Stannous oxides,such as rutile SnO₂ and tetragonal SnO,have received great attention as functional materials,such as catalysts ^{[1, 2]},gas sensors ^{[3, 4]},solar cells ^[5],optoelectronic devices ^{[6, 7]}, and electrode materials of lithium ion batteries ^{[8, 9, 10]},and SnO can be oxidized to SnO₂ at elevated temperature easily ^[11].Synthesis of nano-materials with controlled morphology,size and crystal structure is a key step toward nano-technological applications ^[12]. Particularly,the novel three-dimensional (3D) hierarchical architectures that are assembled by 1D or 2D nanoscale units,have attracted considerable attention because of their improved performance,such as larger surface area,more efficient catalytic activity,superior electrochemical performance and structural stability ^{[12, 13, 14, 15]}. To date,a variety of SnO₂ 2D or 3D structures have been achieved,such as SnO₂ nanorods ^[16], single-layered hollow microspheres and flower like nanospheres have been synthesized through a facile template-free hydrothermal method with the help of PEG400 ^[17]. Hierarchical SnO₂ flower-like architectures consisting of numerous aggregative nanosheets have been successfully synthesized via a facile hydrothermal method with the help of PEG and subsequent calcinations ^[18]. Square SnO₂ nano-sheets have been synthesized by a template-free hydrothermal method based on the reaction between SnCl₂ and NaOH in ethanol/water ^[19] and so on. Certainly,many 1D and 2D SnO distinct morphologies have been successfully synthesized such as ultrathin SnO nanosheets ^[20], single crystalline SnO nanoplatelets ^[9],SnO disk-like structures ^[21],tetragonal SnO nanobranch ^[22]. Recently some 3D SnO structures have been synthesized in a water system. For example, well-controlled novel architectures of SnO with 3D flowerlike microstructures were synthesized in an aqueous solution using SnCl₂.2H₂O and NaOH ^[23],and novel self-assembled 3D hierarchical polygon shaped structures of SnO were synthesized with a template-free hydrothermal method using SnCl₂.2H₂Oand KOH in water ^{[10, 24]}. Compared to SnO₂ ,there have fewer reports in the preparation of novel SnO 3D structures,especially in the ethanol/water system.

In this letter,a hierarchical flower-like SnO consisting of various microsheets was successfully synthesizedviaa template-free and surfactant-free hydrothermal process based on the reaction between SnCl₂ and NaOH in different ratios of ethanol and water. A possible growth mechanism of the 3D structure is discussed. Furthermore,the effect of ethanol and NaOH on the formation of SnO and SnO₂ is discussed too. 2. Experimental

SnCl₂.2H₂O and NaOH were of analytical grade,and needed no further purification. Firstly,35 mmol of NaOH was dissolved in different volume ratios (mL/mL) of ethanol/water with 27:132:1 (symbolized as Sample A),20:20 = 1:1 (Sample B),10:30 = 1:3 (Sample C) at room temperature,respectively. Then 12 mmol of SnCl₂.2H₂O was dropped into each solution under continuous magnetic stirring for another 0.5 h. The obtained mixtures were transferred into a 150 mL Teflon-lined stainless autoclave,sealed and maintained at 180°C for 12 h,and then cooled to room temperature. The obtained precipitates were centrifuged and washed several times with water and ethanol,respectively,until Cl^-ions could not be detected. The products were finally dried in the vacuum at 70°C for 16 h. As comparison,a same synthesis was performed as Sample B but no addition of NaOH was performed, which was symbolized as Sample D.

Structural characterizations of the samples were performed by X-ray diffraction (XRD,Model Y2000,China) with Cu Karadiation (= 1.5406 ) at 30 kV and 20 mA. The structure and morphology of the products were studied using a scanning electron microscope (SEM,Model ZEISS Ultra 55,Germany). 3. Results and discussion

