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Fabrication and Photoluminescence Property of SnO2 Microtowers with Interstitial Tin Ions Ming Fang,*,† Lide Zhang,*,† Xiaoli Tan,‡ Xiaoye Hu,† Weiwei Yan,† and Peisheng Liu§ Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, People’s Republic of China, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China, and Jiangsu Key Laboratory of ASCI Design, Nantong UniVersity, Nantong 226019, People’s Republic of China ReceiVed: March 17, 2009; ReVised Manuscript ReceiVed: April 17, 2009
The tower-like tin oxide microarchitecture with interstitial tin atom (Sni) was fabricated utilizing a hydrothermal method, which is started with metal powders and was expected to be a general path toward high formation energy defects in nano/micro architectures. The low and high valence metal ionic compounds were found forming together in the weak oxidizing solution, and then SnOx (1 e x < 2) and SnO2 codeposited to form a towerlike microarchitechture. The Raman investigation indicates the amorphous SnOx phase is embedded in SnO2. While annealing in the atmosphere, some of the low valence metal ion (Sn2+) moved into the lattice vacant oxygen octahedra to form the Sni defect. Further photoluminescence investigation indicates the process of energy transferring from Sn4+ to the neighbor Sni in the microtower and then giving the narrow emissions due to the electron transitions from p and sp orbits to s orbit in Sni. Introduction Over the last 20 years, researchers have focused their attention on the charming properties of nanomaterials due to the effect of nanoscale.1-4 The efforts to obtain high crystal quality, controllable scale, and morphology of nanomaterials have enriched the knowledge of material science. As an alternative method for the novel properties, defects in nanomaterials have recently attracted great attention,5,6 due to the electronic states of defects being confined to a small volume making the effects on properties more noticeable than in the bulk. But defects with high formation energy are hardly fabricated and seldom reported. Fortunately, the nano/micro system offered the possibility to obtain defects with high formation energy. The material in the nano/micro scale has high system energy (due to the small scale and large surface area), which therefore in theory decreases the formation energy of defects. So, it can be expected that high formation energy defects form in nano/micromaterials.5 However, most of the reported methods are not suitable for the fabrication of high formation energy defects due to the product usually being obtained in a strong oxidizing system or from a precursor with high valence ions. Among the nano-oxides, only the Zn interstitial defect in ZnO was recently reported.7 A new crystal growth route from surface to core has been observed in some core-shell nanostructures.8 A special case is the nano/ micro structures constructed by the low and high valence metal oxide. It can be expected that if the nanostructures were annealed at a proper temperature in the air, the crystal surface will act as a “cage” to limit the expansion of the core (due to the introduction of oxygen atoms from the atmosphere to the low valence oxide core). In this case, the low valence metal ions may like to enter the vacancies in the crystal lattice to be the interstitial defects. * Authors to whom any correspondence should be addressed. E-mail:
[email protected] (M.F.) and
[email protected] (L.Z.). Phone: 86-5515591465. Fax: 86-551-5591434. † Institute of Solid State Physics, Chinese Academy of Sciences. ‡ Institute of Plasma Physics, Chinese Academy of Sciences. § Nantong University.
