Diameter- and shape-controlled ZnS/Si nanocables and Si nanotubes for SERS and photocatalytic applications

ZnS/Si nanocables were synthesized via a simple two-step thermal evaporation method. The shape and diameter of the ZnS/Si nanocables can be controlled by adjusting the morphologies of the ZnS nanostructures (nanowire or nanoribbon) obtained in the first step and the deposition time of the Si shell in the second step, respectively. Furthermore, we obtained polycrystalline Si nanotubes with different shapes and diameters by etching away the inner ZnS core. The as-prepared Si nanotubes were employed as SERS-active substrates, which exhibited a high sensitivity for the detection of R6G. The Si nanotubes also showed effective photocatalytic activity on the decomposition of R6G under the irradiation of visible light.


Introduction
Over the past decade, one-dimensional heterostructures have attracted particular interest due to their unique properties and potential applications in nanoelectronics and nanophotonics [1][2][3][4][5].Among all these heterostructures, the radial nanocable structures have advantages because of its higher efficient injection current in comparison with the axial structures [6][7][8].In particular, Si-based heterostructures are of special interest due to the important role of Si in the modern microelectronics industry.To meet the increasing demands of various applications, different methods have been developed for the fabrication of Si-based nanomaterials, such as the laser-ablation metal-catalytic method [9], chemical vapor deposition [10], chemical etching method [11], and solution growth [12].To date, several Si-based core/shell heterostructures, such as Ge/Si [13,14], Si/SiO 2 [15], Si/SiOx [16], ZnS/Si [17], ZnSe/Si [18], Ag/Si [19], have been fabricated using different growth processes.Since the properties of nanomaterials greatly depend on their structures, shapes, sizes, and chemical compositions [20][21][22][23], the crystalline semiconductor nanocables with controllable structures, shapes, diameters, and chemical compositions would have new applications in many areas, such as photocatalysis, biological and chemical sensing, and nanooptoelectronics.
In this paper, we report a simple thermal evaporation approach for the synthesis of shape-and diameter-controlled Si-based nanocables.Furthermore, polycrystalline Si nanotubes with different shapes and diameters were obtained by etching away the inner ZnS core.Owing to the large specific surface areas, the Si nanotubes would have advantageous applications in biological, chemical, and electrical devices.Here, the Si nanotubes were used as the surfaceenhanced Raman spectroscopy (SERS)-active substrates for the ultrasensitive molecule detection and the photocatalyst for organic dyes degradation.The results indicate that the Si nanotubes might be a promising material for sensing and photocatalytic applications.chemical vapor deposition (CVD) method [24,25].In a typical experiment, 1 g of ZnS powder (Alfa Aldrich, 99.99%) as the source material was placed at the center of a quartz tube.Several pieces of Si wafers (1 cm × 2 cm) coated with 5 nm-thick gold films were used as substrates and placed at about 9 cm downstream of the ZnS powder.The quartz tube was placed in a tube furnace and was evacuated to a pressure of 4.5 × 10 −4 Torr.A carrier gas of high-purity argon premixed with 5% H 2 was continuously fed into the tube at a flow rate of 25 standard cubic centimeters per minute (sccm) and a pressure of 100 Torr throughout the whole experiment.Temperature at the center of the tube was ramped up at 20 • C/min to reach a temperature of 1050 • C and kept at this temperature for 2 hours.Temperatures at the silicon substrates were measured to range from 600 to 800 • C.After the furnace was naturally cooled down to room temperature, ZnS nanowires and nanoribbons were deposited in the temperature region of about 600 • C and 800 • C, respectively, on the Si wafers.

The Synthesis of ZnS/Si
Nanocables.The experiment was then repeated by replacing the ZnS powder with SiO powder (Alfa Aldrich, 99.99%) and adjusting some of the experimental parameters.The as-prepared ZnS nanowires or nanoribbons were placed 7 cm down stream of the SiO powder.High-purity argon with 5% H 2 gas was fed at 50 sccm and a pressure of 300 Torr.Center of the tube was heated to 1350 • C and held for 30 minutes before cooling.

