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Optical properties of surface-modified Bi2O3 nanoparticles
Journal of Physics and Chemistry of Solids 64 (2003) 265–271 www.elsevier.com/locate/jpcs
Optical properties of surface-modified Bi2O3 nanoparticles Wenting Dong*, Congshan Zhu Photon Craft Project Lab, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, People’s Republic of China
Abstract Stearic acid coated Bi2O3 nanoparticles in the size range of 5 – 13 nm were synthesized by the microemulsion method. HRTEM showed that the morphology of Bi2O3 nanoparticles was ellipsoidal. The absorption edge of Bi2O3 nanoparticles showed a blue shift of , 0.45 eV, comparing with that of the bulk Bi2O3. At room temperature, Bi2O3 nanoparticles also showed a strong luminescence at 397 and 420 nm, depending on the excitation wavelength. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Nanoprticles; Surface-modification; D. Luminescence
1. Introduction Semiconductor nanoparticles has been of great interest in recent years because of their unique optical and electrical properties which may be widely used in the area of optoelectronics [1,2]. A large number of investigations on II– VI semiconductor nanoparticles such as CdS, PbS and CdSe have been reported [3– 5] while only a few papers on metal oxide nanoparticles have been published [6 – 8]. Therefore, the knowledge in the area of metal oxide nanoparticles is not as mature as that of II– VI sulfides and selenides. In addition, the practical potonics applications of these nanocrystals are still lacking due to the fact that the surface-related nonradiative recombination dominates in the strong confinement limit [9]. Bulk Bi2O3 is one excellent opto-electronic material [10 – 12] and it is also one of the effective component of third-order nonlinear optical glasses. It was reported that the nonlinear susceptibility of Bi2O3 nanoparticles was 100 times larger than that of the bulk Bi2O3 [13]. However, the microstructure of Bi2O3 nanoparticles and their luminescent properties were seldom reported. In order to eliminate the nonradiative contribution from the surface states, we passivated the Bi2O3 nanoparticles by coating the particles with surfactant. In this paper, we synthesized surfacemodified Bi2O3 nanoparticles by the microemulsion method * Corresponding author. Present Address: Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany. Tel.: þ49-208-3062403; fax: þ 49-208-3062995. E-mail address: [email protected] (W.T. Dong).
and investigated the luminescent properties of the Bi2O3 nanoparticles.
2. Experimental All chemicals used in our experiment are Analytical Reagent and were used as received. The surface-modificated Bi2O3 nanoparticles were prepared by microemulsion method [13]. The preparation procedure is briefly described in Fig. 1. In our preparation, the molar ratio of Bi and St was kept as 1:1.7. Transparent xylene sols of Bi2O3 nanoparticles with different particle sizes were obtained by tuning the concentration of Bi3þ aqueous solutions. In this paper, we will report optical properties of two samples: one is the orange xylene sol of Bi2O3 nanoparticles prepared with 0.005 M Bi3þ aqueous solution, and another is the wine-red prepared with 0.01 M Bi3þ aqueous solution. The theoretical concentration of both Bi2O3 nanoparticles in xylene sols is 0.0059 M. The size of the Bi2O3 nanoparticles was observed in a transmission electron microscope (TEM, JEOL-JEM-2010). Optical absorption spectra of the sol were recorded on an ultraviolet– visible (UV – vis) spectrophotometer (PerkinElmer Lambda 9UV/Vis/Vir) between wavelength of 300 and 800 nm. Double beam scanning method was applied to measure the absorption spectra and xylene solution of St was used as a reference. Luminescence spectra were detected on the luminescence spectrometer (Perkin Elmer LS50B, light source: Xe lamp) at room temperature. The spectral resolution was 5 nm. The optical length of the
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Fig. 1. Schematic diagram of preparation of xylene sol of St coated Bi2O3 nanoparticles (St is Stearic acid).
quartz cuvette we used in the absorption and luminescent measurements is 1 cm.