SnO2 is an n-type wide-band gap (Eg ) 3.6 eV) semiconductor, which has proven to be a key functional material that has been extensively used for optoelectronic devices, gas sensors detecting leakages of both reducing gases (e.g., CO, CH4 and H2), and oxidizing gases (e.g., NOx).9-11 But only the oxygen vacancy defects have been studied in experiments by other researchers,2,3,12-22 as far as we know. We have previously reported a SnO2 hierarchical nanostructure from a hydrothermal method in which the interstitial Sn2+ (Sni) has been proved.23 In this paper, we focus on the elucidation of the influence of the oxidizing ability of the hydrothermal solution on the morphology of the final product. In addition, the main reason for the narrow emissions has also been investigated. Experimental Section The SnO2 microtowers were prepared by utilizing a simple hydrothermal method in the water solvent. In a typical experiment, 1.5 g of tin powder, 2.0 g of sodium nitrate, and 3.0 g of sodium carbonate were dissolved in 50 mL of water. After being shaken by hand for about 5 min to dissolve the carbonate and the nitrate, the mixture was transferred into a Teflon-lined stainless steel autoclave and heated in an electric oven at 180 °C for about 30 h. The autoclave was cooled rapidly by using tap water and the product was harvested by filtration and washed with deionized water several times before drying in the air. X-ray diffraction (XRD) patterns were recorded with a X-ray diffractrometer (Philips X’Pert Pro MPD) with Cu KR (λ ) 1.5406 Å) radiation. The morphological investigation was carried out by using field emission scanning electron microscopy (FESEM; FEI Sirion200), transmission electron microscopy (TEM), and highresolution transmission electron microscopy (HRTEM; JEOL JEM-2010, 200 kV). Raman scattering measurements were performed with a Raman spectrometer (LABRAM-HR) at room temperature. The green line of an Ar laser (514.53 nm) in micro-Raman configuration was used. The photoluminescence (PL) spectrum was recorded from 360 to 800 nm with a resolution of 1 nm, using a photolumi-
10.1021/jp902362k CCC: $40.75 2009 American Chemical Society Published on Web 05/06/2009
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Figure 1. XRD patterns of the microtower.
Figure 3. (a, b) the TEM image of the nanoarchitecture; (c) the HRTEM image to the tip of the microtower (the inset is the corresponding SAED pattern); (d) the sketch map of the exposed facets of the tip.
Figure 2. The FESEM images.
nescence spectrometer (FLUOROLOG-3-TAU type). A highresolution measurement was performed in the range from 581 to 603 nm (with a resolution of 0.3 nm) to obtain a clearer observation on the fine structures and finally took the corresponding place in the spectrum. Results and Discussion A representative XRD pattern is shown in Figure 1. All the diffraction peaks can be indexed to the tetragonal rutile SnO2 (JCPDS card of 41-1445, space group of P42/mnm, a0 ) 4.738 Å, c0 ) 3.187 Å). Figure 2 is the morphology investigation by FESEM. The as-prepared product contains many tower-like microarchitectures. The microtower is about 1.39-2.15 µm in height and 500-800 nm in width, and is constructed of nanosheets (about 25-60 nm in thickness) accumulating layer upon layer. At the bottom, the microtowers jointed together to form bundles. Panels c and d of Figure 2 show the enlarged FESEM images of the tip of the microtower, from which it can be seen that the tower has two symmetrical parts. Further structural investigation is performed by the TEM image, which is shown in Figure 3. The corresponding HRTEM image in Figure 3c clearly
Figure 4. The Raman spectra: (a) the as-prepared sample and (b) the product after 1000 °C annealing. The inset is the corresponding photograph of the samples dispersed in ethanol.
