Tunable band gaps of Co_3−xCu_xO₄ nanorods with various Cu doping concentrations

Abstract

Co_3−xCu_xO₄ nanorods with a crystalline structure were synthesized via a solution process in this study. The Co_3−xCu_xO₄ nanorods exhibit dual direct bandgaps and could be adjusted to 1.3–1.5 eV, and 2.55–3.5 eV by using different Cu-doping concentrations. The Co_3−xCu_xO₄ nanorods show an absorption coefficient of ∼10⁴ cm⁻¹ which increases with the increase of the Cu doping concentration.

---

Introduction

Cobalt oxide (Co₃O₄) based materials have been widely studied because of their potential applications in many technological fields. In particular, their applications in gas sensors,^1,2 electrochromics devices,³ heterogeneous catalysts,⁴ high-temperature solar selective absorbers,⁵ electrocatalytic oxygen evolution reaction (OER),⁶ supercapacitors,⁷ and lithium batteries^2,8,9 have been extensively investigated. These applications are mainly attributed to the high surface area and better electrochemical reactivity of nanostructured Co₃O₄.

Besides, there has been an increasing interest in developing metal-doped Co₃O₄, since the Co₃O₄ properties could be adjusted by the substitution of cobalt ions with foreign divalent metal ions in spinel structure. For example, Li- and Au-doped Co₃O₄ nanowires have been synthesized and exhibited a high storage capacity for lithium batteries.¹⁰ There are a few studies focusing on Cu-doped Co₃O₄ recently.¹¹ The enhancement of Cu-doped Co₃O₄ in catalytic activity, OER,¹¹ and the sensitivity of methane gas sensor¹² were reported in these studies.

In addition, to estimate the electronic structure and band gap value before and after doping metal, theoretical calculation has been made in previous literatures.^13–15

In this study, efforts have been made to synthesize Co_3−xCu_xO₄ nanorods by low temperature hydrothermal process. The morphology, structure and optical property with various Cu doping concentrations were characterized. In addition, the growth of Co_3−xCu_xO₄ nanorods on conductive substrates have also been investigated for the future application on self-assembled electrical devices.

Experimental

Cu-doped Co₃O₄ nanostructure preparation⁹

Co(NO₃)₂·6H₂O (0.1 M) and Cu(CH₃COO)₂·3H₂O (0.02 M, 0.01 M, and 0 M) were loaded into a 500 ml reactor respectively, which was prefilled with 34 ml NH₄OH and 66 ml distilled (DI) water. The reactor was put in a reflux system and remained at 85 °C for 12 h, and then cooled down to room temperature. The reaction temperature and time have been optimized, which were presented in following paragraph. The final products were centrifuged and rinsed with distilled water and ethanol for several times to remove chemical residuals. After rinsing, the product was annealed in the furnace at 400 °C for 2 h to ensure that other chemical residuals were completely eliminated.

Cu-doped Co₃O₄ nanorods optimization

The optimization of Co₃O₄ nanorods at various reaction temperatures and concentrations of reduction agent (ammonia), while all using 10% molar ratio of Cu(CH₃COO)₂ in Co(NO₃)₂, were also investigated in this work as shown in Fig. 1. It could be observed that different processing temperatures (60, 85, and 120 °C) result in different surface morphology (Fig. 1a–c). With processing temperature at 60 °C, the Cu-doped Co₃O₄ nanoparticles with ∼50 nm in diameter were observed (Fig. 1a). When the processing temperature increases to 85 °C, the Cu-doped Co₃O₄ nanorods were observed (Fig. 1b). For the synthesis temperature up to 120 °C, the Cu-doped Co₃O₄ particles with a diameter of 600–1000 nm were observed in Fig. 1c. The synthesis using various NH₄OH concentrations for 12 h at 85 °C result in different morphologies as shown in Fig. 1d–f. Fig. 1d presents the Cu-doped Co₃O₄ nanoparticles with a diameter of 200–400 nm using H₂O to NH₄OH molar ratio 4 : 1. As the H₂O to NH₄OH molar ratio was decreased to 2 : 1, the Cu-doped Co₃O₄ nanorods were synthesized (Fig. 1e). When the H₂O to NH₄OH molar ratio decreases to 1 : 1, the Cu-doped Co₃O₄ nanorods with slightly rough surface were observed (Fig. 1f). As the nanorods morphology could provide larger surface area light collection, the optimized condition was determined to be at 85 °C and H₂O to NH₄OH molar ratio 2 : 1.