3. Results and discussion 3.1. TEM results Figs. 2 and 3 show the TEM pictures of the orange and wine-red Bi2O3 nanoparticles, respectively. It can be seen from Figs. 2(a) and 3(a) that the diffraction patterns of orange and red a-Bi2O3 nanoparticles showed diffuse and faint diffraction rings, indicating the poor crystallinity of the particles. Therefore we only calculated the lattice distances by using the rings near the center of the diffraction pattern. In this way, we could get more exact lattice distances at the lower diffraction angles. The lattice distances calculated ˚ from the diffraction rings in Figs. 2(a) and 3(a) are 3.39 A ˚ (100), 2.72 A ˚ (1¯22, 121), and 2.09 A ˚ (122), (111), 3.25 A etc. These distances coincide with those of the XRD data in
JCPDS No. 41-1449, which is the standard XRD data of bulk a-Bi2O3. Therefore the Bi2O3 nanoparticles was identified to be a-Bi2O3, which belongs to monoclinic system with P21 =c space group. It can be found from Figs. 2(b), 3(b) and (c) that both Bi2O3 particles are not monodisperse and the particles’ shape is ellipsoidal. The long-axial length of the orange and wine-red Bi2O3 nanoparticles are about 5 – 8 and 6 –10 nm, respectively. Fig. 2(c) shows one single Bi2O3 nanoparticle with longaxial length of , 13 nm. The width of the lattice fringes is ˚ , which is a little bit smaller than the determined to be 3.12 A lattice distance of the strongest XRD peak for bulk Bi2O3 ˚ . This difference arises from the experi(120), i.e. 3.26 A mental error, which makes the assignment of the final lattice planes uncertain. Fig. 4 shows the bright field HRTEM view of another big Bi2O3 nanoparticles. It can be seen that the largest length of the Bi2O3 nanoparticle is ,25 nm and the lattice distance is ˚ , which might correspond to the crystal plane (120). 3.15 A One circle was marked at the bottom of the picture, which
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Fig. 2. (a) Electronic diffraction pattern of orange Bi2O3 nanoparticles; (b) TEM view of orange Bi2O3 nanoparticles.
indicates a small particle with size of 8 nm. A rectangle was also marked at the bottom of the big particle, indicating that edge dislocations formed at the interface of the two particles. The other rectangles marked on the big particle also indicate some defects, which we are not sure if they are spiral-dislocation. The St coated Bi2O3 nanoparticles are very stable. No sedimentation or aggregation of the Bi2O3 nanoparticles was observed in the xylene sol after they were kept at room temperature for 2 years. The stability of the Bi2O3 nanoparticles arises from surfactant St, which can prevent Bi2O3 nanoparticles from coagulating. 3.2. Absorption spectra Fig. 5(a) shows the UV – vis absorption spectra of the xylene sol of Bi2O3 nanoparticles. The bulk Bi2O3 is a direct semiconductor with band gap of 2.85 eV [14 – 16]. The direct absorption band gap of the Bi2O3 nanoparticles can be determined by fitting the absorption data to the following
equation [17]
ahn ¼ Bðhn 2 Eg Þ1=2
ð1Þ
in which hn is the photon energy, a is the absorption coefficient, Eg is the absorption band gap and B is a constant relative to the material. Therefore Eg value of the coated Bi2O3 nanoparticles can be determined by the extrapolation of Eq. (1). The Eg of the orange and wine-red Bi2O3 nanoparticles were determined to be , 3.33 and , 3.28 eV, respectively, as can be seen from the curve of ðahnÞ2 , hn in Fig. 5(b). The absorption coefficient can be obtained by the following equation [2,7] 1 I 1 a ¼ 2 ln t ¼ t I0 t
It 1 A I0 ¼ t log e log e
2log
ð2Þ
where t is the thickness of the cuvette, It and I0 are the intensities of transmitted light and incident light, respectively, and A is the absorbance, which can be obtained from the absorption spectra [2,17,18]. Therefore the blue shift of
