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A Facile Route to Heterostructured ZnO:S/ZnO Nanorotors: Structural and Optical Properties Syed Ghazanfar Hussain,*,† Deming Liu,*,† Xintang Huang,‡ Salamat Ali,§ and Muhammad Hassan Sayyad| Opto-Electronics Engineering Department, Huazhong UniVersity of Science and Technology, Wuhan 430074, People’s Republic of China, Department of Physics, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, Department of Physics, GC UniVersity, Lahore 54000, Pakistan, and GIK Institute of Engineering Sciences and Technology, Topi, District Swabi, N.WFP 23460, Pakistan ReceiVed: February 6, 2008; ReVised Manuscript ReceiVed: May 7, 2008
We have synthesized heterostructured ZnO:S/ZnO 6-fold nanorotors through a one-step catalyst-free process during chemical vapor deposition. We performed a series of designed experiments to investigate the effect of growth temperatures, growth time, and the ratios between ZnO and FeS used as starting material on the growth. Optimum conditions where maximum nanorotors were obtained were the following: growth temperatures between the range of 400 and 425 °C; growth time 100 min; and a 1:1 ratio of ZnO + FeS. Each heterostructured nanorotor consisted of a core nanowire with side branches emanating from it. Our studies suggest that the core nanowires were ZnO:S while the nanorods were only ZnO. Furthermore ultraviolet-visible spectroscopy was employed to estimate the excitonic absorption peak of the synthesized nanorotors. The photoluminescence spectrum of the hetrostructured nanorotors showed stronger visible band emission as compared to pure ZnO powder at room temperature. This stronger visible emission in the synthesized nanorotors might be useful as a future UV-excited phosphor for producing bright and broadband visible light. 1. Introduction Semiconducting oxides such as ZnO, In2O3, and SnO2 have attracted increasing interest due to their physical properties and diversity for a wide range of applications in optical devices, solar cells, sensors, photocatalysis, transparent field effect transistors, and bulk acoustic wave devices.1–7 Moreover, oxide nanostructure applications, such as ZnO nanowire nanolasers and nanobelt transistors, have been reported.8–10 Zinc oxide is a multifunctional semiconductor material with a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV.11 ZnO is more resistant to radiation damage relative to other common semiconductors such as Si, CdS, GaN, and GaAs and thus more useful for applications in high-irradiation environments, such as field emission displays in high electric fields.11 A variety of novel nanostructured ZnO materials including nanowires, nanobelts, nanorods, nanocombs, nanosheet, tetrapods, feather-like, and flower-like, have been reported.12–16 To design ZnO-based devices, an important step is the realization of band gap engineering to create barrier layers and quantum wells in device heterostructures.14 The outstanding issue related to optoelectronic applications of ZnO is to optimize the material worth, which is characterized by the defect-related emission and can be achieved by S-doping.15,17 S-doping in ZnO is expected to modify the electrical and optical properties because of the large electronegativity and size difference between S and O (rS/rO ) 1.3). Many groups have synthesized * Corresponding authors. S.G.H.: e-mail:
[email protected], fax +862787556690. D.M.L.: e-mail:
[email protected] and fax +862787556188. † Huazhong University of Science and Technology. ‡ Central China Normal University. § GC University. | GIK Institute of Engineering Sciences and Technology.