Fig. 1 The SEM images of Cu-doped Co₃O₄ nanorods grown on Pt/glass substrates at the growth temperature of (a) 60 °C, (b) 85 °C, and (c) 120 °C with H₂O–NH₄OH = 2 : 1 for 12 h. The samples were prepared using (d) H₂O–NH₄OH = 4 : 1, (e) H₂O–NH₄OH = 2 : 1, and (f) H₂O–NH₄OH = 1 : 1 at a process temperature of 85 °C for 12 h. All above Cu-doped Co₃O₄ samples were, synthesized using 10% molar ratio of Cu(CH₃COO)₂ in Co(NO₃)₂.

Materials characterization

The morphology and structure of the products were characterized by field emission scanning electron microscopy (FESEM, JEOL-JSM 6500F), high resolution transmission electron microscopy (HRTEM, JEOL-JSM 2010) and X-ray diffraction spectroscopy (Shimadzu XRD 6000). The compositions of the materials were analyzed by the inductively coupled plasma mass spectrometry (ICP-MS, PerkinElmer). Raman spectra (HORIBA HR 800, laser excitation λ = 633 nm) were acquired to characterize the structural properties. The chemical composition of Cu-doped CO₃O₄ nanorods was characterized by X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM). The optical properties were measured by an ultraviolet-visible near-infrared spectrophotometer (UV-Vis-NIR, Hitachi U-4100) over the range of 400–2000 nm, and a crygenic cathodoluminescence system (CL, Gatan MonoCL) at room temperature (∼300 K) with an excitation beam with a 30 kV accelerating voltage and ∼18 nA beam current.

Results and discussion

Morphology and structure analysis

The Cu-doped Co₃O₄ nanorods were synthesized by ammonia-evaporation-induced growth.⁹ The glass substrates with and without a Pt layer were immersed in an aqueous solution with Co(NO₃)₂, and different amounts of Cu(CH₃COO)₂ were added into the solution to adjust the Cu-doping level in Co₃O₄ nanorods. The effects of process temperatures and NH₄OH concentrations on the morphology of Cu-doped Co₃O₄ nanorods were investigated. The results in Fig. 1 show the optimum synthesis condition at H₂O–NH₄OH = 2 : 1 and a process temperature of 85 °C for 12 h, which will be used for further study. It could be observed that different concentrations of Cu(CH₃COO)₂ resulted in different Co₃O₄ surface morphologies as shown in the scanning electron microscopy (SEM) images of Fig. 2a–c. The pristine Co₃O₄ nanorods with about 200–500 nm in diameter, and 5–10 μm in length can be observed in SEM image (Fig. 2a). With the increase of the molar ratio of Cu(CH₃COO)₂ in Co(NO₃)₂ from 10% to 20%, the corresponding diameter of Cu-doped Co₃O₄ nanorods increases from 300–600 nm to 500–800 nm as observed in the SEM images of Fig. 2b and c, respectively.

Fig. 2 The SEM images of Co_3−xCu_xO₄ nanorods grown on Pt/glass substrates (a) pristine Co₃O₄, (b) Co_2.98Cu_0.02O₄, and (c) Co_2.94Cu_0.06O₄.

The content of Cu in Co₃O₄ nanorods, synthesized by using 10% molar ratio of Cu(CH₃COO)₂ in Co(NO₃)₂, were characterized by inductively coupled plasma mass spectrometry (ICP-MS spectrometer). The signal of Cu element (0.28 ± 0.01 at%) was measured in Co₃O₄ nanorods, which was calculated to be a compound of Co_2.98Cu_0.02O₄. The content of Cu in Co₃O₄ nanorods, synthesized by using 20% molar ratio of Cu(CH₃COO)₂ in Co(NO₃)₂, were also characterized showing a Cu composition of 0.92 ± 0.02 at%, i.e., a compound of Co_2.94Cu_0.06O₄.

