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Mar 12, 2009 - Department of Physics & Materials Science, City University of Hong Kong, Tat ... These ultrafine nanowire arrays fabricated directly on...
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Synthesis and Growth Mechanism of Quasialigned Ultrafine ZnS Nanowire Arrays Fabricated Directly on Zinc Foils Guixiang Qian,† Kaifu Huo,*,†,‡ and Paul K. Chu*,† Department of Physics & Materials Science, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon, Hong Kong, China, and The Hubei ProVince Key Laboratory of Refractories and Ceramics Ministry - ProVince Jointly-Constructed CultiVation Base for State Key Laboratory, School of Materials and Metallurgy, Wuhan UniVersity of Science and Technology, Wuhan 430081, China ReceiVed: NoVember 20, 2008; ReVised Manuscript ReceiVed: January 22, 2009

Quasialigned ZnS nanowire arrays have been synthesized directly on zinc substrates via a simple and mild one-step solvothermal method. The morphology, structure, and composition of the synthesized materials are evaluated by scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. These ultrafine nanowire arrays fabricated directly on a zinc foil have diameters of 5-15 nm and grow along the [001] direction. The influence of the reaction time, temperature, and solvent on the morphology and composition of the products is investigated systematically and the growth mechanism of the ultrafine nanowire arrays is proposed. This growth model may enable better design and assembly of other chalcogenide nanowire arrays on the appropriate metal substrates for nanodevice applications. 1. Introduction In recent years, II-VI group semiconductor nanowires have inspired much research interest due to their fascinating properties and promising applications in nanoelectronics and optoelectronics devices.1-3 Zinc sulfide (ZnS), one of the most important II-VI semiconductors, has a wide bandgap of 3.7 eV at room temperature and a large exciton binding energy of 40 meV.4 These favorable properties bode well for applications in flat-panel displays, electroluminescent devices, infrared (IR) windows, sensors, and photocatalysts.5-7 Several techniques have been developed to synthesize ZnS nanowires.8-12 For example, Wang et al. reported the synthesis of ZnS nanowires by thermal evaporation of ZnS powders on Au-coated silicon substrates at 900 °C.8 Barrelet et al. prepared ZnS nanowires via thermal decomposition of singlesource molecular precursor Zn(S2CNEt2)2.9 Biswas et al. fabricated ZnS nanowire arrays using zinc foils and sulfur powders as the starting materials by thermal evaporation at 500 °C.10 However, most of these reported ZnS nanowires have diameters of over 30 nm.8,10-12 Semiconductor nanowires with diameters comparable or close to the Bohr diameters of the materials should possess more useful and unique physical and chemical properties due to the quantum confinement effect.13,14 The Bohr diameter of bulk ZnS is about 5 nm, and consequently, the synthesis of ultrafine nanowires with diameters of several nanometers has attracted much attention.15,16 Recently, several groups have reported the fabrication of ultrafine ZnS nanowires with random orientations by means of high-temperature catalyst-assisted growth.17,18 However, the use of catalysts increases the complexity of the synthetic procedures and introduces some nonindigenous * To whom correspondence should be addressed. (K.H.) Tel: +85227844220. Fax: +852-27844185. E-mail: [email protected]; kaifuhuo@ cityu.edu.hk. (P.K.C.) Tel: +852-27887724. Fax: +852-27889549. E-mail: [email protected]. † City University of Hong Kong. ‡ Wuhan University of Science and Technology.

