Facile and Fast Synthesis of Flower-Like ZnO Nanostructures
Facile and Fast Synthesis of Flower-Like ZnO Nanostructures
Facile and Fast Synthesis of Flower-Like ZnO Nanostructures
Materials Letters
journal homepage: www.elsevier.com/locate/matlet
a r t i c l e i n f o a b s t r a c t
Article history: A fast and facile method has been developed to synthesise flower-like ZnO nanostructures at room
Received 8 October 2012 temperature without using any capping agent. ZnO nanoflowers could be easily prepared by adding
Accepted 11 November 2012 ethanol to the precursor solution having higher concentration of hydroxide ions. The amount of ethanol
Available online 20 November 2012
plays an important role in the formation of ZnO nanoflowers. The products were characterized by field
Keywords: emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and transmission electron
ZnO microscopy (TEM). The room-temperature photoluminescence (PL) spectrum of the sample consists of
Flower-like near-band edge emission at 385 nm as well as a strong and broad green–yellow emission band due to
Nanocrystalline materials the presence of defect states in the sample. A reasonable growth mechanism is also proposed for the
Photoluminescence
formation of flower-like ZnO nanostructures.
& 2012 Elsevier B.V. All rights reserved.
n
Corresponding author. Tel.: þ82 41 850 8496; fax: þ82 41 856 8613. Fig. 1 shows the FESEM images and XRD pattern of the
E-mail address: jkim@kongju.ac.kr (J. Kim). as-prepared ZnO nanostructures. A low-magnification view
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.matlet.2012.11.042
P. Ramasamy, J. Kim / Materials Letters 93 (2013) 52–55 53
Fig. 1. FESEM images and XRD pattern of ZnO nanoflowers synthesized at room-temperature for 5 min: (a) low magnification FESEM image (b, c) high-magnification
FESEM images and (d) XRD pattern.
(Fig. 1a) shows the high yield and uniformity of the synthesized When the Zn2 þ /OH molar ratio is 1:20, the ZnO can be dissolved
products. The magnified SEM images in Fig. 1b and c show that the in highly concentrated NaOH solution by Eq. (2), in which the
morphology of ZnO product is well-defined flower-like hierarchical chemical equilibrium tends to proceed towards the left. The
structure. The diameters of the flowers are around 1 mm, assembled reaction can readily be shifted to the right by deactivation of
by many nanosheets as ‘‘petals’’. XRD characterization (Fig. 1d) OH in Eq. (2) using ethanol solvent.
indicated that as synthesized flower-like structures, consisted of
Zn2 þ þ4OH - ZnO22 þ2H2O (1)
pure hexagonal ZnO with wurtzite structure (JCPDS card no. 36-
1451). The flower-like ZnO nanostructures were further confirmed ZnO22 þH2O $ ZnO þ2OH (2)
by TEM results. The typical TEM image (Fig. 2a) confirms the
flower-like ZnO structure with diameter around 1 mm, which Ethanol can effectively shield the OH of Eq. (2) by forming
consisted of large number of nanosheets. The corresponding solvent cages [20], which reduce the OH activity in the reaction
HRTEM image (Fig. 2b) confirmed that the nanoflowers are well and favor the formation of ZnO. When there is sufficient amount
crystalline in structure and the spacing between two lattice planes of ethanol to mask the activity of OH , more ZnO nuclei would
is 0.19 nm, can be ascribed to the adjacent (102) planes of wurtzite form and aggregate together. After that, due to Ostwald ripening
ZnO phase. The chemical stoichiometry of the obtained ZnO nano- more and more spherical sub-micrometer particles were pro-
flowers is further confirmed with the EDS method. Fig. 2c shows the duced at the cost of nano-aggregations [21]. Because of the
typical EDS spectrum which consists of only O and Zn elements, intrinsic anisotropic character of hexagonal ZnO, the aggregated
confirming the formation of pure ZnO. Fig. 3 shows the FESEM nanoclusters would rearrange themselves and orientedly attach
images of the products obtained from different volume ratios of along the c-axis to each other to decrease the energy of the
H2O and EtOH. From Fig. 3, one could find that the volume of system [22]. Further it is speculated that each nanocluster in
ethanol added plays a key role in the formation of ZnO nanoflowers. the nano-aggregations grew preferentially along its c-axis during
In the absence of ethanol the reaction took more than 5 h to form the reaction which results in flower-like ZnO nanostructures.
ZnO and the products appear as rhombus-shaped ZnO particle. The ZnO crystal exhibits the hexagonal wurtzite structure, which
flower-like ZnO started to form when the ratio of H2O:EtOH is 1:3, belongs to the space group C46V. According to the selection rules of
shown in Fig. 3d. On increasing the ratio to 1:4, the flower-like ZnO phonon resonance modes, Raman-active modes for wurtzite ZnO are
structures formed (Fig. 3e). The as synthesized ZnO samples were A1 þ2E2 þE1 [23]. The Raman spectrum of ZnO nanoflowers is shown
nanograined and contained the well developed free surfaces and in Fig. 4a. A dominated and strong intensity peak at 440 cm 1 is
grain boundaries. It has been shown that the presence of defects attributed to the E2 mode of the non-polar optical phonons. The peak
such as grain boundaries have strong influence in the physical at 333 cm 1 is attributed to the 2E2 mode. The peaks at 388 and
properties of nanograined ZnO [18,19]. 585 cm 1 correspond to the polar transverse A1 and longitudinal E1
Based on the above results the formation mechanism of ZnO optical phonon mode, respectively. In general, the E1 (LO) mode is
nanoflowers can be explained as follows. Two kinds of reaction associated with the structural defects such as oxygen vacancies and
can take place in alkali solutions. The initial reaction between zinc interstitials in ZnO [23,24]. The room temperature PL spectrum
Zn(NO3) 6H2O and NaOH leads to the formation of ZnO22 by of the ZnO nanoflowers is shown in Fig. 4b. The PL spectrum shows a
Eq. (1) and that these anions then further react with water to give sharp UV emission peak around 385 nm and broad green–yellow
ZnO nuclei by Eq. (2). As we all know, ZnO is an amphoteric peak centered at 586 nm. The UV emission is usually attributed to
compound, which can be dissolved in acid or alkali solution. recombination of free excitons, that is, near band-edge emission [25].