The crystallographic properties of the Co_2.98Cu_0.02O₄ nanorods were examined by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected-area electron diffraction (SAED) as shown in Fig. 3a–d. The bright-field TEM image in Fig. 3a reveals an individual Co_2.98Cu_0.02O₄ nanorod with a diameter of 200 nm. Fig. 3b presents its dark field image contributed from the (111) diffraction of the Co_2.98Cu_0.02O₄. Fig. 3c shows the corresponding diffraction pattern with the dotted ring-shaped patterns that indicate the polycrystalline structure of Co_2.98Cu_0.02O₄ nanorods. The d-spacings of 0.477 and 0.245 nm as calculated from the HRTEM lattice image of the Co_2.98Cu_0.02O₄ nanorods (Fig. 3d) correspond to the spacing of 0.465 nm (111) and 0.243 nm (311), respectively, for the cubic spinel Co₃O₄ (a = 8.084 nm) according to X-ray JCPDS file no. 653103. It is interesting to observe the d-spacing is slightly larger than that of the JCPDS file reference, which suggests the doping of Cu atoms into Co₃O₄ nanorods lattice which leads to the expansion of Co₃O₄ lattice due to the larger size of Cu atom than Co.

Fig. 3 The transmission electron microscopy (TEM) analysis of Co_2.98Cu_0.02O₄. The (a) bright field image, (b) dark field TEM image contributed from (111) diffraction, (c) the corresponding diffraction pattern taken from (a), and (d) the high resolution TEM lattice image taken from the indicated boxed area of (c).

The crystal structure of Co₃O₄ nanorods with different amounts of Cu doping was also confirmed by X-ray diffraction spectroscopy (XRD), as shown in Fig. 4a. It shows that for pristine Co₃O₄ nanorods all peaks can be indexed to cubic spinel phase Co₃O₄ based on the standard data file (JCPDS file no. 653103). It is interesting to observe that the peaks shift to smaller-angles for both Co_2.98Cu_0.02O₄ and Co_2.94Cu_0.06O₄ compared to pristine Co₃O₄. The peak shift indicates that the lattice is enlarged after Cu-doping in Co₃O₄, which is mostly due to the radius of Cu atom is larger than that of Co. The results are consistent with the lattice enlargement observed in the previous lattice image of HRTEM (Fig. 3d).

Fig. 4 (a) XRD patterns of pristine Co₃O₄, Co_2.98Cu_0.02O₄ and Co_2.94Cu_0.06O₄ nanorods and (b) their Raman spectra.

The structure properties of pristine Co₃O₄, Co_2.98Cu_0.02O₄ and Co_2.94Cu_0.06O₄ were also analyzed by Raman spectra as shown in Fig. 4b. The Co₃O₄ is belonged to spinel structure, Co²⁺(Co³⁺)₂(O²⁻)₄, with space group O⁷_h. Co³⁺ and Co²⁺ placed at octahedral and tetrahedral sites of Co₃O₄, respectively. The vibrational modes of Co₃O₄ at k = 0 in irreducible representations of the factor group O⁷_h are

Γ = A_1g + E_g + 3F_2g + 5F_1u + 2A_2u + 2E_u + 2F_2u (1)

The A_1g, E_g and the three F_2g modes are Raman active. The F_1g, 2A_2u, 2E_u and 2F_2u modes are inactive. In the five F_1u modes, one is an acoustic mode and the other four are infrared active.^16,17 In Raman spectra, the blueshift increases with the increase of Cu doping concentration in Co₃O₄ nanorods. The blueshift is believed to originate from the stress attributed to the lattice expansion.^18,19 It is again consistent with the observation of lattice image HRTEM. The corresponding Raman data and the comparison of their characteristics peaks are summarized in Table 1.

Table 1 The summary of Raman analyses

	F2g (cm^−1)	Eg (cm^−1)	F2g (cm^−1)	F2g′ (cm^−1)	A1g (cm^−1)
Pristine Co3O4 (ref. 2)	194	482	521	618	691
Pristine Co3O4	194	477	520	616	686
Co2.98Cu0.02O4	191	470	510	608	672
Co2.94Cu0.06O4	189	466	509	606	670

---

The purity and composition of Cu-doped Co₃O₄ nanorods were also analyzed using X-ray photoelectron spectroscopy (XPS), and the results show that the Cu has been doped into Co₃O₄ successfully, including Cu⁺ and Cu²⁺, shown in Fig. 5 and Table 2.

Fig. 5 The XPS analyses show (a) the wide scan energy spectra of pristine Co₃O₄, Co_2.98Cu_0.02O₄ and Co_2.94Cu_0.06O₄, and the XPS spectra, including (b) Cu 2p_3/2 spectrum, (c) O 1s spectrum, and (d) Co 2p_3/2 and Co 2p_1/2 spectra, for the Co_2.98Cu_0.02O₄ nanorods.

