September 22, 2011

4:00:07pm

WSPC/175-IJN

FA1

00874

International Journal of Nanoscience Vol. 10, Nos. 4 & 5 (2011) 943
947 # .c World Scienti¯c Publishing Company DOI: 10.1142/S0219581X11008745

SINGLE STEP SYNTHESIS OF HYDROPHOBIC AND HYDROPHILIC NANOPARTICLES VIA THERMAL DECOMPOSITION DIPAK MAITY*,†,‡ and JUN DING*,§ *Department of Materials Science and Engineering National University of Singapore, Singapore 117574 †Department of Chemistry National University of Singapore, Singapore 117574 ‡[email protected] § [email protected] Here, we report single step synthesis of hydrophobic and hydrophilic Fe3 O4 , ZnO, CoO and Y2 O3 :Eu nanoparticles via thermal decomposition of di®erent organometallic complexes in oleylamine (OM) and tri(ethyleneglycol) (TREG) media, respectively. The crystal structure of the as-prepared nanoparticles is identi¯ed using X-ray di®raction, Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Morphology of the nanoparticles is determined by transmission electron microscopy (TEM) while the magnetic properties are measured using vibrating sample magnetometer (VSM). Thermolysis of appropriate precursors in OM and TREG medium are very capable of producing the highly dispresed hydrophobic and hydrophilic nanoparticles with diverse morphologies. Keywords: Iron oxide; zinc oxide; iron oxide; cobalt oxide; nanoparticles; synthesis.

methods, the thermal decomposition method has been well accepted as a promising technique which is able to produce high quality inorganic oxide and °uoride nanocrystals like Cr2 O3 ,5 Mn2 O3 ,6 Fe3 O4 ,7,8 ZnO,9,10 RE2 O3 (RE = La to Lu, Y)11 and NaREF4 (RE: Pr to Lu, Y).3,12 Typically, this method involves thermolysis of organometallic complexes in a high-boiling temperature solvent in the presence of the stabilizing OA and oleylamine (OM) surfactants. The limitation of this method is that the obtained nanoparticles are hydrophobic i.e. soluble only in organic solvent. Recently, the thermal decomposition method has been further modi¯ed to develop solvent-free thermal decomposition technique in which the thermolysis is carried out in absence of solvent.13 The solvent-free technique is

1. Introduction Nanomaterials have been attracted a great attention in the ¯elds of \science and technology" due to their unique chemical and physical properties.1
4 The major di±culty in the synthesis of ultra¯ne particles is to control the particle size and its distribution at the nanosized scale. The di±culty arises from the fact that the nanoparticles form aggregates and continuously grow to minimize the overall surface free energy. Therefore, the search for facile and °exible synthetic routes which are able to produce nanoparticles without particle aggregation is of extremely important. Several synthesis methods including co-precipitation, microemulsion and thermal decomposition have been designed to fabricate dispersible nanoparticles. Among these

943

September 22, 2011

944

4:00:08pm

WSPC/175-IJN

FA1

00874

D. Maity & J. Ding

very promising to fabricate highly dispersed hydrophobic or hydrophilic Fe3 O4 nanoparticles by high temperature decomposition of iron (III) acetylacetonate, Fe (acac)3 in the presence of OM or tri (ethylene glycol) (TREG), respectively.14,15 Here, we have extended the solvent-free technique to synthesize Fe3 O4 , ZnO, CoO and Y2 O3 :Eu nanocrystals using di®erent organometallic precursors.

Structure of the as-prepared nanoparticles was recognized by X-ray di®raction (XRD) (Bruker D8 Advance), FTIR (Varian 3100), and TGA (DMSE SDTQ600). Morphology was determined by TEM (JEOL 2010) and magnetic properties were measured using VSM (Lakeshore, Model 665). Fluorescence spectra of the Y2 O3 :Eu nanoparticles were obtained on a luminescence spectrometer (LS-55, Perkin
Elmer) using the excitation wavelength of 260 nm from a xenon lamp.

