Materials Letters 63 (2009) 350–352

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Materials Letters

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Hydrothermal synthesis of phosphate-mediated ZnO nanosheets

Yanxiang Wang ⁎, Xueyun Fan, Jian SunDepartment of Materials Science and Engineering, JingDeZhen Ceramic Institute, JingDeZhen, 333001, People's Republic of China

⁎ Corresponding author. Tel.: +86 798 8499659; fax:E-mail address: yxwang72@163.com (Y. Wang).

0167-577X/$ – see front matter. © 2008 Elsevier B.V. Adoi:10.1016/j.matlet.2008.10.036

a b s t r a c t

a r t i c l e i n f o

Article history:

ZnO nanosheets were obtai Received 3 September 2008Accepted 10 October 2008Available online 25 October 2008

Keywords:ZnO nanosheetsSodium tripolyphosphateHydrothermal synthesis

ned via a simple hydrothermal synthesis in the presence of sodium tripolypho-sphate. The formation mechanism and effect of sodium tripolyphosphate concentration on the morphologyof ZnO nanosheets have also been reported. Field emission scanning electron microscopy, transmissionelectron microscopy and fourier transform infrared spectroscopy were used to characterize the structurefeatures and chemical compositions. The results show that the replacement of OH− dangling bond on ZnOpositive polar faces (0001) by PO4

3− hinders splicing growth of [Zn(OH)4]2− growth units along the [0001]

direction, which results in the formation of ZnO nanosheets.© 2008 Elsevier B.V. All rights reserved.

1. Introduction

ZnO nanostructures are interesting to study not only because of therecent demonstrations of unique physical properties such as UV lasingemission and photoconductivity properties in nanowire form, but alsobecause a wide variety of morphologies have been prepared. Some ofthe ZnO nanostructures exhibit nanowire, nanorod, nanoribbon,nanoplate, nanotube, tetrapod, and starlike structures [1–4]. Amongof various ZnO nanostructures, relatively few studies on two-dimensional ZnO nanosheets have been reported up to now. ZnOnanosheets have shown superior properties in nanoscale optoelec-tronics, solar cell electrode, catalysis and sensor devices [5,6].

The ZnO nanosheets have been prepared by various methods, suchas, thermal oxidation of zinc powders, carbon-thermal redox of ZnOpowders [7,8] and chemical vapor deposition [9]. These methods needhigh temperature and are also limited by their low yield. However, thehydrothermal route is an important and simple low-temperaturemethod for wet chemistry, and has been employed to fabricate ZnOnanopowders [10].

Here we report the preparation of phosphate-mediated ZnOnanosheets by hydrothermal synthesis method.

2. Experimental

Zinc sulfate heptahydrate (Zn(SO4)2•7H2O), ammonia, Sodiumhydroxide and sodium tripolyphosphate (STPP) were purchasedfrom Shanghai Chemical Reagents Company. All chemicals wereanalytical grade and used as purchased without further purification.

ZnO nanosheets were synthesized by two steps. First, 50 mL of0.06 M zinc sulfate heptahydrate aqueous solution was mixed with50 mL of 2 M ammonia and the resulting mixture were stirred

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thoroughly. Some white precipitates (the precursor) were obtained.Then resulting white precipitate was collected and washed withdeionizedwater several times. Next, precursor powders were added todifferent concentration of STTP solutions. Sodium hydroxide solutionwas added to set the pH 9. The resulting solutions were stirred. Then60 mL of the above solution were transferred to a Teflon-linedautoclave for hydrothermal synthesis at 220 °C in an oven for 28 h. Theautoclave has a capacity of 100 mL. When the reaction time wasreached, the autoclave was removed and cooled to room temperature.After this process, white crystalline product was collected bycentrifugation and thorough washings with ethanol and then driedat 50 °C.

Themorphology of the resulting powder was examined using JSM-6700F field emission scanning electron microscopy and JEM2010transmission electron microscopy. Infrared absorption spectroscopy(IR) spectrum was measured at room temperature on a FTIR spectro-meter (Nicolet 5700) using the KBr Pellet technique.

3. Results and discussion

Fig. 1 shows FSEM and TEM images of the ZnO nanosheets prepared with 0.1 MSTTP. The length and width of ZnO nanosheets were about 900 and 600 nm, while thethickness was 50 nm. Fig. 1(c) clearly depicts an individual nanosheet with width about600 nm. The inset of Fig. 1(c) is the corresponding select area electron diffraction (SAED)pattern. The sharp and bright dots in SAED pattern indicate that the ZnO nanosheet washexagonal single crystalline. The growth direction of nanosheets can be indexed to be[0001] direction.