Table 2 The summary of XPS

	Cu 2p3/2^20–22			Co 2p^23–25			O 1s^20–25			C 1s^20–25
	CuO	Cu2O	Cu multi-complex	Co(III)	Co(II)	Satellite peaks	Co3O4	CuO	Cu2O	C
This work	933.8 eV	932.2 eV	927.6 eV	780.4 eV (2p3/2)	783.4 eV (2p3/2)	786.7 eV	529.9 eV	534.3 eV	531.2 eV	284.7 eV
						790 eV
				795.4 eV (2p1/2)	798.8 eV (2p1/2)	802.6 eV	532.4 eV
						805.8 eV
Reference	933.6 eV	932.5 eV	927.0 eV	779.7 eV	781.3 eV	786.3 eV	531.1 eV	533.0 eV	530.9 eV	284.7 eV
						788.2 eV
				794.8 eV	796.8 eV	802.1 eV	532.2 eV
						805.2 eV

---

The full survey spectra of pristine Co₃O₄, Co_2.98Cu_0.02O₄ and Co_2.94Cu_0.06O₄ are presented in Fig. 5a. The binding energy at 284.7 eV on the C 1s XPS spectrum corresponds to the carbon gel. The peaks located at 284.7, 532, and 780 eV are assigned to the characteristic peaks of C 1s, O 1s, and Co 2p, respectively. The peak located at 929 eV is assigned to Cu 2p, which becomes sharper at higher Cu doping concentration and confirms the existence of Cu in Co₃O₄. The further analysis of the Co_2.98Cu_0.02O₄ nanorods shows the peaks of Cu 2p, O 1s, and Co 2p (Fig. 5b–d). Fig. 5b shows the Cu 2p peaks at 927.6, 932.2, and 933.8 eV, which are attributed to multi-copper complex, Cu⁺, and Cu²⁺, respectively.^20–22 This reveals again that Cu has been doped into Co₃O₄ nanorods successfully, including Cu⁺ and Cu²⁺.

As expected in Fig. 5c, the O 1s peaks at 529.9 and 532.4 eV can be assigned to Co₃O₄, and 531.2 and 534.3 eV to Cu₂O and CuO peaks, respectively. The representative XPS spectrum of Co 2p in Fig. 5d shows that the peaks at 780.6, and 795.6 eV correspond to Co(III) 2p_3/2 and 2p_1/2, while 783.7 and 798.8 eV correspond to Co(II) 2p_3/2 and 2p_1/2, respectively. The weak 2p_3/2 satellite features were found at 786.7 and 790 eV, and those of 2p_1/2 satellite features were found at 802.6 and 805.8 eV,²³⁻²⁵ respectively. Based on above XPS characterization, the Cu-doping in Co₃O₄ nanorods were further confirmed. The XPS results are summarized in Table 2.

Optical property analyses

The optical properties of the Cu-doped Co₃O₄ nanorods were analyzed by UV-Vis and cathodoluminescence (CL) spectra. From the UV-Vis absorption spectra in Fig. 6a for the pristine Co₃O₄, Co_2.98Cu_0.02O₄ and Co_2.94Cu_0.06O₄, it is noteworthy that the concentration of Cu dopant affects the absorption of nanorods significantly. In the visible light region, the absorption of Cu-doped Co₃O₄ nanorods increases with the increase of Cu dopant concentration. In order to quantify the enhancement of Cu-doping, the corresponding absorption coefficient was calculated according to Beer's law from different thicknesses of Cu-doped Co₃O₄. The absorption coefficients at 550 nm wavelength for the pristine Co₃O₄, Co_2.98Cu_0.02O₄ and Co_2.94Cu_0.06O₄ are 2.1 × 10⁴, 3.1 × 10⁴, and 4.5 × 10⁴ cm⁻¹, respectively, which also increase with the increase of Cu dopant concentration. In addition, the optical band gaps of Cu-doped Co₃O₄ nanorods could be adjusted by varying the concentrations of Cu dopant. The pristine Co₃O₄ nanorods exhibit the band gaps of 1.4 and 3.5 eV. With the increase of Cu-doping, the band gaps shift to 1.35 and 2.75 eV for the Co_2.98Cu_0.02O₄ nanorods and to 1.3 and 2.5 eV for Co_2.94Cu_0.06O₄ nanorods. These values were extracted to the energy (= hν) axis at α = 0 according to the linear portion of (αhν)² − hν plots, which were inserted in Fig. 6b–d, where α, h, and ν represent absorption coefficient, Plank constant, and wavelength, respectively.^26,27

Fig. 6 (a) UV-Vis absorption spectrum of the pristine Co₃O₄, Co_2.98Cu_0.02O₄ and Co_2.94Cu_0.06O₄ nanorods. Plots of (αhν)²versus hν show the direct band gap of (b) pristine Co₃O₄, (c) Co_2.98Cu_0.02O₄ and (d) Co_2.94Cu_0.06O₄ nanorods.