2. Experimental Section Absolute ethanol (EtOH), ethyl acetate (EtAc) and acetylacetone (Hacac) were used without puri¯cation. Oleylamine (OM), tri(ethylene glycol), (TREG), iron(II) acetate (Fe(ac)2 Þ, iron(III) citrate (Fe(cit)3 Þ, iron(III) hydroxide oxide (Fe(hyd)), zinc acetylacetonate hydrate (Zn(acac)2  xH2 O), cobalt (III) acetylacetonate (Co(acac)3 Þ, yttrium(III) oxide (Y2 O3 Þ and europium(III) oxide (Eu2 O3 Þ were purchased from Sigma-Aldrich. Yttrium acetylacetonate (Y(acac)3 Þ and europium acetylacetonate (Eu(acac)3 Þ were prepared according to the procedure reported by Si et al.11 ZnO nanoparticles were prepared using the solvent-free thermal decomposition method.14 Typically, 2 mmol of Zn(acac)2  xH2 O precursor was dissolved in a 20 mL of stabilizing media (OM or TREG) and magnetically stirred under a °ow of argon. The solution is dehydrated at 120  C for 1 h, and then quickly heated to 320  C (for OM) or 260  C (for TREG) and kept at this temperature for 1 h. The resulting solution was cooled to room temperature (RT) by removing the heat source. Then, 20 mL of EtOH (for OM) or EtOH/EtAc (1:2 v/v) mixture (for TREG) was added into the solution and the precipitated particles were collected by centrifugation at 8000 rpm followed by three times washing with EtOH or the EtOH/EtAc mixture. Finally, the washed particles were dried overnight in oven to obtain dry ZnO nanoparticles. Fe3 O4 nanoparticles were prepared in OM and TREG following the above mentioned procedure using di®erent precursors like Fe(ac)2 , Fe(cit)3 and Fe(hyd). CoO nanoparticles were prepared in OM following the above mentioned procedure using Co (acac)3 precursor. Y2 O3 :15%Eu nanoparticles were prepared following the procedure that used to synthesize ZnO nanoparticles, except that quantitative Y(acac)3 (1.81 mmol) and Eu(acac)3 (0.19 mmol) were taken as the precursors and added into 20 mL of OM.

3. Results and Discussion 3.1.

Structural characterization

Figure 1 shows the XRD patterns of as-prepared nanoparticles. Position of the di®raction peaks in Figs. 1(a)
1(d) identi¯es the corresponding ZnO (JCPDS ¯le No. 80-0075), Fe3 O4 (JCPDS ¯le No. 19-0629), CoO16 and Y2 O3 :Eu (JCPDS ¯le No. 251011) phase of the nanoparticles, respectively. Figures 2(a) and 2(b) show the FTIR spectra of the ZnO nanoparticles prepared in OM and TREG, respectively. The peaks in Fig. 2(a) at about 2926, 2854, 1591
1501 and 1473
1402 cm
1 are due to C
H stretching, N
H bending and C
N stretching vibration, respectively attributed for chemically adsorbed OM coating to the hydrophobic ZnO nanoparticle surface.13,17,18 On the other hand, the peaks in Fig. 2(b) at about 2962
2809, 1681
1534,

Fig. 1. XRD patterns of (a) ZnO, (b) Fe3 O4 , (c) CoO and (d) Y2 O3 :Eu nanoparticles.

September 22, 2011

4:00:11pm

WSPC/175-IJN

FA1

00874

Single Step Synthesis of Hydrophobic and Hydrophilic Nanoparticles via Thermal Decomposition

945

curves of the ZnO nanoparticles prepared in OM and TREG, respectively. TGA curves represent two-stage weight loss on heating of the samples from room temperature to 800  C. The ¯rst weight loss up to 200  C is due to the evaporation of the surface adsorbed water and the second major weight loss between 200
800  C is due to the decomposition of the organic (OM/TREG) coating. The similar trend in TGA plots were also observed for the OM and TREG coated Fe3 O4 , CoO and Y2 O3 :Eu nanoparticles. Thus, FTIR and TGA studies indicate that surface of the hydrophobic and hydrophilic nanoparticles are adsorbed with OM and TREG coatings, respectively. Fig. 2. FT-IR spectra of (a) OM coated (A) and (b) TREG coated (B) ZnO nanoparticles, respectively.