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the absorption edge of the orange and wine-red Bi2O3 nanoparticles are 0.48 and 0.43 eV, respectively. The blue shift of absorption edge of Bi2O3 nanoparticles can be explained by the effective-mass approximation (EMA) model, which was developed by Brus, Bawendi and Kayanuma [2]. They derived a useful expression which can give energy EðRÞ for the lowest 1 s excited state as a function of cluster radius (R ) of nanoparticles, shown as follows ! "2 p2 1 1 1:786e2 EðRÞ ¼ Eg þ 2 þ 0:248ER þ p p 2 me mh 12 R 2R ð3Þ in which mpe and mph are the electron and hole effective mass, e 2 is the dielectric constant of the microcrystallite, and the ER is the bulk exciton Rydberg energy. The second term is the quantum confinement ‘localization’ for the electrons and holes, which leads to the blue shift; while the third term is the Coulomb term and it leads to red-shift. The fourth term gives the spatial correlation energy which is small and of minor importance. The bulk exciton Bohr radius of Bi2O3 is not available and therefore we cannot decide which confinement region our Bi2O3 nanoparticles belong to, i.e. weak, middle or strong confinement region. But from the blue shift results, we could estimate that the quantum confinement effect of our Bi2O3 nanoparticles should be larger than the Coulomb effect. Further work on theoretical calculations of the energy levels of Bi2O3 nanoparticles should be done in the near future. It can be seen that there is another small absorption peak centered at 484 nm (2.56 eV) besides the UV absorption edge for both Bi2O3 nanoparticles, which probably attributes to exciton absorption. Previous publications about II– VI or III – V semiconductor nanoparticles indicate that this shoulder exciton peak is associated with the size distribution properties of the nanopartices [19– 21]. It is reported that nanoparticles with a size distribution of 5% did show pronounced exciton transitions. However, in samples with a size distribution of 10%, the exciton transitions are broadened. We used a half of full-width-at-half-maximum (FWHM) of this shoulder peak to judge the peak width. For our two Bi2O3 nanoparticles, the half FWHM are 60 – 70 nm, which are broader than those of the monodispersed II– VI nanoparticles [20,21], i.e. 5 – 40 nm. The broad peaks indicate that the size distribution of both Bi2O3 nanoparticles are larger than 10%. On the other hand, the shoulder peak for orange Bi2O3 nanoparticles is less pronounced than that of the wine-red Bi2O3 nanoparticles, suggesting a broader size distribution for orange Bi2O3 nanoparticles than that of wine-red Bi2O3 nanoparticles. Fig. 3. (a) Electronic diffraction pattern of wine-red Bi2O3 nanoparticles; (b) TEM view of wine-red Bi2O3 nanoparticles; (c) HRTEM view of wine-red Bi2O3 nanoparticles.
3.3. Luminescence spectra Figs. 6 and 7 show the photoluminescence (PL) spectra of Bi2O3 nanoparticles at room temperature and the fluorescence characteristics were summarized in Table 1. The luminescence experiment of powder Bi2O3 (Analytical
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Fig. 4. HRTEM view of wine-red Bi2O3 nanoprticles (sphere or ellipsoid: Bi2O3 nanoparticles; rectangle: defect).
Fig. 5. (a) Absorption spectrum of xylene sol of St coated Bi2O3 nanoparticles; (b) curve of ðahnÞ2 , hn of xylene sol of St coated Bi2O3 nanoparticles.
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Fig. 6. Excitation and emission spectra of xylene sol of orange Bi2O3 nanoparticles: (1) excitation spectrum detected with emission wavelength of 420 nm; (2) excitation spectra detected with emission wavelength of 397 nm; (3) emission spectrum excited at 367 nm; (4) emission spectra excited at 387 nm.
