Fabrication and Luminescent Properties of c-Axis Oriented ZnO−ZnS

J. Phys. Chem. C 2007, 111, 5039-5043

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Fabrication and Luminescent Properties of c-Axis Oriented ZnO-ZnS Core-Shell and ZnS Nanorod Arrays by Sulfidation of Aligned ZnO Nanorod Arrays Subhendu K. Panda, Apurba Dev, and Subhadra Chaudhuri* DST Unit on Nano Science and Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: December 6, 2006; In Final Form: February 5, 2007

In this paper, we report the preparation of vertically aligned ZnO-ZnS core-shell and ZnS nanorod arrays with crystallographic orientation along the c-axis by sulfidation of aligned ZnO nanorod arrays. The ZnO nanorod arrays were prepared by a surfactant assisted soft chemical method on a sol-gel derived ZnO thin film. Partial conversion of the ZnO nanorod surface to ZnS at 400 °C in an H2S and argon gas mixture produces ZnO-ZnS core-shell structures, whereas the complete conversion at 600 °C produces well-aligned ZnS nanorods. The influence of the sulfidation time and annealing temperature on the conversion of ZnO nanorods to ZnO-ZnS core-shell nanorods is investigated. TEM studies revealed that the sulfidation process begins at the surface of the ZnO nanorods, which gradually increase inward, retaining the morphology and following the crystal orientation of the ZnO core. The nanostructures showed enhanced UV emission properties. The described procedures open a way to convert a microstructured material into chemically more stable materials with the same morphology, thus leading to a diversification of possible applications.

Introduction Manipulation of size, shape, and orientation of technologically important materials, within the dimension of a few nanometers to a micrometer, has been a great challenge for materials scientists for the last few decades. As a consequence of intense research efforts, the fabrication of wide varieties of nanostructures with a large number of materials has been possible. Among the various nanoforms, one-dimensional (1-D) oriented nanostructures such as nanorods, nanowires, nanotubes, nanopins, etc. have become the focus of intensive research owing to their unique applications in mesoscopic physics and in the fabrication of nanoscale devices.1 The creation of these structures in large arrays with a low-cost method and assembling these outstanding structures into functional devices is the biggest challenge for scientists. Recently, 1-D ZnO and ZnS nanomaterials have attracted much attention because of their potential applications in nanoscale electronics, optics, and other novel devices. As an important II-VI semiconductor, ZnO has a wide band gap energy of 3.37 eV with a large binding energy (60 meV). It possesses unique optical and electronic properties. It has shown a wide range of technological applications including transparent conducting electrodes of solar cells, flat panel displays, surface acoustic wave devices, and chemical sensors.2-7 On the other hand, ZnS has a wide band gap energy of 3.66 eV at room temperature. It is a well-known luminescent material, having prominent applications in flat-panel displays, electroluminescent devices, sensors, and lasers, and also has been applied in photocatalysts, infrared windows, pigments, and nonlinear optical devices.8-13 The nanostructures of luminescent materials are particularly important as the properties of these materials are greatly influenced by various surface states arising out of the higher surface-to-volume ratio. This can be further tuned by deliberately * Corresponding author. Tel.: +9133 24734971, ext. 201. Fax: +9133 24732805. E-mail: [email protected].

modifying the surface of nanostructured materials by other materials such as core-shell nanostructures, which effectively enhance the luminescence properties.14 There are many reports where it was observed that in core-shell structures with higher band gap shell materials, there was an enhancement of optical properties. In this regard, several core-shell materials were explored, but most of the works are based on quantum dots.15-20 Recently, Li et al.21 reported the enhancement of UV emission from ZnS coated ZnO nanowires formed by a self-assembly method. There are also a few reports where the conversion of ZnO to S doped ZnO or ZnS in a 1-D form has been successfully performed by various methods including chemical vapor decomposition, thermal sulfidation, ion exchange in vapor, chemical solution route, thioglycolic acid assisted hydrothermal process, etc., and their optical properties were studied.22-27 However, the ability to choose the crystallographic growth direction of a nanorod array aids in tuning the physical properties of the material, including spontaneous piezoelectric polarization, thermal and electrical conductivity, dielectric constant, lattice strain, and so forth. In this paper, we demonstrate the successful fabrication of c-axis oriented ZnO-ZnS core-shell nanostructures with a controllable shell thickness by partial conversion and also ZnS nanorods by complete conversion of aligned ZnO nanorods. In this sulfidation process, the conversion ratio of ZnS-ZnO can be easily controlled by the reaction time without destroying the alignment of the nanorods. The photoluminescence properties of the nanostructures were studied. Experimental Procedures Preparation of Aligned ZnO Nanorods. The preparation of aligned ZnO nanorods in thin films was reported earlier by our group.28 In brief, first ZnO films were prepared by dip coating from a solution of zinc acetate, ethanol, and DEA. The films were then annealed at 600 °C for 30 min. In the second step, 10 mmol of sodium dodecyl sulfate (SDS) and 1 mmol of