1409, 1143
1051 and 949
842 cm
1 are due to C
H stretching, O
H stretching, C
H bending, C
O stretching and O
H bending vibration, respectively attributed for chemically adsorbed TREG coating to the hydrophilic ZnO nanoparticle The broad band between surface.14,17,18 3000
3600 cm
1 centered at  3400 cm
1 is due to the O
H stretching vibration arising from the water adsorbed to the particle surface. The similar FTIR patterns were observed for the hydrophobic and hydrophilic Fe3 O4 , CoO and Y2 O3 :Eu nanoparticles. The organic coating was further con¯rmed by TGA analysis. Figures 3(a) and 3(b) show the TGA

3.2.

TEM images of the of the Fe3 O4 nanoparticles prepared in OM and TREG using the Fe(acac)3 precursor have been reported elsewhere.14 The resulting nanoparticles were highly dispersed without any agglomeration. Figures 4(a) and 4(b) show the TEM images of the ZnO nanoparticles prepared in OM and TREG using the Zn(acac)2 precursor. Figures 4(c) and 4(d) show the TEM images of the Fe3 O4 nanoparticles prepared in OM and TREG using the Fe(ac)2 precursor. Figures 4(e) and 4(f) show the TEM images of the Fe3 O4 nanoparticles prepared in OM using Fe(cit)3 and Fe(hyd) precursor, respectively. Figures 4(g) and 4(h) show the TEM images of the CoO and Y2 O3 :Eu nanoparticles nanoparticles prepared in OM using Co (acac)3 and Y/Eu(acac)3 precursor, respectively. It can be seen that the highly dispersed di®erent nanoparticles are obtained using the solvent-free thermolysis technique except those agglomerated Fe3 O4 nanoparticles which are prepared in OM using the Fe(cit)3 and Fe(hyd) precursors. Thus, the TEM results indicate that highly dispersible hydrophobic or hydrophilic nanoparticles with diverse morphology can be synthesized by thermolysis of appropriate organometallic precursors in OM or TREG medium, respectively.

3.3.

Fig. 3. TGA curves of (a) OM coated (A) and (b) TREG coated (B) ZnO nanoparticles, respectively.

Morphology

Magnetic properties

Figures 5(a)
5(c) show the magnetization (M
H) curves of OM coated ZnO, Fe3 O4 and CoO nanoparticles, respectively. The corresponding saturation magnetization (MS Þ were measured at 20 kOe as  5  10
3 , 64 and  1 emu/g, respectively.

September 22, 2011

946

4:00:12pm

WSPC/175-IJN

FA1

00874

D. Maity & J. Ding

Fig. 5. (a) Room temperature M
H curves of OM coated (a) ZnO (A), (b) Fe3 O4 (B) and (c) CoO (C) nanoparticles, respectively.

(Fig. 5(b)) con¯rms the superparamagnetic behavior of the Fe3 O4 nanoparticles.18

3.4.

Luminescence properties

Figure 6 shows the °uorescence spectra of the Y2 O3 :Eu nanoparticles. The emission peaks at about 591, 611
624, and 653 nm are due to the 5 D0 ! 7 F1 , 11 5D ! 7F 5D ! 7F The 0 2 and 0 3 transitions. 5 D ! 7 F transition is responsible for the strong red 0 2 emission of the Y2 O3 :Eu phosphor nanoparticles.