exhibits the lattice structure of the (010) plane of rutile SnO2. The spots in the SAED pattern inserted in Figure 3c correspond to the [010] zone axis of the tetragonal rutile SnO2. The microtower is so thick that it is hard to perform a further TEM investigation on the center. Considering the result by Thiel,24 who obtained a macroscopical crystal of SnO2 with similar morphology to the tip of the microtowers, the corresponded crystal facets can be assigned and are shown in Figure 3d. Raman spectroscopy investigation on the as-prepared product was shown in Figure 4, curve a. Four bands25 at 98.6, 475.61, 628, and 771 cm-1 can be assigned as the rutile phase fundamental modes of B1g, Eg, A1g, and B2g, respectively. Abello26 proposed that the relaxation of the k ) 0 selection rule is progressive when the rate of disorder increases or the size decreases. As a result, infrared (IR) modes can become weakly active. So, the weak Raman band at 239 cm-1 is reasonable to be assigned as the IR active Eu(2)LO (LO is the mode of the longitudinal optical phonons) mode. The weak bands peaking at 591, 550, and 690 cm-1 in curve a
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TABLE 1: The Lattice Match Table of SnO2 to SnO dSnO dSnO2
(001) [110]
(001) [100]
(100) [001]
(100) [010]
(110) [110]
(110) [111]
(100) [011]
(001) [110] (001) [100] (110) [111] (110) [001] (110) [110] (100) [010]
19.881% 43.352% 27.655% 15.783% 59.94% 43.352%
13.313% 19.882% 2.318% 19.109% 43.343% 19.882%
43.761% 1.646% 29.811% 51.114% 28.119% 1.646%
13.313% 19.882% 2.318% 19.109% 43.343% 19.882%
19.881% 43.352% 27.655% 15.783% 59.94% 43.352%
115.224% 52.174% 94.34% 126.232% 7.612% 52.174%
83.194% 29.527% 65.418% 92.564% 8.403% 29.527%
can be assigned as bands S1, S2, and S3, respectively, according to the research by Zuo27 and Die´guez.28 However, there are three bands peaking at 133, 169, and 210 cm-1 that can be assigned neither the Raman active modes nor the progressive IR active modes of SnO2. Considering the oxidation process starting from tin powder, and the gray color of the as-prepared sample shown in the inset of Figure 4, these bands should come from the substoichiometric SnOx (1 e x < 2) phase. So, the bands at 133 and 210 cm-1 can be assigned as Eg and A1g modes of SnO,29 respectively. The band at 169 cm-1 indicates an unclear structure of SnOy (1 < y < 2) according to Sangaletti.30 These SnOx phases occur as noncrystal in SnO2 for they could not be identified by XRD, which completely disappeared after annealing at 1000 °C for 2 h (see Figure 4 curve b). It should be pointed out that the three surface modes here are seldom reported in such a scale.27 Also considering the appearance of the IR active Eu(2)LO mode in the microtower, a reasonable explanation can be drawn that the SnOx phase is embedded in the SnO2 phase. The embedded SnOx in SnO2 increase the disorder of the system and the ratio of the interface, which finally result in the appearance of the Eu(2)LO mode and the three surface modes in the Raman spectrum. On the basis of the above analysis, the growth mechanism of the tower-like microarchitecture can be given as follows. In the hydrothermal condition, the sodium carbonate hydrolysis first forms the alkaline environment. Then the metallic Sn powders were oxidized to Sn(OH)42- and Sn(OH)62- in the hydrothermal condition according to the following formulas:
3Sn + 2NO3- + 4OH- + 4H2O f 3Sn(OH)42- + 4NOv (1) 3Sn + 4NO3- + 6OH- f 3Sn(OH)62- + 4NOv
(2) 2-
2-
The produced Sn(OH)6 and Sn(OH)4 then decomposed to SnO2, SnO, and SnOy (according to reactions 3-5).