10.1021/jp068391c CCC: $37.00 © 2007 American Chemical Society Published on Web 03/10/2007

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Figure 1. XRD patterns of (a) aligned ZnO nanorods; (b) ZnO-ZnS core-shell nanorod obtained after 1 h of sulfidation at 400 °C; and (c) ZnS nanorod obtained after 2 h of sulfidation at 600 °C.

Figure 3. SEM images of (a) the vertically aligned ZnO nanorods on a large area; (b) ZnO-ZnS core-shell aligned nanorods obtained after 1 h of sulfidation at 400 °C (the inset shows the top view of the aligned nanorods); (c) ZnS aligned nanorods obtained after 2 h sulfidation at 600 °C; and (d-f) EDAX spectra of aligned ZnO nanorods, ZnOZnS core-shell nanorods, and ZnS nanorods, respectively. The scale bar in the inset of panel b is 100 nm.

Figure 2. XRD patterns of ZnO-ZnS core-shell nanorods obtained at 400 °C with variation of annealing time.

zinc acetate dihydrate were added to 30 mL of xylene and stirred for 1 h. Then, hydrazine hydrate (80%) diluted with ethanol was added to the solution very slowly. After 1 h of stirring, previously sol-gel prepared transparent ZnO films were immersed in the solution and refluxed at 90 °C for 5 h. Finally, the substrates covered with a white layer were washed in distilled water and dried at room temperature. Preparation of Aligned ZnO-ZnS Core-Shell Nanorods. Conversion of the aligned ZnO nanorod films to ZnO-ZnS core-shell nanorods was carried out with an H2S and argon gas mixture in an evacuable quartz tube oven at atmospheric pressure. The sulfidation was carried out at 400 °C for 1-48 h. The ratio of H2S and argon was fixed at 1:20 in all the experiments. The H2S gas was prepared by the reaction of Na2S and HCl. Preparation of Aligned ZnS Nanorods. The complete conversion of ZnO nanorods to ZnS nanorods was carried out at 600 °C in an atmosphere of an H2S and argon mixture of 1:20 at atmospheric pressure. Typically, after 2 h of exposure

to H2S, fully converted ZnS nanorods were obtained without breaking the alignment. Characterization. The crystalline phase of the products was determined by X-ray power diffraction by a Seifert 3000P diffractometer with Cu KR radiation (λ ) 1.54178 Å). The compositional analysis was performed by energy dispersive analysis of X-rays (EDAX, Kevex, Delta Class I). The morphology of the samples was determined with a scanning electron microscope (Hitachi; S-2300). Microstructure and crystal structures of the products were further studied through high-resolution transmission electron microscopy (HRTEM; JEOL 2010). Photoluminescence (PL) measurements were carried out with a fluorescent spectrophotometer (Hitachi; FL 2500). Results and Discussion The XRD pattern (Figure 1a) of the ZnO nanorod arrays shows a strong and sharp diffraction peak at 2θ ) 34.4°, indicating the (002) diffraction peak of ZnO in the wurtzite phase. The strong (002) diffraction peak indicates the preferred orientation of the nanorods along the c-axis. Figure 1b shows the 10× magnified XRD spectra of the sample sulfurized at 400 °C for 1 h. A new diffraction peak centered at 2θ ) 28.5° can be observed, which corresponds to the (002) plane of ZnS in the hexagonal phase (JCPDS-36-1450). When ZnO nanorods were sulfurized at 600 °C for 2 h (Figure 1c), the (002) peak of

ZnO-ZnS Core-Shell and ZnS Nanorod Arrays

Figure 4. TEM images of (a) ZnO nanorod; (b) ZnO-ZnS coreshell nanorod obtained after 1 h of sulfidation at 400 °C; (c) ZnS nanorod obtained after 2 h of sulfidation at 600 °C; and (d-f) HRTEM images of ZnO nanorod, ZnO-ZnS core-shell nanorod, and ZnS nanorods, respectively (the insets represent the corresponding FFT of the lattice fringes); (g) SAED pattern of a ZnO nanorod; and (h) SAED pattern of a ZnO-ZnS core-shell nanorod. (The arrow in the HRTEM images indicates the growth direction of the nanorods.)

