Introduction

Zinc oxide (ZnO), being a wide-band-gap (3.3 eV) wurtzite-phase semiconductor, has attracted much attention here since some of the Zn can be substituted by magnetic transition metal (TM) ions to yield a metastable solid solution (Risbud et al. 2003; Bouloudenine et al. 2005; Thota et al. 2006). TM-doped ZnO has been theoretically proved to be one of the most promising materials for room-temperature ferromagnetism (Wang et al. 2006; Pearton et al. 2003). Zinc oxide is widely used in a number of applications like photocatalysis (Harbour and Hair 1979), gas sensors (Mitra et al. 1998), varistors (Gupta 1990), low-voltage phosphor material (Dijken et al. 2000) and so on. Recent prediction by Dietl et al. (2000) has increased the importance of transition metal-doped ZnO as a potential candidate in spintronics. Recent theoretical predictions (Sato and Katayama-Yoshida 2000; Hammad and Salem 2011) that transition metal (i.e., Mn, Co, Ni, V, Fe)-doped ZnO-diluted magnetic semiconductors (DMS) exhibit ferromagnetic properties at room temperature have led to increasing interest in further experimental studies of the material (Ueda et al. 2001; Lee et al. 2002; Pearton et al. 2007; Guillen et al. 2003; Schwartz et al. 2003). However, the origin of ferromagnetic behavior is not very well-known in these compounds. Recently, it was shown that the ferromagnetism in these materials can be induced by inclusions of nanoscale oxides of transition metals (Kittilstved et al. 2005) and/or nanoparticles containing a large concentration of magnetic ions (Sudakar et al. 2007). In many regards, Co2+: ZnO is the archetypical dopant-sensitized wide-band gap semiconductor. The visible photo-response of Co-doped ZnO has been observed by several researchers using single crystals (Kanai 1968) or polycrystalline of Co2+: ZnO prepared by pellet sintering (Fichou et al. 1985). Zhang et al. (2008) reported a template-free solvothermal method for the formation of transition-metal-doped zinc oxide spheres and hollow spheres. They found that the Co-doped ZnO hollow exhibited ferromagnetism at room temperature, whereas nickel-doped ZnO hollow spheres exhibited only weak ferromagnetism at 300 K. Bhattacharyya and Gedanken (2008) found that the size of ZnO nanocrystals varied with the variation in cobalt concentration. The observed ferromagnetism was intrinsic in nature and not due to any metallic Co segregation or any cobalt oxide phase formation. The Co-doped ZnO nanoparticles of about 5 nm in size were synthesized by Tao Liu et al. (2008). Their results show a good incorporation of cobalt ions into substitutional zinc sites in zinc oxide. Cho et al. (2002) found ferromagnetic behavior in (Co, Fe)-doped ZnO film grown by reactive magnetron co-sputtering, where the effects of a rapid thermal annealing under vacuum lead to an increase in the magnetization and TC. Wei et al. (2006) have deposited Fe-doped ZnO (Zn1-x Fe x O, x = 0–0.07) films onto LiNbO3 substrate by magnetron sputtering and found that Fe substituted the Zn site in the 2+ charge state when x ≤ 0.04 leading to a strong ferromagnetism (TC ~ 400 K). A further increase in the dopant atomic fraction up to 0.07 caused the precipitation of magnetite (Fe3O4), which was conducive to the increase in the magnetic moment of the material. More recently, Kamakar et al. (2007) produced nanocrystalline Fe-doped ZnO powders by a chemical pyrophoric reaction method.

In this work, we report the synthesis and characterization of Co-doped ZnO into spherical superstructure using non-basic simple method with particle size ~12 to ~5 nm, without the assistance of base, template and surfactant. In this work, we also report the changes in the structural, morphologies and optical properties of ZnO nanoparticles brought about successful doping with cobalt.

Experimental procedure

Zinc acetate dihydrate, Zn(Ac)2·2H2O (99.9 %, Sigma Co.), cobalt acetate tetrahydrate, Co(Ac)2.4H2O (99.9 % Sigma Co.) and methanol (99.9 % Sigma Co.) were used as received, without further purification. In a typical experiment, 2.195 g of Zn(Ac)2·2H2O and the respective amount of Co(Ac)2·4H2O were dissolved in 50 mL of methanol under vigorous stirring at room temperature and then ultrasonicating for 10 min. A clear transparent solution was obtained. The solution was heated at 65 °C under reflux for 12 h. The precipitates were carefully collected and washed with absolute ethanol several times to remove the ions possibly remaining in the final products, and then dried at 60 °C for 2 h. A blue solid was obtained from the pink Co2+ solution.