impurities that may adversely influence their properties and further applications. In addition, in many electronic and photonic applications such as field emission and solar cells, it is desirable to fabricate aligned nanowire arrays on the appropriate substrate so that the nanowires can be assembled directly to obtain the desirable properties.19 In this work, quasialigned ultrafine ZnS nanowire arrays are synthesized on zinc foils by a simple one-step solvothermal reaction at 180 °C for 10 h without subsequent thermal or chemical treatments. These ultrafine nanowires which have diameters of 5-15 nm are assembled directly into arrays on the electrically conducting zinc substrate. By systematically studying the influence of the reaction time, temperature, and solvent on the morphology and composition of products, the growth mechanism is revealed and a growth model is proposed. This growth model may provide clues to more effective design and assembly of other chalcogenide nanowire arrays on the appropriate metal substrates in nanodevice applications. 2. Experimental details Thiourea (NH2CdSNH2) and a pure zinc foil (99.9%, 10 × 10 × 1 mm3) were chosen as the sulfur and zinc sources, respectively. Before the experiments, the zinc foil was polished using SiC sandpapers and then ultrasonically cleaned in acetone, ethanol, and deionized water sequentially. Thiourea (5 mmol) and SDS (SDS, C12H25SO4Na, 20 mmol) were added into a 40 mL solution composed of ethylenediamine (en) and deionized water (H2O) with a volume ratio of 1:1. After magnetic stirring for 30 min, the solution was transferred to the 50-mL Teflonlined stainless steel autoclave in which the pretreated zinc foil was immersed in the solution. The autoclave was sealed and maintained at 180 °C for 10 h. After the autoclave cooled naturally to room temperature, the product was taken out from the solution and rinsed with distilled water and absolute ethanol several times, followed by vacuum drying at 60 °C for 4 h. In order to determine the growth mechanism, the effects of the temperature, solvent, and reaction time on the morphology of

10.1021/jp8101892 CCC: $40.75  2009 American Chemical Society Published on Web 03/12/2009

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the synthesized products were investigated by changing the reaction temperature (60-180 °C), reaction time (30 min to 10 h), and volume ratio of en and H2O (1:2, 1:3, pure H2O, and pure en), respectively. The morphology, structure, and composition of the synthesized products were characterized by field-emission scanning electron microscopy (FE-SEM, JSM 6335F, 5kV), transmission electron microscopy (TEM, Philips CM20, 200 kV), highresolution TEM (HRTEM, JEOL 2010F, 200 kV), X-ray diffraction (XRD, Siemens D-500), energy-dispersive X-ray spectroscopy (EDS, Oxford INCA 200), X-ray photoelectron spectroscopy (XPS, Physical electronics PHI-5802), and Fourier transform infrared spectroscopy (FTIR, Perkin-Elmer Spectrum 100). The XRD patterns were recorded using the graphitemonochromatized Cu KR radiation (λ ) 0.154 06 nm) in the 2θ range of 5-80°. Prior to conducting TEM and HRTEM, the products were carefully scraped from the zinc foil and then treated by strong ultrasonic wave for 30 min in ethanol. A drop of the solution was deposited on Cu grids or Ni grids. Fourier transform infrared (FTIR) absorption spectra were acquired on a Perkin-Elmer Spectrum 100 between 400 and 4000 cm-1. The XPS spectra were collected on a Physical electronics PHI-5802 using monochromatic Al KR X-ray as the excitation source. 3. Results and Discussion Figure 1a depicts the SEM micrograph of the product synthesized at 180 °C for 10 h using a 1:1 en to H2O volume ratio. The product is composed of quasialigned nanowire arrays distributed uniformly on the surface of the entire zinc foil. The high-magnification SEM image in the inset discloses that the nanowires lean to each other and coalesce at their tips. These nanowires have ultrafine diameters of about 5-15 nm and lengths up to micrometers. The structure and morphology of the nanowires are further studied by TEM and HRTEM. The TEM image in Figure 1b corroborates that the diameters of the nanowires are indeed 5-15 nm. The EDS spectrum in Figure 1c reveals that the nanowires are composed of Zn and S with an atomic ratio of close to 1:1. The fringe spacing of about 0.308 nm corresponding to the (002) lattice planes of wurtzite ZnS can be clearly observed from the HRTEM image in Figure 1d, confirming that the nanowires grow along the [001] direction. The Cu and C signals originate from the copper grid and carbon film, respectively. In order to fathom the formation mechanism of these ultrafine ZnS nanowire arrays on the zinc foil, a series of experiments were performed to investigate the effects of temperature, solvent, and reaction time on the morphologies and microstructures of the products. 3.1. Influence of Temperature on Morphology. Figure 2 shows the SEM images of the materials produced at different reaction temperatures from 60 to 180 °C for 10 h. As shown in Figure 2a, at 60 °C, a few nanosheets can be observed to sprout from the substrate. At a reaction temperature of 100 °C, the small and thin nanoribbons with coalescent ends appear on the Zn foil surface. The nanoribbions have widths of 100-200 nm and thicknesses of 10-15 nm (Figure 2b). When the temperature is raised to 150 °C, the sample still has a ribbon-like structure about 1-2 µm wide and about 60 nm thick (Figure 2c). At 180 °C, quasialigned nanowire arrays with diameters of about 5 to 15 nm and lengths up to micrometers are found to be uniformly distributed on the surface of the entire zinc foil as shown in Figure 1a. These results indicate that the reaction temperature plays an important role in the morphology of the products.

