transition energies. In cases when ultraviolet lasers are used for excitation, the Raman spectra of ZnO or ZCO are dominated by the signals from LO modes [28]. It is obvious in Fig. 6 that the two E2 modes of ZCO exhibit distinct dependences of phonon energy on the composition. With increased concentration of cobalt, the Elow mode shows a blueshift, 2 while the Ehigh mode exhibits a redshift. Similar result was obtained 2 for Zn1xMgxO [15,29] and Zn1xCoxO [30]. Both Elow and Ehigh 2 2 modes are associated with atomic motions in the ab-plane [15]. low The lower energy mode E2 corresponds mainly (85%) to the vibrations of heavier components, which are the cations in case of ZCO, and conversely the higher energy one Ehigh corresponds 2 mainly to those of lighter components ,oxygen ions [15]. Consequently, the Elow mode energy of ZCO is explicitly affected by the 2 cationic substitution, according to the reduced mass effect. Replacement of Zn with Co will decrease the reduced mass of the oscillator and in turn increase the phonon energy leading to a blueshift. The more the Co2+ concentration, the bigger will be the decrease in the reduced mass of the cation oscillator, giving rise to a greater blueshift. The Elow mode occurs at 101.693, 99.765 and 2 97.836 cm1 for x = 0.0, 0.05 and 0.1 respectively. On the other hand, the change of cation mass should have less influence on the Ehigh mode energy, and its redshift is adequately attributed to 2 the phonon softening caused by the in-plane lattice expansion. As obtained in the structural analysis above, the lattice constant a of ZCO increases monotonically with x giving rise to ab-plane expansion, which appears to account for the observed Ehigh mode 2 behavior. The Ehigh mode occurs at 439.188, 440.36 and 2 441.76 cm1 for x = 0.0, 0.05 and 0.1 respectively.

3.3. Raman spectroscopy Room temperature Raman spectra for Zn1xCoxO (x = 0.05, 0.10 and 0.15) are shown in Fig. 6, along with mode assignments for the observed peaks. The wurtzite lattice, with space group C46v (Hermann–Mauguin symbol P63mc), has four Raman-active phonon modes, A1 + E1 + 2E2. The two E2 (Elow and Ehigh 2 2 ) modes are nonpolar, while the A1 and E1 modes are polarized along the z-axis and in the xy-plane, respectively [25–27]. The polar modes are further split into longitudinal (LO) and transverse (TO) components due to the macroscopic electric field associated with the LO phonons. As shown in Fig. 3, five normal modes are observed around 99.8, 333, 379, 437.7, and 584 cm1 corresponding to Elow 2 , A1(TO),

3.4. XPS analysis X-ray photoelectron spectroscopy (XPS) has been performed to confirm that Co ions have been incorporated in the ZnO lattice. Fig. 7 shows the high-resolution Co 2p spectrum of Zn0.85Co0.15O.

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interactions between the band electrons and the localized d electrons of the Co2+ ions substituting Zn2+ ones. With increased concentration of cobalt, the Elow Raman mode shows a blueshift due 2 to the reduced mass effect caused by cation replacement, while the Ehigh mode exhibits a redshift attributed to the phonon soften2 ing caused by the ab-plane lattice expansion. References

Fig. 7. The high-resolution Co 2p XPS spectrum of Zn0.85Co0.15O.

Four peaks are observed; the 2p3/2 and 2p1/2 doublet resulting from the spin–orbit splitting and their shake-up resonance transitions (satellite). The obtained binding energies of the doublet, 780.6 and 796.2 eV, are comparable to that of the energies of the corresponding photoelectrons of Co2+ in CoO [31,32]. Moreover, the satellite peak at about 786 eV, Fig 7, is considered a feature of Co+2 ions [33]. In addition, the energy difference of 15.6 eV between Co 2p1/2 and Co2p3/2 accords with the data in the published literature on Co2+ in CoO and Co:ZnO [34,35]. This is a strong manifestation that the Co in the present Zn1xCoxO system is present in the oxidized state and the oxidation number is +2. Accordingly, Co ions are successfully incorporated into ZnO lattice which confirms the results obtained from transmittance spectra and XRD analysis. 4. Conclusions Nano-Zn1xCoxO could be prepared as powder and thin film applying a sol–gel technique. Up to x 6 0.12, Co replaces Zn substitutionally yielding ZCO single phase, while for x > 0.12 two phases are formed; ZCO and Co3O4. The microstrain along the direction h0 0 0 1i is found to be much less than along the basal directions  0i. Replacing Zn+2 by Co+2 increases the lattice parameter a h1 0 1 and decreases parameter c with an overall decrease in the ratio c/a of the wurtzite ZCO. The tetrahedral distortion in ZnO is gradually relieved by alloying with CoO. The IR analysis shows red shift entirely depending on Co concentration and is explained as mainly due to sp–d exchange

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