Fig. 4. (a) and (b) are TEM images of OM and TREG coated ZnO nanoparticles prepared using Zn(acac)2 precursor. (c) and (d) are TEM images of OM and TREG coated Fe3 O4 nanoparticles prepared using Fe(ac)2 precursor. (e) and (f) are TEM images of OM coated Fe3 O4 nanoparticles prepared using Fe (cit)3 and Fe(hyd precursor, respectively. (g) and (h) are OM coated CoO and Y2 O3 :Eu nanoparticles using Co(acac)3 and Y/Eu(acac)3 precursor, respectively.

Figure 5(a) depicts the ferromagnetic nature of the ZnO nanoparticles while Fig. 5(c) indicates the combined paramagnetic and ferromagnetic behavior of CoO nanoparticles. Moreover, the zero coercivity and zero remanance on the M
H curves

Fig. 6. Room temperature luminescence emission spectra of Y2 O3 :Eu nanoparticles.

September 22, 2011

4:00:13pm

WSPC/175-IJN

00874

FA1

Single Step Synthesis of Hydrophobic and Hydrophilic Nanoparticles via Thermal Decomposition

4. Conclusion Highly dispersed hydrophobic and hydrophilic nanoparticles with diverse morphologies can be successfully prepared by the single step thermolysis of appropriate organometallic precursors in the OM and TREG medium, respectively.

References 1. A. Henglein, Chem. Rev. 89, 1861 (1989). 2. M. Shen, Y.-K. Du, H.-I. Rong, J.-R. Li and L. Jiang, Colloids Surf. A 257, 439 (2005). 3. H.-X. Mai, Y.-W. Zhang, R. Si, Z.-G. Yan, L.-D. Sun, L.-P. You and C.-H. Yan, Am. Chem. Soc. 128, 6426 (2006). 4. I. Das and S. A. Ansari, J. Sci. Indus. Res. 68, 657 (2009). 5. L. Li, Z. Zhu, X. Yao, G. Lu and Z. Yan, Micropor. Mesopor. Mater. 112, 621 (2008). 6. M. Salavati-Niasari, F. Mohandes, F. Davar and K. Saberyan, Appl. Surf. Sci. 256, 1476 (2009). 7. S. Sun, H. Zeng, D. B. Robinson, S. Raoux, P. M. Rice, S. X. Wang and G. J. Li, J. Am. Chem. Soc. 126, 273 (2004).

947

8. J. Park, K. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, N.-M. Hwang and T. Hyeon, Nat. Mater. 3, 891 (2004). 9. C.-C. Lin and Y.-Y. Li, Mater. Chem. Phys. 113, 334 (2009). 10. C. Li, Y. Li, Y. Wu, B. S. Ong and R. O. Loutfy, Sci. China. Ser. E: Technol. Sci. 51, 2075 (2008). 11. R. Si, Y.-W. Zhang, H.-P. Zhou, L.-D. Sun and C.-H. Yan, Chem. Mater. 19, 18 (2007). 12. J.-C. Boyer, F. Vetrone, L. A. Cuccia and J. A. Capobianco, J. Am. Chem. Soc. 128, 7444 (2006). 13. D. Maity, S.-G. Choo, J. Yi, J. Ding and J.-M. Xue, J. Magn. Magn. Mater. 321, 1256 (2009). 14. D. Maity, J. Ding and J.-M. Xue, Int. J. Nanosci. 8, 65 (2009). 15. D. Maity, S. N. Kale, R. Kaul-Ghanekar, J.-M. Xue and J. Ding, J. Magn. Magn. Mater. 321, 3093 (2009). 16. W. S. Seo, J. H. Shim, S. J. Oh, E. K. Lee, N. Hwi Hur and J. T. Park, J. Am. Chem. Soc. 127, 6188 (2005). 17. J. R. Dyer, Applications of Absorption Spectroscopy of Organic Compounds (Prentice-Hall, Inc., 1965). 18. D. Maity and D. C. Agrawal, J. Magn. Magn. Mater. 308, 46 (2007).