Fig. 1 shows typical XRD patterns of the SnO_x synthesized in different volume ratios of ethanol/water with or without NaOH. Almost all the diffraction peaks in Fig. 1(A-C) are well indexed to the tetragonal SnO standard patterns JCPDS 06-0395 except that a small peak at around 26.958in Fig. 1(B) and (C),which is identified to the intermediate Sn₆O₄(OH)₄ ^{[23, 25]}. Compared with the standard patterns,the intensity of (0 0 2 ) crystal plane is obviously strengthened and (1 0 1) and (1 1 0) planes are significantly depressed for the three samples. There were no descriptions of similar results in water systems at different temperature and time ^{[10, 23, 24, 25]},indicating that ethanol favors the crystal growing toward (0 0 1) crystal planes judged by the intense peaks of the (0 0 1) and (0 0 2) diffractions in the XRD patterns (Fig. 1A-C). The facts that the XRD diffraction intensity of SnO was reduced with the decrease of ethanol amount and the intermediate Sn₆O₄(OH)₄ appeared in sample B and C indicate that ethanol might have enhanced the crystal size and the degree of crystallinity. The XRD patterns of sample D (Fig. 1D) reveal that all the diffraction peaks index to the rutile SnO₂ (JCPDS 41-1445),and the broadened peaks indicate the crystal size of SnO₂ is reasonably small. Compared with the results obtained from sample B,NaOH should have played a crucial role in the formation of SnO_x.

Fig. 1

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Fig. 1. XRD patterns of samples prepared at different volume ratios of ethanol/water with NaOH: (A) 2:1; (B) 1:1; (C) 1:3; and (D) 1:1 without NaOH

Figs. 2 and 3 display the SEM images of the samples at low and high magnification. One can see the morphology of the samples is markedly different from each other. When the volume ratio of ethanol/water was 2:1,several rose flower-like hierarchical 3D architectures with a diameter of about 40mm formed uniformly. Each flower was assembled regularly by a large number of microsheets (Fig. 2a). The enlarged image of an individual architecture in Fig. 3a shows that the unique flower-like hierarchical architecture is regularly composed of numerous well-ordered thin microsheets with an octodecahedral shape. The octodecahedral sheet is composed of two irregular octagons at the bottom and the top,respectively,and sixteen trapezoids two of which have a communal edge at the sides (the front view of the sheets was inserted in Fig. 3a according to the morphology in red circle and rectangle in Fig. 3a). The irregular octagons at the bottom and the top are about 15mm for four long edges and 2-3mm for the other four short edges,and the thickness of the octodecahedral sheet is about 3mm. These microsheets grow in an unexpected fashion in that the flowers bloom naturally and form an open porous structure. Additional high-magnification images reveal the surface of the sheets is rough and composed of numerous smaller nanoparticles (Fig. 3b). As the ratio of ethanol/water was decreased to 1:1,the flower-like structure disappeared and the SnO changed to irregular blocks as a base with about 20-50mm of each edge and 5-10mm of thickness,and many thin nanosheets anchored on the base (Fig. 2b). These thin microsheets are irregular and 10-100 nm in thickness with poor dispersion (Fig. 3c). Additional magnified images illustrate that numerous nanoparticles formed the rough surface of the sheets (Fig. 3d),which is similar to that of Sample A. It is interesting that as the ratio of ethanol/water was decreased to 1:3,most of SnO self-assembled into more regularquasisquare blocks with 20-40mm in length and 2-5mm in thickness as a base, and many nanosheets anchored on the surface of thequasisquare blocks uniformly (Fig. 2c). These nanosheets have good dispersion and form many porous among them (Fig. 3e). In a high magnification image,the sheets seem very thin and smooth,and with about 20 nm in thickness and 1000 nm in length (Fig. 3f). For sample D synthesized without NaOH in Fig. 2d,SnO₂ nano-spheres of 200 nm in diameter are obtained,and the spheres are composed of numerous SnO₂ nanoparticles with smaller than 10 nm diameters (Fig. 3g). Here NaOH is the key factor to the formation of SnO_x. SnO₂ can form directly by the hydrolysis reaction (1) at room temperature and hydrothermal oxidation (2) without NaOH in solution ^[25]:

While with sufficient amount of NaOH,the hydrolysis reactions are as follows:

And then,in the hydrothermal process,Na₂[Sn(OH)_{4] transformed into SnO crystals:}

The total reaction is:

Uchiyama reported that the threshold pH values for the formation of SnO₂ and SnO were 3.3 and 13.1,respectively ^[25].