ZnO totally vanished, and a strong diffraction peak corresponding to the (002) crystal plane of ZnS was observed. This indicates that the complete conversion of ZnO to ZnS occurred, maintaining the c-axis orientation. To the best of our knowledge, this is the first evidence where crystallographic orientation along the c-axis was maintained during the conversion of ZnO nanorods to ZnS nanorods. As the conversion of zinc oxide to zinc sulfide is a diffusionlimited process,29 there are few factors such as (i) the indiffusion of sulfur; (ii) the interchange of sulfur and oxygen at the appropriate site; and (iii) the outdiffusion of oxygen that govern the degree of conversion. To understand the rate of conversion of ZnO to ZnS in the formation of core-shell structures, XRD of different conversion stages controlled by reaction time at 400 °C were examined. When films were sulfurized below

J. Phys. Chem. C, Vol. 111, No. 13, 2007 5041 400 °C, no ZnS peaks were observed. Figure 2 shows the XRD patterns of samples at different conversion periods (1-24 h). From the XRD results, it can be clearly observed that the intensity ratio of the peak for the (002) plane of ZnS to the peak for the (002) plane of ZnO gradually increased with an increase in reaction time. This indicates that the conversion of ZnO crystals to ZnS is continuous with an increased reaction time. So, this approach gives good control over the ZnS shell thickness on the ZnO core by adjusting the reaction time. It was also observed that at 400 °C, extending the duration of the exposure to more than 48 h did not lead to the complete conversion of the ZnO columns to ZnS. This may be due to the fact that the ZnS growth proceeds via ion exchange and indiffusion of S into ZnO. The complete conversion of ZnO to ZnS at 600 °C for 2 h can be explained by the factor that with increasing the reaction temperature, the diffusion coefficients of sulfur and oxygen increase, which is necessary for the replacement of oxygen by sulfur. At high temperatures, oxygen atoms obtain sufficient kinetic energy to leave their lattice sites by thermal disturbance, and sulfur atoms are diffused into the ZnO lattices to occupy the oxygen vacancies. The preferred orientation of the ZnO (002) crystal facet through all conversion stages implies that the conversion from ZnO to ZnS takes place on the surface of ZnO columns and continues across the sidewall of the rods. The ZnO-ZnS core-shell is formed by the direct reaction of H2S with the surface layer of ZnO as ZnO + H2S f ZnS + H2O. As the conversion of ZnO to ZnS is essentially a substitution reaction,24 thus preserving the c-axis orientation of the ZnO core, growth of the ZnS shell takes place. Figure 3a is an SEM image of the aligned ZnO nanorod arrays prepared by a surfactant assisted soft chemical method prepared over quartz substrates, which revealed the uniformity of the nanorods over a large area. From the figure, it can be wellobserved that the orientation of the nanorods is quite good; they are well-separated from each other and grow normal to the substrate. The diameter of the nanorods was calculated to be 100 nm with lengths of ∼3 µm. The surfaces of the nanorods were found to be smooth. Figure 3b shows the side view SEM image of the ZnO-ZnS aligned nanorods. There is no such change in the morphology observed by SEM after the conversion of ZnO nanorods to ZnO-ZnS core-shell nanorods. The inset of Figure 3b is the high magnification top view SEM image of the core-shell nanorods, which clearly shows the hexagonal shape. Figure 3c shows the SEM image of the aligned ZnS nanorod film obtained after exposure to H2S at 600 °C for 2 h. It can be well-observed that after sulfidation, there is a considerable surface roughness, but no break up in the alignment of the nanorods occurred. It is interesting to note that the individual ZnO nanorods were not isolated; instead, they formed small bunches with the neighboring nanorods as shown in Figure 3a. This clumping could be attributed to the amphoteric nature of ZnO and the surface tension effect of longer and thinner nanorods.30 After sulfidation, no such bunches were found, indicating conversion of the ZnO surface to ZnS (Figure 3b,c). To obtain compositional information, an EDAX study of the samples was carried out. Figure 3d-f represents the EDAX spectra of ZnO nanorods, ZnO-ZnS nanorods, and ZnS nanorods, respectively. The pattern of the ZnO nanorods shows only the presence of Zn and O elements. For the ZnO-ZnS nanorods along with Zn and O, the S element was also found to be present, which provides powerful evidence for successful incorporation of S elements into the ZnO nanorods at the surface

