Inorganic Chemistry Communications 6 (2003) 882–885 www.elsevier.com/locate/inoche
Luminescent properties of Mn2þ -doped SnO2 nanoparticles Feng Gu a, Shu Fen Wang a, Meng Kai L€ u a,*, Yong Xin Qi b, Guang Jun Zhou c, a Dong Xu , Duo Rong Yuan a a
State Key Laboratory of Crystal Materials, Shandong University, Shanda Nan Road, Jinan 250100, PR China b College of Materials Science and Technology, Shandong University, Jinan 250100, PR China c Shandong Supervision and Inspection Institute for Product Quality, Jinan 250100, PR China Received 28 January 2003; accepted 9 April 2003
Abstract The room temperature photoluminescent properties of manganese-doped tin oxide nanoparticles are reported. The samples are crystalline with a tetragonal rutile structure of tin oxide. The FTIR and the UV–Vis absorptive spectra of the samples have also been investigated. The photoluminescence spectra are measured at room temperature as a function of different calcining temperatures and the Mn2þ concentration, respectively. The luminescence processes are associated with oxygen vacancies in the host and related with the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction The recent intense research interests in nanosized particles of metals and semiconductors have been mainly attributed to the so-called quantum size effect, i.e., the size-tunable materials properties. For semiconductor nanoparticles, this is distinctly reflected in the rather significant band gap structure that is size-sensitive, and might be manifested by the varied luminescence characteristics [1–3]. Of these, tin oxide (SnO2 ) is a unique semiconductor material, with a rather wide band gap (3.6 eV at 300 K), this band gap can be manipulated by the materialÕs dimensions, reaching a larger electronvolts when SnO2 particles in the nanometer regime are formed. Thus, a great deal of research work has been devoted to the method development for the synthesis of SnO2 particles of varied sizes in a controllable manner. Among these, the most common routes involve using sol–gel [4], chemical vapor deposition (CVD) [5], magnetron sputtering [6], and thermal evaporation method [7] and so forth. And the synthesis of SnO2 with onedimensional (1D) nanostructures has also been reported [8,9].
The optical properties of semiconductor nanoparticles have recently been a subject of great interest. Semiconductor nanoparticles have been explored as potential electroluminescent materials, with applications in optoelectronics [10]. One approach to producing strongly luminescent nanoparticles is to introduce small quantities of an emissive dopant. ZnS nanoparticles have been successfully doped with Mn2þ , and Mn2þ emission has been observed following band gap excitation. This emission has been assigned to the 4 T1 –6 A1 transition of the Mn2þ ion [11,12], which is excited by energy transfer from the ZnS electron/hole state. However, with respect to nanocrystalline SnO2 semiconductor, to the best of our knowledge, no letter reported on the luminescent characteristics of Mn2þ -doped SnO2 nanoparticles. Generally, oxygen vacancies are known to be the most common defect and usually act as radiative centers in luminescence processes in poly- and nano-crystalline oxides. Therefore, it is of interest to study the role of Mn2þ ion in the SnO2 host lattice and the contribution to the luminescence. 2. Experimental
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Corresponding author. Tel.: +86-531-856-4591; fax: +86-531-8565403. E-mail address:
[email protected] (M.K. L€ u).
The experiment was followed by a simple sol–gel method. After dissolving tin chloride (hydrous
1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1387-7003(03)00135-7
F. Gu et al. / Inorganic Chemistry Communications 6 (2003) 882–885
SnCl4 5H2 O (A.R.)) in distilled water, manganese chloride solution (MnCl2 , 0.1 M) was added to it, and the concentration of the Mn2þ ions was varied in the range 0–10 at.% in relation to the Sn content. And then, an aqueous ammonia solution (2 M) was added to the above solution dropwise. The dropping rate must be well controlled for the chemical homogeneity. The resulting opal gel were dried at 80 °C for several hours. Heating treatments of the synthesized SnO2 /Mn2þ nanopowders were conducted at 400, 500, and 600 °C for 2 h, respectively. The X-ray diffraction (XRD) patterns of the sample were measured by using a Japan RigaKu D/MAX 2200PC diffractometer with Cu-Ka radiation (k ¼ 0.15418 nm) and graphite monochromator. Transmission electron micrograph (TEM) images were taken with a JEM-100CX transmission electron microscope. The Fourier transform infrared (FTIR) spectra of the samples were collected using a 5DX FTIR spectrometer. The measurement of the absorption spectra of the samples was recorded on a spectrophotometer (U-3500). The excitation and photoluminescence (PL) spectra of the sample were measured with a Hitachi M-850 fluorescence spectrometer.