Reagent grade) at room temperature was also made and no luminescence was detected. The luminescence of xylene solvent was also measured, which are very weak comparing with that of the Bi2O3 nanoparticles. There are two main reasons for the luminescence of Bi2O3 nanoparticles: the quantum size effect and structural defects in the crystals. It can be seen from Table 1 that the emission peaks of both orange and wine-red Bi2O3 nanoparticles are at 397 nm (3.12 eV) and 420 nm (2.95 eV), depending on different excitation wavelength. The emission peaks at 397 nm for the orange and wine-red Bi2O3 nanoparticles show stoke shifts of 0.21 and 0.16 eV, respectively, which are very near to those of the band-edge emission or near band-edge emission (shallow trap emission) of reported II – VI nanoparticles [22,23]. Therefore, the emission peaks at 397 nm for both samples can be attributed to the band-edge emission or near band-edge emission. As we know, the band-
Fig. 7. Excitation and emission spectra of xylene sol of wine-red Bi2O3 nanoparticles: (1) excitation spectrum detected with emission wavelength of 398 nm; (2) excitation spectrum detected with emission wavelength of 420 nm; (3) emission spectrum excited at 367 nm; (4) emission spectra excited at 387 nm.
edge emission or near band-edge emission is size-dependent (quantum size effect) [2]. Therefore this attribution seems to contradict the fact that both samples with different size ranges show same emission peaks at 397 nm. However, just as we discussed earlier (absorption spectra), the prepared Bi2O3 nanoparticles are not monodisperse and have size distributions. The emission spectra will reflect the emission properties of Bi2O3 nanoparticles of a certain size with largest population. Therefore we could estimate that both Bi2O3 nanoparticles have nearly the same particle size range with largest populations, i.e. within the range of 6–8 nm. For the emission peaks at 420 nm, orange and wine-red Bi2O3 nanoparticles show stoke shifts of 0.38 and 0.33 eV, respectively. This emission peak can be attributed to the recombination from the conduction band to the energy levels of deep-trap or surface state, which normally appears at a longer wavelength and are less size dependent [24]. The deeptrap and surface state originates from the crystal defects inside
Table 1 Data of fluorescence spectra of orange and wine-red Bi2O3 nanoparticles in xylene Sample
Xylene sol of orange Bi2O3 nanoparticles Xylene sol of wine-red Bi2O3 nanoparticles a b c d e f
Excitation spectra
Emission spectra
Curve
lema (nm)
lexb (nm)
FWHMc (nm)
Curve
lexd (nm)
leme (nm)
FWHM (nm)
(1) (2) (1) (2)
420 397 398 420
367 367 314, 367 315, 369, 387(sh)
52 48 79 40
(3) (4) (3) (4)
367 387 367 387
397, 414(shf) 420, 437(sh) 397 420
65 80 63 71
Detected emission wavelength for the excitation spectra. Position of excitation maximum. Full-width-at-half-maximum. Excitation wavelength for the emission spectra. Position of emission maximum. Shoulder peak.
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or at the surface of the crystallites. From Table 1, it can be found that the FWHM of the emission peaks at 420 nm for orange and wine-red Bi2O3 nanoparticles (80 and 71 nm, respectively) are larger than those of the peak at 387 nm (65 and 63 nm, respectively). The effect contributes to this relatively broad width of the emission at longer wavelength is the broad distribution of intra-band states. The energy-levels of Bi2O3 nanoparticles can also be deduced from the excitation spectra. The excitation peaks at 367 nm for the orange and wine-red Bi2O3 nanoparticles are very close to 376 nm (3.3 eV), which is the direct band gap of the Bi2O3 nanoparticles determined by the absorption spectra. It was noticed that the shapes of the absorption spectra and excitation spectra of the Bi2O3 nanoparticles are different. This difference was also shown for the reported II–VI semiconductors [25,26]. The reasons should be investigated further.
4. Conclusion Stearic acid coated Bi2O3 nanoparticles has been prepared by the microemulsion method. The sizes of the Bi2O3 nanoparticles are in the range of 5 – 13 nm. The absorption spectra of Bi2O3 nanoparticles show a blue shift of ,0.45 eV, comparing with that of the bulk Bi2O3. Both Bi2O3 nanoparticles with orange and wine-red color show emission peaks at 397 and 420 nm at room temperature, which are dependent on the excitation wavelength.
Acknowledgments One of the authors, W.T. Dong, would like to thank Dr Qinghua Zeng, for his helpful suggestions and discussions during the revision of the paper.