The reason for band gap narrowing is because Cu dopant replaces Co place and bonds with O. CuO and Cu₂O band gaps (∼1.21 eV and ∼2.10 eV, respectively) are smaller than Co₃O₄ band gaps (1.40 eV and 3.50 eV, respectively). Besides, Cu⁺ and Cu²⁺ could occupy Co²⁺ or Co³⁺ place in Co₃O₄, which generates one more hole carriers.

The absorption coefficient of ∼10⁴ cm⁻¹ and direct band gaps of 1.4, 1.35, and 1.3 eV for Cu-doped Co₃O₄ nanorods suggest that they could serve as an efficient light harvesting agent for potential photovoltaic and photodiode applications.

Their CL properties were determined in Fig. 7a–c. The spectra are basically composed of two peaks and the left peak can be deconvoluted into two sub-peaks (peak 1 and peak 2). The Co₃O₄ absorption bands have been reported to be related to the ligand field transitions of Co³⁺ and Co²⁺ ions in octahedral and tetrahedral coordination, respectively, and the charge transfer process between Co²⁺ and Co³⁺ ions and those from oxygen ligands to Co ions.²⁸ The peak 1 and 2 are related to the transition between Co²⁺ and Co³⁺ bands, which are located at 2.5–2.9 eV and ∼2.0 eV respectively. These peaks could correspond to the larger band gap transition in UV-Vis analyses. Comparing them with the UV-Vis band gap fitting results, the corresponding energies of these peaks are smaller than that of the larger band gap observed in UV-Vis analyses, which could be attributed to the deep-level emissions associated with defects.²⁹ The peak 3 could be corresponded to the band gap (∼1.4 eV) transition of Cu-doped Co₃O₄ nanorods and it is consistent with the previous UV-Vis measurement. It is noteworthy that as the Cu-doping concentration increases, the peak 1 shifts to UV region while peak 2 and peak 3 shifts to infrared light region. The results and their comparison with other works are also summarized in Table 3 with the detailed analysis and description.

Fig. 7 Cathodoluminescence (CL) spectra of (a) pristine Co₃O₄, (b) Co_2.98Cu_0.02O₄ and (c) Co_2.94Cu_0.06O₄ nanorods.

Table 3 The summary of CL analyses

	Center of peak 1 (nm)	Center of peak 2 (nm)	Center of peak 3 (nm)
Pristine Co3O4	488	610	890
Co2.98Cu0.02O4	473	616	891
Co2.94Cu0.06O4	472	617	893

---

Conclusions

In summary, tunable band gaps of Co_3−xCu_xO₄ nanorods, were synthesized via solution process and first reported in this study. Detailed characterizations of the nanorods show a crystalline structure and that the Cu has been doped into Co₃O₄ successfully, including Cu⁺ and Cu²⁺. The optical properties were also carefully examined, showing that the absorption coefficient of nanorods increases with the increase of Cu dopant. The Cu-doped Co₃O₄ nanorods exhibit dual direct bandgaps and could be adjusted at 1.3–1.5 eV, and 2.55–3.5 eV by Cu-doping using different Cu-doping concentration (the higher the doping, the lower the band gap). The Cu-doped Co₃O₄ nanorods also reveal an absorption coefficient about 10⁴ cm⁻¹, which suggests that they could serve as a light harvesting agent for potential applications in photovoltaics and photodiodes.

Acknowledgements

This work was supported by National Science Council under project number NSC 100-2221-E-007-002-MY3. The authors thank C. H. Chen, Y. H. Chang, C. H. Hsu and S. Y. Wei at National Tsing-Hua University for optical property measurements. The authors also thank Prof. J. Y. Gan, C. N. Yeh, C. W. Juan, Y. J. Hong, and Y. M. Hsu for the discussion of the nanorod synthesis.