S-doped ZnO microspheres, films, and different types of nanostructures and have reported their electrical and optical properties. Foreman et al. synthesized S-doped nanowires and compared these with S-doped ZnO powder and undoped ZnO powder.17 They reported that S-doped nanowires show a very weak UV peak with dominant visible band emission as compared to S-doped powder. According to them S-doped powder dominates the S-doped ZnO nanowire because of a larger surface-to-volume ratio of nanowires, which increases the nonradiative transition. These nonradiative transitions compete with the sulfur-induced defects responsible for the bright-visible emission. Yoo et al. synthesized the S-doped film by pulse laser deposition at 700 °C and reported that the energy band gap (Eg) shifted to lower level as sulfur was incorporated.18 Gen et al. reported S-doped ZnO nanowires, where they first synthesized ZnS nanowires by evaporating ZnS nanoparticles in argon atmosphere at 900 °C and subsequently introducing oxygen to make partly oxidized S-doped ZnO nanowires.19 Lao et al. have reported 6-fold nanorotors synthesized at 820-870 °C. The major core was along the [110] or [111] zone axis of In2O3 and all the secondary nanorods grew along the [0001] direction of ZnO.20–22 They have attributed this phenomenon to the presence of elements such as In and Sn. They also reported 6-fold pure ZnO nanostructures with nanonail as the secondary branch. Normally, sapphire and silicon single crystals have commonly been used as substrates with the assistance of metal catalysts.8,9,14 However, the presence of catalysts can be harmful to the application of nanostructures in nanodevices. During the synthesis process, a vacuum environment is generally necessary to ensure that the Zn or ZnOx (x < 1) vapor reactants have a large free path and enough energy to nucleate at their perfect sites on the substrates.20–22
10.1021/jp803323a CCC: $40.75 2008 American Chemical Society Published on Web 07/04/2008
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Figure 1. SEM images of different nanostructures synthesized on silicon substrates at different temperature zones: (a) at 770-800 °C; (b) at 650-675 °C; (c) at 400-425 °C; and (d) at 300-325 °C. Magnified views of typical structures are shown in the inset of each figure. (e and f) EDX patterns of the synthesized nanorotors and the cone-like nanostructures, respectively.
In this paper, we present novel six-sided heterostructured ZnO:S/ZnO nanorotors. These nanorotors were synthesized on Si (111) substrates by a simple thermal evaporation and transport method at atmospheric pressure, which we believe has never been reported before. We have synthesized these heterostructured ZnO:S/ZnO nanorotors at relatively lower temperatures, 400-425 °C, and found that the core nanowires were comprised of Zn, O, and S while outgrowing nanorods contain Zn and O only, which is far different from previously reported nanorotors.20–22 2. Experimental Procedures The method for synthesizing these nanostructures is similar to that explained in our recent report.15 Briefly ZnO and FeS powder (with different ratios: 1:0.25, 1:0.5, 1:0.75, and 1:1) were mixed thoroughly and loaded into a small alumina boat that was then was placed in the center of a quartz tube. Silicon substrates were cleaned with hydrofluoric acid, alcohol, and deionized water in an ultrasonic cleaner and placed downstream in different temperature zones (A, B, C, and D) to collect the
grown products. After the residual air in the quartz tube was eliminated by an Ar flow for 30 min, the furnace was heated to 1150 °C at a rate of 20 deg min-1 and held for 20, 60, 100, and 140 min in different experiments under Ar carrier gas at a constant flow rate of 120 sccm. After synthesis, the furnace was cooled to room temperature and the dark-gray products were found over the Si substrates. The morphology and the composition of the products were examined by scanning electron microscopy (FE-SEM, JEOL JSM-6700F) with an energy dispersive spectrometer (EDS). Crystallinity and phases of the synthesized nanorotors were examined by X-ray diffraction (XRD), using a Y-2000 X-ray diffractometer with Cu KR radiation (λ ) 0.15418 nm). The structure and diffraction pattern of the synthesized nanorotors were investigated by transmission electron microscopy (HRTEM, JEOL 100 CXII) with an energy dispersive spectrometer (EDS). To investigate the optical properties, room temperature ultraviolet-visible (UV-visible) absorption spectra were recorded by a UV-1700 (SHIMADZU) spectrophotometer in the
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Figure 2. SEM images of the synthesized nanostructures at different growth times: (a) at 20 min; (b) at 60 min; (c) at 100 min; and (d) at 140 min.
Figure 3. SEM images of the synthesized nanostructures at different ratios between ZnO and FeS: (a) 1:0.25; (b) 1:0.5; (c) 1:0.75; and (d) 1:1.