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Optical properties of surface-modified Bi2O3 nanoparticles Wenting Dong*, Congshan Zhu Photon Craft Project Lab, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P.O. Box 800-211, Shanghai 201800, People’s Republic of China
Abstract Stearic acid coated Bi2O3 nanoparticles in the size range of 5 – 13 nm were synthesized by the microemulsion method. HRTEM showed that the morphology of Bi2O3 nanoparticles was ellipsoidal. The absorption edge of Bi2O3 nanoparticles showed a blue shift of , 0.45 eV, comparing with that of the bulk Bi2O3. At room temperature, Bi2O3 nanoparticles also showed a strong luminescence at 397 and 420 nm, depending on the excitation wavelength. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Nanoprticles; Surface-modification; D. Luminescence
1. Introduction Semiconductor nanoparticles has been of great interest in recent years because of their unique optical and electrical properties which may be widely used in the area of optoelectronics [1,2]. A large number of investigations on II– VI semiconductor nanoparticles such as CdS, PbS and CdSe have been reported [3– 5] while only a few papers on metal oxide nanoparticles have been published [6 – 8]. Therefore, the knowledge in the area of metal oxide nanoparticles is not as mature as that of II– VI sulfides and selenides. In addition, the practical potonics applications of these nanocrystals are still lacking due to the fact that the surface-related nonradiative recombination dominates in the strong confinement limit [9]. Bulk Bi2O3 is one excellent opto-electronic material [10 – 12] and it is also one of the effective component of third-order nonlinear optical glasses. It was reported that the nonlinear susceptibility of Bi2O3 nanoparticles was 100 times larger than that of the bulk Bi2O3 [13]. However, the microstructure of Bi2O3 nanoparticles and their luminescent properties were seldom reported. In order to eliminate the nonradiative contribution from the surface states, we passivated the Bi2O3 nanoparticles by coating the particles with surfactant. In this paper, we synthesized surfacemodified Bi2O3 nanoparticles by the microemulsion method * Corresponding author. Present Address: Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany. Tel.: þ49-208-3062403; fax: þ 49-208-3062995. E-mail address: [email protected] (W.T. Dong).
and investigated the luminescent properties of the Bi2O3 nanoparticles.
2. Experimental All chemicals used in our experiment are Analytical Reagent and were used as received. The surface-modificated Bi2O3 nanoparticles were prepared by microemulsion method [13]. The preparation procedure is briefly described in Fig. 1. In our preparation, the molar ratio of Bi and St was kept as 1:1.7. Transparent xylene sols of Bi2O3 nanoparticles with different particle sizes were obtained by tuning the concentration of Bi3þ aqueous solutions. In this paper, we will report optical properties of two samples: one is the orange xylene sol of Bi2O3 nanoparticles prepared with 0.005 M Bi3þ aqueous solution, and another is the wine-red prepared with 0.01 M Bi3þ aqueous solution. The theoretical concentration of both Bi2O3 nanoparticles in xylene sols is 0.0059 M. The size of the Bi2O3 nanoparticles was observed in a transmission electron microscope (TEM, JEOL-JEM-2010). Optical absorption spectra of the sol were recorded on an ultraviolet– visible (UV – vis) spectrophotometer (PerkinElmer Lambda 9UV/Vis/Vir) between wavelength of 300 and 800 nm. Double beam scanning method was applied to measure the absorption spectra and xylene solution of St was used as a reference. Luminescence spectra were detected on the luminescence spectrometer (Perkin Elmer LS50B, light source: Xe lamp) at room temperature. The spectral resolution was 5 nm. The optical length of the
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Fig. 1. Schematic diagram of preparation of xylene sol of St coated Bi2O3 nanoparticles (St is Stearic acid).
quartz cuvette we used in the absorption and luminescent measurements is 1 cm.
