Nanoscale heterostructures have attracted more and more attention in recent years owing to the multifunctional and integrated request for the nanodevices^[1]. A great deal of effort has been put in the synthesis of nanoscale heterostructures with controllable composition and properties,such as the superlattices,nanocables and hierarchical nanostructures^[2]. The interface between two different materials is very important for a heterostructure,because it will greatly influence the performance of the devices based on such heterostructures^[3,4,5]. Epitaxial growth is an effect technique to obtain the perfect interface in the heterostructures^[6,7].However,the epitaxial growth generally occured between the materials with the same crystal structures,such as Si/Ge and GaAs/InAs.

Both SnO₂ and ZnO have been intensively studied due to their outstanding properties and potential applications in nanoscale electronics,optics,sensors and other novel devices^{[8,9,10,11,12]}. Recently,the disordered heterostructures composed of ZnO and SnO₂,such as the side-to-side SnO₂/ZnO/SnO₂ triaxial nanobelt and the core-shell ZnO/SnO₂ nanostructures,have been reported^[13,14]. However,it is still a big challenge to prepare the epitaxial growth ZnO-SnO₂ heterostructures due to the quite different crystal structures of these two materials. SnO₂ belongs to the Rutile structure with the lattice constants a=0.4737 nm and c=0.3186 nm,but ZnO has a wurtzite structure with the lattice constants a=0.3249 nm and c= 0.5206 nm. Meanwhile,it is not difficult to find that the atomic stacking is similar in some crystal planes of SnO₂ and ZnO. For example,in the wurtzite ZnO crystal,the atomic stacking along the c axis is the hexagonal closest packing,which is similar for the rutile SnO₂ crystal along the a axis.Such structure similarity makes it possible to epitaxial grow the SnO₂ -ZnO heterostructures,which was proved by a recent report^[15]. However,the two-step process used two different methods,thermal evaporation and the hydrothermal method,make the synthesis process complicated. In addition,it was found that the ZnO seeds are indispensable for the formation of the SnO₂-ZnO epitaxial heterostructures. In this study,we used a simple and fast,two-step thermal evaporation process to synthesize the SnO₂-ZnO epitaxial heterostructures. The ZnO nanrods were directly epitaxial grown on the SnO₂ backbones without using the ZnO seeds. The SnO₂-ZnO heterostructures exhibited good photocatalytic performance for the organic dyes degradation. 1 Experimental Section 1.1 Synthesis of ZnO-SnO₂ nanodendrites

The experiments were carried out in a tube furnace via a two-step thermal evaporation method. First,pure Sn powder was placed at the center of the tube,and several alumina plates served as the substrates were placed at 10 cm downstream. After evacuated,50 sccm Ar and 5 sccm O₂ was introduced into the furnace and kept the vapor pressure at 5 torr. Then the furnace was heated to 850 ℃ and kept for 90 min. After cooled to room temperature,the product of the first step was taken out and used as the substrates for the next step to grow ZnO. The second step of the ZnO growth is a typical method according to the previous report^[16]. After the two-step process,products on the substrates were collected for further characterization. The structures of the products were checked by X-ray diffraction (XRD,M18AHF). The morphologies and microstructure of the products were investigated by scanning electron microscopy (SEM,Hitachi S-4300) and high resolution transmission electron microscopy (HRTEM,JMS 2011). 1.2 Photocatalytic experiments

The photocatalytic experiments were performed under the illumination of a high pressure mercury lamp. The alumina substrate (20×5 mm) with the as-prepared ZnO-SnO₂ nanodendrites was immersed in 50 mL aqueous solution of methyl orange with a concentration of 5 mg/L. Absorbance spectrum was monitored every 20 min. 2 Results and Discussion

The first product studied was obtained in the first thermal evaporation step. The SEM image shown in Figure 1A reveals that the products are large quantities of nanowires. The nanowires are several tens of micrometers in length and 80—120 nm in diameter. Figure 1B shows the XRD patterns recorded from the sample shown in Figure 1A. Most of the diffraction peaks can be indexed to the tetragonal SnO₂(JCPDS No. 88-0287),and the unlabelled peaks are attributed to the Al₂O₂ substrate. Figure 1C and 1D are the typical TEM and HRTEM images of the nanowire. The distances between the clear lattice fringes are 0.26 nm and 0.24 nm,corresponding to the (-101) and (-200) planes of the tetragonal SnO₂,respectively. According to the HRTEM results and the FET image,it can be deduced that the SnO₂ nanowires are single crystal and grow along the [101] direction.

图 1

Fig. 1

Figure 1 (A) SEM image of the SnO2 nanowires synthesized at the first step; (B) XRD pattern of the nanowires; (C) TEM image of the nanowires; (D) HRTEM image and the corresponding FET image(inset) taken from an individual nanowire

Figure 2 shows the morphology and structure analysis of the products obtained in the second step. The XRD results (Figure 2A) confirmed that two different phases,i.e. the tetragonal SnO₂ (JCPDS No. 88-0287) and the hexagonal ZnO (JCPDS No. 36-1451),co-exist in the product. The SEM observation reveals that a hierarchical nanostructure was formed in this step,as shown in Figure 2B,2C and 2D. The hierarchical structure is composed of the SnO₂ nanowire backbone and the numerous ZnO nanorods assembled on the SnO₂ nanowire. The length and diameter of the ZnO nanorods could be controlled by adjusting the duration of the second step. Figures 2B is the SEM image of the as-synthesized products with growth duration of 30 min. After 30 min growth,the central axial SnO₂ nanowires are surrounded by radial oriented nanorods with diameter of about 20 nm and length of about 200 nm. When the ZnO growth time was prolonged to 60 min,as shown in Figure 2C and 2D,the branches evolved from nanorods to nanowires with size of about 50 nm in diameter and 2 μm in length. The branch nanowires were radially grown on the SnO₂ nanowire with a 4-row symmetrical ordered assembly.