wavelength range of 200-800 nm and photoluminescence (PL) studies were conducted with a FP-6500 (JASCO) spectrofluorometer with a Xe lamp at room temperature. The excitation wavelength was 325 nm. 3. Results and Discussion 3.1. Structural Characterization of Heterostructured ZnO: S/ZnO Nanorotors. The morphologies of the synthesized nanostructures were examined with FE-SEM. Figure 1a-d shows SEM images of four samples grown on silicon substrates in different temperature zones keeping the growth time constant at 100 min with a 1:1 ratio of ZnO and FeS. These different temperature zones were A (770-800 °C), B (650-675 °C), C (400-425 °C), and D (300-325 °C). In zone A (770-800 °C), a cone-like nanostructure grew on hexagonal rods (Figure 1a). In zone B (650-675 °C), the rods were longer and nanoteeth started to form (Figure 1b). In zone C (400-425 °C), 6-fold rotor-like nanostructures were formed on a large scale (Supporting Information, Figures S1-4) as shown in Figure 1c. Figure 1c inset exhibits a symmetrical shaped nanorotor. The length of the synthesized nanorotors are as long as tens of micrometers (Supporting Information, Figure S2), whereas the
Hussain et al.
Figure 4. XRD spectrum of the synthesized heterostructured ZnO:S/ ZnO nanorotors on the silicon substrates.
length of the secondary nanorods grown on the core nanowires ranges from 0.7 to a few µm, with their diameters ranging from 20 to 50 nm. In zone D (300-325 °C), incomplete nanorotors developed (Figure 1d). These incomplete nanorotors might be due to lower growth temperature. As is evident from the above discussion, a temperature range between 400 and 425 °C is optimum for growing nanorotors. Magnified views of typical nanostructures are shown in the inset of each figure. To understand the effect of growth time on the morphologies, we have synthesized nanostructures at different growth times at growth temperatures of 400-425 °C with a 1:1 ratio of ZnO and FeS. Figure 2a-d shows the SEM images of the synthesized samples at different growth times. At 20 min growth time, nanoteeth were at their initial stages of growth on the stems (Figure 2a). At 60 min growth time, these nanoteeth grew more and at 100 min uniform and longer nanoteeth developed. When we further increased the growth time (140 min) more dense structures developed as more materials deposited (Supporting Information, Figure S5). To learn and comprehend the effects of different ratios between ZnO and FeS, we performed many experiments for different ratios between ZnO and FeS as starting material at constant growth temperatures of 400-425 °C and a growth time of 100 min. Figure 3a-d shows the SEM images of the synthesized samples at different ratios. At the ratio of 1:0.25 (ZnO:FeS) just a few nanoteeth originated from their corresponding stems and many circular stems also grew. Upon increasing the concentration of FeS (1:0.5) more nanoteeth developed, which became multidirectional at an even higher concentration of FeS (1:0.75), and finally at the ratio of 1:1 (ZnO + FeS), nanorotors were synthesized successfully. In this present work, through a series of designed experiments, we provide evidence that such a difference of morphologies can be attributed to the diverse nucleation-growth process originating from the difference in growth temperatures, growth times, and different ratios between starting materials. Therefore, the nanorotors could be selectively obtained by tuning the location of the substrates in the electric tube furnace (growth temperature) at suitable growth time with suitable ratios of starting materials. The EDX spectrum shown in Figure 1e reveals the nanorotors containing Zn, S, and O elements without other detectable elements in 53.98, 10.68, and 35.34 atom %, respectively. The atom percent amount of Zn (53.98) is more than that of O and S (46.02), which probably indicates the existence of O vacancy in the surface of the sample. This is also consistent with our
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Figure 5. TEM images of heterostructured ZnO:S/ZnO nanorotors: (a) the high-magnification TEM image of a single nanorotor; (b) HRTEM image of the nanocore; (c and d) HRTEM images of the nanorods; and (e and f) the EDX spectra of the core nanowire and nanorods taken from the regions marked “e” and “c” in panel a, respectively.