54 P. Ramasamy, J. Kim / Materials Letters 93 (2013) 52–55
Fig. 2. (a) TEM (b) HRTEM images and (c) EDS spectrum of ZnO nanoflowers.
Fig. 3. FESEM images of the samples synthesized at different volume ratios of H2O/EtOH: (a) 1:0 (b) 1:1 (c) 1:2 (d) 1:3 (e) 1:4 and (f) 0:1.
The broader visible emission peak originates from defect-state interstitials (Oi) in ZnO, and it may also be associated with oxygen
luminescence. It is known that the yellow emission is associated adsorbed in ZnO grain boundaries, which can also act as an electron
with electron acceptor defects such as Zn vacancy (VZn) or O acceptor [26].
P. Ramasamy, J. Kim / Materials Letters 93 (2013) 52–55 55
Fig. 4. (a) Raman spectrum and (b) room temperature PL spectrum of the as synthesized ZnO nanoflowers.
4. Conclusion [3] Guo H, Lin Z, Feng Z, Lin L, Zhou J. J Phys Chem C 2009;113:12546–50.
[4] Thiemann S, Sachnov S, Porscha S, Wasserscheid P, Zaumseil J. J Phys Chem C
2012;116:13536–44.
A facile and fast method has been developed to fabricate [5] Li S, Zhang X, Jiao X, Lin H. Mater Lett 2011;65:2975–8.
uniform flower-like ZnO nanostructures at room temperature. [6] Polarz S, Roy A, Lehmann M, Driess M, Kruis FE, Hoffmann A, et al. Adv Funct
Ethanol was used to precipitate ZnO from the precursor solution Mater 2007;17:1385–91.
having high concentration of hydroxide ions. The amount of [7] Ye F, Peng Y, Chen G-Y, Deng B, Xu A-W. J Phys Chem C 2009;113:10407–15.
[8] Chu D, Masuda Y, Ohji T, Kato K. Langmuir 2009;26:2811–5.
ethanol added plays an important role in the formation of ZnO [9] Lee SH, Minegishi T, Park JS, Park SH, Ha J-S, Lee H-J, et al. Nano Lett
nanoflowers. XRD pattern and Raman measurement confirm that 2008;8:2419–22.
the products have good crystallinity with hexagonal wurtzite [10] Gargas DJ, Moore MC, Ni A, Chang S-W, Zhang Z, Chuang S-L, et al. ACS Nano
2010;4:3270–6.
phase. From the experimental results, a possible formation
[11] Liu X, Zhang J, Wang L, Yang T, Guo X, Wu S, et al. J Mater Chem
mechanism of the ZnO nanoflowers was proposed. The room- 2011;21:349–56.
temperature photoluminescence spectrum of the sample shows a [12] Baxter JB, Aydil ES. Appl Phys Lett 2005;86:053114.
near-band edge emission at 385 nm and a broad defect related [13] Yang P, Yan H, Mao S, Russo R, Johnson J, Saykally R, et al. Adv Funct Mater
2002;12:323–31.
emission around 586 nm. [14] Arnold MS, Avouris P, Pan ZW, Wang ZL. J Phys Chem B 2002;107:659–63.
[15] Vayssieres L. Adv Mater 2003;15:464–6.
[16] Du G, Zhang L, Feng Y, Xu Y, Sun Y, Ding B, et al. Mater Lett 2012;73:86–8.
Acknowledgment [17] Tokumoto MS, Pulcinelli SH, Santilli CV, Briois V. J Phys Chem B
2003;107:568–74.
[18] Straumal BB, Protasova SG, Mazilkin AA, Myatiev AA, Straumal PB, Schutz G,
The authors gratefully acknowledge financial support from the et al. J Appl Phys 2010;108:073923.
Priority Research Center Program (2012-0006682), the Basic [19] Straumal BB, Mazilkin AA, Protasova SG, Myatiev AA, Straumal PB, Goering E,
Science Research Program (2012R1A1A2043731) and the Human et al. Thin Solid Films 2011;520:1192–4.
[20] Dedonder-Lardeux C, Grégoire G, Jouvet C, Martrenchard S, Solgadi D. Chem
Resource Training Project for Regional Innovation through the Rev 2000;100:4023–38.
National Research Foundation of Korea. [21] Li Q, Wang E, Li S, Wang C, Tian C, Sun G, et al. J Solid State Chem
2009;182:1149–55.
[22] Pacholski C, Kornowski A, Weller H. Angew Chem Int Ed 2002;41:1188–91.
References [23] He F-Q, Zhao Y-P. Appl Phys Lett 2006;88:193113.
[24] Chen SJ, Wang GR, Liu YC. J Lumin 2009;129:340–3.
[1] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, et al. Science [25] Qiu Y, Yang S. Adv Funct Mater 2007;17:1345–52.
2001;292:1897–9. [26] Carcia PF, McLean RS, Reilly MH, Crawford MK, Blanchard EN, et al. J Appl
[2] Johnson JC, Knutsen KP, Yan H, Law M, Zhang Y, Yang P, et al. Nano Lett Phys 2007;102:074512.
2003;4:197–204.