The previous literatures have confirmed that ZnO, as a polar crystal, has a polar axis,and the crystallites of ZnO obtained from the hydrothermal solvents inwhich OH−1 wasadded usually appear in elongated form [11–15]. It is well-known that the polar growthof ZnO crystal along (0001) direction proceeds through the adsorption of growth unitsof [Zn(OH)4]2− onto (0001) plane [16]. However, in the present case, STPP was added inthe hydrothermal solution. STPP is a linear polyphosphate. When STPP dissolves inwater, STPP ionizes completely, forming Na+ and PO4

3− ions. The PO43− ion has a good

complex ability with Zn2+ ion. Complex ion stability constant of PO43− ion and Zn2+ ion is

9.7, while complex ion stability constant of ion Zn2+ ion and OH−1 is 4.4. So PO43− will

replace OH− and react with Zn2+. The (0001) and (000̄) faces of ZnO crystal have equalreticular density but differ in the composition of the outermost atomic layer [14]. The

Fig. 1. FSEM and TEM photos of ZnO Nanosheets (a) and (b) FSEM photos, (c)TEM photo, (d) EDX spectroscopy.

351Y. Wang et al. / Materials Letters 63 (2009) 350–352

outermost layer of the positive monohedron (0001) consists of Zn2+ ions. The effectivecharge on this face is close to 2+. The outermost layer of the negativemonohedron (000̄)consists of O2− ions and has a negative charge of the same magnitude. So the PO4

3− ionswill be adsorbed on (0001) face rather than (000 ̄) face, and the attachment of growthunits of [Zn(OH)4]2− onto the (0001) face is hindered. Accordingly, the intrinsicallyanisotropic growth of ZnO along the (0001) direction is substantially suppressed andcrystal growth then proceeds sideways, which results in the formation of ZnOnanosheets.

EDX and IR results suggest that phosphate ions were selectively adsorbed andexisted on the surface of the ZnO sheets. EDX spectroscopy shown in Fig. 1(d) indicatesthat the ZnO nanosheets are mostly composed of Zn, O and P elements. The signal of Cuoriginates from the copper grid, whereas the C peak should come from carbon film onthe Cu grid. The IR spectrum of ZnO nanosheets is shown in Fig. 2. The absorption at~524 and ~435 cm−1 is assigned to Zn–O groups [17].The broad absorption at about3430, 3430 and 1600 cm−1 are assigned to the existence of hydroxyl groups on thesurface of the samples. The sharp peaks at 920 and 809 cm−1 are assigned to the P–Obending vibration. The absorption at 1100 and 1020 cm−1 are assigned to the symmetryor antisymmetry stretching vibration of the P–O bond in the (PO4 )3− tetrahedron.

Fig. 3 illustrates the FSEM images of ZnO powders prepared with different STTPconcentration. Sample prepared with no STTP added was composed of ZnO microrodswith diameter about 1 μm. When STTP concentration were 0.025 M, 0.05 M and 0.1 M,ZnO nanosheets were obtained. The thickness and the 2D size of the samples preparedwith 0.025 M, 0.05 M and 0.1 M are almost the same. The reasons are as follows: to

Fig. 2. IR spectrum of the ZnO nanosheets.

obtain ZnO nanosheets, there is a theoretical STTP concentration value as for a certainamount of zinc salt precursor. When STTP concentration is 0.025 M, 0.05 M and 0.1 M,the amount of PO4

3− from STTP are over the theoretical value, so only a part of PO43− is

absorbed to (0001) plane and redundant PO43− remain in the hydrothermal medium. So

STTP concentration range is at 0.025–0.1 M, the morphologies of ZnO nanosheets arealmost same.

4. Conclusions

ZnO nanosheets with thickness about 50 nm have been success-fully synthesized by hydrothermal synthesis approach under STTPadded into hydrothermal solution. Phosphate ions are selectivelyadsorbed on the (0001) surface of ZnO crystal and restrict the growthof regular ZnO crystal, which result in the ZnO nanosheets formation.

Acknowledgements

This study was supported by The Education Department of JiangxiProvince.

Fig. 3. FESEM photos of ZnO powders prepared with different concentration of STTP (a)0 M, (b) 0.025 M, (c) 0.05 M, (d) 0.1 M.

352 Y. Wang et al. / Materials Letters 63 (2009) 350–352

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