The UV–vis absorption spectra of ethanolic nanoparticle solutions were recorded with Hewlett Packard8453 spectrometer. The photoluminescence (PL) spectra measured at room temperature with a Perkin-Elmer LS 50B luminescence spectrometer and with a 320-nm excitation wavelength. Scanning electron microscopy (SEM) was performed with a Philips X130 ESEMFEG with energy dispersive X-ray (EDAX) spectroscopy. The high-resolution transmission electron microscopy (HRTEM) analysis was done with a Tecnai F300 transmission electron microscope. Crystal structure identification and crystal size analysis were performed with an XDS 2000 X-ray diffractometer, (Scintac Inc.,USA) with CuKα radiation source and scan rate of 2o/min.

Result and discussion

The pure and doped ZnO were synthesized by simply heating methanolic solutions of zinc acetate dihydrate. ZnO solid begins to form after about 8 h of heating and continues to form after the solution cools and sits at room temperature. The temperature of 65 °C is critical to the formation of the materials. The water comes from zinc acetate dihydrate, supplies the oxide oxygen’s and the acetates act as bases, thus no other base, such as hydroxide, is needed.

$$ {\text{Zn}}\left( {{\text{C}}_{2} {\text{H}}_{3} {\text{O}}_{2} } \right)_{2} \, + \,{\text{H}}_{2} {\text{O}}\, \to \,{\text{ZnO}}\left( {\text{s}} \right)\, + \,2{\text{HC}}_{2} {\text{H}}_{3} {\text{O}}_{2} $$
(1)

when the acetate salt of Co2+ was added to the zinc acetate solution during synthesis, ZnO precipitates again formed and the materials were no longer white.

Figure 1 shows the XRD patterns of Co-doped ZnO samples. The XRD showed that all samples exhibited the single crystal of hexagonal wurtzite structure in correspondence with JCPDS database of card number 36-1451, without the appearance of any secondary phase, suggesting that Co2+ incorporated to the Zn+2 sites in the crystal. Cobalt ions introduced as dopants at levels below 7 % shift the diffraction peaks to higher angles, suggesting that the unit cell contracting to occupy the ions (see Fig. 1a–e), and the width broadens due to the formation of smaller average diameters as a result of increase in disorder on Co2+. Interestingly, the relative intensity of the XRD peaks varied with the increase in Co concentration and also with respect to ZnO (a). In b, the increased intensity peak of (002), (110) whereas (002) being as the preferred direction and decreased the intensity peaks of (100) and (101). The (002) peak is strongest in c and all other peaks have decreased. In d and e, the (101) peak is the strongest and (002) is decreased and being broader as compared with a. The interplay of XRD peaks intensities with the Co concentrations is only possible if Co2+ substitutes Zn2+ in the ZnO lattice to form Co-doped ZnO. Our XRD result is in good agreement with reported (Bhattacharyya and Gedanken 2008; Cui and Gibson 2005). The particle size D of the samples was determined using the Scherrer formula (Hammad et al. 2010):

$$ D = \frac{k\lambda }{(B\cos \theta )} $$

where D is the particle size, k a fixed number of 0.9, λ the X-ray wavelength, θ the Bragg’s angle in radians, and B the full width at half maximum of the preferred peak in radians. The calculated average particle size of the Co-doped ZnO samples was found to be around 12–5 nm for Co doping concentration from 0 to 7 %. It seems that the particle size decreased with increasing the dopant concentration of cobalt. The percentages of Co in the doped ZnO samples (Table 1) were obtained by EDX analysis. The EDX data shows that the amount of Co incorporated into the ZnO matrix is less than the actual amount of dopants. Figure 2 depicts the energy-dispersive spectra (EDAX) of 7 % Co-doped ZnO. The spectra reveal that only three elements, Zn and O, and Co element exist in Co-doped ZnO nanoparticles.

Fig. 1
figure 1

a–e XRD patterns of Co-doped ZnO nanoparticles. a 0 %, b 1 %, c 3 %, d 5 % and e 7 % of Co

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Table 1 EDAX analysis of Co-doped ZnO samples
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Fig. 2
figure 2

EDAX spectrum percentage calculated of 7 % Co-doped ZnO nanoparticles

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The morphology of the products was analyzed with SEM and high HRTEM. The SEM morphologies of pure and 7 % of Co-doped ZnO nanoparticles as synthesized are showed in Fig. 3a, b, respectively. The image showed a nearly spherical nanoparticle with homogeneous size distribution of the ZnO nanocrystals is shown in Fig. 3a. Figure 3b presents the obtained spheres containing Co dopant of 7 %. It is seen from this figure that the growth of microspheres on Co doping. Moreover, the SEM indicates that the microspheres were porous with vacancies formed at their centers as shown in the inset of Fig. 2b. It is observed that the nanoparticles are self-aggregated in a closed-pack periodic array toward highly superstructure in size ranging from 600 to 1,300 nm. Figure 4 demonstrates HRTEM images taken for pure (Fig. 4a) and 7 % Co-doped ZnO nanoparticles (Fig. 4b) and corresponding selected area electron diffraction (SAED) patterns. It can be observed from the Fig. 4a that ZnO nanoparticles had a spherical morphology with an average diameter of 12 nm. From HRTEM micrographs, it is observed that Co-doped ZnO nanocrystals were self-aggregated toward porous spherical clustered nanoassemblies (superstructures) during the course of synthesis similar to the Mn-doped ZnO clustered assembly as discussed elsewhere (Barick and Bahadur 2007). Co-doped ZnO clustered nanoassembly is made up of three-dimensionally spatially connected numerous fine nanocrystals of size about 5 nm, which is in good agreement with the crystallite size obtained from XRD line broadening. The inset of Fig. 4 shows the bright field SAED patterns taken from different particles of pure and Co-doped ZnO (0–7 % of Co). These SAED patterns are indexed to the wurtzite ZnO structure, consistent with the above XRD results.