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Figure 5. Room-temperature PL spectra of (a) aligned ZnO nanorods and ZnO-ZnS core-shell nanorods obtained after 1 h of sulfidation at 400 °C, showing enhancement of UV emission; (b) ZnO-ZnS core-shell nanorods obtained after 2 h of sulfidation at 400 °C; and (c) ZnS nanorods.

level. For ZnS nanorods, the EDAX spectra shows only peaks for Zn and S in a stoichiometric ratio. To observe the nanostructures clearly, TEM measurements were preformed. Figure 4a shows the bright field TEM image of a ZnO nanorod. The diameter of the nanorod is 100 nm with a length of 2 µm. Figure 4b shows the bright field TEM image of a ZnO-ZnS core-shell nanorod obtained after 1 h of sulfidation at 400 °C. The TEM image offers evidence of the formation of a ZnO-ZnS core-shell structure, in which the ZnO core and ZnS shell exhibit different microscopy contrasts. The interface between the core and the shell is fairly sharp. The diameter of the core-shell nanorod measured to be ∼100 nm indicates that there is no such size change taking place due to the initial incorporation of S2- ions in the ZnO lattices replacing O2- ions. At the initial stage of transformation, the increase in the diameter due to sulfidation of the ZnO crystal should be negligible, as the cell volume increase by ∼20% when sulfur replaced oxygen in a ZnO crystal to form ZnS.23 Figures 4c shows the TEM image of a ZnS nanorod obtained by sulfidation at 600 °C for 2 h. As compared to the ZnO nanorods, the ZnS nanorod surface is quite rough. The diameter of the ZnS nanorods is ∼115 nm. This change in size is in good agreement with the larger unit cell volume of ZnS.23 Figure 4d shows the HRTEM image of a ZnO nanorod. From the figure, it can be well-observed that the lattice planes were along the growth direction of the nanorods. The lattice spacing was calculated to be around 0.26 nm, which corresponds to the (002) planar spacing of ZnO in the wurtzite phase. The inset shows the FFT of the lattice fringes, which supports the single crystalline wurtzite phase of the ZnO nanorods. HRTEM investigations of individual ZnO-ZnS nanorods (Figure 4e) show that there is a formation of another set of lattice planes, which corresponds to the (002) planar spacing of ZnS in the wurtzite structure. However, some showed growth along the (102) direction of ZnS in the wurtzite structure. The FFT of the lattice fringes shows bright and weak points corresponding to ZnO and ZnS in the wurtzite phase, respectively. The HRTEM image of the ZnS nanorod (Figure 4f) shows the lattice planes, which are grown making an angle to the growth direction of the nanorods. The lattice spacing was calculated to be around 0.31 nm, which corresponds to the (002) planar spacing of ZnS in the wurtzite phase. Figure 4g shows the SAED pattern of a single nanorod, which indicates that the ZnO nanorod is single