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lated from XRD line broadening using Debye–Scherrer equation, which reveals that with increasing calcining temperature, the crystallite size gradually increases. The order of crystallite size ranged between 2.6 and 9.0 nm for 400–600 °C. The TEM micrograph of the SnO2 nanoparticles after calcining for 2 h at 400 °C is shown in Fig. 2. It is clearly observed that the as-prepared SnO2 nanoparticles are extra-fine, with an average grain size of 3 nm. This result is similar to that obtained from XRD analysis. Fig. 3 shows the FTIR spectra of the prepared SnO2 samples before and after calcining at 400 °C. As to the sample un-calcined, an intense, very broad IR peak ranging from 3700 to 2500 cm1 , with one maximum at 3135 cm1 , which may be due to absorbed water and NH3 . After calcination, the peak at 3135 cm1 becomes much weaker. The bands centered at 1737 and 1401 cm1 may also be related to water and NH3 . Increasing calcining temperature results in the decrease of the
3. Results and discussion The XRD patterns of the as-prepared SnO2 /Mn2þ (CMn ¼ 1% in this case) samples at three different temperatures (400, 500, and 600 °C, respectively) for 2 h are shown in Fig. 1. All the diffraction lines are assigned to tetragonal rutile crystalline phases of tin oxide. No characteristic peaks of impurities, such as manganese oxide or other tin oxides, were observed. The crystallite size of the prepared powders is calcuFig. 2. TEM micrograph of the as-prepared SnO2 nanoparticles at 400 °C for 2 h.
Fig. 1. XRD pattern for SnO2 /Mn2þ particles calcined at three different temperatures.
Fig. 3. FTIR spectra for SnO2 : (a) pristine sample and (b) calcined sample at 400 °C.
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intensity of the water band, and after heating at 400 °C, the bands attributed NH3 around 3135 and 1401 cm1 disappear. However, we could still observe a weaker peak at 3239 cm1 , which is probably due to the fact that the spectra were not recorded in situ and the readsorption of water from the ambient atmosphere has occurred. Two intense broad bands at 668 and 562 cm1 of the pristine sample are observed. After calcining at 400 °C, the intensity of these two bands increases remarkably. These bands are assigned to the antisymmetric Sn–O–Sn stretching mode of the surface–bridging oxide formed by condensation of adjacent surface hydroxyl groups. The spectra changes can be easily attributed to changes in the size and the shape of the SnO2 particles. Nanosized semiconductor particles generally exhibits a threshold energy in the optical absorption measurements, due to the size-specific band gap structures [1–3], which is reflected by the blue shift of the absorption edge with decreasing particle size. The optical absorption spectrum of the samples calcined at three different temperatures (400, 500, and 600 °C, respectively) are shown in Fig. 4. Considering the blue shift of the absorption positions from the bulk SnO2 , the absorption onsets of the present samples can be assigned to the direct transition of electron in the SnO2 nanocrystals. In Fig. 4, it can also be observed that the absorption spectra have a red shift with increasing the calcining temperature, which is consistent with the effect of quantum confinement when the semiconductor particles are in nanoscaled range. One of the main objectives of the present investigation is to clarify the effect of Mn2þ ions on the luminescence for the SnO2 host. Fig. 5 shows the excitation and emission spectra of SnO2 /Mn2þ (CMn ¼ 3%) samples
Fig. 4. Absorption spectra for the SnO2 nanoparticles at three different temperatures.