Notes and references

1. J. Wollenstein, M. Burgmair, G. Plescher, T. Sulima, J. Hildenbrand, H. Bottner and I. Eisele, Sens. Actuators, B, 2003, 93, 442–448.
2. W. Y. Li, L. N. Xu and J. Chen, Adv. Funct. Mater., 2005, 15, 851–857.
3. T. Maruyama and S. Arai, J. Electrochem. Soc., 1996, 143, 1383–1386.
4. L. H. Hu, Q. Peng and Y. D. Li, J. Am. Chem. Soc., 2008, 130, 16136–16137.
5. K. Chidambaram, L. K. Malhotra and K. L. Chopra, Thin Solid Films, 1982, 87, 365–371.
6. Y. G. Li, P. Hasin and Y. Y. Wu, Adv. Mater., 2010, 22, 1926–1929.
7. X. H. Xia, J. P. Tu, Y. J. Mai, X. L. Wang, C. D. Gu and X. B. Zhao, J. Mater. Chem., 2011, 21, 9319–9325.
8. X. W. Lou, D. Deng, J. Y. Lee, J. Feng and L. A. Archer, Adv. Mater., 2008, 20, 258.
9. Y. G. Li, B. Tan and Y. Y. Wu, Nano Lett., 2008, 8, 265–270.
10. F. Nobili, F. Croce, R. Tossici, I. Meschini, P. Reale and R. Marassi, J. Power Sources, 2012, 197, 276–284.
11. Q. Zhang, Z. D. Wei, C. Liu, X. Liu, X. Q. Qi, S. G. Chen, W. Ding, Y. Ma, F. Shi and Y. M. Zhou, Int. J. Hydrogen Energy, 2012, 37, 822–830.
12. Z. Sheikhi Mehrabadi, A. Ahmadpour, N. Shahtahmasebi and M. M. Bagheri Mohagheghi, Phys. Scr., 2011, 84, 015801.
13. L. Run, D. Ying and H. J. Baibiao, Phys. Chem. Lett., 2013, 4, 2223–2229.
14. L. Run and N. J. English, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 83, 155209–155213.
15. L. Run and N. J. English, Chem. Mater., 2010, 22, 1626.
16. V. G. Hadjiev, M. N. Iliev and I. V. Vergilov, J. Phys. C: Solid State Phys., 1988, 21(7), L199–L201.
17. H. Shirai, Y. Morioka and I. Nakagawa, J. Phys. Soc. Jpn., 1982, 51(2), 592–597.
18. B. C. Cheng, Y. H. Xiao, G. S. Wu and L. D. Zhang, Appl. Phys. Lett., 2004, 84(3), 416–418.
19. S. K. Sharma and G. J. Exarhos, Solid State Phenom., 1997, 55, 32–37.
20. R. A. Zarate, F. Hevia, S. Fuentes, V. M. Fuenzalida and A. Zuniga, J. Solid State Chem., 2007, 180(4), 1464–1469.
21. E. Abelev, D. Starosvetsky and Y. Ein-Eli, Electrochim. Acta, 2007, 52(5), 1975–1982.
22. J. P. Chang, Z. Zhang, H. Xu, H. H. Sawin and J. W. Butterbaugh, J. Vac. Sci. Technol., A, 1997, 15(6), 2959–2967.
23. S. C. Petitto and M. A. Langell, J. Vac. Sci. Technol., A, 2004, 22(4), 1690.
24. T. He, D. R. Chen, X. L. Jiao, Y. L. Wang and Y. Z. Duan, Chem. Mater., 2005, 17(15), 4023–4030.
25. W. L. Guo, Y. F. E, L. Gao, L. Z. Fan and S. H. Yang, Chem. Commun., 2010, 46(8), 1290–1292.
26. H. Yamamoto, S. Tanaka and K. Hirao, J. Appl. Phys., 2003, 93, 4158–4162.
27. G. X. Wang, X. P. Shen, J. Horvat, B. Wang, H. Liu, D. Wexler and J. Yao, J. Phys. Chem. C, 2009, 113, 4357–4361.
28. A. Llavona, C. Diaz-Guerra, M. C. Sanchez and L. Perez, Mater. Chem. Phys., 2010, 124(2–3), 1177–1181.
29. Z. G. Chen, J. Zou, G. Liu, F. Li, Y. Wang, L. Z. Wang, X. L. Yuan, T. Sekiguchi, H. M. Cheng and G. Q. Lu, ACS Nano, 2008, 2, 2183–2191.

---