Figure 1. (a) SEM images, (b) TEM image, (c) EDS spectrum, and (d) HRTEM image of the product synthesized at 180 °C for 10 h in the 1:1 en and H2O (volume ratio). Inset in Figure 1a is the highmagnification SEM image.

3.2. Effect of Solvent. The solvent is known to affect the morphology of the products in the solvothermal process. In our synthesis, the solvent consists of en and H2O. en is a molecular template which is usually used in the solvothermal process to control the crystal growth. It has been suggested that the en concentration has important effects on the crystal structure and shape of the ZnS.20,21 Hence, the relationship between the composition of the solvent, that is, ratio of en to H2O, and the product is investigated in this work. Figure 3 shows the SEM images of the surface of the zinc foils immersed in different solvents at 180 °C for 10 h. As shown in Figure 1, when the volume ratio of en to H2O is 1:1, quasialigned ultrafine ZnS nanowire arrays are produced on the Zn foil. When the volume ratio of en to H2O is decreased to 1:2, disordered nanowires with diameters of about 10-20 nm and lengths of about 1 µm are obtained (Figure 3a). By further decreasing the volume ratio

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Figure 2. SEM images of the products synthesized for 10 h in the 1:1 en and H2O (volume ratio) at (a) 60 °C, (b) 100 °C, and (c) 150 °C. (SEM image of the product synthesized at 180 °C is shown in Figure 1a.)

to 1:3, very short nanorods with lengths of less than 100 nm are formed on the Zn foil (Figure 3b). If the solvent is pure H2O, the surface of the Zn foil is covered by spherical particles with diameters ranging from hundreds of nanometers to micrometers (Figure 3c). On the contrary, if the zinc foil is treated directly in pure en, the product consists of nanoplates in lieu of nanowires, as shown in Figure 3d. In the pure en solution, Zn2+ ions react easily with en to form ZnS(en)0.5 which is a stable lamellar compound in the en medium and often requires further subsequent thermolysis to release the en molecules to form ZnS nanostrucutures.20,22,23 Hence, the volume ratio of en to H2O, or more precisely the concentration of en, dramatically influences the morphology of the products. Furthermore, the surfactant SDS plays an important role in the nucleation and growth of the ZnS nanostructures which participate in the formation of ultrafine ZnS nanowire arrays. The detailed functions will be discussed in the following part on the growth mechanism.

Figure 3. SEM images of the products under different solutions at 180 °C for 10 h: (a) en: H2O ) 1:2 (volume ratio), (b) en: H2O ) 1:3, (c) pure H2O, and (d) pure en.