Sn(OH)62- f SnO2V+ 2OH- + 2H2O
(3)
Sn(OH)42- f SnOV + 2OH- + H2O
(4)
(y - 1)Sn(OH)62- + (2 - y)Sn(OH)42- f SnOyV + 2OH- + yH2O (5) The codeposition process of SnO2 and SnOx is similar to the epitaxial growth. From eqs 1 and 2, high OH- concentration makes eq 2 apt to happen to produce more SnO2, and thus form the hierarchical nanoarchitecture with only three facets exposed (see ref 23), while low OH- concentration makes eq 1 happen more easily, and then the quantity of SnOx in solution increased. In this case, too much SnOx in the codeposition process
increased the lattice mismatch (see Table 1, for the undetermined structure of SnOy, only the comparison of SnO2 and SnO is presented) of SnO2 to SnO, which directly results in more highenergy crystal facets exposed in the final product. The codeposition process is cyclically slowed and accelerated by the cyclical change of the solute concentration around the sediment. This makes Ostwald ripening31-33 only affect the SnO2 crystal morphology slightly and the kinetic confinement results in the presence of steps on the side surface of SnO2. The photoluminescence investigation is performed on the sample after 1000 °C annealing, which is illuminated with 274 nm wavelength light. From Figure 5a, two broad peaks centered at about 440 and 602 nm can be directly attributed to the oxygen related defects.21,22,33 Besides, eight narrow emissions at 588 593, 598, 609, 617, 627, 658, and 706 nm overlap on the 602 nm broadband. According to our previous work,23,34 these narrow emissions prove the existence of Sni in the annealed SnO2. So, the narrow emissions are due to the electronic transitions from the split p and sp configurations to the s orbit in Sni. Specifically, they can be assigned to D3 (588 nm), D2 (593 nm), D1 (598 nm), C3 (609 nm), C2 (617 nm), C1 (627 nm), B (658 nm), and A (706 nm) bands, respectively. Figure 5b shows the excitation spectra monitored at 588 and 440 nm, respectively. The most effective excitation peak for the narrow emissions centered at 274 nm, but to the oxygen vacancy related emissions it centered at about 440 nm. This result indicates the different origination mechanism for the narrow emissions from the broadband emissions, and then can be the proof for the presence of the Sni. Some researchers have studied the electronic structure of SnO2. According to the calculation by Robertson35 and Munnix,36 only the distance (4.6 eV) between the p orbit (in valence band) and the s orbit (in the conduction band) is closed to the energy (4.5 eV) of the exciting peak at 274 nm. Therefore, it is reasonable to assign the exciting peak centered at 274 nm to the electron transition of Sn p (valence band) f Sn s (conduction band). On the basis of the above analysis, an understanding of the luminescence mechanism of the Sni in SnO2 can be proposed. After being excited by 274 nm wavelength light, electrons belonging to the valence band of the p orbit were excited to the s orbit of the conduction band in Sn4+. This energy then transferred to the neighbor interstitial Sn2+ ions through the nonradiative electron transition. The Sn2+ ions were then brought into the excited states (p and sp configurations, which have fine structures under the effect of the famous Jahn-Teller (JT) effect and the spin-orbit (SO) interaction23) and returned to the ground state with light emissions. Because the ground state of Sni (s orbit) is a little higher than the valence band, the electrons could directly transit to the s orbit to give the narrow emissions rather than to the valence band to give broad emission. The energy levels and the electronic transitions process are schematically shown in Figure 6.
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Figure 5. (a) The emission spectrum of the microtower after annealing at 1000 °C; (b) the room temperature excitation spectra of the microtower after annealing at 1000 °C monitored at (a) 588 and (b) 440 nm, respectively.
Figure 6. Schematic diagram for the energy levels and electronic transitions of interstitial Sn2+ in SnO2 with the data from the emission spectrum.