3. Results and discussion 3.1. TEM results Figs. 2 and 3 show the TEM pictures of the orange and wine-red Bi2O3 nanoparticles, respectively. It can be seen from Figs. 2(a) and 3(a) that the diffraction patterns of orange and red a-Bi2O3 nanoparticles showed diffuse and faint diffraction rings, indicating the poor crystallinity of the particles. Therefore we only calculated the lattice distances by using the rings near the center of the diffraction pattern. In this way, we could get more exact lattice distances at the lower diffraction angles. The lattice distances calculated ˚ from the diffraction rings in Figs. 2(a) and 3(a) are 3.39 A ˚ (100), 2.72 A ˚ (1¯22, 121), and 2.09 A ˚ (122), (111), 3.25 A etc. These distances coincide with those of the XRD data in
JCPDS No. 41-1449, which is the standard XRD data of bulk a-Bi2O3. Therefore the Bi2O3 nanoparticles was identified to be a-Bi2O3, which belongs to monoclinic system with P21 =c space group. It can be found from Figs. 2(b), 3(b) and (c) that both Bi2O3 particles are not monodisperse and the particles’ shape is ellipsoidal. The long-axial length of the orange and wine-red Bi2O3 nanoparticles are about 5 – 8 and 6 –10 nm, respectively. Fig. 2(c) shows one single Bi2O3 nanoparticle with longaxial length of , 13 nm. The width of the lattice fringes is ˚ , which is a little bit smaller than the determined to be 3.12 A lattice distance of the strongest XRD peak for bulk Bi2O3 ˚ . This difference arises from the experi(120), i.e. 3.26 A mental error, which makes the assignment of the final lattice planes uncertain. Fig. 4 shows the bright field HRTEM view of another big Bi2O3 nanoparticles. It can be seen that the largest length of the Bi2O3 nanoparticle is ,25 nm and the lattice distance is ˚ , which might correspond to the crystal plane (120). 3.15 A One circle was marked at the bottom of the picture, which
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Fig. 2. (a) Electronic diffraction pattern of orange Bi2O3 nanoparticles; (b) TEM view of orange Bi2O3 nanoparticles.
indicates a small particle with size of 8 nm. A rectangle was also marked at the bottom of the big particle, indicating that edge dislocations formed at the interface of the two particles. The other rectangles marked on the big particle also indicate some defects, which we are not sure if they are spiral-dislocation. The St coated Bi2O3 nanoparticles are very stable. No sedimentation or aggregation of the Bi2O3 nanoparticles was observed in the xylene sol after they were kept at room temperature for 2 years. The stability of the Bi2O3 nanoparticles arises from surfactant St, which can prevent Bi2O3 nanoparticles from coagulating. 3.2. Absorption spectra Fig. 5(a) shows the UV – vis absorption spectra of the xylene sol of Bi2O3 nanoparticles. The bulk Bi2O3 is a direct semiconductor with band gap of 2.85 eV [14 – 16]. The direct absorption band gap of the Bi2O3 nanoparticles can be determined by fitting the absorption data to the following
equation [17]
ahn ¼ Bðhn 2 Eg Þ1=2
ð1Þ
in which hn is the photon energy, a is the absorption coefficient, Eg is the absorption band gap and B is a constant relative to the material. Therefore Eg value of the coated Bi2O3 nanoparticles can be determined by the extrapolation of Eq. (1). The Eg of the orange and wine-red Bi2O3 nanoparticles were determined to be , 3.33 and , 3.28 eV, respectively, as can be seen from the curve of ðahnÞ2 , hn in Fig. 5(b). The absorption coefficient can be obtained by the following equation [2,7] 1 I 1 a ¼ 2 ln t ¼ t I0 t
It 1 A I0 ¼ t log e log e
2log
ð2Þ
where t is the thickness of the cuvette, It and I0 are the intensities of transmitted light and incident light, respectively, and A is the absorbance, which can be obtained from the absorption spectra [2,17,18]. Therefore the blue shift of