PL spectra (this will be discussed later). EDX spectrum shown in Figure 1f reveals the elemental composition of the cone-like nanostructures. The cone-like nanostructures contain Zn, S, and O elements in 57.17, 16.96, and 25.86 atom %, respectively. The cone-like nanostructures were formed at 22 cm while nanorotors were synthesized at 25 cm from the source material. The atom percent of sulfur in cone-like nanostructures is 16.96 while it is 10.68 in nanorotors. This suggests that the S content in the nanostructures is very sensitive to the distance between the substrate and the source. This phenomenon can be interpreted in terms of S transport. Since element S is very active in the presence of oxygen, the S partial pressure would decrease dramatically with increasing source-substrate distance due to the reaction with residual oxygen. Thus, it is possible to tune the S content in the nanostructures to some extent simply by varying the distance between the substrate and the source. A typical XRD spectrum of the nanorotors is shown in Figure 4, which shows that the sample is a mixture of wurtzite ZnO
and wurtzite ZnS. All peaks in this pattern can be indexed to known wurtzite structures of ZnO (JCPDS No. 36-1451) and ZnS (JCPDS No. 36-1450). The Si (111) peak is attributed to the substrate. No peak of other crystalline impurities was detected. The strong diffraction peaks in Figure 4 correspond to ZnO. Some small peaks corresponding to ZnS indicated the existence of a small amount of ZnS impurity in the synthesized products. However, the characteristic UV emission/absorption from ZnS structure was not observed in our photoluminescence/ UV-vis absorption measurements, indicating that the small amount of ZnS did not contribute significantly to the overall spectrum. To give further understanding of the rotor-like nanostructures, the sample was intensively sonicated for a long time in ethanol. A few individual parts of the synthesized nanostructures could be found in the sonicated samples. We did not obtain an integrated rotor-like nanostructure but some segments of it are shown in Figure 5a. Panels b-d of Figure
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Hussain et al. X-ray (EDX) spectroscopy, attached with TEM. The EDX composition analysis shows that the core nanowire is ZnO:S and all the secondary nanorods are pure ZnO (Figure 5e,f). 3.2. Growth Mechanism of Heterostructured ZnO:S/ZnO nanorotors. The mixture of ZnO and FeS powder was initially heated at high temperature. The following chemical reaction took place:23
ZnO(powder)+FeS(powder) f ZnS(vapors)+ Fe3O4-x(residues in the boat) (1)
Figure 6. Schematic illustration of the possible growth mechanism: (a) the first growth step for making the core nanowire; (b) the second step growth for making the symmetric 6-sided rotor-like nanostructure; and (c) the synthesized nanostructure which is very close to the proposed model.
The above reaction seems to occur during the core nanowire formation. To reduce ZnO, we used FeS as a reducing agent. FeS powder is also a safe and stable sulfur source instead of hydrogen sulfide (H2S) or sulfur (S). According to EDX spectra, iron (Fe) was not found in the synthesized products. As a result of the above reaction, paramagnetic Fe3O 4-x clusters might be produced in the alumina boat.23 At high temperature ZnO vapors decompose into Zn and O2. The solid-vapor conversion and the decomposition processes are expressed as
ZnO(s) T ZnO(g)
(2)
ZnO(g) T ZnO(g)+0.5O2(g)
(3)
ZnO(g) T ZnOx(g)+0.5O2(g)
(4)
and/or
Figure 7. UV-vis absorption spectrum of heterostructurd ZnO:S/ZnO nanorotors. The inserted image is the enlarged portion of the absorption curve from 300 to 500 nm in wavelength.
5 are HRTEM images taken from the core nanowire and the corresponding nanorods of the synthesized nanorotor. The HRTEM image of Figure 5b taken from the region marked “b” on the core nanowire in the Figure 5a, clearly showed the lattice planes. The d-spacing between any two adjacent lattice fringes was 0.26 nm, which matches with that of (0001) planes, and the growth of the core nanowire on this side was along the [0001] direction. The SAED pattern (bottom inset in Figure 5a) of the core nanowire, taken from the region marked “e” in Figure 5a, also confirms that the growth of the core nanowire was along the [0001] direction. The HRTEM image of the one nanorod in Figure 5c was taken from the region marked “c” in Figure 5a, which shows the d-spacing was 0.28 nm, matching that of (101j 0) planes, and the growth of the nanorod on this side was along the [101j 0] direction. The inset image of Figure 5c shows the SAED pattern of the nanorod, which also indicates that this nanorod was grown along the [101j 0] direction. The HRTEM image of the second nanorod in Figure 5d was taken from the region marked “d” in Figure 5a, also revealing that the growth of this nanorod was along the [01j 10] direction. Thus, the growth of the nanorods was along the ([1j 100], ([01j 10], and ([1j 010] directions because all six nanorods are symmetrical in nature (inset in Figure 1c). For further studies about the S-doping location in the synthesized nanostructure, we examined it by energy-dispersive