Fig. 5. (a) Excitation and emission spectra of SnO2 /Mn2þ calcined at three different temperatures for 2 h: (1) kem ¼ 400 nm and (2) kex ¼ 270 nm. (b) Emission spectrum of pure SnO2 calcined at 400 °C.
calcined at three different temperatures (400, 500, and 600 °C, respectively) and the pure SnO2 sample calcined at 400 °C. The excitation spectrum shows one strong band at 300 nm (kem ¼ 400 nm), which is consistent with the results of absorption spectra. The emission spectrum presents two bands at 400 and 430 nm, respectively. From Fig. 5, it can be observed that the addition of Mn2þ to SnO2 host lattice can result in the increment of PL intensity of SnO2 host, while the characteristic peaks of Mn2þ ions could not be collected, which differs from the behavior of Mn2þ in ZnS host. In pure SnO2 host, the emission attributes to electron transition, mediated by defects levels in the band gap, such as oxygen vacancies, tin interstitials and so forth. Probably after introducing Mn2þ into the SnO2 host, the defects still play a dominant role with respect to the luminescence processes. Generally, oxygen vacancies are known to be the most common defects and usually act as radiative centers in luminescence processes. The oxygen vacancies 2þ present in three different charge states: V0o , Vþ o , and Vo 0 [13] in the oxide. As Vo is a very shallow donor, the most oxygen vacancies will be in their paramagnetic Vþ o state under flat-band conditions. And the origin of luminescence is assigned to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band. The temperature behavior of luminescence spectra of the nanoscaled pure SnO2 powders is similar with those from nanocrystals of TiO2 , ZnO, and BaTiO3 [14–16]. After the addition of Mn2þ to the host lattice, it is easy for Mn2þ ion to substitute for Sn4þ ion, which can be due to the fact that the radius [17]) is similar to that of Mn2þ (0.80 A of Sn4þ (0.76 A [17]) ion. In addition, the 2) charge of the Mn2 ion has Sn to be compensated for somewhere in the lattice in the form of oxygen vacancy. That is the reason why the PL
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located on the surface or on the grain boundaries of the nanopowders. 4. Conclusion Thus, we have been successfully in synthesizing pure SnO2 as well as Mn2þ -doped SnO2 nanoparticles by a simple sol–gel method. Doping gave rise to photoluminescence of SnO2 nanoparticles and was maximum for 3 at.% doping. And calcining temperature can also affect the luminescence process. This luminescence emission can be due to the contribution of oxygen vacancies in the SnO2 host. Fig. 6. Mn2þ concentration dependence of relative luminescence intensity at 400 nm of the samples calcined at 400 °C.
intensity becomes larger after introducing Mn2þ to the SnO2 host. From Fig. 5, it can be observed that as the temperature increases, a sharp decrease in PL intensity occurs. As the temperature increases, the grain size of SnO2 /Mn2þ nanocrystal becomes larger, which leads to the decrease of the number of the luminescence centers due to reductions on both the ratio surface area and concentration of oxygen vacancies. Thus decrease in PL intensity can occur. In addition to study the effect of the calcining temperature on the luminescence of the SnO2 nanocrystals, the effect of Mn2þ concentration was also investigated (shown in Fig. 6). It was found that with increasing Mn2þ concentration from 1% to 3%, the PL intensity rose rapidly and reached a maximum. At higher Mn2þ concentrations, the intensity of the 400 nm peak started to decrease slowly. The similar quenching effect with the Mn2þ concentration has been reported previously and it has been a subject of exhaustive studies. Although the exact mechanism for this quenching to occur is still a matter of controversy, it is generally considered that the quenching of the luminescence is associated with interaction among Mn2þ ions at the nearest, the second nearest, and probably even at the third nearest neighbor sites [18], forming small clusters, such as pairs or triplets,
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