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the absorption edge of the orange and wine-red Bi2O3 nanoparticles are 0.48 and 0.43 eV, respectively. The blue shift of absorption edge of Bi2O3 nanoparticles can be explained by the effective-mass approximation (EMA) model, which was developed by Brus, Bawendi and Kayanuma [2]. They derived a useful expression which can give energy EðRÞ for the lowest 1 s excited state as a function of cluster radius (R ) of nanoparticles, shown as follows ! "2 p2 1 1 1:786e2 EðRÞ ¼ Eg þ 2 þ 0:248ER þ p p 2 me mh 12 R 2R ð3Þ in which mpe and mph are the electron and hole effective mass, e 2 is the dielectric constant of the microcrystallite, and the ER is the bulk exciton Rydberg energy. The second term is the quantum confinement ‘localization’ for the electrons and holes, which leads to the blue shift; while the third term is the Coulomb term and it leads to red-shift. The fourth term gives the spatial correlation energy which is small and of minor importance. The bulk exciton Bohr radius of Bi2O3 is not available and therefore we cannot decide which confinement region our Bi2O3 nanoparticles belong to, i.e. weak, middle or strong confinement region. But from the blue shift results, we could estimate that the quantum confinement effect of our Bi2O3 nanoparticles should be larger than the Coulomb effect. Further work on theoretical calculations of the energy levels of Bi2O3 nanoparticles should be done in the near future. It can be seen that there is another small absorption peak centered at 484 nm (2.56 eV) besides the UV absorption edge for both Bi2O3 nanoparticles, which probably attributes to exciton absorption. Previous publications about II– VI or III – V semiconductor nanoparticles indicate that this shoulder exciton peak is associated with the size distribution properties of the nanopartices [19– 21]. It is reported that nanoparticles with a size distribution of 5% did show pronounced exciton transitions. However, in samples with a size distribution of 10%, the exciton transitions are broadened. We used a half of full-width-at-half-maximum (FWHM) of this shoulder peak to judge the peak width. For our two Bi2O3 nanoparticles, the half FWHM are 60 – 70 nm, which are broader than those of the monodispersed II– VI nanoparticles [20,21], i.e. 5 – 40 nm. The broad peaks indicate that the size distribution of both Bi2O3 nanoparticles are larger than 10%. On the other hand, the shoulder peak for orange Bi2O3 nanoparticles is less pronounced than that of the wine-red Bi2O3 nanoparticles, suggesting a broader size distribution for orange Bi2O3 nanoparticles than that of wine-red Bi2O3 nanoparticles. Fig. 3. (a) Electronic diffraction pattern of wine-red Bi2O3 nanoparticles; (b) TEM view of wine-red Bi2O3 nanoparticles; (c) HRTEM view of wine-red Bi2O3 nanoparticles.
3.3. Luminescence spectra Figs. 6 and 7 show the photoluminescence (PL) spectra of Bi2O3 nanoparticles at room temperature and the fluorescence characteristics were summarized in Table 1. The luminescence experiment of powder Bi2O3 (Analytical
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Fig. 4. HRTEM view of wine-red Bi2O3 nanoprticles (sphere or ellipsoid: Bi2O3 nanoparticles; rectangle: defect).
Fig. 5. (a) Absorption spectrum of xylene sol of St coated Bi2O3 nanoparticles; (b) curve of ðahnÞ2 , hn of xylene sol of St coated Bi2O3 nanoparticles.
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Fig. 6. Excitation and emission spectra of xylene sol of orange Bi2O3 nanoparticles: (1) excitation spectrum detected with emission wavelength of 420 nm; (2) excitation spectra detected with emission wavelength of 397 nm; (3) emission spectrum excited at 367 nm; (4) emission spectra excited at 387 nm.