Fig. 2

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Fig. 2. SEM images of (a) sample A (ethanol/water = 2:1 with NaOH), (b) sample B (ethanol/water = 1:1 with NaOH), (c) sample C (ethanol/water = 1:3 with NaOH) and(d) sample D (ethanol/water = 1:1 without NaOH) at low-magnification.

Fig. 3

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Fig. 3. SEM images of the sample A (a and b), sample B (c and d), sample C (e and f) and sample D (g) at different magnification.

In Ref. ^[23],multi-architectured SnO formed by platelets at 30°C through a reaction between SnCl₂.2H₂O and NaOH in water. There are only limited numbers of large microsheets (25mmin length and 1mm in thickness) crossing each other,and all sheet edges are rough and possess multilayers. As the temperature increased,the thickness of the microsheets decreased to thin nanosheets and many closely attached nanosheets with different orientations assembling the 3D structures at 70-90°C. The results showed that the cross sheet structures can form at room temperature (no hydrothermal process) in a water system. In Refs. ^{[24, 26]},SnCl₂.2H₂O was hydrolyzed in water with KOH or NaOH and followed by a hydrothermal process,a SnO 3D structure where many thin microsheets piled up to 8-12mm was achieved. Some nanosheets anchored on the big base block if the reaction was prolonged,which is very similar to the structure of sample B and C in this study. Compared the references ^{[23, 24, 26]} with this study,one can deduce that the high temperature and hydrothermal process is a delamination process of the SnO base block. The thick and rough SnO blocks formed at room temperature can be delaminated by a high temperature hydrothermal process to numerous microsheets or nanosheets. Here,a possible mechanism for the formation of the flower like SnO 3D structure an in ethanol/ water system is proposed based on the structure and morphology of the samples A,B,C and those reported in the literature. The schematic illustration is shown in Fig. 4.

Fig. 4

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Fig. 4. Growth mechanism flow chart of flower-like SnO 3D structures in ethanol/water system.

In a typical growth mechanism the formation of square blocks occurs at the beginning of the reaction in an ethanol/water = 1:3 system (Sample C,the ethanol amount is small and SnCl₂ hydrothermal environment is similar to that of the pure water system ^[24]),after a long (12 h) hydrothermal process at high temperature (180°C),these blocks were delaminated into thin and smooth quasisquare nanosheets and anchored on the residual blocks. As the ratio of ethanol/water increased to 1:1 (Sample B), the polarity and dielectric constant of the solvent would be decreased,which resulted in a slower hydrolysis of SnCl₂ and more irregular growth of the microsheets. The slow hydrolysis will result in larger crystal nucleus and the formation of rougher surface of the microsheets (Fig. 3d) than that of sample C. In addition,ethanol will influence the morphology and the delamination of the base blocks. Compared with Sample C where most of the base blocks and the peeled nanosheets are of regularquasisquare shape,the irregular blocks were peeled and cracked into irregular microsheets,which anchored on the blocks disorderly and nonuniformly. As the ethanol/water ratio increased to 2:1,the SnCl₂ hydrolysis rate was reduced further,and the larger SnO crystals formed the roughest surface of the sheets (Fig. 3b). The large ratio of ethanol accelerates the delamination of the blocks and lead to few large base blocks. Instead,a large number of uniform octodecahedral microsheets formed and the sheets self-assembled as the rose flower-like structures by a hydrothermal process. As a result,abundant surface state and some defects are created in these highly dispersive 2D microsheets and 3D hierarchical structures. In Ref. ^[19],ethanol/water systems with a small amount of NaOH (pH<13) were used to synthesize SnO₂ by a hydrothermal method. The results showed that the size and morphology of the nanosheets also changed with the ratio of ethanol/water. The ethanol content is a crucial factor for the formation of the SnO flower like structures. Further investigation of the formation mechanism of SnO_x will be the next step. 4. Conclusion

Novel rose flower-like SnO 3D hierarchical structures consisting of well-ordered microsheets were synthesized by a simple hydrothermal method in an ethanol/water system. The size and morphology of microsheets can be controlled by changing the ethanol/water ratio. The ratio of ethanol in the solvent plays an important role in the formation of hierarchical flower-like SnO architectures and the NaOH content is critical for the formation of SnO and SnO₂ in this system. Acknowledgment

This study was financially supported by Guangdong Natural Science Foundation (No. S2011010005788).