Fig. 3
figure 3

SEM micrographs of Co-doped ZnO nanoparticles. a 0 % and b 7 % of Co

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Fig. 4
figure 4

a–c TEM micrographs of Co-doped ZnO nanoparticles. a 0 % and b 7 % of Co

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The results of HRTEM characterizations are shown in Fig. 5a, b. From HRTEM images, it is clearly observed that the average particle size of Co-doped ZnO nanoparticles was about 5 nm, which is consistent with the calculation results from XRD. It is also clear from HRTEM results that Co doping in ZnO reduces the particle size. The average interfringe distance of co-doped ZnO nanocrystals was measured to be ~0.28 nm, which corresponds to the (002) planes of ZnO (Wang et al. 2008). According to the results of XRD pattern and HRTEM images, we believed that the Co ions were well incorporated into the crystal lattice of ZnO.

Fig. 5
figure 5

a, b TEM micrographs of 7 % Co-doped ZnO nanoparticles

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The optical properties of the Co-doped ZnO nanostructures were studied by UV–vis absorption spectroscopy. Figure 6 presents the typical absorption spectra of the pure and Co-doped ZnO nanoparticles with different levels of Co. Further, an obvious blueshift of the absorption edges can be observed in the doped ZnO samples from 361 to 340 nm of Co doping levels from 0 to 7 %. The band-gap energy (Eg) values for the samples were evaluated by plotting (αhν)2 versus hν (Fig. 7) (Bahadur et al. 2010). The energy increases from 3.33 eV (pure ZnO) to 4.13 eV (7 % Co), and appears to originate from active transitions involving 3d levels in Co+2 ions and strong sp–d exchange interactions between the itinerant ‘sp’ carriers (band electrons) and the localized ‘d’ electrons of the dopant. It can be observed that Fig. 7 indicates a large blueshift of the band gap corresponding to samples of pure and 7 % Co-doped ZnO nanoparticles, respectively. The increase in the band gap or blueshift can be explained by the Burstein–Moss effect (Nirmala and Anukaliani 2011). This is the phenomenon that the Fermi level merges into the conduction band with an increase in the carrier concentration. Thus, the low energy transitions are blocked. Our results are in good agreement with the results reported by Sakai etal. (2006).

Fig. 6
figure 6

UV–vis spectra of Co-doped ZnO nanoparticles at room temperature

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Fig. 7
figure 7

Variation of band gap of Co-doped ZnO nanoparticles

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Figure 8 illustrates the photoluminescence spectra of the synthesized Co-doped ZnO nanoparticles with different doping concentration at room temperature. As shown in Fig. 8, a weak ultraviolet (UV) emission at 405 nm and a very strong and broad green emission at 508 nm were observed for the pure ZnO nanoparticles. The UV emission at 405 nm is related to the near band-edge emission (Viswanatha et al. 2004), whereas the green is resulted from the recombination of electrons with holes trapped in singly ionized oxygen vacancies (Kong et al. 2001) Compared with the PL spectrum of pure ZnO, the UV-emission peak of Co-doped ZnO samples exhibits a large blueshift (from 410 to 390 nm); this is attributed to the shift of the optical band gap in these samples. A blueshift of the band edge reveals the incorporation of Co in the lattice sites in ZnO. This supports the X-ray results on the incorporation of Co into the ZnO structure.

Fig. 8
figure 8

a, b PL spectra of Co-doped ZnO nanoparticles at room temperature

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Conclusion

We have synthesized Co-doped ZnO nanoparticles by a simple chemical method at low temperature with Co:Zn atomic ratio from 0 to 7 %. The relative intensity of the XRD peaks varied with the increase in Co concentration with respect to ZnO nanoparticles. The average particle size of Co-doped ZnO was found 12–5 nm as calculated by XRD and transmission electron microscope (TEM). The XRD patterns, SEM and HRTEM micrographs of doping of Co in ZnO confirmed the formation of superstructure and indicated that the Co2+ is successfully substituted into the ZnO host structure of the Zn2+ site. The band-gap energy increases largely from 3.32 eV (pure ZnO) to 4.12 eV (7 % Co:ZnO), which is responsible for the blueshifts of wavelength in UV absorption and emission. Thus, the current doping method can be regarded as another effective technique to modulate the optical properties of ZnO nanoparticles.