Reagent grade) at room temperature was also made and no luminescence was detected. The luminescence of xylene solvent was also measured, which are very weak comparing with that of the Bi2O3 nanoparticles. There are two main reasons for the luminescence of Bi2O3 nanoparticles: the quantum size effect and structural defects in the crystals. It can be seen from Table 1 that the emission peaks of both orange and wine-red Bi2O3 nanoparticles are at 397 nm (3.12 eV) and 420 nm (2.95 eV), depending on different excitation wavelength. The emission peaks at 397 nm for the orange and wine-red Bi2O3 nanoparticles show stoke shifts of 0.21 and 0.16 eV, respectively, which are very near to those of the band-edge emission or near band-edge emission (shallow trap emission) of reported II – VI nanoparticles [22,23]. Therefore, the emission peaks at 397 nm for both samples can be attributed to the band-edge emission or near band-edge emission. As we know, the band-
Fig. 7. Excitation and emission spectra of xylene sol of wine-red Bi2O3 nanoparticles: (1) excitation spectrum detected with emission wavelength of 398 nm; (2) excitation spectrum detected with emission wavelength of 420 nm; (3) emission spectrum excited at 367 nm; (4) emission spectra excited at 387 nm.
edge emission or near band-edge emission is size-dependent (quantum size effect) [2]. Therefore this attribution seems to contradict the fact that both samples with different size ranges show same emission peaks at 397 nm. However, just as we discussed earlier (absorption spectra), the prepared Bi2O3 nanoparticles are not monodisperse and have size distributions. The emission spectra will reflect the emission properties of Bi2O3 nanoparticles of a certain size with largest population. Therefore we could estimate that both Bi2O3 nanoparticles have nearly the same particle size range with largest populations, i.e. within the range of 6–8 nm. For the emission peaks at 420 nm, orange and wine-red Bi2O3 nanoparticles show stoke shifts of 0.38 and 0.33 eV, respectively. This emission peak can be attributed to the recombination from the conduction band to the energy levels of deep-trap or surface state, which normally appears at a longer wavelength and are less size dependent [24]. The deeptrap and surface state originates from the crystal defects inside
Table 1 Data of fluorescence spectra of orange and wine-red Bi2O3 nanoparticles in xylene Sample
Xylene sol of orange Bi2O3 nanoparticles Xylene sol of wine-red Bi2O3 nanoparticles a b c d e f
Excitation spectra
Emission spectra
Curve
lema (nm)
lexb (nm)
FWHMc (nm)
Curve
lexd (nm)
leme (nm)
FWHM (nm)
(1) (2) (1) (2)
420 397 398 420
367 367 314, 367 315, 369, 387(sh)
52 48 79 40
(3) (4) (3) (4)
367 387 367 387
397, 414(shf) 420, 437(sh) 397 420
65 80 63 71
Detected emission wavelength for the excitation spectra. Position of excitation maximum. Full-width-at-half-maximum. Excitation wavelength for the emission spectra. Position of emission maximum. Shoulder peak.
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or at the surface of the crystallites. From Table 1, it can be found that the FWHM of the emission peaks at 420 nm for orange and wine-red Bi2O3 nanoparticles (80 and 71 nm, respectively) are larger than those of the peak at 387 nm (65 and 63 nm, respectively). The effect contributes to this relatively broad width of the emission at longer wavelength is the broad distribution of intra-band states. The energy-levels of Bi2O3 nanoparticles can also be deduced from the excitation spectra. The excitation peaks at 367 nm for the orange and wine-red Bi2O3 nanoparticles are very close to 376 nm (3.3 eV), which is the direct band gap of the Bi2O3 nanoparticles determined by the absorption spectra. It was noticed that the shapes of the absorption spectra and excitation spectra of the Bi2O3 nanoparticles are different. This difference was also shown for the reported II–VI semiconductors [25,26]. The reasons should be investigated further.
4. Conclusion Stearic acid coated Bi2O3 nanoparticles has been prepared by the microemulsion method. The sizes of the Bi2O3 nanoparticles are in the range of 5 – 13 nm. The absorption spectra of Bi2O3 nanoparticles show a blue shift of ,0.45 eV, comparing with that of the bulk Bi2O3. Both Bi2O3 nanoparticles with orange and wine-red color show emission peaks at 397 and 420 nm at room temperature, which are dependent on the excitation wavelength.
Acknowledgments One of the authors, W.T. Dong, would like to thank Dr Qinghua Zeng, for his helpful suggestions and discussions during the revision of the paper.
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