crystalline with a wurtzite structure. The SAED pattern of the ZnS-ZnO core-shell nanorod (Figure 4h) shows a few bright spots, indicating the single crystalline ZnO nanorods in the wurtzite phase along with a few weak spots that correspond to ZnS in the wurtzite phase. The diffraction spots of ZnS are much weaker than those of ZnO, which indicates that the quantity of ZnS is very small in the core-shell nanorods.21 The diffraction pattern indicates that the core ZnO and the shell ZnS have an epitaxial relationship with an identical orientation along the (002) plane.27 To investigate the optical properties of these aligned nanostructures, photoluminescence (PL) was performed at room temperature. PL studies are powerful tools to investigate the optical properties as well as crystal defects. The roomtemperature PL spectra of the as-prepared ZnO nanorod arrays are shown in Figure 5a. The strong UV emission at 3.22 eV originated from the excitonic recombination corresponding to the band edge emission of ZnO.28,31 Interestingly, no significant emission in the green region could be detected that corresponds to a transition from a single ionized oxygen vacancy in the ZnO nanorods.32,33 It is known that, in the case of solution phase synthesis, the relative concentration of OH-/Zn2+ regulates the luminescence properties of ZnO crystals and that the green emission intensity decreases with an increase in OH- concentration.34,35 As the ZnO nanorods were prepared in a solution phase having a high pH (10.8-11.6), the possibility of forming an oxygen vacancy in the ZnO crystal was at a minimum. To our knowledge, there are few reports on S doping in ZnO nanostructures, and in their PL properties, it was observed that the visible green emission was greatly enhanced and that the UV emission was weakened.22,33,36-38 Completely different from the earlier reports, the room-temperature PL spectra of ZnOZnS core-shell aligned nanorods obtained by sulfidation of ZnO nanorods at 400 °C for 1 h exhibit distinct enhanced UV emission as shown in Figure 5a. This enhancement in the UV emission can be explained as that the shell material ZnS possesses a higher band gap than the core and that it suppresses the tunneling of the charge carriers from the cores to the newly formed surface atoms of the shell; thus, more photogenerated electrons and holes were confined inside the ZnO core, giving rise to a high quantum yield.15,39 Such an enhanced UV emission in the ZnO-ZnS core-shell nanostructures was also observed by Li et al.21 In addition, the UV emission was observed to be

ZnO-ZnS Core-Shell and ZnS Nanorod Arrays slightly shifted to higher energy as compared to pure ZnO. It is known that when ZnO is doped with S, the excess carriers supplied by the impurities to the conduction band contribute to increase the electrical conductivity of ZnO and thus lead to a blue shift of optical band-to-band transitions, known as the Burstein-Moss effect.40 The PL spectrum of the aligned coreshell ZnO-ZnS arrays obtained by sulfidation of ZnO nanorods at 400 °C for 2 h is shown in Figure 5b, which exhibits the PL properties of both ZnS and ZnO. The spectrum was analyzed by multi-peak Lorentzian fitting, which matches well with the experimental plot and yields two distinguished peaks at 3.38 and 3.28 eV, respectively. The results indicated that the sulfidation process had a great effect on the relative intensity and position of typical PL properties of ZnS-ZnO arrays. So, the PL properties of the core-shell arrays can be tuned by this approach. The completely converted arrayed ZnS exhibits a different emission band centered at 3.38 eV (Figure 5c). The observed UV emission may be from the recombination process due to the interstitial lattice sulfur.41-43 The interstitial sulfur ions are located in between sites occupied in the perfect crystals. Because of the incorporation of an atom into such sites, rearrangement of the nearest neighbors takes place. Since the sulfur ions are larger than the zinc ions, interstitial sulfur induces strain in the lattice, and as a result, the electron levels originating from interstitial sulfur will have small binding energies and should be located closer to the valence band edge.43 So, the UV emission at 3.38 eV for ZnS nanorods may be attributed to the recombination process at the conduction band and interstitial lattice sulfur. Conclusion In conclusion, we have successfully fabricated vertically aligned ZnO-ZnS core-shell and ZnS nanorods arrays in a thin film form by sulfidation of aligned ZnO nanorod arrays. The PL study at room temperature indicated that the high band gap ZnS shell confines the photogenerated carriers inside the ZnO core, and so enhanced UV emission was observed for the ZnO-ZnS core-shell nanorods, when the shell thickness is a few nanometers. With an increase in shell thickness, contributions from both ZnO and ZnS were observed in the PL spectra. The present study provides a good indication of tuning the UV emission of the ZnO-ZnS core-shell nanostructures by controlling the experimental parameters. The nanostructures have possible applications in the fields of luminescence, electronics, and sensors. Since the electromechanical coupling strongly depends on c-axis orientation of the crystals, fabrication of crystallographic oriented ZnS nanorod arrays can be utilized for the fabrication of acoustic wave devices. Further, this coreshell nanostructure can be used for the fabrication of wellaligned hollow ZnS nanorods of desirable thickness. Acknowledgment. The authors thank K. K. Das of IACS for recording the SEM micrographs. The research was supported by DST, New Delhi under the Nano Science and Technology Initiative. References and Notes (1) Law, M.; Goldberger, J.; Yang, P. Annu. ReV. Mater. Res. 2004, 34, 83.

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