Materials Characterization.
Compositions, morphologies, and microstructures of the prepared samples were characterized with X-ray diffraction (XRD, Philips X'Pert), scanning electron microscopy (SEM, HITACHI s-4300), and transmission electron microscopy (TEM, Philips CM20 and CM200 FEG).SERS was measured using a Renishaw 2000 laser Raman microscope equipped with a 633 nm argon ion laser of 2 μm spot size for excitation.UV-vis absorption spectra were acquired with an Aglient 8453 UV-vis Diode Array Spectrophotometer.Structures of the samples shown in Figure 1(c) are further examined with TEM. Figure 2(a) is the TEM image of a typical wire-like structure obtained from the two-step deposition in Figure 1(c) showing that it is a cable with a core/shell structure.The diameter of the inner core is about 60 nm and the thickness of the sheath is around 75 nm.A selected area electron diffraction (SAED) pattern of the cable is shown as an inset in Figure 2(a).The pattern consists of diffraction spots and rings, which can be, respectively, indexed to single-crystalline wurtzite ZnS and polycrystalline Si.   nanocable consisting of many nanocrystallites with a lattice spacing is 0.31 nm which matches well to the (111) spacing of Si.These observations further confirm that the cables have single crystalline cores of wurzite ZnS and shells of polycrystalline Si.

Results and Discussion
The thickness of Si shell could be easily tuned by adjusting duration of the second deposition step.Figure 3 depicts SEM images of the cables obtained with growing duration of 30 to 120 minutes for the second deposition step.It can be seen that diameter of the cables increases from 200 to 900 nm as the growth duration.
Shape-controllable growth is another important issue in nanoscience and nanotechnology, because some properties would be greatly affected by their shapes.In the experiment, we also found that ZnS nanowires and nanoribbons could be formed in the first deposition step at different temperatures.We found that a ZnS/Si core/shell structure can also be formed with the wire and ribbon shapes (Figures 4(a) and 4(c)).The wire-shaped nanocables in Figure 4(a) have a smooth inner surface and small diameter, in contrast to the ribbon-shaped nanocables (Figure 4(c)) with saw-like ZnS nanoribbons as templates which have a saw-like inner surface and bigger diameter.As mentioned before, the core of the cable can be removed by HCl treatment.It can be seen that a typical cable before HCl etching has a dark-contrasted core of ZnS (Figure 4(a)).After the etching, all the cables show a light-contrasted core (Figure 4(b)).EDS analysis is presented in Figure 1(f), which confirms that there is no longer ZnS in the etched sample.The ZnS core has been etched away leaving the hollow tube of polycrystalline silicon.Similarly ribbon-shaped tubes of polycrystalline Si with saw-like inner surfaces can be formed by etching away the ZnS ribbon cores (Figure 4(d)).
The tubular morphology provides the Si nanostructures with even larger specific surface areas, which might further enhance their performance for applications such as sensors and catalyst.As an example, we investigated the sensing properties of the Si nanotubes as the active substrates for the detection of organic molecules by the SERS technique.As is well known that the Raman signals can be significantly enhanced in the presence of nanometer-sized metallic structures [26] because of the plasmonic resonance between We have also tested the photocatalytic activity of the Si nanotubes.The Si nanotubes were used to degrade Rhodamine 6G (R6G) under the visible lights, and control experiment using Si nanowires were also conducted.Equal amount of the Si nanotubes and Si nanowires of the similar diameters were immersed into 70 mL R6G solutions (1.5 × 10 −6 mol/L), respectively, and then irradiated under visible light.After the same time intervals, the Si nanotubes and Si nanowires were taken out, and the absorption spectra of the R6G solutions were recorded.Figure 6 shows the photocatalytic degradation performance of the Si nanotubes and Si nanowires, where C 0 and C are the equilibrium concentration of R6G before and after visible light irradiation, respectively.Without catalyst (curve A), no obvious degradation of R6G was observed after irradiation under visible lights for 5.5 hours.In sharp contrast, both Si nanotubes and nanowires accelerated the degradation significantly, and the Si nanotubes (curve C) showed much higher photocatalytic activity than that of nanowires (curve B), which are considered to be due to the larger specific surface areas of the hollow tubular structures.