The ZnS, ZnOx (zinc suboxide, x < 1), and Zn vapors together with the O2 vapor were transported by Ar carrier gas to a lower temperature region of 400-425 °C. Also ZnOx is reported to have low melting temperature of ∼419 °C.24 These liquid droplets of Zn/ZnOx were the nucleation sites of the grown nanostructures. These droplets can enhance the absorption and diffusion of Zn, ZnO, and ZnS vapors at the tips of the core nanowires during their growth. Due to further oxidation, the concentration of oxygen in the droplets increases, and ZnO:S then condensed and deposits on the interface between the droplets and substrate, resulting in the growth of ZnO:S core nanowires.24 The SAED pattern (bottom inset in Figure 5a) and HRTEM images in Figure 5b have demonstrated that the [0001] orientation is the preferred growth direction for core nanowires of the grown nanostructures. The first step is fast growth along the [0001] direction, forming a core nanowire with the top and bottom surfaces being ((0001) facets and the side surfaces being ((011j0), ((101j0), and ((11j00). The second step is slow growth on each side along the (〈011j0〉 directions. The nanorods grew from the new nucleation sites further growing on six sides of the core nanowires to form symmetrical six-sided rotor-like nanostructures. Figure 6 shows the schematic illustration of the possible growth mechanism. Figure 6a illustrates the first step for making the core nanowire, Figure 6b shows the second step growth for making the symmetric six-sided rotor-like nanostructure, and Figure 6c represents the synthesized nanostructure, which is very close to the proposed model. The angle between any two adjacent nanorods is ∼60°, resulting in a 6-fold symmetric distribution of the nanorods around the core nanowires as shown in the inset of Figure 1c. The reason they grow into 6-fold symmetry is the hexagonal symmetry of the core nanowire. 3.3. Optical Properties of Heterostructured ZnO:S/ZnO Nanorotors. The UV-visible absorption measurement is one of the most powerful tools to reveal the energy structures and optical properties of semiconductor nanocrystals. UV-vis absorption spectra of the heterostructured ZnO:S/ZnO nanorotors with pure ZnO powder (as a reference) were measured at room
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Figure 8. Room temperature photoluminescence spectra of the synthesized heterostructured ZnO:S/ZnO nanorotors with undoped ZnO powder (as a reference).
temperature. In our measurements, the synthesized ZnO:S/ZnO nanorotors and ZnO powder were ultrasonically dispersed in ethanol before examination. Figure 7 shows the UV-vis absorption spectra of the dispersed nanorotors/ZnO powder in ethanol. Pure ethanol was used as a reference. The synthesized ZnO:S/ZnO nanorotors exhibit an excitonic absorption peak at ∼375 nm, which is blue-shifted with respect to the pure ZnO powder (380 nm). Introducing ZnS might have blue-shifted this absorption peak of synthesized nanorotors (inset in Figure 7). To study further optical properties of the nanostructures, PL study is well-known and used by many researchers.15,17,19 PL spectra of the heterostructured ZnO:S/ZnO nanorotors with pure ZnO powder were measured with a fluorescence spectrophotometer using a Xe lamp with an excitation wavelength of 325 nm (3.81 eV), well above the ZnO band gap at room temperature. In Figure 8 the UV emission peak is at 382.89 nm for pure ZnO powder and broadband visible emission consists of two peaks, the blue peak at 481 nm and the green peak at 518 nm. Generally the UV emission peaks are difficult to detect at room temperature due to the thermal quenching effect.12 In the case of heterostructured ZnO:S/ZnO nanorotors, the visible emission peaks were at 475 (blue peak) and 510 nm (green peak) with no detectable UV peak. Introducing ZnS significantly enhances energy transfer from the band edge to the defect states responsible for visible emission.17 The visible band emission