3.3. Influence of Reaction Time. To obtain more information about the formation of the quasialigned ZnS nanowire arrays on the Zn foil, the time-dependent evolution processes are monitored. Figure 4 shows the SEM images and XRD patterns of the samples synthesized for different reaction times of 30 min, 1 h, 2 h, 3 h, and 5 h. After reaction for 30 min, the surface of the zinc foil is covered by a discontinuous and layered film (Figure 4a). The corresponding XRD pattern in the inset of Figure 4a shows a series of equally spaced peaks in the range of 5-35° and a weak peak at 32.8° in addition to the signals from the Zn substrate (JCPDF card, no. 04-0831). The 32.8°

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Figure 4. (a-e) SEM images of the products synthesized at 180 °C in the solution of 1:1 en and H2O (volume ratio) for 30 min, 1 h, 2 h, 3 h, and 5 h, respectively, (f) XRD patterns of the products at different reaction stages: 1, 2, 3, 5, and 10 h. The inset in (a) is the corresponding XRD pattern of 30 min sample. Insets of panels b-d are their corresponding TEM images (the upper) and EDS spectra (the lower).

weak peak corresponds to the (100) plane of β-Zn(OH)2 (JCPDS card, no. 24-1444), and a series of equally spaced peaks in the range of 5-35° can be attributed to the formed layered structures on the Zn foil. It may be composed of zinc hydroxide/ dodecyl sulfate Zn(OH)x(C12H25SO4)y · nH2O (referred to as ZnDS) produced by stacked hexagonal zinc hydroxide intercalated with the DS ions of the surfactant SDS.24,25 This conjecture is supported by the FTIR spectrum (see Figure S1 in the Supporting Information). If the reaction time is increased to 1 h, the layered film transforms into lamellar nanostructures (Figure 4b) and a few nanowires are seen to form between the lamellar nanostructures. The TEM image (the upper inset of Figure 4b) demonstrates that these lamellar structures are curled and aggregate together. In the XRD results (Figure 4f), the equally spaced peaks disappear but the weak peak at 32.8° corresponding to the (100) plane of β-Zn(OH)2 remains. Meanwhile, a new diffraction peak at 29.3° emerges. The EDS spectrum in the lower inset of Figure 4b confirms that the lamellar intermediates are composed of Zn, S, and O with the atomic ratio close to 1.0:0.5:0.8. The high-resolution XPS spectra which disclose the chemical states show the presence of S2- and OH- ions (see Figure S2 in the Supporting Information). These results indicate that the layered structures of ZnDS are destroyed and a new lamellar intermediate (referred to as Zn(OH)xSy26,27) is formed

after 1 h. When the reaction time is increased to 2 h, nanowires with a diameter of about 12 nm are dominant on the surface. They lean to each other but have roots on the surface of the zinc substrate (Figure 4c). Further TEM observation discloses that some incompletely curled lamellar structures (indicated by the arrow) coexist with the nanowires at this stage, as suggested by the upper inset of Figure 4c. The EDS results suggest that the atomic ratio of Zn, S, and O of the nanowires is close to 1.0:0.8:0.4 (the lower inset of Figure 4c), which is different from that of the lamellar nanostructures (1.0:0.5:0.8) produced after 1 h. That is, the S content increases but that of O decreases when the reaction time is prolonged from 1 (lamellar structures) to 2 h (nanowires). The XRD pattern acquired from the 2 h sample (Figure 4f) suggests that the peak at 29.3° corresponding to the lamellar intermediate (1 h) shifts toward a lower Bragg angle of 29.0 ° (2 h), which is closer to the diffraction peak of the (002) plane of ZnS (28.7°). Since the ionic radius of S2ions (0.184 nm) is larger than the thermochemical radius of OH- ions (0.140 nm), 28 the diffraction peak shift from 29.3° to 29.0° arises from the S2- ions that are partially incorporated into the nanowires and replace the OH- ions as the reaction proceeds. A similar phenomenon has been observed in the formation of S-substituted In(OH)3 structures.28,29 Therefore, it can be inferred that the Zn(OH)xSy nanowires may evolve from