Conclusions
In summary, we demonstrated in this work a simple method to synthesize ZnS/Si nanocables and Si nanotubes with controllable geometries.In this approach, single-crystal ZnS nanowires and nanoribbons grown by CVD were employed as templates for the subsequent CVD growth of polycrystalline Si sheath to form a ZnS/Si core-shell nanocable structure.The ZnS cores were removed by chemical etching to construct Si nanotubes.Diameters and morphologies of the ZnS/Si nanocables and the Si nanotubes can be controlled by adjusting the deposition time of the Si sheath and the morphologies of the ZnS template, respectively.Considering the large specific surface areas and stability, Si nanotubes were used as the SERS-active substrates for the highly sensitive detection of organic moleculars and effective photocatalyst for the degradation of organic dyes.

Figure 1 :Figure 2 :
Figure 1: (a) and (b) An SEM image and EDS spectrum of the ZnS nanowires obtained after the first deposition step; (c) and (d) an SEM image and EDS spectrum of the ZnS/Si nanocables after deposition of Si onto the ZnS nanowires; (e) and (f) an SEM image and EDS spectrum of the Si nanotubes obtained by chemical etching of the sample in Figure 1(c); Inset in Figure 1(e), an enlarged SEM image showing the open-ended tubular structures.

Figure 1 (
Figure1(a) shows an SEM image of the product on the Si substrate after the first deposition step using ZnS powder as source.It consists of a high density of nanowires of about 60 nm in diameter and about 3 μm in length.Using an energy dispersive X-ray spectrometer (EDS) attached to the SEM, the sample is shown to consist of only Zn and S in roughly 1 : 1 atomic ratio (Figure1(b)).Figure1(c)shows an SEM image of the sample after the two deposition steps sequentially with ZnS and SiO sources.It can be seen that the morphology of the sample is similar to that shown in Figure1(a) except that the wires are of larger diameter (about 200 nm).Figure1(d)shows a typical EDX spectrum of the sample in Figure1(c).In addition to the Zn and S peaks, an Si peak can be clearly observed.The sample was then immersed in diluted HCl solution for 30 minutes.Figure1(e)and insert shows that the HCl treatment removes cylindrical core from the wire-like structures in Figure1(c) and leads to the formation of hallow tubes.Figure 1(f) is the EDS of the sample in Figure 1(e), which only consists of Si.
Figure1(a) shows an SEM image of the product on the Si substrate after the first deposition step using ZnS powder as source.It consists of a high density of nanowires of about 60 nm in diameter and about 3 μm in length.Using an energy dispersive X-ray spectrometer (EDS) attached to the SEM, the sample is shown to consist of only Zn and S in roughly 1 : 1 atomic ratio (Figure1(b)).Figure1(c)shows an SEM image of the sample after the two deposition steps sequentially with ZnS and SiO sources.It can be seen that the morphology of the sample is similar to that shown in Figure1(a) except that the wires are of larger diameter (about 200 nm).Figure1(d)shows a typical EDX spectrum of the sample in Figure1(c).In addition to the Zn and S peaks, an Si peak can be clearly observed.The sample was then immersed in diluted HCl solution for 30 minutes.Figure1(e)and insert shows that the HCl treatment removes cylindrical core from the wire-like structures in Figure1(c) and leads to the formation of hallow tubes.Figure 1(f) is the EDS of the sample in Figure 1(e), which only consists of Si.

Figure 2 (
b) is an EDS spectrum taken from the tip (as marked by the red box in Figure2(a)) showing the existence of Zn, S, Si, Au, and a minimal content of O.The Au element comes from the gold nanoparticle at the tip of each ZnS/Si nanocable, which worked as catalyst during the ZnS nanowire growth process.

Figure 2 (
c) is a high-resolution TEM image of the inner core of a nanocable.Lattice fringes of 0.31 and 0.33 nm correspond well to spacing of the (002) and (100) planes of wurtzite structured ZnS.

Figure 4 :
Figure 4: TEM images of ZnS/Si nanocables and Si nanotubes of different shapes: (a) circular cable with ZnS nanowire as template, (b) circular tube, (c) cable of ribbon shape with saw-like ZnS nanoribbon as template, and (d) tube of ribbon shape.

Figure 5 :
Figure 5: (a) Si nanotubes coated with Ag nanoparticles; (b) SERS spectra of R6G (at the concentration of 1 × 10 −8 M) collected on Si wafer (curve A) and Si nanotubes (curve B); (c) The EDX spectroscopy of the samples in Figure 5(a)).

Figure 6 :
Figure 6: Degradation rate of R6G in solution with Si nanotubes (curve C) and nanowires.(curve B) as catalyst and without catalyst (curve A).