The geometric morphology of the microtowers may have great responsibility for the formation of the Sni defect while annealing. Structures with geometric figures and a multicrystal core have been studied by Zhou8 recently, which usually give a crystal growth from the surface to the inner while aging. In our case, as the microtower anneals in the air, the recrystallization started from the near of the surface. The substoichiometric oxide SnOx transformed to SnO2 gradually from surface to core to form a rigid crystallized “cage”, which causes the consumption of the lattice vacancies of the nanostructure.23 As all the native vacancies are consumed, two factors may influence the behavior of the low valence ion (Sn2+): (1) the energy needed for the expansion from SnOx to SnO2 (Ee), including the energy for the oxygen atoms in the air getting through the crystallized SnO2 layer to the amorphous SnOx, oxidizing the low valence Sn2+ to Sn4+, and the energy for the broken microtower due to the transformation from SnOx to SnO2; and (2) the energy needed for the Sn2+ (Es) entering the other lattice vacancies, such as the interstitial site. Two meaningful processes happened while Es < Ea < Ee and Ee < Ea < Es (Ea, the energy provided by the annealing). The former probability results in the entrance of Sn2+ into the lattice vacancies to be the intersitital ion, and the latter results in the breakage of the microtower. Our experiment is just in accordance to the former process, and so, the Sni defect formed in the microtower after 1000 °C annealing. Conclusions In summary, a hydrothermal method toward fabricating the Sni defect in nano SnO2 is proposed, for example, the
tower-like SnO2 microarchitecture, which could be obtained by the followed steps: (1) start the hydrothermal method with tin metal powders in solution with appropriated oxidizing ability to get the metal into solution with different valences and (2) make the soluble compounds co-deposit together to form the nano/microarchitectures with geometric figures; after that, appropriate annealing helps the unstable ion get into the lattice vacancies to form defects. As a result, narrow emissions were found at 588 593, 598, 609, 617, 627, 658, and 706 nm, respectively, indicating the existence of Sni in the microtower of SnO2. The fluorescent kinetic investigation indicates the process of energy transfer from Sn4+ to the neighbor interstitial Sn2+ in the microtower and then gives the narrow emissions. It should be pointed out that due to many elements having different valences, the method used here is expected to have application in many other nano oxide systems for high formation defects. Acknowledgment. This work was financially supported by the Ministry of Science and Technology of China (Grant No. 2005CB623603) and the Director Fund of the Institute of Solid State Physics (084NY11311-6). The authors would like to express their thanks to Prof. Jimei Mu and Prof. Xiangke Wang for technical help. References and Notes (1) Goldman, E. R.; Medintz, I. L.; Whitley, J. L.; Hayhurst, A.; Clapp, A. R.; Uyeda, H. T.; Deschamps, J. R.; Lassman, M. E.; Mattoussi, H. J. Am. Chem. Soc. 2005, 127, 6744. (2) Zeng, H.; Cai, W.; Liu, P.; Xu, X.; Zhou, H.; Klingshim, C.; Kalt, H. ACS Nano 2008, 2, 1661. (3) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.; Somorjai, G. A. Nano Lett. 2007, 7, 3097. (4) Chen, W.; Peng, Q.; Li, Y. AdV. Mater. 2008, 20, 2747. (5) Norris, D. J.; Efros, A. L.; Erwin, S. C. Science 2008, 319, 1776. (6) Trancik, J. E.; Barton, S. C.; Hone, J. Nano Lett. 2008, 8, 982. (7) Zeng, H.; Cai, W.; Hu, J.; Duan, G.; Liu, P.; Li, Y. Appl. Phys. Lett. 2006, 88, 171910. (8) Zhou, W. Z. J. Mater. Chem. 2008, 18, 532. (9) Kolmakov, A.; Klenov, D. O.; Lilach, Y.; Stemmer, S.; Moskovits, M. Nano Lett. 2005, 5, 667. (10) Joshi, R. K.; Kruis, F. E. Appl. Phys. Lett. 2006, 89, 152116. (11) Epifani, M.; Dı´az, R.; Arbiol, J.; Siciliano, P.; Morante, J. R. Chem. Mater. 2006, 18, 840. (12) Wang, G.; Lu, W.; Li, J.; Choi, J.; Jeong, Y.; Choi, S.; Park, J.; Ryu, M.; Lee, K. Small 2006, 2, 1436. (13) Wang, Y.; Lee, J. Y.; Deivaraj, T. C. J. Phys. Chem. B 2004, 108, 13589. (14) Yu, J.; Guo, H.; Davis, S. A.; Mann, S. AdV. Funct. Mater. 2006, 16, 2035. (15) Cheng, B.; Russell, J. M.; Shi, W.; Zhang, L.; Samulski, E. T. J. Am. Chem. Soc. 2004, 126, 5972.
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