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Figure 5. Schematic illustration of the formation process of quasialigned ultrafine nanowire arrays: (a) In the initial stage, OH- ions are generated by the reaction between en and H2O. (b) The generated OH- ions react with zinc foil to form the layer of ZnDS and Zn(OH)2 on the surface of the zinc foil with the assistance of DS- ions. Herein, (i) the possible strucutures of ZnDS, which are composed of the layered zinc hydroxide inserted by DS ions. (c) As the reaction temperature increases, ZnDS decomposes to generate Zn(OH)2 slabs into the solution, which react with S2- ions to form lamellar Zn(OH)xSy (ii-iii) and deposits on the surface of Zn(OH)2/Zn. (d-f) At relatively high temperature, lamellar Zn(OH)xSy splits and curls into Zn(OH)xSy nanowires. Then Zn(OH)xSy nanowires transform into ZnS nanowires gradually by the continuous substitution of S2- ions for OH- ions. (g) At low temperature, no curling process can occur and only ZnS nanoribbons are formed.

the Zn(OH)xSy lamellar nanostructures via a curling process, accompanied by gradual incorporation of S2- ions. As the reaction progresses to 3 h and then 5 h, the synthesized products are mainly nanowires which have a tendency to exhibit quasialigned growth on the Zn foil, as shown in Figure 4, panels d and e. The EDS results acquired from the nanowires after a reaction time of 3 h (the lower inset of Figure 4d) indicate that the synthesized nanowires consist of mainly Zn and S with the atomic ratio close to 1:1. In the corresponding XRD patterns, the Zn(OH)xSy and β-Zn(OH)2 peaks disappear and the diffraction peaks can be well indexed to hexagonal ZnS (JCPDS card, no. 36-1450) and ZnO (JCPDS card, no. 36-1451) (Figure 4f). It is reasonable that the XRD patterns of ZnS in Figure 4f originate from the ultrafine nanowires. The intense (002) peak suggests that the ZnS nanostructures grow preferentially along the c axis. The weak ZnO peaks may be assigned to the ZnO layer between the zinc foil and ZnS nanowires as a result of the transformation of the β-Zn(OH)2 layer as mentioned before. The results suggest that the Zn(OH)xSy nanowires completely transform into ZnS nanowires with continuous incorporation of S2- ions and substitution for OH- ions. Meanwhile, the β-Zn(OH)2 between the zinc foil and superficial Zn(OH)xSy film are converted into ZnO. When the reaction time reaches 10 h, as shown in Figures 1 and 4f, quasialigned ultrafine ZnS nanowire arrays are ultimately produced on the Zn foil. 3.4. Growth Mechanism. Based on our results, the growth mechanism and the formation process of the ultrafine ZnS nanowires on the zinc foil are proposed and schematically illustrated in Figure 5. In the solution, en, being a strong nucleophilic agent, reacts with H2O to generate OH- ions.30 The zinc atoms on the surface of Zn foil are oxidized to Zn2+ by the small amount of dissolved O2 and then dissolve in the solution to react with OH- ions forming Zn(OH)2, similar to