of the synthesized nanorotors is also shifted toward higher energy, which is consistent with our previous report.15 Also the intensity of broadband visible emission is larger than that of pure ZnO powder as shown in Figure 8. In our previous work,15 ZnO:S/ZnO nanosaws were reported (with atom % of Zn:S:O at 52.25:11.00:36.75) while in the present work, we report ZnO:S/ZnO nanorotors (with atom % of Zn:S:O at 53.98:10.68:35.34) with more oxygen vacancies as compared to nanosaws. Due to more oxygen vacancies, a more intense visible band should be observed but the intensity decreased. It is because nanorotors possess more surface-to-volume ratio and hence have more chance to increase the nonradiative transitions.17 Hence, these nonradiative transitions compete with increasing oxygen vacancies and the overall
intensity of the visible band in our nanorotors decreases as compared to those of nanosaws. It is generally believed that the visible emissions originate from a transition between a singly charged oxygen vacancy and a photogenerated hole.25,26 The green emission is also related to the lattice defects. The incorporation of ZnS into the ZnO crystal lattice introduces lattice distortion. This influences the energy structure of ZnO which leads to the remarkable enhancement of the green emission band. The stronger intensity of the green emission as compared to pure ZnO powder indicates that there are more singly ionized oxygen vacancies and lattice defects. Many researchers used a mixture of gases (Ar and O2) as a carrier gas8,12,17,18 but we used only argon gas. This gives rise to oxygen vacancies in the synthesized nanostructure, which could be verified by the EDX measurement. The stronger intensity of visible band emission as compared to pure ZnO powder and suppression of the UV band provides strong evidence that ZnS was successfully introduced into ZnO nanostructures. The synthesized nanorotors could be potentially useful in nanoelectromechanical system (NEMS). 4. Conclusions We have synthesized heterostructured ZnO:S/ZnO rotor-like nanostructures by using a one-step catalyst-free thermal evaporation process. Optimum selectivity and maximum nanorotor densities are obtained for growth temperature in the range 400-425 °C, at a growth time of 100 min and with a ratio of 1:1 between starting materials (ZnO and FeS). The core nanowires of the synthesized nanostructures grew along the [0001] direction and its nanorods grew along the ([101j0], ([01j10], and ([1j100] directions. Our study suggests that the core nanowires were ZnO:S while the teeth were only ZnO. The synthesized nanorotors exhibit an excitonic absorption peak at ∼375 nm, which is blue-shifted with respect to the absorption of pure ZnO powder (380 nm). Studies on room temperature PL spectra revealed that the nanorotors showed stronger green emission as compared to the pure ZnO powder.
11168 J. Phys. Chem. C, Vol. 112, No. 30, 2008 Acknowledgment. The above work is supported by the Advanced Research Funds of Key Basic Research Plan (Project no 2005CCA04200). One of the authors (S.G.H.) is thankful to HEC (Higher Education Commission) and the Punjab Education Department of Pakistan for their support. Supporting Information Available: Figures S1-4 showing typical SEM images of the synthesized heterostructured ZnO: S/ZnO nanorotors and Figure S5 showinh the SEM image of the synthesized nanostructures at growth times of 140 min indicating more dense structures developed as more material was deposited. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ohta, H.; Kawamura, K.; Orita, M.; Hirano, M.; Sarukura, N.; Hosono, H. Appl. Phys. Lett. 2000, 77, 475–477. (2) Aoki, T.; Hatanaka, Y.; Look, D, C. Appl. Phys. Lett. 2000, 76, 3257–3258. (3) Hara, K.; Horiguchi, T.; Kinoshita, T.; Sayama, K.; Sugihara, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2000, 64, 115–134. (4) Rodriguez, J. A.; Jirsak, T.; Dvorak, J.; Sambasivan, S.; Fischer, D. J. Phys. Chem. B 2000, 104, 319–328. (5) Yumoto, H.; Inoue, T.; Li, S. J.; Sako, T.; Nishiyama, K. Thin Solid Films 1999, 345, 38–41. (6) Carcia, P, F.; McLean, R. S.; Reilly, M. H.; Nunes, G., Jr. Appl. Phys. Lett. 2003, 82, 1117–1119. (7) Panwar, B. S. Appl. Phys. Lett. 2002, 80, 1832–1834. (8) Yan, H. Q.; He, R. R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. D. J. Am. Chem. Soc. 2003, 125, 4728–4729. (9) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897–1899. (10) Arnold, M. S.; Avouris, P.; Pang, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659–663.
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