图 2

Fig. 2

Figure 2 (A) XRD pattern of products synthesized at the second step; SEM images of the sample with growth duration of (B) 30 min and (C),(D) 60 min,respectively

The morphology and structure of the nanodendrites were further characterized by TEM. A representative TEM image of the nanostructure is shown in Figure 3A. A clear interface can be found between the SnO₂ backbone and the ZnO nanorod branches. The microstructure of the ZnO-SnO₂ heterostructure was studied by HRTEM recorded from different locations. Figures 3B and 3C show the lattice-resolved HRTEM images taken from the sites b and c marked in Figure 3A,respectively. In Figure 3B,the fringe spacing is 0.52 nm,corresponding to the identify distance of the (001) plane in wurtzite ZnO. The SAED pattern (inset of Figure 3B) is accordance with the HRTEM analysis. These results demonstrate that the branch nanorod is single-crystalline ZnO with a growth direction of [001]. The HRTEM image taken from the interface region around the branch nanorod and the backbone nanowire is shown in Figure 3C. It was found that the interface is parallel to the (001) plane of the branch ZnO and the growth direction of SnO₂. The ZnO and SnO₂ have a crystalline orientation relationship as follows: [010]_ZnO//[010]_SnO2,(001)_ZnO//(-101)_SnO2. And the angle between the growth plane of ZnO(100) and SnO₂ (101) is about 157°. Figure 3D is the schematic model of the interface region of the heterojunction. Along the boundary,the lattice constants of the ZnO (100) and SnO₂ (101) are 0.28 nm and 0.26 nm,respectively. There is an angle of 157° between these two planes That deflection could avoid the lattice mismatch of about 5.6% between the ZnO (001) and SnO₂ (101) and ensure the epitaxially growth of the ZnO (001) plane from the SnO₂ (-101). Actually,the planes of the ±(-101) and symmetrical ±(101) of the SnO₂ were exposed during formation of SnO₂ nanowires^[17] and served as active planes to provided preferable sites for ZnO deposition when the ZnO vapor arrived in second step. As a result,the hierarchical nanostructure consisted of 4-rows of secondary ZnO wires grown from ± (-101) and ±(101) planes of SnO₂ trunk is formed.

图 3

Fig. 3

Figure 3 (A) Typical TEM image of single nanodendrite; (B),(C) HRTEM images taken from location b and c marked in Fig. 3A,respectively; (D) A schematic diagram of the heterojunction interface showing the epitaxial and orientation relation

It is reported that the SnO₂-ZnO compound semiconductor is an effective system for the photocatalytic applications. However,it is likely to form some defects at the interfaces of the two semiconductors,which will become the recombination centers for the photo-induced carriers. The epitaxial relationship between the ZnO and SnO₂ nanowires in the present heterostructures can eliminate this drawback. Combing the large surface areas provided by dendritic morphology,the hierarchical ZnO-SnO₂ nanostructure should be a good candidate for photocatalyst. The photocatalytic property of the as-synthesized ZnO-SnO₂ nanostructure was investigated by the photodegradation of methyl orange. Figure 4A shows the UV-Vis absorption spectra of the methyl orange solution after illuminated for different durations. With increasing the irradiating time,the absorption of methyl orange weakened,which means that the concentration of the methyl orange decreased. After 100 min irradiation,the methyl orange was almost totally decomposed. A comparative experiment was done to the pure SnO₂ nanowires under the same condition,which results are shown in Figure 4B. It can be seen that the SnO₂-ZnO heterostructures exhibited a much better photocatalytic performance than that of the SnO₂ nanowires. Two possible reasons might be responsible for the enhanced performance. First,the SnO₂-ZnO heterojunction can enhance the photocatalytic capability by increasing the efficiency of the charge separation and restricting the recombination of the photo-induced carriers. Second,ZnO also has photocatalytic activity under the UV light illumination.

图 4

Fig. 4

Figure 4 (A) UV-Vis absorption spectra of methyl orange solution,the original solution with 5 mg/mol in concentration was treated by the nanodendrites sample for 0,20,40,60,80,100 min; (B) Degradation rate versus time over two samples (with and without the nanodendrites catalyst)

In summary,ZnO-SnO₂ nanodendrites were synthesized via a two-step thermal evaporation process. High oriented 4-rows of the ZnO branches were hetero-epitaxial grown on the SnO₂ trunks. Present ZnO-SnO₂ nanodendrites displayed an evident capability of the photocatalytic decomposition to methyl orange. The heterojunction with special hierarchical structure provide a potential application for photocatalytic degradation of organic pollutants in sewage and air.

This work was supported by Chinese Academy of Sciences,NSFC (Grant Nos. 21103211,51272258,51272302,50902134,and 61025003) and National Basic Research Program of China (973 Program) (Grant Nos. 2010CB934103 and 2012CB932400).