the preliminary stage in the formation of ZnO nanorods on zinc foils.30 In the meantime, the SDS ionizes into negatively charged DS ions and Na+ ions. The negatively charged DS ions can absorb on the zinc-coordinated sites of the Zn(OH)2 layer electrostatically to form a layer of ZnDS and Zn(OH)2,24,31,32 as suggested by the SEM and XRD results (Figure 4a) and schematically indicated in Figure 5b. The possible structure of ZnDS shown in Figure 5i may be composed of layered zinc hydroxide inserted by DS ions. As the reaction temperature increases, the layered structure of ZnDS with low-temperature stability begins to decompose into Zn(OH)2 slabs and DS ions and dissolve in the solution. Meanwhile, the concentration of S2- ions from the decomposition of thiourea increases rapidly. The freshly formed Zn(OH)2 slabs partially combine with the S2- ions quickly to form the Zn(OH)xSy embryo (Figure 5ii and iii), and under a continuous supply of Zn(OH)xSy, the lamellar Zn(OH)xSy intermediate perpendicular to the zinc foil is formed, as schematically illustrated in Figure 5c and supported by the SEM image in Figure 4b. In the subsequent reaction stages, the reaction temperature plays a critical role in the morphology evolution of the lamellar Zn(OH)xSy. If the temperature is high enough (g180 °C in our preparation), the lamellar Zn(OH)xSy transforms into ultrafine nanowires due to thermal stress generated at high temperature, as observed in the formation of Bi, WS2, and titanate nanotubes33-35 schematically shown in Figures 5d and 5e. Meanwhile, S2- ions in the solution continuously diffuse into the Zn(OH)xSy and substitute for OH- ions due to the smaller solubility of the ZnS in comparison with Zn(OH)2 (Ksp,ZnS ) 10-23.8, Ksp,Zn(OH)2 ) 10-15.3).27 As a result, ultrafine ZnS nanowires are produced on the Zn foil. At the same time, the Zn(OH)2 layer between the Zn(OH)xSy film and zinc foil dehydrates completely forming the ZnO film due to the poor thermodynamic stability of Zn(OH)2 in comparison with ZnO, as illustrated in Figure 5f. If the reaction

Quasialigned Ultrafine ZnS Nanowire Arrays temperature is relatively low (below about 150 °C), Zn(OH)xSy nanoribbons instead of nanowires are produced because the temperature is not high enough to curl the nanoribbions to form wire-like structures, as schematically shown in Figure 5g and suggested by Figure 2, panels b and c, respectively. With the continuous incorporation of S2- ions and replacement of OH- ions, Zn(OH)xSy nanoribbons finally transform into ZnS nanoribbions. In the growth process, the en content in the solution is crucial. If the en content is small, the basicity of the solution decreases thereby slowing down the dissociation rate of Zn atoms from the zinc foil. Hence, if the amount of Zn2+ in the solution is small, only short nanowires are formed (Figure 3, panels a and b). 4. Conclusion Ultrafine ZnS nanowire arrays have been synthesized directly on a zinc foil at 180 °C for 10 h via a simple one-step, templatefree, and catalyst-free solvothermal process. These ultrafine nanowires have diameters of 5-15 nm and lengths up to micrometers. A possible growth mechanism and model are proposed to explain the formation of these ultrafine nanowires on the zinc substrate based on systematic investigation of the influence of the reaction time, temperature, and solvent on the morphology and composition of final products. The potential technological importance of the product and the simplicity of the preparation procedure make this study both scientifically and technologically interesting. Acknowledgment. This work was financially supported by Key grant Project of Educational Commission of Hubei Province (Z200711001), Key Project of Chinese Ministry of Education (no. 208087), Hubei Province Natural Science Foundation (no. ZRY0087), and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) no. CityU 112307. Supporting Information Available: FTIR spectrum (Figure S1) of the sample shown in Figure 4a and the high-resolution XPS spectra of the S2p (Figure S2a) and O1s (Figure S2b) shown in Figure 4b. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lao, C. S.; Liu, J.; Gao, P. X.; Zhang, L. Y.; Davidovic, D.; Tummala, R.; Wang, Z. L. Nano Lett. 2006, 6, 263–266. (2) Tang, K. B.; Qian, Y. T.; Zeng, J. H.; Yang, X. G. AdV. Mater. 2003, 15, 448–450. (3) Kumar, S.; Nann, T. Small 2006, 2, 316–329. (4) Jiang, Y.; Meng, X. M.; Liu, J.; Xie, Z. Y.; Lee, C. S.; Lee, S. T. AdV. Mater. 2003, 15, 323–327. (5) Manzoor, K.; Aditya, V.; Vadera, S. R.; Kumar, N.; Kutty, T. R. N. Solid State Commun. 2005, 135, 16–20.

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